Structure of Germanium Films Evaporated onto Clean NaCl Substrates

Abstract

Germanium films, 30–300 Å in thickness, evaporated onto vacuum-cleaved and air-cleaved NaCl substrates held at 25 °–500 °C in ultrahigh vacuum at various deposition rates, 0.1–20 Å/sec, have been investigated by reflection electron diffraction and transmission electron microscopy. It was found that structure and epitaxy of films were markedly influenced by substrate surface conditions and deposition rate as well as substrate temperature. Decoration of imperfections of the substrate surface was obtained with germanium (30–100 Å in thickness) deposited at relatively slow rate (0.1–2 Å/sec). Amorphous to crystalline transition temperature was also influenced by deposition rate. Epitaxial orientation of films: (001)Ge//(001)NaCl and [100]Ge//[100]NaCl, was observed on vacuum-cleaved substrate at 250 °C at the slow deposition rate. Good epitaxial films could be prepared at 400 °–500 °C, although the films included microtwins with the twinning planes of {111} in part. However, films on air-cleaved substrates at 400 °–500 °C showed always polycrystalline structure. Epitaxy and structure of films were also influenced considerably by an electron-beam irradiation applied to the substrate during evaporation.