Tight-binding molecular dynamics study on the structural change of amorphous germanium with the increase of density

The structures of low-density and high-density amorphous germanium (a-Ge) are studied, by using the order-N non-orthogonal tight-binding molecular dynamics method, where N is the number of atoms. The initial amorphous configuration is taken from the results of a glass-transition simulation performed in a separate work, with N = 512. From this initial configuration, we gradually increase the density of the system. In low-density a-Ge, the short-range order (SRO) is characterized essentially by the four-fold configuration although, at lowest densities, the coordination number is smaller than four, thus indicating that quite a few dangling bonds exist. The tetrahedral nature of the SRO in a-Ge is in substance the same as the SRO of a diamond structure which is the low-pressure form of crystalline germanium (c-Ge). On the other hand, the SRO in high-density a-Ge is similar to that of a beta-Sn structure, the latter being the high-pressure form of c-Ge.