FORMATION OF OHMIC CONTACTS TO N-GAAS USING (NH4)2S SURFACE PASSIVATION

Ohmic contacts between Au and sulfur-passivated n-GaAs have been formed without annealing after deposition of the metal. The GaAs samples doped with either 2X10(16) or 3X10(18) cm-3 Si were passivated with (NH4)2S in an aqueous solution, then the passivating sulfur was thermally desorbed in a vacuum. Desorption was followed immediately by in situ thermal evaporation of Au with the substrate at room temperature. Ohmic I-V behavior was measured ex situ. C-V measurements on a sample with a native oxide surface barrier layer showed the surface carrier concentration on the higher doped sample increased from 3X10(18) to 7X10(18) cm-3 after S desorption. Secondary ion mass spectroscopy data showed that S had diffused into the GaAs surface region. Thus S passivation prevented interfacial oxide formation during the exposure to air after the sample was passivated and before the vacuum chamber was pumped to a low pressure. Sulfur apparently diffused into the surface region during flash desorption in vacuum and increased the surface n-doping concentration which led to ohmic contacts without post-annealing of the Au films.