Abstract:
Provided is a semiconductor light emitting element wherein generation of an open failure of the light emitting device can be eliminated by ensuring a current pathway when disconnection is generated in a transparent electrode layer. A semiconductor light emitting element ( 10 ) is provided with: a first semiconductor layer ( 12 ) on a substrate ( 11 ); a light emitting layer ( 13 ) on the first semiconductor layer ( 12 ); a second semiconductor layer ( 14 ) on the light emitting layer ( 13 ); an insulator layer ( 15 ) provided with a hole portion ( 19 ) in a partial region on the second semiconductor layer ( 14 ); a transparent electrode layer ( 16 ) covering the upper surface of the insulator layer ( 15 ) and the second semiconductor layer ( 14 ) without covering the hole portion ( 19 ); and a second pad electrode ( 18 ) brought into contact with the second semiconductor layer ( 14 ) through the hole portion ( 19 ) and faces the insulator layer ( 15 ) with the transparent electrode layer ( 16 ) therebetween. Contact resistance between the second pad electrode ( 18 ) and the second semiconductor layer ( 14 ) is set larger than that between the transparent electrode layer ( 16 ) and the second semiconductor layer ( 14 ).

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage application under 35 USC 371 of International Application No. PCT/JP2009/065043, filed Aug. 28, 2009, which claims priority from Japanese Patent Application No. 2008-222075, filed Aug. 29, 2008, the contents of which prior applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a semiconductor light emitting element which is resistant to an open failure, and a semiconductor light emitting device using the semiconductor light emitting element. 
       BACKGROUND OF THE INVENTION 
       [0003]    A semiconductor light emitting element using nitride semiconductors such as gallium nitride is capable of emitting an ultraviolet light, a blue light, a green light and the like and has a high light emitting efficiency and property of low power consumption, as well as the semiconductor light emitting element is easy to reduce a size, resistant to, for example, mechanical vibrations and has a long life and high reliability. Therefore, applications of the semiconductor light emitting element to a large scale display, a traffic light, a backlight of a liquid crystal display and the like have rapidly progressed, recently. 
         [0004]    The semiconductor light emitting element generally has a stack structure provided with a light emitting layer between a n-type semiconductor layer and a p-type semiconductor layer and emits a light by recombination of an electron and a hole injected into the light emitting layer from the n-type semiconductor layer and the p-type semiconductor layer, respectively. Therefore, a technology of how to efficiently extract the light generated in the light emitting layer is the important technology that determines a characteristic (efficiency) of the light emitting device. 
         [0005]    Hence, a semiconductor light emitting element having a structure provided with an n-type semiconductor layer, an n-side pad electrode disposed on a part of the n-type semiconductor layer, a light emitting layer widely disposed on the n-type semiconductor layer so as to separate from the n-side pad electrode, a p-type semiconductor layer disposed on the light emitting layer, an insulator layer disposed on a part of the p-type semiconductor layer, a transparent electro de layer covering an exposed surface of the p-type semiconductor layer and the insulator layer, and a p-side pad electrode disposed at a position facing the insulator layer across the transparent electrode layer has been known (see, for example, Patent Documents 1 to 5). 
         [0006]    The n-side pad electrode and the p-side pad electrode are connected to an external circuit (power source), respectively by wire bonding or bump bonding in order to apply a voltage between the n-type semiconductor layer and the p-type semiconductor layer. In the semiconductor light emitting element described above, a light emission just below the p-side pad electrode can be suppressed, and a light toward the p-side pad electrode from the light emitting layer is reflected to a side of a light emitting surface (a contact surface between the transparent electrode layer and the p-type semiconductor layer) by the insulator layer to be output from the light emitting surface. As a result, a high light emitting power can be obtained. 
         [0007]    In addition, as an another example, a structure has been proposed, in which an electrode layer having a high contact resistance or a semiconductor layer having a low electrical conductivity is disposed on the p-type semiconductor layer, and the p-side pad electrode is disposed on the electrode layer, while contacting with a transparent electrode layer (see, for example, Patent Documents 6 to 8). In the structure, a light emission just below the p-side pad electrode is suppressed, thereby resulting in high light emitting power. 
         [0008]    However, in the semiconductor light emitting elements disclosed in the Patent Documents 1 to 5 and the Patent Documents 6 to 8, there is a common problem that a disconnection in the transparent electrode layer is likely to be caused. Explanation will be given of the problem in reference to  FIG. 8A  and  FIG. 8B .  FIG. 8A  is a cross sectional view schematically showing a structure in the vicinity of the p-side pad electrode in a conventional semiconductor light emitting element. As shown in  FIG. 8A , a semiconductor light emitting element  110 A has a structure in which an insulator layer, or an electrode layer having a high contact resistance, or a semiconductor layer having a low electrical conductivity (hereinafter, referred to as an insulator layer and the like  112 ) is disposed on a surface of a p-type semiconductor layer  111 , a transparent electrode layer  113 A is disposed so as to cover these layers, and a p-side pad electrode  114 A is disposed at a position facing the insulator layer and the like  112  across the transparent electrode layer  113 A. Since the transparent electrode layer  113 A is generally formed by sputtering, a film thickness of the transparent electrode layer  113 A becomes thin at a step portion S (side face portion of the insulator layer and the like  112 ) of the transparent electrode layer  113 A indicated by dotted lines in  FIG. 8A . As a result, a breakdown or disconnection (so-called open failure) is likely to be caused at the step portion S due to current concentration. 
         [0009]    In order to solve the foregoing problem, another semiconductor light emitting element with a structure schematically shown in  FIG. 8B  has been proposed (see, for example, Patent Documents 9 to 13). A semiconductor light emitting element  110 B has the structure, in which the insulator layer and the like  112  is disposed on a surface of the p-type semiconductor layer  111 , a transparent electrode layer  113 B having a height substantially identical to that of the insulator layer and the like  112  is disposed on the p-type semiconductor layer  111 , and a p-side pad electrode  114 B is disposed so as to cover the insulator layer and the like  112  and a part of the transparent electrode layer  113 B. By setting a contact area between the p-side pad electrode  114 B and the transparent electrode layer  113 B to be large, the contact area is prevented from generating a current concentration.
   [Patent Document 1] JPn. Pat. Appln. KOKAI Publication No. H08-250768   [Patent Document 2] JPn. Pat. Appln. KOKAI Publication No. H09-36431   [Patent Document 3] JPn. Pat. Appln. KOKAI Publication No. H09-129921   [Patent Document 4] JPn. Pat. Appln. KOKAI Publication No. 2004-140416   [Patent Document 5] JPn. Pat. Appln. KOKAI Publication No. H09-129922   [Patent Document 6] JPn. Pat. Appln. KOKAI Publication No. H11-4020   [Patent Document 7] JPn. Pat. Appln. KOKAI Publication No. H11-87772   [Patent Document 8] JPn. Pat. Appln. KOKAI Publication No. 2003-174196   [Patent Document 9] JPn. Pat. Appln. KOKAI Publication No. H10-173224   [Patent Document 10] Pamphlet WO98/42030   [Patent Document 11] JPn. Pat. Appln. KOKAI Publication No. 2000-124502   [Patent Document 12] JPn. Pat. Appln. KOKAI Publication No. 2002-353506   [Patent Document 13] JPn. Pat. Appln. KOKAI Publication No. 2003-124517   
 
       SUMMARY OF THE INVENTION 
       [0023]    However, as the semiconductor light emitting element  110 B shown in  FIG. 8B , if an area of the p-side pad electrode  114 B is enlarged, an area where a light is absorbed by the p-side pad electrode  114 B increases, and as a result, a light emitting area decreases. On the other hand, if the contact area between the p-side pad electrode  114 B and the transparent electrode layer  113 B is reduced, the open failure is likely to be caused by the current concentration as with the semiconductor light emitting element  110 A shown in  FIG. 8A . 
         [0024]    When a light emitting apparatus is manufactured using a light emitting device, generally, a plurality of light emitting devices are connected in series. Therefore, if an open failure occurs in a transparent electrode layer of one of the plurality of light emitting devices, it happens that a current does not flow in all of the light emitting devices, in addition to no light emission of the light emitting device of the open failure, thereby resulting in losing a function as a light emitting apparatus. Therefore, it is important to avoid a generation of the open failure. 
         [0025]    The present invention has been developed in consideration of the foregoing problem, and it is an object of the present invention to provide a semiconductor light emitting element which is capable of avoiding a generation of open failure of the semiconductor light emitting element by securing a current path if a disconnection is generated in a transparent electrode layer. In addition, it is another object of the present invention to provide a semiconductor light emitting device using the semiconductor light emitting element. 
         [0026]    A semiconductor light emitting element according to the present invention includes: a first semiconductor layer; a light emitting layer disposed on the first semiconductor layer; a first pad electrode disposed on the first semiconductor layer so as to separate from the light emitting layer; a second semiconductor layer disposed on the light emitting layer; an insulator layer disposed on one part of areas of the second semiconductor layer and provided with a hole portion passing through in a thickness direction of the second semiconductor layer; a transparent electrode layer disposed continuously from the other part of areas of the second semiconductor layer to a part of an upper surface of the insulator layer; and a second pad electrode which is disposed in contact with the second semiconductor layer through the hole portion of the insulator layer and in contact with the transparent electrode layer at a position facing the insulator layer across the transparent electrode layer. In the semiconductor light emitting element, a contact resistance between the second pad electrode and the second semiconductor layer is larger than a contact resistance between the transparent electrode layer and the second semiconductor layer. 
         [0027]    In the semiconductor light emitting element, when the transparent electrode layer is not disconnected, a current substantially does not flow between the second pad electrode and the second semiconductor layer because the contact resistance between the transparent electrode layer and the second semiconductor layer is different from the contact resistance between the second pad electrode and the second semiconductor layer, and a current flows between the transparent electrode layer and the second semiconductor layer. If the disconnection occurred in the transparent electrode layer, a current flows through a contact surface between the second pad electrode and the second semiconductor layer to form a current path by an overvoltage breakdown of the second semiconductor layer/light emitting layer/first semiconductor layer. Then, when a light emitting apparatus is formed using a plurality of the foregoing semiconductor light emitting elements, even if the disconnection occurred in the transparent electrode layer of one of the semiconductor light emitting elements, the current path is secured and the other semiconductor light emitting elements can be maintained to be capable of light emitting 
         [0028]    In the semiconductor light emitting element according to the present invention, it is preferable that a thickness of the insulator layer is 10 to 500 nm, a thickness of the transparent electrode layer is 20 to 400 nm, and a thickness of the second pad electrode is 400 to 2000 nm. 
         [0029]    By forming the thicknesses as described above, resistances of the transparent electrode layer and the second pad electrode can be made small. In addition, when there is no disconnection in the transparent electrode layer, a generation of current concentration from the second pad electrode toward just below thereof can be avoided. 
         [0030]    In the semiconductor light emitting element according to the present invention, it is preferable that a shape of an opening of the hole portion in the insulator layer is circular or substantially circular, and an area of the opening is not more than 80% of a contact area between the insulator layer and the second semiconductor layer. 
         [0031]    Since the shape of the opening of the hole portion in the insulator layer is a shape of a contact surface between the second pad electrode and the second semiconductor layer, if the transparent electrode layer is disconnected, a distribution of current passing through the contact surface can be homogenized by forming the shape in circular or substantially circular. In addition, by forming the area of the opening of the hole portion in the insulator layer not more than 80% of the contact area between the insulator layer and the second semiconductor layer, a light absorption by the second pad electrode can be made small. 
         [0032]    In the semiconductor light emitting element according to the present invention, it is preferable that an average diameter of the hole portion of the insulator layer is not less than 16 μm. 
         [0033]    By forming the average diameter as described above, the semiconductor light emitting element can be prevented from generating an open failure. 
         [0034]    In the semiconductor light emitting element according to the present invention, it is preferable that the first semiconductor layer is disposed on a predetermined substrate. 
         [0035]    By forming the semiconductor light emitting element on the predetermined substrate, a semiconductor light emitting device provided with a plurality of semiconductor light emitting elements can be easily formed. 
         [0036]    The semiconductor light emitting device according to the present invention includes a plurality of semiconductor light emitting elements each of whose first semiconductor layer is disposed on a predetermined substrate and at least two of the semiconductor light emitting elements are connected in series. 
         [0037]    In addition, another semiconductor light emitting device according to the present invention includes a plurality of the semiconductor light emitting elements disposed on a predetermined substrate and at least two of the semiconductor light emitting elements are connected in series. 
         [0038]    In these semiconductor light emitting devices according to the present invention, even if one semiconductor light emitting element becomes unable to emit a light, the semiconductor light emitting device can be prevented from becoming unable to emit light as a whole. 
         [0039]    According to a semiconductor light emitting element of the present invention, the semiconductor light emitting element can be prevented from generating an open failure even if a disconnection occurs in the transparent electrode layer, because the second pad electrode is in direct contact with the second semiconductor layer and a current flows through the contact surface by forming a current path. Therefore, in a semiconductor light emitting device using a plurality of semiconductor light emitting elements, or in a semiconductor light emitting device using a plurality of semiconductor light emitting elements which are disposed on a single substrate, the semiconductor light emitting device can be prevented from generating the condition that the semiconductor light emitting device does not emit light as a whole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1A  is a top view showing a structure of a semiconductor light emitting element according to a first embodiment of the present invention; 
           [0041]      FIG. 1B  is a cross sectional view showing the structure of the semiconductor light emitting element according to the first embodiment taken along A-A line of  FIG. 1A ; 
           [0042]      FIG. 1C  is a cross sectional view showing the structure of the semiconductor light emitting element according to the first embodiment taken along B-B line of  FIG. 1A ; 
           [0043]      FIG. 2A  is a schematic illustration showing a brief structure of a light emitting apparatus constituted by using a semiconductor light emitting element shown in  FIG. 1A  to  FIG. 1C , which is an example of a connecting structure using a direct current power source; 
           [0044]      FIG. 2B  is a schematic illustration showing a brief structure of a light emitting apparatus constituted by using a semiconductor light emitting element shown in  FIG. 1A  to  FIG. 1C , which is an example of a connecting structure using an alternate current power source; 
           [0045]      FIG. 3  is a top view showing a brief structure of a semiconductor light emitting element according to a second embodiment of the present invention; 
           [0046]      FIG. 4  is a top view showing a brief structure of a semiconductor light emitting element according to a third embodiment of the present invention; 
           [0047]      FIG. 5  is a top view showing a brief structure of a semiconductor light emitting element according to a fourth embodiment of the present invention; 
           [0048]      FIG. 6A  is a cross sectional view showing a brief structure of a semiconductor light emitting element according to the fourth embodiment taken along C-C line of  FIG. 5 ; 
           [0049]      FIG. 6B  is a cross sectional view showing a brief structure of the semiconductor light emitting element according to the fourth embodiment taken along D-D line of  FIG. 5 ; 
           [0050]      FIG. 7  is a graph showing relationships between an open-circuit failure generation voltage (applied voltage) and a breakdown rate as well as an accumulated breakdown rate; 
           [0051]      FIG. 8A  is a cross sectional view showing an example of a structure of a conventional semiconductor light emitting element; and 
           [0052]      FIG. 8B  is a cross sectional view showing an another example of a structure of a conventional semiconductor light emitting element 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0053]    Hereinafter, embodiments of the present invention will be explained in detail by referring to drawings. 
       First Embodiment 
       [0054]    A top view showing a brief structure of a semiconductor light emitting element according to the first embodiment of the present invention is shown in  FIG. 1A , a cross sectional view taken along A-A line in  FIG. 1A  is shown in  FIG. 1B , and a cross sectional view taken along B-B line in  FIG. 1A  is shown in  FIG. 1B . This semiconductor light emitting element  10  includes a substrate  11 , a first semiconductor layer  12 , a light emitting layer  13 , a second semiconductor layer  14 , an insulator layer  15 , a transparent electrode layer  16 , a first pad electrode  17  and a second pad electrode  18 . 
         [0055]    In  FIG. 1A  to  FIG. 1C , one semiconductor light emitting element  10  is formed on a single substrate  11 , but the present invention is not limited to this. For example, a plurality of independent first semiconductor layers  12  may be formed on a surface of the single substrate  11 , and each of the foregoing layers and each of the foregoing electrodes may be formed on each of the first semiconductor layers  12 . An explanation will be given below of each foregoing element of the semiconductor light emitting element  10 . 
       [Substrate] 
       [0056]    As a material of the substrate  11 , the material having a lattice matching which is capable of epitaxially growing a semiconductor (compound semiconductor) constituting the first semiconductor layer  12  is used. For example, Al 2 O 3  (sapphire), MgAl 2 O 4  (spinel), SiC, SiO 2 , ZnS, ZnO, Si, GaAs, C (diamond), LiNbO 3  (lithium niobate), and Nd 3 Ga 5 O 12  (neodymium gallium garnet) may be used. An area and thickness of the substrate  11  are not limited specifically. 
       [First Semiconductor Layer] 
       [0057]    The first semiconductor layer  12  to be formed on a surface of the substrate  11  is constituted by an n-type semiconductor material which is formed by doping an n-type dopant in III-V group compound semiconductors. As the III-V group compound semiconductors, for example, GaN, AlN and InN or In α Al β Ga 1-α-β N (0≦α, 0≦β, 0&lt;α+β≦1) which is a mixed crystal of GaN, AlN and InN, III-V group compound semiconductors which are formed in such a manner that a part of or all of III-group element in the In α Al β Ga 1-α-β N are substituted by, for example, B, or a part of N is substituted by other V-group elements such as P, As and Sb, GaAs-based compound semiconductors (for example, AlGaAs, InGaAs), InP-based compound semiconductors (for example, AlGaInP), and III-V group compound semiconductors such as InGaAsP which is a mixed crystal of GaAs-based compound semiconductor and InP-based compound semiconductor, may be used. In addition, as a n-type dopant, for example, Si, Ge, Sn, S, O, Ti and Zr, which are IV-group element or VI-group element, may be used. 
       [Second Semiconductor Layer] 
       [0058]    The second semiconductor layer  14  to be formed on a surface of the light emitting layer  13  is constituted by a p-type semiconductor material which is formed by doping a p-type dopant in III-V group compound semiconductors. The III-V group compound semiconductors to be used for the second semiconductor layer  14  are identical to the III-V group compound semiconductors to be used for the first semiconductor layer  12 . Then, the descriptions will be omitted. As the p-type dopant, for example, Be, Zn, Mn, Cr, Mg and Ca may be used. 
       [Light Emitting Layer] 
       [0059]    The light emitting layer  13  is formed on a surface of the first semiconductor layer  12  so as to separate from the first pad electrode  17 , while securing a formation area of the first pad electrode  17 , which is connected to a predetermined power source, on the first semiconductor layer  12 . The light emitting layer  13  has a function to radiate energy as a light which is generated by recombination of electrons and holes injected from the first semiconductor layer  12  and the second semiconductor layer  14 , respectively. In order to effectively develop the function, it is preferable that the light emitting layer  13  has a quantum well structure including a well layer and a barrier layer as a quantum structure. 
         [0060]    Specifically, a semiconductor material constituting the light emitting layer  13  may be any one of a non-doped semiconductor, an n-type impurity doped semiconductor and a p-type impurity doped semiconductor. Especially, the non-doped semiconductor or the n-type impurity doped semiconductor is preferably used. Here, an undoped semiconductor may be used for the well layer and the n-type impurity doped semiconductor may be used for the barrier layer. In the quantum well structure, a wavelength of a light to be produced in the light emitting layer  13  can be adjusted by a species and an amount of the dopant doped in the well layer. For example, when the light emitting layer  13  consists of a III-V group compound semiconductor, a light having a wavelength of about 60-650 nm, preferably 380-560 nm, may be emitted. If the well layer contains Al, a light having a wavelength range which is unable to achieve by a conventional well layer of InGaN, specifically, about 365 nm that corresponds to a band gap energy of GaN, or shorter wavelength can be obtained. Then, depending on, for example, an application of the semiconductor light emitting element  10 , a species and an amount of the dopant doped in the well layer may be set in order to adjust a wavelength of the emitting light. 
       Modified Example of First Semiconductor Layer/Light Emitting Layer/Second Semiconductor Layer 
       [0061]    Here, a brief explanation will be given of a modified example of the first semiconductor layer  12 /light emitting layer  13 /second semiconductor layer  14 . As a first modified example, a structure that stacks a contact layer/a clad layer in this order on the substrate  11  may be used as the first semiconductor layer  12 , and similarly, a structure that stacks a clad layer/a contact layer in this order on the light emitting layer  13  may be used as the second semiconductor layer  14 . As a second modified example, a structure that forms a buffer layer between the substrate  11  and the first semiconductor layer  12 , forms the light emitting layer  13  on the buffer layer, in addition, forms a buffer layer on the second semiconductor layer  14  and forms the insulator layer  15  as well as the transparent electrode layer  16  on the buffer layer, may be used. As a third modified example, the first semiconductor layer  12  and the second semiconductor layer  14  each having a multi-layer structure that stacks an undoped semiconductor layer and a doped semiconductor layer alternately may be used. 
       [First Pad Electrode] 
       [0062]    The first pad electrode  17  has a role as a terminal for electrically connecting a predetermined power source and the first semiconductor layer  12 , and a role as a terminal for connecting a plurality of the semiconductor light emitting elements  10  in series (see  FIG. 2A  and  FIG. 2B , which will be described later). In the semiconductor light emitting element  10 , as shown in  FIG. 1A  and  FIG. 1C , the first pad electrode  17  is formed on a step surface which is formed by cutting a part of an upper surface of the first semiconductor layer  12 , in order to separate (not to directly contact with each other) the light emitting layer  13  formed on an upper surface of the first semiconductor layer  12  from the first pad electrode  17 . Meanwhile, the first pad electrode  17  may be disposed on a surface of the first semiconductor layer  12  without forming the step surface separated (electrical insulation) from the light emitting layer  13 . 
         [0063]    The first pad electrode  17  is in contact with the first semiconductor layer  12  with a low resistance. Hereinafter, in the specification, a state that a semiconductor material is in contact with an electrode material with a low resistance within a driving voltage of the semiconductor light emitting element  10  is referred to as “ohmic contact” (Therefore, the first pad electrode  17  is in contact with the first semiconductor layer  12  with ohmic contact). On the other hand, a state of a contact with a resistance higher than the ohmic contact is referred to as “Schottky contact”. There is such a difference of resistance between the ohmic contact and the Schottky contact that when a current flows through the ohmic contact, substantially, no current flows through the Schottky contact in the structure where an ohmic contact and a Schottky contact are formed in parallel. 
         [0064]    From the foregoing point of view, as a material of the first pad electrode  17 , Ti, Al, Cr, Mo, W, Ag, and ITO which have a low contact resistance with the first semiconductor layer  12 , or alloys containing at least one of these metals are preferably used for a layer in contact with the first semiconductor  12 . The layer may be a single layer or a multilayer. Especially, a multilayer such as Ti/Rh/Au, Ti/Pt/Au, Ti/Ir/Au, Ti/Ru/Au, or Al—Si—Cu alloy/W/Au is preferably used because the first pad electrode  17  and the second pad electrode  18  can be formed concurrently. As the multilayer, specifically, the multilayer of Ti/Rh/Au each having a thickness of 2 nm/200 nm/500 nm may be used. 
       [Insulator Layer] 
       [0065]    The insulator layer  15  has a function to reduce a light absorption by the second pad electrode  18  by reflecting the light emitted from the light emitting layer  13 . Therefore, as a material of the insulator layer  15 , the material having a refractive index smaller than that of the second semiconductor layer  14 , for example, SiO 2 , Al 2 O 3 , SiN, MgF 2 , CaF 2 , LiF, AlF 3 , BaF 2 , YF 3 , LaF 3 , CeF 3 , Y 2 O 3 , ZrO 2 , and Ta 2 O 5  may be used. 
         [0066]    In addition, the insulator layer  15  has a function to homogenize a current flowing in the second semiconductor layer  14 . Namely, if the insulator layer  15  is not disposed, a current from the second pad electrode  18  concentrates in an area of the transparent electrode layer  16  located just below the second pad electrode  18 . As a result, the current in the second semiconductor layer  14  becomes inhomogeneous, and accordingly, the luminous efficiency may be decreased due to insufficient utilization of area of the light emitting layer  13 . However, by disposing the insulator layer  15 , the area located just below the second pad electrode  18  can be prevented from generating the current concentration and the lowering of the luminous efficiency can be suppressed. 
         [0067]    A thickness of the insulator layer  15  is preferably set to 10-750 nm. If the thickness is less than 10 nm, it is difficult to suppress the current concentration effectively. On the other hand, if the thickness is more than 750 nm, when the transparent electrode  16  is formed, a thickness of the transparent electrode layer  16  in the vicinity of a side face of the insulator  15  becomes thin due to the thick insulator layer  15 . If the thin portion is formed once in the film of the transparent electrode layer  16  as described above, an open failure is likely to be caused in the thin portion due to a concentrated current from the second pad electrode  18 . The thickness of the insulator layer  15  is, more preferably, set to 250-600 nm. 
         [0068]    The insulator layer  15  is provided with a hole portion  19 . With respect to a role of the hole portion  19  and a shape setting condition thereof, explanations will be given later together with the explanation of a function of a contact surface between the second pad electrode  18  and the second semiconductor layer  14 . 
       [Transparent Electrode Layer] 
       [0069]    The transparent electrode layer  16  is formed to cover an upper surface of the insulator layer  15  except for the hole portion  19  of the insulator layer  15 , and substantially a whole area of an upper surface of the second semiconductor layer  14 , where the insulator layer  15  is not formed. The transparent electrode  16  has a role to electrically connect the second pad electrode  18  and the second semiconductor layer  14  and to supply a current to the second semiconductor layer  14 . In the normal use condition (condition of no disconnection in the transparent electrode layer  16 ) of the semiconductor light emitting element  10 , the transparent electrode layer  16  forms an ohmic contact with the second semiconductor layer  14  so that a current flows between the second pad electrode  18  and the second semiconductor layer  14  through the transparent electrode  16 . 
         [0070]    In addition, the transparent electrode layer  16  has a role to radiate a light emitted from the light emitting layer  13  to outside through thereof. Therefore, especially, a material which has a large light transmission rate in the wavelength range of a light emitted from the light emitting layer  13  is preferably used for the transparent electrode layer  16 . For example, oxides containing at least one selected from In, Zn, Sn, Ga, W and Ti, specifically, ITO, IZO, ZnO, In 2 O 3 , SnO 2  and TiO 2 , and composite oxides thereof are used for the transparent electrode layer  16 . Meanwhile, as the transparent electrode layer  16 , a Ni/Au stack film may also be used. 
         [0071]    A thickness of the transparent electrode layer  16  is preferably set to 20-400 nm for enabling the light emitting layer  13  to emit a light homogeneously in a large area by a current flowing homogeneously in the second semiconductor layer  14  except for the area just below the insulator layer  15 , and for suppressing absorption of light emitted from the light emitting layer  13  by the transparent electrode layer  16 . 
         [0072]    It is noted that a film thickness of the transparent electrode layer  16  in the vicinity of a side face of the insulator layer  15  is formed to be thin in comparison with that of an upper portion of the second semiconductor layer  14  and that of an upper portion of the insulator layer  15 . This is caused by a film thickness of the insulator layer  15  and a film forming method (this will be described later) of the transparent electrode layer  16 . In this sense, the structure has a similar structure to that shown in  FIG. 8A , which was explained as the prior art. 
       [Second Pad Electrode] 
       [0073]    The second pad electrode  18  has a role as a terminal to electrically connect a predetermined power source and the transparent electrode layer  16  and a role as a terminal to connect a plurality of the semiconductor light emitting elements  10  in series or in parallel. In order to prevent a light generated in the light emitting layer  13  from being absorbed by the second pad electrode  18 , the second pad electrode  18  is disposed on a surface of the transparent electrode layer  16  above the insulator layer  15  so that the outer periphery of the second pad electrode  18  is located inside the outer periphery of the insulator layer  15 , or overlap with the outer periphery of the insulator layer  15 . 
         [0074]    The second pad electrode  18  is in contact with the second semiconductor layer  14  through the hole portion  19  of the insulator layer  15 . Here, a contact resistance between the second pad electrode  18  and the second semiconductor layer  14  is larger than that between the second pad electrode  18  and the second semiconductor layer  14  through the transparent electrode layer  16 . Namely, the second pad electrode  18  forms a Schottky contact with the second semiconductor layer  14 . Therefore, in the normal use condition, as described above, a current flows from the second pad electrode  18  to the second semiconductor layer  14  through the transparent electrode layer  16 , however, the current does not flow directly from the second pad electrode  18  to the second semiconductor layer  14  through the hole portion  19  of the insulator layer  15 . 
         [0075]    It is preferable that the second pad electrode  18  has a single layer structure or a multilayer structure including a layer which is in contact with the second semiconductor layer  14  and made of Ti, W, Nb, Al, Sn, Si, Hf, Y, Fe, Zr, V, Mn, Gd, Ir, Pt, Ru, Ta or Cr that is a material having a large contact resistance with the second semiconductor layer  14 , or made of alloys containing at least one of these metals. Especially, if Ti is used in a portion in contact with the second semiconductor layer  14 , Ti forms a Schottky contact with a p-type semiconductor that is used for the second semiconductor layer  14 , while Ti forms an ohmic contact with an n-type semiconductor that is used for the first semiconductor layer  12  and with various kinds of oxides that are used for the transparent electrode layer  16 . Therefore, it is preferable to form the first pad electrode  17  and the second pad electrode  18  concurrently. Accordingly, a multilayer structure such as Ti/Rh/Au, Ti/Pt/Au, Ti/Ir/Au, Ti/Ru/Au and Al—Si—Cu alloy/W/Pt/Au are preferably used. 
       [Function of Schottky Contact Between Second Pad Electrode and Second Semiconductor Layer] 
       [0076]    As described above, a film thickness of the transparent electrode layer  16  is thin in the vicinity of a side face of the insulator layer  15 . Then, a disconnection may occur due to, for example, a current concentration at the thin portion. If the disconnection occurs in the transparent electrode layer  16 , a current does not flow from the second pad electrode  18  to the second semiconductor layer  14  through the transparent electrode layer  16 . However, in the light emitting device  10 , if the disconnection occurs in the transparent electrode layer  16 , a current flows from the second pad electrode  18  to the second semiconductor layer  14  through a Schottky contact surface (hereinafter, simply referred to as Schottky contact) between the second pad electrode  18  and the second semiconductor layer  14 . Due to the current at this time, an overvoltage breakdown is caused in the first semiconductor layer  12 /light emitting layer  13 /second semiconductor layer  14  to form a short circuit, thereby resulting in securing a current path. Therefore, for example, in a light emitting apparatus that connects a plurality of the semiconductor light emitting elements  10  in series, a current path is secured although the semiconductor light emitting element that is disconnected in the transparent electrode layer  16  does not emit a light. As a result, current supplies to the other semiconductor light emitting elements do not stop, and light emissions of the other semiconductor light emitting elements can be maintained. 
         [0077]    A planer shape of the hole portion  19  disposed in the insulator layer  15  is identical to a shape of the Schottky contact. By forming the shape in circular or ellipsoidal, if a disconnection occurs in the transparent electrode layer  16 , a distribution of a current passing through the Schottky contact is likely to be homogeneous, and a current pass directed from the Schottky contact to the first pad electrode  17  can be surely formed when an overvoltage breakdown is caused in the first semiconductor layer  12 /light emitting layer  13 /second semiconductor layer  14 . 
         [0078]    An area of the Schottky contact is identical to an opening area of the hole portion  19  of the insulator layer  15 , and it is preferable that the opening area of the hole portion  19  is not more than 80% of a contact area between the insulator layer  15  and the second semiconductor layer  14 . This is because when the semiconductor light emitting element  10  is normally used, the second pad electrode  18  absorbs a light emitted in the light emitting layer  13  through the Schottky contact. Then, by forming the area of the Schottky contact to be small, the light absorption by the second pad electrode  18  can be made small. 
         [0079]    It is preferable that an average diameter of the hole portion  19  of the insulator layer  15  is not less than 16 μm. Here, the average diameter means that if the planer shape (that is, the shape of the Schottky contact) of the hole portion  19  is not circular, for example, if the shape is ellipsoidal, the average diameter is an average length of the major axis and the minor axis, and if the shape is square, the average diameter is a diameter of a circle having the same area with the square. As shown in the embodiment described later, if the average diameter of the hole portion  19  is not less than 16 μm, when an open failure occurs in the transparent electrode layer  16 , a current path can be surely formed by causing an overvoltage breakdown in the first semiconductor layer  12 /light emitting layer  13 /second semiconductor layer  14  by the current passing through the Schottky contact. 
         [0080]    Meanwhile, a bonding wire is bonded to the second pad electrode  18  in order to connect the second pad electrode  18  to a power source or another semiconductor light emitting element  10 . The boding wire is preferably bonded to the upper center (an area above the hole portion  19  of the insulator  15 ) of the second pad electrode  18 . Then, a current flowing in the second pad electrode  18  can be made homogeneous, and if a disconnection occurs in the transparent electrode layer  16 , a current tends to flow toward the Schottky contact just below the upper center of the second pad electrode  18 . Therefore, it is likely to cause an overvoltage breakdown in the first semiconductor layer  12 /light emitting layer  13 /second semiconductor layer  14 , and likely to form a current path. 
       [Light Emitting Apparatus] 
       [0081]    In  FIG. 2A  and  FIG. 2B , a schematic illustration showing a brief configuration (that is, a connecting structure of semiconductor light emitting elements) of a light emitting apparatus using a semiconductor light emitting element according to the foregoing first embodiment is shown. Here, an example of a connecting structure that uses a direct current is shown in  FIG. 2A , and an example of a connecting structure that uses an alternative current is shown in  FIG. 2B . Meanwhile, since a structure of the semiconductor light emitting element  10  constituting each of the light emitting apparatuses shown in  FIG. 2A  and  FIG. 2B  is obvious from  FIG. 1A  to  FIG. 1C , explanations on elements of the semiconductor light emitting element  10  are omitted in  FIG. 2A  and  FIG. 2B . 
         [0082]    The light emitting apparatus shown in  FIG. 2A  has a structure that connects a plurality (12 pieces are exemplified in  FIG. 2A ) of the semiconductor light emitting elements  10  in series in a line by bonding wires, and the semiconductor light emitting elements  10  can be turned on simultaneously using a direct current power source. The light emitting apparatus shown in  FIG. 2B  has a structure that connects two line units, each consisting of a plurality (6 pieces in  FIG. 2B ) of the semiconductor light emitting elements  10  connected in series in a line by bonding wires, in parallel against an alternative current power source, and currents flowing in the two line units have opposite directions to each other (when a current flows in one line unit, no current flows in the other line unit). Namely, the light emitting apparatus shown in  FIG. 2B  has a structure where the semiconductor light emitting elements  10  in each of the line units alternately emit lights by line unit, depending on a frequency of the alternative current output from the alternative current power source. 
         [0083]    In these light emitting apparatuses, even if a disconnection (breakdown) is generated in the transparent electrode layer  16  of one semiconductor light emitting element  10 , the semiconductor light emitting element  10  is prevented from generating an open failure since a current path passing through the foregoing Schottky contact and the first semiconductor layer  12 /light emitting layer  13 /second semiconductor layer  14  is formed between the second pad electrode  18  and the first pad electrode  17 . Then, even if one semiconductor light emitting element  10  becomes unable to emit a light, the remaining eleven semiconductor light emitting elements  10  can maintain the condition capable of emitting a light. Meanwhile, in  FIG. 2A  and  FIG. 2B , for example, the twelve semiconductor light emitting elements  10  may be disposed on a single substrate. In addition, a plurality of the light emitting apparatus, shown in  FIG. 2A  and  FIG. 2B , consisting of the twelve semiconductor light emitting elements  10  may be further connected in series in order to form another new light emitting apparatus. 
       [Fabrication Method of Semiconductor Light Emitting Element] 
       [0084]    The fabrication method of the semiconductor light emitting element  10  is briefly described by the following steps. 
         [0000]    (1) Formation of the first semiconductor layer  12 , the light emitting layer  13  and the second semiconductor layer  14  on a substrate surface.
 
(2) Formation of the insulator layer  15  and the transparent electrode layer  16 .
 
(3) Etching a part of area in order to form the first pad electrode  17 .
 
(4) Formation of the first pad electrode  17  and the second pad electrode  18 .
 
         [0085]    Explanations of the steps (1) to (4) will be given below. 
       [Formation of First Semiconductor Layer, Light Emitting Layer, and Second Semiconductor Layer] 
       [0086]    The first semiconductor layer, the light emitting layer, and the second semiconductor layer can be formed by growing a semiconductor (compound semiconductor) on a surface of a cleaned substrate  11  using a gas containing, for example, a predetermined semiconductor material and dopants with various kinds of vapor phase epitaxy such as MOVPE (metal-organic vapor phase epitaxy), HDVPE (halide vapor phase epitaxy), MBE (molecular beam epitaxy), and MOMBE (metal-organic molecular beam epitaxy). In this case, according to a composition of the semiconductor layer (first semiconductor layer  12  consisting of n-type semiconductor/light emitting layer  13 /second semiconductor layer  14  consisting of p-type semiconductor) to be formed, a gas species is changed and a growth time is adjusted depending on a film thickness of each of the semiconductor layers, and as a result, these semiconductor layers can be formed continuously. 
       [Formation of Insulator Layer and Transparent Electrode Layer] 
       [0087]    The insulator layer  15  having a planar ring shape is formed on a part of a surface of the second semiconductor layer  14 . For example, the insulator layer  15  may be formed by growing a material composing the insulator layer  15  on a predetermined area by sputtering and the like using a photomask, and removing the photomask thereafter. 
         [0088]    The transparent electrode layer  16  may be formed by growing a conductive oxide containing at least one selected from In, Zn, Sn, and Ga on a whole surface of the insulator layer  15 , for example, after the insulator layer  15  is formed, and subsequently conducting etching on the area (that is, the area of the hole portion  19  and its vicinity of the insulator layer  15  and the area for forming the first pad electrode  17 ) that the transparent electrode layer  16  is unnecessary. 
       [Partial Etching for Forming First Pad Electrode  17 ] 
       [0089]    An etching mask is formed except for an area for forming the first pad electrode  17 , etching is conducted until a mid depth of the first semiconductor layer  12  by, for example, dry etching, and after that, the etching mask is removed. Thus, the area for disposing the first pad electrode  17  can be formed. 
       [Formation of First Pad Electrode and Second Pad Electrode] 
       [0090]    The first pad electrode  17  and the second pad electrode  18  may be formed concurrently in such a manner that, for example, a resist pattern is formed so that areas for forming the first pad electrode  17  and the second pad electrode  18  are exposed, then, Ti/Rh/Au are grown sequentially by using, for example, spattering. After that, the resist pattern is removed. It is noted that the fabrication method of the semiconductor light emitting element  10  is not limited to the foregoing processes. For example, the following processes may be applied to the fabrication method. After the first semiconductor layer  12 /light emitting layer  13 /second semiconductor layer  14  are formed, an area for forming the first pad electrode  17  is formed by etching. Then, the first pad electrode  17  is formed, and subsequently, the insulator layer  15 , the transparent electrode layer  16  and the second pad electrode  18  are formed sequentially. 
       Second Embodiment 
       [0091]      FIG. 3  is a top view showing a brief structure of a semiconductor light emitting element according to a second embodiment of the present invention. An element of a semiconductor light emitting element  10 A shown in  FIG. 3  and having a function identical to that of the semiconductor light emitting element  10  shown in  FIG. 1A  to  FIG. 1C  has the same reference number with that of the semiconductor light emitting element  10  in the drawings and the explanation. This is the same with semiconductor light emitting elements according to a third embodiment and a fourth embodiment, which will be described later. 
         [0092]      FIG. 3  is drawn in a similar manner to  FIG. 1A , and the semiconductor light emitting element  10 A has a shape of substantially square in plan view and includes the substrate  11 , the first semiconductor layer  12  (overlapped with the substrate  11 ) formed on the substrate  11 , the first pad electrode  17  disposed at a corner portion on the first semiconductor layer  12 , the light emitting layer  13  disposed on the first semiconductor layer  12  separated from the first pad electrode  17 , the second semiconductor layer  14  (overlapped with the light emitting layer  13 ) disposed on the light emitting layer  13 , and the insulator layer  15  disposed on the second semiconductor layer  14 . 
         [0093]    The insulator layer  15  is disposed on a part of an upper surface of the second semiconductor layer  14  and includes a nearly circular core portion disposed at a corner portion which is located diagonally to the corner portion where the first pad electrode  17  is disposed and an extending portion extending along a side direction of the second semiconductor layer  14  from the core portion. The foregoing shape of the insulator layer  15  is formed corresponding to a shape of the second pad electrode  18 . The hole portion  19  passing through in the thickness direction is disposed near the center of the core portion. 
         [0094]    In addition, the semiconductor light emitting element  10 A includes the transparent electrode layer  16 , which covers an upper surface of the insulator layer  15  without covering the hole portion  19  of the insulator layer  15  as well as an area where the insulator layer  15  is not formed on the second semiconductor layer  14 , and the second pad electrode  18  which is in contact with the second semiconductor layer  14  through the hole portion  19  of the insulator layer  15  and located at a position facing the insulator layer  15  across the transparent electrode layer  16  so as to come in contact with the transparent electrode layer  16 . 
         [0095]    In the plan view shown in  FIG. 3 , the second pad electrode  18  has a size to fall inside the insulator layer  15 . The second pad electrode  18  includes a core portion  40  disposed on the core portion of the insulator layer  15  and extending portions  41   a ,  41   b  disposed on respective extending portions of the insulator layer  15 . By disposing the extending portions  41   a ,  41   b  as described above, a current in a whole surface of the second semiconductor layer  14  can be made homogeneous. As a result, a light emission that effectively utilizes a light emitting area of the light emitting layer  13  becomes possible. In addition, by adjusting a shape of the insulator layer  15  to that of the second pad electrode  18 , a generation of current concentration just below the second pad electrode  18  can be avoided. 
         [0096]    Meanwhile, with respect to a current flow to the second semiconductor layer  14  from the second pad electrode  18  in the case 
         [0097]    that the second pad electrode  18  is provided with the extending portions  41   a ,  41   b , it is thought that a current flow (current density) to the second semiconductor layer  14  from the core portion  40  is larger than a current flow to the second semiconductor layer  14  from the extending portions  41   a ,  41   b . Therefore, a structure which is provided with the insulator layer  15  only just below the core portion  40  of the second pad electrode  18  may be adopted. 
         [0098]    The transparent electrode  16  forms an ohmic contact with the second semiconductor layer  14 , and the second pad electrode  18  forms a Schottky contact with the second semiconductor layer  14 . Namely, although a planar structure of the semiconductor light emitting element  10 A is different from that of the semiconductor light emitting element  10  in  FIG. 1A  to  FIG. 1C , as described above, a cross sectional structure of the semiconductor light emitting element  10 A is identical to that of the semiconductor light emitting element  10  in  FIG. 1A  to  FIG. 1C  described above. Therefore, if the transparent electrode layer  16  is disconnected, a current flows through the Schottky contact between the second pad electrode  18  and the second semiconductor layer  14  while securing a current path, and as a result, the semiconductor light emitting element  10 A can be prevented from generating an open failure. 
       Third Embodiment 
       [0099]    A top view showing a brief structure of a semiconductor light emitting element according to a third embodiment of the present invention is shown in  FIG. 4 .  FIG. 4  is drawn in a similar manner to  FIG. 1A , and a semiconductor light emitting element  10 B has a shape of substantially square and includes the substrate  11 , the first semiconductor layer  12  (overlapped with the substrate  11 ) formed on the substrate  11 , and the first pad electrode  17  disposed on one end of the first semiconductor layer  12  in the longitudinal direction of the first semiconductor layer  12 . The first pad electrode  17  includes a core portion  42  disposed at an end on the first semiconductor layer  12  in the longitudinal direction of the first semiconductor layer  12  and an extending portion  43  extending form the core portion  42  along a long side of the first semiconductor layer  12 . 
         [0100]    In addition, the semiconductor light emitting element  10 B includes the light emitting layer  13  disposed on the first semiconductor layer  12  separated from the first pad electrode  17 , the second semiconductor layer  14  (overlapped with the light emitting layer  13 ) formed on the light emitting layer  13 , and the insulator layer  15  formed on the second semiconductor layer  14 . The insulator layer  15  includes a core portion disposed on the second semiconductor layer  14  at an end in the longitudinal direction opposite to the first pad electrode  17  and an extending portion extending from the core portion along the long side. The foregoing shape of the insulator layer  15  is formed corresponding to a shape of the second pad electrode  18 . In addition, the hole portion  19  passing through in the thickness direction is disposed near the center of the core portion. 
         [0101]    In addition, the semiconductor light emitting element  10 B includes the transparent electrode layer  16 , which covers an upper surface of the insulator layer  15  without covering the hole portion  19  of the insulator layer  15  and an area where the insulator layer  15  is not formed on the second semiconductor layer  14 , and the second pad electrode  18  which is in contact with the second semiconductor layer  14  through the hole portion  19  of the insulator layer  15  and located at a position facing the insulator layer  15  across the transparent electrode layer  16  so as to come in contact with the transparent electrode layer  16 . 
         [0102]    In the plan view shown in  FIG. 4 , the second pad electrode  18  has a size to fall inside the insulator layer  15 . The second pad electrode  18  includes the core portion  40  disposed on the core portion of the insulator layer  15  and an extending portion  41  disposed on the extending portion of the insulator layer  15 . By disposing the extending portion  41  in the second pad electrode  18  as well as disposing the extending portion  43  in the first pad electrode  17 , a current in a whole surface of each of the first semiconductor layer  12  and the second semiconductor layer  14  can be made homogeneous. As a result, alight emission that effectively utilizes alight emitting area of the light emitting layer  13  becomes possible. In addition, by adjusting a shape of the insulator layer  15  to that of the second pad electrode  18 , a generation of current concentration just below the second pad electrode  18  can be avoided. It is noted that even if the second pad electrode  18  includes the extending portion  41 , the insulator layer  15  may be disposed only just below the core portion  40 . 
         [0103]    The transparent electrode  16  forms an ohmic contact with the second semiconductor layer  14 , and the second pad electrode  18  forms a Schottky contact with the second semiconductor layer  14 . Namely, although a planar structure of the semiconductor light emitting element  10 B is different from that of the semiconductor light emitting element  10  in  FIG. 1A  to  FIG. 1C  as described above, a cross sectional structure of the semiconductor light emitting element  10 B is identical to that of the foregoing semiconductor light emitting element  10  in  FIG. 1A  to  FIG. 1C . Therefore, when the transparent electrode layer  16  is disconnected, a current flows through the Schottky contact between the second pad electrode  18  and the second semiconductor layer  14  while securing a current path, and as a result, the semiconductor light emitting element  10 B can be prevented from generating an open failure. 
       Fourth Embodiment 
       [0104]    A top view showing a brief structure of a semiconductor light emitting element according to a fourth embodiment of the present invention is shown in  FIG. 5 .  FIG. 6A  is a cross sectional view of the semiconductor light emitting element taken along C-C line of  FIG. 5 , and  FIG. 6B  is a cross sectional view of the semiconductor light emitting element taken along D-D line of  FIG. 5 . The semiconductor light emitting element  10 C has a structure provided with two light emitting portions connected in parallel. The semiconductor light emitting element  10 C includes the substrate  11  and the first semiconductor layer  12  formed on the substrate  11 , and areas of the respective light emitting portions are allocated on the common first semiconductor layer  12 . 
         [0105]    The each light emitting portion is provided with the first pad electrode  17  formed on the first semiconductor layer  12 , and the first pad electrode  17  includes a core portion  42  having a nearly circular shape in plan view and an extending portion  43  extending through the core portion  42  in the radial direction. The light emitting layer  13  is formed on the first semiconductor layer  12  so as to separate from the first pad electrode  17 , and the second semiconductor layer  14  is disposed on the light emitting layer  13 . As shown in  FIG. 6A , the light emitting layer  13  is common to the two light emitting portions and the second semiconductor layer  14  is also common to the two light emitting portions. Namely, areas of respective light emitting portions are allocated to the common light emitting layer  13  and to the common second semiconductor layer  14 . 
         [0106]    The semiconductor light emitting element  10 C has such a structure that the second pad electrode  18  surrounds a periphery of the first pad electrode  17 , and the second pad electrodes  18  provided in respective light emitting portions are connected to each other. The second pad electrode  18  includes core portions  40  disposed at two corner portions on the short side of respective light emitting portions and extending portions  41  extending from the core portion  40  along the long side. The insulator layer  15  is formed on the second semiconductor layer  14  corresponding to a shape of the second pad electrode  18 , and a shape of the insulator layer  15  is designed so that the second pad electrode  18  is fallen inside the insulator layer  15  in plan view shown in  FIG. 5 . 
         [0107]    The semiconductor light emitting element  10 C includes the insulator layer  15 , the transparent electrode layer  16 , the first pad electrode  17  and the second pad electrode  18 . The first pad electrode  17  includes the core portion  42  and the extending portion  43 , the second pad electrode  18  includes the core portion  40  and the extending portion  41 , and the insulator layer  15  has a shape corresponding to that of the second pad electrode  18  so that the second pad electrode  18  is fallen inside the insulator layer  15  in plan view shown in  FIG. 5 . The hole portion  19  passing through in the thickness direction is disposed in an area lower than the core portion  40  in the second pad electrode  18  and the insulator layer  15 . 
         [0108]    The each light emitting portion includes the transparent electrode layer  16  which covers an upper surface of the insulator layer  15  without covering the hole portion  19  of the insulator layer  15  and an area where the insulator layer  15  is not formed on the second semiconductor layer  14 . The second pad electrode  18  having the foregoing shape is, as shown in  FIG. 6A , disposed in such a manner that the second pad electrode  18  is in contact with the second semiconductor layer  14  through the hole portion  19  of the insulator layer  15  and located at a position facing the insulator layer  15  across the transparent electrode layer  16  so as to come in contact with the transparent electrode layer  16 . 
         [0109]    By disposing the extending portions  41  in the second pad electrode  18  as well as disposing the extending portion  43  in the first pad electrode  17 , a current in a whole surface of each of the first semiconductor layer  12  and the second semiconductor layer  14  can be made homogeneous. As a result, a light emission that effectively utilizes a light emitting area of the light emitting layer  13  becomes possible. In addition, by adjusting a shape of the insulator layer  15  to that of the second pad electrode  18 , a generation of current concentration just below the second pad electrode  18  can be avoided. It is noted that even if the second pad electrode  18  includes the extending portion  41 , the insulator layer  15  may be disposed only just below the core portion  40 . 
         [0110]    The transparent electrode  16  forms an ohmic contact with the second semiconductor layer  14 , and the second pad electrode  18  forms a Schottky contact with the second semiconductor layer  14 . Therefore, if the transparent electrode layer  16  is disconnected, a current flows through the Schottky contact between the second pad electrode  18  and the second semiconductor layer  14  while securing a current path, and as a result, the semiconductor light emitting element  10 C can be prevented from generating an open failure. 
         [0111]    Explanations for the semiconductor light emitting elements  10 ,  10 A,  10 B and  10 C according to the embodiments 1 to 4 of the present invention have been made. However, the present invention is not limited to these embodiments and, for example, a shape of the light emitting device in plan view may be oval, parallelogram or polygonal, other than square or oblong (rectangle). In addition, in the first to fourth embodiments, the first pad electrode is formed on a side identical to the side of the second pad electrode as seen from the substrate. However, the arrangements of the first pad electrode and the second pad electrode are not limited to this, and a structure having no substrate or having a conductive substrate may be adopted and, for example, the first pad electrode on the first semiconductor layer may be disposed on a side opposite to the semiconductor light emitting element across the semiconductor layers and the second pad electrode. 
       EXAMPLES 
       [0112]    As a semiconductor light emitting element of EXAMPLE 1, a semiconductor light emitting element having a structure shown in  FIG. 1  was fabricated. The semiconductor light emitting element of EXAMPLE 1 was fabricated by the following processes. A first semiconductor layer made of a GaN-based n-type semiconductor, a light emitting layer made of a GaN-based undoped semiconductor and a second semiconductor layer made of a GaN-based p-type semiconductor were sequentially formed on a sapphire substrate by MOCVD. After that, etching was conducted in order to form an area (see  FIG. 1A ) for disposing a first pad electrode, and a part of the first semiconductor layer was exposed. Meanwhile, in order to concurrently fabricate a plurality of semiconductor light emitting elements of EXAMPLE 1, the first semiconductor layer/light emitting layer/second semiconductor layer were formed on the sapphire substrate. 
         [0113]    Here, the first semiconductor layer had the following structure. A buffer layer (film thickness: about 10 nm) made of AlGaN was grown on the sapphire substrate. Subsequently, an undoped GaN layer (1 μm), an n-side contact layer (5 μm) made of GaN containing 4.5×10 18 /cm 3  of Si, an n-side first multilayer (total thickness: 335 nm) consisting of three layers of a bottom layer (300 nm) made of undoped GaN, an interlayer (30 nm) made of GaN containing 4.5×10 18 /cm 3  of Si and an upper layer (5 μm) made of undoped GaN, and an n-side second multilayer (total thickness: 64 nm) that is a superlattice structure where an undoped GaN layer (4 nm) and an undoped In 0.1 Ga 0.9 N layer (2 nm) were alternately stacked ten times for each and further, an undoped GaN layer (4 nm) was stacked, were grown in this order on the buffer layer. 
         [0114]    Next, the light emitting layer was formed of a multiquantum well structure (total thickness: 193 nm) consisting of a barrier layer (25 nm) made of undoped GaN and a layer which was formed by stacking a well layer (3 nm) made of In 0.3 Ga 0.7 N, a first barrier layer (10 nm) made of In 0.02 Ga 0.98 N and a second barrier layer (15 nm) made of undoped GaN alternately six times for each layer. 
         [0115]    In addition, the second semiconductor layer had a structure that sequentially stacked the p-side multilayer (total film thickness: 36.5 nm), which was formed of a superlattice structure formed by stacking a Al 0.15 Ga 0.85 N layer (4 nm) containing 5×10 19 /cm 3  of Mg and an In 0.03 Ga 0.97 N layer (2.5 nm) containing 5×10 19 /cm 3  of Mg alternately five times for each and further stacking another Al 0.15 Ga 0.85 N layer (4 nm) containing 5×10 19 /cm 3  of Mg, and a p-side contact layer (120 nm) made of GaN containing 1×10 20 /cm 3  of Mg, in this order. 
         [0116]    At a predetermined position (see  FIG. 1A ) on a surface of the second semiconductor layer that is a light emitting surface, an insulator layer made of SiO 2  having a flat ring shape which includes a hole portion having an inner diameter of 10 μm and has an outer diameter of 76 μm was grown 500 nm in thickness by sputtering. After that, a transparent electrode layer made of ITO provided with a hole portion having an inner diameter 6 μm larger (that is, inner diameter: 16 μm) than the diameter (hole diameter) of the hole portion of the insulator layer was grown 170 nm in thickness on the insulator layer and the second semiconductor layer. 
         [0117]    In addition, the second pad electrode having a diameter of 70 μm was formed by spattering so as to directly contact with the second semiconductor layer through the hole portion of the insulator. A structure of the second pad electrode was a three-layered structure of Ti/Rh/Au, and thicknesses of the three layers were 1.5 nm/200 nm/500 nm, respectively. In addition, when the second pad electrode was formed, the first pad electrode was formed concurrently with the formation of the second pad electrode with a structure identical to that of the second pad electrode. Here, a shape of the first pad electrode in plan view was nearly circular having an average diameter of 70 μm. Meanwhile, the second pad electrode forms a Schottky contact with the second semiconductor layer (GaN-based p-type semiconductor), and the second pad electrode forms an ohmic contact with the transparent electrode (ITO). The first pad electrode forms an ohmic contact with the first semiconductor layer (GaN-based n-type semiconductor). 
         [0118]    Next, a semiconductor light emitting element having a size of 500 μm×290 μm was cut out by dicing and bonded on a metal lead frame. Then, Au wire was bonded to each of the first pad electrode and the second pad electrode and the semiconductor light emitting element was molded with epoxy resin. According to the processes described above, the semiconductor light emitting element of EXAMPLE 1 was fabricated. 
         [0119]    As a semiconductor light emitting element of EXAMPLE 2, a semiconductor light emitting element having a structure identical to that of the semiconductor light emitting element of EXAMPLE 1 except that a diameter of the hole portion of the insulator layer is 16 μm and that a diameter of the hole portion of the transparent electrode layer corresponding to the hole portion of the insulator layer is 22 μm was fabricated. Similarly, as semiconductor light emitting elements of EXAMPLES 3, 4, 5 and 6, semiconductor light emitting elements having structures identical to that of the semiconductor light emitting element of EXAMPLE 1 except that diameters of the hole portions of the insulator layers of EXAMPLES 3, 4, 5 and 6 are 22 μm, 28 μm, 34 μm and 40 μm, respectively and that diameters of the hole portions of the transparent electrode layers corresponding to the respective hole portions of the insulator layers are 28 μm, 34 μm, 40 μm and 46 μm, respectively were fabricated. In addition, as a semiconductor light emitting element of a COMPARATIVE EXAMPLE having a conventional structure, a semiconductor light emitting element (see  FIG. 8A ) which has no hole portion in the insulator layer, that is, which has no area that the second pad electrode is in direct contact with the second semiconductor layer, was fabricated. 
         [0120]    A voltage to generate an open failure in the semiconductor light emitting elements of the COMPARATIVE EXAMPLE and EXAMPLES 1 to 6 was investigated by applying a voltage of machine model between the first pad electrode and the second pad electrode and investigating an electric conduction between the first pad electrode and the second pad electrode. Meanwhile, generally, the applying a voltage of machine model is to charge up a capacitor of 200 pF at an appropriate voltage and to apply the voltage to a device, and may be conducted using, for example, electrostatic breakdown test equipment (Model: DWP-3000) manufactured by DAITRON TECHNOLOGY CO., LTD. 
         [0121]    A graph showing relations between an open-circuit failure generation voltage (applied voltage) and a breakdown rate as well as an accumulated breakdown rate is shown in  FIG. 7 . It is noted that in  FIG. 7 , the breakdown rate (a ratio of open failure samples to total samples) is indicated by bar charts, and the accumulated breakdown rate is indicated by line charts. In addition, a value of the open-circuit failure generation voltage on the horizontal axis corresponds to the line charts (accumulated breakdown voltage), and the bar charts (breakdown rate) are shown at positions shifted from the actual open-circuit failure generation voltages, while the actual open-circuit failure generation voltages are shown near the bar charts. 
         [0122]    With respect to the semiconductor light emitting element of the COMPARATIVE EXAMPLE, the open failure generation was not observed at applied voltage of 534V (that is, the semiconductor light emitting element was capable of emitting a light). However, at applied voltage of 640V, the open failure was generated in the semiconductor light emitting elements at a rate of 20%. In addition, at applied voltage of 747V, the open failure was generated in the semiconductor light emitting elements at a rate of 60% (The accumulated breakdown rate is 80%), and at applied voltage of 960V, the open failure was generated in the remaining 20% of the semiconductor light emitting elements (The accumulated breakdown rate is 100%). It was proven that the open failure generation in the COMPARATIVE EXAMPLE was caused by a disconnection of the transparent electrode layer. 
         [0123]    On the other hand, in the semiconductor light emitting element of EXAMPLE 1, the open failure generation was not observed at applied voltage of 534 to 854V. Comparing this result with that of the COMPARATIVE EXAMPLE, it was considered that although the transparent electrode layer was disconnected at the applied voltage at 80% of the samples, a current path was secured in the first semiconductor layer/light emitting layer/second semiconductor layer due to a current flow through the Schottky contact between the second pad electrode and the second semiconductor layer. In the semiconductor light emitting element of EXAMPLE 1, the open failure was generated at 80% of the semiconductor light emitting elements by applied voltage of 960V, and at applied voltage of 1096V, the open failure was generated in the remaining 20% of the semiconductor light emitting elements. This was considered that a breakdown was caused by an excess current in the hole portion between the second semiconductor layer and the second pad electrode due to a small diameter of the hole portion disposed in the insulator layer. From the above facts, it can be seen that the semiconductor light emitting element of EXAMPLE 1 has a structure that the open-circuit failure generation voltage is high and the open failure is hardly caused in comparison with the semiconductor light emitting element of the COMPARATIVE EXAMPLE. 
         [0124]    The open failure was not observed in the semiconductor light emitting elements of EXAMPLES 2 to 6 even if 1174V was applied to the devices. Then, the bar charts indicating the breakdown rates of the semiconductor light emitting elements of EXAMPLES 2 to 6 are not shown in  FIG. 7 . This was considered that a current flowed through the Schottky contact between the second pad electrode and the second semiconductor layer and a current path in the first semiconductor layer/light emitting layer/second semiconductor layer was secured by enlarging a diameter of the hole portion disposed in the insulator layer larger than 16 μm, and thereby, the hole portion was also not broken by an excess current within the range of applied voltages of the present tests.