Silicon Single-photon Avalanche Diode Research Articles

Room-temperature photonic quantum computing in integrated silicon photonics with germanium–silicon single-photon avalanche diodes

Most, if not all, photonic quantum computing (PQC) relies upon superconducting nanowire single-photon detectors (SNSPDs) typically based on niobium nitride (NbN) operated at a temperature <4 K. This paper proposes and analyzes 300 K waveguide-integrated germanium–silicon (GeSi) single-photon avalanche diodes (SPADs) based on the recently demonstrated normal-incidence GeSi SPADs operated at room temperature, and shows that their performance is competitive against that of NbN SNSPDs in a series of metrics for PQC with a reasonable time-gating window. These GeSi SPADs become photon-number-resolving avalanche diodes (PNRADs) by deploying a spatially-multiplexed M-fold-waveguide array of M GeSi SPADs. Using on-chip waveguided spontaneous four-wave mixing sources and waveguided field-programmable interferometer mesh circuits, together with the high-metric SPADs and PNRADs, high-performance quantum computing at room temperature is predicted for this PQC architecture.

1064 nm rotational Raman polarization lidar for profiling aerosol and cloud characteristics

The vertical profiles of aerosol or mixed-phase cloud optical properties (e.g. extinction coefficient) at 1064 nm are difficult to obtain from lidar observations. Based on the techniques of rotational Raman signal at 1058 nm described by Haarig et al. [ Atmos. Meas. Tech. 9, 4269 (2016)10.5194/amt-9-4269-2016 ], we have developed a novel rotational Raman polarization lidar at 1064 nm at Wuhan University. In this design, we optimized the central wavelength of the rotational Raman channel to 1056 nm with a bandwidth of 6 nm to increase the signal-to-noise ratio and minimize the temperature dependence of the extracted rotational Raman spectrum. And then separated elastic polarization channels (1064 nm Parallel, P and 1064 nm Cross, S) into near range (low 1064 nm P and 1064 nm S) and far range detection channels (high 1064 nm P and 1064 nm S) to extend the dynamic range of lidar observation. Silicon single photon avalanche diodes (SPAD) working at photon counting mode were applied to improve the quantum efficiency and reduce the electronic noise, which resulted in quantum efficiency of 2.5%. With a power of 3 W diode pumped pulsed Nd:YAG laser and aperture of 250 mm Cassegrain telescope, the detectable range can cover the atmosphere from 0.3 km to the top troposphere (about 12-15 km). To the best of our knowledge, the design of this novel lidar system is described and the mixed-phase cloud and aerosol optical properties observations of backscatter coefficients, extinction coefficients, lidar ratio and depolarization ratio at 1064 nm were performed as demonstrations of the system capabilities.

The impact of spot‐size on single‐photon avalanche diode timing‐jitter and quantum key distribution

AbstractIn free‐space implementations of Quantum key distribution (QKD), the wide adoption of near‐Infrared wavelengths has led to the common use of silicon single‐photon avalanche diodes (Si‐SPAD) for receiver systems. While the impacts of some SPAD properties on QKD have been explored extensively, the relationship of spot‐size and spatial position on the full instrumental response and thus quantum bit error rate (QBER) has been studied little. Changes in spot size and spatial position can result from atmospheric turbulence and pointing and tracking errors. Here, An empirical analysis of that relationship is presented utilising a large active area, 500 μm, free‐space coupled Si‐SPAD designed for free‐space QKD. A baseline full‐width at half‐maximum timing jitter of 182 ps and a QBER contribution of 0.1 % for a 1 GHz clock frequency QKD system and 100 ps time‐gating window are reported. The impacts of spot‐size and spatial position can increase the QBER to over 0.3%. The link between the spot‐size and timing jitter will allow the understanding of tolerancing for the alignment of Si‐SPADs within free‐space QKD receiver systems—an important factor in designing properly engineered practical systems and the equipment needed to compensate for atmospheric turbulence and pointing and tracking.

Timing measurements with silicon single photon avalanche diodes: principles and perspectives [Invited

Picosecond timing of single photons has laid the foundation of a great variety of applications, from life sciences to quantum communication, thanks to the combination of ultimate sensitivity with a bandwidth that cannot be reached by analog recording techniques. Nowadays, more and more applications could still be enabled or advanced by progress in the available instrumentation, resulting in a steadily increasing research interest in this field. In this scenario, single-photon avalanche diodes (SPADs) have gained a key position, thanks to the remarkable precision they are able to provide, along with other key advantages like ruggedness, compactness, large signal amplitude, and room temperature operation, which neatly distinguish them from other solutions like superconducting nanowire single-photon detectors and silicon photomultipliers. With this work, we aim at filling a gap in the literature by providing a thorough discussion of the main design rules and tradeoffs for silicon SPADs and the electronics employed along them to achieve high timing precision. In the end, we conclude with our outlook on the future by summarizing new routes that could benefit from present and prospective timing features of silicon SPADs.

Challenges and prospects for multi-chip microlens imprints on front-side illuminated SPAD imagers.

The overall sensitivity of frontside-illuminated, silicon single-photon avalanche diode (SPAD) arrays has often suffered from fill factor limitations. The fill factor loss can however be recovered by employing microlenses, whereby the challenges specific to SPAD arrays are represented by large pixel pitch (> 10 µm), low native fill factor (as low as ∼10%), and large size (up to 10 mm). In this work we report on the implementation of refractive microlenses by means of photoresist masters, used to fabricate molds for imprints of UV curable hybrid polymers deposited on SPAD arrays. Replications were successfully carried out for the first time, to the best of our knowledge, at wafer reticle level on different designs in the same technology and on single large SPAD arrays with very thin residual layers (∼10 µm), as needed for better efficiency at higher numerical aperture (NA > 0.25). In general, concentration factors within 15-20% of the simulation results were obtained for the smaller arrays (32×32 and 512×1), achieving for example an effective fill factor of 75.6-83.2% for a 28.5 µm pixel pitch with a native fill factor of 28%. A concentration factor up to 4.2 was measured on large 512×512 arrays with a pixel pitch of 16.38 µm and a native fill factor of 10.5%, whereas improved simulation tools could give a better estimate of the actual concentration factor. Spectral measurements were also carried out, resulting in good and uniform transmission in the visible and NIR.

Submerged single-photon LiDAR imaging sensor used for real-time 3D scene reconstruction in scattering underwater environments.

We demonstrate a fully submerged underwater LiDAR transceiver system based on single-photon detection technologies. The LiDAR imaging system used a silicon single-photon avalanche diode (SPAD) detector array fabricated in complementary metal-oxide semiconductor (CMOS) technology to measure photon time-of-flight using picosecond resolution time-correlated single-photon counting. The SPAD detector array was directly interfaced to a Graphics Processing Unit (GPU) for real-time image reconstruction capability. Experiments were performed with the transceiver system and target objects immersed in a water tank at a depth of 1.8 meters, with the targets placed at a stand-off distance of approximately 3 meters. The transceiver used a picosecond pulsed laser source with a central wavelength of 532 nm, operating at a repetition rate of 20 MHz and average optical power of up to 52 mW, dependent on scattering conditions. Three-dimensional imaging was demonstrated by implementing a joint surface detection and distance estimation algorithm for real-time processing and visualization, which achieved images of stationary targets with up to 7.5 attenuation lengths between the transceiver and the target. The average processing time per frame was approximately 33 ms, allowing real-time three-dimensional video demonstrations of moving targets at ten frames per second at up to 5.5 attenuation lengths between transceiver and target.

Quantum bit error rate timing jitter dependency on multi-mode fibers.

The full time-jitter response of a single-photon detector can make a significant contribution to the quantum bit error rate (QBER) of high repetition rate quantum key distribution (QKD) implementations. Although there have been studies into understanding the contribution for single-mode optical fiber coupled single-photon detectors, the contribution of larger, multimode core diameters to the QBER have not been explored in detail. With the growing importance of free-space QKD, which typically use multimode fibers to reduce coupling loss, it is vitally important to understand how the multimode fiber coupling will impact the total QBER. This work studies the impact of the time-jitter contribution to QBER when coupling a commercial off-the-shelf silicon single-photon avalanche diode with various multimode fibers while simulating operating at 1 GHz with empirical measurements taken at 1 MHz repetition rate. It was found that step-index multimode fibers can significantly increase the QBER, while graded-index fibers can provide an QBER contribution similar to a single-mode fiber. The results highlight that there is a significant benefit in using graded index multimode fibers for a free-space QKD receiver, particularly for high repetition rate applications.

3D integration technologies for custom SiPM: From BSI to TSV interconnections

Progress in 3D interconnecting technologies paved the way for a new generation of Silicon Photomultipliers (SiPM) and Single Photon Avalanche Diode (SPAD): hybrid devices which combine the integrated functionalities of the digital SiPM with the high performance of custom technologies, like low noise and high detection efficiency. Recently, Fondazione Bruno Kessler (FBK) has been working on the implementation of recently developed 3D integration technologies, on SiPMs devices, to improve both performances and functionalities by creating backside-illuminated (BSI) devices and Through Silicon Vias (TSV) interconnections.Two different technology platforms have been investigated: a BSI design for near-infrared (NIR) sensitive SiPMs and TSV interconnections for near- and vacuum-ultraviolet (NUV/VUV) sensitive detectors.For NIR applications, electrical characterization of ultra-thin (about 10μm) SiPM wafers with a metal reflector on the frontside has shown an improved photon detection efficiency (PDE) when operated in BSI configuration compared with non-thinned front-side illuminated (FSI) devices, allowing at the same time full high-segmentation access to the SiPM output from the front-side.Instead, for NUV/VUV applications, a FSI stacked approach is considered more suitable since the junction depth needs to be shallower. In this case, TSV interconnections using two different approaches (named Via-Mid and Via-Last) have been implemented allowing the placement of the contacts on the backside of the wafer.

Near Infrared Efficiency Enhancement of Silicon Photodiodes by Integration of Metal Nanostructures Supporting Surface Plasmon Polaritrons

Recent years have witnessed a growing interest in detectors capable of detecting single photons in the near-infrared (NIR), mainly due to the emergence of new applications such as light detection and ranging (LiDAR) for, e.g., autonomous driving. A silicon single-photon avalanche diode is surely one of the most interesting and available technologies, although it yields a low efficiency due to the low absorption coefficient of Si in the NIR. Here, we aim at overcoming this limitation through the integration of complementary metal-oxide-semiconductor (CMOS) -compatible nanostructures on silicon photodetectors. Specifically, we utilize silver grating arrays supporting surface plasmons polaritons (SPPs) to superficially confine the incoming NIR photons and therefore to increase the probability of photons generating an electron-hole pair. First, the plasmonic silver array is geometrically designed using time domain simulation software to achieve maximum detector performance at 950 nm. Then, a plasmonic silver array characterized by a pitch of 535 nm, a dot width of 428 nm, and a metal thickness of 110 nm is integrated by means of the focused ion beam technique on the detector. Finally, the integrated detector is electro-optically characterized, demonstrating a QE of 13% at 950 nm, 2.2 times higher than the reference. This result suggests the realization of a silicon device capable of detecting single NIR photons, at a low cost and with compatibility with standard CMOS technology platforms.

Junction Engineering-Based Modeling and Optimization of Deep Junction Silicon Single-Photon Avalanche Diodes for Device Scaling

A silicon single-photon avalanche diode (Si-SPAD) with a deep multiplication zone was fabricated using 0.11- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -CMOS technology. Deep n-well (DNW) implantation with three energy levels was performed to prepare a deep junction, and the highest breakdown voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {BR}}$ </tex-math></inline-formula> ) and lowest dark count rate (DCR) were obtained at the highest DNW implantation energy. In addition, devices with different structures were manufactured: a shallow junction, deep junction with mask DNW, deep junction with mask DNW without shallow trench isolation (STI), and deep junction with blank DNW. The blank DNW structure exhibited a lower edge breakdown risk and less deviation between samples compared to those of mask DNW structure in terms of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {BR}}$ </tex-math></inline-formula> and DCR. Moreover, a new edge breakdown risk was observed between DNW and p-well (PW) in the DNW blank structure, which could be alleviated by adjusting the PW sizes. According to the measurement results, the STI structure was preferable in terms of fill factor. Measurements were obtained from real fabricated test device structures, and technology computer-aided design (TCAD) for various factors associated with Si-SPADs with deep multiplication structures was performed.

Dynamic-quenching of a single-photon avalanche photodetector using an adaptive resistive switch

One of the most common approaches for quenching single-photon avalanche diodes is to use a passive resistor in series with it. A drawback of this approach has been the limited recovery speed of the single-photon avalanche diodes. High resistance is needed to quench the avalanche, leading to slower recharging of the single-photon avalanche diodes depletion capacitor. We address this issue by replacing a fixed quenching resistor with a bias-dependent adaptive resistive switch. Reversible generation of metallic conduction enables switching between low and high resistance states under unipolar bias. As an example, using a Pt/Al2O3/Ag resistor with a commercial silicon single-photon avalanche diodes, we demonstrate avalanche pulse widths as small as ~30 ns, 10× smaller than a passively quenched approach, thus significantly improving the single-photon avalanche diodes frequency response. The experimental results are consistent with a model where the adaptive resistor dynamically changes its resistance during discharging and recharging the single-photon avalanche diodes.

A Backside-Illuminated Charge-Focusing Silicon SPAD With Enhanced Near-Infrared Sensitivity

A backside-illuminated (BSI) near-infrared enhanced silicon single-photon avalanche diode (SPAD) for time-of-flight (ToF) light detection and ranging applications is presented. The detector contains a 2 $\mu$m wide multiplication region with a spherically-uniform electric field peak enforced by field-line crowding. A charge-focusing electric field extends into a 10 $\mu$m deep absorption volume, whereby electrons generated in all corners of the device can move efficiently towards the multiplication region. The SPAD is integrated with a customized 130 nm CMOS technology and a dedicated BSI process. The device has a pitch of 15 $\mu$m, which has the potential to be scaled down without significant performance loss. Furthermore, the detector achieves a photon detection efficiency of 27% at 905 nm, with an excess bias of 3.5 V that is controlled by integrated CMOS electronics, and a timing resolution of 240 ps. By virtue of these features, the device architecture is well-suited for large format ToF imaging arrays with integrated electronics.

Radiation-Induced Dark Counts for Silicon Single-Photon Detectors in Space

Single-photon detectors operating on satellites for use in a quantum communications network can incur large dark-count-rate increases from the natural radiation environment of space. Displacement damage to the material lattice of a detector from the ionizing radiation can result in a permanent dark-count increase in the detector. In this work, we analyze the radiation-induced dark-count rate of a silicon single-photon avalanche diode onboard a satellite at different orbiting altitudes, as well as, the additional radiation from a nuclear-disturbed environment caused by a high-altitude nuclear explosion. For detectors on low Earth orbit satellites, protons are the biggest source of radiation damage and are best mitigated by choosing an orbit that minimizes exposure when passing through the South Atlantic Anomaly and Polar Cusps. Detectors on medium Earth orbit and geostationary orbit satellites, if shielded by more than 10 mm of aluminum, provide the best platform in terms of the least amount of radiation damage to the detectors. In the event of a high-altitude nuclear explosion, the artificial radiation belts produced by the explosion will cause too much damage to silicon single-photon detectors on low Earth orbit satellites and render them unfit for quantum communications in less than a day. Higher orbit satellites will only suffer minor dark-count-rate increases from the artificial radiation belts.

Superconducting nanowire single-photon sensing of cerebral blood flow

.Significance: The ability of diffuse correlation spectroscopy (DCS) to measure cerebral blood flow (CBF) in humans is hindered by the low signal-to-noise ratio (SNR) of the method. This limits the high acquisition rates needed to resolve dynamic flow changes and to optimally filter out large pulsatile oscillations and prevents the use of large source-detector separations (), which are needed to achieve adequate brain sensitivity in most adult subjects.Aim: To substantially improve SNR, we have built a DCS device that operates at 1064 nm and uses superconducting nanowire single-photon detectors (SNSPD).Approach: We compared the performances of the SNSPD-DCS in humans with respect to a typical DCS system operating at 850 nm and using silicon single-photon avalanche diode detectors.Results: At a 25-mm separation, we detected times more photons and achieved an SNR gain of on the forehead of 11 subjects using the SNSPD-DCS as compared to typical DCS. At this separation, the SNSPD-DCS is able to detect a clean pulsatile flow signal at 20 Hz in all subjects. With the SNSPD-DCS, we also performed measurements at 35 mm, showing a lower scalp sensitivity of with respect to the scalp sensitivity at 25 mm for both the 850 and 1064 nm systems. Furthermore, we demonstrated blood flow responses to breath holding and hyperventilation tasks.Conclusions: While current commercial SNSPDs are expensive, bulky, and loud, they may allow for more robust measures of non-invasive cerebral perfusion in an intensive care setting.

Absolute calibration of a single-photon avalanche detector using a bright triggered single-photon source based on an InGaAs quantum dot.

We apply an InGaAs quantum dot based single-photon source for the absolute detection efficiency calibration of a silicon single-photon avalanche diode operating in Geiger mode. The single-photon source delivers up to (2.55 ± 0.02) × 106 photons per second inside a multimode fiber at the wavelength of 929.8 nm for above-band pulsed excitation with a repetition rate of 80 MHz. The purity of the single-photon emission, expressed by the value of the 2nd order correlation function g(2)(τ = 0), is between 0.14 and 0.24 depending on the excitation power applied to the quantum dot. The single-photon flux is sufficient to be measured with an analog low-noise reference detector, which is traceable to the national standard for optical radiant flux. The measured detection efficiency using the single-photon source remains constant within the measurement uncertainty for different photon fluxes. The corresponding weighted mean thus amounts to 0.3263 with a standard uncertainty of 0.0022.

Simultaneous multi-spectral, single-photon fluorescence imaging using a plasmonic colour filter array.

We present the first realisation of simultaneous multi-spectral fluorescence imaging using a single-photon avalanche diode (SPAD) array, where the spectral unmixing is facilitated by a plasmonic metasurface mosaic colour filter array (CFA). A 64 × 64 pixel format silicon SPAD array is used to record widefield fluorescence and brightfield data from four biological samples. A plasmonic metasurface composed of an arrangement of circular and elliptical nanoholes etched into an aluminium thin film deposited on a glass substrate provides the high transmission efficiency CFA, enabling a bespoke spectral unmixing algorithm to reconstruct high fidelity, full colour images from as few as ∼3 photons per pixel. This approach points the way toward real-time, single-photon sensitive multi-spectral fluorescence imaging. Furthermore, this is possible without additional bulky components such as a filter wheel, prism or diffraction grating, nor the need for multiple sample exposures or multiple detectors.

A Near-Infrared Enhanced Silicon Single-Photon Avalanche Diode With a Spherically Uniform Electric Field Peak

A near-infrared (NIR) enhanced silicon single-photon avalanche diode (SPAD) fabricated in a customized <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.13~\mu \text{m}$ </tex-math></inline-formula> CMOS technology is presented. The SPAD has a depleted absorption volume of approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$15\,\,\mu \text{m}\,\,\times 15\,\,\mu \text{m}\,\,\times 18\,\,\mu \text{m}$ </tex-math></inline-formula> . Electrons generated in the absorption region are efficiently transported by drift to a central active avalanche region with a diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2~\mu \text{m}$ </tex-math></inline-formula> . At the operating voltage, the active region contains a spherically uniform field peak, enabling the multiplication of electrons originating from all corners of the device. The advantages of the SPAD architecture include high NIR photon detection efficiency (PDE), drift-based transport, low afterpulsing, and compatibility with an integrated CMOS readout. A front-side illuminated device is fabricated and characterized. The SPAD has a PDE of 13% at wavelength 905 nm, an afterpulsing probability < 0.1% for a dead time of 13 ns, and a median dark count rate (DCR) of 840 Hz at room temperature. The device shows promising performance for time-of-flight applications that benefit from uniform NIR-sensitive SPAD arrays.

High detection efficiency silicon single-photon detector with a monolithic integrated circuit of active quenching and active reset.

Silicon single-photon detectors (SPDs) are key devices for detecting single photons in the visible wavelength range. Photon detection efficiency (PDE) is one of the most important parameters of silicon SPDs, and increasing PDE is highly required for many applications. Here, we present a practical approach to increase the PDE of silicon SPDs with a monolithic integrated circuit of active quenching and active reset (AQAR). The AQAR integrated circuit is specifically designed for thick silicon single-photon avalanche diodes (SPADs) with high breakdown voltage (250 V-450 V) and then fabricated via the process of high-voltage 0.35-μm bipolar-CMOS-DMOS. The AQAR integrated circuit implements the maximum transition voltage of ∼68 V with 30 ns quenching time and 10 ns reset time, which can easily boost PDE to the upper limit by regulating the excess bias up to a high enough level. By using the AQAR integrated circuit, we design and characterize two SPDs with the SPADs disassembled from commercial products of single-photon counting modules (SPCMs). Compared with the original SPCMs, the PDE values are increased from 68.3% to 73.7% and 69.5% to 75.1% at 785 nm, respectively, with moderate increases in dark count rate and afterpulse probability. Our approach can effectively improve the performance of the practical applications requiring silicon SPDs.

Counting Statistics of Actively Quenched SPADs Under Continuous Illumination

This work presents stochastic approaches to model the counting behavior of actively quenched single-photon avalanche diodes (SPADs) subjected to continuous-wave constant illumination. We present both analytical expressions and simulation algorithms predicting the distribution of the number of detections in a finite time window. We also present formulas for the mean detection rate. The approaches cover recovery time, afterpulsing, and twilight pulsing. We experimentally compare the theoretical predictions to measured data using commercially available silicon SPADs. Their total variation distances range from $10^{-5}$ to $10^{-2}$.

Total dose test with γ-ray for silicon single photon avalanche diodes**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFF0104801).

Gamma-ray (γ-ray) radiation for silicon single photon avalanche diodes (Si SPADs) is evaluated, with total dose of 100 krad(Si) and dose rate of 50 rad(Si)/s by using 60Co as the γ-ray radiation source. The breakdown voltage, photocurrent, and gain have no obvious change after the radiation. However, both the leakage current and dark count rate increase by about one order of magnitude above the values before the radiation. Temperature-dependent current–voltage measurement results indicate that the traps caused by radiation function as generation and recombination centers. Both leakage current and dark count rate can be almost recovered after annealing at 200 °C for about 2 hours, which verifies the radiation damage mechanics.