Formation of vacancy clusters and cavities in He-implanted silicon studied by slow-positron annihilation spectroscopy

The depth profile of open volume defects has been measured in Si implanted with He at an energy of 20 keV, by means of a slow-positron beam and the Doppler broadening technique. The evolution of defect distributions has been studied as a function of isochronal annealing in two series of samples implanted at the fluence of 5×1015 and 2×1016He cm-2. A fitting procedure has been applied to the experimental data to extract a positron parameter characterizing each open volume defect. The defects have been identified by comparing this parameter with recent theoretical calculations. In as-implanted samples the major part of vacancies and divacancies produced by implantation is passivated by the presence of He. The mean depth of defects as seen by the positron annihilation technique is about five times less than the helium projected range. During the successive isochronal annealing the number of positron traps decreases, then increases and finally, at the highest annealing temperatures, disappears only in the samples implanted at the lowest fluence. A minimum of open volume defects is reached at the annealing temperature of 250°C in both series. The increase of open volume defects at temperatures higher than 250°C is due to the appearance of vacancy clusters of increasing size, with a mean depth distribution that moves towards the He projected range. The appearance of vacancy clusters is strictly related to the out diffusion of He. In the samples implanted at 5×1015cm-2 the vacancy clusters are mainly four vacancy agglomerates stabilized by He related defects. They disappear starting from an annealing temperature of 700°C. In the samples implanted at 2×1016cm-2 and annealed at 850–900°C the vacancy clusters disappear and only a distribution of cavities centered around the He projected range remains. The role of vacancies in the formation of He clusters, which evolve in bubble and then in cavities, is discussed.