Abstract:
In a gallium nitride compound semiconductor, making small the thickness of a metal electrode layer in order to enhance the efficiency of taking light out relatively increases the resistance value of the metal electrode layer as measured in a direction that is parallel with this layer compared to that of it in a direction that is vertical with respect thereto. As a result of this, when a voltage has been applied across relevant electrodes, electric current ceases to be sufficiently supplied to the entire metal electrode layer. The semiconductor light emitting device of the invention is equipped, between the metal electrode layer and an active layer, with a superlattice layer for enhancing the efficiency of taking out the light that has been emitted in the active layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The disclosure of Japanese Patent Application No. 2003-207969 filed Aug. 20, 2003 including specification drawings and claims is incorporated herein by reference in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor light emitting device that is comprised of a gallium nitride compound semiconductor and, more particularly, to a semiconductor light emitting device that is comprised of a gallium nitride compound semiconductor that is equipped with a superlattice layer.  
       DESCRIPTION OF THE RELATED ART  
       [0003]     In a semiconductor light emitting device that is comprised of a gallium nitride compound semiconductor that is expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1), that light emitting device being represented by a blue color light emitting diode, a semiconductor substrate that composes a base of it is unable to be manufactured using bulk crystal that is good in quality and large in size. Therefore, ordinarily, a semiconductor substrate is manufactured by causing a gallium nitride compound semiconductor to be crystal-grown onto a substrate consisting of sapphire (Al 2 O 3 ). And, with respect to over this semiconductor substrate, various kinds of process steps are executed to thereby manufacture that device (for example, refer to Japanese Patent Application Laid-Open No. 62-119196).  
         [0004]      FIG. 1  is a schematic view of a conventional semiconductor light emitting device. In  FIG. 1 , a reference numeral  11  denotes a substrate; a reference numeral  12  denotes an n-type semiconductor layer; a reference numeral  13  denotes an active layer that has a luminous region; a reference numeral  14  denotes a p-type semiconductor layer; a reference numeral  16  denotes a metal electrode layer; a reference numeral  18  denotes an electrode on a side of the p-type semiconductor layer; and a reference numeral  19  denotes an electrode on a side of the n-type semiconductor layer.  
         [0005]     The active layer  13  is a layer that has a luminous portion of the semiconductor light emitting device. On a side thereof where the n-type semiconductor layer  12  is located, the light that has been emitted in the active layer  13  is shaded by a base (not illustrated) on which the substrate  11  is placed. Therefore, taking-out of the light emitted in the active layer  13  is performed from the side where the p-type semiconductor layer  14  is located. Therefore, in order to enhance the light taking-out efficiency, it is only necessary to thin the thickness of the metal electrode layer  16  and in addition to make high the light transmittance of that layer  16 . Or, alternatively, it is only necessary to form the electrode  19  on an end of the metal electrode layer  16  to thereby make high the intensity of the light, at around the center of the metal electrode layer  16 , that has been emitted in the active layer  13 . However, when thinning the thickness of the metal electrode layer  16 , the resistance value of the metal electrode layer  16  in the direction that is parallel with the metal electrode layer  16  becomes relatively large as compared with that of the metal electrode layer  16  in the direction that is vertical to the metal electrode layer  16 . Therefore, when having applied a voltage with respect to the electrode  18 , an electric current ceases to be sufficiently supplied to the metal electrode layer  16  as a whole.  
         [0006]     Also, when forming the electrode  18  on the end of the metal electrode layer  16 , the electric current into the entire metal electrode layer  16  has more difficulty being supplied to the entire metal electrode layer  16  than when having formed the electrode  18  at around the center of the metal electrode layer  16 . When an electric current is supplied to part of the metal electrode layer  16 , the electric current flows through only a part of the active layer  13  via the p-type semiconductor layer  14 . As a result of this, the problem arises that emitting of light (luminescence) occurs only from a part of the active layer  13 . On the other hand, when thickning the thickness of the metal electrode layer  16 , the light transmittance of the metal electrode layer  16  becomes low. Also, when forming the electrode  18  near the center of the metal electrode layer  16 , the electrode  18  shades the light that has been emitted in the active layer  13 . As a result of this, the problem arises that the efficiency of taking out the light from the side of the p-type semiconductor layer  14  becomes decreased.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention, in order to solve the above-described problems, has an object to provide a semiconductor light emitting device comprised of gallium nitride compound semiconductor, which is equipped with a superlattice layer that contributes to enhancing the efficiency of taking out the light that has been emitted in the active layer.  
         [0008]     To attain the above object, according to a first aspect of the invention of this application, there is provided a semiconductor light emitting device comprised of a gallium nitride compound semiconductor expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1), which comprises on a substrate at least a first conductivity type semiconductor layer, an active layer having a light emitting region, a second conductivity type semiconductor layer, and a metal electrode layer sequentially in this order from the substrate side, and in which a superlattice layer is located at an arbitrary position between the metal electrode layer and the active layer.  
         [0009]     In the first aspect of the invention of this application, the superlattice layer is a semiconductor layer that consists essentially of a gallium nitride compound semiconductor that is expressed as Al p Ga q In 1-p-q N (where 0≦p≦1, 0≦q≦1, and 0≦p+q≦1) and is the semiconductor layer that has a forbidden band width that is greater than that of the active layer.  
         [0010]     In the first aspect of the invention of this application, the second conductivity type semiconductor layer is a p-type semiconductor layer and the metal electrode layer consists of gold (Au), nickel (Ni), or an alloy comprising these elements.  
         [0011]     According to a second aspect of the invention of this application, there is provided a semiconductor light emitting device comprised of a gallium nitride compound semiconductor expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1), which comprises on a substrate at least a first conductivity type semiconductor layer, an active layer having aluminous region, a second conductivity type semiconductor layer, and an electrode sequentially in this order from the substrate side, and in which a superlattice layer is located at an arbitrary position between the electrode and the active layer.  
         [0012]     In the second aspect of the invention of this application, the superlattice layer is a semiconductor layer that consists essentially of a gallium nitride compound semiconductor that is expressed as Al p Ga q In 1-p-q N (where 0≦p≦1, 0≦q≦1, and 0≦p+q≦1) and is the semiconductor layer that has a forbidden band width that is greater than that of the active layer.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic view of a conventional semiconductor light emitting device;  
         [0014]      FIG. 2  is a schematic view of a semiconductor light emitting device according to an embodiment of the invention of this application;  
         [0015]      FIG. 3 a  schematic view of the semiconductor light emitting device according to another embodiment of the invention of this application; and  
         [0016]      FIG. 4  is an enlarged schematic view of a superlattice layer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Hereinafter, an embodiment of the invention of this application will be explained with reference to the accompanying drawings.  FIG. 2  shows a schematic view of a semiconductor light emitting device that embodies the invention of this application. Also,  FIG. 4  shows a schematic view, enlarged, of a superlattice layer. In  FIG. 2 , a reference numeral  21  denotes a substrate, a reference numeral  22  denotes a first conductivity type semiconductor layer, a reference numeral  23  denotes an active layer that has a luminous region, a reference numeral  24  denotes a second conductivity type semiconductor layer, a reference numeral  26  denotes a metal electrode layer, a reference numeral  28  denotes a second electrode, a reference numeral  29  denotes a first electrode, and a reference numeral  211  denotes the superlattice layer. Also, in  FIG. 4 , a reference numeral  221  denotes a layer whose forbidden band width is narrow, and a reference numeral  222  denotes a layer whose forbidden band width is wide. The invention of this application has a characterizing feature in that the superlattice layer  211  is provided between the metal electrode layer  26  and the active layer  23 . Each of the first conductivity type semiconductor layer  22  and second conductivity type semiconductor layer  24  is an n-type or p-type semiconductor layer, and they are the layers whose polarities of that are opposite to each other.  
         [0018]     As the material of the substrate  21 , there can be used sapphire, SiC or the like. The reason why using sapphire, SiC or the like is in view of the fact that using a GaN substrate is difficult since GaN has the difficulty of being bulk crystal-grown because of the high dissociation pressure of nitrogen. If the substrate is the one that consists of material that is different from GaN, material therefor is not limited to sapphire and SiC. Also, in a case where using a sapphire substrate as the substrate  21 , the principal surface thereof may be a C, R, or A surface.  
         [0019]     Here, although, ordinarily, it is surely not impossible to form bulk crystal of GaN with respect to the sapphire substrate as is, in a case where difficult, it is necessary to perform relevant processing with respect to the substrate  21  for forming the first conductivity type semiconductor layer. Those processing that are performed with respect to the substrate  21  include, for example, forming on the surface made of sapphire, using a growth-at-low-temperature technique, a GaN layer having the thickness of several tens of nano-meters (nm), and forming, a GaN layer the thickness of several micro-meters (&gt;m) using a growth-at-low-temperature technique, after forming an AlGaN layer having a thickness of several tens of nano-meters (nm). These substrates each having formed therein such a GaN layer or AlGaN layer are also included under the category of “substrate” that is referred to in this application.  
         [0020]     As each of the first conductivity type semiconductor layer  22 , active layer  23 , and second conductivity type semiconductor layer  24 , there is used the one that is comprised of a gallium nitride compound semiconductor that is expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1). When applying as the first conductivity type semiconductor layer  22 , active layer  23 , and second conductivity type semiconductor layer  24  the one that is comprised of a gallium nitride compound semiconductor that is expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1), it is possible to cause light emission over a wide range of wavelengths.  
         [0021]     The first conductivity type semiconductor layer  22  may be of a single-layer, or multi-layer, structure. Although in  FIG. 2  that layer consists of a single layer that exhibits both functions of a cladding layer and contact layer that makes ohmic contact with the first electrode  29 , the cladding layer and contact layer may be formed respectively separately. Further, the first conductivity type semiconductor layer  22  may have a layer that has other function such as a hole barrier layer.  
         [0022]     The second conductivity type semiconductor layer  24  may be of a single-layer, or multi-layer, structure. Although in  FIG. 2  that layer  24  consists of a layer that exhibits both functions of a cladding layer and contact layer and the superlattice layer  211 , the cladding layer and contact layer may be formed respectively separately. Further, the second conductivity type semiconductor layer  24  may have a layer that has other function such as an electron barrier layer.  
         [0023]     The active layer  23  may be formed as having a structure that is given in kind, such as a bulk structure, a single quantum well structure, or a multi-quantum well structure. In a case where adopting a single quantum well structure or multi-quantum well structure, it results that as the well layer that composes the single quantum well structure or multi-quantum well structure there is used a layer that is narrow in forbidden band width and as the barrier layer there is used a layer that is wide in forbidden band width. For example, as the well layer, there can be used a layer that consists of material expressed as In 1-a Ga a N (where 0&lt;a≦1), while, as the barrier layer, there can be used a layer that consists of material expressed as Al 1-b Ga b N (where 0&lt;b≦1), provided that a×b&lt;1.  
         [0024]     In the process steps of forming the active layer  22 , it may be constructed in the way that, for example, of the active layer  23 , only a portion having the luminous portion as its central region is left as is, namely, as a mesa shaped semiconductor light emitting device. Or, alternatively, it may be constructed in the way that concentrating the electric current by narrowing thereof to cause this relevant portion to function as a luminous portion. For example, in a DFB laser (distributed feedback laser diode) that is used for long-distance/large-capacity transmission, or fabry-perot laser diode that is used centering the subscriber&#39;s line transmission, the active layer  23  may be constructed as having a BH (Buried Heterostructure) type structure made as a multi-quantum well structure wherein the active layer has formed therein a multi-layer film. Further, the active layer  23  may be constructed as having an FBH (Flat-surface Buried Heterostructure) type structure that has a great effect of narrowing the electric current.  
         [0025]     In a case where utilizing the nature that the electrical conductivity that is measured in the direction that is parallel to the superlattice layer  211  is higher than that which is measured in the direction that is vertical to that layer  211 , if the superlattice layer  211  is disposed at a given position between the metal electrode layer  26  and active layer  23 , the supply of the electric current to the active layer  23  is uniformly performed, even thinning the thickness of the metal electrode layer  26  more than the conventional one of that layer  26  and, further, as illustrated in  FIG. 2 , disposing the second electrode  28  on a terminal end of that layer  26 . As a result of this, it is possible to more enhance, than in the prior art, the effect of taking out the light, which has been emitted in the active layer  23 , from the side where the second conductivity type semiconductor layer  24  is located.  
         [0026]     Here, as illustrated in  FIG. 4 , the superlattice layer  211  may be obtained by superposing a plural number of layer, one upon another, using a hetero-junction. The layer subjected to be superposed is the layer whose thickness is the same as the de Broglie wavelength of electron or hole, or less, such as the layers constitute the superlattice layer  211 , for example, the layers  221  narrow in forbidden band width and the layers  222  wide in forbidden band width. When using this superlattice layer  211 , since in the layer  221  that is narrow in forbidden band width and layer  222  that is wide in forbidden band width the movement of the electrons or holes is quantized by the energy barrier, the electron or hole movement is made two-dimensional. Therefore, it becomes possible to uniformly disperse the electrons in the superlattice layer  211 . As a result of this, it is possible to make large the region in which the light emitted in the active layer  23  becomes uniform.  
         [0027]     As the superlattice layer  211 , it is preferable to use a semiconductor layer which is comprised of a gallium nitride compound semiconductor that is expressed as Al p Ga q In 1-p-q N (where 0≦p≦1, 0≦q≦1, and 0≦p+q≦1) and which has a forbidden band width that is wider than that of the active layer  23 . If, using the semiconductor layer which is comprised of a gallium nitride compound semiconductor that is expressed as Al p Ga q In 1-p-q N (where 0≦p≦1, 0≦q≦1, and 0≦p+q≦1), it is possible to form the superlattice layer  211  by alternately laminating the layer narrow in forbidden band width with the layer wide in forbidden bandwidth. Also, by making the superlattice layer  211  be a semiconductor layer having the forbidden band width of that is wider than that of the active layer  23 , it is possible to efficiently emit the light to outside the semiconductor light emitting device without the light emitted in the luminous region of the active layer  23  being adsorbed into the superlattice layer  211 .  
         [0028]     Furthermore, although, the superlattice layer  211  is disposed at the position that contacts with the active layer  23  in  FIG. 2 , the superlattice layer  211  may be disposed at a position that is arbitrary between the metal electrode layer  26  and the active layer  23 . For example, the superlattice layer  211  may be disposed in direct contact with the metal electrode layer  26  to thereby make the superlattice layer  211  function as a contact layer.  
         [0029]     The term “superlattice” refers to a lattice structure that is formed in such a way that, in general, crystal lattice having a certain length of period is subject to modulation by the periodic structure that is again larger in length of period than that of that crystal lattice. In the invention of this application, the superlattice layer  211  uses a layer that consists of, among the general superlattices, the one that has a structure wherein two layers made of materials the forbidden band widths of that are relatively large in terms of the difference between them are alternately laminated together. In the layer  221  narrow in forbidden band width and layer  222  wide in forbidden band width, which compose the superlattice layer  211 , electrons or holes are in a state of being confined. In the invention of this application, the thickness of the layer  221  narrow in forbidden band width and layer  222  wide in forbidden band width, which compose the superlattice layer  211 , are made to have the thickness of the de Broglie wavelength, or so, of the electrons or holes, thereby limiting the movement of the electrons or holes in the direction that is vertical to the layer  221  narrow in forbidden band width and layer  222  wide in forbidden band width. Further, by making free the movement of the electrons or holes in the direction that is parallel to the layer  221  narrow in forbidden band width and layer  222  wide in forbidden band width, it becomes possible to have electrons or holes uniformly dispersed in those layer  221  and layer  222 . In other words, it is thought that it is possible, in the superlattice layer  211 , to make the electrical conductivity in the parallel direction to the superlattice layer  211  higher than that in the vertical direction to the superlattice layer  211 .  
         [0030]     When forming this superlattice layer, it is necessary that each layer composing it be laminated with its thickness being a critical thickness of approximately 10 nm or less that can resist distortions. By laminating each layer with its thickness being that critical one or less, distortions are mitigated, and crystal defects also are decreased.  
         [0031]     Also, although the superlattice layer  211  is comprised of layers that have the same polarity as the second conductivity type semiconductor layer  24 , doping is not always needed. Namely, since the gallium nitride compound semiconductor that is expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1) becomes n-type unless doping is performed with respect thereto, in a case where making the superlattice  211  an n-type, n-type dopant may be doped, or may not be doped.  
         [0032]     In this embodiment, in a case where the second conductivity type semiconductor layer  24  is of a p-type, it is preferable that, as the metal electrode layer  26 , gold (Au), nickel (Ni), or one of alloys comprising them be applied. It is possible for the metal electrode layer  26  and second conductivity type semiconductor layer  24  to have an ohmic contact therebetween by using gold (Au), nickel (Ni), or one of alloys comprising them as the metal electrode layer  26 . This enables supplying the electric current through the second conductivity type semiconductor layer  24  that is low in resistance. In a case where the second conductivity type semiconductor  24  is of an n-type, it is preferable that, as the metal electrode layer  26 , titanium (Ti), aluminum (Al), or one of alloys comprising them be applied. Whichever material is applied, the resulting layer  26  becomes transparent, or almost transparent, with respect to the light that has been emitted in the active layer  23 .  
         [0033]     It is sufficient that the first electrode  29  is electrically connected to the first conductivity type semiconductor layer  22  and it is the one that can be electrically contacted with the first conductivity type semiconductor layer  22 . In a case where the first conductivity type semiconductor layer  22  is an n-type one, it is preferable that the first electrode  29  be the one that is comprised of titanium (Ti), aluminum (Al), or one of alloys comprising them. In a case where that layer  22  is a p-type one, it is preferable that, as the first electrode  29 , gold (Au), nickel (Ni), or one of alloys comprising them, or an electrode material comprised of ZnO or ITO be applied.  
         [0034]     Furthermore, it is preferable that, as illustrated in  FIG. 2 , part of the first conductivity type semiconductor layer  22  be exposed; and the first electrode  29  be formed on that exposed portion. This is because the manufacturing method involved is made easy. Namely, adopting this structure is preferable in the respect that, after forming all relevant layers, it can be formed only by executing the process steps such as the photolithography, etching or the like. Furthermore, the first electrode  29  is not limited to that position. Needless to say, it would be sufficient if that electrode  29  is provided at a position at which it is electrically connected to the first conductivity type semiconductor layer  22  and which enables exhibiting the effect of the invention of this application.  
         [0035]     Regarding the second electrode  28 , it may be made of any material only if it is electrically connected to the metal electrode layer  26  and can be brought into ohmic contact with the metal electrode layer  26 . For example, as that second electrode  28 , gold (Au) or aluminum (Al) can be applied.  
         [0036]     Accordingly, if the superlattice layer  211  is disposed at a given position between the metal electrode layer  26  and the active layer  23 , supplying the electric current to the active layer  23  becomes uniformly performed. As a result of this, it is possible to make thin the metal electrode layer  26  and to more enhance, than in the prior art, the efficiency of taking out the light, which has been emitted in the active layer  23 , from the side where the second conductivity type semiconductor layer  24  is located. Also, even if the second electrode  28  is disposed on an end portion of the metal electrode layer  26 , it is possible to cause uniform luminescence of the light from within the active layer  23 . As a result of the second electrode  28  being able to be disposed on an end of the metal electrode layer  26 , it is possible to more enhance, than in the prior art, the efficiency of taking out the light, which has been emitted in the active layer  23 , from the side where the second conductivity type semiconductor layer  24  is located. Furthermore, without providing the second electrode  28 , a line of electrode may be bonded directly to the metal electrode layer  26 .  
         [0037]     Next, another embodiment of the invention of this application will be explained using  FIGS. 3 and 4 . The other mode of the invention of this application is a semiconductor light emitting device that has wholly or partly omitted therefrom the metal electrode layer that was provided in the above-described preceding embodiment. If making the most of the function of the superlattice layer  211  which causes the diffusion of the electric current, it is possible to omit the provision of the metal electrode layer wholly or partly. If able to wholly or partly omit the metal electrode layer, it is possible to reduce the manufacturing process steps for the semiconductor light emitting device.  
         [0038]     This other embodiment of the invention of this application will be explained with reference to the accompanying drawings.  FIG. 3  is a schematic view of the semiconductor light emitting device that embodying the other embodiment of the invention of this application. In  FIG. 3 , a reference numeral  21  denotes a substrate, a reference numeral  22  denotes a first conductivity type semiconductor layer, a reference numeral  23  denotes an active layer that has a luminous region, a reference numeral  24  denotes a second conductivity type semiconductor layer, a reference numeral  28  denotes a second electrode, a reference numeral  29  denotes a first electrode, and a reference numeral  211  denotes the superlattice layer. Each of the first conductivity type semiconductor layer  22  and second conductivity type semiconductor layer  24  is an n-type or p-type semiconductor layer, and has a polarity that is opposite to that of the other. The invention of this application has a characterizing feature in that the superlattice layer  211  is provided between the second electrode  28  and the active layer  23 .  
         [0039]     As the material of the substrate  21 , sapphire, SiC or the like, can be applied. That material is not limited to sapphire or SiC if it is material that is different from GaN. Also, in a case where using a sapphire substrate as the substrate  21 , the principal surface thereof may be a C, R, or A surface.  
         [0040]     As each of the first conductivity type semiconductor layer  22 , active layer  23 , and second conductivity type semiconductor layer  24 , there is used the one that is comprised of a gallium nitride compound semiconductor that is expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1). When applying as the first conductivity type semiconductor layer  22 , active layer  23 , and second conductivity type semiconductor layer  24  the one that is comprised of a gallium nitride compound semiconductor that is expressed as Al x Ga y In 1-x-y N (where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1), it is possible to cause light emission over a wide range of wavelengths.  
         [0041]     The first conductivity type semiconductor layer  22  may be of a single-layer, or multi-layer, structure. Although in  FIG. 3  that layer consists of a single layer that exhibits both functions of a cladding layer and contact layer that makes ohmic contact with the first electrode  29 , the cladding layer and contact layer may be formed respectively separately. Further, the first conductivity type semiconductor layer  22  may have a layer that has other function such as a hole barrier layer.  
         [0042]     The second conductivity type semiconductor layer  24  may be of a single-layer, or multi-layer, structure. Although in  FIG. 3  that layer  24  consists of a layer that exhibits both functions of a cladding layer and contact layer and the superlattice layer  211 , the cladding layer and contact layer may be formed respectively separately. Further, the second conductivity type semiconductor layer  24  may have a layer that has other function such as an electron barrier layer.  
         [0043]     The active layer  23  may be formed as having a structure, such as a bulk structure, a single quantum well structure, or a multi-quantum well structure. In a case where adopting a single quantum well structure or multi-quantum well structure, it results that as the well layer that composes the single quantum well structure or multi-quantum well structure there is used a layer that is narrow in forbidden band width and as the barrier layer there is used a layer that is wide in forbidden band width. For example, as the well layer, there can be used a layer that consists of material expressed as In 1-a Ga a N (where 0&lt;a≦1), while, as the barrier layer, there can be used a layer that consists of material expressed as Al 1-b Ga b N (where 0&lt;b≦1), provided that a×b&lt;1.  
         [0044]     In a case where utilizing the nature that the electrical conductivity that is measured in the direction that is parallel to the superlattice layer  211  is higher than that which is measured in the direction that is vertical to that layer  211 , if the superlattice layer  211  is disposed at a given position between the second electrode  28  and active layer  23 , the supply of the electric current to the active layer  23  is uniformly performed, even omitting the use of the metal electrode layer and further disposing the second electrode  28  on the end of the second conductivity type semiconductor layer  24  as illustrated in  FIG. 3 . As a result of this, it is possible to more enhance, than in the prior art, the effect of taking out the light, which has been emitted in the active layer  23 , from the side where the second conductivity type semiconductor layer  24  is located.  
         [0045]     As the superlattice layer  211 , it is preferable to use a semiconductor layer which is comprised of a gallium nitride compound semiconductor that is expressed as Al p Ga q In 1-p-q N (where 0≦p≦1, 0≦q≦1, and 0≦p+q≦1) and which has a forbidden band width that is wider than that of the active layer  23 . If, using the semiconductor layer which is comprised of a gallium nitride compound semiconductor that is expressed as Al p Ga q In 1-p-q N (where 0≦p≦1, 0≦q≦1, and 0≦p+q≦1), it is possible to form the superlattice layer  211  by alternately laminating the layer narrow in forbidden band width with the layer wide in forbidden band width. Also, by making the superlattice layer  211  be a semiconductor layer having the forbidden band width of that is wider than that of the active layer  23 , it is possible to efficiently emit the light to outside the semiconductor light emitting device without the light emitted in the luminous region of the active layer  23  being adsorbed into the superlattice layer  211 .  
         [0046]     Furthermore, although, the superlattice layer  211  is disposed at the position that contacts with the active layer  23  in  FIG. 3 , the superlattice layer  211  may be disposed at a position that is arbitrary between the second electrode  28  and the active layer  23 . For example, the superlattice layer  211  may be disposed in direct contact with the second electrode  28  to thereby make the superlattice layer  211  function as a contact layer.  
         [0047]     It is sufficient if the first electrode  29  is electrically connected to the first conductivity type semiconductor layer  22  and is the one that can be contacted with the first conductivity type semiconductor layer  22 . In a case where the first conductivity type semiconductor layer  22  is an n-type one, it is preferable that the first electrode  29  be the one that is comprised of titanium (Ti), aluminum (Al), or one of alloys comprising them. In a case where that layer  22  is a p-type one, it is preferable that, as the first electrode  29 , gold (Au), nickel (Ni), or one of alloys comprising them, or an electrode material comprised of ZnO or ITO be applied.  
         [0048]     Furthermore, it is preferable that, as illustrated in  FIG. 3 , part of the first conductivity type semiconductor layer  22  be exposed; and the first electrode  29  be formed on that exposed portion. This is because the manufacturing method involved is made easy. Namely, adopting this structure is preferable in the respect that, after forming all relevant layers, it can be formed only by executing the process steps such as the photolithography, etching or the like. Furthermore, the first electrode  29  is not limited to that position. Needless to say, it would be sufficient if that electrode  29  is provided at a position at which it is electrically connected to the first conductivity type semiconductor layer  22  and which enables exhibiting the effect of the invention of this application.  
         [0049]     The second electrode  28  may be electrically connected to the second conductivity type semiconductor layer  24  and can be brought into contact with this layer  24 . In a case where the second conductivity type semiconductor layer  24  is an n-type one, it is preferable that the layer  24  be the one that is comprised of titanium (Ti), aluminum (Al), or one of alloys comprising them. In a case where that layer  24  is a p-type one, it is preferable that, as the layer  24 , gold (Au), nickel (Ni), or one of alloys comprising them, or an electrode material comprised of ZnO or ITO be applied.  
         [0050]     Accordingly, if the superlattice layer  211  is disposed at a given position between the second electrode  28  and the active layer  23 , supplying the electric current to the active layer  23  is uniformly performed. As a result of this, it is possible to omit the use of the metal electrode layer and to more enhance, than in the prior art, the efficiency of taking out the light, which has been emitted in the active layer  23 , from the side where the second conductivity type semiconductor layer  24  is located. Also, even if the second electrode  28  is disposed on an end portion of the second conductivity type semiconductor layer  24 , it is possible to cause uniform luminescence of the light in the active layer  23 . As a result of the second electrode  28  being able to be disposed on an end of the metal electrode layer  26 , it is possible to more enhance, than in the prior art, the efficiency of taking out the light, which has been emitted in the active layer  23 , from the side where the second conductivity type semiconductor layer  24  is located.  
         [0051]     As has been described above, according to the present invention, as a result of its being equipped with the superlattice layer, it becomes possible to make the electrode layer thin and, further, omit the use of it wholly or partly. And, it is possible to more enhance the efficiency of taking out the light emitted in the active layer, than in the prior art. Also, it becomes possible to dispose the second electrode on an end of the electrode layer.