Optical studies of a-Si samples are reported. The dielectric function was determined ellipsometrically from 2.0 to 4.0 eV. Different concentrations of slit-like voids may account for the variation of the optical constants in the investigated films. The depolarization factor L of the voids was estimated to 0.75 5 L 5 0.9. A structure of the optical constants
Introduct,ionAmorphous germanium and silicon (a-Si) have been the subject of many investigations on disordered systems. Among them, a lot of experimental results are consistent with the existence of a network of voids. Structure investigations [l to 31 point to very small holes or density deficiencies with till now uncertain dimensions. Measurements of the paramagnetic resonance [4 to 61 and tlhe dielectric constant [5, 71 may be explained by voids and intimately connected dangling bonds. Void influence may account for the most striking discrepancy between a sharp [8,9] and an exponential ([lo, 111 and references within) absorption edge in a-Ge. The exponential shape is a general consequence of microfields [12] which are plausible within voids with unsaturated bonds. Discrepancies between photoemission curves measured in different laboratories [ 13 to I71 may be connected with sample preparation (and therefore void concentration) which strongly influences the tailing of the density of states into the energy gap. The optical constants of a-Ge for higher photon energies [9, 18 to 201 are strongly influenced by submicroscopic voids. This behaviour was interpreted by an idealized model for an effective dielectric function [21 to 231.The concentration of voids is in all cases a sensitive function of the preparation conditions. It seems a general rule that low pressure during the deposition and a substrate temperature very near to the crystallisation point of the appropriate material yield films with high density. This density is comparable to that in Polk's model [24] and points therefore to small void concentrations.