Abstract:
A semiconductor laser comprises an active section for generating light, and a peripheral section as resonator for producing laser light from the generated light, and includes an InP substrate. The active section has a lower cladding layer formed of AlInAs or AlGaInAs, a core layer including an active layer formed of AlGaInAs or InGaAsP, and an upper cladding layer formed of AlInAs or AlGaInAs. The peripheral section has a first cladding layer formed by oxidizing AlInAs or AlGaInAs, a core layer, and a second clad layer formed by oxidizing AlInAs or AlGaInAs, and a two-dimensional photonic crystal defined by an array of regularly spaced apart holes the peripheral section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor laser of a current injection type having an active section for generating light, and a peripheral section of a resonator for obtaining laser beams from the generated light; and to a method for manufacturing the same. More specifically, the present invention relates to a semiconductor laser that can make the difference in refractive indices between the clad layer and the core layer of the peripheral section large enough for forming a photonic band gap, can lower the resistance of the clad layer in the active section, and can relatively easily manufacture a semiconductor laser of wavelengths of 1.3 μm and 1.55 μm. 
         [0003]    2. Background Art 
         [0004]    Although the speed and capacity of optical communications can be further enhanced by wavelength multiplexing transmission, a wavelength filter, which is a major part of the optical module for wavelength multiplexing transmission is expensive. Therefore, optical devices using photonic crystals that can easily fabricate waveguides, wavelength filters, and the like have been studied. In a semiconductor laser of a current injection type formed in a two-dimensional photonic crystal, outputting light in an in-plane direction and enabling optical coupling with an optical waveguide, a large difference in refractive indices between the core layer and clad layers wherein light is guided and clad layers arranged one above the other is required. If difference in refractive indices is small, optical confinement is weak, loss of light is large, and the semiconductor laser cannot function as a photonic crystal. Therefore, although the air, which has a low refractive index, is often used as clad layers, heat dissipation is poor and mechanical strength is insufficient. 
         [0005]    In a certain semiconductor laser of a current injection type formed in a two-dimensional photonic crystal, a semiconductor wherein a current flows is used as a clad layer in the active section that generates light, and an oxide layer of a low refractive index having a large difference from the refractive index of the core layer is used as clad layers in the peripheral section of the resonator for obtaining laser beams from the generated light (for example, refer to Japanese Patent Application Laid-Open No. 2004-296560). An optical integrated circuit having a semiconductor laser of a current injection type and an optical waveguide formed in a two-dimensional photonic crystal has also been reported (for example, refer to Japanese Patent Application Laid-Open No. 2007-194301). 
       SUMMARY OF THE INVENTION 
       [0006]    Conventionally, AlAs or GaAs was used as a clad layer, and AlAs or GaAs in the peripheral section was oxidized. Thereby, the difference in refractive indices between the clad layers and core layer in the peripheral section could be large enough to form the photonic band gap. However, a current had to be injected via AlAs or GaAs having high resistance (clad layer in the active section). 
         [0007]    Conventionally, since GaAs was used as the material for the substrate, and GaInNAs was used as the material for the active layer, the manufacture of a semiconductor laser of wavelengths of 1.3 μm and 1.55 μm to be applied to an optical module for wavelength multiplexing transmission was technically difficult. 
         [0008]    Conventionally, the core layer in the peripheral section and the p-type light guiding layer in the active layer were simultaneously grown. For injecting current, p-type GaAs was used as the p-type light guiding layer. Therefore, the loss of light occurred by p-type carriers in the core layer in the peripheral section to be the optical waveguide. 
         [0009]    To solve problems as described above, the first object of the present invention is to provide a semiconductor laser that can make the difference in refractive indices between the clad layer and the core layer in the peripheral section large enough to form a photonic band gap, can lower the resistance of the clad layer in the active section, and can relatively easily manufacture a semiconductor laser of wavelengths of 1.3 μm and 1.55 μm; and a method for manufacturing the same. 
         [0010]    The second object of the present invention is to provide a semiconductor laser that can reduce loss of light by carriers in the core layer of the peripheral section; and a method for manufacturing the same. 
         [0011]    According to one aspect of the present invention, a semiconductor laser comprises an active section for generating light, and a peripheral section of a resonator for obtaining laser beams from the generated light, formed on a same substrate, wherein said substrate is an InP substrate; said active section has a lower clad layer formed of AlInAs or AlGaInAs, a core layer including an active layer formed of AlGaInAs or InGaAsP, and an upper clad layer formed of AlInAs or AlGaInAs; said peripheral section has a first clad layer formed by oxidizing AlInAs or AlGaInAs, a core layer, and a second clad layer formed by oxidizing AlInAs or AlGaInAs; and a two-dimensional photonic crystal wherein a plurality of holes are arrayed in a predetermined distance in said peripheral section. 
         [0012]    According to the present invention, the difference in refractive indices between the clad layer and the core layer in the peripheral section can be large enough to form a photonic band gap, can lower the resistance of the clad layer in the active section, and can relatively easily manufacture a semiconductor laser of wavelengths of 1.3 μm and 1.55 μm. 
         [0013]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a sectional view showing a semiconductor laser according to the first embodiment of the present invention. 
           [0015]      FIGS. 2-5  are sectional views for explaining a method of manufacturing a semiconductor laser according to the first embodiment of the present invention. 
           [0016]      FIG. 6  is a photonic band diagram wherein energies at the upper end and the lower end of the photonic band gap (two solid lines) and the energy of the light cone (broken line) are plotted by a plane wave expansion method. 
           [0017]      FIG. 7  is a sectional view showing a semiconductor laser according to the second embodiment of the present invention. 
           [0018]      FIGS. 8-12  are sectional views for explaining a method of manufacturing a semiconductor laser according to the second embodiment of the present invention. 
           [0019]      FIG. 13  is a conceptual diagram showing an optical waveguide optically coupled with a semiconductor laser using a two-dimensional photonic crystal. 
           [0020]      FIG. 14  is a conceptual diagram showing an integrated optical circuit using a semiconductor laser and an optical waveguide according to the third embodiment of the present invention. 
           [0021]      FIG. 15  is a conceptual diagram showing an integrated optical circuit using a semiconductor laser and an optical waveguide according to the fourth embodiment of the present invention. 
           [0022]      FIG. 16  is a sectional view showing a semiconductor laser according to the fifth embodiment of the present invention. 
           [0023]      FIG. 17  is a conceptual diagram of a cut surface in the AlInAsO x  clad layer of an integrated optical circuit using a semiconductor laser and an optical waveguide according to the sixth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0024]      FIG. 1  is a sectional view showing a semiconductor laser according to the first embodiment of the present invention. The semiconductor laser is a semiconductor laser of a current injection type wherein the active section for generating light and the peripheral section, which is a resonator for obtaining laser beams from the generated light, are formed on the same substrate. In the first embodiment, the peripheral section has one or more set of reflective mirrors. 
         [0025]    In the active section, an n-type Al 0.48 In 0.52 As clad layer  12 , an n-type InP light guiding layer  13 , an n-type AlGaInAs light guiding layer  14 , an undoped AlGaInAs strained quantum well active layer  15 , a p-type AlGaInAs light guiding layer  16 , p-type InP light guiding layers  17  and  18 , a p-type Al 0.48 In 0.52 As clad layer  19 , and a p-type InGaAs contact layer  20  are sequentially formed on an n-type InP substrate  11 . 
         [0026]    Here, the n-type InP substrate  11  has an n-type impurity concentration of 1×10 18  cm −3 , and a thickness of 300 μm. The n-type Al 0.48 In 0.52 As clad layer  12  has an n-type impurity concentration of 1×10 18  cm −3 , and a thickness of 1.5 μm. The n-type InP light guiding layer  13  has an n-type impurity concentration of 1×10 18  cm −3 , and a thickness of 0.15 μm. The n-type AlGaInAs light guiding layer  14  has an n-type impurity concentration of 1×10 18  cm −3 , and a thickness of 0.05 μm. The undoped AlGaInAs strained quantum well active layer  15  has a band gap of effectively 0.8 eV, and a thickness of 0.04 μm. The p-type AlGaInAs light guiding layer  16  has a p-type impurity concentration of 5×10 17  cm −3 , and a thickness of 0.05 μm. The p-type InP light guiding layers  17  and  18  have a p-type impurity concentration of 5×10 17  cm −3 , and a thickness of 0.15 μm. The p-type Al 0.48 In 0.52 As clad layer  19  has a p-type impurity concentration of 1×10 18  cm −3 , and a thickness of 1.5 μm. The p-type InGaAs contact layer  20  has a p-type impurity concentration of 1×10 19  cm −3 , and a thickness of 0.3 μm. 
         [0027]    The core layer  21  in the active section has the n-type InP light guiding layer  13 , the n-type AlGaInAs light guiding layer  14 , the undoped AlGaInAs strained quantum well active layer  15 , the p-type AlGaInAs light guiding layer  16 , and the p-type InP light guiding layers  17  and  18  as described above. 
         [0028]    In the peripheral section, an AlInAsO, clad layer  22  formed by oxidizing the n-type Al 0.48 In 0.52 As clad layer  12 , an n-type InP core layer  23 , a p-type InP core layer  24 , and an AlInAsO x  clad layer  25  formed by oxidizing the p-type Al 0.48 In 0.52 As clad layer  19  are sequentially formed on the n-type InP substrate  11 . In the peripheral section, a triangle lattice-shaped two-dimensional photonic crystal, wherein a plurality of holes  26  are arrayed in a predetermined distance, is also formed. 
         [0029]    Here, the n-type InP core layer  23  has an n-type impurity concentration of 1×10 18  cm −3 , and the p-type InP core layer  24  has a p-type impurity concentration of 5×10 17  cm −3 . The total thickness of the n-type InP core layer  23  and the p-type InP core layer  24  is 280 nm. The array distance of the holes  26  is 0.4 μm, and the diameter of the holes  26  is 0.24 μm. 
         [0030]    The core layer  27  in the peripheral section has the n-type InP core layer  23  and the p-type InP core layer  24 . A p-side electrode  28  is formed on the p-type InGaAs contact layer  20 , and an n-side electrode  29  is formed on the back face of the n-type InP substrate  11 . The reference numerals  30  and  31  denote regrown interfaces. 
         [0031]    A method for manufacturing a semiconductor laser according to the first embodiment will be described. First, as shown in  FIG. 2 , the n-type Al 0.48 In 0.52 As clad layer  12 , the n-type InP light guiding layer  13 , the n-type AlGaInAs light guiding layer  14 , the undoped AlGaInAs strained quantum well active layer  15 , the p-type AlGaInAs light guiding layer  16 , and the p-type InP light guiding layer  17  are sequentially formed on the n-type InP substrate  11 . Here, the p-type InP light guiding layer  17  functions as a cap layer for preventing the oxidation of the p-type AlGaInAs light guiding layer  16 . 
         [0032]    Next, as shown in  FIG. 3 , in the state wherein the active section is coated with a resist (not shown) by photolithography, the p-type InP light guiding layer  17 , the p-type AlGaInAs light guiding layer  16 , the undoped AlGaInAs strained quantum well active layer  15 , and the n-type AlGaInAs light guiding layer  14  are etched in the peripheral section. Here, the depth of etching is about 160 nm. 
         [0033]    Next, as shown in  FIG. 4 , the p-type InP light guiding layer  18 , the p-type Al 0.48 In 0.52 As clad layer  19 , and the p-type InGaAs contact layer  20  are sequentially formed. Here, the n-type InP light guiding layer  13  and the p-type InP light guiding layer  18  in the peripheral section correspond to the n-type InP core layer  23  and the p-type InP core layer  24 , respectively. 
         [0034]    Next, as shown in  FIG. 5 , the p-type InGaAs contact layer  20  on the area other than the active section is etched off. Then, by photolithography and dry etching to the upper portion of the n-type InP substrate  11 , the plurality of holes  26  are formed in the peripheral section in a predetermined distance to form a triangle lattice-shaped two-dimension photonic crystal. 
         [0035]    Next, the n-type Al 0.48 In 0.52 As clad layer  12  and the p-type Al 0.48 In 0.52 As clad layer  19  in the peripheral section are selectively oxidized by a distance of about 0.1 μm through the plurality of holes  26  to form AlInAsO x  clad layers  22  and  25  as shown in  FIG. 1 . Then, the p-side electrode  28  is formed on the p-type InGaAs contact layer  20 , and the n-side electrode  29  is formed on the back face of the n-type InP substrate  11 . By the above process, a semiconductor laser according to the first embodiment is manufactured. 
         [0036]    The refractive index of the oxidized AlInAs (AlInAsO x ) is 2.3 to 2.5 (refer to Paragraph 0033 of Japanese Patent Application Laid-Open No. 2001-350039). Therefore, by using the oxidized AlInAs as the clad layer of the peripheral section, the difference in the refractive indices between the clad layer and the core layer in the peripheral section can be large enough for forming the photonic band gap. 
         [0037]    By using AlInAs as the clad layer of the active section, the resistance can be lowered compared with the case when AlAs or GaAs is used. The oxidation rate of AlInAs much depends on film thickness, and does not depend on Al composition within the range between 0.48 and 0.7. For example, when the oxidation temperature is 500° C. and the film thickness is 100 nm, the oxidation rate of AlInAs is 0.5 μm/min 1/2  (refer to Furukawa Electric Co., LTD. News Release No. 107). Therefore, since AlInAs can be selectively oxidized in the same manner as AlAs or GaAs, the clad layer in the peripheral section can be formed by oxidizing the AlInAs layer. 
         [0038]    By using InP as the material for the substrate, and AlGaInAs as the material for the active layer, a semiconductor laser of wavelengths of 1.3 μm and 1.55 μm can be relatively easily manufactured. In this case, device characteristics more excellent in temperature characteristics can be expected than using InGaAsP, which is one of ordinary InP-based materials. 
         [0039]    If InP is used as a material for the substrate, AlGaInAs is used as a material for the active layer and the light guiding layer, and AlInAs or AlGaInAs is used as a material for the clad layer, since the switching of As and P is not required, continuous growth can be performed, and a high-quality crystal can be obtained. 
         [0040]      FIG. 6  is a photonic band diagram wherein energies at the upper end and the lower end of the photonic band gap (two solid lines) and the energy of the light cone (broken line) are plotted by a plane wave expansion method. Here, a lattice constant was selected so that the energy at the center point (dots) of the photonic band gap became 0.8 eV (1.55 μm), the clad layer was assumed to be sufficiently thick, and the core layer and the clad layer were assumed to be an InP layer of a refractive index of 3.4 and an AlInAs oxide (AlInAsO x ) layer, respectively, to calculate. Here, light in the higher energy side than the energy of the light cone leaks for the core layer, the defect level in the vicinity of the center point of the photonic band gap must be in the lower energy side than the energy of the light cone. Therefore, it is known from the result of calculation that the thickness of the InP core layer must be 280 nm or more. Specifically, the thickness of the InP core layer must be at least 70% the array distance of the plurality of holes. 
         [0041]    Therefore, the thickness of each of the n-type Al 0.48 In 0.52 As clad layer  12  and the p-type Al 0.48 In 0.52 As clad layer  19  is made to be 500 nm or more, and the thickness of the core layer  27  in the peripheral section (total thickness of the n-type InP core layer  23  and the p-type InP core layer  24 ) is made to be at least 280 nm, or at least 70% the array distance of the plurality of holes  26 . Thereby, the leakage of light form the core layer can be prevented. 
         [0042]    Although the n-type Al 0.48 In 0.52 As clad layer  12  and the p-type Al 0.48 In 0.52 As clad layer  19  are composed of AlInAs, the present invention is not limited thereto, but AlGaInAs can also be used. In this case, the clad layers  22  and  25  of the peripheral section are formed by oxidizing AlGaInAs. Then, the clad layers  22  and  25  are formed by oxidizing AlInAs or AlGaInAs. The clad layers  22  and  25  can be formed by oxidizing the material which is different from the material of the clad layers  12  and  19  of the active section. Although the undoped AlGaInAs strained quantum well active layer  15  is composed of AlGaInAs, the present invention is not limited thereto, but InGaAsP can also be used. 
         [0043]    By changing the wave length of the resonator, or by providing an electrode to change the refractive index when current is injected in addition to the electrode for laser oscillation, the oscillation wavelength can be changed. Furthermore, by forming an optical waveguide in the two-dimensional photonic crystal, the optical waveguide and the semiconductor laser of the current injection type can be integrally formed on the same substrate. 
         [0044]    InGaAsP can be used as the core layer in the peripheral section, and AlInAs can be used as the light guiding layer. The thicknesses of respective layers are not limited to the thicknesses in the first embodiment. For example, the number of wells in the active layer portion can be increased (to 2 to 15 wells), and the thicknesses of about 30 to 200 nm can also be used. The depth of etching is also changed corresponding to the thicknesses of the layers. 
       Second Embodiment 
       [0045]      FIG. 7  is a sectional view showing a semiconductor laser according to the second embodiment of the present invention. The core layer  27  in the peripheral section has an undoped InP core layer  32 . Other configurations are identical to the configurations of the first embodiment. 
         [0046]    A method for manufacturing a semiconductor laser according to the second embodiment will be described. First, as shown in  FIG. 8 , the n-type Al 0.48 In 0.52 As clad layer  12 , the n-type InP light guiding layer  13 , the n-type AlGaInAs light guiding layer  14 , the undoped AlGaInAs strained quantum well active layer  15 , the p-type AlGaInAs light guiding layer  16 , and the p-type InP light guiding layer  17  are sequentially formed on the n-type InP substrate  11 . Here, the p-type InP light guiding layer  17  functions as a cap layer for preventing the oxidation of the p-type AlGaInAs light guiding layer  16 . 
         [0047]    Next, as shown in  FIG. 9 , in the state wherein the active section is coated with a resist (not shown) by photolithography, the p-type InP light guiding layer  17 , the p-type AlGaInAs light guiding layer  16 , the undoped AlGaInAs strained quantum well active layer  15 , the n-type AlGaInAs light guiding layer  14 , and the n-type InP light guiding layer are etched in the peripheral section. Here, the depth of etching is about 350 nm. 
         [0048]    Next, as shown in  FIG. 10 , the undoped InP core layer  32  is formed on the peripheral section. Then, as shown in  FIG. 11 , the p-type InP light guiding layer  18 , the p-type Al 0.48 In 0.52 As clad layer  19 , and the p-type InGaAs contact layer  20  are sequentially formed. Here, the n-type InP light guiding layer  13  and the p-type InP light guiding layer  18  in the peripheral section correspond to the n-type InP core layer  23  and the p-type InP core layer  24 , respectively. 
         [0049]    Next, as shown in  FIG. 12 , the p-type InGaAs contact layer  20  on the area other than the active section is etched off. Then, by photolithography and dry etching to the upper portion of the n-type InP substrate  11 , the plurality of holes  26  are formed in the peripheral section in a predetermined distance to form a triangle lattice-shaped two-dimension photonic crystal. 
         [0050]    Next, the n-type Al 0.48 In 0.52 As clad layer  12  and the p-type Al 0.48 In 0.52 As clad layer  19  in the peripheral section are selectively oxidized by a distance of about 0.1 μm through the plurality of holes  26  to form AlInAsO x  clad layers  22  and  25  as shown in  FIG. 7 . Then, the p-side electrode  28  is formed on the p-type InGaAs contact layer  20 , and the n-side electrode  29  is formed on the back face of the n-type InP substrate  11 . By the above process, a semiconductor laser according to the second embodiment is manufactured. 
         [0051]    By forming the undoped InP core layer  32  as the core layer  27  in the peripheral section, the loss of light by carriers can be reduced in the core layer  27  in the peripheral section, which becomes an optical waveguide. Furthermore, since the lowering of carrier concentration in the p-type InP light guiding layers  17  and  18  is not required, the concentration can be, for example, 1×10 18  cm −3 . Thereby, the resistance of the clad layer in the active section can be further lowered. 
         [0052]    Equivalent to the first embodiment, the thickness of each of the n-type Al 0.48 In 0.52 As clad layer  12  and the p-type Al 0.48 In 0.52 As clad layer  19  is made to be 500 nm or more, and the thickness of the core layer  27  in the peripheral section (thickness of the undoped InP core layer  32 ) is made to be at least 280 nm. Thereby, the leakage of light form the core layer can be prevented. 
       Third Embodiment 
       [0053]      FIG. 13  is a conceptual diagram showing an optical waveguide optically coupled with a semiconductor laser using a two-dimensional photonic crystal. A semiconductor laser  34  and a waveguide  35  are formed in a two-dimensional photonic crystal  33 . The semiconductor laser  34  corresponds to the active section according to the first or second embodiment. Resonance occurs in the semiconductor laser  34  and the laser oscillates, and the output light of the semiconductor laser  34  can be taken out of the waveguide  35 . In addition, since the wavelength of the standing wave is changed by changing the length of the optical resonator in the semiconductor laser  34 , the oscillation wavelength can be changed. 
         [0054]      FIG. 14  is a conceptual diagram showing an integrated optical circuit using a semiconductor laser and an optical waveguide according to the third embodiment of the present invention. Semiconductor lasers  34   a,    34   b,    34   c  and  34   d,  and a waveguide  35  are integrated in a two-dimensional photonic crystal  33 . The semiconductor lasers  34   a,    34   b,    34   c  and  34   d  correspond to the active section according to the first or second embodiment. However, the lengths of respective optical resonators are different. Thereby, four kinds of light having different wavelengths can be taken out of one waveguide  35 . 
       Fourth Embodiment 
       [0055]      FIG. 15  is a conceptual diagram showing an integrated optical circuit using a semiconductor laser and an optical waveguide according to the fourth embodiment of the present invention. In the semiconductor lasers  34   a,    34   b,    34   c  and  34   d,  in addition to electrodes for laser oscillation, electrodes  36   a,    36   b,    36   c  and  36   d  for changing the oscillation wavelengths are formed in the active section, respectively. Other configurations are same as the configurations of the third embodiment. Since refractive indices change to change the length of resonators by supplying current to the electrodes  36   a,    36   b,    36   c  and  36   d,  the oscillation wavelengths can be changed. 
       Fifth Embodiment 
       [0056]      FIG. 16  is a sectional view showing a semiconductor laser according to the fifth embodiment of the present invention. The p-type Al 0.48 In 0.52 As clad layer  19  in the second embodiment is replaced by a p-type Al 0.48 In 0.52 As clad layer  37 , a p-type Ga 0.7 In 0.3 As tunnel coupling layer  38 , an n-type Ga 0.7 In 0.3 As tunnel coupling layer  39 , and an n-type Al 0.48 In 0.52 As clad layer  40 . The p-type InGaAs contact layer  20  and the upper electrode  28  in the second embodiment are replaced by an n-type InGaAs contact layer  41  and an n-type upper electrode  42  corresponding to the n-type. Other configurations and manufacturing methods are same as those in the second embodiment. 
         [0057]    Both the p-type Ga 0.7 In 0.3 As tunnel coupling layer  38  and the n-type Ga 0.7 In 0.3 As tunnel coupling layer  39  have a thickness of 10 nm, and a carrier concentration of 1×10 20  cm −3 . Since these films are ultra-thin films having ultra-high carrier concentrations, the conductivity type can be changed from p-type to n-type at a low resistance. The p-type Al 0.48 In 0.52 As clad layer  37  has a thickness of 0.08 μm, and a carrier concentration of 1×10 18  cm −3 . The n-type Al 0.48 In 0.52 As clad layer  40  has a thickness of 1.4 μm, and a carrier concentration of 1×10 18  cm −3 . 
         [0058]    Although the non-oxidized p-type Ga 0.7 In 0.3 As tunnel coupling layer  38 , and n-type Ga 0.7 In 0.3 As tunnel coupling layer  39  are present on the upper clad layer in the peripheral section, these are ultra-thin films, and do not cause the loss of light. The AlInAsO x  clad layer  25  in the peripheral section is formed by selectively oxidizing the p-type Al 0.48 In 0.52 As clad layer  37  and the n-type Al 0.48 In 0.52 As clad layer  40 . 
         [0059]    By introducing the tunnel junction in the semiconductor laser as described above, the conductivity type of two electrodes for injecting current and the semiconductor layers contacting thereto can be n-type. Since the mobility of n-type Al 0.48 In 0.52 As is dramatically higher than the mobility of p-type Al 0.48 In 0.52 As, resistance is lowered. Therefore, the resistance of the upper clad layer in the active section, and furthermore, the entire laser element can be significantly lowered. As a result, many advantages, such as the suppression of heat generation of the element and the possibility of high-speed operations, can be obtained. 
         [0060]    In the fifth embodiment, although the tunnel junction is formed in the clad layer, the tunnel junction can be alternatively formed in the core layer. 
       Sixth Embodiment 
       [0061]      FIG. 17  is a conceptual diagram of a cut surface in the AlInAsO x  clad layer of an integrated optical circuit using a semiconductor laser and an optical waveguide according to the sixth embodiment of the present invention. 
         [0062]    A resonator  43  of the semiconductor laser and the waveguide  35  are formed in a two-dimensional photonic crystal  33  wherein a plurality of holes  26  are arrayed in a predetermined distance. The resonator  43  corresponds to the active section of the semiconductor laser according to the first, second or fifth embodiment. 
         [0063]    In the AlInAsO x  clad layer of the resonator  43 , a selectively oxidized AlInAsO x  portion  44  and a non-oxidized AlInAs portion  45  are present. The width of the selectively oxidized AlInAsO x  portion  44  is made to be 230 nm. 
         [0064]    To make the oscillation wavelength of the laser in the 1.3 μm band, the effective band gap of the strained quantum well active layer is made to be 0.95 eV, the array distance of the holes  26  in the two-dimensional photonic crystal  33  is made to be 0.32 μm, and the diameter of each hole  26  is made to be 0.19 μm. 
         [0065]    In the first and second embodiments, the thickness of the core layer in the selectively oxidized portion must be at least 280 μm, specifically, at least 70% the array distance of the holes  26 . While in the sixth embodiment, the thickness of the core layer in the selectively oxidized portion is 256 μm, which is 80% the array distance of the holes  26 . 
         [0066]    The shape of the resonator  43  in the plane of the two-dimensional photonic crystal  33  is circular, and there are no parallel portions. However, laser oscillation occurs in the whispering gallery mode wherein standing waves are generated in the peripheral section of the resonator  43 . Therefore, no so-called reflection mirror is required. Specifically, the resonator  43  surrounded by the two-dimensional photonic crystal  33  does not necessarily require parallel portions, and the shape is not limited as long as the resonance characteristics are sufficiently high. 
         [0067]    The output beams of the semiconductor laser can be taken out of the waveguide  35  in the same manner as the third or forth embodiment of the present invention. Since the wavelength of the standing waves in changed by changing the size of the resonator  43  of the semiconductor laser, the oscillation wavelength can be changed. Therefore, the semiconductor laser according to the sixth embodiment can be applied to integrated optical circuits in the same manner as the third or forth embodiment. 
         [0068]    In the sixth embodiment, since the cut surface area of the resonator  43  can be enlarged, the resistance of the laser element can be lowered. In addition, since the oxidation distance of the AlInAsO, clad layer is large, the entire portion of the waveguide  35  is selectively oxidized. Therefore, ineffective current wherein the current injected from the electrodes leaks into the waveguide  35  can be suppressed. 
         [0069]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0070]    The entire disclosure of a Japanese Patent Application No. 2008-58164, filed on Mar. 7, 2008 and a Japanese Patent Application No. 2009-19129, filed on Jan. 30, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.