Photo-induced current transient spectroscopy of defect clusters in heavily irradiated silicon

We report an experimental study on radiation-induced defects in silicon p+n junctions irradiated with 1MeV neutrons up to a fluence of about 2x10^15 cm^(-2). Heavily irradiated silicon diodes have been studied by means of Photon-Induced Current Transient Spectroscopy (PICTS) technique using a variable filling time. A dominant broad and structured peak has been found in the temperature range 200–300 K. The behavior of this broad peak upon changing the filling time has been analyzed, and it is observed that spectral line-shape broadens toward lower temperature as the filling time is increased. The observed spectra shape modification cannot be explained in terms of isolated point defects being consistent with quasi-continuous distributions of deep levels inside the band gap. We suggest that the investigated broad peak is at least in part, generated by emission from defect clusters.