This talk shows the recent development of linear and Geiger-mode pseudo-planar Ge-on-Si avalanche photodiodes (APDs) in the short-wave infrared region. We demonstrate a 26 µm-diameter Ge-on-Si Geiger-mode APD with an extremely low noise-equivalent-power of 7.7 × 10−17 WHz−½ and a jitter value of 134 ± 10 ps at 1310 nm wavelength and at 100 K operating temperature. We demonstrate that a linear array of Ge-on-Si linear mode APDs comprising of 10 pixels shows high responsivity, highly uniform avalanche breakdown voltage and avalanche gain at 1550 nm wavelength and at room temperature.
KEYWORDS: Single photon avalanche diodes, Passivation, Design, Germanium, Silicon, Monte Carlo methods, Short wave infrared radiation, Ozone, Engineering, Diffusion
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.
Developing single photon avalanche diodes (SPADs) at short-wave infrared (SWIR) wavelengths beyond 1000 nm has attracted interest lately. Numerous quantum technology applications such as light detection and ranging (LIDAR), imaging through obscurants and quantum communications require sensitivity in this region. In quantum communications, operation at the telecoms wavelengths of 1310 nm and 1550 nm is essential. Ge-on-Si SPADs offer potential for lower afterpulsing and higher single photon detection efficiencies in the SWIR in comparison with InGaAs/InP SPADs, at a lower cost due to Si foundry compatibility. In this study, Ge-on-Si devices are fabricated on silicon-on-insulator (SOI) substrates, with a separate absorption, charge and multiplication layer (SACM) geometry and a lateral Si multiplication region. This Si foundry compatible process will allow for future integration with Si waveguides and optical fibres. The Ge is selectively grown inside sub-μm wide SiO2 trenches, reducing the threading dislocation in comparison with bulk Ge; a typical process for integrated Ge detectors. Here we deliberately exposed Ge sidewalls with an etch-back technique, to allow a passivation comparison not normally carried out in selectively grown devices planarised by chemical-mechanical polishing. Reduced dark currents are demonstrated using thermal GeO2 passivation in comparison to plasma-enhanced chemical-vapourdeposition SiO2. The improved passivation performance of GeO2 is verified by activation energy extraction and density of interface trap (Dit) calculations obtained from temperature-dependent capacitance-voltage (CV) and conductance-voltage (GV) measurements. This highlights the benefit of optimal surface passivation on sub-μm wide selectively grown Ge-on-SOI photodetector devices, potentially critical for waveguide integrated SPADs.
KEYWORDS: Single photon avalanche diodes, Electric fields, Monte Carlo methods, Short wave infrared radiation, Design and modelling, TCAD, Device simulation, Germanium, Silicon photonics
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.
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.
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.
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