ABSTRACT
ABSTRACT The electronic properties of oxide films can be described by the semiconductor model in a first approximation. Then, a homogeneous defect concentration N with a defined energy level E is assumed. Due to amorphicity and ionic conductivity of the films, additional defects may be formed at the interfaces and migrate into the film, thereby giving rise to complex dependencies of the defect concentration on energy E, location x, and time t, N(E, x, t). The changes in concentration will vary the reactivity of the films. The course of the related processes is followed by i(t) and q(t) transients, whereas the role of defects is obtained by fast capacity measurements with potentiostatic pulses. Under reasonable assumptions, an evaluation based on the Schottky-Mott model yields donor concentrations and properties of the surface states. In special cases concentration profiles N(x) can be also derived. From the time dependence of the concentration profile N(x, t), the migration of defects can be described. The oxide growth on passive iron is an example for a reaction that strongly depends on the local defect concentration. As a result, either field-dependent or diffusion-limited oxide growth is observed. Intrinsic redox reactions, for example within the passive films on Ti and Ni, can also be followed. The redox process itself is characterized by i(t) transients, whereas C(U) measurements yield corresponding information of the migration of protons. The formation of different types of surface states is obtained during anodic oxygen evolution on Fe and Ni in the dark and on Ti under illumination.
