GB2195823A - Thin film electroluminescent device - Google Patents

Description

GB2195823A 1 SPECIFICATION the EL device produced in Example 2 of this

specification.

Thin film electroluminescent device Detailed Informations of the Invention:

Field of the Invention: 70 The basic material forming the luminescent

This invention relates to a thin film electrolu- layer of the thin film electroluminescent device minescent device which can be utilised in an is an alkaline earth metal chalcogen com electroluminescence display such as a com- pound. Those compounds include, for in puter terminal display. stance, alkaline earth metal sulfide such as 75 SrS, CaS and BaS and alkaline earth metal Background of the Invention: selenicle such as SrSe and CaSe. Into the ba-

In the development of an electroluminescent sic material a luminescence center of lantha- device for multi-coloration, compounds of al- noid series rare earth element such as Ce and kaline earth metal chalcogen have been found Eu is added. Various luminescent colors may to be a desirable basic material of luminescent 80 be obatained depending on the combination of layer. One of them is an alkaline earth metal the basic material and the luminescence cen sulfide such as SrS and CaS, and a thin film ter. For instance, there can be obtained blue electroluminescent device provided with the lu- luminescence from the combination of minescent layer of the sulfide incorporated SrSe/Ce, green luminescence from CaS/Ce with a luminescence center such as Se and Eu 85 and red luminescence from CaS/Eu.

is known. (W.A. Barrow, R.E. Coovent and The present invention is essentially charac- C.N. King; Digest 1984 SID Int. Symp., So- terized by forming the insulating layer with ni ciety for Information Display, Los Angels, tride, which is laminated on the luminescent 1977, p88 and V. Shanker, S. Tanaka, M. layer. The nitride includes BN, AIN, TiN, TaN, Shiiki, H. Deguchi, H. kobayashi and H. Sasa- 90 Si3N4, etc. These films may be prepared by kura; Appl. Phys. Lett., 45, 980(1984)). The the process such as sputtering, ion plating, other is an alkaline earth metal selenide such vapour deposition and CVD.

as SrSe and SaSe, and a thin film electrolumi- The reasons why the brightness is reduced nescent device provided with the luminescent in the case of using the oxide for the insulat layer of the selenide incorporated with the lu- 95 ing layer are not definitely elucidated at the minescence center such as Se and Eu is present time. One of the possible reasons known. thereof is considered so that the oxygen in In these thin film devices, oxides such as the insulating layer is diffused into the lumiY103, Si02, A1103 and Ta20, have been con- nescent layer under the application of high po ventionally used as an insulating layer. Howtential thereto to react the basic material, par ever, those thin film devices have a drawback ticularly, alkaline earth metal,in the luminescent that the luminescent layer interacts with the layer. Accordingly, it is desirable to dispose insulating layer upon applying a high voltage the insulating layer of nitride in contact with thereto, and eventually the brightness is re- the luminescent layer.

duced. 105 In the thin film electroluminescent device, the insulating layer is in general disposed on Object and Summary of the Invention: either the light emitting side or the opposite

The object of the present invention is to side of the luminescent layer, or it may be overcome the foregoing drawback in the prior disposed on both sides of the luminescent art and to provide a multi-color electrolumines110 layer to make a dual insulating structure. In cent (EL) device of high brightness, low vol- the present inveniton, both structures of the tage driving, low cost and high reliability. single side insulation and dual sides insulation The thin film electroluminescent device ac- may be adopted as well. In any insulation cording to the present invention comprises a structure, it is preferable to dispose the insu luminescent layer of alkaline earth metal chal- 115 lating layer of nitride on the side adjacent to cogen, at least one dielectric layer which is the luminescent layer. This can suppress the disposed on the surface of the luminescent reaction of the luminescent layer, particularly, layer and electrode, which is characterised by the alkaline earth metal with the insulating the fact that at least one of the dielectric layer to produce the electroluminescent device layers is made of nitride. 120 of extremely high reliability.

In accordance with the invention, the insulat- Brief Explanations of the Drawings: ing layer is not limitted to a single layer. As Fig. 1 is a schernatical drawing of the thin described in the foregoing, since the reaction film electroluminescent device according to the may be suppressed when disposing the nitride invention. 125 layer in contact with the luminescent layer, it Fig. 2 is a diagrammatic drawing of an is possible to provide the insulating layer con- apparatus for producing the thin film eleGtrolu- sisting of laminated layers, for instance, the minescent device according to the present in- lamination of oxide layer and nitride layer. As vention. the material of the oxide layer, preferred are Fig. 3 is a graph of luminescent spectrum of 130 the dielectric material such as Y203, Si021 2 GB2195823A 2 A1203 and Ta,O, or the ferroelectric material structural unit can be conducted by inserting such as SrTi03, PbTi03 and PbNb20,. an oxide insulating layer, nitride insulating The thin film electroluminescent device of layer, etc.

the present invention may be produced by According to the present invention, the EL successively laminating each layer on a glass 70 device of high reliablity and high performance substrate. A transparent electrode is firstly de- is provided by making the insulating layer with posited on the glass substrate. As the ma- the nitride to suppress the reaction of the al terial of the electrode, preferred is ITO or kaline earth metal compounds in the lumines SnO, added with Sb or F, or alternatively ZnO cent layer.

added with Al. The transparent electrode may 75 Referring to the drawings, Fig. 1 schemati- be formed on the subsrate by sputtering, va- cally illustrates the EL device of the present pour deposition, CVD, etc. and the film thick- invention, in which the glass substrate 1 is ness is suitabley from several hundreds A to superimposed with the transparent electrode several thousands A. 2, the nitride insulating layer 3, the lumines- Then, the transparent electrode thus formed 80 cent layer 4, the nitride insulating layer 3 and -is laminated with the insulating layer or the the back electrode 5 in order.

luminescent layer, as described above. It is Fig. 2 diagrammatically illustrates an appara- preferable to dispose the insultaing layer on tus for producing the EL device of the present the transparent electrode in order to suppress invention. The materials of the electrode, insu the reaction between the luminescent layer 85 lating layer and luminescent layer are charged and the transparent electrode layer. The pre- into the evaporation - sources 6, 7 and 8, re ferred insulating layer is the single nitride layer spectively. A filament 12 and an ionizing elec as described above or the lamination of ni- trode 13 are disposed on the opening of the tride, oxide and carbide layers. In the latter evaporation source.

case, the side adjacent to the luminescent 90 The substrate 9 is disposed above the eva- layer is provided with the nitride layer. The poration sources 6, 7 and 8, the temperature formation of the insulating layer may be con- of which may be regulated by means of a ducted by sputtering, vapour deposition, CVD, heater 10. A shutter 11 is disposed between ion plating or the like. the substrate 9 and each evaporation source.

The EL luminescent layer is superimposed 95 The potential V2 is applied to the ionizing on the insulating layer. As the material of the electrode 13 from an ionizing power source luminescent layer, the alkaline earth metal 14, while the bias potential V, is applied to chalcogens incorporated with the rare earth the substrate 9 from a bias power source 16.

element are exemplified. The suitable film- The molecules evaporated from each evapo- forming process of the luminescent layer in- 100 ration source are ion- plated on the surface of cludes EB vapour deposition, sputtering, reac- the substrate 9. The reference number 15 is a tive vapour deposition, ion plating, etc. Fur- ionizing current meter and the reference num thermore, CVD, MOCVD and the like are also ber 17 is a bias current meter.

applicable to the thin film formation. The film The present invention is further explained by thickness of the luminescent layer preferably 105 reference to Examples hereinafter described, amounts to from several thousands A to sev- which stand only for demonstrating the pre eral microm meters. ferred embodiments of the invention and The luminescent layer thus formed is lami- should not be construed to limit the scope of nated with the insulating layer or a back elec- the invention.

trode. The lamination of the insulating layer 110 may be completed by repeating the foregoing. Example 1 The film thickness of the insulating lEtyer is A thin film EL device provided with an insu- preferably of from several hundreds A to sev- lating layer of silicon nitride (Si,N4) was pro eral micron meters. duced by the following steps.

Then, the back electrode layer is deposited. 115 At first, a film of ITO was formed on a The preferred material of the back electrode is glass substrate by sputtering and the film was Al, Ti, ITO, etc. which are transparent. These grown upto 500 A in thickness. Then, a Si3N4 films may be formed by vapour deposition, insulating layer of 2000 A in thickness was sputtering, CVD and the like and the film formed thereover by way of ion plating EB thickness thereof preferably -amounts to sev- 120 vapour deposition. 4N Si3N4 powder pressed eral hundreds A to several thousands A. and hardened into a tablet-like contour was By the lamination process described in the used as the material of the insulating layer.

foregoings, a monochromic thin film electrolu- The substrate temperature upon film-formation minescent device may be produced. A multi- of the insulating layer was 350 C. Ionizing color electroluminescent device may be pro- 125 process of Si,N, vapours by accelerated ther duced by superimposing a structural unit moelectrons was used in the ion plating. A wherein the luminescent layer of desirable co- bias voltage of -200 V was applied to the lor is sandwiched between the insulating substrate upon film formation. The reaction at layers and in addition between the electrodes. mosphere was nitrogen gas and the pressure In this case, the insulation between each 130 was set to 4 x 10 -2 Pa. The RF power 3 GB2195823A 3 supplied was 100 W. A. The substrate temperature upon film forma- A luminescent layer of SrSe/Ce was vapour tion of the AIN layer was 300 'C. The bias deposited onto the insulating layer thus voltage was -500 V, the reaction atmos formed. The luminescent layer was formed by phere was nitrogen gas, the reaction pressure an ionization vapour depositing process with 7G was 4 X 10-2 Pa and the FIF power supplied accelerated thermoelectrons. The film thick- was 100 W.

ness of the luminescent layer was 1 um. The A luminescent layer of SrSe/Ce was vapour substrate temperature upon film formation was deposited on the thus formed insulating layer set to 400 OC, while the bias voltage (VJ in the same manner as in Example 1. The AIN shown in Fig. 2 was set to -100 V. The 75 insulating layer of 2000 A in thickness was ionization power supplied, which is a product again formed on the luminescent layer. Fur of ionization voltage M) and ionization current ther, an aluminum electrode was vapour de (A2), was set to 200 W. posited on the insulating layer to complete the Another insulating layer of Si,N, was formed EL device.

on the luminescent layer, the film thickness 80 An alternating pulse voltage of 300 V and 5 thereof being 2000 A. The conditions for the KHz was applied to the EL device and a com film formation were the same as those for the pulsory degradation test was conducted at an first insulating layer. Further, an aluminum atmosphere temperature of 80 C. As a result, electrode was vapour deposited on the insu- it was found that the life time was increased lating layer to complete an EL device. 85 by about 3 times as compared with a device The EL device thus produced was subjected in which the insulating layer was made of to a compulsory degradation test at an atmos- Y203. Futher, it was found that the AIN film phere temperature of 80 'C by applying was excellent in the adhesion strength and thereto an alternating pulse voltage of 300 V less peeled off as compared with a Si,N, film.

and 5 KHz. As a result, it was found that the 90 life time estimated from a half-decay value of Example 4 the brightness was doubled as compared with A thin film EL device in which the insulating a device in which the insulating layer was layer was made of a larnianted structure of made Of Y203- silicon oxide POO and titanium nitride (TiN) 95 was produced by the following steps.

Example 2 At first, a film of ITO was formed on a A thin film EL device in which the insulating glass substrate upto 500 A in thickness by layer is made of boron nitride (BN) was pro- way of sputtering. Then, a Si02 insulating duced in the same manner as in Example 1. layer of 1000 A in thickness was formed The thickness of the BN layer was 2000 A. 100 thereover by way f sputtering. A TiN insulat The substrate temperature upon film formation ing layer of 1000 A in thickness was further of the BN layer was 150 'C. The bias voltage formed thereover by way of ion plating EB was -500 V, the reaction atmosphere was vapour deposition. The substrate temperatures nitrogen gas, the reaction pressure was 4 X upon the film formation of each insulating 10-2 Pa and the RF power supplied was 100 105 layer were 150 OC (Si02) and 200 "C (TiN), W. respectively. In the case of SiOz, the atmos- A luminescent layer of SrSe/Ce was vopour phere was a gas mixture of argon and oxy- deposited on the insulating layer in the same gen, the pressure was 1 x 10-1 Pa and the T manner as in Example 1. The BN insulating RF power was 4KW. In the case of TiN, the layer of 2000 A in thickness was again 110 bias voltage was - 500 V, the atmosphere formed on the luminescent layer. Further, an was nitrogen gas, the reaction pressure was 3 aluminum electrode was vapour deposited on X 10-2 Pa and the RIF power supplied was the insulating layer to complete the EL device. 150 W.

An alternating pulse voltage of 300 V and 5 A luminescent layer of SrSe/Ce was vapour KHz was applied to the EL device and a com- 115 deposited on the thus laminated insulating pulsory degradation test was conducted at an layers in the same manner as in Example 1.

atmosphere temperature of 80 'C. As a result, The TiN insulating layer of 1000 A in thick it was found that the life time was increased ness was again laminated on the luminescent by about 3 times as compared with a device layer and also the SiO2 insulating layer of in which the insulating layer was made of 120 1000 A in thickness was formed thereafter.

Y203. The brightness of the device was about Further, an aluminum electrode was vapour 1000 cd/M2 at the maximum value. The lumi- deposited on the outermost indulating layer to nescent spectrum of the device is shown in complete the EL device.

Fig. 3. An alternating pulse voltage of 300 V and 5 125 KHz was applied to the EL device and a com Example 3 pulsory degradation test was conducted at an A thin film EL device in which the insulating atmosphere temperature of 80 'C. As a result, layer was made of aluminum nitride (AIN) was the life time was increased by about twice as produced in the same manner as in Example compared with a device in which the insulat 1. The thickness of the AIN layer was 2000 130 ing layer is made Of Y203. Further, the driving 4 GB2195823A 4 voltage could be reduced by about 30 % as W and the acceleration voltage was 300 V.

compared with the device in which the insulat- A CaS/Ce luminescent layer of 1.2 ym in ing layer was made of SiA. thickness was formed on the thus formed in sulating layer by way of ion plating EB vapour Example 5 70 deposition. The temperature of the substrate A thin film EL device in which silicon oxide was 400 'C.

ON, boron nitride (BN) and aluminum nitride The Si,N, insulating film of 2000 A in thick- (AIN) were used as the insulating, layer was ness was again formed on the luminescent produced by the following steps. layer by way of ion plating and an aluminum At first, an ITO film of 500 A in thickness 75 electrode was further deposited thereover to was formed on a glass substrate by way of complete the EL device.

sputtering. Then a 9102 insulating layer of It was found that the El device using the 1000 A in thickness'was formed thereover insulating layer formed as described above also by way of.sputtering. A BN insulating was excellent as in Example 1 in the life time layer of 1000 A in thickness was further 80 as compared to a conventional device using formed thereover by way of ion plating EB Y203, vapour depositing. In the case of forming the Si02 insulating layer, the substrate temperature Example 7 was 150 'C, the atmosphere was a gas mix- A thin film EL device in which the insulating ture of argon and oxygen, the pressure was 1 85 layer was made of aluminum nitride (AIN) was x 10-1 Pa and the RF power was 4 KW. In produced by the following stepp.

the case of forming the BN insulating layer, At first, an ITO film of 500 A in thickness the substrate temperature was 150 'C, the was formed on the glass substrate. Then, an atmosphere was nitrogen gas, the pressure AIN insulating layer of 2000 A in thickness was 4 x 10-2 Pa and the RF power was 100 90 was formed thereover by way of ion plating W. EB vapour deposition.

A luminescent layer of CaS/Eu was vapour As the material of the insulating layer, 4N deposited on the thus formed insulating layers AIN powder pressed and hardened into a tab in the same manner as in Example 1. The film let contour was used. The temperature of the thickness of the luminescent layer was 1 um. 95 glass substrate bearing the ITO film was main The substrate temperature upon the film for- tained at 350 C, while controlling the reaction mation was 400 C and the bias voltage was pressure upon ion plating to 5 x 10-4 Torr, - 150 V. The ionizing power supplied was the atmosphere to argon, the supplied RF W. power to 50 W and the acceleration voltage An AIN insulating layer of 2000 A in thickto the substrate to 200 V.

ness was formed on the luminescent layer. A CaS/Ce luminescent layer of 1. 2 um in The substrate temperature was 300 'C, the thickness was formed on the thus formed in bias voltage was -500 V, the atmosphere sulating layer by way of ion plating EB vapour was nitrogen gas, the-reaction pressure was 4 deposition. The substrate temperature was X 10-2 Pa and the RF power was 100 W. 105 400 C.

Further, an aluminum electrode was vapour The AIN insulating layer of 2000 A in film deposited on the thus formed insulating layer thickness was again formed on the lumines to complete the EL device. cent layer by way of ion plating. An aluminum An alternating pulse voltage of 300 V and 5 electrode was deposited thereover to com- KHz was applied to the EL device and a com- 110 plete the EL device.

pulsory degradation test was conducted at an An alternating pulse voltage of 300 V and 5 atmosphere temperature of 80 'C. As a result, KHz was applied to the thus produced EL de- le the life time was increased by about three vice and a compulsory degradation test was times as compared with the device in which conducted at an atmosphere temperature of the insulating layer was made of Y,O,. 115 80 'C. It was found that the life time esti mated from a half-decay value of the bright Example 6 ness was doubled as compared with that of A thin film EL device was produced with the Fig. 1 in which the insulating layer was made layer lamination like that in Example 1, in Of Y203- which the insulating layer was made Of SiA, 120 Further, it was found that the life time was and the luminescent layer was made of also excellent when compared with the device CaS/Ce. of using the Si,N, insulating layer (Example 6).

The conditions for forming the insulating layer were as below. As the starting material, Example 8 4N silicon nitride (Si3N4) powder pressed and 125 A thin film EL device with the same layer hardened into a -tablet-like contour was used. lamination as in Example 6, in which the insu The temperature of the glass substrate was lating layer was made of boron nitride (13N), 350 'C. The reaction pressure upon ion plat- was produced.

ing was 3 x 10-4 Torr and the atmosphere The conditions for preparing the insulating was argon. The RF power supplied was 100 130 layer were as below. 4N BN powder pressed GB 2 195 823A 5 and hardened into a tablet-like contour was least one of SrS and SrSe and is incorporated used as the starting material. The temperature with a luminescence center of rare earth ele of the glass substrate was 400 'C. The reac- ment and the dielectric layer is made of at tion pressure upon ion plating was 3 x 10-4 least one of 13N, AIN, TiN, TaN and Si3N41 Torr and the atmosphere was argon. The RF 70 3d A thin film electroluminescent device as power supplied was 150 W and the accelera- defined in any one of the preceding claims 1 tion voltage was 400 V. or 2, wherein the dielectric layer is a lamina- It was found that the EL device using the tion, the side thereof adjacent to the lumines- insulating layer thus formed was excellent as cent layer being formed of the nitride layer as in Example 1 in the life time as compared 75 defined in claim 1.

with a conventional device of using Y203. 7. A thin film electroluminescent device sub- Further, it was found that the life time was stantially as herein before described with ref- also excellent when compared with the device erence to Figure 1 of the accompanying draw using the Si3N4 insulating layer (Example 6). ing.

Claims (1)

1. CLAIMS Published 1988 at The Patent Office, State House, 66/71 High

   Holborn,

   1. A thin film electroluminescent device London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD.

   comprising a luminescent layer of alkaline Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.

   earth metal chalcogen, at least one dielectric layer disposed on the surface of the lumines cent layer and electrode, characterized in that at least one of said dielectric layers is made of nitride.

   2. A thin film electroluminescent device as defined in claim 1, wherein the nitride forming the dielectric layer is selected form the group of 13N, AIN, TiN, TaN and Si,N,.

   3. A thin film electroluminescent device as defined in claim 1 or 2, wherein the lumines cent layer is formed of alkaline earth metal chalcogen sulfide or selenide, incorporated with a luminescence center.

   4. A thin film electroluminescent device as defined in claim 3, wherein the sulfide is se lected form the group of SrS, CaS and BaS, the selenide is selected from the group of SrSe and CaSe, and the luminescence center is selected from lanthanoid series rare earth elements.

   5. A thin film electroluminescent device as defined in any one of the preceding claims 1 to 4, wherein the dielectric layers are dis posed on both surfaces of the luminescent layer.

   6. A thin film electroluminescent device as defined in any one of the preceding claimes 1 to 5, wherein the dielectric layer is a lamina tion, the side thereof adjacent to the lumines cent layer being formed of the nitride layer.

   CLAIMS Amendments to the claims have been filed, and have the following effect:

   Claims 1 to 4 and 6 above have been de- leted or textually amended.

   New or textually amended claims have been filed as follows:

   Claims 5 and 7 above have been re-num- bered as 2 and 4 and their appendancies cor rected.

   1. A thin film electroluminescent device comprises a luminescent layer, at least one dielectric layer disposed on the surface of the luminescent layer and electrode, characterized in that the-luminescent layer is made of at