Spie Press BookSelected Papers on Infrared Detectors: Developments
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Fundamentals of Infrared Detectors
Fundamentals of Focal Plane Arrays
Other Thermal Detectors
InGaSb SL Photodiodes
Thermal Detector Focal Plane Arrays
Other Thermal Detectors
Photon Detector Focal Plane Arrays
Many materials have been investigated in the infrared (IR) field. By observing the history of the development of IR detector technology, a simple theorem, after Norton, can be stated: All physical phenomena in the range of about 0.1 - 1 eV can be proposed for IR detectors. Among these effects are thermoelectric power (thermocouples), a change in electrical conductivity (bolometers), gas expansion (Golay cell), pyroelectricity (pyroelectric detectors), photon drag, the Josephson effect [Josephson junctions, superconducting quantum interference devices (SQUIDs)], internal emission (PtSi Schottky barriers), fundamental absorption (intrinsic photodetectors), impurity absorption (extrinsic photodetectors), low dimensional solids [superlattice (SL) and quantum well (QW) detectors], different types of phase transitions, etc.
The years during World War II saw the origins of modern IR detector technology. The progress in this technology is mainly connected with semiconductor detectors, which are included in the class of photon detectors. In photon detectors the radiation is absorbed within the material by interaction with electrons. The observed electrical output signal results from the changed electronic energy distribution. The class of photon detectors is further subdivided into different types depending on the nature of the interaction. In just a fraction of the last century, photon IR technology was combined with semiconductor material science, photolithography technology was developed for integrated circuits, and the impetus of cold war military preparedness propelled extraordinary advances in IR capabilities.
The second class of IR detectors is composed of thermal detectors. In a thermal detector, the incident radiation is absorbed to change the material temperature, and the resulting change in some physical property is used to generate an electrical output. Thermal effects are generally wavelength independent; the signal depends upon the radiant power (or its rate of change) but not upon its spectral content. In contrast to photon detectors, thermal detectors typically operate at room temperature. They are usually characterized by modest sensitivity and slow response, but they are cheap and easy to use. They have found widespread use in low-cost applications that do not require high performance and speed.
Until the 1990s, thermal detectors were considerably less exploited in commercial and military systems in comparison with photon detectors. The reason for this disparity was that thermal detectors were popularly believed to be rather slow and insensitive in comparison with photon detectors. As a result, the worldwide effort to develop thermal detectors was extremely small relative to that of photon detectors. In the last decade, however, it has been shown that extremely good imagery can be obtained from large thermal detector arrays operating uncooled at TV frame rates. The speed of thermal detectors is adequate for nonscanning imagers using 2D detectors. The moderate sensitivity of thermal detectors can be compensated by a large number of elements in 2D electronically scanned arrays. The development of uncooled IR arrays capable of imaging scenes at room temperature has been an outstanding technical achievement. As a result, a new revolution in thermal imaging is under way.
The 1992 Milestone Series volume Selected Papers on Semiconductor Infrared Detectors (MS 66) collected milestone papers in the field of photon detectors. This volume contains papers that cover outstanding technical achievements during the last decade in the wide field of IR detectors, including both thermal and photon detectors. The research efforts described in these papers were directed toward improving the performance of single-element devices, large electronically scanned arrays, and higher operating temperatures. Another important aim was to make IR detectors cheaper and more convenient to use. All these aspects are discussed in the collected papers. Special effort has been directed toward thermal detectors since this topic is covered in the Milestone Series for the first time. In this way, the volume gives a comprehensive analysis of the latest developments in IR technology and a basic insight into the fundamental processes that are important to developing detection techniques.
How does one select papers for a volume like this from among the thousands that have been published? As in any field, only a few stand the test of time those papers that we keep close at hand and refer to time and time again. These are the papers collected in this volume.
Comments on the Selected Papers
This volume will be of value to scholars, teachers, applied physicists, engineers, and practical optics scientists who use, produce, or simply wish to know more about IR detectors. Background commentaries are provided below for many of the papers.
The volume is divided into seven parts and contains 86 significant papers grouped in the following way: survey and tutorial papers are first, followed by papers on specific types of IR detectors and IR focal plane arrays (FPAs). The final section is devoted to third-generation detectors, which offer an imaging advantage over conventional first- and second-generation systems. Due to the almost daily publication of papers on the subject of IR detectors and the page limitation of this volume, it would have been impossible to avoid omissions in attempting to reproduce the most representative papers on these particular topics.
Infrared radiation was unknown until 203 years ago, when Herschel's experiment with a thermometer was first reported. The first paper in Section One gives a historical overview of the early development of the IR spectral region (Barr 1960, p. 3), and the second paper presents progress in IR detector technologies during the history of their development (Rogalski 2002, p. 16). The early history of IR technology was reviewed more thoroughly about 40 years ago in two well-known monographs[4,5].
Fundamentals of IR Detectors
Section Two provides a tutorial introduction to the technical topics that are fundamental to a thorough understanding of different types of IR detectors. Apart from traditional issues, new trends in the development of IR detectors such as pyroelectric detectors, high-Tc superconductors, bolometers, and internal photoemission are included. Special attention is paid to the physical limits of detector performance and the performance comparison of different types of detectors (Kruse 1995, p. 146; Hanson 2000, p. 178; Kinch 2000, p. 188).
Fundamentals of Focal Plane Arrays
During the 1950s and 1960s, IR detectors were built using single-element cooled lead salt detectors primarily for anti-air missile seekers. At the same time, rapid advances were being made in narrow gap semiconductors that would later prove useful in extending wavelength capabilities and improving sensitivity. These developments paved the way for the highly successful forward-looking airborne systems developed in the 1970s. In the middle of the 1970s the use of charge transfer devices (CTDs) held the key to substantial improvements in thermal imaging. CTD arrays offer significant advantages for focal plane applications. At present there is much research activity directed toward 2D "staring" array detectors consisting of more than 106 elements. Consequently, whereas various 64 x 64 FPAs were available in the early 1980s, several vendors are now producing FPAs in the TV-compatible 2048 x 2048 formats.
The four papers in Section Three provide general theory and an overview of FPA architectures. Lloyd's 1975 paper (p. 199) presents the relevant figure of merit for determining the performance of IR imaging systems. Hewitt at el. 1994 (p. 206) presents the detector readout structures in historical perspective. The fundamental limitations of uncooled FPAs are described by Kruse 1999 (p. 218). And Kozlowski et al. 1999 (p. 224) predicts that CMOS imagers are likely to supplant CCDs for many applications.
Thermal detection mechanisms are defined as mechanisms that change some measurable property of a material due to the temperature rise of that material caused by the absorption of electromagnetic radiation. The most important effects are the resistive bolometric effect, the pyroelectric effect and its modification (as the bias-enhanced pyroelectric effect or the ferroelectric bolometer), and the thermoelectric effect. These are the subjects of Section Four, with the papers presented chronologically.
The first paper of this section (Hornig and O'Keefe 1947, p. 239) gives detailed design criteria for fast thermopiles and possible high detectivity. It is not surprising that a large fraction of the literature on thermal detectors concentrates on methods of shortening their response times, since the response times of thermal detectors are several orders of magnitude longer than those of photon detectors. From the data gathered in this paper it is evident that semiconductors are the most promising materials for use in thermopile detectors. The next paper (Astheimer and Weiner 1964, p. 248) describes the fabrication of thermocouples consisting of bismuth and antimony junctions that are vacuum-deposited on a heat sink. The next two papers (Lahiji and Wise 1982, p. 256; van Herwaarden et al. 1989, p. 265) discuss the design, fabrication, and performance of thermopile IR detectors realized using integrated-circuit process technology. The improved performance of thermopile arrays can be achieved by combining Bi-Sb-Te thermoelectric materials (Volklein et al. 1991, p. 275).
The thermistor bolometer was developed at Bell Telephone Laboratories during World War II. Since that time it has become the most widely used of all thermal detectors for military and space systems. Wormser 1953 (p. 283) describes how a thermistor element is made from a thin flake of sintered semiconductor material chosen to have a large temperature coefficient of resistance. For more information on bolometers, see the next four papers in this section.
A new generation of monolithic Si bolometers was introduced by Downey et al. 1984 (p. 306). This paper presents a bolometer concept in which a thin Si substrate supported by narrow Si legs is micromachined from a Si wafer using the techniques of optical lithography. A conventional Bi film absorber is used on the back of the substrate, but the thermometer is created directly in the Si substrate by implanting P and B ions to achieve a suitable donor density and compensation ratio. However, the performance of the bolometer was not good. Further progress in monolithic Si bolometer technology is fascinating. Honeywell's Sensor and System Development Center in Minneapolis began developing silicon micro-machined IR sensors in the early 1980s. The goal of the work, sponsored by the U.S. Defense Advance Research Projects Agency (DARPA) and the U.S. Army Night Vision and Electronic Sensors Directorate, was aimed at producing low- cost night vision systems amenable to wide use throughout the military, with a noise equivalent temperature difference (NETD) of 0.1 C using f/1 optics. Texas Instruments' Si bolometer arrays and pyroelectric arrays both exceeded that goal. A detailed description of the industry growth in and properties of micromachined semiconductor bolometers is presented here by Unewisse et al. 1995 (p. 313).
The important discovery by Bednorz and Muller of a new class of super-conducting materials, the so called high-temperature superconductors, is undoubtedly one of the major breakthroughs in material science at the end of the twentieth century. However, the usefulness of superconducting IR detectors was limited due to their need for stringent temperature control, their poor radiation absorption characteristics due to being thin, and their fragility. The performance was generally limited by amplifier noise rather than radiation fluctuations. Brasunas and Lakew 1994 (p. 311) discusses some simple improvements of the high Tc superconductor bolometer, which may boost the detectivity above 1010 cmHz1/2/W at 77 K.
Pyroelectric materials are operated in a capacitive mode. In some of these materials, the dielectric constant changes rapidly over a narrow temperature range on either side of the Curie temperature. This effect can be exploited as an addition to the pyroelectric effect, which increases the signal. A readout of this effect requires an electrical bias. When the pyroelectric and dielectric effects are combined in a single device, it is termed a "dielectric bolometer" (Hanel 1961, p. 327). Current developments in the area of pyroelectric materials include the use of dielectric bolometers. The theory of the pyroelectric detector was given initially by Cooper 1962 (p. 332).
The third paper devoted to pyroelectric detectors concerns the pyroelectric vidicon (Holeman and Wreathall 1971, p. 336), which is widely used by firefighting and emergency service organizations. The pyroelectric vidicon tube can be considered analogous to the visible TV camera tube except that the photoconductive target is replaced by a pyroelectric detector and germanium face plate.
The last three papers in Section Four concern the thermal effects that are at present less exploited in thermal detection (the Golay detector or the pneumatic detector, and the p-n junction). Recent advances in micromechanical systems (MEMS) have led to the development of uncooled IR detectors operating as micromechanical thermal detectors as well as micromechanical photon detectors. This novel type of detector is discussed by Wachter et al. 1996 (p. 367).
The topic of photon detectors was also covered in the aforementioned Milestone Series volume, Selected Papers on Semiconductor Infrared Detectors (MS 66). However, the papers gathered in this volume give updated and more comprehensive insight into different types of photon detectors. HgCdTe is the most important semiconductor alloy system for IR detectors in the spectral range between 1 and 25 um. HgCdTe detectors, as the intrinsic photon detectors, absorb the IR radiation across the fundamental energy gap and are characterized by a high optical absorption coefficient and quantum efficiency, and a relatively low thermal generation rate compared to extrinsic detectors, silicide Schottky barriers, and quantum-well IR photodetectors (QWIPs). The operating temperature for intrinsic detectors is, therefore, higher than for other types of photon detectors. The attributes of HgCdTe translate to flexibility and the capability of producing short-wavelength IR (SWIR), middle-wavelength IR (MWIR), and long-wavelength IR (LWIR) detectors (see, e.g., Reine et al. 1993, p. 407). The cryogenically cooled InSb and HgCdTe arrays have comparable array size and pixel yield at the MWIR spectral band. However, wavelength tunability and high quantum efficiency have made HgCdTe the preferred material. Due to the similar band structure of InGaAs and HgCdTe ternary alloys, the ultimate fundamental performances of both types of photodiodes are similar in the wavelength range 1.5 < 8< 3.7 �m (see, e.g., Rogalski and Ciupa 1999, p. 458).
Section Five also contains papers devoted to two novel material systems for IR detectors: InAs/InGaSb strained layer superlattice (SL) and quantum dot IR photodetectors (QDIPs). The InAs/InGaSb type II SL is the most likely candidate to replace the conventional narrow gap semiconductor. Considerable progress in InAs/GaInSb SL photodiodes was achieved by Fuchs et al. 1997 (p. 485). The potential advantages in using QDIPs are presented by Ryzhii et al. 2001 (p. 529). However, at the present stage of their development, the QDIP's performance is clearly inferior to that of the related QWIP (Krishna et al. 2002, p. 532; Ye et al. 2002, p. 543).
Thermal Detector Focal Plane Arrays
The term "infrared focal plane array" (IR FPA) refers to an assemblage of individual IR detector picture elements (pixels) located at the focal plane of an IR imaging system. Although the definition includes 1D (linear) arrays as well as 2D arrays, it is frequently applied only to 2D arrays.
In this section the outstanding technical achievements in uncooled thermal detector FPAs during the last decade are presented. Uncooled IR detector arrays are revolutionizing the IR imaging community by eliminating the need for the expensive and high-maintenance coolers that traditional IR detectors require. Much of the technology was developed under classified military contracts in the U.S., so the public release of this information in 1992 surprised many in the worldwide IR community. There has been an implicit assumption that only cryogenic photon detectors operating in the 8-12 um atmospheric window had the necessary sensitivity to image room-temperature objects. Much recent research has focused on both hybrid and monolithic uncooled arrays and has yielded significant improvements in the detectivity of both bolometric (e.g., Wood 1993, p. 571) and pyroelectric (e.g., Beratan et al. 1994, p. 601) detector arrays. Honeywell has licensed bolometer technology to several companies for the development and production of uncooled FPAs for commercial and military systems. At present, the compact 320 x 240 microbolometer cameras are produced in the U.S. by Raytheon, Boeing, and Lockheed-Martin. The U.S. government allowed these manufacturers to sell their devices to foreign countries but not to divulge manufacturing technologies. In recent years, several countries, including the United Kingdom, Japan, Korea, and France, have picked up the ball, determined to develop their own uncooled imaging systems. As a result, although the U.S. has a significant lead, some of the most exciting and promising developments for low-cost uncooled IR systems may come from non-U.S. companies, such as Mitsubishi Electric's microbolometer FPA with a series p-n junction (described by Ishikawa et al. 1999, p. 637).
Thermopile detectors, while suitable for only limited use in imaging applications, have a combination of characteristics that make them well suited for some low-power applications. They are highly linear, require no optical chopper, and have detectivity values comparable to resistive bolometers and pyroelectric detectors. They operate over a broad temperature range with little or no temperature stabilization. They have no electrical bias, leading to negligible 1/f noise, and no voltage pedestal in their output signal. However, much less effort has been made toward their development because their responsivity to noise is orders of magnitude less; thus, their applications in thermal imaging systems require very low-noise electronics to realize their performance potential. Thermoelectric detectors have found almost no use as matrix arrays in TV frame rate imagers. Instead, they are employed as linear arrays that are mechanically scanned to form an image of stationary or nearly stationary objects.
Photon Detector Focal Plane Arrays
Many architectures are used in the development of IR photon FPAs. In general, they may be classified as hybrid and monolithic, but often these distinctions are not as important as proponents and critics state them to be. The central design questions involve performance advantages versus ultimate producibility. Each application may favor a different approach depending on the technical requirements, projected costs, and production schedule.
Although monolithic structures were proposed for a wide variety of IR detector materials over the past 30 years, only a few have been demonstrated. These included PtSi (Kimata et al. 1998, p. 671), and more recently PbS,9 PbTe, and the uncooled microbolometers presented in Section Five. PtSi, based entirely on silicon technology, has been commercialized for about 15 years, and both monolithic and hybrid versions have been produced. The largest PtSi monolithic detector arrays have been developed by Mitsubishi. However, IR imagers are most commonly built with a hybrid structure combining a detector structure mated to a readout.
The largest arrays have been built principally for astronomy, where dark currents as low as 0.02 electrons/sec are measured at 30 K. The most recent development is the
An alternative hybrid detector for the LWIR spectral region is the QWIP. These detectors are built from alternating layers of GaAs and AlGaAs. A grating structure is incorporated into the contact to scatter the incoming flux so that it has a component of momentum parallel with the layers in order to be absorbed. QWIP detectors have relatively low quantum efficiencies, typically less than 10%. The spectral response band is also narrow for this detector, with a full-width, half-maximum of about 15%. In spite of these limitations, the large flux of LWIR photons from room-temperature objects allows QWIP arrays to achieve good thermal imaging results for typical ambient conditions (Schneider et al. 2000, p. 755; Gunapala et al. 2000, p. 762).
Today the main application of extrinsic silicon detectors is for ground- and space-based far-IR astronomy (Estrada et al. 1998, p. 659). These devices must be cooled to temperatures near 12 K or less, depending upon the background flux, in order to prevent significant thermal ionization.
The definition of third-generation IR systems is not particularly well established. In the common understanding, third-generation IR systems provide enhanced capabilities such as a larger number of pixels, higher frame rates, better thermal resolution as well as multicolor functionality, and other on-chip functions. According to Reago et al., the third generation has been given a label in order to maintain the current advantage enjoyed by the U.S. and allied armed forces. This class of devices includes both cooled and uncooled FPAs[13,14]:
* High-performance, high-resolution cooled imagers having two- or three-color bands, * Medium- to high-performance uncooled imagers, and * Very low-cost, expendable, uncooled imagers.
Both HgCdTe photodiodes and QWIPs offer multicolor capability in the MWIR and LWIR ranges. This section is intended to present the actual status of development toward third-generation HgCdTe and QWIP FPAs. Each of these technologies has its advantages and disadvantages. QWIP technology is based on the well-developed A3B5 material system, which has a large industrial base with a number of military and commercial applications, while the HgCdTe material system is only used for detector applications. Therefore, QWIPs are easier to fabricate with high yield, high operability, good uniformity, and lower cost. On the other hand, HgCdTe FPAs have a higher quantum efficiency, a higher operating temperature, and the potential for the best performance. A more detailed comparison of both technologies has been given by Rogalski.
Antoni Rogalski Institute of Applied Physics Military University of Technology Warsaw, Poland August 2004
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