Atomic Scale STEM/EELS and First Principles Studies of Oxide-Semiconductor Interfaces

Ultrathin transition-meal oxide films on polar substrates have attracted increasing attention in recent years, due to the emergence of novel interfacial phases, not seen in the bulk of either material. In this study, I have combined aberration-corrected atomic-resolution Z-contrast imaging, electron energy loss spectroscopy (EELS) with first-principles density functional theory calculations to examined the atomic and electronic structures of epitaxially grown, ultrathin SrTiO3 (100) films on GaAs (001). I find that the interface is atomically abrupt and no surface reconstruction of the GaAs (001) surface is observed. Using atomic-column resolved EELS, we show that Ti diffuses into the first few monolayers of GaAs and we will present evidence for the formation of As-oxides at the interface depending on the thin film growth conditions. First-principles DFT calculations will be used to analyze the formation energies of Ti-related impurity defects in the bulk and surface regions of GaAs, as well as the stability of any surface reconstruction at the SrTiO3/GaAs interface. These findings are used to explain transport behavior of the SrTiO3 films as a function of deposition conditions. Based on the fundamental understanding of the SrTiO3/GaAs interfaces, the epitaxial properties of ferroelectric BaTiO3 grown on GaAs with a SrTiO3 buffer layer have been extensively studied. The BaTiO3 film is free of defects and exhibits an out of plane polarization, while the charge screening effects at both the BaTiO3/SrTiO3 and SrTiO3/GaAs interfaces have been observed using atomic-resolution HAADF and ABF imaging and EEL spectroscopy. Additionally, the dynamic charge transfer during in-situ polarization switching following the application of an electrical bias to the ferroelectric oxide will be discussed.