Anomalous Photovoltaic Effect in ZnS Single Crystals

Larger-than-band-gap photovoltages have been observed in both cubic and hexagonal crystals of ZnS with stacking faults. Under monochromatic excitation the photovoltage measured parallel to the c axis shows two sign reversals at about 3300 A and 3500 A and peaks at approximately 3250 A and 3400 A. The position of the 3400 A peak shifts by {~}70 A depending on the crystal structure. In the direction perpendicular to the c axis the photovoltage does not change sign and remains smaller than the band gap. Hexagonal crystals free of stacking faults do not show anomalous photovoltages. Heat treatment causes appearance of both. A pyroelectric polarization is found to be correlated with the sign of photovoltage.