Phase equilibria of the Ga–Ni–As ternary system

Abstract

Phase equilibria were investigated in the Ga–Ni–As ternary system, with particular emphasis on the regions of technological importance to Ni/GaAs electrical contacts. A 600 °C Gibbs isotherm was constructed using x-ray-diffraction analysis and electron probe microanalysis of annealed samples. Additionally, three isopleths (NiAs–GaAs, NiGa–NiAs, and NiGa–GaAs) and a partial liquidus projection were established using differential thermal analysis and metallography. These data were utilized to clarify some discrepancies in the literature pertaining to the constitution of the Ga–Ni–As system, particularly questions about the existence of ternary phases. It was demonstrated that at 600 °C, previously reported ternary phases were actually specific compositions of the binary phase, NiAs, which exhibits significant ternary solubility. Additional x-ray-diffraction and differential thermal analysis experiments suggested that superlattice structures based on the NiAs structure may become stable at lower temperatures. A ternary eutectic reaction was shown to occur at 810±5 °C, with eutectic point at the composition Ni0.48Ga0.30As0.22. The existence of this eutectic reaction has important ramifications for the development of Ni-based electrical contacts to GaAs because any metallization scheme with a composition within the region bounded by NiGa, NiAs, and GaAs, as well as elemental Ni, will experience at least partial liquid formation at temperatures greater than 810 °C.