(Invited) Pseudo-Planar Ge-on-Si Avalanche Photodiodes in Geiger and Linear Modes for Short-Wave Infrared Detection
Authors: Kirdoda Jaroslaw , Mirza Muhammad , Smith Charlie , Dumas Derek , Coughlan Conor , McCarthy Charlie , Mowbray Hannah , Fleming Fiona , Yi Xin , Saalbach Lisa , Muir Dave , Modak Mrudul , Jin Xiao , Tian Qingyu , David John , Buller Gerald , Paul Douglas , Millar Ross 
Published: December 2024

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.