Polar Properties of ZnS Crystals and the Anomalous Photovoltaic Effect

Abstract

ZnS platelets grown from the vapor phase, bounded by (2̄11), (011̄), and (111) faces, were used. The crystals showed birefringence bands perpendicular to the [111] axis. X-ray rotation, oscillation, and Laue photographs showed that the bands are due to changes in crystal structure (cubic, hexagonal, and polytypes) and one-dimensional stacking disorder. The polarity of the common [111] axis of the various bands was determined by several techniques. On the (2̄11) faces, heating with H2O2 caused the appearance of triangular etch pits, aligned with bases parallel to the [01̄1] direction and apexes pointing all in the same sense. The (111) face towards which the triangles pointed was attacked faster by the etchant than was the opposite one. The same (111) face became negatively charged if a (2̄11) face was uniformly illuminated by 343 mμ radiation. Furthermore, the same face turned out to be a zinc plane, as revealed by comparing the intensity of two (111) reflections, using wavelengths at both sides of the Zn K-absorption edge. The sense of the crystal growth along the [111] axis was also determined. A correlation was established between the morphology and crystallographic polarity as well. The results show that in spite of the polytypism, twinning, and one-dimensional stacking disorder, the sense of the [111] axis remains constant. Moreover, the one-to-one correspondence between the sign of the photovoltage and the crystallographic polarity restricts the possibilities of interpretations of the anomalous photovoltaic effect observed along the [111] axis in ZnS crystals favoring Neumark's theory [G. F. Neumark, Phys. Rev. 125, 838 (1962)].