Long-wavelength Infrared Detector Research Articles

Multistep InAs/InAsSb staircase nBn long-wavelength infrared detectors with enhanced charge carrier transport

Gallium-free Type-II superlattices (T2SLs) have emerged as promising candidates for high-performance long-wavelength infrared (LWIR) detectors at a high operating temperature. However, InAs/InAsSb barrier detectors suffer from the limited absorption efficiency, resulting in a trade-off between quantum efficiency (QE) and dark current. Here, a nBn LWIR detector based on multistep strain-balanced InAs/InAs0.5Sb0.5 superlattice staircase configuration is proposed. With this configuration, charge carrier transport and collection can be efficiently improved via the multistep staircase valance band, leading to an increased QE from 19.3% to 20.8% at a bias of −0.20 V. As a result, the device with multistep staircase configuration exhibits an improved performance, including a low dark current density of 4.13 × 10−5 A/cm2, high responsivity of 1.23 A/W and peak detectivity reaching 3.10 × 1011 cm·Hz1/2/W. By optimizing absorbing length of the multistep staircase absorber, the peak QE can be further boosted from 20.8% to 28.4% while maintaining a low dark current level, providing an alternative strategy to improve LWIR T2SL device performance.

A compact LWIR borescope sensor for 2D engine component surface temperature measurement

A compact long-wavelength infrared (LWIR) borescope imaging sensor operating in the wavelength range of 8–14 μm is designed and developed for non-contact two-dimensional (2D) surface temperature measurements in gas-turbine engines. The LWIR detection minimizes optical interferences from hot combustion gases and soot (emission within UV–mid IR region). Using simulations, the system is optimized for improving the signal collection efficiency and minimize image aberrations. The LWIR borescope probe is shielded by the custom-build compact water-cooled probe housing (outer diamter (OD) 19 mm, inner diameter (ID) 10 mm), which can sustain flame temperature up to 2400 K at a pressure of 50 bar. It facilitates long-term optical diagnostics inside the actual high-pressure combustion facilities where extreme thermal acoustic perturbation and intense heat fluxes are encountered. The design, construction, and characterization of the LWIR borescope sensor are discussed in detail. We present the 2D surface temperature measurements of a V-gutter bluff-body with simple nitrogen gas cooling in a laboratory testbed. The results indicate the sensor’s ability to accurately measure surface temperature with low background noise and to track transient cooling effect. The water-cooled probe housing was tested to verify the effectiveness of fast heat dissipation in high-temperature combustion environment to avoid unwanted thermal signals from the LWIR probe window. The developed LWIR sensor suite has promising applications in surface temperature measurements of engine components. The findings of this study may aid propulsion system engineers and researchers in designing thermal management systems and optimizing operation.

High-Performance Ultra-Broadband Absorber for Polarized Long-Wavelength Infrared Light Trapping

Conventional infrared polarization imaging and detection systems are normally complex, bulky, and expensive, since rare existing absorber components possess polarization selectivity, especially in the long-wavelength infrared (LWIR) range. In this study, we propose a novel high-performance metamaterial absorber with ultra-broadband high absorption and polarization selectivity in the entire LWIR atmospheric window. The absorber can realize an absorptance higher than 90% with a 4750 nm bandwidth due to different surface plasmons coupled with optical phonons. It exhibits precise polarization-selective absorption at the target wavelength; specifically, it can selectively trap TM-polarized light while restricting the absorption of TE-polarized light. In addition, the designed metamaterial exhibits remarkable spectral stability when the geometric parameters of the microstructure change, which is of great benefit in its manufacturing process. The proposed ultra-thin absorber provides a promising approach to simplify LWIR imaging and detection systems and further improve their performance.

Growth of high material quality InAs/GaSb type-II superlattice for long-wavelength infrared range by molecular beam epitaxy

By optimizing the V/III beam-equivalent pressure ratio, a high-quality InAs/GaSb type-II superlattice material for the long-wavelength infrared (LWIR) range is achieved by molecular beam epitaxy (MBE). High-resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectrometer are used to characterize the material growth quality. The results show that the full width at half maximum (FWHM) of the superlattice zero-order diffraction peak, the mismatching of the superlattice zero-order diffraction peak between the substrate diffraction peaks, and the surface roughness get the best results when the beam-equivalent pressure (BEP) ratio reaches the optimal value, which are 28 arcsec, 13 arcsec, and 1.63 Å, respectively. The intensity of the zero-order diffraction peak is strongest at the optimal value. The relative spectral response of the LWIR detector shows that it exhibits a 100% cut-off wavelength of 12.6 μm at 77 K. High-quality epitaxial materials have laid a good foundation for preparing high-performance LWIR detector.

Nano-thermoelectric infrared bolometers

Infrared (IR) radiation detectors are used in numerous applications from thermal imaging to spectroscopic gas sensing. Obtaining high speed and sensitivity, low-power operation, and cost-effectiveness with a single technology remains to be a challenge in the field of IR sensors. By combining nano-thermoelectric transduction and nanomembrane photonic absorbers, we demonstrate uncooled IR bolometer technology that is material-compatible with large-scale CMOS fabrication and provides fast and high sensitivity response to long-wavelength IR (LWIR) around 10 µm. The fast operation speed stems from the low heat capacity metal layer grid absorber connecting the sub-100 nm-thick n- and p-type Si nano-thermoelectric support beams, which convert the radiation induced temperature rise into voltage. The nano-thermoelectric transducer-support approach benefits from enhanced phonon surface scattering in the beams, leading to reduction in thermal conductivity, which enhances the sensitivity. We demonstrate different size nano-thermoelectric bolometric photodetector pixels with LWIR responsitivities, specific detectivities, and time constants in the ranges 179 V/W–2930 V/W, 1.5 × 107 cm Hz1/2/W–3.1 × 108 cm Hz1/2/W, and 66 µs–3600 µs, respectively. We benchmark the technology against different LWIR detector solutions and show how nano-thermoelectric detector technology can reach the fundamental sensitivity limits posed by phonon and photon thermal fluctuation noise.

Scalable fabrication of long-wave infrared PtSe2-G heterostructure array photodetectors

Two-dimensional (2D) materials with excellent optoelectronic properties have attracted tremendous research interest in recent years. The promising performances of photodetectors based on 2D materials, such as ultrafast photoresponse and ultrahigh photoresponsivity, have been demonstrated in the visible to short-wavelength infrared spectrum range (0.8–2 μm). However, high performance, room temperature operation long-wavelength infrared (LWIR) photodetection is challenging. The detectors based on graphene usually exhibit low photoresponsivity due to the low optical absorption and short carrier lifetime. In addition, the relatively large bandgap of transition metal dichalcogenides limited the photoresponse bandwidth. Here, we report a way to fabricate a scalable device array of room-temperature operation LWIR PtSe2-G heterostructure detectors. The photoresponsivity at 10.6 μm up to ∼300 mA/W is obtained. The long-wave infrared light in the pico-watt range could be detected at room temperature by the PtSe2-G heterostructure detector. This result indicates that the PtSe2-G heterostructure device could be a highly competitive candidate for an uncooled LWIR detector. It also opens a way for a scalable array infrared focus plane device for the LWIR image.

InSbNBi/InSb heterostructures for long wavelength infrared photodetector applications: A 16 band k · p study

The intriguing potential of III-V-N-Bi materials like InSbNBi can lead to pervasive research curiosity in the long wavelength infrared (LWIR) regime. In this article, we have explored numerous prospective possibilities of utilizing InSbNBi for optoelectronic applications using a 16 band k · p Hamiltonian. Considering the lattice-matched condition of InSbNBi with host InSb, we have anticipated the bandgap, spin–orbit coupling splitting energy (ΔSO) and the corresponding operating wavelength of InSb0.9772N0.0028Bi0.02 as 68 meV, 0.824 eV, and around ∼18.23 μm, respectively. At room temperature, a wide range of selective bandgaps and related wavelengths ranging from 160 meV (∼8 μm) to 40 meV (∼30 μm) were obtained for Bi and N concentrations up to 2.5% and 0.35%, respectively. Co-incorporation of N and Bi results in ∼1.5 times reduction in the electron effective mass (0.0091 m0) compared to the host (0.014 m0), which further improves the optical gain of the InSbNBi/InSb quantum well system. The effect of both types of strain (compressive and tensile) on the InSbNBi/InSb system generated due to the deviation from the lattice matched ratio (0.14) of N and Bi offers interesting results. Along with a red shift in optical spectra, compressive strain (∼0.1%) offers a reduction in bandgap, electron effective mass, and enhancement in ΔSO by 44.7 meV, 0.0024 m0, and 12 meV, respectively. On the contrary, tensile strain (∼1.14%) increases the bandgap and the electron effective mass by 26.7 meV and 0.0066 m0, respectively, and reduces ΔSO by 219 meV. Nevertheless, tensile strain beyond 0.25% for a N concentration of 1.3% and fixed Bi concentration (1%) convert the InSbNBi/InSb heterostructure from a type I structure to a type II broken gap structure, which enables the possibility of realizing InSbNBi/InSb material for tunnel junction devices and the intermediate band solar cell along with the LWIR detector.

Design of infrared microspectrometer based on phase-modulated axilenses

We design and characterize a novel axilens-based diffractive optics platform that flexibly combines efficient point focusing and grating selectivity and is compatible with scalable top-down fabrication based on a four-level phase mask configuration. This is achieved using phase-modulated compact axilens devices that simultaneously focus incident radiation of selected wavelengths at predefined locations with larger focal depths compared with traditional Fresnel lenses. In addition, the proposed devices are polarization-insensitive and maintain a large focusing efficiency over a broad spectral band. Specifically, here we discuss and characterize modulated axilens configurations designed for long-wavelength infrared (LWIR) in the 6 µm–12 µm wavelength range and in the 4 µm–6 µm midwavelength infrared (MWIR) range. These devices are ideally suited for monolithic integration atop the substrate layers of infrared focal plane arrays and for use as compact microspectrometers. We systematically study their focusing efficiency, spectral response, and cross-talk ratio; further, we demonstrate linear control of multiwavelength focusing on a single plane. Our design method leverages Rayleigh–Sommerfeld diffraction theory and is validated numerically using the finite element method. Finally, we demonstrate the application of spatially modulated axilenses to the realization of a compact, single-lens spectrometer. By optimizing our devices, we achieve a minimum distinguishable wavelength interval of Δ λ = 240 n m at λ c = 8 µ m and Δ λ = 165 n m at λ c = 5 µ m . The proposed devices add fundamental spectroscopic capabilities to compact imaging devices for a number of applications ranging from spectral sorting to LWIR and MWIR phase contrast imaging and detection.

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.

InAs/GaAsSb Type-II Superlattice LWIR Focal Plane Arrays Detectors Grown on InAs Substrates

InAs/GaAsSb type-II superlattice (T2SL) materials grown at a higher temperature and lattice-matched to InAs substrates are considered to have significant advantages in long wavelength infrared (LWIR) detection. In this work, an InAs/GaAsSb T2SL LWIR focal plane array (FPA) was fabricated and evaluated. The FPA has a format of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$320\times256$ </tex-math></inline-formula> with a pixel pitch of 30 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and exhibits a 100% cutoff wavelength of 9.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> at 80 K. Under a bias of -0.02V, the detectors show a dark current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.7\times 10^{-5}$ </tex-math></inline-formula> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a differential resistance-area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\times 10^{3} \Omega \cdot \text {cm}^{2}$ </tex-math></inline-formula> . The noise equivalent temperature difference and operability of the FPA are 20.7 mK and 99.2% respectively under an integration time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$400~\mu \text{s}$ </tex-math></inline-formula> , a 300 K background and F/2.0 optics. This high-performance FPA further verifies the feasibility of InAs/GaAsSb T2SL in LWIR detection.

Phase-modulated axilenses for infrared multiband spectroscopy.

We design and characterize compact phase-modulated axilens devices that combine efficient point focusing and grating selectivity within four-level phase mask configurations. Specifically, we select and characterize in detail two device configurations designed for long-wavelength infrared (LWIR) operation in the $ 6\,\,\unicode{x00B5}{\rm m}\! -\! 12\,\,\unicode{x00B5}{\rm m} $6µm-12µm wavelength range. These devices are ideally suited for monolithic integration atop the substrate layers of infrared focal plane arrays (IR-FPAs) for use in multiband LWIR photodetection. We systematically study their focusing efficiency, spectral response, and crosstalk ratio, and we demonstrate a single-component microspectrometer. Our design method leverages the Rayleigh-Sommerfeld (RS) diffraction theory that is validated numerically using the finite element method (FEM). The proposed devices are broadband and polarization insensitive and add fundamental spectroscopic capabilities to miniaturized optical components for a number of applications in LWIR detection and spectroscopy.

Double Resonant Cavity of Oxides/Metal/Nitrides for Long Wavelength Infrared Sensing

Sub wavelength double resonant cavity structures of oxides/metal/nitrides (OMN) for bolometric detection in the long wavelength infrared (LWIR) range are studied for the first time. An optimized OMN structure allows for a significant enhancement in effective absorbance (50%-70%) with respect to that of the bare film on an amorphous Al2O3 substrate (1-30%) in the LWIR atmospheric transparent window (8-14 μm). Remarkable improvement in the response range and sensitivity have been achieved by designing the double cavity OMN structure, where the bottom cavity originates from the high reflectance between the transition metal oxide film and the Al2O3 substrate, while the silicon nitride film on the top serves as a quarter wavelength anti-reflection cavity centered at about 8 μm. The OMN structured LWIR detector can be realized by cost effective methods of chemical solution deposition (CSD) or radio frequency (RF) sputtering deposition. A detectivity of 0.18-0.2×108 cm·Hz0.5/W@6-10 Hz was achieved by using the O...

Light-harvesting for high quantum efficiency in InAs-based InAs/GaAsSb type-II superlattices long wavelength infrared photodetectors

Because of the limited absorption coefficient and undesired reflection loss, it has been a challenge to obtain sufficient light absorption for type-II superlattice (T2SL) long wavelength infrared (LWIR) detectors with a thin thickness. Here, we report a greatly enhanced photon absorption by light harvesting in InAs/GaAsSb T2SLs grown on InAs substrates. High quantum efficiencies (QEs) were first observed in a set of InAs/GaAsSb superlattice LWIR photodetectors with different absorption layer thicknesses. Long minority carrier diffusion lengths are also indicated. However, these high QEs are mainly attributed to the large refractive index difference between the n-doped InAs buffer layer and the SL material, which not only improves the optical path length of incident light by multireflection in the SL material but also reduces the surface reflection by dual-layer thin-film interference at some special wavelengths. By further adjusting the doping concentration of the buffer layer, we achieved a LWIR detector with high QE of more than 60% based on InAs/GaAsSb T2SLs with a thin absorption region thickness of 3.55 μm without any antireflection coating. Our result provides an effective strategy for thin-film detectors to boost their photoresponse, especially for materials with low absorption coefficients.

Electronic band structure of InAs/InAsSb type-II superlattice for HOT LWIR detectors

Abstract The InAs/InAs1−xSbx type-II superlattices (T2SLs) grown on GaSb buffer layer and GaAs substrates have recently been applied for detectors for long wavelength infrared (LWIR) range and high operating temperature (HOT) conditions. The detailed modeling of T2SLs minibands structure required for detector’s design optimization process relies on accurate knowledge of the InAs1−xSbx bandgap and band edge position. The k∙p (8 × 8 method) was used to analyze the valence band offset (VBO) between InAs and InAs1−xSbx, and hence the InAs1−xSbx band edge position at required xSb composition. An increase of the VBO leads to the structure transformation from type-IIb to type-II superlattices. The required energy bandgap can be reached by changing the conduction band offset (CBO) and the energy bandgap bowing parameter. The temperature dependence of the experimental results of InAs/InAs1−xSbx T2SLs energy bandgap was found to be comparable with theoretical one when energy bandgap bowing parameter is dependent on temperature. The proper fitting of theoretically calculated and experimentally measured spectral response characteristics was shown.

Dark current improvement due to dry-etch process in InAs/GaSb type-II superlattice LWIR photodetector with nBn structure

The dark current of long wavelength infrared (LWIR) detector should be reduced to improve performance and decrease the system size, weight and power consumption (SWaP). In this work, we present the process to reduce the dark current of T2SL LWIR detector with nBn structure. The nBn structure consists of InAs/GaSb 14/7 monolayers (MLs) contact layer and absorber separated by Al0.2Ga0.8Sb barrier. For T2SL LWIR detector, the barrier structure is often used to reduce the dark current and passivate absorber. However, the barrier itself is very fragile and easily damaged during the device process. The proposed process was developed so that Al0.2Ga0.8Sb barrier is not damaged by any solution from the isolation etch process to the passivation process. In this process, CHF3 plasma treatment after the isolation etch with ICP reduced significantly the dark current. We attribute this to damage reduction of Al0.2Ga0.8Sb barrier by CHF3 plasma treatment. The 15 μm pitch devices fabricated with optimal process conditions showed the dark current density of less than 2.0 × 10−5 A/cm2 at −0.3 V and 80 K.

ICP etching for InAs-based InAs/GaAsSb superlattice long wavelength infrared detectors

In this work, we study and report the dry etching processes for InAs-based InAs/GaAsSb strain-free superlattice long wavelength infrared (LWIR) detectors. The proper etching parameters were first obtained through the parametric studies of Inductively Coupled Plasma (ICP) etching of both InAs and GaSb bulk materials in Cl2/N2 plasmas. Then an InAs-based InAs/GaAsSb superlattice LWIR detector with PπN structure was fabricated by using the optimized etching parameters. At 80 K, the detector exhibits a 100% cut-off wavelength of 12 μm and a responsivity of 1.5 A/W. Moreover, the dark current density of the device under a bias of −200 mV reaches 5.5 × 10−4 A/cm2, and the R0A is 15 Ω cm2. Our results pave the way towards InAs-based superlattice LWIR detectors with better performances.

A high signal-to-noise ratio passive near-field microscope equipped with a helium-free cryostat.

We have developed a passive long-wavelength infrared (LWIR) scattering-type scanning near-field optical microscope (s-SNOM) installed in a helium-free mechanically cooled cryostat, which facilitates cooling of an LWIR detector and optical elements to 4.5 K. To reduce mechanical vibration propagation from a compressor unit, we have introduced a metal bellows damper and a helium gas damper. These dampers ensure the performance of the s-SNOM to be free from mechanical vibration. Furthermore, we have introduced a solid immersion lens to improve the confocal microscope performance. To demonstrate the passive s-SNOM capability, we measured thermally excited surface evanescent waves on Au/SiO2 gratings. A near-field signal-to-noise ratio is 4.5 times the improvement with an acquisition time of 1 s/pixel. These improvements have made the passive s-SNOM a more convenient and higher-performance experimental tool with a higher signal-to-noise ratio for a shorter acquisition time of 0.1 s.

Performance analysis of an N-structure type-II superlattice photodetector for long wavelength infrared applications

We propose a high performance N-structure type-II superlattice (T2SL) photodetector for long wavelength infrared (LWIR) applications. Theoretical study on the electrical and optical properties of the N-structure T2SL LWIR detector with different carrier concentration and different thicknesses in the p-region is carried out. We obtain the relationship of material parameters and performance parameters. With the increasing of the thickness of p-region, quantum efficiency (QE) gradually improves. In contrast, zero-bias resistance area (R0A) product decreases. QE will significantly decreases when the carrier concentration of p-region is too high, while R0A first increases, then decreases. The results show that the performance of the N-structure T2SL LWIR detector can be state of the art by optimizing the carrier concentration and material thickness during the structure growth and device fabrication.

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.