Oxidation properties of II–VI compound surfaces studied by low-energy electron-loss spectroscopy and 21 eV photoemission spectroscopy

Abstract

Oxidation properties of ZnSe, ZnTe, CdSe, and CdTe surfaces were studied. The following combinations of experimental techniques were used: electron-energy-loss spectroscopy on vacuum-cleaved ZnSe(110) surfaces and on Ar+ bombarded-annealed surfaces of ZnSe, ZnTe, CdSe, and CdTe. Photoemission spectroscopy also was used on a vacuum cleaved ZnTe(110) surface. The oxygen uptake is very slow with exposures to molecular oxygen. Thus oxide overlayers were prepared by an electron-beam irradiation technique which greatly enhanced the oxygen uptakes. Changes in loss spectrum detected on oxidation of CdTe can be explained by formation of TeO2. For the case of ZnTe, the shifted Te 4d loss due to TeO2 appears at the oxygen coverage, ϑ, of about 0.5 of a monolayer, and losses characterizing formation of ZnO are seen at ϑ≳1. The oxide overlayer on ZnTe is composed of TeO2 and ZnO and, in addition, an unidentified complex oxide for the heavily oxidized surface. On further oxygen exposures and beam irridation of this oxide, tellurium oxides sublime away from the surface, leaving an oxide layer rich in ZnO. In the case of the Se compounds, the oxygen adsorbs on the surface Se atoms without breaking the back bonds at the initial stages of oxidation (ϑ≳0.5). For ϑ≳0.5, back bonds are broken, resulting in the formation of the bulklike cation oxide and, perhaps, the anion oxide, SeO2. Because of its high vapor pressure, the SeO2 may sublime away from the surface, leaving the overlayer rich in ZnO for ZnSe and rich in CdO for CdSe. Thus, for very heavy oxidation, the following oxides were found; TeO2 on CdTe, CdO on CdSe, and ZnO on ZnTe and ZnSe. For each II–VI the single oxide listed above was that with the largest heat of formations.