IN-SITU STUDIES OF SURFACE FILMS BY PHOTOELECTROCHEMISTRY.

For different types of surface films and depending on thickness, the following cases can be distinguished. The film is p-type semiconducting (a) or n-type semiconducting (b), both under depletion conditions, or the film is insulating (c). The sign of the photocurrent depends on the sign of the electric field at the surface. Simultaneous to an excitation in the film, excitation in the base material (i. e. , in the metal) with a resulting photoemission process can happen as well. Theoretical descriptions of the various possibilities are given together with typical examples of passive films, ion implanted oxide films, and mixed oxide films. The interpretation of photoelectrochemical data allows for an estimate of the energetic positions of the conduction band and the valence band at the surface, the flatband potential, and possibly for a distinction between a crystalline or amorphous structure. By that also an identification of unknown surface film is possible. With these results the surface film can be described in terms of an energy diagram. This helps to interpret electrochemical behavior which is connected to the properties of the surface film.