Enhanced optical output power of blue light-emitting diodes with quasi-aligned gold nanoparticles.

1 MB·English

by  Jin, Yuanhao

Jinetal.NanoscaleResearchLetters2014,9:7
http://www.nanoscalereslett.com/content/9/1/7
NANO EXPRESS Open Access
Enhanced optical output power of blue
light-emitting diodes with quasi-aligned gold
nanoparticles
Yuanhao Jin1,2, Qunqing Li1,2*, Guanhong Li1,2, Mo Chen1,2, Junku Liu1,2, Yuan Zou1,2, Kaili Jiang1,2
and Shoushan Fan1,2
Abstract
The output power of thelight from GaN-based light-emitting diodes(LEDs) was enhanced by fabricating gold(Au)
nanoparticles on the surface of p-GaN. Quasi-aligned Au nanoparticle arrays were prepared by depositing Au thin
film onan aligned suspended carbon nanotube thinfilm surface and then putting the Au-CNT system onthe
surface of p-GaN and thermally annealing the sample. The size and position of the Aunanoparticles were confined
bythecarbon nanotube framework, and no other additional residual Au was distributed on thesurfaceof the
p-GaN substrate. The output power of the light from theLEDs withAu nanoparticleswas enhanced by55.3%for
aninjected current of100mA withtheelectrical property unchangedcomparedwiththeconventional planar LEDs.
The enhancementmay originate from thesurface plasmon effect and scattering effect of theAu nanoparticles.
Keywords: GaN; Light-emitting diodes (LEDs);Au nanoparticles;Carbon nanotube
Background enhance the light emission efficiency in LEDs [12]. Se-
Much research has been devoted towards gallium nitride veral methods have been suggested to fabricate metal
(GaN)-based semiconductor devices, especially in terms nanoparticles (NPs) on LEDs to improve their efficiency.
of applications for light-emitting diodes (LEDs), which These include thermal annealing process [13-16], chem-
operate in the blue and green wavelength regions. GaN- ical synthesis [17], and gas etching technique. For noble
based LEDs have attracted interest for use in full-color metal nanoparticles on GaN surfaces, the collective os-
display panels and solid-state lighting [1] because they cillations of the electrons are localized surface plasmons
have advantages such as low energy consumption, long (LSPs) [18,19]. Under the resonance condition, this LSP
lifetimes, and relatively small sizes. However, in conven- effect enables the metallic nanoparticles to capture the
tional planar LEDs, the light extraction efficiency is lim- trapped light in the p-GaN layer of the LEDs, enhancing
ited by several factors including the high refractive index the extracted efficiency of the light. However, the LSP
of p-GaN (approximately 2.52), leading to a low total in- resonance effect is affected by the geometry and separ-
ternal reflection (TIR) angle [2]. To enhance the output ation of the nanoparticles. When noble metal nanopar-
light power, various approaches, such as surface textur- ticles are fabricated with a thermal annealing process, it
ing [3,4], photonic crystals [5-7], and metal oxide nano- is important to control the distribution and size of the
particles[8-11],havebeen studied. nanoparticles. Furthermore, the residual metal after the
Surface plasmons (excitations on a rough metallic sur- annealing process has a negative influence on the device
face by the interaction between light and the metal performance.
nanoparticles) were suggested as a way to significantly We report a simple method for making quasi-aligned
Au nanoparticle arrays on p-GaN surfaces using supe-
raligned multiwall carbon nanotubes (CNTs). The LED
*Correspondence:[email protected]
1StateKeyLaboratoryofLow-DimensionalQuantumPhysics,Departmentof devices containing quasi-aligned Au particles exhibited
Physics&Tsinghua-FoxconnNanotechnologyResearchCenter,Tsinghua efficient electrical properties, and the optical output po-
University,Beijing100084,China
wer was significantly increased compared with devices
2CollaborativeInnovationCenterofQuantumMatter,Beijing100084,China
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inanymedium,providedtheoriginalworkisproperlycited.
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without Au particles. By eliminating any chemical or distributed along the former CNT path and the quasi-
etching processes, this method has potential for excellent alignedparticlearrayswereformed.TheCNTfilmsplayed
integration with semiconductor technologies. Further- animportantroleinactingasaframeandcouldbeeasily
more,weobservedthatthequasi-alignedAunanoparticle removedwithanannealingprocess.TheAuwasdeposited
arrays also had an effect on the polarization performance around the CNTs, and there was no redundant Au depo-
oftheLEDs. sited on the device surface. Thus, there was no residual
Authatneededtoberemovedaftertheannealingprocess,
Methods preventing any negative impacts on the performance of
The CNT thin films were directly drawn out from the the device from the optical and electrical aspect. Further-
CNT arrays [20], which were composed of CNTs with more, we could control the distribution of the nanoparti-
diameters around 10 nm and were aligned parallel in cles by adjusting the deposition volume. The size and
one direction. This method is convenient for mass pro- densityoftheAunanoparticlesdependedonthethickness
duction of CNT films at a low cost. In our experiment, of the Au film evaporated on the CNTs. Larger particles
the CNT thin films were pulled out from a superaligned andahigherdensitydistributioncouldbefabricatedfrom
CNT array grown on a 4-in.silicon wafer and fixed to the Au-CNT system with a thicker Au film. The size of
metal frames. We then fabricated the Au films using theparticlesandtheirquantitychangedcontinuouslywith
electron beam evaporation on the suspended CNTfilms the Au thickness on the CNT films. Figure 2 shows a
with thicknesses in the range of 1 to 5 nm. The GaN comparisonofthe2-and5-nmAu-CNTsystemstoinves-
LED wafers consisted of a 200-nm-thick p-type GaN tigate the morphology of the Au nanoparticles obtained.
layer, a layer containing InGaN/GaN quantum wells, Compared with the Au nanoparticles derived from the
an n-type GaN layer, and a GaN buffer layer. The as- 2-nmAu-CNTsystem,theAunanoparticlesderivedfrom
prepared Au-CNT films were transferred directly onto the 5-nm Au-CNT system were larger in both size and
the GaN substrates. We used alcohol on the Au-CNT/ quantity.Theaveragediameterswerearound20to25nm
GaN interface to make the carbon nanotubes shrink, and 30 to 35 nm for the nanoparticles derived from the
allowing the film to form a close contact with the sub- 2- and 5-nm Au-CNTsystems, respectively. The heights
strate. Afterwards, the Au-CNT films were thermally were measured using an atomic force microscope (AFM)
annealed at 600°C for 30 min in ambient air, and then acquired with aVeeco Dimension V (Veeco Instruments
the CNTfilms were completely removed because of the Inc., Plainview, NY, USA). The spaces between the nano-
high temperature, inhibiting a decrease in the transmit- particles were from 20 to 70 nm for the 2-nm Au-CNT
tance of the carbon nanotubes. During the annealing system. The spaces between the nanoparticles from the
process,themetalAufilmsintheAu-CNTsystemformed 5-nmAu-CNTsystemwerearound30to70nm.
Au nanoparticles that were bound to the surface. The After fabricating the Au nanoparticles, the GaN wafers
fabrication process of the Au nanoparticles using an were used to fabricate LEDs using standard procedures
Au-CNT system is illustrated in Figure 1. with a mesa area of 1 mm2. A transparent conducting
A scanning electron microscope (SEM) image of a car- layer (TCL) of Ni (2 nm)/Au (5 nm) was deposited on
bon nanotubethin filmisshown in Figure2a. Figure2b,c the p-GaN surface. Ni (5 nm)/Au (100 nm) electrodes
shows top views of the scanning electron microscope im- were then deposited by photolithography exposure and
agesoftheAunanoparticlesonGaNsubstratesthatwere electron-beam evaporation on the n-GaN layer and the
derived from the 2- and 5-nm Au-CNTsystems through TCL as n- and p-pads, respectively. For comparison, a
an annealing process. The schematic representation of a standard LED device was fabricated with a TCL depo-
GaN LED with embedded Au nanoparticles is shown in sited directly on the p-GaN surface with all other fabri-
Figure 2d with a cross-sectional view of the local region. cation processes kept the same as those used for the Au
From Figure 2, it can be seen that the Au nanoparticles nanoparticle LEDs.
Figure1FabricationprocessoftheAunanoparticlesusinganAu-CNTsystem.
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Figure2SEMimagesandschematic3Drepresentation.(a)SEMimageofacarbonnanotubethinfilm(thescalebaris2μm).SEMimagesof
Aunanoparticlesfroma(b)2-nmand(c)5-nmAu-CNTsystemwherethescalebarsare500nm.(d)Schematic3DrepresentationofaGaNLED
withembeddedAunanoparticles.
Results and discussion forward injection currents from 10 to 100 mA at room
To evaluate the optical properties of the as-prepared temperature. Figure 3 shows that the devices with and
LEDs, we performed electroluminescence (EL) spectros- without Au nanoparticles exhibited similar spectra peaks
copyexperimentsforallofthedevices. TheEL spectros- at 470 nm and similar full-width half-maximum values
copy was measured from the top of the samples with of about 18 to 19 nm, demonstrating that the annealing
Figure3ELspectraofLEDs.TheLEDsarewithAunanoparticlesfromthe2-and5-nmAu-CNTsystemswithaninjectioncurrentof100mA
measuredatroomtemperature,usingaplanarLEDasareference.TheinsetshowsopticalmicroscopeimagesoftheLEDs(a)withoutanyAu
nanoparticles,(b)withAunanoparticlesfromthe5-nmAu-CNTsystem,and(c)withAunanoparticlesfromthe2-nmAu-CNTsystem.Allofthe
deviceswereoperatedwithaninjectioncurrentof100mA.
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process used to fabricate the Au nanoparticles on the Figure 4b. The forward voltage for LEDs with Au nano-
p-GaN layers did not damage the GaN-based LED struc- particles on the p-GaN surface was 2.7 V, which is al-
ture. With an injection current of 100 mA, the EL spec- most the same as that of the planar LEDs without any
tra intensities were enhanced by approximately 55.3% Au nanoparticles, indicating that fabricating Au nano-
and 41.3% for the Au nanoparticles fabricated from the particles on the p-GaN surfaces did not cause the elec-
2- and 5-nm Au-CNT systems, respectively, compared trical propertiestodeteriorate.
with the reference conventional planar LEDs. In our EL To further confirm these results, photoluminescence
spectra counting, the peak intensity of LEDs with Au (PL) spectra measurements were taken for all of the
nanoparticles from the 2- and 5-nm Au-CNT systems LEDs. The samples were pumped at a normal incidence
were 290.8 and 264.6, respectively, compared with 187.2 angle with light from a He-Cd laser source (λ=325 nm)
for conventional LEDs. According to the research status with an excitation laser power of 10 mW at room
of surface plasmon in LED devices based on GaN mate- temperature. The polarization direction of the laser was
rials [12-14,16], the improvement in the optical output perpendicular to the Au nanoparticle chains. The laser
power could be attributed to the surface plasmon effect beam penetrated through an attenuator and then was
from the Au nanoparticles. Meanwhile, the Au nanopar- focused on the sample from the top using a 40×UV ob-
ticles on the surface of LED devices could increase the jective lens with a focused spot diameter of approxi-
roughness of the surface. So the enhancement of optical mately 5 μm. Figure 5 shows that the results for the
output power may also originate from the surface scat- devices from the PL measurements were in agreement
tering effect. When comparing the Au nanoparticles with that in the EL experiments. The PL intensity of the
from the 5-nm Au-CNTsystem with the LEDs that had LEDs with Au nanoparticles was much higher than that
Au nanoparticle arrays from the 2-nm Au-CNTsystem, for the planar LEDs. The PL intensity peaks of the
the latter showed more enhanced light emission Optical LEDs with Au nanoparticles were enhanced by 3.3
microscopyimagesofthe LEDs withand without the Au and 2.7 times for the 2- and 5-nm Au-CNT systems,
nanoparticles with an injection current of 100 mA are respectively.
shown in the inset of Figure 3. Further optimization of As the Au nanoparticles were distributed along the
the particle-forming conditions would lead to an even CNT direction, polarization measurements were per-
higher increase in the efficiency of the LEDs with nano- formed on the LEDs with Au nanoparticles for the
particlesfrom themetal-CNTsystem inthefuture. Au-CNTsystem. Figure 6 shows that the P polarization
Figure 4a showsthe optical outputpowerfortheLEDs is defined as the direction that is parallel to the quasi-
with and without Au nanoparticles on p-GaN surfaces aligned Au particle array, while the S polarization in-
versus the injection current(L-I) characteristics for allof dicated the vertical direction of the array. There was
the devices. The enhancement factor in the optical out- almost no difference in the intensity between the S and
put power increased as the injection current increased. P polarizations with respect to the planar LED, which il-
The voltage–current (I-V) characteristics for the LEDs lustratedthattheplanarLEDwasanon-polarizedlighting
with and without an Au nanoparticle layer are shown in source. For the LEDs with embedded Au nanoparticles
Figure4OpticaloutputpowerandI-Vcharacteristics.(a)OpticaloutputpowerasafunctionoftheinjectioncurrentwithAunanoparticles
fromthe2-and5-nmAu-CNTsystems,comparedwithaplanarLED.(b)I-VcharacteristicsofGaNLEDswithAunanoparticlesfromthe2-and
5-nmAu-CNTsystemscomparedwithaplanarLED.
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Figure5Room-temperaturePLspectraofGaNLEDs.TheLEDsarewithAunanoparticlesforthe2-and5-nmAu-CNTsystemswithaplanar
LEDasareference.
derived from the Au-CNT system, polarization was ex- This gives reason for the unsatisfactory polarization mea-
hibited to a certain degree. The polarization degree was surements and also provides an effective method in opti-
approximately2.1and1.3fortheLEDswithAunanopar- mizingtheAunanoparticlesystem.
ticlesderivedfrom the5-and2-nmAu-CNTsystems,re-
spectively. Compared with the Au nanoparticles derived Conclusions
from the 2-nm Au-CNTsystem, the 5-nm Au-CNTsys- In conclusion, the optical output power of the LEDs was
tems could get Au nanoparticles with a more efficient enhanced by employing Au nanoparticles fabricated
morphologyarrayforthepolarizationandarelativelyhigh from an Au-CNTsystem. The enhancement was mainly
density. However, the distance between nanoparticle ar- originated from the surface plasmon effect and surface
rayswasirregular,andinonenanoparticlearray,thespace scattering effect from the Au nanoparticles. The optical
between particles was relatively large in both situations. output power of these LEDs was enhanced up to 55.3%
Figure6PolarizationmeasurementsofLEDswithAunanoparticlesfrom2-and5-nmAu-CNTsystemscomparedwithplanarLED.
Jinetal.NanoscaleResearchLetters2014,9:7 Page6of6
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for an input currentof 100 mA. The Au nanoparticle ar- 14. SungJ-H,KimB-S,ChoiC-H:EnhancedluminescenceofGaN-based
rays also affected the polarization to a certain degree. light-emittingdiodewithalocalizedsurfaceplasmonresonance.
MicroelectronEng2009,86:1120.
Compared with the traditional metal annealing process,
15. KwonM-K,KimJ-Y,KimB-H:Surface-plasmon-enhancedlight-emitting
Au nanoparticles with a more regular distribution and a diodes.AdvMater2008,20:1253–1257.
controllable size in the subwavelength region could be 16. KwonM-K,KimbJ-Y,ParkS-J:EnhancedemissionefficiencyofgreenInGaN/
GaNmultiplequantumwellsbysurfaceplasmonofAunanoparticles.
made using this CNT-based annealing process. This me-
JCrystGrowth2013,370:124.
thod is simple, cheap, and suitable for mass production 17. JangL-W,PolyakovAY,LeeI-H:Localizedsurfaceplasmonenhanced
inthesemiconductorindustry. quantumefficiencyofInGaN/GaNquantumwellsbyAg/SiO2
nanoparticles.OptExpress2012,20(3):2116.
18. HongS-H,ChoC-Y,LeeS-J:Localizedsurfaceplasmon-enhanced
Competinginterests
near-ultravioletemissionfromInGaN/GaNlight-emittingdiodesusing
Theauthorsdeclarethattheyhavenocompetinginterests.
silverandplatinumnanoparticles.OptExpress2013,21(3):3138.
19. SungJ-H,YangJS,KimB-S:Enhancementofelectroluminescencein
Authors’contributions GaN-basedlight-emittingdiodesbymetallicnanoparticles.ApplPhysLett
JYHcarriedoutmostoftheexperimentalworkincludingallthe 2010,96:261105.
measurementsanddraftedthemanuscript.LJKpreparedtheCNTfilm, 20. JiangK,LiQ,FanS:Spinningcontinuouscarbonnanotubeyarns.Nature
andLGHwasinchargeofmetaldeposition.CMandZYcarriedoutthe 2002,419:801.
fabricationofLEDdevices.LQQconductedtheexperimentdesign
andanalysisofalltheexperiments,andrevisedthemanuscriptasa doi:10.1186/1556-276X-9-7
correspondingauthor.JKLandFSSparticipatedinallthediscussiononthis Citethisarticleas:Jinetal.:Enhancedopticaloutputpowerofblue
study.Alloftheauthorsreadandapprovedthefinalmanuscript. light-emittingdiodeswithquasi-alignedgoldnanoparticles.Nanoscale
ResearchLetters20149:7.
Acknowledgments
ThisworkwasfinanciallysupportedbytheNationalBasicResearchProgram
ofChina(2012CB932301)andNationalNaturalScienceFoundationofChina
(90921012).
Received:12October2013Accepted:5December2013
Published:6January2014
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