Electron irradiation and the equilibrium of open core dislocations in gallium nitride

Under electron irradiation, nanopipes in GaN are found to evolve into the so-called bamboo structure and eventually into a chain of voids [Pailloux et al., Appl. Phys. Lett. 86, 131908 (2005)]. Here, the driving mechanism for this morphological evolution is examined using transmission and scanning transmission electron microscopy studies of undoped GaN grown by hydride vapor phase epitaxy. Irradiation at varying beam energies shows that morphological evolution occurs below the threshold for direct knock-on damage, and that voids migrate towards the beam. It is proposed that these observations can be explained by the stimulated diffusion of point defects. In areas relatively unexposed to electrons, a survey of core character, diameter and depth into the layer suggests that a similar process occurs by post-growth annealing, i.e. due to point defects mobile at the growth temperature. The implications of these results for understanding dislocation core structures are assessed. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)