A.T. Blumenau

Dislocations in hexagonal and cubic GaN

The structure and electronic activity of several types of dislocations in both hexagonal and cubic GaN are calculated using first-principles methods. Most of the stoichiometric dislocations investigated in hexagonal GaN do not induce deep acceptor states and thus cannot be responsible for the yellow luminescence. However, it is shown that electrically active point defects, in particular gallium vacancies and oxygen-related defect complexes, can be trapped at the stress field of the dislocations and may be responsible for this luminescence. For cubic GaN, we find the ideal stoichiometric 60° dislocation to be electrically active and the glide set to be more stable than the shuffle. The dissociation of the latter is considered.

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.

Optical bands related to dislocations in Si

First-principles calculations are used to investigate the interaction of self-interstitial aggregates with the 90? partial dislocation in Si. We find that I4 is bound to the line with an energy of around 3?eV. The defect causes deep levels to appear in the band gap and optical transitions between these levels may account for the luminescent bands relating to plastically deformed Si.

The 60° dislocation in diamond and its dissociation

The perfect 60° glide dislocation in diamond serves as an example of how different aspects of dislocations can be modelled in an approach combining continuum elasticity theory with atomistic density-functional-based tight-binding calculations. After investigating the perfect 60° dislocation and its isolated Shockley partials, the energetics of dissociation are discussed. The 60° dislocation is found to dissociate with a substantial lowering of its line energy. However, an energy barrier to dissociation is found. The glide motion of the 30° Shockley partial involved is modelled in a process involving the thermal formation and subsequent migration of kinks along the dislocation line.

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.

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.

Interaction of dislocations in Si with intrinsic defects

Abstract Dislocations in Si are believed to glide by the nucleation and propagation of kinks. However, several experiments and calculations suggest that this simple picture is far from accurate. An alternative theory is that obstacles control the dislocation mobility. Candidates for these are interstitial clusters bound to the line.