Density of amorphous germanium films by spectroscopic ellipsometry

Abstract

Over the last 15 years there has been considerable discussion on the maximum density of a-Ge and a-Si. Part of the significance of this research lies in the central role it plays in structural models of these tetrahedrally bonded amorphous semiconductors. Besides the fact that no detailed model of the homogeneous structure exists, the presence of void networks further complicates the issue. In this study we report on the spectroscopic ellipsometric analysis of an extensive series of a-Ge films prepared by rf sputtering. A systematic analysis of the data using n-layer models, the Bruggeman effective medium approximation, and least squares regression analysis were carried out. The thickness dependence of the bulk film density is shown to be consistent with a fractal model of film morphology and the relative values of density for different sputtering pressures related to film bombardment. The highest film density in this study is 5% greater than crystalline density based upon commonly accepted secondary standard reference data.