Applications of Vapor Deposition in Microelectronics and Dye-Sensitized Solar Cells The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Wang, Xinwei. 2012. Applications of Vapor Deposition in Microelectronics and Dye-Sensitized Solar Cells. Doctoral dissertation, Harvard University. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:9826895 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA © 2012 - Xinwei Wang All rights reserved. Dissertation advisor Author Professor Roy G. Gordon Xinwei Wang Applications of Vapor Deposition in Microelectronics and Dye-sensitized Solar Cells Abstract Over the past decades, vapor deposition of thin films has gained wide interest in both industry and academia, and a variety of its applications have been demonstrated. As one of the most promising vapor deposition techniques, atomic layer deposition (ALD) and its applications in microelectronics and dye- sensitized solar cells are extensively investigated in this dissertation. ALD has many distinct features including low temperature processing, self-limiting growth, and precise control of film composition and thickness. Thus, ALD is considered to be suitable for conformal coating of 3D nanostructures, such as nanoporous structures, high aspect-ratio trench or hole structures, and so forth. Additionally, pulsed chemical vapor deposition (CVD) and its applications in microelectronics are explored in this dissertation. Ruthenium (Ru) is a promising electrode material for next generation microelectronic devices. The ALD and pulsed CVD processes discussed in Chapter 2 provide several approaches to produce smooth, conformal, pin-hole free Ru metal thin films. High-quality Ru films can be made under either oxidizing ambient or reducing ambient, which provides more flexibility for applications in microelectronics. Conductive ruthenium dioxide (RuO2) is also considered as a promising microelectrode material. Chapter 3 demonstrates a pulsed CVD process of depositing pure, smooth RuO2 films with reasonably low iii resistivity. Chapter 3 also demonstrates that RuO2 can be epitaxially grown on rutile TiO2(011) with a high-quality coherent heteroepitaxy structure. III-V MOSFET is now a very active area of growing interest to researchers and engineers in electronic industry and academia. Applications of ALD WN and high-k oxide materials for GaAs and GaN based devices are investigated in Chapters 4 and 5. Taking advantage of the conformal-coating feature of ALD, a stack of gate dielectric and metal gate can be coated uniformly around suspended nanowire structures, which is crucial for well-behaved gate-all-around MOSFETs. III-V MOSFETs also generally lack a suitable dielectric layer that has low interface trap density (Dit). Epitaxial ALD high-k dielectric lanthanum yttrium oxide, grown on GaAs(111)A, is found to have a fairly low Dit, and therefore, the electrical properties are dramatically improved with its inclusion. This finding is very insightful for the applications of next generation III-V MOSFETs. In addition, a few ALD processes of candidate dielectric materials for GaN based devices are discussed. Dye-sensitized solar cells have great potential to compete with conventional p-n junction solar cells due to their relatively low cost. However, their efficiency is limited by the ease with which electrons collected by the nanoparticle framework can recombine with ions in solution. As discussed in Chapter 6, by depositing insulating and transparent SiO2 selectively onto the open areas of nanoparticulate TiO2 surface, while avoiding any deposition of SiO2 over or under the organic dye molecules, the solar cell efficiency can be significantly improved. iv Acknowledgments First and foremost, I would like to present my sincere appreciation to my advisor Prof. Roy G. Gordon for his support, patience and encouragement throughout my PhD life at Harvard. Roy’s enthusiasm and passion in pursuing science has always been an inspiration to me. He is always very kind to share with me his broad knowledge and experience in chemistry, physics, materials science and electronics. I have benefited tremendously from a great number of instructive discussions with Roy over the past four years. I would also like to present my great appreciation to Prof. Frans Spaepen and Prof. Michael J. Aziz for their great support over the past years. In particular, during the past two graduate advising committee (GAC) meetings, Frans and Michael provided me plenty of insightful suggestions and comments on my research, without which this thesis would not have been completed. In addition, Frans taught my first course at Harvard (i.e. AP 282), which was found the most useful course for the following research. I am grateful for working with all the previous and current Gordon Group members. In particular, I would like to thank Dr. Hongtao Wang for mentoring me in the first a few months since I joined this group. I am also indebted to Dr. Zhefeng Li, Dr. Sheng Xu, and Dr. Yiqun Liu for their invaluable support and advice during my early years in this group. My appreciation also goes to Dr. Huazhi Li, Dr. Jun-Jieh Wang, Dr. Youbo Lin, Dr. Harish B. Bhandari, Dr. Bin Xi, Dr. Yeung (Billy) Au, Prasert Sinsermsuksakul, Leizhi Sun, and Jing Yang for their generous help for my research. Furthermore, I would like to particularly thank my new collaborator, Xiabing Lou, for his diligence as a scientist. I also enjoyed working with Dr. Jaeyeong Heo, Dr. Sang Bok Kim, Dr. Sang Woon Lee, Dr. Norifusa Satoh, Dr. Qiang Chen, Helen Park, Lauren Hartle, Kecheng Li, Danny Chua, Ashwin Jayaraman, and Lu Sun in this group. Finally, I would like give my special thanks to Rachel Heasley. She is very kind to help me edit this thesis. v I would also like to thank a number of collaborators, Prof. Shriram Ramananthan and Dr. Yanjie Cui at Harvard SEAS; Prof. Peide D. Ye, Jiangjiang Gu and Lin Dong at Purdue University; Prof. Tomás Palacios and Omair I. Saadat at MIT; and Prof. Joseph T. Hupp, Dr. Ho-Jin Son and Dr. Chaiya Prasittichai at Northwestern University. I have benefited so much from numerous informative discussions with these collaborators who are experts in a variety of areas. In addition, I would also like to appreciate the generous research technical support from Dr. Jiangdong Deng, Dr. Ling Xie, Adam Graham, Hao-Yu (Greg) Lin at Harvard CNS, and Dr. Scott Speakman at MIT CMSE, as well as the administrative support from Teri Howard and Marie Purcell. Lastly, I would like to present my deepest thanks to my parents for their long-time remote support over the years. vi Table of Contents Chapter 1 Introduction ...................................................................................................................... 1 1.1 Overview ................................................................................................................................... 1 1.2 Atomic layer deposition (ALD) .................................................................................................. 4 1.3 Pulsed Chemical Vapor Deposition (Pulsed CVD) ..................................................................... 8 1.4 Atomic layer epitaxy (ALE) ........................................................................................................ 9 1.5 Thesis outline .......................................................................................................................... 10 1.6 References .............................................................................................................................. 12 Chapter 2 Vapor Deposition of Ruthenium ..................................................................................... 14 2.1 Introduction ............................................................................................................................ 14 2.2 ALD of Ru with O2 .................................................................................................................... 16 2.2.1 Further development of ALD Ru process ........................................................................ 16 2.2.2 Ru on W substrates ......................................................................................................... 19 2.2.3 Summary ......................................................................................................................... 25 2.3 Pulsed Chemical Vapor Deposition of Ruthenium in a Reducing Ambient ............................. 25 2.3.1 Pulsed CVD Ru process and film characterization methods ........................................... 26 2.3.2 Properties of pulsed CVD Ru films .................................................................................. 28 2.3.3 Conclusions ..................................................................................................................... 37 2.4 References .............................................................................................................................. 38 Chapter 3 Pulsed Chemical Vapor Deposition of Ruthenium Dioxide and Epitaxy Analysis ........... 39 vii 3.1 Pulsed CVD of RuO2 ................................................................................................................. 39 3.2 High quality epitaxy