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INSTRUMENTATION
Advisory Panel Jonathan W. Amy Glenn L. Booman R o b e r t L. Bowman
Jack W. Frazer G. Phillip Hicks Donald R. Johnson
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H o w a r d V. M a l m s t a d t Marvin M a r g o s h e s William F. Ulrich
Semiconductor Light-Emitting Diodes Richard A. Chapman Texas Instruments Incorporated, Dallas, Tex. 75222
A wide range of light-emitting semiconductor diodes is now available for applications in the visible and infrared. Devices with specific wavelengths of direct interest t o analytical chemists can be developed
Sivhich emit infrnred p-71 junction diodes or visible light ELIICONDU,CTOR
liiivc nt t racted considerable attention ciiiriiig tlie last year. ( I n this article, light cmitter will be used as the generic term for devices which emit radiation in tlie ultraviolet, visible, or infrared.) The noncoherent devices go under various n:uncs such as light-emitting diode (LED) and solid-state lamp (SSL). K i t h proper device design, these diodes c ~ i nh t fabricated as l a s w or used to optic7all:- pump other solid-state lasers. The lioncoherent emitters and lasers emit radiation in a narrow wavelength b m d . Devices have been constructed which opemte in rhe range of wavelcngt h- from 4500A to 30 pm depending 011 the semiconductor used. Commercial interest has centered on visible and near infrared emitters and lasers, but tliere are also some extremely interesting projects using far infr:ired lasers. Interest in semiconductor light, emitters goes back many decades. Research shifted to semiconductor diodes in the mid-1950’s. The discovery in 1962, that, efficient emitters (both lasers a n d noncoherent emitters) could be constructed from Gails, caused considernble R & D activity. Recent advances in materials and in device fabrication have now brought this technology to a position d i i c h should interest analytical chemists in the application of these devices in laboratory mensurements or in process and pollution control. .4 part of the interest in semiconductor light emitters stems from their
unique features in comparison with those of other light sources, such as incandescent, fluorescent, and gas discharge light sources. For instance, the semiconductor emitters are extremely small (typically an m m or less in cross section) and they require small voltages (typically 1 to 2 5’). Pulsed semiconductor lasers have extremely large spectral radiant emittance. Parameters which vary widely with device type are operating temperatures (4.2 to 300”K), bias currents (millamps de to 7 5 A pulsed), linewidths from 1 to 500 A for GaXs devices) and requirements for de us. pulsed operation. High-porver devices available commercially include pulsed GaAs lasers with 1 to 20 W peak power emission a t BO50 A and de-operated Ga.h noncoherent emitters with 200 mTT7 emission at 9350 A. T’isible noncoherent red, gree:i, and yellow light emitters are commercially available. Other than tlie I n h s cmitter n-hich emits at. 3.2 pn, most of tlie longer wavelength emitters and lasers are not yet commercially available. Soncoherent light emitters should be useful in applications requiring single ryavelength-band monitors of reflectance, transmittance, and absorptance. Semiconductor lasers should be part,icularly useful as pulsed sources for luminescence and light scattering or in applications requiring more narrow linewidths than available with noncoherent emitters. I n this article, the a.uthor will attempt to acquaint analytical chemists ~
with the characteristics of available sources. A discussion of theory and device technology necessary for proper application is included. An at,tempt is made to stimulate the interest of the rcnders so that they may in turn direct device development into areas of particular importance to analytical chemistry. Semiconductor Theory and Materials
Before proceeding to a discussion of the devices, it is useful to review terminology for those not active in the field. I n a semiconductor, the atomic wavefunctions have combined in such a manner that a region in energy devoid of states is formed in a perfect crystal. The region is called the forbidden g a p ; this gap separates the valence band from tlie conduction band. I n Figure 1, the lower energy limit of the conduction band, Ec, and the upper limit of the valence band, E,., define the energy gap ( E , - E,). Impurities and other defects in the crystal cause impurity states such as donors (energy E n ) and acceptors (energy E A ) . By adjusting the impurity content, the semiconductor can be made n-type (conductivity dominated by electrons in the conduction band) or p-type (conductivity dominated by holes in the valence band). Genernlly, p-n junctions are formed in semiconductors by impurity diffusion or by changes in doping during crystal growth. Absorption of radiat,ion wit.h energy greater than the band gap ( E , - ET.) causes the generation of excess elec-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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Figure 1. Energy band diagram showing several possible radiative recombination mechanisms between electrons (dots) and holes (open circles)
Figure 2. Three types of noncoherent light emitter structures: (a) flat source, (b) flat using edge emission (laser geometry), and (c) dome emitter. The line through t h e source is the p-n junction i n the flat sources
70A
trons and holes. The excess elect’rons and holes generated may recombine directly as is shown on the left in Figure 1. The electrons and/or holes may be trapped first on donors or acceptors, respectively, and then recombine. TKO examples of such recombinations are also shown in Figure 1. There are many other important recombination mechanisms. An excellent review of recombination processes has been given by P. J. Dean ( 1 ) . If t,he recombination process results in the generation of a photon (with energy E , - Ev,E c Ed: E , - E A , etc.), the process of absorption and recombination results in photoluminescence. If t,he excess holes a,nd electrons have been created b y an incident beam of high-energy electrons, the process is called xthodoluminescence. The unique advant,ages of a semiconductor luminescent’ source are only obtained when p-n junctions are used. When forward bias is applied to the p-n junction, excess electrons are injected into the p-type region and excess holes into the n-type region. Radiative recombination of these excess carriers in their respective regions causes the emission of light. Often only the p-type side is an efficient source of luminescence. Semiconductors of major importance as light, emitters and detectors include compounds of chemical groups 111.4 and VA (the 111-1’ compound semiconductors such as GaAs, etc.), the IT’A-VIA compounds (such as PbTe, etc.), and in some cases the IT’A elemental semiconductors (alloys such as S i c ) . The 11-171 compounds have not been utilized because p-n junctions are difficult or impossible to form in these materials. Alloys of semiconductors are of great technological import,ance. Each semiconductor has a given energy gap which in large part sets the xvavelength of the radiation emitted by the diode. Alloying two similar semiconductors such as GaAs and GaP allows the fabrication of a device with a band gap intermediate between that of the constituents. Light-emitting injection diodes in the blue and ultraviolet are not yet available. The development, of such devices is a formidable task. Large band gap semiconductors typically have highmelting temperatures and poor electrical characteristics, and tend to be more ionic. Blue and ultraviolet semiconductor lasers have been constructed using cathodoluminescence. I n a different approach, infrared emitting diodes have been used to pump green and blue phosphors by the multiple absorption of two or three infrared photons.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
Noncoherent Emitters and Lasers
Both semiconductor diode lasers wit,h coherent emission and semiconductor diode emitters with noncoherent emission are built, I n this section, the characteristics of these two types of LED will be compared. A wide variety of noncoherent emitter designs are available. I n many applicatbns, an emitter and. detector are placed in close proximity without additional optics; in this type of application t,he radiant intensity in watts per steradian is most important. Some available Gahs devices include a parabolic reflector or lens to increase radiant intensity. I n other applications, maximum radiant flux or polver output (watts) is desired. To meet this goal, the emitter must be designed to make the external quantum efficiency (photons emitted per carrier crossing t’he device) approach the internal quantum efficiency (photons created per carrier crossing the device). -4 discussion of the parameters determining external quantum efficiency helps to explain the variety of noncoherent, emitter designs and the relation t o laser design. There are three major loss mechanisms decreasing external quantum efficiency: (1) internal absorption of the created photons can be a large effect vhen the photon energy is close to t,he forbidden gap energy; (2) a fraction of the radiation is internally reflected if the device is not antireflection coated : and in any case, (3) all rays striking the exit surface with angles to the normal greater than the critical angle +c = sin-1 ( l / n ) will be totally reflected i+c 16O for GaAs), where n is the i:idex of refraction in the semiconductor and the index of refraction of air is taken as equal to 1. -4thorough discussion of figure of merit for noncoherent emittere is available ( 2 ) . Figure 2 shows three types of device design. The length of the arrows represents the intensity of the emitted radiation as a function of the external angle 8 , (sin 8 = sin 4). Figure 2a shows an emitter design which uses the emission exiting from the face parallel to the junction. The angular distribution of radiation intensity is almost Lambertian (intensity proportional to cos 6 ) . Figure 2b shows a design which utilizes the light exiting along the p-TI junction periphery. The emitter designs of Figure 2a and 2b are often packaged in a miniature mount which has a parabolic mirror to collect the radiation and increase the radiant intensity. Figure 2c shows a hemispherien1 emitter (the p-n junction is near the center of the hemisphere). This dome greatly reduces the internal total
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Instrumentation
reflection loss and results in a larger external radiant flux and external quant,um efficiency. The highest power output noncoherent emitters use the hemispherical design. Semiconductor diode lasers are constructed using the geomet,ry of Figure 2b with two narrow sides made parallel by cleaving the sa.mple. -4s the current densit,?- is increased in a p-n junction diode, the injected carrier densit'y increases until it. finall>- creates a population inversion in the region near the junction. For currents above this threshold current, the absorpt'ion coefficient becomes negative in the inverted region near the junction. Radiation reflected back and forth betiveen the two cleaved faces can increase in intensity, and if the gain is greater t,lian the losses, lasing will occur with emission from the cleaved faces. T h e rays leave the semiconductor a t angles less than the critical angle helping t'o make the esternal quantum efficiency high in the laser simcture. The angular distribution of radiation from a laser is limited b y diffraction. Unlike other lasers, the inverted region in a semiconductor diode laser is quite narrow (a few micrometers) , thus causing a relatively large far-field diffraction angle. Typically, the radiation from a GaAs laser i: contained in a beam about 5' wide a t half-intensity in the plane perpendicular to the junct,ion and 15' wide in the plane of the junction. This angular distribution is more narrow than that for noncoherent semiconductor emitters which do not have a built-in parabolic mirror, but the 5' by 15' beam is considerably larger than obtained with gas lasers and solid-state lasers such as ruby. The far-field diffraction angle is directly proportional to wvelengt,h, making the diffraction angle larger for the f a r infrared semiconductor lasers. Because of the small emitting area, semiconductor lasers have tremendous radiance (wntts/steradian/mmZ of emitting area). Pulsed Ga.4~ lasers have radiances in the range 105 to 106 peak TFT/steradian/mm2 with a 0.1% duty cycle (pulse duration divided by pulse repetition period), Radiance is the important' factor if the experiment involves focusing the radiation onto a sample to obtain maximum illumination. Soncoherent GaAs hemispherical emitters are available with 20 mTV/steradian/mm2 radiance. A radiance of this value is sufficient for many experiments, and dc operation is often extremely important. Lasing is obtained near the maximum of the noncoherent emission line, and the luminescence from a laser can have a linewidth much smaller than thlat of
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
71 A
Instrumentation
a noncoherent emitter. If the laser is operated with current densities only slightly above threshold, some semiconductor lasers can be operated in one cavity mode which is less than 1 in linewidth for GaAs. More often, however, the device is driven above threshold to obt,ain maximum power out,put; under these circumst.ances, many lower gain cavity modes lase such that numerous lines are observed in a bandwidth in the order of 20 to 40 Detailed information on cavity modes active in GaAs lasers is available ( 3 ) . GaAs noncoherent emitters operated a t room temperature have bandwidths ranging from 250 to 500 A depending on device type and dopant used. Soncoherent' emitters may be operated in a dc mode or in a modulat,ed or pulsed mode. Response times the order of a few nsec for standard GaAs can be obtained and the order of 0.2 psec for GaAs doped witch silicon. When pulsed, a GaAs laser can have a radiant output response time of less than a nsec. Light output degradat.ion with time has been observed to occur in light emitters. *kt least for GaAs, the degradation rate is directly proportional to current density and is, therefore, much more rapid wit.h lasers (which must be driven past' a certlain threshold current density). The required lasing threshold current density increases with increasing temperature. This is one of the reasons that cw room temperature semiconductor lasers have not been developed. Typically, a room temperature GaA4s laser must be operated with a duty cycle of 0.1% or less and a pulse width of 200 nsec.
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Noncoherent emitters can also be used to pump other solid-state laser materials ( 4 ) such as neodymium-doped yttrium aluminum garnet (YA1G:Nd). The high efficiency silicon-doped GaAs emitters are particularly useful in this application. This system has the advantage of better laser optics (smaller diffraction angle and fewer modes), and the prospect of room temperature cw operation since room temperature dc noncoherent emitters are available. Devices and Materials
Table I shows a partial list of semiconductor materials from which lasers on noncoherent emitters have been made. Some entries are or have been commercially available. Of the commercially available devices, Gaiis, Ga[As,P], and InAs are all available as either lasers (including Ga[As,P] at 8600 A) or noncoherent emitters (Ga[As,P] a t 6600 A). The 111-V alloys cover a wide range of wavelengths. Ga[As,P] alloys can be made which emit in the wavelength region between GaAs (9000 k ) and G a P (5600 The quantum efficiency decreases for the shorter wavelength alloys. The alloy system [Ga, Al]As covers about the same wavelength region as Ga[As,P], Because the band gap, E,. changes with temperature, the wavelength, A, of the emission depends on temperature. For the 111-V semiconductors, AE,/(AT) is about -4 x 10-4 x eV/"C. Since AA/(AT) = ( h / E G ) x (AEn/AT), the fractional change in wavelength with temperature is larger for the smaller band gap semicon-
A).
Table I. Lasers and Noncoherent Emitters
Material GaAs:Si-phosphor-coated (Blue) GaAs: Si-phosphor-coated (Greeny GaP (Greeny Sic (Orangey Ga[As,P] (Amber)= GaP (Redy Ga[As,P] (Redp Ga[As,Pp GaAsa GaAs: Si5 I nAs5 lnSb PbSe [Pb,Sn]Te, [Pb,Sn]Se a
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Wavelength 4700 A, 6500 A, 8000 A 5400 9400 A 5600 5900 6100 7000 6600 8600 9000 9200 to 9500 3.1-3.6 prn 5.2-6.2 prn
a, a a a a a a a a
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8.0-22 prn 9.5-30 prn
These devices are commercially available.
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
Nonco. herent emitter response time, sec
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ductors. The entries for InAs and InSb in Table I show the variation of wavelength with temperature from 300 K to 4.2 K. A change of 3 B / ' C is usual for &As. The PbSe laser has been pressure-tuned over S to 22 pm using pressures to 15 atmospheres. The alloys [ Pb,Sn]Te and [ Pb,Sn]Se have a range of wavelengths available, depending on alloy composition. The wavelength of any emitter can be slightly tuned by diode current (a heating effect), The first two entries in Table I are phosphor-coated noncoherent emitters. The wavelength is determined by the phosphor. Several different wavelengths are obtained simultaneously from each phosphor; these are listed for each phosphor-coated device. The linewidth of the green emission is about 30 Visibly bright emission can be obtained if these devices are driven with high intensity infrared emission from the high efficiency GaAs :Si emitters. The last' column in Table I lists the fall-time of commercially available emitters. I t should be emphasized that the response time can be larger than stated if the light emitter is not properly packaged for fast, response. The response t'imes of all injection lasers will be 1 nsec or less. The fall-times of the blue and green phosphor-coated devices are similar and are set by the phosphor response. The maximum dc power output is obtained with silicon-doped GaAs noncoherent hemispherical emitters which emit approximately 200 mW radiant flux at 9300 Aiwith a 350 Ailinewidth. This represents a total radiant emittance of 7.5 W/cm2 of emitting area and a spectral peak emittance of about 210 W/cm2/pm. This total emittance could be obtained with an incandescent tungsten source, but a tungsten filament would have to be operated near its melting point to obtain the same peak spectral radiant emittance at 9300 Room temperature GaAs lasers are available with peak radiant flus of 1 to 20 W peak at 9050 using 10 to 7 5 A peak drive current with a 0.1% to 0,02% duty cycle. The radiance and spectral radiance of these sources should surpass all types of conventional discharge lamps ( 5 ) . -4pplications requiring high illuminance with singlemode dc operation would presently require the use of gas lasers such as the argon-ion laser. The output of visible light emitters is usually given in terms of photometric units which express the response of the human eye to the output emission. For instance, lumens can be cor-
A.
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on Readers' Service Card ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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I CARBON ANALYSIS
related with watts, candela with watts per steradian, and candela/cm2 with watts/steradian/cm2. The reader is referred to the literature on photometric measures ( 6 ) . Information on infrared detectors to be used to detect the emission from semiconductor emitters can be obt’ained from an earlier article by Levinstein ( 7 ) in this column. Both emitters and intrinsic infrared detectors must be operated a t reduced temperatures. The long wavelength [Pb,Sn]Te and InSb lasers are usually operated a t temperatures approaching 4.2 K. InAs is usud l y operated a t 77 K as a detector and incoherent emitter. GaAs and materials with shorter wavelength emission are operated a t room temperature and use detectors such as photomultipliers and silicon photodiode detectors which are also operated a t room temperature.
spectroscopy research of much improved resolution. For instance, a [Pb,Sn]Te laser operating at 10.6 can be tuned over 0.05 pm ( 4 cm-1 wavenumbers) by varying the diode current from 0.7 A to 1.4 A. A current tuned [Pb,Sn]Te laser has been used to study the ammonia absorption line a t 941.75 cm-1 wavenumber and sulfur hexafluoride absorption lines near 10.6 pm wavelength (9). A Doppler broadened linewidth of 0.0027 cm-1 was measured for the 941.75 cm-1 line. This line cannot be resolved with a grating spectrometer having a 0.3 cm-1 resolution at that wavelength. Hundreds of previously unresolved lines are observed in the study of sulfur hexafluoride. Both direct detection and heterodyne detection using the difference signal between a carbon dioxide laser and a [Pb,Sn]Te laser have been used.
Applications Acknowledgments
One of the major scientific applications of noncoherent emitt’ers and lasers has been the use of these devices as sources for opt,ically pumping other materials. The use of silicon-doped GaAs noncoherent emitters to pump solidstate laser materials has already been mentioned. Laser radiabion from GaAs diode lasers has been used optically to pump other semiconductor materials such as InSb. GaAsP lasers have been used to stimulate lasing in GaAs. Radiation from GaAs lasers has also been used to study light scattering in semiconductors ( 8 ); the many modes present in radiation from high-intensity GaA4s lasers can make difficult the interpretation of quantum-loss light scattering experiments. Radiation from either noncoherent emitters or diode lasers can be used to study response times of infrared detectors. Pulsed semiconductor lasers are particularly useful in measuring response times of detectors designed for high-frequency response infrared deACCURATELY determine carbon content of metals tectors. A number of investigators, inand other materials in 2 minutes! No complicated cluding the author, have used GaAs and mathematics; eliminates cnstly time-consuming routines. Samples may be borings, mill chips, [Pb,Sn]Te lasers and InAs emitters and crushed samples, pellets, etc. Dietert-Detroit lasers in the study of detector response testing equipment widely used i n company laboratimes. tories and institutions for over 30 years. Noncoherent-light-emitter siliconSulphur Determinators also available. photodiode-detector pairs can be used to monitor transmittance. The availability of Ga[As,P] alloy emitters ARRY W. DIETERT CO. should Dermit the monitoring” of ab9330 ROSELAWN DETROIT 48204 sorptnnce of materials with absorptance , bands in the near infrared and visible, Send me free 16-page Carbon-Sulphur Bulletin These devices can be operated a t NAME temperature without sophisticated elecCOMPANY tronic pulse equipment. The narrow linewidth of semiconducADDRESS tor lasers and the ability to slightly tune CITY -STATE -ZIP - I the emission wavelength permits ir I
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
The author wishes to thank those manufacturers who supplied requested information on semiconductor lasers and emitters. Product information is available from -4merican Electronics Laboratories, Electronuclear Laboratories, General Electric Miniature Lamp Department, Hewlett Packard, Monsanto Electronic Special Products, RCA Electronic Components Department, Raytheon, Spectronics Incorporated, and Texas Instruments Incorporated-Optoelectronics Departmmt Special thanks are also due to David Hinkley of Lincoln Laboratories for preprints on infrared spectroscopy experiments using [Pb,Sn]Te lasers and for permission to mention results on experiments in progress, References (1) P. J. Dean, Trans. M e t a l . Sac. AZME, 242,
384 (1968).
(2) W. N. Carr, Infrared Phys. 6, 1
(1966). (3) T. H . Zachos, I E E E J . Q u a n t u m Electron., QE5, 29 (1969) : T. L. Paoli. J. E. Ripper, and T. H. Zachos, ibid.,
OE5,271 (1969). (4)- R.’ B. Allen and S. J. Scalise, A p p l . Phvs. L e t t . , 14, 188 (1969). ( 5 ) L. Levi, A p p l . Opt., John Wiley & Sons, Inc., New York, N. Y., 1968, Chap. 1 and Tables 3 and 4. (6) L. Levi, ibid., Chap. 5 .
(7) H. Levinstein, A N A L . CHEM., 41, 81 A (1969). ( 8 ) A. Mooradian, “Advances in Solid State Physics,” Festkorperprobleme I X , edited by 0. E. Madelung, Pergamon Press, New York, N. Y., 1969, pp 73-98. (9) E. D. Hinkley, MIT Lincoln Lab. Opt. Res. Rept., 1969:2 p 3.
