Laser annealing heals radiation damage in avalanche photodiodes

TL;DR

High-power laser annealing effectively repairs radiation damage in avalanche photodiodes, significantly reducing dark count rates while preserving key detection characteristics, thus extending their operational lifetime in space-based quantum communication systems.

Contribution

This study demonstrates that laser annealing can heal radiation damage in APDs, restoring performance and extending their lifespan in space applications, which was not previously established.

Findings

Dark count rates reduced by up to 758 times at -80°C.

Laser annealing does not significantly affect photon detection efficiency.

The method is effective across multiple APD models.

Abstract

Avalanche photodiodes (APDs) are a practical option for space-based quantum communications requiring single-photon detection. However, radiation damage to APDs significantly increases their dark count rates and reduces their useful lifetimes in orbit. We show that high-power laser annealing of irradiated APDs of three different models (Excelitas C30902SH, Excelitas SLiK, and Laser Components SAP500S2) heals the radiation damage and substantially restores low dark count rates. Of nine samples, the maximum dark count rate reduction factor varies between 5.3 and 758 when operating at minus 80 degrees Celsius. The illumination power to reach these reduction factors ranges from 0.8 to 1.6 W. Other photon detection characteristics, such as photon detection efficiency, timing jitter, and afterpulsing probability, remain mostly unaffected. These results herald a promising method to extend the lifetime of a quantum satellite equipped with APDs.

Tables

Table 1. Table 1: Summary of detector samples, applied radiation, previous thermal annealing, and measured results of laser annealing. The detectors are referred to by the given sample IDs throughout the paper. The highest reduction factor is the ratio between the initial dark count rate and the lowest dark count rate after laser annealing—the corresponding laser power for this is also given.

Sample ID	$\mathrm{𝟏𝟎𝟔}\mathrm{𝐌𝐞𝐕}$ proton fluence (${\mathrm{𝐜𝐦}}^{-\mathrm{𝟐}}$)	Equivalent time in 600 km polar orbit (months)	Thermal annealing procedure	Dark count rate at$-\mathrm{𝟖𝟎}\mathrm{°C}$			Annealing power ($𝐖$)	${𝑽}_{\text{excess}}$ ($𝐕$)
				Before ( Hz )	Lowest after ( Hz )	Highest reduction factor
C30902SH-1	${10}^9$	6	None	$347$	$2.3$	150	$0.8$	14
C30902SH-2	${10}^9$	6	None	$363$	$2.64$	137	$1.5$	14
SLiK-1	${10}^8$	0.6	$2h@+100\mathrm{°C}$	$6.71$	$0.16$	41.7	$1.4$	14
SLiK-2	${10}^8$	0.6	$2h@+100\mathrm{°C}$	$2.19$	$0.42$	5.3	$0.8$	14
SLiK-3	$4\times {10}^9$	24	$4h@+80\mathrm{°C}$, $2h@+100\mathrm{°C}$	$43.1$	$2.09$	21	$1.4$	14
SLiK-4	${10}^9$	6	None	$192$	$8.3$	23	$1.0$	20
SLiK-5	$4\times {10}^9$	24 (with bias voltage applied)	$3h@+80\mathrm{°C}$, $2h@+100\mathrm{°C}$	$447$	$58$	7.7	$1.0$	20
SAP500S2-1	$4\times {10}^9$	24	$4h@+80\mathrm{°C}$, $2h@+100\mathrm{°C}$	$1579$	$2.08$	758	$1.4$	20
SAP500S2-2	${10}^8$	0.6	$2h@+100\mathrm{°C}$	$213$	$1.66$	128	$1.6$	20

Table 2. Table 2: Measurement of thermal resistance between APD and cold plate.

${𝑽}_{\text{bias}}$ ($𝐕$)	${𝑽}_{\text{br}}$ ($𝐕$)	$\Delta$T ($\mathrm{°C}$)	Power dissipation ($\mathrm{𝐦𝐖}$)	Thermal resistance ($𝐊/𝐖$)
$340$	$294.65$	$-0.16$	$33.4$	$-4.8$
$350$	$295.05$	$-0.44$	$40.5$	$-10.8$
$360$	$295.82$	$0.11$	$47.4$	$2.3$
$370$	$296.74$	$0.98$	$54.3$	$18.0$
$380$	$297.10$	$0.54$	$61.5$	$8.8$

Full text

I Introduction

Quantum communications protocols, such as quantum key distribution (QKD) Bennett and Brassard (1984); Ekert (1991), quantum teleportation Bennett et al. (1993), and Bell’s inequality tests Bell (1964), are limited to transmission distances of only a few hundred kilometers Yin et al. (2016); Ma et al. (2012); Scheidl et al. (2010) under the restrictions of present technology. For global-scale quantum communications, quantum repeaters Briegel et al. (1998) and quantum satellites Buttler et al. (1998); Rarity et al. (2002); Aspelmeyer et al. (2003) are potential solutions. Unfortunately, quantum repeaters are not ready for deployment as quantum memories with sufficient storage times and fidelities, upon which quantum repeaters depend, are still being developed Simon et al. (2010); Sangouard et al. (2011). On the other hand, quantum communications to satellite platforms are feasible today Gilbert and Hamrick ; Bonato et al. (2009); Ursin et al. (2009); Meyer-Scott et al. (2011); Xin (2011); Nordholt et al. (2002); Takenaka et al. (2011); Higgins et al. (2012); Vallone et al. (2015), with China being the first country to successfully launch a quantum satellite Gibney (2016).

One of many challenges in achieving long-distance quantum communications is the noise floor imposed by detector dark counts Scarani et al. (2009)—false photon detection events caused by thermally excited, tunnelling, and trapped electrons Haitz (1965). A previous study Bourgoin et al. (2013) examined the performance of both downlink and uplink satellite quantum communication designs under various conditions. Uplink communication, where the detectors are placed on the satellite, is particularly interesting because of potentially simpler satellite designs and easy interchangeability of sources at the ground station—for this approach, QKD, quantum teleportation, and Bell tests perform well with a dark count rate up to about $200~{}\mathrm{Hz}$ per detector.

Silicon avalanche photodiodes (APD) are an appropriate choice for the single-photon detector on a satellite because of their low dark count rate, good sensitivity in $400$–$1000~{}\mathrm{n}\mathrm{m}$ wavelength range (covering wavelengths near $785~{}\mathrm{n}\mathrm{m}$ for optimal uplink transmissions Bourgoin et al. (2013)), and no need for cryogenic cooling Cova et al. (2004); Hadfield (2009); Eisaman et al. (2011). However, proton radiation in low Earth orbit significantly increases APD dark count rates over time Sun et al. (1997); Sun and Dautet (2001); Sun et al. (2004); Tan et al. (2013); Tang et al. (2016). In a recent experiment Anisimova et al. (2015a), APD samples (Excelitas C30902SH, Excelitas SLiK, and Laser Components SAP500S2) were irradiated by different fluences of $106~{}\mathrm{M}\mathrm{eV}$ protons to simulate radiation effects over 0.6, 6, 12, and 24 months in a representative low Earth polar orbit. There it was shown that thermal annealing of irradiated APDs at up to $100~{}\mathrm{\SIUnitSymbolCelsius}$ can repair some of the damage, resulting in up to 6.6-fold dark count rate reduction. A separate study showed that laser annealing can lower non-irradiated APD dark count rates by up to 5.4 times Bugge et al. (2014).

Here we perform laser annealing on nine irradiated APDs. We find that laser annealing successfully decreases the dark count rates in all nine irradiated APD samples by a factor ranging from 5.3 to 758 when operated at $-80~{}\mathrm{\SIUnitSymbolCelsius}$. We demonstrate dark count rate reductions due to laser annealing can exceed those from thermal annealing. Notably, we observe that dark count rates are reduced even when laser annealing is applied to APDs that were already thermally annealed. Laser annealing also affects other important photon counting parameters including photon detection efficiency, timing jitter, and afterpulsing probability, but the operation of quantum communications applications should not be significantly influenced by these changes.

II Experimental setup