Diode Lasers Melissa G. D. Baumann, John C. Wright, and Arthur B. Ellis University of Wisconsin-Madison, Madison, WI 53706 Thomas Kuech Department of Chemical Engineering, University of Wisconsin-! dadison, Madison, WI 53706 George C. Lisensky Beloit College, Beloit, WI 53511 Since its discovery in 1960, the laser has revolutionized science and technolow. including - the fields of medical surgery, materials growth, chemical measurement, and consumer electronics. The recent appearance of inexpensive, compact, battery-operated diode lasers promises another expansion in the range of possible applications. Previously, the size, expense, and reliability of conventional lasers has limited their availability to research and large-scale commercial applications. The small diode lasers developed in the past few years have the potential to make this source of light a s common a s the electric light bulb. Already, the rapid appearance of compact-disc players, laser printers and pointers, and fiber-optic telephone transmission indicates the scope of applications that may involve solid-state lasers. I n this article, we will briefly outline the principles by which these remarkable lasers operate.' Lasers and Quantum Mechanics The word "laser" is an acronym for light amplification by stimulatedemission ofradint~on.Produrtion of such coherent electromagnetic radiation requires population inver-
a) Spontaneous emission
b) Absorption
c) Stimulated emission
Fioure 1. Interactions between liaht and electrons in a
--
Absorption and Emission of Light Light, itself a fluctuating electric field, can induce changes in electron orbitals. In the static picture of Figure 2, the two charges exert a net upward force on the distant electron. If the two charges oscillate rapidly, the force exerted on the electron will oscillate. I n this picture, the oscillating pair ofcharges is the light source. The force that is transmitted through empty space is the light wave. If the energy associated-with the light wave is exactly equal to the difference in energy between two electronic orbitals, the light can induce the electron to change shape from one orbital to the other. The process is called "absorption" if the second orbital is higher in energy than the first (Fig. lb). The process is called "stimulated emission" if the second orbitaiis lower in energy, and the difference in energy is emitted a s light (Fig. lc). The emitted light will have the same direction and frequency as the light that stimulated it. Since light also has a particle nature, i t is said that for one photon, or packet, of light enterinc a material, two photbns come o;t that are idintical in f;equency, polarization. direction, and phase. Since all of the photons are identical, they aretermkd coherent. I t is whgrence that differentiates laser light from ordinary, incoherent light. The probability that absorption occurs and the probability that stimulated emission occurs are equal when there are equal numbers of excited and ground state electrons.
C O ~ D O U ~ ~
T& filed c i r ~ ~ e ~ r e ~ r eelectr&s, s e n t and light is represented'by the waves -, I\Snontnneo~rs .~ .- ....emission. . ..- Lioht is - emitted when an electron ~
sion and stimulated emission. The explanation of these terms requires a simple discussion of quantum mechanics. The basic principle of quantum mechanics states that everithine has a wave associated with it. Thus, electrons in atbms are constrained about the nucleus with specific wave shapes or orbitals with specific energies. The wave associate-d with a n electron r i n spnntaneously change shape to become a different orbital with d~ffcrcntenergy. (It is common to use the term "energy levels" when one refers to orbitals.) If the resulting orbital is lower in energy, the loss of energy may appear a s light (Fig. la). This common process is called spontaneous emission. I t is also called fluorescence, phosphoresrenre, and luminescence, depending on the material and the kinds of orbitals. Examples uf'this type of emission are neon lights and iodiurn- or mercury-vapor strret lamps, in which electrical energy promotes a n electmn ofan atom to a higher energy 1rvt:l. Light is then enutted a s the electron falls to a lower energy level.
~~
in an excited energy state fallslo a lower slate Dl Aosorpt~on.An electron 1s rased to a h gher energy eve1 by a~soroinga pnolon. c, St mb atw em ssion When I ghl of me appr0pr:ate energy str kes tne sample, it induces an electron in an excited state to fall to a lower state, emitting light in the process
'General referencesfor this article include the following. (a)Streetman, 0. G. Solid State Electronic Devices; Prentice Hall: Englewood Cliffs. NJ. 1990. (b) O'Shea. D. C.: Callen, W. R.; Rhodes, W. T An introduction to iasers and Theb Applications; Addison-Wesley: Reading, MA, 1977. (c) Hannay, N. 6 . Semiconductors; Reinhold: New York, 1959. Volume 69 Number 2 February 1992
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net force
a
Figure 2. Two oppositely charged particles exert a net upward force on the more distant electron. If the charges are oscillating, the force on the electron will also oscillate. Since there are normally more electrons in the ground state, absorption dominates in most materials. Thus, to obtain the light amplification seen in lasers, there must be more electrons in higher energy levels than in lower ones. This situation is called a population inversion. Laser Construction The construction requirements of a laser, sketched in Fieure 3. do not varv much from svstem to system. There must be an active medium that can support a population invrrsion. There must be a method of"pumpinrl'the active . . medium to obtain the population inversion. Generally, electric or light pulses are used to excite the electrons of the medium to higher energy levels.
-
Pump Source
Active Medium Fully
reflective
Partially reflective
mirror mirror Figure 3. Sketch of laser design. The pump source maintains the
population inversion. Light travels between the ends many times before passing through the panially reflectivemirror. Finally, a laser cavity, which is bounded by two opposing mirrors that trap the light so that it bounces back and forth, is required for greater amplification of the light. By having one of the mirrors be partially reflective (similar to the lenses of silvered sunglasses), light is permitted to leave the cavity. This light is the laser output. The cavity determines the allowed "modes" for the light. A light wave that is "trapped" in a mirrored box is analogous to a string attached to two posts. Not all wave shapes are permitted for such a string because the posts anchor the ends of the string. Several possible wave shapes for the string are shown in Figure 4. Similarly, not all wave shapes or modes are permitted for light because boundary conditions require the amplitude of the light wave to be zero a t the ends of the mirrored box. There will be an infinite number of modes corresponding to all the possible directions and frequencies, but not all frequencies are allowed. Only those modes with a half-integral number of wavelengths between the walls are permitted.
u
N
rn A A A I
I V V
Properties of Laser Light Figure 4. Possible "modes"of Light emitted from a laser vibration for a string attached differs significantly from the to either end of a rigid box. light from more traditional
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Journal of Chemical Education
lamp sources in color, directionality, and beam uniformity. As mentioned above, the light produced in stimulated emission is all of the same frequency, and thus is also of the same color. Lieht that is all one color is called monochro" matic. Unlike light from the sun or an ordinary incandescent bulb. it does not s e ~ a r a t into e different colors when it is passed through a prism. The "directionality" of a laser refers to how much the light beam spreads as it travels. When light is emitted from a normal small source, such as a light bulb, it spreads in all directions. For a laser, the cavity defines the modes in which the laser will emit. and these modes will have their directionality defined b i the multiple trips within the cavitv. The ~arallelmirrors a t either end of the cavity require" that drily waves with parallel wavefronts be amplified, so the output from the laser is light with parallel wavefronts. This type of light does not spread significantly as it travels large distances. The Speckle Pattern The s~eckleoattern that is observed when a laser beam falls on'a rou& surface is another striking feature that is uniaue to laser light. It is due to the coherence exhibited by la& light. As thz waves approach the surface, they are a~ in phase with one another: All of the wave peaks and troughs occur together, so they reinforce one another. However, as the waves approach a rough surface, they travel different distances before being reflected, and they return to your eye with differing phases. Some of the waves will have troughs that coincide with the peaks of other waves, and thus they may cancel one another. Other waves will still be in phase with respect to one another, and they will reinforce one another. When the light waves are in phase on your retina,they look bright. However, as the light waves become increasingly out of phase, they look dimmer. The net result is the speckle pattern. We now need to look in more detail at how the population . . inversion required for lasing is achieved in a sem~conductor and how the ravity for a solid-state laser is construrted. Semiconductor Crystal Structures Electrical conductivitv is based on the motion of electrons through a material. In a metal, there are many mobile electrons that lead to high conductivity. In contrast, nonconductors, or insulators, possess few mobile electrons. Semiconductors, as the name implies, occupy the intermediate ground. Silicon Silicon, the prototypical semiconductor, has the same crystal structure as that of the diamond allotrope of carbon. Figure 5 shows this structure. Si atoms reside at the corners of a cube, in the center of each face of the cube, and in four of the eight interior tetrahedral holes. Each Si atom is bonded to four other Si atoms in a tetrahedral geometry. The illustration shows a unit cell, which is the repeating unit of the solid. Extending the unit cell in all three mutually perpendicular directick creates the entire three-dimensional structure. Although Si has many desirable properties, its optical properties make it unsuitable for a laser material. Zincblende Solids Many of the electrical properties of semiconducting Si are retained in some simple binary structures with the general formula AZ, in whirh atom A is a member ofgroup 13 (poup SAI and atom % is a member of group 15 lgroup 5A1. Thus. AZ has the same overall numberofvalence clertrons as silicon.
I
I Figure 5. Unit cell of the diamond crystal structure possessed by Si. For binarv AZ wmwunds this is called the zincblende struc- certain -twe. the cube. and the darker . .. The .linhter-'mlored = . - itoms ~ are ~ inside ~ ones are on corners or at tne centers of faces.For SI con, a I aloms are S For me Dinary strLctJres. Ihe dark spneres represent Aatoms. and the ghter spheres represent Z aloms or v ce versa ~
~~~
~
~~
~
~~
The crystal structure exhibited by these solids is the zincblende structure shown in Figure 5. This structure is similar to that of diamond, except that it contains two different elements. All A atoms are coordinated by four Z atoms, and all Z atoms are coordinated by four A atoms. Semiconductors with formula AZ are frequently used as laser materials. Examples of semicondu~torsexhibiting this structure are GaAs, Gap, A1As,Alp, and InP. Solid Solutions
The chemical diversity of the zincblende solids can be extended bv their caoability to form "solid solutions", in which two solids are dissolved in one another. For examole. GaAs and Gap can form comoounds of general compoLit& GaAs,P(l_,,, where x has any value 070-1. suchsolutions of variable composition can exist when two conditions are met: The two combining solids must have the same structure, as with GaAs and Gap; and the exchanging atoms must have similar chemistry, as with As and P in these crystal lattices. Structurally, such solids look like the zincblende picture shown in Figure 5, but while each type A atom is Ga, each type Z atom is either As or P with the probability dictated bv the comoosition. For example, with GaAso~Po31,the droup 15 eiements are randomly distributed. There is a 69%chance of finding an As atom and a 31%chance of finding a P atom a t each-2 site. These three-element or temaw solid solutions can be extended to four-element or quaternary solid solutions, such as In,Gql.,,P&cl_,,, where 0 < x < 1and 0 < y < 1. Independent variations in x and y permit even greater flexibility in material properties Semiconductor Electronic Structure The electronic orbitals of atoms in solid structures overlap extensively and create bands, which are tightly spaced electronic enerw "" levels like those sketched in F i m e 6. Bands are required for electrical conductivity This overlap of atomic orbitals throughout the solid vmvides a vathwav for the electronic motion needed for electrical conductivity. ~~~
Band Theory
The number of electrons residing in the bands also determines conductivity. If the band is empty, there are no electrons to carry electrical current. If electrons fill all the en-
Most antibondina
A q a d r a ' s number of s atomic orbitals
Avogadro's number of delocalized molecular o r b t a l ~forming one band
Figure 6. Atomic orbitals in a solid sample may be wmbined to give many delocalized orbitals that are very close in energy. The collection of these orbitals forms a continuous set of energy States called a band. The top of the band is the totally antibonding combination formed from atomic orbitals of alternating signs, which are represented by the shaded and unshaded areas. The bottom of the band is the totally bonding wmbination formed by combining all of the atomic orbitals with the same sign. For example, in elemental sodium, overlap of the valence 3s orbitals gives a half-filled band with metallic properties. e r""w levels in the band. there are no emntv . " orbitals into which the electrons can move. Thus, conductivity requires oartiallv filled bands. Electrons are then easilv vromoted Into the vacant, energetically accessible orbitals of the band by absorbing energy from the applied electric field. Metals possess partially filled bands that are 10-90% full of electrons. thus accounting - for their high electrical wnductivity.
a) No electrons promoted Conduction band
.......... .......... 7
Valence band
.........a
Distance b) Electron-hole pair formation
..... ..........
Conduction band
~a
. 0 0 . 0 . . . 0 .
....o
a.........
Valence band
Distance Figure 7. a)Aconduction band and valence band ofasemiconductor. For a semiconductor the band gap E is on the order of 3 eV or less. Electrons (filledcircles) may easily ge promoted to the wnduction band, as shown in part b. Each promoted electron leaves behind a hole (open circles) in the valence band. Volume 69 Number 2 February 1992
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can substitute the host lattice atoms with other atoms that have more valence electrons (donors)or fewer valence electrons (acceptors).An analogy for n and p in semiconductors can be made with the [Ht1 and [OH-] in aqueous solution (see box on page 95).
In both Si and GaAs, the individual valence s and p atomic orbitals of the constituent atoms give rise to two important bands, as sketched in Figure 7a. These semicouductors have a band that is nearly filled with valence electrons, called the valence band. They also have a band at higher energy that has few electrons, called the conduction band. Neither band is ideal for conductivity: The valence band is too close to being full, and the conduction baud is almost empty.
Doping with Donors
Doping can increase the value of n or p, respectively. Group 15 atoms like P can, for example, replace some Si atoms. Phosphorus has five valence electrons, but needs only four electrons to make the four tetrahedral bonds to the four adjacent Si atoms. The extra electron on P readily leaves the P atom in a process called ionization. This freed electron adds to the conduction-band electron concentration, as depicted in Figure 8a. This increases the value of n, such that n > p, and the material is dubbed an "n-type" semiconductor.
Electrons and Holes
Electrons can be promoted from the valence to the conduction band by absorbing energy. This promotion increases conductivity by increasing the carriers in both hands: The valence band is less filled and the conduction band is more filled, as Figure 7b shows. The "missing" valence band electrons are called "holes". They can formally be treated a s positively charged carriers of electricity, a) Donor in contrast to the negatively charged electrons. Under a n applied voltage, electrons and holes move in opposite directions in the conduction and valence bands, re/ spectively, but they reinforce ,SI, ,Si, each other's contribution to the overall electrical conductivity. The concentration of b) Acceptor mobile electrons in the conduction band is denoted as n, and the concentration of mobile holes in the valence band asp.
I
,\-S/- 1. J I
Conduction Band
- .I
d.4 4 4 ........................... 1 1 1 Donor Level
-$k-
,\-7-I + ,Si,
/Si,
/
............ ...........*............ ............ Valence Band Conduction Band
.
Acceptor Level
The Band Gap
Valence Band The difference in energy he- L tween the top of Figure 8. a) The addition of adonor atom such as phosphorus to a silicon crystal yields an easily ionized band and the the electron that is freeto move through the lanice. In the band diagram, the donor atom donates electrons to conduction band is called the the conduction band by introducing an energy level that is near the bonom of the conduction band. b) An band gap, which is denoted E,. acceptor atom such as aluminum is missing one electron relative to the silicon lanice. Thus, it will accept This is the minimum amount an electron, producing a mobile hole in the lattice. The addition of aluminum to a silicon lattice introduces of energy needed to promote an energy level near the top of the valence band that accepts electrons fromthe valence band, or equivaelectrons, and it can varv sub- lently, releases holes to the valence band. stantiall$ among materials. In Electrons in the conduction band become the majority insulators, the band gap is very large, with an energy in camers, and holes in the valence band become the minorexcess of 300 kJ/mol or about 3 eV. The amount of thermal ity carriers. Compound semiconductors,like GaAs, can be energy needed to promote electrons is thus very large, and rendered n-type by substituting a group 14 element like Si few electrons cross the insulator band gap. Diamond, with for Ga, or by substitutinga group 16 element like Se for As. a band gap greater than 5 eV, is the prototypical example In each case, the additional valence electrons can be reof an insulator. leased to the conduction band. More electrons can cross the hand gap in semiconductors, which have a smaller band gap, as in Si (1.1 eV) or GaAs (1.4 eV). Thus, the electrical conductivity is larger in Doping with Acceptors these materials. Solid solutions of semiconductorsprovide The introduction of acceptors represents a strategy for a way to select or tune the band gap of the material. For increasing the hole concen&ation. ~ l u m i n u mor any g o u p example, the band gaps of the A I , G ~ ,As ~ . alloys increase with increasmg Al content from a minimum valuc of 1.4 eV 13 element that substitutes for a Si atom provides only in GaAs to a maximum value of 2.2 eV in Ah;. The band --three electrons for binding to the four adjacent Si atoms in gaps of GaAs,Pcl_,, alloys increase with increasing P conthe lattice, as shown in Figure 8b. An electron from an adtent from 1.4 eV to 2.3 eV. jacent Si-Si two-electron bond can complete the AI-Si bond, but this creates a mobile hole that is released to the Doping to Alter the Electronic Structure lattice. This process increases the valence-band hole concentration, p. Since every electron promoted from the valence band to Matcnals with p > n are called "p-type" semiconductors. the conduction band leaves a hole in the valence band, the The band diagram in Figure 8h illustrates that holes in the values of n and p in a sample of pure Si or GaAs are idenvalence hand ;ire the majority carriers and electrons in the tical and small. For example, for Si a t room temperature, conduction hand are the minority carriers in a p-type semithe values of n and p are about 10'' charge carrierslcm3. conductor. Compound st:miconductors like GaAs relv on Semiconductors can be "doped" with another element to thc substitution of n Ga alum with a group 12 element like obtain an excess of holes or electrons. The doping process ~~
92
~
Journal of Chemical Education
Zn, or they rely on the substitution of an As atom with a group 14 element like C to form p-type material.
+
=
E~
. . .
..
)
................................,
Ef
The Effect of Doping
I t is noteworthy that these substitutions need n-type P-type n-tYPe only be made a t the part-per-million level to have an enormous effect on electrical conductivity. For distance Equilibrium example, the replacement of only one atom in a million in a Si lattice with P changes the value of Figure 10. Formation of a FH junction. The two semiconductors in equilibrium with n to about 10"/cm3, thus increasing the conduc- one another must have their Fermi levels E,at the same energy. Note that the bandtivity over pure Si by a factor of about lo7. gap energy does not change throughout the solid. Absorption and Emission of Light in Semiconductors A p-type semiconductor,with a lower concentration of electrons, has a lower chemical potential, with the Fermi level The electronic band structure of semiconductorsalso delying energetically close to the valence band. Figure 10 termines the optical properties of the material. The bandenerw diaerams of Fipure 7 predict that an electron can be illustrates these relationships. pron&d From the vAence the conduction band by abWhen a junction 1s formed between an n-type and p-type sorbing a photon of a t least band-gap energy (hv > E,), sernlconductor, electrons flow from the n-type semiconducleaving behind a hole in the valence band. Each photon tor to the p-type semiconductor until thechemical potenabsorbed thus creates one electron-hole pair. tial is equalized in the two materials. The transfer of charge a t the junction leaves a small region of net positive A generic absorption spectrum of semicondudors that charge at the n-type side and a small region of net negative are suitable for laser diodes is sketched in Figure 9. This charge a t the p-type side of the junction. The charge sepaabsorption spectrum is characterized by a sharp increase ration internally builds up a small difference in potential in absorption at the band-gap energy This type of absorpenergy producing a voltage across the junction. tion behavior is due to electronic transitions from one of the many energy states in the valence band to one of the The bending of the band edges in the equilibrium sketch many energy states in the conduction band. of Figure 10 reflects the distribution of the internal or Light of energy E, is emitted from semiconductors when "built-in" voltage throueh the iunction. The chemical DOtential of electrks in tce dono;-doped material is lowered these electron-hole pairs recombine across the band gap. while the chemical potential of electrons in the acceptorThe use of solid solutions or alloy semiconductors, with doped material is raised until the chemical potentials have their variable band-gap energies, provides a means for equilibrated. Then there is no further net transfer of tuning the color of the emitted light. This recombination charge across the interface. explains simple emission from a semiconductor,but it does not provide ;means for obtaining the population inversion needed to observe stimulated emission. Semiconductor Biasing junctions are needed to obtain the population inversion. The application of an external voltage source to induce a current, called biasing, disturbs the equilibrium situation. Semiconductor Junctions Current flow through the junction requires that electrons The Chemical Potential or Fermi Level in the conduction band move from the side with more electmns to the side with fewer electrons. Similarly, holes in The chemical potential of electrons in the semiconductor, the valence band must move from the side with more holes called the Fermi level (EO, is a measure of the average electo the side with fewer holes. Thus, electrons must be given tron energy in the solid. Thus, it depends on the electron enough energy to move over the potential energy barrier concentration in the solid. The chemical potential in an nfrom the n-type side to the p-type side, while holes must be type material lies energetically near the edge of the eonforced to move under the barrier from the p-type side to the duction band due to the large electron concentration there. n-type side during current flow. If the voltage applied to the n-type side is negative relative to that applied to the p-type side, the voltage is referred to as forward bias. The enerm barrier for electrons and holes to flow through the junction is decreasedrelative to the equilibrium energy barrier (Figure l l a ) , and substantial current can flow through the junction. Under reverse bias the situation is reversed. The applied voltage adds to the internal voltage (Figure llh), making current flow even more difficult.This is the essence of diode behavior: The current increases exponentially with forward bias Eg and is nealiaible - - with reverse bias, as sketched in FigWavelength -4 ure llc. Energy Under forward bias, the electrons reaching the p-type side and the holes reaching the n-type side are minority Figure 9. General sketch of the absorption spectrum of a semimncarriers in these regions. he^ can recombine with the maductor. Note that the absorbance increases sharply as the energy of jority carriers that are in abundant supply (Figure 12a). the light approaches the band gap of the semiconductor. Light with This recombination results in emitted light. The color of greater energy than the band gap is absorbed because it correthis luminescence can be controlled by varying the band sponds to a transition from an individual orbital in the valence band gaps of the junction materials (Figure 12c).A p u junction, to another obital in the conduction band. This feature makes some used in this manner, is called a lighbemitting diode or semiconductors useful as optical cutoff filters. LED and emits incoherent light. Stimulated emission is
-
Volume 69 Number 2 February 1992
93
Equilibrium
-A
distance
strongly influence their electrical properties, a s noted above. The techniques most commonly used to prepare and dope the zinchlende semiconductors used in laser diodes are Molecular Beam Epitaxy (MBE) and Metal-Organic Vapor Phase Epitaxy (MOVPE). Both techniques excel in their ability to controllably deposit or grow layered semiconductor structures. Epitaxy refers to the physical nature of the growth process. Epitaxial growth is due to the alignment of the atoms in the growing layer with the underlying atoms. The crystal structure and atomic registry of the atoms is preserved or continued into the growing layer. The grown layers exhibit a high degree of crystalline perfection and are free of defects that would degrade the performance of the laser. These two crystal growth techniques, MBE and MOVF'E, are quite different in practice. The MBE technique forms the growing layers within a high-vacuum chamber. For example in the growth of GaAs, beams of the Ga and As used are formed by heating these elements to a very high temverature. so that thev" beein to vaoorize. The substrate is placed i n front of these sources or beams, and the growing layer is formed by condensing the elements onto the semiconductor wafer. Multilayer structures are grown by shuttering or tumine the desired elemental beams on or off a t the gght momeks. MOVPE is one form of a eeneral techniaue called chemical vapor deposition. The growing layer is formed within a flowing gas environment in which reactive chemicals, such a s trimethyl gallium ((CH&Ga) and arsine (AsH3), that contain the elements of interest. flow over the semiconductor wafer and are decomposed a t or on the surface. This chemical decomvosition causes dewsition of the semiconductor. Changing the composition of the flowing gas by adding new elements or dopailts allows the growth of multilayer structures. Both techniques can produce almost atomically thin lavers that form the internal structure of the diodelaser.
-
-
c Reverse bias Forward bias-
Applied Voltage Figure 11. a) A p-n junction under forward-bias conditions. b) A p-n junction under reverse bias. The dashed lines in a and b show the equilibrium band bending when no bias is applied. c) Current through a p-n junction as a function of applied voltage. not involved in this procrsii. Asketch of acommercial LED, ;~vailablca t any electronics store, 1s shown in Fibwre 12b. Semiconductor Synthesis: Epitaxial Growth Implicit in the description of these semiconductor junctions is their chemical synthesis, which is remarkable in its own right. The state of the art in semiconductor growth is such that the solids can be grown virtually a n atomic layer a t a time! Semiconductors must be prepared in very pure form because small quantities of impurities can
-
Diode Lasers Light production from a diode laser occurs by a process similar to that for the LED'S discussed above. Although an I.I.:l) emits light, it docs not lase since there is no cavity to provide amplification and an insufficient -population inversion is present. The laser cavity of a diode laser is typically constructed by having a t least one pair of opposite faces that are flat and parallel to one another. Parallel faces can readily be created by mechanical cleaving that occurs naturally in a particular crystallographic direction. Due to the change in index of refraction a t the boundary between the crystal and the surrounding air, these parallel surfaces act as the mirrors that hound the diode laser cavity. As seen in the discussion about the string, a p n junction constructed in this manner can support lasing when the cavity length is equal to a half-integral number of wavelengths. The Multilayer Structure
Energy Figure 12. a) A p 4 junction acting as a light-emining diode (LED) under forward-bias conditions. b) Sketch of a commercial LED. c) Emission spectra of various LED'S.Changing the semiconductor can changethe color of the light that is emitted. For all samples discussed here, the peak emission occurs at the band-gap energy of the semiconductor.
94
Journal of Chemical Education
The formation of a semiconductor laser requires, in practice, complex mnltilayer structures (Figure 13a). The active layer will have a p n junction. However, above and below this junction there will be other semiconductor layers, often differing in composition and carrier concentration. These additional layers can serve a variety of purposes. Their two primary purposes are to improve the optical confinement of the emitted light and to improve the recombination of the carriers. Placing layers with a lower index of refraction adjacent to the active region conlines the light to the active region. This construction provides a mechanism for guiding the
-
a) Schematic of triple layer structure
pAI ,Gal,
As
PG~A~
n-Ga~s
b) Equilibrium
-
Placine materials of hieher band eaD . next to the active or lasing region can improve current or carrier confiiement. The higher band gap can cause the formation of potential barriers between the materials, as shown in Figure 13b. The electrons injected into the active region are confined by this potential barrier, causing a buildup of the local electron and hole concentrations in the active region. The increased concentration of electrons and holes imDroves the chance of stimulated emission. Both o~ticaland klectrica~confinement are important for production of high-efficiency diode lasers. Obtaining a Population Inversion
c) High fonvard bias
Figure 13. a) Schematic of a diode laser. The light is emined from the p-n junction area. The cavity length itself must be a half-integral number of the wavelenaths of the liaht emitted. b) Confinement of the electron-hole pairs within the actbe lasing areaof the solid is accomplished by bounding the active region with another semiconductor whose band gap is larger than that of the first semiconductor. In this case, a solid solution of AIfia,, _*As is used as the larger band-gap material, and GaAs is used as the narrower band-gap material, c) Under high forward bias, this construction forces electrons to stay within the GaAs region, where they recombine with the holes that are present, emitting light in the process. light through the active region, thus improving the chances of stimulated emission.
Although the cavity construction is an important part of diode laser function, the laser will not lase, or produce light amplification unless a population inversion is present. The necessarv n can be obtained to .~ . o.~ u l a t i oinversion support stimulated emission if very large quantities of minoritv carriers are iniected across the junction. This is achieved by applying a very high biasto the junction. Thus, many electrons in the wnduction band flow to the p-type side and many holes in the valence band flow to the n-type side, as shown in Figure 13c. Upon the application of a high forward bias, the potential barrier between the n-type GaAs and the p-type GaAs is decreased so much that many electrons can flow across the junction. Once they reach the p-type side, they are prevented from diffising into the solid by the larger band gap of the p-Al,Gql_& material. This creates a buildup of electrons in the conduction band of the D t v ~GaAs. e Stim. --. ulated emission can then effectively compete with absorption for the available Dhotons. and light - amplification is observed. Acknowledgment We thank Professor G. Nathanson for h e l ~ f u lsuggestions and are grateful for support from the ~ a t i o n a f ~ c i ence Foundation throueh erant number USE-9150484 and from the Drevfus ~ o k d g t i o n .
Volume 69 Number 2 February 1992
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