Electronic Properties of the III-VI Layer Compounds GaS, GaSe and InSe, I: Band Structure

The electronic properties of the semiconducting layer compounds GaS, GaSe and InSe are considered comparatively in a series of papers. In this first paper the band structures of the three compounds are computed by using the same method and the same approximations for all of them. The overlap-reduced semi-empirical tight-binding method (ORSTB) is outlined by first. The method consists in scaling only the overlap integrals in the secular equation. In this way it takes into account the localization of the atomiclike functions used in the Bloch sums in a more effective and simpler way than previous tight-binding semiempirical schemes. For GaS, GaSe and InSe bands only two scaling factors are used as matching parameters to reproduce optical and photoemission experimental bands is very good. In particular, the shape of the bonding metal-metal bands which all previous tight-binding calculations failed to reproduce is correctly obtained here. For all the three compounds the top of the valence bands falls at the point Γ and the bottom of the conduction bands is atM. The indirect gap is slightly lower than the direct one in GaSe and InSe, while it is larger in GaS, in agreement with the experiment. The remarkably similar overall shape of the bands, for all the compounds, stems from the similar magnitude of the two-centre integrals. The atomic eigenvalues and the crystal fields are responsible for the main differences in the widths of groups of bands in the three cases. The density of states is computed on the basis of the obtained bands, by using the tetrahedral linear approximation by Lehmann and Taut.