Density of States of Amorphous Selenium by Vacuum Photoemission

The entire valence band of amorphous Se has been probed by vacuum photoemission. The valence-band density of states consists of a large peak 1.7 eV below the upper edge of the band, and a triplet structure with shoulder at 3.8 and peaks at 4.4 and 5.4 eV below the band edge. Although the upper peak is in good agreement with the results of Kramer's calculation, the lower triplet is not. Comparison with molecular-orbital calculations of Chen and Tutihasi and Chen, reveals better but not perfect agreement. It is suggested that a model of amorphous Se as a mixture of rings and chains will produce better agreement with experiment than the disordered trigonal lattice employed in the calculation of Kramer. The interface between Se and various metal electrodes was investigated and an upper limit on the density of states near the Fermi level of 2 × ${10}^{14}$ e${\mathrm{V}}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-3\mathrm{}}$ determined by a new technique. This limit on the density of states is consistent with that obtained by Lanyon from space-charge-limited-current studies.