Thermal evolution of boron irradiation induced defects in predoped Si revealed by positron annihilation experiments

Abstract

The isochronal annealing behavior of high energy (25–72 MeV) boron ion irradiation induced defects in boron-doped silicon is monitored through measurements of positron lifetimes and three distinct defect-evolution stages are identified. The initial boron doping created a defect environment where positrons could sensitively annihilate with the boron electrons, suggesting boron-decorated Si monovacancies as potential trapping sites. The irradiation results in the dissolution of boron from these sites and positrons are then trapped by the empty divacancies of Si. Charge neutralization of divacancies through interaction with boron atoms leads to enhanced positron trapping in the initial stages of isochronal annealing. The divacancies start annealing above 673 K. However, a remarkable defect evolution stage due to the diffusion of the boron atoms beyond their initial depths of penetration is seen above 873 K and it leaves the sample with defects still present even at the highest annealing temperature 1273 K used in this work.