Surface and Coatings Technology 146–147(2001) 508–512

0257-8972/01/$ - see front matter� 2001 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž01.01377-9

(Y O –GeO) phosphor thin-film electroluminescent emitting layers2 3 2

prepared by magnetron sputtering

Tadatsugu Minami*, Toshikuni Nakatani, Toshihiro Miyata, Tetsuya Shirai

Optoelectronic Device System R&D Center, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan

Abstract

In this paper, new multicomponent oxides are shown to be promising as a host material for the phosphor thin-film emittinglayer of thin-film electroluminescent(TFEL) devices. Mn-activated(Y O –GeO) multicomponent oxide phosphor thin films2 3 2

were prepared by r.f. magnetron sputtering and used as the emitting layer of TFEL devices. High luminance multicolor emissionswere obtained in TFEL devices using(Y O –GeO):Mn thin films prepared with a GeO content of 0–100 mol.%. In a yellow2 3 2 2

emitting TFEL device fabricated using a Y GeO phosphor thin film prepared with a GeO content of approximately 50 mol.%,2 5 2

a high luminance of 758 cdym and a luminous efficiency of 1.4 lmyW were obtained when driven by a sinusoidal wave voltage2

at 1 kHz.� 2001 Elsevier Science B.V. All rights reserved.

Keywords: (Y O –GeO); Oxide; Phosphor; Thin film; Electroluminescence; Sputtering; Y GeO2 3 2 2 5

1. Introduction

Recently, we have demonstrated that oxides are verypromising as an alternative to sulfides for practical useas the host material of conventional thin-film electrolu-minescent(TFEL) phosphorsw1–5x. In order to useoxide phosphors as the thin-film emitting layer, it wasnecessary to use a TFEL device structure using a thickceramic sheet acting as the insulating layer as well asthe substrate. TFEL devices using phosphors such asmanganese-activated Ga O (Ga O :Mn) w3,6,7x,2 3 2 3

Ga O :Eu w8,9x, Zn SiO :Mn w1x, ZnGa O :Mn w2x or2 3 2 4 2 4

CaGa O :Mnw3x as the emitting layer produced a high2 4

luminance above 200 cdym when driven at 60 Hz. In2

addition, we demonstrated that the use of multicompo-nent oxide host materials also holds much promise forobtaining higher luminance and multicolor emissionsw10,11x. For example, high luminance muluticolor emis-sions were obtained in TFEL devices using multicom-ponent oxide phosphors such as CaO:Mn-Ga O :Mnw3x,2 3

ZnGa O :Mn-Zn SiO :Mn w10x and Zn SiO :Mn-2 4 2 4 2 4

* Corresponding author. Tel.:q81-76-294-6705; fax:q81-76-294-6707.

E-mail address: minami@neptune.kanazawa-it.ac.jp(T. Minami).

Zn GeO :Mn w12–14x systems prepared with various2 4

chemical compositions as the thin-film emitting layer.In this paper, we propose using a new multicomponent

oxide phosphor composed of Y O :Mn and GeO :Mn,2 3 2

i.e. a Mn-activated Y O –GeO system((Y O –2 3 2 2 3

GeO ):Mn) phosphor, as the emitting layer for TFEL2

devices. Such TFEL devices have been successfullyfabricated on a BaTiO ceramic sheet insulating layer.3

2. Preparation of the thin-film emitting layer

The electroluminescent(EL) characteristics wereinvestigated using a single-insulating-layer TFEL devicestructure as shown in Fig. 1. The TFEL devices werefabricated by depositing phosphor thin films onto thickBaTiO insulating ceramic sheets(thickness of approx.3

0.2 mm) w1–5x. The 1–2-mm-thick (Y O –GeO):Mn2 3 2

phosphor films were prepared by r.f. magnetron sputter-ing using powder targets. The sputter deposition wascarried out under the following conditions: atmosphere,pure argonwAr (purity,99.99%)x; pressure, 6 Pa; r.f.power, 100 W; substrate temperature, 2758C; and target,(Y O –GeO):Mn phosphor powder with a GeO con-2 3 2 2

tent of 0–100 mol.%. The target mixture of Y O ,2 3

GeO and MnO (dopant) powders was calcined at2 2

509T. Minami et al. / Surface and Coatings Technology 146 –147 (2001) 508–512

Fig. 1. Single-insulating-layer type TFEL device.

Fig. 2. L–V characteristics of(Y O –GeO):Mn TFEL devices pre-2 3 2

pared with various GeO contents.2

9008C in an Ar (purity, 99.99%) atmosphere for 1 h.The Mn dopant contentwMny(MnqY) atomic ratioxwas 1 at.%. In order to improve the EL characteristics,all deposited (Y O –GeO):Mn phosphor thin films2 3 2

were post-annealed in an Ar(purity, 99.99%) atmos-phere for 1 h at 800–11008C. In the final procedure, anAl-doped ZnO transparent conducting film and an Alfilm were deposited as the transparent electrode and theback electrode, respectively. The composition(Y, Geand Mn contents, or their atomic ratios) of phosphorthin films, measured by energy dispersive X-ray spec-troscopy(EDX), was found to be approximately equalto that in the target. The EL characteristics of the TFELdevices driven by a 1-kHz or 60-Hz sinusoidal wavevoltage were measured using a Sawyer–Tower circuit,an ac power meter and a conventional luminance meter.The photoluminescence(PL) spectra were measuredusing a spectrofluorophotometer(Shimazu RF-5300PC)at room temperature.

3. Results and discussion

3.1. GeO content dependence of EL characteristics2

EL emission was always observed in TFEL devicesusing (Y O –GeO):Mn thin films prepared with a2 3 2

GeO content of 0, 33, 50, 67 or 100 mol.% and post-2

annealed above approximately 9208C, whereas it wasnot observed when their thin films were in the as-deposited state. PL was also observed in the post-annealed(Y O –GeO):Mn thin films under excitation2 3 2

with a photon energy below approximately 6.2 eV. TheEL characteristics were strongly dependent on the com-position (GeO content) of the (Y O –GeO):Mn thin-2 2 3 2

film emitting layers. In addition to the composition, theywere also dependent on both the sputter deposition andthe post-annealing conditions of the thin-film emittinglayers. X-Ray diffraction analyses showed that as-depos-ited Y O and GeO thin films prepared on BaTiO2 3 2 3

ceramic sheets, i.e. a GeO content of 0 and 100 mol.%,2

respectively, were crystallized. Although as-deposited(Y O –GeO):Mn thin films prepared with a GeO2 3 2 2

content of 33, 50 and 67 mol.% were amorphous, thethin films were crystallized by post-annealing at tem-peratures above approximately 8708C. As mentionedabove, however, EL emission was only obtained in

TFEL devices using(Y O –GeO):Mn thin films that2 3 2

were not only crystallized, but also post-annealed attemperatures above approximately 9208C, irrespectiveof the GeO content. The post-annealing conditions2

necessary to produce higher luminance were alsodependent on the GeO content of the(Y O –GeO):Mn2 2 3 2

thin-film emitting layers. Higher luminances wereobtained in devices using(Y O –GeO):Mn thin-film2 3 2

emitting layers: post-annealed at temperatures of approx-imately 10208C for GeO contents of 0, 33 and 752

mol.%, approximately 9708C for 50 mol.% and approx-imately 10708C for 100 mol.%(GeO film).2

Typical luminance(L) and transferred charge density(Q) as functions of applied voltage(L–V and Q–V)characteristics are shown in Figs. 2 and 3, respectively,for TFEL devices using(Y O –GeO):Mn thin films2 3 2

(thickness of approx. 1.3mm) prepared with variousGeO contents, post-annealed at 10208C, and driven2

with sinusoidal wave driving voltages at 1 kHz. Itshould be noted that the GeO content dependence of2

theL–V characteristics was not related to that of theQ–V characteristics. With a post-annealing for 1 h at atemperature of 10208C, it was found that both highluminance and high luminous efficiency were obtainedin TFEL devices using(Y O –GeO):Mn thin-film emit-2 3 2

ting layers prepared with a GeO content of 50 mol.%.2

3.2. TFEL devices using Y GeO :Mn thin films2 5

As mentioned above, the obtainable EL characteristicswere considerably affected by the post-annealing tem-perature of the(Y O –GeO):Mn thin films used as the2 3 2

emitting layer. Figs. 4 and 5 showL–V and Q–Vcharacteristics as functions of the post-annealing tem-

510 T. Minami et al. / Surface and Coatings Technology 146 –147 (2001) 508–512

Fig. 3. Q–V characteristics of(Y O –GeO):Mn TFEL devices pre-2 3 2

pared with various GeO contents.2

Fig. 5.Q–V characteristics of Y GeO :Mn TFEL devices as a function2 5

of post-annealing temperature.

Fig. 4.L–V characteristics of Y GeO :Mn TFEL devices as a function2 5

of post-annealing temperature.Fig. 6. X-ray diffraction patterns of Y GeO :Mn thin-film emitting2 5

layers as a function of post-annealing temperature.

perature for TFEL devices using(Y O –GeO):Mn thin2 3 2

films prepared with a GeO content of 50 mol.% and2

driven at 1 kHz. TheL–V characteristic and obtainedmaximum luminance also proved to be dependent onthe post-annealing temperature. The highest luminancewas obtained in a device using a(Y O –GeO):Mn thin2 3 2

film post-annealed at 9708C.The crystallinity of the thin-film emitting layer was

also found to be dependent on the post-annealing tem-perature. As an example, Fig. 6 shows X-ray diffractionpatterns as a function of the post-annealing temperaturefor (Y O –GeO):Mn thin-film emitting layers prepared2 3 2

with a GeO content of 50 mol.%. It is well known that2

there are three kinds of ternary compounds, Y GeO ,4 8

Y GeO and Y Ge O , in the multicomponent oxide2 5 2 2 7

Y O –GeO system. As can be seen in Fig. 6, most2 3 2

peaks observed from films post-annealed at temperaturesfrom 970 to 10708C can be identified as the diffractionfrom the Y GeO lattice. It should be noted that the2 5

crystallinity estimated by the diffraction intensity and

511T. Minami et al. / Surface and Coatings Technology 146 –147 (2001) 508–512

Fig. 7. PL emission spectra of Y GeO :Mn thin-film emitting layers2 5

as a function of post-annealing temperature.

Fig. 9. Typical EL characteristics of a Y GeO :Mn TFEL device driv-2 5

en at 60 Hz.

Fig. 8. Typical EL characteristics of a Y GeO :Mn TFEL device driv-2 5

en at 1 kHz. Fig. 10. EL emission spectrum from a Y GeO :Mn TFEL device.2 5

full width at half maximum was relatively unrelated tothe annealing temperature above approximately 8708C,but the diffraction patterns were dependent on theannealing temperature.PL emission was only observed in(Y O –GeO):Mn2 3 2

thin-film emitting layers post-annealed above approxi-mately 8708C, irrespective of the GeO content. How-2

ever, the intensity and spectral shape of PL emissionswere dependent on the GeO content. In addition, the2

emission spectral shape was relatively independent ofthe post-annealing temperature. Fig. 7 shows PL emis-sion spectra from Y GeO :Mn thin film emitting layers2 5

post-annealed at different temperatures. It should benoted that the observed maximum PL intensity wasobtained in a thin-film emitting layer post-annealed at10208C, not correlated to the temperature to obtain themaximum luminance, 9708C, shown in Fig. 4. Thus, it

can be concluded that, in the fabrication of TFELdevices using Y GeO :Mn thin films prepared with a2 5

Mn content of 1 at.%, a higher luminance EL emissioncan be obtained by post-annealing for 1 h at 9708C.Typical L–V and Q–V characteristics are shown in

Figs. 8 and 9 for a TFEL device using an Y GeO :Mn2 5

thin film prepared under optimized preparation condi-tions, and driven at 1 kHz and 60 Hz, respectively:maximum luminances of 758 and 15 cdym , respective-2

ly. A maximum luminous efficiency of 1.4 lmyW wasobtained in the 1 kHz-driven Y GeO :Mn TFEL device.2 5

The emission color from(Y O –GeO):Mn TFEL2 3 2

devices was affected by the GeO content. However, the2

emission spectra were independent of the GeO content2

for TFEL devices using(Y O –GeO):Mn thin films2 3 2

prepared with GeO contents of 0–50 mol.%. For exam-2

512 T. Minami et al. / Surface and Coatings Technology 146 –147 (2001) 508–512

ple, the emission spectrum from a TFEL device using aY GeO :Mn thin film, (Y O –GeO):Mn prepared with2 5 2 3 2

a GeO content of 50 mol%, is shown in Fig. 10. The2

color of EL emission was yellow. It should be notedthat the spectral shape of EL emission observed fromY GeO :Mn TFEL devices was the same as that of PL2 5

emission observed from Y GeO :Mn thin films as shown2 5

in Fig. 7. This yellow emission is attributed to the T–4

A transition in Mn (3d ) ions. The CIE chromaticity6 2q 5

color coordinate was(xs0.428, ys0.552) for theY GeO :Mn TFEL device shown in Fig. 10.2 5

4. Conclusions

Mn-activated (Y O –GeO) multicomponent oxide2 3 2

phosphor thin films were prepared by r.f. magnetronsputtering and used as the emitting layer of TFELdevices. In TFEL devices fabricated using(Y O –2 3

GeO ) phosphor thin films, the obtained EL character-2

istics were strongly dependent on the post-annealingconditions as well as the chemical composition. Higherluminance EL emissions could be obtained using a(Y O –GeO):Mn thin film prepared with a GeO con-2 3 2 2

tent of approximately 50 mol.%, emitting layer compo-sition identified as Y GeO , and post-annealed at2 5

approximately 9008C. High yellow luminances of 758and 15 cdym were obtained for a Y GeO :Mn TFEL2

2 5

device driven by a sinusoidal wave voltage at 1 kHzand 60 Hz, respectively. In addition, a maximum lumi-nous efficiency of 1.4 lmyW was obtained in this devicedriven at 1 kHz. This is the first report of a highluminance emissions from TFEL devices usingY GeO oxide phosphors.2 5

Acknowledgements

The authors would like to thank M. Akasofu, K.Hayashi and M. Yamazaki for their technical assistancein the experiments.

References

w1x T. Minami, T. Miyata, S. Takata, I. Fukuda, Jpn. J. Appl. Phys.30 (1991) L117.

w2x T. Minami, T. Maeno, Y. Kuroi, S. Takata, Jpn. J. Appl. Phys.34 (1995) L684.

w3x T. Minami, H. Yamada, Y. Kubota, T. Miyata, Jpn. J. Appl.Phys. 36(1997) L1191.

w4x T. Minami, H. Yamada, Y. Kubota, T. Miyata, Proc. SPIE SmartElectronics MEMS 3242(1997) 229.

w5x T. Minami, Mater. Res. Soc. Symp. Proc. 558(2000) 29.w6x T. Minami, T. Miyata, Y. Sakagami, Surf. Coat. Technol. 108y

109 (1998) 594.w7x T. Minami, H. Yamada, Y. Kubota, T. Miyata, Y. Sakagami,

Proceedings of the 4th International Display Workshops, 1997,p. 605.

w8x A.H. Kitai, T. Xiao, G. Liu, J.H. Li, SID International Sympo-sium Digest of Technical Papers, 1997, p. 37.

w9x T. Minami, Y. Sakagami, T. Miyata, Ext. Abstract of the 3rdInternational Conference on the Science and Technology ofDisplay Phosphor, 1997, p. 37.

w10x T. Minami, Y. Kuroi, H. Yamada, S. Takata, T. Miyata, Proceed-ings of the 16th International Display Research Conference,1996, p. 215.

w11x T. Minami, Ext. Abstract of the 4th International Conference onthe Science and Technology of Display Phosphor, 1998, p. 195.

w12x A.H. Kitai, T. Xiao, G. Liu, J.H. Li, SID International Sympo-sium Digest of Technical Papers, 1997, p. 415.

w13x T. Minami, Y. Kubota, T. Miyata, H. Yamada, SID InternationalSymposium Digest of Technical Papers, 1998, p. 953.

w14x T. Minami, Y. Kubota, T. Miyata, Proceedings of the 18thInternational Display Research Conference, 1998, p. 1015.