III-Nitride LEDs: From UV to Green (Advances in Optics and Optoelectronics) 9811904359, 9789811904356

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Advances in Optics and Optoelectronics

Shengjun Zhou Sheng Liu

III-Nitride LEDs From UV to Green

Advances in Optics and Optoelectronics Series Editor Perry Ping Shum, Southern University of Science and Technology, Shenzhen, China

The Advances in Optics and Optoelectronics series focuses on the exciting new developments in the fast emerging fields of optics and optoelectronics. The volumes cover key theories, basic implementation methods, and practical applications in but not limited to the following subject areas: AI Photonics Laser Science and Technology Quantum Optics and Information Optoelectronic Devices and Applications Fiber-Based Technologies and Applications Near-infrared, Mid-infrared and Far-infrared Technologies and Applications Biophotonics and Medical Optics Optical Materials, Characterization Methods and Techniques Spectroscopy Science and Applications Microscopy and Adaptive Optics Microwave Photonics and Wireless Convergence Optical Communications and Networks Within the scopes of the series are monographs, edited volumes and conference proceedings. We expect that scientists, engineers, and graduate students will find the books in this series useful in their research and development, teaching and studies.

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Shengjun Zhou · Sheng Liu

III-Nitride LEDs From UV to Green

Shengjun Zhou Wuhan University Wuhan, China

Sheng Liu Wuhan University Wuhan, China

ISSN 2731-6009 ISSN 2731-6017 (electronic) Advances in Optics and Optoelectronics ISBN 978-981-19-0435-6 ISBN 978-981-19-0436-3 (eBook) https://doi.org/10.1007/978-981-19-0436-3 Jointly published with Science Press The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Science Press. © Science Press 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publishers, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publishers nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Over the last three decades, III-nitride LEDs have been widely used in the fields of solid-state lighting and high-resolution display owing to their advantages of high efficiency, low cost, energy-saving, environment-friendliness, long life, and compact size. It is obvious that replacing conventional light source, such as incandescent bulbs and fluorescent lamps, with III-nitride LEDs could significantly reduce energy consumption and alleviate increasingly serious global energy crisis. Furthermore, high-brightness III-nitride mini/micro-LEDs have significant potential to realize the demand of high dynamic range (HDR) for next-generation display. Remarkable breakthroughs have been achieved in recent years for research and development in III-nitride LEDs, such as the growth of high-quality III-nitride epitaxial layers and the fabrication technology of efficient III-nitride LEDs. However, there remain important issues for further boosting the efficiency of III-nitride LEDs. This book aims to provide readers with strategies and methods, which contributes to solving the important issues. It is based on the progresses made by our group in the field of epitaxial growth of III-nitride blue/green/ultraviolet LEDs, design and manufacturing technology of top-emitting LEDs, flip-chip LEDs, vertical LEDs, and high voltage LEDs as well as device reliability and measurement methods of LED characteristic parameters. It is useful for researchers and postgraduate students in the fields of physics and electronic engineering as well as engineers and scientists in various application fields of III-nitride LEDs. Chapter 1 provides an introduction of III-nitride LEDs technologies and their development history. Chapter 2 describes epitaxial growth of III-nitride blue/green/ultraviolet LEDs. A number of technologies, including V-pits, stacked GaN/AlN last quantum barrier, sputtered AlN nucleation layer, and patterned sapphire substrate with silica arrays, were used to improve the quantum efficiency of III-nitride LEDs. Chapter 3 discusses the technologies to realize high-efficiency topemitting III-nitride LEDs in terms of light extraction microstructure (e.g., patterned ITO, roughened surface/sidewall, and air voids structure), current blocking layer, back reflector, low optical loss electrodes, and Ni/Au wire grids transparent conductive electrodes. Chapter 4 overviews the progress of flip-chip LEDs made by our group. Highly reflective low-resistance p-type and via-hole-based n-type ohmic v

vi

Preface

contacts were developed to improve light extraction efficiency and current spreading of flip-chip LEDs, which offer promising strategies to realize high-efficiency flipchip LEDs. Chapter 5 provides fabrication technology of high voltage LEDs and vertical LEDs. Chapter 6 analyzes the mechanisms of forward leakage current and reverse leakage current as well as their effects on device reliability. Meanwhile, transient measurement methods of photometric parameters, colorimetric parameters, and electrical parameters of LEDs are discussed. We are deeply grateful to Dr. Han Ding, academician of Chinese Academy of Sciences and professor of Huazhong University of Science and Technology, Prof. Shunsheng Guo of Wuhan University of Technology, Prof. L. Jay Guo of University of Michigan, Prof. Binhai Yu of South China University of Technology and Dr. Shu Yuan and Shufang Wang of Quantum Wafer Inc. for their constant guidance and support. We would like to thank Bin Tang, Xiaoyu Zhao, Lang Shi, Ziqi Zhang, Xu Liu, Peng Du, Yu Lei, Zehong Wan, and Liyan Gong for their contributions. Besides, we express thanks to Dr. Hongpo Hu from HC SemiTek Corporation and Yingce Liu from Xiamen Changelight Co. Ltd. for their support and suggestion. Both authors appreciate the love and support their families in the past years. Finally, we also acknowledge valuable support from National Natural Science Foundation of China (52075394, 51775387, 51675386, U1501241, 51305266), National Youth Talent Support Program, Natural Science Foundation of Hubei Province (2018CFA091), and National High-tech R&D Program of China (863 Program Grant No. 2015AA03A101). Wuhan, China

Shengjun Zhou Ph.D and Professor, IAAM Scientist Medal and Fellow of VEBLEO Sheng Liu Ph.D and Professor, IEEE Fellow and ASME Fellow

Contents

1 Physics of III-Nitride Light-Emitting Diodes . . . . . . . . . . . . . . . . . . . . . . 1.1 History of III-Nitride LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Mechanisms of III-Nitride LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Radiative Recombination and Non-radiative Recombination . . . . . . 1.4 Internal Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Light Extraction Efficiency and External Quantum Efficiency . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 4 5 9 10

2 Epitaxial Growth of III-Nitride LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 III-Nitride Blue LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 III-Nitride Green LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 InGaN/GaN Superlattice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Stacked GaN/AlN Last Quantum Barrier . . . . . . . . . . . . . . . . 2.3 III-Nitride Ultraviolet LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Sputtered AlN Nucleation Layer . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Effect of PSS on UV LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Patterned Sapphire with Silica Array . . . . . . . . . . . . . . . . . . . . 2.3.4 Isoelectronic Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 InAlGaN/AlGaN Electron Blocking Layer . . . . . . . . . . . . . . . 2.3.6 Graded Al-Content AlGaN Insertion Layer . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 13 19 19 36 41 41 57 61 67 74 78 82

3 High-Efficiency Top-Emitting III-Nitride LEDs . . . . . . . . . . . . . . . . . . . 91 3.1 Light Extraction Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 3.1.1 PSS and Patterned ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 3.1.2 Double Layer ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.1.3 3D Patterned ITO and Wavy Sidewalls . . . . . . . . . . . . . . . . . . 99 3.1.4 Roughened Sidewalls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.1.5 Air Voids Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

vii

viii

Contents

3.2 Current Blocking Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 SiO2 Current Blocking Layer . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Patterned Current Blocking Layer . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Reflective Current Blocking Layer . . . . . . . . . . . . . . . . . . . . . . 3.3 Back Reflector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Low Optical Loss Electrode Structure . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Ni/Au Wire Grid Transparent Conductive Electrodes . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

112 112 117 122 125 135 142 147

4 Flip-Chip III-Nitride LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Via-Hole-Based Two-Level Metallization Electrodes . . . . . . . . . . . . 4.2 Dielectric DBR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Comparison of Flip-Chip LEDs and Top-Emitting LEDs . . . . . . . . . 4.4 Ag/TiW, Ni/Ag and ITO/DBR Ohmic Contacts . . . . . . . . . . . . . . . . . 4.5 High-Power Flip-Chip LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Double-Layer Electrode and Hybrid Reflector . . . . . . . . . . . . . . . . . . 4.7 Mini/Micro-LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1 Prism-Structured Sidewall of Mini-LED . . . . . . . . . . . . . . . . . 4.7.2 Light Extraction Analysis of Micro-LED . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

151 151 158 161 164 175 180 184 184 186 190

5 High Voltage and Vertical LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Direct Current High Voltage LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Alternating Current High Voltage LED . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Comparison of DC-HV LED and AC-HV LED . . . . . . . . . . . . . . . . . 5.4 Vertical LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

193 193 199 201 203 215

6 Device Reliability and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Influence of Dislocation Density on Device Reliability . . . . . . . . . . . 6.2 Forward Leakage Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Reverse Leakage Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Pad Luster Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Transient Measurement of LED Characteristic Parameters . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

217 217 219 222 225 230 238

Chapter 1

Physics of III-Nitride Light-Emitting Diodes

1.1 History of III-Nitride LEDs Light emission caused by electrically pumping was first reported by Henry in 1907 [1]. He found that the silicon carbide (SiC) crystal emitted yellowish light when a voltage was applied between two-point contacts on the SiC surface. The explanation of this phenomenon named electroluminescence was proposed by Kurt Lehovec in 1951 [2]. The electroluminescence process includes carrier injection over the p-n barrier and carrier recombination across the SiC bandgap. The appearance of quaternary alloy AlGaInP in the 1980s is the next important improvement in visible light LED technology [3–6]. The (Alx Ga1-x )0.5 In0.5 P heterostructure can be made to emit from red to green. AlGaInP-based LEDs are now the dominant emitters of red/orange region. However, the emission is limited to long wavelength because the bandgap turns into indirect when the Al concentration is higher than 53%. In addition, higher aluminum content leads to the weaker electronic confinement, resulting in a strong temperature sensitivity for the internal quantum efficiency. Significant improvement in crystal quality of GaN grown on sapphire contributed to the blue LED development. A low-temperature AlN buffer prior to GaN was adopted by using metalorganic chemical vapor deposition (MOCVD) in 1986 [7]. Later, a low-temperature GaN buffer was grown to obtain high-quality GaN film on sapphire using MOCVD and molecular beam epitaxy (MBE) [8, 9]. By employing postgrowth low-energy electron beam irradiation treatment, a breakthrough of p-type Mg-doped GaN was reported in 1989, which encouraged the birth of first p-n junction GaN-based LED. The conductivity of GaN doped with Zn or Mg increased sharply after being irradiated by low energy electron beam. The mechanism of increased conductivity was revealed, and thermal annealing could result in a similar effect on Mg- or Zn-doped GaN [10]. The dopant is passivated by Mg–H complex formation during crystal growth, while low energy electron beam irradiation or thermal annealing can destroy the Mg–H complex. Thermal annealing process is suitable for mass-production, which paves the way to the industrialization of GaN-based p–n junction devices. The high-brightness InGaN/GaN blue LEDs were grown on © Science Press 2022 S. Zhou and S. Liu, III-Nitride LEDs, Advances in Optics and Optoelectronics, https://doi.org/10.1007/978-981-19-0436-3_1

1

2

1 Physics of III-Nitride Light-Emitting Diodes

sapphire using MOCVD in 1993 [11], followed by efficient InGaN/AlGaN blue LED in 1994. Considering the pioneering and critical contributions by Isamu Akasaki, Hiroshi Amano and Shuji Nakamura, they were awarded Nobel Prize in Physics in 2014.

1.2 Mechanisms of III-Nitride LEDs LED is a kind of optoelectronic device based on the p–n junction, which consists of the p-type doped region and the n-type doped region. When two semiconductor materials form a p–n junction, electrons diffuse from the n-region to the p-region, and holes diffuse from the p-region to the n-region. Thus, space charge region (depletion region) is formed at the interface between the p-region and n-region. There exists a built-in electric field whose direction is from the n-region to the p-region. The drift process takes place under the effect of the built-in electric field. When the applied bias is zero, the diffusion and drift of carriers could finally reach a dynamic balance. Figure 1.1a shows the energy band diagram of p–n junction without applied bias. The electrons flow from the n-region with a high Fermi level to the p-region with a low Fermi level. Meanwhile, the holes flow from the p-region to the n-region. Therefore, E Fn keeps moving downward and E Fp keeps moving upward until E Fn equals E Fp . Figure 1.1b shows the energy band diagram of p–n junction when a forward bias is applied. We can find that the value of E Fn is higher than that of E Fp . The electric field induced by the forward bias is opposite to the built-in electric field, resulting in alleviation of electric field in the depletion region. Therefore, diffusion current is higher than drift current. Figure 1.1c shows the energy band diagram of p–n junction when a reverse bias is applied. The value of E Fp is higher than that of E Fn . Besides, the electric field generated by the reverse bias possesses the same direction as the built-in electric field, which enhances the drift process and leads to a larger drift current than diffusion current. The light-emitting region in the initial LED based on homogeneous junction structure is determined by the diffusion length of electrons and holes. Because the carrier diffusion length is generally on the order of micrometers, this will lead to the low electron and hole concentration radiative recombination and thus the low quantum efficiency. The double heterostructure, which consists of a wide bandgap p-type semiconductor, wide bandgap n-type semiconductor and thin interlayer of narrow band gap semiconductor between them, can improve the quantum efficiency of LEDs. Wide bandgap p-type and n-type semiconductor materials provide holes and electrons respectively, while narrow bandgap materials serve as carrier confinement layers. The region where electrons and the holes recombine is defined as the active region. By thinning the active region, internal quantum efficiency of LEDs is improved and the re-absorption of photons in the active region is reduced. When the thickness of active region is close to the de Broglie wavelength of electrons, the energy of carriers is no longer continuous in the direction of movement perpendicular to the surface. MOCVD can be used to grow multiple quantum wells (MQWs) as

1.2 Mechanisms of III-Nitride LEDs

3

Fig. 1.1 Mechanism of p–n junction electroluminescence. a Without bias, b with a forward bias, and c with a reverse bias

active region. The electrons and holes in MQWs recombine and emit photons. The wavelength of photons is determined by composition and thickness of InGaN QW, as shown in Fig. 1.2.

4

1 Physics of III-Nitride Light-Emitting Diodes

Fig. 1.2 Mechanism of electroluminescence for GaN-based LEDs

1.3 Radiative Recombination and Non-radiative Recombination In semiconductor materials, the recombination mechanisms of electrons and holes can be divided into radiative recombination and non-radiative recombination. In Fig. 1.3, radiative recombination indicates that electrons and the holes recombine and release the energy in the form of photons. When electrons and holes recombine to release energy in other ways such as transferring energy to the lattice, increasing the vibration of the lattice or converting into the kinetic energy of free carriers, this process is known as non-radiative recombination. The emission mechanism of LEDs is based on the radiative recombination of electrons and holes. Radiation recombination and non-radiation recombination coexist and compete in LEDs. It is possible to enhance the radiative recombination and to decrease the non-radiative recombination by improving the crystal quality of the epitaxial material and optimizing the design of epitaxial structure.

Fig. 1.3 Radiative recombination and non-radiative recombination

1.4 Internal Quantum Efficiency

5

1.4 Internal Quantum Efficiency The internal quantum efficiency (IQE) is defined as the ratio of the number of photons produced by radiative recombination per unit time to the number of electrons injected per unit time [12], which is given by: ηIQE =

the number of photons produced by radiative recombinationper unit time the number of electrons injected per unit time

=

PIQE /(hv) I /e

(1.1)

where PIQE is the light output power, I is the injecting current, e is the elementary charge (1.6022 × 10−19 C), h is the Planck constant (6.626196 × 10−34 J·s), and v is the frequency of the photon. The radiative recombination lifetime (τr ) and non-radiative recombination lifetime (τnr ) determine the value of recombination rate. The radiative recombination rate is proportional to 1/τr , and the non-radiative recombination rate is proportional to 1/τnr . The ηIQE can be expressed as: ηIQE =

1/τr 1/τr +1/τnr

=

1 1+τr /τnr

=

τnr τnr +τr

(1.2)

It can be seen that when τnr  τr , the possibility of non-radiative recombination is much smaller than that of radiative recombination. For direct bandgap nitride semiconductor materials, the crystal quality of the nitride semiconductors can be improved to obtain higher IQE. Reducing dislocations, point defects, and other non-radiative recombination centers leads to the increased radiative recombination efficiency. (1)

Temperature-dependent photoluminescence (PL) measurement to estimate IQE

In the temperature-dependent PL measurement, it is assumed that the incident intensity of laser is I0 , the light intensity absorbed by the sample is Iab , and the light intensity radiated by the sample is Irad . When the temperature is T, the IQE of the sample can be expressed as [13]: ηIQE =

Irad Iab

=

1/τr 1/τr +1/τnr

=

1 1+τr /τnr

(1.3)

Assuming that during the temperature dependent PL measurement, the light intensity I0 of the incident laser remains unchanged, and the light intensity Iab absorbed by the sample also remains unchanged. When the temperature is close to 0 K, τnr  τr , the IQE is approximately 100%, which is expressed as: ηIQE (∼ 0K) =

Irad (∼0K) Iab

≈1

(1.4)

Therefore, the IQE at room temperature can be derived from the equations above [14–17]: ηIQE (RT) =

Irad (RT) Iab

=

Irad (RT) Irad (∼0K)

(1.5)

6

1 Physics of III-Nitride Light-Emitting Diodes

The integral of the emission wavelength is used to obtain IQE at different temperatures. (2)

ABC model to calculate IQE

The IQE of LED is determined by current injection efficiency and radiative recombination efficiency, which can be defined as: ηIQE = ηinj × ηrad

(1.6)

where ηinj is the current injection efficiency, and ηrad is radiative recombination efficiency. Under high injection current, the injected electron concentration n (cm−3 ) is close to the hole concentration p (cm−3 ). The radiative recombination efficiency can be defined as: ηrad = Bn2 /(An + Bn2 + Cn3 )

(1.7)

where A (s−1 ) is the Shockley–Read–Hall (SRH) non-radiative recombination coefficient, B (cm3 s−1 ) is the radiative recombination coefficient, and C (cm6 s−1 ) is the Auger recombination coefficient. Assuming that ηinj = 1, the IQE can be expressed as [18–20]: ηIQE = Bn2 /(An + Bn2 + Cn3 )

(1.8)

The above-mentioned ABC model has been widely used. Table 1.1 summarizes the recombination coefficients in the ABC model. Table 1.1 Measured values of recombination coefficients (A, B, C) Author

A (s−1 )

Zhang et al. [21]

1.0 ×

107

Shen et al. [22]

5.4 ×

107

Meneghini et al. [23]

2.3 × 107

Laubsch et al. [24]

0.47 ×

Piprek [25]

1.0 × 107

Gardner et al. [26]

5.4 ×

107

Nippert et al. [27]

1.1 ×

107

Shatalov et al. [28]

1.25 × 108

107

B (cm3 s−1 )

C (cm6 s−1 )

2.0 ×

10−11

1.5 × 10−30

2.0 ×

10−11

2.0 × 10−30

1.0 × 10−11

1.0 × 10−30

0.12 ×

10−11

7.0 × 10−11

0.35 × 10−30 2 × 10−30

2.0 ×

10−11

2.0 × 10−30

2.0 ×

10−11

2.3 × 10−30

9.84 × 10−11

5.37 × 10−28

1.4 Internal Quantum Efficiency

(3)

7

Electroluminescence measurement

The carrier concentration in the active region of the LED can be expressed by the carrier rate equation [29]: dn dt

=

I eV

− R(n)

(1.9)

where n is the concentration of injected minority carriers, I is the driving current, V is the volume of the active region, and R(n) is the total recombination rate (including radiative recombination, non-radiative recombination, etc.). For the steady state (i.e.: dn/dt = 0), the injection current efficiency is 100%, and the carrier rate equation can be expressed as [18, 30]: I eV

=

=

I eSd

J ed

= R(n) = An + Bn2 + Cn3

(1.10)

where J is the current density, V is the volume of the active region, S is the crosssectional area of the active region, d is the thickness of the active region. Based on Eqs. (1.8) and (1.10), the relationship between IQE and current density can be obtained by using coefficients A, B and C as fitting parameters. According to Eq. (1.8): ηIQE =

B A/n+B+Cn

≤

B√ B+2 AC

(1.11)

Therefore, the maximum IQE, ηmax is expressed as: ηmax =

B√ B+2 AC

(1.12)

In the relationship curve, we assume that the IQE reaches the maximum value ηmax and the corresponding current density is Jmax . When J = Jmax , the maximum values of IQE, ηmax and current density Jmax satisfy dη/dJ = 0. Equation (1.13) can be obtained by deriving n from Eq. (1.8), which can be expressed as: dη dn

=

2Bn(An+Bn2 +Cn3 )−Bn2 (A+2Bn+3Cn2 )

(An+Bn2 +Cn3 )2

(1.13)

Equation (1.14) can be obtained by deriving n from both sides of Eq. (1.10), which can be expressed as: dJ dn

  = qd A + 2Bn + 3Cn2

(1.14)

When dη/dJ = 0, Eq. (1.15) can be obtained by Eqs. (1.13) and (1.14). It can be expressed as: n2 =

A C

(1.15)

8

1 Physics of III-Nitride Light-Emitting Diodes

Equation (1.16) can be obtained by Eqs. (1.8) and (1.10). It can be expressed as: J =

edBn2 ηIQE

(1.16)

Equation (1.17) can be obtained by Eqs. (1.12) and (1.15). It can be expressed as: Jmax =

edA C

 √  B + 2 AC

(1.17)

From Eqs. (1.12) and (1.17), the relationship between coefficients A, B and C can be expressed by ηmax and Jmax : B=

4ed ηmax A2 Jmax (1−ηmax )2

(1.18)

C=

4e2 d 2 3 2A 2 (1−η Jmax max )

(1.19)

From Eqs. (1.8), (1.10), (1.12) and (1.17), the relationship between IQE and current density can be obtained: ηIQE = 1 − (4)

(1−ηmax ) 2J

 1+

ηIQE J ηmax Jmax



ηIQE JJmax ηmax

(1.20)

Photoluminescence measurement

In the excited state, assuming that the electron concentration n is close to the hole concentration p, the total recombination rate equation can be expressed as [22]: dn dt

= −An − Bn2 − Cn3 + G

(1.21)

where n is the carrier concentration and G (cm−3 s−1 ) is the generation rate. In the steady state (i.e.: dn/dt = 0), Eq. (1.21) can be expressed as: G = An + Bn2 + Cn3

(1.22)

Therefore, the IQE can be expressed through Eqs. (1.8) and (1.22) as: ηIQE =

Bn2 G

(1.23)

In the PL measurement, the carrier concentration n can be calculated by the following equation [31, 32]: nPL =

IPL ×τ (hv)×dMQW

  × e−αGaN dGaN × 1 − e−αMQW dMQW × (1 − R)

(1.24)

where IPL (W/cm2 ) is the power density of the laser, τ (s) is the carrier lifetime, hv (eV) is the energy of the injected photons, dGaN (nm) is the thickness of the GaN capping layer, dMQW (nm) is the thickness of the active region, αGaN (cm−1 ) and

1.4 Internal Quantum Efficiency

9

αMQW (cm−1 ) are the absorption coefficients of GaN and MQWs respectively, and R is the reflectivity of LED wafer. The carrier concentration nPL can be calculated by Eq. (1.24). The SRH nonradiative recombination coefficient A, radiative recombination coefficient B and Auger recombination coefficient C can be substituted into Eq. (1.23) to obtain the ηIQE .

1.5 Light Extraction Efficiency and External Quantum Efficiency Light extraction efficiency (LEE) is an important parameter, which can be defined as [12]: ηLEE =

the number of photons emitted into free spaceper unit time the number of photons emitted from active regionper unit time

=

P/(hv) PIQE /(hv)

(1.25)

where P is the light output power emitting to free space. A number of approaches have been used to enhance LEE of LED, such as chip shaping, patterned sapphire substrate, patterned ITO, reflective current blocking layer, photonic crystals, bottom reflector, air-void structures, surface/sidewall roughening, reflective electrodes and stealth laser scribing, which will be discussed in other chapters. The external quantum efficiency (EQE) of LEDs is defined as: ηEQE =

the number of photons emitting to free space per unit time the number of electrons injected per unit time

=

P/(hν) I /e

(1.26)

where P is the light output power, h is the Planck constant, ν is the frequency of the photon, e is the elementary charge, and I is the injection current, respectively. In electroluminescent (EL) measurement, the relationship between carrier concentration and injection current is given by: nEL =

Iτ eAdMQW

×ρ

(1.27)

where I (mA) is the injection current, τ (ns) is the carrier lifetime. A (cm2 ) is the area of the chip, dMQW (nm) is the thickness of the active region, and ρ is the current injection efficiency, respectively. Besides, the relationship between the ηEQE and the injection current I (mA) is expressed as: ηEQE =

P/(hν) I /e

=

Pλ I

×

e hc

=

Pλ 1240×I

(1.28)

10

1 Physics of III-Nitride Light-Emitting Diodes

where P is the light output power, I is the injection current, and λ is the wavelength of the emitted light, respectively. In addition, the EQE is the product of the IQE and LEE [33]: ηEQE = ηIQE ηLEE

(1.29)

Therefore, the ηEQE can also be calculated by using Eq. (1.29).

References 1. Round HJ (1991) A note on carborundum. Semiconductor devices: pioneering papers. World Scientific, p 879 2. Lehovec K, Accardo CA, Jamgochian E (1951) Injected light emission of silicon carbide crystals. Phys Rev 83(3):603–607 3. Ikdea M, Nakano K, Mori Y et al (1986) MOCVD growth of AlGaInP at atmospheric pressure using triethylmetals and phosphine. J Cryst Growth 77(1):380–385 4. Kobayashi K, Kawata S, Gomyo A et al (1985) Room-temperature CW operation of AlGaInP double-heterostructure visible lasers. Electron Lett 21(20):931–932 5. Ohba Y, Ishikawa M, Sugawara H et al (1986) Growth of high-quality inGaAIP epilayers by MOCVD using methyl metalorganics and their application to visible semiconductors lasers. J Cryst Growth 77(1):374–379 6. Itaya K, Ishikawa M, Uematsu Y (1990) 636 nm room temperature CW operation by heterobarrier blocking structure InGaAlP laser diodes. Electron Lett 26(13):839–840 7. Amano H, Sawaki N, Akasaki I et al (1986) Metalorganic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layer. Appl Phys Lett 48(5):353–355 8. Nakamura S (1991) GaN growth using GaN buffer layer. Appl Phys Lett 30(Part 2, No. 10A):L1705–L7 9. Lei T, Fanciulli M, Molnar RJ et al (1991) Epitaxial growth of zinc blende and wurtzitic gallium nitride thin films on (001) silicon. Appl Phys Lett 59(8):944–946 10. Nakamura S, Iwasa N, Senoh M et al (1992) Hole compensation mechanism of p-type GaN films. Jpn J Appl Phys 31(Part 1, No. 5A):1258–1266 11. Nakamura S, Senoh M, Mukai T (1993) P-GaN/N-InGaN/N-GaN double-heterostructure bluelight-emitting diodes. Jpn J Appl Phys 32(Part 2, No.1A/B):L8–L11 12. Schubert EF (2006) Light emitting diodes. Cambridge University Press, Cambridge, England 13. Chichibu S, Onuma T, Sota T et al (2003) Influence of InN mole fraction on the recombination processes of localized excitons in strained cubic InxGa1-xN/GaN multiple quantum wells. J Appl Phys 93(4):2051–2054 14. Zhou SJ, Hu HP, Liu XT et al (2017) Comparative study of GaN-based ultraviolet LEDs grown on different-sized patterned sapphire substrates with sputtered AlN nucleation layer. Jpn J Appl Phys 56(11):111001 15. Lu TP, Ma ZG, Du CH et al (2014) Temperature-dependent photoluminescence in light emitting diodes. Sci Rep 4:6131 16. Watanabe S, Yamada N, Nagashima M et al (2003) Internal quantum efficiency of highlyefficient Inx Ga1−x N-based near-ultraviolet light emitting diodes. Appl Phys Lett 83(24):4906– 4908 17. Brandt O, Ringling J, Ploog KH et al (1998) Temperature dependence of the radiative lifetime in GaN. Phys Rev B 58(24):R15977–R15980 18. Ryu HY, Kim HS, Shim JI (2009) Rate equation analysis of efficiency droop in InGaN light emitting diodes. Appl Phys Lett 95(8):081114

References

11

19. Binder M, Nirschl A, Zeisel R et al (2013) Identification of nnp and npp Auger recombination as significant contributor to the efficiency droop in (GaIn)N quantum wells by visualization of hot carriers in photoluminescence. Appl Phys Lett 103(7):071108 20. Galler B, Drechsel P, Monnard R et al (2012) Influence of indium content and temperature on Auger-like recombination in InGaN quantum wells grown on (111) silicon substrates. Appl Phys Lett 101(13):131111 21. Zhang M, Bhattacharya P, Singh J et al (2009) Direct measurement of auger recombination in In0.1 Ga0.9 N/GaN quantum wells and its impact on the efficiency of In0.1 Ga0.9 N/GaN multiple quantum well light emitting diodes. Appl Phys Lett 95(20):201108 22. Shen YC, Mueller GO, Watanabe S et al (2007) Auger recombination in InGaN measured by photoluminescence. Appl Phys Lett 91(14):141101 23. Meneghini M, Trivellin N, Meneghesso G et al (2009) A combined electro-optical method for the determination of the recombination parameters in InGaN-based light emitting diodes. J Appl Phys 106(11):114508 24. Laubsch A, Sabathil M, Baur J et al (2010) High-power and high-efficiency InGaN-based light emitters. IEEE Trans Electron Devices 57(1):79–87 25. Piprek J (2010) Efficiency droop in nitride-based light emitting diodes. Phys Status Solidi A 207(10):2217–2225 26. Gardner NF, Müller GO, Shen YC et al (2007) Blue-emitting InGaN-GaN doubleheterostructure light emitting diodes reaching maximum quantum efficiency above 200 A/cm2 . Appl Phys Lett 91(24):243506 27. Nippert F, Tollabi MM, Davies MJ et al (2018) Auger recombination in AlGaN quantum wells for UV light-emitting diodes. Appl Phys Lett 113(7):071107 28. Shatalov M, Chitnis A, Koudymov A et al (2002) Differential carrier lifetime in AlGaN based multiple quantum well deep UV light emitting diodes at 325 nm. Jpn J Appl Phys 41(10B):L1146–L1148 29. Olshansky R, Su C, Manning J et al (1984) Measurement of radiative and nonradiative recombination rates in InGaAsP and AlGaAs light sources. IEEE J Quantum Electron 20(8):838–854 30. David A, Grundmann MJ (2010) Droop in InGaN light emitting diodes: a differential carrier lifetime analysis. Appl Phys Lett 96(10):103504 31. Chiu CH, Lin CC, Han HV et al (2012) High efficiency GaN-based light emitting diodes with embedded air voids/SiO2 nanomasks. Nanotechnology 23(4):045303 32. Lee YJ, Chiu CH, Ke CC et al (2009) Study of the excitation power dependent internal quantum efficiency in InGaN/GaN LEDs grown on patterned sapphire substrate. IEEE J Sel Top Quantum Electron 15(4):1137–1143 33. Piprek J, RÄomer F, Witzigmann B (2015) On the uncertainty of the Auger recombination coefficient extracted from InGaN/GaN light emitting diode efficiency droop measurements. Appl Phys Lett 106(10): 101101

Chapter 2

Epitaxial Growth of III-Nitride LEDs

2.1 III-Nitride Blue LEDs The GaN-based blue LEDs are regarded as the environmental-friendly light sources, due to high electro-optical conversion efficiency, energy saving, long lifetime, and compact size [1]. The epitaxial layers of InGaN/GaN LED are generally grown by MOCVD on the sapphire substrate. Trimethylgallium (TMGa), trimethylindium (TMIn), trimethylaluminum (TMAl) are used as group III sources and ammonia (NH3 ) is used as the group V source. Silane (SiH4 ) and bis-cyclopentadienyl magnesium (Cp2 Mg) are used as the n-dopant and p-dopant sources, respectively. In addition, hydrogen (H2 ) and nitrogen (N2 ) are used as carrier gases. The reaction process of growing GaN epitaxial layers can be expressed by: Ga(CH3 )3 + NH3 → GaN + 3CH4

(2.1)

Due to the complexity of generate reactions of GaN, complex intermediate reaction processes are not considered in the above equation. Some researchers have proposed several reactions that may be involved. (1)

Decomposition reaction of TMGa: Ga(CH3 )3 → Ga(CH3 )2 + Ga(CH3 ) + Ga + CH3

(2)

Decomposition reaction of NH3 : NH3 → NH2 + NH + N + H

(3)

(2.2)

(2.3)

Synthesis reaction of GaN: Ga + N → GaN

© Science Press 2022 S. Zhou and S. Liu, III-Nitride LEDs, Advances in Optics and Optoelectronics, https://doi.org/10.1007/978-981-19-0436-3_2

(2.4)

13

14

(4)

(5)

2 Epitaxial Growth of III-Nitride LEDs

Synthesis reaction of CH4 , H2 and N2 : CH3 + H → CH4

(2.5)

N + N → N2

(2.6)

H + H → H2

(2.7)

Side reaction: Ga(CH3 )3 + NH3 → Ga(CH3 )3 : NH3 + [Ga(CH3 )]n : NH3

(2.8)

Figure 2.1 shows a schematic illustration of the epitaxial structure of GaN-based blue LED grown on patterned sapphire substrate (PSS). The GaN-based blue LED structure consists of a low-temperature GaN nucleation layer (NL), an undoped GaN layer, a Si-doped n-GaN layer, an InGaN/GaN superlattice (SL) strain release layer (SRL), an InGaN/GaN MQW, a low-temperature p-GaN layer, a p-AlGaN electron Fig. 2.1 Schematic illustration of GaN-based blue LED epitaxial structure

2.1 III-Nitride Blue LEDs

15

blocking layer (EBL), a Mg-doped p-GaN layer, and a heavily Mg-doped p+ -InGaN contact layer. The process flow of GaN-based blue LED grown on PSS can be illustrated as follows: (1)

Cleaning of PSS

The surface morphology and cleanliness of the substrate directly affect the subsequent growth of GaN epitaxial layer. It is vital to clean the substrate before the growth of epitaxial layer. To remove impurities, moisture and oxygen on the surface of the substrate, the PSS in the reaction chamber is usually baked at about 1100 °C for 5–10 min in H2 atmosphere. (2)

Low-temperature GaN nucleation layer

There are numerous dislocations in GaN epitaxial layers grown on sapphire substrate because of large mismatch in lattice constant and thermal expansion coefficient between GaN and sapphire substrate. To overcome the lattice mismatch between GaN and sapphire, a low-temperature GaN NL was introduced prior to growth of GaN epilayers at high temperature. It can effectively reduce the dislocation density in the epitaxial layer and improve the crystal quality of the subsequent growth of GaN. At a low temperature of about 530 °C, NH3 is introduced to the reaction chamber continuously for 2–3 min in order to nitride the substrate by replacing the Al–O bond with AlNx O1−x bond. Then TMGa is passed into the reaction chamber to grow a GaN NL with a thickness of about 20 nm. After the growth of a low-temperature GaN NL, the temperature of the reaction chamber is increased. The NL is transformed from polycrystalline or amorphous state to single crystal GaN with wurtzite structure covering the surface of the substrate. In the subsequent high-temperature GaN growth process, GaN grains grow up gradually and coalesce to form a thin layer. Besides in-situ low-temperature GaN/AlN NL, an ex-situ AlN NL with a thickness of about 20 nm can also be deposited on the PSS by reactive magnetron sputtering. In this process, 20 nm-thick AlN film is deposited on sapphire substrate in a commercial reaction magnetron sputtering system, NMC “iTops A330”. The Al sputtering target is 2 inch in diameter and high-purity (at% > 99.999%). The sputtering process is performed at 650 °C by feeding 120 sccm N2 , 1 sccm O2 , and 30 sccm He. Subsequently, the substrate with sputtered AlN NL is thermally annealed at 850 °C for 5 min to further improve the crystalline quality. Figure 2.2 shows the in-situ temperature transient for epitaxial growth of GaN on different NLs. The epitaxial growth of GaN can be divided into three stages: The first stage is the growth of low-temperature GaN NL and recrystallization. GaN is firstly covered on the PSS with cubic or hexagonal phases, and the island structure with increased roughness is formed after high temperature annealing. The second stage is the transition stage from three-dimensional (3D) growth to two-dimensional (2D) growth of GaN. In this stage, GaN islands grow gradually and coalesce with each other. The third stage is 2D growth, in which the GaN epitaxial layer grows at a high temperature to form a smooth and flat film. The growth of GaN on sputtered AlN NL

16

2 Epitaxial Growth of III-Nitride LEDs 1200

With sputtered AlN NL With low-temperature GaN NL

1000

Temperatrue (°C )

Fig. 2.2 In-situ temperature monitoring curves of GaN epitaxial layer grown on sputtered AlN NL and low-temperature GaN NL

800

600

400

0

2000

4000

6000

8000

10000

Time (s)

directly starts from the transition stage. At this stage, GaN grown on the sputtered AlN NL undergoes a mixture of 3D and 2D growth mode. However, using the lowtemperature GaN NL, GaN is mainly grown in a 3D mode and GaN grains with large thickness differences are formed subsequently, leading to a large number of interfaces when these grains coalesce. As a result, the transition time from 3D growth to 2D growth will be prolonged. (3)

Undoped GaN (u-GaN) layer

After the growth of NL, TMGa and NH3 are continuously passed into the reaction chamber to grow u-GaN layer at a high temperature of about 1020 °C. In the early stage of u-GaN growth, 3D growth is promoted by decreasing the growth temperature, increasing the pressure of the reaction chamber and lowering the V/III ratio. Then the GaN growth mode is changed from 3D to 2D by increasing the growth temperature, reducing the growth pressure and increasing the V/III ratio, which promotes crystal grains to coalesce until the surface becomes flat. The growth thickness of the u-GaN layer is generally 2–3 μm. (4)

Si-doped n-GaN layer

The temperature of the reaction chamber is increased to be approximately 1020 °C. TMGa, NH3 , and SiH4 are then passed into the reaction chamber to grow n-GaN layer with a thickness of 2–3 μm (Si-doping = 1 × 1019 cm−3 ). (5)

InGaN/GaN superlattice strain release layer

Figure 2.3 shows the cross-sectional transmission electron microscope (TEM) image of the InGaN/GaN SRL. The lattice mismatch between InN and GaN is about 11%, which results in severe compressive strain in the InGaN QW. This in-plane compressive strain not only leads to piezoelectric polarization induced quantumconfined Stark effect (QCSE) which reduces the probability of electron and hole radiation recombination, but also affects the distribution of In-content and the crystal

2.1 III-Nitride Blue LEDs

17

Fig. 2.3 Cross-sectional TEM image of InGaN/GaN SRL and InGaN/GaN MQW

quality of InGaN/GaN MQW. The crystal quality of InGaN/GaN MQW can be improved by inserting InGaN underlayer or InGaN/GaN SRL. The SRL typically consists of periodically alternating layers of InGaN and GaN. The thickness of InGaN and GaN is a few nanometers, and the growth temperature is about 800 °C. (6)

InGaN/GaN MQW

Figure 2.4 shows the cross-sectional TEM image of InGaN/GaN MQW structure. It is possible to achieve the different InN/InGaN ratio by adjusting the growth temperature, which can form a material system with different bandgap to realize the emissions from ultraviolet to infrared. InGaN layer is usually grown at low temperature to prevent decomposition of InN, while GaN is usually grown at relatively high temperature to obtain good crystal quality. To alleviate the effect of temperature variation on the crystal quality of MQW, we can grow GaN with a thickness of 1–2 nm after the Fig. 2.4 Cross-sectional TEM image of InGaN/GaN MQW

18

2 Epitaxial Growth of III-Nitride LEDs

growth of InGaN QW layer, and the growth temperature of GaN is identical to that of InGaN QW layer. Then increasing the temperature gradually to grow the remaining GaN QB layer. N2 is used as carrier gas instead of H2 during epitaxial growth of InGaN/GaN MQW, because H2 will etch InGaN QW. (7)

Mg-doped low-temperature p-GaN

Growing a heavily Mg-doped p-GaN layer can achieve a higher concentration of holes, which will improve the hole injection efficiency and obtain a higher radiation recombination efficiency [2, 3]. For example, the p-GaN with a thickness of about 20 nm grown at 750 °C is used as the hole injection layer, which can enhance the probability of carrier radiation recombination and thus a higher IQE can be achieved. In addition, the p-GaN layer grown at a low temperature can also inhibit the decomposition of MQW during the subsequent high-temperature growth of p-AlGaN and p-GaN [4, 5]. (8)

Mg-doped p-AlGaN EBL

Figure 2.5 shows the cross-sectional TEM image of p-AlGaN/GaN SL. Electrons have small effective mass, high mobility and high concentration, while holes have large effective mass, low mobility and low concentration. As a result, it is more difficult for injecting holes into InGaN/GaN MQW. Usually, the recombination of electrons and holes takes place in QWs closed to p-GaN layer. But non-radiative recombination also takes place in p-GaN layer, which is caused by electrons overflowing. To effectively block electrons overflowing from n-GaN to p-GaN layer, the p-AlGaN EBL is introduced in the LED epitaxial structure. The potential barrier

Fig. 2.5 Cross-sectional TEM image of p-AlGaN/GaN SL

2.1 III-Nitride Blue LEDs

19

produced by p-AlGaN EBL layer can block electrons and thus reduce the nonradiative recombination of electrons and holes in p-GaN. In addition, the lattice mismatch between p-AlGaN layer and low-temperature p-GaN layer can be alleviated when p-AlGaN layer is replaced by p-AlGaN/GaN SL. For example, by using 6-pair p-AlGaN (2nm)/GaN (2nm) SL grown at around 900 °C, the improved ability of blocking electrons and higher radiation recombination efficiency can be obtained. (9)

Mg-doped p-GaN layer

TMGa, NH3 , and Cp2 Mg are used as precursors. Cp2 Mg is used as the p-dopant source. The ionization energy of Mg acceptor in p-GaN is about 170 meV. The Mgdoped p-GaN grown under the H2 carrier gas exhibits high resistance, due to the formation of Mg-H complex. To improve the hole concentration in p-GaN, subsequent thermal annealing treatment is required to activate the Mg acceptor in p-GaN. For example, a 110 nm-thick p-GaN layer (Mg-doping = 1 × 1020 cm−3 ) can be grown at around 945 °C under the H2 ambient. Then p-GaN is thermally annealed at 700 °C for 20 min in N2 atmosphere to activate the Mg acceptor. Finally, the hole concentration and mobility of the p-GaN layer are 5 × 1017 cm−3 and 15 cm2 /(V s), respectively. (10)

Heavily Mg-doped p-InGaN contact layer

The heavily Mg-doped p-InGaN is deposited on the p-GaN layer to reduce the resistance of ohmic contact by tunneling. For example, the 5 nm-thick p-InGaN layer is grown at 650 °C, and then annealed in N2 atmosphere at 750 °C for 20 min. Figure 2.6 shows Si, Mg, In and Al depth profiles in GaN-based blue LED measured by secondary ion mass spectroscopy (SIMS). It is well known that the optoelectronic performance of GaN-based blue LED can be improved by optimizing epitaxial structure. To improve lateral current spreading and obtain a more uniform current distribution in active region, a Si-doped 200 nm-thick n-GaN layer (L7) can be grown on the n-GaN layer, as shown in Fig. 2.6a. A strong electric field exists in the AlGaN/GaN structure, which prevents electrons overflowing into the p-region and promotes the injection of holes into MQW. To enhance carrier injection efficiency, a thin AlGaN layer (L5) can be deposited on the last QB, as shown in Fig. 2.6b.

2.2 III-Nitride Green LEDs 2.2.1 InGaN/GaN Superlattice Generally, the white LED consists of a blue LED chip and a yellow phosphor coating. However, there exists energy loss when blue light is converted into yellow-green light by phosphor, which limits the further improvement of the luminous efficiency of white LED. In order to solve this problem, the direct color mixing of red, green, and blue LEDs is generally considered as an efficient method to eliminate optical

20

2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.6 Si, Mg, In, and Al depth profiles in GaN-based blue LED epitaxial layers measured by SIMS

conversion. Nevertheless, the LEDs with dominant wavelength ranging from 500 to 600 nm show a significantly lower efficiency, which is described as the “green gap” phenomenon [6, 7, 8]. It has been reported that the “green gap” phenomenon is induced by high threading dislocation density (TDD), which originates from the large lattice mismatch between the GaN and the high-In-content InGaN. The TDs can cause low emission efficiency of green LEDs [9, 10, 11, 12]. In addition, highly strained InGaN/GaN MQWs, because of a large lattice mismatch between the GaN and high-In-content InGaN, exhibit strong QCSE [13, 14]. Over the last few decades, many scientific approaches have focused on improving the optical and electrical properties of green LEDs [15,

2.2 III-Nitride Green LEDs

21

Fig. 2.7 Schematic diagram of epitaxial structure of green LED with SLs

16, 17]. It was found that the InGaN/GaN SL grown at low temperature generates V-pits, which featured an inverted pyramid with six (10–11) sides [18, 19, 20, 21]. Previous studies have reported that the V-pits could affect the optical and electrical performance of LEDs, such as the electrostatic discharge capabilities, leakage current, and the radiative recombination efficiency [22, 23, 24, 25]. The InGaN/GaN SL embedded between the n-GaN and the MQW has been used to release misfit strain [26, 27, 28, 29]. The increasing SL pairs will accumulate strain energy and the partial strain relaxation can promote the formation of V-pits [30]. It was found that the V-pits would form energy barriers around the TDs, which can suppress the diffusion of carriers into TDs [31]. However, the physical mechanism of V-pits for improving LED performance is not clear. In this subchapter, we investigate the effect of V-pits on the optoelectronic performance of green LED [159]. Figure 2.7 shows schematic diagram of the green LED epitaxial structure. In order to elaborate the effect of the InGaN/GaN SLs on optoelectronic properties of the green LEDs, the 24-pair In0.04 Ga0.96 N (3 nm)/GaN (3 nm) SLs were embedded between the In0.25 Ga0.75 N/GaN MQWs and the n-GaN layer. The green LED without the InGaN/GaN SLs was also grown for comparison. Figure 2.8 shows atomic force microscope (AFM) and scanning electron microscope (SEM) images of the samples without and with InGaN/GaN SLs. As shown in Fig. 2.8a, c, the density of V-pits in the sample with InGaN/GaN SLs is higher than that of V-pits in the sample without InGaN/GaN SLs. In addition, it can be observed in Fig. 2.8b, d that the diameter of V-pits in the sample with InGaN/GaN SLs is larger than that of V-pits in the sample without InGaN/GaN SLs. Figure 2.9a and b show the cathodoluminescence (CL) spectra of samples without and with InGaN/GaN SLs, respectively. We observed that the CL spectra at V-pits exhibit two emission components, which consist of a main energy peak located away from the V-pits and a higher-energy component located on the sidewall MQW of the

22

2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.8 AFM images of samples a without the InGaN/GaN SLs and c with the InGaN/GaN SLs. SEM images of samples b without the InGaN/GaN SLs and d with the InGaN/GaN SLs

Fig. 2.9 CL spectra of the sample a without InGaN/GaN SLs and b with InGaN/GaN SLs. c CL spectra of the samples with different acceleration voltages. SEM and CL images of the sample d without InGaN/GaN SLs and e with InGaN/GaN SLs

2.2 III-Nitride Green LEDs

23

Fig. 2.10 XRD ω-scan rocking curves of samples without and with the InGaN/GaN SLs a symmetric (002) and b asymmetric (102)

V-pits. In addition, the height of potential barrier of the sample with InGaN/GaN SLs is higher than that of potential barrier of the sample without InGaN/GaN SLs. In Fig. 2.9c, the CL peak blueshifts with the voltage increasing from 1 to 5 kV. The CL and SEM images of the samples without and with InGaN/GaN SLs are shown in Fig. 2.9d, e. The V-pits can be obviously observed in the SEM images, coinciding with dark spots in the CL images [32]. In order to demonstrate the impact of the InGaN/GaN SLs on the crystalline quality of the epilayers, the X-ray diffraction (XRD) measurement was conducted. Figure 2.10 shows symmetric (002) and asymmetric (102) ω-scan rocking curves of samples without and with InGaN/GaN SLs. It can be observed that the full widths at half maximum (FWHM) of the symmetric (002) and asymmetric (102) rocking curves of sample with InGaN/GaN SLs are both larger than that of the symmetric (002) and asymmetric (102) rocking curves of sample without InGaN/GaN SLs. Figure 2.11a, b demonstrate the temperature-dependent PL spectra of samples without and with InGaN/GaN SLs. It is noted that the PL peak shows a S-shaped variation tendency as the temperature increases, which may be ascribed to the carrier localization and inhomogeneity in the InGaN/GaN MQW [33]. In addition, the peak wavelength shifts of the green LED without and with an InGaN/GaN SL were 3.6

Fig. 2.11 PL spectra of samples a without and b with InGaN/GaN SLs. c Temperature dependence of the normalized PL intensity spectra of samples without and with InGaN/GaN SLs

24

2 Epitaxial Growth of III-Nitride LEDs

and 2.5 nm, respectively, when the temperature increased from 5 K to 300 K. The embedded InGaN/GaN SLs can reduce the strain in the QWs, resulting in the smaller peak wavelength shift of the green LED. Figure 2.11c shows the temperature dependence of the normalized PL intensity, which can be expressed by the empirical Arrhenius equation [34]. I (T ) =

I0   1 + Cex p − kEB aT

(2.9)

where I 0 represents the integrated PL intensity at low temperature, C stands for a constant related to the density of non-radiative recombination centers, E a corresponds to the activation energy of non-radiative recombination centers. By fitting the curves in Fig. 2.11c, the calculated activation energy E a of the samples without and with InGaN/GaN SLs are 15.5 meV and 16.4 meV, respectively. The constant C of the samples without and with InGaN/GaN SLs are 2.4 and 0.8, respectively. These results indicate that a higher emission efficiency and a lower density of non-radiative recombination centers in the sample with InGaN/GaN SLs. Assuming the non-radiative channels are suppressed at low temperature, the IQE of LED is calculated as I QE =

I300K I0K

(2.10)

The I 0K was replaced by I 5K when this experiment was conducted. Accordingly, the IQEs of samples without and with InGaN/GaN SLs are calculated to be 20.07% and 29.35%, respectively. Figure 2.12a shows the light output power (LOP) versus current for the samples. At 20 mA, the calculated EQE of the sample with InGaN/GaN SLs is improved by 29.6% compared with that of the sample without InGaN/GaN SLs. Figure 2.12b shows current versus voltage curves of samples. At 20 mA, the forward voltages of the samples without and with InGaN/GaN SLs are 3.93 and 2.71 V, respectively. Figure 2.12c, d depict reverse leakage current versus voltage characteristics of samples. The reverse leakage current of sample with InGaN/GaN SLs is much lower, compared with that of the sample without InGaN/GaN SLs. The localized states within the bandgap can be responsible for the main channel causing the reverse leakage current [32]. Therefore, screening of TDs by the V-pits can be considered an effective way to reduce the reverse leakage current. Additionally, the larger V-pit with higher Poole–Frenkel barrier plays a significant role in decreasing the reverse leakage current [22]. As mentioned in the preceding, the embedded InGaN/GaN SLs can release the stress in MQWs and improve the IQEs of LEDs due to the formation of V-pits. Thus, we conduct in-depth research into effect of V-pits on optical and electrical properties of GaN-based green LEDs [47]. By altering the growth temperature and the total thickness of InGaN/GaN SLs to form different densities and sizes of V-pits, we explore the relationships between V-pits and performance of green LEDs.

2.2 III-Nitride Green LEDs

25

Fig. 2.12 a LOP versus current and b I-V characteristics of samples without and with InGaN/GaN SLs. Temperature-dependent reverse leakage current versus reverse voltage characteristics of samples c without and d with InGaN/GaN SLs Fig. 2.13 Schematic diagram of the epitaxial structure of the samples

26

2 Epitaxial Growth of III-Nitride LEDs

Three types of green LEDs with same epitaxial structures are prepared except for the period and growth temperature of InGaN/GaN SLs. Three types of LEDs consist of five pairs of InGaN/GaN SLs grown at 850 °C, five pairs of InGaN/GaN SLs grown at 835 °C, and nine pairs of InGaN/GaN SLs grown at 835 °C which are labeled as LED I, LED II, and LED III, respectively. The schematic diagram of the epitaxial structure of the samples is depicted in Fig. 2.13. Figure 2.14 shows the AFM and SEM images of three samples with different InGaN/GaN SLs. According to Fig. 2.14a, c, e, it can be calculated that the densities of V-pits located at MQW surface in LED III are higher than those of V-pits located at MQW surface in LED I and LED II. In addition, as shown in Fig. 2.14b, d, f, the

Fig. 2.14 AFM and SEM images of samples with different SLs. a and b LED I, c and d LED II, e and f LED III

2.2 III-Nitride Green LEDs

27

calculated diameter of V-pits in LED III is larger than the calculated diameter of Vpits in LED I and LED II. According to the above results, increasing SLs periods and decreasing SL growth temperature can realize the higher density and larger size of V-pits. Figure 2.15 shows the SEM-CL images of MQW surface of LED I, LED II and LED III. The dark regions in the CL images correspond to the V-pits, and the size of the dark regions is larger than the size of the V-pits. The V-pit is connected with the dislocation, and the dislocation behaves a non-radiative recombination center [35].

Fig. 2.15 SEM-CL images of samples with different SLs. a and b LED I, c and d LED II, e and f LED III

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2 Epitaxial Growth of III-Nitride LEDs

The carriers around the V-pits can diffuse to the dislocation center, resulting in nonradiative recombination, and the dark regions are larger than the regions of V-pits. By comparing the SEM-CL images of LED I, LED II and LED III, it can be seen that the smaller size of V-pit size is, the larger of the corresponding dark region is. The V-pit generates an energy barrier around the dislocation, which can prevent the carriers from being captured by the dislocations [18, 21]. The smaller size of V-pit is, the lower of the energy barrier is. The carriers around the dislocation are more likely to diffuse to the center of the dislocation, leading to the larger dark regions. Figure 2.16a demonstrates HAADF-STEM image of cross-sectional V-pits. The profile of V-pits can be clearly observed in Fig. 2.16a. Figure 2.16b, c exhibit the cross-sectional TEM images of In0.25 Ga0.75N /GaN MQW and In0.02 Ga0.98 N/GaN SLs. Figure 2.16d show the top-view TEM image of the sample. In order to explore the optoelectronic properties of three samples, the current versus voltage and light intensity versus current curves were obtained as shown in Fig. 2.17. When the injection current is identical, the forward voltage of the LED III

Fig. 2.16 a Cross-sectional HAADF-STEM image of V-pits. b Cross-sectional TEM image of In0.25 Ga0.75 N/GaN MQW. c Cross-sectional TEM image of the In0.02 Ga0.98 N/GaN SL. d Top-view TEM image of the sample

2.2 III-Nitride Green LEDs

29

Fig. 2.17 a Injection current versus forward voltage of the samples. b Light intensity versus injection current of the samples

is the lowest and the light intensity of the LED III is the highest, which imply that the larger V-pits can improve the performance of LEDs. Figure 2.18a shows the EQE versus current for three samples. Firstly, the EQE increases gradually with the increase of injection current. After reaching the peak value, the EQE begins to decrease. This phenomenon is known as efficiency droop. The efficiency droop is defined as   E Q E 350m A η = 1− E Q E max

(2.11)

The efficiency droops of LED I, LED II, and LED III are determined to be 57.6%, 54.9%, and 51.7%, respectively. It can be observed that EQE is the highest and the efficiency droop is the lowest for LED III at a higher current, which may be ascribed to enhanced hole injection efficiency and decreased carrier trapping probability of TDs due to higher potential barrier height of larger V-pit. Figure 2.18b depicts the peak

Fig. 2.18 a EQE versus injection current of three samples. b Peak wavelength versus injection current of three samples

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2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.19 a Schematic diagram of epilayers of the sample. b EL image of a green LED at 20 mA

wavelength of samples as a function of current. With increasing injection current, the peak wavelength of the samples gradually blueshifts. As previously stated, the InGaN/GaN SLs embedded between the MQW and nGaN layer can effectively prevent the carriers from recombining nonradiatively at the TDs and thus improve the IQEs of LEDs. In the following discussion, the different periods of SLs (0, 2, 4, 6, 8, 10) grown at 800 °C and 210 nm-thick GaN grown at 800 °C were inserted into the green LEDs [160]. Figure 2.19 depicts a schematic illustration of the full green LED epitaxial structure as well as the electroluminescence (EL) image of a green LED chip at 20 mA. Figure 2.20 demonstrates the AFM and SEM images of the samples with different InGaN/GaN SLs and low-temperature GaN layer. It can be clearly observed that the diameter of V-pits increases from Fig. 2.20a–f, which implies that the diameter of V-pits increases with the increment of the periods of InGaN/GaN SLs. Although the low-temperature GaN is as thick as the 6 periods of SL, the V-pit diameter of the former (230 nm) is larger than that of the latter (207 nm). We find that the growth rate of low-temperature GaN is higher than that of InGaN/GaN SL during the MOCVD growth process. For the growth of the GaN layer at a low temperature, V-pits having {10–1x} facets with a higher index x are generated, thereby resulting in the formation of V-pits with larger size than those formed by {10–11} facets. Accordingly, we conclude that the growth of low-temperature GaN is more effective for expanding the V-pit size than the growth of InGaN/GaN SL, indicating that lowtemperature GaN could be an alternative to the InGaN/GaN SL for promoting the generation of V-pits. The E2 mode of Raman spectrum was measured to calculate the in-plane strain in different samples. Figure 2.21 shows the Raman spectra of samples with different SLs and low-temperature GaN. Generally, the in-plane strain of the GaN layers can be calculated by σ (Raman) =

ω GPa k

(2.12)

where ω represents the frequency shift of E2 mode, k stands for the Raman strain coefficient (4.2 cm−1 /GPa) for E2 (high) mode of GaN, and σ (Raman) corresponds the strain. As can be seen in Fig. 2.21, the E2 peaks of the samples both blueshift

2.2 III-Nitride Green LEDs

31

Fig. 2.20 AFM and SEM images of the samples with a 0, b 2, c 4, d 6, e 8, f 10 periods of InGaN/GaN SLs and with g a low-temperature GaN layer. h Cross-sectional TEM image of V-pits

compared with the stress-free GaN E2 peak located at 567.36 cm−1 , demonstrating the existence of compressive strain in the samples. According to Eq. (2.12), the inplane strain is directly proportional to the frequency shift of E2 mode so that a larger strain exists in the GaN layer with 10 periods of InGaN/GaN SLs compared with other samples. According to the above results, the compressive strain increases as the periods of InGaN/GaN SLs increase. It can be observed that the compressive strain of the green LED with low-temperature GaN is lower than that of the green LED with 6 periods of InGaN/GaN SLs, despite the low-temperature GaN layer is as thick as the InGaN/GaN SLs with 6 periods. The results seem to be different from the previous studies holding an opinion that the InGaN or InGaN/GaN SLs can act as strain relief layers [14, 29, 36], which can be explained that the ratio of InGaN to GaN in previous studies is significantly larger than that of InGaN to GaN in our SL structure. In order to elaborate the underlying mechanisms of the increased density of Vpits when the periods of the SLs decrease, the first-principle molecular dynamics

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2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.21 Raman spectra of the samples

simulations were conducted to calculate the surface energies of GaN (0001) and GaN (10–11). Figure 2.22a depicts the top and side views of the calculation model. The simulated results for the surface energy versus the compressive stress can be seen in Fig. 2.22b, demonstrating that the surface energies of GaN (0001) or GaN (10–11) both decrease when the compressive strain increases. The results accord with the previous studies [37]. When the periods of SLs decrease, the related compressive stress decreases and thus the surface energy increases, which results in a lower surface diffusion rates and an increased density of V-pits. In addition, the inset of Fig. 2.22b depicts the formation energy versus the compressive stress. Since the

Fig. 2.22 a Top view and side view of molecular dynamics simulation models. b Simulated surface energy versus compressive stress, the inset shows the simulated formation energy versus the compressive stress

2.2 III-Nitride Green LEDs

33

increased formation energy can induce the decreased size of V-pits [38], so the increased periods of SLs can generate V-pits with smaller size. In order to investigate the relationship between the temperature and PL peak, the PL spectra of the samples with different SLs and low-temperature GaN were collected in the temperatue ranges from 5 to 300 K. To simplify the analysis, we only present the PL spectra of the samples with 0 and 6 periods of SLs, as shown in Fig. 2.23a, b. In Fig. 2.23c, it can be observed that the PL peaks show a S-shaped variation tendency as the temperature increases, which may be ascribed to the carrier localization and inhomogeneity in the InGaN/GaN MQW [39, 40, 41]. It is known that the ratio of the integrated PL intensity at 300 K to that of intergrated PL intensity at 5 K can be used to estimate the IQEs of green LEDs. Figure 2.23d shows the relationship between the IQEs and the diameter of V-pits of samples with different SLs and low-temperature GaN. According to the results, the IQE of green LED increases first and then decreases with the increment of SLs. When the periods of SLs increase from 6 to 10 pairs, the V-pits induced the excessive area loss of active region is severer than the IQE improvement induced by the V-pits. In addition, one can find that the variation trend of the IQEs of the samples is consistent with that variation trend of the in-plane compressive strain of InGaN/GaN MQW, which

Fig. 2.23 PL spectra of the samples at different temperatures a involving 0 pairs of SLs, b involving 6 pairs of SLs. c PL peak versus temperature. d IQEs versus diameters of V-pits of samples

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2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.24 TEM and EDX images of the samples

indicates that the in-plane compressive strain of InGaN/GaN MQW also plays an important role on IQEs of green LEDs. HAADF-STEM combined with energy-dispersive X-ray (EDX) was conducted to observe the structure of the V-pits clearly. Figure 2.24 shows cross-sectional TEM and EDX images of the V-pits for the sample with 6 periods of SLs. The V-shaped contours are shown in Fig. 2.24a. Figure 2.24b shows the TEM image of the mixed dislocations running through the V-pits. This phenomenon accords with the previous study proposing that the nucleation rate of V-pits at mixed dislocation is higher than that of V-pits at edge dislocation [42]. Figure 2.24c, d show the cross-sectional bright-field TEM images of the sample under two beams with g = (0002) and (11– 20), respectively. According to the invisibility criteria g·b = 0, the type of dislocations can be clearly classified. Figure 2.24e–j are EDX elemental mapping images of the V-pits and line-scanning analysis along the direction of dotted line as shown in Fig. 2.24a. It can be seen that the presence of Al, Ga, In and N signal near the sidewalls MQW of the V-pits. This indicates that InGaN/GaN MQW, p-AlGaN/GaN SL and p-GaN layers are epitaxially grown on the side plane of V-pits. Compared with (0001) plane of MQW, the thickness of (10–11) plane of MQW is thinner and the In content is lower.

2.2 III-Nitride Green LEDs

35

Fig. 2.25 Plan-view TEM images of the samples

Figure 2.25 shows plan-view TEM images of epilayers of the sample with 6 periods of SLs along the growth direction. The pyramidal-shaped holes on the surface of the sample correspond to V-pits. According to the plan-view TEM image and selected area diffraction (SAD) pattern, it is confirmed that the sidewalls of the Vpit correspond to the {10–11} planes and the hexagonal outline of the V-pit is the intersection lines of the {0001} and {10–11} planes. Figure 2.26 shows optoelectronic properties of the samples with different SLs and low-temperature GaN. As shown in Fig. 2.26a, b, when the current exceeds 40 mA, the light intensity and the EQE in the sample with low-temperature GaN are both the highest. As the previous study reported, the n-type InGaN underlayer can improve the electron injection efficiency because the band gap of InGaN is lower than that of GaN [26]. Therefore, the improved performance may not be ascribed to the enhancement of the electron injection efficiency. We speculated that V-pits not only enhance the injection of holes into the QWs but also improve the uniformity of the hole density distribution in various QWs, and this speculation needs to be further varified. Figure 2.26c shows I-V characteristics of the samples. We can observe that the forward voltages gradually decrease when the periods of SLs increase, under the same injection current. Recently, we have found that the V-pits not only can improve the holes injection, but also improve the distribution uniformity of the holes. Figure 2.26d shows the reverse leakage current versus voltage of the samples with different periods of SLs and low-temperature GaN. One can observed that the reverse leakage current of the samples reduces when the size of V-pits becomes larger. The reverse leakage current of the samples can be ascribed to the electron tunneling through TDs [33]. The effective screening of the TDs can be responsible for the significant reduction of

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2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.26 a Light emission intensity versus injection current of LEDs. b EQE versus injection current of LEDs. c Current density versus forward voltage of LEDs. d Reverse leakage current versus forward voltage of LEDs

the reverse leakage current. Additionally, the larger V-pit with higher Poole–Frenkel barrier plays a significant role in decreasing the reverse leakage current [22].

2.2.2 Stacked GaN/AlN Last Quantum Barrier At present, the bandgap engineering has been considered as an effective method to improve the quantum efficiency especially for green LEDs. Thus, a series of strategies have been proposed such as graded EBL [43], SL EBL [44], and staggered QW [45, 46]. In addition to optimizing the structures of EBL and QW, engineering the structure of QB can be thought an alternative way to improve the efficiency of InGaN-based green LEDs. In this subchapter, a stacked GaN/AlN last quantum barrier (SLQB) was introduced to replace the conventional GaN last quantum barrier (CLQB) [49]. The optical and electrical properties of green LED with SLQB were systematically investigated and the mechanism of performance improvement was discussed in detail by calculating energy band of LED with SLQB. In order to elaborate the effect of SLQB on the performance of green LED, LED chips with size of 228 μm × 356 μm

2.2 III-Nitride Green LEDs

37

Fig. 2.27 Schematic diagram of green LED with CLQB (sample A) or SLQB (sample B)

were fabricated. The green LED with CLQB was labeled as sample A, and the green LED with SLQB was labeled as sample B. A schematic diagram of epitaxial layers of two samples can be seen in Fig. 2.27. Both sample A and B comprise the same epilayers except for the LQB layer. For sample A, the LQB layer consists of GaN with a thickness of 13 nm. While for sample B, the LQB layer is composed of a stacked AlN (2 nm) and GaN (11 nm). The cross-sectional TEM images of epilayers in two samples can be seen in Fig. 2.28a–c. As shown in Fig. 2.28b, the presence of V-pits with large size demonstrates the outstanding performance of our LEDs [47]. The thickness uniformity and abrupt InGaN/GaN MQWs and SLs can also prove the excellent quality of epilayers in two samples. In order to verify the presence of AlN in LQB, the atom probe tomography (APT) was conducted in sample B. The distribution of Al in the AlN layer and AlGaN EBL can be clearly seen in the Fig. 2.28e.

Fig. 2.28 a–c Cross-sectional TEM images of the green LED epitaxial structure grown on PSS. d EL image of green LED at an injection current of 15 mA. e Al atoms distribution in AlN and AlGaN EBL layers of sample B

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2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.29 EL spectra at different injection currents for a sample A and b sample B. c Forward current versus voltage characteristics for two samples. d Light output power versus current characteristics for two samples

The optical and electrical properties of two samples were investigated by EL spectra, I-V and L-I characteristics as shown in Fig. 2.29. As shown in Fig. 2.29a, b, it is noted that the EL peaks of sample A and B both blueshift when the injection current increases from 1 to 80 mA, which is attributed to the combined effects that the free carriers induced screening of QCSE and the band-filling effect [48]. The I-V characteristics of two samples are presented in Fig. 2.29c. When the injection current is 15 mA, the corresponding voltages of sample A and B are 2.39 V and 2.23 V, respectively. Since the higher forward voltage means a lower injection efficiency of holes, the increased hole injection efficiency in sample B can be responsible for the phenomenon that the forward voltage of sample A is higher than that of sample B. As shown in Fig. 2.29d, when the injection current is 15 mA, the LOPs of sample A and B are 9.6 mW and 13 mW, respectively. An enhancement of LOP in sample B compared with sample A may also be associated with the improved efficiency of hole injection in sample B. The detailed mechanisms of performance enhancement will be discussed as follows. In order to explore the physical mechanisms of performance improvement for the green LED with SLQB, the energy band diagrams of sample A and B were both calculated by the software SiLENSe. The detailed parameters setup can be referred to our previous report [49]. The calculated energy band diagrams along [0001] orientation of sample A and B are illustrated in Fig. 2.30 and the corresponding current density is 20 A/cm2 . Figure 2.30a, b provide overall calculated energy band diagrams of sample A and sample B. Compared with sample A, we can observe that a sharp offset of conduction band (Ec ) and valence band (Ev ) at the distance of

2.2 III-Nitride Green LEDs

39

Fig. 2.30 Calculated energy band alignment of a sample A and b sample B along the [0001] orientation. The magnified diagrams of the conduction band around the active region of c sample A and d sample B. Magnified diagrams of the valence band around the active region of e sample A and f sample B

2150 nm in Sample B, which can be ascribed to the presence of AlN layer. In this case, the structure of SLQB can prevent the electrons from overflowing to p-GaN layer effectively. To evaluate the ability of CLQB and SLQB to confine electrons in active region and block electrons overflow to p-GaN layer, the effective potential barrier heights at LQB/p-GaN interface, presented by ϕ1 in sample A and ϕ2 in sample B, are both calculated and the corresponding diagrams are shown in Fig. 2.30c, d. When a part of GaN at LQB is replaced by AlN, the effective potential barrier increases from ϕ1 (362 meV) to ϕ2 (1557 meV), which suggests that the SLQB structure can confine electrons in active region effectively. In addition, the alignment of valence band from

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2 Epitaxial Growth of III-Nitride LEDs

QB to p-GaN for sample A and B is shown in Fig. 2.30e, f. The energy barrier ϕ3 for hole injection from p-GaN to active region is calculated to be −393 meV, which can be ascribed to the polarization induced electric field at the p-GaN/CLQB interface. During the hole injection process from p-GaN to QW, the hole loss their kinetic energy to overcome the energy barrier by the thermionic emission process labeled as P1 in Fig. 2.30e. Then, the energy barrier ϕ4 (575 meV) at the CLQB/QW can promote the hole injection. As a result, the holes can obtain the total energy of ϕ3 + ϕ4 (182 meV) from p-GaN into the active region. Figure 2.30f shows the valence band alignment from p-GaN to QB in sample B. As the GaN is replaced by the AlN, the negative sheet charges induced by the polarization can alleviate the energy barrier such as the ϕ3 at the p-GaN/CLQB interface and enable the emergence of an upward band bending. The polarization-induced electric field provides holes with energy ϕ5 (429 meV). Thus, a higher concentration of holes can be achieved at the point O. Moreover, the thickness of AlN is only 2 nm so that the high local concentration of holes at point O can tunnel across the AlN independent of ϕ6 (−1369 meV) with a higher probability. In addition, the energy band barrier at the SLQB/QW interface ϕ7 is 2592 meV. As a result, when holes transport from p-GaN to the active region, the total energy that holes can be obtained is ϕ5 +ϕ6 +ϕ7 (1652 meV) According to the above analyses, a higher probability of intraband tunneling and a higher energy that holes obtain can be responsible for the enhanced efficiency of injecting holes in sample B. Figure 2.31 shows the carrier concentration and radiative recombination rate distributions in active region of two samples. As can be seen in Fig. 2.31a, the hole concentration of active region is significantly improved when the SLQB is introduced, which can be ascribed to the effect of intraband tunneling promoting the hole concentration. The hole concentration at AlN/p-GaN interface for sample B is two orders of magnitude higher than that for sample A as shown in Fig. 2.31b. Since the electric field induced by the polarization hinders the hole injection at the AlN/p-GaN interface, the hole concentration begins to drop at the CLQB/p-GaN interface for sample B. Figure 2.31c illustrates the electron concentration distribution for sample A and B. By comparing the electron concentration in active region for sample A and B, we can obtain that the SLQB structure can confine the electrons in the active region effectively. Figure 2.31d presents the radiative recombination rate profile for sample A and B. It is noted that the general variation trend of radiative recombination rate is similar as that of hole concentration, which reveals that the radiative recombination rate is dominated by the hole concentration.

2.3 III-Nitride Ultraviolet LEDs

41

Fig. 2.31 a Calculated hole concentration profile in active region for sample A and B. b Calculated hole concentration profile near SLQB/CLQB region for sample A and B. c Calculated electron concentration profile in active region for sample A and B. d Radiative recombination rate profile in active region for sample A and B

2.3 III-Nitride Ultraviolet LEDs 2.3.1 Sputtered AlN Nucleation Layer GaN-based ultraviolet light-emitting diodes (UV LEDs), emitting at wavelength as short as 375 nm, have attracted intensive attention for a series of applications such as sterilization, disinfection, biochemistry, water and air purification, and solid state lighting [50, 51, 52, 53, 54]. However, due to the lack of mass-produced bulk GaN substrate, GaN-based UV LEDs are generally grown on heterogeneous substrates such as sapphire or SiC by MOCVD [55]. The large mismatch between sapphire and GaN results in high TDDs at the GaN/sapphire interface [56, 57], leading to numerous non-radiative recombination centers, thereby deteriorating the optoelectronic properties of GaN-based LEDs [58, 59]. Compared with GaN-based blue LEDs, the efficiency of GaN-based UV LEDs with low In contents is more sensitive to TDrelated non-rediative recombination centers because of the lack of localized states in active region [60, 61]. Therefore, it is crucial to reduce TDDs in GaN epilayers for realizing efficient UV LEDs [62]. In order to minimize the lattice mismatch between sapphire and GaN, a lowtemperature GaN or AlN NL is typically introduced prior to growth of GaN epilayers at a high temperature [63]. In addition, several techniques, such as epitaxial lateral

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2 Epitaxial Growth of III-Nitride LEDs

overgrowth (ELOG) [64, 65, 66], pendeoepitaxy [67], cantilever epitaxy [68], insitu SiNx nanomasks [69], and PSS [70, 71, 72] have been employed to reduce TDDs. Several studies have reported that high-quality GaN epilayers can be obtained by introducing the sputtered AlN NL on sapphire substrate [73, 74, 75, 76]. The crystal quality of GaN epitaxial layer is sensitive to the growth of the NLs. In this subsection, First, a detailed study is conducted to investigate the effects of in-situ lowtemperature GaN/AlGaN NLs and ex-situ sputtered AlN NL on crystalline quality and optoelectronic properties of UV LEDs. Second, the influence of oxygen and thickness of the AlN NLs on the epitaxial growth of AlN films is investigated in detail. What’s more, a method to realize high quality, crack-free AlN films on flat sapphire substrate (FSS) is proposed. In this study [161], a schematic diagram of epilayers grown on the PSS with sputtered AlN NL is shown in Fig. 2.32a. The cross-sectional TEM images of epilayers are shown in Fig. 2.32b–d. The UV LEDs were fabricated on the cone-shaped PSS with GaN/AlGaN/sputtered AlN NLs. The cone-shaped PSS was fabricated by combining a thermally reflowed photoresist technique with the inductively coupled plasma (ICP) etching process [77, 78]. The SEM images of morphological evolution of GaN epilayers grown on the PSS without NL, with low-temperature GaN NL, low-temperature AlGaN NL and

Fig. 2.32 a Schematic diagram of structures of the sample. b–d Cross-sectional TEM images of epilayers

2.3 III-Nitride Ultraviolet LEDs

43

Fig. 2.33 Top-view SEM images of morphological evolution of GaN grown on different NLs. a–d Without NL. e–h Low-temperature GaN NL. i–l Low-temperature AlGaN NL. m–p Sputtered AlN NL

sputtered AlN NL are demonstrated in Fig. 2.33. As demonstrated in Fig. 2.33a–d, GaN grains mainly grow on the inclined sidewalls of PSS rather than on the c-plane sapphire. Hence, it is difficult for GaN grains to coalesce to form GaN film. When GaN epitaxially grows on the low-temperature GaN NL or low-temperature AlGaN NL, the GaN grains grow on both the c-plane sapphire and sidewalls of PSS and then coalesce to form films, as shown in Fig. 2.33 e–h and i–l. However, when GaN epitaxially grows on the sputtered AlN NL, the GaN grains grow on the c-plane sapphire instead of the inclined sidewalls of PSS, as shown in Fig. 2.33 m–p. Thus, the dislocation induced by the GaN grains coalescene from the c-plane sapphire and the sidewalls of GaN island can be inhibited. Figure 2.34 shows the cross-sectional TEM images of the GaN epilayers grown on the low-temperature GaN/low-temperature AlGaN/sputtered AlN NLs. Figure 2.34b, c show the magnified TEM images of the low-temperature GaN NL region marked with the red rectangle in Fig. 2.34a. Figure 2.34e, f show the magnified TEM images of the low-temperature AlGaN NL region marked with the red rectangle in Fig. 2.34d. Figure 2.34h, i show the magnified TEM images of the sputtered AlN NL region marked with the red rectangle in Fig. 2.34g. As shown in Fig. 2.34a, d, there are a large number of GaN islands on the sidewalls of the PSS. It has been reported that a larger misorientation exists on the inclined sidewalls of PSS compared with that exists on the flat c-plane sapphire [79], because the GaN islands on the inclined sidewalls of PSS nucleate at various crystalline planes instead of single c-plane. Therefore, a large number of dislocations will generate when these GaN islands on the inclined

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2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.34 TEM images of GaN grown on different NLs. a–c Low-temperature GaN NL. d–f Lowtemperature AlGaN NL. g–i Sputtered AlN NL

sidewalls of PSS merge with GaN islands on the flat c-plane sapphire [80, 81]. However, as shown in Fig. 2.34g, fewer GaN islands exist on the inclined sidewalls of the PSS with sputtered AlN NL, which suppresses the generation of dislocations by the mergence of GaN islands from the inclined sidewalls of PSS and flat c-plane sapphire. Figure 2.35 shows the cross-sectional TEM images of epilayers grown on PSS with the low-temperature GaN/low-temperature AlGaN/sputtered AlN NLs. In Fig. 2.35a, d, g, the bright-field TEM images of epilayers grown on low-temperature GaN/lowtemperature AlGaN/sputtered AlN NLs are presented. Figure 2.35b, e, h show the bright field TEM of the epilayers grown on the low-temperature GaN/lowtemperature AlGaN/sputtered AlN NLs when g = [0002] and the bright-field TEM images of the epilayers grown on the low-temperature GaN/low-temperature AlGaN/sputtered AlN NLs when g = [11–20] are presented in Fig. 2.35c, f, i. According to the invisibility criteria [82], only the screw (S) and mixed dislocations (M) can be observed in Fig. 2.35b, e, h, and only the edge (E) and mixed dislocations can be observed in Fig. 2.35c, f, i [83, 84]. By comparing the bright field TEM images with g = [11–20] and g = [0002], most of the dislocations are both visible when g = [11–20] and g = [0002] and are thus identified as mixed dislocations. It can be clearly observed that the dislocation density of GaN epitaxial

2.3 III-Nitride Ultraviolet LEDs

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Fig. 2.35 TEM images of UV LEDs grown on different NLs. a–c Low-temperature GaN NL. d–f Low-temperature AlGaN NL. g–i Sputtered AlN NL

layer grown on the low-temperature AlGaN NL is the highest, while the dislocation density of GaN epitaxial layer grown on the sputtered AlN NL is the lowest. Figure 2.36a shows the GaN (002) and (102) rocking curves of UV LEDs grown on the low-temperature GaN/low-temperature AlGaN/sputtered AlN NLs. The TDDs can be estimated from the FWHMs of X-ray rocking curves using the following empirical formula [85, 86]. N=

β2 4.35 × |b|2

(2.13)

where β is the FWHM of XRD rocking curve and b is the Burgers vector of the corresponding dislocations. The FWHMs of (002) and (102) rocking curves are mainly related to the densities of screw and edge dislocations, respectively. By comparing Fig. 2.36 a and b, the density of edge dislocation in epilayer with sputtered AlN NL

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2 Epitaxial Growth of III-Nitride LEDs

Fig. 2.36 a (002) and b (102) XRD rocking curves of UV LEDs grown on the low-temperature GaN NL/low-temperature AlGaN NL/sputtered AlN NL

is the lowest compared with that of edge dislocations in epilayers with GaN/AlGaN NLs, whereas the densities of screw dislocation in these samples are similar. The edge dislocation density of UV LED with sputtered AlN NL exhibits a significant reduction compared to the UV LEDs with low-temperature GaN/AlGaN NLs, whereas the screw dislocation densities of UV LEDs with GaN/AlGaN/sputtered AlN NLs are quite similar. The XRD rocking curves are consistent with the results obtained from cross-sectional TEM analyses. Figure 2.37 shows the LOP-current-voltage characteristics of UV LEDs with lowtemperature GaN/low-temperature AlGaN/sputtered AlN NLs. At 20 mA, the LOPs of UV LEDs with GaN/AlGaN/sputtered AlN NLs are 7.24, 8.56, and 9.53 mW, respectively; the forward voltages of UV LEDs with GaN/AlGaN/sputtered AlN NLs are 3.46, 3.49, and 3.39 V, respectively. The LOP of UV LED with AlGaN NL is 18.2% higher than that of UV LED with GaN NL. The LOP of UV LED with sputtered AlN NL is 11.3% higher than that of UV LED with AlGaN NL.

Fig. 2.37 a LOP versus injection current curves of three UV LED samples. b Injection current versus forward voltage curves of three UV LED samples

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Fig. 2.38 a Schematic diagram of AlN film on sapphire substrate. b Cross-sectional SEM image of the interface between sapphire and AlN. c Image of 2-inch AlN film grown on sapphire substrate

As mentioned above, the sputtered AlN NL is an effective method to reduce TDD of epitaxial layer. However, the oxygen in sputtering chamber can be incorporated into the AlN NL during the sputtering process, which can affect the physical properties of sputtered AlN NL. In addition, the thickness of NL also should be considered since it can affect the crystalline structure of the subsequent AlN flim. Understanding the effect of oxygen-doped sputtered AlN NL on the subsequent AlN film is important to the realization of high-efficiency DUV LEDs [162]. In this study, the low-temperature AlN NL, oxygen-undoped sputtered AlN NL, and oxygen-doped sputtered AlN NL with the same thickness of 25 nm are labelled as A1, B1, and C1, respectively. To investigate the effect of the thickness of oxygenundoped sputtered AlN NL on the crystal quality of AlN film, 15-nm-thick, 25-nmthick, and 35-nm-thick oxygen-undoped sputtered AlN NLs were also prepared and labelled as B2, B1, and B3, respectively. Figure 2.38a shows a schematic diagram of AlN film grown on sapphire substrate with AlN NL. The cross-sectional SEM image in Fig. 2.38b showed a sharp heterointerface between AlN and sapphire substrate. Figure 2.38c displays the microscopy image of 2-in. AlN film grown on c-plane sapphire substrate. The grown AlN film had a smooth and crack-free surface. Figure 2.39a shows the transmittance spectra of samples A1, B1 and C1. It can be seen in Fig. 2.39a that the average transmittance of C1 is higher than that of A1 and B1. The inset shows that the average transmittance of oxygen-doped sputtered AlN NL is higher than that of oxygen-undoped sputtered AlN NL. The previous study reported that the oxygen incorporated into the AlN layer can affect the optical properties of AlN film on c-plane sapphire substrate [87]. Compared with oxygenundoped sputtered AlN NL, oxygen-doped sputtered AlN NL had a larger probability of incorporation of oxygen into AlN film. As a result, the optical transmittance of sample C1 is higher than that of sample B1. Figure 2.39b presents the SIMS depth profiles of oxygen in sample C1. It can be observed that the oxygen concentration of AlN NL is about two orders of magnitude higher than that of AlN film. In order to explore the stress states of samples A1-C1, we conducted the Raman spectra of A1-C1 and the corresponding E2 (high) mode in normalized Raman spectra are depicted in Fig. 2.40. It is noted that all Raman peaks redshift compared with the peak in the stress-free bulk AlN (marked in dotted line), which indicates that the presence of residual tensile stress in samples A1-C1 [88]. In addition, the stress

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Fig. 2.39 a Transmittance spectra of samples A1, B1, and C1. b SIMS depth profiles of oxygen in sample C1

Fig. 2.40 Normalized Raman spectra of E2 (high) mode of samples A1, B1, and C1. The dotted line indicates the E2 (high) mode of stress-free bulk AlN

can be calculated to be 1333 MPa, 100 MPa, and 733 MPa for samples A1-C1 by Eq. (2.12), respectively, where k is 3 cm−1 /GPa for E2 (high) mode of AlN [89]. In order to examine the role of stress states in affecting the surface morphologies of samples A1-C1, the differential interference contrast (DIC) microscopy of AlN film was conducted. Figure 2.41a–c display the DIC images in a plan view of samples A1C1, respectively. The obvious cracks in sample A1 can be observed due to excessive tensile stress. The cracks extend along the [1–210] directions corresponding to the {10–10} cleavage planes. The presence of cracks along {10–10} cleavage planes is ascribed to the surface energy in the {10–10} planes is lower than the surface energy in the {1–210} planes, and the strain energy can be released in the form of crack along {10–10} planes [90]. As shown in Fig. 2.41b, the surface of sample B1 is crack-free, implying that tensile stress can be effectively relaxed in the AlN

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Fig. 2.41 Plan-view DIC images of a sample A1, b sample B1, and c sample C1

film on c-plane sapphire substrate. The cracks along the [1–210] directions also exist in sample C1, as shown in Fig. 2.41c. These results agree well with the calculated tensile stresses of samples A1-C1. It can be seen in Fig. 2.42 that the (002) and (102) ω-scan rocking curves were collected to evaluate the TDDs in the samples A1-C1. As mentioned above, the FWHMs of (002) and (102) rocking curves are mainly related to the screw and edge dislocation densities, respectively [91, 92]. Generally, the smaller FWHM values of rocking curves correspond to the lower TDDs of samples. It can be clearly obtained from Fig. 2.42a, b that the FWHMs of rocking curves for sample B1 and C1 are smaller than those of rocking curves for sample A1. Compared with sample A1 with low-temperature AlN NL, the sputtered AlN NL consists of more uniform grains with better c-axis orientation, leading to a better growth-mode modification in the following AlN film growth [93]. In addition, the FWHMs of (002) and (102) rocking curves for sample B1 are both smaller than those of (002) and (102) rocking curves for C1. Because the incorporated oxygen can form complexes with Al, Al atoms in these complexes may distort the Al sublattice [94, 95]. Therefore, the TDD of sample B1 is lower than that of sample C1.

Fig. 2.42 a (002) and b (102) ω-scan rocking curves of samples A1, B1, and C1

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Fig. 2.43 Normalized Raman spectra of E2 (high) mode of samples B1, B2, and B3. The dotted line indicates the E2 (high) mode of stress-free bulk AlN

Next, the effect of thickness of sputtered AlN NL on the characteristics of AlN film on c-plane sapphire substrate is studied. Figure 2.43 shows the E2 (high) mode in normalized Raman spectra of samples B1-B3. When the thickness of AlN NL increases, the peak of E2 (high) modeI blueshifts. Moreover, especially in sample B3, the peak of E2 (high) mode blueshifts compared with that of stress-free bulk AlN, indicating the presence of compressive stress. The DIC microscopies of AlN film for the samples B1-B3 were also conducted to examine the surface morphologies affected by the stress, as shown in Fig. 2.44. The crack threading network exists in Sample B2, which is induced by the presence of tensile stress. Whereas the smooth surface exists on the surface of sample B1, as presented in Fig. 2.44b. As shown in Fig. 2.44c, a series of hillocks exist on the surface of sample B3. In conclusion, the increase or decrease in thickness of sputtered AlN NL both can lead to uneven surface of AlN film. As depicted in Fig. 2.45, the (002) and (102) ω-scan rocking curves were also collected to evaluate the TDDs in the samples B1–B3. By analyzing the FWHMs of

Fig. 2.44 Plan-view DIC images of a sample B2, b sample B1, and c sample B3

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Fig. 2.45 a (002) and b (102) ω-scan rocking curves of samples B1, B2, and B3

the (002) and (102) rocking curves for three samples, it can be inferred that both the screw and edge dislocation densities of sample B1 are lower than those of sample B2 and B3. Based on the above analyses, the epitaxial AlN film grown on foreign substrates generally suffers from high TDDs because of the large mismatch between the lattice constants and thermal expansion coefficient. In the case of realizing crack-free and high quality AlN films on sapphire substrate, a growth-mode modification process introducing voids into AlN films is proposed. In this discussion [163], AlN films were grown on 2-in. FSS and nano-patterned sapphire substrate (NPSS) by MOCVD. It can be observed in Fig. 2.46a that the NPSS employed in this discussion features hexagonally arranged holes. AlN grows epitaxially on sapphire adopting the following epitaxial relationship: AlN (0002) // Al2 O3 (0006) and AlN (1–100) // Al2 O3 (11– 20), which can be proved by the selected area electron diffraction (SAED) pattern at the interface between AlN and sapphire as shown in Fig. 2.46b. Figure 2.46c shows the schematic diagram of the structures and growth procedures for AlN epilayers on FSS or NPSS. The growth procedures divided into four stages was designated as S1-S4. At S1, the 180 nm-thick high-temperature AlN (HT AlN-1) was grown at the sputtered AlN NL, with the temperature of 1200 °C and V/III ratio of 3350. Then, the 270 nm-thick low-temperature AlN (LT AlN) layer was grown at 960 °C, with V/III ratio of 3350, corresponding to the S2. At S3, the 450 nm-thick AlN layer was grown by the method of pulsed atomic-layer epitaxy (PALE), where the NH3 flow was alternatively turned on/off (5 seconds/5 seconds) and the TMAl flow kept continuous. Finally, at S4, the V/III ratio was controlled as 190 to realize the fast growth of 2.1 μm-thick HT AlN-2 layer at 1200 °C. In order to observe the morphology transitions of AlN epialyers at different stages, the growth interruption was intentionally conducted and the SEM images of the surface on AlN/FSS and AlN/NPSS from S1 to S4 are shown in Fig. 2.47. At stage S1, the c-plane of AlN gradually roughened due to preferred vertical growth induced by a high V/III ratio. A high density of pits on the c-plane of AlN can be observed at stage S2. At S3, the coalescence speed of the c-plane of AlN increased. When

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Fig. 2.46 a Top-view SEM image of the NPSS. b SAED pattern at the interface between the AlN and sapphire. c The growth process of AlN on FSS or NPSS

the growth of PALE AlN completed, a fully coalesced surface can be obtained on AlN/FSS, but the coalescence of AlN/NPSS was incomplete. Finally, the smooth surfaces of the c-plane of AlN on the FSS and NPSS were both observed when the HT AlN-2 layer grown completely. As shown in Fig. 2.48, AFM characterizations were conducted to observe the morphologies of AlN grown on FSS and NPSS. A series of step-like AlN layers can be observed in Fig. 2.48a and b, demonstrating the step-flow growth at S4. Root mean square (RMS) roughness of AlN/FSS and AlN/NPSS are calculated to be 0.263 nm and 0.248 nm, respectively. In conclusion, the atomically flat and crackfree AlN surface on FSS can be achieved by the four-step growth process, which is comparable with that of AlN/NPSS. In contrast, as previous reports studied, when the thicknesses of AlN layer exceeds 1 μm, the crack already exists on the surface of AlN due to the presence of large tensile stress [96, 97, 98, 99, 100]. The cross-sectional STEM images were collected to display the voids embedded in AlN/FSS films. As shown in Fig. 2.49a, b, the STEM images of AlN/FSS and AlN/NPSS were acquired in the condition of g = [11–10] and the edge and mixed dislocations were obviously visible. Compared with the AlN/NPSS, a larger number of TDDs still can be observed when the growth process of LT AlN layer on FSS was completed. When the growth of PALE AlN on FSS started, many voids were

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Fig. 2.47 Morphology evolutions of AlN grown on FSS and NPSS at different stages

introduced in this layer and the corresponding magnified image is shown in Fig. 2.49c. It can be widely seen that the voids existed in AlN films grown on patterned substrates [101], as shown in Fig. 2.49b. The void structure can reduce the TDDs and release the stress, resulting in the growth of high-quality AlN film. We can obtain that the TDDs in AlN/FSS reduce to the same level with that in AlN/NPSS. Based on the above analyses, a possible model was proposed to illustrate the voids embedded in AlN/FSS, as shown in Fig. 2.49d. The epitaxial quality of AlN films from S1 to S4 can be evaluated by the HRXRD rocking curves and the related results are demonstrated in Fig. 2.50. The FWHMs of (002) and (102) rocking curves for AlN/FSS both decline monotonously from S1 to S4, which indicates that the reduction of TDD. Anomalously, the FWHMs of (002)

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Fig. 2.48 AFM images of AlN grown on a FSS and b NPSS

Fig. 2.49 Cross-sectional STEM images under the condition of g = (11−20) of a AlN/FSS b AlN/NPSS and c void in AlN/FSS. d A possible model of voids embedded in AlN/FSS

and (102) rocking curves for AlN/NPSS are broader at stages S1 and S2, which can be ascribed to the misaligned crystallites grown on the sidewalls of NPSS [102]. Due to the presence of thermal coefficient mismatch between AlN and sapphire, wafer cracking becomes a general problem for the epitaxial growth of AlN. As the thickness of AlN grown on the FSS increases to 1 μm, cracks can be observed

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Fig. 2.50 FWHMs of a (002) and b (102) for AlN/FSS and AlN/NPSS at different stages

Fig. 2.51 Wafer curvature versus the thickness of AlN film

[99, 100]. Figure 2.51 shows the variation of wafer curvature from S1 to S4. The relationship between substrate curvature (κ) and the in-plane stress of film (σf ) is expressed as [103]: σf h f =

Ms h 2s κ 6

(2.14)

where hf represents thickness of film, hs represents the thickness of substrate and M s stands for the substrate biaxial modulus. The in-plane stress can be obtained according to the differential transformation: 6σ f dκ = dh f Ms h 2s

(2.15)

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As shown in Fig. 2.51, the tensile stress in AlN/FSS decreases from S2 to S3, and then increases from S3 to S4. A kinetic model was proposed to interpret the variation of stress during the growth of film [104, 105], which elaborates the competition between tensile and compressive mechanisms. For AlN films epitaxially grow on sapphire, the thermal mismatch is partially accountable for the compressive mechanisms, while the morphology evolution is anticipated to be accountable for the tensile mechanisms [106, 107]. The lattice constants a and c from S1 to S4 were obtained by measuring the (002) and (105) reflection peaks via ex-situ HRXRD 2θ-ω scans, as presented in Fig. 2.52a. The evolutions of lattice constants demonstrate that the presence of compressive stress both in AlN/FSS and AlN/ NPSS from S1 to S4, which is primarily attributed to the lattice mismatch between AlN and sapphire. Residual stresses were characterized by the AlN-E2 (high) mode peak in Raman spectra, as depicted in Fig. 2.52b, which agrees well with the evolution of lattice constants. When the growth of AlN/FSS completes, the E2 (high) peak of AlN/FSS closes to that of stress-free AlN [108], which suggests the embedded voids can significantly release the residual compressive stress. Fig. 2.52 a Lattice constants evolution of AlN/FSS and AlN/NPSS at different growth stages. b Raman E2 (high) mode shift and calculated compressive stress of AlN/FSS and AlN/NPSS

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2.3.2 Effect of PSS on UV LED The efficiency of GaN-based UV LEDs is not on par with their visible wavelength counterparts because the presence of high TDDs in GaN epitaxial layers [109, 110, 111, 112, 113]. Thus, improving the crystalline quality of GaN epitaxial layers is urgent for improving the efficiency of GaN-based UV LEDs. A series of techniques have been employed to reduce TDDs in GaN epitaxial layers [114, 115, 116, 117, 118, 119]. Recently, high-efficiency LEDs with reduced TDDs in GaN epitaxial layer were realized by the process of ex-situ sputtered AlN NL on PSS [73, 120, 121, 122, 123, 124]. It can be known that the size of PSS plays a significant role in improving the efficiency of LEDs. In this subsection, firstly, the effects of pattern size and fill factor of PSSs on the crystalline quality of epilayers and optoelectronic properties of UV LEDs were studied intensively. Then, a novel pattern that patterned sapphire with silica arrays (PSSA) is demonstrated. In this study [14], three different sizes of cone-shaped PSSs were fabricated by combining a thermal-reflow photoresist technique and an inductively coupled plasma etching process. Top-view SEM images of the PSSs are shown in Fig. 2.53a–c. It can be seen that the sizes of PSS-I, PSS-II, and PSS-III increase in order. Figure 2.53d–f show the cross-sectional TEM images of the three samples. Figure 2.54 shows the cross-sectional TEM images of epilayers grown on the different-sized PSSs with sputtered AlN NLs. It can be clearly observed from the magnified images that the thickness of the AlN NL at the bottom and sidewalls of PSS is not uniform, where the thickness of AlN NL at bottom of PSS is larger than that of AlN NL at sidewalls of PSS. What’s more, some isolated GaN islands can be observed at the sidewalls of the PSS, which suggests that the coalescence and lateral growth of GaN islands located at the sidewalls of PSS were suppressed.

Fig. 2.53 Top-view SEM images of the morphology of different PSSs: a PSS-I, b PSS-II, c PSS-III. d–f Cross sectional TEM images of epilayers of the sample

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Fig. 2.54 Cross-sectional TEM images of GaN grown on three different PSSs. (a)–(d) PSS-I; (e)–(h) PSS-II; (i)–(l) PSS-III

Figure 2.55 shows the in-situ reflectance and temperature measurements of GaN with different-sized PSSs. The Fig. 2.55 shows that the transition time from 3D to 2D growth mode for GaN grown on PSS-III is the longest, while that for GaN grown

Fig. 2.55 In-situ reflectance and temperature transients of GaN grown on different-sized PSSs

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on PSS-I is the shortest. Considering the epilayers except for PSSs for these samples are same and thus the different transition time is related to the size of cone-shaped PSS. When GaN grows epitaxially on PSS covered with sputtered AlN NL, the selective growth occurs where the growth at the bottom of PSS is allowed but that at the sidewalls of PSS is suppressed. When the size of PSS increases, the left area of the bottom of PSS decreases, resulting in the decrease in the initial density of GaN islands and the lateral growth time becomes longer. In order to investigate the TDDs of three samples, the cross-sectional TEM images of the samples were conducted in Fig. 2.56. Figure 2.56a, d, and g show the brightfield cross-sectional TEM images of three samples grown on PSS-I, PSS-II, and PSS-III, respectively. The corresponding dislocations were also marked. In order to distinguish the type of dislocations, two-beam diffraction images under the conditions of g = [0002] and g = [11–20] were collected and the related images are demonstrated in Fig. 2.56b, e, h and c, f, i. By comparing the dark-field cross-sectional TEM images under the diffraction conditions of g = [0002] and g = [11–20], it can be obtained that the densities of edge and mixed dislocations decrease when the size of PSS increases. Furthermore, counting the etch pits is also an effective way to estimate the TDDs. In this study, the molten NaOH as the etchant was used to produce the etch pits on the surfaces of GaN grown on three different-sized PSS. Figure 2.57 demonstrates the AFM images of etch pits on surfaces of three samples. The density of etch pits decreases from PSS-I to PSS-III. Because one etch pit corresponds to one TD. It is clearly indicated that the TDDs can be effectively suppressed by increasing the size of PSS. As shown in Fig. 2.58, the (002) and (102) ω-scan rocking curves were also collected to evaluate the TDDs in the samples. By analyzing the FWHMs of the (002) and (102) ω-scan rocking curves for three samples, both the screw and edge dislocation densities of GaN grown on PSS-III are lower than those of GaN grown on PSS-I and PSS-II. This result also verifies that the increased size of cone-shaped PSS can improve the crystalline quality of GaN [125]. In order to elaborate the effect of the reduced TDDs on the IQE of the samples, PL spectra collected at low (8 K) and room temperatures (295 K) are demonstrated in Fig. 2.59. Ignoring the spectrally integrated PL intensity differences between 8 and 0 K, the IQEs of three samples can be estimated by the ratio of the integrated PL intensity at room temperature to the integrated PL intensity at low temperature [126, 127, 128]. By calculating the IQEs of three samples, the IQE of UV LEDs grown on PSS-III is the highest among the three samples. In order to investigate the effect of PSS size on the optoelectronic property of UV LED, the LOP versus current and I-V curves of three samples were collected and demonstrated in Fig. 2.60. The LOP of UV LED grown on PSS-III is higher than that of UV LED grown on PSS-I and PSS-II. As shown in Fig. 2.60(b), at 20 mA, the forward voltage of UV LED on PSS-III is slightly higher than that of UV LED on PSS-I or PSS-II, indicating that the number of forward leakage paths caused by TDs could be reduced by improving the crystalline quality of GaN epitaxial layers.

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Fig. 2.56 Cross-sectional TEM images of the three samples: (a)–(c) PSS-I; (d)–(f) PSS-II; (g)– (i) PSS-III

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Fig. 2.57 AFM images of etch pits of the samples. a PSS-I, b PSS-II, c PSS-III

Fig. 2.58 a (002) and b (102) ω-scan rocking curves of the samples

Fig. 2.59 PL spectra of three samples at a 8 K and b 295 K

2.3.3 Patterned Sapphire with Silica Array In this discussion [164], InGaN-based UV LED with PSSA is proposed. Because of the presence of limited refractive index contrast between sapphire (nsapphire = 1.78)

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Fig. 2.60 a LOP versus current of three samples. b I-V characteristics of three samples

and Al(Ga)N (nAlN = 2.16, nGaN = 2.58) in conventional PSS technique, the effective light extraction is not achieved. While in the case of silica (SiO2 ), the smaller refractive index (1.46) enables it become one of the candidates to form larger refractive index contrast between Al(Ga)N and such patterned substrate, facilitating more light to be extracted from the top of flip-chip LED. In order to set up a comparison, the InGaN/AlGaN-based UV LEDs were fabricated on FSS, PSS and PSSA simultaneously. To fabricate PSSA with the pointed-cone-shaped structure, an SiO2 film with thickness of 2 μm was first deposited on the c-plane sapphire through plasma enhanced chemical vapor deposition. A positive photoresist was then spin-coated on the SiO2 film. The photoresist was patterned in a circular shape by optical photolithography and reflowed during the hard baking process at 125 °C to form a cone-shaped pattern, followed by ICP etching using CHF3 /SF6 plasmas. The periodic cone-shaped patterns were finally transferred on the SiO2 film. The residual photoresist decreased ultrasonically using acetone, methanol, and DI water for 5 min in each step and the second ICP etching process using BCl3 /Ar plasmas was performed to etch the underlying sapphire substrate with the remaining SiO2 cone-shaped patterns serving as a mask. The schematic diagram of the fabrication process of PSSA is shown in Fig. 2.61. In order to investigate the crystalline quality of AlGaN epilayers grown on PSS, and PSSA, the XRD and scanning transmission electron microscopy (STEM) characterizations were conducted to estimate the TDDs of AlGaN epilayers. Figure 2.62a and b show the (002) and (102) XRD rocking curves of AlGaN films grown on PSS and PSSA, respectively. It can be seen that the FWHMs of (002) and (102) rocking curves for the AlGaN films grown on PSSA are both smaller than those for the AlGaN films grown on PSS, implying the reduction of the total TDD in the AlGaN film grown on PSSA. However, considering the effects of mosaicity and thickness on the FWHM of XRD rocking curve cannot be neglected [86], the planview STEM characterizations were performed to estimate the TDDs of AlGaN films grown on PSS and PSSA more precisely. By selecting appropriate crystal orientation

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Fig. 2.61 Schematic illustration of the fabrication process of PSSA

and operating vector (g), edge, screw and mixed dislocations are both observed simultaneously in the STEM image [129]. As presented in Fig. 2.62c–l, the lines in the images correspond to the edge dislocations because of their strain fields intersecting the specimen, while black-white spot pairs correspond to the screw dislocations due to their surface strain relaxation. For the mixed dislocation with edge and screw components, a line connected by black-white spot pairs is shown in the image. By comparing the STEM images of AlGaN films grown on PSS and PSSA, the TDDs of AlGaN films on PSSA are lower than those on PSS which agrees well with the results in XRD rocking curves. Therefore, the structure of PSSA can reduce TDDs at the interface between the substrate and epilayers. In order to elaborate the initial growth process difference between AlGaN flims on PSS and PSSA, the in-situ reflectance transients and the top-view SEM images of AlGaN on PSS and PSSA are shown in Fig. 2.63. It can be seen in Fig. 2.63a, the growth process of PSS and PSSA is divided into four stages as denoted. When the temperature increases to 1050 °C, the reflectivity starts to oscillate due to the Fabry–Perot interference phenomenon [130]. Curiously, at the stage of 2D growth, the reflectance on PSS is not saturated compared with the reflectance on PSSA. It can be speculated that the presence of incomplete coalescence. The speculation can be confirmed in top-view SEM images at different growth stages shown in Fig. 2.63b–i. Comparing the SEM images of C1 and S1, a smaller (0001) facet can be obtained on PSSA and thus demonstrating that a preferred vertical growth of AlGaN on PSSA than PSS during the 3D growth stage. According to the principle of dislocation line energy minimization, a preferred vertical growth can induce a closer bending to free surface [131, 132]. It can be seen in Fig. 2.63e, i, a plenty of uncoalesced pits on the surface due to the existence of misoriented crystals on AlGaN-sapphire layers, but no pits can be observed on AlGaN-SiO2 layers. The cross-sectional STEM characterization provides further insights into the growth behavior of AlGaN on PSS and PSSA, as shown in Fig. 2.64. Compared

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Fig. 2.62 a (002) and b (102) XRD rocking curves of the samples grown on PSS and PSSA, respectively. c Plan-view of STEM image of the sample grown on PSS, and the corresponding magnified images in (d)–(g). h Plan-view of STEM image of the sample grown on PSSA, and the corresponding magnified images in (i)–(l)

with AlGaN grown on PSS, the dislocations formed in the initial growth stage of AlGaN grown on PSSA decrease. In addition, during AlGaN grown on PSS, the parasitic crystals formed on the patterns are larger and dislocations are observed on the boundary of parasitic crystals. More details of the parasitic crystals can be seen in the magnified STEM images. In Fig. 2.64c, the magnified STEM image of parasitic crystals on the AlN/Al2 O3 cone-shaped pattern is dominated by bright/dark contrast and moire fringes caused by the misfit and strain dislocation [133]. Different from the condition of AlN/Al2 O3 , parasitic crystals in AlN/SiO2 cone-shaped pattern show columnar grain structure in Fig. 2.64d. Owing to the strongly disturbed crystal symmetry, AlN NLs are difficult to be identified in STEM image [134]. Previous reports indicated that the morphologies of sputtered AlN films on Al2 O3 and SiO2 were different [135, 136]. Sputter-deposited AlN on Al2 O3 cone-shaped patterns results in the formation of misoriented crystals [135]. However, the above

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Fig. 2.63 a In-situ reflectance and temperature curves of the samples grown on PSS and PSSA. b–e Top-view SEM images of AlGaN grown on PSS. f–i Top-view SEM images of AlGaN grown on PSSA

phenomenon is not observed in the amorphous SiO2 cone-shaped patterns, and only an amorphous transition layer was formed with thickness of 20 nm [136]. Subsequently, AlGaN epilayers replicate the large misorientation in AlN NLs grown on AlN/Al2 O3 cone-shaped patterns, and the concentrated stress can be introduced at the coalescence boundary when coalescence happens in the misoriented crystals. The nucleation of AlGaN on the amorphous AlN/SiO2 cone-shaped patterns results in the c-axis preferred orientation due to their minimum surface energy configuration [137]. The L-I-V curves of the samples grown on three kinds of substrates are shown in Fig. 2.65a, b. It can be seen that the I-V curves of three samples overlap closely, implying slight variations in the electrical performance for three kinds of UV LEDs. What’s more, the LOP of the sample with PSSA is higher than that of the samples with FSS and PSS, which is ascribed to the improvement of crystal quality and light extraction efficiency (LEE). According to Fig. 2.65c, d, it is worth noting that the

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Fig. 2.64 Cross-sectional STEM images of epilayers grown on a PSS and b PSSA. c and d Magnified STEM images of the labeled areas in (a) and (b)

peak EQE of the sample grown on PSSA in this discussion is higher than those reported in previous reports [138, 139, 140, 141, 142, 143, 144]. The optical properties of UV LEDs grown on different substrates were simulated by finite-difference time-domain (FDTD). Figure 2.66 depicts the simulated far-field emission patterns of UV LEDs grown on PSS and PSSA. Comparing Fig. 2.66a with b, the UV LED grown on PSSA has a higher radiation intensity along the axial direction. Figure 2.66c illustrates the LEE simulation results at the top, bottom and sidewalls of UV LEDs grown on different substrates. Compared with the UV LED grown on PSS, the LEEs of the UV LED grown on PSSA increase at the top and bottom by 17.0% and 10.7%, respectively, while the LEEs of the UV LED grown on PSSA decrease at the four sidewalls by 2.4%. In summary, UV LEDs grown on PSSA achieved an increasement in total LEE. Figure 2.66d shows the measured far-field radiation patterns of UV LEDs grown on PSS and PSSA. Compared with the UV LED grown on PSS, it can be clearly observed that the UV LED grown on PSSA displays a more concentrated radiation pattern and higher radiation intensity along the axial direction, which are consistent with the simulation results.

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67

Fig. 2.65 a I-V and b LOP versus current curves of the samples. c EQEs of the samples. d Peak EQEs of LEDs of this work and the previous studies

2.3.4 Isoelectronic Doping It was reported that the isoelectronic doping of III-V semiconductor materials was an effective method to improve the crystal quality of epilayers and the optoelectronic performance of semiconductor devices. For example, N or In doping in GaAs [145, 146], and As or In doping in GaN [147, 148, 149]. In this subsection [131], the effect of isoelectronic doping that Al doped in GaN buffer layer on the optoelectronic performance of UV LEDs is investigated. The undoped and isoelectronic Al-doped GaN buffer layers with same thickness on the sputtered AlN NL were prepared by MOCVD. By changing the molar flow ratio of TMAl/(TMAl + TMGa) to control the concentration of doped Al atoms in GaN buffer layer intentionally, the molar flow ratio of TMAl/(TMAl + TMGa) in this experiment was set as 0, 0.02 and 0.04, named as sample A, sample B and sample C, respectively. The schematic diagram of LED structure in this study is shown in Fig. 2.67. The in-situ temperature and reflectance measurements in the epitaxial growth process of undoped and isoelectronic Al-doped GaN buffer layers are shown in Fig. 2.68a. Because the adoption of sputtered AlN NL and PSS can save the growth

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Fig. 2.66 Simulated diagrams for far-field radiation of UV LEDs grown on a PSS and b PSSA. c Simulated LEEs at the top, bottom, and sidewalls of UV LEDs grown on PSS and PSSA. d Measured far-field radiation patterns of UV LEDs grown on different substrates

Fig. 2.67 Schematic diagram of LED structure grown on conventional or Al-doped GaN buffer layers

time of GaN, the growth temperature directly increases from room temperature to 1000 °C and then the epitaxial growth of GaN buffer layers starts at high temperature. At the initial stage of epitaxial growth at high temperature, GaN grows in a 3D island mode on the sputtered AlN NL. When the adjacent GaN islands begin to coalesce, the reflectivity curve starts to oscillate and rises rapidly, demonstrating that the growth mode of GaN buffer layers changes from 3D growth mode to 2D growth mode. When the surface of GaN film is smooth, the amplitude of the reflectivity curve saturates. Compared with the undoped GaN buffer layers (sample A), a longer transition time

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69

Fig. 2.68 a Temperature and reflectance transients during the buffer layers growth of three samples. b TMGa and TMAl mole flow rate transients during the buffer layers growth of three samples

from 3D to 2D growth mode can be obtained in the isoelectronic Al-doped GaN buffer layers (sample B and sample C). Figure 2.68b shows the curves of molar flow rates of TMGa and TMAl during epitaxial growth of samples A, B and C. Figure 2.69 shows the (002) and (102) ω-scan rocking curves of undoped and isoelectronic Al-doped GaN buffer layers. It can be seen from Fig. 2.69a, b that the FWHMs of (002) ω-scan rocking curves for sample B and sample C are both smaller than those of (002) ω-scan rocking curves for sample A. However, there is no obvious difference in the FWHMs of (102) ω-scan rocking curves for samples A, B and C. It is known that the FWHM of (002) ω-scan rocking curve is mainly related to screw and mixed dislocations whereas the FWHM of (102) ω-scan rocking curve is mainly related to edge and mixed dislocations [150, 151]. By analyzing the above results, it can be concluded that the isoelectronic Al-doped GaN buffer layers can reduce the density of screw dislocation rather than that of edge dislocation. In order to further analyze the influence of isoelectronic Al-doped technology on the dislocation type and TDDs of GaN buffer layers, two-beam diffraction images were collected, as shown in Fig. 2.70. This figure shows the bright field TEM foil that

Fig. 2.69 a (002) and b (102) ω-scan rocking curves of undoped and isoelectronic Al-doped GaN buffer layers

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Fig. 2.70 a, d and g Cross-sectional bright field TEM images of GaN buffer layers along the [10– 10] zone axis. b, e and h Bright field TEM images when g = [0002]. c, f and i Bright field TEM images when g = [11–20]

is in the [10–10] zone axis. The screw dislocation (S), edge dislocation (E) and mixed dislocation (M) are marked in Fig. 2.70a, d, g. Figure 2.70b, e, h and Fig. 2.70c, f and i are the two-beam diffraction images under two different diffraction vectors which correspond g = [0002] and g = [11–20], respectively. According to the dislocation invisible criterion (g·b = 0), under the diffraction condition of g = [0002], the screw dislocations and mixed dislocations are visible while the edge dislocations are invisible. Under the diffraction condition of g = [11–20], the edge dislocations and mixed dislocations are visible while the screw dislocations are invisible. By comparing the results in these figures, it can be found that the dislocation density in sample B is the lowest and that in sample A is the highest. The Cross-sectional TEM images of GaN buffer layers cut in the [11–20] are demonstrated in Fig. 2.71. The corresponding screw dislocation (S), edge dislocation

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71

Fig. 2.71 a, d and g Cross-sectional bright field TEM images of GaN buffer layers along the [11– 20] zone axis. b, e and h Bright field TEM images when g = [0002]. c, f and i Bright field TEM images when g = [11–20]

(E) and mixed dislocation (M) are marked in Fig. 2.71a, d, g. Figure 2.71b, e, h show the cross sectional bright field TEM images of sample A, B, and C under the g = [0002]. The cross sectional bright field TEM images of samples A, B and C under the g = [10–10] are depicted in Fig. 2.71c, f, i. By comparing the results in these figures, it can be inferred that the isoelectronic Al-doped technology can significantly reduce the density of screw dislocation.

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The dislocation densities of samples A, B and C are calculated by etching the GaN buffer layers with molten NaOH solution (270 °C). The AFM images of etched sample A, B, and C are shown in Fig. 2.72 and the corresponding scan area is set to be 5 μm × 5 μm. A large number of etching pits located on the surface of GaN buffer layers can be observed in both samples. As demonstrated in Fig. 2.72, the density of dislocations in sample B is lower than that in sample A but larger than that in sample C. According to the above results, we believe that the isoelectronic Al-doped technology can reduce the dislocation density of GaN buffer layers, but an excessive incorporation of Al will lead the dislocation density increased. In order to investigate the effect of isoelectronic Al-doped technology on the optoelectronic performance of devices, the corresponding LOP-I-V curves of the samples were obtained as shown in Fig. 2.73. Figure 2.73a shows the characteristics of LOP versus injection current in three kinds of UV LEDs. It is noted that at the

Fig. 2.72 The 5 × 5 μm2 AFM images a sample A, b sample B, and c sample C, the black dots represent the etching pits

Fig. 2.73 a LOP versus current, and b forward current versus forward voltage characteristics of UV LEDs grown on three samples. The inset shows the reverse leakage current characteristics of UV LEDs

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73

Fig. 2.74 EL peak energy versus applied voltage characteristics of 365 nm UV LEDs grown on sample A, B and C. Inset shows the EL peak energy versus applied voltage characteristics of 395 nm UV/blue/green LEDs

same injection current, the LOP of LED grown on sample B is higher than that of LED grown on sample A because the density of dislocation of sample B is lower than that of sample A. While the concentration of Al increases further, the LOP of LED on sample C is lower than that of LED on sample B, which can be explained that an excessive concentration of Al can reduce the quality of GaN buffer layers. The inset of Fig. 2.73b shows the reverse leakage current in three UV LEDs. By comparing the result from the inset, we can infer that the isoelectronic Al-doped technology can reduce the density of dislocation in GaN buffer layers. Figure 2.74 shows EL peak energy as a function of applied voltage for UV LEDs on sample A, sample B, and sample C at room temperature. For comparison, the EL peak energy of 395 nm UV/blue/green LEDs as a function of applied voltage is demonstrated in the inset. As shown in the inset, the EL peak energy of blue and green LED with high In components increases with the applied voltage increasing, which is caused by the combined effects that the free carriers induced screening of the QCSE and the band-filling of the localized states by excitons. However, the EL peak energy of 365 nm UV LEDs decreases with the forward voltage increasing, which may be caused by the narrowing band gap resulted from many-body effect. Moreover, as the applied voltage increases, the EL peak energy of the 395 nm UV LED increases firstly and then decreases, which indicates that the interband transition energy of the strained InGaN QW is determined by the competition between a redshift phenomenon caused by bandgap narrowing and a blueshift phenomenon caused by carrier-induced screening of the QCSE and band-filling effect.

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2.3.5 InAlGaN/AlGaN Electron Blocking Layer EBL plays an important role in improving carrier injection efficiency (CIE) of AlGaN-based deep ultraviolet LEDs (DUV-LEDs) [152, 153]. Various EBL structures have been reported to enhance CIE of DUV-LEDs, such as gradient EBL [154], graded superlattice EBL [155], and multiquantum-barrier EBL [156]. In this subsection [165], we proposed InAlGaN/AlGaN EBL and InAlGaN/AlGaN superlattice EBL to alleviate electron leakage and enhance hole injection in AlGaN-based DUV-LEDs. The schematic diagram of DUV-LEDs with different EBL structures is shown in Fig. 2.75. The EBL of structure A (named as SA) is a 25 nm-thick Al0.65 Ga0.44 N layer. The EBL of structure B (named as SB) is composed of a 20 nmthick Al0.65 Ga0.44 N layer and a 5 nm-thick Inx Al0.65 Ga0.35−x N layer. The In-content (x) of SB1, SB2, SB3, SB4, and SB5 is 0.01, 0.03, 0.05, 0.07, and 0.09, respectively. The EBL of structure C (named as SC) is composed of five-pair Iny Al0.65 Ga0.35−y N (3 nm) /Al0.65 Ga0.44 N (2 nm) superlattice layers. The In-content (y) of SC1, SC2, SC3, SC4, and SC5 is 0.01, 0.03, 0.05, 0.07, and 0.09, respectively. Figure 2.76 shows the calculated energy band diagrams of SA and SB at 60 mA. We could find that the effective potential barrier height for electrons in SB is higher than the effective potential barrier height for electrons in SA, whereas the effective potential barrier height for holes in SB is lower than the effective potential barrier height for holes in SA. This indicates that SB possesses superior electron blocking capability and hole injection capability in comparison with SA. Among SB1, SB2, SB3, SB4, and SB5, SB3 possesses the highest effective potential barrier height for electrons (330 meV) and the lowest effective potential barrier height for holes (355 meV), as shown in Fig. 2.76d. Figure 2.77 shows the calculated energy band diagrams of SA and SC at 60 mA. Compared with SA, SC possesses higher effective potential barrier height for electrons and lower effective potential barrier height for holes. Among SC1, SC2, SC3,

Fig. 2.75 Schematic diagram of DUV-LEDs with different EBL structures

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75

Fig. 2.76 Energy band diagrams of a SA, b SB1, c SB2, d SB3, e SB4, and f SB5

SC4, and SC5, SC4 reveals the highest effective potential barrier height for electrons (405 meV) and the lowest effective potential barrier height for holes (348 meV), as shown in Fig. 2.77e. Figure 2.78 shows carrier concentrations within active region of SA, SB, and SC at 60 mA. We can find that carrier concentrations of SB and SC are higher than that of SA. Among SB1, SB2, SB3, SB4, and SB5, SB3 reveals the highest carrier concentrations, as shown in Fig. 2.78a, b. Among SC1, SC2, SC3, SC4, and SC5, SC4 reveals the highest carrier concentrations, as shown in Fig. 2.78c, d. Figure 2.79 shows energy bands nearby EBL and carrier current density distributions of SA, SB3, and SC4 at 60 mA. We find that InAlGaN/AlGaN superlattice EBL of SC4 effectively decreases downward energy band bending at the LQB/EBL interface and increases effective potential barrier height for electrons, as shown in

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Fig. 2.77 Energy band diagrams of a SA, b SC1, c SC2, d SC3, e SC4, and f SC5

Fig. 2.79a. Therefore, the electron current density of SC4 is the lowest among SA, SB3, and SC4, as shown in Fig. 2.79c. Meanwhile, InAlGaN/AlGaN superlattice EBL of SC4 effectively alleviates upward band bending at the EBL/p-GaN interface and decreases effective potential barrier height for holes, as shown in Fig. 2.79b. Hence, the hole current density of SC4 is the highest among SA, SB3, and SC4, as shown in Fig. 2.79c. Figure 2.80a shows L-I characteristics of SA, SB3, and SC4. We find that the LOP of SA, SB3, and SC4 is 1.0, 5.2, and 10.1 mW, respectively. Figure 2.80b shows IQE of SA, SB3, and SC4. The maximum IQE of SA, SB3, and SC4 is 29.8%, 43.3%, and 68.0%, respectively. Furthermore, the efficiency droop of SC4 significantly decreases in comparison with that of SB3 and SA. Figure 2.80c shows radiative recombination rate of SA, SB3, and SC4 at 60 mA. The radiative recombination rate of SC4 is the highest among SA, SB3, and SC4. Figure 2.80d shows emission spectra of SA, SB3,

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77

Fig. 2.78 a Electron concentration and b hole concentration within active region of SA and SB at 60 mA. c Electron concentration and d hole concentration within active region of SA and SC at 60 mA

Fig. 2.79 a Conduction bands, b valence bands, c electron current density, and d hole current density of SA, SB3, and SC4 at 60 mA

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Fig. 2.80 a L-I characteristics of SA, SB3, and SC4. b IQE versus current curves of SA, SB3, and SC4. c Radiative recombination rate and d emission intensity of SA, SB3, and SC4 at 60 mA

and SC4 at 60 mA. We find that the emission peak wavelength of SA, SB3, and SC4 is 272 nm. The emission intensity of SB3 and SC4 is improved by 7 and 11 times in comparison with that of SA.

2.3.6 Graded Al-Content AlGaN Insertion Layer Although high-Al-content EBL could effectively alleviate electron leakage, it hinders hole injection in AlGaN-based DUV-LEDs [157, 158]. To solve the issue, we proposed an EBL-free AlGaN-based DUV-LED with graded Al-content AlGaN insertion layer [166]. The schematic diagram and Al-content profiles of DUV-LEDs with EBL and without EBL are shown in Fig. 2.81. The DUV-LED with EBL is named as LED A. The EBL-free DUV-LEDs with graded Al-content AlGaN insertion layers are named as LED B and LED C, as shown in Fig. 2.81. Figure 2.82 shows the calculated energy band diagrams of LED A, LED B, and LED C at 100 mA. ∅B1 , ∅B2 , ∅B3 , ∅B4 , ∅B5 , ∅LQB , and ∅E are effective potential barrier height for electrons in QB1 , QB2 , QB3 , QB4 , QB5 , LQB, and EBL, respectively. We find in Fig. 2.82a that energy band bending at LQB/EBL interface results in low ∅LQB of LED A, leading to severe electron leakage in LED A. The ∅LQB of LED B (281 meV) is significantly larger than that of LED A (50 meV), as shown

2.3 III-Nitride Ultraviolet LEDs

79 LED A 55%

0.65

Al-content

0.50 0.37

MQW

EBL

p-GaN layer p-AlGaN layer

LED B 0.51 0.52 0.53 0.54 0.55

Al0.37Ga0.63N/Al0.5Ga0.5N MQW

n-Al0.55Ga0.45N layer

Al-content

EBL

0.65 0.55 0.50 0.37

MQW QB1 QB2 QB3 QB4 QB5 LQB

AlN Sapphire substrate

LED C

Al-content

0.51 0.52 0.53 0.54 0.55

0.65 0.55 0.50 0.37

QB1 QB2 QB3 QB4 QB5 LQB

Fig. 2.81 Schematic diagram and Al-content profiles of DUV-LEDs with EBL and without EBL

Fig. 2.82 Calculated energy band diagrams of a LED A, b LED B, and c LED C at 100 mA

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in Fig. 2.82b. The result indicates that removing EBL could eliminate energy band bending at LQB/EBL interface of LED B and inserting graded Al-content AlGaN layer into QB of LED B could improve effective potential barrier height for electrons in QB. Compared with LED B, the ∅LQB of LED A (347 meV) is further improved, as shown in Fig. 2.82c. The result indicates that inserting multiple graded Al-content AlGaN layers into LQB of LED C could further enhance electron blocking capability of LED C in comparison with LED B. The detailed values of ∅B1 , ∅B2 , ∅B3 , ∅B4 , ∅B5 , ∅LQB , and ∅E in LED A, LED B, and LED C are shown in Table 2.1. Figure 2.83 shows the valence band diagrams at different interfaces of LED A, LED B, and LED C at 100 mA. In Fig. 2.83a, effective potential barrier height for holes in EBL of LED A is 336 meV. In Fig. 2.83b and c, effective potential barrier Table 2.1 Effective potential barrier height for electrons in LED A, LED B, and LED C at 100 mA ∅B1 (meV)

∅B2 (meV)

∅B3 (meV)

∅B4 (meV)

∅B5 (meV)

∅LQB (meV)

∅E (meV)

LED A

209

209

209

209

209

50

LED B

130

174

196

208

216

281

–

LED C

142

180

204

213

228

347

–

252

Fig. 2.83 Valence band diagrams at different interfaces of a LED A, b LED B, and c LED C at 100 mA

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Fig. 2.84 a Electron concentration, b hole concentration and c radiative recombination rate in active region of LED A, LED B, and LED C at 100 mA

height for holes in LQB of LED B and LED C is 323 and 285 meV, respectively. The result indicates that removing EBL decreases the effective potential barrier height for holes in LED B and LED C. Meanwhile, inserting multiple graded Al-content AlGaN layers into LQB further decreases the effective potential barrier height for holes in LED C. Figure 2.84 shows carrier concentration and radiative recombination rate within active region of LED A, LED B, and LED C at 100 mA. We could find that the electron and hole concentration of LED C is the highest among LED A, LED B, and LED C, as shown in Fig. 2.84a, b. This is because LED C possesses the highest effective potential barrier height for electrons and the lowest effective potential barrier height for holes among LED A, LED B, and LED C. The improvement of electron and hole concentration results in enhancement of radiative recombination rate within active region of LED C, as shown in Fig. 2.84c. Figure 2.85a shows emission spectra of LED A, LED B, and LED C at 100 mA. We can see that the emission peak wavelength of LED A, LED B, and LED C is 287 nm. Compared with LED A, the emission intensity of LED B and LED C is enhanced by 39.8% and 46.8%, respectively. Figure 2.85b shows IQE versus current curves of LED A, LED B, and LED C. The maximum IQE of LED A, LED B, and LED C is 52.9%, 80.5%, and 86.9%, respectively. Besides, the efficiency droop of LED B and LED C significantly decreases in comparison with that of LED A.

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Fig. 2.85 a Emission spectra of LED A, LED B, and LED C at 100 mA. b IQE versus current curves of LED A, LED B, and LED C

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Chapter 3

High-Efficiency Top-Emitting III-Nitride LEDs

3.1 Light Extraction Microstructure 3.1.1 PSS and Patterned ITO Due to large mismatch of lattice constant and thermal expansion coefficient between GaN epilayers and sapphire substrate, there are high dislocation densities existing in the GaN epilayers, thereby decreasing IQE of GaN-based LED [1–4]. Compared to FSS, PSS can effectively decrease dislocation density in the epilayers, thereby improving the crystal quality of epilayers [5–7]. In addition, PSS can manipulate the optical propagation direction to improve the possibility of light scattering, which facilitates light extracting [8–10]. Therefore, PSS technology is anticipated to enhance emission efficiency of LED. ITO is usually deposited on the surface of p-GaN layer as transparent conductive layer (TCL). The refractive index of the ITO and air is 2.08 and 1, respectively. According the Snell’s Law, total internal reflection (TIR) occurs at the ITO/air interface, leading to the low LEE. After roughening ITO TCL, the possibility of light scattering is improved, thereby further enhancing LEE of LED. Wet etching combining with photolithography can be applied to form patterned ITO TCL. Recent research has revealed that intentionally roughening the surface of the ITO TCL is an effective way to improve the LEE of LEDs [11, 12]. We investigated the influence of PSS and patterned ITO on the optoelectronic performance of LED [13]. Figure 3.1a, b show the schematic diagram and optical microscope image of LED grown on PSS. Figure 3.2a shows cross-sectional SEM image of LED grown on PSS. Figure 3.2b, c show top-viewed SEM images of ITO with periodic and non-periodic hexagonal patterns. Diagonal line length, side length and spacing of these patterns are 7.1, 4.3, and 9 μm, respectively. Figure 3.3 shows optical power versus current characteristics of LEDs. LED grown on FSS, LED grown on PSS, LED grown on PSS with non-periodic patterned ITO, and LED grown on PSS with periodic patterned ITO is labeled as FSS-LED, PSS-LED, PSS-LED with non-periodic patterned ITO, and PSS-LED with periodic © Science Press 2022 S. Zhou and S. Liu, III-Nitride LEDs, Advances in Optics and Optoelectronics, https://doi.org/10.1007/978-981-19-0436-3_3

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Fig. 3.1 a Schematic diagram and b optical microscope image of LED grown on PSS

Fig. 3.2 a Cross-sectional SEM image of LED grown on PSS. Top-viewed SEM images of ITO with b periodic and c non-periodic hexagonal patterns

patterned ITO, respectively. At 350 mA, the optical power of FSS-LED, PSS-LED, PSS-LED with non-periodic patterned ITO, and PSS-LED with periodic patterned ITO is 182.8 mW, 208.1 mW, 238.7 mW, and 253.8 mW, respectively. The wallplug efficiency of FSS-LED, PSS-LED, PSS-LED with non-periodic patterned ITO, and PSS-LED with periodic patterned ITO is 15.9%, 18.9%, 21.3%, and 22.3% at 350 mA, respectively. The optical power of PSS-LED is 15.6% higher than that of FSS-LED. The enhancement of optical power is attributed to the increase of both IQE and LEE by improving GaN crystal quality due to the reduction of dislocation density and suppressing TIR at the corrugated interface. After the periodic hexagonal patterns with 4.3 μm side length and 9 μm spacing are formed on the ITO layer, the optical powers of the LEDs with periodic patterned ITO are 20.6% and 36.9% higher than that of the PSS-LED and the FSS-LED, respectively. The reason for further enhancement in optical power when combining PSS with patterned ITO is that the periodic patterned ITO surface provides the photons with multiple opportunities to escape from the LED surface. It is worth noting that the optical power

3.1 Light Extraction Microstructure

93

Fig. 3.3 Optical power versus current characteristic curves of LEDs

of LEDs with periodic patterned ITO is higher than that of LEDs with non-periodic patterned ITO. Figure 3.4 shows the effect of hexagonal pattern’s side length on the optical power and I-V characteristics of LEDs. In Fig. 3.4a, the spacing of two adjacent hexagonal patterns is fixed at 9 μm. When the side length of ITO patterns is 2.3 μm, 3.3 μm, and 4.3 μm, the optical power of PSS-LED with patterned ITO is 236.6 mW, 241.1 mW, and 250.3 mW, respectively. As the side length of ITO patterns increases, the optical power of PSS-LED with patterned ITO increases. Moreover, the 350-mA-driven wall-plug efficiency of the LEDs with patterned ITO is 21.2%, 21.6%, and 22.4%, respectively when the side length of hexagonal pattern is 2.3 μm, 3.3 μm, and 4.3 μm. In Fig. 3.4b, the forward voltages of PSS-LEDs with patterned ITO are slightly higher than that of LED with non-patterned ITO at 350 mA. The slightly higher forward voltage results from the patterned ITO, which affects the current spreading performance. Figure 3.5 shows the effect of hexagonal pattern’s spacing on the optical power and I-V characteristics of LEDs. The side lengths of periodic patterns are fixed at 2 μm. In the Fig. 3.5a, when spacing of two adjacent patterns increases from 6.5 to 8.5 μm, the optical power of LEDs decreases from 246.3 to 230.1 mW at 350 mA. In Fig. 3.5b, when spacing of two adjacent patterns increases from 6.5 to 8.5 μm, the forward voltage of LEDs decreases from 3.215 to 3.191 V at 350 mA.

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Fig. 3.4 a Optical power versus current and b current versus forward voltage characteristic curves of LEDs with different side length of patterns on ITO

3.1 Light Extraction Microstructure

95

Fig. 3.5 a Optical power versus current and b current versus forward voltage characteristic curves of LEDs with different spacing of patterns on ITO

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3.1.2 Double Layer ITO Conventional patterned ITO is not beneficial for the electrical performance of UV LED because it can increase sheet resistance of UV LED. Introducing an annealed ITO layer can avoid electrical degradation and stabilize the current spreading. Thus, we combined two single ITO layers into a double-layer ITO to obtain enhanced LEE and stabilize electrical performance [14]. The first ITO layer is annealed to form lowresistance Ohmic contact with p-GaN. The second ITO layer keeps unannealed and is roughened to increase scattering probability of photons at the ITO/air interface. The periodic pinhole patterns with different intervals are formed on the unannealed ITO layer by laser direct writing. Figure 3.6a, b show the schematic diagrams of UV LED with patterned double-layer ITO. Figure 3.6c shows top-view SEM image of fabricated UV LED with patterned double-layer ITO. Figure 3.7 shows the top-view SEM images of the pinhole patterns with different intervals. In Fig. 3.7a–c, the diameter/interval of the pinhole patterns in UV LEDs is 2 μm/400 nm (UV LED II), 2 μm/600 nm (UV LED III), and 2 μm/800 nm (UV LED IV), respectively. The UV LED with planar double-layer ITO is labeled as UV LED I.

Fig. 3.6 a Schematic diagram of UV LED with patterned double-layer ITO. b Cross-sectional schematic diagram of UV LED with patterned double-layer ITO. c Top-view SEM image of UV LED with patterned double-layer ITO

Fig. 3.7 Top-view SEM images of the pinhole patterns with different intervals

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Fig. 3.8 Transmission spectra of the ITO layer before and after thermal annealing process

We measured the transmittance of annealed and unannealed ITO at the wavelength ranging from 370 to 440 nm. In Fig. 3.8, at the wavelength of 375 nm, the transmittance of the unannealed and annealed ITO is 68.81% and 66.88%, respectively. The transmittance of unannealed ITO is 2.89% higher than that of annealed ITO, revealing that thermal annealing will degrade UV optical transmittance of ITO. We simulated the effect of pattern interval on current spreading of LED with the double-layer ITO. Figure 3.9a–d show the simulated current density distribution of UV LED I, UV LED II, UV LED III, and UV LED IV at 350 mA. The RMS of current density in the active region of UV LED I, UV LED II, UV LED III, and UV LED IV is 35.02, 38.76, 37.27, and 37.09 A/cm2 , respectively. Compared to UV LED I, another three types of UV LEDs have slightly higher RMS of current density, indicating that annealed ITO layer is beneficial for current spreading. In addition, as the interval of the pinhole patterns on the unannealed ITO layer increases, the RMS of current density in the active region decreases. Figure 3.10a shows L-I characteristics of UV LED I, UV LED II, UV LED III and UV LED IV. At 350 mA, LOP of UV LED I, UV LED II, UV LED III and UV LED IV is 32.60 mW, 36.70 mW, 36.38 mW and 34.75 mW, respectively. Compared to UV LED I, the LOP of UV LED II, UV LED III, and UV LED IV is improved by 12.4%, 11.7%, and 6.5%, respectively. Figure 3.10b shows the I-V characteristic of UV LED I, UV LED II, UV LED III and UV LED IV. At 350 mA, the forward voltages of

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Fig. 3.9 Current density distribution in the active region of a UV LED I, b UV LED II, c UV LED III, and d UV LED IV at 350 mA

Fig. 3.10 a L-I and b I-V characteristic curves of UV LED I, UV LED II, UV LED III and UV LED IV

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UV LED I, UV LED II, UV LED III, and UV LED IV are 3.88 V, 4.01 V, 3.99 V, and 3.97 V, respectively. The forward voltage of UV LED with patterned doublelayer ITO is slightly higher than that of UV LED with planar double-layer ITO. The slightly higher forward voltage is originated from the pinhole patterns on the second ITO, which will impede lateral current spreading. Additionally, the forward voltage of UV LED with patterned double-layer ITO increases with decreasing interval of the pinhole patterns.

3.1.3 3D Patterned ITO and Wavy Sidewalls Light is randomly generated in the active region and can be emitted into air from sidewalls. However, TIR will occur at GaN/air interface when light is emitted from sidewalls owing to prominent refractive index contrast between GaN and air, thereby decreasing LEE. The formation of microstructure on sidewalls is an effective way to improve probability of light out-coupling. We investigated the influence of 3D patterned ITO double layers and wavy sidewalls on the optoelectronic performance of LED [15]. We fabricated wavy sidewall structure by using optical photolithography and ICP etching. ICP etching based on BCl3 /Cl2 gas chemistry is used to remove a portion of p-GaN layer and MQW active layer to expose n-GaN layer for the formation of GaN mesa structure. During the mesa fabrication process, a positive photoresist layer is spin-coated onto LED wafer. Then, the photoresist layer is patterned to be a rectangle shape with wavy sidewalls via photolithography process. Finally, GaN epitaxial layer is etched and wavy sidewalls in the photoresist mask layer could be transferred into scribing line along with the etched GaN mesa structure. After forming wavy sidewalls, a 190 nm thick SiO2 current blocking layer (CBL) is subsequently formed on the top of p-GaN layer by plasma enhanced chemical vapor deposition (PECVD) and successive photolithography process and buffer oxide etching. To obtain 3D patterned step-like indium-tin-oxide (ITO), a 230 nm thick ITO is deposited on the SiO2 CBL by using electron beam evaporation. Then, the flat ITO is patterned via using photolithography and etched at 38 °C for 60 s by aqua regia etchant. The etch depth between upper step and lower step is determined to be 130 nm. Then, the circular patterns on upper step are formed with combination of photolithography and etching at 38 °C for 30 s by aqua regia etchant. The etch depth is about 65 nm. Finger-like Cr/Pt/Au (20 nm/50 nm/1.5mm) metallic multilayers are deposited on the 3D patterned step-like ITO and the exposed n-GaN layer as electrodes of LED, respectively. The thickness of LED wafers is thinned down to 150 μm by mechanical grinding. After that, the LED wafers are polished and divided into chips with size of 1 × 1 mm2 . We fabricated four types of high power LEDs. Figure 3.11a shows LED grown on PSS (LED I). Figure 3.11b shows LED grown on PSS with SiO2 CBL (LED II). Figure 3.11c shows LED grown on PSS with SiO2 CBL and 3D patterned step-like ITO (LED III). Figure 3.11d shows LED grown on PSS with SiO2 CBL, 3D patterned step-like ITO, and wavy sidewalls (LED IV).

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Fig. 3.11 Schematic illustration of a LED I, b LED II, c LED III, and d LED IV. The size of chips is 1 mm2

Figure 3.12a, b show the AFM scanning images of the LED III. The scanning area is 50 × 50 μm2 . In Fig. 3.12a, 3D step-like ITO consists of alternating patterned upper/lower steps and pattern circular arrays. The thickness of upper steps and lower steps is 230 and 100 nm, respectively. Figure 3.12b shows the top-view AFM image of 3D step-like ITO double layers in LED III. The diameter of circular patterns ranges from 3.8 to 4.5 μm. Figure 3.12c shows the schematic illustration of light propagation in LED III. It is observed that light generated from MQW active region will experience multiple opportunities to find the escape cone by using the 3D patterned step-like ITO, which can lead to enhanced top LEE. Figure 3.13a, b show the SEM images of etched mesa and wavy sidewalls. The wavy sidewalls can ensure that the photons have a larger probability to be emitted away from LEDs through textured sidewalls, thereby improving horizontal LEE. Figure 3.14a shows L-I characteristics of LED I, LED II, LED III, and LED IV. At 350 mA, the LOP of LED I, LED II, LED III, and LED IV is 430 mW, 486 mW, 554 mW, and 615 mW, respectively. The LOP of LED II is 13% higher than that of LED I, which is attributed to enhanced current spreading performance and

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Fig. 3.12 a AFM image and b top-view AFM image of 3D step-like ITO double layers in LED III; c schematic illustration of light propagation in LED III

Fig. 3.13 a SEM image of etched mesa and the deposited n-electrode. b SEM image of wavy sidewalls

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Fig. 3.14 a L-I and b I-V characteristics of the four types of LEDs

thus more uniform carrier distribution over the entire chip via finger-like p-electrode and underlying finger-like SiO2 CBL. The LOP of LED III is approximately 13.9% higher than that of LED II, revealing that 3D patterned step-like ITO is beneficial for enhancement of top LEE. The LOP of LED IV is 11% higher than that of LED III. This enhancement is originated from the wavy sidewalls, which increases scattering probability of photons at the interface between the GaN sidewalls and the surrounding air, thus improving LEE at the horizontal direction. Figure 3.14b shows the I-V characteristics of LED I, LED II, LED III, and LED IV. At 350 mA, the forward voltages are 3.077 V, 3.102 V, 3.131 V, and 3.137 V for LED I, LED II, LED III, and LED IV, respectively. As the finger-like SiO2 CBL is sandwiched between the ITO and the p-GaN layer, the total ohmic contact area between the ITO layer and the p-GaN layer will decline due to the insulating characteristic of SiO2 CBL, thereby leading to an increase in series resistance. Owing to increased series resistance along the vertical current path, the forward voltage of LED II is higher than that of LED I. The forward voltage of LED III is slightly higher than that of LED II, revealing that the 3D patterned step-like ITO can impede lateral current spreading. In addition, the forward voltages of LED III and LED IV are almost same, indicating that the wavy sidewalls do not degrade electrical property of LED.

3.1.4 Roughened Sidewalls Sidewall roughening is an effective way to enhance LEE of LED. Roughened sidewall structure is generally fabricated by using laser scribing, ICP etching combining with photolithography and wet chemical etching. However, laser scribing can only be used to fabricate trench microstructure. Meanwhile, the debris and thermal damages are inevitable during laser scribing process. ICP etching can generate various patterns on the sidewall by combining with photolithography. However, plasma damage during ICP etching and sidewall contamination of etching products on the active region

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can lead to a severe deterioration of forward and reverse voltage. These disadvantages will restrict further application of laser scribing and ICP etching combined with lithography in roughening sidewalls. By contrast, wet chemical etching is economical and barely damages sidewalls surface, which is suitable for mass-production. We fabricated microstructure on the sidewalls via tetramethylammonium hydroxide (TMAH) wet etching treatment [16]. Figure 3.15a, b show the schematic diagram and SEM image of mini-LEDs. The horizontal arranged mini-LEDs are labeled as mini-LED I and the vertical arranged mini-LEDs are labeled as mini-LED II, as shown in Fig. 3.15a. Figure 3.16 shows sidewall of mini-LEDs with different TMAH etching time. The trigonal prism structure on the sidewall of GaN mesa is the dominant feature for after TMAH etching treatment. The shape of prism structures remains unchanged for different TMAH etching durations while the their size exhibits a significant dependence on the TMAH etching time. Figure 3.16a, b show SEM images of the sidewall of mini-LEDs with 5 min TMAH etching treatment. Figure 3.16c, d show SEM images of the sidewall of mini-LEDs with 10 min TMAH etching treatment. It is found that the size of prism structures on the sidewall varies in large range from nanoscale to a few microns. Figure 3.16e, f shows SEM images of the sidewall of mini-LEDs with 20 min TMAH etching treatment. The morphology of prism structure remains unchanged but their size depends on etching duration. As etching duration increases from 5 to 10 min, the size of prism structure ranges from nanoscale to a few microns. When etching duration is 20 min, the morphology of prism structures is more homogeneous and their average size is about 550 nm. Figure 3.17a shows the I-V characteristics of mini-LED I with different TMAH etching duration. It is observed that curves are almost overlapped. The inset in Fig. 3.17a shows the reverse leakage current versus reverse voltage characteristics of mini-LED I with different TMAH etching duration. At −10 V, the reverse current of mini-LED I with etching duration of 0 min, 5 min, 10 min and 20 min is 5.04

Fig. 3.15 a Schematic of the mini-LED I and mini-LED II grown on the same wafer. b SEM image of the fabricated mini-LED

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Fig. 3.16 SEM images of the sidewall of mini-LEDs along [11–20] direction: a top-view and b cross-section view of sidewall with 5 min TMAH etching; c top-view and d cross-section view of sidewall with 10 min TMAH etching; e top-view and f cross-section view of sidewall with 20 min TMAH etching

Fig. 3.17 a I-V characteristics of mini-LED I with different TMAH etching duration. The inset is reverse current versus reverse voltage characteristics of mini-LED I with different TMAH etching duration; b L-I characteristics of mini-LED I with different TMAH etching duration. The inset is the LOP increments versus etching duration characteristics of mini-LED I and mini-LED II

μA, 4.91 μA, 5.26 μA, and 5.48 μA, respectively. The slight variation in I-V characteristics reveals that TMAH etching barely degrades the electrical performance of mini-LEDs. Figure 3.17b shows L-I characteristics of mini-LED I with different TMAH etching duration. At 130 mA, the LOP of mini-LED I with etching duration of 0 min, 5 min, 10 min and 20 min is 98.05 mW, 100.75 mW, 100.97 mW, and 107.31 mW, respectively. All the mini-LEDs are anticipated to have the same IQEs because

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they possess the identical epitaxial and device structure. Therefore, the increments in LOP of TMAH treated mini-LED I are suggested to be resulted from improved LEE, which is associated with the prism structure on the sidewall. The variation in LOPs of mini-LEDs with different TMAH etching durations indicates that sidewall morphology can exert a moderate influence on the LEE. The inset of Fig. 3.17b shows the LOP increments versus etching duration characteristics of mini-LED I and mini-LED II. In contrast to the untreated mini-LED, the mini-LED I and mini-LED II etched by TMAH exhibit improved LOPs. However, it is noteworthy that LOP of mini-LED I is always higher than that of mini-LED II. That can be attributed to orthogonal arrangement of mini-LED I and mini-LED II on the wafer since they have the same TMAH etching durations., the difference in LOP increments caused by TMAH etching is proposed to be originated from the anisotropic etching property of GaN during the TMAH etching process. Figure 3.18a shows L-I characteristics of mini-LEDs with and without TMAH etching treatment. The LOP is 98.05 mW, 107.31 mW, and 104.89 mW for miniLED without treatment, mini-LED I and mini-LED II with TMAH etching duration of 20 min, respectively. Figure 3.18b shows the light emission distribution of the LEDs at 60 mA. Compared with the emission image of mini-LED without TMAH etching treatment, the most remarkable difference in the emission images of miniLED I and mini-LED II with 20 min TMAH treatment appears in the nearby region of sidewall along [11–20] direction. Such results imply that the textured sidewall functions more effectively in coupling light out than the smooth sidewall. Hence, with the same TMAH etching duration, the better performance of mini-LED I can

Fig. 3.18 a L-I characteristic of mini-LEDs with and without TMAH etching treatment. b Top: light emission mapping image of mini-LED without TMAH treatment. Middle: light emission mapping image of mini-LED I with 20 min TMAH treatment. Bottom: light emission mapping image of mini-LED II with 20 min treatment

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be reasonably explained by its larger textured sidewall surface area owing to the orthogonal arrangement of mini-LED I and mini-LED II on the wafer. Figure 3.19 shows the simulated intensity profile. It can be demonstrated that the roughened sidewall can make more photons propagating through the GaN/air interface, which may arise from light scattering out of the prism structures through multiple reflections and refractions. In addition, it is noted that the light intensity distribution is more uniform, which means regular prism structure on the sidewall is beneficial for light extracting.

Fig. 3.19 Simulated intensity profile for a cross section with each device in the x-y plane: miniLED (left) without TMAH treatment; (middle) with 5 min TMAH treatment; (right) with 20 min TMAH treatment

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3.1.5 Air Voids Structure Light is randomly emitted from active region so that a part of light is inevitably emitted into air from sapphire substrate, which leads to reduced top LEE. An effective way to solve the problem is to embed the air voids at the PSS/GaN interface. Due to prominent refractive index contrast between GaN (nGaN = 2.45) and air (nair = 1), TIR will occurs at n-GaN/air voids interface. It can reduce the bottom light loss and improve top LEE of LED. We embedded air voids at the PSS/GaN interface via laser scribing and H3 PO4 based hot chemical etching [17]. Figure 3.20 shows the fabrication process of the high power LED with embedded air voids. The detailed procedures are as follows: (a) A 3-μm-thick SiO2 protection layer is deposited on LED wafer via using PECVD as the protection layer during wet etching process. (b) A 15-μm-deep and 5-μmwide trench is form on LED wafer by using laser scribing. (c) The scribed samples are etched in the mixture of sulfuric acid (H2 SO4 ) and phosphoric acid (H3 PO4 ) solution (H2 SO4 :H3 PO4 =1:3) at 260 °C for 6 min (LED II) , or 15 min (LED III), or 20 min (LED IV) to form the air voids structure at PSS/GaN interface, and then SiO2 protection layer is removed by using buffer oxide etch solution after the H3 PO4 -based hot chemical etching process. (d) The mesa structure is formed by using ICP etching based on Cl2 /BCl3 /Ar to etch LED samples until the n-GaN layer is exposed. The etching depth is 1.2 μm. (e) A 182-nm-thick SiO2 CBL is subsequently deposited on the p-GaN via PECVD. (f) Then a 100-nm-thick ITO TCL is covered onto the p-GaN and SiO2 CBL via electron beam evaporation in Fig. 3.20f.(g) Cr/Pt/Au (50 nm/50 nm/2100 nm) metal multi-layer films are deposited on exposed n-GaN and ITO to act as n-electrode and p-electrode in Fig. 3.20g. (h) a 69-nm-thick SiO2 passivation layer is deposited on the wafer surface via PECVD in Fig. 3.20h. Finally, LED wafers are thinned to about 150 μm by using backside lapping and polishing, and then broken into chips with size of 45 mil × 45 mil. For comparison, the conventional LED grown on PSS without air voids structure and CBL (LED I) and the LED with air voids structure and SiO2 CBL (LED V) are both prepared. Figure 3.21a–d show schematic diagram of LED II, LED III, LED IV and LED V, respectively. The LED II, LED III, and LED IV have different height of air void structure. The LED V and LED II have the same height of air void structure. The difference between LED II and LED V is that LED V has SiO2 CBL. Figure 3.22 shows the cross-sectional SEM images of LED II, LED III and LED IV. The inverted cone shaped air voids are formed at the PSS/GaN after laser scribing and H3 PO4 based hot chemical etching. TDD in the vicinity of GaN/PSS interface is higher than that away from the interface, especially the vicinity of GaN/PSS peak interface. That leads to the etching rate of GaN near the GaN/PSS interface is much higher than that away from the interface. Through such defect-selective etching, the air voids are formed between PSS and GaN epitaxial layer. As the wet etching time is increased from 6 min to 20 min. The average height of air voids structure is 3 μm (LED II), 4.5 μm (LED III) and 4.8 μm (LED IV), respectively. In addition,

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Fig. 3.20 Fabrication process of the high power LED with embedded air voids structure and SiO2 CBL

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Fig. 3.21 Schematic diagrams of the LED: a LED II; b LED III; c LED IV; d LED V

the cone-shaped PSS is also etched, causing the reduction in the size of cone-shaped PSS. The height of cone-shaped patterns on sapphire substrate is 1.5 μm, 0.7 μm and 0.5 μm for LED II, LED III, and LED IV, respectively. Figure 3.23a shows the simulation model of the air voids formed in GaN/PSS interface. The size of the simulated LED is 1 mm × 1 mm. The thickness of PSS and GaN epilayers is 90 μm and 6.7 μm, respectively. The number of rays emitted from the active region is 600000 and the threshold value of rays tracing is 10−5 . Figure 3.23b shows the simulated LEE of the LED with air voids. As the height of air voids increases from 3 to 4.8 μm, the simulated LEE of the LED decreases from 41.1% to 40.6%. Figure 3.24a shows the I-V characteristics of LED I, LED II, LED III, LED IV, and LED V. At 350 mA, the forward voltage of LED I, LED II, LED III, and LED IV is 3.27, 3.32, 3.35, 3.36, and 3.39 V, respectively. Compared to the LED I, a slightly increased forward voltages for the other four types of LEDs are observed. These results can be attributed to the incomplete covering of the SiO2 protection layer during the H3 PO4 -based hot chemical etching process, thereby leading to surface damage of p-GaN layer and higher serial resistances. Compared with LED II, LED III, and LED IV, a moderate increment in forward

110 Fig. 3.22 Cross-sectional SEM images of a LED II, b LED III, and c LED IV

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Fig. 3.23 a Simulated structure of the air voids. b Simulated LEE of LEDs by the TracePro software

voltage for the LED V is also observed, which is attributed to the reduction in the total area of the p-type metal contact between the ITO layer and the p-GaN layer owing to the presence of the insulating SiO2 layer. Figure 3.24b shows the L-I characteristics of LED I, LED II, LED III, LED IV, and LED V. At 350 mA, LOP of LED I, LED II, LED III and LED IV is 436, 475, 471, and 465 mW, respectively. The LOP of the LED II is 8.9% higher than that of the LED I, which is primarily attributed to strong light reflection and redirection by the air voids. This result can facilitate the top light extraction. The LOP of LED V is 9.1% higher than that of LED II. This enhancement indicates that employing SiO2 CBL can improve current spreading performance. The light output saturation current of LED I, LED II, LED III, LED IV, and LED V is 1021, 1010, 1021, 1000, and 1130 mA, respectively. The LED II and LED IV both exhibit serious efficiency droop. The inconsistency of LED epitaxial structure may be used to explain the occurrence of this phenomenon. Additionally, the light output saturation current of LED V is the highest among the five types of LED. This result is originated from the relieved current crowding around p-electrode by employing SiO2 CBL, thereby facilitating the alleviating in efficiency droop.

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Fig. 3.24 a I-V and b L-I characteristics of LED I, LED II, LED III, LED IV, and LED V. The size of LED chips is 45 mil × 45 mil

3.2 Current Blocking Layer 3.2.1 SiO2 Current Blocking Layer The ITO TCL can be used to improve current spreading in LED. Since sheet resistance of ITO is higher than that of GaN, the current still crowds around p-electrode in lateral LEDs. Localized current crowding will cause serious efficiency droop and degrade reliability of LEDs. Additionally, part of the photons generated from the MQW active region will be absorbed by the opaque metal electrodes, which aggravates the light absorption by metal electrode, thereby degrading LEE of LED. Therefore, we inserted an insulating SiO2 layer between ITO and p-GaN into GaN LED to relieve current crowding around p-electrode [8]. The fabrication and characterization of the LED are described in detail. Figure 3.25 shows fabrication process of LED with SiO2 CBL. The detailed fabrication processes are shown as follows: (1) A epitaxial wafer is etched to expose n-GaN layer via using ICP etching in Fig. 3.25a. In this step, parts of p-GaN and MQW active region are removed to obtain mesa structure. (2) A

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Fig. 3.25 Schematic diagram of fabrication process of LED with SiO2 CBL

70-nm-thick SiO2 CBL is deposited on the p-GaN via uisng PECVD in Fig. 3.25b. (3) A 230-nm-thick ITO is formed on the SiO2 CBL and p-GaN via electron beam evaporation in Fig. 3.25c. (4) a Cr/Ti/Au (50 nm/50 nm/1500 nm) multi-layer metal film is deposited on LED to form p-electrode and n-electrode in Fig. 3.25d. (5) A 90-nm-thick SiO2 passivation layer is deposited on the wafer surface in Fig. 3.25e. It is noted that SiO2 CBL and p-electrode are aligned on the same location in LED by photolithography and buffer oxide etching. Figure 3.26 shows schematic diagram of the four types of LEDs. Figure 3.26a–d show planar LED (P-LED), planar LED with SiO2 CBL (CBL-P-LED), textured LED (T-LED) and textured LED with SiO2 CBL (CBL-T-LED). The size of the tested LEDs is 10 mil × 23 mil. Figure 3.27a shows the L-I characteristics of PLED, CBL-P-LED, T-LED, and CBL-T-LED. At 20 mA, the LOP of P-LED, CBLP-LED, T-LED, and CBL-T-LED is 22.5 mW and 22.8 mW, 24.6 mW, and 27.5 mW, respectively. The LOP of T-LED is 9.3% higher than that of P-LED. This improvement is attributed that the textured surface of T-LED can increase escape probability of photons generated in the MQW active layer. The LOP of CBL-PLED is almost equal to that of P-LED, revealing that SiO2 CBL on p-GaN barely affect the LEE of LED. In addition, LOP of CBL-T-LED is 11.8% higher than that of T-LED, which is attributed to improved current spreading via employing SiO2 CBL, thereby increasing current injection into the MQW active layer of LED. Figure 3.27b shows I-V characteristic of P-LED, CBL-P-LED, T-LED, and CBL-TLED. At 20 mA, the forward voltage of P-LED, CBL-P-LED, T-LED and CBL-TLED is 3.11 V, 3.13 V, 3.16 V and 3.20 V, respectively. Compared to P-LED, T-LED has a slight increment of 0.05 V in forward voltage. In comparison to the T-LED, a slightly increased forward voltage of 0.04 V for is observed in CBL-T-LED, which can be attributed to the reduction in the total area of the p-type metal contact between the ITO layer and the p-GaN layer owing to the presence of the insulating SiO2 CBL. The thickness of the SiO2 CBL will also affect the photoelectric performance of the LED [18]. The cross-sectional SEM images of the deposited ITO and metallic

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GaN buffer layer

GaN buffer layer

Sapphire substrate

Sapphire substrate

GaN buffer layer

GaN buffer layer

Sapphire substrate

Sapphire substrate

Fig. 3.26 Schematic diagram of a P-LED, b CBL-P-LED, c T-LED and d CBL-T-LED

layer on the top of SiO2 CBL with vertical sidewall are shown in Fig. 3.28a, b. when the thickness of the SiO2 CBL is increased to 190 nm, it is difficult to directly deposit a continuous 180-nm-thick ITO layer on the top of SiO2 CBL with vertical sidewall, thereby resulting in significantly increased forward voltage of LEDs. To enable conformal ITO/metal coverage across the sidewalls of SiO2 CBL, a 190nm-thick SiO2 CBL with inclined sidewalls is formed by using a combination of a thermally reflowed photoresist technique and an ICP etching process. Figure 3.28c, d demonstrates the cross-sectional SEM images of the deposited ITO and metallic layers on the top of SiO2 CBL with inclined sidewalls. A 190-nm-thick SiO2 CBL

3.2 Current Blocking Layer

Fig. 3.27 a L-I and b I-V characteristics of P-LED, CBL-P-LED, T-LED and CBL-T-LED

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Fig. 3.28 Cross-sectional SEM images of multilayer thin films with different sidewall structures: a and b vertical sidewall. c and d inclined sidewall

with an oblique angle of about 135º is observed, which enables the ITO and metallic layers to overlay the SiO2 CBL smoothly. Figure 3.29a shows the influence of the thickness of the SiO2 CBL on L-I characteristic of LEDs. At 350 mA, the LOPs of high power LEDs are 399.1, 423.8, 435.9, and 449.8 mW when the thickness of SiO2 CBL is 0, 70, 90, 120, and 190 nm (with inclined sidewall). The LOP of LED with 70-nm-thick SiO2 CBL is 6.2% higher than that of LED without CBL, indicating that SiO2 CBL can relieve current

Fig. 3.29 a L-I and b EQE-I characteristics of high-power LEDs with different thicknesses of SiO2 CBL

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Fig. 3.30 Schematic illustration of the path A and path B with possible current paths in high power LED

crowding around p-electrode. Figure 3.29b shows the influence of the thickness of the SiO2 CBL on EQE-I characteristics of LEDs. At 350 mA, the EQEs of LEDs with the thickness of 0, 70, 90, 120, and 190 nm (with inclined sidewall) are 41.7%, 44.3%, 45.6%, and 47.1%, respectively. It is revealed that the EQEs of LEDs improve with increasing thickness of SiO2 CBL. Compared to EQE of the LED without SiO2 CBL, the EQE of LED with the thickness of 190 nm (with inclined sidewall) is improved by 12.7%. Figure 3.30 shows schematic illustration of the path A and path B with possible current paths in high power LED. Path A is the current transmission path from the p-electrode to the n-electrode when the thickness of the SiO2 CBL is 70 nm. Path B is the current transmission path from the p-electrode to the n-electrode when the SiO2 CBL thickness is 120 nm. Obviously, when the thickness of SiO2 CBL increases, the path length of the current transmitting from p-electrode to n-electrode also gradually increases. It is noted that the length of path B in the vertical direction is longer than the length of path A in the vertical direction, leading to increment of the series resistance in the current transmission.

3.2.2 Patterned Current Blocking Layer Before the deposition of the n-electrode, the epitaxial wafer need to be etched to expose n-GaN layer via the combination of ICP etching and photolithography. As a result, the sidewalls of the mesa structure are exposed to the air, and are very easily contaminated, thereby degrading lifetime and reliability of LEDs. Furthermore, the leakage current and the nonradiative recombination channels through the exposed surface will also deteriorate the optical and electrical properties of the LEDs.

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Therefore, passivation of the exposed area with appropriate dielectrics is usually employed to diminish the negative impact of surface recombination on the properties of LEDs [19–22]. In general, conventional two-level SiO2 layers including CBL and passivation layer are used to relieve current crowding and suppress impact of surface recombination. However, the fabrication of the two-level SiO2 layers needs two lithography processes, which complicates fabrication process and increases fabrication cost. Therefore, we proposed a patterned single SiO2 layer deposited upon ITO layer to act as both CBL and passivation layer. Compared with the conventional two-level SiO2 layers, only a patterned single SiO2 layer is required in our design, which can simplify the fabrication process and reduce the fabrication cost. Moreover, the patterned ITO underneath p-electrode is formed to alleviate light absorption by p-electrode, thus further improving LEE of LEDs [34]. The crystal quality of the grown blue LED wafer is evaluated by PL and XRD measurement. Figure 3.31a, b show the PL mapping of blue LED wafer. It can be observed that the peak wavelength of blue LED is about 450 nm and the full width at half maximum (FWHM) is about 16 nm. Figure 3.31c, d show the XRD measurement of blue LED wafer. It is found that the FWHMs of the symmetric

Fig. 3.31 PL mapping of the grown blue LED wafer: a peak wavelength and b FWHM. X-ray rocking curves of the grown blue LED wafer: c symmetric (002) and d asymmetric (102)

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Fig. 3.32 Top-view SEM images of a LED I, b LED II, and c LED III. Schematic diagram of d LED I, e LED II, and f LED III

(002) and asymmetric (102) of LED wafers are measured to be 123.1 arcsec and 226.5 arcsec, respectively, indicating that the grown GaN epitaxial layers have good crystalline quality. We prepared conventional LED (LED I), LED with patterned SiO2 CBL (LED II) and LED with patterned SiO2 CBL as well as patterned ITO (LED III). Figure 3.32a– c show the top-view SEM image of LED I, LED II and LED III, respectively. The inset in Fig. 3.32b shows the magnified SEM image of the patterned SiO2 CBL underneath the p-electrode of LED II. The inset in Fig. 3.32c shows the magnified SEM image of patterned SiO2 CBL and patterned ITO underneath the p-electrode of LED III. Figure 3.32d–f shows the schematic diagram of LED I, LED II and LED III, respectively. These three types of LED have the same electrode morphology but their device structures are different chip. In LED I, there exists the current crowding around p-electrode because sheet resistance of ITO is higher than that of GaN. In LED II and LED III, via-holes are formed through the SiO2 layer to allow the interconnection between ITO and p-electrode or n-GaN and n-electrode. In addition, the patterned SiO2 CBL can suppress current crowding around p-electrode, thereby leading to a uniform current injection into MQW active layer. Furthermore, in LED III, the ITO with periodic circular patterns is formed underneath p-electrode to enhance the probability of light scattering and to change the direction of the light emanated from the MQW active region, thereby leading to a reduced light absorption by the opaque p-electrode, which facilitates further improving LEE. Figure 3.33a, b show the crosssectional TEM images of n-electrode and p-electrode in LED III. In Fig. 3.33a, the patterned SiO2 CBL is underneath the Cr/Al/Ti/Pt/Ti/Pt/Au multi-layer. To improve the LEE of LED, Al is used as a mirror electrode. Due to the poor mechanical adhesion of Al with n-GaN, a thin Cr metal layer (0.5 nm) is sandwiched between Al and n-GaN to improve adhesion strength, which can improve the LEE of LED. In Fig. 3.33b, the patterned SiO2 CBL and patterned ITO underneath p-electrode are clearly observed. Figure 3.33c, d show the EDX line-scan curves of the n- and p-electrodes along A-A and B-B directions marked in Fig. 3.33a, b.

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Fig. 3.33 a Cross-sectional TEM image of n-electrode in LED III. b Cross-sectional TEM image of p-electrode in LED III. EDX line-scan curves of the n- and p-electrodes along c A-A and d B-B directions

Figure 3.34a–c show the simulated current density distribution of LED I, LED II and LED III at 150 mA. Figure 3.34d–f show the cross-sectional current density distribution of LED I, LED II and LED III along the C-C, D-D and E-E directions which are marked in Fig. 3.34a–c. RMS of current density in the active region of LED I, LED II, and LED III is 73.24, 51.60, and 54.88 A/cm2 at 150 mA, respectively. In LED I, the current will crowd around p-electrode because sheet resistance of ITO is higher than GaN. Compared to LED I, LED II exhibits superior current spreading due to the presence of patterned SiO2 CBL. Patterned SiO2 CBL can suppress the current crowding around p-electrode, thereby leading to uniform current distribution in the MQW active region. Moreover, the current density of LED II and LED III around p-pad is obviously lower than that of LED I. This result is originated from the use of insulating SiO2 CBL, which can block current flowing along vertical direction underneath p-pad. Compared to LED II, the RMS of the current density in LED III is slightly increased owing to a suppression in lateral spreading of current along the p-electrode caused by the patterned ITO. Figure 3.35a–c show light emission intensity distribution images of LED I, LED II and LED III at 100, 150 and 200 mA. LED II has a stronger light emission intensity in

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Fig. 3.34 SimuLED simulation of the current density distribution of the a LED I, b LED II, and c LED III at 150 mA. SimuLED simulation of the cross-sectional current density distribution along directions of d C-C, e D-D, and f E-E

Fig. 3.35 Light emission intensity distribution of LED I, LED II, and LED III at a 100, b 150, and c 200 mA

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Fig. 3.36 a I-V and b L-I and EQE-I characteristics curves of LED I, LED II, and LED III

comparison to LED I owing to the superior current spreading over the entire chip by using patterned SiO2 CBL. Current spreading of LED II is slightly more superior than that of LED III. Nevertheless, the LED III exhibits stronger light emission intensity across the chip area than LED II. This can be attribute to improved LEE owing to the patterned ITO in LED III. Figure 3.36a shows the I-V characteristic of LED I, LED II and LED III. At 150 mA, the forward voltage of LED I, LED II and LED III is 3.22 V, 3.26 V and 3.27 V, respectively. The forward voltages of LED II and LED III are both higher than that of LED I, indicating that patterned SiO2 CBL can enhance electrical performance of LED by increasing the length of current spreading path. Compared with LED II, the slightly higher forward voltage in LED III was attributed to the periodic circular patterns on ITO, which increased the sheet resistance of the ITO transparent conductive layer. Figure 3.36b shows the LOP-I and EQE-I characteristic of LED I, LED II and LED III. At 200 mA, the LOP of LED I, LED II and LED III is 178.6 mW, 196.8 mW and 203.1 mW, respectively. The EQE of LED I, LED II and LED III is 32.6%, 35.9% and 37.1%, respectively. The EQE of LED II is 9.1% higher than that of LED I, indicating that patterned SiO2 CBL can enhance EQE of LED via improving current spreading. Further, the patterned ITO can contribute to reducing the light absorption by the opaque p-electrode, thus resulting in a 13.8% improvement in EQE of LED III in comparison to LED I.

3.2.3 Reflective Current Blocking Layer CBL is deposited underneath the p-electrode to relieve current crowding. But it can not avoid the light absorption by opaque metal electrode. To reduce the light absorption by metal electrode, a highly reflective metal layer is inserted at the ITO/pelectrode interface to form reflective SiO2 /ITO/reflector sandwiched CBL [23–27]. In our design, we choose Al as reflective metal layer to avoid the light absorption [28]. Figure 3.37a–c show the schematic diagram of the fabricated LED without SiO2 CBL, LED with SiO2 CBL, and LED with SiO2 /ITO/Al reflective CBL, respectively. The

3.2 Current Blocking Layer Fig. 3.37 Schematic diagrams of LED a without SiO2 CBL, b with SiO2 CBL, and c with SiO2 /ITO/Al CBL

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Fig. 3.38 TEM image of the LED: a epitaxial layer, b the multi-layer thin film

LED without SiO2 CBL, LED with SiO2 CBL, and LED with SiO2 /ITO/Al reflective CBL are labeled as LED I, LED II and LED III. Figure 3.38a shows the TEM image of the LED epilayers. As shown in the figure, the V-pits can be clearly observed on the p-InGaN/p-GaN surface. Their formation is strongly related to the surface annihilation of threading dislocations. The employing of p-InGaN capping layer grown on p-GaN can improve injected hole concentration into MQW active region, thereby resulting in the formation of low-resistance ITO ohmic contacts on p-InGaN/p-GaN layers. Figure 3.39b shows the TEM image of the SiO2 /ITO/Al/Cr/Pt/Au multi-layer deposited on textured p-GaN. The thickness of SiO2 CBL, ITO, and Al reflector is 196 nm, 133 nm, and 185 nm, respectively. The textured p-GaN surface can enhance the adhesion of the deposited films, which can avoid the Al reflective metal layer peeling. The insulating SiO2 CBL beneath the ITO can be used to deflect the current away from the p-electrode pad, which facilitates achieving of better current spreading. The Al reflective metal layer underneath the p-electrode pad can be used to prevent the light absorption by the opaque metal p-electrode. Figure 3.39 shows the reflectivity of the ITO/Cr/Pt/Au (133 nm/40 nm/65 nm/1500 nm), the SiO2 /ITO/Cr/Pt/Au (196 nm/133 nm/40 nm/65 nm/1500 nm), and the SiO2 /ITO/Al/Cr/Pt/Au (196 nm/133 nm/40 nm/65 nm/1500 nm) multi-layer films. At the incident wavelength of 452.1 nm, the reflectivity of ITO/Cr/Pt/Au, SiO2 /ITO/Cr/Pt/Au, and SiO2 /ITO/Al/Cr/Pt/Au multi-layer films is 9.89%, 7.73%, and 62.19%, respectively. It is revealed that the reflectivity of SiO2 /ITO/Al/Cr/Pt/Au is much higher than that of SiO2 /ITO/Cr/Pt/Au. There is a significant increase in the reflectance when SiO2 CBL is replaced by SiO2 /ITO/Al reflective CBL. Accordingly, the SiO2 /ITO/Al reflective CBL beneath the Cr/Pt/Au p-electrode can effectively prevent the light absorption by the opaque ITO/Cr/Pt/Au electrodes with relatively low reflectivity.

3.2 Current Blocking Layer

Fig. 3.39 Measured reflectivity SiO2 /ITO/Al/Cr/Pt/Au films

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spectra

of

ITO/Cr/Pt/Au,

SiO2 /ITO/Cr/Pt/Au,

and

Figure 3.40a shows L-I characteristics of LED I, LED II and LED III. At 20 mA, the LOP of LED I, LED II and LED III is 25, 27.5, and 29.6 mW, respectively. It is observed that the LOP of LED with reflective CBL is 7.6% and 18.5% larger than those of the LEDs with SiO2 CBL and without SiO2 CBL, respectively. The enhancement in LOP is attributed to the improved current spreading performance via the SiO2 CBL and the wide-angle high reflectivity of Al omnidirectional metal reflector to prevent the light absorption by the opaque p-electrode. Figure 3.40b shows the I-V characteristics of LED I, LED II and LED III. At 20 mA, the forward voltage of LED I, LED II, and LED III is 3.144, 3.197, and 3.202 V, respectively. Slight increments in forward voltages for the LED II and LED III are observed. These results can be attributed to the reduction in the total area of the p-type metal contact between the ITO layer and the p-InGaN/p-GaN layer.

3.3 Back Reflector Light is randomly emitted from active region, and bounces non-directionally in the whole LED. However, a substantial amount (about 50%) of light travels downward, which is not beneficial for top light extraction. Accordingly, employing a

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Fig. 3.40 a LOP-I and b I-V characteristics of LED I, LED II and LED III

back reflector to reflect the light that travels downward is a potential way to improve the LEE of LEDs [29]. The highly reflective metallic mirror such as aluminum (Al) and silver (Ag) can be used as back reflectors of LED. Metallic mirror reflectors have high reflectivity over a broad range of frequencies incident from arbitrary angles. However, they suffers from poor mechanical adhesion with a sapphire substrate. Chromium (Cr) can be used as an adhesion layer to improve the poor

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mechanical adhesion between the metallic mirror and the sapphire substrate at the cost of degrading the reflectivity of the metallic mirror. As substitutions for metallic mirror reflectors, multi-layer dielectric mirrors such as a distributed Bragg reflector (DBR) or an omni-directional reflector (ODR), which are used primarily to reflect a narrow range of frequencies incident from a particular angular range, have also been employed to serve as the LED back reflector. We used commercial software to design single-DBR stack and double-DBR stack for the phosphor-converted LEDs (pc-LEDs) [30]. Figure 3.41a shows the simulated reflectivity spectrum of single-DBR stack as incident angles of light increasing from 0° to 60°. It is noted that the bandwidth of the reflectance band with a high reflectivity (>90%) region for the single DBR stack including quarter wavelength layers of SiO2 /TiO2 can reach up to 140 nm. However, the single-DBR stack exhibits a strong angular dependency. As the incident angle of light increases, the center of the reflectance band is shifted to shorter wavelength (blueshifted) and the bandwidth of the reflectance band is narrowed.This reflectance of the single DBR stack accords with theoretical dispersion curves and the previous investigation for quarter-wave

Fig. 3.41 a Reflectivity spectra of single DBR stack for different angles of light incidence. b Reflectivity spectra of single DBR stack for different central wavelengths. c Reflectivity spectra of ten-pair single DBR stack and ten-pair double DBR stacks. d Reflectivity spectra of the ten-pair double DBR stacks for different angles of light incidence

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dielectric stacks. The reflectivity spectrum of the single DBR stack optimized for a different central wavelength, λ, by altering the thickness of TiO2 /SiO2 dielectric layers is shown in Fig. 3.41b. The reflective bandwidth of the single DBR stack is redshifted with the increasing thickness of the TiO2 /SiO2 dielectric layers. The redshift toward the long wavelength for the single DBR stack with increasing thickness of TiO2 /SiO2 dielectric layers can offset the blueshift toward the short wavelength when the light incident angle increases from the surface normal toward the grazing angle to the DBR stack. Thus, to maximize the redirection of all emitted photons upward, we combine two single DBR stacks, each optimized for a different central wavelength, λ, into double DBR stacks. The first DBR stack contained fivepair TiO2 /SiO2 (57.3 nm/98.1 nm) dielectric layers optimized for central wavelength at 560 nm. The second DBR stack included another five-pair TiO2 /SiO2 (45.8 nm/78.5 nm) dielectric layers optimized for central wavelength at 460 nm. Thus, taking an angle-induced blueshift into account, the double DBR stacks structure should be efficient in both the blue wavelength region and the yellow wavelength region. Figure 3.41c shows the reflectivity spectra of the double-DBR stacks with ten-pair TiO2 /SiO2 dielectric layers. The reflectivity of the ten-pair TiO2 /SiO2 (45.8 nm/78.5 nm) single DBR stack in the blue wavelength region is nearly 100%, whereas it exhibits low reflectivity in the yellow wavelength region. The reflectivity of the ten-pair TiO2 /SiO2 (57.3 nm/98.1 nm) single-DBR stacks in the yellow wavelength region is nearly 100%, whereas the reflectivity in the blue light wavelength is low. Compared to the ten-pair single DBR stack, it is revealed that the ten-pair double DBR stacks, which include a first five-pair TiO2 /SiO2 (57.3 nm/98.1 nm) DBR stack and a second five-pair TiO2 /SiO2 (45.8 nm/78.5 nm) DBR stack, exhibit high reflectivity in both the blue wavelength region and the yellow wavelength region. Figure 3.41d shows the reflectivity spectra of the double-DBR stacks with ten-pair and five-pair TiO2 /SiO2 dielectric layers. Due to comprehensive effect of the dielectric layer thickness and the angular dependence of reflectivity, the doubleDBR stacks with ten-pair TiO2 (57.3 nm)/SiO2 (98.1 nm) dielectric layers can obtain high reflectivity at both the blue and yellow wavelength region over a broad range of incident angles of light. In contrast to the single DBR stack, it was shown that the bandwidth of the reflectance band with high reflectivity (>90%) using the double DBR stacks may be extended. It is demonstrated from Southwell’s work that the omnidirectional bandwidth of quarter-wave dielectric stacks may be extended by the addition of contiguous quarter-wave dielectric stacks. By varying the dielectric layer thicknesses to compensate for the angular dependence of reflection, the reflectivity spectrum of the double DBR stacks, which may obtain high reflectivity in both the blue wavelength region and the yellow wavelength region over a broad range of incident angles, showed less dependence on incident angles of light compared to the single DBR stack. Figure 3.42 shows the SEM image of the ten-pair double DBR stacks formed via using the e-beam evaporation. The fabricated ten-pair double DBR stacks include a first five-pair TiO2 (57.3 nm)/SiO2 (98.1 nm) DBR stack and a second five-pair TiO2 (45.8 nm)/SiO2 (78.5 nm) DBR stack.

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Fig. 3.42 SEM image of the ten-pair double DBR stacks on the bottom of the sapphire substrate

The reflectivity of the DBR can be enhanced by increasing the number of pairs of the DBR. However, it will increase the fabrication cost. One effective way to economically improve reflectivity is to combine the DBR and metallic mirror. The hybrid reflector combining DBR and Al or Ag has been proposed to provide a superior reflective characteristic and decrease the dependence of the reflectivity spectrum on incident angle of light. Owing to higher reflectivity of Ag in comparison to Al, the LEDs with a hybrid reflector combining the DBR with Ag exhibit more superior LOP. However, Ag is easy to be contaminated by the sulfides, chlorides, and oxides in the atmosphere, which can deteriorate its reflectivity. Gold (Au) is rarely used to fabricate the hybrid reflector due to relatively low reflectivity in the visible light region. However, Au is more chemically stable compared with other metals such as Al and Ag. To overcome the disadvantage of the low reflectivity of Au, we combined five-pair double DBR stacks and a Au metallic mirror to improve the reflectivity of the hybrid reflector. Figure 3.43 shows the SEM image of the hybrid

Fig. 3.43 SEM image of the hybrid reflector

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reflector. The SiO2 TIR has a thickness of 535 nm. The TiO2 /SiO2 double DBR stacks on SiO2 TIR layer include a two-periods DBR stack (first DBR stack) and a three-periods DBR stack (second DBR stack). The thickness of TiO2 /SiO2 layer in each period for the first and second DBR stack is 76.9 nm/139.5 nm and 48.9 nm/90.9 nm, respectively. The Cr layer is used to improve adhesion stability at the DBR stacks/Au interface. The light that passes through the SiO2 TIR layer will be further reflected by the double DBR stacks and Au metallic mirror. When light is striking the sapphire/SiO2 interface, the critical angle of TIR is 56° at the interface. Consequently, the SiO2 TIR layer is used to reflect the light that is passing toward the hybrid reflector at angles greater than the critical angle (56°), while the light that passes through the SiO2 TIR layer with incident angles between 0° and 56° are reflected by the double DBR stacks and Au metallic mirror. The advantage of the hybrid reflector on the luminous flux of pc-LEDs packaging modules is shown in Fig. 3.44. The packaged LED chip with ten-pair TiO2 /SiO2 single DBR stack, ten-pair TiO2 /SiO2 double DBR stacks, hybrid reflector consisting of five-pair TiO2 /SiO2 double DBR stacks and Au metallic mirror is labeled as pcLED A, pc-LED B, and pc-LED C, respectively. At 350 mA, the luminous flux of pc-LED A, pc-LED B, and pc-LED C is 145.6 lm, 149.7 lm, and 150.8 lm, respectively. In comparison to pc-LED A, the enhancement of luminous flux in pcLED B is attributed to the introduction of double DBR stacks. Compared to pc-LED

Fig. 3.44 Measured change in luminous flux of pc-LED A, pc-LED B, and pc-LED C during the 168 h temperature/humidity accelerated testing

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Fig. 3.45 Schematic diagram of double-eight periods of TiO2 /SiO2 double-DBR stacks

B, the enhancement of luminous flux in pc-LED C is attributed to the introduction of hybrid reflector structure. The temperature/humidity accelerated lifetime testing of pc-LED A, pc-LED B, and pc-LED C are conducted under the condition of 85°C and 85% humidity at 700 mA. After 168 h temperature/humidity accelerated lifetime testing, the luminous flux of the pc-LED A, pc-LED B, and pc-LED C decreases by 2.26%, 1.2%, and 0.79%, respectively. A single-DBR stack has been widely reported in the previous studies. Generally, the bandwidth of single DBR is less than 170 nm and exhibits strong angular dependence. Narrow bandwidth of single-DBR stack is not beneficial for further application, such as solid-state lighting and backlight for cell phones. Therefore, we design a type of LED with a high-bandwidth hybrid reflector combining a double-DBR stacks structure with metal mirror reflector. Figure 3.45 shows schematic diagram of double-eight periods of TiO2 /SiO2 double-DBR stacks. To investigate influence of the periods of TiO2 /SiO2 on reflectance, periods of TiO2 /SiO2 dielectric layers in a single-DBR stack range from 2 to 16 [31]. As shown in Fig. 3.46a, the reflectance of single-DBR stack increases with increasing of periods of TiO2 /SiO2 dielectric layers. In addition, the maximal reflective bandwidth is 170 nm, which appears at 16 periods of TiO2 /SiO2 dielectric layers. The bandwidth of a single-DBR stack is only 170 nm, which is too narrow for expanded application. Double-DBR stacks with two central wavelengths have been illustrated to serve as back reflectors of LEDs to obtain larger bandwidth. The central wavelengths of double-DBR stacks are set as 500 nm and 600 nm. As a result, the thickness of the TiO2 /SiO2 dielectric layer will be calculated to be 51.61 nm/85.68 nm and 63.8 nm/103.31 nm in double-DBR stacks, respectively. Figure 3.46b shows reflectance spectra of double-DBR stacks with different periods of TiO2 /SiO2 . Double-DBR stacks consist of two single-DBR stacks, which means that double-eight periods of TiO2 /SiO2 double-DBR stacks are equivalent to sixteen periods of a TiO2 /SiO2 single-DBR stack. Similar to singleDBR stack, the reflectance of double-DBR stacks also increases with increasing of periods of TiO2 /SiO2 dielectric layers. However, double-DBR stacks show better reflectance and broader bandwidth in contrast to single-DBR stack. On one hand, double-DBR stacks show higher reflectance than a single-DBR stack with the identical periods of TiO2 /SiO2 layers. On the other hand, the maximal bandwidth of

132 Fig. 3.46 Relationship between reflectance and wavelength in a single DBR stack and b double-DBR stacks. c Comparison between single-DBR stack and double-DBR stacks

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3.3 Back Reflector

133

double-DBR stacks has reached up to 272 nm, which is 102 nm higher than singleDBR stack. Theoretically, the bandwidth of double-DBR stacks should be the sum of bandwidth of two single-DBR stacks, which should be about 340 nm. In fact, the distance of two central wavelengths is 100 nm. It will cause the overlap of bandwidth of two single-DBR stacks, thereby shortening the bandwidth of double-DBR stacks. However, the reflectance could decrease if the distance of two central wavelengths is over 100 nm in double-DBR stacks. As shown in Fig. 3.46c, compared to singleDBR stack, double-DBR stacks show higher reflectance and wider bandwidth. The maximal reflective bandwidth of double-DBR stacks is 272 nm, which appears at 8 periods of TiO2 /SiO2 dielectric layers. Metal mirror reflector such as Ag, Al, or Ni is deposited on the bottom of the DBR stacks to further improve the reflectance of the LED. Ag film shows the best result that its bandwidth and reflectance are both much higher than Al and Ni film. Al film shows better reflectance than Ni film when their thickness is the identical. Simulation results of Ag films deposited on the bottom of single-DBR stack and doubleDBR stacks are shown in Fig. 3.47. The silver film can improve the bandwidth and reflectance on both the single-DBR stack and the double-DBR stacks. It is noted that the reflectance sharply declines in a narrow and certain range of spectrum for both double-DBR stacks and single-DBR stack, which can be explained with optical Tamm states that occurr at the Ag/TiO2 interface. Optical Tamm states can cause high transmittance in the narrow spectrum in the DBR stacks.

Fig. 3.47 Reflectance spectra of single DBR and double DBR with Ag

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Fig. 3.48 Reflectance spectra of double-DBR stacks with silver Ag and Al film

In addition, reflectance of double-DBR stacks with silver film and aluminum film in Fig. 3.48. It is observed that the reflectance of double-DBR stacks with Ag film is higher than that with Al film. Unfortunately, Ag film is not widely applied due to poor adhesion. Although the reflectance of Al is lower than that of Ag, Al film has better adhesion to the double-DBR stacks. Figure 3.49 shows the relationship between thickness of Al and reflectance of single-DBR stack. The reflectance shows a positive effect when the thickness of aluminum increased from 10 nm to 100 nm. Moreover, it can be observed that the film thickness will barely influence on improving the reflectance when the Al film is thicker than 80 nm. The similar tendency can be found in the double-DBR stack. Periods of TiO2 /SiO2 dielectric layers and thickness of Al can both exert influence on the reflectance of the double-DBR stacks. Periods of TiO2 /SiO2 and thickness of aluminum can both affect the reflectance of DBR stacks. The periods of TiO2 /SiO2 and thickness of aluminum are set as 16 and 80 nm, respectively, to compare a single-DBR stack with double-DBR stacks. In Fig. 3.50, the double-DBR stacks with Al film exhibits better reflectance bandwidth than the single-DBR stack. In addition, the average reflectance of double-DBR stacks in the wavelength region of 380–780 nm is 95.09%, which is much higher than the that of single-DBR stack (91.38%). Figure 3.51 shows LOP densities versus current curves of LEDs with a single-DBR stack and double-DBR stacks. Obviously, LOP densities of LEDs increase as injection current increases. At 35 A/cm2 , LOP densities of LEDs with single-DBR stack and double-DBR stacks are 50.33 W/cm2 and 51.84

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Fig. 3.49 Relationship between thickness of aluminum and reflectance of single-DBR stack

W/cm2 . At 35 A/cm2 . The EQE is about 56.14% in double-DBR stacks. LOP density of LED with double-DBR stacks is 3% higher than that of LED with single-DBR stack. This improvement is attributed to high reflectance and broad bandwidth of double-DBR stacks. The simulated reflectance results and experiment reflectance results of double-DBR stacks are shown in Fig. 3.52. The experiment results show lower reflectance than simulation results. The bandwidth of double-DBR stacks in the experiment results is consistent with that of simulation results.

3.4 Low Optical Loss Electrode Structure For high-power LED with the size of 10mil × 23mil, the lateral current spreading distance is very short so that a simple design of the electrode shape can effectively satisfy demand of alleviating current crowding. However, for high-power LED with the size of 45mil × 45mil, the lateral current spreading distance is much longer, leading to more serious current crowding. Therefore, the reasonable design of the electrode geometry should be highlighted in the fabrication processes of high-power LED with the size of 45mil × 45mil. That is because reasonable design of the electrode geometry can make the injection current and temperature uniformly distributed in the active region of the LED, thereby improving the light emission

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Fig. 3.50 Comparison of single DBR and double DBR at optimized thickness of Al

performance of high-power LED. Figure 3.53 shows nine types of low optical loss electrode of high-power LED. In the fabrication process of n-electrode, the formation of n-type electrode needs to expose n-GaN layer. It will reduce the active region area of the LED, leading to decrease of LOP. In order to improve the utilization rate of the active region, we can replace conventional n-electrode structure with electrically interconnected discontinuous n-electrode structure. It can reduce the etching area to save active region area during n-electrode fabrication process, thereby reducing the LOP loss of high-power LED. Figure 3.54a, b show the schematic diagram of the LED with conventional n-electrode structure (LED I) and the LED with discontinuous low optical loss n-electrode structure (LED II) [32]. We inserted Al reflective layer into Cr/Pt/Au multilayer p-electrode to further decrease light absorption of metal electrodes. Figure 3.55 shows the optical microscopy of the LED with reflective p-electrode and discontinuous low optical loss n-electrode structure (LED III). Al/Cr/Pt/Au reflective p-electrode can decrease the light absorption by metal electrode. Stripe width of the reflective p-electrode should be decreased to 3 μm to further relieve undesirable light absorption by metal electrode. The discontinuous n-electrode consists of the periodic separated cutout region and interconnected metal lines. The n-GaN layer is selectively exposed in the

3.4 Low Optical Loss Electrode Structure

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Fig. 3.51 LOP versus current curves of LEDs with a single-DBR stack and double-DBR stacks

separated cutout region to reduce the loss of active region. Compared to the conventional n-electrode, the discontinuous n-electrode structure can save 2.57% active region area. Figure 3.56a shows the TEM image of reflective p-electrode composed of high reflective Al and Cr/Pt/Au multi-layer films. An insulating SiO2 CBL is used to deflect the current away from the p-electrode, which can enhance current spreading. The Cr/Pt/Au (40 nm/65 nm/2100 nm) and the Al/Cr/Pt/Au (185 nm/40 nm/65 nm/2100 nm) films are deposited on the double side polished sapphire wafer, respectively. Al reflector can effectively avoid the light absorption by the opaque electrode. Additionally, the stripe width of the p-electrode is also decreased to 3 μm so as to further reduce the light absorption. However, the cross-sectional area of p-electrode decreases with the reduction of the p-electrode width when the thickness of p-electrode is constant, which will lead to the increased resistance of metal electrodes and thus cause the degraded electrical performance of LED. In order to keep a constant cross sectional area and therefore a constant electrical resistance, the thickness of p-electrode must be proportionally increased while decreasing the p-electrode width. Consequently, the double Cr/Pt/Au layers (Cr/Pt/Au/Cr/Pt/Au) with thickness of 4.2 μm are used for p-electrode.The purpose of the double Cr/Pt/Au is to increase the thickness of the p-electrode, decreasing the resistance of the p-electrode. Figure 3.56b shows reflectivity of Cr/Pt/Au and Al/Cr/Pt/Au multi-layer films. At the wavelength of 455 nm,

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Fig. 3.52 Comparison of simulation results and experiment results in double-DBR stacks

the reflectivity of the Cr/Pt/Au and Al/Cr/Pt/Au multilayer films is 9.71 and 62.82%, revealing that the inserted Al can very effectively improve reflectivity of p-electrode. Figure 3.57a shows LOPs of LED I, LED II and LED III. At 350 mA, the LOPs of LED I, LED II and LED III are 415, 432, and 443 mW. LED II has the LOP which is 4.1% higher than that of LED I, indicating that Al reflector can decrease the light absorption. The LOP of the LED II with reflective p-electrode is 4.1% higher than that of the LED I. This enhancement is partially originated from the highly reflective Al omnidirectional metal reflector, which can relieve light absorption by opaque p-electrode. Meanwhile, narrowing the p-electrode to reduce electrode contact area between the p-electrode and p-GaN layer is also an efficient way to avoid light absorption by the opaque p-electrode. In addition, LED III has a 2.5% higher LOP than LED II, revealing that discontinuous n-electrode can improve LOP by saving active region area. Figure 3.57b shows the I-V characteristics curves of LED I, LED II and LED III. At 350 mA, the forward voltages of LED I, LED II and LED III are 3.27, 3.30, and 3.32 V. The forward voltage of LED II is 0.03 V higher than LED I. revealing that the insertion of an insulating SiO2 CBL cause high series resistance of p-electrode/p-GaN interface. In addition, LED III has 0.02 V higher forward voltage than LED II, revealing that discontinuous n-electrode can reduce the contact area of n-type metal. Additionally, under reverse-bias conditions, the reverse saturation current measured at −5 V is given. The measured reverse saturation current for LED I,

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Fig. 3.53 Nine types of patterned electrodes of high-power LED

Fig. 3.54 a High power LED with conventional n-electrode structure. b High power LED with discontinuous n-electrode structure

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Fig. 3.55 Optical microscopy of the LED with discontinuous n-electrode and reflective p-electrode structure

(a)

(b)

Fig. 3.56 a TEM image of the deposited reflective p-electrode. b Reflectivity spectrums of Cr/Pt/Au and Al/Cr/Pt/Au multi-layer films

LED II, and LED III is all 0.1 μA, which is strongly related with superior device reliability. It is also concluded that the reflective p-electrode and discontinuous n-electrode structure did not degrade the reliability of LED.

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Fig. 3.57 a LOP-I and b I-V characteristics curves of LED I, LED II and LED III

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3.5 Ni/Au Wire Grid Transparent Conductive Electrodes Generally, a TCL formed between the p-electrode and p-GaN is beneficial for improving the current spreading. ITO possesses more than 90% transmittance of light at the visible light wavelength region so that it is widely used as TCL in blue and green LED. However, in the ultraviolet light wavelength region, the light transmittance of ITO will decrease sharply with the decrease of light wavelength. Obviously, ITO TCL is no longer suitable for UV LEDs. In this section, we mainly introduce a kind of Ni/Au wire grid structure fabricated via using the laser direct writing technology. The Ni/Au wire grid structure can be used as transparent conductive electrodes (TCEs) in UV LEDs. Laser direct writing technology has exhibited great advantages in the microstructure fabrication owing to its high resolution and flexibility. It is a kind of maskless photolithography technology and exhibits more tremendous flexibility than conventional photolithography technology. Figure 3.58 shows the diagram of the laser direct writing equipment, which mainly includes an optical module and a servo control platform. The continuous laser emitted by the laser sources enters the optical module through the optical fiber. After the laser beam is focused by the lens group in the optical module, the diameter of focal laser spot is 300 nm. The movement of optical module is omnidirectional. The upward and downward movement is mainly for the purpose of focusing. If the tested sample is relatively thick, the optical module needs to be adjusted to the highest position to avoid collision with the sample. The leftward and rightward movement is mainly for controlling the height of the optical module. The optical module is controlled by the voice coil motor to perform the stepping

Fig. 3.58 Images of a laser direct writer and b the details of the direct writing module

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movement. The stepping distance is usually set to 200 nm. The actual stepping distance can be controlled by software according to the experimental requirements. The servo control platform is controlled by a servo motor to perform the scanning movement in the forward and backward directions. The maximum scanning speed is 400 mm/s. The stepping motion of the optical module and the scanning motion of the servo control platform are alternately executed during the process of laser direct writing. There is a vacuum suction cup in the laser direct writing area on the control platform for fixing the sample. We fabricated Ni/Au wire grids with different periods on UV LED by using laser direct writing [33]. The epitaxial structure of the UV LEDs includes a 2.75-μmthick undoped GaN layer, a 90-nm-thick Si-doped n-AlGaN layer, a 2.23-μm-thick heavily Si-doped n+ -GaN layer, a 30-nm-thick n-AlGaN layer, a 170-nm-thick lightly doped n-GaN layer, a 144-nm-thick InGaN/AlInGaN SLs, an approximately 30nm-thick InGaN/AlInGaN MQW active region including six pairs of 2.8-nm-thick InGaN QW layers and 12-nm-thick AlInGaN quantum barrier layer, a 19-nm-thick AlInGaN layer (last quantum barrier), a 28-nm-thick p-AlInGaN layer, a 26-nm-thick p-AlInGaN/InGaN SLs, a 50-nm-thick p-GaN layer, and a 10-nm-thick heavily Mgdoped p+ -GaN layer. ICP etching based on BCl3 /Cl2 gas chemistry is used to form GaN mesa. The Ni/Au wire gird with different periods is formed on p+ -GaN layer using laser direct writing. For comparison, a full area semi-transparent Ni/Au thin film and 230-nm-thick ITO TCEs are also deposited on p + -GaN layer. Cr/Pt/Au metallic layers are deposited as p- and n-electrodes. Finally, the UV LED wafers are diced into chips with dimension of 305 × 330 μm2 . The peak emission wavelength of UV LED is 395 nm. Figure 3.59 shows a schematic illustration of the laser direct writing process. For laser direct writing, spin coating is the first step. The thickness of the photoresist layer is determined by the rotated speed and coating duration. The values of rotated speed and coating duration are set as 4000 r/min and 1 min, respectively. Then hot baking is used to bake the photoresist. After that, the photoresist layer is exposed by using laser direct writing and the wire grid structure is obtained after developing. The width of the wire grid structure is about 600 nm. The laser focus has two moving directions, including scanning direction and step direction. At the

Fig. 3.59 Schematic illustration of laser direct writing

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Fig. 3.60 a Top-view SEM image of UV LED with Ni/Au wire grid. b Top-view SEM image of Ni/Au wire gird with period of 1.5 μm. c Top-view SEM image of Ni/Au wire gird with period of 2 μm. d Top-view SEM image of Ni/Au wire gird with period of 6 μm

scanning direction, the scanning speed is 400 mm/s. At the step direction, the distance of moving one step is 40 nm. We fabricated three types of Ni/Au wire grid structure with different periods by using laser direct writing. Figure 3.60a shows the SEM image of UV LED with Ni/Au wire grid. Figure 3.60b–d show the SEM images of Ni/Au wire grid with various periods. The periods of the Ni/Au wire grid structures with the identical width fixed at 600 nm in Fig. 3.60b–d are 1.5, 2, and 6 μm. The thickness of the Ni (3 nm)/Au (3 nm) wire grid is 6 nm, which is much thinner than commercial ITO used in mass-produced LEDs. Additionally, nanoscaled Ni/Au wire grid structure with abundant empty area arrays better facilitates light extracting in comparison to Ni/Au thin film. The percentage of empty area in Ni/Au wire grid with periods of 1.5, 2, and 6 μm is 60%, 70%, and 90%,respectively. It is noteworthy that 90% of the electrode area is empty in the Ni/Au wire gird structure with the period of 6 μm, which can remarkably decrease the light absorption. Figure 3.61 shows the transmittance spectra of the UV LEDs with ITO, Ni/Au film and Ni/Au wire grids with periods of 1.5, 2, and 6 μm. In the ultraviolet light wavelength region, transmittance of ITO is lower than that of Ni/Au film and Ni/Au wire grid, revealing that ITO has strong absorption on ultraviolet light. In addition,

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Fig. 3.61 Transmission spectra of UV LEDs with ITO, Ni/Au thin film, and Ni/Au wire grids with periods of 1.5, 2, and 6 μm

as the period of the Ni/Au wire grid increases, the transmittance of the Ni/Au wire grid increases. When the light wavelength is 395 nm, the transmittance of Ni/Au wire grid with periods of 6 μm, 2 μm, and 1.5 μm is 87.9%, 77.1%, 73.1%, respectively, whereas the transmittance of Ni/Au film and ITO is 64.4% and 73%, respectively. Figure 3.62 shows L-I characteristics of UV LEDs with ITO, Ni/Au thin film, and Ni/Au wire grid with different periods. At 20 mA, LOPs of UV LEDs with ITO, Ni/Au thin film, Ni/Au wire grids of 1.5-μm, 2-μm, and 6-μm periods are 30.7, 21.5, 16, 14.3, and 13.0 mW, respectively. The LOP of UV LED with Ni/Au wire grid increase slowly with increasing current, which is much lower than those of UV LED with ITO and Ni/Au thin film. These results are attributed that Ni/Au wire grid can suppress the current spreading, which exerts an adverse influence on the EQE of UV LEDs. Meanwhile, only 10% of the electrode area is occupied in the Ni/Au wire grid with the period of 6 μm, which also extremely degrades current spreading. The transmittance of the UV LEDs with Ni/Au wire grid are higher than that of the UV LED with Ni/Au thin film. However, the LOP of UV LED with Ni/Au film is higher than those of UV LEDs with Ni/Au wire grid. These results confirm that poor LOP is mainly originated from the inferior current spreading rather than light absorption by opaque metal electrode. In addition, when the injection current is below 70 mA, it is observed that the LOP of UV LED with the Ni/Au wire grid of 2-μm period is slightly higher than that of UV LED with the Ni/Au wire grid of 2-μm period, which can be ascribed that the unfilled area of the electrode plays a major role in light extracting. Nevertheless, as the injection current is increased to above 70 mA,

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Fig. 3.62 The LOP of UV LED with ITO, Ni/Au thin film, and Ni/Au wire grids with different periods

the current spreading plays a major role in light extracting, which leading to higher LOP of the UV LED with Ni/Au wire grid of 1.5-μm period than that of UV LED with Ni/Au wire grid of 2-μm period. Figure 3.63 shows the I-V characteristics of the UV LEDs with ITO, Ni/Au film and Ni/Au wire grids with periods of 1.5, 2, and 6 μm. At 20 mA, the forward voltage of the UV LED with 1.5 μm, 2 μm, and 6 μm period Ni/Au wire grid is 4.36 V, 4.53 V and 5.229 V, respectively, whereas the forward voltage of UV LED with ITO and Ni/Au thin film is 3.348 V and 3.338 V, respectively. As periods of Ni/Au wire grid increases, the sheet resistance of Ni/Au wire grid also increases. The forward voltage of UV LED is the highest when period of Ni/Au wire grid is 6 μm, which is attributed to high sheet resistance of 131 /sq. When injection current is more than 50 mA, the forward voltage of UV LED with Ni/Au wire grid with period of 6 μm will have the fastest increasing rate because the UV LED with Ni/Au wire grid of 6-μm period exhibits the highest current density among all the tested UV LEDs. Moreover, prominent thermal effect originated from the high sheet resistance of UV LED with Ni/Au wire of 6-μm period can further increases the sheet resistance of the device, thus leading to faster increase rate of the forward voltage than that of the forward voltage in the UV LED with Ni/Au wire grid of 1.5-μm and 2-μm periods. This phenomenon also is found in the UV LED with Ni/Au thin film.

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Fig. 3.63 I-V characteristics of different UV-LEDs

References 1. Kao CC, Su YK, Lin CL et al (2010) The aspect ratio effects on the performances of GaN-based light-emitting diodes with nanopatterned sapphire substrates. Appl Phys Lett 97(2):023111 2. Gao H, Yan F, Zhang Y et al (2008) Enhancement of the light output power of InGaN/GaN lightemitting diodes grown on pyramidal patterned sapphire substrates in the micro-and nanoscale. J Appl Phys 103(1):014314 3. Lee JH, Oh JT, Kim YC et al (2008) Stress reduction and enhanced extraction efficiency of GaN-based LED grown on cone-shape-patterned sapphire. IEEE Photonics Technol Lett 20(18):1563–1565 4. Chiu CH, Yen HH, Chao CL et al (2008) Nanoscale epitaxial lateral overgrowth of GaN-based light-emitting diodes on a SiO2 nanorod-array patterned sapphire template. Appl Phys Lett 93(8):081108 5. Liao C, Wu YCS (2009) InGaN-GaN light emitting diode performance improved by roughening indium tin oxide window layer via natural lithography. Electrochem Solid State Lett 13(1):J8 6. Fujii T, Gao Y, Sharma R et al (2004) Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening. Appl Phys Lett 84(6):855–857 7. Huang HW, Kao CC, Chu JT et al (2005) Improvement of InGaN-GaN light-emitting diode performance with a nano-roughened p-GaN surface. IEEE Photonics Technol Lett 17(5):983– 985 8. Zhou S, Liu S, Ding H (2013) Enhancement in light extraction of LEDs with SiO2 current blocking layer deposited on naturally textured p-GaN surface. Opt Laser Technol 47:127–130 9. Liu JZ, Charlton MDB, Lin CH et al (2014) Efficiency improvement of blue LEDs using a GaN burried air void photonic crystal with high air filling fraction. IEEE J Quantum Electron 50(5):314–320 10. Ohya M, Naniwae K, Kondo T et al (2015) Improvement of vertical light extraction from GaNbased LEDs on moth-eye patterned sapphire substrates. Phys Status Solidi (a) 212(5):935–940

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11. Wang P, Gan ZY, Liu S (2009) Improved light extraction of GaN-based light-emitting diodes with surface-patterned ITO. Opt Laser Technol 41:823–826 12. Zhang Y, Li J, Wei T et al (2012) Enhancement in the light output power of GaN-based lightemitting diodes with nanotextured indium tin oxide layer using self-assembled cesium chloride nanospheres. Jap J Appl Phys 51(2R): 020204 13. Zhou SJ, Cao B, Liu S, Ding H (2012) Improved light extraction efficiency of GaN-based LEDs with patterned sapphire substrate and patterned ITO. Opt Laser Technol 44(7):2302–2305 14. Zhao J, Zhou SJ et al (2019) Improvement in light output of ultraviolet light-emitting diodes with patterned double-layer ITO by laser direct writing. Nanomaterials 9(2):203 15. Lv JJ, Zheng CJ, Zhou SJ, Fang F, Yuan S (2016) Highly efficient and reliable high power InGaN/GaN LEDs with 3D patterned step-like ITO and wavy sidewalls. Phys Status Solidi A 213(5):1181–1186 16. Tang B, Miao JH, Zhou SJ et al (2019) Insights into the influence of sidewall morphology on the light extraction efficiency of mini-LEDs. IEEE Photonics J 11(4):1–7 17. Zhou SJ, Yuan S, Liu S, Ding H (2014) Improved light output power of LEDs with embedded air voids structure and SiO2 current blocking layer. Appl Surf Sci 305:252–258 18. Zhou S, Liu M, Hu H et al (2017) Effect of ring-shaped SiO2 current blocking layer thickness on the external quantum efficiency of high power light-emitting diodes. Opt Laser Technol 97:137–143 19. Choi WH, You GJ, Abraham M et al (2014) Sidewall passivation for InGaN/GaN nanopillar light-emitting diodes. J Appl Phys 116, 013103 20. Zhou S, Yuan S, Liu Y et al (2015) Highly efficient and reliable high power LEDs with patterned sapphire substrate and strip-shaped distributed current blocking layer. Appl Surf Sci 355:1013–1019 21. Zhou S, Yuan S, Liu S et al (2014) Improved light output power of LEDs with embedded air voids structure and SiO2 current blocking layer. Appl Surf Sci 305:252–258 22. Park JS, Sung YH, Na JY et al (2017) Use of a patterned current blocking layer to enhance the light output power of InGaN-based light-emitting diodes. Opt Express 25(15):17556–17561 23. Kao CC, Su YK, Lin CL et al (2011) Enhancement of light output power of GaN-based lightemitting diodes by a reflective current blocking layer. IEEE Photonic Tech Lett 33(14):986–988 24. Lee J, Seong T-Y, Amano H (2020) Oblique-angle deposited SiO2 /Al omnidirectional reflector for enhancing the performance of AlGaN-based ultraviolet light-emitting diode. ECS J Solid State Sci Technol 9 026005 25. Nakashima T, Takeda K, Shinzato H (2013) Combination of indium–tin oxide and SiO2 /AlN dielectric multilayer reflective electrodes for ultraviolet-light-emitting diodes. Jpn J Appl Phys 52 08JG07 26. Son JH, Kim BJ, Ryu CJ et al (2012) Enhancement of wall-plug efficiency in vertical InGaN/GaN LEDs by improved current spreading. Opt Express 20(102):A287–A292 27. Kao CC, Su YK, Lin CL (2011) Enhancement of light output power of GaN-based light-emitting diodes by a reflective current blocking layer. IEEE Photonics Technol Lett 23(14):986–988 28. Zhou SJ, Fang F, Cao B, Liu S, Ding H (2013) Enhancement in light output power of LEDs with reflective current blocking layer and backside hybrid reflector. Sci China Tech Sci 56(6):1544– 1549 29. Kats MA, Blanchard R, Genevet P et al (2013) Nanometre optical coatings based on strong interference effects in highly absorbing media. Nat Mater 12(1):20–24 30. Zhou SJ, Cao B, Yuan S, Liu S (2014) Enhanced luminous efficiency of phosphor-converted LEDs by using back reflector to increase reflectivity for yellow light. Appl Opt 53(33):8104– 8110 31. Ding XH, Gui CQ, Hu HP, Zhou SJ et al (2017) Reflectance bandwidth and efficiency improvement of light-emitting diodes with double-distributed Bragg reflector. Appl Opt 56(15):4375–4380 32. Zhou SJ, Wang SF, Liu S, Ding H (2013) High power GaN-based LEDs with low optical loss electrode structure. Opt Laser Technol 54:321–325

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Chapter 4

Flip-Chip III-Nitride LEDs

4.1 Via-Hole-Based Two-Level Metallization Electrodes The light absorption of electrode and poor thermal conductivity of sapphire substrate in top-emitting LED hinder further development of high power LEDs [1]. Moreover, the large refractive index difference between GaN (n = 2.45) and air (n = 1) in top-emitting LEDs will limit the LEE, thereby resulting in a low EQE. To solve these issues, flip-chip LEDs are proposed. Flip-chip LEDs are typically bonded to submount (e.g. silicon, ceramic, etc.) with high thermal conductivity, which can dissipate the generated device heat. Meanwhile, the light is extracted through sapphire substrate (n = 1.78) in flip-chip LEDs. However, although LEE and heat spreading performance have been improved, the device performance of flip-chip LEDs is still restricted by the current crowding [2–7]. In this section, we introduce the flip-chip LEDs with uniformly distributed via-hole-based two-level metallization electrodes (TLM-FCLED) and the flip-chip LED with double-layer electrode (DLE-FCLED), which exhibit improved heat spreading, current spreading, and LEE [14, 26]. Figure 4.1 shows the fabrication process of TLM-FCLED. The processing steps are shown as follows: (a) ICP etching was applied to expose the n-GaN layer and form three-dimensional n-via holes. (b) A Ni/Ag was deposited on the p-GaN layer. (c) The first SiO2 insulating layer was deposited on Ni/Ag layer, which was then etched to expose n- and p-via holes by buffer oxide etchant (BOE) wet eching. (d) The first Cr/Pt/Au metallization layer was evaporated on the via holes and the SiO2 insulating layer to form n-electrode, which was then selectively removed by a lift-off process to expose p-via holes. (e) The second SiO2 layer was deposited on the first metallization layer, which was then selectively etched by wet etching for fabrication of a p-contact hole and n-interconnection holes. (f) Cr/Pt/Au was evaporated on SiO2 insulating layer and interconnection holes to form n- and p-pads, respectively. Figure 4.2 shows the fabrication process of DLE-FCLED. The processing steps are shown as follows: (a) ICP etching was applied to expose the n-GaN layer and form the three-dimensional via holes. (b) An ITO layer was deposited on the p-GaN

© Science Press 2022 S. Zhou and S. Liu, III-Nitride LEDs, Advances in Optics and Optoelectronics, https://doi.org/10.1007/978-981-19-0436-3_4

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Fig. 4.1 Schematic fabrication process of TLM-FCLED

Fig. 4.2 Schematic fabrication process of DLE-FCLED

layer, which was then selectively etched to expose via holes. (c) An insulating DBR layer was deposited on the ITO, which was then selectively etched by CHF3 /Ar/O2 based ICP etching to expose the n- and p-contact hole. (d) Cr/Pt/Au metal layer was deposited on the DBR to form the first electrode layer, which was selectively

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removed to form an isolation trench by a lift-off process. (e) A SiO2 insulating layer was deposited on the first electrode layer, which was then etched by wet etching for fabrication of n- and p-interconnection holes. (f) Cr/Pt/Au was deposited on the SiO2 insulating layer to form the second electrode layer. Figure 4.3a shows the schematic diagram of TLM-FCLED. In this structure, viahole-based n-electrodes uniformly distributed on the entire chip surface via introduction of the first metallization layer. The n-pads are connected to the first metallization layer through the n-interconnection holes. The p-pads are connected to the Ni/Ag layer through the p-interconnection holes. Figure 4.3b shows the cross-sectional

Fig. 4.3 a Three-dimensional schematic and b cross-sectional SEM image of the TLM-FCLED

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SEM image of TLM-FCLED. It can be seen that the sidewall of the n-via holes has a certain inclination angle and the SiO2 insulating layer is deposited on the sidewall of via holes to prevent short circuits. In DLE-FCLED, the first p-electrode layer and the second p-electrode layer are connected through the p-interconnection holes, while the first n-electrode layer and the second n-electrode layer are connected through the n-interconnection holes. In Fig. 4.4a, the p-electrode holes are uniformly around the n-contact hole for better current spreading. The isolation trench separates the n-contact and p-contact holes, which can prevent the short circuits. Figure 4.4b, c show the cross-sectional SEM

Fig. 4.4 a Top-view SEM image of the DLE-FCLED. The cross-sectional SEM image of DLEFCLED along b A–A and c B–B direction

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Fig. 4.5 Simulated current density distribution of a CHP-LED and b TLM-FCLED at 350 mA

images along A–A and B–B directions, respectively. The current path marked by the red dashed line shows that the current flows from the second p-electrode layer to the ITO. Subsequently, the current flows from ITO to the second n-electrode layer. Figure 4.5 shows the simulated current density distribution of TLM-FCLED and conventional high power LED (CHP-LED) at 350 mA. The root-mean-square (RMS) values of current density at 350 mA in the active region for CHP-LED and TLMFCLED are 61.69 and 55.36 A/cm2 , respectively. The results indicate that the current spreading performance of TLM-FCLED is much better than that of CHP-LED. Figure 4.6 shows the simulated current density distribution of DLE-FCLED and top-emitting LED at 90 mA. Compared with the top-emitting LED, the RMS value of current density in the DLE-FCLED is smaller. The ITO/DBR layer enables the p-contact holes to uniformly distribute around the three-dimensional n-via holes. This effectively reduces the current spreading distance between the p-electrode and the n-electrode, thereby enhancing the current spreading performance of LEDs. The RMS values of current density at 90 mA in the active region for top-emitting LED and DLE-FCLED are 35.94 and 32.29 A/cm2 , respectively. DLE-FCLED possesses the better current spreading performance and more uniform current distribution in comparison with the top-emitting LED. Figure 4.7 shows the transient junction temperature rise and thermal structure function of CHP-LED and TLM-FCLED at 350 mA. In Fig. 4.7a, the junction temperature of TLM-FCLED and CHP-LED is 49.9 and 60.3 °C, respectively. In Fig. 4.7b, the thermal resistance of TLM-FCLED and CHP-LED is almost equal (~1.2 K/W). The thermal resistance of die attach in TLM-FCLED (~1.5 K/W) is lower than that of die attach in CHP-LED (~10.5 K/W), indicating that the heat dissipation of the TLM-FCLED is improved. Figure 4.8a shows the transient junction temperature rise and thermal structure function of top-emitting LED and DLE-FCLED at 90 mA. In Fig. 4.8a, it can be seen that the junction temperature of DLE-FCLED and top-emitting LED is 34.4 and 45.1 °C, respectively. In Fig. 4.8b, the thermal resistance of DLE-FCLED and

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Fig. 4.6 Simulated current density distribution of a DLE-FCLED b top-emitting LED at 90 mA

top-emitting LED is almost equal (~2.6 K/W). The thermal resistance of die attach in DLE-FCLED (~22.6 K/W) is lower than that of die attach in top-emitting LED (~63.5 K/W), indicating that the heat dissipation of the DLE-FCLED is improved. Figure 4.9 shows the I-V and L-I characteristics of CHP-LED and TLM-FCLED. In Fig. 4.9a, the forward voltage at 350 mA of CHP-LED and TLM-FCLED is 3.20 and 2.93 V, respectively. This is because the electrical conductivity of Ni/Ag in TLM-FCLED is higher than that of ITO in CHP-LED. In Fig. 4.9b, compared with the CHP-LED, the LOP and EQE of the TLM-FCLED are higher. In CHPLED, the saturation current density is 102 A/cm2 and the maximum LOP is 776.7 mW. Whereas, in TLM-FCLED, the saturation current density is 183 A/cm2 and the maximum LOP is 1264 mW. Figure 4.10 shows the I-V and L-I characteristics of top-emitting LED and DLEFCLED. In Fig. 4.10a, the forward voltage at 90 mA of DLE-FCLED and top-emitting

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Fig. 4.7 a Transient junction temperature rise and b cumulative structure functions of CHP-LED and TLM-FCLED at 350 mA

LED is 3.12 and 3.43 V, respectively. At the same injection current, the lower forward voltage of DLE-FCLED is attributed to the better current spreading. In Fig. 4.10b, the LOP and EQE of the TLM-FCLED are higher in comparison with the top-emitting LED. The improved optical and electrical performances of TLM-FCLED and DLEFCLED are mainly attributed to the higher LEE, better current spreading, and heat dissipation.

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Fig. 4.8 a Transient junction temperature rise and b Integral structure functions of top-emitting LED and DLE-FCLED at 90 mA

4.2 Dielectric DBR It has been reported that dielectric distributed Bragg reflector (DBR) could improve the LEE, particularly in blue light wavelength region [8–13]. The dielectric DBR has low optical loss and high reflectance [14, 15]. In this section, we compare the optoelectronic properties of FCLEDs with dielectric TiO2 /SiO2 DBR and without dielectric TiO2 /SiO2 DBR [21].

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Fig. 4.9 a I-V and b L-I curves of CHP-LED and TLM-FCLED

Fig. 4.10 a I-V and b L-I curves of top-emitting LED and DLE-FCLED

Figure 4.11 shows the schematic illustration of the fabrication processes for FCLED with DBR: (a) n-type via holes were obtained using BCl3 /Cl2 /Ar-based ICP etching. (b) SiO2 was deposited on the p-GaN layer, followed by optical photolithography as well as wet etching to form SiO2 current blocking layer (CBL). (c) ITO was deposited on the p-GaN, which was subsequently annealed in N2 atmosphere. (d) Cr/Al/Ti/Pt/Au was deposited on the ITO and n-GaN layer to form electrodes. (e) DBR consisting of TiO2 /SiO2 was deposited on the ITO. (f) Cr/Al/Ti/Pt/Ti/Pt/Au were evaporated on SiO2 layer and p/n-type via holes to form p- and n-pads, respectively. Figure 4.12 presents the schematic diagram of the FCLED with DBR. We conducted numerical analyzation of conventional single TiO2 /SiO2 DBR stack for 465 nm central wavelength. The relationship between reflectance spectra and incident angles of light is exhibited in Fig. 4.13a. The increased thickness of dielectric layers contributes to redshift of the reflective bandwidth of the single DBR stack. Figure 3b shows normal-incident reflectance spectra of the single DBR stack optimized for central wavelength of 465, 545, and 620 nm. With the increasing thickness of dielectric layers, the reflective bandwidth was redshifted toward the long wavelength, which could counteract the blueshift toward the short wavelength when the incident angle of light increased from the surface normal toward the grazing

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Fig. 4.11 Fabrication process of FCLED chip with DBR

Fig. 4.12 Schematic diagram of FCLED with DBR: (a) Top-view image. (b) Cross-section image

angle to the DBR stack. Furthermore, two single DBR stacks were combined into double DBR stacks to achieve a larger bandwidth of reflectance band and less dependence on incident angles of light. In Fig. 4.13c, as the incident light increases, there is slight blueshift of reflectance spectra of double DBR stacks. It implies that the double DBR stacks could effectively reduce the angular dependence. Results in Fig. 4.13d imply the measured results in good agreement with the numerical simulation. Figure 4.14 shows the emission intensity mapping of FCLED with and without DBR under various injection currents. The spatial distribution of emission intensity reflects the distribution of current density. As the injection current density increases, the emission intensity increases correspondingly. When the injection current exceeds 100 mA, we could observe the current crowding around p-electrode, indicating nonuniform light emission intensity in both FCLEDs. Obviously, FCLED with DBR exhibited a stronger light emission intensity due to the high-reflectance DBR.

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Fig. 4.13 a, c Relationship between incident angles of light and reflectance spectra of the single and double DBR stack. b, d Reflectance spectra of single and double DBR stack with different thickness in the case of normal incident light

Figure 4.15 presents the I-V curves for the FCLED with and without DBR. Forward voltages of both FCLEDs with and without DBR are similar, whereas LOP versus current and EQE versus current are much different. Both the LOP and EQE of the FCLED with DBR are higher than those of the FCLED without DBR at 150 mA, benefiting from the high reflectance of DBR in blue light wavelength region. Additionally, in comparison with the FCLED with DBR, the current corresponding to light output saturation is higher in the FCLED without DBR, which could be attributed to the intrinsic low thermal conductivity of DBR.

4.3 Comparison of Flip-Chip LEDs and Top-Emitting LEDs In this section, we compare the optoelectronic properties of FCLED and top-emitting LED (TELED) [15]. Figure 4.16 shows the fabrication process of FCLED with ITO/DBR. Figure 4.17 shows the schematic diagram and optical image of the FCLED with ITO/DBR.

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Fig. 4.14 Emission intensity mapping of a–d FCLED sample without DBR and e–h FCLED sample with DBR at various injection currents

Fig. 4.15 a I-V curves of samples with and without DBR. b LOP and EQE versus injection current of samples with DBR and without DBR

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Fig. 4.16 Fabrication process for the FCLED with ITO/DBR

Fig. 4.17 a Schematic diagram and b optical image of the FCLED with ITO/DBR

To obtain broad reflectance bandwidth, we adopt double DBR stacks consisting of two single Ta2 O5 /SiO2 DBR stacks with different thicknesses. The simulated results of ITO/single DBR and ITO/double DBR as well as the experimental results of ITO/double DBR are presented in Fig. 4.18a. It is found that ITO/double DBR shows broader reflectance bandwidth than ITO/single DBR. Besides, the simulated results match well with experimental results. Figure 4.18b shows the reflectance as a function of incident angle for ITO/single DBR and ITO/double DBR. The average reflectance of double DBR stacks is higher than that of single DBR stack. To alleviate the current crowding, we introduce strip-shaped electrode and SiO2 CBL into FCLED with ITO/DBR. Furthermore, we compare the impact on the current distribution in the LED devices via numerical simulation (Fig. 4.19) . At 150 mA, the RMS value of current density in FCLED with ITO/DBR and p-/n-contact fingers is lower than that of FCLED with ITO/DBR. After adopting SiO2 CBL, RMS value of current density further decreases, indicating that both strip-shaped electrode and

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Fig. 4.18 a Simulated reflectance spectra of ITO/single DBR and ITO/double DBR. b Reflectance spectra of ITO/single DBR and ITO/double DBR at various incident angles

SiO2 CBL contribute to alleviate the current crowding. Besides, RMS of current density FCLED is lower than that of TELED due to the superiority of FCLED. As the emission intensity distribution is associated with current density distribution, we measured the light emission in TELED and FCLED at 90, 150, and 250 mA. In Fig. 4.20, with the current density increasing, emission intensity increases. However, at high injection current, FCLED shows uniform emission intensity distribution and the average intensity is much higher in comparison with TELED. This confirms the better current distribution in FCLED, matching well with the results in Fig. 4.19. Figure 4.21a shows I-V curves and dynamic resistance for FCLED with ITO/DBR, FCLED with Ni/Ag and TELED. At the same current, FCLED with Ni/Ag has a lower forward voltage than that of FCLED with ITO/DBR due to the better electrical conductivity of Ni/Ag. Besides, dynamic resistance of FCLED with ITO/DBR is lower than that of TELED, arising from the more uniform current spreading in FCLED with ITO/DBR. In Fig. 4.21b, due to the higher reflectance of the ITO/DBR, FCLED with ITO/DBR shows larger LOP and EQE in comparison with FCLED with Ni/Ag. Benefiting from the better current distribution and higher reflectance of FCLED, LOP and EQE of FCLED are higher than those of TELED.

4.4 Ag/TiW, Ni/Ag and ITO/DBR Ohmic Contacts This section comprehensively compares Ag/TiW, Ni/Ag and ITO/DBR p-type Ohmic contacts and their influence on the electrical, optical as well as thermal properties in FCLEDs [8, 9]. Figures 4.22 and 4.23 show the fabrication process of FCLED with Ag/TiW and ITO/DBR, respectively. The specific contact resistance was measured by the circular transfer length method (CTLM), as shown in Fig. 4.24a. I-V curves of Ag/TiW and ITO/DBR Ohmic contact annealed at various temperatures are plotted in Fig. 4.24b,

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Fig. 4.19 Current density distribution in MQWs of a FCLED with ITO/DBR, b FCLED with ITO/DBR as well as strip-shaped electrode, c FCLED with ITO/DBR as well as strip-shaped electrode and SiO2 CBL, and d TELED

c, respectively. When annealed at 300–500 °C, the rectifying characteristics could be observed for Ag/TiW contact, while annealing at 600 °C, Ohmic contact is obtained. For ITO/DBR sample, Ohmic contact could be achieved when annealed at both 300 and 600 °C, especially at 300 °C. The obtained specific contact resistances are 9.42

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Fig. 4.20 a–c Emission intensity distribution in TELED at various injection currents. d–f Emission intensity distribution in FCLED with ITO/DBR at various injection currents

Fig. 4.21 a I-V curves and dynamic resistance for the FCLED with ITO/DBR, FCLED with Ni/Ag, and TELED. b LOP and EQE versus current for FCLED with ITO/DBR, FCLED with Ni/Ag, and TELED

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Fig. 4.22 Fabrication process for FCLED with Ag/TiW

Fig. 4.23 Fabrication process for FCLED with ITO/DBR

× 10−2 and 4.46 × 10−4  cm2 for Ag annealed at 600 °C and ITO annealed at 300 °C, respectively. The reflectance spectra of two contacts types before and after annealing are presented in Fig. 4.24d. The value of reflectance for Ag/TiW and ITO/DBR is 94.5% and 92.8%, respectively. The reflectance of the Ag/TiW shows a slight decrease at 453 nm after annealing. Both the current density distribution and the temperature distribution are associated with EL distribution. Figure 4.25 shows the simulation results of the current density distribution of FCLEDs with Ni/Ag and ITO/DBR contacts. FCLED with Ag/TiW shows a smaller RMS value (65.71 A/cm2 ) at 350 mA than that of FCLED with ITO/DBR (95.62 A/cm2 ). It implies that Ag/TiW enables uniformly current spreading. Figure 4.25 shows the current density distribution of FCLED with Ag/TiW and DBR/ITO. From Fig. 4.25b, we find that the RMS value in InGaN/GaN MQWs

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Fig. 4.24 a Schematic of CTLM. b I-V curves of Ag/TiW p-contact at various annealing temperatures. c I-V curves of ITO p-contact at various annealing temperatures. d Reflectance spectra of Ag/TiW and ITO/DBR

at 350 mA is 180 A/cm2 . However, that value dropped to 122 A/cm2 via employing strip-shaped in the sample with ITO/DBR. Adopting the SiO2 CBL underneath the ITO/DBR suppresses the current crowding phenomenon around p-contact. From Fig. 4.26, as the injection current exceeds 150 mA, emission mainly concentrates near the p-contact in the sample with ITO/DBR, matching well with the numerical simulation (Fig. 4.25). The sheet resistance of Ag (0.16 /sq) is much lower than that of ITO (38 /sq). In addition, the sample with Ag/TiW achieves more uniform emission within the entire InGaN/GaN MQWs than sample with ITO/DBR. Figure 4.27a shows the I-V curves of samples with Ag/TiW and ITO/DBR. IV characteristics are associated with the specific contact resistance and spreading resistance. Benefiting from the lower specific contact resistance between Ag and p-GaN (Fig. 4.24), the forward voltages for two samples are similar at low currents. However, as the injection current increases, the spreading resistance of p-type contact dominates in the I-V characteristics. As a result, for injection current above 100 mA, forward voltage of sample with Ag/TiW is markedly lower than that of sample with ITO/DBR. Figure 4.27b shows the LOP and EQE of FCLEDs versus current. At

4.4 Ag/TiW, Ni/Ag and ITO/DBR Ohmic Contacts Fig. 4.25 Current density distribution in the InGaN/GaN MQWs of FCLEDs at 350 mA. a Sample with Ag/TiW. b–d Samples with ITO/DBR

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Fig. 4.26 Measured light emission intensity distributions in the FCLED with a–d Ag/TiW and e–h ITO/DBR at 150, 250, 350, and 500 mA, respectively

400 mA, the LOP and EQE of sample with Ag/TiW are 475 mW and 42.7%, respectively, while those for sample with ITO/DBR are 443 mW and 40.4%, respectively. These results indicate that Ag/TiW contributes to the enhanced current spreading and the higher LEE. Next, we compare the optoelectronic characteristics of FCLED with Ni/Ag and ITO/DBR. Figures 4.28 and 4.29 show the fabrication process of FCLED with Ni/Ag and ITO/DBR, respectively. Five pairs of TiO2 /SiO2 DBR combined with ITO were used as highly reflective ohmic contact. Figure 4.30a shows the measured reflectance spectra of the

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Fig. 4.27 a I-V characteristics for FCLED samples with Ag/TiW and ITO/DBR. b LOP and EQE versus current characteristics for FCLED samples with Ag/TiW and ITO/DBR

Fig. 4.28 Fabrication process for FCLED with ITO/DBR

as-deposited Ni/Ag with various thicknesses of Ni and the ITO/DBR at normal incidence. The reflectance of Ni/Ag decreases remarkably with the increasing thickness of Ni layer. The reflectance of Ni/Ag is 85.6% at 456 nm. The reflectance of ITO/DBR is 91.2% at same light wavelength. Figure 4.30b shows the cross-sectional SEM image of Ni/Ag and ITO/DBR contacts. The sheet resistance of ITO is about 38 /sq for the two types of FCLEDs, while that for Ni/Ag metal layer is about 0.08 /sq. Thus, with the negligible sheet resistance of Ni/Ag, sample with reflective Ni/Ag contact has a larger current spreading length. In Fig. 4.31, compared to FCLED with ITO/DBR, FCLED with Ni/Ag shows a slightly smaller RMS of current density, indicating the uniformity of current spreading.

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Fig. 4.29 Fabrication process for FCLED with Ni/Ag

Fig. 4.30 a Reflectance spectra of the as-deposited Ni/Ag (200 nm) films with various Ni thicknesses and as-deposited ITO (60 nm)/DBR (621 nm). b Cross-sectional SEM image of Ni/Ag and ITO/DBR

Figure 4.32a shows I-V characteristics of sample with pure ITO, ITO/DBR and Ni/Ag. The forward voltages of samples with pure ITO, ITO/DBR and Ni/Ag are 3.09, 3.19 and 3.01 V at 31 A/cm2 (corresponding to 90 mA), respectively. Because of the increased area coverage between ITO and metallization layer, sample with pure ITO shows lower forward voltage than that of sample with ITO/DBR. In addition, owing to the high electrical conductivity of Ni/Ag, Ni/Ag sample has lower forward voltage than that of pure ITO sample. Figure 4.32b shows LOP and EQE of FCLEDs versus current density. The LOP of FCLED with ITO/DBR and FCLED with Ni/Ag is 114.6 and 107.8 mW at 31 A/cm2 (corresponding to 90 mA), respectively. An increase of light extraction induced by the high reflectance DBR leads to higher LOP of FCLED with ITO/DBR, which is higher than that of FCLED with Ni/Ag.

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Fig. 4.31 Simulated current density distribution in the InGaN/GaN MQWs of a sample with ITO/DBR and b sample with Ni/Ag at 90 mA

In the same case, EQEs of samples with reflective ITO/DBR and Ni/Ag contacts are estimated to be 46.8% and 44.0%, respectively. Notably, the current corresponding to light output saturation increases from 196.3 A/cm2 for ITO/DBR sample to 292.8 A/cm2 for Ni/Ag sample. The improvement might result from larger coverage area between Ni/Ag and metallization layer and better heat dissipating performance. We further analyze the temperature distribution of FCLEDs with ITO/DBR and Ni/Ag by using SimuLED software package. In Fig. 4.33a, b, the peak temperature in MQWs for FCLEDs with ITO/DBR and Ni/Ag is 393.3 and 361.1 K at 200 mA, respectively. In Fig. 4.33c, the junction temperature of FCLED with Ni/Ag is lower than that of FCLED with ITO/DBR. The efficiency droop of FCLED with Ni/Ag is

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Fig. 4.32 a I-V characteristics of FCLEDs with pure ITO, ITO/DBR and Ni/Ag; b LOP and EQE versus current for FCLEDs with ITO/DBR and Ni/Ag. The chip size is 381 × 762 μm2

Fig. 4.33 Temperature distribution in InGaN/GaN MQWs of a ITO/DBR FCLED and b Ni/Ag FCLED at 200 mA; c Transient junction temperature rise of FCLED packages at 200 and 400 mA; d Cumulative structure functions of FCLED packages at 200 and 400 mA

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significantly improved at a high injection current, resulting from larger area coverage between Ni/Ag and metallization layer and better heat dissipating performance.

4.5 High-Power Flip-Chip LEDs Flip-chip technology has attracted much attention in improving the heat dissipation and enhancing LEE [16–19]. In this part, we adopt an Ag-based metallic reflector and an ITO/DBR in FCLEDs and investigate the impact on the optoelectronic properties of high-power FCLEDs [16]. Figure 4.34a shows the reflectance spectra of the different as-deposited films at normal incidence before and after annealing. As can be seen, the reflectance of Ag film decreases after annealing due to the agglomeration of Ag. The reflectance of Ag/TiW film shows negligible decrease after annealing, arising from the crack formation on the film during the annealing process. As for Ag/TiW/Pt/TiW/Pt/TiW/Pt, the reflectance before and after annealing is almost the same, indicating that such scheme maintains stability through annealing at 600 °C. Figure 4.48b shows the optical image of CTLM, which was used to measure the contact characteristics. Figure 4.34c shows the I-V curves for Ag contact to the p-GaN annealed at 400, 500 and 600 °C as well as without annealing process. Results show that contact characteristics are improved after annealing and when annealed at 600 °C, ohmic contact is obtained. However, when annealed at 400 and 500 °C, Ag contact to p-GaN shows nonlinear I-V characteristics. According to Fig. 4.34d, the obtained specific contact resistance of Ag contact to p-GaN is 7.33 × 10−2  cm2 when annealed at 600 °C. As reported [20], diffusion phenomenon occurs for metal atoms, which is affected by fabrication conditions. This will further affect the device performance and related properties. To investigate this phenomenon in our designed LED structure, X-ray photoelectron spectroscopy (XPS) measurement was performed for Ag contact to p-GaN before and after annealing at 600 °C. In Fig. 4.34e, f, we find that after annealing, Ag, Ga, and N atoms diffuse as the thicknesses of pure Ag layer and pure p-GaN layer decrease. Undoubtedly, interdiffusion phenomenon occurs for both Ag and p-GaN layer through annealing treatment. In our designed structure, two TiO2 /SiO2 stacks are incorporated in DBR, aiming to obtain broad reflectance bandwidth and more reflected photons. Figure 4.35a shows the reflectance spectra of ITO/DBR at different incidence angles. We compare the reflectance property for ITO/DBR films and Ag/TiW/Pt/TiW/Pt/TiW/Pt films. When the incident angle is 0°, the reflectance of ITO/DBR is higher than that of Ag/TiW/Pt/TiW/Pt/TiW/Pt. However, with the incident angle increasing, the reflectance of ITO/DBR drops, but that of Ag/TiW/Pt/TiW/Pt/TiW/Pt remains almost unchanged. Thus, we conclude that Ag-based reflector contributes to high LEE, suitable for FCLEDs applications. Next, we fabricated three types of FCLEDs, which were denoted as FCLEDs I, II and III and the corresponding fabrication process are shown in Figs. 4.36, 4.37 and 4.38, respectively. All these FCLEDs have the size of 45 × 45 mil2 .

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Fig. 4.34 a Reflectance spectra of the different as-deposited films at normal incidence before and after annealing at 600 °C. b Optical image of CTLM. c I-V curves of the Ag contact to p-GaN annealed at 400, 500 and 600 °C as well as without annealing process. d I-V curves of for various spacing of CTLM patterns after annealing at 600 °C. The XPS depth profile of Ag/p-GaN contact e before annealing treatment and after annealing treatment

In Fig. 4.39, we analyze the current distribution characteristics of three FCLEDs by numerical simulation at 750 mA. As can be seen, current concentrates around pelectrodes in FCLED I, due to the larger sheet resistance of ITO than that of n-GaN. For FCLEDs II and III, current crowding occurs around n-electrodes, which could be attributed to larger sheet resistance of n-GaN than that of Ag reflector. Besides, the RMS value of FCLED III is lower than that of FCLED II, implying that replacing

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Fig. 4.35 a Reflectance spectra of ITO/DBR at different incident angles. b Reflectance spectra of two schemes at different incident angles when the central wavelength is 460 nm

Fig. 4.36 Fabrication process for FCLED I

finger-like n-electrodes with 3D via-hole n-electrodes contributes to better current spreading. Figure 4.40 shows the emission intensity distribution of three samples at various injection currents. Results demonstrate that emission intensity is higher around pelectrodes in FCLED I, while emission intensity is higher around n-electrodes in FCLED II and III. The results match well with the results in Fig. 4.39. At 500 mA, the emission intensity in FCLED I is highest among these samples, because of the severe current crowding in FCLED I. With the current increasing, we find that FCLED III shows the most uniform distribution of light emission in the entire region. Meanwhile, FCLED III has larger region of high emission intensity in comparison with FCLED I and II. It indicates that FCLED with Ag-based reflective p-electrode and 3D via-hole n-electrodes has better high-power performance.

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Fig. 4.37 Fabrication process for FCLED II

Fig. 4.38 Fabrication process for FCLED III

Fig. 4.39 Simulated results for current density distributions in a FCLED I, b FCLED II, and c FCLED III

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Fig. 4.40 Emission intensity distribution of FCLEDs I, II, and III at various injection currents

We performed I-V measurement for three FCLEDs, shown in Fig. 4.41a. At 750 mA, the forward voltage of FCLED III is lower than two other FCLEDs, benefiting from the Ag-based p-type ohmic contact together with 3D via-hole n-electrodes. The inset shows the EL spectra, where the peak wavelength is located at 460 nm. In Fig. 4.41b, FCLED III has the highest LOP and EQE among three samples due to the uniform current distribution. Besides, FCLED III has higher emission intensity in all directions, similar as the results in Fig. 4.40. After the aging test lasting 1000 h at the temperature of 85 °C, LOP of FCLED III is still the highest among three samples and decreases slightly with the increasing stressed time. This implies that FCLED with Ag-based p-type ohmic contact and 3D via-hole n-electrodes has the excellent stability.

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Fig. 4.41 a I-V curves of the different FCLEDs. The inset shows the corresponding EL spectra. b LOP and EQE versus injection current for different FCLEDs. c Far-field emission pattern and d optical degradation test of different LEDs

4.6 Double-Layer Electrode and Hybrid Reflector In this section, we demonstrate that the double-layer electrode and highly reflective hybrid reflector can achieve the green FCLED with high photoelectrical performance [28]. Figure 4.42 shows the fabrication process of green FCLED. The processing steps are shown as follows: (a) ICP etching was applied to expose the n-GaN layer and form n-via holes. (b) An ITO layer was evaporated on the p-GaN layer. (c) A SiO2 TIR layer was deposited on the top of ITO using PECVD. Subsequently, the insulating DBR consists of Nb2 O5 and SiO2 layers was deposited on the SiO2 TIR layer by ion beam sputtering, which was then selectively etched to form n-contact and p-contact holes. (d) A Ti/Rh metallization was deposited on the Nb2 O5 /SiO2 DBR to form first p-electrode and n-electrode. The isolation trench was defined by a lift-off process. (e) The second SiO2 layer was deposited on the first metallization layer, which was then selectively etched by wet etching to form a p-contact holes and n-interconnection holes. (f) Ti/Ni/AuSn was evaporated on the second SiO2 layer to form the second n- and p-electrode. Figure 4.43a, b show the top-view and cross-sectional schematic diagrams of green FCLED, respectively. Figure 4.64c, d show the cross-sectional TEM images of the green FCLED along the A–A and B–B directions, respectively. The first p-electrode and first n-electrode were composed of Ti/Rh multilayer metal stacks. Meanwhile, the

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Fig. 4.42 Schematic fabrication process of green FCLED

Fig. 4.43 a Top-view and b Cross-sectional schematic diagram of green FCLED. Cross-sectional TEM image of green FCLED along c A–A and d B–B direction

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second p-electrode and second n-electrode were composed of Ti/Ni/AuSn multilayer metal stacks. Isolation trench was used to separate the first p-electrode from the first n-electrode to prevent short circuits. In addition, the first n- and p-electrode were connected through n- and p-interconnect holes, respectively. Figure 4.44 shows the simulated current density distribution of green TELED and FCLED at 20 mA. The RMS values of current density in the active region of green TELED and FCLED are 9.87 and 9.05 A/cm2 , respectively. The smaller RMS value of current density in green FCLED indicates a more uniform current spreading, which is attributed to the introduction of double-layer electrode. Figure 4.45a shows the cross-sectional TEM image of hybrid reflector, which consists of SiO2 TIR layer and Nb2 O5 /SiO2 DBR. Figure 4.45b shows the schematic of the optical simulation model used in the TFCalc software for green FCLED with hybrid reflector. The thickness of SiO2 TIR layer (tL ) is determined by the product of factor N and quarter wavelength. Figure 4.45c shows the reflectance spectra of SiO2 TIR layer with various thickness. It is obvious that high reflectance obtained at θ < 15° is attributed to the DBR, which exhibits a large angular dependence. The critical angle (θcat ) of hybrid reflector/GaN interface is decreased with the increase of the factor N, indicating an enhancement of reflectance effect. In the case of N = 6, the

Fig. 4.44 SimuLED simulated current density distribution in the active region of a green TELED and b green FCLED at 20 mA

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Fig. 4.45 a Cross-sectional TEM image of Nb2 O5 /SiO2 DBR and SiO2 TIR layer. b Schematic of the optical simulation model for green FCLED. c Simulated reflectance spectra of SiO2 TIR layer with various thickness (determined by N). d Simulated reflectance spectra of hybrid reflector versus different angle of incidence of light (N = 6)

θcat of hybrid reflector/GaN interface is decreased to 37°. In other words, light with incident angles greater than 37° is reflected by the SiO2 TIR layer, whereas the light with incident angles smaller than 37° is reflected by the DBR. Figure 4.45d shows reflectance spectra of hybrid reflector versus different angle of incidence of light (N = 6). The reflectance bandwidth almost keeps constant when the incident angle of light is smaller than 15°, due to the introduction of SiO2 TIR layer. However, the reflectance bandwidth of DBR is narrowed when incident angle of light exceeded 15°, which is ascribed to large angular dependence of DBR. Figure 4.46a shows the current versus voltage of the green FCLED and TELED. The forward voltage at 20 mA is reduced from the 3.55 V of TELED to the 3.27 V of FCLED, due to the latter exhibits a much more uniform current spreading. Figure 4.46b shows light emission intensity versus current characteristics of the green FCLED and TELED. Owing to the enhanced reflectance of hybrid reflector and current spreading performance, the light emission intensity of the green FCLED (676 mcd) is 18% higher than that of green TELED (571 mcd).

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Fig. 4.46 a Current versus voltage and b light emission intensity versus current characteristics of the green FCLED and green TELED

4.7 Mini/Micro-LED 4.7.1 Prism-Structured Sidewall of Mini-LED To improve the LEE of FC mini-LEDs, tetramethylammonium hydroxide (TMAH)based crystallographic etching method was used to form prism-structured sidewall on FC mini-LEDs [29]. Figure 4.47a shows the optical microscope image of epilayer after ICP etching process. There are two types of mini-LEDs with different sidewall orientations in Fig. 4.47a. Figure 4.47b shows the SEM image of FC mini-LED. Figure 4.47c, d show the SEM images of sidewall on FC mini-LED after TMAH etching process. It could be observed that the prism structure only appears along [1– 210] direction, which is attributed to anisotropic etching feature of TMAH etching.

Fig. 4.47 a Optical microscope image of epilayer after ICP etching process. b Top-view SEM image of FC mini-LED. SEM images of prism-structured sidewalls on FC c mini-LED I and d mini-LED II

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Fig. 4.48 SEM images of the chip sidewall with various etching time. The TMAH etching time is a 0, b 2.5, c 5, d 7.5, e 10, and f 20 min

Figure 4.48 shows SEM images of the chip sidewall with various TMAH etching time. We can see that smooth sidewall surface becomes trigonal prism structure after TMAH etching process. As etching time increases, the sizes of trigonal prism structures in mini-LED vary from nanoscale to a few microns. We used finite-difference time-domain (FDTD) simulation to investigate fundamental mechanism of prism-structure on the enhanced light extraction. Figure 4.49a shows the simulated electric field intensity distribution nearby smooth and prismstructured sidewall of mini-LED. We could find that electric field is mainly emitting out from the center region of smooth sidewall, whereas there is broader and stronger electric field emitting out from the prism-structure of mini-LED. The result exhibits that more photons could escape from the GaN epilayer via prism-structured sidewall. Figure 4.49b shows the relationship between prism size and LEE. The result indicates that prism-structure could effectively enhance the LEE of sidewall. Figure 4.50a shows the I-V characteristics of mini-LEDs with and without TMAH etching. Compared with mini-LED without TMAH etching, there is almost no electric degradation in the mini-LED with TMAH etching, indicating TMAH etching process is a damage-free etching method. Figure 4.50b shows L-I characteristics of miniLEDs with and without TMAH etching. Compared with mini-LED without TMAH etching, the LOP of mini-LED with TMAH etching is improved. Meanwhile, we found that the light emission from sidewall of mini-LED with TMAH etching is significantly improved in comparison with mini-LED without TMAH etching, as shown in inset of Fig. 4.50b.

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Fig. 4.49 a Simulated electric field intensity distribution nearby smooth and prism-structured sidewall of mini-LED. b Relationship between prism size and LEE of sidewall

Fig. 4.50 a I-V characteristics and b L-I characteristics of mini-LEDs with and without TMAH etching

4.7.2 Light Extraction Analysis of Micro-LED We investigated the effects of substrate thickness, refractive index of encapsulation, shapes of surface microstructures on sapphire substrate, sidewall texture, and airvoid array (AVA) on the LEEs of red AlGaInP-based micro-LED and green/blue GaN-based micro-LEDs [30]. Figure 4.51 shows the relationship between substrate thickness and LEE of red AlGaInP-based micro-LED and green/blue GaN-based micro-LEDs. When the

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Fig. 4.51 Relationship between substrate thickness and LEE of red AlGaInP-based micro-LED and green/blue GaN-based micro-LEDs

substrate thickness is less than 28 μm, the total and sidewall LEEs of red, green, and blue (RGB) micro-LEDs increase with the substrate thickness, while the top LEE of RGB micro-LEDs keep unchanged. When the substrate thickness exceeds 28 μm, the top, sidewall, and total LEEs of RGB micro-LEDs are almost constant. Figure 4.52 shows the relationship between refractive index of encapsulation and LEE of encapsulated blue micro-LEDs. As the refractive index of encapsulation increases, the LEEs of encapsulated blue thin film flip-chip (TFFC) micro-LED and FC micro-LED are improved. When the refractive index of encapsulation is 1.7771, the LEE of encapsulated blue FC micro-LED is same with that of blue TFFC micro-LED. This is because refractive index of encapsulation is identical with that of sapphire substrate (~1.7771). When the refractive index of encapsulation exceeds 1.7771, the LEE of blue FC micro-LED is higher than that of blue TFFC micro-LED.

Fig. 4.52 Relationship between refractive index of encapsulation and LEE of encapsulated blue micro-LEDs

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4 Flip-Chip III-Nitride LEDs

Fig. 4.53 a The shapes of surface microstructures on the top surface of sapphire substrate. b The luminous intensity of encapsulated and bare FC micro-LEDs with textured sapphire substrate

To improve the LEEs of FC micro-LEDs, we introduced various shapes of surface microstructures on the top surface of sapphire substrate, as shown in Fig. 4.53a. The method is called as surface texture. Figure 4.53b shows luminous intensity distribution of FC micro-LEDs. We could see that the luminous intensity of FC micro-LED with PSS is larger than that of FC micro-LED with FSS. Benefiting from microstructures on sapphire substrate, more photons could escape from FC micro-LEDs. Therefore, the luminous intensity of FC micro-LED with pyramid, hemisphere, and circular cone as the shape of the microstructures on sapphire substrate is larger than that of FC micro-LED with PSS. We further investigate the effects of inclination angle of circular cone on the LEEs of FC micro-LED with circular cone as the shape of the microstructure on sapphire substrate. In Fig. 4.53c, when inclination angle of circular cone is 38°, the LEE of FC micro-LED is the highest. Figure 4.54a shows the LEEs of bare and encapsulated micro-LEDs with and without sidewall texture. We can find that the total and sidewall LEEs of bare microLED with sidewall texture are higher than that of micro-LED without sidewall texture. However, the total LEE of encapsulated micro-LED with sidewall texture is almost same with that of micro-LED without sidewall texture. Figure 4.54b shows the top light intensity distribution of encapsulated micro-LED with and without sidewall texture. We can see that sidewall texture would decrease the top light intensity of encapsulated micro-LED. Figure 4.55a shows the micro-LEDs with flat substrate, PSS, AVA, and PSS combined with AVA. In Fig. 4.55b, reflecting layer is deposited on the bottom of

4.7 Mini/Micro-LED

189

Fig. 4.54 a LEEs of bare and encapsulated micro-LEDs with and without sidewall texture. b Top light intensity distribution of encapsulated micro-LED with and without sidewall texture

Fig. 4.55 a Micro-LEDs with flat substrate, PSS, air-void array (AVA), and PSS combined with AVA, but without reflecting layer. b Micro-LEDs with reflecting layer. c LEEs of different face of micro-LEDs. d Luminous intensity distribution of micro-LEDs

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4 Flip-Chip III-Nitride LEDs

micro-LEDs. Figure 4.55c shows the LEEs of different face of micro-LEDs. We can find that top, sidewall, bottom, and total LEEs of micro-LED with PSS are higher than those of micro-LEDs with flat sapphire substrate. In addition, for micro-LEDs without reflecting layer, the top and bottom LEEs of micro-LED with AVA are higher than that of micro-LED with PSS, where the sidewall LEE of micro-LED with AVA is lower than that of micro-LED with PSS. The total LEE of micro-LED with reflecting layer is improved in comparison with that of micro-LEDs without reflecting layer. However, adding a reflecting layer could not improve the sidewall LEE of microLED with PSS. Figure 4.55d shows luminous intensity distribution of micro-LEDs. The luminous intensity of micro-LED with PSS and AVA exhibits more uniform distribution.

References 1. Zhang Y, Zheng H, Guo E et al (2013) Effects of light extraction efficiency to the efficiency droop of InGaN-based light-emitting diodes. J Appl Phys 113(1):1274 2. Zhao H, Liu G, Arif RA et al (2010) Current injection efficiency induced efficiency-droop in InGaN quantum well light-emitting diodes. Solid State Electron 54(10):1119–1124 3. Chen ZZ, Liu P, Qi SL et al (2007) Junction temperature and reliability of high-power flip-chip light emitting diodes. Mater Sci Semicond Process 10(4–5):206–210 4. Zakheim DA, Pavluchenko AS, Bauman DA et al (2012) Efficiency droop suppression in InGaN-based blue LEDs: experiment and numerical modelling. Phys Status Solidi A 209(3):456-460 5. Chernyakov AE, Bulashevich KA, Karpov SY et al (2013) Experimental and theoretical study of electrical, thermal, and optical characteristics of InGaN/GaN high-power flip-chip LEDs. Phys Status Solidi A 210(3):466–469 6. Malyutenko VK, Malyutenko OY, Zinovchuk AV et al (2005) Remote temperature mapping of high-power InGaN/GaN MQW flip-chip design LEDs. Proc SPIE 5941:319–325 7. Lee JR, Na SI, Jeong JH et al (2005) low resistance and high reflectance Pt/Rh contacts to p-type GaN for GaN-based flip chip light-emitting diodes. J Electrochem Soc 152(1):G92–G94 8. Zhou S, Liu X, Gao Y et al (2017) Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic contacts. Opt Express 25(22):26615–26627 9. Liu X, Li N, Hu J et al (2018) Comparative study of highly reflective ITO/DBR and Ni/Ag Ohmic contacts for GaN-based flip-chip light-emitting diodes. ECS J Solid State Sci Technol 7(6):Q116–Q122 10. Yamae K, Fukshima H, Fujimoto K (2019) Omnidirectional reflector with total internal reflective interface for light extraction enhancement of solid-state light source. Phys Status Solidi A 216:1700775 11. Hsu YP, Chang SJ, Su YK et al (2003) InGaN/GaN light-emitting diodes with a reflector at the backside of sapphire substrates. J Electron Mater 32(5):403–406 12. Lu TC, Wu TT, Chen SW et al (2011) Characteristics of current-injected GaN-based verticalcavity surface-emitting lasers. IEEE J Sel Topics Quant Electron 17(6):1594–1602 13. Leonard JT, Young EC, Yonkee BP et al (2015) Demonstration of a III-nitride vertical-cavity surface-emitting laser with a III-nitride tunnel junction intracavity contact. Appl Phys Lett 107(9):75–290 14. Zhou S, Zheng C, Lv J et al (2017) GaN-based flip-chip LEDs with highly reflective ITO/DBR p-type and via hole-based n-type contacts for enhanced current spreading and light extraction. Opt Laser Technol 92:95–100

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15. Liu X, Zhou S, Gao Y et al (2017) Numerical simulation and experimental investigation of GaN-based flip-chip LEDs and top-emitting LEDs. Appl Optics 56(34):9502 16. Zhou S, Liu X, Yan H et al (2019) Highly efficient GaN-based high-power flip-chip lightemitting diodes. Opt Express 27(12):A669–A692 17. Yonkee BP, Young EC, Denbaars SP et al (2016) Silver free III-nitride flip chip light-emittingdiode with wall plug efficiency over 70% utilizing a GaN tunnel junction. Appl Phys Lett 109(19):1687 18. Shchekin OB, Epler JE, Trottier TA et al (2006) High performance thin-film flip-chip InGaN– GaN light-emitting diodes. Appl Phys Lett 89(7):L2112 19. Shatalov M, Chitnis A, Yadav P et al (2005) Thermal analysis of flip-chip packaged 280 nm nitride-based deep ultraviolet light-emitting diodes. Appl Phys Lett 86(20):4762–4768 20. Khan MA, Chen H, Qu J et al (2017) Insights into the silver reflection layer of a vertical LED for light emission optimization. ACS Appl Mater Interfaces 9(28):24259–242727 21. Zhou S, Xu H, Liu M et al (2018) Effect of dielectric distributed Bragg reflector on electrical and optical properties of GaN-based flip-chip light-emitting diodes. Micromachines 9(12):650 22. Liu X, Zhou S, Gao Y et al (2017) Numerical simulation and experimental investigation of GaN-based flip-chip LEDs and top-emitting LEDs. Appl Optics 56(34):9502–9509 23. Zhou S, Liu X, Gao Y et al (2017) Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic contacts. Opt Express 25(22):26615 24. Liu X, Li N, Hu J et al (2018) Comparative study of highly reflective ITO/DBR and Ni/Ag ohmic contacts for GaN-based flip-chip light-emitting diodes. ECS J Solid State Sci Technol 7(6):Q116–Q122 25. Zhou S, Liu X, Yan H et al (2019) Highly efficient GaN-based high-power flip-chip lightemitting diodes. Opt Express 27(12): A669-A692 26. Lv J, Zheng C, Chen Q et al (2016) High power InGaN/GaN flip-chip LEDs with via-hole-based two-level metallization electrodes. Phys Status Solidi Appl Mater Sci 213(12):3150–3156 27. Zhou S, Zheng C, Lv J et al (2017) GaN-based flip-chip LEDs with highly reflective ITO/DBR p-type and via hole-based n-type contacts for enhanced current spreading and light extraction. Opt Laser Technol 92:95–100 28. Zhao J, Liu X, Xu H et al (2019) High-performance green flip-chip LEDs with double-layer electrode and hybrid reflector. ECS J Solid State Sci Technol 8(8):Q153 29. Tang B, Miao J, Liu Y et al (2019) Enhanced light extraction of flip-chip mini-LEDs with prism-structured sidewall. Nanomaterials 9(3): 319 30. Lan S, Wan H, Zhao J et al (2019) Light extraction analysis of AlGaInP based red and GaN based blue/green flip-chip micro-LEDs using the Monte Carlo Ray tracing method. Micromachines 10(12): 860

Chapter 5

High Voltage and Vertical LEDs

5.1 Direct Current High Voltage LED Compared to high power LEDs driven by low voltage and high current, high voltage LEDs (HV-LEDs) driven by high voltage and low current can alleviate the efficiency droop and current crowding effect [1–7]. Figure 5.1 demonstrates the layout of HVLED. The LED epitaxial layers grown on PSS consisted of low-temperature GaN nucleation layer, undoped GaN buffer layer, n-GaN layer, InGaN/GaN MQW, p-AlGaN EBL, and p-GaN layer. Then thermal annealing was performed at 750 °C in N2 atmosphere to activate the Mg acceptors in p-GaN. The TEM images are shown in Fig. 5.2. The total thickness of LED epitaxial layers was 7.65 µm. Therefore, in order to form isolation trench, an etch depth around 7.65 µm was needed. Due to the physical hardness and chemical stability of GaN, we adopted ICP etch process to fabricate deep isolation trench. Thick photoresist film was formed on the wafer as etching mask. In Fig. 5.3a, EXP-1520T positive photoresist was spin-coated onto the wafer and patterned to be a rectangle shape. By a hard baking process on an oven chamber at 120 °C for 10 min, the patterned photoresist was thermally reflowed to form tapered profile as shown in Fig. 5.3b. After ICP etching process, the tapered profile was transferred into the deep isolation trench. Figure 5.4 demonstrates the cross-sectional SEM image of the isolation trench with tapered profile. During ICP etching process, 5 sccm BCl3 /115 sccm Cl2 , 5 mTorr pressure and 350 W ICP power/375 W RF power were adopted. Figure 5.5 shows cross-sectional and top-view SEM images of interconnected LED cells by Cr/Pt/Au metal lines. It was observed that the tapered profile enabled the metal line to overlay the isolation trench smoothly. As shown in Fig. 5.5b, the n-pad of one LED cell was successfully connected to the p-pad of adjacent LED cell by metal line [8]. Figure 5.6 shows the fabricated direct current HV LEDs (DC-HV LEDs) [9– 12]. DC-HV LEDs with different working voltages can be obtained by connecting different numbers of LED cells in series. Figure 5.6 demonstrates the top-view SEM © Science Press 2022 S. Zhou and S. Liu, III-Nitride LEDs, Advances in Optics and Optoelectronics, https://doi.org/10.1007/978-981-19-0436-3_5

193

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Fig. 5.1 Schematic illustration of HV-LED layout

Fig. 5.2 Cross-sectional TEM images of a LED epitaxial layers and b InGaN/GaN MQW. c SAD patterns of GaN

5.1 Direct Current High Voltage LED

195

Fig. 5.3 Optical microscope images of photoresist a before and b after baking process

Fig. 5.4 Cross-sectional SEM image of isolation trench

images of 8.5 V DC-HV LED obtained by connecting three LED cells in series and 17.3 V DC-HV LED obtained by connecting six LED cells in series. Figure 5.7 shows the current-voltage characteristics of 8.5 V DC-HV LED and 17.3 V DC-HV LED. Figure 5.8 shows the schematic diagram of light coupling propagation between adjacent LED cells. Through mathematical derivation, equations that clarify the relation of the variants are given by:

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Fig. 5.5 a Cross-sectional and b top-view SEM images of interconnected LED cells

Fig. 5.6 Top-view SEM images of a 8.5 V and b 17.3 V DC-HV LEDs Fig. 5.7 I-V characteristics of two kinds of DC-HV LEDs

5.1 Direct Current High Voltage LED

197

Fig. 5.8 Schematic diagram of light coupling propagation between adjacent LED cells

tan α =

H −h H +h+W

(5.1)

h h+W

(5.2)

tan β =

θ =α+β

(5.3)

The light absorption ratio can be calculated as follows: η=

θ π

(5.4)

From the mathematical formula above, we can draw a conclusion that for point A, the light absorption decreases with the increase of the bottom width of the isolation trench. In other words, when the bottom width of isolation trench is narrowed, more light will be absorbed by adjacent LED cells, resulting in lower LEE. However, with the increase of the bottom width of isolation trench, the loss of the active region area also increases, leading to a reduction in the LOP of DC-HV LED. As a result, there is a trade-off between the LEE and the active region area. Figure 5.9 demonstrates the top-view SEM images of the deep isolation trench formed using ICP etching process. It was observed that the GaN epilayers were entirely removed and the PSS was exposed. To investigate the influence of isolation trench width on the optical performance of DC-HV LED, DC-HV LEDs with different isolation trench widths were fabricated, including 3.81, 6.38, 12.30 and 40.49 µm. Figure 5.10 demonstrates I-V characteristics of DC-HV LEDs with different isolation trench widths. At 10 mA, the forward voltages of DC-HV LEDs with isolation trench width of 3.81 µm (HV LED I), 6.38 µm (HV LED II), 12.30 µm (HV LED III) and 40.49 µm (HV LED IV) were 8.57, 8.42, 8.49 and 8.51 V. With the increase of isolation trench width, the forward voltage remained almostly consistent, indicating that there was no significant leakage current occurring in these HV-LEDs. This testified that complete isolation and reliable interconnection were obtained through isolation trench with tapered profile. As the isolation trench width increases, the coupling propagation of light between adjacent LED cells can be effectively suppressed. Hence, at 10 mA, the LOP of DCHV LEDs was increased from 170 to 185 mW when the isolation trench width was

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Fig. 5.9 Top-view SEM images of isolation trenches with width of a 3.81 µm, b 6.38 µm, c 12.30 µm and d 40.49 µm

Fig. 5.10 I-V characteristics of DC-HV LEDs with different isolation trench widths

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199

increased from 3.81 to 12.30 µm. However, the loss of active region area resulting from the formation of isolation trench decreased the LOP of DC-HV LEDs. As a result, the LOP of DC-HV LEDs was decreased from 185 to 150 mW when the isolation trench width was increased from 12.30 to 40.49 µm.

5.2 Alternating Current High Voltage LED Alternating current HV LEDs (AC-HV LEDs) can be operated under alternating current, thereby avoiding energy loss caused by AC-DC conversion and reducing the cost of LED lighting system [25]. Figure 5.11 shows circuit diagrams of traditional AC-HV LED with LED cells arranged in reverse parallel circuit and AC-HV LED with LED cells arranged in Wheatstone Bridge circuit. Compared to traditional AC-HV LED, AC-HV LED with LED cells arranged in Wheatstone Bridge circuit exhibited superior performance due to larger emitting area [13–17]. In Fig. 5.11b, four groups of LEDs in the perimeter serve as rectifiers and only two of them can be operated at the same time. The LED cells in the center can work all the time. To investigate the performance of AC-HV LED with Wheatstone Bridge circuit, we fabricated six types of AC-HV LEDs with different layout configurations. Figure 5.12 demonstrates circuit diagrams and corresponding optical microscopy images of the AC-HV LEDs. The total numbers of LED cells in AC-HV LEDs I-VI were 6, 10, 14, 12, 8 and 10. The numbers of working LED cells in AC-HV LEDs I-VI were 4, 6, 8, 8, 6 and 8. The numbers of LED cells and layout arrangement of the AC-HV LEDs were illustrated in Table 5.1. At 20 mA, the luminous intensity distributions of the AC-HV LEDs I-VI were demonstrated in Fig. 5.13. It was observed that half of the rectifier cells were sacrificed to transform the AC to DC for all the AC-HV LEDs Figure 5.14 demonstrates I-V and L-I characteristics of AC-HV LEDs I-VI. Compared with AC-HV LED I, II and IV, AC-HV LED III, V and VI with more Fig. 5.11 Circuit diagram of AC-HV LEDs with LED cells arranged in a reverse parallel circuit and b Wheatstone Bridge circuit

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Fig. 5.12 Circuit diagrams and corresponding optical microscopy images of AC-HV LED I-VI

Table 5.1 Layout arrangement of AC-HV LEDs I-VI

Name

Total number of LED cells

Number of working LED cells

HV AC LED I

6

4

HV AC LED II

10

6

HV AC LED III

14

8

HV AC LED IV

8

6

HV AC LED V

12

8

HV AC LED VI

10

8

working cells exhibited higher forward voltage and larger LOP. Additionally, ACHV LED VI had the highest LOP resulting from the largest ratio of emitting area to chip area. According to Fig. 5.13, the ratios of emitting area for AC-HV LED I–VI were 58.5%, 56.1%, 51.4%, 67.4%, 56.0% and 72.4%, respectively. Therefore, the

5.2 Alternating Current High Voltage LED

201

Fig. 5.13 Luminous intensity distributions of AC-HV LEDs I-VI

conclusion can be drawn that the optical performance of AC-HV LED is co-affected by the number of working cells and the ratio of emitting area to chip area. Figure 5.15 demonstrates the Wall-Plug Efficiency (WPE) versus current curves of AC-HV LEDs I-VI. With the increase of current, the WPEs of AC-HV LEDs I-VI obviously decreased. Compared to AC-HV LEDs II-VI, AC-HV LED I had larger WPE due to larger emitting area and fewer LED cells.

5.3 Comparison of DC-HV LED and AC-HV LED To investigate the performance of DC-HV LED and AC-HV LED, we fabricated DC-HV LED and AC-HV LED with the same working cells. Figure 5.16 shows the SEM images of the fabricated DC-HV LED and AC-HV LED. The DC-HV LED consisted of eight LED cells in series while the AC-HV LED consisted of ten LED cells arranged in Wheatstone Bridge circuit. For the AC-HV LED, only two of the four rectifier cells can work at the same time. Hence, both the DC-HV LED and AC-HV LED have eight working cells at the same time. Figure 5.17a illustrates the I-V characteristics of DC-HV LED and AC-HV LED. At 15 mA, the forward voltages of DC-HV LED and AC-HV LED were 23.8 and 24.3 V, respectively. The L-I of DC-HV LED and AC-HV LED were shown in Fig. 5.17b. At 15 mA, the LOP of DC-HV LED was 9.7% higher than that of AC-HV LED. Figure 5.18 illustrates the spatial distribution of light emission intensity of the DC-

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Fig. 5.14 a I-V and b L-I characteristics of AC-HV LEDs I-VI

HV LED and AC-HV LED at 15, 20, 25 and 30 mA. Compared with AC-HV LED, the spatial distribution of light emission intensity of the DC-HV LED was more uniform due to larger emitting area.

5.4 Vertical LEDs

203

Fig. 5.15 WPE versus current of AC-HV LED I–VI

Fig. 5.16 SEM images of a DC-HV LED and b AC-HV LED

5.4 Vertical LEDs Compared with top-emitting LEDs and flip-chip LEDs, vertical LEDs are more eligible for future high-power lighting devices due to superior device architecture [18, 19]. Firstly, vertical LEDs have larger emitting area without the loss of active layer for n-contact [20, 21]. Secondly, transferring the GaN epitaxial layers from sapphire substrate to a substrate with superior heat conductivity improves the heat dissipation ability of vertical LEDs [18]. Lastly, the surface of vertical LEDs can be easily roughened by wet chemical etching due to exposed N-polar GaN [22]. Figure 5.19 shows the fabrication process of vertical LEDs. The LED epilayers grown on sapphire substrate consisted of low-temperature GaN nucleation layer, undoped GaN buffer layer, n-GaN layer, InGaN/GaN MQWs, EBL, and p-GaN

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Fig. 5.17 a I-V and b L-I characteristics of DC-HV LED and AC-HV LED

layer. ICP etching was adopted to create channels for electric isolation. A SiO2 CBL was deposited by PECVD and then patterned by wet-etching processes. Ag film was deposited on the p-GaN as metal reflector. TiW alloys were deposited onto Ag film as diffusion barrier. Then 5-pair Pt/Ti capping layers were deposited on the TiW alloys. To improve the Ag/p-GaN Ohmic contact, rapid thermal annealing was performed at 600 °C. Ti/Pt/Au multilayers were deposited on the p-Si wafer and Ti/Pt capping layers respectively, followed by the thermal evaporation of an In layer as bonding layer onto the p-Si wafer. Then the LED wafer was bonded to the p-Si wafer by thermal compression at 230 °C. The laser lift-off (LLO) process was performed to remove the sapphire substrate and expose the undoped GaN layer. The undoped GaN layer was removed to expose n-GaN surface by ICP etching process. The LEDs were then dipped into KOH solution to roughen the n-GaN surface. Finally, Cr/Pt/Au multilayers were deposited on the backside of p-Si wafer and the n-GaN surface respectively to form the p- and n-electrodes. Figure 5.20a shows the cross-section TEM image of the vertical LED. The red lines marked as L1 and L2 in the TEM image denoted the bonding interface and protective layers, respectively. Figure 5.20b shows the EDX line scanning result along L1. Besides acting as an adhesion layer, the Ti film also served as barrier layer. We think that the Ti can react with the oxide forming Ti oxides plus silicon and possibly Ti silicide, which permitted reasonably intimate contact between the Ti and Si as indicated in Fig. 5.20b. As shown in Fig. 5.20c, the atomic ratio of Au/In in the bonding layer was about 1/1.87, indicating that the bonding layer was a mixture of AuIn and AuIn2 . The Ag concentration kept at over 90% along the scanning path and declined sharply at the Ag/TiW interface. Figure 5.21 shows the X-ray photoelectron spectroscopy (XPS) Ga 3d core level spectra obtained from the interface region of Ag/p-GaN before and after thermal annealing at 600 °C. Thermal annealing treatment caused the Ga 3d core level shifts toward the lower bindingenergy side around 0.15 eV [23]. Following the relieved downward surface band bending, the Schottky barrier height between p-GaN and Ag decreased, resulting in a better p-type contact. The optical microscope image of conventional multilayer metallization stacks was shown in Fig. 5.22a. Telephone cord buckles were obvious, which can be viewed

5.4 Vertical LEDs

205

Fig. 5.18 Spatial distribution of light emission intensity of a DC-HV LED and b AC-HV LED

as a form of environmentally assisted fracture. We deposited the Pt/Ti capping layers surrounding the Ag/TiW films as shown in the inset scheme in Fig. 5.22b for protecting the interface from environmental humidity. After undergoing the same environment, no buckle occurred in the optical microscope image as shown in Fig. 5.22b. We performed high humidity-temperature aging test to investigate the reliability of vertical LEDs. Two groups of vertical LEDs, as illustrated in Fig. 5.22, are investigated. Vertical LEDs were placed into a life-test humidity chamber and stressed

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5 High Voltage and Vertical LEDs

Fig. 5.19 a–f Schematic illustration of the fabrication process of vertical LEDs. g SEM image of the exposed n-GaN surface. h Cross-section SEM image of vertical LEDs bonded on Si wafer. i Photograph of fabricated vertical LEDs on 4-inch Si wafer

under the constant condition of 85 °C and 85% RH. Figure 5.23 shows the LOP maintenance of vertical LEDs after high temperature–humidity aging test. Vertical LEDs adopting the metallization scheme design of Ag/TiW films with lateral protection suffered negligible optical degradation after an aging time of 1008 h, showing the superior reliability. However, vertical LEDs that adopted the metallization scheme design of Ag/TiW films without lateral protection exhibited an obvious degradation as aging time prolonged. Figure 5.24a demonstrates the atomic structure model of the GaN/InGaN/GaN quantum well. Figure 5.24b demonstrates the schematic illustration of the energy band structure. The energy band structures of the single quantum well (SQW) structure under different stress states were calculated by the SimuLED software. Figure 5.24c demonstrates the calculated energy band diagram of the SQW structure for different relaxation cases. The decrease of compressive stress flattens out the energy bands resulting from the reduction of piezoelectric polarization. As the degree of relaxation increased, the calculated wavelength of peak emission blue-shifted, indicating that the reduction of compressive stress can relieve the QCSE [26]. We used Raman spectroscopy to investigate the stress states before and after LLO. Figure 5.24d demonstrates normalized Raman spectra of the E2 (high) mode before and after LLO. The Raman peaks were located at 568.5 and 567.4 cm–1 before and after LLO, respectively. The corresponding compressive stress before and after LLO

5.4 Vertical LEDs

207

Fig. 5.20 a Cross-section TEM image of the vertical LED. The element concentration profile b along L1 and c along L2

was 458 and 182 MPa, respectively. Figure 5.24e demonstrates PL spectra (T = 300 K) of the LEDs before and after LLO. Due to the relaxation of the compressive stress, a shift in peak emission wavelength from 471 to 456 nm can be observed. Thus, we can conclude that transferring LED epitaxial layers from the sapphire substrate onto the Si substrate can relieve the adverse impacts of the QCSE. Roughening the surface of vertical LEDs is an effective method for improving LEE. The advantage of vertical LEDs in roughening the surface lies on that the exposed N-polar GaN can be easily roughened in KOH or H3 PO4 etchant. Figure 5.25a, b demonstrate the textured surface morphology of vertical LEDs after wet etching using KOH or H3 PO4 solution, respectively. Integrated surface textures consisted of periodic hemispherical dimples and hexagonal pyramids were formed after wet etching using KOH solution. The hemispherical dimples were caused by the removal of PSS. KOH wet etching created hexagonal pyramids structures across

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5 High Voltage and Vertical LEDs

Fig. 5.21 The XPS spectra of Ga 3d core level before and after thermal annealing

Fig. 5.22 Optical microscope images of different metallization scheme designs suffering from moisture in the air: a multilayer metallization stacks without lateral protection; b multilayer metallization stacks with lateral protection

the surface of n-GaN. After wet etching using H3 PO4 solution, there were only dodecagonal pyramid structures on the surface. We adopted FDTD method to calculate the LEE of vertical LEDs with integrated surface textures. Figure 5.26a–c shows schematic illustrations of the emitting surface morphologies of vertical LEDs with a flat surface, single micro-scale surface texture, and integrated surface textures, respectively. Figure 5.26d–f demonstrate the normalized light-induced electric field intensity distributions on the emitting surface of vertical LEDs with different surface textures. The vertical LED with integrated surface textures exhibited the largest intensity of electric field, indicating the largest luminous intensity. Compared with that of a vertical LED without surface textures, the LEE of a vertical LED with single micro-scale surface texture increased by

5.4 Vertical LEDs

209

Fig. 5.23 LOP maintenance of vertical LEDs after high humidity–temperature aging test

Fig. 5.24 a Atomic structure model and b schematic energy band structure of the quantum well c Calculated energy band diagram structure for different relaxation cases. d Normalized Raman spectra and e PL spectra of the LED epitaxial layers

116%. The integrated surface textures further enhanced the LEE by scattering light. Compared with that of vertical LED with single micro-scale surface texture, the LEE of vertical LED with integrated surface textures increased by 10.9%. Figure 5.27 exhibits more evidence of the influence of surface textures on the performance of a vertical LED. The I-V and L-I characteristics are presented in

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5 High Voltage and Vertical LEDs

Fig. 5.25 SEM image of the surface of vertical LEDs after wet etching a using KOH solution and b using H3 PO4 solution

Fig. 5.26 Schematic of the surface morphologies of a flat surface, b single micro-scale surface texture, and c integrated surface textures. Normalized light-induced electric field intensity distributions in vertical LEDs with d flat surface, e single micro-scale surface texture, and f integrated surface textures

Fig. 5.27a, b. The vertical LED with integrated surface textures exhibited larger LOP and lower forward voltage. At 350 mA, the LOP for the vertical LEDs with single micro-scale surface texture was 420.1 mW with forward voltage of 2.90 V. However, the LOP at 350 mA for the vertical LEDs with integrated surface textures was 468.9 mW with forward voltage of 2.88 V. The slightly lower forward voltage was due to better Ohmic contact between the n-GaN and the n-electrode. The enhancement factor of the LOP was 11.4%. Given equal internal quantum efficiency, the experiment results match well with the simulation results of LEE. Figure 5.27c, d demonstrate the far-field radiation profiles at 350 mA. Vertical LEDs with integrated

5.4 Vertical LEDs

211

Fig. 5.27 a L-I and b I-V characteristics of the two types of vertical LEDs. Far-field radiation profiles of the vertical LEDs with c integrated surface textures and d single micro-scale surface texture

surface textures have a more convergent emission pattern resulting from scattering light by nano-scale structures. Figure 5.28a shows the top-view SEM image of the vertical LED. Figure 5.28b shows the light emission intensity distribution of vertical LED at 350 mA. Figure 5.28c, d demonstrate the current density distribution of flip-chip LED and vertical LED at 350 mA. The RMS value and maximum current density of flip-chip LED were 48.06 and 100.61 A/cm2 . It was obvious that the current was crowded around the n-contact in flip-chip LED, causing inhomogeneous current distribution. Compared with flip-chip LED, the RMS value of vertical LED was reduced to 34.96 A/cm2 . The maximum current density of vertical LED was only 49.29 A/cm2 . Therefore, it revealed that the current spreading of vertical LED was better than that of flip-chip LED [27]. Thermal issue of the flip-chip LED and vertical LED was investigated using COMSOL Multiphysics. Numerical simulation about the temperature distribution of flip-chip LED and vertical LED was performed. The simulation results were shown in Fig. 5.29a–h. At 200, 350, 500 and 700 mA, the maximum temperatures of flip-chip LED were 35.90, 53.71, 74.52 and 107.41 °C, respectively. Vertical LED generated less heat as a result of the low resistance of current path. Furthermore, vertical LED exhibited better heat dissipation resulting from high thermal conductivity of silicon substrate. As a result, the corresponding maximum temperatures of vertical LED were only 31.78, 44.83, 59.90 and 83.11 °C, respectively.

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Fig. 5.28 a Top-view SEM image of vertical LED. b Light emission intensity distributions of vertical LED. c Current density distribution of flip-chip LED d Current density distribution of vertical LED

The LEE of flip-chip LED and vertical LED was calculated using FDTD method. Figure 5.30a, b show the normalized electric field intensity distribution of flip-chip LED and vertical LED. The electric field distribution of flip-chip LED was broader than that of vertical LED, which meant a more divergent emission from the top surface. Figure 5.30c demonstrates the calculated LEE of different emitting surfaces. The LEE in 4-sides of vertical LED was lower than that of flip-chip LED. However, the LEE in top surface of vertical LED was superior than that of flip-chip LED due to textured n-GaN surface. The total LEE of two kinds of vertical LED was almost the same. In Fig. 5.30d, compared to flip-chip LED, the vertical LED shows a near Lambertian emission pattern, indicating that the Fabry-Perot interference was broken down due to the textured n-GaN surface. In addition, the emission of vertical LED is more concentrated in the top surface than that of FCLED. Figure 5.31a demonstrates the I-V characteristics for flip-chip LED and vertical LED. Under 350 mA injection current, forward voltage of flip-chip LED and vertical

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Fig. 5.29 a–d Temperature distributions of flip-chip LED at 200, 350, 500, and 700 mA, respectively. e–h Temperature distributions of vertical LED at 200, 350, 500, and 700 mA, respectively

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Fig. 5.30 Normalized electric field distribution of a flip-chip LED and b vertical LED. c LEEs of top and four-sides of flip-chip LED and vertical LED. d Far-filed radiation patterns of flip-chip LED and vertical LED

LED was 2.89 and 2.60 V, respectively. Figure 5.31b shows dynamic resistance versus injection current for vertical LED and flip-chip LED. The slightly lower series resistance of vertical LED resulted from the superior current path and the less current crowding effect around the metal electrodes. The electroluminescence spectrum of flip-chip LED and vertical LED at 350 mA is demonstrated in Fig. 5.31c. The peak wavelengths of flip-chip LED and vertical LED were 453 and 454 nm, respectively. Figure 5.31d shows the LOP and EQE versus current for flip-chip LED and vertical LED. At 350 mA, the LOPs of flip-chip LED and vertical LED were 479.9 and 497.6 mW, respectively. Additionally, the LOP of flip-chip LED saturated at 1280 mA, while there was not light output saturation below 1400 mA in vertical LED.

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Fig. 5.31 a I-V characteristics for flip-chip LED and vertical LED. b Dynamic resistance versus current for flip-chip LED and vertical LED. c EL spectra of flip-chip LED and vertical LED. d LOP and EQE versus current for high-power VLED and FCLED

References 1. Wang CH, Lin DW, Lee CY et al (2011) Efficiency and droop improvement in GaN-based high-voltage light-emitting diodes. IEEE Electron Dev Lett 32(8):1098–1100 2. Horng RH, Shen KC, Kuo YW et al (2012) Effects of cell distance on the performance of GaN high-voltage light emitting diodes. ECS Solid State Lett 1(5):R21–R23 3. Li S, Lam KT, Huang WC et al (2015) Effects of microcell layout on the performance of GaN-based high-voltage light-emitting diodes. J Photon Energy 5(1):057605 4. Hwu FS, Yang C H, Chen JC (2011) Method for measuring the mean junction temperature of alternating current light-emitting diodes. Measur Sci Technol 22(4):045701 5. Lin YS, Hsiao SY, Tseng CL et al (2017) Effect of a cooling step treatment on a high-voltage GaN LED during ICP dry etching. J Electron Mater 46(2):941–946 6. Song XB, Ji X, Li M et al (2014) A review on development prospect of CZTS based thin fllm solar cells. Int J Photoenergy 2014:1–11 7. Lee HK, Yu JS (2011) Optoelectronic and thermal characteristics of GaN-based monolithic light emitting diode arrays. Semicond Sci Technol 26(9):095006 8. Zhou SJ, Zheng CJ, Lv JJ et al (2016) Effect of profile and size of isolation trench on the optical and electrical performance of GaN-based high-voltage LEDs. Appl Surf Sci 366:299–303 9. Zhou SJ, Cao B, Liu S (2011) Optimized ICP etching process for fabrication of oblique GaN sidewall and its application in LED. Appl Phys A 105(2):369–377

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10. Rawal DS, Arora H, Sehgal BK et al (2014) Comparative study of GaN mesa etch characteristics in Cl2 based inductively coupled plasma with Ar and BCl3 as additive gases. J Vac Sci Technol A: Vac Surf Films 32(3):031301 11. Zhou SJ, Cao B, Liu S (2010) Dry etching characteristics of GaN using Cl2 /BCl3 inductively coupled plasmas. Appl Surf Sci 257(3):905–910 12. Yang GF, Guo Y, Zhu HX et al (2013) Fabrication of nanorod InGaN/GaN multiple quantum wells with self-assembled Ni nano-island masks. Appl Surf Sci 285:772–777 13. Yeh WY, Yen HH, Chan YJ (2011) The development of monolithic alternating current lightemitting diode. Proc SPIE 7939:793910 14. Cho J, Jung J, Chae JH et al (2007) Alternating-current light emitting diodes with a diode bridge circuitry. Jpn J Appl Phys 46(12L):L1194–L1196 15. Ao J, Sato H, Mizobuchi T et al (2015) Monolithic blue LED series arrays for high-voltage AC operation. Phys Status Solidi A 194(2):376–379 16. Yen HH, Kuo HC, Yeh WY (2008) Characteristics of single-chip GaN-based alternating current light-emitting diode. Jpn J Appl Phys 47(12R):8808–8810 17. Sadaf SM, Ra YH, Nguyen HPT et al (2015) Alternating-current InGaN/GaN tunnel junction nanowire white-light emitting diodes. Nano Lett 15(10):6696–6701 18. Chu CF, Cheng CC, Liu WH et al (2010) High brightness GaN vertical light-emitting diodes on metal alloy for general lighting application. Proc IEEE 98(7):1197–1207 19. Griffith AA (1921) VI. The phenomena of rupture and flow in solids. Philos Trans Roy Soc Lond A 221(582–593):163–198 20. Oh SH, Lee TH, Son KR et al (2019) Fabrication of HfO2 /TiO2 –based conductive distributed Bragg reflectors: Its application to GaN-based near-ultraviolet micro-light-emitting diodes. J Alloy Compd 773:490–495 21. Khan MA, Chen H, Qu J et al (2017) Insights into the silver reflection layer of a vertical LED for light emission optimization. ACS Appl Mater Interfaces 9(28):24259–24272 22. Zhmakin AI (2011) Enhancement of light extraction from light emitting diodes. Phys Rep 498(4–5):189–241 23. Hasanov N, Zhu B, Sharma VK et al (2016) Improved performance of InGaN/GaN flip-chip light-emitting diodes through the use of robust Ni/Ag/TiW mirror contacts. J Vac Sci Technol B 34(1):011209 24. Zhou SJ, Xu HH, Tang B et al (2019) High-power and reliable GaN-based vertical light-emitting diodes on 4-inch silicon substrate. Opt Express 27(20):A1506 25. Zhou SJ, Gao YL, Zheng CJ et al (2018) A comparative study of GaN-based direct current and alternating current high voltage light-emitting diodes. Phys Status Solidi A 215(10): 1700554 26. Lei Y, Wan H, Tang B et al (2020) Optical characterization of GaN-based vertical blue lightemitting diodes on p-type silicon substrate. Crystals 10(7):621 27. Zhao Q, Miao JH, Zhou SJ et al (2019) High-power GaN-based vertical light-emitting diodes on 4-inch silicon substrate. Nanomaterials 9(8):1178

Chapter 6

Device Reliability and Measurement

6.1 Influence of Dislocation Density on Device Reliability The device reliability of LEDs is significantly affected by the leakage current including forward and reverse leakage current [1]. It is well known that both the forward leakage current and reverse leakage current are closely related to the dislocation density in LEDs [2, 3]. In this subsection, we investigated the influence of dislocation density on device reliability of LEDs [43]. The dislocation density of GaN epilayers grown on FSS and PSS was investigated using XRD rocking curve methods. The FWHMs of symmetry (002) rocking curves of GaN epilayers grown on FSS and PSS were 300 and 249 arcsec, respectively. The FWHMs of asymmetry (102) rocking curves of GaN epilayers grown on FSS and PSS were 310 and 270 arcsec, respectively. For GaN epilayers grown on FSS, the calculated densities of screw and edge dislocation were 1.88 × 108 cm−2 and 5.4 × 108 cm−2 , respectively; For GaN epilayers grown on PSS, the calculated densities of screw and edge dislocation were 1.29×108 cm−2 and 4.1×108 cm−2 , respectively. Figure 6.1 shows cross-sectional TEM images of GaN epilayers grown on FSS and PSS. Figure 6.1a shows bright field TEM image of GaN epilayers grown on FSS. It can be observed that edge, screw, and mixed dislocations exist in GaN epilayers grown on FSS. Figure 6.1b, c show dark field TEM images of GaN epilayers grown on FSS when g = [0002] and g = [11-20], respectively. The screw and mixed dislocations can be observed in Fig. 6.1b. The edge and mixed dislocations can be observed in Fig. 6.1c. In Fig. 6.1d, it can be clearly seen that the dislocation of GaN epilayers grown on PSS is lower than that of GaN epilayers grown on FSS. Figure 6.1e, f show the weak beam dark field TEM images of GaN epilayers grown on PSS when g = [0002] and g = [11 − 20], respectively. In Fig. 6.1e, screw and mixed dislocations can be observed. In Fig. 6.1f, edge and mixed dislocations can be observed. The screw, edge, and mixed dislocation densities of GaN epilayers grown on PSS are significantly lower than those of GaN epilayers grown on FSS.

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Fig. 6.1 Cross-sectional TEM images of GaN epilayers grown on FSS and PSS. a Bright field TEM image of GaN epilayers grown on FSS. b Weak beam dark field TEM image of GaN epilayers grown on FSS when g = [0002]. c Weak beam dark field TEM image of GaN epilayers grown on FSS when g = [11 − 20]. d Bright field TEM image of GaN epilayers grown on PSS. e Weak beam dark field TEM image of GaN epilayers grown on PSS when g = [0002]. f Weak beam dark field TEM image of GaN epilayers grown on FSS when g = [11 − 20]. S, E, and W indicate screw, edge, and mixed dislocations, respectively. g is reciprocal lattice vector

There are five critical electrical characteristics in LEDs, namely forward voltage one (VF1), forward voltage two (VF2), forward voltage three (VF3), forward voltage four (VF4), and reverse leakage current (IR ). We measured the VF1, VF2, VF3, and VF4 at 350 mA, 10 µA, 5 µA, and 1 µA, respectively. The IR was measured under a bias voltage of −7 V. In Fig. 6.2a, the VF2, VF3, and VF4 of LED grown on FSS (FSS-LED) are 2.36 V (@10 µA), 2.31 V (@5 µA), and 2.15 V (@1 µA), respectively. In Fig. 6.2b, the VF2, VF3, and VF4 of LED grown on PSS (PSS-LED) are 2.40 V (@10 µA), 2.35 V (@5 µA), and 2.19 V (@1 µA), respectively. The forward voltages of PSS-LED are larger than those of FSS-LED. Figure 6.2c shows reverse leakage current distribution of FSS-LED in a wafer. It can be found that the reverse leakage current of ~34.5% LEDs is lower than 0.25 µA in the FSS-LED wafer. Figure 6.2d shows reverse leakage current distribution of PSS-LED in a wafer. It can be found that the reverse leakage current of ~82.2% LEDs is lower than 0.25 µA in the PSS-LED wafer. The average value of reverse leakage

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Fig. 6.2 Forward voltages and reverse leakage current. The VF2, VF3, and VF4 of a FSS-LED and b PSS-LED; The reverse leakage current distribution in the LED wafers grown on c FSS and d PSS

current in PSS-LED is lower than that of reverse leakage current in FSS-LED. This is because the dislocation density of GaN epilayers grown on PSS is lower than that of GaN epilayers grown on FSS. The PSS-LED and FSS-LED samples were placed into high-temperature test chamber to conduct high temperature operation life test at 350 mA under the condition of 85 °C. The PSS-LED and FSS-LED samples were removed from the hightemperature test chamber at a specific interval to measure their LOPs. Figure 6.3 shows normalized LOP degradation of high-power PSS-LED and FSS-LED. After high temperature operation life test (288 h), the LOP of PSS-LED decreases by 0.41%, whereas the LOP of FSS-LED decreases by 0.71%. The result indicates that PSS-LED has less optical degradation and better device reliability owing to its lower reverse leakage current.

6.2 Forward Leakage Current Forward leakage current is correlated with device reliability. A number of studies have been conducted through experimental and numerical methods [4–8]. The schematic diagram of the LED structure is shown in Fig. 6.4a and corresponding cross-sectional

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Fig. 6.3 Normalized LOP degradation of high-power PSS-LED and FSS-LED during high temperature operation life test process

Fig. 6.4 a Schematic diagram of the LED structure. b Corresponding cross-sectional TEM image

TEM image is shown in Fig. 6.4b. We performed temperature-dependent I-V characteristic measurement for both unstressed and stressed LEDs. Fig. 6.5 shows the I-V characteristics of the unstressed LED at temperatures ranging from 100 to 400 K. The curves can be divided into three regions on the basis of their slopes, i.e., the low-bias region (~2.2 V), the medium-bias region (2.2-2.6 V), and the high-bias region (above 2.6 V). The slope in low-bias region shows temperature-independent property, while the slope in medium-bias region is strongly related to temperature. The phenomenon indicates that the carrier transport mechanism varies from the low-bias region to the medium-bias region. Figure 6.5(b) shows the extracted characteristic energies and ideality factors for the low-bias region and medium-bias region. The ideality factor serves as an indicator

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Fig. 6.5 a Temperature-dependent I-V characteristics of unstressed LED. b Extracted characteristic energies (ET1 and ET2 represent low-bias region and medium-bias region, respectively) and ideality factors (n1 and n2 represent low-bias region and medium-bias region, respectively)

of the carrier transport mechanism. An ideality factor of one (n = 1) is well related to radiative recombination, an ideality factor of two (n = 2) is a result of Shockley– Read–Hall (SRH) recombination,and an ideality factor larger than two (n > 2) indicates the tunneling mechanism. In the low-bias region, as the temperature increases, characteristic energy varies slightly, whereas the ideal factor decreases from 8.1 to 2.2. This indicates that tunneling current is dominant in the low-bias region, matching with the results in Fig. 6.5a. In medium-bias region, as the temperature increases, the ideal factor varies between 1 and 2. It indicates that diffusion-recombination current dominates the excess current. The I-V characteristics of stressed LED from 100 to 400 K are shown in Fig. 6.6a. As can be seen, there also exist three regions in the I-V characteristics of stressed LED based on the slopes. For comparison, temperature-dependent I-V characteristics for stressed and unstressed LEDs are shown in Fig. 6.6b. When the temperature ranges from 100 to 400 K, the medium-bias regionand high-bias region of stressed LED are consistent with those of the unstressed LED. Nevertheless, the stressed LED exhibits higher current in low-bias region, implying that defect-assisted tunneling

Fig. 6.6 a Temperature-dependent I-V characteristics of stressed LED. b Temperature-dependent I-V characteristics of stressed and unstressed LEDs. c Characteristic energies and ideality factors of low-bias regions of unstressed and stressed LEDs versus temperature

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Fig. 6.7 Reverse I-V characteristics for unstressed and stressed LEDs

process becomes stronger. Moreover, according to I-V characteristics, carrier transport behavior is different between low-bias region and medium-bias region. We conclude that low-bias region is dominated by tunneling current and medium-bias region is dominated by diffusion-recombination current. Besides, in Fig. 6.6c, ET varies slightly for both unstressed and stressed samples, and nT tends to decrease with the increasing temperature. Figure 6.7 shows reverse I-V characteristics for unstressed and stressed LEDs. We find that stressed LED has higher reverse leakage current in comparison with unstressed LED at the same temperature. The result is attributed to the higher defect density in the stressed LED.

6.3 Reverse Leakage Current Reverse leakage current in LEDs is closely related to localized states formed by deep-level centers, which determine device reliability. Meneghini et al. [9] compared LEDs with different dislocation densities, and found that LEDs with a high dislocation density possessed higher reverse leakage current. Ferdous et al. [10] used the photoelectrochemical etching method to measure the threading dislocation density in MQWs of LEDs, and found that the reverse leakage current increased exponentially with the increase of threading dislocation density. Hsu et al. [11] observed that reverse-bias leakage region was correlated with the pure screw dislocation via scanning current-voltage microscope. Cao et al. [12] found that V-pits, which were connected with screw and mixed dislocations, were responsible for the high reverse leakage current in GaN-based LEDs. Although the dislocation density of MOWs is more than 108 /cm2 , GaN-based blue LEDs could achieve an IQE more than 85%. Researchers believed that high efficiency of GaN-based LEDs was attributed to the localization induced by In composition fluctuations. The carriers were captured

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by the localized energy states in the QWs, so as to avoid the non-radiative recombination of carriers captured by dislocations [13, 14]. In addition, researchers thought that the V-pits formed a potential barrier around the dislocations, which suppressed the diffusion of carriers toward dislocations and reduced the non-radiative recombination, thereby improving the luminous efficiency of LEDs [15–17]. Several models of reverse leakage current, mainly including Shockley-Read-Hall (SRH) current [18], band-to-band tunneling current [19], and defect-assisted tunneling current [20–25] have been proposed. The reverse leakage currentis generally characterized by temperature-dependent current-voltage (I-V) measurement [26–31]. Variable-range hopping (VRH) conduction [32], nearest-neighbor hopping (NNH) conduction [25, 33], field-emission tunneling [24], interband tunneling [34], and Poole-Frenkel emission [28] have been suggested as the mechanisms of reverse leakage current in GaN-based LEDs. Researchers believed that the reverse leakage current was closely related to the defect density. However, the carrier transport mechanisms behind reverse leakage current needed to be revealed clearly. We investigate and compare the reverse leakage current characteristics of near-ultraviolet (NUV)/blue/green LEDs. In Fig. 6.8a, we observe that the reverse leakage current of green LED is higher than that of blue and NUV LEDs. The result is ascribed to the higher dislocation density in the green LED. It is well known that as the In content increases, the number of defects increases in the active region [35]. To elucidate the underlying physical mechanism of reverse leakage current in the NUV/blue/green LEDs, we conducted temperature-dependent I-V measurement of these LEDs. Figure 6.8(b) shows temperature-dependent I-V characteristics of the NUV LED. As the temperature increases, the reverse leakage current of NUV LED increases. Similarly, the reverse leakage current increases as the reverse voltage increases. Currents lower than 5 × 10−10 A are strongly influenced by noise and apparatus in measuring process. Therefore,we neglect currents lower than 5 × 10−10 A. Drift-diffusion current, generation-recombination current, and interband tunneling current under reverse voltage were too small to be the main mechanism causing the

Fig. 6.8 a I-V characteristics of NUV/blue/green LEDs. b Temperature-dependent I-V characteristics of NUV LED

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Fig. 6.9 Arrhenius plots of the reverse leakage current at a series of fixed reverse bias of a NUV LED, b blue LED, and c green LED

excess reverse leakage current owing to the wide bandgap of GaN.Hence, hopping conduction may be the main mechanism causing the reverse leakage current [36, 42]. Figures 6.9(a)-6.9(c) show the relationships of reverse leakage current and reciprocal temperature (1/T) for the NUV/blue/green LEDs. We divide the curves into region I (100–240 K) and region II (300–400 K) according to their slopes. For the region I, the reverse leakage current of NUV/blue/green LEDs shows slight dependence of the temperature, which is related to VRH conduction. In the VRH process, owing to small thermal energy at low temperature, electrons easily hop between remote localized states with small energy difference, rather than close localized states with large energy difference. Nevertheless, for the region II, as the temperature increases, electrons are thermally activated, and most likely, hopping occurs between close localized states. Thus, NNH conduction is the most probable dominant mechanism at high temperature (T≥300 K). In the NNH process, electrons transport from the valence band of p-GaN to the conduction band of n-GaN by multistep hopping as well as thermal activation, and electrons hop between close localized states with large energy difference. Owing to the existence of electric field, thermal activation in deep states can be enhanced by reducing activation energy, which is called as Poole-Frenkel effect. Figure 6.10a shows dependences of capacitance and average internal electric field strength on reverse voltage for green/blue/NUV LEDs. Figure 6.10b shows the relationship between square root of the average internal electric field strength in the depletion region for green/blue/NUV LEDs and thermal activation energy. We find in Fig. 6.10(b) that as the square root of the average internal electric field (F 1/2 ) increases, thermal activation energies (E a ) decreases, which can be attributed to the influence of Poole-Frenkel effect. The NNH conduction current is inversely proportional to the average distance (d nn ) between localized states as well as to the E a . The NUV LED has a higher E a and a larger d nn , resulting in less probability for carrier to hop between localized states. Hence, reverse leakage current of NUV LED is lower than that of blue and green LEDs. Although the E a of green LED is slightly higher than that of blue LED, defect density is much higher in green LED, resulting in smaller average distance between localized states. This leads to a larger reverse leakage current in green LED in comparison with blue LED.

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Fig. 6.10 a Dependences of capacitance and average internal electric field strength on reverse voltage for green/blue/NUV LEDs. b Relationship between thermal activation energy and square root of the average internal electric field strength in the depletion region for green/blue/NUV LEDs

6.4 Pad Luster Consistency Although roughened p-GaN surface can effectively improve the LEE of LEDs, it will result in a luster difference between p-pad and n-pad. Consequently, the luster inconsistency of the pads will decrease the rate and accuracy of image recognition during the wire bonding process. The fundamental reason for luster inconsistency of the pads is that the roughness difference between n-GaN layer and ITO layer on top of p-GaN layer. To solve this issue, we proposed an etching method called “under-etching process”.

Fig. 6.11 Schematic of the fabrication process for LED A

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Figure 6.11 shows the schematic fabrication process of conventional LED (denoted as LED A). The processing steps are shown as follows: (a) A lowtemperature p-GaN growth process (850 °C) was applied to form the V-shaped pits for roughed surface. (b, c) The SiO2 insulating layer was deposited on the roughed p-GaN surface by using PECVD, and positive photoresist (EPG516) was spin-coated as a mask for the formation of mesa structure. Then the exposed SiO2 insulating layer was removed by immersing the LED wafer into a buffered oxide etch (BOE) solution (a mixed solution of NH4 F and HF) for 30 s. (d) ICP etching was used to expose the n-GaN layer and define the mesa structure. (e) The SiO2 CBL was prepared by standard photolithography and BOE wet etching process. (f) ITO transparent conductive layer was deposited on roughened p-GaN surface, and Cr/Pt/Au multi-layer were deposited onto the n-GaN layer and the ITO layer to form the n-electrode and pelectrode, respectively. The fabrication processes of LED B and LED C are identical to LED A, except for the growth temperature of p-GaN layer and wet etching time. In LED A, p-GaN layer with roughed surface was epitaxially grown at 850 °C, and the BOE wet etching time was 30 s; In LED B, p-GaN layer with smooth surface was epitaxially grown at 1000 °C, and the BOE wet etching time was 30 s; In LED C, the p-GaN with roughed surface was epitaxially grown at 850 °C, and the BOE wet etching time was 5 s. Figure 6.12 a, b are the schematic illustrations of LED B and LED A. The top-view SEM images of the p-GaN layers of LED A and LED B are shown in Fig. 6.12c and d, respectively. It can be seen that the p-GaN surface in LED B is smooth, whereas the

Fig. 6.12 Schematic illustrations of a LED B and b LED A; SEM images of c LED B and d LED A

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Fig. 6.13 I-V characteristics and optical microscopy images of LED A and LED B

p-GaN surface in LED A has numerous V-shaped pits. Such a rough p-GaN surface is most likely the result of the samller diffusion length of GaN atoms [40]. As the growth temperature is decreased, the Ga atoms on the surface probably do not have enough energy for migration to proper locations; Hence, the lateral GaN growth is depressed. The V-shaped pits at the p-GaN surface are believed to originate from threading dislocations during the low temperature p-GaN growth process or during the cooling stage after the low-temperature p-GaN growth process. Figure 6.13 shows the LOP of LED A and LED B. At 20 mA, the LOP of LED A and LED B is 14.6 and 12.4 mW, respectively. The higher LOP of LED A is attributed to the roughed p-GaN surface, which increases the LEE of LED A. However, there exists a luster inconsistency between the p-pad and the n-pad in LED A, whereas LED B exhibits luster consistency between p-pad and n-pad, as shown in the inset of Fig. 6.13. The luster inconsistency between the p-pad and the n-pad takes the form of different reflected light intensities between the p-pad and the n-pad during the wire bonding process, reducing the rate and accuracy of image recognition in the process of wire bonding. This decreases the yield of products and the reliability of LEDs. Figure 6.14a–d show the surface morphologies of the p-GaN, SiO2 , ITO, and n-GaN surface of LED A measured by AFM. The RMS surface roughness of the p-GaN, SiO2 , ITO, and n-GaN surface in LED A is 112.0, 94.4, 72.9, and 109.0 nm, respectively. In LED A, since mesa-etching process transfers the morphology of p-GaN surface to n-GaN surface, the RMS surface roughness of the n-GaN surface (109.0 nm) is similar to that of the p-GaN surface (112.0 nm). However, since the RMS surface roughness of the ITO surface (72.9 nm) is much lower than that of the n-GaN surface (109.0 nm). This will result in severe luster inconsistency between the p-pad deposited on ITO layer and n-pad deposited on n-GaN layer, as shown in the inset of Fig. 6.13. Figure 6.15a–d show the surface morphologies of the p-GaN, SiO2 , ITO, and n-GaN surface of LED C measured by AFM. The RMS surface roughness of the p-GaN, SiO2 , ITO, and n-GaN surface in LED C is 112.0, 95.1, 74.8, and 73.1 nm,

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Fig. 6.14 AFM scans of a p-GaN, b SiO2 , c ITO and d n-GaN of LED A

Fig. 6.15 AFM scans of a p-GaN, b SiO2 , c ITO and d n-GaN of LED C

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respectively. The RMS surface roughness of the p-GaN, SiO2 , and ITO in LED C is similar to that of the p-GaN, SiO2 , and ITO in LED A, as shown in Fig. 6.14. This indicates that the under-etching process does not influence the surface morphologies of the p-GaN, SiO2 , and ITO. The RMS surface roughness of the n-GaN surface (73.1nm) in LED C is much lower than that of the n-GaN surface (87.9 nm) in LED A. Since the RMS surface roughness of the n-GaN surface (73.1 nm) and ITO surface (74.8 nm) is similar, the surface roughness difference between n-GaN and ITO in LED C is effectively reduced, resulting in the improvement of the luster consistency between n-pad and p-pad. A number of LEDs were fabricated to prove the effectiveness and the repeatability of the under-etching process. Figure 6.16a, b show the optical microscope images of LED A selected randomly from different part of the same LED wafer. It can be seen that luster inconsistency is universal for LED A with a naturally textured p-GaN surface. We can find that the p-pad is much brighter than the n-pad. Figures 6.16(c) and 6.16(d) show the optical microscope images of LED C, which was fabricated by using under-etching process. We can find that luster consistency between the ppad and the n-pad in LED C selected from different parts of the same LED wafer is improved. Thus, we can conclude that the under-etching process is effective for improving the luster consistency by decreasing the surface roughness of n-GaN. Figure 6.17 shows L-I and V-I characteristics of LED A, LED B, and LED C. It can be seen that the electrical performance of all LEDs is almost the same. The result indicates that p-GaN texturing and the under-etching process do not degrade the

Fig. 6.16 Optical microscopy images of a–b LED A and c–d LED C

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Fig. 6.17 L-I and V-I characteristics of LED A, LED B, and LED C

electrical performance of LEDs. At 20 mA, the LOPs of LED A, LED B, and LED C are 14.6, 12.4, and 15.0 mW, respectively. The LOPs of LED A and LED C are 17.7% and 21.0% higher than that of LED B. The improvements are attributed to the naturally textured p-GaN surface, which increases the LEE of LEDs. Moreover, the L-I characteristics of LED A is concident with those of LED C. This indicates the under-etching process will not degrade the optical performance of LEDs, while improving the luster consistency between the p-pad and the n-pad.

6.5 Transient Measurement of LED Characteristic Parameters The characteristic parameters of LEDs, including photometric parameters, colorimetric parameters, and electrical parameters, are important for quality management of LED industry [37]. A fast and accurate measurement instrument to test the photometric parameters, colorimetric parameters, and electrical parameters of LEDs is demanded for LED industry [38, 39]. In this subsection, we developed an instrument to realize transient measurement of characteristic parameters of LEDs [41]. The instrument is composed of a computer with control software, a multichannel spectrometer with charge-coupled device (CCD) sensor, a data acquisition card (DAQ), a human photopic vision detector, and signal processing circuit, as shown in Fig. 6.18. Optical element, including spectrometer and detector, captures the light emitted from LEDs under testing. Subsequently, signal obtained from optical element would be analyzed automatically by using signal processing circuit. The characteristic parameters of LEDs would be finally measured. Figure 6.19 shows the testing platform and corresponding mechanical structure. A rotating plate driven by servo motor is served as testing platform. There are sixteen vacuum suction nozzles uniformly distributing on the periphery of rotating plate, as shown in Fig. 6.19b. The LED samples are located on the vacuum suction nozzles.

6.5 Transient Measurement of LED Characteristic Parameters

231

Fig. 6.18 Layout of the instrument

Fig. 6.19 a Testing platform and b corresponding mechanical structure

The fixed optical element from instrument is on the top of rotating plate. The electrical connection with LED is achieved by two probes. Once the LED samples are firmly clamped by the probe, the light emitted from the measured LED is captured by spectrometer and detector. To measure electrical parameters of LED sample, a programmable constant current/voltage source is designed for power supply of LED sample. Figure 6.20a shows electronic circuit design for measuring forward voltage of LED sample.

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6 Device Reliability and Measurement

Fig. 6.20 Electronic circuit designs for measuring. a Forward voltage and b reverse leakage current of LED sample

The programmable constant current source to drive LED sample is controlled by anolog output port (Ao1) of PCI-9112 DAQ. The anolog output of PCI-9112 DAQ in the range of 0 to 10 V, which is adjusted in terms of software programming, is converted to current in the range of 0 to 99 mA via operational amplifier (KA324) and PNP transistor (B566A). Once LED sample is driven with a constant current, the measured voltage between two pins of LED sample is sent to analog input port (Ai6 and Ai7) of PCI-9112 DAQ. The forward voltage of LED sample is finally calculated by control software. Figure 6.20b shows electronic circuit design for measuring reverse leakage current of LED sample. We use a programmable constant voltage source to drive LED sample. The measured reverse leakage current signal of LED sample is converted to voltage signal using operational amplifiers (LM365N), as shown in Fig. 6.20b. Then, the voltage signal was processed by the next stage operational amplifier (LM356N). Finally, the voltage signal is sent to analog input port (Ai5) of PCI-9112 DAQ through operational amplifier (KA324). After

6.5 Transient Measurement of LED Characteristic Parameters

233

being processed by software, the reverse leakage current of measured LED sample can be calculated in terms of the collected voltage signal by PCI-9112 DAQ. We measure the spectral power distribution (SPD) of LED sample by using spectrometer and PCI-6023E DAQ. Figure 6.21a shows the relative spectral intensity of measured LED sample before calibrating and filtering. We can find that there exists noise signal in the relative spectral intensity of measured LED. Figure 6.21b shows the relative spectral intensity of noise signal. The noise signal is harmful to the measurement. Thus, we remove the average relative intensity of noise signal from the relative spectral intensity of measured LED sample. Furthermore, the pixel numbers could be related to wavelengths of LED sample after calibrating. Therefore, we can calculate the peak wavelength of LED sample in terms of the pixel related to the maximum relative spectral intensity. Figure 6.21c shows the SPD of measured LED sample after

Fig. 6.21 a Relative spectral intensity of measured LED sample before calibrating and filtering. b Relative spectral intensity of noise signal. c SPD of measured LED sample after calibrating and filtering

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6 Device Reliability and Measurement

calibrating and filtering. We can calculate the chromaticity coordinates according to SPD of measured LED sample. The SPD is used to calculate colorimetric parameters of measured LED sample. The calculation process of colorimetric parameters is given by: ⎧ 780  ⎪ ⎪ X = k P(λ)x(λ)λ ⎪ ⎪ ⎪ λ=380 ⎪ ⎨ 780  P(λ)y(λ)λ Y =k ⎪ λ=380 ⎪ ⎪ ⎪ 780  ⎪ ⎪ ⎩Z =k P(λ)z(λ)λ

(6.1)

λ=380

where X, Y, and Z are tristimulus values; k is a normalizing constant; P(λ) is the SPD function of measured LED sample; x(λ), y(λ), and z(λ) are CIE 1931 colormatching functions; λ is the wavelength gap. Then, the chromaticity coordinates are given by: ⎧ ⎪ x = X +YX +Z ⎪ ⎪ ⎨ y = X +YY +Z ⎪ z = X +YZ +Z ⎪ ⎪ ⎩ x+y+z =1

(6.2)

where x, y, and z are chromaticity coordinates of measured LED sample. We record the luminous intensity of LED sample by using human photopic vision detector. The luminous intensity is converted to photocurrent. Then, the photocurrent is converted to voltage signal through operational amplifiers LM365N. The voltage signal is collected by PCI-9112 DAQ. Finally, we calculate the luminous intensity of LED sample by control software. The calculation process is as follows: We have measured the luminous intensity of calibration source, which is defined as IvS . The luminous intensity of the LED sample is defined as IvL . The IvS and IvL are calculated as: IvS = K 1

780 

PS (λ)V (λ)dλ

(6.3)

PL (λ)V (λ)dλ

(6.4)

380

IvL = K 1

780  380

where PS (λ) is the measured SPD function of calibration source; PL (λ) is the SPD function of LED sample; V (λ) is spectral luminous efficiency function of the human eye. When light emitted from calibration source and LED sample is captured by the human photopic vision detector, the photocurrent signal is converted to voltage signal

6.5 Transient Measurement of LED Characteristic Parameters

235

VS and VL . The VS and VL are calculated as: VS = K 2

∞

PS (λ)S(λ)dλ

(6.5)

PL (λ)S(λ)dλ

(6.6)

0

VL = K 2

∞ 0

where S(λ) is relative spectral sensitivity of the detector. Combining equations (6.3)–(6.5) with equation (6.6), the luminous intensity of the LED sample is calculated as: IvL = K c IVvSS VL

(6.7)

where K c is the correction factor for detector. It is defined as: Kc =

 780 380 780 380

∞ PL (λ)V (λ)dλ× 0 PS (λ)S(λ)dλ ∞ PS (λ)V (λ)dλ× 0 PL (λ)S(λ)dλ

(6.8)

We use the instrument to measure the chromaticity coordinates (x, y) of LED sample. The equal-energy point (WE ) with coordinates of (0.333314, 0.333288) is as the reference of white light. We connect the chromaticity coordinates (x, y) with equal-energy point (WE ) to form a straight line. The intersection point of the straight line with periphery of the chromaticity diagram is dominant wavelength of LED sample. The process is shown in Fig. 6.22.

Fig. 6.22 1931 CIE-XYZ chromaticity diagram shows process of determining dominant wavelength of LED sample

236

(1)

6 Device Reliability and Measurement

Curve fitting method

We use cubic spline function to fit periphery of the chromaticity diagram. When the wavelength in periphery of chromaticity diagram is in the range of 380–507 nm, the function is defined as: y = 0.64 − 9.94x + 57.92x 2 − 125.54x 3

(6.9)

When the wavelength in periphery of chromaticity diagram is in the range of 508–520 nm, the function is defined as: y = 0.67 + 7.58x − 127.10x 2 + 767.33x 3

(6.10)

When the wavelength in periphery of chromaticity diagram is in the range of 520–540 nm, the function is defined as: y = 0.81 + 0.89x − 7.55x 2 + 11.87x 3

(6.11)

When the wavelength in periphery of chromaticity diagram is in the range of 540–700 nm, the function is defined as: y = −0.9956x + 0.9968

(6.12)

We can obtain a linear function via chromaticity coordinates (x, y) and equalenergy point. Combining the linear function and the fitting function, the dominant wavelength of LED sample can be calculated. (2)

Look-up table method

The chromaticity coordinates (x, y) corresponding to dominant wavelength can be obtained in CIE 1931 2° chromaticity coordinates of table with wavelength interval of 1 nm. Then, the slope of dominant wavelength in periphery of chromaticity diagram and equal-energy point can be calculated. To obtain monotone decreasing value of slope, we divide the periphery of chromaticity diagram into four regions, which are defined as follows: Region 1: 360 nm ≤ λd ≤ 491 nm; x < 0.333314; y < 0.333288. Region 2: 492 nm ≤ λd ≤ 554 nm; x < 0.333314; y > 0.333288. Region 3: 555 nm ≤ λd ≤ 610 nm; x > 0.333314; y > 0.333288. Region 4: 611 nm ≤ λd ≤ 700 nm; x > 0.333314; y < 0.333288. Figure 6.23 shows the slope versus dominant wavelength curves. The slopes in the four regions are monotone decreasing. Hence, we can obtain the dominant wavelength of LED sample via the value of slope. Then, we use the method of linear interpolation to locate dominant wavelength with a resolution as precise as 0.1 nm. The process is shown in Fig. 6.24. We assume the slope between chromaticity coordinate of LED sample and equal-energy point is k, which is between k 1 and k 2 .

6.5 Transient Measurement of LED Characteristic Parameters

237

Fig. 6.23 Slope versus dominant wavelength curves. a Dominant wavelength in the range of 360 to 491 nm. b Dominant wavelength in the range of 492–554 nm. c Dominant wavelength in the range of 555–610 nm. d Dominant wavelength in the range of 611–700 nm Fig. 6.24 Method of linear interpolation

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6 Device Reliability and Measurement

Therefore, dominant wavelength (λ0 ) of LED sample locates between λ1 and λ2 . Dominant wavelength of LED sample is calculated as: −λ1 λ0 = λ1 λd12 +d d1 2

(6.13)

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