Abstract:
A light-emitting element, a light-emitting element unit and a light-emitting element package are provided, which are each reduced in reflection loss and intra-film light absorption by suppressing multiple light reflection in a transparent electrode layer and hence have higher luminance. The light-emitting element  1  includes a substrate  2 , an n-type nitride semiconductor layer  3 , a light-emitting layer  4 , a p-type nitride semiconductor layer  5 , a transparent electrode layer  6  and a reflective electrode layer  7 , and the transparent electrode layer  6  has a thickness T satisfying the following expression (1): 
     
       
         
           
             
               
                 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.30 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   T 
                   ≤ 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.45 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     wherein λ is the light-emitting wavelength of the light-emitting element  4 , and n is the refractive index of the transparent electrode layer  6.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a Continuation of U.S. application Ser. No. 14/966,535, filed on Dec. 11, 2015, and allowed on May 1, 2017, which was a Continuation of U.S. application Ser. No. 14/147,639, filed on Jan. 6, 2014, and issued on Jan. 5, 2016 as U.S. Pat. No. 9,231,162, which was a Continuation of U.S. application Ser. No. 13/350,013, filed on Jan. 13, 2012, and issued on Feb. 18, 2014 as U.S. Pat. No. 8,653,551. Furthermore, this application claims the benefit of priority of Japanese Patent Application No. 2011-006065, filed on Jan. 14, 2011. The disclosures of these prior U.S. and foreign applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a light-emitting element, a light-emitting element unit including the light-emitting element, and a light-emitting element package including the light-emitting element unit and a resin package encapsulating the light-emitting element unit. 
       Description of the Related Art 
       [0003]    One of prior-art semiconductor light-emitting elements is disclosed, for example, in JP-2008-263130A. 
         [0004]    A semiconductor light-emitting element disclosed in FIG. 4 of JP-2008-263130A includes a sapphire substrate through which light is extracted, a light-emitting layer, a transparent electrode and a reflective metal layer. 
         [0005]    When the light-emitting layer emits light, the light is mostly extracted through the sapphire substrate, but partly passes through the transparent electrode to be reflected on the reflective metal layer, and further passes through the transparent electrode to be extracted through the sapphire substrate. 
         [0006]    In the prior-art semiconductor light-emitting element, therefore, the light emitted from the light-emitting layer is extracted at a lower light extracting efficiency, making it difficult to provide a sufficient luminance. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore a principal object of the present invention to provide a light-emitting element having a higher luminance. 
         [0008]    According to the present invention, there is provided a light-emitting element having a front surface and a back surface and adapted to extract light from the back surface with the front surface facing down, the light-emitting element including: a substrate transparent to a light-emitting wavelength λ and defining the back surface; an n-type nitride semiconductor layer provided on the substrate; a light-emitting layer which is provided on the n-type nitride semiconductor layer and emits light at the light-emitting wavelength λ; a p-type nitride semiconductor layer provided on the light-emitting layer; a transparent electrode layer which is provided on the p-type nitride semiconductor layer and is transparent to the light-emitting wavelength λ of the light-emitting layer; and a reflective electrode layer which is provided on the transparent electrode layer and reflects light passing through the transparent electrode layer toward the back surface; wherein the transparent electrode layer has a thickness T satisfying the following expression (1): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.30 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   T 
                   ≤ 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.45 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein λ is the light-emitting wavelength of the light-emitting element, and n is the refractive index of the transparent electrode layer. 
         [0009]    The term “light-emitting wavelength λ” means the peak wavelength of the spectrum of the light emitted from the light-emitting layer. The expression “transparent to the light-emitting wavelength λ” means that the transmittance at the light-emitting wavelength λ is, for example, not less than 60%. 
         [0010]    It is generally considered that, if the thickness of the transparent electrode layer is equal to an integer multiple of λ/4n (wherein λ is the light-emitting wavelength and n is the refractive index of the transparent electrode layer), the reflectance of an interface between the transparent electrode layer and the p-type nitride semiconductor layer can be reduced. Therefore, it is a conventional practice to optically design the transparent electrode layer so that the thickness thereof is equal to an integer multiple of λ/4n. The design is based on a precondition that the light reflected on the reflective electrode layer includes only a perpendicular light component that is incident perpendicularly on the transparent electrode layer. 
         [0011]    In reality, however, the light reflected on the reflective electrode layer includes not only the perpendicular light component but also an oblique light component that is incident obliquely on the transparent electrode layer. Therefore, even if the transparent electrode layer is designed to have a thickness equal to an integer multiple of λ/4n, the oblique light component is reflected on the interface between the transparent electrode layer and the p-type nitride semiconductor layer, and the reflected light is further reflected in the transparent electrode layer multiple times. That is, there is a limitation in improving the light extracting efficiency simply by designing the transparent electrode layer so that the thickness of the transparent electrode layer is equal to an integer multiple of λ/4n. 
         [0012]    According to the present invention, the thickness T of the transparent electrode layer satisfies the above expression (1). Therefore, the reflection of the light on the interface between the transparent electrode layer and the p-type nitride semiconductor layer can be reduced as compared with the case in which the transparent electrode layer is designed to have a thickness equal to an integer multiple of λ/4n. As a result, multiple light reflection in the transparent electrode layer can be reduced to improve the light extracting efficiency. Thus, the light-emitting element has a higher luminance. 
         [0013]    The transparent electrode layer preferably includes a first electrode sublayer provided on the p-type nitride semiconductor layer in contact with the p-type nitride semiconductor layer and having a first thickness t 1 , and a second electrode sublayer provided on the first electrode sublayer and having a second thickness t 2  that is greater than the first thickness t 1 . 
         [0014]    The transparent electrode layer is formed, for example, by depositing an electrode material imparted with a predetermined level of energy on the p-type nitride semiconductor layer. Therefore, an excessively high energy level is disadvantageous, because the p-type nitride semiconductor layer is likely to be significantly damaged. 
         [0015]    The damage to the p-type nitride semiconductor layer can be reduced by depositing an electrode material at a lower energy level to form the first electrode sublayer in contact with the p-type nitride semiconductor layer. If the transparent electrode layer included only the first electrode sublayer formed by the deposition at the lower energy level, however, the transparent electrode layer would have a poorer quality with a lower density. Therefore, the first electrode sublayer is first formed as having a smaller thickness t 1  and then the second electrode sublayer is formed as having a thickness t 2  by depositing an electrode material at a higher energy level to adjust the overall thickness T of the transparent electrode layer, whereby the quality of the entire transparent electrode layer can be improved. 
         [0016]    The first electrode sublayer and the second electrode sublayer may have different light absorbances. 
         [0017]    The transparent electrode layer preferably comprises ITO or ZnO. The light-emitting wavelength λ is preferably 450 nm. 
         [0018]    The reflective electrode layer preferably comprises an alloy comprising silver, a platinum group metal and copper, and is preferably provided on the transparent electrode layer in contact with the transparent electrode layer. 
         [0019]    The interface between the transparent electrode layer and the reflective electrode layer of the alloy comprising silver, the platinum group metal and copper has a proper light reflecting ability, which is comparable to that of an interface between the reflective electrode layer and an insulative layer which may be provided between the transparent electrode layer and the reflective electrode layer. 
         [0020]    Since the reflective electrode layer is provided on the transparent electrode layer in contact with the transparent electrode layer, heat generated by the emission of the light from the light-emitting layer is conducted directly to the reflective electrode layer from the transparent electrode layer to be thereby efficiently released from the reflective electrode layer to the outside of the light-emitting element. 
         [0021]    This improves the heat releasing efficiency and the light extracting efficiency. 
         [0022]    The reflective electrode layer is preferably provided in the same pattern as the transparent electrode layer on the transparent electrode layer, and a surface of the reflective electrode layer opposed to the transparent electrode layer is preferably entirely kept in contact with the transparent electrode layer. 
         [0023]    With this arrangement, the reflective electrode layer, the transparent electrode layer and contact portions of these layers completely overlap each other as seen in a stacking direction (in a substrate thicknesswise direction) in which these layers are stacked. That is, the reflective electrode layer and the transparent electrode layer have no non-overlapping portions and, therefore, are free from irregularities which may otherwise occur due to the non-overlapping portions. 
         [0024]    Thus, the light emitted from the light-emitting layer and passing through the transparent electrode layer can be reliably incident on the interface between the transparent electrode layer and the reflective electrode layer, and efficiently reflected on the interface. The reflective electrode layer and the transparent electrode layer have the same size as seen in the stacking direction, so that the interface (reflective surface) between the transparent electrode layer and the reflective electrode layer has the greatest possible size. Thus, the light passing through the transparent electrode layer can be efficiently reflected on the interface. This further improves the light extracting efficiency. 
         [0025]    In the absence of the irregularities described above, a connection surface of the light-emitting element to be connected to an external wiring element is flat, and a connection area between the light-emitting element and the wiring element can be increased. This further improves the efficiency of heat release from the light-emitting element to the wiring element. 
         [0026]    In the light-emitting element, no insulative layer is preferably provided between the transparent electrode layer and the reflective electrode layer. 
         [0027]    According to this arrangement, the reflective electrode layer of the alloy permits proper light reflection on the interface between the reflective electrode layer and the transparent electrode layer even without the provision of the insulative layer. In the absence of the insulative layer, reduction in heat releasing efficiency and light extracting efficiency can be prevented which may otherwise occur due to the insulative layer. 
         [0028]    The platinum group metal is preferably platinum or palladium. The substrate preferably comprises sapphire, GaN or SiC. 
         [0029]    The substrate preferably has a plurality of projections provided on a contact surface thereof kept in contact with the n-type nitride semiconductor layer. 
         [0030]    With this arrangement, light traveling from the n-type nitride semiconductor layer toward the substrate is substantially prevented from being reflected on the contact surface toward the n-type nitride semiconductor layer. This correspondingly improves the light extracting efficiency. 
         [0031]    The projections are preferably discretely arranged. In this case, the projections may be arranged in a matrix array, or may be arranged in staggered relation. 
         [0032]    The light-emitting layer preferably comprises In. 
         [0033]    The light-emitting element preferably further includes a connection layer comprising silver, solder or AuSn and provided on the reflective electrode layer. This makes it possible to provide a light-emitting element unit which includes the light-emitting element and a wiring element connected to the connection layer to apply a voltage from the wiring element to the light-emitting element. 
         [0034]    The light-emitting element preferably further includes an n-type electrode layer which connects the wiring element to the n-type nitride semiconductor layer. With this arrangement, the light is emitted from the light-emitting layer by applying the voltage between the n-type electrode layer and the reflective electrode layer. 
         [0035]    The light-emitting element preferably further includes an isolation insulative layer which isolates and insulates the n-type electrode layer and the reflective electrode layer from each other. 
         [0036]    A light-emitting element package is also provided, which includes the light-emitting element unit and a resin package encapsulating the light-emitting element unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a schematic sectional view of a light-emitting element according to one embodiment of the present invention. 
           [0038]      FIG. 2  is a schematic plan view of the light-emitting element. 
           [0039]      FIG. 3  is a schematic perspective view of the light-emitting element. 
           [0040]      FIG. 4A  is a schematic perspective view showing an exemplary structure of a substrate. 
           [0041]      FIG. 4B  is a schematic perspective view showing another exemplary structure of the substrate. 
           [0042]      FIG. 5A  is a schematic sectional view showing a method of producing the light-emitting element shown in  FIG. 1 . 
           [0043]      FIG. 5B  is a schematic sectional view showing a step subsequent to the step shown in  FIG. 5A . 
           [0044]      FIG. 5C  is a schematic sectional view showing a step subsequent to the step shown in  FIG. 5B . 
           [0045]      FIG. 5D  is a schematic sectional view showing a step subsequent to the step shown in  FIG. 5C . 
           [0046]      FIG. 5E  is a schematic sectional view showing a step subsequent to the step shown in  FIG. 5D . 
           [0047]      FIG. 5F  is a schematic sectional view showing a step subsequent to the step shown in  FIG. 5E . 
           [0048]      FIG. 5G  is a schematic sectional view showing a step subsequent to the step shown in  FIG. 5F . 
           [0049]      FIG. 6  is a schematic sectional view of a wiring element. 
           [0050]      FIG. 7  is a schematic plan view of the wiring element. 
           [0051]      FIG. 8  is a schematic sectional view of a light-emitting element package. 
           [0052]      FIG. 9  is a schematic plan view of the wiring element, illustrating the connection state of the wiring element and the light-emitting element. 
           [0053]      FIG. 10  is a schematic perspective view of the light-emitting element package. 
           [0054]      FIG. 11  is a graph showing a relationship between the luminance change ratio of the light-emitting element and the thickness of an ITO film. 
           [0055]      FIG. 12  is a diagram showing a variation of the light-emitting element shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0056]    An embodiment of the present invention will hereinafter be described in detail with reference to the attached drawings. 
         [0057]      FIG. 1  is a schematic sectional view of a light-emitting element according to one embodiment of the present invention.  FIG. 2  is a schematic plan view of the light-emitting element.  FIG. 3  is a schematic perspective view of the light-emitting element. 
         [0058]    The light-emitting element  1  includes a substrate  2 , an n-type nitride semiconductor layer  3 , a light-emitting layer  4 , a p-type nitride semiconductor layer  5 , a transparent electrode layer  6 , a reflective electrode layer  7 , an n-type electrode layer  8 , an isolation insulative layer  9  and connection layers  10 . 
         [0059]    The n-type nitride semiconductor layer  3 , the light-emitting layer  4 , the p-type nitride semiconductor layer  5 , the transparent electrode layer  6 , the reflective electrode layer  7 , the n-type electrode layer  8 , the isolation insulative layer  9  and the connection layers  10  are provided on the substrate  2 . 
         [0060]    The substrate  2  is made of a material (e.g., sapphire, GaN or SiC) transparent to a light-emitting wavelength λ (e.g., 450 nm) of the light-emitting layer  4  at which light is emitted from the light-emitting layer  4 . The substrate  2  has a thickness of, for example, 400 μm. The substrate  2  has a back surface  2 A defined by a lower surface thereof in  FIG. 1 , and a front surface  2 B defined by an upper surface thereof in  FIG. 1 . The back surface  2 A serves as a light extracting surface from which the light is extracted, and defines a back surface of the light-emitting element  1 . The front surface  2 B serves as a contact surface of the substrate  2  kept in contact with the n-type nitride semiconductor layer  3 . 
         [0061]    The n-type nitride semiconductor layer  3  is provided on the substrate  2 . The n-type nitride semiconductor layer  3  covers the entire front surface  2 B of the substrate  2 . The n-type nitride semiconductor layer  3  is made of n-type GaN, and is transparent to the light-emitting wavelength λ of the light-emitting layer  4 . The n-type nitride semiconductor layer  3  has a back surface  3 A defined by a lower surface thereof covering the front surface  2 B of the substrate  2  in  FIG. 1 , and a front surface  3 B defined by an upper surface thereof opposite from the back surface  3 A in  FIG. 1 . A left portion of the front surface  3 B in  FIG. 1  projects, so that the front surface  3 B has a step. 
         [0062]    The light-emitting layer  4  is provided on the n-type nitride semiconductor layer  3 . The light-emitting layer  4  is formed by a dry etching method. The light-emitting layer  4  covers the left projecting portion of the front surface  3 B of the n-type nitride semiconductor layer  3  in  FIG. 1 . The light-emitting layer  4  is made of an In-containing nitride semiconductor (e.g., InGaN). The light-emitting layer  4  has a back surface  4 A defined by a lower surface thereof covering the left portion of the front surface  3 B of the n-type nitride semiconductor layer  3  in  FIG. 1 , and a front surface  4 B defined by an upper surface thereof opposite from the back surface  4 A in  FIG. 1 . 
         [0063]    The p-type nitride semiconductor layer  5  is provided on the light-emitting layer  4 . The p-type nitride semiconductor layer  5  covers the entire front surface  4 B of the light-emitting layer  4 . The p-type nitride semiconductor layer  5  is formed together with the light-emitting layer  4  by the dry etching method. The p-type nitride semiconductor layer  5  is made of p-type GaN, and is transparent to the light-emitting wavelength λ of the light-emitting layer  4 . The total thickness of the n-type nitride semiconductor layer  3 , the light-emitting layer  4  and the p-type nitride semiconductor layer  5  is, for example, 6.5 μm at a maximum. The p-type nitride semiconductor layer  5  has a back surface  5 A defined by a lower surface thereof covering the front surface  4 B of the light-emitting layer  4  in  FIG. 1 , and a front surface  5 B defined by an upper surface thereof opposite from the back surface  5 A in  FIG. 1 . 
         [0064]    The transparent electrode layer  6  is provided on the p-type nitride semiconductor layer  5 . The transparent electrode layer  6  covers substantially the entire front surface  5 B of the p-type nitride semiconductor layer  5 . The transparent electrode layer  6  is formed, for example, by a lift-off method. The transparent electrode layer  6  is made of ZnO (zinc oxide) or ITO (indium tin oxide), and is transparent to the light-emitting wavelength λ of the light-emitting layer  4 . In this embodiment, the transparent electrode layer  6  is made of ITO. The transparent electrode layer  6  has a thickness T which satisfies the following expression (1): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.30 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                   ≤ 
                   T 
                   ≤ 
                   
                     
                       
                         3 
                          
                         λ 
                       
                       
                         4 
                          
                         n 
                       
                     
                     + 
                     
                       0.45 
                       × 
                       
                         ( 
                         
                           λ 
                           
                             4 
                              
                             n 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein λ is the light-emitting wavelength of the light-emitting layer  4 , and n is the refractive index of the transparent electrode layer  6 . 
         [0065]    For example, ITO has a refractive index n of 2.0 and, where the light-emitting wavelength λ of the light-emitting layer  4  is 450 nm, the thickness T of the transparent electrode layer  6  is about 1850 Å to about 1950 Å. 
         [0066]    The transparent electrode layer  6  has a back surface  6 A defined by a lower surface thereof covering the front surface  5 B of the p-type nitride semiconductor layer  5  in  FIG. 1 , and a front surface  6 B defined by an upper surface thereof opposite from the back surface  6 A in  FIG. 1 . 
         [0067]    The reflective electrode layer  7  is provided in the same pattern as the transparent electrode layer  6  on the transparent electrode layer  6 . The reflective electrode layer  7  covers the entire front surface  6 B of the transparent electrode layer  6  so as not to protrude from the front surface  6 B in  FIG. 1 . The reflective electrode layer  7  has a back surface  7 A defined by a lower surface thereof covering the front surface  6 B of the transparent electrode layer  6  in  FIG. 1 , and a front surface  7 B defined by an upper surface thereof opposite from the back surface  7 A. The back surface  7 A of the reflective electrode layer  7  serves as an opposed surface which is opposed to the front surface  6 B of the transparent electrode layer  6 , and is entirely kept in contact (surface contact) with the front surface  6 B of the transparent electrode layer  6 . Therefore, nothing (e.g., no insulative layer) is present between the transparent electrode layer  6  and the reflective electrode layer  7 . 
         [0068]    In this embodiment, the reflective electrode layer  7  is made of an alloy containing silver, a platinum group metal and copper. Usable examples of the platinum group metal include platinum and palladium. The proportions of silver, the platinum group metal and copper in the alloy are about 98%, about 1% and about 1%, respectively. The reflective electrode layer  7  typically has a thickness of, for example, 50 nm to 500 nm, preferably 350 nm. 
         [0069]    The n-type electrode layer  8  is provided on a right portion of the front surface  3 B of the n-type nitride semiconductor layer  3  in  FIG. 1 . The n-type electrode layer  8  is provided on a portion of the front surface  3 B of the n-type nitride semiconductor layer  3  which is recessed toward the rear surface  3 A to form the step described above. The n-type electrode layer  8  is made of Al and Cr. In this embodiment, the n-type electrode layer  8  is formed by forming an Al sublayer in contact with the n-type nitride semiconductor layer  3  and then forming a Cr sublayer on the Al sublayer. The n-type electrode layer  8  has a thickness of, for example, about 26000 Å. The n-type electrode layer  8  has a back surface  8 A defined by a surface thereof in contact with the front surface  3 B of the n-type nitride semiconductor layer  3  in  FIG. 1 , and a front surface  8 B defined by a surface thereof opposite from the back surface  8 A. 
         [0070]    Side surfaces of the n-type nitride semiconductor layer  3 , the light-emitting layer  4 , the p-type nitride semiconductor layer  5 , the transparent electrode layer  6  and the reflective electrode layer  7  are covered with the isolation insulative layer  9  which is made of, for example, SiO 2 . Thus, the light-emitting layer  4 , the p-type nitride semiconductor layer  5 , the transparent electrode layer  6  and the reflective electrode layer  7  are isolated and insulated from the n-type electrode layer  8 . Sin, AN, Al 2 O 3  or SiON may be used instead of SiO 2  as a material for the isolation insulative layer  9 . The isolation insulative layer  9  has a thickness of 500 Å to 50000 Å, for example, 1000 Å. In  FIGS. 2 and 3 , the isolation insulative layer  9  is not shown. 
         [0071]    The connection layers  10  are respectively provided on the reflective electrode layer  7  and the n-type electrode layer  8 . The connection layers  10  each comprise, for example, Ag, Ti or Pt, or an alloy of any of these metals. The connection layers  10  may each comprise solder or AuSn. The connection layers  10  may each include a Pt sublayer for suppression of diffusion of the connection layer materials from the connection layers  10  to the reflective electrode layer  7  and the n-type electrode layer  8 . In this embodiment, the connection layers  10  each include a Ti sublayer, a Pt sublayer and an AuSn sublayer stacked in this order from the reflective electrode layer  7  and the n-type electrode layer  8 . The connection layers  10  each have a back surface  10 A defined by a lower surface thereof in contact with the reflective electrode layer  7  or the n-type electrode layer  8  in  FIG. 1 , and a front surface  10 B defined by an upper surface thereof opposite from the back surface  10 A in  FIG. 1 . The front surfaces  10 B of the connection layers  10  serve as a front surface of the light-emitting element  1 . 
         [0072]    The front surface  10 B of the connection layer  10  contacting the reflective electrode layer  7  serves as a p-type electrode portion  12 , and the front surface  10 B of the connection layer  10  contacting the n-type electrode layer  8  serves as an n-type electrode portion  13 . The p-type electrode portion  12  and the n-type electrode portion  13  respectively have flat surfaces, which are located at the same height level to be flush with each other (also see  FIG. 3 ). Since the reflective electrode layer  7  and the n-type electrode layer  8  are isolated and insulated from each other by the isolation insulative layer  9  as described above, the p-type electrode portion  12  of the connection layer  10  on the reflective electrode layer  7  and the n-type electrode portion  13  of the connection layer  10  on the n-type electrode layer  8  are isolated and insulated from each other by the isolation insulative layer  9 . 
         [0073]    As seen in plan, the p-type nitride semiconductor layer  5 , the transparent electrode layer  6 , the reflective electrode layer  7  and the p-type electrode portion  12  of the connection layer  10  each have, for example, a generally E-shape, and the n-type electrode layer  8  and the n-type electrode portion  13  of the connection layer  10  each have a generally I-shape (see  FIG. 2 ). The n-type electrode layer  8  has two extension portions  8 C which respectively extend into two spaces defined by the E-shapes of the p-type nitride semiconductor layer  5 , the transparent electrode layer  6 , the reflective electrode layer  7  and the connection layer  10  (p-type electrode portion  12 ) (see  FIGS. 2 and 3 ). 
         [0074]    When a forward voltage is applied between the p-type electrode portion  12  and the n-type electrode portion  13 , the light is emitted from the light-emitting layer  4  at a light-emitting wavelength λ of 440 nm to 460 nm in the light-emitting element  1 . The light passes through the n-type nitride semiconductor layer  3  and the substrate  2  in this order to be extracted from the back surface  2 A of the substrate  2 . Light traveling from the light-emitting layer  4  toward the p-type nitride semiconductor layer  5  passes through the p-type nitride semiconductor layer  5  and the transparent electrode layer  6  in this order to be reflected on an interface between the transparent electrode layer  6  and the reflective electrode layer  7 . The reflected light passes through the transparent electrode layer  6 , the p-type nitride semiconductor layer  5 , the light-emitting layer  4 , the n-type nitride semiconductor layer  3  and the substrate  2  in this order to be extracted from the back surface  2 A of the substrate  2 . 
         [0075]    A plurality of projections  11  are provided on the front surface  2 B of the substrate  2  as projecting toward the n-type nitride semiconductor layer  3 . 
         [0076]      FIGS. 4A and 4B  are schematic perspective views showing exemplary structures of the substrate. 
         [0077]    The projections  11  are discretely arranged. More specifically, the projections  11  may be spaced from each other to be arranged in a matrix array (see  FIG. 4A ), or may be arranged in staggered relation (see  FIG. 4B ). The projections  11  are each made of SiN. 
         [0078]    With the provision of the projections  11  of SiN, light rays incident at different angles are substantially prevented from being totally reflected on the front surface  2 B of the substrate  2 . Thus, light rays traveling from the n-type nitride semiconductor layer  3  toward the substrate  2  are substantially prevented from being reflected on the interface between the n-type nitride semiconductor layer  3  and the substrate  2  toward the n-type nitride semiconductor layer  3 . This improves the light extracting efficiency. 
         [0079]      FIGS. 5A to 5H  are schematic sectional views showing a method of producing the light-emitting element shown in  FIG. 1 . 
         [0080]    First, a substrate  2  is prepared as shown in  FIG. 5A . 
         [0081]    Then, a layer of SiN (SiN layer) is formed on a front surface  2 B of the substrate  2 . The SiN layer is etched with the use of a resist pattern (not shown) as a mask to be thereby divided into a plurality of projections  11  as shown in  FIG. 5B . 
         [0082]    In turn, a layer of n-type GaN (n-GaN layer) is formed over the front surface  2 B of the substrate  2 . The n-GaN layer serves as an n-type nitride semiconductor layer  3  on the substrate  2 , and covers all the projections  11 . 
         [0083]    Subsequently, as shown in  FIG. 5C , an In-containing nitride semiconductor layer (e.g., In x Ga 1-x N layer) is formed on a front surface  3 B of the n-type nitride semiconductor layer  3 . This layer serves as a light-emitting layer  4  on the n-type nitride semiconductor layer  3 . The light-emitting wavelength λ of the light-emitting layer  4  is adjusted to 440 nm to 460 nm by controlling the composition ratio of In and Ga. 
         [0084]    Then, a layer of p-type GaN (p-GaN layer) is formed as a p-type nitride semiconductor layer  5  on a front surface  4 B of the light-emitting layer  4 . A p-AlGaN layer containing Al or a layered structure including a p-GaN sublayer and a p-AlGaN sublayer may be employed as the p-type nitride semiconductor layer  5 . 
         [0085]    In turn, a resist pattern (not shown) having an opening in a region to be formed with a transparent electrode layer  6  is formed on the p-type nitride semiconductor layer  5 . Subsequently, an ITO material is deposited on the p-type nitride semiconductor layer  5  via the resist pattern, for example, by a sputtering method. Then, an unnecessary portion of the ITO material is lifted off together with the resist pattern. Thus, a layer of ITO (ITO layer) is formed on a selected portion of a front surface  5 B of the p-type nitride semiconductor layer  5  as shown in  FIG. 5D . The ITO layer serves as a transparent electrode layer  6 . In the formation of the transparent electrode layer  6 , a heat treatment may be performed to improve electrical connection and adhesion between the p-type nitride semiconductor layer  5  and the transparent electrode layer  6 . In this case, the heat treatment is performed, for example, at a temperature of 500° C. to 700° C. 
         [0086]    Subsequently, a layer of an alloy containing silver, a platinum group metal and copper (alloy layer) is formed over the front surface  6 B of the transparent electrode layer  6  and the front surface  5 B of the p-type nitride semiconductor layer  5 , and etched with the use of a resist pattern (not shown) as a mask, whereby a reflective electrode layer  7  is formed in the same pattern as the transparent electrode layer  6  on the transparent electrode layer  6  as shown in  FIG. 5D . 
         [0087]    Then, parts of the p-type nitride semiconductor layer  5 , the light-emitting layer  4  and the n-type nitride semiconductor layer  3  are selectively etched off as shown in  FIG. 5E  with the use of a resist pattern (not shown) as a mask. 
         [0088]    In turn, as shown in  FIG. 5F , an n-type electrode layer  8  is formed on the front surface  3 B of the n-type nitride semiconductor layer  3  by a lift-off method using a resist pattern (not shown). The n-type electrode layer  8  may be made of Al, or may have a layered structure including a Ti sublayer and an Al sublayer. In the formation of the n-type electrode layer  8 , a heat treatment may be performed for improvement of adhesion and electrical connection between the n-type electrode layer  8  and the n-type nitride semiconductor layer  3 . 
         [0089]    Subsequently, as shown in  FIG. 5G , an isolation insulative layer  9  of SiO 2  is formed. The isolation insulative layer  9  is formed so as to cover a part of the front surface  3 B of the n-type nitride semiconductor layer  3  to be located adjacent a p-type electrode portion  12  (see  FIG. 1 ), side surfaces of the light-emitting layer  4 , the p-type nitride semiconductor layer  5 , the transparent electrode layer  6  and the reflective electrode layer  7 , and a part of a front surface  7 B of the reflective electrode layer  7 . The formation of the isolation insulative layer  9  is achieved by a lift-off method using a resist pattern (not shown) or an etch-off method. SiN, AlN, Al 2 O 3  or SiON may be used instead of SiO 2  as the material for the isolation insulative layer  9 . 
         [0090]    Then, connection layers  10  are formed on the front surface  7 B of the reflective electrode layer  7  and the front surface  8 B of the n-type electrode layer  8  by a lift-off method using a resist pattern (not shown). In this embodiment, the connection layers  10  each include a sublayer of AuSn (AuSn sublayer). The connection layers  10  each further include a Pt sublayer for protection of the reflective electrode layer  7  and the n-type electrode layer  8  from diffusion of AuSn from the AuSn sublayers. The connection layers  10  each further include a Ti sublayer for improvement adhesion between the connection layer  10  and the reflective electrode layer  7  and between the connection layer  10  and the n-type electrode layer  8 . In this embodiment, therefore, the Ti sublayer, the Pt sublayer and the AuSn sublayer are stacked in this order on each of the reflective electrode layer  7  and the n-type electrode layer  8  to form the connection layer  10 . 
         [0091]    A structure shown in  FIG. 1  is formed by the process sequence described above. 
         [0092]    In practice, the process sequence described above with reference to  FIGS. 5A to 5G  is performed as a semiconductor wafer process for forming a plurality of light-emitting elements  1  each having the structure shown in  FIG. 1  on a wafer. After this process, the thickness of the wafer is adjusted to, for example, 300 μm by a grinding/polishing process. Then, the wafer is divided into the plurality of light-emitting elements  1  (chips) by an element isolating process (scribing/breaking process). 
         [0093]      FIG. 6  is a schematic sectional view of a wiring element. The light-emitting element  1  is connected to the wiring element  20  via the connection layers  10  thereof. 
         [0094]    Referring to  FIG. 6 , the wiring element  20  includes a base substrate  21 , an insulative layer  22 , electrode layers  23  and connection layers  24 . 
         [0095]    The base substrate  21  is made of, for example, Si, and has a thickness of, for example, 130 μm. The insulative layer  22  is made of, for example, SiO 2 , and covers the entire front surface (upper surface in  FIG. 6 ) of the base substrate  21 . The insulative layer  22  has a thickness of, for example, 1000 Å. A back surface (lower surface in  FIG. 6 ) of the base substrate  21  defines a back surface of the wiring element  20 . 
         [0096]    The electrode layers  23  are made of, for example, Al, and each have a thickness of, for example, 25000 Å. Two electrode layers  23  are provided in two positions on the insulative layer  22  in laterally spaced relation in  FIG. 6 . 
         [0097]    The connection layers  24  are respectively provided on the electrode layers  23 . The connection layers  24  each have a double layered structure including a Ti sublayer  25  provided closer to the base substrate  21  and an Au sublayer  26  provided on the Ti sublayer  25 . The Ti sublayer  25  is made of Ti, and has a thickness of, for example, 1000 Å. The Au sublayer  26  is made of Au, and has a thickness of, for example, 10000 Å. The connection layers  24  each have a front surface  24 A defined by a surface (upper surface in  FIG. 6 ) thereof opposite from a surface contacting the electrode layer  23 . The front surfaces  24 A of the connection layers  24  are flat, and define a front surface of the wiring element  20 . 
         [0098]      FIG. 7  is a schematic plan view of the wiring element. 
         [0099]    As seen in plan in  FIG. 7 , the connection layers  24  include a generally E-shaped connection layer conformal to the p-type electrode portion  12  and a generally I-shaped connection layer conformal to the n-type electrode portion  13  in combination (also see  FIG. 2 ). 
         [0100]      FIG. 8  is a schematic sectional view of a light-emitting element package.  FIG. 9  is a schematic plan view of the wiring element, illustrating the connection state of the wiring element and the light-emitting element.  FIG. 10  is a schematic perspective view of the light-emitting element package. The light-emitting package  50  includes the light-emitting element unit  30 , a support board  31  and a resin package  40 . 
         [0101]    As shown in  FIG. 6 , the wiring element  20  is placed with the front surfaces  24 A of the connection layers  24  thereof facing up. The light-emitting element  1  shown in  FIG. 1  is held with the front surfaces  10 B (the p-type electrode portion  12  and the n-type electrode portion  13 ) of the connection layers  10  thereof facing down (in an attitude turned upside down from that shown in  FIG. 1 ), and brought into opposed relation to the wiring element  20  assuming an attitude of  FIG. 6  from the above. 
         [0102]    By moving the light-emitting element  1  toward the wiring element  20 , the front surfaces  10 B of the connection layers  10  are brought into surface contact with the front surfaces  24 A of the connection layers  24  as shown in  FIG. 8 . More specifically, the p-type electrode portion  12  and the n-type electrode portion  13  of the connection layers  10  are brought into surface contact with the front surfaces  24 A of the left connection layer  24  and the right connection layer  24 , respectively, in  FIG. 8 . In this state, a heat treatment process is performed to bond the connection layers  10  to the connection layers  24 , respectively, by fusion and solidification. As a result, the light-emitting element  1  and the wiring element  20  are combined together to provide the light-emitting element unit  30 . 
         [0103]    In the completed light-emitting element unit  30 , the front surfaces  10 B of the connection layers  10  are respectively laid on and bonded to the front surfaces  24 A of the connection layers  24  as indicated by hatching in  FIG. 9 . The front surfaces  10 B of the connection layers  10  and the front surfaces  24 A of the connection layers  24  are flat and flush with each other and, therefore, have no portion uninvolved in the bonding. Thus, the entire front surfaces  10 B of the connection layers  10  are respectively bonded to the entire front surfaces  24 A of the connection layers  24 . The connection layers  10  are also electrically connected to the connection layers  24 , respectively, by the bonding. 
         [0104]    Referring to  FIG. 8 , the light-emitting element unit  30  is connected to the support board  31 . The support board  31  includes an insulative substrate  32  supporting the light-emitting element unit  30 , and a pair of metal leads  33  exposed along opposite edges of the insulative substrate  32  for electrical connection between the light-emitting element  1  and an external element. 
         [0105]    Where the light-emitting element unit  30  assumes a reference attitude as shown in  FIG. 8 , the substrate  2  of the light-emitting element  1  is located at the uppermost position, and the base substrate  21  of the wiring element  20  is located at the lowermost position. In this state, the base substrate  21  is bonded to the insulative substrate  32  from the above. Then, the electrode layer  23  (left electrode layer  23  in  FIG. 8 ) on which the connection layer  24  connected to the p-type electrode portion  12  is provided is connected to one of the leads  33  located adjacent this electrode layer  23  via a bonding wire  34 . Further, the electrode layer  23  (right electrode layer  23  in  FIG. 8 ) on which the connection layer  24  connected to the n-type electrode portion  13  is provided is connected to the other lead  33  located adjacent this electrode layer  23  via a bonding wire  34 . 
         [0106]    The resin package  40  is a case filled with a resin. The light-emitting element unit  30  is accommodated (or encapsulated) in the resin package  40  for protection thereof and, in this state, fixed to the support board  31 . The resin package  40  has a reflective portion  40 A on its side wall (opposed to the light-emitting element unit  30 ) to reflect light emitted from the light-emitting element  1  of the light-emitting element unit  30  for extraction of the light. 
         [0107]    The resin of the resin package  40  may contain a fluorescent material and a reflective material. Where the light-emitting element  1  emits blue light, for example, a yellow fluorescent material is added to the resin to cause the light-emitting element package  50  to emit white light. A multiplicity of such light-emitting element packages  50  may be combined together for use as an lighting device such as an electric lamp, a back light for a liquid crystal TV and a head lamp for a motor vehicle. 
         [0108]    The light-emitting element package according to the present invention is not limited in structure to the light-emitting element package  50 , but may be constructed such that the light-emitting element unit  30  is connected to a pair of lead frames respectively electrically connected to the p-type electrode portion  12  and the n-type electrode portion  13  of the light-emitting element unit  30  by a flip bonding method. 
         [0109]    In the light-emitting element  1 , as described above, the light emitted from the light-emitting layer  4  mostly passes through the n-type nitride semiconductor layer  3  to be extracted from the substrate  2 , but partly passes through the p-type nitride semiconductor layer  5  and the transparent electrode layer  6  to be reflected on the interface between the transparent electrode layer  6  and the reflective electrode layer  7  and then extracted from the substrate  2 . 
         [0110]    Since the thickness T of the transparent electrode layer  6  of the light-emitting element  1  satisfies the above expression (1), the light reflection on the interface between the transparent electrode layer  6  and the p-type nitride semiconductor layer  5  and on the interface between the transparent electrode layer  6  and the reflective electrode layer  7  can be reduced as compared with the case in which the transparent electrode layer  6  is designed to have a thickness T equal to an integer multiple of λ/4n. As a result, multiple light reflection in the transparent electrode layer  6  is suppressed to improve the light extracting efficiency. This increases the luminance of the light-emitting element  1 . More specifically, this is explained with reference to  FIG. 11 . 
         [0111]      FIG. 11  is a graph showing a relationship between the luminance change ratio of the light-emitting element and the thickness of an ITO film. 
         [0112]    Light-emitting elements (each employing an ITO film as the transparent electrode layer  6  and GaN films as the nitride semiconductor layers) having the same construction as the light-emitting element  1  shown in  FIG. 1  were produced, and it was checked how luminance observed when light was emitted from the light-emitting layer at a light-emitting wavelength λ of 450 nm was changed with respect to the thickness of the ITO film (ITO film thickness). 
         [0113]    Luminance observed when the ITO film thickness was 2050 Å was used as a reference luminance value (1.000 under reference conditions) by way of example, and the change ratio of an experiment luminance value relative to the reference luminance value was determined for evaluation. For example, a luminance change ratio of 1.100 indicates that the experiment luminance value was 10% greater than the reference luminance value. 
         [0114]      FIG. 11  indicates that, when the ITO film thickness T satisfies the above expression (1), the experiment luminance value is 2% to 13% greater than the reference luminance value. Experiment values providing such a luminance increasing effect are plotted to provide an approximate curve indicated by a broken line in  FIG. 11 . 
         [0115]    The approximate curve is phase-shifted in an ITO film thickness increasing direction with respect to a curve (indicated by a solid line in  FIG. 11 ) showing a change in the light transmittance of the ITO film determined in consideration of only a light component incident perpendicularly on the ITO film. In the solid line curve, the luminance is increased as the light transmittance increases, because the multiple light reflection in the ITO film is reduced. 
         [0116]    That is, where not only the perpendicular light component but also an oblique light component incident obliquely on the ITO film is taken into consideration, an optimum value of the ITO film thickness (at a vertex of the broken line curve) is shifted in the ITO film thickness increasing direction. 
         [0117]    The description made with reference to  FIG. 11  is directed to a case in which the light-emitting wavelength λ is 450 nm, but the light-emitting wavelength λ is not limited to 450 nm in the present invention. 
         [0118]    The interface between the transparent electrode layer  6  and the reflective electrode layer  7  of the alloy containing silver, the platinum group metal and copper has a proper light reflecting ability, which is comparable to that of an interface between the reflective electrode layer  7  and an insulative layer (not shown) which may be provided between the transparent electrode layer  6  and the reflective electrode layer  7 . 
         [0119]    Since the reflective electrode layer  7  is provided on the transparent electrode layer  6  in contact with the transparent electrode layer  6 , heat generated by the emission of the light from the light-emitting layer  4  is conducted directly to the reflective electrode layer  7  from the transparent electrode layer  6  to be thereby efficiently released from the reflective electrode layer  7  to the outside of the light-emitting element  1  (wiring element  20 ). 
         [0120]    This further improves the heat releasing efficiency and the light extracting efficiency. 
         [0121]    The reflective electrode layer  7  is provided in the same pattern as the transparent electrode layer  6  on the transparent electrode layer  6 , and the entire back surface  7 A of the reflective electrode layer  7  opposed to the transparent electrode layer  6  is kept in contact with the front surface  6 B of the transparent electrode layer  6 . Therefore, the back surface  7 A of the reflective electrode layer  7  and the front surface  6 B of the transparent electrode layer  6  completely overlap each other, so that the reflective electrode layer  7  and the transparent electrode layer  6  are free from irregularities which may otherwise occur due to non-overlapping portions thereof. 
         [0122]    Thus, the light emitted from the light-emitting layer  4  and passing through the transparent electrode layer  6  can be efficiently reflected on the interface between the transparent electrode layer  6  and the reflective electrode layer  7  to be extracted without hindrance by the irregularities. The reflective electrode layer  7  and the transparent electrode layer  6  have the same pattern as seen in the stacking direction, so that the interface between the transparent electrode layer  6  and the reflective electrode layer  7  has the greatest possible area. Thus, the light passing through the transparent electrode layer  6  can be efficiently reflected on the interface. This further improves the light extracting efficiency. 
         [0123]    In the absence of the irregularities described above, the connection surface of the light-emitting element  1  to be connected to the external wiring element  20  (the front surfaces  10 B of the connection layers  10 ) is flat, and a connection area between the light-emitting element  1  and the wiring element  20  can be increased. The connection area herein means the area of connection between the front surfaces  10 B of the connection layers  10  and the front surfaces  24 A of the connection layers  24  of the wiring element  20  (see  FIG. 9 ). With the increased connection area, the efficiency of heat release from the light-emitting element  1  to the wiring element  20  can be improved. 
         [0124]    Further, no insulative layer is provided between the transparent electrode layer  6  and the reflective electrode layer  7 . Even without the provision of the insulative layer, the reflective electrode layer  7  of the aforementioned alloy permits proper light reflection on the interface between the transparent electrode layer  6  and the reflective electrode layer  7 . In the absence of the insulative layer, reduction in heat release efficiency and light extracting efficiency can be prevent which may otherwise occur due to the presence of the insulative layer. 
         [0125]    Since the plurality of projections  11  are provided on the front surface  2 B of the substrate  2 , the light traveling from the n-type nitride semiconductor layer  3  toward the substrate  2  is substantially prevented from being reflected on the front surface  2 B of the substrate  2  toward the n-type nitride semiconductor layer  3 . This correspondingly improves the light extracting efficiency. 
         [0126]    While the embodiment of the present invention has thus been described, the invention may be embodied in other ways. 
         [0127]    For example, the transparent electrode layer  6  may have a layered structure including a first electrode sublayer  61  provided on the p-type nitride semiconductor layer  5  in contact with the p-type nitride semiconductor layer  5  and having a first thickness t 1 , and a second electrode sublayer  62  provided on the first electrode sublayer  61  and having a second thickness t 2  greater than the first thickness t 1 . The first thickness t 1  is, for example, 5 Å to 500 Å, and the second thickness t 2  is, for example, 1400 Å to 1900 Å. The first electrode sublayer  61  has a light absorbance of, for example, 0 to 5% (at a light-emitting wavelength of 450 nm), and the second electrode layer  62  has a light absorbance of, for example, 0 to 2% (at a light-emitting wavelength of 450 nm). 
         [0128]    When the transparent electrode layer  6  is formed as having the structure shown in  FIG. 12  (in the step shown in  FIG. 5D ), the first electrode sublayer  61  is formed by deposition at a lower energy level, whereby a damage to the p-type nitride semiconductor layer  5  is reduced. In addition, the overall thickness T of the transparent electrode layer  6  may be adjusted by forming the first electrode sublayer  61  to the smaller thickness t 1  and then forming the second electrode sublayer  62  by deposition at a higher energy level, thereby improving the quality of the overall transparent electrode layer  6 . 
         [0129]    The material for the reflective electrode layer  7  is not limited to the alloy containing silver, the platinum group metal and copper, but other examples of the material include silver (Ag) and Rh (rhodium). 
         [0130]    The reflective electrode layer  7  is not necessarily required to be kept in direct contact with the transparent electrode layer  6 , but an insulative layer, for example, may be provided between the reflective electrode layer  7  and the transparent electrode layer  6 . 
         [0131]    It should be understood that the embodiment of the present invention is merely illustrative of the technical principles of the invention but not limitative of the invention. The spirit and scope of the present invention are to be limited only by the appended claims.