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There is increasing interest in development of high speed, low noise and readily fieldable near infrared (NIR) single
photon detectors. InGaAs/InP Avalanche photodiodes (APD) operated in Geiger mode (GM) are a leading choice for
NIR due to their preeminence in optical networking. After-pulsing is, however, a primary challenge to operating
InGaAs/InP single photon detectors at high frequencies1. After-pulsing is the effect of charge being released from traps
that trigger false ("dark") counts. To overcome this problem, hold-off times between detection windows are used to
allow the traps to discharge to suppress after-pulsing. The hold-off time represents, however, an upper limit on detection
frequency that shows degradation beginning at frequencies of ~100 kHz in InGaAs/InP. Alternatively, germanium (Ge)
single photon avalanche photodiodes (SPAD) have been reported to have more than an order of magnitude smaller
charge trap densities than InGaAs/InP SPADs2, which allowed them to be successfully operated with passive quenching2
(i.e., no gated hold off times necessary), which is not possible with InGaAs/InP SPADs, indicating a much weaker dark
count dependence on hold-off time consistent with fewer charge traps. Despite these encouraging results suggesting a
possible higher operating frequency limit for Ge SPADs, little has been reported on Ge SPAD performance at high
frequencies presumably because previous work with Ge SPADs has been discouraged by a strong demand to work at
1550 nm. NIR SPADs require cooling, which in the case of Ge SPADs dramatically reduces the quantum efficiency of
the Ge at 1550 nm. Recently, however, advantages to working at 1310 nm have been suggested which combined with a
need to increase quantum bit rates for quantum key distribution (QKD) motivates examination of Ge detectors
performance at very high detection rates where InGaAs/InP does not perform as well. Presented in this paper are
measurements of a commercially available Ge APD operated at relatively short GM hold-off times to examine whether
there are potential advantages to using Ge for 1310 nm single photon detection. A weaker after-pulsing dependence on
frequency is observed offering initial indications of the potential that Ge APDs might provide better high frequency
performance.
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