Formation of vacancy clusters and cavities in He-implanted silicon studiedby 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 Do ppler broadening technique. The evolution of defect distributions has been studied as a function of isochronal annealing in two series of samples impl anted at the fluence of 5 x 10(15) and 2 x 10(16) He cm(-2). A fitting proc edure has been applied to the experimental data to extract a positron param eter characterizing each open volume defect. The defects have been identifi ed by comparing this parameter with recent theoretical calculations. In as- implanted samples the major part of vacancies and divacancies produced by i mplantation is passivated by the presence of He. The mean depth of defects as seen by the positron annihilation technique is about five times less tha n the helium projected range. During the successive isochronal annealing th e number of positron traps decreases, then increases and finally, at the hi ghest annealing temperatures, disappears only in the samples implanted at t he lowest fluence. A minimum of open volume defects is reached at the annea ling temperature of 250 degrees C in both series. The increase of open volu me defects at temperatures higher than 250 degrees C is due to the appearan ce of vacancy clusters of increasing size, with a mean depth distribution t hat moves towards the He projected range. The appearance of vacancy cluster s is strictly related to the out diffusion of He. In the samples implanted at 5 x 10(15) cm(-2) the vacancy clusters are mainly four vacancy agglomera tes stabilized by He related defects. They disappear starting from an annea ling temperature of 700 degrees C. In the samples implanted at 2 x 10(16) c m(-2) and annealed at 850-900 degrees C the vacancy clusters disappear and only a distribution of cavities centered around the He projected range rema ins. The role of vacancies in the formation of He clusters, which evolve in bubble and then in cavities, is discussed.