Abstract:
A semiconductor laser comprises: a semiconductor substrate and a lower cladding layer, an active layer, and an upper cladding layer on the semiconductor substrate. The layers form a resonator having opposed end surfaces. A ridge includes part of the upper cladding layer. The upper cladding layer in the ridge, proximate the resonator end surfaces, is thicker than the upper cladding layer in the ridge at a central part of the resonator.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor laser capable of preventing end surface deterioration and easy to manufacture. 
         [0003]    2. Background Art 
         [0004]    The amount of absorption of light and the current density at end surfaces of a resonator can be reduced by increasing the bandgap of an active layer in the vicinities of the end surfaces of the resonator relative to the bandgap of the active layer at a center of the resonator of a semiconductor laser. Prevention of end surface deterioration is thus enabled (see, for example, Japanese Patent Laid-Open Nos. 2005-191588 and 10-256645). 
       SUMMARY OF THE INVENTION 
       [0005]    Semiconductor lasers have been manufactured by performing selective growing using a mask. For manufacturing semiconductor lasers in such a way, a complicated manufacturing process and strict control of growing conditions are required. 
         [0006]    In view of the above-described problem, an object of the present invention is to provide a semiconductor laser capable of preventing end surface deterioration and easy to manufacture. 
         [0007]    According to one aspect of the present invention, a semiconductor laser comprises: a semiconductor substrate; and a lower clad layer, an active layer and an upper clad layer formed one on another on the semiconductor substrate, wherein a ridge is formed on the upper clad layer, and the thickness of the upper clad layer in the ridge in the vicinities of resonator end surfaces is larger than the thickness of the upper clad layer in the ridge at a resonator center. 
         [0008]    According to the present invention, a semiconductor laser capable of preventing end surface deterioration and easy to manufacture can be obtained. 
         [0009]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a sectional view of a semiconductor laser according to a first embodiment of the present invention taken in a direction along a resonator. 
           [0011]      FIG. 2  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 1 . 
           [0012]      FIG. 3  is a sectional view at a center of the resonator shown in  FIG. 1 . 
           [0013]      FIGS. 4-6  are sectional views for explaining a method of manufacturing a semiconductor laser according to a first embodiment of the present invention. 
           [0014]      FIG. 7  is a sectional view of a semiconductor laser according to a second embodiment of the present invention taken in a direction along a resonator. 
           [0015]      FIG. 8  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 7 . 
           [0016]      FIG. 9  is a sectional view at a center of the resonator shown in  FIG. 7 . 
           [0017]      FIGS. 10-13  are sectional views for explaining a method of manufacturing a semiconductor laser according to a second embodiment of the present invention. 
           [0018]      FIG. 14  is a sectional view of a semiconductor laser according to a third embodiment of the present invention taken in a direction along a resonator. 
           [0019]      FIG. 15  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 14 . 
           [0020]      FIG. 16  is a sectional view at a center of the resonator shown in  FIG. 14 . 
           [0021]      FIGS. 17-20  are sectional views for explaining a method of manufacturing a semiconductor laser according to a third embodiment of the present invention. 
           [0022]      FIG. 21  is a sectional view of a semiconductor laser according to a fourth embodiment of the present invention taken in a direction along a resonator. 
           [0023]      FIG. 22  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 21 . 
           [0024]      FIG. 23  is a sectional view at a center of the resonator shown in  FIG. 21 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0025]      FIG. 1  is a sectional view of a semiconductor laser according to a first embodiment of the present invention taken in a direction along a resonator.  FIG. 2  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 1 .  FIG. 3  is a sectional view at a center of the resonator shown in  FIG. 1 . 
         [0026]    An n-AlGaN clad layer  12  (lower clad layer), a guide layer  13 , a quantum well active layer  14  (active layer), a guide layer  15  and a p-AlGaN clad layer  16  (upper clad layer) are formed one on another on an n-GaN substrate  11  (semiconductor substrate). A ridge  17  is formed on the p-AlGaN clad layer  16 . A p-GaN contact layer  18  and a p-electrode  19  are formed on the ridge  17 . An n-electrode  20  is formed on the lower surface of the n-GaN substrate  11 . An insulating film  21  is formed so as to cover side wall surfaces of the ridge  17  and the p-AlGaN clad layer  16  in outside of the ridge  17 . 
         [0027]    A feature of the present embodiment resides in that, in the ridge  17 , the thickness of the p-AlGaN clad layer  16  in the vicinities of resonator end surfaces is larger than the thickness of the p-AlGaN clad layer  16  at a resonator center (oscillation section). In the outside of the ridge  17 , in the present embodiment, the thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces and the thickness of the p-AlGaN clad layer  16  at the resonator center are equal to each other. 
         [0028]    The process of manufacturing the semiconductor laser according to the present invention will be described. First, as shown in  FIG. 4 , the n-AlGaN clad layer  12 , the guide layer  13 , the quantum well active layer  14 , the guide layer  15  and the p-AlGaN clad layer  16  are formed on the n-GaN substrate  11 . 
         [0029]    Next, as shown in  FIG. 5 , the ridge  17  is formed by etching the p-AlGaN clad layer  16 . For this etching, a resist  22  patterned by photolithography is used as a mask. 
         [0030]    Subsequently, as shown in  FIG. 6 , a resist  23  is formed in the vicinities of the resonator end surfaces by photolithography. The p-AlGaN clad layer  16  is etched by using this resist  23  as a mask so that the thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces is larger than the thickness of the p-AlGaN clad layer  16  at the resonator center. 
         [0031]    Subsequently, as shown in  FIGS. 1 to 3 , the p-GaN contact layer  18  and the p-electrode  19  are formed on the ridge  17 . The n-electrode  20  is formed on the lower surface of the n-GaN substrate  11 . The insulating film  21  is formed so as to cover the side wall surfaces of the ridge  17  and the p-AlGaN clad layer  16  in the outside of the ridge  17 . The semiconductor laser according to the present embodiment is manufactured by the above-described process. 
         [0032]    In the semiconductor laser according to the present embodiment, as described above, the thickness of the p-AlGaN clad layer  16  in the ridge  17  in the vicinities of the resonator end surfaces is larger than the thickness of the p-AlGaN clad layer  16  in the ridge  17  at the resonator center. Because compressive strain is caused in the quantum well active layer  14  having a lattice constant larger than that of the p-AlGaN clad layer  16 , the bandgap of the quantum well active layer  14  in the vicinities of the resonator end surfaces is increased relative to the bandgap of the quantum well active layer  14  at the resonator center (oscillation section) by constructing the semiconductor laser according to the present embodiment. The amount of absorption of light at each resonator end surface is reduced thereby, thus enabling prevention of end surface deterioration. 
         [0033]    In the vicinities of the resonator end surfaces where the thickness of the p-AlGaN clad layer  16  is large, a strain of the quantum well active layer  14  with respect to the p-AlGaN clad layer  16  due to the difference between the lattice constants is large. Therefore, polarized charge due to a piezoelectric effect increases at the end surfaces. For this reason, a current spreads laterally from positions below the side walls of the ridge  17 , and the current density in the vicinities of the resonator end surfaces is reduced, thus enabling prevention of end surface deterioration. 
         [0034]    The semiconductor laser according to the present embodiment can be easily manufactured by etching the p-AlGaN clad layer  16  at the resonator center. 
         [0035]    In the semiconductor laser according to the present embodiment, the positions at which the structure of the p-AlGaN clad layer  16  is changed are at a substantial distance from the quantum well active layer  14 . Therefore, changes in the light distribution and current distribution in the direction along the resonator are gradual. As a result, stable laser oscillation can be achieved. 
       Second Embodiment 
       [0036]      FIG. 7  is a sectional view of a semiconductor laser according to a second embodiment of the present invention taken in a direction along a resonator.  FIG. 8  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 7 .  FIG. 9  is a sectional view at a center of the resonator shown in  FIG. 7 . 
         [0037]    In outside of the ridge  17 , the thickness of the p-AlGaN clad layer  16  in the vicinities of resonator end surfaces is larger than the thickness of the p-AlGaN clad layer  16  at the resonator center. In other respects, the construction in the second embodiment is the same as that in the first embodiment. 
         [0038]    The process of manufacturing the semiconductor laser according to the present embodiment will be described. First, as shown in  FIG. 10 , the n-AlGaN clad layer  12 , the guide layer  13 , the quantum well active layer  14 , the guide layer  15  and the p-AlGaN clad layer  16  are formed on the n-GaN substrate  11 . 
         [0039]    Next, as shown in  FIG. 11 , a resist  23  is formed in the vicinities of the resonator end surfaces by photolithography. The p-AlGaN clad layer  16  is etched by using this resist  23  as a mask so that the thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces is larger than the thickness of the p-AlGaN clad layer  16  at the resonator center. Thereafter, the p-GaN contact layer  18  is formed on the p-AlGaN clad layer  16 . 
         [0040]    Subsequently, as shown in  FIGS. 12 and 13 , the ridge  17  is formed by etching the p-GaN contact layer  18  and the p-AlGaN clad layer  16 . For this etching, a resist  22  patterned by photolithography is used as a mask. Since the thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces is larger than the thickness of the p-AlGaN clad layer  16  at the resonator center, the thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces in the outside of the ridge  17  can also be made larger than the thickness of the p-AlGaN clad layer  16  at the resonator center in the outside of the ridge  17  by uniformly performing etching. 
         [0041]    Subsequently, as shown in  FIGS. 7 to 9 , the p-electrode  19  is formed on the ridge  17 . The n-electrode  20  is formed on the lower surface of the n-GaN substrate  11 . The insulating film  21  is formed so as to cover the side wall surfaces of the ridge  17  and the p-AlGaN clad layer  16  in the outside of the ridge  17 . The semiconductor laser according to the present embodiment is manufactured by the above-described process. 
         [0042]    In the semiconductor laser according to the present embodiment, as described above, the thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces in the outside of the ridge  17  is larger than the thickness of the p-AlGaN clad layer  16  at the resonator center in the outside of the ridge  17 . Because of this construction, a strain of the quantum well active layer  14  in the vicinities of the resonator end surfaces with respect to the p-AlGaN clad layer  16  is increased. Also, the thickness of the p-AlGaN clad layer in the vicinities of the resonator end surfaces is increased in the outside of the ridge  17  relative to that in the first embodiment. Therefore, a current larger than that in the first embodiment spreads laterally from positions below the side walls of the ridge  17 , and the current density in the vicinities of the resonator end surfaces is further reduced, thus enabling prevention of end surface deterioration with improved reliability. 
       Third Embodiment 
       [0043]      FIG. 14  is a sectional view of a semiconductor laser according to a third embodiment of the present invention taken in a direction along a resonator.  FIG. 15  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 14 .  FIG. 16  is a sectional view at a center of the resonator shown in  FIG. 14 . 
         [0044]    An n-AlGaN clad layer  12  (lower clad layer), a guide layer  13 , a quantum well active layer  14  (active layer), a guide layer  15 , a p-AlGaN clad layer  16  (upper clad layer) and a p-GaN contact layer  18  (contact layer) are formed one on another on an n-GaN substrate  11  (semiconductor substrate). 
         [0045]    A feature of the present embodiment resides in that a p-AlGaN clad layer  24  (second upper clad layer) is formed on the p-GaN contact layer  18  only in the vicinities of resonator end surfaces but is not formed on at a resonator center. A ridge  17  is formed on the p-AlGaN layers  16  and  24  and the p-GaN contact layer  18 . A p-electrode  19  is formed on the ridge  17 . An n-electrode is formed on the lower surface of the n-GaN substrate  11 . An insulating film  21  is formed so as to cover side wall surfaces of the ridge  17  and the p-AlGaN clad layer  16  in outside of the ridge  17 . 
         [0046]    The process of manufacturing the semiconductor laser according to the present invention will be described. First, as shown in  FIG. 17 , the n-AlGaN clad layer  12 , the guide layer  13 , the quantum well active layer  14 , the guide layer  15  and the p-AlGaN clad layer  16 , the p-GaN contact layer  18  and the p-AlGaN clad layer  24  are formed on the n-GaN substrate  11 . 
         [0047]    Next, as shown in  FIG. 18 , a resist  23  is formed in the vicinities of the resonator end surfaces by photolithography. The p-AlGaN clad layer  24  is etched by using this resist  23  as a mask. The p-AlGaN clad layer  24  at the resonator center is removed thereby. 
         [0048]    Subsequently, as shown in  FIGS. 19 and 20 , the ridge  17  is formed by etching the p-AlGan clad layer  24 , the p-GaN contact layer  18  and the p-AlGaN clad layer  16 . For this etching, a resist  22  patterned by photolithography is used as a mask. 
         [0049]    Subsequently, as shown in  FIGS. 14 to 16 , the p-electrode  19  is formed on the ridge  17 . The n-electrode  20  is formed on the lower surface of the n-GaN substrate  11 . The insulating film  21  is formed so as to cover the side wall surfaces of the ridge  17  and the p-AlGaN clad layer  16  in the outside of the ridge  17 . The semiconductor laser according to the present embodiment is manufactured by the above-described process. 
         [0050]    In the semiconductor laser according to the present embodiment, as described above, the p-AlGaN clad layer  24  is formed only in the vicinities of the resonator end surfaces. Because of this construction, the bandgap of the quantum well active layer  14  in the vicinities of the resonator end surfaces is increased relative to the bandgap of the quantum well active layer  14  at the resonator center (oscillation section). Also, a current spreads laterally from positions below the side walls of the ridge  17 , and the current density in the vicinities of the resonator end surfaces is reduced, thus enabling prevention of end surface deterioration. 
         [0051]    The semiconductor laser according to the present embodiment can be easily manufactured by etching the p-AlGaN clad layer  24  at the resonator center. There is no need for regrowing of the contact layer or the like after etching. Therefore, growing can be performed with improved reproducibility. 
         [0052]    In the semiconductor laser according to the present embodiment, the positions at which the structure of the p-AlGaN clad layer  24  is changed are at a substantial distance from the quantum well active layer  14 . Therefore, changes in the light distribution and current distribution in the direction along the resonator are gradual. As a result, stable laser oscillation can be achieved. 
         [0053]    It is desirable that the proportion of Al in the composition of the p-AlGaN clad layer  24  be larger than the proportion of Al in the composition of the p-AlGaN clad layer  16 . If the proportion of Al is increased, the compressive strain with respect to the quantum well active layer  14  becomes larger. Therefore, prevention of end surface deterioration is enabled with improved reliability by increasing the proportion of Al. 
         [0054]    In the present embodiment, the thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces in the outside of the ridge  17  is larger than the thickness of the p-AlGaN clad layer  16  at the resonator center in the outside of the ridge  17 . However, the present invention is not limited to this. The thickness of the p-AlGaN clad layer  16  in the vicinities of the resonator end surfaces in the outside of the ridge  17  and the thickness of the p-AlGaN clad layer  16  at the resonator center in the outside of the ridge  17  may be equal to each other. 
       Fourth Embodiment 
       [0055]      FIG. 21  is a sectional view of a semiconductor laser according to a fourth embodiment of the present invention taken in a direction along a resonator.  FIG. 22  is a sectional view in the vicinity of an end surface of the resonator shown in  FIG. 21 .  FIG. 23  is a sectional view at a center of the resonator shown in  FIG. 21 . 
         [0056]    A p-electrode  19  (electrode) is formed on a p-GaN contact layer  18  but is not formed on the p-AlGaN clad layer  24 . In other respects, the construction is the same as that in the third embodiment. 
         [0057]    The semiconductor laser thus constructed can be manufactured by selectively growing the p-electrode  19  on the p-GaN contact layer  18  while covering the p-AlGaN clad layer  24  with a mask or by forming the p-electrode  19  on the entire surface and thereafter etching the p-electrode  19  on the p-AlGaN clad layer  24 . 
         [0058]    This construction enables reducing the current flowing in the vicinities of the resonator end surfaces, thus enabling prevention of end surface deterioration with improved reliability. 
         [0059]    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. 
         [0060]    The entire disclosure of a Japanese Patent Application No. 2008-017309, filed on Jan. 29, 2008 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.