Amorphous silicon studied by ab initio molecular dynamics: Preparation, structure, and properties

We present a first-principles molecular-dynamics study of pure amorphous silicon obtained by simulated quench from the melt. A cooling rate of ${10}^{14}$ K/s is sufficient to recover a tetrahedral network starting from a well-equilibrated metallic liquid having average coordination larger than 6. Dramatic changes in physical properties are observed upon cooling. In particular, a gap forms in the electronic spectrum, indicating a metal-to-semiconductor transition. The as-quenched structure has average coordination very close to 4, but contains several coordination defects as well as a large fraction of distorted bonds. Subsequent annealing reduces the amount of strain and the number of defects present in our system. The average structural, dynamical, and electronic properties of our sample are in good agreement with the available experimental data. We report a detailed analysis of the structural relaxation processes accompanying annealing and compare our findings with recent experiments.