C. J. Fall

Straight and kinked 90° partial dislocations in diamond and 3C-SiC

Density-functional based calculations are used to investigate low energy core structures of 90° partial dislocations in diamond and 3C-SiC. In both materials dislocation glide is analysed in terms of kink formation and migration and the fundamental steps to kink migration are investigated. We find the C terminated core structure in SiC to be more mobile than the Si core. However, the Si partial is electrically active and this opens the possibility of recombination-enhanced glide under ionizing conditions or an enhanced mobility in doped material.

Electron spectroscopy of carbon materials: experiment and theory

We present a comparative spectroscopic study of carbon as graphite, diamond and C60 using C1s K-edge electron energy-loss spectroscopy (EELS), X-ray emission spectroscopy, and theoretical modelling. The first principles calculations of these spectra are obtained in the local density approximation using a self-consistent Gaussian basis pseudo-potential method. Calculated spectra show excellent agreement with experiment and are able to discriminate not only between various carbon hybridisations but also local variation in environment. Core-hole effects on the calculated spectra are also investigated. For the first time, the EEL spectrum of carbyne is calculated.

Electron energy loss studies of dislocations in GaN thin films

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 sta...

Dislocation-induced electronic states and point-defect atmospheres evidenced by electron energy loss imaging

Electronic band gap states connected with individual dislocations in diamond and GaN are revealed, using highly spatially resolved electron energy loss (EEL) spectrum mapping. Comparison with calculations of low EEL spectra from first-principle methods allows the identification of the joint density of states of different dislocation core types. Also presented is evidence for instances where point defects/impurities have accumulated in the strain field or segregated to the core of dislocations.

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

Abstract 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.

Calculated and experimental low-loss electron energy loss spectra of dislocations in diamond and GaN

First-principles calculations of electron energy loss (EEL) spectra for bulk GaN and diamond are compared with experimental spectra acquired with a scanning tunnelling electron microscope offering ultra-high-energy resolution in low-loss energy spectroscopy. The theoretical bulk low-loss EEL spectra, in the Eg to 10 eV range, are in good agreement with experimental data. Spatially resolved spectra from dislocated regions in both materials are distinct from bulk spectra. The main effects are, however, confined to energy losses lying above the band edge. The calculated spectra for low-energy dislocations in diamond are consistent with the experimental observations, but difficulties remain in understanding the spectra of threading dislocations in GaN.

Modelling electron energy-loss spectra of dislocations in silicon and diamond

Abstract Electron energy-loss spectroscopy (EELS) performed near dislocation cores is one of the few experimental techniques that can yield valuable information about the electronic levels associated with dislocations. In this study, using ab initio calculations, we model and predict low-loss and core-excitation EELS spectra acquired on various dislocation cores in silicon and diamond, and compare the results with bulk spectra. In diamond, we consider in particular 90° partial glide, undissociated 60° shuffle, and 30° partial dislocations. We find evidence of empty states localized on diamond shuffle dislocation cores and positioned below the bulk band edge, which modify the EELS spectrum. In silicon, we find changes—analogous to those seen experimentally—in core-excitation EELS near stacking faults and partial glide dislocations.

Small aggregates of interstitials and models for platelets in diamond

By examining the structure of small clusters of self-interstitials in diamond using local-density-functional techniques, we have developed models for the planar defects called platelets. We present the structures, energies and vibrational properties.

Theory of Electron Energy Loss Spectroscopy and its Application to Threading Edge Dislocations in GaN

The electronic structure of dislocations in GaN is controversial. Several experimental techniques such as carrier mobility studies and cathodoluminescence experiments have indicated that dislocations are charged while theoretical studies point to intrinsic states and/or point defect accumulation along the core as a source of electrical activity. Electron Energy Loss Spectroscopy (EELS) studies have the ability to probe the electronic structure of extended defects. Here we report first principles calculations of the EELS spectrum applied to edge dislocations in GaN. It is found that the electrostatic potential at N atoms in the vicinity of the dislocation varies by the order of a volt and casts doubt on any simple interpretation of core loss spectroscopy. On the other hand, low loss spectroscopy leads directly to detailed information about any gap states. The low loss spectrum obtained by the theory is in good agreement with recent experimental work and indicates that threading dislocations in p-type GaN possess acceptor levels in the upper half of the gap.