Structure and Oxidation of Thin Gold-Nickel Alloy Films

Abstract

The structure and oxidation of thin alloy films of composition Au-15 at.% Ni have been investigated by means of x-ray and electron diffraction, and the oxide growth rate determined by measuring changes in the electrical resistivity of the films during oxidation. The films are prepared by sputtering and, even at low substrate temperatures, comprise single-phase homogeneous structures. Oxidation in the temperature range 250°–310°C shows a two-stage logarithmic growth rate (or resistivity change) with time similar to that observed for Ni, Cu, and Fe. This behavior is consistent with the formation of oxide only at the alloy film surface, and can be interpreted qualitatively in terms of the electron-transfer model proposed by Uhlig et al., for the oxidation of pure Ni. Epitaxial alloy films with (001) and (111) orientation are produced by sputtering onto the cleaved surfaces of NaCl and mica at about 400°C. On annealing these films at and above 500°C, pre-precipitation effects are observed in the electron-diffraction patterns. These are associated with the formation, in succession, of the superlattice structures, NiAu3 and NiAu. True precipitation of Ni-rich alloy occurs at temperatures above 600°C. Oxidation of epitaxial films of the disordered alloy at temperatures below 250°C results in the growth of poorly crystallized, non-oriented NiO. At 280°C both non-oriented and also three differently oriented oxide products, are formed, the NiO orientations corresponding to those reported by Collins and Heavens for oxide growth on Ni films. At 400°C only oxide orientations parallel to the metal film are detected.