Electronic structure and properties of point defects in diamond: A quantum chemical approach

Sanjay Chawla, University of Pennsylvania

Abstract

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.

Subject Area

Condensation|Molecules|Chemistry

Recommended Citation

Chawla, Sanjay, "Electronic structure and properties of point defects in diamond: A quantum chemical approach" (1996). Dissertations available from ProQuest. AAI9627899.
https://repository.upenn.edu/dissertations/AAI9627899

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