Valence band structures and core-electron energy levels in the monochalcogenides of gallium. Photoelectron spectroscopic study

Abstract

X-ray induced photoemission studies on single crystals of GaS, GaSe and GaTe reveal the details of the occupancy of the valence-bands in these solids. As expected from their electronic and crystallographic (layer) structures, GaS and GaSe have remarkably similar valence-band profiles: peaks occur (for GaS) at 4, 9 and 14.5 eV and (for GaSe) at 3.5, 8 and 14 eV below the respective valence-band edges. U.-v. photoemission in ultrahigh vacuum was used to locate the precise positions of the Fermi levels and valence band edges. These occur, for GaS, respectively at 5.8 and 6.5 eV and, for GaSe, at 5.2 and 5.4 eV below the vacuum level. Agreement between the experimentally determined peak shapes and positions and the values predicted theoretically on the basis of a semi-empirical tight-binding approach is good. No theoretically estimated band structure appears to be available for comparison with the observed GaTe band profile. Core-level binding energies have been evaluated for levels extending from ca. 40 to 1010 eV. There is evidence that the currently accepted values for the core-level binding energies in elemental tellurium are in error.