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We use positron annihilation to study vacancy defects in GaAs grown at low temperatures ͑LT-GaAs͒. The vacancies in as-grown LT-GaAs can be identified to be Ga monovacancies, V Ga , according to their positron lifetime and annihilation momentum distribution. The charge state of the vacancies is neutral. This is ascribed to the presence of positively charged As Ga ϩ antisite defects in vicinity to the vacancies. Theoretical calculations of the annihilation parameters show that this assignment is consistent with the data. The density of V Ga is related to the growth stoichiometry in LT-GaAs, i.e., it increases with the As/Ga beam equivalent pressure ͑BEP͒ and saturates at 2ϫ10 18 cm Ϫ3 for a BEPу20 and a low growth temperature of 200°C. Annealing at 600°C removes V Ga . Instead, larger vacancy agglomerates with a size of approximately four vacancies are found. It will be shown that these vacancy clusters are associated with the As precipitates formed during annealing.

We have used pulsed-slow-positron-beam-based positron lifetime spectroscopy to investigate the nature of acceptors and charge states of vacancy-type defects in low-energy proton-implanted 6H-SiC(H). We can infer from the temperature dependence of the lifetime spectra that neutral and negatively charged vacancy clusters exist in the track region. Depending on annealing, they give rise to positron lifetimes of 257±2, 281±4, and 345±2 ps, respectively. The 281 ps cluster likely has an ionization level near the middle of the band gap. By comparison with theory, the 257 and 280 ps are identified as (VC–VSi)2 and (VC–VSi)3 clusters, respectively. In addition, other acceptors of ionic type act as strong trapping centers at low temperature (T<150 K). Neutral monovacancy-like complexes are also detected with a lifetime of 160±2 after 900 °C annealing.

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