Electron Avalanche Photodiode Research Articles

Emu: a case study for TDI-like imaging for infrared observation from space

A wide-field zenith-looking telescope operating in a mode similar to time-delay-integration (TDI) or drift scan imaging can perform an infrared sky survey without active pointing control, but it requires a high-speed, low-noise infrared detector. Operating from a hosted payload platform on the International Space Station (ISS), the Emu space telescope employs the paradigm-changing properties of the Leonardo SAPHIRA electron avalanche photodiode array to provide powerful new observations of cool stars at the critical water absorption wavelength (1.4 μm) largely inaccessible to ground-based telescopes due to the Earth’s own atmosphere. Cool stars, especially those of spectral-type M, are important probes across contemporary astrophysics, from the formation history of the Galaxy to the formation of rocky exoplanets. Main sequence M-dwarf stars are the most abundant stars in the Galaxy and evolved M-giant stars are some of the most distant stars that can be individually observed. The Emu sky survey will deliver critical stellar properties of these cool stars by inferring oxygen abundances via measurement of the water absorption band strength at 1.4 μm. Here, we present the TDI-like imaging capability of Emu mission, its science objectives, instrument details, and simulation results.

Enhanced Performance of HgCdTe Midwavelength Infrared Electron Avalanche Photodetectors With Guard Ring Designs

In this article, the performance of midwavelength infrared (MWIR) HgCdTe electron avalanche photodiodes (e-APDs) with variable annealing processes and guard ring design is investigated. Carrier distribution in the device is performed by simulation methods to better understand the mercury interstitial dynamic transport mechanism. It is found that the device with suitable annealing time can effectively reduce the dark current while maintaining the bandwidth at a high level. Surface leakage current and the effect of guard ring design in ion implanted-type HgCdTe e-APDs are clarified by the electric-field distribution. The simulated results show that the dark current, excess noise factor, and mean square noise can be decreased with a guard ring design. In addition, the temperature-dependent performance of HgCdTe e-APD with guard ring is simulated, indicating optimization operating window. This article demonstrates the significance of guard ring designs for HgCdTe APDs, and the simulation method will provide an effective way to determine the source of the current generation mechanism.

Sensitivity Calculations of High-Speed Optical Receivers Based on Electron-APDs

Sensitivity of high-speed optical receivers is heavily influenced by the performance of the optical detectors used in the receivers, the data rate, and the target bit-error-rate (BER). A simulation model for sensitivity of optical receivers based on electron-avalanche photodiodes (e-APDs) is presented. It allows for the optimization of avalanche width and operating voltage to achieve the optimum receiver sensitivity for given bit rate and target BER. The effects modelled include inter-symbol interference (ISI), various dark current components (tunnelling, diffusion, and generation), current impulse duration, avalanche gain, and amplifier's noise. The model was demonstrated through simulations of Indium Arsenide (InAs) e-APDs. For $10^{-12}$ target BER, the receiver's sensitivities were found to be −30.6, −22.7, −19.2, and −16.6 dBm, for 10, 25, 40, and 50 Gb/s data rate, respectively. Desirable avalanche properties of InAs e-APDs are counteracted by detrimental effects of high dark currents. Hence InAs e-APDs with lower dark currents are required to be more competitive with other optical detector technologies for high-speed optical receivers. The data reported in this article is available from the ORDA digital repository (DOI: 10.15131/shef.data.9959468).

Modeling the e-APD SAPHIRA/C-RED ONE camera at low flux level

Context. We implement an electron avalanche photodiode (e-APD) in the MIRC-X instrument, which is an upgrade of the six-telescope near-infrared imager MIRC, at the CHARA array. This technology should improve the sensitivity of near-infrared interferometry. Aims. We aim to characterize a near-infrared C-RED ONE camera from First Light Imaging (FLI) using an e-APD from Leonardo (previously SELEX). Methods. We first used the classical mean-variance analysis to measure the system gain and the amplification gain. We then developed a physical model of the statistical distribution of the camera output signal. This model is based on multiple convolutions of the Poisson statistic, the intrinsic avalanche gain distribution, and the observed distribution of the background signal. At low flux level, this model independently constrains the incident illumination level, the total gain, and the excess noise factor of the amplification. Results. We measure a total transmission of 48 ± 3% including the cold filter and the Quantum Efficiency. We measure a system gain of 0.49 ADU/e, a readout noise of 10 ADU, and amplification gains as high as 200. These results are consistent between the two methods and therefore validate our modeling approach. The measured excess noise factor based on the modeling is 1.47 ± 0.03, with no obvious dependency with flux level or amplification gain. Conclusions. The presented model allows the characteristics of the e-APD array to be measured at low flux level independently of a preexisting calibration. With < 0.3 electron equivalent readout noise at kilohertz frame rates, we confirm the revolutionary performances of the camera with respect to the PICNIC or HAWAII technologies. However, the measured excess noise factor is significantly higher than what is claimed in the literature (< 1.25), and explains why counting multiple photons remains challenging with this camera.

Short-Wave Infrared HgCdTe Electron Avalanche Photodiodes for Gated Viewing

Short-wave infrared (SWIR) HgCdTe electron avalanche photodiodes (eAPDs) with different doping profiles have been characterized for use in SWIR gated viewing systems. Gated viewing offers enhanced image contrast in scenes with clutter from the foreground or background. HgCdTe-based eAPDs show exponential gain–voltage characteristics and low excess noise and are, therefore, well suited for active imaging applications. The gain achievable at a fixed reverse voltage varies with the bandgap of the Hg1−xCdxTe detector material. We analyze current–voltage and gain–voltage plots measured on SWIR Hg1−xCdxTe eAPDs with x = 0.45, corresponding to a cutoff wavelength of 2.55 μm at 150 K. The cutoff has been chosen as a trade-off between achievable APD gain and operating temperature for SWIR gated-viewing systems with target distances of about 1000 m. Focal plane arrays with a readout-integrated circuit featuring a fast internal clock have been built and their performance with respect to gated viewing applications has been evaluated on a laboratory demonstrator for short distances. Future plans for a field demonstrator for distances up to 1000 m are described briefly at the end.

Impulse response measurement in the HgCdTe avalanche photodiode

HgCdTe based mid-wave infrared focal plane arrays (MWIR FPAs) are being developed for high resolution imaging and range determination of distant camouflaged targets. Effect of bandgap grading on the response time in the n+/ν/p+ HgCdTe electron avalanche photodiode (e-APD) is evaluated using impulse response measurement. Gain normalized dark current density of 2 × 10−9 A/cm2 at low reverse bias for passive mode and 2 × 10−4 A/cm2 at −8 V for active mode is measured in the fabricated APD device, yielding high gain bandwidth product of 2.4 THZ at the maximum gain. Diffusion of carriers is minimized to achieve transit time limited impulse response by introducing composition grading in the HgCdTe epilayer. The noise equivalent photon performance less than one is achievable in the FPA that is suitable for active cum passive imaging applications.

Influencing Sources for Dark Current Transport and Avalanche Mechanisms in Planar and Mesa HgCdTe p-i-n Electron-Avalanche Photodiodes

In this paper, both planar and mesa homojunction p-i-n HgCdTe electron-avalanche photodiodes (e-APDs) are fabricated and investigated to better understand the dark current transport and electron-avalanche mechanisms of the devices and optimize the structures. The experiment results are agreed well by simulated ${I}$ – ${V}$ characteristics based on established numerical models. Our results show that the multiplication region fabrication process leads to enormous characteristic difference between planar and mesa diodes. Shockley–Read–Hall and trap-assisted tunneling current are the main components of dark current for the planar/mesa junction under low bias voltage, and dark current is mainly influenced by band-to-band tunneling and avalanche current when higher reverse bias is added. In addition, we found that the difference between the uniformity of the electric field distributions in multiplication regions is the primary reason for the differences of the dark current. It was also proved that the dark current of planar e-APDs is dramatically affected by the junction corners, and mesa e-APDs dark current is found to be greatly dependent on the multiplication region thickness. Our work provides a great deal of theoretical basis for dark current formation and the avalanche mechanism of HgCdTe e-APDs.

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness.

Dark Current Transport and Avalanche Mechanism in HgCdTe Electron-Avalanche Photodiodes

HgCdTe electron avalanche photodiodes (e-APDs) have been widely used for low-flux and high-speed application. To better understand the dark current transport and electron-avalanche mechanism of the devices and optimize the structures, we performed accurate numerical simulations of the current–voltage characteristics and multiplication factor in planar homojunction (p-i-n) HgCdTe APDs. Based on the Okuto–Crowell avalanche model, an efficient physical model has been obtained by concerning the major generation-recombination processes, such as trap-assisted tunneling and band-to-band tunneling (BBT) recombination. Simulated current–voltage characteristics were in good agreement with available data in the literature, which demonstrates the validity of the proposed model. The origins of dark current in high reverse voltages are jointly dominated by BBT and the avalanche mechanism. It is proved to be effective for reducing BBT by improving the uniformity of the electric field distribution across the junction. The electric performance of p-i-n e-APD can be improved by optimizing the APD structure, such as eliminating the sharp corners of junctions, light doping, and the appropriate thickness in multiplication region. Our works provide a good deal of insight into the fundamental carrier transport processes involved in HgCdTe e-APDs.

Design of a Coherent Receiver Based on InAs Electron Avalanche Photodiode for Free-Space Optical Communications

In this paper, a coherent receiver based on InAs electron avalanche photodiode (e-APD) is designed for free-space optical (FSO) communications. According to the numerical simulations, it is found that the coherent receiver based on InAs e-APD can operate nearly in its shot-noise limit condition with a lower local oscillator (LO) power than traditional coherent receivers and get a better sensitivity improvement due to the gain provided by the coherent gain from the LO power and the internal gain of the e-APD. Hence, the multiple diversity receiver systems can be easier implemented and the number of the independent beams can be increased. In addition, this system is not sensitive to the change of the thermal noise of the coherent receiver and the dark current of the InAs e-APD that makes it more suitable for space environment. Furthermore, as the requirement of the internal gain for the InAs e-APD in this type of coherent receiver is not very high, the depletion region of InAs e-APD can be reduced. Hence, the coherent receiver based on InAs e-APD may become a potential choice in much higher speed FSO communication applications.

High Performance of Midwave Infrared HgCdTe e-Avalanche Photodiode Detector

This letter reports on the design and fabrication of HgCdTe electron-avalanche photodiode (e-APD) for low dark current and high gain for imaging applications. HgCdTe e-APD photodiodes were fabricated in the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -ν-p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> configuration for FPA at 30 μm × 30 μm pitch. Process for creating the required carrier profile and compositional grading in the absorption and multiplication regions was developed. Graded bandgap profile in the absorption region has been introduced. Shallow mesa etch isolation and effective passivation of side walls were introduced to control lateral currents. High quantum efficiency of 65% makes these APDs suitable for applications like quantum encryption. These measures helped in achieving high avalanche gain of 5550 at 8 V reverse bias in HgCdTe MWIR e-APD for the first time.

A Highly Sensitive Multi-element HgCdTe e-APD Detector for IPDA Lidar Applications

An HgCdTe electron avalanche photodiode (e-APD) detector has been developed for lidar receivers, one application of which is integrated path differential absorption lidar measurements of such atmospheric trace gases as CO2 and CH4. The HgCdTe APD has a wide, visible to mid-wave-infrared, spectral response, high dynamic range, substantially improved sensitivity, and an expected improvement in operational lifetime. A demonstration sensor-chip assembly consisting of a 4.3 μm cutoff HgCdTe 4 × 4 APD detector array with 80 μm pitch pixels and a custom complementary metal–oxide–semiconductor readout integrated circuit was developed. For one typical array the APD gain was 654 at 12 V with corresponding gain normalized dark currents ranging from 1.2 fA to 3.2 fA. The 4 × 4 detector system was characterized at 77 K with a 1.55 μm wavelength, 1 μs wide, laser pulse. The measured unit gain detector photon conversion efficiency was 91.1%. At 11 V bias the mean measured APD gain at 77 K was 307.8 with σ/mean uniformity of 1.23%. The average, noise-bandwidth normalized, system noise-equivalent power (NEP) was 1.04 fW/Hz1/2 with a σ/mean of 3.8%. The measured, electronics-limited, bandwidth of 6.8 MHz was more than adequate for 1 μs pulse detection. The system had an NEP (3 MHz) of 0.4 fW/Hz1/2 at 12 V APD bias and a linear dynamic range close to 1000. A gain-independent quantum-limited SNR of 80% of full theoretical was indicative of a gain-independent excess noise factor very close to 1.0 and the expected APD mode quantum efficiency.

Temperature dependence of gain and excess noise in InAs electron avalanche photodiodes

Measurement and analysis of the temperature dependence of avalanche gain and excess noise in InAs electron avalanche photodiodes (eAPDs) at 77 to 250 K are reported. The avalanche gain, initiated by pure electron injection, was found to reduce with decreasing temperature. However no significant change in the excess noise was measured as the temperature was varied. For avalanche gain > 3, the InAs APDs with 3.5 µm i-region show consistently low excess noise factors between 1.45 and 1.6 at temperatures of 77 to 250 K, confirming that the eAPD characteristics are exhibited in the measured range of electric field. As the dark current drops much more rapidly than the avalanche gain and the excess noise remains very low, our results confirmed that improved signal to noise ratio can be obtained in InAs eAPDs by reducing the operating temperature. The lack of hole impact ionization, as confirmed by the very low excess noise and the exponentially rising avalanche gain, suggests that hole impact ionization enhancement due to band "resonance" does not occur in InAs APDs at the reported temperatures.

High speed InAs electron avalanche photodiodes overcome the conventional gain-bandwidth product limit

High bandwidth, uncooled, Indium Arsenide (InAs) electron avalanche photodiodes (e-APDs) with unique and highly desirable characteristics are reported. The e-APDs exhibit a 3dB bandwidth of 3.5 GHz which, unlike that of conventional APDs, is shown not to reduce with increasing avalanche gain. Hence these InAs e-APDs demonstrate a characteristic of theoretically ideal electron only APDs, the absence of a gain-bandwidth product limit. This is important because gain-bandwidth products restrict the maximum exploitable gain in all conventional high bandwidth APDs. Non-limiting gain-bandwidth products up to 580 GHz have been measured on these first high bandwidth e-APDs.

Avalanche Multiplication and Excess Noise in InAs Electron Avalanche Photodiodes at 77 K

The findings of a study of impact ionization, avalanche multiplication and excess noise in InAs avalanche photodiodes at 77 K are reported. It is shown that hole impact ionization is negligible in practical devices which continue to operate as electron avalanche photodiodes, as they do at room temperature. A new electron ionization coefficient capable of modeling multiplication at 77 K is presented and it is shown that significant multiplication can be achieved in practical devices without excessive tunneling currents. The characteristic changes observed between room temperature and 77 K are discussed. This paper helps to demonstrate the potential for practical InAs electron avalanche photodiodes, operating cooled.

Impact Ionization in InAs Electron Avalanche Photodiodes

A systematic study of impact ionization, avalanche multiplication, and excess noise in InAs diodes has been carried out, confirming that avalanche multiplication is dominated by the impact ionization of electrons. This results in highly desirable “electron avalanche photodiode” characteristics previously only demonstrated in HgCdTe diodes, which are discussed in detail. The suppression of excess noise by nonlocal effects, to levels below the local model minimum of F = 2, is explained. An electron ionization coefficient is calculated and shown to be capable of modeling the electron impact ionization, which differs characteristically from that in wider bandgap III-V materials.

Shockley–Haynes Characterization of Minority-Carrier Drift Velocity, Diffusion Coefficient, and Lifetime in HgCdTe Avalanche Photodiodes

A combined study of the avalanche gain characteristics of HgCdTe electron-avalanche photodiodes (e-APDs) and of the minority electron properties in the p-type absorber using Shockley–Haynes (SH) measurements is presented for various Cd compositions xCd. Ideal gain performance associated with a low excess noise factor F = 1.2 have been measured at T = 80 K down to cutoff wavelengths of λc = 2.9 μm. The observation of both a record high, exponentially increasing gain of M = 600 in short-wave e-APDs and a low excess noise factor proved that the exclusive electron multiplication is stable down to xCd = 0.4. Zero-flux measurements at 80 K confirmed that the dark current tends to decrease at constant gain as xCd increases. Measurements using a readout integrated circuit allowed us to establish a new record in sensitivity for APDs: Ieq_in = 2 aA, corresponding to 12 e/s at gain of M = 24 in an e-APD with λc = 2.9 μm. SH measurements enabled direct estimation of the electron diffusion coefficient, drift velocity, and lifetime in the p-type absorber of the e-APDs as a function of electric field at temperatures between 80 K and 200 K. Measurements at 80 K yielded lifetimes consistent with the values expected for the nominal doping of the samples. The low-field electron drift mobility, estimated from the drift velocity, was found to be a factor of 0.4 to 0.5 lower than the mobility in n-type material with the same composition. In mid-wave (MW) infrared samples with λc = 5.3 μm, the mobility was observed to be μep = 15 kcm2/Vs to 20 kcm2/Vs, being less than μen ≈ 40 kcm2/Vs to 50 kcm2/Vs. The reduction in mobility can, in part, be attributed to scattering by ionized acceptors and heavy holes. The diffusion mobility, estimated from the diffusion coefficient, was systematically higher than the drift mobility, indicating diffusion of hot electrons with a temperature higher than that of the lattice. The saturation velocity, vsat_ep = 2 × 106 cm/s to 6 × 106 cm/s, did not correlate with the Cd composition in the samples. The measured saturation velocities made it possible to estimate the timing jitter in p-type absorbers with a built-in electric field. Jitter below 100 ps was estimated for SW and MW APDs with absorbing layer thicknesses up to 4 μm.

Extremely Low Excess Noise in InAs Electron Avalanche Photodiodes

Measurements of the avalanche multiplication noise in InAs p-i-n and n-i-p diodes at room temperature demonstrate unambiguously that the avalanche multiplication process is dominated by impact ionization of electrons. This results in the excess noise factor for electron initiated multiplication asymptotically approaching a maximum value just less than two and becoming virtually gain-independent for higher gains. Measurements for predominantly hole initiated multiplication show corresponding high excess noise factors suggesting the electron to hole ionization coefficient ratios are comparable to those reported for Hg1-xCdxTe electron avalanche photodiodes.

Study of the Transit-Time Limitations of the Impulse Response in Mid-Wave Infrared HgCdTe Avalanche Photodiodes

The impulse response in frontside-illuminated mid-wave infrared HgCdTe electron avalanche photodiodes (APDs) has been measured with localized photoexcitation at varying positions in the depletion layer. Gain measurements have shown an exponential gain, with a maximum value of M = 5000 for the diffusion current at a reverse bias of V b = 12 V. When the light was injected in the depletion layer, the gain was reduced as the injection approached the N+ edge of the junction. The impulse response was limited by the diode series resistance–capacitance product, RC, due to the large capacitance of the diode metallization. Hence, the fall time is given by the RC constant, estimated as RC = 270 ps, and the rise time is due to the charging of the diode capacitance via the transit and multiplication of carriers in the depletion layer. The latter varies between t 10–90 = 20 ps (at intermediate gains M < 500) and t 10–90 = 70 ps (at M = 3500). The corresponding RC-limited bandwidth is BW = 600 MHz, which yields a new absolute record in gain–bandwidth product of GBW = 2.1 THz. The increase in rise time at high gains indicates the existence of a limit in the transit-time-limited gain–bandwidth product, GBW = 19 THz. The impulse response was modeled using a one-dimensional deterministic model, which allowed a quantitative analysis of the data in terms of the average velocity of electrons and holes. The fitting of the data yielded a saturation of the electron and hole velocity of v e = 2.3 × 107 cm/s and v h = 1.0 × 107 cm/s at electric fields E > 1.5 kV/cm. The increase in rise time at high bias is consistent with the results of Monte Carlo simulations and can be partly explained by a reduction of the electron saturation velocity due to frequent impact ionization. Finally, the model was used to predict the bandwidth in diodes with shorter RC = 5 ps, giving BW = 16 GHz and BW = 21 GHz for x j = 4 μm and x j = 2 μm, respectively, for a gain of M = 100.

Evolution of IR detection and possible outcomes for astrophysics

Infrared focal plane technology has evolved rapidly in the past two decades. The detector format has grown from single pixel detectors to 4K × 4K pixel arrays, and a mosaic of 4 × 4 2K × 2K pixel arrays and six mosaics of 2 × 2 2K × 2K pixel arrays are already in operation at ground based telescopes. The quantum efficiency of these arrays is close to 100%, the readout noise a few electrons rms and the dark current a few electrons per hour. However, the latent image problem has not been solved yet. A persistence model and a mitigation strategy will be discussed briefly. Specialized ASIC readout chips which digitize the multichannel video signals of large arrays directly on the focal plane are becoming available for large format arrays and they will eventually replace conventional controllers. In the mid infrared a high flux version of the MIRI detector is being developed for ground based applications. Fast low noise infrared sensors are needed for wavefront sensors of adaptive optics (AO) systems. To meet both the speed and the noise requirements two different technical approaches are being followed. One solution is the use of high gain CTIA amplifiers in the detector unit cell, the other solution is to exploit the noiseless gain mechanism of HgCdTe electron avalanche photodiode (eAPD) arrays.