Electron-diffraction and spectroscopical characterisation of ultrathin ZnS films grown by molecular beam epitaxy on GaP(0 0 1)

ZnS films were grown by molecular beam epitaxy employing a single compound effusion cell on GaP(0 0 1) substrate at different temperatures, and characterised by means of low energy electron diffraction, X-ray and ultra-violet photoelectron spectroscopy, angle-resolved ultra-violet photoelectron spectroscopy and X-ray emission spectroscopy. The GaP(0 0 1) substrate exhibits a (4 2) reconstruction after Ar ion sputtering and annealing at 370 8C. Crystal quality of the ZnS films depends on both film thickness and growth temperature. Thinner films grown at higher temperatures and thicker films grown at lower temperatures have better crystal quality. The layer-by-layer growth mode of the ZnS films at lower (25, 80 and 100 8C) temperatures changes to layer-by-layer-plus-island mode at higher temperatures (120, 150 and 180 8C). A chemical reaction takes place and is confined to the interface. The valence band offset of the ZnS–GaP heterojunction was determined to be 0:8 0:1 eV. Sulphur L2,3 emission spectra of ZnS powder raw material and the epitaxial ZnS films display the same features, regardless of the existence of the Ga–S bonding in the film samples. # 2003 Elsevier B.V. All rights reserved