Photovoltage inversion effect and its application to semiconductor surface studies: CdS

Abstract

Photovoltage inversion, a new surface photoelectric effect, was discovered in CdS, and was manifested as an increase in surface barrier height under illumination of sub-bandgap energy. A small, but measurable, decrease in surface conductance was found to be associated with such barrier height increase; the time constants of the barrier and conductance changes were found to be identical and inversely proportional to the intensity of illumination. These findings could not be explained on the basis of the classical photovoltage theories, but were found consistent with a model involving photo-stimulated population of surface states. Photovoltage inversion, in conjunction with surface photovoltage spectroscopy 1), allows the study of surface states via two different processes, i.e., optical excitation of carriers into and from existing surface states. This approach is particularly effective in the case of wide bandgap materials, where standard methods are not readily applicable. Experimental results on the surface states of the basal plane of CdS are presented and discussed in the light of a photovoltage inversion model.