Band Structure of Cadmium Sulfide—Photoemission Studies

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Photoemission measurements have been made at photon energies between 7.2 and 11.6 eV on single crystals of CdS which had been cleaved in high vacuum at pressures ${10}^{\ensuremath{-}9}$ Torr. Additional measurements have been made from 6 to 21.2 eV using surfaces cleaved and tested in pressures of about ${10}^{\ensuremath{-}4}$ Torr. The electron affinities are found to be 4.8\ifmmode\pm\else\textpm\fi{}0.3 and 3.8\ifmmode\pm\else\textpm\fi{}0.4 eV for the high- and low-vacuum-cleaved samples, respectively. Important features of the density of states are deduced from the energy distributions. Maxima in the density of states are located in the conduction band at 6.7\ifmmode\pm\else\textpm\fi{}0.3, 8.2\ifmmode\pm\else\textpm\fi{}0.3 eV and (with the aid of reflectivity data) at 4.4\ifmmode\pm\else\textpm\fi{}0.5 eV. A single maximum in the valence-band density of states is found at -1.2\ifmmode\pm\else\textpm\fi{}0.3 eV and the $d$ band of Cd is located at -9.4\ifmmode\pm\else\textpm\fi{}0.5 eV. All energies are referred to the top of the valence band. These features agree qualitatively with the calculated band structure of Herman and Skillman and, in the case of the $d$ band, with x-ray data. Details of the density of states are deduced from the electron distributions with the aid of quantum-yield and reflectivity data by assuming (1) that direct conservation of k is not an important selection rule and (2) constant matrix elements. The optical conductivity and quantum yield calculated from the experimentally determined density of states are in first-order agreement with the measured values. The effects of inelastic scattering due to pair production are discussed. The contribution to the energy distribution due to single pair-production events were calculated for simple models. The results agree qualitatively with the measured energy distribution.