Abstract:
A semiconductor laser diode is provided, including: an active layer; an upper clad layer formed above the active layer; a first lower clad layer formed below the active layer; a second lower clad layer formed under the first lower clad layer; and a substrate formed under the second lower clad layer, wherein a refractive index of the first lower clad layer is identical with a refractive index of the upper clad layer and is lower than a refractive index of the second lower clad layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims the benefit of Korean Patent Application No. 2004-35864, filed on May 20, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       1. FIELD OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor laser diode, and more particularly, to an improved semiconductor laser diode including a clad layer having an asymmetric refractive index.  
       2. DESCRIPTION OF THE RELATED ART  
       [0003]     Generally, a semiconductor laser diode has a comparatively small size and a threshold current for laser oscillation, which is less than a threshold current of a general laser device. Therefore, semiconductor laser diodes are widely used as an element for high-speed data recording and reading in a communication field or players in which optical disks are used.  
         [0004]      FIG. 1A  is a cross-sectional view of a conventional semiconductor laser diode.  FIG. 1B  illustrates a refractive index profile and optical field distribution of the semiconductor laser diode of  FIG. 1A .  
         [0005]     Referring to  FIGS. 1A and 1B , a conventional semiconductor laser diode has a configuration in which an n-clad layer  11 , an n-waveguide layer  12 , an active layer  30 , a p-waveguide layer  22 , and p-clad layers  21  are sequentially deposited on a substrate  10 . An etch stop layer to form a ridge is interposed between the p-clad layers  21 . A p-electrode layer  40  is formed on the top surface of the p-clad layer  21 , and an n-electrode layer  50  is formed on the bottom surface of the substrate  10 .  
         [0006]     Here, the p-clad layer  21  formed above the active layer  30  and the n-clad layer  11  formed below the active layer  30  have lower refractive indexes n upper  and n lower , respectively than the refractive index n active  of the active layer  30 , and the refractive index n upper  of the p-clad layer  21  is identical with the refractive index n lower  of the n-clad layer  11 .  
         [0007]     However, in a conventional optical storage device, a semiconductor laser diode is required to have a smaller far field vertical beam divergence angle than approximately 19°. A semiconductor laser diode having a configuration as described above cannot obtain a small far field vertical beam divergence angle if a thin waveguide layer is formed to obtain a large output of power.  
         [0008]     A refractive index profile of a semiconductor laser diode including a clad layer having an asymmetric refractive index to solve the problem described above is illustrated in  FIG. 2A . Referring to  FIG. 2A , a p-clad layer and an n-clad layer are formed on both sides of an active layer respectively, and a refractive index of the n-clad layer is higher than a refractive index of the p-clad layer. Accordingly, the p-clad layer and the n-clad layer have asymmetric refractive indexes with the active layer as a center.  
         [0009]     A semiconductor laser diode having the configuration as described above has an advantage to decrease a far field vertical beam divergence angle by dispersing a near field. However, asymmetry of the near field increases significantly which leads to deflection of a laser beam according to an output power as illustrated in  FIG. 2B , and potential barrier energy, which is inversely proportional to a refractive index, is reduced in an n-clad layer, thereby suffering more carrier overflow.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides an improved semiconductor laser diode including a clad layer having an asymmetric refractive index.  
         [0011]     According to an aspect of the present invention, there is provided a semiconductor laser diode comprising: an active layer; an upper clad layer formed above the active layer; a first lower clad layer formed below the active layer; a second lower clad layer formed under the first lower clad layer; and a substrate formed under the second lower clad layer, wherein a refractive index of the first lower clad layer is identical with a refractive index of the upper clad layer and is lower than a refractive index of the second lower clad layer.  
         [0012]     In this case, an etch stop layer may be formed in the upper clad layer.  
         [0013]     First and second electrode layers are provided on the top surface of the upper clad layer and the bottom surface of the substrate, respectively.  
         [0014]     The active layer may have one of a multiple quantum wells configuration and a single quantum well configuration.  
         [0015]     The active layer may be composed of a compound semiconductor of GaInP series.  
         [0016]     The upper clad layer may be composed of a compound semiconductor of p-AlGaInP series, and the first and second lower clad layers may be composed of compound semiconductors of n-AlGaInP series.  
         [0017]     An upper waveguide layer and a lower waveguide layer may be further formed between the active layer and the upper clad layer and between the active layer and the first lower clad layer, respectively. In this case, the upper waveguide layer and the lower waveguide layer may be composed of compound semiconductors of p-AlGaInP series and n-AlGaInP series, respectively. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0019]      FIG. 1A  is a cross-sectional view of a conventional semiconductor laser diode;  
         [0020]      FIG. 1B  is a diagram illustrating a refractive index profile and optical field distribution of the semiconductor laser diode of  FIG. 1A ;  
         [0021]      FIG. 2A  is diagram illustrating a refractive index profile of another conventional semiconductor laser diode;  
         [0022]      FIG. 2B  is a diagram illustrating deflection of a laser beam of the semiconductor laser diode of  FIG. 2A ;  
         [0023]      FIG. 3  is a cross-sectional view of a semiconductor laser diode according to an embodiment of the present invention;  
         [0024]      FIG. 4  is a diagram illustrating a refractive index profile and optical field distribution of the semiconductor laser diode of  FIG. 3 ; and  
         [0025]      FIG. 5  is a boxplot comparatively illustrating far field vertical beam divergence angles of the semiconductor laser diode according to an embodiment of the present invention and a conventional semiconductor laser diode. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Hereinafter, a preferred embodiment of the present invention will now be described with reference to the attached drawings.  
         [0027]     A semiconductor laser diode according to an embodiment of the present invention is not restricted to a following embodiment, and other embodiments including other compound semiconductor material of III-V group are possible.  
         [0028]      FIG. 3  is a cross-sectional view of the semiconductor laser diode according to an embodiment of the present invention.  FIG. 4  is a diagram illustrating a refractive index profile and optical field distribution of the semiconductor laser diode of  FIG. 3 .  
         [0029]     Referring to  FIGS. 3 and 4 , the semiconductor laser diode according to an embodiment of the present invention including a substrate  110  formed of GaAs, and a lower clad layer  111 , an active layer  130 , and upper clad layers  121  are sequentially deposited on the substrate  110 . Here, the lower clad layer  111  is composed of a first lower clad layer  111   a  and a second lower clad layer  111   b . On the other hand, a lower waveguide layer  112  can be formed between the active layer  130  and the first lower clad layer  111   a , and an upper waveguide  122  can be formed between the active layer  130  and the upper clad layer  121 . A first electrode layer  140  and a second electrode layer  150  are formed on the top surface of the upper clad layer  121  and the bottom surface of the substrate  110 , respectively.  
         [0030]     A refractive index n lower2  of the second lower clad layer  111   b  is higher than a refractive index n lower 1  of the first lower clad layer  111   a , and the second lower clad layer  111   b  is formed of a compound semiconductor layer. For the sake of this, the second lower clad layer  111   b  formed on the substrate  110  can be composed of n-(Al 0.68 Ga 0.32 ) 0.5 In 0.5 P compound semiconductor, and the first lower clad layer  111   a  formed on the second lower clad layer  111   b  can be composed of n-(Al 0.7 Ga 0.3 ) 0.5 In 0.5 P compound semiconductor. The first and second lower clad layer  111   a  and  111   b  can be formed by epitaxially growing a compound semiconductor of AlGaInP series on the substrate  110  to change the amount of Al. On the other hand, the first and second lower clad layer  111   a  and  111   b  can be composed of another compound semiconductor of III-V group.  
         [0031]     The lower waveguide layer  112 , the active layer  130 , and the upper waveguide layer  122  are sequentially formed on the top surface of the first lower clad layer  111   a . Here, the lower waveguide layer  112  and the upper waveguide layer  122 , which guide laser oscillation are formed of compound semiconductor layers having refractive indexes which are higher than refractive indexes of the lower and upper clad layers  111  and  121 . For the sake of this, the lower waveguide layer  112  and the upper waveguide layer  122  can be composed of n-(Al 0.5 Ga 0.5 ) 0.5 In 0.5 P compound semiconductor and p-(Al 0.5 Ga 0.5 ) 0.5 In 0.5 P compound semiconductor, respectively.  
         [0032]     The active layer bringing about laser oscillation is formed of a compound semiconductor layer having a refractive index n active  which is higher than the refractive indexes of the lower and upper waveguide layer  112  and  122 . For the sake of this, the active layer  130  is composed of Ga 0.5 In 0.5 P compound semiconductor. Here, the active layer  130  has a configuration of a multiple quantum well or a single quantum well.  
         [0033]     The upper clad layer  121  formed on the top surface of the upper waveguide layer  122  is formed of a compound semiconductor layer having a refractive index n upper  identical with the refractive index n lower1  of the first lower clad layer  111   a . For the sake of this, the upper clad layer  121  can be composed of p-(Al 0.7 Ga 0.3 ) 0.5 In 0.5 P compound semiconductor. On the other hand, an etch stop layer  123  may be formed in the upper clad layer  121  and when the upper portion of the upper clad layer  121  is etched to form a ridge, the etch stop layer  123  is required to accurately form a ridge having a designated height.  
         [0034]     The first electrode layer  140  that is a p-electrode layer is formed on the top surface of the upper clad layer  121 , and the second electrode  150  that is an n-electrode layer is formed on the bottom surface of the substrate  110 .  
         [0035]     As described above, in the semiconductor laser diode according to an embodiment of the present invention, the lower clad layer  111  is divided into the first and second lower clad layers  111   a  and  111   b , the refractive index n lower2  of the second clad layer  111   b  is greater than the refractive index n lower1  of the first lower clad layer  111   a , the refractive index n lower1  of the first lower clad layer  111   a  is identical with the refractive index n upper  of the upper clad layer. As described above, when the upper and lower clad layers  121  and  111  have asymmetric refractive indexes with the active layer  130  as a center, thereby dispersing a near field to reduce a far field vertical beam divergence angle. Also, because of the first lower clad layer  111   a  having a refractive index identical with a refractive index of the upper clad layer  121 , a reduction level of an optical confinement factor can be comparatively decreased, after all, an unavoidable rate of increase of a threshold current according to a beam divergence angle increase can be comparatively decreased. Also, distinctively from a conventional refractive index asymmetric configuration, because the refractive index of the first lower clad layer  111   a  is identical with the refractive index of the upper clad layer  121 . Therefore, deflection of a laser beam can be prevented, and since potential barrier energy of the first lower clad layer  111   a  is not reduced, the confinement of a carrier is effectively performed.  
         [0036]     Table 1 and  FIG. 5  are a table and a boxplot, respectively, in which experimental values of far field vertical beam divergence angles of a conventional semiconductor laser diode, including upper and lower clad layers having symmetric refractive indexes and a semiconductor laser diode according to an embodiment of the present invention, including upper and lower clad layers having asymmetric refractive indexes, are comparatively shown.  
                                         TABLE 1                                   Average value of far field               vertical beam divergence               angle (in degrees)   Standard deviation                                    Conventional   21.80°   1.19       semiconductor       laser diode       Semiconductor   17.48°   0.66       laser diode according       to an embodiment of       the present invention                  
 
         [0037]     Referring to Table 1 and  FIG. 5 , an average value of a far field vertical beam divergence angle of a conventional semiconductor laser diode is 21.80°, and an average value of a far field vertical beam divergence angle of a semiconductor laser diode according to an embodiment of the present invention is 17.48°. Accordingly, the far field vertical beam divergence of the semiconductor laser diode according to an embodiment of the present invention is reduced approximately 20% from the far field vertical beam divergence of the conventional semiconductor laser diode.  
         [0038]     As described above, the semiconductor laser diode according to an embodiment of the present invention has following advantages.  
         [0039]     First, since upper and lower clad layers have asymmetric refractive indexes with an active layer as a center, a far field vertical beam divergence angle is reduced by dispersion of a near field.  
         [0040]     Second, since a lower clad layer of two lower clad layers, which is adjacent to an active layer, has the same refractive index of an upper clad layer, reduction of optical confinement factor is comparatively decreased, after all, a threshold current can be reduced.  
         [0041]     Third, since potential barrier energy of a lower clad layer is not reduced, the confinement of a carrier is more effectively performed.  
         [0042]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.