The complex dielectric functions, in ( omega )= in 1( omega )+ in 2( omega ), are calculated for the semiconductors Ge, GaAs, InSb, and CdTe in order to explain new ellipsometric measurements. The dependence on hydrostatic pressure of the dielectric functions of GaAs and Ge is also investigated. The band structures and optical transition matrix elements are obtained from the relativistic self-consistent linear muffin-tin orbitals scheme. In order to correct for the too low values of the gaps as obtained within the local-density approximation, shifts are introduced by adding external potentials that are sharply peaked at the atomic sites. These external potentials are kept invariant under pressure. In general, the authors' calculation enables a consistent assignment of the structure in the experimental spectra and also of the pressure dependence of the peak positions. Their band-structure calculations show that the indirect band gaps of GaAs decrease under pressure at a rate of -1.15 eV/Mbar whereas in the case of Ge both direct and indirect band gaps increase with pressure. These shifts of band gaps are in good agreement with experiment.
 

Physical Review B, 37 1167-79, 1988.