Abstract:
A monolithic semiconductor laser having plural semiconductor lasers having different emission wavelengths from each other, including: a semiconductor substrate; a first double hetero-structure formed within a first area on the semiconductor substrate and having first clad layers disposed above and below a first active layer; and a second double hetero-structure formed within a second area on the semiconductor substrate and having second clad layers disposed above and below a second active layer. The first and second active layers are made of different semiconductor materials from each other. The first clad layers above and below the first active layer are of approximately the same semiconductor materials and the second clad layers above and below the second active layer are of approximately the same semiconductor materials.

Description:
This application is a Divisional of U.S. application Ser. No. 10/898,196, filed Jul. 26, 2004, claiming priority of Japanese Application No. 2003-339768, filed Sep. 20, 2003, the entire contents of each of which are hereby incorporated by reference. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     Related patent application is commonly assigned Japanese Patent Application No. 2003-339768 filed on Sep. 30, 2003, which is incorporated by reference into the present patent application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a monolithic semiconductor laser and a method of manufacturing the same, and more particularly, to a monolithic semiconductor laser comprising a stripe structure (current confining structure) and a method of manufacturing the same. 
     2. Description of the Related Art 
     An information reader/writer apparatus or the like for DVDs and CDs uses a monolithic semiconductor laser in which the single substrate seats multiple of semiconductor lasers which have different emission wavelengths. In such a monolithic semiconductor laser, the single substrate seats a semiconductor laser whose emission wavelength is 650 nm and a semiconductor laser whose emission wavelength is 780 nm, for example. 
     After one of the semiconductor lasers is formed on the substrate, the location of the other one of the semiconductor lasers is determined using alignment marks which are provided on the substrate, to thereby fabricate the monolithic semiconductor laser (K. Nemoto and K. Miura: “A Laser Coupler for DVD/CD Playback Using a Monolithic-integrated Two-wavelength Laser Diode,” JSAP International, No. 3, pp. 9-14 (January 2001)). 
     SUMMARY OF THE INVENTION 
     In an information reader/writer apparatus or the like for DVDs and CDs, a monolithic semiconductor laser and other components such as a lens are assembled together and an optical system is accordingly obtained. 
     However, in a monolithic semiconductor laser fabricated in accordance with a conventional manufacturing method, the distance between emission points of two semiconductor lasers is dependent upon the accuracy of aligning locations as described above. Between different production batches therefore, the distance between the emission points, namely relative locations of stripe structures, is different and it is therefore necessary to finely adjust the locations of the lens, etc. 
     The fine adjustment makes the assembling step complex and increases a manufacturing cost. 
     An object of the present invention is to provide a monolithic semiconductor laser in which relative locations of stripe structures, namely the distance between emission points, remain constant and a method of manufacturing the same. 
     The present invention is directed to a monolithic semiconductor laser having plural semiconductor lasers having different emission wavelengths from each other, including: a semiconductor substrate; a first double hetero-structure formed within a first area on the semiconductor substrate and having first clad layers disposed above and below a first active layer; and a second double hetero-structure formed within a second area on the semiconductor substrate and having second clad layers disposed above and below a second active layer. The first and second active layers are made of different semiconductor materials from each other. The first clad layers above and below the first active layer are of approximately the same semiconductor materials and the second clad layers above and below the second active layer are of approximately the same semiconductor materials. 
     In the monolithic semiconductor laser according to the present invention, the gap between stripe structures (current confining structures) of plural semiconductor lasers, i.e., the distance between emission points is always approximately constant. 
     “The same materials” herein referred to are semiconductor materials which are the same in terms of material and/or composition. “Different materials” herein referred to are semiconductor materials which are different in terms of material and/or composition. 
     In another aspect of the present invention, a method of manufacturing a monolithic semiconductor laser having plural semiconductor lasers having different emission wavelengths is provided comprising preparing a semiconductor substrate, stacking within a first area on the semiconductor substrate a first semiconductor layer including a first double hetero-structure in which an active layer is disposed between an upper and a lower clad layers, and stacking within a second area on the semiconductor substrate a second semiconductor layer including a second double hetero-structure in which an active layer is disposed between an upper and a lower clad layers. An etching-resistant pattern film is formed on the first double hetero-structure which is formed within the first area and the second double hetero-structure which is formed within the second area. A plurality of first stripes in an upper surface of the first double hetero-structure and a plurality of second stripes in an upper surface of the second double hetero-structure are simultaneously formed using the pattern film. The first stripes are defined by a plurality of first trenches formed adjacent the first stripes and the second stripes are defined by a plurality of second trenches formed adjacent the second stripes. The second trenches are formed to a greater depth than a depth of the first trenches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1G  show cross sectional views of the steps of manufacturing the monolithic semiconductor laser according to the embodiment 1 of the present invention; 
         FIGS. 1H and 1I  show cross sectional views of the steps of manufacturing other monolithic semiconductor laser according to the embodiment 1 of the present invention; 
         FIGS. 2A-2C  show cross sectional views of the steps of manufacturing the monolithic semiconductor laser according to the embodiment 2 of the present invention; 
         FIGS. 3A-3C  show cross sectional views of the steps of manufacturing the monolithic semiconductor laser according to the embodiment 3 of the present invention; and 
         FIGS. 4A-4C  show cross sectional views of the steps of manufacturing the monolithic semiconductor laser according to the embodiment 4 of the present invention. 
         FIG. 5  shows a flowchart of a method of manufacturing a monolithic semiconductor laser according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIGS. 1A-1G  are cross sectional views of steps of manufacturing a monolithic semiconductor laser according to this embodiment which is generally denoted at  100 . The monolithic semiconductor laser  100  has a first semiconductor laser whose emission wavelength is 780 nm and a second semiconductor laser whose emission wavelength is 650 nm (See  FIG. 1G ). 
     The steps of manufacturing the monolithic semiconductor laser  100  will now be described with reference to  FIGS. 1A-1G . These steps include the following steps 1 through 7. 
     Step 1: As shown in  FIG. 1A , a GaAs substrate  1  of the n-type is prepared. On the GaAs substrate  1 , an n-AlGaInP lower clad layer  2 , an AlGaAs active layer (having the emission wavelength of 780 nm)  3  and a p-AlGaInP upper clad layer  4  are then stacked one atop the other. A GaInP etching stopper layer  5  is inserted in the p-AlGaInP upper clad layer  4 . Further, on the p-AlGaInP upper clad layer  4 , a p-GaAs cap layer  6  is formed. 
     The AlGaAs active layer  3  has such a composition which provides the emission wavelength of 780 nm. 
     The respective semiconductor layers  2  through  6  are formed using an MOCVD method for instance. 
     Step 2: As shown in  FIG. 1B , by means of a photolithographic technique and an etching technique, thus formed semiconductor layers  2  through  6  are removed except for within the area where the first semiconductor laser is to be formed. 
     Step 3: As shown in  FIG. 1C , by an MOCVD method for instance, semiconductor layers for forming the second semiconductor laser are stacked one atop the other. These semiconductor layers have the same compositions, the same impurity concentrations and the same film thicknesses as those of the semiconductor layers which are used to form the first semiconductor laser, with an exception of an AlGaInP active layer  13 . The AlGaInP active layer  13  has such a composition which provides the emission wavelength of 650 nm. 
     Step 4: As shown in  FIG. 1D , by means of a photolithographic technique and an etching technique, thus formed semiconductor layers  2  through  6  are removed except for within the area where the second semiconductor laser is to be formed, which is similar to the step 2 ( FIG. 1B ). This results in a stacked structure as that shown in  FIG. 1D . 
     Step 5: As shown in  FIG. 1E , photoresist layers  7  are formed and patterned on the semiconductor layers which are used as the first semiconductor laser and the second semiconductor laser. At this step, the photoresist layers  7  on the both sets of semiconductor layers are patterned simultaneously using a common photomask. 
     Instead of the photoresist layers  7 , other patterned films which are resistant against etching may be used. 
     Step 6: As shown in  FIG. 1F , the p-GaAs cap layer  6  is removed through wet etching which uses a solution which is obtained by mixing a tartaric acid and hydrogen peroxide. A wet etchant using a solution of a tartaric acid allows an improvement of etching selectivity between As-containing semiconductors and P-containing semiconductors. Because of this, the etching almost stops upon arrival at the P-containing semiconductor surface (which is the p-AlGaInP upper clad layer  4 ). 
     Step 7: As shown in  FIG. 1G , using an etchant containing a sulfuric acid, the p-AlGaInP upper clad layer  4  is etched until the GaInP etching stopper layer  5  gets exposed. 
     Use of a sulfuric acid-based etchant increases etching selectivity between the AlGaInP layer (the upper clad layer), the GaInP (the etching stopper layer) and the GaAs layer (the cap layer). Hence, as the p-AlGaInP upper clad layer  4  is etched using the p-GaAs cap layer  6  as a mask, the etching approximately stops when the GaInP etching stopper layer  5  gets exposed. 
     Insertion of the GaInP etching stopper layer  5  thus controls the shapes of ridge-type stripes in the depth direction. 
     Through the steps 1 to 7 described above, the monolithic semiconductor laser  100  is fabricated which includes the first semiconductor laser  101  and the second semiconductor laser  102  which have different emission wavelengths. 
     Metal electrodes are formed on the back surface of the GaAs substrate  1  and the front surface of the p-GaAs cap layer  6 , which will not be described here. 
     When the manufacturing method according to the embodiment is used, ridge structures for the two semiconductor lasers  101  and  102  having ridge-type stripe structures are fabricated at the same time. This ensures that the relative distance between the emission points A and B at which the 650/780 nm lasers are emitted always remains constant. Even if the location of the photomask gets deviated at the step 5, the relative distance stays unchanged while the locations of the emission points A and B are displaced. 
     In general, the location of an optical system containing a lens and the like is finely adjusted depending upon the locations of emission points within an optical apparatus which incorporates a monolithic semiconductor laser. 
     However, in the monolithic semiconductor laser  100  which is fabricated in accordance with the manufacturing method according to the embodiment, since the relative distance between the two emission points remains constant, such fine adjustment of the optical system is not necessary. 
     It is to be noted in particular that the first semiconductor laser and the second semiconductor laser are etched concurrently, which lessens the manufacturing steps. 
       FIGS. 1H and 1I  show cross sectional views of steps of manufacturing other monolithic semiconductor laser  150  according to the embodiment. In  FIGS. 1H and 1I , the same reference symbols as those in  FIGS. 1A-1G  denote the same or corresponding portions. 
     In accordance with this manufacturing method, the respective semiconductor layers are formed following the steps 1 through 4 described above ( FIGS. 1A-1D ). However, the GaInP etching stopper layer  5  is not formed. 
     After a resist mask  18  is formed, protons  19  are then implanted by an ion implantation method as shown in  FIG. 1H . 
     As a result, as shown in  FIG. 1I , the resistance values become high in parts of the p-AlGaInP upper clad layer  4  and the p-GaAs cap layer  6 , thereby forming high-resistance layers  8 . Metal electrodes (not shown) are formed on the front and the back surfaces after removal of the resist mask  18 , and the monolithic semiconductor laser  150  is thus completed. 
     In the monolithic semiconductor laser  150  as well, the relative distance between the stripe structures of two semiconductor lasers  103  and  104 , i.e., the relative distance between emission points C and D is constant. 
     Further, at one step, the high-resistance layers are formed at the same time for the first and the second semiconductor lasers. 
     Although this embodiment requires that the lower clad layers and the upper clad layers of the first and the second semiconductor lasers  101  and  102  are all made of the semiconductor material having the same material and the same composition, different semiconductor materials having different material and/or different compositions may be used between the first semiconductor laser  101  and the second semiconductor laser  102 . This remains similar in the following embodiments. 
     To be more specific, the first semiconductor laser  101  has a structure in which the active layer  3  which is a mono-layer or multi-layer of Al x1 Ga 1-x1 As (0≦x1≦1) is located between the lower and the upper clad layers  2  and  4  of (Al x2 Ga 1-x2 ) y2 In 1-y2 P (0≦x2≦1, 0≦y2≦1), while the second semiconductor laser  102  has a structure in which the active layer  13  which is a mono-layer or multi-layer of (Al x3 Ga 1-x3 ) y3 In 1-y3 P (0≦x3≦1, 0≦y3≦1) is located between the lower and the upper clad layers  2  and  4  of (Al x2 Ga 1-x2 ) y2 In 1-y2 P (0≦x2≦1, 0≦y2≦1). The lower and the upper clad layers of the first semiconductor laser  101  may be made of (Al x2 Ga 1-x2 ) y2 In 1-y2 P (0≦x2≦1, 0≦y2≦1) and the lower and the upper clad layers of the second semiconductor laser  102  may be made of (Al 4 Ga 1-x4 ) y4 In 1-y4 P (0≦x4≦1, 0≦y4≦1) which is different from the material of the first semiconductor laser  101 . This is the same in the following embodiments. 
     As the materials of the first and the second semiconductor lasers, an AlGaN-based material, a GaInNAs-based material or an AlGaInNAs-based material may be used in addition to an AlGaAs-based material and an AlGaInP-based material. As the materials of the active layers, an AlGaInAsP-based material or an AlGaInAs-based material may be used in addition to an AlGaAs-based material and an AlGaInP-based material. The active layers may be mono-layer or multi-layer. This is the same in the following embodiments. 
     Embodiment 2 
       FIGS. 2A-2C  show cross sectional views of steps of manufacturing a monolithic semiconductor laser according to this embodiment which is generally denoted at  200 . The monolithic semiconductor laser  200  has two semiconductor lasers  201  and  202  which have different emission wavelengths (The emission wavelengths are 780 nm and 650 nm for example) ( FIG. 2C ). 
     In the monolithic semiconductor laser  200  according to the embodiment 2, the compositions and the like of other layers than active layers are different between the two semiconductor lasers  201  and  202 . Use of stopper layers makes it possible to etch at a high accuracy also in these structures. 
     Through approximately the same steps ( FIGS. 1A-1D ) as those exercised in the embodiment 1 described above, a stacked structure as that shown in  FIG. 2A  is fabricated on an n-GaAs substrate  1 . 
     As the first semiconductor laser, on the n-GaAs substrate  1 , an n-AlGaInP lower clad layer  21 , an AlGaAs active layer  22 , a GaInP etching stopper layer  23 , a p-AlGaInP upper clad layer (first upper clad layer)  24  and a p-GaAs cap layer  25  are stacked one atop the other. 
     Meanwhile, as the second semiconductor laser, on the n-GaAs substrate  1 , an n-AlGaInP lower clad layer  31 , an AlGaInP active layer  32 , a GaInP etching stopper layer  23 , a p-AlGaInP upper clad layer (second upper clad layer)  24 , p-AlGaAs upper clad layers  33 ,  34 , a p-GaAs cap layer  25  are stacked one atop the other. The layers denoted at different reference symbols (those denoted at  33  and  34  for instance) have different compositions. Further, the respective semiconductor layers are formed by an MOCVD method for example. 
     Following this, photoresist layers are formed and patterned on the semiconductor layers which are used as the first and the second semiconductor lasers  201  and  202 , thereby forming a resist mask  30 . At this step, the photoresist layers on the both sets of semiconductor layers are patterned simultaneously using a common photomask. 
     Using the resist mask  30  as an etching mask, the structures are thus etched halfway through into the p-AlGaInP upper clad layers  24 . 
     In the first semiconductor laser  201  and the second semiconductor laser  202 , as described above, the semiconductor lasers above the etching stopper layers  23  have different compositions and different film thicknesses. 
     Nevertheless, when the manufacturing method according to this embodiment is used, even the semiconductor lasers different compositions are etched by such an etching method which realizes approximately equal etching speeds. 
     To be more specific, during ECR etching which uses a mixture gas of a chloride gas and an oxygen gas as an etching gas, the AlGaAs-based semiconductor lasers and the AlGaInP-based semiconductor lasers are etched at approximately equal etching speeds. 
     Through ECR etching for instance, the structures are etched halfway through into the p-AlGaInP upper clad layers  24  as shown in  FIG. 2B . 
     After removal of the resist mask  30 , the p-AlGaInP upper clad layers  24  are etched using a sulfuric acid-based etchant until the GaInP etching stopper layers  23  get exposed. In this case, the GaAs cap layers  25  serve as an etching mask. 
     As described above, during the etching using a sulfuric acid-based etchant, the etching almost stops when the GaInP etching stopper layers  23  get exposed. 
     Through these steps, ridge-type stripe structures as those shown in  FIG. 2C  are fabricated. Following this, metal electrodes (not shown) are formed on the front and the back surfaces, whereby the monolithic semiconductor laser  200  is completed. 
     In this monolithic semiconductor laser  200 , the gap between the ridge-type stripes of the two semiconductor lasers  201  and  202 , namely, the relative distance between emission points E and F is constant. 
     According to this embodiment, the first semiconductor laser  201  and the second semiconductor laser  202  include the GaInP etching stopper layers  23  and the upper clad layers  24  which are formed immediately on the GaInP etching stopper layers  23  and exhibit high etching selectivity against the GaInP etching stopper layers  23 . Hence, it is possible to accurately control the etching of the semiconductor lasers above the GaInP etching stopper layers  23 . 
     Meanwhile, since the semiconductor lasers below the GaInP etching stopper layers  23  are not etched, the compositions of them may be chosen without considering the etching step. 
     Alternatively, in the monolithic semiconductor laser  200 , instead of forming the ridge-type stripes, high-resistance layers may be formed to obtain the stripe structures. 
     Embodiment 3 
       FIGS. 3A-3C  show cross sectional views of steps of manufacturing a monolithic semiconductor laser according to this embodiment which is generally denoted at  300 . The monolithic semiconductor laser  300  comprises two semiconductor lasers  301  and  302  which have different emission wavelengths (The emission wavelengths are 780 nm and 650 nm for example) ( FIG. 3C ). 
     In the monolithic semiconductor laser  300 , ridge-type stripes are formed so that the ridge-type stripes have different depths between the two semiconductor lasers  301  and  302 , and the refractive indices of the ridge-type stripes are adjusted. 
     In the monolithic semiconductor laser  300 , first, through approximately the same steps ( FIGS. 1A-1D ) as those exercised in the embodiment 1 described earlier, a stacked structure as that shown in  FIG. 3A  is fabricated on an n-GaAs substrate  1 . 
     As the first semiconductor laser, on the n-GaAs substrate  1 , an n-AlGaInP lower clad layer  41 , an AlGaAs active layer  42 , a GaInP etching stopper layer  43 , a p-AlGaInP upper clad layer (first upper clad layer)  44  and a p-GaAs cap layer  45  are stacked one atop the other. 
     On the other hand, as the second semiconductor laser, on the n-GaAs substrate  1 , an n-AlGaInP lower clad layer  51 , an AlGaInP active layer  52 , an n-AlGaAs upper clad layer  53 , a GaInP etching stopper layer  43 , a p-AlGaInP upper clad layer (second upper clad layer)  44 , p-AlGaAs upper clad layers  54 ,  55 , a p-GaAs cap layer  45  are stacked one atop the other. The layers denoted at different reference symbols (those denoted at  54  and  55  for instance) have different compositions. Further, the respective semiconductor layers are formed by an MOCVD method for example. 
     Following this, photoresist layers are formed and patterned on the semiconductor layers which are used as the first and the second semiconductor lasers  301  and  302 , thereby forming a resist mask  40 . At this step, the photoresist layers on the both sets of semiconductor layers are patterned simultaneously using a common photomask. 
     Using the resist mask  40  as an etching mask, the structures are etched halfway through into the p-AlGaAs upper clad layers  44 , as shown in  FIG. 3B . This etching may be ECR etching which is used in the embodiment 2 described above. 
     As in the embodiment 2, the p-AlGaInP clad layers  44  are then etched using a sulfuric acid-based etchant until the GaInP etching stopper layers  43  get exposed after removal of the resist mask  40 . In this case, the GaAs cap layers  45  serve as an etching mask. The etching almost stops when the GaInP etching stopper layers  43  get exposed. 
     Through these steps, ridge-type stripe structures as those shown in  FIG. 3C  are fabricated. Following this, metal electrodes (not shown) are formed on the front and the back surfaces, whereby the monolithic semiconductor laser  300  is completed. 
     In this fashion, when the manufacturing method according to this embodiment is used, the semiconductor lasers  301  and  302  in which the ridge-type stripes have different depths are fabricated on the same substrate in such a manner that the gaps between the ridges are maintained approximately constant. 
     Embodiment 4 
       FIGS. 4A-4C  show cross sectional views of steps of manufacturing a monolithic semiconductor laser according to this embodiment which is generally denoted at  400 . The monolithic semiconductor laser  400  has two semiconductor lasers  401  and  402  which have different emission wavelengths (The emission wavelengths are 780 nm and 650 nm for example) ( FIG. 4C ). 
     In the monolithic semiconductor laser  400 , the two semiconductor lasers  401  and  402  have different heights (namely, the heights from the surface of a GaAs substrate  1  up to the surfaces of GaAs cap layers  50 ). 
     In the monolithic semiconductor laser  400 , first, through approximately the same steps ( FIGS. 1A-1D ) as those exercised in the embodiment 1 described earlier, a stacked structure as that shown in  FIG. 4A  is fabricated on the n-GaAs substrate  1 . 
     As the first semiconductor laser  401 , on the n-GaAs substrate  1 , an n-AlGaInP lower clad layer  61 , an AlGaAs active layer  62 , a GaInP etching stopper layer  63 , a p-AlGaInP upper clad layer (first upper clad layer)  64  and a p-GaAs cap layer  65  are stacked one atop the other. 
     On the other hand, as the second semiconductor laser  402 , on the n-GaAs substrate  1 , an n-AlGaInP lower clad layer  71 , an AlGaInP active layer  72 , an n-AlGaAs upper clad layer  73 , a GaInP etching stopper layer  63 , a p-AlGaInP upper clad layer (second upper clad layer)  64 , p-AlGaAs upper clad layers  74 ,  75 , a p-GaAs cap layer  65  are stacked one atop the other. The layers denoted at different reference symbols (those denoted at  54  and  55  for instance) have different compositions. Further, the respective semiconductor layers are formed by an MOCVD method for example. 
     As shown in  FIG. 4A , according to this embodiment, the first semiconductor laser  401  and the second semiconductor laser  402  have different heights, and there is thus a stepped surface. 
     Following this, photoresist layers are formed and patterned on the semiconductor layers which are used as the first and the second semiconductor lasers  401  and  402 , thereby forming a resist mask  50 . At this step, the photoresist layers on the both sets of semiconductor layers are patterned simultaneously using a common photomask. In this manner, even when the first and the second semiconductor lasers have different heights and there is a surface which is stepped to a certain extent, the both photoresist layers are patterned simultaneously. 
     Using the resist mask  50  as an etching mask, the structures are etched halfway through into the p-AlGaAs upper clad layers  64 , as shown in  FIG. 4B . This etching may be ECR etching which is used in the embodiment 2 described earlier. 
     As in the embodiment 2, the p-AlGaInP clad layers  64  are then etched using a sulfuric acid-based etchant until the GaInP etching stopper layers  63  get exposed after removal of the resist mask  50 . In this case, the GaAs cap layers  65  serve as an etching mask. The etching almost stops when the GaInP stopper layers  63  get exposed. 
     Through these steps, ridge-type stripe structures as those shown in  FIG. 4C  are fabricated. Following this, metal electrodes (not shown) are formed on the front and the back surfaces, whereby the monolithic semiconductor laser  400  is completed. 
     In this fashion, when the manufacturing method according to this embodiment is used, the semiconductor lasers  401  and  402  in which the ridge-type stripes have different heights are fabricated on the same substrate. 
     Alternatively, in the monolithic semiconductor laser  400 , instead of forming the ridge-type stripes, high-resistance layers may be formed to obtain the stripe structures. 
     While the foregoing has described the embodiments 1 through 4 in relation to a monolithic semiconductor laser which comprises two semiconductor lasers, the present invention is applicable to a monolithic semiconductor laser which comprises three or more semiconductor lasers. In addition, although the foregoing has described such semiconductor lasers which have emission wavelengths of 780 nm and 650 nm, the present invention is applicable also to semiconductor lasers which have other emission wavelengths.