uv Photoemission Measurements of the UpperdLevels in the IIB-VIA Compounds

Photoemission measurements have been made of the upper $d$ levels in ZnO, ZnSe, ZnTe, CdS, CdSe, CdTe, HgSe, and HgTe using 21.2-eV (584-Å) and 16.8-eV (740-Å) radiation. All the samples except for HgSe and HgTe were cleaved and measured in an oil-free ion-pumped vacuum system at a pressure in the ${10}^{-7\mathrm{}}$-${10}^{-8\mathrm{}}$ Torr range. The results agree extremely well with values obtained by x-ray-induced-electron-emission spectroscopy. The spectra obtained for HgSe and HgTe make it possible to positively verify the identification of certain peaks observed in photoemission spectra of the IIB-VIA compounds as being due to the upper $d$ levels. The photoemission results reported in this study are also compared with both reflectivity and energy-loss measurements to obtain information about the optical density of states in the conduction bands of these compounds.