III-Nitride Photonic Integrated Circuit: Multi-Section Gan Laser Diodes For

III-nitride Photonic Integrated Circuit: Multi-section GaN Laser Diodes for

Smart Lighting and Visible Light Communication

Dissertation by

Chao Shen

In Partial Fulfillment of the Requirements

For the Degree of

Doctor of Philosophy

King Abdullah University of Science and Technology

Thuwal, Kingdom of Saudi Arabia

April, 2017 2

EXAMINATION COMMITTEE PAGE

The dissertation of Chao Shen is approved by the examination committee.

Committee Chairperson: Prof. Boon S. Ooi Committee Member: Prof. Kazuhiro Ohkawa, Prof. Lain-Jong Li, Prof. Jung Han (Yale University).

Chao Shen

ABSTRACT

III-nitride Photonic Integrated Circuit: Multi-section GaN Laser Diodes for

Smart Lighting and Visible Light Communication

Chao Shen

The past decade witnessed the rapid development of III-nitride light-emitting diodes

(LEDs) and laser diodes (LDs), for smart lighting, visible-light communication (VLC), optical storage, and internet-of-things. Recent studies suggested that the GaN-based LDs, which is free from efficiency droop, outperform LEDs as a viable high-power light source.

Conventionally, the InGaN-based LDs are grown on polar, c-plane GaN substrates.

However, a relatively low differential gain limited the device performance due to a significant polarization field in the active region. Therefore, the LDs grown on nonpolar m-plane and semipolar (2021)-plane GaN substrates are posed to deliver high-efficiency owing to the entirely or partially eliminated polarization field. To date, the smart lighting and VLC functionalities have been demonstrated based on discrete devices, such as LDs, transverse-transmission modulators, and waveguide photodetectors. The integration of

III-nitride photonic components, including the light emitter, modulator, absorber, amplifier, and photodetector, towards the realization of III-nitride photonic integrated circuit (PIC) offers the advantages of small-footprint, high-speed, and low power consumption, which has yet to be investigated. This dissertation presents the design, fabrication, and characterization of the multi-section InGaN laser diodes with integrated 5 functionalities on semipolar (2021)-plane GaN substrates for enabling such photonic integration. The blue-emitting integrated waveguide modulator-laser diode (IWM-LD) exhibits a high modulation efficiency of 2.68 dB/V. A large extinction ratio of 11.3 dB is measured in the violet-emitting IWM-LD. Utilizing an integrated absorber, a high optical power (250mW), droop-free, speckle-free, and large modulation bandwidth (560MHz) blue-emitting superluminescent diode is reported. An integrated short-wavelength semiconductor optical amplifier with the laser diode at ~404 nm is demonstrated with a large gain of 5.32 dB at 6 V. A high-performance waveguide photodetector integrated LD at 405 nm sharing the single active region is presented, showing a significant large modulation bandwidth of 230 MHz. Thus these seamlessly integrated elements enable photonic IC at the visible wavelength for many important applications, such as smart lighting and display, optical communication, switching, clocking, and interconnect. The findings are therefore significant in developing an energy-saving platform technology that powers up human activities in a safe, health- and environmental-friendly manner.

ACKNOWLEDGEMENTS

My sincere appreciation goes to my supervisor, Prof. Boon S. Ooi, a dedicated scientist, researcher, entrepreneur, leader, and badminton player, always with great passion and insights. His supervision and support helped me accomplish this dissertation research.

I would also like to express my appreciation to our center manager, Dr. Tien Khee Ng, who is always helpful and accommodating. He spent so much time on discussing with me and has given numerous suggestions during my dissertation research. Besides, I would like to thank the great and lovely people I met at KAUST Photonics Laboratory: Dr. Zhao Chao, Dr. Liu Guangyu, and Dr. Mohd Sharizal Bin Alias provided continuous support and helped in my experiments. I learnt a lot from work done by MBE team, including Aditya Prabaswara, Bilal Janjua, Davide Priante, and Pawan Mishra. I enjoyed a lot when working closely with Hassan Oubei, Dr. It Ee Lee, Guo Yujian, Chen Rui, Zhang Zhenyu, and Sun Xiaobin on free-space and underwater optical communications. I would also thank Chun Hong Kang for the discussions related to GaN-based material processing. Very important, I highly appreciate the help and support from Ho Kang-Ting, Abeer AlSaggaf, Edgars Stegenburgs, Jorge Holguin Lerma, and Ram Chandra Subedi on device characterization and system testing. I would like to acknowledge many other lab mates and colleagues, such as Dr. AbdulMajid Mohammed, Dr. Mohammed Zahed Mustafa Khan, Dr. Damián San Román, Dr. Sakandar Rauf, Dr. Malleswararao Tangi, Dr. Mohamed Ebaid, Dr. Mohammad Khaled Shakfa, Dr. Rozalina Zakaria, Dr. Ahmed Ali AlJabr, Dr. Ahmed Ben Slimane, Dr. Hala Al-Hashim, Rami EiAfandy, Abdullah A. Alatawi, Abdullah A. Alhamoud, Liang Jian-Wei, Amanda Ooi, Liao Hsien-Yu, Aigerim Tankimanova, Zhang Huafan, Michele Conroy, and Yang Wenjian for sharing various experiences and making my days enjoyable and amusing. Special thanks to Ms. Khathijah Syed Osman, our center assistant, who provides highly efficient admin support to make the dissertation research goes smoothly.

My appreciation also goes to my friends and colleagues and the department faculty and staff for making my time at KAUST a great experience. I would like to thank Prof. Osman Bakr, and his team members, including Mr. Ibrahim Dursun and Mr. Peng Wei, for the collaboration on the study of novel nanomaterials for optoelectronic applications. I would also like to thank Prof. Mohamed-Slim Alouini, Prof. He Jr-Hau, and Prof. Li Xiaohang, for the encouragement during my research. I have benefited from discussions with Dr. Ki- Hong Park on optical communications and Dr. Cheng Yingchun, especially his in-depth understanding of solid-state physics and experience in stimulating material properties in semiconductor materials. In addition, I would like to thank Dr. Sun Haiding, Dr. Jose Ramon Duran Retamal, Mr. Yang Shuai, Ms. Su Zhen, Mr. Farhan Abdul Ghaffar, Mr. Mahmoud Ouda, Mr. Li Ting-You, Mr. Lin Chun-Ho, Ms. Fu Hui-Chun, Mr. Wu Feng, Mr. Nasir Alfaraj, Mr. Li Kuang-Hui, Mr. Lin Ronghui in CEMSE division and Mr. Wang Zhenwei, Dr. Li Yangyang, Dr. Pan Jun in PSE division, who shared their research experience and provided useful information on equipment and lab operation. 7

Many of the experiments in my research would not be completed smoothly without the help from the scientists and engineers from KAUST’s core-laboratories. I would like to offer my great acknowledgement to Dr. Yang Yang for micro-Raman and PL measurements; Dr. Wang Xianbin, Dr. Wang Zhihong, and Mr. Basil Chew for lithography; Dr. Yue Weisheng, Mr. Li Jun, Mr. Wang Qingxiao, and Ms. Wei Nini for SEM imaging and FIB optimization; Dr. Chen Longqing and Dr. Max Chen for wet chemical process; Dr. Yao Yingbang and Mr. Kim Chong Wong for thermal annealing process; Mr. Ahad Syed for keeping ICP-RIE at very good condition; Dr. Li Liang for discussion of strain effect in semiconductors.

I would express my great appreciations to the external collaborators at the University of California, Santa Barbara (UCSB) for growing the epitaxial structures and providing many help and support in the joint research project. Especially, the leading PIs of Solid State Lighting & Energy Electronics Center (SSLEEC), Prof. Shuji Nakamura, Prof. Steven P. DenBaars and Prof. James S. Speck, have been giving continuous support and encouragement during my dissertation research. I would also like to thank Dr. John T. Leonard and Mr. Changmin Lee for their help in preparing epitaxial materials and running some simulations. Besides, I also benefited from the discussions with Dr. Robert Farrell, Dr. Zhao Yuji, Dr. Erin Young, Dr. Arash Pourhashemi, and Dr. Anisa Myzaferi.

I also want to extend my gratitude to the King Abdullah University of Science and Technology (KAUST) baseline funding, King Abdulaziz City for Science and Technology (KACST) Technology Innovation Center (TIC) for Solid State Lighting (KACST TIC R2-FP- 008), and the KACST-KAUST-UCSB Solid-State Lighting Program, which financially supports the research project.

Finally, my heartfelt gratitude is extended to my family for their unconditional love and unweaving morale support. I would also thank many of my friends, who had enriched my life experience during my Ph.D. study.

Shen Chao

April, 2017

Thuwal, Kingdom of Saudi Arabia

TABLE OF CONTENTS

Page

EXAMINATION COMMITTEE PAGE ...... 2 COPYRIGHT PAGE ...... 2 ABSTRACT ...... 4 ACKNOWLEDGEMENTS ...... 6 TABLE OF CONTENTS...... 8 LIST OF ABBREVIATIONS ...... 11 LIST OF ILLUSTRATIONS...... 12 Chapter 1: Introduction ...... 17 1.1 Solid-State Lighting and White Light Generation ...... 18 1.2 Visible-Light Based Optical Communication ...... 20 1.3 III-Nitride Light Emitters ...... 24 1.3.1 InGaN-based light-emitting didoes ...... 24 1.3.2 Edge emitting laser diodes ...... 28 1.3.3 Vertical cavity surface emitting lasers ...... 29 1.4 Passive and Active Optical Components ...... 30 1.4.1 Optical modulators ...... 30 1.4.2 Photodetector ...... 31 1.4.3 Semiconductor optical amplifier ...... 32 1.5 Novelty Statement ...... 34 1.6 Structure and Organization of the Dissertation ...... 35 Chapter 2: InGaN-based Laser Diode on Non-c-Plane Oriented GaN Substrates ...... 36 2.1 Overview ...... 36 2.2 Optical Gain and Absorption of Laser Diode Grown on m-Plane GaN Substrate 37 2.2.1 Theory ...... 37 9

2.2.2 Device fabrication ...... 38 2.2.3 Results and discussions ...... 39 2.3 Characterization of Laser Diode on Semipolar GaN Substrate ...... 42 2.3.1 Characterization of semipolar InGaN/GaN QW laser diode at 450 nm...... 43 2.3.2 Characterization of semipolar InGaN/GaN QW laser diode at 410 nm...... 47 2.4 Summary ...... 51 Chapter 3: III-Nitride based Integrated Waveguide Modulator-Laser Diode ...... 53 3.1 Overview ...... 53 3.2 Integrated Waveguide Modulator-Laser Diode (IWM-LD) at 448 nm ...... 54 3.2.1 Device fabrication ...... 55 3.2.2 Electrical characterization of IWM-LD at 448 nm ...... 57 3.2.3 Electroabsorption and photocurrent characteristics of the waveguide modulator ...... 60 3.2.4 High speed modulation of IWM-LD at 448 nm ...... 64 3.3 Integrated Waveguide Modulator-Laser Diode at 404 nm ...... 65 3.3.1 Experimental details ...... 66 3.3.2 Characterization of electroabsorption modulation effect ...... 67 3.3.3 High-speed performance of the IWM-LD ...... 69 3.4 Summary ...... 71 Chapter 4: III-Nitride Superluminance Diode ...... 72 4.1 Overview ...... 72 4.2 GaN-based Superluminance Diode with Integrated Absorber Configuration at 450 nm ...... 75 4.2.1 Structure and device fabrication ...... 75 4.2.2 Electro-optical properties of 450 nm emitting SLDs ...... 76 4.2.3 White light generated using SLDs ...... 80 4.2.4 Small signal modulation of InGaN-based SLDs ...... 83 4.3 Violet-emitting GaN-based Superluminance Diode with Tilted Facet Configuration at 405 nm ...... 85 4.3.1 Electro-optical properties of the violet-emitting SLD ...... 86 4.3.2 High-frequency performance of the 405 nm SLD ...... 92 10

4.3.3 SLD as the transmitter for VLC applications ...... 94 4.4 Summary ...... 95 Chapter 5: III-Nitride based Integrated Semiconductor Optical Amplifier-Laser Diode... 97 5.1 Overview ...... 97 5.2 Device Fabrication ...... 98 5.3 Characterization and Discussions ...... 100 5.4 Summary ...... 106 Chapter 6: III-Nitride based Laser Diode with Integrated Waveguide Photodetector ... 107 6.1 Overview ...... 107 6.2 Device Fabrication ...... 108 6.3 Characterization and Discussions ...... 110 6.4 Summary ...... 115 Chapter 7: Conclusions and Outlook ...... 116 7.1 Closing remarks ...... 116 7.2 Future research ...... 119 BIBLIOGRAPHY ...... 121 APPENDICES ...... 137 Appendix A: High-speed modulation performance of violet-blue nonpolar GaN-based vertical-cavity surface-emitting lasers (VCSELs) ...... 137 Appendix B: Standard Operation Procedure of Keithley 2510 TEC Controller ...... 143 Appendix C: Development of Bi-Layer Photoresist Process for self-aligned fabrication applications ...... 150 Appendix D: Ridge waveguide GaN laser diode fabrication process flow sheet ...... 154 Appendix E: publication list during PhD ...... 156 Patents ...... 156 Journal Papers ...... 156 Peer-reviewed Conference Papers ...... 158 Posters and Presentations ...... 159 Manuscript in preparation...... 160

LIST OF ABBREVIATIONS

EAM Electroabsorption Modulator EL Electroluminescence EQE External Quantum Efficiency FIB Focused Ion Beam FWHM Full Width at Half Maximum HRTEM High Resolution Transmission Electron Microscopy IQE Internal Quantum Efficiency ITO Indium Tin Oxide LD Laser Diode LED Light Emitting Diode MBE Molecular Beam Epitaxy MOCVD Metal – Organic Chemical Vapor Deposition MQW Multiple Quantum Well OLED Organic Light Emitting Diode PD Photodetector PIC Photonic Integrated Circuit PL Photoluminescence µPL Micro Photoluminescence QCSE Quantum Confined Stark Effect QW Quantum Well RWG Ridge Waveguide SCH Separate Confinement Heterostructure SEM Scanning Electron Microscope SOA Semiconductor Optical Amplifier SLD Superluminescent Diode SSL Solid State lighting TEM Transmission Electron Microscope TIR Total Internal Reflection UWOC Underwater Wireless Optical Communication VLC Visible Light Communication

LIST OF ILLUSTRATIONS

Figure 1. Three approaches of generating white light from LEDs [28]: (a) combining red, green and blue (RGB) LEDs. (b) combining UV LED and RGB phosphor. (c) combining blue LED and yellow phosphor...... 19

Figure 2. Illustration of indoor and outdoor applications for laser-based VLC...... 20

Figure 3. The modulation bandwidths of LED and LD-based white lighting system considering the phosphor effect [42]...... 23

Figure 4. Schematic epitaxial structure of an InGaN/GaN LED wafer...... 25

Figure 5. Schematic of a typical epitaxial structure for a GaN-based ridge waveguide edge- emitting LD, from [84]...... 28

Figure 6. Schematic of the ﬂip-chip nonpolar VCSEL incorporating an ion implanted aperture, from [96]...... 30

Figure 7. Schematics of the layer structure of the nitride SOAs in [127]...... 33

Figure 8. Schematic diagram of the fabricated multi-section device structure for gain measurement using the segmented contact method...... 37

Figure 9. SEM image of FIB etched multi-section laser structure for gain and absorption measurement. (Scale bar is 100 µm)...... 38

Figure 10. Optical emission spectrum of the laser diode...... 39

Figure 11. Net modal gain (훤푔 − 훼𝑖) spectra obtained using the segmented contact method. Inset: the peak modal gain (훤푔푝푒푎푘) as a function of injection current density...... 40

Figure 12. Net modal absorption spectra obtained using the segmented contact method...... 41

Figure 13. Gain curves at a given current (0.2A) for devices with form factors, L/w, of 20, 15, and 10, with the respective peak gain of 13.8 cm-1, 14.3 cm-1 and 19.4 cm-1, respectively...... 42

Figure 14. SEM image of the facet of the fabricated laser diode on semipolar GaN substrate...... 43

Figure 15. Light output power vs. injection current (L-I) relation of the 2-µm-wide ridge waveguide laser diode with different cavity length ranging from 600 µm to 1800 µm. . 44 13

Figure 16. Plot of 1/differential quantum efficiency vs. cavity length and its linear fitting to derive the internal quantum efficiency (휂𝑖) and internal loss (훼𝑖)...... 45

Figure 17. Plot of threshold current density (퐽푡ℎ) vs. 1/cavity length (1/L) and its linear fitting to derive the transparency current density (퐽0)...... 47

Figure 18. Photo of the violet-emitting LD chip under current injection...... 48

Figure 19. L-I relation for the 3-µm-wide RWG violet-emitting laser diodes...... 48

Figure 20. Electroluminescence emission spectrum of the violet-emitting laser diode at an injection current of 400 mA...... 49

Figure 21. Schematic of the small signal modulation response measurement setup...... 50

Figure 22. Small signal modulation response of the violet-emitting LD at an injection current of 400 mA. The LD shows a -3 dB modulation bandwidth of ~ 3.1 GHz...... 51

Figure 23. Schematic structure of an InGaN/GaN based IWM-LD...... 56

Figure 24. Intensity vs. wavelength showing the lasing spectrum with a peak emission wavelength of 448 nm and a full-width half-max (FWHM) of 0.8 nm...... 57

Figure 25. SEM top down view of the fabricated multi-section laser diode with an integrated modulator...... 58

Figure 26. Output power vs. injection current (L-I) relations of IWM-LD with varying bias voltage applied to the IM section. The light output power at ILD = 500 mA is indicated. 58

Figure 27. The light output power at an injection of 500 mA in the gain region (Pout-500 mA) as a function of the applied modulation bias in the IM section (VIM). Insets: photographs of the fabricated IWM-LD operating in on and off states...... 59

Figure 28.Measured change in absorption versus wavelength under applied reverse bias (VIM) from -1 V to -6 V with respect to the absorption at zero bias...... 60

Figure 29.Photocurrent (PC) measurements. PC spectra of (a) semipolar 2021 and (b) c- plane 0001 InGaN/GaN quantum well modulators with an applied DC bias of -4 V to 0 V...... 62

Figure 30. Small signal modulation of IWM-LD under injection current of 500 mA and IM bias of -3.5V shows ~ 1GHz -3 dB bandwidth...... 64

Figure 31. Schematic of the EAM-integrated LD device structure at 404 nm...... 66

Figure 32. The emission spectrum of the violet-emitting integrated waveguide modulator- laser diode (IWM-LD)...... 67 14

Figure 33. Optical power vs. injection current (ILD) of the EAM-integrated LD at different modulation bias (VIM). Inset: current vs. voltage of the device...... 67

Figure 34. Plot pf optical power at 650 mA vs. VIM. Inset: photos of the device emissions at ON and OFF states...... 68

Figure 35. Small signal frequency response of the violet-emitting integrated waveguide modulator-laser diode under ILD of 650 mA and VIM of -1V...... 69

Figure 36. Eye diagrams of (a) 1 Gbps data rate, and (b) 1.7 Gbps data rate using OOK modulation...... 70

Figure 37. 3D schematic of the demonstrated InGaN/GaN QW SLD on semipolar GaN substrate. The length of the SLD gain region and the passive absorber are 700 µm and 490 µm, respectively...... 75

Figure 38. Photo of the amplified spontaneous emission from the blue-emitting SLD with integrated absorber configuration under DC injection...... 76

Figure 39. Optical power vs. injection current and voltage vs. injection current relations of the SLD. The photodetector was placed both on top and at the edge of the SLD to measure the optical powers from the top surface of the device (spontaneous emission, SE only), and from the edge of the device (amplified spontaneous emission and spontaneous emission, ASE+SE), respectively...... 77

Figure 40. Emission spectra collected from the edge of the blue-emitting SLD under different injection currents...... 78

Figure 41. Emission peak full-width at half-maximum (FWHM) and wavelength as a function of injection current...... 79

Figure 42. Schematic of the SLD-based, phosphor converted white light source...... 80

Figure 43. Spectra of the white light generated using blue-emitting LED, SLD, and LD with yellow YAG phosphor...... 81

Figure 44. The chromaticity coordinates of the white light generated using blue-emitting LED, SLD, and LD with yellow YAG phosphor...... 82

Figure 45. Schematic of the small signal modulation response measurement setup for blue-emitting SLDs...... 83

Figure 46. Measured frequency response of the SLD. A -3 dB bandwidth of 560 MHz is observed at 600 mA...... 84 15

Figure 47. Schematic of the layered structure of the 405-nm emitting superluminescent diode on semipolar GaN substrate with tilted facet configuration. The device has InGaN/GaN multi-quantum-wells (MQWs) as the active region...... 86

Figure 48. 3D illustration of the 405-nm emitting superluminescent diode on semipolar GaN substrate with tilted facet configuration. The length of the device is 590 µm...... 87

Figure 49. Electroluminescence (EL) spectra of the 405 nm SLD under current injection of 50 mA - 400 mA...... 88

Figure 50. Comparison of EL spectra from an LD, an SLD, and a LED at 400 mA...... 89

Figure 51. FWHM and peak wavelength of the SLD as a function of injection current at room temperature...... 89

Figure 52. Plot of the optical power vs. injection current from the top emission (SE) and edge emission (ASE+SE) of the SLD. Inset: Photo of the edge-emitting SLD operating at 300 mA...... 90

Figure 53. Voltage vs. current relation of the SLD. Inset: Capacitance vs. voltage characteristics of the SLD...... 91

Figure 54. Modulation response from the violet-emitting SLD at different current...... 92

Figure 55. -3 dB bandwidth vs. current of the 405-nm emitting SLD...... 93

Figure 56. Eye diagrams of the VLC link using SLD as the transmitter operating at a data rate of: (a) 622 Mbit/s, and (b) 1 Gbit/s using on-off keying (OOK) modulation scheme.95

Figure 57. Cross-sectional layered structure of the 405-nm emitting integrated semiconductor optical amplifier-laser diode (SOA-LD) on semipolar GaN substrate...... 98

Figure 58. 3D illustration of the 405-nm emitting integrated semiconductor optical amplifier-laser diode (SOA-LD) on semipolar GaN substrate. The device involves four pairs of In0.1Ga0.9N/GaN multi-quantum-wells (MQWs) as the active region and a pair of InGaN separate confinement heterostructure (SCH) waveguide layers. The length of the SOA and LD is 300 µm and 1190 µm, respectively...... 99

Figure 59. Photo of the device operating at room temperature...... 100

Figure 60. The emission spectrum of the SOA-LD at LD current of 250 mA and zero SOA driving voltage with a peak emission of ~ 404.3 nm...... 100

Figure 61. Light output power vs. LD injection current (ILD) relation of SOA-LD with varying SOA driving voltage (VSOA) at room temperature...... 101

Figure 62. Plots of optical power at ILD = 250 mA vs. VSOA and Ith vs. VSOA...... 102 16

Figure 63. Plots of gain vs. ILD relation of the SOA-LD at different VSOA...... 103

Figure 64. Plots of gain at ILD = 250 mA vs. VSOA...... 104

Figure 65. Emission spectra of the SOA-LD at ILD = 200 mA with VSOA = 0 V and 6.25 V. The amplification ratio (Ramp) is also plotted with a peak Ramp of 18.4 at 404 nm...... 105

Figure 66. (a) Epitaxial layer structure, (b) schematic of the InGaN/GaN MQW based waveguide photodetector - laser diode (WPD-LD), and (c) photo of the laser emission from the front facet of the WPD-LD...... 109

Figure 67. (a) Plot of light output power collected from the front facet vs. current injection and voltage vs. current injection relations in the laser diode (LD). (b) Comparison of LD output power vs. current in LD and photocurrent from the waveguide PD (WPD) at zero bias vs. current in LD...... 110

Figure 68. (a) Measured photocurrent in the WPD as a function of injection current in the LD region under different reverse biases applied to the WPD. (b) Calculated photocurrent as a function of the received optical power in the WPD...... 112

Figure 69. Measured responsivity as a function of the reverse bias in the waveguide PD...... 113

Figure 70. Modulation response of the waveguide photodetector at zero bias...... 114

Figure 71. Graphic summary of the dissertation...... 116

Chapter 1: Introduction

InGaN/GaN quantum well (QW)-based light-emitting diodes (LEDs) and laser diodes

(LDs) are fundamental components of solid-state lighting (SSL) [1-4]. There are increasing potentials in using such emitters together with modulators, waveguides, amplifiers, and detectors, for visible light communication (VLC) [5-7]. Instead of utilizing discrete components, the eventual development of III-nitride photonic integrated circuit (PIC) will lead to a small-footprint, low-cost, energy-efficient, and multi-functionality solution to many potential applications in the visible color regime, such as smart lighting and display, optical wireless communications, optical switching and clocking, and optical interconnect.

Solid-state lighting based on GaN LEDs has been successfully commercialized with the potential to be the dominant standard for general lighting since the cost of LEDs drops, and the efficiency is improved in the last decade [8-13]. However, LEDs currently suffer a significant drop in external quantum efficiency (EQE) as operating current increases, which is known as “efficiency droop” [14-23]. The origins of the droop is still under investigation and Auger recombination has been proposed to be one of the possible mechanisms [16, 24]. Because the loss occurs as a form of heat, increased temperature causes an additional decrease in efficiency [15]. In contrast to LEDs, laser diodes do not suffer the efficiency droop due to the carrier clamping in the cavity after threshold point of lasing [25]. As a result, the EQE of laser diodes linearly increases with increasing operating current until reaching the thermal roll-over [3]. Owing to these advantages of LDs, my dissertation is focusing on the laser diode based photonic integration for SSL and VLC functionalities. 18

In this chapter, the concepts and advances of SSL, VLC, III-nitride light emitters, GaN- based active and passive optical devices and the structure of the dissertation will be introduced.

1.1 Solid-State Lighting and White Light Generation

Lighting consumes a significant amount of energy, but the current technology is extremely inefficient. For example, the incandescent light bulbs convert about 5% of the electricity they use into visible light [26-28]. Even energy-saving compact fluorescent lamps (a phosphor coated gas discharge tube) have an energy conversion efficiency of up to 20%

[29]. Besides, it is not widely realized that lighting contributes significantly to greenhouse gas emissions. The energy consumed to supply lighting throughout the world entails greenhouse gas emissions of 1900 megatons (Mt) of CO2 per year. The estimation is based on an energy mix according to the 2005 world electricity generation values of 40% from coal, 20% from natural gas, 16% from hydropower, 15% from nuclear, 7% from oil, and

2% from renewables other than hydro [30]. This is equivalent to 70% of the emissions from the automobile globally [31].

Solid-state lighting is a type of lighting that uses semiconductor light emitters, such as

LEDs, as sources of illumination [28, 32]. SSL can revolutionize the lighting market by offering a versatile, incredibly energy-efficient light source with high color stability and growing lumen efficacy [33, 34]. Besides, the LEDs are nontoxic, whereas the fluorescent lamps may contain small quantities of mercury. Owing to the highly toxic nature of the fluorescent lamps, there are strong environmental pressures to develop “mercury-free” 19 lamps. Further, GaN-based white LEDs emit only visible light, which referred to as “clear” light without UV or infrared radiation [33, 35].

Figure 1. Three approaches of generating white light from LEDs [28]: (a) combining red, green and blue (RGB) LEDs. (b) combining UV LED and RGB phosphor. (c) combining blue LED and yellow phosphor.

In general, there are three approaches to generate white light using LEDs as illustrated in

[28, 36]. Firstly, white light can be generated by directly mixing the light from three (or more) monochromatic sources, such as red, green and blue (RGB) LEDs. The second method involves an ultraviolet (UV) LED to pump a combination of red, green and blue phosphors. The third approach integrates the yellow light-emitting phosphor into the phosphor-converted LED package. In this design, part of the blue light is converted into the red and green portions of the spectrum after passing through the phosphor material while the unconverted part contributes to the blue portion of the spectrum. Each of these approaches has its advantages and challenges, but the last approach is widely used in manufacturing due to is relatively simple structure and low cost. 20

1.2 Visible-Light Based Optical Communication

There has been a growing demand in visible-light based optical communication for indoor and outdoor applications (Figure 2) as data-rate demands are exponentially growing due to increasing mobile use, exceeding the capacity existing radio frequency (RF) spectrum in the near future.

Figure 2. Illustration of indoor and outdoor applications for laser-based VLC.

Due to many potential advantages, the high-speed laser-based visible light communication features a number of opportunities:

 High speed: Many applications such as high resolution live video streaming,

large capability broadcasting, vehicle autopilot for smart traffic, and real-time 21

telesurgery, require large bandwidth wireless communications. This can be

enabled by laser based VLC, which offers multi-giga bps data rates.

 Energy efficient: As laser diodes are not associated with efficiency droop,

laser-based VLC system is capable of maintaining high efficiency at high

current density. Thus laser-based VLC system becomes an energy efficient

solution for free-space optical wireless communications.

 High power: The state-of-the-art violet-blue laser diodes are capable of

generating an optical power exceeding 1 W. A laser-based white light source

generating a luminous flux of above 1000 lumens was demonstrated.

Compared to LEDs, laser diodes can generate high power sustaining

comparatively high modulation bandwidth, suggesting laser-based VLC system

is capable for outdoor applications, which require high power.

 High security: Laser-based VLC will enable line-of-sight optical network with

non-penetrating nature of light through the walls, offering a high level of

privacy and security in free-space data communication. This is in contrast to

the conventional RF-based communication techniques, including Wi-Fi,

Bluetooth, and cell phone network, where the network has a high risk of being

hacked. This is of particular importance for commercial and military

applications.

 Long distance: Laser-based lighting has been demonstrated to outperform

LEDs in transmission distance. This offers another advantage for laser-based 22

VLC system to be a promising candidate for applications requires long

transmission channel distance.

 Small footprint: The laser diode is a compact device, which has a much higher

emission power per unit area, making it easier to be implanted for high

mobility applications. Also, the small footprint feature help reduces the

packaging cost as well.

There have been a number of LED-based VLC research reports in the past decade. Some early work presents the road-to-vehicle communication using LED based traffic light and high-speed frame camera [37]. The small signal modulation measurement of a commercial blue-emitting LEDs at 440 nm showed a bandwidth of ~ 10 MHz for blue component and ~ 2 MHz for overall white emission [38].

Though many studies on LED based VLC have been done, only a few reports have been published on laser-based VLC [7, 39, 40]. This might be due to the fact that the laser diodes were not widely accepted as the lighting source yet, but only for fiber optics in long wavelength. Recently, the LD based VLC has gained increasing attention because laser diodes are operated with photon density in lasing mode. As a result, the modulation speed of laser diodes is governed by the photon lifetime (~ ps) instead of the carrier lifetime (~ ns) in LEDs. In addition to the advantage in the photon lifetime, droop-free characteristics at high current density is favorable for not only solid state lighting, but also higher signal amplitude level to the background noise compared to reduced power in

LEDs. A recent study reported an 1 GHz VLC system using 450 nm laser diode with the 23 concept of white light free-space transmission [41]. Using a 422 nm blue laser, a high- speed VLC has been reported with a bandwidth of 1.4 GHz and a data rate of 2.5 Gbit/s

[6]. This is almost three times higher than the recorded bandwidth of the blue LED-based

VLC system. It should be noted that the bandwidth of LD-based VLC system is reported to be limited by the response of the photodetector (PD). Denault et al. demonstrated 442 nm laser based white lighting using YAG:Ce phosphor with a luminous efficacy of 76 lm/W

[3]. Therefore, it casts the possibility of a high-speed laser-based optical communication link that still leverages on a white lighting system. LD-based VLC will less be affected by the phosphor response than that in LED-based systems due to its larger inherent bandwidth than LEDs. Figure 3 compares the modulation bandwidths of GaN-based LED and LD, and the white lighting system with phosphor effect based on LED and LD. Hence,

III-nitride laser diodes are the promising transmitter for VLC application.

Figure 3. The modulation bandwidths of LED and LD-based white lighting system considering the phosphor effect [42]. 24

In additional to the free-space communications, the laser diode based optical communication system can enable underwater applications as well [43-46]. The rapid growing commercial, military, and scientific activities undersea demand for high-speed underwater wireless communications for real-time video streaming, data upload, and remote control [47]. The laser-based underwater wireless optical communication technology has recently been studied to outperform acoustic and radio-frequency communications owing to its large modulation bandwidth, energy efficiency, and small footprint [44, 46].

1.3 III-Nitride Light Emitters

1.3.1 InGaN-based light-emitting didoes

The revolution in solid-state lighting industry dates back to 1994 when Nakamura reported the ﬁrst high-brightness gallium nitride (GaN) based blue LED, and a white LED through combining the blue LED with a YAG phosphor was reported a few years later [48,

49]. Since then, intensive research on improving the performance of white LEDs has been impressive [50], beating by far the luminous efﬁcacy of other light sources including the compact ﬂuorescent lamps. The current development of GaN-based LEDs is likely to replace our current lighting devices [2].

In EU, Photoinics21 European Technology Platform is established to coordinate the R&D activity with a total investment of approximately €110 million till the year of 2012. In the area of LED-based SSL technology, there are a number of projects currently underway – in the framework of EU's Seventh Framework Program for Research (FP7) including 25

FAST2Light, SSL4EU, OLED100.eu, SCOOP, LAMP, IMOLA, CSSL, ENLIGHT, SMASH and

THERMOGRIND [51]. China has identified LED manufacturing as a strategic market and has provided significant financial incentives, over US$1.7 billion, for companies and institutions. Thirteen industrial science parks have been established in mainland China for

SSL R&D, and Industrial Technology Research Institute (ITRI) was set up in Taiwan province. Patent activity in mainland China has increased significantly in the past few years with 28,912 LED-related patents at the end of 2009, including 59% on applications and 13% on packaging. In South Korea, private sectors such as Samsung Electronics, LG

Electronics, Hynix and Hyundai, play a key role in promoting R&D of SSL with a total amount of €90-120 million dedicated.

Figure 4. Schematic epitaxial structure of an InGaN/GaN LED wafer.

A typical InGaN-based LED structure involves multiple quantum wells as the active layer to achieve better electron confinement [52-54]. Quantum well is the structure that a thin 26

InGaN (< 5 nm) is sandwiched by GaN barrier (~10 nm). The active layer is embedded by a top layer of Mg-doped p-type GaN and a bottom layer of Si-doped n-type GaN. Undoped

GaN grain layer is introduced before the epitaxy was grown on a c-plane sapphire substrate to reduce the strain and defects. The epitaxy of a typical blue InGaN LED is illustrated in Figure 4, with a 12 stack of InGaN/GaN MQWs as the active layer. DC sputtered Ni and RF sputtered ITO are typically used as the p contact layer, or the current spreading layer. However, new materials, such as carbon nanotube, graphene, and hybrid cellulose nanopaper have been explored as the transparent conductive electrode as well

[55, 56]. A post deposition RTP process proved to enhance the transparence of ITO layer and reduce the resistance [57].

The low extraction efficiency in current LEDs limits the wall-plug efficiency of LED-based white light. Due to the large difference of the refractive indexes between GaN (n=2.5) based LED structure and the external medium, the critical angle of total internal reflection is about 23° according to Snell’s law if the external medium is air (n=1) [58]. Therefore, only a small proportion of emitting light is capable of escaping outside the LED. Many methods have been proposed to increase the extraction efficiency, such as photonic crystal [59], patterned substrate [60] and surface roughening [61].

The improvement of internal quantum efficiency suffers from several problems in current

LED technology. The inﬂuence of the GaN material quality, especially the structural defects, on LED performance is discussed in [62]. It is demonstrated that a reduction of defects will lead to higher internal quantum efficiency. Besides, the biaxial strain resulted 27 from a mismatch of lattice constant and thermal expansion coefficient, introduces a piezoelectric field inside the active layer. As a result, the band profile is inclined and a decrease in oscillator strength of electron–hole pairs would be expected, which will consequently reduce the internal quantum efficiency [63]. Thus engineering strain would be effective in improving the device efficiency [64-68].

There is another issue limits the light output power in GaN-based LEDs, namely efficiency droop effect. Typically, the quantum efficiency reaches its peak value at low current density and rapidly decreases with increasing injection current under the influence of the effect [69]. The issue of efficiency droop has become an impediment to the developing of high power devices in addition to the factors discussed before. The origin of efficiency droop has been widely discussed, and direct measurement of the Auger electrons using

Faraday cup in an ultra-high vacuum chamber suggests the Auger recombination contributors to the droop effect, in addition to the carrier overflow [24, 70]. So it turns out to be an equally important issue to alleviate the droop effect when designing high brightness GaN-based LEDs, which requires sustaining high EQE under large drive current.

Some researchers studied the utilization of nanowire and nanorod structures to address the above-mentioned concerns [11, 12, 71-75]. However, there remain challenges in the epitaxial and processing technology for utilizing nanowires in device manufacturing.

Current LED manufacturers are still focusing on the optimization and production of conventional planer LED wafers. 28

1.3.2 Edge emitting laser diodes

Traditionally, edge emitting laser diodes (LDs) are used for optical systems which require very compact sources and small emission spots as well as for those applications that require high modulation rates [76-81]. Recent studies suggested that III-nitride LDs are interesting light sources for SSL applications owing to the advantages including a higher power conversion efficiency than blue-emitting LEDs at high current densities [81-83].

The relative low efficiency from the LEDs is a result of the Auger recombination-related efficiency droop discussed in the previous section. Therefore, substituting LDs for LEDs as an SSL source is a promising approach for circumventing efficiency droop.

Figure 5. Schematic of a typical epitaxial structure for a GaN-based ridge waveguide edge- emitting LD, from [84]. 29

In GaN-based LDs, the population inversion necessary for lasing is achieved by means of a double heterostructure with cladding design [78, 85-87]. At sufﬁciently high carrier injection, the stimulated emission in the QW exceeds absorption, and the light is ampliﬁed. Lasing can be achieved by adding two opposing mirrors which form a Fabry-

Perot cavity around the gain medium. The mirrors can be obtained by controllable cleaving or etching of the semiconductor epitaxy [88-92]. A typical layer structure of ridge waveguide (RWG) GaN-based LDs is featured in Figure 5.

1.3.3 Vertical cavity surface emitting lasers

As compared to the conventional edge-emitting lasers in the violet-blue color regime,

[93-95]. Recent advances in nonpolar III-nitrides suggest their potential for low threshold

VCSELs with 100% polarization ratio due to higher peak material gain, lower transparency carrier density and anisotropic gain characteristics [96-98]. Figure 6 presents a 419 nm emitting VCSELs containing a cavity sandwiched by two dielectric distributed Bragg reflectors (DBRs), tunnel-junction contact, and ion-implanted aperture [96]. 30

Figure 6. Schematic of the ﬂip-chip nonpolar VCSEL incorporating an ion implanted aperture, from [96].

Although much work needs to be done for achieving high performance III-nitride VCSELs, those devices are attractive due to their reduced active region volume, shorter cavity length, circular beam proﬁle, low beam divergence [94, 97, 99]. The reduced active region volume allows for high modulation frequencies as well. Therefore, the GaN-based VCSELs have potential for single longitudinal and lateral mode operation with emission normal to the substrate and plastic optical ﬁber and free space data transmission applications [98].

1.4 Passive and Active Optical Components

1.4.1 Optical modulators

To date, LDs have been used in the VLC systems based on direct current modulation [6,

39] for implementing signal modulation schemes, such as non-return-to-zero on-off-keying

(NRZ-OOK) [39], and orthogonal-frequency division multiplexing (OFDM) [7]. Because blue emitters (λ = 440-460 nm) are widely accepted as fundamental components for white light communications [7], it is anticipated that there will be a growing demand for a blue- 31 emitting integrated waveguide modulator-laser diode (IWM-LD), which has a small- footprint and a low-power consumption, as well as multi-functionality by nature. Thus far, visible light modulators, either broadband or narrowband, were implemented using transverse-transmission modulators based on novel low-dimensional materials[100] ,

ZnSe-based QWs [101] and III-nitrides, including InGaN/GaN QW [102, 103],

GaN/AlGaN QW [102, 104] and GaN bulk film [104]. However, the integration of a waveguide electroabsorption (EA) modulator with a laser diode in the blue-green regime has yet to be demonstrated. The lack of interest in this effort originates from the fundamental issues of large spontaneous and piezoelectric polarization fields in c-plane

InGaN/GaN QWs. As a result, it requires a large bias to offset the internal field, in the order of MV/cm [105], making it a highly inefficient compared to the phosphide- and arsenide-based counterparts [106, 107].

1.4.2 Photodetector

Photodetectors (PDs) are devices used for the detection of light, which typically has a metal-semiconductor-metal (MSM) structure, p–n junction or p–i–n structure [108-113].

In PDs, the light is absorbed in a depletion region and generates a photocurrent in response. Such devices can be of small footprint, fast response, highly linear, and exhibit a high quantum efficiency. In particular, the merits of high quantum efﬁciency, low noise, and sharp, the tunable band edge makes GaN-based PDs popular for practical applications in NUV, violet, and blue color regime [108, 114]. There have been various types of GaN- based PDs proposed, and p-i-n PD exhibits high responsivity, low dark currents, and high 32 breakdown voltage, making it attractive when comparing with Schottky barrier counterparts [108, 114-118].

1.4.3 Semiconductor optical amplifier

The Semiconductor optical amplifier (SOA) is an important component in optical communication systems [119]. Numerous reports regarding this type of device have been published since its first demonstration in the 1960s. An SOA can be used as a booster amplifier or a pre-amplifier. In addition, SOAs can also be used in other functional devices, including wavelength converters or optical switches which utilize its nonlinear nature.

One of the main advantages of the SOA is its ability to be monolithically integrated with other devices [119, 120]. An SOA can also be applied as a gain medium to produce short optical pulses in conjunction with a saturable absorber [121]. Additionally, SOAs can be used for boosting the short optical pulses generated by a mode-locked laser for high peak power applications, two-photon bio-imaging as an example [122, 123].

For pulsed emission generation, the majority of light sources consist of large-size, bulky solid-state lasers, such as the widely used mode-locked Ti:Sapphire laser. As for optical pulses generation at NUV, violet, and blue regime [124-126], wavelength conversion process including the second- or third-order harmonic of an IR pulse is required for available pulse sources. As a result, the laser system is typically having a larger footprint, complicated structure, leading to a low wall-plug efficiency. Therefore, the direct generation of light pulses in NUV, violet and blue color region is highly desirable, and thus, 33 making the GaN-based SOA an important device for short wavelength optical pulses generation [127].

There are Fabry–Perot (F-P) type and the travelling-wave type SOA. An F-P SOA has nonzero facet reflectivity so that signal light is amplified as the result of multiple passes through the amplifier. In contrast, a travelling-wave SOA has negligible facet reflectivity and thus, the signal passes through only once. The travelling-wave SOA has a much simple structure, and a typical structure of GaN-based SOAs is shown in Figure 7.

Figure 7. Schematics of the layer structure of the nitride SOAs in [127].

The output power of an SOA is limited by its saturation power, Psat, which is given by

ℎ휈푑푤 푃 = (1) 푠푎푡 훤훼휏 where h is the Plank constant, ν is the optical frequency, d and w are the active region thickness and width, respectively. Γ is the confinement factor, α is the differential gain, and τ is the carrier lifetime. Therefore, for high power applications, the SOA structure should have a larger active region cross section, a lower confinement factor, a lower differential gain, and a reduced carrier lifetime. 34

1.5 Novelty Statement

The presented dissertation features the following points of novelty:

 Measurement and comparison of the net modal gain, internal loss, and differential

modal gain in non-c-plane InGaN/GaN quantum wells since the gain and absorption

spectra are essential to understanding the electronic transition process and device

performance.

 Demonstrate >Gbit/s white light communication link using a single blue laser diode

and YAG:Ce phosphor, achieving >50 lm luminous flux.

 Demonstrate large extinction ratio, high modulation efficiency electroabsorption

modulator integrated with non-c-plane InGaN-based LDs. Investigate the

modulation response and data transmission using integrated modulator with the

laser diode.

 Study the integration of absorber for fabrication of high optical power non-c-plane

superluminescent diodes and its potential application in SSL and VLC. Their spectral

and frequency response properties are first to be reported.

 Demonstrate the integration of semiconductor optical amplifier (SOA) on non-c-

plane LDs and its dynamic property for SSL-VLC applications.

 Investigate the integration of waveguide photodetector (WPD) with ridge

waveguide LD for power monitoring and on-chip communication. 35

1.6 Structure and Organization of the Dissertation

Chapter one is the introduction, which summarizes the background of the study. Chapter two is the discussion of device characteristics of InGaN/GaN quantum well based laser diodes grown on non-c-plane GaN substrates. It features the optical gain and absorption measurement of GaN-based laser diodes on m-plane GaN and the electrical and optical properties of GaN-based laser diodes on semipolar plane bulk GaN substrate. Chapter three presents the integration of a waveguide electroabsorption modulator (EAM) with a laser diode and the high-speed modulation using the integrated waveguide modulator (IWM).

Chapter four introduces the III-nitride superluminance diodes (SLDs), which is fabricated by integrating a passive absorber with gain region using a laser diode epi-structure. The optoelectrical performance of the SLD and the white light generated using SLD are discussed. Chapter five reports the integrated semiconductor optical amplifier-laser diode

(SOA-LD). The amplification effect of the integrated III-nitride semiconductor optical amplifier (SOA) at visible wavelength is measured and analyzed. Chapter six presents the integration of waveguide photodetector (WPD) with laser diode on same InGaN/GaN QW epitaxial structure. The fabrication and characterization of such integrated photonic system are included. Chapter seven provides a summary of the dissertation research and an outlook of the research topic.

Chapter 2: InGaN-based Laser Diode on Non-c-Plane Oriented GaN

Substrates

2.1 Overview

Development of c-plane GaN-based violet and blue laser diode (LD) technology in the past two decades stands to satisfy a number of application needs including high-speed visible light communication (VLC) and laser based solid state lighting (SSL) [78, 128]. However, those devices on the polar plane suffer from strong polarization fields, which reduces their internal quantum efficiency [129]. Recently demonstrated m-plane (nonpolar) and semipolar plane LDs show the potential to outperform c-plane devices due to the absence or reduction of built-in polarization-related electric fields [130, 131].

While the gain and absorption spectra are essential to understanding the electronic transition process and device performance, there are few related studies on c-plane LDs, and even more limited reports in that on m-plane devices [132, 133]. Using the Hakki-

Paoli method, T. Melo et al. reported the gain spectra of m-plane LDs, but only at below lasing threshold conditions [132]. In this chapter, the gain and absorption characteristics of m-plane InGaN/GaN LDs are measured both at below and above lasing threshold using the segmented contact method [134].

The light output power vs. injection current relation is the basic characteristics of a laser diode. The slope efficiency and threshold current can be derived accordingly. The 37 emission spectrum is another important factor to be measured and evaluated. The characterization of violet and blue emitting LDs on semipolar (2021) GaN substrate is presented and discussed.

2.2 Optical Gain and Absorption of Laser Diode Grown on m-Plane GaN

Substrate

2.2.1 Theory

Figure 8. Schematic diagram of the fabricated multi-section device structure for gain measurement using the segmented contact method.

The segmented contact method was performed by electrical injection of the devices with variable length [135]. The schematic drawing of the structure used for gain measurement is illustrated in Figure 8. The multi-section LD consists of two sections, denoted as A and

B, with the same length (L) near the front facet, followed by a relatively long absorber region, which serves to suppress the photon round trip feedback and therefore the stimulated emission. By three successive measurement of the front facet light intensities, i.e.I(A), I(B) and I(AB), which correspond to: (a) pumping only section A, (b) pumping only section B, and (c) pumping both sections together, the net modal gain spectrum (훤푔 − 38

〈훼𝑖〉) and the modal absorption spectrum (−훤푔 − 〈훼𝑖〉) can be calculated according to equations (2) and (3):

1 퐼(퐴퐵) 훤푔 − 〈훼 〉 = 푙푛 ( − 1) (2) 𝑖 퐿 퐼(퐴)

1 퐼(퐵) −훤푔 − 〈훼 〉 = 푙푛 ( ) (3) 𝑖 퐿 퐼(퐴)

Here, the 훤 is the confinement factor; 푔 is the material gain; 〈훼𝑖〉 is the internal model loss.

2.2.2 Device fabrication

Figure 9. SEM image of FIB etched multi-section laser structure for gain and absorption measurement. (Scale bar is 100 µm).

The 420 nm emission laser diode structure was grown using metal-organic chemical vapor deposition (MOCVD) on m-plane GaN substrate, and fabricated into ridge-waveguide LD.

The epitaxial structure consists of a Mg-doped p-GaN contact layer, a Mg-doped p-InGaN separate confinement heterostructures (SCH) cladding layer, an AlGaN electron blocking layer (EBL), a four periods of In0.12Ga0.88N/GaN multi-quantum wells, a Si-doped n-InGaN

SCH cladding layer, and a Si-doped n-GaN contact layer. The facets were defined using inductively-coupled plasma (ICP) etching. The three-section were then patterned using 39 focused-ion beam (FIB) with section A and B each having length (L) of 100 µm, 150 µm or

200 µm (see Figure 9). The LDs were measured under 5 µs pulsed current at 1% duty cycle.

2.2.3 Results and discussions

Figure 10 shows the emission from the front facet of a laser diode with 5 µm ridge width and 1250 µm cavity length. A peak emission wavelength of 425.15 nm was measured.

Figure 10. Optical emission spectrum of the laser diode.

Figure 11 and Figure 12 show the gain (훤푔 − 〈훼𝑖〉) and absorption (−훤푔 − 〈훼𝑖〉) spectra from the device operating at various injection currents, using L = 200 µm segmented pads.

2 The lasing threshold current density (Jth) for this device is 5.17 kA/cm , and we obtained the gain characteristics at below threshold (5 kA/cm2) and above threshold (7.5, 15 and

2 -1 18.8 kA/cm ) conditions. The internal loss 〈훼𝑖〉 = 13.46 cm is derived from the stable plateau formed at the long wavelength side of the gain spectrum. This value is in agreement with the internal loss of 13.40 cm-1 obtained from the analysis of slope 40 efficiencies and threshold current densities from a set of LDs with different cavity lengths on the same die.

Figure 11. Net modal gain (훤푔 − 〈훼𝑖〉) spectra obtained using the segmented contact method. Inset: the peak modal gain (훤푔푝푒푎푘) as a function of injection current density.

-1 Now considering the mirror loss 〈훼푚〉, which was calculated to be -17.8 cm for the tested

- sample without facet coating, the gain overcomes total loss ( 훤푔 − 〈훼𝑖〉 − 〈훼푚〉 = 3.8 cm

1 at 420 nm ) at 7.5 kA/cm2, indicating the presence of lasing action. The linewidth of the gain curve increases at above threshold, and a blue-shifting of peak gain with increasing injection current was observed, resulting from the band-filling effect. Both gain and absorption curves converge to comparable internal modal loss values of -13.46 cm-1 and

-12.54 cm-1, and hence validating the measurements. 41

Figure 12. Net modal absorption spectra obtained using the segmented contact method.

-1 Comparing the peak modal gain (훤푔푝푒푎푘) of m-plane LDs (29.2 cm ) with c-plane devices

(20.5 cm-1 as reported in [136]) at the injection of 5 kA/cm2, the LDs on non-polar substrate show higher gain due to reduced carrier separation. The differential modal gain

(훤푔푑𝑖푓푓), yielded from a linear fitting of the peak modal gain curves in the inset of Figure

2(a), describes the growth of modal gain with increasing injection. For LDs operating above threshold, 훤푔푑𝑖푓푓 is calculated to be 4.6 cm/kA, which is significantly lower than those below threshold (12.35cm/kA as in [136]). We attribute this observation to a rapid increase of recombination rate overcoming the growth of gain per carrier when devices started to lase. 42

Figure 13. Gain curves at a given current (0.2A) for devices with form factors, L/w, of 20, 15, and 10, with the respective peak gain of 13.8 cm-1, 14.3 cm-1 and 19.4 cm-1, respectively.

It is noted that the reliability of this measurement is affected by the device form factor, defined as the ratio of the section length and ridge width, L/w. The gain curves measured with various form factors (see Figure 13) show consistency in measurement with L/w = 15 and 20, while a 20% deviation was found in the case of L/w = 10, indicating a form factor at or greater than 15 is favored for a reliable measurement in this study.

2.3 Characterization of Laser Diode on Semipolar GaN Substrate

In this section, the electrical and optical characterization of blue-emitting and violet- emitting semipolar InGaN laser diodes are presented and discussed. 43

2.3.1 Characterization of semipolar InGaN/GaN QW laser diode at 450 nm.

The blue-emitting laser diode contains a four period of In0.2Ga0.8N/GaN quantum wells as the active region. The epitaxial structure was grown using MOCVD technique. A series of

LDs with different ridge width and cavity length were fabricated on the same chip. In one die, there are LDs with 2, 3, 4, and 7.5 µm ridge waveguide. Besides, the cavity length ranges from 600, 900, 1200, 1500, to 1800 µm. By analyzing the device performance of

LDs with different cavity length, the transparency conditions of the LDs can be derived.

Figure 14. SEM image of the facet of the fabricated laser diode on semipolar GaN substrate.

Figure 14 shows the SEM image of the fabricated 2-µm-wide RWG laser diode with a clear etched facet. The laser bar and p-metal pad are labeled, respectively. 44

Figure 15. Light output power vs. injection current (L-I) relation of the 2-µm-wide ridge waveguide laser diode with different cavity length ranging from 600 µm to 1800 µm.

The electrical characteristics of the 2-µm-wide ridge waveguide (RWG) LDs were measured using Keithley 2520 diode laser testing system with calibrated Si photodetector installed in an integrating sphere at room temperature. Figure 15 shows the light output power vs. injection current (L-I) relation of the LDs with various cavity lengths, increasing from 600 µm to 1800 µm. The laser emission was first dominated by spontaneous emission which increases very gradually until it enters the stimulated radiation regime as the injection current increased. For a 600-µm-long LD, the rapid increase of light output power was observed at a current greater than 100 mA, which is typically referred to as the threshold current (Ith). Generally, it is desirable that the LDs show a small Ith, resulting in a more efficient device. However, Ith also depends on the design and area of the laser device. With increasing cavity length, the LD exhibited a higher Ith as observed in Figure 45

15. Hence, the threshold current is one important parameter used to quantify the performance of a laser diode.

Figure 16. Plot of 1/differential quantum efficiency vs. cavity length and its linear fitting to derive the internal quantum efficiency (휂𝑖) and internal loss (훼𝑖).

Besides the threshold current, the efficiency of the device is another important parameter for an LD. It is desirable to have a rapid increase in the light output yet with a slow increase of input current. In other words, a laser diode which has a good conversion rate of input electric power to output optical power is a well-performing device. A direct measurement of the efficiency is the slope efficiency, which is the slope of the L-I curve above the threshold current point. The slope is typically denoted as ΔP/ΔI, and has the units of

Watts per Amperes (W/A). The external differential quantum efficiency (ηd) is defined as the efficiency of a laser device in converting the injected electrons to the emitted photons. It can be calculated from the slope efficiency. An ideal hypothetical LD converts

100 percent of the injected current into emitted photons without any waste in heat 46 generation. However, the LDs, in reality, may be associated a number of loss mechanisms, including crystal defects and surface states, leading to an external differential quantum efficiency < 100%. Figure 16 shows the plot of inverse external differential quantum efficiency vs. the cavity length for the semipolar LDs. The curve is useful to derive many important internal parameters for the LDs, including the internal quantum efficiency (휂𝑖) and internal loss (훼𝑖). The former is a measure of the efficiency of an LD in converting injected electron-hole pairs into photons within the active region. The latter is a measure of the losses of light that propagating through the waveguide before emitted from the facet. The ηi and αi can be determined according to the following relation,

1 1 훼 = [1 + 푖 퐿] (3) 휂푑 휂푖 푙푛(1/푅) where the R refers to the reflectivity of the facets and the L is the cavity length.

Based on the linear fitting as shown in Figure 16, the internal quantum efficiency (휂𝑖) of

-1 the 2-µm-wide RWG semipolar LDs is 79.74% and the internal loss (훼𝑖) is 10.327 cm . 47

Figure 17. Plot of threshold current density (퐽푡ℎ) vs. 1/cavity length (1/L) and its linear fitting to derive the transparency current density (퐽0).

The transparency threshold current density, usually denoted by the symbol J0, is another significant parameter that could be extracted. Since the threshold current depends upon the cavity design of the LD, it is not accurate to be used to compare the material quality of LDs from different wafers. The transparency threshold current density (퐽0), which can be thought of as the threshold current density of a theoretical LD having an infinitely long optical cavity with no loss in the mirror facets, is thus been used as a parameter independent of the device geometry. Figure 17 shows the threshold current density (퐽푡ℎ)

2 as a function of the inverse cavity length (1/L). The 퐽0 is calculated to be 1.72 kA/cm based on the linear fitting of the curve.

2.3.2 Characterization of semipolar InGaN/GaN QW laser diode at 410 nm.

The violet-emitting LDs have similar epitaxial structure to the blue-emitting LDs, expect for a lower In composition in the active region, which is In0.1Ga0.9N/GaN QWs. The LDs 48 were also grown on (2021) plane GaN substrate using MOCVD technique. A series of LDs with different geometry were fabricated on the same chip without facet coating.

Figure 18. Photo of the violet-emitting LD chip under current injection.

Figure 18 shows the photo of the laser light emitted from the violet-emitting LD chip under current injection. The device under test is the as-fabricated LD chip without dicing and TO-can packaging.

Figure 19. L-I relation for the 3-µm-wide RWG violet-emitting laser diodes. 49

The light output power vs. injection current (L-I) relation of the 3-µm-wide RWG violet- emitting LDs is shown in Figure 19. The LDs with 1500- and 1800-µm-long cavity length exhibit smooth L-I curves, but the LDs with 900- and 1200-µm-long cavity length show the efficiency roll-over and unexpected output power reduction at high injection current after threshold. This may be attributed to fabrication related concerns and thermal roll-over.

Hence, the following study will focus on the 1500- and 1800-µm-long LDs. A threshold current of 274 mA and a slope efficiency of 0.33 W/A were measured from the 1500-µm- long LD. The 1800-µm-long LD has a threshold current of 294 mA and a slope efficiency of

0.31 W/A.

Figure 20. Electroluminescence emission spectrum of the violet-emitting laser diode at an injection current of 400 mA.

Figure 20 presents the electroluminescence emission spectrum of the 1500-µm-long violet-emitting laser diode at an injection current of 400 mA using an Ando AQ6315A optical spectrum analyzer. A peak emission wavelength of 411.3 nm was measured. The

LD shows a single emission peak with a narrow peak full-width at half-maximum (FWHM) 50 of 0.527 nm, which is attributed to the effective mode confinement as a result of the narrow RWG design.

Figure 21. Schematic of the small signal modulation response measurement setup.

To study the high-frequency performance of the LD, a small signal modulation response measurement was performed using an Agilent E8361C network analyzer. Figure 21 shows the schematic of the measurement setup. The LD was probed using a custom-made prober with a high-frequency ground-signal (GS) RF probe. The setup involves a Keithley

2400 source-meter as the DC power supply, a Tektronix PSPL 5580 15 GHz broadband bias-tee, and an ALPHALAS 7 GHz UPD-50-UP high-speed Si photodetector (PD). In typical

InGaN/GaN based quantum well LEDs, a relative small -3 dB modulation bandwidth of few to tens of megahertz (MHz) has been reported [137]. Recently, the development of micro-

LEDs has shown improvement in extending the modulation bandwidth to hundreds of megahertz albeit a relative low emission power (1~2 mW) [5, 138, 139]. Therefore, there 51 is a strong motivation to develop a light source with both high optical power and large modulation bandwidth, which is favorable for practical VLC systems.

Figure 22. Small signal modulation response of the violet-emitting LD at an injection current of 400 mA. The LD shows a -3 dB modulation bandwidth of ~ 3.1 GHz.

In semipolar LDs, a significant large -3 dB modulation bandwidth of > 3 GHz was measured from the 1500-µm-long violet-emitting laser diode at a driving current of 400 mA (Figure

22). The resonance peak can be observed in Figure 22 as well. Benefiting from the stimulated emission process, the LDs outperform LEDs in high-frequency response.

Therefore, the GaN-based LD is the fundamental component in nitride photonic IC for SSL and VLC applications.

2.4 Summary

In this chapter, the gain and absorption spectra of m-plane InGaN LDs both at below and above the lasing threshold were measured and analyzed. Segmented contact method was 52 utilized to measure the gain spectra, showing a comparatively higher peak modal gain of

29.2 cm-1. An increase in peak modal gain compared with reported c-plane LDs as well as the reduction in differential modal gain in LDs operating above the threshold was reported.

The optical, electrical and high-frequency characterizations of semipolar laser diodes emitting at both violet and blue color regimes have been presented and discussed. The

InGaN/GaN QW based LDs grown on (2021) plane GaN substrate exhibits a high quantum efficiency, leading to a high optical power, together with a large modulation bandwidth, enabling high-speed data transmission. Hence, the development of III-nitride

PIC at visible wavelength will base on semipolar GaN-based laser diodes.

Chapter 3: III-Nitride based Integrated Waveguide Modulator-Laser

Diode

3.1 Overview

To date, solid-state lighting (SSL), visible light communication (VLC) and optical clock generation functionalities in the blue-green color regime have been demonstrated based on discrete devices, including light-emitting diodes, laser diodes, and transverse- transmission modulators. However, the development of small-footprint, high-speed, low- power-consumption optical interconnect and communication system requires chip-level integration of photonic components in III-nitrides, which has not been investigated.

This chapter presents the design, fabrication and characterization of integrated waveguide modulator-laser diodes (IWM-LDs) at 448 nm and 405 nm. For the blue- emitting IWM-LD, a high modulation efficiency of 2.68 dB/V derived from a large extinction ratio of 9.4 dB and a low operating voltage range of 3.5 V were measured. The electroabsorption characteristics reveal that the modulation effect, as observed from the red-shifting of the absorption edge, results from the external field-induced quantum- confined-Stark-effect (QCSE). A comparative analysis of the photocurrent versus wavelength spectra in semipolar and polar plane InGaN/GaN quantum wells (QWs) was conducted, which confirms that the IWM-LD based on semipolar (2021) QWs is able to operate similarly to other III-V materials typically used in optical telecommunications due to the reduced piezoelectric field. The experimental investigation highlighted the 54 advantage of the IWM-LD implementation on the same semipolar QW epitaxy in enabling a high-efficiency platform toward SSL-VLC functionalities.

3.2 Integrated Waveguide Modulator-Laser Diode (IWM-LD) at 448 nm

InGaN/GaN quantum well (QW)-based light-emitting diodes (LEDs) and laser diodes (LDs) are fundamental components of solid-state lighting (SSL) [1-3, 140]. The eventual widespread usage of such emitters is expected to usher in prospects of functional diversification into visible light communication (VLC), which has recently attracted increasing attention [5-7]. To date, LDs have been used in the VLC systems based on direct current modulation [6, 80] for implementing signal modulation schemes, such as non-return-to-zero on-off-keying (NRZ-OOK) [44] and spectrally efficient orthogonal-frequency division multiplexing (OFDM) [141]. The advent of the SSL-VLC dual-function lamp dictates the further development of LD devices with an energy-efficient, compact device architecture to address the potential drawbacks that limit the modulation performance of the device, including the transient heating effect [104] and RC delay. In this work, we present an alternate electroabsorption (EA) modulation approach based on the integration of optical waveguide modulators with laser diodes, in which the above drawbacks can be further alleviated.

Although broadband and narrowband transverse-transmission modulators in the visible range have recently been demonstrated in novel low-dimensional materials [142] and III-nitrides, including InGaN/GaN QW [102, 103], GaN/AlGaN QW [102, 104] and GaN bulk film [104], the integration of a waveguide EA modulator with a laser diode in the blue-green regime remains 55 unexamined. Because blue emitters (λ = 440-460 nm) are widely accepted as fundamental components for white light communications [7], it is anticipated that there will be a growing demand for a blue-emitting integrated waveguide modulator-laser diode (IWM-LD) with a small- footprint, low-power consumption and multi-functionality nature. Moreover, the conventional

GaAs and InP-based near-infrared (NIR) LDs are not compatible with CMOS-based Si photodetectors (PDs) due to their long absorption length. Thus, our investigation into GaN-based

IWM-LD at 448 nm has the potential for further implementation in high-speed optical interconnects (OIs) and photonic integrated circuits (PICs) [143-145]. In order to achieve the above platform, one has to examine the polarization properties of the materials of interest.

Because of the large spontaneous and piezoelectric polarization fields in InGaN/GaN QWs grown on conventional polar c-plane sapphire substrates, the absorption edge blue-shifts with increasing negative bias [146]. As a result, the device requires a large bias to offset the internal field on the order of MV/cm [105], which makes it difficult to build a highly efficient modulator, such as those made of other III-V materials including InGaAsP [106] and InGaP [107]. In this work, we investigate the utilization of InGaN/GaN QWs on semipolar bulk GaN substrates for achieving an effective and efficient IWM-LD. This approach is advantageous due to the reduced polarization fields in semipolar (2021) InGaN/GaN QWs [147, 148].

3.2.1 Device fabrication

The InGaN/GaN MQW laser diode epi-structure was grown on semipolar (2021) substrates provided by Mitsubishi Chemical Corporation using the metal-organic chemical 56 vapor deposition (MOCVD) technique. The epitaxial structure consisted of a 2 µm Si- doped n-GaN template, a 60 nm Si-doped n-In0.05Ga0.95N separate confinement heterostructure (SCH) waveguiding layer, a 4-period undoped multiple quantum well

(MQW) active region with 3.6 nm-wide In0.2Ga0.8N QWs and 7.5 nm-wide GaN barriers, a

18 nm Mg-doped p-Al0.15Ga0.85N electron blocking layer (EBL), a 60 nm low Mg-doped p-

17 -3 In0.05Ga0.95N SCH waveguiding layer ([Mg]=7.5 × 10 cm ), a 400 nm standard Mg-doped p-GaN cladding layer ([Mg]=1.5 × 1018 cm-3), and a 12 nm highly Mg-doped p-GaN contact layer ([Mg]=1 × 1020 cm-3).

Figure 23. Schematic structure of an InGaN/GaN based IWM-LD.

The studied multi-section EA modulator-integrated laser diode is a three-terminal device consisting of a reverse-biased waveguide modulator section and a forward-biased gain section (Figure 23). The RWG multi-section laser diode was defined using UV photolithography and inductively coupled plasma (ICP) etching. The isolation trench between the IM region and gain region is 10 µm wide and was etched into the InGaN 57 cladding layer. The etching of the isolation trench is carefully controlled; the metal contact layer and highly doped GaN layer were removed, providing good electrical isolation and maintaining optical coupling. Facets were dry etched along the a-direction without dielectric coating. Pd/Au and Ti/Al/Ti/Au metallization layers were deposited using sputter as p- and n-electrodes, respectively.

3.2.2 Electrical characterization of IWM-LD at 448 nm

Figure 24. Intensity vs. wavelength showing the lasing spectrum with a peak emission wavelength of 448 nm and a full-width half-max (FWHM) of 0.8 nm.

The IWM-LD is made of a 2-µm-wide ridge waveguide to achieve single-mode lasing at

448 nm (Figure 24). Each fabricated device consists of a 200-µm-long integrated modulator (IM) section and a 1.29 mm long gain section. Both sections share the same

QW active region layer structure and are optically coupled, allowing the emitted beam from the gain section to be modulated by the IM. It is noted that in InGaAs-InGaAsP based devices, additional bandgap change induced by annealing is required to achieve multiple 58 sections with different transition energy levels to reduce absorption loss [149]. In the group-III nitride, however, the modulator can be built utilizing the inherent polarization field, where the extra fabrication and processing is not required.

Figure 25. SEM top down view of the fabricated multi-section laser diode with an integrated modulator.

Figure 25 shows the scanning electron microscopy (SEM) image of the fabricated IWM-

LD. Owing to the high lateral resistance of the InGaN waveguiding layer and AlGaN electron blocking layer (EBL), the IM section and gain section were electrically separated, enabling the independent operation of the two sections. The isolation resistance between the two sections was 1.2 MΩ, which is five orders of magnitude higher than the series resistance for the LD.

Figure 26. Output power vs. injection current (L-I) relations of IWM-LD with varying bias voltage applied to the IM section. The light output power at ILD = 500 mA is indicated. 59

Figure 26 shows the optical output power (L) vs. injection current in the IWM-LD gain region (ILD) with various reverse biases applied to the waveguide modulator (VIM from 0 V to -4 V). Without any modulation bias (VIM = 0 V), the IWM-LD exhibits a threshold current

(Ith) of 435 mA. Further increase in injection current to 500 mA at the gain region (ILD =

1.15 Ith) resulted in an optical output power of 15.9 mW under DC operation. The operating current was limited to 600 mA as the thermal roll-over became significant when

ILD > 600 mA. With increasing |VIM| applied to the IM region, decreasing optical power and increasing lasing threshold current were observed in the IWM-LD device due to increasing loss. For example, the optical power of IWM-LD at ILD = 500 mA was reduced to 9.3, 4.6, and 1.8 mW at VIM = -2, -3, and -3.5 V, respectively.

The strong VIM-dependence in optical power is evident in Figure 27, indicating the amplitude modulation effect. The modulation bias applied to the IM section controls the 60 light output by switching the absorption from a low value to a high value. With a constant driving current (ILD) of 500 mA in the gain region, the lasing was suppressed at VIM = -3.5

V, which represented the Off state. The IWM-LD has its maximum emission power at VIM

= 0 V, which is the On state. The modulation effect can be clearly observed from the emission spot in the captured photograph (see insets in Figure 27). At ILD = 500 mA, the

IWM-LD exhibited a high extinction ratio (Ron/off = PON/POFF) of 8.8 (9.4 dB) with a relatively small bias of 0 / -3.5 V, compared to approximately 7 V required in c-plane modulators

[105]. Our device exhibits a high modulation efficiency (Ron/off/ΔV) of 2.68 dB/V, which is more than twice the value of that in c-plane modulators (approximately 1.11 dB/V) [105].

3.2.3 Electroabsorption and photocurrent characteristics of the waveguide modulator

Figure 28.Measured change in absorption versus wavelength under applied reverse bias (VIM) from -1 V to -6 V with respect to the absorption at zero bias. 61

To study the EA modulation effect in the IWM-LD, the changes in absorption (Δα) of semipolar InGaN/GaN QWs were obtained from the electrotransmission measurements with different modulation bias voltages applied to the QW. The spectra of absorption changes shown in Figure 28 were derived based on the transmission spectrum at zero modulation bias, according to the relationship:

1 ∆훼 = − 푙푛(푃 /푃 ) (4) 푑 푉퐼푀 0푉 where d is the total thickness of the InGaN QW layers. 푃푉퐼푀 refers to the transmitted optical power when modulation bias is applied to the device and 푃0푉 refers to the transmitted optical power at zero modulation bias. The external field-induced absorption changes occur for photon energies near the transition energy of QWs within the space charge region of the p-i-n junction. Because the active layer for our device consists of

InGaN QWs embedded within GaN barriers, the EA signature for the InGaN layers, with a total thickness (d) of 14.4 nm, can be observed separately at 448 nm.

The absorption of the InGaN QWs can be strongly modulated around the lasing wavelength by the applied external field. With increasing modulation bias |VIM| from -1

V to -6 V, a broadening and redshift of the absorption edge can be identified in a semipolar

(2021) InGaN/GaN waveguide modulator. This trend, in which the increasing modulation bias leads to a growing internal field within the entire space charge region, is similar to the behavior of AlGaAs/GaAs-based heterostructures, which do not present an internal piezoelectric field [150]. With increasing |VIM|, the peak in the absorption change spectrum shifts from 441 nm (VIM = -1 V) to 446 nm (VIM = -6 V) and the absorption change 62

-1 -1 at the lasing wavelength (λ = 448 nm) increases from 200 cm (VIM = -1 V) to 3200 cm

(VIM = -6 V). The monotonic increase in the absorption change and red-shifting of the absorption edge are attributed to the external field-induced quantum-confined-Stark- effect (QCSE), which is utilized to modulate the absorption in IWM-LDs, as shown in Figure

26.

Figure 29.Photocurrent (PC) measurements. PC spectra of (a) semipolar (2021) and (b) c-plane (0001) InGaN/GaN quantum well modulators with an applied DC bias of -4 V to 0 V.

The photocurrent measurements were performed to further investigate the external field-dependent optical absorption response in semipolar (2021) InGaN/GaN QWs.

Figure 29a illustrates the photocurrent spectrum collected around the absorption edge of the device at room temperature. For comparison purposes, we also measured the photocurrent spectrum from c-plane InGaN/GaN QWs with a similar transition energy, shown in Figure 29b. As expected, the polar QW exhibits a monotonic blue-shifting absorption edge with an increasing applied electric field due to the reversed QCSE with

VIM from 0 V to -4 V. With regard to the semipolar QW, similar blue-shifting was observed when a small negative bias was applied (0 V to -1 V). Interestingly, the absorption edge of 63 the semipolar QW shows a red-shifting trend when an increasing negative bias (> 2 V) is applied. The red-shifting clearly indicates the occurrence of a QCSE-induced redshift in the absorption edge. The change is due to the applied external field on the IM canceling the built-in polarization-induced electric fields in the active region and thus manifesting itself as in conventional GaAs-based materials. Due to a reduced piezoelectric field in semipolar QWs, the significant shifting of absorption edges in the IM region in response to modulation bias is effective in modulating the optical output power of the IWM-LD.

In c-plane InGaN/GaN QWs, there exists a strong piezoelectric field (approximately 3.1

MV/cm in an In0.2Ga0.8N layer) [151] due to the large total polarization discontinuity (as

2 high as 0.03 C/m in an In0.2Ga0.8N layer) [152]. Moreover, the directions of the piezoelectric field and the p-n junction built-in field in c-plane InGaN/GaN QWs are opposite. As a result, when a reverse modulation bias is applied to the QW grown on c- plane GaN, it will compensate the piezoelectric field in the InGaN QWs before introducing a net electric field in the direction of the built-in field of the QWs. Therefore, the applied modulation bias will first reverse the piezoelectric field-induced QCSE effect, leading to a blue-shifting and narrowing of the absorption edge. Only when the applied modulation bias-induced external field exceeds the piezoelectric field can the effect of broadening and red-shifting of the absorption edge be achieved. For typical 450 nm-emitting InGaN

QWs containing 4× 3.6-nm-thick In0.2Ga0.8N layers, an additional bias voltage of approximately 4.5 V is required to create an external field to compensate for the piezoelectric field. Consequently, a high modulation voltage is expected in blue-green 64 emitting IWM-LDs based on c-plane InGaN/GaN QWs, resulting in low modulation efficiency and high power consumption. The modulation voltage required for the semipolar (2021) InGaN/GaN QW-based IWM-LD is considerably smaller due to the significantly reduced piezoelectric field compared to the QWs grown on a polar c-plane.

For example, a major change in absorption at the lasing wavelength, as large as 1800 cm-

1, was obtained with a small modulation bias of -4 V according to the measurement shown in Figure 28.

3.2.4 High speed modulation of IWM-LD at 448 nm

Figure 30. Small signal modulation of IWM-LD under injection current of 500 mA and IM bias of - 3.5V shows ~ 1GHz -3 dB bandwidth.

For a proof-of-concept demonstration of AC modulation using the proposed IWM-LD scheme, we performed a small-signal modulation measurement by applying a -10 dBm

AC signal to the integrated modulator while pumping the gain region with a constant 65 driving current (500 mA). Figure 30 illustrates the frequency response of the tested IWM-

LD, in which a -3 dB bandwidth of approximately 1 GHz was measured with |VIM| = 3.5 V.

The maximum bandwidth of the IWM-LD is expected to exceed 1 GHz due to the 1 GHz bandwidth limitation of the photodetector used in the measurement setup. Nevertheless, our demonstration proves the feasibility of using IWM-LD for data transmission.

3.3 Integrated Waveguide Modulator-Laser Diode at 404 nm

InGaN-based violet-emitting laser diodes have attracted increasing research attention due to its application for high color rendering index (CRI) white lighting system [3], high- speed visible light communication [39] as well as avoiding the health-related concerns associated with blue laser diodes [153]. Compared to the direct modulation of LDs, the modulation utilizing integrated electroabsorption modulator (EAM) is advantageous due to low power consumption and suppression of transient heating [105, 154].

This section presents the waveguide EAM- integrated LD, i.e. the integrated waveguide modulator-laser diode, emitting at 404 nm, where the EAM and LD are fabricated using the same InGaN/GaN quantum well structure, grown on semipolar (202̅1̅) plane GaN substrate. A large extinction ratio of 11.3 dB is measured in our device with a modulation voltage as low as -2.5 V to achieve On/Off switching, which are the best results achieved in III-nitride system to the best of our knowledge. This is attributed to the small polarization field in the device grown on semipolar substrates. By modulating the EAM- integrated LD, a -3 dB bandwidth of ~1GHz, which is limited by the photodetector, and 66 data rate of 1.7 Gbps is measured, suggesting it to be a novel platform for visible light communications.

3.3.1 Experimental details

Figure 31. Schematic of the EAM-integrated LD device structure at 404 nm.

The schematic of the EAM-integrated LD fabricated is shown in Figure 31, which is a three- terminal device consisting of 7.5-µm-wide RWG. The 100-µm-long integrated modulator and the 1390-µm-long gain section were optically coupled but electrically isolated (153 kΩ resistance was measured between the two sections). The device was tested using

Keithley 2520 diode laser tester and Keithley 2400 source-measure unit with calibrated Si photodetector (PD). The spectrum was measured using Ocean Optics HR4000 spectrometer. For bandwidth measurement and data transmission using on-off keying

(OOK) modulation scheme, an Agilent E8361C network analyzer, an Agilent N4903B J-

BERT, an Agilent DCA-86100C digital communication analyzer, and a Menlo Systems APD-

210 Si avalanche PD (APD) were used in the setup. 67

Figure 32. The emission spectrum of the violet-emitting integrated waveguide modulator-laser diode (IWM-LD).

At 700-mA injection current in the gain section, the violet-emitting integrated waveguide modulator-laser diode lased at 404 nm as shown in Figure 32.

3.3.2 Characterization of electroabsorption modulation effect

Figure 33. Optical power vs. injection current (ILD) of the EAM-integrated LD at different modulation bias (VIM). Inset: current vs. voltage of the device. 68

Figure 33 shows the optical power vs. injected current in the gain section (L-I) and the characteristics of the EAM-integrated LD under the applied bias from 0 V to -3 V. The current vs. voltage (I-V) relation can be found in the inset of Figure 33. Without modulation bias (VIM=0), the device shows a threshold current (Ith) of 590 mA, and an optical power of 5.2 mW at 650 mA in the gain region (ILD=1.1×Ith). With increasing |VIM|, a decreasing optical power and increasing Ith were achieved owing to the quantum- confined Stark effect induced electroabsorption effect [154].

Figure 34. Plot pf optical power at 650 mA vs. VIM. Inset: photos of the device emissions at ON and OFF states.

The optical power of the EAM-integrated LD was 5.2 mW, 2.7 mW, 1.3 mW, 0.56 mW and

0.38 mW at VIM = 0V, -1 V, -1.5 V, -2 V, -2.5 V, respectively, as shown in Figure 34, suggesting the strong VIM-dependence in optical power. The lasing action was suppressed at |VIM| > 2 V, representing the off state at ILD = 650 mA. The maximum optical power was 69 at |VIM| = 0 V, which is the on state. A large extinction ratio (Ron/off=PON/POFF) of 13.53 (~

11.3 dB), and relatively small modulation bias of 0V/-2.5V were measured in our EAM- integrated LD, where a high modulation efficiency of 4.5 dB/V was derived. This is significantly higher than that of the c-plane modulator (~ 1.1 dB/V).

3.3.3 High-speed performance of the IWM-LD

Figure 35. Small signal frequency response of the violet-emitting integrated waveguide modulator-laser diode under ILD of 650 mA and VIM of -1V.

To demonstrate the utilization of the integrated waveguide modulator-laser diode for visible light communications, the small signal modulation measurement was performed by applying -10 dBm AC signal to the EAM while the gain section was driven at constant

ILD of 650 mA). A -3dB bandwidth of ~ 1GHz was measured as shown in Figure 35 with VIM

= -1 V. The frequency response is limited by the 1-GHz bandwidth of the APD.

Nonetheless, the feasibility of utilizing EAM-integrated LD for data transmission has been confirmed. 70

Figure 36. Eye diagrams of (a) 1 Gbps data rate, and (b) 1.7 Gbps data rate using OOK modulation.

Subsequently, the violet-emitting integrated waveguide modulator-laser diode was used to transmit a pseudorandom binary sequence (PRBS 210-1) data stream using OOK modulation scheme. A clear open-eye diagram at 1-Gbps data rate [Figure 36(a)] was observed, and a relatively low bit-error rate (BER) of 1.1×10-6 was measured. A high data rate of 1.7 Gbps using the EAM-integrated LD was achieved [Figure 36(b)] with BER of

3.1×10-3, still passing the forward error correction (FEC) limit (3.8×10-3). The data rate 71 could be further improved by further optimizing the system, and employing complex modulation scheme, such as orthogonal frequency-division multiplexing.

3.4 Summary

In summary, the monolithic integration of an electroabsorption waveguide modulator with a laser diode and measured the DC and AC modulation characteristics of the device, which is grown on a (2021) plane GaN substrate was demonstrated. By alternating the modulation voltages at -3.5 V and 0 V, the laser output power was tuned from 1.8 to 15.9 mW, respectively, leading to an On/Off ratio of 9.4 dB. The results clearly demonstrated that modulation could be achieved using a semipolar EA-modulator that consumes less power compared to c-plane devices.

The violet-emitting the integrated waveguide modulator-laser diode showing large extinction ratio of 11.3 dB and high modulation efficiency of 4.5 dB/V was demonstrated.

The device has a -3 dB bandwidth of ~ 1 GHz, enabling 1.7 Gbps data rate using OOK modulation scheme. The results show that the integrated waveguide modulator-laser diode on semipolar plane GaN substrates are favorable for low power consumption visible light communications. The presented work in this chapter discloses the physical mechanism of QCSE-induced absorption change, leading to the promising deployment of the blue-green-red electromagnetic spectrum for utilization in compact SSL-VLC devices.

Chapter 4: III-Nitride Superluminance Diode

4.1 Overview

The development of InGaN/GaN quantum well (QW) based violet-blue-green light- emitting diodes (LEDs) [55, 64, 65, 152, 155, 156] and laser diodes (LDs) [76, 157-159] enables efficient solid-state lighting (SSL) technology for a wider range of applications, such as general illumination, automotive lighting, display and horticulture [2, 26]. Besides, the utilization of III-nitride LEDs and LDs as the transmitter for free-space visible light communications (VLC) and underwater wireless optical communications (UWOC) has been demonstrated and investigated recently [41, 42, 46, 160]. Compared to the conventional radio frequency (RF) communication techniques, VLC shows many potential advantages, such as unregulated channels, avoiding electromagnetic interference (EMI), high security and low-cost [41, 42]. Hence, by combining GaN-based LEDs or LDs with phosphors, the white light bulb can be achieved for SSL and VLC functionalities [2, 42].

Though high-efficient GaN-based LEDs and LDs have been studied for high-power and high-speed SSL-VLC applications, there are performance-limiting concerns to be resolved.

For example, InGaN/GaN QW LEDs suffers from the “efficiency droop” effect, resulting in a rapidly reduced efficiency at high current injections [64, 65]. Also, a relatively small modulation bandwidth of LEDs limits the data communication rate in LED-based VLC links.

As for LDs, the efficiency droop is not presented, and a much higher modulation bandwidth is measured. However, the LD-based light bulb is associated with speckle noise 73 and safety concerns [161]. The GaN superluminescent diodes (SLDs), which combine the advantages of both LEDs and LDs, have shown great potentials for SSL-VLC applications

[162].

As a time incoherent but a spatial coherent light source, SLDs, which operate in the amplified spontaneous emission (ASE) regime, have been used for a number of attractive applications, such as optical coherence tomography (OCT), fiber optic gyroscope (FOG), and fiber-optic sensor [162-164].

Table 1. Summary of advances of InGaN-based SLDs.

Emission Substrate Configuration Waveguide Optical power Referenc wavelength material design e Semipolar 405 nm tilted facet 4 µm ridge 20 mW (cw) [165] GaN 0.65 mW 405 nm c-GaN tilted waveguide 3 µm ridge [166] (pulse) 405 nm c-GaN passive absorber 3/10 µm ridge 25 mW [167] 405 nm c-GaN tilted waveguide 3/10 µm ridge 125 mW [167] 405 nm c-GaN “j-shape” waveguide curved ridge 350 mW (cw) [168] 408 nm c-GaN “j-shape” waveguide “j-shape” ridge 200 mW (cw) [169] 410 ~ 445 30 ~ 55 mW c-GaN tilted waveguide 2 µm ridge [170] nm (cw) 2 mW (cw) 420 nm c-GaN tilted facet 2 µm ridge [171] 100 mW (pulse) “j-shape” waveguide 420 nm c-GaN 3 µm ridge 200 mW (cw) [172] AR/HR coating 439 nm m-GaN facet roughening 4 µm ridge 5 mW (pulse) [173] 443 nm c-GaN curved waveguide 2 µm ridge 100 mW (cw) [174] 445 nm c-GaN oblique facet 5 µm ridge - [175] Semipolar 447 nm passive absorber 7.5 µm ridge 256 mW (cw) [161] GaN 500 nm c-GaN curved waveguide 2 µm ridge 4 mW (pulse) [176]

More recently, the violet-blue emitting InGaN/GaN QW based superluminescent diodes have been reported, which are the essential components for SSL system. Table 1 74 summarizes the design and performance of the demonstrated GaN-based SLDs, comparing the emission wavelength, substrate material, configuration, waveguide design, and the maximum light output power reported in each case.

Since most of InGaN/GaN QW SLDs are grown on a polar, c-plane GaN substrate, there is a growing interest to develop high efficient violet-blue SLDs on nonpolar or semipolar substrates owing to a reduced polarization field presented in the QW structure [152].

Studies on semipolar and nonpolar GaN-based LEDs and LDs have revealed that the enhanced electron and hole wavefunction overlap is expected for InGaN/GaN QWs grown on nonpolar (m-plane) and semipolar GaN substrates, leading to an enhanced internal quantum efficiency [152, 156]. Therefore, the presented study focuses on SLDs grown on semipolar GaN substrate.

In this chapter, the electrical and spectral characteristics of the semipolar InGaN/GaN QW based SLDs will be presented and discussed. The SLD is utilized to generate white light by exciting a commercially available YAG:Ce phosphor for SSL applications. The emission properties were discussed by comparing the performance with that of the LED and LD based white light. Finally, the utilization of SLD as a high-speed transmitter for VLC system is demonstrated. 75

4.2 GaN-based Superluminance Diode with Integrated Absorber

Configuration at 450 nm

The studied blue-emitting SLD was fabricated using integrated passive absorber configuration. The 446-nm emitting SLD having a 490-µm absorber region and 1000-µm gain region showed a broad spectral linewidth of 8.4 nm at an injection current of 500 mA

(6.67 kA/cm2). The SLD generated > 200 mW optical power in the amplified spontaneous emission (ASE) regime and the phosphor-converted white light exhibited a color rendering index of 64.4 and a color temperature of 4094 K.

4.2.1 Structure and device fabrication

Figure 37. 3D schematic of the demonstrated InGaN/GaN QW SLD on semipolar GaN substrate. The length of the SLD gain region and the passive absorber are 700 µm and 490 µm, respectively.

The SLD is constructed using an integrated passive absorber configuration, so the oscillations of light in the resonating cavity structure are suppressed to avoid lasing. 76

Figure 37 shows the 3D schematic of the device structure. The epitaxial structure of demonstrated SLD structure was grown on a semipolar (2021) bulk GaN substrate from

Mitsubishi Chemical Corporation using metal-organic chemical vapor deposition

(MOCVD) technique. It consisted of a pair of GaN contact layers, a pair of GaN cladding layers, a pair of InGaN separate confinement heterostructure (SCH) waveguiding layers, an AlGaN electron blocking layer (EBL), and a 4-period of In0.2Ga0.8N multiple QW active region. The detailed layer structure is similar to that in other reports [159, 161]. The device has a 700-µm-long gain region and a 490-µm-long passive absorber. The SLD is driven by injecting current into the gain region without biasing the absorber.

4.2.2 Electro-optical properties of 450 nm emitting SLDs

Figure 38. Photo of the amplified spontaneous emission from the blue-emitting SLD with integrated absorber configuration under DC injection.

The SLD is tested in a customer made prober at room temperature. The setup involves a

Keithley 2520 diode tester with a calibrated Si photodetector and an integrating sphere 77 for electrical characterization. The emission spectra were collected using Ocean Optics

HR 4000 high-resolution spectrometer. Figure 38 shows the optical image of the amplified spontaneous emission of the SLD. All the measurements were carried out under DC current injection.

Figure 39 presents the optical power – injection current – voltage (L-I-V) characteristic of the SLD. The photodetector was placed both at the facet, in-plane to the waveguide, and above the device, normal to the waveguide, in order to measure the optical power of the edge emission and the surface emission, respectively. The former approach measured the amplified spontaneous emission together with the spontaneous emission (ASE+SE) of the 78

SLD while the latter approach measured only the uncoupled, spontaneous emission (SE) from the SLD. By comparing the two curves, a significant divergence of the intensities of

ASE+SE and SE is observed at an injection current (ISLD) of approximately 250 mA, which is the onset of superluminescence. The SLD exhibits a superlinear increase in the intensity at ISLD > 250 mA, which is different from the behavior in LDs, where a linear increase of optical power above the threshold condition is expected. The corresponding forward voltage at ISLD = 250 mA is 6 V. The optical powers of the SLD are 78.0 mW, 122.6 mW, and 202.5 mW at ISLD = 400 mA, 500 mA and 550 mA, respectively. The high optical power is partially attributed to the improved optical gain of the SLD, originating from a high material gain in InGaN/GaN QWs grown on semipolar GaN substrate.

Figure 40. Emission spectra collected from the edge of the blue-emitting SLD under different injection currents. 79

The emission spectra of the SLD collected at the edge of the device at an increasing ISLD from 100 mA to 550 mA are plotted in Figure 40. The SLD has a peak emission wavelength of ~ 450 nm. With increasing ISLD, a narrowing of the emission peak, as well as a slight blueshift of the peak, was observed.

Figure 41. Emission peak full-width at half-maximum (FWHM) and wavelength as a function of injection current.

The reducing of full-width at half-maximum (FWHM) of the emission peak from 17.3 nm,

11.7 nm, 7.6 nm to 4.0 nm at an increasing ISLD from 100 mA, 300 mA, 500 mA to 550 mA is evidenced in Figure 41. Our SLD shows a large spectral linewidth at 500mA, with EL peak

FWHM of > 7 nm and optical power > 120 mW, which is an advantage for micro-projection and SSL applications. Since the peak FWHM of an InGaN/GaN QW based LD at ~ 450 nm is typically < 1 nm, the SLD is confirmed to be operating at ASE regime at an ISLD up to 550 mA. A blueshift of the emission peak from 451 nm to 446 nm at an increasing ISLD from 80

100 mA to 550 mA is also measured. This blue shift is attributed to the band filling effect in the InGaN/GaN QW SLD.

4.2.3 White light generated using SLDs

Following the electrical characterization of the SLD, we investigated the utilization of SLD for the white light generation in solid-state lighting applications by using the blue InGaN

SLD to excite a YAG:Ce3+ phosphor embedded in a silicone rubber. Figure 42 shows the schematic of the setup.

Figure 42. Schematic of the SLD-based, phosphor converted white light source.

The spectrum and white light characteristics of the generated white light were measured and analyzed using a GL Spectis 5.0 Touch spectrometer. Figure 43 exhibits the white light spectra generated by the SLD and the comparison with that generated by a blue-emitting

LED and LD. It can be observed that both LED, SLD, and LD are effective in exciting the yellow YAG phosphor. The generated white light is a combination of the InGaN

LED/SLD/LD emitted blue light and the phosphor converted yellow light. 81

Figure 43. Spectra of the white light generated using blue-emitting LED, SLD, and LD with yellow YAG phosphor.

The color rendering index (CRI) and correlated color temperature (CCT) are important parameters to evaluate the quality of white light. The CRI is a quantitative measure of the ability of a light source to accurately render all visible wavelength in the color spectrum when compared to perfect reference light, such as the sunlight. The CCT is a specification of the color appearance of the light emitted by a light source, relating its color to the color of light from a reference source when heated to a particular temperature, which is measured in degrees Kelvin (K). Besides, the CIE 1931 color space is a widely used method to quantitatively define the color of emitted light and its link with perceived colors in human vision. Hence, the x-y coordinates in the CIE 1931 space chromaticity diagram are of interest in the study the emission property of a light source. The CRI, CCT and CIE 1931 coordinates of the three white light scenarios are summarized in Table 2.

Table 2. Comparison of CRI, CCT and CIE 1931 coordinates of white light generated based on a blue LED, SLD, and LD.

CIE 1931 CRI CCT coordinates

LED based white light 69.3 4620 K (0.3594, 0.3775)

SLD based white light 68.9 4340 K (0.3711, 0.3895)

LD based white light 64.7 4243 K (0.3779, 0.4038)

Figure 44. The chromaticity coordinates of the white light generated using blue-emitting LED, SLD, and LD with yellow YAG phosphor.

The corresponding chromaticity diagram (CIE 1931) coordinates of the three white light scenarios are shown in Figure 44. All three scenarios generate cool white light and the 83

SLD based white light shows an enhanced CRI compared to LD based white light. Both LED and SLD based white light exhibits a similar CRI. The CRI can be further improved by engineering the phosphor mixture, such as combining green and red phosphors, and introducing a broadband phosphor material. However, nonetheless, the results suggest that GaN-based SLDs outperforms LDs in generating high-quality white light using the same phosphor.

4.2.4 Small signal modulation of InGaN-based SLDs

Figure 45. Schematic of the small signal modulation response measurement setup for blue- emitting SLDs.

To study the frequency response properties of the SLD, we measured the small signal modulation under CW operation using the setup featured in Figure 45. The -10 dBm modulation signal was generated using an Agilent E8361C PNA network analyzer and a

Keithley 2400 source meter was used as the DC power supply. The setup also involves a

Picosecond Pulse Labs 5543 bias tee and a Menlo Systems APD 210 high-speed Si avalanche photodetector with a bandwidth of 1 GHz. 84

Figure 46. Measured frequency response of the SLD. A -3 dB bandwidth of 560 MHz is observed at 600 mA.

The measured frequency response of the SLD is shown in Figure 46. The SLD exhibits a -3 dB bandwidth of 430 MHz, 490 MHz, and 560 MHz at a DC driving current of 400 mA, 500 mA, and 600 mA, respectively. The SLD shows a significantly higher modulation bandwidth than LEDs [177], which is attributed to the fast stimulated recombination of carriers due to the amplified spontaneous emission process [178].

The > 0.5GHz (560 MHz) modulation bandwidth measured in this SLD indicates that a ~

Gbit/s data communication rate is expected using InGaN-based SLDs and towards ~10

Gbit/s or beyond by employing spectrally efficient modulation techniques, such as OFDM

[138]. Overall, the modulation performance of this device demonstrates the feasibility of using SLD for data transmission. 85

4.3 Violet-emitting GaN-based Superluminance Diode with Tilted Facet

Configuration at 405 nm

InGaN-based violet-blue light-emitting diodes (LEDs) has been widely used as the fundamental component for solid-state lighting (SSL) due to its advantages such as high efficiency, long lifetime, reduced heat generation, and fast turn-on [2, 26, 64, 147]. In addition to the illumination application, the LEDs were recently demonstrated as transmitters in visible-light communication (VLC) system, advantages of which include license-free spectrum range, high security and free from electromagnetic interference [5,

179, 180].

The modulation bandwidth of conventional III-nitride LEDs, however, is limited to 10 ~

100 MHz owing to a relative large RC delay and a relative long carrier lifetime, which is originating from the spontaneous emission (SE) process [137]. Recently, micrometer- sized LED pixels were demonstrated to achieve a high modulation bandwidth of 225 MHz

[177] and 462 MHz [5], albeit with a relatively low optical power of 1 ~ 2 mW. Although

GaN-based laser diode (LD) has shown a modulation bandwidth of ~ 2.6 GHz [181] and a data rate of ~ 2 Gbps for white light communication [39], further development is required to address the limited etendue, speckle noise, and eye-safety related concerns [81, 182].

The superluminescent diode (SLD) has lately been studied as a high-brightness, speckle- free light source for SSL, combining the advantages of both LEDs and LDs [164, 170, 171,

183]. However, there is a limited investigation on the high-speed operation of SLDs for

VLC application. As white light with high color rendering index (CRI) can be achieved using 86 violet LEDs pumped phosphors [2], the investigation into violet-emitting SLD constitutes an important research topic.

This section presents a high-speed, absorber-free InGaN-based SLD on semipolar (2021)

GaN substrate, emitting at 405 nm. The 405-nm emitting SLD having a 590-µm long tilted facet configuration showed a spectral linewidth of 9 nm at 400 mA (16.9 kA/cm2). The SLD exhibits > 800 MHz modulation bandwidth with an optical power of > 20 mW, benefiting from the small RC delay, the short lifetime associated with the amplified spontaneous emission (ASE) process, and the reduced polarization field in semipolar quantum-wells

(QWs) [152]. A data rate of 1.3 Gbps was achieved for SLD transmitter with bit-error rate

(BER) of 2.9×10-3 The findings are significant in presenting the violet-emitting SLD as a promising light source for SSL-VLC dual-functionalities.

4.3.1 Electro-optical properties of the violet-emitting SLD

The SLD was grown on a semipolar (2021) bulk GaN substrate using metal-organic chemical vapor deposition (MOCVD). It consists of 4 periods of In0.1Ga0.9N/GaN multi- quantum-wells (MQWs) sandwiched between InGaN separate confinement heterostructure (SCH) waveguiding layers, low doped GaN cladding layers, highly doped

GaN contact layers (Figure 47). An Al0.18Ga0.82N electron blocking layer (EBL) was also included in the epitaxial structure.

Figure 48. 3D illustration of the 405-nm emitting superluminescent diode on semipolar GaN substrate with tilted facet configuration. The length of the device is 590 µm.

The 4-µm RWG SLD has 45ᵒ tilted facet on one-side to suppress feedback oscillation, without the need of using an integrated absorber (Figure 48). The Pd/Au and Ti/Al/Ni/Au stacks are deposited as the p- and n-contacts, respectively, for ground-signal (GS) probing.

The ridge waveguide structure and device mesa were defined using UV photolithography and plasma etching. The self-aligned SiO2 layer was sputtered for passivation of ridge sidewalls. 88

The fabricated SLD was tested using a radio frequency (RF) prober with a Picoprobe

Model-10 microwave probe. For emission spectra measurement, the device was driven using Keithley 2400 source meter and the spectra were collected using Ocean Optics

HR4000 spectrometer. The optical power–current (L-I) measurement was performed using Keithley 2520 diode-laser test system with calibrated integrating sphere from

Labsphere. The current–voltage (I-V) and capacitance–voltage (C-V) measurements were carried out using Keithley 4200 semiconductor characterization system with 4225-RPM remote amplifier/switch module. All the measurements were performed under continuous wave (CW) operation.

Figure 49. Electroluminescence (EL) spectra of the 405 nm SLD under current injection of 50 mA - 400 mA. 89

The spectral characteristics of the SLD and its comparison with that of a LED and an LD are important characteristics for an SLD. The electroluminescence (EL) emission spectra with injection current from 50 mA to 400 mA at room temperature, collected from the tilted facet, is shown in Figure 49.

Figure 50. Comparison of EL spectra from an LD, an SLD, and a LED at 400 mA.

Figure 51. FWHM and peak wavelength of the SLD as a function of injection current at room temperature. 90

At 400 mA, the emission spectrum from the SLD is compared with that of a LED and an

LD, as shown in Figure 50. The full-width at half-maximum (FWHM), and the peak wavelength of SLD as a function of injecting current are summarized in Figure 51. The violet-emitting SLD has a peak wavelength of ~405 nm. A slight red shift is observed with increasing injection current, likely due to junction heating. The SLD has a peak FWHM of

~16 nm at 50 mA and 100 mA injection current, which is comparable to that of the spontaneous emission from conventional violet LEDs. With increasing injection current of

>100 mA, the peak FWHM of the device reduces from ~16 nm (at 100 mA) to ~9 nm (at

400 mA), indicating the onset of amplified spontaneous emission at ~ 100 mA. For the

LED, SLD and LD with a similar epitaxial design operating, the emissions at 400 mA exhibit an FWHM of 16.3 nm, 9 nm, and 2 nm, respectively. This further confirms the achievement of ASE mode in the 405-nm SLD.

Figure 52. Plot of the optical power vs. injection current from the top emission (SE) and edge emission (ASE+SE) of the SLD. Inset: Photo of the edge-emitting SLD operating at 300 mA. 91

The optical power versus current (L-I) relation of the SLD is shown in Figure 52. The photodetector was placed either at the edge, or on top of the SLD device to measure the coupled ASE-SE components, or SE component alone, respectively. As expected, the superluminescence is observed at ~ 100 mA and beyond, which is consistent with the spectral characteristics discussed previously. The SLD has an optical power of 20.5 mW at

400 mA, which can be further improved using a heat sink, and with proper packaging.

Figure 53. Voltage vs. current relation of the SLD. Inset: Capacitance vs. voltage characteristics of the SLD.

Figure 53 shows the measured current versus voltage (I-V), and capacitance versus voltage

(C-V) characteristics of the SLD. The C-V measurement was carried out at 1 MHz. Our device exhibits a turn-on ~ 3 V, a series resistance of 5.9 Ω, and a capacitance of ~ 35 pF at -4V. As a result, the RC time-constant for the SLD is ~0.2 ns, corresponding to the RC- limited bandwidth of ~800 MHz. 92

4.3.2 High-frequency performance of the 405 nm SLD

The frequency response was measured using an Agilent E8361C PNA network analyzer, a

Picosecond 5543 bias tee, and a Menlo Systems APD 210 Si avalanche photodetector. The same setup was used for on-off keying (OOK) data transmission, involving an Agilent

N4903B J-BERT and an Agilent DCA-86100C digital communication analyzer.

Figure 54. Modulation response from the violet-emitting SLD at different current.

The frequency response of the SLD operating at different current is presented in Figure

54. The 3-dB bandwidth is measured to be 333 MHz, 616 MHz, 784 MHz, and 807 MHz at an injection current of 100 mA, 200 mA, 300 mA, and 400 mA, respectively. 93

Figure 55. -3 dB bandwidth vs. current of the 405-nm emitting SLD.

The 3-dB bandwidth as a function of the injection current is plotted in Figure 55. It suggests that the bandwidth of SLD is limited by the RC delay, rather than the carrier lifetime, at the driving current above 300 mA. The SLD shows a considerably high bandwidth compared to conventional LEDs, which is attributed to the rapid recombination of carriers during the ASE process. The following rate equation describes the alternating current (AC) operation of a SLD [178]:

푑푛 퐽 푛 = − − 푅푎푠푒 (5) 푑푡 푞푑 휏푛 where n is the carrier density, J is the current density, 휏푛 is the carrier lifetime for SE process, q is the elementary charge, d is the thickness of the active layer. The first term on the right-hand side of equation (5) describes the generation of carriers by current injection. The second term presents the carriers consumed by radiative recombination 94 during the SE process, which is similar to those in LEDs. The last term features the carriers consumed via ASE process in SLDs, leading to a reduced effective carrier lifetime, 휏푒.

The 3 dB bandwidth can then be expressed using the power transfer function as

√3 푓3푑퐵 = (6) 2휋휏푒

Therefore, the effective carrier lifetime (휏푒 ) at 300 mA for the SLD demonstrated is extracted to be ~ 0.35 ns, and would be even smaller at a higher current. This value is shorter than the minority carrier lifetime of 0.6 ~ 6 ns in micro-LED pixels [5, 184].

The SLD exhibits a much higher modulation bandwidth of > 500 MHz [161], suggesting

SLD as a promising transmitter in VLC systems.

4.3.3 SLD as the transmitter for VLC applications

To evaluate the violet-emitting SLD based VLC link, the SLD was utilized as a transmitter for optical communication using on-off keying (OOK) modulation scheme. A 210-1 pseudorandom binary sequence (PRBS) data stream was generated using the pattern generator in the Agilent J-BERT N4903B high-performance serial bit error rate tester

(BERT). The data stream was amplified and combined with DC bias using a bias-tee to modulate the SLD. The emitted light was received by a Si avalanche photodetector and analyzed using the BERT and an Agilent 86100C digital communication analyzer. 95

Figure 56. Eye diagrams of the VLC link using SLD as the transmitter operating at a data rate of: (a) 622 Mbit/s, and (b) 1 Gbit/s using on-off keying (OOK) modulation scheme.

At a data rate of 622 Mbit/s and 1 Gbit/s, the VLC link using SLD as the transmitter exhibits a BER of 4 × 10-4 and 8.4 × 10-4, respectively, well passes the forward error correction (FEC) limit of 3.8 × 10-3. The corresponding eye diagrams in Figure 56 show a clear open eye, suggesting the SLD works well for Gbit/s data communication link. Clear open-eye is achieved at data rates up to 1.3 Gbps with the BER of 2.1×10-3, below the FEC limit, thus demonstrating the significant advantage of high-speed SLD for VLC applications.

4.4 Summary

In summary, the recent advances of III-nitride violet-blue emitting superluminescent diodes, a novel InGaN/GaN QW based SLD on semipolar (2021) GaN substrate and its application for solid-state lighting and visible-light communication systems have been presented in this chapter. The presented 450 nm emitting SLD has a high optical power of > 200 mW with a broad emission peak FWHM of > 7 nm at an optical power of 100 mW. This lead to a white light source with a CRI of 68.9 and a CCT of 4340 K when using the blue emitting SLD exciting the yellow YAG phosphor plate. The SLD based white light 96 offers improved white light quality compared to that of the LD-based counterpart, from the aspects of CRI, and CCT.

A high-speed InGaN/GaN QW based superluminescent diode was also demonstrated using a tilted facet configuration emitting at 405 nm. Benefiting from the amplified spontaneous emission process induced short carrier lifetime and a low RC delay, the SLD exhibits a large -3 dB modulation bandwidth of > 800 MHz. As a transmitter in VLC system, the SLD is able to support much higher data rate (> Gbit/s) for free-space communication compared to the system based on LEDs. Therefore, the GaN-based violet-blue SLD, in standalone or array form, offers a combination of the advantages of both LEDs and LDs, making it a promising candidate for the best light source.

Chapter 5: III-Nitride based Integrated Semiconductor Optical

Amplifier-Laser Diode

5.1 Overview

The past decade witnessed the rise of III-nitride optoelectronic devices, such as violet- blue-green light-emitting diodes (LEDs), laser diodes (LDs), modulators and photodetectors (PDs), owing to its applications in white lighting, display, optical data storage, and sensing [2, 26, 108, 159]. Recently, InGaN/GaN quantum well (QW) based LEDs and LDs were demonstrated as transmitters for high-speed visible light communications (VLC) and underwater wireless optical communications (UWOC) [5, 42, 46, 161]. However, the functions of light generation, amplification, modulation and detection at visible wavelength are currently enabled using discrete III-nitride components. Since the on-chip integration of various photonic devices offers the advantages of small footprint, low-cost and multi-functionality, it is of great interest to develop III-nitride photonic integrated circuit (PIC) at the visible wavelength. Though GaAs- and InP-based PIC at the near-infrared wavelength has been studied [185], such photonic integration at the visible wavelength is still not available in GaN-based material system. Recently, a GaN-based integrated waveguide modulator- laser diode (IWM-LD) was demonstrated [159]. In addition, the integration of GaN nanowires based LED and PD was reported for an intrachip optical interconnect [186]. As an important optical component, the III-nitride semiconductor optical amplifier (SOA) and its integration with LD, however, are yet to be developed for the eventual realization of III-nitride PIC.

This chapter reports on the high gain semiconductor optical amplifier (SOA) based on InGaN/GaN quantum wells (QWs) at 404 nm, which is an important building block for photonic IC at the visible wavelength. The device features the seamless integration of an SOA and an LD (SOA-LD) in the form of a 2-µm ridge-waveguide fabricated on a semipolar

GaN substrate. At an SOA driving voltage (VSOA) of 6.25V, the amplification effect was 98 evidenced by a significantly increased light output power from 8.2 mW to 30.5 mW at

ILD=250mA. The corresponding gain of 5.7 dB and a peak amplification ratio of 18.4 confirmed the achievement of III-nitride integrated SOA-LD. The findings are significant in presenting an integrated photonic system at the visible wavelength for smart lighting and visible-light communications (VLC). The same technology may also benefit the applications of optical clocking, optical switching, and optical interconnect.

5.2 Device Fabrication

Figure 57. Cross-sectional layered structure of the 405-nm emitting integrated semiconductor optical amplifier-laser diode (SOA-LD) on semipolar GaN substrate.

The layer structure of the SOA-LD as shown in Figure 57 was grown using metal-organic chemical vapor deposition (MOCVD) technique. The device consists of four pairs of

In0.1Ga0.9N/GaN QWs, 600 nm/350 nm p-GaN/n-GaN cladding layers, 60 nm/60 nm p-

InGaN/n-InGaN separate confinement heterostructure (SCH) waveguide layers, a 16 nm 99 p-Al0.18Ga0.82N electron blocking layer (EBL), and highly doped p-GaN/n-GaN contact layers.

Figure 58. 3D illustration of the 405-nm emitting integrated semiconductor optical amplifier- laser diode (SOA-LD) on semipolar GaN substrate. The device involves four pairs of In0.1Ga0.9N/GaN multi-quantum-wells (MQWs) as the active region and a pair of InGaN separate confinement heterostructure (SCH) waveguide layers. The length of the SOA and LD is 300 µm and 1190 µm, respectively.

Figure 58 shows the structure of the SOA-LD device. The Pd/Au and Ti/Al/Ni/Au metallization stacks form the p- and n-electrodes, respectively, for the 300-µm-long SOA and the 1190-µm-long LD. The uncoated facets and 2-µm-wide RWG were defined using

UV photolithography and plasma etching. The SOA and the LD have their Mg-doped p-

GaN contact layer etched off to form an isolation trench for electrical insulation at the p- contacts. Thus, this allows independent control of the two sections electrically, while retaining the seamless optical coupling of the LD light to the integrated SOA for light amplification. 100

5.3 Characterization and Discussions

Figure 59. Photo of the device operating at room temperature.

Figure 60. The emission spectrum of the SOA-LD at LD current of 250 mA and zero SOA driving voltage with a peak emission of ~ 404.3 nm.

The fabricated SOA-LD was tested under continuous wave (CW) operation at room temperature as shown in Figure 59. The measurement setup consists of a Keithley 2520 diode laser testing system with a calibrated Si PD and an integrating sphere from

Labsphere; a Keithley 2400 source meter for providing SOA driving voltage, an Ando AQ- 101

6315A optical spectrum analyzer for spectra measurement. Without biasing the SOA, the

LD is lasing at a peak wavelength of 404.3 nm as shown in Figure 60.

Figure 61. Light output power vs. LD injection current (ILD) relation of SOA-LD with varying SOA driving voltage (VSOA) at room temperature.

Figure 61 presents the optical output power of the SOA-LD vs. LD injection current (L-ILD) characteristics at a various driving voltage applied to the waveguide SOA (VSOA from 0 V to 6.26 V). The onset of lasing is identified by the rapid increase of light output power collected from the front facet of the SOA-LD when ILD goes beyond the threshold current

(Ith). An increase of the slope of L-ILD characteristics is observed with increasing VSOA beyond 4 V, suggesting the amplification of light emission owing to the integrated SOA. 102

Figure 62. Plots of optical power at ILD = 250 mA vs. VSOA and Ith vs. VSOA.

In particular, at a constant ILD of 250 mA, the optical power of the SOA-LD was enhanced from 8.2 mW (at VSOA = 0 V) to 9.0 mW, 17.5 mW, 28.0 mW, and 30.5 mW at VSOA of 4 V,

5 V, 6 V, and 6.25 V, respectively (Figure 62). In the meantime, an apparent decrease of

Ith from 229 mA (at VSOA = 0 V) to 209 mA, 155 mA, 138 mA, and 135 mA at VSOA of 4 V, 5

V, 6 V, and 6.25 V, respectively, was observed (Figure 62). The amplification of light output as a response to increasing SOA bias was measured. 103

Figure 63. Plots of gain vs. ILD relation of the SOA-LD at different VSOA.

The VSOA-dependent amplification effect origins from the switching of the SOA gain from a low value to a high value when VSOA increases. The gain of the SOA-LD is defined as the ratio of the optical power at an SOA driving voltage of VSOA over the optical power at zero

SOA driving voltage, which can be expressed as,

gain P P VVSOA 0 (7)

Figure 63 shows the gain as a function of ILD at different VSOA. Gain saturation occurred at an LD current of ~ 250 mA. With increasing VSOA from 4 V to 6.25 V, an increasing gain was observed. Basically, a positive net gain is expected when the InGaN/GaN QW active region is biased beyond the transparency condition. Since the SOA and LD share the same epitaxial layers, the injection current density required to reach transparency is identical 104 for both SOA and LD. Hence, the transparency current density (J0) can be derived from the fitting of LD performance measured from a series of LDs with different cavity length

2 as discussed in section 2.3.2. A J0 of 900 A/cm was measured, suggesting a transparency current (I0) of 5.4 mA in the SOA. This corresponds to a bias voltage (V0) of 3.5 V at transparency, which is consistent with the gain measurement, showing a positive net gain at VSOA ≥ 4 V.

Figure 64. Plots of gain at ILD = 250 mA vs. VSOA.

At ILD = 250 mA, the gain increases from 1.1 dB to 5.7 dB in response to increasing VSOA from 4 V to 6.25 V, at a slope of 2.46 dB/V (Figure 64). The high gain observed in the device is partially attributed to the large electron-hole wavefunction overlap in

InGaN/GaN QWs grown on semipolar (2021) GaN substrate, which exhibits reduced polarization field compared to that in conventional c-plane devices [187]. 105

Figure 65. Emission spectra of the SOA-LD at ILD = 200 mA with VSOA = 0 V and 6.25 V. The amplification ratio (Ramp) is also plotted with a peak Ramp of 18.4 at 404 nm.

The amplification effect of the integrated SOA can be further analyzed from the spectral characteristics. At ILD = 200 mA, the emission spectra of the SOA-LD at VSOA of 0 V to 6.25

V were plotted in Figure 65. The amplification ratio, which is defined as the intensity ratio between the two emission spectra, was also plotted. Compared to the emission spectra at zero SOA driving voltage, the emission peak after amplification (with VSOA = 6.25 V) exhibits a significant enhancement in intensity as well as a narrowing of the peak full width at half maximum (FWHM) from 3 nm to 0.6 nm. Hence, with the amplification originating from the integrated SOA, the device was switched from spontaneous emission mode (at VSOA = 0 V) to stimulated emission regime (at VSOA = 6.25 V), which supports the observation in L-ILD measurements as in Figure 61. A peak amplification ratio of 18.4 has been observed at the lasing wavelength of 404 nm. The spectral characteristics of the

SOA-LD further confirm the amplification effect from the integrated SOA. 106

5.4 Summary

To sum up, this chapter demonstrated an InGaN/GaN QW based integrated semiconductor optical amplifier - laser diode (SOA-LD) fabricated on a semipolar (2021)

GaN substrate. The high-gain (5.7 dB) integrated SOA can effectively enhance light output and reduce the threshold current of the LD section. The presented III-nitride SOA-LD constitute an important building block for eventual realization of III-nitride PIC at the visible wavelength for a plethora of emerging applications, such as smart lighting, optical communication, visible light based optical switching, and optical interconnect.

107

Chapter 6: III-Nitride based Laser Diode with Integrated Waveguide

Photodetector

6.1 Overview

The on-chip integration of III-nitride photonic devices with versatile functionalities offers a compact and low-cost solution for emerging applications, such as smart lighting, visible light communication and optical interconnect. Group III-nitride materials, mainly (In,

Ga)N, have been mostly studied for active devices, including light-emitting diodes (LEDs)

[188, 189], superluminescent diodes (SLDs) [161, 165], and laser diodes (LDs) [84, 190], in the violet-blue-green color regime. Those light emitters based on InGaN/GaN quantum- well (QW) structures have been used in applications for energy-efficient solid-state lighting (SSL) [33, 191] and high-speed visible light communications (VLC) [39, 192].

Additionally, the same structure can be configured for passive devices, serving the functionalities of transverse transmission modulators [193] and surface photodetectors

(SPDs) [194].

The combination of both active and passive components enables the possible on-chip integration of optoelectronic devices with versatile functionalities [195]. However, the combination of SPDs with surface light emitters, such as LEDs, is not a cost-effective and mass-producible approach due to the fabrication complexity. Hence, the utilization of edge emitting laser diodes together with light modulators or receivers based on the same active region becomes a promising approach. The major challenge is the large separation 108 between the absorption and emission peak, i.e. the Stokes shift, induced by the strong polarization field in conventional c-plane orientated InGaN/GaN QWs [196]. To achieve an LD based on-chip photonic system, it is of great interest to develop a high-performance waveguide photodetector (WPD) and investigate its integration with laser diodes.

In this chapter, a high-performance waveguide photodetector (WPD) integrated with a laser diode (LD) at 405 nm sharing the single InGaN/GaN quantum well active region on semipolar GaN substrate is demonstrated. A significant large modulation bandwidth of

230 MHz has also been measured in the WPD. To the best of our knowledge, this is the first report of such photonic integration enabled using the same InGaN/GaN quantum- well active region at visible wavelength. The findings are significant in presenting the on- chip integration of III-nitride light emitters and receivers, for the eventual realization of

III-nitride photonic integrated circuit (PIC).

6.2 Device Fabrication

The epitaxial structure of the device contains a highly Mg-doped p-GaN contact layer, a

600 nm Mg-doped p-GaN cladding layer, a 120 nm Mg-doped p-InGaN separate confinement heterostructure (SCH) waveguiding layer, a 16 nm Mg-doped p-Al0.18Ga0.82N electron blocking layer (EBL), 4-period 3.6 nm/7 nm In0.1Ga0.9N/GaN multiple QWs active region, a 120 nm Si-doped n-InGaN SCH waveguiding layer, a 350 nm Si-doped n-GaN cladding layer, and a highly Si-doped n-GaN contact layer, as shown in Figure 66(a). It is grown on a semipolar (2021) orientated free-standing GaN substrate using the metal- 109 organic chemical vapor deposition (MOCVD) technique. The Pd/Au and Ti/Al/Ni/Au metal stacks are deposited as p- and n-GaN contacts, respectively.

The fabricated WPD-LD as shown in Figure 66(b) consists of a 90-µm long WPD placed at the rear facet of a 505-µm long LD, with a separation distance of ~ 5 µm. The 2-µm wide

RWG and the facets are defined using UV photolithography and plasma etching, without facet coating. The isolation trench is etched using focused ion beam (FIB) milling technique. The isolation resistance between the WPD and the LD is measured to be ~ 1

MΩ, which is more than five orders of magnitude higher than the junction series resistance, enabling the independent operation of the two components. 110

6.3 Characterization and Discussions

The LD is characterized using a Keithley 2520 diode laser testing system with a calibrated

Si photodetector and an integrating sphere for accurate measurements of the optical power vs. current (L-I) characteristics. Figure 66(c) shows the laser emission from the front facet.

The WPD is biased using a Keithley 2400 source meter. The frequency response measurement setup involves an Agilent E8257D analog signal generator, a set of microwave probe, a bias-tee, and an Agilent DSO5034A oscilloscope. The LD is modulated by sinusoidal waveform signals at different frequencies, and the received signals from the

WPD are collected and analyzed using the oscilloscope.

Figure 67(a) shows the light-current-voltage characteristics of 405-nm emitting LD under continuous wave (CW) operation, which is measured at the front facet using a standard calibrated Si PD at room temperature. The LD shows a threshold current (Ith) of 130 mA and a slope efficiency of ~ 0.4 W/A. Figure 67(b) compares the LD light output power

(collected from the front facet) and the photocurrent from the WPD at zero bias that is plotted against the LD injection current. It can be observed that the WPD current measured at zero WPD bias is very well correlated with the emitted optical power by the

LD. The onset of significant increase in the WPD current is measured at an LD current of

130 mA, that matches the threshold current of the LD. With increasing LD injection current beyond Ith, the LD starts to lase, and thus, more light is received by the WPD, leading to a clear increase in WPD current. Therefore, the integrated WPD can be utilized for on-chip power monitoring. 112

Enhanced optical responsivity is achieved from the biased WPD as shown in Figure 68(a).

To minimize the heating effect of the LD, pulsed current operation of the LD with a pulse width of 5 µs and a duty cycle of 10% was used for the further characterization of WPD-

LD. With an increasing reverse bias applied to the WPD, the width of the depletion junction in the active region increases, resulting in an increased absorption and photocurrent. For instance, at a constant LD current (ILD) of 200mA, the WPD 113 photocurrent (IPD) is 63.5 µA, 80.7 µA, 112.3 µA, and 130.4 µA, at a WPD reverse bias voltage (VPD) of 0 V, 2 V, 4 V, and 6 V respectively. Figure 68(b) presents the WPD current as a function of the received optical power. The results are derived based on the assumptions that the amount of light emitted from the uncoated front and back facets of the LD is identical, and all the light emitted has been received by the WPD. In Figure 68 , the slope of the WPD response curves is growing with an increasing reverse bias, suggesting a clear power-dependent PD response. The enhanced responsivity indicates that the WPD operating in the photoconductive mode is advantageous for weak signal detection.

Figure 69. Measured responsivity as a function of the reverse bias in the waveguide PD.

The responsivity (R) can be calculated according to the ratio of IPD over the incident optical power. The measured R vs. VPD relation is plotted in Figure 69. With increasing reverse bias voltage from 0 V to 10 V, the R of the WPD increases from 0.018 A/W to 0.051 A/W. 114

Considering the fact that the WPD and LD are sharing the same active layer design without the need of epitaxial regrowth, the presented WPD outperforms other PDs utilizing

InGaN/GaN QWs on c-plane orientated substrates for simultaneously light emission and detection [195, 197-199]. The high responsivity of the WPD is attributed to an enhanced overlap between the absorption peak of the WPD and the emission peak of the LD. The enhancement in the overlap is originating from a reduced polarization field in QWs that are grown on a semipolar GaN substrate.

Figure 70. Modulation response of the waveguide photodetector at zero bias.

The frequency response of the unbiased WPD-LD has been measured to study the performance of the device for on-chip communication as shown in Figure 70. The emitted light from LD was modulated by the applied small signal and was received by the WPD at

VPD = 0 V. Since the modulation bandwidth of the LD is beyond GHz [200], the measured signal amplitude decay is thus due to the WPD bandwidth. A 3-dB bandwidth of ~ 230 115

MHz is measured, suggesting a significantly improved modulation performance than the reported GaN Schottky barrier PDs (5.4 MHz) [197] and GaN p-i-n PDs (10 ~ 20 MHz) [195].

The increased cut-off frequency of the WPD is associated with the reduced device size owing to the narrow ridge design, and the improved responsivity of the semipolar plane

WPDs. The high-speed WPD suggests its potential as an integrated receiver for on-chip communication and VLC applications.

6.4 Summary

In summary, an integrated waveguide photodetector – laser diode has been designed, fabricated and demonstrated. Sharing the same InGaN/GaN QW structure, the semipolar plane WPD exhibits a high responsivity of 0.051 A/W at -10 V, 405 nm and a large modulation bandwidth of 230 MHz. As LDs outperforms LEDs in terms of efficiency droop and modulation bandwidth, LD based integrated photonic platform becomes more promising for both SSL and VLC applications. Therefore, the presented work of WPD-LD is an important step towards the eventual realization of III-nitride on-chip photonic system with integrated functionalities. 116

Chapter 7: Conclusions and Outlook

7.1 Closing remarks

In this dissertation, the design, fabrication, and characterization of the on-chip integration of III-nitride photonic components, including the integrated short-wavelength waveguide modulator, passive absorber, semiconductor optical amplifier and waveguide detector with the laser diode, were presented, demonstrated and discussed. Figure 71 illustrates the core concepts, motivation, innovation and applications of the presented dissertation research. As a fundamental investigation towards the development of photonic integrated circuit (PIC) at the visible wavelength, the original work presented in this dissertation paved the way towards the eventual realization of a compact, small- footprint, low-cost, and energy-efficient platform offering multi-functionalities.

Figure 71. Graphic summary of the dissertation. 117

To sum up, the dissertation features the following significant results and advances:

 The optical, electrical and high-frequency characterizations of non-c-plane laser

diodes emitting at both violet and blue color regimes have been presented and

discussed. Segmented contact method was utilized to measure the gain spectra of

the nonpolar GaN laser diode, showing a comparatively higher peak modal gain of

29.2 cm-1. An increase in peak modal gain compared with reported c-plane LDs as

well as the reduction in differential modal gain in LDs operating above the

threshold was reported. The InGaN/GaN QW based LDs grown on semipolar

(2021) plane GaN substrate exhibits a high quantum efficiency, and a large

modulation bandwidth, making it a promising light emitter for III-nitride PIC.

 Integrating light-generation and modulation functionalities achieved by

employing a small foot-print, low material polarization field integrated waveguide

modulator-laser diode (IWM-LD) configuration, fabricated seamlessly on

semipolar GaN substrate. The fabricated blue-emitting IWM-LD exhibited a large

extinction ratio of 9.4 dB and a low operating voltage range of 0 to 3.5 V, leading

to a high modulation efficiency of 2.68 dB/V. A large extinction ratio of 11.3 dB is

measured in the violet-emitting IWM-LD with a modulation voltage as low as -2.5

V to achieve On/Off switching. By modulating the waveguide modulator, a -3 dB

bandwidth of ~1GHz, which is limited by the photodetector, and data rate of 1.7

Gbps is achieved, suggesting it to be a novel platform for visible light

communications. 118

 Utilizing an integrated absorber, a high optical power (> 250 mW), droop-free,

blue-emitting superluminescent diode (SLD) was demonstrated as a unique light-

emitting component. The phosphor-converted white light based on the SLD

showed a speckle-free emission owing to its time incoherence. Benefiting from

the reduced carrier lifetime (τe<0.35 ns) through the amplified spontaneous

emission, the violet-emitting SLD exhibits a significantly large 3-dB bandwidth of

807 MHz. Using SLD as a transmitter, data transmission at 1.3 Gbps was achieved

with bit-error rate (BER) of 2.9×10-3 (below the FEC limit of 3.8×10-3) using on-off

keying (OOK) modulation scheme. The findings are significant for simultaneous

implementation of solid-state lighting and visible-light communications, as well as

for florescence and biomedical imaging applications.

 The first report of the integrated short-wavelength semiconductor optical

amplifier with the laser diode at ~ 404 nm based on InGaN/GaN MQWs. The device

exhibited an efficient amplification effect, showing a reducing threshold current

from 229 mA to 138 mA at VSOA = 6 V, and an increasing optical power from 9.0

mW to 28 mW at ILD = 250 mA, leading to a large gain of 5.32 dB at VSOA = 6 V.

 The first report of a high-performance waveguide photodetector (WPD)

integrated with a laser diode (LD) at 405 nm sharing the single InGaN/GaN

quantum well active region. The photocurrent of the integrated WPD is effectively

tuned by the emitted optical power from the LD. The responsivity ranges from

0.018 A/W to 0.051 A/W by an increasing reverse bias from 0 V to 10 V. A 119

significant large modulation bandwidth of 230 MHz has been measured in the

WPD.

The study of multi-section InGaN-based laser diodes and the integration with light modulator, absorber, amplifier and detector enables both high power light source for smart lighting and display, and as an efficient transmitter for visible light communication.

Besides, the development of III-nitride PIC may also be utilized for other potential applications, such as optical switching, clocking and optical interconnect.

7.2 Future research

Towards next generation high-brightness smart lighting and high-speed visible-light communications, laser-diode based solution is a promising approach that caught the attention of both academia and industry [201, 202]. Moving forward towards the multi- functionality integration at the visible wavelength, there are few research and development opportunities to be further investigated:

- The bulk crystal growth technology: produce low-cost and high-quality GaN

substrate.

- The material epitaxy technology: effective defect filtering and strain engineering

to grow high quality InGaN-based active region.

- Doping engineering in epitaxial growth: high-quality p-type doping to reduce the

series and contact resistance.

- Facet coating technique: high-reflection facet coating to reduce the mirror loss. 120

- Full integration of III-nitride photonic components: integrate three and more

devices.

- Device packaging: proper heat sink, light guiding and packaging to improve the

reliability and device lifetime.

121

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APPENDICES

Appendix A: High-speed modulation performance of violet-blue nonpolar

GaN-based vertical-cavity surface-emitting lasers (VCSELs)

As compared to the conventional edge-emitting lasers in the violet-blue color regime,

GaN-based vertical-cavity surface-emitting lasers (VCSELs) are excellent for high-density display, pico-projection, solid-state lighting and high-speed data communication applications [98, 203]. VCSELs are advantages owing to their small device footprint, short cavity length, circular emission beam, low beam divergence, and reduced active region volume [93-95]. Recent advances in nonpolar III-nitrides suggest their potential for low threshold VCSELs with 100% polarization ratio due to higher peak material gain, lower transparency carrier density and anisotropic gain characteristics [96, 97].

In this section, the emission characteristics and modulation response of current injected nonpolar InGaN/GaN quantum-well (QW) VCSELs will be reported. A single-longitudinal mode lasing at 419 nm was observed from the VCSEL with a 10-μm aperture at a threshold current (Ith) of 18 mA. The device has a sub-pF capacitance (~ 0.85 pF) and a -3 dB modulation bandwidth (BW) of ~ 1 GHz, which is limited by the BW of the avalanche photodetector (APD), suggesting the VCSELs to be a competing candidate for high-speed visible light communication applications. 138

Figure A-1. L–I–V characteristics of a 10-μm-aperture VCSEL.

The VCSELs presented herewith contain a cavity sandwiched by two dielectric distributed

Bragg reflectors (DBRs), tunnel-junction contact, and ion-implanted aperture. The substrate was removed via photoelectrochemical etching, and the device was flip-chip bonded to a Copper heat sink. Figure A-1 shows the light output vs. current and voltage vs. current (L-I-V) characteristics of the 10-μm aperture VCSEL under pulsed current injection (pulse width of 0.5 μs and duty cycle of 0.1 %) to avoid thermal roll-over. A threshold current of 18 mA is measured, corresponding to a threshold voltage of 7.5 V.

The VCSEL is capable of generating > 200 μW emission output power with a slope efficiency of 7.74 μW/mA. 139

Figure A-2. The 2D (a), and 3D (b) emission profiles from a 10 μm aperture VCSEL operating at 1.2 Ith (22.5 mA). The corresponding 2D (c), and 3D (d) emission profile from a 12 μm aperture VCSEL operating at 1.2 Ith (25 mA).

Figure A-2 (a)~(d) show the 2D and 3D emission profiles of the VCSELs with 10 μm and 12

μm apertures, measured at 22.5 mA (1.2 Ith) and 25 mA (1.2 Ith), respectively, using an

Ophir SP620U beam profiling camera. The far-field pattern, which shows the linearly- polarized (LP) mode shape, of the 10-μm-aperture VCSEL is significantly different from that of the 12-μm-aperture VCSEL. 140

Figure A-3. Single-peak EL spectrum of a 10 μm aperture VCSEL. Inset: high-resolution EL spectrum fitted using a Gaussian function, with peak-emission (λpeak) of 419 nm and FWHM of 0.6 nm.

Figure A-3 presents the electroluminescence (EL) spectrum of the 10-μm-aperture VCSEL at 25 mA collected using an Ocean Optics HR4000 spectrometer where a single emission peak at 419 nm was observed. The single-longitudinal mode emission is further confirmed by examining the high-resolution spectrum as shown in the inset of Figure A-3, which is measured using an Ando AQ6315A optical spectrum analyzer with a spectral resolution of

0.05 nm. A Gaussian fitting with a coefficient of determination (R2) of 0.998 was achieved for the EL peak of the 10-μm-aperture VCSEL, giving a full-width at half-maximum (FWHM) of 0.6 nm. 141

Figure A-4. Modulation response of 10 μm aperture VCSEL showing a -3 dB bandwidth of 1 GHz, which is limited by the frequency response of APD. Inset: The C-V characteristics of the 10 μm aperture VCSEL.

Figure A-4 shows the small signal frequency response of the 10 μm aperture VCSEL, which is measured using an Agilent E8361A PNA network analyzer and a Menlo Systems APD210

1-GHz Si APD. A -3 dB bandwidth of 1 GHz is measured, which reaches the BW limit of the

APD. We further investigate the frequency response of the VCSEL from the angle of carrier-lifetime and RC delay. Considering a much smaller cavity length and active region volume compared to edge-emitting lasers, the resonance frequency of VCSELs is significantly higher than 1 GHz because of a short photon lifetime.

To verify the RC delay of the fabricated VCSEL, the capacitance vs. bias voltage (C-V) relation was measured at 1 MHz as shown in the inset of Figure A-4. The measured capacitance of ~0.85 pF is significantly lower than that of a typical ridge-waveguide laser 142 diode. As a result, the RC delay-limited frequency is calculated to be > 10 GHz. Therefore, the maximum frequency response of the presented 10-μm-aperture VCSEL is expected to exceed 1 GHz, although the current result is limited by the BW of the APD.

In summary, the modulation response of a nonpolar InGaN/GaN QW VCSELs emitting at

419 nm was experimentally measured. The single-longitudinal mode emission with a peak

FWHM of 0.6 nm was measured for the VCSEL with 10-μm aperture. The device presented has a -3 dB bandwidth of 1 GHz and a capacitance of < 1pF, and thus prove its feasibility for high-speed visible-light communication.

143

Appendix B: Standard Operation Procedure of Keithley 2510 TEC Controller

1. Hazards

 This TEC controller is used in the laser diode testing system. So kindly read through the SOP for laser diode tester system for general safety warnings. Adhere to the safety precautions mentioned in the "Laboratory safety plan".  During operation, the surface of TEC and substrate mount might be cold or hot. Use proper tweezers to handle the sample to avoid cold injury or burning of the skin. 2. PPE

 Always wear a clean pair of gloves, and a lab coat when performing the measurements.  Always wear proper goggles based on the wavelength range of interest when a laser diode is under testing.  For more details of the PPE, refer to the page. 13, 25, and 43 of "Laboratory safety plan" 3. Precautions

 The device under test may be at temperature extremes. Improper use of the temperature controller may cause personal injury.  Allow sufficient time for the device under test to return to a safe temperature after testing before making personal contact. Use a heat sink or physical barriers as necessary to avoid contact.  The TEC module, thermistor, heat sink and sample holder should be assembled without a gap, and proper thermal paste should be used to make a good heat transfer between the layers.  Use a temperature meter to measure the temperature at the surface of sample holder if you would like to confirm the temperature at the surface. 4. Characteristics

 The Model 2510 has the following basic control characteristics: o Control capabilities — Control thermoelectric cooler (TEC) power to maintain a constant temperature, current, voltage, or thermistor resistance. o Output range — ±10V DC at up to ±5A DC. o Setpoint range — -50°C to 225°C. o Setpoint resolution — ±0.001°C. o Over temperature limit — 250°C maximum. o Under temperature limit — -50°C minimum. 144

o Current limit — 1A to 5.25A 5. Connections

 Connect the power cord to Keithley 2510. (The Model 2510 operates from a line voltage in the range of 100 to 240V at a frequency of 50 to 60Hz. Line voltage and line frequency are automatically sensed. Therefore, there are no switches to set.)  The input/output connector is located on the rear panel of Keithley 2510 (see figure B-1). Connect the 2510-CAB cable assembly to the port.

Figure B-1 Rear panel of Keithley 2510.

 The Keithley 2510 supports 2-wire and 4-wire connections. Terminals include: o OUTPUT terminals: F+, F- (force), S+, and S- (sense) connections to thermoelectric cooler. o INPUT terminals: F+, F- (force), S+, and S- (Kelvin sense) connections to temperature sensor.  We are currently using TEC+thermistor in 2-wire connections. The configurations can be found in figure B-2.

145

Figure B-2 Two-wire input/output connections.

 The Model 2510 assumes that a positive output current is a heating current.  Connect TEC module (TB-99-1.4-0.8 as an example): o TEC- red end to the red cable of Keithley 2510 (F- of output). o TEC- black end to the white cable of Keithley 2510 (F+ of output).  Connect thermistor to input ends of Keithley 2510 (F – and F+, black and green cables) 6. Operation procedure

 The front panel of Keithley 2510 is shown in figure B-3. Power on the equipment, setup the parameters before turning on the output.

Figure B-3 Keithley 2510 front panel 146

Configuring temperature 1. Press CONFIG then T to access the temperature configuration menu.

2. Select PROTECTION, then press ENTER.

3. Select CONTROL, then press ENTER.

4. Choose ENABLE, then press ENTER.

5. Select LOLIM, then press ENTER. Set the low temperature limit using the

EDIT keys, then press ENTER again.

6. Choose HILIM, then press ENTER, set the high temperature limit using the

EDIT keys, then again press ENTER.

7. Press EXIT, choose SENSOR-TYPE, then press ENTER.

8. Choose the desired temperature sensor (THERMISTOR, RTD, I-SS, or V-SS),

then press ENTER.

9. Program the settings for the selected temperature sensor. For example, for

a thermistor sensor, choose the following:

• RANGE: Set thermistor range (100Ω, 1k Ω, 10kΩ, 100k Ω).

• A, B, C: Set A, B and C coefficients. (critical step)

• I-SRC: Set thermistor current (AUTO). 147

• SENSE-MODE: Choose 2-WIRE.

10. Press EXIT as needed to return to the CONFIG TEMPERATURE menu.

11. Select UNITS, press ENTER, then select the desired units (°C, °F, or Kelvin).

Configuring voltage 1. Press CONFIG then V to access the voltage configuration menu.

2. Select PROTECTION, then press ENTER.

3. Using the EDIT keys, set the voltage limit to the desired value, then press

ENTER. (Maximum for Keithley 2510 is 10.5 V, the TEC module installed is TB-

99-1.4-0.8, which can take up to 12.3 V. So we can set the limit to 10 V)

Configuring current 1. Press CONFIG then I to enter the current configuration menu.

2. Select PROTECTION, then press ENTER.

3. Using the EDIT keys, set the current limit to the desired value, then press

ENTER. (Maximum for Keithley 2510 is 5.25 A, the TEC module installed is TB-

99-1.4-0.8, which can take up to 11.3 V. So we can set the limit to 5 A)

 To retain these tuning constants after cycling power, be sure to save them as power-on defaults by using the SAVESETUP selection in the main MENU. 7. TEC and thermistor SPECIFICATIONS THERMOELECTRIC MODULE TB-99-1.4-0.8 148

Thermistor MP 3176

MP 3176 (5 kΩ) uses the TS-141 material. The A, B and C coefficients are calculated to be:

A: 1.28E-3

B: 2.36184E-4

C: 9.29325E-8 Thermistor MP 2444

MP 2444 (15 kΩ) uses the TS-67 material. The A, B and C coefficients are calculated to be:

A: 1.03E-3 149

B: 2.33884E-4

C: 7.90348E-8 8. Supplementary materials: Steinhart–Hart equation

The Steinhart–Hart equation is a model of the resistance of a semiconductor at different temperatures:

T is the temperature (in Kelvins)

R is the resistance at T (in ohms)

A, B, and C are the Steinhart–Hart coefficients which vary depending on the type and model of the thermistor and the temperature range of interest.

150

Appendix C: Development of Bi-Layer Photoresist Process for self-aligned fabrication applications

Bi-layer photoresist (PR) based photolithography and lift-off technique offers significant advantages in resolution, removal, process simplicity, undercut control and yield over conventional single-layer lift-off process [204-206].The bi-layer lifts-off has been widely used in fabricating source, drain or T-shaped gate Ohmic contacts for gallium arsenide

(GaAs), GaN, InP and other semiconductor devices [207, 208].

The LOL-2000 Lift-Off Layer is a non-photosensitive material which dissolves in photoresist developer in a controlled way. It is typically used as the bottom layer in the bi-layer PR process. However, it is not available at KAUST cleanroom by the time the devices were fabricated. Therefore, the bi-layer PR process utilizing the PRs available at

KAUST nanofab has been developed.

Typical PRs used in KAUST cleanroom and the reference recipes have been summarized:

AZ1505

Positive tone

Softbake: 100 °C 60 s hotplate

Thickness @ 4000 rpm: 0.50 µm

Exposure: 25 mJ/cm2 broadband

Develop: AZ726 60 s 151

AZ1512HS

Positive tone

Softbake: 100 °C 60 s hotplate

Thickness @ 4000 rpm: 1.2 µm

Exposure: 60 mJ/cm2 broadband

Develop: AZ726 25 s

AZ ECI3027

Positive tone

Softbake: 90 °C 60 s hotplate

Thickness @ 4000 rpm: 2.7 µm

Exposure: 200 mJ/cm2 broadband

Develop: AZ726 60 s

AZ 9260

Positive tone

Softbake: 110 °C 160 s hotplate

Thickness @ 3000 rpm: 8.80 µm

Exposure: 1500 mJ/cm2 broadband

Develop: AZ400K 1:4 180 s spray

AZ5214E

Positive tone, image reversal capable

Softbake: 110 °C 60 s hotplate 152

Thickness @ 4000 rpm: 1.4 µm

Exposure: 70 to 110 mJ/cm2 broadband

Reversal bake: 120 °C 120 s hotplate (critical step)

Flood exposure: > 200 mJ/cm2

Develop: AZ726 60 s

The bi-layer PR process for self-aligned ridge formation and lift-off process has been summarized in the following table:

Table C-1. Bilayer PR exposure and develop process for self-aligned ridge formation and lift-off

Bench/equipment Process Wet Bench Sample cleaning using Acetone, IPA and Di water o Dehydration bake, 2 min 110 C Spin HMDS, 3000 rpm, 30 s PR Bench Spin AZ1505, 4000rpm, 30 s (500nm) EBR removal of PR on the backside of the sample using swab Softbake 100 oC , 60 s 2 Contact Aligner Flood expose, 30 mJ/cm , soft contact Spin ECI3027, 4000 rpm, 30 s (2.7um) PR Bench EBR removal of PR on the backside of the sample using swab Softbake 100 oC , 60 s 2 Contact Aligner Expose, 200 mJ/cm , hard contact Develop AZ726 for 70 – 80 s Develop Bench 2 min DI rinse flowing, N dry 2 Hot plate Hard bake 110 oC, 60 s.

After the exposure and develop, the undercut can be observed from the image collected using the optical microscope (Figure C-1). 153

Figure C-1 Optical image of the bi-layer PR pattern after develop. The undercut in the PR stack can be observed.

154

Appendix D: Ridge waveguide GaN laser diode fabrication process flow sheet

Procedure Step Resource Recipe

Wafer Cleaning Acetone / IPA rinsing Solvent bench blow dry using N2 gun. LIT-016 baking 110 °C, 1 minute

Waveguide Spin-coating HMDS (Recipe 2) LIT-001 Photolithography Step1: speed 800 rpm, ramp:1000 rmp/s time using bi-layer PR 3 s Step2: speed 1500 rpm, ramp:1500 rmp/s time 3 s Step3: speed 3000 rpm, ramp:3000 rmp/s time 30 s Spin-coating AZ 1505, 4000 rpm to get 500 nm LIT-001 EBR removal of PR on the backside of the sample Softbake on hotplate 100 °C for 60 s UV flood exposure for 30 mJ/cm2, soft contact EVG 6200 Spin-coating ECL3027 (4000 rpm to get 2.7 um) LIT-001 Mask 2 Softbake on hotplate 100 °C for 60 s UV exposure for 200 mJ/cm2, hard contact, EVG 6200 SC 3027 separation 60 um, Develop with AZ726 MIF solution for 60-80s Check with optical microscope to confirm undercut Hard bake 110 °C for 60 seconds

Waveguide ICP-RIE etching of GaN: Etch time:1 min 20 s PE-004B Chao- etching (T=30C, P= 7.3 mTorr ICP:RF=500:100W, GaN etch Cl2:Ar=15:3 sccm) target: ~ 500-550 nm. 6 Measurement of thickness using profiler

Oxide isolation Sputter SiO2 as isolation layer: 15 mins * 3 DEP-002 self- cycles with 5-10 minutes break, H=190 aligned process Lift-off: Acetone rinse

155

P- Metal pad Spin-coating nLOF AZ 2020 (Recipe2 to get 2.2 LIT-001 Mask: 3a Photolithography um) metal Step1: speed 800 rpm, ramp:1000 rmp/s time contact 3 s Step2: speed 1500 rpm, ramp:1500 rmp/s time 3 s Step3: speed 3000 rpm, ramp:3000 rmp/s time 30 s Softbake on hotplate 110 °C for 120 s UV exposure for 65 mJ/cm2 EVG 6200 SC-2020 Post exposure bake on hotplate 110 °C for 60 s Develop sample using AZ726 MIF solution for 120 s Oxygen discum using RIE HCl surface washing @ 80C in wet bench for 5 minutes

Metal pad E-beam evaporator: Pd/Au: 5nm:200nm DEP-008 Depoistion Lift-off: Acetone rinse

Substrate Mount the sample with wax thinning Polishing the substrate to ~120 um Polisher Cleaning use Acetone rinse

N-Metal E-beam evaporator: DEP-008 Depoistion Ti/Al/Ti/Au: 15nm:60nm:35nm:150nm

Contact RTA: 600 °C in N2 atmosphere RTP annealing

Laser bar Cleaving Loomis cleaving

156

Appendix E: publication list during PhD

Patents

[1]. B. S. Ooi, C. Shen, T. K. Ng. “An Apparatus Comprising A Waveguide-Modulator And Laser-Diode And A Method Of Manufacture Thereof”. US patent (provisional application 62/237,523). [2]. I. Dursun, C. Shen, O. Bakr, B. S. Ooi, “Perovskite Nanocrystals For Visible Light Communication”. US patent (provisional application 62/335,936). [3]. B. S. Ooi, C. Shen, T. K. Ng. “Integrated Semiconductor Optical Amplifier And Laser Diode At Visible Wavelength”. US patent (provisional application 62/426,819). [4]. B. S. Ooi, B. Janjua, C. Shen, C. Zhao, T. K. Ng. “Ultrabroad Linewidth Orange-Emitting Nanowires LED For High CRI Laser Based White Lighting And Gigahertz Communications”. US patent (provisional application 62/375,748). [5]. B. S. Ooi, C. Shen, H. M. Oubei, B. Janjua, T. K. Ng. “Systems and Methods for Underwater illumination, Survey and Wireless Optical Communications”. US patent (provisional application 62/379,565).

Journal Papers

[1]. C. Shen, C. Lee, E. Stegenburgs, J. H. Lerma, T. K. Ng, S. Nakamura, J. S. Speck, S. P. DenBaars, A. Y. Alyamani, M. M. Eldesuki, and B. S. Ooi, “Semipolar III-nitride quantum well waveguide photodetector integrated with laser diode for on-chip photonic system”. Applied Physics Express, 10, 042201 (2017).  Highlighted by Semiconductor Today. [2]. B. Janjua, T. K. Ng, C. Zhao, A. Prabaswara, G. B. Consiglio, D. Priante, C. Shen, R. T. ElAfrandy, D. H. Anjum, A. A. Alhamoud, A. A. Alatawi, Y. Yang, A. Y. Alyamani, M. M. El-desouki and B. S. Ooi, “True Yellow Light-emitting Diodes as Phosphor for Tunable Color-Rendering Index Laser-based White Light”. ACS Photonics, 3(11), 2089–2095 (2016).  Highlighted by EE Times Europe. [3]. C. Shen, Y. Guo, H. M. Oubei, T. K. Ng, G. Liu, K.-H. Park, K.-T. Ho, M.-S. Alouini, and B. S. Ooi, “20-meter underwater wireless optical communication link with 1.5 Gbps data rate”, Optics Express, 24 (22), 25502-25509 (2016). [4]. C. Shen, C. Lee, T. K. Ng, S. Nakamura, J. S. Speck, S. P. DenBaars, A. Y. Alyamani, M. M. El-desouki and B. S. Ooi, “High-speed 405-nm superluminescent diode (SLD) with 807-MHz modulation bandwidth”, Optics Express, 24 (18), 20281-20286 (2016). 157

[5]. C. H. Kang, C. Shen, M. S. M. Saheed, N. M. Mohamed, T. K. Ng, B. S. Ooi, and Z. A. Burhanudin, “Carbon nanotube-graphene composite film as transparent conductive electrode for GaN-based light-emitting diodes”. Applied Physics Letters, 109, 081902 (2016). [6]. B. Janjua, T. K. Ng, C. Zhao, H. M. Oubei, C. Shen, A. Prabaswara, M. S. Alias, A. A. Alhamoud, A. A. Alatawi, A. M. Albadri, A. Y. Alyamani, M. M. El-desouki and B. S. Ooi, “Ultrabroad Linewidth Orange-emitting Nanowires LED for High CRI Laser- based White Lighting and GigaHertz Communications.”, Optics Express, 24 (17), 19228-19236 (2016). [7]. C. Zhao, T. K. Ng, R. T. ElAfandy, A. Prabaswara, G. B. Consiglio, I. A. Ajia, I. S. Roqan, B. Janjua, C. Shen, J. Eid, A. Y. Alyamani, M. M. El-Desouki, and B. S. Ooi, “Droop- Free, Reliable, and High-Power InGaN/GaN Nanowire Light-Emitting Diodes for Monolithic Metal-Optoelectronics”, Nano Letters, 16(7), 4616-4623 (2016) [8]. I. Dursun*, C. Shen*, M. Parida, J. Pan, S. Sarmah, D. Priante, N. Alyami, J. Liu, M. Saidaminov, M. S. Alias, A. Abdelhady, T. K. Ng, O. Mohammed, B. S. Ooi, and O. Bakr, “Perovskite Nanocrystals as a Color Converter for Visible Light Communication”, ACS Photonics, 3(7), 1150-1156 (2016) (*equal contributor)  Most read article in Jun. 2016, Jul. 2016 and Year 2016  Highlighted by ACS Chemical & Engineering News, Phys.Org, IET E&T magazine, LaserFocusWorld, Optics & Photonics News, Futurism, EE Times Europe, EE Times Taiwan, Lux Review, and AlJazeera. [9]. X. Xu, J. Zhou, L. Jiang, G. Lubineau, T. K. Ng, B. S. Ooi, H. Liao, C. Shen, L. Chen, and J. Zhu, “Highly Transparent, Low-Haze, Hybrid Cellulose Nanopaper as Electrodes for Flexible Electronics”, Nanoscale, 8, 12294-12306 (2016). [10]. P. Mishra, B. Janjua, T. K. Ng, D. H. Anjum, R. T. Elafandy, A. Pabaswara, C. Shen, A. Salhi, A. Y. Alyamani, M. M. El-Desouki, B. S. Ooi, “On the optical and microstrain analysis of graded InGaN/GaN MQW based on plasma assisted molecular beam epitaxy” Optics Material Express, 6(6), 2052-2062 (2016). [11]. C. Shen, T. K. Ng, J. T. Leonard, A. Pourhashemi, S. Nakamura, S. P. DenBaars, J. S. Speck, A. Y. Alyamani, M. M. Eldesouki, and B. S. Ooi, "High brightness semipolar (2021̅̅̅̅) blue InGaN/GaN superluminescent diodes for droop-free solid-state lighting and visible-light communications” Optics Letters, 41(11), 2608-2611 (2016). [12]. C. Shen, T. K. Ng, J. T. Leonard, A. Pourhashemi, H. M. Oubei, M. S. Alias, S. Nakamura, S. P. DenBaars, J. S. Speck, A. Y. Alyamani, M. M. Eldesouki, and B. S. Ooi, "High-modulation-efficiency, integrated waveguide modulator-laser diode at 448 nm", ACS Photonics, 3 (2), pp 262–268 (2016).  Most read article in Feb. 2016. [13]. C. Zhao, T. K. Ng, N. Wei, A. Prabaswara, M. S. Alias, B. Janjua, C. Shen, and B. S. Ooi, "Facile formation of high-quality InGaN/GaN quantum-disks-in-nanowires on bulk- metal substrates for high-power light emitters", Nano Letters, 16 (2), pp 1056–1063 (2016).  Most read article in Jan. 2016. 158

[14]. C. Lee, C. Shen, H. M. Oubei, M. Cantore, B. Janjua, T. K. Ng, R. M. Farrell, M. M. El- Desouki, J. S. Speck, S. Nakamura, B. S. Ooi, and S. P. DenBaars, "2 Gbit/s data transmission from an unfiltered laser-based phosphor-converted white lighting communication system", Optics Express, 23(23), 29779-29787 (2015). [15]. P. Mishra, B. Janjua, T. K. Ng, C. Shen, A. Salhi, A. Alyamani, M. El-Desouki, and B. S. Ooi. “Achieving uniform carriers distribution in MBE grown compositionally graded InGaN multiple-quantum-well LEDs” IEEE Photonics Journal, 7(3), 2300209 (2015). [16]. C. Shen, T. K. Ng, B. S. Ooi. “Enabling area-selective potential-energy engineering in InGaN/GaN quantum wells by post-growth intermixing”. Optics Express, 23(6), 7991-7998 (2015).

Peer-reviewed Conference Papers

[1]. C. Shen, C. Lee, T. K. Ng, S. Nakamura, J. S. Speck, S. P. DenBaars, A. Y. Alyamani, M. M. El-desouki and B. S. Ooi, “High gain semiconductor optical amplifier – laser diode at visible wavelength”, IEEE International Electron Devices Meeting (IEDM), p. 22.4 (2016). [2]. C. Shen, C. Lee, T. K. Ng, J. S. Speck, S. Nakamura, S. P. DenBaars, A. Y. Alyamani, M. M. El-Desouki, and B. S. Ooi, “GHz modulation enabled using large extinction ratio waveguide-modulator integrated with 404 nm GaN Laser Diode” IEEE Photonics Conference (IPC), p. ThE1.3 (2016). [3]. C. Shen, J. T. Leonard, E. C. Young, T. K. Ng, S. P. DenBaars, J. S. Speck, S. Nakamura, A. Y. Alyamani, M. M. El-Desouki, and B. S. Ooi, “GHz modulation bandwidth from single-longitudinal mode violet-blue VCSEL using nonpolar InGaN/GaN QWs” Conference on Lasers and Electro-Optics (CLEO), p. STh1L.2 (2016).  Awarded Incubic/Milton Chang Travel Grant for CLEO. [4]. C. Zhao, T. K. Ng, N. Wei, A. Prabaswara, M. S. Alias, B. Janjua, C. Shen, G. B. Consiglio, A. Y. Alyamani, M. M. El-Desouki, and B. S. Ooi, “Direct Growth of High-Power InGaN/GaN Quantum-Disksin-Nanowires Red Light-Emitting Diodes on Polycrystalline Molybdenum Substrates” CLEO 2016, p. Stu3R.8 (2016). [5]. J. T. Leonard, E. C. Young, B. P. Yonkee, D. A. Cohen, C. Shen, T. Margalith, T. K. Ng, S. P. DenBaars, B. S. Ooi, J. S. Speck, and S. Nakamura, “Comparison of Nonpolar III- nitride Vertical-Cavity Surface-Emitting Lasers with Tunnel Junction and ITO Intracavity Contacts” Proc. SPIE, OPTO, 97481B-13 (2016). [6]. C. Shen, J. T. Leonard, A. Pourhashemi, H. Oubei, M. S. Alias, T. K. Ng, S. Nakamura, S. P. DenBaars, J. S. Speck, A. Y. Alyamani, M. M. Eldesouki and B. S. Ooi. “Low Modulation Bias InGaN-based Integrated EA-Modulator-Laser on Semipolar GaN Substrate.” IEEE Photonics Conference (IPC), p. TuE2.4, (2015). [7]. C. Shen, T. B. Susilo, I. Khan, P. Mishra, B. Janjua, T. K. Ng, H. Althib, M. A. Alsunaidi, and B. S. Ooi. “Emission characteristics in soft potential profile InGaN/GaN quantum 159

wells” 11th International Conference on Nitride Semiconductors (ICNS), p. TuOO16 (2015). [8]. P. Mishra, B. Janjua, D. H. Anjum, T. K. Ng, C. Shen, and B. S. Ooi. “Microscopic strain analysis of graded InGaN/GaN quantum well grown using nitrigon plasma assisted molecular beam epitaxy” 11th ICNS, p. WeGO14 (2015). [9]. T. B. Susilo, C. Shen, B. S. Ooi and M. A. Alsunaidi, “Intermixing effects on emission properties of InGaN/GaN coupled quantum wells,” 8th IEEE-GCC, pp. 1-4 (2015).  Best paper award, 2nd prize. [10]. C. Shen, T. K. Ng, B. Janjua, A. Alyamani, M. El-Desouki, J. Speck, S. DenBaars, and B. S. Ooi, "Optical gain and absorption of 420 nm InGaN-based laser diodes grown on m-Plane GaN substrate", 2014 Asia Communications and Photonics Conference (ACP), p. AW4A (2014). [11]. P. Mishra, T. K. Ng, B. Janjua, C. Shen, J. Eid, A. Y. Alyamani, M. M. El-Desouki, B. S. Ooi. “Extending quantum efficiency roll-over threshold with compositionally graded InGaN/GaN LED.” 2014 IEEE Photonics Conference (IPC), p. 22-23 (2014). [12]. T. K. Ng, C. Zhao, C. Shen, S. Jahangir, B. Janjua, A. Ben Slimane, C. H. Kang, A. A. Syed, J. Li, A. Y. Alyamani, M. M. El-Desouki, P. Bhattacharya, B. S. Ooi, “Red to Near- infrared Emission from InGaN/GaN Quantum-disks-in-nanowires LED” The Conference on Lasers and Electro-Optics (CLEO), p. SM2J.2 (2014).

Posters and Presentations

[1]. C. Shen, T. K. Ng, C. Lee, J. T. Leonard, H. M. Oubei, Y. Guo, B. Janjua, A. Y. Alyamani, M. M. Eldesuki, J. S. Speck, S. P. DenBaars, S. Nakamura, and B. S. Ooi, “Energy efficient laser-based solution enabling high-speed white light communications” 2nd place winner at 2016 IEEE Photonics Society Student and Young Professional Poster Competition. (Oct. 2016) [2]. B. S. Ooi, C. Shen, H. M. Oubei, B. Janjua, J. R. D. Retamal, C. Li, K.-H. Park, T. K. Ng, J.- H. He, M.-S. Alouini, “Group-III-nitride laser for Gbps Visible Light / Underwater Communications”. IEEE WCNC Workshop 2016. (invited talk) [3]. C. Shen, C. Lee, T. K. Ng, J. T. Leonard, H. M. Oubei, B. Janjua, A. Y. Alyamani, M. M. Eldesouki, J. S. Speck, S. P. Denbaars, S. Nakamura, and B. S. Ooi. “More-than-WiFi: laser based high-speed visible light communications”. Poster at KAUST-NSF Conference on Electronic Materials, Devices and Systems for Sustainable Future 2016. (Mar. 15th, 2016)  Awarded best student poster. [4]. C. Shen, T. K. Ng, J. T. Leonard, C. Lee, H. M. Oubei, B. Janjua, S. Nakamura, S. P. DenBaars, J. S. Speck, A. Y. Alyamani, M. M. Eldesouk, B. S. Ooi. “Energy efficient lasers for smart lighting and optical communications”. Poster at 2016 KAUST WEP Graduate 160

Student & Postdoc Poster Session. (Jan. 19th, 2016) [5]. T. K. Ng, C. Zhao, B. Janjua, A. Gasim, A. Prabaswara, U. Buttner, Y. Yang, D. Priante, P. Mishra, M. S. Alias, C. Shen, H. Liao, M. Z. M. Khan, and B. S. Ooi, "Quantum-disk nanowires array emitters grown using molecular beam epitaxy - potential for closing the "green gap"", 5th Advances in Optoelectronics and Micro/nano-optics 28-31 Oct 2015, Hangzhou, China. (invited talk) [6]. C. Shen, T. K. Ng, B. S. Ooi. “Optical Properties of Non-c-plane InGaN-based Laser Diodes”. Poster at KAUST-NSF Conference on Electronic Materials, Devices and Systems for Sustainable Future 2015. [7]. C. Shen, T. K. Ng, C. H. Kang, Y. Yang, D. Cha, B. S. Ooi. “High performance InGaN/GaN Micro-light emitting diodes”. Poster at 2015 KAUST WEP Graduate Student & Postdoc Poster Competition. [8]. C. Shen, T. K. Ng, M. S. Alias, B. S. Ooi. “Micro-LED: Origin of Improvement in Electro- Optical Performance”. Poster at the 3rd Saudi International Nanotechnology Conference 2014, and KACST-KAUST-UCSB Workshops on Solid State Lighting, KACST Headquarters, Riyadh, Saudi Arabia, 1 - 3 December 2014.

Manuscript in preparation

[1]. C. Shen, T. K. Ng, C. Lee, S. Nakamura, J. S. Speck, S. P. DenBaars, A. Y. Alyamani, M. M. Eldesuki, and B. S. Ooi, “III-nitride Integrated Semiconductor Optical Amplifier - Laser Diode at visible wavelength” Manuscript in preparation. [2]. C. Shen, C. Lee, T. K. Ng, S. Nakamura, J. S. Speck, S. P. DenBaars, and B. S. Ooi, “Electrical field dependent optical absorption in polar, nonpolar and semipolar InGaN- based quantum heterostructures” Manuscript in preparation.