Radiation hardness of silicon detectors based on pre-irradiated silicon

Radiation hardness of planar detectors processed from pre-irradiated and thermo-annealed n-type FZ silicon substrates, and standard FZ as a reference, was studied. The high purity n-Si wafers with carrier concentration 4.8x1011 cm-3 were pre-irradiated in Kiev’s nuclear research reactor by fast neutrons to fluence of about 1016 neutrons/cm2 and thermo-annealed at a temperature of about 850 1C. Silicon diodes were fabricated from standard and pre-irradiated silicon substrates by IRST (Italy). All diodes were subsequently irradiated by fast neutrons at Kiev and Ljubljana nuclear reactors. The dependence of the effective doping concentration as a function of fluence (Neff = f(F)) was measured for reference and pre-irradiated diodes. Pre-irradiation of silicon improves the radiation hardness by decreasing the acceptor introduction rate (b), thus mitigating the depletion voltage (Vdep) increase. In particular, b in reference samples is about 0.017 cm-1, and for pre-irradiated samples is about 0.008 cm-1. Therefore, the method of preliminary irradiation can be useful to increase the radiation hardness of silicon devices to be used as sensors or detectors in harsh radiation environments.