Insights into fractal feature evolution from Au/Ge thin films after annealing

We report a perplexing behavior of fractal shape transition that results from a change in the annealing temperature and time or the film thickness ratio. We find that a compact-to-open fractal shape transition can be induced by increasing the annealing temperature and time or decreasing the thickness ratio of the Au and Ge films. This behavior is not completely consistent with what is predicted by theories based on diffusion-limited aggregation and previous experimental observations. In this new system, we find that fractal shape transitions are truly dominated by the random-successive nucleation and growth mechanism.