ANALYSIS OF EUROPIUM DOPED LUMINESCENT BARIUM THIOALUMINATE

J OURNAL OF RARE EARTHS,Vol.28,No.2,Apr.2010,p.

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Analysis of eur opium doped luminescent bar ium thioaluminate

ZHANG Dongpu (张东璞)1,YU Zhinong (喻志农)1,XUE Wei (薛唯)1,ZHANG Ting (章婷)1,DING Zhao (丁瞾)1,WANG Wuyu (王武育)2

(1.School of Optoelectronics,B eijing Institute of Technology,B eijing 100081,China;2.Division ofMineral &Metallurgical Materials,B eijing General R esearch Institute for Non-Ferrous Metals,B eijing 100088,China)Received 6June 2009;revised 12October 2009

Abstract:Europium-doped barium thioaluminate sputtering target was synthesized by powder sintering method and thin film was deposited by radio frequency (RF)sputtering.X-ray diffractometer (XRD)pattern indicated that the main compound of the target was BaAl 4S 7.Oxygen was the main impurity which led to the formation of BaAl 2O 4.It was shown that both BaAl 4S 7and BaAl 2S 4were contained in the as-grown thin films and a 471.7nm emission peak in the PL spectra appeared due to a combination of BaAl 4S 7:Eu 2+and BaAl 2S 4:Eu 2+.In addition,the product of oxidation in the film was BaSO 4instead of BaAl 2O 4and led to an emission peak at 415.2nm in the PL spectra assigned to the f →f transition of Eu 2+in the BaSO 4host.

Keywords:inorganic electroluminescence;Ba-Al-S:Eu;powder-sintering;RF-sputtering;photoluminescence;rare earths

The observation of blue emission from europium-doped BaAl 2S 4(BaAl 2S 4:Eu)was reported more than thirty years ago [1].However,the material did not attract much attention from practical viewpoint.Until 1999,Miura et al.demon-strated that electroluminescence (EL)from thin film of BaAl 2S 4:Eu had a relatively high brightness of 65cd/m 2and EL device exhibited excellent color definition [2].A success-ful proto type for commercial application was demonstrated by iFire,which developed a pilot industrial process produc-ing electroluminescent full color flat panel display [3].The color-by-blue (CBB)scheme which iFire used had only one high luminescence blue phosphor in combination with green and red color conversion layers.Their thick dielectric elec-troluminescent (TDEL)display technology allowed fabrica-tion of high-definition TV display with a brightness ap-proaching 1000cd/m 2.Although current product cannot compete with LCD and PDP in terms of color depth and contrast,the relatively simple technology and very low manufacturing cost make iFire ’s TDEL display technology extremely attractive for the next generation of flat-panel dis-play [4].

Blue fluorescence of BaAl 2S 4:Eu was discovered by Donohue and Hanlon [1].The fluorescence properties of the compounds in M-Al-S (where M=Ca,Sr,and Ba)systems were studied systematically by Le Thi et al [5].More than four barium thioaluminates were found by them.As a result they pointed out that BaAl 2S 4:Eu demonstrated the strongest brightness among the materials investigated.BaAl 2S 4:Eu phosphor was synthesized by solid-state reaction between metal sulfides in evacuated ampoules [1,5].The thin films for EL devices are usually prepared either by two-source elec-

tron beam deposition using Al 2S 3and BaS:EuS precur-sors [2,6,7]or by sputtering reaction of Ba-Al-Eu alloy with H 2S [3].In this work,we reported the properties of Ba-Al-S:Eu target prepared by powder sintering method and Ba-Al-S:Eu film deposited by RF sputtering with the target.

1Experimental

The starting materials for fabrication of target were bar-ium sulfide (BaS),europium sulfide (EuS)and aluminum sulfide (Al 2S 3).The molar ratio of BaS,EuS and Al 2S 3was 0.97:0.03:1.The fabrication process consisted of five steps:(1)preparing BaS,Al 2S 3,and EuS powders;(2)grinding sul-fide powders in ceramic vessel in Ar airflow until the pow-ders were as fine as 1–10m;(3)mixing selective powders according to stoichiometric relation;(4)pressing the mixture upon plumbaginous underlay surrounded by Ar as protective atmosphere (0.4MPa),agglomerating the mixture under a pressure of 20MPa in 1100C and keeping the sample in this environment for 28h;(5)precisely abrading the target into required shape by carborundum.

Ba-Al-S:Eu thin film was deposited by RF sputtering us-ing the as-prepared target.The fabrication parameters in-cluded base pressure of 5×10–3

Pa,sputtering power of 250W,argon flux of 34.5ml/min and substrate temperature of 20°C.The crystallinity of Ba-Al-S:Eu target powder and the as-grown film were analyzed using X-ray diffractometer (XRD-6000,SHIMADZU)operated at 40kV and 30mA using Cu K α(λ=0.15406nm)radiation.Photoluminescence (PL)spectra of the powder were measured by F-4500Fluo-rescence Spectrophotometer (Hitachi Co.)with 290,300and

ound at ion ite m:Pro ect supported b the ational El eventh ive-ea r Pre-rese arc h P ro ec t o China 1020020Corre sponding a uthor :hinong E-ma il:n u https://www.sodocs.net/doc/b610165840.html, el.:8-1-89129-11:10.101/1002-0721090077-

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320nm excitation wavelength,respectively.The excited ul-traviolet for the thin film was 220nm.The X-ray photoelec-tron spectroscopy (XPS)was measured using PHI5300ESCA system.

2Results and discussion

2.1Analysis of the sputter ing target

Fig.1shows the components of the target.The components include barium tetraaluminum sulfide (BaAl 4S 7),barium sulfide (BaS),europium sulfide (EuS),barium aluminum oxide (BaAl 2O 4)and aluminum sulfide (Al 2S 3).As shown in the figure,europium contained in as-fabricated target is in the form of EuS.The main com-pound in the target is BaAl 4S 7,formed by a combination of BaS and Al 2S 3.Because Al 2S 3is subjected to oxidation,BaAl 2O 4forms when target is being fabricated.Part of BaS and Al 2S 3are kept the same in the target.It was reported that BaS and Al 2S 3reacted to produce BaAl 2S 4at temperatures starting from 800°C [8].The relative low melting point of Al 2S 3compared to BaS and EuS was prone to result in the preferred binary interactions between BaS-Al 2S 3and EuS-Al 2S 3.As a result,eutectic product of BaS and Al 2S 3might exist in the target because the eutectic product of “BaS Al 2S 3”would form in the range of 800–1000°C [9].But there is no obvious evidence indicating the existence of ei-ther BaAl 2S 4or BaS Al 2S 3.

X-ray photoelectron spectroscopy (XPS)was used to analysis the chemical components of the target.Fig.2shows the bending energy of sulfur and oxygen in Ba-Al-S:Eu tar-get.According to the result of XPS pattern,fit curves corre-sponding to Ba,Al,S and O are obtained and the integral area of each curve is calculated.The relative molar ratio of Ba,Al,S and O is about 9:33:41:17.The exact molar per-centages are shown in Table 1.

Fig 3presents PL spectra of the as-fabricated target pow-der.A peak centered at 470nm corresponding to 4f 6(7F)5d

→

Fig 1XRD pattern of as-fabricated target

Table 1Relative molar ratio of main elements in target

B S O M R 3%

355%

6%

6%

4f 7(8S 7/2)transition of Eu 2+occurs regardless of the excita-tion wavelength used.This emission peak represents the presence of BaAl 4S 7:Eu as this material has an emission maximum at 470nm [5].Its emission will overlap with the fluorescence peak of BaAl 2S 4:Eu at 475nm.Generally,the Ba-Al-S system contains at least seven ternary compounds.All of these ternary compounds can be doped at the Ba site with Eu [5,10].However,only the cubic BaAl 2S 4:Eu compound has practical value as an EL phosphor.The existence of other phases may affect the color definition of the emitted blue light.The other compounds in the Ba-Al-S system ac-tivated by Eu 2+emit light at a longer wavelength.Their presence usually result in red-shift of the emission peak up to 485–490nm except for BaAl 4S 7:Eu.2.2Thin films deposition using the as-fabr icated target Fig.4shows XRD patterns observed in as-grown Ba-Al-S:Eu thin films on glass substrate using as-fabricated target.BaAl 2S 4phase is observed.The three diffraction peaks cor-responding to this phase are located at 22.386°,30.685°and 32.363°,respectively.BaAl 2S 4film on glass is crystallized in cubic.BaS is included in thin film.The positions of broad peaks at 24.12°and 27.97°correspond well to orthorhombic BaAl 4S 7.850°C is the lowest temperature at which cubic BaAl 2S 4form completely.As the deposition temperature is below 850°C,part of BaS does not react and is deposited on the substrate.So the crystal lack of Ba and S orthorhombic BaAl 4S 7but not cubic BaAl 2S 4.Oxidation can not be avoided during the deposition process,so the product is BaSO

4

Fig.2Bending energy of Ba,Al,S and O in

target

F 3f B S Elements a Al ole atio

9.2 2.41.117.0ig.Photoluminescence spectra o a-Al-:Eu target powder

ZHANG Dongpu et al.,Analysis of europium doped luminescent barium thioaluminate 187

instead of BaAl 2O 4.

Fig.5indicates analysis of the components in the as-fabricated target and thin https://www.sodocs.net/doc/b610165840.html,pared with the com-ponents in the target,Al 2S 3and EuS disappear in the Ba-Al-S:Eu thin film.But there is no change to the molar ra-tio of BaS.The data indicates that BaS and Al 2S 3which do not become BaAl 2S 4or BaAl 4S 7are changed into parts of these two compounds.This can be proved by the increasing trends of their relative molar ratio.Almost all of Eu 2+in EuS forms the adulterant of the host materials including BaAl 4S 7,BaAl 2S 4and BaSO 4.This can be proved by the following analysis of PL spectrum.

Fig.6presents PL spectra of the film.Two obvious emis-sion peaks position at 415.2and 440.0nm.The full width

at

Fig.4BaAl 2S 4:Eu thin film on glass

substrate

Fig.5Components analysis of the as-fabricated target and

film

F 6L f B S f half maximum (FWHM)of the peaks are 14.68and 48.30nm,respectively.The broad peak at 415.2nm can be as-signed to f →f transition of Eu 2+.The electronic configura-tion of the Eu 2+belongs to 4f 7which is one of the lowest ex-cited states (another is 4f 65d).Because the state energy of 4f 65d is lower than that of 4f 7,d →f transition is usually per-mitted and f →f transition is forbidden in most of the host materials [11,12].In the as-grown Ba-Al-S:Eu film,part of Ba is contained in BaSO 4which is the product of oxygenation.The 4f 65d state energy of the Eu 2+is affected by the ionic ra-dius of Ba 2+in the BaSO 4host material (0.175nm)and the excitation band become broader [13].It means that the up-per-limit of Eu 2+4f 65d band shows blue shift and the lower-limit red shift.As a result,the bottom of Eu 2+4f 65d excited band descends from a position higher than 6P 7/2level to a lower one.This leads to the blue shift of the emission peak to 415.2nm.

Another weak emission peak is located at 471.7nm,which corresponds to the 4f 6(7F)5d →4f 7(8S 7/2)transition of Eu 2+and indicates the formation of BaAl 2S 4:Eu phase in as-grown film because this material has the emission maxi-mum at 475nm.In addition,the emission of BaAl 4S 7:Eu is slightly red shifted as a result of the presence of BaAl 2S 4:Eu.So the emission band is broadened and the peak in 475nm is covered.The other compounds in Ba-Al-S system activated by Eu 2+emit light at a longer wavelength.Their presence usually results in deep red-shift of the emission peak center to 485–490nm.No obvious evidence can prove the exis-tence of them here.

Electroluminescent performance of the as-fabricated film was measured.The Ba-Al-S:Eu phosphor film exhibited EL-luminance at the CIE color coordinates of X =0.241and Y =0.060.The luminance exceeded about 35cd/m 2driven by 1kHz 150V sinusoidal alternating current.The peak emis-sion of the phosphor was located at about 468.8nm.Al-though the emission corresponds to the 4f 6(7F)5d →4f 7(8S 7/2)transition of Eu 2+,the electroluminescent performance is not satisfied to EL panel.Oxygen is contained in BaSO 4,which is the impurity in the host material and leads to poor blue-emitting performance.Additionally,BaAl 4S 7is another reason for the shift of emission peak though it is a member of Ba-Al-S compounds.

3Conclusions

Europium doped barium thioaluminate target for sputter-ing was fabricated by powder sintering method.XRD pattern indicated that the main compound belonging to Ba-Al-S sys-tem in the target was BaAl 4S 7.Though it was reported that BaAl 2S 4formed starting from 800°C and “BaS Al 2S 3”prod-uct maybe existed when the temperature was in the range of 800–1000°C,there was no evidence to prove the existence of these two compounds.Oxygen was the main impurity f f B O T f x ,33,y T ig.P spectra o aAl 24:Eu ilm on glass substrate

which led to the ormation o aAl 24.he emission peaks were located at 470nm when the wavelengths o the e cited ultraviolet were 29000and 20nm respectivel .his in-

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dicated that the as-fabricated target could be used in sputter-ing.

With the target fabricated using powder sintering method, Ba-Al-S:Eu thin film was deposited successfully.The com-pound in the film was cubic BaAl2S4:Eu with impurities of BaSO4and BaAl4S7which affected PL spectra of the film. BaS was another impurity in as-fabricated film but this part of BaS had no impact on the formation of blue emitting layer. BaAl4S7would make the emission peak blue shift and cover the PL emission peak of BaAl2S4:Eu.Impurity of oxygen was inevitable in fabricating process when the film was ex-posed in the atmosphere because Al2S3is easily hydrolyzed in moist air.In the as-grown thin film,BaSO4was the result of oxidation.With the impact of the BaSO4,part of the ex-cited Eu2+had the transition corresponding to f→f instead of 4f6(7F)5d→4f7(8S7/2)which resulted in blue shift of PL emis-sion peak to the position at415.2nm.

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