Patent Publication Number: US-2006018352-A1

Title: Ridge-type semiconductor laser and method of fabricating the same

Description:
BACKGROUND OF THE INVENTION  
      This application claims the priority of Korean Patent Application No. 2004-56417, filed on Jul. 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  
      The present invention relates to a semiconductor laser and a fabrication method thereof, and more particularly, to a ridge-type semiconductor laser and a method of fabricating the same.  
      2. Description of the Related Art  
      Generally, a ridge-type semiconductor laser has advantages of simple fabrication processes and a high production yield because it can be fabricated without complicated etch process and regrowth process in comparison with a buried heterostructure (BH) laser.  
       FIG. 1  is a sectional view illustrating an example of a conventional ridge-type semiconductor laser.  
      In specific, an InGaAsP active layer  3  and a p-InP layer  5  are formed on an n-InP substrate  1 . An InGaAsP etch stop layer  7  is formed on the p-InP layer  5 , and a p-InP ridge  9  is formed on the InGaAsP etch stop layer  7 .  
      The p-InP ridge  9  is formed using the InGaAsP etch stop layer  7  during the formation process. An InGaAs electrode contact layer  11  is formed on the p-InP ridge  9 , and a passivation layer  13  is formed on both sidewalls of the p-InP ridge  9 . An electrode metal layer  15  is formed on the InGaAs electrode contact layer  11  and the passivation layer  13 .  
      In the ridge-type semiconductor laser, the optical mode is determined by the p-InP ridge  9 . Particularly, the dimension of the optical mode in the lateral direction is determined by the width of the ridge. In the ridge-type semiconductor laser, the electrode metal layer  15  is applied with an anode, and the n-InP substrate  1  is applied with a cathode. Thus, the holes injected by the InGaAs electrode contact layer  11  go into the InGaAsP active layer  3  along the p-InP ridge  9 . Further, the electrons coming into the InGaAsP active layer  3  along the n-InP substrate  1  are recombined, so as to flow an electric current. When the stimulated emission by light is increased with recombination, a laser starts lasing. Since the diffusion length of electrons is greater than that of holes, the width of the active region in the InGaAsP active layer  3  is determined by the diffusion of holes. Thus, the width of the active region in the InGaAsP active layer  3  is determined by the width W of the ridge.  
      However, in the ridge-type semiconductor laser of  FIG. 1 , a significant amount of current may be lost, not serving to produce light, because the extent that current is spread along the InGaAsP active layer  3  is much greater than the dimension of the optical mode. Therefore, the ridge-type semiconductor laser of  FIG. 1  has threshold currents higher than the BH laser.  
       FIG. 2  is a sectional view illustrating an example of a conventional ridge-type semiconductor laser. In specific, an n-GaN layer  23  is formed on a substrate  21 , and an active layer  25  is formed on the n-GaN layer  23 . A p-AlGaN/GaN layer  27  and a p-AlGaN/GaN ridge  29  are sequentially formed on the active layer  25 . Depletion layers  31  are formed on the active layer  25  on both sides of the p-AlGaN/GaN layer  27 .  
      A p-GaN electrode contact layer  33  is formed on the p-AlGaN/GaN ridge  29 . Passivation layers  35  are respectively formed on both sidewalls and the upper surface of the p-AlGaN/GaN ridge  29 , and on the upper surface of the depletion layer  31 , to expose a partial surface of the p-GaN electrode contact layer  33 , and a partial surface of the depletion layer  31 . A current inflow electrode  37  is formed on a partial surface of the exposed p-GaN electrode contact layer  33 , and an electrode for current inflow path control  39  is formed on a partial surface of the exposed depletion layer  31 .  
      The ridge-type semiconductor laser of  FIG. 2  includes the electrode for current inflow path control  39  for reducing a width that current is spread in order to lower the threshold current of a ridge-type semiconductor laser. That is, the electrode for current inflow path control  39  controls the width of the path through which a current flows on the bottoms of the both sides of the p-AlGaN/GaN ridge  29 . The electrode for current inflow path control  39  makes current flow just with a constant width when a reverse direction of voltage is applied to form the depletion layer  31  as shown in  FIG. 2 . By controlling the reverse voltage applied to the electrode for current inflow path control  39  to change the thickness of the depletion layer  31 , the width of the path through which current flows can be controlled. Therefore, the ridge-type semiconductor laser of  FIG. 2  can lase just with one optical mode even in the case that a ridge has a great width, by narrowing the current inflow path width, and lower a threshold current by reducing the current spreading in the active layer  25 .  
      However, the ridge-type semiconductor laser of  FIG. 2  can be applied to a GaN group of a semiconductor laser which is difficult to make the width of the ridge I less than a few micron, but may have a disadvantage to be applied to an InP group of a semiconductor laser having a few micron of a ridge width because two more electrodes must be formed very close to each other in the fabrication process.  
      Further, in the ridge-type semiconductor laser of  FIG. 2 , the electrode for current inflow path control  39  determines the width of the path through which current flows, and concurrently, greatly affects the optical mode. Therefore, in the ridge-type semiconductor laser of  FIG. 2 , the extent the extent that current is spread in the space, and the extent that optical mode is spread in the space cannot be controlled separately, which is disadvantageous.  
     SUMMARY OF THE INVENTION  
      The present invention provides a ridge-type semiconductor laser for improving the characteristics of a laser by separately controlling the extent the extent that current is spread in the space, and the extent that optical mode is spread in the space, to maximize the coincidence of the respective space distributions of current and optical mode.  
      The present invention provides a method of fabricating a ridge-type semiconductor laser for separately controlling the extent that current is spread in the space, and the extent that optical mode is spread in the space.  
      According to an aspect of the present invention, there is provided a ridge-type semiconductor laser including an active layer formed on a substrate, and a pattern for a current inflow path control formed on the active layer and having an opening thereinside controlling a current inflow path with a width W 1 .  
      The ridge-type semiconductor laser also includes a ridge formed on the pattern for a current inflow path control with a width W 2  greater than W 1 , and burying the opening with a width W 1  and controlling an optical mode. An electrode contact layer pattern is formed on the ridge, and a passivation layer is formed on both sidewalls of the ridge and on the active layer. An electrode metal layer is formed on the electrode contact layer pattern and the passivation layer.  
      Preferably, the substrate may be formed of an n-substrate, the ridge may be formed of a p-semiconductor layer, and the pattern for a current inflow path control may be formed of an n-semiconductor layer. The ridge may be formed of a p-InP layer, and the pattern for a current inflow path control may be formed of an n-InP layer. The active layer may be formed of an InGaAsP layer. The active layer under the ridge may be formed of a p-InGaAsP layer, and the active layer other than that may be formed of an n-InGaAsP layer.  
      According to another aspect of the present invention, there is provided a method of fabricating a ridge-type semiconductor laser, which includes forming an active layer on an n-substrate, and forming an etch stop layer on the active layer. After forming an n-semiconductor layer on the etch stop layer, the n-semiconductor layer and the etch stop layer are patterned, thereby forming an n-semiconductor layer pattern having an opening thereinside with a width W 1 , and an etch-stop layer pattern.  
      After forming a p-semiconductor layer on the n-semiconductor layer pattern, burying the opening, an electrode contact layer is formed on the p-semiconductor layer. The electrode contact layer, the p-semiconductor layer, and the n-semiconductor layer are patterned, thereby forming an electrode contact layer pattern, a ridge with a width W 2  greater than the width W 1 , and a pattern for a current inflow path width control having an opening thereinside with a width W 1 . After forming a passivation layer on both sidewalls of the ridge and the etch stop layer pattern, an electrode metal layer is formed on the electrode contact layer pattern and the passivation layer.  
      As described above, the ridge-type semiconductor laser of the present invention improves the characteristics of a laser by separately controlling the extent that current is spread in the space, and the extent that optical mode is spread in the space, and maximably coinciding the respective space distributions of current and optical mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      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:  
       FIGS. 1 and 2  are sectional views of a conventional ridge-type semiconductor laser;  
       FIGS. 3 through 5  are graphs explaining the theory of a ridge-type semiconductor laser of the present invention;  
       FIG. 6  is a sectional view of a ridge-type semiconductor laser according to one embodiment of the present invention;  
       FIG. 7  is a sectional view of a ridge-type semiconductor laser according to another embodiment of the present invention; and  
       FIGS. 8 through 11  are sectional views illustrating a method of fabricating the ridge-type semiconductor laser of  FIG. 6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout the specification.  
       FIGS. 3 through 5  are graphs explaining the theory of a ridge-type semiconductor laser of the present invention.  
      In specific, G. J. Letel, et. al. disclosed an extent of the current spread in the active layer of the conventional ridge-type semiconductor laser of  FIG. 1  in the paper entitled “Determination of active-region leakage currents in ridge-waveguide strained-layer quantum-well lasers by varying the ridge width” (IEEE J. of Quantum Electronics, vol. 34, No. 3, pp. 512-518, 1998). That is, the space distribution of holes as carriers may be presented by following Formulas, and it can be respectively presented as Formulas 1 and 2 in the outside and the inside of the ridge width determining a current inflow path. In the conventional technology, a current inflow path width and a waveguide mode width can be determined by a ridge width concurrently, but according to the present invention, the current inflow path width W 1  is different from the ridge width W 2 , and the current inflow width used in the Formulas 1 and 2 is the current inflow path width W 1 , not the ridge width. 
 
 p ( x )=[(Jin τ)/( q d )]exp( −|x|/LD ) sin  h ( W   1 / 2   LD ) | x |&gt;( W   1 / 2 )   &lt;Formula 1&gt;
 
 p ( x )=[(Jin·τ)/( q·d )][1−exp( −W   1 / 2   LD )·cos  h ( x /2 LD )]| x |&lt;( W   1 / 2 )   &lt;Formula 2&gt;
 
      Herein, p(x) is a hole density profile in the lateral direction, Jin is an injection current density, d is a thickness of an active layer, q is a charge amount of electrons, τ is a life time of holes, and LD is a diffusion length of holes.  
      Further,. an average recombination current density, J′ in the inside of the ridge width W 2  where the optical intensity of the optical mode is high may be presented by Formula 3 as follows.  
               J   ′     =       dq   W2     ⁢       ∫       -   w2     2       w2   2       ⁢         p   ⁡     (   X   )       τ     ⁢     ⅆ   x                   &lt;     Formula   ⁢           ⁢   3     &gt;             
 
      As described above, W 1  is a current inflow path width in the inner width of the ridge, and W 2  is a ridge width determining a dimension of the optical mode in the outer width of the ridge.  
       FIG. 3  illustrates the space distribution of the optical intensity of the optical mode in accordance with the ridge width W 2 .  FIG. 4  illustrates the space distribution of the carrier density of holes in accordance with the current inflow path width W 1 . The “LD” is 1 μm. As shown in  FIGS. 3 and 4 , the space distribution of the holes is much wider than the space distribution of the optical intensity of the optical mode with a same width.  
      From Formulas 1 and 2 presenting the space distribution of holes, and Formula 3, J′, a ratio of the current useful in the laser relative to inflow current can be calculated. Calculation results based on these formulas with W 2  and W 1  are presented in  FIG. 5 .  
      The solid lines of  FIG. 5  present threshold currents in accordance with the current inflow path width W 1  with respectively given ridge widths W 2 . The line connecting solid square marks presents the structure of the ridge-type semiconductor laser when W 1  and W 2  are identical. It can be known that the threshold current is low with the ridge width narrower. In  FIG. 5 , the star mark presents the case that the ridge width W 2  is 3 μm, and the current inflow path width W 1  is 1 μm, and the threshold current is reduced by 50% or more than that of the case that W 1  and W 2  are identical as 1 μm. Therefore, the ridge-type semiconductor device of the present invention is structured such that the ridge width W 2  is 3 μm, and the current inflow path width W 1  is 1 μm smaller than W 2 .  
       FIG. 6  is a sectional view of a ridge-type semiconductor laser according to one embodiment of the present invention.  
      In specific, an active layer  102  and the p-clad layer  103  are sequentially formed on the n-substrate  101 . The n-substrate  101  is formed of an n-substrate, for example, n-InP substrate, and the active layer  102  is formed of an InGaAsP layer of a quantum well structure, or an InGaAsP layer of a non-quantum well structure, and the p-clad layer  103  is formed of a p-semiconductor layer, for example, p-InP layer. The p-clad layer  103  is formed with a thickness of 0.1 μm. The p-clad layer  103  may not be formed if unnecessary.  
      An etch stop layer  105   a  having an opening thereinside with a width W 1  is formed on the p-clad layer  103 . The etch stop layer pattern  105   a  is formed of an InGaAsP layer. The etch stop layer  105  is formed with a thickness of 300′.  
      A pattern for a current inflow path width control  107   b  is formed on the etch stop layer pattern  105   a , having an opening  111  with a width equal to the W 1 , and partially exposing the surface of the etch stop layer pattern  105   a . Using the opening  111  with the width W 1  formed inside the pattern for a current inflow path width control  107   b , a current inflow path can be controlled. That is, the pattern for a current inflow path width control  107   b  functions to stop the flow of holes and not to allow a current to pass through. The pattern for a current inflow path width control  107   b  is formed of an n-semiconductor layer, for example, an n-InP layer. The pattern for a current inflow path width control  107   b  is formed with a thickness of 0.2 μm.  
      A ridge  113   a  is formed on the etch stop layer pattern  105   a  and the pattern for a current inflow path width control  107   b , to bury the opening having the width W 1 , with a width W 2  greater than the width W 1  for controlling an optical mode. The ridge width W 2  determines a dimension of an optical mode. The ridge  113   a  is formed of a p-semiconductor layer, for example, a p-InP layer. The ridge  113   a  is formed with a thickness of about 1.5 μm.  
      An electrode contact layer pattern  115   a  is formed on the ridge  113   a . The electrode contact layer pattern  115   a  is formed of an InGaAs layer. The electrode contact layer pattern  115   a  is formed with a thickness of 0.3 μm.  
      A passivation layer  117  is formed on both sidewalls of the ridge  113   a  and on the etch stop layer pattern  105   a . The passivation layer  117  is formed of an SOG (spin-on-glass) or polyimide. An electrode metal layer  119  is formed on the electrode contact layer pattern  115   a  and the passivation layer  117 .  
      The ridge-type semiconductor laser of the present invention controls a current inflow path, using the width W 1  of the opening formed inside the pattern for current inflow path width control  107   b , and controls an optical mode using the ridge width W 2  thereby maximably coinciding the respective space distributions of a current and an optical mode. Therefore, the ridge-type semiconductor laser of the present invention can reduce a threshold current, for example, as one of the characteristics of a laser. In the embodiment, the current inflow path width W 1  is 1 μm, and the ridge width W 2  is 3 μm.  
       FIG. 7  is a sectional view of a ridge-type semiconductor laser according to another embodiment of the present invention.  
      In specific, like reference numerals of  FIG. 7  refer to like elements of  FIG. 6 . The ridge-type semiconductor laser of  FIG. 7  is the same as that of  FIG. 6 , just except that the active layer  102  is formed to be separated into a p-active layer  102   a  and an n-active layer  102   b . The p-active layer  102   a  is formed under the ridge  113   a , and other portion is formed of the n-active layer  102   b . The p-active layer  102   a  is formed of a p-InGaAsP layer, and the n-active layer  102   b  is formed of an n-InGaAsP layer. The p-active layer  102   a  is formed by forming the n-active layer  102   b  on the substrate, and diffusing p-dopants during the growth of p-clad layer  103 . Therefore, the ridge-type semiconductor laser of  FIG. 7  can improve the laser characteristics since the p-n junction formed inside the active layer  102  stops the diffusion of holes during the operation.  
       FIGS. 8 through 11  are sectional views illustrating a method of fabricating the ridge-type semiconductor laser of  FIG. 6 .  
      Referring to  FIG. 8 , an active layer  102  and a p-clad layer  103  are formed on an n-substrate  101 . The n-substrate  101  uses an n-InP substrate, and the active layer  102  uses an InGaAsP layer with a quantum well structure or an InGaAsP layer with a non-quantum well structure. The p-clad layer  103  uses a p-InP layer. The p-clad layer  103  is formed with a thickness of 0.1 μm. In this embodiment, the p-clad layer  103  is formed to facilitate easy performance of a post-process, that is, a process of forming a p-semiconductor layer  113  ( FIG. 10 ) using a MOCVD (metal organic chemical vapor deposition) method, but may not be formed.  
      An etch stop layer  105  is formed on the p-clad layer  103 . The etch stop layer  105  is formed of an InGaAsP layer. The etch stop layer  105  is formed with a thickness of 300′.  
      An n-semiconductor layer  107  for controlling a current inflow path width is formed on the etch stop layer  105 . The n-semiconductor layer  107  is formed of an n-InP layer. The n-semiconductor layer  107  is formed with a thickness of 0.2 μm.  
      A mask layer  109  is formed on the n-semiconductor layer  107  using a photolithography process, and the mask layer  109  opens the central portion of the n-semiconductor layer  107  as much as a width W 1 . The mask layer  109  is formed of a silicon nitride layer or a silicon oxide layer. The mask layer  109  is formed by forming a silicon nitride layer or a silicon oxide layer on the n-semiconductor layer  107 , and etching the layer as much as a predetermined width W 1  using a photolithography process. The width W 1  is the current inflow path width.  
      Referring to  FIG. 9 , using the mask layer  109  as an etch mask, the n-semiconductor layer  107  is etched using an etchant, for example, a mixture of HCl and H 3 PO 4 , thereby forming an n-semiconductor layer pattern  107   a . Then, the etch stop layer  105  is etched using the mask layer  109  as an etch mask, and using an etchant, for example, a mixture of H 2 SO 4 , H 2 O 2 , and H 2 O, thereby forming an etch stop layer pattern  105   a . Thus, an opening  111  is formed inside the n-semiconductor layer pattern  107   a  and the etch stop layer pattern  105   a  with a width W 1 .  
      Referring to  FIG. 10 , the mask layer  109  used as the etch mask is removed. Then, a p-semiconductor layer  113  to be a ridge later is formed on the n-semiconductor layer pattern  107   a  while burying the opening  111 . The p-semiconductor layer  113  uses a p-InP layer. The p-semiconductor layer  113  is formed with a thickness of about 1.5 μm. The p-semiconductor layer  113  is formed using a MOCVD (metal organic chemical vapor deposition) method. An electrode contact layer  115  is formed on the p-semiconductor layer  113 . The electrode contact layer  115  uses an InGaAs layer. The electrode contact layer  115  is formed with a thickness of 0.3 μm.  
      Referring to  FIG. 11 , the electrode contact layer  115 , the p-semiconductor layer  113 , and the n-semiconductor layer pattern  107   a  are patterned by a photolithography and etching processes, thereby forming an electrode contact layer pattern  115   a , a ridge  113   a  with a width W 2  and a pattern for current inflow path width control  107   b  having an opening with a width W 1 .  
      Then, as shown in  FIG. 4 , a passivation layer  117  is formed on both sidewalls of the ridge  113   a  and on the etch stop layer pattern  105   a . The passivation layer  117  is formed of SOG (spin-on-glass) or polyimide. An electrode metal layer  119  is formed on the electrode contact layer pattern  115   a  and the passivation layer  117 . As such, in the ridge-type semiconductor layer of the present invention, the ridge width W 2  and the current inflow path width W 1  can be freely controlled by a photolithography process, thereby reducing a threshold current.  
      As described above, the present invention provides an advantage of improving the characteristics of the ridge-type semiconductor laser by separately controlling the extent that current is spread in the space, and the extent that optical mode is spread in the space, thereby to maximize the coincidence of the respective space distributions of current and optical mode.  
      That is, according to the present invention, the coincidence of the respective space distributions of the current and the optical mode can be maximized by controlling the current spread using the pattern for current inflow path width control having an opening with a width W 1 , and by controlling the dimension of the optical mode in the space only by the ridge width, thereby improving the characteristics of the semiconductor laser, for example, lowering the threshold current.  
      Further, the method of fabricating a ridge-type semiconductor laser according to the present invention can use the conventional fabrication method thereof, and reduces a threshold current as characteristics of the semiconductor laser.  
      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.