Patent Publication Number: US-2007108467-A1

Title: Vertical GaN-based light emitting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 2005-108872 filed with the Korean Industrial Property Office on Nov. 15, 2005, the disclosure of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a vertical (vertical electrode type) gallium nitride (GaN)-based light emitting diode (LED) and a method of manufacturing the same. The vertical GaN-based LED can increase the light extraction efficiency, thereby improving the external quantum efficiency.  
      2. Description of the Related Art  
      Generally, GaN-based LEDs are grown on a sapphire substrate. The sapphire substrate is a rigid nonconductor and has a low thermal conductivity. Therefore, it is difficult to reduce the size of the GaN-based LED for cost-down or improve the optical power and chip characteristics. Particularly, heat dissipation is very important for the LEDs because a high current should be applied to the GaN-based LEDs so as to increase the optical power of the GaN-based LEDs. To solve these problems, a vertical GaN-based LED has been proposed. In the vertical GaN-based LED, the sapphire substrate is removed using a laser lift-off (hereinafter, referred to as LLO) technology.  
      The vertical GaN-based LED according to the related art will be described below with reference to  FIG. 1 .  
      Referring to  FIG. 1 , the conventional vertical GaN-based LED includes an n-type bonding pad  110 , an n-type reflective electrode  120 , an n-type transparent electrode  130 , an n-type GaN layer  140 , an active layer  150 , a p-type GaN layer  160 , a positive electrode (p-electrode )  170 , and a support layer  190 , which are sequentially formed under the n-type bonding pad  110 . The n-type transparent electrode  130  is used for improving the current diffusion efficiency.  
      In  FIG. 1 , a reference numeral  180  represents a plating seed layer used as a plating crystal nucleus when the support layer  190  is formed by electroplating or electroless plating.  
      In the conventional vertical GaN-based LED, however, because the p-electrode formed under the p-type GaN layer is formed of Cr/Au, it absorbs or totally reflects some of light emitted from the active layer. Thus, an entire luminous efficiency of the LED is degraded.  
     SUMMARY OF THE INVENTION  
      An advantage of the present invention is that it provides a vertical GaN-based LED in which a p-electrode is formed of a transparent layer with an uneven surface so that the external quantum efficiency is maximized and a current spreading effect is improved so as to secure a high power characteristic.  
      Additional aspect and advantages of the present general inventive concept will be set forth 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 vertical GaN-based LED includes: an n-type bonding pad; an n-type reflective electrode formed under the n-type bonding pad; an n-type transparent electrode formed under the n-type reflective electrode; an n-type GaN layer formed under the n-type transparent electrode; an active layer formed under the n-type GaN layer; a p-type GaN layer formed under the active layer; a p-electrode formed under the p-type GaN layer, the p-electrode having an uneven profile at a surface which does not come in contact with the p-type GaN layer; a p-type reflective electrode formed along the uneven surface of the p-type electrode; and a support layer formed under the p-type reflective electrode.  
      According to another aspect of the present invention, the p-type electrode is formed of a transparent layer, more preferably, TCO or Ni/Au. The TCO is a mixture made by adding at least one element selected from the group consisting of Sn, Zn, Ag, Mg, Cu, and Al to indium oxide.  
      According to a further aspect of the present invention, the vertical GaN-based LED further includes an adhesive layer formed at an interface between the p-type GaN layer and the p-electrode.  
      According to a still further aspect of the present invention, the adhesive layer is a transparent layer and is formed of a material different from that of the p-electrode.  
      According to a still further aspect of the present invention, the adhesive layer is formed of a mixture made by adding at least one element selected from a group consisting of Sn, Zn, Ag, Mg, Cu, and Al to indium oxide, and the element added to the adhesive layer is different from the element added to the TCO. Moreover, it is preferable that an amount of the element added to the adhesive layer is different from an amount of the element added to the TCO forming the p-electrode.  
      According to a still further aspect of the present invention, it is preferable that the adhesive layer has a thickness of 1˜200 Å because its transmissivity decreases as its thickness increases.  
      According to a still further aspect of the present invention, the p-type reflective electrode has the uneven profile at the surface that does not contact the support layer, and thus the support layer is formed using a plating seed by electroplating or electroless plating. 
    
    
     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 of a vertical GaN-based LED according to the related art; and  
       FIG. 2  is a sectional view of a vertical GaN-based LED according to the present invention. 
    
    
     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, a vertical GaN-based LED according to the present invention will be described in detail with reference to  FIG. 2 .  
       FIG. 2  is a sectional view of a vertical GaN-based LED according to the present invention.  
      Referring to  FIG. 2 , an n-type bonding pad  110  for electrical connection to an external device is formed at the uppermost portion of the vertical GaN-based LED.  
      An n-type reflective electrode  120  for improving the luminous efficiency is formed under the n-type bonding pad  110 .  
      An n-type GaN layer  140  is formed under the n-type reflective electrode  120 . More specifically, the n-type GaN layer  140  may be formed of an n-doped GaN layer or an n-doped GaN/AlGaN layer.  
      In order to improve the current spreading effect, an n-type transparent electrode  130  is further formed at the interface between the n-type reflective electrode  120  and the n-type GaN layer  140 .  
      An active layer  150  and a p-type GaN layer  160  are sequentially formed under the n-type GaN layer  140 , thereby forming a GaN-based LED structure.  
      In the GaN-based LED structure, the active layer  150  may be formed in a multi-quantum well structure with InGaN/GaN layer. Like the n-type GaN layer  140 , the p-type GaN layer  160  may be formed of a p-doped GaN layer or a p-doped GaN/AlGaN layer.  
      A p-electrode  170  is formed under the p-type GaN layer  160  of the GaN-based LED structure. The p-electrode  170  is preferably formed of a transparent layer, more preferably, transparent conductive oxide (TCO) or Ni/Au. The TCO is a mixture made by adding at least one element selected from the group consisting of Sn, Zn, Ag, Mg, Cu, and Al to indium oxide.  
      Unlike the conventional p-electrode formed of Cr/Au, the p-electrode  170  according to the present invention is formed of a transparent layer such as TCO or Ni/Au. Therefore, an amount of light emitted from the active layer and absorbed by the p-type electrode is minimized, thereby improving both the luminous efficiency and the current spreading effect.  
      Further, the bottom surface of the p-electrode  170 , which does not come in contact with the p-type GaN layer  160 , has an uneven profile. Therefore, the light emitted from the active layer is scattered by the uneven surface, thereby maximizing the external quantum efficiency.  
      Although not shown, an adhesive layer may be further formed at an interface between the p-electrode  170  and the p-type GaN layer  160  so as to increase their adhesive strength. As the thickness of the adhesive layer increases, its transmittance decreases. Therefore, it is preferable that the adhesive layer has a thickness of 1˜200 Å.  
      The adhesive layer is formed of a transparent layer. However, it is preferable that the adhesive layer is formed of a material different from that of the p-electrode  170 . More specifically, in order to obtain the excellent adhesive strength, the adhesive layer is formed of a mixture made by adding at least one element selected from the group consisting of Sn, Zn, Ag, Mg, Cu, and Al to indium oxide. It is preferable that the element added to the adhesive layer is different from the element added to the TCO, or an amount of the element added to the adhesive layer is different from an amount of the element added to the TCO.  
      A p-type reflective electrode  200  is formed along the uneven surface of the p-electrode  170 . Therefore, the p-type reflective electrode  200  also has the uneven profile, so that the light emitted from the active layer  150  can be prevented from being totally reflected and lost.  
      Under the p-type reflective electrode  200 , a support layer  190  is formed of a plating layer. The plating slayer is formed using a plating seed layer  180  by electroplating or electroless plating.  
      Although the support layer  190  is provided with the plating layer formed by using the plating seed layer  180  as a crystal nucleus, the present embodiment is not limited thereto. That is, the support layer may be formed of a Si substrate, a GaAs substrate, a Ge substrate, or a metal layer, which can serve as a support layer of a final LED and an electrode.  
      In addition, the metal layer may be formed using thermal evaporator, e-beam evaporator, sputter, and chemical vapor deposition (CVD).  
      As described above, the p-electrode is formed to have the uneven surface, so that the light emitted from the active layer can be prevented from being absorbed or scattered by the p-electrode. Then, it is possible to improve the light extraction efficiency and maximizing the external quantum efficiency.  
      Furthermore, the current spreading effect is improved by forming the p-electrode of the transparent layer, thereby obtaining the high power characteristic.  
      Consequently, the present invention can improve the characteristics and reliability of the vertical GaN-based LED.  
      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.