The design, modeling, fabrication, and characterization of single-photon avalanche diode detectors with an epitaxial Ge absorption region grown directly on Si are presented. At 100 K, a single-photon detection efficiency of 4% at 1310 nm wavelength was measured with a dark count rate of ∼6 megacounts/s, resulting in the lowest reported noiseequivalent power for a Ge-on-Si single-photon avalanche diode detector (1 × 10 −14 WHz −1/2 ). The first report of 1550 nm wavelength detection efficiency measurements with such a device is presented. A jitter of 300 ps was measured, and preliminary tests on after-pulsing showed only a small increase (a factor of 2) in the normalized dark count rate when the gating frequency was increased from 1 kHz to 1 MHz. These initial results suggest that optimized devices integrated on Si substrates could potentially provide performance comparable to or better than that of many commercially available discrete technologies.Index Terms-Detector, germanium on silicon, single-photon avalanche diode, single-photon counting.

Single-photon avalanche diode (SPAD) detector arrays generally suffer from having a low fill-factor, in which the photo-sensitive area of each pixel is small compared to the overall area of the pixel. This paper describes the integration of different configurations of high efficiency diffractive optical microlens arrays onto a 32 × 32 SPAD array, fabricated using a 0.35 µm CMOS technology process. The characterization of SPAD arrays with integrated microlens arrays is reported over the spectral range of 500-900 nm, and a range of f-numbers from f/2 to f/22. We report an average concentration factor of 15 measured for the entire SPAD array with integrated microlens array. The integrated SPAD and microlens array demonstrated a very high uniformity in overall efficiency.

Arrays of single-photon avalanche diode (SPAD) detectors were fabricated, using a 0.35 μm CMOS technology process,\ud for use in applications such as time-of-flight 3D ranging and microscopy. Each 150 x 150 μm pixel comprises a 30 μm\ud active area diameter SPAD and its associated circuitry for counting, timing and quenching, resulting in a fill-factor of\ud 3.14%. This paper reports how a higher effective fill-factor was achieved as a result of integrating microlens arrays on\ud top of the 32 x 32 SPAD arrays. Diffractive and refractive microlens arrays were designed to concentrate the incoming\ud light onto the active area of each pixel. A telecentric imaging system was used to measure the improvement factor (IF)\ud resulting from microlens integration, whilst varying the f-number of incident light from f/2 to f/22 in one-stop\ud increments across a spectral range of 500-900 nm. These measurements have demonstrated an increasing IF with fnumber,\ud and a maximum of ~16 at the peak wavelength, showing a good agreement with theoretical values. An IF of 16\ud represents the highest value reported in the literature for microlenses integrated onto a SPAD detector array. The results\ud from statistical analysis indicated the variation of detector efficiency was between 3-10% across the whole f-number\ud range, demonstrating excellent uniformity across the detector plane with and without microlenses

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