Some aspects on an improved stability of a-Si:H and a-Ge:H films with respect to their microstructure

In this paper we present investigations concerning the influence of the microstructure on the stability of amorphous hydrogenated silicon (a-Si:H) films deposited by the conventional glow discharge and the 'hot wire' CVD (chemical vapor deposition) techniques. It is demonstrated that the results obtained for a-Si:H films are, in general, also applicable to our high quality de magnetron sputtered a-Ge:H films (sp-a-Ge:H). Spectroscopic ellipsometry is used to determine microstructural properties of the films. We have used a novel data interpretation based on a tetrahedron model, which considers hydrogen incorporation into the amorphous network. This data interpretation yields information about the void volume fraction and additionally allows an accurate determination of the film density. We show that the saturated defect density after 20 keV electron irradiation or 1000 h of AM1 light soaking respectively, is directly connected to film density.