  The crosstalk and afterpulsing in Hamamatsu silicon photomultipliers, called
Multi-Pixel Photon Counters (MPPCs), have been studied in depth. Several
components of the correlated noise have been identified according to their
different possible causes and their effects on the signal. In particular, we
have distinguished between prompt and delayed crosstalk as well as between
trap-assisted and hole-induced afterpulsing. The prompt crosstalk has been
characterized through the pulse amplitude spectrum measured at dark conditions.
The newest MPPC series, which incorporate isolating trenches between pixels,
exhibit a very low prompt crosstalk, but a small component remains likely due
to secondary photons reflected on the top surface of the device and
photon-generated minority carriers diffusing in the silicon substrate. We
present a meticulous procedure to characterize the afterpulsing and delayed
crosstalk through the amplitude and delay time distributions of secondary
pulses. Our results indicate that both noise components are due to minority
carriers diffusing in the substrate and that this effect is drastically reduced
in the new MPPC series as a consequence of an increase of one order of
magnitude in the doping density of the substrate. Finally, we have developed a
Monte Carlo simulation to study the different components of the afterpulsing
and crosstalk. The simulation results support our interpretation of the
experimental data. They also demonstrate that trenches longer than those
employed in the Hamamatsu MPPCs would reduce the crosstalk to a much greater
extent.
