Solid -state dewetting in Pd/a-Ge bilayers

The post-growth morphology and the chemical composition of sputter-deposited Pd/a-Ge films were studied by in situ atomic force microscopy and X-ray photoelectron spectroscopy under UHV conditions. The polycrystalline films consist of locally epitaxial Pd/Pd$_2$Ge grains on an amorphous germanium buffer layer. We found that the films show a thickness-dependent tendency for solid-state dewetting, which occurs at the interface between the Pd and Pd$_2$Ge layers and for Pd thicknesses below 6 nm. The post-growth dewetting is correlated with the real-time stress evolution during deposition. Real-time x-ray reflectivity measurements confirm that the dewetting is kinetically hindered during deposition, while individual grains dissolve after deposition, leaving deep holes. The room-temperature dewetting of Pd/a-Ge combines characteristic features of epitaxial and polycrystalline dewetting mechanisms. We propose that it is driven by the misfit strain between Pd and Pd$_2$Ge, while the stability of individual grains is determined by the local grain boundary configuration.