Speaker
Description
We present a study of the forward and reverse current in silicon pad diodes irradiated to extreme neutron fluences of up to $5 \times 10^{17}\,n_{eq}/$cm$^2$, corresponding to expected fluences at the innermost radii of tracking detectors at a future circular hadron collider.
At such fluences, the low-doped silicon bulk and the highly doped implant no longer behave like a typical pn diode. Excess free carriers get trapped at radiation-induced deep defects, compensating ionized shallow defects like residual doping in the bulk. Consequently, the carrier concentrations in the bulk decrease and become similar to those in intrinsic silicon, increasing the resistivity of the bulk. At the same time, the radiation-induced defects lead to narrow space charge regions (SCR) with high space charge concentrations and, due to the short lifetimes, the resistivity of the SCR may be less than the bulk resistivity. Hence, the bulk resistivity dominates the current-voltage characteristic for lower forward and reverse bias voltages and the electric field extends into the entire bulk. The bulk resistivity depends on the carrier concentrations and the carrier mobilities, which enables us to determine the decrease to the low-field carrier mobilities due to ionized impurity scattering as a function of the fluence.
The current-voltage characteristics were measured for fluences of $2.3 \times 10^{17}\,n_{eq}/$cm$^2$ and $5 \times 10^{17}\,n_{eq}/$cm$^2$ at different temperatures and are combined with previous measurements of proton-irradiated sensors to extend the fluence range down to $9 \times 10^{15}\,n_{eq}/$cm$^2$.
These results are relevant for modeling signal formation in silicon sensors at potential future hadron colliders and in other harsh radiation environments, and for understanding conditions under which silicon can still be used as a particle detector.
