Densities of valence states of amorphous and crystalline ${\mathrm{As}}_2$${\mathrm{S}}_3$, ${\mathrm{As}}_2$${\mathrm{Se}}_3$, and ${\mathrm{As}}_2$${\mathrm{Te}}_3$: X-ray photoemission and theory

The density of valence states (DOVS) of amorphous and crystalline ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Te}}_3$ and of amorphous ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ have been investigated with monochromatized x-ray photoemission spectroscopy (XPS). All three compounds exhibit similar XPS spectra consisting of a pronounced peak near the Fermi energy associated with the nonbonding $p$ orbital of the chalcogen atom followed by a shoulder 4 eV wide, ascribed to the bonding $p$ orbitals, and lower $s$ bands situated about 13 eV below the top of the valence band. These XPS spectra are well described by DOVS synthesized from those of the constituent elements. The similarity in the DOVS of the crystalline and amorphous forms of ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ is presumably attributable to the similarity of the local structural configurations of the two forms. In the case of ${\mathrm{As}}_2$ ${\mathrm{Te}}_3$, where the crystalline and amorphous structures are quite different, the very similar XPS spectra for the two forms are argued to be a consequence of the chemical composition and the maintenance of chemical order in the amorphous state. A band-structure calculation for a model of an isolated arsenic chalcogenide layer is performed using only important nearest-neighbor interactions. The few parameters in the calculation are derived from XPS spectra for the constituent elements and the resulting band structure is in good agreement with the gross features of the XPS DOVS for the arsenic chalcogenides. From this semiempirical band structure, the $s-p$ hybridization is estimated to be about 5%. This $s-p$ interaction, however, is sufficient to shift the $s$ and $p$ energy bands by 1 eV.