CRYSTALLIZATION OF ENCAPSULATED VERY THIN AMORPHOUS-GE LAYERS

The crystallization processes and morphologies of a-Ge have been studi ed 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 we re also covered by a thin SiO(x) layer. The samples were annealed in f orming gas and examined by transmission electron microscopy. The morph ological stability of the layers depends on the film thickness and sur face and interface diffusion processes during crystal growth resulting in ribbon shaped crystals in the thinnest layers. The single crystall ine, ribbon-shaped crystals are 100 nm wide and can grow to the length of several mum. The morphological instability of the very thin amorph ous layer close to the crystalline phase is proved by the formation of empty regions along the ribbons and around crystalline nuclei. The te mperature of crystallization increases with decreasing film thickness in the thickness range 1-5 nm.