Electron beam deposition was employed to deposit thin Fe-Au bilayers (of 3nm and 9nm in thickness, respectively) on basal-plane sapphire substrates. The Fe-Au bilayers were single crystalline in the as-deposited state, with the <111> normal perpendicular to the substrate. The agglomeration of the bilayers was studied by annealing at 650°C, under reducing ambient conditions (Ar\H2 flow). Both atomic force microscopy (AFM) and cross-section transmission electron microscopy (TEM) were employed to study the agglomeration process.
The combination of composition, annealing temperature and cooling rate leads to the formation of a single (Au) phase, and the dissolution of Fe occurs very rapidly compared to the annealing time. The single crystalline films exhibit much-improved stability compared to polycrystalline Fe-Au and Au films of the same thickness. Their agglomeration begins with the formation of faceted hexagon-shaped pinholes which grow laterally upon annealing. Over time the pinholes change their shapes from perfect hexagons towards triangles with faceted corners. This suggests further anisotropy of the facet's velocities which is related to the difference in mobilities of {111} and {100} facets.
A model was developed, which demonstrates that surface-energy and diffusivity anisotropies govern this process. The model allowed determining the effective self-diffusion coefficients of Au atoms along the facets by comparing the calculated and experimentally measured hole growth rates.