A comparison on radiation tolerance of 〈1 0 0〉 and 〈1 1 1〉 silicon substrates of microstrip detectors

A comparison between silicon microstrip detectors with the same geometry built on 〈1 0 0〉 and 〈1 1 1〉 substrates have been carried out. Three sets of structures—〈1 0 0〉 low resistivity, 〈1 1 1〉 low resistivity and 〈1 1 1〉 high resistivity—have been electrically characterized. Leakage current, depletion voltage, interstrip capacitance and resistance have been measured before and after neutron irradiation. The samples have been irradiated at five different fluences, up to ≃1.5×1014 n/cm2. The measurements show that the leakage current does not depend, at a given fluence, on crystal orientation and on silicon resistivity. At high irradiation fluences the interstrip resistance decreases for all structures to few tens MΩ. The low resistivity substrates, after type inversion, have a lower depletion voltage than the high resistivity ones. The interstrip capacitance is much less sensitive to radiation effects in 〈1 0 0〉 than in 〈1 1 1〉 structures. We conclude that 〈1 0 0〉 low resistivity sensors show, after irradiation, better performances with respect to standard 〈1 1 1〉 high resistivity devices.