Highly disordered amorphous selenium studied by ultraviolet photoemission spectroscopy

Abstract

Highly disordered amorphous (HDA) selenium films were prepared by vapor deposition onto a cold substrate held at 77 K under ultrahigh vacuum. HeI photoemission spectra of these films were found to be considerably different from the previously observed spectrum of the amorphous film prepared at room temperature (RT film). Characteristic features in the spectra of the HDA films compared with those in the spectrum of the RT film are as follows: (1) the width of the $4p$ nonbonding band is remarkably narrower; (2) the intensity of the upper $4p$ bonding band is decreased, and the intensity ratio of the upper and lower $4p$ bonding bands is reversed; (3) the upper $4p$ bonding band is located at higher initial energy; and (4) a part of the $4s$ band is well resolved. The HDA films underwent thermal annealing effects and at room temperature gave a photoemission spectrum similar to that observed in earlier work. In order to interpret the experimental results, CNDO molecular-orbital calculations were made on a newly proposed model for amorphous selenium, in which distorted infinite chains consist of small spiral units including a few selenium atoms. On this model, dependence of the density of states on the following factors was studied: (1) the number of atoms in a unit cluster, (2) the dihedral angle for the bonds on the junction of the units, and (3) the bond angle. The results of the calculations well explained the spectrum of HDA selenium and its thermally induced changes. The importance of the above three factors for the structure of amorphous selenium was stressed.