Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J.
07079
These articles are intended to serue the readers O ~ T H I SJOURNAL by calling attention to new developments in the t h e w , deaign, or availability of chemical laboratory instrumentation, or ly presenting useful insights and ezplanations of topics that are of practical imporlance lo those who use, or leach the use of, modern instrumentation and instrumental techniques. The editor invites correspondence from prospective contributors.
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PN
LIX. Infrared Detectors GALEN W. EWlNG Introduction There are many devices available which act as transducers to convert the information carried by a beam of infrared radiation into usable electrical signals. They can generally be classified into two broad groups: (1) those which depend on quantum effects, in which a n individual infrared photon contains sufficient energy to effect the transition of a n electron from one level to another in the receptor; and (2) those designated as thermal detectors, which depend on the integrated enerev of a lame numher of ~ h o t o n st o produce a measurable response via their heating effect. I n general the photon detectors are faster and more sensitive, hut severely restricted with respect t o the range of wavelengths to which they can respond. Thermal detectors, on the other hand, are usable over a wide range of wavelengths, hut suffer from relatively low sensitivity and slow response. This review is directed principally toward detectors for use in laboratory spectraphotometers utilizing scanning techniques That is to say, the detector is called upon to observe the spectrum one narrow wavelength hand a t a time, rather than the whole spectrum simultaneously. This eliminates from considerstion spectrographio methods using photographic det,ection. Before turning to specific deteetors, we should consider the parameters by which they can he evahmted. Of foremost importance, of course, is the wavelength or frequency range. Our discussion will cover to some extent the entire infrared from 0.75 to 1000 pm, which corresponds to 1.3 X 104 to 10 em-$ wavenumber. Ilecsll that wavenumber, although often loosoly called "frequency," is not such, hut. rather is proportional to frequency, the proportionality factor being the velocity of light. The greatest activity in chemical applications of in-
represent the voltage or current developed a t the output. Unfortunately all detectors generate noise of various types, which limits their ability to observe very small signals. The noise can he expressed as its equivalent in radiant power, the amount of radiation which would produce the same electrical signal. This quantity, PN, (often called the noise equivalent power and designated NEP) is given by
frared is in the region from about 2 to 20 pm (5000 to 500 em-'). The sensitivity of a detector can be expres3ed in a, numher of ways. The most basic is the responsivity, R, which can be thought of as a transfer coefficient, the ratio of output to input. It may be expressed a s either
R
=
dV/dP
or
R
=
dZ/dP
=
VNIR
where V N is the root-mean-square noise voltage produced by the detector. The value of V N is dependent on many factars, including the effective area of the detector, the amhient temperature and the wavelength. The noise in a detector covers a. very wide frequency range, whereas the signal information is carried a t a single frequency, that a t which the beam of radiation is chopped. This is why the signal-to-noise ratio is so vastly improved by the use of an amplifier sensitive to the chopping frequency only (i.e., a lock-in amplifier) ( 1 ) . I n view of the many variables involved, the NEP for a detector can be specified only for carefully described conditions. I t is commonly given for a. hypothetical
where P denotes r d i a n t power (in watts) incident on the detector, and V and I
.-
Op-Amp
TO
Amplifier
-
-
1, Q1
Q2 Neg.
J1
Feedback
-
J2
--
i
e ur.ng Feld-effect Ironsstor, ond an operatonol omdifier. Figwe I . 1 h e r m o ~ o ~ p lprcampl'fer, Simplmed from the rchemotic for the Beckman IR-33, b y permtnon. 1. or the rrodmted ~~nct'on. 1. the reference 11.n~8on.0,ond 0 1 ore FET', n a p a ' r ' connguroton hype 2h53921; the omplifler is a air child type pA741C, or equivalent.
(Continuedon page A 6 W Volume 48, Number 9, September 1971
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Chemical Instrumentation to keep them taut. The assembly is usually mounted in an evacuated housing (with a small iinfrsred-t1,ansmitting window) to increase the responsivity by minimizing conductive heat loss. The "cold" junction of the couple actually consists of the heavy lead wires (usually copper) i n contart wit,h the thermocouple wires. Since the detecbor only needs to respond to chopped radiation, and to give a n AC output, only change8 of temperature are significant, hence the actual temperature of the "cold /unrtioiP is unimportant. Thermocouple detectors are low-impedance devices, and are usually coupled to a preamplifier through a high turns ratio transformer (as high as 1000:l) with good Low-frequency response. The transformer must be protected from inductive noise pick-up by an efficient magnetic shield. Alternatively, the thermocouple may he connected directly to a F S T stage, ~s in Figure 1. The detector itself should never he tested for continuity with an ohmmeter, as the fine wires would burn out instantaneously. A thermopile is an array of thermocouple junctions connected so that t.he voltage produced by eseh junction is mult,iplied by the number of junctions. I n it,s classical form it has a fairly high sensitivity but s, time constant of a second or so. Hence it is unsuited for chopped operation. It is, however, one of the most. reliable devices for energy moasnrements and cnlihration purposes. Thermopiles are also avaihhle for spect,rophotometry, with three or four junctions spaced so as to utilize efficiently the radiation from a narrow dit. Barnes Engineering offers novel thermopiles of 20, 120, and 308 junctions, made by integrated circuit techniques, and intended parbicnlady for rocket and satellite applications.
Bolometers
A bolometer is a miniature resistance thermometer usr~slly made of metal or sernicond~~ctor.The sensitive component in a metsllio bolometer is nsuslly platinum or nickel; the latter has the largest ternpernt.ure coefficient of resistanre of any metal. Thermistors (3) are about five times more sensitive, and can he fshricated so as to have a smdler time constant. Room temperature bolometers appear not to he slandnrd equipment in any cornmereid spectrophotometers now available. They were used in t,he several models formerly m n u f a e t ~ ~ r eby $ BairdAtomic. A small flake of lightly doped germanium or silicon, cooled with liquid helium, shows nn unnsually large lemperature coefficient of resistance. This property can be made the basis of very effective bolometers (.3). Cryogenic holnmeters of both Ge and Si are commercially available. The resistance of a bolometer, can he measured by conventional circuits. Two which have been widely used we shown in Figores 2 and 3, where one bolometer A524
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Amplifier
Figure 3. Wheomone bridge circuit for a bolometer. RI and Ra ore equal re3irtorr of zero temperature coefficient; R, and Rs are as in Figwe 2; Rr is a m o l l variable resistor for initial balancing.
element is exposed to the radiation while an identical one is shielded, but subject to the same amhient conditions. For both circuits, the output voltage is proportional t o the ratio of the resistances of the two elements, with the effects of changes in ambient temperature essentially eliminated. Figure 4 shows s, somewhat more elaborate circuit built mound operational amplifiers. Circuit analysis shows that the output is proportional to the diffe~ence between R, and R, rather than their ratio. This means that the nominal resistance values drop out, giving a voltage output directly proportional to the temperature rise. Ferroelectric and Pyroelectric Bolometers
Certain crystals, described as ferroeleclrics, possess a permanent dipole moment. If mounted between two parallel electrodes, they respond to heating as though they were temperature-sensitive capacitors, and so they can be nsed for temperature measurement. I n addition, these and a few similar materials develop a potential difference between the electrodes as the result of heating. This is called the pyroeleetrie effect. Barium titanate and triglycine sulfate (TGS) are both ferroeleetric and pyroelectrie; lithium niohate is an example of s. material whieh is pyroelect,ric but not ferraelectrie. Both types have been made commercially avd.ilable in mountings suitable for speetrophotemetric detection. They are referred to ss "bolometers," even though they are not resistive devices. A field-effect transistor is usually mounted immediately adjacent to t,he detector, as the first stage of amplifie&m. Pneumatic Detectors A very reliable and sensitive infrared detector can he made by causing the radiation to heat a small enclosed portion of gas, then detecting the resulting pressure change. This is the principle of the Golay cell (4), whieh is produced by
and impinges on an absorbing film immersed in a small volume of xenon gas. The rising pressnre produced by heating deforms R. silvered diaphragm, and this modulates a secondary optical sg-stem. The infrared spectral informat.ion is (Cmtimn7red m nnne AAZ6I Volume 48, Number 9, September 1971
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Chemical Instrumentation transferred to an electrical form, via a photocell. The Golay detector is about equivalent to a good thermocouple through the near and mid-infrared, and is seldom used in these areas because of greater expense. I t is significantly superior in the far infrared, beyond about 50 pm, and is the customary detector in this region.
R
Amplifier
Photon Detectors Infrared photons of wavelengths greater than about 2 pm do not contain sufficient energy to excite electronic transitions in insulators, corresponding to, for example, absorption and fluorescence transitions
-
-.
circuit for o bolometer detector, using three operational ompiiFigwe 4. A suggested flerr. R, and Ra hove their previous rigniflcanse; all other resirtor, are equal. The output voltage is given by Et, = Ei./R IRs - R A
in the ultraviolet and visible regions. Nor do they have energy enough to cause photoelectric emission of electrons from a.
slly observable efflcts is i n semiconductors where it is sometimes oossihle to Dromote an electron from the valence band to the conduction hand by absorption of an infrared photon. Figure 6 shows several possible mechanisms for absorption of infrared photons which can produce electronic transitions. I n each, the diagram shows three energy regions: the valence hand, filled with electrons involved in covalent bonding, a forbidden band, which constitutes an energy harrier, and the conduction band, nearly unpopulsted. At room temper* ture, a. predictable fraction of the electrons will have sufficient thermal energy to move into the conduction band. The higher the temperature, the greater the populat,ion in the upper hand, hence the lower the resistance. (This e x ~ l a i n s the large negative temperature coefficient of resistance of thermistors.) At the shorter infrared wavelengths (up to about 10 pm) the dominant mechanism is the so-called intrinsic phataconduction (Fie. .. 6a). . The energy of the incident photon goes to promatean electron from valence to conduction band, leaving a. "hole" behind. Both the electron and the hole are now ahle to conbrihute to electric conduction, the phatooonductive effect. If the absorption occurs in the vicinity of a pn junction (Fig. 6b), t,he electron will immediately he swept away from t,he n to the p aide of the junction, to a point of lower energy. This gives rise to a potential, t h e photovoltaic effect. At intermediate wwelengths (10-120 pm) the photons have insufficient energy for the intrinsic mechminism. I n order to permit photocondnrtion in this region, the semiconductor must be "doped" with impurity atoms (Fig. 6c). The effect of t,he impurities is to provide some energy levels intermediate between the valence and conduction hands. Absorption of smaller-energy quanta can then cause an electron to jump from a. high-lying intermediate level into the conduction band, or frnm the valence band into a lower in-
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MoirP Grid
Absorbing Membrane
\. .
Gas Chamber
\
Photo tube
F l e x i b l e Mirror
Figwe 5. Golay pneurndk infrared detector. With the flexible mirror in itr rest position, on image of half the Moirk grid folk on the other half so that no light p a r e s through; flexing of the mirror mover the image of the grid loterolly so that varying amounts of light can reosh the phototube.
termediate level. Either will result in increased conductivity. At still longer wavelengths (the far infrared, 100-10,000 pm, i.e., 0.1-10 mm) the only mechanism available (Fig. 6d) is "free-carrier absorption," whereby the distribution of electrons within the conduction band is altered, thereby affecting the conductance. The sensitivity of some of these devices, particularly the last described, can he improved by subjecting the detector to a static magnetic field. The effect is to tend to move the electrons and holes in opposite directions ss they are formed, thus decreasing the chance of immediate recombination. This is called photoelectromagnetic detection. Because of the low energies involved, most of these semiconductor effectswould be swamped out by thermal electrons if operated a t room temperature. This source of excessive noise can only he eliminated by cooling. Often liquid helium (4.2%) is required, hut in some detectors liquid nitrogen (77'K) is sdequate. In the near infrared, the intrinsic process is sometimes sufficientlynoise-free to operste at ambient temperature. Semiconductors useful as infrared detectors include the sulfides, selenides, tellurides, arsenides, and ant.imonides of such metals as Ga, In, TI, Cd, Sn, and Ph. Also important is germanium doped wit,hAu, Hg, Cu, Cd, or In. Figure 7 shows a comparison of the theoretically attainable detectivity of thermal and photoconductive detectors at room temperature. Since the thermal detectors are affected only by heat, their response should be uniform throughout the spectrum. Real detectors are less than the ideal to the extent thst they fail to absorb all wwavelengths uniformly. The minimum a t about 10 pm for the photoconductor corresponds to the maximum in black-body radiation at room temperature. I t is here that noise due to radiation from the surroundings is a maximum, hence reducing the S/N ratio. The picture can be improved greatly by cooling the detector and its surroundings.
Future Prospects Of all the detectors discussed above, the one most likely to displace the thermoVolume 48, Number 9, September 1977
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couple is the pyroelectrie detector. I t has a. considerably better speed, with about the same detectivity and does not require cooling. The increased speed (lower time constant) means that a spectrum from 2 to 20 pm can be recorded in, say, 10 seconds instead of 10minutes. One spectrophotometer is already on the market with this capability (Model RS-1, OCLI Instruments, South Norwalk, Conn. 06854). The speed is not merely a. timesaver, but permits on-line operation, for example in series with s. gas chromatograph. Probing further afield (blue skies department) one might speculate on the possibilities of heterodyne detection with s laser as local oscillator. The method would be the optical analog of heterodyne radio circuitry, in which the signal to be (Cmtinued a page A688)
Chemical Instrumentation
available oscillator is a laser, which can only operate a t a single frequency. Hence the infrared would be translated t o a frequency in another region, which is scanned with the techniques suitable to that region. Up-tritnslation is possible, whereby infrared is converted to visible
\
or ultraviolet, which could be observed with a photomultiplier tube. It is also possible to down-translate, and generate a microwave signal, observable with standard wwe-guide techniques. Experiments are under way in a number of laboratories t o explore these possibilities.
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Photoconductive
Figure 6. Mechanisms for photoconduction in an n-type remiconductor: (01 intrinxic; ( b ) same, in vicinity of 0 pn-junction M e n-region i s to the left of the dircmtinuity, p to the right); (c) impurity photoconduction; Id) free corrier photoconduction.
Detector
Thermal Detector
I observed is translated from its origins1 frequency to one more convenient. to handle in the electronics, by beating with a laeslly generated signal. While in radio, the local oscillator is variable in frequency, in the infrared case, the best
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I
10 100 Wavelength ( p u )
I
1000
Figure 7. Theoretisol limit* of D* for photocondvctive and thermal detectors a t 300°K.
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Table 1.
T w Thermocouple Bolometer, metal Bolometer, thermistor Golay (pneumatic) Ge bolometer Ge bolometer Si bolometer Si (photovoltaic)* (Ba,Sr)TiO. (ferroelectrio) Perovsklte (pyroelectrio) TGS (pyroelectric) InSh (photoeonduetive) InSb (photoconductive) PbS (photaeonductive)~ Ge(Hg) (photoconduct~ve) Ge(Cu) (photoconductive) (Cd,Hg)Te (photoconductive) InSb (photoelectromagnetic)
Summarv of Infrared Detectors. Operating Temp. ("K) 300 300 300 300 2 4.2 4.2 300 300 300 300 77 300 300 35 4.2 77 300
Max. Useful Chopping Rangeb freq. (w) (HZ) 1-1000 20 1-1000 20 1-1000 100 1-1000 10 1-20 20 >50 13 >50 O.&l 250 loo 1-1000 20 1-1000 500 2-25 500 1-56 1000 -7.5 1000 1000 1-3 1000 2-13 2-25 1000 3-15 1000 1.5-6.9 1000
Manufacturers (Code)
Log D*O
9 V 9 G 9 D, W, X 9 C, G, M, R, S 10.8T 10.5 T
2
12 l1 I 8.7 K 8 D, N, 0, P X 0 11 A, B, D, L, 0 8 P, Q, U 10.7 E, 0 , P 10 U 0, P, Q, U 10 J, 0, P 9 7.5 L
Figures are suggestive only, as there is much variation between manufacturers.
* The notation "1-1000" does not signify limits, but merely that coverage extends over
the whole region covered by this review. a Given instead of the more usual D* to conserve space. d Representative of many devices from various manufacturers, far which the maximum response is outside the wavelength region covered in this review. The fundamental phenomena of frequency trandation have been observed, hut as yet do not have sufficient sensitivity for spectrophotometric use. I t will be exciting to watch future announcements in these directions.
REFERENCES (1) Coon, T.. T ~ l sJOURXAL. 45, A533, A583 (1968). 44, , A935 (2) R o u c e ~ n .E. A,. Tnrs J o u n l * ~ ~ (1967). (3) B*c"M*NN, R.. KXABC", H. C.. and GERIZLE. T . H . , Re". Sei. Instrum., 4 1 , 517 (1970). (4) G O U T , M. J. E., Reu. s c i . I n r t m m . , 18, 357 (1947).
General References ALI.snr. N. L.,K m s m . m.E., and S Z Y M A N ~ I . H. A,. " I R : Thaory and P ~ a c U c sof I n l m m d Spsctroscopy," Plenum Press, N e w York, 2nd ed., 1970: p 42 e t aeq. KIMMITT, M. F., "For-Injiored Techniques;' Pion, Ltd., London. 1970: P. 60 et se". (The discussion is not limited to the far IR regions.) STEWART. J. E., "Inlimed S p e c t o s e o g v E w e r + mental Methods and Techniques," Dekker. New York. 1970: D. 363 e t seq.
Sources of Defectors (A) Advanced Kinetics, Inc. 1231 Victoria St. Costa Mesa, Calif. 92626 (B) American Electronics Lab.. Ino. P. 0 . Box 552 Lansdale, Pa. 19446 (C) Angenieux Corp. of America 55 1Merriek Rd. Oceanside, N.Y. 11572 (D) Bmnes Eng~neeringCo. 30 Commerce Rd. Stamford, Conn. 06902 (E) Borg-Warner Thermoelectronios Wolf and Algonquin Rds. Des Plaines. Ill. 60018 (F) Electro-Nuclear Laboratories, Inc. 115 Independence Drive Menlo Park, Calif. 94025 (G) The Eppley Laboratory, Inc. 12 Sheffield Ave. Newport, R.I. 02841 (H) Far Infrared div. of Malectron Corp. 930ThompsonPlsce Sunnyvale, Calif. 94086
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~ a r s h a wChemical Co. 6801 Cochran Rd. Solon, Ohio 44139 Honeywell, Inc., Radiation Center 2 Forbes Rd. Lexington, Mass. 02173 Huggins Laboratories 999 E. Arques Ave. Sunnyvale, Calif. 94086 Infrared Industries, Inc., Photoconductor div. P.O. Box 42 Waltham, Mass. 02154 Jobin-Yvon 94 Arcueil, France (via Angenieux) Laser Precision Corp. 5 W. Whitesboro St. Yorkville. N.Y. 13495 Mullard, 1n'c. 100 Finn Court Farmingdale, N.Y. 11735 Optoelectronics, Inc. 1309 Dynamic St. Petaluna, Calif. 94952 Philco-Ford, Microelectronics div. 500 S. Main St. Spring City, Pa. 19475 Philips Electronic Instruments 750 S. Fulton Ave. Mount Vernon, N.Y. 10550 Pye-Unicrtm, Ltd. York St. Cambridge, England (via Philips) Quantum Electronics Corp. 1106 Wisterwood Houston, Texas 77043 Raytheon Company, Special Microwave Devices Operation 130 Second Ave. Waltbsm. Mass. 02154 Charles M. keeder Co., Inc. 196 Victor Ave. Detroit, Mich. 48203 Semo Corp. of America. 111 New South Rd. Hicksville, N.Y. 11802 Victory Engineering Corp. P.O. Box 187 Springfield, N.J. 07081
