Abstract:
In a semiconductor laser device, an active region, including a quantum well layer sandwiched between upper and lower optical waveguide layers, is formed on a substrate. A near-edge portion of the active region is etched down to a mid-thickness of the lower optical waveguide layer. A non-absorbing layer, made of a semiconductor material having a bandgap greater than photon energy of laser light generated in the active region, is formed over the active region. An etching stop layer is formed at the mid-thickness location in the lower optical waveguide layer so as to selectively stop the etching of the near-edge portion of the active region. An electron barrier layer, made of a semiconductor material having a bandgap greater than the bandgap of the upper optical waveguide layer, is formed at a mid-thickness location in the upper optical waveguide layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser device having an end-facet window structure, i.e., a structure which makes an end facet nonabsorbent of oscillation light. 
     2. Description of the Related Art 
     J. K. Wade et al. (“6.1 W continuous wave front-facet power from Al-free active-region (λ=805 nm) diode lasers,” Applied Physics Letters, vol. 72, No. 1 (1998) pp.4-6) disclose a semiconductor laser device which emits light in the 805 nm band. The semiconductor laser device comprises an Al-free InGaAsP active layer, an InGaP optical waveguide layer, and InAlGaP cladding layers. In addition, in order to improve the characteristics in the high output power range, the semiconductor laser device includes a so-called large optical cavity (LOC) structure in which the thickness of the optical waveguide layer is increased so as to reduce the light density, and increase the maximum light output power. However, when the optical power is maximized, currents generated by optical absorption in the vicinity of end facets generate heat, i.e., raise the temperature at the end facets. In addition, the raised temperature reduces the bandgap at the end facets, and therefore the optical absorption is further enhanced to damage the end facet. That is, a vicious cycle is formed. This damage is the so-called catastrophic optical mirror damage (COMD). When the optical power reaches the COMD level, the optical output deteriorates with time. Further, the semiconductor laser device is likely to suddenly break down due to the COMD. Therefore, the above semiconductor laser device is not reliable when the semiconductor laser device operates with high output power. 
     In addition, T. Fukunaga et al. (“Highly Reliable Operation of High-Power InGaAsP/In 0.48 Ga 0.52 P/AlGaAs 0.8 μm Separate Confinement Heterostructure Lasers,” Japanese Journal of Applied Physics, vol. 34 (1995) L1175-L1177) disclose a semiconductor laser device which comprises an Al-free active layer, and emits light in the 0.8 μm band. In the semiconductor laser device, an n-type AlGaAs cladding layer, an intrinsic (i-type) InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-type InGaP optical waveguide layer, a p-type AlGaAs cladding layer, and a p-type GaAs cap layer are formed on an n-type GaAs substrate. S. O&#39;Brien, H. Zhao, and R. J. Lang report, in Electronics Letters, vol. 34, No. 2 (1998) p.184, that the maximum output power of the above semiconductor laser device disclosed by Fukunaga et al, is 1.8 W. They also report the maximum breakdown light output power of multiple-transverse-mode semiconductor laser devices having a stripe width of 50 micrometers or greater. For example, at the wavelength of 0.87 micrometers, the maximum breakdown light output power of a multiple-transverse-mode semiconductor laser device having a stripe width of 100 micrometers is reported to be 1.3 W, and the maximum breakdown light output power of a multiple-transverse-mode semiconductor laser device having a stripe width of 200 micrometers is reported to be 16.5 W. 
     Further, the present inventor, T. Hayakawa, and others report, in Applied Physics Letters, Vol. 75, No. 13 (1999) p. 1839, that the practical light output power of 1.5 W is achieved in continuous oscillation of a semiconductor laser device having a stripe width of 50 micrometers when the semiconductor laser device is designed to increase the beam width in the direction perpendicular to the active layer, lower the peak optical strength, and minimize the temperature raise at a light-exit end facet. However, it is difficult to increase the reliability and the practical light output power of the semiconductor laser device by a large amount. 
     In order to solve the above problems, the Japanese Patent Application No. 11(1999)-348527 and the copending U.S. patent application Ser. No. 09/731,702, “HIGH-POWER SEMICONDUCTOR LASER DEVICE IN WHICH NEAR-EDGE PORTIONS OF ACTIVE LAYER ARE REMOVED”, corresponding to the Japanese patent application and being filed on Dec. 8, 2000 by Toshiaki Fukunaga and assigned to the same assignee as the present patent application, disclose a semiconductor laser device in which transparent regions are formed in vicinities of end facets with Al-free material. However, the transparent regions are required to be formed by crystal regrowth, and the crystal regrowth is initiated from a surface of an optical waveguide layer located near to the quantum well active layer. That is, portions of the regrowth boundary are near the quantum well. In addition, there is no energy barrier between the quantum well active layer and the other portions of the regrowth boundary. In the semiconductor laser device formed as above, carriers leaked from the active layer and diffused to the regrowth boundary cause non-radiative recombination at the regrowth boundary. Therefore, the efficiency of the semiconductor laser device decreases, and degradation is promoted. Further, before the regrowth, the regions in the vicinities of the end facets must be etched to the depth of a crystal layer which is located immediately below the active layer. Therefore, it is not easy to control the depth of the etching. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a semiconductor laser device in which regions in vicinities of end facets are made of a material being non-absorbent of oscillation light so that non-radiative recombination at a regrowth boundary is prevented, and the reliability and performance of the semiconductor laser device are improved. 
     According to the present invention, there is provided a semiconductor laser device comprising a substrate, an active region formed above the substrate, and a non-absorbing layer formed over the active region, and made of a semiconductor material having a bandgap greater than the photon energy of laser light which oscillates in the semiconductor laser device. The active region includes a first lower optical waveguide layer formed above the substrate, an etching stop layer formed on the first lower optical waveguide layer except for near-edge areas of the first lower optical waveguide layer, a second lower optical waveguide layer formed on the etching stop layer, a quantum well active layer formed on the second lower optical waveguide layer, a first upper optical waveguide layer formed on the quantum well active layer, an electron barrier layer formed on the first upper optical waveguide layer and made of a semiconductor material having a bandgap greater than a bandgap of the first upper optical waveguide layer, and a second upper optical waveguide layer formed on the electron barrier layer, where the near-edge areas are located adjacent to opposite end facets which are perpendicular to the direction of the laser light. The etching stop layer has such a chemical property that the etching stop layer can be maintained when the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, and the electron barrier layer are etched, and the first lower optical waveguide layer can be maintained when the etching stop layer is etched. 
     When the semiconductor laser device according to the present invention is produced, first, a first lower optical waveguide layer, an etching stop layer, a second lower optical waveguide layer, a quantum well active layer, a first upper optical waveguide layer, an electron barrier layer, and a second upper optical waveguide layer are formed above the substrate in this order. Next, near-edge portions (i.e., portions in vicinities of end facets which are perpendicular to the light axis) of the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, the electron barrier layer, and the second upper optical waveguide layer are removed by selective etching. Then, near-edge portions of the etching stop layer are also removed by selective etching. The etching stop layer has such a chemical property that the etching stop layer can be maintained when the second lower optical waveguide layer, the quantum well active layer, the first upper optical waveguide layer, the electron barrier layer, and the second upper optical waveguide layer are etched, and the first lower optical waveguide layer can be maintained when the etching stop layer is etched. Therefore, the etching of the above near-edge portions can be stopped at the upper surface of the first lower optical waveguide layer with high accuracy. Thereafter, a non-absorbing layer, which is made of a semiconductor material having a bandgap greater than the photon energy of laser light which oscillates in the semiconductor laser device, is formed over the active region. Since the upper surface of the first lower optical waveguide layer which is apart from the quantum well active layer is a regrowth boundary, carriers leaked and diffused from the quantum well active layer do not cause non-radiative recombination at the regrowth boundary. Therefore, it is possible to prevent the decrease in the efficiency due to the non-radiative recombination and the degradation of the end facet due to heat generation. Thus, the performance and reliability of the semiconductor laser device can be improved. 
     In addition, since the electron barrier layer is formed between the first and second upper optical waveguide layers, and made of a semiconductor material having a bandgap greater than the bandgap of the first upper optical waveguide layer, it is possible to prevent leakage of carriers from the active layer to a regrowth boundary located above the second upper optical waveguide layer. Therefore, at the regrowth boundary, no non-radiative recombination is caused by the carriers leaked from the active layer. Thus, the decrease in the efficiency due to the non-radiative recombination and the degradation of the end facet due to heat generation can be prevented. 
     Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) and (ii). 
     (i) The quantum well active layer may be made of an aluminum-free semiconductor material. It is well known that when an active layer does not contain aluminum, composition change due to oxidation of aluminum can be prevented, and the reliability of the semiconductor laser device can be increased. However, in particular, when the quantum well active layer in the semiconductor laser device according to the present invention is made of an aluminum-free semiconductor material, the reliability of the semiconductor laser device can be remarkably increased. 
     (ii) The non-absorbing layer and a semiconductor layer immediately under the non-absorbing layer may be made of an aluminum-free semiconductor material. In this case, both of the regrown layer and the base layer of the regrowth are aluminum-free. Therefore, the reliability of the semiconductor laser device can be remarkably increased. 
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B and  1 C are cross-sectional views of a semiconductor laser device as the first embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of a semiconductor laser device as the second embodiment of the present invention. 
     FIG. 3 is a cross-sectional view of a semiconductor laser device as the third embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of a semiconductor laser device as the fourth embodiment of the present invention. 
     FIGS. 5A,  5 B and  5 C are cross-sectional views of a semiconductor laser device as the fifth embodiment of the present invention. 
     FIGS. 6A,  6 B and  6 C are cross-sectional views of a semiconductor laser device as the sixth embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained in detail below with reference to drawings. 
     First Embodiment 
     The construction of a semiconductor laser device  1  as the first embodiment of the present invention and a process of producing the construction are explained below with reference to FIGS. 1A to  1 C, which are cross-sectional views of the semiconductor laser device  1  as the first embodiment. FIG. 1A is a first cross-sectional view illustrating a first cross section (the C-C′ cross section indicated in FIG. 1B) parallel to a resonator axis of the semiconductor laser device  1 , FIG. 1B is a second cross-sectional view illustrating a second cross section (the A-A′ cross section indicated in FIG.  1 A), and FIG. 1C is a third cross-sectional view illustrating a third cross section (the B-B′ cross section indicated in FIG.  1 A). 
     As illustrated in FIG. 1A, the right and left ends are cleaved mirror surfaces (end facets). In the first MOCVD (metal organic chemical vapor deposition) stage, an n-type GaAs buffer layer  12  being doped with 5×10 17  cm −3  Si and having a thickness of 0.5 micrometers, an n-type Al x Ga 1-x As graded buffer layer  13  being doped with 5×10 17  cm −3  Si and having a thickness of 0.2 micrometers (where x gradually increases from 0.1 to 0.63), an n-type Al 0.63 Ga 0.37 As lower cladding layer  14  being doped with 5×10 17  cm −3  Si and having a thickness of 1.5 micrometers, an n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  15  being doped with 5×10 17  cm −3  Si and having a thickness of 0.4 micrometers, an undoped AlGaAs etching stop layer  16  having a thickness of 20 nm, an undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  17  having a thickness of 0.1 micrometers, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  18  having a thickness of 10 nm, an undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  19  having a thickness of 0.1 micrometers, an undoped AlGaAs electron barrier layer  20  having a thickness of 20 nm, a p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  21  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.05 micrometers are formed in this order by reduced-pressure MOCVD on an n-type GaAs substrate  11  which is doped with 2×10 18  cm −3  Si. 
     Next, near-edge portions W (i.e., portions in vicinities of end facets) of the above layered structure are removed by photolithography and chemical etching, as explained below. The width of the near-edge portions W is 25 micrometers. Since, in practice, the above layered structure is formed on a wafer for concurrently producing a plurality of semiconductor laser devices, stripe regions each having a width of 50 micrometers and straddling a boundary between the semiconductor laser devices are removed from the layered structure on the wafer. First, stripe areas of the p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  21  are etched off with HCl, and stripe areas of the undoped AlGaAs electron barrier layer  20  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. Next, stripe areas of the undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  19  are etched off with HCl, stripe areas of the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  18  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O, and stripe areas of the undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  17  are etched off with HCl. Then, the remaining resist and the like are removed, and the wafer is washed. Finally, stripe areas of the undoped AlGaAs etching stop layer  16  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. Since the n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  15  exposed by the removal of the stripe areas of the undoped AlGaAs etching stop layer  16  are not etched, the exposed surfaces of the stripe areas of the n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  15  are cleaned by the washing process after the etching. 
     Thereafter, in the second MOCVD stage, a p-type In 0.48 Ga 0.52 P third upper optical waveguide layer  22  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.35 micrometers, a p-type Al 0.63 Ga 0.37 As upper cladding layer  23  being doped with 7×10 17  cm −3  Zn and having a thickness of 2 micrometers, and a p-type GaAs cap layer  24  being doped with 2×10 19  cm −3  Zn and having a thickness of 0.1 micrometers are formed in this order. Then, near-edge portions of the p-type GaAs cap layer  24  are selectively removed by etching with a mixed solution of NH 4 OH and H 2 O 2 , as illustrated in FIG.  1 A. As illustrated in FIG. 1C, the p-type GaAs cap layer  24  does not appear on the end facet. 
     Next, a pair of stripe grooves each having a width of 10 micrometers are formed in the p-type GaAs cap layer  24  and the p-type Al 0.63 Ga 0.37 As upper cladding layer  23  by photolithography and chemical etching with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O so that a ridge stripe structure having a width of 50 micrometers is formed between the pair of stripe grooves as illustrated in FIG.  1 B. Due to the use of the mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O, the etching automatically stops just above the p-type In 0.48 Ga 0.52 P third upper optical waveguide layer  22 . Then, a SiO 2  insulation film  25  is formed on the above layered structure by plasma CVD, and then a portion of the SiO 2  insulation film  25  on the top surface of the ridge stripe structure, except for the near-edge portions under which the p-type GaAs cap layer  24  is removed, is removed by using diluted HF. 
     Thereafter, a (Ti/Pt/Ti/Pt/Au) p electrode  26  is formed by evaporation and heat treatment, and the bottom surface of the n-type GaAs substrate  11  is polished until the total thickness of the layered structure becomes about 100 micrometers. Then, an (AuGe/Ni/Au) n electrode  27  is formed by evaporation and heat treatment. 
     Next, a laser bar having a length of about 1 cm and a resonator length of 1.5 mm is cut out from the wafer formed as above by scribing with a diamond needle and cleaving, and optical coatings are provided on the light-exit end facet and the opposite end facet so that the light-exit end facet has a reflectance of 8%, and the opposite end facet has a reflectance of 95%. Then, discrete laser chips having a width of about 500 micrometers are cut out by scribing with a diamond needle and cleaving. Finally, the p-electrode side of each laser chip is bonded to a copper heatsink with indium solder having a thickness of 4 to 5 micrometers, and the performance of the semiconductor laser device is evaluated. The semiconductor laser device is oscillated at the wavelength of about 809 nm above a threshold current of about 120 mA at room temperature. As a result, no kink is observed in the current-light output characteristic, and it is found that the semiconductor laser device as the first embodiment of the present invention can operate with high output power of 5 W or higher. 
     In the semiconductor laser device as the first embodiment of the present invention, the Al composition of the undoped AlGaAs electron barrier layer  20  can be determined to be in such a range that the energy gap of the undoped AlGaAs electron barrier layer  20  is greater than the energy gaps of the undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  19  and the p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  21 . 
     In addition, the Al composition of the undoped AlGaAs etching stop layer  16  may be identical to the Al composition of the undoped AlGaAs electron barrier layer  20 . 
     Further, the selective, chemical etching of each of the n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  15 , the undoped AlGaAs etching stop layer  16 , and the undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  17  is possible regardless of the Al composition of the undoped AlGaAs etching stop layer  16 . In this regard, the composition of the undoped AlGaAs etching stop layer  16  can be expressed as Al v Ga 1-v As (0≦v≦1). 
     Since, in the first embodiment, the undoped AlGaAs etching stop layer  16  is arranged between the n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  15  and the undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  17 , which realize a lower optical waveguide layer, the controllability of the etching is very high. Therefore, the n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  15 , which is apart from the quantum well layer, can be the base of the regrowth. Thus, the semiconductor laser device as the first embodiment is free from the influence of the non-radiative recombination caused by electrons leaked from the active layer. For example, the semiconductor laser device as the first embodiment is less prone to the efficiency reduction, degradation, and the like. That is, the performance and reliability are improved. 
     Furthermore, since the undoped AlGaAs electron barrier layer  20  is formed between the undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  19  and the p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  21 , which realize an upper optical waveguide layer, the leakage electrons which reach the regrowth boundary (i.e., the boundary between the p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  21  and the p-type In 0.48 Ga 0.52 P third upper optical waveguide layer  22 ) can be reduced. 
     Second Embodiment 
     The construction of a semiconductor laser device  2  as the second embodiment of the present invention and a process of producing the construction are explained below with reference to FIG. 2, which is a cross-sectional view of the semiconductor laser device  2  as the second embodiment. FIG. 2 is a cross-sectional view illustrating a cross section parallel to a resonator axis of the semiconductor laser device. The semiconductor laser device as the second embodiment has a full-face-electrode structure. 
     As illustrated in FIG. 2, in the first MOCVD stage, an n-type GaAs buffer layer  32  being doped with 5×10 17  cm −3  Si and having a thickness of 0.5 micrometers, an n-type In 0.48 (Ga 0.5 Al 0.5 ) 0.52 P lower cladding layer  33  being doped with 5×10 17  cm −3  Si and having a thickness of 1.5 micrometers, an n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  34  being doped with 5×10 17  cm −3  Si and having a thickness of 0.3 micrometers, an undoped AlGaAs etching stop layer  35  having a thickness of 20 nm, an undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  36  having a thickness of 0.1 micrometers, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  37  having a thickness of 10 nm, an undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  38  having a thickness of 0.1 micrometers, an undoped AlGaAs electron barrier layer  39  having a thickness of 20 nm, a p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  40  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.05 micrometers are formed in this order by MOCVD on an n-type GaAs substrate  31  which is doped with 2×10 18  cm −3  Si. 
     Next, near-edge portions (i.e., portions in vicinities of end facets) of the above layered structure are removed by photolithography and chemical etching, as explained below. First, stripe areas of the p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  40  are etched off with HCl, and stripe areas of the undoped AlGaAs electron barrier layer  39  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. Next, stripe areas of the undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  38  are etched off with HCl, stripe areas of the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  37  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O, and stripe areas of the undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  36  are etched off with HCl. Then, the remaining resist and the like are removed, and the wafer is washed. Finally, stripe areas of the undoped AlGaAs etching stop layer  35  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. 
     Thereafter, in the second MOCVD stage, a p-type InGaAsP third upper optical waveguide layer  41  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.3 micrometers, a p-type In 0.48 (Ga 0.5 Al 0.5 ) 0.52 P upper cladding layer  42  being doped with 7×10 17  cm −3  Zn and having a thickness of 2 micrometers, a p-type In 0.48 Ga 0.52 P layer  43  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.1 micrometers, and a p-type GaAs cap layer  44  being doped with 2×10 19  cm −3  Zn and having a thickness of 0.1 micrometers are formed in this order. 
     Next, near-edge portions of the p-type GaAs cap layer  44  are selectively removed by etching with a mixed solution of NH 4 OH and H 2 O 2 , as illustrated in FIG.  2 . Thereafter, a (Ti/Pt/Au) p electrode  45  is formed by evaporation and heat treatment, and the bottom surface of the n-type GaAs substrate  31  is polished until the total thickness of the layered structure becomes about 100 micrometers. Then, an (AuGe/Ni/Au) n electrode  46  is formed by evaporation and heat treatment. 
     Finally, laser bars are cut out, end facets are coated, and laser chips are cut out. Thus, the semiconductor laser device as the second embodiment is completed. The semiconductor laser device as the second embodiment oscillates at the wavelength of 809 nm. 
     Third Embodiment 
     The construction of a semiconductor laser device  3  as the third embodiment of the present invention and a process of producing the construction are explained below with reference to FIG. 3, which is a cross-sectional view of the semiconductor laser device  3  as the third embodiment. FIG. 3 is a cross-sectional view illustrating a cross section parallel to a resonator axis of the semiconductor laser device. The semiconductor laser device as the third embodiment also has a full-face-electrode structure. 
     As illustrated in FIG. 3, in the first MOCVD stage, an n-type GaAs buffer layer  52  being doped with 5×10 17  cm −3  Si and having a thickness of 0.5 micrometers, an n-type AlxGa 1-x As graded buffer layer  53  being doped with 5×10 17  cm −3  Si and having a thickness of 0.2 micrometers (where x gradually increases from 0.1 to 0.5), an n-type Al 0.5 Ga 0.5 As lower cladding layer  54  being doped with 5×10 17  cm −3  Si and having a thickness of 1.5 micrometers, an n-type In 0.13 Ga 0.87 As 0.75 P 0.25  first lower optical waveguide layer  55  being doped with 5×10 17  cm −3  Si and having a thickness of 0.4 micrometers, an undoped In 0.48 Ga 0.52 P etching stop layer  56  having a thickness of 20 nm, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  second lower optical waveguide layer  57  having a thickness of 0.1 micrometers, an undoped In 0.16 Ga 0.84 As quantum well layer  58  having a thickness of 7 nm, an undoped In 0.13 Ga 0.87 As 0.75 P 0.2 S first upper optical waveguide layer  59  having a thickness of 0.1 micrometers, an undoped AlGaAs electron barrier layer  60  having a thickness of 20 nm, a p-type In 0.13 Ga 0.87 As 0.75 P 0.25  second upper optical waveguide layer  61  being doped with 7×10 17  cm −1  Zn and having a thickness of 0.05 micrometers are formed in this order by MOCVD on an n-type GaAs substrate  51  which is doped with 2×10 18  cm −3  Si. 
     Next, near-edge portions W (i.e., portions in vicinities of end facets) of the above layered structure are removed by photolithography and chemical etching, as explained below. First, stripe areas of the p-type In 0.13 Ga 0.87 As 0.75 P 0.25  second upper optical waveguide layer  61 , the undoped AlGaAs electron barrier layer  60 , the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  first upper optical waveguide layer  59 , the undoped In 0.16 Ga 0.84 As quantum well layer  58 , and the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  second lower optical waveguide layer  57  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. Then, the remaining resist and the like are removed, and the wafer is washed. Finally, stripe areas of the undoped In 0.48 Ga 0.52 P etching stop layer  56  are etched off with HCl. 
     Thereafter, in the second MOCVD stage, a p-type In 0.13 Ga 0.87 As 0.75 P 0.25  third upper optical waveguide layer  62  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.3 micrometers, a p-type In 0.48 (Ga 0.5 Al 0.5 ) 0.52 P upper cladding layer  63  being doped with 7×10 17  cm −3  Zn and having a thickness of 2 micrometers, and a p-type GaAs cap layer  64  being doped with 2×10 19  cm −3  Zn and having a thickness of 0.1 micrometers are formed in this order. Then, near-edge portions of the p-type GaAs cap layer  64  are selectively removed by etching with a mixed solution of NH 4 OH and H 2 O 2 , as illustrated in FIG.  3 . 
     Next, a (Ti/Pt/Au) p electrode  65  is formed by evaporation and heat treatment, and the bottom surface of the n-type GaAs substrate  51  is polished until the total thickness of the layered structure becomes about 100 micrometers. Then, an (AuGe/Ni/Au) n electrode  66  is formed by evaporation and heat treatment. 
     Finally, laser bars are cut out, end facets are coated, and laser chips are cut out. Thus, the semiconductor laser device as the third embodiment is completed. The semiconductor laser device as the third embodiment oscillates at the wavelength of 980 nm. 
     Fourth Embodiment 
     The construction of a semiconductor laser device  4  as the fourth embodiment of the present invention and a process of producing the construction are explained below with reference to FIG. 4, which is a cross-sectional view of the semiconductor laser device  4  as the fourth embodiment. FIG. 4 is a cross-sectional view illustrating a cross section parallel to a resonator axis of the semiconductor laser device. The semiconductor laser device as the fourth embodiment has the most simple full-face-electrode structure. 
     As illustrated in FIG. 4, in the first MOCVD stage, an n-type GaAs buffer layer  72  being doped with 5×10 17  cm −3  Si and having a thickness of 0.5 micrometers, an n-type In 0.48 Ga 0.52 P lower cladding layer  73  being doped with 5×10 17  cm −3  Si and having a thickness of 1.5 micrometers, an n-type In 0.13 Ga 0.87 As 0.75 P 0.25  first lower optical waveguide layer  74  being doped with 5×10 17  cm −3  Si and having a thickness of 0.3 micrometers, an undoped In 0.48 Ga 0.52 P etching stop layer  75  having a thickness of 20 nm, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  second lower optical waveguide layer  76  having a thickness of 0.1 micrometers, an undoped In 0.13 Ga 0.87 As quantum well layer  77  having a thickness of 6 nm, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  first upper optical waveguide layer  78  having a thickness of 0.1 micrometers, an undoped GaAs 0.75 P 0.25  electron barrier layer  79  having a thickness of 12 nm, a p-type In 0.13 Ga 0.87 As 0.75 P 0.25  second upper optical waveguide layer  80  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.05 micrometers are formed in this order by MOCVD on an n-type GaAs substrate  71  which is doped with 2×10 18  cm −3  Si. 
     Next, near-edge portions (i.e., portions in vicinities of end facets) of the above layered structure are removed by photolithography and chemical etching, as explained below. First, stripe areas of the p-type In 0.13 Ga 0.87 As 0.75 P 0.25  second upper optical waveguide layer  80 , the undoped GaAs 0.75 P 0.25  electron barrier layer  79 , the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  first upper optical waveguide layer  78 , the undoped In 0.13 Ga 0.87 As quantum well layer  77 , and the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  second lower optical waveguide layer  76  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. Then, the remaining resist and the like are removed, and the wafer is washed. Finally, stripe areas of the undoped In 0.48 Ga 0.52 P etching stop layer  75  are etched off with HCl. 
     Thereafter, in the second MOCVD stage, a p-type In 0.13 Ga 0.87 As 0.75 P 0.25  third upper optical waveguide layer  81  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.3 micrometers, a p-type In 0.48 Ga 0.52 P upper cladding layer  82  being doped with 7×10 17  cm −3  Zn and having a thickness of 2 micrometers, and a p-type GaAs cap layer  83  being doped with 2×10 19  cm −3  Zn and having a thickness of 0.1 micrometers are formed in this order. 
     Next, near-edge portions of the p-type GaAs cap layer  83  are selectively removed by etching with a mixed solution of NH 4 OH and H 2 O 2 , as illustrated in FIG.  4 . Thereafter, a (Ti/Pt/Au) p electrode  84  is formed by evaporation and heat treatment, and the bottom surface of the n-type GaAs substrate  71  is polished until the total thickness of the layered structure becomes about 100 micrometers. Then, an (AuGe/Ni/Au) n electrode  85  is formed by evaporation and heat treatment. 
     Finally, laser bars are cut out, end facets are coated, and laser chips are cut out. Thus, the semiconductor laser device as the fourth embodiment is completed. The semiconductor laser device as the fourth embodiment oscillates at the wavelength of 950 nm. 
     Fifth Embodiment 
     The construction of a semiconductor laser device  5  as the fifth embodiment of the present invention and a process of producing the construction are explained below with reference to FIGS. 5A to  5 C, which are cross-sectional views of the semiconductor laser device  5  as the fifth embodiment. FIG. 5A is a first cross-sectional view illustrating a first cross section parallel to a resonator axis of the semiconductor laser device, FIG. 5B is a second cross-sectional view illustrating a second cross section (the A-A′ cross section indicated in FIG.  5 A), and FIG. 5C is a third cross-sectional view illustrating a third cross section (the B-B′ cross section indicated in FIG.  5 A). 
     As illustrated in FIG. 5A, in the first MOCVD (metal organic chemical vapor deposition) stage, an n-type GaAs buffer layer  92  being doped with 5×10 17  cm −3  Si and having a thickness of 0.5 micrometers, an n-type AlxGa 1-x As graded buffer layer  93  being doped with 5×10 17  cm −3  Si and having a thickness of 0.2 micrometers (where x gradually increases from 0.1 to 0.45), an n-type Al 0.45 Ga 0.55 As lower cladding layer  94  being doped with 5×10 17  cm −3  Si and having a thickness of 1.5 micrometers, an n-type In 0.13 Ga 0.87 As 0.75 P 0.25  first lower optical waveguide layer  95  being doped with 5×10 17  cm −3  Si and having a thickness of 0.4 micrometers, an undoped In 0.48 Ga 0.52 P etching stop layer  96  having a thickness of 20 nm, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  second lower optical waveguide layer  97  having a thickness of 0.1 micrometers, an undoped In 0.16 Ga 0.84 As quantum well layer  98  having a thickness of 7 nm, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  first upper optical waveguide layer  99  having a thickness of 0.1 micrometers, an undoped Al 0.5 Ga 0.5 As electron barrier layer  100  having a thickness of 20 nm, a p-type In 0.13 Ga 0.87 As 0.75 P 0.25  second upper optical waveguide layer  101  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.05 micrometers are formed in this order by reduced-pressure MOCVD on an n-type GaAs substrate  91  which is doped with 2×10 18  cm −3  Si. 
     Next, near-edge portions (i.e., portions in vicinities of end facets) of the above layered structure are removed by photolithography and chemical etching, as explained below. First, stripe areas of the p-type In 0.13 Ga 0.87 As 0.75 P 0.25  second upper optical waveguide layer  101 , the undoped Al 0.5 Ga 0.5 As electron barrier layer  100 , the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  first upper optical waveguide layer  99 , the undoped In 0.16 Ga 0.84 As quantum well layer  98 , and the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  second lower optical waveguide layer  97  are etched off with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. Then, the remaining resist and the like are removed, and the wafer is washed. Finally, stripe areas of the undoped In 0.48 Ga 0.52 P etching stop layer  96  are etched off with HCl. 
     Thereafter, in the second MOCVD stage, a p-type In 0.13 Ga 0.87 As 0.75 P 0.25  third upper optical waveguide layer  102  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.35 micrometers, an n-type In 0.48 Ga 0.52 P etching stop layer  103  being doped with 1×10 18  cm −3  Si and having a thickness of 10 nm, an n-type Al 0.55 Ga 0.45 As current confinement layer  104  being doped with 1×10 18  cm −3  Si and having a thickness of 0.8 micrometers, and an n-type GaAs cap layer  105  being doped with 1×10 18  cm −3  Zn and having a thickness of 10 nm are formed in this order. Then, stripe areas of the n-type GaAs cap layer  105  and the n-type Al 0.55 Ga 0.45 As current confinement layer  104 , corresponding to a stripe oscillation region of the undoped In 0.16 Ga 0.84 As quantum well layer  98 , are removed by photolithography and etching with a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O. In addition, a stripe area of the n-type In 0.48 Ga 0.52 P etching stop layer  103 , corresponding to the stripe oscillation region of the undoped In 0.16 Ga 0.84 As quantum well layer  98 , is also removed by etching with HCl. As illustrated in FIG. 5B, the above stripe areas of the n-type GaAs cap layer  105 , the n-type Al 0.55 Ga 0.45 As current confinement layer  104 , and the n-type In 0.48 Ga 0.52 P etching stop layer  103  do not include near-edge portions (i.e., portions in vicinities of end facets) corresponding to the removal of the near-edge portions of the undoped In 0.16 Ga 0.84 As quantum well layer  98 . Thus, a current non-injection portions are formed in the vicinities of the end facets. As illustrated in FIG. 5C, the current injection portion does not appear on the end facet. 
     Thereafter, in the third MOCVD stage, a p-type Al 0.45 Ga 0.55 As upper cladding layer  106  being doped with 7×10 17  cm −3  Zn and having a thickness of 1.5 micrometers and a p-type GaAs cap layer  107  being doped with 2×10 19  cm −3  Zn and having a thickness of 0.1 micrometers are formed in this order. Then, a (Ti/Pt/Au) p electrode  108  is formed by evaporation and heat treatment, and the bottom surface of the n-type GaAs substrate  91  is polished until the total thickness of the layered structure becomes about 100 micrometers. Then, an (AuGe/Ni/Au) n electrode  109  is formed by evaporation and heat treatment. 
     Finally, laser bars are cut out, end facets are coated, and laser chips are cut out. Thus, the semiconductor laser device as the fifth embodiment is completed. The semiconductor laser device as the fifth embodiment oscillates at the wavelength of 980 nm. 
     As illustrated in FIG. 5C, in the near-edge portions, the active region except for the n-type In 0.13 Ga 0.87 As 0.75 P 0.25  first lower optical waveguide layer  95  is removed, and the p-type In 0.13 Ga 0.87 As 0.75 P 0.25  third upper optical waveguide layer  102  is formed on the n-type In 0.13 Ga 0.87 As 0.75 P 0.25  first lower optical waveguide layer  95 . Therefore, a structure which is nonabsorbent of oscillation light is formed in the near-edge portions. 
     When the width of the stripe oscillation region is about 3 micrometers, the semiconductor laser device as the fifth embodiment can oscillate in a single transverse mode with high output power. Further, when the width of the stripe oscillation region is 50 micrometers, the semiconductor laser device as the fifth embodiment can oscillate with high output power of 5 W or more. 
     Sixth Embodiment 
     The construction of a semiconductor laser device  6  as the sixth embodiment of the present invention and a process of producing the construction are explained below with reference to FIGS. 6A to  6 C, which are cross-sectional views of the semiconductor laser device  6  as the sixth embodiment. FIG. 6A is a first cross-sectional view illustrating a first cross section parallel to a resonator axis of the semiconductor laser device, FIG. 6B is a second cross-sectional view illustrating a second cross section (the A-A′ cross section indicated in FIG.  6 A), and FIG. 6C is a third cross-sectional view illustrating a third cross section (the B-B′ cross section indicated in FIG.  6 A). 
     As illustrated in FIG. 6A, in the first MOCVD stage, an n-type GaAs buffer layer  112  being doped with 5×10 17  cm −3  Si and having a thickness of 0.5 micrometers, an n-type In 0.48 (Ga 0.4 Al 0.6 ) 0.52 P lower cladding layer  113  being doped with 5×10 17  cm −3  Si and having a thickness of 1.5 micrometers, an n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  114  being doped with 5×10 17  cm −3  Si and having a thickness of 0.4 micrometers, an undoped Al 0.5 Ga 0.5 As etching stop layer  115  having a thickness of 20 nm, an undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  116  having a thickness of 0.1 micrometers, an undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  117  having a thickness of 10 nm, an undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  118  having a thickness of 0.1 micrometers, an undoped Al 0.5 Ga 0.5 As electron barrier layer  119  having a thickness of 20 nm, a p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  120  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.05 micrometers are formed in this order by reduced-pressure MOCVD on an n-type GaAs substrate  111  which is doped with 2×10 18  cm −3  Si. 
     Next, near-edge portions (i.e., portions in vicinities of end facets) of the above layered structure are removed by photolithography and chemical etching alternately using HCl and a mixed solution of H 2 SO 4 , H 2 O 2 , and H 2 O as an etchant until near-edge portions of the n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  114  are exposed. 
     Thereafter, in the second MOCVD stage, a p-type In 0.48 Ga 0.52 P third upper optical waveguide layer  121  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.35 micrometers, an n-type GaAs etching stop layer  122  being doped with 1×10 18  cm −3  Si and having a thickness of 10 nm, an n-type In 0.48 (Ga 0.6 Al 0.4 ) 0.52 P current confinement layer  123  being doped with 1×10 18  cm −3  Si and having a thickness of 0.8 micrometers, and an n-type In 0.48 Ga 0.52 P layer  124  being doped with 1×10 18  cm −3  Zn and having a thickness of 10 nm are formed in this order. Then, stripe areas of the n-type In 0.48 Ga 0.52 P layer  124  and the n-type In 0.48 (Ga 0.6 Al 0.4 ) 0.52 P current confinement layer  123 , corresponding to a stripe oscillation region of the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  117 , are removed by photolithography and etching with HCl. In addition, a stripe area of the n-type GaAs etching stop layer  122 , corresponding to the stripe oscillation region of the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  117 , is removed by etching with a mixed solution of NH 4 OH and H 2 O 2 , as illustrated in FIGS. 6A and 6B. 
     Subsequently, in the third MOCVD stage, a p-type In 0.48 (Ga 0.4 Al 0.6 ) 0.52 P upper cladding layer  125  being doped with 7×10 17  cm −3  Zn and having a thickness of 1.5 micrometers and a p-type In 0.48 Ga 0.52 P cap layer  126  being doped with 7×10 17  cm −3  Zn and having a thickness of 0.1 micrometers are formed in this order. Next, a (Ti/Pt/Au) p electrode  128  is formed by evaporation and heat treatment, and the bottom surface of the n-type GaAs substrate  111  is polished until the total thickness of the layered structure becomes about 100 micrometers. Then, an (AuGe/Ni/Au) n electrode  129  is formed by evaporation and heat treatment. 
     Finally, laser bars are cut out, end facets are coated, and laser chips are cut out. Thus, the semiconductor laser device as the sixth embodiment is completed. 
     As illustrated in FIG. 6C, in the near-edge portions, the undoped Al 0.5 Ga 0.5 As etching stop layer  115 , the undoped In 0.48 Ga 0.52 P second lower optical waveguide layer  116 , the undoped In 0.13 Ga 0.87 As 0.75 P 0.25  quantum well layer  117 , the undoped In 0.48 Ga 0.52 P first upper optical waveguide layer  118 , the undoped Al 0.5 Ga 0.5 As electron barrier layer  119 , and the p-type In 0.48 Ga 0.52 P second upper optical waveguide layer  120  are removed, and the p-type In 0.48 Ga 0.52 P third upper optical waveguide layer  121  is formed on the n-type In 0.48 Ga 0.52 P first lower optical waveguide layer  114 . Therefore, a structure which is nonabsorbent of oscillation light is formed in the near-edge portions. 
     The semiconductor laser device as the sixth embodiment oscillates at the wavelength of 810 nm. 
     Additional Matters 
     (i) For similar reasons to the first embodiment, the performance and reliability of the semiconductor laser devices as the second to sixth embodiments are improved. 
     (ii) The present invention is not limited to the semiconductor laser devices as the first to sixth embodiments, and can be applied to every type of semiconductor laser device having any construction and composition. When the semiconductor laser devices have the features of the present invention, the characteristics and reliability of the semiconductor laser devices are improved for similar reasons to the first embodiment. 
     (iii) Since the semiconductor laser device according to the present invention comprises, in vicinities of end facets, a reliable window structure which is nonabsorbent to oscillation light, the semiconductor laser device according to the present invention can be used as a light source in the fields of high-speed, information processing, image processing, communications, measurement, medicine, printing, and the like.