Patent Publication Number: US-2023140710-A1

Title: Nitride-based semiconductor light-emitting element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part application of PCT International Application No. PCT/JP2022/018699 filed on Apr. 25, 2022, designating the United States of America, which is based on and claims priority of Japanese Patent Application No. 2021-114115 filed on Jul. 9, 2021. The entire disclosures of the above-identified applications, including the specifications, drawings and claims are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to a nitride-based semiconductor light-emitting element. 
     BACKGROUND 
     Conventionally, a nitride-based semiconductor light-emitting element is used as a light source of a processing device, for instance. There has been a demand for a light source of a processing device to output higher power and to have higher efficiency. In order to improve efficiency of a nitride-based semiconductor light-emitting element, for example, technology for lowering an operating voltage has been known (for example, Patent Literature (PTL) 1). 
     Citation List 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2018-50021 
     SUMMARY 
     Technical Problem 
     It is effective to reduce the thickness of a P-type cladding layer in order to lower the operating voltage of a nitride-based semiconductor light-emitting element, other than the technology stated in PTL 1. However, along with a reduction in the thickness of the P-type cladding layer, a peak of a light intensity distribution in a stack direction (that is, a direction in which semiconductor layers are grown) shifts from an active layer toward an N-type cladding layer. Accordingly, a coefficient of confinement of light in the active layer decreases, and due to this, a heat saturation level of light output decreases. Thus, it is difficult to increase output power of a nitride-based semiconductor light-emitting element. The present disclosure is to address such problems, and is to provide a nitride-based semiconductor light-emitting element having a lowered operating voltage and an increased coefficient of confinement of light in an active layer. 
     Solution to Problem 
     In order to address the above problems, a nitride-based semiconductor light-emitting element according to an aspect of the present disclosure is a nitride-based semiconductor light-emitting element including: a semiconductor stack body. The nitride-based semiconductor light-emitting element emits light from an end surface that faces in a direction perpendicular to a stack direction of the semiconductor stack body. The semiconductor stack body includes: an N-type first cladding layer; an N-side guide layer provided above the N-type first cladding layer; an active layer provided above the N-side guide layer, the active layer including a well layer and a barrier layer and having a quantum well structure; a P-side guide layer provided above the active layer; and a P-type cladding layer provided above the P-side guide layer. Band gap energy of the P-side guide layer monotonically increases with an increase in distance from the active layer. The P-side guide layer includes a portion in which the band gap energy continuously increases with an increase in distance from the active layer. An average of the band gap energy of the P-side guide layer is greater than or equal to an average of band gap energy of the N-side guide layer. Band gap energy of the barrier layer is less than or equal to a smallest value of the band gap energy of the N-side guide layer and a smallest value of the band gap energy of the P-side guide layer, and Tn &lt; Tp is satisfied, where Tp denotes a thickness of the P-side guide layer, and Tn denotes a thickness of the N-side guide layer. 
     Advantageous Effects 
     According to the present disclosure, a nitride-based semiconductor light-emitting element that has a lowered operating voltage, and an increased coefficient of confinement of light in an active layer can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG.  1    is a schematic plan view illustrating the overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  2 A  is a schematic cross sectional view illustrating the overall configuration of the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  2 B  is a schematic cross sectional view illustrating a configuration of an active layer included in the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  3    is a schematic diagram illustrating, in a simplified manner, a light intensity distribution in a stack direction of the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  4    is a graph showing coordinates at positions in the stack direction of the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  5    is a schematic graph showing a distribution of band gap energy in the active layer and layers in the vicinity thereof in the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  6    illustrates graphs showing refractive index distributions and light intensity distributions in the stack direction of nitride-based semiconductor light-emitting elements according to Comparative Examples 1 to 3 and the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  7    illustrates graphs showing simulation results of distributions of valence band potentials and hole Fermi levels in the stack direction of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1 to 3 and the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  8    illustrates graphs showing simulation results of distributions of carrier concentrations in the stack direction of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1 to 3 and the nitride-based semiconductor light-emitting element according to Embodiment 1. 
         FIG.  9    is a graph showing simulation results of a relation between the thickness of an N-side guide layer according to Embodiment 1 and a light confinement coefficient. 
         FIG.  10    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 1 and waveguide loss. 
         FIG.  11    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 1 and an effective refractive index difference. 
         FIG.  12    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 1 and position P1. 
         FIG.  13    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 1 and difference ΔP. 
         FIG.  14    is a graph showing simulation results of a relation between the thickness of a P-type cladding layer according to Embodiment 1 and a light confinement coefficient. 
         FIG.  15    is a graph showing simulation results of a relation between the thickness of the P-type cladding layer according to Embodiment 1 and waveguide loss. 
         FIG.  16    is a graph showing simulation results of a relation between the thickness of the P-type cladding layer according to Embodiment 1 and an effective refractive index difference. 
         FIG.  17    is a graph showing simulation results of a relation between the thickness of the P-type cladding layer according to Embodiment 1 and position P1. 
         FIG.  18    is a graph showing simulation results of a relation between the thickness of the P-type cladding layer according to Embodiment 1 and difference ΔP. 
         FIG.  19    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 2. 
         FIG.  20    is a schematic graph showing a distribution of band gap energy in an active layer and layers in the vicinity thereof in the nitride-based semiconductor light-emitting element according to Embodiment 2. 
         FIG.  21    is a graph showing simulation results of distributions of a valence band potential and a hole Fermi level in the stack direction of the nitride-based semiconductor light-emitting element according to Embodiment 2. 
         FIG.  22    is a graph showing simulation results of distributions of carrier concentrations in the stack direction of the nitride-based semiconductor light-emitting element according to Embodiment 2. 
         FIG.  23    is a graph showing simulation results of a relation between an average In composition ratio of a P-side guide layer  206  according to Embodiment 2 and waveguide loss. 
         FIG.  24    is a graph showing simulation results of a relation between an average In composition ratio of the P-side guide layer according to Embodiment 2 and a light confinement coefficient. 
         FIG.  25    is a graph showing simulation results of a relation between the thickness of an N-side guide layer according to Embodiment 2 and a light confinement coefficient. 
         FIG.  26    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 2 and waveguide loss. 
         FIG.  27    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 2 and an effective refractive index difference. 
         FIG.  28    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 2 and position P1. 
         FIG.  29    is a graph showing simulation results of a relation between the thickness of the N-side guide layer according to Embodiment 2 and difference ΔP. 
         FIG.  30    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 3. 
         FIG.  31    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 4. 
         FIG.  32 A  is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 5. 
         FIG.  32 B  is a cross sectional view illustrating a configuration of an active layer included in the nitride-based semiconductor light-emitting element according to Embodiment 5. 
         FIG.  33    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 6. 
         FIG.  34    is a schematic graph showing a distribution of band gap energy in an active layer and layers in the vicinity thereof in the nitride-based semiconductor light-emitting element according to Embodiment 6. 
         FIG.  35    is a graph showing simulation results of a relation between an average In composition ratio of an N-side guide layer according to Embodiment 6 and a light confinement coefficient. 
         FIG.  36    is a graph showing simulation results of a relation between an average In composition ratio of the N-side guide layer according to Embodiment 6 and an operating voltage. 
         FIG.  37    illustrates graphs showing relations of a position in the stack direction of the nitride-based semiconductor light-emitting element according to Embodiment 1 with a piezo polarization charge density, a piezo polarization electric field, and a conduction band potential. 
         FIG.  38    illustrates graphs showing relations of a position in the stack direction of the nitride-based semiconductor light-emitting element according to Embodiment 6 with a piezo polarization charge density, a piezo polarization electric field, and a conduction band potential. 
         FIG.  39    illustrates graphs showing simulation results of a relation between an average In composition ratio of the N-side guide layer in the nitride-based semiconductor light-emitting element according to Embodiment 6 and a light confinement coefficient. 
         FIG.  40    illustrates graphs showing simulation results of a relation between an average In composition ratio of the N-side guide layer in the nitride-based semiconductor light-emitting element according to Embodiment 6 and waveguide loss. 
         FIG.  41    illustrates graphs showing simulation results of a relation between an average In composition ratio of the N-side guide layer in the nitride-based semiconductor light-emitting element according to Embodiment 6 and an operating voltage. 
         FIG.  42    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 7. 
         FIG.  43    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 8. 
         FIG.  44    is a graph showing a distribution of an Al composition ratio in the stack direction of an electron barrier layer according to Embodiment 8. 
         FIG.  45    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Embodiment 9. 
         FIG.  46    is a schematic graph showing a distribution of band gap energy in an active layer and layers in the vicinity thereof in the nitride-based semiconductor light-emitting element according to Embodiment 9. 
         FIG.  47    is a graph showing a relation between a ridge width and an effective refractive index difference necessary to reduce kinks. 
         FIG.  48    is a schematic graph showing a distribution of band gap energy in an active layer and layers in the vicinity thereof in a nitride-based semiconductor light-emitting element according to Embodiment 10. 
         FIG.  49    is a schematic graph showing a distribution of band gap energy in an active layer and layers in the vicinity thereof in a nitride-based semiconductor light-emitting element according to Embodiment 11. 
         FIG.  50    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Variation 1. 
         FIG.  51    is a schematic cross sectional view illustrating an overall configuration of a nitride-based semiconductor light-emitting element according to Variation 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present disclosure, with reference to the drawings. Note that the embodiments described below each show a particular example of the present disclosure. Accordingly, the numerical values, shapes, materials, elements, the arrangement and connection of the elements, and others stated in the following embodiments are mere examples, and therefore are not intended to limit the present disclosure. 
     The drawings are schematic diagrams, and do not necessarily provide strictly accurate illustration. Accordingly, scales, for instance, are not necessarily the same throughout the drawings. In the drawings, the same numeral is given to substantially the same configuration, and a redundant description thereof may be omitted or simplified. 
     In the present specification, the terms “above” and “below” do not indicate vertically upward and vertically downward in the absolute space recognition, but are rather used as terms defined by a relative positional relation based on the stacking order in a stacked configuration. Furthermore, the terms “above” and “below” are used not only when two elements are spaced apart from each other and another element is present therebetween, but also when two elements are disposed in contact with each other. 
     Embodiment 1 
     A nitride-based semiconductor light-emitting element according to Embodiment 1 is to be described. 
     1-1. Overall Configuration 
     First, an overall configuration of the nitride-based semiconductor light-emitting element according to the present embodiment is to be described with reference to  FIG.  1   ,  FIG.  2 A , and  FIG.  2 B .  FIG.  1    and  FIG.  2 A  are a schematic plan view and a schematic cross sectional view, respectively, which illustrate the overall configuration of nitride-based semiconductor light-emitting element 100 according to the present embodiment.  FIG.  2 A  illustrates a cross section taken along line II-II in  FIG.  1   .  FIG.  2 B  is a schematic cross sectional view illustrating a configuration of active layer  105  included in nitride-based semiconductor light-emitting element  100  according to the present embodiment. Note that the drawings show the X axis, the Y axis, and the Z axis orthogonal to one another. The X axis, the Y axis, and the Z axis form a right-handed orthogonal coordinate system. A stack direction of nitride-based semiconductor light-emitting element  100  is parallel to the Z-axis direction, and a principal direction in which light (laser beam) is emitted is parallel to the Y-axis direction. 
     As illustrated in  FIG.  2 A , nitride-based semiconductor light-emitting element  100  includes semiconductor stack body  100 S that includes nitride-based semiconductor layers, and emits light from end surface  100 F (see  FIG.  1   ) in a direction perpendicular to the stack direction of semiconductor stack body  100 S (that is, the Z-axis direction). In the present embodiment, nitride-based semiconductor light-emitting element  100  is a semiconductor laser element having two end surfaces  100 F and  100 R that form a resonator. End surface  100 F is a front end surface from which a laser beam is emitted, and end surface  100 R is a rear end surface having a higher reflectance than that of end surface  100 F. In the present embodiment, the reflectance of end surfaces  100 F and the reflectance of  100 R are 16% and 95%, respectively. The length of the resonator (that is, a distance between end surface  100 F and end surface  100 R) of nitride-based semiconductor light-emitting element  100  according to the present embodiment is about 1200 µm. 
     As illustrated in  FIG.  2 A , nitride-based semiconductor light-emitting element  100  includes semiconductor stack body  100 S, current block layer  112 , P-side electrode  113 , and N-side electrode  114 . Semiconductor stack body  100 S includes substrate  101 , N-type first cladding layer  102 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  105 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  109 , P-type cladding layer  110 , and contact layer  111 . 
     Substrate  101  is a plate shaped member serving as a base for nitride-based semiconductor light-emitting element  100 . In the present embodiment, substrate  101  is an N-type GaN substrate. 
     N-type first cladding layer  102  is an example of an N-type cladding layer provided above substrate  101 . N-type first cladding layer  102  has a smaller refractive index and greater band gap energy than those of active layer  105 . In the present embodiment, N-type first cladding layer  102  is an N-type Al 0.026 Ga 0.974 N layer having a thickness of 1200 nm. N-type first cladding layer  102  is doped with Si having a concentration of 1×10 18  cm -3 , as an impurity. 
     N-type second cladding layer  103  is an example of an N-type cladding layer provided above substrate  101 . In the present embodiment, N-type second cladding layer  103  is provided above N-type first cladding layer  102 . N-type second cladding layer  103  has a smaller refractive index and greater band gap energy than those of active layer  105 . In the present embodiment, N-type second cladding layer  103  is an N-type GaN layer having a thickness of 100 nm. N-type second cladding layer  103  is doped with Si having a concentration of 1×10 18  cm -3 , as an impurity. The band gap energy of N-type second cladding layer  103  is less than that of N-type first cladding layer  102  and is greater than or equal to the greatest value of the band gap energy of P-side guide layer  106 . 
     N-side guide layer  104  is a light guide layer provided above N-type second cladding layer  103 . N-side guide layer  104  has a greater refractive index and less band gap energy than those of N-type first cladding layer  102  and N-type second cladding layer  103 . In the present embodiment, N-side guide layer  104  is an undoped In 0.04 Ga 0.96 N layer having a thickness of 160 nm. 
     Active layer  105  is a light-emitting layer provided above N-side guide layer  104  and having a quantum well structure. In the present embodiment, active layer  105  includes well layers  105   b  and  105   d , and barrier layers  105   a ,  105   c , and  105   e , as illustrated in  FIG.  2 B . 
     Barrier layer  105   a  is provided above N-side guide layer 104, and functions as a barrier of the quantum well structure. In the present embodiment, barrier layer  105   a  is an undoped In 0.05 Ga 0.95 N layer having a thickness of 7 nm. 
     Well layer  105   b  is provided above barrier layer  105   a , and functions as a well of the quantum well structure. Well layer  105   b  is provided between barrier layer  105   a  and barrier layer  105   c . In the present embodiment, well layer  105   b  is an undoped In 0.18 Ga 0.82 N layer having a thickness of 3 nm. 
     Barrier layer  105   c  is provided above well layer  105   b , and functions as a barrier of the quantum well structure. In the present embodiment, barrier layer  105   c  is an undoped In 0.05 Ga 0.95 N layer having a thickness of 7 nm. 
     Well layer  105   d  is provided above barrier layer  105   c , and functions as a well of the quantum well structure. Well layer  105   d  is provided between barrier layer  105   c  and barrier layer  105   e . In the present embodiment, well layer  105   d  is an undoped In 0.18 Ga 0.82 N layer having a thickness of 3 nm. 
     Barrier layer  105   e  is provided above well layer  105   d , and functions as a barrier of the quantum well structure. In the present embodiment, barrier layer  105   e  is an undoped In 0.05 Ga 0.95 N layer having a thickness of 5 nm. 
     In the present embodiment, band gap energy of each barrier layer is less than or equal to the smallest value of the band gap energy of N-side guide layer  104  and P-side guide layer  106 . Thus, the refractive index of each barrier layer is greater than the refractive indices of N-side guide layer  104  and P-side guide layer  106 . Thus, a coefficient of confinement of light in active layer  105  can be increased. 
     P-side guide layer  106  is a light guide layer provided above active layer  105 . P-side guide layer  106  has a greater refractive index and less band gap energy than those of P-type cladding layer  110 . The band gap energy of P-side guide layer  106  monotonically increases with an increase in distance from active layer  105 . Here, a configuration in which band gap energy monotonically increases includes a configuration in which a region where band gap energy is constant in the stack direction is present. P-side guide layer  106  includes a portion in which band gap energy monotonically increases with an increase in distance from active layer  105 . Here, a configuration in which band gap energy continuously increases does not include a configuration in which band gap energy discontinuously changes in the stack direction. In the present disclosure, the configuration in which band gap energy continuously and monotonically increases means a configuration in which a discontinuous increase in band gap energy in the stack direction at a certain position is less than 2% of the magnitude of band gap energy at the position. For example, the configuration in which band gap energy continuously increases does not include a configuration in which band gap energy increases stepwise by 2% or more in the stack direction, but includes a configuration in which band gap energy increases stepwise by less than 2% in the stack direction. In the present embodiment, band gap energy of entire P-side guide layer  106  continuously increases with an increase in distance from active layer  105 , yet the configuration of P-side guide layer  106  is not limited thereto. For example, a proportion of the thickness of a portion of P-side guide layer  106  having band gap energy that continuously increases with an increase in distance from active layer  105  may be 50% or more of the thickness of entire P-side guide layer  106 . The proportion may be 70% or more, or may be 90% or more. 
     An amount of increase (ΔEgp) in band gap energy of P-side guide layer  106  in the stack direction may be preferably greater than or equal to 100 meV. Here, an amount of increase in band gap energy of P-side guide layer  106  in the stack direction is defined by a difference between band gap energy of P-side guide layer  106  at and in the vicinity of an edge surface on the side close to active layer  105  and band gap energy of P-side guide layer  106  at and in the vicinity of an edge surface on the side close to P-side cladding layer  110 , for example. The magnitude of band gap energy that continuously increases may be 70% or more of ΔEgp. The percentage may be 80% or more, or may be 90% or more. 
     When P-side guide layer  106  consists essentially of In xp Ga 1-xp N, In composition ratio Xp of P-side guide layer  106  may monotonically decrease with an increase in distance from active layer  105 . Accordingly, the band gap energy of P-side guide layer  106  monotonically increases with an increase in distance from active layer  105 . Here, the configuration in which In composition ratio Xp continuously and monotonically decreases also includes a configuration in which a region where In composition ratio Xp is constant in the stack direction is present. P-side guide layer  106  includes a portion in which In composition ratio Xp continuously decreases with an increase in distance from active layer  105 . Here, the configuration in which In composition ratio Xp continuously decreases does not include a configuration in which In composition ratio Xp discontinuously changes in the stack direction. In the present disclosure, the configuration in which the In composition ratio continuously decreases means a configuration in which an amount of discontinuous decrease in In composition ratio Xp in the stack direction at a certain position in P-side guide layer  106  is less than 20% of In composition ratio Xp at the position. 
     Average band gap energy of P-side guide layer  106  is greater than or equal to average band gap energy of N-side guide layer  104 . Stated differently, an average of the In composition ratio of N-side guide layer  104  is greater than or equal to an average of the In composition ratio of P-side guide layer  106 . In the present embodiment, an average of the In composition ratio of N-side guide layer  104  is greater than the average of the In composition ratio of P-side guide layer  106 . The following relation is satisfied, where Tp denotes the thickness of P-side guide layer  106 , and Tn denotes the thickness of N-side guide layer  104 : 
     
       
         
           
             Tn &lt; Tp 
           
         
       
     
     Further, the greatest value of the In composition ratio of P-side guide layer  106  is less than or equal to the In composition ratio of each barrier layer. 
     In the present embodiment, P-side guide layer  106  is an undoped In xp Ga 1-xp N layer having a thickness of 280 nm. More specifically, P-side guide layer  106  has a composition represented by In 0.04  Ga 0.96  N at and in the vicinity of the interface on the side close to active layer  105 , and has a composition represented by GaN at and in the vicinity of the interface on the side far from active layer  105 . In composition ratio Xp of P-side guide layer  106  decreases at a certain rate of change with an increase in distance from active layer  105 . 
     Intermediate layer  108  is provided above active layer  105 . In the present embodiment, intermediate layer  108  is provided between P-side guide layer  106  and electron barrier layer  109 , and having bandgap energy less than the bandgap energy of electron barrier layer  109  and greater than or equal to the bandgap energy of P-side guide layer  106 . Intermediate layer  108  decreases a stress generated due to a difference in lattice constant between P-side guide layer  106  and electron barrier layer  109 . Accordingly, the occurrence of crystal defect in nitride-based semiconductor light-emitting element  100  can be reduced. In the present embodiment, intermediate layer  108  is an undoped GaN layer having a thickness of 20 nm. 
     Electron barrier layer  109  is a nitride-based semiconductor layer that is provided above active layer  105  and includes at least Al. In the present embodiment, electron barrier layer  109  is provided between intermediate layer  108  and P-type cladding layer  110 . Electron barrier layer  109  is a P-type Al 0.36 Ga 0.64 N layer having a thickness of 5 nm. Electron barrier layer  109  is doped with Mg having a concentration of 1×10 19  cm -3 , as an impurity. Electron barrier layer  109  can prevent leakage of electrons from active layer  105  to P-type cladding layer  110 . 
     P-type cladding layer  110  is provided above active layer  105 . In the present embodiment, P-type cladding layer  110  is provided between electron barrier layer  109  and contact layer  111 . P-type cladding layer  110  has a smaller refractive index and greater band gap energy than those of active layer  105 . P-type cladding layer  110  may have a thickness less than or equal to 460 nm. Accordingly, the electrical resistance of nitride-based semiconductor light-emitting element  100  can be decreased. Thus, the operating voltage of nitride-based semiconductor light-emitting element  100  can be lowered. Since self-heating of nitride-based semiconductor light-emitting element  100  during operation can be reduced, temperature characteristics of nitride-based semiconductor light-emitting element  100  can be enhanced. Thus, nitride-based semiconductor light-emitting element  100  can perform high-power operation. Note that in nitride-based semiconductor light-emitting element  100  according to the present embodiment, the thickness of P-type cladding layer  110  may be preferably greater than or equal to 200 nm, in order to sufficiently exhibit functionality of P-type cladding layer  110  as a cladding layer. The thickness of P-type cladding layer  110  may be greater than or equal to 250 nm. In the present embodiment, P-type cladding layer  110  is a P-type Al 0.026 Ga 0.974 N layer having a thickness of 450 nm. P-type cladding layer  110  is doped with Mg as an impurity. An impurity concentration at an edge portion of P-type cladding layer  110  on the side close to active layer  105  is lower than the impurity concentration at an edge portion of P-type cladding layer  110  on the side far from active layer  105 . Specifically, P-type cladding layer  110  includes a P-type Al 0.026 Ga 0.974 N layer that is provided on the side close to active layer  105 , doped with Mg having a concentration of 2×10 18  cm -3 , and has a thickness of 150 nm, and a P-type Al 0.026 Ga 0.974 N layer that is provided on the side far from active layer  105 , doped with Mg having a concentration of 1×10 19  cm -3 , and has a thickness of 300 nm. 
     Ridge  110 R is formed in P-type cladding layer  110  of nitride-based semiconductor light-emitting element  100 . Further, two grooves  110 T are formed in P-type cladding layer  110 , which are provided along ridge  110 R and extend in the Y-axis direction. In the present embodiment, ridge width W is about 30 µm. As illustrated in  FIG.  2 A , dp denotes a distance between active layer  105  and a lower edge of ridge  110 R (that is, the bottoms of grooves  110 T). Further, dc denotes the thickness of P-type cladding layer  110  at the lower edge of ridge  110 R (that is, a distance between the lower edge of ridge  110 R and the interface between P-type cladding layer  110  and electron barrier layer  109 ). 
     Contact layer  111  is provided above P-type cladding layer  110 , and is in ohmic contact with P-side electrode  113 . In the present embodiment, contact layer  111  is a P-type GaN layer having a thickness of 60 nm. Contact layer  111  is doped with Mg having a concentration of 1×10 20  cm -3 , as an impurity. 
     Current block layer  112  is an insulating layer provided above P-type cladding layer  110  and having transmissivity for light from active layer  105 . Current block layer  112  is provided in a region above an upper surface of P-type cladding layer  110  except an upper surface of ridge  110 R. In the present embodiment, current block layer  112  is an SiO 2  layer. 
     P-side electrode  113  is a conductive layer provided above contact layer  111 . In the present embodiment, P-side electrode  113  is provided above contact layer  111  and current block layer  112 . P-side electrode  113  is a single-layer film or is a multi-layer film formed using at least one of Cr, Ti, Ni, Pd, Pt, or Au, for example. 
     N-side electrode  114  is a conductive layer provided below substrate  101  (that is, on a principal surface of substrate 101 on the side different from the principal surface above which N-type first cladding layer  102 , for instance, is provided). N-side electrode  114  is a single-layer film or is a multi-layer film formed using at least one of Cr, Ti, Ni, Pd, Pt, or Au, for example. 
     Since nitride-based semiconductor light-emitting element  100  has such a configuration as above, effective refractive index difference ΔN is generated between a portion below ridge  110 R and portions below grooves  110 T, as illustrated in  FIG.  2 A . Accordingly, light generated in the portion of active layer  105  below ridge  110 R can be confined in the horizontal direction (that is, the X-axis direction). 
     1-2. Light Intensity Distribution and Stability of Light Output 
     Next, a light intensity distribution and stability of light output of nitride-based semiconductor light-emitting element  100  according to the present embodiment is to be described. 
     First, a light intensity distribution in the stack direction (the Z-axis direction in the drawings) of nitride-based semiconductor light-emitting element  100  according to the present embodiment is to be described with reference to  FIG.  3   .  FIG.  3    is a schematic diagram illustrating, in a simplified manner, a light intensity distribution in the stack direction of nitride-based semiconductor light-emitting element  100  according to the present embodiment.  FIG.  3    illustrates a schematic cross sectional view of nitride-based semiconductor light-emitting element  100 , and a graph showing, in a simplified manner, a light intensity distribution in the stack direction at positions corresponding to ridge  110 R and grooves  110 T. 
     Generally, in a nitride-based semiconductor light-emitting element, light is generated in an active layer, yet a light intensity distribution in the stack direction relies on the stack structure, and a peak of the light intensity distribution is not necessarily located in the active layer. The stack structure of nitride-based semiconductor light-emitting element  100  according to the present embodiment differs in the portion below ridge  110 R and portions below grooves  110 T, and thus the light intensity distribution also differs in the portion below ridge  110 R and the portions below grooves  110 T. As illustrated in  FIG.  3   , P1 denotes a peak position of the light intensity distribution in the stack direction in the center of the horizontal direction (that is, the X-axis direction) in the portion below ridge  110 R. P2 denotes a peak position of the light intensity distribution in the stack direction in the portion below groove  110 T. Here, positions P1 and P2 are to be described with reference to  FIG.  4   .  FIG.  4    is a graph showing coordinates at positions in the stack direction of nitride-based semiconductor light-emitting element  100  according to the present embodiment. As illustrated in  FIG.  4   , coordinates of a position of an N-side edge surface of well layer  105   b  of active layer  105 , that is, coordinates of a position in the stack direction of the edge surface of well layer  105   b  on the side close to N-side guide layer  104  are set to zero, a downward direction (a direction toward N-side guide layer  104 ) is a negative direction in the coordinate system, and an upward direction (a direction toward P-side guide layer  106 ) is a positive direction in the coordinate system. An absolute value of a difference between position P1 and position P2 is difference ΔP at a peak position. 
     In the following, a light intensity distribution in the stack direction of nitride-based semiconductor light-emitting element  100  according to the present embodiment is to be described with reference to  FIG.  5   .  FIG.  5    is a schematic graph showing a distribution of band gap energy in active layer  105  and layers in the vicinity thereof in nitride-based semiconductor light-emitting element  100  according to the present embodiment. 
     In nitride-based semiconductor light-emitting element  100  according to the present embodiment, the thickness of P-type cladding layer  110  is set to be relatively thin, in order to lower the operating voltage. Along therewith, the height of ridge  110 R (that is, a height of ridge  110 R from the bottom surfaces of grooves  110 T) is set to be relatively low. In a semiconductor light-emitting element having such a configuration, a peak position of the light intensity distribution in the stack direction shifts from active layer  105  toward N-type second cladding layer  103 . Accordingly, a coefficient of confinement of light in the active layer decreases, and due to this, a heat saturation level of light output decreases. Thus, it is difficult for the semiconductor light-emitting element to perform high-power operation. In the present embodiment, average band gap energy of P-side guide layer  106  is greater than or equal to average band gap energy of N-side guide layer  104 , as illustrated in  FIG.  5   . On the other hand, thickness Tp of P-side guide layer  106  is greater than thickness Tn of N-side guide layer  104  (see the inequality (1) above). In this manner, by increasing the thickness of P-side guide layer  106  having a greater refractive index than those of the cladding layers, a light intensity distribution can be shifted from active layer  105  toward P-side guide layer  106 . Thus, according to nitride-based semiconductor light-emitting element  100  according to the present embodiment, a peak of the light intensity distribution in the stack direction can be controlled so that the peak is located in active layer  105 . 
     Furthermore, in the present embodiment, P-side guide layer  106  includes a portion in which band gap energy continuously and monotonically increases with an increase in distance from active layer  105 . Thus, P-side guide layer  106  has a portion in which a refractive index continuously and monotonically increases with a decrease in distance from active layer  105 . In this manner, the refractive index of P-side guide layer  106  increases with a decrease in distance from active layer  105 , and thus the peak of the light intensity distribution in the stack direction can be located closer to active layer 105. 
     In the present embodiment, barrier layers  105   a ,  105   c , and  105   e  each consist essentially of In Xb Ga 1-Xb N, and the relations as below are satisfied with regard to In composition ratios Xb, Xn, and Xp of the barrier layers, N-side guide layer  104 , and P-side guide layer  106 : 
     
       
         
           
             Xp  
             ≤ 
              Xb 
           
         
       
     
     
       
         
           
             Xn  
             ≤ 
              Xb 
           
         
       
     
      Accordingly, band gap energy of each barrier layer is less than or equal to the smallest value of band gap energy of N-side guide layer  104  and band gap energy of P-side guide layer  106 . Thus, the refractive indices of the barrier layers can be made greater than or equal to the greatest values of the refractive indices of P-side guide layer  106  and N-side guide layer  104 . Accordingly, a peak of the light intensity distribution in the stack direction can be brought close to active layer  105 . Furthermore, the light intensity distribution can be prevented from excessively shifting from active layer  105  toward P-type cladding layer  110 . The effects can be further enhanced by making the refractive indices of the barrier layers greater than the greatest values of the refractive indices of P-side guide layer  106  and N-side guide layer  104 . 
     With the above configuration, in the present embodiment, position P1 of a peak of the light intensity distribution in the stack direction in the portion below ridge  110 R can be placed at 1.3 nm. Thus, the peak of the light intensity distribution can be located in well layer  105   b  of active layer  105  (see  FIG.  4   ). Further, ΔP can be reduced to 5.6 nm. Accordingly, a coefficient of confinement of light in active layer  105  can be increased to about 1.49%. 
     As described above, according to nitride-based semiconductor light-emitting element  100  according to the present embodiment, a peak of the light intensity distribution in the stack direction can be located in active layer  105 . Note that the expression that a peak of a light intensity distribution in the stack direction is located in active layer  105  means a state in which a peak of a light intensity distribution in the stack direction is located in active layer  105  at at least one position in the horizontal direction of nitride-based semiconductor light-emitting element  100 , and thus is not limited to a state in which a peak of a light intensity distribution in the stack direction is located in active layer  105  at all the positions in the horizontal direction. 
     As in the present embodiment, if a peak of a light intensity distribution in the stack direction is located in active layer  105 , a proportion of a portion of light located in P-type cladding layer  110  can be increased as compared with the case where the peak of the light intensity distribution is located in N-side guide layer  104 . Here, P-type cladding layer  110  includes impurities having a concentration higher than those of N-type first cladding layer  102  and N-type second cladding layer  103 , and thus if the proportion of a portion of light located in P-type cladding layer  110  increases, an increase in loss of free carrier in P-type cladding layer  110  is concerned. However, in the present embodiment, P-side guide layer  106  is an undoped layer, and thickness Tp of P-side guide layer  106  is made relatively great, which can increase a proportion of a portion of a light intensity distribution located in the undoped layer. Thus, an increase in free carrier loss can be reduced. Specifically, in the present embodiment, waveguide loss can be reduced to about 3.2 cm -1 . 
     In nitride-based semiconductor light-emitting element  100  according to the present embodiment, in order to decrease the divergence angle of emitted light in the horizontal direction (that is, the X-axis direction), effective refractive index difference ΔN between the portion below ridge  110 R and the portions below grooves  110 T is set to a relatively small value. Specifically, effective refractive index difference ΔN is determined by adjusting distance dp (see  FIG.  2 A ) between current block layer  112  and active layer  105 . Here, effective refractive index difference ON decreases with an increase in distance dp. In the present embodiment, effective refractive index difference ΔN is about 2.1×10 -3 . Thus, in the present embodiment, the number of high-order modes (that is, high-order transverse modes) that can propagate within a waveguide formed by ridge  110 R is less than in the case where effective refractive index difference ΔN is greater than 2.1×10 -3 . Accordingly, a proportion of the high-order modes is relatively high, out of all the transverse modes included in light emitted from nitride-based semiconductor light-emitting element  100 . Thus, an amount of change in a coefficient of confinement of light in active layer  105  due to increase/decrease in the number of modes and mode coupling is relatively great. Accordingly, when the number of modes increases or decreases and modes are coupled in nitride-based semiconductor light-emitting element  100 , linearity of characteristics (so-called IL characteristics) of light output relative to a supplied current decreases. Stated differently, a portion that is not linear (so called a kink) is generated in a graph showing IL characteristics. Along therewith, stability of light output of nitride-based semiconductor light-emitting element  100  may decrease. 
     A decrease in stability of light output as stated above is to be described in the following. In nitride-based semiconductor light-emitting element  100 , a basic mode (that is, a zero-order mode) is dominant in the light intensity distribution in the portion below ridge  110 R, whereas a high-order mode is dominant in the light intensity distribution in the portion below groove  110 T. Accordingly, when nitride-based semiconductor light-emitting element  100  has great difference ΔP between position P1 of a peak of the light intensity distribution in the stack direction in the portion below ridge  110 R and position P2 of a peak of the light intensity distribution in the stack direction in the portion below groove  110 T, if the number of modes increases or decreases and modes are coupled, a coefficient of confinement of light in active layer  105  changes and thus stability of light output decreases. 
     For example, when the number of high-order modes decreases, a peak of a light intensity distribution resulting from adding the light intensity distributions in the portions below both ridge  110 R and groove  110 T shifts to a position close to position P1. Accordingly, as difference ΔP between position P1 and position P2 is greater, a change in coefficient of confinement of light in active layer  105  when the number of modes changes is greater. Thus, stability of light output decreases. 
     Nitride-based semiconductor light-emitting element  100  according to the present embodiment includes N-side guide layer  104  and P-side guide layer  106  having configurations as stated above, and thus the peaks of the light intensity distributions in both the portion below ridge  110 R and the portion below groove  110 T can be located in active layer  105 . Thus, difference ΔP between position P1 and position P2 of the light intensity distributions can be decreased. Accordingly, even if the number of modes increases or decreases or modes are coupled, a shift in the stack direction of the position of a peak of a light intensity distribution resulting from adding the light intensity distributions in the portions below both ridge  110 R and groove  110 T can be decreased. Thus, stability of light output can be enhanced. 
     Note that as described above, in order to set effective refractive index difference ΔN to a relatively small value, distance dp is set to a relatively large value. When distance dp is determined, if the lower edge of ridge  110 R (that is, the bottom of groove  110 T) is determined to be positioned below electron barrier layer  109 , since electron barrier layer  109  has great band gap energy, holes injected from contact layer  111  readily leak out of ridge  110 R from the side wall of ridge 110R when passing through electron barrier layer  109 . As a result, holes flow to a position below groove  110 T. Along therewith, since a light distribution intensity is low in active layer 105 below groove  110 T, a probability of radiative recombination of electrons and holes injected to active layer  105  decreases, and nonradiative recombination increases. Nitride-based semiconductor light-emitting element  100  tends to deteriorate due to an increase in such non-radiative recombination. In order to reduce such deterioration, the lower edge of ridge  110 R is set to be positioned above electron barrier layer  109 . If distance dc (see  FIG.  2 A ) between the lower edge of ridge  110 R and electron barrier layer  109  is excessively long, holes flow from ridge  110 R into a position between groove  110 T and electron barrier layer  109  to cause a leakage current. In order to reduce an increase in such a leakage current, distance dc is set to a value as small as possible. Distance dc is in a range from 10 nm to 70 nm, for example. 
     1-3. Effects 
     11. Guide Layers 
     Effects of the above-described guide layers of nitride-based semiconductor light-emitting element  100  according to the present embodiment are to be described in comparison with nitride-based semiconductor light-emitting elements according to comparative examples, with reference to  FIG.  6    to  FIG.  8   .  FIG.  6    illustrates graphs showing refractive index distributions and light intensity distributions in the stack direction of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1 to 3 and nitride-based semiconductor light-emitting element  100  according to the present embodiment. Graphs (a), (b), and (c) in  FIG.  6    illustrate refractive index distributions and light intensity distributions of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1, 2, and 3, respectively. Graph (d) in  FIG.  6    illustrates a refractive index distribution and a light intensity distribution of nitride-based semiconductor light-emitting element  100  according to the present embodiment. The graphs in  FIG.  6    show refractive index distributions with solid lines and light intensity distributions with broken lines. 
       FIG.  7    illustrates graphs showing simulation results of distributions of valence band potentials and hole Fermi levels in the stack direction of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1 to 3 and nitride-based semiconductor light-emitting element  100  according to the present embodiment. Graphs (a), (b), and (c) in  FIG.  7    illustrate distributions of valence band potentials and hole Fermi levels of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1, 2, and 3, respectively. Graph (d) in  FIG.  7    illustrates distributions of a valence band potential and a hole Fermi level of nitride-based semiconductor light-emitting element  100  according to the present embodiment. The graphs in  FIG.  7    show valence band potentials with solid lines and hole Fermi levels with broken lines. 
       FIG.  8    illustrates graphs showing simulation results of distributions of carrier concentrations in the stack direction of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1 to 3 and nitride-based semiconductor light-emitting element  100  according to the present embodiment. Graphs (a), (b), and (c) in  FIG.  8    illustrate distributions of carrier concentrations of the nitride-based semiconductor light-emitting elements according to Comparative Examples 1, 2, and 3, respectively. Graph (d) in  FIG.  8    illustrates distributions of carrier concentrations of nitride-based semiconductor light-emitting element  100  according to the present embodiment. The graphs in  FIG.  8    show electron concentration distributions with solid lines and hole concentration distributions with broken lines. 
     The nitride-based semiconductor light-emitting elements according to Comparative Examples 1 to 3 are different from nitride-based semiconductor light-emitting element  100  according to the present embodiment in the configurations of the N-side guide layer and the P-side guide layer. The nitride-based semiconductor light-emitting element according to Comparative Example 1 illustrated in graph (a) in  FIG.  6    includes N-side guide layer  1104  that is an undoped In 0.04 Ga 0.96 N layer having a thickness of 280 nm, and P-side guide layer  1106  that is an undoped In 0.04 Ga 0.96 N layer having a thickness of 160 nm. The nitride-based semiconductor light-emitting element according to Comparative Example 2 illustrated in graph (b) in  FIG.  6    includes N-side guide layer  1204  that is an undoped In 0.04 Ga 0.96 N layer having a thickness of 160 nm, and P-side guide layer  1206  that is an undoped In 0.04 Ga 0.96 N layer having a thickness of 280 nm. The nitride-based semiconductor light-emitting element according to Comparative Example 3 illustrated in graph (c) in  FIG.  6    includes N-side guide layer  1304  that is an undoped In 0.04 Ga 0.96 N layer having a thickness of 160 nm, and P-side guide layer  1306  having a thickness of 280 nm. P-side guide layer  1306  of the nitride-based semiconductor light-emitting element according to Comparative Example 3 includes P-side first guide layer  1306   a  that is an undoped In 0.04 Ga 0.96 N layer provided above active layer  105  and having a thickness of 140 nm, and P-side second guide layer  1306   b  that is an undoped In 0.02 Ga 0.98 N layer provided above P-side first guide layer  1306   a  and having a thickness of 140 nm. 
     In the nitride-based semiconductor light-emitting element according to Comparative Example 1, N-side guide layer  1104  and P-side guide layer  1106  have the same composition, and N-side guide layer  1104  has a thickness greater than that of P-side guide layer  1106 . Thus, in the nitride-based semiconductor light-emitting element according to Comparative Example 1, a peak of the light intensity distribution in the stack direction is located in N-side guide layer  1104 , as illustrated in graph (a) in  FIG.  6   . Accordingly, a light confinement coefficient of the nitride-based semiconductor light-emitting element according to Comparative Example 1 is 1.33%, which is a small value. As illustrated in graph (a) in  FIG.  7   , in P-side guide layer  1106 , the hole Fermi level increases from the interface of P-side guide layer  1106  on the side far from active layer  105  to the interface thereof on the side close to active layer  105 , in order to conduct holes from P-side guide layer  1106  to active layer  105 . On the other hand, the valence band potential is substantially constant in the stack direction in P-side guide layer  1106 . Accordingly, a difference between the hole Fermi level and the valence band potential in P-side guide layer  1106  is greater with a decrease in distance from active layer  105 . Accordingly, as illustrated in graph (a) in  FIG.  8   , concentrations of holes and electrons of P-side guide layer  1106  in the stack direction, that is, a free carrier concentration increases with an increase in distance from active layer  105 . In this manner, in the nitride-based semiconductor light-emitting element according to Comparative Example 1, the free carrier concentration of P-side guide layer  1106  in the stack direction cannot be reduced, and thus free carrier loss and a probability of non-radiative recombination cannot be reduced. In the nitride-based semiconductor light-emitting element according to Comparative Example 1, effective refractive index difference ΔN is 3.6×10 -3 , positions P1 and P2 of light intensity distributions are -34.1 nm and -75.6 nm, respectively, and difference ΔP is 41.5 nm. Further, waveguide loss is 4.5 cm -1 , and free carrier loss (hereinafter, also referred to as “guide-layer free carrier loss”) in each of N-side guide layer  1104  and P-side guide layer  1106  is 2.8 cm -1 . 
     In the nitride-based semiconductor light-emitting element according to Comparative Example 2, the thickness of P-side guide layer  1206  is greater than the thickness of N-side guide layer  1204 , and thus as illustrated in graph (b) in  FIG.  6   , a peak of a light intensity distribution in the stack direction is closer to active layer  105  than that in the nitride-based semiconductor light-emitting element according to Comparative Example 1. Accordingly, in the nitride-based semiconductor light-emitting element according to Comparative Example 2, the light confinement coefficient is 1.37%, and is slightly improved as compared with the nitride-based semiconductor light-emitting element according to Comparative Example 1. However, as illustrated in graph (b) in  FIG.  7   , a difference between the hole Fermi level and the valence band potential in P-side guide layer  1206  increases with a decrease in distance from active layer  105 , similarly to Comparative Example 1. Accordingly, as illustrated in graph (b) in  FIG.  8   , concentrations of holes and electrons, that is, a free carrier concentration of P-side guide layer  1206  in the stack direction increases with an increase in distance from active layer  105 . In this manner, in the nitride-based semiconductor light-emitting element according to Comparative Example 2, the free carrier concentration of P-side guide layer  1206  in the stack direction cannot be reduced, and thus free carrier loss and a probability of non-radiative recombination cannot be reduced. In the nitride-based semiconductor light-emitting element according to Comparative Example 2, effective refractive index difference ΔN is 3.3×10 -3 , positions P1 and P2 of light intensity distributions are 31.3 nm and 10.8 nm, respectively, and difference ΔP is 20.5 nm. Further, waveguide loss is 5.2 cm -1 , and guide-layer free carrier loss is 3.6 cm -1 . 
     In the nitride-based semiconductor light-emitting element according to Comparative Example 3, a refractive index of P-side second guide layer  1306   b  that is a region farther from active layer  105  is made smaller than the refractive index of P-side first guide layer  1306   a  that is a region closer to active layer  105 . Accordingly, as illustrated in graph (c) in  FIG.  6   , a peak of a light intensity distribution in the stack direction is still closer to active layer  105  than the nitride-based semiconductor light-emitting element according to Comparative Example 2. Accordingly, in the nitride-based semiconductor light-emitting element according to Comparative Example 3, the light confinement coefficient is 1.47%, and is further improved as compared with the nitride-based semiconductor light-emitting element according to Comparative Example 2. However, in a hetero barrier at a boundary surface between P-side first guide layer  1306   a  and P-side second guide layer  1306   b , a region in a spike shape is generated in a distribution of a valence band potential due to piezoelectric polarization charge, as illustrated in graph (c) in  FIG.  7   . Accordingly, as illustrated in graph (c) in  FIG.  8   , an electron concentration of P-side guide layer  1306  in the stack direction increases in a spiking manner in a portion in which a valence band potential discontinuously changes. A concentration of holes in P-side guide layer  1306  also exceeds 1×10 17  cm -3 . In this manner, in the nitride-based semiconductor light-emitting element according to Comparative Example 3, the free carrier concentration of P-side guide layer  1306  in the stack direction cannot be reduced, and thus free carrier loss and a probability of non-radiative recombination cannot be reduced. In the nitride-based semiconductor light-emitting element according to Comparative Example 3, effective refractive index difference ΔN is 2.5×10 -3 , positions P1 and P2 of light intensity distributions are 10.7 nm and 4.4 nm, respectively, and difference ΔP is 6.3 nm. Further, waveguide loss is 3.93 cm -1 , and guide-layer free carrier loss is 2.56 cm -1 . 
     In nitride-based semiconductor light-emitting element  100  according to the present embodiment, as illustrated in graph (d) in  FIG.  6   , the refractive index of P-side guide layer  106  increases with a decrease in distance from active layer  105 , and thus the peak of the light intensity distribution in the stack direction can be brought close to active layer  105 . Accordingly, in nitride-based semiconductor light-emitting element  100  according to the present embodiment, the light confinement coefficient is 1.49%, and is further improved as compared with the nitride-based semiconductor light-emitting element according to Comparative Example 3. Since the band gap energy of P-side guide layer  106  continuously and monotonically increases with an increase in distance from active layer  105 , and thus as illustrated in graph (d) in  FIG.  7   , a valence band potential continuously decreases with an increase in distance from active layer  105 . Accordingly, a difference between the hole Fermi level and the valence band potential can be made substantially constant in P-side guide layer  106 . In this manner, as illustrated in graph (d) in  FIG.  8   , concentrations of holes and electrons of P-side guide layer  106  in the stack direction can be decreased and made substantially constant. Here, an amount of increase (ΔEgp) in band gap energy of P-side guide layer  106  in the stack direction is small, effects thereof are small, and thus ΔEgp may be preferably greater than or equal to 100 meV. On the contrary, if ΔEgp is excessively increased, band gap energy at an edge of P-side guide layer  106  on the side close to active layer  105  may be small, out of the band gap energy of P-side guide layer  106 . In this case, a valence band potential of P-side guide layer  106  has an excessively great slope, and thus holes injected to active layer  105  leak to N-side guide layer  104  so that a leakage current is generated. Accordingly, ΔEgp may be less than or equal to 400 meV. 
     In this manner, in nitride-based semiconductor light-emitting element  100  according to the present embodiment, the free carrier concentration of P-side guide layer  106  in the stack direction can be reduced, and thus free carrier loss and a probability of non-radiative recombination can be reduced. In nitride-based semiconductor light-emitting element  100  according to the present embodiment, effective refractive index difference ΔN is 2.1×10 -3 , positions P1 and P2 of light intensity distributions are 1.3 nm and -4.3 nm, respectively, and difference ΔP is 5.6 nm. In this manner, in the present embodiment, position P1 and difference ΔP can be reduced, and thus a nonlinear portion is not readily generated in the graph showing IL characteristics. Further, waveguide loss is 3.20 cm -1 , and a guide-layer free carrier loss is 1.8 cm -1 . Accordingly, in the present embodiment, waveguide loss and free carrier loss can be reduced, as compared with the comparative examples. 
     Next, effects of a relation between the thicknesses of N-side guide layer  104  and P-side guide layer  106  according to the present embodiment are to be described with reference to  FIG.  9    to  FIG.  13   .  FIG.  9    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and a light confinement coefficient ( ┌ v).  FIG.  10    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and waveguide loss.  FIG.  11    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and effective refractive index difference ΔN.  FIG.  12    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and position P1.  FIG.  13    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and difference ΔP. In the simulations as shown by  FIG.  9    to  FIG.  13   , the thicknesses of N-side guide layer  104  and P-side guide layer  106  are changed while a total of the thicknesses of N-side guide layer  104  and P-side guide layer  106  is maintained constant at 440 nm. The In composition ratio of N-side guide layer  104  is 4%, whereas the In composition ratio of P-side guide layer  106  is 4% at and in the vicinity of the interface on the side close to active layer  105  and is 0% at and in the vicinity of the interface on the side far from active layer  105 . The In composition ratio of P-side guide layer  106  is changed at a certain rate of change in the stack direction.  FIG.  9    to  FIG.  13    also illustrate simulation results in an example in which the In composition ratio of a P-side guide layer is 2% as a comparative example, using broken lines. 
     As illustrated in  FIG.  9   , the light confinement coefficient can be increased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  106 . Note that thickness Tn of N-side guide layer  104  may be greater than or equal to 100 nm. In this manner, an excessive shift of a light intensity distribution from active layer  105  to P-side guide layer  106  due to thickness Tn of N-side guide layer  104  being excessively thin can be reduced. As illustrated in  FIG.  9   , also when the In composition ratio of P-side guide layer  106  is maintained constant at 2%, the light confinement coefficient can be increased by making the thickness of N-side guide layer  104  smaller than the thickness of P-side guide layer  106 . But nevertheless, the light confinement coefficient can be further increased by continuously and monotonically decreasing the In composition ratio with an increase in distance from active layer  105 , as with P-side guide layer  106  according to the present embodiment. 
     As illustrated in  FIG.  10   , as with P-side guide layer  106  according to the present embodiment, waveguide loss can be more decreased in the case where the In composition ratio is continuously and monotonically decreased with an increase in distance from active layer  105  than in the case where the In composition ratio is maintained constant at 2%. In the present embodiment, even if the thickness of N-side guide layer  104  is changed, waveguide loss can be maintained substantially constant at 3.5 cm -1  or less. 
     As illustrated in  FIG.  11   , effective refractive index difference ΔN can be decreased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  106 . As illustrated in  FIG.  11   , as with P-side guide layer  106  according to the present embodiment, effective refractive index difference ΔN can be more decreased in the case where the In composition ratio is continuously and monotonically decreased with an increase in distance from active layer  105  than in the case where the In composition ratio is maintained constant at 2%. 
     As illustrated in  FIG.  12   , the absolute value of position P1 can be decreased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  106 . Note that thickness Tn of N-side guide layer  104  may be in a range from 100 nm to 190 nm. Stated differently, the thickness of N-side guide layer  104  may be set to a value in a range from 23% to 43% of a total of the thicknesses of N-side guide layer  104  and P-side guide layer  106 . In this manner, position P1 can be placed at a position in a range from -7 nm to 18 nm, that is a peak of a light intensity distribution can be located within active layer  105 . When the thickness of N-side guide layer  104  is set to a value in a range from 23% to 43% of a total of the thicknesses of N-side guide layer  104  and P-side guide layer  106  and distance dc is set to 40 nm, effective refractive index difference ΔN can be maintained in a range from 2×10 -3  to 2.2×10 -3 , as illustrated in  FIG.  11   . 
     As illustrated in  FIG.  12   , also when the In composition ratio of P-side guide layer  106  is maintained constant at 2%, the absolute value of position P1 can be decreased by making the thickness of N-side guide layer  104  smaller than the thickness of P-side guide layer  106 . But nevertheless, the absolute value of position P1 can be further decreased by continuously and monotonically decreasing the In composition ratio with an increase in distance from active layer  105  as with P-side guide layer  106  according to the present embodiment, when the thickness of N-side guide layer  104  is greater than or equal to 160 nm. 
     As illustrated in  FIG.  13   , difference ΔP can be decreased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  106 . In particular, difference ΔP can be made 20 nm or less by setting the thickness of N-side guide layer  104  to a value in a range from 23% to 43% of a total of the thicknesses of N-side guide layer  104  and P-side guide layer  106 . As illustrated in  FIG.  13   , also when the In composition ratio of P-side guide layer  106  is maintained constant at 2%, difference ΔP can be decreased by making the thickness of N-side guide layer  104  smaller than the thickness of P-side guide layer  106 . But nevertheless, difference ΔP can be further decreased by continuously and monotonically decreasing the In composition ratio with an increase in distance from active layer  105 , as with P-side guide layer  106  according to the present embodiment, when the thickness of N-side guide layer  104  is greater than or equal to 160 nm. 
     12. Barrier Layers 
     Next, effects of the configurations of the barrier layers of active layer  105  according to the present embodiment are to be described in comparison with a comparative example. In the present embodiment, as described above, band gap energy of each barrier layer is less than or equal to the smallest value of band gap energy of N-side guide layer  104  and P-side guide layer  106 . Here, as a comparative example, a simulation result of a nitride-based semiconductor light-emitting element according to Comparative Example 4 is shown in which the composition of the barrier layers is undoped GaN, band gap energy of each barrier layer is made greater than or equal to the smallest value of band gap energy of N-side guide layer  104  and P-side guide layer  106 , and the other configuration is the same as that of nitride-based semiconductor light-emitting element  100  according to the present embodiment. In the nitride-based semiconductor light-emitting element according to Comparative Example 4, a light confinement coefficient is 1.39%, effective refractive index difference ΔN is 2.3×10 -3 , positions P1 and P2 of light intensity distributions are 0.35 nm and -21.9 nm, respectively, and difference ΔP is 22.3 nm. Further, waveguide loss is 3.4 cm -1 , and free carrier loss of the N-side guide layer and the P-side guide layer is 1.84 cm -1 . Accordingly, in the nitride-based semiconductor light-emitting element according to Comparative Example 4, band gap energy of the barrier layers is great, or stated differently, the refractive indices of the barrier layers are small, and thus a light confinement coefficient is smaller than that of nitride-based semiconductor light-emitting element  100  according to the present embodiment. Along with this, other evaluation indices of the nitride-based semiconductor light-emitting element according to Comparative Example 4 are not as good as those of nitride-based semiconductor light-emitting element  100  according to the present embodiment, except position P1. 
     As described above, in nitride-based semiconductor light-emitting element  100  according to the present embodiment, the light confinement coefficient can be increased by setting band gap energy of the barrier layers to a value less than or equal to the smallest value of band gap energy of N-side guide layer  104  and P-side guide layer  106 . Along with this, since difference ΔP can be decreased, a nonlinear portion is not readily generated in a graph showing IL characteristics. 
     13. P-Type Cladding Layer 
     Next, the thickness of P-type cladding layer  110  according to the present embodiment is to be described with reference to  FIG.  14    to  FIG.  18   .  FIG.  14    is a graph showing simulation results of a relation between the thickness of P-type cladding layer  110  according to the present embodiment and a light confinement coefficient (┌v).  FIG.  15    is a graph showing simulation results of a relation between the thickness of P-type cladding layer  110  according to the present embodiment and waveguide loss.  FIG.  16    is a graph showing simulation results of a relation between the thickness of P-type cladding layer  110  according to the present embodiment and effective refractive index difference ΔN.  FIG.  17    is a graph showing simulation results of a relation between the thickness of P-type cladding layer  110  according to the present embodiment and position P1.  FIG.  18    is a graph showing simulation results of a relation between the thickness of P-type cladding layer  110  according to the present embodiment and difference ΔP.  FIG.  14    to  FIG.  18    also illustrate simulation results of two comparative examples in which the In composition ratios of the P-side guide layer are maintained constant at 2% and 4%, as comparative examples.  FIG.  14    to  FIG.  18    also illustrate simulation results of nitride-based semiconductor light-emitting element  400  according to Embodiment 4 described later. 
     As illustrated in  FIG.  14   , in nitride-based semiconductor light-emitting element  100  according to the present embodiment, a light confinement coefficient can be made greater than those of the nitride-based semiconductor light-emitting elements according to the comparative examples. In the present embodiment, owing to the configurations of the guide layers and the barrier layers described above, the light confinement coefficient does not decrease even if the thickness of P-type cladding layer  110  is decreased to 250 nm. 
     As illustrated in  FIG.  15   , in nitride-based semiconductor light-emitting element  100  according to the present embodiment, waveguide loss can be made smaller than that of the nitride-based semiconductor light-emitting elements in the comparative example. In nitride-based semiconductor light-emitting element  100  according to the present embodiment, a great increase in waveguide loss can be reduced even when the thickness of P-type cladding layer  110  is decreased to about 300 nm. 
     As illustrated in  FIG.  16   , in nitride-based semiconductor light-emitting element  100  according to the present embodiment, effective refractive index difference ΔN can be made smaller than that of the nitride-based semiconductor light-emitting elements in the comparative examples. 
     As illustrated in  FIG.  17    and  FIG.  18   , in nitride-based semiconductor light-emitting element  100  according to the present embodiment, the absolute value of position P1 and difference ΔP can be made smaller than those of the nitride-based semiconductor light-emitting elements in the comparative examples. 
     As described above, in nitride-based semiconductor light-emitting element  100  according to the present embodiment, the thickness of P-type cladding layer  110  can be decreased, and thus the operating voltage can be lowered. 
     Embodiment 2 
     A nitride-based semiconductor light-emitting element according to Embodiment 2 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  100  according to Embodiment 1 in the band gap energy distribution of the P-side guide layer. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  100  according to Embodiment 1. 
     2-1. Overall Configuration 
     First, an overall configuration of the nitride-based semiconductor light-emitting element according to the present embodiment is to be described with reference to  FIG.  19    and  FIG.  20   .  FIG.  19    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  200  according to the present embodiment.  FIG.  20    is a schematic graph showing a distribution of band gap energy in active layer  105  and layers in the vicinity thereof in nitride-based semiconductor light-emitting element  200  according to the present embodiment. 
     As illustrated in  FIG.  19   , nitride-based semiconductor light-emitting element  200  according to the present embodiment includes semiconductor stack body  200 S, current block layer  112 , P-side electrode  113 , and N-side electrode  114 . Semiconductor stack body  200 S includes substrate  101 , N-type first cladding layer  102 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  105 , P-side guide layer  206 , intermediate layer  108 , electron barrier layer  109 , P-type cladding layer  110 , and contact layer  111 . 
     In P-side guide layer  206 , similarly to P-side guide layer  106  according to Embodiment 1, band gap energy of P-side guide layer  206  monotonically increases with an increase in distance from active layer  105 . P-side guide layer  206  includes a portion in which band gap energy continuously increases with an increase in distance from active layer  105 . In the present embodiment, P-side guide layer  206  is an undoped In xp Ga 1-xp N layer, and an average rate of change in the In composition ratio of P-side guide layer  206  in the stack direction in a region from the interface on the side close to active layer  105  to a center portion in the stack direction is greater than an average rate of change in the In composition ratio of P-side guide layer  206  in the stack direction in a region from the center portion to the interface on the side close to P-type cladding layer  110 . Stated differently, a curve showing a relation between a position in the stack direction and the In composition ratio of P-side guide layer  206  has a downward convex shape. Further stated differently, a curve showing a relation between a position in the stack direction and band gap energy of P-side guide layer  206  has an upward convex shape (see  FIG.  20   ). 
     In the present embodiment, P-side guide layer  206  includes P-side first guide layer  206   a  and P-side second guide layer  206   b . P-side first guide layer  206   a  is an undoped In xp Ga 1-xp N layer having a thickness of 140 nm. More specifically, P-side first guide layer  206   a  has a composition represented by In Xp1 Ga 1-Xp1 N at and in the vicinity of the interface on the side close to active layer  105 , and has a composition represented by In Xpm Ga 1-Xpm N at and in the vicinity of the interface on the side far from active layer  105 . In composition ratio Xp of P-side first guide layer  206   a  decreases at a certain rate of change with an increase in distance from active layer  105 . P-side second guide layer  206   b  is an undoped In xp Ga 1-xp N layer having a thickness of 140 nm. More specifically, P-side second guide layer  206   b  has a composition represented by In Xpm Ga 1-Xpm N at and in the vicinity of the interface on the side close to active layer  105 , and has a composition represented by In Xp2 Ga 1-Xp2 N at and in the vicinity of the interface on the side far from active layer  105 . In composition ratio Xp of P-side second guide layer  206   b  decreases at a certain rate of change with an increase in distance from active layer  105 . In the present embodiment, Xp1 = 0.04, Xpm = 0.02, and Xp2 = 0. 
     2-2. Effects 
     21. Free Carrier Loss 
     Next, effects on reduction in free carrier loss, which are yielded by nitride-based semiconductor light-emitting element  200  according to the present embodiment, are to be described with reference to  FIG.  21    and  FIG.  22   .  FIG.  21    is a graph showing simulation results of distributions of a valence band potential and a hole Fermi level in the stack direction of nitride-based semiconductor light-emitting element  200  according to the present embodiment.  FIG.  22    is a graph showing simulation results of distributions of carrier concentrations in the stack direction of nitride-based semiconductor light-emitting element  200  according to the present embodiment. 
     As illustrated in  FIG.  21   , in nitride-based semiconductor light-emitting element  200  according to the present embodiment, a curve showing a valence band potential in P-side guide layer  206  has a downward convex shape. Here, a curve showing a hole Fermi level in P-side guide layer  206  has a downward convex shape. Accordingly, since a curve showing a valence band potential in P-side guide layer  206  is given a downward convex shape, a difference between a hole Fermi level and a valence band potential in P-side guide layer  206  can be maintained more uniform than that in P-side guide layer  106  according to Embodiment 1. Thus, as illustrated in  FIG.  22   , a concentration of holes particularly in a region of P-side guide layer  206  close to active layer  105  can be reduced. In this manner, free carrier loss in P-side guide layer  206  can be still further reduced. Specifically, in the present embodiment, guide-layer free carrier loss can be reduced down to 1.7 cm -1 , and waveguide loss can be reduced down to 3.1 cm -1 . 
     In nitride-based semiconductor light-emitting element  200  according to the present embodiment, effective refractive index difference ΔN is 1.9×10 -3 , positions P1 and P2 of light intensity distributions are -3.8 nm and -15.8 nm, respectively, and difference ΔP is 12 nm. In this manner, in the present embodiment, position P1 and difference ΔP can be reduced, and thus a nonlinear portion is not readily generated in the graph showing IL characteristics. 
     22. In Composition Ratio Distribution 
     Next, effects of the In composition ratio distribution in P-side guide layer  206  of nitride-based semiconductor light-emitting element  200  according to the present embodiment are to be described with reference to  FIG.  23    and  FIG.  24   .  FIG.  23    and  FIG.  24    are graphs showing simulation results of a relation between (i) an average In composition ratio of P-side guide layer  206  according to the present embodiment and (ii) waveguide loss and a light confinement coefficient (┌v), respectively.  FIG.  23    and  FIG.  24    illustrate waveguide loss and a light confinement coefficient, respectively, when In composition ratio Xp1 of P-side guide layer  206  at and in the vicinity of the interface on the side close to active layer  105  is 4%, In composition ratio Xp2 of P-side guide layer  206  at and in the vicinity of the interface on the side far from active layer  105  is 0%, and the In composition ratio is continuously and monotonically decreased with an increase in distance from active layer  105 . More specifically,  FIG.  23    and  FIG.  24    illustrate waveguide loss and a light confinement coefficient, respectively, when the average In composition ratio of P-side guide layer  206  is changed by changing In composition ratio Xpm in a center portion of P-side guide layer  206  in the stack direction. In the examples illustrated in  FIG.  23    and  FIG.  24   , when the average In composition ratio is less than 2%, a curve showing a relation between the position of P-side guide layer  206  in the stack direction and the In composition ratio has a downward convex shape. For example, the case where the average In composition ratio is 1.5% corresponds to nitride-based semiconductor light-emitting element 200 according to the present embodiment, and the case where the average In composition ratio is 2% corresponds to nitride-based semiconductor light-emitting element  100  according to Embodiment 1.  FIG.  23    and  FIG.  24    also illustrate simulation results when the In composition ratio of the P-side guide layer is uniform, using broken lines. 
     As illustrated in  FIG.  23    and  FIG.  24   , waveguide loss can be more decreased and a light confinement coefficient can be more increased in the case where the In composition ratio of P-side guide layer  206  is continuously and monotonically decreased with an increase in distance from active layer  105  than in the case where the In composition ratio of P-side guide layer  206  is uniform. When the average In composition ratio is less than 2%, waveguide loss can be still more decreased, and the light confinement coefficient can be still more increased. 
     23. Relation of Thicknesses of Guide Layers 
     Next, effects of a relation between the thicknesses of N-side guide layer  104  and P-side guide layer  206  according to the present embodiment are to be described with reference to  FIG.  25    to  FIG.  29   .  FIG.  25    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and a light confinement coefficient (┌v).  FIG.  26    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and waveguide loss.  FIG.  27    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and effective refractive index difference ΔN.  FIG.  28    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and position P1.  FIG.  29    is a graph showing simulation results of a relation between the thickness of N-side guide layer  104  according to the present embodiment and difference ΔP. In the simulations as shown by  FIG.  25    to  FIG.  29   , the thicknesses of N-side guide layer  104  and P-side guide layer  206  are changed while a total of the thicknesses of N-side guide layer  104  and P-side guide layer  206  is maintained constant at 440 nm. The In composition ratio of N-side guide layer  104  is 4%, whereas the In composition ratio of P-side guide layer  106  is 4% at and in the vicinity of the interface on the side close to active layer  105 , is 0% at and in the vicinity of the interface on the side far from active layer  105 , and is 1% in a center portion of P-side guide layer  206  in the stack direction.  FIG.  25    to  FIG.  29    also illustrate simulation results in an example in which the In composition ratio of the P-side guide layer is constant at 1.5% as a comparative example, using broken lines. 
     As illustrated in  FIG.  25   , the light confinement coefficient can be increased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  206 . Note that thickness Tn of N-side guide layer  104  may be greater than or equal to 100 nm. In this manner, an excessive shift of a light intensity distribution from active layer  105  toward P-side guide layer  206  due to thickness Tn of N-side guide layer  104  being excessively thin can be reduced. As illustrated in  FIG.  25   , also when the In composition ratio of P-side guide layer  206  is maintained constant at 1.5%, the light confinement coefficient can be increased by making the thickness of N-side guide layer  104  smaller than the thickness of P-side guide layer  206 . But nevertheless, the light confinement coefficient can be further increased by continuously and monotonically decreasing the In composition ratio with an increase in distance from active layer  105  as with P-side guide layer  206  according to the present embodiment. 
     As illustrated in  FIG.  26   , as with P-side guide layer  206  according to the present embodiment, waveguide loss can be more decreased in the case where the In composition ratio is continuously and monotonically decreased with an increase in distance from active layer  105  than in the case where the In composition ratio is maintained constant at 1.5%. In the present embodiment, even if the thickness of N-side guide layer  104  is changed, waveguide loss can be maintained substantially constant at 3.2 cm -1  or less. 
     As illustrated in  FIG.  27   , effective refractive index difference ΔN can be decreased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  206 . As illustrated in  FIG.  27   , as with P-side guide layer  206  according to the present embodiment, effective refractive index difference ΔN can be more decreased in the case where the In composition ratio is continuously and monotonically decreased with an increase in distance from active layer  105  than in the case where the In composition ratio is maintained constant at 1.5%. 
     As illustrated in  FIG.  28   , the absolute value of position P1 can be decreased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  206 . Note that thickness Tn of N-side guide layer  104  may be in a range from 100 nm to 165 nm. Stated differently, the thickness of N-side guide layer  104  may be set to a value in a range from 23% to 38% of a total of the thicknesses of N-side guide layer  104  and P-side guide layer  206 . In this manner, position P1 can be placed at a position in a range from -7 nm to 18 nm, that is, a peak of a light intensity distribution can be located within active layer  105 . When the thickness of N-side guide layer  104  is set to a value in a range from 23% to 38% of a total of the thicknesses of N-side guide layer  104  and P-side guide layer  206  and distance dc is set to 40 nm, effective refractive index difference ΔN can be maintained in a range from 1.85×10 -3  to 2.0×10 -3 , as illustrated in  FIG.  27   . 
     As illustrated in  FIG.  28   , also when the In composition ratio of P-side guide layer  206  is maintained constant at 1.5%, the absolute value of position P1 can be decreased by making the thickness of N-side guide layer  104  smaller than the thickness of P-side guide layer  206 . But nevertheless, the absolute value of position P1 can be further decreased by continuously and monotonically decreasing the In composition ratio with an increase in distance from active layer  105 , as with P-side guide layer  206  according to the present embodiment, when the thickness of N-side guide layer  104  is greater than or equal to 160 nm. 
     As illustrated in  FIG.  29   , difference ΔP can be decreased by setting thickness Tn of N-side guide layer  104  to a thickness less than 220 nm, or stated differently, by making thickness Tn smaller than thickness Tp of P-side guide layer  206 . In particular, the thickness of N-side guide layer  104  is set to a value in a range from 23% to 38% of a total of the thicknesses of N-side guide layer  104  and the P-side guide layer  206 , thus making difference ΔP 13 nm or less. As illustrated in  FIG.  29   , also when the In composition ratio of P-side guide layer  206  is maintained constant at 1.5%, difference ΔP can be decreased by making the thickness of N-side guide layer  104  smaller than the thickness of P-side guide layer  206 . But nevertheless, difference ΔP can be further decreased by continuously and monotonically decreasing the In composition ratio with an increase in distance from active layer  105 , as with P-side guide layer  206  according to the present embodiment. 
     24. Barrier Layer 
     Next, effects of the configurations of the barrier layers of active layer  105  according to the present embodiment are to be described in comparison with a comparative example. In the present embodiment, band gap energy of each barrier layer is less than or equal to the smallest value of band gap energy of N-side guide layer  104  and P-side guide layer  206 . Here, as a comparative example, a simulation result of a nitride-based semiconductor light-emitting element according to Comparative Example 5 is shown in which the composition of the barrier layers is undoped GaN, band gap energy of the barrier layers is made greater than or equal to the smallest value of the band gap energy of N-side guide layer  104  and P-side guide layer  206 , and the other configuration is the same as that of nitride-based semiconductor light-emitting element  200  according to the present embodiment. In the nitride-based semiconductor light-emitting element according to Comparative Example 5, a light confinement coefficient is 1.37%, effective refractive index difference ΔN is 2.7×10 -3 , positions P1 and P2 of light intensity distributions are 28.1 nm and 9.2 nm, respectively, and difference ΔP is 18.9 nm. Further, waveguide loss is 4 cm -1 , and free carrier loss of the N-side guide layer and the P-side guide layer is 2.5 cm -1 . Accordingly, in the nitride-based semiconductor light-emitting element according to Comparative Example 5, band gap energy of the barrier layers is great, or stated differently, the refractive indices of the barrier layers are small, and thus a light confinement coefficient is smaller than that of nitride-based semiconductor light-emitting element  200  according to the present embodiment. Along with this, other evaluation indices of the nitride-based semiconductor light-emitting element according to Comparative Example 5 are not as good as those of nitride-based semiconductor light-emitting element  200  according to the present embodiment, except position P2. 
     As described above, in nitride-based semiconductor light-emitting element  200  according to the present embodiment, the light confinement coefficient can be increased by setting band gap energy of the barrier layers to a value less than or equal to the smallest value of band gap energy of N-side guide layer  104  and P-side guide layer  206 . Along with this, since difference ΔP can be decreased, a nonlinear portion is not readily generated in a graph showing IL characteristics. 
     Embodiment 3 
     A nitride-based semiconductor light-emitting element according to Embodiment 3 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  100  according to Embodiment 1 in the relation between the Al composition ratios of the N-type first cladding layer and the P-type cladding layer and the configuration of the electron barrier layer. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  100  according to Embodiment 1, with reference to  FIG.  30   . 
       FIG.  30    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element 300 according to the present embodiment. 
     As illustrated in  FIG.  30   , nitride-based semiconductor light-emitting element  300  according to the present embodiment includes semiconductor stack body  300 S, current block layer  112 , P-side electrode  113 , and N-side electrode  114 . Semiconductor stack body 300S includes substrate  101 , N-type first cladding layer  302 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  105 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  309 , P-type cladding layer  110 , and contact layer  111 . 
     N-type first cladding layer  302  according to the present embodiment is an N-type Al 0.036 Ga 0.964 N layer having a thickness of 1200 nm. N-type first cladding layer  302  is doped with Si having a concentration of 1×10 18  cm -3 , as an impurity. 
     P-type cladding layer  110  according to the present embodiment is a P-type Al 0.026 Ga 0.974 N layer having a thickness of 450 nm, as stated above. 
     In the present embodiment, N-type first cladding layer  302  and P-type cladding layer  110  each include Al, and the following relation is satisfied, where Tnc denotes the Al composition ratio of N-type first cladding layer  302 , and Ypc denotes the Al composition ratio of P-type cladding layer  110 : Ync &gt; Ypc (4) 
     Here, at least one of N-type first cladding layer  302  or P-type cladding layer  110  has a superlattice structure, composition ratios Ync and Ypc each show an average Al composition ratio. For example, when N-type first cladding layer  302  includes a plurality of GaN layers each having a thickness of 2 nm, a plurality of AlGaN layers each having a thickness of 2 nm and an Al composition ratio of 0.07, and the GaN layers and the AlGaN layers are alternately stacked, Ync is 0.035 that is an average Al composition ratio of entire N-type first cladding layer  302 . When P-type cladding layer  110  includes a plurality of GaN layers each having a thickness of 2 nm, a plurality of AlGaN layers each having a thickness of 2 nm and an Al composition ratio of 0.07, and the GaN layers and the AlGaN layers are alternately stacked, Ypc is 0.035 that is an average Al composition ratio of entire P-type cladding layer  110 . 
     Accordingly, the refractive index of N-type first cladding layer  302  can be made smaller than the refractive index of P-type cladding layer  110 . Thus, even if the thickness of P-type cladding layer  110  is reduced in order to lower the operating voltage of nitride-based semiconductor light-emitting element  300 , the refractive index of N-type first cladding layer  302  is smaller than the refractive index of P-type cladding layer  110 , and thus a shift of a peak of a light intensity distribution in the stack direction from active layer  105  toward N-type first cladding layer  302  can be decreased. 
     Electron barrier layer  309  is a nitride-based semiconductor layer that is provided above active layer  105  and includes at least Al. In the present embodiment, electron barrier layer  309  is provided between intermediate layer  108  and P-type cladding layer  110 . Electron barrier layer  309  is a P-type AlGaN layer having a thickness of 5 nm. Electron barrier layer 309 includes an Al composition ratio increasing region in which the Al composition ratio monotonically increases with a decrease in distance from P-type cladding layer  110 . Here, a configuration in which the Al composition ratio monotonically increases includes a configuration that includes a region in which the Al composition ratio is constant in the stack direction. For example, the configuration in which the Al composition ratio monotonically increases also includes a configuration in which the Al composition ratio increases stepwise. In electron barrier layer  309  according to the present embodiment, entire electron barrier layer  309  is the Al composition ratio increasing region, and has an Al composition ratio that increases at a certain rate of change in the stack direction. Specifically, electron barrier layer  309  has a composition represented by Al 0.02 Ga 0.98 N at and in the vicinity of the interface thereof with intermediate layer  108 . The Al composition ratio of electron barrier layer  309  monotonically increases with a decrease in distance from P-type cladding layer  110 . Electron barrier layer  309  has a composition represented by Al 0.36 Ga 0.64 N, at and in the vicinity of the interface thereof with P-type cladding layer  110 . Electron barrier layer  309  is doped with Mg having a concentration of 1×10 19  cm -3 , as an impurity. 
     Electron barrier layer  309  can prevent leakage of electrons from active layer  105  to P-type cladding layer  110 . Further, electron barrier layer  309  has the Al composition ratio increasing region in which the Al composition ratio monotonically increases, and thus a potential barrier in a valence band of electron barrier layer  309  can be decreased. Accordingly, holes readily flow from P-type cladding layer  110  to active layer  105 . Thus, as in the present embodiment, also when P-side guide layer  106  that is an undoped layer has a great thickness, an increase in electrical resistance of nitride-based semiconductor light-emitting element  300  can be reduced. In this manner, the operating voltage of nitride-based semiconductor light-emitting element  300  can be lowered. Since self-heating of nitride-based semiconductor light-emitting element  300  during operation can be reduced, temperature characteristics of nitride-based semiconductor light-emitting element  300  can be enhanced. Thus, nitride-based semiconductor light-emitting element  300  can perform high-power operation. 
     According to the present embodiment, nitride-based semiconductor light-emitting element  300  can be produced, in which effective refractive index difference ΔN is 1.9×10 -3 , position P1 is 5.3 nm, difference ΔP is 4.2 nm, a coefficient of confinement of light in active layer  105  is 1.55%, waveguide loss is 3.6 cm -1 , and guide-layer free carrier loss is 2.4 cm -1 . 
     Embodiment 4 
     A nitride-based semiconductor light-emitting element according to Embodiment 4 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  300  according to Embodiment 3 mainly in that a light-transmitting conductive film is provided above a contact layer at a ridge. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  300  according to Embodiment 3, with reference to  FIG.  31   . 
       FIG.  31    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  400  according to the present embodiment. As illustrated in  FIG.  31   , nitride-based semiconductor light-emitting element  400  according to the present embodiment includes semiconductor stack body  400 S, current block layer  112 , P-side electrode  113 , N-side electrode  114 , and light-transmitting conductive film  420 . Semiconductor stack body  400 S includes substrate  101 , N-type first cladding layer  302 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  105 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  309 , P-type cladding layer  410 , and contact layer  411 . 
     P-type cladding layer  410  according to the present embodiment is provided between electron barrier layer  309  and contact layer  411 . P-type cladding layer  410  has a smaller refractive index and greater band gap energy than active layer  105 . In the present embodiment, P-type cladding layer  410  is a P-type Al 0.026 Ga 0.974 N layer having a thickness of 330 nm. P-type cladding layer  410  is doped with Mg as an impurity. An impurity concentration at an edge portion of P-type cladding layer  410  on the side close to active layer  105  is lower than the impurity concentration at an edge portion of P-type cladding layer  410  on the side far from active layer  105 . Specifically, P-type cladding layer  410  includes a P-type Al 0.026 Ga 0.974 N layer provided on the side close to active layer  105 , doped with Mg having a concentration of 2×10 18  cm -3 , and having a thickness of 150 nm, and a P-type Al 0.026 Ga 0.974 N layer provided on the side far from active layer  105 , doped with Mg having a concentration of 1×10 19  cm -3 , and having a thickness of 180 nm. 
     Similarly to nitride-based semiconductor light-emitting element  300  according to Embodiment 3, ridge  410 R is formed in P-type cladding layer  410 . Further, two grooves  410 T are formed in P-type cladding layer  410 , which are provided along ridge  410 R and extend in the Y-axis direction. 
     Contact layer  411  is provided above P-type cladding layer  410 , and is in ohmic contact with P-side electrode  113 . In the present embodiment, contact layer  411  is a P-type GaN layer having a thickness of 10 nm. Contact layer  411  is doped with Mg having a concentration of 1×10 20  cm -3 , as an impurity. 
     Light-transmitting conductive film  420  according to the present embodiment is a conductive film that is provided above P-type cladding layer  410 , and transmits at least a portion of light generated in nitride-based semiconductor light-emitting element  400 . As light-transmitting conductive film  420 , for example, an oxide film can be used which has visible light transmissivity and low-resistance electrical conductivity, such as a tin-doped indium oxide (ITO) layer, a Ga-doped zinc oxide layer, an Al-doped zinc oxide layer, or an In- and Ga-doped zinc oxide layer. 
     It is sufficient if light-transmitting conductive film  420  is formed above at least P-type cladding layer  410 , and light-transmitting conductive film  420  may be formed between current block layer  112  and P-side electrode  113 . 
     Nitride-based semiconductor light-emitting element  400  according to the present embodiment also yields equivalent effects to those yielded by nitride-based semiconductor light-emitting element  100  according to Embodiment 1, as illustrated in  FIG.  14    to  FIG.  18    described above. 
     Furthermore, also in the present embodiment, light-transmitting conductive film  420  provided above P-type cladding layer  410  is included, and thus loss of light that propagates above P-type cladding layer  410  can be decreased. As illustrated in  FIG.  15   , this effect is noticeable particularly when P-type cladding layer  410  has a small thickness. A great increase in waveguide loss can be reduced even if the thickness of P-type cladding layer  410  is reduced down to 0.32 µm.Furthermore, even if the thickness of P-type cladding layer  410  is reduced down to 0.25 µm, an amount of increase in waveguide loss can be reduced to at most 0.8 cm -1 , as compared with the case where P-type cladding layer  410  has a thickness of 0.6 µm.It can be seen that this amount of increase is reduced to at most a half of an amount of increase in waveguide loss of nitride-based semiconductor light-emitting element  100  according to Embodiment 1 in which light-transmitting conductive film  420  is not used. Furthermore, the thickness of P-type cladding layer  410  can be still further decreased, and thus electrical resistance of nitride-based semiconductor light-emitting element  400  can be still further decreased. As a result, slope efficiency of nitride-based semiconductor light-emitting element  400  can be increased, and furthermore the operating voltage thereof can be lowered. 
     According to the present embodiment, nitride-based semiconductor light-emitting element  400  can be produced, in which effective refractive index difference ΔN is 2.0×10 -3 , position P1 is 1.4 nm, difference ΔP is 4.0 nm, a coefficient of confinement of light in active layer  105  is 1.51%, waveguide loss is 3.8 cm -1 , and free carrier loss is 1.9 cm -1 . 
     Embodiment 5 
     A nitride-based semiconductor light-emitting element according to Embodiment 5 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  300  according to Embodiment 3 in the configuration of the active layer. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  300  according to Embodiment 3, with reference to  FIG.  32 A  and  FIG.  32 B . 
       FIG.  32 A  is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  500  according to the present embodiment.  FIG.  32 B  is a cross sectional view illustrating a configuration of active layer  505  included in nitride-based semiconductor light-emitting element  500  according to the present embodiment. 
     As illustrated in  FIG.  32 A , nitride-based semiconductor light-emitting element  500  according to the present embodiment includes semiconductor stack body  500 S, current block layer  112 , P-side electrode  113 , N-side electrode  114 , and light-transmitting conductive film  420 . Semiconductor stack body  500 S includes substrate  101 , N-type first cladding layer  302 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  505 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  309 , P-type cladding layer  110 , and contact layer  111 . 
     Active layer  505  according to the present embodiment has a single quantum well structure, and includes single well layer  105   b , and barrier layers  105   a  and  105   c  between which well layer  105   b  is provided, as illustrated in  FIG.  32 B . Well layer  105   b  has the same configuration as that of well layer  105   b  according to Embodiment 1, and barrier layers  105   a  and  105   c  have the same configuration as that of barrier layers  105   a  and  105   c  according to Embodiment 1. 
     Nitride-based semiconductor light-emitting element  500  according to the present embodiment also yields equivalent effects to those yielded by nitride-based semiconductor light-emitting element  300  according to Embodiment 3. In particular, in nitride-based semiconductor light-emitting element  500  having such a single quantum well structure as stated above, active layer  505  includes single well layer  105   b . In this manner, also in nitride-based semiconductor light-emitting element  500  that includes a less number of well layer  105   b  having a great refractive index, a peak of a light intensity distribution in the stack direction can be located in active layer  505  or in the vicinity thereof, owing to the configurations of N-side guide layer  104  and P-side guide layer  106 , for instance. Thus, a light confinement coefficient can be increased. 
     According to the present embodiment, nitride-based semiconductor light-emitting element  500  can be produced, in which effective refractive index difference ΔN is 2.1×10 -3 , position P1 is 1.1 nm, difference ΔP is 6.0 nm, a coefficient of confinement of light in active layer  505  is 0.75%, waveguide loss is 3.8 cm -1 , and guide-layer free carrier loss is 2.4 cm -1 . Note that in the present embodiment, a total thickness of active layer  505  is smaller than active layer  105  according to Embodiment 3 by 8 nm, and thus the light confinement coefficient is smaller than that in Embodiment 3. 
     Embodiment 6 
     A nitride-based semiconductor light-emitting element according to Embodiment 6 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  100  according to Embodiment 1 mainly in the configuration of the N-side guide layer. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  100  according to Embodiment 1. 
     6-1. Overall Configuration 
     First, an overall configuration of the nitride-based semiconductor light-emitting element according to the present embodiment is to be described with reference to  FIG.  33    and  FIG.  34   .  FIG.  33    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  600  according to the present embodiment.  FIG.  34    is a schematic graph showing a distribution of band gap energy in active layer  105  and layers in the vicinity thereof in nitride-based semiconductor light-emitting element  600  according to the present embodiment. 
     As illustrated in  FIG.  33   , nitride-based semiconductor light-emitting element  600  according to the present embodiment includes semiconductor stack body  600 S, current block layer  112 , P-side electrode  113 , and N-side electrode  114 . Semiconductor stack body  600 S includes substrate  101 , N-type first cladding layer  602 , N-type second cladding layer  103 , N-side guide layer  604 , active layer  105 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  109 , P-type cladding layer  610 , and contact layer  111 . 
     N-type first cladding layer  602  according to the present embodiment is an N-type Al 0.035 Ga 0.965 N layer having a thickness of 1200 nm. N-type first cladding layer  602  is doped with Si having a concentration of 1×10 18  cm -3 , as an impurity. 
     P-type cladding layer  610  according to the present embodiment is provided between electron barrier layer  109  and contact layer  111 . P-type cladding layer  610  has a smaller refractive index and greater band gap energy than those of active layer  105 . In the present embodiment, P-type cladding layer  610  is a P-type Al 0.035 Ga 0.965 N layer having a thickness of 450 nm. P-type cladding layer  610  is doped with Mg as an impurity. An impurity concentration at an edge portion of P-type cladding layer  610  on the side close to active layer  105  is lower than an impurity concentration of P-type cladding layer  610  at an edge portion on the side far from active layer  105 . Specifically, P-type cladding layer  610  includes a P-type Al 0.035 Ga 0.965 N layer provided on the side close to active layer  105 , doped with Mg having a concentration of 2×10 18  cm -3 , and having a thickness of 150 nm, and a P-type Al 0.035 Ga 0.965 N layer provided on the side far from active layer  105 , doped with Mg having a concentration of 1×10 19  cm -3 , and having a thickness of 300 nm. 
     Similarly to nitride-based semiconductor light-emitting element  100  according to Embodiment 1, ridge  610 R is formed in P-type cladding layer  610 . Further, two grooves  610 T are formed in P-type cladding layer  610 , which are provided along ridge  610 R and extend in the Y-axis direction. 
     N-side guide layer  604  according to the present embodiment is a light guide layer provided above N-type second cladding layer  103 . N-side guide layer  604  has a greater refractive index and less band gap energy than those of N-type first cladding layer  602  and N-type second cladding layer  103 . As illustrated in  FIG.  34   , the band gap energy of N-side guide layer  604  continuously and monotonically increases with an increase in distance from active layer  105 . 
     When N-side guide layer  604  consists essentially of In Xn Ga 1-Xn N, In composition ratio Xn of N-side guide layer  604  may continuously and monotonically decrease with an increase in distance from active layer  105 . Accordingly, band gap energy of N-side guide layer  604  continuously and monotonically increases with an increase in distance from active layer  105 . 
     N-side guide layer  604  is an N-type In Xn Ga 1-Xn N layer having a thickness of 160 nm. More specifically, N-side guide layer  604  has a composition represented by In Xn1 Ga 1-Xn1 N at and in the vicinity of the interface on the side close to active layer  105 , and has a composition represented by In Xn2 Ga 1-Xn2 N at and in the vicinity of the interface on the side far from active layer  105 . In the present embodiment, In composition ratio Xn1 of N-side guide layer  604  at and in the vicinity of the interface on the side close to active layer  105  is 4%, and In composition ratio Xn2 thereof at and in the vicinity of the interface on the side far from active layer  105  is 0%. In composition ratio Xn of N-side guide layer  604  decreases at a certain rate of change with an increase in distance from active layer  105 . 
     6-2. Effects 
     61. In Composition Ratio Distribution 
     Next, effects of the In composition ratio distribution in N-side guide layer  604  of nitride-based semiconductor light-emitting element  600  according to the present embodiment are to be described with reference to  FIG.  35    and  FIG.  36   .  FIG.  35    and  FIG.  36    are graphs showing simulation results of relations between (i) an average In composition ratio of N-side guide layer  604  according to the present embodiment and (ii) a light confinement coefficient (┌v) and an operating voltage, respectively. 
       FIG.  35    and  FIG.  36    illustrate a light confinement coefficient and an operating voltage, respectively, when In composition ratio Xn1 of N-side guide layer  604  at and in the vicinity of the interface on the side close to active layer  105  is 4%, In composition ratio Xn2 thereof at and in the vicinity of the interface on the side far from active layer  105  is 0%, 1%, 2%, 3%, and 4%, and the In composition ratio is decreased at a certain rate of change with an increase in distance from active layer  105 . Note that the drawings illustrate the operating voltage when the supply current amount is 3 A.  FIG.  35    and  FIG.  36    also illustrate simulation results when the In composition ratio of the N-side guide layer is uniform, using broken lines. 
     As illustrated in  FIG.  35    and  FIG.  36   , a high refractive index region of N-side guide layer  604  can be located closer to active layer  105  in the case where the In composition ratio of N-side guide layer  604  is continuously and monotonically decreased with an increase in distance from active layer  105  than in the case where the In composition ratio of N-side guide layer  604  is uniform, and thus a light confinement coefficient can be more increased and an operating voltage can be lowered. When the average In composition ratio is less than 2%, waveguide loss can be still more decreased, and the light confinement coefficient can be still more increased. 
     Next, effects on reduction in the operating voltage of nitride-based semiconductor light-emitting element  600  according to the present embodiment are to be described with reference to  FIG.  37    and  FIG.  38   , in comparison with nitride-based semiconductor light-emitting element  100  according to Embodiment 1.  FIG.  37    illustrates graphs showing relations of a position in the stack direction of nitride-based semiconductor light-emitting element  100  according to Embodiment 1 with a piezo polarization charge density, a piezo polarization electric field, and a conduction band potential.  FIG.  38    illustrates graphs showing relations of a position in the stack direction of nitride-based semiconductor light-emitting element  600  according to the present embodiment with a piezo polarization charge density, a piezo polarization electric field, and a conduction band potential. Graphs (a), (b), and (c) in  FIG.  37    and  FIG.  38    each show a relation of a position in the stack direction of the nitride-based semiconductor light-emitting element with a piezo polarization charge density, a piezo polarization electric field, and a conduction band potential, respectively. Note that graphs (c) in  FIG.  37    and  FIG.  38    each also show a hole Fermi level using a broken line. 
     As illustrated in graph (a) in  FIG.  37   , a piezo polarization charge density of N-side guide layer  104  in nitride-based semiconductor light-emitting element  100  according to Embodiment 1 is constant in the stack direction. Accordingly, there are great differences in piezo polarization charge density at an interface between N-side guide layer  104  and N-type second cladding layer  103  and an interface between N-side guide layer  104  and active layer  105 . Due to this, piezo polarization charge is locally formed at an interface between N-side guide layer  104  and N-type second cladding layer  103  and an interface between N-side guide layer  104  and active layer  105 . Accordingly, great piezo polarization electric fields are generated. Thus, as illustrated in graph (b) in  FIG.  37   , a piezo polarization electric field having a spiking shape is generated at each of the interface between N-side guide layer  104  and N-type second cladding layer  103  and the interface between N-side guide layer  104  and active layer  105 . As a result, holes are attracted to and in the vicinity of the interface between N-side guide layer  104  and N-type second cladding layer  103  and the interface between N-side guide layer  104  and active layer  105 , and conduction band potentials at the interfaces increase (see ΔE1 shown in graph (c) in  FIG.  37   ). 
     On the other hand, as illustrated in graph (a) in  FIG.  38   , the polarization charge density of N-side guide layer  604  of nitride-based semiconductor light-emitting element  600  according to the present embodiment monotonically decreases as approaching from the interface on the side close to active layer  105  to the interface on the side far from active layer  105 . Accordingly, differences in piezo polarization charge density at the interface between N-side guide layer  604  and N-type second cladding layer  103  and the interface between N-side guide layer  604  and active layer  105  are reduced. Accordingly, piezo polarization charge is dispersed in the stack direction in N-side guide layer  604 . Thus, as illustrated in graph (b) in  FIG.  38   , a piezo polarization electric field can be reduced at each of the interface between N-side guide layer  604  and N-type second cladding layer  103  and the interface between N-side guide layer  604  and active layer  105 . As a result, as illustrated in graph (c) in  FIG.  38   , an increase in conduction band potential (see ΔE1 shown in graph (c) in  FIG.  38   ) due to holes being attracted can be reduced at the interface between N-side guide layer  604  and N-type second cladding layer  103  and the interface between N-side guide layer  604  and active layer  105 . Accordingly, in nitride-based semiconductor light-emitting element  600  according to the present embodiment, conductivity of electrons that flow from N-type second cladding layer  103  toward active layer  105  can be increased, and thus an operating voltage can be lowered. 
     62. Impurity in N-Side Guide Layer 
     Next, effects of impurities in N-side guide layer  604  according to the present embodiment are to be described with reference to  FIG.  39    to  FIG.  41   .  FIG.  39   ,  FIG.  40   , and  FIG.  41    illustrate graphs showing simulation results of a relation between (i) an average In composition ratio of N-side guide layer  604  in nitride-based semiconductor light-emitting element  600  according to the present embodiment and (ii) a light confinement coefficient (┌v), waveguide loss, and an operating voltage, respectively. Graphs (a), (b), (c), and (d) in  FIG.  39    to  FIG.  41    show simulation results when the concentrations of an impurity (Si) in N-side guide layer  604  are 0 (that is, undoped), 3×10 17  cm -3 , 6×10 17  cm -3 , and 1×10 18  cm -3 , respectively. Note that  FIG.  41    illustrates the operating voltage when the supply current amount is 3 A. 
       FIG.  39    and  FIG.  41    illustrate a light confinement coefficient and an operating voltage, respectively, when In composition ratio Xn1 of N-side guide layer  604  at and in the vicinity of the interface on the side close to active layer  105  is 4%, In composition ratio Xn2 thereof at and in the vicinity of the interface on the side far from active layer  105  is 0%, 1%, 2%, 3%, and 4%, and the In composition ratio is decreased at a certain rate of change with an increase in distance from active layer  105 .  FIG.  39    and  FIG.  41    also illustrate simulation results when the In composition ratio of N-side guide layer  604  is uniform, using broken lines. 
     As illustrated in  FIG.  39   , in nitride-based semiconductor light-emitting element  600  according to the present embodiment, a light confinement coefficient can be made greater than that of the nitride-based semiconductor light-emitting element according to the comparative example in which the In composition ratio of the N-side guide layer is uniform. Furthermore,  FIG.  39    also shows that the light confinement coefficient hardly depends on an impurity concentration, in nitride-based semiconductor light-emitting element  600  according to the present embodiment. 
     As illustrated in  FIG.  40   , in nitride-based semiconductor light-emitting element  600  according to the present embodiment, waveguide loss can be reduced more than that of the nitride-based semiconductor light-emitting element according to the comparative example in which the In composition ratio of the N-side guide layer is uniform, except when an impurity is not added. This is considered to be caused by a decrease in hole concentration due to an energy band gap distribution in N-side guide layer  604  in the stack direction although an electron concentration is increased by the addition of an impurity. 
     As illustrated in  FIG.  41   , in nitride-based semiconductor light-emitting element  600  according to the present embodiment, an operating voltage can be made lower than that of the nitride-based semiconductor light-emitting element according to the comparative example in which the In composition ratio of the N-side guide layer is uniform. An electron concentration in N-side guide layer  604  can be increased by increasing a concentration of an impurity added to nitride-based semiconductor light-emitting element  600 , and thus an operating voltage can be still further lowered. 
     As shown in  FIG.  40    and  FIG.  41   , in nitride-based semiconductor light-emitting element  600  according to the present embodiment, a great increase in waveguide loss can be reduced and an operating voltage can be lowered, by setting the impurity concentration in N-side guide layer  604  to a value in a range from 1×10 17  cm -3  to 6×10 17  cm -3 . 
     As described above, according to the present embodiment, nitride-based semiconductor light-emitting element  600  can be produced, in which effective refractive index difference ΔN is 2.9×10 -3 , position P1 is 15.9 nm, difference ΔP is 6.2 nm, a coefficient of confinement of light in active layer  105  is 1.44%, waveguide loss is 3.4 cm -1 , and guide-layer free carrier loss is 1.45 cm -1 . 
     Embodiment 7 
     A nitride-based semiconductor light-emitting element according to Embodiment 7 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  100  according to Embodiment 1 mainly in the configuration of the P-type cladding layer. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  100  according to Embodiment 1, with reference to  FIG.  42   . 
       FIG.  42    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  700  according to the present embodiment. As illustrated in  FIG.  42   , nitride-based semiconductor light-emitting element  700  according to the present embodiment includes semiconductor stack body  700 S, current block layer  112 , P-side electrode  113 , and N-side electrode  114 . Semiconductor stack body  700 S includes substrate  101 , N-type first cladding layer  102 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  105 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  709 , P-type cladding layer  710 , and contact layer  111 . 
     Electron barrier layer  709  according to the present embodiment is a P-type Al 0.36 Ga 0.64 N layer having a thickness of 1.6 nm. Electron barrier layer  709  is doped with Mg having a concentration of 1.5×10 19  cm -3 , as an impurity. 
     P-type cladding layer  710  according to the present embodiment is provided between electron barrier layer  709  and contact layer  111 . P-type cladding layer  710  has a smaller refractive index and greater band gap energy than active layer  105 . Similarly to P-type cladding layer  110  according to Embodiment 1, ridge  710 R is formed in P-type cladding layer  710 . Further, two grooves  710 T are formed in P-type cladding layer  710 , which are provided along ridge  710 R and extend in the Y-axis direction. 
     P-type cladding layer  710  is a P-type Al 0.026 Ga 0.974 N layer having a thickness of 450 nm. P-type cladding layer  710  is doped with Mg as an impurity. In the present embodiment, an impurity concentration at an edge portion of P-type cladding layer  710  on the side close to active layer  105  is lower than the impurity concentration at an edge portion on the side far from active layer  105 . An impurity concentration of P-type cladding layer  710  includes a region in which the concentration monotonically increases with an increase in distance from active layer  105 . Here, the configuration in which an impurity concentration monotonically increases includes a configuration in which a region where an impurity concentration is constant in the stack direction is present. Specifically, P-type cladding layer  710  includes: a P-type Al 0.026 Ga 0.974 N layer provided closest to active layer  105 , doped with Mg having a concentration of 2×10 18  cm -3 , and having a thickness of 150 nm; a P-type Al 0.026 Ga 0.974 N layer provided thereabove, doped with Mg having a concentration of 1×10 19  cm -3 , and having a thickness of 180 nm; and a P-type Al 0.026 Ga 0.974 N layer provided thereabove, doped with Mg having a concentration of 1.3×10 19  cm -3 , and having a thickness of 120 nm. As described above, in the present embodiment, P-type cladding layer  710  includes a first layer closest to active layer  105 , a second layer having an impurity concentration higher than that of the first layer, and a third layer having an impurity concentration higher than that of the second layer. 
     In the present embodiment, the thickness of P-side guide layer  106  is greater than the thickness of N-side guide layer  104 . In this case, a peak of a light intensity distribution in the stack direction is located in a region of active layer  105  or in the vicinity thereof, and thus spread of light to P-type cladding layer  710  can be reduced. Thus, the light intensity in P-type cladding layer  710  is weak. Accordingly, an increase in waveguide loss can be reduced even if an Mg concentration of P-type cladding layer  710  in a region in the vicinity of contact layer  111  is increased. In addition, a series resistance of nitride-based semiconductor light-emitting element  700  (that is a resistance between P-side electrode  113  and N-side electrode  114 ) can be decreased by increasing an Mg concentration. 
     For example, when the thickness of P-side guide layer  106  is greater than or equal to 200 nm, a light intensity is sufficiently low so as to reduce an increase in waveguide loss even if an Mg concentration is increased to and above 1.3×10 19  cm -3 , in a region of P-type cladding layer  710  within a range of 0.15 µm from the interface with contact layer  111 . In this manner, a series resistance of nitride-based semiconductor light-emitting element  700  can be decreased by increasing an Mg concentration of P-type cladding layer  710 . Note that the Mg concentration of P-type cladding layer  710  may be less than or equal to 1.6×10 19  cm -3 . Accordingly, an increase in series resistance can be reduced since a decrease in mobility of carriers due to an excessive increase in Mg concentration can be reduced. 
     When a thickness of P-side guide layer  106  is greater than or equal to 250 nm, a light intensity of P-type cladding layer  710  is further decreased, and thus an increase in waveguide loss can be reduced even if the thickness of a low-concentration region of P-type cladding layer  710  in which the Mg concentration is the lowest is set to 20 nm or less. 
     The Mg concentration in P-type cladding layer  710  does not need to be changed stepwise, and may be changed continuously. For example, the Mg concentration in P-type cladding layer  710  may have a configuration as follows. The Mg concentration in P-type cladding layer  710  at the interface on the side close to active layer  105  is substantially the same as the Mg concentration of 1.5×10 19  cm -3  in electron barrier layer  709 . The Mg concentration may be decreased monotonically as an increase in distance from the interface so that the Mg concentration reaches a range from 1×10 18  cm -3  to 3×10 18  cm -3  in a region of P-type cladding layer  710  within a range of 100 nm from the interface. In this manner, P-type cladding layer  710  may include a concentration decreasing region in which an impurity concentration monotonically decreases with an increase in distance from active layer  105 , in a region closest to active layer  105 . Furthermore, P-type cladding layer  710  may include a low concentration region which is provided above the concentration decreasing region and in which a change in Mg concentration in the stack direction is small and the Mg concentration is the lowest in P-type cladding layer  710 . In the low concentration region, for example, the Mg concentration is in a range from 1×10 18  cm -3  to 3×10 18  cm -3 . Furthermore, P-type cladding layer  710  may include a concentration increasing region which is provided above the low concentration region and in which the Mg concentration monotonically increases with an increase in distance from active layer  105 . In the concentration increasing region, for example, the Mg concentration monotonically increases from the range from 1×10 18  cm -3  to 3×10 18  cm -3  to reach 1.3×10 19  cm -3 . 
     The concentration increasing region may include a high increasing-rate region provided on the side close to active layer  105  and a low increasing-rate region provided above the high increasing-rate region. A rate of change in the Mg concentration in the stack direction in the high increasing-rate region is greater than a rate of change in the Mg concentration in the stack direction in the low increasing-rate region. 
     According to the present embodiment, nitride-based semiconductor light-emitting element  700  can be produced, in which effective refractive index difference ΔN is 1.9×10 -3 , position P1 is 3.6 nm, difference ΔP is 2.8 nm, a coefficient of confinement of light in active layer  105  is 1.54%, waveguide loss is 3.6 cm -1 , and guide-layer free carrier loss is 2.4 cm -1 . 
     Embodiment 8 
     A nitride-based semiconductor light-emitting element according to Embodiment 8 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  700  according to Embodiment 7 in the configuration of the electron barrier layer. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  700  according to Embodiment 7, with reference to  FIG.  43    and  FIG.  44   . 
       FIG.  43    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  800  according to the present embodiment.  FIG.  44    is a graph showing a distribution of an Al composition ratio in the stack direction of electron barrier layer  809  according to the present embodiment. The horizontal axis of the graph illustrated in  FIG.  44    indicates position x in the stack direction, whereas the vertical axis thereof indicates an Al composition ratio. In  FIG.  44   , distributions of the Al composition ratio in a part of intermediate layer  108  and a part of P-type cladding layer  710 , together with the distribution in electron barrier layer  809 . 
     As illustrated in  FIG.  43   , nitride-based semiconductor light-emitting element  800  according to the present embodiment includes semiconductor stack body  800 S, current block layer  112 , P-side electrode  113 , and N-side electrode  114 . Semiconductor stack body  800 S includes substrate  101 , N-type first cladding layer  102 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  105 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  809 , P-type cladding layer  710 , and contact layer  111 . 
     Electron barrier layer  809  according to the present embodiment is a P-type AlGaN layer. Electron barrier layer  809  is doped with Mg having a concentration of 1.5×10 19  cm -3 , as an impurity. Electron barrier layer  809  includes an Al composition ratio increasing region in which the Al composition ratio monotonically increases with a decrease in distance from P-type cladding layer  110 , and an Al composition ratio decreasing region which is provided above the Al composition ratio increasing region and in which the Al composition ratio monotonically decreases with a decrease in distance from P-type cladding layer  710 . Here, the configuration in which the Al composition ratio monotonically decreases includes a configuration that includes a region in which the Al composition ratio is constant in the stack direction. For example, the configuration in which the Al composition ratio monotonically decreases also includes a configuration in which the Al composition ratio decreases stepwise. In the graph as illustrated in  FIG.  44   , position x = Xs indicates an interface of electron barrier layer  809  with intermediate layer  108 , and position x = Xe indicates an interface of electron barrier layer  809  with P-type cladding layer  710 . Note that position x = Xs may be defined by an edge portion of a region in which the Al composition ratio increases as a decrease in distance from P-type cladding layer  710 . Position x = Xe may be defined by an edge portion of a region in which the Al composition ratio decreases with a decrease in distance from P-type cladding layer  710 , or in other words, an edge portion of a region in which the Al composition ratio is constant in the stack direction. Position x = Xm is a position at which the Al composition ratio is the highest in electron barrier layer  809 . A region from position x = Xs to position x = Xm is an Al composition ratio increasing region, and a region from position x = Xm to position x = Xe is an Al composition ratio decreasing region. 
     The thickness of electron barrier layer  809  is less than or equal to 5 nm. The thickness of the Al composition ratio increasing region is less than or equal to 2 nm. The thickness of the Al composition ratio decreasing region is greater than the thickness of the Al composition ratio increasing region. The thickness of a region of electron barrier layer  809  in which the Al composition ratio is the highest is less than or equal to 0.5 nm. Here, the region in which the Al ratio is the highest means a region in which the Al composition ratio is greater than or equal to 95% of the greatest value of the Al composition ratio of electron barrier layer  809 . 
     The graph illustrated in  FIG.  44    shows straight lines g(x) and h(x) together with curve f(x) showing a distribution of the Al composition ratio with respect to a position in electron barrier layer  809  in the stack direction. Straight line g(x) passes through a point on curve f(x) at position x = Xs and a point on curve f(x) at position x = Xm. Straight line h(x) passes through a point on curve f(x) at position x = Xm and a point on curve f(x) at position x = Xe. As illustrated in  FIG.  44   , curve f(x) is convex downward in a range from position x =Xs to position x = Xm. Curve f(x) is convex downward in a range from position x =Xm to position x = Xe. In other words, f(Xd1) &lt; g(Xd1) at position x = Xd1 corresponding to an intermediate point between position x = Xs and position x = Xm. Further, f(Xd2) &lt; h(Xd2) at position x = Xd2 corresponding to an intermediate point between position x = Xm and position x = Xe. 
     As described above, by tilting the Al composition ratio of electron barrier layer  809  on the side close to active layer  105 , positive piezo-polarization charge formed at the interface of electron barrier layer  809  with intermediate layer 108 can be distributed in the Al composition ratio increasing region. Along with this, a concentration of electrons attracted by positive piezo-polarization charge is decreased at the interface of electron barrier layer  809  with intermediate layer  108 . As a result, a decrease in potential energy in a valence band at the interface of electron barrier layer  809  with intermediate layer  108  can be reduced. Accordingly, a potential barrier against holes that flow from P-type cladding layer  710  to active layer  105  is reduced, so that the operating voltage is lowered. 
     By setting the thickness of a region in which the Al composition ratio is the highest to less than or equal to 0.5 nm, a potential barrier against holes in the valence band can be reduced, and the operating voltage can be lowered. 
     Further, by setting the thickness of electron barrier layer  809  to less than or equal to 5 nm, a width (that is, the thickness) of a potential barrier in the valence band, which is formed in electron barrier layer  809 , can be decreased. As a result, a barrier against electrical conduction from P-type cladding layer  710  to active layer  105  by holes can be decreased, and thus the operating voltage is lowered. Here, if the thickness of electron barrier layer  809  is smaller than 2 nm, more electrons flow from active layer  105  to P-type cladding layer  710  over electron barrier layer  809 , and thus the thickness of electron barrier layer  809  needs to be greater than or equal to 2 nm. 
     If the thickness of the Al composition ratio increasing region is less than or equal to 2 nm, generation of electrons that flow from active layer  105  to P-type cladding layer  710  over electron barrier layer  809  can be reduced by making the thickness of the Al composition ratio decreasing region of electron barrier layer  809  greater than the thickness of the Al composition ratio increasing region while maintaining the thickness of electron barrier layer  809  less than or equal to 5 nm. 
     More positive piezo-polarization charge formed at and in the vicinity of the interface of electron barrier layer  809  with intermediate layer  108  is found in a region in which a rate of change in Al composition ratio is relatively great than in a region in which a rate of change in Al composition ratio is relatively small and which is close to intermediate layer  108 . By making the shape of curve f(x) in the Al ratio increasing region in electron barrier layer  809  downward convex, a rate of change in Al composition ratio at and in the vicinity of the interface of electron barrier layer  809  with intermediate layer  108  can be made small, and thus positive piezo polarization charge can be decreased at the interface. Thus, a concentration of electrons attracted by positive piezo-polarization charge is decreased at the interface. Along with this, a potential barrier against holes in the valence band at the interface of electron barrier layer  809  with intermediate layer  108  can be decreased. 
     Furthermore, since the shape of curve f(x) in the Al composition ratio decreasing region of electron barrier layer  809  is made downward convex, the thickness of a region in which the Al composition ratio is high at or in the vicinity of position x = Xm can be made thin, and the width of a potential barrier in the valence band, which is formed in electron barrier layer  809 , can be further narrowed. As a result, a barrier against electrical conduction from P-type cladding layer  710  to active layer  105  by holes can be decreased, and the operating voltage can be lowered. 
     The Mg concentration in electron barrier layer  809  according to the present embodiment may be less than or equal to 1.5 × 10 19  cm -3 . Since the Al composition ratio tilts in a region (that is, the Al composition ratio increasing region) of electron barrier layer  809  on the side close to active layer  105 , a potential barrier against holes in electron barrier layer  809  can be decreased even if the Mg concentration is set to 1.5 × 10 19  cm -3  or less. Accordingly, even if the Al composition ratio of electron barrier layer  809  is increased to 30% or higher, an increase in the operating voltage can be reduced. 
     Furthermore, since the shape of curve f(x) in a region (that is, the Al composition ratio increasing region) of electron barrier layer  809  on the side close to active layer  105  is made downward convex, an effect on prevention of a potential barrier from being formed in the valence band is increased, and the Mg concentration can be set to 1 × 10 19  cm -3  or less. Note that the Mg concentration may be set to 0.7×10 18  cm -3  or more. Accordingly, an excessive decrease in potential of electron barrier layer  809  in the valence band can be reduced. 
     In the present embodiment, electron barrier layer  809  has a composition expressed by Al 0.02 Ga 0.98 N at and in the vicinity of the interface with intermediate layer  108 , and has an Al composition ratio that monotonically increases with a decrease in distance from P-type cladding layer  710 . Electron barrier layer  809  has a composition expressed by Al 0.36 Ga 0.64 N at position x=Xm distant from the interface of electron barrier layer  809  with intermediate layer  108  by 1 nm. The Al composition ratio of electron barrier layer  809  monotonically decreases with a decrease in distance from position x=Xm to P-type cladding layer  710 . Electron barrier layer  809  has a composition expressed by Al 0.026 Ga 0.974 N at and in the vicinity of the interface with P-type cladding layer  710 . 
     According to the present embodiment, nitride-based semiconductor light-emitting element  800  can be produced, in which effective refractive index difference ΔN is 1.9×10 -3 , position P1 is 3.6 nm, difference ΔP is 2.8 nm, a coefficient of light confinement in active layer  105  is 1.54%, waveguide loss is 3.6 cm -1 , and guide-layer free carrier loss is 2.4 cm -1 . 
     Embodiment 9 
     A nitride-based semiconductor light-emitting element according to Embodiment 9 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  100  according to Embodiment 1 mainly in the configuration of the ridge. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  100  according to Embodiment 1, with reference to  FIG.  45    and  FIG.  46   . 
       FIG.  45    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  900  according to the present embodiment.  FIG.  46    is a schematic graph showing a distribution of band gap energy in active layer  105  and layers in the vicinity thereof in nitride-based semiconductor light-emitting element  900  according to the present embodiment. 
     As illustrated in  FIG.  45   , nitride-based semiconductor light-emitting element  900  according to the present embodiment includes semiconductor stack body  900 S, current block layer  112 , P-side electrode  113 , and N-side electrode  114 . Semiconductor stack body  900 S includes substrate  101 , N-type first cladding layer  102 , N-type second cladding layer  103 , N-side guide layer  104 , active layer  105 , P-side guide layer  106 , intermediate layer  108 , electron barrier layer  109 , P-type cladding layer  910 , and contact layer  111 . 
     P-type cladding layer  910  according to the present embodiment is provided between electron barrier layer  109  and contact layer  111 . P-type cladding layer  910  has a smaller refractive index and greater band gap energy than those of active layer  105 . 
     P-type cladding layer  910  is a P-type Al 0.026 Ga 0.974 N layer having a thickness of 315 nm. P-type cladding layer  910  is doped with Mg as an impurity. In the present embodiment, an impurity concentration of P-type cladding layer  910  at an edge portion on the side close to active layer  105  is lower than the impurity concentration thereof at an edge portion on the side far from active layer  105 . P-type cladding layer  910  includes a P-type Al 0.026 Ga 0.974 N layer provided on the side close to active layer  105 , doped with Mg having a concentration of 2×10 18  cm -3 , and having a thickness of 150 nm, and a P-type Al 0.026 Ga 0.974 N layer provided thereabove, doped with Mg having a concentration of 1×10 19  cm -3 , and having a thickness of 165 nm. 
     Ridge  910 R is formed in P-type cladding layer  910 . Further, two grooves  910 T are formed in P-type cladding layer  910 , which are provided along ridge  910 R and extend in the Y-axis direction. 
     One method of reducing kinks is to increase effective refractive index difference ΔN of nitride-based semiconductor light-emitting element  900 . Here, a relation between ridge width W and effective refractive index difference ΔN necessary to reduce kinks is to be described with reference to  FIG.  47   .  FIG.  47    is a graph showing a relation between ridge width W and effective refractive index difference ΔN necessary to reduce kinks.  FIG.  47    illustrates a relation obtained by simulation of a semiconductor light-emitting element having a stack structure similar to that of nitride-based semiconductor light-emitting element  900  according to the present embodiment. 
     As illustrated in  FIG.  47   , effective refractive index difference ΔN necessary to reduce kinks increases with a decrease in ridge width W. In the present embodiment, a relation between ridge width W [µm] and effective refractive index difference ΔN necessary to reduce kinks is ΔN = 11.4 exp (-0.039 W) × 10 -3 . Thus, in order to reduce kinks, effective refractive index difference ΔN may be preferably greater than or equal to 11.4 exp (-0.039 W) × 10 -3 . In this manner, if ridge width W is small, kinks are readily generated in a graph showing IL characteristics. In particular, ridge width W is smaller than ridge width W according to Embodiment 1, and kinks are readily generated when ridge width W is less than or equal to 30 µm. 
     In order to achieve such effective refractive index difference ΔN that reduces kinks, in the present embodiment, as illustrated in  FIG.  45    and  FIG.  46   , a ridge is formed in P-type cladding layer  910  and electron barrier layer  109 , and lower edge Rb of ridge  910 R (that is, the bottoms of grooves  910 T) is positioned between active layer  105  and electron barrier layer  109 . Accordingly, effective refractive index difference ΔN can be increased without reducing a distance between electron barrier layer  109  having a low refractive index and active layer  105 . Thus, effective refractive index difference ΔN can be increased while a decrease in a light confinement coefficient is reduced, and position P1 is placed in active layer  105 . Ridge width W according to the present embodiment is less than or equal to 45 µm. Ridge width W can be made smaller than ridge width W according to Embodiment 1, and can be made 30 µm or less. In other words, a beam spot size in the horizontal direction can be decreased. 
     In the present embodiment, the thickness of P-type cladding layer  910  is 315 nm, and lower edge Rb of ridge  910 R is positioned in P-side guide layer  106 . More specifically, lower edge Rb of ridge  910 R is at a position distant from the interface between P-side guide layer  106  and intermediate layer  108  by 70 nm (or in other words, the distance from the interface between P-side guide layer  106  and active layer  105  is 210 nm). In this manner, since ridge  910 R is formed in P-side guide layer  106 , an average refractive index of P-side guide layer  106  can be decreased, and thus position P1 can be shifted from active layer  105  toward N-side guide layer  104 . Accordingly, light absorption by P-side electrode  113  can be decreased. Thus, waveguide loss can be decreased. 
     In the present embodiment, as described above, position P1 can be shifted from active layer  105  toward N-side guide layer  104 . Accordingly, by using P-type cladding layer  910  having a small thickness, even if a distance between P-side electrode  113  and active layer  105  is short, light absorbed by P-side electrode  113  can be decreased. Here, if Ag is used for P-side electrode  113 , a refractive index of light having a wavelength in a range from 360 nm to 800 nm can be decreased down to 0.2 or less, even if the thickness of P-type cladding layer  910  is set to 300 nm or less, loss of light absorbed by P-side electrode  113  is small, so that an increase in waveguide loss is not caused. Thus, the thickness of a P-type cladding layer having a high impurity doping concentration and great free carrier loss can be made further thinner. If the thickness of P-type cladding layer  910  is made excessively thin, a light intensity distribution in the stack direction shifts toward the substrate, which leads to a decrease in light confinement coefficient, so the thickness may be greater than or equal to 100 nm. 
     Further, a light-transmitting conductive film may be provided as a portion of an electrode, between P-type cladding layer  910  and P-side electrode  113  according to the present embodiment. Similarly to light-transmitting conductive film  420  according to Embodiment 4, light-transmitting conductive film  420  according to the present embodiment is a conductive film that is provided above P-type cladding layer  910 , and transmits at a portion of light generated in nitride-based semiconductor light-emitting element  900 . As light-transmitting conductive film  420 , for example, an oxide film can be used which has visible light transmissivity and low-resistance electrical conductivity, such as a tin-doped indium oxide (ITO) layer, a Ga-doped zinc oxide layer, an Al-doped zinc oxide layer, or an In- and Ga-doped zinc oxide layer. 
     It is sufficient if light-transmitting conductive film  420  is formed above at least P-type cladding layer  410 , and light-transmitting conductive film  420  may be formed between current block layer  112  and P-side electrode  113 . 
     Light-transmitting conductive film  420  has high transmissivity for light having a wavelength in a range from 400 nm to 1 µm, and has a refractive index of about 2 (for example, 2.1 in the case of an ITO layer). From this, a light intensity distribution in a waveguide propagation mode can spread in light-transmitting conductive film  420 , and thus light-transmitting conductive film  420  can be used as a portion of a cladding layer. Light-transmitting conductive film  420  has low loss of absorbed light having a wavelength greater than or equal to 420 nm, and can be used as a low-loss cladding layer for a nitride-based semiconductor light-emitting element having a wavelength range from 420 nm to 600 nm. In this case, even if the thickness of P-type cladding layer  910  is reduced down to 300 nm or less, waveguide loss does not increase, and a low-loss waveguide can be produced, which reduces kinks owing to an increase in effective refractive index difference ΔN even in an element having ridge width W that is less than or equal to 30 µm. Furthermore, light-transmitting conductive film  420  has functions as a cladding layer as described above, and even if a distance between light-transmitting conductive film  420  and active layer  105  is decreased, an intensity peak position of a light intensity distribution in the stack direction can be placed in active layer  105  or in the vicinity thereof, and thus a high light confinement coefficient can be obtained. The thickness of P-type cladding layer  910  may be zero nm (or stated differently, P-type cladding layer  910  may not be provided). Yet, the refractive index of P-type cladding layer  910  has a magnitude between the refractive index of active layer  105  and the refractive index of light-transmitting conductive film  420 , and better control for placing an intensity peak position of a light intensity distribution in the stack direction in active layer  105  or in the vicinity thereof can be achieved if P-type cladding layer  910  is present between active layer  105  and light-transmitting conductive film  420 . As a result, the thickness of P-type cladding layer  910  may be preferably greater than or equal to 10 nm. If the thickness of light-transmitting conductive film  420  is too thin, attenuation of a light intensity distribution in the stack direction in light-transmitting conductive film  420  is insufficient, and waveguide loss is generated when an electrode such as an Ag electrode having a low refractive index as described above is used as P-side electrode  113 . In contrast, if the thickness of light-transmitting conductive film  420  is excessively thick, a series resistance of a nitride-based semiconductor light-emitting element increases, and thus the thickness of light-transmitting conductive film  420  may be in a range from 100 nm to 500 nm. Here, if Ag is used for P-side electrode  113 , loss of light absorbed by P-side electrode  113  is decreased, and thus the thickness of light-transmitting conductive film  420  may be in a range from 50 nm to 500 nm. 
     According to the present embodiment, nitride-based semiconductor light-emitting element  900  can be produced, in which effective refractive index difference ΔN is 8.4x10 -3 , position P1 is 1.9 nm, a coefficient of confinement of light in active layer  105  is 1.46%, and waveguide loss is 3.44 cm -1 . 
     Embodiment 10 
     A nitride-based semiconductor light-emitting element according to Embodiment 10 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  900  according to Embodiment 9 mainly in the position of the lower edge of the ridge. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from nitride-based semiconductor light-emitting element  900  according to Embodiment 9, with reference to  FIG.  48   . 
       FIG.  48    is a schematic graph showing a distribution of band gap energy in active layer  105  and layers in the vicinity thereof in the nitride-based semiconductor light-emitting element according to the present embodiment. 
     The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element  900  according to Embodiment 9 in the configurations of intermediate layer  1008  and P-type cladding layer  910  and the position of lower edge Rb of ridge  910 R. 
     Intermediate layer  1008  according to the present embodiment is a layer provided between P-side guide layer  106  and electron barrier layer  109 , and having band gap energy less than that of electron barrier layer  109  and greater than that of P-side guide layer  106 , similarly to intermediate layer  108  according to Embodiment 9. In the present embodiment, intermediate layer  1008  is an undoped GaN layer having a thickness of 50 nm. 
     P-type cladding layer  910  according to the present embodiment includes a P-type Al 0.026 Ga 0.974 N layer provided on the side close to active layer  105 , doped with Mg having a concentration of 2×10 18  cm -3 , and having a thickness of 150 nm, and a P-type Al 0.026 Ga 0.974 N layer provided thereabove, doped with Mg having a concentration of 1×10 19  cm -3 , and having a thickness of 215 nm. 
     As illustrated in  FIG.  48   , in the nitride-based semiconductor light-emitting element according to the present embodiment, lower edge Rb of ridge  910 R is positioned in intermediate layer  1008 . More specifically, lower edge Rb of ridge  910 R is at a position distant from the interface between P-side guide layer  106  and intermediate layer  1008  by 10 nm (or in other words, the distance from the interface between intermediate layer  1008  and electron barrier layer  109  is 40 nm). Accordingly, a shift in position P2 from active layer  105  toward N-side guide layer  104  can be reduced. Thus, in the graph showing IL characteristics, generation of kinks can be reduced. 
     In the present embodiment, intermediate layer  1008  has a first constant region in which band gap energy is constant in the stack direction, and lower edge Rb of ridge  910 R is positioned in the first constant region. Accordingly, even if the position of lower edge Rb of ridge  910 R changes due to a manufacturing error, for instance, a change in characteristics of the nitride-based semiconductor light-emitting element can be reduced. Thus, an individual difference of the nitride-based semiconductor light-emitting element can be reduced. In particular, lower edge Rb of ridge  910 R is positioned outside P-side guide layer  106 , and thus influence exerted by a change in position of lower edge Rb of ridge  910 R on light guide effects can be reduced. Thus, an individual difference in effect of the nitride-based semiconductor light-emitting element on a light guide can be reduced. Note that in the present embodiment, entire intermediate layer  1008  is the first constant region. Note that intermediate layer  1008  may have a region in which band gap energy is not constant in the stack direction. 
     According to the present embodiment, the nitride-based semiconductor light-emitting element can be produced, in which effective refractive index difference ON is 3.83×10 -3 , position P1 is 1.4 nm, a coefficient of confinement of light in active layer  105  is 1.49%, and waveguide loss is 3.32 cm -1 . 
     Embodiment 11 
     A nitride-based semiconductor light-emitting element according to Embodiment 11 is to be described. The nitride-based semiconductor light-emitting element according to the present embodiment is different from nitride-based semiconductor light-emitting element according to Embodiment 1 mainly in the configuration of the intermediate layer. The following mainly describes differences of the nitride-based semiconductor light-emitting element according to the present embodiment from the nitride-based semiconductor light-emitting element according to Embodiment 10, with reference to  FIG.  49   . 
       FIG.  49    is a schematic graph showing a distribution of band gap energy in active layer  105  and layers in the vicinity thereof in the nitride-based semiconductor light-emitting element according to the present embodiment. 
     The nitride-based semiconductor light-emitting element according to the present embodiment is different from the nitride-based semiconductor light-emitting element according to Embodiment 10 in the configurations of intermediate layer  1008  and P-type cladding layer  910  and the position of lower edge Rb of ridge  910 R. 
     Intermediate layer  1108  according to the present embodiment is a layer provided between P-side guide layer  106  and electron barrier layer  109 , and having band gap energy less than that of electron barrier layer  109  and greater than that of P-side guide layer  106 , similarly to intermediate layer  1008  according to Embodiment 10. In the present embodiment, intermediate layer  1108  includes first constant region  1108   a  in which band gap energy is constant in the stack direction, and second constant region  1108   b  which is provided above first constant region  1108   a , and having band gap energy greater than that in first constant region  1108   a  and constant in the stack direction. In the present embodiment, first constant region  1108   a  is an undoped GaN layer having a thickness of 10 nm, and second constant region  1108   b  is an undoped Al 0.02 GaN 0.98 N layer having a thickness of 10 nm. 
     P-type cladding layer  910  according to the present embodiment includes a P-type Al 0.026 Ga 0.974 N layer provided on the side close to active layer  105 , doped with Mg having a concentration of 2×10 18  cm -3 , and having a thickness of 150 nm, and a P-type Al 0.026 Ga 0.974 N layer provided thereabove, doped with Mg having a concentration of 1×10 19  cm -3 , and having a thickness of 250 nm. 
     As illustrated in  FIG.  49   , in the nitride-based semiconductor light-emitting element according to the present embodiment, lower edge Rb of ridge  910 R is positioned in second constant region  1108   b  of intermediate layer  1108 . More specifically, lower edge Rb of ridge  910 R is at a position distant from the interface between first constant region 1108a and second constant region  1108   b  by 5 nm (stated differently, 5 nm from the interface between second constant region  1108   b  and electron barrier layer  109 ). Accordingly, a shift in position P2 from active layer  105  toward N-side guide layer  104  can be reduced, similarly to Embodiment 10. Thus, generation of kinks can be reduced in the graph showing IL characteristics. 
     Also in the present embodiment, similarly to Embodiment 10, lower edge Rb of ridge  910 R is positioned in a region in which band gap energy is constant in the stack direction. Accordingly, even if the position of lower edge Rb of ridge  910 R changes due to a manufacturing error, for instance, a change in characteristics of the nitride-based semiconductor light-emitting element can be reduced. In particular, lower edge Rb of ridge  910 R is positioned outside P-side guide layer  106 , and thus influence exerted due to the position of lower edge Rb of ridge  910 R on light guide effects can be reduced. Thus, an individual difference in effect of the nitride-based semiconductor light-emitting element on light guide can be reduced. 
     Note that in the present embodiment, lower edge Rb of ridge  910 R is positioned in second constant region  1108   b , but may be positioned in first constant region  1108   a . Further, intermediate layer  1108  includes two regions in which band gap energy is constant in the stack direction, but may include three or more of such regions. Moreover, intermediate layer  1008  may have a region in which band gap energy is not constant in the stack direction. 
     According to the present embodiment, the nitride-based semiconductor light-emitting element can be produced, in which effective refractive index difference ΔN is 3.7×10 -3 , position P1 is 1.2 nm, a coefficient of confinement of light in active layer  105  is 1.49%, and waveguide loss is 3.52 cm -1 . 
     Variations and Others 
     The nitride-based semiconductor light-emitting elements according to the present disclosure have been described above based on the embodiments, yet the present disclosure is not limited to the above embodiments. 
     For example, the embodiments have shown examples in which the nitride-based semiconductor light-emitting elements are semiconductor laser elements, yet the nitride-based semiconductor light-emitting elements are not limited to semiconductor laser elements. For example, the nitride-based semiconductor light-emitting element may be a super luminescent diode. In this case, a reflectance of an edge surface of the semiconductor stack body included in the nitride-based semiconductor light-emitting element with respect to emitted light from the semiconductor stack body may be less than or equal to 0.1%. Such a reflectance can be obtained by forming an anti-reflection film that includes, for instance, a dielectric multilayer film on the edge surface, for example. Alternatively, if a tilting stripe structure is adopted in which ridges serving as waveguides cross a front end surface in a state of tilting 5 degrees or more relative to the normal line direction, a percentage of a component that is guided light, which is reflected off the front end surface, coupled to the waveguides again, and guided, can be decreased to a small value that is less than or equal to 0.1%. In particular, if the wavelength of emitted light is caused to fall within a band from 430 nm to 455 nm, the thickness of each of well layers  105   b  and  105   d  is less than or equal to 35 Å. In this case, even if a reflectance of the edge surface is reduced, light amplification gain can be ensured owing to effects on reduction in waveguide loss and effects on an increase in a coefficient of confinement of light in active layer  105 , which are yielded by the nitride-based semiconductor light-emitting element according to the present disclosure. If such a nitride-based semiconductor light-emitting element is provided inside an external resonator that includes a wavelength selection element, self-heating of the nitride-based semiconductor light-emitting element can be reduced, and a change in wavelength of emitted light can be decreased, and thus oscillation at a desired selected wavelength can be more readily caused. 
     In Embodiments 1 to 4 and 6 described above, the nitride-based semiconductor light-emitting element has a structure in which two well layers are included as the structure of active layer  105 , yet a structure in which only a single well layer is included may be adopted. In this manner, also when the active layer includes only one well layer having a high refractive index, controllability of a position in light intensity distribution in the stack direction can be enhanced if the N-side guide layer and the P-side guide layer according to the present disclosure are used, and thus a peak of the light intensity distribution in the stack direction can be located in the well layer or in the vicinity thereof. Thus, the nitride-based semiconductor light-emitting element that has a low oscillation threshold, a low waveguide loss, a high light confinement coefficient, and current—light output (IL) characteristics with excellent linearity can be produced. 
     In the above embodiments, the nitride-based semiconductor light-emitting element has a single ridge, but may include a plurality of ridges. Such a nitride-based semiconductor light-emitting element is to be described with reference to  FIG.  50   .  FIG.  50    is a schematic cross sectional view illustrating an overall configuration of nitride-based semiconductor light-emitting element  1200  according to Variation 1. As illustrated in  FIG.  50   , nitride-based semiconductor light-emitting element  1200  according to Variation 1 includes a configuration in which nitride-based semiconductor light-emitting elements  100  according to Embodiment 1 are provided in an array in the horizontal direction. In  FIG.  50   , nitride-based semiconductor light-emitting element  1200  has a configuration in which three nitride-based semiconductor light-emitting elements  100  are integrally provided, but the number of nitride-based semiconductor light-emitting elements  100  included in nitride-based semiconductor light-emitting element  1200  is not limited to three. The number of nitride-based semiconductor light-emitting elements  100  included in nitride-based semiconductor light-emitting element  1200  may be preferably two or more. Nitride-based semiconductor light-emitting elements  100  each include light emitter  100 E that emits light. Light emitter  100 E is a portion of active layer  105  that emits light and corresponds to a portion of active layer  105  positioned below ridge  110 R. In this manner, nitride-based semiconductor light-emitting element  1200  according to Variation 1 includes a plurality of light emitters  100 E provided in an array. Accordingly, a plurality of light beams are emitted from single nitride-based semiconductor light-emitting element  1200 , and thus high-power nitride-based semiconductor light-emitting element  1200  can be produced. Note that in Variation 1, nitride-based semiconductor light-emitting element  1200  includes plural nitride-based semiconductor light-emitting elements  100 , yet the plural nitride-based semiconductor light-emitting elements included in nitride-based semiconductor light-emitting element  1200  are not limited thereto, and may be nitride-based semiconductor light-emitting elements according to a different embodiment. 
     As shown in nitride-based semiconductor light-emitting element  1200   a  according to Variation 2 illustrated in  FIG.  51   , each adjacent pair of light emitters  100 E may be separated by separation groove  100 T having a width (a size in the X-axis direction) in a range from 8 µm to 20 µm and a depth (a size in the Z-axis direction) in a range from 1.0 µm to 1.5 µm. By adopting such a structure, even when the spacing between adjacent light emitters  100 E is narrowed down to 300 µm or less, thermal interference caused by self-heating of light emitters  100 E while operating can be reduced. 
     In the semiconductor laser device according to the present disclosure, ΔN is small and a horizontal spread angle can be decreased, and thus even if the distance between centers of light emitters  100 E illustrated in  FIG.  50    and  FIG.  51    is shortened, light beams emitted from light emitters  100 E are not readily interfered, so that the distance between the centers of light emitters  100 E can be shortened down to 250 µm or less. In Variation 2, the distance is 225 µm. 
     The nitride-based semiconductor light-emitting elements according to the above embodiments each include N-type second cladding layer  103 , intermediate layer  108 , electron barrier layer  109 , and current block layer  112 , but do not necessarily include those layers. 
     Furthermore, P-type cladding layers  110 ,  410 , and  610  are layers having a uniform Al composition ratio, yet the configurations of the P-type cladding layers are not limited thereto. For example, the P-type cladding layers may each have a superlattice structure in which AlGaN layers and GaN layers are alternately stacked. Specifically, the P-type cladding layers may each have a superlattice structure in which AlGaN layers each having a thickness of 1.85 nm and an Al composition ratio of 0.052 (5.2%) and GaN layers each having a thickness of 1.85 nm are alternately stacked. In this case, the Al composition ratio of each P-type cladding layer is defined by an average Al composition ratio of 0.026 (2.6%) in the superlattice structure. 
     The present disclosure also encompasses embodiments as a result of adding, to the embodiments, various modifications that may be conceived by those skilled in the art, and embodiments obtained by combining elements and functions in the embodiments in any manner as long as the combination does not depart from the spirit of the present disclosure. 
     For example, the configuration of the cladding layers according to Embodiment 1 may be applied to the nitride-based semiconductor light-emitting elements according to Embodiment 3 and Embodiment 4. For example, the light-transmitting conductive film according to Embodiment 3 may be applied to the nitride-based semiconductor light-emitting elements according to Embodiment 1 and Embodiment 4. 
     Although only some exemplary embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. 
     Industrial Applicability 
     The nitride-based semiconductor light-emitting elements according to the present disclosure are applicable to, for instance, light sources for processing machines, as high-power highly efficient light sources, for example.