This electronic thesis or dissertation has been downloaded from Explore Bristol Research, http://research-information.bristol.ac.uk Author: Zhou, Yan Title: Thermal/Mechanical/Structural Properties of Polycrystalline Diamond and Novel Layered Materials for Electronic Devices General rights Access to the thesis is subject to the Creative Commons Attribution - NonCommercial-No Derivatives 4.0 International Public License. A copy of this may be found at https://creativecommons.org/licenses/by-nc-nd/4.0/legalcode This license sets out your rights and the restrictions that apply to your access to the thesis so it is important you read this before proceeding. Take down policy Some pages of this thesis may have been removed for copyright restrictions prior to having it been deposited in Explore Bristol Research. However, if you have discovered material within the thesis that you consider to be unlawful e.g. breaches of copyright (either yours or that of a third party) or any other law, including but not limited to those relating to patent, trademark, confidentiality, data protection, obscenity, defamation, libel, then please contact [email protected] and include the following information in your message: •Your contact details •Bibliographic details for the item, including a URL •An outline nature of the complaint Your claim will be investigated and, where appropriate, the item in question will be removed from public view as soon as possible. Thermal/Mechanical/Structural Properties of Polycrystalline Diamond and Novel Layered Materials for Electronic Devices Yan Zhou A thesis submitted to the University of Bristol in accordance with the requirements for award of the degree of Doctor of Philosophy at the H. H. Wills Physics Laboratory School of Physics, Faculty of Science May 2018 Main text word count: ~62,000 Declaration of authorship I declare that the work in this dissertation was carried out in accordance with the requirements of the University’s Regulations and Code of Practice for Research Degree Programmes and that it has not been submitted for any other academic award. Except where indicated by specific reference in the text, the work is the candidate’s own work. Work done in collaboration with, or with the assistance of, others, is indicated as such. Any views expressed in the dissertation are those of the author. Signed: …………………………………………………… Date: ……………………………………………………… Abstract Under the ever-increasing requirements of higher power and higher radio-frequency in future power electronics and telecommunication applications, the thermal management of gallium nitride (GaN) based devices becomes crucial, this can be significantly improved by integrating high thermal conductivity diamond into the devices to enhance the extraction of waste heat. An important consideration is the thermal boundary resistance (TBR) of the GaN/diamond interface, which forms a bottleneck for heat transport. For incorporation as a substrate, the thermal properties of this interface and of the polycrystalline diamond (PCD) grown onto GaN using various controlled barrier layers under different growth conditions are investigated and systematically compared; SiN barrier layers were found normally producing lower TBR with a smoother interface formed. For integration of PCD as a top-side heat spreader onto AlGaN/GaN-on-Si HEMT, its thermal performance was systematically evaluated by time-domain thermoreflectance and ANSYS simulation; at best a 15% reduction in peak temperature was obtained when only the source-drain opening of a passivated AlGaN/GaN-on-Si HEMT is overgrown with PCD. Meanwhile, next generation higher compacted electronics for future communications or computing require sub-10-nm or even atomic dimension scaling, incorporation of a new two- dimensional (2D) materials channel then has emerged as a highly attractive solution to address this challenge. Gallium telluride (GaTe) is a 2D layered material that recently raised considerable interests due to its unique optoelectronic properties but is still under extensive exploration of its fundamental properties for potential applications. The pressure-dependent solid-state properties of GaTe multilayers up to 46 GPa were firstly investigated. A strong Raman mode anisotropic splitting started at ~6.5 GPa originating from phase transition was first-time revealed and understood through first-principles calculations. Then the thermal properties of free-standing GaTe multilayers were studied mainly by micro-Raman opto- thermography, displaying an anisotropic and very low thermal conductivities along the in- plane armchair and zigzag orientations. Moreover, the mechanical properties of both SiO2/Si substrates supported and free-standing GaTe multilayers were investigated mainly through nanoindentation. Concurrence of multiple pop-ins and load-drops in the loading curve were found, likely originating from interlayer sliding within the GaTe multilayer. These pressure- tuned behaviors, thermal and mechanical properties of GaTe multilayers enable new insights for investigating and manipulating the anisotropic solid-state properties of potential device applications and other low symmetry layered materials. i ii To my dear family and friends 致 我的家人和朋友们 “We don’t get a chance to do that many things, and everyone should be really excellent. Because this is our life.” Steve Jobs (1955-2011) Acknowledgements First, I would like to thank my supervisor, Prof. Martin Kuball, who supported and advised me throughout my PhD study. Then, I would like to acknowledge Dr. James Pomeroy, Dr. Julian Anaya and Dr. Huarui Sun who mentored me in thermal theory, characterization, simulation and writing, Dr. Dong Liu who helped me the skills in nanoindentation based mechanical analysis, and Dr. Yong Xie who helped me in the fabrication skills of layered materials. Finally, I want to acknowledge the collaboration work with Mr. Rajesh Rameneti and Prof. Ken Hannen in University of Hasselt, Qorvo Company, Mr. Qinghua Zhao in Northwestern Polytechnical University and Dr. Jianbo Liang in Osaka City University. I would also warmly thank everyone in the Centre of Device Thermography and Reliability Group, Materials and Devices for Energy and Communication Group for making the laboratory a stimulating environment to work in. A special thank you to my dear family and friends for supporting me. Table of Contents Abstract ...................................................................................................................................... i Table of Contents ....................................................................................................................... I List of Figures ........................................................................................................................... V List of Tables ......................................................................................................................... XIII List of Abbreviations .............................................................................................................. XV List of Publications .............................................................................................................. XVII Chapter 1. Introduction ............................................................................................................. 1 1.1 A brief history for semiconductor transistors ............................................................ 1 1.2 Challenges and improvements in AlGaN/GaN based devices and 2D devices ......... 2 1.3 The structure of this work .......................................................................................... 5 Chapter 2. Properties of Materials and Devices ........................................................................ 7 2.1 Properties of GaN semiconductors and HEMT devices ............................................ 7 2.1.1 Crystal structure .................................................................................................... 7 2.1.2 Electronic band structure ....................................................................................... 9 2.1.3 Phonon modes and phonon dispersion ................................................................ 11 2.1.4 Thermal conductivity .......................................................................................... 13 2.1.5 Growth ........................................................................................................ 18 2.1.6 AlGaN/GaN high electron mobility transistors (HEMTs) .................................. 21 2.2 Properties of diamond .............................................................................................. 26 2.2.1 Crystal structure .................................................................................................. 26 2.2.2 Electronic band structure ..................................................................................... 27 2.2.3 Phonon dispersion ............................................................................................... 28 2.2.4 Thermal conductivity .......................................................................................... 29 I Table of Contents 2.2.5 Growth of CVD diamond .................................................................................... 31 2.2.6 Thermal boundary resistance ............................................................................... 38 2.3 Properties of layered materials and GaTe ...............................................................