Experimental Determination of the Impact Ionization Coefficients in Irradiated Silicon

We present new results on the influence of radiation-induced damage on the electron Impact Ionization (I.I.) coefficient α, suggesting a small but distinct reduction of α at high fluence with respect to unirradiated silicon. Experiments on thick (1.5 μm) and thin (1 μm) epitaxial silicon samples confirm that such a reduction of α is expected even in cases where impact ionization is not simply a field driven process because of strongly non local transport conditions. A consistent increase on the breakdown voltage of a 3D radiation detector has been evaluated by means of TCAD simulations using the experimentally extracted I.I. coefficient for irradiated silicon. These results clarify the impact of radiation damage on some of the key model parameters for TCAD simulations and allow for improved accuracy toward predictive breakdown simulations of silicon particle detectors, e.g., for the ATLAS experiment.