In situ X-ray diffraction study of melting in gold contacts to gallium arsenide

By in situ X-ray diffraction, we have obtained direct evidence for the distinction in melting temperature between the first and subsequent heating in ～ 1000 Å gold contacts to GaAs. During the first heating, melting occurred at 456–500°C and is attributed to the melting of a reaction product tentively identified as the orthorhombic AuGa compound (50 at.% Ga), which formed in the solid state. The minimum temperature required for the solid-state reaction and the melting temperature decreased with decreasing hydrostatic pressure. During second and subsequent heating, melting occurred at 410–415°C and is attributed to the melting of β AuGa (or Au₇Ga₂), which formed after the first melting-solidification cycle. At high cooling rates (e.g. 40°C/min) during the first solidification, β was observed together with a phase (tentative Au₂Ga) which increased in proportion with increasing cooling rate.