QWIP ( Quantum Well Infrared Photodetector : infra-red quantum well photodetector) is, as its name indicates it, a detector of radiation Infrarouge. The range wavelengths currently covered (2007) extends from 3 µm to more than 30 µm. This type of detector is sensitive to the thermal radiation emitted by any body whose temperature is not null (law of radiation of the black Corps, introduced by max Planck).
The QWIP is a quantum detector: the Absorption of the incidental radiation is done via an electronic transition. This transition takes place between the fundamental level of the quantum well and the first excited level. If only one confined level exists, the transition takes place between the fundamental level and the continuum of delocalized states.
The rules of selection governing the interaction between electromagnetic radiation and electrons lead to two very important characteristics of this component:
- absorption is resonant: the detector absorbs in a narrow band wavelengths, its absorption is appeared as a peak characterized by the position of its maximum and its width to middle height. For a centered peak with 9 µm, the width with middle height of the peak is close to 1 µm;
- the absorption of the radiation arriving at normal incidence is weak (theoretically null for a detector of infinite surface). It is consequently necessary to provide each detecting element (pixel) with a structure of coupling optical, for example a diffraction pattern functioning in reflection.
The QWIP is a cooled detector. Indeed, the concerned electronic transitions can also be excited by the Phonon S, i.e. by the vibrations of the crystal lattice. To reduce these vibrations the detector should be cooled. The operating temperature depends on the wavelength. For a detecting QWIP with 9 µm, the operating temperature exceeds 77 K today (- 196 °C, temperature of liquid nitrogen).
The QWIP is a unipolar detector: the electric current is only with the electrons. It is about a major difference compared to other technologies in detection, which exploit photodiodes (junctions pn). In a photodiode the incidental photons excite the electrons of the valence band towards the band of conduction of a semiconductor. The current is carried at the same time by the electrons and the holes. A photodiode is consequently component bipolar.
Technology QWIP exploits the industrial die of semiconductors III-V: GaAs and its alloys (AlGaAs, InGaAs). QWIPs profit from all the advantages of this dual technology, developed for the applications of the micro-electronics (e.g cellphones) and the opto-electronic (telecommunications on fiber):
- availability of substrates of big size. The wafers (English wafers) GaAs reach today diameters of 6 inches (15 cm). In comparison, the substrates used for the mercatel (HgCdTe, material of reference for infra-red detection) do not exceed 5 cm in diameter. The availability of great substrates makes it possible to undertake the manufacture of matrices of detectors of big size;
- excellent uniformity and reproducibility of material. This point is very important for the detectors with the matric format and an production activity at reduced cost;
- very low rate of defects. In a matrix of detectors each pixel is important. It is essential to reduce to the maximum the full number of defective pixels as well as the number of clusters (groups of defective pixels). The current state of the art is of a few tens of nonexploitable pixels for a matrix with the format 640x512 (either a rate of operability higher than 99.97%);
- very high output of manufacture. The availability of large wafers makes it possible to carry out several tens of matrices in parallel. The low number of defects makes it possible to reach outputs of manufacture close to 90%, with relatively modest technological means;
- excellent temporal stability of the image. Detectors QWIP are characterized indeed by a noise low frequency (known under the name of noise in 1/f) negligible. Contrary to other technologies of infra-red imagery, it is not necessary to carry out a correction of the image with regular intervals. This makes it possible to conceive systems easier to implement for the final utilistor.
QWIPs are particularly well adapted for the applications of fast imagery (more than 50 images a second), long distance (several kilometers), requiring a high resolution and a great sensitivity. Here a nonexhaustive list of the possible applications:
- telemonitoring by any time (night, fog). Some examples: monitoring of the littoral, monitoring of pipelines;
- assistance with the landing of the planes by bad conditions weather (fog);
- medical applications: detection of breast cancer, helps visual at the time of significant surgical operations…
- remote sensing of hazardous substances (explosive, pollutant gases);
- meteorological satellite;
- imagery and space spectroscopy (astronomy);
- military applications: sights of tank, cameras embarked on vehicles, twin portable, detection of hidden mines…
In addition to the traditional thermal imagery, in a given spectral band, QWIPs open the way with other applications, called of third generation:
- imagery high-resolution (format high-definition, 1280x1024 pixels);
- multispectral imagery. Thanks to their resonant absorption QWIPs make it possible to detect within the same pixel several wavelengths. To date demonstrations were made for the bands 3-5 µm/8-10 µm and 8-10 µm/10-12 µm. A multispectral imagor gives access the absolute temperature objects, like with their chemical constitution;
- polarimetric imagery. A polarimetric imagor is sensitive on the one hand to the intensity of the radiation, on the other hand with the polarization of this radiation. QWIPs are currently only technology in the world making it possible to integrate in a monolithic way a matrix of detectors and a matrix of polarizers. It is the structure of coupling optical (e.g lamellate network 1D) which plays the part of polarizer. Such a thermal imagor would allow for example to detect the objects manufactured in an unspecified environment;
In France, QWIPs are promoted mainly by the research laboratory of the company THALÈS, old Thomson. Research, the development as well as the production are ensured by Groupement d'Interet Economique (GIE) Alcatel-Thalès III-V Lab. Cameras infra-red containing QWIPs are produced in France by Thalès and Cedip. Sofradir is another important actor in the infra-red range as well in France as abroad.
On a world level, the quantum well detectors are studied and produced in the United States, Germany, Sweden, Israel, Canada, Australia. More and more of country are interested in it (Turkey, India, Corrée of the South) thanks to the accessibility of technology III-V (research can be undertaken in an university laboratory).
Realization of the detectorThe realization of a quantum well detector requires four principal stages:
- structural design quantum (left theoretical and simulation)
- realization of the quantum structure, by a technique of epitaxy
- realization of the matrix of detectors
- characterization of the performances of the detector
Quantum structural designThe active layer of the detector is a heterostructure containing semiconductors. Today the most used materials are the alloys AlGaAs (arsenide of Gallium and Aluminum) and InGaAs (indium and gallium arsenide). The structure is obtained by carrying out a periodic succession of layers of different chemical nature. For example, one can carry out a AlGaAs alternation - GaAs - AlGaAs - GaAs -… In this structure the AlGaAs layers have a less large affinity for the electrons. The latter will be confined in the GaAs layers, which thus will constitute wells. The AlGaAs layers play the part of containments. The electrons are introduced into the structure by doping certain layers with an element donor (the Silicium).
The quantum wells are sandwiched between two layers of doped semiconductor, playing the part of electrical contacts. These contacts make it possible to apply a potential difference to the component and to recover a current.
The structural design consists in choosing the thickness, as well as the composition of the various layers, the number of period, the doping of the wells, the thickness and the doping of the contacts, etc
Example: a structure whose peak of absorption is centered to 9 µm corresponds to GaAs broad wells of approximately 5 Nm (either 18 atomic layers). The AlGaAs barriers contain 25% of aluminum and their thickness is of approximately 40 Nm. A detector includes/understands a few tens of well.
Epitaxy of the structureThe realization of the active layer is made by a technique of epitaxy, the such epitaxy by molecular jets (EJM). This technique employs single-crystal substrates, having the shape of a wafer (diameter from 5 to 10 cm, thickness 0.5 mm), on which one deposits atoms, sleep atomic by atomic layer. It is the substrate which imposes the space organization of the atoms deposited. The crystalline growth takes place vacuum (10^-10 atmospheres) and at high temperature (500-600 °C).
Advantages of the technique:
makes it possible to change the chemical nature of the atoms deposited in the course of growth and obtaining artificial structures, qualified hétérostuctures to mean that the chemical composition is not homogeneous;
the speed of growth being very weak (typically 0.2 nm/seconde), this technique makes it possible to control the interfaces with full-course atomic near;
thanks to technological developments of the 10-15 last years, this technique of growth guarantees an excellent uniformity of composition on large surfaces as well as a very great purity of synthesized material (less 10^15 impuretés/cm^3, to compare with 10^23 atomes/cm^3, typical density of a crystal).
Realization of the matrix of detectorsOnce the active layer obtained, one proceeds to the realization of the matrices of detectors. Work is carried out in clean room. The quality of the matrices (percentage of pixels usable) depends directly on the quality of the atmosphere and the chemicals used. As example, is regarded as impurity criticizes all that exceeds a size of 1 µm.
Several technological processes are necessary:
photolithography using photosensitive resins. It makes it possible to define reasons in the surface of the semiconductor, which will be then engraved;
technical of engraving dries: reactive ion engraving, engraving plasma;
of deposit of metal layers, allowing to carry out electrical contacts;
The realization of the matrix is done in several stages:
engraving of the structure of coupling optical, which is integrated in the higher contact;
engraving of the pixels;
realization of the electrical contacts.
One thus obtains matrices with the format TV (640x512 pixels) or TV/4 (384x288 pixels), even recently with format HDTV (1280x1024). The pixels have dimensions going from 15 to 25 µm.
The matrix of detectors is connected using balls of indium to a similar matrix, composed of small read-out circuits out of silicon. These circuits make it possible to polarize the detector and to collect the signal. The operation of assembly is called hybridization.
Realization of a cameraThe hybrid obtained is assembled in small a Cryostat (recall: the detector functions at low temperature), which is then coupled with a small cold machine, functioning on the principle of the compression-relaxation. Cold the cryostat-machine unit forms the detecting block. This one is very compact: a dozen centimetres for a few hundred grams.
The detecting block is integrated in a camera, which contains also optics making it possible to form the image and the electronics of reading and treatment.
Characterization of the performancesWhen one speaks about performances, it is necessary to distinguish between performances from the detector and performances from the camera. We will speak here about the performances of the detector.
The electro-optical performances of an infra-red detector are evaluated using the following physical sizes:
spectral answer (in Amps per Watt). This size makes it possible to know the spectral beach of sensitivity of the detector. It is defined as the relationship between the number of electrons recovered in the contacts (a current) and the number of incidental photons (a power).
dark current (in Amps). This current is generated by physical processes others that absorption of the photons to be detected. In QWIPs, the principal component of the dark current is activated thermically (law of Arrhenius).
spectral concentration of noise while running (in Amps per root of Hertz).
specific spectral detectivity.
One meets also other physical sizes, such as for example the NETD (Noise Equivalent Temperature Difference: difference in temperature of scene corresponding to a signal are equivalent to the noise). Utilizing external parameters with the detector, it cannot be used to estimate the performances of a detector. For the complete thermal imagor the relevant parameter will be the NETD, or the range (in km). Today QWIPs make it possible to reach thermal resolutions of 10 - 30 millikelvin (1 to 3 hundredths of degree), according to the configuration of the imagor.
External bondsSite of the GIE Alcatel-Thalès III-V Lab
Site IRNova, Sweden
AIM Infrarot Modulates, Germany
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