Structures and Electronic Properties of Amorphous Molybdenum-Germanium Alloys from Molecular Dynamics Simulations and the Tight-Binding Approximation.
We have developed a set of two-body and three -body potentials for modelling the structure of solid phases of germanium. The potential is of the same functional form as that of Stillinger-Weber for silicon, but it has different values of the parameters. The potential gives an excellent structural representation of amorphous solid germanium as well as crystalline Ge and gives good results for several thermodynamic properties of the crystalline phase and the phonon dispersion relations of the crystal. A set of effective two-body and three-body empirical interatomic potentials are proposed for describing the short range structure of amorphous alloys of molybdenum and germanium. Molecular dynamics computer simulation calculations were performed for these alloys for a wide range of compositions using these potentials. The resulting structures have radial distribution functions, differential distribution functions, and partial distribution functions that are in very good agreement with recent x-ray scattering experiments of Kortright and Bienenstock for all the compositions studied. The experimental observation is confirmed that Mo atoms do not substitute into the germanium random network at low Mo concentrations. The calculations predict that at low concentrations the metal atoms tend to cluster together and that they significantly distort the random tetrahedral network of the germanium atoms in their vicinity. Electronic structure calculations are performed on the amorphous MoGe alloys, using Harrison's universal linear-combination-of-atomic-orbitals parameters to generate hamiltonians for configurations obtained from the molecular dynamics simulations. The calculated density of states for liquid germanium has no band gap, indicating its metallic behavior in agreement with experimental observations, whereas the calculated density of states of amorphous germanium showed a distinct pseudogap, although with an appreciable density of states at the minimum. As the concentration of molybdenum atoms increases, the pseudogap of the germanium density of states is gradually filled up. The density of states at the Fermi energy calculated for our model agrees quite well with that determined by Yoshizumi, Geballe and co-workers. The localization index for the states at the Fermi energy in the gap is a decreasing function of molybdenum concentration in the range of 2-14 atom % Mo. The localization length for these states is an increasing function of Mo concentration. These results are consistent with the experimental observation of an insulator-metal transition at about 10% molybdenum.
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- Physics: Condensed Matter