Ultrafast Single Photon Detector with Double Epitaxial Structure for Minimum Carrier Diffusion Effect

Avalanche photodiodes designed to work biased above the breakdown voltage are employed for timing single photons with picosecond resolution. The time resolution curve of these devices (called Single Photon Avalanche Diodes SPADs) shows a fast peak and an undesirable tail, due to carrier diffusion. We had, previously, designed an epitaxial SPAD structure that succeeded in reducing the effect to a short, exponential-like tail with 1.7 ns lifetime. In order to achieve a further improvement, we have devised and fabricated a double epitaxial structure. The measurements performed on these new devices show a drastic reduction of the diffusion effect. The time resolution curve of the new detector is characterized by a peak with better than 45 pS fullwidth at half-maximum (FWHM) and a short, exponential tail, with 270 ps lifetime.