Abstract:
A semiconductor laser device having a smooth cleavage plane is provided. The provided laser device includes a current injection ridge and force distribution ridges formed adjacent to the current injection ridge, which protrudes from an upper surface of a mesa structure. The mesa structure is formed of multi-semiconductor material layers including a laser resonance layer and cladding layers disposed above and below the resonance layer. The current injection ridge and the force distribution ridges distribute a scribing force when cleaving the laser device so that the smooth cleavage planes are obtained. Defects are prevented in the current injection ridge due to the distribution of force when bonding flip chips.

Description:
[0001]    This application claims the priority of Korean Patent Application No. 2003-35601, filed on Jun. 3, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a semiconductor laser device, and more particularly, to a laser device having a smooth cleavage plane.  
           [0004]    2. Description of the Related Art  
           [0005]    A semiconductor laser is widely used to transfer, record, and read data in the field of communications, such as optical communications, or in devices, such as a compact disk player (CDP) and a digital versatile disk player (DVDP).  
           [0006]    Since a semiconductor laser device may maintain oscillation characteristics of laser beam in a limited space, may be formed to a small scale, and requires a small critical current for laser oscillations, the semiconductor laser device is widely used. As the number of industrial fields to which the semiconductor laser is applied increases, demand for semiconductor laser devices having a smaller critical current increases. In other words, semiconductor laser devices having excellent characteristics, such as oscillating at a low current, and the ability to pass a lifespan test are needed.  
           [0007]    In order to decrease the operating power and increase the output of the laser device, a smooth light-exiting surface, through which light exits the laser device, perpendicular to a laser oscillation layer is required. The light-exiting surface, which is formed by etching or scribing, is referred to as a facet or a cleavage plane.  
           [0008]    When the light-exiting surface is formed by dry etching, the light-exiting surface is rough, resulting in a large optical loss and low reproducibility. However, when the cleavage plane is formed by scribing, the optical loss is reduced. A nitride semiconductor laser device, such as gallium nitride (GaN) uses the cleavage plane as the light-exiting surface. However, the crystal structures of GaN grown on a sapphire substrate and the sapphire substrate are different so that it is technically difficult to form a smooth cleavage plane and the yield is low.  
           [0009]    [0009]FIG. 1 is a sectional view of a conventional nitride semiconductor laser device.  
           [0010]    Referring to FIG. 1, an n-GaN lower contact layer  12 , which is divided into a first region R 1  and a second region R 2 , is stacked on a sapphire substrate  10 . A multi-layered semiconductor material layer with a mesa structure exists on the lower contact layer  12 . In other words, on the first region R 1 , an n-GaN/AlGaN lower cladding layer  24 , an n-GaN lower wave guide layer  26 , a InGaN active layer  28 , a p-GaN upper wave guide layer  30 , and a p-GaN/AlGaN upper cladding layer  32  are sequentially stacked on the n-GaN lower contact layer  12 . The refractive indexes of the n-GaN/AlGaN lower cladding layer  24  and the p-GaN/AlGaN upper cladding layer  32  are smaller than the refractive indexes of the n-GaN lower wave guide layer  26  and the p-GaN upper wave guide layer  30 . In addition, the refractive indexes of the n-GaN lower wave guide layer  26  and the p-GaN upper wave guide layer  30  are smaller than the refractive index of the active layer  28 . In the mesa structure, a protruding ridge  32 a having a predetermined width is formed at the center of the upper portion of the p-GaN/AlGN upper cladding layer  32 , providing a ridge wave guide structure, and a p-GaN upper contact layer  34  is formed on the ridge  32   a.  A buried layer  36 , which acts as a passivation layer having a contact hole  36   a  is formed on the p-GaN/AlGaN upper cladding layer  32 . The contact hole  36   a  of the buried layer  36  is located over the upper contact layer  34  that is formed on the ridge  32   a,  and edge of the contact hole  36   a  overlaps the edge of the upper surface of the upper contact layer  34 .  
           [0011]    A p-type upper electrode  38  is formed on the buried layer  36 . The p-type electrode  38  contacts the upper contact layer  34  through the contact hole  36   a  of the buried layer  36 . In the second region R 2 , an n-type lower electrode  37  is formed on the lower contact layer  12 , whose height is lower in the second region R 2  than in the first region R 1 .  
           [0012]    The ridge wave guide structure formed on the upper cladding layer  32  limits currents that are injected to the active layer  28  in order to limit a width of a resonance area for laser oscillation in the active layer  28 . Thus, a transverse mode is stabilized and the operating current is lowered.  
           [0013]    In the process of manufacturing the conventional nitride semiconductor laser device, the multi-layered GaN semiconductor material layer is formed on the sapphire substrate, and the ridge corresponding to a current injection area is formed by dry etching. Then, a mesa structure is formed on the n-GaN lower contact layer in order to expose the n-GaN lower contact layer and form the resonance surface. Such a mesa structure is formed as an array type on the sapphire substrate, and is then divided into unit devices by scribing. FIG. 2 is a plane view illustrating two mesa structures corresponding to two unit devices that are formed on the n-GaN contact layer  12 . The mesa structures are interconnected by a connection unit  40  and share the ridge  32   a,  which crosses the connection unit  40 . The mesa structures and the substrate, which supports the mesa structures, are divided into the unit devices along a scribing line A-A′ that intersects the connection unit  40 .  
           [0014]    As described above, the mesa structures are divided into the unit devices by scribing, and the cleavage planes from which a laser beam exits are formed at the edges resulting from the scribing. A GaN c-plane formed on a sapphire-c plane is tilted by about 30° toward the sapphire-c plane. Since the sapphire-c plane and the GaN c-plane are tilted, it is difficult to form a smooth cleavage plane perpendicular to the laser oscillation layer. In order to form the smooth cleavage planes perpendicular to the laser oscillation layer on the GaN semiconductor material layer, the cleavage plane of the sapphire substrate should be precisely divided by scribing. When the scribing force is transferred from the sapphire substrate to the lower portion of the mesa structure and the ridge at the upper portion of the mesa structure, the scribing force should not be concentrated at a specific location of the mesa structure, but should be evenly distributed.  
           [0015]    The light-exiting surfaces, in other words, the cleavage planes, of the semiconductor material layer formed by the conventional method have little uniformity. In other words, the shapes of the cleavage planes are different from chip to chip even when the chips are manufactured under the same scribing conditions. The yield of laser devices proper for transmitting light, in other words, having the smooth cleavage plane perpendicular to the oscillation layer, is about 65%.  
           [0016]    The following is an analysis of the laser device with the inferior light-exiting surface. When scribing the mesa structure by transferring the scribing force from the sapphire substrate to the mesa structure, the scribing force is concentrated at a lower corner of the mesa structure so that cracks occur at the lower corner of the mesa structure as shown in the dotted rectangle of FIG. 3. Here, the cracks are transferred to the light-exiting surface. Another inferior light-exiting surface is caused by cracks in a GaN coalescence formed by epitaxial lateral overgrowth (ELOG), which is disclosed in U.S. Pat. No. 6,348,108. Referring to FIG. 4, when the scribing force is transferred from the sapphire substrate to the GaN, cracks occur at the GaN coalescence. The cracks are transferred to a ridge wave guide formed on the mesa structure as shown in the dotted rectangle of FIG. 4, so that a rough cleavage plane is formed.  
           [0017]    The cracks and the rough cleavage plane result in a decrease in optical output and an increase in operating current.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention provides a laser device having an excellent laser exiting surface and a method of manufacturing the same.  
           [0019]    The present invention also provides a semiconductor laser device having a low operating current and an improved laser oscillation efficiency, and a method of manufacturing the same.  
           [0020]    According to an aspect of the present invention, there is provided a semiconductor laser device, which includes a multi-semiconductor material layered mesa structure having a laser resonance layer on a substrate and cladding layers formed above and below the resonance layer, comprising a current injection ridge and force distribution ridges at the both sides of the current injection ridge formed on an upper portion of the mesa structure and protruding from the surface of an upper surface of the mesa structure.  
           [0021]    According to another aspect of the present invention, there is provided a semiconductor laser device, which includes a multi-semiconductor material layered mesa structure having a laser resonance layer on a substrate and cladding layers formed above and below the resonance layer, comprising rounded corners connected to the substrate, in a lower portion of the mesa structure, and a current injection ridge and force distribution ridges formed in an upper portion of the mesa structure and protruding from an upper surface of the mesa structure.  
           [0022]    The upper and the lower cladding layers may be a p-GaN/AlGaN layer and an n-GaN/AlGaN layer, respectively.  
           [0023]    The resonance layer may include a lower wave guide layer stacked on the lower cladding layer and having a greater refractive index than the lower cladding layer, an active layer stacked on the lower wave guide layer that generates a laser beam, and an upper wave guide layer stacked on the active layer.  
           [0024]    The refractive indexes of the upper and the lower wave guide layers may be less than the refractive index of the active layer, and the upper and the lower wave guide layers may be GaN based group III-V compound semiconductor layers.  
           [0025]    The active layer may be a semiconductor layer made of a GaN based group III-V nitride compound expressed as In x Al y Ga 1-x-y N where 0≦x≦1, 0≦y≦1, and x+y≦1. The ridges may be formed on the upper cladding layer, and a second compound semiconductor layer may be formed on the current injection ridge. The second compound semiconductor layer may be a p-GaN based group III-V nitride semiconductor layer.  
           [0026]    The first compound semiconductor substrate further may include an n-type electrode on the upper surface, and the substrate may be a sapphire substrate having a GaN semiconductor material layer or a freestanding GaN substrate.  
           [0027]    Both sides of the mesa structure may be inclined toward the substrate, and the width of the mesa structure may increase toward the substrate.  
           [0028]    The force distribution ridges may be formed at the both edges of the mesa structure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The above aspects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:  
         [0030]    [0030]FIG. 1 is a sectional view of a conventional semiconductor laser device;  
         [0031]    [0031]FIG. 2 is a plane view of a substrate on which undivided unit laser devices are formed when manufacturing a conventional semiconductor laser device;  
         [0032]    [0032]FIGS. 3 and 4 are scanning electron microscope (SEM) photographs illustrating irregular surfaces on a cleavage plane of a mesa structure of a conventional semiconductor laser device;  
         [0033]    [0033]FIG. 5 is a sectional view illustrating a semiconductor laser device according to an embodiment of the present invention;  
         [0034]    [0034]FIG. 6 is a plane view of a substrate on which undivided unit laser devices are formed when manufacturing a semiconductor laser device according to the embodiment of the present invention;  
         [0035]    [0035]FIG. 7 is an SEM photograph illustrating a lower structure of a mesa structure of a semiconductor laser device according to the embodiment of the present invention; and  
         [0036]    [0036]FIG. 8 is an SEM photograph illustrating a smooth cleavage plane formed on a mesa structure of a semiconductor laser device according to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    The present invention will now be described more fully with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown.  
         [0038]    [0038]FIG. 5 is a sectional view of a semiconductor laser device according to an embodiment of the present invention. FIG. 6 is a plane view of an n-GaN contact layer  121  on which two mesa structures corresponding to two unit laser devices are formed. The semiconductor laser device includes a substrate  100 , and a lower material layer  120 , a resonance layer  130 , and an upper material layer  140 , which are grown on the substrate  100 .  
         [0039]    The lower material layer  120  includes a first compound semiconductor layer  121  as a lower contact layer, which is stacked on the substrate  100  and has a step, and a lower cladding layer  122  stacked on the first compound semiconductor layer  121 . An n-type lower electrode  153  is disposed on the step of the first compound semiconductor layer  121 .  
         [0040]    A sapphire substrate or a freestanding gallium nitride (GaN) substrate is used for the substrate  100 . The first compound semiconductor layer  121  is an n-GaN based group III-V nitride compound semiconductor layer, and it is preferable that the first compound semiconductor layer  121  is an n-GaN layer. However, the first compound semiconductor layer  121  may be another group III-V compound semiconductor layer that can oscillate laser, in other words, lasing. It is preferable that the lower cladding layer  122  is an n-GaN/AlGaN layer having a predetermined refractive index, but may be formed of another compound that can oscillate laser.  
         [0041]    The resonance layer  130  includes a lower wave guide layer  131 , an active layer  132 , and an upper wave guide layer  133 , which are sequentially stacked on the lower cladding layer  122 . The upper and lower wave guide layers  131 , 133  are formed of a material having a smaller refractive index than the active layer  132 . It is preferable that the upper and lower wave guide layers  131  and  133  are GaN based group Ill-V compound semiconductor layers. The lower wave guide layer  131  is an n-GaN layer, and the upper wave guide layer  133  is a p-GaN layer. The active layer  132  is formed of a lasing material, preferably a material oscillating laser beam that has a small critical current and a stable traverse mode characteristic. It is preferable that the active layer  132  is formed of a GaN based group III-V nitride compound semiconductor material such as In x Al y Ga 1-x-y N (0≦x≦1, 0≦y≦1, x+y&lt;1). The active layer  132  may have a multi-quantum well structure or a single quantum well structure, and the structure of the active layer  132  does not limit the scope of the present invention.  
         [0042]    The upper material layer  140 , which is the characteristic of the present invention, includes an upper cladding layer  141  and a second compound semiconductor layer  142 . The upper cladding layer  141  is stacked on the upper surface of the upper wave guide layer  133  and has a protruded current injection ridge  141   a  at its center and protruded force distribution ridges  141   b  adjacent to the current injection ridge  141   a.  The second compound semiconductor layer  142  acts as an ohmic contact layer and is stacked on the current injection ridge  141   a.  When the lower cladding layer  122  is an n-type compound semiconductor layer, the upper cladding layer  141  is a p-type compound semiconductor layer. When the lower cladding layer  122  is a p-type compound semiconductor layer, the upper cladding layer  141  is an n-type compound semiconductor layer. In other words, when the lower cladding layer  122  is the n-GaN/AlGaN layer, the upper cladding layer  141  is a p-GaN/AlGaN layer. Similarly, when the first compound semiconductor layer  121  is an n-type compound semiconductor layer, the second compound semiconductor layer  142  is a p-type compound semiconductor layer, and when the first compound semiconductor layer  121  is formed of n-GaN, the second compound semiconductor layer  142  is formed of p-GaN. A passivation layer  151  is formed on the ridges  141   a  and  141   b.  The passivation layer  151  includes a contact hole  151   a  that exposes the current injection ridge  141   a,  and a p-type upper electrode  152  is formed therein.  
         [0043]    The mesa structure includes the resonance layer  130 , the upper material layer  140 , and the lower cladding layer  122  of the lower material layer  120 . The lower portions of the mesa structure have rounded corners  121   a.  The rounded corners  121   a  of the mesa structure prevent the concentration of a scribing force when dividing the unit devices along a line B-B′ in FIG. 6.  
         [0044]    It is preferable that the force distribution ridges  141   b  are parallel with the current injection ridge  141   a  and are symmetrical about the current injection ridge  141   a.  In addition, it is preferable that the width of each of the force distribution ridges  141   b  is equal to or greater than the width of the current injection ridge  141   a.  The force distribution ridges  141   b  prevent cracks in a GaN coalescence, which are caused by the scribing force, from being concentrated to the current injection ridge  141   a.  In other words, the cracks are vertically transferred in the mesa structure, and are not transferred to the current injection ridge  141   a,  and then, the light-exiting surface is not affected from the cracks.  
         [0045]    [0045]FIG. 7 is a scanning electron microscope (SEM) photograph illustrating the lower structure of the mesa structure of the semiconductor laser device according to the embodiment of the present invention, and FIG. 8 is an SEM photograph illustrating the current injection ridge  141   a  of the mesa structure.  
         [0046]    Referring to FIG. 7, a smooth cleavage plane is formed at the rounded corner formed in the lower portion of the mesa structure, which contrasts the conventional cleavage plane shown in FIG. 3. Here, the smooth cleavage plane could be formed by not concentrating the cracks.  
         [0047]    Referring to FIG. 8, since the current injection ridge is formed at the center of the mesa structure and the force distribution ridges are formed adjacent to the current injection ridge, the cracks of the coalescence are vertically transferred. Accordingly, the ridge has the smooth cleavage plane.  
         [0048]    According to the present invention, rounded corners are formed in lower portions of a mesa structure, and force distribution ridges are disposed adjacent to a current injection ridge in an upper portion of the mesa structure. Accordingly, a smooth cleavage plane perpendicular to the oscillation surface is obtained by scribing, with a high yield. Because of the smooth cleavage plane, the laser oscillation efficiency is improved and the operating current of the laser device is lowered. The force distribution ridge may also distribute a load applied to the current injection ridge when bonding flip chips.  
         [0049]    Such a laser device can be applied to a laser diode, in particular, a GaN laser diode having a mesa structure. While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.