Effects of Ni and Cu Antisite Substitution on the Phase Stability of CuGa₂ from Liquid Ga/Cu-Ni Interfacial Reaction

Ga and Ga-based alloys have received significant attention for applications in the liquid state and also for their potential as a bonding material in microelectronic assemblies. This study investigates the phase stability of the CuGa₂ phase as a product of the interfacial reaction between liquid Ga and Cu-10Ni substrates at room temperature. In the binary Ga-Cu system, CuGa₂ is decomposed into liquid Ga and Cu₉Ga₄ as the temperature increases to around 260 °C, which prevents the widespread application of this alloy. In contrast to CuGa₂ grown from a pure Cu substrate, CuGa₂ from the Cu-10Ni substrate shows an increase in the decomposition temperature during heating from 25 to 300 °C. According to our first-principle calculations, there is only a minor difference in the total free energy between Ni solute at the Cu sublattice and the Ga sublattice in the tetragonal CuGa₂ crystal structure. This result indicates that both of the sublattices can accommodate the dilute Ni solute with comparable probability. Regardless of the sublattice where the Ni impurities are located, the presence of diluted Ni in the matrix stabilizes the CuGa₂ system by inducing some localized Ni 3d states at energy levels near the Fermi level. It is also shown that the formation of Cu antisite defects, which also stabilizes CuGa₂, is preferable if the CuGa₂ matrix is grown on a Ni-containing substrate.