Abstract:
The semiconductor light emitting device having a protrusion and recess structure includes: a lower clad layer disposed on a substrate; an active layer formed on one portion of a top surface of the lower clad layer; an upper clad layer formed on the active layer; a first electrode formed on the upper clad layer; and a second electrode that is formed on a protrusion and recess structural pattern region formed on a portion of the top surface of the lower clad layer not occupied by the active layer.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     Priority is claimed to Korean Patent Application No. 10-2005-0052044, filed on Jun. 16, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     This application is a Div. of Ser. No. 12/215,407 filed on Jun. 27, 2008, now U.S. Pat. No. 7,785,910 which is a Div. Fo Ser. No. 11/453,866 filed on Jun. 16, 2006, abandonment 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure relates to a semiconductor light emitting device, and more particularly, to a semiconductor light emitting device having a protrusion and recess structure that is disposed at an interface between a lower clad layer and a side electrode to improve the light extraction efficiency and is formed using a simple manufacturing process and a method of manufacturing the same. 
     2. Description of the Related Art 
     A conventional light emitting diode (LED) produces a signal in the form of infrared rays, visible rays, or light from electrical energy using characteristics of a compound semiconductor. A light emitting diode can be electroluminescent (EL) diodes or light emitting diodes using Group III-V compound semiconductor, which are in practical use. 
     A Group III nitride-based compound semiconductor is a direct transition type semiconductor, and can be used to achieve a more stable operation at a high temperature than other devices using different semiconductors. Therefore, Group III nitride-based compound semiconductors are widely used in light emitting devices such as LEDs and laser diodes (LDs). In general, such Group III nitride-based compound semiconductors are formed on sapphire (Al 2 O 3 ) substrates. In order to improve light extraction efficiency, research has been conducted on LEDs with various structures, especially on a protrusion and recess structure disposed in light extraction regions of a light emitting device. 
     At an interface of material layers having different refractive indexes, the transmission of light is dependent on the refractive indexes of the material layers. In the case of a flat interface, when the light is incident from a semiconductor layer with a high refractive index (n=2.5) to an air layer with a low refractive index (n=1), the light is transmitted when the angle of incidence is below a predetermined angle (θ). When the angle of incidence is greater than a predetermined angle, the light is totally reflected at the flat interface, progressed in a lateral direction, and is finally trapped in the LED. Thus, the light extraction efficiency is greatly decreased. The use of a protrusion and recess structure at an interface has been proposed to address this problem. 
       FIGS. 1 and 2  are cross-sectional views of conventional semiconductor light emitting devices having the protrusion and recess structure. 
     The semiconductor light emitting device illustrated in  FIG. 1  has a PSS (patterned sapphire substrate) structure. Referring to  FIG. 1 , a GaN buffer layer  11  and a GaN layer  12  are sequentially formed on the sapphire substrate  10 . An n-GaN clad layer  13  is formed on the GaN layer  12  and an active layer  14  having a MQW (multi quantum well) structure, a p-GaN clad layer  15  and first and second p-electrode layers  16  and  17  are sequentially formed on the upper surface of one portion of the n-GaN clad layer  13 . The first and second p-electrode layers  16  and  17  are respectively a mesh-type p-electrode  16  and a p-electrode pad  17 . An n-electrode layer  18  is formed on the upper surface of the other portion of the n-GaN clad layer  13 . 
     Here, the protrusion and recess structure is formed on the sapphire substrate  10 . The GaN buffer layer  11  conforms to the structure of the sapphire substrate  10  and the GaN layer  12  has a lower surface that corresponds to the protrusion and recess structure of the GaN buffer layer  11 , and a flat upper surface. The light emitting device having the protrusion and recess structure has an advantage in that the light generated from the active layer  14  can be scattered toward the sapphire substrate  10  rather than being trapped. However, a process of forming the protrusion and recess structure on the sapphire substrate  10  is complicated and has a low yield. 
     In the semiconductor light emitting device of  FIG. 2 , an n-GaN clad layer  21  is formed on a sapphire substrate  20  and an active layer  22 , a p-GaN clad layer  23 , a p-type transparent electrode  24  and a p-electrode pad  25  are formed on the upper surface of one portion of the n-GaN clad layer  21 . An n-electrode layer  26  is formed on the other portion of the n-GaN clad layer  21 . Here, the protrusion and recess structure exists on the surface of the p-GaN clad layer  23 . In this structure, the light emitting device has an advantage in that the light generated from the active layer  22  can be scattered toward the p-type transparent electrode  24  rather than being trapped. However, since the thickness of the p-GaN clad layer  23  where the protrusion and recess structure is formed is approximately 0.2 μm, the efficiency of scattering is relatively low. In addition, on the surface of the p-GaN clad layer  23  where the protrusion and recess structure is locally formed, ohmic contact resistance is high, and thus, an operating voltage of the light emitting device is high. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides a semiconductor light emitting device having a protrusion and recess structure that can improve light extraction efficiency and can be manufactured through a simple process, and a method of manufacturing the same. 
     According to an aspect of the present disclosure, there is provided a semiconductor light emitting device having a protrusion and recess structure, the device including: a lower clad layer disposed on a substrate; an active layer formed on one portion of a top surface of the lower clad layer; an upper clad layer formed on the active layer; a first electrode formed on the upper clad layer; and a second electrode that is formed on a protrusion and recess structural pattern region formed on the remaining portion of the top surface of the lower clad layer. 
     The substrate may be a sapphire substrate. 
     The lower clad layer and the upper clad layer may be formed of gallium nitride-based materials. 
     The first electrode may include at least one material selected from the group consisting of Ag, Al, Rh, Pd, Ni, and Pt. 
     The second electrode may include at least one material selected from the group consisting of Al, Cr, Ti, and Au. 
     The thickness of the lower clad layer may be 1 to 10 μm. 
     The width of the protrusion and recess structure may be 0.1 to 10 μm and the height of the protrusion and recess structure may be 0.1 to 10 μm. 
     According to another aspect of the present disclosure, there is provided a method of manufacturing a semiconductor light emitting device having a protrusion and recess structure, the method including: forming a lower clad layer on a substrate; forming an active layer and an upper clad layer on the lower clad layer; forming a protrusion and recess structural pattern on a portion of the top surface of the upper clad layer by etching a portion of the lower clad layer; and forming a first electrode on the upper clad layer and forming a second electrode on the protrusion and recess structural pattern of the lower clad layer. 
     The forming of the protrusion and recess structural pattern may include: exposing the portion of the lower clad layer using a first etching process; and forming the protrusion and recess structure using a second etching process. 
     The second etching process may be one of a dry etching process and a wet etching process. 
     The wet etching process may be performed using one of an aqueous H 3 PO 4  solution and an aqueous KOH solution. 
     The wet etching process may be performed in the range of 100 to 300° C. 
    
    
     
       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 cross-sectional views of conventional semiconductor light emitting devices having a protrusion and recess structure; 
         FIG. 3  is a cross-sectional view of a light emitting device having a protrusion and recess structure according to an embodiment of the present disclosure; 
         FIGS. 4A through 4D  illustrate a manufacturing method of a light emitting device having a protrusion and recess structure according to an embodiment of the present disclosure; 
         FIG. 5  is an image showing the protrusion and recess structure formed according to the method illustrated in  FIGS. 4A through 4D ; 
         FIG. 6  is a plane view of a semiconductor light emitting device formed according to an embodiment of the present disclosure; 
         FIGS. 7A and 7B  are images of protrusion and recess structures taken after being etched under different conditions in the manufacturing of semiconductor light emitting devices according to the present disclosure; and 
         FIG. 8  is a graph of light output power versus width ratio of an electrode region of the portion of a light emitting device having a protrusion and recess structure according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. 
       FIG. 3  is a cross-sectional view of a light emitting device having a protrusion and recess structure according to an embodiment of the present disclosure. 
     Referring to  FIG. 3 , a lower clad layer  31  is formed on a substrate  30  and an active layer  32 , an upper clad layer  33  and a first electrode  34  are formed sequentially on a portion of the upper surface of the low clad layer  31 . The protrusion and recess structure is formed on the upper surface of the other portion of the lower clad layer  31 . A second electrode  35  is formed on the upper surface of the protrusion and recess structure. In the present embodiment, the protrusion and recess structure is formed at the interface between the lower clad layer  31  and the second electrode  35 . The protrusion and recess structure is formed between the lower clad layer  31  and the second electrode  35  to scatter the light that arrives at the interface between the lower clad layer  31  and the second electrode  35  when the light generated from the active layer  32  is trapped between the lower clad layer  31  and the upper clad layer  33  due to total reflection. 
     According to an embodiment of the present disclosure, materials forming the substrate  30 , the lower clad layer  31 , the active layer  32 , the upper clad layer  33 , the first electrode  34  and the second electrode  35  of the semiconductor light emitting device having the protrusion and recess structure are not restricted, and may be any materials used in general conventional semiconductor light emitting devices. Particularly, material used in a gallium nitride-based compound semiconductor device can be used. For example, the substrate  30  can be a semiconductor substrate or a sapphire substrate and the lower clad layer  31  and the upper clad layer  33  can be formed of n-GaN or p-GaN. Also, the active layer  32  can be a MQW. The first electrode  34  can be formed of a conventional material for an electrode such as Ag, Al, Rh, Pd, Ni or Pt, and the second electrode  35  can be formed of Al, Cr, Ti or Au. 
     Hereinafter, referring to  FIGS. 4A through 4D , a method of manufacturing the semiconductor light emitting device having the protrusion and recess structure according to an embodiment of the present disclosure will be described in detail. 
     Referring to  FIG. 4A , the substrate  30  is prepared. The substrate is a semiconductor substrate or a sapphire substrate. 
     Referring to  FIG. 4B , the lower clad layer  31 , the active layer  32  and the upper clad layer  33  are sequentially formed on the substrate  30 . The lower clad layer  31  and the upper clad layer  33  are formed of commonly used materials including GaN. For example, the lower clad layer  31  is formed of n-GaN and the upper clad layer  33  is formed of p-GaN. 
     Referring to  FIG. 4C , portions of the lower clad layer  31 , the active layer  32  and the upper clad layer  33  on the sapphire substrate  30  are etched. In detail, first, the upper clad layer  33  and the active layer  32  are etched vertically. This can be performed conveniently using a dry etching process such as reactive ion etching (RIE). Therefore, the lower clad layer  31  is exposed. Then, an etching process is performed on the exposed lower clad layer  31  to form the protrusion and recess structure. At this time, a dry etching process or a wet etching process can be used. In more detail, a wet etching process is performed using H 3 PO 4  and KOH aqueous solutions in the range of 100 to 300° C. for a few minutes to tens of minutes. The etching time can be selected according to the thickness of the lower clad layer  31 . The lower clad layer  31  can be formed to a thickness of 1 to 10 μm and the width and the height of the protrusion and recess structure can each be formed to 0.1 to 10 μm. 
       FIG. 5  is a sectional image of the protrusion and recess structure formed on the exposed surface of the lower clad layer  31  using the etching process described with reference to  FIG. 4C . In  FIG. 5 , the wet etching was performed at 220° C. for about 10 minutes using an aqueous H 3 PO 4  solution. Referring to  FIG. 5 , a trapezoid-shape groove was made in the lower clad layer  31  through the etching process. 
     Referring to  FIG. 4D , the semiconductor light emitting device is completed by forming the first electrode  34  on the upper clad layer  33  and the second electrode  35  on the protrusion and recess structure. 
       FIG. 6  is a plan view of a light emitting device having the protrusion and recess structure and a surface area of 320×320 μm 2  according to an embodiment of the present disclosure. Referring to  FIG. 6 , the second electrode  35  having the protrusion and recess structure takes up 30% of the total surface area of the device. That is, when the light generated from the active layer  32  is trapped inside the light emitting device and totally reflected, the region to which the light can be scattered is not small. 
       FIGS. 7A and 7B  are images of the surface of the lower clad layer  32  after performing an etching process for forming the protrusion and recess structure under different conditions according to embodiments of the present disclosure. 
       FIG. 7A  is an image of the surface of the lower clad layer  32  after performing the etching process at 200° C. using an aqueous H 3 PO 4  solution for approximately 10 minutes. Here, the width of the etched region (pit) was approximately 2 μm and a depth was less than approximately 1.5 μm. 
       FIG. 7B  is an image of the surface of the lower clad layer  32  after performing the etching process at 220° C. using an aqueous H 3 PO 4  solution for approximately 10 minutes. Here, the width of the etched region (pit) was approximately 5 μm and a depth was less than approximately 2.5 μm. 
     Referring to  FIGS. 7A and 7B , when forming the protrusion and recess structure under similar conditions but at slightly different temperatures, the dimensions of the formed protrusion and recess structure showed a big difference. Thus, the size of the protrusion and recess structure can be easily controlled by making a simple change in the condition of the wet etching process. 
       FIG. 8  is a graph of light output power versus current for semiconductor light emitting devices having the protrusion and recess structure manufactured using the etching processes described with reference to  FIGS. 7A and 7B  and a conventional semiconductor light emitting device without the protrusion and recess structure. 
     Referring to  FIG. 8 , 3 samples were used. The first sample did not have the conventional protrusion and recess structure, and is indicated by squares in  FIG. 8 . The second sample was the semiconductor light emitting device for which the etching process for forming the protrusion and recess structure was performed for approximately 10 minutes at 200° C. using an aqueous H 3 PO 4  solution, as illustrated in  FIG. 7A . The third sample was the semiconductor light emitting device for which the etching process for forming the protrusion and recess structure was performed for approximately 10 minutes at 220° C. using an aqueous H 3 PO 4  solution, as illustrated in  FIG. 7B . As indicated by the results of the third sample, when the size (width and height) of the protrusion and recess structure formed at the interface between the lower clad layer  31  and the second electrode  35  using the etching process was large, the light output power was high. At 20 mA, the second sample showed 25% higher light output power than the first sample and the third sample showed 46% higher light output power than the first sample. 
     According to the present disclosure, the protrusion and recess structure formed at the interface between the lower clad layer and the second electrode of the semiconductor light emitting device greatly improves the light extraction efficiency from the active layer, and the semiconductor light emitting device having the protrusion and recess structure at the interface between the lower clad layer and the second electrode is simpler to manufacture than a conventional semiconductor light emitting device having the protrusion and recess structure. 
     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.