THIN FILM ELECTROLUMINESCENT DEVICE AND METHOD OF MANUFACTURING THE SAME

Abstract: The thin film EL device includes a luminescent layer (21) made of column polycrystals formed by independently evap­ orating luminescent host material and an activator (Im) and then combining evaporated substances on a substrate. Electrons (E) in the luminescent layer (21) are accelerated by electric field applied from outside efficiently collide against the activator (Im) without being intercepted by inter­ faces between crystalline particles. The thin film EL device can be used for display, illumination, writing, reading out and erasure of signals of photo-recording medium.

Description: This invention relates to a thin film electroluminescent (EL) element and more particularly an improved high brightness low voltage drive thin film EL device, and a method of manufacturing the same. A prior art dispersion type EL device using a powder of zinc sulfide (Zn S) type fluorescent substance could not be used as a light source of illumination because of its low brightness. In recent years, a thin film type EL device utilizing a thin film of fluorescent substance has become noted because of the high brightness. The thin film EL device is now widely used for vehicles, display apparatus of a computer terminal element or the like and a source of illumination because the thin film EL device comprises a light emitting layer made of a transparent film which prevents dispersion of light incident from outside or emitted by an internal luminescent layer thereby decreasing halation and blur and providing clear display of high contrast. For example, a thin film EL device utilizing Mn as an activator in ZnS has a dual dielectric structure wherein a transparent electrode made of a tin oxide layer (Sn O₂), a first insulating layer, a crystalline film utilizing Zns as a host material and Mn acting as an activator, that is a ZnS: Mn luminescent layer, a second insulating layer and a back electrode made of aluminum or the like are sequentially laminated on a transparent substrate. Light is emitted as follows. When voltage is impressed across the transparent electrode and the back electrode, electrons that have been trapped at an interface level by electric field induced in the luminescent layer are released and imparted with energy sufficient to accelerate the electrons so that these electrons collide against the orbit electrons of Mn (luminescent center) so as to excite the orbit electrons. Light is emitted when the excited activator returns to the base state. As disclosed in Japanese Published Patent Specification Nos. 10358/1978 and 8080/1979, the electron beam vapor deposition method has been used for forming such luminescent layer at ZnS: Mn of the thin film EL device. Such thin film EL device has been formed in a vacuum chamber 1 by irradiating a pellet 2 formed by sintering a mixture of ZnS and 0.1-1 at.% of Mn with an electron beam 4 emitted by an electron gun 3 as shown in Fig. 10, so as to heat and vaporize the pellet and cause the vaporized pellet to deposit on a substrate 5. With this method, however, since the vapor pressure of the host material comprising the luminescent layer, the vapor pressure of an element comprising the host material, and the vapor pressure of the activator (for example PZnS, PZn, PS, PMn) differ greatly (PZnS < PMn < PZn < PS), there are such problems that the host material of the laminated luminescent layer deviates from stoichiometric composition, thus degrading crystalline structure and making nonuniform the distribution of the activator due to nonuniform evaporation and reevaporation of elements once deposited on the substrate. In the foregoing description PZnS, PMn, PZn and PS respectively represent vapor pressures of ZnS, Mn, Zn and S. Consequently, as shown in Fig. 9, the luminescent layer formed by the electron beam vapor deposition method has a particulate polycrystalline structure wherein at the initial stage of growth many small crystal particles are formed, that is a structure in which so called dead layers are present. In a thin film EL device utilizing such luminescent layer, electrons E in the luminescent layer accelerated by electric field applied from outside collide against the activator Im with the result that the electrons will be dispersed at the interfaces B of the crystal particles before the electrons contribute to luminescence. Thus, the electric field applied from outside does not efficiently contribute to luminescence. From EP-A-0 141 116 it is known that a thin film EL device should show a substantially stoichiometric composition of the light emitting layer in order to avoid the requirement of a stabilizing treatment after manufacture of the thin film EL device and to achieve high reliability thereof in use. The light emitting layer 14 is prepared by electron beam evaporation of a sintered pellet which is made of a compound semi-conductor material (ZnS) as its base material and an activator (Mn) added thereto. From DE-A-1 939 994 it is known that in the preparation of light emitting layers activator/host material incompatibilities can be reduced so that higher activator concentrations can be achieved if activator and host material are deposited from separate evaporation sources so that the activator remains unassociated and is uniformly dispersed in the host material. However, this known method also provides for evaporation of the compound host material to produce a thin layer thereof, and such method does not always produce the desired substantially stoichiometric composition of the light emitting layer. It is therefore an object of this invention to provide a novel thin film EL device with improved properties comprising a luminescent layer having a well defined and stable composition, and a method of manufacturing such thin film EL device. According to one aspect of this invention, a thin film EL device comprising a transparent electrode, an insulating layer, a luminescent layer consisting of a host material and an activator added thereto in even distribution, wherein said luminescent layer as a whole is present in substantially stoichiometric composition, and a back electrode is provided, such EL device being characterized in that said luminescent layer is constituted by column polycrystals. According to another aspect of this invention, there is provided a method of manufacturing a thin film EL device having the stated properties, such method comprising the steps of forming a transparent electrode on a substrate, forming an insulating layer on said transparent electrode, forming a luminescent layer on said insulating layer by means of a multi-source evaporation method whereby the host material and the activator are evaporated from respectively separated evaporating sources in such manner that the activator is doped into the host material and uniformly distributed therein, and forming a back electrode on said luminescent layer, such method being characterized in that the luminescent layer (21) is formed by evaporating the elements constituting the host material from respectively separated evaporating sources (13, 14) and by evaporating the activator from an evaporating source (12) in such a manner that they are combined in the luminescent layer (21) as a whole in a stoichiometric composition and form column polycrystals by controlling the temperatures of the evaporating sources. Further embodiments of the invention will be apparent from the following description and claims. In the accompanying drawings: Fig. 1 shows the crystalline structure of the luminescent layer of a thin film EL device embodying the invention; Fig. 2 is a diagram showing the principle of the method of forming the luminescent layer according to this invention; Fig. 3 is a side view showing one embodiment of the thin film EL device according to this invention; Fig. 4 shows an equivalent circuit of the element shown in Fig. 3; Figs. 5a through 5e are side views showing successive steps of manufacturing the element shown in Fig. 3; Figs. 6a and 6b are graphs showing the result of X-ray diffraction of the luminescent layer of the film EL element of this invention and of the prior art element; Fig. 7 is a graph for comparing the voltage - brightness characteristics of the thin film EL device of this invention and of the prior art; Fig. 8 shows the principle of the method of manufacturing the luminescent layer of the prior art thin film EL device; and Fig. 9 shows the crystalline structure of the prior art luminescent layer. Briefly stated, according to this invention a column polycrystalline film is used for luminescent layer of a thin film EL device. Further, the host material of the luminescent layer or the elements constituting the base material and an activator are evaporated from independent sources and the evaporated substances are then combined on the substrate. Thus as shown in Fig. 1, where column polycrystals are used as the luminescent layer, the electrons E in the luminescent layer are accelerated by the electric field applied from outside and collide against the actuator Im, thereby enabling efficient luminescence. Referring to Fig. 2, to form the luminescent layer, constituting elements of host material and activator, for example, Mn, S and Zn are put in separate crucibles 12, 13 and 14 in a vacuum chamber 11 which is maintained at a vacuum of 10⁻³ - 10⁻⁷ Torr, the temperatures of the crucibles are independently controlled so that the luminescent layer will have a stoichiometric composition. Thus, column crystals having a uniform distribution of the activator can be deposited on a substrate 15 according to the following steps. Suppose now that substances A and B are formed on a substrate by multisource vapor deposition process in which independent evaporation sources comprising independent crucibles respectively containing substances A and B are used. Let us denote the vapor pressure of the substance A at a given temperature TA by PA. Then, when the degree of vacuum PO (pressure) in the vacuum chamber is PO < PA, by selecting TS in a relation TS > TA, the substrate temperature TS will satisfy a relation PAS > PA where PAS represents the vapor pressure of substance A at a substrate temperature TS so that even if the substance A is evaporated, it is not deposited on the substrate. On the other hand, when the vapor pressure PABS of a compound of AB at a temperature TS of substances A and B is selected such that PABS < PO, when substance B is present on the substrate (PBS < PO), the substances A and B react with each other on the substrate whereby crystals of a compound AB grows. At this time, when element B (or A) present on the substrate combines with arriving element A (or B), the elements are deposited at a position having lowest potential with the result that only the crystalline surfaces grow thereby forming column crystals. Where the activator is constituted by a compound, a more efficient luminescent layer can be formed by evaporating respective elements of the compound in independent crucibles and then independently controlling the quantity of evaporation of respective elements. For example, respective constituting elements Zn and S of ZnS utilized as the host material of the luminescent layer, and constituting elements Tb and F of the activator TbF₃ are put in independent crucibles (Zn, S, Tb, TbF₃). When the temperature of the crucibles are independently controlled so as to control evaporation quantities such that the resultant luminescent layer will have a stoichiometric structure, it is possible to deposit on the substrate column crystals having a uniform distribution of the activator. The embodiment shown in Fig. 3 has a laminated construction wherein a luminescent layer 21 is made of a thin layer of a thickness of 5000Å of column polycrystalline structure made of host material ZnS containing 0.4 at.% Mn acting as the activator (hereinafter merely termed ZnS: 0.4 at.% Mn). More particularly, on a transparent glass substrate 22 having a thickness of 1 micron are sequentially laminated a transparent electrode 23 having a thickness of 0.3 micron and made of tin oxide (SnO₂) on the like, a first insulating layer 24 having a thickness of 0.5 micron and made of tantalum oxide (Ta₂O₅), the luminescent layer 21, a second insulating layer 25 having a thickness of 0.5 micron and made of Ta₂O₅ and a back electrode 26 made of an aluminum film having a thickness of 0.5 micron. As shown in Fig. 4, the equivalent circuit of this thin film EL device can be shown by three serially connected capacitors respectively constituted by the first insulating layer 24, the luminescent layer 21 and the second insulating layer 25. A method of manufacturing the thin film EL device will now be described as follows. As shown in Fig. 5A, a transparent electrode 23 consisting of Sn O₂ is formed on a transparent glass substrate by sputtering. Then as shown in Fig. 5B, a first insulating layer 24 consisting of a tantalum oxide layer is formed on the electrode layer 23 by sputtering. Then a vapor deposition apparatus as shown in Fig. 2 is used and Zn, S and Mn are put in discrete crucibles and the vacuum in the vacuum chamber is selected to be 10⁻⁵ Torr. Then the temperatures of the three crucibles are independently controlled to evaporate Zn, S and Mn such that the luminescent layer would have a stoichiometric structure while at the same time the temperature TS of the glass substrate is selected to a suitable value in a range of 100 - 1000°C so as to form a luminescent layer 21 (see Fig. 5c) consisting of ZnS column polycrystals having a uniform distribution of the activator Mn. After that, as shown in Fig. 5d, a second insulating layer 25 consisting of tantalum oxide is formed on the luminescent layer 21 by sputtering. Finally as shown in Fig 5e, back electrodes 26 are formed by forming an aluminum film by vacuum vapor deposition technique followed by patterning with lithophoto-etching process. The luminescent layer of the thin film EL device has an excellent crystalline nature as evidenced by the result of X-ray diffraction shown in Fig. 6a. For comparison, the result of X-ray difraction of a ZnS: Mn layer formed by prior art electron beam vapor deposition method is shown in Fig. 6b. Comparison of the characteristics shown in Figs. 6a and 6b shows that the luminescent layer formed by the method of this invention has better crystalline property. The thin film EL device of this invention is driven by applying an AC electric field across the transparent electrode and the back electrode, and its voltage brightness characteristic a is shown in Fig. 7 together with that b of a prior art thin film EL device. Comparison of these characteristics shows that the thin film EL device of this invention can produce the same brightness with a voltage of about 1/2 of that required for the prior art thin film EL device. In other words, the element of this invention can produce a high brightness with low voltage. Where an insulating film whose dielectric constant is much greater than that of the luminescent layer 21, for example, barium titanate BaTiO₃, is used for the first insulating layer 24 and the second insulating layer 25 the voltage = brightness characteristic is shown by a curve c in Fig. 7 showing that the element can be driven with a voltage of 1/3 or 1/4 of the voltage necessary for the prior art thin film EL device, as shown by curve b in Fig. 7. Denoting the dielectric constants of the first and second dielectric layers 24 and 25 and of the luminescent layer 21 by εr1, εr2 and εℓ respectively, the relation among these dielectric constants becomes εr1 εr2 >> εℓ (see Fig. 4). Consequently, the relation among their capacitances expressed by Cr1, Cr2 and Cℓ becomes Cr1, Cr2 >> Cℓ so that almost all voltage applied to the device from outside would be applied across the luminescent layer. As above described with the thin EL element of this invention a low voltage of less than about 100V is sufficient to obtain a brightness of about 20ft-L (foot Lambert) which is valuable for practical use. Although in the foregoing embodiment a ZnS: Mn film was used as the luminescent layer, it should be understood that the invention is not limited to such specific materials. For example, such other columnar polycrystals can be used as ZnS: 0.1-1 at.% TbF₃, ZnS: 0.1-1 at.% SmF₃ column polycrystals wherein ZnS is used as the host material and only the activator is substituted by terbium fluoride (TbF₃) or samarium fluoride (SmF₃) and such other column polycrystals as calcium sulfide (CaS): 0.1-1 at.% europium (Eu), strontium sulfide (SrS): 0.1-1 at.% cerium fluoride (CeF₃) can also be used. In general, compounds of appropriate elements of groups II and VI of the periodic system (II - VI compounds) are useful as host material. In the foregoing embodiment, for the purpose of forming a column polycrystalline film of ZnS: Mn three crucibles respectively containing Zn, S and Mn were used as the sources of vapor deposition, any combination of ZnS, S, Mn; Zn, S, MnS and Zn, S, ZnS, Mn can be used. Where such compound as TbF₃ is used as the activator, the constituting elements of the host material and the constituting elements of an activator and compound of the constituting elements Zn, S, Tb, TbF₃ can be put in independent crucibles so as to evaporate Zn, S, Tb and TbF₃ by independently controlling the temperatures of the crucibles such that host material and activator of the luminescent layer will have the stoichiometric composition and that the impurity concentration would have a predetermined value. At the same time, the substrate temperature is selected to a suitable value in a range of 100 - 1000°C (for the crucibles containing Tb and TbF₃, temperatures of 100° - 110°C are preferred). Then a luminescent layer consisting of ZnS column polycrystals having a uniform distribution of the activator TbF₃ can be formed. Preferably, the crucibles have a shutter, that can be opened and closed. With such method, it is possible to obtain a luminescent layer having constant and stable concentration distribution. Furthermore, the thin film, EL device can be driven by applying an alternating current electric field across the transparent electrode and the back electrode but it can produce the same brightness under a voltage of 1/2 of that of the prior art film EL. In other words, a higher brightness can be contained with lower voltage. Thus a green thin film EL device can for the first time be obtained according to this invention. The activator is not limited to TbF₃, and such other compounds as SmF₃, Sm₂ S₃ and Tb₂ S₃ can also be used. Further, other substances then ZnS can be used as the host material. Although in the foregoing embodiment, for the purpose of preparing column polycrystalline film of ZnS: TbF₃, four crucibles respectively containing Zn,S, Tb, and TbF₃ were used as the vapor deposition sources, four crucibles respectively containing Zn, TbS, TbF₃ and S can also be used. Furthermore the thin film EL device of this invention can be used as a light source for display, illumina- tion, writing, reading and erasing of signals into and out of light recording medium.

Claims: A thin film electroluminescent (EL) device comprising a transparent electrode (23), an insulating layer (24), a luminescent layer (21) consisting of a host material and an activator added thereto in even distribution, wherein said luminescent layer as a whole is present in substantially stoichiometric composition, and a back electrode (26), characterized in that said luminescent layer (21) is constituted by column polycrystals. A thin film electroluminescent (EL) device according to claim 1, further comprising a second insulating layer (25) between said luminescent layer (21), and said back electrode (26). A thin film EL device according to claim 1 or 2, wherein said luminescent layer (21) is made of material consisting of a II-VI compound acting as a host material and an activator (Im) added to said host material. A thin film EL device according to claim 3, wherein said activator is constituted of a single element. A thin film EL device according to claim 3, wherein said activator is constituted of a compound. A thin film EL device according to claim 3, wherein said luminescent host material is ZnS. A thin film EL device according to claim 4, wherein said activator is Mn. A thin film EL device according to claim 5, wherein said activator is selected from the group consisting of terbium fluoride (TbF₃), samarium fluoride (SmF₃) and cerium fluoride (CeF₃). A method of manufacturing a thin film EL device according to claim 1 on a substrate (22) comprising the steps of forming a transparent electrode (23) on said substrate; forming an insulating layer (24) on said transparent electrode; forming a luminescent layer (21) on said insulating layer (24) by means of a multi-source evaporation method whereby the host material and the activator are evaporated from respectively separated evaporating sources in such manner that the activator is doped into the host material and uniformly distributed therein, and forming a back electrode (26) on said luminescent layer (21), characterized in that the luminescent layer (21) is formed by evaporating the elements constituting the host material from respectively separated evaporating sources (13, 14) and by evaporating the activator from an evaporating source (12) in such a manner that they are combined in the luminescent layer (21) as a whole in a stoichiometric composition and form column polycrystals by controlling the temperatures of the evaporating sources. A method of manufacturing a thin film EL device according to claim 1 on a substrate (22) comprising the steps of forming a back electrode (26) on said substrate; forming a luminescent layer (21) on said back electrode (26) by means of a multi-source evaporation method whereby the host material and the activator are evaporated from respectively separated evaporating sources in such manner that the activator is doped into the host material and uniformly distributed therein, forming an insulating layer (24) on said luminescent layer (21) and forming a transparent electrode (23) on said insulating layer (24), characterized in that the luminescent layer (21) is formed by evaporating the elements constituting the host material from respectively separated evaporating sources (13, 14) and by evaporating the activator from an evaporating source (12) in such a manner that they are combined in the luminescent layer as a whole in a stoichiometric composition and form column polycrystals by controlling the temperatures of the evaporating sources. A method of manufacturing a thin film EL device according to claim 9, further comprising the step of forming a second insulating layer (25) on said luminescent layer (21) before said step of forming a back electrode (26). A method of manufacturing a thin film EL device according to claim 10, further comprising the step of forming a second insulating layer (25) on said luminescent layer (21) before said step of forming a transparent electrode (23). A method according to any of claims 9 to 12 wherein said multi-source deposition method is performed in a vacuum of 10⁻³ - 10⁻⁷ Torr. A method according to any of claims 9 to 13 wherein the stoichiometric composition of said host material and a concentration of said activator in said host material are determined by independently controlling quantities of said elements constituting the host material, and the activator which are evaporated from said evaporating sources by controlling the temperatures of the evaporating sources.

Inventors: NIRE, TAKASHI, WATANABE, TAKEHITO
Applicants: KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1987
Main classification: H05B33
IPC codes: C09K11, H05B33