Abstract:
A method of manufacturing an optical semiconductor device including: forming a mesa structure including a first conductivity type cladding layer, an active layer and a second conductivity type cladding layer in this order on a first conductivity type semiconductor substrate, an upper most surface of the mesa structure being constituted of an upper face of the second conductivity type cladding layer; growing a first burying layer burying both sides of the mesa structure at higher position than the active layer; forming an depressed face by etching both edges of the upper face of the second conductivity type cladding layer; and growing a second burying layer of the first conductivity type on the depressed face of the second conductivity type cladding layer and the first burying layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 13/094,117, filed on Apr. 26, 2011, which is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-102729, filed on Apr. 27, 2010 and Japanese Patent Application No. 2011-057014, filed on Mar. 15, 2011, which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to an optical semiconductor device and a method of manufacturing an optical semiconductor device. 
         [0004]    (ii) Related Art 
         [0005]    Japanese Patent Application Publication No. 2000-174389 discloses a semiconductor laser in which a p-type InP, an n-type InP and a p-type InP bury a mesa stripe including an active layer. The semiconductor laser may be manufactured through a process of burying the mesa stripe by laminating the p-type InP, the n-type InP and the p-type InP after forming the mesa stripe. 
       SUMMARY 
       [0006]    It is effective to narrow a hole leak path, in order to reduce a threshold current of a semiconductor laser. In concrete, two ways of arranging an n-type InP burying layer closer to a p-type cladding layer and reducing a thickness of the p-type cladding layer are effective. 
         [0007]    However, it is difficult to make a distance between the n-type InP burying layer and the p-type cladding layer constant in a wafer face, because of temperature distribution in the wafer face and a decomposition rate difference of material gas, and so on. This may result in variation of a narrowed width. A mask is formed on the p-type cladding layer when growing the burying layer selectively. The mask may cause a distortion of the active layer when the thickness of the p-type cladding layer is reduced. Therefore, the thickness of the p-type cladding layer must be larger. Accordingly, it is difficult to narrow the hole leak path. 
         [0008]    It is an object of the present invention to provide an optical semiconductor device of which leak path is narrowed, and a method of manufacturing the optical semiconductor device. 
         [0009]    According to an aspect of the present invention, there is provided a method of manufacturing an optical semiconductor device including: forming a mesa structure including a first conductivity type cladding layer, an active layer and a second conductivity type cladding layer in this order on a first conductivity type semiconductor substrate, an upper most surface of the mesa structure being constituted of an upper face of the second conductivity type cladding layer; growing a first burying layer burying both sides of the mesa structure at higher position than the active layer; forming a depressed face by etching both edges of the upper face of the second conductivity type cladding layer; and growing a second burying layer of the first conductivity type on the depressed face of the second conductivity type cladding layer and the first burying layer. 
         [0010]    According to another aspect of the present invention, there is provided an optical semiconductor device including: a mesa structure having a first conductivity type cladding layer, an active layer and a second conductivity type cladding layer in this order on a first conductivity type semiconductor substrate; a first burying layer burying both sides of the mesa at higher position than the active layer; a depressed face provided at both edges of an upper face of the second conductivity type cladding layer; and a second burying layer provided on the depressed face and the first burying layer, the second burying layer being the first conductivity type. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  through  FIG. 1D  illustrate a method of manufacturing a semiconductor laser in accordance with a comparative embodiment; 
           [0012]      FIG. 2A  through  FIG. 2D  illustrate a method of manufacturing a semiconductor laser in accordance with a first embodiment; 
           [0013]      FIG. 3A  through  FIG. 3D  illustrate the method of manufacturing the semiconductor laser in accordance with the first embodiment; 
           [0014]      FIG. 4  illustrates the method of manufacturing the semiconductor laser in accordance with the first embodiment; 
           [0015]      FIG. 5  illustrates an enlarged view of a mesa stripe; 
           [0016]      FIG. 6  illustrates current characteristics of a semiconductor laser; 
           [0017]      FIG. 7A  illustrates a schematic cross sectional view of a semiconductor laser in accordance with a second embodiment; 
           [0018]      FIG. 7B  illustrates a method of manufacturing the semiconductor laser in accordance with the second embodiment; 
           [0019]      FIG. 8  illustrates a schematic cross sectional view of a semiconductor laser in accordance with a third embodiment; and 
           [0020]      FIG. 9  illustrates current characteristics of a semiconductor laser. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    A description will be given of a best mode for carrying the present invention. 
       Comparative Embodiment 
       [0022]      FIG. 1A  through  FIG. 1D  illustrate a method of manufacturing a semiconductor laser in accordance with a comparative embodiment. As illustrated in  FIG. 1A , a mesa stripe is formed on an n-type InP substrate  10 . The mesa stripe has a structure in which an active layer  20  and a p-type cladding layer  30  are provided on an n-type cladding layer  11 . A mask  40  made of SiO 2  is formed on the p-type cladding layer  30  of the mesa stripe. 
         [0023]    Next, as illustrated in  FIG. 1B , a first burying layer  50  and a second burying layer  60  are grown on the n-type InP substrate  10  in this order on both sides of the mesa stripe. In this case, the first burying layer  50  is grown so that an end of the first burying layer  50  on the side of the mesa stripe is higher than an upper face of the active layer  20 . The first burying layer  50  is made of p-type semiconductor. The second burying layer  60  is made of n-type semiconductor. 
         [0024]    Then, as illustrated in  FIG. 1C , a third burying layer  70  made of p-type InP is grown so as to cover an upper face of the p-type cladding layer  30  and an upper face of the second burying layer  60 , after removing the mask  40 . The p-type cladding layer  30  and the third burying layer  70  act as a p-type cladding layer. A contact layer  80  made of InGaAs or the like is grown on the third burying layer  70 . After that, a needed electrode is provided. With the processes, the semiconductor laser in accordance with the comparative embodiment is manufactured. 
         [0025]    In the semiconductor laser in accordance with the comparative embodiment, an contact area between the p-type cladding layer  30 , the third burying layer  70  and the first burying layer  50  gets larger. Therefore, an amount of hole leak from the p-type cladding layer  30  and the third burying layer  70  to the first burying layer  50  is enlarged. In this case, a threshold current is increased, and direct modulation property is degraded. So, the hole leak may be restrained by arranging the n-type InP burying layer  60  closer to the p-type cladding layer  30  and reducing a thickness of the p-type cladding layer  30 . 
         [0026]    However, it is difficult to keep a distance between the n-type InP burying layer  60  and the p-type cladding layer  30  constant in a wafer face, because of temperature distribution in the wafer face, a decomposition rate difference of raw material gas, or the like. This may result in variation of narrowed width. It is necessary to provide the mask  40  on the p-type cladding layer  30  in order to grow the first burying layer  50  and the second burying layer  60  on an area except for the mesa stripe. When the thickness of the p-type cladding layer  30  is reduced, the mask  40  causes a strain in the active layer  20 . Therefore, the thickness of the p-type cladding layer  30  must be increased in the manufacturing method in accordance with the comparative embodiment. It is therefore difficult to narrow the hole leak path. 
       First Embodiment  
       [0027]    A description will be given of a method of manufacturing a semiconductor laser in accordance with a first embodiment.  FIG. 2A  through  FIG. 4  illustrate the method of manufacturing the semiconductor laser in accordance with the first embodiment. As illustrated in  FIG. 2A , the n-type cladding layer  11 , the active layer  20  and the p-type cladding layer  30  are grown on the n-type InP substrate  10 . Next, the mask  40  is formed in a stripe shape on an area of the p-type cladding layer  30  where the mesa stripe is to be formed. 
         [0028]    The n-type InP substrate  10  is, for example, made of n-type InP in which Sn (tin) of 1.0×10 18 /cm 3  is doped. The n-type cladding layer  11  is, for example, made of n-type InP having a thickness of 0.5 μm in which Si (silicon) of 1.0×10 18 /cm 3  is doped. For example, the active layer  20  has an InGaAsP-based multiple quantum well structure. The p-type cladding layer  30  is, for example, made of p-type InP having a thickness of 0.2 μm in which Zn (zinc) of 1.0×10 18 /cm 3  is doped. For example, the mask  40  is made of SiO 2 . 
         [0029]    Next, as illustrated in  FIG. 2B , the p-type cladding layer  30 , the active layer  20  and the n-type cladding layer  11  are subjected to a dry etching process with use of the mask  40  as an etching mask. Thus, a mesa stripe is formed on the n-type InP substrate  10 . For example, RIE (Reactive Ion Etching) method using SiCl 4  may be used as the dry etching process. A height of the mesa stripe without the mask  40  is, for example, 1.5 μm to 2.0 μm. 
         [0030]    Then, as illustrated in  FIG. 2C , the first burying layer  50  and the n-type burying layer  61  are grown on the n-type InP substrate  10  on both sides of the mesa stripe. In this case, the first burying layer  50  and the n-type burying layer  61  are selectively grown on an area except for the mask  40 . The first burying layer  50  is grown so that an end of the first burying layer  50  on the side of the mesa stripe is higher than an upper face of the active layer  20 . The first burying layer  50  is p-type semiconductor layer or highly-resistive semiconductor layer in which impurity (deep acceptor) such as Fe, Ti or Co generating deep acceptor level is doped. For example, the first burying layer  50  may be made of InP having a thickness of 1.3 μm in which Zn (Zinc) of 5.0×10 17 /cm 3  is doped or made of InP having a thickness of 1.3 μm in which Fe (iron) of 7.0×10 16 /cm 3  is doped. The n-type burying layer  61  is, for example, made of n-type InP having a thickness of 0.2 μm in which S (sulfur) of 1.0×10 19 /cm 3  is doped. 
         [0031]    Next, as illustrated in  FIG. 2D , the mask  40  is subjected to an etching process. Thus, the upper face of the mask  40 , and both end portions of the mask  40  on the side of the first burying layer  50  is etched. Thus, both end portions of the p-type cladding layer  30  on the side of the first burying layer  50  are exposed. A BHF (Buffered Hydrofluoric Acid) may be used in the etching process of  FIG. 2D . 
         [0032]    Then, as illustrated in  FIG. 3A , the exposed face of the p-type cladding layer  30  is subjected to an etching process. In this case, a face lower than the upper face of the mesa stripe (depressed face) is formed on both sides of the mesa stripe. For example, the p-type cladding layer  30  has only to be etched by approximately 0.1 μm. A liquid (NH 3 :H 2 O 2  is 1:1) may be used as the etching liquid. 
         [0033]    Next, as illustrated in  FIG. 3B , an n-type burying layer  62  is grown so as to cover the area of the p-type cladding layer  30  removed through the etched area of the p-type cladding layer  30  and the n-type burying layer  61 . The n-type burying layer  62  is, for example, made of the same material as the n-type burying layer  61 . The n-type burying layer  62  is, for example, made of n-type InP having a thickness of 0.25 μm in which S (sulfur) of 1.0×10 19 /cm 3  is doped. 
         [0034]    Then, as illustrated in  FIG. 3C , the third burying layer  70  is grown so as to cover an upper face of the p-type cladding layer  30  and an upper face of the n-type burying layer  62 . Further, a contact layer  80  is grown so as to cover an upper face of the third burying layer  70 . The third burying layer  70  is made of p-type semiconductor. The third burying layer  70  is, for example, made of the same material as the p-type cladding layer  30 . The third burying layer  70  is, for example, made of p-type InP having a thickness of 2.0 μm in which Zn (Zinc) of 1.2×10 18 /cm 3  is doped. The contact layer  80  is made of a material having a band gap that is narrower than that of the third burying layer  70 . The contact layer  80  is, for example, made of p-type InGaAs having a thickness of 0.5 μm in which Zn (zinc) of 1.5×10 19 /cm 3  is doped. As illustrated in  FIG. 3D , the p-type cladding layer  30  and the third burying layer  70  act as a p-type cladding layer  75 . The n-type burying layer  61  and the n-type burying layer  62  act as the second burying layer  60 . 
         [0035]    Next, as illustrated in  FIG. 4 , an n-type electrode  91  is formed on a bottom face of the n-type InP substrate  10 . A passivation film  92  is formed on the contact layer  80  except for an area above the mesa stripe. And, a p-type electrode  93  is formed so as to cover the exposed area of the contact layer  80  and the passivation film  92 . The n-type electrode  91  is, for example, made of AuGeNi. The passivation film  92  is made of an insulating material such as SiO 2 . The p-type electrode  93  is, for example, made of TiPtAu. 
         [0036]    With the processes, a semiconductor laser  100  is manufactured. A MOVPE (Metal Organic Vapor Phase Epitaxy) method may be used when growing above-mentioned semiconductor layers. Growth temperature in the MOVPE method may be approximately 600 degrees C. The InP is made from trimethyl indium and phosphine. Dimethyl zinc may be used for when doping Zn (zinc). Ferrocene may be used for when doping Fe (iron). Hydrogen sulfide may be used for when doping S (sulfur). Disilane may be used for when doping Si (silicon). 
         [0037]    In the embodiment, the processes of  FIG. 2D  and  FIG. 3A  are performed after growing the n-type burying layer  61 . However, the manufacturing method is not limited to the embodiment. For example, in the process of  FIG. 2C , the n-type burying layer  61  may not be grown. The second burying layer  60  may be grown after the etching process of  FIG. 3A . 
         [0038]      FIG. 5  illustrates an enlarged view around of the mesa stripe. As illustrated in  FIG. 5 , a thickness of a part contacting area of the p-type cladding layer  75  with the first burying layer  50  is reduced through the etching process. Thus, the hole leak path is narrowed. Therefore, the threshold current is reduced, and the direct modulation property is improved. The thickness of the contacting area is controlled better in the etching process than in the growth method. Thus, the thickness variation of the contacting area in a wafer face may be restrained. Therefore, variation of the narrowed width is restrained. And, the distortion of the active layer  20  caused by the mask  40  is restrained because the area of the p-type cladding layer  30  on where the mask  40  is provided is relatively thick. 
         [0039]      FIG. 6  illustrates current characteristics of the semiconductor laser. In  FIG. 6 , a horizontal axis indicates a current provided to the semiconductor laser, and a vertical axis indicates an outputting power of the semiconductor laser.  FIG. 6  illustrates the current characteristics of the semiconductor laser  100  in accordance with the first embodiment and the semiconductor laser in accordance with the comparative embodiment. An element length L is 200 μm. A measuring temperature is 75 degrees C. 
         [0040]    As illustrated in  FIG. 6 , the threshold current of the semiconductor laser  100  was lower than that of the semiconductor laser in accordance with the comparative embodiment. The outputting power of the semiconductor laser  100  was higher than that of the semiconductor laser in accordance with the comparative embodiment. This is because the hole leak path is narrowed in the semiconductor laser  100 . 
       Second Embodiment  
       [0041]    The first burying layer  50  may have a structure in which a highly resistive semiconductor layer and a p-type semiconductor layer are laminated.  FIG. 7A  illustrates a schematic cross sectional view of a semiconductor laser  100   a  in accordance with a second embodiment. The semiconductor laser  100   a  is different from the semiconductor laser  100  of  FIG. 4  in a point that a burying layer in which a highly resistive semiconductor layer  52  is laminated on a p-type semiconductor layer  51  is provided instead of the first burying layer  50 . Impurity such as Fe, Ti or Co generating deep acceptor level is doped in the highly resistive semiconductor layer  52 . With the structure, an element capacity may be reduced more, compared to a case where a p-type InP is used as the first burying layer  50 . Thus, the frequency characteristics of the semiconductor laser  100   a  are improved. 
         [0042]      FIG. 7B  illustrates a method of manufacturing the semiconductor laser  100   a . As illustrated in  FIG. 7B , instead of the first burying layer  50 , the p-type semiconductor layer  51  and the highly resistive semiconductor layer  52  are grown in this order on the n-type InP substrate  10  in the process of  FIG. 2C  when manufacturing the semiconductor laser  100   a . In this case, the highly resistive semiconductor layer  52  is grown so that an end of the p-type semiconductor layer  51  on the side of the mesa stripe is higher than the upper face of the active layer  20 . The p-type semiconductor layer  51  is, for example, made of InP having a thickness of 0.5 μm in which Zn of 5.0×10 17 /cm 3  is doped. The highly resistive semiconductor layer  52  is, for example, made of InP having a thickness of 0.7 μm in which Fe (iron) of 7.0×10 16 /cm 3  is doped. 
         [0043]    A MOVPE (Metal Organic Vapor Phase Epitaxy) method may be used when growing the p-type semiconductor layer  51  and the highly resistive semiconductor layer  52 . Growth temperature in the MOVPE method may be approximately 600 degrees C. The InP is made from trimethyl indium and phosphine. Dimethyl zinc may be used for when doping Zn (zinc). Ferrocene may be used for when doping Fe (iron). 
       Third Embodiment  
       [0044]      FIG. 8  illustrates a schematic cross sectional view of a semiconductor layer  100   b  in accordance with a third embodiment. The same components as those illustrated in  FIG. 8  have the same reference numerals as  FIG. 4 . In the embodiment, “W” and “h” of a region between the active layer  20  and the second burying layer  60  are researched. The “h” is a height from the active layer  20  to a lower face of the second burying layer  60  formed in the process of  FIG. 3A . The “W” is a width of the depressed face of the second burying layer  60  above the active layer  20 . 
         [0045]    Samples 1 to 3 of Table 1 were manufactured having a different combination of “W” and “h”. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 HEIGHT h 
                 WIDTH W 
                   
               
               
                   
                 (nm) 
                 (nm) 
                 W/h 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SAMPLE 1 
                 120 
                 200 
                 1.7 
               
               
                   
                 SAMPLE 2 
                 100 
                 200 
                 2.0 
               
               
                   
                 SAMPEL 3 
                 80 
                 160 
                 2.0 
               
               
                   
                   
               
             
          
         
       
     
         [0046]      FIG. 9  illustrates the current characteristics of the samples 1 to 3 of the semiconductor laser  100   b . In  FIG. 9 , a horizontal axis indicates a current provided to the semiconductor lasers, and a vertical axis indicates outputting power of the semiconductor lasers. An element length L is 200 μm. A measuring temperature is 75 degrees C. The width of the active layer  20  is 1.2 μm. As illustrated in  FIG. 9 , an operating current Iop@15 mW of the samples 2 and 3 at an outputting power of 15 mW is lower than the sample 1. This means that the rising efficiency or slope efficiency (mW/mA) is increased, compared to the sample 1. 
         [0047]    In the sample 3, the “W” is reduced further than in the sample 2, and the “h” is smaller than in the sample 2. The amount of hole leak has a correlation with the region defined by the “h” and the “W”. Increasing of the resistance value of the region may cause the reduction of the hole leak. The resistance value of the area defined by the “W” and the “h” is the same in the samples 2 and 3. However, in accordance with  FIG. 9 , a maximum optical outputting power of the sample 3 is larger than that of the sample 2. This is because the reduction of the “W” causes a reduction of an area shaded by the first burying layer  60  over the active layer  20 . That is, the reduction of the “W” causes an enlargement of a hole current clearance Wp with respect to the active layer  20 . Thus, conductance of the hole current is increased. The clearance Wp is defined with the second burying layer  60  formed on both sides of the mesa stripe. 
         [0048]    According to the research with the samples 1 to 3, it is preferable that the “h” is reduced in order to reduce the hole leak. And, it is preferable that the “W” is optimally defined with the correlation between the hole leak and the hole conductance with respect to the active layer  20 . The present inventors have confirmed that it is preferable that the “h” is preferably 100 nm or less, 1.8&lt;W/h, and the “Wp” is 500 nm or more. It is more preferable that the “h” is 80 nm or less. 
         [0049]    In the above mentioned embodiments, an active layer is provided on an n-type cladding layer, and a p-type cladding layer is provided on the active layer. However, the structure is not limited to the embodiments. For example, the p-type cladding layer, the active layer and the n-type cladding layer are provided in this order on a p-type semiconductor substrate. 
         [0050]    In the above-mentioned embodiments, the semiconductor laser is used as one example of an optical semiconductor device of the present invention. However, the optical semiconductor device is not limited to the semiconductor laser. For example, another optical semiconductor device such as a semiconductor optical amplifier (SOA) is used as the optical semiconductor device. 
         [0051]    The present invention is not limited to the specifically disclosed embodiments and variations but may include other embodiments and variations without departing from the scope of the present invention.