Hamamatsu Photonics introduced a new generation of their Multi-Pixel PhotonCounters in 2013 with significantly reduced after-pulsing rate. In this paper,we investigate the causes of after-pulsing by testing pre-2013 and post-2013devices using laser light ranging from 405 to 820nm. Doing so we investigatethe possibility that afterpulsing is also due to optical photons produced inthe avalanche rather than to impurities trapping charged carriers produced inthe avalanches and releasing them at a later time. For pre-2013 devices, weobserve avalanches delayed by ns to several 100~ns at 637, 777nm and 820 nmdemonstrating that holes created in the zero field region of the silicon bulkcan diffuse back to the high field region triggering delayed avalanches. On theother hand post-2013 exhibit no delayed avalanches beyond 100~ns at 777nm. Wealso confirm that post-2013 devices exhibit about 25 times lower after-pulsing.Taken together, our measurements show that the absorption of photons from theavalanche in the bulk of the silicon and the subsequent hole diffusion back tothe junction was a significant source of after-pulse for the pre-2013 devices.Hamamatsu appears to have fixed this problem in 2013 following the preliminaryrelease of our results. We also show that even at short wavelength the timingdistribution exhibit tails in the sub-nanosecond range that may impair the MPPCtiming performances.
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