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.
The performance of the photodetector is often the primary limiting factor affecting a free space communication or LiDAR system's sensitivity. Avalanche photodiodes (APDs) can be used to improve the signal to noise ratio (SNR) compared to conventional p-i-n photodiodes. Our study focuses on demonstrating an APD operating in the eye-safe short-wave infrared (SWIR) spectrum (>1400 nm) with high multiplication (M>1200) and low excess noise (F<7 at M=200) at room temperature. This device utilizes GaAsSb and Al0.85Ga0.15AsSb in a separate absorber, charge, and multiplication (SACM) configuration on an InP substrate. Notably, this device exhibits more than 40 times improvement in maximum achievable multiplication and 6.5 times lower excess noise at M=25 compared to commercially available InGaAs/InP devices.
Avalanche photodiodes (APDs) capable of operating at telecommunication wavelengths usually utilize an InGaAs absorber and a multiplication region of InP or InAlAs. Since the electron and hole ionization coefficients (α and β respectively) in these multiplication regions are very similar they suffer from high excess noise, limiting their sensitivity. In recent years, there have been a number of reports of Sb containing III-V semiconductor alloys that appear to show very low excess noise characteristics, similar to or better than that obtained in silicon. These reports show that AlInAsSb grown on GaSb appears to show a β/α ratio of ~0.015. Both AlAsSb and Al0.85Ga0.15As0.56Sb0.44 grown lattice matched on InP also show β/α values that vary from 0.005-0.01. The exception to this appears to be AlGaAsSb grown lattice matched on GaSb where a β/α ratio of ~2.5 has been seen. This paper reviews the published results in this area.
Avalanche photodiodes (APDs) are used in high-speed data communication and light detection and ranging (LIDAR) systems due to their high sensitivity and high speed. However, InAlAs and InP based APDs have relatively high excess noise because they have relatively similar electron and hole ionization coefficients (α and β respectively). Here, we report on an ultra-low excess noise material Al0.85Ga0.15As0.56Sb0.44 (hereafter AlGaAsSb) with a k value (β/α) of 0.01. The excess noise and multiplication measurements were performed on both random alloy (RA) p+-i-n+ and digital alloy (DA) grown p+-i-n+ diodes with depletion regions of 1020nm and 890nm respectively. The excess noise was found to be broadly similar in both RA and DA grown structures.
A series of AlAsSb p+-i-n+ and n+-i-p+ diodes with varying i-region thickness from 0.08μm to 1.55μm have been used to determine the temperature dependent impact ionization coefficients by performing avalanche multiplication measurements from 210K to 335K. The increase in electron and hole ionization coefficients as the temperature decreases is much smaller when compared to InAlAs and InP. This leads to a much smaller avalanche breakdown variation of 13mV/K in a 1.55μm p+- i-n+ diode. For a 10Gb/s InGaAs/AlAsSb separate absorption and multiplication avalanche photodiode (SAM-APD), the variation in breakdown voltage is predicted to be only 15.58 mV/K.
In this work we report on the simulation of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs). We simulate an etched mesa structure (substrate emission) with the substrate acting as the current spreading layer. The effect of contact misalignment on the carrier distribution within the active element is explored and confirms the validity of the model in describing the carrier distribution. We go on to discuss the effects of the substrate thickness and trench depth on the intensity profile. Simulation results show that a thicker substrate and a trench partially etched into the substrate may improve the intensity profile in future devices.
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