Influence of Sr deficiency on structural and electrical properties of SrTiO thin films grown by metal-organic vapor phase epitaxy.

Homoepitaxial growth of SrTiO thin films on 0.5 wt% niobium doped SrTiO (100) substrates with high structural perfection was developed using liquid-delivery spin metal-organic vapor phase epitaxy (MOVPE). Exploiting the advantage of adjusting the partial pressures of the individual constituents independently, we tuned the Sr/Ti ratio of the gas phase for realizing, stoichiometric, as well as Sr deficient layers. Quantitative energy dispersive X-ray spectroscopy in a scanning transmission electron microscope confirm Sr deficiency of up to 20% in nominally off-stoichiometrically grown films. Our MOVPE process allows to grow such layers in phase pure state and without extended defect formation. Indications for oxygen deficiency could not be identified. Sr deficient layers exhibit an increased permittivity of ɛ = 202 and a larger vertical lattice parameter. Current-voltage characteristics (IVCs) of metal-oxide-semiconductor (Pt/SrTiO/SrTiO:Nb) structures reveal that Sr deficient SrTiO films show an intrinsic resistive switching with on-off ratios of three orders of magnitude at RT and seven orders of magnitude at 10 K. There is strong evidence that a large deviation from stoichiometry pronounces the resistive switching behavior. IVCs conducted at 10 K indicate a defect-based mechanism instead of mass transport by ion diffusion. This is supported by in-situ STEM investigations that show filaments to form at significant higher voltages than those were resistive switching is observed in our samples.