Direct Observation of Film Polarization and Oxygen Vacancies at the BaTiO$_{3}$/SrTiO$_{3}$/GaAs Interface

We report successful growth of BaTiO$_{3}$ thin films on GaAs (001) with a SrTiO$_{3}$ buffer layer using oxide molecular beam epitaxy (MBE), and investigate the oxide/semiconductor interface using atomic-resolution imaging, electron energy loss spectroscopy (EELS) and first principles density functional theory (DFT). Atomic-resolution Z-contrast and annular bright field (ABF) images of BaTiO$_{3}$/SrTiO$_{3}$/GaAs reveal atomically sharp interfaces and show no sign of interfacial diffusion or extensive sensitivity to the electron beam. ABF images also show that the first SrO monolayer in contact with the GaAs substrate is highly oxygen deficient, and the SrTiO$_{3}$ buffer layer has an out of plane polarization due to the presence of oxygen vacancies, which can be directly observed by the displacement between the Ti and O columns. The Ti $L_{2,3}$ and O $K$ edge spectra from the SrTiO$_{3}$/GaAs interfacial Ti columns indicate the presence of oxygen vacancies and a distortion of the TiO$_{6}$ octahedra. DFT calculations show that O vacancies form preferentially at the SrTiO$_{3}$/GaAs interface, where they polarize the SrTiO$_{3}$, and, in turn, inhibit the ferroelectric switching in the BaTiO$_{3}$.