UNIVERSITY OF CALIFORNIA Los Angeles Growth and Characterization of Zr and ZrC Thin Films on Al2O3(0001) A thesis submitted in partial satisfaction of the requirements of the degree Master of Science in Materials Science and Engineering by Joshua Paul Fankhauser 2016 ABSTRACT OF THESIS Growth and Characterization of Zr and ZrC Thin Films on Al2O3(0001) by Joshua Fankhauser Master of Science in Materials Science and Engineering University of California, Los Angeles, 2016 Professor Suneel Kodambaka, Chair I report the growth of epitaxial Zr(0 0 0 1) and ZrC(1 1 1) thin films on Al2O3(0 0 0 1) via dc magnetron sputtering in an ultra-high vacuum deposition system equipped with facilities for chemical vapor deposition, low-energy electron diffraction, and Auger electron spectroscopy. Zr layers with a nominal thickness up to 270 nm are deposited at a rate of ~0.07 nm/s in 10 mTorr Ar (99.999%) atmosphere. ZrC layers with a nominal thickness up to 110 nm are deposited at a rate of ~0.06 nm/s in 10 mTorr atmosphere composed of 1 mTorr C2H4 (99.999%) and 9 mTorr ii Ar. As part of my thesis work, I investigate the effect of substrate temperature during sputter- deposition on the composition and crystallinity of the Zr and ZrC films. The as-deposited films are characterized in situ using Auger electron spectroscopy and low energy electron diffraction and ex situ using x-ray diffraction, transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. I deposited Zr thin films at temperatures between 600 °C and 900 °C. My x-ray diffraction studies reveal that increasing the substrate temperature during sputter-deposition of Zr leads to the growth of polycrystalline hexagonal close-packed structure Zr films. Cross-sectional transmission electron microscopy images reveal columnar growth and the formation of an interfacial layer, whose thickness increased with increasing temperature. Energy dispersive x-ray spectra obtained from this region reveal the presence of both Zr and Al. I attribute the formation of this interfacial layer to plasma-induced substrate decomposition followed by interdiffusion of Al and Zr at the film-substrate interface during sputtering. I deposited ZrC layers at temperatures between 800 °C and 1400°C. X-ray diffraction data acquired from my samples indicate that the crystallinity improves with increasing temperature. X-ray photoelectron spectra reveal that all of my films contain excess carbon, whose content decreases with increasing temperature. Based upon my results, I identify optimal growth temperatures for obtaining single-crystalline and epitaxial Zr and ZrC layers. iiiiii The thesis of Joshua Paul Fankhauser is approved. Jenn-Ming Yang Mark Goorsky Suneel Kodambaka, Committee Chair University of California, Los Angeles 2016 iv Table of Contents Chapter 1. Introduction ………………………………………………….……... 1 1.1 Transition Metal Carbides ………………………………………………… 1 1.2 Properties of Zirconium and Zirconium Carbide …………………………. 1 1.3 Applications of Zirconium ………………………………………………... 2 1.4 Applications of Zirconium Carbide ………………………………………. 2 1.5 Growth of Zirconium ……………………………………………………... 3 1.5.1 Magnetron Sputtering ………………………………………………... 3 1.5.2 Pulsed laser Deposition ………………………………………………. 4 1.6 Growth of Zirconium Carbide ……………………………………………. 4 1.6.1 Chemical Vapor Deposition …………………………………………. 4 1.6.2 Thermal Evaporation ………………………………………………… 5 1.6.3 Pulsed Laser Deposition ………………………………………………5 1.6.4 Magnetron Sputtering ……………………………………………….. 5 Chapter 2. Experimental ……………………………………………………….. 7 2.1 Zr Thin Film Deposition ………………………………………………….. 7 2.2 ZrC Thin Film Deposition ………………………………………………… 9 2.3 Characterization …………………………………………………………. 10 2.3.1 Low Energy Electron Diffraction & Auger Electron Spectroscopy ... 10 v 2.3.2 X-ray Photoelectron Spectroscopy …………………………………..10 2.3.3 X-ray Diffraction …………………………………………………….11 2.3.4 Transmission Electron Microscopy ………………………………….12 Chapter 3: Zr Thin Film Results ………………………………………………13 3.1 In situ Characterization ………………………………………………….. 13 3.2 Growth of Single-Crystalline Zr/Al2O3(0001) …………………….…….. 14 3.3 Film-Substrate Interfacial Layer ………………………………..………. 17 3.4 Outlook ………………………………………………………………….. 22 Chapter 4: ZrC Thin Film Results ………………………………………….... 23 4.1 X-ray Diffraction Data ………………..………………………………… 23 4.2 X-ray Photoelectron Spectroscopy of ZrC Thin Films ………………….. 24 4.3 Microstructure ………………………………………..…………………. 25 Chapter 5: Conclusions ………………………………………………….……. 27 References …………………………………………………………………….... 28 vi Acknowledgements I would like to thank Prof. Kodambaka for his guidance and express my appreciation for his help in completing these experiments. In addition, I would like to thank Dr. Abbas Ebnonnasir for his patience and assistance with teaching me the principles of ultra-high vacuum (UHV). Without his help, I would never have been able to operate and understand the capabilities of a UHV system. I cannot thank Dian Yu enough for all of her time spent performing transmission electron microscopy, without which I would not have completed this thesis. In addition, I would like to thank Prof. Goorsky and Chao Li for their assistance with the acquisition and analysis of my x-ray diffraction data. My lab group of Abbas Ebnonnasir, Pedro Arias, Denisa Vlasache, Masaki Sato, and Taichi Nakao, kept me going through my entire time at UCLA and made every day interesting. I would like to thank my better half Kellie Brown for all the time and support she has given me over the past six years. Finally, I would like to thank my family for their support in my pursuit of this M.S. degree and throughout my life. vii Chapter 1: Introduction 1.1 Transition Metal Carbides The aerospace industry has long been reliant on coating technologies. Spacecraft, rockets, and hypersonic vehicles all operate under extreme environments experiencing high temperatures (>2200 °C), drastic thermal cycling, and both neutral and oxidizing atmospheres. In order to survive such environments, materials are selected with desirable properties such as oxidation resistance, a low vapor pressure at operating temperatures, good mechanical properties, and chemical stability.1 One class of materials is favored as a potential candidate for these coatings, transition-metal carbides (TMC). This class of materials has some of the highest melting temperatures and generally exhibit high strengths and hardnesses, as well as good corrosion resistances.2 One such TMC is ZrC, which has many of these desirable characteristics as will be described below. 1.2 Properties of Zirconium and Zirconium Carbide The parent element in ZrC is the group IVB transition-metal Zr. Elemental Zr exhibits a hexagonal close-packed (hcp) crystal structure at room temperature with bulk in-plane (a) and out-of-plane (c) lattice parameters of 0.3233 nm and 0.5149 nm, respectively.3 At a temperature T > 863 °C,4 Zr undergoes a phase transition from hcp to body-centered cubic (bcc) and retains the same structure until its melting point (1852 °C).5 Pure Zr has a tensile strength of 150 – 270 MPa and an elastic modulus of 72 GPa and is considered to be one of the more ductile metals due to the fact that it deforms on three independent slip planes and 1 multiple twinning planes.5, 6 In addition, Zr also has a low absorption cross section of 0.18 ± 0.02 barns for thermal neutrons.6 As a transition-metal refractory carbide, ZrC is known primarily for its physical properties such as: high hardness (25-35 GPa), high melting temperature of ~3500 °C, chemical inertness to strong aqueous acids and bases, metal-like resistivity (5 × 10-7 Ω•m), and relatively low work function (4.0 eV) compared to materials such as Si and Mo.7-11 ZrC exhibits NaCl structure with a bulk lattice parameter of 0.46976 nm and typically contains 0.5-2 at.% Hf impurities.12 1.3 Applications of Zirconium Zr is primarily used in nuclear reactors as a structural material due to its high corrosion resistance to acids and bases, good mechanical properties, and low absorption cross section.6, 13 Zr is also used as a fuse, as a primer, as a getter in vacuum tubes, in chemical processing 5 14 plants, and as an alloying element. Zirconium based compounds such as zircon (ZrSiO4) are used as a refractory material in glasses and steels, ZrC and ZrN as hard protective layers on 15, 16 cutting tools, and ZrB2 as thermally-resistive hard coatings in hypersonic vehicles. Metallic Zr thin films are used in multi-layer nuclear fuel particles, as high-strength and diffusion-barrier layers,17 as corrosion resistant coatings on both high carbon steel cutting tool tips,18 and on “First Mirrors”, used for optical diagnosis of the nuclear fusion process.19 1.4 Applications of Zirconium Carbide ZrC is commonly used in the aerospace industry, for example in hypersonic vehicles, due to its thermochemical and thermomechanical stabilities in extreme operational 2 environments.20 ZrC thin films are considered as potential replacements for SiC as the main pressure barrier in multi-layer nuclear fuel particles due to the fact that ZrC does not undergo any phase transitions, has very little reaction with Pd, and has good fission product retention capability.10 The tribological properties of ZrC are attractive for a variety of applications. For example, ZrC coatings can be used on sliding electrical contacts due to their high hardness and conductivity, in micro-electromechanical devices, in circuit breakers, and in motor vehicle starters.21 1.5 Growth of Zirconium Zirconium thin films are deposited using a few different methods (outlined below) all within the category of physical vapor deposition (PVD). Zr is highly reactive and readily forms compounds upon exposure to air. Therefore, pure Zr films are best grown in a high or an ultra-high vacuum (UHV) chamber and inert atmospheres. 1.5.1 Magnetron Sputtering One approach to the growth of Zr thin films is to use magnetron sputtering. In this method a metallic target is placed on a magnetron in an inert gaseous atmosphere. The magnetron generates plasma at the surface of the target and the gas ions present in the discharge knock off the target material, some of which is collected on the substrate surface.