IEEETRANSACTIONS ON INDUSTRY APPLICATIONS, VOL.50,NO.2...

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 2, MARCH/APRIL 2014 1469

Rare-Earth-Free Direct-Emitting Light-EmittingDiodes for Solid-State Lighting

Christian Wetzel, Senior Member, IEEE, and Theeradetch Detchprohm

Abstract—The advent of a wide-bandgap GaN p-n junctionhas enabled highly efficient blue light-emitting diodes (LEDs)in GaInN/GaN heteroepitaxy. The system also enables a muchwider range of emission wavelengths. We summarize progress inepitaxial materials development of green, yellow, and orange di-rect-emitting LEDs that bypass the steps of external phosphorconversion to achieve higher stability, efficiency, and higher overallcolor rendering quality for wider adoption of one of the widestpower savings resources so far identified.

Index Terms—LED lamps, light emitting diodes, semiconductordiodes, semiconductor epitaxial layers, semiconductor growth,wide band gap semiconductors, III-V semiconductor materials.

I. INTRODUCTION

SUNLIGHT means vegetation, which means life, and ourlanguage full of allegories to light reflects it. Artificial

light brings safety from intruders throughout human evolution.Today, artificial light allows education after sunset when theday’s work is done, and more than ever, education seems thebest handle to stabilize the Earth’s human population andthe depletion of resources that goes along with it. Consequently,the availability of artificial lighting is a prime factor in humancultural and economical development.

Existing lighting technology is also a major sink for primaryenergy generation worldwide [1]. Within the U.S., some 20%is consumed in lighting, and numbers are much higher forcountries catching up economically. The single most potentmeasure to cut primary energy consumption indeed is solid-state lighting. Its full implementation allows that fraction to becut into half. This amounts to gigantic carbon dioxide savingsworldwide, and its realization is within reach.

Manuscript received June 13, 2013; revised August 14, 2013; acceptedAugust 14, 2013. Date of publication August 22, 2013; date of current versionMarch 17, 2014. Paper 2012-ILDC-711.R1, presented at the 2012 InternationalSymposium on the Science and Technology of Lighting, Troy, NY, June 24–29,and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY

APPLICATIONS by the Industrial Lighting and Display Committee of the IEEEIndustry Applications Society. This work was supported in part by the Na-tional Science Foundation Smart Lighting Engineering Research Center (EEC-0812056) and in part by a Department of Energy/National Energy TechnologyLaboratory (DOE/NETL) Solid-State Lighting Contract of Directed Researchunder Grant DE-EE0000627.

C. Wetzel is with Rensselaer Polytechnic Institute, Troy, NY 12180 USA(e-mail: [email protected]).

T. Detchprohm was with Rensselaer Polytechnic Institute, Troy, NY 12180USA. He is now with the School of Electrical and Computer Engineering,Georgia Institute of Technology, Atlanta, GA 30332 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/TIA.2013.2279192

Fig. 1. Schematic. (a) Blue LED combined with a yellow and red in compar-ison to the midday solar spectrum. (b) Corresponding color gamut. (c) Colorcombination approach of four direct-emitting LEDs. (d) Corresponding colorgamut. (e) Eye sensitivity curves for cone vision.

Fig. 2. (a) Conventional polar c-plane LEDs widely change color with drivecurrent. (b) LEDs developed in nonpolar crystal growth remain highly stable incolor with current change (color schematic).

The quasi-continuum of natural sunlight may be the overallreference for lighting [see Figs. 1(a) and 2]. Throughout theday, weather, and season, there is cyclical and random vari-ation in intensity and spectral distribution as path length and

0093-9994 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

1470 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 2, MARCH/APRIL 2014

atmospheric conditions modulated the spectral flux. Then, overthe last 130 years, the very broad and continuous spectrum ofa heated tungsten filament, tunable only along the blackbodytemperature locus, has grown familiar to the consumer inincandescent lamps.

The primary approach to energy efficiency in lighting is toreduce the emission spectrum of the source to the limits of thehuman vision (400–∼680 nm) [see Fig. 1(e)] [2], [3]. Some95% of the power spectrum of the incandescent lamp lies in theinfrared where it does not aid the human vision at all. To cut thisBoltzmann tail of thermal distribution, narrow-band emittersare being sought.

Then, there are mercury and sodium vapor lamps with aset of discrete narrow spectral lines (Δλ ∼ 1−2 nm), latervaried and broadened to bands in high pressure and metal halidelamps (Δλ ∼ 10−20 nm). Their high efficiency has madethem the standard in large-area illumination. The human brainseems very generous in labeling as white whatever baseline itperceives. The impression of contrasted object colors whosewavelength is not included in the light source, however, willvastly differ from those seen in regular sunlight. The currentbest explanation is a poor overlap of the source spectrum withthe spectral sensitivity of the various eye receptors. Due to suchpoor color rendering, the consumer generally does not acceptsuch lamps in any social setting.

Therefore, the extent to which the offered spectrum canbe compromised between the extremes of full-spectrum in-candescent and narrow-band metal halide band lamps is upto a lively negotiation between science, technology, and theconsumer. The spectroscopic analysis is easy, and the photo-metric analysis is possible; the physiological impact, however,is difficult, costly, and time consuming to assess. This perceiveduncertainty leaves ample room for product diversification andcontinuous lighting quality improvement.

For spectral linewidths in the Δλ = 20− 50 nm range,semiconductor light-emitting diodes (LEDs) are the preferredemitter. Opting for narrower lines (Δλ < 1 nm), semicon-ductor laser diodes are available, while for broader spectra(Δλ ∼ 100 nm), phosphors of variable strong photon-latticecoupling are common. What is crucial, however, is the rangeof wavelengths covered by either technologies to accommodatethe lighting consumer’s dictate.

Laser diodes are well known for their highly directed emis-sion pattern, yet due to their rather complex device structures,they are currently not deemed cost effective for the high volumeproduction demanded for general lighting. Phosphors, on theother hand, achieve their broad emission spectrum by theinternal down conversion of higher energy photons, wherebythe unwanted differential photon energy is deposited as low-temperature heat.

The development of the wide-bandgap semiconductors, mostprominently GaN [4], [5], has led to a highly stable highlyefficient p-n junction for high-flux electro-optical energy con-version. In principle, the metallurgical alloying of GaN withInN into Ga1−xInxN allows the variation of the emission wave-length all across the visible spectrum. However, the highestefficiency has so far been limited to the 440–460-nm spectralregion in the blue. [2] With development effort, LED perfor-

mance over a wider range of emission wavelengths into thenear UV (260–400 nm) and the green (500–540 nm) spectralranges has improved tremendously [6]. This, to the most part,however, comes from collateral benefits in the developmentof blue LEDs, i.e., by the shared benefit of improved epitaxyequipment and refined techniques of electrical contact and lightextraction.

Therefore, solid-state lighting’s current first generation ap-proach for white-light LEDs relies on 450-nm narrow-bandblue GaInN/GaN LEDs in combination with a phosphor op-tically excited by the same to fill in the spectrum with lowerenergy photons [see Fig. 1(a)] [7]. The concept resembles thatof fluorescent lighting where narrow-band UV light is down-converted in a phosphor to a broad emission of visible light.[For a schematic of the achieved color gamut, see Fig. 1(b).]

One shortcoming of this approach is any variation of thephosphor and its direct comparison to the blue excitationsource will strongly affect the perceived color performanceof the resulting lamp. Such variations are further worsenedby environmental conditions of moisture or temperature aswell as photon flux and operation age. In particular, in directcomparison between different LED lamps, such nonuniformityvariations will displease the critical end user.

Moreover, lighting users have become critical since the heav-ily promoted compact fluorescent lamps have left more than aspurious fraction of consumers dissatisfied.

An improvement to this behavior is expected from an in-visible UV excitation LED combined with broader spectrumphosphors so that no direct visual comparison to the excitationsource is made.

The photon energy conversion within the phosphor mostnaturally is a lossy process. This holds even if a 100% quantumefficiency could be maintained. This simply lies in the factthat the quantized photon energy Eout of an emitted photon(e.g., λout = 550 nm) is lower than that of the excitationphoton Ein(λin = 450 nm). Therefore, dependent on the actualquantum efficiency of the phosphor, a minimum Stokes conver-sion loss of (Ein − Eout)/Ein = 1− λin/λout = 18% is lostas heat in the phosphor.

The resulting heating of the phosphor conversion LED lightsource has at least two aspects. First, it is an indication ofwasted energy, and second, most aspects of LED efficiency turnworse with an increase of the operation temperature. Since allknown phosphors’ efficiency is higher at lower lattice tempera-ture, such heat has to be extracted by thermal conduction. Dueto the low temperature differential to the environment, coolingby radiation, as known for incandescent lamps, is not a relevantprocess here. Therefore, tremendous effort has been paid forpassive—and, in some cases, active—cooling elements as partof the LED lamp’s casing.

A second important shortcoming in the use of phosphors liesin their primary dependence on rare earth elements. Similarto other clean technologies, the solid-state lighting of the firstgeneration therefore becomes critically dependent on those rawmaterials, primarily mined in China under highly polluting con-ditions [8]. Other approaches of nanocrystal phosphor materialsmostly use alloys of toxic elements such as Cd. The high useof rare earth phosphors in fluorescent tubes has only partially

WETZEL AND DETCHPROHM: RARE-EARTH-FREE DIRECT-EMITTING LEDs FOR SOLID-STATE LIGHTING 1471

been reduced in the white-light LEDs of the first generation. Inparallel, due to high demand, the cost of rare earth elements hasshot up rapidly in recent times.

In the second generation of solid-state lighting, the goal mustbe to bypass the usage of rare earth element phosphors. Infact, bypass the use of phosphors altogether, and so avoid theinherently lossy mechanism of photon Stokes shifting betweenoptical excitation and emission wavelengths.

Full-spectrum white LEDs should therefore be achieved bya combination of LEDs of different emission wavelengths tocompose a spectrum that could approximate the solar spectrumor incandescent lamp to an arbitrary fine detail [see Fig. 1(c)].Naturally, the winning solution will be a competitive compro-mise of spectral portions delivered by individual LEDs, theresulting color fidelity [see exemplary schematic in Fig. 1(d)],and the technical complexity, e.g., the supply of various biasvoltages.

One example could be the combination of four LEDs, cov-ering blue, green, amber, and red [see Fig. 1(c)]. A case forsuch an approach, but utilizing very narrow band emitters, hasbeen made by the group of J. Tsao of Sandia [9]. Examplesof using an even wider range of LEDs have been given byPaolini [10].

A chosen spectral composition could be impinged by individ-ual power controls or readily built in by serial wiring schemesof dies of various sizes and relative powers. Such compositedirect-emitting white LEDs could not be driven in parallel froma shared power supply or controller. This is in contrast to single-type LED excitation with phosphor conversion. Therefore, theneed for multiple drivers may add cost, but the additional colortunability would be a desirable aspect of product differenti-ation that could justify the extra expense. Moreover, in thelong run, the stabilized serial connection of proper balancedLEDs for higher voltage operation is likely to obsolete theproblem.

The R&D goal on the materials and device level there-fore must be to drive any wavelength of the visible spectrumwith only a minimum of overvoltage, a feat typical for well-developed LEDs.

With today’s AlGaInP technology, the spectral regions ofamber (580–595 nm) and red (600–650 nm) can be generated indirect-emitting LEDs, albeit only under significant compromisein efficiency as well as in stability and controllability. Eventhe most advanced AlGaInP LEDs, with high efficiency inthe red, suffer widely reduced efficiency when expanded toshort wavelength amber and yellow [2]. This is due to theunavailability of a higher barrier bandgap compound that couldconfine carriers to the quantum well recombination areas. Theensuing spillover of carriers leads to a rapid drop in efficiencywith higher carrier and device temperature. This variably issubstantially different from the behavior of the GaN/GaInN-based 450-nm blue LED and therefore a poor match for thehigher performing AlGaInN group-III nitride competitor.

The opportunity to tune the Ga1−xInxN alloys’ emission[11], [12] across the entire visible spectrum is therefore amost natural choice to expand the market reach of the widelydeployed workhorse of all solid-state lighting, the existingGa1−xInxN/GaN blue LED. All that is needed is a continued

research and development effort in a distinct but related sidebranch of current blue LED development.

II. DIRECT-EMITTING LEDs

The approach of direct-emitting LEDs therefore alleviatesthe need for any rare earth element or Cd-containing phos-phors by developing efficient longer wavelength green, amber,and red LED dies. Those, when complemented with existingblue LEDs, form a second-generation full-spectrum solid-statewhite-light source of unprecedented efficiency, low cost poten-tial, and color quality.

What follows is an account of major challenges and spe-cific opportunities in the wide-bandgap semiconductor crystalgrowth to achieve those envisioned all-spectrum direct-emittingLEDs in high performance.

The crystal growth of the GaN p-n junction is most efficientlyperformed in metal organic vapor phase epitaxy (MOVPE)at temperature above 1000 ◦C [5]. The active layers containquantum wells of Ga1−xInxN/GaN that, by tuning the alloybandgap, primarily define the peak emission wavelength andspectrum. To reach the long wavelength red portion of thespectrum, high alloy fractions x of InN need to be incorporated.For InN in turn, molecular beam epitaxy at 450 ◦C achieves thebest results. The formation of the Ga1−xInxN alloy thereforeis a compromise between both growth conditions. In addition,in thermodynamical equilibrium, a separation of the alloy intoGaN-rich and InN-rich phases is expected [13]. The challengein advanced heteroepitaxy therefore lies in the stabilizationof an alloy composition against the driving forces of thermo-dynamic decomposition. It is for that reason that advancedheteroepitaxy always thrives to grow layers far from suchequilibria.

A particular aspect of the group-III nitrides is its wurtzitecrystal structure that causes significant distinction betweenepitaxial growth conditions along the different crystallographicdirections and resulting layered device properties. [14] Forexample, epitaxy along its c-axis, as most commonly used, willinduce huge piezoelectric polarization charges and associatedlarge electric fields at every heterointerface [15]. By the sametoken, structural line defects, once initiated by large latticemismatch to common sapphire or Si substrates, will extendalong this growth direction, typically through the entire devicestructure. The symmetries of the material make it very unlikelythat they could be bent away by any means from reachingthe relevant active layers of the device where they deteriorateperformance.

Remarkable performance of GaN-based LEDs has beenachieved, particularly in the blue spectral region [5], whereonly low InN fractions are needed, despite the presence of highdensities of substrate-induced threading dislocations. However,our research into longer wavelength green and yellow emittingGaInN/GaN LEDs has always found a drastic decrease inperformance directly associated with the occurrence of suchline defects in their various forms [6]. Our development efforttherefore has concentrated on epitaxial means to continuouslyreduce the number of such defects that would reach the activequantum well region.


A. Typical Construction of a LED

The vast majority of GaN-based LEDs are prepared in epi-taxial wafer scale thin film processes on dissimilar substrate ofsapphire (0001), Si (111), or 6H-SiC (0001). The initiation ofthe wurtzite GaN layer growth therefore requires advanced lat-tice mismatch accommodation techniques, the most importantof which is the low-temperature deposition of a typically 30-nmthin GaN or AlN buffer layer [5] combined with a subsequentdelicately tuned growth recovery sequence to then form ann-type GaN:Si doped buried contact layer. This layer maylater either be contacted from the top growth surface in lateraldevice geometry or exposed from the substrate side in verticaldevice geometries after substrate removal. The total epitaxiallayer stack at that stage may be of the order of 3–4 μm.Thereafter, a series of 3 to 10 quantum wells of GaInN alloyembedded in GaN barriers with or without any doping is grown.The proper sequence and procedures for the growth of theselayers are of utmost impact on the final product performance,and various vendors prefer different optimization proceduresto achieve their resulting product specifications. The stack ofquantum wells is mostly concluded by a higher gap AlGaNlayer that should prevent electrons from direct injection intothe subsequent p-type layers. This layer may be of the orderof 30 nm and is mostly Mg doped. The p-type contact layerthen is mostly performed in GaN:Mg and may reach a thick-ness of some 200 nm before either transparent or reflectivecontact patterns are applied. For light extraction enhancement,all surface and even interfaces between the substrate and epi-taxial layer may be patterned or roughened, either during orafter epitaxy.

B. LEDs on Bulk GaN

Of particular appeal in this context is the use of bulk GaNsubstrate, as available in limited volume by various processes.Here, dislocation densities had been reduced to levels in themid 106 cm−2 [16], some two to three orders of magnitudebelow the common sapphire-based approaches. While thesereductions immediately transformed to performance gains [17]in the blue and near UV wavelength regions, no significantimprovement could be observed within the 500–540 nm greenspectral region [18]. In fact, from detailed structural defectcharacterization, we observed the generation of new defectsright within the highly strained high InN fraction quantumwells—even without the need of any apparent precursor de-fect [19]. To fulfill the defect geometry requirements, suchdislocations appeared in pairs of opposing Burgers vectors andpropagated in a highly regular V-shaped pattern throughout thefurther grown layers. Such inclination of the defects, by some20◦, came as a big surprise but could be well described byperiodic jumps of the dislocation core in every third atomiclayer. Despite the low defect density of the substrate, thesenew-formed defects reach densities into mid 109 cm−2 [19],the same values achieved on common sapphire substrate. Theinclination of the defect is explained by a progressive butgradual lattice relaxation as the primary driving force for thedefect generation.

C. Non c-Plane LEDs

The availability of such bulk GaN substrates also offered theopportunity to prepare other crystal orientations as substrate forepitaxial overgrowth [16]. Here, in particular, the preparationof c-axis grown GaN bulk templates affords the opportunity tocontinue growth in those new directions, leaving all threadingdislocations behind. At the same time, growing any heterostruc-tures along a direction perpendicular to the polar c-axis avoidsall polarization effects along the growth direction [20]. Inthis way, nonpolar a-axis [21] and m-axis [22] grown het-erostructures can be achieved without any dislocation defectsdiscernible in cross-sectional transmission electron microscopy(TEM) (equivalent to densities < 106 cm−2).

D. Wavelength-Stable Green LEDs

The absence of piezoelectric polarization comes with impor-tant advantages and some additional challenges. One of thebenefits is that LED emission wavelength remains extremelystable under a wide variation of current and, therefore, lightoutput power. This essentially became the proof for the piezo-electric polarization dominating the electronic band lineup ofthe common c-plane structures. [23] It is the interaction of thestatic polarization charges with the injected carriers that wouldshift the wavelength around in polar structures. A side effect isthat, for increasing quantum well width, the radiative transitionprobability would rapidly decrease since electrons and holesmove in opposite directions in the static electric field [15].The same does not hold for the nonpolar structures, essentiallyallowing, in principle, rather wide quantum wells to reduce therole of interface effects.

In our approaches on homoepitaxial growth on squaremillimeter-sized nonpolar side planes of bulk GaN, green LEDshave been achieved. However, light output power in the sub-milliwatt range widely trails performances achieved in conven-tional c-plane growth. The extremely limited amount of totalavailable substrate area amounting to far less than a single 2-inc-plane wafer on sapphire attests to a very limited opportunityto properly dial in dopant flows and orientation-specific growthparameters. Also, in these cases of nonpolar m- and a-axisgrowth, a significant light output power drop with longer wave-length always correlated with a large increase in the density ofline defects as seen in cross-sectional TEM [22]. These defectsspecifically start in the quantum wells themselves, suggestingthe need for the design of better interfaces between GaN barri-ers and GaInN quantum well layers. Such specific optimizationhas so far not been possible due to the very limited availabilityof corresponding substrate material. Progress in this directionis expected once bulk GaN processes mature to provide largerarea nonpolar and semipolar GaN substrates.

The highly advantageous aspect of wavelength-stable emis-sion in the green will justify such work. We expect that thebenefit can be extended to even longer wavelengths.

E. Polarized LEDs

LED structures of such orientation furthermore emit pri-marily linear polarized light from their top surface, a direct


Fig. 3. Linear polarized cyan LED prepared in GaInN/GaN homoepitaxy on m-plane bulk GaN. Due to the valence band symmetry, the top emission spectrumis vastly dominated by light polarized along the a-axis of the crystal.

Fig. 4. Polarization contrast analysis of individual elongated 25-nm Auparticles as seen in the light of the polarized LED. Under variation of theanalyzer angle, the preferential orientation becomes apparent (interferometricreflectance imaging sensing).

consequence of the uniaxial nature of the GaN material. Thisparticularly lends itself to any application with liquid crystaldisplay modulators, which can accept only linear polarizedlight. Therefore, LEDs that do not waste any excitation into apolarization state of light that cannot be used in the modulatorcan enhance system efficiency up to a factor of two. Addi-tional efficiency gains are realized by the reduction of the heatload in the consequence. A well-characterized linear polarizedGaInN/GaN LED emitting in the cyan spectral range is shownin Fig. 3 [24].

Polarized light sources, in general, are also useful in particledetection spectroscopy where, e.g., the shape of tiny nanopar-ticles like viruses needs to be analyzed. Our LEDs have suc-cessfully been employed in interferometric reflectance imagingsensing [25]. As a model for viruses, 35-nm Au particles areanalyzed under the polarized light of the LED. The ellipticityand orientation can be identified from the imaging contrast (seeFig. 4).

F. Nanopatterned Epitaxy

The strict correlation of light output power with the densityof threading dislocations in the active region exemplifies thelingering challenges in heteroepitaxy.

Patterning of the LED material’s outer surfaces has primarilybeen pursued to enhance the light extraction efficiency [26].By introducing surfaces that avoid total internal reflection, lightoutput power typically is enhanced over twofold. This pattern-ing may be performed on length scales above the wavelengthof light and can well be described and simulated in ray tracingoptics. Patterning on length scales below that limit rather needsdescription by light scattering and requires simulation basedon the full wave propagation equations. From the intent ofenhanced light extraction, a patterning of an interface internalto the LED does not seem intuitive. In such a case, the aspectsof scattering and beam redirection would only apply to theportion of light that cannot be extracted upon first incidenceto the surface. While that portion is intentionally high in laserdevices, it is beneficial to keep that portion low in LEDs whereno coherent interference needs to be established.

Nevertheless, with the intention of enhancing light extrac-tion from the outer surfaces, approaches were developed ofpatterning the sapphire substrate before the epitaxial growthof the GaN-based LED structures [27], [28]. Various worksreport on the successful enhancement of light output power,e.g., lens-shaped patterns of several micrometers in size. Thisled to the introduction of micropatterned sapphire substrate.Some works also noticed a crystalline defect reduction. Suchpatterned substrate was found to induce a higher rate of defectbending and annihilation than standard planar substrates [29].

The patterning of the substrate with the goal of reducingthreading dislocation defect densities had also been tried beforewith lateral epitaxial overgrowth [30]. In such approaches, nu-cleation on vertical pattern surfaces would induce GaN growthfronts to progress primarily within the substrate plane beforecoalescence and subsequent vertical progression.

Indeed, regions first induced in such lateral growth typicallyshow significantly reduced dislocation density within the activeregion layers compared to the window areas where only verticalgrowth had occurred. This benefit, on the other hand, came atthe cost of additional new defect formation in the imperfectcoalescence of laterally progressed growth fronts. In fact, fre-quently, a slight tilt of opposing growth fronts was observed


Fig. 5. Interface of GaN epitaxially grown on nanopatterned sapphire in cross-sectional TEM. The virtual absence of threading dislocations generated in theinterface is evidence of the tremendous potential of this heteroepitaxy approach(after [32]).

that would disallow a perfect coalescence [31]. Overall, theapproach has proven most beneficial if the device mesa areacould be fit in and aligned to the good portions of the laterallyovergrown portions. Due to its complexity, the process is pri-marily being applied to laser diodes only.

All these problems, however, seem to disappear in ap-proaches where such substrate patterning is performed on thenanometer length scale, i.e., at dimensions below 1 μm. Wefound superb defect reduction in c-plane sapphire substratethat had been etched into a hexagonal arrangement of circularcolumns 200 nm in diameter [32]. The pattern was convenientlycreated in large-area nanoimprint lithography and subsequen-tially transferred into the substrate by reactive ion etching.

A TEM analysis in cross-sectional view (see Fig. 5) re-veals strikingly fewer threading dislocations when compared tostandard direct low-temperature buffer-layer-induced epitaxy.In panning across wider areas of the material, some bunchingof threading dislocations above the patterns is seen. Overall,however, there are very wide areas without any discerniblevertically propagating defects. In some locations, lateral defectsare seen. These most likely are stacking faults that do notpropagate to the higher layers of the structure where the activeregion is located. Some open areas within the depth of thepattern suggest a lateral growth initiation from the sides ofthe columns to dominate over vertical growth initially. Mostimportantly, the convergence of opposing lateral growth frontsseems to induce no discernible defects. In particular, unlike thecase of patterning on the above-micrometer length scale, novertically propagating defects are observed. Accounting for thespecimen thickness, threading dislocation densities of the orderof 106 cm−2 are estimated for these lower portions of the filmstructure.

Green LEDs developed on the basis of such nanopatternedepitaxy exhibit substantial performance gains as seen in quan-tum efficiencies and light output power (see Fig. 6). By ourfurther analysis, about half of the enhancement could eachbe attributed to enhanced light generation and enhanced lightextraction, respectively, both being direct consequences of thenanopatterning on the substrate side interface.

The apparent rapid recovery of the GaN layer quality withina very thin layer should allow a reduction of the overall grown

LED layer thicknesses. This would allow the implementation ofnew device concepts and also translate to major cost reductions.In particular, the opportunity arises to expand the concept to awider range of low-cost substrates that have previously beendismissed due to larger lattice mismatch with the target GaNlattice.

Our recent results further show that such nanopatterning alsobenefits the growth on nonpolar planes and should thereforecatalyze the further extension to longer emission wavelengthat progressively increasing efficiency.

G. Reaching From Green to Yellow and Amber LEDs

The possibility of GaInN-based LEDs across the visiblespectrum has long been shown. However, to achieve relevantlight output powers and efficiency is subject to continuousdevelopment. Combining our approaches of continuous defectreductions has provided us significant progress in this endeavor,e.g., extension from the green into the yellow and amber spec-tral regions (see Fig. 7, insets) at increasing efficiency. Our tech-nology benefits from the continuously evolving blue GaInN/GaN LED processes yet requires distinctive special develop-ment efforts. Therefore, we expect that, once fully developed,it should be transferable to the existing and highly deployedMOVPE-based nitride epitaxy and LED fabrication technolo-gies without major retooling.

Data of sapphire-based c-plane LEDs are summarized inFig. 7. The internal quantum efficiency (IQE) is typicallyassessed by monitoring the temperature dependence of photo-luminescence efficiencies. For comparison to LED operationconditions, the photoexcitation density at 405 nm was variedover a wide range at room temperature. At such wavelength,excitation occurs directly in the quantum wells without priorabsorption in the GaN layers. In order to identify the highestpossible reference signal, the same was also performed at lowtemperature of 4 K. The resulting maximum IQE data as afunction of excitation power density at room temperature isshown in Fig. 4(a). Values for yellow (560 nm) reach as highas 49% and drop to 43% at an excitation level corresponding toa current density of 35 A/cm2. Values for the amber (590 nm)material reach 34% in its maximum which coincides with theexcitation level of a current density of 35 A/cm2.

These very high values evidence the excellent performanceof these materials as a rare earth-free Cd-free down-conversionelement for the combination with near UV or blue LEDexcitation.

In striking contrast to other approaches involving down-conversion phosphor elements, this use of the GaInN alloysystem provides important system integration advantages. Pre-pared in the same materials, it can inherently be integrated tothe LED source, closer than any other material, and therebyshare a continuity in refractive index, thermal expansion, anddurability aspects.

However, most importantly, the system furthermore holds thepromise of direct electric injection. While our initial experi-ments yield light output powers around 1 mW at 13 A/cm2,the performance should significantly improve with a dedi-cated effort in advancing carrier injection schemes. These


Fig. 6. Results for green LEDs on nanopatterned substrate (red) in comparison to un-patterned reference LEDs (black): (a) IQE as a function of optical excitationpower density. (b) Partial external quantum efficiency as a function of current density. (c) Partial light output Power (after [32]).

Fig. 7. (a) IQE as a function of photoexcitation density derived in the low-temperature photoluminescence (PL) method. Very high values are observed for both560-nm yellow and 590-nm orange LED structures on c-plane sapphire. For the first, we find an upper limit of IQE with 43% at 35 A/cm2, and for the latter,a value of 34% was found. The excitation levels roughly equivalent to current densities of 13 and 35 A/cm2 are indicated. The insets show the respective LEDwafers under electroluminescence. (b) Light output power performance of 590-nm amber LED versus drive current at stage temperatures of 25 ◦C and 100 ◦C.Power drops to as much as 78% of its initial value. This demonstrates the superior performance of the GaN-based approach.

extraordinary numbers under photon excitation provide evi-dence of the high promise of the approach.

H. Extending Full-Spectrum Performance

The achievement of the wavelength efficiency and power en-hancement naturally is a combination of numerous approachesand aspects: high In incorporation, defect reduction, polariza-tion control, wafer uniformity, carrier injection, light extraction,device fabrication, packaging, and so on.

Playing to the strengths of our team, we paid particularemphasis on enhancing the light generation efficiency while rel-egating light extraction to other experts. This separation allowsfor a particular well-defined interface of materials transfer andperformance assessment, namely, that of the plain device wafersurface. Under such premise, approaches of nanopatterningand polarization control under polar, nonpolar, and semipolarcrystal growth in homoepitaxy in comparison with sapphire-based polar c-plane growth were explored and advanced. Eitherof those approaches led to specific advantages and performanceenhancements that, to a large extent, could also be combinedinto a unified overall enhanced technology.

So far, we have not been able to combine all approachesinto a single structure. This primarily reflects the practicallimitations of throughput in university-based research. It alsoreflects the fact that widely disparate optimization stages, e.g.,large-area low-cost substrate approaches, have an inherent de-

velopment advantage over small-area high-cost substrates thatdo not reflect a fundamental limitation per se.

As a result, we can present here a direct comparison of therelative performance of various technologies. To standardizethe comparison, all data corresponds to wafer-level measure-ments at injection current density of 13 A/cm2 as measuredfrom the substrate side into a 1-cm-diameter detector.

Wafer-level LED performance achieved in sapphire-basedc-plane growth is summarized in Fig. 8. Open red trianglesrepresent results before the avoidance of V-defect generationwas achieved, while full blue and orange circles show resultswith the benefit of these improvements. Dashed lines guide theeye to different development stages with the latest generationdiscussed above (orange dots) indicating a power of 1 mW at580-nm yellow that reaches 1/6 of that achieved at 500 nm.

Fig. 8(b) provides a direct comparison of light output powerachieved in polar, nonpolar a-plane, nonpolar m-plane, andsemipolar growth orientations versus peak wavelength. Com-mon to all is the reduction of efficiency with the extension of thewavelength. Not surprisingly, the highest performance is stillbeing achieved in polar c-plane growth. However, the rate ofrelative drop varies substantially. Per our analysis, this directlycorrelates with the rate of new structural defect generation inthe various growth orientations. Most interestingly, the semipo-lar series show the least wavelength-related reduction and maypredestine such growth orientation for higher gains in the longwavelength range with further development.


Fig. 8. Bare wafer light output power as measured through the substrate sides versus dominant wavelength. (a) Polar c-plane LED structures. Open triangles showwafer-typical values for epitaxy approaches resulting in V-defect generation. Full dots represent approaches avoiding V-defects. Orange circles refer to yellowand amber structures discussed here. (b) Equivalent comparison with LEDs obtained in various nonpolar and semipolar crystal growth as well as nanopatternedgrowth as achieved in the authors’ group. Note the wide discrepancies in growth optimization iterations between the different substrate sizes.

III. CONCLUSION

The development of wide-bandgap p-n junction has enabledelectronics to reach the realms of power electronics—the powerof light output in GaN-based p-n junctions that challenges allhitherto conventional light sources and that of electric/electricpower conversion and reforming as a next step. The abilityof color tuning the light source by semiconductor alloying inGaInN has added product diversification by fundamental lineextension progressively across the entire visible spectral region.By devising a multitude of approaches for superior defectreduction by means of homoepitaxy, nanopattern templating,and quantitative polarization control, ever higher performanceat emission wavelengths reaching from the blue beyond thegreen and into the yellow and amber spectral regions havebecome possible. The combination of these approaches bearthe promise to enable full-spectrum white-light direct-emittingLEDs without the need of any lossy rare earth element ortoxic external phosphor elements. Our approach thereby heedsto the promise of higher energy efficiency at higher colorrendering performance that ultimately benefits society by pleas-ing light suitable for a wide range of social and occupationfunctions.

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Christian Wetzel (SM’07) received the Diplom-Physiker degree in technical physics and the Dr. rer.nat. degree in physics from the Technical UniversityMunich, Munich, Germany, in 1988 and 1993,respectively.

In 1994, he joined the University of Californiaat Berkeley, Berkeley, CA, USA, and the LawrenceBerkeley National Laboratory, Berkeley, CA, USA,as a Visiting Scientist. He joined the High TechResearch Center at Meijo University, Nagoya, Japan,in 1997 and Uniroyal Optoelectronics, Tampa, FL,

USA, in 2000. In March 2004, he joined the Department of Physics, AppliedPhysics, and Astronomy at Rensselaer Polytechnic Institute, Troy, NY, USA,as an Associate Professor and Wellfleet Career Development ConstellationProfessor, Future Chips. Since 2011, he has been a Professor. He is a foundingmember of the Future Chips Constellation and a Thrust Leader in the SmartLighting Engineering Research Center. His work concerns energy efficiencyby wide-bandgap semiconductors, primarily group-III nitrides for solid-statelighting. His recent research interests include materials and device developmentof green light-emitting diodes by GaInN epitaxy.

Dr. Wetzel was the Chair of the International Conference on Nitride Semi-conductors (ICNS-10 2013), the International Workshop on Nitride Semicon-ductors (IWN 2010), and the Electronic Materials Conference (EMC 2012and 2013). His work was awarded the Illuminating Ideas Award by the U.S.Department of Energy for significant achievements in solid-state lighting R&Din 2009 and 2011.

Theeradetch Detchprohm was born in Sukhothai,Thailand, in 1967. He received the B.S., M.S., andPh.D. degrees in electrical and electronic engineer-ing from Nagoya University, Nagoya, Japan, in 1991,1993, and 1996, respectively.

In 1996, he joined Alpha-TI Semiconductor,Bangkok, Thailand, as a Senior Process Engineer,and in 1998, he joined Meijo University, Nagoya, asa Postdoctoral Researcher developing (Al, Ga, In)N-based light-emitting devices and UV photodetec-tors. From 2001 to 2004, he was with Uniroyal

Optoelectronics, Tampa, FL, USA, as an R&D Scientist and was later promotedto epitaxy Department Manager. In 2004, he joined Blue Photonics, Walnut,CA, USA, as a Senior EPI Engineer. His work at Uniroyal Optoelectronics,and Blue Photonics, involved improving performance of GaInN-based light-emitting diodes (LEDs). From December 2004 to January 2013, he was withRensselaer Polytechnic Institute, Troy, NY, USA, with his last appointment asa Research Associate Professor. He recently joined the School of Electrical andComputer Engineering, Georgia Institute of Technology, Atlanta, GA, USA,as a Research Engineer. He is a highly accomplished expert in vapor phaseepitaxy and device development of group-III nitrides with a particular emphasison GaInN and AlGaN alloys and defect reduction techniques for full-spectrumsolid-state lighting and solar cells.

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