The influence of the local oxygen vacancy concentration on the piezoresponse of strontium titanate thin films

In this study, the influence of the local oxygen vacancy concentration on piezoresponse force microscopy (PFM) measurements was investigated. Ultra-thin single-crystalline SrTiO₃ thin films were deposited on niobium doped SrTiO₃ substrates and analyzed using a combined PFM and local conductive atomic force microscopy (LC-AFM) measurement setup. After applying different polarization voltages between ±2 V and ±5 V to the thin films, we simultaneously observed an anomalous contrast in the piezoresponse amplitude and phase signal as well as a changed local conductivity in the exact same region. Since classic ferroelectricity can be excluded as the reason for the observed contrast, an influence of the local oxygen vacancy concentration on the piezoresponse is considered. Additionally, the surface potential was measured using Kelvin probe force microscopy (KPFM) revealing a change in surface potential in the regions of the applied voltage. The observed relaxation of the surface potential over time was fitted to a local oxidation reaction of the previously reduced regions of the ultra-thin SrTiO₃ film. We propose a model that relates the local oxygen vacancy concentration to the surface potential. The influence of the oxygen vacancy concentration on the PFM measurements is explained.