Estimating and Mitigating Effects of Space Radiation Damage in Single-Photon Avalanche Diodes

Satellite-based quantum key distribution necessitates practical and robust detectors capable of withstanding the harsh environment of space. Single-photon avalanche diodes (SPADs) have been shown to fulfill the required criteria, especially with their excellent detection characteristics and easy of integration. However, these detection characteristic can be degraded when exposed to space radiation. While there is a significant number of studies on the radiation hardness of Silicon based SPADs, there is little information available for such detectors with an Indium-Gallium-Arsenide (InGaAs) substrate. Here, we present results of a ground radiation test of commercial-off-the-shelf InGaAs SPADs in the context of their viability in future satellite-based QKD applications. The expected radiation over the lifetime of such a satellite is modelled using the open-source European Space Agency Space Environment Information System (SPENVIS). Orbit altitudes, solar cycle period and detector substrates are varied to model the expected damage equivalent to 100 MeV protons. The results of the modelling are used to draw a ground radiation testing plan for eight InGaAs-based SPADs with a final target fluence amounting to 10-years in low-Earth orbit. The SPADs' characteristics (breakdown voltage, current-voltage curve and dark count rate) are measured before and after each radiation step. We find that breakdown voltage and the current-voltage curve does not change significantly, but dark count rate is elevated in most devices, albeit with variability between SPAD devices. Later measurements reveal that detector efficiency also decreases after radiation exposure. In a separate study, we explore using the method of laser annealing to reduce radiation-induce dark count rate in highly irradiated silicon SPADs. We expose previously irradiated detectors to various annealing powers ranging from 50 mW to 2.3 W, as well as at various annealing duration, from 10 s to 16 min. We find no significant dark count rate reduction occurred below 1 W, and employment of higher annealing power leads to steeper decreases in dark count rate. Annealing duration plays a smaller role in dark count rate, with exposures as short as 10 s able to incur some alleviation of elevated dark count rate when coupled with a high annealing power. Results of these two studies can be applied to estimate the feasibility of use of InGaAs SPADs in the space environment, as well as create annealing protocols for mitigation of radiation-induced dark count rate in future space-based quantum experiment requiring the use of SPADs

CubeSat Single-Photon Detector Module for Performing In-Orbit Laser Annealing to Heal Radiation Damage

Silicon-based single-photon avalanche photodiodes (SPADs), widely considered for satellite-based quantum communications, suffer a constant increase of dark count rate (DCR) from radiation-induced proton displacement damage in their active areas. When this accumulated damage causes the DCR to exceed a certain threshold (for example, 10,000 counts per second), the SPADs become unreliable for quantum communications, limiting mission lifetime. Previous ground experiments showed that radiation-induced DCR of synthetically irradiated SPADs could be significantly improved by high-power laser annealing, a localized heating of SPADs’ active areas using a focused laser beam. The next step is therefore to demonstrate realtime laser annealing on constantly irradiated SPADs in actual low-Earth-orbit is viable. To facilitate this study, the University of Waterloo team built a miniaturized software controllable SPAD module as part of the annealing payload on CAPSat (Cool Annealing Payload Satellite), a 3U CubeSat satellite developed by a team from the University of Illinois Urbana-Champaign. We present the concept of in-orbit laser annealing and the electronic platform of the SPAD module containing four detectors supporting thermal and laser annealing and detector characterization. The CAPSat, launched and deployed in a low-Earth orbit at 400 km altitude from the International Space Station in October 2021, was intended to assess the viability of this approach before incorporating SPADs in future quantum satellite missions, especially in quantum receivers

Protocols for healing radiation-damaged single-photon detectors suitable for space environment

Single-photon avalanche detectors (SPADs) are well-suited for satellite-based quantum communication because of their advantageous operating characteristics as well as their relatively straightforward and robust integration into satellite payloads. However, space-borne SPADs will encounter damage from space radiation, which usually manifests itself in the form of elevated dark counts. Methods for mitigating this radiation damage have been previously explored, such as thermal and optical (laser) annealing. Here we investigate in a lab, using a CubeSat payload, laser annealing protocols in terms of annealing laser power and annealing duration, for their possible later use in orbit. Four Si SPADs (Excelitas SLiK) irradiated to an equivalent of 10 years in low Earth orbit exhibit very high dark count rates (>300 kcps at -22 C operating temperature) and significant saturation effects. We show that annealing them with optical power between 1 and 2 W yields reduction in dark count rate by a factor of up to 48, as well as regaining SPAD sensitivity to a very faint optical signal (on the order of single photon) and alleviation of saturation effects. Our results suggest that an annealing duration as short as 10 seconds can reduce dark counts, which can be beneficial for power-limited small-satellite quantum communication missions. Overall, annealing power appears to be more critical than annealing duration and number of annealing exposures.Comment: 6 pages, 9 figures, work presented at IEEE Nuclear and Space Radiation Effects Conference 2022, prepared for submission to IEEE Transactions on Nuclear Scienc