Annealing effects in n+–p strip detectors irradiated with high neutron fluences

Miniature micro-strip detectors made by implanting n-type readout strips on p-type silicon bulk (n+–p) were irradiated with reactor neutrons up to fluences of 5×1015 neq/cm2. Their charge collection properties were measured with signals caused by fast electrons from 90Sr source and read out by SCT128A chip. Collected charge and detector current were measured up to high bias voltages (1400 V) at which signs of charge multiplication can be observed. Detectors were submitted to successive annealing steps at 60 °C up to total time of 5040 min. Increase of collected charge after long annealing times was measured at high bias voltages. A similar effect was observed in the leakage current, which at high voltages increased with reverse-annealing time. Reverse annealing leads to higher space charge concentrations and therefore to higher values of electric field near the p–n junction. The consequence is larger multiplication resulting in increase of collected charge and leakage current.