Results from the development of substrate illuminated planar Ge on Si Single Photon Avalanche Diodes (SPAD) imaging arrays will be presented operating at short wave infrared wavelengths. Simulations have been used to optimize the designs aiming to reduce dark count rates and increase the number of absorbed photons aiming for Pelter cooler operation whilst also minimizing cross talk. To date the highest performance of Ge on Si SPADs has been demonstrated at 125 K with 38% single photon detection efficiencies and a noise equivalent power of 8e-17 W/√Hz. Surface illuminated devices have demonstrated single photon detection efficiencies up to 38% for 1 μm thick Ge absorbers and the present work will present results from 2 μm and 3 μm thick Ge absorbers aiming to increase the absorption of incident photons. The paper will describe the compromises between absorbing more photons compared to dark count rates and jitter. Examples of single photon LiDAR applications at 1310 to 1550 nm will be presented and the performance from Ge on Si SPADs will be compared to InGaAs SPAD technology in terms of single photon detection efficiency, dark count rates, afterpulsing, jitter and operating temperatures. Afterpulsing measurements demonstrate significant reductions compared to InGaAs SPADs operated under nominally identical conditions by a factor of 5 to 10. The performance of the surface illuminated SPADs in linear mode as avalanche photodetectors will also be presented. Operation at 1550 nm wavelengths at room temperature has demonstrated responsivities at unity gain of 0.41 A/W, maximum avalanche gain of 101 and an excess noise factor of 3.1 at a gain of 20 for 50 μm diameter photodetectors.
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.
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