Electronic structure and properties of point defects in diamond: A quantum chemical approach
There is recent interest in diamond as an electronic and optical material. The aim of this thesis is to use and develop techniques to study, at a quantitative level, the electronic structure and properties of point defects in diamond. Our study is carried out using cluster-based ab initio quantum chemical methods, and focusses on the elemental impurity--hydrogen, and an intrinsic defect--the carbon vacancy. Although H is a common impurity in polycrystalline diamond, little is known of its binding sites or properties. In silicon, H forms an electrically active bond-center (BC) defect. Our calculations of the energetics of BC H and its complexes in diamond suggest that both isolated BC defects and H dimers should form in strained bonds at grain boundaries. We have implemented a new multi-configurational (MC) perturbative method to reliably include dominant spin polarization contributions to its hyperfine couplings (HFCs). Our method is economical compared to the more extensive CI approach; yet gives HFCs for BC muonium in bulk diamond in very good agreement (within 15%) with experiment. We also calculate the HFCs and vibrational frequencies for H in pre-strained bonds. Next we examine two issues related to the vacancy. We first treat the distortions at the neutral vacancy (V$\sp0$). To this end we have implemented a novel hybrid (quantum + classical) cluster approach. Test calculations on substitutional nitrogen and the negative vacancy (V$\sp-$) show that fairly reliable geometries and HFCs are obtained. Our results for V$\sp0$ using HF and MC-SCF methods show that electron correlations, missing in mean-field (HF, LDA) approaches, qualitatively affect its Jahn-Teller distortion. We present theoretical evidence in favor of a dynamic Jahn-Teller effect, well-known experimentally for this defect. We then calculate, using MC-CI methods, the splittings between the ground and low-lying excited states of V$\sp0$ and V$\sp-$, and address previously unresolved issues. We show that while delocalization effects are small ($\approx$10%), dynamic polarization (screening) contributions dramatically stabilize the charge-transfer states. Similar results are obtained for V$\sp-$, for which reasonable agreement with experiment is obtained. Our results stress the importance of a proper inclusion of correlation and non-local exchange in calculations of excitation energies at defects.
Chawla, Sanjay, "Electronic structure and properties of point defects in diamond: A quantum chemical approach" (1996). Dissertations available from ProQuest. AAI9627899.