A modification to the random path length technique (RPL) with a hard-threshold dead space (Eth) is demonstrated to be able to calculate the electron initiated avalanche multiplication (Me) and excess noise (Fe) in GaAs p+–i–n+ structures with n- and p-type background doping levels up to 1 × 1017 cm−3 in the multiplication region. This model’s Me, Fe, and position dependent ionization probabilities are compared with the results obtained from a multi-valley analytical band Monte Carlo model, enabling us to quantify how the ionization process is affected when the electric field changes rapidly. The RPL results for Me show excellent agreement with the Monte Carlo model, suggesting that the simple hard dead-space correction to the ionization probability distribution function works as well with a varying electric field up to 1400 kV/cm/μm as in a constant electric field and that the “history dependence” effects of a varying electric field are not very significant once the initial carrier dead space is allowed for. The modified RPL Fe also agrees well with the Monte Carlo model for p+–n−–n+ structures; however, it is underestimated for p+–p−–n+ structures. This is attributed to the hard dead space of the feedback carriers having a disproportionate effect on the variance in the multiplication.

Near-infrared single photon avalanche diodes (SPADs) are practical single photon detectors, particularly for applications requiring high operating temperatures. Compared to established InP SPADs, AlGaAsSb SPAD offers superior thermal stability but currently exhibit lower single photon detection efficiency. To improve their performance, origin(s) of AlGaAsSb SPAD's dark count rate (DCR) and detection efficiency versus overbias characteristics should be investigated. We explore these by performing extensive DCR and SPDE measurements on InGaAs/AlGaAsSb SPADs in gated mode at relevant temperatures (200-290 K). The maximum SPDE at 1550 nm wavelength was 31 (4 V overbias) and 20% (3 V overbias) at 200 and 250 K, respectively, higher than earlier reports of AlGaAsSb SPADs. DCR data analyses show that the tunneling current is a dominant DCR mechanism over the range studied. It most likely originated from the InGaAs absorber, despite an attempt to reduce the electric field in the design. It is therefore necessary to ensure the entire InGaAs absorber does not experience a relatively high electric field in future designs of AlGaAsSb SPADs. If AlGaAsSb SPADs without tunneling current can be achieved, their single photon detection performance is expected to match that of InP SPADs at least, while exhibiting superior thermal stability.

To realise high-speed free-space optical communication links in harsh space environments, it is crucial to consider the link's operating wavelength, the performance of the optical receiver, and the radiation hardness of the avalanche photodiode (APD)-optical detectors in the optical receivers. In this work, we experimentally evaluated the radiation hardness of 2.5 Gb/s receivers based on InGaAs/AlGaAsSb APDs integrated with Ommic CGY2102UH/C2 transimpedance amplifiers. Proton energy (62 MeV) and fluence (up to 3.8 x 10(10) p/cm(2)) representative of space environments were used to irradiate multiple receivers, ensuring rigour. After irradiation, the receivers maintained their avalanche gain and photocurrent, while exhibiting bandwidths exceeding 1.5 GHz. Despite a slight increase in APD's dark current at high reverse bias, there was no degradation of the receiver's bit error rate. At 2.5 Gb/s data rate and 1550 nm wavelength, the irradiated receivers achieved a bit error rate of 10(-9) with an average optical power of -38.2 dBm, outperforming selected commercial receivers by similar to 3 dB. Since the displacement damage dose induced by the proton radiation levels used in this work are representative of those in Low Earth, Geostationary and Global Positioning System orbits, we demonstrated that InGaAs/AlGaAsSb APDs have sufficient radiation hardness to be employed as optical detectors of high-speed optical links in harsh space environments.

ABSTRACT There is an increasing demand for high speed 25 Gb/s low noise APDs. Semiconductors with disparate ionisation coefficients are known to provide low noise and high gain‐bandwidth product. In this work, we performed a comprehensive simulation of 400 combinations Al 0.85 Ga 0.15 As 0.56 Sb 0.44 (AlGaAsSb) avalanche width ( w m ) and InGaAs absorption width ( w a ) to identify a design to achieve high sensitivity at 25 Gb/s. Our model, which was benchmarked using experimental data in the literature, predicts that design options ranging from w m = 100 nm with w a = 700 nm to w m = 700 nm with w a = 100 nm will produce a bandwidth of at least 18.7 GHz and a gain of 10. Reducing the avalanche width to 100 nm will provide a gain‐bandwidth product > 200 GHz, without suffering from excessive band to band tunnelling current. Results from a large number of APD designs show that a sensitivity of −26.6 dBm at a wavelength of 1310 nm and a bit error rate of 1 × 10 −12 at 25 Gb/s can be achieved using a 100 nm AlGaAsSb avalanche region with a 1000 nm hybrid InGaAs absorption region (consisting of a 600 nm p‐doped and a 400 nm depleted region). This is ∼5.5 and 3.8 dB better than a commercial III–V APD and Ge/Si APD, respectively, suggesting the great potential of using AlGaAsSb avalanche photodiodes in very high bit rate optical communication.

Planar indium arsenide (InAs) electron avalanche photodiodes (e-APDs) can provide significant avalanche gain with negligible excess noise. Reported InAs e-APDs are so far all top-side illuminated p-i-n diodes. Yet, back-side illuminated n-i-p diodes are needed to be compatible with focal plane arrays (FPAs) (bump-bonding process). This work reports n-type ion implantation into i-InAs layer grown on p-InAs layers, forming n-i-p diodes for the first time. Electrical and optical characteristics of S-and Si-implanted mesa and planar photodiodes are investigated both experimentally and through simulation. The mesa InAs n-i-p diodes fabricated from implanted samples exhibit similar dark current densities to previously reported Be implanted mesa InAs p-i-n diodes. A peak responsivity of 1.09 A/W at 2004 nm wavelength was demonstrated using S implanted detectors after rapid thermal annealing at 600 C-degrees for 30 s. The simple planar diodes exhibit higher dark current compared to Be implanted planar InAs p-i-n diodes. This is attributed to poor junction isolation resulting from n-type unintentional doping in intrinsic InAs layers. This can be mitigated by adding isolation trenches around the diodes or introducing p-type isolation implant. Therefore, we have demonstrated a promising approach for fabricating bump-bonding compatible and back-illuminated InAs n-i-p planar diodes.

Indium arsenide (InAs) is an exceptional material for absorbing infrared photons with wavelengths up to 3500 nm, making it ideal for mid-infrared detection. However, the development of high-performance separate sbsorption and multiplication avalanche photodiodes (SAM APDs) has been hindered by the absence of suitable low-noise avalanche materials compatible with InAs absorbers. In this study, we investigate the potential of Al0.8In0.2As0.31Sb0.69 (lattice matched to InAs) as a low-noise avalanche material. We have performed comprehensive Al0.8In0.2As0.31Sb0.69 excess noise measurements using three optical signal wavelengths on a large number of p-i-n and n-i-p diodes. Under pure electron injection, Al0.8In0.2As0.31Sb0.69 p-i-n diodes exhibit very low excess noise factors similar to 4 at high gain of 100, corresponding to an effective k of 0.030.03. In contrast, a small gain of 3 produces very high excess noise factors (>17) when using hole injection in the n-i-p diodes. The contrasting behavior indicates that in Al0.8In0.2As0.31Sb0.69 electron ionization coefficient is much larger than hole ionization coefficient. As a consequence, low-noise Al0.8In0.2As0.31Sb0.69 avalanche regions emerge as a promising candidate for the avalanche region of SAM-APDs designed for mid-infrared applications, such as methane gas sensing and imaging through fog. The design of such SAM-APDs should ensure electrons rather than holes are injected into the Al0.8In0.2As0.31Sb0.69 avalanche regions to achieve the lowest possible excess noise factors.

There has been considerable interest over many years in finding a way by which we can amplify optical signals at the telecommunication wavelengths of 1300 and 1550 nm using the impact ionization process, much as in the manner that silicon does at visible wavelengths but with low excess noise, high sensitivity and high speed. To date, AlxGa1-xAs0.56Sb0.44 is a promising avalanche material which can be grown lattice-matched to an InP substrate and therefore use InGaAs or GaAsSb as the absorption region. In this paper, we compare the excess noise in the AlxGa1-xAs0.56Sb0.44 alloy lattice matched to InP. In 2019, excess noise values of F = 2.2 in AlAs0.56Sb0.44 based APDs were obtained at gains of M = 40 significantly better than those achievable with InP or InAlAs. We very recently demonstrated sub-McIntyre characteristics with F remaining below the k = 0 limit of 2 up to avalanche gains of M > 60, and then increasing linearly to F = 2.45 at M = 90 from thick Al0.75Ga0.25As0.56Sb0.44 grown on InP substrates. We also demonstrated that the excess noise factor in the lower-aluminium Al0.55Ga0.45As0.56Sb0.44 alloy, with a bandgap of similar to 1.24 eV, is higher than in the higher- aluminium composition alloys studied to date, but comparable to some commercial Silicon APDs and about half that of equivalent InAlAs.

This work presents a high-sensitivity shortwave infrared (SWIR) photoreceiver, designed using a high-gain In0.53Ga0.47As/Al0.85Ga0.15As0.56Sb0.44 Avalanche Photodiode (APD) with an extremely low excess noise factor of <3.5 at a gain of 100. The transimpedance amplifier (TIA) and input circuitry were rigorously optimized for precise APD gain control. Under investigation with APDs of 30, 80, and 200 mu m active diameters, the receiver demonstrated a record-low room-temperature Noise Equivalent Power (NEP). An extremely low NEP of 21.2 fW/root Hz was achieved with a 30-mu m-diameter APD, over a 440 MHz signal bandwidth, at an APD gain of 230 and the wavelength of 1550 nm. Current commercial APD-TIA modules typically exhibit NEPs of >100 fW/root Hz to 10's of pW/root Hz. Compared to its best-in-class 80-mu m counterpart, this work's receiver demonstrated a 6.5x sensitivity improvement at 2x the operating bandwidth, with an NEP of 32.5 fW/root Hz. These results are of great significance for SWIR applications including extending the range of LiDAR systems, for which optimal performance requires the maximal-sensitivity detection of few-nanosecond optical pulses.

Near-infrared Single Photon Avalanche Diodes (SPADs) are the dominant, practical single photon detectors for quantum applications and low-level optical sensing. Although some infrared SPADs can operate at room temperature, thermoelectric coolers are still essential, increasing complexity (operation and device packaging) and power consumption. Passively-cooled SPADs could be realized by avalanche materials exhibiting better temperature stability. A promising candidate is the InGaAs/AlGaAsSb SPAD, because the AlGaAsSb multiplier is highly stable with temperature. In this work, we report single photon detection performance of InGaAs/AlGaAsSb SPADs at room temperature and 1550 nm wavelength using multiple devices for each type of measurements. With 0.1 photons per pulse and 15 mu m diameter devices, the maximum SPDE was 14% at DCR of 30 Mc.s(-1), respectively. The best NEP value is around an order of magnitude higher than InGaAs/InP SPADs, but are comparable to InGaAs/InAlAs SPADs. Within the relevant overbias range and repetition rate up to 1 MHz, the DCR was unaffected by afterpulsing. Timing jitters were as low as 150 ps, matching InGaAs/InP SPADs. The results of this work are much more competitive than the previous report of InGaAs/AlGaAsSb SPAD, which required cooling to 200 K to detect single photons. Further research could help InGaAs/AlGaAsSb SPADs progressing towards passively-cooled single photon detectors for room temperature operation.

At a 1550 nm wavelength, the optical sensitivity of conventional indium gallium arsenide (InGaAs)-based avalanche photodiodes (APDs) is restricted by their high excess noise, hindering their performance in long-range free-space optical communication links. Al0.85Ga0.15As0.56Sb0.44 (AlGaAsSb), lattice-matched to indium phosphide (InP) substrates, has a much lower excess noise factor than InP, the conventional avalanche material. In this work, we evaluated the performance of optical APD-TIA receivers utilizing InGaAs/AlGaAsSb APDs through simulations and experiments. Simulations confirmed their optimum gain is much higher than conventional APDs. InGaAs/AlGaAsSb APD dies and transimpedance amplifier (TIA) chips were integrated, yielding four optical receivers for experimental evaluation. At 2.5 Gb/s and BER = 10−9, these receivers operated at a high optimal gain of 56 (as predicted in simulations) and produced a mean sensitivity of −38.5 dBm, with the best sensitivity at −39.2 dBm. These sensitivity values are at least 2.7 (or, in the best case, 3.4) dB better than those of typical commercial receivers with InGaAs APDs. This work quantifies the significant performance improvement that InGaAs/AlGaAsSb APDs provide to long-range free-space optical communication links.

Separate - Absorption - Multiplication Avalanche Photodiode (SAM-APD) made from Al0.85Ga0.15As0.56Sb0.44 (AlGaAsSb) avalanche region and InGaAs absorption region exhibits low dark current, excess noise, and Noise Equivalent Power (NEP) at room temperature. However, its performance at elevated temperatures has not been reported. In this work we performed comprehensive measurements of dark current, avalanche gain and noise spectrum of three InGaAs/AlGaAsSb SAM-APDs at 22 to 52 degrees C. We observed a weak temperature dependence of dark current and breakdown voltage has a temperature coefficient of 12.9 (+/- 0.5) mV/degrees C, which is 6 - 15 times better than a number of commercial 1550 nm APD modules. The best NEP ranges between 78 and 92 fW/vHz at 22 to 52 degrees C with NEP at 52 degrees C outperforming a number of commercial 1550 nm APD modules at 22 degrees C. The low NEP value enable the InGaAs/AlGaAsSb SAM-APDs to detect optical pulse with as few as 78 photons per pulse at 22 degrees C, increasing to 100 photons at 52 degrees C. These results demonstrate that InGaAs/AlGaAsSb SAM-APDs have a superior temperature tolerance to maintain gain, NEP and low photon detection compared to typical 1550 nm SAM-APDs. Therefore, they can potentially increase the range and sensitivity of gas sensing, free space optical communication and ranging instruments.

Near-infrared linear mode Al $_{\text{0.85}}$ Ga $_{\text{0.15}}$ As $_{\text{0.56}}$ Sb $_{\text{0.44}}$ avalanche photodiodes (APDs) exhibit excellent temperature stability, potentially simplifying Geiger mode operation. We have carried out the first experimental evaluation of In $_{\text{0.53}}$ Ga $_{\text{0.47}}$ As/Al $_{\text{0.85}}$ Ga $_{\text{0.15}}$ As $_{\text{0.56}}$ Sb $_{\text{0.44}}$ APDs in Geiger mode. Characterization on multiple devices included temperature-dependent dark current, avalanche multiplication, dark count rate (DCR), afterpulsing, and single photon detection efficiency (SPDE). The temperature coefficient of breakdown voltage extracted from avalanche multiplication data was 13.5 $\text{mV}\cdot\text{K}^{-\text{1}}$ , much lower than InGaAs/InP Geiger mode APDs, reducing changes in operation voltage and offering possible protection from high optical power thermal attack in communication systems. At 200 K, SPDE were 5%–16% with DCR of 1–20 $\text{Mc}\cdot\text{s}^{-\text{1}}$ , comparable to InAlAs and early InP-based Single Photon APDs. The afterpulsing at 200 K was negligible for hold-off time $>$ 50 $\mu$ s (reducing to 5 $\mu$ s at 250 K). These are similar to the performance of InGaAs/InAlAs and some InGaAs/InP Geiger mode APDs. The data reported in this article is available from the ORDA digital repository (https://doi.org/10.15131/shef.data.24125721).

It is well known that avalanche photodiode can enhance the signal to noise ratio of a detection system, when the excess avalanche noise is low. The excess noise factor, F which characterizes the excess noise of an APD can be calculated using the established noise theory from R. McIntyre. When the ratio of hole to electron ionization coefficients, k = 0, F similar to 2 is achieved at high gain. This means the avalanche gain, M, can be increased without the penalty of increased excess noise factor. In this work, we will present the progress in APDs incorporating InAs and AlGaAsSb as the avalanche regions. Both show k similar to 0 and therefore F similar to 2. The former can be used for low photon detection at wavelengths beyond InGaAs, while the latter can be combined with InGaAs to provide low noise APD for wavelengths up to 1700 nm. Our work demonstrated that low photons of < 100 photon within a 50 microsecond pulse can be detected using InAs APDs. We also achieved single photon detection at 1550 nm using AlGaAsSb APD.

Sensing of weak photon fluxes in the short to mid-wave infrared is important for a variety of applications such as optical communication systems, light detection and ranging (LiDAR) and remote gas sensing. For the most demanding applications, avalanche photodiodes (APDs) are regularly employed due to the enhanced sensitivity afforded by their internal avalanche gain. Indium Arsenide (InAs) is a material which exhibits near ideal avalanche multiplication properties and is capable of detecting infrared radiation up to 3 mu m at 77 K. Due to exclusive multiplication of electrons, it exhibits incredibly low excess noise factors below 2, regardless of the magnitude of avalanche gain. Furthermore, unlike most APD technologies, its bandwidth is not limited by its avalanche gain, allowing it to operate at high speeds with high gains. Using our recently developed planar process, we report InAs avalanche photodiodes which exhibit high gains in excess of 100 and external quantum efficiencies at 1550 nm of 56 %. Our liquid nitrogen cooled detectors are combined with a low noise current amplifier and the performance of the system is analyzed. Detection of weak 1550 nm laser pulses corresponding to <70 photons per pulse is demonstrated. The detector's noise current is shown to be background limited, hence, detection of lower optical powers could be achieved through further set-up optimization.

There is an increased demand for low noise avalanche photodiodes (APDs) for infrared wavelengths at 1550 nm for long range Light Detection and Ranging applications. Here we present two classes of APD that produce high avalanche gain but with extremely low excess noise factors, F similar to 2. InAs APDs show F < 2 and offer detection wavelength up to 3500 nm, although this drops to similar to 3000 nm when cooled. For reducing effects of scattering in atmosphere, InAs could be an attractive option. In addition InAs APDs are based on a simple homojunction design, which is relatively easy to grow epitaxially. AlGaAsSb when combined with InGaAs, provides a direct replacement for the traditional InGaAs/InP APDs. It is therefore capable of room temperature performance with excess noise performance similar to Si APDs but operates at 1550 nm. We will present results that show noise equivalent power as low as 69 fW/Hz(0.5).

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.

Electron-only avalanche multiplication and low excess noise has previously been established in InAs avalanche photodiodes (APDs). However, there is currently a lack of experimental investigations into the noise and low photon detection capability of planar InAs APDs. Here, the noise equivalent power (NEP) of planar InAs APDs operating with a low-noise transimpedance amplifier is investigated for the first time. Our devices have a responsivity of 0.7 A/W and excellent linearity at the wavelength of 1550 nm. Using these APDs, a very low NEP of 45 $\text{fW/}\sqrt {\text{Hz}}$ is achieved at a gain of 54. Modeling of the NEP suggests that the excess factor is close to 1.6. This low NEP result is corroborated by the detection of weak optical pulses corresponding to $<$ 70 photons per 1550-nm laser pulse. Our analysis suggests that this performance can be enhanced through the suppression of the background component of the photocurrent as well as reducing the unintentional doping in the devices.

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.

Avalanche photodiodes (APDs) are widely used in near-infrared optical receivers to detect weak and/or high-speed optical signals. Emerging high-order optical signal modulation formats require the APD's photocurrents to vary linearly with the signal power. There is, however, a lack of comprehensive understanding of the linearity of APD's photocurrent and gain versus optical power characteristics underpinned by experimental results. An experimental study was carried out on the linearity of near-infrared APD's photocurrent and avalanche gain with optical signal power, covering a wide range of optical power and APD's operating voltage. The work utilized thin 200-nm Al0.85Ga0.15As0.56Sb0.44 (AlGaAsSb) avalanche region, exploiting their excellent temperature stability compared to thick structures and other commonly used avalanche materials. Three types of linearity behaviors were identified and explained: 1) around the punchthrough voltage; 2) higher reverse bias and moderate gains; and 3) close to the breakdown voltage and large gains. The best linearity performance, tested under optical power from 0.08 to 750 mu W, was achieved under high reverse bias ( > 18 V) but with moderate gain ( < 10). Our findings of linearity performance are also applicable to near-infrared APDs with other avalanche materials. Furthermore, AlGaAsSb-based APDs exhibit better linearity performance compared to a commercial non-AlGaAsSb APD. At a gain of 10, a 10% attenuation was observed at the output current of 34 mu A in the commercial APD compared to 670 mu A (20 times higher) in our APD, suggesting the potential of our detector for optical communication links utilizing high-order signal modulation formats. The data reported in this article are available from the ORDA digital repository (https://figshare.com/s/34f0f27e42de168c5c41).