On the Critical Layer Thickness of Strained-Layer Heteroepitaxial CoSi₂ Films on 〈111〉Si

Abstract

The technique of Double Crystal X-Ray Diffractometry (DXD) and ion beam channeling are applied to investigate, as a function of thickness, the average perpendicular strain and crystal quality of CoSi₂ layers grown by MBE on 〈111〉Si. The results show that thin layers (from 20 to 30 nm) are partially relaxed but with a strain greater than that expected for a free CoSi₂ lattice. For layers thicker than 30nm the magnitude of the CoSi₂ strain incrgases to 1.7%, somewhat less than the maximum magnitude strain expected for coherent growth (2.1%). For layers thicker than 50 nm, the perpendicular strain relaxes very slowly, with the strain at 225 nm only 5% less than that at 50nm. It was concluded that a coherent epitaxial layer does not form initially and the relaxation of the strained layers is not consistent with a planar growth mechanism of the CoSi₂ epilayers. Therefore the concept of a critical thickness, below which the epilayers are strained and above which the epilayers are relaxed, cannot be applied to our CoSi₂/Si system.