Amorphous germanium II. Structural properties

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Abstract
The coherent X-ray scattering for momentum transfer, k, between 0·025 and 15·0 Å−1 has been measured for a series of sputtered amorphous Ge films prepared at various substrate temperatures, T s, between 0 and 350°C. Differences in the radial distribution function (RDF) of films of different T s have been determined by an accurate differential scattering technique. The small angle scattering (SAS) of the films is less than 100 electron units for k < 1 Å−1. From a combination of SAS, RDF and scanning electron microscope studies, it is concluded that an observed increase in film density with increasing T s occurs through a reduction in the number of voids about 7 Å or less in diameter. No variation of bond length with T s is found. With increasing T s, there is an increase in first and second-neighbour coordination and a reduction in bond angle distortion. The rate of change of coordination, C, with density, ρ0, is found to be d ln C/d ln ρ0 = 0·6±0·2. Using a new, general theory of the dependence of the RDF on the dihedral angle distribution, P(θ), it is shown that with increasing T s there is an increased probability of dihedral angles corresponding to the staggered configuration. For all films, the experimental RDF between r = 4·5 and 6·2 Å agrees with a nearly random P(θ) distribution. Comparison of experimental RDF's of crystalline and amorphous Ge indicates the static distortion of the first-neighbour bond length has a standard deviation of only about 0·04 Å.

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