Patent Publication Number: US-8525196-B2

Title: Nitride-based semiconductor light emitting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 12/967,558, filed on Dec. 14, 2010, now U.S. Pat. No. 7,994,525 which is a Continuation of U.S. patent application Ser. No. 12/153,842, filed on May 27, 2008, now U.S. Pat. No. 7,893,447, which is a Divisional of U.S. patent application Ser. No. 11/543,798, filed Oct. 6, 2006, now U.S. Pat. No. 7,977,134 and claims the benefit of Korean Patent Application No. 2005-94453 filed with the Korea Industrial Property Office on Oct. 7, 2005, the disclosures of each of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a nitride-based semiconductor light emitting diode (LED). In the nitride-based semiconductor LED, an area around a p-electrode pad, in which light is preferentially emitted, is expanded so as to enhance light extraction efficiency, and local current crowding is prevented so as to reduce a driving voltage. 
     2. Description of the Related Art 
     Because group III-V nitride semiconductors such as GaN have excellent physical and chemical properties, they are considered as essential materials of light emitting devices, for example, light emitting diodes (LEDs) or laser diode (LDs). The LEDs or LDs formed of the group III-V nitride semiconductors are widely used in the light emitting devices for obtaining blue or green light. The light emitting devices are applied to light sources of various products, such as household appliances, electronic display boards, and lighting devices. Generally, the group III-V nitride semiconductors are comprised of gallium nitride (GaN) based materials having an compositional formula of In X Al Y Ga 1-X-Y N (0≦X, 0≦Y, X+Y≦1). 
     Hereinafter, a conventional nitride-based semiconductor LED will be described in detail with reference to  FIGS. 1 and 2 . 
       FIG. 1  is a sectional view illustrating the conventional nitride-based semiconductor LED, and  FIG. 2  is a plan view illustrating the conventional nitride-based semiconductor LED. 
     As shown in  FIG. 1 , the nitride-based semiconductor LED  100  includes a sapphire substrate  101  for growing nitride-based semiconductor materials, an n-type nitride semiconductor layer  102 , an active layer  103 , and a p-type nitride semiconductor layer  104 , which are sequentially formed on the sapphire substrate  101 . Portions of the p-type nitride semiconductor layer  104  and the active layer  103  are removed by a mesa etching process, so that the n-type nitride semiconductor layer  102  is partially exposed. 
     On the p-type nitride semiconductor layer  104  which has not been etched by the mesa etching process, a p-electrode pad  106  is formed. On the n-type nitride semiconductor layer  102 , an n-electrode pad  107  is formed. 
     Since the p-type nitride semiconductor layer  104  has larger specific resistance than the n-type nitride semiconductor layer  102 , a difference in resistance between the p-type nitride semiconductor layer  104  and the n-type nitride semiconductor layer  102  causes a current spreading effect to be reduced. As such, when a current spreading effect decreases, light extraction efficiency also decreases so that the brightness of the nitride semiconductor LED  100  is reduced. Accordingly, in order to enhance a current spreading effect in the related art, a transparent electrode  105  is formed on the p-type nitride semiconductor layer  104  so as to increase an injection area of current which is injected through the p-electrode pad  106 . 
     In the above-described nitride-based semiconductor LED  100 , the transparent electrode  105  is further provided on the p-type nitride semiconductor  104  so as to obtain an enhanced current spreading effect. However, when a difference in surface resistance between the transparent electrode  105  and the n-type nitride semiconductor layer  102  is large, a current spreading effect is still small. For example, when a commonly-used ITO (indium tin oxide) is used as the transparent electrode  105 , local current crowding occurs in the vicinity (refer to reference numeral ‘A 1 ’) of the p-electrode pad because of high surface resistance of the ITO. 
     In the nitride-based semiconductor LED  100 , the p-electrode pad  106  is formed as close to the outer edge line of the p-type nitride semiconductor layer  104  as possible, the outer edge line being a mesa line. Further, the p-electrode pad  106  and the n-electrode  107  is spaced at the maximum distance from each other so as to secure the maximum light emitting area therebetween. Then, an optical output is expected to be enhanced. In this case, however, local current crowding increases in the vicinity (A 1 ) of the p-electrode pad  106 , thereby degrading the reliability of the diode. 
     The vicinity (A 1 ) of the p-electrode pad  106  is a region (hereinafter, referred to as ‘preferential light emission region’) in which light is preferentially emitted. When the p-electrode pad  106  is formed close to the mesa line, there is a limit in securing an area in the vicinity (A 1 ) of the p-electrode pad  106  which is a preferential light-emission region of which the luminous density is high. Such a limit makes it difficult to enhance the light extraction efficiency of the entire chip. In the meantime, a dotted line of  FIG. 1  represents a current path. 
     SUMMARY OF THE INVENTION 
     An advantage of the present invention is that it provides a nitride-based semiconductor light emitting diode (LED) in which an area around a p-electrode pad is expanded so as to enhance light extraction efficiency, and local current crowding is prevented so as to reduce a driving voltage, in order to enhance the reliability of the diode. 
     Additional aspect and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     According to an aspect of the invention, a nitride-based semiconductor LED comprises a substrate that is formed in a rectangle shape and of which a ratio of the width and the length is equal to or more than 1.5; an n-type nitride semiconductor layer that is formed on the substrate and is composed of an n-type semiconductor material having a compositional formula of In X Al Y Ga 1-X-Y N (0≦X, 0≦Y, and X+Y≦1); an active layer and a p-type nitride semiconductor layer that are sequentially formed on a predetermined region of the n-type nitride semiconductor layer, the active layer being composed of a semiconductor material having a compositional formula of In X Al Y Ga 1-X-Y N (0≦X, 0≦Y, and X+Y≦1) and the p-type nitride semiconductor layer being composed of a p-type semiconductor material having a compositional formula of In X Al Y Ga 1-X-Y N (0≦X, 0≦Y, and X+Y≦1); a transparent electrode that is formed on the p-type nitride semiconductor layer so as to be spaced at a predetermined distance from the outer edge line of the p-type nitride semiconductor layer; a p-electrode pad that is formed on the transparent electrode so as to be spaced at a distance of 50 to 200 μm from the outer edge line of the p-type nitride semiconductor layer composed of a p-type semiconductor material having a compositional formula of In X Al Y Ga 1-X-Y N (0≦X, 0≦Y, and X+Y≦1); and an n-electrode pad that is formed on the n-type nitride semiconductor layer. 
     The nitride-based semiconductor LED further comprises a buffer layer that is formed between the substrate and the n-type nitride semiconductor layer and is composed of AlN/GaN. The substrate is a sapphire substrate. 
     Preferably, the n-type nitride semiconductor layer is a GaN layer or GaN/AlGaN layer doped with any one n-type conductive impurity selected from the group consisting of Si, Ge, and Sn, the p-type nitride semiconductor layer is a GaN layer or GaN/AlGaN layer doped with any one p-type conductive impurity selected from the group consisting of Mg, Zn, and Be, and the active layer is composed of an InGaN/GaN layer with a multi-quantum well structure. 
     Preferably, the transparent electrode is an ITO (Indium Tin Oxide) material. 
     Preferably, the p-electrode pad and the n-electrode pad are formed of Au or Au/Cr. 
     When the p-electrode pad is spaced at a distance of 50 to 200 μm from the outer edge line of the p-type nitride semiconductor layer, optical power may increase. 
     When the p-electrode pad is spaced at a distance of more than 200 μm from the outer edge line of the p-type nitride semiconductor layer, optical power may decrease. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a sectional view illustrating a conventional nitride-based semiconductor LED; 
         FIG. 2  is a plan view illustrating the conventional nitride-based semiconductor LED; 
         FIG. 3  is a sectional view illustrating a nitride-based semiconductor LED according to an embodiment of the present invention; 
         FIG. 4  is a plan view illustrating the nitride-based semiconductor LED according to the embodiment of the invention; 
         FIGS. 5A to 5D  are sectional views for explaining a method of manufacturing the nitride-based semiconductor LED according to an embodiment of the invention; 
         FIG. 6  is a graph illustrating a change in Po (optical power) in accordance with a separation distance of a p-electrode pad; 
         FIG. 7  is a graph illustrating a change in a driving voltage in accordance with a separation distance of the p-electrode pad; and 
         FIG. 8  is a color photograph showing a state where the p-electrode pad is spaced from a mesa-line by 55 μm. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     Structure of Nitride-Based Semiconductor LED 
     Referring to  FIGS. 3 and 4 , a nitride-based semiconductor LED according to an embodiment of the invention will be described in detail. 
       FIG. 3  is a sectional view illustrating the nitride-based semiconductor LED, and  FIG. 4  is a plan view illustrating the nitride-based semiconductor LED. 
     As shown in  FIG. 3 , the nitride-based semiconductor LED  200  according to the embodiment of the invention includes a sapphire substrate  201  for growing nitride-based semiconductor materials, a buffer layer (not shown), an n-type nitride semiconductor layer  202 , an active layer  203 , and a p-type nitride semiconductor layer  204 , which are sequentially formed on the sapphire substrate  201 . Portions of the p-type nitride semiconductor layer  204  and the active layer  203  are removed by a mesa etching process, so that the upper surface of the n-type nitride semiconductor layer  202  is partially exposed. 
     The buffer layer is grown on the sapphire substrate  201  so as to enhance the lattice matching between the sapphire substrate  210  and the n-type nitride semiconductor layer  202 . The buffer layer may be formed of AlN/GaN or the like. 
     The n-type and p-type nitride semiconductor layers  202  and  204  and the active layer  203  can be formed of a semiconductor material having a compositional formula of In X Al Y Ga 1-X-Y N (here, 0≦X, 0≦Y, and X+Y≦1). More specifically, the n-type nitride semiconductor layer  202  can be formed of a GaN or GaN/AlGaN layer doped with n-type conductive impurities. For example, the n-type conductive impurity may be Si, Ge, Sn and the like, among which Si is preferably used. Further, the p-type nitride semiconductor layer  204  can be formed of a GaN or GaN/AlGaN layer doped with p-type conductive impurities. For example, the p-type conductive impurity may be Mg, Zn, Be and the like, among which Mg is preferably used. The active layer  203  can be formed of an InGaN/GaN layer with a multi-quantum well structure. 
     On the p-type nitride semiconductor layer  204  which has not been removed by the mesa-etching process, a transparent electrode  205  is formed of an ITO material. As shown in  FIG. 3 , the transparent electrode  205  is spaced at a predetermined distance from the outer edge line of the p-type nitride semiconductor layer  204 . On the transparent electrode  205 , a p-type electrode pad  206  is formed so as to be spaced at a predetermined distance from the outer edge line of the p-type nitride semiconductor layer  204  which is a mesa line. On the n-type nitride semiconductor layer  202  exposed by the mesa etching process, an n-type electrode pad  207  is formed. At this time, it is preferable that the p-type electrode pad  206  is formed so as to be spaced from the outer edge line of the p-type nitride semiconductor layer  204  by 50 to 200 μm, in consideration of the size of a general nitride-based semiconductor LED chip. 
     As shown in  FIG. 4 , the plan shape of the substrate  201  is formed in a rectangle shape. In this case, it is preferable that a ratio of the width to the length of the rectangle is 1:1.5. 
     In the meantime, when a commonly-used ITO is used as the transparent electrode  205  as described above, local current crowding can occur in the vicinities of the p-type electrode pad  206  because of high surface resistance of the ITO. In this embodiment, however, the p-type electrode pad  206  is spaced at a predetermined distance from the mesa line, which makes it possible to reduce local current crowding. Accordingly, it is possible to enhance the reliability of the diode (for example, a driving voltage can be reduced) and to expand an area around the p-electrode pad  206  which is a preferential light emitting region (refer to reference numeral ‘A 2 ’ of  FIG. 3 ). Therefore, it is possible to enhance the overall light emission efficiency of the chip. Meanwhile, a dotted line of  FIG. 3  shows a current path. 
     Method of Manufacturing Nitride-Based Semiconductor LED 
     Hereinafter, a method of manufacturing a nitride-based semiconductor LED according to an embodiment of the invention will be described. 
       FIGS. 5A to 5D  are sectional views illustrating the method of manufacturing a nitride-based semiconductor LED. 
     First, as shown in  FIG. 5A , a buffer layer (not shown), an n-type nitride semiconductor layer  202 , an active layer  203 , and a p-type nitride semiconductor layer  204  are sequentially formed on a sapphire substrate  201  for growing nitride-based semiconductor materials. The buffer layer may be omitted, and the n-type nitride semiconductor layer  202 , the active layer  203 , and the p-type nitride semiconductor layer  203  can be formed of a semiconductor material having a compositional formula of In X Al Y Ga 1-X-Y N (here, 0≦X, 0≦Y, and X+Y≦1). In general, they may be formed through such a process as a metal organic chemical vapor deposition (MOCVD) method. 
     Next, as shown in  FIG. 5B , portions of the p-type nitride semiconductor layer  204 , the active layer  203 , and the n-type nitride semiconductor layer  202  are mesa-etched so as to partially expose the n-type nitride semiconductor layer  202 . 
     As shown in  FIG. 5C , a transparent electrode  205  is formed on the p-type nitride semiconductor layer  204 . In general, the transparent electrode  205  is formed of an ITO. 
     As shown in  FIG. 5D , a p-electrode pad  206  is formed on the transparent electrode  205  spaced at a predetermined distance from the outer edge line of the p-type nitride semiconductor layer  204 , and an n-electrode pad  207  is formed on the n-type nitride semiconductor layer  202 . The p-electrode pad  206  and the n-electrode pad  207  may be formed of metal such as Au or Au/Cr. 
     As described above, current crowding can occur in the vicinities of the p-electrode pad  206  because of high surface resistance of an ITO used as the transparent electrode  205 . In this embodiment, however, the p-electrode pad  206  is spaced at a predetermined distance from the mesa line, which makes it possible to reduce local current crowding, Therefore, a driving voltage can be reduced, and an area around the p-electrode pad  206  which is a preferential light emitting region can be expanded (refer to reference numeral ‘A 2 ’ of  FIG. 5D ), which makes it possible to enhance the overall light emission efficiency of the chip. 
       FIG. 6  is a graph illustrating a change in Po (optical power) in accordance with a separation distance of the p-electrode pad, and  FIG. 7  is a graph illustrating a change in a driving voltage in accordance with a separation distance of the p-electrode pad. 
     Referring to  FIG. 6 , while the p-electrode pad  206  is spaced from the mesa line by 50 to 200 μm, Po tends to increase. As the p-electrode pad  206  is spaced from the mesa line by more than 200 μm, Po decreases. Therefore, it is most preferable that the p-electrode pad  206  is spaced from the outer edge line of the p-type nitride semiconductor layer  204  as the mesa line by 50 to 200 μm. Further, referring to  FIG. 7 , as the p-electrode pad  206  is spaced at a predetermined distance from the mesa line, that is, as the distance between the p-electrode pad  206  and the n-electrode pad  207  is reduced, a driving voltage is reduced. 
       FIG. 8  is a color photograph showing a luminous state when the p-electrode pad is spaced from the mesa line by 55 μm. 
     When the p-electrode pad  206  is spaced from the mesa line by 55 μm, a uniform luminous effect can be obtained in the entire chip, as shown in  FIG. 8 . Further, an area around the p-electrode pad  206 , which is a preferential light emitting region, can be expanded, so that the overall luminous efficiency of the chip can be further enhanced. 
     Preferably, the plan shape of the sapphire substrate  201  is formed in a rectangle shape. This is because, when the sapphire substrate  201  is rectangular, it is advantageous to secure a margin of distance where the p-electrode pad  206  can be spaced from the mesa line, compare with when the sapphire substrate  201  is formed in a square shape. In this case, it is preferable that a ratio of the width to the length of the rectangle is 1:1.5. This is because, when a ratio of the width to the length of the rectangle is less than 1.5, the p-electrode pad  206  spaced from the mesa line becomes so close to the n-electrode pad  207  that a current spreading effect can be reduced. 
     According to the nitride-based semiconductor LED and the method of manufacturing the same of the present invention, the p-electrode pad is spaced at a predetermined distance from the mesa line, and an area around the p-electrode pad, in which light is preferentially emitted, is expanded so as to enhance light extraction efficiency of a chip. Further, local current crowding is reduced so as to reduce a driving voltage, thereby enhancing the reliability of the diode. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.