Medium-range structure of amorphous silicon studied by the Voronoi-Delaunay method

Structural inhomogeneities and their effect on the dynamics are investigated for a molecular dynamics model of quenched amorphous silicon. The structure of the model is analysed with the help of the Voronoi-Delaunay approach, which is a convenient tool for this purpose. The silicon structure is found to be rather homogeneous. Only localized defects are found, and no medium-range regions of 'imperfect' structure as were observed in a Lennard-Jones glass by V. A. Luchnikov, N. N. Medvedev, Yu. I. Naberukhin and V. N. Novikov (1995, Phys. Rev. B, 51, 15 569). It is assumed that the inhomogeneity in glasses is the result of competition between the tendency of atoms to pack locally into the most favourable arrangements and the necessity to realize space filling structures. For spherical atoms a regular tetrahedron of four atoms is the densest local configuration; however, such units cannot cover the whole space. As a result, Lennard-Jones glasses have regions of structural inhomogeneity with an extension of 3-5 interatomic distances. Amorphous silicon is not governed by close packing: its structure is a tetrahedral bond network. The aforementioned conflict is not relevant here, because the disordered tetrahedral network can fill the space with minor distortions of bond lengths and tetrahedral angles.