Abstract
We report on first-principles calculations of electron energy-loss (EEL) spectra for bulk group-III nitrides and of spatially resolved EEL spectra for a variety of edge and screw dislocations in GaN. The theoretical bulk low-loss EEL spectra, in the 0-10 eV range, are in good agreement with recent experimental data. We find gap states associated with undecorated full-core dislocations that lead to absorption below the bulk onset energy in low-loss EEL spectroscopy. It is also found that the electrostatic potential at N atoms in the vicinity of an edge dislocation varies by an order of a volt and casts doubt on any simple interpretation of core-loss spectroscopy.