Patent Publication Number: US-2023163236-A1

Title: Light emitting element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims priority to Japanese Patent Application No. 2021-189590, filed on Nov. 22, 2021. The entire contents of these applications are incorporated herein by reference. 
     FIELD 
     The present disclosure relates to a light emitting element. 
     BACKGROUND 
     Japanese Patent Publication No. 2017-157667, for example, discloses a light emitting element that comprises nitride semiconductor layers including a tunnel junction layer. 
     SUMMARY 
     For a light emitting element that comprises nitride semiconductor layers including a tunnel junction layer, there is a need to improve the emission efficiency. An object of the present invention is to provide a light emitting element with improved emission efficiency. 
     According to an embodiment of the present disclosure, a light emitting element includes, successively from a lower side to an upper side, a first light emitting part having a first active layer, a tunnel junction part, and a second light emitting part having a second active layer. The first active layer includes a plurality of first well layers, and a first barrier layer positioned between two adjacent first well layers among the first well layers and having a larger band gap than the band gaps of the first well layers. The second active layer includes a plurality of second well layers, and a second barrier layer positioned between two adjacent second well layers among the second well layers and having a larger band gap than the band gaps of the second well layers. The second barrier layer includes a nitride semiconductor layer containing aluminum and gallium and having a higher aluminum composition ratio than the aluminum composition ratio of the first barrier layer. The aluminum composition ratio peak of the second barrier layer is located on the first light emitting part side. 
     According to certain embodiments of the present disclosure, a light emitting element with improved emission efficiency can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view of a light emitting element according to a first embodiment. 
         FIG.  2    is a cross-sectional view of a first superlattice layer according to the first embodiment. 
         FIG.  3    is a cross-sectional view of a first active layer according to the first embodiment. 
         FIG.  4    is a cross-sectional view of a second superlattice layer according to the first embodiment. 
         FIG.  5    is a cross-sectional view of a second active layer according to the first embodiment. 
         FIG.  6    is a cross-sectional view of a portion of the second active layer of a variation of the first embodiment. 
         FIG.  7    is a cross-sectional view of a light emitting element according to a second embodiment. 
         FIG.  8    is a cross-sectional view of a second active layer according to the second embodiment. 
         FIG.  9    is a cross-sectional view of a second superlattice layer according to the second embodiment. 
         FIG.  10    is a cross-sectional view of a light emitting element according to a third embodiment. 
         FIG.  11 A  is a graph showing the forward voltage measurement results of the light emitting elements according to the first embodiment. 
         FIG.  11 B  is a graph showing the light output measurement results of the light emitting elements according to the first embodiment. 
         FIG.  12 A  is a graph showing the forward voltage measurement results of the light emitting elements according to the second embodiment. 
         FIG.  12 B  is a graph showing the light output measurement results of the light emitting elements according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be explained below with reference to the accompanying drawings. In the drawings, the same reference numerals are used to denote the same constituents. The drawings are schematic representations of the embodiments. As such, the scale, spacing, or positional relationships of the members might be exaggerated, or a certain portion of a member omitted. An end face view showing only a cut section might be used as a cross-sectional view. 
     First Embodiment 
       FIG.  1    is a cross-sectional view of a light emitting element  1  according to a first embodiment. 
     The light emitting element  1  has a substrate  10 , a semiconductor structure  20 , a p-side electrode  11 , and an n-side electrode  12 . 
     A substrate  10  supports a semiconductor structure  20 . For the material for the substrate  10 , for example, sapphire, silicon, SiC, GaN, or the like can be used. In the case of using a sapphire substrate as the substrate  10 , the semiconductor structure  20  is disposed on C-plane of the sapphire structure  20 . 
     A semiconductor structure  20  is a stack structure in which a plurality of nitride semiconductor layers are stacked. Nitride semiconductors can include all semiconductors obtained by varying the composition ratio x and y within their ranges in the chemical formula In x Al y Ga 1-x-y N (0≤x≤1, 0≤y≤1, x+y≤1). For example, the semiconductor structure  20  can be epitaxially grown on the substrate  10 . 
     In the present specification, the lower side is closer to the substrate  10  relative to the upper side. A semiconductor structure  20  includes, successively from the lower side to the upper side, a first light emitting part  21 , a tunnel junction part  30 , and a second light emitting part  22 . 
     A first light emitting part  21  has an n-side nitride semiconductor layer  41  positioned on the substrate  10 , a first superlattice layer  50  positioned on the n-side nitride semiconductor layer  41 , a first active layer  60  positioned on the first superlattice layer  50 , and a first p-side nitride semiconductor layer  42  positioned on the first active layer  60 . 
     A second light emitting part  22  has a second superlattice layer  70  positioned on a tunnel junction part  30 , a second active layer  80  positioned on the second superlattice layer  70 , and a second p-side nitride semiconductor layer  43  positioned on the second active layer  80 . 
     An n-side nitride semiconductor layer  41  has an n-type layer containing an n-type impurity. The n-type layer contains, for example, silicon (Si) as an n-type impurity. Alternatively, the n-type layer may contain germanium (Ge) as an n-type impurity. The n-side nitride semiconductor layer  41  only needs to have the function of supplying electrons, and may include an undoped layer formed without intentionally doping with an n-type impurity or a p-type impurity. In the case in which an undoped layer is adjacent to a layer intentionally doped with an n-type impurity and/or a p-type impurity, the undoped layer might contain the n-type impurity and/or the p-type impurity diffused from the adjacent layer. 
     A first p-side nitride semiconductor layer  42  and a second p-side nitride semiconductor layer  43  each have a p-type layer containing a p-type impurity. The p-type layer contains, for example, magnesium (Mg) as a p-type impurity. The first p-side nitride semiconductor layer  42  and the second p-side nitride semiconductor layer  43  only need to have the function of supplying positive holes, and may include an undoped layer. 
     A first active layer  60  and a second active layer  80 , as described later, have a multi-quantum well structure having a plurality of well layers and a plurality of barrier layers. The first active layer  60  and the second active layer  80  can emit, for example, blue light or green light. The peak emission wavelength of blue light is 430 nm to 490 nm. The peak emission wavelength of green light is 500 nm to 540 nm. The peak emission wavelength of the first active layer  60  and the peak emission wavelength of the second active layer  80  may be the same or different. The first active layer  60  and the second active layer  80  can emit light having a shorter peak emission wavelength than that of blue light, or light having a longer peak emission wavelength than that of green light. 
     A tunnel junction part  30  is made of a nitride semiconductor. The tunnel junction part  30  forms a tunnel junction with the first p-side nitride semiconductor layer  42 . The tunnel junction part  30  has at least one semiconductor layer of a p-type layer and a n-type layer. The p-type layer is disposed in contact with the upper face of the first p-side nitride semiconductor layer  42 . The n-type layer, if a p-type layer is disposed, is disposed in contact with the upper face of the p-type layer. If no p-type layer is disposed, the n-type layer is disposed in contact with the upper face of the first p-side nitride semiconductor layer  42 . 
     A first superlattice layer  50  is positioned between the n-side nitride semiconductor layer  41  and the first active layer  60 . A second superlattice layer  70  is positioned between the tunnel junction part  30  and the second active layer  80 . Providing a first superlattice layer  50  and a second superlattice layer  70  can reduce lattice mismatch between the substrate  10  and the semiconductor structure  20 , thereby reducing crystal defects in the semiconductor structure  20 . 
     The n-side nitride semiconductor layer  41  has an n-side contact face  41   a  on which no semiconductor layer is disposed. On the n-side contact face  41   a,  an n-side electrode  12  is disposed. The n-side electrode  12  is electrically connected to the n-side nitride semiconductor layer  41 . 
     On the upper face of the second p-side nitride semiconductor  43 , a p-side electrode  11  is disposed. The p-side electrode  11  is electrically connected to the second p-side nitride semiconductor layer  43 . 
     A forward voltage is applied across the p-side electrode  11  and the n-side electrode  12 . At this time, a forward voltage is applied across the second p-side nitride semiconductor layer  43  of the second light emitting part  22  and the n-side nitride semiconductor layer  41  of the first light emitting part  21 , supplying positive holes and electrons to the first active layer  60  and the second active layer  80  thereby allowing the first active layer  60  and the second active layer  80  to emit light. 
     According to a light emitting element  1  of the first embodiment, in which a second active layer  80  is provided above a first active layer  60 , the per unit area output can be increased as compared to a light emitting element having a single active layer. 
     When a forward voltage is applied across the p-side electrode  11  and the n-side electrode  12 , a reverse voltage would be applied to the tunnel junction formed by the tunnel junction part  30  and the first p-side nitride semiconductor layer  42 . Accordingly, allowing the p-type layer and the n-type layer that form the tunnel junction to respectively have high p-type and n-type impurity concentrations can narrow the width of the depletion layer formed by the junction between the tunnel junction part  30  and the first p-side nitride semiconductor layer  42 . This allows for the tunneling of the electrons present in the valence band in the p-type layer to the conduction band of the n-type layer to thereby facilitate the electric current flow to the tunnel junction part  30 . 
     The first superlattice layer  50 , the first active layer  60 , the second superlattice layer  70 , and the second active layer  80  will be explained in detail below. 
     First Superlattice Layer 
     As shown in  FIG.  2   , a first superlattice layer  50  has a plurality of first nitride semiconductor layers  51  and a plurality of second nitride semiconductor layers  52 . The first superlattice layer  50  can have, for example, 15 to 25 pairs of a first nitride semiconductor layer  51  and a second nitride semiconductor layer  52 . The first superlattice layer  50  can have, for example, twenty first nitride semiconductor layers  51  and twenty second nitride semiconductor layers  52 . In the first superlattice layer  50 , a second nitride semiconductor layer  52  is in the lowest position (the lowermost layer), and a first nitride semiconductor layer  51  is in the highest position (the uppermost layer). From the second nitride semiconductor layer  52 , the lowermost layer, to the first nitride semiconductor layer  51 , the uppermost layer, the second nitride semiconductor layers  52  and the first nitride semiconductor layers  51  are formed alternately. 
     The second nitride semiconductor layers  52  include a second nitride semiconductor layer  52  positioned between two adjacent first nitride semiconductor layers  51  among the first nitride semiconductor layers  51 . A second nitride semiconductor layer  52  is also interposed between the lowermost first nitride semiconductor layer  51  and the n-side nitride semiconductor layer  41 . 
     The first nitride semiconductor layers  51  and the second nitride semiconductor layers  52  contain gallium (Ga). The composition of a first nitride semiconductor layer  51  differs from the composition of a second nitride semiconductor layer  52 . The first nitride semiconductor layers  51  can further contain indium (In). The first nitride semiconductor layers  51  are, for example, undoped InGaN layers. The In composition ratio in each InGaN layer can be set in a range of 5% to 10%. The second nitride semiconductor layers  52  are, for example, undoped GaN layers. The n-type impurity concentration of the first nitride semiconductor layers  51  and the second nitride semiconductor layers  52  can be set, for example, to 1×10 17  cm 3  to 1×10 18  cm 3 . The n-type impurity concentration of the first nitride semiconductor layers  51  and the second nitride semiconductor layers  52  refers to the highest n-type impurity concentration among all concentrations in the first nitride semiconductor layers  51  and the second nitride semiconductor layers  52 . 
     The first nitride semiconductor layers  51  are smaller in thickness than the second nitride semiconductor layers  52 . For example, the thickness of each first nitride semiconductor layer  51  can be set in a range of 0.5 nm to 1.5 nm. For example, the thickness of each second nitride semiconductor layer  52  can be set in a range of 1.5 nm to 3 nm. 
     First Active Layer 
     As shown in  FIG.  3   , a first active layer  60  has a plurality of first well layers  61  and at least one first barrier layer  65 . The first active layer  60  has, for example, three or more first well layers  61  and two or more first barrier layers  65 . The first active layer  60  can have, for example, seven first well layers  61  and six first barrier layers  65 . Each first barrier layer  65  is positioned between two adjacent first well layers  61  among the first well layers  61 . The first active layer  60  can further have a fourth barrier layer  63  in the lowest position in the first active layer  60  and a fifth barrier layer  64  in the highest position in the first active layer  60 . A first well layer  61  is disposed between the fourth barrier layer  63  and the first barrier layer  65  that has the lowest position among the first barrier layers  65 . A first well layer  61  is disposed between the fifth barrier layer  64  and the first barrier layer  65  that has the highest position among the first barrier layers  65 . Between the fourth barrier layer  63  and the fifth barrier layer  64 , the first well layers  61  and the first barrier layers  65  are alternately provided. 
     The band gaps of the first barrier layers  65 , the fourth barrier layer  63 , and the fifth barrier layer  64  are larger than the band gaps of the first well layers  61 . The first well layers  61 , the first barrier layers  65 , the fourth barrier layer  63 , and the fifth barrier layer  64  contain gallium. The first well layers  61  contain gallium and indium. For example, the first well layers  61  are undoped InGaN layers. In the case in which the first well layers  61  are InGaN layers, the In composition ratio can be set in a range of 12% to 18%. For example, the first barrier layers  65 , the fourth barrier layer  63 , and the fifth barrier layer  64  are undoped GaN layers. The first well layers  61  may contain aluminum. 
     The first barrier layers  65  and the fifth barrier layer  64  are larger in thickness than the first well layers  61 . For example, the thickness of a first well layer  61  can be set in a range of 2.5 nm to 4 nm. For example, the thicknesses of the first barrier layers  65  and the fifth barrier layer  64  can be set in a range of 3 nm to 5 nm. The thickness of the fourth barrier layer  63  can be set in a range of 3 nm to 5 nm. 
     Second Superlattice Layer 
     As shown in  FIG.  4   , a second superlattice layer  70  has a plurality of third nitride semiconductor layers  71  and a plurality of fourth nitride semiconductor layers  72 . The second superlattice layer  70  can have, for example, 15 to 25 pairs of a third nitride semiconductor layer  71  and a fourth nitride semiconductor layer  72 . The second superlattice layer  70  can have, for example, twenty third nitride semiconductor layers  71  and twenty fourth nitride semiconductor layers  72 . In the second superlattice layer  70 , a fourth nitride semiconductor layer  72  is in the lowest position (the lowermost layer), and a third nitride semiconductor layer  71  is in the highest position (the uppermost layer). From the fourth nitride semiconductor layer  72 , the lowermost layer, to the third nitride semiconductor layer  71 , the uppermost layer, the fourth nitride semiconductor layers  72  and the third nitride semiconductor layers  71  are formed alternately. 
     The fourth nitride semiconductor layers  72  include a fourth nitride semiconductor layer  72  positioned between two adjacent third nitride semiconductor layers  71  among the third nitride semiconductor layers  71 . A fourth nitride semiconductor layer  72  is also interposed between the lowermost third nitride semiconductor layer  71  and the tunnel junction part  30 . 
     The third nitride semiconductor layers  71  and the fourth nitride semiconductor layers  72  contain gallium and an n-type impurity. The composition of a third nitride semiconductor layer  71  differs from the composition of a fourth nitride semiconductor layer  72 . The third nitride semiconductor layers  71  can further contain indium. The third nitride semiconductor layers  71  are, for example, silicon-doped InGaN layers. In the case in which the third nitride semiconductor layers  71  are InGaN layers, the In composition ratio can be set in a range of 5% to 10%. The fourth nitride semiconductor layers  72  are, for example, silicon-doped GaN layers. The n-type impurity concentration of the third nitride semiconductor layers  71  and the fourth nitride semiconductor layers  72  can be set, for example, to 1×10 17  cm 3  to 1×10 20  cm 3 . The n-type impurity concentration of the third nitride semiconductor layers  71  and the fourth nitride semiconductor layers  72  refers to the highest n-type impurity concentration among all concentrations in the third nitride semiconductor layers  71  and the fourth nitride semiconductor layers  72 . 
     The third nitride semiconductor layers  71  are smaller in thickness than the fourth nitride semiconductor layers  72 . For example, the thickness of a third nitride semiconductor layer  71  can be set in a range of 0.5 nm to 1.5 nm. For example, the thickness of a fourth nitride semiconductor layer  72  can be set in a range of 1.5 nm to 3 nm. 
     Second Active Layer 
     As shown in  FIG.  5   , a second active layer  80  has a plurality of second well layers  81  and at least one second barrier layer  82 . The second active layer  80  has, for example, three or more second well layers  81  and two or more second barrier layers  82 . The second active layer  80  can have, for example, seven second well layers  81  and six second barrier layers  82 . Each second barrier layer  82  is positioned between two adjacent second well layers  81  among the second well layers  81 . 
     The second active layer  80  can further have a sixth barrier layer  83  in the lowest position in the second active layer  80  and a third barrier layer  84  in the highest position in the second active layer  80 . A second well layer  81  is disposed between the sixth barrier layer  83  and the second barrier layer  82  that has the lowest position among the second barrier layers  82 . A second well layer  81  is disposed between the third barrier layer  84  and the second barrier layer  82  that has the highest position among the second barrier layers  82 . Between the sixth barrier layer  83  and the third barrier layer  84 , the second well layers  81  and the second barrier layers  82  are alternately provided. 
     The band gaps of the second barrier layers  82 , the sixth barrier layer  83 , and the third barrier layer  84  are larger than the band gaps of the second well layers  81 . The second well layers  81 , the second barrier layers  82 , the sixth barrier layer  83 , and the third barrier layer  84  are, for example, undoped nitride semiconductor layers. 
     The second well layers  81  contain gallium and indium. For example, the second well layers  81  are undoped InGaN layers. In the case in which the second well layers  81  are InGaN layers, the In composition ratio can be set in a range of 12% to 18%. The second well layers  81  may contain aluminum. 
     The second barrier layers  82  contain aluminum and gallium. The second barrier layers  82  include a nitride semiconductor layer having a higher aluminum composition ratio than the aluminum composition ratio of the first barrier layers  65  of the first active layer  60 . 
     The aluminum composition ratio peak of at least one second barrier layer  82  is located on the first light emitting part  21  side. The aluminum composition ratio peak is preferably located on the first light emitting part  21  side in all second barrier layers  82 . 
     At least one second barrier layer  82  has, successively from the first light emitting part  21  side, a first layer  82   a  and a second layer  82   b.  All second barrier layers  82  preferably have a first layer  82   a  and a second layer  82   b.  A second barrier layer  82  is formed by forming a first layer  82   a,  followed by forming a second layer  82   b  on the first layer  82   a.  For example, a second barrier layer  82  is formed by forming an undoped AlGaN layer as a first layer  82   a,  followed by forming on the first layer  82   a  an undoped GaN layer as a second layer  82   b.  The aluminum composition ratio of each first layer  82   a  is higher than the aluminum composition ratio of each second layer  82   b.  The aluminum composition ratio peak in a second barrier layer  82  is positioned in the first layer  82   a.  The aluminum composition ratio of a first layer  82   a  can be set in a range of 3% to 5%. 
     The third barrier layer  84  contains gallium. The aluminum composition ratio of the third barrier layer  84  is lower than the aluminum composition ratio of the second barrier layers  82 . For example, the third barrier layer  84  is a GaN layer. 
     The sixth barrier layer  83  contains gallium. The aluminum composition ratio of the sixth barrier layer  83  is lower than the aluminum composition ratio of the second barrier layers  82 . For example, the sixth barrier layer  83  is, for example, a GaN layer. 
     The second barrier layers  82  and the third barrier layer  84  are larger in thickness than the second well layers  81 . For example, the thickness of a second well layer  81  can be set in a range of 2.5 nm to 4 nm. For example, the thicknesses of the second barrier layers  82  and the third barrier layer  84  can be set in a range of 3 nm to 5 nm. The thickness of the sixth barrier layer  83  can be set in a range of 3 nm to 5 nm. 
     The thickness of the first layer  82   a  of a second barrier layer  82  can be set in a range of 2% to 25% of the thickness of the second barrier layer  82 . In other words, the thickness of the first layer  82   a  is smaller than the thickness of the second layer  82   b.  For example, the thickness of a first layer  82   a  is preferably set in a range of 0.1 nm to 1 nm, more preferably in a range of 0.5 nm to 1 nm. The thickness of a second layer  82   b  is more preferably set in a range of 0.5 nm to 4 nm. 
     In a light emitting element in which a second light emitting part is formed on a first light emitting part via a tunnel junction part, the crystalline quality and the flatness of the second light emitting part tend to worsen as compared to the first light emitting part. Particularly, the diameters of V-pits formed in the semiconductor layers originating from dislocations that are crystal defects readily become larger in the second light emitting part than in the first light emitting part. This reduces the region of the active layer in the second light emitting part that contributes to emission to thereby easily reduce the emission efficiency of the second light emitting part as compared to the first light emitting part. As a method for improving the flatness of the upper face of a semiconductor layer to be formed, the growth temperature for the semiconductor layer can be increased. However, raising the growth temperature for the semiconductor layer would allow for the diffusion of magnesium, for example, contained in a high p-type impurity concentration p-type layer disposed for forming a tunnel junction, which broadens the depletion layer formed in the tunnel junction part to reduce the tunneling probability. 
     According to the first embodiment, the second barrier layers  82  of the second active layer  80  of the second light emitting part  22  have a higher aluminum composition ratio than the first barrier layers  65  of the first active layer  60  of the first light emitting part  21 , and the aluminum composition ratio peaks in the second barrier layers  82  are located on the first light emitting part  21  side. Aluminum is incorporated into V-pits with priority to initiate crystal growth in V-pits. This can readily fill V-pits thereby improving the flatness without having to increase the growth temperature. This, as a result, can increase the region of the second active layer  80  of the second light emitting part  22  that contributes to emission, thereby improving the emission efficiency of the light emitting element  1 . 
     Increasing the aluminum composition ratio of the first barrier layers  62  in the first active layer  60  on the first light emitting part  21  side, which is less prone to flatness degradation than the second light emitting part  21  side, does not tend to lead to any increase in the emission efficiency of the light emitting element  1 . This is believed to be because the flattening effect on the first barrier layers  65 , although achieved to some extent, is less than that achieved on the second light emitting part  22  side, and the increased band gaps of the first barrier layers  65  reduce the emission efficiency. 
     If the aluminum composition ratio peaks in the second barrier layers  82  are located on the second p-side nitride semiconductor layer  43  side, instead of the first light emitting part  21  side, aluminum would mix into the second well layers  81  formed on the second barrier layers  82  to degrade the crystalline quality of the second well layers  81  that might reduce the output of the light emitting element  1 . If the aluminum composition ratio peaks in the second barrier layers  82  are located on the second p-side nitride semiconductor layer  43  side, moreover, the internal electric field might have an adverse effect to reduce the efficiency of injecting electrons from the first light emitting part  21  into the second well layers  81 . 
     Next, the forward voltage and light output measurement results of the samples of the light emitting element  1  according to the first embodiment will be explained. The thickness of the first layer  82   a  in each second barrier layer  82  that included a first layer  82   a  and a second layer  82   b  was varied among the samples to be 0.1 nm, 0.3 nm, 0.5 nm, and 1 nm, to measure their forward voltages and light outputs. 
     Each of the samples of the light emitting element  1  had the constituents described below. 
     The substrate  10  was a sapphire substrate. 
     The n-side nitride semiconductor layer  41  contained silicon as a n-type impurity. The silicon concentration of the n-side nitride semiconductor layer  41  was about 1×10 19  cm 3 . The silicon concentration of the n-side nitride semiconductor layer  41  refers to the highest silicon concentration in the n-side nitride semiconductor layer  41 . The thickness of the n-side nitride semiconductor layer  41  was about 5 μm. 
     The first superlattice layer  50  had twenty first nitride semiconductor layers  51  and twenty second nitride semiconductor layers  52 . The first nitride semiconductor layers  51  were undoped InGaN layers. The In composition ratio of each InGaN layer was about 7%. The first nitride semiconductor layers  51  were about 1 nm in thickness. The second nitride semiconductor layers  52  were undoped GaN layers. The second nitride semiconductor layers  52  were about 2 nm in thickness. 
     The first active layer  60  had seven first well layers  61  and six first barrier layers  65 . The first active layer  60  further had a fourth barrier layer  63  positioned lowest in the first active layer  60 , and a fifth barrier layer  64  positioned highest in the first active layer  60 . The first well layers  61  were undoped InGaN layers. The In composition ratio of each first well layers  61  was about 15%. The first well layers  61  were about 3.5 nm in thickness. The first barrier layers  65  were undoped GaN layers. The first barrier layers  65  were about 4 nm in thickness. The fourth barrier layer  63  included, successively from the first superlattice layer  50  side, a silicon-doped InGaN layer and an undoped GaN layer. The thickness of the fourth barrier layer  63  was about 3.5 nm. The fifth barrier layer  64  was an undoped GaN layer. The thickness of the fifth barrier layer  64  was about 4 nm. 
     The first p-side nitride semiconductor layer  42  contained magnesium as a p-type impurity. The magnesium concentration of the first p-side nitride semiconductor  42  was about 5×10 20  cm 3 . The magnesium concentration of the first p-side nitride semiconductor  42  refers to the highest magnesium concentration in the first p-side nitride semiconductor  42 . The thickness of the first p-side nitride semiconductor  42  was about 80 nm. 
     The tunnel junction part  30  included a silicon-doped n-type GaN layer. The silicon concentration of the n-type GaN layer was about 5×10 20  cm 3 . The thickness of the n-type GaN layer was about 150 nm. 
     The second superlattice layer  70  had twenty third nitride semiconductor layers  71  and twenty fourth nitride semiconductor layers  72 . The third nitride semiconductor layers  71  were silicon-doped InGaN layers. The In composition ratio of each third nitride semiconductor layer  71  was about 7%. The third nitride semiconductor layers  71  were about 1 nm in thickness. The fourth nitride semiconductor layers  72  were silicon-doped GaN layers. The fourth nitride semiconductor layers  72  were about 2 nm in thickness. The silicon concentration of the third nitride semiconductor layers  71  and the fourth nitride semiconductor layers  72  was about 1×10 19  cm 3 . 
     The second active layer  80  had seven second well layers  81  and six second barrier layers  82 . The second active layer  80  further had a sixth barrier layer  83  that was positioned lowest in the second active layer  80 , and a third barrier layer  84  that was positioned highest in the second active layer  80 . The second well layers  81  were undoped InGaN layers. The In composition ratio of each second well layer  81  was about 15%. The second well layer  81  were about 3.5 nm in thickness. Each second barrier layer  82  had a first layer  82   a  and a second layer  82   b.  The first layers  82   a  were undoped AlGaN layers. The Al composition ratio in each first layer  82   a  was about 4%. The second layers  82   b  were undoped GaN layers. The second barrier layers  82  were about 4 nm in thickness. The thickness of the first layer  82   a  in each second barrier layer  82  was varied among samples to be 0.1 nm, 0.3 nm, 0.5 nm, and 1 nm. The sixth barrier layer  83  included, successively from the second superlattice layer  70  side, a silicon-doped InGaN layer and an undoped GaN layer. The thickness of the sixth barrier layer  83  was about 3.5 nm. The third barrier layer  84  was an undoped GaN layer. The thickness of the third barrier layer  84  was about 4 nm. 
     The second p-side nitride semiconductor layer  43  contained magnesium as a p-type impurity. The magnesium concentration of the second p-side nitride semiconductor layer  43  was about 5×10 20  cm 3 . The magnesium concentration of the second p-side nitride semiconductor layer  43  refers to the highest magnesium concentration in the second p-side nitride semiconductor layer  43 . The thickness of the second p-side nitride semiconductor layer  43  was about 100 nm. 
       FIG.  11 A  is a graph showing the forward voltage measurement results of the light emitting element  1  samples when the forward current applied was 120 mA. In  FIG.  11 A , the horizontal axis represents the thickness (nm) of the first layer  82   a  in a second barrier layer  82 . In  FIG.  11 A , the vertical axis represents the amount of change in the forward voltage (V) as compared to the forward voltage (0.00 V) assumed in the case in which the thickness of a first layer  82   a  was zero. The thickness of a first layer  82   a  being zero is the case in which the second barrier layers  82  were composed only of GaN layers. 
     In  FIG.  11 A , black circles show the measurement results of the samples of the light emitting element  1  according to the first embodiment. In  FIG.  11 A , white squares show the measurement results of the light emitting element samples in the first comparative example in which the aluminum composition ratio peaks in the second barrier layers  82  were not located on the first light emitting part  21  side, but were located on the second p-side nitride semiconductor layer  43  side. The light emitting element samples in the first comparative example were produced to have the same structure as the light emitting element  1  of the first embodiment except for the difference in the second barrier layers  82 . The light emitting element samples in the first comparative example were also produced such that the thickness of the first layer  82   a  in each second barrier layer  82  was varied to be 0.1 nm, 0.3 nm, 0.5 nm, and 1 nm. 
     The results in  FIG.  11 A  show that the forward voltage of the sample of light emitting element  1  according to the first embodiment where the first layers  82   a  were 0.5 nm in thickness was lower than those of the sample without first layers  82   a  and the first comparative example sample where the first layers  82   a  were 0.5 nm in thickness. Moreover, the forward voltage of the sample of the light emitting element  1  according to the first embodiment where the first layers  82   a  were 1 nm in thickness was lower than that of the sample without first layers  82   a,  but similar to that of the first comparative example sample where the first layers  82   a  were 1 nm in thickness. 
       FIG.  11 B  is a graph showing the light output measurement results of the samples of the light emitting element  1  when the forward current applied was 120 mA. In  FIG.  11 B , the horizontal axis represents the thickness (nm) of the first layer  82   a  in a second barrier layer  82 . In  FIG.  11 B , the vertical axis represents the relative light output value as compared to the light output (1.00) assumed in the case in which the first layers  82   a  were zero in thickness. 
     In  FIG.  11 B , black circles show the measurement results of the samples of the light emitting element  1  according to the first embodiment. In  FIG.  11 B , white squares show the measurement results of the light emitting element samples in the first comparative example described above. 
     The results in  FIG.  11 B  show that the light outputs of the samples of the light emitting element  1  according to the first embodiment where the first layers  82   a  were 0.1 nm, 0.3 nm, 0.5 nm, and 1 nm in thickness were higher than the sample without first layers  82   a  and all of the light emitting element samples in the first comparative example. Accordingly, the first layers  82   a  are preferably 0.1 nm to 1 nm in thickness in order to increase the light output. Furthermore, considering together the results shown in  FIG.  11 A , the first layers  82   a  are preferably 0.5 nm to 1 nm in thickness in order to increase the light output while lowering the forward voltage. 
     As shown in  FIG.  6   , a second barrier layer  82  can further have a third layer  82   c  that is positioned closer to the first light emitting part  21  than the first layer  82   a  is.  FIG.  6    is a cross-sectional view of a portion of the second active layer  80  in a variation of the first embodiment.  FIG.  6    shows two adjacent second well layers  81  and the second barrier layer  82  interposed between the second well layers  81 . 
     A second barrier layer  82  can be formed on a second well layer  81  by forming a third layer  82   c  on the second well layer  81 , forming a first layer  82   a  on the third layer  82   c,  and forming a second layer  82   b  on the first layer  82   a.  For example, a second barrier layer  82  is formed by forming an undoped GaN layer as a third layer  82   c,  forming an undoped AlGaN layer as a first layer  82   a  on the third layer  82   c,  and forming an undoped GaN layer as a second layer  82   b  on the first layer  82   a.  Providing a third layer  82   c  can reduce the internal electric field generated at the interface between the second well layer  81  and the second barrier layer  82 , thereby increasing the emission efficiency. The aluminum composition ratio of the second layer  82   b  and the aluminum composition ratio of the third layer  82   c  are lower than the aluminum composition ratio of the first layer  82   a.  The third layer  82   c  is smaller in thickness than the second layer  82   b.  The thickness of the third layer  82   c  can be set in a range of 0.1 nm to 1 nm. 
     Second Embodiment 
       FIG.  7    is a cross-sectional view of a light emitting element  2  according to a second embodiment. 
       FIG.  8    is a cross-sectional view of a second active layer  180  of the second embodiment. 
       FIG.  9    is a cross-sectional view of a second superlattice layer  170  of the second embodiment. 
     In a light emitting element  2  according to the second embodiment, the second active layer and the second superlattice layer are different from the light emitting element  1  according to the first embodiment. 
     Second Active Layer 
     As shown in  FIG.  8   , a second active layer  180  has a plurality of second well layers  81  and at least one second barrier layer  85 . The second active layer  180  has, for example, three or more second well layers  81  and two or more second barrier layers  85 . The second active layer  180  can have, for example, seven second well layers  81  and six second barrier layers  85 . Each second barrier layer  85  is interposed between two adjacent second well layers  81  among the second well layers  81 . Furthermore, the second active layer  180  can have a sixth barrier layer  83  positioned lowest in the second active layer  180  and a third barrier layer  84  positioned highest in the second active layer  180 . A second well layer  81  is interposed between the second barrier layer  85  positioned lowest among the second barrier layers  85  and the sixth barrier layer  83 . A second well layer  81  is interposed between the second barrier layer  85  positioned highest among the second barrier layers  85  and the third barrier layer  84 . Between the sixth barrier layer  83  and the third barrier layer  84 , the second well layers  81  and the second barrier layers  85  are alternately provided. 
     The band gaps of the second barrier layers  85 , the sixth barrier layer  83 , and the third barrier layer  84  are larger than the band gaps of the second well layers  81 . For the materials for the second well layers  81 , the same materials as those for the first well layers  61  in the first embodiment can be used. For the materials for the sixth barrier layer  83  and the third barrier layer  84 , the same materials as those for the first barrier layers  65  in the first embodiment can be used. 
     In the second active layer  180  of the second embodiment, a portion of each second barrier layer  85  on the first light emitting part  21  side does not contain aluminum, and the second barrier layers  85  are, for, example, undoped GaN layers. The second barrier layers  85  are larger in thickness than the second well layers  81 . For example, the thickness of each second barrier layer  85  may be set in a range of 3 nm to 5 nm. 
     Second Superlattice Layer 
     As shown in  FIG.  9   , a second superlattice layer  170  has a plurality of third nitride semiconductor layers  71  and a plurality of fourth nitride semiconductor layers  73 . The second superlattice layer  170  can have, for example, 15 to 25 pairs of a third nitride semiconductor layer  71  and a fourth nitride semiconductor layer  73 . The second superlattice layer  170  can have, for example, twenty third nitride semiconductor layers  71  and twenty fourth nitride semiconductor layers  73 . In the second superlattice layer  170 , a fourth nitride semiconductor layer  73  is in the lowest position (the lowermost layer), and a third nitride semiconductor layer  71  is in the highest position (the uppermost layer). From the fourth nitride semiconductor layer  73 , the lowermost layer, to the third nitride semiconductor layer  71 , the uppermost layer, the fourth nitride semiconductor layers  73  and the third nitride semiconductor layers  71  are formed alternately. 
     The fourth nitride semiconductor layers  73  include a fourth nitride semiconductor layer  73  interposed between two third nitride semiconductor layers  71  among the third nitride semiconductor layers  71 . A fourth nitride semiconductor layer  73  is also provided between the third nitride semiconductor layer  71  that is the lowermost layer and the tunnel junction part  30 . 
     For the materials for the third nitride semiconductor layers  71 , the same materials as those used in the first embodiment can be used. For example, the third nitride semiconductor layers  71  are silicon-doped InGaN layers. The n-type impurity concentration of the third nitride semiconductor layers  71  can be set in a range of 1×10 17  cm 3  to 1×10 20  cm 3 . 
     The composition of a fourth nitride semiconductor layer  73  differs from the composition of a third nitride semiconductor layer  71 . The fourth nitride semiconductor layers  74  contain aluminum and gallium, and have a higher aluminum composition ratio than the aluminum composition ratio of the second nitride semiconductor layers  52  of the first superlattice layer  50  in the first light emitting part  21 . The aluminum composition ratio peak in at least one of the fourth nitride semiconductor layers  73  is located on the first light emitting part  21  side. The aluminum composition ratio peak is preferably on the first light emitting part  21  side in all fourth nitride semiconductor layers  73 . 
     The fourth nitride semiconductor layers  73  are, for example, silicon-doped n-type nitride semiconductor layers. The silicon concentration of the fourth nitride semiconductor layers  73  can be set, for example, to 1×10 17  cm 3  to 1×10 20  cm 3 . 
     At least one of the fourth nitride semiconductor layers  73  has, successively from the tunnel junction part  30  side, a fourth layer  73   a  and a fifth layer  73   b.  All fourth nitride semiconductor layers  73  preferably have a fourth layer  73   a  and a fifth layer  73   b.  A fourth nitride semiconductor layer  73  is formed by forming a fourth layer  73   a,  followed by forming a fifth layer  73   b  on the fourth layer  73   a.  For example, a fourth nitride semiconductor layer  73  is formed by forming an AlGaN layer as a fourth layer  73   a,  followed by forming a GaN layer as a fifth layer  73   b  on the fourth layer  73   a.  The aluminum composition ratio of a fourth layer  73   a  is higher than the aluminum composition ratio of a fifth layer  73   b.  The aluminum composition ratio peak in a fourth nitride semiconductor layer  73  is located in the fourth layer  73   a.  The aluminum composition ratio of a fourth layer  73   a  can be set in a range of 3% to 5%. 
     The fourth nitride semiconductor layers  73  are smaller in thickness than the third nitride semiconductor layers  71 . For example, the thickness of a third nitride semiconductor layer  71  can be set in a range of 0.5 nm to 1.5 nm. The thickness of a fourth nitride semiconductor layer  73  can be set in a range of 1.5 nm to 3 nm. 
     According to the second embodiment, the fourth nitride semiconductor layers  73  in the second superlattice layer  170  of the second light emitting part  22  contain aluminum at a higher composition ratio than that of the second nitride semiconductor layers  52  in the first superlattice layer  50  of the first light emitting part  21 . Forming the fourth nitride semiconductor layers  73  containing aluminum allows aluminum to be incorporated into V-pits with priority to initiate crystal growth in V-pits thereby readily filling V-pits to improve the flatness without having to increase the growth temperature. This can also improve the flatness of the second active layer  180  formed on the second superlattice layer  170 , increasing the region of the second active layer  180  that contributes to emission, thereby improving the emission efficiency of the light emitting element  2 . 
     Increasing the aluminum composition ratio of the second nitride semiconductor layers  52  of the first superlattice layer  50  on the first light emitting part  21  side, which is less prone to flatness degradation than the second light emitting part  21  side, does not tend to lead to any increase in the emission efficiency of the light emitting element  2 . This is believed to be because the flattening effect on the second nitride semiconductor layers  52 , although achieved to some extent, is less than that achieved on the second light emitting part  22  side, and the increased band gaps of the second nitride semiconductor layers  52  reduce the emission efficiency. 
     Next, the forward voltage and light output measurement results of the samples of the light emitting element  2  according to the second embodiment will be explained. The thickness of the fourth layer  73   a  in each second barrier layer  82  that included a fourth layer  73   a  and a fifth layer  73   b  was varied among samples to be 0.1 nm, 0.3 nm, and 0.5 nm to measure their forward voltages and light outputs. 
     Each of the samples of the light emitting element  2  had the constituents described below. 
     The substrate  10  was a sapphire substrate. 
     The n-side nitride semiconductor layer  41  contained silicon as a n-type impurity. The silicon concentration of the n-side nitride semiconductor layer  41  was about 1×10 19  cm 3 . The silicon concentration of the n-side nitride semiconductor layer  41  refers to the highest silicon concentration in the n-side nitride semiconductor layer  41 . The thickness of the n-side nitride semiconductor layer  41  was about 5 μm. 
     The first superlattice layer  50  had twenty first nitride semiconductor layers  51  and twenty second nitride semiconductor layers  52 . The first nitride semiconductor layers  51  were undoped InGaN layers. The In composition ratio of each InGaN layer was about 7%. The first nitride semiconductor layer  51  were about 1 nm in thickness. The second nitride semiconductor layers  52  were undoped GaN layers. The second nitride semiconductor layers  52  were about 2 nm in thickness. 
     The first active layer  60  had seven first well layers  61  and six first barrier layers  65 . The first active layer  60  further had a fourth barrier layer  63  positioned lowest in the first active layer  60 , and a fifth barrier layer  64  positioned highest in the first active layer  60 . The first well layers  61  were undoped InGaN layers. The In composition ratio of each first well layer  61  was about 15%. The first well layers  61  were about 3.5 nm in thickness. The first barrier layers  65  were undoped GaN layers. The first barrier layers  65  were about 4 nm in thickness. The fourth barrier layer  63  included, successively from the first superlattice layer  50  side, a silicon-doped InGaN layer and an undoped GaN layer. The thickness of the fourth barrier layer  63  was about 3.5 nm. The fifth barrier layer  64  was an undoped GaN layer. The thickness of the fifth barrier layer  64  was about 4 nm. 
     The first p-side nitride semiconductor layer  42  contained magnesium as a p-type impurity. The magnesium concentration of the first p-side nitride semiconductor  42  was about 5×10 20  cm 3 . The magnesium concentration of the first p-side nitride semiconductor  42  refers to the highest magnesium concentration in the first p-side nitride semiconductor  42 . The thickness of the first p-side nitride semiconductor  42  was about 80 nm. 
     The tunnel junction part  30  included a silicon-doped n-type GaN layer. The silicon concentration of the n-type GaN layer was about 5×10 20  cm 3 . The thickness of the n-type GaN layer was about 150 nm. 
     The second superlattice layer  70  had twenty third nitride semiconductor layers  71  and twenty fourth nitride semiconductor layers  73 . The third nitride semiconductor layers  71  were silicon-doped InGaN layers. The In composition ratio of each third nitride semiconductor layer  71  was about 7%. The third nitride semiconductor layers  71  were about 1 nm in thickness. Each fourth nitride semiconductor layer  73  had a fourth layer  73   a  and a fifth layer  73   b.  The fourth layers  73   a  were silicon-doped AlGaN layers. The Al composition ratio of each fourth layer  73   a  was about 4%. The fifth layers  73   b  were silicon-doped GaN layers. The fourth nitride semiconductor layers  73  were about 2 nm in thickness. The thicknesses of the fourth layers  73   a  were varied among samples to be 0.1 nm, 0.3 nm, and 0.5 nm. The silicon concentration of each of the third nitride semiconductor layers  71  and the fourth nitride semiconductor layers  73  was about 1×10 19  cm 3 . 
     The second active layer  180  had seven second well layers  81  and six second barrier layers  85 . The second active layer  180  further had a sixth barrier layer  83  that was positioned lowest in the second active layer  180 , and a third barrier layer  84  that was positioned highest in the second active layer  180 . The second well layers  81  were undoped InGaN layers. The In composition ratio of each second well layer  81  was about 15%. The second well layers  81  were about 3.5 nm in thickness. The second barrier layers  85  were undoped GaN layers. The second barrier layers  85  were about 4 nm in thickness. The sixth barrier layer  83  included, successively from the second superlattice layer  70  side, a silicon-doped InGaN layer and an undoped GaN layer. The thickness of the sixth barrier layer  83  was about 3.5 nm. The third barrier layer  84  was an undoped GaN layer. The thickness of the third barrier layer  84  was about 4 nm. 
     The second p-side nitride semiconductor layer  43  contained magnesium as a p-type impurity. The magnesium concentration of the second p-side nitride semiconductor layer  43  was about 5×10 20  cm 3 . The magnesium concentration of the second p-side nitride semiconductor layer  43  refers to the highest magnesium concentration in the second p-side nitride semiconductor layer  43 . The thickness of the second p-side nitride semiconductor layer  43  was about 100 nm. 
       FIG.  12 A  is a graph showing the forward voltage measurement results of the samples of the light emitting element  2  when the forward current applied was 120 mA. In  FIG.  12 A , the horizontal axis represents the thickness (nm) of the fourth layer  73   a  in a fourth nitride semiconductor layer  73 . In  FIG.  12 A , the vertical axis represents the amount of change in the forward voltage (V) as compared to the forward voltage (0.00 V) assumed in the case in which the thickness of a fourth layer  73   a  was zero. The thickness of a fourth layer  73   a  being zero is the case in which the fourth nitride semiconductor layers  73  were composed only of GaN layers. 
     In  FIG.  12 A , black circles show the measurement results of the samples of the light emitting element  2  according to the second embodiment. In  FIG.  12 A , white squares show the measurement results of the light emitting element samples in the second comparative example in which the aluminum composition ratio peaks in the fourth nitride semiconductor layers  73  were not located on the tunnel junction part  30  side, but were located on the second active layer  180  side. The light emitting element samples in the second comparative example were made to have the same structure as the light emitting element  2  of the second embodiment except for the difference in the fourth nitride semiconductor layers  73 . The light emitting element samples in the second comparative example were also produced such that the thickness of the fourth layer  73   a  in each fourth nitride semiconductor layer  73  was varied to be 0.1 nm, 0.3 nm, and 0.5 nm. 
     The results in  FIG.  12 A  show that the forward voltages of the samples of light emitting element  2  according to the second embodiment where the fourth layers  73   a  were 0.3 nm and 0.6 nm in thickness were lower than those of the sample without fourth layers  73   a  and the second comparative example samples where the fourth layers  73   a  were 0.3 nm and 0.5 nm in thickness. Moreover, the forward voltage of the sample of the light emitting element  2  according to the second embodiment where the fourth layers  73   a  were 0.1 nm in thickness was lower than the sample without fourth layers  73   a,  but was similar to the sample in the second comparative example where the fourth layers  73   a  were 0.1 nm in thickness. 
       FIG.  12 B  is a graph showing the light output measurement results of the samples of the light emitting element  2  when the forward current applied was 120 mA. In  FIG.  12 B , the horizontal axis represents the thickness (nm) of the fourth layer  73   a  in a fourth nitride semiconductor layer  73 . In  FIG.  12 B , the vertical axis represents the relative light output value as compared to the light output (1.00) assumed in the case in which the thickness of a fourth layer  73   a  was zero. 
     In  FIG.  12 B , black circles show the measurement results of the samples of the light emitting element  2  according to the second embodiment. In  FIG.  12 B , white squares show the measurement results of the light emitting element samples in the second comparative example. 
     The results in  FIG.  12 B  show that the light outputs of the samples of the light emitting element  2  according to the second embodiment where the fourth layers  73   a  were 0.1 nm, 0.3 nm, and 0.5 nm in thickness were higher than the sample without fourth layers  73   a  and all of the light emitting element samples in the second comparative example. Accordingly, the fourth layers  73   a  are preferably 0.1 nm to 0.5 nm in thickness in order to increase the light output. Furthermore, considering together the results shown in  FIG.  12 A , the fourth layers  73   a  are preferably 0.3 nm to 0.5 nm in thickness in order to increase the light output while lowering the forward voltage. 
     Third Embodiment 
       FIG.  10    is a cross-sectional view of a light emitting element  3  according to a third embodiment. 
     A light emitting element  3  according to a third embodiment includes as a second active layer the second active layer  80  of the first embodiment and as a second superlattice layer the second superlattice layer  170  of the second embodiment. Accordingly, the light emitting element  3  according to the third embodiment can achieve the effect of the second active layer  80  of the first embodiment as well as the effect of the second superlattice layer  170  of the second embodiment. 
     In the foregoing, certain embodiments of the present invention have been explained with reference to specific examples. The present invention, however, is not limited to these specific examples. All forms implementable by a person skilled in the art by suitably making design changes based on any of the embodiments of the present invention described above also fall within the scope of the present invention so long as they encompass the subject matter of the present invention. Furthermore, various modifications and alterations within the spirit of the present invention that could have been made by a person skilled in the art also fall within the scope of the present invention.