Structural Instability of Annealed a-Si/a-Ge Nanostructures

It is shown that heat treatments cause remarkable structural instability in hydrogenated nanostructures made of alternating 3 nm thick layers of a-Si and a-Ge deposited by sputtering. Upon annealing surface bumps are formed. Their size and density increase with increasing H content. They appear due to the presence of H bubbles in the samples, which even blow up for the highest H content. The H bubbles are produced by accumulation of H2 molecules made possible by the break of the Si-H and Ge-H bonds driven by energy supplied by the heat treatment and the recombination of thermally generated carriers.