Electron energy loss studies of dislocations in GaN thin films

Abstract

We present studies of the effects of dislocations in epitaxial GaN films on the low loss and the core loss electron energy loss spectrum. Electron microscopy samples were prepared in cross-sectional and plan-view geometry and investigations carried out in a dedicated cold field emission scanning transmission electron microscope. We consider the relative dislocation signal strength in both orientations on grounds of geometric considerations, and from comparison with the experimental signal, deduce that the scattering cross section for bulk and dislocation related scattering in the core loss energy regime are similar. The low loss results suggest that the scattering cross sections for dislocation related scattering in the band-gap regime are significantly smaller than for bulk scattering processes, also the localization of scattering events in this energy regime is an order of magnitude less than in core loss spectroscopy. The experimental low loss spectra provide evidence for dislocation related energy states below band edge, in accordance with predictions from calculated low loss spectra. Core excitation losses show dislocation related changes in the s−pz hybridized states. A peak in the energy range around 2.5 eV, which is not related to energy states introduced by the dislocation cores but presumably due to point defects, increases in strength for measurements towards the thin film surface, and in the vicinity of dislocations.