ABSTRACT
Photon counting in one or in arrays of pixels is useful in a number of applications, from more traditional ones, such as low-light-level and fluorescence imaging, superresolution microscopy, 3D time-of-flight sensing, and NIR optical tomography [1–6], to niche applications, such as voltage-sensitive-dye (VSD)-based imaging [7,8], particle image velocimetry [9], instantaneous gas imaging [10,11], fluorescence-based time-resolved imaging (both single- and multiphoton, and lifetime, and correlation based) [12–14], and applications based on time domain interferometry, like the Hanbury Brown–Twiss interferometry, for the analysis of microlight and macrolight sources as well as for stellar bodies [15–17]. In this context, and in the context of consumer applications, compactness, weight, and power consumption have prompted a boost to the development of solid-state photon-counting sensors, where 186photoelectrons are multiplied in single-sided abrupt junctions operating at high electric fields that can sustain impact ionization. These structures are generally based on a technology that is known as Geiger-mode avalanche photodiode or single-photon avalanche diode (SPAD), whereas the first devices of this kind were vertically integrated in dedicated processes, e.g., reach-through avalanche photodiode (RAPD) based on p+-π-p-n [18].
