HgTe-CdTe superlattice subband dispersion

Abstract

The results of band-structure calculations for (001) and (111) HgTe-CdTe superlattices using a multiband tight-binding model are presented. The band structures of superlattices in the semiconducting and semimetallic regimes are found for directions both parallel and perpendicular to the growth direction. The tight-binding model automatically incorporates the correct space-group symmetry of the superlattice. Band mixing, band crossings or anticrossings, degeneracies, and spin splittings are therefore correctly produced. For semiconducting superlattices the strain-induced reversal of light- and heavy-hole subbands depends on growth orientation as well as layer thicknesses. The light-hole subband, as defined by the effective mass in the growth direction, is found to be higher than the heavy-hole subband for the (001) 50-Å–40-Å superlattice by 9.2 meV, but lower for the (111) superlattice with similar layer thicknesses. This feature is somewhat sensitive to the precise deformation potentials and bulk valence-band parameters assumed in the calculation. The semi- metallic superlattice which had been the subject of magnetoabsorption experiments is studied in detail. In agreement with the calculation of Wu and McGill, it is found that the inclusion of strain significantly distorts the valence-band-edge band structure, implying that a reappraisal of the 40-meV HgTe-CdTe valence-band-offset determination is needed. It is found that the stress opens up a gap at the Brillouin-zone center, but that the superlattice is still semimetallic due to a stress-induced indirect valence-band maximum with energy above that of the conduction-band minimum. The conduction-band-minimum wave function has a large interfacial component. The relative magnitude of the amplitude of this component increases approximately with the inverse square root of the energy of the state.