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.

The rising concentration of greenhouse gases, especially methane and carbon dioxide, is driving global temperature increases and exacerbating the climate crisis. Monitoring these gases requires detectors that operate in the extended short-wavelength infrared range (similar to 2.4 mu m), covering methane (1.65 mu m) and carbon dioxide (2.05 mu m) wavelengths. Here, we present a high-performance linear mode avalanche photodetector (APD) with an InGaAs/GaAsSb type-II superlattice absorber and an AlGaAsSb multiplier, matched to InP substrates. This APD achieves a room temperature gain of 178, an external quantum efficiency of 3560% at 2 mu m, low excess noise (less than 2 at gains below 20), and a small temperature coefficient of breakdown (7.58 mV/K center dot mu m). These results indicate that a manufacturable semiconductor material-based APD could significantly advance high-sensitivity receivers for greenhouse gas monitoring, potentially enabling their commercial production and widespread use.

Extended shortwave infrared (eSWIR) detectors capable of detecting wavelengths between 1.7 and 2.7 mu m are useful for a wide range of applications, such as remote sensing and monitoring, but most of these detectors require cooling to reduce the dark currents. Identifying a suitable material that extends the wavelength range to well beyond 2 mu m with minimal cooling is therefore important. The overall sensitivity of such a detector can be enhanced by using it in conjunction with a wide bandgap multiplication region which can increase the photocurrent via impact ionization. In this work, a systematic study of avalanche multiplication in seven Al( 0.9 )Ga( 0.1 )A(s 0.08) Sb( 0.92 )diodes lattice matched to GaSb shows that the electron impact ionization coefficient (alpha) is larger than the hole impact ionization coefficient (beta), especially at low electric fields. Using In (0.22) Ga- 0.78 A(s 0.19 )Sb( 0.89) (bandgap = 0.45 eV) as the absorber and Al 0.9 Ga 0.1 As 0.08 Sb- 0.92 (bandgap = 1.6 eV) as the multiplier in a separate absorption, charge, and multiplication region avalanche photodiode configuration enabled room temperature optical detection up to 2.75 mu m with a peak external quantum efficiency (EQE) of > 50% at the punch-through voltage (V-pt) similar to 2 mu m wavelength. This device demonstrates a low excess noise of F = 4.5 at a multiplication of M = 20, giving rise to a noise equivalent power for an unoptimized device of 1.69 x 10(-12) W/root Hz. A maximum multiplied EQE of > 2000% at 2 mu m is achieved before a low breakdown voltage of 18.9 V, obtained using a novel undepleted absorber design. This work shows the possibility of a high sensitivity eSWIR detector capable of operating at room temperature.

Germanium-on-Silicon (Ge-on-Si) avalanche photodiodes (APDs) are of considerable interest as low intensity light detectors for emerging applications.The Ge absorption layer detects light at wavelengths up to ≈ 1600 nm with the Si acting as an avalanche medium, providing high gain with low excess avalanche noise.Such APDs are typically used in waveguide configurations as growing a sufficiently thick Ge absorbing layer is challenging.Here, we report on a new vertically illuminated pseudo-planar Ge-on-Si APD design utilizing a 2 µm thick Ge absorber and a 1.4 µm thick Si multiplication region.At a wavelength of 1550 nm, 50 µm diameter devices show a responsivity of 0.41 A/W at unity gain, a maximum avalanche gain of 101 and an excess noise factor of 3.1 at a gain of 20.This excess noise factor represents a record low noise for all configurations of Ge-on-Si APDs.These APDs can be inexpensively manufactured and have potential integration in silicon photonic platforms allowing use in a variety of applications requiring high-sensitivity detectors at wavelengths around 1550 nm.

The presence of large bismuth (Bi) atoms has been shown to increase the spin-orbit splitting energy in bulk GaAsBi, reducing the hole ionization coefficient (beta) and thereby reducing the excess noise seen in avalanche photodiodes. In this study, we show that even very thin layers of GaAsBi introduced as quantum wells (QWs) in a GaAs matrix exhibit a significant reduction of beta while leaving the electron ionization coefficient, alpha, largely unchanged. The optical and avalanche multiplication properties of a series of GaAsBi/GaAs multiple quantum well (MQW) p-i-n structures with nominally 5 nm thick, 4.4% Bi GaAsBi QWs, varying from 5 to 63 periods and corresponding barrier widths of 101 to 4 nm were investigated. From photoluminescence, omega-2 theta X-ray diffraction, and cross section transmission electron microscopy measurements, the material was found to be of high quality despite the strain introduced by the Bi in all except the samples with 54 and 63 QW periods. Photomultiplication measurements undertaken with different wavelengths showed that alpha in these MQW structures did not change appreciably with the number of QWs; however, beta decreased significantly, especially at lower values, the noise factor, F, is reduced by 58% to 3.5 at a multiplication of 10, compared to a similar thickness bulk GaAs structure without any Bi. This result suggests that Bi-containing QWs could be introduced into the avalanching regions of APDs as a way of reducing their excess noise.

The application of an electric field to a semiconductor can alter its absorption properties. This electroabsorption effect can have a significant impact on the quantum efficiency of detector structures. The photocurrents in bulk InGaAs and GaAsSb p-i-n photodiodes with intrinsic absorber layer thicknesses ranging from 1 to 4.8 mu m have been investigated. By using phase-sensitive photocurrent measurements as a function of wavelength, the absorption coefficients as low as 1 cm(-1) were extracted for electric fields up to 200 kV/cm. Our findings show that while the absorption coefficients reduce between 1500 and 1650 nm for both materials when subject to an increasing electric field, an absorption coefficient of 100 cm(-1) can be obtained at a wavelength of 2 mu m, well beyond the bandgap energy when they are subject to a high electric field. The results are shown to be in good agreement with theoretical models that use Airy functions to solve the absorption coefficients in a uniform electric field.

The avalanche multiplication and noise characteristics of Al0.55Ga0.45As0.56Sb0.44p–i–n and n–i–p structures grown lattice matched on InP have been investigated. From measurements undertaken using 530 nm illumination on several devices, the electron (α) and hole (β) impact ionization coefficients have been determined. While α only shows a relatively small increase compared to the higher Al composition alloys of AlxGa1−xAsSb, β is found to increase significantly. Although the β/α ratio is increased to ∼0.125–0.2, higher than the ∼0.003–0.02 seen in the higher-Al alloys, a relatively low excess noise factor of 2.2 was measured in the p-i-n with electron-initiated multiplication of 20. This noise performance is significantly lower than that predicted using a local-field model and comparable to some commercial silicon APDs. This avalanching material with a bandgap of ∼1.24 eV will have the advantages of a smaller band discontinuity with the absorber region and should also operate at a lower voltage.

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.

This paper reports on laser excitation power dependent photoluminescence (PL) studies on epitaxial GaAs1−xBix (2.3% < x < 10.4%) layers with thicknesses of 30–40 nm which are compressively strained onto GaAs substrates. Such materials when used as optical active regions in semiconductor lasers offer the possibility of suppressing the efficiency-limiting Auger recombination losses and improving laser performance in the telecommunication range (1.3–1.5 μm). These experimental investigations on GaAsBi allow us to verify the extent to which GaAs1−xBix provides the optimised band structure as predicted, and secondly to provide the first evidence of the influence of this band structure on optical efficiency and carrier recombination processes. An analysis of the dependence of PL intensity on excitation power was employed to identify the recombination mechanisms in GaAs1−xBix alloys. Temperature tuning the samples with bismuth concentration ~ 8.5% and 1.4% provides tentative evidence for the suppression of Auger recombination losses in this material system highlighting its potential for efficient telecoms laser applications.

Low noise avalanche photodiodes (APDs) detecting 1550 nm wavelength play a crucial role in optical communication and LiDAR systems. These APDs utilize a separate absorption, charge, and multiplication (SACM) architecture with an absorber for 1400–1650 nm detection and a low noise, high gain multiplier that can be independently optimized for a high signal-to-noise ratio. Recently, GaAs0.5Sb0.5/Al0.85Ga0.15As0.56Sb0.44 SACM APDs have demonstrated ultra-high gain and extremely low noise, possibly improving sensitivity over Si and InGaAs/InP commercial APDs. This accomplishment was achieved using a GaAsSb absorber instead of a conventional InGaAs absorber, mitigating band discontinuities between the absorber and the multiplier. However, further optimization is required to reduce noise due to tunneling and impact ionization from the GaAsSb absorber, which occurs at a high electric field region. This paper focuses on the study of the high-field characteristics of GaAsSb photodiodes (PDs). The tunneling phenomenon is analyzed through current density-voltage measurements, and the impact ionization behavior is evaluated by measuring the multiplication of p-i-n GaAsSb PDs. The result suggests that when designing a SACM APD with a GaAsSb absorber, the electric field in the absorber can be increased to 175 kV/cm without the detrimental effects of ionization occurring in the absorber. The findings from this investigation will assist in optimizing GaAsSb-based SACM APDs and promoting further advancements in the 1550 nm APD technology.

A series of GaAsBi/GaAs multiple quantum well p-i-n diodes was grown using molecular beam epitaxy and the opto-electrical characterisations are presented. The result shows that devices experience low carrier extractions when light is absorbed due to hole trapping in the valence band. Carrier enhancement can be achieved by applying slight reverse bias when the measurement was taken. The absorption coefficient of the devices is confirmed to be similar with other Bi-based work. GaAsBi/GaAs multiple quantum well do have a lot of room for improvement especially on growth, structure and strain level of the material. If these components can be catered, GaAsBi can be a competitive alternative for 1 eV junction in multiple junction solar cells.

High sensitivity avalanche photodiodes (APDs) operating at eye-safe infrared wavelengths (1400–1650 nm) are essential components in many communications and sensing systems. We report the demonstration of a room temperature, ultrahigh gain ( M = 278 , λ = 1550 n m , V = 69.5 V , T = 296 K ) linear mode APD on an InP substrate using a G a A s 0.5 S b 0.5 / A l 0.85 G a 0.15 A s 0.56 S b 0.44 separate absorption, charge, and multiplication (SACM) heterostructure. This represents ∼ 10 × gain improvement ( M = 278 ) over commercial, state-of-the-art InGaAs/InP-based APDs ( M ∼ 30 ) operating at 1550 nm. The excess noise factor is extremely low ( F < 3 ) at M = 70 , which is even lower than Si APDs. This design gives a quantum efficiency of 5935.3% at maximum gain. This SACM APD also shows an extremely low temperature breakdown sensitivity ( C b d ) of ∼ 11.83 m V / K , which is ∼ 10 × lower than equivalent InGaAs/InP commercial APDs. These major improvements in APD performance are likely to lead to their wide adoption in many photon-starved applications.

Lattice-mismatched InGaAs has appeared to be emerging semiconductor materials for sensors and photovoltaic applications. The absorption coefficients of the materials are crucial in designing high-performance semi-conductor devices. Nevertheless, the absorption coefficient of lattice-mismatched InGaAs were not comprehen-sively studied to cater for the 2000-3000 nm applications. This study aims to determine the absorption coefficients of lattice-mismatched In0.73Ga0.27As and In0.83Ga0.17As semiconductor materials through photo -current measurement which enables the absorption tail information to be extracted. In addition, this work demonstrates the incorporation of an innovative artificial intelligence-based method in solving the absorption coefficient of lattice-mismatched InGaAs, considering the detailed information of the structure design and ma-terial parameters. By selecting the best gene for the next iteration, the utilization of Genetic Algorithm has significantly reduced the number of iterations from a maximum of 10 000 to 300. Validation of the algorithm was conducted, showing a good agreement of absorption coefficient result compared to the published work on In0.72Ga0.28As. The absorption coefficient of In0.83Ga0.17As with an extended cutoff wavelength near 2.6 mu m is newly reported in this paper. In addition, the extrapolation of the obtained absorption results demonstrates energy gaps of 0.475 eV for In0.73Ga0.27As and 0.55 eV for In0.83Ga0.17As, which are compatible with the reported bandgaps of these materials. The extracted absorption coefficient information can be used in the design of semiconductor devices for emerging technologies such as focal plane array, short wave infrared sensing and thermophotovoltaic.

Accurate detection of weak optical signals is a key function for a wide range of applications. A key performance parameter is the receiver signal-to-noise ratio, which depends on the noise of the photodetector and the following electrical circuitry. The circuit noise is typically larger than the noise of photodetectors that do not have internal gain. As a result, a detector that provides signal gain can achieve higher sensitivity. This is accomplished by increasing the photodetector gain until the noise associated with the gain mechanism is comparable to that of the output electrical circuit. For avalanche photodiodes (APDs), the noise that arises from the gain mechanism, impact ionization, increases with gain and depends on the material from which the APD is fabricated. Si APDs have established the state-of-the-art for low-noise gain for the past five decades. Recently, APDs fabricated from two Sb-based III-V compound quaternary materials, AlxIn1-xAsySb1-y and AlxGa1-xAsySb1-y, have achieved noise characteristics comparable to those of Si APDs with the added benefit that they can operate in the short-wave infrared (SWIR) and extended SWIR spectral regions. This paper describes the materials and device characteristics of these APDs and their performance in different spectral regions.

Gallium arsenide bismide (GaAsBi) is a potential candidate to replace InGaAs as the middle-junction structure in a multiple junction solar cell due to its lower level of lattice strain in the structure. The level of strain in the lattice structure is challenging as it can manipulate the total current collected in a solar cell, affecting the total output power. In this study, a GaAsBi multiple quantum wells with GaAs barriers (GaAsBi/GaAs) p-i-n structure was grown by using MBE machine and fabricated. A current-voltage (I-V) under illumination measurements were conducted to study the effect of biasing with the presence of light. This result is compared with a strained-balance InGaAs/GaAsP device. From the photocurrent output, it is shown that GaAsBi can achieve a longer cut-off wavelength, around 1053nm compared to InGaAs/GaAsP, with only 930 nm cut-off wavelength. The result also shows that the device is extracting low carriers from the photon sweeps at zero bias. A small amount of reverse bias is needed to allow carrier enhancement and increase the number of carriers collected by the device. As the forward bias is applied, photocurrent value drops as the forward dark current dominates the total current output. In conclusion, GaAsBi/GaAs MQW can be a competitive alternative to InGaAs in achieving a 1eV material system for photovoltaic, especially when the growth and structural component is optimized, improving its strain level and dark current density.

We studied the electroluminescence (EL) properties of an optically pumped GaAsBi–GaAs heterojunction p–i–n diode. GaAsBi–GaAs quantum well excitonic transitions dominate the EL except at low temperatures, where the luminescence from Bi-induced localized states also influences the luminescence. When the diode is optically pumped, the EL exhibits negative thermal quenching, and for a certain range of optical pump powers, we obtained the room-temperature EL intensity higher than that at the lowest temperature (22 K). We explain this by considering the thermally induced tunneling of photo-generated carriers from the n + and p + regions into the GaAsBi QW in the i -region of the p–i–n diode.

Al 0.85 Ga 0.15 As 0.56 Sb 0.44 has recently attracted significant research interest as a material for 1550 nm low-noise short-wave infrared (SWIR) avalanche photodiodes (APDs) due to the very wide ratio between its electron and hole ionization coefficients. This work reports new experimental excess noise data for thick Al 0.85 Ga 0.15 As 0.56 Sb 0.44 PIN and NIP structures, measuring low noise at significantly higher multiplication values than previously reported ( F = 2.2 at M = 38). These results disagree with the classical McIntyre excess noise theory, which overestimates the expected noise based on the ionization coefficients reported for this alloy. Even the addition of ‘dead space’ effects cannot account for these discrepancies. The only way to explain the low excess noise observed is to conclude that the spatial probability distributions for impact ionization of electrons and holes in this material follows a Weibull–Fréchet distribution function even at relatively low electric-fields. Knowledge of the ionization coefficients alone is no longer sufficient to predict the excess noise properties of this material system and consequently the electric-field dependent electron and hole ionization probability distributions are extracted for this alloy.

For short-wavelength infrared (SWIR) avalanche photodiodes, a separate absorption, charge, and multiplication design is widely used. AlInAsSb on an InP substrate is a potential multiplication layer with a lattice match to absorber candidates across the SWIR. Our new measurements demonstrate that AlInAsSb on InP is a promising multiplier candidate with a relatively low dark current density of 10−4 A/cm2 at a gain of 30; a high gain, measured up to 245 in this study; and a large differentiation of electron and hole ionization leading to a low excess noise, measured to be 2.5 at a gain of 30. These characteristics are all improvements over commercially available SWIR detectors incorporating InAlAs or InP as the multiplier. We measured and analyzed gain for multiple wavelengths to extract the ionization coefficients as a function of an electric field over the range 0.33–0.6 MV/cm.