In this thesis, the theoretical electronic structures of diamond-like amorphous carbons (DLC) are studied using the total energy code "Fast Structure" based on local orbitals, Harris functional and local density approximation (LDA) molecular dynamics simulations. The carbon atom can exhibit different bonding configurations including <I>sp<SUP>3</SUP>, sp<SUP>2</SUP></I> and <I>sp<SUP>1</SUP></I> hybridizations. Firstly, the structural trend of the amorphous carbon (a-C) shows an increasing <I>sp<SUP>3</SUP></I> fraction with the increase of mass densities. The existence of both <I>sp<SUP>3</SUP></I> and <I>sp<SUP>2</SUP></I> sites in a-C accentuates the band-tailing process with σ and π bonding. The smaller band gap found in the DLC is mainly due to the distortions of π-bonded clusters. The large distortions of π dihedral angles cause the entire π band to be localized. The localization of π states causes the mobility gap to exceed the optical gap, which accounts for the low carrier mobility and the flat photoluminescence excitation spectrum. The stabilities of various nitrogen doping configurations in a-C are also studied. At low N contents, N dopants exhibit σ bonding and are in favour of forming non-doping trivalent coordination N<SUP>0</SUP><SUB>3 </SUB>sites. At high N contents, the non-doping π-bonded sites are preferred. These results account for the low doping efficiency in a-C. Surface properties of the a-C structures show the formation of the <I>sp<SUP>2</SUP>-</I>rich surface layers compared to the predominant <I>sp<SUP>3</SUP></I> regions. These structures are further applied to the study of defect formation energies for vacancies and interstitials in the near surface region. The considerably low defect formation energy found in a-C supports the possibility of deposition mechanism described by the subplantation model.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:597527 |
| Date | January 1998 |
| Creators | Chen, C. W. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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