Bond-length relaxation in Si1-xGex alloys

We have measured and quantified the effect of alloy composition on the atomic bonding in relaxed molecular-beam-epitaxy-deposited crystalline Si1-xGex alloys. X-ray-absorption fine structure (XAFS) and x-ray diffraction were used to examine how the atomic bonding in Si1-xGex is affected by changes in alloy composition. In this study, the Ge-Ge and Ge-Si bond lengths were measured using XAFS and compared with the conflicting results of existing analytical models and previous XAFS studies. The measured Ge-Ge and Ge-Si bond lengths were found to be in good agreement with the analytical models, which predict that the Ge-Ge, Ge-Si, and Si-Si bonds maintain distinctly different lengths which change linearly with alloy composition. The topological rigidity parameter a** was used to quantify the linear dependence of the bond lengths on alloy composition and a value of a**=0.63 was calculated from the measured bond lengths. An extensive XAFS error analysis was performed and the error in the topological rigidity parameter a**=0.63-0.13+0.08 was determined. This value of a**, which is notably different from 0 or 1, indicates that both the bond lengths and bond angles are distorted by changes in composition.