Charge collection studies of proton-irradiated n- and p-type epitaxial silicon detectors

Proton-irradiated epitaxial pad diodes of 75, 100 and 150μm thickness and different oxygen concentrations were studied as an option to withstand the extreme radiation environment in the innermost tracking region of the future Super-LHC. With a new TCT setup using red laser light, time-resolved current signals could be measured in 150μm thin diodes. Thus the charge correction method (CCM) could be used to extract the effective trapping times. Similar results compared to previously investigated materials were obtained if the standard model of trapping is assumed, which is based on a constant trapping time at each fluence and neglects detrapping and charge multiplication. Charge collection efficiency (CCE) measured with 5.8 MeV α-particles showed an increase for decreasing thickness, but no dependence on impurity concentration was seen. CCE simulations based on the effective trapping time constants determined with the CCM resulted in systematically lower values than the measurements. This is the case for the CCE of both α-particles, red and infrared laser light. To account for this, possible modifications of the trapping model including voltage- or field-dependent trapping times will be discussed. Moreover, at high fluences and voltages anomalously high CCE>1 was observed, which indicates charge multiplication effects.