AG Ardakani

Chien-Chung Huang

HS Roh

HW Suh

JH Lim

K Prabakar

LC Chen

Lung-Chien Chen

M Esmaeilpour Ganji

N Zhang

S Dhara

SC Su

T Mukai

WC Li

XH Wang

XH Zhang

Y Danhara

YC Shen

YN Xu

Nanoscale Research Letters

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Chen and Huang Nanoscale Research Letters 2012, 7:627http://www.nanoscalereslett.com/content/7/1/627NANO EXPRESS Open AccessOptoelectronic characteristics of YAGphosphor-incorporated ZnO films deposited byultrasonic spray pyrolysisLung-Chien Chen* and Chien-Chung HuangAbstractThis work presents a novel white light device. An yttrium aluminum garnet (YAG) phosphor-incorporated zinc oxide(ZnO) film is deposited on a slide glass substrate by ultrasonic spray pyrolysis. A nanoflower consisting of ahexagonal nanopetal is formed on the surfaces of the samples, and the sizes of the nanopetal are approximately200 to 700 nm. Additionally, the nanopetal becomes blunted with an increasing incorporated amount of YAG. Asthe incorporated amount is 1.5 and 2.5 wt.%, the photoluminescence color of the YAG-incorporated ZnO film isnearly white, possibly contributing to the YAG emission and the band-to-deep level transition in the ZnO film.Keywords: YAG, ZnO, Photoluminescence, NanoflowerBackgroundSolid state lighting technology is the next generationlight source, owing to its potential luminescence effi-ciency. GaN-based white light-emitting diode (LED) isthe most widely used solid state light source because ofits long lifetime, high energy efficiency, small size, abilityto produce color light directly without filtering, and in-tegration with other semiconductor electronic elements[1-6]. However, prohibitively high manufacturing costs,unstable production in run-to-run and wafer-to-wafer,inadequate light uniformity, and inferior color renderingindex represent major obstacles to GaN-based LED as amain light source in the future. The first two obstacles areattributed to the expensive growth technology of metal-organic chemical vapor deposition and the Ga-N bondingmechanism. The latter two obstacles are caused by amixture of blue and yellow lights. White LED consists of ablue LED chip and a yellow phosphor coating. A non-conformal phosphor coating causes thickness of phosphorcoating layer variation and inadequate color uniformity.Therefore, this study describes a novel white lightdevice for light source. ZnO is commonly used as amaterial for optical device applications in the UV rangeowing to its wide direct bandgap (3.37 eV) [7-10]. An* Correspondence: ocean@ntut.edu.twDepartment of Electro-optical Engineering, National Taipei University ofTechnology, 1, sec.3, Chung-Hsiao E. Road, Taipei 106, Taiwan© 2012 Chen and Huang; licensee Springer. ThCommons Attribution License (http://creativecoreproduction in any medium, provided the orign-ZnO/p-GaN heterostructure LED was reported [11].However, that is a UV range structure, and no any opticalcharacteristics were demonstrated. In this work, an yt-trium aluminum garnet (YAG) phosphor-incorporatedZnO film is deposited by ultrasonic spray pyrolysis. Theoptoelectronic characteristics are also studied. Addition-ally, the crystallinity of YAG phosphor-incorporated intothe ZnO films is studied using X-ray diffraction (XRD)analysis.MethodsYAG phosphor-incorporated ZnO film was depositedby ultrasonic spray pyrolysis on slide glass substrates atatmospheric pressure in nitrogen (N2) gas, at a flow rateof 100 sccm for 30 min. The YAG phosphor-incorporatedZnO film (YAG phosphor at 0, 0.5, 1.5, and 2.5 wt.%;NYAG4156 phosphor, INTEMATIX, Fremont, CA,USA) was produced by spraying aqueous solutions.The spraying aqueous solution preparation is as fol-lows: (1) a solution consisted of Zn(CH3COO)2·2H2O(0.2 M) and CH3COONH4·2H2O (0.2 M) with 1:3 pro-portional ratio which were used as sources of ZnO, and(2) the NYAG4156 phosphor powder was added into theZnO source solution to form the spraying aqueous solu-tion, where the NYAG4156 powder is suspendible. Aslide glass was used as the substrate, which was etchedwith HCl for 5 min before deposition. An aerosol of theis is an Open Access article distributed under the terms of the Creativemmons.org/licenses/by/2.0), which permits unrestricted use, distribution, andinal work is properly cited.Chen and Huang Nanoscale Research Letters 2012, 7:627 Page 2 of 5http://www.nanoscalereslett.com/content/7/1/627precursor solution was then generated using a com-mercial ultrasonic nebulizer. Next, the morphology ofthe film was studied by field-emission scanning electronmicroscope (FESEM). The resistivity and the mobility ofthe film were studied by Hall measurement. The crystal-linity was investigated by XRD using a rotating anodeRigaku X-ray diffractometer (Mac Science Corporation,Yokohama, Japan) with Cu-Kα1 radiation at a wavelengthof 1.54 Å, where the radiation was generated at 45 kVand 40 mA. Notably, the film had a polycrystalline struc-ture. Additionally, photoluminescence (PL) was mea-sured at room temperature (RT). The excitation sourcefor photoluminescence was a frequency-quadrupled Nd:YAG laser, which emitted 266 nm, 6 ns pulses at a 5-Hzrepetition rate.Results and discussionFigure 1 shows the FESEM micrographs of the YAGphosphor-incorporated ZnO films with various incorpo-rated amounts. Following a close examination of thecross-sectional and top-view images of the samples,the micrographs indicate that the nanoflower consistsof hexagonal nanopetal on the surface of the film. Nonanorod and nanofiber structures were observed asother published articles [12,13]. The nanopetal sizes wereapproximately 200 to 700 nm. The nanopetal becomesblunted with an increasing incorporated amount of YAGfrom 0.5% to 2.5%. The blunted nanopetal might beresponsible for the degradation of the nanocrystalline(c) 1.5%(a) 0%Figure 1 FESEM micrographs. YAG-incorporated ZnO films with various iZnO grains because the lattice constant of the YAG islarger than that of the ZnO [14,15]. Despite the attentionpaid to the ZnO nanoflower in the literature, the nano-flower and nanopetal processes still remain unclear[10,16-19]. However, the origin of the hexagonal nano-flower may contribute to the decomposition and randomnucleation of the solution precursor leading to the for-mation of the three-dimensional ZnO nuclei [10]. Asthe growth process is terminated, the three-dimensionalgrowth becomes two-dimensional owing to the reductionof the source and the aggregation of the residue pre-cursor, subsequently leading to the formation of thehexagonal nanopetal on the surface of the sample.Figure 2a shows the resistivity as a function of theincorporated amount of YAG phosphor. The resistivity isnearly constant, ranging from 6 to 8 Ω cm. Figure 2bplots both the carrier concentration and the mobility as afunction of the incorporated amount of YAG phosphor.As the incorporated amount of YAG phosphor increasesto 2.5 wt.%, the carrier concentration likely increasesto 2.7 × 1018 cm−3, and simultaneously, the mobilitydecreases to 0.3 cm2/Vs. The decrease of mobility and theincrease of carrier concentration, as shown in Figure 2b,may be attributed to the degradation of the ZnO filmcrystallinity and the increasing defects caused by theincorporation of the YAG phosphor.Figure 3 shows a typical XRD pattern of the YAGphosphor-incorporated ZnO film deposited on a sapphiresubstrate prepared by the ultrasonic spraying pyrolysis(d) 2.5%(b) 0.5%ncorporated amounts: (a) 0%, (b) 0.5%, (c) 1.5%, and (d) 2.5%.Figure 2 Resistivity, carrier concentration, and mobility. (a) The resistivity as a function of the incorporated amount of YAG phosphor. (b) Thecarrier concentration and the mobility as a function of the incorporated amount of YAG phosphor.Chen and Huang Nanoscale Research Letters 2012, 7:627 Page 3 of 5http://www.nanoscalereslett.com/content/7/1/627method. The XRD patterns are normalized to the samemain peak intensity. Three dominant diffraction peaks,i.e., ZnO(100) (2θ = 31.76°), ZnO(002) (2θ = 34.46°), andZnO(101) (2θ = 36.27°), are observed. Experimental resultsindicate that the lattice parameters are a = 3.244 Å andc = 5.199 Å. The film demonstrates a polycrystallinestructure. The sample with YAG of 0.5 wt.% has themaximum ZnO(101) diffraction peak height. As theincorporated amount of YAG phosphor increases morethan 1.5 wt.%, the intensity in ZnO(100) and ZnO(101)diffraction peaks decreases. A slight shift in the XRD pat-terns were observed from 34.445° to 34.425° when theYAG phosphor was incorporated into the ZnO films.This may be attributed to the lattice constant of incorpo-rated YAG larger than that of the ZnO [14,15].Figure 4 presents the RT PL spectra of the ZnO filmswith various incorporated amounts of YAG phosphor.The inset shows the photoexcited luminescent photo-graphs. According to Figure 4, the RT PL spectrum ofthe ZnO film without a YAG phosphor reveals one peak,denoted as peak A, i.e., at approximately 3.28 eV (378 nm)and a broad weak band denoted as peak B at approxi-mately 2.557 eV (485 nm). Peak A has the shortest wave-length and, therefore, is interpreted as being associatedwith free-exciton or band-to-band recombination in theZnO. Additionally, its position is reasonably close to thatof the bandgap of ZnO at RT, which is approximately3.285 eV (377.5 nm) [7-9,20]. Peak B may be attributed tothe band-to-deep level transition in the ZnO film. As theYAG phosphor incorporated into the ZnO films is moreFigure 3 X-ray diffraction patterns of the ZnO films with various incorporated amounts of YAG phosphor.Chen and Huang Nanoscale Research Letters 2012, 7:627 Page 4 of 5http://www.nanoscalereslett.com/content/7/1/627than 1.5 wt.%, peak C emerges at around 540 nm. Peak Cis associated with the emission of the YAG phosphor. Asthe incorporated amount is 1.5 and 2.5 wt.%, the color ofphotoluminescence is nearly white, as shown in Figure 4.The white light may contribute to the wide emission bandranging from 420 to 650 nm, as merged by peaks B and C.ConclusionIn summary, an YAG phosphor-incorporated ZnOfilm is deposited on a slide glass substrate by ultrasonic350 400 450 50PL INTENSITY (arb.units) WAVELENAB0%0.5%Figure 4 RT PL spectra of ZnO films with various incorporated amounphotographs,spray pyrolysis. A nanoflower consisting of a hexagonalnanopetal is formed on the surface of the samples; inaddition, the sizes of the nanopetal are approximately200 to 700 nm. The origin of the hexagonal nanoflowermay contribute to the three-dimensional growth becomingtwo-dimensional, owing to the reduction of the sourceand aggregation of the residue precursor as the growthprocess is terminated. Consequently, the hexagonalnanopetal is formed on the surface of the sample. Thisstudy also examines the PL spectra of the samples. As0 550 600 650GTH (nm) 2.5% 1.5% 0.5% 0%C1.5% 2.5%ts of YAG phosphor. The inset shows the photoexcited luminescentChen and Huang Nanoscale Research Letters 2012, 7:627 Page 5 of 5http://www.nanoscalereslett.com/content/7/1/627the incorporated amount of the YAG is 1.5 and 2.5 wt.%,the photoluminescence color of the YAG-incorporatedZnO films is nearly white. This color may contribute tothe wide emission band ranging from 420 to 650 nm, ascaused by the YAG emission and the band-to-deep leveltransition in the ZnO film.Competing interestsThe authors declare that they have no competing interests.Authors' contributionsLCC wrote the paper, designed the experiments, and analyzed the data. CCHgrew the samples and did all the measurements. Both authors read andapproved the final manuscript.AcknowledgmentsFinancial support of this work was provided by the National Science Councilof the Republic of China under contract number NSC 101-2221-E-027-054.Received: 4 October 2012 Accepted: 1 November 2012Published: 15 November 2012References1. Shen YC, Wierer JJ, Krames MR, Ludowise MJ, Misra MS, Ahmed F, Kim AY,Mueller GO, Bhat JC, Stockman SA, Martin PS: Optical cavity effects inInGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes.Appl Phys Lett 2003, 82:2221.2. 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Ardakani AG, Pazoki M, Mahdavi SM, Bahrampour AR, Taghavinia N:Ultraviolet photodetectors based on ZnO sheets: the effect of sheet sizeon photoresponse properties. Appl Surf Sci 2012, 258:5405–5411.20. Wang XH, Yao B, Shen DZ, Zhang ZZ, Li BH, Wei ZP, Lu YM, Zhao DX,Zhang JY, Fan XW, Guan LX, Cong CX: Optical properties of p-type ZnOdoped by lithium and nitrogen. Solid State Commum 2007, 141:600–604.doi:10.1186/1556-276X-7-627Cite this article as: Chen and Huang: Optoelectronic characteristics ofYAG phosphor-incorporated ZnO films deposited by ultrasonic spraypyrolysis. Nanoscale Research Letters 2012 7:627.Submit your manuscript to a journal and beneﬁ t from:7 Convenient online submission7 Rigorous peer review7 Immediate publication on acceptance7 Open access: articles freely available online7 High visibility within the ﬁ eld7 Retaining the copyright to your article    Submit your next manuscript at 7 springeropen.com

Optoelectronic characteristics of YAG phosphor-incorporated ZnO films deposited by ultrasonic spray pyrolysis

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Optoelectronic characteristics of YAG phosphor-incorporated ZnO films deposited by ultrasonic spray pyrolysis

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