Analysis of nonlinear Mott-Schottky plots obtained from anodically passivating amorphous and polycrystalline TiO₂ films

Abstract

The present work is concerned with analysis of the nonlinear Mott-Schottky plots obtained from the measured impedances of anodically passivating amorphous and polycrystalline TiO₂ films including multiple donor levels. The passivating amorphous TiO₂ films were prepared on titanium in 0.5 M H₂SO₄ solution galvanostatically at 2 mA cm⁻² at formation potentials of 2, 3 and 5 V/SCE, and the polycrystalline films at 5 mA cm⁻² at potentials of 20 and 30 V/SCE. The analysis was made by obtaining a numerical solution to a modified Mott-Schottky equation by using the Euler method. Based upon the analysis, the applied potential dependence of the donor distribution across the amorphous and polycrystalline TiO₂ films was determined for various film thicknesses and measuring frequencies. Ionized donor concentration increases with increasing applied potential, regardless of film thickness and measuring frequency. The thinner film and the lower measuring frequency enhance the donor concentration. It is suggested from the analysis of the nonlinear Mott-Schottky plots that the applied potential dependence of the donor distribution is attributable to the presence of multiple donor levels and that the donor concentration as a function of applied potential is definitely determined by the film thickness and measuring frequency.