History of infrared detectors The oxidation may be carried out by using additives nth bath. by post-depos The effect of the oxidant istointroduce sensitizing cen 3.Classification of infrared detectors 0b ewill be proposed for IR detectors"Among these effect cinypoelecdiedie s).photon drag Jose rption (extrin odetect (SL h Fig.5.Cashman nology.Recent success in applying intr ared technology t tube on which electrical lead ddaceoae 411 such 3.1.Photon detectors importan when he In photon detectors the radiation is absorbed within the joined Lock material by interac on with electro ndolanic ut signal results from the ch salt photoconductors was usually elect The photon detectors sho pes.Unlike most others miconductor IR detectors.lead salt a good signal-to-noise nce and approx 284 Opto-Electron.Rev.0.no.3.2012 2012 SEP.Warsaw
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)
Gen.Scan to image PA-ROIC FPA-ROIC 194 195 196M 1980 1990 2000 201 ent of infrared de nd systems.Three generation systems can b canning systems).2 cally scanned)an Conduction The spectral current responsivity of photon detectors is (1) (c) wavelength the Planck's the contacts of the device is noisy due to the statistica sorption he noise current Photon detector 月=2g2g2(Gp+Gh+M4. Fig.8.Relative spectral response for a photon and thermal detector D=A)次 NEP Opto-Electron.Rev.20.no.3.2012 A.Rogalski 285
generation of charge carriers. The thermal transitions com− pete with the optical ones, making non−cooled devices very noisy. The spectral current responsivity of photon detectors is equal to R hc qg i , (1) where is the wavelength, h is the Planck’s constant, c is the velocity of light, q is the electron charge, and g is the photoelectric current gain. The current that flows through the contacts of the device is noisy due to the statistical nature of the generation and recombination processes – fluc− tuation of optical generation, thermal generation, and radia− tive and nonradiative recombination rates. Assuming that the current gain for the photocurrent and the noise current are the same, the noise current is I qg G G R f n op th 2 22 2 ( ) , (2) where Gop is the optical generation rate, Gth is the thermal generation rate, R is the resulting recombination rate, and f is the frequency band. It was found by Jones [42], that for many detectors the noise equivalent power (NEP) is proportional to the square root of the detector signal that is proportional to the detector area, Ad. The normalized detectivity D* (or D−star) sug− gested by Jones is defined as D A NEP d ( )1 2 . (3) Opto−Electron. Rev., 20, no. 3, 2012 A. Rogalski 285 Fig. 6. History of the development of infrared detectors and systems. Three generation systems can be considered for principal military and civilian applications: 1st Gen (scanning systems), 2nd Gen (staring systems – electronically scanned) and 3rd Gen (multicolour functionality and other on−chip functions). Fig. 7. Fundamental optical excitation processes in semiconductors: (a) intrinsic absorption, (b) extrinsic absorption, (c) free carrier ab− sorption. Fig. 8. Relative spectral response for a photon and thermal detector.
History of infrared detectors Detectivity,D'is the main parameter to characterize noise performance of detectors and (7 D-B4140p tors,photoemissive (Schottky barriers).Different typeso Atthe generation and recombination ates for a number of commercially available IR detectors. D' 2hd(Gr) 3.2.Thermal detectors The ond ao e he incident radiation is al orbed to change the material =2BAu2g24 nge in som pended on legs which are connected to the heat sink.The performance) 30k0 Fig.9.Compa lakle detectors whenop102)Golaycell(10Hz)anrol re of 300 K.Th s for the b 286 Opto-Electron.Rev.20.no.3.2012 2012 SEP.Warsaw
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.
雪 Metal 11 material types 1950s of the la change)bur not the war.c munications.fire control face coating,the spectral response can be very d.Atten n search systems began to stimulate a strong developmen on is direct rd three approaches The therr ment-cooled lead salt detectors,primarily for anti pile is one of the oldest IR detectors.and isac tion o s conne 4).The missile entered service with the United States measured.whe eters a change in【 electrica opments of the semiconductor technology,they can be ned flake whose impedance is highly ter ture de into several types.The most com fourth is the This bolometer operates on a cor mgsionnwhebte dep principles of thermal detecto s are described in many books: sce e.g.Refs.5.6,41,and 43. 4.Post-War activity It was inevitable that the military would recognize the potential Opto-Electron.Rev.20.no 3 2012 A.Rogalski 287
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)
History of infrared detectors techniques for controled impurity introduction became cuc opet ercary. m is required to avoidthick detectors sing lin oled lead cction mechanism was based on sulphide photoconductive detector From the AIM-9D Side age cooler to operate at 25 however.the two-stage After60 years.low-cost versatile PbS and PbSe poly ctor-on d3-5 d by Soref 52 the state nd lowe ration cro section for hisher quantum eff and lo ance). hes eeded to bring it to the level of the by then,highly deve oped Ge ctors.After being dormant for abou n year on chip. nade in nan ful in extend wavelength capabilitie cor family. nique.The end of the 1950s and the beginning of the 1960 SIV-VIPSn.Te) naterial systems.These alloys allowed the bandgap of th 288 Opto-Electron.Rev.20.no.3.2012 2012 SEP.Warsaw
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)