Nanostructured Silicon for Photonics [1 ed.] 9783038131045, 9780878494880

Citation preview

Nanostructured Silicon for Photonics

Preface

Using light to convey signals around electronic chips could solve several current problems in microelectronic evolution, including power dissipation, interconnect bottleneck, input/output of the chip to optical communication channels, signal bandwidth, and so forth. Unfortunately, silicon is not a good photonic material: it has poor light-emission efficiency and negligible electrooptic effect. Silicon photonics is a field with the objective of improving the physical properties of silicon, thus turning it into a photonic material and allowing full convergence between electronics and photonics. Various research directions based on the use of nano-sized silicon are described in this book. We tried to review the field and to report on recent research results achieved in Trento. In view of the large extent of the field encompassed, it is clear that these results have been possible only with many collaborations worldwide and with the support of many funding agencies. In particular: The work on silicon gain has been mostly carried out in collaboration with F. Priolo and coworkers (Catania), M. Zacharias and coworkers (Halle), P. Fauchet and coworkers (Rochester), I. Pelant and coworkers (Prague), L. Khriachtchev and coworkers (Helsinki). This work was supported by INFM and by EC through the project SEMINANO. The work on Er-coupled silicon nanocrystals has been mostly carried out in the framework of the EC project SINERGIA and done in collaboration with R. Rizk and coworkers (Caen), B. Garrido and coworkers (Barcellona), L. Cognolato and coworkers (Torino), E. Borsella and coworkers (Padova). The work on silicon waveguides has been mostly carried out with the help of the ITC-irst (P. Bellutti, A. Lui and G. Pucker) and supported by PAT through the PROFIL project. The work on photonic crystals has been mostly carried out in collaboration with L. C. Andreani and coworkers (Pavia), E. Di Fabrizio and coworkers (Trieste) and in the framework of FIRB (“Sistemi Miniaturizzati per Elettronica e Fotonica” and “Nanostrutture molecolari ibride organiche-inorganiche per fotonica”) and COFIN (Silicon-based photonic crystals: technology, optical properties and theory and Silicon-based photonic crystals for the control of light propagation and emission) projects.

The work on complex system has been mostly carried out in collaboration with D.S. Wiersma and coworkers (Florence), N.E. Capuj and coworkers (Tenerife) and was supported by INFM. We thank all of them for the enjoying collaborations and for sharing with us the enthusiasm about this exciting research field.

Povo, July 2005 Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana, and L. Pavesi

Table of Contents Preface ................................................................................................................... i 1. Introduction ......................................................................................................1 What is photonics? .........................................................................................1 Measurement of light .....................................................................................3 Photonics as a dominant technology of this millennium ...............................4 Light as energy carrier .....................................................................6 Light as information carrier ...........................................................10 Silicon as the enabling materials .................................................................18 References ....................................................................................................19 2. Silicon nanocrystals fundamentals ...............................................................25 Low dimensional structures .........................................................................25 Si nanostructures ..........................................................................................26 Si nanocrystals growth .................................................................................26 PECVD technique..........................................................................27 Porous silicon.................................................................................29 Structural properties .....................................................................................33 Optical properties .........................................................................................34 Luminescence models....................................................................35 References ....................................................................................................40 3. Nanoprobe technique to characterize Silicon nanocrystals .......................45 The Rationale of SPM development ............................................................45 Nanoprobe techniques .................................................................................47 STM ...........................................................................................47 AFM ...........................................................................................49 Lateral force microscopy ..............................................................52 Force modulation microscopy .......................................................52 Magnetic force microscopy ...........................................................53 Electrostatic force microscopy ......................................................53 Scanning thermal microscopy SNOM ...........................................................................................54 Nanoprobe characteristics and limits ...........................................................57 SPM characterization of Silicon nanocrystal ..............................................59 References ....................................................................................................61

4. Silicon based light emitting diode .................................................................65 Silicon LED .................................................................................................65 Bulk Silicon LED.........................................................................................65 SiGe LED .....................................................................................................69 Porous Silicon LED .....................................................................................71 Nanocrystals LED .......................................................................................75 Erbium Doped LED, Silicon Erbium Organic LED ....................................77 References ....................................................................................................81 5. Optical gain in silicon and the quest for a silicon laser ..............................87 Basic on light amplification and gain ..........................................................87 Limitation of Silicon for light amplification ...............................................89 Various approaches to a silicon laser...........................................................91 Silicon Raman laser ...........................................................................91 Bulk silicon light emitting diodes ......................................................93 Optical gain in silicon nanocrystals ...................................................96 Light amplification in Er coupled Si nanoclusters ..........................101 Si/Ge quantum cascade structures ...................................................104 Conclusions ................................................................................................107 References ..................................................................................................107

6. Er coupled Si nanocrystal optical amplifiers ............................................113 Motivations ................................................................................................113 Spectroscopy of the Er3+ ion ....................................................................114 Lifetimes .........................................................................................115 Cross sections ..................................................................................116 Optical amplification in a three level system ............................................118 Er3+ and Si-Nc interactions. Models and experimental ...........................121 evidences Waveguide amplification studies ...............................................................127 Conclusions ...............................................................................................128 References ..................................................................................................128

7. Design of silicon based optical components ...............................................131 Introduction ................................................................................................131 Simulation .................................................................................................131 Plane Wave Expansion method ..................................................135 Finite difference time domain method ........................................139 Eigen Mode Expansion method...................................................142 Examples ....................................................................................................149 References ..................................................................................................153 8. Si-based waveguides.....................................................................................157 Introduction ................................................................................................157 Si-based waveguides ..................................................................................157 Silicon oxynitride and silicon nitride waveguides ......................158 Silicon on insulator waveguides ..................................................159 Silicon nanocrystals waveguides ................................................ 159 Propagation Losses in optical waveguides ...............................................160 Scattering losses ..........................................................................160 Interface scattering ......................................................................161 Absorption losses .......................................................................................163 Molecular bond absorption ..........................................................163 Interband absorption ....................................................................165 Free carrier absorption .................................................................165 Coupling losses for butt and end-fire coupling ........................................166 Reflection from waveguide facet.................................................167 Numerical aperture mismatch ......................................................168 Misalignment ...............................................................................168 Spot size of light beam and waveguide size ................................169 Measurement of propagation losses in optical waveguide .......................171 Methods .......................................................................................171 Experimental results for different kinds of waveguides ..............173 Conclusion .................................................................................................178 References .................................................................................................178 9. Silicon based photonic crystals ...................................................................183 Engineering photon states ..........................................................................183 Band structure and electronic analogy .......................................................184 One dimensional PC ..................................................................................186 The multilayer structure...............................................................186

Complete gap and omnidirectional reflector ...............................187 Introducing a defect: the cavity ...................................................188 Apodized filters ...........................................................................191 Porous silicon 1D photonic structures .........................................191 Two dimensional PC ..................................................................................194 Perfect 2D PC ..............................................................................194 Defects: cavities and waveguides ................................................197 Silicon based 2D structures .........................................................200 Three dimensional PC ................................................................................201 Complete control of light propagation .......................................201 3D fabrication techniques ............................................................201 Advanced properties ..................................................................................203 References ..................................................................................................203 10. Silicon based complex dielectric systems .................................................209 Introduction ...............................................................................................209 PS multilayers: a controlled growth ..........................................................211 Light propagation in optical superlattices..................................................216 Time-resolved photonic Bloch oscillations .................................216 Resonant Zener tunneling of light ...............................................223 Light transport in Fibonacci Quasicrystals ................................................229 References ..................................................................................................235

Table of Contents Preface Table of Contents Chapter 1: Introduction Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 2: Silicon Nanocrystals Fundamentals Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 3: Nanoprobe Technique to Characterize Silicon Nanocrystals Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 4: Silicon Based Light Emitting Diode Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 5: Optical Gain in Silicon and the Quest for a Silicon Laser Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 6: Er Coupled Si Nanocrystal Optical Amplifiers Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 7: Design of Silicon Based Optical Components Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 8: Si--Based Waveguides and Optical Switching Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 9: Silicon Based Photonic Crystals Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi Chapter 10: Silicon Based Complex Dielectric Systems Z. Gaburro, P. Bettotti, N. Daldosso, M. Ghulinyan, D. Navarro, M. Melchiorri, F. Riboli, M. Saiani, F. Sbrana and L. Pavesi

1 25 45 65 87 113 131 157 183 209

Materials Science Foundations Vols. 27-28 (2006) pp 1-24 © (2006) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSFo.27-28.1

1.

INTRODUCTION “I will never stop pondering on the question of the essence of light." A. Einstein (1879-1955) [1]

1.1

WHAT IS PHOTONICS?

Photonics – from ancient Greek φῶς, φοτός (light) – is “the study or application of electromagnetic energy whose basic unit is the photon, incorporating optics, laser technology, electrical engineering, materials science, and information storage and processing”. [2] With relaxation of formal precision, we intend photonics as the science and technology of light. In its original meaning, light is the limited subset of electromagnetic waves which is perceived by human eye, i.e. whose vacuum wavelength ranges from about 350 to 750 nm. However, since a lot of applications of photonics are not restricted to visible light, we flexibly extend the definition to some of Near UltraViolet (NUV) and Near InfraRed (NIR) radiation.

Figure 1. The electromagnetic spectrum. After [3].

Before the 1990s, the definition as “the science and technology of light” was assigned to optics. The word photonics has become popular in the last decade in analogy to electronics. By referring to photon, it emphasizes the role of quantization of light. Indeed, the physical mechanisms of lasers, LEDs and

2

Nanostructured Silicon for Photonics

detectors critically depend on the energy discreteness of electromagnetic energy. Despite the analogies, however, photons and electrons have also quite different properties. For example, photonic wave functions are vector fields, as opposed to scalar electronic wave functions. This leads to a degree of freedom for light, the polarization, which is absent for electrons. Another difference is in the mechanisms of energy exchange. Free electrons, as charged and massive particles, can store mechanical energy continuously, for example, by accelerating under electric fields, and can exchange it with each other. Photons do not interact unless interaction is mediated by materials. Yet, interaction phenomena usually involve either elastic scattering – i.e., with no energy exchange – or photon creation and annihilation processes1. Our capability of exploiting coherence, which requires conservation of frequency2, depends indeed on the scattering being usually elastic. Yet, anelastic photonic scattering also occurs, leading to interesting effects and useful applications. A well known example is Raman scattering, which can be exploited both for material analysis and optical amplification (the reader is directed to Ref. [4] for an exhaustive introduction to Raman effect and its applications). Finally, electrons and photons differ also in the range of their wavelength λ. Typical wavelengths3 are in nanometer (10-9 m) range for electrons and in the micron (10-6 m) range for photons. Diffraction and wave-like character become apparent in materials at size scale of the order of the wavelength and below. For example, in devices with size of the order of a µm (a frequent case in electronic and photonic technologies), the dominating character of propagation is wave-like for light and particle-like for electrons.

1

As opposed to electrons, the number of photons is not a conserved physical quantity.

2

The conservation of frequency of light ν has the same meaning as the conservation of photonic energy E=h ν, where h is Planck’s constant. 3

The wavelength λ of electron depends on its energy E = h2/(2mλ2), where h is Planck’s constant and m is the electron mass. In principle, the energy of electrons can assume values over a large dynamic range, but in the context of electronic devices, the typical values are E ≈ 1 eV, implying λ ≈ 1 nm. Photonic energy is E=hc/λ, where c is the speed of light in vacuum.

Materials Science Foundations Vols. 27-28

1.2

3

MEASUREMENT OF LIGHT

Light can be measured according to its electromagnetic energy (radiometric units), or its perception by human eye (photometric units). For the latter units, a “standard” response of the human eye has been defined, from suitable experimental characterizations and averages, by the International Commission on Illumination (CIE, from its French title Commission Internationale de l'Eclairage). As the eye sensitivity adjusts according to the intensity of illumination, two sensitivity spectral curves have been defined, the photopic response – for daylight illumination – and the scotopic4 response – for low intensity levels. Both curves are shown in Figure 2. The reader interested in measurement of color is directed to Refs. [3, 5, 6] Human vision (CIE standard) 3

10

scotopic photopic

Efficiency (lm/W)

2

10

1

10

0

10 10

-1

10

-2

10

-3

400

500

600

700

800

Wavelength (nm)

Figure 2. Spectral response of average human eye, according to the Commission Internationale de l'Eclairage (CIE). The plot shows both the photopic (as established in 1924) and the scotopic (1951) responses.

The following table is a summary of the main units of both systems. Radiometric

4

Photometric

Quantity

Unit

Quantity

Unit

Radiant Flux (Power)

Watt (W)

Luminous Flux (Power)

Lumen (lm)

Radiant Intensity

W/sr

Luminous Intensity

Candela (cd) 1 cd = 1 lm/sr

Irradiance

W/m2

Illuminance

Lux (lx) 1 lx = 1 lm/m2

Radiance

W/(sr m2)

Luminance

Nit 1 nit = 1 lm/(sr m2)

The word scotopic is derived from ancient Greek σκότος (darkness).

4

Nanostructured Silicon for Photonics

A wave with vacuum wavelength 555 nm and 1 W radiometric power carries 683 lumens (lm), both in photopic and scotopic regimes (see Figure 2). 1.3

PHOTONICS AS A DOMINANT TECHNOLOGY OF THIS MILLENNIUM

Light – as electromagnetic radiation in general – is energy in motion. One can exploit this energy either directly, or as a vehicle of information. Both ideas are as old as mankind. According to Greek historian Polybius, Archimedes knew how to burn the ships of the Roman general Marcellus by focusing sun’s light onto them, during the siege of Syracuse (212-215 BC)5, [7] and prehistoric men left graffiti, which rely on visual perception, to communicate. There are, however, three reasons why we consider photonics as a technology of this millennium. First, starting from the 19th century (and especially in the last 50 years), many crucial discoveries about light generation, processing and detection have dramatically changed the perspective on how to exploit light. The most important enabling invention has been the laser (Light Amplification by Stimulated Emission of Radiation)6. In conveying energy or information, one faces the question of efficiently transmitting energy in a directional fashion, from a source to a receiver7. Light sources based on spontaneous emission are inefficient with this respect: acting as dipole-like sources (e.g. atoms in an excited state), they radiate energy over all directions. On the contrary, the stimulated emission of a laser is concentrated in one direction, and since it is by nature spatially coherent, it can be made diffraction-limited (i.e., divergence angle θ can be kept to its fundamental limit). [8] This minimizes optical loss associated with sending energy in unwanted directions. Finally, lasers are monochromatic: this minimizes wasted energy in useless spectral bands. The reader can find recent reviews for historical and general overviews on lasers technology, [9, 10] as well as on laser applications. [11] The second reason is about semiconductor technology. Semiconductors are materials with electronic energy bandgaps of the same order of magnitude as photonic energy (≈ 1 eV). In semiconductors, valence electrons can be excited 5

Ways to focus sun’s light were known also earlier (e.g. Aristophanes, “The Clouds”, 420 BC). 6

Scientists Townes, Basov and Prokhorov have been awarded the Nobel prize in 1964 “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle”.

7

There are also different scenarios, such as broadcasting, where information is transmitted in all directions.

Materials Science Foundations Vols. 27-28

to conduction states by energy absorbtion, provided that the available amount is at least as large as the bandgap. Analogously, conduction electrons relax to valence states by emitting energy quanta sized as the bandgap. Such energy exchanges naturally occur by absorption and generation of light photons. Efficient semiconductor light sources and detectors have been invented. The semiconductor laser was invented in 1962. Semiconductor laser invention and history has been recently reviewed. [12, 13] Finally, photonics has been so far providing the best solution in terms of communication bandwidth (vide infra), which qualifies it as core technology of the new economy (Figure 3).

Figure 3. Performance of communication technologies in terms of distance and bandwidth. Source: Bell Laboratories, Lucent, Stand: 2003.

Predicted Internet traffic for next years (over 5,000 petabits – or 5x1018 bits – per day by the end of 2007, according to International Data Corp., Framingham, MA) suggests that photonics will maintain a key role in digital communication technology. Nowadays, photonics is a very wide discipline, which encompasses applications in numerous and diverse field of technology. Many applications get barely noticed in our daily life. The unnoticeable presence is the signature of its success. In the following paragraphs, we enumerate the application branches of photonics.

5

6

Nanostructured Silicon for Photonics

1.3.1 LIGHT AS ENERGY CARRIER PHOTOVOLTAICS Photovoltaics8 (PV) is the science and technology of electricity generated by light. Its main application is conversion of the radiation from the sun (Figure 4) in electrical energy.

Figure 4. The solar spectrum outside the atmosphere and at ground level. Data from [14].

A recent review (2003) of photovoltaics can be found in Ref. [15]. The photovoltaic effect was first observed in 1839 by Becquerel. [16] Generally, PV involves a solid or liquid system, but in practice the most common converting device is a semiconductor pn-junction, possibly with anti-reflecting coating and/or a textured surface, which optimize light trapping (Figure 5). For PV, the semiconductor must be carefully grown with low defect density to avoid unwanted electron-hole recombinations. In fact, once the electron-hole pairs are generated by photons, carriers generated in proximity of the pn-junction that do not recombine are drifted by the built-in electric field towards opposite directions. Charge separation leads to usable electrostatic potential.

8

The suffix “voltaic” derives from Alessandro Volta, Italian scientist (1745-1827), who first devised apparatus for developing electric currents by chemical action. The photovoltaic effect gets electric energy from light.

Materials Science Foundations Vols. 27-28

Figure 5. Schematic representation of a basic semiconductor pn-junction solar cell. Textured surface improves conversion efficiency by increasing photon trapping inside the cell.

The theoretically available solar energy, according to International Energy Annual (IEA) agency, is about 10,000 times the world total primary energy demand. Getting energy directly from the sun is becoming a top technological priority: there is simply no alternative in the long term to developing renewable energies. [17] The role of solar cell technology in energy production is expected to increase in the years to come (Figure 6). 10000

World's Energy Sources (Mtoe)

1000

100

Conventional Photovoltaic Other renewable

10

1

0.1 2001

2010

2020

2030

2040

Year

Figure 6. Projected world’s energy demand, up to year 2040, in millions of tonnes of oil equivalent (Mtoe). Data source: European Renewable Energy Council (EREC). [17]

PHOTOCATALYSIS When light excites electron-hole pairs inside a semiconductor, energy can be released to the outside chemically, provided that carriers are trapped by a suitable scavenger or by surface states. Holes in the valence band (electrons in the conduction band) can act as oxidants (reductants), and can promote chemical reactions, in a process called “photocatalysis” (Figure 7). To increase active surface, it is advantageous to use porous semiconductors, whereas

7

8

Nanostructured Silicon for Photonics

semiconductor nanoparticles are an interesting option to minimize effects of band bending. [18]

Figure 7. Schematic of the mechanism of photocatalysis in semiconductor nanocrystals.

MEDICINE AND BIOSCIENCE Laser in medicine is therapeutically beneficial in tissue repair and pain control. In 1968, Endre Mester, a Hungarian physician, while attempting to destroy, with ruby laser light, some tumors cells implanted under the skin of laboratory rats, observed that skin incisions appeared to heal faster with laser treatment. [19] Later, he discovered that defects, burns, ulcers arising from diabetes, venous insufficiency, infected wounds, and bedsores also healed faster in response to his laser treatment. [20] Other reported mechanisms of light-induced beneficial effects include modulation of prostaglandin levels, alteration of somatosensory evoked potential and nerve conduction velocity, and hyperemia of treated tissues. Clinical benefits include pain relief in conditions such as carpal tunnel syndrome (CTS), bursitis, tendonitis, ankle sprain and temporomandibular joint (TMJ) dysfunction, shoulder and neck pain, arthritis, and post-herpetic neuralgia, as well as tissue repair in cases of venous ulcer, mouth ulcer, fractures, tendon rupture, ligamentous tear, torn cartilage, and nerve injury. Evidence indicates that cells absorb photons and transform their energy into adenosine triphosphate (ATP), the form of energy that cells utilize. The resulting ATP is then used to power metabolic processes; synthesize DNA, RNA, proteins, enzymes, and other products needed to repair or regenerate cell components; foster mitosis or cell proliferation; and restore homeostasis. [21] The reader is redirected to a recent review for extended information. [22]

Materials Science Foundations Vols. 27-28

MANUFACTURING Power laser beams provide highly collimated energy beams, useful for laserassisted forming, joining, machining and surface engineering (Figure 8). An exhaustive review can be found in [11].

Figure 8. Classification of laser material processing. After [11].

Applications can be classified according to the power density and interaction time between light and the target material (Figure 9).

Figure 9. Laser power density, specific energy and interaction times for various laser processing regimes. After [23].

While CO2 lasers9 have been used since late 60’s for industrial applications because of their power efficiency, over the past decade neodymium ion-doped

9

CO2 emission wavelength is 10.6 µm, whereas Nd:YAG’s fundamental emission is at wavelength 1.063 µm, with frequency doubled emission at 532 nm.

9

10

Nanostructured Silicon for Photonics

yttrium aluminum garnet (Nd:YAG) solid-state lasers9 have become increasingly popular for manufacturing applications, due to better absorptivity and the ability to use fiber optics for beam transport. Q-switched Nd:YAG are standard tools for manufacturing under pulsed regimes. [23] A recent review on pulsed manufacturing (laser shot peening and shock processing) is available as Ref. [24]. 1.3.2 LIGHT AS INFORMATION CARRIER STORAGE Up to early 19th century, information storage and retrieval was only associated with human perception. The idea of storage for computing machines was formalized by John von Neumann less than a century ago. Von Neumann architecture [25] is based on a memory and a central processing unit (CPU), where information is transferred and processed. The leap towards the miniaturization of storage media has been led by Feynman’s visionary indications. [26] The information areal density of magnetic disks has reached levels larger than 100 Gb/in2 (gigabyte per square inch), for less than a US dollar/Gb. [27,28] In recent years, optical disks have substituted magnetic media in a few segments of storage technology. In optical disks, bits are recorded as small spots which modulate a readout beam. Modulation can be on the phase, polarization or intensity. Optical disc technology was invented in 1966 by James T. Russell, who was able to store data as micron-wide dark and light spots. [29] Today, a Compact Disc (CD) provides a typical areal data density of 0.39 Gb/in2 and data rate of about 100 kbps. [30] The Digital Versatile Disk (DVD) offers a 4.7 Gb capacity (per side) with 2.77 Gb/in2 areal density and 10 Mb/s data rate. [30] The wavelength was reduced from 780 nm (CD) to 650 nm (DVD) and the numerical aperture10 (NA) was increased from 0.45 (CD) to 0.6 (DVD). [30,31] The decrease of wavelength and the increase of NA have been driven by the quest to larger areal density. As the spot size on the medium is diffraction limited, the spot diameter d is proportional to beam wavelength λ and inversely proportional to NA. The spot area is proportional to the square of the diameter, and the areal density D is inversely proportional to spot area.

10

The Numerical Aperture is defined as NA = n sinθ, where θ is the vertex angle of the largest cone of meridional (i.e. crossing the optical axis) rays that can enter or leave an optical system or element, and n is the refractive index of the medium in which the vertex of the cone is located.

Materials Science Foundations Vols. 27-28

d ∝ (λ/ΝΑ);

D ∝ (NA/λ)2

11

(1)

Additional technologies include Ultra Density Optical (UDO) disks [32] and Magneto-Optic (MO) disks. UDO standard offers a recording density of 7.4 Gb/in2. It has been anticipated that the capacity of media will grow to 120 Gb (3rd generation, expected in 2007-2009). Beam wavelength of UDO technology is 405 nm. [32] In MO systems, the recording is performed by heating the media with a laser beam, and locally orienting the magnetic domains by simultaneously applying a magnetic field. Magnetic domain orientation only occurs in a region smaller than the diffraction limit. During the reading, domains cause a modulation of polarization of the readout beam. [30,33] At present, the areal density of optical disks lags behind magnetic media. Reliability and removability remain optics’ main advantages. Both stem from the possibility of larger distance of optical heads from the media surface, implying no wear associated with the access to data (reliability) and larger mechanical tolerances (removability). Current optical media materials offer excellent reliability at very low cost. A disadvantage is slow access time: removability needs slow rotational disk speeds, and the inertia of optical heads is large. Typically, an optical head weights some tens of grams, as compared to few grams of magnetic head assemblies. Slow access and low areal density of current optical technology have restricted the market of optical disks to segments where reliability and removability are critical, such as software distribution, entertainment, large archival storage, medical and biological imaging. Opportunities for semiconductor materials in optical storage are connected to ability of integrating photonics: the simplest approach to increase the areal density is to reduce dimensions of devices. Integration has also the advantage of reducing the inertia of optical heads, thus facilitating the improvement of data rates. Further opportunities are devised in disruptive solutions such as magnetic super resolution, two photon absorption and holographic storage. [34] TRANSMISSION The oscillating frequency ν=c/λ of light waves, where c is the velocity of light and λ is the wavelength, is in the range of 1014÷1015 Hz. This is several orders of magnitude larger than electric and radio signals. A modulated optical wave has an enormous capability of handling the signal bandwidths. In fact, for a bandwidth ∆ν much smaller than the oscillation frequency ν, the respective wavelength band ∆λ is

12

Nanostructured Silicon for Photonics

∆λ ≈

λ2 dλ ∆ν = ∆ν dν c

(2)

For example, at λ ≈ 1.5 µm, a signal with a frequency bandwidth of the order of 100 GHz has a wavelength bandwidth of just about a nanometer. Optical properties of transmission media are fairly homogeneous over such bandwidth, leading to small distortion and homogeneous attenuation. The idea of Wavelength Division Multiplexing (WDM) precisely stems from the fact that with optical data transmission one can squeeze one channel per nm (with hundred GHz bandwidth per channel) in a single transmission medium which can accept signals over several nm. The advantage of having several channels over a single medium has made data transmission over low-attenuation optical fibers particularly appealing for long haul networking. Optical fibers are transmission media based on total internal reflection (TIR), an effect which was first demonstrated by J. Tyndall in 1870. [35] In the last decades, we have witnessed the rapid progress of single mode silica fibers, starting from signal attenuation larger than 20 dB/km (R. Maurer, D. Keck, and P. Schultz at Corning in 1970), to the present values, less than 0.2 dB/km at 1.5 µm wavelength (Figure 10). WDM technology over fiber optics has benefited by other significant breakthroughs, such as the invention of Erbium Doped Fiber Amplifier (EDFA) and, for the second window (1.3 µm region, see Figure 10), the less known Praseodymium Doped Fluoride Fiber Amplifier (PDFFA). Opportunities for semiconductor technology in the field of telecommunications are wide. A good review which includes both inorganic an organic semiconductor technology has been published. [36] Elaborated signal processing is possible by exploiting all-optical circuits. [37,38] In this decade, information technology is entering its “tera-era”: each person will have more than 1 Terabyte of information storage capability, computation capacity in excess of 1 Teraoperations per second, and a communication bandwidth in excess of 1 Terabit per second. The world market of optical storage alone has reached € 30 billion in 2003 and the Optoelectronics Industrial Development Association (OIDA) forecasts € 240 billion ten years from now.

Materials Science Foundations Vols. 27-28

Figure 10. Attenuation in fiber optics.

LIGHTING Lighting is the science and technology of generation and processing of optical radiation to be perceived by human eye. Its purpose is to supply information by illuminating passive objects, e.g. for reading, r by directly providing information, e.g. as in cars’ taillights, in traffic lights, or in advertising signs.

US Consumption (TWh/yr)

In 2001, world-wide sales were more than 13 billion US dollars for lamp market only, and approximately 40 billion US dollars including all the applications. [39] Energy consumption for lighting in the United States is shown in Figure 11; predictions are for a consumption increase at the rate of 1.5% per year [40]. Energy

10000

Electricity

1000 Lighting Lighting including Solid State Lighting

1970

1980

1990

2000

2010

2020

year

Figure 11. Total energy, electricity and lighting consumptions and future projections of United States. Adapted from [40].

Modern lighting technology started with the invention of arc lamp in the 19th century. In arc lamps, emission takes place by radiative decay of gas atoms. [41] Excitation is provided by highly energetic electrons accelerated by the field between non-touching electrodes. In contrast, incandescence lamps [42] emit by heating a filament, which glows as a blackbody radiator

13

14

Nanostructured Silicon for Photonics

with continuous spectrum. In high-voltage discharge gas-containing tubes, lighting results from radiative decay of excited atoms as in arc lamps, but at lower gas pressure. Fluorescent lamps are similarly gas filled tubes; however, primary emission is at shorter (UV) wavelength, and it is converted to visible light by coating the inner walls of tubes with suitable UV absorbing materials, which reemit visible radiation. Solid state (incoherent) lighting is generated in Light Emitting Diodes (LEDs), which are pn-junctions. LEDs are not dissimilar in principle form the photovoltaic device shown in Figure 5, except that they operate by converting electrical energy into light: hence, LEDs are optimized to maximize radiative electron-hole recombinations. Usually, LEDs look different from solar cells; however, forward biased solar cells can actually work as LEDs and can be very efficient. [43] Electroluminescence (EL) in inorganic semiconductor pn-junctions was first observed in Ge and Si in 1952, [44] whereas the first reported EL effect in organic materials is dated 1953. [45] Organic LEDs have been recently reviewed [46-49]. In Figure 12, history and projections of luminous efficacy (the amount of outcoming photometric power per injected electrical power) are reported.

Figure 12. Historical evolution and projections of luminous efficacy of several lighting technologies. The thick black line represents a prediction of the efficacy attainable with LEDs, according to 2002 OIDA Roadmap. [49,50] Adapted from [40].

An important subclass of lighting includes self-illuminated displays, widely used for cell phones, cars, wristwatches, calculators, computers, TVs, and projection systems. Display technology is a fundamental enabler for imaging (vide infra). Commercial displays are currently Cathode Ray Tubes (CRTs, Figure 13) [3] and Liquid Crystals Displays (LCDs). Emerging technology are Organic LED (OLED) displays, [46] and Surface-conduction Electron-emitter

Materials Science Foundations Vols. 27-28

Displays (SEDs). The SED’s principle is similar to CRT’s, but it makes use of an array of tiny electron emitters measuring just several nanometers wide, each producing an individual pixel. [51,52] A special issue of Proceedings of IEEE has been entirely dedicated to flat panel technologies. [53]

Figure 13. Schematic diagram of operation of a color Cathode Ray Tube. The colors are the additive result of emission by three different phosphors, which are struck by three independent electron guns.

IMAGING Imaging is information coded as a function of position11. The interest in imaging stems from the global features that are associated with the topological distribution of information. An example of such features is the idea of “shape”. A photograph is a 2D representation of a set of light emitting or scattering sources, distributed in the 3D space. In its simplest form, this representation is built as a map of brightness associated with space; color pictures carry also information about color mapping. Movies, being a 2D representation of the 3D space with the additional dimension of time, are 3D images of space and time phenomena. Analogously, holography12 and tomography13 are 3D images of the 3D space. Although nowadays imaging techniques rely on a very wide spectrum of electromagnetic radiation, ranging from frequencies of the order of 50 MHz

11

A more strict definition would require the definition of a topological space (i.e., a space where one can define the neighborhood of a point), an object (a function defined on such topological space), a second topological space (possibly coincident with the first) and a homomorphism between the two spaces. 12

Holography, from Greek ὅλος, “whole”, and γραφείν, “write”, was firs proposed by Dennis Gabor (1948), [54] who won a Nobel prize for his idea in 1971. [55]

13

from Greek τόμος, “slice”.

15

16

Nanostructured Silicon for Photonics

(used in very long-range surveillance radars) up to X and gamma rays (used for medical imaging14), automatic imaging was born from visible light with photography in 1839, [56] by mimicking the behavior of human eye. An example of a present-day acquisition tool for optical imaging is the Charged Coupled Device (CCD), which was invented by Smith and Boyle at Bell Labs in 1969. [57]. Medical imaging [58] techniques working at optical wavelengths are reviewed in Ref. [59]. Additional applications include astrophysics, aviation and navigation, security, surveillance systems and military, entertainment, material analysis, seismology, oceanography, archaeology, just to name a few. The development of large volume data storage and multimedia technology has motivated a large interest in digital processing of images. A basic and practical introduction to digital imaging can be found in Ref. [60]. ANALYSIS Optical properties of materials reflect their internal electronic structure. The optical response of a material is described by its complex dielectric function. The science and technology of the determination of the dielectric function of materials by looking at their optical properties is called optical spectroscopy. Optical spectroscopy is one of the oldest tools to study material properties. [61] Its seeds were sown by Fraunhofer, who observed that the dispersed sun’s spectrum is crossed by a large number of fine dark lines (1814), now known as Fraunhofer lines. Kirchhoff later established that materials can be identified by looking at their spectrum. [62]

14

Medical imaging began in 1895 with Röntgen’s discovery ox X-rays; contemporary medical imaging began in the 1970s with the advent of computer tomography. [58]

Materials Science Foundations Vols. 27-28

Figure 14. Drawing of the original apparatus (1860) used by Kirchhoff and Bunsen for the observation of spectra. The little pearl held on support E represents the material under study. [62]

The fields of applications of optical spectroscopy include medicine, biology, physics, chemistry, astrophysics, materials science, and forensics. Common optical analytical techniques include luminescence, optical transmission, optical reflection, Raman scattering, Brillouin scattering, ellipsometry, nonlinear spectroscopy, time-resolved spectroscopy. Many general introductions to solid-state spectroscopy (not limited to optics) are available (e.g. [41, 63, 64, 65]). The reader can find an exhaustive introduction to semiconductor optics in Ref. [66] and a review of spectroscopy of semiconductor surfaces and interfaces in Ref. [67]. METROLOGY The appealing characteristic of optical measurement is the extremely high accuracy achievable with interferometry. Michelson and Morley could exploit such accuracy to prove that something was seriously wrong with the ether concept. [68] Even though their interferometer was invented over a century ago, it still remains unsurpassable in several measurement contexts, such as for gravitational waves. [69] With laser Doppler velocimetry, one can construct velocity maps of fluids in motions. [70] There are several areas in science and engineering where it is important to determine the flow field of fluids at the micron scale. Examples include µm-scale supersonic nozzles used as microthrusters on micro-satellites, [71] inkjet printers, and biomedical industry, where microfluidic devices are used for patient diagnosis, patient monitoring, drug delivery, and drug discovery. [72] Optical metrology is an extensive topic. Interested readers are directed to a recent book. [3]

17

18

Nanostructured Silicon for Photonics

1.4

SILICON AS THE ENABLING MATERIAL

Given the vastness of applications, it is not possible to imagine a single technology as the most effective for photonics in its generality. In this work, we focus on applications where solid state solutions are superior because of integration, costs and energy consumption issues. All these requirements have been the winning ingredients that drove the semiconductor industry, and, in particular, silicon (Si) technology. Indeed, the success of today’s microelectronic industry is based on • a single material (Si), which is widely available, can be purified to an unprecedented level, is easy to handle and to manufacture; [73] • Si native oxide (SiO2), which effectively passivates the surface of Si and is an excellent insulator; • a single dominating processing technology (Complementary MetalOxide-Semiconductor, or CMOS, technology), which accounts for more than 95% of the whole market of semiconductor chips; [74] • the possibility to integrate more and more devices, with a single transistor size which is decreasing (gate lengths of 180 nm are in production while 15 nm have been demonstrated); [75] • the ability of the Si industry to introduce improvements when the technology is hitting the so-called red brick wall, e.g. introduction of low k-materials and of Cu to reduce RC delays; • an accepted common roadmap which is dictating the technology evolution; [75] • the presence of large companies which define standards and trends (almost 90% of the market is shared by ten companies). In recent years some concerns about the evolution of this industry have been raised, related to fundamental materials and processing aspects, [76] such as the limitations of the operating speed of microelectronic devices due to the interconnect. [77] An option to overcome the electronic limitations is the introduction of optical functionality in Si technology. The question is whether this is possible and advantageous. If one compares the characteristics of photonic industry with microelectronics one can see many differences, • a variety of different materials are used, for example InP as substrate for light sources, silica as material for fibers, lithium niobate for modulators, and organic materials in many of these devices;

Materials Science Foundations Vols. 27-28

• no single material or single technology is leading the market, with some convergence appearing towards the use of InP as the substrate material, though with some issues related to costs and backward compatibility; • many small companies participate to specific market segments, with no large company dominating; • technology is still in its infancy; chip scale integration of optical components, standardization of technology and packaging, which enable low cost and high reproducibility, are not yet achieved; • roadmaps to dictate and forecast the evolution of photonics are only now being elaborated [78]. It is commonly accepted that the industrial model of microelectronics, if applied to photonics, will be a booster to the development and implementation of photonics. [79] All the big players of microelectronics have aggressive programs to develop microphotonics, mostly based on Si. It was predicted in the early 1990s that silicon based optoelectronics would be a reality before the end of the century. [80-82] Indeed, all the basic components have already been demonstrated, [83] including a silicon Raman laser. [84] REFERENCES 1. W. Pauli, “Albert Einstein in der Entwicklung der Physik,” Universitas, 13 (1958) p. 593-598. 2. The American Heritage® Dictionary of the English Language, Fourth Edition. Copyright © 2000 by Houghton Mifflin Company. Published by the Houghton Mifflin Company. 3. K. J. Gasvik: Optical Metrology, 3rd Edition (ISBN: 0-470-84300-4, Wiley, New York, 2002). 4. J. R. Ferraro, “Introductory Raman Spectroscopy”, Academic Press; 2 edition (October 28, 2002). 5. G. Sharma, and H. J. Trussell, “Digital Color Imaging”, IEEE Trans. Image Proc. 6 (1997), 901. 6. G. Sharma, M. J. Vrehl, and H. J. Trussell, “Color Imaging for Multimedia”, P. IEEE 86 (1998), 1088. 7. D. L. Simms, “Archimedes and burning mirrors”, Physics Education 10 (1975) 517-521. 8. O. Svelto, “Principles of lasers”, 2nd edition, Plenum Press: New York, 1982. 9. J. L. Bromberg, Phys. Today 41 (1988), 26.

19

20

Nanostructured Silicon for Photonics

10.R. E. Slusher, “Laser technology”, Reviews of Modern Physics 71 (Centenary 1999), S471. 11.J. D. Majumdar, and I. Manna, “Laser processing of materials”, Sadhana-Academy Proceedings in Engineering Science 28, Parts 3 & 4, June/August 2003, pp. 495–562. 12.N. Holonyak, “The semiconductor laser: a thirty-five-year perspective”, Proceedings of the IEEE 85 (1997), 1678. 13.R. D. Dupuis, “The Diode Laser – the First Thirty Days Forty Years Ago”, IEEE Leos Newsletter (February 2003), 3. 14.S. L. Valley (Ed.), “Handbook of geophysics and space environments”, McGraw-Hill Book Company Inc. 1965. 15.A. Goetzberger, C. Hebling, and H.-W. Schock, “Photovoltaic materials, history, status and outlook”, Materials Science and Engineering R 40 (2003) 1–46. 16.A. E. Becquerel, “On Electric Effects under the Influence of Solar Radiation”, Comt. Rend. Acad. Sci. 9 (1839) 561. 17.“Renewable Energy Scenario to 2040”, European Renewable Energy Council (EREC), 2004. 18.D. Bahnemann, “Photocatalytic water applications”, Solar Energy 77 (2004) 445.

treatment:

solar

energy

19.E. Mester, M. Ludany, and M. Seller: Laser Rev. 1 (1968), 3. 20.E. Mester, A. F. Mester, and A. Mester: Lasers Surg Med. 5 (1985), 31. 21.T. Karu: Laser Life Sci. 2, No. 1 (1988), 53. 22.A. Schindl, M. Schindl, H. Pernerstorfer-Schon, L. Schindl: J. Invest. Med. 48, No. 5 (2000), 312. 23.K. Nagarathnam, and K. M. B. Taminger, AIP Conference Proceedings Vol 552(1) pp. 153-160. February 2, 2001. 24.C. S. Montross, T. Wei, L. Ye, G. Clark, and Y. W. Mai, Int. J. of Fatigue 24 (10): 1021-1036 Oct 2002. 25.J. von Neumann, “First Draft of a Report on the EDVAC”, draft for the Moore School of Electrical Engineering, University of Pennsylvania, 1945. 26.R. P. Feynman, “There's Plenty of Room at the Bottom”, the Annual Meeting of the American Physical Society (1959).

Materials Science Foundations Vols. 27-28

27.M. Yamagishi, aper BA 01, Intermag Conf., Amsterdam, April 2002. 28.N. Yeh et al., paper BA 02, Intermag Conf., Amsterdam, April 2002. 29.J. T. Russell, “Analog to digital to optical photographic recording and playback system”, US Patent No. 3,501,586, filed in 1966, granted in 1970. 30.WTEC Panel Report on “The Future of Data Storage Technologies”, International Technology Research Institute, World Technology (WTEC) Division, June 1999. 31.NSIC-OIDA Optical Disk Storage Roadmap, National Storage Industry Consortium and Optoelectronics Industry Development Association, San Diego, 1997. 32.http://www.udo.com 33.M. Mansuripur, “The physical principles of Magneto-optical recording”, Cambridge University Press, 1995. 34.H. Coufal, and G. W. Burr: Optical data storage, in A. Guenther (ed.), International Trends in Optics (SPIE, Bellingham, Washington, 2002). 35.D. R. Goff, “Fiber Optic Reference Guide”, 3rd ed., Focal Press: Woburn, Massachusetts, 2002. 36.L. Eldada, Rev. Sci. Instr. 75 (2003), 575. 37.L. Thylen, M. Qiu, and S. Anand, Chemphyschem 5 (2004), 1268. 38.L. Pavesi, and D. J. Lockwood (Eds.): Silicon Photonics (Series: Topics in Applied Physics Vol. 94, Springer-Verlag, Berlin, 2004). 39.D. Gall, “Current Topics in Light Source Technology for Lighting and Radiation”, Advanced Engineering Materials 3 (2001), 775. 40.J. Tsao, “Solid-State Lighting: The Promise and the Potential”, American Vacuum Society Meeting (invited talk), November, 2003. 41.H. Kuzmany, “Solid state spectroscopy – An introduction”, SpringerVerlag: Berlin, 1998. 42.T. A. Edison, “Electric Lamp”, US Patent No. 223,898, filed on January 27, 1880. 43.M. A. Green, J. Zhao, A. Wang, P. J. Reece, and M. Gal, “Efficient silicon light-emitting diodes”, Nature 412 (2001), 805. 44.J. R. Haynes and H. B Briggs, “Radiation Produced in Germanium and Silicon by Electron-Hole Recombination”, Phys Rev. 86 (1952) 647. 45.A. Bernanose, M. Comte, and P. Vouaux, J. Chim. Phys. 50 (1953), 64.

21

22

Nanostructured Silicon for Photonics

46.J. R. Sheats, “Manufacturing and commercialization issues in organic electronics”, J. Mat. Res. 19 (2004), 1974. 47.L. S. Hunga, C. H. Chen, “Recent progress of molecular organic electroluminescent materials and devices”, Mat. Sci. and Eng. R 39 (2002) 143. 48.M. T. Bernius, M. Inbasekaran, J. O'Brien, and W. Wu, “Progress with Light-Emitting Polymers”, Adv. Mater. 12 (2000), 1737. 49.A. Kraft, A. C. Grimsdale, and A. B. Holmes, “Electroluminescent Conjugated Polymers–Seeing Polymers in a New Light”, Angew. Chem. Int. Ed. 37 (1998), 402. 50.J.Y. Tsao, Ed., Light Emitting Diodes (LEDs) for General Illumination Update 2002 (Optoelectronics Industry Development Association, Sep 2002). 51.J. Knisley, “New Strides in LED Technology Could Be a Sign of Things to Come”, EC&M (http:\\www.ecmweb.com), Nov 1, 2003. 52.J. Boyd, “Canon and Toshiba Go Their Own Way in Flat Panels”, IEEE Spectrum 41 (Nov. 2004), 24. 53.B. R. Chalamala, F. E. Libsch, R. H. Reuss, and B. E. Gnade, “Scanning the issue - special issue on flat-panel display technology”, Proc. of IEEE 90, 4 (2002) 447. 54.D. Gabor, “A New Microscopic Principles”, Nature 161, No. 4098, 777778 (1948). 55.D. Gabor, “Holography, 1948-1971”, Nobel Prize Lecture, 1971. 56.Sir John Herschel,“Note on the art of Photography, or The Application of the Chemical Rays of Light to the Purpose of Pictorial Representation,” presented to the Royal Society on 14 March 1839. 57.W. S. Boyle, and G. E. Smith, “Charge coupled semiconductor devices”, Bell Syst. Tech. J. 49:587-93, 1970. 58.Z. H. Cho, J. P. Jones, and M. Singh, “Foundations of Medical Imaging”, Wiley-Interscience (September 17, 1993). 59.X. Intes, and B. Chance, “Non-PET functional imaging techniques: optical”, Radiologic Clinics of North America 43 (2005) p. 221. 60.M. Galer, L. Horvat, “Digital Imaging: Essential Skills”, Focal Press; 2 edition (December 12, 2002).

Materials Science Foundations Vols. 27-28

61.E. F. Nichols, “A Study of the Transmission Spectra of Certain Substances in the Infra-Red”, Phys. Rev. (Series I) 1 (1893) p.1. 62.G. Kirchhoff and R. Bunsen, “Chemical Analysis by Observation of Spectra”, Annalen der Physik und der Chemie 110 (1860) pp. 161-189. 63.F. Wooten, “Optical Properties of Solids”, Academic Press, New York and London, 1972. 64.M. Dressel and G. Grüner, “Electrodynamics of Solids”, University Press, Cambridge, England, 2002. 65.J. L. Skinner, and W. E. Moerner, “Structure and dynamics in solids as probed by optical spectroscopy”, J. of Physical Chemistry 100 (1996) pp. 13251-13262. 66.N. Peyghambarian, S. W. Koch, and A. Mysyrowicz, “Introduction to Semiconductor Optics”, Prentice-Hall, New Jersey, 1993. 67.J. F. McGilp, “Optical characterisation of semiconductor surfaces and interfaces”, Progress in Surface Science 49 (1995) pp. 1-106. 68.A. A. Michelson, and E. W. Morley, The American Journal of Science No: 203, Vol. 134 (1887) 333. 69.L. Ju, D. G. Blair and C. Zhao, Rep. Prog. Phys. 63 (2000), 1317. 70.Coupland, J.M., ''Laser Doppler and Pulsed Laser Velocimetry in Fluid Mechanics'', in Photomechanics for Engineers, Pramod Rastogi (ed.), Springer-Verlag, 2000, pp 373-412, ISBN 3-450-65990-0 . 71.R. L. Bayt, A. A. Ayon, and K. S. Breuer: A performance evaluation of MEMS-bases micronozzles. AIAA Paper 97-3169, 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Seattle, 1997. 72.K. S. Yun, and E. Yoon: Biotechnol. Bioproc. E 9, No. 2 (2004), 86; P. R. Selvaganapathy, E. T. Carlen, and C. H. Mastrangelo: Proc. IEEE 91, No. 6 (2003), 954; B. H. Weigl, R. L. Bardell, and C. R. Cabrera: Adv. Drug Deliver Rev. 55, No. 3 (2003), 349. 73.J. D. Plummer, M. D. Deal, and P. B. Griffin: Silicon VLSI Technology, (Prentice-Hall, Upper Saddle River, New Jersey, 2000). 74.J. T. Clemens, Bell Lab. Tech. J. Autumn 1997, 76. 75.International Technology Roadmap for Semiconductors. 76.L. Risch: Mater. Sci. Eng. C 19, 2002, 363. 77.T. N. Theis: IBM J. Res. Dev. 44, 2000, 379.

23

24

Nanostructured Silicon for Photonics

78.http://mph-roadmap.mit.edu. 79.L. Kimerling: Appl. Surf. Sci. 159/160 (2000), 8. 80.R. A. Soref: Proc. IEEE 81 (1993), 1687. 81.O. Bisi, S. U. Campisano, L. Pavesi, and F. Priolo (eds.): Silicon Based Microphotonics: from Basics to Applications (IOS Press, Amstardam, 1999); L. Pavesi and D. Lockwood: Silicon Photonics, Topics in Applied Physics vol. 94 (Springer-Verlag, Berlin, 2004). 82.S. Ossicini, L. Pavesi and F. Priolo, Light Emitting Silicon for Microphotonics (Springer Tracts in Modern Physics Vol. 194, Berlin, 2003). 83.G. Masini, L. Colace, and G. Assanto: Mater. Sci. Eng. B 89 (2002), 2. 84.H. Rong, R. Jones, A. Liu, O. Cohen, D. Hak, A. Fang, and M. Paniccia, “A continuous-wave Raman silicon laser”, Nature 433, 725-728 (2005).

Materials Science Foundations Vols. 27-28 (2006) pp 25-44 © (2006) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSFo.27-28.25

2.

SI NANOCRYSTALS FUNDAMENTALS

2.1

LOW DIMENSIONAL STRUCTURES

The electronic and optical properties of molecules and crystals are determined by the constituent atoms, their interactions (bonding) and their spatial arrangements. The evolution from single atoms to crystals is a consequence of the increasing strength of the interactions and of the number of constituents. Two main states of the matters can be individuated: clusters (few atoms) and crystals (many atoms). In this chapter we are interested in clusters. As the cluster size increases, its properties can be described in terms of particle size and shape, instead of dealing with the number of atoms and their spatial configuration. When clusters assume nanometer dimensions, they are named nanosystems or quantum systems. When nanosystems are ordered domains of atoms, the word nanocrystals or nanocrystallites is widely used. Low dimensional systems may be classified according to their dimensions as quantum wells (two-dimensional confinement), quantum wires (onedimensional confinement) and quantum dots (quasi zero-dimensional confinement). Quantum wells and quantum wires still possess a translational symmetry in one or two dimensions, and a statistically large number of electronic excitations can be created. In quantum dots the translational symmetry is totally broken, and only a finite number of electrons and holes can be created. Quantum dots or nanocrystals are synonymous, though usually one uses the expression quantum dots when direct gap semiconductors are involved, and nanocrystals or nanoclusters in the other cases. Nanocrystals are fabricated by means of techniques borrowed from glass technology, colloidal chemistry and other techniques non-conventionally used for bulk crystal growth. This chapter deals with nanocrystals and nanoclusters dispersed in a transparent (dielectric) host environment. These nanocrystals exhibit a variety of guesthost phenomena, which are well known for molecular structures. Each nanocrystal ensemble has inhomogeneously broadened absorption and emission spectra due to distribution of sizes, shape fluctuations, environmental inhomogeneities and other features. Hence, the study of the optical properties of nanocrystals forms a strongly interdisciplinary field. The most studied nanocrystal systems are semiconductors. The St. Petersburg school in Russia [1,2] (1982) and independently the Murray Hill group in the United States [3] (1983) were the first to outline the size-dependent properties of nanocrystals due to quantum confinement (QC) effects. Since then, great progress in the field has been achieved due to extensive studies throughout the world. The materials considered are II-VI (CdSe, CdTe, CdS), III-V (GaAs, InAs), I-VII (CuCl, CuBr, AgBr) compounds and group IV (Si and Ge) semiconductors.

26

Nanostructured Silicon for Photonics

2.2

SI NANOSTRUCTURES

The interest for the optoelectronic properties of Si rose after the observation of photoluminescence at room temperature in porous Si (PS). [4-9] PS is usually formed by electrochemical etching of Si under controlled conditions, a procedure which leads to a skeleton of nanocrystalline Si, where QC effects induce a band gap increase and an enhanced radiative transition rate. The resulting material is named PS due to its morphology composed of a disordered web of pores, with feature size in the nanometer scale (quantum sponge [9]). It can be permeated by a variety of chemicals. Reactivity of PS is large because of its specific surface. Microelectronics compatibility of PS has been demonstrated and integration of driving circuits with light emitting element has been performed. At the same time microcavities with improved light emission properties have been produced. Although the interest in PS originated from its emission properties, the potential application areas are now wider than light emission devices. In comparison to PS, Si nanocrystals [10] embedded in amorphous silica (aSiO2) are better candidates for optoelectronic and nanophotonic applications, because of their robustness and stability and their better compatibility with the mainstream Complementary Metal Oxide Semiconductor (CMOS) technology. The generation of visible light from Si nanocrystals embedded in a-SiO2 matrix has been extensively studied to obtain optically tunable quantum systems by modifying the dimensions of the nanoparticles. The physical mechanism underlying high quantum efficiency for photoluminescence is mainly the QC of excitons in a nanometer scale crystalline structure. However, the Si/SiO2 interface is also thought to play an important role in both the surface passivation and formation of radiative states. Despite many experimental and theoretical efforts performed up to now, a clear picture of the emission and absorption processes in still lacking. In particular, a discrepancy exists about the size dependence of the experimental value of the transition energies and theoretical estimations based on a simple QC theory as well as for PS. [11-15] Individual nanocrystals participate differently to the radiative recombination processes, according to their size and interface characteristics. Therefore, the interpretation of experimental techniques assessing whole nanocrystal systems (for example, optical absorption) must be performed carefully. [16,17] Interface and/or defect centers can act as intermediate states for electron hole trapping and optical recombination. [18,19]

Materials Science Foundations Vols. 27-28

2.3 SI NANOSTRUCTURES GROWTH Si nanocrystals can be produced by different techniques either directly from a gas phase or indirectly by recrystallization within a matrix:[20-23] ion implantation of Si in SiO2, [24-26] laser ablation [27,28] gas evaporation, [29] sputter deposition [30,31] and Si/SiO2 multilayers formation [32] (Figure 1). The common feature of recrystallization techniques are the deposition or formation of sub-stoichiometric silica films, with a large excess of Si, followed by a high temperature annealing. The annealing causes a partial phase separation between the two constituent phases, Si and SiO2, with the formation of small Si nanocrystals. The size and density of the Si nanocrystals can be controlled by the deposition and the annealing parameters. The degree of phase separation depends on the annealing temperature and time. Recently, fine control of size distribution has been achieved by annealing of amorphous Si/SiO2 superlattices, and an almost monodispersed size distribution has been demonstrated.

Figure 1: Si nanocrystals fabrication techniques. The three lower techniques produce nanocrystals in a SiO2 matrix. Courtesy of J. Linnros.

In the following, we discuss two techniques in greater detail. 2.3.1 PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION (PECVD) Plasma enhanced chemical vapor deposition (PECVD) of substoichiometric Si oxide (SiOx) followed by high-temperature annealing that stimulates precipitation and phase separation represents a powerful method to produce Si nanocrystals. [33] The PECVD cell consists of an ultrahigh vacuum chamber (typically 10-9 Torr) and a RF generator, connected to the top electrode of the reactor. The RF forms a plasma which is used to crack the precursors. The sample holder is electrical-

27

28

Nanostructured Silicon for Photonics

ly grounded and acts as bottom electrode. The substrates, consisting either of Czochralski Si or quartz wafers, are usually heated at low temperature (200500 °C) during the deposition. The source gases (precursors) are high purity (99.99 % or higher) SiH4 and N2O. The N2O/SiH4 flow ratio γ is varied to introduce an excess of Si in the film, while keeping the total gas flow rate constant. After deposition, the SiOx thin films are annealed at high temperature (800-1250 °C) in ultra pure nitrogen atmosphere to induce precipitation and phase separation. By varying the flow ratio of the gaseous precursors, it is possible to change the stoichiometry of the film and, therefore, the size of the Si nanocrystals for a fixed annealing temperature. Moreover, one can also control the size of Si nanocrystals by changing temperature of the annealing process for a fixed Si concentration. As SiOx samples fabricated with γ = 15 or larger are stoichiometric (about 33 at. % of Si), for substoichiometric oxides the deposition must be performed with γ < 15 values. The structure of the substoichiometric SiOx films can be described as a random Si/SiO2 mixture [random mixture model (RMM)]; [34] this model neglects the existence of Si intermediate oxidation states. Alternatively, it can be considered as a mixture of all the Si oxidation states, under the form of Si-SixO4-x tetrahedra (0>σEr. In the case of sufficiently low powers σNCτΦ