SWIR Detector Research Articles

Theoretical Study of Quaternary nBp InGaAsSb SWIR Detectors for Room Temperature Condition.

This paper presents a theoretical analysis of an nBp infrared barrier detector's performance intended to operate at a room temperature (300 K) based on AIIIBV materials-In1-xGaxAsySb1-y quaternary compound-lattice-matched to the GaSb substrate with a p-n heterojunction ternary Al1-xGaxSb barrier. Numerical simulations were performed using a commercial Crosslight Software-package APSYS. The band structure of the nBp detector and the electric field distribution for the p-n heterojunction with and without a potential barrier were determined. The influence of the barrier-doping level on the detector parameters was analyzed. It was shown that Shockley-Read-Hall (SRH) recombination plays a decisive role in carrier transport for lifetimes shorter than 100 ns. The influence of the absorber/barrier thickness on the detector's dark current density and photocurrent was investigated. It was shown that valence band offset does not influence the device's performance. The quantum efficiency reaches its maximum value for an absorber's thickness of ~3 μm. The performed simulations confirmed the possibility of the detector's fabrication exhibiting high performance at room temperature based on quaternary compounds of AIIIBV materials for the short wavelength infrared range.

(Invited) emerging Designs for Organic Infrared Photodetectors and Energy Storage

The shortwave infrared spectral region (SWIR: 1-3 mm) is particularly powerful for health and environmental monitoring, enabling greater penetration depth and improved resolution in comparison to visible light. However, conventional SWIR sensors are limited by complex die transfer and bonding processing. This presentation will discuss the advances made in organic infrared detectors to overcome the issues in conventional SWIR detectors. The organic devices show photo-response spanning from the visible to 1.7 micron, using a new generation of semiconducting polymers that are processed by solution processing techniques and allow simple direct deposition. We will discuss the recent progress in identifying avenues that will improve sensor detector, such as understanding the origin of efficiency losses and materials optimization. Several demonstrations will show the various potential applications of organic infrared imagers, particularly for an up-conversion structure that combine photo-sensing and display in a compact structure without the need for pixilation. The imaging active area of 2 cm2 enabled concurrent recording of blood vessel location and blood flow pulses.In addition to visualisation of infrared radiation, we are also developing the integration of redox polymers into supercapacitor structures to serve as energy supplies for wireless electronics. A new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated enhanced charge delocalization and thus improved stability dramatically. The supercapacitors were patterned by electrodeposition and showed a high areal power density of 227 mW/cm2, enabling rapid charging and high power output. The capacitance retention was 84% after 11,000 full redox cycles, offering the critical benefit of long cycle life. This work demonstrated the application of a new class of stable redox-active materials suitable to meet the energy storage needs for short-range wireless electronics.

Spectral responsivity of mosaic SWIR detectors

Spectral responsivity of mosaic SWIR detectors

Model and Characterization of Persistence in HgCdTe SWIR Detectors for Astronomy Application

Persistence is the remnant signal that plagues HgCdTe infrared detectors used for astronomy applications after a bright illumination. Briefly, any perturbation on these detectors generates a nonlinear signal with higher amplitude than a dark current and lasts for hours. The traditional hypothesis used to explain this phenomenon is based on trapping/emission processes from deep-level defects in the space charge region (SCR) of the diode. Inspired by deep-level transient spectroscopy formalism, we have developed an analytical model describing the trap emission current from the SCR of the photodiode. We also take into account the intrinsic non-linearity of the source follower per detector ROIC architecture. Compared to data obtained on detectors built in-house at CEA-LETI, the model allows the estimation that a trap density on the order of the residual doping is enough to explain the persistence amplitude. A graded trap density in the SCR is in addition necessary to explain the persistence measurement as a function of the stress amplitude. Limits of the model are also underlined in the case of higher persistence amplitude. In this case, trap density should be close to the doping. This implies that N doping of the diode would be compensated, which is an extreme scenario out of the scope of this model.

Systematic optical study of high-x InxGa1-xAs/InP structures for infrared photodetector applications

Optical and structural properties in high-x InxGa1-xAs (x > 0.65) samples with varying indium concentration grown on InP (100) substrate are reported. By increasing the indium fraction, it was found by the high-resolution X-ray diffraction (HR-XRD) study that the dislocation density in the InxGa1-xAs epitaxial layer significantly increased, and the surface quality deteriorated remarkably. Photoreflectance (PR) spectra show the presence of Franz-Keldysh Oscillations (FKOs) features above the InxGa1-xAs energy bandgap. The strain-induced electric field is then estimated directly from the FKOs periods. Temperature-dependent photoluminescence (TDPL) measurements from 10 K to 300 K showed carrier locations (S-shape). This abnormal behavior is due to the dislocation density associated with fluctuations in the indium concentration. A quasi-stationary rate equation model for the temperature-dependent luminescence spectra of the localized state material system is proposed to interpret the band gap emission process quantitatively. Low-temperature (10 K) time-resolved PL measurements show the increase of lifetime with increasing the indium concentration. Yet, the addition of only 1.7% of indium concentration results in a strong enhancement of PL lifetime by ∼ 80%.All these results reveal a more precise picture of the localization and recombination mechanisms of photogenerated carriers in the InGaAs layer, which could be the crucial factors in controlling the performance of high indium content InGaAs SWIR detector.

Dual Mode High Speed Uncooled Short Wave Infrared Camera Based on FPGA

This paper introduces a short-wave infrared camera logic design based on FPGA. The hardware design adopts the architecture of Sofradir Tecless SWIR detector SW640+Xilinx FPGA xc4vsx55, which has a dual working mode of 640×512 at 25 FPS and 64×48 at 4000 FPS. The function of high precision target tracking can be realized by high frequency capture of laser signal. In this design, the non-uniform correction coefficient table in SDRAM cache Flash is controlled by FPGA to correct the image of full resolution mode, and the table of 64×48 is cached by Bram in FPGA to achieve fast image correction and processing. The exposure time is controlled by the external synchronous signal, and the 14 bit precision image data are collected by four ADCs for restoration. The output data is processed by image bad point correction, non-uniform correction, median filtering, histogram equalization, and then the image is packaged and sent by the specified protocol.

Highly sensitive SWIR detector array based on nanoscale phototransistors integrated on CMOS readout

Ultra-sensitive and fast infrared imaging has become increasingly important in applications that require high frame rates at low light levels, such as exoplanet imaging. The sensitivity of conventional short-wave infrared cameras is limited by their readout noise level. This limitation can be addressed by the internal gain of the sensors, but only if fast response time and low dark current are achieved simultaneously. Recent theoretical predictions suggested that reducing the internal capacitance of detectors with internal gain can increase their sensitivity. Here, we show the experimental validation of this prediction for III–V heterojunction phototransistors. We have fabricated a 320 × 256 array of InGaAs/InP infrared phototransistors integrated with a conventional silicon readout circuit. The array is made of two groups of pixels: 50% are devices with a 1 μm base diameter and the other 50% with a 2 μm base diameter. Characterization of a large number of pixels shows that 1 μm devices have significantly higher sensitivity than 2 μm devices. These have an average noise equivalent photon sensitivity of about 20 photons at a camera frame rate of ∼500 frames per second, which is better than the best existing infrared cameras with a similar cutoff wavelength and frame rate. Interestingly, the processing variation in the 1 μm devices resulted in variation in sensitivity, and a good number of devices show sensitivity to less than 10 photons. These results suggest that the proposed phototransistors are promising for ultra-sensitive short-wavelength infrared cameras.

The modeling of a SWIR type-II InAs/AlSb superlattice using an ETBM and interface engineering

A short-wavelength infrared InAs/AlSb type-II superlattice photodetector has been modeled with 15 Mono Layer (ML) of InAs and 4 Mono Layer (ML) of AlSb. In this work, the Empirical Tight-Binding Method (ETBM) was used to calculate the band structure and predict the material of the interfaces. In addition, the effective mass of the heavy hole, light hole, and electron was calculated for each type of the interfaces by tight binding s p 3 s ∗ modeling. Based on the results, the material of the interfaces plays a critical role in lattice mismatch and cutoff wavelength. The detector cutoff wavelength can be changed by atomic engineering of the superlattice interfaces and changing the thicknesses of InAs and AlSb. The simulation results are in good agreement with the experimental values and prove that the [ ( I n A s ) 15 − I n S b − ( A l S b ) 4 − A l A s ] is the most suitable structure. In order to match the simulation results with experimental results, the effect of the segregation on the band structure was evaluated. In this case, the cutoff wavelength of the SWIR detector is 3.27 μ m and the lattice mismatch between the superlattice and the GaSb (100) substrate is 0.63%. • A SWIR InAs/AlSb T2SL photodetector has been modeled with 15 ML of InAs and 4 ML of AlSb. • (ETBM) is used to calculate and predict the material of the interfaces. • The effective mass of the heavy hole, light hole, and electron was calculated for each type of the interfaces by tight binding s p 3 s ∗ modeling. • The flowchart of extracting the band structure by ETBM is presented. • The migration of atoms in interfaces is investigated.

InGaAs nBn SWIR detector design with lattice-matched InAlGaAs barrier

Dark current optimization with band gap engineering has been numerically studied for InGaAs nBn type infrared photodetectors. Undoped InAlGaAs grading layers are utilized in constructing the barrier and dipole delta-doped layers are placed in both sides of the graded layers for eliminating valence band offset. As a result, the high band gap barrier layer blocks the majority carriers and allows minority carrier flow while minimizing various dark current components, as expected from an nBn detector. Substantial improvement has been shown in the dark current level without compromising any photoresponse compared to the conventional pn junction and recently proposed all InGaAs nBn type photodetectors.

Fully Depleted InP Nano-Layer for In-Device Passivation of InGaAs SWIR Detectors

We designed a p-n InGaAs/InP heterojunction photodiode with a novel passivation approach that employs a thin and fully depleted in-device (embedded in the p-n structure) p-InP layer. We comparatively characterized mesa-type detector pixels and experimentally observed expected passivating behavior. Characterization results under illumination indicated that fully depleted p-InP layer increases photo-current as well due to increasing device active area. Dark current analysis of detector pixels with different areas resulted in a suppression of surface dark current by nearly three times.

On the Sensitivity of Electron-Injection Detectors at Low Light Level

We present the signal-to-noise performance of a short-wave infrared detector, which offers an internal avalanche-free gain. The detector is based on a similar mechanism as the heterojunction phototransistor and takes advantage of a type-II band alignment. Current devices demonstrate a noise-equivalent sensitivity of ∼670 photons at 260 K and over a linear dynamic range of 20 dB. While this level of sensitivity is about an order of magnitude better than an ideal p-i-n detector attached to the same low-noise amplifier, it was still limited by the amplifier noise (∼2600 electrons root mean square) due to the insufficient device gain. Performance comparison with other SWIR detector technologies demonstrates that the so-called electron-injection detectors offer more than three orders of magnitude better noise-equivalent sensitivity compared with state-of-the-art phototransistors operating at similar temperature.

HgCdTe p+‐n structures grown by MBE on Si (013) substrates for high operating temperature SWIR detectors

AbstractElectrophysical properties of multilayered heteroepitaxial structures of Hg1‐xCdxTe with x=0.3‐0.4 grown by molecular beam epitaxy on silicon substrates are presented. The passivating effect of thin CdTe layers grown on top of the structures in single process is demonstrated. Comparison between experimental and theoretical temperature dependencies of reverse currents in n‐on‐p and p‐on‐n diodes fabricated by boron and arsenic ion implantation in vacancy‐doped p‐type and In‐doped n‐type Hg1‐xCdxTe films, respectively, are presented. The influence of p‐n junction position in double‐layer heterostructure on temperature dependencies of reverse currents is examined. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anneal treatment to improve the performance of extended wavelength In0.83Ga0.17As photodetectors

For the purposes of improving the performance of the SWIR detector, two types of annealing treatment for extended wavelength In0.83Ga0.17As photodiodes with PIN structure are studied. In contrast to the sample A (rapid thermal annealing is performed after mesa etching for fabricating the In0.83Ga0.17As photodiodes), the sample B (rapid thermal annealing is performed before mesa etching for fabricating the In0.83Ga0.17As photodiodes) has a lower dark current at the same test temperature 220K and 300K, respectively. The different anneal treatment improves quality of the material by decreasing the defects of devices structure. The voltage–current curves have been used to measure the dark current of different devices. Plane-view electron beam induced current (PV-EBIC) and secondary electron (SE) images have been used to evaluate the performance correlated imperfection or defect features of In0.83Ga0.17As photodiodes structures and the defect density is 2.33×107cm−2 for sample A, 8.14×106cm−2 for sample B.

Detection and Restoration of Defective Lines in the SPOT 4 SWIR Band

This paper presents the categorization and restoration of defective lines developed in pushbroom images. About 100 of the 3000 SPOT 4 SWIR detectors malfunction, which degrades image quality. Conventional methods have difficulties in effectively detecting and restoring defective lines, because they ignore the heterogeneity of the ground surface and the presence of sporadically unstable detectors with gain and offset that vary during a scan. While all defective lines have previously been considered as a single type, here they are categorized into three types according to the variation pattern in the scanning direction: constant defective lines, irregular defective lines, and irrecoverable defective lines. The detection procedure utilizes summed data and standard deviation data that consist of abnormal peaks originating from defective lines and a slowly varying baseline reflecting the surface characteristics within the image. The defective lines are detected by finding abnormal peaks, and classified and restored by using either a moment-matching method or interpolation, depending upon their types. Three SPOT 4 images were used to test and evaluate the performance of the proposed method. From the test results, the constant defective line was the most common type, comprising about 60%, while the irregular defective lines caused serious image degradation because of the difficulty of detecting and classifying them. Commission and omission errors were less than 10% and detection accuracy was higher than 90%. The analysis of signal-to-noise ratio (SNR) showed that the low SNR created by the defective lines was effectively removed. Our method gave a significant improvement of the detection and restoration capability.

Enhanced diffusion and interdiffusion in HgCdTe from fermi-level effects

Excessive dopant or compositional mixing (interdiffusion) during the processing of HgCdTe photodiodes can lead to significant reductions in device performance. With the advent of multi-color and wider bandgap detectors, processes developed for single color LWIR and MWIR devices may not be transferable to the more complex structures. An important factor to account for in processing multicolor and wider gap HgCdTe is the effect of the Fermi level on point defect (PD) concentrations. In general, the density of PDs that have donor states in the band gap will be boosted in the presence of acceptors through the energy gained by the donor state electrons dropping into the vacant acceptor states. The density of PDs that have acceptor states in the band gap will be boosted in the presence of donors through a similar compensation mechanism. This Fermi-level effect is increasingly more important as the band gap is widened. Since almost all diffusion is mediated by either native and/or dopant point defects, and the intrinsic carrier concentration is relatively low at typical processing temperatures, significant broadening of composition and dopant profiles can occur in moderately and heavily doped HgCdTe. In this paper, we illustrate the Fermi-level effect on diffusion with two examples: compositional interdiffusion in multicolor detectors and diffusion of indium in MWIR and SWIR detectors.

VSWIR to VLWIR MBE grown HgCdTe material and detectors for remote sensing applications

The molecular beam epitaxy (MBE) growth technology is inherently flexible in its ability to change the Hg1−xCdxTe material’s bandgap within a growth run and from growth run to growth run. This bandgap engineering flexibility permits tailoring the device architecture to the various specific requirements. Material with active layer x values ranging from ∼0.198 to 0.570 have been grown and processed into detectors. This wide range in x values is perfectly suited for remote sensing applications, specifically the National Polar Orbiting Environmental Satellite System (NPOESS) program that requires imaging in a multitude of infrared spectral bands, ranging from the 1.58 to 1.64 µm VSWIR (very short wave infrared) band to the 11.5 to 12.5 µm LWIR (longwave infrared) band and beyond. These diverse spectral bands require high performance detectors, operating at two temperatures; detectors for the VSWIR band operate near room temperature while the SWIR, MWIR (mid wave infra red), LWIR and VLWIR (very long wave infrared) detectors operate near 100K, because of constraints imposed by the cooler for the NPOESS program. This paper uses material parameters to calculate theoretical detector performance for a range of x values. This theoretical detector performance is compared with median measured detector optical and electrical data. Measured detector optical and electrical data, combined with noise model estimates of ROIC performance are used to calculate signal to noise ratio (SNR), for each spectral band. The SNR are compared with respect to the meteorological NPOESS system derived focal plane. The derived system focal plane requirements for NPOESS are met in all the spectral bands.

Mercury Cadmium Telluride Short- And Medium-Wavelength Infrared Staring Focal Plane Arrays

Short-wavelength (1 to 2.5 µm) and medium-wavelength (1 to 4.0 µm and 1 to 4.8 µm) 64 x 64 hybrid focal plane arrays (FPAs) have been developed. The short-wavelength (SWIR) arrays were developed for use in a prototype Airborne Imaging Spectrometer system. The medium-wavelength (MWIR) arrays are suitable for tactical missile seekers and strategic surveillance systems. The detector material is HgCdTe LPE grown on a sapphire substrate. The unit cell size is 52 µm x 52 µm, and the detectors are ion-implanted planar structures. The multiplexer is a surface- or buried-channel, four-phase charge-coupled device. The unit cell has a direct-injection FET and a storage capacitor. The current-voltage characteristics of the 64 x 64 detector arrays were measured at near-zero background using Si fan-out chips. For the detector arrays with a 2.5 µm cutoff, the mean RoA for 29 random elements was 2.7 x 106 cm2 at T = 150 K. The mean RoA was 3 x 104 cm2 at 120 K for detector arrays with a 4.6 µm cutoff. Both results were adequate for specific system applications. The arrays were also characterized at different temperatures, and the results are presented herein. The 64 x 64 SWIR and MWIR detector arrays were mated to a multiplexer, and the resulting FPAs were characterized under different operating conditions. Detailed characterization results are presented.