SiPM Avalanche Size and Crosstalk Measurements with Light Emission Microscopy

Diodes operating in reverse bias emit visible light. This light emission process is important in silicon photomultipliers where microcells, arranged in a matrix, can trigger avalanches in adjacent microcells. One of the main goals toward improving silicon photomultipliers as light detectors is to design sensors with significantly higher photon detection efficiency than the classical photomultipliers. To achieve this goal, one needs to operate the silicon photomultiplier under a high relative overvoltage, saturating its Geiger efficiency. This means operating it under high gain and a correspondingly high level of optical crosstalk. In this paper, we describe a new and precise visual method for measuring the crosstalk as well as the transverse size of avalanches in a silicon photomultiplier microcell. This size turns out to be significantly smaller than the usually used microcell size, practically independent from the overvoltage or type. It also has implications for designing small, fast microcells that could reach a naturally lower size limit.