Electroluminescent device and method of producing the same

[0002] This application is based upon and claims the benefit of priority of the prior Japanese Patent Applications No. 9-49178 filed on Mar. 4, 1997, and No. 9-341462 filed on Dec. 11, 1997, the contents of which are incorporated herein by reference.

[0003] 1. Field of the Invention

[0004] The present invention relates to electroluminescent (hereinbelow referred to as EL) devices that are used for various instruments of emissive-type segment displays and matrix displays, displays of various information terminals, and the like. The present invention also relates to methods for producing the same.

[0005] 2. Related Arts

[0006] An EL device is typically formed by laminating a first electrode, a first insulating layer, a luminescent layer, a second insulating layer, and a second electrode on an insulating glass substrate in this order. The first and second insulating layers are made of silicon dioxide (SiO₂), silicon nitride (SiN), silicon oxynitride (SiON), ditantalum pentaoxide (Ta₂O₅) or the like, and formed by a sputtering method, a vapor deposition method, or the like.

[0007] Further, JP-B2-64-5440 proposes an insulating layer having an aluminum oxide (Al₂O₃)/titanium oxide (TiO₂) laminated structure (herebelow referred to as Al₂O₃/TiO₂laminated layer) to improve a withstand voltage of an EL device. The Al₂O₃/TiO₂laminated layer is formed by alternately laminating Al₂O₃layers and TiO₂layers by an Atomic Layer Epitaxy (ALE) method.

[0008] Each of the Al₂O₃layers has insulating characteristics and each of the TiO₂layers has conductive characteristics. Accordingly, the Al₂O₃/TiO₂laminated layer withstands a high voltage. However, TiO₂forming the TiO₂layers is very sensitive to a temperature, and a resistivity of TiO₂decreases enormously as the temperature rises. When the resistivity of TiO₂decreases, each resistance of the TiO₂layers in the Al₂O₃/TiO₂laminated layer also decreases. This results in substantial deterioration of the voltage withstanding performance of the Al₂O₃/TiO₂laminated layer.

[0009] The present invention has been made in view of the above-mentioned problem and an object of the present invention is to provide an EL device having a high withstand voltage stable against a change in temperature and a method of producing the same. More specifically, the object of the present invention is to provide an insulating layer with a laminated structure for an EL device having a withstand voltage stable against a change in temperature and a method of producing the same.

[0010] To achieve the above-mentioned object, an EL device according to the present invention includes an insulating layer having a laminated structure of first and second layers. The first layer includes an insulating material, and the second layer includes titanium oxide added with an element for mitigating a change in resistivity of the insulating layer relative to a change in temperature. Accordingly, the EL device has a high withstand voltage stable against the change in the temperature. Preferably, an atomic ratio of the element relative to the titanium in the second layer is in a range of 0.01 to 1. The element can be selected from a group consisting of calcium, barium, strontium, aluminum, lead, and magnesium. More preferably, the insulating material of the first layer has a resistivity larger than 1×10¹⁰Ωcm.

[0011] The second layer may include a titanium oxide type material having a positive temperature coefficient of resistivity. In this case, the EL device has a high withstand voltage stable against the change in the temperature as well. The titanium oxide type material can be selected from calcium titanate, barium titanate, and strontium titanate.

[0012] The insulating layer of the present invention can be produced by alternatively laminating the first and second layers by the ALE method.

[0013] These and other objects and features of the present invention will become more readily apparent from a better understanding of preferred embodiments described below with reference to the following drawings, in which:

[0014] FIG. 1 is a cross-sectional view of an EL device according to a first embodiment of the present invention;

[0015] FIG. 2 is a graph illustrating resistivity versus temperature characteristics of a TiO₂layer added with barium (TiOx:Ba) and a TiO₂layer with a desired thickness;

[0016] FIG. 3 is a graph illustrating breakdown voltage versus temperature characteristics of the EL device including an Al₂O₃/TiOx:Ba laminated layer in the first embodiment and of an EL device including an Al₂O₃/TiO₂laminated layer as a comparative example;

[0017] FIG. 4 is a graph illustrating breakdown voltage versus temperature characteristics of an EL device including an Al₂O₃/strontium titanate (SrTiO₃) laminated layer in a second embodiment; and

[0018] FIG. 5 is a partially enlarged cross-sectional view of the EL device of FIG. 1, in which Al₂O₃/TiOx:Al laminated layers are employed as first and second insulating layers as a modified example of the first embodiment.

[0019] (First Embodiment)

[0020] Referring to FIG. 1, an EL device in a first preferred embodiment is composed of an insulating glass substrate 1, and a first electrode 2, a first insulating layer 3, a luminescent layer 4, a second insulating layer 5 and a second electrode 6, which are laminated on the insulating glass substrate 1 in this order. In the EL device, at least the first electrode 2 and the first insulation layer 3, or the second insulating layer 5 and the second electrode 6, which are disposed on a light emitting side of the luminescent layer 4, are made of transparent materials. All of the first and second electrodes 2 and 6 and the first and second insulating layers 3 and 5, which are provided on the both sides of the luminescent layer 4, may be made of transparent materials. In this case, light is taken out from the both sides of the luminescent layer 4.

[0021] The first and second insulating layers 3 and 5 are respectively formed by alternately laminating Al₂O₃layers (first layers) and TiO₂layers added with barium (Ba) (second layers). The TiO₂layers added with Ba are referred to as TiOx:Ba layers, and the laminated layer of the Al₂O₃layers and the TiOx:Ba layers are referred to as an Al₂O₃/TiOx:Ba laminated layer. In the TiOx:Ba layers, because barium titanate (BaTiO₃) is produced from added Ba and TiO₂, a stoichiometric ratio of titanium (Ti) to oxygen (O) decreases slightly from 2. Therefore, the subscript x of TiOx is slightly smaller than 2 and varies depending on formation conditions of the TiOx:Ba layers.

[0022] Next, resistivity versus temperature characteristics of one of the TiOx:Ba layers will be explained compared to a TiO₂layer without including Ba. The TiOx:Ba layer and the TiO₂layer used in this explanation have a specific thickness respectively. Referring to FIG. 2, the resistivity of the TiO₂layer decreases as the temperature rises. For example, when the temperature rises from a room temperature (25° C.) to 80° C., the resistivity of the TiO₂layer decreases to approximately one tenth of the original resistivity. As opposed to this, a change in resistivity of the TiOx:Ba layer is small compared to that of the TiO₂layer. Accordingly, it is confirmed that adding Ba to the TiO₂layer mitigates the change in the resistivity of the TiO₂layer with respect to the change in the temperature.

[0023] Next, characteristics of a breakdown voltage with respect to a temperature of the EL device in this embodiment were measured compared to an EL device including Al₂O₃/TiO₂laminated layers as first and second insulating layers 3 and 5. As mentioned above, the EL device in this embodiment includes the Al₂O₃/TiOx:Ba laminated layers as the first and second insulating layers 3 and 5. The breakdown voltage is a voltage applied to the EL device when breakdown occurs. Each of the Al₂O₃/TiOx:Ba laminated layer and the Al₂O₃/TiO₂laminated layer includes each 30 layers of Al₂O₃and TiOx:Ba (TiO₂) with a thickness of 5 nm.

[0024] As shown in FIG. 3, the change in the breakdown voltage to the change in the temperature of the EL device with the Al₂O₃/TiOx:Ba laminated layer is less than that of the EL device with the Al₂O₃/TiO₂laminated layer. In other words, a voltage withstanding performance of the EL device with the Al₂O₃/TiOx:Ba laminated layer is stable against the change in the temperature. Therefore, the EL device with the Al₂O₃/TiOx:Ba laminated layer can provide an excellent voltage withstanding performance even when the EL device generates heat due to light emission therefrom, resulting in high reliability.

[0025] Next, a method of producing the EL device will be described with reference to FIG. 1. First, the first electrode 2 made of transparent indium tin oxide (ITO) is deposited on the glass substrate 1 by a sputtering method. Next, the first insulating layer 3 composed of the Al₂O₃/TiOx:Ba laminated layer is formed on the first electrode 2 by alternately laminating Al₂O₃layers and the TiOx:Ba layers by the ALE method.

[0026] In detail, as a first step, an Al₂O₃layer is formed on the first electrode 2 by the ALE method using aluminum trichloride (AlCl₃) gas and water vapor (H₂O). The AlCl₃gas and the water vapor serve as source gases for aluminum (Al) and oxygen (O) respectively. The source gases are introduced into a forming chamber alternately so that only one atomic layer is formed at a time. That is, the AlCl₃gas is introduced into the forming chamber with argon (Ar) carrier gas for one second, and then the chamber is purged so that the AlCl₃gas in the chamber is sufficiently ventilated. Next, water vapor is introduced into the chamber with the Ar carrier gas for one second, and then the chamber is purged so that the water vapor in the chamber is sufficiently ventilated. The Al₂O₃layer with a desired thickness is formed by repeating the above-mentioned introducing and purging cycle.

[0027] Next, as a second step, an TiO₂layer is formed on the Al₂O₃layer using titanium tetrachloride (TiCl₄) gas and water vapor. The TiCl₄gas and the water vapor serve as source gases for titan (Ti) and O respectively. As in the first step, first, the TiCl₄gas is introduced into the forming chamber with the Ar carrier gas for one second, and then the chamber is purged so that the TiCl₄gas in the chamber is sufficiently ventilated. Next, the water vapor is introduced into the chamber with the Ar carrier gas for one second, and then the chamber is purged so that the H₂O gas in the chamber is sufficiently ventilated. The TiO₂layer with a desired thickness is formed by repeating the above-mentioned introducing and purging cycle as well.

[0028] When forming the TiO₂layer, it is noted that Ba is added to the TiO₂layer so that the resistivity of the TiO₂layer becomes stable against the change in the temperature. To add Ba to the TiO₂layer, a bisdipivaloymethanate barium (Ba(C₁₁H₁₉O₂)₂) gas, which is a source gas for Ba, is introduced into the forming chamber with the Ar carrier gas immediately before introducing the TiCl₄gas. The amount of Ba to be added is controlled by adjusting an introducing time of the Ba(C₁₁H₁₉O₂)₂gas or by controlling the number of times of introducing the Ba(C₁₁H₁₉O₂)₂gas relative to that of introducing the TiCl₄gas.

[0029] As another method of adding Ba to the TiO₂layer, the Ba(C₁₁H₁₉O₂)₂gas and the TiCl₄gas may be introduced into the chamber simultaneously. In this method, the amount of Ba to be added is controlled by setting temperatures of the both source gases based on an introduction ratio therebetween. The amount of Ba to be added can be also controlled by controlling a flow rate of the Ar carrier gas.

[0030] The Al₂O₃/TiOx:Ba laminated layer as the first insulating layer 3 with a desired thickness is formed by repeating the above-described first and second steps for an appropriate times. In this first embodiment, the Al₂O₃and TiOx:Ba layers are alternately laminated with each other to respectively have 30 layers, thereby forming the Al₂O₃/TiOx:Ba laminated layer. Each thickness of the Al₂O₃and TiOx:Ba layers is preferably 5 nm. Although the top and bottom layers of the Al₂O₃/TiOx:Ba laminated layer may be either the Al₂O₃layer or the TiOx:Ba layer, it is more preferable that each of the top and bottom layers is the TiOx:Ba layer.

[0031] Next, the luminescent layer 4 made of zinc sulfide (ZnS) as a host material added with manganese (Mn) as a luminescent center (ZnS:Mn) is deposited on the insulating layer 3 by the vapor deposition method. Further, the second insulating layer 5 is formed on the luminescent layer 4 in the same way as the first insulating layer 3. The second electrode 6 made of ITO is formed on the second insulating layer 5 in the same way as the first electrode 2. The luminescent layer 4 may be deposited by the ALE method. In this case, the first insulating layer 3, the luminescent layer 4 and the second insulating layer 5 are formed successively. Thus, the EL device shown in FIG. 2 is manufactured.

[0032] When alternatively depositing the Al₂O₃layers and the TiOx:Ba layers by the ALE method to form the insulating layers 3 and 5, the Al₂O₃layers having a thickness less than 0.5 nm do not work as insulating layers. Further, the Al₂O₃layers and the TiOx:Ba layers respectively having a thickness more than 100 nm decrease the withstand voltage of the EL device. Therefore, the thickness of the Al₂O₃layers and the TiOx:Ba layers is preferably between 0.5 nm and 100 nm, and more preferably between 1 nm and 10 nm.

[0033] When the amounts of Ba included in the TiOx:Ba layers are insufficient, the stability of the resistivity of the Al₂O₃/TiOx:Ba laminated layer against the change in the temperature deteriorates. Therefore, an atomic ratio of Ba to Ti in the TiOx:Ba layers should be 0.01 or more. When the atomic ratio of Ba to Ti is larger than 1 that is a stoichiometric ratio of BaTiO3, water-soluble barium oxide (BaO) is generated due to an excessive amount of Ba. As a result, the Al₂O₃/TiOx:Ba laminated layer can not sufficiently work as an insulating layer. Therefore, the atomic ratio of Ba to Ti in the TiOx:Ba layer should be 1 or less. In addition to Ba, other elements such as calcium (Ca), strontium (Sr), Al, lead (Pb) and magnesium (Mg) can be used as an additive for the TiO₂layers.

[0034] Each of the Al₂O₃layers provides an excellent insulating performance even with a thickness of a few nm. However, other insulating materials such as magnesium oxide (MgO) and silicon oxide (SiO₂) may be used in place of the Al₂O₃, provided that the materials have a resistivity larger than 10¹⁰Ωcm and can be formed into an extremely thin layer by the ALE method.

[0035] The above-mentioned various materials and elements can be combined as the insulating material for the first layer and the additives for the second layer for the first and second insulating layers 3 and 5. For example, an Al₂O₃layer and a TiO₂layer added with Al (TiOx:Al layer) may be employed as the first and second layers respectively. Then, a method of forming a laminated layer composed of alternatively laminating Al₂O₃and TiOx:Al layers (i.e., an Al₂O₃/TixO:Al layer), which is applicable for the first and second insulating layers 3 and 5, will be described below.

[0036] First, as a first step, the Al₂O₃layer is formed by the ALE method using the AlCl₃gas and water vapor as the source gases for Al and O respectively in substantially the same procedures as mentioned above. Next, as a second step, the TiO₂layer is formed on the Al₂O₃layer, using the TiCl₄gas and water vapor as the source gases for Ti and O respectively in substantially the same procedures as mentioned above. At that time, Al is added to the TiO₂layer so that the resistivity of the TiO₂layer becomes stable against the change in the temperature. As a method of adding Al to the TiO₂layer, the AlCl₃gas is introduced into the forming chamber immediately before introducing the TiCl₄gas. An amount of Al to be added is controlled according to the introducing time of the AlCl₃gas or the number of times of introducing the AlCl₃gas relative to that of introducing the TiCl₄gas. Thus, when Al is used as the additive for the TiO₂layer, the AlCl₄gas is used not only for forming the Al₂O₃layer but also for adding Al to the TiO₂layer. This reduces production cost composed of material cost, equipment cost and the like. By repeating the Al₂O₃/TiOx:Al laminated layer is obtained.

[0037] In this embodiment, employed Al₂O₃/TiOx:Al laminated layers as the first and second insulating layers 3 and 5 respectively include 121 layers of Al₂O₃with a thickness of 2 nm and 120 layers of TiOx:Al with a thickness of 2.5 nm. The bottom layer in contact with the first electrode 2 and the top layer in contact with the luminescent layer 4 are the Al₂O₃layers (see FIG. 5).

[0038] Employing the Al₂O₃layer as the top and bottom layers of the first insulating layer 3 prevents invasion of elements, which can deteriorate the withstand voltage of the EL device. The elements include heavy metals such as alkaline earth metals, alkaline metals, iron, and the like, which are contained in source materials for the first electrode 2 and the luminescent layer 4 (e.g., a pellet of ZnS, a source material of ITO). The reason that the Al₂O₃layers prevent the invasion of the elements is because the Al₂O₃layers are insulative. As a result, deterioration of the withstand voltage of the insulating layer 3 is prevented. The top and bottom layers of the second insulating layer 5 are also the Al₂O₃layers for the same reason.

[0039] If the TiOx:Al layers are employed as the top and bottom layers of the first insulating layer 3, the above-mentioned elements easily diffuse into the TiOx:Al layers to decrease the withstand voltage of the first insulating layer 3. This is because the TiOx:Al layers have electrical conductivity and are likely to have oxygen vacancies therein. Specifically, when the EL device is activated, the above-mentioned elements diffuse under an electric field. The ions of the above-mentioned elements easily diffuse into the TiOx:Al layers due to conductivity of the TiOx:Al layers. Further, it is considered that ion paths are easily provided in the TiOx:Al layers due to the oxygen vacancies to increase diffusion speeds of the ions.

[0040] The first layers for the first and second insulating layer 3 and 5 can be made of other insulating materials such as MgO or SiO₂instead of Al₂O₃. In these cases, MgO or SiO₂layers are used for the top and bottom layers of the first and second insulating layers 3 and 5 to prevent deterioration of the voltage withstanding performance of the insulating layers 3 and 5.

[0041] (Second Embodiment)

[0042] In a second preferred embodiment, Al₂O₃/strontium titanate (SrTiO₃) laminated layers are employed as the first and second insulating layers 3 and 5. The other features are the same as those in the first embodiment, and detail descriptions thereof will not be reiterated.

[0043] The first and second insulating layers 3 and 5 in the second embodiment are formed in the following way. First, as a first step, an Al₂O₃layer is formed as a first layer by the ALE method in the same procedures as the first embodiment. Next, as a second step, a SrTiO₃layer is formed on the Al₂O₃layer as a second layer, using TiCl₄gas, bisdipivaloymethanate strontium (Sr(C₁₁H₁₉O₂)₂) gas and water vapor as source gases for Ti, Sr and O respectively.

[0044] The second step for forming the SrTiO₃layer will be described in more detail. First, the TiCl₄gas is introduced into a forming chamber with an Ar carrier gas for one second, and then the chamber is purged so that the TiCl₄gas in the chamber is ventilated sufficiently. Next, the water vapor is introduced into the forming chamber with the Ar carrier gas for one second, and then the chamber is purged so that the water vapor in the chamber is ventilated sufficiently. Thereafter, the Sr(C₁₁H₁₉O₂)₂gas is introduced into the forming chamber with the Ar carrier gas for one second, and then the chamber is purged so that the Sr(C₁₁H₁₉O₂)₂gas in the chamber is ventilated sufficiently. Further, the water vapor is introduced into the chamber with the Ar carrier gas for one second, and then the chamber is purged so that the water vapor in the chamber is ventilated sufficiently. By repeating the above-mentioned introducing and purging cycle for an appropriate number of times, the SrTiO₃layer with a desired thickness is formed.

[0045] The Al₂O₃/SrTiO₃laminated layer with a desired thickness is formed by repeating the above-mentioned first and second steps for an appropriate number of times to function as the first or second insulating layer 3 or 5. The top and bottom layers of the first and second insulation layers 3 and 5 may be either the Al₂O₃layers or the SrTiO₃layers.

[0046] Characteristics of the above-mentioned Al₂O₃/SrTiO₃laminated layer can be improved by a heat treatment for crystallizing SrTiO₃. This heat treatment can be conducted simultaneously with a heat treatment for the luminescent layer 4. In the thus manufactured EL device, the SrTiO₃layers have a positive temperature coefficient of resistivity. That is, as the temperature rises, the resistivity of the SrTiO₃layers becomes large. Therefore, the withstand voltage of the EL device hardly deteriorates with respect to the temperature as shown in FIG. 4. In this embodiment, the second layer of the first and second insulating layers 3 and 5 is made of SrTiO₃. However, the second layer can be made of other materials such as calcium titanate (CaTiO₃) and barium titanate (BaTiO₃) to obtain the same effects as those of SrTiO₃. CaTiO₃and BaTiO₃layers can be formed by the above-mentioned method. In this embodiment, each thickness of the first and second layers for the first and second insulating layers 3 and 5 is preferably between 0.5 nm and 100 nm as in the first embodiment.

[0047] In the first and second embodiments, sufficient insulating characteristics of the insulating layers 3 and 5 can be achieved when the numbers of the first and second layers of the insulating layers 3 and 5 are at least 10 respectively. The larger the number of the first and second layers become, the more effective the insulating characteristics of the first and second insulating layers 3 and 5 become. However, the preferable numbers of the first and second layers are respectively under 300 to prevent the EL device from being too thick.

[0048] In the first and second embodiments, the first and second electrodes 2 and 6 are made of transparent material. However, the electrode on the side opposite to the light emitting side can be made of opaque material such as metal. In this case, the light is taken out from only one side of the luminescent layer 4. The luminescent layer 4 can be made of ZnS added with terbium (Tb) or strontium sulfide (SrS) added with cerium (Ce), in addition to ZnS added with Mn. The present invention is applied not only to the EL device having the insulating layers on the both sides of the luminescent layer but also to an EL device having an insulating layer only on one side of a luminescent layer.

[0049] While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.