Energy loss spectroscopy of dislocations in GaN and diamond: A comparison of experiment and calculations

We present results of electron energy loss spectroscopy (EELS), carried out with nanometer spatial resolution in a dedicated scanning transmission electron microscope (STEM), of thin films of GaN and diamond. We are able to extract the scattering intensity down to energy losses of 2 eV with an energy resolution of 0.36 eV. This is achieved by using a high spectrometer dispersion. We specifically attempt to determine changes in the electronic bandstructure, reflected in the inelastic low loss scattering distribution at dislocations. Ab initio calculations within the local density approximation to density functional theory of the bandstructure used to simulate low electron energy loss spectra reveal bandgap states associated with all dislocation types in GaN as well as in diamond [Phys Rev B65 (2002) 245-304; Phys Rev B 65 (2002) 205-206]. Core loss calculations of GaN predict potential shifts at edge dislocations due to the piezoelectric effect. The theoretical findings were in very good agreement with the measurements.