A relation between the Au-surfactant effect and the chemical mixing duringthe epitaxial growth of Ni on Au(001)

We report far the first time direct experimental evidence of a chemical mix ing between Au and Ni atoms, when depositing a Ni film on Au(0 0 1). This a lloying effect, which has been put forward previously by several authors fr om 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 conce ntration profile inside the Ni layers has been measured by Anger electron s pectroscopy (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 t he Ni incoming atoms with a constant probability of similar to0.9: at the b eginning of the growth, Au atoms 'floats' at the free surface and are subse quently incorporated into the Ni layers as the growth continues. This inter mixing could act as a strain-relieving mechanism for the first growth stage s, where coherency between Ni and Au is observed despite the 14% lattice mi smatch. The chemical homogeneity of a MBE co-deposited Au0.5Ni0.5 alloy has also been investigated by AES. It is shown that the Au atoms still segrega te 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 du e to the continuous contribution of the Au atoms during the co-deposition, (C) 2001 Elsevier Science B.V. All rights reserved.