Patent Publication Number: US-2021167582-A1

Title: Semiconductor laser element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. continuation application of PCT International Patent Application Number PCT/JP2019/027857 filed on Jul. 16, 2019, claiming the benefit of priority of Japanese Patent Application Number 2018-141109 filed on Jul. 27, 2018, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a semiconductor laser element, 
     2. Description of the Related Art 
     Conventionally, a semiconductor laser element has been known as a small high-power light source (for example, see WO 2002/21578, etc.). In the design of such a semiconductor laser element, there is a demand for reduction in operating voltage to reduce power consumption or the like. In order to reduce the operating voltage, the semiconductor laser element disclosed in WO 2002/21578 is configured to have a larger energy band gap in a barrier layer included in an active layer than in an n-type cladding layer. With this, it is intended to suppress an increase in operating voltage by reducing a possible spike-like hetero barrier generated at the interface between the active layer and the n-type cladding layer. 
     SUMMARY 
     Here, in an AlGaAs-based semiconductor, it is known that the  refractive index decreases as the energy band gap increases, but the refractive index increases as the energy band gap decreases. 
     Accordingly, when the active layer has an energy band gap larger than that of the n-type cladding layer to reduce the spike-like hetero barrier at the interface between the active layer and the n-type cladding layer, the n-type cladding layer has a refractive index higher than that of the active layer. This decreases the effect of confining light in the active layer, and thus the light emission characteristics deteriorate. 
     The present disclosure is conceived to solve such a problem. An object of the present disclosure is to provide a semiconductor laser element capable of achieving both low operating voltage and high light confinement effect. 
     In order to achieve the above object, a semiconductor laser element according to an aspect of the present disclosure includes: an n-type cladding layer disposed above an n-type semiconductor substrate; an active layer disposed above the n-type cladding layer; and a p-type cladding layer disposed above the active layer, in which the active layer includes a well layer and a barrier layer, an energy band gap of the barrier layer is larger than an energy band gap of the n-type cladding layer, and a refractive index of the barrier layer is higher than a refractive index of the n-type cladding layer. 
     Moreover, in the semicond ctor laser element according to an aspect of the present disclosure, the n-type cladding layer may contain Al x1 Ga 1-x1-y1 In y1 As 1-z1 P z1 , the barrier layer may contain Al x2 Ga 1-x2-y2 In y2 As 1-z2 P z2 , and z1&gt;z2 may be satisfied. 
     Moreover, in the semiconductor laser element according to an aspect of the present disclosure, the n-type cladding layer may contain Al x1 Ga 1-x1-y1 In y1 P, and the barrier layer may contain Al x2 Ga 1-x2-y2 In y2 As, 
     Moreover, in the semiconductor laser element according to an aspect of  the present disclosure, x1&lt;x2 may be satisfied. 
     Moreover, in the semiconductor laser element according to an aspect of the present disclosure, the n-type semiconductor substrate may contain GaAs. 
     Moreover, the semiconductor laser element according to an aspect of the present disclosure may further include an n-side light guide layer disposed between the n-type cladding layer and the active layer and having a refractive index higher than the refractive index of the n-type cladding layer. 
     Moreover, the semiconductor laser element according to an aspect of the present disclosure may further include a hole barrier layer disposed between the n-type cladding layer and the active layer and having an energy hand gap larger than the energy hand gap of the n-type cladding layer. 
     Moreover, in the semiconductor laser element according to an aspect of the present disclosure, an energy hand gap of the p-type cladding layer may be larger than the energy hand gap of the barrier layer. Moreover, in the semiconductor laser element according to an aspect of the present disclosure, the energy band gap of the n-type cladding layer may be larger than an energy hand gap of the n-type semiconductor substrate. 
     With the present disclosure, it is possible to provide a semiconductor laser element capable of achieving both low operating voltage and high light confinement effect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages and features of the disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present disclosure, 
         FIG. 1  is a perspective view schematically illustrating the exterior of a  semiconductor laser element according to Embodiment 1; 
         FIG. 2  is a first sectional view schematically illustrating the structure of the semiconductor laser element according to Embodiment 1; 
         FIG. 3  is a second sectional view schematically illustrating the structure of the semiconductor laser element according to Embodiment 1; 
         FIG. 4  is a partially enlarged view of  FIG. 3 ; 
         FIG. 5  is a schematic sectional view of a substrate, illustrating the outline of a step of forming a semiconductor layer of the semiconductor laser element according to Embodiment 1; 
       FIG. GA is a schematic sectional view of a substrate and a semiconductor layer according to Embodiment 1; 
         FIG. 6B  is a schematic sectional view of the substrate and the semiconductor layer, illustrating a method of forming a window region according to Embodiment 1; 
         FIG. 7  is a schematic sectional view of the substrate, illustrating the outline of a step of forming a waveguide according to Embodiment 1; 
         FIG. 8  is a schematic sectional view of the substrate, illustrating the outline of a step of forming a first protective film according to Embodiment 1; 
         FIG. 9  is a graph illustrating the relationship between refractive index and Al con position ratio with respect to an AlGaInP-based semiconductorand an AlGaAs-based semiconductor; 
         FIG. 10  is a graph illustrating the relationship between energy land gap and Al composition ratio with respect to an AlGaInP-based semiconductor and an AlGaAs-based semiconductor; 
         FIG. 11  is a diagram illustrating the energy band structure (band diagram) and the refractive index distribution of the semiconductor laser element according to Embodiment 1;  
         FIG. 12  is a diagram illustrating the energy band structure and the refractive index distribution of a semiconductor laser element according to Variation 1 of Embodiment 1; 
         FIG. 13  is a diagram illustrating the energy hand structure and the refractive index distribution of a semiconductor laser lenient according to Variation 2 of Embodiment 2; 
         FIG. 14  is a table illustrating the layer structure of a semiconductor laser element according to Embodiment 2; 
         FIG. 15  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to Embodiment 2; 
         FIG. 16  is a table illustrating the layer structure of a semiconductor laser element according to Embodiment 3; 
         FIG. 17  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to Embodiment 3; 
         FIG. 18  is a table illustrating the layer structure of a semiconductor laser element according to Embodiment 4; 
         FIG. 19  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to Embodiment 4; 
         FIG. 20  is a top view illustrating the shape of a waveguide of a semiconductor laser element according to Embodiment 5; 
         FIG. 21  is a table illustrating the layer structure of the semiconductor laser element according to Embodiment 5; 
         FIG. 22  is a diagram illustratingle energy band structure and the refractive index distribution of the semiconductor laser element according to  Embodiment 5; 
         FIG. 23  is a table illustrating the layer structure of a semiconductor laser element according to Embodiment 6; and 
         FIG. 24  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to Embodiment 6. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described. with reference to the drawings. It should be noted that each of the embodiments described below shows a specific example of the present disclosure. Therefore, numerical values, shapes, materials, structural components, the arrangement and connection of the structural components, etc. shown in the following embodiments are mere examples, and are not intended to limit the scope of the present disclosure. Furthermore, among the structural components in the following embodiments, components not recited in the independent claims which indicate the broadest concept of the present disclosure are described as arbitrary structural components. 
     In addition, each of the diagrams is a schematic diagram and thus is not necessarily strictly illustrated. Therefore, the scale sizes and the like are not necessarily exactly represented in each of the diagrams. In each of the diagrams, substantially the same structural components are assigned with the same reference signs, and redundant descriptions will be omitted or simplified. 
     Moreover, in this Specification, the terms “above” and “below” do not refer to the upward (vertically upward) direction and downward (vertically downward) direction in terms of absolute spatial recognition, and are used as terms defined by relative positional relationships based on the stacking order of  a stacked structure. Furthermore, the terms “above” and “below” are applied not only when two structural components are disposed with a gap therebetween and another structural component is interposed between the two structural components, but also when two structural components are disposed in contact with each other. 
     Embodiment 1 
     A semiconductor laser element according to Embodiment 1 will be described. 
     [1.-1. Overall Structure] 
     Firstly, the overall structure of a semiconductor laser element according to the present embodiment will be described with reference to  FIG. 1  through  FIG. 4 . 
       FIG. 1  is a perspective view schematically illustrating the exterior of semiconductor laser element  14  according to the present embodiment.  FIG. 2  is the first sectional view schematically illustrating the structure of semiconductor laser element  14  according to the present embodiment.  FIG. 2  shows an enlarged sectional view of the vicinity of the waveguide WG taken along II-II of  FIG. 1 .  FIG. 3  is the second sectional view schematically illustrating the structure of semiconductor laser element  14  according to the present embodiment.  FIG. 3  shows the sectional view taken along III-III of  FIG. 1 .  FIG. 4  is a partially enlarged view of  FIG. 3 .  FIG. 4  shows an enlarged view of the IV part surrounded by the dotted line in  FIG. 3 . 
     As shown in  FIG. 2 , semiconductor laser element  14  includes chip-like substrate  24  and semiconductor layer  100  disposed on first surface P 1  of chip-like substrate  24 . 
     Chip-like substrate  24  is a substrate on which semiconductorlayer  100  of semiconductor laser element  14  is stacked. In the present embodiment,  chip-like substrate  24  is an n-type semiconductor substrate containing GaAs, particularly, an n-GaAs substrate in which the plane direction is inclined by 10 degrees from the (100) plane toward the (011) plane. The plane direction of first surface P 1  is the 10-degree off-angled (100) plane which is inclined by 10 degrees toward the (11) plane. 
     Semiconductor layer  100  includes: first semiconductor layer  30  including an n-type layer; active layer  40 ; and second semiconductor layer  50  including a p-type layer, which are stacked on chip-like substrate  24  in this order. In the present embodiment, semiconductor layer  100  mainly includes: n-type cladding layer  32  disposed above chip-like substrate  24 ; active layer  40  disposed above n-type cladding layer  32 ; and a p-type cladding layer disposed above active layer  40 . 
     As shown in  FIG. 3 , semiconductor laser element  14  includes: p-side lower electrode  151  and p-side upper electrode  152  which are disposed on semiconductor layer  100 ; and n-side electrode  160  disposed on a surface of chip-like substrate  24  opposite to semiconductor layer  100 . 
     Semiconductor layer  100  of semiconductor laser element  14  also includes waveguide WG formed in a direction along semiconductor layer  100 . In the present embodiment, in semiconductor layer  100 , waveguide WG having a ridge structure is formed. As shown in  FIG. 1 , waveguide WG extends in the first direction. 
     Moreover, as shown in  FIG. 3 , both end faces of semiconductor laser element  14  in the first direction are each cleavage end face  121 . Two cleavage end faces  121  serve as resonator faces of semiconductor laser element  14 , and second protective films  132 F and  132 R each serving as a reflectance control film are formed. Second protective films  132 F and  132 R not only serve as reflectance control films on a front side (i.e., an end-face side from which laser  light is mainly emitted) and a rear side of the resonator, respectively, but also have a function for protecting cleavage end faces  121 . 
     Moreover, as shown in  FIG. 3  and  FIG. 4 , semiconductor layer  100  has window regions  80  formed at both ends of waveguide WG. In the present embodiment, window region  80  in which light absorption in active layer  40  is reduced is formed in the vicinity of each of two cleavage end faces  121  serving as resonator faces. 
     Moreover, in the present embodiment, the resonator length is 280 μm. Here, the resonator length of semiconductor laser element  14  is set to 300 μm or less, and thus a ratio of a region other than window region  80  to waveguide WG can be reduced, thereby reducing saturation light output power. Accordingly, it is possible to prevent end-face damage of semiconductor laser element  14 . 
     The following describes each of the components in semiconductor laser element  14 . 
     Chip-like substrate  24  is a substrate obtained by dividing into chip forms, and semiconductor layer  100  is stacked thereon. The structure of chip-like substrate  24  is not particularly limited as long as it is an n-type semiconductor substrate. In the present embodiment, as described above, chip-like substrate  24  is the n-GaAs substrate. 
     First semiconductor layer  30  is a semiconductor layer including an n-type layer. In the present embodiment, as shown in  FIG. 2 , first semiconductor layer  30  includes n-type buffer layer  31 , n-type cladding layer  32 , and n-side light guide layer  33 . N-type buffer layer  31  is an n-GaAs layer having a film thickness of 0.4 μm. N-type cladding layer  32  contains Al x1 Ga 1-x1-y1 In y1 As 1-z1 P z1 . In the present embodiment, n-type cladding layer  32  is an n-(A 1   0.16 Ga 0.84 ) 0.5 In 0.5 P layer having a film thickness of 4.7 μm. N-side  light guide layer  33  is a Ga 0.5 In 0.5 P layer having a film thickness of 0.09 μm. 
     Active layer  40  is a layer serving as a light emitting unit of semiconductor laser element  14 . Active layer  40  has a well layer and a harrier layer. The barrier layer contains Al x2 Ga 1-x2-y2 In y2 As 1-z2 P z2 . 
     In the present embodiment, active layer  40  is an active layer of a multiple quantum well including: a barrier layer containing Al 0.59 Ga 0.41 As and having a film thickness of 0.03 μm; a well layer containing GaAs and having a filar thickness of 0.0065 μm; a barrier layer containing Al 0.59 Ga 0.41 As and having a film thickness of 0.004 μm; a well layer containing GaAs and having a film thickness of 0.0065 μm; and a barrier layer containing Al 0.59 Ga 0.41 As and having a film thickness of 0.021 μm, from a side facing to n-side light guide layer  33 . 
     Second semiconductor layer  50  is a semiconductor layer including a p-type layer. In the present embodiment, as shown in  FIG. 2 , second semiconductor layer  50  includes p-side light guide layer  51 , p-type first cladding layer  52 , p-type second cladding layer  53 , p-type third cladding layer  54 , p-type interlayer  55 , and p-type contact layer  56 . 
     P-side light guide layer  51  is a Ga 0.5 In 0.5 P layer having a film thickness of 0.07 μm. 
     P-type first cladding layer  52  is a p-(Al 0.30 Ga 0.70 ) 0.5 In 0.5 P layer having a film thickness of 0.17 μm. P-type second cladding layer  53  is a p-(Al 0.60 Ga 0.40 ) 0.5 In 0.5 P layer having a film thickness of 0.4 μm. P-type third cladding layer  54  is a p-(Al 0.30 Ga 0.70 ) 0.5 In 0.5 P layer having a film thickness of 0.6 μm. It is to be noted that p-type first cladding layer  52 , p-type second cladding layer  53 , and p-type third cladding layer  54  are each an example of the p-type cladding layer according to the present embodiment. 
     P-type interlayer  55  is a p-(Al 0.1 Ga 0.9 ) 0.5 In 0.5 P layer having a film  thickness of 0.106 μm. The Al composition ratio of p-type interlayer  55  is lower than that of p-type third cladding layer  54 . P-type contact layer  56  is a p-GaAs layer having a film thickness of 0.23 μm. 
     It is to be noted that the cladding layer including the above-mentioned n-type cladding layer  32  and p-type cladding layer refers to a layer having a function of confining light in the stacking direction of semiconductor layer  100 , and further having a film thickness of 0.1 μm or more and a refractive index lower than an effective refractive index for light confined in the stacking direction. 
     As shown in  FIG. 2 , first protective film  131  is formed on: the sides and a part of the top of the ridge serving as waveguide WG; trenches TRs; and flat portions at both sides. In the top of the ridge, first protective film  131  has an opening which exposes the top of the ridge. As shown in  FIG. 4 , the vicinity of cleavage end face  121  including window region  80  is covered with first protective film  131 . First protective film  131  is not particularly limited as long as it is a dielectric film, and SiO 2 , SiN, TiO 2 , ZrO 2 , Al 2 O 3 , Nb 2 O 5 , Ta 2 O 5 , etc. may be used. In the present embodiment, first protective film  131  is a SiN film having a film thickness of approximately 180 nm. 
     P-side lower electrode  151  shown in  FIG. 2  through  FIG. 4  is a patterned metal film. In the present embodiment, p-side lower electrode  151  includes: a Ti film having a film thickness of approximately 50 nm; a Pt film having a film thickness of approximately 150 nm: and an Au film having a film thickness of approximately 50 nm, which are stacked on semiconductor layer  100  in this order. P-side lower electrode  151  is in contact with p-type contact layer  56  in the opening of first protective film  131 . 
     P-side upper electrode  152  shown in  FIG. 3  is an Au film having a film thickness of at least 2.0 μm and at most 5.0 μm.  
     In the present embodiment, n-side electrode  160  shown in  FIG. 3  includes: an AuGe film having a film thickness of 90 nm; a Ni film having a film thickness of 20 nm; an Au film having a film thickness of 50 nm: a Ti film having a film thickness of 100 nm; a Pt film having a film thickness of 50 nm; a Ti film having a film thickness of 50 nm; a Pt film having a film thickness of 100 mn: and an Au film having a film thickness of 500 nm, which are stacked on chip-like substrate  24  in this order. 
     In the present embodiment, second protective film  132 F employed on the front side is a dielectric multilayer film in which one or multiple pairs of an Al 2 O 3  film having a film thickness of 50 nm and a Ta 2 O 5  film having a film thickness of  55  nm are stacked on cleavage end face  121 . Moreover, second protective film  132 R employed on the rear side is a dielectric multilayer film in which an Al 2 O 3  film having a film thickness of λ/8n λ , a SiO 2 . film having a film. thickness of λ/8n S , and a Ta 2 O 5  film having a film thickness of λ/4n T  are stacked on cleavage end face  121  in this order, and one or multiple pairs of a SiO 2  film having a film thickness of λ/4n S  and a Ta 2 O 5  film having a film thickness of λ/4n T  are further stacked. It is to be noted that X is the oscillation wavelength of semiconductor laser element  14 , and n A , n T , and n S  are the refractive indexes of the Al 2 O 3  film, the Ta 2 O 5  film, and the SiO 2  film for light having wavelength λ, respectively. In the present embodiment, λ is approximately 860 nm, and an Al 2 O 3  film having a film thickness of 65 nm, a SiO 2  film having a film thickness of 74 nm, and a Ta 2 O 5  film having a film thickness of 102 nm are stacked on cleavage end face  121  in this order, and multiple pairs of a SiO 2  film. having a film thickness of  147  nm and a Ta 2 O 5  film having a film thickness of 102 nm are further stacked. 
     Window region  80  is formed by diffusion of impurity such as Zn for forming a window region. Window region  80  is a region in which the quantum  well structure of active layer  40  is changed into a disordered structure by diffusing the impurity in the vicinity of the resonator faces of semiconductor laser element  14 , and thereby the energy hand gap of active layer  40  is increased. By increasing the energy band gap,it is possible to form window region  80  in which absorption of laser oscillation light is reduced. With this, damage by melting at the end face of semiconductor laser element  14  can be reduced, and thus it is possible to achieve highly reliable semiconductor laser element  14 . 
     [1-2. Method of Manufacturing Semiconductor Laser Element] 
     Next, a method of manufacturing the semiconductor laser element according to the present embodiment will be described. In the present embodiment, each of the steps in the method. of manufacturing the semiconductor laser element described above as an example of the semiconductor laser element will be described. 
     [1-2-1. Step of Forming Semiconductor Layer] 
     A step of forming the semiconductor layer according to the present embodiment; will be described with reference to the drawings.  FIG. 5  is a schematic sectional view of substrate  20 , illustrating the outline of the step of forming the semiconductor layer of the semiconductor laser element according to the present embodiment. 
     Firstly, as shown in  FIG. 5 , substrate  20  having first surface P 1  and second surface P 2  is prepared, and semiconductor layer  100  including active layer  40  is formed on first surface P 1  of substrate  20 . Each layer included in semiconductor layer  100  is stacked using, for example, metalorganic chemical vapor deposition (MOCVD). In the present embodiment, first semiconductor layer  30  including an n-type layer, active layer  40 , and second semiconductor layer  50  including a p-type layer are formed on substrate  20  in this order as  semiconductor layer  100 . 
     Subsequently, in the present embodiment, a so-called window region is formed in the vicinity of the resonator face of the semiconductor laser element. The following describes a method of forming the window region with reference to  FIG. 6A  and  FIG. 6B .  FIG. 6A  is a schematic sectional view of substrate  20  and semiconductor layer  100  according to the present embodiment.  FIG. 6B  is a schematic sectional view of substrate  20  and semiconductor layer  100 , illustrating the method of forming window region  80  according to the present embodiment.  FIG. 6A  and  FIG. 6B  each show the sectional view of substrate  20  and semiconductor layer  100  along the first direction. 
     For example, as shown in  FIG. 6B , window region  80  may be formed by thermally diffusing Zn into the p-type contact layer included in second semiconductor layer  50 . More specifically, a ZnO film serving as a diffusion source and a SiN or SiO film for preventing vaporization of Zn are formed above the p-type contact layer in this order, and Zn is diffused into the vicinity of the resonator face of the semiconductor laser element through a thermal treatment. Thus, the energy band gap of active layer  40  is increased. With this, it is possible to form window region  80  in which light absorption in active layer  40  is reduced. Such window region  80  can prevent the deterioration in the vicinity of the resonator face of semiconductor laser element  14 . 
     [1-2-2. Step of Forming Waveguide] 
     Next, a step of forming a waveguide will be described with reference to the drawings.  FIG. 7  is a schematic sectional view of substrate  20 , illustrating the outline of the step of forming the waveguide according to the present embodiment. 
     As shown in  FIG. 7 , waveguides WGs each of which uses a ridge between a pair of trenches TRs are formed by forming multiple pairs of  trenches TRs in second semiconductor layer  50  formed above substrate  20 , in a direction perpendicular to the paper of  FIG. 7 . In this manner, multiple waveguides WGs extending in the first direction are formed in semiconductor layer  100 . The width of each waveguide WG is approximately 3 μm, for example. 
     The method of forming waveguide WG is not particularly limited. In the present embodiment, in order to form the ridge, a photolithographic technique is used to form a mask using SiO 2  or the like. Subsequently, the pair of trenches TRs, i.e., the ridge, is formed by non-selective etching such as dry etching. In doing so, the dry etching is performed on the p-type contact layer, the p-type interlayer, the p-type third cladding layer, and the p-type second cladding layer, and the p-type second cladding layer is removed not completely but to the middle. 
     Next, a protective film made of SiO 2  or the like is formed on the entire top surface of semiconductor layer  100  in which the ridge is formed. 
     Next, SiO 2  protective film is removed by dry etching only at the bottom of trench TR. In doing so, the side wall of the ridge and the top of the ridge are covered by the protective film. 
     Subsequently, the p-type second cladding layer is removed completely by selective etching such as wet etching. In this manner, the p-type first cladding layer is exposed at the bottom of trench TR. As described above, waveguide WG can be formed in semiconductor layer  100 . 
     Here, the dry etching technique employed in the present embodiment may be anisotropic plasma etching. A method using, for example, inductively coupled plasma (hereinafter referred to as ICP) or electron cyclotron resonance (hereinafter referred to as ECR) plasma is taken as an example of the dry etching.  
     Moreover, gas mixture of SiCl 4  and Ar, or the like is used as etching gas, but chlorine gas, boron trichloride gas, or the like may be used instead of SiCl 4 . 
     In the present embodiment, the ICP method is employed as the dry etching technique, and the gas mixture of SiCl 4  and Ar is used as the etching gas. Etching conditions are as follows: the volume ratio of SiCl 4  to the gas mixture is at least 5% and at most 12%; the temperature of the lower electrode on which the semiconductor substrate is provided is at least 150 degrees C. and at most 200 degrees C.; the pressure in chamber is at least 0.1 Pa and at most 1 Pa; the bias power of the lower electrode is at least 50 W and at most 150 W; and the ICP power is at least 200 W and at most 300 W. However,the etching conditions are not limited to these, and may be selected as needed. 
     [1-2-3. Step of Forming First Protective Film] 
     Next, the step of forming a first protective film will be described with reference to  FIG. 8 . 
       FIG. 8  is a schematic sectional view of substrate  20 , illustrating the outline of the step of forming the first protective film according to the present embodiment.  FIG. 8  is the enlarged view of the region within dotted box VIII shown in  FIG. 7 , illustrating the step of forming the first protective film. As shown in  FIG. 8 , first protective film  131  is formed on semiconductor layer  100  excluding a part of the top of the ridge. The part of the top of the ridge in which first protective film  131  is not formed is a region in contact with the p-type lower electrode to be formed later. 
     The method of forming first protective film  131  is not particularly limited. In the present embodiment, first protective film  131  made of SiN and having a film thickness of approximately 180 nm is formed on the sides and a part of the top of the ridge, trenches TRs, and flat portions at both sides, 
     [1-2-4. Step of Forming Electrode] 
     Next, the step of forming an electrode will be described. The electrode formed in this step is a p-type electrode, an n-type electrode, or the like for supplying electric power to the semiconductor laser element manufactured using the manufacturing method according to the present embodiment. 
     P-side lower electrode  151  is formed above semiconductor layer  100  including the top of the ridge and trenches TRs. P-side upper electrode  152  is further formed on p-side lower electrode  151 . P-side lower electrode  151  is in contact with second semiconductor layer  50  in the opening of first protective film  131  provided on the ridge. 
     The structures of p-side lower electrode  151  and p-side upper electrode  152  and the forming method thereof are not particularly limited. In the present embodiment, a mask is formed by photolithography using a resist, and a Ti film, a Pt film, and an Au film are formed in this order using the vapor deposition method after a pre-process of wet etching. 
     Next, a pattern for p-side upper electrode  152  is formed by photolithography using a resist mask, and an Au film having a film thickness of at least 2.0 μm and at most 5.0 μm is formed by an electrolytic plating method. Next, the resist is removed using a lift-off method to form patterned p-side upper electrode  152 . 
     Next, substrate  20  is ground until a thickness from second surface P 2  of substrate  20  to p-side upper electrode  152  becomes approximately 100 μm (this grinding step is not shown). Subsequently, a resist mask is formed on second surface P 2  using photolithography, and an Aute film, a Ni film, an Au film, a Ti film, a Pt film, a Ti film, a Pt film, and an Au film are formed in this order using the vapor deposition method after a pre-process of wet etching. Subsequently, the resist is removed using the lift-off method to form patterned n-side electrode  160 .  
     With these steps, substrate  20  on which semiconductor layer  100  is stacked is formed. 
     [1-2-5. Step of Cleaving Substrate] 
     Next, the step of cleaving the substrate will be described with reference to the drawings. At this step, substrate  20  on which semiconductor layer  100  formed as described above is stacked is cleaved at a face corresponding to the resonator face of the semiconductor laser element. In other words, as shown in  FIG. 3 , substrate  20  is cleaved such that window region  80  is located at the resonator face. In the present embodiment, substrate  20  is cleaved such that the resonator length of semiconductor laser element  14  is 300 μm or less. With this, it is possible to form a bar-like substrate on which semiconductor layer  100  is formed. 
     [1-2-6. Step of Forming Second Protective Film] 
     Next, the step of forming a second protective film will be described. At this step, the second protective film is formed on cleavage end face  121  formed at the above-mentioned step of cleaving the substrate, using an ECR chemical vapor deposition method or the like. It is to be noted that the structures of second protective films  132 F and  132 R and the forming method thereof are not particularly limited. 
     The light reflectance of front-side second protective film  132 F is approximately 30%, and the light reflectance of rear-side second protective film  132 R is 90% or more. Semiconductor laser element  14  according to the present embodiment can be formed by further dividing the bar-like substrate formed as described above into chip-like substrates. 
     [1-3. Function and Advantageous Effect of Semiconductor Laser Element] 
     Next, the function and advantageous effect of semiconductor laser element  14  according to the present embodiment will be described.  
     The refractive indexes and the energy band gaps of an AlGaInP-based semiconductor and an AlGaAs-based semiconductor will be described with reference to  FIG. 9  and  FIG. 10 .  FIG. 9  is a graph illustrating the relationship between refractive index and Al composition ratio x with respect to an (Al x Ga 1-x ) 0.5 In 0.5 P-based semiconductor and an Al x Ga 1-x As-based semiconductor.  FIG. 10  is a graph illustrating the relationship between energy band gap and Al composition ratio x with respect to the (Al x Ga 1-x ) 0.5 In 0.5 P-based semiconductor and the Al x Ga 1-x As-based semiconductor. As shown in  FIG. 9  and  FIG. 10 , in both the AlGaInP-based semiconductor and the AlGaAs-based semiconductor. it is found that the energy band gap decreases as the refractive index increases. 
     Accordingly, in the case where both the cladding layer and the barrier layer are formed using an AlGaInP-based material, or the case where both the cladding layer and the barrier layer are formed using an AlGaAs-based material, the barrier layer has a refractive index lower than that of the cladding layer in the both cases when the barrier layer has an energy band gap larger than that of the cladding layer. On the other hand, it is found that the barrier layer may have an energy band gap larger than that of the cladding layer and a refractive index higher than that of the n-type cladding layer by forming the barrier layer and the cladding layer using Al x2 Ga 1-x2 As and (Al x Ga 1-x ) 0.5 In 0.5 P, respectively. 
     The semiconductor laser element according to the present embodiment includes an AlGaInP-based semiconductor layer and an AlGaAs-based. semiconductor layer. The energy band structure and the refractive index distribution of semiconductor laser element  14  according to the present embodiment will be described with reference to  FIG. 11 .  FIG. 11  is a diagram illustrating the energy band structure (band diagrams and the refractive index distribution of semiconductor laser element  14  according to the present embodiment.  
     The structure of semiconductor laser element  14  according to the present embodiment allows barrier layers  41 ,  43 , and  45  to have an energy band gap larger than that of n-type cladding layer  32  and a refractive index higher than that of n-type cladding layer  32 . In doing so, n-type cladding layer  32  contains Al x1 Ga 1-x1-y1 In y1 As 1-z1 P z1 , barrier layers  41 ,  43 , and  45  contain Al x2 Ga 1-x2-y2 In y2 As 1-z2 P z2 , and z1&gt;z2 is satisfied. Furthermore, z1=1, and z2=0 may be satisfied, or z1=1, z2=0, and x1&lt;x2 may be satisfied. 
     According to the layer structure of semiconductor laser element  14  according to the present embodiment, the energy band gap of each of barrier layers  41 ,  43  and  45  containing AlGaAs (2.042 eV) is larger than that of n-type cladding layer  32  (1.970 eV). Moreover, the conduction hand position of n-type cladding layer  32  is lower than that of each of barrier layers  41 ,  43  and  45 . Accordingly, the energy of the conduction band of semiconductor laser element  14  is the highest at barrier layers  41 ,  43  and  45 , and is lower at n-type cladding layer  32  and chip-like substrate  24  in this order. With this band structure, it is possible to allow electrons injected from chip-like substrate  24  to arrive at active layer  40  at the minimum voltage. Accordingly, semiconductor laser element  14  according to the present embodiment can reduce the operating voltage. Moreover, as shown in  FIG. 11 , the refractive index of each of barrier layers  41 ,  43  and  45  (3.211) is higher than that of n-type cladding layer  32  (3.208). With this, the light generated in well layers  42  and  44  can be efficiently confined in active layer  40 , and thus it is possible to enhance the efficiency of light emission. 
     Moreover, in the present embodiment, chip-like substrate  24  contains GaAs. Accordingly, the energy band gap of n-type cladding layer  32  containing Al x1 Ga 1-x1-y1 In y1 As 1-z1 P z1  is larger than that of chip-like substrate  24 . With this, the energy of the conduction band of chip-like substrate  24  can be  reduced, and thus it is possible to prevent a hetero barrier between chip-like substrate  24  and n-type cladding layer  32 . Accordingly, it is possible to more reliably reduce the operating voltage of semiconductor laser element  14 . 
     Moreover, in the present embodiment, the energy band gap of the p-type cladding layer (p-type first cladding layer  52 , p-type second cladding layer  53 , and p-type third cladding layer  54 ) is larger than that of the harrier layer. With this, electron leakage from active layer  40  to p-type cladding layer can be reduced, and thus it is possible to enhance the effect of confining electrons in active layer  40 . Accordingly, semiconductor laser element  14  according to the present embodiment can achieve high efficiency of light emission even under high-temperature operation. 
     Moreover, semiconductor laser element  14  according to the present embodiment includes n-side light guide layer  33  that is disposed between n-type cladding layer  32  and active layer  40  and has a refractive index higher than that of n-type cladding layer  32 . With this, it is possible to enhance the effect of confining light in active layer  40 . 
     In addition, n-side light, guide layer  33  may be made of Al x3 Ga 1-x3-y3 In y3 P (0≤x3). Furthermore, when n-type cladding layer  32  is made of Al x1 Ga 1-x1-y1 In y1 P, x3&lt;x1 is satisfied. In comparison with the n-type light guide layer made of an AlGaAs-based material, Al composition ratio x3 can be reduced when n-side light guide layer  33  is made of Al x3 Ga 1-x3-y3 In y3 P. With this, it is possible to reduce a nonradiative recombination center caused by the oxidized Al in n-side light guide layer  33  at cleavage end face  121 , and thus end-face deterioration due to the increase in the nonradiative recombination center can be prevented. Accordingly, it is possible to achieve highly reliable semiconductor laser element  14 . Moreover, when window region  80  is formed by diffusing impurities for window region formation such as Zn, an  AlGaInP-based material can be used for n-side light guide layer  33  to enhance the Zn diffusion. With this, it is possible to form window region  80  using small amounts of impurities for window region formation, and thus the free carrier loss due to impurities can be reduced. Accordingly, it is possible to achieve high efficiency of light emission. Furthermore, p-side light guide layer  51  may be made of Al x4 Ga 1-x4-y4 In y4 P (0≤x4). With this, it is possible to prevent Al in p-side light guide layer  51  at cleavage end face  121  from being oxidized. 
     As described above, semiconductor laser element  14  according to the present embodiment can achieve both low operating voltage and high light confinement effect. 
     [1-4. Variation 1] 
     Next, a semiconductor laser element according to Variation 1 of Embodiment 1 will be described. The semiconductor laser element according to the present variation differs from semiconductor laser element  14  according to Embodiment 1 in that a hole barrier layer and an electron barrier layer are provided, and the rest is the same. The following describes the semiconductor laser element according to the present variation with reference to  FIG. 12 , mainly in terms of differences from semiconductor laser element  14  according to Embodiment 1. 
       FIG. 12  is a diagramillustratingthe energy band structure and the refractive index distribution of the semiconductor laser element according to the present variation. The semiconductor laser element according to the present variation further includes hole barrier layer  91  and electron barrier layer  92  in addition to the components in semiconductor laser element  14  according to Embodiment 1. 
     The semiconductor laser element according to the present variation includes hole barrier layer  91  that is disposed between n-type cladding layer  32   and active layer  40  and has an energy band gap larger than that of n-type cladding layer  32 . In the example shown in  FIG. 12 , the semiconductor laser element includes hole barrier layer  91  between n-side light guide layer  33  and active layer  40 . Hole barrier layer  91  includes an n-(Al 0.4 Ga 0.6 ) 0.5 In 0.5 P film. having a film thickness of 0.05 μm. Hole barrier layer  91  is included in first semiconductor layer  30 . Hole barrier layer  91  has an energy hand gap larger than that of each of barrier layers  41 ,  43 , and  45  made of Al 0.59 Ga 0.41 As, and a film thickness that has no effect on the distribution of light confined in the stacked direction. For example, hole barrier layer  91  has a film thickness of 0.1 μm or less, and may be thinner than n-side light guide layer  33 . 
     In this manner, hole barrier layer  91  having a large energy band gap is provided between n-side light guide layer  33  and active layer  40 , thereby reducing hole leakage from active layer  40  to n-side light guide layer  33  and enhancing the effect of confining holes in active layer  40 . Accordingly, it is possible to achieve high efficiency of light emission even under high-temperature operation. 
     The semiconductor laser element according to the present variation includes electron barrier layer  92  that is disposed between the p-type cladding layer and active layer  40  and has an energy band gap larger than that of the p-type cladding layer. In the example shown in  FIG. 12 , the semiconductor laser element includes electron barrier layer  92  between p-side light guide layer  51  and active layer  40 . Electron barrier layer  92  includes a p-(Al 0.6 Ga 0.4 ) 0.5 In 0.5 P film having a film thickness of 0.05 μm. Electron barrier layer  92  is included in second semiconductor layer  50 . Electron barrier layer  92  has an energy band gap larger than that of each of barrier layers  41 ,  43 , and  45  made of Al 0.59 Ga 0.41 As, and a film thickness that has no effect on the distribution of light confined in the stacked direction. For example, electron  barrier layer  92  has a film thickness of 0.1 μm or less, and may be thinner than p-side light guide layer  51 . 
     In this manner, electron barrier layer  92  having a large energy hand gap is provided between p-side light guide layer  51  and active layer  40 , thereby reducing electron leakage from active layer  40  to p-side light guide layer  51  and enhancing the effect of confining carriers in active layer  40 . Accordingly, it is possible to achieve high efficiency of light emission even under high-temperature operation. 
     It is to be noted that in the semiconductor laser element according to the present variation, the p-type cladding layer, electron barrier layer  92 , p-side light guide layer  51 , and n-side light guide layer  33  may contain AlGaAs. 
     [1-5. Variation 2] 
     Next, a semiconductor laser element according to Variation 2 of Embodiment 1 be described. The semiconductor laser element according to the present variation differs in the arrangement of a hole barrier layer from the semiconductor laser element according to Variation 1 of Embodiment 1, and the rest is the same. The following describes the semiconductor laser element according to the present variation with reference to  FIG. 13 , mainly in terms of differences from the semiconductor laser element according to Variation 1 of Embodiment 1. 
       FIG. 13  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to the present variation. As shown in  FIG. 13 , in the semiconductor laser element according to the present variation, hole harrier layer  91  is disposed. between n-type cladding layer  32  and n-side light guide layer  33 . 
     Hole barrier layer  91  according to the present variation may have an energy band gap larger than that of n-type cladding layer  32 , and a film  thickness that easily allows for electron tunneling. For example, hole barrier layer  91  has a film thickness of 0.1 μm or less, and may be thinner than n-side light guide layer  33 . 
     In this manner, hole barrier layer  91  having a large energy band gap is provided between n-type cladding layer  32  and n-side light guide layer  33 , thereby reducing hole leakage from n-side light guide layer  33  to n-type cladding layer  32  and enhancing the effect of confining holes in active layer  40 . 
     In the semiconductor laser elements according to Variation 1 and Variation 2, the hole barrier layer may contain AlGaAas. As described in Variation 1 and Variation 2, however, the hole barrier layer is made of AlGaInP, and thus a higher barrier of the valence band and a lower barrier of the conduction band are allowed. With this, it is possible to easily inject electrons into active layer  40  and simultaneously further enhance the effect of confining holes in active layer  40 . 
     It is to be noted that in the semiconductor laser elements according to Variation 1 and Variation 2, the p-type cladding layer, electron barrier layer  92 , p-side light guide layer  51 , and n-side light guide layer  33  may contain AlGaAs. 
     Embodiment 2 
     A semiconductor laser element according to Embodiment 2 will be described. The semiconductor laser element according to the present embodiment mainly differs in the structure of an active layer from semiconductor laser element  14  according to Embodiment 1. The following describes the semiconductor laser element according to the present embodiment with reference to  FIG. 14  and  FIG. 15 , mainly in terms of differences from semiconductor laser element  14  according to Embodiment 1. 
       FIG. 14  is a table illustrating the layer structure of the semiconductor laser element according to the present embodiment.  FIG. 15  is a diagram  illustratingthe energy band structure and the refractive index distribution of the semiconductor laser element according to the present embodiment. As shown in  FIG. 15 , the semiconductor laser element according to the present embodiment includes chip-like substrate  224 , first semiconductor layer  230 , active layer  240 , and second semiconductor layer  250  in the same manner as semiconductor laser element  14  according to Embodiment 1. First semiconductor layer  230  includes n-type buffer layer  231 , n-type cladding layer  232 , and n-side light guide layer  233 . Active layer  240  includes well layers  242  and  244 , and barrier layers  241 ,  243 , and  245 . Second semiconductor layer  250  includes p-side light guide layer  251 , p-type first cladding layer  252 , p-type second cladding layer  253 , p-type third cladding layer  254 , p-type interlayer  255 , and p-type contact layer  256 . 
     Moreover, the semiconductor laser element according to the present embodiment differs in the structures of barrier layer  241 , well layers  242  and  244 , and n-type cladding layer  232  from semiconductor laser element  14  according to Embodiment 1. 
     Barrier layer  241  according to the present embodiment is an Al 0.59 Ga 0.41 As layer having a film thickness of 0.024 μm. Well layers  242  and  244  are each an In 0.03 Ga 0.97 As layer having a film thickness of 0.0055 μm N-type cladding layer  232  is an n-(Al 0.17 Ga 0.83 ) 0.5 In 0.5 P layer having a film. thickness of 4.7 μm. 
     The semiconductor laser element according to the present embodiment includes active layer  240  as described above, thereby achieving TE-mode oscillation with an oscillation wavelength of at least 830 nm and at most 860 nm. 
     Also in the semiconductor laser element according to the present embodiment, like semiconductor laser element  14  according to Embodiment 1,  the energy band gap of each of barrier layers  241 ,  243 , and  245  (2.042 eV) is larger than that of n-type cladding layer  232  (1.976 eV), and the refractive index of each of barrier layers  241 ,  243 , and  245  (3.211) is higher than that of n-type cladding layer  232  (3.204). Accordingly, the semiconductor laser element according to the present embodiment can produce the same effect as semiconductor laser element  14  according to Embodiment 1. 
     Embodiment 3 
     A semiconductor laser element according to Embodiment 3 will be described. The semiconductor laser element according to the present embodiment mainly differs in the structure of an active layer from semiconductor laser element  14  according to Embodiment 1. The semiconductor laser element according to the present embodiment can implement TM-mode oscillation. The following describes the semiconductor laser element according to the present embodiment with reference to  FIG. 16  and.  FIG. 17 , mainly in terms of differences from semiconductor laser element  14  according to Embodiment 1. 
       FIG. 16  is a table illustrating the layer structure of the semiconductor laser element according to the present embodiment,  FIG. 17  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to the present embodiment. As shown in  FIG. 17 , the semiconductor laser element according to the present embodiment includes chip-like substrate  324 , first semiconductor layer  330 , active layer  340 , and second semiconductor layer  350  in the same manner as semiconductor laser element  14  according to Embodiment 1. First semiconductor layer  330  includes n-type buffer layer  331 , n-type cladding layer  332 , and n-side light guide layer  333 . Active layer  340  includes well layers  342 ,  344 , and  346 , and barrier layers  341 ,  343 ,  345 , and  347 . Second  semiconductor layer  350  includes p-side light guide layer  351 , p-type first cladding layer  352 , p-type second cladding layer  353 , p-type third cladding layer  354 , p-type interlayer  355 , and p-type contact layer  356 . 
     As described above, the semiconductor laser element according to the present embodiment differs from semiconductor laser element  14  according to Embodiment 1 in the structures of harrier layers  341 ,  343 ,  345 , and  347  and well layers  342 ,  344 , and  346 , and the structures of n-type cladding layer  332  and p-type first cladding layer  352 . 
     In active layer  340  according to the present embodiment, the number of barrier layers  341 ,  343 ,  345 , and  347  and the number of well layers  342 ,  344 , and  346  are each greater than that of semiconductor laser element  14  according to Embodiment 1. Moreover, the material forming each well layer is different from that of semiconductor laser element  14  according to Embodiment 1. Barrier layer  341  according to the present embodiment is an Al 0.59 Ga 0.41 As layer having a film thickness of 0.04 μm. Barrier layer  343  and barrier layer  345  are each an Al 0.59 Ga 0.41 As layer haying a film thickness of 0.008 μm. Barrier layer  347  is an Al 0.59 Ga 0.41 As layer having a film thickness of 0.021 μm. Well layers  342 ,  344 , and  346  are each a GaAs 0.84 P 0.16  layer having a film thickness of 0.0065 μm. 
     N-type cladding layer  332  is an n-(Al 0.17 Ga 0.83 ) 0.5 In 0.5 P layer having a film thickness of 4.7 μm. P-type first cladding layer  352  is an n-(Al 0.17 Ga 0.83 ) 0.5 In 0.5 P layer having a film thickness of 0.16 μm. 
     Also in the semiconductor laser element according to the present embodiment, like semiconductor laser element  14  according to Embodiment 1, the energy band gap of each of barrier layers  341 ,  343 ,  345 , and  347  (2.042 eV) is larger than that of n-type cladding layer  332  (1.986 eV), and the refractive index of each of barrier layers  341 ,  343 ,  345 , and  347  (3.211) is higher than that  of n-type cladding layer  332  (3.198). Accordingly,the semiconductor laser element according to the present embodiment can produce the same effect as semiconductor laser element  14  according to Embodiment 1. 
     Embodiment 4 
     A semiconductor laser element according to Embodiment 4 will be described. The semiconductor laser element according to the present embodiment differs from semiconductor laser element  14  according to Embodiment 1 in that a graded buffer layer and a graded interlayer are provided. The following describes the semiconductor laser element according to the present embodiment with reference to  FIG. 18  and  FIG. 19 , mainly in terms of differences from semiconductor laser element  14  according to Embodiment 1. 
     Moreover, in Embodiment 4, the resonator length is 260 μm. Here, the resonator length of semiconductor laser element  14  is set to  260  μm, and thus a ratio of a region other than window region  80  to waveguide WG can be reduced., thereby reducing saturation light output power. Accordingly, it is possible to prevent end-face damage of semiconductor laser element  14 . 
       FIG. 18  is a table illustrating the layer structure of the semiconductor laser element according to the present embodiment.  FIG. 19  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to the present embodiment. As shown in  FIG. 19 , the semiconductor laser element according to the present embodiment includes chip-like substrate  424 , first semiconductor layer  430 , active layer  440 , and second semiconductor layer  450  in the same manner as semiconductor laser element  14  according to Embodiment 1. First semiconductor layer  430  includes n-type buffer layer  431 , graded buffer layer  434  n-type cladding layer  432 , and n-side light guide layer  433 . Active layer   440  includes well layers  442  and  444 , and barrier layers  441 ,  443 , and  445 . Second semiconductor layer  450  includes p-side light guide layer  451 , p-type first cladding layer  452 , p-type second cladding layer  453 , p-type third cladding layer  454 , p-type interlayer  455 , graded interlayer  45   7 , and p-type contact layer  456 . 
     Graded buffer layer  434  according to the present embodiment is an Al x Ga 1-x As layer having a film thickness of 0.075 μm, and the Al composition ratio gradually changes in the stacked direction. More specifically, from the interface with n-type buffer layer  431  to the interface with n-type cladding layer  432 , the Al composition ratio of graded buffer layer  434  gradually changes from x=0.05 to x=0.35. With this, it is possible to smooth a spike-like hetero barrier generated between chip-like substrate  424  and n-type cladding layer  432 . Accordingly, it is possible to reduce the operating voltage of the semiconductor laser element. 
     Moreover, graded interlayer  457  according to the present embodiment is a p-Al x Ga 1-x As layer having a film thickness of 0.05 μm, and the Al composition ratio gradually changes in the stacked direction. More specifically, from the interface with p-type interlayer  455  to the interface with p-type contact layer  456 , the Al composition ratio of graded interlayer  457  gradually changes from x=0.55 to x=0.05. With this, it is possible to smooth a spike-like hetero barrier generated between p-type interlayer  455  and p-type contact layer  456 . Accordingly, it is possible to reduce the operating voltage of the semiconductor laser element. 
     The semiconductor laser element according to the present embodiment also differs in the structures of n-side light guide layer  433  and p-side light guide layer  451  from the semiconductor laser element according to Embodiment 1. N-side light guide layer  433  according to the present  embodiment is an (Al 0.04 Ga 0.96 ) 0.5 In 0.5 P layer having a film thickness of 0.09 μm. It is to be noted that the Al composition ratio of aside light guide layer  433  may gradually change in the stacked direction. For example, n-side light guide layer  433  may be an (Al x Ga 1-x ) 0.5 In 0.5 P layer, and from the interface with n-type cladding layer  432  to the interface with active layer  440 , the Al composition ratio may gradually change from x=0.18 to x=0.02. 
     P-side light guide layer  451  according to the present embodiment is an (Al 0.04 Ga 0.96 ) 0.5 In 0.5 P layer having a film thickness of 0.07 μm. It is to be noted that the Al composition ratio of p-side light guide layer  451  may gradually change in the stacked direction. For example, p-side light guide layer  451  may be an (Al x Ga 1-x ) 0.5 In 0.5 Player, and from the interface with active layer  440  to the interface with p-type first cladding layer  452 , the Al composition ratio may gradually change from x=0.02 to x=0.30. 
     Also in the semiconductor laser element according to the present embodiment, like semiconductor laser element  14  according to Embodiment 1, the energy band gap of each of barrier layers  441 ,  443 , and  445  (2.042 eV) is larger than that of n-type cladding layer  432  (1.981 eV), and the refractive index of each of barrier layers  441 ,  443 , and  445  (3.211) is higher than that of n-type cladding layer  432  (3.201). Accordingly, the semiconductor laser element according to the present embodiment can produce the same effect as semiconductor laser element  14  according to Embodiment 1. 
     Embodiment 5 
     A semiconductor laser element according to Embodiment 5 will be described. The semiconductor laser element according to the present embodiment mainly differs in the shape of a waveguide and the structure of an active layer from the semiconductor laser element according to Embodiment 4. The semiconductor laser element according to the present embodiment  achieves an oscillation wavelength of 980 nm. The following describes the semiconductor laser element according to the present embodiment, mainly in terms of differences from the semiconductor laser element according to Embodiment 4. 
     Firstly, the shape of the waveguide of the semiconductor laser element according to the present; embodiment will be described with reference to  FIG. 20 .  FIG. 20  is a top view illustrating the shape of the waveguide of the semiconductor laser element according to the present embodiment.  FIG. 20  illustrates waveguide WG when viewed from the p-sid.e upper electrode in the semiconductor laser element according to the present embodiment. It is to be noted that in the present embodiment, the resonator length is 2500 μm 
     As shown in  FIG. 20 , waveguide WG in the semiconductor laser element according to the present embodiment includes first region. R 1  through fifth region R 5  from the front side (from which light is emitted) toward the rear side. Firstly, first region R 1  has a length a length in the resonance direction) of 1000 μm and a constant waveguide width (i.e., a dimension in a direction perpendicular to the resonance direction and parallel to the main surface of chip-like substrate  524 ) of 4.5 μm. Second region R 2  has a length of 320 μm and a waveguide width that continuously decreases from 4.5 μm to 2.0 μm from first region R 1  toward third region R 3  from the front side toward the rear side). Third region R 3  has a length of 800 μm and a constant waveguide width of 2.0 μm. Fourth region R 4  has a length of 320 μm and a waveguid.e width that continuou.sly increases from 2.0 μm to 4.5 μm from third region. R 3  toward fifth region R 5  (i.e., from the front side toward the rear side). Fifth region R 5  has a length of 60 μm and a constant waveguide width of 4.5 μm. 
     In this manner, the waveguide width is widened on the front side of  waveguide WG, and thus it is possible to increase an amount of injected current, thereby achieving high light output power. Moreover, a single mode operation is implemented by providing, between the front side and the rear side, a waveguid.e width region (third region R 3 ) where transverse higher order mode light can be cut off. Moreover, a region having a waveguide width of 4.5 μm and a region having a waveguide width of 2.0 μm are connected by a 320 μm length region having a continuously changing waveguide width, and thus it is possible to reduce propagation loss of laser light due to a change in the waveguide width. 
     Accordingly, it is possible to achieve a single mode semiconductor laser element capable of performing a high-power operation. 
     Next, the following describes the structure of the active layer in the semiconductor laser element according to the present embodiment with reference to  FIG. 21  and  FIG. 22 .  FIG. 21  is a table illustrating the layer structure of the semiconductor laser element according to the present embodiment.  FIG. 22  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to the present embodiment. As shown in  FIG. 22 , the semiconductor laser element according to the present embodiment includes chip-like substrate  524 , first semiconductor layer  530 , active layer  540 , and second semiconductor layer  550  in the same manner as the semiconductor laser element according to Embodiment 4. First semiconductor layer  530  includes n-type buffer layer  531 , graded buffer layer  534 , n-type cladding layer  532 , and n-side light guide layer  533 . Active layer  540  includes well layers  542  and  544 , and barrier layers  541 ,  543 , and  545 . Second semiconductor layer  550  includes p-side light guide layer  551 , p-type first cladding layer  552 , p-type second cladding layer  553 , p-type third cladding layer  554 , p-type interlayer  555 , graded interlayer  557 ,  and p-type contact layer  556 . 
     Moreover, the semiconductor laser element according to the present embodiment differs in the structures of active layer  540 , n-side light guide layer  533 , p-side light guide layer  551 , and p-type first cladding layer  552  from the semiconductor laser element according to Embodiment 4. 
     Well layers  542  and  544  in active layer  540  according to the present embodiment are each an In 01.7 Ga 0.83 As layer having a film thickness of 0.008 μm. Barrier layer  541  is an Al 0.59 Ga 0.41 As layer having a film thickness of 0.03 μm. Barrier layer  543  is an Al 0.59 Ga 0.41 As layer having a film thickness of 0.007 μm. Barrier layer  545  is an Al 0.59 Ga 0.41 As layer haying a film thickness of 0.021 μm. The semiconductor laser element according to the present embodiment includes active layer  540  as described above, thereby achieving an oscillation wavelength of 980 nm. 
     N-side light guide layer  533  according to the present embodiment is an (Al 0.04 Ga 0.96 ) 0.5 In 0.5 Player having a film thickness of 0.085 μm. It should be noted that the Al composition ratio of n-side light guide layer  533  may gradually change in the stacked direction in the same manner as n-side light guide layer  433  according to Embodiment 4. 
     P-side light guide layer  551  according to the present embodiment is an (Al 0.04 Ga 0.96 ) 0.5 In 0.5 P layer having a film thickness of 0.13 μm. It is to be noted that the Al composition ratio of p-side light guide layer  451  may gradually change in the stacked direction in the same manner as p-side light guide layer  451  according to Embodiment 4. 
     P-type first cladding layer  552  according to the present embodiment is a p-(Al 0.04 Ga 0.96 ) 0.5 In 0.5 P layer having a film thickness of 0.20 μm. 
     Also in the semiconductor laser element according to the present embodiment, like the semiconductor laser element according to Embodiment 4,  the energy band gap of each of barrier layers  541 ,  543 , and  545  (2.042 eV) is larger than that of n-type cladding layer  532  (1.973 eV), and the refractive index of each of barrier layers  541 ,  543 , and  545  (3.211) is higher than that of n-type cladding layer  532  (3.206). Accordingly, the semiconductor laser element according to the present embodiment can produce the same effect as the semiconductor laser element according to Embodiment 4. 
     Embodiment 6 
     A semiconductor laser element according to Embodiment 6 will be described. The semiconductor laser element according to the present embodiment differs in the structures of a p-type first cladding layer, a p-side light guide layer, a barrier layer, an n-side light guide layer, and an n-type cladding layer from the semiconductor laser element according to Embodiment 4, and the other structures are the same. The following describes the semiconductor laser element according; to the present embodiment with reference to  FIG. 23  and  FIG. 24 , mainly in terms of differences from the semiconductor laser element according to Embodiment 4. 
       FIG. 23  is a table illustrating the layer structure of the semiconductor laser element according to the present embodiment.  FIG. 24  is a diagram illustrating the energy band structure and the refractive index distribution of the semiconductor laser element according to the present embodiment. As shown in  FIG. 24 , the semiconductor laser element according to the present embodiment includes chip-like substrate  624 , first semiconductor layer  630 , active layer  640 , and second semiconductor layer  650  in the same manner as the semiconductor laser element according to Embodiment 4. First semiconductor layer  630  includes n-type buffer layer  631 , graded buffer layer  634 , n-type cladding layer  632 , and n-side light guide layer  633 . Active layer  640  includes well layers  642  and  644 , and barrier layers  641 ,  643 , and  645 .  Second semiconductor layer  650  includes p-side light guide layer  651 , p-type first cladding layer  652 , p-type second cladding layer  653 , p-type third cladding layer  654 , p-type interlayer  655 , graded interlayer  657 , and p-type contact layer  656 . 
     As shown in  FIG. 23 , p-type first cladding layer  652  according to the present embodiment is a p-(Al 0.29 Ga 0.71 ) 0.5 In 0.5 P layer, and has the Al composition ratio lower than that of p-type first cladding layer  452  according to Embodiment 4. With this, p-type first cladding layer  652  has a refractive index higher than that of p-type first cladding layer  452  according to Embodiment 4, and thus it is possible to enhance the effect of confining light in active layer  640 . The Al composition ratio of p-type first cladding layer  652  may be lower than that of p-type second cladding layer  653  and that of p-type third cladding layer  654 , for example. With this, it is possible to produce the above-mentioned light confinement effect. 
     P-side light guide layer  651  according to the present embodiment has a film thickness of 0.13 μm which is greater than that of p-side light guide layer  451  according to Embodiment 4. With this, the center portion of the light distribution in the stacked direction is located closer to active layer  640 , and thus it is possible to enhance the effect of confining light in active layer  640 . 
     Among the barrier layers in active layer  640  according to the present embodiment, barrier layer  641  closest to first semiconductor layer  630  has a film thickness of 0.02 μm which is smaller than that of barrier layer  441  according to Embodiment 4. Moreover, n-side light guide layer  633  according to the present embodiment has a film thickness of 0.05 μm which is smaller than that of n-side light guide layer  433  according to Embodiment 4. Moreover, n-type cladding layer  632  according to the present embodiment has a film thickness of 3.2 μm which is smaller than that of n-type cladding layer  432   according to Embodiment 4. As described above, the distance between chip-like substrate  624  and the center portion of active layer  640  in the stacked direction can be reduced by decreasing the film thickness of barrier layer  641  on first semiconductor layer  630  side of active layer  640  and the film thickness of each layer in first semiconductor layer  630 . Accordingly, it is possible to increase a part of the tail portion of the light distribution which is located in chip-like substrate  624 . Here, the impurity concentration of chip-like substrate  624  is higher than that of n-type cladding layer  632  or the like, and thus light loss is high in chip-like substrate  624 . Here, this light distribution may include transverse higher order mode light in addition to transverse base mode light. The effective refractive index of the transverse higher order mode light is lower than that of the transverse base mode light, and thus the light distribution spreads toward chip-like substrate  624 . Accordingly, in the semiconductor laser element according to the present embodiment, the loss of transverse higher order mode light in chip-like substrate  624  is greater than that of the semiconductor laser element according to Embodiment 4, and thus the transverse higher order mode light can be attenuated. With this, it is possible to suppress the instability of light output power due to the transverse higher order mode light and the instability of the shape of the light emission distribution. 
     Moreover, n-type cladding layer  632  according to the present embodiment is an n-(Al 0.155 Ga 0.845 ) 0.5 In 0.5 P layer, and has an Al composition ratio lower than that of n-type cladding layer  432  according to Embodiment 4. With this, n-type cladding layer  632  has a refractive index higher than that of n-type cladding layer  432  according to Embodiment 4, and thus it is possible to enhance the effect of confining light in active layer  640 . 
     As described above, in comparison to the semiconductor laser element  according to Embodiment 4, the semiconductor laser element according to the present embodiment can further enhance the effect of confining light in active layer  640  and reduce the transverse higher order mode light. (Variation, etc.) 
     The foregoing has described the semiconductor laser element or the like according to the present disclosure based on the embodiments, yet the present disclosure is not limited to the embodiments described above. 
     For example, the present disclosure also includes embodiments as a result of adding various modifications that may be conceived by those skilled in the art to the embodiments, 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. 
     Moreover, in the semiconductor laser elements according to the above-mentioned embodiments and their variations, the waveguide is formed using a ridge structure. However, the method of forming the waveguide is not limited to this, and an embedded structure or the like is possible. 
     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 semiconductor laser element according to the present disclosure is particularly applicable to various light sources that are required to reduce power consumption.