A first principles study of paramagnetic defects in diamond

Abstract

Diamond is a radiation-hard material with a wide band-gap which is typically electrically insulating. A wide range of diamond-based material is available, including natural gemstones and synthetic diamondmanufactured by chemical vapour deposition (CVD) and high pressure and temperature (HPHT) techniques. The wide band-gap and electrical insulation properties mean that defects introduced by irradiation and plastic deformation, during growth or otherwise, may exist in more than one charge and/or electronic-spin state. Of these, defects with unpaired electrons give rise to characteristic spectroscopic signatures in electron paramagnetic resonance (EPR) experiments. In favourable situations these spectra provide chemical specificity, defect symmetry, effective spin-state, and wave function character (such as sp hybridisation), and when combined with temperature effects can yield data relating to thermal stability (for example binding and migration energies) and temperature dependent symmetry effects. Recent advances in quantum chemical simulation allow for reasonable accuracy in simulated hyperfine interaction (HFI) tensors (electron-spin and nuclear-spin interactions). Indeed, many assignments for EPR centres are tentative in nature. Where the proposed model yields calculated hyperfine values that differ substantially from those measured, in addition to refuting the current model, an examination of a wide range of candidate defects may lead to more realistic models. Defects observed in natural and synthetic diamonds provide a finger-print of their differing growth conditions, as well as the thermal and mechanical processes they have experienced. A challenge that has existed for decades in the effort to produce diamond suitable for electronics is in overcoming the difficulty in production of low resistivity n-type material. N, a dopant one might naively assume would be effective, undergoes a significant structural relaxation, rendering it a very deep donor at around Ec − 1.7 eV. Phosphorus is a better candidate since the substantial difference in atomic radius relative to the host suppresses large structural relaxations associated with nitrogen. However, with a donor level at around 0.6 eV below the conduction band, it has a low ionisation fraction at room temperature. This thesis presents the results of calculations performed with the ab initio modelling (AIMPRO) code. The initial stages studied the stability, defect symmetry, Kohn Sham orbitals and the hyperfine interaction of di-nitrogen substitutionals diamond. Analysis was then conducted of nitrogen pair defects relating to published theoretical and experimental models of W24, N1, W7, M2, N4 and M3 electron-paramagnetic-resonance centres in diamond in which pairs of nitrogen donors are separated by different numbers of intervening host sites, both in ionised S = 1/2 and neutral S = 1 forms. Using density functional techniques, these models are confirmed, but in order to do so for the N4 centre, for example, it is shown to be essential that extremely low energy reorientation takes place. Charge exchange and chemical re-bonding effects also provide an explanation for the distinct forms of the S = 1 neutral configurations observed. Other proposed models were then considered, including a detailed analysis of the hyperfine interactions formuoniumcentres, including the effects of vibration. This was combined with the experimental examination of N-containing diamond, where muon-spin relaxation experiments suggest a strong interaction between N-aggregates and the implanted muon. Structures made from two arrangements of pairs of substitutional nitrogen and oxygen showed no direct evidence for the involvement of the oxygen. The proposed model for N3 broadly agreed with experimental results but not for the OK1 centre. Despite its high ionisation energy, the n-type dopant of choice in diamond is currently phosphorus. Chemical vapour deposited diamond can be grown with high concentrations of P, and the substitutional donor has been characterised via a number of experimental techniques including electron paramagnetic resonance. Substitutional P undergoes a Jahn-Teller distortion at low temperature, where the EPR tensors reflect a tetragonal symmetry. Other P-containing EPR centres have also been detected but their structure remains uncertain.