Positron annihilation was used to characterize vacancy-type defects in two
types of polycrystalline Si grown at temperatures above ~800 °C
by chemical vapour deposition. The majority of vacancies (80%) consisted
of monovacancies, and their thermal stability indicated them to be trapped
at grain boundaries or at dislocations. Annealing above 500 °C caused a
significant reduction in the monovacancy concentration, and an increase in
divacancy concentration. Divacancies started to anneal above 1200 °C.
Measurements between 8 and 293 K indicated that vacancies were neutral
before as well as after annealing at 1380 °C. Fz-grown Si from one of
these materials contained vacancy clusters with an average size of six to
ten vacancies which persisted to 1380 °C. The cluster concentration
corresponded to a monovacancy concentration of 1015 to 1016 cm−3, which is at least one order of magnitude larger than estimates
based on voids [R. Falster, V.V. Voronko, F. Quast, Phys. Status Solidi B
222, 219 (2000)].