Papers by Author: M.I. Heggie

Abstract: First-principles calculations are used to investigate the partial dislocations in 4H-SiC. We have shown that the Peierls barriers are strongly dependent on the dislocation core structures. Our results have revealed that the asymmetric reconstruction does not possess midgap states while the symmetric reconstructions, characterized by dangling bond on like atoms along the dislocation line, are always electrically active. We suggested that under forward bias, the free energies of the symmetric reconstructions are dynamically lowered by continuous electron-hole transitions between the respective deep levels and valence/conduction bands.

Abstract: Carbon nanotubes present interesting potential applications especially in nanoelectronics. Their electrical properties are known to be a function of their chirality. It happens that 1/3 of CNs are metallic and 2/3 are semiconductors. Narrow nanotubes are expected to be wide-band gap semiconductors. Several experimental results have shown that the thickness of a multi-wall nanotube along the axis can change, while the interlayer spacing remains fairly constant. These observations suggest the coexistence in the same tube of a scroll structure and a multi-wall nested tube. We explain this defect as a screw dislocation which by gliding transforms between these two forms. In this paper, we present a density functional theory study of the structure and energetics of screw dislocations in AA and ABC graphite, and we discuss their role in the scroll-to-nanotube transformation in multi-wall nanotubes.

Abstract: First-principles calculations are used to investigate the partial dislocations in 4H-SiC. We show that the stability of the dislocation cores and the Peierls barriers of the first kind are chargestate dependent. In intrinsic bulk the partials are stable in the neutral asymmetric reconstructions. These reconstructions have no deep states and are characterized by high Peierls barriers. In strongly doped regime the symmetric reconstructions can become more stable. These reconstructions are always electrically active with a half filled band across the band gap. In particular the symmetric reconstructions of the 30° partial have a lower Peierls barriers than the respective asymmetric ones and could be the cause of the 1.8 eV electroluminescence peak observed under carrier injection conditions.

Abstract: 90 Shockley partial dislocations in GaN are investigated by first-principles calculations. This work is focussed on the electrical properties of dislocation cores, and on investigating the electrical fields around these defects. The band structure analysis shows that both the  and  core partials possess a midgap state. The -core dislocations give rise to a donor level Ev +0:87 eV that might explain the absorption peak at 2.4 eV revealed by energy loss spectroscopy measurements. The acceptor level Ev + 1:11 eV localized at the -core dislocations might contribute to the yellow luminescence. These dislocations experience a substantial charge polarization along the [0001] growth axis. In addition, we show that these dislocations tend to charge in a high stress field.

Abstract: We use DFT calculations to investigate the problem of hydrogen aggregation in silicon. We study atomic structures of finite hydrogen aggregates containing four or more hydrogen atoms. Beyond four hydrogen atoms, complexes consisting of Si-H bonds are likely to form, rather than aggregates of H2 molecules, which are the most stable diatomic hydrogen complex. Our calculations show that the basic structural unit of such complexes is a hydrogenated dislocation loop, which is formed spontaneously by a structural transformation of two H∗2 complexes. Hydrogen-induced formation of dislocation loops may account for the experimental observations of dislocation loops in proton-implanted or hydrogen plasma-treated silicon. We indicate the routes leading from H∗2 aggregates and hydrogenated dislocation loops to twodimensional hydrogen-induced platelets. We discuss the effect of hydrogen-catalysed formation of dislocation loops on the plasticity of silicon.