GaN layers grown by means of heteroepitaxial techniques contain a huge amount of threading dislocations with typical density of 10¹⁰cm^-2. The majority of these dislocations are (a) edge and the remaining ones are both (c) screw and mixed (a+c). A great improvement in layers quality has been achieved with ELO (Epitaxial Lateral Overgrowth), by lowering this dislocation density with several orders, laser diodes with lifetime reaching l0.000h has been realized. However, it’s well known that in the ELO technique, dislocations have the tendency to bend and then some of them can change their character for screw to edge and vice-versa. In this work, we explore the issue of what could be the core structure of a c screw dislocation (b = <0001>) when it bends and changes its character to edge (b=<0001>, l=<11-20>).

In the framework of self-consistent tight-binding methods (SCC-DFTB), the core geometries of four configurations of the c edge dislocation are presented and their structures analyzed in terms of bonds angles and lengths. The dislocations are modeled in pseudo hydrogen terminated clusters of about 720 atoms, periodic along the dislocation line direction <1-210>. The equilibrium core configurations were obtained by relaxing the clusters in a procedure based on a conjugate gradient algorithm. The two core configurations without wrong bonds display a 4/8 atoms rings like structure while those with wrong bonds display a 5/8/5 atoms ring like one.

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4. Energy and electronic structure of gallium and nitrogen interstitials in GaN Tilt Boundaries
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