Double Zinc Diffusion Optimization for Charge Persistence Reduction in InGaAs/InP SPADs

We present an in-depth characterization of the impact of the double zinc diffusion 2D profile on the charge persistence effect in InGaAs/InP Single-Photon Avalanche Diodes (SPADs). Charge persistence might be mitigated by increasing the operating temperature, so as to reduce minority carriers' lifetime, as well as by increasing the number of grading layers. However, the first approach leads to higher dark count rate due to stronger thermal generation of dark carriers, while the latter is technologically challenging. We show that, by adjusting the depth and radius of the shallow zinc diffusion, the electric field profile in the InGaAs layer outside the active area can be optimized in order to effectively mitigate charge persistence, achieving also its complete suppression in specific structures. Our study is based on both TCAD simulations and experimental measurements of different SPADs, whose only difference is in the geometrical shallow diffusion parameters

Performance analysis of a noncontact plastic fiber optical fiber displacement sensor with compensation of target reflectivity

peer-reviewedAn inexpensive fiber-based noncontact distance sensor specific for monitoring short-range displacements in micromachining applications is presented. To keep the overall costs low, the sensor uses plastic optical fibers and an intensiometric approach based on the received light intensity after the reflection from the target whose displacement has to be measured. A suitable target reflectivity compensation technique is implemented to mitigate the effects due to target surface nonuniformity or ageing.The performances of the sensor are first evaluated for different fiber configurations and target reflectivity profiles and positions using a numerical method based on Monte Carlo simulations. Then, experimental validations on a configuration designed to work up to 1.5mm have been conducted. The results have confirmed the validity of the proposed sensor architecture, which demonstrated excellent compensation capabilities, with errors below 0.04mm in the (0-1)mm range regardless the color and misalignment of the target

SPICE Electrical Models and Simulations of Silicon Photomultipliers

We present and discuss a comprehensive electrical model for Silicon Photomultipliers (SiPMs) based on a microcell able to accurately simulate the avalanche current build-up and the self-quenching of its Single-Photon Avalanche Diode (SPAD) “pixel” with series-connected quenching resistor. The entire SiPM is modeled either as an array of microcells, each one individually triggered by independent incoming photons, or as two macrocells, one with microcells all firing concurrently while the other one with all quiescent microcells; the most suitable approach depends on the light excitation conditions and on the dimension (i.e. number of microcells) of the overall SiPM. We validated both models by studying the behavior of SiPMs in different operating conditions, in order to study the effect of photons pile-up, the deterministic and statistical mismatches between microcells, the impact of the number of firing microcells vs. the total one, and the role of different microcell parameters on the overall SiPM performance. The electrical models were developed in SPICE and can simulate both custom-process and CMOS-compatible SiPMs, with either vertical or horizontal current-flow. The proposed simulation tools can benefit both SiPM users, e.g. for designing the best readout electronics, and SiPM designers, for assessing the impact of each parameter on the overall detection performance and electrical behavior

Low-cost and compact single-photon counter based on a CMOS SPAD smart pixel

We present a single-photon counter based on a silicon Single-Photon Avalanche Diode (SPAD) fabricated in a 0.35 μm CMOS technology. The detector is monolithically integrated with a front-end circuit and a digital pulse output driver. External components are kept to a minimum and the resulting instrument is low-cost, low-power and compact, being housed into an industry-standard 1-inch aluminum optical mounting tube. It features a maximum power consumption of just 250 mW from an USB link. The embedded 50 μm diameter SPAD has high photon detection efficiency in the visible range (55 % at 420 nm), low noise (< 100 cps at room temperature), low timing jitter (< 100 ps full-width at half maximum), and very low afterpulsing probability (down to 1 % with 60 ns hold-off time). The high performance, compactness and low cost enable many unexplored applications in life sciences, personal health care, industrial quality check, quantum physics and others, where it is required to count single photons and to measure their arrival time

Time-to-digital converter card for multichannel time-resolved single-photon counting applications

We present a high performance Time-to-Digital Converter (TDC) card that provides 10 ps timing resolution and 20 ps (rms) timing precision with a programmable full-scale-range from 160 ns to 10 mu s. Differential Non-Linearity (DNL) is better than 1.3% LSB (rms) and Integral Non-Linearity (INL) is 5 ps rms. Thanks to the low power consumption (400 mW) and the compact size (78 mm x 28 mm x 10 mm), this card is the building block for developing compact multichannel time-resolved instrumentation for Time-Correlated Single-Photon Counting (TCSPC). The TDC-card outputs the time measurement results together with the rates of START and STOP signals and the number of valid TDC conversions. These additional information are needed by many TCSPC-based applications, such as: Fluorescence Lifetime Imaging (FLIM), Time-of-Flight (TOF) ranging measurements, time-resolved Positron Emission Tomography (PET), single-molecule spectroscopy, Fluorescence Correlation Spectroscopy (FCS), Diffuse Optical Tomography (DOT), Optical Time-Domain Reflectometry (OTDR), quantum optics, etc

InGaAs/InP SPAD with Monolithically Integrated Zinc-Diffused Resistor

Afterpulsing and optical crosstalk are significant performance limitations for applications employing near-infrared single-photon avalanche diodes (SPADs). In this paper, we describe an InGaAs/InP SPAD with monolithically integrated resistor that is fully compatible with the planar fabrication process and provides a significant reduction of the avalanche charge and, thus, of afterpulsing and optical crosstalk. In order to have a fast SPAD reset (<50 ns), we fabricated quenching resistors ranging from 10 to 200 k\Ω, smaller than what is available in the literature. The resistor, fabricated with the zinc diffusions already used for avoiding premature edge-breakdown, promptly reduces the avalanche current to a low value ∼ 100~ μ A in less than 1 ns, while an active circuit completes the quenching and enforces a well-defined hold-off. The proposed mixed-quenching approach guarantees an avalanche charge reduction of more than 20 times compared with similar plain SPADs, enough to reduce the hold-off time down to 1 μ s. Finally, a compact single-photon counting module based on this detector and featuring 70-ps photon-timing jitter is presented