Theory of hydrogen in diamond

Ab initio cluster and supercell methods are used to investigate the local geometry and optical properties of hydrogen defects in diamond. For an isolated impurity, the bond-centered site is found to be lowest in energy, and to possess both donor and acceptor levels. The neutral defect possesses a single local mode with a very small infrared effective charge, but the effective charge for the negative charge state is much larger. ${\mathrm{H}}^{+}$ is calculated to be very mobile with a low activation barrier. Hydrogen dimers are stable as ${\mathrm{H}}_2^{*}$ defects, which are also found to be almost IR inactive. The complex between B and H is investigated and the activation energy for the reaction $\mathrm{B}-\overrightarrow{H}{\mathrm{B}}^{-}+{\mathrm{H}}^{+}$ found to be around 1.8 eV in agreement with experiment. We also investigate complexes of hydrogen with phosphorus and nitrogen. The binding energy of H with P is too low to lead to a significant codoping effect. A hydrogen-related vibrational mode of the N-H defect, and its isotopic shifts, are close to the commonly observed $3107-{\mathrm{cm}}^{-1}$ line, and we tentatively assign this center to the defect. Hydrogen is strongly bound to dislocations which, together with ${\mathrm{H}}_2^{*},$ may form part of the hydrogen accumulation layer detected in some plasma studies.