Reversibility of Fermi level pinning

Abstract

By depositing monolayer coverage of Ag on freshly cleaved, clean GaAs(110) surface, the surface Fermi level was found to pin at 0.45±0.05 eV above valence band maximum (VBM) for p-GaAs and 0.65±0.05 eV above VBM for n-GaAs, due to the formation of midgap electronic states. Upon annealing to 500 °C, the Ag on the surface clustered into islands and the surface Fermi level moved back to within 0.2 eV of the bulk position [conduction band minimum (CBM) and VBM for n and p types, respectively]. Further deposition of Ag on the annealed surface moved the surface Fermi level back to the previously pinned positions. The surface Fermi level movement was monitored using Ga 3d and As 3d photoemission signals. The Ag clustering was seen both by a dramatic decrease in the Ag 4d photoemission signal and by scanning Auger microscopy. Scanning electron microscopy also revealed that the Ag clustering formed hemispherical-like islands with a size distribution up to ∼500 Å in diameter. The average island separation is about a few thousand angstroms (∼2000 Å). As a comparison with a reactive metal, similar annealing experiments were performed for monolayer coverage of Al on GaAs. For the Al case, the metal remained unclustered after annealing and the surface Fermi level remained pinned.