Signal development in irradiated silicon detectors

This work provides a detailed study of signal formation in silicon detectors, with the emphasis on detectors with high concentration of irradiation induced defects in the lattice. These defects give rise to deep energy levels in the band gap. As a consequence, the current induced by charge motion in silicon detectors is signifcantly altered. Within the framework of the study a new experimental method, Charge correction method, based on transient current technique (TCT) was proposed for determination of effective electron and hole trapping times in irradiated silicon detectors. Effective carrier trapping times were determined in numerous silicon pad detectors irradiated with neutrons, pions and protons. Studied detectors were fabricated on oxygenated and non-oxygenated silicon wafers with different bulk resistivities. Measured effective carrier trapping times were found to be inversely proportional to fuence and increase with temperature. No dependence on silicon resistivity and oxygen concentration was observed. In terms of charge trapping charged hadrons seem to be more damaging than neutrons. An increase of hole trapping and a decrease of electron trapping was revealed during annealing. The formation of induced current depends strongly on electricfield which is determined by effective dopant concentration. The latter was checked for non- uniformity during the reverse annealing. In high resistivity neutron irradiated detectors no significant deviations from uniform effective doping concentration was observed. The increase of efective dopant concentration with fluence becomes the most important limiting factor for successful operation of heavily irradiated detectors. A significant reduction of effective dopant concentration was achieved by continuous illumination of n+ detector side with red light. In that way enhanced hole concentration in detector bulk is established causing changes in deep levels' contribution to the space charge. At given temperature and bias voltage illumination intensity can be fine tuned to reach optimal point of operation where effective dopant concentration is zero. In this way heavily irradiated detectors thicker than usual 300 m can be fully efficient. An attempt was also made to determine the dominant electron and hole trap levels and their generation rates by combining the results of effective dopant concentration measurements in the presence of enhanced carrier concentration and measurements of effective carrier trapping times. The observation of signals from 90Sr source in the p+-n-n+ micro-strip detectors connected to SCT32A read-out chip showed that voltage much higher than full depletion voltage is needed for high charge collection efficiency. This was checked throughout the beneficial and reverse annealing. The most probable signal decreased with time as expected from the measurements of the effective trapping times. A detailed simulation was employed aiming to predict the influence of effective trapping times on induced charge. The electronic processing of the induced current was also taken into account thus enabling the simulation of strip detectors connected to read-out chip. A good agreement between measured and simulated data was found. A number of other results were obtained, among which the most interesting ones are: prediction of ballistic deficit importance, comparison of strip detectors iv with p+ and n+ strips, detector thickness optimization and prediction of induced charge appearance on neighboring strips. Finally, appearance of bistable damage upon applying the bias to an irradiated detector was confirmed by TCT measurements. The results are in agreement with those previously measured with the C-V technique.