Long-wavelength Infrared Detector Research Articles

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.

Low operating bias InAs/GaSb strain layer superlattice LWIR detector

Minimization of operating bias and generation–recombination dark current in long wavelength infrared (LWIR) strained layer superlattice (SLS) detectors, consisting of a lightly doped p-type absorber layer and a wide band gap hole barrier, are investigated with respect to the band alignment between the wide band gap barrier and absorber layers. Dark current vs. bias, photoresponse, quantum efficiency, lifetime, and modeling are used to correlate device performance with the wide gap barrier composition. Decreases in dark current density and operating bias were observed as the conduction band of the wide gap barrier was lowered with respect to the absorber layer. The device achieved 95% of its maximum quantum efficiency at 0V bias, and 100% by 0.05V. This study demonstrates key device design parameters responsible for optimal performance of heterojunction based SLS LWIR detectors.

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.

A performance assessment of type-II interband In0.5Ga0.5Sb QD photodetectors

Self-assembled quantum-dot (QD) structures with type-II band alignment to the surrounding matrix material have been proposed as a III/V material approach to realize small-bandgap device structures suitable for photon detection and imaging in the long-wavelength infrared (LWIR) band. Here, we analyze the photoresponse of In0.5Ga0.5Sb/InAs QD photodiodes and estimate the system performance of type-II QD –based photodetectors. A review of alternative design approaches is presented and the choice of matrix material is discussed in terms of band alignment and its effect on the photoresponse.Photodiodes were fabricated consisting of 10 layers of In0.5Ga0.5Sb QDs grown on InAs (001) substrates with metal–organic vapor-phase epitaxy (MOVPE). The photoresponse and dark current were measured in single pixel devices as a function of temperature in the range 20–230K. The quantum efficiency shows an Arrhenius type behavior, which is attributed to hole trapping. This severely limits the detector performance at typical LWIR sensor operating temperatures (60–120K). A device design with the matrix material InAs0.6Sb0.4 is proposed as a mean to improve the performance by reducing the barrier for hole transport. This can potentially allow type-II InGaSb QDs to be a competitive sensor material for LWIR detection.

Applications of the Infrared Measurement Analyzer: Hydrogenated LWIR HgCdTe Detectors

Low-cost silicon-based alternative substrates are an attractive choice for next-generation large-area high-resolution multicolor infrared (IR) detector arrays. However, the high density of dislocations formed during molecular-beam epitaxy growth of HgCdTe/CdTe/Si limits the performance of IR arrays, especially in the long-wavelength infrared (LWIR) region. Atomic hydrogen introduced by inductively coupled plasma (ICP) into HgCdTe is expected to passivate dislocations, bulk and surface defects, removing their contributions to dark current. Passivation using ICP hydrogenation can have different effects on HgCdTe photodiode performance, depending on which class of defects is being passivated. The infrared measurement analyzer (IRMA) was used to deconvolute the effects of hydrogenation on LWIR HgCdTe photodiodes through a reverse-modeling fit of the current–voltage (I–V) characteristic. This approach results in a fit with fewer false minima, low parameter error and bias, and high confidence in extracted device parameters. A description of this tool and its application to hydrogenated HgCdTe LWIR detectors is presented. Lower dark currents have been observed after hydrogenation of fully fabricated devices. Model-fits performed on a wide variety of LWIR HgCdTe photodiodes suggest that hydrogenation provides both surface and bulk quality improvements. These benefits of ICP hydrogenation have been retained over several months.

Modeling of HgCdTe LWIR detector for high operation temperature conditions

Abstract The paper reports on the photoelectrical performance of the long wavelength infrared (LWIR) HgCdTe high operating temperature (HOT) detector. The detector structure was simulated with commercially available software APSYS by Crosslight Inc. taking into account SRH, Auger and tunnelling currents. A detailed analysis of the detector performance such as dark current, detectivity, time response as a function of device architecture and applied bias is performed, pointing out optimal working conditions.

Optimization of thickness and doping of heterojunction unipolar barrier layer for dark current suppression in long wavelength strain layer superlattice infrared detectors

Suppression of generation-recombination dark current and bias stability in long wavelength infrared (LWIR) strained layer superlattice (SLS) detectors, consisting of a lightly doped p-type absorber layer and a wide bandgap hole barrier, are investigated with respect to the wide bandgap barrier layer thickness and doping profile. Dark current IV, photoresponse, and theoretical modeling are used to correlate device performance with the widegap barrier design parameters. Decreased dark current density and increased operating bias were observed as the barrier thickness was increased. This study also identifies key device parameters responsible for optimal performance of heterojunction based SLS LWIR detector.

Probing thermal evanescent waves with a scattering-type near-field microscope**This paper was presented at ISMQC-2010, the 10th International Symposium on Measurement and Quality Control, held in Osaka, Japan, on 5–9 September 2010. Two other papers from that meeting also appear in this issue.

Long wavelength infrared (LWIR) waves contain many important spectra of matters like molecular motions. Thus, probing spontaneous LWIR radiation without external illumination would reveal detailed mesoscopic phenomena that cannot be probed by any other measurement methods. Here we developed a scattering-type scanning near-field optical microscope (s-SNOM) and demonstrated passive near-field microscopy at 14.5 µm wavelength. Our s-SNOM consists of an atomic force microscope and a confocal microscope equipped with a highly sensitive LWIR detector, called a charge-sensitive infrared phototransistor (CSIP). In our s-SNOM, photons scattered by a tungsten probe are collected by an objective of the confocal LWIR microscope and are finally detected by the CSIP. To suppress the far-field background, we vertically modulated the probe and demodulated the signal with a lock-in amplifier. With the s-SNOM, a clear passive image of 3 µm pitch Au/SiC gratings was successfully obtained and the spatial resolution was estimated to be 60 nm (λ/240). The radiation from Au and GaAs was suggested to be due to thermally excited charge/current fluctuations and surface phonons, respectively. This s-SNOM has the potential to observe mesoscopic phenomena such as molecular motions, biomolecular protein interactions and semiconductor conditions in the future.

Doping dependence of minority carrier lifetime in long-wave Sb-based type II superlattice infrared detector materials

Theoretical modeling and experimental evidence show that the dark currents of antimonide (Sb)-based Type-II Superlattice (T2SL) detectors sensitive to long-wavelength infrared (LWIR) radiation are limited by the minority carrier lifetimes associated with the Shockley Read-Hall (SRH) generation-recombination process. Despite the low minority carrier lifetimes, the present photodiode dark current performance is achieved by increasing the background doping density of the absorber layers to NA ∼ 2×1016 cm−3. This paper will discuss the measured minority carrier lifetimes of T2SL LWIR detectors with various background doping densities. To make comparisons, we designed several T2SL absorber layers sensitive in the LWIR range and p-type doped up to various densities. We find the minority carrier lifetimes are dominated by the SRH generation-recombination process. By analyzing minority carrier lifetime data as a function of excess carrier densities, we estimate SRH lifetimes and actual ionized acceptor densities of the absorber layers. The data are compared with the measured impurity doping densities of the detectors. In addition, we predict the type of flaws responsible for the SRH recombination process and narrow down the flaw energy level relative to the Fermi energy.

Long-wavelength infrared quantum-dot based interband photodetectors

We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley–Read–Hall generation–recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal–organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 × 10 11 cm −2, 1 × 10 12 cm −2 and 3 × 10 10 cm −2 were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6–12 μm. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II interband transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.

Overview of alternative infrared detectors and focal plane arrays for LWIR applications

For a variety of scientific, space and defence applications, there is an increasing demand for long-wavelength infrared (LWIR) detector focal plane arrays and compact infrared instruments. In the first part, we present an overview of alternative detectors to standard mercury cadmium telluride photodiodes for LWIR detection, such as the HgCdTe avalanche photodiode, the quantum-well infrared photo-detectors, the superlattice detectors and the carbone nanotubes-based bolometers. In the second part, we focus on new concepts developed to meet the requirement of miniaturization of infrared instruments. Original IRFPA-based micro-optical assemblies have been achieved, demonstrating several optical functions such as imagery, spectral filtering, spectrometry and wavefront sensing.

Towards dualband megapixel QWIP focal plane arrays

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) 1024×1024 pixel quantum well infrared photodetector (QWIP) focal planes have been demonstrated with excellent imaging performance. The MWIR QWIP detector array has demonstrated a noise equivalent differential temperature (NEΔT) of 17mK at a 95K operating temperature with f/2.5 optics at 300K background and the LWIR detector array has demonstrated a NEΔT of 13mK at a 70K operating temperature with the same optical and background conditions as the MWIR detector array after the subtraction of system noise. Both MWIR and LWIR focal planes have shown background limited performance (BLIP) at 90K and 70K operating temperatures respectively, with similar optical and background conditions. In addition, we have demonstrated MWIR and LWIR pixel co-registered simultaneously readable dualband QWIP focal plane arrays. In this paper, we will discuss the performance in terms of quantum efficiency, NEΔT, uniformity, operability, and modulation transfer functions of the 1024×1024 pixel arrays and the progress of dualband QWIP focal plane array development work.

Integrated HBT/QWIP structure for dual color imaging

An integrated two-terminal structure of quantum well infrared photodetector (QWIP) and heterojunction bipolar phototransistor (HBT) for dual band imaging is presented. The integrated device functions as a voltage switchable dual band detector for long wavelength infrared (LWIR) and near infrared (NIR) imaging. The device consists of series integration of floating base HBT and a QWIP in a two terminal configuration. Switching between the two spectral bands is achieved by changing the applied bias voltage. At a low bias voltage the HBT operates in the saturation mode and, due to its higher impedance, it is the only portion of the device that responds to NIR radiation. With the increasing bias voltage the HBT enter the breakdown region to activate the operation of the QWIP as LWIR detector. In order to achieve NIR response up to about 1.064 μm, strained layers of InGaAs/GaAs quantum wells were implemented in the sub-collector of the HBT. The NIR photocurrent acts as the base current of the HBT and is amplified inherently by the transistor gain. Furthermore, the breakdown of the HBT is designed to follow the punch-through mode so that one can engineer the breakdown voltage to be of the order of a few volts. In addition, this mode allows fast, reliable and nondestructive switching of the integrated device. Standard design of GaAs/AlGaAs QWIPs layers have been used to achieve LWIR detection. The integrated device, which is based on the GaAs technology, allows fabrication of large focal plane array (FPA) that can be operated using a commercial two-terminal readout integrated circuit (ROIC). Such FPA configuration would allow multispectral applications such as simultaneous imaging of a NIR laser spot superimposed on a thermal imaging scene (SEE SPOT).

Development of mid-wavelength and long-wavelength megapixel portable QWIP imaging cameras

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) megapixel quantum well infrared photodetector (QWIP) focal plane arrays have been demonstrated with excellent imaging performance. The MWIR detector array has shown noise equivalent temperature difference (NETD) of 17 mK at 95 K operating temperature with f/2.5 optics at 300 K background and the LWIR detector array has given NETD of 13 m K at 70 K operating temperature with the same optical and background conditions as the MWIR array. Two portable prototype infrared cameras were fabricated using these two focal planes. The MWIR and the LWIR prototype cameras with similar optics have shown background limited performance (BLIP) at 90 K and 70 K operating temperatures respectively, at 300 K background. In this paper, we will discuss their performance in quantum efficiency, NETD, uniformity, and operability.

1024 × 1024 pixel mid-wavelength and long-wavelength infrared QWIP focal plane arrays for imaging applications

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) 1024 × 1024 pixel quantum well infrared photodetector (QWIP) focal planes have been demonstrated with excellent imaging performance. The MWIR QWIP detector array has demonstrated a noise equivalent differential temperature (NEΔT) of 17 mK at a 95 K operating temperature with f/2.5 optics at 300 K background and the LWIR detector array has demonstrated a NEΔT of 13 mK at a 70 K operating temperature with the same optical and background conditions as the MWIR detector array after the subtraction of system noise. Both MWIR and LWIR focal planes have shown background limited performance (BLIP) at 90 K and 70 K operating temperatures respectively, with similar optical and background conditions. In this paper, we will discuss the performance in terms of quantum efficiency, NEΔT, uniformity, operability and modulation transfer functions.

HgCdTe/Si materials for long wavelength infrared detectors

The heteroepitaxial growth of HgCdTe on large-area Si substrates is an enabling technology leading to the production of low-cost, large-format infrared focal plane arrays (FPAs). This approach will allow HgCdTe FPA technology to be scaled beyond the limitations of bulk CdZnTe substrates. We have already achieved excellent mid-wavelength infrared (MWIR) and short wavelength infrared (SWIR) detector and FPA results using HgCdTe grown on 4-in. Si substrates using molecular beam epitaxy (MBE), and this work was focused on extending these results into the long wavelength infrared (LWIR) spectral regime. A series of nine p-on-n LWIR HgCdTe double-layer heterojunction (DLHJ) detector structures were grown on 4-in. Si substrates. The HgCdTe composition uniformity was very good over the entire 4-in. wafer with a typical maximum nonuniformity of 2.2% at the very edge of the wafer; run-to-run composition reproducibility, realized with real-time feedback control using spectroscopic ellipsometry, was also very good. Both secondary ion mass spectrometry (SIMS) and Hall-effect measurements showed well-behaved doping and majority carrier properties, respectively. Preliminary detector results were promising for this initial work and good broad-band spectral response was demonstrated; 61% quantum efficiency was measured, which is very good compared to a maximum allowed value of 70% for a non-antireflection-coated Si surface. The R0A products for HgCdTe/Si detectors in the 9.6-µm and 12-µm cutoff range were at least one order of magnitude below typical results for detectors fabricated on bulk CdZnTe substrates. This lower performance was attributed to an elevated dislocation density, which is in the mid-106 cm−2 range. The dislocation density in HgCdTe/Si needs to be reduced to <106 cm−2 to make high-performance LWIR detectors, and multiple approaches are being tried across the infrared community to achieve this result because the technological payoff is significant.

Investigation of a multistack voltage-tunable four-color quantum-well infrared photodetector for mid- and long-wavelength infrared detection

A four-color quantum-well infrared photodetector (QWIP) for mid- and long-wavelength infrared (MWIR and LWIR) detection has been demonstrated in this work. Four stacks of QW structures with four different detection wavelengths are sandwiched between three highly doped contact layers. Peak detection wavelengths of this device are centered at 4.7, 8.5, 9, and 12.3 /spl mu/m, respectively. The 4.7- and 8.5-/spl mu/m stacks are separated from the 9- and 12.3-/spl mu/m stacks by a middle contact layer, and the peak detection wavelength within these two double-stack QWIP's can be tuned by the applied bias. Four different combinations of two-color simultaneous readings can be achieved. By using a small number of QW's and balancing the impedance between the stacks, we are able to use all four stacks for voltage-tunable multicolor detection with two terminals. The bias and temperature dependence of the dark current and peak responsivity as well as the dynamic impedance in this four-color QWIP were systematically studied over a wide range of bias and temperature. In spite of using four different stacks of strained InGaAs-AlGaAs and GaAs-AlGaAs materials, the device shows excellent material quality and performance.

The growth and characterization of two new P-type compressively strained layer InGaAs/AlGaAs/GaAs quantum well infrared photodetectors for mid- and long-wavelength infrared detection

Investigation of two p-type compressively-strained layer (PCSL) InGaAs/AlGaAs/GaAs quantum well infrared photodetectors (QWIPs) grown on (10 0) semi-insulating (SI) GaAs substrate has been carried out. The first detector uses a step-bound-to-miniband (SBTM) transition scheme for long wavelength infrared (LWIR) detection and has a detection peak at 10.4 μm with a full width at half-maximum bandwidth, Δλ/λ p = 20%. A responsivity of 28 mA/W was obtained for this detector at T = 65 K and V = 3.0 V, with a spectral detectivity D* = 1.4 x 10 9 cm Hz 1/2 W -1 at T = 65 K and V = 1.0 V. The detector was under the background limited performance (BLIP) at T = 40K and V ≤ 2.0 V. The second detector is a two-color stacked QWIP composed of a PCSL In 0.2 Ga 0.8 As/Al 0.3 Ga 0.7 As QWIP for the MWIR detection at λ p = 4.8 μm and a PCSL In 0.15 Ga 0.85 As/Al 0.1 Ga 0.9 As for the LWIR detection at λ p = 10 μm. The peak responsivity for the LWIR QWIP was found to be 25 mA/W at V b = 2 V, T = 40 K, λ p = 10 μm, a FWHM of Δλ/λ p = 40%, and the calculated detectivity was found to be D* = 1.1 x 10 10 cm Hz 1/2 W -1 . Two response peaks for the MWIR QWIP were found to be at 4.8 and 5.4 μm with maximum responsivities of 12 and 19 mA/W (at T = 77 K and V b = 5 V), and spectral bandwidths of 21 and 26%, respectively. The detectivity (D*) for the MWIR stack determined at λ p = 5.4 μm, V b = 1.0 V and T = 77 K was found to be 5.5 x 10 11 cm Hz 1/2 W -1 .

Enhanced responsivity of Hg 0.80Cd 0.20Te infrared photoconductors using MBE grown heterostructures

A long wavelength infrared (LWIR) n-type Hg 1− x Cd x Te photoconductor device ( λ cutoff = 14.8 μm) has been developed using two-layer heterostructures grown by in-situ molecular beam epitaxy (MBE). The experimental results indicate that incorporating a wider bandgap capping layer ( x = 0.24, 2 μm thick) on an LWIR absorbing layer ( x = 0.20, 10 μm thick) results in improved responsivity, particularly at higher applied bias, when compared with single-layer LWIR detectors. A comprehensive device model is used to extract effective contact recombination velocities of 235 cm/s for two-layer photoconductors, and 820 cm/s for single-layer devices. The corresponding excess carrier lifetimes are 0.64 μs and 0.55 μs, respectively, compared with the calculated value of 1.16 μs for bulk recombination limited by Auger and radiative mechanisms. The experimental data clearly demonstrates the advantages of employing an MBE-grown Hg 1− x Cd x Te heterostructure to reduce contact recombination, and as a technology for forming in-situ grown surface passivation layers.

Improved device technology for epitaxial Hg1-xCdxTe infrared photoconductor arrays

The performance of Hg1-xCdxTe infrared photoconductors is strongly dependent on the semiconductor surface conditions and, in particular, the degree to which the surface contributes to recombination of photogenerated excess carriers. Although published photoconductor fabrication processes based on bulk Hg1-xCdxTe address this issue by fully passivating both major surfaces (i.e. front and back) with anodically grown native oxide, passivation of the sidewalls is neglected. In this paper it is shown both theoretically and experimentally that leaving the sidewalls unpassivated can result in approximately a factor of two reduction in responsivity for long-wavelength infrared (LWIR) detectors used in high-resolution thermal imaging systems. Detector arrays are typically fabricated on x=0.23 Hg1-xCdxTe representing a cut-off wavelength of 9.4 mu m and use individual element sizes of approximately 50*50 mu m2. We describe in detail for the first time a device technology which enables the fabrication of Hg1-xCdxTe photoconductor arrays such that the entire surface of the semiconductor is effectively passivated, including the sidewalls. Of particular interest is the fact that this improved device technology is compatible with present-day Hg1-xCdxTe epitaxial growth processes. This is in contrast to current photoconductor technology which is primarily based on bulk Hg1-xCdxTe. Experimental results are presented which compare device performance of LWIR detectors fabricated using the improved photoconductor technology with current published photoconductor technology. These results clearly indicate that detectors fabricated on liquid phase epitaxially (LPE) grown x=0.23 Hg1-xCdxTe material using the improved photoconductor device technology achieve much higher responsivities and detectivities. Furthermore, it is shown that only a fully passivated device structure is capable of exploiting any future improvements in bulk minority carrier lifetime as it approaches the Auger recombination limit.