Abstract:
A semiconductor laser diode is provided. The semiconductor laser diode includes a substrate; a lower clad layer on a substrate; an active layer on the lower clad layer; and an upper clad layer on the active layer and having a ridge that protrudes in a vertical direction. In the upper clad layer, impurity layers are formed by diffusing impurities at both sides of the ridge to suppress high-order traverse-mode lasing. The impurities are Ga-ions free vacancies or Zn ions.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims the priority of Korean Patent Application No. 10-2004-0108030, filed on Dec. 17, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE DISCLOSURE  
       [0002]     1. Field of the Invention  
         [0003]     The present disclosure relates to a semiconductor laser diode, and more particularly, to a semiconductor laser diode, in which impurity regions are disposed on both sides of a ridge to suppress lasing in high-order traverse modes.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, a semiconductor laser diode is widely used to transmit data or write and read data at high speed in the field of communications or optical disk players because it is comparatively small-sized and requires a smaller threshold current for lasing than other typical laser devices.  
         [0006]     A laser diode for an optical disk player requires not only high optical efficiency and a long lifetime, but also a stable single transverse mode laser operating property (i.e., a kink free property). In particular, since a laser diode for a digital versatile disk (DVD) should operate at high speed, it requires a high power output characteristic.  
         [0007]      FIG. 1  is a cross sectional view of a conventional semiconductor laser diode.  
         [0008]     Referring to  FIG. 1 , the conventional semiconductor laser diode includes an n-clad layer  20 , a resonant layer  30 , and a p-clad layer  40 , which are sequentially stacked on a substrate  10 . The resonant layer  30  includes an n-waveguide layer  32 , an active layer  34 , and a p-waveguide layer  36 . An etch stop layer  42  for forming a ridge  44  may be interposed in the p-clad layer  40 . A p-contact layer  50  is disposed on the ridge  44 , and a current blocking layer  60  covers a top surface of the p-clad layer  40  and an edge of the p-contact layer  50 . A p-type electrode  70  is formed to contact a portion of the p-contact layer  50 , which is exposed by the current blocking layer  60 . Also, an n-type electrode  80  is disposed on a bottom surface of the substrate  10 .  
         [0009]      FIGS. 2A and 2B  are schematic cross-sectional views for explaining lasing modes of the semiconductor laser diode shown in  FIG. 1 , and  FIG. 3  is a graph showing an optical output characteristic of a conventional semiconductor laser diode.  
         [0010]     Referring to  FIG. 2A , in a fundamental mode of the semiconductor laser diode having the ridge  44 , a peak of the optical field is formed at a central region under the ridge  44  to generate laser-beams. In such a fundamental mode, the power of emitted laser beams is constantly increased at a threshold current or higher.  
         [0011]     Meanwhile, referring to  FIG. 2B , in a first-order mode (a high-order mode) of the semiconductor laser diode, lasing areas are mostly formed at both sides of the ridge  44 , and peaks of optical field exist at both sides of the ridge  44 . When laser beams are emitted in the first-order mode, since optical power in the fundamental mode is partially converted into optical power in the high-order mode, output power is not changed linearly at a predetermined current as shown in  FIG. 3  (this is generally termed a “kink level”). When the kink level is below a predetermined optical power (e.g., 250 mW), a laser diode for a high-speed DVD cannot normally operate.  
         [0012]     To overcome this drawback, it is necessary to suppress lasing in high-order traverse modes including a first-order mode.  
       SUMMARY OF THE DISCLOSURE  
       [0013]     The present invention may provide a semiconductor laser diode, in which impurity regions for increasing loss in high-order traverse-mode regions are disposed on both sides of a ridge to suppress lasing in high-order traverse modes.  
         [0014]     According to an aspect of the present invention, there may be provided a semiconductor laser diode including a substrate; a lower clad layer on a substrate; an active layer on the lower clad layer; and an upper clad layer on the active layer and having a ridge that protrudes in a vertical direction. Herein, the upper clad layer includes impurity layers, which are formed by diffusing impurities at both sides of the ridge to suppress high-order traverse-mode lasing.  
         [0015]     In one embodiment, the impurities may be vacancies formed in the upper clad layer.  
         [0016]     The vacancies may be formed by a depletion of Ga ions in the upper clad layer.  
         [0017]     In another embodiment, the impurities may be Zn ions doped into the upper clad layer.  
         [0018]     The impurity layers may be spaced at least 0.5 μm apart from both lateral surfaces of the ridge.  
         [0019]     The semiconductor laser diode may further include an etch stop layer formed in the upper clad layer under the ridge.  
         [0020]     The semiconductor laser diode may be formed of one of a GaAs-based semiconductor compound and a GaP-based semiconductor compound. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     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:  
         [0022]      FIG. 1  is a cross-sectional view of a conventional semiconductor laser diode;  
         [0023]      FIGS. 2A and 2B  are schematic cross-sectional views for illustrating lasing modes of the semiconductor laser diode shown in  FIG. 1 ;  
         [0024]      FIG. 3  is a graph showing an optical output characteristic of a conventional semiconductor laser diode;  
         [0025]      FIG. 4  is a cross-sectional view of a semiconductor laser diode according to an exemplary embodiment of the present invention;  
         [0026]      FIG. 5  is a diagram illustrating a process of forming impurity regions according to the present invention;  
         [0027]      FIG. 6  is a graph of photoluminescence (PL) peak versus the annealing temperature to confirm a difference in energy bandgap between a region where a vacancy is formed and a region where no vacancy is formed; and  
         [0028]      FIGS. 7A and 7B  are graphs of PL peak versus wavelength of a quantum well (QW) to confirm a difference between a Zn undoped region and a Zn doped region in a laser diode formed of AlGaInP. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0029]     The present invention will now be described more fully with reference to the accompanying drawings, in which an exemplary embodiment of the invention is shown. A semiconductor laser diode according to the invention should not be construed as being limited to a stacked structure of the embodiment set forth herein and may be embodied as different structures formed of other III-V group compound semiconductor materials.  
         [0030]      FIG. 4  is a cross-sectional view of a semiconductor laser diode according to an exemplary embodiment of the present invention.  
         [0031]     Referring to  FIG. 4 , the semiconductor laser diode according to the exemplary embodiment of the present invention includes an n-clad layer  120 , a resonant layer  130 , and a p-clad layer  140 , which are sequentially stacked on a substrate  110 . The resonant layer  130  includes an n-waveguide layer  132 , an active layer  134 , and a p-waveguide layer  136 . An etch stop layer  142  for forming a ridge  144  may be interposed in the p-clad layer  140 . A p-contact layer  150  is disposed on the ridge  144 , and a current blocking layer  160  covers a top surface of the p-clad layer  140  and an edge of the p-contact layer  150 . A p-type electrode  170  is formed to contact a portion of the p-contact layer  150 , which is exposed by the current blocking layer  160 . Also, an n-type electrode  180  is disposed on a bottom surface of the substrate  110 .  
         [0032]     The substrate  110  may be formed of a p-GaAs or n-GaP conductive material.  
         [0033]     The n-clad layer  120  may be formed of an n-(Al 0.7 Ga 0.3 ) 0.5 In 0.5 P compound semiconductor. In this case, the n-clad layer  120  may be obtained by epitaxially growing an AlGaInP-based compound semiconductor on the substrate  110  while varying the Al content.  
         [0034]     The n-waveguide layer  132 , the active layer  134 , and the p-waveguide layer  136  are sequentially formed on a top surface of the n-clad layer  120 . In this case, the n-waveguide  132  and the p-waveguide layer  136 , which guide lasing, are formed of compound semiconductors having higher refractive indexes than those of the n- and p-clad layers  120  and  140 . For example, the n-waveguide  132  and the p-waveguide layer  136  may be formed of an n-(Al 0.53 Ga 0.47 ) 0.5 In 0.5 P compound semiconductor and a p-(Al 0.53 Ga 0.47 ) 0.5 In 0.5 P compound semiconductor, respectively.  
         [0035]     Also, the active layer  134 , which causes lasing, is formed of a compound semiconductor having a higher refractive index than those of the n- and p-waveguide layers  132  and  136 . For example, the active layer  134  may be formed of a Ga 0.5 In 0.5 P compound semiconductor. Here, the active layer  134  may have one of a multiple quantum well (MQW) structure and a single quantum well (SQW) structure.  
         [0036]     The p-clad layer  140  disposed on a top surface of the p-waveguide layer  134  is formed of a compound semiconductor having the same refractive index as that of the n-clad layer  120 . For example, the p-clad layer  140  may be formed of a p-(Al 0.7 Ga 0.3 ) 0.5 In 0.5 P compound semiconductor. Meanwhile, the etch stop layer  142 , which is disposed in the p-clad layer  140 , assists in forming the ridge  144  to a precisely desired height while the ridge  144  is being formed by etching an upper portion of the p-clad layer  140 .  
         [0037]     In the meantime, predetermined impurity regions  146  are formed in the p-clad layer  140  on both sides of the ridge  144 . The impurity regions  146  include Ga-ions free vacancies or Zn ions as impurities. The impurity regions  146  induce scattering of an optical field region formed in a first-mode region and increase loss in the first-mode region, thus suppressing lasing. Preferably, the impurity regions  146  are spaced about 0.5 μm apart from both lateral surfaces of the ridge  144 . When the ridge  144  has a width of about 1 to 2 μm, because fundamental-mode lasing happens in a region that reaches 0.5 μm from the both lateral surfaces of the ridge  144 , the impurity regions  146  may be formed outside the region. The impurity regions  146  may be limited to the depth of regions  146  as illustrated with dotted lines in  FIG. 4  or may be formed to a greater depth such that impurities diffuse into the underlying layers.  
         [0038]     In the present embodiment, the semiconductor laser diode including the n- and p-clad layers  120  and  140  and the resonant layer  130  is formed of an AlGaInP compound, but the present invention is not limited thereto. That is, the semiconductor laser diode can be formed of other GaAs-based or GaP-based III-V group compound semiconductors.  
         [0039]      FIG. 5  is a diagram illustrating an example of a process of forming impurity regions according to the present invention.  
         [0040]     Referring to  FIG. 5 , a p-clad layer  140  and a p-contact layer  150  are sequentially formed on a resonant layer  130  using a known method. Thereafter, a diffusion control mask  210  is formed on the ridge forming portion  144  at a region that reaches 0.5 μm from both lateral surfaces of the ridge forming portion  144 . The mask  210  may be formed of silicon nitride. In this case, an etch stop layer  142  may be formed in the p-clad layer  140 . Subsequently, an absorption layer  220  is formed to cover the mask  210  on the p-clad layer  140  in order to generate impurities using a diffusion process. The absorption layer  220  may be formed of SiO 2  using a sputtering process. After that, a passivation layer  230  is formed on the absorption layer  220 .  
         [0041]     Thereafter, when the p-clad layer  140  is heated to a temperature of about 600 to 800° C., Ga ions diffuse from the p-clad layer  140  toward the absorption layer  220  so that Ga-ions free vacancies are formed in a portion of the p-clad layer  140 , which is not covered by the mask  210 . Typically, in order to control a diffusion region, bandgaps of quantum wells (QWs) of an impurity diffusion region (at both sides of the ridge forming portion  144 ) and an impurity non-diffusion region (corresponding to the ridge forming portion  144 ) are measured and compared with each other. Thus, as QWs in a region where a vacancy is formed are intermixed due to the diffusion, the bandgap of the QWs becomes greater and the wavelength of the QWs becomes shorter. Accordingly, it can be confirmed that vacancies are formed in regions that are not covered by the mask  210 . After the diffusion process is finished, the diffusion control mask  210 , the remaining absorption layer  220  and the passivation layer  230  are removed, and subsequent processes for fabricating a laser diode are performed.  
         [0042]     In order to confirm a difference in energy bandgap between a region where a vacancy is formed and a region where no vacancy is formed, a photoluminescence (PL) peak relative to an annealing temperature was measured, and results of the measurement was illustrated in  FIG. 6 . Referring to  FIG. 6 , the vacancy resulting from a raise in the annealing temperature of the p-clad layer  140  of the laser diode composed of AlGaInP can be observed from a difference of PL peak and PL peak shift.  
         [0043]     In the meantime, when a ZnO layer is formed in place of the SiO 2  absorption layer  220  in  FIG. 6 , Zn ions diffuse into a portion of the p-clad layer  140 , which are not covered by the mask  210 , thus forming impurities.  FIGS. 7A and 7B  are graphs of PL peak versus wavelength in QWs of a Zn undoped region and a Zn doped region in a laser diode composed of AlGaInP, respectively. In this case, the p-clad layer  140  was annealed at a temperature of about 510 ° C. for 30 minutes.  
         [0044]     Referring to  FIGS. 7A and 7B , PL reached a peak at a wavelength of 645.7 nm in the QW of the Zn undoped region, whereas PL reached a peak at a wavelength of 615.7 nm in the QW of the Zn doped region. A reduction in wavelength at which PL reaches a peak in a certain region indirectly shows that impurities diffuse into the region. In other words, impurity regions, in which Zn ions are doped or vacancies are formed from a scattering layer, and the scattering layer increases loss in high-order traverse-mode including a first-order mode. Thus, lasing can be suppressed in the high-order modes except a fundamental mode in which there are no or few portions that overlap impurity regions. As a result, a kink level caused by high-order lasing can be elevated to a desired high level.  
         [0045]     As described above, in a semiconductor laser diode according to the present invention, impurity regions formed at both sides of a ridge can increase loss in a high-order traverse mode region so that lasing can be suppressed in high-order traverse modes. Hence, an optical power at which a kink level is generated can be elevated, thus resulting in an excellent semiconductor laser diode, in which the kink level is not generated in a predetermined optical output region.  
         [0046]     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.