Surface properties of II-VI compounds

Single crystals of ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe have been cleaved in ultrahigh vacuum ($p\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ Torr) and measurements have been made of photoemission, contact potential, and surface photovoltage. In some cases, metal contacts were deposited onto the clean surfaces and the Schottky barrier heights determined. Photoelectric thresholds were similar for compounds having the same chalcogen, being higher for chalcogens with higher electronegativity. For a given chalcogen, the electron affinity of the crystal was about 0.8 eV smaller for the zinc compound than for the cadmium compound. Band bending observed was consistent with a band of surface acceptors near the conduction-band edge and a band of surface donors near the valence-band edge. Schottky barrier heights were close to the value given by the difference between the semiconductor electron affinity and the metal work function, provided the work function used was that for the specific metal-substrate system in question.