Energy Bands of Hexagonal II-VI Semiconductors

Energy bands for hexagonal ZnO and ZnS have been calculated along the main symmetry axes of the hexagonal Brillouin zone with the relativistic mass velocity and Darwin correction considered. The band structure of ZnO differs from the ZnS band structure in that $d$ bands occur closely below the upper valence bands, and $p$-like conduction bands lie 17 eV above the valence bands. Thus, the ZnO band structure exhibits a very broad lowest conduction band. For hexagonal ZnS, the relativistic corrections increase the separation between $p$-like valence and conduction bands. This effect is discussed in connection with relativistic band calculations for cubic ZnS along the $\Lambda$ axis, and a revised interpretation of higher interband transitions is given. At k = 0, full relativistic calculations yielded good agreement with experimental values for crystal-field and spin-orbit splittings for ZnS, CdS, and CdSe, and gave a negative spin-orbit splitting for ZnO. Deformation potentials for CdS were calculated in good accord with experiment.