List of Contributors Ian .M BAKER, EAB Systems Infrared Ltd., P.O. xoB 217, Southampton, Hampshire OS 15 0EG, KU .S .V BANDARA, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, AC 91 09, ASU .L ,ELKRUB Fraunhofer Institut f/ir Angewandte Festk6rperphysik, Tullastrasse 72, D-79108 Freiburg, Germany Henri-Jean DROUHIN, Laboratoire de Physique de la Mati~re Condens~e (UMR 7643-CNRS), Ecole Polytechnique, 9 128 Palaiseau cedex, France .F FUCHS, Fraunhofer Institut fiir Angewandte Festk6rperphysik, Tullastrasse 72, D-79108 Freiburg, Germany S.D. GUNAPALA, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, AC 91109, ASU .M HENINI, Department of Physics and Astronomy, University of Nottingham, Nottingham, KU Chris van HOOF, IMEC Kapeldreef 75, B-30()1 Haverlee, Belgium and ESAT- SYSNI Department, University of Leuven, Belgium .J E. JENSEN, LRH Laboratories, 3011 Malibu Canyon Road, Malibu, AC 90265. ASU .J JIANG, Center for Quantum Devices, Electrical and Computer Engineering Department, Northwestern University, Evanston, Illinois 60208, ASU vix Handbook of Infrared Detection seiq,olonhceT Masafumi KIMATA, Senior Technology Department, Advanced Technology, D&R Center, Mitsubishi Electric Corporation, 8-1-1, Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661. Japan Randolph .E LONGSHORE, Raytheon Missile Systems, P.O. Box 1137, MS 840/7 Tuscon, ZA 85734, ASU Terry de LYON, HRL Laboratories. 3Oll Malibu Canyon Road, Malibu, AC 90265, ASU H. MOHSENI, Center for Quantum Devices. Electrical and Computer Engineering Department, Northwestern University. Evanston. Illinois 60208. ASU Piet de MOOR, IMEC, Kapeldreef 7 .5 -B )()(3 1 Heverlee, Belgium Hartmut PRESTING, DaimlerChrysler Research (REM/C), Dep. FT2/H, Wilhelm- Runge Strasse 11, D-89081 ULM, Germany R. D. RAJAVEL, HRL Laboratories, 3011 Malibu Canyon Road. Malibu, AC 90265, ASU Manijeh RAZEGHI, Center for Quantum Devices. Electrical and Computer Engineering Department, Northwestern University. Evanston. Illinois 6()2()8, ASU Antoni ROGALSKI, Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str.. ()()-908 Warsaw. Poland .J A. ROTH, HRL Laboratories, 3011 Malibu Canyon Road, Malibu, AC 90265. ASU Chapter 1 Introduction M. Razeghi and M. Henini Nature has provided numerous examples of efficient detection systems. Almost all types of life, from bacteria, to plants, to human beings, have evolved some type of optoelectronic detection system for perceiving the world around them. These systems have had millions of years to develop, and demonstrate a seamless integration of optoelectronics with biological systems. The jewel beetle alihponaleM( )atanimuca thrives on the remnants of forest fires. Its larva feed on the dead wood, which gives evolutionary incentive for the beetle to find dead wood before other species. Towards this end, the beetles have developed an infrared detection system which allows them to sense a 10 hectare forest fire from up to 12 km away. As shown in Figure 1.1, a pit organ, called a sensilla, is located on either side of the beetle's thorax, which allows both intensity and directional information to be obtained. Absorption of infrared (2.4-4 mxl wavelength) light triggers a mechanical expansion which triggers nerve impulses. Obviously, this system must be small and easy to use. Further, as Figure 1.1 The jewel beetle and its infrared sensor. 2 Handbook of Infrared Detection seigolonhceT a beetle does not have a large built-in power supply or cryogen, the system must be power efficient and be uncooled. Our eyes are also excellent examples. Nature has provided a multi-spectral detection system based on microscopic variation in detector design. These differentiated detector cells add another dimension to the versatility of the eye. With a broadband detector, there is no way to differentiate between the intensity of a source and its emissivity at different wavelengths. This is akin to trying to pick out a matching wardrobe with a black and white camera. Multispectral systems allow separate waveband analysis of objects, which allows faster and more accurate identification to be made. On an evolutionary perspective, this ability allows more efficient target identification, allowing faster response to a potentially hazardous situation. The goal of science is to enhance our senses and better understand the universe around us. Infrared detectors broaden our vision into the realm of heat, allowing remote sensing of an object's temperature. This has had a dramatic impact on how we perceive our environment, and has led to many types of thermal imaging, including night vision, infrared astronomy, medical diagnostics, and failure analysis. These newfound abilities have spurred the development of many new systems, as shown in Figure 1.2. Infrared detectors have seen a remarkable surge in interest over the past several decades. This is thanks in part to the successful development of high- performance devices which have become the core of all the infrared systems listed above. The natural progression of these systems is a multispectral, uncooled, infrared camera, which can, by itself, address most of these applications. As in nature, a good system should be flexible, power efficient, lightweight, and easy to use. While we cannot expect to match the sophistication of natural systems, we can be inspired by them. Figure 1.2 Examples of mainstream thermal imaging systems. Introdl~ction 3 One inspiration involves the exploitation of quantum size effects for higher efficiency and added functionality. Most infrared photon detectors have a limited photocarrier lifetime and peak detection wavelength that is fixed by the bandgap of the material. Without changing the chemical composition of the material, patterning on an atomic scale can allow an increase in carrier lifetime and tuning of the peak detection wavelength. This type of effect has already been demonstrated in the form of the type-II InAs/GaSb semiconductor detector. Used in another way, similar to the eye, microscopic alterations can be made to the lateral size of individual detectors to demonstrate multispectral sensitivity in a single focal plane array. The purpose of this book si to present current methods and future directions in infrared detection. By bringing together experts in physics, material science, fabrication technology, and application, we will develop a well-rounded view of how far we have progressed towards the goal of an integrated, versatile, infrared detection system. Chapter 2 nosirapmoC fo notohp dna lamreht rotceted ecnamrofrep A. Rogalski 1.2 Introduction At present, HgCdTe is the most widely used variable gap semiconductor for infrared (IR) photodetectors. Over the last forty years it has successfully fought ffo major challenges from extrinsic silicon and lead-tin telluride devices, but despite that it has more competitors today than ever before. These include Schottky barriers on silicon, SiGe heterojunctions, A1GaAs multiple quantum wells, GaInSb strain layer superlattices, high temperature superconductors and especially two types of thermal detectors: pyroelectric detectors and silicon bolometers. It is interesting, however, that none of these competitors can compete in terms of fundamental properties. They may promise to be more manufacturable, but never to provide higher performance or, with the exception of thermal detectors, to operate at higher or even comparable temperatures. The main motivations to replace HgCdTe, are technological problems of this material. One of them is a weak Hg-Te bond, which results in bulk, surface and interface instabilities. Uniformity and yield are still issues. The slow progress in the development of large photovoltaic HgCdTe infrared imaging arrays and the rapid achievements of novel semiconductor heterostructure systems have made it more difficult to predict what types of arrays will be readily available for future systems applications. For spaceborne surveillance systems, low background IR seeker/tracker systems, reliable and affordable sensors with long life are needed which can function effectively at temperatures higher than the 20-30K currently required by bulk photon detectors. The only alternative to HgCdTe that had been available so far was extrinsic ,iS which operates at much lower temperatures where a problematic three-stage cryocooler would be required. Improvement in surveillance sensors and interceptor seekers requires large area size, highly uniform and multicolour (or multispectral) IR focal plane arrays (FPAs) involving long wavelength IR (LWIR) and very long wavelength IR 6 koobdnaH of derarfnI noitceteD seigolonhceT (VLWIR) regions. Among the competing technologies are the quantum well infrared photoconductors (QWIPs) based on lattice matched GaAs/A1GaAs and strained layer InGaAs/A1GaAs material systems. In comparison with photon detectors, thermal detectors have been considerably less exploited in commercial and military systems. The reason for this disparity si that thermal detectors were popularly believed to be rather slow and insensitive in comparison with photon detectors. As a result, the world-wide effort to develop thermal detectors has been extremely small relative to that of photon detectors. In the last ten years, however, it has been shown that extremely good imagery can be obtained from large thermal detector arrays operating uncooled at VT frame rates. The speed of thermal detectors is quite adequate for non-scanned imagers with two-dimensional detectors. At present, uncooled, monolithic FPAs fabricated from thermal detectors, revolutionise the development of low cost thermal imagers. In this paper, we discuss the performance of photon detectors as compared to thermal detectors. In comparative studies, more attention si paid to a wide family of photon detectors, especially to HgCdTe photodiodes and QWIPs. The potential performance of different materials used for photon detectors si examined utilizing the o~/G ratio, where zc si the absorption coefficient and G si the thermal generation. Different types of detectors operated as single element devices, are considered. Also such FPA issues as array size, uniformity, operability, multicolour capability and cost of systems, are discussed. 2.2 Fundamental limits to infrared detector performance Spectral detectivity curves for a number of available IR detectors are shown in Figure 2.1. Interest has centered mainly on the wavelengths of the two atmospheric windows 3-5 ~m middle wavelength IR (MWIR) and 8-14 pm (LWIR region) (atmospheric transmission si the highest in these bands and the emissivity maximum of the objects at T,~3()()K si at the wavelength ;.~10 micron), though in recent years there has been increasing interest in longer wavelengths stimulated by space applications. Depending on the detection mechanism, nature of interaction and material properties, the various types of detectors have their own characteristics. These characteristics result in advantages and disadvantages when the detectors are used in field applications 1-4. Table 2.1 shows a comparison of various IR detectors. Progress in IR detector technology si connected with semiconductor IR detectors, which are included in the class of photon detectors. In this class of detectors the radiation si 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. They exhibit both perfect signal-to-noise performance and a very fast response. But to nosirapmoC of photon dna thermal srotceted ecnamrofrep 7 2101 ,101 10'~ o D" 901 801 1 5.1 2 3 4 5 6 7 8 9 01 51 20 30 40 Wavelength (lum) Figure 2.1 Comparison of the *D of various infrared detectors when operated at tile 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 p!lroelectric detector (10 Hz). Each detector is assumed to view a hemispherical surround at a temperature of 300 .K Theoretical curves for the background- limited D'for ideal photovoltaic and photoconductive detectors and thermal detectors are also shown. achieve this, the photon detectors require cryogenic cooling. Photon detectors having long-wavelength limits above about 3 pm are generally cooled. This is necessary to prevent the thermal generation of charge carriers. The thermal transitions compete with the optical ones, making non-cooled devices very noisy. Cooling requirements are the main obstacle to the more widespread use of IR systems based on semiconductor photodetectors, making them bulky, heavy, expensive and inconvenient to use. Depending on the nature of the interaction, the class of photon detectors is further sub-divided into different types as shown in Table 2.1. The most important are: intrinsic detectors, extrinsic detectors, photoemissive detectors (PtSi Schottky barriers), and quantum well detectors. Depending on how the electric or magnetic fields are developed, there are various modes such as photoconductive, photovoltaic, photoelectromagnetic (PEM), and photoemissive ones. Each material system can be used for different modes of operation. In this paper we focus on photodiodes. Photodiodes with their very low power dissipation, easy multiplexing on focal plane silicon chip and less stringent noise requirements for the readout devices and circuits, can be assembled in two- dimensional (2D) arrays containing a very large number of elements, limited only by existing technologies. Current cooled IR detector systems use material such as HgCdTe, InSb, PtSi, and doped .iS OWIP is a relatively new technology for IR applications. Among these cooled IR detector systems, PtSi FPAs are highly uniform and 8 Handbook eq E ~ 0 k., of Infrared Detection L (cid:14)9 ~ ...... L --~ (cid:14)9 .~ :~o ~ ::>~ Z. ~ ~ ,..,., ~ Technologies .2.' ~ "V_. ~.~ r-. ,~ " ~ .2.' ,,... -3 =~ N .~ <: ~'- .o ~ ..., 8. ~ "7_. ,-~ t. t_ K ~ ""0 L ,,> t. "0 "0 2~ ~-, .,..., ~8 ,...,, V. nosirapmoC of photon dna thermal srotceted ecnamrofrep 9 manufacturable, but have very low quantum efficiency and can only operate in the MWIR range. The InSb FPA technology is mature with very high sensitivity, but it also can operate in the MWIR spectral range. Doped iS has a wide spectral range from 0.8 to 30 pm and it can only operate at very low temperatures. PtSi, InSb, and doped iS detectors do not have wavelength tunability or multicolour capabilities. Both QWIPs and HgCdTe offer high sensitivity with wavelength flexibility in the MWIR, LWIR and VLWIR regions, as well as multicolour capabilities. HgCdTe can also operate in the short wavelength IR (SWIR) region, while QWIP has to go to a direct band-to-band scheme for SWIR operation. The second class of IR detectors is composed of thermal detectors. In a thermal detector the incident radiation is absorbed to change the temperature of the material, and the resultant change in some physical property is used to generate an electrical output. The detector is suspended on lags, which are connected to the heat sink. The signal does not depend upon the photonic nature of the incident radiation. Thus, thermal effects are generally wavelength independent; the signal depends upon the radiant power (or its rate of change) but not upon its spectral content. This assumes that the mechanism responsible for the absorption of the radiation is itself wavelength independent, which is not strictly true in most instances. Attention is directed toward three approaches which have found the greatest utility in infrared technology; namely, bolometers, pyroelectric and thermoelectric effects. 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 si measured. In contrast to photon detectors, the thermal detectors are typically operated at room temperature. They are usually characterized by modest sensitivity and slow response (because heating and cooling of a detector element is a relatively slow process), but they are cheap and easy to use. They have found widespread use in low cost applications, which do not require high performance and speed. Being unselective, they are frequently used in IR spectrometers. Uncooled FPAs fabricated currently from thermal detectors revolutionize the development of thermal imagers s'6. 2.2.1 Photon detectors The photodetector is a slab of homogeneous semiconductor with the actual 'electrical' area, A ,e that is coupled to a beam of infrared radiation by its optical area, Ao (Figure 2.2). Usually, the optical and electrical areas of the device are the same or close. The use of optical concentrators can increase the Ao/Ae ratio. The current responsivity of the photodetector is determined by the quantum efficiency, ,/r and by the photoelectric gain, .g The quantum efficiency value describes how well the detector is coupled to the radiation to be detected. It is usually defined as the number of electron-hole pairs generated per incident photon. The idea of photoconductive gain, ,g was put forth by Rose 7 as a simplifying concept for the understanding of photoconductive phenomena and is now widely used in the field. The photoelectric gain si the number of carriers passing contacts per one generated pair. This value shows how well the