光电成像理论与技术（书籍文献）红外图像传感器资料——History of infrared detectors.pdf_大学文库

History of infrared detectors A. ROGALSKI* Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00–908 Warsaw, Poland This paper overviews the history of infrared detector materials starting with Herschel’s experiment with thermometer on February 11th, 1800. Infrared detectors are in general used to detect, image, and measure patterns of the thermal heat radia− tion which all objects emit. At the beginning, their development was connected with thermal detectors, such as ther− mocouples and bolometers, which are still used today and which are generally sensitive to all infrared wavelengths and op− erate at room temperature. The second kind of detectors, called the photon detectors, was mainly developed during the 20th Century to improve sensitivity and response time. These detectors have been extensively developed since the 1940’s. Lead sulphide (PbS) was the first practical IR detector with sensitivity to infrared wavelengths up to ~3 μm. After World War II infrared detector technology development was and continues to be primarily driven by military applications. Discovery of variable band gap HgCdTe ternary alloy by Lawson and co−workers in 1959 opened a new area in IR detector technology and has provided an unprecedented degree of freedom in infrared detector design. Many of these advances were transferred to IR astronomy from Departments of Defence research. Later on civilian applications of infrared technology are frequently called “dual−use technology applications.” One should point out the growing utilisation of IR technologies in the civilian sphere based on the use of new materials and technologies, as well as the noticeable price decrease in these high cost tech− nologies. In the last four decades different types of detectors are combined with electronic readouts to make detector focal plane arrays (FPAs). Development in FPA technology has revolutionized infrared imaging. Progress in integrated circuit design and fabrication techniques has resulted in continued rapid growth in the size and performance of these solid state arrays. Keywords: thermal and photon detectors, lead salt detectors, HgCdTe detectors, microbolometers, focal plane arrays. Contents 1. Introduction 2. Historical perspective 3. Classification of infrared detectors 3.1. Photon detectors 3.2. Thermal detectors 4. Post−War activity 5. HgCdTe era 6. Alternative material systems 6.1. InSb and InGaAs 6.2. GaAs/AlGaAs quantum well superlattices 6.3. InAs/GaInSb strained layer superlattices 6.4. Hg−based alternatives to HgCdTe 7. New revolution in thermal detectors 8. Focal plane arrays – revolution in imaging systems 8.1. Cooled FPAs 8.2. Uncooled FPAs 8.3. Readiness level of LWIR detector technologies 9. Summary References 1. Introduction Looking back over the past 1000 years we notice that infra− red radiation (IR) itself was unknown until 212 years ago when Herschel’s experiment with thermometer and prism was first reported. Frederick William Herschel (1738–1822) was born in Hanover, Germany but emigrated to Britain at age 19, where he became well known as both a musician and an astronomer. Herschel became most famous for the discovery of Uranus in 1781 (the first new planet found since antiquity) in addition to two of its major moons, Tita− nia and Oberon. He also discovered two moons of Saturn and infrared radiation. Herschel is also known for the twenty−four symphonies that he composed. W. Herschel made another milestone discovery – discov− ery of infrared light on February 11th, 1800. He studied the spectrum of sunlight with a prism [see Fig. 1 in Ref. 1], mea− suring temperature of each colour. The detector consisted of liquid in a glass thermometer with a specially blackened bulb to absorb radiation. Herschel built a crude monochromator that used a thermometer as a detector, so that he could mea− sure the distribution of energy in sunlight and found that the highest temperature was just beyond the red, what we now call the infrared (‘below the red’, from the Latin ‘infra’ – be− Opto−Electron. Rev., 20, no. 3, 2012 A. Rogalski 279 OPTO−ELECTRONICS REVIEW 20(3), 279–308 DOI: 10.2478/s11772−012−0037−7 * e−mail: rogan@wat.edu.pl

low) – see Fig. 1(b) [2]. In April 1800 he reported it to the Royal Society as dark heat (Ref. 1, pp. 288–290): Here the thermometer No. 1 rose 7 degrees, in 10 minu− tes, by an exposure to the full red coloured rays. I drew back the stand, till the centre of the ball of No. 1 was just at the vanishing of the red colour, so that half its ball was within, and half without, the visible rays of the sun. And here the thermometer No. 1 rose, in 16 minutes, 83 4 degrees, when its centre was 1 2 inch out of the visible rays of the sun. Now, as before we had a rising of 9 de− grees, and here 83 4 the difference is almost too trifling to suppose, that this latter situation of the thermometer was much beyond the maximum of the heating power; while, at the same time, the experiment sufficiently indi− cates, that the place inquired after need not be looked for at a greater distance. Making further experiments on what Herschel called the ‘calorific rays’ that existed beyond the red part of the spec− trum, he found that they were reflected, refracted, absorbed and transmitted just like visible light [1,3,4]. The early history of IR was reviewed about 50 years ago in three well−known monographs [5–7]. Many historical information can be also found in four papers published by Barr [3,4,8,9] and in more recently published monograph [10]. Table 1 summarises the historical development of infrared physics and technology [11,12]. 2. Historical perspective For thirty years following Herschel’s discovery, very little progress was made beyond establishing that the infrared ra− diation obeyed the simplest laws of optics. Slow progress in the study of infrared was caused by the lack of sensitive and accurate detectors – the experimenters were handicapped by the ordinary thermometer. However, towards the second de− cade of the 19th century, Thomas Johann Seebeck began to examine the junction behaviour of electrically conductive materials. In 1821 he discovered that a small electric current will flow in a closed circuit of two dissimilar metallic con− ductors, when their junctions are kept at different tempera− tures [13]. During that time, most physicists thought that ra− diant heat and light were different phenomena, and the dis− covery of Seebeck indirectly contributed to a revival of the debate on the nature of heat. Due to small output vol− tage of Seebeck’s junctions, some μV/K, the measurement of very small temperature differences were prevented. In 1829 L. Nobili made the first thermocouple and improved electrical thermometer based on the thermoelectric effect discovered by Seebeck in 1826. Four years later, M. Melloni introduced the idea of connecting several bismuth−copper thermocouples in series, generating a higher and, therefore, measurable output voltage. It was at least 40 times more sensitive than the best thermometer available and could de− tect the heat from a person at a distance of 30 ft [8]. The out− put voltage of such a thermopile structure linearly increases with the number of connected thermocouples. An example of thermopile’s prototype invented by Nobili is shown in Fig. 2(a). It consists of twelve large bismuth and antimony elements. The elements were placed upright in a brass ring secured to an adjustable support, and were screened by a wooden disk with a 15−mm central aperture. Incomplete version of the Nobili−Melloni thermopile originally fitted with the brass cone−shaped tubes to collect ra− diant heat is shown in Fig. 2(b). This instrument was much more sensi− tive than the thermometers previously used and became the most widely used detector of IR radiation for the next half century. The third member of the trio, Langley’s bolometer appea− red in 1880 [7]. Samuel Pierpont Langley (1834–1906) used two thin ribbons of platinum foil connected so as to form two arms of a Wheatstone bridge (see Fig. 3) [15]. This instrument enabled him to study solar irradiance far into its infrared region and to measure the intensity of solar radia− tion at various wavelengths [9,16,17]. The bolometer’s sen− History of infrared detectors 280 Opto−Electron. Rev., 20, no. 3, 2012 © 2012 SEP, Warsaw Fig. 1. Herschel’s first experiment: A,B – the small stand, 1,2,3 – the thermometers upon it, C,D – the prism at the window, E – the spec− trum thrown upon the table, so as to bring the last quarter of an inch of the read colour upon the stand (after Ref. 1). Inside Sir Frederick William Herschel (1738–1822) measures infrared light from the sun – artist’s impression (after Ref. 2). Fig. 2 . The Nobili−Meloni thermopiles: (a) thermopile’s prototype invented by Nobili (ca. 1829), (b) incomplete version of the Nobili− −Melloni thermopile (ca. 1831). Museo Galileo – Institute and Museum of the History of Science, Piazza dei Giudici 1, 50122 Florence, Italy (after Ref. 14)

Opto−Electron. Rev., 20, no. 3, 2012 A. Rogalski 281 Table 1. Milestones in the development of infrared physics and technology (up−dated after Refs. 11 and 12) Year Event 1800 Discovery of the existence of thermal radiation in the invisible beyond the red by W. HERSCHEL 1821 Discovery of the thermoelectric effects using an antimony−copper pair by T.J. SEEBECK 1830 Thermal element for thermal radiation measurement by L. NOBILI 1833 Thermopile consisting of 10 in−line Sb−Bi thermal pairs by L. NOBILI and M. MELLONI 1834 Discovery of the PELTIER effect on a current−fed pair of two different conductors by J.C. PELTIER 1835 Formulation of the hypothesis that light and electromagnetic radiation are of the same nature by A.M. AMPERE 1839 Solar absorption spectrum of the atmosphere and the role of water vapour by M. MELLONI 1840 Discovery of the three atmospheric windows by J. HERSCHEL (son of W. HERSCHEL) 1857 Harmonization of the three thermoelectric effects (SEEBECK, PELTIER, THOMSON) by W. THOMSON (Lord KELVIN) 1859 Relationship between absorption and emission by G. KIRCHHOFF 1864 Theory of electromagnetic radiation by J.C. MAXWELL 1873 Discovery of photoconductive effect in selenium by W. SMITH 1876 Discovery of photovoltaic effect in selenium (photopiles) by W.G. ADAMS and A.E. DAY 1879 Empirical relationship between radiation intensity and temperature of a blackbody by J. STEFAN 1880 Study of absorption characteristics of the atmosphere through a Pt bolometer resistance by S.P. LANGLEY 1883 Study of transmission characteristics of IR−transparent materials by M. MELLONI 1884 Thermodynamic derivation of the STEFAN law by L. BOLTZMANN 1887 Observation of photoelectric effect in the ultraviolet by H. HERTZ 1890 J. ELSTER and H. GEITEL constructed a photoemissive detector consisted of an alkali−metal cathode 1894, 1900 Derivation of the wavelength relation of blackbody radiation by J.W. RAYEIGH and W. WIEN 1900 Discovery of quantum properties of light by M. PLANCK 1903 Temperature measurements of stars and planets using IR radiometry and spectrometry by W.W. COBLENTZ 1905 A. EINSTEIN established the theory of photoelectricity 1911 R. ROSLING made the first television image tube on the principle of cathode ray tubes constructed by F. Braun in 1897 1914 Application of bolometers for the remote exploration of people and aircrafts ( a man at 200 m and a plane at 1000 m) 1917 T.W. CASE developed the first infrared photoconductor from substance composed of thallium and sulphur 1923 W. SCHOTTKY established the theory of dry rectifiers 1925 V.K. ZWORYKIN made a television image tube (kinescope) then between 1925 and 1933, the first electronic camera with the aid of converter tube (iconoscope) 1928 Proposal of the idea of the electro−optical converter (including the multistage one) by G. HOLST, J.H. DE BOER, M.C. TEVES, and C.F. VEENEMANS 1929 L.R. KOHLER made a converter tube with a photocathode (Ag/O/Cs) sensitive in the near infrared 1930 IR direction finders based on PbS quantum detectors in the wavelength range 1.5–3.0 μm for military applications (GUDDEN, GÖRLICH and KUTSCHER), increased range in World War II to 30 km for ships and 7 km for tanks (3–5 μm) 1934 First IR image converter 1939 Development of the first IR display unit in the United States (Sniperscope, Snooperscope) 1941 R.S. OHL observed the photovoltaic effect shown by a p−n junction in a silicon 1942 G. EASTMAN (Kodak) offered the first film sensitive to the infrared 1947 Pneumatically acting, high−detectivity radiation detector by M.J.E. GOLAY 1954 First imaging cameras based on thermopiles (exposure time of 20 min per image) and on bolometers (4 min) 1955 Mass production start of IR seeker heads for IR guided rockets in the US (PbS and PbTe detectors, later InSb detectors for Sidewinder rockets) 1957 Discovery of HgCdTe ternary alloy as infrared detector material by W.D. LAWSON, S. NELSON, and A.S. YOUNG 1961 Discovery of extrinsic Ge:Hg and its application (linear array) in the first LWIR FLIR systems 1965 Mass production start of IR cameras for civil applications in Sweden (single−element sensors with optomechanical scanner: AGA Thermografiesystem 660) 1970 Discovery of charge−couple device (CCD) by W.S. BOYLE and G.E. SMITH 1970 Production start of IR sensor arrays (monolithic Si−arrays: R.A. SOREF 1968; IR−CCD: 1970; SCHOTTKY diode arrays: F.D. SHEPHERD and A.C. YANG 1973; IR−CMOS: 1980; SPRITE: T. ELIOTT 1981) 1975 Lunch of national programmes for making spatially high resolution observation systems in the infrared from multielement detectors integrated in a mini cooler (so−called first generation systems): common module (CM) in the United States, thermal imaging common module (TICM) in Great Britain, syteme modulaire termique (SMT) in France 1975 First In bump hybrid infrared focal plane array 1977 Discovery of the broken−gap type−II InAs/GaSb superlattices by G.A. SAI−HALASZ, R. TSU, and L. ESAKI 1980 Development and production of second generation systems [cameras fitted with hybrid HgCdTe(InSb)/Si(readout) FPAs]. First demonstration of two−colour back−to−back SWIR GaInAsP detector by J.C. CAMPBELL, A.G. DENTAI, T.P. LEE, and C.A. BURRUS 1985 Development and mass production of cameras fitted with Schottky diode FPAs (platinum silicide) 1990 Development and production of quantum well infrared photoconductor (QWIP) hybrid second generation systems 1995 Production start of IR cameras with uncooled FPAs (focal plane arrays; microbolometer−based and pyroelectric) 2000 Development and production of third generation infrared systems

sitivity was much greater than that of contemporary thermo− piles which were little improved since their use by Melloni. Langley continued to develop his bolometer for the next 20 years (400 times more sensitive than his first efforts). His latest bolometer could detect the heat from a cow at a dis− tance of quarter of mile [9]. From the above information results that at the beginning the development of the IR detectors was connected with ther− mal detectors. The first photon effect, photoconductive ef− fect, was discovered by Smith in 1873 when he experimented with selenium as an insulator for submarine cables [18]. This discovery provided a fertile field of investigation for several decades, though most of the efforts were of doubtful quality. By 1927, over 1500 articles and 100 patents were listed on photosensitive selenium [19]. It should be mentioned that the literature of the early 1900’s shows increasing interest in the application of infrared as solution to numerous problems [7]. A special contribution of William Coblenz (1873–1962) to infrared radiometry and spectroscopy is marked by huge bib− liography containing hundreds of scientific publications, talks, and abstracts to his credit [20,21]. In 1915, W. Cob− lentz at the US National Bureau of Standards develops ther− mopile detectors, which he uses to measure the infrared radi− ation from 110 stars. However, the low sensitivity of early in− frared instruments prevented the detection of other near−IR sources. Work in infrared astronomy remained at a low level until breakthroughs in the development of new, sensitive infrared detectors were achieved in the late 1950’s. The principle of photoemission was first demonstrated in 1887 when Hertz discovered that negatively charged par− ticles were emitted from a conductor if it was irradiated with ultraviolet [22]. Further studies revealed that this effect could be produced with visible radiation using an alkali metal electrode [23]. Rectifying properties of semiconductor−metal contact were discovered by Ferdinand Braun in 1874 [24], when he probed a naturally−occurring lead sulphide (galena) crystal with the point of a thin metal wire and noted that current flowed freely in one direction only. Next, Jagadis Chandra Bose demonstrated the use of galena−metal point contact to detect millimetre electromagnetic waves. In 1901 he filed a U.S patent for a point−contact semiconductor rectifier for detecting radio signals [25]. This type of contact called cat’s whisker detector (sometimes also as crystal detector) played serious role in the initial phase of radio development. How− ever, this contact was not used in a radiation detector for the next several decades. Although crystal rectifiers allowed to fabricate simple radio sets, however, by the mid−1920s the predictable performance of vacuum−tubes replaced them in most radio applications. The period between World Wars I and II is marked by the development of photon detectors and image converters and by emergence of infrared spectroscopy as one of the key analytical techniques available to chemists. The image con− verter, developed on the eve of World War II, was of tre− mendous interest to the military because it enabled man to see in the dark. The first IR photoconductor was developed by Theodore W. Case in 1917 [26]. He discovered that a substance com− posed of thallium and sulphur (Tl2S) exhibited photocon− ductivity. Supported by the US Army between 1917 and 1918, Case adapted these relatively unreliable detectors for use as sensors in an infrared signalling device [27]. The pro− totype signalling system, consisting of a 60−inch diameter searchlight as the source of radiation and a thallous sulphide detector at the focus of a 24−inch diameter paraboloid mir− ror, sent messages 18 miles through what was described as ‘smoky atmosphere’ in 1917. However, instability of resis− tance in the presence of light or polarizing voltage, loss of responsivity due to over−exposure to light, high noise, slug− gish response and lack of reproducibility seemed to be inhe− rent weaknesses. Work was discontinued in 1918; commu− nication by the detection of infrared radiation appeared dis− tinctly unpromising. Later Case found that the addition of oxygen greatly enhanced the response [28]. The idea of the electro−optical converter, including the multistage one, was proposed by Holst et al. in 1928 [29]. The first attempt to make the converter was not successful. A working tube consisted of a photocathode in close proxi− mity to a fluorescent screen was made by the authors in 1934 in Philips firm. In about 1930, the appearance of the Cs−O−Ag photo− tube, with stable characteristics, to great extent discouraged further development of photoconductive cells until about 1940. The Cs−O−Ag photocathode (also called S−1) elabo− History of infrared detectors 282 Opto−Electron. Rev., 20, no. 3, 2012 © 2012 SEP, Warsaw Fig. 3. Longley’s bolometer (a) composed of two sets of thin plati− num strips (b), a Wheatstone bridge, a battery, and a galvanometer measuring electrical current (after Ref. 15 and 16)

rated by Koller and Campbell [30] had a quantum efficiency two orders of magnitude above anything previously studied, and consequently a new era in photoemissive devices was inaugurated [31]. In the same year, the Japanese scientists S. Asao and M. Suzuki reported a method for enhancing the sensitivity of silver in the S−1 photocathode [32]. Consisted of a layer of caesium on oxidized silver, S−1 is sensitive with useful response in the near infrared, out to approxi− mately 1.2 μm, and the visible and ultraviolet region, down to 0.3 μm. Probably the most significant IR development in the United States during 1930’s was the Radio Corporation of America (RCA) IR image tube. During World War II, near−IR (NIR) cathodes were coupled to visible phosphors to provide a NIR image converter. With the establishment of the National Defence Research Committee, the develop− ment of this tube was accelerated. In 1942, the tube went into production as the RCA 1P25 image converter (see Fig. 4). This was one of the tubes used during World War II as a part of the ”Snooperscope” and ”Sniperscope,” which were used for night observation with infrared sources of illumination. Since then various photocathodes have been developed including bialkali photocathodes for the visible region, multialkali photocathodes with high sensitivity ex− tending to the infrared region and alkali halide photocatho− des intended for ultraviolet detection. The early concepts of image intensification were not basically different from those today. However, the early devices suffered from two major deficiencies: poor photo− cathodes and poor coupling. Later development of both cathode and coupling technologies changed the image in− tensifier into much more useful device. The concept of image intensification by cascading stages was suggested independently by number of workers. In Great Britain, the work was directed toward proximity focused tubes, while in the United State and in Germany – to electrostatically focused tubes. A history of night vision imaging devices is given by Biberman and Sendall in monograph Electro−Opti− cal Imaging: System Performance and Modelling, SPIE Press, 2000 [10]. The Biberman’s monograph describes the basic trends of infrared optoelectronics development in the USA, Great Britain, France, and Germany. Seven years later Ponomarenko and Filachev completed this monograph writ− ing the book Infrared Techniques and Electro−Optics in Russia: A History 1946−2006, SPIE Press, about achieve− ments of IR techniques and electrooptics in the former USSR and Russia [33]. In the early 1930’s, interest in improved detectors began in Germany [27,34,35]. In 1933, Edgar W. Kutzscher at the University of Berlin, discovered that lead sulphide (from natural galena found in Sardinia) was photoconductive and had response to about 3 μm. B. Gudden at the University of Prague used evaporation techniques to develop sensitive PbS films. Work directed by Kutzscher, initially at the Uni− versity of Berlin and later at the Electroacustic Company in Kiel, dealt primarily with the chemical deposition approach to film formation. This work ultimately lead to the fabrica− tion of the most sensitive German detectors. These works were, of course, done under great secrecy and the results were not generally known until after 1945. Lead sulphide photoconductors were brought to the manufacturing stage of development in Germany in about 1943. Lead sulphide was the first practical infrared detector deployed in a variety of applications during the war. The most notable was the Kiel IV, an airborne IR system that had excellent range and which was produced at Carl Zeiss in Jena under the direction of Werner K. Weihe [6]. In 1941, Robert J. Cashman improved the technology of thallous sulphide detectors, which led to successful produc− tion [36,37]. Cashman, after success with thallous sulphide detectors, concentrated his efforts on lead sulphide detec− tors, which were first produced in the United States at Northwestern University in 1944. After World War II Cash− man found that other semiconductors of the lead salt family (PbSe and PbTe) showed promise as infrared detectors [38]. The early detector cells manufactured by Cashman are shown in Fig. 5. Opto−Electron. Rev., 20, no. 3, 2012 A. Rogalski 283 Fig. 4. The original 1P25 image converter tube developed by the RCA (a). This device measures 115×38 mm overall and has 7 pins. It opera− tion is indicated by the schematic drawing (b)

After 1945, the wide−ranging German trajectory of research was essentially the direction continued in the USA, Great Britain and Soviet Union under military sponsorship after the war [27,39]. Kutzscher’s facilities were captured by the Russians, thus providing the basis for early Soviet detector development. From 1946, detector technology was rapidly disseminated to firms such as Mullard Ltd. in Southampton, UK, as part of war reparations, and some− times was accompanied by the valuable tacit knowledge of technical experts. E.W. Kutzscher, for example, was flown to Britain from Kiel after the war, and subsequently had an important influence on American developments when he joined Lockheed Aircraft Co. in Burbank, California as a research scientist. Although the fabrication methods developed for lead salt photoconductors was usually not completely under− stood, their properties are well established and reproducibi− lity could only be achieved after following well−tried reci− pes. Unlike most other semiconductor IR detectors, lead salt photoconductive materials are used in the form of polycrys− talline films approximately 1 μm thick and with individual crystallites ranging in size from approximately 0.1–1.0 μm. They are usually prepared by chemical deposition using empirical recipes, which generally yields better uniformity of response and more stable results than the evaporative methods. In order to obtain high−performance detectors, lead chalcogenide films need to be sensitized by oxidation. The oxidation may be carried out by using additives in the deposition bath, by post−deposition heat treatment in the presence of oxygen, or by chemical oxidation of the film. The effect of the oxidant is to introduce sensitizing centres and additional states into the bandgap and thereby increase the lifetime of the photoexcited holes in the p−type material. 3. Classification of infrared detectors Observing a history of the development of the IR detector technology after World War II, many materials have been investigated. A simple theorem, after Norton [40], can be stated: ”All physical phenomena in the range of about 0.1–1 eV will be proposed for IR detectors”. Among these effects are: thermoelectric power (thermocouples), change in elec− trical conductivity (bolometers), gas expansion (Golay cell), pyroelectricity (pyroelectric detectors), photon drag, Jose− phson effect (Josephson junctions, SQUIDs), internal emis− sion (PtSi Schottky barriers), fundamental absorption (in− trinsic photodetectors), impurity absorption (extrinsic pho− todetectors), low dimensional solids [superlattice (SL), quantum well (QW) and quantum dot (QD) detectors], different type of phase transitions, etc. Figure 6 gives approximate dates of significant develop− ment efforts for the materials mentioned. The years during World War II saw the origins of modern IR detector tech− nology. Recent success in applying infrared technology to remote sensing problems has been made possible by the successful development of high−performance infrared de− tectors over the last six decades. Photon IR technology com− bined with semiconductor material science, photolithogra− phy technology developed for integrated circuits, and the impetus of Cold War military preparedness have propelled extraordinary advances in IR capabilities within a short time period during the last century [41]. The majority of optical detectors can be classified in two broad categories: photon detectors (also called quantum detectors) and thermal detectors. 3.1. Photon detectors In photon detectors the radiation is absorbed within the material by interaction with electrons either bound to lattice atoms or to impurity atoms or with free electrons. The observed electrical output signal results from the changed electronic energy distribution. The photon detectors show a selective wavelength dependence of response per unit incident radiation power (see Fig. 8). They exhibit both a good signal−to−noise performance and a very fast res− ponse. But to achieve this, the photon IR detectors require cryogenic cooling. This is necessary to prevent the thermal History of infrared detectors 284 Opto−Electron. Rev., 20, no. 3, 2012 © 2012 SEP, Warsaw Fig. 5. Cashman’s detector cells: (a) Tl2S cell (ca. 1943): a grid of two intermeshing comb−line sets of conducting paths were first pro− vided and next the T2S was evaporated over the grid structure; (b) PbS cell (ca. 1945) the PbS layer was evaporated on the wall of the tube on which electrical leads had been drawn with aquadag (after Ref. 38)

Detectivity, D*, is the main parameter to characterize normalized signal−to−noise performance of detectors and can be also defined as D RA f I i d n ( ) 1 2 . (4) The importance of D* is that this figure of merit permits comparison of detectors of the same type, but having diffe− rent areas. Either a spectral or blackbody D* can be defined in terms of corresponding type of NEP. At equilibrium, the generation and recombination rates are equal. In this case D hc Gt 2 1 2 ( ) . (5) Background radiation frequently is the main source of noise in a IR detector. Assuming no contribution due to recombination, I A qg f n Bd 2 22 2 , (6) where B is the background photon flux density. Therefore, at the background limited performance conditions (BLIP performance) D hc BLIP B 1 2 . (7) Once background−limited performance is reached, quan− tum efficiency, , is the only detector parameter that can influence a detector’s performance. Depending on the nature of the interaction, the class of photon detectors is further sub−divided into different types. The most important are: intrinsic detectors, extrinsic detec− tors, photoemissive (Schottky barriers). Different types of detectors are described in details in monograph Infrared Detectors [41]. Figure 9 shows spectral detectivity curves for a number of commercially available IR detectors. 3.2. Thermal detectors The second class of detectors is composed of thermal detec− tors. In a thermal detector shown schematically in Fig. 10, the incident radiation is absorbed to change the material temperature and the resultant change in some physical prop− erty is used to generate an electrical output. The detector is suspended on legs which are connected to the heat sink. The signal does not depend upon the photonic nature of the inci− dent radiation. Thus, thermal effects are generally wave− length independent (see Fig. 8); the signal depends upon the History of infrared detectors 286 Opto−Electron. Rev., 20, no. 3, 2012 © 2012 SEP, Warsaw Fig. 9. Comparison of the D* of various available detectors when operated at the indicated temperature. Chopping frequency is 1000 Hz for all detectors except the thermopile (10 Hz), thermocouple (10 Hz), thermistor bolometer (10 Hz), Golay cell (10 Hz) and pyroelectric detec− tor (10 Hz). Each detector is assumed to view a hemispherical surrounding at a temperature of 300 K. Theoretical curves for the back− ground−limited D* (dashed lines) for ideal photovoltaic and photoconductive detectors and thermal detectors are also shown. PC – photoconductive detector, PV – photovoltaic detector, PEM – photoelectromagnetic detector, and HEB – hot electron bolometer.

radiant power (or its rate of change) but not upon its spectral content. Since the radiation can be absorbed in a black sur− face coating, the spectral response can be very broad. Atten− tion is directed toward three approaches which have found the greatest utility in infrared technology, namely, bolom− eters, pyroelectric and thermoelectric effects. The thermo− pile is one of the oldest IR detectors, and is a collection of thermocouples connected in series in order to achieve better temperature sensitivity. In pyroelectric detectors a change in the internal electrical polarization is measured, whereas in the case of thermistor bolometers a change in the electrical resistance is measured. For a long time, thermal detectors were slow, insensitive, bulky and costly devices. But with developments of the semiconductor technology, they can be optimized for specific applications. Recently, thanks to con− ventional CMOS processes and development of MEMS, the detector’s on−chip circuitry technology has opened the door to a mass production. Usually, a bolometer is a thin, blackened flake or slab, whose impedance is highly temperature dependent. Bolom− eters may be divided into several types. The most com− monly used are metal, thermistor and semiconductor bolom− eters. A fourth type is the superconducting bolometer. This bolometer operates on a conductivity transition in which the resistance changes dramatically over the transition tempera− ture range. Figure 11 shows schematically the temperature dependence of resistance of different types of bolometers. Many types of thermal detectors are operated in wide spectral range of electromagnetic radiation. The operation principles of thermal detectors are described in many books; see e.g., Refs. 5, 6, 41, and 43. 4. Post-War activity It was inevitable that the military would recognize the potential of night vision. However, the military IR technology was in its infancy at the end of World War II. The IR hardware activities at the beginning of 1950s of the last century involved mainly simple radiometric instruments (see Fig. 12) and passive night vision technology (see Fig. 13) capable of allowing vision under ambient starlight conditions. Immediately after the war, communications, fire control and search systems began to stimulate a strong development effort of lead salt detector technology that has extended to the present day. The IR systems were built by using sin− gle−element−cooled lead salt detectors, primarily for anti− −air−missile seekers. The Sidewinder heat−seeking infrared− −guided missiles received a great deal of public attention [46]. The missile entered service with the United States Opto−Electron. Rev., 20, no. 3, 2012 A. Rogalski 287 Fig. 10. Schematic diagram of thermal detector. Fig. 11. Temperature dependence of resistance of three bolometer material types. Fig. 12. Spectral radiometer used for early measurements of infrared terrain signatures using a PbTe detector (after Ref. 44)

Navy in the mid−1950s and variants and upgrades remain in active service with many air forces after six decades. Early Sidewinder models (see Fig. 13 [47]) used uncooled lead sulphide photoconductive detector. From the AIM−9D Side− winder on, the PbS detector was cooled, which reduced the self generated noise in the detector material. First generation imagers utilized scanned single−element detectors and linear arrays. In the MWIR region (3–5 μm) apart from PbSe, early systems employed InSb. After 60 years, low−cost versatile PbS and PbSe poly− crystalline thin films remain the photoconductive detectors of choice for many applications in the 1–3 μm and 3–5 μm spectral range. Current development with lead salts is in the focal plane arrays (FPAs) configuration. The first extrinsic photoconductive detectors were re− ported in the early 1950s [48–50] after the discovery of the transistor, which stimulated a considerable improvement in the growth and material purification techniques. Since the techniques for controlled impurity introduction became available for germanium at an earlier date, the first high per− formance extrinsic detectors were based on germanium. Extrinsic photoconductive response from copper, mercury, zinc and gold impurity levels in germanium gave rise to devices using in the 8− to 14−μm long wavelength IR (LWIR) spectral window and beyond the 14− to 30−μm very long wavelength IR (VLWIR) region. The extrinsic photo− conductors were widely used at wavelengths beyond 10 μm prior to the development of the intrinsic detectors. They must be operated at lower temperatures to achieve perfor− mance similar to that of intrinsic detectors and sacrifice in quantum efficiency is required to avoid thick detectors. The discovery in the early 1960s of extrinsic Hg−doped germanium [51] led to the first forward looking infrared (FLIR) systems operating in the LWIR spectral window using linear arrays. Ge:Hg with a 0.09−eV activation energy was a good match to the LWIR spectral window, however, since the detection mechanism was based on an extrinsic excitation, it required a two−stage cooler to operate at 25 K. The first real production FLIR program based upon Ge:Hg was built for the Air Force B52 Aircraft in 1969 [10]. It used a 176−element array of Ge:Hg elements and provided excel− lent imaging, however, the two−stage cooler had limited lifetime and high system maintenance. In 1967 the first comprehensive extrinsic Si detector−ori− ented paper was published by Soref [52]. However, the state of extrinsic Si was not changed significantly. Although Si has several advantages over Ge (namely, a lower dielectric constant giving shorter dielectric relaxation time and lower capacitance, higher dopant solubility and larger photoioni− zation cross section for higher quantum efficiency, and lo− wer refractive index for lower reflectance), these were not sufficient to warrant the necessary development efforts needed to bring it to the level of the, by then, highly deve− loped Ge detectors. After being dormant for about ten years, extrinsic Si was reconsidered after the invention of charge− −coupled devices (CCDs) by Boyle and Smith [53]. In 1973, Shepherd and Yang [54] proposed the metal−silicide/silicon Schottky−barrier detectors. For the first time it became pos− sible to have much more sophisticated readout schemes both detection and readout could be implemented in one common silicon chip. Beginning in the 1950’s, rapid advances were being made in narrow bandgap semiconductors that would later prove useful in extending wavelength capabilities and improving sensitivity. The first such material was InSb, a member of the newly discovered III−V compound semi− conductor family. The interest in InSb stemmed not only from its small energy gap, but also from the fact that it could be prepared in single crystal form using a conventional tech− nique. The end of the 1950s and the beginning of the 1960s saw the introduction of narrow gap semiconductor alloys in III−V (InAs1–xSbx), IV−VI (Pb1–xSnxTe), and II−VI (Hg1–xCdxTe) material systems. These alloys allowed the bandgap of the semiconductor and hence the spectral response of the detec− tor to be custom tailored for specific applications. In 1959, History of infrared detectors 288 Opto−Electron. Rev., 20, no. 3, 2012 © 2012 SEP, Warsaw Fig. 13. TVS−4 Night Observation Device – 1st generation intensi− fier used only at the night sky illumination. It had an 8 “aperture and was 30” long (after Ref. 45). Fig. 14. Prototype Sidewinder−1 missile on an AD−4 Skyraider during flight testing (after Ref. 47)