(SI)3/(GE)4 SUPERLATTICES - DIRECT-GAP SEMICONDUCTORS

Theoretical calculations for the energy band structure as well as for the optical transitions at the centre-GAMMA of the superlattice Brillouin zone are presented for (Si)3/(Ge)4 superlattices grown on SixGe1-x(001) substrates. These superlattices belong to the D4h19 symmetry (n + m = odd). The influence of the alloy buffer (on which the superlattice is considered to be grown) on the transition energies and probabilities and the nature of the gap (direct/indirect) is investigated. It is found that for some Si concentrations x in the buffer alloy the superlattices become direct-gap semiconductors with significant transition probabilities. Moreover, the dependence of the preceding features on the valence band offset is also examined and it is found that the latter does not influence them drastically.