On the role of Fe in the growth of single crystalline heteroepitaxial Au thin films on sapphire

Thin Au-Fe bilayers were deposited on c-plane sapphire (α-Al ₂O₃) substrates at room temperature employing the electron beam deposition method. The layers were found to be single crystalline (i.e. the grain size was much larger than the film thickness), with a [1 1 1] and [1 1 0] texture for Au and Fe, respectively, and strong heteroepitaxy to the substrate. Au films deposited on sapphire and Au-Fe bilayers deposited on amorphous SiO₂ were polycrystalline and exhibited random in-plane orientation of the grains. The effects of Fe and the Fe-sapphire interface on the microstructure of the Au film were investigated and discussed in terms of the orientation relationships, in-plane strain, interface energy and adhesion. The microstructures of annealed and as-deposited films were very similar, indicating that as-deposited films are close to thermodynamic equilibrium in terms of the orientation relationship with the substrate. This is uncommon for non-equilibrium thin film deposition processes, which usually result in a high density of defects in the as-deposited films.