CRYSTALLIZATION OF ENCAPSULATED VERY THIN AMORPHOUS-GE LAYERS

The crystallization processes and morphologies of a-Ge have been studied in the thickness range 1-30 nm. The thin Ge layers were evaporated under ultrahigh vacuum (5 x 10(-9) Torr) onto SiO(x) substrates and were also covered by a thin SiO(x) layer. The samples were annealed in forming gas and examined by transmission electron microscopy. The morphological stability of the layers depends on the film thickness and surface and interface diffusion processes during crystal growth resulting in ribbon shaped crystals in the thinnest layers. The single crystalline, ribbon-shaped crystals are 100 nm wide and can grow to the length of several mum. The morphological instability of the very thin amorphous layer close to the crystalline phase is proved by the formation of empty regions along the ribbons and around crystalline nuclei. The temperature of crystallization increases with decreasing film thickness in the thickness range 1-5 nm.