A Surface Infrared Study of the Liquid Methanol-Gallium Arsenide Interface

The (100) surface of gallium arsenide was examined during and after exposure to liquid methanol and 0.05N methanolic potassium hydroxide, using surface infrared spectroscopy in the multiple internal reflection mode. Liquid methanol dissolves components of the natural oxide present at the semiconductor surface (Ga₂O₃, 690 cm⁻¹, and a sub-oxide of gallium or arsenic, 765 cm⁻¹) and leaves behind a persistent physisorbed layer following evaporation. The natural oxide regrows at a rate of approximately 10A per hour, while methanol is still adsorbed at the surface, indicating that the layer is non-protective. Methanolic potassium hydroxide etches the gallium arsenide surface, most likely via a two-step oxidation/dissolution process. Methanolic KOH also leaves a layer of crystalline potassium methoxide on the surface following evaporation, as evidenced by absorption bands at 1059 cm⁻¹ (C—O stretch) and 2812 and 2924 cm⁻¹ (CH₃ stretches).