He-implantation induced defects in Si studied by slow positron annihilation spectroscopy

Open volume defect profiles have been obtained by performing Doppler broadening measurements with a slow positron beam on p-type Si samples implanted near liquid nitrogen temperature with He ions at 20 keV and at 5 x 10(15) and 2 x 10(16) cm(-2) fluence. The evolution of the defect profiles was studied as a function of isothermal annealing at 250 degrees C. The fraction of released He was measured by thermal programmed desorption. The defects could be identified as a coexistence of monovacancies stabilized by He-related defects and divacancies. The number of defects decreases for annealing time of a few minutes, then increases at longer annealing times. The mean depth of the defect profiles in the as-implanted samples was found to be very near the surface. After annealing, the mean depth increases to less than one half of the projected He range. This complex dynamics has been interpreted as due to passivation of vacancies by He during the implantation process and the first annealing step when no appreciable He is lost, and to subsequent depassivation during He desorption. (C) 1999 American Institute of Physics. [S0021-8979(99)02103-9].