VUV-ellipsometry on Be x Zn 1−x Se and BeTe

We have determined the dielectric function of BeTe and BexZn1−xSe for the full composition range (0≤x≤1) by spectroscopic ellipsometry in the ultraviolet- and vacuum ultraviolet-range. The spectra show pronounced features in the photon energy range between 2.7 and 25 eV. For BeTe, we assign the main structures to interband transitions from the heavy and light hole bands to the lowest conduction band at high symmetry points in the Brillouin zone. The spectral positions of the interband critical points agree with quasiparticle excitations obtained by a GW calculation. We extracted the dielectric function of BeTe in the transparent regime below the fundamental gap E0 at 4.2 eV from our experimental data using the three-phase model (vacuum/BeTe/GaAs). By comparing the ZnSe spectra (x=0) to literature data, we assign the observed structures to interband transitions at specific points or regions of the Brillouin zone. The corresponding assignment for BeSe is made by tracking the dependence of these structures on Be content x from x=0 to x=1. E0 has the steepest slope dE0/dx and exhibits a bowing vs x. For samples with a large Zn content (x<0.2) two structures show up at high energies (15.7 and 16.9 eV) which have not been observed before. We assign these structures to transitions from the Zn 3d bands to a higher conduction band at the L point and at the γ point.