Control of Fermi level pinning and recombination processes at GaAs surfaces by chemical and photochemical treatments

The effects of photochemical oxidation in water, Na2 S deposition and exposure to NH3 and HCl gas ambients on the Fermi level pinning, and the surface recombination process at GaAs surfaces were studied by measuring the band edge photoluminescence (PL) intensity, x-ray photoelectron spectroscopy spectra, and surface current transport(SCT). Computer simulation of the surface recombination process was also made on the basis of the disorder-induced gap state model. Marked increase of PL intensity was observed after both photochemical oxidation and deposition of Na2 S as previously reported. However, SCT measurements on n-type materials detected increase of surface band bending in the dark. Exposure to NH3 resulted in a slight reduction in the band bending with little change in the PL intensity. Exposure to HCl, on the other hand, resulted in marked reduction of the band bending with marked increase of the PL intensity. The computer simulation shows that the contradictory behavior of the PL intensity and band bending, as observed after photochemical oxidation and Na2 S deposition, is explicable by a shift of the Fermi level pinning position towards the valence band edge due to a fixed negative interface charge which reduces the effective recombination velocity. On the other hand, exposure to HCl reduces the surface-state density and weakens the pinning remarkably.