Very Long Wavelength Infrared Research Articles

Wet etching and passivation of GaSb-based very long wavelength infrared detectors

The etching and passivation processes of very long wavelength infrared (VLWIR) detector based on the InAs/GaSb/AlSb type-II superlattice have been studied. By studying the effect of each component in the citric acid solution (citric acid, phosphoric acid, hydrogen peroxide, deionized water), the best solution ratio is obtained. After comparing different passivation materials such as sulfide + SiO2, Al2O3, Si3N4 and SU8, it is found that SU8 passivation can reduce the dark current of the device to a greater degree. Combining this wet etching and SU8 passivation, the R 0 A of VLWIR detector with a mesa diameter of 500 μm is about 3.6 Ω⋅cm2 at 77 K.

1/f Noise in InAs/InAsSb Superlattice Photoconductors

This article concerns the temperature dependence of low-frequency noise measured in the temperature range of 77–300 K in InAs/InAsSb superlattice photoconductors optimized for long-wavelength infrared (LWIR) and very long-wavelength infrared (VLWIR) spectral range. The differences in the T-dependence and the magnitude of 1/f noise, observed for LWIR and VLWIR photoconductors, are connected with different scattering mechanisms. Using the Hooge empirical model of 1/f noise and modeling of the total mobility, it is suggested that either the phonon-related or the defect-related mobility fluctuations can be responsible for the measured noise depending on the mechanism which determines the total mobility. In the VLWIR photoconductors, the defect-related 1/f noise prevails, while the phonon-related component dominates in the LWIR detector. The latter is best described by the Melkonyan et al. ’s mobility fluctuation model, which assumes the phonon scattering as the origin of 1/f noise. It roughly explains the T-dependence and predicts the value to within the one order of magnitude.

Space Application Requirement Breakdown and Sensor Concept Implementation for MCT-Based LWIR and VLWIR 2D High-Performance Focal Plane Detector Arrays at AIM

Future demanding scientific space and earth observation missions such as for exoplanet atmospheric spectral analysis or earth climate monitoring will define new benchmarks in technological feasibility and come along with increasingly stringent requirements on instrument subsystems. The requirements imposed on infrared detectors are dependent on the type of optical instrument, the chosen observation approach and the spectral range to be covered. With advanced detector architectures on hand and suitable␣technology approaches in mind, various electrical and electro-optical quantities have to be traded off against each other and possibly be prioritized, reflecting the specific instruments’ needs. From such considerations, implications on the detector material, design and processing arise. In this methodological paper, we will illustrate specific trade-off considerations for focal plane detector arrays (FPAs) based on mercury cadmium telluride (MCT) as detector material, e.g. with respect to modulation transfer function (MTF), detection efficiency (QE), noise level, defect density, etc. As an illustration, we will discuss simulation and implementation approaches and solutions found for a presently ongoing cornerstone pre-development activity in the long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR). In particular, we will consider the thermal infrared (TIR) channels of the European Space Agency (ESA) Land and Surface Temperature Monitoring Mission (LSTM) observation instrument, for which we will present a photodiode array design approach balanced for key electro-optical FPA performance parameters.

Behavior of the Reststrahlen Band in the 17-25 μm Spectral Region in the Diffuse Reflection Spectra of Sand and Silt Mixtures.

Studies on the reflectance spectra of different mixtures of sand and silt were performed in the infrared spectral region of 7-25 µm to explore the behavior of a second reststrahlen band occurring between 17 and 25 µm with respect to different mixtures of sand and silt. The reflectance spectrum of all samples exhibited reststrahlen bands within both the long wavelength infrared (LWIR) and the very long wavelength infrared (VLWIR) regions of the reflectance spectrum. Results demonstrate that both LWIR and VLWIR reststrahlen bands increase in area as the fraction of sand increases to 80%. More importantly, the data demonstrate that the VLWIR reststrahlen band, like that in LWIR, exhibits a significant and reproducible decrease in reflectivity for mixed soil versus weathered soil.

Carrier reduction studies of type-II superlattice materials for very long wavelength infrared sensing

There are continuing efforts to develop type-II superlattice (SL) materials for very long wavelength infrared (VLWIR) detector applications. However, the SLs have high residual electron background doping densities that depend on SL growth conditions, which lead to shorter minority carrier lifetime and lower performance parameters than theoretically predicted. In this study, the authors compare the technical advantages of using InAs/GaInSb over InAs/GaSb SL with respect to reducing the electron doping levels. Our temperature-dependent electrical transport measurements show that the InAs/GaInSb SL design has a lower electron density than the InAs/GaSb SL with the same bandgap and have electron densities (mobilities) on the order of the mid 1011 cm−2 (25 000 cm2/V s). Since small period InAs/GaInSb SLs also produce greater Auger recombination suppression for a given VLWIR gap than the large period InAs/GaSb SL, the InAs/GaInSb SL appears to be a better candidate for long lifetime IR materials for future very long wavelength infrared devices.

Bilayer graphene/HgCdTe based very long infrared photodetector with superior external quantum efficiency, responsivity, and detectivity.

We present a high-performance bilayer graphene (BLG) and mercury cadmium telluride (Hg1−xCdx=0.1867Te) heterojunction based very long wavelength infrared (VLWIR) conductive photodetector. The unique absorption properties of graphene enable a long carrier lifetime of charge carriers contributing to the carrier-multiplication due to impact ionization and, hence, large photocurrent and high quantum efficiency. The proposed p+-BLG/n-Hg0.8133Cd0.1867Te photodetector is characterized and analyzed in terms of different electrical and optical characteristic parameters using computer simulations. The obtained results are further validated by developing an analytical model based on drift-diffusion, tunneling and Chu's methods. The photodetector has demonstrated a superior performance including improved dark current density (∼1.75 × 10−14 µA cm−2), photocurrent density (∼8.33 µA cm−2), internal quantum efficiency (QEint ∼ 99.49%), external quantum efficiency (QEext ∼ 89%), internal photocurrent responsivity (∼13.26 A W−1), external photocurrent responsivity (∼9.1 A W−1), noise equivalent power (∼8.3 × 10−18 W), total noise current (∼1.06 fA), signal to noise ratio (∼156.18 dB), 3 dB cut-off frequency (∼36.16 GHz), and response time of 9.4 ps at 77 K. Furthermore, the effects of different external biasing, light power intensity, and temperature are evaluated, suggesting a high QEext of 3337.70% with a bias of −0.5 V near room temperature.

Progress of MCT Detector Technology at AIM Towards Smaller Pitch and Lower Dark Current

We present our latest results on cooled p-on-n planar mercury cadmium telluride (MCT) photodiode technology. Along with a reduction in dark current for raising the operating temperature (T op), AIM INFRAROT-MODULE GmbH (AIM) has devoted its development efforts to shrinking the pixel size. Both are essential requirements to meet the market demands for reduced size, weight and power and high-operating temperature applications. Detectors based on the p-on-n technology developed at AIM now span the spectrum from the mid-wavelength infrared (MWIR) to the very long wavelength infrared (VLWIR) with cut-off wavelengths from 5 μm to about 13.5 μm at 80 K. The development of the p-on-n technology for VLWIR as well as for MWIR is mainly implemented in a planar photodetector design with a 20-μm pixel pitch. For the VLWIR, dark currents significantly reduced as compared to ‘Tennant’s Rule 07’ are demonstrated for operating temperatures between 30 K and 100 K. This allows for the same dark current performance at a 20 K higher operating temperature than with previous AIM technology. For MWIR detectors with a 20-μm pitch, noise equivalent temperature differences of less than 30 mK are obtained up to 170 K. This technology has been transferred to our small pixel pitch high resolution (XGA) MWIR detector with 1024 × 768 pixels at a 10-μm pitch. Excellent performance at an operating temperature of 160 K is demonstrated.

Very long wavelength infrared focal plane arrays with 50% cutoff wavelength based on type-II InAs/GaSb superlattice**Project supported by the National Basic Research Program of China (Grant Nos. 2013CB932904 and 2011CB922201), the National Special Funds for the Development of Major Research Equipment and Instruments, China (Grant No. 2012YQ140005), and the National Natural Science Foundation of China (Grant Nos. 61274013, 61290303, and

A very long wavelength infrared(VLWIR) focal plane array based on InAs/GaSb type-II super-lattices is demonstrated on a GaSb substrate. A hetero-structure photodiode was grown with a 50% cut-off wavelength of 15.2 m, at 77 K. A 320×256 VLWIR focal plane array with this design was fabricated and characterized. The peak quantum efficiency without an antireflective coating was 25.74% at the reverse bias voltage of −20 mV, yielding a peak specific detectivity of cm·Hz ·W−1. The operability and the uniformity of response were 89% and 83.17%. The noise-equivalent temperature difference at 65 K exhibited a minimum at 21.4 mK, corresponding to an average value of 56.3 mK.

MCT-Based LWIR and VLWIR 2D Focal Plane Detector Arrays for Low Dark Current Applications at AIM

We present our latest results on n-on-p as well as on p-on-n low dark current planar mercury cadmium telluride (MCT) photodiode technology long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR) two-dimensional focal plane arrays (FPAs) with quantum efficiency (QE) cut-off wavelength >11 μm at 80 K and a 512 × 640 pixel format FPA at 20 μm pitch stitched from two 512 × 320 pixel photodiode arrays. Significantly reduced dark currents as compared with Tennant’s “Rule 07” are demonstrated in both polarities while retaining good detection efficiency ≥60% for operating temperatures between 30 K and 100 K. This allows for the same dark current performance at 20 K higher operating temperature than with previous AIM INFRAROT-MODULE GmbH (AIM) technology. For p-on-n LWIR MCT FPAs, broadband photoresponse nonuniformity of only about 1.2% is achieved at 55 K with low defective pixel numbers. For an n-on-p VLWIR MCT FPA with 13.6 μm cut-off at 55 K, excellent photoresponse nonuniformity of about 3.1% is achieved at moderate defective pixel numbers. This advancement in detector technology paves the way for outstanding signal-to-noise ratio performance infrared detection, enabling cutting-edge next-generation LWIR/VLWIR detectors for space instruments and devices with higher operating temperature and low size, weight, and power for field applications.

Investigations of quantum efficiency in type-II InAs/GaSb very long wavelength infrared superlattice detectors

In this paper, we have investigated the quantum efficiency (QE) of InAs/GaSb T2SL very long wavelength Infrared (VLWIR) photodetectors with 50% cutoff of 12.7 μm. Due to the small depletion width and similar absorption coefficient in the T2SL material system, the minority-carrier diffusion length was determined as the key element to improve the QE of VLWIR T2SL photodetectors. The minority-carrier diffusion length was estimated by a comparison of the experimental data with the Hovel model. Our result suggest that the short hole diffusion length (Lh ∼ 520 nm) and the large its ratio to the width of this region (xn/Lh) are considered against the photo-excited carrier collection in the T2SL photodetectors. In addition, the influence of surface recombination velocity (Sh) on the QE of the T2SL photodetectors is also studied. The change of QE with Sh is not so significant due to the relatively low absorption coefficient and short hole diffusion length in our photodetector.

Demonstration of long minority carrier lifetimes in very narrow bandgap ternary InAs/GaInSb superlattices

Minority carrier lifetimes in very long wavelength infrared (VLWIR) InAs/GaInSb superlattices (SLs) are reported using time-resolved microwave reflectance measurements. A strain-balanced ternary SL absorber layer of 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb, corresponding to a bandgap of ∼50 meV, is found to have a minority carrier lifetime of 140 ± 20 ns at ∼18 K. This lifetime is extraordinarily long, when compared to lifetime values previously reported for other VLWIR SL detector materials. This enhancement is attributed to the strain-engineered ternary design, which offers a variety of epitaxial advantages and ultimately leads to a reduction of defect-mediated recombination centers.

Latest Developments in Long-Wavelength and Very-Long-Wavelength Infrared Detection with p-on-n HgCdTe

We report recent developments at Commissariat a l’Energie Atomique-Laboratoire d’Electronique des Technologies de l’Information Infrared Laboratory on the processing and characterization of p-on-n HgCdTe (MCT) planar infrared focal plane arrays (FPAs) operating in the long-wavelength infrared (LWIR) and very-long-wavelength infrared (VLWIR) spectral bands. The active layers in these FPAs were grown by liquid phase epitaxy (LPE) on a lattice-matched CdZnTe substrate. This technological process results in lower dark current and lower serial resistance than for n-on-p vacancy-doped architecture and thus is better adapted for lower flux detection or higher operating temperature. This architecture was evaluated for space applications in the LWIR and VLWIR spectral bands with cutoff wavelengths from 10 to 17 μm at 78 K. Innovations have been introduced to the technological process to form a heterojunction by use of an LPE growth technique. The initial objective was to reduce the dark current at low temperatures, by reducing the transition temperature from diffusion-limited to depletion-limited dark current. Another advantage is that the wider bandgap obtained in the vicinity of the junction ensures less sensitivity to the defects present at the interface between MCT and passivation layers. Electro-optical characterization of p-on-n photodiodes was performed on quarter video graphics array format FPAs with 25 and 30 μm pixel pitches. The results revealed excellent operabilities in current and responsivity, with low dispersion, and noise limited by current shot noise. Studies performed on the dark current show that dark current densities at 78 K are consistent with the heuristic prediction law “Rule 07”. Below this temperature, dark current varies as a pure diffusion current for a variety of devices from different manufacturers, introducing a temperature range limitation in the description of the “Rule 07” law.

Modeling and Characterization of MTF and Spectral Response at Small Pitch on Mercury Cadmium Telluride

Space applications are challenging infrared (IR) technologies, demanding the best system performance achievable. This requires covering the entire IR spectrum from short-wavelength infrared (SWIR) to very long-wavelength infrared (VLWIR) for various pixel sizes, which is possible thanks to a well-mastered mercury cadmium telluride technology. Because of its adjustable gap, it can be operated in all the IR bands. Nevertheless, technology optimization requires deep understanding of physical mechanisms. This paper presents computations by finite-element modeling of two aspects of electrooptical performance: spectral response and modulation transfer function (MTF). Computations and characterizations for all IR bands demonstrate the accuracy of our simulations and the state-of-the-art nature of our technology, which performs according to theory. This paper also highlights the capability to measure MTF at very small pitch (10 μm) by a nondestructive method.

Growth optimization studies to develop InAs/GaInSb superlattice materials for very long wavelength infrared detection

In order to develop ternary antimonide-based superlattice (SL) materials for very long wavelength infrared (VLWIR) detection, systematic growth optimization studies were performed to produce high quality ternary materials. For the studies, a SL structure of 47.0Å InAs/21.5ÅGa0.75In0.25Sb was selected to create a very narrow band gap. Results indicate that an epitaxial process developed can produce a precisely controlled band gap around 50meV, but the material quality of grown SL layers is particularly sensitive to growth defects formed during the growth process. Since Group III antisites and strain-induced dislocations are the dominant structural defects responsible for the low radiative efficiencies, our optimization strategies to eliminate these defects have focused on stabilizing III/V incorporation during surface reconstruction by manipulating the growth surface temperature and balancing the residual strain of the SLs by adjusting the As/Sb flux ratio. The optimized ternary SL materials exhibited an overall strong photoresponse over a wide wavelength range up to ∼15μm that is important for developing VLWIR detectors. A quantitative analysis of the lattice strain, performed at the atomic scale by aberration corrected transmission electron microscopy, provided valuable information about the strain distribution at the interfaces that was important for optimizing the strain balancing process during SL layer growth.

Interband cascade infrared photodetectors with long and very-long cutoff wavelengths

We report our recent studies of interband cascade infrared photodetectors (ICIPs) with 100% cutoff wavelengths of 16.0μm and 9.2μm at 78K. The very-long-wavelength infrared (VLWIR) detectors were able to operate at temperatures up to 143K. Relatively high dark current densities were observed from these VLWIR detectors. This was attributed to the very narrow bandgap and possibly to defects in the materials. In addition, the photo-response of these detectors was strongly dependent on bias, indicating that further research and device optimization are necessary. Two-stage ICIPs in the long-wavelength infrared (LWIR) spectrum (8–12μm) were able to operate at higher temperatures (up to 250K) with an extended cutoff wavelength of ∼12μm. At 78K, these LWIR detectors had a bias-independent photocurrent, with an R0A of 115Ωcm2. This corresponded to a Johnson-noise-limited D∗ of 3.7×1010cmHz1/2/W at 8.0μm.

Higher Dislocation Density of Arsenic-Doped HgCdTe Material

There is a well-known direct negative correlation between dislocation density and optoelectronic device performance. Reduction in detector noise associated with dislocations is an important target for improvement of mercury cadmium telluride (Hg1−xCdxTe)-based material in order to broaden its use in the very long-wavelength infrared (VLWIR) regime. The lattice mismatch and differences in physical properties between substrates and the epitaxial Hg1−xCdxTe layers cause an increased threading dislocation density. As demonstrated in this work, the presence of arsenic impurities via p-type doping in molecular beam epitaxy (MBE)-grown epitaxial crystal structure increases the etch pit density (EPD) of Hg1−xCdxTe grown on Si substrates but not on CdZnTe substrates. This EPD increase is not observed in indium n-type-doped Hg1−xCdxTe grown on either Si or CdZnTe substrates. This trend is also seen in layers with different cadmium compositions. All of the EPD variations of the structures studied here are shown to be independent of the MBE machine used to grow the structure. The fundamentals of this higher EPD are not yet completely understood.

Infrared detectors for space applications

Abstract The motivation and intended benefits for the use of infrared (IR) detectors for space applications are highlighted. The actual status of state-of-the-art IR detectors for space applications is presented based on some of AIM’s currently ongoing focal plane detector module developments covering the spectral range from the short-wavelength IR (SWIR) to the long-wavelength IR (LWIR) and very long-wavelength IR (VLWIR), where both imaging and spectroscopy applications will be addressed. In particular, the integrated detector cooler assemblies for a mid-wavelength IR (MWIR) push-broom imaging satellite mission, for the German hyperspectral satellite mission EnMAP and the IR detectors for the Sentinel 3 SLSTR will be elaborated. Additionally, dedicated detector modules for LWIR/VLWIR sounding, providing the possibility to have two different PVs driven by one ROIC, will be addressed.

Narrow-band long-/very-long wavelength two-color type-II InAs/GaSb superlattice photodetector by changing the bias polarity

We report on a narrow-band two-color photodetector using type-II InAs/GaSb superlattices (SLs) in the long-/very-long wavelength infrared (VLWIR) ranges by changing the polarity of the bias. The narrow-band photoresponse is achieved by sequentially growing the doped SL structure that has a shorter cutoff wavelength as a low-pass filter for the absorption layers that has a longer cutoff wavelength. At 77 K, the 50% cutoff wavelength of the photodiode is 10 μm when the applied bias voltage is –0.1 V and is 16 μm at +40 mV. The δλ/λ is 44% for the LWIR band and is 46% for the VLWIR band.

Status of p-on-n Arsenic-Implanted HgCdTe Technologies

In this paper recent developments made by the French Atomic Energies and Alternative Energies Commission (CEA) at the Electronics and Information Technology Laboratory (LETI) on the fabrication of planar p-on-n HgCdTe photodiodes are reported. Results obtained on long-wavelength infrared (LWIR) liquid-phase epitaxy (LPE) and mid- and short-wavelength infrared (MWIR/SWIR) molecular beam epitaxy (MBE) have been previously published. For these photodiodes, p-type doping is obtained by arsenic implantation followed by diffusion and activation under Hg-overpressure annealing. The active layer is n-type doped by indium incorporation during growth. Control of the p-on-n junctions is one of the key points of this technology, requiring good knowledge of the arsenic implantation and diffusion and the evolution of implantation-induced defects. Concerning implantation, the impact of dose (from 2 × 1014 at./cm2 to 2 × 1015 at./cm2) and energy (from 50 keV to 500 keV) on As profiles is considered. The profiles after implantation are modeled using Pearson IV moments. Realistic descriptions of arsenic distributions are obtained, and the evolution of moments with implantation conditions is fitted. In addition, implantation damage is examined by transmission electron microscopy (TEM) and the evolution of defects is studied depending on implantation conditions and Hg-overpressure annealing. Previous results obtained on 30-μm-pitch LPE LWIR and MBE MWIR/SWIR showed state-of-the-art detector performance. Since these first results, progress has been made to decrease the pixel pitch to 15 μm and increase the focal-plane array (FPA) format. In this way, 640 × 512 LPE LWIR FPAs have been processed and characterized. In addition we report results obtained on our first p-on-n very long-wavelength infrared (VLWIR) photodiode fabricated at CEA–LETI with λc = 13.35 μm at 50 K. These latest results demonstrate the viability of our technology and materials.

Ultimate performance of quantum well infrared photodetectors in the tunneling regime

Thanks to their wavelength diversity and to their excellent uniformity, Quantum well infrared photodetectors (QWIP) emerge as potential candidates for astronomical or defense applications in the very long wavelength infrared (VLWIR) spectral domain. However, these applications deal with very low backgrounds and are very stringent on dark current requirements. In this paper, we present the full electro-optical characterization of a 15 μm QWIP, with emphasis on the dark current measurements. Data exhibit striking features, such as a plateau regime in the I( V) curves at low temperature (4–25 K). We show that present theories fail to describe this phenomenon and establish the need for a fully microscopic approach.