A. Hairie

An empirical potential for the calculation of the atomic structure of extended defects in wurtzite GaN

Abstract The Stillinger–Weber potential has been parametrized for GaN with bond-type dependent parameters to allow efficient atomic simulations of large systems containing wrong, dangling and extra bonds. The input data for the fit are the experimental elastic constants of wurtzite structure and wrong bond energies deduced from ab initio calculation. The potential in then applied to zincblende structure and to planar defects. The predicted values are compared to experimental observation and previous computations.

Free energies of interfaces from quasi-harmonic theory: A critical appraisal

A critical assessment is given of the quasi-harmonic approximation, and various approximations to the quasi-harmonic approximation, with regard to predicting the free energy and atomic structure of grain boundaries in silicon at elevated temperatures. The quasi-harmonic results are compared with those obtained by molecular dynamics and thermodynamic integration. It is found that the quasi-harmonic approximation yields accurate excess free energies and atomic structures of grain boundaries at 1,000 K. The anharmonic contribution to the free energy that is absent in the quasi-harmonic contribution is virtually the same at a grain boundary in Si and in the perfect crystal. The second-moment and Einstein approximations to the full quasi-harmonic theory yield unreliable free energies, but reasonably accurate atomic structures. However, excess free energies are quite well described by the Einstein model. It is concluded that the quasi-harmonic approximation works remarkably well in silicon. The simplest approximations to the phonon density of states lead to unreliable results for the free energy, but cancellation of errors occurs to a large extent when excess free energies are computed.

Determination of the high c/a ratio of hexagonal metals with a semi-empirical tight-binding method

Abstract A semi-empirical tight-binding potential is developed to reproduce the high c/a ratio of zinc and cadmium. In this scheme, we show that the calculation of the electronic energy by the standard recursion method with the Harrison model for the hopping integrals and their radius dependence is not able to predict c/a values higher than the ideal one γ 0 = 8/3 =1.633 . Only, the recursion method limited to first neighbours has permitted to get the right c/a ratio of zinc and cadmium. The calculated energies of different structures (2H, 4H, 6H and 3C) show that the hexagonal compact structure (2H) is always most stable.

Energy of the Two Variants 11A and 11B in Silicon and Germanium by the Semi-Empirical Tight-Binding Method

A semi-empirical tight-binding model was used to study the total energy of Σ = 11 tilt grain boundaries in silicon and germanium. At low temperature the structures A in silicon and B in germanium were found to be more stable in agreement with the experimental results.

Comparison of two O(N) methods for total-energy semi-empirical tight-binding calculation

We compare two O(N) methods, to calculate the total energy of a system, based on a local evaluation of the density matrix (DM) via the Lanczos-Haydock recursion scheme. We have recently introduced the first method, in which an approximated DM is directly computed from the local tridiagonalized hamiltonian (LTH). In the second method, the DM is obtained from the diagonalized LTH without any further approximation. The use of the first method is restricted to temperatures higher than 1000 K, but there is no limitation for the second one. The methods are compared when applied to a grain boundary in silicon and germanium.

Potential energy of two structures of Σ = 11〈0 1 1〉 tilt grain boundary in silicon and germanium with empirical potentials and tight-binding methods

Abstract Two atomic structures A and B of the Σ = 11〈0 1 1〉 grain boundary were observed in silicon and germanium. We have performed a complete study of the stability of these two grain boundaries using some empirical potentials and also the semiempirical, tight-binding (TB) method. The TB method has confirmed the experimental observations at low temperatures. The A structure is more stable in silicon whereas for germanium the B structure is obtained. The empirical potentials, such as those of Keating (1966), Baraff et al. (1980) and of Stillinger and Weber (1985), give the A structure as the most stable for both germanium and silicon. The non-ability of these empirical potentials to make a difference between germanium and silicon and the advantage of TB method are discussed.

Analysis of Distortions in 〈110〉 Tilt Silicon Bicrystals

The empirical potentials used for defect simulation in silicon are fitted on elastic or phonon properties. Usually they are not able to take into account all the distortions present in a defected material. On the basis of the potential proposed by Vanderbilt, Taole and Narasimhan a method is presented to separate the contribution of distortions linked to elastic properties from the contribution of distortions linked to phonon properties. The method is applied to grain boundaries in silicon simulated by potentials in the harmonic approximation. The phonon contribution is found slightly predominant with respect to the elastic one.

The volume expansion of the {112̄0} planar defect in 2H–GaN/6H–SiC (0001)Si grown by MBE

Abstract In an attempt to complete the determination of the atomic structure of the {1120} planar defects which are found systematically in the interfacial area of (2H) GaN–AlN/6H–SiC, high resolution electron microscopy and atomistic calculations were used and an adequate model was chosen. It is found that the atomic structure corresponds to the ideal 1/2〈1101〉{1120} stacking fault model, with no volume expansion.