Hydrogen- and helium-implanted silicon: Low-temperature positron-lifetime studies

High-purity single-crystal samples of float-zoned Si have been implanted with 6.95-MeV protons and with 25-MeV ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^2{}_{}{}^{}$ ions at 15 K, and the positron-lifetime technique has been used to identify the defects created in the samples, and to study the effects of H and He on the annealing of point defects in Si. The results have been compared with those of proton-irradiated Si. A 100–300-K annealing stage was clearly observed in hydrogen (${\mathrm{H}}^{+}$) -implanted Si, and this stage was almost identical to that in the p-irradiated Si. The final annealing state of the ${\mathrm{H}}^{+}$-implanted Si started at about 400 K, and it is connected to annealing out of negatively charged divacancy-oxygen pairs. This stage was clearly longer than that for the p-irradiated Si, probably due to the breakup of Si-H bonds at about 550 K. The 100-K annealing stage was not seen with the He-implanted samples. This has been explained by assuming that almost all vacancies contained He after the irradiation with ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. Helium is suggested to be released from vacancies at about 600 K, and small He bubbles seem to have grown at temperatures above 800 K. The specific positron-trapping rate for negatively charged monovacancy-type defects in ${\mathrm{H}}^{+}$-implanted Si has been found to have a ${\mathit{T}}^{\mathrm{-}0.5}$ dependence, whereas for neutral divacancies and monovacancies in He-implanted Si no dependence on temperature has been observed.