Photoemission investigation of polycrystalline and amorphous tellurium

Photoemission measurements of amorphous and polycrystalline Te films have been made in the photon energy range 4.8-11.6 eV in an ultrahigh vacuum of pressures less than ${10}^{-10\mathrm{}}$ Torr. In contrast with the group-IV semiconductors Ge and Si, the photoemission spectra for both ordered and disordered films is remarkably similar with two broad peaks in the valence-band density of states located 1.7 and 4.6 eV below the top of the valence band. In addition, direct transitions do not appear to be very important in determining the photoemission spectra of either the amorphous or polycrystalline films. Differences in the density of states are second order and are brought out by measuring the second derivative of the photoemission spectra. Valence-band structure for the polycrystalline films lying within about 0.5 eV of the band edge is lost in the amorphous phase; however, weak conduction-band structure located 5.4 eV above the top of the valence band is observed in both films. Higher lying conduction-band structure appears to be smeared out in the amorphous films. The absolute photoelectric yield was measured and no evidence for density of states tailing below the $d$-like second conduction band, such as reported by Laude et al., was found in either amorphous or polycrystalline films. The separation of the upper two peaks in the valence-band density of states is in good agreement with a molecular-orbital calculation which we present for trigonal Te; however, the calculated valence-band width is much too small by about 7.5 eV.