A study of electronic states of trigonal and amorphous Se using ultraviolet photoemission and inverse-photoemission spectroscopies

Abstract

Valence-band and conduction-band densities of states (DOSs) of trigonal and amorphous Se have been investigated by in situ measurements of ultraviolet photoemission and inverse-photoemission spectroscopies (UPS and IPES), respectively. The IPES spectra reveal, for the first time, a measure of conduction-band DOS, while the UPS spectra are fully consistent with earlier results. From comparison between theoretical DOS and a whole spectrum for trigonal Se obtained by a connection of the UPS and IPES spectra at the Fermi level, it has been shown that the coupling strength between the hybrid 4p-like orbitals of different atoms should be increased. On the basis of a comparison between IPES and core-absorption spectra for trigonal Se, a prominent peak and weak structures in the 1 - 5 and 5 - 10 eV regions above the valence-band maximum in the IPES spectrum are experimentally assigned to the 4p-like antibonding and 4d and/or 5s states, respectively. In amorphous Se, the peak at shallower energy in the 4p-like bonding bands with two peak structures is more intense in contrast to the case of the trigonal one. Such a change is attributed to a chain structure with the dihedral angle varying randomly in sign along the chain. Increases of the bonding - antibonding splitting energy of the 4p-like bands and band-gap energy in amorphous Se are ascribed to an increase of intrachain coupling strength between the hybrid 4p-like orbitals and a simultaneous decrease of the interchain coupling strength.