Patent Publication Number: US-8114691-B2

Title: Semiconductor light emitting device having textured structure and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application is a divisional of U.S. application Ser. No. 11/293,273, filed on Dec. 5, 2005, U.S. Pat. No. 7,655,959, issued on Feb. 2, 2010, which claims the benefit of Korean Patent Application No. 10-2004-0103112, filed on Dec. 8, 2004, in the Korean Intellectual Property Office, the entire disclosure of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure relates to a semiconductor light emitting diode, and more particularly, to a semiconductor light emitting diode that improves light extraction efficiency using a textured structure and a method of manufacturing the same. 
     2. Description of the Related Art 
     A light emitting diode (LED) is a device used for converting electrical energy into infra red rays, visible light, or other light using the characteristics of a compound semiconductor. The light emitting diode is a type of electro luminescent (EL) device, and presently, the light emitting diodes that employ an III-V group compound semiconductor are being practically utilized. 
     The III-V group compound semiconductor is a direct transition semiconductor, and is widely used for LEDs or laser diodes (LDs) since it provides stable operation at a higher temperature than devices that use other semiconductors. The III-V group compound semiconductor is typically formed on a substrate formed of sapphire Al 2 O 3  or SiC. To improve light emission efficiency, or light extraction efficiency, a variety of LED structures have been studied. Presently, a study is being carried out to improve the light extraction efficiency by forming a textured structure on a light extraction region of the LED. 
     Light is hindered at an interface of material layers having different refractive indexes according to the refractive index of each of the material layers. In the case of a flat interface, when the light passes from a semiconductor layer having a greater refractive index (n=2.5) into an air layer having a smaller refractive index (n=1), the light must enter the flat interface at less than a predetermined angle with respect to the normal. If the light enters at an angle greater than the predetermined angle, the light totally is internally reflected at the flat interface, thereby greatly reducing the light extraction efficiency. To avoid the total internal reflection of light, a method of incorporating a textured structure at the interface has been attempted. 
       FIGS. 1A and 1B  are cross-sectional views illustrating a conventional light emitting diode having a textured structure. Referring to  FIG. 1A , a p-GaN layer  102 , an active layer  103 , an n-GaN layer  104  are sequentially formed on a p-electrode  101 , and an n-electrode  105  is formed on the n-GaN layer  104 . When light generated by the active layer  103  is extracted upward through the n-GaN layer  104 , to change the incidence angle of the light, a textured structure  106  is incorporated at an interface between the n-GaN layer  104  and the air layer. 
     Referring to  FIG. 1B , an n-GaN layer  112  is formed on a sapphire substrate  111 , and an n-AlGaN layer  113 , an active layer  114 , a p-AlGaN layer  115 , a p-GaN layer  116 , and a p-electrode  117  are sequentially formed on a region of the n-GaN layer  112 . An n-electrode  118  is formed on a region of the n-GaN layer  112  where the n-AlGaN layer  113  is not formed. This is a flip-chip structure in which light generated by the active layer  114  is mainly extracted through the transparent sapphire substrate  111 . Here, the light extraction efficiency is improved by forming a textured structure  120  on the surface of the sapphire substrate  111 . 
     A conventional semiconductor light emitting diode incorporates the textured structure  120  to improve the light extraction efficiency. However, particularly as depicted in  FIG. 1B , when the textured structure  120  is incorporated by patterning the sapphire substrate  111 , the growth of a semiconductor layer with a uniform quality is difficult, because there is a great possibility of generating defects in the semiconductor layer due to unmatched crystal structure between the sapphire substrate  111  and the semiconductor layer formed on the sapphire substrate  111 . As a result, the light extraction efficiency is reduced due to the internal crystal defects. 
     SUMMARY OF THE DISCLOSURE 
     The present invention may provide a semiconductor light emitting diode having a structure that can improve light extraction efficiency and reduce internal crystal defects of the semiconductor light emitting diode and a method of manufacturing the semiconductor light emitting diode. 
     According to an aspect of the present invention, there may be provided a semiconductor light emitting diode comprising: a first semiconductor layer formed to a textured structure; an intermediate layer formed between the textured structures of the patterned first semiconductor layer; and a second semiconductor layer, an active layer, and a third semiconductor layer sequentially formed on the first semiconductor layer and the intermediate layer. 
     The substrate may be a sapphire substrate. 
     The intermediate layer may be formed of a transparent insulating material or a transparent conductive material having a refractive index of 2.5 or less. 
     The intermediate layer may be formed of a transparent insulating material including at least one of SiO 2 , SiN x , Al 2 O 3 , HfO, TiO 2 , or ZrO. 
     The intermediate layer may be formed of a transparent conductive material, such as ZnO or an In oxide that includes at least one additive selected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La. 
     The first semiconductor layer, the second semiconductor layer, and the third semiconductor layer may be formed of GaN. 
     The first semiconductor layer and the intermediate layer may be formed on the sapphire substrate. 
     The semiconductor light emitting diode may further comprise a first electrode formed on the third semiconductor layer, and a second electrode formed on a region of the second semiconductor layer in which the active layer is not formed. 
     The width of the textured structure of the first semiconductor layer pattern may be gradually narrowed as it goes upward. 
     According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor light emitting diode having a textured structure, the method comprising: forming a first semiconductor layer on a sapphire substrate; exposing a portion of the sapphire substrate while forming the textured structure by etching the first semiconductor layer; forming an intermediate layer on the exposed sapphire substrate between the textured structures of the first semiconductor layer; and sequentially forming a second semiconductor layer, an active layer, and a third semiconductor layer on the first semiconductor layer and the intermediate layer. 
     The exposing of a portion of the sapphire substrate while forming the textured structure by etching the first semiconductor layer may comprise: performing a first etching to form etch pits on the surface of the first semiconductor layer; and performing a second etching to expose the surface of the sapphire substrate by etching the etch pits of the first semiconductor layer. 
     The first etching may be performed using H 3 PO 4  and the second etching may be performed using KOH. 
     The forming of an intermediate layer on the exposed sapphire substrate between the textured structures of the first semiconductor layer comprises: coating a optical transmittance material on the exposed sapphire substrate and the textured structure of the first semiconductor layer; and forming the intermediate layer by leveling the optical transmittance material to expose the surface of the first semiconductor layer. 
     The method may further comprise annealing after the optical transmittance material is coated on the exposed sapphire substrate and the textured structure of the first semiconductor layer. 
     The method may further comprise performing a third dry etching of the exposed surface of the sapphire substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will be described in greater detail in exemplary embodiments thereof with reference to the attached drawings in which: 
         FIGS. 1A and 1B  are cross-sectional views of conventional semiconductor light emitting diodes having a textured structure; 
         FIGS. 2 and 3  are cross-sectional views of semiconductor light emitting diodes having a textured structure according to the present invention; 
         FIGS. 4A through 4E  are cross-sectional views for illustrating a method of manufacturing a semiconductor light emitting diode according to an embodiment of the present invention; 
         FIGS. 5A through 5E  are cross-sectional views for illustrating a method of manufacturing a semiconductor light emitting diode according to another embodiment of the present invention; 
         FIGS. 6A through 6D  are SEM images of the semiconductor light emitting diode according to an embodiment of the present invention; and 
         FIG. 7  is a graph showing the light extraction efficiencies of a conventional semiconductor light emitting diode having a textured structure and a semiconductor light emitting diode having a textured structure according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present invention will now be described more fully with reference to the accompanying drawings in which exemplary embodiments of the invention are shown. 
       FIGS. 2 and 3  are cross-sectional views of semiconductor light emitting diodes having a textured structure according to the present invention. 
       FIG. 2  shows a textured structure applied to flip-chip type semiconductor light emitting diodes and  FIG. 3  shows a textured structure applied to vertical type semiconductor light emitting diodes. 
     Referring to  FIG. 2 , a first semiconductor layer  22  and an intermediate layer  23  are formed in a textured structure on a transparent substrate  21 , and a second semiconductor layer  24  is formed on the first semiconductor layer  22  and the intermediate layer  23 . An active layer  25 , a third semiconductor layer  26  and a first electrode  27  are sequentially formed on a first region of the second semiconductor layer  24 . A second electrode  28  is formed on a second region of the second semiconductor layer  24 . 
     The materials used for forming the layers are described hereafter. The transparent substrate  21  can be a widely used sapphire Al 2 O 3  substrate, and the first semiconductor layer  22  and the second semiconductor layer  24  can be formed of p-GaN. The intermediate layer  23  may be formed of a transparent insulating material or a transparent conductive material having a refractive index of 2.5 or less. For example, the transparent insulating material can be SiO 2 , SiN x , Al 2 O 3 , HfO, TiO 2 , or ZrO, and the transparent conductive material can be ZnO or an In oxide that includes at least one additive selected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La. Here, it is seen that the intermediate layer  23  is formed of a transparent material. The active layer  25  can be formed of a material typically used for forming a semiconductor light emitting diode or a laser emitting diode in a multi-layer structure of a multi-quantum well barrier structure. The third semiconductor layer  26  can be formed of p-GaN, and, at this time, the first electrode  27  is formed of a p-type conductive material and the second electrode  28  is formed of an n-type conductive material. 
     As shown in  FIG. 2 , in the textured structure according to an embodiment of the present invention, the intermediate layer  23  is formed within a region where the first semiconductor layer  22  is patterned into the textured structure. Here, the distance between the pattered textured structures of the first semiconductor layer  22  is not uniform, but is determined according to crystal defects in the first semiconductor layer  22 , particularly to screw dislocation which will be described later with reference to a subsequent manufacturing process. According to the semiconductor light emitting diode having the above structure, the intermediate layer  23  is formed in the crystal defect region of the first semiconductor layer  22 , and the internal crystal defects can be reduced by forming the second semiconductor layer  24  on the first semiconductor layer  22 . Accordingly, the extraction efficiency of light generated by the active layer  25  can be increased by incorporating the textured structure. 
       FIG. 3  is a cross-sectional view of a vertical type semiconductor light emitting diode having a textured structure according to an embodiment of the present invention. Referring to  FIG. 3 , a first electrode  32 , a third semiconductor layer  33 , an active layer  34 , and a second semiconductor layer  35  are sequentially formed on a lower structure  31 . A first semiconductor layer  37  patterned into a textured structure layer and an intermediate layer  36  are formed on the second semiconductor layer  35 . Also, a second electrode  38  is formed on the first semiconductor layer and the intermediate layer  36 . 
     The materials for forming each of the layers that constitute the vertical type semiconductor light emitting diode are as follows. The first semiconductor layer  37  and the second semiconductor layer  35  can be formed of p-GaN. The intermediate layer  36  may be formed of a transparent insulating material or a transparent conductive material having a refractive index of 2.5 or less. For example, the transparent insulating material can be SiO 2 , SiN x , Al 2 O 3 , HfO, TiO 2 , or ZrO, and the transparent conductive material can be ZnO or an In oxide that includes at least one additive selected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La. The active layer  34  can be formed of a material typically used for forming a semiconductor light emitting diode or a laser emitting diode in a multi-layer structure of a multi-quantum well barrier structure. The third semiconductor layer  33  can be formed of p-GaN, and, at this time, the first electrode  32  is formed of a p-type conductive material and the second electrode  38  is formed of an n-type conductive material. 
     As shown in  FIG. 3 , in the textured structure according to an embodiment of the present invention, the intermediate layer  36  is formed within a region where the first semiconductor layer  37  is patterned into the textured structure. Here, the distance between the patterned textured structures of the first semiconductor layer  37  is not uniform, but is determined according to crystal defects in the first semiconductor layer  37 , particularly, to screw dislocation. According to the semiconductor light emitting diode having the above structure, the intermediate layer  36  is formed in the crystal defect region of the first semiconductor layer  37 , and the internal crystal defects can be reduced by forming the second semiconductor layer  35  on the first semiconductor layer  37 . Accordingly, the extraction efficiency of light generated by the active layer  33  can be increased by incorporating the textured structure. 
     A method of manufacturing a semiconductor light emitting diode having a textured structure according to an embodiment of the present invention will now be described with reference to  FIGS. 4A through 4E . 
     Referring to  FIG. 4A , a first semiconductor layer  42  is formed on a substrate  41 . Here, the substrate  41  is a sapphire substrate having a refractive index of 1.78, and the first semiconductor layer  42  is formed of n-GaN. After the first semiconductor layer  42  is applied, a first etching process for etching the surface of the first semiconductor layer  42  is performed using H 3 PO 4 . Here, internal crystal defects may be generated since the sapphire has a different crystal structure than GaN. Particularly, internal crystal defects which grow vertically from the sapphire substrate  41  toward the first semiconductor layer  42 , such as screw dislocation  43 , can be formed. When the surface of the first semiconductor layer  42  is wet etched using H 3 PO 4 , etch pits are formed at screw dislocation  43  regions since the etching occurs mainly at the screw. The wet etching progresses downward in the direction of the screw dislocations  43  and in the lateral directions as well.  FIG. 6A  is a SEM image of the first semiconductor layer  42  after the wet etching process as depicted in  FIG. 4A  with respect to the first semiconductor layer  42  is performed using H 3 PO 4 . 
     Referring to  FIG. 4B , a second etching is performed with respect to the first semiconductor layer  42  using KOH. When the second etching is performed using KOH, the etching progresses vertically downward along the screw dislocations  43  of the first semiconductor layer  42 . The etching direction of the first semiconductor layer  42  by KOH is vertically downward unlike that obtained by the use of H 3 PO 4 . Consequently, the surface of the sapphire substrate  41  is exposed, and the cross-section of the first semiconductor layer  42  becomes a textured structure patterned into a trapezoidal shape. Reference numeral  42   a  represents a region etched by H 3 PO 4  and KOH.  FIG. 6B  is a SEM image for showing the result of etching the first semiconductor layer  42 . Referring to  FIG. 6B , the cross-section of the first semiconductor layer  42  has a trapezoidal shape, that is a textured structure, as a result of etching the first semiconductor layer  42  using KOH until the surface of the sapphire substrate  41  is exposed. 
     Referring to  FIG. 4C , an intermediate layer  44  is formed on the first semiconductor layer  42  patterned into a textured structure on the sapphire substrate  41 . The intermediate layer  44  may be formed of a material having a high light transmittance since light emitted by an active layer is extracted to the outside through the textured structure. The intermediate layer  44  may be formed of a transparent insulating material or a transparent conductive material having a refractive index of 2.5 or less. For example, the transparent insulating material can be SiO 2 , SiN x , Al 2 O 3 , HfO, TiO 2 , or ZrO, and the transparent conductive material can be ZnO or an In oxide that includes at least one additive selected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La. These materials have refractive indexes in the range of approximately 1.4 to 1.8.  FIG. 6C  is a SEM image of the intermediate layer  44  formed on the first semiconductor layer  42  patterned into the textured structure. The intermediate layer  44  is formed in the etched region of the first semiconductor layer  42  as well as on the first semiconductor layer  42 . After the intermediate layer  44  is applied, an annealing process can further be performed. A MOCVD process an be performed at 1100° C. for approximately 1 hour under a H 2  atmosphere. 
     Referring to  FIG. 4D , to expose the upper part of the first semiconductor layer  42  patterned into the textured structure, a leveling process is performed to remove the upper part of the intermediate layer  44 . Accordingly, the intermediate layer  44  remains only between the textured structures of the first semiconductor layer  42 . 
     Referring to  FIG. 4E , a second semiconductor layer  45  is formed on the exposed first semiconductor layer  42  and the remaining intermediate layer  44 . The second semiconductor layer  45  may be formed of the same material as the first semiconductor layer  42 , such as n-GaN. In this case, since the second semiconductor layer  45  is grown on the first semiconductor layer  42  which has fewer crystal defects than the sapphire substrate  41 , the crystal defects in the second semiconductor layer  45  are greatly reduced compared to the instance when the second semiconductor layer  45  is grown directly on the sapphire substrate  41 . 
     The textured structure according to an embodiment of the present invention can be formed in a semiconductor light emitting diode by the processes described with reference to  FIGS. 4A through 4E . An active layer and a third semiconductor layer formed on the second semiconductor layer  45  can be readily formed using a conventional process. The textured structure can be used as a flip-chip structure, or as a vertical structure after the sapphire substrate  41  is removed and electrodes are further formed. 
     The textured structure formed in the semiconductor light emitting diode using the processes described above, unlike in the conventional art, not only improves the light extraction efficiency but also reduces crystal defects, thereby allowing stable operation and extending the lifetime of the device. 
     A semiconductor light emitting diode having a textured structure according to another embodiment of the present invention will now be described with reference to  FIGS. 5A through 5E . 
     Referring to  FIG. 5A , a first semiconductor layer  52  is formed on a substrate  51 . Here, the substrate  51  is a sapphire substrate, and the first semiconductor layer  52  is formed of n-GaN. After the first semiconductor layer  52  is applied, a first etching process is performed using H 3 PO 4  to etch the surface of the first semiconductor layer  52 . This forms etch pits at screw dislocation  43  regions, since the etching occurs mainly at the screw dislocation  43  regions. The wet etching progresses downward in the direction of the screw dislocations  43 , and in the lateral directions as well. 
     Referring to  FIG. 5B , a second etching is performed on the first semiconductor layer  52  using KOH. This etches vertically downward along the screw dislocations  43  of the first semiconductor layer  52 . The etching direction of the first semiconductor layer  52  by KOH is vertically downward, unlike that obtained by the use of H 3 PO 4 . Consequently, the surface of the sapphire substrate  51  is exposed, and the cross-section of the first semiconductor layer  52  becomes a textured structure patterned into a trapezoidal shape. Reference numeral  52   a  represents a region etched by H 3 PO 4  and KOH. At this time, after etching the first semiconductor layer  52  by KOH, etching is performed on a region of the substrate  51  exposed by the dry etching. Accordingly, grooves are formed by etching the exposed regions of the substrate  51 .  FIG. 6D  is a SEM image of the substrate  51  obtained dry etching after the etching of the first semiconductor layer  52  using KOH. 
     Referring to  FIG. 5C , an intermediate layer  54  is formed on the first semiconductor layer  52  patterned into a textured structure on the sapphire substrate  51 . The intermediate layer  54  may be formed of a transparent insulating material or a transparent conductive material having high light transmittance and a refractive index of 2.5 or less, since light emitted by an active layer is extracted to the outside through the textured structure. The transparent insulating material can be SiO 2 , SiN x , Al 2 O 3 , HfO, TiO 2 , or ZrO, and the transparent conductive material can be ZnO or an In oxide that includes at least one additive selected from the group consisting of Mg, Ag, Zn, Sc, Hf, Zr, Te, Se, Ta, W, Nb, Cu, Si, Ni, Co, Mo, Cr, Mn, Hg, Pr, and La. After the intermediate layer  54  is coated, an annealing process can further be performed. The annealing can be performed at approximately 1100° C. for approximately 1 hour under a H 2  atmosphere. 
     Next, referring to  FIG. 5D , to expose the upper part of the first semiconductor layer  52  patterned into the textured structure, a leveling process is performed to remove the upper part of the intermediate layer  54 . Accordingly, the intermediate layer  54  remains only between the textured structures of the first semiconductor layer  52 . 
     Referring to  FIG. 5E , a second semiconductor layer  56  is formed on the exposed first semiconductor layer  52  and the remaining intermediate layer  54 . The second semiconductor layer  56  may be formed of the same material as the first semiconductor layer  52 , such as n-GaN. In this case, since the second semiconductor layer  56  is grown on the first semiconductor layer  52  which has fewer crystal defects than the sapphire substrate  51 , the crystal defects in the second semiconductor layer  56  are greatly reduced compared to the instance when the second semiconductor layer  56  is grown directly on the sapphire substrate  51 . An active layer and a third semiconductor layer formed on the second semiconductor layer  56  can be readily formed using a conventional process. 
       FIG. 7  is a graph showing the light extraction efficiencies of a conventional semiconductor light emitting diode having a textured structure and a semiconductor light emitting diode having a textured structure according to an embodiment of the present invention. 
     Referring to  FIGS. 2 and 3 , the textured structure of the first semiconductor layer  22  has a hexagonal trapezoidal shape or a hexagonal cylindrical shape, or an inverse of these shapes. Patterns are prepared, each with a diameter of 1 μm, and a height of 0.5 μm, and with the distance between the patterns being 1 μm, and the light extraction efficiency of the patterns is investigated. The results show that the semiconductor light emitting diode having the textured structure (dielectric embedded nitride structure, n=1.4) according to an embodiment of the present embodiment has a maximum of 85% higher light extraction efficiency than a conventional planar structure semiconductor light emitting diode, and a maximum of 77% higher light extraction efficiency than a semiconductor light emitting diode Ref  2  having a conventional trapezoidal textured structure (patterned sapphire substrate (PSS), n=1.78). 
     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 detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 
     According to the present invention, the extraction efficiency of light emitted by an active layer can be greatly improved, and the crystal defects in the semiconductor device can be reduced, by forming a textured structure pattern in a semiconductor layer of a semiconductor light emitting diode, thereby enabling stable operation and increasing the lifespan of the semiconductor light emitting diode.