Ge-on-Si single-photon avalanche diode detectors for short-wave infrared wavelengths

Germanium-on-silicon (Ge-on-Si) based single-photon avalanche diodes (SPADs) have recently emerged as a promising detector candidate for ultra-sensitive and picosecond resolution timing measurement of short-wave infrared (SWIR) photons. Many applications benefit from operating in the SWIR spectral range, such as long distance light detection and ranging, however, there are few single-photon detectors exhibiting the high-performance levels obtained by all-silicon SPADs commonly used for single-photon detection at wavelengths <1 μm. This paper first details the advantages of operating at SWIR wavelengths, the current technologies, and associated issues, and describes the potential of Ge-on-Si SPADs as a single-photon detector technology for this wavelength region. The working principles, fabrication and characterisation processes of such devices are subsequently detailed. We review the research in these single-photon detectors and detail the state-of-the-art performance. Finally, the challenges and future opportunities offered by Ge-on-Si SPAD detectors are discussed.