Abstract:
A semiconductor laser device having an active region including alternating layers of at least one quantum well layer and a plurality of barrier layers, where two of the plurality of barrier layers are the outermost layers of the alternating layers. Each of the at least one quantum well layer has a compressive strain, and each of the plurality of barrier layers has a tensile strain. In the active region, a strain buffer layer having an intermediate strain is formed between each quantum well layer and each of two barrier layers adjacent to the quantum well layer. Interfacial strain is thus reduced, improving high-output-power characteristics.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser device having at least one compressive-strain quantum well layer and tensile-strain barrier layers. 
     2. Description of the Related Art 
     Conventionally, semiconductor laser devices having an active region in which at least one compressive-strain quantum well layer and tensile-strain barrier layers are alternately laminated have been proposed. 
     For example, Japanese Unexamined Patent Publication, No. 8(1996)-78786 discloses a stress-compensation semiconductor laser device in which compressive-strain quantum well layers and tensile-strain barrier layers are alternately laminated so that the overall average of the strains in the active region is a compressive strain. However, when the active region has such a construction, the difference in the strain between the compressive-strain quantum well layers and tensile-strain barrier layers increases with the increase in the compressive strains. Therefore, a great interfacial strain occurs at the boundaries between the compressive-strain quantum well layers and tensile-strain barrier layers. Thus, it becomes difficult to realize high crystallinity without an interfacial defect. 
     Further, T. Fukunaga et al. (“Reliable Operation of Strain-Compensated 1.06 μm InGaAs/InGaAsP/GaAs Single Quantum Well Lasers,” Applied Physics Letters, vol. 69, No. 2, 1996, pp. 248-250) report that the reliability of a semiconductor laser device including an InGaAs compressive-strain quantum well active layer and a GaAs substrate is increased by providing tensile-strain barrier layers adjacent to the quantum well layer and compensating for the strain. However, the reliability and high-output-power characteristics of the above semiconductor laser device are not yet practicable. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a semiconductor laser device including in an active region at least one compressive-strain quantum well layer and tensile-strain barrier layers, in which an interfacial strain is reduced, and high-output-power characteristics are improved. 
     (1) According to the present invention, there is provided a semiconductor laser device comprising an active region which includes alternating layers of at least one quantum well layer and a plurality of barrier layers, where two of the plurality of barrier layers are outermost layers of the alternating layers, each of the at least one quantum well layer has a first thickness da and a compressive strain Δa, and each of the plurality of barrier layers has a second thickness db and a tensile strain Δb. In the active region, a strain buffer layer is formed between each of the at least one quantum well layer and each of two barrier layers adjacent to the quantum well layer, where each strain buffer layer has a third thickness dr and an intermediate strain Δr which is between the compressive strain Δa and the tensile strain Δb. The first thickness da, the compressive strain Δa, the second thickness db, the tensile strain Δb, the third thickness dr, and the intermediate strain Δr satisfy a relationship,
 
0 ≦N·Δa·da +( N +1) ·Δ b·db +2 N·Δr·dr ≦0.08 nm,
 
where N is the number of the at least one quantum well layer.
 
     The intermediate strain Δr between the compressive strain Δa and the tensile strain Δb means that Δa&lt;Δr&lt;Δb. Preferably, the intermediate strain Δr is a compressive strain. 
     The strains of the quantum well layer, the barrier layer, and the strain buffer layer are respectively defined as Δa=(c a −c s )/c s , Δb=(c b −c s )/c s , and Δr=(c r −c s )/c s , where c s , c a , c b , and c r  are the lattice constants of a (GaAs) substrate, the quantum well layer, the barrier layer, and the strain buffer layer, respectively. 
     When the above relationship is satisfied, the probability of occurrence of lattice defects due to the strain in the quantum well layer can be reduced. Therefore, the semiconductor laser device can oscillate in the fundamental transverse mode with high repeatability even when the output power is high. Thus, the reliability of the semiconductor laser device can be increased. 
     Since the tensile-strain barrier layer, which has a great band gap, is provided in the semiconductor laser device according to the present invention, leakage current can be reduced. In addition, since the strain buffer layer is provided between the tensile-strain barrier layer and the compressive-strain quantum well layer, the quality of the quantum well layer can be improved. Therefore, non-radiative recombination components can be reduced. Thus, it is possible to realize a reliable semiconductor laser device. 
     For example, in the case of a single quantum well structure, i.e., N=1, 0≦Δa·da+2Δb·db+2 Δr·dr≦0.08 nm. Preferably, 0.01 nm≦Δa·da+2Δb·db+2 Δr·dr≦0.06 nm. More preferably, 0.012 nm≦Δa·da+2Δb·db+2 Δr·dr≦0.04 nm. 
     Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (x). 
     (i) The at least one quantum well layer may be made of In x1 Ga 1-x1 As 1-y1 P y1 , each of the plurality of strain buffer layers may have a thickness of approximately 1 to 5 nm, and may be made of In x2 Ga 1-x2 As 1-y2 P y2 , and each of the plurality of barrier layers may have a thickness of approximately 5 to 20 nm, and may be made of In x3 Ga 1-x3 As 1-y3 P y3 , where 0.4≦x 1 &gt;0.49y 1 , 0≦y 1 ≦0.1, 0≦x 2 ≦0.4, 0≦y 2 ≦0.5, 0≦x 3 &lt;0.49y 3 , and 0&lt;y 3 ≦0.5. 
     (ii) The semiconductor laser device according to the present invention may further comprise an upper optical waveguide layer formed on or above the active region, and a lower optical waveguide layer formed under the active region. Each of the upper optical waveguide layer and the lower optical waveguide layer may be made of In x4 Ga 1-x4 As 1-y4 P y4  or Al z3 Ga 1-z3 As, where x 4 =(0.49±0.01)y 4 , 0≦x 4 ≦0.3, and 0≦z 3 ≦0.3. 
     (iii) The semiconductor laser device according to the present invention may further comprise an upper cladding layer formed on or above the upper optical waveguide layer, and a lower cladding layer formed under the lower optical waveguide layer. Each of the upper cladding layer and the lower cladding layer may be made of In x7 (Al z7 Ga 1-z7 ) 1-x7 P or Al z1 Ga 1-z1 As or In x8 Ga 1-x8 P, where x 7 =0.49±0.01, 0≦z 7 ≦1, 0.2≦z 1 ≦0.7, and x 8 =0.49±0.01. 
     (iv) The semiconductor laser device according to the present invention may have a stripe structure. 
     (v) The stripe structure in the feature of (iv) may be realized by a current confinement layer formed above the active region, where the current confinement layer has an opening having a stripe shape and realizing a current injection window. 
     (vi) In the semiconductor laser device having the additional feature of (v), the current injection window may have a width equal to or greater than 2 micrometers and less than 4 micrometers, and realize a difference in an equivalent refractive index between a portion of the active region under the current injection window and another portion of the active region under the current confinement layer except for the current injection window may be in a range from 1.5×10 −3  to 7×10 −3 . 
     (vii) In the semiconductor laser device having the additional feature of (v), the current injection window may have a width equal to or greater than 4 micrometers, and realize a difference in an equivalent refractive index between a portion of the active region under the current injection window and another portion of the active region under the current confinement layer except for the current injection window may be equal to or greater than 1.5×10 −3 . 
     (viii) The stripe structure in the feature of (iv) may be realized by a ridge structure formed above the active region, where the ridge structure includes a current path. 
     (ix) In the semiconductor laser device having the additional feature of (viii), the current path may have a width equal to or greater than 2 micrometers and less than 4 micrometers, and realize a difference in an equivalent refractive index between a portion of the active region under the current path and another portion of the active region which is not located under the current path may be in a range from 1.5×10 −3  to 7×10 −3 . 
     (x) In the semiconductor laser device having the additional feature of (viii), the current path may have a width equal to or greater than 4 micrometers, and realize a difference in an equivalent refractive index between a portion of the active region under the current path and another portion of the active region which is not located under the current path may be equal to or greater than 1.5×10 −3 . 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a semiconductor laser device as the first embodiment. 
         FIG. 2A  is a magnified cross-sectional view of an active region of a semiconductor laser device including a single quantum well layer. 
         FIG. 2B  is a magnified cross-sectional view of an active region of a semiconductor laser device including multiple quantum well layers. 
         FIG. 3  is a graph indicating the dependency of the maximum light output power on the thickness of the strain buffer layer in the case where the average strain of the active region is a compressive strain. 
         FIGS. 4A  to  4 C are cross-sectional views of representative stages in the process of producing a semiconductor laser device as the second embodiment. 
         FIG. 5  is a cross-sectional view of a semiconductor laser device as the third embodiment. 
         FIG. 6  is a cross-sectional view of a semiconductor laser device as the fourth embodiment. 
         FIG. 7  is a cross-sectional view of a semiconductor laser device as the fifth embodiment. 
         FIG. 8  is a cross-sectional view of a semiconductor laser device as the sixth embodiment. 
         FIG. 9  is a cross-sectional view of a semiconductor laser device as the seventh embodiment. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained in detail below with reference to drawings. 
     First Embodiment 
     The construction of a semiconductor laser device as the first embodiment of the present invention and a process for producing the semiconductor laser device are explained below with reference to  FIG. 1 , which is a cross-sectional view of the semiconductor laser device as the first embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. 
     As illustrated in  FIG. 1 , first, an n-type In x8 Ga 1-x8 P lower cladding layer  2  (x 8 =0.49±0.01), an n-type or i-type (intrinsic) In x4 Ga 1-x4 As 1-y4 P y4  lower optical waveguide layer  3  (x 4 =(0.49±0.01)y 4 , 0≦x 4 ≦0.3), an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  4  (0≦x 3 &lt;0.49y 3 , 0&lt;y 3 ≦0.5) having a thickness of approximately 5 to 20 nm, an In x2 Ga 1-x2 As 1-y1 P y2  strain buffer layer  5  (0≦x 2 ≦0.4, 0≦y 2 ≦0.5) having a thickness of approximately 1 to 5 nm, an In x1 Ga 1-x1 As 1-y1 P y1  compressive-strain quantum well layer  6  (0.4≧x 1 &gt;0.49y 1 , 0≦y 1 ≦0.1) having a thickness of approximately 3 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  7 , an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  8 , a p-type or i-type In x4 Ga 1-x4 As 1-y4 P y4  upper optical waveguide layer  9 , a p-type In x8 Ga 1-x8 P upper cladding layer  10 , and a p-type GaAs contact layer  11  are formed on an n-type GaAs substrate  1  by organometallic vapor phase epitaxy. Then, a SiO 2  film  12  is formed over the p-type GaAs contact layer  11 , and a stripe area of the SiO 2  film  12  having a width of about 50 micrometers and extending in the &lt;011&gt; direction is removed by a conventional lithography technique. Next, a p electrode  13  is formed over the above layered construction. In addition, the exposed surface of the substrate  1  is polished, and an n electrode  14  is formed on the polished surface of the substrate  1 . Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device as illustrated in FIG.  1 . 
     The above construction is an oxide-stripe type semiconductor laser device. However, instead, an index-guided structure realized by an internal stripe structure or a ridge structure may be formed. 
     The active region of the semiconductor laser device as the first embodiment is explained below.  FIG. 2A  is a magnified cross-sectional view of an active region of a semiconductor laser device which includes a single quantum well layer. As described above, the semiconductor laser device as the first embodiment has a single quantum well structure. That is, the quantum well layer  6  is sandwiched between the strain buffer layers  5  and  7 , and the quantum well layer  6  and the strain buffer layers  5  and  7  are further sandwiched between the barrier layers  4  and  8 . 
     When the thicknesses of the quantum well layer, each barrier layer, and each strain buffer layer are respectively indicated by da, db, and dr, and the lattice constants of the GaAs substrate, the quantum well layer, each barrier layer, and each strain buffer layer are respectively indicated by c s , c a , c b , and c r , the amounts of strains Δa, Δb, and Δr of the quantum well layer, each barrier layer, and each strain buffer layer are respectively indicated as Δa=(c a −c s )/c s , Δb=(c b −c s )/c s , and Δr=(c r −c s )/c s . 
     A relationship between the maximum light output power and the thickness of each strain buffer layer is indicated in  FIG. 3  based on five concrete examples of the semiconductor laser device as the first embodiment. In the five concrete examples, the quantum well layer and the barrier layer are fixed, and the thickness of the strain buffer layer is varied. That is, the compositions of the quantum well layer is arranged as x 1 =0.3 and y 1 =0, and the strain Δa and the thickness da of the quantum well layer are respectively Δa=2.1% and da=7 nm. The composition of the barrier layer is arranged as x 3 =0 and y 3 =0.20, and the strain Δb and the thickness db of the barrier layer are respectively Δb=−0.7% and db=10 nm. The compositions of the strain buffer layer is arranged as x 2 =0.05 and y 2 =0, the strain Δr of the strain buffer layer is Δr=0.35%, and the thicknesses dr of the strain buffer layers of the five concrete examples are respectively arranged as dr=0, 1, 2, 5, and 10 nm. In these cases, the average strains of the active regions (i.e., a product sum of the strains and thicknesses of the respective layers) are a compressive strain. As indicated in  FIG. 3 , the maximum light output power increases due to the provision of the strain buffer layers. However, when the thickness of each strain buffer layer reaches approximately 10 nm, the effect of the strain buffer layer diminishes. This is considered to be because the effective strain in the quantum well layer increases. Practically, in consideration of the maximum light output power and the control of the thickness, the preferable thickness of the strain buffer layer is about 1 to 5 nm. 
     In order to avoid occurrence of a defect due to the strains of the crystals, it is preferable that the sum of the first product of the strain Δa and the thickness da of the quantum well layer, the second product of the strain Δb and the thickness db of the barrier layer, and the third product of the strain Δr and the thickness dr of the strain buffer layer is 0.08 nm or smaller. That is, a preferable relationship between the strains and thicknesses of the respective layers of the active region is,
 
0 ≦Δa·da +2 Δb·db +2 Δr·dr ≦0.08 nm.
 
     In addition, the amount of the strain of the strain buffer layer is between the amounts of the strains of the quantum well layer and the barrier layer, i.e., Δb&lt;Δr&lt;Δa. Further, the strain of the strain buffer layer is a compressive strain, i.e., Δr&gt;0. 
     Furthermore, the active region may have a multiple quantum well structure.  FIG. 2B  is a magnified cross-sectional view of an example of the active region of a semiconductor laser device including multiple quantum well layers. Similar to the single quantum well structure, the barrier layers B are formed in alternation with the quantum well layers A so that both of the outermost layers of the alternating layers are the barrier layers B, and a barrier layer R is formed between each quantum well layer A and each of two barrier layers B adjacent to the quantum well layer A. In the case where the number of the quantum well layers A is N, a preferable relationship between the strains and thicknesses of the respective layers of the active region is,
 
0 ≦N·Δa·da +( N+ 1) ·Δ b·db+ 2 N·Δr·dr≦ 0.08 nm.
 
     The above relationship is also preferable in the semiconductor laser devices as the second to seventh embodiments, which are explained below. 
     Second Embodiment 
     The construction of a semiconductor laser device as the second embodiment of the present invention and a process for producing the semiconductor laser device are explained below with reference to  FIGS. 4A  to  4 C, which are cross-sectional views of representative stages in the process of producing a semiconductor laser device as the second embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     As illustrated in  FIG. 4A , first, an n-type In x8 Ga 1-x8 P lower cladding layer  22  (x 8 =0.49±0.01), an n-type or i-type (intrinsic) In x4 Ga 1-x4 As 1-y4 P y4  lower optical waveguide layer  23  (x 4 =(0.49±0.01)y 4 , 0≦x 4 ≦0.3), an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  24  (0≦x 3 &lt;0.49y 3 , 0&lt;y 3 ≦0.5) having a thickness of approximately 5 to 20 nm, an In x2 Ga 1-x2 AS 1-y2 P y2  strain buffer layer  25  (0≦x 2 &lt;0.4, 0≦y 2 &lt;0.5) having a thickness of approximately 1 to 5 nm, an In x1 Ga 1-x1 As 1-y1 P y1  compressive-strain quantum well layer  26  (0.4≧x 1 &gt;0.49y 1 , 0≦y 1 ≦0.1) having a thickness of approximately 3 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  27 , an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  28 , a p-type or i-type In x4 Ga 1-x4 As 1-y4 P y4  upper optical waveguide layer  29 , a p-type GaAs first etching stop layer  30 , a p-type In x5 Ga 1-x5 P second etching stop layer  31  (0≦x 5 ≦1) having a thickness of approximately 5 to 20 nm, an n-type Al z2 Ga 1-z2 As current confinement layer  32  (0&lt;z 2 ≦0.8), and an n-type GaAs cap layer  33  having a thickness of approximately 10 nm are formed on an n-type GaAs substrate  21  by organometallic vapor phase epitaxy. Then, a SiO 2  film  34  is formed over the n-type GaAs cap layer  33 , and a stripe area of the SiO 2  film  34  having a width of about 2 to 4 micrometers and extending in the &lt;011&gt; direction is removed by a conventional lithography technique. 
     Next, in order to form a stripe groove as illustrated in  FIG. 4B , the n-type GaAs cap layer  33  and the n-type Al z2 Ga 1-z2 As current confinement layer  32  are etched with a sulfuric acid etchant by using the SiO 2  film  34  as a mask until a stripe area of the p-type In x5 Ga 1-x5 P second etching stop layer  31  is exposed. Then, the exposed area of the p-type In x5 Ga 1-x5 P second etching stop layer  31  is etched with a hydrochloric acid etchant until a stripe area of the p-type GaAs first etching stop layer  30  is exposed. 
     Thereafter, as illustrated in  FIG. 4C , the remaining areas of the SiO 2  film  34  are removed by a fluoric acid etchant. Then, a p-type Al z2 Ga 1-z1 As upper cladding layer  35  (0&lt;z 1 ≦0.7 and z 1 &lt;z 2 ) and a p-type GaAs contact layer  36  are formed over the above construction. Next, a p electrode  37  is formed on the p-type GaAs contact layer  36 . In addition, the exposed surface of the substrate  21  is polished, and an n electrode  38  is formed on the polished surface of the substrate  21 . Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device. 
     In the above construction, the p-type or i-type In x4 Ga 1-x4 As 1-y4 P y4  upper optical waveguide layer  29  has such a thickness that oscillation in the fundamental transverse mode can be maintained even when the semiconductor laser device operates with high output power. In addition, a difference in an equivalent refractive index between a portion of the active region under the stripe groove and another portion of the active region which is not located under the stripe groove is in a range from 1.5×10 −3  to 7×10 −3 . Alternatively, the n-type lower cladding layer  22  may be made of Al z1 Ga 1-z1 As (0&lt;z 1 ≦0.7 and z 1 &lt;z 2 ). 
     Third Embodiment 
     The construction of a semiconductor laser device as the third embodiment of the present invention and a process for producing the semiconductor laser device are explained below with reference to  FIG. 5 , which is a cross-sectional view of the semiconductor laser device as the third embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. 
     As illustrated in  FIG. 5 , first, an n-type Al z1 Ga 1-z1 As lower cladding layer  42  (0&lt;z 1 ≦0.7), an n-type or i-type (intrinsic) Al z3 Ga 1-z3 As lower optical waveguide layer  43  (0≦z 3 ≦0.3, z 3 &lt;z 1 ), an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  44  (0≦x 3 &lt;0.49y 3 , 0&lt;y 3 ≦0.5) having a thickness of approximately 5 to 20 nm, an In x2 Ga 1-x2 AS 1-y2 P y2  strain buffer layer  45  (0≦x 2 ≦0.4, 0≦y 2 ≦0.5) having a thickness of approximately 1 to 5 nm, an In x1 Ga 1-x1 As 1-y1 P y1  compressive-strain quantum well layer  46  (0.4≧x 1 &gt;0.49y 1 , 0≦y 1 ≦0.1) having a thickness of approximately 3 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  47 , an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  48 , a p-type or i-type Al z3 Ga 1-z3 As upper optical waveguide layer  49 , a p-type Al z1 Ga 1-z1 As first upper cladding layer  50 , a p-type GaAs first etching stop layer  51 , a p-type In x5 Ga 1-x5 P second etching stop layer  52  (0≦x 5 ≦1) having a thickness of approximately 5 to 20 nm, an n-type Al z2 Ga 1-z2 As current confinement layer  53  (z 1 &lt;z 2 ≦0.8) having a thickness of 1 micrometer, and an n-type GaAs cap layer  54  having a thickness of approximately 10 nm are formed on an n-type GaAs substrate  41  by organometallic vapor phase epitaxy. Then, a SiO 2  film (not shown) is formed over the n-type GaAs cap layer  54 , and a stripe area of the SiO 2  film having a width of about 2 to 4 micrometers and extending in the &lt;011&gt; direction is removed by a conventional lithography technique. 
     Next, in order to form a stripe groove, the n-type GaAs cap layer  54  and the n-type Al z2 Ga 1-z2 As current confinement layer  53  are etched with a sulfuric acid etchant by using the SiO 2  film as a mask until a stripe area of the p-type In x5 Ga 1-x5 P second etching stop layer  52  is exposed. Then, the exposed area of the p-type In x5 Ga 1-x5 P second etching stop layer  52  is etched with a hydrochloric acid etchant until a stripe area of the p-type GaAs first etching stop layer  51  is exposed. 
     Thereafter, the remaining areas of the above SiO 2  film are removed by a fluoric acid etchant. Then, a p-type Al z1 Ga 1-z1 As second upper cladding layer  56  and a p-type GaAs contact layer  57  are formed over the above construction. Next, a p electrode  58  is formed on the p-type GaAs contact layer  57 . In addition, the exposed surface of the substrate  41  is polished, and an n electrode  59  is formed on the polished surface of the substrate  41 . Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device. 
     In the above construction, the total thickness of the p-type or i-type Al z3 Ga 1-z3 As upper optical waveguide layer  49  and the p-type Al z1 Ga 1-z1 As first upper cladding layer  50  is so arranged that oscillation in the fundamental transverse mode can be maintained even when the semiconductor laser device operates with high output power. In addition, a difference in an equivalent refractive index between a portion of the active region under the stripe groove and another portion of the active region which is not located under the stripe groove is in a range from 1.5×10 −3  to 7×10 −3 . 
     Fourth Embodiment 
     The construction of a semiconductor laser device as the fourth embodiment of the present invention and a process for producing the semiconductor laser device are explained below with reference to  FIG. 6 , which is a cross-sectional view of the semiconductor laser device as the fourth embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. 
     As illustrated in  FIG. 6 , first, an n-type Al z1 Ga 1-z1 As lower cladding layer  62  (0&lt;z 1 ≦0.7), an n-type or i-type (intrinsic) Al z3 Ga 1-z3 As lower optical waveguide layer  63  (0≦z 3 ≦0.3 and z 3 &lt;z 1 ), an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  64  (0≦x 3 &lt;0.49y 3 , 0&lt;y 3 ≦0.5) having a thickness of approximately 5 to 20 nm, an In x2 Ga 1-x2 AS 1-y2 P y2  strain buffer layer  65  (0≦x 2 ≦0.4, 0≦y 2 ≦0.5) having a thickness of approximately 1 to 5 nm, an In x1 Ga 1-x1 As 1-y1 P y1  compressive-strain quantum well layer  66  (0.4≧x 1 &gt;0.49y 1 , 0≦y 1 ≦0.1) having a thickness of approximately 3 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  67 , an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  68 , a p-type or i-type Al z3 Ga 1-z3 As first upper optical waveguide layer  69 , a p-type GaAs first etching stop layer  70 , a p-type In x5 Ga 1-x5 P second etching stop layer  71  (0≦x 5 ≦1) having a thickness of approximately 5 to 20 nm, an n-type Al z2 Ga 1-z2 As current confinement layer  72  (z 1 &lt;z 2 ≦0.8) having a thickness of 1 micrometer, and an n-type GaAs cap layer  73  having a thickness of approximately 10 nm are formed on an n-type GaAs substrate  61  by organometallic vapor phase epitaxy. Then, a SiO 2  film (not shown) is formed over the n-type GaAs cap layer  73 , and a stripe area of the SiO 2  film having a width of about 2 to 4 micrometers and extending in the &lt;011&gt; direction is removed by a conventional lithography technique. 
     Next, in order to form a stripe groove, the n-type GaAs cap layer  73  and the n-type Al z2 Ga 1-z2 As current confinement layer  72  are etched with a sulfuric acid etchant by using the SiO 2  film (not shown) as a mask until a stripe area of the p-type In x5 Ga 1-x5 P second etching stop layer  71  is exposed. Then, the exposed area of the p-type In x5 Ga 1-x5 P second etching stop layer  71  is etched with a hydrochloric acid etchant until a stripe area of the p-type GaAs first etching stop layer  70  is exposed. 
     Thereafter, the remaining areas of the above SiO 2  film are removed by a fluoric acid etchant. Then, a p-type Al z3 Ga 1-z3 As second upper optical waveguide layer  75 , a p-type Al z1 Ga 1-z1 As upper cladding layer  76 , and a p-type GaAs contact layer  77  are formed over the above construction. Next, a p electrode  78  is formed on the p-type GaAs contact layer  77 . In addition, the exposed surface of the substrate  61  is polished, and an n electrode  79  is formed on the polished surface of the substrate  61 . Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device. 
     In the above construction, the p-type or i-type Al z3 Ga 1-z3 As first upper optical waveguide layer  69  has such a thickness that oscillation in the fundamental transverse mode can be maintained even when the semiconductor laser device operates with high output power. In addition, a difference in an equivalent refractive index between a portion of the active region under the stripe groove and another portion of the active region which is not located under the stripe groove is in a range from 1.5×10 −3  to 7×10 −3 . 
     Fifth Embodiment 
     The construction of a semiconductor laser device as the fifth embodiment of the present invention and a process for producing the semiconductor laser device are explained below with reference to  FIG. 7 , which is a cross-sectional view of the semiconductor laser device as the fifth embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. 
     As illustrated in  FIG. 7 , first, an n-type Al z1 Ga 1-z1 As lower cladding layer  82  (0&lt;z 1 ≦0.7), an n-type or i-type (intrinsic) In x4 Ga 1-x4 As 1-y4 P y4  optical waveguide layer  83  (x 4 =(0.49±0.01)y 4 , 0≦x 4 ≦0.3), an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  84  (0≦x 3 &lt;0.49y 3 , 0&lt;y 3 ≦0.5) having a thickness of approximately 5 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  85  (0≦x 2 ≦0.4, 0≦y 2 ≦0.5) having a thickness of approximately 1 to 5 nm, an In x1 Ga 1-x1 As 1-y1 P y1  compressive-strain quantum well layer  86  (0.4≧x 1 &gt;0.49y 1 , 0≦y 1 ≦0.1) having a thickness of approximately 3 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  87 , an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  88 , a p-type or i-type In x4 Ga 1-x4 As 1-y4 P y4  optical waveguide layer  89 , a p-type In x5 Ga 1-x5 P etching stop layer  90  (0≦x 5 ≦1) having a thickness of approximately 5 to 20 nm, a p-type Al z1 Ga 1-z1 As upper cladding layer  91 , and a p-type GaAs contact layer  92  are formed on an n-type GaAs substrate  81  by organometallic vapor phase epitaxy. Then, a SiO 2  first insulation film (not shown) is formed over the p-type GaAs contact layer  92 , and parallel stripe areas of the first insulation film, each having a width of about 10 micrometers, are removed by a conventional lithography technique. Next, in order to form a ridge stripe structure, the parallel stripe areas of the above layered structure are etched to the depth of the upper surface of the p-type In x5 Ga 1-x5 P etching stop layer  90  by wet etching using the remaining areas of the first insulation film as a mask. When a solution of sulfuric acid and hydrogen peroxide is used as an etchant, the etching automatically stops at the upper boundary of the p-type In x5 Ga 1-x5 P etching stop layer  90 . Thereafter, the remaining areas of the first insulation film are removed, and then a second insulation film  94  is formed over the ridge stripe structure. Next, a stripe portion of the second insulation film  94  on the top of the ridge stripe structure is removed by a conventional lithography technique so as to expose a stripe area of the p-type GaAs contact layer  92  and form a current injection window. Next, a p electrode  95  is formed on the exposed stripe area of the p-type GaAs contact layer  92 . In addition, the exposed surface of the substrate  81  is polished, and an n electrode  96  is formed on the polished surface of the substrate  81 . Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device. 
     In the above construction, the p-type or i-type In x4 Ga 1-x4 As 1-y4 P y4  optical waveguide layer  89  has such a thickness that oscillation in the fundamental transverse mode can be maintained even when the semiconductor laser device operates with high output power. In addition, a difference in an equivalent refractive index between a portion of the active region under the current injection window and another portion of the active region which is not located under the current injection window is in a range from 1.5×10 −3  to 7×10 −3 . 
     Sixth Embodiment 
     The construction of a semiconductor laser device as the sixth embodiment of the present invention and a process for producing the semiconductor laser device are explained below with reference to  FIG. 8 , which is a cross-sectional view of the semiconductor laser device as the sixth embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. 
     As illustrated in  FIG. 8 , first, an n-type Al z1 Ga 1-z1 As lower cladding layer  102  (0&lt;z 1 ≦0.7), an n-type or i-type (intrinsic) Al z3 Ga 1-z3 As optical waveguide layer  103  (0≦z 3 ≦0.3 and z 3 &lt;z 1 ), an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  104  (0≦x 3 &lt;0.49y 3 , 0&lt;y 3 ≦0.5) having a thickness of approximately 5 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  105  (0≦x 2 ≦0.4, 0≦y 2 ≦0.5) having a thickness of approximately 1 to 5 nm, an In x1 Ga 1-x1 As 1-y1 P y1  compressive-strain quantum well layer  106  (0.4≧x 1 &gt;0.49y 1 , 0≦y 1 ≦0.1) having a thickness of approximately 3 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  107 , an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  108 , a p-type or i-type Al z3 Ga 1-z3 As optical waveguide layer  109 , a p-type Al z1 Ga 1-z1  As first upper cladding layer  110 , a p-type In x5 Ga 1-x5 P etching stop layer  111  (0≦x 5 ≦1) having a thickness of approximately 5 to 20 nm, a p-type Al z1 Ga 1-z1 As second upper cladding layer  112 , and a p-type GaAs contact layer  113  are formed on an n-type GaAs substrate  101  by organometallic vapor phase epitaxy. Then, a first insulation film (not shown) is formed over the p-type GaAs contact layer  113 , and parallel stripe areas of the first insulation film, each having a width of about 10 micrometers, are removed by a conventional lithography technique. Next, in order to form a ridge stripe structure, the parallel stripe areas of the above layered structure are etched to the depth of the upper surface of the p-type In x5 Ga 1-x5 P etching stop layer Ill by wet etching using the remaining areas of the first insulation film as a mask. When a solution of sulfuric acid and hydrogen peroxide is used as an etchant, the etching automatically stops at the upper boundary of the p-type In x5 Ga 1-x5 P etching stop layer  111 . Thereafter, the remaining areas of the first insulation film are removed, and then a second insulation film  115  is formed over the ridge stripe structure. Next, a stripe portion of the second insulation film  115  on the top of the ridge stripe structure is removed by a conventional lithography technique so as to expose a stripe area of the p-type GaAs contact layer  113  and form a current injection window. Next, a p electrode  116  is formed on the exposed stripe area of the p-type GaAs contact layer  113 . In addition, the exposed surface of the substrate  101  is polished, and an n electrode  117  is formed on the polished surface of the substrate  101 . Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device. 
     In the above construction, the total thickness of the p-type or i-type Al z3 Ga 1-z3 As optical waveguide layer  109  and the p-type Al z1 Ga 1-z1 As first upper cladding layer  110  is so arranged that oscillation in the fundamental transverse mode can be maintained even when the semiconductor laser device operates with high output power. In addition, a difference in an equivalent refractive index between a portion of the active region under the current injection window and another portion of the active region which is not located under the current injection window is in a range from 1.5×10 −3  to 7×10 −3 . 
     Seventh Embodiment 
     The construction of a semiconductor laser device as the seventh embodiment of the present invention and a process for producing the semiconductor laser device are explained below with reference to  FIG. 9 , which is a cross-sectional view of the semiconductor laser device as the seventh embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. 
     As illustrated in  FIG. 9 , first, an n-type Al z1 Ga 1-z1 As lower cladding layer  122  (0&lt;z 1 ≦0.7), an n-type or i-type (intrinsic) Al z3 Ga 1-z3 As optical waveguide layer  123  (0≦z 3 ≦0.3 and z 3 &lt;z 1 ), an In x3 Ga 1-y3 As 1-y3 P y3  tensile-strain barrier layer  124  (0≦x 3 &lt;0.49y 3 , 0&lt;y 3 ≦0.5) having a thickness of approximately 5 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  125  (0≦x 2 ≦0.4, 0≦y 2 ≦0.5) having a thickness of approximately 1 to 5 nm, an In x1 Ga 1-x1 As 1-y1 P y1  compressive-strain quantum well layer  126  (0.4≧x 1 &gt;0.49y 1 , 0≦y 1 &lt;0.1) having a thickness of approximately 3 to 20 nm, an In x2 Ga 1-x2 As 1-y2 P y2  strain buffer layer  127 , an In x3 Ga 1-x3 As 1-y3 P y3  tensile-strain barrier layer  128 , a p-type or i-type Al z3 Ga 1-z3 As first upper optical waveguide layer  129 , a p-type In x5 Ga 1-x5 P etching stop layer  130  (0≦x 5 ≦1) having a thickness of approximately 5 to 20 nm, a p-type Al z3 Ga 1-z3 As second upper optical waveguide layer  131 , a p-type Al z1 Ga 1-z1 As upper cladding layer  132 , and a p-type GaAs contact layer  133  are formed on an n-type GaAs substrate  121  by organometallic vapor phase epitaxy. Then, a first insulation film (not shown) is formed over the p-type GaAs contact layer  133 , and parallel stripe areas of the first insulation film, each having a width of about 10 micrometers, are removed by a conventional lithography technique. Next, in order to form a ridge stripe structure, the parallel stripe areas of the above layered structure are etched to the depth of the upper surface of the p-type In x5 Ga 1-x5 P etching stop layer  130  by wet etching using the remaining areas of the first insulation film as a mask. When a solution of sulfuric acid and hydrogen peroxide is used as an etchant, the etching automatically stops at the upper boundary of the p-type In x5 Ga 1-x5 P etching stop layer  130 . Thereafter, the remaining areas of the first insulation film are removed, and then a second insulation film  135  is formed over the ridge stripe structure. Next, a stripe portion of the second insulation film  135  on the top of the ridge stripe structure is removed by a conventional lithography technique so as to expose a stripe area of the p-type GaAs contact layer  133  and form a current injection window. Next, a p electrode  136  is formed on the exposed stripe area of the p-type GaAs contact layer  133 . In addition, the exposed surface of the substrate  121  is polished, and an n electrode  137  is formed on the polished surface of the substrate  121 . Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device. 
     In the above construction, the p-type or i-type Al z3 Ga 1-z3 As first upper optical waveguide layer  129  has such a thickness that oscillation in the fundamental transverse mode can be maintained even when the semiconductor laser device operates with high output power. In addition, a difference in an equivalent refractive index between a portion of the active region under the current injection window and another portion of the active region which is not located under the current injection window is in a range from 1.5×10 −3  to 7×10 −3 . 
     Additional Matters 
     (i) In the constructions of the first to seventh embodiments, the respective cladding layers may be made of In x7 (Al z7 Ga 1-z7 ) 1-x7 P (x 7 =0.49±0.01, 0≦z 7 ≦1). 
     (ii) Although the present invention is applied to the index-guided semiconductor laser devices in the first to seventh embodiments, the present invention can also be applied to other semiconductor laser devices having a diffraction lattice, and further to optical integrated circuits. 
     (iii) Although n-type GaAs substrates are used in the constructions of the first to seventh embodiments, instead, p-type GaAs substrates may be used. When the GaAs substrate is a p-type in each embodiment, the conductivity types of all of the other layers in the construction of the embodiment should be inverted. 
     (iv) In the second to seventh embodiments, the processes for producing semiconductor laser devices which oscillate in a fundamental transverse mode are explained. However, the processes disclosed for the second to seventh embodiments can also be used in production of broad-stripe, index-guided semiconductor laser devices which have a stripe width of 4 micrometers or greater and an equivalent refractive index of 1.5×10 −3  or greater, and oscillate in multiple modes. Since the above semiconductor laser devices which oscillate in multiple modes have a low-noise characteristic, it is possible to realize devices which can be used in excitation of solid-state lasers or the like. 
     (v) Due to the In x1 Ga 1-x1 As 1-y1 P y1  compressive strain quantum well active layers, the oscillation wavelengths of the semiconductor laser devices as the first to seventh embodiments can be controlled in the range of 900 to 1,200 nm. 
     (vi) Each layer in the constructions of the first to seventh embodiments may be formed by molecular beam epitaxy using solid or gas raw material.