Studies of in-plane anisotropic physical properties in a-plane MgXZn1-XO

MgxZn1-xO is a compound semiconductor material formed by alloying ZnO with MgO. The larger direct bandgap of MgxZn1-xO (~4eV for Mg0.33Zn0.67O) renders it useful as a barrier layer in ZnO/MgxZn1-xO based heterostructures and quantum wells. Conventionally grown c-oriented ZnO based quantum wells suffer from piezoelectric and spontaneous polarization fields, leading to lower quantum efficiency. Non-polar a-plane and m-plane ZnO based heterostructures avoid such problems. The non-polar MgxZn1-xO films also possess in-plane anisotropic optical, acoustic and electrical properties, useful for novel polarization sensitive devices. However, as-grown non-polar MgxZn1-xO films show rougher surface compared to its c-plane counterpart, introducing serious difficulty in growth of high quality heterostructures. This dissertation addresses growth optimization and comprehensive characterization of a-plane MgxZn1-xO (0[less than or =] x [less than or =] 0.33) films on r-sapphire substrates using Metalorganic Chemical Vapor Deposition (MOCVD). Angle Resolved Auger electron spectroscopy (ARAES) indicates that Mg replaces Zn in ZnO lattice. Mg composition and films' crystal properties are characterized using transmission spectroscopy and x-ray diffraction (HRXRD), respectively. The in-plane strain along c-axis is compressive, while perpendicular to c-axis is tensile. Strain anisotropy reduces with increase in film thickness due to relaxation, with complete relaxation at film thickness of ~2µm. An increase in Mg composition increases a-axis lattice parameter and reduces c-axis lattice parameter, resulting in higher strain in the films. Scanning tunneling microscopy (STM) shows long and elongated terraces on films surface due to anisotropic surface diffusion and strain-relaxation. The direction of terraces is dependent on the miscut direction of r-sapphire substrates. The effects of film thickness, Mg composition, and deposition temperature on surface morphology have been analyzed. A red shift in the optical transmission edge is observed for electric field polarized perpendicular to in-plane c-axis as compared to that along in-plane c-axis. An increase in Mg composition reduces the separation in transmission edge. In-plane anisotropy in Hall mobility is analyzed, with mobility measured perpendicular to c-axis being higher than that along c-axis. The ZnO thin film transistors (TFTs) fabricated with devices aligned along and perpendicular to c-axis. The TFTs show high on-off ratio up to ~109. The field-effect mobility and transconductance of ~35.5cm2/Vs and 1.09mS/mm are obtained.Ph.D.Includes bibliographical references (p. 118-126)