Positron annihilation investigation of vacancies in as-grown and electron-irradiated diamonds

Vacancy-type defects in the four main types of diamond (la, Ib, IIa and IIb) were investigated using positron lifetime, Doppler broadening and optical absorption spectroscopies. In unirradiated samples vacancy clusters were found in all types, synthetic as well as natural. These clusters are situated in highly defected regions, rather than homogeneously distributed, and their concentration varies significantly from sample to sample. For synthetic Ib diamonds vacancy clusters were investigated as a function of nitrogen content. The bulk lifetime for diamond is calculated to be: 98 +/- 2 ps and the bulk Doppler S parameter is estimated to be 25% lower than that far silicon. Electron irradiation (2.3 MeV) produced neutral monovacancies in IIa diamond and the positron data correlated well, as a function of dose, with the GR1 optical zero-phonon fine; the introduction rate was estimated to be 0.5 +/- 0.2 cm(-1). In Ib diamond, monovacancies were found to be negatively charged. The positron lifetime for monovacancies was (40 +/- 6)% larger than the bulk lifetime and the Doppler S parameter increased by (8 +/- 1)%. At-temperature Doppler measurements between 30 and 770 K indicated that irradiation-produced neutral monovacancies can convert to the negatively charged state above 400 K but this was dependent on diamond type. Isochronal annealing of irradiated Ib diamonds showed that the complex of a substitutional nitrogen and a vacancy, formed upon annealing close to 600 degrees C, undergoes two detectable modifications between 600 and 870 degrees C reaching a configuration stable to 1170 degrees C. Key conclusions based on positron and optical data are in mutual accord. (C) 2000 Elsevier Science S.A. All rights reserved.