Si-Ge-based Oxynitrides: From Molecules to Solids

Density functional theory simulations are used to predict ground state crystal structures, electronic properties, and thermodynamic stability of a new class of Si1-xGexN2O oxynitride materials with potential applications as tunable dielectrics. Thermochemical simulations are also used to explore their possible synthetic routes via reactions of ammonia with (a) mixtures of (SiH3)(2)O and (GeH3)(2)O, and (b) a single-source heteronuclear analogue SiH3OGeH3. To obtain quantitative values for the above reaction energies we implement a consistent computational methodology to simulate the structural and thermochernical properties of both molecular and solid state reactants and products at finite-temperature. In the case of the well-known (SiH3)(2)O and (GeH3)(2)O compounds our calculated molecular structures and vibrational spectra are in excellent agreement with experiment. The hypothetical SiH3OGeH3 molecule is predicted to possess an intermediate molecular structure and energy, with stability differences on the order of 1-2 kcal/mol between SiH3OGeH3 and mixtures of (SiH3)(2)O/(GeH3)(2)O. For the solids we predict two new ordered structures: (i) an alpha-SiGeN2O phase composed of a uniform distribution of SiN3O and GeN3O tetrahedra, and (ii) a "pseudo-lamellar" form beta-SiGeN2O in which the SiN3O and GeN3O units occupy alternating layers. The structural, electronic, and thermoelastic properties of the latter are then systematically compared to those of Si2N2O and Ge2N2O. Here again, small energy differences comparable to those in the molecular case are found between the SiGeN2O polytypes and their Si2N2O/Ge2N2O analogues. The enthalpy of formation of a-SiGeN2O, beta-SiGeN2O, and a random SiGeN2O alloy are predicted to be comparable, indicating that mixing entropy should favor the disordered solid at high temperatures. Collectively, a remarkable consistency is found for the bond-lengths and bond-angles across molecular and solid-state forms. From an experimental perspective, the recent development of industrial scale synthesis for (SiH3)(2)O suggests that the Ge-based analogues proposed here might be accessed using similar approaches, opening the door to new chemically compatible Si-Ge-O-N high-k gate materials for high mobility Si-Ge based applications.