Structural effects of heating gold-based contacts to gallium phosphide

The formation of α′ AuGa was observed by X-ray diffraction in situ upon heating Au/Ni/Au-Ge/GaP(111) in nitrogen to ≥ 438 ± 12°C and subsequent cooling to 300°C. No AuGa phase formation was observed without cooling, suggesting that the reaction product was a liquid at the reaction temperature. The amount of α′ increased with increasing reaction temperature for the same reaction time. The peritectic transformation of α′ to α and liquid AuGa solution was observed reversibly upon subsequent heating to 388 ± 12°C. After the AuGa phase formation in Au/Ni/Au-Ge/GaP(111), scanning electron microscopy and X-ray spectroscopy revealed ∼ 20 μm interconnected Au-rich islands and ∼ 2μm Ni-rich islands in a matrix which was Ga- and P-rich. In Au/GaP, Au-Ga phase formation was observed ex situ after heating in hydrogen to ≥ 475 ± 25°C, and a change from Schottky to near ohmic behavior was observed ex situ after heating in hydrogen to ≥ 400°C; neither AuGa phase formation nor appreciable change in the Schottky characteristic was observed when heated in nitrogen up to 550°C.