Photodetector Characteristics in Visible Light Communication Dissertation by Kang-Ting Ho In Partial Fulfillment of the Requirements For the Degree of Master of Science King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia © April, 2016 Kang-Ting Ho All Rights Reserved 2 EXAMINATION COMMITTEE APPROVALS FORM The dissertation of Kang-Ting Ho is approved by the examination committee: Committee Chairperson: Dr. Jr-Hau He Committee Member: Dr. Boon S. Ooi Committee Member: Dr. Lance Li Committee Member: Dr. Daniel Lau 3 ABSTRACT Photodetector Characteristics in Visible Light Communication Kang-Ting Ho Typically, in the semiconductor industry pn heterojunctions have been used as either light-emitting diodes (LED) or photodiodes by applying forward current bias or reverse voltage bias, respectively. However, since both devices use the same structure, the light emitting and detecting properties could be combine in one single device, namely LED-based photodetector. Therefore, by integrating LED-based photodetectors as either transmitter or receiver, optical wireless communication could be easily implemented for bidirectional visible light communication networks at low-cost. Therefore, this dissertation focus on the investigation of the photodetection characteristics of InGaN LED-based photodetectors for visible light communication in the blue region. In this regard, we obtain external quantum efficiency of 10 % and photoresponse rise time of 71 µs at 405-nm illumination, revealing high-performance photodetection characteristics. Furthermore, we use orthogonal frequency division multiplexing quadrature amplitude modulation codification scheme to enlarge the operational bandwidth. Consequently, the transmission rate of the communication is efficiently enhanced up to 420 Mbit/s in visible light communication. 4 ACKNOWLEDGEMENTS Earnestly thank King Abdullah Bin Abdulaziz Al Saud, and KAUST’s proceeding and current management who realize his dream and vision. I will always cherish the tremendous experiences gained in the past one and a half years. I sincerely thank my supervisor, Prof. Jr-Hau He, a dedicated scientist and entrepreneur with great passion and insights. His leadership and his constant support helped me accomplish this thesis dissertation. I would also like to express my appreciation to our laboratory manager, Dr. Jose Ramon Duran Retamal, who always help me push me to achieve high-performance as a top-quality engineer and scientific worker. Also thanks for all the valuable discussions and creative solutions. Special thanks to our senior lab member Dr. Cheng-Fang Kang who introduced me to this exciting field. Thanks to Prof. Boon S. Ooi for his suggestions and discussions during my research. He also kindly provided me with a high-technical environment to perform the experiments. Thanks also to Dr. Thien Khee Ng and Chao Shen for all the support provided during the experiments. Thanks to Prof. Hao-Chung Kuo form National Chiao Tung University for providing me with the devices, and to Dr. Yu-Lin Tsai for giving me a lot of suggestion about conceptualizing the project. Thanks to Prof. Gong-Ru Lin from National Taiwan University for elaborating the project design. Additional thanks to Dr. Yu-Chieh Chi, Cheng-Ting Tsai, and Huai-Yung Wang for their explanations and lectures about OFDM measurements. 5 TABLE OF CONTENTS Page EXAMINATION COMMITTEE APPROVALS FORM ....................................................... 2 ABSTRACT ................................................................................................................... 3 ACKNOWLEDGEMENTS ............................................................................................. 4 TABLE OF CONTENTS.................................................................................................. 5 LIST OF ABBREVIATIONS ............................................................................................ 7 LIST OF ILLUSTRATIONS.............................................................................................. 8 LIST OF TABLES ........................................................................................................... 9 1. Chapter One: Background ...................................................................................... 10 1.1 Introduction........................................................................................... 10 1.2 Motivation ............................................................................................. 10 1.3 Multiple quantum wells InGaN/GaN LED .............................................. 12 1.4 LED-based photodetector ..................................................................... 13 1.4.1 Responsivity & external quantum efficiency .......................... 14 1.4.2 Response time ........................................................................ 15 1.4.3 Frequency spectrum............................................................... 16 1.5 Orthogonal Frequency Division Multiplexing ....................................... 17 2. Chapter Two: Measurement Setup........................................................................ 19 2.1 Current–voltage characterization ......................................................... 19 2.2 Light source and responsivity calculation ............................................. 19 2.3 Light source and RC time constant measurement ................................ 20 2.4 Frequency spectrum measurement ...................................................... 21 2.5 Orthogonal Frequency Division Multiplexing ....................................... 22 3. Chapter Three: Devices Characterization .............................................................. 23 3.1 External Quantum Efficiency ................................................................. 23 3.2 Response Time ...................................................................................... 24 3.3 Frequency Spectrum ............................................................................. 25 3.4 Orthogonal Frequency Division Multiplexing ....................................... 26 3.4.1 Bias Voltage Depend .............................................................. 26 3.4.2 Start Frequency ...................................................................... 27 6 3.4.3 Bandwidth .............................................................................. 28 3.4.4 Summary ................................................................................ 30 4. Chapter Four: Conclusion....................................................................................... 31 BIBLIOGRAPHY……………………………………………………………………………………………………….. 32 7 LIST OF ABBREVIATIONS VLC Visible light communication OWC Optical wireless communication RF Radio frequency Li-Fi Light Fidelity LED Light-emitting diode PD Photodetector SSL Solid-state lighting OOK On-off keying YAG Y3Al5O12:Ce MQWs Multiple quantum wells OOK On-off keying FWHM Full width at half maximum OFDM Orthogonal frequency-division multiplexing FDM Frequency division multiplexing BER Bit error rate SNR Signal-to-noise ratio 4-QAM Quadrature phase shift keying FEC Forward error correction 8 LIST OF ILLUSTRATIONS Figure 1. White light spectrum of a coating YAG phosphor illuminated by blue light LED……………………………………………………............................................................................ 11 Figure 2. The band diagram of AlGaN/GaN/InGaN MQWs structure.............................. 13 Figure 3. Output wave forms of the LED-based PD.......................................................... 14 Figure 4. (a) The current-voltage characteristics of perovskite solution-processed hybrid perovskite PDs…………...................................................................................................... 15 Figure 5. The relation between the optical signal and electrical response of PD…......... 16 Figure 6. The spectrum efficiency is improved by OFDM modulation............................. 18 Figure 7. The different bit-to-symbol mapping for OFDM…............................................ 18 Figure 8. The profile shows the size of light spot and power intensity distribution which is uniform that we can calculate the power illuminated on devices............................... 19 Figure 9. The profile shows the size of laser spot and power intensity distribution which is Gaussian beam that we can calculate the power illuminated on devices…................. 21 Figure 10. The LED is used gold wire bonding bonded on a designed mount…............... 22 Figure 11. The structure of InGaN/GaN MQW LED…....................................................... 23 Figure 12. The photonics-electronics performance of LED-based PD.............................. 23 Figure 13. The time response measurement illuminated 405 nm laser diode which maximum power is 900 mW……………………………………………............................................. 24 Figure 14. The temporal characteristics of LED-based PD............................................... 25 Figure 15. The frequency spectrum of LED-based PD is applied in different bias........... 26 Figure 16. The OFDM testing depends on different bias voltage……………………............... 26 Figure 17. The BER and SNR plot shows start frequency only 2.34 GHz pass the FEC limit………………………………………………………………………………………………………………............... 28 Figure 18. The start frequency of bandwidth testing is 2.34 GHz…………………................. 29 9 LIST OF TABLES Table 1. The BER and SNR value is measured in