Infrared Detectors

INSTRUMENTATION

Advisory Panel Jonathan W . Amy Glenn L. Booman Robert L. Bowman

Jack W . Frazer Howard V. Malmstadt William F. Ulrich

Infrared Detectors Users of currently available infrared detectors have considerable choice. Sensitive areas may range anywhere from 0.01 mm 2 to 1 cm 2 with thresholds from the visible to the millimeter wave region and time constants shorter than 1 nsec Henry Levinstein Syracuse University, Syracuse, N.Y, 13210

voltage when charge carriers are sepa rated by a built-in field. The different their spectral response, speed of re principles of operation of the two sponse, and the minimum amount of classes of detectors are evidenced by radiant power they can detect. In ad differences in their properties. Ther dition to these basic parameters, the mal detectors usually have a spectral user is generally interested in several response determined only by the char other characteristics, such as the tem acteristics of the absorber. For con perature of operation, the magnitude stant incident energy this might be and type of signal produced by the in wavelength invariant over a wide spec cident radiation and the type of circuit tral range. Ideal photon detectors which best couples the detector to a have a response which for constant in readout system. cident energy rises linearly with wave length, then drops abruptly. This is Detectors may be placed into two broad categories determined b y t h e based on the assumption that the num ber of charge carriers excited per pho physical principles underlying their op ton is constant and independent of eration : thermal detectors and photon photon energy until the photon energy detectors. I n thermal detectors, the in is no longer large enough to excite a cident energy is absorbed by a tempera charge carrier. The speed of response ture-sensitive material or by an absorb of thermal detectors is determined by ing layer in contact with the tempera the rate at which the sensing layer ture-sensitive material. The receiving heats and cools. This is partly a mat layer usually consists of materials, such ter of construction detail involving the as "blacks," whose absorption is uni dimensions of the sensing element and form over a wide spectral range. Tem perature-sensitive materials m a y be its thermal contact with the environ ment and partly its specific heat. Since either metal or semiconductor layers, in photon detectors the speed of re whose resistance is temperature depen sponse is determined by the time re dent (bolometers) ; junctions of unlike quired for the excited charge carriers metals (thermocouples) ; ferroelectric to become immobilized through recom materials, where incident radiation af bination, they usually respond faster. fects the polarization of the sensing Furthermore, in the vicinity of the element (pyroelectric detectors) ; or peak of their spectral response, photon gases, where pressures are temperature detectors may be used to detect a con dependent (pneumatic cells). siderably lower radiant flux than ther Photon detectors are usually semi mal detectors. T o make possible a conductor devices, where the incident comparison among commercially avail photon flux interacts with electrons in able detectors, a more precise specifica bound states, exciting them to a free tion of measurement techniques and (conducting) state. This may result parameters is necessary. in a decreased resistance or a photo DETECTORS m a y be charac I NFRARED terized b y three basic parameters:

Measurement Techniques

The minimum detectable power, noise equivalent power ( N E P ) , is usually measured by placing a black body of known temperature (usually 500 °K) at a convenient distance from the detec tor. Its radiation is interrupted peri odically b y means of a rotating sec tored disk, such that a detector signal of constant frequency is obtained. The radiant power received a t the detector may be calculated. The detector noise is measured, usually with a narrow band amplifier, when the detector is shielded from the radiation source. The noise equivalent power (NEP) is calculated from the relation NEP=P/(S/iV) where Ρ is the power received by the detector and S and Ν are signal and noise voltage, respectively. I t is often more convenient to specify the reciprocal of N E P , the detectivity, D, or in the comparison of detectors, a normalized quantity, D*. D * is the detectivity of a detector of a 1 cm 2 area whose noise is rediiced to that ob tained with an amplifier of 1 Hz band width. D*

PD \ A )

where Δ/ is the amplifier bandwidth, A the detector area, and PD the power density at the detector. The assump tions made in the normalization pro cess require that detector noise varies as AV· and Afv". The first assumption is valid for photon detectors as long as the detector areas do not vary by more than an order of magnitude. I t may

ANALYTICAL CHEMISTRY, VOL. 4 1 , NO. 14, DECEMBER 1969

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INSTRUMENTATION

not be valid for certain thermal detec tors. The second approximation re quires either that noise is frequency in variant over the amplifier bandwidth, or that it is measured over such a narrow band that its variation is insig nificant. The spectral response of a detector is determined by measuring its signal, as it is exposed to radiation over a range of wavelengths obtained from a monochromator. In this process it is impor tant that the energy received at the detector be held constant or that its variation be known. When its magni tude is measured by means of a cali brated detector, the absolute spectral response may be calculated directly by comparison. Otherwise, absolute re sponse at any wavelength may be cal culated from the response to a black body at known temperature and a knowledge of the shape of the relative response. The speed of response of a detector is usually determined by exposing the detector to square radiation pulses and then observing the rise and decay of the signal on an oscilloscope. The ra diation pulses are obtained either me chanically by rotating a sector disk in front of a line radiation source or op tically by various modulating methods. Since the speed of response may be de pendent on the wavelength and the in tensity of incident radiation, conditions of measurement should approximate those under which the detector is ac tually being used. Characteristics of Thermal Detectors

Uncooled thermal detectors have val ues of D* in the 10 8 -10 9 -cm Hz1/*w a t t - 1 range. Their time constant is usually no less than several millisec onds. Shorter time constants usually result in decreased detectivity. The most commonly used thermal detectors are thermocouples and thermistor bo lometers. Thermocouples, because of their low impedance ( — 10Ω) are usu ally matched to an amplifier by means of specially designed preamplifiers or by transformers. Their time constant is of the order of 10 msec and responsitivities vary from 2 to 7 V / W . Thermistor bolometers have a resis tance in the megohm range. They con sist of thin flakes from oxides of man ganese, nickel, and cobalt. Materials are selected on the basis of their high temperature coefficient of resistance. Another of the conventional thermal detectors is the pneumatic cell. It de pends, in its operation, on mechanical rather than electrical properties of ma terials. As radiation strikes an absorb-

ing layer in thermal contact with an enclosed gas, the increase in tempera ture of the layer produces an increase in gas pressure. This, in turn, causes a displacement of a diaphragm which may be monitored by various optical or electrical techniques. One of the versions, which employs optical moni toring of the diaphragm, is the Golay cell. Most recently, two new types of ther mal detectors have been introduced: pyroelectric detoctors and cooled bo lometers. The pyroelectric detector consists of a slice of a ferroelectric ma terial, usually single crystal tryglycine sulfate (TGS), whose polarization is temperature dependent. The device is fabricated in the same manner as a capacitor. Two electrodes, one of them transparent, are formed on opposite sides of a TGS slice. Radiation is re ceived through the transparent elec trode. The voltage generated within the crystal is usually applied directly to a field effect transistor which is an in tegral part of the detector package. The cooled bolometer consists of a ger manium crystal with impurities having low charge carrier activation energies. It is cooled to a temperature of be tween 1 and 4 °K, where its resistance variation with temperatures is large and its thermal capacity is low. Detoc tors of this type have detectivities sev eral orders of magnitudes larger and time constants considerably shorter than the uncooled thermal detectors and, if coated with a good absorber, have a response which extends into the far infrared. Characteristics of Photon Detectors

While thermal detectors have been in use in one form or another from the beginning of infrared in 1800, photon detectors are of relatively recent vin tage, having been developed during the past 25 years. Their operation requires that the incident radiation signal lib erate charge carriers from either the crystal lattice (intrinsic detectors) or from impurities which have been in tentionally added to the host crystal (extrinsic detectors). There is thus associated with these detectors a wave length threshold determined by the minimum energy required to free these charge carriers. Since charge carriers may be liberated also by thermal ra diation from the background and by vibrations of the crystal lattice, as de termined by the temperature of the crystal, detectivities depend on these factors. The limiting noise source of a detec tor is G-R (generation-recombination) noise. It is associated with fluctuations in the density of free charge carriers, produced either by lattice vibrations, when the detector is not cooled suffi-

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ANALYTICAL CHEMISTRY, VOL 4 1 , NO 14, DECEMBER 1969

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INSTRUMENTATION

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Figure 1. Detectivity at spectral peak as a function of long wavelength threshold for background limited (BLIP) detectors. Solid curves represent photoconductive detectors with 180°, 60° and 30° angular fields of view, dotted curve represents photovoltaic detector with 180° field of view

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WAVELENGTH Figure 2. 180°)

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Spectral Response of typical intrinsic detectors (angular field of view is


eiently or by the random arrival of photons from the background. When the latter is the dominant source of noise, the detector is said to be back ground limited ( B L I P ) . With no other source of noise, this condition is reached by cooling the detector to the point where the number of charge carriers freed by lattice vibrations is consider ably fewer than the number freed by the background. A reduction in the amount of back ground radiation, obtained by a smaller field of view or the use of cooled nar row band filters, usually requires lower detector temperatures for B L I P opera tion. I n general, detectors with a- re sponse to about 3μ and 180° field of view looking into thermal background (300 ° K ) require little if any cooling. Detectors prepared from materials with lower activation energy require cooling to temperatures as low as liquid helium for materials with a response beyond 30μ. Detectors with a response in the intermediate I R , 5-10ju, may be oper ated at liquid nitrogen temperature. Because background-limited detec tivities are dependent only on the amount of background reaching the de tector, their detectivities may be cal culated as a function of the number and energy distribution of background pho tons. Figure 1 shows B L I P detectivi ties as a function of detector threshold wavelength, for various fields of view, assuming unity quantum efficiency. Of course, actual values of detectivities are lower, since part of the incident energy is reflected from the surface of the detector element and some of it is transmitted through the element with out being absorbed. Absorption may be increased by selecting optimum ele ment thickness; reflection m a y be re duced by the use of nonreflecting coat ings. Detectivity is also reduced by excess noise. The most commonly ob served excess noise varies inversely as the square root of the frequency ( 1 / / noise). I t has been made negligible above 100-1000 Hz for most detectors. With proper care in the construction of detectors, those most advanced reach their ideal detectivity within 10-20%. When photon detectors are of the photoconductive variety, they are con nected in series with a dc power source and a load resistor. Optimum supply voltage must be determined before the detector may be used effectively. Too high a voltage may produce excess noise and may occasionally damage the de tector; too low a voltage may produce a noise level below that of the ampli fier. Photovoltaic detectors, depend ing on their resistance, may require the


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INSTRUMENTATION

The Universal Humidity Transducer . . . still eludes us. For years we searched for a humidity transducer as versatile as the thermistor temperature transducer. No luck! So we've designed new "state-of-the-art" into instruments for all three classical humidity measurement methods:

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use of a transformer or a specially de signed preamplifier. PbS, PbSe, and P b T e were among the earliest I B photon detectors developed. They consist of evaporated or chemi cally deposited layers sensitized by various techniques involving the use of oxygen in one form or another. Pro duction of these detectors is more of an art than a science. PbS and PbSe are commercially available. PbS is oper ated a t room temperature. I t responds to 3μ, has a time constant of several hundred microseconds and a peak JD* of about 10 11 cm H z ^ W " 1 . When cooled to liquid nitrogen temperature, D* for specially prepared detectors may increase by as much as an order of magnitude, but at the expense of time constants, which are. lengthened considerably. PbSe has a response ex tending to 5μ, when cooled to dry ice temperature, a value of JD* of the order of 101U, and a time constant of about 50/zsec. Liquid nitrogen cooling shifts the response to slightly longer wave lengths and makes possible the use of material with element resistance of the order of 1 AfG instead of 100 MSI of PbSe materials designed for operation in the dry iee temperature range. The spectral curves of the lead salt detec tors are shown in Figure 2. The development, of crystal growing and material preparation techniques has

led to the preparation of InSb and liiAs detectors. These are prepared by wellunderstood techniques from single crys tals of the material. InSb can be pre pared to operate in the photoconductive mode or by the formation of a p-n junction in the photovoltaic mode. These detectors have reached such a state of perfection t h a t Z>* approaches its theoretical limit (10 1 1 at a spectral peak of about. 5μ) closer than any other material. Time constants are of the order of l^sec for photovoltaic detec tors and lOjusec for the photoconductive detectors. InAs, because its re gion of response overlaps that of PbS, but requires cooling to at least dry ice temperature, has not found as many ap plications as PbS. I t is available only in the photovoltaic mode. When cooled to dry ice temperature, it has a detec tivity which may exceed that of uncooled PbS, especially near the spec tral cutoff wavelength, and a time con stant considerably shorter than PbS. Several approaches have been used in the construction of detectors with a response beyond 6μ. No single ele ment or binary compound with charge carrier activation energy less than 0.2 eV has so far been found. One method for producing detectors with a threshold at longer wavelengths has consisted of using germanium with an activation en ergy of about 0.7 eV as the host lattice and adding selected impurities. The materials so prepared have a composite response, one due to excitation of charge carriers from the germanium with a response to about ί.7μ, the other

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Figure 3. Spectral Response of typical impurity activated germanium detectors. All detectors with the exception of Ge:B have a 60" field of view; Ge:B has a 10° field of view


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Infrared Detectors

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