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.WarsawAfter 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)