Synchrotron radiation photoemission spectroscopy of III-VI compounds

Abstract

The valence and conduction bands of GaSe, GaS, and InSe have been studied with several synchrotron-radiation photoelectron-spectroscopy techniques. Energy distribution curves (EDC's) in the photon energy range 16-36 eV show four main valence-band density of states features between the top of the valence band and the cation $d$ core levels. The nature of these features is discussed in terms of the two kinds of intralayer chemical bonds, cation-cation bonds and anion-cation bonds. Photon polarization effects show that the top of the valence band has a ${p}_{z}$-like character and this character extends to lower energies for GaS than for GaSe and InSe. Photon-energy effects reveal a fine structure of most valence-band features which is particularly pronounced for InSe. Constant-initial-state (CIS) spectra have been taken employing several EDC peaks as initial states. Most of the CIS data can be straightforwardly interpreted in terms of the conduction-band density of states. Good agreement is found with pseudopotential band-structure calculations for GaSe and a direct interpretation is derived for the observed features in the reflectivity spectra.