Abstract:
An omni-directional reflector having a transparent conductive low-index layer formed of conductive nanorods and a light emitting diode utilizing the omni-directional reflector are provided. The omni-directional reflector includes: a transparent conductive low-index layer formed of conductive nanorods; and a reflective layer formed of a metal.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims the benefit of U.S. patent application Ser. No. 60/704,884, filed on Aug. 3, 2005 in the United States Patent and Trademark Office and Korean Patent Application No.10-2005-0089473, filed on Sep. 26, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE DISCLOSURE  
       [0002]     1. Field of the Disclosure  
         [0003]     The present disclosure relates to a conductive omni-directional reflector and a light emitting diode adopting the same, and more particularly, to a reflector having a high electro-optic characteristic and a light emitting diode adopting the same.  
         [0004]     2. Description of the Related Art  
         [0005]     Reflectors used in LEDs must have high conductivities as well as high reflectivities. High reflective metal electrodes formed of Ag or Al have been used as existing mono metal reflectors. Such a metal reflector cannot obtain a reflectivity beyond a predetermined limit due to the refractive index and the extinction coefficient that are characteristics of the metal itself. As shown in  FIG. 1 , an omni-directional reflector (ODR) is suggested as shown in  FIG. 1  to overcome a limit of such a metal reflectivity (Ag: about 86%, Al: about 92%). The ODR has a structure in which a low-index layer and a metal layer formed of Ag or Al are sequentially stacked on a semiconductor layer. A thickness th of the low-index layer must be proportional to ¼n (n: refractive index) of a wavelength λ so that the ODR achieves a high reflectivity. The low-index layer is formed of a material such as SiO 2  or Si 3 N 4  having a low reflectivity. The metal layer is formed of a material having a high extinction coefficient, for example, a metal such as Ag or Al. However, in the structure of the ODR, the material from which the low-index layer has been formed is generally a nonconductor. Thus, the low-index layer may not be formed of an active element injecting a current.  
         [0006]     U.S. Pat. No. 6,784,462 discloses a light emitting diode having high light extraction efficiency. A reflector is positioned between a substrate and a light emitter and includes a transparent layer formed of a low-index material such as SiO 2 , Si 3 N 4 , MgO, or the like and a reflective layer formed of Ag, Al, or the like. The light emitting diode is characterized by a plurality of micro-ohmic contacts are arrayed on the transparent layer of the reflector so as to inject a current. The transparent layer is formed of the low-index material such as SiO 2 , Si 3 N 4 , MgO or the like, and the reflector is formed of Ag or Al. However, the disclosed light emitting diode uses micro-ohmic contacts having a limited area. Thus, the contact resistance is large, and thus the operation voltage is high. Also, a process of piercing the transparent layer to a micro-size is not suitable for mass-production and requires highly elaborate patterning and etching processes.  
         [0007]     A refractive index of a low-index layer is required to be minimized to obtain a high-quality ODR because a reflectivity is increased with a low refractive index.  FIGS. 2A and 2B  are graphs illustrating variations in reflectivities of an Ag ODR and an Al ODR with respect to a refractive index of a low-index layer. The Ag ODR includes an Ag reflector having a thickness of approximately 2,000 Å, and the Al ODR includes an Al reflector having a thickness of approximately 2,000 Å  
         [0008]     As shown in  FIGS. 2A and 2B , the reflectivity is increased with the low refractive index. The reflectivity of the Al ODR is much higher than that of the Ag ODR at a wavelength of 400 nm. Thus, a high reflectivity of 92% or more can be obtained within a refractive index range between 1.1 and 1.5, the refractive index being usable in an ODR. In other words, a refractive index of a low-index layer is required to be minimized to obtain a high-quality ODR. Furthermore, transparency and conductivity are required to be high.  
       SUMMARY OF THE DISCLOSURE  
       [0009]     The present invention may provide an ODR utilizing a low-index layer having a high electric conductivity and a very low refractive index so as to secure a high electric characteristic and high light extraction efficiency and a light emitting diode utilizing the ODR.  
         [0010]     According to an aspect of the present invention, there may be provided an omni-directional reflector including: a transparent conductive low-index layer formed of conductive nanorods; and a reflective layer formed of a metal.  
         [0011]     According to another aspect of the present invention, there may be provided a light emitting diode including: a light emitting region comprising an active layer and upper and lower semiconductor layers; a transparent conductive low-index layer comprising a plurality of conductive nanorods formed on one of the upper and lower semiconductor layers of the light emitting region; and a metal reflective layer formed on the transparent conductive low-index layer.  
         [0012]     The plurality of conductive nanorods may be formed of a transparent conducting oxide or a transparent conducting nitride.  
         [0013]     The transparent conducting oxide may be formed of In, Sn, or Zn oxide and selectively include a dopant. The dopant may be Ga, Cd, Mg, Be, Ag, Mo, V, Cu, Ir, Rh, Ru, W, Co, Ni, Mn, Pd, Pt, or La.  
         [0014]     The transparent conducting nitride may include Ti and N and be formed of TiN, TiON, or InSnON.  
         [0015]     A thickness of the transparent conductive low-index layer may be proportional to a ¼ n (n: refractive index) of a peak wavelength of the light emitting region. The metal reflective layer may be formed of Ag, Ag 2 O, Al, Zn, Ti, Rh, Mg, Pd, Ru, Pt, and Ir.  
         [0016]     The conductive nanorodes may be formed using sputter or e-beam oblique angle deposition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above and other features and advantages of the present invention are described in detailed exemplary embodiments thereof with reference to the attached drawings in which:  
         [0018]      FIG. 1  is a view illustrating a stack structure of a general ODR of the prior art;  
         [0019]      FIGS. 2A and 2B  are graphs illustrating variations in reflectivities of ODRs with respect to variations in refractive indexes of low-index layers of the ODRs;  
         [0020]      FIG. 3A  is a schematic cross-sectional view illustrating a stack structure of a light emitting diode according to an embodiment of the present invention;  
         [0021]      FIG. 3B  is a scanning electron micrograph (SEM) of a sample corresponding to portion A shown in  FIG. 3A ;  
         [0022]      FIG. 4  is a cross-sectional view illustrating a stack structure of a conventional light emitting diode adopting a simple metal reflector;  
         [0023]      FIG. 5A  is a graph illustrating current (I)-voltage (V) characteristics of the light emitting diode of the present invention shown in  FIG. 3A  and the conventional light emitting diode shown in  FIG. 4 ;  
         [0024]      FIG. 5B  is a graph illustrating light output with respect to variations in currents of the light emitting diode of the present invention shown in  FIG. 3A  and the conventional light emitting diode shown in  FIG. 4 ;  
         [0025]      FIG. 6  is an SEM of a nanorod low-index layer manufactured in an ODR according to an embodiment of the present invention;  
         [0026]      FIG. 7  is a view illustrating a method of forming a nanorod low-index layer using e-beam oblique angle deposition according to an embodiment of the present invention;  
         [0027]      FIG. 8  is an SEM of a manufactured sample showing a flux incidence angle and an oblique angle of nanorods formed using e-beam oblique angle deposition;  
         [0028]      FIG. 9  is a graph illustrating variations in a refractive index of a low-index layer formed of SiO 2  nanorods on a silicon substrate to a thickness of 150.8 nm with respect to a wavelength;  
         [0029]      FIG. 10A  is an SEM of an ITO nanorod low-index layer;  
         [0030]      FIG. 10B  is an AFM of an ITO nanorod low-index layer;  
         [0031]      FIG. 11A  is an SEM of a CIO(CulnO) nanorod low-index layer; and  
         [0032]      FIG. 11B  is an AFM of a surface of the CIO nanorod low-index layer.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0033]     Hereinafter, an ODR and a light emitting diode utilizing an ODR according to a preferred embodiment of the present invention will be described in detail with reference to the attached drawings.  
         [0034]      FIG. 3A  is a schematic cross-sectional view of a light emitting diode having an ODR according to an embodiment of the present invention, and  FIG. 3B  is an SEM of a substantially manufactured ODR corresponding to portion A shown in  FIG. 1 . As shown in  FIG. 3A , a light emitting region including a lower semiconductor layer  21 , an active layer  22 , and an upper semiconductor layer  23  is formed on a transparent sapphire substrate  10 . An ODR  30  including one of the lower and upper semiconductors  21  and  23  as one component, i.e., the upper semiconductor layer  23  in the present embodiment, is formed on the light emitting region  20 . As shown in  FIGS. 3A and 3B , the ODR  30  includes the upper semiconductor layer  23 , a low-index layer  31  formed of conductive nanorods on the upper semiconductor layer  23 , a metal reflective layer  32  formed on the low-index layer  31 .  
         [0035]     The conductive nanorods may be formed of transparent conducting oxide (TCO) or transparent conducting nitride (TCN) The TCO may be In, Sn, or Zn oxide that may selectively include a dopant. Here, a usable dopant may be Ga, Cd, Mg, Be, Ag, Mo, V, Cu, Ir, Rh, Ru, W, Co, Ni, Mn, Pd, Pt, or La.  
         [0036]     The TCN includes Ti or/and N, that is, at least one of Ti and N, in detail, may be formed of TiN, TION, or InSnON.  
         [0037]     A thickness of the low-index layer  31  is proportional to ¼n of a peak wavelength of the light emitting region  20 . The metal reflective layer  32  is formed of Ag, Ag 2 O, Al, Zn, Ti, Rh, Mg, Pd, Ru, Pt, Ir, or the like.  
         [0038]      FIG. 4  is a cross-sectional view of a reference sample to be compared with the light emitting device of the present invention, i.e., a light emitting device in which an Ag reflector is directly formed on an upper semiconductor layer without a low-index layer.  
         [0039]      FIG. 5A  is a graph illustrating I-V characteristics of the light emitting device of the present invention shown in  FIG. 3A  and the light emitting device shown in  FIG. 4 . Referring to  FIG. 5A , the light emitting diode shows a very high current at a voltage relatively lower than that of the reference sample. In particular, a considerable increase of a current appears in a voltage range between 3V and 4V. However, the reference sample requires a considerably higher driving voltage. In particular, the reference sample requires a higher driving voltage to obtain a high current. As shown in  FIG. 5A , the light emitting diode of the present invention shows a very high current at a low voltage. Also, the voltage shows little change when compared to the current.  
         [0040]      FIG. 5B  is a graph illustrating light intensity with respect to variations in currents of the light emitting device of the present invention shown in  FIG. 3A  and the reference sample shown in  FIG. 4 , i.e., variations in output voltages of photodetectors. The results of  FIG. 5B  may be estimated through the results of  FIG. 5A . In other words, the light emitting diode of the present invention shows a very high light intensity at the same current compared to the reference sample.  
         [0041]      FIG. 6  is an SEM of a manufactured conductive low-index layer. A lower portion of the SEM shows a cross-section of the conductive low-index layer, and an upper portion of the SEM shows a surface of the low-index layer.  
         [0042]     The conductive low-index layer shown in  FIG. 6  is SiO 2  nanorodes formed on a silicon substrate using e-beam oblique angle deposition. A SiO 2  flux is incident at an oblique angle of 85° with respect to the silicon substrate as shown in  FIG. 7  so as to form the SiO 2  nanorodes. The SiO 2  nanorods are formed at an oblique angle of 45° with respect to a substrate by such oblique angle deposition. In this case, self-shadowing regions are formed. The self-shadowing regions refer to a phenomenon in which subsequently deposited materials cannot reach predetermined portions due to initially randomly deposited materials.  
         [0043]      FIG. 8  is a view illustrating an incidence angle θ of the SiO2 vapor flux and an oblique angle θ t  of the SiO 2  nanorods. As shown in  FIG. 8 , when the incidence angle of the SiO 2  vapor flux is about 85°, the oblique angle of the SiO 2  nanorods is about 45°.  
         [0044]      FIG. 9  is a graph illustrating variations in a refractive index of a low-index layer of the SiO 2  nanorods formed on the silicon substrate to a thickness of 150.8 nm with respect to a wavelength. The refractive index was measured using an ellipsometry model. Referring to  FIG. 9 , the refractive index is about 1.090 at a wavelength of 400 nm. This is a very epoch-making result in terms of an original refractive index of SiO 2 .  
         [0045]      FIG. 10A  is an SEM of a low-index layer formed of ITO nanorods using e-beam oblique angle deposition, and  FIG. 10B  is an AFM of a surface of the low-index layer shown in  FIG. 10A .  FIG. 11A  is an SEM of a low-index layer formed of CIO(CulnO) nanorods, and  FIG. 11B  is an AFM of a surface of the low-index layer shown in  FIG. 11A .  
         [0046]     A surface roughness of the low-index layer formed of the ITO nanorods is 6.1 nm/rms (root means square), and a surface roughness of the low-index layer formed of the CIO nanorods is 6.4 nm/rms.  
         [0047]     A refractive index of the low-index layer formed of the ITO nanorods is 1.34 at a wavelength of 461 nm, and a refractive index of the low-index layer formed of the CIO nanorods is 1.52 at the wavelength of 461 nm. The low refractive indexes of the low-index layers are epoch-making results in terms of respective refractive indexes “2.05” and “1.88” of ITO and CIO thin films. A low-index layer formed of ITO or CIO nanorods using e-beam oblique angle deposition has a very low refractive index and a very high electric conductivity. Thus, the low-index layer formed of the ITO or CIO nanorods may be effectively used as a low-index layer of an ODR without an additional conductor such microcontact layers.  
         [0048]     As described above, an ODR according to the present invention has high conductivity and reflectivity. As a result, a light emitting diode having higher luminance and light extraction efficiency than a conventional light emitting diode can be obtained. The light emitting diode of the present invention does not require an additional element such as microcontacts for an additional conductive path. Thus, the light emitting diode can be readily manufactured on an economical basis.  
         [0049]     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.