1028 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM...

1028 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 4, JULY/AUGUST 2009

LEDs for Solid-State Lighting: PerformanceChallenges and Recent Advances

Mary H. Crawford, Member, IEEE

(Invited Paper)

Abstract—Over the past decade, advances in LEDs have enabledthe potential for wide-scale replacement of traditional lighting withsolid-state light sources. If LED performance targets are realized,solid-state lighting will provide significant energy savings, impor-tant environmental benefits, and dramatically new ways to utilizeand control light. In this paper, we review LED performance tar-gets that are needed to achieve these benefits and highlight some ofthe remaining technical challenges. We describe recent advancesin LED materials and novel device concepts that show promise forrealizing the full potential of LED-based white lighting.

Index Terms—Energy conservation, LEDs, lighting, semicon-ductor devices.

I. INTRODUCTION

SOLID-STATE lighting (SSL) based on LEDs is an emergingtechnology with potential to greatly exceed the efficiency

of traditional lamp-based lighting systems. Whereas energy ef-ficiency is the primary motivation behind SSL, LEDs are alsoanticipated to bring entirely new functionalities to lighting sys-tems, greatly enhancing the ways in which we use light. LEDshave already replaced traditional lamps in a number of light-ing systems, including traffic lights, signs, and displays. Manyof these applications require monochrome light and the narrow-band emission properties of LEDs present a clear advantage overfiltered-lamp approaches. However, the greatest impact of SSLwill likely be in general illumination applications that demanda high-quality white-light source. Coupled with the need forhigh efficiency, the stringent color requirements of white-lightillumination place new challenges on LED technology, many ofwhich have yet to be overcome.

One of the obvious challenges specific to white lighting isthe need to have highly efficient light emission spanning thevisible spectrum. Whether the light is generated from inorganicLEDs, organic LEDs, or phosphors, this requirement demandsdevelopment of a range of materials and/or material alloys withexceptional optical performance. In addition, many illumination

Manuscript received December 19, 2008. First published May 27, 2009;current version published August 5, 2009. This work was supported in partby the Department of Energy Office of Basic Energy Sciences and in part bySandia’s Laboratory Directed Research and Development Program. Sandia is amultiprogram laboratory operated by Sandia Corporation, a Lockheed MartinCompany, for the U.S. Department of Energy’s National Nuclear Security Ad-ministration, under Contract DE-AC04-94AL85000.

The author is with Sandia National Laboratories, Department of Semicon-ductor Material and Device Sciences, Albuquerque, NM 87185 USA (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2009.2013476

applications require significantly higher brightness levels thanindicator applications, which has motivated the development oflarge-area, high-power LEDs [1]. A major challenge for LEDs isdelivering highest efficiency performance at the current densitiesand temperatures relevant to high-power operation. LED-basedillumination systems must also provide efficient and flexibleapproaches to extract and distribute light. As LEDs have entirelydifferent form factors and dielectric properties than traditionallamps, new strategies for efficiently extracting light from theLED chip as well as controlling the light emission pattern arebeing explored.

While there are certainly other challenges to LED-based whitelighting, these requirements present some of the most significantchallenges on the LED chip level. In this paper, we examinethese roadblocks to realizing the full potential of LED-basedwhite lighting, and highlight recent research advances towardovercoming them. In keeping with the multidisciplinary andglobal nature of SSL research, our review encompasses insightsand advances in materials development, device physics, andnovel device concepts, as accomplished by many groups aroundthe world.

The remainder of this paper is organized into three sections.Section II provides a brief background on solid-state whitelighting, presenting LED performance targets and describingapproaches to generating white light from LED-based systems.A review of LED performance challenges and recent researchadvances is given in Section III. To provide sufficient technicaldetail, we have limited the scope of that section to inorganicsemiconductor LEDs, omitting organic LEDs, which have re-cently been reviewed in [2]. Finally, Section IV provides a sum-mary and outlook for LED-based white lighting.

II. SOLID-STATE WHITE LIGHTING

The potential for SSL, and indeed the optimism that it willsucceed, is based on dramatic LED performance advances overthe past two decades. AlGaInP semiconductor alloys, developedand matured throughout the 1990s [3], have emerged as the mostefficient LED materials in the yellow, orange, and red regions,with peak external quantum efficiencies (EQEs) as high as 55%at 650 nm [4]. The commercialization of blue LEDs based onInGaN semiconductor alloys was a major breakthrough in theearly 1990s, providing, for the first time, the potential for effi-cient and reliable blue and green LEDs [5]. Extensive develop-ment of InGaN LEDs over the past 15 years has yielded com-mercial high-power blue LEDs with EQEs as high as 56% [6].And, as described in the following section, high-power white

1077-260X/$25.00 © 2009 IEEE

CRAWFORD: LEDs FOR SOLID-STATE LIGHTING: PERFORMANCE CHALLENGES AND RECENT ADVANCES 1029

Fig. 1. Approaches for generating white light from LEDs and representativepower spectra. (a) RB GB B system that employs a blue LED to pump red andgreen phosphors. (b) Blue LED pumping a green phosphor combined with a redLED (RGB B). (c) Four-color RYGB all-LED system.

LEDs based on InGaN blue LEDs with phosphor coatings havealready surpassed the luminous efficacy1 of incandescent lampsand are approaching that of fluorescents [6].

For SSL to realize decisive energy conservation benefits, how-ever, even greater performance must be achieved. The U.S.Department of Energy’s Multi-Year Program Plan for SSLhas targeted a white LED luminous efficacy of 163 lm/W by2015, which would surpass those of incandescent and fluores-cent lamps by ∼10× and ∼2×, respectively [8]. Realizing thisLED performance would have a tremendous impact, as wide-scale adoption of such LEDs would have the potential to reduceelectricity consumption for lighting by 2× or more, with pro-jected year 2025 savings in the U.S. alone of 620 TWh/year(or roughly $42 billion/year at current prices). Translated intoenvironmental benefits, these energy savings would serve to re-duce carbon-equivalent emissions by about 100 Megatons peryear [9]. These benefits have provided substantial motivationfor research toward advancing LED performance.

A major challenge of reaching these goals is that LED-basedwhite lighting must achieve such high efficiencies while si-multaneously delivering exceptional color quality at low cost.Fig. 1 illustrates several approaches that are being explored toachieve these qualities in LED-based white lighting. Presently,the most common white LED design employs an InGaN LEDto provide blue emission and to pump phosphors that emit atlonger wavelengths where LED performance is deficient. These“phosphor-converted” or pc-LEDs are illustrated in Fig. 1(a).The simplest pc-LEDs combine a blue LED (λ ∼ 440–460nm) with a YAG:Ce3+ phosphor (λ ∼ 560 nm) to create a

1Luminous efficacy, measured in lumens per Watt of electrical power, is ametric defining the power conversion efficiency of the LED weighted by thehuman eye response. Incandescent and fluorescent lamps have values of ∼14and ∼75 lm/W, respectively [7].

two-color white LED with a color rendering index (CRI)2 of70–80 and a correlated color temperature (CCT)3 of 4000–8000K. While suitable for less demanding applications, includingoutdoor lighting, these LEDs are not adequate for indoor illu-mination applications that generally require CRI > 80 [6].

The color characteristics may be substantially improved byadding a second, red-emitting phosphor, creating a three-colorsystem that we designate as RBGBB in Fig. 1(a) (the subscriptB indicating that the red and green light is produced via op-tical pumping by the primary blue LED). Recently developedcommercial white LEDs employing nitrodosilicate red-emittingphosphors have demonstrated CRI of 90, with luminous effica-cies of 55 lm/W and CCT of 3000–3600 K (350 mA, 1 mm ×1 mm chip [6]). As mentioned before, the luminous efficacyof these “warm white” LEDs have already surpassed that ofincandescents (∼14 lm/W [7]) and are approaching that of fluo-rescents (∼75 lm/W [7]). Moving beyond this performance willrequire further optimization of phosphors, particularly narrow-band phosphors in the red region, as well as improvements inblue LED efficiencies.

In contrast to phosphor approaches, the “multichip” approachutilizes only LEDs to generate white light [Fig. 1(c)]. Three-color systems with components at red, green, and blue (RGB)wavelengths are limited to CRI values of 85 or less [12], whereasa four-color (RYGB) system can achieve CRI > 95 [13]. As de-scribed in the following section, highly efficient LED emissionhas not yet been demonstrated across the entire visible spectrum,and significant semiconductor material advances are needed torealize efficient multichip white LEDs. As present-day LEDperformance is most limited in the green–yellow region, hybridapproaches, employing a green phosphor in combination withred and blue LEDs [Fig. 1(b)], are also being explored.

In assessing the merits of these LED architectures, we notethat pc-LEDs may provide a simpler and lower cost solution.Requiring only a blue LED, the pc-LED avoids some of the com-plexities of multichip systems, including aspects of assembly,color mixing, and feedback circuitry to maintain color quality,given different thermal and degradation properties of the indi-vidual LEDs. However, in terms of energy efficiency, the mul-tichip approach offers a clear advantage. By providing directemission at the necessary visible wavelengths, multichip LEDsavoid the absorption and emission losses of the phosphor as wellas down-conversion losses associated with generating lower en-ergy phosphor emission from a higher energy blue source. Themultichip approach also has greater potential for actively con-trolling the light’s spectral distribution, providing “smart” light-ing capabilities far beyond traditional lamp systems. Thus, al-though success of the multichip approach requires overcomingsignificant LED materials challenges, the compelling benefitshave inspired concerted pursuit of that design.

2Color rendering index (CRI) describes the ability of a light source to ac-curately render the colors of objects in the environment as seen by the humaneye [10]. The maximum CRI of 100 is achieved by incandescent lamps; a CRIof 90 is considered excellent for illumination applications.

3Correlated color temperature (CCT) is defined by the chromaticity positionalong the Planckian locus of the CIE chromaticity diagram [11]. Incandescentand fluorescent lamps are typically in the 3000–4100 K range.


Fig. 2. State-of-the-art EQEs for LEDs (Tj = 25 ◦C) emitting at various wave-lengths and made from two material systems. The first material system is InGaN:(1) InGaN TFFC LEDs, 350 mA; (2) InGaN vertical thin-film (VTF) LED,1000 mA; and (3) InGaN conventional chip (CC) LEDs employing patternedsubstrates. The second material system is AlGaInP: (4) Production performanceAlGaInP truncated-inverted-pyramid (TIP) LEDs, Philips Lumileds LightingCompany, 350 mA. V (λ) is the luminous eye response curve from CIE. Dashedlines are guides to the eye. (reprinted from [6], IEEE).

III. LED PERFORMANCE CHALLENGES AND

RECENT ADVANCES

All of the approaches described in the previous section relyon LED emission for a major part of the white-light production.This section discusses LED performance limitations that mustbe overcome to realize the full potential of SSL. Our first topicexamines the LED material challenges in developing multichipwhite LED platforms. We next examine the present limitationsof achieving high-efficiency LED operation at high operatingcurrents and output powers, relevant to all of the white LEDplatforms under consideration. Finally, we review approachesto achieve high light extraction efficiencies from LEDs and therelated topic of advanced photon management. Each of theseareas brings unique challenges, but notable advances are beingmade, as described next.

A. The “Green–Yellow Gap”

A challenge for SSL is the fact that high optical efficienciesmust be achieved across the visible spectrum to match the colorquality of traditional white-light sources. While much progresshas been made, semiconductors and down-conversion materialsstill show major performance limitations in particular spectralregions. The most notable challenge for semiconductor materi-als is the efficiency gap in the green–yellow region of the spec-trum. As shown in Fig. 2, InGaN LEDs exhibit high EQEs in theviolet and blue regions, but longer wavelength devices, achievedby increasing indium concentration in the InGaN active region,show a notable drop in efficiency. This efficiency loss could beattributed to a number of material issues, but the dominant causeis still unclear. AlGaInP-based LEDs, on the other hand, haveshown impressive performance in the red region, but lose effi-ciency at shorter wavelengths. An indirect-bandgap crossover in

the green–yellow region provides an explanation for efficiencyloss in these AlGaInP alloys, and their improved performancein the green–yellow region is therefore far less likely.

Improving LED performance within the “green–yellow gap”is a major focus of SSL research efforts, and a number of ap-proaches are being explored. To attain the long-term SSL perfor-mance targets [8], these efforts will need to achieve high internalquantum efficiencies (IQEs) of ≥90% at particular green andyellow wavelengths. The ultimate solution may include innova-tive approaches to improved InGaN materials, the explorationof alternative III–V or II–VI semiconductors [14], [15], or thedevelopment of suitable broadband green phosphors to replacegreen semiconductor LEDs [13]. In this paper, we narrow ourscope to InGaN materials, given the tremendous potential thatthey have demonstrated to date.

InGaN alloys are largely believed to be the most promis-ing semiconductors for bridging the green–yellow gap. UnlikeAlGaInP alloys, they possess a direct energy bandgap acrossthe visible spectrum and present no intrinsic roadblock to high-efficiency optical emission. While efficiency clearly drops off inthe green region, state-of-the-art commercial InGaN LEDs near530 nm have now demonstrated 100 lm output and 80 lm/Wperformance (at 35 A/cm2 [16]), and IQE values have been esti-mated to be greater than 30% (at 50 A/cm2 [6]). These achieve-ments have provided strong motivation for pushing the limits ofhigh-performance InGaN to longer visible wavelengths.

The efficiency loss in the green–yellow region is inextricablylinked with the evolution of InGaN material properties with in-creasing indium content of the alloy. Substantial research overthe past decade has revealed InGaN alloys to have a complexarray of properties that evolve with indium composition andnegatively impact material quality and optical efficiency. Here,we focus on two of the most significant factors that influencehigh-In-content InGaN alloys in LED heterostructures: polar-ization effects and strain effects.

1) Polarization Effects: One factor that has long beenthought to limit the efficiency of green InGaN materials is thestrong polarization effects that arise from their wurtzite crystalstructure. In particular, piezoelectric and spontaneous polariza-tion along the polar [0 0 0 1] (c-axis) crystal direction leadsto large (>1 MV/cm) electrostatic fields in the quantum well(QW) active regions of InGaN LEDs. These fields cause a spa-tial separation of electron and hole wavefunctions in the QWs,thereby reducing radiative recombination rates and related radia-tive efficiency. Because the piezoelectric polarization dependson strain, the reduction in radiative recombination rate is largerfor longer emission wavelengths (moving toward the green andyellow) where higher In-concentration InGaN is grown coher-ently strained on GaN.

Polarization effects also impact LED emission wavelength aswell as the stability of the emission wavelength with injectioncurrent. Interestingly, polarization-induced electrostatic fieldsin InGaN QWs produce a red shift of the emission wavelength,which is beneficial as we seek to achieve longer wavelengthswith a given alloy composition. However, this red shift is primar-ily realized at lower currents and is counteracted at increasingcurrents by carrier-induced screening of the internal fields. The


Fig. 3. Piezoelectric polarization of an InGaN QW grown pseudomorphicallyon GaN as a function of crystal orientation. Zero degree represents c-plane(0 0 0 1) growth, (1 1 2 2) is a semipolar crystal plane, and m-plane(1 1 0 0) is a nonpolar crystal plane. P ′

z is the polarization perpendicular tothe growth plane (reprinted from [17]).

overall effect is a blue shift of emission wavelength with increas-ing current, which is problematic for achieving a color-stablewhite-light source.

Given these undesirable effects, significant effort has been ex-pended in developing InGaN LEDs on GaN templates with non-polar [e.g., m-plane (1 1 0 0) or a-plane (1 1 2 0)] or semipolar[e.g., (1 1 2 2)] crystal orientations. As shown in Fig. 3, movingto these nontraditional crystal orientations affords the opportu-nity to greatly reduce, and even eliminate, polarization effectsin wurtzite nitride materials [17], [18]. However, early attemptsto achieve nonpolar GaN templates by heteroepitaxial growthon foreign substrates, such as a-plane GaN on r-plane sapphire,resulted in unacceptably high densities of extended defects, in-cluding threading dislocations (>1010 cm−2) and stacking faults(>105 cm−1) [19]. More recently, the commercial availabilityof high-quality, free-standing, nonpolar GaN substrates grownby hydride vapor-phase epitaxy has enabled major advances.These substrates are formed by slicing small-area wafers withnonpolar orientation from thick, high-quality, c-plane-orientedsubstrates [20]–[22]. By employing such m-plane GaN sub-strates with threading dislocation densities <5 × 106 cm−2 , theUniversity of California at Santa Barbara group reported re-markable progress in LED performance: an EQE of 45% at402 nm, comparable to the best results from polar (c-plane)InGaN LEDs [23].

With these advances, nonpolar InGaN LEDs have reached thematerial quality where their advantages and limitations can bemore clearly examined. Many reports have confirmed that non-polar InGaN LEDs show improved wavelength stability withincreasing current compared to c-plane LEDs [23]–[25]. And,as mentioned before, efficiencies of the best m-plane InGaNLEDs at near-UV wavelengths have met or exceeded that ofc-plane LEDs [23]. However, indium incorporation efficiencyin nonpolar orientations is reported to be 2–3× less than forc-plane, making it more difficult to reach blue and longer wave-

Fig. 4. Electroluminescence output powers at 20 mA dc drive current as afunction of emission wavelength of m-plane InGaN LEDs. Circle and trianglesymbols represent InGaN LEDs with 2.5 and 8.0 nm QW thickness, respectively(reprinted from [26]).

lengths [26]. Increased indium incorporation could be achievedby using lower growth temperatures than those used for c-planegrowth, but this approach is expected to reduce material quality.Therefore, various active region designs, including wide QWsand thick barrier layers, have been employed to achieve longerwavelengths. These efforts have enabled blue (468 nm) LEDson m-plane GaN substrates with EQEs of 16.8% at 20 mA [27].

The development of nonpolar InGaN LEDs with reasonableefficiencies into the blue region provides important insights intothe green–yellow gap problem. In Fig. 4, we show wavelength-dependent output powers reported for a series of m-planeInGaN LEDs with different indium compositions and with twodifferent QW widths [26]. Similar to the c-plane LED EQE per-formance in Fig. 2, the nonpolar LED output power is seen topeak in the near-UV/violet region and is reduced significantlywith increasing indium composition and emission wavelength.Notably, this strong drop in LED output power at longer wave-lengths is present even in the absence of polarization effects,indicating that polarization effects alone do not account for thegreen–yellow efficiency gap. Yamada et al. hypothesize thatnonradiative centers caused by strain may be a remaining factor[26].

It should be noted that semipolar LEDs provide an interestingcompromise for achieving emission in the green–yellow region.As shown in Fig. 3, the semipolar (1 1 2 2) plane has substantiallyreduced piezoelectric polarization compared to the c-plane, andindium incorporation efficiency on the (1 1 2 2) plane is reportedto be comparable to that of c-plane [28]. These factors have en-abled the demonstration of (1 1 2 2) LEDs at blue, green, andamber wavelengths [29]. Recent performance advances includeEQEs of 10.5% at 20 mA in the green region (516 nm) [28]and EQEs as high as 9.5% at 20 mA in the green–yellow region(562 nm) [30]. While the results in the green are not yet com-petitive with c-plane LEDs, the green–yellow LED performancereportedly surpasses that of any LEDs in this wavelength region,representing an important advance.


Ultimately, nonpolar and semipolar nitrides may play a rolein enabling both LEDs and laser diodes in the green–yellowregion. Continued improvements in longer wavelength deviceefficiencies as well as sustained development of larger-area,high-quality, nonpolar substrates [23], will be critical for theircommercial success.

2) Strain Effects: As discussed before, other factors, beyondpolarization effects, likely contribute to the low efficiencies ofInGaN LEDs at longer wavelengths. One candidate is the latticemismatch strain between InGaN QWs and commonly employedGaN templates that increases with increasing indium composi-tion of the QWs. Strain is, of course, directly responsible forthe piezoelectric polarization properties described before. But anumber of additional phenomena are related to material strain.First, it is well known that the growth of higher indium com-position InGaN alloys requires lower growth temperatures dueto reduced thermal stability [31]. At a given growth tempera-ture, the strain in an InGaN/GaN heterolayer limits the furtherincorporation of indium by so-called “composition pulling” ef-fects [32], [33]. In this way, strain inhibits the higher indiumconcentrations needed for longer wavelength devices and con-tributes to employing even lower growth temperatures for ade-quate indium incorporation. The combination of lower growthtemperatures and increasing strain energy enhances the forma-tion or incorporation of a number of structural defects (e.g., pointdefects [34], impurities [35], V-defects [36], [37]) with accom-panying degradation of optical efficiency. Strain may also playa role in the increased compositional instabilities that have beenobserved in higher indium composition alloys [38]. Thus, be-yond contributing piezoelectric effects, strain is a major factorthat limits the emission wavelength and optical performance ofInGaN LEDs.

An effective solution to mitigating these strain-related fac-tors is to replace GaN templates with InGaN templates that aremore closely lattice-matched to overlying InGaN QW struc-tures. While this approach sounds simple, the need to real-ize thick, strain-relaxed InGaN templates with both low defectdensities and smooth surface morphologies presents a tremen-dous challenge. Given that bulk InGaN substrates are not yetavailable, alternative substrates or growth strategies must beemployed.

To date, several alternative substrates have been explored tofind a suitably lattice-matched condition for InGaN growth.ZnO is an interesting candidate for green materials, havinga similar wurtzite crystal structure to InGaN and providing alattice-matched condition for In0.18Ga0.82N alloys grown in the[0 0 0 1] crystal orientation [39]. However, a major roadblock toachieving high-quality InGaN on ZnO is the thermal incompat-ibility of the two materials, as metal–organic vapor-phase epi-taxy (MOVPE) of InGaN is ideally performed at temperatures>700 ◦C for higher material quality, whereas ZnO decomposi-tion and formation of interfacial alloys is seen at temperatures<600 ◦C [39].

Although this incompatibility seems formidable, two recentareas of exploration have been fruitful. First, nonpolar ZnOsubstrates have been shown to resist decomposition up to highergrowth temperatures than c-plane ZnO. Accordingly, MOVPE-

grown In0.25Ga0.75N films on both a-plane and m-plane ZnOhave shown no degradation in structural quality up to 650 ◦C[40]. Second, the application of pulsed-layer deposition (PLD)has demonstrated distinct advantages for InGaN/ZnO growth.Here, laser ablation of an InxGa1−x eutectic target in the pres-ence of atomic nitrogen provides the source materials for InGaNgrowth. As one benefit, the ablation process provides enhancedkinetic energy of group III atoms on the growing surface, en-abling improved material quality at lower growth temperatures.Notably, atomically smooth InGaN films on ZnO have been re-alized by PLD at room temperature [39]. PLD growth of GaNon m-plane ZnO further suggests that GaN or InGaN epilayersdeposited at room temperature are viable buffer layers, suppress-ing ZnO decomposition for subsequent growth temperatures upto 700 ◦C [41]. Overall, these reports present clear progress to-ward higher temperature growth conditions, but structural datasuggest considerable advances in InGaN material quality arestill needed.

Given continuing challenges of high-quality InGaN growthon lattice-matched substrates, a greater emphasis has beenplaced on studies to examine the limits of InGaN growth onlattice-mismatched GaN templates. The goal of these efforts isto gain insight into strain relaxation mechanisms and to devisestrategies to relax strain while mitigating the defect formationthat is typically present. One of the recent advances in thisarea is the application of epitaxial lateral overgrowth (ELO)techniques to InGaN alloys [42]. ELO approaches involve epi-taxial growth on templates (e.g., GaN) that are patterned, typ-ically with dielectric stripes or by etching a stripe pattern di-rectly into the template. Vertical growth combined with lateralgrowth between the stripes results in a coalesced epitaxial filmand provides a mechanism for turning or redirecting threadingdislocations away from the overlying active region. When ap-plied to GaN or AlGaN materials, such strategies have enabledthick, coalesced films with dislocation densities <5 × 107 cm−2

[43], [44].The aforementioned challenges of strained InGaN epilayers

have made it more difficult to effectively apply ELO strategiesto InGaN; however, InGaN ELO on periodically grooved GaNtemplates has recently shown promise [45]. The ELO processemploys GaN templates with low dislocation density regions(<107 cm−2) achieved through sidewall lateral epitaxial over-growth methods [46]. Subsequent reactive ion etching yieldsperiodically grooved structures on the GaN surface for InGaNnucleation, growth, and coalescence into a planar film. The se-lection of m-plane GaN templates has proven to be a key factor inenabling a flat and smooth coalesced InGaN film, as c-plane anda-plane GaN orientations led to faceted InGaN surfaces [42]. Bycombining these components of low-dislocation m-plane GaNtemplates with InGaN lateral overgrowth, Iwaya et al. have re-alized a 7-µm-thick coalesced In0.07Ga0.93N film with almostcomplete strain relaxation and threading dislocation densities<108 cm−2 in selected regions (Fig. 5) [45]. While ELO addscomplexity and cost, this effort is an important advance towardrealizing high-quality strain-relaxed InGaN templates. Furtherefforts to extend this approach to higher indium compositionsand/or growth of InGaN LED structures should yield valuable


Fig. 5. Cross-sectional transmission electron micrograph of a strain-relaxedIn0 .07 Ga0 .93 N film on a periodically grooved m-plane GaN template. The GaNtemplate itself was grown by sidewall epitaxial lateral overgrowth to provideregions of low threading dislocation density The crystal orientation of the sampleis shown in the inset (reprinted from [45]).

insights into the strain limitations of present-day green–yellowInGaN LEDs.

Finally, we note that the growth of InGaN alloys as lowerdimensional structures is another promising strategy for pro-viding strain relaxation and defect reduction. 1-D nanowires(or nanorods) with small radial dimensions (∼10–200 nm) andtypical axial dimensions >1 µm, are one such candidate thatis currently under investigation (Fig. 6). Unlike 2-D planar het-erostructures that are laterally constrained during growth, 1-Dnanowires can relieve strain by lateral relaxation, and thereforeaccommodate higher mismatch strains before misfit dislocationsare formed [48]. Single-crystal nanowires can be grown entirelyfree of threading dislocations on a wide range of substrates,including silicon [49]. These benefits, coupled with potentiallight extraction benefits of the 1-D geometry, may ultimately beapplied to enhance both IQE and EQE of InGaN LEDs over awide compositional range.

Recent research efforts on InGaN nanowires have alreadyconfirmed some of these impressive properties. Through theapplication of halide vapor-phase epitaxy, Kuykendall et al.have demonstrated single-crystal InxGa1−xN nanorods over theentire compositional range [50]. Notably, the nanowire photo-luminescence intensity did not show a dramatic drop at longerwavelengths, as only a 1.7× drop in photoluminescence in-tensity for x = 0–0.6 (spanning near-UV to red wavelengths)was reported. Other growth techniques, including RF-plasma-assisted molecular beam epitaxy [49] and MOVPE [51], haveyielded InGaN nanowires at green and longer wavelengths.

Transitioning these nanostructured materials into a viableLED device presents obvious challenges. Nevertheless, severalproof-of-concept device geometries and fabrication strategieshave been demonstrated. One such strategy involves growth of3-nm-thick InGaN “quantum disks” along the growth directionof a GaN nanowire followed by lateral growth and coalescenceof the overlying p-GaN layers to present a planar top layer for

Fig. 6. Cross-sectional scanning electron micrograph showing a high-densityand highly aligned GaN nanowire array catalyzed by a 0.8 A layer of Ni onan r-plane sapphire substrate (reprinted from [47], 2008, American Institute ofPhysics).

electrical contacting [49]. Similar axially grown LEDs use aspin-on-glass layer to encase the nanowire array, thereby pro-viding a support structure for top metal contacting [52]. Finally,LEDs with a radial geometry, employing a GaN core and anInGaN-QW/p-AlGaN/p-GaN shell structure, have been demon-strated at wavelengths into the yellow region (577 nm) [51].Electroluminescence characterization of individual core/shellLEDs employing a single InGaN QW has yielded 3.9% EQEin the 540-nm green region, and improved performance is ex-pected with multi-QW (MQW) structures and optimized lightextraction.

Despite still being largely in the research realm, these demon-strations show the potential for nanostructured InGaN LED so-lutions in the green–yellow region. In the near term, lower di-mensional structures present a unique opportunity for studyingthe properties of InGaN heterostructures with light emissionacross the entire visible region, and with distinctly different de-fect, strain, and polarization properties compared to 2-D planarstructures.

B. “Efficiency Droop” of InGaN LEDs

As described in Section II, InGaN-based LEDs promise to bekey elements in any white-LED system, whether based on themultichip or phosphor conversion approach. One of the mostcritically important challenges to achieving ultraefficient SSLrelates to the loss of radiative efficiency of InGaN LEDs at highinjected carrier densities. As shown in Fig. 7, although nitrideLEDs have demonstrated high efficiencies at low currents, high-power LEDs can suffer more than 70% loss of optical efficiencyat desired operating currents [53].

This phenomenon, called “efficiency droop,” is not due tosimple heating, but occurs under pulsed as well as continuouswave (CW) conditions. While seen in virtually all InGaN-basedLEDs, the effect is found to be less severe in shorter wave-length (near-UV) InGaN LEDs and more pronounced at longerwavelengths. Nonradiative mechanisms proposed to explain


Fig. 7. External quantum efficiency of commercial green LEDs (Philips Lu-mileds’ Luxeon line) as a function of operating current. Maximum rated operat-ing currents of three different Luxeon models are indicated [53]. Note that morerecent device designs have yielded improved efficiency droop, as described inthe text.

efficiency droop are varied, and include carrier delocaliza-tion [54], screening of excitons [55], defect-assisted tunnel-ing of carriers out of the active region [56], Auger recombi-nation [57], [58], and inefficient carrier injection, particularlydue to internal polarization fields [59], [60]. Despite some im-provements in efficiency droop, a consensus on the mechanismbehind it remains elusive.

Of the many possible theories that have been proposed, twohypotheses have gained considerable momentum over the pastyear and are now supported by multiple research groups. Thefirst hypothesis, proposed by Philips Lumileds, identifies Augerrecombination as the dominant nonradiative mechanism leadingto efficiency loss at high carrier densities [57]. This proposalwas not without controversy, as wide-bandgap semiconductorsare thought to have small Auger coefficients, indeed some fourorders of magnitude lower than for near-IR materials such asInGaAsP. Further, the Auger process, with its higher order (∼n3)dependence on carrier density, was not expected to be dominantat the relatively low carrier densities of typical LED operation.

Several factors, however, may support the hypothesis of anAuger-related nonradiative mechanism in InGaN heterostruc-tures. As pointed out by Hader et al. [61], while the couplingbetween electron and hole bands is strongly reduced with in-creasing bandgap, the low dielectric constants of wide-bandgapmaterials contribute to enhanced Coulomb interaction, as seenin exciton binding energies that are ∼10× stronger than fornear-IR materials. Given these competing effects, the proba-bilities of electron–hole scattering processes underlying Augerrecombination are not intuitively obvious, leaving open the pos-sibility for reasonably large Auger losses in InGaN materials.Furthermore, it has been shown that low hole mobility and largehole confinement energies lead to nonuniform hole distributionacross the QWs in InGaN LEDs, and primarily population of theQW closest to the p-side of the junction [62]. Such a build-up ofcarriers in an ultrathin (∼2.5 nm) QW translates to fairly highcarrier densities of >1018 cm−3 at currents as low as 50 mA instandard high-power LEDs.

Efforts to test the Auger recombination hypothesis have beencarried out on both theoretical and experimental fronts. Theapplication of microscopic many-body models by the Univer-sity of Arizona and Philipps University Marburg groups hasprovided theoretical predictions of the impact of Auger re-combination, and initial reports predict that direct Auger re-combination is negligible in InGaN LEDs under relevant op-erating conditions [61]. However, indirect processes, includingphonon-assisted or defect-assisted Auger recombination, wereproposed as potentially yielding higher losses and are now be-ing evaluated as possible mechanisms behind efficiency droop[61].

Experimental investigations of Auger processes have in-volved both photoluminescence and electroluminescence stud-ies of InGaN heterostructures. Notably, photoluminescencestudies employing direct excitation of the InGaN epilayersthrough selective optical pumping was reported to show effi-ciency droop with increasing pump powers [57]. This observa-tion is consistent with an intrinsic recombination mechanism,such as Auger recombination, while being contrary to proposedcarrier transport and injection mechanisms. We note, however,that other groups have reported no efficiency droop in suchmeasurements [59], [63], with opposing conclusions, and con-sensus on the carrier densities achieved in these measurementsis needed. LED development to mitigate the effects of Augerrecombination has focused on new heterostructure designs tobetter distribute hole populations, including double heterostruc-tures designs with thick (∼10 nm) InGaN active layers [58] aswell as MQW designs that enhance hole injection into multiplewells in the QW stack [16]. These strategies have succeeded inreducing efficiency droop and further advances are anticipatedwith optimized designs.

Coexisting with the Auger recombination theory is a sec-ond hypothesis that more directly attributes droop to carriertransport and injection limitations, without invoking Auger pro-cesses [60], [64]. At the core of this hypothesis are the relatedphenomena of ineffective hole injection into the active regionand electron leakage out of the active region; both of which leadto parasitic carrier recombination on the p-side of the device.The cause and effect relationship of these phenomena at high in-jection currents—whether poor hole injection drives increasedelectron leakage or whether electron leakage further limits holeinjection—is not entirely clear, but both effects may contributeto reduced radiative recombination in the active region at highcurrent densities [65].

As a major proponent of this hypothesis, a group fromRensselaer Polytechnic Institute (RPI) has identifiedpolarization-induced electron leakage out of the active regionas a major contributor to efficiency loss at high carrier densi-ties [59], [64], [65]. The phenomenon is depicted in Fig. 8, whichshows the substantial modification of InGaN LED bandstruc-ture due to interfacial polarization charges and related internalfields (occurring for polar crystal orientations as discussed in theprevious section). Triangular band profiles of QWs and barriersas well as a reduced effective bandgap of electron block layerson the p-side of the device are all manifestations of these strongpolarization effects.


Fig. 8. Schematic of the conduction band (Ec ) and valence band (Ev ) energiesof an InGaN-based LED heterostructure, including n-type GaN, InGaN MQWs,AlGaN EBL, and p-type GaN regions. Arrows indicate regions of electron andhole injection and recombination (figure courtesy of E. F. Schubert).>

This carrier leakage hypothesis predicts that the excess biasvoltage needed to overcome the triangular barrier profiles as wellas the reduced effectiveness of the electron block layer (EBL)combine to yield increased carrier leakage and reduced LEDefficiency at high injection currents. The RPI team has proposedthat mitigation of these polarization effects through polarization-matched barriers and EBLs is a method to eliminate efficiencydroop. Recently, they have succeeded in reducing efficiencydroop by ∼18% in blue LEDs that employ AlInGaN quaternarybarriers to independently engineer bandgap and polarization[65]. Further benefits, including reduced operating voltages andrelated 25% improvement in wall plug efficiency, were attainedwith these polarization-engineered designs.

Other groups have emphasized a more direct improvementin hole injection as an approach to eliminating efficiency droop[60], [63], [66]. In particular, improved performance is pre-dicted from a p-type MQW structure, or a single p-type InGaNactive layer, which would ideally mitigate both hole injectionand hole transport limitations [63]. The practical challenges ofrealizing such a structure include the fact that Mg dopants actas nonradiative centers and reduce the radiative efficiency of theInGaN active layer. As a compromise, p-type doping of only thebarrier layers has been explored [63], which yielded peak ef-ficiencies at much higher current densities (∼900 A/cm2) thanstandard LEDs (∼35 A/cm2). However, the proximity of Mgdopants to the QWs still leads to some reduction of radiative ef-ficiency, and an optimal solution will have to mitigate thosenonradiative effects. Various new device designs, includingp-type MQW devices with thin barriers or lower bandgapInGaN barriers to promote hole transport [67], as well as n-p-ndevice structures [60], are examples of related solutions beingexplored.

Interestingly, the study of efficiency droop in LEDs grown innonpolar crystal orientations, where deleterious effects of polar-ization are avoided, has not sufficiently clarified the efficiencydroop mechanism. Early nonpolar LED studies demonstratedminimal droop, potentially confirming polarization effects as thecause of droop in standard c-plane LEDs [68]. However, these

studies were carried out on LEDs at shorter (407 nm) wave-lengths where droop is less pronounced, and further employedrelatively thick (8 nm) QWs, which would serve to reduce car-rier density and thus Auger recombination. More recent studieson nonpolar near-UV LEDs have verified increased efficiencydroop for narrower (∼2.2 nm) near-UV QWs [69], consistentwith Auger processes. However, electroluminescence studies ofLEDs over a range of temperatures suggest hole transport lim-itations may be the cause of efficiency loss at high currents inboth c-plane and nonpolar LEDs [70].

Overall, the origin of efficiency droop is still under activeinvestigation, but improved hole distribution in the LED QWactive region is a common strategy to avoid both higher ordernonradiative processes and carrier transport limitations, and willlikely have continued emphasis in future LED designs. Resolu-tion of the cause of efficiency droop will require further appli-cation of advanced modeling to quantify Auger recombinationlosses, including treatment of indirect (e.g., phonon-assisted)Auger processes. In addition, reevaluation of selective opticalpumping studies of InGaN MQW structures at high carrier den-sities is in order, as contradictory results are undoubtedly con-tributing to the sustained efficiency-droop controversy.

C. Advanced Photon Management

Beyond requiring highly efficient generation of light withinthe semiconductor material, advanced LED devices must em-ploy approaches to efficiently extract photons and control lightdistribution from the chip. Light extraction solutions must sur-mount the significant trapping of light that occurs within thechip due to the relatively high (n ∼ 2.4–3.5) refractive indexesof semiconductors compared to surrounding encapsulants (n ∼1.3–1.5) or air. Requirements for controlling light propertiesare highly dependent on the particular application and may in-clude achieving highly directional (and high radiance) sourcesfor projection systems as well as polarization-controlled sourcesfor display applications. In some cases, the compatibility of LEDmaterials with nanoscale patterning affords unique opportuni-ties for “advanced” photon management, which may simulta-neously enhance photon generation (and IQE) and extraction,and further allow for tailored light distributions. In this section,we review recent progress in LED light extraction and highlightnovel strategies for reaping the benefits of advanced photonmanagement.

1) Thin-Film Flip-Chip (TFFC) LEDs: The latest advancedchip design adopted by the SSL industry is the TFFC LED [71].This design has been applied to both InGaN [72], [73] andAlInGaP LEDs [74], and employs several features to enhancelight extraction. InGaN-based thin-film LEDs include PhilipsLumileds’ TFFC [72] and Osram’s “Thin-GaN” structures [73],and in Fig. 9, we illustrate the general features of these de-signs. Device fabrication includes flip-chip mounting of theInGaN-based LED followed by removal of the sapphire orSiC substrate. Texturing of the n-GaN surface improves out-coupling of light while the thin epilayer design restricts thenumber of modes that are trapped within the LED by total in-ternal reflection. The design also employs a low-loss reflective


Fig. 9. General design of TFFC InGaN LEDs. Enhanced light extraction isachieved through substrate removal, n-GaN surface texturing, and use of areflective p-contact.

p-contact underneath the InGaN epilayers to further enhancelight extraction. Notably, these designs have achieved LED lightextraction efficiencies as high as 80% [6].

Given the success of these approaches, relatively little im-provement is now needed to reach the 90% extraction efficien-cies targeted in the U.S. Department of Energy Multi-Year Pro-gram Plan for SSL [8]. However, surface texturing approachesmay not bridge this final gap and do not provide highly tailoredlight distributions or the potential for enhanced IQE. In addition,the 80% extraction efficiency has been achieved with the use ofencapsulants that degrade at higher currents (temperatures), andimproved extraction designs that avoid encapsulants are of in-terest. A number of alternative designs are being explored tomeet these needs, and next, we specifically highlight progressin photonic crystal LEDs and surface-plasmon-enhanced LEDs.

2) Photonic Crystal LEDs (PXLEDs): PXLEDs are fabri-cated by periodic patterning of the semiconductor material,or an overlying dielectric layer, with feature sizes on the or-der of the wavelength of emitted light [75]. Consistent withthe concept of “advanced” photon management, the photoniccrystal (PX) structure has the potential to improve LED perfor-mance in several ways. First, it enables enhanced light extrac-tion through scattering of waveguided (trapped) light into radi-ation modes, and inhibits the emission of particular waveguidedmodes through the creation of a “photonic bandgap” [75], [76].Modification of the photonic density of states by the PX struc-ture can also enhance spontaneous recombination rates (andrelated IQE) by the so-called “Purcell effect” [77]. Lastly, PXscan enable control of the light emission pattern, and in particu-lar, can be employed to enhance forward-directed light for LEDapplications that require high radiance [78]. Each of these en-hancements has been demonstrated in LEDs, but achieving allof them in the same design has proven difficult.

The most widely used PX design for LEDs is a 2-D structureemploying periodic patterns of holes (∼100–200 nm diameter)created by nanolithography and plasma etching. Early theoret-ical modeling predicted that 2-D PXs could enable extractionefficiencies as high as 90% [76]. These model structures consistof a semiconductor slab with a 2-D PX penetrating the entireslab, providing substantial control of light emission and extrac-tion. High extraction efficiencies and Purcell enhancement havealso been demonstrated experimentally, through photolumines-cence of a similarly designed PX [69].

SSL applications require electrically injected devices and thisadds challenges to employing PX structures. Since PXs are typi-cally etched into the semiconductor, there is potential for degra-dation of LED performance through current leakage, surfacerecombination, or damage to the QW layers. To avoid theseeffects, PX patterning in InGaN LEDs is often limited to thetopmost (typically p-type) epilayers of the LED structure, leav-ing the QWs intact. To date, there have been a number of reportsof InGaN-based PXLEDs using this approach [78], [80]. Thesurface patterning leads to fairly poor interaction between con-fined optical modes and the PX, but has enabled the demonstra-tion of ∼1.5× increased light extraction efficiency compared tononpatterned planar structures [78]. More recently, it has beenshown that a reduction in the number of optical modes andincreased optical mode–PX interaction can be achieved by in-corporating a lower index AlGaN confining layer beneath theInGaN MQW region [81], [82] or using thin-film structures[83].

The realization of PX structures that are embedded in theLED heterostructure, rather than limited to surface layers, isanticipated to have several advantages, including stronger modeinteraction with the PX structure, avoidance of QW etch dam-age, and simplified electrical injection by maintaining a planartop layer. Recently, InGaN LEDs with embedded PXs havebeen explored through lateral overgrowth of GaN on periodi-cally patterned SiO2 layers [84], [85]. These strategies achievefull planarization of the periodic structure, allowing subsequentplanar growth of high-quality QW and p-GaN layers. Here, theembedded SiO2 acts as the PX structure as well as a lower indexconfining layer. Although further optimization is needed, pre-liminary LED results include up to 33% increased output powerat 480 nm with the implementation of embedded 2-D SiO2 pillarpatterns [85].

Importantly, the enhanced light extraction and radianceachieved by PXLEDs can eliminate the need for low-refractive-index encapsulants, resulting in longer LED operational life-times at high currents. To date, commercialization of PXLEDsis not yet widespread, and little data on photonic-crystal-enabledperformance enhancements compared to conventional designsare available. However, existing PXLED products highlight theadvantages of long life and high brightness [86].

To enable 90% extraction efficiencies from LEDs, futurePXLED designs may control light propagation in all directionsthrough the growth or assembly of 3-D PX structures [87] withembedded light emitting layers. Strategies to etch 2-D PX struc-tures through the entire LED heterostructure, without degrada-tion of QW efficiency, should also enable further advances overexisting PXLED designs.

3) Surface plasmon (SP) enhanced LEDs: SP-enhancedLEDs are also being explored for achieving enhanced light ex-traction and improved IQE at visible wavelengths. As shownschematically in Fig. 10(a), these devices circumvent radiativerecombination in the QW region, and instead, invoke ultrafastenergy transfer from the QW to SP modes in a neighboring metalsurface coating [88]. With appropriate nanoscale structure in themetal, the SP excitations can then scatter, lose momentum, andcouple to radiation modes [Fig. 10(b)].


Fig. 10. (a) Schematic of an SP-enhanced InGaN LED structure employinga thin p-GaN contact layer for effective QW–SP coupling and metal patterningfor surface plasmon scattering. (b) Surface plasmon energy dispersion curve fora metal interface with air and a dielectric, yielding surface plasmon frequenciesωair and ωn , respectively. Scattering of the surface plasmon mode enablesmomentum loss (g) and photon emission (figure courtesy of A. J. Fischer).

Through this SP-mediated process, LED efficiency can po-tentially be enhanced in multiple ways. Coupling to SP modesaids in extracting photons that would have otherwise sufferedtotal internal reflection [89]. In addition, the fast energy transferto the SP, promoted by the high SP density of states at resonance,may increase IQE by allowing more effective competition withnonradiative processes in the QW.

Whether SPs ultimately enhance or reduce the overall emis-sion efficiency is critically dependent upon the losses of the SPmode and the efficiency with which radiation can be extractedfrom SP modes compared to the intrinsic radiative efficiencyof the semiconductor material. The greatest enhancements aretherefore expected for QW structures with low radiative effi-ciency, where SP-mediated emission can enable a clear advan-tage [90]. Conversely, the advantages of SP-mediated emissionare reduced for structures with higher IQE, and are likely to beminimal for state-of-the-art blue LEDs (IQE ∼ 70%). Given thelandscape of LED efficiencies depicted in Fig. 2, we see thatSSL materials near the green–yellow-gap region have the mostto gain from SP-mediated emission strategies.

As a first step to achieving SP-enhanced InGaN LEDs, sev-eral groups have explored optical pumping of InGaN QWs withvarious metal coatings [91]–[93]. Ag and Au metals on GaN sur-faces are expected to have SP resonances at∼440 and∼560 nm,respectively [94], and are therefore particularly well-matched toInGaN visible emitters. To date, application of Ag coatings toInGaN QWs has shown evidence of SP-enhanced emission, in-cluding a 6.8× enhancement of IQE, from 6% to 41%, at awavelength of 470 nm [93]. Time-resolved photoluminescencestudies of similar structures have further verified spontaneousemission rate enhancement up to 32× compared to emissiondirectly in air [95], establishing effective QW–SP coupling.

Transitioning these optical pumping results to electrically in-jected structures adds formidable challenges. Unlike InGaN QWstructures with thin (∼10 nm) GaN cap layers, InGaN LEDs typ-ically employ thick (>100 nm) p-type layers between the QWsand the surface metal. Substantially thinner p-type layers areneeded to place the QW within the fringing field of the SP mode(∼40 nm for Ag on GaN and λ = 425 nm [91]); however, thinp-type layers can lead to device shorting and electrical perfor-mance degradation. In spite of these challenges, a few reports ofSP emission in InGaN-based LEDs employing thin p-type layers

have been made [96], [97]. In particular, nanostructured Ag filmswere applied to green (550 nm) InGaN LEDs with an originallylow IQE of 7%. Electroluminescence intensity enhancements of2.2× were observed and attributed to SP-mediated effects [97].

At this stage, dramatic efficiency improvements have not yetbeen realized for InGaN LEDs employing SP-mediated emis-sion. Significant opportunity exists for demonstrating greaterefficiency enhancements through improved QW–SP coupling,reduced SP losses, and more effective extraction of light from theSP mode. However, it remains to be seen whether SP-enhancedapproaches can yield the ultrahigh IQE and EQE necessary toreach SSL performance targets.

IV. SUMMARY AND OUTLOOK

In summary, the recent efforts of many groups worldwidehave continued the impressive advancement of LED perfor-mance. A major emphasis has been on the development ofInGaN-based materials and heterostructures to overcome LEDefficiency limitations. At this stage, there is still much workto be done to achieve ultrahigh (≥90%) IQEs from LED ma-terials at all desired wavelengths. Blue LEDs have particularimportance, as they are a likely component of either multichipor phosphor-converted white LED approaches. Given their highIQEs to date (∼70%), and expected advances in nitride mate-rials, that goal appears to be attainable. The greater challenge,necessary for realizing the benefits of multichip approaches, isto fill the green–yellow gap where present IQEs are 30% andlower. In this paper, we have highlighted a number of innova-tive materials strategies, including nonpolar crystal orientations,strain-relaxed InGaN templates, and nanowire-based emitters,that show particular promise for overcoming the challenges oflonger wavelength InGaN LEDs. Most compelling is the pos-sibility that these and other approaches may ultimately yieldhigh-performance InGaN emitters all the way to the red region,providing performance advantages over AlInGaP LEDs (e.g.,efficiency at elevated temperatures) and enabling a multichipplatform based on a single-alloy system.

The loss of InGaN LED efficiency at high operating cur-rents is relevant to both multichip and phosphor-converted whiteLEDs, and has been a topic of active investigation. Many mech-anisms for efficiency droop have been proposed and both Augerrecombination and carrier injection/leakage mechanisms haveemerged as strong candidates. LED heterostructure designs in-spired by these insights have already reduced efficiency droop.It is now very likely that focused theoretical and experimentalefforts will enable final consensus on the underlying mechanismin the near future, clarifying the path to even greater performanceadvances.

LED light extraction efficiencies have seen notable improve-ment through the application of TFFC LED designs, comingwithin reach of the 90% extraction efficiency targets. Althoughnot yet fully optimized for high efficiency, advanced photonicstructures, including PXLEDs and SP-enhanced LEDs, are be-ing explored to provide combined IQE and EQE benefits andnew strategies for controlling light emission.


In this paper, we have narrowed our scope to primarily InGaN-based materials and LEDs, and the significant advances achievedby many groups throughout the past few years. SSL effortsare of course widespread and encompass many technical areas.Important light-emitting materials research and developmenttopics that we have not emphasized include alternative green-to-red-emitting semiconductors, high-performance phosphors,quantum dots and other down-conversion materials, as well asorganic LED materials. Similarly, research on next-generationLED device concepts for SSL covers many additional topics,including systems employing strong light–matter interactionsfor tailoring the light emission process and novel nanoscienceapproaches to white-light generation. Details on these and otherkey SSL research topics may be found in this journal editionand also in [9].

Given the vital importance of energy efficiency, our emphasisin this paper has been on roadblocks to achieving SSL efficiencygoals. Sustained advances, particularly in LED IQEs, are essen-tial for wide-scale adoption of SSL and realizing the potentialfor tremendous energy and environmental benefits.

ACKNOWLEDGMENT

The author would like to thank J. Tsao, M. Coltrin, J. Wierer,S. Lee, G. Wang, A. Fischer, A. Allerman, J. Cederberg,D. Koleske, L. Rohwer, M. Krames, E. F. Schubert, C. Wetzel,and T. Detchprohm for technical discussions and editorialcomments.

REFERENCES

[1] J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Lu-dowise, G. Christenson, Y.-C. Shen, C. Lowery, P. S. Martin, S. Subra-manya, W. Gotz, N. F. Gardner, R. S. Kern, and S. A. Stockman, “High-power AlGaInN flip-chip light-emitting diodes,” Appl. Phys. Lett., vol. 78,pp. 3379–3381, 2001.

[2] F. So, J. Kido, and P. E. Burrows, “Organic light-emitting diodes forsolid-state lighting,” MRS Bull., vol. 33, pp. 663–669, 2008.

[3] C. P. Kuo, R. M. Fletcher, T. D. Ostenkowski, M. C. Lardizabal,M. G. Craford, and V. M. Robbins, “High performance AlGaInP visi-ble light-emitting diodes,” Appl. Phys. Lett., vol. 57, pp. 2937–2939,1990.

[4] M. R. Krames, M. Ochiai-Holcomb, G. E. Hofler, C. Carter-Coman,E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J.-W. Huang,S. A. Stockman, F. A. Kish, M. G. Craford, T. S. Tan, C. P. Kocot,M. Hueschen, J. Posselt, B. Loh, G. Sasser, and D. Collins, “High-power truncated-inverted-pyramid (Alx Ga1−x )0 .5 In0 .5 P/GaP light-emitting diodes exhibiting >50% external quantum efficiency,” Appl.Phys. Lett., vol. 75, pp. 2365–2367, 1999.

[5] S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightnessInGaN/AlGaN double-heterostructure blue-light-emitting diodes,” Appl.Phys. Lett., vol. 64, pp. 1687–1689, 1994.

[6] M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou,G. Harbers, and M. G. Craford, “Status and future of high-power light-emitting diodes for solid-state lighting,” J. Disp. Technol., vol. 3, pp. 160–175, 2007.

[7] Navigant Consulting, U. S. Department of Energy. (2002, Sep.).U.S. Lighting Market Characterization, Vol. 1: National LightingInventory and Energy Consumption Estimate [Online]. Available:http://www.netl.gov/ssl/

[8] U.S. Department of Energy. (2008). Multi-Year Program Plan FY’09–FY’14, Solid-state Lighting Research and Development, prepared forthe U.S. Department of Energy [Online]. pp. 53–66. Available:http://www.netl.doe.gov/ssl/

[9] U.S. Department of Energy, Office of Basic Energy Sciences. (2006). BasicResearch Needs for Solid-state Lighting [Online]. pp. 167–169. Available:http:/www.sc.doe.gov/bes/reports/files/SSL_rpt.pdf

[10] CIE, Method of Measuring and Specifying Colour Rendering Propertiesof Light Sources 13.3-1995, Commission Internationale de l’Eclairage,Wien, Austria, 1995.

[11] M. S. Rea, The IESNA Lighting Handbook. New York: IlluminationEngineering Society of North America, 2000.

[12] A. Zukauskas, R. Vaicekauskas, F. Ivanauskas, R. Gaaska, and M. S. Shur,“Optimization of white polychromatic semiconductor lamps,” Appl. Phys.Lett., vol. 80, pp. 234–236, 2002.

[13] J. M. Phillips, M. E. Coltrin, M. H. Crawford, A. J. Fischer, M. R. Krames,R. Mueller-Mach, G. O. Mueller, Y. Ohno, L. E. S. Rohwer, J. A. Simmons,and J. Y. Tsao, “Research challenges to ultra-efficient inorganic solid-statelighting,” Laser Photon. Rev., vol. 1, pp. 307–333, 2007.

[14] I. Nomura, Y. Nakai, K. Hayami, T. Saitoh, and K. Kishino, “Long lifeoperations over 5000 hours of BeZnSeTe/MgZnCdSe visible light emittingdiodes on InP substrates,” Phys. Status Solidi B, vol. 243, no. 4, pp. 924–928, 2006.

[15] A. Walsh and S.-H. Wei, “Theoretical study of stability and electronicstructure of Li(Mg,Zn)N alloys: A candidate for solid state lighting,”Phys. Rev. B, vol. 76, pp. 195208-1–195208-8, 2007.

[16] M. Peter, A. Laubsch, P. Strauss, A. Walter, J. Baur, and B. Hahn, “GreenThinGaN power-LED demonstrates 100 lm,” Phys. Status Solidi C, vol. 5,no. 6, pp. 2050–2052, 2008.

[17] U. T. Schwarz and M. Kneissl, “Nitride emitters go nonpolar,” Phys.Status Solidi (RRL), vol. 1, pp. A44–A46, 2007.

[18] P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ram-steiner, M. Reiche, and K. H. Ploog, “Nitride semiconductors free of elec-trostatic fields for efficient white light-emitting diodes,” Nature, vol. 406,pp. 865–868, 2000.

[19] M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, “Struc-tural characterization of nonpolar (1 1 2 0) a-plane GaN thin films grownon (1 1 0 2) r-plane sapphire,” Appl. Phys. Lett., vol. 81, pp. 469–471,2002.

[20] T. Paskova, R. Kroeger, S. Figge, D. Hommel, V. Darakchieva, B. Mone-mar, E. Preble, A. Hanser, N. M. Williams, and M. Tutor, “High-qualitybulk a-plane GaN sliced from boules in comparison to heteroepitaxi-ally grown thick films on r-plane sapphire,” Appl. Phys. Lett., vol. 89,pp. 051914-1–051914-3, 2006.

[21] K. Okamoto, H. Ohta, D. Nakagawa, M. Sonobe, J. Ichihara, andH. Takasu, “Dislocation-free m-plane InGaN/GaN light-emitting diodeson m-plane GaN single crystals,” Jpn. J. Appl. Phys., vol. 45, pp. L1197–L1199, 2006.

[22] K. Fujito, K. Kiyomi, T. Mochizuki, H. Oota, H. Namita, S. Nagao,and I. Fujimura, “High-quality nonpolar m-plane GaN substrates grownby HVPE,” Phys. Status Solidi A, vol. 205, no. 5, pp. 1056–1059,2008.

[23] K.-C. Kim, M. C. Schmidt, H. Sato, F. Wu, N. Fellows, M. Saito, K. Fujito,J. S. Speck, S. Nakamura, and S. P. DenBaars, “Improved electrolumines-cence on nonpolar m-plane InGaN/GaN quantum wells LEDs,” Phys.Status Solidi (RRL), vol. 1, no. 3, pp. 125–127, 2007.

[24] H. Behmenburg, T. C. Wen, Y. Dikme, C. Mauder, L. R. Khoshroo, M. M.C. Chou, M. V. Rzheutskii, E. V. Lutsenko, G. P. Yablonskii, J. Woitok,H. Kalisch, R. H. Jansen, and M. Heuken, “m-plane GaN/InGaN/AlInNon LiAlO2 grown by MOVPE,” Phys. Status Solidi B, vol. 245, no. 5,pp. 893–895, 2008.

[25] C. Chen, V. Adivarahan, J. Yang, M. Shatalov, E. Kuokstis, andM. A. Khan, “Ultraviolet light emitting diodes using non-polar a-planeGaN-AlGaN multiple quantum wells,” Jpn. J. Appl. Phys., vol. 42,pp. L1039–L1040, 2003.

[26] H. Yamada, K. Iso, M. Saito, H. Masui, K. Fujito, S. P. DenBaars,and S. Nakamura, “Compositional dependence of nonpolar m-planeInx Ga1−x N/GaN light emitting diodes,” Appl. Phys. Exp., vol. 1,pp. 041101-1–041101-3, 2008.

[27] K. Iso, H. Yamada, H. Hirasawa, N. Fellows, M. Saito, K. Fujito, S. P. Den-Baars, J. S. Speck, and S. Nakamura, “High brightness blue InGaN/GaNlight emitting diode on nonpolar m-plane bulk GaN substrate,” Jpn. J.Appl. Phys., vol. 46, no. 40, pp. L960–L962, 2007.

[28] H. Sato, A. Tyagi, H. Zhong, N. Fellows, R. B. Chung, M. Saito, K. Fujito,J. S. Speck, S. P. DenBaars, and S. Nakamura, “High power and highefficiency green light emitting diode on free-standing semipolar (1 1 2 2)bulk GaN substrate,” Phys. Status Solidi (RRL), vol. 1, no. 4, pp. 162–164,2007.

[29] M. Funato, M. Ueda, Y. Kawakami, Y. Narukawa, T. Kosugi, M. Takahashi,and T. Mukai, “Blue, green and amber InGaN/GaN light-emitting diodeson semipolar {1 1 2 2} GaN bulk substrates,” Jpn. J. Appl. Phys., vol. 45,pp. L659–L662, 2006.


[30] H. Sato, R. B. Chung, H. Hirasawa, N. Fellows, H. Masui, F. Wu, M. Saito,K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Opticalproperties of yellow light-emitting diodes grown on semipolar (1 1 2 2)bulk GaN substrates,” Appl. Phys. Lett., vol. 92, pp. 221110-1–221110-3,2008.

[31] I. Akasaki and H. Amano, “Crystal growth of column III nitrides andtheir applications to short wavelength light emitters,” J. Cryst. Growth,vol. 146, pp. 455–461, 1995.

[32] F. D. Larche and J. W. Cahn, “Stress effects in III–V solid–liquid equilib-ria,” J. Appl. Phys., vol. 62, pp. 1232–1239, 1987.

[33] M. Shimizu, Y. Kawaguchi, K. Hiramatsu, and N. Sawaki, “MOVPEgrowth of thick homogeneous InGaN directly on sapphire substrate usingAlN buffer layer,” Solid State Electron., vol. 41, no. 2, pp. 145–147,1997.

[34] Y. J. Sun, O. Brandt, and K. H. Ploog, “Photoluminescence intensityof GaN films with widely varying dislocation density,” J. Mater. Res.,vol. 18, pp. 1247–1250, 2003.

[35] D. D. Koleske, A. E. Wickenden, R. L. Henry, and M. E. Twigg, “Influenceof MOVPE growth conditions on carbon and silicon concentrations inGaN,” J. Cryst. Growth, vol. 242, pp. 55–69, 2002.

[36] C. J. Sun, M. Z. Anwar, Q. Chen, J. W. Yang, M. A. Khan, M. S. Shur,A. D. Bykhovski, Z. Liliental-Weber, C. Kisielowski, M. Smith, J. Y. Lin,and H. X. Jiang, “Quantum shift of band-edge stimulated emission inInGaN–GaN multiple quantum well light-emitting diodes,” Appl. Phys.Lett., vol. 70, pp. 2978–2980, 1997.

[37] F. Scholz, A. Sohmer, J. Off, V. Syganow, A. Dornen, J.-S. Im,A. Hangleiter, and H. Lakner, “In incorporation efficiency and compo-sition fluctuations in MOVPE grown GaInN/GaN heterostructures andquantum wells,” Mater. Sci. Eng. B, vol. 50, pp. 238–244, 1997.

[38] I. Ho and G. B. Stringfellow, “Solid phase immiscibility in GaInN,” Appl.Phys. Lett., vol. 69, pp. 2701–2701, 1996.

[39] A. Kobayashi, J. Ohta, and H. Fujioka, “Low temperature epitaxial growthof In0 .25 Ga0 .75 N on lattice-matched ZnO by pulsed laser deposition,” J.Appl. Phys., vol. 99, pp. 123513-1–123513-4, 2006.

[40] Y. Kawai, S. Ohsuka, M. Iwaya, S. Kamiyama, H. Amano, and I. Akasaki,“Thick InGaN growth on several crystal planes of ZnO substrate by met-alorganic vapor phase epitaxy,” Proc. SPIE, vol. 6889, pp. 688904-1–688904-9, 2008.

[41] A. Kobayashi, S. Kawano, Y. Kawaguchi, J. Ohta, and H. Fujioka,“Room temperature epitaxial growth of m-plane GaN on lattice-matchedZnO substrates,” Appl. Phys. Lett., vol. 90, pp. 041908-1–041908-3,2007.

[42] R. Senda, A. Miura, T. Hayakawa, T. Kawashima, D. Iida, T. Nagai,M. Iwaya, S. Kamiyama, H. Amano, and I. Akasaki, “Realization ofhigh-crystalline-quality thick m-plane GaInN film on 6H-SiC substrate byepitaxial lateral overgrowth,” Jpn. J. Appl. Phys., vol. 46, pp. L948–L950,2007.

[43] O. Nam, M. D. Bremser, T. S. Zheleva, and R. F. Davis, “Lateral epitaxyof low defect density GaN layers via organometallic vapor phase epitaxy,”Appl. Phys. Lett., vol. 71, pp. 2638–2640, 1997.

[44] T. Detchprohm, S. Sano, S. Mochizuki, S. Kamiyama, H. Amano, andI. Akasaki, “Growth mechanism and characterization of low-dislocation-density AlGaN single crystals grown on periodically grooved substrates,”Phys. Status Solidi A, vol. 188, pp. 799–802, 2001.

[45] M. Iwaya, A. Miura, R. Senda, T. Nagai, T. Kawashima, D. Iida,S. Kamiyama, H. Amano, and I Akasaki, “Control of stress and crys-talline quality in GaInN films used for green emitters,” J. Cryst. Growth,vol. 310, pp. 4920–4922, 2008.

[46] D. Iida, A. Miura, Y. Okadome, Y. Tsuchiya, T. Kawashima, T. Nagai,M. Iwaya, S. Kamiyama, H. Amano, and I. Akasaki, “One-step lateralgrowth for reduction in defect density of a-plane GaN on r-plane sapphiresubstrate and its application to light emitters,” Phys. Status Solidi A,vol. 204, pp. 2005–2009, 2007.

[47] Q. Li and G. Wang, “Improvement in aligned GaN nanowire growth usingsubmonolayer Ni catalyst films,” Appl. Phys. Lett., vol. 93, pp. 043119-1–043119-3, 2008.

[48] E. Ertekin, P. A. Greaney, D. C. Chrzan, and T. D. Sands, “Equilibriumlimits of coherency in strained nanowire heterostructures,” J. Appl. Phys.,vol. 97, pp. 114325-1–114325-10, 2005.

[49] K. Kishino, A. Kikuchi, H. Sekiguchi, and S. Ishizawa, “InGaN/GaNnanocolumn LEDs emitting from blue to red,” Proc. SPIE, vol. 6473,pp. 64730T-1–64730T-12, 2007.

[50] T. Kuykendall, P. Ulrich, S. Aloni, and P. Yang, “Complete compositiontenability of InGaN nanowires using a combinatorial approach,” Nat.Mater., vol. 6, pp. 951–956, 2007.

[51] F. Quan, S. Gradecak, Y. Li, C.-Y. Wen, and C. M. Lieber, “Core/multishellnanowire heterostructures as multi-color, high-efficiency light-emittingdiodes,” Nano Lett., vol. 5, pp. 2287–2291, 2005.

[52] H.-M. Kim, Y.-H. Cho, H. Lee, S. I. Kim, S. R. Ryu, D. Y. Kim, T. W. Kang,and K. S. Chung, “High-brightness light emitting diodes using dislocation-free indium gallium nitride multiquantum-well nanorod arrays,” NanoLett., vol. 4, pp. 1059–1062, 2004.

[53] W. Goetz, “InGaN-based high-power LEDs: Challenges and applications,”presented at the International Symposium on Blue Laser and Light Emit-ting Diodes (ISBLLED), Montpellier, France, May 2006.

[54] S. F. Chichibu, A. Uedono, T. Onuma, B. A. Haskell, A. Chakraborty,T. Koyama, P. T. Fini, S. Keller, S. P. DenBaars, J. S. Speck, U. K. Mishra,S. Nakamura, S. Yamaguchi, S. Kamiyama, H. Amano, I. Akasaki, J. Han,and T. Sota, “Origin of defect-insensitive emission probability in In-containing (Al, In, Ga)N alloy semiconductors,” Nat. Mater., vol. 5,pp. 810–816, 2006.

[55] S. F. Chichibu, T. Sota, K. Wada, and S. Nakamura, “Exciton localizationat InGaN quantum well devices,” J. Vac. Sci. Technol. B, vol. 16, pp. 2204–2214, 1998.

[56] B. Monemar and B. E. Sernelius, “Defect related issues in the “currentroll-off” in InGaN based light emitting diodes,” Appl. Phys. Lett., vol. 91,pp. 181103-1–181103-1, 2007.

[57] Y. C. Shen, G. O. Mueller, S. Watanabe, N. F. Gardner, A. Munkholm,and M. R. Krames, “Auger recombination in InGaN measured by photo-luminescence,” Appl. Phys. Lett., vol. 91, pp. 141101-1–141101-3, 2007.

[58] N. F. Gardner, G. O. Mueller, Y. C. Shen, G. Chen, S. Watanabe, W. Gotz,and M. R. Krames, “Blue-emitting InGaN-GaN double-heterostructurelight-emitting diodes reaching maximum quantum efficiency above200 A/cm2 ,” Appl. Phys. Lett., vol. 91, pp. 243506-1–243506-3,2007.

[59] M. H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F. Schubert, J. Piprek, andY. Park, “Origin of efficiency droop in GaN-based light-emitting diodes,”Appl. Phys. Lett., vol. 91, pp. 183507-1–183507-3, 2007.

[60] I. V. Rozhansky and D. A. Zakheim, “Analysis of processes limitingquantum efficiency of AlGaInN LEDs at high pumping,” Phys. StatusSolidi A, vol. 204, pp. 227–230, 2007.

[61] J. Hader, J. V. Moloney, B. Pasenow, S. W. Koch, M. Sabathil, N. Linder,and S. Lutgen, “On the importance of radiative and Auger losses in GaN-based quantum wells,” Appl. Phys. Lett., vol. 92, pp. 261103-1–261103-3,2008.

[62] A. David, M. J. Grundmann, J. F. Kaeding, N. F. Gardner, T. G. Mi-hopoulos, and M. R. Krames, “Carrier distribution in (0 0 0 1) InGaN/GaNmultiple quantum well light-emitting diodes,” Appl. Phys. Lett., vol. 92,pp. 053502-1–053502-3, 2008.

[63] J. Xie, X. Ni, Q. Fan, R. Shimada, U. Ozgur, and H. Morkoc, “On theefficiency droop in InGaN multiple quantum well blue light emittingdiodes and its reduction with p-doped quantum well barriers,” Appl.Phys. Lett., vol. 93, pp. 121107-1–121107-3, 2008.

[64] M. F. Schubert, S. Chhajed, J. K. Kim, E. F. Schubert, D. D. Koleske, M.H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, “Effectof dislocation density on efficiency droop in GaInN/GaN light-emittingdiodes,” Appl. Phys. Lett., vol. 91, pp. 23114-1–23114-3, 2007.

[65] M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M. H. Kim, S. Yoon,S. M. Lee, C. Sone, T. Sakong, and Y. Park, “Polarization-matchedGaInN/AlGaInN multi-quantum-well light-emitting diodes with reducedefficiency droop,” Appl. Phys. Lett., vol. 93, pp. 041102-1–041102-3,2008.

[66] M. L. Reed, E. D. Readinger, H. Shen, M. Wraback, A. Syrkin, A. Usikov,O. V. Kovalenkov, and V. A. Dmitriev, “n-InGaN/p-GaN single het-erostructure light emitting diode with p-side down,” Appl. Phys. Lett.,vol. 93, pp. 133505-1–133505-3, 2008.

[67] X. Ni, Q. Fan, R. Shimada, U. Ozgur, and H. Morkoc, “Reduction ofefficiency droop in InGaN light emitting diodes by coupled quantumwells,” Appl. Phys. Lett., vol. 93, pp. 171113-1–171113-3, 2008.

[68] M. C. Schmidt, K.-C. Kim, H. Sato, N. Fellows, H. Masui, S. Nakamura, S.P. DenBaars, and J. S. Speck, “High power and high external efficiency m-plane InGaN light emitting diodes,” Jpn. J. Appl. Phys., vol. 46, pp. L126–L128, 2007.

[69] K. Kim, M. C. Schmidt, H. Sato, F. Wu, N. Fellows, Z. Jia, M. Saito,S. Nakamura, S. P. DenBaars, J. S. Speck, and K. Fujito, “Study of non-polar m-plane InGaN/GaN multiquantum well light emitting diodes byhomoepitaxial metal–organic chemical vapor deposition,” Appl. Phys.Lett, vol. 91, pp. 181120-1–181120-3, 2007.

[70] H. Masui, H. Kroemer, M. C. Schmidt, K.-C. Kim, N. N. Fellows, S. Naka-mura, and S. P. DenBaars, “Electroluminescence efficiency of (1 0 1 0)-


oriented InGaN-based light-emitting diodes at low temperature,” J. Phys.D, vol. 41, pp. 082001-1–082001-5, 2008.

[71] I. Schnitzner, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer,“30% external quantum efficiency from a surface textured, thin-filmlight emitting diode,” Appl. Phys. Lett., vol. 63, pp. 2174–2176,1993.

[72] O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steigerwald,M. O. Holcomb, P. S. Martin, and M. R. Krames, “High performancethin-film flip-chip InGaN–GaN light emitting diodes,” Appl. Phys. Lett.,vol. 89, pp. 071109-1–071109-3, 2006.

[73] V. Haerle, B. Hahn, S. Kaiser, A. Weimar, S. Bader, F. Eberhard, A. Plossl,and D. Eisert, “High brightness LEDs for general lighting applicationsusing the new ThinGaNTM -technology,” Phys. Status Solidi A, vol. 201,pp. 2736–2739, 2004.

[74] R. Wirth, S. Illek, C. Karnutsch, I. Pietzonka, A. Ploessl, P. Strauss,W. Stein, W. Wegleiter, R. Windisch, H. Zull, and K. P. Streubel, “Re-cent progress of AlGaInP thin-film light emitting diodes,” Proc. SPIE,vol. 4996, pp. 1–9, 2003.

[75] E. Yablonovitch, “Inhibited spontaneous emission in solid-state physicsand electronics,” Phys. Rev. Lett., vol. 58, pp. 2059–2062, 1987.

[76] S. Fan, P. R. Villeneuve, J. D. Joannopoulos, and E. F. Schubert, “Highextraction efficiency of spontaneous emission from slabs of photonic crys-tals,” Phys. Rev. Lett., vol. 78, pp. 3294–3297, 1997.

[77] E. M. Purcell, H. C. Torrey, and R. V. Pound, “Spontaneous emissionprobabilities at radio frequencies,” Phys. Rev., vol. 69, pp. 37–38, 1946.

[78] J. J. Wierer, M. R. Krames, J. E. Epler, N. F. Gardner, M. G. Craford, J.R. Wendt, J. A. Simmons, and M. M. Sigalas, “InGaN/GaN quantum-wellheterostructure light-emitting diodes employing photonic crystal struc-tures,” Appl. Phys. Lett., vol. 84, pp. 3885–3887, 2004.

[79] M. Boroditsky, R. Vrijen, T. F. Krauss, R. Coccioli, R. Bhat, andE. Yablonovitch, “Spontaneous emission extraction and Purcell enhance-ment from thin-film 2D photonic crystals,” J. Lightw. Technol., vol. 17,no. 11, pp. 2096–2112, Nov. 1999.

[80] K. Orita, S. Tamura, T. Takizawa, T. Ueda, M. Yuri, S. Takigawa,and D. Ueda, “High-extraction-efficiency blue light-emitting diode usingextended-pitch photonic crystal,” Jpn. J. Appl. Phys., vol. 43, pp. 5809–5813, 2004.

[81] A. David, T. Fujii, R. Sharma, K. McGroddy, S. Nakamura, S. P. DenBaars,E. L. Hu, C. Weisbuch, and H. Benisty, “Photonic-crystal GaN light-emitting diodes with tailored guided modes distribution,” Appl. Phys.Lett, vol. 88, pp. 061124-1–061124-3, 2006.

[82] K. McGroddy, A. David, E. Matioli, M. Iza, S. Nakamura, S. DenBaars,J. S. Speck, C. Weisbuch, and E. L. Hu, “Directional emission control andincreased light extraction in GaN photonic crystal light emitting diodes,”Appl. Phys. Lett., vol. 93, pp. 103502-1–103502-3, 2008.

[83] A. David, T. Fujii, B. Moran, S. Nakamura, S. P. DenBaars, C. Weisbuch,and H. Benisty, “Photonic crystal laser lift-off GaN light-emitting diodes,”Appl. Phys. Lett., vol. 88, pp. 133514-1–133514-3, 2006.

[84] A. David, B. Moran, K. McGroddy, E. Matioli, E. L. Hu, S. P. Den-Baars, S. Nakamura, and C. Weisbuch, “GaN/InGaN light emitting diodeswith embedded photonic crystal obtained by lateral epitaxial overgrowth,”Appl. Phys. Lett., vol. 92, pp. 113514-1–113514-3, 2008.

[85] M. K. Kwon, J.-Y. Kim, I.-K. Park, K. S. Kim, G.-Y. Jung, S.-J. Park, J.W. Kim, and Y. C. Kim, “Enhanced emission efficiency of GaN/InGaNmultiple quantum well light-emitting diode with an embedded photoniccrystal,” Appl. Phys. Lett., vol. 92, pp. 251110-1–251110-3, 2008.

[86] [Online]. Available: http://www.luminus.com[87] J. G. Fleming and S. Y. Lin, “3D-silicon photonic-lattices—Cornerstone

of an emerging photonics revolution,” Proc. SPIE, vol. 3877, pp. 9–18,1999.

[88] J. Vuckovic, M. Loncar, and A. Scherer, “Surface plasmon enhanced light-emitting diode,” IEEE J. Quantum Electron., vol. 36, no. 10, pp. 1131–1144, Oct. 2000.

[89] A. Kock, E. Gornik, M. Hauser, and W. Beinstingl, “Strongly directionalemission from AlGaAs/GaAs light-emitting diodes,” Appl. Phys. Lett.,vol. 57, pp. 2327–2329, 1990.

[90] G. Sun, J. B. Khurgin, and R. A. Soref, “Practicable enhancement ofspontaneous emission using surface plasmons,” Appl. Phys. Lett., vol. 90,pp. 111107-1–111107-2, 2007.

[91] I. Gontijo, M. Boroditsky, E. Yablonovitch, S. Keller, U. K. Mishra, andS. P. DenBaars, “Coupling of InGaN quantum-well photoluminescence tosilver surface plasmons,” Phys. Rev. B, vol. 60, pp. 11564–11567, 1999.

[92] A. Neogi, C.-W. Lee, H. O. Everitt, T. Kuroda, A. Tackeuchi, andE. Yablonovitch, “Enhancement of spontaneous recombination rate ina quantum well by resonant surface plasmon coupling,” Phys. Rev. B,vol. 66, pp. 153305-1–153305-4, 2002.

[93] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer,“Surface-plasmon-enhanced light emitters based on InGaN quantumwells,” Nat. Mater., vol. 3, pp. 601–605, 2004.

[94] K. Okamoto, I. Niki, A. Shvartser, G. Maltezos, Y. Narukawa, T. Mukai,K. Nishizuka, Y. Kawakami, and A. Scherer, “Surface plasmon enhancedInGaN light emitter,” Proc. SPIE, vol. 5733, pp. 94–102, 2005.

[95] K. Okamoto, I. Niki, A. Scherer, Y. Narukawa, T. Mukai, andY. Kawakami, “Surface plasmon enhanced spontaneous emission rate ofInGaN/GaN quantum wells probed by time-resolved photoluminescencespectroscopy,” Appl. Phys. Lett., vol. 87, pp. 071102-1–071102-3, 2005.

[96] D.-M. Yeh, C.-F. Huang, C.-Y. Chen, Y.-C. Lu, and C. C. Yang, “Sur-face plasmon coupling effect in an InGaN/GN single-quantum-well light-emitting diode,” Appl. Phys. Lett., vol. 91, pp. 171103-1–171103-3, 2007.

[97] D.-M. Yeh, C.-F. Huang, C.-Y. Chen, Y.-C. Lu, and C. C. Yang, “Localizedsurface plasmon-induced emission enhancement of a green light-emittingdiode,” Nanotechnology, vol. 19, pp. 345201-1–345201-4, 2008.

Mary H. Crawford (M’05) received the B.A. degreefrom Holy Cross College, Worcester, MA, in 1985,and the M.S. and Ph.D. degrees from Brown Univer-sity, Providence, RI, in 1987 and 1993, respectively,all in physics. Her Ph.D. research involved gain spec-troscopy and device physics of II–VI semiconductorlaser diodes and LEDs.

In 1993, she joined Sandia National Laboratories,Albuquerque, NM, where she is currently a Distin-guished Member of Technical Staff in the Semicon-ductor Material and Device Sciences Department.

Her work at Sandia has involved the development of vertical-cavity surface-emitting lasers (VCSELs), including AlInGaP-based red VCSELs, and the studyof wide-bandgap nitride materials for deep UV and visible LED applications.From 2000–2002, she was a Senior Scientist and the Director of Researchand Development at Uniroyal Optoelectronics, a start-up company developingInGaN and AlInGaP LEDs for a range of applications. Her current research in-terests include optical spectroscopy of wide-bandgap semiconductor materialsand the development of novel semiconductor light-emitting devices.

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