Abstract:
In a semiconductor laser device: an n-type lower cladding layer; a lower optical waveguide layer; a compressive strain quantum well active layer made of In x3 Ga 1−x3 As 1−y3 P y3 , where 0&lt;x3≦0.4 and 0≦y3≦0.1; an upper optical waveguide layer; a p-type In 0.49 Ga 0.51 P first upper cladding layer; an etching stop layer made of In x1 Ga 1−x1 As 1−y1 P y1 , where 0≦x1≦0.3 and 0≦y1≦0.6; an n-type current confinement layer made of In 0.49 (Al z1 Ga 1−z1 ) 0.51 P, where 0≦z1≦0.1; an In 0.49 Ga 0.51 P cap layer; a p-type second upper cladding layer made of In x4 Ga 1−x4 As 1−y4 P y4 , where x4=(0.49±0.01)y4 and 0.9≦y4≦1; and a p-type contact layer are formed on an n-type GaAs substrate in this order. Each of the etching stop layer, the current confinement layer, and the cap layer has a stripe-shape opening realizing a current injection window filled with the second upper cladding layer. The absolute value of the product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; and the absolute value of the product of the strain and the thickness of the etching stop layer is equal to or smaller than 0.25 nm. Each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the cap layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser device having an internal current confinement structure, and a process for producing a semiconductor light emitting device having an internal current confinement structure. 
     2. Description of the Related Art 
     (1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 μm band, a current confinement structure and an index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that the semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.102 discloses a semiconductor laser device which emits light in the 0.98 μm band. This semiconductor laser device is formed as follows. 
     On an n-type GaAs substrate, an n-type Al 0.48 Ga 0.52 As lower cladding layer, an undoped Al 0.2 Ga 0.8 As optical waveguide layer, an Al 0.2 Ga 0.8 As/In 0.2 Ga 0.8 As double quantum well active layer, an undoped Al 0.2 Ga 0.8 As optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al 0.67 Ga 0.33 As etching stop layer, a p-type Al 0.48 Ga 0.52 As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al 0.67 Ga 0.33 As etching stop layer by normal photolithography and selective etching, and an n-type Al 0.7 Ga 0.3 As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device. 
     However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult. 
     (2) In addition, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp.1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 μm band. This semiconductor laser device is formed as follows. 
     On an n-type GaAs substrate, an n-type Al 0.4 Ga 0.6 As lower cladding layer, an undoped Al 0.2 Ga 0.8 As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al 0.2 Ga 0.8 As optical waveguide layer, a p-type Al 0.4 Ga 0.5 As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al 0.4 Ga 0.6 As upper cladding layer by normal photolithography and selective etching, and an n-type In 0.5 Ga 0.5 P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction. 
     However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having a different V-group component, on such an upper cladding layer. 
     (3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.189 discloses an all-layer-Aluminum-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 μm band. This semiconductor laser device is formed as follows. 
     On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by normal photolithography and selective etching, and an n-type In 0.5 Ga 0.5 P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized. 
     The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width. 
     (4) Furthermore, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp.1889 discloses an internal striped structure semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.8 μm band. This semiconductor laser device is formed as follows. 
     On an n-type GaAs substrate, an n-type AlGaAs lower cladding layer, an AlGaAs/GaAs triple quantum well active layer, a p-type AlGaAs first upper cladding layer, an n-type AlGaAs current confinement layer, and an n-type AlGaAs protection layer are formed in this order. Next, a narrow-stripe groove is formed, by normal photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Next, over the above structure, a p-type AlGaAs second upper cladding layer and a p-type GaAs contact layer are formed. 
     In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in a fundamental transverse mode can be realized by the difference in the refractive index between the n-type AlGaAs current confinement layer and the p-type AlGaAs second upper cladding layer. However, the above semiconductor laser device also has a drawback that it is difficult to form an AlGaAs layer on another AlGaAs layer since the AlGaAs layers are prone to oxidation. 
     As described above, in the conventional current semiconductor laser devices which include an internal current confinement structure, oscillate in a fundamental transverse mode, and emit light in the 0.9 to 1.1 μm band, it is difficult to form an upper layer on a current confinement layer when aluminum exists near the boundary of the current confinement layer and the upper layer, since the AlGaAs layers are prone to oxidation. Even if the upper layer can be formed, defects occur at the boundary of the current confinement layer and the upper layer for the same reason. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high. 
     Another object of the present invention is to provide a process for producing a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high. 
     (1) According to the first aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of In x3 Ga 1−x3 As 1−y3 P y3 , and formed on the lower optical waveguide layer, where 0&lt;x3≦0.4, 0≦y3≦0.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer; a first upper cladding layer made of In 0.49 Ga 0.51 P of a second conductive type, and formed on the upper optical waveguide layer; an etching stop layer made of In x1 Ga 1−x1 As 1−y1 P y1 , and formed on the first upper cladding layer other than a stripe area of the first upper cladding layer so as to form a first portion of a stripe groove realizing a current injection window, where 0≦x1≦0.3, 0≦y1≦0.6, and an absolute value of a second product of a strain and a thickness of the etching stop layer is equal to or smaller than 0.25 nm; a current confinement layer made of In 0.49 (Al z1 Ga 1−z1 ) 0.51 P of the first conductive type, and formed on the etching stop layer so as to form a second portion of the stripe groove, where 0≦z1≦0.1; a cap layer made of In 0.49 Ga 0.51 P, and formed on the current confinement layer so as to form a third portion of the stripe groove; a second upper cladding layer made of In x4 Ga 1−x4 As 1−y4 P y4  of the second conductive type, and formed on the current confinement layer and the stripe area of the first upper cladding layer, where x4=(0.49±0.01)y4 and 0.9≦y4≦1; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the cap layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate. 
     The first conductive type is different in polarity of carriers from the second conductive type. That is, when the first conductive type is n type, and the second conductive type is p type. 
     The strain of the compressive strain quantum well active layer is defined as (c a −c s )/c s , where c s  and c a  are the lattice constants of the GaAs substrate and the compressive strain quantum well active layer, respectively. 
     The strain of the etching stop layer is defined as (c s −c s )/c s , where c s  and c s  are the lattice constants of the GaAs substrate and the etching stop layer, respectively. 
     When a layer grown over the substrate has a lattice constant c, and an absolute value of the amount (c−c s )/c s  is equal to or smaller than 0.003, the layer is lattice-matched with the substrate. That is, in the semiconductor laser devices according to the first and second aspects of the present invention, the absolute value of the amount (c−c s )/c s  is equal to or smaller than 0.003 for each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, and the second upper cladding layer. 
     Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (vi). 
     (i) The semiconductor laser device may further include first and second tensile strain barrier layers both made of In x5 Ga 1−x5 As 1−y5 P y5 , and respectively formed above and below the compressive strain quantum well active layer, where 0≦x5≦0.3 and 0&lt;y5≦0.6, and the absolute value of a sum of the first product and a third product of the strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm. The strain of the first and second tensile strain barrier layers is defined as (c b −c s )/c s , where c b  is the lattice constant of the first and second tensile strain barrier layers. 
     (ii) The etching stop layer may be one of the first and second conductive types. 
     (iii) The first cap layer may be one of the first and second conductive types and an undoped type. 
     (iv) The lower optical waveguide active layer may be the first conductive type, and the upper optical waveguide active layer is the second conductive type. 
     (v) The compressive strain quantum well active layer may have a multiple quantum well structure. 
     (vi) The stripe groove may have a width equal to or greater than 1 μm. 
     (2) According to the second aspect of the present invention, there is provided a process for producing a semiconductor laser device, including the steps of: (a) forming a lower cladding layer of a first conductive type on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of In x3 Ga 1−x3 As 1−y3 P y3 , on the lower optical waveguide layer, where 0&lt;x3≦0.4, 0≦y3≦0.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming an upper optical waveguide layer on the In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer; (e) forming a first upper cladding layer made of In 0.49 Ga 0.51 P of a second conductive type, on the upper optical waveguide layer; (f) forming an etching stop layer made of In x1 Ga 1−x1 As 1−y1 P y1 , on the first upper cladding layer, where 0≦x1≦0.3 and 0≦y1≦0.6, and the absolute value of a second product of the strain and the thickness of the etching stop layer is equal to or smaller than 0.25 nm; (g) forming a current confinement layer made of In 0.49 (Al z1 Ga 1−z1 ) 0.51 P of the first conductive type, on the etching stop layer, where 0≦z1≦0.1; (h) forming a first cap layer made of In 0.49 Ga 0.51 P, on the current confinement layer; (i) removing a stripe area of the first cap layer and a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (j) removing a stripe area of the etching stop layer so as to form a second portion of the stripe groove; (k) forming a second upper cladding layer made of In x4 Ga 1−x4 As 1−y4 P y4  of the second conductive type so that the stripe groove is covered with the second upper cladding layer, where x4=(0.49±0.01)y4 and 0.9≦y4≦1; and (l) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the first cap layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate. 
     That is, the semiconductor laser device according to the first aspect of the present invention can be produced by the process according to the second aspect of the present invention. 
     Preferably, the process according to the second aspect of the present invention may also have one or any possible combination of the following additional features (vii) to (xi). 
     (vii) The process may further include, after the step (h), the steps of, (hi) forming a second cap layer made of GaAs, and (h2) removing a stripe area of the second cap layer. In addition, in the step (j), a remaining area of the second cap layer is removed together with the stripe area of the etching stop layer so as to form an additional portion of the stripe groove. 
     (viii) The etching stop layer may be one of the first and second conductive types. 
     (ix) The first cap layer may be one of the first and second conductive types and an undoped type. 
     (x) The second cap layer may be one of the first and second conductive types and an undoped type. 
     (xi) The process may further include the steps of, (b1) after the step (b), forming a first tensile strain barrier layer made of In x5 Ga 1−x5 As 1−y5 P y5 , on the lower optical waveguide layer, where 0≦x5≦0.3 and 0&lt;y5≦0.6, and (c1) after the step (c), forming a second tensile strain barrier layer made of In x5 Ga 1−x5 As 1−y5 P y5 , on the compressive strain quantum well active layer, where an absolute value of a sum of the first product and a third product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm. 
     (3) The first and second aspects of the present invention have the following advantages. 
     (a) When aluminum exists near a boundary surface on which the second upper cladding layer is formed, the boundary surface is prone to oxidation. However, in step (k) of the process according to the second aspect of the present invention, aluminum exists in only the In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer, which appears in only the side surfaces of the stripe groove. In addition, the aluminum content in the In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer does not exceed 10%. Therefore, it is easy to form the second upper cladding layer, and the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved. 
     (b) In the semiconductor laser device according to the first aspect of the present invention, the current confinement layer is made of In 0.49 (Al z1 Ga 1−z1 ) 0.51 P, and the second upper cladding layer is made of In x4 Ga 1−x4 As 1−y4 P y4 . Therefore, the difference in the refractive indexes between the current confinement layer and the second upper cladding layer realizes a difference of about 1.5×10 −3  to 7×10 −3  in the equivalent refractive index of the active layer between the portion under the current confinement layer and the portion under the stripe groove, with high accuracy, and it is possible to cut off oscillation in higher modes. Thus, oscillation in the fundamental transverse mode can be maintained even when the output power becomes high. 
     (c) The etching stop layer is made of InGaP. Therefore, when a hydrochloric acid etchant is used, InGaAsP layers under the etching stop layer are not etched. That is, when a hydrochloric acid etchant is used, it is possible to accurately stop etching at the lower boundary of the first upper cladding layer. Thus, the stripe width can be accurately controlled, and the index-guided structure can be built in with high accuracy. 
     (d) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced. 
     (e) When the tensile strain barrier layers are provided as described in the paragraphs (1)(i), various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased. 
     (f) When the GaAs second cap layer is used as described in paragraph (2)(vii), it is possible to prevent formation of a natural oxidation film on the InGaP first cap layer, and metamorphic change in the InGaP first cap layer, which may occur when a resist layer is formed directly on the InGaP first cap layer. In addition, since the GaAs second cap layer is removed before the second upper cladding layer is formed, it is possible to remove a residue left on a boundary surface on which the second upper cladding layer is formed, and prevent occurrence of crystal defects. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 D are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the first embodiment. 
     FIGS. 2A to  2 D are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the second embodiment. 
     FIGS. 3A to  3 D are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the third embodiment. 
     FIGS. 4A to  4 C are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the fourth embodiment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained in detail below with reference to drawings. 
     First Embodiment 
     FIGS. 1A to  1 D show cross sections of the representative stages in the process for producing a semiconductor laser device as the first embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     First, as illustrated in FIG. 1A, an n-type In 0.49 Ga 0.51 P lower cladding layer  12 , an n-type or i-type (intrinsic) In x2 Ga 1−x2 As 1−y2 P y2  optical waveguide layer  13  (x2=(0.49±0.01)y2, 0≦x2≦0.3), an In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer  14  (0&lt;x3≦0.4, 0≦y3≦0.1), a p-type or i-type (intrinsic) In x2 Ga 1−x2 As 1−y2 P y2  optical waveguide layer  15  (x2=(0.49±0.01)y2, 0≦x2≦0.3), a p-type In 0.49 Ga 0.51 P first upper cladding layer  16 , an n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  17  (0≦x1≦0.3, 0≦y1≦0.6) having a thickness of, for example, 20 nm, an n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  18  (0≦z1≦0.1) having a thickness of, for example, 1 μm, an n-type In 0.49 Ga 0.51 P cap layer  19 , and a GaAs cap layer  20  having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate  11  by organometallic vapor phase epitaxy. Then, a SiO 2  film  21  is formed over the GaAs cap layer  20 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction, of the SiO 2  film  21  is removed by normal photolithography. 
     Next, as illustrated in FIG. 1B, the GaAs cap layer  20  is etched with a sulfuric acid etchant by using the SiO 2  film  21  as a mask. Then, the stripe areas of the n-type In 0.49 Ga 0.51 P cap layer  19  and the n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  18  are etched with a hydrochloric acid etchant until the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  17  is exposed. 
     Thereafter, as illustrated in FIG. 1C, the remaining areas of the SiO 2  film  21  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  20  and the exposed area of the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  17  are removed by etching using a sulfuric acid etchant. 
     Finally, as illustrated in FIG. 1D, a p-type In x4 Ga 1−x4 As 1−y4 P y4  second upper cladding layer  21  (x4=(0.49±0.01)y4, 0.9≦y4≦1) and a p-type GaAs contact layer  22  are formed over the construction of FIG.  1 C. Then, a p electrode  24  is formed on the p-type GaAs contact layer  23 . In addition, the exposed surface of the substrate  11  is polished, and an n electrode  25  is formed on the polished surface of the substrate  11 . 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 In 0.49 Ga 0.51 P first upper cladding layer  16  has such thickness that oscillation in a fundamental transverse mode can be maintained even when output power becomes high. In addition, since a current confinement structure and a real refractive index structure are realized by the provision of the p-type In x4 Ga 1−x4 As 1−y4 P y4  second upper cladding layer  21  and the n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  18 , it is possible to realize a difference of about 1.5×10 −3  to 7×10 −3  in the equivalent refractive index of the active layer between the portion under the current confinement layer and the portion under the stripe area. Therefore, oscillation in a fundamental transverse mode can be maintained even when the output power becomes high. 
     Second Embodiment 
     FIGS. 2A to  2 D show cross sections of the representative stages in the process for 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. 
     First, as illustrated in FIG. 2A, an n-type In 0.49 Ga 0.51 P lower cladding layer  32 , an n-type or i-type In x2 Ga 1−x2 As 1−y2 P y2  optical waveguide layer  33  (x2=(0.49±0.01)y2, 0≦x2≦0.3), an In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layer  34  (0≦x5≦0.3, 0&lt;y5≦0.6), an In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer  35  (0&lt;x3≦0.4, 0≦y3≦0.1), an In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layer  36  (0≦x5≦0.3, 0&lt;y5≦0.6), a p-type or i-type In x2 Ga 1−x2 As 1−y2 P y2  optical waveguide layer  37  (x2=(0.49±0.01)y2, 0≦x2≦0.3), a p-type In 0.49 Ga 0.51 P first upper cladding layer  38 , an n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  39  (0≦x1≦0.3, 0≦y1≦0.6) having a thickness of, for example, 20 nm, an n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  40  (0≦z1≦0.1) having a thickness of, for example, 1 μm, an n-type In 0.49 Ga 0.51 P cap layer  41 , and an n-type GaAs cap layer  42  are formed on an n-type GaAs substrate  31  by organometallic vapor phase epitaxy. Then, a SiO 2  film  43  is formed over the n-type GaAs cap layer  42 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction, of the SiO 2  film  43  is removed by normal photolithography. 
     Next, as illustrated in FIG. 2B, the n-type GaAs cap layer  42  is etched with a sulfuric acid etchant by using the SiO 2  film  43  as a mask. Then, the stripe areas of the n-type In 0.49 Ga 0.51 P cap layer  41  and the n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  40  are etched with a hydrochloric acid etchant until the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  39  is exposed. 
     Thereafter, as illustrated in FIG. 2C, the remaining areas of the SiO 2  film  43  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  42  and the exposed area of the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  39  are removed by etching using a sulfuric acid etchant. 
     Finally, as illustrated in FIG. 2D, an In x4 Ga 1−x4 As 1−y4 P y4  second upper cladding layer  44  (x4=(0.49±0.01)y4, 0.9≦y4≦1) and a p-type GaAs contact layer  45  are formed over the construction of FIG.  2 C. Then, a p electrode  46  is formed on the p-type GaAs contact layer  45 . In addition, the exposed surface of the substrate  31  is polished, and an n electrode  47  is formed on the polished surface of the substrate  31 . 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 In 0.49 Ga 0.51 P first upper cladding layer  38  has such a thickness that oscillation in a fundamental transverse mode can be maintained even when output power becomes high. 
     In the construction of the second embodiment, the In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer  35  is sandwiched between the In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layers  34  and  36 . Therefore, compared with the first embodiment, characteristics are improved (e.g., the threshold current is lowered), and reliability is increased. 
     Third Embodiment 
     FIGS. 3A to  3 D show cross sections of the representative stages in the process for producing a semiconductor laser device as the third embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     First, as illustrated in FIG. 3A, an n-type Al z1 Ga 1−z1 As lower cladding layer  52  (0.35≦z1≦0.7), an n-type or i-type Al z2 Ga 1−z2 As optical waveguide layer  53  (0≦z2≦0.2), an In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layer  54  (0≦x5≦0.3, 0&lt;y5≦0.6), an In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer  55  (0&lt;x3≦0.4, 0≦y3≦0.1), an In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layer  56  (0&lt;x5≦0.3, 0≦y5≦0.6), a p-type or i-type Al z2 Ga 1−z2 As optical waveguide layer  57  (0≦z2≦0.2), a p-type In 0.49 Ga 0.51 P first upper cladding layer  58 , an n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  59  (0≦x1≦0.3, 0≦y1≦0.6) having a thickness of, for example, 20 nm, an n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  60  (0≦z1≦0.1) having a thickness of, for example, 1 μm, an n-type In 0.49 Ga 0.51 P cap layer  61 , and an n-type GaAs cap layer  62  having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate  51  by organometallic vapor phase epitaxy. Then, a SiO 2  film  63  is formed over the n-type GaAs cap layer  62 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction, of the SiO 2  film  63  is removed by normal photolithography. 
     Next, as illustrated in FIG. 3B, the n-type GaAs cap layer  62  is etched with a sulfuric acid etchant by using the SiO 2  film  63  as a mask. Then, the stripe areas of the n-type In 0.49 Ga 0.51 P cap layer  61  and the n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  60  are etched with a hydrochloric acid etchant until the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  59  is exposed. 
     Thereafter, as illustrated in FIG. 3C, the remaining areas of the SiO 2  film  63  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  62  and the exposed area of the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  59  are removed by etching using a sulfuric acid etchant. 
     Finally, as illustrated in FIG. 3D, an In x4 Ga 1−x4 As 1−y4 P y4  second upper cladding layer  64  (x4=(0.49±0.01)y4, 0.9≦y4≦1) and a p-type GaAs contact layer  65  are formed over the construction of FIG.  3 C. Then, a p electrode  66  is formed on the p-type GaAs contact layer  65 . In addition, the exposed surface of the substrate  51  is polished, and an n electrode  67  is formed on the polished surface of the substrate  51 . 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 In 0.49 Ga 0.51 P first upper cladding layer  58  has such thickness that oscillation in a fundamental transverse mode can be maintained even when output power becomes high. 
     In the construction of the third embodiment, the In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer  55  is also sandwiched between the In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layers  54  and  56 . Therefore, compared with the first embodiment, characteristics are improved (e.g., the threshold current is lowered), and reliability is increased. 
     Fourth Embodiment 
     FIGS. 4A to  4 C show cross sections of the representative stages in the process for producing a semiconductor laser device as the fourth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     First, as illustrated in FIG. 4A, an n-type In 0.49 Ga 0.51 P lower cladding layer  72 , an n-type or i-type In x2 Ga 1−x2 As 1−y2 P y2  optical waveguide layer  73  (x2=(0.49±0.01)y2, 0≦x2≦0.3), an In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layer  74  (0≦x5≦0.3, 0&lt;y5≦0.6), an In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layer  75  (0&lt;x3≦0.4, 0≦y3≦0.1), an In x5 Ga 1−x5 As 1−y5 P y5  tensile strain barrier layer  76  (0≦x5≦0.3, 0&lt;y5≦0.6), a p-type or i-type In x2 Ga 1−x2 As 1−y2 P y2  optical waveguide layer  77  (x2=(0.49±0.01)y2, 0≦x2≦0.3), a p-type In 0.49 Ga 0.51 P first upper cladding layer  78 , an n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  79  (0≦x1≦0.3, 0≦y1≦0.6) having a thickness of, for example, 20 nm, an n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  80  (0≦z1≦0.1) having a thickness of, for example, 1 μm, and an n-type In 0.49 Ga 0.51 P cap layer  81  are formed on an n-type GaAs substrate  71  by organometallic vapor phase epitaxy. Then, a SiO 2  film  82  is formed over the n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  80 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction, of the SiO 2  film  82  is removed by normal photolithography. 
     Next, as illustrated in FIG. 4B, the n-type In 0.49 Ga 0.51 P cap layer  61  and the n-type In 0.49 (Al z1 Ga 1−z1 ) 0.51 P current confinement layer  80  are etched with a hydrochloric acid etchant by using the SiO 2  film  82  as a mask until the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  79  is exposed. Then, the remaining areas of the SiO 2  film  82  are removed by etching using a fluoric acid etchant, and the exposed area of the n-type or p-type In x1 Ga 1−x1 As 1−y1 P y1  etching stop layer  79  is removed by etching using a sulfuric acid etchant. 
     Thereafter, as illustrated in FIG. 4C, an In x4 Ga 1−x4 As 1−y4 P y4  second upper cladding layer  83  (x4=(0.49±0.01)y4, 0.9≦y4≦1) and a p-type GaAs contact layer  84  are formed over the construction of FIG.  4 B. Then, a p electrode  85  is formed on the p-type GaAs contact layer  84 . In addition, the exposed surface of the substrate  71  is polished, and an n electrode  86  is formed on the polished surface of the substrate  71 . 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 In 0.49 Ga 0.51 P first upper cladding layer  78  has such a thickness that oscillation in a fundamental transverse mode can be maintained even when output power becomes high. 
     As described above, it is possible to produce a semiconductor laser apparatus according to the present invention without forming a GaAs cap layer on the n-type In .49 Ga 0.51 P cap layer. 
     Additional Matters 
     (i) Due to the In x3 Ga 1−x3 As 1−y3 P y3  compressive strain quantum well active layers (0&lt;x3≦0.4, 0≦y3≦0.1), the oscillation wavelengths of the semiconductor laser devices as the first to fourth embodiments can be controlled in the range of 900 to 1,200 nm. 
     (ii) The constructions of the first to fourth embodiments can be used not only in index-guided structure semiconductor laser devices, but also in other semiconductor laser devices having a diffraction grating, as well as in optical integrated circuits. 
     (iii) The constructions of the first to fourth embodiments can be used not only in semiconductor laser devices oscillating in a fundamental transverse mode, but also in wide-stripe index-guided semiconductor laser devices oscillating in multiple modes and having a stripe width of 3 μm or more. 
     (iv) Although n-type GaAs substrates are used in the constructions of the first to fourth embodiments, instead, p-type GaAs substrates may be used. When the GaAs substrate is a p-type, the conductivity types of all of the other layers in the constructions of the first to fourth embodiments should be inverted. 
     (v) Although the constructions of the first to fourth embodiments have a so-called single-quantum-well separate-confinement heterostructure (SQW-SCH) which includes a single quantum well and an optical waveguide made of a material having fixed composition, instead, a multiple quantum well structure made of a plurality of quantum wells may be used. 
     (vi) Each layer in the constructions of the first to fourth embodiments may be formed by molecular beam epitaxy using solid or gas raw material. 
     (vii) In addition, all of the contents of the Japanese Patent Application No. 11(1999)-222171 are incorporated into this specification by reference.