Tight-binding study of the electronic states in GaSe polytypes

The electronic band structures of the beta , epsilon and gamma polytypes of GaSe are studied by the tight-binding method. The calculation takes into account interlayer interactions and the generally neglected lowest excited atomic states. The energy levels of the three polytypes differ from each other by 0.3 eV at most. Inclusion of excited atomic states strongly modifies the conduction bands but has little effect on all valence states. The calculated valence band structure compares well with both angular-resolved and integrated photoemission data. The electronic charge-density distribution resulting from tight-binding Block states confirms the chemical bonding picture proposed in recent pseudopotential investigations.