Ga-free InAs/InAsSb T2SL XBn detector is now a reliable candidate for high-performance focal plane arrays in the MWIR (3-5μm) domain. However, this T2SL is a very anisotropic quantum structure having a type-IIb band offset alignment where electrons are rather delocalized all over the structure while holes are strongly confined in deep InAsSb quantum well. This configuration could penalize the absorption and the hole minority carrier transport but MWIR detector device without anti reflection coating shows quantum efficiency higher than 50%. Considering results of specific measurements and band structure calculation, possible carrier transport scenario is presented to explain such performance.
Monolithic integration of InAs/InAsSb type-II superlattice (T2SL) photodetector on large-scale Si wafers would allow the development of a low-cost, high-quality, Si-readout integrated circuit compatibility focal plane array (FPA). In this study, we compare the performances of MWIR InAs/InAsSb T2SL samples grown on Si and GaSb substrates. The material quality is investigated with High-Resolution X-ray Diffraction, Atomic Force Microscopy, and Photoluminescence (PL). A minority carrier lifetime of 800 ns at 150 K is extracted from time-resolved PL on the sample grown on GaSb/Si templates with dislocation filtering layers. The device performances will be reported at the conference.
Achieving higher operating temperatures is a key-point in the current infrared photodetection research. One promising way to achieve this goal is through the reduction of the thickness of the active region and the use of optical resonators to compensate the consequent loss of absorption. Herein we present simulation results of the absorption in a thin LWIR T2SL photodetector, capped with heavily doped semiconductors nanostructures and the benches used to measure their properties.
In this communication, we report on electrical and electro-optical characterizations of InAs/InAsSb Type-II superlattice (T2SL) MWIR photodetector, showing a cut-off wavelength at 5 μm. The device, made of a barrier structure in XBn configuration, was grown by molecular beam epitaxy (MBE) on GaSb substrate. At 150K, dark current measurements shows a device in the Shockley-Read-Hall (SRH) regime but with an absolute value comparable to the state-of-the-art. A quantum efficiency of 50% at the wavelength of 3 μm for a 3 μm thick absorption layer is found in simple pass configuration and front-side illumination. Combined with lifetime measurements performed on dedicated samples through time resolved photoluminescence (TRPL) technique, mobility is extracted from these measurements by using a theoretical calculation of the quantum efficiency thanks to Hovel’s equations. Such an approach helps us to better understand the hole minority carrier transport in Ga-free T2SL MWIR XBn detector and therefore to improve its performance.
In this paper, we study the influence of three different etching depths on electrical and electro-optical properties of nonpassivated T2SL nBn Ga-free pixel detector having a 5μm cut-off wavelength at 150 K. The study shows the strong influence of lateral diffusion length on the shallow etched pixel properties and therefore, the need to perform etching through the absorber layer to avoid lateral diffusion contribution. The lowest dark current density was recorded for a deep-etched detector, on the order of 1 × 10-5 A/cm2 at 150 K and operating bias equal to – 300 mV. The quantum efficiency of this deep-etched detector is measured close to 55 % at 150 K, without anti-reflection coating. A comparison between electro-optical performances obtained on the three etching depths demonstrates that the etching only through the middle of the absorber layer (Mid-etched) allows eliminating lateral diffusion contribution while preserving a good uniformity between the diode’s performance. Such result is suitable for the fabrication of IR focal plane arrays (FPA).
Stability over time has recently become a figure of merit of major importance to compare the performances of infrared focal plane arrays (FPA) of different technologies. Indeed, this parameter dictates how often the calibration of operational electro-optical systems has to be done, and thus reflects the availability of the system during an operational mission. The stability over time is generally estimated through fixed pattern noise (FPN) and residual fixed pattern noise (RFPN) measurements after a two-point correction. However, each laboratory or industrial has its own protocols and criteria, such that published results cannot be easily compared. Recent studies also showed that random telegraph signal (RTS) noise, which leads to flickering pixels, can strongly affect the image quality, so the question arises as to wether these RTS pixels have an effect on RFPN. In this paper, we describe our experimental protocol to evaluate the stability over time of an FPA and to count up/classify flickering pixels. We then present the results obtained on a T2SL MWIR Integrated Detector Dewar Cooler Assembly (IDDCA) provided by IRnova. Our measurements show that the stability over time of the T2SL MWIR IDDCA are excellent: first, in terms of FPN/RFPN; then, in terms of RTS noise with only a few blinking pixels. We also show that the RTS pixels having an effect on the RFPN are fully detected by the algorithm used to rule out defective pixels before calculating RFPN.
Type-II InAs/GaSb superlattice (T2SL) has recently matured into a commercially available technology addressing both MWIR and LWIR spectral domains. As the prerequisites such as Quantum Efficiency (QE) and dark current were met, more advanced figures of merits related to the ElectroOptic (EO) system as a whole can now be studied in order to position this technology. In this paper, we focus on modulation transfer function (MTF) measurements. Knowing the MTF of a detector is indeed of primary importance for the EO system designers, since spatial filtering affects the system range. We realized MTF measurements on a 320x256 MWIR T2SL FPA provided by IRnova, using a Continuously Self Imaging Grating (CSIG). The advantage of this experimental configuration is that no high performance projection optics is required. Indeed, the CSIG exploits the self-imaging property (known as Talbot effect) to project a pattern with known spatial frequencies on the photodetector. Such MTF measurements have never been done in Integrated Detector Dewar Cooler Assembly (IDDCA) configuration, so we had to study the effect of the vibrations induced by the cryocooler. Vibrations indeed affect the MTF measurement in the same way electrical diffusion would do. Using three accelerometers we optimized our experimental setup and extracted MTF measurements with reduced vibrations. The pixel size is 26μm for a pitch of 30μm.
In recent years, Type-II InAs/GaSb superlattice (T2SL) has emerged as a new material technology suitable for high performance infrared (IR) detectors operating from Near InfraRed (NIR, 2-3μm) to Very Long Wavelength InfraRed (LWIR, λ > 15μm) wavelength domains. To compare their performances with well-established IR technologies such as MCT, InSb or QWIP cooled detectors, specific electrical and radiometric characterizations are needed: dark current, spectral response, quantum efficiency, temporal and spatial noises, stability… In this paper, we first present quantum efficiency measurements performed on T2SL MWIR (3-5μm) photodiodes and on one focal plane array (320x256 pixels with 30μm pitch, realized in the scope of a french collaboration ). Different T2SL structures (InAs-rich versus GaSb-rich) with the same cutoff wavelength (λc= 5μm at 80K) were studied. Results are analysed in term of carrier diffusion length in order to define the optimum thickness and type of doping of the absorbing zone. We then focus on the stability over time of a commercial T2SL FPA (320x256 pixels with 30μm pitch), measuring the commonly used residual fixed pattern noise (RFPN) figure of merit. Results are excellent, with a very stable behaviour over more than 3 weeks, and less than 10 flickering pixels, possibly giving access to long-term stability of IR absolute calibration.
This paper reports studies on spatial characteristics of Mid Wave InfraRed (MWIR) InAs/GaSb superlattices (T2SL) photodetectors. Modulation Transfer Function (MTF) measurements on commercial T2SL MWIR Focal Plane Array (FPA) are reported, using a Continuously Self Imaging Grating (CSIG). We find that measurements of the pixel size can be reliably achieved thanks to a new approach of data processing. Next, a new class of radiometric characterization, called "correctability", or ability for FPA pixels to durably keep the same behavior when exposed to a given radiometric flux, has been investigated on. Gain and offset corrections and Residual Fixed Pattern Noise (RFPN) measurements have also been made. The results obtained confirm the potentiality of high performance T2SL infrared photodetectors.
InAs/GaSb superlattice (SL) is a peculiar quantum system for infrared detection, where electrical and optical
properties are directly governed by the composition and the periodicity of the InAs/GaSb cell. Indeed, several
structures with different InAs to GaSb thickness ratios in each SL period, can target the same cut-off wavelength.
Likewise, the type of conductivity of the non-intentionally doped SL structure is also linked to the InAs/GaSb
SL period. The objective of this communication is to use the flexibility properties of InAs/GaSb SL to design
and then to fabricate by MBE a pin photodiode where the active zone is made of different SL periods. Electrical
and electro-optical characterizations are reported. The results show that SL structure for the MWIR domain can
be designed by combining the best of each SL periods.
We first present an electro-optical characterization of the radiometric performances of a type-II InAs/GaSb superlattice (T2SL) pin photodiode operating in the mid-wavelength infrared domain. This photodiode was grown with an InAs-rich structure. We focused our attention on quantum efficiency and responsivity: quantum efficiency of mono-pixel device reaches 23% at λ = 2.1 μm for 1 μm thick SL structure and 77K operating temperature. Then we measured the angular response of this photodiode: the response of the photodiode doesn’t depend on the angle of incidence of the flux. We also report the QE of 2μm-thick InAs-rich T2SL pin 320×256 pixels focal plane array, which reaches 61% at λ = 2.6 μm.
Today, both military and civilian applications require miniaturized and cheap optical systems. One way to achieve this trend consists in decreasing the pixel pitch of focal plane arrays (FPA). In order to evaluate the performance of the overall optical systems, it is necessary to measure the modulation transfer function (MTF) of these pixels. However, small pixels lead to higher cut-off frequencies and therefore, original MTF measurements that are able to extract frequencies up to these high cut-off frequencies, are needed. In this paper, we will present a way to extract 1D MTF at high frequencies by projecting fringes on the FPA. The device uses a Lloyd mirror placed near and perpendicular to the focal plane array. Consequently, an interference pattern of fringes can be projected on the detector. By varying the angle of incidence of the light beam, we can tune the period of the interference fringes and, thus, explore a wide range of spatial frequencies, and mainly around the cut-off frequency of the pixel which is one of the most interesting area. Illustration of this method will be applied to a 640×480 microbolometer focal plane array with a pixel pitch of 17µm in the LWIR spectral region.
In this communication, we examine the influence of the SL period of InAs/GaSb superlattice (SL), with diverse InAs to
GaSb thickness ratio, on the material and device properties of midwave infrared pin photodiodes. Three SL devices made
of three different periods, but exhibiting the same cut-off wavelength at 5 μm at 77K, were grown by molecular beam
epitaxy on p-type GaSb substrates. Optical and electrical characterizations (photoluminescence, current-voltage,
capacitance-voltage, and photoresponse measurements) were performed and analyzed in order to explain the results
obtained. Our investigations show the strong influence of the SL composition on both the material and photodetector
properties, such as residual doping concentration, shape of the response spectra and dark current values.
In this communication, we studied the influence of the SL period on the electrical performances of MWIR
pin photodiodes, fabricated by MBE on p-type GaSb substrate. These SL structures are made of symmetric or
asymmetric SL period designs and exhibited cut-off wavelength around 5μm at 77K. Experimental
measurements carried out on several SL pin photodiodes show the superiority, in terms of dark current
density, of the asymmetric SL structure composed of 7 InAs monolayers (MLs) and 4 GaSb MLs. As a result,
the 7/4 SL diode exhibits dark current density values as low as 40nA/cm2 and R0A product greater than
1.7x106 Ohm.cm2 at 77K, one decade larger than the value obtained with equivalent symmetric 10/10 SL diode. This result obtained demonstrates the strong influence of the SL period design on the performances,
and then on temperature operation, of MWIR SL photodiodes.
InAs/GaSb superlattice pin photodiodes showing asymmetrical period design were fabricated by MBE on ptype
GaSb substrate. These SL structures exhibited cut-off wavelength in the midwave infrared domain
(MWIR) at 5μm at 80K. Electrical characterizations including dark current and capacitance-voltage
measurements were performed on single detectors in the temperature range [77K-300K]. The SL photodiode
measurements revealed carrier concentrations of about 6x1014 cm-3 at 77K, dark current densities
J= 4x10-8 A/cm2 at 77K, J = 0.19A/cm2 at 200K and J = 10A/cm2 at 300K for Vbias =-50mV. The measured
R0A product is higher than 1.5x106Ω.cm2 at 77K and equal to 1x10-2Ω.cm2 at T=300K, for cut-off device
equal to 5μm and 6.05μm, respectively. These results are compared with the ones obtained by symmetrical
SL structure and show that the differential resistance area product is improved by more than one order of
magnitude. These results obtained help us to define the optimized SL pin structure design suitable for high
temperature operation in the MWIR domain.
We report the full electrooptical characterization of two MWIR InAs/GaSb superlattice (SL) pin
photodiodes. The first one features a symmetrical period with 8 InAs monolayers (MLs) and 8 GaSb MLs, while
the second one relies on an asymmetrical period with 7.5 InAs MLs and 3.5 GaSb MLs. This asymmetrical
design was recently proposed by IES to both decrease the dark current (since it decreases the intrinsic carrier
concentration) and increase the quantum efficiency (since it increases the wavefunctions overlap).
We present dark current, noise, spectral response and quantum efficiency measurements. Our results
confirm that the asymmetrical design allows to greatly improve the performance of MWIR SL pin photodiodes,
with an improvement of more than one decade in terms of dark current and an improvement of a factor 1.5 in
terms of quantum efficiency. The noise measurements under dark conditions show that the symmetrical
(asymmetrical) sample remains Schottky noise-limited up to a bias voltage of -600mV (resp -800mV) and that
1/f noise remains very low.
Symmetric and asymmetric mid-wavelength infrared (MWIR) InAs/GaSb superlattice (SL) pin photodiode were
fabricated by Molecular Beam Epitaxy (MBE) on p-type GaSb substrate and characterized as a function of temperature.
The symmetric SL structure was made of 8 InAs monolayers (MLs) and 8 GaSb MLs and exhibits at 80K a cut-off
wavelength (λc) of 4.5μm, while the asymmetric SL design was composed of 7.5 InAs MLs and 3.5 GaSb MLs for
λc = 5.5μm at 80K.
Optical characterizations made of photoluminescence as a function of temperature and room temperature absorption
spectra were performed on these two kinds of structures. Several electrical characterizations including dark current and
capacitance-voltage measurements were also carried out on single detectors in the temperature range [77K-300K].
Results obtained were compared and analyzed in order to define optimized SL structure design for the high performance
in the MWIR domain.
We report the full electrooptical characterization of a MWIR InAs/GaSb superlattice (SL) pin
photodiode, including dark current, noise, spectral response and quantum efficiency measurements. The SL
structure was made of 8 InAs monolayers (MLs) and 8 GaSb MLs, with a total thickness of 3μm. It exhibits a
cut-off wavelength of 4.55 μm at 77K. Dark current measurements reveal a diffusion-limited behavior for
temperatures higher than 95K, and a R0A value of 1x106Ωcm2 at 77K. Noise measurements were performed
under dark conditions and are interpreted in this paper. The results show that the SL detector remains Schottky
noise-limited up to a bias voltage of -600mV and that 1/f noise is not present above 6Hz. Spectral response
revealed that the cut-off wavelength increases from 4.48μm to 4.91μm when the temperature increases from 12K
to 170K. The quantum efficiency in photovoltaic mode and at 77K is 25% (3μm-thick active zone device, single
pass and without any antireflection coating). All these electrooptical performances confirm the high quality of
the MWIR SL pin photodiode under test.
Mid-wavelength infrared (MWIR) InAs/GaSb superlattice (SL) pin photodiodes were fabricated by molecular Beam
Epitaxy on p-type GaSb substrate. Dark current measurements as a function of temperature were performed on single SL
detectors with two different period designs: one made of standard symmetric 8 InAs monolayers (MLs) / 8 GaSb MLs
SL period, another made of alternative design with asymmetric 7.5 InAs MLs / 3.5 GaSb MLs SL period. Comparison of
results revealed the predominance of the asymmetric SL design showing an improvement of the differential resistance
area product of nearly two orders of magnitude. Spectral response measurements performed on asymmetric SL showed
that the quantum efficiency was more than doubled.
Infrared detectors based on InAs/GaSb superlattices (SL) have recently emerged as a promising technology for high
performance infrared (IR) imaging systems. In this paper, we present the results of dark current and noise measurements
realized on MWIR superlattice single detectors. The SL structure was made of 8 InAs monolayers (MLs) and 8 GaSb
MLs, for a total thickness of 2μm. This structure exhibits a cut-off wavelength of 4.8μm at 77K. An original chemical
etching solution was designed to obtain smooth mesa sidewalls, followed by a simple passivation technique. Dark
current measurements were carried out to prove the good quality of both the etching and the passivation steps. The
measured R0A product reaches the state-of-the-art values at 80K. Noise measurements were also performed under dark
conditions. The detectors under test proved to be Schottky-limited on a range of bias voltage of 200mV typically, which
confirms the very good quality of the technological process.
Electrical properties of non-intentionally doped (nid) InAs/GaSb Superlattice (SL) structures and p-nid-n detectors
grown by Molecular Beam Epitaxy on GaSb substrate are reported. The SL structures were made of 600 periods of 8
InAs monolayers (MLs) and 8 GaSb MLs, for a total thickness of 3ìm. This structure exhibited a cutoff wavelength in
the midwave infrared (MWIR) domain, near 4.7μm at 80K. Electrical transport measurements, based on resistivity and
Hall Effect measurements, were performed on SL structure after removing the conducting GaSb substrate with an
appropriate technological process. Carrier concentrations and mobilities carried out as a function of temperature (77-
300K) for magnetic fields in the 0-1 Tesla range are analyzed. A change in type of conductivity is observed. The nid SL
layers is p-type at liquid Nitrogen temperature while is n-type at room temperature. These results are completed with
diode characterizations based on current-voltage (I-V) and capacitance-voltage (C-V) measurements performed on p-nidn
devices with identical InAs/GaSb SL active zone.
QWIPs are nowadays considered as rather good candidates for multispectral imaging. Indeed, their narrow spectral bandwidth can be tailored by band gap engineering, and they rely on the use of the mature GaAs technology. However, a light coupling scheme, such as 2D-grating for example, is required to obtain normal incidence absorption. The geometrical parameters of this 2D-grating (grating period, etch depth of the grating) need to be optimized. This optimization is particularly difficult for dual-band pixels, since a compromise is required between the two wavelengths. This paper deals with the influence of the etch depth of the grating on the angular response of quantum well infrared photodetectors. We will first present the experimental setup which was specifically developed at ONERA to perform angular response measurements on infrared photodetectors from different technologies. The results obtained on "low-noise" QWIP samples provided by Fraunhofer-IAF will be presented and analyzed, with emphasis on the effect of the etch depth of the grating on the angular response.
A test bench has been developed at ONERA in order to measure the spectral responses of infrared focal plane arrays (IRFPAs). This test bench can deliver hyperspectral cartographies with rather good resolutions (better than 16 cm-1) on large spectral ranges (from 1.3 μm to 20 μm). The principle of this test bench will be described. Using this technique, tests have been performed on a large format (640x512) IRFPA of quantum-well technology operating in the 8- to 10-μm spectral range. The prototype tested had several small defects that produce spectacular hyperspectral cartographies. To explain the hyperspectral structures observed across the array, an empirical model based on Fourier optics will be presented.
As far as calibrated radiometric imaging is concerned, a complete prediction of oblique incidence effect on the FPA pixels’ response is required. Since a light coupling scheme needs to be used in QWIP detectors, this effect is particularly complicated to understand. This article presents two complementary test benches which allow to quantify the effect of oblique incidence on cooled infrared detectors issued from different technologies. The first test bench performs measurements over a wide angular range with low background emission levels, but gives spectrally integrated measurements. The second one delivers spectrally resolved responses for incident angles lower than 30°. In order to validate both experimental concepts, we studied QWIPs equipped with 2D periodic gratings. Relatively large pixels (100x100μm2) were chosen to ease comparison with models. Calculations based on the modal expansion method reveal that diffraction off an infinite grating does not account very well for the observed spectral responses.
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