Charge multiplication in irradiated segmented silicon detectors with special strip processing

Abstract

Charge multiplication in severely irradiated silicon detectors is now a well proven effect that enhances their charge collection and makes them operable up to the doses anticipated for future super-colliders (like the high luminosity LHC at CERN). The effect is well documented but not completely understood. The multiplication is caused by impact ionisation due to hot electrons moving in the high electric field that develops near the junction in the irradiated sensors. The details of the electric field profile in the silicon bulk are not available due to the unknown spatial distribution of the inhomogeneous effective space charge in the hadron irradiated silicon bulk. The gradient of the effective space charge distribution is crucial for the formation of high electric field regions where the multiplication takes place. The electric field might be influenced by the implant forming the n–p junction and by a non-homogenous bulk space charge near the junction. Deep n+ structures (junction) could enhance or reduce the multiplication effect. Also an altered doping gradient (obtainable for example by a graded p-doping between the junction and the p-bulk) could achieve the same results. In order to achieve enhanced charge multiplication in microstrip detectors the junction has been shaped by mean of deep p-doping diffusion and of trenches etched in the silicon bulk and filled with n+ doped polysilicon to create a deep junction. We report here the result obtained with these methods before and after various doses of neutron irradiation.