Free-energy model for bonding in amorphous covalent alloys

A free-energy model (FEM) for bonding in amorphous covalent alloys is developed and applied to the ternary a-Siₓ{N}yHz alloy system. The Gibbs free energy of mixing GM=HM-TSM is obtained with use of the quasichemical approach to the thermodynamics of regular solutions. The enthalpy of mixing, HM, is expressed in terms of the nearest-neighbor bond energies, while the entropy of mixing, SM, is given by the number of possible bonding configurations in Si-centered tetrahedra. Chemical ordering (CO) in these alloys is shown to correspond to the preference for Si-N and Si-H bonds at the expense of Si-Si and N-H bonds. The predictions of the FEM are shown to be in good agreement with experimentally determined bond concentrations in a-Siₓ{N}yHz alloy films where, in general, neither the CO (T=0 K) nor random-bonding limits are valid.