INSTRUMENTATIΟΝ
Advisory Panel J o n a t h a n W. Amy Glenn L. Booman Robert L. Bowman
J a c k W. Frazer G. Phillip Hicks Donald R. J o h n s o n
Howard 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. C h a p m a n Texas I n s t r u m e n t s Incorporated, Dallas, Tex. 7 5 2 2 2
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 to analytical chem ists can be developed
SEMICONDUCTOR p-n junction diodes which emit infrared or visible light have attracted considerable attention during the last year. (In this article, light emitter will be used as the generic term for devices which emit radiation in the ultraviolet, visible, or infrared.) The noncoherent devices go under vari ous names such as light-emitting diode (LED) and solid-state lamp (SSL). With proper device design, these diodes can be fabricated as lasers or used to optically pump other solid-state lasers. The noncoherent emitters and lasers emit radiation in a narrow wavelength band. Devices have been constructed which operate in the range of wave lengths from 4800 Â to 30 ^m depending on the semiconductor used. Commercial interest has centered on visible and near infrared emitters and lasers, but there are also some extremely interesting projects using far infrared lasers. Interest in semiconductor light emitters goes back many decades. Research shifted to semiconductor diodes in the mid-1950's. The disco\rery in 1962, that efficient emitters (both lasers and noncoherent emitters) could be constructed from GaAs, caused considerable R&D activity. Recent advances in materials and in device fabrication have now brought this technology to a position which should interest analytical chemists in the application of these devices in laboratory measurements or in process and pollution control. A 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 mm or less in cross section) and they require small voltages (typically 1 to 2 V). 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 dc to 75 A pulsed), linewidths from 1 to 500 À for GaAs devices), and requirements for dc vs. pulsed operation. High-power devices available commercially include pulsed GaAs lasers with 1 to 20 W peak power emission at 9050 A and dc-operated GaAs noncoherent emitters with 200 mW emission at 9350 A. Visible noncoherent red, green, and yellow light emitters are commercially available. Other than the InAs emitter which emits at 3.2 μπ\, most of the longer wavelength emitters and lasers are not yet com mercially available. Noncoherent light emitters should be useful in applications requiring single wavelength-band monitors of reflec tance, transmittance, and absorptance. Semiconductor lasers should be particu larly useful as pulsed sources for lumi nescence and light scattering or in ap plications requiring more narrow linewidths than available with noncoherent emitters. In this article, the author will at tempt to acquaint analytical chemists
with the characteristics of available sources. A discussion of theory and device technology necessary for proper application is included. An attempt is made to stimulate the interest of the readers so that they may in turn direct device development into areas of par ticular importance to analytical chem istry. Semiconductor Theory and Materials
Before proceeding to a discussion of the devices, it is useful to review ter minology for those not active in the field. In a semiconductor, the atomic wavefunctions have combined in such a manner that a region in energy de void of states is formed in a perfect crystal. The region is called the for bidden gap ; this gap separates the val ence band from the conduction band. In Figure 1, the lower energy limit of the conduction band, Ec, and the upper limit of the valence band, Er, define the energy gap (Ec — Ev). Impurities and other defects in the crystal cause impurity states such as donors (energy ED) and acceptors (energy EA). By adjusting the impurity content, the semiconductor can be made ?i-type (conductivity dominated by electrons in the conduction band) or p-type (con ductivity dominated by holes in the valence band). Generally, p-n junc tions are formed in semiconductors by impurity diffusion or by changes in doping during crystal growth. Absorption of radiation with energy greater than the band gap (E0 — Ev) causes the generation of excess elec-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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Instrumentation
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 ge ometry), and (c) dome emitter. The line through the source is the p-n junction in the flat sources 70 A ·
trons and holes. The excess electrons and holes generated may recombine di rectly 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. Two 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 the recombina tion process results in the generation of a photon (with energy Ec — Ev, Ec — EA, ED — EA, etc.), the process of ab sorption and recombination results in photoluminescence. If the excess holes and electrons have been created by an incident beam of high-energy electrons, the process is called cathodoluminescence. The unique advantages of a semicon ductor luminescent source are only ob tained when p-n junctions are used. When forward bias is applied to the p-n junction, excess electrons are in jected into the p-type region and excess holes into the η-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 ΠΙΑ and VA (the III-V compound semi conductors such as GaAs, etc.), the IVA-VIA compounds (such as PbTe, etc.), and in some cases the IVA ele mental semiconductors (alloys such as SiC). The II-VI 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 importance. Each semi conductor has a given energy gap which in large part sets the wavelength of the radiation emitted by the diode. Alloying two similar semiconductors such as GaAs and GaP allows the fab rication of a device with a band gap in termediate between that of the con stituents. Light-emitting injection diodes in the blue and ultraviolet are not yet avail able. The development of such de vices is a formidable task. Large band gap semiconductors typically have highmelting temperatures and poor electri cal characteristics, and tend to be more ionic. Blue and ultraviolet semiconduc tor lasers have been constructed vising cathodoluminescence. In a different ap proach, 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 with coherent emission and semiconductor diode emitters with noncoherent emis sion are built. In this section, the characteristics of these two types of LED will be compared. A wide variety of noncoherent emit ter designs are available. In many ap plications, an emitter and. detector are placed in close proximity without addi tional optics; in this type of applica tion the radiant intensity in watts per steradian is most important. Some available GaAs devices include a para bolic reflector or lens to increase radiant intensity. In other applications, maxi mum radiant flux or power output (watts) is desired. To meet this goal, the emitter must be designed to make the external quantum efficiency (pho tons emitted per carrier crossing the device) approach the internal quan tum efficiency (photons created per carrier crossing the device). A dis cussion of the parameters determining external quantum efficienc3' helps to explain the variety of noncoherent emitter designs and the relation to laser design. There are three major loss mecha nisms decreasing external quantum effi ciency: (1) internal absorption of the created photons can be a large effect when the photon energy is close to the 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 nor mal greater than the critical angle φ0 — sin -1 (1/n) will be totally re flected (φ0 ~ 16° for GaAs), where η is the index of refraction in the semi conductor and the index of refraction of air is taken as equal to 1. A thorough discussion of figure of merit for nonco herent emitters is available [β). Figure 2 shows three types of de vice design. The length of the arrows represents the intensity of the emitted radiation as a function of the external angle Θ, (sin θ = η sin φ). Figure 2a shows an emitter design which uses the emission exiting from the face parallel to the junction. The angular distribu tion of radiation intensity is almost Lambertian (intensity proportional to cos Θ). Figure 2b shows a design which utilizes the light exiting along the p-n 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 in tensity. Figure 2c shows a hemispheri cal emitter (the p-n junction is near the center of the hemisphere). This dome greatly reduces the internal total
Instrumentation
reflection loss and results in a larger external radiant flux and external quan tum efficiency. The highest power out put noncoherent emitters use the hemi spherical design. Semiconductor diode lasers are con structed using the geometry of Figure 2b with two narrow sides made parallel by cleaving the sample. As the current density is increased in a p-n junction diode, the injected carrier density in creases until it finally creates a popula tion inversion in the region near the junction. For currents above this threshold current, the absorption co efficient becomes negative in the in verted region near the junction. Radia tion reflected back and forth between the two cleaved faces can increase in in tensity, and if the gain is greater than the losses, lasing will occur with emis sion from the cleaved faces. The rays leave the semiconductor at angles less than the critical angle helping to make the external quantum efficiency high in the laser structure. The angular distribution of radiation from a laser is limited by 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 dif fraction angle. Typically, the radiation from a GaAs laser is contained in a beam about 5° wide at half-intensity in the plane perpendicular to the junction 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 di rectly proportional to wavelength, making the diffraction angle larger for the far infrared semiconductor lasers. Because of the small emitting area, semiconductor lasers have tremendous radiance (watts/steradian/mm 2 of emitting area). Pulsed GaAs lasers have radiances in the range 105 to 106 peak W/steradian/mm 2 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. Noncoherent GaAs hemispherical emit ters are available with 20 mW/steradian/mm 2 radiance. A radiance of this value is sufficient for many ex periments, and dc operation is often ex tremely 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 that of
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ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 8, JULY 1 9 7 0 .
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a noncoherent emitter. If the laser is operated with current densities only slightly above threshold, some semicon ductor lasers can be operated in one cavity mode which is less than 1 A in linewidth for GaAs. More often, how ever, the device is driven above threshold to obtain maximum power output; under these circumstances, many lower gain cavity modes lase such that numerous lines are observed in a bandwidth in the order of 20 to 40 A. Detailed information on cavity modes active in GaAs lasers is available (S). GaAs noncoherent emitters operated at room temperature have bandwidths ranging from 250 to 500 A depending on device type and dopant used.
Noncoherent emitters can also be used to pump other solid-state laser materials (4) such as neodymium-doped yttrium aluminum garnet (YAlG:Nd). The high efficiency silicon-doped GaAs emitters are particularly useful in this application. This system has the ad vantage 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 semi conductor materials from which lasers on noncoherent emitters have been made. Some entries are or have been commercially available. Of the com mercially available devices, GaAs, Ga[As,P], and InAs are all available as either lasers (including Ga[As,P] at 8600 A) or noncoherent emitters (Ga[As,P] at 6600 λ). The I I I - V alloys cover a wide range of wave lengths. Ga[As,P] alloys can be made which emit in the wavelength region be tween GaAs (9000 Â) and GaP (5600 A). 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].
Noncoherent emitters may be oper ated in a dc mode or in a modulated or pulsed mode. Response times the order of a few nsec for standard GaAs can be obtained and the order of 0.2 ,«sec for GaAs doped with silicon. When pulsed, a GaAs laser can have a radiant output response time of less than a nsec. Light output degradation with time has been observed to occur in light emitters. At least for GaAs, the deg radation rate is directly proportional to current density and is, therefore, much more rapid with lasers (which must be driven past a certain threshold current density). The required lasing threshold current density increases with increasing temperature. This is one of the reasons that cw room tem perature semiconductor lasers have not been developed. Typically, a room temperature GaAs laser must be op erated with a duty cycle of 0.1 % or less and a pulse width of 200 nsec.
Because the band gap, Ea, changes with temperature, the wavelength, λ, of the emission depends on temperature. For the III-V semiconductors, AEa/(\T) is about - 4 X 10-* X eV/°C. Since Δλ/(ΔΤ) = (\/Εβ) χ (ΔΕα/ΔΤ), the fractional change in wavelength with temperature is larger for the smaller band gap semicon-
Table 1. Lasers and Noncoherent Emitters
Material GaAs:Si-phosphor-coated (Blue) GaAs:Si-phosphor-coated (Green)" GaP (Green)" SiC (Orange)" Ga[As,P] (Amber)» GaP (Red)" Ga[As,P] (Red)" Ga[As,P]« GaAs" GaAs: Si» InAs» InSb PbSe [Pb,Sn]Te, [Pb,Sn]Se
Nonco herent emitter response time, sec
Wavelength 4700 Â, 6500 Â, 8000 Â 5400 Â, 9400 Â 5600 Â 5900 Â 6100 Â 7000 Â 6600 Â 8600 Â 9000 Â 9200 Â to 9500 Â 3.1-3.6 Mm 5.2-6.2 Mm 8.0-22 M m 9.5-30 Mm
2 X 10"3 10-»
