The Laser Interferometry Space Antenna (LISA) Mission is a project organized by the European Space Agency, in which three satellites will be sent to space with the goal of detecting gravitational waves. To achieve this, laser interferometry will be used, and to this end, photodiodes will play a very important role in converting optical signals into electrical ones. In space, semiconductor devices are exposed to radiation from many sources such as the Sun and Galactic Cosmic Rays. To better understand the expected radiation effects, electro-optical characteristics of the photodiodes must be analyzed before and after irradiation. Photodiodes developed by Hamamatsu in Japan have been tested through proton, gamma and electron irradiation and characterized more than one year ago.
A setup to study the characteristics of these photodiodes at the Observatoire de la Côte d'Azur (OCA) was implemented as many analyses were and will be performed not only on the before mentioned photodiodes, but
also on future candidates for the LISA Mission. The quality of the measurements allowed us to study the annealing effects on the irradiated devices.
IV measurements showed that there is a significant effect on the photodiodes caused by proton, electron and gamma irradiation as compared to pre-irradiation measurements. Due to the amount of time between measurements, annealing effects were observed for electron irradiation although no annealing was detected for proton irradiation. The gamma ray irradiated photodiode had undergone an annealing process, and further annealing effects were not observed on it.
CV measurements showed a slight increase in capacitance after irradiation, but differences were so low and within standard deviations between the 4 quadrants of each photodiode, that conclusions regarding the radiation effects cannot be drawn, and it is even possible to say that no radiation effects were observed on the capacitances of the photodiodes.
Experimental damage factors were obtained from the dark current values, for proton and electron irradiated photodiodes, showing once again no annealing from proton irradiation while electron irradiation did present annealing. A slight tendency is observed in which the photodiode size affects the damage factor, being lower at larger photodiode diameters. These damage factors were used to obtain the Non-Ionizing Energy Loss (NIEL) of each measurement, by using the NIEL scaling approach, showing a tendency from proton irradiated photodiodes to have higher coulombic interactions at higher energies as compared to nuclear interactions.
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