Strain Relaxation Mechanisms and Local Structural Changes in Si1−xGex Alloys

In this work, we address issues pertinent to the understanding of the structural and electronic properties of Si1−xGex alloys, namely, (i) how does the lattice constant mismatch between bulk Si and bulk Ge manifests itself in the alloy system? and (ii) what are the relevant strain release mechanisms? To provide answers to these questions, we have carried out an in-depth study of the changes in the local geometric and electronic structures arising from the strain relaxation in Si1−xGex alloys. We first compute the optimized lattice constant for different compositions (x) by fully relaxing the system and by minimizing the total energy with respect to the lattice constant at each composition, using an ab initio molecular dynamics scheme. The optimized lattice constant, while exhibiting a general trend of linear dependence on the composition (Vegard’s law), shows a negative deviation from Vegard’s law in the vicinity of x=0.5. We delineate the mechanisms responsible for each one of the above features. We show that the radial-strain relaxation through bond stretching is responsible for the overall trend of linear dependence of the lattice constant on the composition. On the other hand, the negative deviation from Vegard’s law is shown to arise from the angular-strain relaxation.