Passivation of GaAs by electrochemical sulfur treatments

Simple chemical sulfur treatments of GaAs have been shown to passivate the GaAs surface. These treatments result in lower surface state density, lower surface recombination velocity, and shifting or unpinning of the Fermi level, in addition to improvement in the performance of GaAs devices. Electrochemical sulfur treatment, however, has only recently been explored and pursued as a method of growing anodic surface layers which have good passivating characteristics on semiconductors. In this study, using surface infrared reflection spectroscopy (SIRS), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, we have investigated the electrochemical sulfidation of GaAs as a method to produce a GaAs surface that has good electronic properties and is stable chemically and electronically. We have found that anodic treatments with Na₂S and (NH₄)₂S solutions resulted in the removal of the pre-existing oxide of GaAs and in the formation of films comprising sulfur, sodium carbonate, ammonium thiosulfate, and various sulfide and sulfur-oxygen compounds of arsenic. The surface state density of this anodically treated surface was significantly better than that of untreated GaAs. Rinsing the GaAs with water removed the bulk of the film, leaving behind a surface on which only arsenic sulfide was detected. The surface state density after rinsing has degraded slightly, however, but it was still better than that of an untreated GaAs.