  The Silicon Photomultiplier (SiPM) is a promising photo-detector for a
variety of applications. However, the high dark count rate (DCR) of the SiPM is
still a contemporary problem. Decreasing the DCR would significantly broaden
the range of possible applications. In this work we present a novel method for
the spatially resolved characterization of crystal defects in SiPMs. The
contribution of crystal defects to the DCR is evaluated by exploiting the
effect of "hot carrier luminescence" (HCL), which is light that is emitted
during the Geiger mode operation of avalanche photodiodes (SiPM micro-cells).
Spatially confined regions with an enhanced light emission intensity (hotspots)
are identified within the active areas of SiPM micro-cells. By correlating the
detected light intensity and the DCR, a significant contribution of up to 56 %
of the DCR can be attributed to less than 5 % of the micro-cells. The analysis
of the temperature dependence of the emitted light identifies the
Shockley-Read-Hall-Generation to be the dominant mechanism responsible for the
occurrence of hotspots. The motivation of this work is to generate a deeper
understanding of the origin of hotspots in order to suppress their contribution
to the DCR of SiPMs.
