Simultaneously Enhancing Light Emission and Suppressing EfficiencyDroop in GaN Microwire-Based Ultraviolet Light-Emitting Diode bythe Piezo-Phototronic EffectXingfu Wang,†,§,∥ Wenbo Peng,†,∥ Ruomeng Yu,† Haiyang Zou,† Yejing Dai,† Yunlong Zi,†

Changsheng Wu,† Shuti Li,*,§ and Zhong Lin Wang*,†,‡

†School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States‡Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China 100083§Laboratory of Nanophotonic Functional Materials and Devices, Institute of Optoelectronic Materials and Technology, South ChinaNormal University, Guangzhou, China 510631

*S Supporting Information

ABSTRACT: Achievement of p−n homojuncted GaN enables the birthof III-nitride light emitters. Owing to the wurtzite-structure of GaN,piezoelectric polarization charges present at the interface can effectivelycontrol/tune the optoelectric behaviors of local charge-carriers (i.e., thepiezo-phototronic effect). Here, we demonstrate the significantlyenhanced light-output efficiency and suppressed efficiency droop inGaN microwire (MW)-based p−n junction ultraviolet light-emitting diode(UV LED) by the piezo-phototronic effect. By applying a −0.12% staticcompressive strain perpendicular to the p−n junction interface, the relativeexternal quantum efficiency of the LED is enhanced by over 600%.Furthermore, efficiency droop is markedly reduced from 46.6% to 7.5%and corresponding droop onset current density shifts from 10 to 26.7 Acm−2. Enhanced electrons confinement and improved holes injectionefficiency by the piezo-phototronic effect are revealed and theoreticallyconfirmed as the physical mechanisms. This study offers an unconventional path to develop high efficiency, strong brightness andhigh power III-nitride light sources.

KEYWORDS: Piezo-phototronic effect, light emission, efficiency droop, GaN, UV LED

Group III-nitride-based ultraviolet (UV) light-emittingdiodes (LEDs) have attracted considerable interest due

to a wide range of applications such as water treatment, high-density optical data storage, sterilization of medical equipment,and biological imaging.1 Although III-nitride LEDs have beencommercialized and widely used in backlighting and generalillumination, quantum efficiency enhancement and light outputimprovement is still necessary to further reduce the cost andexpand their applications. Especially, UV LED sources sufferfrom relatively low quantum efficiencies that are still far behindthose of the visible region.2,3 Ideally, increasing currentinjection enables the improvement of internal quantumefficiency (IQE) and light output in LEDs. Unfortunately,this straightforward route to cost-reduction is not readilyavailable in practical GaN-based LEDs due to a phenomenon,generally called “efficiency droop” that makes IQE to decline athigh injection current density.4,5 The efficiency droop has longbeen an obstacle to develop high efficiency and high powerLEDs. Although the physical origin of the droop effect is stillbeing debated, many proposals have been forwarded such asJoule heating,6,7 poor holes injection efficiency,8,9 electronsleakage,10,11 and so forth.

The large ionic component of the Ga−N bonds, combinedwith the deviation of their equilibrium lattice structure fromideal wurtzite crystals, give rise to giant spontaneous polar-ization fields in III-nitride semiconductors.12,13 The intrinsicpolarizations in the nitrides play essential roles in correspond-ing devices. For example, high-density two-dimensionalelectron gas (2DEG) forming at AlGaN/GaN interfaces causedby spontaneous and lattice-mismatch-induced piezoelectricpolarizations enable the performances of nitride high-electronmobility transistors to surpass transistors made from any othersemiconductor family in radio frequency power performance.14

Moreover, current commercial GaN LEDs grown along c-axisare negatively impacted by the quantum confined Stark effect(QCSE) due to large polarization-related spontaneous andpiezoelectric fields.15 Therefore, several technological mod-ifications, aiming to modify the polarization field in GaN-basedLEDs, have been proposed as droop remedies, such as varying

Received: March 8, 2017Revised: April 25, 2017Published: May 10, 2017

Letter

pubs.acs.org/NanoLett

© XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.7b01004Nano Lett. XXXX, XXX, XXX−XXX

the volume of active region,9,16 exploring semipolar or nonpolardevices,17,18 engineering the energy band profile by insertinginterlayers.19 Most of these methods were realized bycontinuously optimizing material growth parameters/processes,which inevitably add the cost and complexity of devicesfabrication and also may degrade the crystal quality caused bythe introduction of dislocations. Therefore, an alternativestrategy for efficiency enhancement and droop suppression istherefore highly desirable.Owing to lacking inversion symmetry in wurtzite-structured

semiconductors, piezoelectric polarizations can be inducedunder external deformation and then utilized to modulate thecharge carrier transport processes in piezoelectric semi-conductors, which are referred to as the piezotronic effect.20,21

Furthermore, the piezo-phototronic effect22,23 is introduced asa three-way coupling among piezoelectric polarization, semi-conductor property and optical excitation by applying piezo-electric charges (piezo-charges) presented at the vicinity of thelocal interface to control/tune generation, separation, recombi-nation, and transport of charge carriers within piezoelectricsemiconductors. These two emerging effects have drawnincreasing research interests recently and have been utilizedto optimize the performances of GaN-based devices, such astuning the photoluminescence properties of InGaN/GaNquantum structure,24 enhancing the photoresponsivity of singleGaN microwire,25 and modulating the 2DEG transportproperties in AlGaN/AlN/GaN heterostructured microwires.26

In the present work, the piezo-phototronic effect is utilized tosimultaneously enhance quantum efficiency and suppressefficiency droop in GaN microwire (MW)-based p−n junction

UV LED. By applying a −0.12% static compressive strainperpendicular to the p−n junction interface, the relativeexternal quantum efficiency (EQE) of GaN-based UV LED isenhanced by over 600%. An obvious efficiency droop isobserved in the GaN-based LEDs as increasing injectioncurrent beyond certain value under strain-free condition. Whenthe piezo-phototronic effect is introduced by applying a−0.12% static compressive strain, the efficiency droop issignificantly reduced from 46.6% to 7.5% and correspondingdroop onset current density shifts from 10 to 26.7 A cm−2.Performances optimization of our GaN MW-based LEDs isattributed to the enhanced electrons trapping and reducedelectrons leakage within the p−n junction region and uniformcarriers injection due to energy bands tilt across the bulk region(n- and p-type region). Theoretical calculations are systemati-cally performed to confirm the proposed working mechanisms.This study presents in-depth understandings about the piezo-phototronic effect in p−n homojunctions and offers anunconventional path for the development of high efficiency,strong brightness, and high-power III-nitride visible/UV LEDs/LDs.GaN MWs used in this work were synthesized by a metal−

organic chemical vapor deposition (MOCVD) system asdescribed elsewhere.27 The synthesized GaN MWs are alongthe nonpolar ⟨211 0⟩ (i.e., a-axis) direction with their c-axispointing across its thickness direction, which has been fullycharacterized in our previous work.28 The correspondingatomic structure models (overall and cross-section view) ofthe GaN MW are shown in Figure 1a1. A Mg-doped p-typeGaN film grown on a sapphire substrate and an indium tin

Figure 1. General characteristics of GaN MW-based LEDs. (a) Atomic structure model (overall and cross-section view) of the GaN MW (a1) andschematic image of the fabricated LED (a2). (b) Schematic image showing the light emission escaped from the distal end of the GaN MW. Upperinset: light spot of the MW-based LED observed under microscope. Lower inset: SEM image of the trapezoid-shaped end surface of GaN MW. (c)I−V characteristic of the as-fabricated LED. Inset: photograph of a real MW-based LED device. (d) Room-EL spectrum collected from the MW-based LED device and corresponding peak-deconvolution with Gaussian functions of the EL spectrum.

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oxide (ITO)-coated sapphire substrate were used to fabricatep−n junction LED with a single n-GaN MW (SupportingInformation Method S1). Figure 1a2 schematically shows theGaN-based p−n junction LED. Under external bias above turn-on voltage, light emission occurs at p−n junction interface andescapes from the distal end of the GaN MW (Figure 1b) due tothe Fabry−Perot waveguide behavior.29,30 A bright light spot(upper right, inset in Figure 1b) from the end of the GaN MWwas observed and collected by a microscope objective. Thescanning electron microscope (SEM) image of the trapezoid-shaped end surface of GaN MW (lower right, inset in Figure1b) indicates that its polar c-axis points away from p-GaN filmand −c plane contacted with p-GaN to form p−n junction. TheI−V characteristic of the as-fabricated LED under strain-freecondition (Figure 1c) exhibits a strong rectification behaviorwith quite low reverse leakage current. The low turn-on voltageestimated to be ∼2.8 V implies high p−n junction quality of theLEDs and good Ohmic contact formed at two electrodes(Supporting Information Figure S1a). A digital image of the as-fabricated p−n junction GaN MW-based LED device is shownin the inset of Figure 1c. Room-temperature electrolumines-cence (EL) spectra under various injection currents are shownin Supporting Information Figure S1b. A strong UV lightemission (Figure 1d) centered at around 382 nm is obtainedunder 30 μA injection current and its full width at half-maximum (fwhm) is ∼42 nm. Peak-deconvolution withGaussian functions exhibits that the broad spectrum consistsof six distinct bands and each emission band corresponds to aparticular recombination process. Emission origin analyses areillustrated in Supporting Information Note S1 and confirmed

by room-temperature photoluminescence (PL) spectrum takenfrom n-GaN MW (Supporting Information Figure S1c).To explore the piezo-phototronic effect on the performances

of the as-fabricated GaN NW-based LED devices, a homemademeasurement system (Figure 2a) consisting of an invertedmicroscope, a piezo-nanopositioning stage, and a fiber opticalspectrometer was utilized to simultaneously collect theelectrical characteristics, output light spectra and brightness oflight spot of the GaN MW-based LEDs under a series ofuniform external strains (Supporting Information Method S1and S2). At a fixed external bias, the output currents of the LEDdevices increase gradually with increasing the compressivestrain from 0.00% to −0.12% (Figure 2b). Associated physicalmechanisms will be discussed later. Under a fixed injectioncurrent of 20 μA, the significantly enhanced light intensity byapplied compressive strain can be directly observed in opticalimages of the light spot recorded by a CCD (upper panel inFigure 2c) and corresponding light output spectrum under eachcompressive strain (lower panel in Figure 2c). The integrationarea of each emitted spectra is further calculated (magenta,Figure 2d) to represent the relative light intensity Ψ of theLED, indicating the emitted light intensity is graduallyenhanced by increasing the external compressive strain. Therelative EQE (ηex) of the GaN MW-based LED can berepresented by ψ/J (Supporting Information Note S2). Thecalculated ηex (blue, Figure 2d) is enhanced by over 450%under −0.12% applied compressive strain at the fixed injectioncurrent density J of 33.3 A cm−2.Efficiency droop is another main restrictive factor for

achieving high power-conversion efficiency and high-powerGaN-based LEDs. Generally, the physical origin of efficiency

Figure 2. Enhancement of emitted light intensity by piezo-phototronic effect. (a) Schematic diagram of the homemade measurement system forcharacterizing the performances of MW-based LEDs under compressive strain. (b) I−V characteristic of the as-fabricated LED at forward bias withthe variation of the applied compressive strain. (c) Optical spectra collected from the MW-based LED at 20 μA injection current under variouscompressive strains (lower panel) and corresponding CCD images recorded from the emitting end of the GaN MW under different applied strain.(d) Change of the integrated EL intensity (magenta) and relative EQE (blue) of the MW-based LED with increasing externally compressive strain.

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droop phenomenon in GaN-based LEDs is attributed tothermal and/or nonthermal mechanisms.31 In order to rule outthat thermal effect is the dominant mechanism of efficiencydroop here, two distinct current injection modes, continuousmode and pulsed mode (duty cycle of 20%), are utilized todrive our MW-based LEDs. Figure 3a shows EL spectra undervarious injection current densities from 3.3 to 83.3 A cm−2

under continuous and pulsed operation mode. Correspondingpeak locations (Figure 3b) are extracted and indicate that thespectra peak wavelength first blueshift to shorter wavelengthunder low injection current for both continuous and pulsedmodes. Then it exhibits an obvious redshift of about 10 nmunder continuous mode as further increasing the currentdensity whereas it remains almost constant for pulsed mode.The blueshift of the spectra peak wavelength under lowinjection current arises from the Fermi-level band fillingeffect32,33 and the redshift under high injection current iscaused by Joule-heating.34,35 No obvious peak wavelength shiftwas observed as increasing the injection current under pulsedcurrent injection mode, indicating that the thermal effect in thep−n junction of GaN MW-based LEDs is largely relieved at thissituation.Efficiency droop in the GaN MW-based LEDs under various

external compressive strains and a series of injection currents isthen systematically studied under pulsed operation mode.Relative EL intensity (Ψ) of the MW LED devices as a functionof injection current density (J) at various compressive strains(Supporting Information Figure S1d) indicate the light

intensity increases with increasing the injection currentdensity/externally applied compressive strain under a fixedcompressive strain/injection current density condition. Corre-sponding ηex of the MW LED devices derived by Ψ/J at eachinjection current density and each compressive strain conditionare further calculated (Figure 3c). Under strain-free condition(black curve, Figure 3c), ηex first increases with increasing theinjection current density, and then significantly drops ascontinuous increase of the current density after reaching itsmaximum value at the current density of 10 A cm−2. This highdroop onset current density is impressive compared to thepreviously reported planar structured GaN LEDs,36,37 which isattributed to high light extraction efficiency due to 1Dgeometry of n-GaN MW and high crystal quality of our n-GaN MW and p-GaN film. Furthermore, ηex significantly dropsdown by 46.6% when the pulsed current density increases to83.3 A cm−2, which indicates that thermal effect is not thedominant mechanism of efficiency droop in our GaN MW-based LEDs. As the externally applied compressive strainincreases from −0.00% to −0.12%, the efficiency droop ismarkedly reduced from 46.6% to 7.5% and correspondingdroop onset current density gradually shifts from 10 to 26.7 Acm−2 (Figure 3d). Various technological modifications38−40

have been reported and utilized to suppress the efficiencydroop in InGaN multiquantum well (MQW) LEDs byregulating internal polarization. On the basis of theexperimental observations above, the piezo-phototronic effect,

Figure 3. Suppression of efficiency droop by piezo-phototronic effect. (a) EL spectrum under various injection current density from 3.3 to 83.3 Acm−2 under continuous (left) and pulsed (right) current injection mode. (b) Evolution of the peak wavelength as increasing injection current densityunder continuous (magenta) and pulsed (blue) mode. (c) Corresponding relative EQE (ηex) of the MW LED devices represented by Ψ/J at eachinjection current density (J) point under various compressive strains. (d) Droop onset current densities and the change of efficiency drop values as afunction of the external applied strains. (e) Relative change of ηex at each injection current density condition.

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as an unconventional approach, can effectively relieves thedroop efficiency in the p−n junction GaN MW-based LED.Further study of the piezo-phototronic effect on the

efficiency enhancement was performed by calculating therelative change of ηex which is represented by [(Ψ/J)strain=−0.12%− (Ψ/J)strain=−0.00%)]/(Ψ/J)strain=−0.00% at each injection currentdensity condition (Figure 3e). Obviously, ηex is enhanced byover 600% at certain injection current density conditions undera −0.12% compressive strain. At low J less than 10 A cm−2, therelative variation of ηex decreases as increasing J due to theincrease of J dominant ηex in our GaN MW-based LEDs. As Jfurther increasing, the relative change of ηex gradually increases,indicating that ηex of the LED is largely enhanced and efficiencydroop is effectively suppressed under high injection currentdensity by the piezo-phototronic effect.To fully understand and analyze the piezo-phototronic effect

on the performances of our GaN MW-based LED, piezo-potential distribution across the LED under compressive strainswere systematically calculated (Method S2a, SupportingInformation). Under a −0.12% static compressive strain, theoverall view and cross-section view of piezo-potentialdistribution within the LED are simulated and presented inFigure 4a and inset of Figure 4b, respectively. Corresponding

piezo-potential values across the LED device are extracted andplotted in Figure 4b. Obviously, negative piezo-charges isinduced at the top surface (+c plane) of n-GaN MW andpositive at the bottom surface of p-GaN film. The netpolarization charges at the p−n junction is simulated to bepositive as a result of electrostatic coupling between theinduced positive piezo-charges at −c plane of n-GaN MW andnegative piezo-charges at p-GaN surface. The built-in electricfield, energy band profile within the depletion region of the p−n junction, and corresponding device performances of theLEDs are naturally and inevitably modulated by the inducedpiezo-charges.The generation of piezo-potential and corresponding energy

band profile of the LED devices is carefully analyzed understrain-free and compressive strain conditions to illustrate thephysical mechanism of the performances optimization in ourp−n junction LEDs (Figure 4c). Under strain-free condition,depletion region and built-in electric field (Eb) are generated atthe p−n junction interface with the direction of Eb pointingaway from n- to p-type region (upper left). The correspondingenergy band diagram of the p−n junction at forward bias(lower left) exhibits the charge carriers (electrons and holes)inject into the junction region from bulk region and recombine

Figure 4. Theoretical simulation results and physical mechanisms. (a) Under a −0.12% compressive strain, the overall view of piezo-potentialdistribution. (b) Piezo-potential values across the LED device normal to the p−n junction interface; inset: cross-section view of piezo-potentialdistribution within the GaN MW-based LEDs under −0.12% compressive strain. (c) Generation of piezo-potential and corresponding energy bandprofile of the LED devices under strain-free (left) and compressive strain (right) conditions.

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with each other. During this process, the electrons from n-typeregion go through depletion region and leak into p-GaN regionwith strong possibility due to their high mobility (up to ∼200cm2 (V·S)−141), leading to severe nonradiative recombinationdue to the existence of a large number of nonradiativerecombination centers within p-GaN. On the other hand, poorholes mobility in GaN (in the order of 10 cm2 (V·S)−1) leads tononuniform/asymmetric carriers injection (insufficient holeinjection) in junction region. Therefore, electrons leakage andnonuniform/asymmetric carriers injection results in low powerconversion efficiency and efficiency droop in the p−n junctionLED, especially under high current injection condition.Under externally applied compressive strain (upper right),

two extra piezoelectric fields Fpzn and Fpz

p with opposite directionwith Fb are induced within n-GaN MW and p-GaN film,respectively. The generation of Fpz

n and Fpzp also weakens Fb and

tilts the energy band across n- and p-type region, leading toeffective energy band gap (Eg) shrinking for n- and p-typeregion.42 The magnitude of Fb is equal to the barrier height(Φb) of the p−n junction, which can be expressed by

Φ = qVb d (1)

where Vd is the contact potential difference of the p−njunction43 and

=⎛⎝⎜

⎞⎠⎟V

k Tq

N Nn

lni

db A D

2(2)

in which NA and ND are concentration of acceptors and donors.ni2 is the product of electrons and holes concentration innondegenerate semiconductor under equilibrium, which can bewritten as

= −⎛⎝⎜

⎞⎠⎟n N N

E

k Texpi

2C V

g

b (3)

where NC and NV are the effective density of states at thebottom of conductive band and top of the valence band,respectively. According to above equations, Vd is proportionalto Eg that is reduced under compressive strain, and thus Vd andΦb can also be effectively reduced by externally appliedcompressive strains. Details about the quantitative calculationsare founded in Supporting Information (Method S2b andFigure S3). The induced Fpz

p within p-GaN film and the reducedFb effectively promote the holes injection from p-type intojunction region, leading to more uniform carrier injection andhigher radiative combination efficiency with electrons. Moreimportantly, an electron potential dip is formed as a result ofthe local positive piezo-potential within depletion regioninduced by external compressive strain (lower right). Theelectrons injected from n-GaN can be temporarily trapped andaccumulated in the potential dip and thus facilitates theradiative recombination with holes at p−n junction region.Furthermore, the formed electron potential dip also acts as anatural electron-blocking barrier to prevent the leakage ofelectrons into p-GaN region, giving rise to significantly reducednonradiative combination in p-GaN region. According to thediscussion above, the enhancement of light emission efficiencyand suppression of efficiency droop in GaN-MW based LEDscan be effectively and simultaneously achieved by engineeringthe energy band profile induced by piezo-phototronic effect.In order to further confirm our experimental results, the c-

axis orientation of n-GaN MW was inverted by micro-

manipulation with its c-axis pointing to p-GaN film and +cplane contacted with p-GaN film (Figure S4a). EL character-istics of this kind of LEDs collected under various compressivestrains (Figure S4b) exhibit that the light emission intensitydecreases gradually with increasing the externally appliedcompressive strain. Theoretical simulations of this kind ofLEDs were also conducted under a −0.12% compressive strainto understand and confirm the physical mechanisms (FigureS4c,d, Note S3). Experimentally, 28 devices were fabricated bymanipulating the c-axis orientation of GaN MW pointing to p-GaN (downward) or away from p-GaN (upward). 18 LEDsamong them with c-axis of GaN MW pointing upward exhibitenhanced light emission and reduced efficiency droop undercompressive strain, exhibiting the same trends and approximatemagnitude of change. The other 10 LEDs with c-axis of GaNMW pointing downward exhibit obviously reduced emittinglight intensity as increasing the externally applied compressivestrain. This fact indicates that the observed performancesoptimization in our GaN p−n junction LEDs is dominated bypolar piezo-potential effect rather than any nonpolar effectssuch as change in contact-area and/or piezoresistance.In conclusion, the piezo-phototronic effect is utilized to

simultaneously enhance the quantum efficiency and suppressthe efficiency droop in GaN MW-based p−n homojunction UVLEDs. By applying a −0.12% compressive strain perpendicularto the p−n junction interface, the relative EQE of the LEDs isenhanced by over 600%. The efficiency droop is significantlyreduced from 46.6% to 7.5% and corresponding droop onsetcurrent density shifts from 10 to 26.7 A cm−2 when the piezo-phototronic effect is introduced by applying a −0.12% staticcompressive strain. The corresponding physical mechanismsare carefully proposed and fully confirmed by theoreticalcalculations. This study not only presents in-depth under-standings about the piezo-phototronic effect in p−n homo-junctions but also offers a novel approach to develop highefficiency, strong brightness and high power III-Nitride visible/UV light emitters.

■ ASSOCIATED CONTENT*S Supporting InformationThis material is free of charge via the Internet at TheSupporting Information is available free of charge on the ACSPublications website at DOI: 10.1021/acs.nanolett.7b01004.

More detailed information about device fabrication andexperimental setup; calculation of strain, piezo-potentialdistribution; origin analysis of EL spectra; character-ization of the as-fabricated LED; discussion about thetheoretical simulation results; electrical transport andoptical properties of the as-fabricated micro-LED;experimental and theoretical simulation results of theGaN MW-based LEDs with inverted c-axis orientationfor n-GaN MW (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: zhong.wang@mse.gatech.edu.*E-mail: lishuti@scnu.edu.cn.ORCIDYunlong Zi: 0000-0002-5133-4057Zhong Lin Wang: 0000-0002-5530-0380Author Contributions∥X.W. and W.P. contributed equally to this work.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Research was supported by U.S. Department of Energy, Officeof Basic Energy Sciences (Award DE-FG02-07ER46394), theNational Key R & D Project from Minister of Science andTechnology (2016YFA0202704) and the “thousands talents”program for pioneer researcher and his innovation team, China.This work is also supported by supported by the NationalNatural Science Foundation of China (Grants 11474105 and51172079), the Science and Technology Program ofGuangdong Province, China (Grants 2015B090903078 and2015B010105011) the Science and Technology Project ofGuangzhou City (No. 201607010246).

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Nano Letters Letter

DOI: 10.1021/acs.nanolett.7b01004Nano Lett. XXXX, XXX, XXX−XXX

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