| Photodiodes capable of detecting very weak intensities of light, down to single photon levels, have been investigated for a long time. Single photon detectors have numerous applications, including quantum cryptography, three-dimensional imaging, atmospheric monitoring, and time resolved spectroscopy. Conventionally, photomultiplier tubes have been the detectors of choice for low intensity light applications. However, due to their limitations such as high operating voltages, bulkiness, and limited sensitivity in the infrared, alternative solutions are sought. Avalanche photodiodes (APDs) operated above their breakdown voltage, can achieve single photon sensitivity. APDs were fabricated for single photon detection at wavelengths ranging from ultraviolet to infrared. Linear mode characterization experiments such as contact resistance and current-voltage were performed to determine the viability of a detector as a photon counter. Single photon detectors are often operated at low temperatures to minimize the dark noise. The temperature dependence of forward and reverse characteristics of the APDs was studied. A cryogenic system was established and calibrated for characterization of photodetectors at very low levels of illumination. The effect of various experimental parameters such as temperature, excess voltage, and discriminator threshold on Geiger mode APD performance was studied. Single photon detection at 1.54 mum was demonstrated using a separate-absorption-charge-multiplication avalanche photodetector with an In0.52Al0.48As multiplication layer. Various dark count generation mechanisms were modeled to understand the temperature dependence of dark counts in the In0.53Ga0.47As/In0.52Al 0.48As APDs. The experimental observations were explained using band-to-band tunneling in the multiplication layer as the dominant dark count mechanism. Based on the modeling and experimental results, design guidelines for single photon counting avalanche diodes are suggested. An APD structure with thicker In0.52Al0.48As multiplication layer and punchthrough voltage close to the breakdown voltage was designed. In addition, SiC and GaAs based APDs were evaluated as photon detectors in the ultraviolet and near infrared regions, respectively. Single photon counting at 325nm was achieved using 4H-SiC APDs. Although MBE grown GaAs/AlGaAs APDs with high quantum efficiencies and low dark currents showed promise for single photon counting at 830nm, further experiments are needed to understand their inability to function as single photon detectors. |
