Whitelight emitting thin film electroluminescent devices with stacked SrS:Ce/CaS:Euactive layersYoshimasa A. Ono, Moriaki Fuyama, Kenichi Onisawa, Katsumi Tamura, and Masahiko Ando Citation: Journal of Applied Physics 66, 5564 (1989); doi: 10.1063/1.343661 View online: http://dx.doi.org/10.1063/1.343661 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Violet light emitting SrS/SrCl:Eu thinfilm electroluminescent devices Appl. Phys. Lett. 67, 7 (1995); 10.1063/1.115514 White light emitting SrS:Pr electroluminescent devices fabricated via atomic layer epitaxy Appl. Phys. Lett. 66, 419 (1995); 10.1063/1.114042 Luminescence and electron paramagnetic resonance studies of whitelight emitting SrS: Pr, F thin filmelectroluminescent devices J. Appl. Phys. 75, 1754 (1994); 10.1063/1.356366 Bright whitelight electroluminescence in SrS:Pr,K thin films Appl. Phys. Lett. 52, 2102 (1988); 10.1063/1.99548 Bright whitelight electroluminescence based on nonradiative energy transfer in Ce and Eudoped SrS thin films Appl. Phys. Lett. 51, 1661 (1987); 10.1063/1.98535

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White-light emitting thin fUm eiectroluminescent devices with stacked SrS:Ce/CaS:Eu active layers

Yoshimasa A. Ono. Moriaki Fuyama, Ken-ichi Onisawa, Katsumi Tamura, and Masahiko Ando Hitachi Research Laboratory, Hitachi, Ltd., 4026 Kuji-cho, Hitachi-shi, Ibaraki-ken 319-12, Japan

(Received 12 May 1989; accepted for publication 28 August 1989)

By .st~cking blue-green emitting SrS:Ce and red-emitting CaS:Eu active layers, white-light emIttmg electroluminescent (EL) devices were fabricated. Luminance improvement and EL characteristics of SrS:Ce and CaS:Eu EL devices were discussed. The electrooptical characteristics of white-light emitting EL devi.ces with stacked SrS:Ce/CaS:Eu active layers w~re presen~e~ .. Color cha?ged from bl~e-green to white by changing the voltage or frequency. Fm~ny, f~aslblhty of multlColor EL devIces by using the fabricated white-light emitting EL devices with color filters were discussed.

I. INTRODUCTION

Recently interest in multicolor thin film electrolumines­cent (EL) devices has been growing rapidly and new active layer materials, such as CaS and SrS, have been em­ployed 1-28 to achieve higher luminance or better color purity than ZnS based EL devices. Especially, CaS:Eu EL devices emit red color with better color purity than that of orange­color emitting ZnS:Sm,F EL devices, and SrS:Ce EL devices have higher luminance than that of blue-color emitting ZnS:Tm,F EL devices, although the emission color is blue­green.

Furthermore, white-light emitting thin film EL devices are getting renewed interest,29-33 since from the ergonomics study of data display cathode-ray tubes (CRTs), white color was found to be easy on the eyes and less tiring compared with other colors. To realize white-light emitting EL de­vices, several combinations of active layers have been dis­cussed: A SrS:Ce/ZnS:Mn stacked active layer EL device29

emits a yellowish white color, a SrS:Ce,Eu EL device30 and a SrS:Ce/SrS:Eu stacked active layer EL device3l emits an eggshell-white color, and a SrS:Pr EL device32 emits a white color,

Multicolor or full-color EL devices can be constructed from the white-light emitting EL cells combined with color filters. 34 To realize this, EL emission spectrum must have red, green, and blue spectral components. The combinations of active layers discussed above are not good enough for this purpose.

This paper proposes a new combination of active layers, stacked SrS:Ce/CaS:Eu active layers, in order to obtain pa­per white EL emission with red, green, and blue spectral components. Luminance improvement and EL characteris­tics of SrS:Ce and CaS:Eu EL devices are discussed. Then electro-optical characteristics of white-light emitting EL de­vices with stacked SrS:Ce/CaS:Eu active layers are de­scribed. Finally, feasibility of multicolor EL devices by uti­lyzing the the present white-light emitting EL devices with color filters are discussed.

II. IMPORTANCE OF WHITE COLOR FOR SCREENS OF OA EQUIPMENT

In the ergonomics study of data display terminals for office automation (OA) equipment, such as word proces-

sors, personal computers, and work stations, it was found35

,36 that emission colors play an important role in man-machine interface characteristics, such as easiness on the eyes and less-tiring conditions. In the display of word processors, especially, reverse-mode representation is often employed, where alphanumeric characters are displayed in dark color against a bright background. In using word processors, one has to look at the text and the screen back and forth very often for a long time, it is desirable to have the similar representation on the display to that of the text, where characters are written in black against white paper. Therefore paper white color should be appropriate as the background color of the displays, for example CR Ts and flat panel displays.

Figure 1 shows the Commision Internationale de l'Edairage (CIE) color coordinates offour kinds of white colors: paper white color, eggshell-white color, bluish-white color, and conventional white color. In the figure, the area surrounded by the dotted line indicates the white color re­gion.

In the ergonomics study on CR Ts, 35 the following re­sults were obtained for favorable white colors under two it-

0.8

0.6 BLUISH WHITE

Y ~ EGGSHELL WH1rE -/-' 0.4 ,... ~

" -)

~ 0,2

, .. Jf ~PER WHITE

CONVENTIONAL WHIT

0 0 0.2 0.4 0.6 0.8

X

FIG. I. CIE chromaticity diagram. Color coordinates off our kinds ofwhite colors are shown.

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luminating conditions with different fluorescent lamps. Un­der the white fluorescent lamp, favorable white colors in descending order were paper white, eggshell white, conven­tional white, and bluish white. On the other hand, under the daylight-color fluorescent lamp, favorable white colors in descending order were paper white, conventional white, egg­shell white, and bluish white. Therefore under both condi­tions, paper white was found to be the most favorable color.

In the study on the visual display terminal (VDT) guidelines undertaken at Industrial Products Research In­stitute of Japan,36 the following results were obtained as fa­vorable colors under the criteria of easiness on the eyes and less tiring conditions. From the experiments in the reverse­mode representation ofCRTs, the region offavorable back­ground colors were estimated, which are shown in two encir­cled areas in the CIE chromaticity diagram in Fig. 2. One with the CRT brightness of 17.1 cd/m2( = 5 ft L), and the other with the CRT brightness of 51.4 cd/m2

( = 15 ft L). Both areas cover most of the white region enclosed by dotted line with shades.

From these studies, white color, especially paper white color, is found to be a favorable color for screens of OA equipment, such as word processors and personal comput­ers.

III. SrS:Ce BLln::~GREEN EL DEVICES AND CaS:Eu RED ElDEVICES

Ao Active layer combinations to obtain white El devices

White-light emission can be obtained from the following two conditions: (1) Combination of three primary color emissions, Le., blue, green, and red emissions. (2) Combina­tion of complementary color emissions, such as red and blue­green emissions, or yellow and blue emissions.

In this study, we consider white EL emission by combin­ing EL emissions from SrS:Ce blue-green EL active layer

0.8

0.6

Y 0.4

0.2

0 0 0,2 0.4 0,6 0.8

X

FIG. 20 CIEchromatidty diagramo Regions offavorable background colors in the reverse-mode representation of CRTs are shown: One with the CRT brightness of 1701 cd/m"( = 5 ft L) and the other with the CRT brightness of51.4cd/m'( = 15ftL)0

5565 Jo AppL Physo, VoL 66, Noo 11,1 December 1989

400

CaS:Eu

4f6(7F)Sd

500 600

WAVELENGTH (nm)

FIG. 3. Emission spectra of CaS:Eu and SrS:Ce EL devices.

700

and CaS:Eu red EL active layer. Figure 3 shows EL emission spectra of these EL devices. EL emission from CaS:Eu is due to parity-allowed transition of Eu2+ with a peak at 650 nm. EL emission from SrS:Ce consists ofa blue component with a peak at 475 nrn and a broad green component with a peak around 520-550 nrn. These emissions are due to parity-al­lowed transitions of Ce3 + ions. The CIE color coordinates of these EL emissions are shown in Fig. 4, where dosed squares with R, G, and B indicate the positions of red, green, and blue color CRTs, respectively. In the graph, color coordi­nates of CaS:Ce green EL device, ZnS:Mn orange-yellow EL device, ZnS:Tb,F green EL device, ZnS:Sm,F orange EL de­vice, and ZnS:Tm,F blue EL device are shown for reference.

Bo luminance improvement

In order to obtain high-luminance white-emitting EL devices, it is necessary to increase the luminance of SrS:Ce EL devices as well as that of CaS:Eu EL devices. From the study on the deposition conditions of active layers, the fol­lowing conditions were found to be very effective in lumi-

0.8

0.6

Y 0.4

0,2

0.2 0.4 Q.6 0.8 X

FIGo 40 CIE chromaticity diagram. Color coordinates of color EL devices arc shown, together with the color coordinates of CRT.

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104 SrS:Ce

103 5kHz SINUSOIDAL WAVE DRIVE 4.5 x 10-2 Po

"i= ..... 102 LO x 10-2 Po "0

U

-l

~ 101 7.0xlO-3 pa :z:::

<C z III • II - ---::IE: :3 100

10-1 0 200 300 400

VOLTAGE V (V)

FIG. 5. Luminance vs voltage characteristics of SrS:Ce blue-green EL de­vices with pressure as a parameter. The pressures 7 X 10-3 Pa, I X 10-2 Pa, and4.SX 10-- 2 Pa correspond to the case of no sulfur, 1 gofsulfur, and 5 gof sulfur, respectively.

nance improvement; For SrS;Ce, sulfur coevaporation,24 and for CaS:Eu, both the increase of the substrate tempera­ture and sulfur coevaporation.

In the following, experimental results are briefly dis­cussed. Figure 5 shows the luminance versus voltage charac­teristics of SrS:Ce EL devices with 0.1 mol % CeC13 as a luminescent center. The pressures during the deposition were varied by changing the added amount of sulfur, and the voltage was measured in zero-to-peak value. The pressures 7 X 10-3 Pa, 1 X 10--2 Pa, and 4.5 X 10-2 Pa correspond to the cases of no added sulfur, 1 g of added sulfur, and 5 g of added sulfur, respectively. The SrS:Ce active layer with 0.5 pm thickness was formed by the electron-beam (EB) evapo­ration method with the substrate temperature of 500 ·C and the insulating layers of stacked Ta20s/Si02 were formed by the multitarget rf sputtering method. As can be seen from the graph, sulfur addition increased the luminance level by two orders of magnitudes. Auger electron spectroscopy (AES) analysis of the active layers indicated that by sulfur coevaporation, oxygen contents in SrS:Ce decreased very much to the level below the measuring limit.

On the other hand, Figs. 6 and 7 indicate luminance improvement results of CaS:Eu, where active layers with 1.1-1.4 f.lm thickness were deposited by the EB evaporation method with 0.1 mol % EuS as a luminescent center. The deposition rate of the active layers was controlled to be 27-33 A/s. The insulating layers of stacked Ta20slSi02 were formed by the multitarget rfsputtering method as was in the case of SrS:Ce EL devices. Figure 6 indicates the substrate temperature dependence of the maximum luminance. The results show that with the increase of the substrate temper­ture the maximum luminance increased almost linearly. X­ray analysis showed that crystallinity of CaS:Eu increased with (222) alignment becoming dominant with the increase of substrate temperature. Figure 7 shows the effect of sulfur

5566 J. Appl. Phys" Vol. 66, No. 11, 1 December 1989

- 3

~ CaS:Eu <0,1 mol%) "C U -j200 W U z: <t: Z - 100 :E: ::::! -l

:e: => :E: -X 0 ~ 0 300 400 500

SUBSTRATE TEMPERATURE T (OC)

FIG. 6. Substrate temperature dependence ofluminance in CaS:Eu red EL devices.

coevaporation on the luminance, where sulfur powder of 0.1-1.0 g was placed in Ta crucible and was evaporated by the resistive heating method. The substrate temperature was set at 470°C during the deposition. The maximum lumi­nance increased with the increase of the active layer thick­ness because Eu contents in the active layer increased. The more important feature is that the maximum luminance lev­el almost doubled by coevaporating sulfur during the depo­sition.

N E ....... '0 U

400

i 100

CaS: Eu <0,1 molX) ,

•

ACTIVE LAYER THICKNESS dAL q.lI11)

FIG. 7. Effects of active layer thickness and sulfur coevaporation to lumi­nance in CaS:Eu red EL devices.

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104 ....... SINUSOIDAL WAVE DRIVE

"1: ~ ul 5 kHz

u

-l 10

2

w u z ~ 10t ..... ::IE: ::;) ....I

1rP 50 100 150 200 250 300

VOLTAGE V (V)

FIG. 8. Luminance vs voltage characteristics ofSrS:Ce and CaS:Eu EL de­vices.

c. Optical properties of SrS:Ce and CaS:Eu EL devices

Figure 8 shows the luminance versus voltage (L- V) characteristics ofSrS:Ce and CaS:Eu EL devices. Active lay­ers of these devices were deposited at the same time when acti.ve layers of the stacked-structure white-light emitting EL devices were deposited. In the figure, the results for 5 and 1 kHz sinusoidal wave drive conditions are shown with the voltage being measured in zero-to-peak value. The maxi­mum luminances under 5 kHz drive were 1300 cd/m2 for SrS:Ce and 350 cd/m2 for CaS:Eu. From the figure we ob· serve that the luminance level of SrS:Ce EL devices is 5 to 6 times larger than that of CaS:Eu EL devices.

As was shown in Fig. 3, emission spectrum of SrS:Ce blue-green EL devices consists of a blue component and a green component. Therefore, by using color filters we can separate blue and green colors. Figure 9 shows the transmis~ sion spectrum of color filters used in the following discus­sion. With the use of the blue filter, the color of the EL emission changed from blue-green with x = 0.194, Y = 0.382, a principal wavelength of 500 nm, and a purity of 0.39 to blue with x = 0.104, Y = 0.190, a principal wave­length of 485 nrn, and a purity of 0.87. The luminance, how­ever, decreased to 25% of the original luminance. On the other hand, with the use of the green filter, the color of the EL emission changed from blue-green to green with x = O.246,y = 0.661, a principal wavelength of 542 nm, and a purity of 0.78. The luminance decreased to 75% of the original luminance. These results of color changes are shown in the CIE chromaticity diagram in Fig, 10.

From Fig. 10, we find that by combining the EL emis­sions from CaS:Eu and SrS;Ce EL devices we can obtain the coiors indicated by the solid line connecting CaS:Eu and SrS:Ce color coordinate points. Therefore, white color, espe­cially paper white color, can be obtained by combining CaS:Eu and SrS:Ce emissions. In the figure the color coordi­nates ofSrS:Eu orange EL devices is also shown. From this, an eggshell-white color, rather than a paper white color, can be obtained when EL emissions from SrS:Ce and SrS:Eu are combined.

5567 J. Appl. Phys., Vol. 66, No. 11, 1 December 1989

FILTER

50

o 400 500 600 700

WAVELENGTH (nm)

~ 100 BLUE FILTER

400 500 600 700 WAVELENGTH (nm)

FIG. 9. Transmission characteristics of the blue and green filters.

When blue and green colors are realized separately by using the color filters discussed above, colors represented in the triangle enclosed by dashed lines can be reproduced. This color triangle is very close to the CRT color triangle en­closed by dotted lines.

Figure 11 shows the luminance versus frequency (L-/) characteristics. The SrS:Ce EL device has similar character-

0.8

0,6

y

0.4

0.2

o o 0.2 0.4 0.6 0,8

x

FIG. 10. cm chromaticity diagram. Color coordinates of SrS:Ce and CaS:Eu EL devices and color changes of SrS:Ce due to color filters are shown,

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104

SINUSOIDAL WAVE DRIVE

'1: 103 ........ U u

....J 10l w u z <C :z: ..... 101 :e: :::I ....I

105

FREQUENCY f (Hz)

FIG. 11. Luminance vs frequency characteristics ofSrS:Ce and CaS:Eu EL devices.

istics to those of ZnS:Mn and the relation L 0::./ holds over a wide frequency range, and above 1 kHz luminance tends to saturate. On the other hand, the CaS:Eu EL device has quite a different L.Jbehavior: At a high voltage region, the L 0::./ relation holds only in the frequency range 500 Hz.if< 5 kHz. At a low voltage region, the L o:/relation does not hold at all, and luminance does not respond to frequency until vol­tage with a certain frequency ofthe order of 100 Hz to 1 kHz is impressed. This is due to the fact that at a low drive vol­tage, transferred charge within the active layer decreases very rapidly with the decrease offrequency. This behavior is • detrimental to the matrix drive at low frequencies and must be remedied.

I -AI

T0205(Q·5j.lm)

S rS : Ce (Q • 90~m)

CaS:Eu(O.98~m)

T0205(Q·5~m)

-ITO

t GLASS SUBSTRATE

FIG. 12. Cross-sectional device structure of a white-light emitting EL de­vice with stacked SrS:Ce/CaS:Eu active layers.

5568 J. Appl. Phys .. Vol. 66, No. 11, 1 December 1989

104

........ SINUSOIDAL WAVE DRIVE NEE

1cP SrS:Ce/CaS:Eu ....... '0 U - 1 kHz -l

LiJ U z <t: z

101 0-0

E; ::::) ...J

1cP 0 150 200 250 300 350 400

VOLTAGE V (V)

FIG. 13. Luminance vs voltage characteristics of a SrS:Ce/CaS:Eu white­light emitting EL device.

IV. WHITE~LIGHT EMITTING EL DEVICE WITH STACKED SrS:Ce/CaS:Eu ACTIVE LAYERS

By stacking SrS:Ce and CaS:Eu active layers, a white­light emitting EL device was fabricated. Figure 12 shows the cross-sectional device structure. Active layers SrS:Ce/Ca­S:Eu were fabricated by the EB evaporation method and the insulating layers Ta20 S were fabricated by the rf sputtering method.

Luminance versus voltage (L- V) characteristics are shown in Fig. 13. With the increase of voltage the emission color changed from blue-green to white and remained white over the wide voltage region. With the decrease of frequency the voltage region with white emission narrowed down. The

'1 ...... SINUSOIDAL WAVE DRIVE

N SrS:Ce/CaS:Eu E ""'- 103 "0 u - 250 V -' w u 240 V z <t: Z ...... :E: :::::I ...J

lOS f

FIG. 14. Luminance vs frequency characteristics of a SrS:Ce/CaS:Eu white-light emitting EL device.

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a,s

0.6

Y 0,4

0.2

0 () 0.2 0.4 0.6

X (c) V=240V

0.8

0.6

Y 0.4

0.2

0 () 0.2

X

(b) V = 250 V

0.8

0.6

Y 0.4

0.2

0 0 0.2 0.4 0.6

X (e) V = 330V

FIG. 15. CIE chromaticity diagram. Color changes as a function offrequen­cy in a SrS:Ce/CaS:Eu white-light emitting EL device are shown for the drive voltages of (a) 240, (b) 250, and (c) 330 V.

maximum luminance at 5-kHz sinusoidal wave drive condi­tion was 1400 cd/m2.

Luminance versus frequency (L-f) characteristics are shown in Fig. 14. The relation L rx/ holded for the whole voltage region. The emission color was white at a high vol­tage regardless of the frequency, but at a low voltage the emission color changed from white to blue-green with the decrease of frequency. This is described in the cm chroma­ticity diagrams in Fig. 15 for the drive voltages of 240, 250, and 330 V.

These behaviors can be explained from the electro-opti-

5569 J. Appl. Phys" Vol. 66, No. 11 , 1 December 1989

cal characteristics of SrS:Ce and CaS:Eu active layers given by Figs. 8 and 11. First we consider the L-V characteristics. Since the threshold voltage of the SrS:Ce active layer is lower than that of the CaS:Eu active layer, at low voltage only the SrS:Ce part of the stacked active layer of the white-light emitting EL device is excited, resulting in only blue-green emission. With the increase of voltage, CaS:Eu starts to be excited and the emission color becomes white.

Next we consider L-f characteristics. Since the lumi­nance response of CaS:Eu is very slow at a low frequency as shown in Fig. 11, only the blue-green emission from the SrS:Ce part of the stacked active layers is observed at a low frequency in the white-light emitting EL device. With the increase of frequency, CaS:Eu begins to respond and the emitting color becomes white.

Figure 16 shows the col.or separation results of a white~ light emitting EL device by using color filters, whose trans­mission characteristics are shown in the middle column of the figure. The original EL emission characteristics are shown in the left-hand side and the emission characteristics after transmitting color filters are shown in the right-hand side of the figure. The percentage in the right-upper corner of the transmitted emission characteristics diagram indicates the luminance ratio of respective primary color with respect to the original luminance. In the lower-left corner, the color coordinates of the original emission and those after transmit­ting color filters are shown in the eIE chromaticity diagram.

Comparing these results with Fig. to, we find that color positions of red, green, and blue of the filter-transmitted emissions are close to the respective position of CRT. Fur­thermore, the luminance ratio of red, green, and blue is R:G:B = 3:6:1, which again is close to that of CRT. These results indicate that the present white-light emitting EL de­vice is capable of making multicolor EL devices by employ­ing color filters.

\/. CONCLUSIONS

The luminance improvement of BB deposited SrS:Ce and CaS:Eu EL devices were discussed. White-light emitting EL devices were fabricated by stacking SrS:Ce and CaS:Eu active layers, and electro-optical characteristics are dis­cussed. In forming the SrS:Ce active layer, sulfur coevapora­tion was very effective in increasing the luminance. In the case of the CaS:Eu active layer forming, an increase of sub­strate temperature and sulfur coevaporatioll was effective in improving luminance, At high-substrate temperatures, the crystallinity of CaS:Eu active layers was improved with strong alignment along the (222) direction. This is in accor­dance with the results obtained in Ref. 5.

The emission color of the fabricated EL devices with stacked SrS:Ce/CaS:Eu active layers was paper white. At a high voltage, the emission color was white regardless of the frequency, but at a low voltage, the emission color changed from white to blue-green with the decrease of frequency. This is due to the fact that the luminance response ofCaS:Eu is slow at a low voltage and a low frequency,

By using color filters, it was possible to obtain red, green, and blue emissions with the luminance ratio of R:G:B = 3:6:1. Therefore the fabricated white-light emit-

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~lOO " 100

"" w RED FILTER ( u 30 % ZN l- e( ,

z '"""' 0 0 0 <C x - SO I

. oc 50 V')

~ V') - -' i: e(

100 0:: " V') l- lK

UJ z: u U

e( j w 0

IX --L C- o ZN I- 0 ..;. V> a - 0 400 500 600 700 400 500 600 0 700 <C( >< WAVELENGTH A (nm) 0:: WAVELENGTH A (nm)

-' c; 50 ..;

100 IX " ~100 w I- "" ..., u U .... a Z M 58 %

"- a I-<:( ,

VI - 0 0 Z

0

400 c( >< 0 0:: .... 50 0 50

WAVELENGTH A (om) VI -- -' .; VI <C( .... 0: " ::E I- "" Vl U Z .... 0 «: Q. 0

"" 0 VI I- a

0.8 400 500 600 700 400 500 600 700 WIWELENGTH A (nm)

WAVELENGTH A (om) 0.6

100 Y BLUE FILTER w

UM 9% 0.4 I- z, z ::; 0 a 0 x - 50 e(

0.2 VI - 0:: c; 50 I- -VI - -' .....

j :IE: <:( VI "" " 0 z: I-c( U

.., \ 0 D.2 0.4 c::

0 w 0

X I- 0.. 0 a t

400 500 600 700 V)

WAVELENGTH A (nm) 400 500 600 700

WAVELENGTH A (nm)

FIG. 16. Color separation results of a SrS:Ce/CaS:Eu white-light emitting EL device by using color filters. The spectral radiance in the EL emission spectrum is measured in units ofW Isr/m2/nm. The percentage in the right-upper corner of tile transmitted emission characteristics diagram indicates the luminance ratio of respective primary color with respect to the original luminance.

ting EL device is capable of making multicolor EL devices by employing color filters. This structure of white-light emit­ting EL devices should have large application fields as flat panel displays of OA equipment.

ACKNOWLEDGMENTS

We would like to thank Dr. Y. Sugita of Hitachi Re­search Laboratory, Hitachi, Ltd., and H. Kawakami of Mobara Works, Hitachi, Ltd. for suggestions and encour­agements.

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