The depth profiles of open-volume defects were determined, in samples which had been implanted with 20keV He ions, by using slow-positron beams and Doppler broadening techniques. The evolution of defect distributions was studied in 2 series of samples which had been implanted to fluences of 5 x 1015 or 2 x 1016/cm2. A fitting procedure was applied to the data in order to extract a positron parameter which characterized each open-volume defect. The defects were identified by comparing this parameter with theoretical calculations. In as-implanted samples, most of the vacancies and divacancies which were produced by implantation were passivated by the presence of He. The mean depth of defects, as indicated by the positron annihilation technique, was some 5 times smaller than the He projected range. During successive isochronal annealing, the number of positron traps decreased, then increased and finally, at the highest annealing temperature, disappeared only in samples which had been implanted to the lower fluence. A minimum number of open-volume defects was attained at an annealing temperature of 250C, for both series. The increase in open-volume defects at temperatures higher than 250C was due to the appearance of vacancy clusters of increasing size; with a mean depth distribution that moved towards the He projected range. The appearance of vacancy clusters was related to the out-diffusion of He. In samples which were implanted to 5 x 1015/cm2, the vacancy clusters were mainly 4-vacancy agglomerates which were stabilized by He-related defects. They disappeared above an annealing temperature of 700C. In samples which were implanted to 2 x 1016/cm2, and annealed at 850 to 900C, the vacancy clusters disappeared and only a distribution of cavities - centered around the He projected range - remained.

Formation of Vacancy Clusters and Cavities in He-Implanted Silicon Studied by Slow-Positron Annihilation Spectroscopy R.S.Brusa, G.P.Karwasz, N.Tiengo, A.Zecca, F.Corni, R.Tonini, G.Ottaviani: Physical Review B, 2000, 61[15], 10154-66