Thermal evolution of defects in H-implanted Si studied by monoenergetic positrons

The behavior of hydrogen-vacancy defects in silicon implanted with H ions (1×10¹⁶/cm², 60 keV) has been investigated by means of a slow positron beam, in which both the positron lifetime and the Doppler broadening have been measured. It has been found that the positron-trapping defect associated with H is formed within the H implantation profile. This gives rise to a long positron lifetime, which is close to that characteristic of a monovacancy. However, the Doppler broadening parameter S of this defect is not as large as that of the monovacancy, since positrons annihilate electrons of H within the defect. Upon annealing at up to 400 °C, the defects located within 350 nm of the surface are agglomerated and also hydrogenated, whereas the H-decorated defects within the H implantation profile are unchanged. Hydrogenation stabilizes and immobilizes the defects in this temperature range. Annealing at 500 °C leads to the agglomeration of the H-decorated defects in the vicinity of the peak of the hydrogen implantation profile. Annealing at 600 °C leads to hydrogen release from these agglomerates. It has been found that a high density of hydrogen-stabilized defects is present within the implantation profile upon annealing at 400 °C. This high density allows for the formation of larger vacancy clusters upon higher temperature annealing.