Infrared imaging is a method of introscopy(*) which makes it possible to visualize objects invisible to the naked eye with the aid of infrared rays reflected or emitted by such objects. This is a common technique used in studying various substances, in flaw detection (e.g. in the construction industry) and in controlling all kinds of technological processes.
The first infrared imagers appeared in 1941. By the present-day standards, they were quite primitive in design and possibilities. It was only in 1975 that it became possible to make direct precursors of the present-day apparatuses (with photodetectors as the centerpiece) on the basis of semiconductor materials synthesized by French scientists. Even though much has been done to improve such semiconductors ever since, they no longer conform to the present standards of infrared imaging.
Our research scientists from the Joint Institute of Semiconductor Physics (RAS Siberian Branch, Novosibirsk) have come up with an adequate technology. They have designed a new material and equipment for growing CMT (cadmium/ mercury/tellurium) films on gallium arsenide substrates by the method of molecular- beam epitaxy(*) which experts assess as a real breakthrough. Our scientists began this research back in 1979. And the first practical results were already on hand in 1986 when Angara and Katun units for the production of superfine films were put into operation. Assisting the "birth" of this innovative technology were likewise experts from other arms of the Siberian Branch of the Russian Academy of Sciences-namely from the G. Budker Institute of Nuclear Physics, the Institute of Applied Microelectronics and its pilot plant.
At first-about fifteen years ago or so-the use of molecular-beam epitaxy for making CMT films was being questioned, for the growth rates were then only at 1 um/h. For photodetectors the material should be 10-12 urn thick, however. So, growing a film of desired parameters took about 24 hours (preliminary operations included). But our Siberian researchers upped the growth rate to 5 um/h, and the whole process was brought down to 6 hours. Besides, it is fully automated: an operator monitors the features and composition of the film on a display. Let's join him and see how it all works.
First, a thin layer of zinc telluride is coated on the gallium arsenide substrate; next comes cadmium telluride, and then, CMT with a photosensitive working layer. The process should be controlled all along so as to get the required thickness of the layers. Such kind of control poses problems since the table on which the substrate is placed keeps rotating (to ensure the homogeneity of the powdered layers). Our experts coped with that too by designing a circular device for coating the materials, the world's first. As a consequence, the working surface stays motionless,
*Introscopy- visualization of objects or processes within optically opaque bodies, in opaque media.- Ed.
* Epitaxy-oriented growth of one monocrys-tal on the surface of another (substrate).- Ed.
which enables measurements in the very process of work. The device employed for such measurements, an ellipsometer, is also Novosibirsk-designed.
Yet another parameter, the temperature of the substrate, has a substantial effect on the CMT material. This temperature should be sustained with high accuracy Quite a problem too, for the process takes place in a vacuum, with no adequate instruments available up to now. Our experts have found a way out by designing an infrared pyrometer for measurements.
All these innovations have been applied in a multichamber plant dubbed Ob which allows to grow films on a substrate up to 75 mm in diameter (and we are hoping to bring it up to 200 mm). That's a very important point: we can obtain photodetectors with images not inferior to those on our home TV screens. After this plant has attained its rated capacity, it will be producing enough films to cater to the country's present needs in the manufacture of photodetectors and IR imagers.
Another very important characteristic of CMT materials is that they can be made sensitive to any wavelength. In the atmosphere there are transparency windows between 3 and 5 um, and also within 8 and 14 um. Peeping through like windows, an IR eye can see pretty far and make out objects at huge distances, atmospheric interferences notwithstanding. And since the earth and various objects on it (trees, aircraft and the like, and man too) give off 10 urn radiation, they can be visualized from outer space. Our IR imagers are custom-designed for two spectral bands-3-5 urn and ca. 10 um simultaneously-to enable different modes of visualization. A computer will then synthesize the imagery and evaluate it.
Today only the United States and Japan are growing CMT films by means of molecular-beam epitaxy and making high-efficiency quantum photodetectors on their basis. This problem is being tackled also in China, India and Brazil, but with little success thus far. IR imagers are likewise installed on military-related hardware. Since Russia is up to the mark in this respect, we shall be concentrating on peaceful uses of IR technology in power engineering and metallurgy, in the oil and gas industries, in ecological monitoring and so forth.
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