Abstract:
A method for forming a transparent electrode on a visible light-emitting diode is described. A visible light-emitting diode element is provided, and the visible light-emitting diode element has a substrate, an epitaxial structure and a metal electrode. The metal electrode and the epitaxial structure are located on the same side of the substrate, or located respectively on the different sides of the substrate. An ohmic metal layer is formed on a surface of the epitaxial structure. The ohmic metal layer is annealed. The ohmic metal layer is removed to expose the surface of the epitaxial structure. A transparent electrode layer is formed on the exposed surface. A metal pad is formed on the transparent electrode layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/938,309 filed Sep. 9, 2004, which is incorporated herein in its entirety by this reference thereto. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a light-emitting diode (LED), and more particularly, to the method for manufacturing an improved electrode on a visible LED.  
       METHOD FOR BACKGROUND OF THE INVENTION  
       [0003]     An LED is a p-n junction diode that can emit ultraviolet, visible and infrared light. A visible LED is usually used as the light source of the operation panel for electric appliances such as, for example, the light source of a camera with an auto-focus function and the light source of a bar code reader.  
         [0004]     A visible LED is an LED that can emit visible light with a wavelength of 400 nm to 700 nm. A visible LED can be manufactured by utilizing III-V semiconductor materials having energy gaps within the range of 1.36 eV to 3.26 eY, such as GaP, Ga 1-x Al x As, GaN, and GaAs 1-y , P y .  
         [0005]     The brightness of a visible LED is the most important quality for use thereof. Some manufacturing steps can be performed to enhance the brightness of a visible LED, such that a transparent electrode layer is added before forming a metal electrode during the manufacturing process of a visible LED. Suitable materials for the transparent electrode layer are used, such as InO, SiO, ZnO, or ITO (Indium Tin Oxide). The transparent electrode layer can be used not only to form ohmic contact between the LED devices, but also to diffuse the electricity flowing therethrough to enhance the brightness of the visible LED. However, the conventional procedure for forming the transparent electrode layer, by depositing suitable materials directly on the LED element, forms a proper ohmic contact between the LED devices only with difficulty.  
         [0006]     To resolve the problem, some III-V semiconductor materials, such as GaAs, GaP, or GaAsP have been used to form a p-type ohmic contact film with heavy doping between the LED element and the transparent electrode layer to reduce the resistance of the transparent electrode layer. However, when GaAs is doped into the AlGaInP-based LED element to form the ohmic contact film, most of the visible light emitted from the LED element can be absorbed by the GaAs because the energy gap of the GaAs, about 1.35 eV, is less than 1.63 eV to 3.26 eV, which is the energy gap of visible light. Although using other materials avoids the energy gap problem, electric resistance still increases.  
         [0007]     On the other hand, a hybrid superlattice structure of the contact layer of the LED element has been used as an alternative way to reduce the electric resistance of the transparent electrode layer. However, the structure still absorbs visible light.  
         [0008]     A problem in this technique is that it can be difficult to fabricate a transparent electrode with low resistance that also does not absorb visible light.  
       SUMMARY OF THE INVENTION  
       [0009]     Therefore, the objective of the present invention is to provide a transparent electrode of a visible LED, where the transparent electrode has a low resistance and does not absorb visible light absorption. An improved manufacturing method is also provided to resolve the drawback of the prior art, and an improved visible LED with high brightness is thereby fabricated.  
         [0010]     According to the aforementioned objectives, the present invention provides a manufacturing method for fabricating an improved transparent electrode of a visible LED. The manufacturing method comprises several steps. First, an LED element is formed by a prior technique. Then, an ohmic metal layer is deposited on the LED element. Before removing the ohmic metal layer, a thermal annealing is performed on the ohmic metal layer, such that the ohmic metal ion can diffuse onto the surface of the LED element. An etching step is conducted for removing the ohmic metal layer. A transparent electrode layer is deposited onto the surface of the LED element. Finally, a metal pad is formed on the transparent electrode to complement an LED device.  
         [0011]     Accordingly, the problems of prior art can be overcome by reducing the resistance between the transparent electrode and the LED element, and through the present invention an improved visible LED device with a high degree of brightness can be obtained simultaneously. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawing, wherein:  
         [0013]      FIGS. 1-1C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the first embodiment of the present invention;  
         [0014]      FIGS. 2-2C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the second embodiment of the present invention;  
         [0015]      FIGS. 3-3C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the third embodiment of the present invention;  
         [0016]      FIGS. 4-4C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the fourth embodiment of the present invention;  
         [0017]      FIGS. 5-5C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the fifth embodiment of the present invention; and  
         [0018]      FIGS. 6-6C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the sixth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     The present invention discloses a manufacturing method for fabricating a transparent electrode of a visible LED.  
         [0020]     The present invention is characterized by the thermal diffusion of ohmic metal ions to the surface of the LED element to reduce the resistance between the transparent electrode and the LED element. In order to make the illustration of the present invention more explicit and complete, the following description is stated with reference to some preferred embodiments of the present invention.  
         [0021]     According to present invention, an LED element is formed by a prior technique. Then, an ohmic metal layer is deposited over the LED element. Before removing the ohmic metal layer, a thermal annealing step is performed on the ohmic metal layer, such that the ohmic metal ion can diffuse onto the surface of the LED element. An etching step is conducted for removing the ohmic metal layer. A transparent electrode layer is deposited onto the surface of the LED element. Finally, a metal pad is formed on the transparent electrode to complement an LED device.  
         [0022]     Prior techniques can be used for forming the LED element; for example, an epitaxial structure is grown on a substrate by metal organic chemical vapor deposition (MOCVD). The material of the epitaxial structure may be III-V semiconductor materials having energy gaps of 1.63 eV to 3.26 eV, such as GaP, Ga 1-x Al x As, GaN, and GaAs 1-y  P y .  
         [0023]      FIGS. 1-1C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the second embodiment of the present invention.  FIG. 1A  illustrates an AlGaInP element comprising a substrate  102  and an epitaxial structure  100  over the substrate  102 . Preferably, the substrate  102  is made of n-type GaAs. In the embodiment, the epitaxial structure  100  comprises a buffer layer, an n-type cladding layer, an active layer, and a p-type cladding layer, deposited in sequence. The preferable material of the buffer layer is n-type GaAs. The preferable material of the n-type cladding layer is n-type GaAs with a wider energy gap. The preferable material of the active layer is n-type GaAs with a narrower energy gap or n-type GaAs with multiple quantum wells (MQW). The preferable material of the buffer layer is n-type GaAs. The preferable material of the p-type cladding layer is p-type GaAs with a wider energy gap.  
         [0024]     The LED element further comprises a metal electrode  112 . The preferable metal material  112  of the electrode is Ti, Al, or GeAu alloy. In the first embodiment of present invention, the metal electrode  112  and the epitaxial structure  100  are formed respectively on opposite sides of the substrate  102 .  
         [0025]      FIG. 1A  illustrates a cross-sectional structure of the LED element after the ohmic metal layer  114  is formed over the epitaxial structure  100 . The process, such as thermal evaporation, electron enhanced evaporation, or sputtering deposition may be used to deposit ohmic metal over the epitaxial structure  100 . The preferred material of the ohmic metal may be PdIn, Zn, Ni, Au, or AuBe alloy. The preferred thickness of the ohmic metal layer  114  may be greater than 10 Å. In the first embodiment of present invention, the metal electrode  112  and the ohmic metal layer  114  are formed respectively on opposite sides of the epitaxial structure  100 .  
         [0026]     Then, a thermal annealing step is conducted on the ohmic metal layer, such that the ohmic metal ion can diffuse onto the surface of the epitaxial structure  100 . In the first embodiment of present invention, the annealing temperatures are, for example, 200 to 700° C., and more preferably to 300 to 500° C. After the annealing step, an etching step is conducted for removing the ohmic metal layer to form the structure illustrated in  FIG. 1B .  
         [0027]      FIG. 1C  illustrates a cross-sectional structure of the LED element after the transparent electrode layer  116  is formed over the epitaxial structure  100 . After removal of the ohmic metal layer  114 , a transparent electrode layer  116  is deposited over the epitaxial structure  100 . The transparent electrode layer  116  is a layer of conductive material, such as InO, CdSiO, ZnO, MgO, SiO, TiWN or ITO, and preferably ITO.  
         [0028]     Finally, a metal pad  118  is formed on the transparent electrode layer  116  to complement an LED device.  
         [0029]      FIGS. 2-2C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the second embodiment of the present invention.  FIG. 2  illustrates an AlGaInP element comprising a substrate  202  and an epitaxial structure  200  over the substrate  202 . In the second embodiment of present invention, the LED element further comprises a metal electrode  212  over a first surface of the epitaxial structure  200 . The preferable material of the metal  212  electrode is Ti, Al, or GeAu alloy; and the metal electrode  212  and the epitaxial structure  200  are formed respectively on the same side of the substrate  202 .  
         [0030]     Generally, the materials used to fabricate the AlGaInP-Based LED element are the same as those used for the AlGaInP-Based LED element described in the first embodiment of present invention, with the exception of the material of substrate  202 . The substrate  202  is made of sapphire rather than n-type GaAs. Furthermore, the structures of the first embodiment and the second embodiment are not the same. In the first embodiment of present invention, the metal electrode  112  and the epitaxial structure  100  are formed respectively on opposite sides of the substrate  102 . In contrast, in the second embodiment of present invention, the metal electrode  212  and the epitaxial structure  200  are formed respectively on the same side of the substrate  202 . In addition the metal electrode  212  is located at least on a portion of the surface of the first surface of epitaxial structure  200 .  
         [0031]     Referring to  FIG. 2 , the epitaxial structure  200  comprises a buffer layer, an n-type cladding layer, an active layer, and a p-type cladding layer, formed in sequence. The preferable material of the buffer layer is n-type GaAs. The preferable material of the n-type cladding layer is n-type GaAs with a wider energy gap. The preferable material of the active layer is n-type GaAs with a narrower energy gap or n-type GaAs with multiple quantum wells (MQW). The preferable material of the buffer layer is n-type GaAs. The preferable material of the p-type cladding layer is p-type GaAs with a wider energy gap.  
         [0032]      FIG. 2   a  illustrates a cross-sectional structure of the LED element after the ohmic metal layer  214  is formed over a second surface of the epitaxial structure  200 . A suitable process, such as thermal evaporation, electron enhanced evaporation, or sputtering deposition is used to deposit ohmic metal over the epitaxial structure  200 . The preferred material of the ohmic metal is PdIn, Zn, Ni, Au, or AuBe alloy. The preferred thickness of the ohmic metal layer  214  is greater than 10 Å. In the second embodiment of present invention, the metal electrode  212  and the ohmic metal layer  214  are formed respectively on the same side of the substrate  202 .  
         [0033]     Then, a thermal annealing step is conducted on the ohmic metal layer, such that the ohmic metal ions diffuse onto the second surface of the epitaxial structure  200 . In the second embodiment of present invention, the annealing temperatures are  200  to 700° C., and more preferably 300 to 500° C. After the annealing step, an etching step is conducted to remove the ohmic metal layer and form the structure illustrated in  FIG. 2B .  
         [0034]      FIG. 2C  illustrates a cross-sectional structure of the LED element after the transparent electrode layer  216  is formed over the second surface of the epitaxial structure  200 . After removing the ohmic metal layer  214 , a transparent electrode layer  216  is deposited over the second surface of the epitaxial structure  200 . The transparent electrode layer  216  is a layer of conductive materials, such as InO, CdSiO, ZnO, MgO, SiO, TiWN or ITO, and preferably ITO.  
         [0035]     Finally, a metal pad  218  is formed on the transparent electrode layer  216  to complement an LED device.  
         [0036]      FIGS. 3-3C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the third embodiment of the present invention.  FIG. 3  illustrates an AlInGaN-based LED element comprising a substrate  302  and an epitaxial structure  300  over the substrate  302 . In the embodiment, the substrate  302  is made of n-type GaAs. Generally, the structure of the AlInGaN-based LED element described in the third embodiment of present invention is the same as that of the AlGaInP-based LED described in the first embodiment. The materials of the epitaxial structure used in both embodiments are, however, quite different. In the first embodiment of present invention, the base material of the epitaxial structure  100  is AlGaInP, but in the third embodiment of present invention, the base material of the epitaxial structure  300  is AlInGaN.  
         [0037]     Referring to  FIG. 3 , the epitaxial structure  300  comprises a buffer layer, an n-type cladding layer, an active layer, and a p-type cladding layer deposited in sequence. The preferable material of the buffer layer is n-type GaAs. The preferable material of the n-type cladding layer is n-type GaAs with a wider energy gap. The preferable material of the active layer is n-type GaAs with a narrower energy gap or n-type GaAs with multiple quantum wells (MQW). The preferable material of the buffer layer is n-type GaAs. The preferable material of the p-type cladding layer is p-type GaAs with a wider energy gap.  
         [0038]     In the third embodiment of present invention, the LED element further comprises a metal electrode  312 . The preferred material of the metal electrode  312  is Ti, Al, or GeAu alloy; and the metal electrode  312  and the epitaxial structure  300  are formed respectively on opposite sides of the substrate  302 .  
         [0039]      FIG. 3A  illustrates a cross-sectional structure of the LED element after the ohmic metal layer  314  is formed over the epitaxial structure  300 . A suitable process, such as thermal evaporation, electron enhanced evaporation, or sputtering deposition is used to deposit ohmic metal over the epitaxial structure  300 . The preferred material of the ohmic metal is PdIn, Zn, Ni, Au, or AuBe alloy. The preferred thickness of the ohmic metal layer  314  is greater than 10 Å. In the second embodiment of present invention, the metal electrode  312  and the ohmic metal layer  314  are formed respectively on opposite sides of the epitaxial structure  300 .  
         [0040]     Then, a thermal annealing step is conducted on the ohmic metal layer, such that the ohmic metal ions diffuse onto the surface of the epitaxial structure  300 . In the third embodiment of present invention, the annealing temperatures are 200 to 700° C., and more preferably 300 to 500° C. After the annealing step, an etching step is conducted for removing the ohmic metal layer to form the structure illustrated in  FIG. 3   b.    
         [0041]      FIG. 3C  illustrates a cross-sectional structure of the LED element after the transparent electrode layer  316  is formed over the epitaxial structure  300 . After removing the ohmic metal layer  314 , a transparent electrode layer  316  is deposited over the epitaxial structure  300 . The transparent electrode layer  316  is a layer of conductive materials, such as InO, CdSiO, ZnO, MgO, SiO, TiWN or ITO, and preferably ITO.  
         [0042]     Finally, a metal pad  318  is formed on the transparent electrode layer  316  to complement an LED device.  
         [0043]      FIGS. 4-4C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the fourth embodiment of the present invention.  FIG. 4  illustrates an AlInGaN-based element comprising a substrate  402  and an epitaxial structure  400  over the substrate  402 . In the fourth embodiment of present invention, the LED element further comprises a metal electrode  412  formed over a first surface of the epitaxial structure  400 . The preferable material of the metal electrode  412  is Ti, Al, or GeAu alloy, and the metal electrode  412  and the epitaxial structure  400  are formed respectively on the same side of the substrate  402 .  
         [0044]     Generally, the materials used for the AlInGaN-based LED element described in the fourth embodiment of present invention are the same as those used for the AlInGaN-based LED element described in the third embodiment, with the exception of the material of substrate  402 . The substrate  402  is made of sapphire rather than n-type GaAs. Furthermore, the structures of the third embodiment and the fourth embodiment are not the same. In the third embodiment of present invention, the metal electrode  312  and the epitaxial structure  300  are formed respectively on opposite sides of the substrate  302 . In contrast, in the fourth embodiment of present invention, the metal electrode  412  and the epitaxial structure  400  are formed respectively on the same side of the substrate  402 . In addition the metal electrode  412  is at least located on portion of the first surface of the epitaxial structure  400 .  
         [0045]     Referring to  FIG. 4 , the epitaxial structure  400  comprises a buffer layer, an n-type cladding layer, an active layer, and a p-type cladding layer deposited in sequence. The preferable material of the buffer layer is n-type GaAs. The preferable material of the n-type cladding layer is n-type GaAs with a wider energy gap. The preferable material of the active layer is n-type GaAs with a narrower energy gap or n-type GaAs with multiple quantum wells (MQW). The preferable material of the buffer layer is n-type GaAs. The preferable material of the p-type cladding layer is p-type GaAs with a wider energy gap.  
         [0046]      FIG. 4A  illustrates a cross-sectional structure of the LED element after the ohmic metal layer  414  is formed over a second surface of the epitaxial structure  400 . A suitable process, such as thermal evaporation, electron enhanced evaporation, or sputtering deposition is used to deposit ohmic metal over the second surface of the epitaxial structure  400 . The preferred material of the ohmic metal is PdIn, Zn, Ni, Au, or AuBe alloy. The preferred thickness of the ohmic metal layer  414  is greater than 10 Å. In the fourth embodiment of present invention, the metal electrode  412  and the ohmic metal layer  414  are formed respectively on the same side of the substrate  402 .  
         [0047]     Then, a thermal annealing step is conducted on the ohmic metal layer, such that the ohmic metal ions diffuse onto the second surface of the epitaxial structure  400 . In the fourth embodiment of present invention, the annealing temperature is 200 to 700° C., and more preferably 300 to 500° C. After the annealing step, an etching step is conducted for removing the ohmic metal layer to form the structure illustrated in  FIG. 4B .  
         [0048]      FIG. 4C  illustrates a cross-sectional structure of the LED element after the transparent electrode layer  416  is formed over the second surface of the epitaxial structure  400 . After removing the ohmic metal layer  414 , a transparent electrode layer  416  is deposited over the second surface of the epitaxial structure  400 . The transparent electrode layer  416  is a layer of conductive material, such as InO, CdSiO, ZnO, MgO, SiO, TiWN or ITO, and preferably ITO.  
         [0049]     Finally, a metal pad  418  is formed on the transparent electrode layer  416  to complement an LED device.  
         [0050]     FIGS.  5  to  FIG. 5C  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the third embodiment of the present invention.  FIG. 5  illustrates an MgZnSSe-based LED element comprising a substrate  502  and an epitaxial structure  500  over the substrate  502 . In the embodiment, the substrate  502  is made of n-type GaAs. Generally, the epitaxial structure  500  used to build the MgZnSSe-based LED element described in the fifth embodiment of present invention is the same as the epitaxial structure  100  described in the first embodiment. But the materials of the epitaxial structure used in both embodiments are quite different. In the first embodiment of present invention, the base material of the epitaxial structure  100  is AlGaInP, but in the fifth embodiment of present invention, the base material of the epitaxial structure  500  is MgZnSSe.  
         [0051]     Referring to  FIG. 5 , the epitaxial structure  500  comprises a buffer layer, an n-type cladding layer, an active layer, and a p-type cladding layer, deposited in sequence. The preferable material of the buffer layer is n-type GaAs. The preferable material of the n-type cladding layer is n-type GaAs with a wider energy gap. The preferable material of the active layer is n-type GaAs with a narrower energy gap or n-type GaAs with multiple quantum wells (MQW). The preferable material of the buffer layer is n-type GaAs. The preferable material of the p-type cladding layer is p-type GaAs with a wider energy gap.  
         [0052]     In the fifth embodiment of present invention, the LED element further comprises a metal electrode  512 . The preferred material of the metal electrode  512  is Ti, Al, or GeAu alloy, and the metal electrode  512  and the epitaxial structure  500  are formed respectively on opposite sides of the substrate  502 .  
         [0053]      FIG. 5   a  illustrates a cross-sectional structure of the LED element after the ohmic metal layer  514  is formed over the epitaxial structure  500 . A suitable process, such as thermal evaporation, electron enhanced evaporation, or sputtering deposition is used to deposit ohmic metal over the epitaxial structure  500 . The preferred material of the ohmic metal may be PdIn, Zn, Ni, Au, or AuBe alloy. The preferred thickness of the ohmic metal layer  514  is greater than 10 Å. In the second embodiment of present invention, the metal electrode  512  and the ohmic metal layer  514  are formed respectively on opposite sides of the epitaxial structure  500 .  
         [0054]     Then, a thermal annealing step is conducted on the ohmic metal layer, such that the ohmic metal ions diffuse onto the surface of the epitaxial structure  500 . In the third embodiment of present invention, the annealing temperature is 200 to 700° C., and more preferably 300 to 500° C. After the annealing step, an etching step is conducted for removing the ohmic metal layer to form the structure illustrated in  FIG. 5B .  
         [0055]      FIG. 5C  illustrates a cross-sectional structure of the LED element after the transparent electrode layer  516  is formed over the epitaxial structure  500 . After removing the ohmic metal layer  514  a transparent electrode layer  516  is deposited over the epitaxial structure  500 . The transparent electrode layer  516  is a layer of conductive materials, such as InO, CdSiO, ZnO, MgO, SiO, TiWN or ITO, and preferably ITO.  
         [0056]     Finally, a metal pad  518  is formed on the transparent electrode layer  516  to complement an LED device.  
         [0057]     Referring to  FIG. 6  to  FIG. 6c  illustrate a series of cross-sectional structures showing the manufacturing processes for fabricating a transparent electrode of a visible LED in accordance with the fourth embodiment of the present invention.  FIG. 6  illustrates an MgZnSSe-based element comprising a substrate  602  and an epitaxial structure  600  over the substrate  602 . In the fourth embodiment of present invention, the LED element further comprises a metal electrode  612  formed over a first surface of the epitaxial structure  600 . The preferable material of the metal electrode  612  is Ti, Al, or GeAu alloy, and the metal electrode  612  and the epitaxial structure  600  are formed respectively on the same side of the substrate  602 .  
         [0058]     Generally, the material of the MgZnSSe-based LED element described in the sixth embodiment of present invention is the same as the MgZnSSe-Based LED element described in the fifth embodiment, with the exception of the material of substrate  602 . The substrate  602  is made of sapphire rather than n-type GaAs. Furthermore, the structures of the sixth embodiment and the fifth embodiment are not the same. In the fifth embodiment of present invention, the metal electrode  512  and the epitaxial structure  500  are formed respectively on opposite sides of the substrate  502 . In contrast, in the sixth embodiment of present invention, the metal electrode  612  and the epitaxial structure  600  are formed respectively on the same side of the substrate  602 . In addition, the metal electrode  612  is at least located on portion of the first surface of the epitaxial structure  600 .  
         [0059]     Referring to  FIG. 6 , the epitaxial structure  600  comprises a buffer layer, an n-type cladding layer, an active layer, and a p-type cladding layer, deposited in sequence. The preferable material of the buffer layer is n-type GaAs. The preferable material of the n-type cladding layer is n-type GaAs with a wider energy gap. The preferable material of the active layer is n-type GaAs with a narrower energy gap or n-type GaAs with multiple quantum wells (MQW). The preferable material of the buffer layer is n-type GaAs. The preferable material of the p-type cladding layer is p-type GaAs with a wider energy gap.  
         [0060]      FIG. 6A  illustrates a cross-sectional structure of the LED element after the ohmic metal layer  614  is formed over the second surface of the epitaxial structure  600 . A suitable process, such as thermal evaporation, electron enhanced evaporation, or sputtering deposition is used to deposit ohmic metal over the second surface of the epitaxial structure  600 . The preferred material of the ohmic metal is PdIn, Zn, Ni, Au, or AuBe alloy. The preferred thickness of the ohmic metal layer  614  is greater than 10 Å. In the sixth embodiment of present invention, the metal electrode  612  and the ohmic metal layer  614  are formed respectively on the same side of the substrate  602 .  
         [0061]     Then, a thermal annealing step is conducted on the ohmic metal layer, such that the ohmic metal ions diffuse onto the second surface of the epitaxial structure  600 . In the sixth embodiment of present invention, the annealing temperature is 200 to 700° C., and more preferably 300 to 500° C. After the annealing step, an etching step is conducted for removing the ohmic metal layer to form the structure illustrated in  FIG. 6B .  
         [0062]      FIG. 6C  illustrates a cross-sectional structure of the LED element after the transparent electrode layer  616  is formed over the second surface of the epitaxial structure  600 . After removing the ohmic metal layer  614 , a transparent electrode layer  616  is deposited over the second surface of the epitaxial structure  600 . The transparent electrode layer  616  is a layer of conductive materials, such as InO, CdSiO, ZnO, MgO, SiO, TiWN or ITO, and preferably ITO.  
         [0063]     Finally, a metal pad  618  is formed on the transparent electrode layer  616  to complement an LED device.  
         [0064]     Accordingly, the method provided by present invention utilized thermal annealing to diffuse ohmic metal ions onto the surface of the epitaxial structure, thereby reducing the resistance between the transparent electrode and the visible LED element. Additionally, the method avoids the problems of visible light absorption.  
         [0065]     As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrated of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements; for example the various structures known in the art and any materials within the range of the energy gap (1.36 eV to 3.26 eV) are included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.