A INTRODUCTION

No data from reliable experimental measurements (e.g. ultrasonic measurements of bulk single crystals) of the elastic constants of Si-Ge alloys have been published to date. However, the data that do exist suggest that the elastic constants can be adequately determined using simple linear rule-of-mixtures (ROM) interpolations from the elastic constants of pure Si and Ge, which are very well documented. Furthermore, there is a variety of indirect evidence, both experimental and theoretical, which suggests that deviations from ROM values are small.

This possibility is not surprising considering the close similarity of Si-Si and Ge-Ge bonds and the very low ionicity of Si-Ge bonds. For covalently bonded materials, the stiffness can be related to bond length. The lattice parameters of Si and Ge differ by only 4% and the average lattice parameters of Si-Ge alloys follow the ROM prediction with only small deviations [1].

Si-Ge alloys crystallise in the diamond cubic form. Thus the elastic stiffness tensor, c _ij, is reduced to the three independent stiffness constants c ₁₁, c ₁₂ and c ₄₄. TABLE 1 shows typical values of these constants for pure Si and Ge.