Scanning Electron Microscopy Observations of Fractal Pattern Formation in Al/Ge Bilayer Films

The fractal pattern formation on the free surface of annealed Al/amorphous-Ge bilayer film deposited on a SiO2 substrate (Al/Ge/SiO2) was investigated with scanning electron microscopy (SEM). When the Al/Ge/SiO2 bilayer film is annealed at lower temperatures than the crystallization temperature of amorphous Ge itself (e.g. 383-418 K), amorphous Ge crystallizes, i.e. the so-called metal-mediated-crystallization (MMC) takes place. In the course of MMC, crystalline Ge aggregates appear on the free surface, which results in the formation of fractal patterns with branching. The morphology of fractal patterns and hence the fractal dimensions are different depending on the relative difference in thickness between *a-Ge and Al layers. In-situ SEM observations indicate that the aggregation of Ge atoms takes place at the points of protruding branches, then the protruded branches run out further and further but the morphological changes can scarcely be observed at the portions other than the growing points.