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;x 3 ≦0.4 and 0≦y 3 ≦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-y3 P y1 , where 0≦x 1 ≦0.3 and 0≦y 1 ≦0.6; an n-type In 0.49 Ga 0.51 P current confinement layer; a p-type second upper cladding layer made of In x4 Ga 1-x4 As 1-y4 P y4 , where x 4 =(0.49±0.01)y 4  and 0.4≦x 4 ≦0.46; and a p-type contact layer are formed on an n-type GaAs substrate in this order. At least the current confinement 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; 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; and 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 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 a current confinement structure and an index-guided structure, and a process for producing a u semiconductor light emitting device having a current confinement structure and an index-guided 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 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 0.6 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 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 ten 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 or 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, the conventional current semiconductor laser devices which include a current confinement structure and an index-guided structure, oscillate in a fundamental transverse mode, and emit light in the 0.9 to 1.1 μm band with high output power, are unreliable, or uneasy to produce, or have poor characteristics. 
     (5) Alternatively, in many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 μm band, a current confinement structure is provided in crystal layers which constitute the semiconductor laser devices so as to oscillate in a fundamental transverse mode. For example, Proceedings of SPIE, vol. 3628, 1999, pp.38-45 discloses an internal striped structure 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 x Ga 1-x As lower cladding layer, an n-type GaAs optical waveguide layer, an InGaAs quantum well active layer, a p-type GaAs first upper optical waveguide layer, and an n-type Al y Ga 1-y As current confinement 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 GaAs second optical waveguide layer, a p-type AlGaAs 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 GaAs 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. In addition, since the optical waveguide layer is made of GaAs, there is large current leakage, and the threshold current becomes high, although AlGaAs leak-current protection layers are provided on both sides of the active layer. 
     As in the above example, the conventional current semiconductor laser devices which contain a current confinement structure, and oscillate in a fundamental transverse mode are also unreliable, or uneasy to produce, or have poor characteristics, and the above conventional current semiconductor laser devices cannot oscillate in a fundamental transverse mode when output power is high. 
     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;x 3 ≦0.4, 0≦y 3 ≦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 Py 3  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≦x 1 ≦0.3, 0≦y 1 ≦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; a current confinement layer made of In 0.49 Ga 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; a second upper cladding layer made of In x4 Ga 1-y4 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 x 4 =(0.49±0.01)y 4  and 0.4≦x 4 ≦0.46; 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 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 carrier polarity 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 e −c s )/c s , where c s  and c e  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 (iv). 
     (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≦x 5 ≦0.3 and 0&lt;y 5 ≦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 stripe groove may have a width equal to or greater than 1 μm. 
     (iv) The compressive strain quantum well active layer may include a plurality of quantum wells. 
     (2) According to the second 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;x 3 ≦0.4, 0≦y 3 ≦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 0.85 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  of the second conductive type, and formed on the first upper cladding layer, where 0≦x 1 ≦0.3, 0≦y 1 ≦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; a current confinement layer made of In 0.49 Ga 0.51 P of the first conductive type, and formed on the etching stop layer other than a stripe area of the etching stop layer so as to form a stripe groove realizing a current injection window; a second upper cladding layer made of In x4 Ga 1-x4 A 1-y4 P y4  of the second conductive type, and formed on the current confinement layer and the stripe area of the etching stop layer, where x 4 =(0.49±0.01)y 4  and 0.4≦x 4 ≦0.46; 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 second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate. 
     Preferably, the semiconductor laser device according to the second aspect of the present invention may also have one or any possible combination of the following additional features (v) to (vii). 
     (v) 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≦x 5 ≦0.3 and 0&lt;y 5 ≦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. 
     (vi) The stripe groove may have a width equal to or greater than 1 μm. 
     (vii) The compressive strain quantum well active layer may include a plurality of quantum wells. 
     (3) According to the third 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;x 3 ≦0.4, 0≦y 3 ≦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 Py 3  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≦x 1 ≦0.3 and 0≦y 1 ≦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 Ga 0.51 P of the first conductive type, on the etching stop layer; (h) removing a stripe area of the current confinement layer so as to form a stripe groove for realizing a current injection window; (i) 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 x 4 =(0.49±0.01)y 4  and 0.4≦x 4 ≦0.46; and (j) 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 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 second aspect of the present invention can be produced by the process according to the third aspect of the present invention. 
     Preferably, the process according to the third aspect of the present invention may also have one or any possible combination of the following additional features (viii) to (xi). 
     (viii) 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≦x 5 ≦0.3 and 0&lt;y 5 ≦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 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. 
     (ix) The process may further include, after the step (g), the steps of: (g1) forming a cap layer made of GaAs; and (g2) removing a stripe area of the cap layer; and the process may further include, after the step (h), the step of (h1) removing a remaining area of the cap layer and a stripe area of the etching stop layer so as to form an additional portion of the stripe groove. That is, the semiconductor laser device according to the first aspect of the present invention can be produced by the process according to the third aspect of the present invention when the process includes the above steps (g1), (g2), and (h1). When a GaAs cap layer is used as in the above steps (g1), (g2), and (h1), it is possible to prevent formation of a natural oxidation film on the InGaP current confinement layer, and metamorphic change in the InGaP current confinement layer, which may occur when a resist layer is formed directly on the InGaP current confinement layer. In addition, since the GaAs 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 the occurrence of crystal defects. 
     (x) The cap layer may be one of the first and second conductive types. 
     (xi) The etching stop layer may be one of the first and second conductive types. 
     (4) The first and second aspects of the present invention have the following advantages. 
     (a) In the semiconductor laser devices according to the first and second aspects of the present invention, the current confinement layer is made of In 0.49 Ga 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 a 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. 
     (b) When aluminum exists near a boundary surface on which the second upper cladding layer is formed, the boundary surface is prone to oxidation, and it is difficult to realize desired characteristics in the semiconductor laser device. However, the In 0.49 Ga 0.51 P first upper cladding layer, the In x1 Ga 1-x1 As 1-y1 P y1  etching stop layer, or the In 0.49 Ga 0.51 P current confinement layer, which can be exposed at a boundary surface on which the second upper cladding layer is formed, do not include aluminum. Therefore, it is easy to form the second upper cladding layer. In addition, since no crystal defect due to oxidation of aluminum occurs, the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved. 
     (c) 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. 
     (d) Since the etching stop layer is made of InGaAsP, controllability of the stripe width by wet etching is enhanced. 
     (e) Although the InGaAsP etching stop layer does not have a stripe opening for the current injection window in the semiconductor laser device according to the second aspect of the present invention, the semiconductor laser device according to the second aspect of the present invention has the same advantages as the semiconductor laser device according to the first aspect of the present invention. 
     (f) When the tensile strain barrier layers are provided as described in the paragraphs (1)(i) and (2)(v), various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased. 
     (g) When the stripe width is equal to or greater than 1 μm, as described in paragraphs (1)(iii) and (2)(vi), the semiconductor laser device can oscillate in multiple modes with high output power and low noise. 
     (5) According to the fourth 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, and 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;x 3 ≦0.4 and 0≦y 3 ≦0.1; a first upper optical waveguide layer made of In x2 Ga 1-x2 As 1-y2 P y3 , and formed on the In x3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer, where x 2 =(0.49±0.01)y 2  and 0≦x 2 ≦0.3; a first etching stop layer made of In x6 Ga 1-x6 P of a second conductive type, and formed on the first upper optical waveguide layer, where 0.2≦x 6 ≦0.8; a second etching stop layer made of In x4 Ga 1-x1 As 1-y1 P y1 , and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where 0≦x 1 ≦0.3 and 0≦y 1 ≦0.6; a current confinement layer made of In 0.49 Ga 0.51 P of the first conductive type, and formed on the second etching stop layer so as to form a second portion of the stripe groove; a second upper optical waveguide layer made of In x2 Ga 1-x2 As 1-y2 P y2  of the second conductive type so as to cover the stripe groove, where x 2 =(0.49±0.01)y 2  and 0≦x 2 ≦0.3; an upper cladding layer made of In x4 Ga 1-x4 As 1-y4 P y4  of the second conductive type, and formed over the second upper optical waveguide layer, where x 4 =(0.49±0.01)y 4  and 0.9≦y 4 ≦1; and a contact layer of the second conductive type, formed on the upper cladding layer. In the semiconductor laser device, 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, the absolute value of a sum of a second product and a third product is equal to or smaller than 0.25 nm, where the second product is a product of the strain and the thickness of the first etching stop layer, and the third product is a product of the strain and the thickness of the second etching stop layer, and each of the lower cladding layer, the lower optical waveguide layer, the first and second upper optical waveguide layers, the current confinement layer, the upper cladding layer, and the contact layer is formed to have such composition as to lattice-match with the GaAs substrate. 
     Preferably, the semiconductor laser device according to the fourth aspect of the present invention may also have one or any possible combination of the following additional features (xii) to (xv). 
     (xii) 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≦x 5 ≦0.3 and 0&lt;y 5 ≦0.6, and the absolute value of a sum of the first product and a fourth 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. 
     (xiii) The second etching stop layer may be one of the first and second conductive types. 
     (xiv) The stripe groove may have a width equal to or greater than 1 μm. 
     (xv) The compressive strain quantum well active layer may include a plurality of quantum wells. 
     (6) According to the fifth 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;x 3 ≦0.4, 0≦y 3 ≦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 a first upper optical waveguide layer made of In x2 Ga 1-x2 As 1-y2 P y2 , on the In x3 Ga 1-x3 AS 1-y3 P y3  compressive strain quantum well active layer, where x 2 =(0.49±0.01)y 2  and 0≦x 2 ≦0.3; (e) forming a first etching stop layer made of In x6 Ga 1-x6 P of a second conductive type, on the first upper optical waveguide layer, where 0.2≦x 6 ≦0.8; (f) forming a second etching stop layer made of In x1 Ga 1-x1 As 1-y1 P y1 , on the first upper cladding layer, where 0≦x 1 ≦0.3 and 0≦y 1 ≦0.6; (g) forming a current confinement layer made of In 0.49 Ga 0.51 P of the first conductive type, on the second etching stop layer; (h) removing a stripe area of the current confinement layer and a stripe area of the second etching stop layer so as to form a stripe groove realizing a current injection window; (i) forming a second upper optical waveguide layer made of In x2 Ga 1-x2 As 1-y2 P y2  of the second conductive type, so as to cover the stripe groove, where x 2 =(0.49±0.01)y 2  and 0≦x 2 ≦0.3; (j) forming an upper cladding layer made of In x4 Ga 1-x4 As 1-y4 P y4  of the second conductive type, over the second upper optical waveguide layer, where x 4 =(0.49±0.01)y 4  and 0.9≦y 4 ≦1; (k) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, the absolute value of a sum of a second product and a third product is equal to or smaller than 0.25 nm, where the second product is a product of the strain and the thickness of the first etching stop layer, and the third product is a product of the strain and the thickness of the second etching stop layer, and each of the lower cladding layer, the lower optical waveguide layer, the first and second upper optical waveguide layers, the current confinement layer, the upper cladding layer, and the contact layer is formed to have such a composition as to lattice-match with the GaAs substrate. 
     That is, the semiconductor laser device according to the fourth aspect of the present invention can be produced by the process according to the fifth aspect of the present invention. 
     Preferably, the process according to the fifth aspect of the present invention may also have one or any possible combination of the following additional features (xvi) to (xix). 
     (xvi) The process may further include the steps of: (b1) after the step (b), forming a first tensile strain barrier layer made of In x   5 Ga 1-x5 As 1-y5 P y5 , on the lower optical waveguide layer, where 0≦x 5 ≦0.3 and 0&lt;y 5 ≦0.6; and (c1) after the step (c), forming a second tensile strain barrier layer made of In x5 Ga 1-x5 As 1-x5 P y5 , on the compressive strain quantum well active layer, where the absolute value of a sum of the first product and a fourth 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. 
     (xvii) The process may further include, after the step (g), the step of (g1) forming a cap layer made of GaAs, and the step (h) comprising the substeps of: (h1) removing a stripe area of the cap layer and the stripe area of the current confinement layer, and (h2) removing a remaining area of the cap layer and the stripe area of the second etching stop layer. When a GaAs cap layer is used as in the above steps (g1) and (h1), it is possible to prevent formation of a natural oxidation film on the InGaP current confinement layer, and metamorphic change in the InGaP current confinement layer, which occurs when a resist layer is formed directly on the InGaP current confinement layer. In addition, since the GaAs cap layer is removed before the second upper optical waveguide layer is formed, it is possible to remove a residue left on the boundary surface on which the second upper optical waveguide layer is formed, and prevent the occurrence of crystal defects. 
     (xviii) The second etching stop layer may be one of the first and second conductive types. 
     (xix) The cap layer may be one of the first and second conductive types. 
     (7) The fifth aspect of the present invention has the following advantages. 
     (a) In the semiconductor laser device according to the third aspect of the present invention, the current confinement layer is made of In 0.49 Ga 0.51 P, and the second upper optical waveguide layer is made of In x4 Ga 1-x4 As 1-y4 P y4 . Therefore, the difference in the refractive index between the current confinement layer and the second upper optical waveguide layer realizes a difference of 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. 
     (b) Deterioration of light exit end surfaces of semiconductor laser devices which emit laser light with high output power can be effectively prevented by increasing the thickness of the optical waveguide layer, since the peak optical density is reduced when the thickness of the optical waveguide layer is increased. However, in the conventional semiconductor laser devices having a current confinement structure and an index-guided structure, it is impossible to increase the thickness of a portion between the current confinement layer and the active layer since a fundamental transverse mode must be achieved. In particular, the thickness of the optical waveguide layer is limited since the optical waveguide layer is located between the current confinement layer and the active layer on the other hand, in the semiconductor laser device according to the fifth aspect of the present invention, the thickness of the optical waveguide layer is substantially increased by arranging the In x1 Ga 1-x1 As 1-y1 P y1  second upper optical waveguide layer above the current confinement layer, where the second upper optical waveguide layer has the same composition as the first upper optical waveguide layer. Therefore, it is possible to reduce the peak optical density and the deterioration of a light exit end surface which is caused by high optical density. Thus, reliability is increased. 
     (c) Compared with the conventional semiconductor laser device, the difference in the band gap between the active layer and the In x1 Ga 1-x1 As 1-y1 P y1  second upper optical waveguide layer can be increased by provision of the In x1 Ga 1-x1 As 1-y1 P y1  second upper optical waveguide layer. Therefore, it is possible to prevent leakage currents, and efficiently confine the carriers. Thus, the threshold current can be lowered. 
     (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) The first etching stop layer is made of In x6 Ga 1-x6 P y1 , and the second etching stop layer made of In x1 Ga 1-x1 As 1-y1 P y1  is formed on the first etching stop layer. Therefore, when a sulfuric acid etchant is used, only the In x1 Ga 1-x6 As 1-y1 P y1  second etching stop layer is etched, and the In x6 Ga 1-x6 P first etching stop layer is not etched. That is, it is possible to stop etching accurately on the surface of the first etching stop layer, and thus the stripe width can be accurately controlled by wet etching. 
     (f) Since the In 0.49 Ga 0.51 P first upper cladding layer, the In x1 Ga 1-x1 As 1-y1 P y1  etching stop layer, or the In 0.49 Ga 0.51 P current confinement layer, which can be exposed at a boundary surface on which the second upper optical waveguide layer is formed, does not include aluminum, it is easy to form the second upper optical waveguide layer. In addition, since no crystal defect due to oxidation of aluminum occurs, the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved. 
     (g) when the tensile strain barrier layers are provided as described in paragraphs (5) (xii), various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased. 
     (h) When the stripe width is equal to or greater than 1 μm, as described in paragraphs (5) (xiv), the semiconductor laser device can oscillate in multiple modes with high output power and low noise. 
    
    
     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. 
     FIGS. 5A to  5 D are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the fifth embodiment. 
     FIGS. 6A to  6 D are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the sixth embodiment. 
     FIGS. 7A to  7 D are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the seventh embodiment. 
     FIGS. 8A to  8 D are cross-sectional views of representative stages in the process for producing a semiconductor laser device as the eighth 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  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), an In x3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer  14  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), a p-type or i-type (intrinsic) In x2 Ga 1-x2 As 1-y2 P y2  optical waveguide layer  15  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦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≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, 20 nm, an n-type In 0.49 Ga 0.51 P current confinement layer  18  having a thickness of, for example, 1 μm, and an n-type GaAs cap layer  19  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  20  is formed over the n-type GaAs cap layer  19 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction of the SiO 2  film  20  is removed by normal photolithography. 
     Next, as illustrated in FIG. 1B, the n-type GaAs cap layer  19  is etched with a sulfuric acid etchant by using the SiO 2  film  20  as a mask. Then, the exposed area of the n-type In 0.49 Ga 0.51 P current confinement layer  18  is 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  20  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  19  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  (x 4 =(0.49±0.01)y 4 , 0.4≦x 4 ≦0.46) and a p-type GaAs contact layer  22  are formed over the construction of FIG.  1 C. Then, a p electrode  23  is formed on the p-type GaAs contact layer  22 . In addition, the exposed surface of the substrate  11  is polished, and an n electrode  24  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 a 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 Ga 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  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  34  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), an In x3 Ga 1-x3 ,As 1-y3 P y3  compressive strain quantum well active layer  35  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  36  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), a p-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  optical waveguide layer  37  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦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≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, 20 nm, an n-type In 0.49 Ga 0.51 P current confinement layer  40  having a thickness of, for example, 1 μm, and an n-type GaAs cap layer  41  are formed on an n-type GaAs substrate  31  by organometallic vapor phase epitaxy. Then, a SiO 2  film  42  is formed over the n-type GaAs cap layer  41 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction of the SiO 2  film  42  is removed by normal photolithography. 
     Next, as illustrated in FIG. 2B, the n-type GaAs cap layer  41  is etched with a sulfuric acid etchant by using the SiO 2  film  42  as a mask. Then, the exposed area of the n-type In 0.49 Ga 0.51 P current confinement layer  40  is 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  42  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  41  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, a p-type In x4 Ga 1-x4 As 1-y4 P y4  second upper cladding layer  43  (x 4 =(0.49±0.01)y 4 , 0.4≦x 4 ≦0.46) and a p-type GaAs contact layer  44  are formed over the construction of FIG.  2 C. Then, a p electrode  45  is formed on the p-type GaAs contact layer  44 . In addition, the exposed surface of the substrate  31  is polished, and an n electrode  46  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≦z 1 ≦0.7), an n-type or i-type Al z2 Ga 1-z2 As optical waveguide layer  53  (0≦z 2 ≦0.2), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  54  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), an In x   3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer  55  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  56  (0&lt;x 5 ≦0.3, 0&lt;y 5 ≦0.6), a p-type or i-type Al z2 Ga 1-z2 As optical waveguide layer  57  (0≦z 2 ≦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≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, 20 nm, an n-type In 0.49 Ga 0.51 P current confinement layer  60  having a thickness of, for example, 1 μm, and an n-type GaAs cap layer  61  having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate  51  by organometallic vapor phase epitaxy. Then, a Si 0   2  film  62  is formed over the n-type GaAs cap layer  61 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction of the SiO 2  film  62  is removed by normal photolithography. 
     Next, as illustrated in FIG. 3B, the n-type GaAs cap layer  61  is etched with a sulfuric acid etchant by using the SiO 2 , film  62  as a mask. Then, the exposed area of the n-type In 0.49 Ga 0.51 P current confinement layer  60  is 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  62  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  61  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, a p-type In x4 Ga 1-x4 As 1-y4 P y4  second upper cladding layer  63  (x 4 =(0.49±0.01)y 4 , 0.4≦x 4 ≦0.46) and a p-type GaAs contact layer  64  are formed over the construction of FIG.  3 C. Then, a p electrode  65  is formed on the p-type GaAs contact layer  64 . In addition, the exposed surface of the substrate  51  is polished, and an n electrode  66  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 a 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  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  74  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), an In x3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer  75  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  76  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), a p-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  optical waveguide layer  77  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), a p-type In 0.49 Ga 0.51 P first upper cladding layer  78 , a p-type In x1 Ga 1-x1 As 1-y1 P y1  etching stop layer  79  (0≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, 20 nm, and an n-type In 0.49 Ga 0.51  P current confinement layer  80  having a thickness of, for example, 1 μm 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 Ga 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 current confinement layer  80  is etched with a hydrochloric acid etchant by using the SiO 2  film  82  as a mask until the 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. 
     Thereafter, as illustrated in FIG. 4C, a p-type In x4 Ga 1-x4 As 1-y4 P y4  second upper cladding layer  83  (x 4 =(0.49±0.01)y 4 , 0.4≦x 4 ≦0.46) 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, in the fourth embodiment, a stripe area of the p-type In x1 Ga 1-x1 As 1-y1 P y1  etching stop layer  79  is not removed from the above construction. Alternatively, the p-type In x4 Ga 1-x4 As 1-y4 P y4  second upper cladding layer  83  and the p-type GaAs contact layer  84  may be formed after the p-type In x1 Ga 1-x1 As 1-y1 P y1  etching stop layer  79  is removed. 
     Fifth Embodiment 
     FIGS. 5A to  5 D show cross sections of the representative stages in the process for producing a semiconductor laser device as the fifth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     First, as illustrated in FIG. 5A, an n-type In 0.49 Ga 0.51 P lower cladding layer  212 , an n-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  optical waveguide layer  213  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), an In x3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer  214  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), a p-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  215 , a p-type In x6 Ga 1-x6 P first etching stop layer  216  (0.2≦x 6 ≦0.8) having a thickness of, for example, about 10 nm, a p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  217  (0≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, about 10 nm, an n-type In 0.49 Ga 0.51 P current confinement layer  218  having a thickness of, for example, 1 μm, and an n-type GaAs cap layer  219  having a thickness of, for example, about 10 nm are formed on an n-type GaAs substrate  211  by organometallic vapor phase epitaxy. Then, a SiO 2  film  220  is formed over the n-type GaAs cap layer  219 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction of the SiO 2  film  220  is removed by normal photolithography. 
     Next, as illustrated in FIG. 5B, the n-type GaAs cap layer  219  is etched with a sulfuric acid etchant by using the SiO 2  film  220  as a mask. Then, the exposed area of the n-type In 0.49 Ga 0.51 P current confinement layer  218  is etched with a hydrochloric acid etchant until the p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  217  is exposed. 
     Thereafter, as illustrated in FIG. 5C, the remaining areas of the SiO 2 , film  220  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  219  and the exposed area of the p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  217  are removed by etching using a sulfuric acid etchant until the p-type In x6 Ga 1-x6 P first etching stop layer  216  is exposed. 
     Finally, as illustrated in FIG. 5D, a p-type In x2 Ga 1-x2 As 1-y2 P y2  second upper optical waveguide layer  221  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), a p-type In 0.49 Ga 0.51 P upper cladding layer  222 , and a p-type GaAs contact layer  223  are formed over the construction of FIG.  5 C. Then, a p electrode  224  is formed on the p-type GaAs contact layer  223 . In addition, the exposed surface of the substrate  211  is polished, and an n electrode  225  is formed on the polished surface of the substrate  211 . 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 x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  215  has such a thickness that oscillation in a fundamental transverse mode can be maintained even when output power becomes high. 
     In addition, in the construction of the fifth embodiment, the n-type GaAs cap layer  219  may be dispensed with. 
     Sixth Embodiment 
     FIGS. 6A to  6 D show cross sections of the representative stages in the process for producing a semiconductor laser device as the sixth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     First, as illustrated in FIG. 6A, an n-type In 0.49 Ga 0.51 P lower cladding layer  232 , an n-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  optical waveguide layer  233  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  234  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), an In x3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer  235  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  236 , a p-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  237 , a p-type In x6 Ga 1-x6 P first etching stop layer  238  (0.2≦x 6 ≦0.8) having a thickness of, for example, 10 nm, an n-type or p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  239  (0≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, 10 nm, an n-type In 0.49 Ga 0.51 P current confinement layer  240  having a thickness of, for example, 1 μm, and an n-type GaAs cap layer  241  having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate  231  by organometallic vapor phase epitaxy. Then, a SiO 2  film  242  is formed over the n-type GaAs cap layer  241 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction of the SiO 2  film  242  is removed by normal photolithography. 
     Next, as illustrated in FIG. 6B, the n-type GaAs cap layer  241  is etched with a sulfuric acid etchant by using the SiO 2  film  242  as a mask. Then, the exposed area of the n-type In 0.49 Ga 0.51 P current confinement layer  240  is etched with a hydrochloric acid etchant until the n-type or p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  239  is exposed. 
     Thereafter, as illustrated in FIG. 6C, the remaining areas of the SiO 2  film  242  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  241  and the exposed area of the n-type or p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  239  are removed by etching using a sulfuric acid etchant until the p-type In x6 Ga 1-x6 P first etching stop layer  238  is exposed. 
     Finally, as illustrated in FIG. 6D, a p-type In x2 Ga 1-x2 As 1-y2 P y2  second upper optical waveguide layer  243  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), a p-type In x4 Ga 1-x4 As 1-y4 P y4  upper cladding layer  244  (x 4 =(0.49±0.01)y 4 , 0.9≦y 4 ≦1), and a p-type GaAs contact layer  245  are formed over the construction of FIG.  6 C. Then, a p electrode  246  is formed on the p-type GaAs contact layer  245 . In addition, the exposed surface of the substrate  231  is polished, and an n electrode  247  is formed on the polished surface of the substrate  231 . 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 x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  237  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 sixth embodiment, the compressive strain quantum well active layer is sandwiched between the tensile strain barrier layers. Therefore, compared with the fifth embodiment, characteristics are improved (e.g., the threshold current is lowered), and reliability is increased. 
     Seventh Embodiment 
     FIGS. 7A to  7 D show cross sections of the representative stages in the process for producing a semiconductor laser device as the seventh embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     First, as illustrated in FIG. 7A, an n-type Al z1 Ga 1-z1 As lower cladding layer  252  (0.35≦z 1 ≦0.7), an n-type or i-type Al z2 Ga 1-z2 As optical waveguide layer  253  (0≦z 2 ≦0.2), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  254  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), an In x3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer  255  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  256 , a p-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  257  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), a p-type In x6 Ga 1-x6 P first etching stop layer  258  (0.2≦x 6 ≦0.8) having a thickness of, for example, about 10 nm, an n-type or p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  259  (0≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, about 10 nm, an n-type In 0.49 Ga 0.51 P current confinement layer  260  having a thickness of, for example, 1 μm, and an n-type GaAs cap layer  261  having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate  251  by organometallic vapor phase epitaxy. Then, a SiO 2  film  262  is formed over the n-type GaAs cap layer  261 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction of the SiO 2  film  262  is removed by normal photolithography. 
     Next, as illustrated in FIG. 7B, the n-type GaAs cap layer  261  is etched with a sulfuric acid etchant by using the SiO 2  film  262  as a mask. Then, the exposed area of the n-type In 0.49 Ga 0.51 P current confinement layer  260  is etched with a hydrochloric acid etchant until the n-type or p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  259  is exposed. 
     Thereafter, as illustrated in FIG. 7C, the remaining areas of the SiO 2  film  262  are removed by etching using a fluoric acid etchant. Then, the n-type GaAs cap layer  261  and the exposed area of the n-type or p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  259  are removed by etching using a sulfuric acid etchant until the p-type In x6 Ga 1-x6 P first etching stop layer  258  is exposed. 
     Finally, as illustrated in FIG. 7D, a p-type In x2 Ga 1-x2 As 1-y2 P y2  second upper optical waveguide layer  263  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), a p-type In x4 Ga 1-x4 As 1-y4 P y4  upper cladding layer  264  (x 4 =(0.49±0.01)y 4 , 0.9≦y 4 ≦1), and a p-type GaAs contact layer  265  are formed over the construction of FIG.  7 C. Then, a p electrode  266  is formed on the p-type GaAs contact layer  265 . In addition, the exposed surface of the substrate  251  is polished, and an n electrode  267  is formed on the polished surface of the substrate  251 . 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 x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  257  has such a thickness that oscillation in a fundamental transverse mode can be maintained even when output power becomes high. 
     Eighth Embodiment 
     FIGS. 8A to  8 D show cross sections of the representative stages in the process for producing a semiconductor laser device as the eighth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device. 
     First, as illustrated in FIG. 8A, an n-type In 0.49 Ga 0.51 P lower cladding layer  272 , an n-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  optical waveguide layer  273  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  274  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), an In x3 Ga 1-x3 As 1-y3 P y3  compressive strain quantum well active layer  275  (0&lt;x 3 ≦0.4, 0≦y 3 ≦0.1), an In x5 Ga 1-x5 As 1-y5 P y5  tensile strain barrier layer  276  (0≦x 5 ≦0.3, 0&lt;y 5 ≦0.6), a p-type or i-type In x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  277  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), a p-type In x6 Ga 1-x6 P first etching stop layer  278  (0.2≦x 6 ≦0.8) having a thickness of, for example, about 10 nm, an n-type or p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  279  (0≦x 1 ≦0.3, 0≦y 1 ≦0.6) having a thickness of, for example, about 10 nm, and an n-type In 0.49 Ga 0.51 P current confinement layer  280  having a thickness of, for example, 1 μm are formed on an n-type GaAs substrate  271  by organometallic vapor phase epitaxy. Then, a SiO 2  film  281  is formed over the n-type In 0.49 Ga 0.51 P current confinement layer  280 , and a stripe area having a width of about 3 μm and extending in the &lt;011&gt; direction of the SiO 2  film  281  is removed by normal photolithography. 
     Next, as illustrated in FIG. 8B, the n-type In 0.49 Ga 0.51 P current confinement layer  280  is etched with a hydrochloric acid etchant by using the SiO 2  film  281  as a mask until the p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  279  is exposed. 
     Thereafter, as illustrated in FIG. 8C, the remaining areas of the SiO 2  film  281  are removed by etching using a fluoric acid etchant. Then, the exposed area of the p-type In x1 Ga 1-x1 As 1-y1 P y1  second etching stop layer  279  is removed by etching using a sulfuric acid etchant as a mask until the p-type In x   6 Ga 1-x6 P first etching stop layer  278  is exposed. 
     Finally, as illustrated in FIG. 8D, a p-type In x2 Ga 1-x2 As 1-y2 P y2  second upper optical waveguide layer  282  (x 2 =(0.49±0.01)y 2 , 0≦x 2 ≦0.3), a p-type In x4 Ga 1-x4 As 1-y4 P y4  upper cladding layer  283  (x 4 =(0.49±0.01)y 4 , 0.9≦y 4 ≦1), and a p-type GaAs contact layer  284  are formed over the construction of FIG.  8 C. Then, a p electrode  285  is formed on the p-type GaAs contact layer  284 . In addition, the exposed surface of the substrate  271  is polished, and an n electrode  286  is formed on the polished surface of the substrate  271 . 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 x2 Ga 1-x2 As 1-y2 P y2  first upper optical waveguide layer  277  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 eighth embodiment, no cap layer is provided. Namely, the semiconductor laser device according to the present invention can be produced without a 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;x 3 ≦0.4, 0≦y 3 ≦0.1), the oscillation wavelengths of the semiconductor laser devices as the first to eighth embodiments can be controlled in the range of 900 to 1,200 nm. 
     (ii) The constructions of the first to eighth embodiments can be used not only in index-guided structure semiconductor laser devices, but also in other semiconductor laser devices having a diffraction lattice, as well as in optical integrated circuits. 
     (iii) Although n-type GaAs substrates are used in the constructions of the first to eighth 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 eighth embodiments should be inverted. 
     (iv) Although the constructions of the first to eighth 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 a fixed composition, instead, a multiple quantum well structure made of a plurality of quantum wells may be used. 
     (v) Each layer in the constructions of the first to eighth embodiments may be formed by molecular beam epitaxy using solid or gas raw material. 
     (vi) The stripe width in the constructions of the first to fourth embodiments may be 1 μm or more. 
     (vii) The constructions of the fifth to eighth 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 1 μm or more. 
     (viii) In addition, all of the contents of the Japanese patent applications Nos. 11(1999)-222168 and 11(1999)-222169 are incorporated into this specification by reference.