Patent Publication Number: US-7897988-B2

Title: Electroluminescent device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of application Ser. No. 11/941,807 filed Nov. 16, 2007, now U.S. Pat. No. 7,625,769, and for which priority is claimed under 35 U.S.C. 120; and this application claims priority of application Ser. No. 095147369, filed in Taiwan, Republic of China on Dec. 18, 2006, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE PRESENT INVENTION 
     1. Field of Invention 
     The present invention relates to an electroluminescent device and a fabrication method thereof, and more particularly to an electroluminescent device and a fabrication method thereof with high efficiency. 
     2. Related Art 
     The light emitting diode (LED) is a cold light emitting element, which releases lights when energy released after electrons and holes are combined in a semiconductor material. According to different used materials, various monochromatic lights with different wavelengths are outputted. The LEDs may be mainly classified into a visible light LED and an invisible light (infrared) LED. Compared with the conventional lighting manner of a light bulb or lamp, the LED has advantages of power save, vibration resist, and high flicker speed, so that the LED has become an indispensable and important element in the daily life. 
     With reference to  FIG. 1 , a conventional LED device  1  is made by at least one LED element  10  attached to a transparent substrate  11 . The LED element  10  includes a n-type semiconductor layer  101 , a light emitting layer  102  and a p-type semiconductor layer  103  formed in sequence. A first contact electrode  104  is connected with the n-type semiconductor layer  101 . A second contact electrode  105  is connected with the p-type semiconductor layer  103 . When voltages are respectively applied to the semiconductor layers  101  and  103  to generate currents, the electrons and the holes of the n-type semiconductor layer  101  and the p-type semiconductor layer  103  are combined together so that electric power is converted into optical energy. As shown in  FIG. 1 , the LED element  10  is attached to the transparent substrate  11  by a transparent adhering layer  12 . To raise the current distribution efficiency, the junction surface between the LED element  10  and the transparent adhering layer  12  further uses a transparent conduction layer  13 . The overall brightness of the LED device  1  is increased by evenly distributing the current. 
     As shown in  FIG. 1 , the first contact electrode  104  is formed on the n-type semiconductor layer  101 . The second contact electrode  105  is formed on the transparent conduction layer  13 . That is, the first contact electrode  104  and the second contact electrode  105  are disposed at the same side of the transparent substrate  11 . Therefore, the production of the LED device must involve the process of removing part of the LED element  10  by etching, for example, the area A in  FIG. 1  is used for disposition of the second contact electrode  105 . However, in addition to increase the complexity of the production process, this step may cause a leakage current of the LED element  10  due to its uneven surface as a result of the bad control in etching. This will lower the yield of the LED device  1 . 
     In the prior art, it is common to utilize the epitaxial substrate as the transparent substrate and utilize an organic adhering material to form the transparent adhering layer. Since the epitaxial substrate and the organic adhering material have low thermal conductivity coefficients, they cannot provide a better heat dissipation path for the LED element  10 . As a result, the heat generated from the operating LED device  1  accumulates and affects the light emitting efficiency thereof. 
     Current researches in the LED put emphases on how to extract the photons generated from the LED element  10  in order to reduce the unnecessary heat caused by repeated reflections and absorptions of the photons therein. It is thus important to lower the operating temperature of the LED device  1  in order to dissipate heat generated from the LED element  10 . 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides an electroluminescent device and a fabrication method thereof that involve simple production processes, possess a uniform electrical current distribution, and are capable of reducing heat accumulation. 
     A fabrication method of an electroluminescent device according to the present invention includes the steps of: providing a plate; forming at least one light emitting diode (LED) element on the plate, wherein the LED element includes a first semiconductor layer, a light emitting layer and a second semiconductor layer in sequence formed on the plate; forming a patterned transparent conduction layer on the LED element; forming a reflection layer on the patterned transparent conduction layer; attaching a conduction substrate to the reflection layer; removing the plate; and forming a first contact electrode and a second contact electrode at one side of the first semiconductor layer and one side of the conduction substrate, respectively. 
     An electroluminescent device according to the present invention includes a conduction substrate, a reflection layer, a patterned transparent conduction layer, at least one light emitting diode (LED) element, a first contact electrode and a second contact electrode. The reflection layer is disposed on the conduction substrate. The patterned transparent conduction layer is formed on the reflection layer. The LED element is formed on the patterned transparent conduction layer, the LED element includes a first semiconductor layer, a light emitting layer and a second semiconductor layer in sequence, and the second semiconductor layer is located on the patterned transparent conduction layer and the reflection layer. The first contact electrode is disposed at one side of the first semiconductor layer. The second contact electrode is disposed at one side of the conduction substrate. 
     An electrode substrate according to the present invention includes a conduction substrate, a reflection layer, a patterned transparent conduction layer and a contact electrode. The reflection layer is disposed on the conduction substrate. The patterned transparent conduction layer is formed on the reflection layer. The contact electrode is disposed at one side of the conduction substrate. 
     As mentioned above, an electroluminescent device and a fabrication method thereof according to the present invention utilize a conduction substrate that connects the contact electrodes of the LED element disposed at opposite sides of the conduction substrate. In comparison with the prior art, because the etching process is avoided, the production processes are thus simplified and the yield is raised. In addition, the patterned transparent conduction layer with a plurality of island-like structures is formed by for example the etching method so that the electrical current imposed to the LED element can be uniformly distributed so as to prevent current clogging. Moreover, the deposition of the reflection layer has a good ohmic contact with the patterned transparent conduction layer and provides a scattering and reflection interface. It effectively raises external light extraction and light emitting efficiency. Also, since the conduction substrate and the reflection layer are highly thermally conductive, the heat generated from the LED element can be more effectively dissipated than the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below illustration only, and thus is not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic diagram showing a conventional LED device; 
         FIGS. 2A to 5B  are a set of schematic diagrams showing an electroluminescent device according to a preferred embodiment of the present invention; 
         FIG. 6  is a flowchart showing a fabrication method of an electroluminescent device according to a preferred embodiment of the present invention; and 
         FIGS. 7A and 7B  are schematic diagrams showing the steps in the fabrication method of the electroluminescent device according to the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
     Please refer to  FIGS. 2A ,  2 B,  3 A and  3 B. An electroluminescent device  2  according to a preferred embodiment of the present invention includes a conduction substrate  21 , a reflection layer  22 , a patterned transparent conduction layer  23 , at least one light emitting diode (LED) element  24 , a first contact electrode  25  and a second contact electrode  26 . 
     In this embodiment, the conduction substrate  21  can be made of a semiconductor material or a metal that having a high coefficient of thermal conductivity. The semiconductor material is selected from the group consisting of Si, GaAs, GaP, SiC, BN and their combinations. The metal is selected from the group consisting of Al, Cu, Ag, Au, Ni and their combinations. 
     The reflection layer  22  is formed on the conduction substrate  21 . The patterned transparent conduction layer  23  is formed on the reflection layer  22 . As shown in  FIGS. 2A to 3B , the reflection layer  22  has an uneven surface. The patterned transparent conduction layer  23  is filled with the concave parts of the reflection layer  22  so that the patterned transparent conduction layer  23  includes a plurality of island-like structures being independent or continuous. That is, the island-like structures can be separate (as shown in  FIG. 2A ) or connected (as shown in  FIG. 3A ). Of course, the patterned transparent conduction layer  23  can be a combination of the above-mentioned two island-like structures. The cross section of the island-like structure can be rectangular, circular, polygonal or irregular. 
     In this embodiment, the reflection layer  22  is made of a metal with high reflectivity. Along with the uneven surface, the reflection layer  22  provides good reflecting and scattering effects to enhance external light extraction. The metal of the reflection layer  22  is selected from the group consisting of Pt, Au, Ag, Cr, Ni, Pd, Ti, Al and their combinations. Besides, the reflection layer  22  and the patterned transparent conduction layer  23  are connected to form a good ohmic contact, thereby lowering its resistance and raising the light emitting efficiency of the electroluminescent device  2 . 
     The LED element  24  is formed on the patterned transparent conduction layer  23 . The LED element  24  includes a first semiconductor layer  241 , a light emitting layer  242  and a second semiconductor layer  243 . These layers are formed in the order of the second semiconductor  243 , the light emitting layer  242  and the first semiconductor layer  241  on the patterned transparent conduction layer  23  and the reflection layer  22 . In this embodiment, the LED element  24  can be formed on the patterned transparent conduction layer  23  with independent island-like structures. In this case, the second semiconductor layer  243  is in contact with the patterned transparent conduction layer  23  and the reflection layer  22  (as shown in  FIG. 2A ). Besides, the LED element  24  can also be formed on the patterned transparent conduction layer  23  with continuous island-like structures. In this case, the second semiconductor layer  243  is in contact with the patterned transparent conduction layer  23  (as shown in  FIG. 3A ). The patterned transparent conduction layer  23  thus achieves a uniform electrical current distribution in the LED element  24  so as to prevent the possibility of current clogging. 
     In this embodiment, the first semiconductor layer  241  can be an n-type semiconductor layer and the second semiconductor layer  243  can be a p-type semiconductor layer. However, this is only an example and the present invention is not restricted to this embodiment. Whether the first semiconductor layer  241  and the second semiconductor layer  243  are n-type and p-type semiconductor layers or the other way around depends upon the practical requirement. 
     The first contact electrode  25  is disposed at one side of the first semiconductor layer  241 . The second contact electrode  26  is disposed at one side of the conduction substrate  21 . That is, the first contact electrode  25  and the second contact electrode  26  are disposed at opposite sides of the conduction substrate  21 , which forming an electroluminescent device with a perpendicular structure. Thereby, the present invention can avoid the etching process for disposing contact electrodes at the same side. This simplifies the production process and raises the yield. 
     Please refer to  FIG. 4A . In addition to the first contact electrode  25  is formed on the first semiconductor layer  241 , the first semiconductor layer  241  can further form with a rough structure or an anti-reflection layer  245  on a light-emitting surface of the LED element  24  for guiding the light out. That is, the rough structure or the anti-reflection layer  245  is formed at the side of the first semiconductor layer  241  and on a position without the first contact electrode  25  so that the light emitting efficiency is effectively raised. 
     As shown in  FIG. 4B , the electroluminescent device  2  can further include an adhering layer  27  disposed between the reflection layer  22  and the conduction substrate  21 . The adhering layer  27  is highly thermally conductive and the adhering layer  27  may serve as a bonding layer. The material of the adhering layer  27  is silver paste, tin paste, tin-silver paste, a eutectic bonding material, metal or a conductive adhering material, which is a material containing lead or containing no lead. Since the coefficient of thermal conductivity of the adhering layer  27  in this embodiment is higher than that of the organic adhering material in the prior art, it can more effectively dissipate heat. 
     As described above, the electroluminescent device  2  utilizes the adhering layer  27  and the conduction substrate  21  that having high thermal conductivity coefficients to fully lower the operating temperature of the LED element  24 . Moreover, it has the advantages of capable of carrying a large current and suitable for making a large-area device. Thus, overall light emitting efficiency is greatly raised. 
     In addition to the structures of direct contact between the conduction substrate  21  and the reflection layer  22  shown in  FIGS. 2A ,  3 A,  4 A and  5 A, an adhering layer  27  with high thermal conductivity can be inserted therebetween (as shown in  FIGS. 2B ,  3 B,  4 B and  5 B). The material of the adhering layer  27  is silver paste, tin paste, tin-silver paste, a eutectic bonding material, metal or a conductive adhering material, which is a material containing lead or containing no lead. Since the coefficient of thermal conductivity of the adhering layer  27  in this embodiment is higher than that of the organic adhering material in the prior art, it can more effectively dissipate heat. 
     To further understand the contents of the electroluminescent device  2  according to the present invention, please refer to  FIGS. 6 ,  7 A and  7 B. A fabrication method of the electroluminescent device includes the steps of S 1  to S 7 . In step S 1 , a plate  20  is provided. In step S 2 , at least one LED element  24  is formed on the plate  20 . The LED element  24  includes in sequence a first semiconductor layer  241 , a light emitting layer  242 , and a second semiconductor layer  243 , with the first semiconductor layer  241  formed on the plate  20 . In step S 3 , a patterned transparent conduction layer  23  is formed on the LED element  24 . In step S 4 , a reflection layer  22  is formed on the patterned transparent conduction layer  23 . In step S 5 , a conduction substrate  21  is attached to the reflection layer  22 . In step S 6 , the plate  20  is removed. In step S 7 , a first contact electrode  25  and a second contact electrode  26  are formed at one side of the first semiconductor layer  241  and one side of the conduction substrate  21 , respectively. 
     In step S 1 , the plate  20  is provided as a temporary substrate for making the LED element  24 . The material of the plate  20  can be, for example aluminum oxide (e.g., Al 2 O 3 ). After the plate  20  is appropriately cleaned, a subsequent process of growing the epitaxy layer of the LED element  24  is performed. 
     In step S 2 , the LED element  24  is formed on the plate  20 . That is, the first semiconductor layer  241 , the light emitting layer  242  and the second semiconductor layer  243  are disposed in sequence on the plate  20  to form the LED element  24 . In this embodiment, the first semiconductor layer  241  can be an n-type semiconductor layer and the second semiconductor layer  243  can be a p-type semiconductor layer. However, this is only an example and the present invention is not restricted to this embodiment. 
     In step S 3 , the patterned transparent conduction layer  23  is formed on the LED element  24 . In this embodiment, the material of the patterned transparent conduction layer  23  is selected from the group consisting of ITO, Cd—Sn oxide, Sb—Sn oxide, Be, Ge, Ni, Au and their combinations. It is formed on the second semiconductor layer  243  of the LED element  24  by deposition. Afterwards, it is patterned by photolithography and etching processes. The etching process can be either dry etching or wet etching, accompanied by physical and/or chemical etching. In this embodiment, the patterned transparent conduction layer  23  includes a plurality of island-like structures. By etching in different depths, such as etching stop at different depth, the patterned transparent conduction layer  23  can include continuous island-like structures (as shown in  FIG. 7A ). Alternatively, the etching can be performed until the entire patterned transparent conduction layer  23  is etched through and stops at the second semiconductor layer  243  of the LED element  24 . The patterned transparent conduction layer  23  thus includes independent island-like structures (as shown in  FIG. 7B ). The cross section of the island-like structure is not restricted, that can be rectangular, circular, polygonal or irregular. 
     In step S 4 , the reflection layer  22  is formed on the patterned transparent conduction layer  23 . In this embodiment, the material of the reflection layer  22 , which having high reflectivity, is selected from the group consisting of Pt, Au, Ag, Pd, Ni, Cr. Ti, Al and their combinations. According to the patterned structure of the patterned transparent conduction layer  23 , the reflection layer  22  thereon is uneven in surface such as ups and downs. Moreover, the reflection layer  22  forms an ohmic contact with the patterned transparent conduction layer  23 , which enhancing the external light extraction and lowering the resistance. Thus, overall light emitting efficiency is effectively raised. 
     In step S 5 , the conduction substrate  21  is attached to the reflection layer  22 . In this embodiment, the conduction substrate  21  is attached to the reflection layer  22  via an adhering layer  27 . Here the adhering layer  27  can be applied on the reflection layer  22  or the conduction substrate  21  before the conduction substrate  21  is attached. The adhering layer  27  can cover parts of the reflection layer  22  or the entire surface of the reflection layer  22 . The conduction substrate  21  and the adhering layer  27  are highly thermally conductive. The conduction substrate  21  is made of a semiconductor mater (such as Si, GaAs, GaP, SiC, BN or their combinations) or a metal, such as Al, Cu, Ag, Au, Ni or their combinations. The material of the adhering layer  27  is silver paste, tin paste, tin-silver paste, a eutectic bonding material, metal or a conductive adhering material which is a material containing lead or containing no lead. 
     Step S 5  is followed by step S 51 . In step S 51 , the electroluminescent device is flipped for the subsequent step of removing the temporary substrate. 
     In step S 6 , the plate  20  is removed. That is, the temporary substrate for growing the LED element  24  is removed. In this embodiment, the step of flipping the electroluminescent device can be executed after step S 6  as well. 
     In step S 7 , the first contact electrode  25  and the second contact electrode  26  are formed at the side of the first semiconductor layer  241  and the side of the conduction substrate  21  so as to form electrical connections with the first semiconductor layer  241  and the second semiconductor layer  243 , respectively. When the first contact electrode  25  and the second contact electrode  26  are imposed with a voltage, the electrons and holes in the first semiconductor layer  241  and the second semiconductor layer  243  start to combine and release optical energy. Beside, a rough stricture, an anti-reflection layer  245  or a transparent conduction layer can be formed at the side of the first semiconductor layer  241  (i.e., the light-emitting surface) and on a position without the first contact electrode  25 . This helps guiding the lights out. 
     In summary, an electroluminescent device and a fabrication method thereof according to the present invention utilize a conduction substrate that connects the contact electrodes of the LED element disposed at opposite sides of the conduction substrate. In comparison with the prior art, because the etching process is avoided, the production processes are thus simplified and the yield is raised. In addition, the patterned transparent conduction layer with a plurality of island-like structures is formed by for example the etching method so that the electrical current imposed to the LED element can be uniformly distributed so as to prevent current clogging. Moreover, the deposition of the reflection layer has a good ohmic contact with the patterned transparent conduction layer and provides a scattering and reflection interface. It effectively raises external light extraction and light emitting efficiency. Also, since the conduction substrate and the reflection layer are highly thermally conductive, the heat generated from the LED element can be more effectively dissipated than the prior art. 
     Although the present invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the present invention.