Abstract:
A laser diode without a ridge and a method of fabricating the same are provided. The laser diode includes an active layer and upper and lower clad layers. A current blocking layer formed of a semiconductor material is formed on the upper clad layer, and a current passing region is formed using doping through the current blocking layer. The current passing region diffuses down into the upper clad layer. Since the laser diode includes no ridge, it can be fabricated in a simple fabrication process at a low production cost.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims the benefit of Korean Patent Application No. 10-2004-0083582, filed on Oct. 19, 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 Disclosure  
         [0003]     The disclosure relates to a laser diode and a method of fabricating the same, more particularly, to a laser diode, which is structurally simple and fabricated in a simple process, and a method of fabricating the same.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, a ridge waveguide laser diode has a ridge structure in which the injection of current into a top portion of a crystalline layer is locally restrained. The ridge structure is typically formed in an upper clad layer, and a passivation layer or current blocking layer is formed on both sides of the ridge structure to block current flow.  
         [0006]     An example of a conventional nitride semiconductor laser device will now be described.  
         [0007]     Referring to  FIG. 1 , an n-GaN lower contact layer  12  is stacked on a sapphire substrate  10 . The n-GaN lower contact layer  12  is divided into a first region R 1  and a second region R 2 . A multiple semiconductor material layer is disposed as a mesa structure on the lower contact layer  12 . Specifically, an n-GaN/AlGaN lower clad layer  24 , an n-GaN lower waveguide layer  26 , an InGaN active layer  28 , a p-GaN upper waveguide layer  30 , and a p-GaN/AlGaN upper clad layer  32  are sequentially stacked on a top surface of the n-GaN lower contact layer  12  in the first region R 1 . In this case, the refractive indexes of the n- and p-GaN/AlGaN lower and upper clad layers  24  and  32  are lower than those of the n- and p-GaN lower and upper waveguide layers  26  and  30 , respectively, and each of the refractive indexes of the n- and p-GaN lower and upper waveguide layers  26  and  30  is lower than that of the InGaN active layer  28 . In this mesa structure, a protruding ridge  32   a  with a predetermined width, which provides a ridge waveguide structure, is disposed on a top central portion of the p-GaN/AlGaN upper clad layer  32 , and a p-GaN upper contact layer  34  is disposed on a top surface of the ridge  32   a . A buried layer  36  having a contact hole  36   a  is disposed as a passivation layer on the p-GaN/AlGaN upper clad layer  32 . The contact hole  36   a  disposed in the buried layer  36  corresponds to a top portion of the upper contact layer  34  disposed on the top surface of the ridge  32   a , and an edge portion of the contact hole  36   a  overlaps an edge portion of a top surface of the upper contact layer  34 .  
         [0008]     A p-type upper electrode  38  is disposed on the buried layer  36  such that it contacts the upper contact layer  34  through the contact hole  36   a  disposed in the buried layer  36 . An n-type lower electrode  37  is disposed on the second region R 2  of the n-GaN lower contact layer  12 , which forms a lower top surface than the first region R 1 .  
         [0009]     The ridge waveguide structure, which is prepared on the upper clad layer  32 , restricts the flow of current into the active layer  28  so that a resonant region of the active layer  28  for laser oscillation is limited in width to stabilize a transverse mode and reduce an operating current.  
         [0010]     Fabrication of the above-described conventional nitride semiconductor laser device involves forming a multiple GaN-based semiconductor material layer corresponding to a pair of unit devices on a sapphire substrate  10  as shown  FIG. 2  forming a ridge  32   a  corresponding to a current injection region using dry etching, and performing a facet etching process to form a mesa structure on an n-GaN lower contact layer  12  so that the n-GaN lower contact layer  12  is exposed and a facet surface is formed along A-A′ line. The facet etching process should be followed by formation of a buried layer on both sides of the ridge  32   a  and formation of a contact hole corresponding to a top portion of the ridge in the buried layer.  
         [0011]     As described above, since a conventional laser diode makes use of a ridge to restrict the flow of current, its fabrication involves complicated process operations of during formation, for example, the presence of the ridge, a buried layer, and a contact hole to constrain the injection of current into the ridge. In various aspects, there is a strong need for research to minimize of the complicated process operations as much as possible.  
       SUMMARY OF THE DISCLOSURE  
       [0012]     The present invention may provide a laser diode with a new-type of current injection structure and a method of fabricating the same.  
         [0013]     The present invention also provides a laser diode, which is fabricated in a simple process at a low production cost, and a method of fabricating the same.  
         [0014]     According to an aspect of the present invention, there may be provided a laser diode, which includes a crystalline layer disposed on a substrate, the crystalline layer in which a sandwich of an upper clad layer and a lower clad layer is separated by a laser resonant layer; a current blocking layer disposed on the crystalline layer; and an impurity current passing region disposed through respective portions of the current blocking layer and the upper clad layer.  
         [0015]     According to another aspect of the present invention, there may be provided a method of fabricating a laser diode. The method includes forming a crystalline layer on a substrate, the crystalline layer in which a sandwich of an upper clad layer and a lower clad layer is separated by a resonant layer; forming a current blocking layer on the crystalline layer; and forming a current passing region through respective portions of the current blocking layer and the upper clad layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     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:  
         [0017]      FIG. 1  is a cross-sectional view of a conventional semiconductor laser device;  
         [0018]      FIG. 2  is a plan view of a substrate illustrating an operation for fabricating a conventional semiconductor laser device, in which unit laser devices are not separated from each other;  
         [0019]      FIG. 3  is a cross-sectional view of a laser diode according to the present invention; and  
         [0020]      FIGS. 4A through 7  are cross-sectional views illustrating exemplary operations for fabricating a laser diode according to the present invention.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0021]     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0022]     Referring to  FIG. 3 , an n-GaN lower contact layer  112  may be stacked on a sapphire substrate  111 . An n-type lower electrode  118   b  may be disposed on a portion of the lower contact layer  112 , and a mesa structure may be disposed using a multiple semiconductor material layer on the other portion thereof. That is, an n-GaN/AlGaN lower clad layer  113 , an InGaN active layer  114 , and a p-GaN/AlGaN upper clad layer  115  are sequentially stacked on a top surface of the n-GaN lower contact layer  112 . In the above-described structure, an upper waveguide layer and a lower waveguide layer, which are prepared on and under the active layer  114 , are omitted here to simplify the explanation.  
         [0023]     In the mesa structure, the p-GaN/AlGaN upper clad layer  115  has a planar top surface on which a current blocking layer  116  is formed using a semiconductor material. The present invention is characterized by the current blocking layer  116 . In addition, the present invention is also characterized by a current passing region  119 , which is formed on the current blocking layer  116  through the diffusion or injection of impurity ions. The current passing region  119  extends to the upper clad layer  115  by diffusion of impurity ions. The current blocking layer  116  may be formed of a material having a reverse polarity to the p-CaN/AlGaN upper clad layer  115 , for example, n-AlGaN. Thus, the current blocking layer  116  serves as a current blocking barrier for blocking current flow between the upper clad layer  115  and a p + -GaN contact layer  117 . In another embodiment, the current blocking layer  116  may be formed of a semiconductor material having a very high electric resistance, for example, undoped AlGaN. Some materials have n- or p-type physical properties while they are being undoped. For the present invention, the current blocking layer  116  at least must not have the same polarity as the upper clad layer  115 . In other words, it will be understood that the current blocking layer  116  should not be a p-type layer, as might be the case in forming a p-type upper clad layer, and should not be an n-type layer, as might be the case in forming an n-type upper clad layer. The current passing region  119  is about 0.5 to about 50 microns in width.  
         [0024]     The current passing region  119  extends also into the sufficiently doped p + -GaN contact layer  117 . An upper electrode  118   a  is disposed over the current blocking layer  116 .  
         [0025]     Since the above-described laser diode according to the present invention does not have a conventional ridge structure, the fabrication of such ridge structure is unnecessary. The present invention constrains the injection of current through a high resistance or a current blocking barrier and allows the supply of current to the active layer  114  through a highly conductive diffusion (or implantation) region (i.e., the current passing region  119 ). The laser diode of the present invention has a gain waveguide structure in place of the conventional ridge waveguide structure.  
         [0026]     Hereinafter, exemplary operations for fabricating a laser diode according to the present invention will be described.  
         [0027]     Formation of a Crystalline Layer for a Laser Diode  
         [0028]     Referring to  FIG. 4A , an n-GaN lower contact layer  112 , a GaN-based III-V group nitride compound semiconductor layer  114  as an active layer formed of In x Al y Ga 1-x-y N(0≦x≦1, 0≦y≦1x+y≦1), and a p-GaN/AlGaN upper clad layer  115  are sequentially grown on a sapphire substrate  111  by a known method.  
         [0029]     Referring to  FIG. 4B , a current blocking layer  116  for blocking the flow of current is formed on the upper clad layer  115 . The current blocking layer  116  is formed of undoped AlGaN (un-AlGaN) or doped n-GaN.  
         [0030]     Referring to  FIG. 4C , a sufficiently doped p + -GaN contact layer  117  is formed over the current blocking layer  115 .  
         [0031]     Formation of a Current Passing Region  
         [0032]     The formation of the current passing region can be performed using a diffusion process or an impurity implantation process as described below.  
         [0033]     1. Diffusion  
         [0034]     Referring to  FIG. 5A , a Zn (or Si) diffusion material layer  120  for forming a current passing region  119  is formed on the contact layer  117 . The position of the diffusion material layer  120  substantially corresponds to the position of a conventional ridge.  
         [0035]     Referring to  FIG. 5B , an annealing process is carried out in a furnace so that the diffusion material layer  120  diffuses into the underlying semiconductor material layer. In this case, the diffusion material layer  120  thermally diffuses into a portion of the underlying semiconductor material layer in a vertical direction, thus the current passing region  119  is formed from the contact layer  117  to the upper clad layer  115 .  
         [0036]     2. Implantation  
         [0037]     Referring to  FIG. 6 , Zn (or Si) ions are implanted into a top surface of the crystalline layer down to the upper clad layer  115  using an ion implantation apparatus, thereby forming the current passing region  119 .  
         [0038]     Formation of a Mesa Structure and Electrodes  
         [0039]     Referring to  FIG. 7A , the above-described stacked structure is patterned so that a mesa structure with a multiple semiconductor stacked layer and a stepped portion  112   a  are obtained. The stepped portion  112   a  is formed in the lower contact layer  112 .  
         [0040]     Referring to  FIG. 7B , an upper electrode  118   a  and a lower electrode  118   b  are formed on the mesa structure (i.e., on the upper contact layer  117 ) and the lower contact layer  112 , respectively.  
         [0041]     As explained thus far, the present invention does not involve the formation of a ridge and the formation of electrodes using patterns, which are utilized in a conventional method. Thus, a laser diode can be fabricated using a monolithic growth process, which is performed in a more straightforward manner than the conventional method. Also, the laser diode of the present invention has no ridge so that a top surface of a crystalline layer generally is planar.  
         [0042]     According to the method of the present invention, a current injection region can be effectively controlled as [to] an active layer through the adjustment of the size of a material pattern or an ion implantation region. The control of the current injection region facilitates ideal single transverse-mode oscillation of the laser diode.  
         [0043]     The method of the present invention can be applied to laser diodes formed of various materials, such as an AlGaN-based laser diode or an InGaAlP-based laser diode.  
         [0044]     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.