Diffusion in amorphous solids - Metallic alloys and elemental semiconductors

In this article an overview is presented of our present knowledge on diffusion in two groups of non-crystalline solids, namely metallic glasses and amorphous semiconductors. Whereas for metallic glasses emphasis is laid on self-diffusion, in the case of amorphous semiconductors diffusion of transition metals is considered. In both cases, diffusion is thermally activated like in crystalline solids. However, the diffusion mechanisms differ considerably from those operating in crystals. In metallic glasses, there is a change from vacancy-controlled diffusion in the as-produced amorphous state to direct collective diffusion in the relaxed amorphous state. In amorphous semiconductors, transition metals diffuse by interstitial-like hopping which is retarded by saturable traps, presumably dangling-bond-related extra-broad meshes in the covalent amorphous network. Finally attention has been paid to diffusion in undercooled melts of metallic alloys just above the glass-transition temperature, since from this an understanding of the transition from solid-type to fluid-type diffusion is expected.