Recombination Luminescence in Irradiated Silicon

Radiative recombination of optically injected carriers has been observed in ${\mathrm{Co}}^{60}-\gamma$-ray- and neutronirradiated silicon. All samples were irradiated at room temperature and stored at room temperature following irradiation. The luminescent spectrum is complex and exhibits sharp structure at liquid-helium temperature. In both $n$- and $p$-type float-zone-grown material, the spectrum is dominated by a very intense sharp band located at 0.971 eV. In both $n$- and $p$-type pulled crystals the same spectrum appears, but an additional spectrum is found that is shifted to lower energies by 0.180 eV, with a very intense sharp band located at 0.791 eV. The major features of both spectra are independent of the type and amount of doping impurities of valence III or V, and the type and amount of irradiation. The defect associated with the lower-energy spectrum anneals at room temperature, while the defect associated with the higher-energy spectrum is stable at room temperature. An additional band associated with a third defect is found at 0.794 eV in $\gamma$-irradiated pulled crystals under certain conditions. Measurements of the luminescence at liquid-nitrogen temperature are consistent with those at liquid-helium temperature. It is suggested that the luminescence of the two major portions of the spectra are due to both phononless and phonon-assisted recombination of free holes and trapped electrons in the ground states of two different defects. Subtraction of the energies of the sharp bands found in the luminescence spectra from the band-gap energy yields 0.194 and 0.374 eV as the energies of the traps below the conduction-band minima.