GaN green LED was grown on Si (111) substrate by MOCVD. To enhance the quality of InGaN/GaN MQWs, same-temperature (ST) GaN protection layers with different thickness of 8 Å, 15 Å, and 30 Å were induced after the InGaN quantum wells (QWs) layer. Results show that a relative thicker cap layer is benefit to get InGaN QWs with higher In percent at fixed well temperature and obtain better QW/QB interface. As the cap thickness increases, the indium distribution becomes homogeneous as verified by fluorescence microscope (FLM). The interface of MQWs turns to be abrupt from XRD analysis. The intensity of photoluminescence (PL) spectrum is increased and the FWHM becomes narrow

Changda Zheng

Chunlan Mo

Fengyi Jiang

Li Wang

Wenqing Fang

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Effect of Same-Temperature GaN Cap Layer on the InGaN/GaN Multiquantum Well of Green Light-Emitting Diode on Silicon Substrate

GaN green LED was grown on Si (111) substrate by MOCVD. To enhance the quality of InGaN/GaN MQWs, same-temperature (ST) GaN protection layers with different thickness of 8 Å, 15 Å, and 30 Å were induced after the InGaN quantum wells (QWs) layer. Results show that a relative thicker cap layer is benefit to get InGaN QWs with higher In percent at fixed well temperature and obtain better QW/QB interface. As the cap thickness increases, the indium distribution becomes homogeneous as verified by fluorescence microscope (FLM). The interface of MQWs turns to be abrupt from XRD analysis. The intensity of photoluminescence (PL) spectrum is increased and the FWHM becomes narrow.</jats:p

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Hindawi Publishing CorporationThe Scientific World JournalVolume 2013, Article ID 538297, 4 pageshttp://dx.doi.org/10.1155/2013/538297Research ArticleEffect of Same-Temperature GaN Cap Layer onthe InGaN/GaN Multiquantum Well of Green Light-EmittingDiode on Silicon SubstrateChangda Zheng, Li Wang, Chunlan Mo, Wenqing Fang, and Fengyi JiangNational Engineering Technology Research Center for LED on Si Substrate, Nanchang University, Nanchang 330047, ChinaCorrespondence should be addressed to Changda Zheng; zhengchangda@ncu.edu.cnReceived 25 October 2013; Accepted 24 November 2013Academic Editors: W. Hu, S. Jit, and F. YueCopyright © 2013 Changda Zheng et al.This is an open access article distributed under the Creative CommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.GaN green LED was grown on Si (111) substrate by MOCVD. To enhance the quality of InGaN/GaN MQWs, same-temperature(ST) GaN protection layers with different thickness of 8 A˚, 15 A˚, and 30 A˚ were induced after the InGaN quantum wells (QWs)layer. Results show that a relative thicker cap layer is benefit to get InGaN QWs with higher In percent at fixed well temperatureand obtain better QW/QB interface. As the cap thickness increases, the indium distribution becomes homogeneous as verified byfluorescencemicroscope (FLM).The interface ofMQWs turns to be abrupt fromXRD analysis.The intensity of photoluminescence(PL) spectrum is increased and the FWHM becomes narrow.1. IntroductionAlthough GaN-based blue/green LEDs have been commer-cially used in color digital display, liquid crystal display back-lighting, traffic lamps, vehicle lamps, and general lightingareas as the significant progress achieved in the materialgrowth and device manufacture, on the way to widely sub-stitute the incandescent and fluorescent lamps as the generallighting source, cost lowing and electricity to light convertefficiency improvement are still required for III-nitride LEDs[1, 2].Generally estimated, substrates cost 25 to 35 percent ofthe LED chips. So, how to reduce the cost of substrates is oneof the key tasks for researchers. Due to lack of high quality,inexpensive, and large size GaN single-crystal substrate,commercially used GaN LED structures are mostly growneither on sapphire or SiC substrates. But there still existssome deficiency for both substrates. For example; they bothare relatively expensive (especially SiC substrates) and hard,which introduces great complexity and trouble to the devicefabrication processes and ultimately improves the devicescost. Therefore, silicon is used by some researchers as sub-strate to grow GaN-based LEDs and reduce device cost for itslow cost, availability of large size, high surface quality, highconductivity, and well-established processing techniques [3–5]. Today GaN blue LEDs have already been grown on siliconsubstrate by several groups, which provide one of the low-costsolutions for solid state lighting [6–8].Otherwise, the increase in efficiency and output of GaNLEDs is still required. The difficulty to grow high efficientLEDs is mainly due to the fundamental problem associatedwith the growth of InGaN/GaNMQWs [9, 10]. Especially forgreen LEDs, high-Indium content InGaN layers with goodcrystal quality are necessary [11–13]. Hence, on the one hand,InGaN QWs must be gained with homogeneous indiumincorporation.On the other hand, InGaNMQWswith abruptinterface are also needed [14–16]. In this study, based onthe successful growth of crack free GaN epilayer on siliconsubstrate [7], we researched the effect of cap layer thicknesson the quality of the InGaNQWmaterial and the well/barrierinterface.2. ExperimentThe InGaN/GaN MQWs green LED structure (as shown inFigure 1) was grown on silicon (111) substrate by ThomasSwan 6 × 2󸀠󸀠 MOCVD with close-coupled showerheadreactor. Before being loaded into reactor, silicon substrates2 The Scientific World Journal130nm GaN:MgGaN barrier layer (16nm)×60.3um AlN/AlxGa1−xN multilayerInxGa1−xN well layer (3nm)GaN LT-cap layer (8 A˚, 15 A˚, 30 A˚)3𝜇m GaN:SiSi (111) substrateFigure 1: Epilayer structure of green InGaN/GaN light emittingdiodes on silicon (111) substrate.were degreased by H2SO4: H2O2= 3 : 1 mixture solution andetched with diluted HF (5%) solution to remove the surfacecontamination and oxide layer. After 30 minutes H2in situheating at 1100∘C in reactor, only TMAl carried by H2wasfirstly injected into reactor for 2 minutes at about 800∘C toforma thinAl filmon silicon surface and avoid silicon refusedby metal gallium and/or nitrificated to SiN by ammonia(NH3). A 0.3 𝜇m AlN/Al𝑥Ga(1−𝑥)N stress releasing layer and3 𝜇m GaN:Si layer were then grown. 200 nm InGaN strainprerelief layer and 6 periods InGaN/GaNMQWswere grownon 𝑛GaN. 130 nm GaN:Mg layer was lastly capped on theMQWs. For MQWs, every period was composed of InGaNwell layer, GaN cap layer at well temperature, andGaNbarrierlayer at temperature 120∘C higher than well. The cap layerswere designed with three thicknesses of 8 A˚, 15 A˚, and 30 A˚and those samplesweremarked as sample “A,” sample “B,” andsample “C,” respectively. Double crystal X-ray diffractometry(DCXRD) was used to characterize the structure quality ofMQWs. Fluorescence microscope (FLM) was used to getfluorescence image of QWs under the optional excitationwavelength from 380 nm to 420 nm.The selected wavelengthof dichroic mirror is 430 nm and barrier filter is 450 nm inthis study. Photoluminescence (PL) spectrum was used tocharacterize the optical performance of InGaN/GaN MQWsexcitated with He-Ge 325 nm laser.3. Results and DiscussionXRD Omega/2theta rocking scan was induced to check thequality of MQWs. The scan range is from −5000arcsec to+5000arcsec. The diffraction peak of GaN (0002) plan is seton the symmetry scanning center of the rocking curve andis located as 0 position of the 𝑥-coordinate. As shown inFigure 2, satellite peaks clearly observed for samples B andC on the left side of diffraction curves, indicating that fineQW/QB periodic structure and abrupt QW/QB interface areformed forMQWwith 15 A˚ and 30 A˚ cap layer. 1st to 4th ordersatellite peaks are also appeared for these two samples. Still, itis found that 3rd and 4th order peaks widths of sample B areeven larger compared with those of sample C, showing thatsample B has relative worse QW/QB interface. Furthermore,when the cap thickness is reduced to 8 A˚ (Sample A), thesatellite peaks become not easy to be distinguished from−4500 −3000 −1500 0001500 3000 4500Sample ASample BSample C−1−2−3−4−1−2−3−4−1−2−3−4Omega (Arcs)Intensity (log a.u.)AlN + AlGaN + satelliteFigure 2: DCXRD Omega-2theta rocking curve on GaN (0002)plan for three samples.The center peak is defined asGaN (002) peak.InGaN diffraction peak is marked as zero order peak.the left side of diffraction curve. This means that clearQW/QB periodic structure was not formed for the well beingdestroyed when temperature ramping from well to higherbarrier temperature with only 8 A˚ thin cap layer. For samplesB and C, an overall peak can be differentiate from the leftof GaN (0002) peak, which is assigned to 0 order peak fromthe well In𝑥Ga(1−𝑥)N (0002) diffraction. As the insufficientlyresolution of the double crystal X-ray diffractionmeter, thispeak is not so clearly and encapsulated in GaN peak with asmall signal that appeared on the left shoulder of GaN (0002)peak. For sample A, no 0 order peak can be resolved fromthe curve.The reasonmay also be stem from the heat-damageeffect and heterogeneous In fraction of QW for sample A. Forall three samples, there are no clearly symmetric satellitepeaks on the right side of the diffraction curves as which lieson the left side. One reason is that the X-ray intensitydecreases as the diffraction angle increases. Another reason isthat AlN and AlGaN multilayer was grown to release thestress between Si substrate and GaN layer and the multilayerdiffraction peaks are also laid on the right of GaN (002) peak.These peaks superpose with MQWs satellite peak, whichbrings on an indecipherable overall peak.The FLM images of the samples are showed in Figure 3. Itis found that sample A has an uneven FL image. Extra-brightpoints are distributed on the picture.This is caused by thewellInGaN decomposed and segregated when ramping to growbarrier and p-GaN at high temperature. For image of sampleB, the uneven points become not so serious. No extremelybright points can be found from sample B. But some localuneven areas still exist on the image. When cap thicknessincreased to 30 A˚, the image becomes more uniform and nobig bright points appear on it. The FL images are usuallyused to reflect the Indian distribution in the well. It canbe concluded that the MQWs have a much more uniformfluorescence microscope images as increasing the cap layerthickness. Thicker cap would protect the well from beingdecomposed. The results are also in accordance with theconclusions from XRD test.The Scientific World Journal 3Sample A(a)Sample B(b)Sample C(c)Figure 3: The FLM images for three samples. The selected wavelength of dichroic mirror is 430 nm and barrier filter is 450 nm in the test.706050403020100400 450 500 550 600 650Sample CSample BSample AWavelength (nm)Intensity (cps)(a)MQWn-GaNp-GaN(b)Figure 4: (a) PL spectra of the three samples are excitated by He-Ge 325 nm laser at room temperature. (b) Schematic diagram of opticalcavity effects of the MQWs light from the surface and GaN/Si interface reflection.The lighting characteristic of MQWs was taken by photo-luminescence (PL) measurement with He-Ge 325 nm laser asexcitated source. The PL spectra of three samples are shownin Figure 4(a). The peak wavelength is all loaded in greenwavelength ranged from 500 nm to 535 nm. All the spectraare not smooth curve but have interference candy stripesaccompanying with the curve. Those stripes are from opticalcavity effects [17]. As shown in Figure 4(b), green light isgenerated from MQW excitated by laser. Some fraction oflight is directly extracted out the wafer surface. Some fractionof light is firstly reflected by GaN/Si interface and thenextracted out the wafer surface. The two lights have opticalpath difference, forming regions of high and low intensities.As the cap increases from 8 A˚ to 30 A˚, the intensity of the PLspectra increases and the peak wavelength is red-shifted from500 nm to 530 nm. At the same time, the full width of halfmaximum (FWHM) of the spectra becomes narrow.This canalso be explained by the well protection effect leading toincreasing the In component and uniformity with the thickercap layer.4. ConclusionsIn conclusion, GaN-based green LED structure with InGaN/GaN MQWs has been grown on Si (111) substrate byMOCVD. InGaN QWwas found to decompose at the highertemperature when growing GaN barrier and Mg-dopedGaN:Mg layer, which results in heterogeneous indium com-ponent in InGaN QW-layer and poor InGaN/GaN interface.Same-temperatureGaN cap layer after InGaNQW is effectivein preventing the InGaNdecomposition. As the cap thicknessincreases, the indium distributions become homogeneous asverified by FL.The interface of MQWs turns out to be abruptfromXRD analysis.The intensity of PL spectrum is increasedand FWHMbecomes narrow.Thus, the cap layer is one of thekey optional tuning parameters to improve GaN-based greenMQWs quality and further to obtain high-efficiency LEDs.AcknowledgmentsThis work was supported by the National Natural ScienceFoundation of China (no. 51072076), by the National HighTechnology Research and Development program of China(no. 2011AA03A101 and no. 2012AA041002), by the Fundfor Less Developed Regions of the National Natural ScienceFoundation of China (Grant no. 11364034), by the Key Pro-gram of the National Natural Science Foundation of China(Grant no. 61334001), and the National Key Technol-ogy Research and Development Program of China (no.2011BAE32B01).4 The Scientific World JournalReferences[1] E. F. Schubert and J. K. 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Krames et al., “Optical cavityeffects in InGaN/GaN quantum-well-heterostructure flip-chiplight-emitting diodes,”Applied Physics Letters, vol. 82, no. 14, pp.2221–2223, 2003.Submit your manuscripts athttp://www.hindawi.comHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014High Energy PhysicsAdvances inThe Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014FluidsJournal of Atomic and  Molecular PhysicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Advances in  Condensed Matter PhysicsOpticsInternational Journal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014AstronomyAdvances inInternational Journal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014SuperconductivityHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Statistical MechanicsInternational Journal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014GravityJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014AstrophysicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Physics Research InternationalHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Solid State PhysicsJournal of Computational  Methods in PhysicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Soft MatterJournal ofHindawi Publishing Corporationhttp://www.hindawi.comAerodynamicsJournal ofVolume 2014Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014PhotonicsJournal ofHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014Journal ofBiophysicsHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014ThermodynamicsJournal of

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Effect of Same-Temperature GaN Cap Layer on the InGaN/GaN Multiquantum Well of Green Light-Emitting Diode on Silicon Substrate

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