Structural and vibrational properties of amorphous Si_1-xGe_x alloys: An ab initio molecular-dynamics study

The simulated quenching procedure in ab initio molecular-dynamics approach has been used to generate amorphous Si_1-xGe_x alloys for various Ge compositions. The generated structures show a random covalent network rather than a chemically ordered network. The bond-length distributions and the vibrational properties of the generated networks do not follow a simple Vegard’s rule, suggesting the presence of inhomogeneity in the networks. It has been found that the local defect structures are governed by the dangling bonds at the Ge sites and the Si-centered floating bonds for a wide range of Ge compositions of x>~0.15. The backbonding states of the Ge dangling-bond sites increase with the Ge composition. The Si-related high-frequency modes are softened at a low Ge composition of x=0.15. Results and discussion on the calculated structural and vibrational properties of the Si_1-xGe_x alloys are presented.