Epitaxial growth of SrCaTiO3 films on GaN by molecular beam epitaxy with a TiO2 buffer layer

We demonstrate the epitaxial growth of (111)-oriented Sr1-xCaxTiO3 (SCTO) thin films on (0002) GaN using a thin (100) TiO2 buffer layer by RF-plasma-assisted oxide molecular beam epitaxy. We explore the growth window of SCTO for both x=0 (i.e., SrTiO3) and x=0.24 with various TiO2 buffer layer thicknesses and find that a substrate temperature of 650 degrees C and TiO2 thickness of 1nm produce the highest quality films, as determined by reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy. SCTO films deposited without a buffer layer are polycrystalline, confirming the necessity of the TiO2 buffer layer to facilitate (111)-oriented epitaxial growth of SCTO on GaN. Transmission electron microscopy performed on the samples shows that the SCTO films grown with a TiO2 buffer layer are highly textured, and selected-area electron diffraction patterns indicate that some domains in the oxide exhibit cation ordering along the (111) direction. We also show that the growth mode changes from 2D to 3D as the thickness of SCTO increases past 10nm, which leads to film relaxation and a rougher surface morphology.