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 3He2 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 (H+) -implanted Si, and this stage was almost identical to that in the p-irradiated Si. The final annealing state of the 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 3He. 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 H+-implanted Si has been found to have a T-0.5 dependence, whereas for neutral divacancies and monovacancies in He-implanted Si no dependence on temperature has been observed.