Impurity effects on the structure of amorphous silicon and germanium prepared in various ways

Abstract

Thin film transmission electron diffraction patterns from amorphous silicon, prepared by ion-implantation, RF sputtering and vapour deposition (evaporation) are strikingly similar in most details except in the excact position of the diffuse peaks. For high purity silicon, either evaporated or ion-implanted, the first diffuse ring, for example, occurs consistently at a value of s(= 4π sin θ/λ) slightly lower than that of the (111) Bragg peak of the crystallized film. For sputtered films this diffuse ring appears at a higher angle than the (111) peak both in thin film electron diffraction and in x-ray scattering from thick films. The reversal is attributed to impurities, most probably oxygen or nitrogen. An analysis shows that the position of the diffuse peak in question can be correlated with the separation of second nearest-neighbours which is therefore deduced to be contracted in the impure samples. No changes in the diffraction pattern were observed on annealing, although the optical absorption edge moves to higher energiers. Similar measurements on amorphous germanium films show little difference between the position of the first diffuse ring in evaporated and sputtered films; in both cases it occurs at a lower angle than does the (111) reflection of the crystallized samples. However, in severely oxygen-contaminated amorphous germanium, the diffuse peak shifts to lower angle with annealing while the absorption edge position shows a shift to higher energies. In germanium of a purity comparable with the sputtered silicon, there is a negligible change in position or shape of diffuse peaks on annealing although, as with silicon, there are still changes in optical absorption.