Electrochemical impedance analysis of electric field dependence of the permittivity of SrTiO3 and TiO2 single crystals

Electric field dependence of the permittivity of Nb-doped SrTiO3(001) (Nb:STO) and Nb-doped TiO2 (110) (Nb:TiO2) were investigated by using an electrochemical impedance spectroscopy method. The plots for Nb:STO could be well fitted to extrapolate the flat-band potential and the donor density, provided that a model of the electric field dependence of the permittivity of STO is incorporated into the conventional Mott–Schottky equation. The constants that represents a degree of the electric field dependence of the permittivity were calculated from the fitting parameters, and they were in good agreement with the reported values for the Nb:STO-based solid-state Schottky junction. In a similar way, the electric field dependence of the permittivity of Nb:TiO2 was revealed but it was not so strong as the case of Nb:STO, which is a basis for the validity of the linear fitting approximation with the conventional Mott–Schottky equation for TiO2.