Photoconductivity evolution due to carrier trapping by defects in 17 MeV-proton irradiated silicon

Damage concentration dependence of dark and photoconductivity has been studied in crystalline Si, irradiated with 17 MeV proton. The photoconductivity spectra consist of a broad peak due to a band-to-band transition and a tail spectrum on the lower energy side. Two kinds of tail spectra are observed in impurity-doped and/or damaged specimens, and the latter threshold is deeper than the former. While the main peak of the doped specimen does not greatly change up to a certain irradiation fluence φC, it steeply decreases beyond the φC, which depends on the shallow-impurity concentration. The tail spectrum of the shallow impurity simultaneously vanishes at the φC and another tail spectrum grows above the φC. As compared to fluence dependence of photocurrent decay time, it is clarified that the drastic decrease in the main peak results from a drop in the decay time or the carrier density, not in the carrier mobility. The defect density for the critical fluence φC has good correlation with the dopant concentration. It is concluded from these results that a competitive process between shallow impurities and deep defects dominates the optical transition and transport, and that the critical fluence φC corresponds to the damage concentration required to capture all the doped carriers. Mechanisms for the photoconductivity evolution and defect trapping efficiency are discussed as is a comparison with electron-induced damage.