Periodic properties in a family of common semiconductors

Feb 1, 1992 - ... connection between periodic trends in physical/chemical properties ... A Guided-Inquiry Laboratory for Introductory High School Chem...
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Periodic Properties in a Family of Common Semiconductors Experiments with Light Emitting Diodes George C. Lisensky and Rona Penn Beloit College, Beloit, WI 53511 Margret J. Geselbracht and Arthur B. Ellis University of Wisconsin-Madison, Madison, WI 53706 Light-emitting diodes (LEDs) are inexpensive semiconductor structures that find extensive use in a multitude of digital readout devices in electronic clocks, balances, calculators, and stereo equipment. Their prevalence and low cost make LEDs ideal for classroom demonstrations or laboratory experiments showing the connection between periodic trends in physicaVchemical properties and a common high-tech device. Semiconductor p-n junctions or "solidstate concentration cells" are the basis for LEDs and, when fashioned into optical cavities, for diode lasers. ( 1 ) In this paper, we will demonstrate that LEDs are eye-catching devices that are useful for surveying the stoichiometry, structure, bonding, and spectroscopy of a family of common semiconductors, and we will present a n experiments using LEDs. Periodic Trends In Band Gap Diamond Structure The band gap of a solid, E,, is a fundamental property of the material that reflects its structure and bonding. We can understand periodic trends in the band gaps of solids with the same structure by considering a simplified description of the bonding interactions that lead to the band gap. For the solids of group 14 exhibiting the diamond

structure, each atom in the structure is surrounded by four adjacent atoms in a tetrahedral coordination as in Figure 1. A simple bonding picture is presented in Figure 2. The four equivalent valence orbitals of the central atom (sp3 hybrid orbitals) can be combined with an equivalent hybrid orbital from each of the four adjacent atoms, leading to four bonding and four antihonding molecular orbitals. Interactions among the orbitals of the central atom and interactions with the equivalent orbitals from all of the other atoms in the solid cause the bonding molecular orbitals to broaden into a valence band of tightly spaced, delocalized orbitals. (This is a simplification; the interested reader is referred to reference 2.)Likewise, the antibonding molecular orbitals broaden into a higher energy conduction band of tightly spaced, delocalized orbitals. The total number of molecular orbitals preserves the original number of atomic orbitals. For a large number (N, on the order of Avogadro's number) of atoms in the solid, with each atom using four valence orbitals for bonding, there will be 2N orbitals comprising the valence band and 2N orbitals comprising the wnduction band. Each atom has four valence electrons, with one electron placed in each of its four valence orbitals to make four two-electron bonds, one with each neighboring atom. These 4N valence electrons of the solid can be entirely acwmmodated by the 2N orbitals of the valence band, since each orbital holds two electrons by the Pauli principle. The result, at a temperature of absolute zero, is a filled valence band and an empty conduction band, shown in Figure 2. The valence band edge in a solid is analogous to the highest occupied molecular orbital (HOMO) in a discrete molecular species; the conduction band edge is analogous to the lowest unoccupied molecular orbital (LUMO).

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Conduction Band

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Figure 1. Perspective drawing of a unit cell of the diamond crystal lattice when all of the atoms are the same. For materials of AZ stoichiometry, when the lighter colored spheres are different from the darker spheres, this structure is called zinc blende.

Valence Band

Figure 2. Schematic bonding picture of a group 14 tetrahedral solid. The energies ofthe four equivalent hybrid orbitals ofthecentral atom, A, are shown on the left, and one overlapping hybrid odital from each of the four adjacent atoms, A, to &, is shown on the right. A singleheatled arrow indicates one electron. In a solid, the energy levels broaden into bands. The resulting filled valence band and empty conduction band are drawn as shaded and unshaded rectangular boxes, respectively. Volume 69 Number 2 February 1992

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Insulator

Semiconductor

Metal

Figure 3. Schematic band structure diagrams for an insulator, asemiconductor, and a metal. The band gap, Eg, shown as the doubleended arrow, is the energy separation between the top of the valence band and the bottom of the conduction band. The size of the band gap decreases in passing from an insulator to a semiconductor to a metal. Electron-hole pairs are also shown in the diagram for a semiconductor as filled circles in the conduction band (electrons) and open circles in the valence band (holes). The band gap,'^,, is the energy separation between the top of the valence band and the bottom of the conduction band. The magnitude of the band gap is derived from two factors: the streneth of the interaction that leads to the separation of the ;onding from the antibonding molecular orbitals, and the energy spread of each band. The terms insulator, semiconductor, and metal are used to classify solids based on the size of the band gap and the effect of the band gap energy on electrical conductivity. Figure 3 schematically illustrates these three classes of solids. Electrical conductivity arises from a partially filled energy band ( I ) . In a n electrical insulator, the valence band-is essentially filled with electrons, and the conduction band is essentiallv emotv. The band eaD of an insulator is large (E, > 3 ev-= 300 kJlmol; h < 406 nm), and the relatively small amount of thermal energy available a t room temperature (KT,where k is Boltzmann's constant, is -0.025 eV = 2.4 kJImol; A 50000 nm) ensures that few electrons will be promoted thermally across the hand gap. A semiconductor h a s a smaller band gap (E, < 3 eV =360 kJlmol: A > 400 nm). and electrons are more easily promoted, either thermally or by the absorption of visi"bie or near-IR light, into the conduction band. One type of metal exhibits a n electronic structure in which the valence band and conduction band overlap with no real hand gap; thermal energy provides a large concentration of mobile electrons, resulting in a high electrical conductivity. I n the group 14 solids exhibiting the diamond structure, there is a progressive decrease in E, down the group. Table

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Table 1. Periodic Properties of the Group 14 Solids Possessing the Diamond Structure. Element

Lattice Parameter (Ala

DO (~llimol)~ 4,eV (1,nm)c

'Re~eatdistance, a, of the cubic unit cell at 300 K obtained from X-rav d firact on oala Tnese va ~ e are s * n o w togreater orecsor, 0.1 are ro-noed OY nere lor 5 mp c !, and are l a e n from referewe 3 'Dono o ssoc a.on enerm oeferm nco from neafs of a~ornmfoon aata from refyen,, 4 Band gap energy at 300 K from reference 3.

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Journal of Chemical Education

Figure 4. Portion of the periodic table emphasizing the formation of AZ solids that are isoelectronicwith the group 14 solids. Complementary pairs are indicated with similar shading; e.g., Ge, GaAs, ZnSe, and CuBr.

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1 lists the m o u 14 ~ solids. C through a-Sn. alone with their cubic l a t t s e parameters; estimated bond dissociation energies, and band gap energies (300 K). The lattice parameter scales with interatomic distance in the solid and follows the expected periodic trend for the atomic radii. Note that shorter, stronger bonds result in a larger band gap energy. reflecting its Diamond has a lattice constant of 3.57 exceptionally strong covalent bonds that result from excellent orbital overlap. The resulting band gap of -5.5 eV = 580 kJlmol ( A = 230 nm) makes diamond a good electrical insulator. Promotion of a n electron across the band gap, which corresponds to ionizing one of the electrons out of the directional covalent bonds comprising the lattice, is s t r o n ~ l ydisfavored by the streneth of the C-C covalent bond; The optical properties of &arnond can also be expla~nedby the hand gap, which giws the threshold e n e r n for an rlrctronic transltlon from the valence to the conduction hand. The E, value of 5.5 tV f h = 230 nm is in the ultraviolet portion of the clcctromagnrric spectrum rather than in the visible portion, which ranges lkom violet light at -3.1 eV1).=400nm;.toredlilrht at -1.7eV .A= 730nms. making diamond transparent t o visible light. In descendin6gronp 14 to silicon, the lattice constant expands to 5.43 A, and the longer, weaker S i S i bonds result in a substantially smaller band gap of -1.1 eV= 110 kJlmol ( A = 1100 nm). Electrons are more easily promoted by thermal energy across the band gap, and silicon is a semiconductor. Crystalline samples of silicon absorb the entire visible spectrum and thus appear black. The modest increase in lattice constant passing from Si to Ge (5.66 A). ~resumablvreflectine the interoosition of the transition eiements, results in o k y a smalirednction in the hand can to 0.66 eV = 64 kJlmol ( A = 1900 nm). Germanium is 2s; a black semiconductor.^ larger change in lattice constant is seen in passing to a-Sn. This phase of tin, which exhibits the diamond structure, is only stable below -13 'C. The lattice constant of a-Sn is 6.49 A, and a-Sn has a band gap of less than 0.1 eV = 9.6 kJlmol ( A > 12,000 nm). Finally, although Pb does not have the diamond structure, if Pb were to adopt this structure, it would he predicted to be a metal from the trend toward longer. weaker bonds. The conduction and valence bands would overlap, facilitating promotion of valence electrons into the unfilled conduction band. I t should be emphasized that these correlations of band gap energy with bond length and strength reflect both

A,

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Table 2. Periodic Properties of a Family of isoelectronic, Tetrahedral Semiconductors?

Material

Laltice Parameter (A)b

ZnSe ciBr

Eg,eV (hnm)d

AxC

Ge 5.66 0.0 0.66 (1900) GaAs 5.65 0.4 1.4 (690) ZnSe 5.67 0.8 2.7 (460) CuBr 5.69 0.9 2.9 (430) 'The indicated materials either have the diamond structure (Ge)orthe zinc blende structure of AZ stoichiometry in which all Aatoms are bonded to four Z atoms and all Z atoms are bonded to four Aatoms in a tetrahedral geometry. The sol~dsare listed sequentially as isoelectronic semiconductorsfrom groups 14 13and 15, 12and 16, and 11 and 17 inthe periodictable (see Fig. 4). b~epeatdinance, a, of the cubic unit cell at 300 K obtained from X-ray diffraction data. These values are known to oreater wecision, but are rounded o f f p t e for simplicity and are taken fromrefknce Difference In Pauling electronegativities lor the interatomic bonds in the lanice. 'sand gap energy at 300 K from reference 3,

l

.

GaAs

>

interatomic and intra-atomic orbital interactions.' The net result, a s we have seen, i s the dramatic decrease in the band gap energy moving from diamond to a-Sn.

Figure 5. Periodic properties for a group of isoelectronic semiconductors. The band gap depends on the ionic contributions to bonding, represented here as the difference in Pauling electronegativities for the interatomic bonds in the lattice.

Zinc Blende Structure

The total valence electron count of elements having the diamond structure i s preserved i n compounds of AZ stoichiometm where and Z are elements that svmmetricallv flank theu&up 1 4 elements i n the periodic tahe. In ~ i g u r k 4. complementarv AZ airs are indicated with similar shading. One struiture in which all of the A atoms are tetrahedrally coordinated to Z atoms and all of the Z atoms a r e tetrahedrally coordinated to A atoms is called zinc blende.' The zinc blende structure i s identical to the diamond structure but with two different atoms,.A and Z (see Figure 1). Therefore, solids such a s Ge. GaAs. ZnSe, and C;B~ can be considered to be isoelectronic andisostr&tural to a first approximation. Trends i n periodic properties can also be u s i d to rationalize band gap trends i n this group of materials. InTable 2, the cubiclattice parameters, the Paulingelectronegativity differences, and the band gap energies a t 300 K are presented for Ge, GaAs, ZnSe, and CuBr. Structurally, in passing from Ge to CuBr, t h e lattice parameter is essentially constant a t 5.67 _+ 0.02 A. At the same time, however, the bonding i n this series acquires a n increasing ionic contribution: the electronegativity differences increase from 0.0 to 0.9 along this series. I n comparing heteronuclear bonds with their homonuclear counterparts, the heteronuclear bond is stronger than the averaged corr e s ~ o n d i n ehomonuclear bonds because of ionic contributiois. or &le, the estimated H-C1 bond energy of 428 kJ/mol i s larger than the averaee of the H-H (432 Wlmol) and CI-CI ( 2 i 0 kJ/mol, bond en&"es. 14 nand gaps of the A% series reveal a snnilar effect. As shown in F'irmre 5 and Table 2, band gaps monotonically increase with ionic bonding contribution from Ge to C a r . Zinc Blende Solid Solutions

The ability to form solid solutions provides a chemical means for tuning band gap energies that would be lacking were we restricted to the AZ stoichiometries obtainable with the elements i n the periodic table. Preparation of solid solutions having tunable stoiehiometry requires two solids with the same crystal structure of similar size and similar chemistry. G a p and GaAs satisfy these criteria. The smaller atomic radius of P relatiye to As leads to a smaller lattice constant for G a p (5.45 A) relative to GaAs (5.65 A). Also, P has a slightly larger electronegativity than

F ~ g ~6r eTrends n the cuoe lance parameter, a (A). (1, ed clrcesf and me oand gap at 300 K (opentr ang esf as a bnct on 01 compos !.on, x, lor !he sold soldtlon ser es GaP.As,., Tne nmk n tne band gap data at x = 0.45 corresponds to a changefrom a direct band gap to an indirect band gap; see footnote 3. Data taken from reference 7.

As. Both internuclear distance and ionic contributions to

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the bondinefactor into the lareer band gap for G a p (2.3 eV: h = 540 nmj than for GaAs ( 1 . 4 e ~h; 890 nm). hel lattice constant decreases and t h e band gap energy increases with x in solid solutions of formula GaPAsl, (0 < x < 1) a s shown i n Figure 6. These are disordered solid solutions with the stoichiometric fractions x and ( I -XI denoting the 'The widths of the valence and conduction bands for the group 14 solids are directly proportional to the initial energy separation between the valence s and p orbitals, E, - Ep (see reference 2). The energy difference,E,- E,, drops from -8.5 eV (820 kJlmol) for carbon to -6.6 eV (640 kJlrnol) for tin, suggesting that the widths of the valence and conduction bands are getting smaller. If the widths of the valence and conduction bands decrease, this would seem to widen the band gap down the group. However, this effectis ovelwhelmed by the great reduction in the separation between the bonding and antibonding orbitals with intemuclear distance, which occurs down the orouo. The other common oossibilitv is the- wurtzite structure. The.. differ.... , - - ~,~ ence oetweentne m c o ende strJclJre and the w~rtzIC slrLctdre ies m tne par6 ng symmetries of me Z atoms tpn n de cha cogemde or nal oe,. in 2 nc o ende, The Z atoms pack in c ~ cbc osesl-pac6eolayers, while in the wurtzite structure, the Z atoms pack in hexagonal closest-packed layers. For a further discussion of these structures, see refeience 4. ~

Volume 69 Number 2

February 1992

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Table 3. Spectroscopic Properties of Some Common LEDsa

GaPo.~oAso60 Red

660 (1.88) 639 (1.94) 32F1462

GaPos5Aso~:NOrange

635 (1.95) 615 (2.02) 13F9733

GaPo.ssAso.ts:N Yellow

578 (2.15) 570 (2.18) 13F9731 590 (2.10)

GaPmAso.oo:N Green

556 (2.23) 550 (2.25)"4~653 570 (2.16)

a)

b

Lens

socket # LED

Leads

*Thistable summarizesthe spectrosmpic data shown in Figure 9, using the

circuit shown in Figure 7b.

b~omposition of the semiconductor used for the p n junction of the LED. Fractional stoichiometries represent solid solutions of GaAs and Gap The desionation :N sionifies that nitrwen atoms have been added as doDanis to the sen~cono.ctor w e footnote 4 Coor referslo'he coor of em neo ght not tne color of the pastlc ens on these corrrnerc a 1 aud laole LEDs Campar8son of LED3 of me same cornom ton DJI nar nq ccar or cooreo Past c ewes snow n e vanat on n obsdrved mlar or wav&ngths. mission band maximum at 300 K. 'Emission band maxlmum at 77K. obtained bv immersing the LED in liquid nitrogen. 'part numberfor the LED from Newark Electronics catalog. %ee footnote6. probabilities that an atom on a pniwgen site in the zinc blende crystal structure is P or As, respectively. Experimental Procedure Materials

Commercially available members of the G a P A s , solid solution series (see Table 3) were obtained a s Quality Technologies discrete LED lamps from Newark Electronics (6321 North Avondale Ave., Chicago, I L 60631-1924). These LEDs have 9V off-axis lenses, making light from the LED easy to see from a wider range of viewing positions. Because the semiconductors i n the LEDs in Table 3 are covered by colored translucent lenses, i n order to see the actual semiconductor (Figure 7a) students viewed LEDs with a clear flat-top case obtained from DigiKey (parts P431 to P434, 701 Brooks Ave. South, P.O. Box 677, Thief River Falls, MN 56701). The single LED holder shown i n Figure 7b was constructed by slicing sections wntaining two sockets from larger integrated circuit sockets (Radio Shack single inline IC socket, 276-1975, or Newark MPSIP socket terminal strip, 89N6182). One lead was soldered to a 1-kR resistor and then to a 9-V battery snap (Radio Shack, 270-325, or Newark, 38F1270); the other lead is soldered to the other battery connection. An additional set of LEDs was used to fashion a reference strip. In order to keep the LEDs on the reference strip aligned, the leads were shortened and soldered directly to a circuit board (a portion of a Radio Shack IC Perfboard, 276.150) after verifying the polarity of the diode. Each LED was connected to a resistor and then to the battery a s shown in Figure 7c. The red LED i s less intense than the other LEDs for the same current, so a smaller resistor was used to increase the current and achieve a comparable brightness. The relative wavelengths of the LED colors are obtained by observing the reference strip through a 35-mm slide mount containing a diffraction grating (Edrnund Scientific Co., R1,307-101, E. Gloucester Pike, Barrington, N J 08007) or a holographic prism filter (Flinn Scientific, AP1714, P.O. Box 219,131 Flinn St., Batavia, IL 60510). 154

Journal of Chemical Education

Figure 7. Experimental set-up for the LED experiment. (a) Schematic oicture.o-f a-~commerciallv showino the location of the -- ,available LED. -~ semiconductor chip. (b) Circuit diagram'for a single LED. (c) Circuit diagram for the reference strip of LEDs.

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Solid Solutions and Band Gap Energy

Students were asked to consider the relative atomic radii and electronegativities of phosphorus and arsenic and the relations hi^ between orbital over la^ and e n e r w difference between bonding and antibonding molecula;orbitals, in order to predict the order of increasing band gap energy for t h e following compositions: GaP~ooAso.~, GaPo.ssAs~.ls, GaPa.6sAso.ss,and GaPo4&soso. After making the predictions, LEDs with these compositions were wnnected to the single socket in the electrical circuit pictured i n Figure 7b, emitting light corresponding approximately to the band gap energy The students were asked if the orientation of the LED i n the socket mattered (it does, since an LED is a diode ( I ) )and to record the color of emitted light each LED produced. After assigning a range of wavelengths for each color from their textbooks, students listed the LEDs i n order of increasing band gap energy A diffraction grating was then used to examine the reference strip of LEDs, Figure 8; the spacing of the images to each side i s proportional to the wavelength of the emitted light. This information was used to experimentally list the LEDs in order of increasing band gap energy. Temperature and Band Gap Energy

The students were asked to predict what will happen to the bond distances when these solids are cooled. Applying

Proportional to h

0

0

red 0

0

orange 0

0

yellow 0

0

green F g ~ r e8 Schematc d agram of the -ED reference slrlp v ewed tnro~ghtne o lfract on grat ng Tne spl nmg of tne offfracteaspofsfor eacn LED IS a rect y propotllonal to tne wave engtn of emlned lgnt for small diffraction angles. See cover for a color photo of the results

identify the color shiR of the red LED to orange as it is cooled in an expanded polystyrene cup filled with liquid nitrogen. Discussion of Optical Properties

Mechanism

Wavelength, nni

An LED is simply a p-n junction. ( I ) Application of an electric field in one direction (forward bias) by connecting a battery to the p-n junction promotes the proximity and radiative recombination of conduction band electrons and valence band holes near the junction, producing photons of approximately band gap energy. (If the direction of the electric field is reversed, no current flows and no light is droduced.) Although there are many possible energy levels In the conductionband that may bcpopulated, the emission peak is relatively narrow (Fig. 9; typical peak width a t half the maximum height of -30 nm, -0.09 eV). Electrons that populate the higher energy levels rapidly lose energy to the crystalline lattice through quantized lattice vibrations and fall to the bottom of the conduction band prior to recombination. Likewise, holes created within the valence band rapidly rise to the top of the valence band (the lowest energy configuration for holes), prior to recombination. The most probable emission corresponds to band-edge recombination of electron-hole pairs, producina a narrow spectrum of emitted light. By tuning the handgap energy throueh the use of solid solutions, the color of the emitted light can be continuously varied. For Gap&_- diodes described herein. the colors and - - - the ~-~~ band gaps range from green to yellow to orange to red (Table 3). To enhance the low emissive efficienciesof some GaP&, samples,3they are doped with nitrogen atoms at oom concentrations. i.e.. a small fraction of the PlAs sites have been filled with N atoms! Such samples are denoted as GaP,Asl, :N.

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Figure 9. Spectral distribution of emitted light for several commercially available LEDs at 300 K (lessintense) and 77 K (moreintense). The spectra were recorded at the same scale for a given LED and correctly show the relative intensity at the two temperatures. Peak wavelengths are listed in Table 3. this information to the GaPAsl, solid solution series, they were asked to predict further what will happen to the band gap energy and wavelength of emitted light when a n LED is cooled. A lighted LED in the single socket was dipped into an expanded polystyrene cup ("Styrofoam") containing liquid nitrogen (77 K), and changes in the color and intensity of the LED were noted. The cold, lit LED in the single socket was held in line with the reference strip of LEDs, which were used as a "ruler", and this array of LEDs was examined with the diffraction grating. ARer the LED in the single socket had warmed to rooitemperature again, the diffraction grating and the reference strip of LEDs were again usedas a "ruler" to determine ifthe temperature effects were reversible. Finally, the students were asked to relate the wavelength increase or decrease of the cold LED to the predicted change in the band gap energy. Emission spectra in Figure 9 were obtained by clamping the LEDs in the sample position of a Shimadzu RF5000 spectrofluorimetbr with the excitation beam blocked, an emission slit width of 1.5 nm, a neutral density filter in front of the sample, and a low sensitivity setting. Emission spectra for a given LED were recorded at room temperature and a t &id nitrogen temperature without moving the sample. Lecture Demonstration We have used the described Drocedure as a short laboratory experiment illustrating concepts in spectroscopy and oeriodic oro~erties.However. the cooling - of a single - socket

.. LED in liquid nitrogen also serves as a dramatic lecture

demonstration. The lieht intensitv of a cooled LED is bhght enough to light up the expanded polystyrene cup. In alarge, darkened lecture hall, the students can easily

A

Temperature Effects Both the energy and the intensity of the light emitted by LED'S are generally affected by temperature. Use of liquid nitrogen to cool t h e LED can provide dramatic illustrations of both effects. With regard to the spectral distribution, cooling the solid would beexpected &reduce the internuclear distance. For example, in Gap, the cubic lat3When pnolons are absorbed or em ttea, wth energy and momentun in the sol~dmust be conserveo. For GaAs and As-r cn Gap&., samples (0.00< x s 0.45),momentum associated with absorption or emission of a photon can be conserved exclusively by production or recombination of an electron-hole pair, respectively. Such solids. called "direct band gap" materials, absorb light strongly at the band gap energy and emit band gap energy light with high efficiency.In contrast, Gap and P-rich GaP+s,,samples (0.45S x < 1.00)cannot conserve momentum during an electronictransition at the band gap energy unless the absorption or emission of a photon is also accompanied by a change in crystal lattice momentum, corresponding to absorption or emission of quantized lanicevibrationscalled phonons. This is a less likely process, and these ''indirect band gap" materials absorb light more weakly and yield less efficientradiative recombination, making them inferiorLED materials. The effect of doping with nitrogen is that the nitrogen atoms, which are isovalent with but more electronegative than P or As, can easily trap conduction band electrons in localized regions of the lattice. By the Heisenberg uncertainty principle, if the electron is more localized in space, it has a greater uncertainty in momentum. The hroadsr ranoe of momentum values affordsthe electron the ~robabilit" nf matchinn the hole momentum .. and - -~therebv,eliminates the need for a phonon in the recombination process and dramatically boosts the radiative efficiency. The emitted photon will be slightly red-shifted relative to the band gap energy, however, because the electronic trap state introduced by the nitrogen dopant lies at an energy slightly below the conduction band edge. ~

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Volume 69 Number 2 Februaly 1992

155

tice parameter shrinks from 5.451 Aat 300 K to 5.447 Aat 77 K (5). . . Reducing the internuclear distance increases the orbital overlap and thus raises the band gap e n e ~ g yover ;~ the same temperature range, E, of Gap increases from 2.27 eV (A = 550 nm) at 300 K to 2.33 eV (A = 530 nm) at 77 K (6).Several of the LEDs examined do exhibit such a blue shift: the most visually dramatic being the shift of the GaP0.4,,Aso.so LED output from red at mom temperature to orange at 77 K. Spectra recorded on a spectmfluorimeter, Figure 9, agree with the qualitative observations: the emission peak moves to shorter wavelengths and the iptensity increases. All of the LEDs are more emissive a t 77 K relative to room temperature. At any temperature, radiative recombination of electron-hole pairs competes with nonradiative recombination processes that produce heat through quantized lattice vibrations, called phonons. At lower temperatures, the lattice vibrations play a less effective role, leading to significant enhancements of t h e radiative recombination process. LED efficiency enhancements of one to two orders of magnitude are common in this experiment.

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Conclusions In this paper, we have described a quick, inexpensive series of experiments designed for introductory chemistry courses utilizing commercially available LEDs. We have found that these ex~erimentsc a ~ t u r estudents' interest and enhance traditikal discussidns of spectroscopy, electronic transitions, and the interrelationships between color, wavelength, and energy. In addition, they provide an excellent foundation for the discussion of periodic properties in a family of common semiconductors. But most importantly, the use of LEDs in the classroom opens a link between the fundamentals learned in general chemistry and an understanding device commonlv en- of a high-tech countered. Acknowledgment The authors would like to thank Jeremy Burdett, Clark Landis, Tom Mallouk, John Moore, and Ned Tabatabaie for helpful discussions and the Dreyfus Foundation and the National Science Foundation (grant USE-9150484) for financial support. Literature Cited 1. Baumann, M. G. D.; Wright, J. C.; Ellis, A. B.; Ku-h.

'The decrease in lattice vibrations with temoerature will also decrease the width of the band, contributingto thd increase in the band gap. Momentum considerations are also important;the indirect band gap materials generally show smaller band gap changes with temperature than do the direct band gap materials. "1 is important to note that a clear shift in wavelength of emitted liaht with temperature was only observed for LEDs which had a single emission peak at room temperature. LEDs with multiple emissibn peaks gave more complex temperature effects.

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Journal of Chemical Education

Fdur 1-2

T;Limnsky, G C. J. Cham.

fi9 SQ.96

.... . . 4. Huhecy, J. E. Inorgonlc Chemistry.3rd d.; Harper & Row: NewYork 1983. 5. D-, P.; H. A.P~YS. sol. rai 1983.~0,~~9.102.

valand.u.;schneider, stat. 6. Panish, M. B.:Casey J r , H . C. J A p p l . Phys. 1989,40,163-167. 7. (a1 Thornp8on.A. G.: Cardona, M.:Shaklee, K L.:Woolley, J. C.Phyr. Re". 1966.2d6, 601410. lb) Mathieu, H.; Merle, P.: Ameriane, E. L.Phw. Rev. 1977, B15, 20482052. (ci Strammanis. M. E.; Krumrne, J P.; Rubenstein, M. J. Eledrochsm. Soc 1967,114,M0441.