Photoemission Studies of Wurtzite Zinc Oxide

Photoemission measurements have been made at photon energies between 7.8 and 11.6 eV on single crystals of wurtzite ZnO cleaved at pressures ${10}^{-10\mathrm{}}$ Torr. Additional measurements have been made from 11.6 to 21.2 eV in pressures of about ${10}^{-4\mathrm{}}$ Torr. Important features of the density of states are deduced from the energy distributions of the photoemitted electrons. With respect to the top of the valence band, maxima in the conduction-band density of states are located at 8.5 ± 0.2 eV and 10.4 ± 0.2 eV, while two maxima in the valence-band density of states are found at -1.6 ± 0.2 eV and -2.8 ± 0.2 eV. The $3d$ band of Zn is located at -7.5 ± 0.2 eV. In addition, the valence band appears to be about 5 eV wide in contrast to widths of about 2-3 eV for other II-VI compounds such as ZnS, CdS, and CdSe. These features are in qualitative agreement with both a recent pseudopotential calculation by Bloom and, excepting the valence-band width and the location of the $3d$ levels, with the Green's-function (Korringa-Kohn-Rostoker) calculation of Rössler. The $d$ bands are placed some 3 eV lower than predicted by Rössler's calculation; however, our assignment is in good agreement with recent x-ray photoemission measurements and is shown to be consistent with the low value of the effective number of free electrons $n_{\mathrm{eff}}$ calculated from the reflectivity. The effect of inelastic scattering due to pair production is also discussed.