From amorphous to nanocrystalline germanium— the role of twinning studied by high resolution electron microscopy

Abstract

The structure of amorphous thin films of germanium obtained by physical vapor deposition at temperatures well below one half of the melting temperature has been studies by high resolution electron microscopy. Crystallization of the films was achieved during deposition by appropriate heating of the substrate or after deposition by annealing at elevated temperature, respectively. Therewith, very small crystallites extending from nearly 3 to about 25 nm in size are formed, at which twinning is frequently observed. In particular, multiply-twinned structures with five-fold twin junctions occur at various stages of the crystallization. The temperature at which the crystallization starts can be markedly lowered, and the crystallinity of the films obtained can be substantially increased by capping the crystalline substrates with amorphous carbon layers. A combination of substrate capping and post-deposition annealing leads to almost completely crystalline films which exhibit a heavily twinned structure.