Free energy model for the analysis of bonding in a-SixNyHz alloys

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Abstract
A free energy model (FEM) for the analysis of the bonding in amorphous covalent alloys is developed and applied to ternary a-SixNyHz alloys. The quasichemical approach to the thermodynamics of regular solutions is used to obtain the Gibbs free energy of mixing GM=HM−TSM for the alloys. The enthalpy of mixing HM is determined by the nearest-neighbor bond energies while the entropy of mixing SM is given by the number of possible bonding configurations in Si-centered tetrahedra. The concentrations of Si–N, Si–Si, Si–H, and N–H bonds have been determined as functions of temperature and composition by minimizing the free energy of the a-SixNyHz alloy. It is shown that chemical ordering (CO) in these alloys corresponds to a 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-SixNyHz alloy films and it is demonstrated that, in general, neither the CO nor random bonding limits are valid.
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