Pyroelectric Materials: Physics and Applications 3527351019, 9783527351015

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Pyroelectric Materials

Pyroelectric Materials Physics and Applications

Ashim Kumar Bain Prem Chand

Authors Prof. Ashim Kumar Bain

University of Birmingham Electronic, Electrical & Systems Engin. B15 2TT Edgbaston United Kingdom

All books published by WILEY-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Prof. Prem Chand

IIT Kanpur Department of Physics 208016 Kanpur India Cover Image: © Jose A. Bernat

Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

Bacete/Getty Images Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2023 WILEY-VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-35101-5 ePDF ISBN: 978-3-527-83972-8 ePub ISBN: 978-3-527-83973-5 oBook ISBN: 978-3-527-83974-2 Typesetting

Straive, Chennai, India

v

Contents Preface ix 1 1.1 1.1.1 1.1.2 1.1.2.1 1.1.2.2 1.1.2.3 1.1.2.4 1.1.3 1.1.4 1.1.5 1.1.6 1.1.7 1.1.8 1.1.8.1 1.1.8.2 1.1.9 1.1.9.1 1.1.9.2 1.1.10

2 2.1 2.2 2.3 2.4 2.5 2.6

Fundamentals of Dielectrics 1 Dielectrics 1 Polarization of Dielectrics 2 Dispersion of Dielectric Polarization 3 Electronic Polarization 3 Ionic Polarization 4 Orientation Polarization 4 Space Charge Polarization 4 Dielectric Relaxation 5 Debye Relaxation 6 Molecular Theory of Induced Charges in a Dielectric 6 Capacitance of a Parallel Plate Capacitor 7 Electric Displacement Field, Dielectric Constant, and Electric Susceptibility 9 Local Field in a Dielectric 10 Lorentz Field, E2 11 Field of Dipoles Inside Cavity, E3 11 Dielectrics Losses 12 Dielectric Loss Angle 13 Total and Specific Dielectric Losses 14 Dielectrics Breakdown 15 References 16 Pyroelectricity 19 Introduction 19 History of Pyroelectricity 21 Theory of Pyroelectricity 32 Simple Model of Pyroelectric Effect 33 Pyroelectric Crystal Symmetry 36 Piezoelectricity 37

vi

Contents

2.7 2.7.1 2.7.2 2.7.3 2.7.4 2.7.4.1 2.7.4.2

Ferroelectricity 39 Ferroelectric Phase Transitions 40 Ferroelectric Domains 42 Ferroelectric Domain Wall Motion 42 Soft Mode 45 Zone-center Phonons 46 Zone-boundary Phonons 46 References 47

3 3.1 3.2 3.3 3.4 3.4.1 3.4.1.1 3.4.1.2 3.4.1.3 3.4.1.4 3.4.2 3.4.2.1 3.4.2.2 3.4.3 3.4.4 3.4.5 3.4.6 3.4.6.1 3.4.6.2 3.4.6.3

Pyroelectric Materials and Applications 55 Introduction 55 Theory of Pyroelectric Detectors 57 Material Figure-of-Merits 62 Classification of Pyroelectric Materials 62 Single Crystals 63 Triglycine Sulphate 63 Lithium Tantalate (LT) and Lithium Niobate (LN) Barium Strontium Titanate (BST) 72 Strontium Barium Niobite (SBN) 75 Perovskite Ceramics 77 Modified Lead Zirconate (PZ) 78 Modified Lead Titanate (PT) 85 Organic Polymers 87 Ceramic-Polymer Composites 90 Lead-Free Ceramics 96 Other Pyroelectric Materials 97 Aluminum Nitride (AlN) 98 Gallium Nitride (GaN) 102 Zinc Oxide (ZnO) 103 References 108

4 4.1 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.4.1 4.2.4.2 4.2.4.3 4.2.5 4.2.5.1 4.2.5.2 4.2.5.3

Pyroelectric Infrared Detector 119 Introduction 119 Device Configurations 120 Thick Film Detectors 120 Thin Film Detectors 123 Hybrid Focal Plane Array Detector 126 Linear Array Detector 127 Detector Chip Technology 127 Detector Assembly 129 Camera System 129 Periodic Domain TFLTTM Detector 131 TFLTTM Pyroelectric Detector Fabrication 132 TFLTTM Attached to Metalized Silicon 134 TFLTTM on Ceramic 135

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Contents

4.2.5.4 4.2.5.5 4.2.6 4.2.7 4.2.7.1 4.2.7.2 4.2.7.3 4.2.8 4.2.9 4.2.9.1 4.2.9.2 4.2.10 4.2.10.1 4.2.10.2 4.2.11 4.2.11.1 4.2.11.2 4.2.11.3 4.2.11.4 4.2.12 4.2.12.1 4.2.12.2 4.2.13 4.2.13.1 4.2.13.2

Large Aperture Devices 136 Domain Engineered TFLTTM Device 137 Terahertz Thermal Detector 139 PVDF Polymer Detector 140 Self-absorbing Layer Structure 140 PVDF Pyroelectric Sensor Assembly 141 Sensor Array Specification and Performance 142 TFP Polymer Detector 143 Tetraaminodiphenyl (TADPh) Polymer Detector 146 Detector Design 147 Detector Sensitivity 147 Integrated Resonant Absorber Pyroelectric Detector 148 Detector Design 149 Detector Sensitivity 150 Resonant IR Detector 150 Principles of Operation of Resonant Detector 151 IR Absorbing Coatings and Structures 152 Differential Operation and Detector Arrays 154 Performance of GaN Resonators 155 Plasmonic IR Detector 155 Structure Design 156 Fabrication and Performance of the Detector 158 Graphene Pyroelectric Bolometer 161 Device Architecture 163 Device Performance 164 References 165

5 5.1 5.2 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.1.3 5.4 5.5 5.5.1 5.5.1.1 5.5.1.2 5.5.1.3 5.5.1.4 5.5.2 5.5.2.1 5.5.2.2

Pyroelectric Energy Harvesting 173 Introduction 173 Theory of Pyroelectric Energy Harvesting 175 Pyroelectricity in Ferroelectric Materials 178 Thermodynamic Cycles of PyEH 178 Carnot Cycle 178 Ericsson Cycle 179 Olsen Cycle 180 Pyroelectric Generators 181 Pyroelectric Nanogenerators 183 Polymer-Based Pyroelectric Nanogenerators 183 PyNGs Driven by Various Environmental Conditions 183 Development of Pyroelectric Materials 186 Wearable Pyroelectric Nanogenerators 188 Hybrid Pyroelectric Nanogenerators 191 Ceramic-Based Pyroelectric Nanogenerators 198 ZnO-Based Pyroelectric Nanogenerators 198 PZT-Based Pyroelectric Nanogenerators 201

vii

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Contents

5.5.2.3 5.5.3 5.5.4

Lead-Free Ceramic-Based Pyroelectric Nanogenerators 204 Thermal Nanophotonic-Pyroelectric Nanogenerators 208 Challenges and Perspectives of Pyroelectric Nanogenerators 210 References 211

6 6.1 6.2 6.3 6.4 6.4.1 6.4.2

Pyroelectric Fusion 221 Introduction 221 History of Pyroelectric Fusion 221 Pyroelectric Neutron Generators 224 Pyroelectric X-ray Generators 229 Applications 231 Features 231 References 233 Index 237

ix

Preface Pyroelectricity was probably first observed by the Greeks more than 24 centuries ago. The philosopher Theophrastus wrote that lyngourion (most likely the mineral tourmaline) had the property of attracting straws and bits of wood. For two millennia, the peculiar properties of tourmaline were more a part of mythology than of science. In the eighteenth century, pyroelectric studies made a major contribution to the development of our understanding of electrostatics. In the nineteenth, research on pyroelectricity added to our knowledge of mineralogy, thermodynamics, and crystal physics. The field of pyroelectricity flourished in the twentieth century with many applications, particularly in infrared detection and thermal imaging. Pyroelectric sensors have been carried in many space missions and have contributed significantly to our astronomical knowledge. Recently, pyroelectricity has become one of the most studied phenomena in the scientific community due to the various applications of pyroelectric materials in electronic devices. This book describes the basic physical properties, structure, and applications of pyroelectric materials. The first chapter of this book deals with the basic concepts of dielectrics. Chapter 2 describes the basic concepts of pyroelectricity, theory of pyroelectricity, and history of pyroelectricity. Chapter 3 presents the physical properties, structure, and applications of different pyroelectric materials developed up to the recent time. Chapter 4 provides up-to-date information on the design and applications of various pyroelectric IR detectors. Chapter 5 gives an overview of the progress in the development of pyroelectric nanogenerators (PyNGs) for an energy harvesting system that uses environmental or artificial energies such as the sun, body heat, and heaters. The last chapter discusses the latest research results on pyroelectric fusion and provides information on newly designed pyroelectric neutron generators and X-ray generators (prototype portable). I sincerely hope that this book will be very useful for scientific community including students, teachers, and researchers working in this field. Finally, I would like to thank the Wiley-VCH publishing team for their outstanding support. Birmingham, UK I.I.T. Kanpur, India July 2022

Ashim Kumar Bain Prem Chand

1

1 Fundamentals of Dielectrics 1.1 Dielectrics A dielectric material is a substance that is a poor conductor of electricity. On the basis of band structure, the dielectric materials have an energy gap of 3 eV or more. This large magnitude of energy gap precludes the possibility of electrons being excited from the valence band to the conduction band by thermal means. In electromagnetism, a dielectric (or dielectric material or dielectric medium) is an electrical insulator that can be polarized by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor, but instead only slightly shift from their average equilibrium positions, causing dielectric polarization (Figure 1.1). Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field (e.g. if the field is moving parallel to the positive x-axis, the negative charges will shift in the negative x-direction). This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized but also reorient so that their symmetry axes align to the field. The study of dielectric properties concerns storage and dissipation of electric and magnetic energy in materials [1, 2]. Dielectrics are important for explaining various phenomena in electronics, optics, solid-state physics, and cell biophysics [3, 4]. Although the term insulator implies low electrical conduction, dielectric typically means materials with a high polarizability. The latter is expressed by a number called the relative permittivity. The term insulator is generally used to indicate electrical obstruction, while the term dielectric is used to indicate the energy-storing capacity of the material (by means of polarization). A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor. The polarization of the dielectric by the applied electric field increases the capacitor’s surface charge for the given electric field strength. The term dielectric was coined by William Whewell (from dia + electric) in response to a request from Michael Faraday [5, 6]. A perfect dielectric is a material with zero electrical conductivity (cf. perfect conductor infinite electrical conductivity), thus exhibiting only a displacement current; therefore, it stores and returns electrical energy as if it were an ideal capacitor. Pyroelectric Materials: Physics and Applications, First Edition. Ashim Kumar Bain and Prem Chand. © 2023 WILEY-VCH GmbH. Published 2023 by WILEY-VCH GmbH.

2

1 Fundamentals of Dielectrics

Charge +Q

Figure 1.1 material.

–Q

A polarized dielectric

Dielectric

Electric field E

Plate area A

Plate separation d

1.1.1

Polarization of Dielectrics

The factors contributing to the polarization of dielectric molecules are as follows: the formation of dipole moments and their orientation relative to the electric field. If, in a dielectric, the molecules forming elementary dipole moments are composed of neutral particles such as atoms, the electric field shifts the electric charge of an atomic shell against the direction of field and the nucleus is moved in with the field. Thus, the center of gravity of the positive and negative charges is displaced from the center of the atom and an “induced dipole moment” is produced, as shown in Figure 1.2a. This part of polarization of molecules is called electronic (Pe ). The electronic polarization is independent of temperature, but it is directly proportional to the field strength. If the molecule producing an elementary dipole moment is made of ions of opposite signs, the following process occurs when the dielectric is placed into an electric field: the positive ions leave their equilibrium positions and move in the direction of field, and the negative ions are displaced against the direction of field. This displacement of ions or their groups in a dielectric initiates an ionic polarization (Pi ) of molecules, as shown in Figure 1.2b. The ionic polarization is also independent of temperature, but it depends on the binding energy of particles in the molecule and in the lattice of the dielectric.

+

No field

–

+

(a)

+

(b) (c)

(d)

–

–

E

Figure 1.2 Polarization processes: (a) electronic polarization, (b) ionic polarization, (c) orientational polarization, and (d) space charge polarization.

– +

+

1.1 Dielectrics

The asymmetric distribution of charge between different atoms in a molecule produces permanent dipole moments in the molecules of a dielectric. Under the action of an electric field, these permanent dipoles are rotated into the direction of the field and thus contribute to polarization. In this case, we speak about the orientational polarization (Po ), as shown in Figure 1.2c. The orientational polarization is dependent on temperature. With increasing temperature, the thermal energy tends to randomize the alignment of the permanent dipoles inside the materials. In real dielectrics, free charges may exist, which, under the action of an electric field, move through the dielectric and are captured by various defects within the dielectric without coming into contact with the electrodes. The free charges then form regions with a surface or a space charge, which in turn produces a dipole moment, also contributing to the polarization of a dielectric. This mechanism initiates a space (surface) charge polarization (Ps ) inside the dielectric, as shown in Figure 1.2d. Like the orientational polarization, the space charge polarization is also a function of temperature, which, in most cases, increases with temperature. The total polarization of a dielectric may simultaneously involve all the four mechanisms. If we assume that they are independent, we can write the total polarization of a dielectric material as the sum of the contributions from the four sources described earlier: Ptotal = Pe + Pi + Po + Ps

(1.1)

where the subscripts on the right refer to the four types: electronic, ionic, orientational, and space charge polarization, respectively.

1.1.2

Dispersion of Dielectric Polarization

In general, a material cannot polarize instantaneously in response to an applied field. The dielectric polarization process can be expressed as a function of temperature. )] [ ( t (1.2) P(t) = P 1 − exp − tr where P is the maximum polarization attained on application of the electric field, and tr is the relaxation time for the particular polarization process. The relaxation time tr is the time taken for a polarization process to reach 63% of the maximum value. The relaxation time varies widely on different polarization processes. There are a number of polarization mechanisms, as shown in Figure 1.3. The most common, starting from high frequencies, are given in the subsequent section. 1.1.2.1 Electronic Polarization

This process occurs in an atom when the electric field displaces the electron density relative to the nucleus it surrounds. Electronic polarization may be understood by assuming an atom as a point nucleus surrounded by spherical electron cloud of uniform charge density. Electrons have very small mass and are therefore able to follow the high-frequency fields up to the optical range. It is an extremely rapid process and is essentially complete at the instant the voltage is applied. Even when the

3

1 Fundamentals of Dielectrics

ε′, ε″

4

Relaxations Space charge

Resonances Dipoles

ε′

Ions

Electrons

ε″ 1 10

104

108

1012

1016

1020 Frequency (Hz)

Figure 1.3

Frequency dependence of polarization dispersion.

frequency of the applied voltage is very high in the optical range (∼1015 Hz), the electronic polarization occurs during every cycle of the applied voltage. 1.1.2.2 Ionic Polarization

This process is associated with the relative motions of cations and anions in an electric field. Ionic polarization is slower than electronic polarization, as the displacement involved here is that of much heavier ion as compared to the electron cloud. The frequency with which ions can be displaced over a small fraction of the interatomic distance will be of the same order as the lattice vibration frequency (∼1013 Hz). If an electric field of frequency in the optical range (∼1015 Hz) is applied, the ions do not respond at all, as the time required by an ion for one vibration is 100 times larger than the period of the applied voltage. Therefore, there is no ionic polarization at optical frequencies. 1.1.2.3 Orientation Polarization

It is slower than ionic polarization. Orientation polarization arises from the rotation of molecular dipoles in the field. It is easier for the polar molecules to reorient themselves in a liquid than in solid. Orientation polarization occurs when the frequency of the applied voltage is in the audio range. 1.1.2.4 Space Charge Polarization

It is the slowest process, as it involves the diffusion of ions over several interatomic distances. The relaxation time for this process is related to the frequency of successful jumps of ions under the influence of the applied field. Space charge polarizations often occur in the kilohertz range or even lower. Referring to Figure 1.3, all the four types of polarization are present at machine frequencies. As the frequency increases, space charge, orientation, and ionic polarization become inoperative in that order. When several polarization processes occur in a material, it follows that the dielectric constant will decrease with increasing frequency of the applied voltage. When the period of the applied voltage is much larger than the relaxation time of a polarization process, the polarization is completed at

1.1 Dielectrics

any instant during each cycle, and when the period of the applied voltage is much shorter than the relaxation time for a polarization process, the polarization does not occur at all. But when the period of the applied voltage is in the same range as the relaxation time, resonance occurs. At high frequencies, usually microwave and beyond – the processes that take place are undamped and are called “resonances.” Real dielectric materials have several such resonances due to ionic and electronic polarization. At frequencies below microwaves, the polarization processes are heavily damped and are called “relaxations.” In physics, dielectric dispersion is the dependence of the permittivity of a dielectric material on the frequency of an applied electric field. This is because there is a lag between changes in polarization and changes in the electric field. The permittivity of the dielectric is a complicated function of frequency of the electric field. Dielectric dispersion is very important for the applications of dielectric materials and for the analysis of polarization systems. This is one instance of a general phenomenon known as material dispersion: a frequency-dependent response of a medium for wave propagation. When the frequency becomes higher: ●

●

●

dipolar polarization can no longer follow the oscillations of the electric field in the microwave region around 1010 Hz ionic polarization and molecular distortion polarization can no longer track the electric field past the infrared or far-infrared region around 1013 Hz electronic polarization loses its response in the ultraviolet region around 1015 Hz.

In the frequency region above ultraviolet, permittivity approaches the constant 𝜀0 in every substance, where 𝜀0 is the permittivity of the free space. Because permittivity indicates the strength of the relation between an electric field and polarization, if a polarization process loses its response, permittivity decreases. The effect of temperature on the relative permittivity of a material can be twofold. In orientation polarization, the randomizing action of thermal energy decreases the tendency for the permanent dipoles to align themselves in the applied field. This results in a decrease in the relative permittivity with increasing temperature. The other effect of temperature is to facilitate the diffusion of ions in space charge polarization. Thermal energy may aid in overcoming the activation barrier for the orientation of relatively large polar molecules in the direction of the field.

1.1.3

Dielectric Relaxation

Dielectric relaxation is the momentary delay (or lag) in the dielectric constant of a material. This is usually caused by the delay in molecular polarization with respect to a changing electric field in a dielectric medium (e.g. inside capacitors or between two large conducting surfaces). Dielectric relaxation in changing electric fields could be considered analogous to hysteresis in changing magnetic fields (e.g. in inductor or transformer cores). Relaxation in general is a delay or lag in the response of a linear system, and therefore, dielectric relaxation is measured relative to the expected linear steady state

5

6

1 Fundamentals of Dielectrics

(equilibrium) dielectric values. The time lag between electrical field and polarization implies an irreversible degradation of Gibbs free energy. In physics, dielectric relaxation refers to the relaxation response of a dielectric medium to an external, oscillating electric field. This relaxation is often described in terms of permittivity as a function of frequency, which can, for ideal systems, be described by the Debye equation. On the other hand, the distortion related to ionic and electronic polarization shows behavior of the resonance or oscillator type. The character of the distortion process depends on the structure, composition, and surroundings of the sample.

1.1.4

Debye Relaxation

Debye relaxation is the dielectric relaxation response of an ideal, noninteracting population of dipoles to an alternating external electric field. It is usually expressed in the complex permittivity 𝜀 of a medium as a function of the field’s frequency 𝜔: Δ𝜀 (1.3) 1 − i𝜔𝜏 where 𝜀∞ is the permittivity at the high-frequency limit (i.e., 𝜔 → ∞), Δ𝜀 = 𝜀s − 𝜀∞ where 𝜀s is the static, low-frequency (i.e., 𝜔 → 0) permittivity, and 𝜏 is the characteristic relaxation time of the medium. Separating into the real part 𝜀′ and the imaginary part 𝜀′′ of the complex dielectric permittivity yields [7]: 𝜀 −𝜀 𝜀′ = 𝜀∞ + s 2∞2 (1.4) 1+𝜔 𝜏 (𝜀 − 𝜀∞ )𝜔𝜏 (1.5) 𝜀′′ = s 1 + 𝜔2𝜏 2 𝜀(𝜔) = 𝜀∞ +

These are the Debye equations, and we find that they are reasonably applicable to most dispersions at electrical frequencies. The dielectric loss is also represented by: tan 𝛿 =

(𝜀 − 𝜀∞ )𝜔𝜏 𝜀′′ = s 𝜀′ 𝜀s + 𝜀∞ 𝜔2 𝜏 2

(1.6)

This relaxation model was introduced by and named after the physicist Peter Debye [8]. It is characteristic for dynamic polarization with only one relaxation time.

1.1.5

Molecular Theory of Induced Charges in a Dielectric

A dielectric contains no free charges; then how it is possible for an induced charge to appear on the surface of a dielectric when placed in an electric field? This can be explained by the molecular viewpoint of dielectric. The dielectrics are classified into polar and nonpolar. A nonpolar molecule is one in which the center of gravity of positive and negative charges normally coincides, while a polar molecule is one where they do not coincide. Polar molecules, therefore, have permanent dipole moments. In the absence of an external field, these dipoles are oriented at random. But strong field orients more dipoles in the direction of the field. The charges of a nonpolar molecule suffer a small displacement when placed in an electric field.

1.1 Dielectrics

Figure 1.4 The depolarization field E 1 is opposite to P. The fictitious surface charges are indicated: the field of these charges is E 1 within the ellipsoid.

E0

E1

P

The molecules are said to become polarized by the field and are called induced dipoles. Therefore, the dielectrics, both polar and nonpolar, behave in the same way under the influence of external electric field. We can imagine that these dipoles in the applied electric field can have excess negative charges on one surface and positive charges on the opposite surface, as shown in Figure 1.4. These charges are not free, but each is bound to a molecule lying in or near the surface. The net charge per unit volume within the rest of the dielectric medium is zero. The electric field E1 set up by the induced charge always opposes the applied field E0 . The resultant field E is the vector sum of these two. That is, (1.7)

E = E𝟎 + E𝟏

The field E1 is called the depolarization field; this is because within the body, it tends to oppose the applied field E0 as shown in Figure 1.4. The resultant field E points to the same direction as E0 but is smaller in magnitude. This leads to the conclusion that if a dielectric is placed in an electric field, the induced surface charges appear, which tend to weaken the original field within the dielectric. Thus, we can define the dielectric constant (k) or relative permittivity (𝜀r ) as the ratio of the magnitude of the applied field E0 to the resultant field E. Then, E0 V = 0 = k = 𝜀r (1.8) E V where V 0 is the potential difference without any medium and V is the same with a dielectric medium in between the capacitor plates. Therefore, for same charges Q, the ratio of capacitance with dielectric C and capacitance without dielectric (for free space) C0 will be C = C0

Q V Q V0

=

E V0 = 0 = k = 𝜀r V E

(1.9)

From the above definition of k, the dielectric constant or permittivity for free space is unity. Obviously, k is a dimensionless quantity.

1.1.6

Capacitance of a Parallel Plate Capacitor

If a constant voltage V 0 is applied to a plane condenser with a vacuum capacity C0 , a charge Q of density 𝜎 = Q/A is set up on the condenser with area A and distance of separation d between the plates (Figure 1.4). From the application of Gauss’s law, we know that the electric field intensity between two plates with a vacuum is E = 𝜎/𝜀0 .

7

8

1 Fundamentals of Dielectrics

The potential difference V 0 is the work done in carrying a unit charge from one plate to the other. Hence, ) ( ( ) Qd 𝜎 (1.10) d= V0 = Ed = 𝜀0 𝜀0 A Rearranging the relation (1.9), we can write ) ( 𝜀0 A Q = V0 d The capacitance C0 can be written as: ) ( 𝜀0 A Q C0 = = V0 d

(1.11)

(1.12)

The capacitance C of the capacitor with dielectric medium can be written as: A (1.13) C=𝜀 d where 𝜀 is the permittivity (absolute permittivity) of dielectric medium between the capacitor plates. The permittivity (𝜀) is often represented by the relative permittivity (𝜀r ), which is the ratio of the absolute permittivity (𝜀) and the vacuum permittivity (𝜀0 ). 𝜀 k = 𝜀r = (1.14) 𝜀0 Rearranging relations (1.13) and (1.14), we can write ( ) ( ) A A C = 𝜀 r 𝜀0 = k𝜀0 (1.15) d d Relation (1.15) can be expressed in the rationalized form in the SI system by the formula: ( ) ( ) A A = 𝜀r (8.854 × 10−12 ) F (1.16) C = 𝜀r 𝜀 0 d d where d is in meters and A is in square meters. Normalized units in the cgs electrostatic system can be expressed by the formula: ( )( ) A 1 cm (1.17) C = 𝜀r 4𝜋 d where d is in centimeters and A in square centimeters. Sample Problem 1.1 A parallel-plate capacitor of area A = 4 × 10−2 m2 and plate separation d = 2 × 10−2 m is raised to a potential difference V 0 = 100 V by connecting a battery when there is no dielectric in between the plates. (a) Calculate the capacitance C0 of the capacitor. (b) What is the free charge appeared on the plates? (a) From Eq. (1.12), the capacitance of the capacitor: 𝜀0 A (8.85 × 10−12 F∕m)(4 × 10−2 m2 ) = d 2 × 10−2 m −12 = 17.8 × 10 F = 17.8 pF (Answer).

C0 =

1.1 Dielectrics

(b) From Eq. (1.12), the free charge: Q = C0 V0 = 17.8 × 10−12 F × 100 V = 17.8 × 10−10 C (Answer). Sample Problem 1.2 Calculate the dielectric constant of a barium titanate crystal, which when inserted in a parallel plate capacitor of area A = 10 mm × 10 mm and distance of separation of d = 2 mm, gives a capacitance of 10−9 F. From Eq. (1.15), the dielectric constant of the medium between the capacitor plates: ( ) 10−9 F × 2 × 10−3 m C d = = 2259 (Answer). k= 𝜀0 A 8.854 × 10−12 F∕m × 102 × 10−6 m2 Sample Problem 1.3 A capacitor of 1 nF is required. If a dielectric material of thickness 0.1 mm and relative permittivity 5.4 is available, determine the required plate area. From Eq. (1.15), the area of the capacitor plates: A=

1 × 10−9 F × 0.1 × 10−3 m Cd = 0.00209 m2 = 20.9 cm2 (Answer). = 𝜀0 𝜀r 8.854 × 10−12 F∕m × 5.4

1.1.7 Electric Displacement Field, Dielectric Constant, and Electric Susceptibility In physics, the electric displacement field (denoted by D) or electric induction is a vector field that appears in Maxwell’s equations. It accounts for the effects of free and bound charge within materials. “D” stands for “displacement,” as in the related concept of displacement current in dielectrics. In free space, the electric displacement field is equivalent to flux density, a concept that lends the understanding of Gauss’s law. In a dielectric material, the presence of an electric field E causes the bound charges in the material (atomic nuclei and their electrons) to slightly separate, inducing a local electric dipole moment. The electric displacement field “D” is defined as: D = 𝜀0 E + P

(1.18)

where 𝜀0 is the vacuum permittivity (also called permittivity of free space), and P is the (macroscopic) density of the permanent and induced electric dipole moments in the material, called the polarization density. In a linear, homogeneous, isotropic dielectric with instantaneous response to changes in the electric field, P depends linearly on the electric field, P = 𝜀0 χE

(1.19)

where the constant of proportionality 𝜒 is called the electric susceptibility of the material. Now, rearranging relations (1.18) and (1.19), we can write D = 𝜀0 (1 + 𝜒)E = 𝜀E

(1.20)

9

10

1 Fundamentals of Dielectrics

where 𝜀 = 𝜀0 𝜀r is the permittivity, and 𝜀r = (1 + 𝜒) is the relative permittivity of the material. In a linear, homogeneous, and isotropic media, 𝜀 is a constant. However, in a linear anisotropic media, it is a tensor, and in nonhomogeneous media, it is a function of position inside the medium. It may also depend upon the electric field (nonlinear materials) and have a time-dependent response. Explicit time dependence can arise if the materials are physically moving or changing in time (e.g. reflections off a moving interface give rise to Doppler shifts). A different form of time dependence can arise in a time-invariant medium, as there can be a time delay between the imposition of the electric field and the resulting polarization of the material. In this case, P is a convolution of the impulse response susceptibility 𝜒 and the electric field E. Such a convolution takes on a simpler form in the frequency domain: by Fourier transforming the relationship and applying the convolution theorem, one obtains the following relation for a linear time-invariant medium: D(𝜔) = 𝜀(𝜔)E(𝜔)

(1.21)

where 𝜔 is the frequency of the applied field. The constraint of causality leads to the Kramers–Kronig relations, which place limitations upon the form of the frequency dependence. The phenomenon of a frequency-dependent permittivity is an example of material dispersion. In fact, all physical materials have some material dispersion because they cannot respond instantaneously to applied fields, but for many problems (those concerned with a narrow enough bandwidth), the frequency dependence of ε can be neglected.

1.1.8

Local Field in a Dielectric

We now develop an expression for the local field at a general lattice site, not necessarily of cubic symmetry. To evaluate Eloc , we must calculate the total field acting on a certain typical dipole; this field is due to the external field as well as all other dipoles in the system. This was done by Lorentz as follows: the dipole is imagined to be surrounded by a spherical cavity whose radius R is sufficiently large that the matrix lying outside it may be treated as a continuous medium as far as the dipole is concerned (Figure 1.5a). The interaction of our dipole with the other dipoles lying inside the cavity is, however, to be treated microscopically, which is necessary since the discrete nature of the medium very close to the dipoles should be taken into account. The local field, acting on the central dipole, is thus given by the sum Eloc = E0 + E1 + E2 + E3

(1.22)

where E0 is the external field; E1 is the depolarization field, that is, the field due to the polarization charges lying at the external surfaces of the sample; E2 is the field due to the polarization charges lying on the surface of the Lorentz sphere (Figure 1.5b), which is known as Lorentz field; and E3 is the field due to other dipoles lying within the sphere. It is important to note that the part of the medium between the sphere and the external surface does not contribute anything since the volume polarization charges

1.1 Dielectrics

ε0 Central dipole

ε1

R

R

θ

ε2 (a)

(b)

Figure 1.5 (a) The procedure for computing the local field. (b) The procedure for calculating E 2 , the field due to the polarization charge on the surface of the Lorentz sphere.

compensate each other, resulting in a zero net charge in this region. The contribution E1 + E2 + E3 to the local field is nothing but the total field at one atom caused by the dipole moments of all the other atoms in the specimen. Dipoles at distances greater than perhaps 10 lattice constants from the reference site make a smoothly varying contribution. It is convenient to let the interior surface be spherical. 1.1.8.1 Lorentz Field, E 2

The polarization charges on the surface of the Lorentz cavity may be considered as forming a continuous distribution. The field due to the charge at a point located at the center of the sphere is, according to Coulomb’s law, given by E𝟐 =

P 3𝜀0

(1.23)

1.1.8.2 Field of Dipoles Inside Cavity, E 3

The field E3 due to the dipoles within the spherical cavity is the only term that depends on the crystal structure. For a reference site with cubic surroundings in a sphere, E3 = 0 if all the atoms may be replaced by point dipoles parallel to each other. The total local field at a cubic site is then, Eloc = E𝟎 + E𝟏 +

P P =E+ 3𝜀0 3𝜀0

(1.24)

This is known as Lorentz relation. The difference between the Maxwell’s field E and the Lorentz field Eloc is as follows: the field E is macroscopic in nature and is an average field. On the other hand, Eloc is a microscopic field and is periodic in nature. This is quite large at molecular sites, indicating that the molecules are more effectively polarized than they are under the influence of Maxwell’s field E. If there are n molecules or atoms per unit volume in a dielectric, then the electric dipole moment per unit volume is n𝛼Eloc , represented by P, known as polarization. Therefore, P = n𝛼Eloc

(1.25)

11

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1 Fundamentals of Dielectrics

where 𝛼 is a constant and is known as polarizability of the dielectric material. Rearranging relations (1.24) and (1.25), we can write ) ( P (1.26) P = n𝛼Eloc = n𝛼 E + 3𝜀0 The polarization is induced by electric field, and therefore, it is a function of electric field. The relationship is written in the following way: P = 𝜀0 𝜒E

(1.27)

where 𝜒 is called the dielectric susceptibility. In general, 𝜒 is a tensor and depends on the electric field. The dielectric susceptibility 𝜒 is defined in terms of relative permittivity 𝜀r of the material. 𝜀r = (1 + 𝜒)

(1.28)

Now, rearranging relations (1.26–1.28), we can write an expression in terms of the relative permittivity: 𝜀r − 1 n𝛼 = 𝜀r + 2 3𝜀0

(1.29)

The above equation is known as Clausius–Mossotti relation. This relates the relative permittivity to polarizability of the dielectric material. The total polarizability 𝛼 can be written as the sum of four terms, representing the most important contributions to the polarization, that is, 𝛼 = 𝛼 e + 𝛼 i + 𝛼 o + 𝛼 s , where 𝛼 e , 𝛼 i , 𝛼 o , and 𝛼 s are the electronic, ionic, orientational, and space charge polarizabilities, respectively. Since 𝜀r = k = n2 , we can rewrite relation (1.29) as follows: n𝛼 n2 − 1 = n2 + 2 3𝜀0

(1.30)

This is the Lorentz–Lorenz equation. It connects the index of refraction (n) with the polarizability.

1.1.9

Dielectrics Losses

When an electric field acts on any matter, the latter dissipates a certain quantity of electric energy that transforms into heat energy. This phenomenon is known as the loss of power, meaning an average electric power dissipated in matter during a certain interval of time. As a rule, the loss of power in a specimen of a material is directly proportional to the square of the electric voltage applied to the specimen. If a metal conductor is first connected to direct voltage and then to alternating voltage, the acting magnitude of which is equal to direct voltage, the loss of power P in watts will be the same in both cases in conformity with the Joule–Lenz law and equal to V2 (1.31) R where V is the voltage in volts and R is the resistance of the conductor in ohms. P=

1.1 Dielectrics

As distinct from conductors, most of the dielectrics display a characteristic feature: under a given voltage, the dissipation of power in the dielectrics depends on the voltage frequency; the expense of power at an alternating voltage is markedly higher than that at a direct voltage; rapidly grows with an increase in frequency, voltage, and capacitance; and depends on the material of the dielectric. The power losses in a dielectric under the action of the voltage applied to it are commonly known as dielectric losses. This is the general term determining the loss of power in an electrical insulation at both a direct and an alternating voltage. Dielectric losses at a direct voltage can be found from relation (1.31) where R stands for the resistance of the insulation, while the losses under the alternating voltage are determined by more intricate regularities. Actually, the dielectric losses mean the losses of power under an alternating voltage. 1.1.9.1 Dielectric Loss Angle

The phase diagram of currents and voltages in a capacitor energized by an alternating voltage is shown in Figure 1.6. If the power was not dissipated at all in the dielectric of the capacitor (ideal dielectric), the phase of current I through the capacitor would be ahead of the phase of voltage V by 90∘ , and the current would be purely reactive. In actual fact, the phase angle 𝜙 is slightly less than 90∘ . The total current I through the capacitor can be resolved into two components: active I a and reactive I r currents. Thus, the phase angle describes a capacitor from the viewpoint of losses in a dielectric. Since the phase angle 𝜙 is very close to 90∘ in a capacitor with a high-quality dielectric, the angle 𝛿 (i.e. 𝛿 = 90∘ − 𝜙) is a more descriptive parameter, which is called the dielectric loss angle. The tangent of the angle is equal to the ratio of the active currents to the reactive currents: tan 𝛿 = Ia ∕Ir

(1.32)

or the ratio of active power P (power loss) to the reactive power Pr : tan 𝛿 = P∕Pr

(1.33)

The dielectric loss angle is an important parameter for the dielectric materials. This parameter is usually described by the loss tangent tan 𝛿. Sometimes, the quality factor of an insulation portion is determined, that is, the value is reciprocal of the loss tangent: 1 = tan 𝜙 (1.34) Q= tan 𝛿 Figure 1.6 Phase diagram of current and voltage in a capacitor with a dielectric material.

V

I, Z

Ia

φ δ

Ir

O

13

14

1 Fundamentals of Dielectrics

The values of tan 𝛿 for the best electrical insulating materials employed in high-frequency and high-voltage engineering practice are of the order of thousands and even tenths of thousands of fractions. 1.1.9.2 Total and Specific Dielectric Losses

The value of dielectric losses P in an insulating material having a capacitance C is described from relation (1.31) as follows: P = VIa = VIr tan 𝛿 Inserting the intensity of the capacitive current through an insulation portion with a capacitance of C, we get (1.35)

Ir = V𝜔C

Since 𝜔 = 2𝜋f , the angular frequency, the dielectric losses P can be expressed as follows: P = V 2 𝜔C tan 𝛿 = 2𝜋fCV 2 tan 𝛿

(1.36)

Inserting the value of effective length Λ = A/d in Eq. (1.36) and replacing 𝜀0 by its numerical value 10−9 F∕m 36𝜋 the expression of dielectric losses can be formulated as: 𝜀0 ≈

P = 5.56 × 10−11 V 2 f Λ𝜀r tan 𝛿

(1.37)

Formulas (1.36) and (1.37) have a broad field of application. They hold for any size and shape of an insulated portion. The knowledge of total amount of dielectric losses in the insulated portion is not enough, and it is necessary to study the distribution of dielectric losses at the separate points of insulation. Let us consider a cube with edge dx inside the insulated portion in which we are interested so that the lines of forces pierce the cube entering and leaving it through two opposite faces in the direction perpendicular to these faces (Figure 1.7).

Charge

Electric field

Charge

+Q

–Q

dx dx dx Plate separation d

Plate area A

Figure 1.7 Electric field pierces a cube with edge dx in an insulated portion.

1.1 Dielectrics

The capacitance of the capacitor formed by the cube according to relation (1.15) with d = dx and A = (dx)2 is ( ) A C = 𝜀r 𝜀0 = 𝜀r 𝜀0 dx d and the voltage across the cube is V = E dx. Inserting these values into Eq. (1.36), we get dP = E2 𝜔𝜀0 𝜀r tan 𝛿(dx)3

(1.38)

whence the specific dielectric losses are the losses per unit volume of the dielectric, p=

dP dP = dV (dx)3

where V = (dx)3 is the volume of the cube. So, the specific dielectric loss p is expressed as: p = E2 𝜔𝜀0 𝜀r tan 𝛿

(1.39)

Now substituting 𝜔 = 2𝜋f and replacing 𝜀0 by its numerical value in Eq. (1.39), 10−9 F∕m 36𝜋 We have the expression for specific dielectric losses: 𝜀0 ≈

p = 5.56 × 10−11 E2 f 𝜀r tan 𝛿

(1.40)

Formulas (1.39) and (1.40) are suitable for any pattern of field that possesses unlike properties at different places. The product 𝜀r tan 𝛿 is called the dielectric loss index (factor).

1.1.10 Dielectrics Breakdown At high electric fields, a material that is normally an electrical insulator may begin to conduct electricity – that is, it ceases to act as a dielectric. This phenomenon is known as dielectric breakdown. The mechanism behind dielectric breakdown can best be understood using the band theory. Essentially, there are two “bands” in every material that the electrons within the material may occupy: the valence band and the higher energy conduction band (Figure 1.8). Electrons in the valence band can be conducted as being bound in place, whereas electrons in the conduction band may act as mobile charge carriers. In dielectrics, the two bands are separated by a certain energy gap Eg , corresponding to energies that are forbidden to the electrons. Since the valence band is lower in energy, electrons will preferentially occupy this band. Therefore, in a dielectric under normal conditions, the conduction band will be empty. If an electron in the valence band is supplied with energy greater than or equal to Eg , for example, from a high energy photon, it may be promoted to the conduction band. An electric field of sufficient strength can supply enough energy to promote many electrons to the conduction band at once. Since electrons in the conduction band act as charge carriers, the material now conducts charge rather than storing it. For

15

1 Fundamentals of Dielectrics

Conduction band (empty)

Energy

16

Energy gap Eg

Valence band (occupied)

(a)

Conduction band (occupied)

Before break down

e– e– e– e– e– e– e– e– e–e– e– e– e– e– e– e– e– e– e– e– e– e– e– e– e– e– e– Valence band (occupied)

(b) High electric field promotes dielectric break down

Figure 1.8 (a) Band structure before dielectric breakdown. (b) band structure after dielectric breakdown.

each material, there is a characteristic field strength needed to cause dielectric breakdown. This is referred to as the breakdown field or dielectric strength. Typically, values of the dielectric strength lie in the range 106 –109 Vm−1 . The exact value of the dielectric strength depends on many factors – most obviously, the size of the energy gap, the geometry and microstructure of the sample, and the conditions it is subjected to. The dielectric breakdown is associated with the formation in a dielectric crystal of a conducting path in which the current density is substantially higher than the average for the specimen. The Joule heat generated because of the high-density current in the path leads to the destruction of the material, including melting, the appearance of an air channel as a result of volatilization, and the extensive formation of crystal defects or cracking. Thus, dielectric breakdown is an irreversible phenomenon. Dielectric breakdown is often associated with the failure of solid or liquid insulating materials used inside high-voltage transformers or capacitors in the electricity distribution grid, usually resulting in a short circuit or a blown fuse. It can also occur across the insulators that suspend overhead power lines and within underground power cables or lines arcing to nearby branches of trees.

References 1 Thoms, E., Sippel, P., Reuter, D. et al. (2017). Dielectric study on mixtures of ionic liquids. Sci. Rep. 7 (1): 7463. 2 Belkin, A., Bezryadin, A., Hendren, L., and Hubler, A. (2017). Recovery of alumina nanocapacitors after high and low voltage breakdown. Sci. Rep. 7 (1): 932. 3 Hossain, S. (2020). Malignant cell characterization via mathematical analysis of bio impedance and optical properties. Electromagn. Biol. Med. 40 (1): 65–83. 4 Hossain, S. (2020). Biodielectric phenomenon for actively differentiating malignant and normal cells: an overview. Electromagn. Biol. Med. 39 (2): 89–96. 5 Daintith, J. (1994). Biographical Encyclopedia of Scientists, 943. CRC Press.

References

6 James, F.A.J.L. (ed.) (1996). The Correspondence of Michael Faraday, Volume 3, 1841–1848. Letter 1798, William Whewell to Faraday, p. 442. Archived from the original on 2016-12-23. Retrieved 2012-05-18. London, United Kingdom: The Institution of Electrical Engineers. 7 Kao, K.C. (2004). Dielectric Phenomena in Solids, 92–93. London: Elsevier Academic Press. 8 Debye, P. (1913). Ver. Deut. Phys. Gesell. 15: 777; reprinted 1954 in collected papers of Peter J.W. Debye. Inter Science, New York.

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2 Pyroelectricity 2.1 Introduction Pyroelectricity (from the two Greek words pyr meaning fire and electricity) is a property of certain crystals that are naturally electrically polarized and as a result contain large electric fields. Pyroelectricity can be described as the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to a voltage across the crystal. If the temperature stays constant at its new value, the pyroelectric voltage gradually disappears due to the leakage of current. The leakage can be due to either electrons moving through the crystal, ions moving through the air, or current leaking through a voltmeter attached across the crystal. If there is a small temperature change ΔT, uniform over the crystal, the change in the polarization vector ΔPi is described by the following relation: ΔPi = pi ΔT where the vector pi (i = 1, 2, 3) are the three pyroelectric coefficients and, by Neuman’s principle, ought to remain invariant under all the symmetry operations of the crystal [1]. Hence, pyroelectricity can be exhibited only by crystals belonging to the 10 polar classes, namely, 1, 2, 3, 4, 6, m, mm 2, 3 m, 4 mm, and 6 mm. The equation for the pyroelectric effect may be written as follows: dPi = pi dT Since Di = k0 Ei + Pi

(2.1)

We have, dPi = dDi − k0 dEi If we now specify that the temperature change is to be carried out with the electric field in the crystal held constant, we have dDi = pi dT (E − constant)

(2.2)

Pyroelectric Materials: Physics and Applications, First Edition. Ashim Kumar Bain and Prem Chand. © 2023 WILEY-VCH GmbH. Published 2023 by WILEY-VCH GmbH.

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It may be pointed out here that in some crystal, polarization may appear under the influence of hydrostatic pressure, and this is a special case of piezoelectric effect. Since hydrostatic pressure is a scalar-like temperature, this polarization linearly proportional to the hydrostatic pressure can also be treated on par with pyroelectricity. Thus, the piezoelectric effect under hydrostatic pressure can occur only in the abovementioned 10 polar classes. In general, the piezoelectric effect has the mathematical character of a third-rank tensor and is restricted only to 20 non-centrosymmetric classes, excluding the class 432. Thus, the symmetry permits all the pyroelectric crystals to be piezoelectric, while the converse is not true. Therefore, it is possible to imagine two possible contributions to pyroelectricity in the following way. Experimentally to observe pyroelectricity, one can heat the crystal and observe the change in polarization. The experiment could be performed in two ways. Either the shape and size of the crystal can be kept fixed during the heating or the crystal may be released so that thermal expansion can occur quite freely. Obviously, the magnitude of the effect observed in the two cases will be different. In the first case, the crystal is clamped, and the observed effect may be regarded as the primary pyroelectricity. In the second case, in addition to the primary effect, there is pyroelectric effect due to the variation of piezoelectrically induced polarization with temperature. This is known as the secondary pyroelectricity. The secondary pyroelectricity is found to contribute substantially to the total effect. Hence, for a free crystal, the total pyroelectric effect could be written as follows: ( ) ( ) ( ) ( ) dD d𝜖 dD dD = + (E − constant) (2.3) dT 𝜎 dT 𝜖 d𝜖 T dT 𝜎 where 𝝈, 𝝐, and E denote the stress, strain, and electric field, respectively (Figure 2.1). Eq. (2.3) could be written as follows: ( ) ( ) ( ) d𝜎 d𝜖 dD T = p𝜖 + dTijk Cjklm 𝛼jk𝜎 (E − constant) (2.4) p 𝜎 = p𝜖 + d𝜎 T d𝜖 T dT 𝜎 where p𝝈 is the total pyroelectric coefficient; p𝝐 is the primary pyroelectric coefficient; and dijk , Cjklm , and 𝜶 jk denote, respectively, the piezoelectric moduli, elastic compliance coefficients, and coefficients of thermal expansion. Clearly, the secondary pyroelectric coefficient is given by the product of dijk , Cjklm , and 𝜶 jk . Though secondary pyroelectricity is due to piezoelectricity, only those piezoelectric crystals that belong to the 10 polar classes are permitted by crystal symmetry to exhibit secondary pyroelectricity. Figure 2.1 Primary and secondary pyroelectricity. The full line illustrates the primary effect (with the strain 𝜖 constant); the broken lines illustrate secondary effects that can occur when the crystal is free to deform. Here, E – electric field, 𝜎 – stress, T – temperature, 𝜖 – strain, D – displacement, and S – entropy.

E

D

ε σ

S T

2.2 History of Pyroelectricity

So far, we have tacitly assumed that the temperature of the crystal is the same at all points. Uneven heating causes temperature gradients, which by thermal expansion give rise to nonuniform stress and strain. Under such conditions, it is possible for piezoelectric crystals like α-quartz, which does not belong to the 10 polar classes, to exhibit secondary pyroelectricity. This secondary pyroelectricity due to nonuniform heating is called the tertiary pyroelectric effect. The tensorial pyroelectricity refers to the production of quadrupole or higher electric moments on heating.

2.2 History of Pyroelectricity This treatment of pyroelectricity in terms of a change in net dipole moment emerged in modern times. But as a phenomenon, the pyroelectric effect has been known for 24 centuries – the Greek philosopher Theophrastus probably wrote the earliest description of the pyroelectric effect, as exhibited by the mineral tourmaline [2]. He described a stone called lyngourion in Greek or lyncurium in Latin, which had the property of attracting straws and bits of wood. Those attractions were no doubt the effects of electrostatic charges produced by temperature changes, most probably in the mineral tourmaline. Theophrastus and other writers of the two millennia that followed were far more interested in the origin of the stone and its possible therapeutic properties than they were in physical explanations. Theophrastus proposed that lyngourion was formed from the urine of a wild animal, later identified by Pliny the Elder as a lynx (Figure 2.2) [3–5]. This was not the cat known today, but a doglike animal. Two thousand years after Theophrastus, tourmaline’s unusual physical properties were reintroduced to Europe through the publication in 1707 of a book entitled Curiöse Speculationes bey Schlaflosen Nächten (Curious Speculations During Sleepless Nights) [6]. Its author, Johann Georg Schmidt, using the pen named Immer Gern Speculirt (Always Gladly Speculating), wrote a series of 48 dialogs, one of which contained a section describing hard and glassy bodies that were not magnetic. He described the experiences of Dutch gem cutters when they tested the durability of tourmaline in a fire: The ingenious Dr. Daumius, chief physician to the Polish and Saxon troops on the Rhine, told me that, in the year 1703, the Dutch first brought from Ceylon in the East Indies a precious stone called tourmaline, turmale, or trip, which had the property of not only attracting the ashes from the warm or burning coals, as the magnet does iron, but also repelling them again. In 1717, the physician and chemist Louis Lemery wrote the first scientific description of pyroelectricity in a journal and exhibited a tourmaline crystal before the Academy of Sciences of Paris [7]. In 1747, the naturalist Carl von Linné (Linnaeus) was the first to relate the pyroelectric property of tourmaline to electricity; he called the mineral lapis electricus (electric stone) [8]. The first serious scientific study of the electrical properties of tourmaline was presented to the Royal Academy of Sciences

21

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2 Pyroelectricity

Figure 2.2 The mythological origin of lyncurium (tourmaline, most likely) according to Greek philosopher Theophrastus, with lynx at lower right. Source: Lang [3–5].

in Berlin by Dr. Franz Ulrich Theodor Aepinus in 1756 [9]. His major observations were that tourmaline became electrified by being warmed (rather than by friction which was the common method in use at the time) and that the crystal acquired opposite electric charges on two opposing faces. The usefulness of this new way of generating electric charges and its relevance to the rapidly developing understanding of electricity and magnetism induced many others to experiment on tourmaline, including Johann Karl Wilcke [10], Benjamin Wilson [11], Joseph Priestley [12], John Canton [13], and Torben Bergman [14].

2.2 History of Pyroelectricity

Figure 2.3

Experimental apparatus of Bergman [14].

An elaborate drawing of the apparatus of Bergman is shown in Figure 2.3 [14]. Canton was apparently the first person to observe that cooling of tourmaline caused its electrical polarity to be the reverse of that found on heating. He also devised a very novel experiment demonstrating that the quantities of positive and negative charges were equal. A more quantitative understanding of pyroelectricity emerged during the nineteenth century as more sophisticated research techniques were developed. In 1824, David Brewster, famous for his work in optics, was the first author to use the term “pyroelectricity” [15]. One of the materials he studied was a “tartrate of soda and potash” – Rochelle salt – the same material in which Joseph Valasek discovered ferroelectricity almost exactly a century later. Quantitative electrometers for charge measurement were developed by A. C. Becquerel [16], James D. Forbes [17], and Wilhelm Gottlieb Hankel [18]. A drawing of the electroscope developed by Forbes is shown in Figure 2.4. Shortly after Antoine Becquerel and others developed the electrometer, John-Mothée Gaugain made the first precise measurements of pyroelectric charges in 1859 [19]. He reached some important conclusions: the total quantity of

23

24

2 Pyroelectricity

Figure 2.4 Electroscope developed by James D. Forbes. Source: Based on Forbes [17].

electricity produced by a crystal of tourmaline depends uniquely upon the limits within which its temperature is varied; within those limits, the amount of electricity produced during heating is the same as that produced during cooling, but with the signs of the charges reversed; and the amount of charge produced is proportional to the cross-sectional area of the crystal and is independent of its length. William Thomson and Lord Kelvin published the first major theoretical treatment of pyroelectricity in 1878; their paper included a prediction of the electrocaloric effect [20], the converse effect of pyroelectricity. A much-used technique for determining the charge distribution on a crystal was developed by Kundt in 1883 [21]. A mixture of red lead oxide and sulfur was dusted onto the crystal. Lead oxide adhered to the negative parts of the crystal and sulfur to the positive parts. Figure 2.5 illustrates some typical results [22]. Jacques and Pierre Curie proposed that the electrical effects due to nonuniform heating of quartz crystals might have been caused by pressure, which led to their discovery of piezoelectricity in 1880 [23]. During the latter part of the nineteenth century and the early decades of the twentieth century, seven Nobel laureates – Wilhelm Röntgen, Pierre Curie, Gabriel Lippman, Heike Kammerlingh Onnes, Erwin Schrödinger, Archer J.P. Martin, and Max Born – published papers

2.2 History of Pyroelectricity

Figure 1

Figure 2

Figure 4a

Figure 3

Figure 4b

Figure 5

Figure 6a

Figure 6b

Figure 11 Figure 7

Figure 8

Figure 9

Figure 10

Figure 12

Figure 2.5 Kundt powder patterns [22]: materials shown are D-tartaric acid (Figure 1), L-tartaric acid (Figure 2), calamine (Figure 3), struvite (Figure 4 a, b), tourmaline (Figure 5), left and right-handed quartz (Figure 6 a and b, respectively), quartz plates (Figures 7–10), boracite (Figure 11), topaz plate (Figure 12). Source:Groth [22].

25

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2 Pyroelectricity

on pyroelectricity [24]. These scientists are, of course, much better known for their research in other fields. Few important papers on pyroelectricity were published in the first two decades of the twentieth century. Joseph Valasek studied the properties of Rochelle salt and discovered ferroelectricity in 1920. Subsequently, interest in pyroelectricity virtually vanished. In 1938, Yeou Ta published a paper that initiated the great growth that continues in the field today [25]. In his paper for the first time, it proposed that tourmaline crystals could be used as infrared (IR) sensors in spectroscopy. Some research was conducted on pyroelectric IR detectors during and immediately after World War II in the United Kingdom, the United States, and Germany, but the results appeared only in classified documents. In 1962, J. Cooper made the first detailed analysis of the behavior of fast IR detectors and conducted experiments using BaTiO3 [26–28]. In that year, S.B. Lang proposed the use of pyroelectric devices for measuring temperature changes as small as 0.2 μK [29]. An explosive growth in theoretical studies, basic measurements, and applications had begun: more than 8500 papers on pyroelectricity have been published since 1960 [30]. In 1965, Hadni proposed the use of pyroelectric elements for thermal imaging [31]. Although early interest was in military and security applications, thermal television devices have had a significant impact in nonsecurity areas. As an example, a thermal imaging sensor has been built into helmets worn by firefighters that enables them to see through smoke and dust to locate the sources of fires and possible victims. An illustration of the helmet is shown in Figure 2.6 [24]. Some of the highest resolution pyroelectric imaging devices have been developed at the UK Defence Evaluation and Research Agency. A 256 × 128 pixel image and a 384 × 288 pixel image are shown in Figure 2.7 [24]. Pyroelectric devices have been used for applications in space beginning with the vertical temperature profile radiometer launched into an earth orbit on the ITOS-D Helmet mounted display (HMD)

Counterweight Rechargeable battery

Sensor

Processor/power module

Figure 2.6

Frangible cable link (FCL) Video port

Pyroelectric IR imager in firefighter’s helmet. Source: Lang [24].

2.2 History of Pyroelectricity

Figure 2.7 High-resolution pyroelectric images: 256 × 128 pixels (above), 384 × 288 pixels (below).Source: Lang [24].

spacecraft in 1972 [32]. One of the most recent space applications was in the Galileo mission, which was launched on 18 October 1989. Included in its instrumentation was a photopolarimeter-radiometer used to determine thermal radiation on Jupiter and its moons [33]. The temperature distribution around the Great Red Spot of Jupiter is shown in Figure 2.8. A probe carrying the net flux radiometer (NFR) was released into the atmosphere of Jupiter on 13 July 1995 [34]. This device measured the vertical distribution of net flux of solar energy and planetary emission in order to help determine the chemical composition and structure of the Jovian atmosphere. The optical head and the detector/hybrid assembly of the NFR are shown in Figure 2.9. In the eighteenth century, pyroelectricity emerged from two millennia of fable and myth into early studies of electricity, mineralogy, thermodynamics, and crystal physics. It gave birth to piezoelectricity and ferroelectricity and has given rise to a large body of scientific knowledge and a host of applications. It continues to be a vibrant and active field of research today.

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121 K

135 K

–242 F

–216 F

Figure 2.8 Galileo photopolarimeter (upper left) and pyroelectric images of Great Red Spot of Jupiter. Source: Hunten [33], NASA.

Recently, there have been many excellent individuals and institutions that have been involved in the research, development, and application of these very interesting materials. A chronological listing of many of the more notable specific events in the history of pyroelectric materials is given in Table 2.1. Progress has been made in creating artificial pyroelectric materials, usually in the form of a thin film, using gallium nitride (GaN), cesium nitrate (CsNO3 ), polyvinyl fluorides, derivatives of phenylpyridine, and cobalt phthalocyanine. Lithium tantalate (LiTaO3 ) is a crystal exhibiting both piezoelectric and pyroelectric properties, which has been used to design prototype portable X-ray generator [83] and pyroelectric neutron generators [45–49]. A pyroelectric can be repeatedly heated and cooled (analogously to a heat engine) to generate usable electrical power. Possible advantages of pyroelectric generators for generating electricity (as compared to the conventional heat engine plus electrical generator) include potentially lower operating temperatures, less bulky equipment, and fewer moving parts. Recently, many research groups have designed pyroelectric nanogenerators [60–87] using pyroelectric polymers and ceramic materials, but such generators do not appear to be anywhere close to commercialization. Very small changes in temperature in pyroelectric materials can produce a pyroelectric IR detector. Over the past few years, many pyroelectric IR sensors are designed with pyroelectric materials such as DLaTGS-based fourier transform infrared (FTIR) spectrometer [37], lithium tantalate (LT)-based pyroelectric detector [35], barium strontium titanate (BST)-based pyroelectric detector [39], PVDF-based pyroelectric sensor [36], AlN-based pyroelectric sensor [55], PZT thin-film IR sensor [42], dual-element PZT pyroelectric IR detector [50], hybrid focal plane array detector [40, 41], high-resolution linear array detector based on LiTaO3 [44], periodic domain TFLTTM (thin film lithium tantalate) detector [51],

2.2 History of Pyroelectricity

Rear bearing Mounting flange

Rotor gear

Flex circuits

Upward aperture (for viewing downward flux) Diamond window Shroud Front housing

Detector package

Forward bearing

Toroidal mirror Folding mirror

Stepper motor

Condensing cone

Flex circuit Alignment pins A

Beryllium stiffener Hybrid preamp Detector board

Detector plane C E

Filter

Upper filter frame A EMI shield Filter frames

Lower filter frame Section A-A

Detector temperature diode

Figure 2.9 Detector assembly of Galileo probe net flux radiometer: optical head (above) and detector/hybrid assembly (below). Source: Sromovsky et al. [34], figure 2, 5 (pp. 237–240)/Springer Nature.

THz detector based on ultra-thin LiTaO3 crystal [53], integrated 16 × 16 PVDF pyroelectric sensor [43], large-area TFP (thin film pyroelectric) polymer detector [54], Tetraaminodiphenyl (TADPh) polymer detector [58], integrated resonant absorber pyroelectric detector [56], GaN-based resonant IR detector [52], plasmonic IR detector [59], and graphene pyroelectric bolometer [57].

29

30

2 Pyroelectricity

Table 2.1

Notable events in the history of pyroelectricity.

Time line

Events

314BC

Greek philosopher Theophrastus first observed the pyroelectric effect in the stone called lyngourion in Greek or lyncurium in Latin (most likely the mineral tourmaline) [2].

1707

Johann Georg Schmidt rediscovered the physical properties of tourmaline through the publication of a book entitled Curiöse Speculationes bey Schlaflosen Nächten (Curious Speculations During Sleepless Nights) [6].

1717

Louis Lemery wrote the first scientific description of pyroelectricity in a journal and exhibited a tourmaline crystal before the Academy of Sciences of Paris [7].

1747

Carl von Linné (Linnaeus) first related the pyroelectric property of tourmaline to electricity; he called the mineral lapis electricus – electric stone [8].

1756

Franz Ulrich Theodor Aepinus presented first serious scientific study of the electrical properties of tourmaline to the Royal Academy of Sciences in Berlin [9].

1759

John Canton was the first person to observe that cooling of tourmaline caused its electrical polarity to be the reverse of that found on heating. He also devised a very novel experiment demonstrating that the quantities of positive and negative charge were equal [13].

1824

David Brewster was the first author to use the term “pyroelectricity.” It appeared in his paper entitled “Observation on the pyro-electricity of minerals” [15].

1859

Jean-Mothée Gaugain made the first precise measurements of pyroelectric charges [19].

1878

William Thomson and Lord Kelvin published the first major theoretical treatment of pyroelectricity; their paper included a prediction of the electrocaloric effect [20], the converse effect of pyroelectricity.

1901

Wilhelm Röntgen was awarded Nobel prize for his work on pyroelectric and piezoelectric investigations [24].

1903

Pierre Curie was awarded Nobel prize for his works on quartz and tourmaline [24].

1908

Gabriel Lippman was awarded Nobel prize for his work “Principles of conservation of electricity (theory of electrocaloric effect)” [24].

1913

H. Kammerlingh Onnes was awarded Nobel prize for his work “Piezoelectric and pyroelectric properties of quartz at low temperatures down to liquid hydrogen” [24].

1933

Erwin Schrödinger was awarded Nobel prize for his work “Kinetics of dielectrics: melting point, pyro- and piezoelectricity” [24].

1938

Yeou Ta published a paper [25] and proposed that tourmaline crystals could be used as infrared (IR) sensors in spectroscopy.

1952

Archer J. P. Martin was awarded Nobel prize for his work “New method for detecting pyroelectricity” [24].

1954

Max Born was awarded Nobel prize for his work “Quantum theory of pyroelectricity” [24]. (continued)

2.2 History of Pyroelectricity

Table 2.1

(Continued)

Time line

Events

1962

J. Cooper made the first detailed analysis of the behavior of fast IR detectors and conducted experiments using BaTiO3 [26–28].

1965

A. Hadni proposed the use of pyroelectric elements for thermal imaging [31].

1972

Pyroelectric device was used for applications in space beginning with the vertical temperature profile radiometer launched into an earth orbit on the ITOS-D spacecraft [32].

1989

Photopolarimeter-radiometer was used in the Galileo mission to determine thermal radiation on Jupiter and its moons [33].

1990

Lithium tantalate (LT)-based pyroelectric detector [35].

1990

PVDF-based pyroelectric sensor [36].

1992

DLaTGS-based Fourier Transform Infrared (FTIR) spectrometer [37].

1992

Pyroelectric X-ray generator [38].

1995

The Galileo net flux radiometer (NFR) was released into the atmosphere of Jupiter to measure the vertical distribution of net flux of solar energy and planetary emission [34].

1995

Barium strontium titanate (BST)-based pyroelectric detector [39].

1996

Hybrid focal plane array detector [40, 41].

1997

PZT thin-film IR sensor [42].

2000

Integrated 16 × 16 PVDF pyroelectric sensor [43].

2001

High-resolution linear array detector based on LiTaO3 [44].

2003

COOL-X pyroelectric x-rays generator (www.amptek.com/coolx.html).

2005

Pyroelectric neutron generator [45].

2007–2010

Pyroelectric neutron generators [46–49].

2009

Dual-element PZT pyroelectric IR detector [50].

2012

Periodic domain TFLT detector [51].

2014

GaN-based resonant IR detector [52].

2015

High-performance THz detector based on ultra-thin LiTaO3 crystal [53].

2015

Large-area TFP polymer detector [54].

2016

AlN-based pyroelectric sensor [55].

2016

Integrated resonant absorber pyroelectric detector [56].

2017

Graphene pyroelectric bolometer [57].

2019

TADPh polymer detector [58].

2019

Plasmonic IR detector [59].

2013–2018

PVDF polymer-based nanogenerators [60–78].

2012–2017

ZnO ceramic-based nanogenerators [79, 80].

2012–2017

PZT ceramic-based nanogenerators [81–85].

2012

KNbO3 ceramic-based nanogenerator [86].

2018

BTO ceramic-based nanogenerator [87].

31

32

2 Pyroelectricity

2.3 Theory of Pyroelectricity To understand pyroelectricity in crystals, we have to consider the various mechanisms of the spontaneous polarization (such as ionic, electronic, orientational, or surface charge) and study their variation with temperature. The pyroelectric behavior is fully described by the temperature variation of pyroelectric coefficient. Actually, it is convenient to consider the temperature variation of primary and secondary pyroelectric coefficients separately. The temperature variation of secondary pyroelectric coefficient is governed by those of piezoelectric moduli, elastic compliance constants, and thermal expansion coefficients. Both piezoelectric moduli and elastic compliance constants are not strong functions of temperature in those crystals that do not undergo any phase transition. Hence, the temperature variation of secondary pyroelectric coefficient is akin to those of thermal expansion coefficients. Conceptually, the origin of primary pyroelectric effect and its variation with temperature are more difficult to visualize and have invoked considerable discussion in the literature [88]. The lattice dynamical theory of primary pyroelectricity for the ease of ionic crystals was first formulated by Max Born in the year 1945 [89]. In this paper, Born had indicated that primary pyroelectric coefficient would be proportional to T, though in his later treatise on lattice dynamics, he predicted the T 3 as a law for the pyroelectric coefficient [90]. Szigeti [91, 92] has brought out clearly many salient features of the theory, especially the role of mechanical and electrical anharmonicity in primary pyroelectricity, which in fact was overlooked by Born. In the case of ionic crystals, there are two important mechanisms of polarization. One being responsible for absorption in the IR, that is, the lattice or ionic polarization, and the other in the ultraviolet, that is, the electronic polarization. In the very simplest and crude model known as the rigid ion model, the electron cloud around the ion is assumed to be rigid and, consequently, there is no contribution from electronic polarization. For such a model, the total dipole moment of the crystal is given by ∑ 𝛼a Qa M= a

where Qa are the active normal coordinates, that is, those that produce uniform polarization in the direction of M, and 𝛼 a are suitable normalizing constants. However, in reality, the electrons are deformed during the lattice vibrations as they experience short-range forces and also the dipolar field due to ions. The electron deformation is not only linearly proportional to lattice displacements but also involves higher terms. Hence, when one takes into consideration the electronic polarization also, the dipole moment of the crystal is given by ∑ ∑ 𝛼a Qa + 𝛽jj′ Qj Qj′ + … (2.5) M= a

jj′

An important point to note is that 𝛼 a includes both the effects due to lattice displacements and the first-order effects due to electron deformation. The above expression for the dipole moment clearly implies the presence of electrical anharmonicity in the crystal.

2.4 Simple Model of Pyroelectric Effect

For the case of harmonic crystal, the potential energy is proportional to the square of the normal coordinates. In reality, crystals are not harmonic, and the mechanical anharmonicity has to be taken into account. For such a crystal, the potential energy involves the cubic and higher powers of the normal coordinates. 1∑ 2 2 ∑ 𝜔Q + b ′ ′′ Q Q ′ Q ′′ + (2.6) W= 2 j j j jj′ j′′ jj j j j j The primary pyroelectric coefficients p𝜖 could be expressed in terms of coefficients that appear in the expansion of dipole moment and potential energy (i.e. Eqs. 2.5 and 2.6). ( ) ∑ ∑ 𝛼a bajj Cj 𝜖 (2.7) 𝛽jj − p = 𝜔2a 𝜔2j a j Here, Cj is the contribution of the jth mode to the specific heat and is given by ℏ𝜔j (𝜕ñj /𝜕T) and ñj is the average occupation number of the phonons with energy ℏ𝜔j . From the above expression, it could be shown that under Debye’s approximation (which actually involves invoking some properties of 𝛽 jj and bajj in the long wavelength limit), the primary pyroelectric coefficient obeys T 3 law. But as has been rightly pointed out by Szigeti, in the case of complex polyatomic crystals, optic modes can have very low frequencies, and hence, the T 3 behavior is observed only at very low temperatures. At this stage, it is also necessary to emphasize the role of both electrical and mechanical anharmonicity in primary pyroelectricity. Since the primary pyroelectric effect does not involve thermal expansion, for quite some time, it was regarded that only the electrical anharmonicity was responsible for primary pyroelectricity. It may be mentioned that thermal expansion is the result of mechanical anharmonicity. However, from the expression for primary pyroelectric coefficient, it is evident that the part played by mechanical anharmonicity cannot be neglected.

2.4 Simple Model of Pyroelectric Effect Microscopically, the pyroelectric effect occurs because of the asymmetric environment experienced by electrically charged atoms within the crystal structure of the material. This can be viewed schematically as presented in Figure 2.10, which shows a two-dimensional lattice of cations and anions. The cations are displaced relative to the unit cells “center of gravity” to give rise to an electrical dipole moment (or spontaneous polarization PS ) along the line (x1 − x2 ). The potential energy of any cation along this line will be an asymmetric form as illustrated in Figure 2.11. Any excitation caused by an increase in lattice temperature will make it change its quantized energy level (E1 to En ) within the well and lead to a change in its mean equilibrium position in the lattice along the line A–B in Figure 2.11. This gives a change in the overall electrical dipole moment, which appears as the macroscopic pyroelectric effect [93].

33

2 Pyroelectricity

–

– +

+

–

–

X1

Figure 2.10 Schematic two-dimensional presentation of pyroelectricity. Source: Whatmore [93], figure 1 (p. 1337)/IOP Publishing.

–

+

0

X2

– +

–

–

–

Ps

Figure 2.11 Potential energy of cation in lattice of Figure 2.10 along the line x1 –x2 , E 1 to E n represent the quantized energy levels for the cation and the locus A–B is the change in its equilibrium position with change in energy. Source: Whatmore [93], figure 2 (p. 1338)/IOP Publishing.

Potential energy

34

B

En E1 X1

A

Distance 0X2

In dielectrics exhibiting pyroelectricity, the dipole moment can arise as a consequence of the packing in an ionic crystal (e.g. in zincite ZnO, hexagonal 6 mm) because of the alignment of polarized covalent bonds in molecular crystals or crystalline polymers (such as in the carbon–fluorine bond in the polar form of polyvinylidene fluoride, PVDV orthorhombic 2 mm) or because of atomic displacements controlled by the position of hydrogen ions in a hydrogen bonded crystal (such as potassium dihydrogen phosphate, KDP, tetragonal 4 mm below 122 K) [93]. To appreciate the meaning of that definition and the nature of the pyroelectric effect, we can consider a simple example: a thin, parallel-sided sample of material, such as a tourmaline crystal or a ceramic disk of barium titanate, is cut so that its crystallographic symmetry axis is perpendicular to the flat surfaces. The unit cells of pyroelectric materials have a dipole moment. The dipoles are packed so that the components of the dipole moment in each unit cell add up in the direction normal to the flat surfaces. The dipole moment per unit volume of the material is called the spontaneous polarization PS . Being always nonzero in a pyroelectric material, PS exists in the absence of an applied electric field and is equivalent to a layer of bound charge on each flat surface of the sample.

2.4 Simple Model of Pyroelectric Effect

– – – – + + + + + + + + + + + – – – Ps – – – – – – – – + + + +

dT = 0 dt

– – – – + + + + + + – – – – – – + + + +

– – – + + + + – Ps – – – + + +

– +

Ions

Dipoles

–

0

+

– +

Ammeter

Electrodes e– – +

+

– +

– +

– +

–

Ps – +

– + + –

+ –

dT > 0 dt

– +

– +

Current –

0

+

Ammeter

Figure 2.12 If a pyroelectric crystal with an intrinsic dipole moment (top) is fashioned into a circuit with electrodes attached on each surface (middle), an increase in temperature T prompts the spontaneous polarization P S to decrease as the dipole moments, on average, diminish in magnitude. The horizontal tilting of the dipoles, pictured at bottom, signifies the effect. A current flow to compensate for the change in bound charge that accumulates on the crystal edges. Source: Lang [94], figure 1 (p. 245)/Taylor & Francis.

Nearby free charges such as electrons or ions will be attracted to the sample (Figure 2.12). Imagine that conductive electrodes are then attached to the surfaces and connected through an ammeter having a low internal resistance. If the temperature of the sample is constant, then so is PS and no current flows through the circuit. But in most single crystals and ceramics, an increase in temperature causes the net dipole moment and, consequently, the spontaneous polarization to decrease. The quantity of bound charge then decreases, and the redistribution of free charges to compensate for the change in bound charge results in a current flow – the pyroelectric current – in the circuit. If the sample had been cooled instead of heated, the current’s sign would be reversed. Note that the pyroelectric effect is only observable during the period in which the temperature changes. In an open circuit, the free charges would simply remain on the electrodes, and the voltage could be measured. A large number of pyroelectric materials exist, including minerals such as tourmaline, single crystals such as triglycine sulfate, ceramics such as lead zirconate titanate, polymers such as polyvinylidene fluoride, and even biological materials such as collagen.

35

36

2 Pyroelectricity

2.5 Pyroelectric Crystal Symmetry According to Neumann’s principle, polarization P must conform to the point-group symmetry of the crystal. It follows immediately that a pyroelectric effect cannot exist in a crystal possessing a center of symmetry, a fact that provides a practical method of testing for the absence of a center. A little thought shows that a pyroelectric moment can only lie along a direction in a crystal that is unique, in the sense that it is not repeated by any symmetry element. If there should exist in the point group a unique direction, which is an axis of symmetry (twofold, threefold, fourfold, or sixfold), this will necessarily be the direction of P. But the presence of such a unique symmetry axis is not essential for the existence of a pyroelectric effect. It may be noted that a unique direction, as defined above, is not synonymous with a polar direction. A polar direction is any direction of which the two ends are not related by any symmetry element of the point group. Thus, a diad axis in class 32 is a polar direction, but not a unique direction. All unique directions are polar, but only some polar directions are unique. The direction of the polarization vector P and the form of its components in the 21 noncentrosymmetrical classes are depicted in Table 2.2.

Table 2.2

Crystal symmetry and direction of polarization P.

System

Symmetry class

Polarization components

Direction of polarization

Triclinic

1

P1 P 2 P 3

no symmetry restricted on the direction of P

Monoclinic

2

𝟎 P𝟐 𝟎

x2 parallel to the diad axis, rotation or inverse

Monoclinic

m

P𝟏 𝟎 P𝟑

P has any direction in the symmetry plane

Orthorhombic

mm2

𝟎 𝟎 P𝟑

P parallel to the diad axis

Orthorhombic

222

𝟎 𝟎 𝟎

Tetragonal, trigonal, hexagonal

4, 4mm, 3, 3m, 6, 6mm

𝟎 𝟎 P𝟑

Tetragonal, trigonal, hexagonal

4, 42m, 422, 32, 6, 6m2, 622

𝟎 𝟎 𝟎

Cubic

432, 43m, 23

𝟎 𝟎 𝟎

P parallel to the 4, 3 or 6 axes

2.6 Piezoelectricity

Thus, the following 10 classes may theoretically show pyroelectricity under uniform heating or cooling: 1

2

3

4

6

m

mm2

3m

4 mm

6 mm

They are called the polar classes.

2.6 Piezoelectricity In 1880, the piezoelectric effect was discovered by the French physicists Pierre Curie and Paul-Jean Curie; it is the appearance of electric charges on the surfaces of some crystals when they are acted upon by external mechanical stress [95]. Crystalline substances that display a piezoelectric effect are called piezoelectrics. The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in solid materials [96]. It also exhibits the reverse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electrical field). The nature of the piezoelectric effect is closely related to the occurrence of electric dipole moments in solids. The latter may be either induced for ions on crystal lattice sites with asymmetric charge surroundings or directly be carried by molecular groups. The dipole density or polarization may easily be calculated for crystals by summing up the dipole moments per volume of the crystallographic unit cell. As every dipole is a vector, the dipole density P is a vector field. Dipoles near each other tend to be aligned in regions called Weiss domains. The domains are usually randomly oriented but can be aligned using the process of poling, a process by which a strong electric field is applied across the material, usually at elevated temperatures. Not all piezoelectric materials can be poled. Of decisive importance for the piezoelectric effect is the change of polarization P when applying a mechanical stress. This might be caused by either a reconfiguration of the dipole-inducing surrounding or reorientation of molecular dipole moments under the influence of the external stress. Piezoelectricity may then manifest in a variation of the polarization strength, its direction, or both, with the details depending on (i) the orientation of P within the crystal, (ii) the crystal symmetry, and (iii) the applied mechanical stress. The change in P appears as a variation of surface charge density upon the crystal faces, that is, as a variation of the electrical field extending between the faces, since the units of surface charge density and polarization are the same. However, piezoelectricity is not caused by a change in charge density on the surface but by dipole density in the bulk. The polarization vector Pi is related to the stress tensor 𝜎 jk by the linear equation. Pi = dijk 𝜎jk

(2.8)

The quantities dijk , known as piezoelectric moduli, constitute a tensor of the third order having 33 = 27 components. Since the stress tensor is symmetric and dijk = dikj ,

37

38

2 Pyroelectricity

relation (2.8) can be abbreviated to a matrix form (Pi = dij 𝜎 j , where i = 1, 2, 3 and j = 1, 2, 3, 4, 5, 6), which is more useful and simpler for both calculation and consideration.

⎛P ⎞ ⎛d d d d d ⎜ 1 ⎟ ⎜ 11 12 13 14 15 ⎜P ⎟ = ⎜d d d d d ⎜ 2 ⎟ ⎜ 21 22 23 24 25 ⎜P ⎟ ⎜d ⎝ 3 ⎠ ⎝ 31 d32 d33 d34 d35

⎛𝜎1 ⎞ ⎜ ⎟ ⎜𝜎 ⎟ d16 ⎞ ⎜ 2 ⎟ ⎟ ⎜𝜎 ⎟ 3 d26 ⎟ ⎜ ⎟ ⎟ ⎜𝜎 ⎟ 4 d36 ⎟⎠ ⎜ ⎟ ⎜𝜎5 ⎟ ⎜ ⎟ ⎝𝜎6 ⎠

The matrix elements dij , which are used more often than the tensor elements dijk , are also called piezoelectric moduli. According to the classical theory, 18 piezoelectric moduli are necessary to describe piezoelectric properties. Their number decreases with the increasing symmetry of the crystal. Of the 32 crystal classes, 21 are non-centrosymmetric (not having a center of symmetry), and of these, 20 exhibit direct piezoelectricity (the 21st is the cubic class 432). Ten of these represent the polar crystal classes, which show a spontaneous polarization without mechanical stress due to a nonvanishing electric dipole moment associated with their unit cell and which exhibit pyroelectricity. If the dipole moment can be reversed by the application of an electric field, the material is said to be ferroelectric. ● ●

Polar crystal classes: 1, 2, m, mm2, 4, 4mm, 3, 3m, 6, 6mm Piezoelectric crystal classes: 1, 2, m, 222, mm2, 4, 4, 422, 4mm, 42m, 3, 32, 3m, 6, 6, 622, 6mm, 62m, 23, 43m

For polar crystals, for which P ≠ 0 holds without applying a mechanical load, the piezoelectric effect manifests itself by changing the magnitude or the direction of P or both. For the nonpolar, but piezoelectric crystals, on the other hand, a polarization P different from zero is only elicited by applying a mechanical load. For them, the stress can be imagined to transform the material from a nonpolar crystal class (P = 0) to a polar one, having P ≠ 0. For the converse piezoelectric effect, strain 𝜀 is related to electric field E. The matrix form of the converse coefficient dij (electrostriction coefficients) is the transpose of the direct effect matrix. 𝜀i = dij Ej

(2.9)

Written out, the converse effect is ⎛𝜀 ⎞ ⎛d ⎜ 1 ⎟ ⎜ 11 ⎜𝜀 ⎟ ⎜d ⎜ 2 ⎟ ⎜ 12 ⎜𝜀 ⎟ ⎜d ⎜ 3 ⎟ = ⎜ 13 ⎜𝜀 ⎟ ⎜d ⎜ 4 ⎟ ⎜ 14 ⎜𝜀 ⎟ ⎜d ⎜ 5 ⎟ ⎜ 15 ⎜𝜀 ⎟ ⎜d ⎝ 6 ⎠ ⎝ 16

d21 d31 ⎞ ⎟ d22 d32 ⎟ ⎟ ⎛E ⎞ 1 d23 d33 ⎟ ⎜ ⎟ ⎟ ⎜E ⎟ 2 d24 d34 ⎟ ⎜ ⎟ ⎟ ⎜E ⎟ ⎝ 3⎠ d25 d35 ⎟⎟ d26 d36 ⎟⎠

2.7 Ferroelectricity

For the matrix elements of electrostriction coefficients dij , the same notation is used as for piezoelectric moduli, as the two quantities are equal to one another and, therefore, are expressed in the same units C/N (Coulomb/Newton) or m/V.

2.7 Ferroelectricity Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field [97, 98]. All ferroelectrics are pyroelectric, with the additional property that their natural electrical polarization is reversible. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle salt by Valasek [99]. Thus, the prefix ferro, meaning iron, was used to describe the property despite the fact that most ferroelectric materials do not contain iron. Materials that are both ferroelectric and ferromagnetic are known as multiferroics. Ferroelectrics are the most typical nonlinear dielectrics. Apart from the dependence of permittivity on electric field intensity, the most essential features of this class of dielectrics are hysteresis under the action of an alternative voltage. It is possible to obtain for a ferroelectric a loop of electric hysteresis, displacement–electric field intensity D–E. This loop is similar to a loop of magnetic hysteresis: magnetic induction–magnetic field intensity B–H for ferromagnetic materials. The presence of spontaneous polarization (Ps ) without an external electric field usually acts on the ferroelectric materials. Ferroelectric crystals possess regions with uniform polarization called ferroelectric domains. Within a domain, all the electric dipoles are aligned in the same direction. There may be many domains in a crystal separated by interfaces called domain walls. A ferroelectric single crystal, when grown, has multiple ferroelectric domains. A single domain can be obtained by domain wall motion made possible by the application of an appropriate electric field. A very strong field could lead to the reversal of the polarization in the domain, known as domain switching. The main difference between pyroelectric and ferroelectric materials is that the direction of the spontaneous polarization in ferroelectrics can be switched by an applied electric field. The polarization reversal can be observed by measuring the ferroelectric hysteresis as shown in Figure 2.13. As the electric field strength is increased, the domains start to align in the positive direction, giving rise to a rapid increase in the polarization (OB). At very high field levels, the polarization reaches a saturation value. The polarization does not fall to zero when the external field is removed. At zero external field, some of the domains remain aligned in the positive direction; hence, the crystal will show a remnant polarization Pr (OD). The crystal cannot be completely depolarized until a field of magnitude Ec is applied in the negative direction. The external field needed to reduce the polarization to zero is called the coercive field strength Ec . If the field is increased to a more negative value, the direction of polarization flips, and hence, a hysteresis loop is obtained. The value of the spontaneous polarization Ps (OE) is obtained by extrapolating the curve onto the polarization axes (BE).

39

40

2 Pyroelectricity

P

Figure 2.13 A polarization vs. electric field (P–E) hysteresis loop for a typical ferroelectric crystal.

B

E D Ps –Es

Pr

A

–Ec

E Ec

O

Es

H G

2.7.1

Ferroelectric Phase Transitions

A phase transition is the transformation of thermodynamic system from one phase or state of matter to another. It is a collective phenomenon in which the critical behavior depends on small number of parameters and is universal for many systems. During a phase transition of a given medium, certain properties of the medium change, often discontinuously, as a result of some external conditions such as temperature and pressure. Phase transition involves some change of symmetry. According to Paul Ehrenfest, phase transitions can be divided into two major groups called first- and second-order transitions, depending on whether the transition is discontinuous or continuous, respectively. Paul Ehrenfest classified phase transitions based on the behavior of the thermodynamic free energy as a function of other thermodynamic variables. Under this scheme, phase transitions were labeled by the lowest derivative of the free energy that is discontinuous at the transition. First-order phase transitions exhibit a discontinuity in the first derivative of the free energy with respect to some thermodynamic variable [100]. Second-order phase transitions are continuous in the first derivative but exhibit discontinuity in a second derivative of the free energy [100]. Though useful, Ehrenfest’s classification has been found to be an inaccurate method of classifying phase transitions, for it does not take into account the case where a derivative of free energy diverges (which is only possible in the thermodynamic limit). For instance, in the ferromagnetic transition, the heat capacity diverges to infinity. In the modern classification scheme, phase transitions are divided into two broad categories, named similarly to the Ehrenfest classes. First-order phase transitions are those that involve a latent heat. During such a transition, a system either absorbs or releases a fixed (and typically large) amount of energy. During this process, the temperature of the system will stay constant as heat is added: the system is in a “mixed-phase regime” in which some parts of the system have completed the transition and others have not. Familiar examples are the melting of ice or the boiling of water (the water does not instantly turn into vapor but forms a turbulent mixture of liquid water and vapor bubbles). Second-order phase transitions are also called continuous phase transitions. They are characterized by a divergent susceptibility, an infinite correlation length, and a power-law decay of correlations near criticality.

2.7 Ferroelectricity

Figure 2.14

Schematic potential well.

Energy

Polarization

(a)

Polarization (P)

Polarization (P)

Examples of second-order phase transitions are the ferromagnetic transition, superconducting transition, and the superfluid transition. In ferroelectrics, two common types of phase transition are identified. These are named depending on how the order parameter (polarization) changes during the transition. It is common to observe that as the temperature is raised, the bulk polarization decreases and vanishes abruptly at temperature T c . This is a phase transition, just as in a ferromagnet raised above its Curie temperature or a solid raised above its melting point. It arises microscopically because as temperature is raised, the thermal vibrations of the atoms in the solid cause fluctuations, which overcome the potential barrier between the two (or more) wells. It is most easily understood in a molecular crystal such as NaNO2 , where one can imagine that each molecule can fluctuate between two configurations. Each of which has a double potential as shown in Figure 2.14 and some interactions between the dipoles that tend to align them. The detailed microscopic theory of how this happens will differ from material to material, but the macroscopic properties of the phase transition will be similar across many different classes of materials. A first-order transition is one that has a discontinuity in the order parameter itself, while a second-order transition is one that has a discontinuity in the first derivative of the order parameter. In a first-order transition, the polarization varies continuously, until the Curie temperature (T c ) at which there is a discontinuity (Figure 2.15a). In a second-order transition, the order parameter itself is a continuous function of temperature, but there is a discontinuity in its first derivative at T c (Figure 2.15b).

Temperature (T)

Tc

(b)

Temperature (T)

Tc

Figure 2.15 Plots of spontaneous polarization vs. temperature. (a) First-order transition and (b) second-order transition.

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Figure 2.16 Ideal domain configuration in a single crystal of cubic ferroelectric material where the coupling to strain is negligible. On the right is the configuration adopted when strain effects are important.

2.7.2

Ferroelectric Domains

The spontaneous polarization arises inside the ferroelectric materials with equal probability along any of the crystallographic direction when the material is cooled from the paraelectric to ferroelectric phase transition temperature. The directions along which the polarization will develop depend on the electrical and mechanical boundary conditions imposed on the sample. The regions of the crystal with uniform oriented spontaneous polarization are called ferroelectric domains (Figure 2.16). The region between two domains is called the domain wall. In a newly grown single crystal, there will be many domains with individual polarizations such that there is no overall polarization. They often appear: the polarization of individual domains is organized such that positive heads are held near negative tails. This leads to a reduction in stray field energy, because there are fewer isolated heads and tails of domains. This is analogous to the strain energy reduction found in dislocation stacking. The walls that separate domains with oppositely oriented polarization are called 180∘ walls and those that separate regions with mutually perpendicular polarization are called 90∘ walls. In a polycrystal (one with more than one crystallographic grain), the arrangement of domains depends on grain size. If the grains are fine (≪1 μm), then there is usually found to be one domain per grain. In larger grains, there can be more than one domain in each grain. Figure 2.17 displays a micrograph showing the 90∘ domains in a single crystal of BaTiO3 . In this grain, the domains are twinned in such a way as to reduce the overall stray electric field energy. As each domain possesses its own dipole moment, we may switch dipole moments in order to encode information. The types of domain walls that can occur in a ferroelectric crystal depend on the symmetry of both the nonferroelectric and ferroelectric phases of the crystal. In the rhombohedral phase of lead zirconate titanate Pb(Zr,Ti)O3 , the direction of the polarization develops along the body diagonals (direction ) of the paraelectric cubic unit cell. This gives eight possible directions of the spontaneous polarization with 180∘ , 71∘ , and 109∘ domain walls. Observations by transition electron microscopy show that domain walls in ferroelectric thin films are on the order of 1–10 nm.

2.7.3

Ferroelectric Domain Wall Motion

The most interesting characteristic of ferroelectric materials is the polarization reversal or switching of domain wall by an electric field. The domain wall switching in

2.7 Ferroelectricity

Figure 2.17 Optical micrograph of 90∘ domains in a single crystal of BaTiO3 . Source: Reproduced from Figure 4.5 from the book “Ferroelectrics: Principles and Applications, Wiley-VCH, 2017.

ferroelectric materials is the occurrence of the polarization hysteresis loop as shown in Figure 2.18. The polarization reversal can be accomplished by either the growth of existing domains antiparallel to the applied field by domain wall motion or the nucleation and growth of new antiparallel domains. Nucleation usually occurs at particular locations in a sample where a particular polarization direction is favored over another, and domains will nucleate at the same sites each time the sample switches (i.e. nucleation is inhomogeneous). The time scale for domain growth varies strongly on the strength of the applied field and the material and sample geometry; however, it is usually seen that the forward growth of domains occurs first and is much faster than the subsequent sideways growth. The domains can grow either along the polar direction or by sideways motion of 180∘ domain walls as shown in Figure 2.19. The domain structure and the properties of the domain boundaries play an important role in the performance of many ferroelectric devices, such as the field-effect transistors (FeFETs) [102, 103] and the fast high-density nonvolatile random access memory (FeRAMs) [104]. Mechanical and electrical characteristics such as the permittivity, coercive field, and piezoelectric constants are often significantly influenced. In particular, the thickness and the interfacial energy of the domain walls are important parameters in understanding the switching kinetics and fatigue mechanism in ferroelectric materials. The width affects the wall mobility and the energy determines how easily new domain walls may be introduced during polarization reversal processes. The domain nucleation and growth process are affected strongly by extended structural defects (domain walls, dislocations, grain boundaries, etc.) and point defects (vacancies). As a consequence, the switching kinetics is significantly influenced by the density of these defects [105–107]. The

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100

3.04 Switched area (%)

44

–800

4.67 (–40, 9.3) –307

227 316 –81.4

–600

–400

–200

200

0

400

600

800

–1

Electric field (kV cm )

Figure 2.18 Asymmetry of the P–E hysteresis loop. The ferroelectric hysteresis loop and corresponding footage of the TEM images recorded at various stages of polarization switching (ash line for positive and black line for negative switching) after poling (black line) to the P[001] polarization. Source: Lee et al. [101]. Figure 4 (p.9)/With permission of Elsevier.

(a)

(b)

E –1 (kV cm )

A

318 319

326

331

E (kV cm–1)

A

–218 –249

B

–327

B

–545

Figure 2.19 (a) Series of TEM images and schematic drawing illustrates the domain nucleation at the PZT/Ni interface followed by forward-limited switching and (b) sideways growth-limited switching. Source: Lee et al. [101]. Reproduced from Figure 4.7 from the book “Ferroelectrics: Principles and Applications, Wiley-VCH, 2017.

2.7 Ferroelectricity

domain evolution is determined by the spatial distribution of local field produced by various charged systems with wide range of the relaxation times. The knowledge of kinetics of screening processes and sources of the fields driving the domain evolution is the foundation for the progress in domain engineering. Thus, for a thorough understanding of the physical processes associated with the switching and fatigue behavior of a ferroelectric material, an accurate microscopic description of the underlying domain walls and their dynamics is very important. Application of high-resolution techniques such as scanning probe microscopy (SPM) in conjunction with conventional electrical measurements provides a unique opportunity to achieve microscopic insight into the physical processes occurring in ferroelectric single crystals and thin films. The discovery of electrical conductivity in specific types of ferroelectric domain walls gave rise to “domain wall nanoelectronics,” a technology in which the wall (rather than the domain) stores information. This paradigm shift critically hinges on precise nanoengineering of reconfigurable domain walls. Domain walls are spatially mobile, can be controllably shunted from point to point, and can be spontaneously created, or made to disappear. This unique “now-you-see-it, now-you-don’t” dynamic property could radically alter the way in which we think about the integration of functional materials into devices and the way in which device functionality is enabled: functionally active domain walls themselves could be introduced or removed as the primary mechanism in device operation. Conductive ferroelectric domain walls (EFDWs) were predicted as far back as the 1970s [108]. However, critical advances made recently in the understanding of their nature and conduction properties have enabled the knowledge required to realize FEDW memory and logic devices based on these nanoscale elements [109–113]. For example, the ability to precisely manipulate FEDW conductivity [114–117], modulate and gate FEDW transport [109–113], and create FEDW interconnects has been pivotal milestones toward the development of a functioning memory.

2.7.4

Soft Mode

In 1960, W. Cochran proposed the soft-mode concept to describe the mechanism of the ferroelectric structural phase transitions [118–120]. This concept is based on assuming that the crystal becomes unstable against a particular normal vibration of the lattice (phonon) whose frequencies tend to zero (soften) if the crystal approaches a structural phase transition on changing the external thermodynamic force (e.g. temperature, pressure, and field). In the high-temperature phase, there exists a certain unstable phonon (the “soft mode”) whose frequency decreases as the temperature approaches T c (the phonon “softens”) and reaches zero at T c. It means that the corresponding lattice vibration become “frozen” at this temperature and produce a structure of another symmetry with a finite dipole moment. Since the soft modes in ferroelectrics lead to electrical polarization, they are optically active and can be detected by means of optical spectroscopy in the spectra of dielectric permittivity (real and imaginary parts). The behavior of the parameters of the soft mode is governed by three fundamental laws:

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The static dielectric permittivity produced by the soft mode obeys the Curie–Weiss law: 𝜀0 ∼ (T − Tc )−1

●

The eigen frequency 𝜔t of the soft mode follows the Cochran behavior: 𝜔t ∼ (T − T0 )1∕2

where T 0 is the soft mode condensation temperature. The static dielectric constant and the soft-mode frequency are related via the Lyddane–Sachs–Teller relation: 𝜀inf 𝜀0 (T)

=

𝜔2t (T) 𝜔2L

where 𝜀inf is the high-frequency dielectric constant and 𝜔L is the longitudinal frequency of the corresponding vibration. Normally, the phonons that soften exhibit frequencies below 100–200 cm−1 well above the transition temperature. When softening while approaching T c , these modes shift to lower frequencies, finally leaving the range where the conventional IR spectrometers can operate and entering the submillimeter-millimeter wavelength domain. At the phase transition, the phonons become highly anharmonic at the precise Brillouin zone point, but the crystal as a whole remains rather harmonic and the thermal expansion anomalies are typically small. Below the phase transition, something must happen to restore the quasi-harmonic character of all lattice vibrations, and this depends on details of phase transition. 2.7.4.1

Zone-center Phonons

In this case, the optical zone-center phonon softens completely at the phase transition and then hardens again below it, as the system finds a new dynamical equilibrium around the distorted structure. The periodicity of the structure is unchanged through the phase transition. 2.7.4.2

Zone-boundary Phonons

When the distortion is driven by a zone-boundary phonon, the distorted structure will have a larger unit cell (the translational symmetry is broken). The zone boundary point will then fold to the new zone center and the soft phonon will harden below the phase transition to become a new zone center phonon. The atomic displacements associated with the soft mode are the same as the deformation of the structure in the low-temperature phase. For example, in the high-temperature phase of PbTiO3 , the soft mode involves the Pb+ and Ti+ cations moving along the c-axis in one direction and the O anions moving in the opposite direction [121]. This is the distortion that freezes into the structure in the low-temperature phase. There is a soft mode in the low-temperature phase that corresponds to the same atomic motions but now vibrating about new mean positions. In the case of PbTiO3 , the soft mode in the high-temperature phase is degenerate; thus, two corresponding modes are observed in the low-temperature phase.

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97 Känzig, W. (1957). Ferroelectrics and antiferroelectrics. In: , Solid State Physics, vol. 4 (ed. F. Seitz, T.P. Das, D. Turnbull and E.L. Hahn), 5. Academic Press. ISBN: 978-0-12-607704-9. 98 Lines, M. and Glass, A. (1979). Principles and Applications of Ferroelectrics and Related Materials. Oxford: Clarendon Press. ISBN: 978-0-19-851286-8. 99 Valasek, J. (1920). Piezoelectric and allied phenomena in Rochelle salt. Phy. Rev. 15 (6): –537; and Valasek, J. (1921). Piezo-electric and allied phenomena in rochelle salt. Phy. Rev. 17 (4): 475. 100 Blundell, S.J. and Katherine, M.B. (2008). Concepts in Thermal Physics. Oxford University Press. 101 Lee, J.K., Shin, G.Y., Song, K. et al. (2013). Direct observation of asymmetric domain wall motion in a ferroelectric capacitor. Acta Mater. 61: 6765–6777. 102 Naber, R.C.G., Tanase, C., Blom, P.W.M. et al. (2005). High-performance solution-processed polymer ferroelectric field-effect transistors. Nat. Mat. 4 (3): 243–248. 103 Tokumitsu, E., Fujii, G., and Ishiwara, H. (1999). Nonvolatile ferroelectric-gate field-effect transistors using /SiON/Si6 O2 /Pt/SrTa9 O2 Ta2 SrBi structures. Appl. Phys. Lett. 75: 575. 104 International Technology Roadmap for Semiconductors (ITRS), Update p. 23, 2006. 105 Nagarajan, V., Roytburd, A., Stanishevsky, A. et al. (2003). Dynamics of ferroelectric domains in ferroelectric thin films. Nat. Mater. 2: 43. 106 Chu, M.W., Szafraniak, I., Scholz, R. et al. (2004). Impact of misfit dislocations on the polarization instability of epitaxial nanostructured ferroelectric perovskites. Nat. Mater. 3: 87. 107 Roelofs, A., Pertsev, N.A., Waser, R. et al. (2002). Depolarizing field-mediated 180∘ switching in ferroelectric thin films with 90∘ domains. Appl. Phys. Lett. 80: 1424. 108 Vul, B.M., Guro, G.M., and Ivanchik, I.I. (1973). Encountering domains in ferroelectrics. Ferroelectrics 6: 29–31. 109 Whyte, J.R., McQuaid, R.G.P., Sharma, P. et al. (2014). Ferroelectric domain wall injection. Adv. Mater. 26: 293–298. 110 McGilly, L.J., Yudin, P., Feigl, L. et al. (2015). Controlling domain wall motion in ferroelectric thin films. Nat. Nanotechnol. 10: 145–150. 111 Whyte, J.R. and J.M. (2015). Gregg A diode for ferroelectric domain-wall motion. Nat. Commun. 6: 7361. 112 McGilly, L.J., Feigl, L., Sluka, T. et al. (2016). Velocity control of 180∘ domain walls in ferroelectric thin films by electrode modification. Nano Lett. 16: 68–73. 113 Li, L., Britson, J., Jokisaari, J.R. et al. (2016). Giant resistive switching via control of ferroelectric charged domain walls. Adv. Mater. 28: 6574–6580. 114 Seidel, J., Maksymovych, P., Batra, Y. et al. (2010). Domain wall conductivity in La-doped BiFeO3 . Phys. Rev. Lett. 105: 197603. 115 Crassous, A., Sluka, T., Tagantsev, A.K., and Setter, N. (2015). Polarization charge as a reconfigurable quasi-dopant in ferroelectric thin films. Nat. Nanotechnol. 10: 614–618.

References

116 Vasudevan, R.K., Morozovska, A.N., Eliseev, E.A. et al. (2012). Domain wall geometry controls conduction in ferroelectrics. Nano. Lett. 12: 5524–5531. 117 Stolichnov, I., Feigl, L., McGilly, L.J. et al. (2015). Bent ferroelectric domain walls as reconfigurable metallic-like channels. Nano. Lett. 15: 8049–8055. 118 Cochran, W. (1959). Crystal stability and the theory of ferroelectricity. Phys. Rev. Lett. 3: 412–414. 119 Cochran, W. (1960). Crystal stability and the theory of ferroelectricity. Adv. Phys. 9: 387–423. 120 Cochran, W. (1961). Crystal stability and the theory of ferroelectricity part II. Piezoelectric crystals. Adv. Phys. 10: 401. 121 Shirane, G., Axe, J.D., Harada, J., and Remeika, J.P. (1970). Soft ferroelectric mode in lead titanate. Phys. Rev. B2: 155–159.

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3 Pyroelectric Materials and Applications 3.1 Introduction The pyroelectric effect is the phenomenon where a change in temperature in a polar dielectric engenders a change in its electrical dipole moment. It can be used to create a flow of current in an external circuit connected to a piece of the material. The pyroelectric effect has been known for many years, but it is only since about 1960 that its technological applications have been seriously considered. These applications have been almost entirely used in the field of the detection of electromagnetic radiation, especially in the two “atmospheric window” infrared (IR) bands of 3–5 and 8–14 μm. The ambient temperature operation of pyroelectric detectors, leading to low power consumption, low cost, compactness, and lack of any requirement for logistical support in the form of cooling fluids or high-pressure gas gives them a number of important advantages over photon detectors used in these wavebands, such as mercury cadmium telluride or indium antimonide. These detectors must be cooled to cryogenic temperatures to obtain optimum performance. Hence, pyroelectric detectors have found a huge number of applications in products ranging from fire alarms to intruder detectors, in instrumentation such as gas analysis and laser beam characterization, and in military/paramilitary applications such as thermal imaging. Pyroelectricity is a property of polar dielectric materials and is described by a change in the spontaneous polarization of the material due to a change in temperature with time. Under constant electric field and stress, the polarization change is described by a vector – the pyroelectric coefficient – given by: ) ( ) ( 𝜕P 𝜕D = (3.1) p= 𝜕T 𝜎,E 𝜕T 𝜎,E where D is the dielectric displacement, P is the polarization, and 𝜎 and E denote constant stress and constant electric field, respectively. Physically, the internal structure of the electric dipoles in the pyroelectric material is modified, such as the rotation or reorientation of molecular dipoles or a shift in the atomic arrangement of displacive-type ferroelectric crystals, when the material is exposed to a temperature gradient through heating or cooling. This causes a change in the spontaneous polarization (Ps ) and a subsequent change in the surface charge of the material. Pyroelectric Materials: Physics and Applications, First Edition. Ashim Kumar Bain and Prem Chand. © 2023 WILEY-VCH GmbH. Published 2023 by WILEY-VCH GmbH.

56

3 Pyroelectric Materials and Applications Electrode (area A, emissivity η)

VS

Incident radiation (power W(t)) Vo CA

Polar axis of element CE

ip

RG

RL

d 0V

Thermal conductance (GT)

Figure 3.1 Schematic diagram of a pyroelectric detector element. Source: Whatmore and Watton [1]/Springer Nature.

Basically, the pyroelectric detector is a capacitor – formed by adding top and bottom electrodes to a pyroelectric material – and is connected to an external circuit (Figure 3.1). Incident radiation absorbed by the pyroelectric material is converted into heat, resulting in a temperature variation dT, thus changing the magnitude of spontaneous polarization (Ps ). Changes in polarization alter the surface charge of the electrodes. To maintain neutrality, charges are expelled from the surface, resulting in a pyroelectric current in the external circuit. This current is proportional to the rate of change of polarization with respect to temperature as given by: ( ) dT (3.2) ip = Ap dt where A is the effective area of the capacitor, p is the pyroelectric coefficient (in this case, the component of vector Ps perpendicular to the electrode surface), and (dT/dt) is the rate of change of the pyroelectric material’s temperature with time and thus pyroelectric devices are coupled to the changes of any input energy flux that generates a change in element temperature. This energy input is usually in the form of electromagnetic radiation absorbed in the material (or a coating that is “black” at the wavelength of interest). The largest pyroelectric effects are generally observed in the class of polar dielectric materials known as ferroelectrics [2]. In these materials, the dipole moment appears at a phase transition from a higher to a lower symmetry state at the temperature known as Curie temperature (T c ). The direction of the dipole moment can be switched between two or more stable directions which would be crystallo-graphically equivalent in the higher temperature phase by the application of an electric field greater than a certain value (the coercive field, Ec ). A major advantage of ferroelectrics is that they can be used in a polycrystalline form. While a random assembly of crystals of a polar dielectric cannot exhibit the polar symmetry necessary for pyroelectricity, the application of a field greater than Ec will re-orient the polar axes of the crystallites so that they have a component parallel to it. This produces a net spontaneous polarization and thus a net pyroelectric effect.

3.2 Theory of Pyroelectric Detectors

It is worthwhile to explain here that the two modes of operation for a pyroelectric detector are “pyroelectric” and “dielectric” (bolometer). The pyroelectric mode of operation is in the pyroelectric or ferroelectric state of the material (i.e. below the T c of the material). The dielectric mode becomes operative through the application of a biasing field near T c . It can also be operated in both ferroelectric and para-electric phases of the material but near the T c . In the pyroelectric mode, large changes in the spontaneous polarization (Ps ) with temperature near ferroelectric phase transitions lead to large p; thus, the detector sensitivity increases. In the dielectric bolometer mode, a larger p can be achieved for operation with an electric field and low losses, which are typically realized by the application of an electric field that impedes domain boundary motion. The optimum bolometer detector response is a function of the applied electric field and temperature. However, additional equipment is required to control the temperature near T c . Ferroelectric materials are required by symmetry considerations to be also piezoelectric and pyroelectric. The combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors very useful, for example, for sensor applications. Ferroelectric capacitors are used in high-quality IR detectors, where the IR image is projected onto a two-dimensional array of ferroelectric capacitors that are capable of detecting temperature differences as small as millionths of a degree Celsius.

3.2 Theory of Pyroelectric Detectors In 1938, Y. Ta [3] first suggested the use of pyroelectric materials as IR detectors. Later on, a number of scientists [4–11] presented the theory of IR detectors. Figure 3.1 shows a schematic arrangement of a typical pyroelectric detector. The element has a thickness d, has electrodes with a cross-sectional area A and an emissivity 𝜂, and has thermal capacity H and thermal conductance to its surroundings GT . The thermal time constant is 𝜏 T = H/GT . The element is exposed to a sinusoidally modulated radiation flux, W = W 0 ei𝜔t where i is the imaginary operator and 𝜔 is the radial modulation frequency. The temperature difference between the element and its surroundings 𝜃 is given by the differential equation, 𝜂W = H

d𝜃 + GT 𝜃 dt

(3.3)

which has the solution, 𝜃=

𝜂W0 ei𝜔t GT + i𝜔H

(3.4)

The output electrical signal of the pyroelectric detector is amplified using a high input impedance field-effect transistor (FET) as a source follower amplifier as shown in Figure 3.1. The pyroelectric element generates a current I p = pA(d𝜃/dt), where p is the pyroelectric coefficient. The electrical time constant of the circuit is 𝜏 E = RG (CE + CA ), where RG is the gate resistor, CE is the capacitance of the element,

57

3 Pyroelectric Materials and Applications

and CA is the capacitance of the amplifier. The pyroelectric current ip , generated per watt of input power (the current responsivity RI ) is represented as: RI =

ip W0

𝜂pA𝜔 ( )1 GT 1 + 𝜔2 𝜏T2 2

=

(3.5)

At low frequencies (𝜔 ≪ 𝜏T−1 ), RI is proportional to 𝜔. For frequencies much greater than 𝜏T−1 , RI is constant and is: 𝜂p 𝜂pA = ′ (3.6) H cd where c′ is the volumetric specific heat. The voltage responsivity of the detector shown in Figure 3.1 is simply derived from the pyroelectric current and the electrical admittance (Y ) presented to ip . Ignoring, for the moment, the AC conductance of the pyroelectric element Y = R−1 + i𝜔C., where C = CE + CA . The pyroelectric voltG age (V p ) is generated on the gate of the electrical fast transient (EFT); therefore, the output voltage V 0 for unity amplifier gain is given by ip /Y and the voltage responsibility is RV : RI =

RV =

RG 𝜂pA𝜔 VP = ( )1∕2 ( )1∕2 W0 2 1 + 𝜔2 𝜏E2 GT 1 + 𝜔 𝜏T2

(3.7)

The voltage responsivity is shown in Figure 3.2. At low frequencies, RV is proportional to 𝜔 and at high frequencies, it is proportional to 𝜔−1 . RV is the maximum at 𝜔 = (𝜏 T 𝜏 E )−1/2 with the value: RV (max ) = 𝜂pA

RG 1 GT (𝜏T + 𝜏E )

(3.8)

At the frequencies 𝜔 = 𝜏E−1 and 𝜔 = 𝜏T−1 : 1

RV (T) = 2− 2 RV (max )

(3.9)

RV (Max) RV (T)

log RV

58

τ–1 T

(τE τT)–1/2 log ω

τ–1 E

Figure 3.2 Frequency variation of voltage responsivity. Source: Whatmore [6], figure 6 (p. 1342)/IOP Publishing.

3.2 Theory of Pyroelectric Detectors

Hence, between the limits set by 𝜏E−1 and 𝜏T−1 , the frequency response is reasonably flat, deviating by only 3 dB from the maximum value given by Eq. (3.8) over this range. For a given pyroelectric element, the responsivity can be maximized by reducing H (i.e. by making the element thin) and by minimizing GT (i.e. thermally isolating it from its surroundings). The high frequency (𝜔 ≫ 𝜏E−1 ; 𝜔 ≫ 𝜏T−1 ) dependence of RV is given by: RV =

c′ d(C

𝜂p E + CA )𝜔

(3.10)

If the element capacitance is large compared with CA , then Eq. (3.10) reduces to: RV =

𝜂p 0 A𝜔

c′ 𝜀𝜀

(3.11)

where 𝜀0 is the permittivity of the free space and 𝜀 is the relative permittivity of the pyroelectric material. For such a detector, the physical properties of the pyroelectric can be combined to create a voltage figure-of-merit (FoM), FV =

p c′ 𝜀0 𝜀

(3.12)

This only contains parameters describing properties of the pyroelectric material and is therefore an FoM that can be used to compare different materials for their potential voltage responsibility performances. If, however, in Eq. (3.10), CA ≫ CE the current FoM can be described as: p FI = ′ (3.13) c which is a different material FoM. Hence, it can be seen that the choice of the appropriate material will depend both on the amplifier to be used (JFET [junction-gate field-effect transistor] for example, have higher input capacitances than MOSFET [metal-oxide-semiconductor field effect transistor]) and on the size of the detector element. Small area detector elements will tend to favor high dielectric constant materials and vice versa to produce a good match between CA and CE . To optimize the behavior of a pyroelectric detector, it is necessary to consider the effects of intrinsic and extrinsic noise signals and compare them with the response as well. The primary noise sources are thermal noise (statistical fluctuations in the power flow from the detector element to the heat sink), Johnson noise in the equivalent circuit resistance, amplifier current noise, amplifier voltage noise, and piezoelectric or microphony noise. These noise sources were described in detail in the reference [6]. They can be combined to yield a total root mean squares (RMS) noise voltage, ΔV N . A useful parameter is the noise equivalent power (NEP) for an amplifier of bandwidth Δf . It is usually given for a unity bandwidth and is expressed −1∕2 in W HZ , NEP =

ΔVN (Δf )1∕2 RV

(3.14)

Both thermal noise and Johnson noise are proportional to the square root of the electrode area. This gives rise to one of the most frequently cited parameters, the

59

60

3 Pyroelectric Materials and Applications

area normalized detectivity or specific detectivity, A1∕2 (3.15) NEP Frequently, Johnson noise is the dominant source of noise in pyroelectric detectors. Thus, a noise FoM can be defined as follows: p (3.16) FD = ′ c (𝜀𝜀0 tan 𝛿)1∕2 D∗ =

where tan 𝛿 is the loss tangent of the pyroelectric materials. There is an increasing trend toward mounting pyroelectric detectors directly on an integrated circuit substrate. Bauer et al. [12, 13] developed analogous FoMs for a pyroelectric material mounted on a (heat sink) substrate whose thermal conductivity was infinite. These FoM values are p FV,sin k = (3.17) 𝜀𝜀0 k p (3.18) FI,sin k = k p FD,sin k = (3.19) k(𝜀𝜀0 tan 𝛿)1∕2 where k is the thermal conductivity of the pyroelectric materials. A dielectric bolometer mode of operation for a pyroelectric detector was introduced by Watton [14]. A conventional pyroelectric detector is operated below the Curie temperature T c of the material. A radiation-induced change in the detector temperature results in a change of polarization that is equivalent to the flow of a surface charge. In the dielectric bolometer mode, the pyroelectric is biased with a DC field. This charges the element and the heating due to the incident radiation results in an incremental change of permittivity and hence a signal voltage. The pyroelectric behavior of either type can be described in terms of a generalized pyroelectric coefficient, ) E( dPS d𝜀 p= + 𝜀0 dE (3.20) ∫0 dT dT E In equation (3.20), the first term is the conventional pyroelectric coefficient at zero applied field (below T c ) and the second term is an induced pyroelectric coefficient (above T c ). As shown in Figure 3.3, the two terms are in opposition below T c , and only the second term is nonzero above T c . It is not possible to obtain extremely high signal voltages by increasing the bias field because both the dielectric peak and d𝜀/dT are depressed with the increasing field as shown in Figure 3.4. The FoM analysis given previously is valid for the dielectric bolometer mode as well. Because it is desirable that T c should be above ambient temperatures for a conventional pyroelectric and below ambient temperatures for the dielectric bolometer mode, different pyroelectric materials must be used. Two extensively studied materials for the latter mode of operation are barium strontium titanate, Ba1−x Srx TiO3 , (BST) and lead magnesium niobate, Pb(Mg1/3 Nb2/3 )O3 , (PMN). The induced pyroelectric coefficient for BST with x = 0.34 is shown in Figure 3.5. The T c of this material is about 17 ∘ C.

3.2 Theory of Pyroelectric Detectors

Pyroelectric operation

Dielectric operation

Polarization

Dielectric constant with bias Eb Polarization Ps

Tc

Temperature

Figure 3.3 Two operating modes for ferroelectric materials as IR detectors. Source: Watton [14], figure 3 (p. 90)/Taylor & Francis. 10 ε V/micron

8

GEC(EEV)– BST

34/1

0

× 10–3 0.3

6

0.6 4

0.9 1.5

2

0

2.7 3.9 0

10

20 30 Temperature (°C)

40

50

Figure 3.4 Permittivity versus temperature at various bias fields for Ba0.66 Sr0.34 TiO3 . Source: Watton [14], figure 11 (p. 100)/Taylor & Francis. Figure 3.5 Induced pyroelectric coefficient of Ba0.66 Sr0.34 TiO3 . Source: Watton [14], figure 14 (p. 102)/Taylor & Francis.

30 20 °C GEC(EEV)– BST 34/1 P

18 °C

20

28 °C

10−4 C m−2 K−1 10

13 °C

38 °C

48 °C 0

0

1

2

3

4

Applied field (V/micron)

5

61

62

3 Pyroelectric Materials and Applications

3.3 Material Figure-of-Merits From a materials perspective, it is important to develop criteria that can be used to assess (quantifiably) the performance of a material for a particular application based solely on material properties. Typically, material FoMs are developed that allow material assessment outside of geometric and/or engineering constraints for material selection. However, in the assessment of pyroelectric materials, there is no single global FoM that can be applied to all situations, and the correct FoM must conform to the intended application of the material. This extends to the type of amplifier used at the output of the pyroelectric, the operation frequency of the detector, and the intended size of the pyroelectric element. Since pyroelectrics are essentially thermal transducers, both the electrical and thermal properties must be taken into account. Whatmore [6] has reviewed the necessary analysis to derive the current and voltage response and subsequent FoMs for pyroelectric materials: p FI = ′ for high current detectivity, (3.21) c p FV = ′ ′ for high voltage responsibility (3.22) c𝜀 and p FD = √ for high detectivity (3.23) c′ 𝜀′′ where p is the pyroelectric coefficient, c′ is the volumetric specific heat, 𝜀′ is the real part of the dielectric constant, and 𝜀′′ is the imaginary part of the dielectric constant (dielectric loss). Bauer et al. [12, 13] developed the following FoMs when the pyroelectric material is placed on a substrate that acts as a heat sink (i.e. whose thermal conductivity is infinite): p FI = for high current detectivity (3.24) k p FV = ′ for high voltage responsibility (3.25) k𝜀 and p for high detectivity (3.26) FD = √ k 𝜀′′ where k is the thermal conductivity of the pyroelectric materials.

3.4 Classification of Pyroelectric Materials Pyroelectric materials can be classified into five main areas: single crystals, perovskite ceramics, polymers, thin-film materials, and nanoparticles. General requirements of pyroelectric materials are a high pyroelectric coefficient, low relative permittivity, physical and chemical stability, low piezoelectric response, low

3.4 Classification of Pyroelectric Materials

cost, high quality, ease of processing, and if the material is ferroelectric, stability against depoling. A large number of pyroelectric materials have been extensively studied for their large potential applications, and the pyroelectric coefficient, dielectric properties, and the FoM for these materials will be given in here.

3.4.1

Single Crystals

The well-known single crystals for pyroelectric applications are triglycine sulfate (TGS), lithium tantalate (LiTaO3 ), lithium niobate (LiNbO3 ), BST (Bax Sr1−x TiO3 ), and strontium barium niobite (Srx Ba1−x Nb2 O6 , 0.25 ≤ x ≤ 0.75). Most of the aforementioned crystals are discussed in detail as follows. 3.4.1.1 Triglycine Sulphate

TGS crystal is considered as one of the potential materials for its wide range of applications, namely ultraviolet (UV) tunable laser, second harmonic generation (SHG), and pyroelectric IR sensors, due to its high pyroelectric coefficient, optical transmission, and reasonably low dielectric constant. It is a chemical compound with the formula (NH2 CH2 COOH)3 ⋅H2 SO4 [15]. The empirical formula of TGS does not represent the molecular structure, which contains protonated glycine moieties and sulfate ions. Figure 3.6 shows the crystal structure of TGS where hydrogen atoms are not shown [16]. TGS with protons replaced by deuterium is called deuterated TGS or DTGS (deuterated triglycine sulphate); alternatively, DTGS may refer to doped TGS. TGS and DTGS crystals are pyroelectric and ferroelectric and have been used as detector elements in IR spectroscopy. TGS crystals may be formed by evaporation of an aqueous solution of sulfuric acid, which is containing greater than threefold excess of glycine [17]. They belong to the polar space group P21 and therefore are pyroelectric and ferroelectric at room temperature, exhibiting spontaneous polarization along the b-axis [010] direction. The T c of the ferroelectric second-order phase transition is 49 ∘ C for TGS and 62 ∘ C for DTGS. The crystal structure consists of SO4 2− , 2(N+ H3 CH2 COOH) (G1 and G2 in the crystal-structure diagram), and + NH CH COO− (G ) species held together by hydrogen bonds [18]. These bonds 3 2 3 Figure 3.6 Crystal structure of TGS. Hydrogen atoms are not shown. Source: Balakumar and Zeng [16], figure 1 (p. 651)/Royal Society of Chemistry.

G2

S

G3

N G1

O

G1

C

G2 G3 G1

G1

b β a

c

63

3 Pyroelectric Materials and Applications

are easily broken by the polar molecules of water that explains the hygroscopicity of TGS – its crystals are easily etched by water. Along the b-axis, the G1 –SO4 and G2 –G3 layers are stacked alternately. The nearest two neighboring layers with identical chemical composition are rotated 180∘ around the b-axis against each other [16, 19]. Many modifications to TGS have been evaluated while maintaining the same basic crystal structure. These include deuteration (DTGS); substitution of sulfuric with fluoroberyllic acid to give triglycine fluoroberyllate (TGFB) without and with deuteration (to give DTGFB [deuterated triglycine fluoroberyllate]) [20] and substitution of glycine either by L-alanine or D-alanine (ATGS [alanine triglycine sulphate]) which has the effect of introducing an “internal” electric bias and stabilizing the spontaneous polarizations [21]. This is because the alanine molecules possess an extra methyl group which prevents them from rotating within the lattice. The dipole that it possesses is thus fixed with respect to the crystal structure and does not disappear at T c . ATGS crystals can be thermally cycled through T c and will retain their spontaneous polarization. Some promising results have been reported from crystals grown from solutions doped with alanine and phosphoric or arsenic acids [22, 23] to give ATGSP or ATGSAS crystals. Deuteration of these crystals provides little improvement in performance at room temperature but marginally increases T c . The temperature dependence of F V for the TGS, DTGS, and TGFB materials calculated from the data given by Felix et al. [24] is shown in Figure 3.7. It can be seen that deuteration of TGS makes a marked improvement to the pyroelectric properties and increases T c , which is desirable as it increases the temperature range of operation. Even so, provision is usually made in devices such as pyroelectric vidicon tubes which use DTGS for poling the targets in situ-Goss et al. [25]. Marginally higher performance and a further 12 ∘ C increase in T c are offered by TGFB. The development 0.6

0.5 DTGS

FV (m2 C–1)

64

0.4

TGFB

TGS

0.3

0.1

0 20

30

40

50

60

70

80

Temperature (°C)

Figure 3.7 Temperature dependence of the merit figure F v for some members of the TGS family. Source: Based on Felix et al. [24].

3.4 Classification of Pyroelectric Materials

of the As-doped materials may offer still further performance improvements (about 30% in F V relative to DTGS). It is interesting to note that the TGS family offers much higher values of F V than any of the oxide or polymer materials discussed in the following sections. However, most of them also possess dielectric losses that are an order of magnitude higher. As a consequence, the values of F D are comparable. An exception to this may be ATGSAS, which is reported to possess tan 𝛿 ≤ 0.01. It has been reported [26, 27] that the large anisotropy in the principal dielectric constant tensor coefficients can be used to obtain an improved p/𝜀 ratio from the TGS family by taking oblique crystal cuts in which the normal to the electrode faces of the detector is no longer parallel to the polar axis. This is because the effective pyroelectric coefficient exhibits a cosine dependence on the angle of rotation, while the dielectric constant depends on a cosine-squared relationship. A factor of two improvements in p/𝜀 for DTGFB can be obtained in this way, as shown in Figure 3.8. The TGS family provides the highest voltage responses, handling difficulties associated with their water solubility, hygroscopic nature and fragility have confined their use to those single-element detectors and vidicons, where sensitivity is of prime importance. Various techniques have been explored to prepare thin films as an alternative way to lapping and polishing thin slices and long crystal growth processes. Whipps and Bye [28] have prepared thin polycrystalline films (10–40 μm thick) by the sedimentation from a fluid suspension of freeze-dried crystallites of TGS onto glass slices, followed by compression to remove porosity. However, the pyroelectric performance from the films was lower than from single crystal devices of similar dimensions. Hadni et al. [29] have developed a technique by which oriented thin films of TGS can be grown by nucleation of crystals in an array of sub-microscopic holes etched in a substrate. Hadni and Thomas [30] and Hadni [31] have shown that it is possible to grow an epitaxial layer of TGS on a gold (Au) electrode with a thickness smaller than 1 mm and thus make a pyroelectric detector. The pyroelectric 2

FV (m2 C–1)

Figure 3.8 Temperature dependence of F v in DTGFB at a normal cut and in a cut perpendicular to a direction that forms an angle of 74∘ with the pyroelectric axis. Source: Shaulov [26], figure 4 (p. 181)/AIP Publishing.

74° cut

1

Normal

0 30

40

50 60 Temperature (°C)

70

65

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detectivity was found to be better than bulk TGS detectors at 40 ∘ C. They have also shown that the detectivity of TGSe epitaxial detectors grown on bulk TGS crystals is six times better than that of the best detector made by Mullard, UK, under similar conditions. Several studies have been attempted with different organic and inorganic dopants to achieve effective internal bias to stabilize the domains and desired pyroelectric and ferroelectric properties of TGS crystals [32–37]. Alkali halides such as NaBr and KBr-doped TGS crystals were grown, and the effects of the dopant were investigated [38, 39]. Metal ion dopants have been added to modify the properties of TGS crystal [40, 41]. In the literature, only limited information is available about the behavior of TGS doped with lithium [42]. Lithium reacts with water easily and noticeably with less energy than other alkali metals. Li+1 ion has 90 p.m. ionic radius; hence, it can easily reside into the lattice site of the TGS crystal and modify the electrical, mechanical, thermal, and surface morphology of TGS crystals. Khanum and Podder [43] have grown pure TGS and LiSO4 -doped TGS crystals from aqueous solution by natural evaporation method. The grown crystals were characterized by UV–vis spectroscopy, electrical conductivity (𝜎 dc ) measurement, dielectric studies, microhardness, and thermogravimetry/differential thermal analysis. Pure TGS and LiSO4 -doped TGS crystals were found highly transparent and full faced. The direct current conductivity was found to increase with temperature as well as dopant concentrations. T c remains the same for pure and doped crystals, but dielectric constant and dielectric loss increased with dopant concentration. The Vicker’s microhardness of the LiSO4 -doped TGS crystals along (001) face was found to be higher than that of pure TGS crystals. Etching studies illustrated the quality of the doped crystal. The high optical transparency, thermal stability, low dielectric constant, and microhardness properties exhibited the optical quality and suitability of the as-grown TGS crystal doped with LiSO4 for SHG and IR sensor applications. Batra et al. [44] have grown modified DTGS single crystals from the deuterated aqueous solution simultaneously doped with L-alanine and neodymium sulfate in the ferroelectric phase using temperature lowering technique. The effects of different dopants on the growth and dielectric and pyroelectric properties were investigated. Doped crystals exhibited higher material FoMs for pyroelectric IR sensing devices and vidicons applications with wider operating temperature as compared to virgin DTGS crystals. Applications The basic material of pyroelectric detectors with the highest performance level is Deuterated and L-alanine-doped Triglycine Sulfate (DLaTGS). DLaTGS detectors have a high detectivity, even at high frequencies, and a wide spectral sensitivity range from UV to THz wavelengths (Figure 3.9). The devices are manufactured in standard transistor outline (TO) style packages that can be fitted with a range of windows including CaF2 , KBr, BaF2 , and ZnSe-Ar (https://www .lasercomponents.com/uk/product/pyroelectric-dlatgs-detectors/). Thermoelectric coolers (TECs) can also be supplied in TO-99 and TO-37 packages. The TEC can be used to tune the detector temperature to maintain maximum

3.4 Classification of Pyroelectric Materials

Figure 3.9 DLaTGS Pyroelectric Detectors (https://www.lasercomponents.com/uk/product/ pyroelectric-dlatgs-detectors/). Source: LASER COMPONENTS.

responsiveness. fourier transform infrared spectroscopy (FTIR) spectroscopy was developed to overcome the slow scanning limitations encountered with dispersive elements, and so, ideally, the interferogram needs to be processed quickly. The broad spectral response and short time constant of DLaTGS makes it ideal for this application. Compared to the standard DLaTGS detector mounted on a FT-IR for macro measurements, Norlab (https://www.norlab.com/library/technical-note/11145) has developed a dedicated DLaTGS detector for IR microscopy, especially focusing on an increased sensitivity (Figure 3.10). 3.4.1.2 Lithium Tantalate (LT) and Lithium Niobate (LN)

The behavior of lithium tantalate (LT) (LiTaO3 ) and lithium niobate (LN) (LiNbO3 ) was first discovered in 1949. The materials belong to a small group of ferroelectric materials with similar structures of the type ABO3 . The neighboring oxygen octahedrals are connected to each other through an oxygen ion that serves a common “tie-end” as shown in Figure 3.11. The symmetry of both crystals belongs to the point group 3 m in the trigonal ferroelectric phase at room temperature, and it changes in the paraelectric phase above T c . At room temperature, the lattice parameters of the trigonal unit cell are a = 5.15052 Å, c = 13.86496 Å in LN, and a = 5.154 Å, c = 13.783 Å in LT. Sometimes, it is more convenient to choose a hexagonal cell for description, with Li+ and Ta5+ ions occupying two-thirds of the octahedral interstices between the layers. LT also possesses a high melting point (1650 ∘ C) and is insoluble in water [45]. These factors make the material one of the most stable pyroelectrics with a very wide temperature range of operation. This material is generally used in a single-crystal form; crystals are grown by the Czochralski method, and wafers of more than 50 mm diameter are commercially available. It possesses a moderate pyroelectric effect and dielectric constant that combine to give a response figure about one-quarter of TGS. However, very low

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Figure 3.10 DLATGS detector for IRT-5000/7000 infrared microscopes (https://www.norlab .com/library/technical-note/ 11145). Source: norlab.

Figure 3.11 The structure of LiTaO3 crystals (https://en.wikipedia.org/wiki/Lithium_tantalate). Source: Wikimedia Commons/Public Domain.

c a

b

Li+

Ta5+

O 2−

values of dielectric loss (about 10−4 ) have been reported for this material, which give it a potential F D that is about four times greater than that of DTGS, although the loss of commercially available material is rather higher. However, with the lowest loss materials, high D* figures can be achieved with low-noise amplifiers. By using ion beam milling to fabricate very thin (about 2–5 μm) elements in structures

3.4 Classification of Pyroelectric Materials

and packages in which thermal conductance had been minimized, Stokowski et al. demonstrated D* figures as high as 2 × 109 cm Hz1/2 W−1 at 10 Hz [46]. Roundy [47] has also used LT for one-dimensional detector arrays interfaced to a charge coupled device (CCD) multiplexing chip. A disadvantage that LT possesses for this application is the high thermal diffusivity, which reduces the array’s minimum resolvable temperature difference at high spatial frequencies. The robustness, good performance, and wide availability of this material, including its ability to withstand high energy IR radiation with a fast response time of 0.5 ns make it a popular choice for commercial detectors. LT thin films (≈0.5 μm) have been successfully deposited on Pt(111)/SiO2 /Si(100) substrates by means of the sol–gel spin-coating technology. The FOMs for IR detectors were studied for the LT thin films. High F V of 2.1 × 10−10 Ccm J−1 and F D of 2.4 × 10−8 Ccm J−1 exist because of the relatively low 𝜀 of 35 and high p of 4 × 10−8 C cm−2 K−1 of the films. The Passive Infrared (PIR) detector fabricated by the LT thin film exhibits an RV of 4584 v W−1 at 20 Hz and a high specific D* of 4.23 × 107 cm Hz1/2 W−1 at 100 Hz [48]. An LT thin film has been deposited by RF magnetron sputtering with an Li-enriched target composed of Li2 O2 /Ta2 O5 (55 : 45 wt %) on membranes of SiNx with the aim of improving the performance of thermal microsensors. The best p of LT films (400 nm) obtained was equal to 40 μC m−2 K−1 for a growth temperature of 620 ∘ C and a pressure of 0.67 Pa [49]. Zhao et al. [50] designed pyroelectric detector based on ultra-thin (≈20 μm) LiTaO3 wafer and carbon black IR coating for gas detection. They introduced electrospray technique to fabricate a carbon black IR coating for the detectors. The coating shows uniform and porous microstructure and has the absorbance more than 98% at the common gas absorption wavelength (2.5–10 μm) when the optimized electrospray time is about 20 mintues. By using the black coating, the voltage response of the detector was increased by about 2.6 times. Three voltage mode pyroelectric detectors including single-element detector, dual-element detector with compensation and four-element detector with compensation and reference channel were investigated. The specific detectivity of the single-element detector is 1.78 × 108 cm Hz1/2 W−1 . The signal-to-noise ratio (SNR) of the detector with compensation is doubled compared to that of the single-element detector. The pyroelectric detector with compensation was used in a non-dispersive infra-red (NDIR) gas detection system. The testing result of SO2 gas indicates that the detection limit is about 1 × 10−6 . LN crystals display larger “secondary pyroelectricity,” which is caused by a change in the polarization owing to thermal expansion. The Ps of 2.3 × 10−9 C cm−2 K−1 , which is found to be 25% of the first p (8.3 × 10−9 C cm−2 K−1 ), has been found [51]. Due to this property, it has found limited commercial application. Gebre et al. [52] have extensively investigated the general p of pure and doped LN crystals found that iron (Fe)-doped LN crystals are attractive for use in detectors. Lehman et al. [53] have fabricated a bicell detector consisting of a single freestanding film of single-crystal lithium niobate (LiNbO3 ) 10 μm in thickness, having two adjacent domains of opposite spontaneous polarization, and hence, two adjacent pyroelectric detector regions of equal and opposite sensitivity. The film was created by applying the process of crystal ion slicing and electric field poling (domain

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engineering) to a Z-cut LiNbO3 wafer. The detector’s noise equivalent power was 6 nW Hz−1/2 at 16 Hz, and the ambient temperature dependent variation of the detector’s response near room temperature was 0.1% K−1 . The acoustic noise sensitivity measured at 100 Hz was 24 dB relative to that of a single-domain detector. Suen et al. [54] proposed the concept of and demonstrated a new architecture that uses a multifunctional metamaterial absorber to directly absorb the incident longwave IR (8–12 μm) energy in a thin-film LN layer and also to function as the contacts for the two-terminal detector. The device achieves a narrow band (560 nm FWHM at 10.73 μm), yet highly efficient (86%) absorption. The metamaterial creates a high field concentration, which reduces temperature fluctuation noise and lowers device capacitance and loss of tangent noise. The metamaterial design paradigm applied to detectors thus results in a very fast planar device with a thermal time constant of /√ Hz. 28.9 ms with a room temperature detectivity, D∗ = 107 cm W Applications LN and LT possess a combination of unique electro-optical, acoustic, piezoelectric, pyroelectric, and non-linear optical properties, thus making it a suitable material for applications in acoustic, electro-optical and non-linear optical devices, high-temperature acoustic transducers, receivers-transmitters of acoustic vibrations, air force acceleration meters, acoustic wave delay lines, deflectors, generators of non-linear distorted waves, acoustic filters, electro-optical Q-modulators (Q-switch), encoders-decoders, filters in television receivers, video-recorders and decoders, converters, frequency doublers and resonators in laser systems, non-linear elements in parametric light generators, etc. An indispensable condition of some of these applications is a high degree of optical uniformity of LN crystals used for fabrication of active elements. Crystal growth technology by low temperature-gradient Czochralsky method allows the growth of large-size high-quality LN (up to 1–1.5 kg) and LT single crystals for such non-conventional applications. It should be noted that both crystals are non-hygroscopic, colorless, and water-insoluble and have low transmission losses. For some applications, LT crystals are more advantageous than LN crystals. LT is the most widely used pyroelectric material in many non-dispersive applications and as power monitors for pulsed laser systems due to its relatively high performance and low cost compared to other thermal detectors. The LiTaO3 pyroelectric detectors are readily available as single-channel and multi-channel version. Laser Components Pyro Group (https://www.lasercomponents.com/uk/product/multi-channellitao3-pyroelectric-detector/) manufactured pyroelectric detectors that use LiTaO3 as an active material. Pyroelectric LiTaO3 single-channel detectors (Figure 3.12a) are available for current mode (CM) and voltage mode (VM). These detectors can be tailored to applications by integrating bandpass filters or broadband windows. Multichannel pyrodetectors (dual-channel, triple-channel, and quad-channel versions), as well as current mode and voltage mode models are also available (Figure 3.12b). Multichannel pyroelectric detectors (multicolor detectors) are mostly used for gas analysis. Aside from a reference filter, each measurement channel is tuned to a gas absorption line by integrating a bandpass filter. Microtech Instruments, Inc. (www.mtinstruments.com) designed terahertz pyroelectric detector based on LiTaO3 crystal for the registration of modulated

3.4 Classification of Pyroelectric Materials

(a)

(b)

Figure 3.12 (a) Single-channel LT pyroelectric detector and (b) Multichannel LT pyroelectric detectors (https://www.lasercomponents.com/uk/product/multi-channellitao3-pyroelectric-detector/). Source: LASER COMPONENTS. Figure 3.13 Lithium Tantalate (LT) Terahertz pyroelectric detector (www.mtinstruments.com). Source: Microtech Instruments.

electromagnetic radiation in the millimeter and sub-millimeter wavelength range as shown in Figure 3.13. At square wave modulation of radiation, the saw-tooth signal voltage from the detector is proportional to the intensity of radiation. A black filter is mounted on the detector input to prevent parasitic illumination in visible and infrared regions. The detector is equipped with a standard Bayonet Neill–Concelman (BNC) connector. Key features of the detector include: ● ● ● ●

Operating spectral range: 0.02–3 THz Typical responsibility: 1000 V W−s Dynamic range: 1 μW − 10 mW Optimal modulation frequency: 5–30 Hz

Pyroelectric technology is used in laser sensors and beam analyzers. Ophir Photonics (https://www.ophiropt.com/laser--measurement/knowledge-center/article/ 8104) uses pyroelectric detectors in a number of their products, both for beam profiling and for laser power measurement. PyrocamTM III (Figure 3.14) is a pyroelectric

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Figure 3.14 Spiricon PyrocamTM III camera (https://www.ophiropt .com/laser--measurement/ knowledge-center/article/8104). Source: Ophir Optronics Solutions Ltd.

array camera that can be used to profile lasers of very short wavelength UV light or IR from the near IR wavelengths to the very far IR and even terahertz wavelengths. The Pyrocam detector consists of a LiTaO3 pyroelectric crystal mounted with indium bumps to a solid-state readout multiplexer. This sensor, developed for the original Pyrocam I, has proven to be the most rugged, stable, and precise IR detector array available. Light impinging on the pyroelectric crystal is absorbed and converted to heat, which creates charge on the surface. The multiplexer then reads this charge onto the video line. For use with short laser pulses, the firmware of the camera creates a very short electronic shutter to accurately capture the thermally generated signal. Pyrocam III measures the beam profile of both pulsed and continuous wave (CW) lasers. Since the pyroelectric crystal is an integrating sensor, pulses from femtosecond to 12.8 ms can be measured. The pyroelectric crystal only measures changes in intensity, and therefore, it is relatively immune to ambient temperature changes. Pyrocam III is an essential tool in the maintenance of industrial IR lasers, especially CO2 . The beam profiler replaces non-electronic mode burns and acrylic blocks by providing higher definition electronic recording of data and analysis of short-term fluctuations. 3.4.1.3 Barium Strontium Titanate (BST)

Barium Strontium Titanate (Bax Sr1−x TiO3 ) is a solid solution between BaTiO3 and SrTiO3 that possesses different ferroelectric Curie temperature (T c ) depending on the Ba : Sr ratio. The isovalent additive strontium (Sr+2 ) has a high solid solubility

3.4 Classification of Pyroelectric Materials

Figure 3.15 The BST unit cell in ferroelectric and paraelectric phases. Source: [61] Microwave Journal.

Barium, strontium Oxygen Titanium

P

P E

Ferroelectric phase

E

Paraelectric phase

and has the same valency as the replaced barium ion. The addition of Sr+2 ion shifts the Curie temperature close to room temperature. The BST structure is shown in Figure 3.15. BST has applications in electronic, microwave, and pyroelectric devices. When the Ba : Sr ratio equals 0.7 : 0.3, the tetragonal to cubic phase is near room temperature. BST ceramic and single crystals have the highest pyroelectric coefficient (p) ever obtained from pyroelectric materials (23 μC cm−2 K−1 ) [55]. Many efforts have been made to grow BST thin films with higher pyroelectric coefficient (p) value. Zhang and Ni [56] prepared BST (Ba0.64 Sr0.36 TiO3 ) thin films by the sol–gel method on a Pt-coated Si substrate. The resulting thin films show very good dielectric and pyroelectric properties. The dielectric constant and dissipation factor for the Ba0.64 Sr0.36 TiO3 thin film at a frequency of 200 Hz are 592 and 0.028, respectively. The peak pyroelectric coefficient(p) at 30 ∘ C is 1080 μC m−2 K−1 and at room temperature (25 ∘ C) is 1860 μC m−2 K−1 . The FOM of this film is 37.4 μC m−3 K−1 . The high pyroelectric coefficient (p) and the greater FOMs of Ba0.64 Sr0.36 TiO3 thin films make them possible to be used for thermal IR detection and imaging. Lee et al. [57] obtained BST(Ba0.66 Sr0.34 TiO3 ) films with a considerably high pyroelectric coefficient (p) of 240 nC cm−2 K−1 . Zhu et al. [58] prepared BST (Ba1–x Srx TiO3 ) ferroelectric thin films by metal organic decomposition (MOD) on Pt/Ti/SiO2 /Si substrates and on a micromachined wafer with an aim to fabricate a dielectric bolometer-type IR sensor. The uniformity of the BST film on a micro-machined Si wafer has also been confirmed to be good enough for the operation of a sensor array. Chopperless operation has been attained and IR response evaluation of the fabricated sensor has also been carried out with an RV of 0.4 kV W−1 and D* of 1 × 108 cm Hz1/2 W−1 . Sengupta et al. [59] investigated the ceramic compositions of BST for applications of pyroelectric sensors. The material has been fabricated in tape-cast and thin film forms. In general, the materials demonstrated high pyroelectric coefficients (p) accompanied with low dielectric constant and low dissipation factor. Doping BST

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with MgO has been shown to reduce the dielectric constant and loss tangent (over a very wide frequency range from 100 Hz–10 GHz) of the material, thereby increasing the pyroelectric FoM. Cheng et al. [60] prepared tetragonal BST (Ba0.8 Sr0.2 TiO3 ) thin films with large columnar grains 100–200 nm in diameter on Pt/Ti/SiO2 /Si substrates using a 0.05 M solution precursor by sol–gel processing. The ferroelectric phase transition in the prepared BST thin films is broadened and suppressed to 40 ∘ C with a maximum dielectric constant of 𝜀r (100 kHz) = 680. The observed low dissipation factor (tan 𝛿 = 2.6%) and high pyroelectric coefficient p = 4.586 × 10−4 C m−2 K−1 at 33 ∘ C render the prepared BST thin films promising for uncooled IR detector and thermal imaging applications. (Ba0.6 Sr0.3 Ca0.1 )TiO3 powders were prepared by the sol–gel method using a solution of Ba, Sr and Ca acetate, and Ti-isopropoxide, and the specimens doped with MnCO3 (0.1 mol%) and Y2 O3 (0.5 mol%) were fabricated by the cold isostatic press method. The dielectric constant and the dielectric loss at Curie temperature (T c ) were 16 600 and 1.2%, respectively. The specimen under a 4 kV cm−1 DC bias field showed the maximum pyroelectric coefficient (p) of 550 × 10−9 C cm−2 K−1 at Curie temperature (T c ). FoM for specific detectivity (D* ) of the specimen, applied with DC 8 kV cm−1 bias field, showed the highest value of 17.6 × 10−9 C cm J−1 at T c [61]. Liu et al. [62] prepared SiO2 -doped Ba0.85 Sr0.15 TiO3 (SBST) glass–ceramic (g–c) films with perovskite structure on Pt/Ti/SiO2 /Si substrates by the sol–gel technique. It is found that both the loss tangent and leakage current density decrease with increasing SiO2 content. The temperature coefficient of dielectric (TCD) and the pyroelectric coefficient (p) of the films are measured. The results show that TCD and the pyroelectric coefficient (p) of SBST film at 20–25 ∘ C temperature respectively, are 4.6% ∘ C−1 and 8.1 × 10−8 C cm−2 K−1 , which is about 2/3 value of the pure BST films. The SBST g–c film is hopeful to be the advanced candidate material for uncooled infrared focal plane arrays (UFPAs) applied at near room temperature. Liu et al. [63] prepared the BST (Ba1–x Srx TiO3 ) thin films with x = 0, 0.05, 0.1, and 0.15 (BST, BST5, BST10, and BST15, respectively) on Pt/Ti/SiO2 /Si substrate by the improved sol–gel method. The resulting thin films show very good dielectric and pyroelectric properties. At 25 ∘ C and 10 kHz, the dielectric constants of BST, BST5, BST10, and BST15 thin films were 320, 375, 400, and 425, respectively. Their loss tangents were found to be 0.035, 0.041, 0.024, and 0.01, respectively. The maximum pyroelectric coefficient (p) for BST10 film was found to be 12 × 10−8 C cm−2 K−1 at 35 ∘ C. Application Sciencetech Inc. (http://www.sciencetech-inc.com/broadband-pyroelectric-infrared-area-detector-2-14um.html) designed broadband pyroelectric IR are a detector with a sensor size of 320 × 240 pixels and a spectral range of 2–14 μm as shown in Figure 3.16. The pixel size is 48.5 μm2 . The sensor is an uncooled pyroelectric BST array. Because of the nature of pyroelectric sensors, there is an internal Archimedes spiral chopper that recalibrates the image. However, this calibration system, called an auto image calibration, is actually a benefit for the user. The detector self-calibrates

3.4 Classification of Pyroelectric Materials

Figure 3.16 BST Broadband Pyro-Electric Infrared Area Detector (http://www .sciencetech-inc.com/broadbandpyro-electric-infrared-areadetector-2-14um.html). Source: Sciencetech.

to a reference (https://www.ophiropt.com/laser--measurement/knowledge-center/ article/8104) times per second, which causes the system to be extremely stable and to have very little fixed pattern noise. 3.4.1.4 Strontium Barium Niobite (SBN)

The well-known ferroelectric relaxor strontium barium niobite (Srx Ba1−x Nb2 O6 , 0.25 ≤ x ≤ 0.75) crystals belong to tetragonal tungsten bronze structure are of significant interests for a variety of applications due to their excellent piezoelectric, electro-optic, photorefractive, and pyroelectric properties [64–66]. The crystal structure, described by Jamieson et a1. [67], consists of corner-sharing Nb06 octahedra forming a framework within which there are three types of interstitial sites, two of which are occupied by Ba/Sr ions (Figure 3.17). The structure is tetragonal both above and below T c (which varies from 195 ∘ C for x = 0.72 to 53 ∘ C for x = 0.25), possessing the point groups 4 mm below T c and 4/mmm above T c . The crystal strontium barium niobite (SBN) possesses a spontaneous polarization of 32 μC cm−2 (Sr/Ba = 75 : 25) and a polar axis along the c-axis of the tetragonal lattice. Zhanh et al. [68] studied the influence of the Sr : Ba ratio on the dielectric b a

A2 A1

Figure 3.17 View along the polar c-axis of the strontium barium niobate tetragonal tungsten bronze structure (https://www.intechopen.com/books/ferroelectrics-materialaspects/strontium-barium-niobate-thin-films-for-dielectric-and-electro-opticapplications). Rings made of five NbO6 octahedra form three types of interstitial sites. The tetragonal (A1) and pentagonal (A2) positions are partially occupied by Sr and Ba atoms (5/6) and partially vacant (1/6). Source: Cuniot-Ponsard.

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and pyroelectric properties of Srx Ba1−x Nb2 O6 (with x = 0.50, 0.45, 0.40, and 0.30) ceramics. The dielectric characteristics showed that SBN ceramic was a relaxor with strong diffuse phase transition and weak frequency dispersion. As Sr molar fraction x increases from 0.30 to 0.50, the transition temperature T C decreases linearly while the diffusivity parameter 𝛾 declines from 1.88 to 1.68. The saturated polarization, remnant polarization, and pyroelectric coefficient were also found to be enhanced in SBN ceramics with the increasing Sr content. The pyroelectric coefficient (p) of about 2 × 10−8 C cm−2 K−1 can be obtained for polarized SBN50 ceramics at room temperature. Yao et al. [69] studied the pyroelectric properties of calcium-doped SBN ceramics Sr0.65 − x Cax Ba0.35 Nb2 O6 (SCBN, x = 0.05–0.425). X-ray diffraction patterns and Raman spectra showed that calcium was completely dissolved into the tetragonal tungsten-bronze (TTB) crystal structure up to x = 0.20, above which second phase CaNb2 O6 (CN) was formed. Diffuse phase transitions were observed in all the samples. The Curie temperature of the SCBN (calcium doped strontium barium niobate) ceramic was increased from 60 to 103 ∘ C with the calcium dopant. Frequency (2–200 mHz) and temperature (25–110 ∘ C) dependence of the pyroelectric coefficient were measured. From the frequency profile of the pyroelectric coefficient, the thermal diffusivity was determined. Due to the low Curie temperature, all samples experienced serious thermal depolarization and the pyroelectric coefficient was greatly decreased during the cooling round. The sample with x = 0.125 shows the largest pyroelectric coefficient of 237 μC m2 K. The piezoelectric coefficient (d33 ) was also increased after calcium doping. Yao et al. [70] studied the effects of various rare-earth (RE) dopants (Y3+ , La3+ , Ce3+ , Pr3+ , Nd3+ , Sm3+ , Eu3+ , Gd3+ , Tm3+ , Dy3+ , Er3+ , and Yb3+ ) on the dielectric, ferroelectric, and pyroelectric properties of Sr0.5 Ba0.5 Nb2 O6 (SBN50) ceramics. In the studies, the doping concentrations of all the RE dopants were fixed at 1 mol%. Their potential usefulness in pyroelectric applications was discussed based on their measured pyroelectric detectivity FOM. On the basis of studies, for RE dopants with atomic numbers smaller than Nd, their dielectric constants were greatly increased, while for RE dopants with atomic numbers larger than Sm, their dielectric constants as well as dielectric losses became smaller. Among various dopants, Eu-doped SBN showed the most improved ferroelectric properties. Its remnant polarization (Pr ) was increased to 4.86 μC cm−2 as compared with 3.23 μC cm−2 obtained in undoped SBN50. On the other hand, Gd-doped SBN exhibited the largest pyroelectric coefficient of 168 μC m−2 K−1 , which was more than three times the undoped sample (49 μC m−2 K−1 ). The work shows that Gd-doped SBN exhibits the greatest potential for pyro-applications because it bears the largest FOM of 0.45 × 10−5 Pa. Jayalakshmy and Philip [71] studied the pyroelectric property of strontium barium niobite (Sr0.3 Ba0.7 Nb2 O6 , abbreviated SBN30)/polyurethane (PU) nanocomposites prepared in the form of cast films with varying the volume fraction of SBN30 with particle size in the range 40–90 nm following an aqueous organic gel route. Pyroelectric coefficients of poled samples, determined by Byer and Roundy method, increase from 81 to 385 μC m−2 K−1 as the volume fraction of SBN30 increases from 0 to 0.25. The FoMs for the material to act as a thermal/IR detector, current sensitivity F I ,

3.4 Classification of Pyroelectric Materials

voltage responsivity F V and detectivity F D are found to increase respectively from 48 to 283, 6.83 to 20.99, and 191 to 792, respectively, all in units of 10−3 μC m J−1 , as the volume fraction of SBN30 increases from 0 to 0.25. These composites hold promise as potential materials to develop moldable thermal/IR detectors. V’yukhin and Ivanov [72] reported the result of an experimental study of a thin-film pyroelectric detector of radiation based on SBN for detecting nanosecond radiation pulses. The possibility of recording 30-ns radiation pulses with the sensitivity of 1 V W−1 (NEP = 8 × 10−7 W Hz−1/2 ) was demonstrated.

3.4.2

Perovskite Ceramics

Many perovskite-structured ferroelectric ceramics are used commercially for pyroelectric device fabrication. They are a large family of oxygen octahedra structures (Figure 3.18) with the general formula of ABO3 , where A is a monovalent or divalent metal, B is a tetra- or pentavalent metal, and O is oxygen. The structure can be described as a three-dimensional network of corner-sharing BO6 octahedra within which there is a 12-fold coordinated site occupied by A cations. The A and B cations can be occupied with a wide range of ions to give a ferroelectric structure. Most of the ferroelectrics with the perovskite-type structure are compounds of either the A2+ B4+ O2− or A1+ B5+ O2− type formulas. 3 3 The perovskite ferroelectric ceramics are relatively low cost to manufacture in large areas using a standard mixed-oxide process. These ceramics are robust in nature, remain stable in high humid or vacuum conditions, and are thermally more stable than the polymers. So that they can be readily processed into thin wafers for device fabrication, and they possess high Curie temperatures (T c ). So that there is no danger of depoling during normal usage over a wide range of ambient temperatures (e.g. −50 ∘ C to 100 ∘ C). Also, their properties can be readily modified by the inclusion of selected dopant elements into the lattice. These ceramic materials can not only control the parameters as p, 𝜀, and tan 𝛿, which have a direct effect on the FoM, but they can also be used to alter the electrical conductivity, which can be extremely useful in the control of T c , electrical impedance in the amplifier circuit, and grain size which is important in the determination of mechanical properties. The basic cubic “aristotype” shown in Figure 3.18 undergoes distortion to give rhombohedral, tetragonal, or orthorhombic structures. Two groups of perovskites have been extensively studied for their pyroelectric properties: rhombohedral Figure 3.18 (a) Cubic ABO3 perovskite-type unit cell and (b) three-dimensional network of corner-sharing of O2− ions. Source: Reproduced Figure 4.8 from the book: Ferroelectrics: Principles and Applications, Wiley-VCH, 2017.

B O A

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structures based on modified lead zirconate (PZ) and tetragonal structures based on modified lead titanate. The properties of these two types of pyroelectric ceramic are now described in detail. 3.4.2.1 Modified Lead Zirconate (PZ)

The PZ is an antiferroelectric ceramic. However, when modified with lead titanate (PT), a number of ferroelectric phases are formed. The PZ–PT phase diagram is shown in Figure 3.19. The lead zirconate-lead titanate (1−x)PbZrO3 –xPbTiO3 –PZT perovskite solid solution has been explored for its possible applications in pyroelectric devices. The compositions of greatest interest are located either close to the PbZrO3 end of the phase diagram (Figure 3.19) or close to the PbTiO3 end. By changing the compositional ratio of PZT, various pyroelectric materials can be produced with a dielectric constant of 200–400 and a T c range of 250–450 ∘ C. Since PZT has a polycrystalline composition structure, it features high resistance to mechanical distortion caused by sudden temperature changes and offers extremely high stability against temperature fluctuations, allowing highly reliable operation as pyroelectric detectors. The pyroelectric properties of a hot-pressed PLZT (8/40/60) were investigated in some detail to evaluate the potential of PLZT as a prospective pyroelectric IR detector material [73]. The results of measurements show that the PLZT composition (8/40/60) has a relatively high dielectric constant, a high pyroelectric coefficient, and favorable thermal and mechanical characteristics comparable with composition (8/65/35). The calculated FoM based on the measured properties were compared with other PLZTs of (8/65/35), (12/40/60), and (14/40/60) compositions and LiTaO3 single crystal. The results indicate that (8/40/60) is a promising material for small 500

Pe AT

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400 Temperature (°C)

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Mole % PbTiO3

Figure 3.19 The PbZrO3 -PbTiO3 phase diagram. Here A0 = antiferroelectric, orthorhombic; F R = ferroelectric, rhombohedral; LT = low temperature; HT = high temperature; F T = ferroelectric; tetragonal. Source: Reproduced Figure 4.22 from the book: Ferroelectrics: Principles and Applications, Wiley-VCH, 2017.

3.4 Classification of Pyroelectric Materials

area voltage mode IR detectors. This material has a permanent bias to its polarization; hence, repoling is not necessary following a temperature excursion above the Tc . Czekaj et al. [74] prepared multicomponent ferroelectric ceramic materials, on the basis of solid solutions of PbTiO3 –PbZrO3 –PbNb2/3 Zn1/3 O3 –PbSb2/3 Mn1/3 O3 – PbW1/2 Mg1/2 O3 (system I) and PbTiO3 –PbZrO3 –PbNb2/3 Zn1/3 O3 –PbSb2/3 Mn1/3 O3 – PbNb2/3 Mn1/3 O3 (system II), by means of hot-pressing technique and investigated for compositions from both tetragonal and rhombohedral phase areas, as well as from the morphotropic phase boundary region at room temperature. The pyroelectric coefficient for the compositions from the morphotropic region was 𝛾 1 = 4 × 10−4 Cm−2 K and 𝛾 2 = 5 × 104 Cm−2 K for systems I and II, respectively. It was stated that the materials under investigation were characterized by high merit numbers, which made them prospective candidates for pyroelectric detectors. Suaste-Gómez et al. [75] studied the pyroelectric properties of Pb0.88 Ln0.08 Ti0.98 Mn0.02 O3 (Ln = La, Sm, Eu) ferroelectric ceramic system. This type of ferroelectric ceramic presents high values of the following characteristics: dielectric constant, Curie temperature, electromechanical anisotropy, and high frequencies of operation, which make them useful for applications such as ultrasonic transducers in biomedical applications. The relationship between dielectric constant and temperature measurements as well as pyroelectric measurements using the technique of Byer and Roundy were performed. Values of the pyroelectric coefficient and FoM for IR detector materials were obtained to use these ceramics in the detection of IR radiation, laser power measurements, and solar energy technology. Guggilla et al. [76] studied the pyroelectric properties of various ceramics such as PLZT, PLZT + MnO2 and modified PZ to examine its applications in IR sensor materials and compared with other candidate materials. The materials are calculated from measured parameters (dielectric constants and pyroelectric coefficient) for their use in pyroelectric IR detectors. From the viewpoint of pyro sensor applications, the investigated ceramics PLZT (STPZT-1), PLZT + MnO2 (STPZT-2), and modified PZ (BM 740) are remarkably attractive due to large pyroelectric coefficient and low dielectric constant and loss. It is worthy to mention that the materials investigated are capable of high temperature applications. Deb et al. [77] prepared very thin films of PZT (40/60), (0.26 μm or less) on Pt-coated oxidized Si substrates (Pt/Ti/SiO2 /Si) by a sol–gel process. These films were of high density with fine grains of about 0.2 μm and were annealed in the range of 600 to 700 ∘ C in an oxygen atmosphere. The X-ray diffraction patterns obtained from this film showed a single-phase perovskite-type structure. The authors investigated the influence of poling treatment on the dielectric and pyroelectric properties, as well as dielectric constants and pyroelectric properties. Dielectric constants and pyroelectric coefficients at room temperature were determined as 1300 and 840 nC cm−2 K−1 , and 68.0 and 10.3 nC cm−2 K−1 for the poled and unpoled PZT films, respectively. The remanent polarization was 37.8 μC cm−2 , and a coercive field was 146 kV cm−1 at a switching voltage of 16 V peak-to-peak and a frequency of 200 kHz. The remanent polarization and coercive field were found to vary slowly

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with temperature change. The material was also difficult to depole. The authors suggested that these films would be suitable for IR detector applications. Kobune et al. [78] deposited highly c-axis-oriented PLZT films with compositions of (Pb0.925 La0.075 )(ZryTi1−y )0.981 O3 , where y = 0.2–0.4, on Pt(100)/MgO(100) by the rf-magnetron sputtering, applying an intermittent deposition comprising the repetition of deposition and nondeposition. It was confirmed that the highest FoMs for the pyroelectric IR sensors were obtained by using powders with excess PbO of 20 mol% added to PLZT, as sputtering targets in the preliminary experiment. The PLZT film with y = 0.2 exhibited the highest FOM for voltage responsivity RV of approximately 5.3 × 10−13 Cm J−1 among the three samples studied, which was around 1.8 times larger than that of PLZT ceramic with y = 0.2. The top-electrode sizes of around 500 and 250 μm diameters were suitable for the measurement of pyroelectric properties, judging from the characteristics of the materials and the accuracy of the measuring instruments. Taniguchi et al. [79] designed a new pyroelectric IR sensor made of a thin Pb(Zr, Ti)O3 (PZT) ceramic plate mounted on a diaphragm fabricated by the Si micromachining method. The prototype sensor is about 10 μm in thickness and has a sensing area of 1 × 2 mm2 . The new sensor has an Rv value of 36.3 to 37.9 mV μW which is about five times higher than a pyroelectric sensor with a 100-μm thick PZT bulk ceramic plate. The amount of sensor noise was expected to be reduced by half. However, the amount of sensor noise increased by a factor of two. Because of the increase in the sensor noise, the values of NEP ranged from 1.8 to 3.3 × 10−2 μW, which is a factor of two to three smaller than the NEP value of the bulk sensor. Furthermore, the rise time decreased to 31–35 ms, which is 10–20% shorter than that of the bulk sensor. De Cicco et al. [80] studied the pyroelectric properties of PZT-based layers prepared with standard procedures of thick film technology (950 ∘ C peak temperature) on an alumina substrate and with buried interdigital electrodes. The investigation was aimed at setting adequate procedures and at identifying sample configurations for measuring the relevant pyroelectric quantities rather than optimizing the material pyroelectric performances. The results show that the pyroelectric coefficient of the porous layers (∼1.2 ± 0.1 × 10−4 C m−2 K−1 ) is lower than that of ceramic (∼3.8 × 10−4 C m−2 K−1 ) and dense thick films (∼2 × 10−4 C m−2 K−1 ) of comparable composition, but the thermal properties of the samples (low heat capacity and thermal conductivity) as well as the low relative dielectric constant result in FoMs better than the corresponding values either for ceramic PZT or other pyroelectric materials (including TGS and LiTaO3 ). Hence, porous ferroelectric thick films are a good candidate for new pyroelectric devices. Bruchhaus et al. [81] used a planar multi-target sputtering technology to deposit highly (111) oriented Pb(Zrx Ti1−x )O3 (PZT) thin films with x ranging from 0 to 0.6. The preparation of a stable Pt/ZrO2 electrode is described and analyzed in terms of stress and stress-temperature behavior. The PZT films with low Zr content are under compressive stress after deposition. The dielectric constant and loss peaks occur at a composition close to the morphotropic phase boundary. Films on the tetragonal side of the phase diagram with a Zr content up to about 25% exhibited a

3.4 Classification of Pyroelectric Materials

strong self-polarization and strong voltage shifts in the C(V) curves. High pyroelectric coefficients of >2 × 10−4 C (m2 K)−1 have been measured on these films without additional poling. The self-polarization fades out with increasing Zr content. The low values of the pyroelectric coefficient for the PZT film with 60% Zr are discussed in terms of the possible crystallographic variants after distortion and the tensile stress state during the phase transition. Based on the systematic study of stress and electrical properties of PZT films with a wide range of compositions presented in this paper, films with a Zr content up to about 25% yielded the best properties for use in pyroelectric IR detector arrays. To develop a high-performance pyroelectric IR detector, Liu et al. [82] deposited Pb1.1 (Zr0.3 Ti0.7 )O3 /PbTiO3 (PZT/PT) multilayer thin films onto the top of a Pt/Ti/Si3 N4 /SiO2 membrane by a modified sol–gel process. For the comparison purpose, Pb1.1 (Zr0.3 Ti0.7 )O3 (PZT) thin films were also prepared with the identical method under the same conditions. X-ray diffraction measurement revealed that the diffraction pattern of the multilayer film was the superimposition of the PZT and PT patterns. At 1 kHz, the dielectric constant of 389 and 558 and the dielectric loss of 1.2 and 1.1% were obtained for the PZT/PT and PZT thin films, respectively. The PZT/PT film showed a lower dielectric constant as expected and a similar dielectric loss compared with those of the PZT film, which is beneficial to use the multilayer thin films as the pyroelectric IR detecting element. Pyroelectric coefficients for the PZT/PT film and the PZT film were correspondingly 380 and 400 μC m−2 K−1 . The calculated detectivity FoMs for the PZT/PT and PZT thin films were 20.3 × 10−6 Pa−1/2 , and 18.7 × 10−6 Pa−1/2 , and values of the voltage response FoMs were 0.038 and 0.028 m2 C−1 , respectively. At 20 Hz, the dynamic pyroelectric voltage responsivity of 132 V W−1 (in rms) was obtained for the PZT/PT film and 98 V W−1 (in rms) for PZT film with the same element size of 240 × 360 μm2 . The high response of the multilayer thin film was ascribed to its relatively lower dielectric constant when compared with the PZT thin films. Experimental results showed the PZT/PT multilayer thin film is a good candidate material for developing high-performance IR detectors. Shi et al. [83] prepared epitaxially grown and polycrystalline PbTiO3 (PT), (Pb, La)TiO3 (PLT), and Pb(Zr, Ti)O3 (PZT) thin films with thicknesses from 1 to 2 μm on Pt/Ti/SiO2 /Si substrates by means of a sol–gel spin-coating technique. The ferroelectric thin films have good crystallization behavior and excellent dielectric, and pyroelectric properties. The pyroelectric coefficients of PT, PLT, and PZT thin films are 2.9 × 10−8 C cm−2 K−1 , (3.37–5.25) × 10−8 C cm−2 K−1 , and 6.10 × 10−8 C cm−2 K−1 , respectively. The FoMs for voltage responsivity of PT, PLT, and PZT thin films are 0.60 × 10−10 C cm J−1 , (0.59–0.78) × 10−10 C cm J−1 , and 0.63 × 10−10 C cm J−1 , respectively. The current responsivity values of these films are 9.0 × 10−9 C cm J−1 , (10.5–16.0) × 10−9 C cm J−1 and 19.1 × 10−9 C cm J−1 , and the detectivity values are 0.74 × 10−8 C cm J−1 , (0.79–1.13) × 10−8 C cm J−1 and 0.83 × 10−8 C cm J−1 , respectively. Zhang and Whatmore [84] prepared thin films of ferroelectric PZT (PbZr0.3 Ti0.7 O3 PZT30/70) and manganese doped PZT [(Pb(Zr0.3 Ti0.7 )1 − x Mnx ]O3, where x = 0.01, PM01ZT30/70 and x = 0.03, PM03ZT30/70) using sol–gel processing techniques. These materials can be used as pyroelectric thin films in uncooled IR detectors.

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Films deposited on Pt/Ti/SiO2 /Si substrates and annealed on a hot plate at 530 ∘ C for 5 minutes were observed to fully crystallize into the required perovskite phase and showed excellent ferroelectric behavior, demonstrated by reproducible hysteresis loops (Pr = 33–37 μC cm−2 , Ec (+) = 70–100 kV cm−1 , Ec (−) = −170–140 kV cm−1 ). The pyroelectric coefficients (p) were measured using the Byer–Roundy method. At 20 ∘ C, p was 2.11 × 10−4 Cm−2 K−1 for PZT30/70, 3.00 × 10−4 Cm−2 K−1 for PM01ZT30/70, and 2.40 × 10−4 Cm−2 K−1 for PM03ZT30/70 thin films. The detectivity figures-of-merit (F D ) were 1.07 × 10−5 Pa−0.5 for PZT30/70, 3.07 × 10−5 Pa−0.5 for PM01ZT30/70, and 1.07 × 10−5 Pa−0.5 for PM03ZT30/70. These figures were compared well with values reported previously. Zhang and Whatmore [85] investigated the effects of Mn doping on the ferroelectric and pyroelectric properties of Pb(Zr0.3 Ti0.7 )O3 (PZT) thin films on substrates Pt/Ti/SiO2 /Si. The Mn-doped (1 mol%) PZT (PMZT) showed almost no hysteretic fatigue up to 1010 switching bipolar pulse cycles, coupled with excellent retention properties. The authors presented evidence that while a low permittivity interfacial layer forms between the Pt electrode and PZT films, this does not occur in PMZT. They proposed that Mn dopants could reduce oxygen vacancy mobility in PZT films and Mn2+ ions consume the oxygen vacancies generated during repeated switching, forming Mn4+ ions. These mechanisms are probably responsible for their low observed fatigue characteristics. Mn doping brings additional benefits to the electrical properties of PZT films. The relevant pyroelectric coefficients (p) of a 700 nm thick film are 3.52 × 10−4 Cm−2 K−1 and the detectivity figures of merit F D = 3.85 × 10−5 Pa−0.5 at 33 Hz for Mn-doped PZT, compared with p = 2.11 × 10−4 Cm−2 K−1 and F D = 1.07 × 10−5 Pa−0.5 for the undoped PZT films. This means that the Mn-doped PZT thin films are excellent candidates as device materials for both memory and pyroelectric applications. Sun et al. [86] compared sol–gel Pb(Zr0.3 Ti0.7 )O3 (PZT) film and Pb(Zr0.3 Ti0.7 )O3 / PbTiO3 (PZT/PT) multilayer film for pyroelectric IR detector application. The microstructure, dielectric, ferroelectric, and pyroelectric properties of the pure PZT and PZT/PT multilayer thin films were investigated. The results indicate that the multilayer thin film exhibits lower dielectric constant and lower dielectric loss, which is beneficial for a higher FoM. Although the pyroelectric coefficient of the PZT/PT multilayer thin film is 14% lower than that of the pure PZT thin film, the F D of PZT/PT multilayer thin film is 17% higher than that of the pure PZT thin film due to the reduced dielectric constant and dielectric loss tangent. It is evident that the PZT/PT multilayer thin film is a promising candidate for IR detection application. Sun et al. [87] studied the poling of multilayer Pb(Zr0.3 Ti0.7 )O3 /PbTiO3 thin films. Poling temperature and electrical field are optimized to achieve a high pyroelectric coefficient. The pyroelectric coefficient is found to saturate when a sufficient high poling electric field (∼350 kV cm−1 ) is applied at room temperature. Further improvement of the pyroelectric properties is achieved with the increasing of poling temperature (up to 180 ∘ C). Pyroelectric coefficient of the poled PZT/PT film (at 350 kV cm−1 , 180 ∘ C) is measured using a modified Byer–Roundy method to be 250 μC m−2 K−1 . Compared with the pure Pb(Zr0.3 Ti0.7 )O3 thin film, the multilayer PZT/PT thin film has a lower pyroelectric coefficient; however, its dielectric constant

3.4 Classification of Pyroelectric Materials

and dielectric loss are reduced. Thus, the multilayer PZT/PT film has higher FoMs for pyroelectric applications. Irzaman et al. [88] studied thin films of PZT[PbZr0.525 Ti0.475 O3 )] and 1% tantalum-oxide-doped lead zirconium titanate [Pb0.9950 (Zr0.525 Ti0.465 Ta0.010 ) O3 (PTZT)] on Pt(200)/SiO2 /Si(100) using the dc unbalanced magnetron sputtering (DC-UBMS) method. The films were grown at deposition temperatures in the range of 300 to 750 ∘ C for 3 hours, followed by annealing at 700 ∘ C for 1 hour for each deposited film. Observations by X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the physical and pyroelectric properties of the grown films. The films show tetragonal structures with preferred orientation to the (100) and (200) crystal planes. The calculated lattice constants are: a = b = 4.056 Å, c = 4.105 Å for PZT thin film; and a = b = 4.056 Å, c = 4. 068 Å for PTZT thin film. The physical properties extracted from the I–V curves show that the PZT and PTZT films are insulators or dielectric materials. The conductivity (𝜎) of PZT films is between 2.24 × 10−13 (Ω m)−1 and 5.00 × 10−12 (Ω m)−1 , whereas the conductivity (𝜎) of PTZT films is between 1.02 × 10−9 (Ω m)−1 and 1.90 × 10−8 (Ω m)−1 for films deposited at the mentioned temperature ranges. The voltage responsivity (r v ) measured at chopper frequency of 2000 Hz and at the wavelength of 947 nm is 62.1–80 μV W−1 and 61–76.4 μV W−1 for PZT and PTZT thin films, respectively. Meanwhile, the pyroelectric coefficient (p) is in the range of 9.54 × 10−4 to 12.3 × 10−4 C m−2 K−1 for PZT thin films and 9.35 × 10−4 to 11.7 × 10−4 C m−2 K−1 for PTZT thin films. These results show that PZT and PTZT thin films are suitable for use as pyroelectric IR sensors. Guggilla et al. [76] studied the pyroelectric properties of various ceramics such as PLZT, PLZT + MnO2, and modified PZ for their applications to IR sensor materials and compared with other candidate materials. The materials are calculated from measured parameters (dielectric constants and pyroelectric coefficient) for their use in pyroelectric IR detectors. From the viewpoint of pyro sensor applications, the investigated ceramics PLZT (STPZT-1), PLZT + MnO2 (STPZT-2), and modified PZ (BM 740) are remarkably attractive due to large pyroelectric coefficient and low dielectric constant and loss. It is worthy to mention that the materials investigated are capable of high temperature applications. Thakur et al. [89] synthesized a modified PZT system with compositional formula Pb1−x Smx (Zr0.58 Fe0.18 Mn0.02 Nb0.2 Ti0.02 )O3 where 0 ≤ x ≤ 0.025 by conventional solid-state reaction method. The materials were characterized for their properties like dielectric and pyroelectric coefficients. Hysteresis loop was recorded at room temperature. The sample with 2 mol% samarium (Sm) substitutions was found to be more promising for sensor application based on its high material’s FoM. Sensors fabricated with this material were integrated with a FET amplifier. The devices configured with compensating elements were evaluated for different chopping frequencies. The values of the material’s FOM, F D , and detectivity (D*) were determined from measured parameters and were 3.6 × 10−5 Pa−1/2 and 2 × 108 cm Hz1/2 W−1 , respectively. Han et al. [90] deposited Pb(Zr0.2 Ti0.8 )1 − x Nbx O3 (PNZT) (x = 0%, 1%, 2%, 3%, 4%) films with 1 μm thickness on a platinized silicon using the chemical solution

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deposition (CSD) method followed by rapid thermal annealing (RTA). X-ray patterns indicate that introduction of the first seeding layer significantly enhances the crystallization of films after RTA processing. Electrical characterization was performed on Pt/PNZT/Pt capacitors. In order to precisely determine the pyroelectric coefficients of the PNZT films, trapezium and sinusoidal temperature modulations were applied to Pt/PNZT/Pt capacitors while measuring the pyroelectric current signal. The pyroelectric voltage response of these films was measured with chopped IR radiation from a blackbody source tuned to an equivalent of 400 ∘ C. Results show that doping PZT with Nb significantly improves the ferroelectric and pyroelectric properties of these films. Films with a doping level of 1 mol% of Nb exhibit the highest values of remanent polarization (49.33 μC cm−2 ) and pyroelectric coefficient (4.6 × 10−4 C m−2 K−1 ), and FoM (1.89 × 10−5 Jm3 K−1 ), which indicates that Pb(Zr0.2 Ti0.8 )0.99 Nb0.01 O3 would be a good candidate for PIR sensor applications. Liu et al. [91] investigated the dielectric, ferroelectric, and pyroelectric properties of Mn-doped 0.74Pb(Mg1/3 Nb2/ 3)O3 –0.26PbTiO3 crystals. Compared with pure PMN-0.26PT, Mn substitutions resulted in reduced dielectric loss and enhanced coercive field. Furthermore, the pyroelectric coefficient and F D were enhanced to 17.2 × 10−4 C m−2 K−1 and 40.2 × 10−5 Pa−1/2 , respectively, which were the highest values so far reported among intrinsic pyroelectric materials with rhombohedral-tetragonal phase transition temperature greater than 90 ∘ C. The specific detectivity of IR detector based on Mn-doped crystals was 1.07 × 109 cm Hz1/2 W−1 , which was approximately double that of commercial LiTaO3 crystals-based, thus making these crystals promising candidates for IR detector applications. Wu et al. [92] fabricated porous PZT (PbZr0.3 Ti0.7 O3 , PZT30/70) thick films and detectors for pyroelectric applications on alumina substrates by screen-printing technology. Low-temperature sintering of PZT thick films has been achieved at 850 ∘ C by using Li2 CO3 and Bi2 O3 sintering aids. The microstructure of PZT thick film has been investigated by XRD and SEM. The dielectric properties were measured using HP 4284 at 1 kHz under 25 ∘ C. The permittivity and loss tangent of the thick films were 94 and 0.017, respectively. T C of PZT thick film was 425 ∘ C as revealed by dielectric constant temperature measurement. The pyroelectric coefficient was determined to be 0.9 × 10−8 Ccm−2 K−1 by dynamic current measurement. IR detector-sensitive element of dual capacitance was fabricated by laser direct-write technology. Detectivity of the detectors was measured using mechanically chopped blackbody radiation. Detectivity ranging from 1.23 × 108 to 1.75 × 108 (cm Hz1/2 W−1 ) was derived at a frequency range from 175.5 to 1367 Hz, and D* ’s − 3 dB cut-off frequency bandwidth was 1.2 kHz. The results indicate that the IR detectors based on porous thick films have great potential applications in fast and wide-band frequency response conditions. Zhao et al. [93] fabricated a pyroelectric IR detector array using lead zirconate titanate (PbZr0.3 Ti0.7 O3 , PZT) thin film as a sensing layer on Si3 N4 /SiO2 /Si substrate by a microelectromechanical system (MEMS) technique. Si3 N4 /SiO2 layers, used as the etching mask and supporting layer, were grown on both sides of a 4-in. double-side polished silicon wafer (100, p-type). The silicon window was etched

3.4 Classification of Pyroelectric Materials

by a series of processes for photo- etching, dry-etching, and wet-etching. Pt/TiO2 bottom electrode was deposited and patterned by using photoetching and RF magnetron sputtering. Furthermore, 800-nm-thick PZT thin film was deposited at 600 ∘ C by RF magnetron sputtering. The Au top electrode and the IR absorption layer of black gold were deposited and patterned by photoetching and DC sputtering. The dielectric constant and loss tangent of PZT film are 440 and 0.021 at 1 kHz, respectively. The remanent polarization (Pr ) and the coercive field (Ec ) were 25 μC cm−2 and 40 kV cm−1 , respectively. The pyroelectric coefficient of the film was measured by a quasi-static method and was 300 μC m−2 K−1 . The pyroelectric IR detector array has potential applications in safety monitoring and smart appliances. 3.4.2.2 Modified Lead Titanate (PT)

Lead titanate is a ferroelectric material, possessing a tetragonal structure at room temperature with a high Curie point (490 ∘ C) and a high spontaneous polarization (up to 75 μC cm−2 ). Although the growth of single crystals by flux and top-seeded solution growth is possible, the growth of crystals more than 1 cm in size is very difficult, again restricting the material’s practical application to ceramic form. The high spontaneous lattice strain associated with the phase transition makes the fabrication of pure PT ceramics impractical, as they are mechanically unstable at room temperature. A wide range of dopants, e.g. Mn, rare earth ions, Ca, and Pb[(Co1/2 W1/2 )O3 ], have been explored to stabilize the material’s properties. Ichinose et al. [94] studied the pyroelectric properties of (Pb, Ca) (Co1/2 W1/2 )TiO3 ceramics by changing the Ca concentration. With an increase in Ca concentration, the pyroelectric coefficient p and figure of merit F V increased and reached 4.43 × 10−8 C cm−20 C−1 , and 0.61 × 10−10 C cm J−1 , respectively. The pyroelectric characteristics have been improved to twice the values for the PbTiO3 ceramics. The maximum responsivity RV value versus chopping frequency is obtained in a less than 1 Hz frequency range. Responsivity RV and specific detectivity D* are √ 780 V W−1 , and 1.1 × 108 cm Hz W−1 at 1 Hz, respectively. The sensor is applicable for use as a human body sensor. Using an rf-magnetron sputtering method, Iijima et al. [95] fabricated highly c-axis-oriented La-modified PbTiO3 (PLT) thin films with compositions of Pb1 − x Lax Ti1 − x/4 O3 , where x = 0.05, 0.10, and 0.15, on MgO single-crystal and epitaxial Pt thin-film substrates under conditions of low gas pressure and low deposition rate. The degree of c-axis orientation of the PLT films decreases, and the tetragonality of PLT becomes smaller with increasing La content. The Curie point of the films decreases at the rate of 18 ∘ C/atom % of La with an increase of La content. D–E hysteresis loops were observed, and these became slim and symmetric with increasing La. It was found that significant pyroelectric currents are detected on all as-grown specimens even without a poling treatment, and the directions are the same in all specimens. The relative dielectric constant 𝜀r decreases and the pyroelectric coefficient p increases with increasing degree of c-axis orientation. The 𝜀r and p became large with an increase of La content in the films. Considering the FoM for pyroelectric IR detection and the temperature coefficient of p, it was concluded that PLT films with x = 0.10 were good material for pyroelectric IR detectors.

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Wang et al. [96] fabricated high-performance pyroelectric IR detectors without the backside etching process using La-modified lead titanate (PLT) thin films. The PLT thin films were deposited on (111)-oriented Pt thin film on SiO2 /Si(100) substrates by a diol-based sol–gel process. The randomly oriented PLT thin film exhibits a relatively small dielectric constant and a large pyroelectric coefficient without poling treatment. The pyroelectric characteristics of point detectors with various La contents as a function of modulation frequency were compared. PLT(10) detector showed a large voltage responsivity of 3330 (V W−1 ) at 20 Hz. The specific detectivity (D* ) at 100 Hz is 6.2 × 107 cm Hz1/2 W−1 . The results showed that Pb1 − x Lax Ti1 − x/4 O3 thin film with x = 0.10 [PLT(10)] was most suitable for use as a pyroelectric IR detector. Wang et al. [97] fabricated calcium-modified lead titanate (PCT) thin-film pyroelectric IR detectors on Pt(111)/SiO2 [100] substrates using a diol-based sol–gel process. The randomly oriented PCT thin film, which has not undergone the poling treatment, exhibits a relatively small dielectric constant and a large pyroelectric coefficient. The pyroelectric properties of PCT thin-film point detectors with various Ca contents on modulation frequency are measured and compared. The optimized composition of PCT thin film IR detectors was found to be the PCT (25), which exhibits a large voltage responsivity of 4533 (V W−1 ) at 20 Hz and a specific detectivity (D*) of 6.45 × 107 cm Hz1/2 W−1 at 100 Hz. The results show that Pb1−−x Cax TiO3 thin film with x = 0.25 [PCT (25)] was most suitable for application to the pyroelectric thin film IR detector. Batra et al. [98] fabricated the modified PT-based thick films using the thick film transfer (TFT) technique. The pyroelectric and dielectric behavior ascertained plausible usage as IR detectors for these films. The conduction processes in these films are represented by the development of an equivalent circuit model that portrays a near-perfect dielectric material containing traps. Various materials’ FoMs of modified PT films are determined for their potential use as an IR detector. Based on existing information, these material properties are found to be reasonable. Ahmed and Butler [99] fabricated the lead zirconium titanate (PbZr0.4 Ti0.6 O3 ) (PZT) and lead calcium titanate (Pb0.7 Ca0.3 TiO3 ) (PCT) thin films on a gold electrode with a gold top electrode to form a capacitor structure. Pulsed laser deposition was utilized to deposit the thin films. The thin films were characterized for their dielectric constant and loss tangent variation with temperature as well as the pyroelectric coefficient. The thin films were poled to improve the pyroelectric effect. The highest pyroelectric coefficient was found to be 280 μC m−2 K−1 for PZT film and 400 μC m−2 K−1 for the PCT film after poling. Chrostoski et al. [100] demonstrated the deposition and characterization of calcium modified lead titanate (Pb1−x Cax TiO3 , PCT) thin films for using them as materials of pyroelectric thermal detectors. The PCT thin films were sputtered using an RF sputter system in Ar:O2 environment at room temperature. The thin films were grown on an Au electrode. The capacitance was formed by using Au electrodes on top of PCT thin films which were fabricated by sputtering and lift-off. The PCT films were annealed at 450, 500, 550, and 600 ∘ C in an O2 environment for 15 minutes. Energy dispersive spectroscopy was done to determine the atomic composition of

3.4 Classification of Pyroelectric Materials

PCT films. Variations of capacitance, pyroelectric voltage, loss tangent, and pyroelectric current between the temperature range 303 to 353 K were determined. The PCT films annealed at 550 ∘ C showed the highest value of pyroelectric current and pyroelectric coefficient of 2.45 × 10−12 A and 1.99 μC m−2 K−1 , respectively, at room temperature. Mafi et al. [101] reported the deposition and characterization of pyroelectric PCT thin films for using them to fabricate pyroelectric IR detectors. PCT films were deposited on both silicon and Si/SiN/Ti/Au substrates at 13 mTorr pressure by 200 W Radio Frequency (RF) sputtering in an Ar + O2 environment for 4 hours. Substrates were kept at variable temperatures starting from 550 to 800 ∘ C during the deposition. The PCT films were annealed at 550, 600, 650, and 700 ∘ C in an O2 environment for 15 minutes. XRD results confirm the polycrystalline nature of these films. The energy dispersive spectroscopy (EDS) function of the SEM was used to determine the elemental composition of PCT films. The EDS result reveals the presence of the elements such as calcium, lead, titanium, and oxygen in the thin films. Moreover, it shows that the films are stoichiometric (Ca0.43 Pb0.57 )TiO3 (Ca/Ti = 0.5, Pb/Ti = 0.66). The film thicknesses were measured using a Dektak model XT profilometer which ranges from ∼250 to 400 nm. The surface morphology obtained from SEM and atomic force microscopy confirms the crack-free nature of the films as well as their smoothness and low surface roughness. Temperature dependence of capacitance, pyroelectric current, and pyroelectric coefficient were investigated for different PCT films. The results show that films deposited at 550 and 600 ∘ C demonstrate better quality and larger values of pyroelectric coefficient. On the other hand, the capacitance fabricated on the PCT films at 550 ∘ C showed the highest value of pyroelectric current and pyroelectric coefficient, which are 14 pA and 50 μC m−2 K−1 , respectively, at a higher temperature.

3.4.3

Organic Polymers

In 1969, Kawai [102] discovered the phenomenon of piezoelectricity in polyvinyl fluoride (PDF) and polyvinylidene fluoride (PVDF), and shortly after, Bergman et al. [103] observed strong pyroelectricity in the same polymers. The PVDF has received the most attention from the scientific community for its unique electro-active properties, including piezo-, pyro-, and ferroelectric properties as well as its other useful properties such as its flexibility, lightweight, and long-term stability under high electric fields [104, 105]. It is a semi-crystalline polymer with typical crystallinity of 50%, whose molecular structure consists of the repeated monomer unit (—CH2 —CF2 —)n . It is well known that PVDF has five distinct crystallite polymorphs [104]. The most common polymorph of PVDF is the 𝛼-phase, which has a monoclinic unit cell with ˘ (T = trans, G = gauche +, G ˘ = gauche −). The piezoelectric crystallization TGTG polymorph is the 𝛽-phase which has an all-trans (TTTT) conformation, with an orthorhombic unit cell. The 𝛾-phase also has an orthorhombic unit cell, with a ˘ chain conformation (Figure 3.20). The other two (𝛿 and 𝜀) polymorphs TTTGTTTG are the polar and antipolar analogs of the 𝛼 and 𝛾 forms, respectively [105].

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Fluorine Hydrogen Carbon

Alpha structure

Beta structure

Gamma structure

Figure 3.20 Diagrams of the chain conformation for 𝛼, 𝛽, and 𝛾 crystalline phases of PVDF. Source: Ruan et al. [106]/figure 1 (p. 2)/MDPI/Licensed under CC BY 4.0.

The first two conformations (the 𝛼-phase and the 𝛽-phase) are the most general ˘ conformation (the 𝛼-phase conformation), the and important ones. In the TGTG dipole is inclined relative to the normal axis; hence, the average dipole moment for each monomer is much reduced. Furthermore, the unit cell of the 𝛼-PVDF ˘ conformation (Figure 3.20), whose dipole lattice consists of two chains in a TGTG components normal to the chain axis are antiparallel, thus neutralizing each other [107]. As a result, the 𝛼-phase can be described as non-polar, non-piezoelectric, and non-pyroelectric. On the other hand, the 𝛽-phase, which is in all-trans (TTTT) conformation (Figure 3.20), has all of its dipoles aligned in the same direction normal to the chain axis. Its unit cell consists of two all-trans chains packed with their dipoles pointing in the same direction. The molecular dipoles in the 𝛽-phase are thus entirely aligned in one direction; this crystal form can therefore generate the largest spontaneous polarization and can exhibit strong ferroelectric and piezoelectric properties. These unique 𝛽-phase-derived properties of PVDF make it useful in a wide range of applications, including actuators, biosensors, energy harvesting materials, audio devices, transducers, and non-volatile memories [104, 105, 107–109]. Based on the pyroelectric properties, Glass et al. [110] and Yamaka [111] were among the first to use PVDF for IR thermal sensing. To date, several research groups have investigated the pyroelectric properties of PVDF and P(VDF-TrFE) for applications in thermal radiation sensing Bauer and Lang [112], Hammes and Regtien [113], Navid et al. [114], and Setiadi et al. [115, 116]. They typically exhibit pyroelectric coefficients of about 25 and 40 μC m−2 K−1 , respectively, which is about 10 μC m−2 K−1 lower than the coefficient (380 μC m−2 K−1 ) measured for one of the

3.4 Classification of Pyroelectric Materials

most commonly used classes of ceramics: lead zirconate titanate Pb(Zr, Ti)O3 ) (PZT) Bauer and Lang [112]). Researchers have, therefore, explored the possibilities of using composite materials to take advantage of the high flexibility of polymers and high performance of ceramics Malmonge et al. [117] and Sakamoto et al. [118, 119]. Polymers have a high coefficient of thermal expansion as compared to metals and semiconductors. Most polymers absorb IR radiation because of the vibrational resonance modes present in their organic bonds. Temperature-induced morphological changes such as conformational changes of polymer molecules and/or rearrangements in their crystal structure may also result in additional energy transduction Mueller [120]. Setiadi et al. [115, 116] developed a polymer-based pyroelectric IR sensor. The sensor comprised a conductive polymer (PEDOT: PSS) as an absorber layer and front electrode, a pyroelectric material (PVDF film), and a nickel–aluminum (Ni-Al) metal film as a reflector layer and rear electrode. The practical use of metal-polymer combinations of pyroelectric sensors is hindered by the poor adhesion between the front electrode (metal film) and sensing layer (polymer-based pyroelectric film). Setiadi et al. [115, 116] used a conductive polymer (PEDOT: PSS) for effective adhesion of the front electrode to the sensing material (pyroelectric PVDF) film beneath it. The measured IR response was shown to be 10 times higher than that of commercially available PVDF films with Ni-Al front and back electrodes. In 2019, Wu et al. [121] prepared a PVDF film by the solution cast on hydrophilically treated substrates (Figure 3.21). The obtained PVDF films exhibit fairly good H

Glass

PVDF

C

H bonds

PVDF

–OH groups

O

PDMS Aluminum

F

Substrate

Si

Step 1: Substrate treatment

Step 2: PVDF casting Step 3

Step 5

Device fabrication

PVDF film peel-off and cut

Step 4

Electrode preparation

Figure 3.21 Illustration of PVDF film and device preparation process. Source: Wu et al. [121]/figure 1 (p. 2)/Springer Nature/Licensed under CC BY 4.0. Step-1, the glass substrate was soaked in piranha solution for 2–8 hours. Step-2, a well-stirred PVDF solution was cast on the substrate and dried at 80 ∘ C for 10 hours. Step-3, the PVDF film was peeled off from the substrate, and the edge was cut off to remove the edge effect. Step-4, aluminum was evaporated onto both sides of the film as electrodes. Step-5, the bilayer device was fabricated by using PDMS pillars supported between the two layers as separators. Also indicated were schematics of the hydroxyl groups bonded on the surface of the glass substrate after treatment, hydrogen bonds formation after PVDF casting and orderly arrangement of the “ultra-thin layer” at the bottom of PVDF film.

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pyroelectricity comparable to those fabricated by thermal poling, indicating the film is self-polarized. This result is attributed to the hydrogen-bonding-induced orderly arrangement of the first sub-nanolayer at the bottom, which serves as a “seed layer” and triggered alignment of the rest of the film in a layer-by-layer approach. Additionally, to suppress the piezoelectric noise, a pyroelectric sensor with a novel bilayer structure is developed using the as-prepared PVDF film (Figure 3.22). Compared with the conventional monolayer sensor, the SNR of the bilayer one is drastically improved to 38 dB from 18 dB. The above results provide immense possibilities for achieving a high-performance wearable pyroelectric sensor with reduced cost and simple procedures.

3.4.4

Ceramic-Polymer Composites

The ceramic-polymer composites consist of ferroelectric ceramics particles such as PT, PZT, barium titanate (BT), and TGS embedded in polymer matrix host materials such as PVDF or polyvinylidene fluoride-trifluoro-ethylene [P(VDF-TRFe)] in an ordered manner following the concept of connectivity proposed by Newnham et al. [122]. In recent decades, a large number of ceramic-polymer electronic composites have been introduced for medical, telecommunication, and microelectronics applications, and for devices ranging from micromechanical systems (MEMS; Bio-MEMS) through sensors and actuators [123]. The composites have a unique blend of polymeric properties such as mechanical flexibility, high strength, design flexibility and formability, and low cost, with the high electro-active functional properties of ceramic materials. In these materials, it is thus possible to tailor physical, electronic, and mechanical properties catering to a variety of applications. As a result, the composite as a whole is described using a set of microstructural characteristics, for example, connectivity, volume fractions of each component, the spatial distribution of the components, percolation threshold, and other parameters. Thus, the response of an electronic composite (electro-ceramic-polymer) to an external excitation (electric field, temperature, stress, etc.) depends upon the response of individual phases, their interfaces as well as the type of connectivity. As a result, an electronic composite can broadly be described as exhibiting electromagnetic, thermal, and/or mechanical behavior while maintaining structural integrity [123]. The dielectric constants, dielectric losses, and pyroelectric coefficients of the TGS-PVDF composite with different component proportions have been measured, and their variations with temperature were observed [124]. The D* value of the pyroelectric devices made from this composite even reach (5–7) × 107 cm Hz1/2 W−1 . The results have shown that TGS-PVDF composite has bright prospects in applications as a pyroelectric composite material. Changshui et al. [125] fabricated a novel ATGS-PVDF composite film by exerting an external electric field on the composite during preparation. XRD spectra of the films indicate that they have a high orientation ratio along the external electric field. The permittivity, pyroelectric coefficient, and the pyroelectric FoM increase with the applied field. With a field of 10 kV cm, better results for dielectric constant ∼10 and pyroelectric

Irradiation power on top surface φ = 10 sin(2πt) (W m–2)

Applied acceleration on device a = 10 cosθ (m s–2) (θ = 0~π)

6.0 × 10–12

0.25

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4.0 × 10–12

dT/dt (K s–1)

Piezoelectric response (Nm3)

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6

Figure 3.22 Simulation and measurement results of the bilayer-structured pyroelectric sensor. Source: Wu et al. [121]/figure 4 (p. 6)/Springer Nature/Licensed under CC BY 4.0. (a) Explored schematic of the device structure. (b) Model and results of piezoelectric response simulation. (c) Model and results of the thermal simulation. (d) Optical photo of the fabricated device. (e) Piezoelectric response at different frequencies. (f) Responses of the bilayer and conventional monolayer devices when simultaneously stimulated by mechanical vibration (5 Hz) and thermal irradiation (1 Hz). Source: Wu et al. [121].

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coefficient ∼ 30 μC m−2 K−1 at about 30 ∘ C have been reported. Yang et al. [126] performed an extensive study on TGS: P(VDFTrFE) composites with various volume fractions (0.05–0.43) of TGS embedded in P(VDFTrFE). The pyroelectric coefficient varied from 32 to 102 μC m−2 K−1 , the dielectric constant increased from 9.66 to 12.27, and the dielectric loss decreased from 0.021 to 0.008. The two phases of samples were polled in the same direction, and the pyroelectric coefficient was reinforced, while the piezoelectric contribution was partially canceled out. The low piezoelectric activity in the pyroelectric composite is an asset as it reduces vibration-induced noise. Thus, TGS: P (VDF-TrFE) composite is a good candidate for sensing elements in pyroelectric IR detecting devices. Chan et al. [127] studied the pyroelectric and piezoelectric properties of PT-P (VDF-TrFE) 0–3 composites. Two groups of poled samples have been prepared: one with only the ceramic phase polarized and the other with both phases polarized. The value of pyroelectric coefficient (40.7 μC m2 K), dielectric constant (57.3), and FoM (0.71) was obtained with 54% volume fraction of the particles when only the ceramic phase was poled. However, when both the phases were poled, then the pyroelectric coefficient (68.2 μC m2 K), dielectric constant (55), and FoM (1.24) were obtained with 54% volume fraction of the particles. Chen et al. [128] prepared nanocrystalline lead titanate (PbTiO3 or PT) powder by the sol–gel process followed by annealing at 700 ∘ C. Using the spin-coating technique, 0–3 nanocomposite thin films of PT powder dispersed in a vinylidene fluoride–trifluoro ethylene P(VDF–TrFE) copolymer matrix have been fabricated on glass substrates. The permittivity, pyroelectric coefficient, specific heat, and pyroelectric FoMs have been measured as functions of the volume fraction (𝜑) of ceramic for 𝜑 up to 0.12. All three FoMs (current, voltage, and detectivity) were found to increase with increasing 𝜑. Nanocrystalline calcium and lanthanum-modified lead titanate (PCLT) powder prepared by a sol–gel process was incorporated into a polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] copolymer matrix to form PCLT–P(VDF-TrFE) nanocomposite thin films with 0.11 volume fraction of ceramic [129]. The relative permittivity and pyroelectric coefficient of the P(VDF-TrFE) copolymer and nanocomposite films were measured as functions of the poling electric field. After poling under the same conditions, the nanocomposite film was found to have a higher pyroelectric coefficient (by ∼35%) and figures of merit than those of the P(VDF-TrFE) film of a similar thickness. Lead magnesium niobate–lead titanate (PMN-PT with 35 mol% PT) ceramic powder fabricated using the Columbite method has been incorporated into a polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE) 70/30 mol%) copolymer matrix to form 0–3 composites [130]. With the composition near the morphotrophic phase boundary (MPB) region, PMN-PT has high piezoelectric properties as a result of the enhanced polarizability arising from the coupling between two equivalent energy states. P(VDF-TrFE) ferroelectric copolymer films can be poled to give piezoelectric and pyroelectric performance without prior mechanical stretching. The composites with both phases being ferroelectric were prepared using solvent casting to disperse the ceramic powder homogeneously in the copolymer matrix. Several composites with PMN-PT volume fraction 𝜙 ranging from 0.05 to 0.4 were fabricated. The piezoelectric and pyroelectric coefficients of the composites were

3.4 Classification of Pyroelectric Materials

studied as a function of ceramic volume fraction 𝜙 under different poling conditions. Both the piezoelectric and pyroelectric coefficients of the PMN-PT/P(VDF-TrFE) 0–3 composites were found to be higher than that of the PZT/P(VDF-TrFE) 0–3 composites. Thick films of 0–3 composites of lead-zirconate-titanate ceramic and polyvinylidene-trifluoroethylamine copolymer have been produced by spin coating on gold-coated silicon wafers [131]. The dielectric properties were investigated as a function of ceramic volume fraction and temperature. Pyroelectric measurements were undertaken by temperature modulation with a Peltier element. Additionally, the pyroelectric response has been investigated up to 3000 Hz using a modulated laser. The piezoelectric response of the composites obtained by using a laser vibrometer is also reported. It is shown that the dielectric constant increases with the increasing volume fraction of ceramic, reaching a maximum at a temperature in the range of 65–70 ∘ C due to the ferroelectric-paraelectric phase transition of the polymer matrix. The pyroelectric coefficient increases to 92 μC m−2 K−1 at a ceramic volume fraction of 20%. Furthermore, the effective piezoelectric charge coefficient d33 of the composite almost vanishes at this composition. These composites show relatively high pyroelectric figures of merit and may be a potential candidate for pyroelectric sensor applications. de Campos Fuzari Jr. et al. [132] produced composite films of PZT ceramic particles coated with polyaniline and poly(vinylidene fluoride) – PZT-PAni/PVDF by hot pressing the powder mixtures in the desired ceramic volume fraction. The ceramic particles were coated during the polyaniline synthesis and the conductivity of the conductor polymer was controlled by different degrees of protonation. Composites were characterized by FTIR, SEM, ac and dc electrical measurements, the longitudinal d33 piezo coefficient, and the photopyroelectric response. Results showed that the presence of nanocomposites of polyaniline (PANI) increased the dielectric permittivity of the composite and allowed better efficiency in the poling process, which increased the piezo- and pyroelectric activities of the composite film and reduced both the poling time and the poling electric field. The thermal sensing of the material was also analyzed, showing that this composite can be used as a pyroelectric sensor. The thick films of PZT/PVDF-TrFE composites with different PZT particles (different calcined temperatures) were produced by casting PZT/PVDF-TrFE suspensions onto the indium-tin-oxide (ITO)-coated glass substrates by Wu et al. [133]. It was found that the calcining treatment of PZT powders showed significant impact on the electric properties of PZT/PVDF-TrFE composite thick films. The highest pyroelectric coefficient obtained in the sample using 700 ∘ C calcined PZT powders was 96 μCm−2 K−1 , which was 20% higher than the composites made of uncalcined powders. Additionally, the highest F D of the composite was 1.36 × 10−5 Pa−1/2 , which increased about 13.5% compared to the one using uncalcined powders. So, the composite made by 700 ∘ C calcined PZT has the highest value of F D , providing excellent electric properties for IR applications. The fabrication method and the pyroelectric response of a single element IR sensor, based on PZT particles and a PVDF-TrFE copolymer composite thick film was reported by Wu et al. [134]. A special thermal insulation structure, including

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polyimide (PI) thermal insulation layer and thermal insulation tanks, was used in this device. The thermal insulation tanks were fabricated by the laser micro-etching technique. Voltage responsivity (RV ), noise voltage (V noise ), noise equivalent power (NEP), and detectivity (D* ) of the PZT/P(VDF-TrFE)-based IR sensor are 1.2 × 103 V W−1 , 1.25 × 10−6 V Hz1/2 , 1.1 × 10−9 W and 1.9 × 108 cm Hz1/2 W−1 at 137.3 Hz modulation frequency, respectively. The thermal time constant of the IR sensor 𝜏 T was about 15 ms. The results demonstrate that the composite IR sensor shows a high detectivity at high chopper frequency, which is an essential advantage in IR detectors and some other devices. Dietze and Es-Souni [135] processed 0–3 composites of PNN/PVDF-TRFE (lead nickel niobate-lead zirconate titanate) relaxor type ceramic powder (PZ21) from Meggitt A/S (Denmark) and PVDF-TrFE copolymer powder (56 : 44, Solvay Solexis S.A.S, France) with different volume fractions of ceramic powder using solution deposition techniques. Free-standing thick films of up to 100 μm thickness were fabricated by the doctor blade method, while thin films of up to 10 μm thickness were deposited on silicon wafers using spin coating. The samples investigated show that the phase transition temperatures decrease with increasing ceramic volume content. The dielectric properties depend on the ceramic volume fraction. Higher dielectric constants together with lower dielectric loss are obtained for higher ceramic volume fractions. The dielectric properties also depend on fabrication processes with the tape-cast samples being characterized by superior properties which may be explained by the better microstructure. The dependence of the imaginary part of the complex permittivity on temperature shows two dielectric relaxation peaks that can be attributed to the ferroelectric-paraelectric transition at a higher temperature and the glassy-to-rubbery state transition of the amorphous polymer at a lower temperature. The pyroelectric coefficient shows a maximum at 20 vol% PNN-PZT. The spin-coated samples have higher pyroelectric coefficients, due to the better poling behavior of the thinner films. The FoM shows comparable values to those of PZT thick films. The high-frequency pyroelectric response of spin-coated films shows a maximum at higher frequencies, due to the higher time constants of the films. Satapathy et al. [136] studied the PVDF and LT nano-composites for their pyroelectric and dielectric properties as an IR detector. The choice of LT was decided by the fact that LT has a much lower poling field requirement and larger pyroelectric coefficient, with a comparable dielectric constant to PVDF. Studies show an increase in pyroelectric voltage sensitivity in LT/PVDF nanocomposite film compared to pure PVDF film. As the volume fraction of LT (f LT ) increases from 0.0 to 0.17 in LT/PVDF nanocomposite, the ferroelectric polarization increases from 0.014 to 2.06 μC cm−2 at an applied field of 150 kV cm−1 . Similarly, the increase of f LT from 0.00 to 0.17 results in an increase of pyroelectric voltage sensitivity from 3.93 to 18.5 V J−1 . LT oxide (LiTaO3 , LT), the pyroelectric ceramic powder, has been incorporated into a polyvinylidene fluoride–trifluoroethylene [P(VDF–TrFE) 70/30 mol%] copolymer matrix to form 0–3 composites films [137]. The films were prepared using the solvent casting (SC) method to disperse the ceramic powder homogeneously in the P(VDF–TrFE) copolymer matrix with various wt% of LT powder. In order to derive high pyroelectric performance, the samples were poled. Electric properties such as the dielectric constant, dielectric loss, and pyroelectric coefficient were measured

3.4 Classification of Pyroelectric Materials

as a function of temperature and frequency. In addition, material FoMs that are very important factors for assessing many sensor applications have also been calculated. The results show that the fabricated lead-free lithium tantalite: P(VDF–TrFE) composite materials have a good potential for uncooled pyroelectric IR sensor applications operating at moderate temperatures. LiTaO3 nanoparticles are synthesized and dispersed in PVDF at varying volume fractions, and composite materials are cast in the form of films for measurements [138]. The 𝛽-phase of PVDF is confirmed from powder XRD, FT-IR, and Differential Scanning Calorimetry (DSC) measurements. The dielectric properties, shore D hardness, and pyroelectric coefficients of the cast films are measured. The thermal conductivity and specific heat capacity of the films are measured following a photothermal technique. From these data, the pyroelectric figures of merit of the composite films have been determined and values compared with that of pure PVDF film. It is found that, in general, the pyroelectric FoM increases with the concentration of LiTaO3 ; however, it is at the expense of the mechanical flexibility of the material. This chapter aims at providing guidelines to strike the right balance between detection sensitivity and material flexibility (or mouldability), depending on the application. Padmaja et al. [139] have used the FT-IR, Raman spectral analysis, and dielectric measurements to determine the characteristic properties of multi-walled carbon nanotubes (MWCNTs) doped the lead-free pyroelectric material; PVDF; and LT nanocomposites. These nanocomposite thin films dielectric behavior was characterized as a function of temperature and frequency. Deterministic characteristics of the FT-IR, Raman spectrum for both the nanocomposites, doped and undoped, were observed. Values of both 𝜀′ and 𝜀′′ are high at a lower frequency and decrease with an increase of frequencies due to polarization effects. Samples, ranging from 15 to 280 μm in thickness, were measured in the temperature range from room temperature 20 to 90 ∘ C. All results showed that MWNTs were covalently linked with the blend through C—C bonds. The temperature-dependent pyro-voltage and figure of merits derived from pyroelectric coefficient and dielectric measurements were presented. The addition of MWCNTs increases and enhances the conductivity attributed to charge carrier build-up and increases the segmental mobility of polymeric chains. The samples were prepared by the solution casting technique. Measures indicate that as the temperatures vary both the dielectric constants increase with the MWCNT doping these quantities increase at least 10 times. The MWCNTs-doped PVDF + LT thin films yield higher figures of merit compared to PVDF + LT thin films, and the results indicate an efficient usage of MWCNTs-doped PVDF + LT thin films in multi-device applications. Jayalakshmy and Philip [140] studied the pyroelectric properties of polymerceramic nanocomposites of LiNbO3 /PVDF (abbreviated LN/PVDF) for thermal/IR sensing applications. The composites are prepared by dispersing nanoparticles of LiNbO3, with particle size in the range of 45–65 nm in the β-PVDF matrix at varying volume fractions, and cast in the form of flexible films by solvent-cast technique. The electro-active 𝛽-phase of PVDF is confirmed by powder XRD, FT-IR, and DSC analyses. The thermal properties, thermal conductivity, and specific heat capacity of the composites are determined by a photothermal technique. The prepared films

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have been poled in a high dc electric field, and their pyroelectric and dielectric properties are measured by direct methods. From these data, the pyroelectric FoMs of the composite films have been determined and their values compared with corresponding values for pure PVDF film. The shore hardness of the films has been measured to estimate the extent to which the flexibility of the films is affected by the addition of ceramic. Significant enhancement in pyroelectric sensitivity has been obtained with an increase in the volume fraction of LiNbO3 nanoparticles. However, this enhancement is at the expense of the flexibility of the composite; therefore, one has to strike a balance between the two while selecting a suitable composition for the development of pyroelectric sensors with these materials. The results of this work provide guidelines for this selection. The (K0.5 Na0.5 )NbO3 (KNN)/[P(VDF-TrFE)70: 30] composite thick films with different KNN weight ratios have been fabricated, and the effects of KNN mass content on the material structure and properties have been studied [141]. Properties of the IR sensor-based KNN/[P(VDF-TrFE)70:30] composite thick film were also systematically studied. It was found that the sample containing 30 wt% KNN show optimal properties for pyroelectric appliance and the highest pyroelectric coefficient was 63 μCm−2 K−1 . IR sensors using 30 wt% KNN-70 wt% [P(VDF-TrFE)70:30] show the highest detectivity (D* = 3.21 × 108 cm Hz1/2 W−1 ) at 137.3 Hz, indicating it is a promising candidate in lead-free quick response IR detectors.

3.4.5

Lead-Free Ceramics

The use of lead-free materials has recently become a very important issue in the environmental protection of the earth. Ferroelectric ceramics such as PT and PZT are widely used in sensor, actuator, and transducer applications due to their superior piezoelectric and pyroelectric properties. However, the use of lead-based ceramics has caused serious environmental pollution. The toxicity of these materials either during the manufacturing process (evaporation of lead) or after making the device is of serious concern. Therefore, there is an urgent need to develop lead-free ceramics for replacing lead-containing ceramics in various applications. In the recent past, a number of BNT-based lead-free ceramics such as Bi0.5 Na0.5 TiO3 –BaTiO3 (BNT-BT), Bi0.5 Na0.5 TiO3 –Bi0.5 K0.5 TiO3 (BNKT), and Bi0.5 Na0.5 TiO3 – NaNbO3 (BNT-NN), have been developed with a view to improving their performance [142–144]. Lang et al. [145] reported the piezoelectric and pyroelectric properties of the K0.5 Na0.5 NbO3 (KNN)-based ceramic. The d33 and p coefficient were found to be 289 pC N and 160 μC m2 K, respectively. Lau et al. [146] studied two groups of lead-free ceramics, (K0.5 Na0.5 )NbO3 -based (KNN)and Bi1 − y (Nax K1 − x )y TiO3 -based(BNKT), for their thermal, dielectric, and pyroelectric properties as candidates for pyroelectric sensor applications. Four different systems of ceramic powder were fabricated and the chemical formulas are listed below: (a) [(K0.5 Na0.5 )0.96 Li0.04 ](Nb0.8 Ta0.2 )O3 (KNLNT); (b) [(K0.5 Na0.5 )0.96 Li0.04 ](Nb0.84 Ta0.1 Sb0.06 )O3 (KNLNTS);

3.4 Classification of Pyroelectric Materials

Table 3.1 samples.

Pyroelectric coefficient and figures of merit of the PZT and lead-free ceramic

Sample

tan 𝜹 𝝆 𝜺r (at cv (×106 J p (𝛍C m−2 F I (×10−12 F V (m2 F D 100 Hz) (at 100 Hz) (kgm−3 ) m−3 K−1 ) K−1 ) m V−1 ) C−1 ) (𝝁Pa1/2 )

KNLNT

1230

0.0182

5081

0.263

165

KNLNTS 1520

0.0181

4556

0.448

BNKBT

0.0278

5800

0.288

BNKLBT 858

0.0294

5770

PZT

0.0140

7700

853 1990

123.5

0.011

8.82

190

93.1

0.007

5.98

325

194.6

0.026

13.43

0.283

360

220.5

0.029

14.75

0.380

414

141.5

0.008

9.01

Source: Lau et al. [146], table 1 (p. 2)/AIP Publishing.

(c) [Bi0.5 (Na0.95 K0.05 )0.5 ]0.95 Ba0.05 TiO3 (BNKBT); (d) [Bi0.5 (Na0.94 K0.05 Li0.016 )0.5 ]0.95 Ba0.05 TiO3 (BNKLBT). The BNKT-based ceramic, [Bi0.5 (Na0.94 K0.05 Li0.016 )0.5 ]0.95 Ba0.05 TiO3 (BNKLBT), shows excellent pyroelectric properties when compared with KNN-based ceramic and lead zirconate titanate. Its properties were measured as follows: pyroelectric coefficient p = 360 μC m2 K, pyroelectric figure of merit of current, voltage, and detectivity F I = 221 p.m. V−1 , F V = 0.030 m2 C−1 , and F D = 14.8 μPa−1/2 . Table 3.1 shows the pyroelectric coefficient and figures of merit of all these lead-free ceramic samples. For comparison, the pyroelectric properties of the PZT are also given in Table 3.1. With these outstanding pyroelectric properties, the BNKLBT lead-free ceramic samples were used as a promising material for pyroelectric sensor applications. Sensing elements of different thicknesses (i.e. 0.3, 0.5, and 0.7 mm) were prepared. A chromium-gold (Cr/Au) electrode of a diameter of 8 mm was sputtered on both surfaces of the samples. To enable a high absorption in the spectral region of interest, the surface of the element to be irradiated was coated with a thin graphite layer. The element was then mounted in a holder and put in the shielding box, as shown in Figure 3.23. The current responsivity of the sensors was evaluated as functions of frequency. The current responsivity increases with the frequency and reaches the maximum at 180 Hz. At a higher frequency, the current responsivity decreases because the heat transfer from the graphite absorber and top electrode becomes less efficient. By reducing the element thickness, the current responsivity increases significantly.

3.4.6

Other Pyroelectric Materials

The electronic transport in semiconductors that possess high internal spontaneous and piezoelectric polarization opens up a new field of pyroelectronics and pyrosensors. The wurtzite structures of the group III-nitride (III-N) compounds (such as AlN and GaN) and ZnO have been suggested as possible candidates for pyroelectric sensors. Calculations by Bernardini et al. [147] and Zoroddu et al. [148] have predicted

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Blackened layer

Sample holder

Top electrode Bottom electrode

Sensing element BNC output

Figure 3.23 Schematic diagram of the pyroelectric sensor. Source: Lau et al. [146], figure 2 (p. 3)/AIP Publishing.

that III-N compounds should have significantly higher piezoelectric constants and pyroelectric coefficients 5–20 times higher than other III-V and II-VI compounds. 3.4.6.1 Aluminum Nitride (AlN)

Research is being conducted on aluminum nitride (AlN) as a pyroelectric material for use in detecting applications. AlN is being investigated because of its high pyroelectric coefficient, thermal stability, and high Curie temperature. Pyroelectric properties of high-quality (0001) AlN films grown on (111) Si were studied by Fuflyigin et al. [149]. The pyroelectric coefficient p was measured using the dynamic method. The value was in the range of 6–8 μC m−2 K−1 , yielding a p/𝜀 FOM of 0.8–0.95. The pyroelectric coefficient was independent of temperature and applied bias. The leakage current as low as 1–2 × 10−9 A cm−2 was measured at 5 V on large-area devices. The obtained results indicate that AlN films can be used in pyroelectric thin-film devices. Kebede et al. [150] determined the suitability of the pyroelectric properties of AlN for use as a detector, testing of several devices was conducted. These devices were fabricated using MEMS fabrication processes. The devices were also designed to allow for voltage and current measurements. The deposited AlN films used were 150–300 nm in thickness. The design used was a face-electrode bridge that provides thermal isolation that minimizes heat loss to the substrate, thereby increasing the operating frequency of the pyroelectric device. A thermal stimulus was applied to the pyroelectric material, and the response was measured across the electrodes. A thermal imaging camera was used to monitor the temperature changes. Throughout the testing process, the annealing temperatures, type of layers, and thicknesses were also varied. These changes resulted in improved MEMS designs, which were fabricated to obtain an optimal design configuration for achieving a high pyroelectric response. A pyroelectric voltage response of 38.9 mVp-p was measured without filtering, 12.45 mVp-p was measured in the IR region using a Si filter, and 6.38 mVp-p

3.4 Classification of Pyroelectric Materials

was measured in the short wavelength IR region using a long-pass filter. The results showed that AlN’s pyroelectric properties can be used in detecting applications. The crystallographic, microstructural, and pyroelectric properties of 900 nm thick pyroelectric Al1 − x Scx N (AlScN) thin films (x = 0 to 0.3) were investigated by Kurz et al. [151]. All films have a wurtzite-type crystal structure and are highly c-axis oriented. The pyroelectric coefficient in the temperature range of 20–80 ∘ C was determined by measuring the generated current as a function of sinusoidal temperature excitation. The effective pyroelectric coefficient of AlScN increases by approximately 85% in the investigated composition range. Moreover, no temperature dependence of the effective pyroelectric coefficient was observed for all the studied samples. The increase of the effective pyroelectric coefficient is explained qualitatively by considering the contribution of primary and secondary pyroelectric effects as well as the substrate clamping. The electric and pyroelectric properties of AlN layers deposited on Si substrates with different resistivities were investigated by Stan et al. [152]. The dielectric constant was found to be around 12, while the conductance determined from dc current measurements was found to be in the 10−9 –10−10 S range. The pyroelectric measurements were performed in voltage mode using two types of IR sources: a laser diode with 800 nm wavelength and a black body at 700 ∘ C. A peculiar behavior was observed for the signal recorded when the laser diode was used as an IR source. It was found that the Si substrate is introducing a signal component, due to the photogenerated carriers, which add to the pyroelectric signal generated by the AlN layer. This component is strongly dependent on the resistivity of the Si substrate. For strongly doped Si (Si++ ), the signal generated into the substrate represents only 10% of the recorded pyroelectric voltage. For electronic grade Si, the signal generated into the substrate is about 100 times larger than the pyroelectric signal generated in the AlN layer. This effect can be used as an optical amplification of the pyroelectric signal. The frequency dependence observed for the pyroelectric signal recorded when the black body is used as an IR source is typical for a pyroelectric detector. A value as large as 12.4 μC m−2 K−1 was obtained for the pyroelectric coefficient using for estimation the constant signal at low modulation frequencies of the IR beam. However, the value of the pyroelectric coefficient is strongly affected by the electrical conductance of the AlN layer. As the conductance is frequency-dependent, it results that the value of the pyroelectric coefficient is frequency-dependent, the value from above being valid only for very small frequencies of the temperature variation. It was also found that the electric and pyroelectric properties are dependent on the crystalline quality of the AlN layer. Goldsmith et al. [153] presented a pyroelectric detector for selective detection in the long-wavelength IR region. Pyroelectric AlN forms a dielectric layer separating a gold plasmonic hole array from a refractory metal, titanium nitride. IR radiation incident on the hole- array forms a gap plasmon mode whose electric field is confined to the pyroelectric spacer layer. Relaxation of the mode heats the pyroelectric spacer layer, producing a pyroelectric voltage. Using this technique, 18.9 dB of spectral selectivity at 𝜆 = 9 μm was achieved with a peak responsivity of 1.08 V W−1 .

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Ranacher et al. [154] presented a CMOS (complementary metal-oxidesemiconductor) compatible pyroelectric detector which was devised as a mid-IR detector, comprising AlN thin film as the pyroelectric material and fabricated using semiconductor mass fabrication processes. The detector comprises a phosphorous-doped poly-Si bottom electrode and an AlSiCu top electrode with an AlN layer sandwiched in between. The poly-Si bottom electrode also acts as an IR radiation-absorbing layer. The absorbed radiation induces a temperature change in the poly-Si layer and also in the AlN layer if the two layers are in sufficient thermal contact. Due to the thermal detection approach, it is crucial to thermally decouple the detector from the substrate to avoid transfer of the heat to the substrate and maximize the induced temperature change in the AlN layer. Therefore, to achieve the thermal decoupling from the substrate, the devised detector is located on a free-standing Si3 N4 /SiO2 membrane. To electrically connect the detector, both electrodes feature the necessary contact pads. Figure 3.24 shows a schematic quarter cut of the devised detector. The SiO2 layer that is located below the Si3 N4 layer is necessary for the evanescent field absorption sensing, using a Si waveguide in order to suppress the coupling of an electromagnetic mode into the substrate. In the final application, for further thermal decoupling, it would be beneficial to remove the SiO2 layer below the detector area as this is expected to result in a higher response of the detector. Within this work, various detector/membrane dimensions were tested. The size of the membrane was varied from 0.14 to 1.44 mm2 , and the size of the detector (more specifically of the AlN pad) was varied from 0.03 to 1 mm2 . Among the tested ones, the highest response was achieved with a detector area of 0.25 and 0.50 mm2 , respectively and a membrane size of 1.44 mm2 . The noise equivalent power of the detector with an area

Top electrode (AlSiCu) Bottom electrode (doped poly-Si) Pyroelectric layer (AIN) Nitride layer (Si3N4)

Contact pad (AlSiCu)

Oxide layer (SiO2) Substrate (Si) Radiation

Figure 3.24 Quarter cut schematic representation of the pyroelectric detector. Source: Ranacher et al. [154]/figure 1 (p. 3)/MDPI/Licensed under CC BY 4.0.

3.4 Classification of Pyroelectric Materials

/√ of 0.25 mm2 was investigated and was as low as NEP = 5.3 × 10−9 W Hz for the investigated demonstrator devices and a measurement bandwidth of 1 Hz, which √ corresponds to a detectivity of D* = 9.4 × 106 cm Hz W−1 . The study on the development of a polycrystalline AlN thin film-based bulk micromachined pyroelectric IR sensor was presented by Gaur et al. [155]. Structural optimization of IR sensors has been carried out using 3D finite element modeling (FEM) and simulations. A 1.0-μm-thick thermally grown SiO2 layer used for thermal isolation also serves as a diaphragm to hold the fabricated IR sensor. The rate of temperature change (dT/dt) of the sensor under dynamic heating is 0.12–0.15 K s−1 and agrees well with the simulated value of 0.1 K s−1 . High pressure, N2 ambient sputtered Au film of 160 nm thickness has been used to enhance IR absorptivity. IR absorptivity of the sensor on medium to long wave (2.5–25 μm) radiations is nearly 67% and creates a thermal gradient of 0.23 K between sensor and substrate. The developed pyroelectric IR sensor exhibits response time 8.0 ms, pyroelectric coefficient (𝜌) 0.32 × 10−4 C m−2 K−1 , 𝜌/𝜀 FOM 3.0 μC m−2 K−1 , and pyroelectric current responsivity (Ri ) of 2.5 × 10−6 A W−1 . Ng et al. [156] designed a MEMS pyroelectric IR detector using CMOS compatible AlN and 8-in. semiconductor wafer technology. The pyroelectric sensing material used is AlN deposited at a temperature of around 200 ∘ C. Figure 3.25 shows a schematic of the AlN pyroelectric detector design layer structure. This AlN pyroelectric detector detects IR over wavelength ranging from 5 to 14 μm. The detector is designed to have added mechanical stiffness for improved device integrity. Pyroelectric detectors fabricated with different sensing areas are measured,

Al

SiO2 / SiN / SiO2 (absorber stack)

Al

TiN (top electrode) AIN (sensing layer) Mo (bottom electrode) SiO2

Si substrate

SiO2 ribs for added mechanical stiffness

Si substrate

Figure 3.25 Schematic of the AlN pyroelectric detector showing the structure layers. Source: Doris et al. [156]], figure 1 (p. 2)/IEEE/Licensed under CC BY 4.0. The absorber stack is the topmost layer, and the AlN sensing layer is between the top and bottom electrodes. An array of SiO2 ribs is fabricated under the bottom electrode to increase the mechanical stiffness of this membrane device. Al is used for the metal contacts.

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and the results are compared. So far, a sensing area of 0.29 mm2 has shown√the / best performance out of the 4 sensing areas with the NEP ∼ 8.87 × 10−9 W Hz √ and D* ∼ 6.04 × 106 cm Hz W−1 . These pyroelectric detectors are designed and built with the consideration to enable ease of monolithic integration with other components to form an integrated gas sensor system. This includes enabling detection of illumination from the front side and using an absorber stack that consists of CMOS dielectric layers. The results from these pyroelectric detectors will form a crucial part in the design, realization and monolithic integration of architectures for CMOS-compatible miniature photonics-based gas sensors, manufactured at an 8-in. wafer-level scale. 3.4.6.2 Gallium Nitride (GaN)

Gallium Nitride (GaN)-based materials are usually grown in the [0001] direction (when they have the wurtzite crystal structure) and in the [111] direction (when they have the zinc blende crystal structure). These are polar axes, and therefore, GaN-based materials exhibit strong lattice polarization effects. These effects are uniquely suited for applications in high-temperature piezo-electronics and applications in pyroelectric sensors. Bykhovski et al. [157] reported on the measurements of the pyro-effect in wurtzite n-type GaN films deposited over basal plane sapphire substrates. They measured the voltage drop between the contacts while the sample was subjected to uniform heating or cooling. The pyroelectric voltage coefficient extracted from the data is comparable to that of the pyroelectric ceramics (∼104 V m−1 K−1 ). The results show that the pyroelectric effect in GaN is a combination of a fast response to an initial heat flow and a slower response related to a change in the sample temperature. Shur et al. [158] reviewed the pyroelectric and piezoelectric properties of GaN-based materials. Pyroelectric effects in GaN have been studied in two different regimes: (i) uniform sample heating regime and (ii) under applied temperature gradient along with the sample. The modeling results show that the pyroelectric coefficient, Pv , in GaN (for c-axis along with the contacts) can reach 7 × 105 V (m-K)−1 (compared to Pv = 5 × 105 V m−1 K−1 for the best-known high temperature pyroelectric/piezoelectric material LiTaO3 ). This points to a high potential of GaN-based sensors for high-temperature pyro-electronics. Piezoelectric effects strongly affect the performance of electronic and light-emitting devices based on III-N materials. Piezoelectrically induced charge in heterostructures can be as large as 3 to 4 × 1013 cm−2 . Hence, strong lattice polarization effects provide unique possibilities for utilizing GaN-based materials in high-temperature piezo-electronics and for their applications in pyroelectric detectors. Jachalke et al. [159] reported the temperature-dependent pyroelectric coefficient of free-standing and strain-free GaN grown by hydride vapor phase epitaxy (HVPE). The Sharp–Garn method is applied to extract the pyroelectric coefficient from the electrical current response of the crystals subjected to a sinusoidal temperature excitation in a range of 0 to 160 ∘ C. To avoid compensation of the pyroelectric response by an internal conductivity, insulating GaN crystals were used by applying C, Mn, and Fe doping during HVPE growth. The different pyroelectric coefficients observed at room temperature due to the doping correlate well with the change of the lattice

3.4 Classification of Pyroelectric Materials

parameter c. The obtained data are compared to previously published theoretical and experimental values of thin-film GaN and discussed in terms of a strained lattice. Rais-Zadeh [160] reported on a novel technology for low-noise un-cooled detection of IR radiation using a combination of piezoelectric, pyroelectric, electrostrictive, and resonant effects. The architecture consists of a parallel array of high-Q GaN micro-mechanical resonators coated with an IR absorbing nanocomposite. The nanocomposite absorber converts the IR energy into heat with high efficiency. The generated heat causes a shift in frequency characteristics of the GaN resonators because of the pyroelectric effect. IR detection is achieved by sensing the shift in the resonance frequency and amplitude of the exposed GaN resonator as compared to a reference resonator that is included in the array. Figure 3.26 shows a general schematic of the resonant GaN IR detector. This architecture offers an improved SNR compared with conventional pyroelectric detectors as the resonant effect reduces the background noise and improves sensitivity, enabling IR detection with the noise equivalent of temperature difference (NETD) below 5 mK at room temperature. GaN is chosen as the resonant material as it possesses high pyroelectric, electrostrictive, and piezoelectric coefficients and can be grown on silicon substrates for low-cost batch fabrication. 3.4.6.3 Zinc Oxide (ZnO)

Zinc oxide (ZnO) is a unique material that possesses such properties as semi-conductivity (II-VI compound semiconductors), piezoelectricity, and pyroelectricity without the pooling process. It also has low cost, low toxicity, and is environmentally friendly. Interesting properties, including a large bandgap (∼3.3 eV), high exciton binding energy (60 meV), good transmittance, excellent thermal stability, and n- or p- conductivity, are attributes of ZnO material. Wide bandgap wurtzite phase ZnO has attracted attention due to its versatility in many applications, such as blue and UV light emitters, transparent conductors, solar cell windows, gas sensors, photovoltaic devices, pyroelectric sensors, surface

Top electrode

ion

rpt

so

b Ra

I

Admittance

IR absorber

GaN

GaN resonant sensor

GaN reference resonator

Bottom electrode

Frequency

Figure 3.26 A schematic view showing the GaN IR sensor architecture. A reference resonator is used to reduce noise and cancel the frequency shift because of other changes such as an increase in the ambient temperature. The detector response time is governed by the thermal time constant (𝜏 th = Rth C th ), where Rth and C th are the thermal resistance of the tethers and the thermal capacity of the resonator, respectively. Source: Rais-Zadeh [160].

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acoustic wave (SAW) devices, film bulk acoustic resonators (FBARs), catalysis, optoelectronics, and photo-electrochemical devices [161–163]. ZnO films have been synthesized by numerous methods such as metal–organic chemical vapor deposition, molecular beam epitaxy, magnetron sputtering, pulsed laser deposition, atomic layer deposition, spray pyrolysis, filtered cathodic vacuum arc technique, sol–gel process, and aerosol deposition (AD). The quality of ZnO films obtained by the above methods depends on the specific growth methods and conditions. Thus, the preferential orientation of ZnO films depends on the growth conditions. The pyroelectricity of ZnO is attributable to non-centrosymmetrical crystals, and thus, it has a specific polar axis along the direction of spontaneous polarization [6, 164–166]. The most densely packed and thermodynamically favorable growth orientation in a ZnO wurtzite structure is one in which the c-axis is perpendicular to the substrate. ZnO films with the c-axis normal to the substrate are preferred in many applications, such as ZnO pyroelectric devices [165, 166] and film bulk acoustic resonators [167]. When ZnO is subjected to temperature variations, its internal polarization will produce an electric field. Pyroelectric materials respond to changes in temperature, which cause internal strain and in turn result in an electrical charge on the material surface. Therefore, increasing the responsivity of a ZnO pyroelectric sensor depends on increasing the temperature variation rate of the ZnO layer, adopting a ZnO film with a strongly preferred orientation toward the c-axis, and using a high-performance thermal-isolation structure. Moreover, a partially covered top electrode sensor exhibits at least 4 times higher responsivity in the frequency range of 10–1000 Hz than the full-electrode sensors, because it allows the pyroelectric layer to make direct contact with the heat source, thus increasing the temperature variation rate [168]. In 2008, Hsiao et al. [165] proposed a two-step RF sputtering process to form a ZnO film for pyroelectric sensors. It is shown that the two-step sputtering process with a lower power step followed by a higher power step can significantly improve the voltage responsivity of the ZnO pyroelectric sensor. The improvement is attributed mainly to the formation of ZnO film with a strongly preferred orientation toward the c-axis. Furthermore, a nickel film deposited onto the uncovered parts of the ZnO film can effectively improve the voltage responsivity at higher modulating frequencies since the nickel film can enhance the incident energy absorption of the ZnO layer. In 2012, Hsiao and Yu [169] fabricated a three-dimensional ZnO film by the AD rapid process using the shadow mask method, integrated with a comb-like top electrode, enhanced the voltage response of ZnO pyroelectric devices (Figure 3.27). At a high frequency of about 3000 Hz (33 μs), the ZnO pyroelectric device with the comb-like electrode possessed a voltage response about four times greater than that with the fully covered electrode. The three-dimensional ZnO film did indeed induce lateral temperature gradients on the sidewalls of the ZnO layer, thereby increasing the temperature variation rate of the responsive element, enhancing the voltage response and reducing the response time of ZnO pyroelectric devices. In 2014, Hsiao and Liu [170] proposed a methodology for designing a multi-frequency band pyroelectric sensor which can detect subjects with various

3.4 Classification of Pyroelectric Materials Bottom electrode

Silicon nitride

Silicon substrate

Top electrode

Bottom electrode

Silicon substrate Laser irradiated ZnO film

Laser irradiated ZnO film

(a)

Silicon nitride

Top electrode

(b) 1 μm thick ZnO film above bottom electrode 2 μm thick ZnO film with a comb-like form

Bottom electrode

(c)

Figure 3.27 Fabricated ZnO pyroelectric devices: (a) the fully covered electrode with the single ZnO layer; (b) the comb-like electrode with the three-dimensional ZnO film; (c) the patterned device before top electrode deposited. Source: Hsiao and Yu [169].

frequencies or velocities. The sensor consists of a thinner and a thicker ZnO pyroelectric layer and top and bottom electrodes was built on a silicon substrate with a thermal insulation (silicon nitride) layer to reduce heat and electric loss. The thinner ZnO pyroelectric layer was named the sputtered ZnO layer, and the thicker ZnO pyroelectric layer was named the aerosol ZnO layer. Figure 3.28 shows the schematic diagram of the multi-frequency band pyroelectric sensor. The sputtered ZnO layer acted as a producer of the responsivity at higher frequency bands, while the aerosol ZnO layer detected the signals of the sensors at lower frequency bands. Sputtering was used to deposit the thinner ZnO films with high quality. Furthermore, the thicker ZnO layer was to detect the signals of the sensors at lower frequency bands, and deposited by the AD. The fabricated multi-frequency band pyroelectric sensor with a 1-μm-thick sputtered ZnO layer and a 20-μm-thick aerosol ZnO layer can sense a frequency band from 4000 to 40 000 Hz without tardy response and low voltage responsivity. Room-temperature UV detection has attracted tremendous attention for defense technology, aerospace technique, flame warning, and chemical/biological analysis [171]. ZnO has become one of the ideal materials for room temperature UV sensing applications [172–176]. Various ZnO-based UV photodetectors (PDs) have been reported with high photoresponsivity and/or detectivity; however, the typical response and recovery time of these PDs are ranging from several tens of microseconds to few hours in most cases [177–183]. Moreover, the response/recovery time is highly dependent on the ambient gas conditions due to the trapping mechanism controlled by the process of oxygen adsorption to and desorption from the ZnO nanowires (NWs) surface [173, 181, 184]. Normally, an even longer

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Sputtered ZnO layer

5500

5500

Top electrode(Au/Cr)

A

A

Aerosol ZnO layer

Bottom electrode(Au/Cr)

Vertical view

TPZ

0.1

Silicon substrate

1500

0.1

Silicon nitride layer

1500

1 500 1 TAZ

106

A-A cross section

Figure 3.28 Schematic diagram of the multi-frequency band pyroelectric sensor (unit: μm). Source: Hsiao and Liu [170]/figure 2 (p. 22185)/MDPI/Licensed under CC BY 4.0.

response time is expected for ZnO-based UV sensors operating in a vacuum or oxygen-deficient environments [173, 184]. This issue has long been an obstacle to improving the ZnO-based UV sensing performances and prevented them from practical applications [185]. Therefore, it is essential to develop a new working mechanism for ultrafast response/recovery UV sensing devices based on ZnO micro/nanostructures. For wurtzite-structured ZnO NWs, pyroelectric polarizations are induced by changing temperatures across the material due to the non-central-symmetric crystal structures [169, 186]. Upon light illuminations, a rapid increase in temperature is naturally induced within ZnO NWs, leading to a distribution of pyroelectric potentials along the crystal with pyro-polarization charges presenting at both ends of the NW [166, 169]. By forming a p-n heterojunction between n-ZnO and p-type semiconductors, these light-induced pyroelectric charges can effectively and rapidly tune/control the charge transport across the interface/junction and modulate the optoelectronic processes of local carriers, such as generation, separation, diffusion, and recombination. This is the pyro-phototronic effect [187], a three-way coupling effect among pyroelectric effect, photonic excitations, and semiconductor properties, which provides an alternative to achieve ultrafast photo-sensing performances through optoelectronic processes. In 2016, Wang et al. [188] designed a UV nano-sensor based on p-Si/n-ZnO NWs heterojunctions, and carefully studied and systematically characterized. The light-self-induced pyro-phototronic effect in ZnO is utilized to modulate the optoelectronic processes and thus enhance the performances of UV sensing in different ambient gas conditions (i.e. air and vacuum). The response time of these p-Si/n-ZnO UV PDs is improved from 59 to 19 μs at the rising edge and 40 to 22 μs

3.4 Classification of Pyroelectric Materials

at the falling edge by the pyro-phototronic effect in the air. The photoresponsivity R is enhanced by up to 599% regarding the relative changes of transient current in the air. Similar UV sensing performances are derived in a vacuum as well. This work provides a novel design mechanism to achieve ultrafast UV sensing performances at room temperature in air/vacuum. The demonstrated ultrafast UV sensing devices (as shown in Figure 3.29) may find broad applications in photosensing, optothermal detections, health monitoring, and other optoelectronic processes. p-Si

ZnO

Cu

ITO

500 nm

(b1)

(a) Evac (eV)

–4.35

–4.65 –5.22

325 nm

–7.71 ITO E (eV)

–4.05

h+ h

–40

0

+

ZnO

Si

Cu

40 2

(c)

1

0 Voltate (V)

(d) hν

p-Si

500 nm

3.7E-3 W cm–2 2.5E-3 1.5E-3 1.0E-3 6.0E-4 4.0E-4 2.8E-4 2.0E-4 5.0E-5 3.0E-5 dark

–80 Current (µA)

–4.4

(b2)

c axis

Depletion

–2

–1 c axis

p-Si

n-ZnO

Depletion

e–

n-ZnO

e– Ec

hν

Ec hν Ef

Ef

h+

h+

ΔEc = 0.30 eV ΔEv = 2.49 eV

(e)

ΔEc = 0.30 eV ΔEv = 2.49 eV

Ev

(f)

Ev

Figure 3.29 Structure, characterization, and working mechanism of pyro-phototronic effect-based UV sensors. Source: Wang et al. [188]. (a) Schematic demonstration of the structure of pyro-phototronic effect-based UV sensors. (b1, b2) Scanning electron microscopy (SEM) images of ZnO nanowires (NWs) array: (b1) side view and (b2) top view. (c) Energy band diagram of pyro-phototronic effect-based UV sensors. Energies are expressed in electron volts, using the electron energy in a vacuum as a reference. (d) I–V characteristics of the pyro-phototronic effect-based PD under dark and 325 nm laser illumination with different power densities. The arrow points out the increasing direction of laser power density. (e, f) Schematic band diagrams of p-Si/n-ZnO heterojunction: (e) as turning on and (f) turning off light to illustrate the working mechanism of pyro-phototronic effect-based PDs.

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139 Padmaja, G., Courtney, W., Matthew, E. et al. (2017). Dielectric behaviour of lithium tantalate (LiTaO3 )/poly(vinylidene fluoride) (PVDF) nanocomposites doped with MWCNTs. Adv. Sci. Eng. Med. 9 (3): 204–208(5). 140 Jayalakshmy, M.S. and Philip, J. (2015). Enhancement in pyroelectric detection sensitivity for flexible LiNbO3 /PVDF nanocomposite films by inclusion content control. J. Polym. Res. 22: Article number: 42. 141 Zhang, W.L., Yu, Y.C., Luo, W.B. et al. (2017). Lead free KNN/P(VDF-TrFE) 0–3 pyroelectric composite films and its infrared sensor. Infrared Phys. Technol. 80: 100–104. 142 Takenaka, T., Maruyama, K.I., and Sakata, K. (1991). (Bi1/2 Na1/2 )TiO3 -BaTiO3 system for lead-free piezoelectric ceramics. Jpn. J. Appl. Phys., Part 1 30: 2236. 143 Sasaki, A., Chiba, T., Mamiya, Y., and Otsuki, E. (1999). Dielectric and piezoelectric properties of (Bi0.5 Na0.5 )TiO3 –(Bi0.5 K0.5 )TiO3 systems. Jpn. J. Appl. Phys., Part 1 38: 5564. 144 Takenaka, T., Okuda, T., and Takegahara, K. (1997). Lead-free piezoelectric ceramics based on (Bi1/2 Na1/2 )TiO3 -NaNbO3 . Ferroelectrics 196: 175–178. 145 Lang, S.B., Zhu, W.Y., and Cross, L.E. (2006). Piezoelectric and pyroelectric properties of (K0.5 Na0.5 )1-x (Nb1-y Tay )O3 ceramics. Ferroelectrics 336: 15–21. 146 Lau, S.T., Cheng, C.H., Choy, S.H. et al. (2008). Lead-free ceramics for pyroelectric applications. J. Appl. Phys. 103: 104105. 147 Bernardini, F., Fiorentini, V., and Vanderbilt, D. (1997). Spontaneous polarization and piezoelectric constants of III-V nitrides. Phys. Rev. B 56: R10024. 148 Zoroddu, A., Bernardini, F., Ruggerone, P., and Fiorentini, V. (2001). First-principles prediction of structure, energetics, formation enthalpy, elastic constants, polarization, and piezoelectric constants of AlN, GaN, and InN: comparison of local and gradient-corrected density-functional theory. Phys. Rev. B 64: 45208. 149 Fuflyigin, V., Salley, E., Osinsky, A., and Norris, P. (2000). Pyroelectric properties of AlN. Appl. Phys. Lett. 77: 3075–3077. 150 Kebede, B., Coutu, R.A., and Starman, L. (2014). Optimal microelectromechanical systems (MEMS) device for achieving high pyroelectric response of AlN. Proceedings of the Micromachining and Microfabrication Process Technology XIX, San Francisco, CA, USA, 1–6 February 2014. Bellingham, WA, USA: International Society for Optics and Photonics; Volume 8973, p. 89730I. 151 Kurz, N., Lu, Y., Kirste, L. et al. (2018). Temperature dependence of the pyroelectric coefficient of AlScN thin films. Phys. Status Solidi A 215 (13): 1700831. 152 Stan, G.E., Botea, M., Boni, G.A. et al. (2015). Electric and pyroelectric properties of AlN thin films deposited by reactive magnetron sputtering on Si substrate. Appl. Surf. Sci. 353: 1195–1202. 153 Goldsmith, J.H., Vangala, S., Hendrickson, J.R. et al. (2017). Long-wave infrared selective pyroelectric detector using plasmonic near-perfect absorbers and highly oriented aluminum nitride. J. Opt. Soc. Am. B: Opt. Phys. 34 (9): 1965–1970. 154 Ranacher, C., Consani, C., Tortschanoff, A. et al. (2019). A CMOS compatible pyroelectric mid-infrared detector based on aluminium nitride. Sensors 19: 2513.

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155 Gaur, S.P., Rangra, K., and Kumar, D. (2019). MEMS AlN pyroelectric infrared sensor with medium to long wave IR absorber. Sens. Actuators, A 300: 111660. 156 Ng, D.K.T., Wu, G., Zhang, T.-T. et al. (2020). Considerations for an 8-inch wafer-level CMOS compatible AlN pyroelectric 5–14 μm wavelength IR detector towards miniature integrated photonics gas sensors. J. Microelectromech. Syst. 29 (5): 1199–1207. 157 Bykhovski, A.D., Kaminski, V.V., Shur, M.S. et al. (1996). Pyroelectricity in gallium nitride thin films. Appl. Phys. Lett. 69: 3254. 158 Shur, M.S., Bykhovski, A.D., and Gaska, R. (1999). Pyroelectric and piezoelectric properties of GaN-based materials. MRS Internet J. Nitride Semicond. Res. 4 (S1): 57–68. 159 Jachalke, S., Hofmann, P., Leibiger, G. et al. (2016). The pyroelectric coefficient of free standing GaN grown by HVPE. Appl. Phys. Lett. 109: 142906. 160 Rais-Zadeh, M. (2012). Gallium nitride micromechanical resonators for IR detection. Proceedings of SPIE 8373, Micro- and Nanotechnology Sensors, Systems, and Applications IV, 83731M (3 May 2012). 161 Winfield, R.J., Koh, L.H.K., O’Brien, S., and Crean, G.M. (2007). Excimer laser processing of ZnO thin films prepared by the sol-gel process. Appl. Surf. Sci. 254: 855–858. 162 Amézaga-Madrid, P., Antúnez-Flores, W., Ledezma-Sillas, J.E. et al. (2011). Synthesis microstructural characterization and optical properties of undoped, V and Sc doped ZnO thin films. J. Alloys Compd. 509: S490–S495. 163 Kim, J.J., Bak, J.Y., Lee, J.H. et al. (2010). Characteristics of laser-annealed ZnO thin film transistors. Thin Solid Films 518: 3022–3025. 164 Porter, S.G. (1980). A brief guide to pyroelectric and detectors. Ferroelectrics 33: 193–216. 165 Hsiao, C.C., Huang, K.Y., and Hu, Y.C. (2008). Fabrication of a ZnO pyroelectric sensor. Sensors 8: 185–192. 166 Hsiao, C.C., Huang, S.W., and Chang, R.C. (2012). Temperature field analysis for ZnO thin-film pyroelectric devices with partially covered electrode. Sens. Mater. 24: 421–441. 167 Park, S.H., Seo, B.C., Park, H.D., and Yoon, G. (2000). Film bulk acoustic resonator fabrication for radio frequency filter applications. Jpn. J. Appl. Phys. 39: 4115–4119. 168 Wei, C.S., Lin, Y.Y., Hu, Y.C. et al. (2006). Partial-electrodedZnO pyroelectric sensors for response improvement. Sens. Actuators, A 128 (1): 18–24. 169 Hsiao, C.-C. and Yu, S.-Y. (2012). Improved response of ZnO films for pyroelectric devices. Sensors 12: 17007–17022. 170 Hsiao, C.-C. and Liu, S.-Y. (2014). Multi-frequency band pyroelectric sensors. Sensors 14: 22180–22198. 171 Monroy, E., Omnes, F., and Calle, F. (2003). Wide-bandgap semiconductor ultraviolet photodetectors. Semicond. Sci. Technol. 18: R33. 172 Kim, D.C., Jung, B.O., Kwon, Y.H., and Cho, H.K. (2012). Highly sensible ZnO nanowire ultraviolet photodetectors based on mechanical Schottky contact. J. Electrochem. Soc. 159: K10.

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173 Soci, C., Zhang, A., Xiang, B. et al. (2007). ZnO nanowire UV photodetectors with high internal gain. Nano Lett. 7: 1003–1009. 174 Kind, H., Yan, H.Q., Messer, B. et al. (2002). Nanowire ultraviolet photodetectors and optical switches. Adv. Mater. 14: 158–160. 175 Wang, Z., Yu, R., Wen, X. et al. (2014). Optimizing performance of silicon-based p–n junction photodetectors by the piezo-phototronic effect. ACS Nano 8: 12866–12873. 176 Yang, Q., Guo, X., Wang, W.H. et al. (2010). Enhancing sensitivity of a single ZnO micro−/nanowire photodetector by piezo-phototronic effect. ACS Nano 4: 6285–6291. 177 Ahn, S.E., Lee, J.S., Kim, H. et al. (2004). Photoresponse of sol-gel-synthesized ZnO nanorods. Appl. Phys. Lett. 84: 5022. 178 Keem, K., Kim, H., Kim, G.T. et al. (2004). Photocurrent in ZnO nanowires grown from Au electrodes. Appl. Phys. Lett. 84: 4376. 179 Jeong, M.C., Oh, B.Y., Lee, W., and Myoung, J.M. (2005). Optoelectronic properties of three-dimensional ZnO hybrid structure. Appl. Phys. Lett. 86: 103105. 180 Hu, Y.F., Zhou, J., Yeh, P.H. et al. (2010). Supersensitive, fast-response nanowire sensors by using Schottky contacts. Adv. Mater. 22: 3327–3332. 181 Cheng, G., Wu, X.H., Liu, B. et al. (2011). ZnO nanowire Schottky barrier ultraviolet photodetector with high sensitivity and fast recovery speed. Appl. Phys. Lett. 99: 203105. 182 Wang, Z., Yu, R., Pan, C. et al. (2015). Piezo-phototronic UV/visible photosensing with optical-fiber–nanowire hybridized structures. Adv. Mater. 27: 1553–1560. 183 Bai, S., Wu, W.W., Qin, Y. et al. (2011). High-performance integrated ZnO nanowire UV sensors on rigid and flexible substrates. Adv. Funct. Mater. 21: 4464–4469. 184 Li, Q.H., Gao, T., Wang, Y.G., and Wang, T.H. (2005). Adsorption and desorption of oxygen probed from ZnO nanowire films by photocurrent measurements. Appl. Phys. Lett. 86: 123117. 185 Razeghi, M. and Rogalski, A. (1996). Semiconductor ultraviolet detectors. J. Appl. Phys. 79: 7433. 186 Yang, Y., Guo, W.X., Pradel, K.C. et al. (2012). Pyroelectric nanogenerators for harvesting thermoelectric energy. Nano Lett. 12: 2833–2838. 187 Wang, Z., Yu, R., Pan, C. et al. (2015). Light-induced pyroelectric effect as an effective approach for ultrafast ultraviolet nanosensing. Nat. Commun. 6: 8401. 188 Wang, Z., Yu, R., Wang, X. et al. (2016). Ultrafast response p-Si/n-ZnO heterojunction ultraviolet detector based on pyro-phototronic effect. Adv. Mater. 28 (32): 1–7.

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4 Pyroelectric Infrared Detector 4.1 Introduction The detection of long-wavelength infrared (IR) radiation is of great interest in a wide range of applications. For example, in the analysis of absorption spectra for qualitative and quantitative analysis of gases, detection of flames for fire alarms, intruder detection, and many applications of imaging. There are two “windows” in the atmosphere where the absorption due to water vapor and the scattering due to dust are minimal, from 3 to 5 μm and from 8 to 14 μm, respectively. The latter window is especially important because the peak in the emission of the black body curve for objects at 300 K occurs around 10 μm. There are two general classes of IR detectors: quantum or photon detectors and thermal detectors. Quantum detectors are based on the photoelectric effect and are produced from III-V or II-VL semiconductors such as gallium arsenides (GaAs) and Hgx Cd1−x Te. These materials are difficult to grow and fabricate into devices and require cooling, usually to 77 K, for operation in the long-wavelength IR spectrum. It is true that excellent performance has been obtained with such materials, but the devices are expensive and power consumptive. Their cost and requirement for cooling make them unsuitable for most untended applications and professional or consumer systems so the main users of these applications are the military. Even here, these factors tend to confine their use for the most part to high value, high power systems such as armored fighting vehicles, aircraft, or ships. Thermal detectors convert the energy of the IR photons into heat and are usually operated at ambient temperature. They do not require cooling and are not restricted to use in limited wavebands as the quantum or photon detectors are. Thermal devices are much cheaper, but they are less sensitive. The most important type of thermal detector is a pyroelectric one and this is the major area of application for pyroelectric devices. Pyroelectric thermal detectors have five main advantages which make them suitable for many applications: ●

Sensitivity over a very large spectral bandwidth – in principle, over the entire electromagnetic spectrum, depending on the absorption characteristics of the pyroelectric material and its electrodes.

Pyroelectric Materials: Physics and Applications, First Edition. Ashim Kumar Bain and Prem Chand. © 2023 WILEY-VCH GmbH. Published 2023 by WILEY-VCH GmbH.

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●

● ●

Sensitivity over a very wide temperature range – from a few degree kelvin to hundreds, depending on pyroelectric material. Low power requirements – just enough to drive the field-effect transistor that amplifies the signal. Fast response, over periods as short as picoseconds. Low-cost manufacture from inexpensive materials.

Because pyroelectric devices only respond to changes in temperature, they are used to observe moving objects in a scene or stationary objects through a light chopper run at video frequencies (usually 25 to 60 Hz). The light chopper provides a continuous source of temperature variation that allows the material to generate charge while a scene is monitored. IR detectors can consist of single pyroelectric elements or one or two-dimensional arrays of elements. Densely spaced 2D arrays generate a thermal image, similar to the visible-light image of a television. Some of the applications of IR pyroelectric detectors include intrusion detectors and burglar alarms, flame and fire detectors, IR spectrometers, laser detectors, and pollution monitors. Other major areas of pyroelectric research and applications unrelated to IR detection include electron-emission devices and the measurement of thermal and optical properties of materials.

4.2 Device Configurations The most important component of any pyroelectric device is the detector material. Triglycine sulfate (TGS) and its isomorphs have very favorable properties, including high pyroelectric coefficients and relatively low permittivity. Despite their hygroscopic nature, these materials are favorite for high-sensitivity applications. Lithium tantalite (LT) is very stable because of its high Curie temperature and insensitivity to humidity and vacuum, and is often used for space applications. Polyvinylidene fluoride polymer(PVDF) polymer and its copolymers have low pyroelectric coefficients, but their low thermal conductivity and dielectric constant make them useful for large-area detectors and arrays. Ceramics based on the lead zirconate titanate (PZT) system are probably the most widely used materials; they are relatively cheap to manufacture, and are both mechanically and chemically robust. Variation in the Zr/Ti ratio and the addition of dopants permit great variation in the physical properties of these ceramics. Prior to 1990, almost all pyroelectric IR detectors were made from a single crystal, a thin slice of ceramic, or a polymer foil encapsulated with an amplifier circuit. Since then, rapid advances in thin-film technology led to the development of pyroelectric devices directly coupled to integrated circuits. As the reliability of the thin-film devices improves, they will probably displace the older devices for most applications. Here, a brief description of a different array of pyroelectric IR detectors is presented as follows.

4.2.1

Thick Film Detectors

Pyroelectric IR detectors define the leading edge of motion detection technology, offering a variety of both digital and analog pyroelectric IR detectors to serve a wide

4.2 Device Configurations

Figure 4.1 Photograph of a single element pyroelectric detector (PLT522D) on a TO5 header. Source: Whatmore [1]. A pyroelectric detector using a 2 × 2-mm platinum-blacked LiTaO3 element (in the center). The output from the element is taken via the wire bond to a discrete JFET amplifier (small square at top of the device). The other components are bias resistors.

range of applications in smart home, smart city, Internet of Things (IOT), burglary detection, and security systems. A typical high sensitivity single element LiTaO3 pyroelectric IR detector with a low thermal mass platinum-black as an absorbing layer is illustrated in Figure 4.1 [1]. It is a Plessey PLT522 detector on a TO5 header designed for gas analysis and flame detector applications. It uses a low noise junction field-effect transistor (JFET) amplifier. To give high sensitivity at low operating frequencies, the element is freely suspended so as to minimize the conduction of heat to the surroundings. The thermal capacity of the element is minimized to maximize the response and the electrical capacitance is high to reduce the electrical noise. Typical values of 𝜏 T and 𝜏 E are in the range of 0.05–10 s, leading to an RV proportional to 𝜔−1 for operating frequencies above about 20 Hz. The output signal produced by square wave irradiation will approximate a sawtooth curve. A disadvantage of a single element is its sensitivity to drift in ambient temperature or piezoelectric microphonic noise. Pyroelectric elements can also be configured to attenuate noise from ambient temperature changes and from vibrations. In the configuration shown in Figure 4.2, for instance, two elements are connected in electrical opposition and one is shielded from the radiation source. A germanium window is often chosen for devices because it is opaque to visible light but transparent at wavelengths near 10 μm. In this device, the voltage responsivity is about 3500 VW−1 in the range from 6 to 14 μm with a pyroelectric element made of either LiTaO3 or modified PbTiO3 [2]. The specific detectivity (D* ) of PbTiO3 elements is about 1.6 × 108 cm Hz−1/2 W−1 and slightly higher with LiTaO3 . The temperature sensitivity of the PbTiO3 is about 1% K−1 and that of LiTaO3 is about three times lower than that of PbTiO3 (because of the high Curie temperature of the latter material). Other materials can also be used as detector elements as well. Neumann grew crystals of deuterated TGS (DTGS) with l-𝛼-alanine doping [3]. This increased the Curie temperature (Tc ) to 61 ∘ C, but the high internal bias field permitted operation at even higher temperatures. Single 2 × 2 mm elements which were 20 μm in thickness had a voltage responsivity of 1500 VW−1 and a D* of 1.5 × 109 cm Hz−1/2 W−1 . It is known that a pyroelectric element gives an electrical response only while its temperature is changing. This is achieved automatically if an IR detector views

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Electronics container

Figure 4.2 Design of compensated pyroelectric detectors. Source: Wersing [2].

Optical filter 4.8 × 5.8 mm

Metal shield

FET Pyroelectric detectors Gate resistor Circuit board

a scene in which there is motion (e.g. a moving person). If the scene is static, it is necessary to modulate or chop the incoming radiation. This is most frequently accomplished with a rotating blade external to the detector. The ferroelectric-hybrid focal-plane array comprises a lens, usually made of germanium to block visible light, as shown in Figure 4.3 [4]. The incident IR radiation is periodically blocked by a chopper, to ensure that a temperature variation is measured. Detector elements made of lead strontium titanate (PST) or barium strontium titanate (BST) are commonly used. These materials have T c close to room temperature (RT). The devices operate at temperatures above the Curie level (in the so-called paraelectric phase) and an applied electric field induces the pyroelectric effect. Laser-assisted chemical etching produces an array with a large number of detector pixels. The elements on that array are soldered (or “bump bonded”) to a silicon multiplexer. Contemporary versions have as many as 384 × 288 pixels and an noise-equivalent temperature difference (NETD) as low as 75 mK. Applications exist in areas as diverse as fire-fighting, law enforcement and border patrol, land mine

4.2 Device Configurations

Rotating chopper Hybrid focal-plane array IR lens Ferroelectric wafer

Silicon multiplexer

Image difference processor

Solder bump bonding

Figure 4.3

A ferroelectric-hybrid focal-plane IR detector. Source: Watton [4].

detection, building surveillance, process control, vision testing, facial recognition, and traffic control.

4.2.2

Thin Film Detectors

Pyroelectric thin films allow parallel structures (2D arrays) to be made easily and offer the possibility of integrating the readout electronics and reducing the cost of a complex system. Heat conduction to the substrate is reduced by micromachining the silicon wafer. Figure 4.4 shows an example of a packaged pyroelectric thin-film point detector obtained by bulk micromachining. The pyroelectric capacitor is supported by a ceramic membrane of SiO2 and Si3 N4 . While micromachining allows for good thermal insulation from the silicon frame holding the membrane, air convection to the surrounding walls (housing, IR optics) may greatly contribute to thermal bridging if the respective distances are small, or if the device is not operated in a vacuum. Figure 4.4 Schematic illustration of a thin film pyroelectric sensor. Source: Muralt [5].

Infrared optics

Electrodes

Absorbing layer Pyroelectric film

Micromachined substrate

Housing

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IR absorbing layer Top electrode Pb(Ti,Zr)O3 Pt Si3N4/SiO2 Etched silicon cavity Silicon

Figure 4.5 Typical structure of pyroelectric elements of a linear array on a thin membrane fabricated by means of micromachining. Source: Muralt et al. [6].

A typical design of a pyroelectric array for an IR spectrometer is shown in Figure 4.5. A stress-compensated membrane layer of Si3 N4 /SiO2 , or low-stress nitride is coated on both sides of a double-side polished wafer. This coating fulfills the following functions. First, it serves as a mask for back-side etching in KOH or in an equivalent base. Second, it serves as a support of the pyroelectric elements (membrane) exhibiting a low thermal conductivity. The bottom electrode and pyroelectric film (PZT15/85) are deposited by sputtering and sol–gel, respectively. The top electrode is deposited and patterned by a lift-off technique before a quartz layer is sputter-deposited for reduction of parasitic capacitance below the contact pads [7]. Windows down to the top electrodes are opened by a CF4 reactive ion etching. The PZT elements on the membrane part are etched free in an HCl: F solution, leaving only narrow bridges between the elements and the bulk silicon part, as needed for separation of bottom and top conductor. The platinum bottom electrode is removed between the elements by electrochemical etching. This etching technique does not attack the membrane material. After deposition and patterning of the conductor lines, pads (Au/Cr), and absorbing layer, the silicon is removed below the elements by back-side etching, as defined by a window in the back-side nitride layer, in order to obtain the result shown in Figures 4.5 and 4.6. The 0.9-μm-thick membrane with a specific conductivity of 2 Wm−1 K−1 gives a fairly good thermal insulation, which allows for rather high voltage responses at 1 Hz of 800 V W−1 in the air [6]. The membrane roughly doubles the heat capacity of the pyroelectric element. Rather long thermal time constants of 28 ms in air and 104 ms in vacuum have been obtained with such devices [9]. Recently, a new Au Pt–PZT–Pt IR detecting structure on a silicon substrate with a micro bridge was designed by Tan et al. [10] as shown in Figure 4.7a. A 150 nm gold black layer was deposited on the top of the sensing elements and the top electrode to serve as the radiant absorption layer [11]. After the above process, the PZT thin films were protected, and the KOH(potassium hydroxide) etching process was used to etch off the backside silicon substrate under the sensing area to form a Pt/Ti/Si3 N4 /SiO2 membrane.

4.2 Device Configurations

Figure 4.6 Top view of 50 element array with 200 μm period obtained with bulk micromachining, membrane size: 2 × 11 mm. The black platinum absorbers, the Cr–Au contact lines, the membrane layers between the elements, and the SiO2 layer for reduction of parasitic capacitance are clearly visible. Source: Willing et al. [8].

Au

Pt PZT Pt Ti Si3N4

GND

Output signal

SiO2 Si

(a)

(b)

Figure 4.7 (a) Schematic of an Au Pt–PZT–Pt pyroelectric IR detector on the silicon substrate with a micro bridge, (b) Dual-element PZT pyroelectric IR detector design. Source: Qiu-lin et al. [10].

To obtain high detectivity, pyroelectric thin films with low dielectric loss, moderate dielectric constant, and high pyroelectric coefficient are desired. To achieve high device sensitivity, an effective thermal isolation structure is as important as a large pyroelectric coefficient [12]. In this study, a micro bridge of silicon substrate is presented, and it minimizes the thermal conductance of the device. In order to decrease heat loss through the substrate, Si3 N4 /SiO2 film is chosen as the transition layer. As the incident IR ray into PZT thin film layer can decrease heat loss, the sensitivity of the device has been improved. Another major problem in the design of the PZT pyroelectric detector lies in its sensitivity to mechanical stress and vibration, as all pyroelectrics are also piezoelectrics. The change in ambient temperature and sunlight also brings superimposed interference to the output voltage signal of the detector. The response/reference dual-element configuration as illustrated in Figure 4.7b, where two front Pt electrodes of identical dimensions are connected in series, which can reject the common-mode interferences such as acceleration sensitivity brought by

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JFET (2SK303V3)

V+

Signal output

Filter – + Dual-element Pyroelectric PZT film

Figure 4.8

+ – GND

Equivalent circuit of the IR sensor. Source: Qiu-lin et al. [10].

mechanical vibration due to the piezoelectric property of PZT material, because the directions of the induced polarization in the response/reference dual-element, as connected in series, are opposite. Based on the same ideas, the interferences bought about by the ambient temperature change and sunlight have also been canceled out. So, the dual-element design is very effective to improve its detectivity. To characterize an IR sensor device, sensor response and reference dual-element was mounted on a TO-5 metal vessel with a discrete JFET, the narrowband filters (in order to realize selective absorption to the IR radiation) and a 47 KΩ resistance for read-out circuit as shown in Figure 4.8. The dielectric properties of the sol–gel-prepared PbZr0.3 Ti0.7 O3 thin film were measured with evaporated Pt dots as a front electrode and the Pt substrate as the bottom electrode. The pyroelectric coefficient of 1.12 × 10−5 C/m2 K at 50 ∘ C was measured as a function of temperature. Voltage response experimental results of the PbZr0.3 Ti0.7 O3 device are obtained by an IR device measurement system and indicate that the response voltage is about 42 VW−1 at a modulation frequency of 1 Hz. A gas calibration experiment shows that this IR detector has high sensitivity and good stability.

4.2.3

Hybrid Focal Plane Array Detector

The GEC-Marconi and the Texas Instruments [13, 14] presented quite complex hybrid FPAs (focal plane arrays) with 384 × 288 and 328 × 245 pixels. These arrays work either with pyroelectric PZT ceramic or with field-induced pyroelectricity (dielectric bolometer) of BaSrTiO3 (BST) or PbSc1/2 Ta1/2 O3 . The version of Texas Instruments operates with BST at a stabilized temperature near the ferroelectric critical temperature around 20 ∘ C. It is called an uncooled imaging device because the necessary heater/Peltier element combination consumes much less power than that required for standard cooling devices. The starting point of fabrication is a ceramic BST wafer. After reticulation by laser or ion milling processes, polishing down, and electrode and absorber layer deposition, the obtained BST array is bonded to a readout IC via a mesa structure (Figure 4.9) (GEC-Marconi utilizes solder bumps). The absorber layer is a transparent organic 𝜆/4 layer, sandwiched between a semi-transparent metal

4.2 Device Configurations Semi-transparent metal

Backside contact

λ/4 Organic layer BST

Reflecting metal (common electrode)

Mesa

Silicon read-out IC

Figure 4.9 Schematic cross-section through FPA obtained by reticulation of ceramic BST wafer that is ground down and bonded to the readout chip by a mesa structure (TI uncooled FR hybrid technology. Source: Owen et al. [13]). The pixel thickness (BST) amounts to 10–18 μm, the pitch is 48.5 μm.

layer and the common electrode. The mesa structure is grown on the readout IC. The latter is either a charge-coupled device (CCD) or exhibits amplifiers and sample-and-holds for each pixel, combined with a multiplexed reading for the output. Such devices achieve a NETD of 0.1 K or less.

4.2.4

Linear Array Detector

A new generation of LT linear arrays with better spatial and thermal resolution than in the former detector generation [15–17] has been developed by Köhler et al. [18]. Figure 4.10 shows the fundamental structure of linear pyroelectric arrays. In general, LiTaO3 detectors are hybrid components. The introduced linear arrays are an arrangement of a pyroelectric detector chip, printed circuit board (PCB), anisotropically etched silicon aperture, and a complementary metal oxide semiconductor (CMOS) read-out circuit in the 0.8 μm technique. Interconnections between detector pixels and read-out circuits are carried out by wire bonding. All components are mounted in hermetically sealed 16 pin metal packages. 4.2.4.1 Detector Chip Technology

The introduced linear arrays have 256 responsive elements with an area of 40 × 50 μm2 arranged in a pitch of 50 μm. Starting materials are polarized single-crystal LiTaO3 wafers with a diameter of 2.5′′ and a thickness of 500 μm. Φs

Sensitive area As

a

c tp

b

Pyroelectric chip Common electrode Pixeled electrode

y x

Vs′ z

Figure 4.10

Multiplexer

Read-out circuit

Output driver

Fundamental structure of a linear pyroelectric array. Source: Köhler et al. [18].

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After mechanical processing, the responsive elements are ion beam etched down to 5 μm. The LiTaO3 chip has a 25-μm-thick supporting frame and a 5 μm thin area for highly sensitive responsive elements. The top electrode metallization system is evaporated and lift-off structured again. The responsive elements themselves are coated with low reflecting 10 nm Ni80 Cr20 . An additional gold layer reinforces all other conductive tracks. In order to minimize the thermal cross-talk, small trenches are ion beams etched between the responsive elements. The results of trench etching are two types of thermally insulated pixels – a fence-like geometry and a reed-like one. Both types of pixels are represented in Figure 4.11. In both pictures, the pitch of the pixels is 50 μm. The fence-like pixel structure is characterized by higher mechanical stability while the reed structure is better thermally insulated. The applied ion beam etching parameters are strongly dependent on the etching depth, the structure size, and the desired sidewall angle. The etched 5 μm thin LiTaO3 reeds in Figure 4.12 demonstrate the outstanding performance of the ion beam milling process. For etching, mask photo resist AZ 4562 has been used. The two ion beam etching processes described above, simultaneously separate the 10 linear array detector chips placed on one sub-wafer. Finally, the chips are detached and cleaned. In order to cure damages, which were caused by ion beam etching processes, the chips are annealed by a soft

Figure 4.11 Detector pixels with ion beam etched trenches (left: reed geometry, right: fence geometry). Source: Köhler et al. [18].

Figure 4.12

SEM image of LiTaO3 reeds (50 μm pitch). Source: Köhler et al. [18].

4.2 Device Configurations

thermal treatment. The 75 mW consuming read-out circuit is specially designed for detectors with 256 pixels. The circuit is produced in 0.8 μm CMOS technology. It consists of an analog and a digital part and is optimized for low noise and high linearity. 4.2.4.2 Detector Assembly

As mentioned above, the detectors are hybrid arrangements. An adhesive onto a micromachined silicon carrier bonds the fragile LiTaO3 chips. Simultaneously, this carrier acts as an optical aperture, that is, as an optical shield against heat absorption in the pixel surroundings. Afterward, both the sandwich construction and the read-out circuit are mounted onto PCB with a conductive adhesive. For the connections between the read-out circuit and PCB, thermal compression bonding is used whereas the connections between the pyroelectric chip and input stages of the read-out circuit are realized by ultrasonic fine pitch bonding. A special bonding wedge is used together with an AlSi wire with a diameter of 17.5 μm in order to produce bonds with a pitch of 50 μm. For optimum heat absorption, broadband absorbing black coating or a special 𝜆/4 thin-film absorber can be used. Regardless of the application of an absorber on top of the responsive elements, the silicon precision aperture must be black coated in order to avoid heat radiation transmission and reflection inside the package. Before this black coating, the PCB with the parts has to be mounted face down into the case header. The connections between PCB and package pins are bonded. An AD590 temperature detector is integrated onto the bottom of the housing as well. Now the absorber can be deposited. A silver black coating has been optimized for the LiTaO3 detectors. The coating with a thickness of about 5 μm is electrically conductive and is distinguished by a band absorbance of about 0.92 for a 500 K black radiator in the range of 2–20 μm [19]. Optional with the application, the spectral range can be determined within 1.2 and 20 μm by a band pass window in the lid of the package. Finally, the package is sealed hermetically. In addition, a detector type without absorption black and silicon aperture enables more easy assembling but doesn’t have as high performance as the more complicated one. In this case, PCB with detector chip is not face-down mounted. In Figure 4.13, both types are represented schematically. Figure 4.14 shows completely mounted (except the lid) arrays of both 256 pixels arrays. All detector measurements have been performed in unsealed packages, without black coating and with an antireflection-coated 8–14 μm Ge filter. Its voltage responsivity amounts up to more than 600 000 V/W at a rectangular modulation frequency f mod = 128 Hz. Noise equivalent power (NEP) amounts without software averaging to 1.1 nW at 128 Hz for a detector with trenches and a thickness of the sensitive elements of 4.8 μm. The higher NEP of the detector without trenches is caused by the thicker elements (5.5 μm). All the introduced detectors are optimized for modulation frequencies of 128 Hz or higher. 4.2.4.3 Camera System

The main applications are line cameras for temperature measurement in industrial use [20–22]. The camera system PYROLINE 256 was developed for use with

129

130

4 Pyroelectric Infrared Detector

Support Pyroelectric Read-out circuit chip IR window

Black coating

Ceramic carrier

Support

Read-out circuit IR window

PCB Silicon aperture

Black coating

Pyroelectric chip

Figure 4.13 Sectional view of 256 pixels LiTaO3 linear arrays (left: without silicon aperture and absorption black, right: with silicon aperture and absorption black). Source: Köhler et al. [18].

Figure 4.14 LiTaO3 arrays in the metal package without lid (left: standard assembly, right: face down assembly). Source: Köhler et al. [18].

pyroelectric linear 256-element arrays 256-LT-I. Figure 4.15 shows the complete camera. The PYROLINE 256 cameras provide continuous, non-contact measurement of linear temperature distributions. Operation in conjunction with the IR_LINE software provides data recording, real-time graphical analysis, process integration, and camera-control capabilities. It consists of a robust, industrial housing that can be equipped with integrated water-cooling and air purge for lens system. The camera includes the pyroelectric array, a chopper module (chopper frequency 545 Hz), the IR-optics as well as the entire analog and digital signal processing. The standard temperature ranges include temperatures of 50–1300 ∘ C. Spectral ranges are 8–14 μm for low-temperature applications, 3–5 μm for measurement of medium temperatures, 4.8–5.2 μm for glass-temperature applications, and 1.4–1.8 μm for high-temperature measurements.

4.2 Device Configurations

Figure 4.15

4.2.5

Infrared line camera PYROLINE 256. Source: Köhler et al. [18].

Periodic Domain TFLTTM Detector

Pyroelectric thermal detectors are excellent candidates for broadband THz detection. Such detectors utilize permanently poled ferroelectric crystals LT to generate a charge as the crystal heats up by absorbing THz radiation. The charge, which results in a current output when connected to an external electrical circuit, is directly proportional to the rate of change of temperature of the crystal. It is therefore important to maximize the temperature change through thermal isolation of the crystal and by using absorbing coatings with low thermal mass to rapidly transfer heat to the crystal. The fundamental approach toward enhancing the pyroelectric detector response is to form the pyroelectric material into a thin film. An elegant approach for producing bulk quality thin films of pyroelectric materials is by crystal ion slicing. Instead of resorting to the laborious process of polishing and wafer thinning, SRICO Inc. [23] has combined the emerging technologies of wafer bonding and ion slice layer transfer to achieve uniform and thin mono-crystalline lithium tantalate films on device carrier substrates. The thin-film lithium tantalate (TFLT) layers have ranged in size from a few millimeters to 25 mm. In addition to thin-film formation, SRICO Inc. has developed periodic domain engineered patterns in the thin films for the purpose of suppressing noise sources from the film outside the detector aperture. Domain engineered periodic patterns effectively and reliably de-pole the film. Judicious use of domain engineering may also be applied within the device aperture to suppress standing wave acoustic noise. Using TFLT technology, SRICO Inc. [23] has demonstrated record-breaking 20 times detector performance improvement relative to state-of-the-art commercial

131

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4 Pyroelectric Infrared Detector

products. Further reductions in thickness are expected to enable room-temperature pyroelectric detector response approaching 100 times that of current commercially available devices. By application of suitable THz absorbers, the new TFLT pyroelectric detectors are operable over the entire terahertz spectrum of 100 GHz to 30 THz. Applications of the TFLT pyroelectric detector technology include non-destructive evaluation and inspection of aerospace materials and structures, imaging, and spectroscopy. Deployment of these newly developed detector devices would result in the successful implementation of a large number of important defenses, industrial, medical, and environmental applications that could benefit from terahertz radiation technologies. Pyroelectric detectors may also be used as a transfer standard in the calibration of other terahertz detectors. 4.2.5.1 TFLTTM Pyroelectric Detector Fabrication

The TFLT process begins with a 3-in. bulk wafer of LT. Depending on the application, a domain pattern may be formed in the wafer prior to the TFLT process. A full wafer ion implant is performed to create a highly stressed implant layer at a depth determined by the implant energy. This depth defines the film thickness and is typically between 5 and 10 μm. The implanted wafer is then temporarily bonded face down to a handle substrate for further processing. Either selective wet etching or a high-temperature step is used to separate or “slice” the seed wafer along the ion implant plane, leaving a thin portion of the seed wafer still attached to the handle wafer. Polishing is used to smoothen the slice surface and annealing is performed to remove any residual implant stress. The bulk quality TFLT film may be further processed for the specific application or transferred directly to a device carrier. Figure 4.16 illustrates the fabrication process of the TFLT. Device carriers are typically comprised 0.5 mm thick ceramic or quartz discs with center aperture openings of from 2 to 5 mm diameter. In other cases, the carrier was a silicon wafer, either with or without an open window. After transferring the TFLT film, metal coatings and wire leads are applied to the top and to the bottom. If the TFLT layer is domain patterned over the entire film, an additional poling step may be performed to activate the aperture region. The thinness of the ion sliced films enables in situ poling of devices to activate pyroelectric response only within the desired aperture. Noise contributions normally present outside the aperture are nullified by the periodic domain pattern. An illustration of the domain engineered TFLT pyroelectric detector is shown in Figure 4.17. The carbon nanotube layer ensures efficient detection at wavelengths from visible into the THz regime. The entire assembly is packaged in a standard TO-can for testing. Like any other pyroelectric material, LT films cannot absorb the entire incident THz radiation. This will be especially true for TFLT. As in other technologies, an additional absorbing layer enhances radiation capture and conversion. In addition to standard gold-black coatings[24], there have been promising reports of carbon nanotubes (CNT) for this application [25]. These carbon materials provide large broadband absorption while maintaining thermal conductivity and low heat capacity. These thermal properties promote faster thermal response, which is critical for highly sensitive pyroelectric detection. Recent reports indicate that multi-walled

Temporary bond

Seed wafer LiTaO3

LiTaO3

Implant layer

Figure 4.16

Polish surface TFLTTM

Handle substrate (a)

Ion slice

Handle substrate (b)

TFLTTM fabrication process. Source: Stenger et al. [23].

Handle substrate (c)

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4 Pyroelectric Infrared Detector

Bond wire

Ag epoxy

Carbon nanotube absorber

Top electrode

Domain patterned TFLTTM UV epoxy

Carrier Bottom electrode

Figure 4.17

Poled TFLTTM

Carrier

TFLTTM domain engineered pyroelectric detector. Source: Stenger et al. [23].

nanotubes may even be better for this application. CNTs are commercially available in powder form and can be applied using spray-on techniques. One recent development has resulted in the columnar growth of nanotubes to about 7 mm height using a chemical vapor deposition (CVD) process [26]. Lehman et al. have reported on vertically aligned CNT growth on LT [27]. With suitable radiation absorbing layer, about two orders of magnitude improvement are expected for the thin film detector over currently available commercial pyroelectric detectors. In addition to suppressing noise sources outside the detector aperture, domain engineering is being explored for the suppression of acoustic noise within the device open aperture. The approach starts with the experimental determination of resonant modes in a fabricated device. The magnitude of aperture acoustic noise will depend on the level of clamping achieved in the device [28]. Theoretically, perfectly clamped films will exhibit essentially zero standing wave-induced noise. Modeling of the device is performed to determine the domain pattern that results in the minimum standing acoustic wave noise. The operating frequency response of the device and associated electronics are taken into account during optimization. Typically, the lowest frequency vibration signal component is suppressed. The trade-off for acoustic suppression within the detector aperture is reduced responsivity and is used when a window or vacuum sealing is not possible. 4.2.5.2 TFLTTM Attached to Metalized Silicon

TFLT devices were produced on closed aperture silicon carriers and on 2 mm and 5 mm open aperture sapphire and ceramic carriers. A total of six assembled detector chips were prepared. Some chips were then assembled into a TO-5 package as shown in Figure 4.18a. Testing was carried out by solder installing the device in the Spectrum Detector Mach 5 probe PCB (shown in Figure 4.18b). The voltage responses for a 25-μm film reference device and the TFLT device were 800 V J−1 and 6.12 kV J−1 , respectively. Correcting for detector areas, the results indicated about a 3× improvement for the ion sliced device relative to the state-of-the-art 25 μm film device. This result has proven that the silicon-mounted films have a performance advantage even without backside window formation in the silicon substrate.

4.2 Device Configurations

Bond wire Top electrode

TFLTTM

Si carrier

(a)

(b)

Figure 4.18 TFLTTM on silicon detector device (a) mounted in TO-5 package, (b) enclosure for testing packaged chips. Source: Stenger et al. [23].

4.2.5.3 TFLTTM on Ceramic

Ceramic mounted TFLT films at various stages of assembly are shown in Figure 4.19. The ceramic open window aperture was 2 mm. A chopped He-Ne laser was used to test the devices. Room temperature test results for three device samples are given in Table 4.1. From the data shown in Table 4.1, it can be seen that the devices exhibited consistent performance across multiple devices. Some devices were hybrid packaged with an integrated transimpedance amplifier (TIA) circuit to

TFLTTM

Top electrode

Carrier Aperture

(a)

Bond wire

(b)

CNT coating

(c)

Figure 4.19 Sliced TFLTTM films on ceramic carriers in various stages of assembly, (a) after backside and front side deposition, (b) after packaging in TO can, (c) after spray-on CNT THz absorber. Source: Stenger et al. [23]. Table 4.1

Typical room temperature test data for TFLTTM pyroelectric detector devices.

Parameter

Sample 1

Sample 2

Sample 3

Laser power

570

570

572

Voltage (V 0 )

2.43

2.48

2.60

V

Current responsibility (Ri )

4.26

4.35

4.50

μA/W

Capacitance (Cd )

210

217

211

pF

Source: Stenger et al. [23]

Units

μW

135

136

4 Pyroelectric Infrared Detector

determine noise-limited performance. Hybrid packaged devices exhibited NEP of √ less than 200 pW Hz−1 at a voltage responsivity of more than 0.8 MV W−1 . Typical √ D* values were greater than 2 × 109 cm Hz W−1 for these 2 mm aperture devices. TFLT films were also transferred to sapphire in a manner similar to that used for ceramic carriers. Room temperature performance for sapphire-mounted devices was comparable to that measured for ceramic carriers. 4.2.5.4 Large Aperture Devices

A process was developed for handling and assembly of larger TFLT films. Figure 4.20 shows one of the TFLT on 5 mm aperture sapphire carrier structures prior to electrode metallization. Metallization and packaging steps were performed as the next step to form complete devices. Metal Cr front and Cr-Au backside electrodes were deposited by shadow masked e-beam thermal evaporation. After metallization, the detector device structure was packaged into a TO-8 can for testing. An example of a fully packaged device is shown in Figure 4.21. Room temperature current responsivity for the device as shown in Figure 4.21 was measured to be 4.62 μA/W, which was similar to that measured for 2 mm aperture devices. Noise performance for 5 mm TFLT devices was determined by measuring low frequency zero stability of the amplified device output when mounted in a Gentec-EO TPR-D-65 probe head.

Sapplure carrier

Figure 4.20 Image of TFLTTM film on 5-mm aperture sapphire, before electrode metallization. Source: Stenger et al. [23].

TFLTTM

5-mm aperture Kapton

5 mm aperture

TO package

Figure 4.21 TO-packaged 5-mm aperture TFLTTM pyroelectric detector. Source: Stenger et al. [23].

4.2 Device Configurations

Figure 4.22 Image of a 5 mm aperture TFLTTM detector mounted in a TPR-D-65 probe head. Source: Stenger et al. [23].

A mounted 5 mm TFLT detector device is shown in Figure 4.22. The probe head incorporated a TIA. A cover plate was used to shield the device from ambient wind currents and acoustic waves. For comparison, zero stability measurements were also taken for a standard 5 mm aperture 50 μ film LT detector device. From the measurements, it can be seen that the TFLT device exhibited almost two times the zero drift of the reference device. However, the responsivity of the TFLT device was measured to be more than six times higher than for the reference device. The detectivity of the TFLT device, when mounted in the TPR-D-65 probe head, was thus more than three times that of the standard 50 μ thick film device. The intrinsic detectivity of the device cannot be extracted from this measurement because the noise performance was limited by electronic (TIA) noise. However, test results for hybrid packaged devices have confirmed the approximately 10× improvement for 5 mm aperture TFLT devices over state-of-the-art 25 μ devices. Hybrid packaged devices integrate low noise JFET electronics to achieve D* closer to the theoretical maximum for the pyroelectric film. The response uniformity of the 5 mm device was measured by scanning a 632 nm radiation source beam across the aperture and measuring the voltage response. The beam diameter was about 0.6 mm. From the result of the uniformity measurement, it can be inferred that interference effects are responsible for the periodic undulation in the spatial response. Interference fringes were visible to the naked eye prior to metal electrode coatings. These fringes are the result of finite thickness non-uniformity along the radius of the film. This non-uniformity is inherent in the wet etch ion slicing process, as the center of the film will tend to be etched less than the edges. Two fringes at 632 nm correspond to a thickness non-uniformity of about 2500 Angstroms in lithium tantalate or less than 3% of the 9-μ film thickness. It is anticipated that the application of CNT or other absorber coatings would eliminate these interference effects and ensure response uniformity at all wavelengths.

4.2.5.5 Domain Engineered TFLTTM Device

The poling process was tuned to produce a 35 μm period and 50% duty cycle domain pattern in lithium tantalate. The TFLT process was used to form 9 μ thin and 3 mm

137

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4 Pyroelectric Infrared Detector

Sliced LT film Aperture

Figure 4.23 Packaged domain patterned LT thin film pyroelectric device. Source: Stenger et al. [23].

Position 1 Position 2

PPLT region

(a)

(b)

Figure 4.24 Pyroelectric test results for the (a) un-patterned vs. (b) patterned portions of the TFLTTM device. Source: Stenger etr al. [23].

size domain patterned films on sapphire carriers. After the top and bottom metallization, the carrier-mounted films were TO packaged and tested. An example packaged device is shown in Figure 4.23. The device depicted in Figure 4.24 is specifically designed to have one portion of the active area as domain patterned, and another portion un-patterned. This represented the ideal control and test case. A focused visible laser beam was used to independently probe the response in the patterned and un-patterned portions of the detector. It was anticipated that the patterned region of the device would exhibit a highly suppressed response relative to the un-patterned region. Figure 4.24 shows the test results for the detector of Figure 4.23. A helium-neon (He-Ne) laser chopped at 125 Hz was used as the signal source. The un-patterned region of the device yielded a current responsivity of 4 μA/W, which was typical of devices fabricated from previous un-patterned and sliced 9 μm LT films. From Figure 4.24b, the patterned portion of the device was found to exhibit a highly suppressed response relative to the un-patterned region. Any signal present in Figure 4.24b may be attributed to finite thermal isolation from the un-patterned portion and to finite laser spot size. The data indicated conclusively that periodic domain patterning does result in high pyroelectric suppression. It is anticipated from these results that domain patterning would be just as effective at the suppression of piezoelectric and other noise sources.

4.2 Device Configurations

4.2.6

Terahertz Thermal Detector

The typical pyroelectric terahertz (THz) thermal detector is structured with LiTaO3 material in the central layer between two metal electrodes. In a thermal detector, the incident radiation is absorbed to change the material temperature, and the resultant change in some physical properties is used to generate an electrical output. The detector is suspended on lags, which are connected to the heat sink. The signal does not depend upon the photonic nature of the incident radiation. Thus, thermal effects are generally wavelength independent [29], and the signal depends upon the radiant power (or its rate of change) but not upon its spectral content. Since the radiation can be absorbed in a black surface coating, the spectral response can be very broad. To enhance the absorption in the THz region, a thin-film THz absorber structure for thermal detectors is covered on the top LiTaO3 crystal surface by a special method. Because the terahertz detector is a kind of thermal sensitive detector, the heat conduction of the device needs to be considered. The structure of the THz crystal detector, from top to bottom, is THz gold black absorb coating/dielectric layer/ferrite layer/gold black absorb coating/LiTaO3 crystal (2 mm × 2 mm thickness 10 μm)/gold black reflector layer. It is an effective way to improve the performance of the THz detector based on LiTaO3 crystal with high response and the lowest NEP value. The gold black films were deposited on the LT crystal of the THz detector by magnetron sputtering. Films with 10 nm thicknesses were prepared. The biggest challenge in implementing a THz detector based on LiTaO3 is high fragility, which is particularly intrinsic to ultra-thin (10 μm) LiTaO3 wafer. In all previous implementations of the pyroelectric THz radiation with optical readout, the incident THz radiation goes through the solid Si substrate. This imposes several limitations on the detector performance. The most significant limitation is related to thermal dissipation. Zhiqing Liang et al. [30] proposed to surmount this limitation by removing the substrate material underneath the absorbing area of the THz detector. In addition to providing an unobstructed optical path for THz radiation, this design eliminates the shortest pathway for heat transfer between the absorber and the substrate. Therefore, the thermal isolation of the detector can be improved. The THz detector can be considered as a small capacitor with two conducting electrodes mounted perpendicularly to the direction of spontaneous polarization. During incident radiation, the change in polarization appears as a charge on the capacitor and a current is generated, the magnitude of which depends on the temperature rise and the pyroelectric coefficient of the LiTaO3 crystal material. The signal must be chopped or modulated. The detector sensitivity is limited either by amplifier noise or by loss−tangent noise. Response speed can be engineered making THz detectors useful for fast laser pulse detection, however, with a proportional decrease in sensitivity. The THz detectors fabricated with the ultra-thin (10 μm) LT crystal and device structure as shown in Figure 4.25a, b from top to bottom are THz gold black absorb coating/dielectric layer/ferrite layer/gold black absorb coating/LiTaO3 crystal/gold black reflector layer. When a heat flux of 4.7 × 10−11 W μm−2 is applied

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4 Pyroelectric Infrared Detector

(a)

(b)

(c)

Figure 4.25 Three-dimensional device structures and temperature changing map at fixed energy flux of THz detector (a) absorption layer structures, (b) device structures with LiTaO3 crystal slice, (c) simulation result of the temperature changing of the THz detector. Source: Liang et al. [30].

to the detectors, the temperature map of the devices is shown in Figure 4.25c. It is obvious the temperature increasing value of device structure and the temperature uniformity are good. It is an effective way to improve the performance of the THz detector based on ultra-thin LiTaO3 crystal. Because the detector is a direct detection detector, 2.52 THz laser radiation acted as a radiation source. The detecting frequency could be controlled precisely by chopper. The response voltage (V s ) signal and the noise voltage (V n ) of the THz detector were measured with a lock-in amplifier (SR850) at RT after removing ambient radiation. The responsivity for the THz detector tested by lock-in amplifier reaches 8.38 × 104 V W−1 and the lowest NEP value reaches 3.25 × 10−12 W at 20 Hz operating frequency by laser radiation source at 2.52 THz. Meanwhile, this processing method provides a feasible approach for fabricating high responsivity THz detector.

4.2.7

PVDF Polymer Detector

The development of pyroelectric IR sensors based on low-cost, thin-film, pyroelectric polymers offers the feasibility of low-cost infrared focal plane arrays (IRFPAs) with a significantly simpler manufacturing process. Pyroelectric polymers are suited to array implementation as they have low thermal conductivity, are broadband absorbers, and have a reasonably fast response. In last 30 years, there has been much interest in the pyroelectric polymer poly-vinylidene fluoride (PVDF) due to its high pyroelectric coefficient, robustness, wide availability, and ease of use. 4.2.7.1

Self-absorbing Layer Structure

The fabrication of an uncooled IR sensor array based on pyroelectric polymer thin film and a commercially fabricated silicon integrated circuit potentially offers good sensitivity, low noise, thermal stability, and minimum electromagnetic and noise interference. Binnie et al. [31] developed a fully integrated PVDF-on-silicon pyroelectric sensor array. In this integrated sensor, the surface electrode/absorber and the pyroelectric layer are directly bonded to an integrated silicon substrate. Then the influence of the silicon substrate on the optical properties is minimal as long as a selective absorber structure is provided with a metal reflector near the surface of

4.2 Device Configurations

Figure 4.26 Sectional view of a pixel showing sensor structure. Source: Binnie et al. [31].

Infrared radiation PEDT/PSS PVDF SiO2 Metal 2 CSA

the silicon. The sensor structure is shown in Figure 4.26. The top metal layer (termed a metal 2 layer in the CMOS process) fulfills this requirement and the pyroelectric sensor element can be directly integrated onto a silicon integrated circuit. This metal 2 layer is reserved solely for the lower pixel electrode and its patterning defines the position and size of each pixel. Neither the conductive surface layer nor the PVDF film is patterned. 4.2.7.2

PVDF Pyroelectric Sensor Assembly

The PVDF film is prepoled and both metal surface coatings are removed. A 1-μm-thick layer of non-conducting adhesive (Technical product datasheet 358, Curing Acrylic epoxy, Publication TDS-358, Loctite Holding Ltd., Welwyn, Garden City, UK AL7 1JB, 1990) is used to bond the PVDF film to the silicon substrate. The thermal conductivity of this dielectric adhesive film is lower than that of PVDF film, which thus acts as a thermal insulator. The thermal time constant and the responsibility of the sensor both increase with increasing adhesive thickness, which is thus an important design parameter. Initially, the measured quantity of nonconducting adhesive is applied to a “dummy” silicon substrate using a spin-coating technique and left uncured. A 9-μm prepoled PVDF pyroelectric film, supplied by Atochem, is then mechanically pressed onto the “dummy” substrate. A thin film of Poly 3,4- ethylenedioxythiophene/polystyrenesulphonate (PEDT/PSS) of approximately 1-μm thickness is then spin-cast from its dispersion onto the PVDF supported by the silicon substrate. The PVDF film, coated with PEDT/PSS on the top and UV curing acrylic on the bottom, is then transferred from the dummy silicon substrate to the integrated silicon circuit containing the readout electronics. The sample was then placed under a strong UV lamp at a slightly raised temperature to cure the adhesive. This technique gives an even, consistent bond layer. This process is compatible with commercially produced CMOS integrated circuits. Manufacturing of the integrated PVDF pyroelectric sensor requires only five maskless postprocessing steps after CMOS fabrication. Thus, it is a very simple and low-cost process. After processing, the sensor array, which occupies a silicon area of 3.854 mm × 3.854 mm, is mounted on a 121 pin PGA and wire bonded. Figure 4.27 shows the layout of the 16 × 16 pyroelectric sensor array.

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4 Pyroelectric Infrared Detector

Figure 4.27 Layout of the 16 × 16 PVDF-on-silicon pyroelectric sensor array. Source: Binnie et al. [31].

4.2.7.3

Sensor Array Specification and Performance

The data given in Table 4.2 gives the full PVDF pyroelectric sensor array specification, and the data presented in Table 4.3 summarizes the measured performance of the 16 × 16 PVDF pyroelectric sensor with a PEDT/PSS conductive polymer as a front electrode/absorber. The voltage sensitivity of the sensor is 10 times higher than that of a PVDF on a silicon array coated with a nickel-aluminum front electrode. The relative performance of the integrated pyroelectric sensor is assessed in terms of NEP and D* which √ is independent of the sensor area. A NEP of 10−11 W Hz−1 at a modulation frequency of 100 Hz has been achieved. With a sensor area of 105 × 105 μm2 , the specific √ detectivity of the integrated PVDF pyroelectric sensor is 4.4 × 108 cm Hz W−1 at a modulation frequency of 100 Hz. Table 4.2

Properties of the PVDF thin film.

Properties

Value

Unit

Pixel array

16 × 16

Pixel size

105 × 105

μm2

Detector element capacitance

7

pF

Array area

2.1 × 2.1

mm2

Pitch

130

μm

Output noise at 100 Hz

60

Current

850

√ nV/ Hz

Operation temperature

0 to 60

μA ∘C

Power supply

±2

V

Power consumption

3.4

mW

Source: Binnie et al. [31]

4.2 Device Configurations

Table 4.3

Performance of the PVDF-on-silicon sensor coated with a PEDT/PSS.

Parameter (at 100 Hz)

Value

Unit

Voltage sensitivity

2200

Noise voltage

60

NEP

2.4 × 10−11

D*

4.4 × 108

V/W √ nV/ Hz √ W/ Hz √ cm Hz/W

NETD (𝜀t = 1 and F/1)

80 × 10−3

K

Source: Binnie et al. [31], Table IV (p. 1417)/IEEE.

NETD is recognized as a figure of merit for appraising the thermal resolution and sensitivity of the sensor. The NEDT is defined as the scene temperature difference, 𝛥T (black body with a mean temperature of 300 K), which causes a signal-to-noise ratio (SNR) of unity at the output. Assuming no optical loss (the aperture of the sensor optic taken as F/1) and a frequency bandwidth of 500 Hz, the minimum of the NEDT for the integrated PVDF pyroelectric sensor is 80 mK. Each pixel was serially addressed via integrated row and column MOSFET switches. Measurements were then taken using a phase-sensitive detector. The mean pixel voltage sensitivity is 3500 V W−1 with a standard deviation and 878 V W−1 . These results offer a promising prospect for a very low-cost uncooled IR sensor array.

4.2.8

TFP Polymer Detector

The national metrology institute of Germany, the Physikalisch-Technische Bundesanstalt (PTB), together with the company Sensor and Lasertechnik (SLT), develops pyroelectric detectors for radiation in the terahertz (THz) spectral range [32]. The intention of this development is to deliver a highly sensitive, accurately calibrated detector for power measurement in the power range of time-domain spectroscopy (TDS) systems. This work reports about a large-area thin-film pyroelectric (TFP) detector applicable within a wide spectral range from 300 GHz to 30 THz and its radiometric characterization by PTB’s THz radiation sources. Applying coherent synchrotron radiation from the Metrology Light Source (MLS), laser radiation from a molecular gas laser and blackbody radiation from a water-heated blackbody to this detector reveals their potential to be capable of spanning an even wider THz frequency range than covered by TDS systems. The TFP polymer detector consists of pyroelectric PVDF foil coated on both sides with a thin layer of metal oxide. These layers are conductive and serve two objectives: Each layer is one electrode to catch the charge generated by the pyroelectric effect of the foil when the temperature changes; both layers, together, compose the absorber for the THz radiation. The physical principle of the absorption process can be derived from Maxwell’s equations and was first described in the 1930s [33]. The sheet resistance of both layers can be carefully adjusted via their combined thicknesses to match half the vacuum impedance. In this case, 50% of the power

143

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4 Pyroelectric Infrared Detector

Figure 4.28 Photograph of the thin-film pyroelectric (TFP) detector with the aperture mounted in front of it. The current amplifier is seen to the right. Source: Müller et al. [32].

is absorbed in the metal oxide layers, whereas 25% of the power is reflected and 25% transmitted. Following Maxwell’s theory, this absorbance is frequency independent in a wide spectral range at least from the low GHz range to several THz depending on the resistance of the metal oxide used for the coating. This is why these TFP detectors are assumed to be suitable for TDS power measurements. The detector is shown in Figure 4.28. Bonding wires on each layer gives access to amplify and measure the current generated by the pyroelectric effect when the foil is heated by absorbed radiation in the coating layers. This current is converted into a voltage output by a specifically designed amplifier with adjustable amplification of 107 V A−1 , 108 V A−1 , 109 V A−1 , or 1010 V A−1 . The amplifier has a cutoff frequency of 50 Hz limiting the chopper frequency to approximately 20 Hz. The pyroelectric effect is known to be linear to the temperature change over many orders of magnitude [34]. In order to be able to generate a continuously refreshed signal, periodic temperature changes have to be applied to the detector, that is, chopped radiation is necessary. Therefore, an initial study on the time performance of the detector was conducted (Figure 4.29). It is also of great interest to understand up to which size the detector can be fabricated. The sensitive area of the detector was defined by a high precision circular aperture with 31 mm in diameter just in front of the detector foil; the diameter of the foil itself was 36 mm. It is therefore a large-area detector. The spatial uniformity of its spectral responsivity is depicted in Figure 4.30. The maximum responsivity deviation between different positions on the sensitive area is 8%. Good uniformity in combination with good linearity indicates that the detector signal is independent of the spot size of the radiation and only depends on the power. It is important to determine the NEP of the detector because this limits the minimum detectable power, a crucial point as TDS systems only have a total power of

4.2 Device Configurations

0.08

0.04 Step response

Signal voltage (V)

0.06

0.02 0.00 –0.02 –0.04 –0.06 –0.08 –0.05

0.00

0.05

0.10

Time (s)

Figure 4.29 Amplified output signal of the TFP detector irradiated with chopped THz radiation. The THz radiation frequency was 1.4 THz, the chop frequency was 20 Hz. The step response signal is proportional to the absorbed power and is the basis to calculate the spectral responsivity. The beginning decay of the signal during the irradiated and dark sections of the chopper wheel is due to the temperature relaxation of the absorber foil. Source: Müller et al. [32].

100.0 95.00

30

90.00 85.00

25

80.00

Vertical (mm)

75.00 70.00 65.00

20

60.00 55.00

15

50.00

10

5 0 0

5

10

15

20

25

30

Horizontal (mm)

Figure 4.30 Two-dimensional diagram of the normalized spatial uniformity of the spectral responsivity of the TFP detector including the front aperture, given in %. The measurement was performed using a focused THz beam at 2.52 THz with a focus diameter of approximately 2 mm and scanning this focus across the sensitive area of the detector. The smooth edge reproduces the beam size [32].

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some microwatts at best. The detector noise is strongly dependent on the environmental conditions as it is sensitive to vibration and acoustic disturbance. This is evident because it is a widely spanned thin foil acting just as a microphone. The NEP was therefore measured in an almost perfectly silent and vibration-free envi√ ronment. Using the responsivity determined below, it is 200 nW Hz−1 . A large-area TFP detector with a diameter of 31 mm was characterized for its spectral responsivity over a spectral range from 300 GHz to 30 THz by applying three different THz radiation sources. This investigation revealed that ●

●

●

the spectral responsivity measured with different radiation sources in the frequency range from 300 GHz to 30 THz remains flat within the respective uncertainty interval. the spectral responsivity is (351 ± 14) nA W−1 calculated as the mean value of the measurements. the spatial uniformity within the sensitive area is better than 8% and the time response is faster than 4 ms.

In this chapter, the main emphasis was on the flatness of the spectral responsivity that was measured using all three THz radiation sources that are available at PTB. The flatness was theoretically predicted and experimentally confirmed for a wide spectral range from 300 GHz to 30 THz. Spatial uniformity and time response meet the requirements for power measurement of TDS systems. However, the spectral responsivity of the detector limits its application for power measurement of TDS systems to specialized high-power devices. Further work is warranted to increase the detectivity of the detector, that is, √ decreasing the NEP below 200 nW Hz−1 currently achieved for this detector. The pyroelectric foil might be fabricated even thinner to decrease the NEP. This will finally pave the way to a TFP detector capable of measuring the power of standard TDS systems strongly needed for further development of this widely used spectroscopy technique in the THz frequency range.

4.2.9

Tetraaminodiphenyl (TADPh) Polymer Detector

In recent years, the interest in THz radiation has been growing, including medical diagnosis of dangerous diseases, due to a specificity of electromagnetic wave interactions with biological tissues in THz and IR ranges. Numerous attempts have been made to realize the high sensitivity of pyroelectric sensors in this spectral region [35–39]. The structures based on thin films of PZT, vinylidene fluoride copolymers, organic polycyclic polymers of tetraaminodiphenyl (TADPh), lithium tantalate (LiTaO3 ), and niobate (LiNbO3 ), KTaNbO3 , PbZnNbO3 , PbScTaO3 , and Sr0.5 Ba0.5 Nb2 O6 compounds, as well as organic films with pyroelectric properties, are used as a photosensitive material. To ensure the high sensitivity and speed of the detector, it is necessary to use as thin as possible pyroelectric films with minimal total heat capacities. The pyroelectric TADPh films have the smallest working thickness of 0.6 to 1.0 μm among other pyroelectrics, which have a thickness of about 20 μm [35–40].

4.2 Device Configurations

Therefore, this material is the most promising for creating a highly sensitive and high-speed detector. Paulish et al. [41] designed a pyroelectric photodetectors (PD based on a TADPh film in the spectral range from visible to millimeter-wave (MMW) radiation to determine the possibility of its use as an ultrawideband detector for visible, IR, THz, and MMW spectroscopy. 4.2.9.1 Detector Design

The scheme of pyrodetector samples is shown in Figure 4.31a. The photosensitive element is a capacitor in which a 1-μm-thick TADPh layer is used as a dielectric. A semitransparent aluminum layer with the thickness of 0.01 μm was used as a top electrode, and a 0.07-μm-thick aluminum layer was used as a bottom electrode. The capacitor is placed on a self-support film of polyarylate-epoxy varnish with a thickness of 0.3 μm, fixed at the edges on a 4 × 4 × 1-mm ceramic substrate with a hole 2-mm in diameter in the center. This design minimizes the heat sink from the pyroelectric film, which is heated by the radiation absorption that increases the detector sensitivity. A preamplifier was also placed on the ceramic plate. The size of the photosensitive area was 1 × 1 mm. This structure was housed in a sealed metal case with power and output leads (Figure 4.31b). The input window was made of a sapphire plate with a thickness of about 170 μm, which is supposedly transparent in the visible, IR, and THz ranges. 4.2.9.2 Detector Sensitivity

In this work, the spectral sensitivity characteristics of a new pyroelectric radiation detector based on 1-μm-thick TADPh layers in the wavelength ranges of 0.4 to 10 μm and 300 to 3000 μm, as well as at the local frequencies of 3.0 THz (𝜆 = 100 μm) and 3.7 THz (𝜆 = 81 μm), were studied. It is shown that the volt–Watt sensitivity of such a pyroelectric detector has a relatively nonselective characteristic in the entire measured range. The resulting NEP was ≈6 × 10−10 W Hz−1/2 , which is 2 to 8 times lower than for the known pyroelectric detectors and the Golay cell. At the Tetraaminodiphenyl film Top electrode Bottom electrode

Polymer carrier film

Input window

(b) (a) Preamplifier

Ceramic substrate

Figure 4.31 The TADPh film-based pyroelectric detector: (a) scheme and (b) overview through the input window. Source: Andrey et al. [41].

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4 Pyroelectric Infrared Detector

frequency of 3.0 THz, the volt–watt dependence was equal to 2 × 106 V W−1 and NEP = 2 × 10−8 W Hz−1/2 . At the frequency of 3.7 THz, the sensitivity was equal to 1 × 106 V W−1 and NEP = 3 × 10−8 W Hz−1/2 . The amplitude-frequency characteristic showed that the pyroelectric detector passband is about 500 Hz at the level of 0.5, is determined by the preamplifier on the sensor chip, and can be expanded by changing the preamplifier parameters. The use of compact secondary amplifiers and a multichannel power supply will allow the construction of relatively inexpensive systems for high-speed IR- and THz-spectroscopy in scientific and technological research activities, including medical applications.

4.2.10 Integrated Resonant Absorber Pyroelectric Detector Pyroelectric detectors have proven to be one of the most effective, compact, and relatively inexpensive types of uncooled thermal sensors widely used for IR detection. Nowadays, following fast progress in terahertz (THz) and MMW technologies, a lot of efforts are being made to improve the sensitivity of pyroelectric detection at longer wavelengths 𝜆 far beyond the IR range. In particular, the short MMW and sub-MMW bands (𝜆 ≈ 0.5–3 mm) remain highly attractive for various applications, including security imaging and surveillance, non-destructive evaluation and quality control of materials, etc. due to an opportunity to combine the options of relativity high penetrability of such waves through the atmosphere and different non-metallic objects versus high-frequency THz, IR, and optical radiation, as well as attainability of spatial resolution of the order of several millimeters acceptable for imagining of concealed targets. When deploying active systems in these tasks, mostly monochromatic and linearly polarized MMW/sub-MMW radiation is normally used for object illumination. Such radiation is to be detected by a highly sensitive, compact, and reliable sensor optimized for maximal performance at a prescribed wavelength and wave polarization. Moreover, selective MMW/sub-MMW sensors are required when multi-spectral techniques and polarization discrimination are involved in the data processing. It should be highlighted that direct application of conventional IR pyro-sensors to the MMW/sub-MMW range faces a problem of a noticeable degradation in the detector’s sensitivity that makes them appreciably inferior to Golay cells [42]. This problem appears due to the rapidly diminishing absorption of long-wave radiation in a thin pyroelectric film when the wavelength increases. Currently, noticeable technological efforts are being undertaken to create a sensitive broadband pyro-detector applicable to spectroscopy tasks that are achieved via the integration of absorptive metallic, oxide, or CNT coatings with pyroelectric films [43, 44]. On the other hand, in spectrally selective detection the detector’s sensitivity can be maximized if the radiation is absorbed by a thin metamaterial structure whose heating is further sensed with a pyroelectric. It is noteworthy that meta-surface absorbers enable perfect or near-to-perfect absorption within a narrow frequency band under the condition d/𝜆 ≪ 1 that makes them suitable as the radiation-sensitive elements in the selective MMW/sub-MMW bolometric detectors. Such detectors enable a

4.2 Device Configurations

relatively simple adjustment of their spectral and polarization characteristics upon fabrication through a proper modification of the metamaterial structure. 4.2.10.1 Detector Design

Kuznetsov et al. [45] reported on the development of a compact pyroelectric detector optimized for selective sensing of the MMW radiation within a narrow spectral band centered at 140 GHz (𝜆 = 2.14 mm). The detector is realized by combining a commercial IR pyro-sensor with a resonant meta-surface absorber attached to the pyroelectric layer. To demonstrate the efficient frequency- and polarization-selective detection of millimeter waves, the developed 15 μm-PP (polypropylene) film absorber was integrated with the commercial discrete pyroelectric sensor MG33 from the Russian manufacturer “Vostok” [46]. Originally optimized for sensing IR radiation within the wavelength range of 2–20 μm, this kind of sensor exhibits the typical voltage responsivity of 105 V/W at the NEP around 1.0 × 10−9 W Hz−1/2 . The basic scheme of the pyroelectric sensor combined with the resonant absorber is depicted in Figure 4.32a. In this work, the resonant absorber is directly attached to the top electrode of the pyroelectric film and fixed around the periphery by a heat-conducting paste. The resulting packaged structures are shown in Figure 4.32b. Due to fabrication constraints, two detectors with slightly different absorbing areas were prepared for testing. The absorber’s dimensions were correspondingly chosen to be 2.47 × 2.30 mm2 (Prototype #1) and 2.47 × 1.54 mm2 (Prototype #2) that included only 3 × 6 and 3 × 4 resonant patches in the FSS (Frequency selective surface) pattern, respectively. From the fundamental point of view, it is of great interest to ascertain the influence of such a small quantity of resonant FSS elements interacting with the incident electromagnetic radiation on the detector performance.

Resonant absorber

Top electrode

Bottom electrode

Carrier

(a)

Support Pyroelectric layer film

Carrier

(b) (c)

Figure 4.32 Pyroelectric sensor. (a) Sketch of the pyroelectric sensor with an integrated resonant absorber. (b) Photo of the sensor structure through the sapphire window. (c) The appearance of the accomplished sensor in the standard KT-3 package. Source: Kuznetsov et al. [45].

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4.2.10.2 Detector Sensitivity

In this work, the voltage responsivity of the detector prototypes was measured using the absolute THz power meter from TK Instruments, Ltd. [47]. Due to its large radiation-sensitive area, this device is optimal for determining the power of free-space beams with a diameter up to 30 mm. In the experiments with CW (continuous wave) illumination from the backward wave oscillator (BWO) source, the total mean power for the MMW flux focused onto the input window of the pyro-detector at the frequency of 140 GHz and 23 Hz-modulation optimal for the power meter operation were found to be 2.6 mW. At 23 Hz of the modulation frequency, the resulted voltage responsivities of the detectors were found to be 56 kV W−1 for Prototype #1 and 40 kV W−1 for Prototype #2, while their NEP was estimated as 2.0 × 10−9 W Hz−1/2 . It is noteworthy that, when examined with the same detection system, the IR responsivity and NEP values of the original MG33 detector before its integration with the resonant absorber were estimated to be 90–110 kV W−1 and 1.0 × 10−9 W Hz−1/2 respectively. Thus, the presence of the 15 μm-PP-absorber does not lead to a noticeable deterioration of the detector performance in the MMW regime, which is characterized by the degradation factor of 2. The magnitudes of the output signals from the pyroelectric detectors were also compared with the similar response of the Golay cell from the BWO-spectrometer kit using the same lock-in detection system. A difference in dimensions of the radiation-sensitive areas of the detectors was taken into account. As a result, at the chopper frequency of 23 Hz, the pyroelectric signals generated under 140 GHz-illumination were found to exceed the Golay cell signals by the factors of 3.8 and 3.0 referred to the Prototype #1 and Prototype #2 respectively. The response time of the pyroelectric detectors was identified via inspecting the time behavior of the signals under CW 140 GHz-illumination modulated by a 23 GHz-meander-waveform pulse with the unit on–off time ratio. For this purpose, a fast-speed PIN modulator attached to the output flange of the BWO providing the wavefront rise/decay time around 1 ns was used. The experimental signals from the pyro-detectors were mathematically processed by fitting the falling edges in their time characteristics with a damped exponential function exp(t/𝜏). The resulted response time 𝜏 was evaluated to be 2.3 ms for Prototype #1 and 3.0 ms for Prototype #2. For comparison, the original MG33 detector free from the resonant absorber showed the larger value of 𝜏 estimated as 4.1 ms. As a result, the developed device appears to surpass a commercial Golay cell in similar characteristics that pave the way for realizing highly efficient compact selective sensors for the MMW/sub-MMW and THz bands, which can be produced at relatively moderate technological expenses.

4.2.11 Resonant IR Detector This research proposes a relatively new and unexplored technology for uncooled micromechanical IR sensors using a combination of piezoelectric, pyroelectric, electrostrictive, and resonant effects. It can offer high sensitivity and a large SNR

4.2 Device Configurations

with a small sensing area, along with the fast response time required for continuous video imaging. In the proposed sensor, Gokhale et al. [48] used micromechanical resonators that respond to IR radiation by a shift in their resonant frequency. This technology has been proposed by Vig and Kim [49, 50] to be a way to make high sensitivity, low noise IR detectors. Unlike resistive microbolometers, the limiting noise in micromechanical resonators operating in the 1 MHz – 1 GHz frequency range is due to the thermomechanical noise arising from random thermal fluctuations in the body of the resonator. The critical figure of merit for noise in IR detectors is the NETD, which denotes the noise-limited minimum resolvable temperature. Prototypes of such resonant IR detectors were fabricated using quartz in the past [51–53], but this technology was not amenable to making resonator arrays with element size and performance comparable to that of conventional microbolometer arrays. Furthermore, quartz, and in fact, most of the materials that are used for making resonators are dielectrics or wide-bandgap semiconductors and are thus not inherently sensitive to IR radiation. Decoupling the resonator operation from the efficient absorption of IR radiation is the best available solution for resonant IR detection. This can be achieved by coating the resonator surface with a thin film absorber that has a high absorption efficiency. 4.2.11.1 Principles of Operation of Resonant Detector

The resonant IR detector comprises two major functional components (Figure 4.33): the resonant structure and the absorber. The resonator is a mechanically suspended structure (Figure 4.33a) that undergoes mechanical vibration as a result of either self-sustaining and stable oscillation conditions or a driving signal at the natural frequency of resonance. The natural frequency of resonance for micromechanical Infrared radiation

IR radiation

IR

tor

a son

Re

(a)

Gtether

Resonator

2

Cth

Dark response

Temperature change

Amplitude

r

be sor Ab r e lay

Illuminated res ponse

Absorber

Frequency change

(b)

(c)

Frequency

Figure 4.33 (a) The resonant IR detector is a thin-film micromechanical resonator mechanically suspended by thin tethers. The resonator body accounts for most of the thermal mass of the device, while the tethers isolate the resonator thermally. The resonator is coated with an IR absorber layer that efficiently absorbs incoming radiation. (b) The transduction mechanism can be separated as the absorption of IR radiation resulting in a temperature shift, resulting in a proportional frequency change for the resonator. (c) A depiction of the reduction of frequency of a resonator due to IR illumination. For most materials, increased temperature causes a reduction in frequency. Source: Gokhale [54].

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resonators (Eq. (4.1)) is set by the material properties and dimensions of the structure. √ √ keff n n E ∝ fn = (4.1) 2𝜋 meff 2D 𝜌 where f n is the natural frequency of resonance, n is the mode of vibration for the resonator, keff is the effective elastic stiffness of the resonator, meff is the effective vibrating mass of the resonator, L is the critical dimension of vibration of the resonator, E is the effective Young’s modulus of the material, and 𝜌 is the mass density of the material. Since both the material properties and dimensions are temperature-dependent, a change in the temperature of the device results in a proportional change in the frequency of the resonator. The transduction mechanism can be one of a number of mechanisms: capacitive, piezoelectric, thermal, magnetic, etc. In this work, Gokhale et al. [48] used piezoelectric actuation, primarily using GaN for high sensitivity, but also other similar materials and combinations (GaN-on-Si, AlN, AlN-on-Si, and PZT) can be used. Most materials used as resonators respond to temperature changes through the temperature coefficient of frequency (TCF), which is in the range of −20 ppm/K to +80 ppm/K depending on the material. GaN is used as the material of choice since it is not only piezoelectric but also a strong pyroelectric and electrostrictive material. Changes in temperature cause fast and large changes in the net strain in a GaN layer due to the pyroelectric-electrostrictive mechanisms, leading to frequency shifts as large as −2000 ppm K−1 . The tethers mechanically and thermally connect the resonator to the substrate. The thermal conductance of the tethers (Gtether ) is an important property for optimizing the thermal performance of the device. For high sensitivity and low NETD, the resonator needs to be thermally isolated from the substrate, and thus Gtether should be as low as possible. However, a high Gtether also slows down the response time. Therefore, the tether design needs to be optimized for the application in question. The thermal capacitance of the body of the resonator (Cth ) is also significant for determining the response time, as well as the level of thermomechanical noise in the system. In addition, the material, size, and dimensions of the resonator and its tethers also determine its basic performance as a resonator, and thus it is critical to optimize the mechanical performance of the device as a resonator and its thermal and noise performance as an IR detector. 4.2.11.2 IR Absorbing Coatings and Structures

The thin-film absorber coating is needed to provide efficient absorption of incident radiation and conversion into thermal energy [55–58]. Absorber layers for resonant IR detectors require the following attributes [58]: (i) high and polarization-independent absorption [59], (ii) absorption bandwidth that can be designed to specification and can be made narrow-band or broadband as desired, (iii) low thermal and inertial mass, (iv) good thermal conduction to the resonant detector, (v) stable and reproducible properties, and (vi) process-compatible with detector and electronics fabrication technologies. Figure 4.34 shows a schematic of

4.2 Device Configurations

Reflection Incident radiation Absorption

Transmission

Substrate

Figure 4.34 Schematic showing the absorption, transmission and reflection of infrared radiation by an IR absorber coating on the resonator substrate. The absorption can be broadband over a large spectrum or designed to be narrowband at a specific wavelength, based on the intended application. Within the absorption band, the goal is to increase absorption while reducing transmission and reflection. Source: Gokhale [54].

the thin-film absorber layer on the resonator substrate, with the fraction of energy absorbed by the layer given by Eq. (4.2). A(𝜆) = 1 − R(𝜆) − T(𝜆)

(4.2)

where 𝜆 is the wavelength of radiation, A, R and T are the fractional absorption, reflection, and transmission. Silicon nitride is widely used as the IR absorbing layer as it is CMOS-compatible and can be deposited at low temperatures. However, its absorbance is at best ∼80% and limited to a small region of the far-IR spectrum. In order to boost the efficiency of IR detection and to widen the spectral range of the sensor, Gokhale et al. [48] developed a new material that is highly absorptive (>95%) across large parts of the near, mid, and far IR spectra. This material is a nanocomposite comprised of CNTs and nano-diamonds (NDs) mixed into a polymer matrix for mechanical stability. Additionally, such absorbers can be used for solar-thermal energy harvesting [60]. For surveillance applications and thermography, high-efficiency narrow-spectrum absorbers are needed that can isolate certain thermal phenomena (e.g., long-wavelength infrared [LWIR] absorption to distinguish human forms against a cold background). However, recently, interest is growing in using multi-spectral and hyperspectral imaging that combines data across various spectra to get a better understanding of the target. This is useful for both surveillance and for remote sensing/astronomy. For photonic sensors, this can be achieved by using materials with different bandgaps. For thermal sensing, it can be achieved by multiple pixels each having an IR absorber layer sensitive to different narrow spectral bands. For this purpose, Gokhale et al. [48] designed plasmonic metamaterial-based IR absorbers that can be designed to absorb a specific wavelength. The absorbed wavelength is based on the dimensions and material properties of the metamaterial and can be designed according to the application. Measured results have demonstrated selectivity for a particular wavelength (with the absorption of >45% and a pass band of 1.5 μm centered around 10 μm) and the ability to integrate these designs on a resonant detector platform.

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4.2.11.3 Differential Operation and Detector Arrays

In order to boost the sensitivity of detection, and at the same time, eliminate the effects of interfering signals such as slow changes in ambient temperature, acceleration, rotation, pressure changes, and background radiation, Gokhale et al. [48] used a differential measurement scheme with a resonant sensor, paired with a reference resonator that is invariant to IR radiation. The sense resonator is coated with the IR absorber, while the reference resonator is coated with a reflective metal that does not absorb IR radiation, and thus, it does not undergo significant frequency shift in response to IR radiation (Figure 4.35a). A number of readout schemes can be implemented to achieve differential sensing for these sense/reference pairs (Figure 4.35 b). A major advantage of thin-film technology and standard microelectromechanical systems (MEMS) processing techniques is the ability to fabricate not just single resonant IR detector elements (unlike quartz-based resonators), but to batch fabricate large arrays (Figure 4.36) of identical resonant IR detector “pixels.” This is a critical step in achieving parity with conventional microbolometer-based FPAs. While this work has not reached the maturity of industrial FPA design and fabrication, Gokhale et al. [48] demonstrated small-format arrays of up to 16 elements. Important practical aspects to be considered while designing arrays are the pixel footprint, the pixel pitch, the routing, and the readout/signal processing scheme. The current limiting factor for the number of elements (in a research lab environment) is the routing design optimization, and not the design or fabrication of the actual resonant detectors. With mature industrial processes such as through-wafer via or flip-chip bonding to a readout IC, it is possible to extend these designs to larger formats.

Top electrode

A

IR absorber

fs

Mixer LPF

fB = |fs − fr|

GaN

fr

GaN resonant sensor

(a)

GaN reference resonator

Bottom electrode

A

(b)

Figure 4.35 (a) Schematic of the conceptual approach to differential resonant IR sensing. A sense reference resonator pair is shown. Upon IR irradiation, the sense resonator, which is coated with an IR absorbing layer, undergoes a frequency shift, while the reference resonator does not absorb radiation and remains frequency-invariant. (b) A representative circuit that can be used for differential measurement. Both resonators are used in identical feedback loops as self-sustaining oscillators. The oscillator signals are mixed together and the beat frequency is extracted. The beat frequency is a function of the IR signal power but eliminates common mode signals such as temperature drift. The effective sensitivity of the differential signal is larger than that of a single sensor. Source: Gokhale [54].

4.2 Device Configurations

Focusing optics

(a)

(b)

Figure 4.36 (a) A scanning electron microscope (SEM) image of a fabricated array of 4 × 4 resonators. Bilayer routing is seen here and can be replaced by flip-chip bonding with a readout IC. (b) A schematic showing the FPA composed of a number of resonant IR detectors. The focusing optics are standard equipment for FPAs and depend on the spectral range to be detected. Source: Gokhale [54].

4.2.11.4 Performance of GaN Resonators

This work demonstrates the first experimental prototype monolithically integrated thin-film arrays of resonant IR detectors, made from GaN and other materials. These resonant IR detectors are capable of achieving a small footprint, high sensitivity, and extremely low noise levels, as evidenced by the designed NETD of 10 mK. The resonators are characterized for their RF and thermal performance and exhibit a radiant responsivity of 1.68%/W, the thermal time constant on the order of 556 μs, and an average IR responsivity of −1.5% when compared with a reference resonator, for a 100 mK radiation-induced temperature rise. In order to develop resonant IR detectors with high sensitivity, this work has pioneered high-frequency GaN-based micromechanical resonators based on extensional modes of vibration. It allows reaching frequencies ranging from 10 MHz to several gigahertz. As part of this work, a number of GaN resonators were designed, fabricated, and characterized. Important properties of GaN such as the acoustic velocity, piezoelectric coupling coefficient, TCF, and acoustic loss were characterized. Characterizing these properties is critical to ensure that GaN resonators can be implemented reliably with high performance. Apart from their use as IR detectors, GaN resonators can be integrated homogenously with GaN electronics to provide a high-frequency, high-power electromechanical signal processing system.

4.2.12 Plasmonic IR Detector Over the last two decades, the advent of plasmonic metamaterials, which are artificially structured materials with periodic subwavelength unit cells, has offered great freedom to tailor the absorption spectra [61–65]. The absorption peaks can be precisely controlled and manipulated by carefully designing the geometrical parameters of the unit cells. As the field of MEMS has rapidly advanced, plasmonic perfect

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absorbers can be directly integrated on micromachined pyroelectric transducers to create compact, high-performance yet low-cost multi-wavelength detectors that operate at RTs. Doan et al. [66] proposed and implemented a quad-wavelength pyroelectric detector with four distinct plasmonic absorbers to selectively detect light in the mid-IR region. For non-dispersive infra- red (NDIR) multi-gas sensing applications, the four resonance wavelengths were determined at 3.3, 3.7, 4.1, and 4.5 μm, which corresponded to the centered absorption band of CH4 , H2 S, CO2 , and N2 O, respectively [67, 68]. The spectral selectivity was achieved by the coupling of incident IR light to resonant modes of Al-disk-array/Al2 O3 /Al perfect absorbers with various disk sizes. The top patterned resonators were hexagonal arrays of disks used to achieve wide-angle acceptance and polarization-insensitivity, which are highly desirable for many sensing applications. Doan et al. [66] chose Al as the plasmonic base metal because it is abundant on Earth and it is industry-compatible while still exhibiting low-loss plasmonic properties similar to noble metals such as Au, Ag in the IR region [69]. The model of the Al-disk-array/Al2 O3 /Al perfect absorber was first constructed in a computer-aided design (CAD) layout (Rsoft CAD, Synopsys’s Rsoft, Synopsys, Inc.) [70]. The absorptivities, electric field, and magnetic field distribution of the absorbers were simulated and optimized using the commercial rigorous coupled-wave analysis (RCWA) package and the FullWAVE package from Synopsys’ Rsoft [70], which is a highly sophisticated tool for studying the interaction of light and photonic structures, including integrated wavelength-division multiplexing (WDM) devices [71, 72], as well as nanophotonic devices such as metamaterial structures [63, 73], and photonic crystals [74]. The sensing areas were designed as floating membranes above a void space to minimize thermal conduction, thereby improving the responsivity of the detector. The electromagnetic energy at the resonance wavelengths induced heat on the upper surface of the zinc oxide layer, which features pyroelectricity in a thin-film form. Due to the pyroelectric effect, a signal voltage was generated at the resonance wavelengths for each absorber. The on-chip design of the proposed quad-wavelength pyroelectric detector demonstrated the feasibility of integrating micro-detectors of different selective wavelengths into arrays with good CMOS compatibility. This opens the possibility of developing miniaturized and robust multi-color spectroscopic devices. 4.2.12.1 Structure Design

The schematic diagram given in Figure 4.37a illustrates the design layout of the proposed quad-wavelength detector. Four individual sensing elements were directly integrated on the same CMOS platform with a size of 0.5 × 1.0 cm2 to selectively detect IR radiation at four resonant wavelengths of 3.3, 3.7, 4.1, and 4.5 μm. The structural design of a single sensing element is illustrated in Figure 4.37b. From the top to bottom, it consisted of an Al-disk-array/Al2 O3 /Alperfect absorber structure with an active area of 200 × 200 μm2 , a 300-nm-thick pyroelectric zinc oxide thin film sandwiched between the Al back plate of the absorber and a 100 nm Pt/10 nm Ti bottom electrode, and a membrane-based CMOS substrate. A 300 nm-thick layer of silicon nitride was deposited on both sides of the silicon substrate to supply

4.2 Device Configurations

Al top electrode

ZnO

(Al-disk-array)–Al2O3–Al plasmonic resonance absorber

E

Z X

k

Y

Thermal Isolation arms Pt bottom electrode

(a)

(c) (b) Al disk array Al2O3

Side view Al top electrode Thermal isolation arms

Top electrode Al ZnO

Pt/Tf

Bottom electrode

Pt bottom electrode Top view

Si3N4

Thermal isolation arms

Si Si3N4

(d)

(e)

Back view

Figure 4.37 (a) Schematic illustration of the proposed quad-wavelength detector. (b) Illustration of the structural design of a single sensing element. (c) Illustration of the plasmonic perfect absorber with the indicated field lines at resonance (electric field E, red arrows; magnetic field H, blue arrows). (d) Exploded view of a single sensing element. (e) Side view, top view, and bottom view of a single sensing element. Source: Anh Tung Doan [66].

adequate mechanical strength for the membrane structure. The silicon wafer was 380 μm thick. The width and the length of the supporting arms were 20 and 400 μm, respectively. The cross-sectional profile of the unit cell is shown in Figure 4.37c. The Al-disk-array/Al2 O3 /Al perfect absorber consisted of an Al disk array as the resonator, an Al2 O3 layer as the middle insulator, and an Al film as the back reflector. The thicknesses of the three layers were identical and were represented as “t.” The Al disks of diameter “d” were arranged in a periodic hexagonal array of periodicity “p.” After some numerically computational efforts based on the RCWA, the optimized geometrical dimensions for the perfect absorbers were taken as p = 2.0 μm, t = 100 nm, and d = 0.97, 1.25, 1.35, and 1.60 μm for the individual sensing elements at resonance wavelengths of 3.3, 3.7, 4.1, and 4.5 μm, respectively. When an incident IR radiation impinged on the top surface of the device, its external oscillating electric field-induced electric dipoles inside the Al disks, which excited anti-parallel currents between the disk array and the back reflector. These circular currents produced a magnetic flux opposing the external magnetic field,

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resulting in magnetic resonance. By adjusting the geometrical parameters of the Al disk array, Doan et al. [66] could tailor the electromagnetic response of the structure to the external electromagnetic field to achieve selective absorption. The exploded view of a single sensing element is shown in Figure 4.37d. It was shown that the bottom layer of the plasmonic metamaterial perfect absorber was also utilized as the top electrode of the pyroelectric detector. In this design, the zinc oxide layer was stacked below and in intimate contact with the plasmonic resonance, absorber to fully exploit the spectrally selective radiation. It is also worth noting that Doan et al. [66] chose zinc oxide as the pyroelectric material due to its advantages of being nontoxic and compatible with the semiconductor process. This design is also applicable for other pyroelectric materials such as PZT, LT, lithium niobate, BST, deuterated triglycine sulfate (DTGS), and others. The side view, top view, and back view of the single sensing elements are shown in Figure 4.37e, indicating the micromachined floating membrane design of the sensing element. 4.2.12.2 Fabrication and Performance of the Detector

Figure 4.38a shows a photo of a fabricated MEMS-based hybrid plasmonic– pyroelectric detector, which had a width of 0.5 cm and length of 1 cm. It clearly demonstrates that multiple hybrid plasmonic–pyroelectric sensing elements were easily integrated on a standard CMOS platform to achieve multispectral selectivity without any additional bulky optical filters. Figure 4.38b shows the optical microscopy images of the top view and back view of a single sensing element with an active area of 200 × 200 μm2 . It verifies that the sensing area is suspended by the long thermal isolation arms. The suspended area remained flat without any additional stress-reducing process. While the thicknesses of the sputtered layers could be precisely controlled by establishing optimum sputtering conditions and deposition rates, the process of transferring the shape and size of the Al disk arrays poses further challenges due to the complex multi-stage photolithography of the sub-micron patterning. The residual photoresist and non-uniformity may result in degradation from the expected performance. Figure 4.39a–d shows the scanning electron microscope 100 μm

Top view (a)

100 μm

Back view

(b)

Figure 4.38 (a) Photo of a fabricated MEMS-based hybrid plasmonic–pyroelectric detector. (b) Microscope images of the top view and back view of the fabricated sensing element. Source: Anh Tung Doan [66].

4.2 Device Configurations

Absorptivity

1.0

0.6 0.4 0.2 0.0 1.0

(b)

Absorptivity

λ2 = 3.71 µm

(a)

Simulation

0.8

(e) λ1 = 3.30 µm

λ1 λ2 λ3 λ4

Experiment

0.8 0.6 0.4 0.2

λ3 = 4.09 µm

(c)

λ4 = 4.54 µm

(d)

Responsivity (mV W−1)

(f) 0.0 125

Experiment

100

(g)

75 50 25 0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Wavelength (µm)

Figure 4.39 (a–d) Scanning electron microscope (SEM) images of the fabricated disk array patterns at four resonance wavelengths. (e, f) Simulated and measured absorptivities of the perfect absorbers at four resonance wavelengths. (g) Measured responsibility of the quad-wavelength detector. Source: Anh Tung Doan [66].

(SEM) images of the hexagonal arrays of the Al disk resonators with diameters of 0.97, 1.25, 1.35, and 1.60 μm, respectively. It was shown that the Al disk arrays fabricated on top of the membrane structures were well-defined and homogeneously distributed, indicating that the patterning process was precisely implemented. Because the Al disk arrays were patterned using electron-beam lithography with sub-10 nm resolution, there was a tolerance of a few nanometers in the diameter of the disks. The reflectance spectrum of each sensing element was measured using a Fourier transform infrared spectrometer (FTIR) (Thermo Scientific Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA), coupled with a microscope (Nicolet Continuum FTIR Microscope, Thermo Nicolet). The reflectance spectra were normalized with respect to the reflectance from a gold film. Given that the transmittance was zero for the thick back reflector, the absorptivity spectra were calculated using the formula A = 1 − R, where A denotes the absorptivity, and R denotes the reflectivity. The simulated and measured absorptivity spectra are shown in Figure 4.39e, f. The reflectance spectra were normalized with respect to the reflectance from a gold film. The four sensing elements exhibited absorptivity peaks of 0.92, 0.93, 0.85, and 0.87 at 3.32, 3.74, 4.06, and 4.51 μm, respectively, which were highly consistent with the corresponding simulated absorptivity peaks of 0.94, 0.99, 0.94, 0.98 at 3.33, 3.72,

159

4 Pyroelectric Infrared Detector

4.05, and 4.50 μm, respectively. The precise pattern transfer in the fabrication process resulted in small shifts of only a few tens nm and slightly lower magnitudes of absorption peaks. The FWHMs of the measured absorptivity curves were 0.35, 0.45, 0.49, and 0.68 μm, compared to those of the simulated curves, which were 0.30, 0.44, 0.47, and 0.63 μm. Together with the defined patterns and uniformity observed in the SEM images, the excellent agreement between the simulated and experimentally measured absorptivity proved the quality of the optimized fabrication method. The performance of the detector was evaluated by measuring its spectral responses to the IR radiation from a wavelength-tunable pulsed laser system in the range of 2.5–6.0 μm. The pulse width of the laser was 104 fs, and the repetition rate was 1 kHz. The IR laser beam was guided to the sensing area of the detector. The output electrical signal was amplified with a preamplifier and demodulated with a lock-in amplifier. The spectrally selective absorption of laser pulses at resonant wavelengths was due to the excitation of highly localized magnetic and electric dipole resonances, which was evidenced by the simulated field distributions as shown in Figure 4.40b–d. Such strong resonances efficiently confined the electromagnetic energy and provided sufficient time to convert it into resistive heat within

λ1 = 3.72 µm

Z (µm)

0.75

p = 2.0 µm d = 1.25 µm t = 0.1 µm

0.50 0.25

0.3 0.0 –0.3

–2

k

–1

Z (µm)

(b) 0.3 0.0 –0.3

0.75 p = 2.0 µm d = 1.6 µm t = 0.1 µm

0.50 0.25

0.00 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Wavelength (µm)

60

10

40

5

20

0

(e)

3 4 5 Wavelength (µm)

6

1

2

0 X (µm)

1

2 Ez

0.3 0.0 –0.3 –1

(d) 2.0

0

1

Al2O3

5E-04

1.0 0.5 3 4 5 Wavelength (µm)

2

–22.2

–0.1 –12.6

22 –22

1E-03

1.5

0.0

(f)

0 X (µm)

22.2

6

Imaginary index

Real index

Al

–1

–2

–2

Imaginary index

15

(c)

0 X (µm)

Ex

Hy

Real index

0.00 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 λ2 = 4.50 µm 1.00

(a)

E

λ1 = 3.72 µm

Z (µm)

Reflectivity Absorptivity Transmissivity

1.00

Reflectivity Absorptivity Transmissivity

160

0E-00

Figure 4.40 (a) Reflectivity, absorptivity, and transmissivity characteristics of the plasmonic perfect absorber at the resonance wavelengths of 3.7 and 4.50 μm. The simulated amplitude plots of (b) electric field in x-direction, (c) magnetic field in y-direction, (d) electric field in z-direction at the resonance wavelength of 3.7 μm, illustrates the highly localized characteristics of the plasmonic perfect absorber. The refractive index dispersion data of (e) Al and (f) Al2 O3 used in the simulations. Source: Anh Tung Doan [66].

4.2 Device Configurations

the Al disks and the continuous Al back plate. Al2 O3 underwent almost no loss in the mid-infrared region (Figure 4.40f); that is, dielectric losses in the spacer layer were negligible in contrast to the plasmonic perfect absorbers operating in lower frequencies, such as in the microwave and terahertz region, in which the absorption was mainly due to dielectric losses. The heat at the continuous Al back plate that was associated with spectrally selective absorption was directly transferred to the zinc oxide thin film, which was among the materials that were electrically polarized due to their c-axis-oriented textured crystal structure [75–77]. The zinc oxide film consequently became electrically polarized with changes in the temperature, and the amount of electric charges was also proportional to the temperature fluctuation. The electrically polarized zinc oxide film hence produced a voltage between the Al and Pt electrodes. Pt was chosen as the bottom electrode because it has been reported that the hexagonal arrangement of the atoms in the (111) plane of Pt facilitated the nucleation and growth of hexagonal zinc oxide oriented along the [001] direction [78]. In Figure 4.39g, the measured spectral responsivity curves of the four sensing elements are plotted. As seen from the results, the spectral responses of the four sensing elements were 125, 150, 126, 128 mV W−1 at 3.30, 3.71, 4.09, and 4.54 μm, respectively. The excellent measured spectral responses proved that the detector exhibited low thermal drift and thermal noise due to the low mass of the membrane-based design. The measured FWHMs of the spectral responses were 0.94, 1.02, 1.10, 1.20 μm, respectively, which were approximately two times broader than those of the absorptivity measured using the FTIR spectrometer. The broadened spectra could be partly attributed to the spectral bandwidth of the IR pulsed laser and the fluctuation of the incident angles and positions of the laser pulses. These experimental results clearly demonstrated that the proposed multispectral hybrid plasmonic–pyroelectric detectors can be fabricated and easily integrated into existing IR spectroscopic devices such as miniature NDIR sensors, chemicaland bio-sensors, photoacoustic imaging systems, and thermographic cameras. As an example, on-chip multi-spectral infrared detectors may offer an opportunity to develop miniaturized and robust photoacoustic computed tomography [79, 80]. Specifically, in all-optical photoacoustic imaging systems, shifts in the wavelengths of light are able to be detected and analyzed using an array of multi-spectral micro-detectors.

4.2.13 Graphene Pyroelectric Bolometer Bolometers are another class of uncooled thermal PDs, where T variations due to incoming photons produce a change in the resistance (R) of a sensing element. A bolometer can be a thin metal layer [81], a semiconductor [82], or a superconductor [83]. Common metallic bolometers for RT operation are made of Ti [84], Ni [85] or Pt [81]. Polysilicon [83, 86], amorphous silicon [87] or vanadium oxide [82] are usually exploited for semiconducting bolometers. For fixed bias, V d , the resistance change of the sensing element translates in a measurable change in current (I). The temperature coefficient of resistance (TCR in units of %K−1 ) is a key performance

161

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4 Pyroelectric Infrared Detector

indicator for a bolometer and is defined as [86]: 1 dI 1 dR =− . . TCR(R0 ) = R0 dT I0 dT

(4.3)

The TCR represents the percentage change in resistance per Kelvin around the operating point R0 and corresponds in a module to the normalized current change per Kelvin around the operating current I 0 (Eq. (4.3)). The TCR in metallic bolometers is ∼0.4%K−1 [86], whereas for semiconducting bolometers it is ∼2–4%K−1 [83, 86]. It follows that the output of a bolometer (measured current) is proportional to T, in contrast to the output of a pyroelectric detector (measured current) that depends on the derivative of T [88]. Graphene is ideally suited for photonic and optoelectronic applications [89–91], with a variety of PDs in the visible [92–96], near-infrared [90, 91] and THz reported to date [97], as well as Mid-infrared Spectroscope (MIR) thermal detectors [98–100]. Refs. [101–103] previously reported graphene-based bolometers at low T ( C2 . The perimeter of the pads defining C3 sets the overall pixel size, from which only the source and drain contacts stem out to interface with the measurement electronics. In the first approximation, the generated pyroelectric charge 𝛥Q is uniformly distributed on the substrate on a T variation [83, 86]. Therefore, the direct effect from C1 does not depend on the channel area AC1 , as the bottom-gate field depends on the pyroelectric polarization, which is constant over any area. For the floating gate in Figure 4.41b, 𝛥Q accumulating on C3 depends on area as (from the equation: 𝛥P = p⋅𝛥T) 𝛥Q = p𝛥TAC3 . Being the structure electrically floating and free from external parasitic capacitances, 𝛥Q is entirely provided by C2 , because of the conservation of charge. A charged C2 generates for the SLG channel an effective top-gate voltage (in the module): ΔVTG =

ΔQ pΔTt AC3 = C2 𝜀0 𝜀r AC2

(4.4)

where C2 = 𝜀0 𝜀r AC2 t−1 , 𝜀0 and 𝜀r are the vacuum and relative permittivity, and t is the oxide thickness. Hence, for fixed t and 𝛥T, the geometrical ratio AC3/AC2 controls the gain of the integrated SLG amplifier and therefore the TCR (ΔI I −1 = gm ΔV TG I −1 , where I is the current and gm is the transconductance of the GFET). Figure 4.41c shows an optical micrograph of a device with patterned pads. Sassi et al. [112] illuminated this device with MIR radiation at 1100 cm−1 (∼9 μm) using a laser spot matching the pixel size (300 × 300 μm2 ). The resulting modulation of the channel drain current is shown in Figure 4.41dover nine ON/OFF laser cycles. A responsivity ∼0.27 mA W−1 is obtained for a drain current in the dark (I OFF ) ∼1.3 μA (V d = 10 mV). The responsivity can be increased by applying a larger V d , which, in turn, increases I OFF . The normalized current responsivity (%W−1 ) is defined as [86]: Rph,N =

ION −IOFF .100 IOFF

Pin

(4.5)

where I ON is the current under illumination and Pin is the optical power of the incoming radiation. Rpn,N is a better parameter to compare photoconductive detectors. Rpn,N in Figure 4.41d is ∼2 × 104 %W−1 , over two orders of magnitude higher than Ref. [107], where only the direct effect was exploited (∼1.2 × 102 %W−1 ). 4.2.13.2 Device Performance

The graphene-based pyroelectric bolometer operates at RT with TCR up to ∼900%K−1 for a device area ∼300 × 300 μm2 (i.e., two orders of magnitude larger than state-of-the-art IR PDs having any similar or larger area [ 83, 86, 106, 105 ]) able to resolve temperature variations down to 15 μK at 1 Hz. For smaller devices, the TCR scales sub-linearly with area, due to an enhancement of the collected pyroelectric charge in close proximity to the metallic edges. When used as MIR PDs, the devices deliver very promising performance (in terms of responsivity, speed, and NEP) even on bulk substrates and are capable to detect warm bodies in their proximity. Spectral selectivity can be achieved by patterning resonant structures

References

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87 Tissot, J.L., Rothan, F., Vedel, C. et al. (1998). LETI/LIR’s amorphous silicon uncooled microbolometer development. Proc. SPIE 3379: 139–144. 88 Whatmore, R.W. (1986). Pyroelectric devices and materials. Rep. Prog. Phys. 49: 1335–1386. 89 Bonaccorso, F., Sun, Z., Hasan, T., and Ferrari, A.C. (2010). Graphene photonics and optoelectronics. Nat. Photon. 4: 611–622. 90 Ferrari, A.C., Bonaccorso, F., Fal’ko, V. et al. (2015). Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems. Nanoscale 7: 4587–5062. 91 Koppens, F.H.L., Mueller, T., Avouris, P. et al. (2014). Photodetectors based on graphene, other two-dimensional materials and hybrid systems. Nat. Nanotechnol. 9: 780–793. 92 Xia, F., Mueller, T., Lin, Y.-M. et al. (2009). Ultrafast graphene photodetector. Nat. Nanotechnol. 4: 839–843. 93 Mueller, T., Xia, F., and Avouris, P. (2010). Graphene photodetectors for high-speed optical communications. Nat. Photonics 4: 297–301. 94 Echtermeyer, T.J., Britnell, L., Jasnos, P.K. et al. (2011). Strong plasmonic enhancement of photovoltage in graphene. Nat. Commun. 2: 458. 95 Echtermeyer, T.J., Nene, P.S., Trushin, M. et al. (2014). Photo thermoelectric and photoelectric contributions to light detection in metal–graphene–metal photodetectors. Nano Lett. 14: 3733–3742. 96 Echtermeyer, T.J., Milana, S., Sassi, U. et al. (2015). Surface plasmon polariton graphene photodetectors. Nano Lett. 16: 8–20. 97 Vicarelli, L., Vitiello, M.S., Coquillat, D. et al. (2012). Graphene field effect transistors as room-temperature Terahertz detectors. Nat. Mater. 11: 865–871. 98 Hsu, A.L., Herring, P.K., Gabor, N.M. et al. (2015). Graphene-based thermopile for thermal imaging applications. Nano Lett. 15: 7211–7216. 99 Yao, Y., Shankar, R., Rauter, P. et al. (2014). High-responsivity mid-infrared graphene detectors with antennaenhanced photocarrier generation and collection. Nano Lett. 14: 3749–3754. 100 Badioli, M., Woessner, A., Tielrooij, K.J. et al. (2014). Phonon-mediated mid-infrared photoresponse of graphene. Nano Lett. 14: 6374–6381. 101 Vora, H., Kumaravadivel, P., Nielsen, B., and Du, X. (2012). Bolometric response in graphene based superconducting tunnel junctions. Appl. Phys. Lett. 100: 153507. 102 Yan, J., Kim, M.H., Elle, J.A. et al. (2012). Dual-gated bilayer graphene hot-electron bolometer. Nat. Nanotechnol. 7: 472–478. 103 Han, Q. et al. (2013). Highly sensitive hot electron bolometer based on disordered graphene. Sci. Rep. 3: 1–6. 104 Shao, Q., Liu, G., Teweldebrhan, D., and Balandin, A.A. (2008). High-temperature quenching of electrical resistance in graphene interconnects. Appl. Phys. Lett. 92: 202108. 105 Bae, J.J. et al. (2015). Sensitive photo-thermal response of graphene oxide for midinfrared detection. Nanoscale 7: 15695–15700.

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5 Pyroelectric Energy Harvesting 5.1 Introduction The ever-growing energy demand, escalating energy prices, and environmental concerns such as global warming compel us to look for cleaner and more sustainable energy sources. Thermal energy harvesting is a promising method for capturing freely available heat and converting it to a more usable form such as mechanical or electrical energy. The “free” heat is available to us primarily in two forms: waste heat and nature heat. It has been reported that more than half of the energy produced from various renewable and nonrenewable resources worldwide is rejected to the environment, mostly in the form of waste heat [1]. In addition, natural resources such as geothermal heat, volcanic heat, and solar heat are the enormous energy resources that remain untapped. Plenty of such thermal energy reserves exist all over the world, which release thousands of joules of energy every second into the ambient atmosphere [2]. Unarguably, a cost-effective method for recovering waste heat and utilizing natural heat to generate electricity can revolutionize the production of renewable energy. Based upon the temperature, heat is usually classified into three categories: high-grade (1200 ∘ F/649 ∘ C and higher), medium-grade (450 ∘ F/232 ∘ C–1200 ∘ F/ 649 ∘ C), and low-grade (450 ∘ F/232 ∘ C and lower) [3]. Normally, high- and medium-grade heat are easy to recover, whereas low-grade waste heat, which constitutes more than 50% of the total waste heat, is unfortunately the most difficult to recover [3]. Considering hot and cold reservoirs at 232 ∘ C and 25 ∘ C, respectively, it can be calculated that the Carnot efficiency of a low-grade heat recovery engine cannot be more than 41%. The Carnot efficiency decreases to 24% when the operating temperature is between 150 ∘ C and 50 ∘ C. Such low efficiency makes the recovery process economically unviable using traditional power cycles. In the past few decades, immense efforts have been made to explore and develop alternative technologies to capture low-grade heat. The low-grade thermal energy harvesting technologies are currently based upon the thermoelectric, pyroelectric, thermomagnetic, and thermoelastic effects (Figure 5.1). Among the above forms of stray energy, heat is ubiquitous and serves as a low-grade waste [5]. To convert thermal energy into usable electricity, thermoelectricity and pyroelectricity can Pyroelectric Materials: Physics and Applications, First Edition. Ashim Kumar Bain and Prem Chand. © 2023 WILEY-VCH GmbH. Published 2023 by WILEY-VCH GmbH.

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Thermoelectric

Requirements: temperature fluctuation (dT/dt ≠ 0) and alternating electric field Material: ferroelectrics E ∫o max pdE η= Th ∫ Emax pdE c+ (T –T ) o h

Pyroelectric

Requirements: thermal gradient (dT/dt ≠ 0) Phenomenon: seedbeck effect Materials: semiconductors

η=

Thermal energy harvesting

(Th–Tc) [√1 + ZT )–1] Th [√1 + ZT) + Tc ] Th

Thermomagnetic

Requirements: temperature fluctuation (dT/dt ≠ 0) and alternating magnetic field Materials: ferromagnetics μ ∮ η ≌ To H dM · p∫ h C (T)dT p Tc

c

Thermoelastic

Requirements: temperature fluctuation (dT/dt ≠ 0) and alternating stress Materials: shape memory alloys

η=

(Th–Tc)Δs–2A ThΔs+Cp(Th–Tc)–A

Figure 5.1 Relevant thermal energy harvesting technologies: thermoelectricity, pyroelectricity, thermomagneticity, and thermoelasticity. Source: Kishore and Priya [4]/figure 1 (p. 4)/MDPI/Licensed under CC BY 4.0.

be utilized. Thermoelectric materials have been employed to convert the spatial thermal gradient into electrical energy, that is, the Seebeck effect [4]. Meanwhile, pyroelectricity is a phenomenon in which temperature fluctuations in the environment are converted into electrical energy [6]. Pyroelectric materials need a temporal temperature gradient just as thermoelectric materials need a spatial gradient [4, 7]. Variation in the temperature of the pyroelectric material causes a net dipole moment, which further results in the accumulation of charges at the electrode, separating application targets such as small-scale microgenerators with small dimensions (small enough for spatial temperature fluctuations). Thermoelectric materials have a lower ZT (“Z” is Ioffe’s figure of merit) at room temperature. Thus, pyroelectric energy harvesting (PyEH) is preferable for harvesting low-grade thermal energy and at low temperatures. The typical application arena of pyroelectric energy conversion is a thermal sensor, which can detect thermal signals at the moment. However, it can also be used as a thermal energy harvester if the conversion efficiency and total converted energy are sufficiently high to charge electrical energy storage devices such as a supercapacitor or battery. Although the PyEH concept was introduced in the 1960s, it remains a comparatively less studied area [8–14]. The reports estimate that in 2009, over 50% of the total consumed energy was wasted as heat, which is mostly from electrical power generation and automobile systems [15]. In addition, a large amount of heat energy is lost through electrical appliances such as refrigerators, air conditioning systems, and heat pumps. Although the study of PyEH accounts for a just small fraction of the total amount of studies on pyroelectric materials, it can be noted from Figure 5.2 that

5.2 Theory of Pyroelectric Energy Harvesting

500 Others 400

Energy harvesting applications

300

7.03%

200

data till Jan, 2019

Number of papers

92.97%

100

0

2001

2004

2007

2010 Year

2013

2016

2019

Figure 5.2 The histogram distribution of the number of papers on the pyroelectric materials published in the last two decades. The inset image shows a pie chart for the number of papers reporting the application of thermal energy harvesters (red) through pyroelectricity. Source: Thakre et al. [16]/figure 1 (p. 2)/MDPI/Licensed under CC BY 4.0.

over the past few years, the number of research articles focused on it has increased. Further study of PyEHs would clearly benefit the utilization of this wasted heat. In the twenty-first century, wearable and implantable electronics have gained considerable attention [17–25]. To power these electronics, which are necessarily small, flexible, and endurable, on-board power sources are required. As with other autonomous devices, PyEHs could be the optimal powering solution for such devices. Several reports have demonstrated significant generated output power densities (in the range of ∼ μW cm−3 to mW cm−3 ) using pyroelectric energy conversion, which can be used to drive devices, such as liquid color displays, light-emitting diodes (LED), and wireless devices [26, 27]. Among various energy harvester candidates, although PyEHs have great potential for such applications, they are the least explored area. In addition, energy harvesters are clearly necessary for the flexibility of wearable and implantable devices. In recent years, many excellent individuals and institutions have been involved in the research and development of flexible pyroelectric or hybrid pyroelectric–piezoelectric energy harvesters [28–38].

5.2 Theory of Pyroelectric Energy Harvesting Pyroelectricity is a phenomenon in which the temperature fluctuation of pyroelectric material induces a change in polarization, which is used to generate electricity. Here, special attention should be paid to the term “temperature fluctuation”: it refers

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Current

Dipoles

dT >0 dt

Ps (a)

Ps

Ammeter

(c) Current

dT =0 dt

(b)

Ps

dT 0, then the thermal vibration causes a disturbance in the dipole alignment, and therefore, spontaneous polarization (Ps ) decreases and vice versa. Consequently, as shown in Figure 5.3c, the change in temperature of the material alters the quantity of bound charges. The redistribution of free charges to compensate the change in bound charge results in a current flow, termed as pyroelectric current in the circuit [39]. Also, when the pyroelectric material is cooled, that is, dT/dt < 0, the dipoles realign themselves, which further results in an enhancement in the Ps , as shown in Figure 5.3d. When the pyroelectric capacitive structure is connected to the load, a corresponding electric current flows (opposite direction) through the load [40]. Therefore, by cycling the temperature of pyroelectric materials, we can generate AC.

5.2 Theory of Pyroelectric Energy Harvesting

Assuming that there is a pyroelectric material with a temperature gradient dT, then the polarization change dP occurring in materials and pyroelectric coefficient p can be defined as [41]: ( ) dPs p= (5.1) dT 𝜎,E where 𝜎 and E denote the stress and electric field, respectively, and the letters in subscripts correspond to constant conditions. Ps is the electrical polarization of the pyroelectric material that sandwiched between two metal electrodes, forming a parallel-plate capacitive structure with the poling direction normal to the electrode plates and cross-sectional area A. Assuming a homogeneous pyroelectric material (constant pyroelectric coefficient) for which the temperature T at any time is uniform, the electric current generated (Figure 5.3c, d) from the pyroelectric effect is given as [41, 42]: ip =

dQ dT = pA dt dt

(5.2)

where Q denotes the pyroelectric charge, ip represents the pyroelectric current, A is the surface area of the pyroelectric material, and dT/dt is the rate of temperature change. It should be noted that the current obtained from Eq. (5.2) is under a short-circuit condition and the electrodes of the capacitor are positioned as normal to the polar direction. In addition, Eq. (5.2) shows that the pyroelectric current is proportional to the surface area of the material and is independent of its thickness. This happens because the current is simply the rate of change of surface charge, Q, and has no relationship with the volume of the material. Using Eq. (5.2), the net charge developed on the electrodes of the capacitor in Figure 5.3 can be calculated as: Q=

∫

ip dt = pAΔT

(5.3)

where ΔT is the temperature change. The equivalent capacitance (C) of the capacitor is given by [43]: C=

A𝜀𝜎33 h

(5.4)

where 𝜀𝜎33 is the permittivity in the polarization direction at constant stress and h is the thickness of the material. Using Eqs. (5.3, 5.4), we can calculate open-circuit voltage, V OC , and total energy, TE, stored in a capacitor as: Voc =

Q phΔT = 𝜎 C 𝜀33

(5.5)

TE =

1 p2 Ah(ΔT)2 1 2 CVoc = 2 2 𝜀𝜎33

(5.6)

and

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5.3 Pyroelectricity in Ferroelectric Materials As we categorize the different types of dielectrics, all pyroelectric materials are piezoelectric, but not all piezoelectric materials are pyroelectric. Also, all ferroelectric materials are both pyroelectric and piezoelectric [43]. Ferroelectric materials can switch the orientation of their spontaneous polarization when the direction of the applied alternating electric field is reversed. However, reversal is often accompanied with ferroelectric hysteresis (Figure 5.4a). Most ferroelectric materials exhibit a transition temperature (called Curie temperature, T c ) above which they transform to nonferroelectric or paraelectric phase. At Curie temperature, the spontaneous polarization of a ferroelectric material ceases to zero. Ferroelectric materials are of special interest in the field of pyroelectrics because the pyroelectric coefficient (gradient of polarization versus temperature, dP/dT) is very high near Curie temperature (as shown in Figure 5.4b).

5.3.1

Thermodynamic Cycles of PyEH

There are variety of thermodynamic cycles that have been proposed in the literature [44–46] for PyEH. Some of the important PyEH cycles, such as Carnot cycle [47, 48], Ericson cycle [49, 50], and Olsen cycle [4, 16], are briefly described here. 5.3.1.1 Carnot Cycle

The Carnot cycle is considered an ideal cycle in thermodynamics, and it imposes a maximum limit on the efficiency of a heat engine. The pyroelectric Carnot cycle, such as the gas Carnot cycle, consists of two isothermal processes and two adiabatic processes. As shown in Figure 5.5a, paths 1–2 and 3–4 represent isothermal processes and paths 2–3 and 4–1 represent adiabatic processes. Here, state 1 represents the initial condition of the pyroelectric material under no external electric field and p(T) =

T>TC Electric field (E)

(a)

(b)

dP dT

E increasing

E=0

T 0)

0.2

0.6 0.4 Time (ms)

0.8

1

Figure 5.16 (a) Structure diagram of an ultrathin (45 μm) p-Si/n-ZnO heterojunction PD. (b) Optical image of the flexible PD. (c) Short-circuit photoresponse current of the ultrathin (45 μm) p-Si/n-ZnO heterojunction device under zero bias by periodically turning on/off 325-nm UV illumination with a frequency of 1 kHz. (d) Dynamic response characteristics under 1064-nm NIR illumination. (e) Schematic diagram of a PENG powering different loads. Source: Chen et al. [111].

The output current through the transformer can be used to power LEDs (inset in Figure 5.16e). The rated power of the green LED is 0.06 W and the corresponding rated current is 20 mA. The rated power of the red LED is about 0.04 W, and the corresponding rated current is also 20 mA. The self-powered PDs exhibit high responsivity (1200 mA/W), high detectivity (1013 Jones), and fast response speed (𝜏 r = 18 μs and 𝜏 f = 25 μs) under UV illumination. High and stable short-circuit output currents at each wavelength illumination range from UV to NIR demonstrate that the device can realize full-spectrum optical communication. Based on the investigation of directly powering the LEDs, it can be concluded that the p-Si/n-ZnO NWs heterojunction device not only can realize self-powered full-spectrum (UV–visible–NIR) optical sensing but can also serve as a power source (PENG) transforming thermal energy into electrical energy to power other loads. 5.5.2.2 PZT-Based Pyroelectric Nanogenerators

Self-powering nanotechnology has been developed since 2005 with the aim to build self-powered systems that can operate independently and wirelessly without the use of a battery or other energy storage/supply systems. Yang et al. [112] demonstrate a PyNG based on a PZT film, which has a pyroelectric coefficient of about −80 nC cm−2 K−1 (Figure 5.17a). A 300-nm-thick Ni layer as the electrode was deposited on both the top and bottom of the PZT film. They applied a high voltage of larger than 4 kV across the PZT film for electric poling at room temperature. Two Cu wires were used to connect the device and the electrical measurement system,

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5 Pyroelectric Energy Harvesting

(a)

(b)

(c)

(d)

(e)

Figure 5.17 PZT-based PyNGs. (a) Photo and SEM image of PZT-based PyNG. (b) Schematic illustration of single PZT microwire-based PyNG. (c) Real photo and schematic image of PZT-based hybrid PENG and PyNG and XRD analysis of PZT thin film. (d) Schematic image of the hybrid PDMS nanowire-based TENG and PZT film-based PyNG. (e) Schematic diagram of one structure-based hybrid NG. Source: Ryu and Kim [61].

where the Cu wires were fixed on the surface of the device by the silver paste. A Kapton tape was used to cover the silver paste to keep a good contact between the Cu wire and the Ni electrode. The TiO2 nanotubes were grown by electrochemically anodizing Ti foil in ethylene glycol solution, which was used as the anode material of the Li-ion battery. The cathodes are LiCoO2 /conductive carbon/binder mixtures on aluminum foils. The polyethylene film was used as the separator material. For a temperature change of 45 K at a rate of 0.2 K/s, the output open-circuit voltage and short-circuit current density of the PENG reached 22 V and 171 nA cm−2 , respectively, corresponding to a maximum power density of 0.215 mW cm−3 . The theoretical calculation and analysis indicate that the high-output performance of the PENG is associated with the large pyroelectric coefficient, the change in temperature, and the thickness of the film. A single electrical output pulse can directly drive an LCD for longer than 60 seconds. A Li-ion battery was charged by the PENG at different working frequencies from 0.005 to 0.02 Hz, which was used to drive a green LED. The demonstrated PENG shows potential applications in wireless sensors. Yang et al. [113] demonstrated the first application of a PZT micro/NW-based PyNGas a self-powered sensor (or active sensor) for detecting a change in temperature of a fingertip (Figure 5.17b).A PZT micro/nanowire was achieved by pressing of a bulk PZT, and a PZT micro/nanowire was poled by the applied voltage of 3.5 kV. Two ends of a PZT microwire were fixed by Ag paste on a glass substrate, and PDMS layer was covered on the device to avoid atmosphere, contamination, and corrosive effect. Peak output voltage and current of the PyNG were 60 mV and 0.6 nA under 1.5 K s−1 temperature change condition, respectively. To utilize the PyNG as a sensor application, it was attached on the metallic body and contact/separate with

5.5 Pyroelectric Nanogenerators

heat source. The response time and reset time of the sensor were about 0.9 and 3 seconds, respectively. The PyNG was evaluated under different temperature conditions of heat source. It was able to expect that output voltage increased with an increasing heat source temperature. The linear relationship indicated good performance for temperature sensor and linear change of the output voltage was good to calibrate temperature change rate. They successfully demonstrate the detection of a finger touch and moving out and measured the surface temperature of the finger. If temperature of heat source was too high, the output voltage of the PyNG was over 3 V, which was able to drive LCD using a single PZT micro/NW. The self-powered temperature sensors developed have potential applications in temperature measurements in environmental sciences, safety monitoring, medical diagnostics, and more. Ko et al. [114] reported a flexible PZT film-based hybrid PPENG for simultaneously scavenging a mechanical vibration and thermal fluctuation in ambient and harsh environments (Figure 5.17c). A highly flexible Ni − Cr metal foil substrate with a conductive LaNiO3 bottom electrode enables the growth of flexible PZT film having high piezoelectric (140 pC N−1 ) and pyroelectric (50 nC cm−2 K−1 ) coefficients at room temperature. As a result of the simultaneous mechanical bending with cooling and unbending with heating, the PZT film-based hybrid NG successfully scavenges mechanical and thermal energies from various power sources including a human finger and cold/hot winds in ambient atmosphere. While the hybrid NG was touched, the temperature change was 1.5 K with the rate of 1.K s−1 . Peak output voltage and current of the PyNG were 0.05 V and 0.1 μA cm−2 , respectively. Peak output voltage and current of the PENG were 0.3 V and 0.3 μA cm−2 , respectively, under 0.4% strain condition. The hybrid NG generated the peak voltage and current of 0.34 V and 0.34 μA cm−2 , respectively. Output performance of the wind-driven hybrid NG was almost linearly increased with wind velocity and temperature change rate. The PZT film-based hybrid NG stably also operates even at high humidity of 70%, strong pH of 13, and elevated temperature of 100 ∘ C, that is, in harsh environments; under a strong UV source; however, the hybrid NG generates slightly lowered power, mainly due to the reduction of piezoelectric component rather than pyroelectric component. As proof of power generation under harsh environments, Ko et al. [114] demonstrated the generation of extremely high current at the exhaust pipe of a car, where hot CO and CO2 gases are rapidly expelled to air. This work expands the application of flexible PZT film-based NG for the scavenging mechanical vibration and thermal fluctuation energies even at extreme conditions. Yang et al. [115] demonstrated a PZT-based hybrid NG, which can simultaneously/individually harvest the mechanical and thermal energies for self-powered degradation of methyl orange (MO) without using an external power source (Figure 5.17d). The hybrid energy cell was fabricated by PZT-based PENG and PDMS -based TENG. By using the flexible PDMS nanowire array, the fabricated TENG at the top was used to harvest the mechanical energy, and a PZT film-based PENG at the bottom was used to harvest the thermal energy. Peak output voltage and current of the TENG were 12 V and 0.2 μA, respectively, and peak output current of the PENG was 0.3 μA. The mechanical energy produced by the TENG was stored

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in a Li-ion battery, which can be used for degradation of MO by electrocatalytic oxidation. Moreover, the PENG was directly used for degradation of MO, where the degradation percentage is up to 80% after 144 hours. The fabricated hybrid energy cells have the potential electrochemical applications in the self-powered electrodeposition, pollutant degradation, corrosion protection, and water splitting. Zhang et al. [116] designed a one-structure-based multieffects-coupled NG with high-degree integration of pyroelectric, photoelectric, triboelectric, and piezoelectric effects to simultaneously scavenge the thermal, solar, and mechanical energies whenever and wherever one or all of the energy resources are available (Figure 5.17e). Integration of a PyNG, solar cell, TENG, and PENG into one device was able to efficient environmental energy harvesting. PZT block was used as an active material of piezoelectric, pyroelectric, and photoelectric; polyamide was used as flexible vibrating film for triboelectric material and applied strain to PZT; Ag, ITO, and Ag NW-imbedded PDMS film were used as electrodes; thermoelectric module was integrated to convert thermal energy into electric energy. Due to one structure and same output electrodes, simultaneous or individual outputs were integrated. The PyNG generated peak voltage and current of 100 V and 480 nA, respectively. The solar cell generated voltage and current of 60 V and 890 nA with the illumination-induced heat effect, respectively, but the solar cell without the illumination-induced heat effect generated voltage and current of 48 V and 170 nA, respectively. The TENG and PENG hybrid NG generated a peak current of 3.8 μA under an airflow speed of 15 m s−1 , but the voltage was not measurable. When all energy harvesting systems harvest energy, they were able to provide a charge of 71 μC. For practical application, they charged a 10-μF capacitor to 5.1 V in 90 seconds. Therefore, one structure based on various integrated energy harvesting systems realizes maximizing energy scavenging from environment. 5.5.2.3 Lead-Free Ceramic-Based Pyroelectric Nanogenerators

To achieve wearable PyNG applications, the nontoxic property of pyroelectric materials is crucial for biocompatibility. Thus, PZT-based PyNGs have high-output performance due to its superior piezoelectric and pyroelectric properties, but the development of lead-free pyroelectric materials is inevitably required. Yang et al. [117] reported a single crystalline lead-free KNbO3 NW/PDMS polymer composite-based PyNG for the first time (Figure 5.18). The KNbO3 nanowires were synthesized by hydrothermal process. For flexible property of the PyNG, PDMS and KNbO3 nanowires were mixed with a volume ratio of 7 : 3. The mixed PDMS was spin coated on ITO substrate and Ag was deposited as a top electrode. A transmission electron microscopy (TEM) measurement and corresponding selected area electron diffraction (SAED) pattern analysis confirmed the phase of the perovskite structure. In the combined SAED patterns and HR-TEM image, the crystal direction of nanowire was along the [011] direction. The mixed nanowires were randomly oriented and dispersed in PDMS without aggregations. Peak output voltage and current of the PyNG were 10 mV and 120 pA under a temperature change rate of 2 K s−1 , respectively. Although PDMS decreases the volume efficiency of the PyNG, it realized the flexibility of the PyNG, which was important for practical applications.

5.5 Pyroelectric Nanogenerators

Ag 50 nm

[011] [011]

ITO (a)

dT/dt ≠ 0

(b)

[011]

2 nm

500 nm (d)

200 µm

(c)

(e)

2 µm

Figure 5.18 (a) Schematic diagram showing the structure of the pyroelectric nanogenerator. (b) TEM image of a single KNbO3 nanowire, the corresponding SAED pattern of the nanowire, and the corresponding HR-TEM image of the nanowire. (c) SEM image of a bent KNbO3 -PDMS composite film. (d) SEM image of a single KNbO3 nanowire. (e) SEM image of the enlarged cross section of the KNbO3 -PDMS composite film in (c) [61, 117]. Source: Yang et al. [117].

The output voltage/current of the fabricated nanogenerators can be modulated in a controlled manner due to the change of ferroelectric domains tuned by electric fields. The voltage/current outputs of the nanogenerators under heating and cooling conditions show an opposite change and can be enhanced by increasing the change in temperature. By using the nanogenerators, energy harvesting from sunlight induced heat and hybrid nanogenerators with a solar cell were demonstrated, indicating that the PyNGs may have extensive applications in self-powered nanodevices and nanosystems. Isakov et al. [118] reported a flexible PyNG based on hybrid ferroelectric 1,4-diazabicyclo[2.2.2]octane perrhenate (dabcoHReO4 ) nanofibers (Figure 5.19). The ferroelectric nanoparticles are embedded into the fibers being naturally aligned with the major polarization component along the fiber axis. The crystalline structure of dabcoHReO4 was examined by XRD, and the most intensive Bragg reflection was (200) followed by (110) in the fibers. Thus, the orientations of the dabcoHReO4 fibers were all aligned in one direction and had the same polarization. Raman spectroscopy also confirmed a single crystal orientation of dabcoHReO4 fibers. Peak current output performance of the PyNG was 200 pA at the temperature change rate of 0.2 K s−1 and provides a voltage above 100 mV under a moderate strain level. To evaluate the pyroelectric coefficient, the PyNG was constant heating

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300

Z, nm

0

0

2.5 X, µm

5 50

deff, a.u.

206

0

–50 0

1

2 0 length, µm

1

2

–20

0 UDC, V

20

Figure 5.19 Surface morphology and piezoresponse force microscopy analysis of the dabcoHReO4 nanofiber-based PyNG. Source: Ryu and Kim [61].

and cooling, and the corresponding pyroelectric coefficient was 8.5 μC m−2 K−1 . The results show that the nanofibers based on dabcoHReO4 have a great potential for pyroelectric and piezoelectric autonomous energy harvesting with natural advantages such as biocompatibility, flexibility, low cost, and easy fabrication. Ji et al. [119] reported a multieffect-coupled hybrid NG based on ferroelectric BTO ceramic (Figure 5.20). It promotes the ability to simultaneously scavenging thermal, solar, and mechanical energies. The BTO ceramic disk was fabricated by dry press process for piezoelectric, pyroelectric, and photoelectric material. BTO was covered by PDMS as a protective layer and a FEP film was attached on the PDMS as a triboelectric material; a nylon film attached on acrylic was used as counter triboelectric material; commercial thermoelectric module was used for thermal energy harvester. XRD patterns of the BTO confirmed tetragonal phase of BTO, which was having ferroelectric property. By integration of a PyNG, a photovoltaic cell, and a TENG–PENG in one structure with only two electrodes, multieffects interact with each other to alter the electric output, and a complementary power source with peak current of ≈1.5 μA, peak voltage of ≈7 V, and platform voltage of ≈6 V were successfully achieved. The calculated pyroelectric coefficient of the hybrid NG was 26 nC cm−2 K−1 . The coupling effect between PyNG and solar cell

5.5 Pyroelectric Nanogenerators

Multi-effects coupled nanogenerator (PENG + PVC + TPiENG)

PENG + PVC

PENG (pyroelectric effect) PENG + TPiENG

PVC (photovoltaic effect)

TPiENG (triboelectricpiezoelectric effect)

5 mm (a)

PVC + TPiENG

(c)

Nylon PDMS

ITO BTO

FEP

Ag

TE module Acrylic

(b)

Figure 5.20 et al. [119].

Schematic and real photo images of BTO-based hybrid NG [61, 119]. Source: Ji

was evaluated by simultaneous heating/cooling and light irradiation. The enhancement ratio of the output current was 86% by coupling effect. They successfully demonstrated charging a 0.33-μF capacitor up to 1.1 V in 10 seconds using the coupled NG. Compared with traditional hybridized nanogenerators with stacked architectures, the one-structure-based multieffect-coupled NG is smaller, simpler, and less costly, showing prospective in practical applications and represents a new trend of all-in-one multiple energy scavenging. Qi et al. [120] reported a photovoltaic-pyroelectric coupling effect-based self-powered PD using BFO (BiFeO3 ) ferroelectric material (Figure 5.21). BFO powders were synthesis by hydrothermal method, and XRD measurement confirmed perovskite phase of the BFO. The Schottky barrier was formed at the interface of ITO/BFO and BFO/Ag. When the temperature change rate was 0.2 K s−1 under 450-nm light illumination condition, the ITO/BFO/Ag hybrid NG generated 0.13 V and 8.8 nA, but the photovoltaic current was 2 nA. Due to the differing optimum load resistances of the PyNG and solar cell, the hybrid NG had two optimum load resistances, which were the same as the optimum load resistance of PyNG and solar cell. The responsibility of the hybrid NG was 978% larger than normal solar cell, and the recovery time of the hybrid NG was 0.8 seconds after 450-nm light irradiation. They successfully demonstrated 3 × 3 photodetectors array for real-time signals mapping on a BFO disk with high resolution. Therefore, lead-free pyroelectric materials are also able to be used in various applications instead of PZT.

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BFO

Sputtering Ag Polarization P Bi2Fe4O9 875 °C – 1 h

850 °C – 1 h

Intensity (a.u.)

208

Ag Polishing

850 °C – 1 h 750 °C – 1 h As-grown

P Ag Sputtering

JCPDS NO.74–2016

BiFeO3

20

30

40

50 60 2θ (Deg.)

70

80

ITO

P

Ag

Figure 5.21 SEM and XRD analysis of BFO and schematic fabrication process of BFO-based PyNG [61, 120]. Source: Qi et al. [120].

5.5.3

Thermal Nanophotonic-Pyroelectric Nanogenerators

At present, there are various limitations to harvesting ambient waste heat, which include the lack of economically viable material and innovative design features that can efficiently recover low-grade heat for useful energy conversion. Wang et al. [121] designed a thermal nanophotonic-pyroelectric (TNPh-pyro) scheme consisting of a metamaterial multilayer and pyroelectric material, which performs synergistic waste heat rejection and photothermal heat-to-electricity conversion (Figure 5.22).

5.5 Pyroelectric Nanogenerators

Figure 5.22 Schematic fabrication process of the thermal nanophotonic and pyroelectric coupled TNPh, and practical application of TNPh-pyro device [61, 121]. Source: Wang et al. [121].

Unlike any other pyroelectric configuration, this conceptual design deviates from the conventional by deliberately employing back-reflecting NIR to enable waste heat reutilization/recuperation to enhance pyroelectric generation, avoiding excessive solar heat uptake and also retaining high visual transparency of the device. Typically, PyNGs had limited thermal fluctuation [122], but TNPh-pyro effect was able to deviate the problem by deliberate channeling of the reflected NIR heat onto PVDF [123]. The TNPh structure consisted of periodically layered titanium dioxide (TiO2 ) and mesoporous silicon dioxide (SiO2 ) films and topmost layer of mesoporous TiO2 /Cu. A ferroelectric PVDF film was a representative as an outdoor solar heat harvester and the TNPh layer reflected NIR irradiation, absorbed UV and transmitted visible light for cooling, air purification, and lighting [124]. The crystal structures of TiO2 and SiO2 were confirmed by XRD characterization and the NIR

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reflection characteristic of TNPh-pyro device showed that the NIR transmittance was 35% and that the visible light transmittance was 69.5%, which satisfied the typical transparency requirements of a window. Compared with bare glass and the TNPh-pyro device under 100 mW cm−2 solar irradiation, bare glass heated up to 33.1 ∘ C and the TNPh-pyro device heated up to 29 ∘ C, which was able to save up to 45% of energy for the building’s cooling [125]. In addition, UV irradiation on TiO2 /Cu film decreased ethanol, IPA, and formaldehyde, and the TNPh-pyro device generated peak voltage and current of 8.2 V and 23.5 nA, respectively, that was 50% enhanced output performance than pristine PVDF film. Wang et al. [121] successfully demonstrated that a TNPh-pyro-roofed house had 4.2 ∘ C lower internal temperature than glass-roofed house with harvesting peak voltage and current of 32 V and 126 nA, respectively. Another requirement of the improving PyNGs is innovation of pyroelectric material. Although rapid temperate change rate control is able to increase output performance, the most important factor is pyroelectric coefficient, which is the unique property of material. To date, the PyNGs have shown wide discrepancies in theoretical and experimental output performances due to imperfect material structure and property. Thus, heat transfer, internal polarization change, defects, working temperature condition, and other physical parameters are necessarily improved.

5.5.4

Challenges and Perspectives of Pyroelectric Nanogenerators

PyEH systems have recently received substantial attention for their potential applications as power generators. In particular, the pyroelectric effect, which converts thermal energy into electrical energy, has been utilized as an IR sensor, but upcoming sensor technology that requires a miniscule amount of power is able to utilize PyNGs as a power source. Herein, an overview of the progress in the development of PyNGs for an energy harvesting system that uses environmental or artificial energies such as the sun, body heat, and heaters is described. Polymer-based PyNGs are flexible, stretchable, and biocompatible, so it is more suitable to use as wearable applications. Ceramic-based PyNGs have higher pyroelectric coefficient; thus, its output power performance is much better than polymer-based PyNGs, and more suitable for self-powered IoT systems. To increase flexibility of PyNGs, micropatterned structure, NW structure, and fiber structure are effective approaches. Pyroelectric ceramic nanoparticle/NW embedded in flexible polymer is also able to have high flexibility. With the future development of pyroelectric materials, PyNGs may become important new energy harvesting technologies, but there remain the following challenges: Devices should ensure long cycle thermal and sun irradiation stability. If substrate or electrode cannot maintain the mechanical property by thermal and sun light induced damage, the life of system will be limited even pyroelectric material can endure the thermal and sun irradiation induced damage. In addition, extremely high humidity in the atmosphere can react with pyroelectric materials and degrade its performance. Furthermore, spontaneous polarization of pyroelectric materials is depleted at

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79 Tamang, A., Ghosh, S.K., Garain, S. et al. (2015). DNA-assisted β-phase nucleation and alignment of molecular dipoles in PVDF film: a realization of self-poled bioinspired flexible polymer nanogenerator for portable electronic devices. ACS Appl. Mater. Interfaces 7: 16143–16147. 80 Grim, S.O. and Matienzo, L.J. (1975). X-ray photoelectron spectroscopy of inorganic and organometallic compounds of molybdenum. Inorg. Chem. 14: 1014–1018. 81 Seyama, H. and Soma, M. (1986). X-ray photoelectron spectroscopic study of the effect of heating on montmorillonite containing sodium and potassium cations. Clays Clay Miner. 34: 672–676. 82 Kim, Y.J. and Park, C.R. (2002). Analysis of problematic complexing behavior of ferric chloride with N, N-dimethylformamide using combined techniques of FT-IR, XPS, and TGA/DTG. Inorg. Chem. 41: 6211–6216. 83 Sugama, T., Kukacka, L.E., Carciello, N., and Hocker, N.J. (1989). Study of interactions at water-soluble polymer/Ca (OH)2 or gibbsite interfaces by XPS. Cem. Concr. Res. 19: 857–867. 84 Xue, H., Yang, Q., Wang, D. et al. (2017). A wearable pyroelectric nanogenerator and self-powered breathing sensor. Nano Energy 38: 147–154. 85 Mokhtari, F., Latifi, M., and Shamshirsaz, M. (2016). Electrospinning/electrospray of polyvinylidene fluoride (PVDF): piezoelectric nanofibers. J. Text. Inst. 107: 1037–1055. 86 Lee, J.H., Lee, K.Y., Gupta, M.K. et al. (2014). Highly stretchable piezoelectric-pyroelectric hybrid nanogenerator. Adv. Mater. 26: 765–769. 87 Ghosh, S., Calizo, I., Teweldebrhan, D. et al. (2008). Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits. Appl. Phys. Lett. 92: 151911. 88 Balandin, A.A., Ghosh, S., Bao, W. et al. (2008). Superior thermal conductivity of single-layer graphene. Nano Lett. 8: 902–907. 89 You, M.-H., Wang, X.-X., Yan, X. et al. (2008). A self-powered flexible hybrid piezoelectric–pyroelectric nanogenerator based on non-woven nanofiber membranes. J. Mater. Chem. A 6: 3500–3509. 90 Sun, J.-G., Yang, T.-N., Wang, C.-Y., and Chen, L.-J. (2018). A flexible transparent one-structure tribo-piezo-pyroelectric hybrid energy generator based on bio-inspired silver nanowires network for biomechanical energy harvesting and physiological monitoring. Nano Energy 48: 383–390. 91 Zhang, Q., Liang, Q., Zhang, Z. et al. (2018). Electromagnetic shielding hybrid nanogenerator for health monitoring and protection. Adv. Funct. Mater. 28: 1703801. 92 Ding, Y., Zhang, Z., Luo, B. et al. (2017). Investigation on the broadband electromagnetic wave absorption properties and mechanism of Co3 O4 -nanosheets/reduced-graphene-oxide composite. Nano Res. 10: 980–990. 93 Genuis, S.J. (2008). Fielding a current idea: exploring the public health impact of electromagnetic radiation. Public Health 122: 113–124.

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110 Wang, X., Dai, Y., Liu, R. et al. (2017). Light-triggered pyroelectric nanogenerator based on a pn-junction for self-powered near-infrared photosensing. ACS Nano 11: 8339–8345. 111 Chen, L., Dong, J., He, M., and Wang, X. (2020). A self-powered, flexible ultra-thin Si/ZnO nanowire photodetector as full-spectrum optical sensor and pyroelectric nanogenerator. Beilstein J. Nanotechnol. 11: 1623–1630. 112 Yang, Y., Wang, S., Zhang, Y., and Wang, Z.L. (2012). Pyroelectric nanogenerators for driving wireless sensors. Nano Lett. 12: 6408–6413. 113 Yang, Y., Zhou, Y., Wu, J.M., and Wang, Z.L. (2012). Single micro/nanowire pyroelectric nanogenerators as self-powered temperature sensors. ACS Nano 6: 8456–8461. 114 Ko, Y.J., Kim, D.Y., Won, S.S. et al. (2016). Flexible Pb(Zr0.52 Ti0.48 )O3 films for a hybrid piezoelectric-pyroelectric nanogenerator under harsh environments. ACS Appl. Mater. Interfaces 8: 6504–6511. 115 Yang, Y., Zhang, H., Lee, S. et al. (2013). Hybrid energy cell for degradation of methyl orange by self-powered electrocatalytic oxidation. Nano Lett. 13: 803–808. 116 Zhang, K., Wang, S., and Yang, Y. (2017). A one-structure-based piezo-tribo-pyro-photoelectric effects coupled nanogenerator for simultaneously scavenging mechanical, thermal, and solar energies. Adv. Energy Mater. 7: 1601852. 117 Yang, Y., Jung, J.H., Yun, B.K. et al. (2012). Flexible pyroelectric nanogenerators using a composite structure of lead-free KNbO3 nanowires. Adv. Mater. 24: 5357–5362. 118 Isakov, D., de Matos Gomes, E., Almeida, B. et al. (2014). Energy harvesting from nanofibers of hybrid organic ferroelectric dabco HReO4. Appl. Phys. Lett. 104: 032907. 119 Ji, Y., Zhang, K., and Yang, Y. (2017). A one-structure-based multieffects coupled nanogenerator for simultaneously scavenging thermal, solar, and mechanical energies. Adv Sci (Weinh) 5 (2): 1700622. 120 Qi, J., Ma, N., and Yang, Y. (2018). Photovoltaic–pyroelectric coupled effect based nanogenerators for self-powered photodetector system. Adv. Mater. Interfaces 5: 1700622. 121 Wang, X.Q., Tan, C.F., Chan, K.H. et al. (2017). Nanophotonic-engineered photothermal harnessing for waste heat management and pyroelectric generation. ACS Nano 2017 (11): 10568–10574. 122 Ugur, G., Chang, J., Xiang, S. et al. (2012). A near-infrared mechano responsive polymer system. Adv. Mater. 24: 2685–2690. 123 Shi, X., Jeong, H., Oh, S.J. et al. (2016). Unassisted photoelectrochemical water splitting exceeding 7% solar-to-hydrogen conversion efficiency using photon recycling. Nat. Commun. 7: 11943.

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6 Pyroelectric Fusion 6.1 Introduction Pyroelectric fusion refers to the technique of using pyroelectric crystals to generate high strength electrostatic fields to accelerate deuterium ions (tritium might also be used someday) into a metal hydride target also containing deuterium (or tritium) with sufficient kinetic energy to cause these ions to undergo nuclear fusion. It was reported in April 2005 by a team at the University of California, Los Angeles (UCLA). The scientists used a pyroelectric crystal heated from −34 to 7 ∘ C (−29 to 45 ∘ F), combined with a tungsten needle to produce an electric field of about 25 GV m−1 to ionize and accelerate deuterium nuclei into an erbium deuteride (ErD2 ) target. Though the energy of the deuterium ions generated by the crystal has not been directly measured, the authors used 100 keV (a temperature of about 109 K) as an estimate in their modeling [1]. At these energy levels, two deuterium nuclei can fuse together to produce a helium-3 nucleus, a 2.45-MeV neutron, and bremsstrahlung. Although it makes a useful neutron generator, the apparatus is not intended for power generation since it requires far more energy than it produces (UCLA Crystal Fusion. http://rodan.physics.ucla.edu/pyrofusion/).

6.2 History of Pyroelectric Fusion The process of light ion acceleration using electrostatic fields and deuterium ions to produce fusion in solid deuterated targets was first demonstrated by Cockcroft and Walton in 1932 (https://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_ generator) (Cockcroft–Walton generator). Indeed, the process is used today in thousands of miniaturized versions of their original accelerator, in the form of small sealed tube neutron generators, in the petroleum exploration industry. The process of pyroelectricity has been known from ancient times [2]. The first use of a pyroelectric field to accelerate deuterons was in 1997 in an experiment conducted by Drs. V.D. Dougar Jabon, G.V. Fedorovich, and N.V. Samsonenko [3]. This group was the first to utilize a lithium tantalate (LiTaO3 ) pyroelectric crystal in fusion experiments. The novel idea with the pyroelectric approach to fusion is in its application Pyroelectric Materials: Physics and Applications, First Edition. Ashim Kumar Bain and Prem Chand. © 2023 WILEY-VCH GmbH. Published 2023 by WILEY-VCH GmbH.

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of the pyroelectric effect to generate the accelerating electric fields. This is done by heating the crystal from −34 ∘ C to +7 ∘ C over a period of a few minutes. In April 2005, a UCLA team headed by chemistry professor James K. Gimzewski and physics professor Seth Putterman utilized a tungsten probe attached to a pyroelectric crystal to increase the electric field strength [1]. Brian Naranjo, a graduate student working under Putterman, conducted the experiment demonstrating the use of a pyroelectric power source for producing fusion on a laboratory bench top device [4] as shown in Figure 6.1. The device used a LiTaO3 pyroelectric crystal to ionize deuterium atoms and to accelerate the deuterons toward a stationary ErD2 target. When deuterium nuclei hit target, some of them collide head-on with innocent neutral deuterium atoms embedded in the target. Some of these collisions result in a violent nuclear reaction: nuclear fusion. The electrostatic field of the crystal is used to generate and accelerate a deuteron beam (>100 keV and >4 nA), which, upon striking a deuterated target, produces a neutron flux over 400 times the background level. The presence of neutrons from the reaction D + D → 3 He (820 keV) + n (2.45 MeV) within the target is confirmed by pulse shape analysis and proton recoil spectroscopy. As further evidence for this fusion reaction, the team used a novel time-of-flight technique to demonstrate the delayed coincidence between the outgoing 𝛼-particle and the neutron. Although the reported fusion is not useful in the power-producing sense, they anticipated that the system will find application as a simple palm-sized neutron generator. A team at Rensselaer Polytechnic Institute (RPI), led by Yaron Danon and his graduate student Jeffrey Geuther, improved upon the UCLA experiments using a device with two pyroelectric crystals and capable of operating at noncryogenic temperatures [5, 6]. Nuclear D–D fusion driven by pyroelectric crystals was proposed by Naranjo and Putterman in 2002 [7]. It was also discussed by Brownridge and Shafroth in 2004 [8]. The possibility of using pyroelectric crystals in a neutron production device (by D–D fusion) was proposed in a conference paper by Geuther and Danon in 2004 [9] and later in a publication discussing electron and ion acceleration by pyroelectric crystals [10]. None of these later authors had prior knowledge of the earlier 1997 experimental work conducted by Dougar Jabon, Fedorovich, and Samsonenko, which mistakenly believed that fusion occurred within the crystals [3]. The key ingredient of using a tungsten needle to produce sufficient ion beam current for use with a pyroelectric crystal power supply was first demonstrated in the 2005 Nature paper, although in broader context tungsten emitter tips have been used as ion sources in other applications for many years. In 2010, it was found that tungsten emitter tips are not necessary to increase the acceleration potential of pyroelectric crystals; the acceleration potential can allow positive ions to reach kinetic energies between 300 and 310 keV [11]. Pyroelectric fusion has been hyped in the news media [12], which has overlooked the earlier experimental work of Dougar Jabon, Fedorovich, and Samsonenko [3]. Pyroelectric fusion is not related to the earlier claims of fusion reactions, having been observed during sonoluminescence (bubble fusion) experiments conducted under the direction of Rusi Taleyarkhan of Purdue University [13]. In fact, Naranjo of the

6.2 History of Pyroelectric Fusion

Lithium tantalate crystal 1.5 mm lead shield

Deuterated target

Copper mesh Macor ring Thermocouple 1 µm

1 cm

(a)

(b)

Heater

(c)

(d)

Figure 6.1 Experiment geometry. (a) Calculated equipotential and D+ trajectories for a crystal charged to 100 kV; calculations were performed using finite-element methods. The grounded copper mesh (85% open area, 19.8 mm wire; vertical dashed line) shields the Faraday cup (right). The cup and target are connected to a Keithley 6485 pico-ammeter and biased to +40 V to collect secondary electrons and help prevent avalanche discharges. (b) Same trajectories shown near the tip. Using a shorter tip reduces the beam’s angular spread. (c) Vacuum chamber cut-away view. D2 pressure was set using a leak valve and monitored with a D2 compensated Pirani gauge. The target was a molybdenum disc coated with ErD2 . (d) Arrangement of neutron and X-ray detectors (Amptek XR-100T-CdTe). To better resolve the bremsstrahlung end point, a 2.5-cm aluminum filter (not shown) was placed between the X-ray detector and the view port. The vacuum chamber’s thick stainless-steel walls and lead sheet shielded the neutron detector from X-rays. Source: Naranjo et al. [1] and Naranjo [4]. Republished with permission of Springer Nature, from Observation of nuclear fusion driven by a pyroelectric crystal, B. Naranjo, J.K. Gimzewski & S. Putterman, volume 434, 2005; permission conveyed through Copyright Clearance Center, Inc.

UCLA team has been one of the main critics of these earlier prospective fusion claims from Taleyarkhan [13]. The first successful results with pyroelectric fusion using a tritiated target was reported in 2010 [14]. The UCLA team of Putterman and Naranjo worked with T. Venhaus of Los Alamos National Laboratory to measure a 14.1-MeV neutron signal far above background. This was a natural extension of the earlier work with deuterated targets.

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6.3 Pyroelectric Neutron Generators The pyroelectric neutron generator (PNG) is a compact neutron generator in which the electrical field is created on the pyroelectric crystal surface by changing the crystal temperature. The most commonly used crystal is LiTaO3 . The ionizer is mounted on the pyroelectric crystal surface and can be either a tungsten tip with the pin diameter of several hundred nanometers or a carbon nanotubes array. In 2001, Shafroth [15] first discussed the possibility of achieving fusion and generating neutrons using pyroelectric crystals at a meeting of the American Physical Society. He suggested that pyroelectric crystals could be used to achieve D–D fusion through ionization of deuterium gas and acceleration of deuterium D+2 or D+ ions into a deuterated target. However, at the time, the lack of both neutron detection equipment and a target put a stop to this research. Since then, there have been two main research groups who have led the major developments in pyroelectric neutron generation. The lead research groups are UCLA [1, 16–20] and RPI [5, 6, 9, 21–32]. In 2005, Brian Naranjo et al. [1] at UCLA published the first professional publication describing pyroelectric-generated neutron production. They reported that gently heating (240 to 265 K, heating rate 0.2 K s−1 ) a pyroelectric crystal LiTaO3 in a deuterated atmosphere can generate nuclear fusion under desktop conditions. It was reported that, although the fusion was not useful in a power-producing sense, it was anticipated that the system would find application as a simple palm-sized neutron generator. Figure 6.1 shows the UCLA vacuum chamber cut-away view and the arrangement of neutron, ion current (Faraday cup), and X-ray detectors. The observed neutron count reported was 15 300 neutrons per cycle (800 per second) with 2.45 MeV energy efficiency. In 2007, researchers at UCLA [18] collaborated with Lawrence Livermore National Laboratory (LLNL) and reported the first results from the operation of the LLNL Crystal Driven Neutron Source (CDNS) based on the pyroelectric effect. The goal of this work was to raise the neutron output by increasing the beam energy and current using different crystal configurations. They incorporated feedback temperature control, socket mounts for rapid changing of field emitters, and the ability of the assembly to traverse the beam axis permitting maximum experimental flexibility. The LLNL CDNS is operational and has produced record ion currents of ∼10 nA and neutron output of 1.9 (±0.3) × 105 per thermal cycle using a crystal heating rate of 0.2 ∘ C s−1 from 10 to 110 ∘ C. This neutron yield is still the highest yield to date. A 3-cm-diameter by 1-cm-thick LiTaO3 crystal with a socket-secured field emitter tip is thermally cycled with feedback control for ionization and acceleration of deuterons onto a deuterated target to produce D–D fusion neutrons. Figure 6.2 graphically illustrates the concept along with a typical Faraday cup setup for current measurement and neutron production. The entire crystal and temperature system is mounted on a bellows that allows movement of the crystal along the beam axis and is completely contained on a single small vacuum flange (Figure 6.3). The modular crystal assembly permitted experimental flexibility. Operationally, flashover breakdowns along the side of the crystal and poor emitter tip characteristics can limit the neutron source. The experimental

6.3 Pyroelectric Neutron Generators

Crystal Conductor

Faraday cup (GND mesh & D target)

Field emitter

A

D+, D2+ Beam Heat

Figure 6.2 Schematic of the pyroelectric fusion concept and a typical Faraday cup–target setup for ion current measurement. The current collector or target plate behind the grounded mesh is loaded with deuterium for D–D fusion and connected to an ammeter for current measurement. The collector is operated with a positive bias to prevent the escape of secondary electrons. Source: Republished with permission of American Institute of Physics, from Tang et al. [18], permission conveyed through Copyright Clearance Center, Inc.

Cu Heatsink

Crystal 3

He array

Nal X-ray detector

Coolant TEC Clamp Pt coating Tip in socket

Modular crystal assembly

Crystal

Faraday cup (mesh and target) Z or beam axis

Cu Base

Figure 6.3 Schematic of the CDNS with close-up views of the modular crystal assembly and one of the crystals. The crystal can be moved back and forward along the beam axis. The thermocouple for feedback control is sandwiched between the TEC and the copper backplate on the rear of the crystal. A groove cut into the backplate allows the thermocouple to be sandwiched snuggly without causing a gap between the TEC and the backplate. Source: Tang et al. [18]. Republished with permission of American Institute of Physics, from Neutron production from feedback controlled thermal cycling of a pyroelectric crystal, V. Tang, G. Meyer, J. Morse, G. Schmid, C. Spadaccini, P. Kerr, B. Rusnak, S. Sampayan, B. Naranjo and S. Putterman, volume 78, 2007; permission conveyed through Copyright Clearance Center, Inc.

neutron results extend earlier published works by increasing the ion current and pulse length significantly to achieve a factor-of-two higher neutron output per thermal cycle. This neutron yield is still the highest yield to date. Between 2007 and 2009, the researchers at RPI [28–32] demonstrated a pairedcrystal device that generated an acceleration potential of over 200 keV, which is the highest energy produced using this technology (Figure 6.4). They developed a portable protype vacuum system that enhances reproducibility of the neutron

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Amptek XR-100T-CdTe

226

5″ Diameter × 3″ Thick Eljen proton-recoil detector

5

Cu disk

CD2

5

Tip

1

2

3 4

DIP socket

4 3

2

1

Vacuum chamber

BF3 thermal neutron detector

Figure 6.4 Schematic of the two-crystal system inside the vacuum chamber: (1) CPU cooling fans, (2) aluminum vacuum flanges, (3) TECs (Laird Technologies part number 430141–501), (4) lithium tantalate crystals, and (5) type T thermocouples. The distance between the two crystals is ∼7 cm. The distance from the target crystal to the face of the proton-recoil detector is ∼6.55 cm and to the BF3 detector is ∼7.65 cm. The stainless-steel chamber is ∼0.17 cm thick. Source: Gillich et al. [31], figure 3 (p. 307)/with permission of Elsevier.

yields. Experiments were undertaken that produced ∼1.0 × 104 neutrons with a standard deviation of 14% for each thermal cycle for a two-crystal system. The group had the ability to reproduce experimental results using a new thermal management system consisting of two thermoelectric module temperature controllers, meaning thermal cycles could replicated from one experiment to another. Research at RPI had focused on the practical application of a portable pyroelectric neutron source with emphasis on improvements in the neutron production yield, reproducibility, and controlled emission length. Such sources could find uses in neutron detector calibration, research and education, and security applications. The RPI researchers [33] used thin films of vertically aligned tungsten nanorods to enhance field ionization in pyroelectric crystal D–D fusion experiments resulting in increased neutron production (Figure 6.5). The tungsten nanorods were deposited on a single LiTaO3 crystal using sputter deposition at glancing angles. The combination of a single tungsten tip with a thin film of nanorods on the face of the crystal yielded about four times the number of neutrons than did either a single tip or nanorods alone.

6.3 Pyroelectric Neutron Generators

(+z face)

(–z face)

(–z face)

(a)

Cooling dT/dt < 0 (+z face)

Hot crystal dT/dt = 0 (+z face)

Heating dT/dt > 0 (–z face)

(–z face)

(+z face)

Cool crystal dT/dt = 0

Thermal cycle Thermocouple

Thermocouple CPU fan

CPU fan

Pyroelectric crystal (tip)

Pyroelectric crystal (target)

CD2 W nanorods (–Z face) (+Z face) Vacuum chamber (b)

AI flange

Thermoelectric heater/cooler

Thermoelectric heater/cooler

AI flange

Figure 6.5 (a) Schematic diagram of the change in charge formation on the face of a pyroelectric crystal charge during heating and cooling. (b) Diagram of RPI’s two-crystal system. The crystals are approximately 7 cm apart from each other. The distance from the target crystal to the face of the proton-recoil neutron detector is ∼6.55 cm. A 70 nm radius tungsten tip was added to the center of the crystal coated with nanorods for some of the experiments. Source: Gillich et al. [33], figure 1 (p. 228)/with permission of Elsevier.

In 2010, Tornow et al. [34] demonstrated neutron production with a pyroelectric double-crystal assembly without nano-tip as shown in Figures 6.6 and 6.7. Two cylindrical LiTaO3 crystals (2.5 cm long and 2.5 cm diameter) facing each other’s deuterated circular face were exposed to deuterium gas at an ambient pressure of 6 mTorr. With a distance of about 4 cm between the z+ and z− cut crystal faces, neutrons were produced via the 2 H(d,n)3 He fusion reaction upon the heating and cooling of the crystals (130 to 0 ∘ C). The 2.5-MeV neutrons were detected with organic liquid scintillation detectors equipped with neutron-gamma pulse-shape discrimination electronics to reject pulses generated by the intense X-ray flux. During the cooling phase of naked crystals, deuterium ion-beam (D+2 ) energies of up to 400 keV were obtained as deduced from the associated electron bremsstrahlung end point energy. The highest electron-beam energy observed during the heating phase was 360 keV. With a layer of deuterated polyethylene evaporated on the front face of the crystals, the maximal energies were about 10% lower. In this study, an electric field enhancing nano-tip was not employed. Neutron yields up to 500 per

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Neutron detector

Pyroelectric crystal Vacuum chamber

Planar HPGe Detector

Copper support structure

Neutron detector

Figure 6.6 Cartoon of top view of chamber containing the double-crystal arrangement for neutron generator. Two neutron detectors and the HPGe detector are also shown, all positioned in the horizontal plane. Source: Tornow et al. [34], figure 2 (p. 700)/with permission of Elsevier. Neutron detector

Geiger counter

Plastic flange

Si surface barrier detector

Aluminum cylinder

Disk with apertures D2 gas 1″ × 1″ LiTaO3

Aluminum flange

Figure 6.7 Cartoon of side view of chamber containing the double-crystal arrangement for neutron generator. The two neutron detectors and the end-window Geiger counter placed on top of the chamber are also shown. The disk above the crystals is used to measure the electron and positive ion currents. Source: Tornow et al. [34], figure 4 (p. 702)/with permission of Elsevier.

6.4 Pyroelectric X-ray Generators

thermal cycle were observed, resulting in a total neutron production yield of about 1.6 × 104 neutrons per thermal cycle. However, uncontrolled discharges limit the usefulness of the present experimental setup, although, again for the first time, this setup allows for the continuous production of neutrons (above the natural background level) during the constant temperature phase over a time period of at least 30 min. The neutron yield can be enhanced by at least a factor of 10 by optimizing the geometry and by the admixture of certain gases to the pure deuterium gas presently used, which could result in considerably less frequent discharges. In 2011, RPI research [22, 24] focused on improving the neutron yield and emission reproducibility and shortening the heating cycle. They showed that D–D and D–T neutrons could theoretically produce ∼107 n cycle−1 and could reach ∼107 n s−1 for the D–T reaction. They demonstrated that the neutron emission during heating or cooling could be shortened to ∼30 seconds during a typical heating/cooling cycle of about 2–3 minutes. Pulsed operation at low repetition rate was also demonstrated by the LLNL group [19]. They also developed an educational accelerator device in collaboration with the US Military Academy and the Défense Threat Reduction Agency at West Point [21]. They discussed a method to optimize the acceleration potential by precisely controlling the thermal cycle of the crystals through a proportional thermal controller implemented in Labview. The current state of this technology allows production of a low-cost sealed D–D or D–T neutron source with neutron yield that is sufficient for testing of fast neutron detectors. In 2011, researchers at UCLA [14] collaborated with the Los Alamos National Laboratory demonstrated a compact 14.1-MeV neutron source by which deuterons are accelerated into a tritiated target through the action of a pyroelectric crystal. At the ion currents they measured, the expected flux is about 3 × 105 neutrons per second. Although the measured flux was much less than expected, spectral analysis of the neutron signal unambiguously verified a 14.1-MeV neutron signal far above background. The experimental fusion rate is limited by surface flashover and the time development of a surface barrier on the target due to chemical contamination. Various improvements in the management of flashover originating from triple points and stabilization of field ionization tips are discussed. In 2014, Tornow [35] demonstrated neutron production of 14 MeV by changing the temperature of a LiTaO3 pyroelectric double-crystal arrangement in a deuterium gas environment, where deuterium ions were produced and accelerated toward a tritiated target via the 3 H(d,n)4 He nuclear fusion reaction. The pyroelectric effect was used to provide the acceleration potential by heating or cooling the crystals. The low power needed for accomplishing the temperature changes can easily be produced by a small solar panel. As a result, solar energy can be recovered into nuclear fusion energy. The present approach can be improved and scaled up considerably, far-reaching practical applications could be envisioned.

6.4 Pyroelectric X-ray Generators The pyroelectric effect consists on the generation of electric charges on the surface of the pyroelectric crystal, perpendicular to the polar z-axis, during a change

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6 Pyroelectric Fusion

of the crystal temperature. Such property is found in both crystalline and ceramic materials. The charge polarity depends on the temperature gradient (cooling or heating) and the crystal z-axis orientation. The generated charge is compensated by free charge from air in a very short time at atmospheric pressure. However, if the charge is not compensated in low-pressure conditions, then high electric potential up to 100 kV can be generated on the crystal surface. This is the cause of the polarization of molecules from the residual gas as well as of the emission and acceleration of electrons from the pyroelectric crystal surface. In 1992, Brownridge [36] discovered the possibility of X-ray generation using the pyroelectric effect for LiNbO3 and LiTaO3 pyroelectric crystals experimentally for the first time. Two different possibilities can be realized for X-ray production when a grounded target is located in front of the charged surface of the crystal in vacuum. If the pyroelectric crystal surface has a negative polarity as it is presented in Figure 6.8a, then the electrons from polarized molecules and the crystal surface are accelerated to the grounded target producing bremsstrahlung and characteristic radiation from the target. When the crystal has a positive charge polarity as it is shown in Figure 6.8b, the electrons from polarized molecules are accelerated toward the crystal surface producing bremsstrahlung and characteristic radiation from the crystal surface. It should be noted that the ions of the residual gas are also accelerated in the opposite direction relative to the electron propagation, but the ions X-ray yield is substantially smaller in comparison with the yield produced by the electrons. Several geometries can be realized to develop pyroelectric X-ray sources [38–40]. In 2003, Amptek [40] introduced the first commercially produced miniature X-ray generator “COOL-X” with pyroelectric crystal (Figure 6.9). This miniature device gives a new opportunity to put forward the radiographic X-ray technology. The COOL-X is a novel, miniature X-ray generator that uses a pyroelectric crystal to generate energetic electrons that produce X-rays in the target material (Cu). The hermetically sealed package has a thin beryllium window that allows the X-rays to be transmitted. The COOL-X does not use radioisotopes or high-power X-ray tubes. It is a self-contained, solid-state system that generates X-rays when the crystal is thermally cycled. The COOL-X is unique and should not be compared with other X-ray tubes. It is thermally cycled between 2 and 5 minutes and does not produce a constant flux of X-ray Crystal

X-ray Crystal

Target

Z

(a)

Target

Z

(b)

Figure 6.8 The two cycles of X-ray generation by pyroelectric crystal: (a) crystal negative polarity; (b) crystal positive polarity. Source: Chepurnov et al. [37], figure 1 (p. 2)/IOP Publishing/Licensed under CC BY 3.0.

6.4 Pyroelectric X-ray Generators

Be window X-rays Low pressure gas X-ray production copper target

Temperature sensor e–

e–

e–

Pyroelectric crystal Heater/Cooler

TO-8 Package Temperature

+V

Ground

Figure 6.9 Amptek “COOL-X” miniature X-ray generator. Source: Amptek [40], Miniature X-ray generator with pyroelectric crysta, AMPTEK INC. Retrieved from: https://www.amptek .com/-/media/ametekamptek/documents/products/coolx.pdf.

X-rays. The X-ray flux varies throughout the cycle and may vary from cycle to cycle. The practical applications and features of the X-ray generator are as follows:

6.4.1 ● ● ● ● ●

Portable X-Ray Instrumentation Teaching Laboratories Instrument Calibration Radiography (X-Ray Film Imaging) Research

6.4.2 ● ● ● ● ● ● ●

Applications

Features

Miniature size – 0.6′′ dia × 0.4′′ , 15 mm dia × 10 mm Low Power: