Helium ion implantation-induced defects in silicon probed with variable-energy positrons

Lifetime and Doppler broadening S parameter measurements using variable-energy positrons have been applied to Si implanted with 1x10(15) He+/cm(2) at 60 keV. Two kinds of defective layers are compared: a vacancy-rich region in the near-surface layer less than 300 nm from the top surface, and a He-vacancy-coexisting region in the He implanted layer of 300-600 nm depth. In order to enhance the depth resolution of the positron beam spectroscopy, the positron measurements have been combined with a controlled removal of thin layers of the sample by chemical etching. For the as-irradiated sample, the predominance of divacancies in the near-surface layer is indicated. In the He implanted layer the defects are identified as divacancies coupled with He. Annealing of the samples at 300 degreesC results in a diffusion of He atoms and a passivation of divacancies by He even in the near-surface layer. Upon annealing at 500 degreesC, He atoms effuse completely from the sample and V-2-He complexes are transformed into V-4. The vacancies decorated with He become positron trapping sites and the stability of He-related defects strongly depends on the formation of larger vacancy clusters.