Delayed avalanches in Multi-Pixel Photon Counters

Hamamatsu Photonics Multi-Pixel Photon Counters (MPPCs) achieve high photo-detection efficiency with relatively low dark noise. The MPPC gain is comparable to Photomultipliers but pixel avalanches tend to trigger others either in neighboring pixels (cross-talk) or in the same pixel at a later time (after-pulsing). We investigate the possibility that after-pulsing avalanches are due to visible photons created in the avalanches and absorbed in the silicon bulk. In this case, the charge carriers created in the silicon bulk may diffuse back to the high field region, producing delayed avalanches. The MPPCs were illuminated with laser light at wavelengths ranging from 404 to 820 nm and the timing distribution of the avalanches was measured. Delayed avalanches were observed at 637 and 820 nm showing that some holes do diffuse to the high field region. Simulations show that optical photons are responsible for at least half of the after-pulsing avalanches.