Photovoltaic properties of ZnS crystals and a comparative study with luminescence

Abstract

In this paper we first analyze the problem of obtaining reproducible photovoltage measurements in ZnS crystals grown from the vapor phase in H₂S atmosphere. We show that the photovoltaic spectrum of such crystals can be as follows: (a) the crystal may exhibit two peaks at about 335 and 341 nm; (b) the crystal may exhibit only the peak at about 335 nm; (c) the crystal may exhibit only the peak at 341 nm. We find that only crystals of type (a) and (c) exhibit storage effects, i.e., the photovoltaic spectrum obtained with increasing wavelength is different from the spectrum obtained with decreasing wavelength. To investigate the photovoltage build up, we perform simultaneously fluorescence and photovoltage measurements, and we show that a close peak‐to‐peak correspondence exists only when an excitation of an impurity level localized near the valence band or an excitation between a localized impurity level and the conduction band is involved. This analysis is supported also from the behavior of these spectra when laser light is added. We show that among the various models proposed to explain photovoltage in ZnS crystals, the one which postulates negative internal fields in the hexagonal region and positive internal fields in the cubic region appears to be the most suitable.