A major challenge in fabricating/optimizing group-III nitride based devices is the huge dislocation density commonly observed in this materials system. A thorough understanding of the structure and particularly of the electrical activity of dislocations is thus crucial. However, so far even an agreement whether dislocations in GaN are electrically active or not is lacking. From a theoretical point of view a major challenge in describing dislocations is the large range of different length scales: While the core structure is rather localized the surrounding strain field is long range. Combining elements of density functional theory (DFT), empirical potentials, and continuum elastic theory we were able to describe edge dislocations consisting of a few 10⁵ atoms with ab initio accuracy. Using this approach we were able to (i) identify a hitherto not considered dislocation structure in GaN[1], (ii) demonstrate that the huge strain field around edge dislocations in GaN may cause deep gap states independent of the specific core structure, and (iii) understand the role of defects (vacancies) on the structure and electrical activity of dislocations. Based on these results the effect dislocations have on the optical properties of epitaxial films grown at various conditions and by different techniques.

[1] L. Lymperakis et al., Phys. Rev. Lett. 93, 196401 (2004).

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1. First ab initio determination of the phase transformations in Ni₂MnGa
2. Ab initio based growth simulations of group-III-nitrides
3. Multiscale modelling of group-III nitride growth