There is significant interest in short-wave infrared (SWIR) avalanche photodiodes (APDs) for imaging and optical communications. SWIR detection enables long-range, eye-safe LIDAR with improved transmission through obscurants and also compatibility with telecoms networks operating around lambda=1550 nm. Si-based platforms are cost-effective and integrate with CMOS electronics but are limited to wavelengths < 1 mu m. Ge absorber layers extend detection to SWIR wavelengths but typical mesa-based designs typically have edge effects that limit available gain. Here we demonstrate the benefits of a pseudo-planar design that mitigates these effects and enables high gains of 101 to be demonstrated in surface normal APDs with low excess noise factors of 3.1 at a gain of 20; a record for Ge-on-Si devices. Devices have a unitygain responsivity of 0.41 A/W at lambda=1550 nm wavelength.

Short-wave infrared (SWIR) light offers several advantages over visible light, including high eye-safety threshold, reduced atmospheric transmission attenuation, and lower solar background interference. These properties make SWIR light valuable in various applications, such as in optical communications, imaging technologies and automotive industry. Avalanche photodiodes (APDs) are particularly suited for detecting low levels of light due to their internal avalanche gain. Silicon (Si) is favoured as an avalanche material because of its low excess avalanche noise, while germanium (Ge) is an excellent candidate for detecting SWIR light. The use of cost-effective CMOS process technology and the potential for integration with existing Si photonics further enhance the appeal of this material system. Our previous work demonstrated a 50 mu m-diameter Ge-on-Si linear-mode APD with high avalanche gain and low excess noise at 1550 nm wavelength and at room temperature. However, a high dark current still remains one of the main challenges. In this paper, we will present the ongoing development of SWIR Ge-on-Si APDs, focusing on reducing the device dark current by reducing device active area and extending device operating wavelength by employing GeSn as photon absorption region. A 10 mu m-diameter Ge-on-Si APD shows 8 times dark current reduction at room temperature c.f. the 50 mu m-diameter Ge-on-Si APD. Furthermore, we designed and fabricated GeSn-on-Si photodiodes with a GeSn thickness of 1 mu m to investigate device spectrum response at different temperatures.

Germanium‐containing short‐wave infrared (SWIR) avalanche photodiode (APD) arrays on silicon platforms have the potential for monolithic integration into complementary metal‐oxide‐semiconductor (CMOS) integrated circuits, making them mass‐manufacturable, high‐performance, arrayed optical detectors operating at wavelengths beyond the silicon cut‐off wavelength. Here, the first high‐performance, surface‐illuminated, 10‐pixel linear array of pseudoplanar geometry germanium‐on‐silicon (Ge‐on‐Si) APDs operating at 1550 nm wavelength and at temperatures up to 378 K are demonstrated. At room temperature, the dark current, avalanche gain, responsivity, and avalanche breakdown of the devices show good uniformity. Array A exhibits a mean dark current density of 198 ± 62 mA cm −2 at 90% of the breakdown voltage. The excess noise factor is less than half that of InP‐based SWIR APD arrays, which allows Ge‐on‐Si devices to operate at a higher avalanche gain. A responsivity of 8.2 A W −1 at a gain of 20 and excess noise of 3.3 is achieved when illuminated with 1550 nm wavelength light. The detector array also demonstrates stable performance at 378 K with a maximum avalanche gain of 24. This device architecture will be applicable for the design of large‐scale APD arrays on Si platforms for SWIR detection which can be used in imaging, sensing, and optical communication applications.

Simulations of single photon avalanche diodes (SPADs) based on the Ge-on-Si material platform are presented, highlighting the potential performance enhancement achievable with nano-hole array photonic crystal structures. Such structures can be used to enhance photon absorption and therefore increase single photon detection efficiencies (SPDE). However, there is yet to be a study of these structures in application to Ge-on-Si SPADs to determine if the optical enhancements can be realized as SPDE or to evaluate the change in dark count rate due to the nano-holes that form the photonic crystal. This work establishes an optimization and analysis platform for investigating photonic crystal structures on SPAD devices. Both a direct Ge etch method, and an etched amorphous Si design are compared to a reference device with an optimized anti-reflection coating. Finite difference time domain simulations were used to optimize the photonic crystal parameters for these structures, finding a potential absorption of up to 37.09 % at wavelengths of 1550 nm for a 1 µm absorption layer, compared to 11.33 % for the reference device. Subsequently, TCAD simulations and custom code were used to calculate the effective enhancement to SPAD performance metrics, as a function of material and passivation quality, showing up to 2.41x higher SPDE and 2.57x better noise-equivalent power is achievable provided etched surfaces are sufficiently well passivated.

Single Photon Avalanche Diodes (SPADs) are semiconductor devices capable of accurately timing the arrival of single photons of light. Previously, we have demonstrated a pseudo-planar Ge-on-Si SPAD that operates in the short-wave infrared, which can be compatible with Si foundry processing. Here, we investigate the pseudo-planar design with simulation and experiment to establish the spatial contributions to the dark-count rate, which will ultimately facilitate optimisation towards operation at temperatures compatible with Peltier cooler technologies.

Germanium-on-Silicon (Ge-on-Si) avalanche photodiodes (APDs) are of considerable interest as low intensity light detectors for emerging applications.The Ge absorption layer detects light at wavelengths up to ≈ 1600 nm with the Si acting as an avalanche medium, providing high gain with low excess avalanche noise.Such APDs are typically used in waveguide configurations as growing a sufficiently thick Ge absorbing layer is challenging.Here, we report on a new vertically illuminated pseudo-planar Ge-on-Si APD design utilizing a 2 µm thick Ge absorber and a 1.4 µm thick Si multiplication region.At a wavelength of 1550 nm, 50 µm diameter devices show a responsivity of 0.41 A/W at unity gain, a maximum avalanche gain of 101 and an excess noise factor of 3.1 at a gain of 20.This excess noise factor represents a record low noise for all configurations of Ge-on-Si APDs.These APDs can be inexpensively manufactured and have potential integration in silicon photonic platforms allowing use in a variety of applications requiring high-sensitivity detectors at wavelengths around 1550 nm.

Germanium-on-silicon (Ge-on-Si) single photon avalanche diodes (SPADs) operating in the short-wave infrared (SWIR) have various applications such as long-range eye-safe LIDAR, quantum imaging, and quantum key distribution. These SPADs offer compatibility with Si foundries and potential cost advantages over existing InGaAs/InP devices. However, cooling is necessary to reduce dark-count rates (DCR), which limits photon absorption at 1550 nm wavelength. To address this, we propose integrating a photonic crystal (PC) nano-hole array structure on the Ge absorber layer. While this technique has shown enhanced responsivity in linear Ge detectors, its potential in Ge-on-Si SPADs remains unexplored. Our simulations consider temperature dependence and the impact of electric-field hot-spots on dark count rates. Through these simulations, we have identified means of enhancing single-photon detection efficiency (SPDE) without adversely affecting DCR. We predict significant improvements in performance, including at least a 2.5x enhancement in absorption efficiency.

Single photon avalanche diodes (SPADs) are a key underpinning technology to many existing and emerging applications, including LIDAR for 3D imaging as well as quantum imaging, quantum encryption and quantum information applications. There is a growing demand for low-cost LIDAR systems for autonomous vehicles, particularly in the short-wave infrared (SWIR) spectral range, which enables long-range measurements whilst complying with eye-safety regulations and offers enhanced transmission through atmospheric obscurants like smoke and haze compared to systems operating in the near-infrared. Furthermore, for quantum-key distribution, single photons must be measured at telecoms wavelengths for compatibility with optical fibre networks. Ge-on-Si SPADs offer significant potential for low-cost SWIR single photon detection, with the ability to meet the price points required for these large emerging markets thanks to Si foundry compatibility. This contrasts with state-of-the-art SWIR SPADs based on InGaAs/InP, which are not only expensive but suffer from the effects of afterpulsing. Here, we present an overview of our work on the design, fabrication and characterisation of pseudo-planar Ge-on-Si detectors, with both operation in the Geiger mode (i.e. SPADs), as well as linear mode for avalanche photodiodes (APDs). The pseudo-planar SPAD design resulted in a step-change in performance compared to mesa-based SPADs, leading to high single-photon detection efficiencies (SPDEs) of 38 % at 1310 nm wavelength, and ultimately low noise equivalent powers of 7.7x10-17 W/Hz0.5 in 26 µm diameter pixels at 125 K, with single photon detection demonstrated up to 165 K. This was achieved by local ion-implantation of a charge-sheet layer, used to mediate the electric field between the Si avalanche layer and the Ge absorber, which in conjunction with a local p+Ge etched contact layer enabled a reduced E-field at etched sidewalls. These devices demonstrated over 100 X improvement in NEP compared to the most comparable Ge-on-Si mesa devices, and showed reduced afterpulsing compared to commercial InGaAs/InP devices when run in nominally identical operating conditions. In order to further optimise the technology, and gain insight into the device dynamics, we have simulated SPADs using TCAD process and device simulators, and developed custom-code to solve triggering probabilities using McIntyre’s model to understand the sensitivities of the detectors to the device design. Simulations are compared to experimental DCR measurements to probe sensitivities to surface passivation and geometry scaling and reveal that surfaces do not appear to be the limiting factor on performance, therefore validating the pseudo-planar architecture. Furthermore, with these simulation techniques, we have investigated potential enhancements achievable by the inclusion of an etched photonic crystal nano-hole array, which is known to enhance absorption and will therefore enhance SPDE. Here, we demonstrate the design trade-offs between enhanced SPDE performance, and the degradation of DCR that can be induced by etching surfaces into the device active area. Finally, we present recent results on surface normal Ge-on-Si APDs using the pseudo-planar architecture in devices with 2 µm thick Ge absorbers. Here, the benefits of the device architecture are demonstrated to be applicable to high performance in the linear mode. Devices are measured at room-temperature, and demonstrate ~0.40 A/W responsivity at unity gain at 1550 nm wavelength, with a maximum avalanche gain of ~100, and excess noise of 3.1 at a gain of 20; to our knowledge a record for any Ge-on-Si APD.

Single photon avalanche diodes (SPADs) are semiconductor photodiode detectors capable of detecting individual photons, typically with sub-ns precision timing. We have previously demonstrated novel pseudo-planar germanium-on-silicon SPADs with absorption into the short-wave infrared, which promise lower costs and potentially easier CMOS integration compared to III-V SPADs. Here we have simulated the dark count rate of these devices, using a custom solver for McIntyre's avalanche model and a trap assisted tunnelling generation model. Calibration and fitting have been performed using experimental data and the results have highlighted areas in which the technology can be optimised.

In this letter, we investigate afterpulsing in 26 and $100~\mu \text{m}$ diameter planar geometry Ge-on-Si single-photon avalanche diode (SPAD) detectors, by use of the double detector gating method with a gate width of 50 ns. Ge-on-Si SPADs were found to exhibit a 1% afterpulsing probability at a delay time of $200~\mu \text{s}$ and temperature of 78 K, and $130~\mu \text{s}$ at a temperature of 150 K. These delay times were measured with an excess bias of 3.5% applied, which corresponded to a single-photon detection efficiency of 15% at $1.31~\mu \text{m}$ . We demonstrate that reducing the detector diameter can also be an effective way to restrict afterpulsing in this material system.

Ge-on-SOI (silicon-on-insulator) single photon avalanche diodes (SPADs) have been fabricated with exposed sidewalls allowing variation of passivation techniques. Reduced dark currents and density of surface states are demonstrated with thermal oxide passivation, demonstrating the benefit of optimal passivation of low aspect ratio selectively grown Ge.

Semiconductor based single-photon avalanche diode (SPAD) detectors are widely used in quantum technology applications, which focus on the arrival time of single photons. Using germanium as the absorption region in a Separate Absorption and Multiplication design solves the operating limitation beyond the spectrum range of silicon, i.e. typically at a wavelength of ~ 1000 nm. Our first-generation planar geometry Ge-on-Si single-photon avalanche diodes utilised a 1000 nm Germanium absorption region and showed extremely low noise-equivalent-power of 7.7 × 10−17 WHz−½ at a wavelength of 1310 nm. We demonstrate new structures designed to achieve high single-photon detection efficiency at a wavelength of 1550 nm.

Germanium-on-silicon (Ge-on-Si) based single-photon avalanche diodes (SPADs) have recently emerged as a promising detector candidate for ultra-sensitive and picosecond resolution timing measurement of short-wave infrared (SWIR) photons. Many applications benefit from operating in the SWIR spectral range, such as long distance light detection and ranging, however, there are few single-photon detectors exhibiting the high-performance levels obtained by all-silicon SPADs commonly used for single-photon detection at wavelengths <1 µ m. This paper first details the advantages of operating at SWIR wavelengths, the current technologies, and associated issues, and describes the potential of Ge-on-Si SPADs as a single-photon detector technology for this wavelength region. The working principles, fabrication and characterisation processes of such devices are subsequently detailed. We review the research in these single-photon detectors and detail the state-of-the-art performance. Finally, the challenges and future opportunities offered by Ge-on-Si SPAD detectors are discussed.

Single-Photon Avalanche Diode (SPAD) detectors are of significant interest for a range of applications [1] , in particular for quantum technologies (e.g. quantum-key distribution, quantum information processing), and light detection and ranging (LIDAR) for defence, terrain mapping, and autonomous vehicles. These applications either require, or benefit from, operation at wavelengths in the short-wave infrared (SWIR). Previous SWIR single-photon LIDAR has typically used InGaAs/InP SPAD detector technology, which has relatively low efficiency and suffers from afterpulsing. Previously, a pseudo-planar design for a Ge-on-Si SPAD was demonstrated [2] , yielding a huge improvement in performance for Ge-on-Si SPADs at 1310 nm and demonstrating the potential for Si foundry compatible SWIR SPADs. Furthermore, reduced afterpulsing was demonstrated compared to a commercial InGaAs/InP device when measured in nominally identical conditions. Here we present a further step change in performance, with reduced dark count rate (DCR), record low noise-equivalent-power (NEP) and low jitter by scaling the technology and developing 26 µm diameter pixels [3] .

Mid-infrared spectroscopy in the fingerprint region of the electromagnetic spectrum is demonstrated using a Ge on Si photonic platform technology. Components that can be integrated into complete lab-on-a-chip sensors for spectroscopic identification of healthcare and security analytes is presented capable of parts per billion sensitivity.

We present a pseudo-planar geometry 26µm diameter Ge-on-Si single-photon avalanche diode (SPAD) detector with temperature insensitive single photon detection efficiency of 29.4% at 1310nm wavelength for applications including free-space LIDAR. A record low dark count rate of 104 counts/s at 125K at an excess bias of 6.6% is demonstrated, with temporal jitter reaching 134ps. The noise-equivalent power is measured to be 7.7x10-17WHz-12 which is a 2 orders of magnitude reduction when compared to comparable 25µm mesa devices. This device represents the state-of-the-art for Ge-on-Si SPADs, and highlights that these Si foundry compatible devices have enormous potential for SWIR single-photon applications.

In the last decade, silicon photonics has undergone an impressive development driven by an increasing number of technological applications. Plasmonics has not yet made its way to the microelectronic industry, mostly because of the lack of compatibility of typical plasmonic materials with foundry processes. In this framework, we have developed a plasmonic platform based on heavily n-doped Ge grown on silicon substrates. We developed growth protocols to reach n-doping levels exceeding 1020 cm-3, allowing us to tune the plasma wavelength of Ge in the 3-15 μm range. The plasmonic resonances of Ge-on-Si nanoantennas have been predicted by simulations, confirmed by experimental spectra and exploited for molecular sensing. Our work represents a benchmark for group-IV mid-IR plasmonics.

Single-photon avalanche diode (SPAD) detectors are of significant interest for numerous applications, including light detection and ranging (LIDAR), and quantum technologies such as quantum-key distribution and quantum information processing. Here we present a record low noise-equivalent-power (NEP) for Ge-on-Si SPADs using a pseudo-planar design, showing high detection efficiency in the short-wave infrared; a spectral region which is key for quantum technologies and hugely beneficial for LIDAR. These devices can leverage the benefits of Si avalanche layers, with lower afterpulsing compared to InGaAs/InP, and reduced cost due to Si foundry compatibility. By scaling the SPAD pixels down to 26μm diameter, a step change in performance has been demonstrated, with significantly reduced dark count rates (DCRs), and low jitter (134ps). Ge-on-Si SPADs were fabricated using photolithography techniques and characterised using time-correlated single-photon counting. The DCR reaches as low as kilocount/s at 100K for excess bias up to ~5%. This reduction in DCR enables higher temperature operation; e.g. the DCR of a 26μm diameter pixel at 150 K is approximately equivalent to a 100 μm diameter pixel at 77 K (100s of kilocounts/s). These low values of DCR, coupled with the relatively temperature independent single photon detection efficiencies (SPDE) of ~29% (at 1310nm wavelength) leads to a record low NEP of 7.7×10−17WHz−1/2. This is approximately 2 orders of magnitude lower than previous similarly sized mesa-geometry Ge-on-Si SPADs. This technology can potentially offer a lowcost, Si foundry compatible SPAD operating at short-wave infrared wavelengths, with potential applications in quantum technologies and autonomous vehicle LIDAR.