Radiation tolerance of epitaxial silicon detectors at very large proton fluences

Detectors processed on epitaxial silicon are a promising solution for the extreme radiation levels in the innermost tracking layers at future particle physics experiments as in the upgraded (S-LHC). In order to systematically investigate their radiation tolerance, sets of 25 and 50-μm-thick diodes had been irradiated with 24 GeV/c protons up to fluences of Φeq=1016 cm−2. The full depletion voltage was measured during full isothermal annealing cycles at both 60 and 80 °C. A voltage of only 290 V was measured for the 50 μm diodes after Φeq=1016 cm−2. In contrast to likewise thin float zone (FZ) diodes, the depletion voltage in epi-devices decreases with annealing during foreseen operational and storage periods. Low-temperature storage during beam-off periods as inevitable for FZ devices could be avoided. The reverse annealing components reveal an overall similar functional time dependence as observed in FZ diodes. However, the most important stable component is drastically different. In contrast to the well-known negative space charge build up observed in FZ diodes, the behavior in epi-diodes can only be explained by an additional formation of positive space charge. The immense improvement of the radiation tolerance of epi-detectors is thus attributed to a compensation effect between acceptor generation and donor creation unique for the epi-material. Charge collection efficiency drops only to about 80% after Φeq=6×1015 cm−2 proving that epi-detectors are indeed adequate candidates for the tracking area at extremely large radiation levels.