Structural and dielectric properties of epitaxial perovskite titanate thin films

Abstract

In recent years, tunable and affordable high-frequency microwave devices have been widely studied for the next generation of communication and radar system. Perovskite titanates such as SrTiO3 have relatively large and field-dependent dielectric constant and low loss, which are considered to be one of the most promising candidates for such application. Structural and dielectric properties of perovskite titanates are important in comprising those materials into devices. In the present thesis, the multilayer of SrTiO3/YBa2Cu3Oy was deposited using laser molecular beam epitaxy technique. The growth mode of the films was investigated using in-situ reflection high energy electron diffraction. The crystal structure in SrTiO3/YBa2Cu3Oy multilayer was measured by X-ray diffraction measurement. The morphology was determined with atomic force microscope. The Raman scattering measurement is employed to study the lattice dynamic properties of the heterostructure in the temperature range from 10 to 300 K. The effects of temperature and frequency on the dielectric properties of SrTiO3 thin films were evaluated. Secondly, epitaxial SrTiO3 films were grown on GaAs (001) substrates without any buffer layers using laser molecular beam epitaxy technique. The reflection high energy electron diffraction observations have revealed that a layer-by-layer growth of SrTiO3 was achieved by optimizing deposition process. The crystalline orientation of the as-grown SrTiO3 (001) films rotates 45o in plane with respect to the GaAs (001) substrates. Atomic force microscope studies show that these films possess atomically flat surfaces. The SrTiO3/GaAs interface with a sharp interface was investigated by transmission electron microscopy. The dielectric properties of the heterostructure were also investigated. Our results have clearly demonstrated the practicality of integrating perovskite oxide thin films with GaAs substrates. Thirdly, Ferroelectric BaTiO3 thin films were epitaxially grown on (001) GaAs substrate using SrTiO3 as a buffer layer by laser molecular beam epitaxy. The interface structure was analyzed by means of reflection high energy electron diffraction and transmission electron microscopy. The perovskite SrTiO3 buffer layer presents a body centered cubic structure by forming 10nm-thick interfacial layer, with the [100]SrTiO3//[110]GaAs in-plane relationship. Thereupon, a highly c-oriented BaTiO3 was grown on SrTiOs/GaAs in a layer by layer mode with a sharp interface and a significant reduction of mismatch-related defects. The BaTiO3 (150nm)/SrTiO3(10nm)/GaAs heterostructure demonstrates ferroelectric hysteresis behavior with a remnant polarization of 2.5C/cm 2 and a small leakage current density of 2.9x10-7 A/cm2 at 200 kV. The observation shows that it is feasible for integrated device applications by introducing SrTiO3 buffer layer at the interface between film and GaAs.