Obliquely deposited amorphous Ge films. I. Growth and structure

The structure and growth of amorphous Ge films deposited at an angle of incidence ranging from 0° to 80° have been studied by electron microscopy and electron diffraction as a function of the film thickness, deposition temperature, deposition rate, and annealing temperature. Like crystalline films, the growth of amorphous films proceeds via random nucleation centers and is controlled by the small but finite adatom mobility. The small adatom mobility results in the formation of a large concentration of vacancies, vacancy clusters, voids, and associated dangling bonds in a-Ge films. The adatom mobility is enhanced by the elevated deposition temperatures and oblique incidence and, consequently, the concentration of vacancy clusters and voids decreases. The formation of columns and decoration of structural defects associated with the cleaved NaCl single-crystal substrates during the growth of a-Ge films are a direct consequence of the enhanced adatom mobility. The observed thickness dependence of the resistivity and the variation of density, crystallization temperature, and short-range order with the deposition parameters are in accord with the structural model as deduced from our observations of the growth behavior of a-Ge films. The marked structural changes in a-Ge films on deposition at an angle of incidence result in significant changes in their physical, electrical, and optical properties.