Defects in diamond

Diamond is one of the remarkable forms of carbon. For example, it can be found in nature with an impurity content of less than 1 ppm, or it can be synthesized either as a single-crystal or as a polycrystalline deposit on various surfaces. Furthermore, boron doping makes it a p-type semiconductor, and even superconducting. Diamond is also one of the few materials where optical absorption (OA) yields relevant information on defects, including the omnipresent nitrogen impurity. This is so because in diamond many defects have sharp zero-phonon lines. Irradiation produces vacancies and interstitials both of which have been identified by OA and electron paramagnetic resonance, and also by positron annihilation (vacancy only). Irradiation-produced defects are remarkable stable, with the monovacancy becoming mobile at 700 °C, and the divacancy surviving to at least 1000 °C. In this contribution are reviewed relevant aspects of diamond physics and what positron annihilation has contributed so far. Diamond provides the opportunity for a correlation between optical absorption and positron results, including the difficult issue of the electrical charge of a defect. Such combined experiments are presently searching for the elusive positively charged monovacancy in irradiated B doped single crystalline diamond. Future positron experiments could with benefit be aimed at investigating vacancy impurity complexes, such as the one claimed by OA as a four nitrogen-vacancy complex. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)