A relation between the Au-surfactant effect and the chemical mixing during the epitaxial growth of Ni on Au(0 0 1)

Abstract

We report for the first time direct experimental evidence of a chemical mixing between Au and Ni atoms, when depositing a Ni film on Au(0 0 1). This alloying effect, which has been put forward previously by several authors from indirect experiments, is out-of-equilibrium since the AuNi system phase-separates in the bulk state. It is ascribed to the dynamical segregation of Au atoms during the growth of Ni on Au. In the present study, the Au concentration profile inside the Ni layers has been measured by Auger electron spectroscopy (AES) on a Ni wedge film grown by molecular beam epitaxy (MBE) and whose thickness varies continuously from 0 to 50 monolayers (ML). It is found that the Au atoms of the topmost surface planes are exchanged with the Ni incoming atoms with a constant probability of ∼0.9: at the beginning of the growth, Au atoms ‘floats’ at the free surface and are subsequently incorporated into the Ni layers as the growth continues. This intermixing could act as a strain-relieving mechanism for the first growth stages, where coherency between Ni and Au is observed despite the 14% lattice mismatch. The chemical homogeneity of a MBE co-deposited Au 0.5Ni 0.5 alloy has also been investigated by AES. It is shown that the Au atoms still segregate at the surface, but contrary to the case of the growth of a pure Ni film, the Au concentration reaches an equilibrium value after 5 deposited ML due to the continuous contribution of the Au atoms during the co-deposition.