Radiation damage on silicon after very high neutron fluence irradiation

Silicon wafers with two different resistivities were used to produce p(+)n junction and bulk samples. Irradiations were performed at high neutron flux and with fluence ranging from 10(13) n/cm(2) to 4 10(15)n/cm(2). Bulk resistivity and Hall coefficient were measured on bulk samples using the four contact probe method. Measurements of capacitance and current versus voltage characteristics (CV-IV), Thermally Stimulated Currents (TSC), laser induced current Deep Level Transient Spectroscopy (I-DLTS) and Transient Currents by Transient Currents Technique (TCT) were performed on irradiated diodes showing a strong degradation of the detector performances after very high fluences of irradiation. Above 10(15) n/cm(2) the well known monotonic increase of the full depletion voltage seems to have a quadratic dependence on the fluence. In this extreme fluence range, the values calculated for the space charge concentration are of the order of 10(15) cm(-3) : the same order of magnitude was found by TSC and I-DLTS analyses for the concentration of the deepest radiation induced traps. Moreover, TCT measurements suggest that it is practically impossible to deplete the most irradiated detectors and that there is virtually no difference between front and back contacts.