THIN FILM ELECTROLUMINESCENCE DISPLAY DEVICE

Abstract: In a thin film EL display device wherein a transparent electrode (2), a first dielectric layer (3), an EL emission layer (5), a second dielectric layer (7) and a back electrode (8) are laminated in order on a transluscent substrate, 10nm--200nm thickness thin films (4,6) each being made of calcium sulfide or of a mixture containing calcium sulfide which is formed by an electron beam vapor deposition method are provided between the first dielectric layer (3) and the EL emission layer (5) and between the EL emission layer (5) and the second dielectric layer (7), thereby obtaining a thin film EL display device which maintains a stable operation for a long period even when it is driven by A.C. pulses being asymmetry in the time relationship of the driving pulses or being different in amplitude in a positive side and a negative side.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT 1. FIELD OF THE INVENTION The present invention relates to an electroluminescence cell (hereinafter referred to as EL display device), and more particularly to a thin film EL display device to be driven by an alternating current. This light emitting device has a feature of realizing a flat panel display and is suitable for terminal display for displaying characters and graphics of a personal computer or the like, and is widely applied to a field of office automation apparatus. 2. DESCRIPTION OF THE RELATED ART Heretofore, an X-Y matrix display has been known as a flat panel display using an electroluminescence phosphor. In the X-Y matrix display, horizontal parallel electrodes group and vertical parallel electrodes group are arranged on both sides of an electroluminescence light emission layer (hereinafter referred to as EL emission layer), in a manner to intersect to each other with right angle in plan view. Electric signal is applied across these electrodes groups from feeder through switches, thereby making light emission at the parts where the horizontal electrodes group and vertical electrodes group intersect to each other (hereinafter the each small element part of the EL emission layer at the electrode intersection part to be driven to emit light is referred to as pixel.), and then by combining the light-emitting pixel, letters, symbols, figures or the like are indicated. A display panel of this display is generally made as follows: First, transparent front side parallel electrodes group are provided on a transluscent substrate such as a glass plate, and then a first dielectric layer, the EL emission layer and a second dielectric layer are laminated thereon one after another, and further, back side parallel electrodes group are provided thereon in a manner to intersect the underlying transparent parallel electrodes group with right angle. The transparent parallel electrode is generally formed by applying tin oxide on a smooth glass substrate. The back electrode is generally formed by vacuum deposition of aluminum or the like. Materials having large dielectric constant and large dielectric breakdown electric field are suitable for the first and the second dielectric layers to be driven by low voltage. Having large dielectric constant is necessary for efficiently applying large portion of voltage, which is applied from the transparent electrode and the back electrode, to the EL emission layer, thereby to lower necessary driving voltage. Large dielectric breakdown electric field is required for safe operation without causing dielectric breakdown. As such a dielectric layer for constituting a thin film electroluminescence cell and (hereinafter referred to as thin film EL display device) splendid in stability, oxide dielectric films of large dielectric constant is more suitable than silicon oxide or silicon nitride, which has small dielectric constant. Therefore, wide researches are being made on the thin film EL display device using the oxide dielectric film. When the thin film EL display device having matrix electrode is driven with an addressing method that sequentially scans the rows from the top to the bottom of the device and after each row of the device has been scanned, a refresh pulse is applied to the rows, thereby making twice light emissions in one scanning period, in each pixel between the transparent electrodes and the back electrodes, period from start of application of positive pulse to start of application of negative pulse is not equal to the period from start of application of negative pulse to start of application of positive pulse. That is, driving pulses are asymmetry in the time relationship. When the conventional thin film EL display device is driven for a long time under such condition, there is a problem that in the pixels driven to emit light, light emission threshold voltage changes by several volts in comparison with the picture elements which has not been lit. OBJECT AND SUMMARY OF THE INVENTION The present invention aims to the obtain a thin film EL display device capable of stable operation for long time even when it is driven by A.C. pulses of asymmetric with respect to time relationship of positive and negative pulses and/or having different amplitudes in positive side and negative side. The said aim is obtained according to the invention by a Thin film EL (Electro-Luminescence) display device comprising: an EL emission layer, first and second thin films of one member selected from the group consisting of calcium sulfide and a mixture containing calcium sulfide sandwiching said EL emission layer therebetween, a transparent electrode and a back electrode for application of voltage thereacross, a first dielectric layer provided between said first thin film and said transparent electrode, and a second dielectric layer provided between said second thin film and said back electrode. Researches revealed that decrease in the threshold voltage comes from formation of various depth of trap level at interface between the EL emission layer and the dielectric layers and reaction between the EL emission layer and the dielectric layers. In the present invention, from intensive experimental researches, it is confirmed that the formation of the trap level and the reaction between the EL emission layer and the dielectric layers are suppressed by providing the calcium sulfide thin film or the mixture film containing calcium sulfide between the EL emission layer and the dielectric layers by an electron beam vapor deposition method. As a result, thin film EL display device capable of stable operation for long time becomes obtainable. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a cross sectional view showing a constitution of an thin film EL display device embodying the present invention. FIG.2 is a chart of a driving voltage waveform for driving the thin film EL display device. FIG.3 is a characteristics diagram showing a change of light emission threshold voltage with the passage of time. FIG.4 is a sectional view showing a constitution of the thin film EL display device of another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.1 shows a sectional construction of a thin film EL display device embodying the present invention. As a glass substrate 1, Corning #7059 glass is used. A 200 nm thick thin film of indium oxide containing tin is formed on the glass substrate 1 by a sputtering method, and is worked into a plurality of parallel strips by a photolithography, thereby forming a transparent electrode 2. Then strontium zirconium titanate [Sr(Ti x Zr 1-x )O₃] is sputtered on the transparent electrode 2 under the condition of 400°C of substrate temperature, thereby forming an oxide dielectric layer 3 having 600nm thickness as a first dielectric layer. Furthermore, a calcium sulfide thin layer 4 having 50nm thickness is formed on the first dielectric layer 3 by electron beam vapor deposition method, under the condition of 300°C of substrate temperature, using a calcium sulfide pellet as a vaporization source. Onto the calcium sulfide thin layer 4, a 400nm thick EL emission layer 5 made of zinc sulfide containing manganese is formed by electron beam deposition method using a zinc sulfide pellet and manganese flakes as a vaporization source at 200°C of the substrate temperature. On the EL emission layer 5, a calcium sulfide layer 6 having 50nm thickness is formed by performing the electron beam vapor deposition under a 300°C of substrate temperature with using the calcium sulfide pellet as vaporization source, and after one hour of heat treatment at 500°C in a vacuum, sintered barium tantalate [BaTa₂O₆] is sputtered on the calcium sulfide layer 6 under the condition of 100°C of substrate temperature, thereby forming a 200nm thick oxide dielectric thin film 7 as a second dielectric layer. Moreover, a 150nm thick aluminum layer is formed on the second dielectric layer by vacuum vapor deposition, and was worked into a plurality of parallel strips intersecting the transparent electrode 2 in right angle, thereby forming a back electrode 8. Thus, a thin film EL display device embodying the present invention is obtained. Then, A.C. pulse voltage which is asymmetric with respect to time relationship of positive and negative pulses as shown in FIG.2 was applied across the transparent electrode 2 and the back electrode 8 of the thin film EL display device, thereby making light emission, and change of a light emission threshold voltage (driving voltage producing a brightness of 1 cd/m²) is observed. For comparison, a similar test was made with respect to the conventional thin film EL display device which does not have the calcium sulfide thin films 4 and 6. Test Results are shown in FIG.3 with that of the comparison test. As shown by curve "a" for the conventional thin film EL display device, the light emission threshold voltage decreases about 6% after 100 hours of light emission. On the other hand, as shown by curve "b" the thin film EL display device of the present invention shows shift of the light emission threshold voltage under 1%. FIG.4 shows a closs section of another embodiment of the present invention. In FIG.4, a glass substrate 21 is made of Corning #7059 glass. A 300nm thick thin film of indium oxide containing tin is formed on the glass substrate 21 by sputtering method, and thereafter, it is worked into a plurality of parallel strips by photolithography, thereby forming a transparent electrode 22. Then sintered barium tantalate [BaTa₂O₆] is sputtered on the transparent electrode 22 under the condition of 200°C of substrate temperature, thereby forming a 300nm thick oxide dielectric layer 23 as the first dielectric layer. Next, a mixture thin film 24 containing calcium sulfide having 50nm thickness is formed on the first dielectric layer 23 by the electron beam vapor deposition, using a mixture pellet of calcium sulfide and zinc sulfide as a vaporization source, under a condition of 180°C of substrate temperature. This mixture thin film 24 contains about 10% of calcium sulfide. On the mixture thin film containing calcium sulfide 24, a 500nm thick thin film EL emission layer 25 made of zinc sulfide containing 1mol% of manganese is formed by electron beam deposition method using a zinc sulfide pellet and manganese flakes as a vaporization source under a condition of 180°C of the substrate temperature. After one hour of heat treatment at 570°C in a vacuum, a 60nm thick mixture thin film containing calcium sulfide 26 is formed on the EL emission layer 25 by electron beam vapor deposition, under a condition of 180°C of the substrate temperature, using the mixture pellet of calcium sulfide and zinc sulfide as the vaporization source. Then, sintered barium tantalate [BaTa₂O₆] is sputtered on the mixture thin film 26 under a condition of 100°C of the substrate temperature, thereby forming a 200nm thick oxide dielectric thin film 27 as a second dielectric layer. Further, 150nm thick aluminum layer was formed on the second dielectric layer 27 by vacuum vapor deposition, and is worked into a plurality of parallel strips intersecting the underlying transparent electrode 22 in right angle, thereby forming a back electrode 28. Thus, a thin film EL display device as another embodiment of the present invention is obtained. Next, characteristics of this thin film EL display device is observed. A.C. pulse voltage which is asymmetric with respect to time relationship of positive and negative pulse as shown in FIG.2 is applied across the transparent electrode 22 and the back side electrode 28 to make light emission. Then deterioration of the light emission threshold voltage is observed. For comparison, similar test was made with respect to the thin film EL display device which does not has the mixture thin films 24 and 26 which contain calcium sulfide. Test results are is shown by curves "c" and "d" in FIG.3. As shown by the curve "c" of FIG.3, for the comparison thin film EL display device, the light emission threshold voltage decreases about 6% after 100 hours of light emission, while, as shown by the curve "d" for the thin film EL display device of the present invention, decrease in the threshold voltage is only about 1 to 2%. When a thickness of calcium sulfide thin film or mixture thin film containing calcium sulfide is under 10nm, effect of suppressing undesirable lowering of the light emission threshold voltage becomes small; and when the thickness thereof is above 200nm, voltage for driving the thin film EL display device becomes too high since a dielectric constant of calcium sulfide is small. Therefore, 10--200nm thickness is preferable. Moreover it is preferable that the calcium sulfide thin film is formed by the electron beam vapor deposition method, because the experimental results showed that when other methods such as sputtering method are used, the effect of suppressing undesirable lowering of the light emission threshold voltage is substantially lost. Particularly, such tendency becomes remarkable as the heat treatment temperature of the EL emission layer is high. With respect to amount of calcium sulfide contained in the mixture thin film which contains calcium sulfide, the larger amount the better. When the thin layer consists of pure calcium sulfide, it is most effective for suppressing the lowering of the light emission threshold voltage with the passage of time. However, considering a adhesive force with other layers and a manufacturing process, the thin film may contain other substance. When amount of the other substance is more than about 5%, a practical effect can be obtained. There is no limitation with respect to the other substance to be mixed with calcium sulfide, so far as it does not ruin characteristics of the EL display device. Sulfides generally brings an excellent result and particularly, and zinc sulfide is most effective. Additionally, it was tried to use a nitride film such as silicon nitride film, a carbide film such as silicon carbide film and a fluoride film such as magnesium fluoride film substituting for the thin film of calcium sulfide or mixture containing the calcium sulfide. However, they were not effective for suppressing the drop of light emission threshold voltage. As a material for the EL emission layer, zinc sulfide (ZnS) containing activator is usable. Mn, Cu, Ag, Au, TbF₃, SmF₃, ErF₃, TmF₃, DyF₃, PrF₃, EuF₃ or the like are suitable for the activator. Moreover, other substances than zinc sulfide containing the activator are usable for the EL emission layer, and substances showing a electroluminescence, for example SrS and CaS containing the activator may be used. The heat treatment of the EL emission layer is carried out to improve a characteristic of light emission thereof. The temperature of the heat treatment is preferably above 500°C, since high brightness is obtainable. Temperature of above 650°C is not practical, since deformation of the glass substrate is induced. When a thickness of the oxide dielectric film used as the first dielectric layer is thicker than the second dielectric layer, stability against dielectric breakdown is high. The larger is the dielectric constant of the dielectric layer, the more preferable the using of thicker first dielectric layer. And as a result of the experiment, it is found that above 15 of the dielectric constant is preferable. When the dielectric constant is smaller than 15, it is difficult to form the thin film EL display device which can be driven stably under a voltage of 100--180V. As the oxide dielectric layer having above 15 of the dielectric constant, thin films having perovskite structure is preferable from the viewpoint of dielectric breakdown voltage. Among them, thin films made of strontium titanium binary oxide dielectrics such as SrTiO₃, Sr x Mg 1-x TiO₃, SrTixZr 1-x O₃, Sr x Mg 1-x Ti y Zr 1-y O₃ are preferable. And by using them as the first dielectric layer, thin film EL display device showing high stability is obtainable. Thin films made of barium tantalum binary oxide dielectrics such as BaTa₂O₆ are suitable for the second dielectric layer. By using them, it becomes possible to suppress a propagation dielectric breakdown, and as a result, the thin film EL display device having high reliability is obtained. The thin films made of barium tantalum binary oxide dielectrics also show excellent characteristics as the first dielectric layer, and therefore it is possible to form stable thin film EL display device showing high dielectric breakdown voltage by using it as the first dielectric layer.

Claims: Thin film EL (Electro-Luminescence) display device comprising: an EL emission layer (5), first (4) and second (6) thin films of one member selected from the group consisting of calcium sulfide and a mixture containing calcium sulfide sandwiching said EL emission layer (5) therebetween, a transparent electrode (2) and a back electrode (8) for application of voltage thereacross, a first dielectric layer (3) provided between said first thin film (4) and said transparent electrode (2), and a second dielectric layer (7) provided between said second thin film (6) and said back electrode (8). Thin film EL display device in accordance with claim 1 further comprising: a transparent substrate (1) provided on said transparent electrode (2). Thin film EL display device in accordance with claim 1 wherein: said first (4) and second (6) thin films have 10 nm -- 200 nm thickness, respectively, and said films are formed by an electron beam vapor deposition method. Thin film EL display device in accordance with claim 1 or 2 wherein: said first dielectric layer (3) is made of an oxide dielectric film having more than 15 of dielectric constant. Thin film EL display device in accordance with claim 1 or 2 wherein: said second dielectric layer (7) is made of an oxide dielectric film. Thin film EL display device in accordance with claim 1 wherein: said first (4) and second (6) thin films are made of a mixture of calcium sulfide and zinc sulfide. Thin film EL display device in accordance with claim 1 or 2 wherein: said first dielectric layer (3) is made of an oxide dielectric having perovskite structure. Thin film EL display device in accordance with claim 1 or 2 wherein: said first dielectric layer (3) is made of strontium titanium binary oxide dielectrics. Thin film EL display device in accordance with claim 1 or 2 wherein: said second dielectric layer (7) is made of barium tantalum binary oxide dielectrics. Thin film EL display device in accordance with claim 1 wherein: said EL emission layer (5) is made of zinc sulfide activated by manganese.

Inventors: ABE, ATSUSHI, FUJITA, YOSUKE, KUWATA, JUN, MATSUOKA, TOMIZO, NISHIKAWA, MASAHIRO, TOHDA, TAKAO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: H05B33
IPC codes: H05B33