Table Of ContentList 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
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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
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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
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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
