Numerical simulation of radiation damage effects in p-type silicon detectors

In the framework of the CERN-RD50 Collaboration, the adoption of p-type substrates has been proposed as a suitable approach to optimize the long-term radiation hardness of silicon detectors. In this work, we present a numerical model for the simulation of radiation damage effects in p-type silicon, developed within the general-purpose device simulator DESSIS. The model includes radiation-induced deep-level recombination centers in the semiconductor band-gap and the Shockley-Read-Hall statistics. In particular, two deep-level defects have been introduced: one located at E-C-0.42eV, corresponding to a single charge state divacancy and a second one located at E-C-0.46 eV, corresponding to a single charge state tri-vacancy. For simulation purposes we have considered a simple, two-dimensional test structure, consisting of a single diode of 40 mu m width and 300 mu m depth, surrounded by a 6 mu m wide guard ring. The n + implant region depth is 1 mu m, with donor concentration of ND = 10(18) cm(-3) implanted on a high-resistivity p-type substrate (N-A = 5 x 10 1 2 cm(-3)). The results of simulations adopting the proposed radiation damage model for p-type substrate have been compared with experimental measurements carried out on similar test structures irradiated with neutrons at high fluence. A good agreement with the experimental data has been obtained for the depletion voltage and diode leakage current. The simulated current damage constant (alpha = 3.75 x 10(-17) A cm(-1)) is in satisfactory agreement with values reported in the literature. A preliminary study of charge collection efficiency as a function of the fluence is also reported.