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 f ound in all types, synthetic as well as natural. These clusters are situate d in highly defected regions, rather than homogeneously distributed, and th eir concentration varies significantly from sample to sample. For synthetic Ib diamonds vacancy clusters were investigated as a function of nitrogen c ontent. 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 sil icon. 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 negative ly 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 ir radiation-produced neutral monovacancies can convert to the negatively char ged state above 400 K but this was dependent on diamond type. Isochronal an nealing of irradiated Ib diamonds showed that the complex of a substitution al nitrogen and a vacancy, formed upon annealing close to 600 degrees C, un dergoes two detectable modifications between 600 and 870 degrees C reaching a configuration stable to 1170 degrees C. Key conclusions based on positro n and optical data are in mutual accord. (C) 2000 Elsevier Science S.A. All rights reserved.