Anomalous photovoltaic effect and disorder in ZnS crystals

Abstract

Some crystals of ZnS are known to produce an anomalously high photovoltage, up to several hundred volts per cm, when illuminated by UV light in the absorption edge region. This has been attributed to the presence of alternate regions of hexagonal and cubic packing with charged dislocations at the interfaces producing built-in electric fields. Differential absorption of the incident light in the hexagonal and cubic regions is believed to create the necessary asymmetry in the built-in fields, causing an addition of tiny photovoltages at a series of interfaces which finally results in the abnormally high photovoltages observed. This paper investigates the possible mechanism by which disordered ZnS crystals containing alternating regions of cubic and hexagonal packing can result. X-ray diffraction studies show that such a disordered configuration results during the 2H to 3C phase transformation in ZnS. It is suggested that the transformation occurs by the non-random nucleation of deformation faults wherein the probability (α) of random nucleation of the faults is much less than the probability (β) for the faults to occur at two-layer separations.