©2017 WEN-CHIANG HONG ALL RIGHTS RESERVED Magnesium Zinc Oxide High Voltage Thin Film Transistors By WEN-CHIANG HONG A dissertation submitted to the Graduate School-New Brunswick Rutgers, The State University of New Jersey In partial fulfillment of the requirements For the degree of Doctor of Philosophy Graduate Program in Electrical and Computer Engineering Written under the direction of Professor Yicheng Lu And approved by ______________________________________ ______________________________________ ______________________________________ ______________________________________ New Brunswick, New Jersey May, 2017 ABSTRACT OF THE DISSERTATION Magnesium Zinc Oxide High Voltage Thin Film Transistors By WEN-CHIANG HONG Dissertation Director: Prof. Yicheng Lu Energy is one of the most important topics in the 21st century, and solar energy has been a leading technology in the search to replace fossil-fuel energy as a sustainable and clean energy source. In order to provide an energy-efficient, less expensive, and reliable energy source, the PV system on glass (PV SOG) is emerging as an attractive concept. It integrates solar cells, solar inverters, and controller circuits on a single glass substrate. This dissertation focuses on development of the novel oxide-based high voltage thin film transistor (HVTFT) on glass technology, which is one of the core devices for solar inverter of the PV-SOG. Currently, the inverter counts for more than 10% of the total cost of an entire PV system. The solar inverter will be the major challenge of PV-SOG because the conventional solar inverters are bulky and could not be directly built on glass substrates. In particular, its key device, high voltage transistor, is not only pricy but also requires high process temperature which is incompatible with glass substrates. In comparison of several semiconductor materials, such as polycrystalline silicon, amorphous silicon, SiC and GaN, ZnO based materials have several ii promising features suitable for HVTFT on glass technology, including wide bandgap, high thermal conductivity, high mobility, and low deposition temperature. However, the thin film transistor (TFT) made up of the pure ZnO generally suffers from poor stability and reliability due to high defect density in the material. Because energy source is a basic unit of the infrastructure, it’s critical for a solar energy system to have a long lifespan. The first important issue of this dissertation research was to improve the TFT stability by adding a small amount of Mg into ZnO to form the ternary oxide, MgXZn1-XO (MZO, X<0.03) as the TFT channel. The density of oxygen vacancies in MZO was reduced so that after negative bias stress (NBS) the threshold voltage shift of MZO TFT was 30% smaller in comparison with the shift of ZnO TFT counterpart. Based on the solid foundation of stable MZO TFTs, MZO high voltage TFT (MZO HVTFT) on glass technology was designed and developed. To eliminate the electrical field crowding around the corners of the conventional TFT with a rectangular channel, a symmetric circular-shape transistor was adapted. From the simulation result, the peak electrical field is reduced by 50% in the symmetric circular structure than in the conventional rectangular structure. However, the MZO HVTFT with the circular configuration only showed a blocking voltage of 92V. To further enhance the device performances, especially the blocking voltage, we developed a modified MZO (m-MZO) HVTFT, which had an ultrathin MZO transition layer (MZO-TL) using the in-situ modulation doping in the channel- dielectric interface. The comprehensive characterizations using X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy iii (EDS) were conducted to study depth profiles of elements across the channel-gate dielectric interface. It was proved that this interface engineering effectively suppressed the interdiffusion of Zn and Si between the channel and dielectric layers, resulted in the reduction of the interface states and the oxide trapped charges. The combination of the interface engineering with the symmetric device design significantly increased the blocking voltage of the m-MZO HVTFT on glass. As a result, the regular m-MZO HVTFT (channel length=10µm) has on/off ratio of 3.5×1010 and blocking voltage of 305V, which is suitable for the regular AC 110V power system. The m-MZO HVTFT with a channel length=25µm has on/off ratio of 3.3×109 and blocking voltage of 609V which is suitable for the regular AC 220V power system. Finally, in order to expand the HVTFT technology from glass to the flexible substrate, the ZnO-based HVTFTs on plastic substrate were explored. By adopting low temperature even room temperature process, such as sputtering and atomic layer deposition, the flexible HVTFT consisting of the ZnO based channel with 9 Al2O3 dielectric layer showed an on/off current ratio of 10 and blocking voltage of 92V. The MZO HVTFT technology opens opportunities for cost-effective and highly efficient power management systems for many applications. The HVTFTs on glass will serve for the inverter in novel Building-integrated photovoltaics (BIPV) and smart glass while the flexible HVTFT is promising for the emerging self- powered wearable systems. iv Dedication To my family and mentor Chien-Yu Lin, who always loves me unconditionally, takes care of me and gives me strength. Professor Yicheng Lu, who teaches me the true importance of life and research. Chin-Kuo Hong and Hsiu-Chen Chen, who raised me and support the pursuit of my dream. Yi-Ting Wu and Shih-Kuei Lin, who give my family unreserved support and love. Zachery Hong and Katherine Hong, who are my angels. v Acknowledgement I would like to show my deepest gratitude to my dissertation advisor, Professor Yicheng Lu, for his limitless support in both my Ph.D. work and my daily life. His novel and insightful ideas inspired my creativity, and his precise and logical critiques polished my skills of logic. His patience and open-mindedness gave me great freedom to explore my passion. Having him as my advisor is my most fortunate opportunity in life. I also would like to thank my dissertation committee members, Prof. Jaeseok Jeon, Prof. Mehdi Javanmard, and Prof. Ming Lu for spending their valuable time to review my dissertation. Their profound questions and suggestions improved my understanding of the topics and helped to promote the improvement of my dissertation. It’s my honor to have had the chance to work with colleagues in Prof. Lu's research group, who have provided indispensable help to further my work. Dr. Chieh-Jen Ku gave me comprehensive training in device fabrication and set a solid foundation for my dissertation. Dr. Pavel Reyes guided me through writing papers, making proposals, and reviewing scientific articles. Dr. Rui Li gave professional and limitless support of material growth and analysis. My thanks also go to all other members: Dr. Ziqing Duan, Dr. Yang Zhang, Mr. Tengfei Xu, Mr. Tanvir Mohsin, Mr. Keyang Yang, Ms. Szu-Ying Wang, Ms. Ke Tang, Mr. Guangyuan Li, Mr. Yuxuan Li, Mr. Hongfei Ye, Ms. Navila Alim, and other visiting scholars and students. Their great assistance in the research as well as their friendship gave vi me the momentum and courage to solve difficulties in both scientific work and daily life. A special thank has to be given to Mr. Robert Lorber, our guardian of the cleanroom. Your talented mechanical skills keep our tools running. It has been an unforgettable experience to work with you and Dr. Pavel Reyes to manage the laboratory. My friends have given me great backing in my life. Ms. Carol Blumenthal, Dr. Jeremy Sykes, and Mrs. Tari Lee Sykes have always been there for my family and me. Your support helped us to stand on our own feet in the United States, and your wise advice guided us through the seemingly hopeless days. I really appreciate your great care and generous support. Above all, I would like to thank my family. My wife, Chien-Yu Lin, always supports me unconditionally, encourages me consistently, and loves me unreservedly. My parents, Chin-Kuo Hong and Hsiu-Chen Chen, always give me great freedom and support in my path to explore my passion. My parents-in-law, Yi-Ting Wu and Shih-Kuei Lin, always gives my wife, my children, and me the greatest love and care. This work has been supported by the National Science Foundation under Grant No. CBET-1264508. Research carried out (in part) at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No DE-SC0012704. vii Table of Contents ABSTRACT OF THE DISSERTATION ................................................................ ii Dedication ........................................................................................................... v Acknowledgement ............................................................................................. vi Table of Contents ............................................................................................. viii Lists of Tables .................................................................................................. xiii List of Illustrations ........................................................................................... xiv Chapter 1 Introduction .................................................................................. 1 1.1. Motivation ..............................................................................................