Abstract
We present results of highly localized electron energy loss spectroscopy carried out using scanning transmission electron microscopy in the energy loss regime at band gap and at the nitrogen near-edge structure of cross-sectional GaN, InGaN, and Al(Ga)N structures. We specifically attempt to determine changes in the intensity distribution and the onset energy of the inelastic scattering (the band-gap-related energy), of heterointerfaces, of quantum wells and of dislocations. We have used ab initio calculations within the local-density approximation to density-functional theory of the GaN and AlN band structure to simulate low electron energy loss spectra. Tests in which these were compared to experimental low loss spectra of pure GaN and AlN show good agreement in the position and shape of the spectral features. We then compare the positions of the onset energies on traversing interfaces of single AlN and AlGaN quantum wells as well as of GaInN and GaAlN multiple quantum well structures to the pure GaN and AlN spectra. We have been able to map relative changes in band-gap related energies of isolated interfaces and quantum wells, while the energy loss near edge structure allowed us to monitor relative changes in multiple layered structures of less than 5-nm separation. Reasons for the different sensitivities towards the above features, when measured in different energy regimes, are discussed. Following on from this we study the scattering intensity around onset and the position of the onset energy in locations along projection lines of isolated dislocations. Low loss spectrum calculations of dislocated regions reveal band-gap states associated with all dislocation types in GaN. The related pre-band-gap scattering intensity at 3.3 eV of the simulated spectra, in particular for the full core screw dislocation is in qualitative agreement with the experimental findings. An absorption peak at 2.4 eV found in certain regions in the vicinity of dislocations was not reproduced in the calculations and therefore was thought not to be produced by the dislocation but by impurity segregation.