Increase of the photoconductivity quantum yield in silicon irradiated by neutrons to extremely high fluences

An enhanced quantum yield observed in silicon ionizing radiation detectors, neutron-irradiated to extremely high fluences, could be attributed to impact ionization via deep levels. The quantum yield was investigated by the intrinsic photoconductivity optical spectroscopy in silicon irradiated by neutrons to a wide range of fluences up to $1 \times 10^{17}$ neutron cm$^{-2}$. An increase of quantum yield was observed in highly irradiated samples. We have demonstrated that the quantum yield enhancement could be attributed to the impact ionization via deep levels, this process being presumably related to disordered defect clusters regions in Si. The proposed mechanism explains the observed decrease of the impact ionization energy by at least an order of magnitude at low temperature. The impact ionization energy values of up to 0.30–0.36 eV and less, and 0.38–0.40 eV were determined at T ∼ 21–33 K and at T = 195 K, respectively.