Thermal Imaging Applications Research Articles

STRUCTURE AND FERROELECTRICITY OF Ba0.8Sr0.2TiO3 THIN FILMS PREPARED BY A MODIFIED SOL-GEL PROCESSING

Quality Ba0.8Sr0.2TiO3 (BST) thin films with a remnant polarization of about 3.5μC/cm2,and a coercive field of about 53kV/cm have been successfully prepared by a sol-gel processing using a 0.05mol/L precursor solution.X-ray diffraction and field emission scanning electron microscope investigations show that the BST films exhibit a tetragonal structure (c/a=1.009) and consist dominantly of large columnar grains of about 150nm in diameter.The observed good ferroelectricity and insulating property render the sol-gel derived BST thin films promising for uncooled infrared detector and thermal imaging applications.

Pyroelectric Ba0.8Sr0.2TiO3 thin films derived from a 0.05 M solution precursor by sol–gel processing

Tetragonal Ba0.8Sr0.2TiO3 thin films with large columnar grains 100–200 nm in diameter have been prepared on Pt/Ti/SiO2/Si substrates using a 0.05 M solution precursor by sol–gel processing. The ferroelectric phase transition in the prepared Ba0.8Sr0.2TiO3 thin films is broadened, and suppressed to 40 °C with a maximum dielectric constant of εr (100 kHz)=680. The observed low dissipation factor tan δ=2.6% and high pyroelectric coefficient p=4.586×10−4 C/m2 K at 33 °C render the prepared Ba0.8Sr0.2TiO3 thin films promising for uncooled infrared detector and thermal imaging applications.

Update: a protein microbolometer for focal plane arrays

The current infrared microbolometer technology using vanadium oxide material is not quite as sensitive as a cooled platinum silicide or cooled mercury cadmium telluride or indium antimonide array. The performance factor of the present IR bolometer needs to be improved by at least an order of magnitude to make a bolometric IR device comparable to the cooled detector arrays. I recently reported [1] that thin films of globular proteins such as cytochrome c exhibit a high temperature coefficient of resistance (TCR) value of about 10% as compared with a value of 2.0 to 2.5% normally observed in VOx films. The measurements that were initially reported were taken two weeks after the thin films had been prepared. The measurement of the TCR values presented in this report were taken within three days of the films being prepared. These TCR values are very high, approximately 35% for temperatures between 25 and 60°C, and they have a sheet resistance around 1 to 2 Ω/cm2 with a current of 1 mA. The values of TCR were very uniform over the wafer surface, but the values varied slightly from wafer to wafer. It appears that a cytochrome c monolayer on metal oxide semiconductor (CMOS) electronics could generate miniaturized, uncooled IR detector arrays that would be suitable for military and commercial applications that require very high performance and low-power drain. With these high TCR values, biological uncooled devices will have the potential for long-range thermal imaging applications comparable to cooled detectors.I have also examined the sheet resistance of the protein layer with an increase in the bias current near ambient temperature. I found that the sheet resistance of cytochrome c rapidly goes towards the negative values as the bias current is increased. On the other hand, TCR characteristics remained unchanged with an increase in bias current. This is a very interesting phenomenon that has never been reported in biomolecules. I am making further explorations into the electrical properties of many proteins for possible device applications.

Molecular beam epitaxial growth of HgCdTe midwave infrared multispectral detectors

Molecular beam epitaxy (MBE) has been utilized to fabricate high performance HgCdTe infrared detectors with sensitivity to midwave infrared radiation in adjacent spectral bands for two-color thermal imaging applications. Growth of a multilayer HgCdTe device structure by MBE enables the use of an n-p-n device architecture that facilitates pixel-level registration of images in two separate spectral bands. Device structures were grown on CdZnTe(211)(B) substrates using CdTe, Te, and Hg sources with in situ In and As doping. The composition of the HgCdTe alloy layers was adjusted to achieve detection of infrared radiation in adjacent spectral bands in the 3.5–4.5 μm wavelength range. As-grown device structures were characterized with x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. Mesa type devices were patterned using reactive ion etching and ohmic contacts were made to the two n-type layers for operation of the detectors in a sequential detection mode. The spectral response characteristics of the devices are highly uniform across a 64×64 element array, with standard deviation in cutoff wavelength less than 0.01 μm and external quantum efficiencies greater than 70% in both bands. Sharp detector cutoffs enable spectral crosstalk less than 1% to be obtained for spectral bands with as little as 0.6 μm separation. Junction reverse-breakdown voltages in excess of 500 mV and 80 K dynamic resistance-area products for each component diode in excess of 1×106 Ω cm2 at ±100 mV operating bias have been demonstrated.

Thermal Wave Imaging For Characterizing Structures In Aging Aircraft

ABSTRACTWe present examples of the application of thermal wave imaging to the detection of structural defects in aging aircraft. Examples include the imaging of bonded and disbonded internal doublers, together with quantitative corrosion thinning measurements made from thermal wave images of aircraft fuselages and wing skins. We also show example images of impact damage, disbonds and delaminations in composite aircraft materials.

Infrared spectroscopic chemical imaging

Infrared (IR) spectroscopy is a powerful, widely used technique for identifying materials or chemical compounds. An IR spectrum often provides a specific fingerprint for a given molecular component or species. IR frequencies, intensities, and line widths are also extremely sensitive to environmental perturbations and changes in molecular structure. Infrared spectroscopic images recorded through a Fourier transform infrared (FT-IR) microscope attachment have traditionally been constructed by translating a mapping stage a single pixel at a time through the sample area of interest; this is a very tedious and time-consuming procedure. Recently, a technique for rapidly performing high-fidelity FT-IR imaging spectroscopy using an indium antimonide (InSb) focal-plane array (FPA) detector coupled to an IR microscope and a step-scanning FT-IR spectrometer has been developed. These multichannel IR detectors were originally developed for thermal-imaging applications (mainly in the military), but they have tremendous potential as chemical imaging detectors when used as part of a spectrometer. The multiple detector elements enable images from all pixels to be collected simultaneously for each mirror retardation position of the interferometer. Use of an interferometer allows the entire IR spectrum over some wavelength range to be measured. The combination of a step-scanning FT-IR microscope and an InSb FPA detector provides unprecedented speed and image quality, limited only by the diffraction limit and/or the number of detector elements on the array.

Ferroelectric IR bolometers - from ceramic hybrid arrays to direct thin film integration

Abstract Bolometer elements in large arrays, at a pitch from 100 down to 40μm, offer uncooled and simple operation in thermal imaging applications. A hybrid array technology, exploiting the pyroelectric property of ferroelectric ceramic materials in the bolometer elements, has produced a range of successful linear and 2-D arrays. High merit for the ceramic has been coupled to fabrication technologies including solder bump bonding of the elements to the multiplexer IC. However, other designs will compete for low costs in the large arrays required for civil applications. In particular, direct deposition of thin film ferroelectric material onto suitable thermal microstructures on the silicon readout IC, could provide substantial reductions in costs and improved performance. This integration will require processes compatible with the silicon IC. Already, lead based perovskite films are showing considerable potential. The successful technology will form the basis for a rapid growth of commercial IR imaging and...

IR-sensor array fabrication in Pb 1− xSn xSe-on-Si heterostructures

Large line or area arrays of photovoltaic infrared sensors are desired for thermal imaging and spectroscopic applications. The sensor arrays should be arranged on Si-substrates because of size, costs and the possibility to integrate the read-out electronics of large arrays directly into the Si-substrate. We therefore grow narrow gap Pb 1− x Sn x Se layers by molecular beam epitaxy (MBE) on Si(111)-substrates. A CaF 2 epitaxial buffer layer is used for compatibility reasons. Photovoltaic infrared sensor arrays for the 8–12 μm atmospheric window are fabricated in the 2–4 μm thick layers by photolithographic techniques with 8 mask levels and 6 simple wet etching steps. The operability of the sensor arrays is demonstrated by recording thermal images with a simple camera.

Si1?XGeX/Si quantum well infrared photodetectors

P-type Si1−XGeX/Si quantum well infrared photodetectors for thermal imaging applications have been grown by low pressure vapour phase epitaxy for the first time. Good control of periodicity in these pseudomorphic multiple quantum well structures on Si substrates was demonstrated by real-time ellipsometry during growth and subsequently confirmed by transmission electron microscopy. The photoresponse spectrum was tuned in the 8–13 Μm waveband by varying the Ge fraction in the Si1−XGeX quantum wells. Dark currents were reduced by doping only the centre regions of the quantum wells.

Fluoride glasses for modern optics

A very small number of multicomponent fluoride melts are associated with the formation of stable vitreous materials. When these unconventional glasses are sufficiently resistant towards devitrification, they can be shaped into sophisticated optical objects such as optical fibres. These original IR waveguides have found practical applications in remote spectroscopy, laser surgery and thermal imaging. Fluoride glasses also represent a unique optical host for rare earth ions, the low phonon energy of the lattice enabling high efficiency in laser emission and the observation of up-conversion phenomena. Very efficient blue-green fibre-laser emission and optical amplification for telecommunications have been recently demonstrated and are under development.

Integrated quantum well intersub-band photodetector and light emitting diode

The authors propose and demonstrate the integration of a quantum well intersub-band photodetector (QWIP) and a light emitting diode (LED) for making large two-dimensional focal plane arrays for thermal imaging applications. The newly developed QWIP technology is combined with the well established LED technology both based on GaAs and related epitaxially grown alloys, such as AlGaAs and InGaAs.

GaAs/AlGaAs quantum well infrared photodetector arrays for thermal imaging applications

The performance of GaAs/AlGaAs multiple quantum well infrared detectors is studied theoretically and experimentally, with special emphasis on 8-12 μm thermal imaging applications. The dependence of detector performance on various factors like light coupling configurations (one and two dimensional reflection gratings or 45° polished edge), detector temperature, response wavelength and quantum well doping density is dealt with. An absorption quantum efficiency of 87% is demonstrated using a crossed grating and a waveguide (CGW). It is also found that an optimised 34 μm × 34 μm detector (a detector size suitable for large staring arrays, i.e. 256 × 256 or larger) with 9.0 μm cut-off wavelength, f# = 2 optics and 70% optical transmission reaches background limited operation at 74 K detector temperature.The potential of making highly uniform staring arrays utilising the mature GaAs material and processing technology is demonstrated by uniformity measurements of detector dark current. The experiments show that a metalorganic vapour phase epitaxy (MOVPE) grown structure can have a dark current standard deviation to mean value ratio over a 10mm long linear detector array of less than 2%.The staring array performance in terms of noise equivalent temperature difference (temporal NETD) is calculated to NETD < 20 mK at 77 K detector temperature and NETD < 10 mK at 70 K detector temperature.

PyI11: Pyroelectric properties of thin film lead scandium tantalate

Abstract Electrical measurements on thin film ferroelectric lead scandium tantalate, deposited from solution, are described. It has good pyroelectric properties, approaching those of the bulk ceramic material. Its potential for thermal imaging applications is discussed. A theoretical specific detectivity of 4 × 108cmHz1/2W−1 has been calculated using measured film properties in a simple detector array structure.

PyX3: Ir bolometers and thermal imaging: The role of ferroelectric materials

Abstract Large arrays of bolometer elements have considerable potential for thermal imaging applications, offering uncooled operation, and a performance which challenges the cooled semiconductor detectors. A hybrid array technology, exploiting pyroelectric materials, is the basis of a successful range of linear and 2-D arrays. Ceramic uniformity for large area arrays is good but wafer processing is demanding. Other technologies will compete for cost-effectiveness in large area devices. Ferroelectric thin films have recently shown marked improvement in the merit figures and, if compatible with the silicon IC, may allow a more direct array fabrication. Thin film resistance bolometers have advantages in ease of fabrication, but there are also some inherent disadvantages, when compared with the ferroelectric operation.

Fabrication procedures of photovoltaic lead-chalcogenide-on-silicon infrared sensor arrays for thermal imaging

Epitaxial Pb1−xSnxSe layers have been grown onto Si(111) substrates with the aid of an intermediate CaF2/BaF2 buffer layer by MBE. Photovoltaic infrared sensor arrays with up to 256 elements for thermal imaging applications have been fabricated in the narrow gap lead chalcogenide layers. The whole growth and fabrication procedure was done at temperatures never exceeding 450°C on Si substrates containing prefabricated integrated circuits with standard Al-metallization for the first time.

Photovoltaic lead-chalcogenide IR-sensor arrays on Si for thermal imaging applications

Abstract Linear arrays of photovoltaic infrared sensors for thermal imaging applications are fabricated in narrow gap semiconductor layers grown heteroepitaxially on Si. Epitaxy is achieved using stacked intermediate BaF 2 SrF 2 CaF 2 buffers to overcome the large lattice-as well as thermal expansion mismatch. The arrays consist of 66 elements and cover cut-off wavelengths ranging from 3 to above 12 μm. Extrapolated resistance-area products of the best PbTe sensors (cut-off wavelengths5.7 μm) on Si are up to 20 000 Ω cm 2 at 77 K. They approach those of similar HgCdTe sensors fabricated in bulk or epitaxial material on CdTe substrates. Mean detectivities of whole PbTe on Si arrays at 90 K are as high as D JNL ∗ = 1.5×10 12 cm √ Hz / W .

Photovoltaic infrared sensors in heteroepitaxial PbTe on Si

Arrays of photovoltaic infrared sensors for thermal imaging applications have been fabricated in narrow gap PbTe grown heteroepitaxially on Si substrates. PbTe epitaxy was achieved with the aid of intermediate CaF2 /BaF2 buffer layers of only ≊2000 Å thickness. Blocking Pb contacts on about 3-μm-thick p-PbTe layers form the active areas of the sensors. Cutoff wavelengths are 5.6 μm, and resistance-area products are up to R0A=400 Ω cm2 at 85 K with mean value R0A≊150 Ω cm2 for 66 element linear arrays, well above the room-temperature photon field background noise limit. The temperature dependence of R0A indicates a depletion-limited noise current behavior between 250 and 100 K.

PHOTOVOLTAIC INFRARED SENSOR ARRAY IN HETEROEPITAXIAL NARROW GAP LEAD-CHALCOGENIDES ON SILICON

Linear arrays of photovoltaic IR-sensors for thermal imaging applications have been fabricated for the first time in narrow gap semiconductor layers grown heteroepitaxially on Si-substrates. Heteroepitaxy was achieved using intermediate stacked CaF 2 -BaF 2 bilayers to overcome the large lattice- and thermal expansion mismatch between Si and lead-chalcogenides. Sensors fabricated in narrow gap PbTe have ?5.7 ?m cut-off wavelength at 90K and quantum efficiencies around 70%. Resistance-area products are up to 500 ?cm2 with mean value of ?150 ?cm2 for 66 element linear arrays, well above the room temperature photon background noise limit. Sensor arrays with shorter cut-off wavelength were fabricated in the same manner in epitaxial Pb 1-x Eu x Se on fluoride covered Si-substrates.

Heteroepitaxial IV–VI infrared sensors on Si-substrates with fluoride buffer layers

Epitaxial layers of IV–VI narrow gap semiconductors (PbTe, PbSe, PbSnSe) have been grown onto Si-wafers with the aid of graded epitaxial CaF 2-BaF 2 buffer films by molecular beam epitaxy (MBE). Photovoltaic IR sensors for the 3–5 μm and 8–12 μm range have been fabricated in these structures. They exhibit near background noise limited sensitivity (for room temperature radiation) at their operating temperature of about 77 K. The results open up the possibility to construct large heteroepitaxial monolithic integrated circuits, e.g. staring IR focal plane arrays for thermal imaging applications: IR sensors are integrated in the narrow gap semiconductor layer and the Si-substrate is used for signal processing circuitry. Furthermore, the graded fluoride buffer technique offers more general applicability for the growth of heteroepitaxial stacks of different nonlattice matched semiconductor materials. Future integrated circuits and sensors fabricated in such stacks on a common substrate will combine and take advantage of the different properties of the different materials.

Time-Delay-Domain and Pseudorandom-Noise Photoacoustic and Photothermal Wave Processes: A Review of the State of the Art

The application and development of correlation and spec- tral analysis methods to photoacoustic and photothermal wave detec- tion are examined. The purposes of this review are to 1) describe the two most prominent techniques available for photothermal correlation and spectral processing, namely the pseudorandom noise and the fre- quency-modulation (FM) time-delay (or frequency sweep) optical ex- citation; 2) compare the technical features of these techniques, such as measurement dynamic range properties, amount of measurement time required and instrumentation requirements; and 3) present advantages of the techniques over the widely used conventional frequency domain and pulsed laser excitation, as well as a detailed comparison between themselves. Attention is focused on the conceptual and mathematical details of signal generation, processing, and interpretation. Subtle but significant differences in the signal dynamic range, impulse response, and transfer function are shown to determine the limits of each of the two major techniques and are corroborated by the available experi- mental evidence to date. A review of the steadily increasing literature on spectroscopic and thermal imaging applications of these techniques is a powerful indicator of the promise they hold for conducting fun- damental and applied studies, primarily due to 1) the superior wealth of information obtained through pseudorandom photothermal exci- tation, as compared to the conventional frequency dispersive tech- niques and 2) the less destructive nature of the pseudorandom im- pulse response, compared to pulsed laser excitation.