Effects of the TiO2 buffer thickness on SrTiO3 (111) epitaxial films grown on GaN (0002)

SrTiO3 (STO) films have been grown on TiO2-buffered GaN(0002) substrates. The deposition process was in situ monitored by reflective high energy electron diffraction (RHEED). The deposition rate and in-plane lattice parameter of TiO2 were calculated from the oscillation curve and RHEED patterns, respectively. It was found that the TiO2 lattice parameter changed as the thickness increased, which indicated a strain relaxation process of TiO2 buffer layers during the deposition. We show that the thickness of TiO2 can significantly influence the STO growth mode, surface morphology, and crystalline quality. As the TiO2 thickness increased, the STO growth mode is changed from 3D island to 2D growth mode and finally to SK mode as revealed by RHEED. The growth mode evolution shows close relation with the surface morphology and crystalline quality of STO. STO films deposited on 2 nm thick TiO2 film show 2D growth mode and have smoothest surface and smallest full width at half maximum of the STO rocking curve. Our investigation hints towards to a general approach to optimize the crystalline quality of STO, which can be optimized by adjusting the strain state of TiO2 buffer layer.