Ordered structures in III-Nitride ternary alloys

T Pavloudis and J Kioseoglou and T Karakostas and P Komninou, COMPUTATIONAL MATERIALS SCIENCE, 118, 22-31 (2016).

DOI: 10.1016/j.commatsci.2016.02.036

An efficient evolutionary structure prediction algorithm in combination with ab initio calculations is implemented in order to reveal energetically favorable superstructures of the III-Nitride ternary alloys. Several 2 x 2 x 2 32-atom supercells are used to explore the full range of concentrations, from x = 0 to 1. The formation enthalpies, bandgaps, clustering and/or ordering of the atoms are investigated and the results are discussed. The formation enthalpy plots show local minima at specific concentrations, namely for x = 0.25, 0.50 and 0.75, that correspond to ordered structures. The valance band maxima, conduction band minima, bandgaps and the composition-independent bowing parameters for 2nd order Vegard's equation are calculated. The bandgap deviations from 1st order Vegard's law show total maxima at specific concentrations. The formation enthalpies and the bandgaps cannot be accurately described by single composition-independent bowing parameters, but the bandgaps are sufficiently described by composition- dependent bowing parameters, that are established. In order to verify the rationality of the results against the size of the ab initio supercells, molecular dynamics calculations of 14 x 14 x 14 supercells of similar to 10(4) atoms using bond-order interatomic potentials are performed. The obtained local minima of the formation enthalpy for the specific alloy compositions concur with those predicted by ab initio calculations proving the results are not influenced by the super-cell size. (C) 2016 Elsevier B.V. All rights reserved.

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