Browsing by Author "Yao, Yao"
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Item Enhancing Thermoelectric Performance in Single-Crystal-Like Semiconducting Films by Tuning the Carrier Scattering Mechanism(2017) Singh, Shivkant; Dutta, Pavel; Rathi, Monika; Yao, Yao; Gao, Ying; Sun, Sicong; Khatiwada, Devendra; Selvamanickam, Venkat; Mavrokefalos, AnastassiosItem Hetero-Epitaxial Growth of Ge and ZnO on Metal Substrates by Reel-to-Reel Physical Vapor Deposition for Flexible Device Fabrication(2017-12) Yao, Yao; Selvamanickam, Venkat; Ardebili, Haleh; Bao, Jiming; Ryou, Jae-Hyun; Meen, James K.Flexible electronic has attracted tremendous interest because of its extensive applications in various areas. Due to limitations in growth technologies, flexible electronics device fabrication currently relies on organic semiconductors or amorphous inorganic semiconductors. However, these materials suffer from issues such as low stability and poor electronic performance. To fabricate low-cost, high-efficient, sustainable and multi-functional flexible devices with high-quality crystalline inorganic semiconductors, methods for epitaxial deposition on flexible substrates need to be developed. In this dissertation, crystalline Ge and ZnO thin films are deposited on flexible metal substrates by reel-to-reel radio-frequency magnetron sputtering for use as functional semiconductors as well as buffer layers for the growth of other semiconductors such as Si, GaAs, InGaZnO4 and GaN. Firstly, Ge(001)//CeO2(001) epitaxial structure is developed on Hastelloy with multi-layer oxide buffers LaMnO3//epi-MgO//Ion-Beam-Assisted-Deposition (IBAD) MgO//α-Y2O3//α-Al2O3 as the template and diffusion barrier. In this study, it is found that defect density affects the crystalline quality of the Ge layer. Two origins for the defects are discussed. One is the twin defects caused by the lattice mismatch between Ge and CeO2, which can be decreased by expanding CeO2 lattice. Another one is the dislocations owing to the oxygen diffusion in the Ge layer. The growth temperature, deposition power and other parameters are optimized to reduce the density of defects and improve the Ge crystal quality. Hall mobility of the optimized sample reached 121 cm2/Vs. Moreover, the electron carrier concentration is successfully decreased to 1017 cm-3. Then, the growth of Ge(001)//CeO2(001) epitaxial structure on Ni-5at%W biaxially-textured tape is studied to reduce the procedures of buffer layer deposition. A two-step CeO2 growth method is applied to avoid the oxidation of Ni-5at%W surface. The atmosphere, temperature and deposition power are optimized to achieve the high-quality growth. Crystalline wurtzite ZnO(001) is successfully fabricated on CeO2(001)// LaMnO3//epi-MgO// IBAD MgO//α-Y2O3//α-Al2O3//Hastelloy. Two-domain ZnO growth happens to accommodate the symmetry mismatch between ZnO and CeO2. With nitrogen doping, the ZnO thin film can work as a p-type semiconductor with carrier concentration of 3.48×1018 cm-3 and mobility of 61.5 cm2/Vs. ZnO(100) is deposited on LaMnO3(001) surface as well, providing another possibility for crystalline ZnO growth. This study optimized the epitaxial growth condition for flexible Ge and ZnO, a narrow band gap semiconductor and a wide band gap semiconductor respectively. The outcome of this work provides an advanced crystalline inorganic semiconductor platform for flexible multifunctional electronics, emphasizing low-cost along with high material quality.Item Study of Electromagnetic Properties of Zr-Doped REBa2Cu3O7-X Coated Conductors Fabricated by Metal Organic Chemical Vapor Deposition over a Wide Range of Temperature & Magnetic Fields(2012-05) Yao, Yao; Selvamanickam, Venkat; Sun, Li; Ardebili, HalehZr-doped REBa2Cu3O7-x (REBCO and RE = Gd, Y in this thesis) coated conductors fabricated by the Metal Organic Chemical Vapor Deposition (MOCVD) method has demonstrated excellent in-field performance at 77K and a low magnetic field in previous research. However, the most important and promising applications of HTS coated conductors requires excellent in-field performance at various cryogenic temperatures between 77K and 20K, or even lower temperatures, with a high external magnetic field. In this thesis, REBCO coated conductor samples with different Zr contents (0%-15%) fabricated by the MOCVD method were systematically studied in a series of electromagnetic measurements at a wide range of temperatures and fields. Interesting pinning effects of various Zr-doping levels were observed across the temperature and field range. In order to interpret these pinning phenomena, microstructural analysis was done using SEM and TEM methods to reveal the influence of temperature, field and chemical composition on the performance and microstructure of Zr-doped REBCO coated conductors.