Band Structure of Cadmium Sulfide—Photoemission Studies

Abstract

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 <${10}^{-9\mathrm{}}$ Torr. Additional measurements have been made from 6 to 21.2 eV using surfaces cleaved and tested in pressures of about ${10}^{-4\mathrm{}}$ Torr. The electron affinities are found to be 4.8±0.3 and 3.8±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±0.3, 8.2±0.3 eV and (with the aid of reflectivity data) at 4.4±0.5 eV. A single maximum in the valence-band density of states is found at -1.2±0.3 eV and the $d$ band of Cd is located at -9.4±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.