Structural Units and Energy of Grain Boundaries in GaN
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Structural Units and Energy of Grain Boundaries in GaN Jun Chen1, Pierre Ruterana and Gerard Nouet Laboratoire Structure des Interfaces et Fonctionnalité des Couches Minces, UMR CNRS 6176, ENSICAEN, 6 Boulevard du Maréchal Juin 14050 Caen Cedex, France 1 Laboratoire Universitaire de Recherche Scientifique d’Alençon, Institut Universitaire de Technologie, 61250 Damigny, France ABSTRACT The energy of coincidence grain boundaries in wurtzite structure was investigated with the Stillinger-Weber potential after modification for GaN. A boundary can have two interfaces with a different period corresponding to the edge and diagonal of the unit cell of coincidence site lattices for rotations around [0001] in the range 0- 60°. Their energy depends mainly on the atomic structure of the dislocation cores used for the boundary plane reconstruction. As a function of the structure units in the period, the energy of the edge and diagonal shows two minima which are observed for the edge boundaries.
INTRODUCTION The properties of the materials depend on the nature and density of defects. The excellent optical properties of the nitride semiconductors could be explained by considering the influence of the defects on the states inside the band gap. The description of the defects at the atomic scale is the first stage to approach the electronic structure of materials containing defects. GaN layers grown on any substrates by molecular beam epitaxy or by metal-organic chemical vapour deposition contain a lot of structural defects with a very high concentration of threading dislocations. They may form low-angle grain boundaries giving rise to a mosaic structure. The adjacent islands are generally twisted by a few degrees about the axis and the prismatic boundaries are made of 1/3 pure edge dislocations. In some cases, high angle grain boundaries may be formed with the same dislocations. The atomic structure of these edge dislocations was investigated by high-resolution electron microscopy and atomistic calculations. It was shown using anisotropic elasticity calculations that their atomic structure is multiple with cores containing either 4, 5/7 or 8 atoms. Experimentally, these core configurations were found, 5/7 and 8 for the dislocations, and the three in the high angle grain boundaries [1]. More recently, energetic calculations were performed to analyse the potential energy of the dislocations and grain boundaries. The calculations were carried out with the Stillinger-Weber potential after modifications of the parameterisation in order to take into account the Ga-Ga and N-N wrong bonds [2]. The atomic structures of the grain boundaries with the smallest period, edge, were determined for the lowest energy configuration. They are described in terms of a limited number of structural units corresponding to the above cores [3]. In this paper, we have calculated the energies of the edge and diagonal of the coincidence site lattice unit cell for two complementary rotations corresponding to Θ and 60°- Θ and we have compared their energ
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