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Infrared Detectors for the Year 2000

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Infrared Detectors for the Year 2000 !!~:: :~:+ ~:~ ::~:::':::$:::: :" ~: :'::$: :$: ::::::::: ::: :::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::~i$?.~::$ ~:;'~~:::::~:::;~$:$ ~:::::.<::.~:~ ::'¢:':::'::::::::'~ ~k x ~::::::~:~:::::::::::::::::::::::::::::::::::::::::::::?~:~:::~: 3::::: ~;:~: :: $::: :::::::::::::::::::::::::::::: :?$~: ::::::::::::::::::::::::::::: :::] ~::~i~i~::~::~!~ii~i~::~!::~::~!~::~i~::i~::~::~~~i~i~::~i~::::~::~::~::~i~::::~::~::::~::~¢:~::!::~::~ii::~i~::!~::~ii::!i::!::::~::~::i::i~::~:::i~i~;~::~:~::~i~::~::~::~::~::~::~::~i::~::~::~::~::~ Infrared Detectors for the Year 2000 I.M. Baker, M.P. Hastings, L.G. Hipwood, C.L. Jones, and P. Knowles GEC-Marconi Infra-Red Limited GEC-Marconi Infra-Red Limited (GMIRL) is Europe's largest supplier of infrared detectors. Its leading position in this field is illustrated in this article by describing the key detector technologies that will supply major projects into the year 2000, and beyond. he last two decades have seen rapid development in the T technology of thermal ima- ging, and the introduction into ser- vice of many new infrared detector types for military, industrial, and space applications. The semiconduc- tor cadmium mercury telluride (CMT) has been employed in the great majority of high-performance detectors, and continues to be the material of choice for advanced infrared detectors from nearly all major suppliers. For the highest performance, CMT detectors require cooling to operational temperatures around 80K (-193°C). This intro- duces the need for miniaturized Figure la. CMT bulk crystal and slices. cryogenic cooling techniques and a rugged vacuum-insulated encapsula- tion. silicon read-out circuit, allowing a Expk~sk~l det(mtOf Conventional thermal imaging sys- much greater array size than conven- // / Gor~e oh~ tems employ a variety of photocon- tional photoconductors, but with an ray ductor arrays as detectors. Among increase in complexity of detector ard these, the TED or SPRITE detector, manufacture. The successful manu- ~es manufactured exclusively by GMIRL facture of these advanced devices m and its licensees, has achieved con- depends critically on bringing to- ~e siderable success in the UK Common gether the technologies of crystal Module programme and in many growth, array processing, and multi- other countries. Its range of applica- plexer design. m=c tions has been broadened by improv- The aim of GMIRL's research, ~ket ing performance over time and which is strongly supported by links extending the range of array formats with UK Government research agen- and encapsulations in which it is cies, is in the direction of higher- star supplied. performance, lower-cost detector ar- lion The drive towards higher perfor- rays and its scope embraces "flip- nyo mance in thermal imaging has stimu- chip" hybrids, heterostructures, and late lated the development of Focal Plane multiple quantum wells. Photodiode Arrays. These detectors combine arrays of photodiodes of an Figure lb. Bridgman infrared sensitive material with a furnace for crystal growth. 0961-1290/96/$15.00 ©1996 Elsevicr Science Ltd. PageS0li~i~iii l!ii!i~ii~:/:~iii.i"]VolgNo2 !i ;~i!',::,j!ii~,,~&~,!!~ ~,-ii~i~ ........ ................. :~ .**:~::::~..,. :~.:. :. :. :. ,.,. :.:. :.:.~. :~..~: ................. ~............. .~...:~ ..................... ~................. ~. ~,~ ~A :~ ~~.~- ~!~-#~ •~.~-~..~ ..................................................................... • -.-.-..-.-- --.,-- -.---.--.-----.--.-- ...-.-.-:.-...-.~ ~-.-....~ .~ ..... :::::::::::::::::::::::::::::::::::::::: =================================================:::::.:::~::~<~::~4~:~::::~::~::~;;~4~:.:::~ j Some basic principles In the absence of light from the sun or artificial sources, the predominant source of electromagnetic radiation in the terrestrial scene is the uni- versally-present thermal radiation which is emitted naturally from all objects, and is described in an idea- lized form as black body radiation. Real objects depart from the ideal and their thermal signatures are characterized by a mixture of tem- perature, emissivity and reflectivity. Except at very short range, the absorption properties of the atmo- sphere restrict the useful range of wavelengths for the observation of thermal emissions to two bands -- the medium (3-5 I~m) and long wa- velength (8-12 I~m) infrared bands. Figure 2. CMT photoconductor array. The 48 elements are .50 ~m square. These are distinct from the short wavelength (c. 1 I~m) or near-infra- red band, which borders the visible task of camouflage very difficult erates excess mobile charge carriers range and covers most applications of compared with the visible. This is (electron-hole pairs) which, in the fibre-optic communications and do- because of a mixture of effects. In the most sensitive types, persist for per- mestic infrared remote controls. outdoors, temperature variations oc- iods up to several microseconds CMT infrared detectors exhibit cur throughout the 24-hour cycle of before recombining. The transient strong absorption of radiation up to solar heating, and objects of interest change in conductivity can be de- a characteristic cut-off wavelength may be either warmer or cooler than tected and amplified into a signal which corresponds to the semicon- their surroundings. Climatic varia- proportional to the infrared intensity. ductor band-gap between the va- tions induce a wide range of back- The construction of simple photo- lence and conduction bands. In ground temperatures, and effective conductor arrays generally involves CMT, this cut-off wavelength can be sky temperatures (which influence the mounting of a CMT die on a engineered through the alloy compo- reflections) vary widely with weather sapphire carrier, delineation of ele- sition of the semiconductor crystal, conditions. Finally, human skin is ments, and the application of ohmic to match the precise requirement of effectively a black body in the long contacts that extend to remote wire- the application, and can be extended wavelength infrared and, as a natu- bonding pads on the carrier (Fig. 2). out to the longer wavelengths of rally warm surface, is highly visible. For imaging applications, this type of interest by virtue of its narrow The crystal growth of CMT is detector needs the image to bc band-gap properties. problematic, and no single growth mechanically scanned, in swathes, to The long wavelength band contains method has emerged which domi- build up a picture in TV format. a large fraction of the total thermal nates the industry/11. In GMIRL, the The detecting elements of focal radiation from objects in the ambient older methods of bulk crystal growth plane photodiode arrays (FPPAs) arc temperature range and is unsur- by the vertical Bridgman technique p-n junction photodiodes. This type passed for the detection and imaging and the accelerated crucible rotation of detector, although technologically of small temperature or emissivity technique (ACRT) are still used to more complex, has the low power differences in the ambient scene. The provide a lowcost, volume source for dissipation and high impedance ne- medium waveband contains only a the production of photoconductor cessary for the realization of large few per cent of this radiation, but arrays. The in-house growth capabil- arrays of many thousands of ele- also contains the peak emissions ity includes zone refining and distilla- ments, interfaced by a direct injec- from artificial sources at a few tion of the elemental starting tion scheme to a silicon multiplexer hundred degrees C. The medium ingredients. These methods produce circuit. Time division multiplexing of waveband is therefore an excellent a cylindrical ingot which is sawn into the diode outputs means that the medium for the detection of hot slices and mechanically polished to silicon chip can be serviced by bodies against a cooler background. device thickness before division into relatively few electrical connections. In addition, the small fraction of individual detector dies (Fig. 1). In GMIRL, CMT material of the ambient scene radiation in this band All of GMIRL's CMT detectors are required crystal perfection for these can be converted into sensitive ima- photoconductor or photodiodc devices is supplied by the technique gery by the most advanced types of types. The simpler photoconductor of liquid phase epitaxy (LPE). Diode large detector arrays. arrays, manufactured in a wide vari- formation techniques are described The intimate combination of tem- ety of-formats and sizcs, share a in later sections, but in general, for perature and emissivity which deter- common operating principle. The detector types currently in develop- mines infrared signatures makes the absorption of infrared radiation gen- ment, diode formation proceeds after Page 51 [~r|~/ VOl 9 No 2 achieved by tuning the detector bias to equalize the image scan velocity and the drift velocity of the photo- generated minority carriers (holes). As a result, TDI is performed intern- ally and the integrated signal is read- out between a pair of contacts at one end of the element. In effect, the whole area of the long element is useful in gathering signal while preserving the spatial resolu- tion characteristic of a small element. This results in an eight-channel sys- tem of equivalent performance to one using many tens of conventional elements, bias supplies, and ampli- tiers. Figure 3a. Common module
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