Nonpolar and Semipolar III-Nitride Light-Emitting Diodes: Achievements and Challenges

88 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 1, JANUARY 2010

Nonpolar and Semipolar III-Nitride Light-EmittingDiodes: Achievements and Challenges

Hisashi Masui, Member, IEEE, Shuji Nakamura,Steven P. DenBaars, Fellow, IEEE, and Umesh K. Mishra, Fellow, IEEE

(Invited Paper)

Abstract—It has been several years since InGaN/GaNlight-emitting diodes (LEDs) on nonpolar and semipolarorientations were first demonstrated. Prominent performance andinherent potential of these crystallographic orientations have beenrevealed as bulk-GaN substrates of arbitrary orientations becameavailable for epitaxial device growth. At this point in time, weintend to survey the progress made to date and prospect the futurerequirements for further device improvements. The discussionbegins with a historical background: how nonpolar/semipolarorientations were introduced to III-nitride LEDs and why theyare beneficial. The discussion then provides information onelementary crystallography and piezoelectricity in addition to theelectronic band structure of wurtzite crystals. Later in this paper,LED reports are collected to develop comprehensive knowledgeof the past research efforts and trends. Nonpolar and semipolarorientations provide not only high LED performances, e.g., opticaloutput power and wavelength ranges, but also unique functions,e.g., polarized light emission, which will explore new fields ofapplications.

Index Terms—Light-emitting diodes (LEDs), optical polariza-tion, piezoelectricity, quantum well (QW) devices, semiconductorepitaxial layers.

I. INTRODUCTION

L IGHT-EMITTING diodes (LEDs) are being used in ourdaily life all around us: they are seen in traffic lights,

automobile lamps, architecture illumination, household locallighting, liquid-crystal display (LCD) backlighting, mobileelectronic devices, etc. The LED is not only a new type oflight source but also a prospective contributor to global energysavings. When we remember our first encounter with solid-stateluminescence, either GaAs-, AlInGaP-, ZnSSe-, SiC-, or GaN-based material system, or other nonsemiconductor materials,we certainly appreciate the fast and great progress in solid-statetechnologies, from basic scientific understandings to commer-cial applications.

Manuscript received June 2, 2009; revised August 19, 2009. First publishedNovember 13, 2009; current version published December 23, 2009. This paperwas supported by the Solid State Lighting and Energy Center, University ofCalifornia, Santa Barbara. The review of this paper was arranged by EditorG. Meneghesso.

The authors are with the Solid State Lighting and Energy Center, MaterialsDepartment, University of California, Santa Barbara, CA 93106-5055 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/TED.2009.2033773

The purpose of this paper is to provide comprehensive infor-mation, in addition to research accomplishments, on nonpolar-and semipolar-oriented InGaN-based LEDs for readers withvarious backgrounds. A large part of this paper has been de-voted to explain how nonpolar and semipolar orientations areunique and attractive based on fundamental material properties,followed by a summary of past research work.

The following three sections are dedicated to introducingthe basic role of III–nitride semiconductors in optoelectron-ics based on their properties, discussing developments ofnonpolar/semipolar nitride LEDs, and prospecting future po-tentials and requirements.

II. BACKGROUND AND FUNDAMENTALS

A. Historical Movements

Upon achieving the commercialization of the AlGaAs- andAlInGaP-based red LEDs by the late 1980s, optoelectronicresearch was confronted by the desire for shorter wavelengthluminescence, namely, blue light. Three material systems werecompeting to conquer the blue spectral range until the early1990s. The ZnSe-based system (II–VI compounds) was themost researched candidate at that time. GaAs wafers were usedas epitaxial substrates due to a relatively small lattice mismatchand devices were grown via metalorganic chemical vapor depo-sition (MOCVD) or molecular beam epitaxy. After large efforts,material quality was improved, and laser operation at 490 nmwas finally demonstrated in 1991 [1]. The largest concern wasdevice lifetime. II–VI compounds are rather soft materials,and it was concluded that the seen dislocation multiplicationwas caused by emitted photons. Thus, device degradation wasinevitable and was not easily overcome. Although ZnSe bulkcrystal growth was intensively researched [2] and ZnSe wafersbecame commercially available during the first half of the1990s, it had a minimal impact on device research activi-ties. Cadmium was sometimes used as a constituent, whichbecame more difficult with time due to global environmentalmovements. SiC (IV–IV compound) was another candidate [3].Blue LEDs (Schottky structure) were commercialized by thebeginning of 1990s, although they were not very bright: approx-imately 10 mcd per LED lamp. This is because of its indirectbandgap nature, and further improvements were not expectedto meet industrial requirements. The GaN-based system was

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MASUI et al.: NONPOLAR AND SEMIPOLAR III-NITRIDE LIGHT-EMITTING DIODES: ACHIEVEMENTS AND CHALLENGES 89

considered to be a third candidate, although only a few elec-troluminescence (EL) reports were available.

Optical properties of GaN were investigated as early as1971 [4]. For the following 15 years, a few research groupsactively worked on GaN and looked for a technique to growsingle-crystal GaN epitaxially on sapphire. Amano, a graduatestudent from Akasaki’s group at Nagoya University, reported atechnique using a thin AlN layer, called the buffer layer, in 1986[5]. This technique established a stepping stone for the nextstage in development. Nakamura [6], [7] reported GaN bufferlayers, which have been widely adapted in today’s industry.As single crystals became available as a result of the bufferlayer techniques, the next challenge became p-type doping.Magnesium was the most common p-type dopant as Be is toxicand Zn is considered a deep level in GaN. Background dopingconcentration of undoped GaN films was decreased to lowlevels, nevertheless, p-type conduction was not demonstratedfor some time. It is known today that during a scanning electronmicroscope experiment, Amano noticed the Mg-doped GaNsample started to emit blue light. Electron beam irradiationwas established to be a technique to activate Mg in GaN [8].The mechanism was at that time not understood. Althoughthermal effects due to the beam irradiation were discardedby Amano, Nakamura [9] later demonstrated that thermalannealing of Mg-doped GaN activated p-type conductivity.This method has become the standard industrial technique.Nakamura et al. [10] concluded via later studies that H attachedto Mg deactivated Mg as an acceptor. Thus, the GaN p-njunction was realized, yet InGaN growth was an obstacle tothe blue LED industrialization. InGaN growth was found tobe very sensitive to growth conditions and Nakamura et al.showed that high-quality InGaN crystals could be grown bycarefully selecting growth conditions [11], [12]. This is due tothe large difference in dissociation temperature for GaN andInN and their immiscibility. High-quality InGaN was grown upto a few percent of In. Nakamura et al. [13] employed dopants(Si and Zn) in the active layers, and donor–acceptor pairemission was used for blue emission. These LEDs were an-nounced [14] and commercialized late in 1993. The blue lightemission was astonishingly bright and sufficient for industrialapplications.

Commercial green LEDs were still only GaP based, whichhad dim and yellowish luminescence, and GaN-based greenLEDs were not realized. Research focused on growing high-quality InGaN. Elaborate growth optimization improved InGaNquality, nevertheless, thick layers, which are usually desired toincrease the volume of the light-emitting material, were stilldifficult to grow since the lattice mismatch with the substratewas not considered. Additionally, it was observed that LEDswith thick active layers were dimmer than those with thin activelayers. This is now understood as the quantum-confined Starkeffect (QCSE). LED structures preferred quantum wells (QWs)due to carrier confinement, and hence, pure green InGaN-basedLEDs were realized [15], [16].

At about the same time, GaN-based laser diodes weredemonstrated [17]. To date, all GaN device structures weregrown on c-oriented sapphire. Sapphire is hard to cleave andthe GaN epitaxial layer is rotated by 30◦ with respect to

sapphire [18]. These conditions make formation of cleavedfacet mirrors difficult. It was also predicted that strain did notmodify the valence band structure, which was thought to bedisadvantageous [19]. For these reasons, in 1996, Horino et al.[20], [21] began to grow GaN epitaxial layers in the m ori-entation on m-oriented SiC. It was discussed in 1996 that thesignificant blue shift of InGaN QW LEDs were caused partiallyby the QCSE [22], [23], which also explained the low efficiencywhen InGaN active layers were made 3 nm and thicker. Toeliminate the QCSE, which had a negative impact on nitrideoptoelectronic devices, nonpolar orientations suddenly began toattract attention [24], [25]. Despite such great interest, we hadto wait until 2004 for an a-plane LED [26]. This portrays thedifficulty of nonpolar growth, which still exists today.

Meanwhile, theorists found inclined planes (planes betweenc and nonpolar orientations) would have properties which re-duce the QCSE [27]. Baker et al. [28] called these inclinedplanes “semipolar” planes upon their planar film fabricationon foreign substrates. Semipolar LEDs were reported in 2005[29], yet none of these nonpolar- and semipolar-oriented LEDssatisfactorily achieved high optical power output.

Great improvement was brought by the emergence of bulk-GaN substrate in 2006. By that time, c-plane GaN waferswere commercially available, but were limited in supply at ahigh cost. By making c-oriented hydride vapor-phase epitaxy(HVPE) GaN grow thick enough (∼5 mm), it is possible toslice it in other orientations, thereby obtaining nonpolar andsemipolar GaN substrates [30]. Given these substrates, not onlywere high-performance LEDs obtained, but also other charac-teristics, e.g., optical polarization, QCSE, growth conditionsand structural properties, laser diodes, transistors, etc., wereinvestigated. We have since to learn inherent characteristics ofnonpolar and semipolar GaN-based materials and devices.

B. Problems With Conventional c-Plane LEDs and Motivationfor Nonpolar and Semipolar Orientations

As the spectral range of light emission was extended fromblue to green, we encountered a problem in luminescenceefficiency, which is strongly related to InGaN material qualityand QW structure.

The material issue is that InGaN alloys tend to show spinodaldecomposition [31], [32]. Furthermore, underlying GaN layerscause pseudomorphic growth of InGaN, thus, high In contentis difficult and tends to introduce misfit dislocations and othertypes of defects. This problem is related to thermodynamicsand growth kinetics, hence growth techniques and optimizationwill improve the materials. This aspect of the problem is com-mon to all crystallographic orientations; nevertheless, differentorientations may have different degrees of tolerances. Theempirically found trend of high In incorporation into (112̄2)-oriented InGaN films may be related to this aspect. Furtherinvestigations are expected.

The QW structure related challenge is due to the QCSE(Fig. 1). This effect is inevitable as long as strained QWstructures are used in the polar orientation. In this case, a crys-tallographic solution may be given: employment of nonpolarorientation to have the polarization vectors lie in the plane,


Fig. 1. QW structures on (a) polar and (b) nonpolar orientations and their banddiagrams. Polarization charges appear at interfaces of the polar-oriented QWand induce electric fields that spatially separate electrons and holes in the QW.In nonpolar orientations, polarization charges do not affect the band structure.Because of the internal electric fields in the polar-oriented QW, transitionenergy occurs to be smaller than that of the nonpolar-oriented QW (QCSE).When the QW is embedded in the common +c-oriented LED structure,the internal electric fields are increased as LED positive bias is increased.After S. F. Chichibu et al. [25].

Fig. 2. Computed polarization charge (spontaneous and piezoelectric) densityin InGaN/GaN QWs as a function of tilt angle of the growth plane withrespect to the c-plane. Internal field strength becomes zero around θ = 45◦.After A. E. Romanov et al. [34].

thus in the device direction, so no strain-induced electric fieldsappear. This is the main reason for the nonpolar orientation.Once we eliminate the QCSE, thicker active layers, as thick asthe critical thickness can be employed, which offer advantages,e.g., a larger active region volume. In addition to nonpolarorientations, it has been found by computation that semipo-lar orientations can provide similar effects in eliminating theQCSE. These calculations depend on material parameters suchas elastic constants. Some of the necessary parameters are notwell known to date, particularly those of InN [33]. These un-certainties introduce variations in the calculated results; Fig. 2is an example result of these studies [27], [34]. Nevertheless,it has been believed that the advantage will be obtained for an-gles around 45◦–60◦. These angles include (101̄1) and (112̄2)orientations.

Fig. 3. Hexagonal prism representing a GaN crystal unit cell with commonnomenclature of planes and axes. Coloring of the planes is for clarity and doesnot indicate any physical significance.

In addition, extra properties such as optical polarization areconsidered to be advantages over c-plane devices in someapplications.

C. Crystallography and Piezoelectricity

A significant difference in nitride semiconductors from otherconventional optoelectronic semiconductors is the crystal struc-ture. Nitride semiconductor materials crystallize in wurtzitestructures, while other typical optoelectronic semiconductormaterials crystallize in zinc blende structures under normalconditions. Hexagonal lattices are expressed by using theMiller–Bravais index [35], [36]. Fig. 3 shows a hexagonalprism representing a GaN crystal. The (0001) plane is theconventional growth plane and is called the +c plane. All thecurrent commercial products are fabricated via the c-orientationgrowth. The c axis [0001] is an electrically polar axis in wurtzitecrystals. The (0001̄) plane is often called the −c plane; thezinc blende equivalence is considered to be the {111} B plane.Vertical planes are parallel to [0001], thus electrically nonpolar,and they are called nonpolar planes. Low-index nonpolar planesare {112̄0} and {11̄00}, called a and m planes, respectively.It is a convention that the z axis of a 3-D spatial coordinatesystem is defined to be parallel to the c axis. It is commonpractice that the x and y axes are defined to be parallel tothe a and m axes, respectively. Note that Cartesian coordinatesare conventionally used in wurtzite crystals. In some literature,the x and y axes are defined to be parallel to the m anda axes, respectively, as these two crystallographic axes arechemically different but equivalent in the sense of continuummechanics [37]. Planes that make angles with respect to thec axis are called semipolar planes. Fig. 4 shows three low-index semipolar planes in relation to the two nonpolar planes,which are important in InGaN LEDs. Semipolar planes areseldom atomically flat planes containing atomic steps. Detaileddiscussions are given in [38].

It is calculated from the closely packed structure of spheresthat the ideal wurtzite structure possesses a c/a ratio of 1.633,where c and a are the conventional lattice constants in the c anda crystallographic directions, respectively. In reality, GaN, for


Fig. 4. Important semipolar planes (bottom three, painted in gray) shownalong with the low-index nonpolar planes (top two). {101̄1}, {101̄3}, and{112̄2} planes are at 62◦, 32◦, and 58◦ from the basal plane for GaN,respectively.

example, has a ratio of 1.627, which implies that the crystal iscompressed in the c direction under standard conditions (roomtemperature, 1 atm). Consequently, the centers of positive andnegative charges from the ionicity of the Ga–N bond aredisplaced from each other, and electric charges theoreticallyappear at the opposite surfaces of the crystal. This is calledspontaneous polarization. It is also possible to intentionallystress crystals (e.g., coherently grown heteroepitaxial films).The c/a ratio changes from the unstrained state, and additionalpolarization charges will result, which is called piezoelectricpolarization. In any case, semiconductor crystals are not ableto sustain voltages larger than their bandgaps, as carriers startshifting within the crystal to neutralize polarization charges[Fig. 5(a)], or ions in the atmosphere may adhere to the crystalsurface to neutralize the polarization charges [39]. As a result,electric fields induced by spontaneous/piezoelectric polariza-tion are irrelevant in large homogeneous materials [Fig. 5(b)].However, in heterostructures, this is not the case. There willbe polarization-charge discontinuities at the interfaces, and netpolarization charges induce internal electric fields. These elec-tric fields are relevant in small size heterostructures, e.g., QWs,resulting in the QCSE (Fig. 1). The strength of the internalelectric fields is determined by the projection (i.e., cos θ) ofthe polarization vector (i.e., strain state, c/a deviation from1.633) onto the plane of interest. Nonpolar orientations utilizecos θ = 0 and semipolar orientations try to attain c/a = 1.633(although strained semipolar crystals do not necessarily remainwurtzite, rigorously speaking; see [34] for details).

Electrical polarity of the growth surface is often of interestin growth and electronic device operation. This is, in fact, nottrivial to determine in arbitrarily orientated crystals since it isinfluenced by strain, with the two major effects being 1) surfacechemistry (anion and cation) and 2) mechanical conditions(strain-induced electric polarization).

Surface Chemistry: The (0001) plane is the so-called Ga-polar face and is believed to be terminated by Ga atoms under

Fig. 5. Effects of polarization fields on carrier generation and field screening.(a) Idealized case of intrinsic semiconductors described by the band diagramand charge distribution. As a crystal grows on an insulating substrate, EV

meets EF as a result of polarization fields. Holes are created at EV andelectrons are swept toward the substrate by the polarization fields. Polarizationcharges (+Qπ and −Qπ) are located at the interface and growth surface andare screened by carriers as the film grows. (b) More realistic case of thick n-type semiconductor films. Generated electrons have gathered at the substrateinterface (−n) and neutralized the polarization charges. At the film surface,EF is pinned at a deep donor level (EDD), and a space charge layer iscreated where donors are charged (ND). Polarization charges at the surfaceare neutralized by the charged donor and deep donors (NDD).

common ambient conditions. In this case, the surface may bethought to be positively charged as a result of cation exposure.In real GaN crystals, however, the Ga face is considered to benegatively charged as a result of spontaneous polarization (inthe idealized case where the polarization charge is not neutral-ized). When the crystal is rotated so a semipolar plane is thesurface, the situation changes. The (101̄1) plane, for example,appears to be an anion-terminated plane (hence, negativelycharged) as a result of crystal symmetry [38]. Spontaneous po-larization is nevertheless still negative; thus, the (101̄1) surfaceis probably strongly negatively charged.

Mechanical Conditions: Electrical polarization appears par-allel to the c axis as a result of crystal distortion along the polaraxis. Although long-range electrical polarization is neutralizedby adhered charged species and/or free carrier accumulation, itmay become significant in heterostructure interfaces/surfaces.For example, the growth surface of (0001)-oriented InGaNlayers coherently grown on GaN tend to be positively chargedas a result of strain, which may affect the chemistry of theMOCVD growth front. Another example is MOCVD’s epitax-ial preference of {11̄01̄}-oriented GaN growth over {11̄01}growth while (0001̄) GaN growth is far more difficult than(0001) growth [40], [41]. The latter example may indicate thatthe MOCVD preference is predominantly determined by thesurface chemistry, and mechanical conditions are insignificant.

Comprehensive understanding has not yet been established inclassifying these effects. We leave the aforementioned questionunanswered since this topic needs more discussion.

D. Electronic Band Structure

The electronic band structure of GaN reflects the crystalstructure as widely studied during the 1990s [42]–[44]. Fig. 6shows the commonly used sketch for the electronic band struc-ture of wurtzite GaN. It is known that the strong ionicity ofGaN results in a conduction band consisting of 4s orbitals,


Fig. 6. Schematic band structure of wurtzite GaN, after Suzuki andUenoyama [43].

predominantly from the Ga atom, thus, the excited state canbe considered to be isotropic in real space. The valence bandconsists of 2p orbitals from the N atoms. The three mutuallyorthogonal p orbitals are degenerate in free space. In a wurtzitecrystal, the |Z〉 state (corresponding to the pz state) becomesunique because of the crystal field generated by the wurtzitestructure, whereas the other two states |X〉 and |Y 〉 (corre-sponding to the px and py states) are degenerate. The |Z〉 stateforms a subband called the crystal-field-splitting hole band. Theenergy difference between the two states (one of the two isdegenerate) at the Γ point (k = 0) is called the crystal-field-splitting energy (Δcr) and has been determined to be approx-imately 10 meV [33], although theoretical calculated valuesare often larger. Crystal-field splitting is an important notion inconsidering optically polarized characteristics of nitride LEDs.

There is spin–orbit coupling. The state |X + iY, ↑〉 hasparallel magnetic moments from orbiting electrons and elec-tron spins (the same holds for |X − iY, ↓〉; these states formthe heavy hole band). The state |X + iY, ↓〉 has antiparallelmagnetic moments (and the same in |X − iY, ↑〉; these statesform the light hole band); hence, the energy of the latter twostates becomes slightly lower than the former two states. Thespin–orbit splitting energy has been obtained to be approxi-mately 17 meV [33] but is neglected in LED experiments.

When strain is introduced in the GaN crystal, the bandstructure changes. InGaN active layers are almost alwaysstrained by the virtue of heteroepitaxial growth on GaN layers.In the c-plane growth, nevertheless, in-plane biaxial strain isisotropic, which does not change the crystal symmetry, andthe band structure remains essentially the same. In nonpolarand semipolar growth, however, in-plane strain is anisotropic,and crystal symmetry is changed. |X〉 and |Y 〉 are no longerdegenerate. Under anisotropic compressive strain, the topmostvalence subband (sometimes called the A or T1 band) has itsdipole moment parallel to the plane. Hence, the predominantoptical polarization of emitted light is parallel to the plane,which can be observed in the direction normal to the plane.

III. FABRICATION AND PERFORMANCE OF NONPOLAR-AND SEMIPOLAR-ORIENTED LEDs

A. Early Work via Foreign Substrates

GaN substrates with arbitrary orientations were not availablewhen nonpolar- and semipolar-oriented devices were proposed.

For this reason, research began with fabricating thick nonpolarand semipolar GaN films on foreign substrates, called GaNtemplates [45].

r (and R)-plane sapphire and a-plane SiC were used fora-oriented GaN growth [46], [47]. Availability of inexpensivesapphire substrates with acceptable lattice mismatch was theadvantage of a-plane GaN growth. m-oriented GaN was notgrown on sapphire because of intolerable lattice mismatch;better options were m-plane SiC [20], [21] and (100) γ-LiAlO2

[24], [48], [49]. m-plane SiC was expensive, and LiAlO2

was not chemically stable at common GaN growth conditions.Nevertheless, research was encouraged by realization of free-standing 2-in m-oriented GaN wafers fabricated via HVPEusing LiAlO2 substrates [50]. As a result of these efforts,nonpolar LEDs were fabricated on an a-plane template in2004 by Chakraborty et al. [26]. In 2005, m-oriented LEDswere reported by Gardner et al. [51] on m-plane SiC andby Chakraborty et al. [55] on free-standing m-plane GaN.EL characteristics of these LEDs, in addition to other LEDsreported to date, have been tabulated in Table I.

Semipolar orientations were attracting less attention whilenonpolar templates were researched, probably because of theircomplexity and variety of choices compared to nonpolar ori-entations. In 2005, Baker et al. [28] demonstrated planar filmgrowth of semipolar orientations on foreign substrates. Theyemployed spinel as epitaxial substrates and fabricated (101̄1̄)and (101̄3̄) GaN templates. Baker et al. [52] then fabricated(101̄3̄) and (112̄2) templates on sapphire in 2006. Conse-quently, semipolar-oriented LEDs grown on these templateswere reported: (101̄1̄) and (101̄3̄) blue LEDs by Chakrabortyet al. and (101̄3̄) green LEDs by Sharma et al. in 2005, followedby (112̄2) LEDs by Masui et al. [53], [54] in 2006. Semipolar-oriented LEDs were also fabricated on Si substrates [57].

In response to these device demonstrations, recent researchis active in fabricating large-area high-quality templates and ismotivated by future cost reduction in competition with GaNbulk-crystal growth. The research group led by Amano [58],[59] is an active group developing GaN templates during thelast several years. Okada et al. [60], [61] have recently demon-strated a unique technique to grow m-plane GaN films on sap-phire and LEDs have been demonstrated by Saito et al. [62] onm-plane GaN templates. (112̄2)-oriented templates have beengrown via lateral epitaxial overgrowth techniques [63], [64].Use of Si and SiC substrates has been demonstrated to prepare(112̄2)- and (101̄1̄)-oriented templates [65]–[67]. Studies ofsemipolar (112̄2) films were recently reported independentlyby a joint group of University of Manchester and University ofCambridge [68] and a group of the present authors [54].

In addition to planar device structures, ridge structures onc-plane GaN were fabricated [69], [70]. These semipolar facetLEDs did not received strong attention, probably because thesemicro-3-D structures require modified techniques in metal con-tact formation and discrete device fabrication.

B. Recent Work Using Homoepitaxial Substrates

Recent work may be defined as LED fabrication on bulk-GaN substrates, which became available commercially in 2006.


TABLE IHETEROEPITAXIALLY GROWN NONPOLAR AND SEMIPOLAR LEDs REPORTED TO DATE. SHOWN PERFORMANCES WERE DATA TAKEN AT 20 mA,

UNLESS MENTIONED OTHERWISE

TABLE IIREPORTED PERFORMANCES OF NONPOLAR AND SEMIPOLAR LEDs PREPARED ON BULK-GaN SUBSTRATES. SHOWN DATA WERE TAKEN AT 20 mA,

UNLESS MENTIONED OTHERWISE. THE COLUMN FOR STRUCTURE INDICATES THE THICKNESS AND NUMBER OF InGaN QWs

These bulk-GaN substrates are grown in the c orientation viaHVPE and sliced in desired orientations [30]. Hence, the size ofthe sliced wafers depends on how thick c-oriented GaN crystalswere grown and the current HVPE growth technology en-ables to provide approximately 10-mm-thick crystals. Althoughharvested nonpolar/semipolar substrates are small in view ofcommercial LED production, they are large enough to carryout basic research to investigate the potential of nonpolar- andsemipolar-oriented LEDs. The performance of homoepitaxialLEDs is summarized in Table II.

Most nonpolar LEDs reported to date have been fabricatedon the m orientation. LED performances were drastically im-proved by two research groups via homoepitaxy [71]–[73].Kim et al. [74] claimed an advantage of growing thick QWs dueto the absence of the QCSE. The external quantum efficiencywas determined to be ∼40% in the early study [71]. The currentdesire is to achieve luminescence of longer wavelengths [75],as only one report can be found on LEDs beyond 500 nmwith reasonable optical output power [76]. We may be ableto have an optimistic view on achieving green light emissionfrom m-plane LEDs, as a similar obstacle was confronted by c-plane LEDs during the 1990s. Other problems include hillockformation on epitaxial m-plane films. These hillocks are clearlyseen in EL (Fig. 7). The mechanism of hillock formation andtheir effects on LED characteristics are still unknown. Hillockformation can be avoided by employing vicinal substrates [72],

Fig. 7. Electroluminescence of an m-plane LED [75] operated at 2 μA.

[77], [78]. Tsuda et al. [79] reported smooth film growthwithout hillocks.

Although a study by Yamada et al. [80] indicated that the morientation had a higher potential for light-emitting devices thanthe a orientation, recent accomplishments on a-plane LEDsemerged in 2008 have demonstrated competitive performances[81], [82]. Green light emission was achieved, yet output powerwas comparable to heteroepitaxial LEDs.

Despite the advantage of arbitrary slicing GaN crystals,experimental studies on semipolar LEDs have been limited


Fig. 8. Light intensity modulation experiments using an LCD unit (twisted nematic cell with two polarization films) combined with m- and c-plane LED.(a) Experimental setup. Results of (b) m-plane and (c) c-plane LEDs.

to (101̄1̄) and (112̄2) orientations [45]. These two low-indexplanes share a tilt angle of approximately 60◦ with respect to thec-plane; thus, they are considered to be advantageous in termsof the QCSE. Green, yellow, and amber light emission havebeen realized on (112̄2) with reasonable optical output [83]–[85]. The (112̄2)-oriented InGaN yellow LEDs were reportedto have more stable performances against changes in ambienttemperature than AlInGaP yellow LEDs [85]. (101̄1̄)-orientedLEDs have exhibited strong blue light emission [86], [87]with 34% external quantum efficiency and longer wavelengthemission has now been sought. As far as green emitters areconcerned, the (112̄2) orientation seems to have advantagesamong reported nonpolar/semipolar orientations to date. Animportant achievement on laser diodes to be mentioned herewas reported by Enya et al. [88] in July 2009. Pure green laserdiodes were demonstrated on the {202̄1} plane, which has notbeen used for LEDs.

It is not intuitive that if the m-plane is preferred to the a-plane in device fabrication, why does the (112̄2)-plane (aninclined a-plane) seem to have advantages over the (101̄1̄)-plane (an inclined m-plane). We do not know yet whether theseare inherent properties of these crystallographic planes or if itis only a matter of growth techniques and conditions.

C. Polarized Light Emission

Optically polarized characteristics of GaN were observed byDingle et al. in 1971 and are considered to be unique amongcommon semiconductor materials. The anisotropic character-

istics become relevant when LEDs are fabricated on nonpolarand semipolar orientations [49], [89]. Gardner et al. reportedin 2005 that spontaneous light emission from m-plane LEDswas polarized. The study on polarized EL was followed byother groups [90]–[94]. These groups showed that light emis-sion along the surface normal to m-plane LEDs was partiallylinearly polarized in the direction parallel to the a-axis. Thedegree of polarization tended to increase as the EL wavelengthwas increased, while the polarization degree was reduced ascurrent was increased. It has been attempted to extract Δcr

values from experimental measurements on InGaN/GaN QWs.Reported values (40–90 meV) are, in fact, greater than mostcomputed values (10–20 meV) on GaN.

Applications of polarized LEDs include LCDs. Preliminarydemonstrations of polarized light sources combined with anLCD unit have been performed by Masui et al. [95]. Fig. 8shows an experiment setup and results comparing m- and c-plane LEDs as LCD light sources. When a commercial LCDunit (twisted nematic cell with two polarization films) wasinserted in the light path between the m-plane LED and detectorin such a way that the direction of polarization was mutuallyparallel [blue and green curves in Fig. 8(b)], light intensitywas only slightly reduced by absorption of the polarizationfilms. The same measurement with the c-plane LED resultedin a loss of more than half of the light intensity because ofunpolarized light emission. When the LCD unit was biased,intensity modulation was achieved in both cases with a betterextinction ratio for the m-plane LED. Imperfect extinction (ex-tinction ratio ∼0.2) was due mostly to imperfect functioning of


the LCD unit. Employment of polarized light sources in LCDsis believed to contribute to energy savings. Demonstrations onhue control by utilizing polarized light emission have beenreported [96].

Although similar polarization characteristics are predictedon a-plane LEDs as those of m-plane LEDs from a theoret-ical viewpoint, experimental studies have not been reported.Among the semipolar orientations, polarization characteristicsof (112̄2)-oriented LEDs have been relatively well investigated.A counterintuitive observation on optical polarization of LEDsprepared on a (112̄2)-oriented GaN template was made in 2006by Masui et al. [53]. In 2008, Ueda et al. [97] found a simi-lar phenomenon on (112̄2)-oriented homoepitaxial LEDs andcalled it polarization switching: in-plane optical polarizationparallel to the m-axis was weaker than the in-plane orthogo-nal direction [1̄1̄23]. Occurrence of polarization switching on(112̄2)-oriented LEDs has also been observed by Fellows et al.[98], [99], Masui et al. [100], and Sizov et al. [101]. Ueda et al.have attributed the cause of polarization switching to a verylarge deviation of the deformation potentials of InN from thoseobtained by the quasi-cubic approximation, which has becomecontroversial as many material parameters of InN have not beenwell determined in general.

D. QCSE

It is the QCSE that has motivated us to employ nonpolarand semipolar orientations for InGaN/GaN QW LEDs; we areinterested in experimentally confirming whether these orien-tations suppress the QCSE. Although many reports claimedupon their demonstrations of nonpolar/semipolar QW LEDsthat employment of those orientations resulted in the re-duction of luminescence peak shifts due to suppression ofthe QCSE, multiple effects on peak shifts, e.g., band filling,localized-state filling, and joule heating, were not deconvoluted.Koyama et al. [102] and Onuma et al. [103] showed exper-imentally via electrical-bias-applied photoluminescence (PL)techniques that the QCSE was, indeed, absent in nonpolar-oriented QW LEDs.

Semipolar orientations have also been investigated. Fourexperimental reports on planar (112̄2)-oriented QW structuresdo not quite agree on their conclusions. Hylton et al. [68]concluded that internal fields were weaker than those in (0001)-oriented QWs, while Masui et al. [104] and Shen et al. [105]reached the same conclusion that built-in fields of the p-njunctions were strengthened (compared to nonpolar orientedQW LEDs) by piezoelectric polarization, resulting in strongerinternal fields than nonpolar cases. Garrett et al. [106] onlymentioned, based on their time-resolved PL measurements, thatcarrier dynamics were different in c, m, and (112̄2) orienta-tions. Three experimental reports on QW structures fabricatedon {101̄1} facets or planar films are not in agreement. WhileFeneberg et al. [107] extracted positive values of piezoelectricpolarization, Masui et al. [54] and Shen et al. [108] reportednegative values. Experimental determination of piezoelectricfields in semipolar-oriented QW structures is controversial atthe moment, and no conclusion is made on effects of semipolarorientations on the QCSE as of today.

E. Efficiency Droop

Efficiency droop is a phenomenon that the luminescenceefficiency of InGaN LEDs is degraded as current density isincreased to the practical level of LED operation. Efficiencydroop has been considered to be a serious problem in c-planeLEDs since demands for high-current operation is desired insolid-state lighting applications. Piezoelectric polarization andcarrier overflow have been nominated as the causes [109]–[111]. Consequently, nonpolar and semipolar orientations arebelieved to be a solution to the efficiency droop problems.In 2007, Kim et al. [74] reported on m-oriented QW LEDsthat thick QWs were effective in suppressing efficiency droop.In later reports, however, efficiency droop is observed [75],[76]. Possibilities of Auger processes have been discussed onc-plane LEDs since 2007 [112]–[115]. Research groups fromindustrial organizations have employed wide c-plane QW struc-tures to reduce carrier densities during high-current operation.This was not known to be effective at the early time whenNakamura’s choice of QWs thinner than 3 nm was reported dur-ing 1993–1995 [15], [16], [116]. It seems that further researchefforts are required from various standpoints.

IV. FUTURE CHALLENGES

A. Substrates and Templates

Although high-quality GaN substrates with arbitrary orienta-tions have been supplied commercially, they are small in size,costly, and limited in production volume, thus, not viable forcommercial LED production. This fact prevents easy access tosuch attractive materials for many interested researchers, bothin academia and industry. Large-scale production of GaN sub-strates, particularly nonpolar [48] and semipolar orientations,is therefore high in demand. Current HVPE techniques havelimitations in growth duration caused by parasitic reaction ofprecursors and limited amount of precursor load [117]. BouleGaN (and AlN) growth techniques have been developed andadvanced in the past years [118], [119], yet are often notpractical in many countries because of restrictions by local lawand safety regulations on the extreme pressures needed in thegrowth chambers.

For these reasons, template growth is attractive. As previ-ously discussed, popular nonpolar and semipolar orientations(ones shown in Fig. 4) of GaN templates have been producedusing low-cost high-quality large-area optically transparentsubstrates, i.e., sapphire and spinel. Further improvements intemplate material quality is widely expected.

In either case, bulk substrates or templates, nitride LEDs areconsidered to be chemically nontoxic, conforming to the globaltrend of material safety.

B. Epitaxial Growth

Indium incorporation in QWs and Mg doping for p-typeconduction are always concerns in InGaN-based visible LEDs.Although long-wavelength (yellow and beyond) electrolumi-nescence and relatively low operation voltages have beenreported via nonpolar and semipolar LEDs, long-wavelength


c-plane LEDs have not been commercialized because of thepresence of superior AlInGaP LEDs. Nonpolar and semipolarLEDs will need to challenge AlInGaP LEDs in commercial vi-ability, and to do this, fundamental material and device studiesare needed. The good news is, InGaN laser diodes on variousorientations have been achieving records in lasing wavelengthsduring the last few years. Knowledge gained during laser dioderesearch is expected to provide new insights to LED research.Near-UV is another spectral range of interest for tricolor phos-phor excitation for white LED applications, although it has beenattracting less attention.

Through the aforementioned device demonstrations, thecommon metal contact scheme does not seem to be universal toall device orientations. Doping and electrical property studieson nonpolar/semipolar orientations are not abundant to datepartially due to substrate constraints.

A specific problem in m-plane growth is natural hillockformation, while (112̄2)-oriented films appears to be mostlysmooth. As employment of m-plane vicinal substrates is asolution, the mechanism of hillock formation remains to be ofscientific interest for growers.

C. Light Extraction

For high-power LED applications, light extraction efficiencyis no less important than luminescence efficiencies. Researchefforts performed in the past include chip shaping [120], sur-face roughening [121], fused ZnO cones [122], and spher-ical encapsulation [123]. As some of these techniques relyon the c-plane material properties, they may not be readilytransferred to nonpolar and semipolar materials. The idea ofsurface roughening has been implemented in semipolar LEDsby dry-etching techniques [124]. In the near future, when themajority of device designs utilize vertical current flow (twoelectrical contacts placed at the opposite sides of the LEDchip) by using conductive GaN substrates [125], conventionallight-extraction techniques developed for AlInGaP LEDs maybecome applicable.

D. Direct Evidence of Excellence

The best numbers in terms of LED performances to datehave been reported on conventional c-plane LEDs [126]. This ispartly because of the immature technologies due to the smallerand newer community of nonpolar/semipolar experimental re-search. It is the researchers’ task to show the excellency ofnonpolar and semipolar LEDs.

V. SUMMARY

The former half of this paper has been devoted to elementaryand tutorial discussions to introduce the motivation of employ-ing nonpolar and semipolar orientations for LED fabrication.The QCSE originally discussed in 1996 has undesired effects inc-plane QW LEDs, and nonpolar/semipolar orientations wereproposed to suppress the QCSE. Mechanisms of electricalpolarization in wurtzite semiconductors were described to fa-miliarize readers with the QCSE. Polarized light emission has

been considered to be a unique property of nonpolar/semipolarLEDs, which was explained by the electronic band structure ofwurtzite GaN.

The latter half was dedicated to summarizing LED researchefforts made to date and future prospects of nonpolar/semipolarLEDs. Heteroepitaxially grown LEDs on templates have ad-vantages in prospective cost-effective LED production and areunder continuous investigation, while homoepitaxially grownLEDs on bulk-GaN substrates explore ultimate performance ofnonpolar-/semipolar-oriented LEDs. The key to this competi-tion between heteroepitaxial and homoepitaxial techniques isthe substrate; templates need improvements in material qualityand bulk substrates need large-scale production and cost effec-tiveness. The nonpolar/semipolar LED will have to compete notonly with the c-plane counterpart, but also with the AlInGaPLED in the long-wavelength ranges to enable us to rely on onenontoxic material system for the coverage of the entire visiblespectrum.

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[73] H. Yamada, K. Iso, M. Saito, H. Masui, K. Fujito, S. P. DenBaars,and S. Nakamura, “Compositional dependence of nonpolar m-planeInxGa1−xN/GaN light emitting diodes,” Appl. Phys. Express, vol. 1,no. 4, p. 041101, 2008.

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[78] H. Yamada, K. Iso, H. Masui, M. Saito, K. Fujito, S. P. DenBaars, andS. Nakamura, “Effects of off-axis GaN substrates on optical properties of

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[84] 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 powerand high efficiency green light emitting diode on free-standing semipo-lar (112̄2) bulk GaN substrate,” Phys. Stat. Sol. (RRL), vol. 1, no. 4,pp. 162–164, Jul. 2007.

[85] 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, “Opti-cal properties of yellow light-emitting diodes grown on semipolar (112̄2)bulk GaN substrates,” Appl. Phys. Lett., vol. 92, no. 22, p. 221 110,Jun. 2008.

[86] A. Tyagi, H. Zhong, N. N. Fellows, M. Iza, J. S. Speck, S. P. DenBaars,and S. Nakamura, “High rightness violet InGaN/GaN light emittingdiodes on semipolar (101̄1̄) bulk GaN substrates,” Jpn. J. Appl. Phys.,vol. 46, no. 4–7, pp. L129–L131, 2007.

[87] H. Zhong, A. Tyagi, N. N. Fellows, F. Wu, R. B. Chung, M. Saito,K. Fujito, J. S. Speck, S. P. DenBaars, and S. Nakamura, “High powerand high efficiency blue light emitting diode on freestanding semipolar(101̄1̄) bulk GaN substrate,” Appl. Phys. Lett., vol. 90, no. 23, p. 233 504,Jun. 2007.

[88] Y. Enya, Y. Yoshizumi, T. Kyono, K. Akita, M. Ueno, M. Adachi,T. Sumitomo, S. Tokuyama, T. Ikegami, K. Katayama, andT. Nakamura, “531 nm green lasing of InGaN based laser diodeson semi-polar {202̄1} free-standing GaN substrates,” Appl. Phys.Express, vol. 2, no. 8, p. 082101, 2009.

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[90] H. Masui, A. Chakraborty, B. A. Haskell, U. K. Mishra, J. S. Speck,S. Nakamura, and S. P. DenBaars, “Polarized light emission fromnonpolar InGaN light-emitting diodes grown on a bulk m-plane GaNsubstrate,” Jpn. J. Appl. Phys., vol. 44, no. 42–45, pp. L1 329–L1 332,2005.

[91] H. Masui, H. Yamada, K. Iso, S. Nakamura, and S. P. DenBaars, “Opticalpolarization characteristics of InGaN/GaN light-emitting diodes fabri-cated on GaN substrates oriented between (101̄0) and (101̄1̄) planes,”Appl. Phys. Lett., vol. 92, no. 9, p. 091105, Mar. 2008.

[92] H. Masui, H. Yamada, K. Iso, S. Nakamura, and S. P. DenBaars, “Opti-cal polarization characteristics of m-oriented InGaN/GaN light-emittingdiodes with various indium compositions in single-quantum-well struc-ture,” J. Phys. D, Appl. Phys., vol. 41, no. 22, p. 225 104, Nov. 2008.

[93] H. Tsujimura, S. Nakagawa, K. Okamoto, and H. Ohta, “Characteristicsof polarized electroluminescence from m-plane InGaN-based light emit-ting diodes,” Jpn. J. Appl. Phys., vol. 46, no. 41–44, pp. L1 010–L1 012,2007.

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[95] H. Masui, H. Yamada, K. Iso, J. S. Speck, S. Nakamura, andS. P. DenBaars, “Non-polar-oriented InGaN light-emitting diodes forliquid-crystal-display backlighting,” J. Soc. Inf. Displays, vol. 16,pp. 571–578, 2008.


[96] N. N. Fellows, H. Sato, Y. Lin, R. B. Chung, S. P. DenBaars, andS. Nakamura, “Dichromatic color tuning with InGaN-based light-emitting diodes,” Appl. Phys. Lett., vol. 93, no. 12, p. 121 112,Sep. 2008.

[97] M. Ueda, M. Funato, K. Kojima, Y. Kawakami, Y. Narukawa,and T. Mukai, “Polarization switching phenomena in semipolarInxGa1−xN/GaN quantum well active layers,” Phys. Rev. B, Condens.Matter, vol. 78, no. 23, p. 233 303, Dec. 2008.

[98] N. Fellows, H. Sato, H. Masui, S. P. DenBaars, and S. Nakamura,“Increased polarization ratio on semipolar (112̄2) InGaN/GaN light-emitting diodes with increasing indium composition,” Jpn. J. Appl.Phys., vol. 47, no. 10, pp. 7854–7856, 2008.

[99] N. Fellows, H. Sato, H. Masui, S. P. DenBaars, and S. Nakamura,“Erratum: Increased polarization ratio on semipolar (112̄2) InGaN/GaNlight-emitting diodes with increasing indium composition,” Jpn. J. Appl.Phys., vol. 48, no. 4, p. 049201, 2009.

[100] H. Masui, H. Asamizu, A. Tyagi, N. F. DeMille, S. Nakamura, andS. P. DenBaars, “Correlation between optical polarization and lumines-cence morphology of (112̄2)-oriented InGaN/GaN qunatum-well struc-tures,” Appl. Phys. Express, vol. 2, no. 7, p. 071002, 2009.

[101] D. S. Sizov, R. Bhat, J. Napierala, C. Gallinat, K. Song, and C. Zah,“500-nm optical gain anisotropy of semipolar (112̄2) InGaN quantumwells,” Appl. Phys. Express, vol. 2, no. 7, p. 071001, 2009.

[102] T. Koyama, T. Onuma, H. Masui, A. Chakraborty, B. A. Haskell,S. Keller, U. K. Mishra, J. S. Speck, S. Nakamura, S. P. DenBaars,T. Sota, and S. F. Chichibu, “Prospective emission efficiency and in-plane light polarization of nonpolar m-plane InxGa1−xN/GaN bluelight emitting diodes fabricated on freestanding GaN substrates,” Appl.Phys. Lett., vol. 89, no. 9, p. 091906, Aug. 2006.

[103] T. Onuma, H. Amaike, M. Kubota, K. Okamoto, H. Ohta, J. Ichihara,H. Takasu, and S. F. Chichibu, “Quantum-confined Stark effects in them-plane In0.15Ga0.85N/GaN multiple quantum well blue light-emittingdiode fabricated on low defect density freestanding GaN substrate,”Appl. Phys. Lett., vol. 91, no. 18, p. 181 903, 2007.

[104] H. Masui, H. Asamizu, T. Melo, H. Yamada, K. Iso, S. C. Cruz,S. Nakamura, and S. P. DenBaars, “Effects of piezoelectric fields onoptoelectronic properties of InGaN/GaN quantum-well light-emittingdiodes prepared on nonpolar (101̄0) and semipolar (112̄2) orientations,”J. Phys. D, Appl. Phys., vol. 42, no. 13, p. 135 106, Jul. 2009.

[105] H. Shen, M. Wraback, H. Zhong, A. Tyagi, S. P. DenBaars, S. Nakamura,and J. S. Speck, “Determination of polarization field in a semipolar(112̄2) InGaN/GaN single quantum well using Franz–Keldysh oscilla-tions in electroreflectance,” Appl. Phys. Lett., vol. 94, no. 24, p. 241 906,Jun. 2009.

[106] G. A. Garrett, H. Shen, M. Wraback, A. Tyagi, M. C. Schmidt,J. S. Speck, S. P. DenBaars, and S. Nakamaura, “Comparison of time-resolved photoluminescence from InGaN single quantum wells grownon nonpolar and semipolar bulk GaN substrates,” Phys. Stat. Sol. (C),vol. 6, no. S2, pp. S800–S803, Jun. 2009.

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[108] H. Shen, M. Wraback, H. Zhong, A. Tyagi, S. P. DenBaars, S. Nakamura,and J. S. Speck, “Unambiguous evidence of the existence of polariza-tion field crossover in a semipolar InGaN/GaN single quantum well,”Appl. Phys. Lett., vol. 95, no. 3, p. 033503, Jul. 2009.

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[111] H. Masui, H. Kroemer, M. C. Schmidt, K.-C. Kim, N. N. Fellows,S. Nakamura, and S. P. DenBaars, “Electroluminescence efficiency of(101̄0)-oriented InGaN-based light-emitting diodes at low temperature,”J. Phys. D, Appl. Phys., vol. 41, no. 8, p. 082001, Apr. 2008.

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Hisashi Masui (M’09) was born in Tokyo, Japan.He received the M.S. degree in electrical engineeringfrom Ibaraki University, Hitachi, Japan, in 1991 andthe Ph.D. degree in materials from the University ofCalifornia, Santa Barbara (UCSB), in 2007.

He was with Stanley Electric Company Ltd.,Yokohama, Japan as a Researcher. He was also a Vis-iting Research Scholar at UCSB under the directionof Prof. S. P. DenBaars. He is currently a Postdoc-toral Researcher with the Solid-State Lighting andEnergy Center at UCSB. His research interests in-

clude III-nitride semiconductors, electroluminescence, and solid-state lighting.Dr. Masui is a member of the Optical Society of America, the Society of

Information Displays, and the Japan Society of Applied Physics.


Shuji Nakamura received the M.S. and Ph.D. de-grees from Tokushima University, Tokushima, Japan,in 1979 and 1994, respectively.

He was with Nichia Corporation, Tokushima. In2000, he joined the University of California, SantaBarbara (UCSB) as a Professor. He is currently aCodirector of the Solid-State Lighting and EnergyCenter, Materials Department, UCSB. His currentresearch interests include III-nitride thin-film tech-nologies and growth of bulk GaN crystals.

Dr. Nakamura was the recipient of the IEEELasers and Electro-Optics Society (LEOS) Engineering Achievement Award(1996), the IEEE Jack A. Morton Award (1998), the LEOS DistinguishedLecturer Award (2001), and the LEOS Quantum Electronics Award (2002), andthe 2006 Millennium Technology Prize of the Finnish Prize Foundation.

Steven P. DenBaars (M’91–SM’03–F’05) receivedthe Ph.D. degree in electrical engineering from theUniversity of Southern California, Los Angeles, in1988, under the direction of Prof. P. D. Dapkus.

From 1988 to 1991, he was a Member of TechnicalStaff of Hewlett-Packards Optoelectronics Division.He is currently a Professor with and a Codirectorof the Solid-State Lighting and Energy Center, Ma-terials Department, University of California, SantaBarbara. His current research interests include met-alorganic chemical vapor deposition (MOCVD) of

III–V compound semiconductor materials and devices, specifically the growthof III-nitride semiconductors and their application to blue LEDs and lasers andhigh-power electronic devices.

Dr. DenBaars is a Fellow of the IEEE Lasers and Electro-Optics Society. Hereceived the National Science Foundation Young Investigator Award in 1994.

Umesh K. Mishra (F’95) received the M.S. degreein electrical engineering from Lehigh University,Bethlehem, PA, in 1981 and the Ph.D. degree in elec-trical engineering from Cornell University, Ithaca,NY, in 1984.

He is a Professor with the Department of Electricaland Computer Engineering, University of California,Santa Barbara. He made major contributions in thearea of high-speed field effect transistors at everylaboratory and academic institution that he was with,including North Carolina State University, Raleigh,

Hughes Research Laboratories, Malibu, CA, University of Michigan, AnnArbor, and General Electric, Syracuse, NY. His research interests include elec-tronics and photonics: high-speed transistors, semiconductor device physics,quantum electronics, optical control, design and fabrication of millimeter-wavedevices, in situ processing, and integration techniques.

Dr. Mishra was the recipient of the 2007 IEEE David Sarnoff Award.

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Nonpolar and Semipolar III-Nitride Light-Emitting Diodes: Achievements and Challenges

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