Surface chemistry of GaAs treated with buffered HF and NH4F solutions: Slow reactions of process residuals

The surface chemistry of GaAs after treatment with NH4F and with HF buffered with NH4F (BHF) was studied using surface infrared spectroscopy in the multiple internal reflection mode. These two treatments not only dissolved the GaAs native chemical oxide, but also deposited a thin inorganic film comprised of ammonium salts (NH4F and NH4FHF) that further react through a series of complex reactions with the semiconductor substrate to form ammonium hexafluorogallate [(NH43GaF6]. These reactions are apparently driven by favorable thermodynamics, but occur very slowly. The reactions on BHF-treated GaAs proceed over approximately one day, while reactions on the NH4F-treated GaAs can extend for more than three days. Although the fluoride and bifluoride salts are quite soluble in alcohols as well as in aqueous solutions, the hexafluorogallate is completely insoluble in alcohols, and can only be removed by a thorough water rinse.