Analysis of europium doped luminescent barium thioaluminate

Available online at www.sciencedirect.comJOURNALOF“ScienceDirectRARE EARTHS ,EL SEVIERJOURNAL OF RARE EARTHS, Vol. 28, No. 2, Apr. 2010, p. 185www.e-journal.com/en/Analysis of europium doped luminescent barium thioaluminateZHANG Dongpu (张东璞), YU Zhinong (喻志农)', XUE Wei (薛唯), ZHANG Ting (章婷), DING Zhao(丁墨),WANG Wuyu (王武育)(1. School of Optoelectronics, Beijing Institute of Technology, Bejing 100081, China; 2. Division of Mineral & Melallurgical Materials, Bejing General Research Institutefor Non-Ferrous Metals, Beiing 10088 China)Received 6 June 2009; revised 12 October 2009Abstract: Europium-doped barium thioaluminate sputtering target was synthesized by powder sintering method and thin film was depositedby radio frequency (RF) sputtering. X-ray diffractometer (XRD) pattern indicated that the main compound of the target was BaAl4S7. Oxygenwas the main impurity which led to the formation of BaAl2O4 It was shown that both BaAl4S- and BaAl2S4 were contained in the as grownthin films and a 471.7 nm emission peak in the PL spectra appeared due to a combination of BaAl4S:Eu2+ and BaAl2S4:Eu2 . In addition, theproduct of oxidation in the film was BaSO4 instead of BaAl2O. and led to an emission peak at 415.2 nm in the PL spectra assigned to the f-→ftransition of Eu+ in the BaSO4 host.Keywords: inorganic electroluminescence; Ba-Al-S:Eu; powder-sintering; RF-sputering; photoluminescence; rare earthsThe observation of blue emission from europium-dopedtron beam deposition using Al2S3 and BaS:EuS precur-BaAl2S4 (BaAl2S4:Eu) was reported more than thirty yearssorsl2-6.1 or by sputtering reaction of Ba-Al-Eu alloy withago". However, the material did not attract much attentionH2Sb. In this work, we reported the properties of Ba-Al-S:from practical viewpoint. Until 1999, Miura et al. demon-Eu target prepared by powder sintering method andstrated that electroluminescence (EL) from thin film ofBa-Al-S:Eu film deposited by RF sputtering with the target.BaAl2S4:Eu had a relatively high brightness of 65 cd/m2 andEL device exhibited excellent color definition!2]. A success-1 Experimentalful proto type for commercial application was demonstratedby iFire, which developed a pilot industrial process produc-The starting materials for fabrication of target were bar-ing electroluminescent full color flat panel display'. Theium sulfide (BaS), europium sulfide (EuS) and aluminumcolor-by-blue (CBB) scheme which iFire used had only onesulfide (Al2Ss3). The molar ratio of BaS, EuS and Al2S3 washigh luminescence blue phosphor in combination with green0.97:0.03:1. The fabrication process consisted of five steps:and red color conversion layers. Their thick dielectric elec-(1) preparing BaS, Al2Ss, and EuS powders; (2) grinding sul-troluminescent (TDEL) display technology allowed fabrica-fide powders in ceramic vessel in Ar airflow until the pow-tion of high-definition TV display with a brightness ap-ders were as fine as 1-10 pum; (3) mixing selective powdersproaching 1000 cd/m2. Although current product cannotcompete with LCD and PDP in terms of color depth andupon plumbaginous underlay surrounded by Ar as protectivecontrast, the relatively simple technology and very lowatmosphere (0.4 MPa), agglomerating the mixture under amanufacturing cost make iFire's TDEL display technologypressure of 20 MPa in 1100 °C and keeping the sample inextremely attractive for the next generation of flat-panel dis-this environment for 28 h; (5) precisely abrading the targetplay"I.into required shape by carborundum.Blue fluorescence of BaAlS4:Eu was discovered byBa-Al- S:Eu thin film was deposited by RF sputtering us-Donohue and Hanlon' ". The fluorescence properties of theing the as-prepared target. The fabrication parameters in-compounds in M-AI-S (where M=Ca, Sr, and Ba) systemscluded base pressure of 5x 10 3 Pa, sputtering power of250 W,were studied systematically by Le Thi et al'. More than fourargon flux of 34.5 ml/min and substrate temperature of 20 °C.barium thioaluminates were found by them. As a result theyThe crystallinity of Ba-AI-S:Eu target powder and the as-pointed out that BaAl2S:Eu demonstrated the strongestgrown film were analyzed using X-ray diffractometerbrightness among the materials investigated. BaAl2S4Eu(XRD-6000, SHIMADZU) operated at 40 kV and 30 mAphosphor was synthesized by solid-state reaction betweenusing Cu Ka (2=0.15406 nm) radiation. Photoluminescencemetal sulfides in evacuated ampoulsfl-.]. The thin films for(PL) spectra of the powder were measured by F-4500 Fluo-EL devices are usually prepared either by two-source elec-rescence Spec中国煤化ivith 290, 300 andCNMH GFoundation item: Project supported by the National Eleventh Five-year Pre-research Project of China (TMYHCorresponding author: YU Zhinong (E-mail: zmnyu@bit.edu.cn; Tel: +86-1-68913259-11)DOI: 10.01/0-2072109)6077-6186JOURNAL OF RARE EARTHS, Vol. 28, No. 2, Apr. 2010320 nm excitation wavelength, respectively. The excited ul-4f7(S72) transition of Eu2+ occurs regardless of the excita-traviolet for the thin film was 220 nm. The X-ray photoelec-ion wavelength used. This emission peak represents thetron spectroscopy (XPS) was measured using PHI5300presence of BaAl4S:Eu as this material has an emissionESCA system.maximum at 470 nm5. Its emission will overlap with thefluorescence peak of BaAl2S4:Eu at 475 nm. Generally, the2 Results and discussionBa-AI-S system contains at least seven termary compounds.All of these ternary compounds can be doped at the Ba sitewith Eu5.10. However, only the cubic BaAl2S4:Eu compound2.1 Analysis of the sputtering targethas practical value as an EL phosphor. The existence ofFig.1 shows the components of the target. Theother phases may affect the color definition of the emittedcomponents include barium tetraaluminum sulfide (BaAl4S),blue light. The other compounds in the Ba-Al-S system ac-barium sulfide (BaS)， europium sulfide (EuS)， bariumtivated by Eu2+ emit light at a longer wavelength. Theiraluminum oxide (BaAl2O4) and aluminum sulfide (Al2S3).presence usually result in red-shift of the emission peak up toAs shownthe figure,europium contained in485- 490 nm except for BaAl4S-:Eu.as- fabricated target is in the form of EuS. The main com-2.2 Thin films deposition using the as-fabricated targetpound in the target is BaAl$S7, formed by a combination ofBaS and Al2S3. Because Al2S3 is subjected to oxidation,Fig. 4 shows XRD patterns observed in as- grown Ba-Al-S:BaAl2O4 forms when target is being fabricated. Part of BaSEu thin films on glass substrate using as-fabricated target.and Al2S; are kept the same in the target. It was reported thatBaAl2S4 phase is observed. The three diffraction peaks cor-BaS and AlS3 reacted to produce BaAl2S4 at temperaturesresponding to this phase are located at 22.386°, 30.6859 andstarting from 800 °C8. The relative low melting point of32.3639, respectively. BaAl2S4 film on glass is crystallized inAl2S3 compared to BaS and EuS was prone to result in thecubic. BaS is included in thin film. The positions of broadpreferred binary interactions between BaS-Al2S3 andpeaks at 24.12° and 27.97° correspond well to orthorhombicEuS-Al2S3. As a result, eutectic product of BaS and Al2S3BaAl4S7. 850 °C is the lowest temperature at which cubicmight exist in the target because the eutectic product ofBaAl2S4 form completely. As the deposition temperature is“BaS- Al2S," would form in the range of 800- 1000 °C!9. Butbelow 850 °C, part of BaS does not react and is deposited onthere is no obvious evidence indicating the existence of ei-the substrate. So the crystal lack of Ba and S orthorhombicther BaAl2S4 or BaSAl2S3.BaAl4S7 but not cubic BaAl2S4. Oxidation can not be avoidedX-ray photoelectron spectroscopy(XPS) was used toduring the deposition process, so the product is BaSO4analysis the chemical components of the target. Fig. 2 shows7000the bending energy of sulfur and oxygen in Ba-Al-S:Eu tar-a(3d,)Ba(d.aget. According to the result of XPS pattern, fit curves corre-sponding to Ba, Al, S and O are obtained and the integralarea of each curve is calculated. The relative molar ratio ofBa, Al, S and O is about 9:33:41:17. The exact molar per-0(1s)centages are shown in Table 1.S(2p3)Fig 3 presents PL spectra of the as-fabricated target pow-der. A peak centered at 470 nm corresponding to 4f*('F)5d-→Al(2p)S(2p1)50010000200 400 600 800 100000-BaAl SBending energy/eVA BaAIO.Fig. 2 Bending energy of Ba, Al, s and O in target6000T5500--λ=290nm005000.... λ=300 nm45001---λ =320 nm0o-全4000willlii.s 3500-信300000 102030405060708090250020/(°)20001500.Fig 1 XRD pattern of as-fabricated target中国煤化工Table 1 Relative molar ratio of main elements in targetMYHCNM H G00550Elements_B_AIwavelengun/ nmMole Ratio9.23%32.55%41.16%17.06%Fig. 3 Photoluminescence spectra of Ba-A1-S:Eu target powderZHANG Dongpu et al, Analysis of europium doped luminescent barium thioaluminate187instead of BaAl2O4half maximum (FWHM) of the peaks are 14.68 and 48.30Fig.5 indicates analysis of the components in thenm, respectively. The broad peak at 415.2 nm can be as-as-fabricated target and thin film. Compared with the com-signed to f-→f transition of Eu-*. The electronic configura-ponents in the target, Al2S3 and EuS disappearin thetion of the Eu2+ belongs to 4f7 which is one of the lowest ex-Ba-AI-S:Eu thin film. But there is no change to the molar ra-cited states (another is 4f5d). Because the state energy oftio of BaS. The data indicates that BaS and Al2S3 which do4f^5d is lower than that of 4f', d-→f transition is usually per-not become BaAl2S4 or BaAl4S7 are changed into parts ofmitted and f- →f transition is forbidden in most of the hostthese two compounds. This can be proved by the increasingmterisl. In the as-grown Ba-Al-S:Eu film, part of Batrends of their relative molar ratio. Almost all of Eut+ in EuSis contained in BaSO4 which is the product of oxygenation.forms the adulterant of the host materials including BaAl4S7,The 4f5d state energy of the Eu2+ is afcted by the ionic ra-BaAl2S4 and BaSO4. This can be proved by the followingdius of Ba2+ in the BaSO4 host material (0.175 nm) and theanalysis of PL spectrum.excitation band become broader'5l. It means that the up-Fig. 6 presents PL spectra of the film. Two obvious emis-per-limit of Eu2+ 4f5d band shows blue shift and thesion peaks position at 415.2 and 440.0 nm. The full width atlower-limit red shift. As a result, the bottom of Eu2+ 4f5dexcited band descends from a position higher than °P72 level●Bato a lower one. This leads to the blue shift of the emission▼BaAl,S,peak to 415.2 nm.▲BaSO,Another weak emission peak is located at 471.7 nm,8基which corresponds to the 4f"(F)5d- →4f(8S72) transition ofEu2+ and indicates the formation of BaAl2S4:Eu phase inas-grown film because this material has the emission maxi-mum at 475 nm. In addition, the emission of BaAl4S:Eu isslightly red shifted as a result of the presence of BaAl2S4:Eu.So the emission band is broadened and the peak in 475 nm iscovered. The other compounds in Ba-Al-S system activated0 10 203040506070 80 90by Eu2+ emit light at a longer wavelength. Their presence20/()usually results in deep red-shift of the emission peak centerFig. 4 BaAl2S4:Eu thin film on glass substrateto 485- 490 nm. No obvious evidence can prove the exis-50%「厂 47%tence of them here.■TargetElectroluminescent performance of the as-fabricated film0% t日Thin filmwas measured. The Ba-Al-S:Eu phosphor film exhibited35%EL-luminance at the CIE color coordinates of X=0.241 and .30%Y=0.060. The luminance exceeded about 35 cd/m2 driven by24%1 kHz 150 V sinusoidal alternating current. The peak emis-19% .sion of the phosphor was located at about 468.8 nm. Al-20%16%15%149%14% 13%though the emission corresponds to the 4r'('F)5d- >4f(*Sn2)transition of Eu2*, the electroluminescent performance is not10%satisfied to EL panel. Oxygen is contained in BaSO4, whichis the impurity in the host material and leads to poor0% I■BaAlS, BaAlS. BaSO。、 BaS AIlS,EuSblue-emitting performance. Additionally, BaAl4S7 is anotherreason for the shift of emission peak though it is a memberFig. 5 Components analysis of the as- fabricated target and filmof Ba-Al-S compounds.-PL spectra of thin film160-... Sum of 3 separate lines140-.... Peak at 415.227 nm3 ConclusionsPeak at 440.000 nm201Peak at 471.772 nmEuropium doped barium thioaluminate target for sputter-100-ing was fabricated by powder sintering method. XRD patternindicated that the main compound belonging to Ba-Al-S sys-30-tem in the target was BaAl4S7. Though it was reported that50一BaAl2S4 formed starting from 800 °C and“BaS:Al2S3”prod-o-uct maybe existed when the temperature was in the range of20 |800- 1000。C中国煤化工ove the existenceof these two; main impurity380 400 420 440 460 480 500 520which led to uHhMHcNMHGeeissonpeaksWavelength 1 nmwere located at 470 nm when the wavelengths of the excitedFig. 6 PL spectra of BaAl2S:Eu film on glass substrateultraviolet were 290, 300 and 320 nm, respectively. This in-188JOURNAL OF RARE EARTHS, Vol. 28, No. 2, Apr. 2010dicated that the as-fabricated target could be used in sputter-posium Digest of Technical Papers. USA, Palisades Conven-ing.tion Management Inc, 2003.108.With the target fabricated using powder sintering method,[5] Le Thi K T, Garcia A, Guillen F, Fouassier C. Investigation ofBa-Al-S:Eu thin film was deposited sucessfully. The com-the MS-Al2S3 system (M=Ca, Sr, Ba) and luminescence prop-erties of europium-doped thioaluminates. Material Science andpound in the film was cubic BaAl2S4:Eu with impurities ofEngineering B, 1992, 14(4): 393.BaSO4 and BaAlS7 which affected PL spectra of the film.[6] Inoue Y, Tanaka I, Tanaka K, Izumi Y, Okamoto S, KawanishiBaS was another impurity in as-fabricated film but this partM, Barada D, Miura N, Matsumoto H, Nakano R. Atomicof BaS had no impact on the formation of blue emitting layer.composition and structural properties of blue emittingBaAlS7 would make the emission peak blue shift and coverBaAlS4Eu electroluminescent thin films. Japanese Journal ofthe PL emission peak of BaAl2S4:Eu. Impurity of oxygenApplied Physics Part 1, 2001, 40(4A): 2451.was inevitable in fabricating process when the film was ex-I7] SmetP F, Van Hacke J E, Van Meirhaghe R L, Poelman D.posed in the atmosphere because Al2S3 is easily hydrolyzedCrystallographic and luminescent properties of orthorhombicin moist air. In the as-grown thin film, BaSO4 was the resultBaAlS4: Eu powder and thin films. Journal ofApplied Physics,of oxidation. With the impact of the BaSO, part of the ex-2005, 98(4): 043512.cited Eu2+ had the transition corresponding to f-→f instead of[8] Petrykin V, Kakihana M. Synthesis of BaAl2S4Bu phosphor4f(F)5d- >42(S7/) which resulted in blue shift of PL emis-using BaS:Eu precursor prepared by the polymerizable com-plex method. Journal of the Ceramic Society of Japan, 2007,sion peak to the position at 415.2 nm.115(10): 615.[9] Petrykin V, Kakihana M. Synthesis of BaAl2S4:Eu2+ elec-References:troluminescent material by the polymerizable complex methodcombined with CS2 sulfurization. Journal of the American[1] Donohue P C, Hanlon J E. Synthesis and PhotoluminescenceCeramic Society, 2009, 92: S27.of M"M2"(S,Se)4 Journal of Electrochemical Society, 1974,[10] Stiles J A R, Kamkar M. Polymorphic barium thioaluminate121: 137.electroluminescent phosphor materials. Journal of Applied[2] Miura N, Kawanishi M, Matsumoto H, Nakano R. High-lu-Phnysics, 100: 0745081.minance blue-emiting BaAl2S4:Eu thin-film electrolumines-. [] Arakawa T, Takata T, Adachi G Y, Shiokawa J. Luminescencecence devices. Japanese Journal of Applied Physics Part 2,properties of Eu ion-exchanged Y-type Zeolite. Journal of Lu-1999, 38: 1291.minescence, 1979, 20(3): 325.[3] Xin Y, Hunt T, Acchione J. Pressure Controlled Pulse DC Re-[12] Zhao W, Song Z F, Su M Z. Electron Transition of Eu2+ andactive Sputtering of BaAl2S4:Eu Film for TDEL Display. Pro-energy transfer in crystals BaFX:Eu2 ,Eu+ (X=CI, Br). Jour-ceedings of the 13" International Workshop on Inorganic andnal of the Chinese Rare Earth Society, 1992, 10(4): 241.Organic EL and 2006 Interational Conference on the Science[13] GongX, WuP F, Chan W K, Chen W J. Effect of y-ray irar-and Techonology of Emissive Display and Lighting. Korea:diation on structures and luminescent properties of nanocrys-Jeju, 2006. 446.tlline MSO4xEu* (M=Ca, Sr, Ba; x=0.001-0.005). Journal[4] Wu X, Carkner D. Technology Evolution in Inorganic Elec-of Plhysics and Chemistry of Solids, 2000, 61: 115.troluminescence. Jay Morreale. 2003 SID Intermational Sym-中国煤化工MYHCNMH G