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.
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.
We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centred at 3.4 and 4.9 THz. . Different strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design employs a vertical optical transition and the observed spectral features are well described by non-equilibrium Green’s function calculations. We observe two emission peaks that are due to a non-selective injection in the upper state of the radiative transition. Comparison with similar III-V emitters is used to deduce radiative efficiencies. We will present new results from 4 quantum well Ge/SiGe emitters based on diagonal transitions in real space.
A terahertz intersubband emitter based on silicon is presented. The emission originates from n-type Ge/SiGe quantum cascade structures. We designed a strain-compensated single quantum active region based on a vertical optical transition and tensile-strained Si0.15Ge0.85 barriers. The 51 quantum cascade periods (corresponding to 4.2 μm) were grown on a Si1-xGex reverse graded virtual substrate on Ge/Si(001) substrates. Deeply etched diffraction gratings were processed and the surface emitting devices were characterized at 5 K with a Fourier transform infrared spectrometer. We observed two distinct peaks at 3.4 and 4.9 THz with a line broadening of 20%. This is an important step towards the realization of an Ge/SiGe THz quantum cascade laser.
We report electroluminescence at 14meV and 20meV from a n-type Ge/Si0.15Ge0.85 quantum cascade heterostructure on Si substrate grown by ultra-high vacuum chemical vapour deposition. The electroluminescence signal of the single quantum well active region design, extracted through diffraction gratings from mesa structures, is compared with its GaAs counterpart.The spectral features agree well with modeling based on Non-equilibrium Green's function calculations. The observed electroluminescence peaks show a full width at half maximum of 3meV and 4meV. These results are an important step towards the realization of an n-type THz quantum cascade laser on a non-polar material system.
We present innovative planar geometry Ge-on-Si single-photon avalanche diode (SPAD) detectors. These devices provide picosecond timing resolution for applications operating in the short-wave infrared wavelength region such as quantum communication technologies and three-dimensional imaging. This new planar design successfully reduces the undesirable contribution of surface defects to the dark current. This has allowed for the use of large excess biases, resulting in a single-photon detection efficiency of 38% when operated at 125 K using 1310 nm wavelength illumination. A record low noise equivalent power of 2 × 10-16 WHz-1/2 was achieved, more than a fifty-fold improvement compared to the previous best Ge-on-Si mesa geometry SPADs when operated under similar conditions. These Ge-on-Si SPAD detectors have operated in the range of 77 K to 175 K, and we will discuss ways in which the operating temperature can be raised to that consistent with Peltier cooling. We will present analysis of Ge-on-Si SPADs, which has revealed much reduced afterpulsing compared with SPAD detectors in other material systems. Laboratory trials have demonstrated these Ge-on-Si SPAD devices in short-range LIDAR and depth profiling measurements. Estimations of the performance of these detectors in longer range measurements will be presented. We will discuss the potential for the development of high efficiency arrays of Ge-on-Si SPADs for the use in eye-safe automotive LIDAR and quantum technology applications.
KEYWORDS: Silicon, Germanium, Sensors, Waveguides, Single photon, 3D image enhancement, Quantum key distribution, LIDAR, 3D image processing, Quantum information processing
Single photon avalanche detectors (SPADs) operating in gated-Geiger mode at near infrared wavelengths have applications in quantum key distribution (QKD), eye-safe light detection and ranging (LIDAR), 3D image sensing, quantum enhanced imaging and photonic based quantum information processing. Whilst InGaAs SPADs are commercially available, the high cost and lack of integrated SPADs limit the applications. We have previously demonstrated vertical Geiger mode Ge on Si SPADs at 1310 and 1550 nm operating at 100 K where the Ge is used as an absorber and the lower noise Si is used as the avalanche gain region. At 100 K and 1310 nm a single photon detection efficiency of 4% was demonstrated with a dark count rate (DCR) of 5 MHz.
Here we present first results on Ge on Si SPADs grown on top of silicon-on-insulator (SOI) substrates. Both vertical photodetectors and waveguide coupled detectors were investigated with designs aimed to reduce the DCR over previous results. Waveguides and avalanche regions were patterned in the top Si of a SOI wafer before being coated with silicon dioxide. Holes were then etched in the oxide to allow selective area growth of Ge inside these windows and on top of the Si waveguides for the waveguide coupled Ge SPADs. This approach reduces the threading dislocation density compared to bulk Ge growths which aids the reduction of the DCR. The fabricated devices have been tested at both 1310 nm and 1550 nm wavelengths and demonstrate improved performance over previous published results.
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