Electronic Properties of the III-VI Layer Compounds GaS, GaSe and InSe, III: The reflectivity from 4 to 32 eV

High-resolution reflectivity spectra of the layer compounds GaS, GaSe and InSe have been measured at RT between 4 and 32 eV by using synchroton radiation. The function (ħω)2 ε2 has been obtained by Kramers-Kronig analysis of the reflectivity spectra. Above 20 eV in GaS and GaSe and 17.5 eV in InSe the excitation of the Ga3d and In4d electrons, respectively, generates sharp structures associated with the density of states of the conduction bands, except for the first prominent peak (and its spin-orbit partner) that has been associated with a core exciton, the binding energy of which is (0.5 ± 0.2) eV in GaS and (0.3 ± 0.2) eV in GaSe, while in InSe the core exciton is not resolved from the In4d ionization continuum. The valence band excitations have been analysed in detail and the relevant structures have been associated with transitions beginning in deeper and deeper valence bands and terminating mostly in the lower groups of conduction bands.