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

Abstract

Permission has been granted to the Libnry of The University of Manitoba to lend or sen copies of this thesislpracticum, to the National Library of Canada to microfilm this thesis/practicum and to lend or seil copies of the film, and to Dissertations Abstracts international to pubiisb an abstract of this thesis/prrcticum.The author reserves other publication rights, and neither this thesis/prrcticum nor extensive extracts from it m a y be printed or otherwisc reproduccd without the author's written permission. Abstract ABSTRACTVacancy-type defects in the four main types of diamond (Ia, Ib, IIa and IIb) were investigated using positron lifetime, Doppler broadening and optical absorption spectroscopies. In unirradiated sarnples vacancy clusters were found in al1 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 Tt, diamonds vacancy clusters were investigated as a function of nitrogen content. The bulk positron lifetime for diamond is calculated to be 98 + 2 ps and the bulk Doppler S parameter is estimated to be 25% lower than that for 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 line (the optical absorption at 740nm arise fiom neutral monovacancies in diarnond); the introduction rate was estimated to be 0.5 k 0.2 cm-'. In Ib diamond monovacancies were found to be negatively charged whereas they were neutral in IIa diamonds. The positron lifetime for monovacancies was (40 t 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 diarnonds showed that the complex of a substitutional nitrogen and a vacancy, formed upon annealing close to 6 0 0°c , undergoes two detectable modifications between 600 and 870 OC reaching a configuration stable to 1 170 OC . Key conclusions based on positron and optical data are in mutual accord.

Vacancy-type defects in brown diamonds investigated by positron annihilation

“…The value for the t 2 lifetime component is a bit smaller than for monovacancies, 14575 ps [4], but clearly arises from a defect with an open volume close to that for a monovacancy, and the other lifetime of 434 ps is clearly associated with large vacancy clusters. No theoretical calculations for positron lifetimes in diamond as a function of vacancy cluster size have been reported, but they have for SiC [5] and Si [6].…”

“…The two lifetime components shown in Fig. 1 are larger than the bulk lifetime of 100 ps for diamond [4] and arise, therefore, from vacancy-type defects. There is also another resolved lifetime, t 1 ; which is shorter than the bulk lifetime.…”

“…Lifetime data were accumulated to 1:5 Â 10 7 counts per spectrum, but were otherwise accumulated and analysed as described in detail in an earlier publication [4].…”

Defects in diamond

“…The bulk lifetime is established very well experimentally from several sources [24][25][26] at 98 ± 2 ps, and its temperature dependency between 10 and 293 K is imperceptible. The monovacancy, neutral as well as negatively charged, gives rise to a lifetime of 140-145 ps, and the trapping rate for V o is proportional to the GR1 optical absorption from V o in irradiated IIa samples [26].…”

“…The monovacancy, neutral as well as negatively charged, gives rise to a lifetime of 140-145 ps, and the trapping rate for V o is proportional to the GR1 optical absorption from V o in irradiated IIa samples [26]. The percentage increase in lifetime (40-45%) is significantly higher than for silicon (~20%), and the same is the case for the relative increase in the Doppler S parameter, 8% in diamond [26] as compared to ~ 3.5% estimated for Si [27]. Angular correlation data showed significant anisotropy of the electron momentum distribution [28] and, as expected, a very wide angular distribution.…”