Middle Wavelength Infrared Research Articles

Characterizations of bismuth telluride/gallium nitride heterojunction photovoltaic detector for MWIR detection under room temperature

Uncooled infrared detectors have aroused keen interest in narrow bandgap semiconductor research. We propose to use Bismuth Telluride (Bi2Te3) bandgap energy of about 0.15 eV for middle wavelength IR (MWIR) detection. In this work, a heterojunction structure of Bi2Te3/GaN/Al2O3 was fabricated to form a photovoltaic detector. The structural properties of Bi2Te3 thin films were characterized by high resolution x-ray diffraction spectroscopy. Crystallinity of Bi2Te3 thin films was found to be strongly dependent on deposition temperatures. The spectral current responsivity for this heterojunction in NIR-to-MWIR region was evaluated under back-side illumination. The devices were also characterized by studying their low-frequency noise spectra measurement. The experimental results suggest that, the noise level of the devices is correlated to the crystallinity of the Bi2Te3 thin films.

Moxa sticks: Thermal properties and possible implications for clinical trials

The aim of the study was to examine the infrared spectrum and the irradiance of different types of moxa sticks to develop a basis for a moxa stick control therapy. An IR radiometer was used to measure the spectral infrared irradiance of seven glowing moxa sticks of different types in dependence of distance to the heat. All sticks investigated showed a similar spectral distribution of the emitted IR radiation with maxima of about 83.5-87.5% in the range long-wavelength IR radiation (IR-C) and with small contributions of both short wavelength IR radiation (IR-A) between 2.2% and 5.5% and of middle wavelength IR radiation (IR-B) between 9.0% and 12.1% of the total IR emission. Study results showed that only a small proportion of the IR radiation emitted by moxa sticks is capable of affecting subepidermal tissue. This finding indicates that thermal effects of moxa sticks are caused primarily by superficial effects on the skin. Because most heat receptors are located in the superficial skin, it thus appears impossible to separate the effects of moxa sticks from the sensation of heat. These results should be taken into account while developing placebo or sham moxibustion devices.

Electrical and optical properties of bismuth telluride/gallium nitride heterojunction diodes

Semiconducting bismuth telluride has a band gap energy of about 0.15eV at room temperature and is a good material for middle wavelength IR (MWIR) detection [S.K. Mishra, S. Satpathy, O. Jepsen, J. Phys. Condens. Matter, p. 461]. We have grown bismuth telluride thin films on gallium nitride (on sapphire) by pulsed laser deposition at a substrate temperature of 300°C. The structural characteristics and surface morphology of the films were studied by X-ray diffraction and scanning electron microscopy, respectively. The chemical composition of the as-deposited bismuth telluride thin films was determined by X-ray photoelectron spectroscopy and was found to be different from that of the bulk target, changing from stoichiometric Bi2Te3 to bismuth rich. A bismuth rich p-Bi2Te3/n-GaN/Al2O3 heterojunction was fabricated for photovoltaic detection of low energy photons. The wide band gap semiconducting n-GaN layer and the Al2O3 substrate act as a window for IR transmission. A sensitive IR photoresponse of the heterojunction is obtained by back-side illumination. The photocurrent measured is 0.18μA according to the irradiation change in the current–voltage characteristics. The current–voltage characteristics allow us to evaluate the series resistance (Rs), zero-bias resistance (R0) and ideality factors (n) of the junctions. Our results have suggested that bismuth telluride is a potential candidate for photovoltaic mid-infrared detection.

Impact assessment of Aqua MODIS band-to-band misregistration on snow index

The MODerate Resolution Imaging Spectroradiometer (MODIS) is a key instrument for the NASA Earth Observing System (EOS) mission. It was successfully launched onboard the Terra satellite in December 1999 and Aqua satellite in May 2002. MODIS senses the Earth's surface in thirty-six spectral bands which are distributed on four Focal Plane Assemblies (FPAs): Visible (VIS), Near-Infrared (NIR), Short-and Middle-wavelength IR (SMIR), and Long-wavelength IR (LWIR). It was found from sensor pre-launch measurements that Aqua MODIS SMIR/LWIR FPAs had a large misalignment or misregistration relative to the VIS/NIR FPAs in both along-scan and along-track directions. The misregistration of the two FPA groups has remained nearly the same during its on-orbit operation. Consequently this has been a major concern for Aqua MODIS performance since it could affect the quality of MODIS products which utilize bands from both the VIS/NIR and SMIR/LWIR FPAs, for example, the snow index. This paper focuses on investigating the impact of Aqua MODIS FPA-to-FPA or band-to-band misregistration on its snow index (NDSI) derived from measurements made by VIS band 4 and SWIR band 7. Preliminary results show that shifting one pixel (500 m) forward in the along-track direction of band 7 can improve the band-to-band registration between bands 4 and 7 and, therefore, the quality of Aqua MODIS snow mapping. This study will help MODIS data users to understand the potential impact of band-to-band misregistration on MODIS science products, and also be useful for the future sensor design.

Molecular beam epitaxy growth of HgCdTe on Ge for third-generation infrared detectors

The Leti-Lir has studied II–VI compounds for infrared (IR) detection for more than 20 years. The need to reduce the production cost of IR focal plane arrays (FPAs) sparked the development of heteroepitaxy on large-area substrates. Germanium has been chosen as the heterosubstrate for the third generation of IR detectors. First, we report on the progress achieved in HgCdTe growth on 3-in. and 4-in. (211)B CdTe/Ge. Then, we discuss the choice of a new machine for larger size and better homogeneity. Finally, we present the latest results on third-generation IR multicolor and megapixel devices. First-time results regarding a middle wavelength infrared (MWIR) dual-band FPA, with a reduced pitch of 25 µm, and a MWIR 1,280×1,024 FPA will be shown. Both detectors are based on molecular beam epitaxy (MBE)-grown HgCdTe on Ge. The results shown validate the choice of Ge as the substrate for third-generation detectors.

InAs/GaInSb superlattices as a promising material system for third generation infrared detectors

Hitherto, two families of multielement detectors have been used for infrared applications: scanning systems (first generation) and staring systems (second generation). Third generation systems are being developed nowadays. In the common understanding, third generation IR systems provide enhanced capabilities like larger number of pixels, higher frame rates, better thermal resolution as well as multicolour functionality and other on-chip functions. In the class of third generation infrared photon detectors, two main competitors, HgCdTe photodiodes and AlGaAs/GaAs quantum well infrared photoconductors (QWIPs) are considered. However, in the long wavelength infrared (LWIR) region, the HgCdTe material fail to give the requirements of large format two-dimensional (2-D) arrays due to metallurgical problems of the epitaxial layers such as uniformity and number of defective elements. A superlattice based InAs/GaInSb system grown on GaSb substrate seems to be an attractive alternative to HgCdTe with good spatial uniformity and an ability to span cut-off wavelength from 3 to 25 μm. The recently published results have indicated that high performance middle wavelength infrared (MWIR) InAs/GaInSb superlattice focal plane arrays can be fabricated. Also LWIR photodiodes with the R 0 A values exceeding 100 Ωcm 2 even with a cut-off wavelength of 14 μm can be achieved. Based on these very promising results it is obvious now that the antimonide superlattice technology is competing with HgCdTe dual colour technology with the potential advantage of standard III–V technology to be more competitive in costs and as a consequence series production pricing.

QWIP products and building blocks for high performance systems

Standard GaAs/AlGaAs quantum well infrared photodetectors (QWIP) are coming out from the laboratory. In this paper we demonstrate that production and research cannot be dissociated in order to make the new generation of thermal imagers benefit as fast as possible from the building blocks developed by researchers. Since 2002, the THALES group has been manufacturing sensitive arrays using QWIP technology based on GaAs techniques through THALES Research and Technology Laboratory. This QWIP technology allows the realization of large staring arrays for thermal imagers (TI) working in the IR band III (8–12 μm). A review of the current QWIP products is presented. In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and was the key parameter for the production initiation. Another advantage widely claimed also for QWIPs was the so-called band-gap engineering, allowing the custom design of quantum structure to fulfill the requirements of specific applications like very long wavelength or multispectral detection. In this paper, we present the performances for Middle Wavelength InfraRed (MWIR) detections and demonstrate the ability of QWIP’s to cover the two spectral ranges (3–5 μm and 8–20 μm). Last but not least, the versatility of the GaAs processing appeared for QWIPs as an important gift. This assumption was well founded. We give here some results achieved on building blocks for two color QWIP pixels. We also report the expected performances of focal plane arrays that we are currently developing with the CEA-LETI-SLIR.

MBE P-on-n Hg1−xCdxTe heterostructure detectors on silicon substrates

The capability of growing state-of-the-art middle wavelength infrared (MWIR)-HgCdTe layers by molecular beam epitaxy (MBE) on large area silicon substrates has been demonstrated. We have obtained excellent compositional uniformity with standard deviation of 0.001 with mean composition of 0.321 across 1.5″ radii. R0A as high as 5 × 107 ω-cm2 with a mean value of 7 × 106 Θ-cm2 was measured for cut-off wavelength of 4.8 µm at 77K. Devices exhibit diffusion limited performance for temperatures above 95K. Quantum efficiencies up to 63% were observed (with no anti-reflection coating) for cut-off wavelength (4.8–5.4) µm @ 77K. Excellent performance of the fabricated photodiodes on MBE HgCdTe/CdTe/Si reflects on the overall quality of the grown material in the MWIR region.

Heterojunction GexSi1-x/Si infrared detectors and focal plane arrays

Heterojunction Ge_<i>x</i>Si_1-<i>x</i>/Si internal-photoemission infrared detectors are being developed for multispectral imaging in the middle-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral bands. The detectors, which are fabricated by molecular beam epitaxy of degenerately doped Ge_<i>x</i>Si_1-<i>x</i> heteroepitaxial layers on Si, exhibit high responsivity uniformity, low dark-current noise, tunable cutoff wavelength out to 25 μm, and excellent producibility. High-quality MWIR and LWIR thermal imagery has been obtained for 320-x 244- and 400- x 400-element focal plane arrays consisting of Ge_<i>x</i>Si_1-<i>x</i>/Si detectors with cutoff wavelength of ~10 μm and monolithic CCD readout circuitry.