Abstract:
An electroluminescent device and a method for manufacturing the same are provided to achieve a highly luminous electroluminescent device that can be used as a backlight for an LCD monitor. The electroluminescent device includes a substrate, a lower electrode layer having a surface of a plurality of convex shapes formed on the substrate, an insulating layer, a light-emitting layer, and an upper electrode layer sequentially formed on the lower electrode layer, and a passivation layer formed on the upper electrode layer. The method for manufacturing an electroluminescent device includes forming a lower electrode layer having a surface of a plurality of convey shapes on a substrate, sequentially forming an insulating layer, a light-emitting layer, and an upper electrode layer over the lower electrode layer to have substantially corresponding surface shapes as the lower electrode layer, and forming a passivation layer on the upper electrode layer.

Description:
This application claims the benefit of Korean Application No. P 2000-83098, filed in Korea on Dec. 27, 2000, which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a display device, and more particularly, to an electroluminescent device and a method for manufacturing the same.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Ultra thin sized flat panel displays having a display screen with a thickness of several centimeters, especially liquid crystal display (LCD) devices, are widely used for monitors in notebook computers, spacecraft, aircraft, etc.  
           [0005]    LCD panels are in general non-luminous and require a backlight at the rear of the liquid crystal panel as a light source. The conventional, backlight is not satisfactory because of its large weight, power consumption, and thickness. In this respect, it is desirable to replace the conventional backlight with a thinner, lighter, less-power consuming alternative. Currently, thin and light electroluminescent devices are under research and development.  
           [0006]    Electroluminescent devices can be divided into two types: a light-emitting diode (LED) and an electroluminescent diode (ELD), depending on the operational principles. The light emission of LEDs is based on a radiant transition due to electron-hole recombination near a P-N junction. Recently, a rapid development of an LED based on an organic material is in progress.  
           [0007]    On the other hand, tie light emission of ELDs is based on luminescence that takes place when high energy electrons generated in a light-emitting layer excite a phosphor upon impact, Electrons within the light-emitting layer acquire energy from a high electric field and turn into hot electrons. The hot electrons then excite an activator to generate light.  
           [0008]    ELDs are manufactured by thick-film printing of a mixture of resin and light-emitting powder or by thin film printing. ELDs are also divided into two types: the AC type and the DC type, depending on the driving modes.  
           [0009]    An electroluminescent device of the related art will be described with reference to FIG. 1. FIG. 1 is a schematic perspective view of a related art electroluminescent device. As shown in FIG. 1, the related art electroluminescent device includes a substrate  11  and a transparent electrode layer  13  on the substrate  11 . The transparent electrode layer  13  is formed in a predetermined pattern, such as in a stripe pattern. The transparent electrode  13  is formed of indium tin oxide (ITO), for example. A lower insulating layer  15  of SiO X , SiN X , or BaTiO 3  is formed on the transparent electrode layer  13 , and a light-emitting layer  17  of a ZnS based light-emitting material is formed on the lower insulating layer  15 . The related art device further includes an upper insulating layer  19  made of SiO X , SiN X , or Al 2 O 3  on the light-emitting layer  17 . It further includes a metal electrode layer  21  made of a metal, such as Al, on the upper insulating layer  19 , and a surface passivation layer  23  on the metal electrode layer  21 .  
           [0010]    In this related art electroluminescent device, when an AC voltage is applied between the transparent electrode layer  13  and the metal electrode layer  21 , a high electric field in the order of 10 6  V/cm is built within the light-emitting layer  17 . Electrons generated in the interface between the upper insulating layer  19  and the light-emitting layer  17  tunnel into the light-emitting layer  17 .  
           [0011]    The tunneling electrons are accelerated by the high electric field in the light-emitting layer  17 . The accelerated electrons collide with activators (Cu and/or Mn) within the light-emitting layer  17  to excite electrons in the ground state to excited states. When electrons at a higher energy level transit to the vacant sites in a lower energy level state created by the excitation—e.g., when the excited electrons transit to the ground state (or to other lower energy level states), light having a wavelength corresponding to the energy difference is emitted. The color of the emitted light thus depends on the energy difference.  
           [0012]    A method for manufacturing the related art electroluminescent device will now be described in more detail. The transparent electrode layer  13  is formed on the glass substrate  11 . Specifically, a thin ITO film having a high conductivity end a good transparent physical characteristic is deposited on the substrate  11 . The thin ITO film is then patterned by photolithography into a stripe shape to form transparent electrodes, which are collectively referred to as “transparent electrode layer  13 .” 
           [0013]    A BaTiO 3  based lower insulating layer  15  is formed on the transparent electrode layer  13  by RF reactive sputtering. The light-emitting layer  17  is then formed on the lower insulating layer  15 . The light-emitting layer  17  may be formed via electron-beam deposition by cold pressing a powder of a Cu or Mn doped ZnS material and by generating small grains. Alternatively, the light-emitting layer  17  nay be formed by sputtering using a target.  
           [0014]    The upper insulating layer  19  of SiO X , SiN X , or Al 2 O 3  is formed on the light-emitting layer  17  by sputtering or chemical vapor deposition (CVD). The metal electrode layer  21  is formed on the upper insulating layer  19 . Specifically, a thin Al or Ag film is formed on the upper insulating layer  19  by thermal deposition and is patterned into stripe-shaped metal electrodes that extend perpendicularly to the transparent electrodes of the transparent electrode layer  13  underneath. Finally, the surface passivation layer  23  is formed on the metal electrode layer  21 . This completes the manufacture of the related art electroluminescent device.  
           [0015]    However, the related art electroluminescent device have several drawbacks. As briefly explained above because a thin film transistor (TFT) liquid crystal display (LCD) panel (TFT-LCD panel) for notebook computers and monitors has no self-luminous function, a light-emitting device such as a backlight is required. Since the conventional backlight is constructed by combining a light-guiding plate, a light-diffusion plate, and a prism with a cold cathode fluorescent lamp, the manufacturing cost is high, and the manufacturing process is undesirably complicated. Moreover, the large thickness of the backlights increases the thickness of the resultant monitor devices, which is undesirable. To substitute for such a conventional backlight, the related, art electroluminescent device has been proposed. Although the manufacturing cost and thickness of the related art electroluminescent device have been somewhat reduced recently, it is still expensive. Moreover, the related art electroluminescent device still has an insufficient luminance to be used as a light source for LCDs.  
         SUMMARY OF THE INVENTION  
         [0016]    Accordingly, tie present invention is directed to an electroluminescent device and a method for manufacturing the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.  
           [0017]    An object of the present invention is to provide an electroluminescent device and a method for manufacturing the same, in which a sufficiently high luminance can be obtained so that the device can be used as a backlight for LCD panels.  
           [0018]    Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the scheme particularly pointed, out in the written description and claims hereof as well as the appended drawings.  
           [0019]    To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the electroluminescent device according to a first aspect includes a lower electrode layer having a surface of a plurality of convex shapes, formed on the substrate, an insulating layer, a light-emitting layer, and an upper electrode layer sequentially formed on the lower electrode layer, and a passivation layer formed on the upper electrode layer.  
           [0020]    In another aspect, the prevent invention provides a method for manufacturing an electroluminescent device, the method including forming a lower electrode layer having a surface of a plurality of convex shapes on a substrate, sequentially forming an insulating layers a light-emitting layer, and all upper electrode layer over the lower electrode layer to have the same shapes as the lower electrode layer, and forming a passivation layer on the upper electrode layer.  
           [0021]    In another aspect, the prevent invention provides an electroluminescent device including a substrate; a lower electrode layer over the substrate, having a plurality of convex shapes in its surface; an insulating layer over the lower electrode layer; a light-emitting layer over the insulating layer; an upper electrode layer over the light-emitting layer; and a passivation layer over the upper electrode layer, wherein the insulating layer, the light-emitting layer, and the upper electrode layer are formed in succession.  
           [0022]    In another aspect, the present invention provides a method for manufacturing an electroluminescent device, the method including forming, over a substrate, a lower electrode layer having a plurality of convex shapes in its surface; forming, over the lower electrode layer, an insulating layer, a light-emitting layer, and an upper electrode layer in succession so that the insulating layer, the light-emitting layer, and the upper electrode layer have substantially the same surface profile as the lower electrode layer; and forming a passivation layer over the upper electrode layer.  
           [0023]    In a further aspect, the present invention provides an electroluminescent device including a substrate; a lower electrode layer over the substrate, having an uneven surface profile; an insulating layer over the lower electrode layer, having an uneven surface profile substantially corresponding to the uneven surface profile of tie lower electrode layer; a light-emitting layer over the insulating layer, having an uneven surface profile substantially corresponding to the uneven surface profile of the insulating layer; and an upper electrode layer over the light-emitting layer, having an uneven surface profile substantially corresponding to the uneven surface profile of the light-emitting layer.  
           [0024]    It is to be understood that both the foregoing general description and die following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0026]    In the drawings;  
         [0027]    [0027]FIG. 1 is a perspective view of a related art electroluminescent device;  
         [0028]    [0028]FIG. 2 is a sectional view of an electroluminescent device according to a first embodiment of the present invention;  
         [0029]    [0029]FIGS. 3A to  3 D are sectional views illustrating process steps for manufacturing the electroluminescent device according to the first embodiment of the present invention;  
         [0030]    [0030]FIG. 4 is a sectional view of an electroluminescent device according to a second embodiment of the present invention; and  
         [0031]    [0031]FIGS. 5A to  5 D are sectional views illustrating process steps for manufacturing the electroluminescent device according to the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated, in the accompanying drawings.  
         [0033]    First Embodiment  
         [0034]    [0034]FIG. 2 is a sectional view of air electroluminescent device according to a first embodiment of the present invention. FIGS. 3A to  3 D are sectional views illustrating process steps for manufacturing the electroluminescent device according to the first embodiment of the present invention.  
         [0035]    As shown in FIG. 2, the electroluminescent device according to the present invention includes a substrate  31 , and a lower electrode layer  33  formed on the substrate  31 . The lower electrode layer  33  has a surface profile of a plurality of convex shapes. An insulating layer  35  is formed on the lower electrode layer  33 , and a light-emitting layer  37  is formed on the insulating layer  35 . The electroluminescent device of FIG. 2 further includes an upper electrode layer  39  formed on the light-emitting layer  37  and a passivation layer  41 , formed on the upper eletrode layer  39 .  
         [0036]    If desired, the lower electrode layer  33  and/or upper electrode layer  39  may be patterned into a plurality of stripes crossing each other in a manner similar to the related art device of FIG. 1. In such a case, FIGS. 2 and 3A- 3 D illustrate a portion of the cross-sections where the stripes of the upper electrode cross the stripes of the lower electrode. (The same is true for FIGS. 4 and 5A- 5 D below.)  
         [0037]    The lower electrode layer  33  has a layered structure made of either a pair of a polysilicon layer  33   a  and a metal layer  33   b  or a pair of a tungsten layer  33   a  and a metal, layer  33   b.  If the lower electrode layer  33  is to have a layered, structure made of the polysilicon layer  33   a  and the metal layer  33   b,  the polysilicon layer  33   a  is preferably formed by low pressure chemical vapor deposition (LPCVD), and the metal layer  33   b  is preferably formed by thermal deposition. If the lower electrode layer  33  is to have a layered structure made of the tungsten. layer  33   a  and the metal layer  33   b,  the tungsten layer  33   a  is preferably formed by chemical vapor deposition (CVD) to create the uneven surface profile. Here, the insulating layer  35  is formed of a BaTiO 3  based material having a high dielectric constant, and the upper electrode layer  39  is formed of a transparent conductive material such as indium tin oxide (ITO).  
         [0038]    In this example, because the surface of the lower electrode layer  33  has a plurality of convex shapes, the insulating layer  35 , the light-emitting layer  37 , and the upper electrode layer  39 , which ale formed over the lower electrode layer  33  in succession, also have the substantially the same surface profile of a plurality of convex shapes. The uneven surface profile of the lower electrode layer  33  thus helps increase the luminance of the resultant device by increasing the surface areas of the light-emitting layer  37 .  
         [0039]    A method for manufacturing the electroluminescent device according to the first embodiment of the present invention will now be described with reference to FIGS. 3A to  3 D. As shown in FIG. 3A, the lower electrode layer  33  is formed on the substrate  31  (e.g., glass substrate). Specifically, a polysilicon layer  33   a,  for example, is growl Oil the substrate  31  by LPCVD at a temperature between about 560° C. and about 610° C. Then, a metal layer  33   b  having an excellent light reflecting characteristic, such as Al or Ag, is formed on the polysilicon layer  33   a  by thermal deposition.  
         [0040]    When, the polysilicon layer  33   a  is grown by LPCVD at a temperature between about 560° C. and about 610° C., the resultant polysilicon layer  33   a  exhibits a surface profile having a plurality of convex shapes each of which resembles a hemispheric shape. The metal layer  33   b  is then formed along the uneven surface of the polysilicon layer  33   a.  Accordingly, the resultant metal layer  33   b  exhibits substantially tie same surface profile as the polysilicon layer  33   a.    
         [0041]    Instead of the polysilicon, layer  33   a,  a tungsten layer may be grown by CVD as the layer  33   a.  In this case, the tungsten layer  33   a  exhibits all uneven surface profile having a plurality of convex shapes each of which has a shape similar to a hemispheric shape although the resemblance to the hemispheric shape is in general not so strong as compared to the case of polysilicon layer  33   a.    
         [0042]    After the surface of the lower electrode layer  33  is formed to have a plurality of convex shapes, as shown in FIG. 3B, the insulating layer  35  is formed on the lower electrode layer  33 . The insulating layer  35  is formed of, for example, a BaTiO 3  based material by sputtering or CVD. The surface of the insulating layer  35  also exhibits a plurality of convex shapes because of the uneven surface profile of the metal layer  33   b  thereunder.  
         [0043]    As shown in FIG. 3C, the light-emitting layer  37  is then formed on the insulating layer  35 . The light-emitting layer  37  is formed of, for example, a ZnS based material by electron beam deposition or sputtering. Again, because the surface of the resultant insulating layer  35  has an uneven surface profile having a plurality of convex shapes, the surface of the resultant light-emitting layer  37  also exhibits an uneven surface profile having a plurality of convex shapes.  
         [0044]    Referring to FIG. 3D, the upper electrode layer  39  is formed on the light-emitting layer  37  by sputtering. The upper electrode layer  39  is formed of a transparent material, e.g. an indium tin oxide (ITO) material, having a high conductivity. The upper electrode layer  39  is then patterned by photolithography. Thereafter, the passivation layer  41  is formed on the patterned upper electrode layer  39  to protect the surface thereof. This completes the manufacture of the electroluminescent device according to the first embodiment of the present invention.  
         [0045]    In the electroluminescent device of the first embodiment, a metal having an excellent reflecting characteristic, such as Al or Ag, is used in the lower electrode layer, and a polysilicon or tungsten layer having a significantly uneven surface profile (e.g., having a plurality of convex shapes) is formed under the metal layer in order to increase the surface area of the metal layer thereabove. This construction helps increase upward convergence effects upon light, and thus improves the luminance of the device. Further, as shown in FIG. 2, because no upper insulating layer is formed on the light-emitting layer  37 , a voltage drop due to the upper insulating layer is eliminated, thereby lowering the driving voltage, which is desirable.  
         [0046]    Second Embodiment  
         [0047]    [0047]FIG. 4 is a sectional view of an electroluminescent device according to a second embodiment of the present invention. FIGS. 5A to  5 D are sectional views illustrating process steps for manufacturing the electroluminescent device according to the second embodiment of the present invention.  
         [0048]    As described above, in the first embodiment of the present invention, the lower electrode layer is formed of a layered structure of either a polysilicon layer and a metal layer, or a tungsten layer and a metal layer. In contrast, in the second embodiment of the present invention, the lower electrode layer is formed essentially of a single layer of metal only.  
         [0049]    As shown in FIG. 4, the electroluminescent device according to the second embodiment of the present invention includes a substrate  51  and a lower electrode layer  53  formed on the substrate  51 . The lower electrode layer  53  has an uneven surface profile having a plurality of convex shapes. The device of the second embodiment further includes an insulating layer  55 , a light-emitting layer  57 , an upper electrode layer  59 , and a passivation layer  61 . The insulating layer  55 , the light-emitting layer  57 , the upper electrode layer  59 , and the passivation layer  61  are formed over the lower electrode layer  53  in succession.  
         [0050]    A metal having all excellent reflecting characteristic, such as Al or Ag, is used as the lower electrode layer  53 . If the lower electrode layer  53  is to be formed by thermal deposition or like process, its surface does not normally exhibit an uneven profile. In this example, the surface of the lower electrode layer  53  is engraved by wet etching, dry etching, or both wet and dry etching processes in order to form an uneven surface having a plurality of convex shapes.  
         [0051]    A method for manufacturing the electroluminescent device according to the second embodiment of the present invention will now be described with reference to FIGS. 5A to  5 D. As shown in FIG. 5A, the lower electrode layer  53  is formed on the substrate  51 . (e.g., glass substrate). Specifically, after a metal layer  53   a  having an excellent light reflecting characteristic, such as Al or Ag, is formed on the substrate  51  by thermal deposition, a photoresist pattern  54  is formed on the metal layer  53   a.    
         [0052]    The photoresist pattern  54  serves as a mask when the lower metal layer  53  is etched for the purpose of forming an uneven surface profile having a plurality of convex shapes. A dry etching process find a wet etching process are successively performed using the photoresist pattern  54  as a mask. As a result, as shown in FIG. 5B, the metal layer  53   a  turns into the lower electrode layer  53  raving a plurality of convex shapes in its surface.  
         [0053]    Subsequently, as shown in FIG. 5C, an insulating material having a high dielectric constant, such as BaTiO 3  based material, is deposited over the lower electrode layer  53  by sputtering to form the insulating layer  55 . A ZnS based light-emitting maternal is deposited on the insulating layer  55  by electron beam deposition or sputtering to form the light-emitting layer  57 .  
         [0054]    Referring to FIG. 5D, a transparent conductive material, such as ITO, is deposited on the light-emitting layer  57 . The transparent conductive material is then patterned by photolithography to form the upper electrode layer  59 . The passivation layer  61  is then formed on the upper electrode layer  59 . This completes the manufacture of the electroluminescent device according to the second embodiment of the present invention.  
         [0055]    In the electroluminescent device according to the second embodiment of the present invention, when an AC voltage of a sufficient amplitude is applied between the lower electrode layer  53  and the upper electrode layer  59 , a high electric field in the order of 10 6  V/cm is built within the light-emitting layer  57 . Electrons generated in the interface between the insulating layer  55  and the light-emitting layer  57  tunnel into the light-emitting layer  57 . The tunneling electrons are accelerated by the high electric field within the light-emitting layer  57 . The accelerated, electrons collide with activators in the light-emitting layer  57  to excite electrons in the ground state to some excited states. When electrons at a higher energy level transit to the vacant sites in a lower energy level state created by the excitation—e.g., when the excited electrons transit to the ground state (or to other lower energy level states), light having a wavelength corresponding to the energy difference is emitted.  
         [0056]    The electroluminescent device and the method for manufacturing the same according to the present invention have, among others, the following advantages. Because Al or Ag having an excellent light reflecting characteristic is used as the lower electrode layer, the luminance of the resulting device is significantly improved because of the upward convergence effects upon light. Furthermore, because no upper insulating layer is formed on the light-emitting layer, a voltage drop due to the upper insulating layer is eliminated, thereby lowering the driving voltage, which is desirable.  
         [0057]    When the polysilicon layer is to be grown by LPCVD at a temperature between about 560° C. and about 610° C., the resulting polysilicon layer exhibits an uneven surface profile having a plurality of hemispheric shaped bumps. Particularly in, this case, the effective surface area of the polysilicon layer significantly increases, which in turn results in a significant increase in the surface area of the light-emitting layer. This contributes to a drastic improvement of the light luminance.  
         [0058]    It will be apparent to those skilled in the art that various modifications and variations can be made in the separating method and apparatus of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of thus invention provided they come within the scope of the appended claims and their equivalents.