Optical properties of GaSe andGaSxSe1−xmixed crystals

Reflectivity and wavelength-modulated reflectivity measurements on GaSe and $\mathrm{Ga}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ mixed crystals are presented for energies ranging from 3 to 6 eV. Additional electroreflectance measurements are used to investigate the excitonic character of the structure at 3.4 eV in GaSe. The results are discussed on the basis of a new pseudopotential band-structure calculation for GaSe. The prominent spectral structures of GaSe are identified in terms of location in $\overrightarrow{\mathrm{k}}$ space and atomic character of the states involved in the transitions. The main structures in the optical-response function of GaSe are found to result from excitations of the Ga-Ga bond and from atomiclike transitions between nonbonding Se $p$ and $s$ states. These main transitions are located in $\overrightarrow{\mathrm{k}}$ space close to symmetry determined "special $\overrightarrow{\mathrm{k}}$ points." The observed mixed-crystal spectra can be decomposed into three classes of transitions which exhibit zero, intermediate, and strong energy shifts with anion substitution. Theoretically, the three classes of transitions are found to take place between states with zero or little, intermediate, and large charge concentrations on the anion sites. Thus, the characteristic energy shifts, observed experimentally can directly be related to atomic spectral properties of sulphur and selenium.