Radiation tests of photodiodes for the ATLAS SCT and PIXEL opto- links

In previous research, epitaxial Si PIN photodiodes produced by Centronic which will be used in the ATLAS semiconductor tracker have been irradiated with 1 MeV neutrons and 24 GeV protons with fluences up to an equivalent of $10^{15}$ 1 MeV neutrons (1,2) . In this work 30 MeV proton beams were used to irradiate Centronic and Truelight epitaxial Si PIN diodes with accumulated fluences of up to 2.1 multiplied by $10^{14}$-30 MeV p $cm^{-2}$, an equivalent of 5.7 multiplied by $10^{14} cm^{-2}$ 1 MeV neutrons, to reach the pixel radiation environment. The responsivity was measured with different levels of fluence in order to study the responsivity behaviour of two different types of photodiodes. The responsivity behaviour of these two photodiodes was similar: a linear degradation at large fluences, greater than $10^{14}$ 30 MeV p $cm^{-2}$, but with different slopes. The response of the Centronic PIN diode showed a degradation to 73% after a proton fluence of $10^{13}$ p $cm^{-2}$ of 30 MeV and a linear degradation at a rate of -0.32% per $10^{13}$ p $cm^{-2}$ for fluences larger than $10^{13}$ 30 MeV p $cm^{-2}$. The Truelight PIN degraded to 84% after a fluence of $10^{13}$ p $cm^{-2}$, with a linear degradation rate of -1.8% per $10^{13}$ p $cm^{-2}$. Annealing of both photodiodes with a bias voltage of -10 V for 180 h showed a 7% recovery for the Truelight PIN diode and a 2% recovery for the Centronic PIN diode after irradiation fluences of 2.1 multiplied by $10^{14}$ p $cm^{-2}$. A gamma source was also used to irradiate 6 Truelight PIN diodes with a total dose larger than 600 kGy; no obvious responsivity change was observed in this case.