Abstract:
Disclosed are a light emitting diode employing an array of nanorods and a method of fabricating the same. The light emitting diode comprises an array of semiconductor nanorods positioned on a substrate. An upper electrode layer is deposited on the array of the nanorods such that an empty space remains between adjacent ones of the nanorods. Since the space between adjacent ones of the nanorods is not filled with an insulating material, the light extraction efficiency of a light emitting diode can be improved and a method of fabricating the light emitting diode can be simplified.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a U.S. national phase application of PCT International Application No. PCT/KR2006/002148, filed Jun. 5, 2006, which claims priority of Korean Patent Application No. 10-2005-0076205, filed Aug. 19, 2005, the contents of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a light emitting diode and a method of fabricating the same, and more particularly, to a light emitting diode employing an array of nanorods and a method of fabricating the light emitting diode. 
       BACKGROUND OF THE INVENTION 
       [0003]    In general, a light emitting diode is formed by sequentially stacking an n-GaN layer doped with n-type impurities, an InGaN active layer and a p-GaN layer doped with p-type impurities on a sapphire substrate. Since there are a great deal of threading dislocations caused by lattice mismatching due to physical properties of GaN or limitations on growth methods of GaN, such a laminated light emitting diode has limitations on light-emission efficiency. Further, the laminated light emitting diode has disadvantages such as a broad spectrum width and a large output variation in addition to the threading dislocations. 
         [0004]    To overcome these disadvantages of the laminated light emitting diode, there have been conducted studies on a light emitting diode having a nanostructure in which a p-n junction is formed of one-dimensional nanorods or nanowires. Nanorods grown through the bottom-up scheme greatly reduce threading dislocations that might be produced in a laminated light emitting diode, resulting in improved light-emission efficiency. A light emitting diode employing an array of such nanorods secures higher light output as compared with a laminated light emitting diode. 
         [0005]    Meanwhile, to supply electric power to the array of the nanorods, electrodes should be connected to upper and lower portions of the nanorods. Although the nanorods may be grown on a lower electrode so that the nanorods can be electrically connected to the lower electrodes, upper electrodes are formed on the nanorods through an additional process. 
         [0006]    An example of a method of forming upper electrodes on an array of nanorods has been disclosed in U.S. Pat. No. 6,882,051 entitled “Nanowires, nanostructures and devices fabricated therefrom” by Majumdar et al. Further, an example of a method of selectively depositing nanometer-sized metal on specific regions of nanorods has been disclosed in International Publication No. WO2004/109815 entitled “Contacts fabric using heterostructure of metal/semiconductor nanorods and fabrication method thereof” by YI et al. 
         [0007]    In U.S. Pat. No. 6,882,051 (see FIG. 30), nanowires are formed and then are embedded in a polymer matrix by flowing a polymer solution. Subsequently, the polymer is cured, the polymer matrix is etched until the nanowires are exposed, and then metal is deposited thereon. Accordingly, a metallic bonding pad, i.e., an upper electrode, is formed. According to this method, the nanowires with weak mechanical strength are embedded in the polymer matrix to provide enhanced mechanical strength. 
         [0008]    However, in a case where such an array of nanowires is employed to form a light emitting diode, light absorption of the polymer matrix leads to light loss. Thus, light extraction efficiency that relates to extraction of light out of the array of the nanowires is reduced. Additionally, the processes for forming the polymer matrix and etching the polymer matrix are added, resulting in complicated fabrication processes of a light emitting diode. 
         [0009]    Meanwhile, in the International Publication No. WO2004/109815, semiconductor nanorods are grown on a predetermined substrate, and metal is deposited on predetermined regions of the nanorods by means of a method such as sputtering or a thermal or electron beam evaporation method. The nanorods and the metal are contacted together such that an ohmic or schottky property is exhibited. 
         [0010]    According to the International Publication No. WO2004/109815, a nanometer-sized metal electrode is formed on a certain portion of each of the nanorods so as to provide an electrode structure applicable to various devices. 
         [0011]    However, as the nanometer-sized metal electrode is formed on a certain portion of each of the nanorods, an additional metal layer is required to simultaneously drive an array of nanorods. A process of forming such an additional metal layer makes the fabricating method of a light emitting diode complicated. 
         [0012]    An object of the present invention is to provide a light emitting diode employing an array of nanorods, which has improved light extraction efficiency. 
         [0013]    Another object of the present invention is to provide a light emitting diode employing an array of nanorods, which can be fabricated through simplified processes. 
         [0014]    A further object of the present invention is to provide a method of fabricating a light emitting diode employing an array of nanorods and having improved light extraction efficiency. 
         [0015]    To achieve these objects of the present invention, the present invention provides a light emitting diode employing an array of nanorods and a method of fabricating the same. A light emitting diode according to an aspect of the present invention comprises an array of semiconductor nanorods positioned on a substrate. An upper electrode layer is deposited and positioned on the array of nanorods such that an empty space remains between adjacent ones of the nanorods. According to the aspect of the present invention, since the space between adjacent ones of the nanorods is not filled with an insulating substance such as a polymer, the light extraction efficiency of the light emitting diode can be improved. Further, since the upper electrode layer is formed by being deposited directly on the array of the nanorods, a method of fabricating the light emitting diode can be simplified. 
         [0016]    Each of the nanorods may comprise a first semiconductor nanorod of a first conductive type and a second semiconductor nanorod of a second conductive type. Further, an active layer may be interposed between the first and second semiconductor nanorods. 
         [0017]    As the interval between adjacent ones of the second semiconductor nanorods becomes smaller, the upper electrode layer may be formed using a variety of deposition methods. 
         [0018]    Preferably, the second semiconductor nanorods may be spaced apart from one another at an interval of 200 nm or less on the average. 
         [0019]    The upper electrode layer may be made of a transparent electrode material. Further, an upper electrode pad may be formed on the upper electrode layer. Since the upper electrode layer is made of the transparent electrode material, light emitted from the nanorods can be radiated to the outside by passing through the upper electrode layer. 
         [0020]    Additionally, a lower electrode layer of the first conductive type may be interposed between the substrate and the array. The nanorods are grown on the lower electrode layer. 
         [0021]    Meanwhile, the lower electrode layer may have an extension portion extending from a side of the array of the nanorods. A lower electrode pad may be formed on the extension portion. Accordingly, bonding wires can be connected to the lower and upper electrode pads to supply electric power so that the array of the nanorods can be driven. 
         [0022]    On the contrary, in a case where the substrate is conductive, the lower electrode pad may be formed on a bottom surface of the substrate. Accordingly, a larger number of nanorods can be disposed in one array so that light output can be increased. 
         [0023]    A method of fabricating a light emitting diode according to another aspect of the present invention comprises growing nanorods on a substrate. An upper electrode layer is deposited on the nanorods such that an empty space remains between adjacent ones of the nanorods. Accordingly, it is possible to fabricate a light emitting diode with improved light extraction efficiency. 
         [0024]    The upper electrode layer may be formed using a variety of deposition techniques such as physical vapor deposition, chemical vapor deposition, electroplating and electroless plating techniques, with considering stepcoverage characteristics thereof. For example, the physical vapor deposition technique may be sputtering, a thermal evaporation method or an electron beam evaporation method. Sputtering, or the thermal or electron beam evaporation method is advantageous to mass production of light emitting diodes, and reduces fabricating costs thereof. 
         [0025]    Prior to the growth of the nanorods, a lower electrode layer may be formed on the substrate. The lower electrode layer may be a semiconductor layer of the first conductive type. 
         [0026]    Additionally, the lower electrode layer may be exposed by patterning the upper electrode layer and the nanorods. A lower electrode pad may be formed on the exposed portion of the lower electrode layer. Further, an upper electrode pad may be formed on the upper electrode layer. 
         [0027]    The nanorods may be grown using for example, MOCVD, MBE, MOHVPE or the like. The nanorods may be grown using such a technique without using a catalytic metal or a template. 
         [0028]    According to the present invention, the upper electrode layer is deposited such that an empty space remains between adjacent ones of the nanorods, thereby preventing light absorption by a polymer matrix or the like and thus enhancing the light extraction efficiency of a light emitting diode. Further, since the upper electrode layer can be formed using a simple process, processes of fabricating a light emitting diode can be simplified and fabricating costs can be reduced. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a perspective view illustrating a light emitting diode according to an embodiment of the present invention. 
           [0030]      FIGS. 2 to 6  are sectional views illustrating a method of fabricating the light emitting diode according to the embodiment of the present invention. 
           [0031]      FIG. 7  is a sectional view illustrating a method of fabricating a light emitting diode according to other embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided only for illustrative purposes so that those skilled in the art can fully understand the spirit of the present invention. Therefore, the present invention is not limited to the following embodiments but may be implemented in other forms. In the drawings, the widths, lengths, thicknesses and the like of elements are exaggerated for convenience of illustration. Like reference numerals indicate like elements throughout the specification and drawings. 
         [0033]      FIG. 1  is a perspective view illustrating a light emitting diode according to an embodiment of the present invention. 
         [0034]    Referring to  FIG. 1 , an array of nanorods  27  is positioned on a substrate  21 . The substrate  21  may be a sapphire, glass, Si, SiC or ZnO substrate, and may be an insulative or conductive substrate. 
         [0035]    Each of the nanorods  27  comprises a first semiconductor nanorod  27   a  of a first conductive type, and a second semiconductor nanorod  27   c  of a second conductive type. 
         [0036]    Here, first and second conductive type semiconductors refer to n-type and p-type semiconductors, or p-type and n-type semiconductors, respectively. An active layer  27   b  is interposed between the first and second semiconductor nanorods  27   a  and  27   c . Each of the first and second nanorods  27   a  and  27   c  may be made of Al x In y Ga (1-x-y) N (0≦x≦1, 0≦y&lt;1 and 0≦x+y≦1) or ZnO. The active layer  27   b  may be made of (Al x In y Ga (1-x-y) )N (0≦x&lt;1, 0≦y≦1 and 0≦x+y≦1). The bandgap of GaN is varied according to the content of Al and In. Thus, the content of Al and/or In contained in the active layer is selected depending on a required wavelength of light, and the content of Al and/or In contained in each of the first and second nanorods  27   a  and  27   c  is selected such that each of the first and second nanorods has a bandgap larger than that of the active layer  27   b.    
         [0037]    The active layer  27   b  may be a single quantum well or multi quantum well. The multi quantum well may be formed by alternately growing substances that have large and small bandgaps. 
         [0038]    The first semiconductor nanorod  27   a , the active layer  27   b  and the second semiconductor nanorod  27   c  may be consecutively grown using a conventional vapor-liquid-solid (VLS) process as disclosed in U.S. Pat. No. 6,882,051 or using an MOCVD, MBE or MOHVPE technique. If the MOCVD, MBE or MOHVPE technique is used, a nanorod may be grown without using a catalytic metal or a template. 
         [0039]    Meanwhile, a lower electrode layer  25  may be interposed between the substrate  21  and the array of the nanorods. The lower electrode layer  25  is used as an electrode electrically connected to the first nanorods  27   a  to supply electric power thereto. Particularly, in a case where the substrate  21  is an insulative substrate, the lower electrode layer  25  is interposed to supply electric power to the nanorods  27 . The lower electrode layer  25  may be a layer made of the same conductive type semiconductor as the first semiconductor nanorod  27   a.    
         [0040]    The lower electrode layer  25  may have an extension portion extending from the array of the nanorods  27 , and a lower electrode pad  31   a  may be formed on the extension portion. The electrode pad  31   a  may be formed of, but not limited to, a Ti/Al layer, and a wire (not shown) may be bonded thereto. 
         [0041]    Additionally, a buffer layer  23  may be interposed between the lower electrode layer  25  and the substrate  21 . The buffer layer  23  may be used to alleviate the lattice mismatch between the substrate  21  and the lower electrode layer  25 . 
         [0042]    In a case where the substrate  21  is a conductive substrate, the buffer layer  23  and the lower electrode layer  25  may be omitted, and the substrate  21  may serve as a lower electrode. 
         [0043]    Meanwhile, an upper electrode layer  29  is positioned on the array of the nanorods  27 . The upper electrode layer  29  is formed through deposition on the array such that an empty space remains between adjacent ones of the nanorods  27 . The upper electrode layer  29  may be deposited using a physical vapor deposition, chemical vapor deposition, electroplating or electroless plating technique, Particularly, the upper electrode layer may be deposited using a physical vapor deposition technique such as sputtering, a thermal evaporation method or an electron beam evaporation method. 
         [0044]    The upper electrode layer  29  may be made of various metallic materials or a conductive oxide film, particularly, a transparent electrode material, e.g., Ni/Au or indium tin oxide (TTO). The upper electrode layer  29  is commonly in ohmic or schottky contact with the second nanorods  27   c.    
         [0045]    An upper electrode pad  31   b  to be connected to a bonding wire may be formed on the upper electrode layer  29 . 
         [0046]      FIGS. 2 to 6  are sectional views illustrating a method of fabricating the light emitting diode according to the embodiment of the present invention. 
         [0047]    Referring to  FIG. 2 , a substrate  21  is prepared. The substrate  21  may be a sapphire, glass, Si, SiC or ZnO substrate, and may be an insulative or conductive substrate. 
         [0048]    A lower electrode layer  25  may be formed on the substrate  21 . Particularly, in a case where the substrate  21  is an insulative substrate, a conductive lower electrode layer  25  is formed on the substrate  21 . The lower electrode layer  25  may be formed of a first conductive type semiconductor. For example, the lower electrode layer  25  may be made of Al x In y Ga (1-x-y) N (0≦x≦1, 0≦y≦1 and 0≦x+y≦1) or ZnO. In a case where the lower electrode layer  25  is Al x In y Ga (1-x-y) N, it can be formed into a p-type semiconductor layer by doping Mg, or an n-type semiconductor layer by doping Si or without artificial doping. Meanwhile, in a case where the lower electrode layer  25  is ZnO, Mg may be added. 
         [0049]    Meanwhile, prior to the formation of the lower electrode layer  25 , a buffer layer  23  may be formed. The buffer layer  23  may be used to alleviate the lattice mismatch between the lower electrode layer  25  and the substrate  21 . 
         [0050]    In a case where the substrate is a conductive substrate, the buffer layer  23  and/or the lower electrode layer  25  may be omitted. 
         [0051]    Referring to  FIG. 3 , nanorods  27  are formed on the lower electrode layer  25 . Each of the nanorods  27  comprises a first semiconductor nanorod  27   a  of a first conductive type, and a second semiconductor nanorod  27   c  of a second conductive type, and an active layer  27   b  is interposed between the first and second semiconductor nanorods  27   a  and  27   c.    
         [0052]    The first and second nanorods  27   a  and  27   c  may be made of Al x In y Ga (1-x-y) N (0≦x≦1, 0≦y&lt;1 and 0≦x+y≦1) or ZnO. In a case where the nanorods  27   a  and  27   c  are made of ZnO, Mg may be added. Meanwhile, the active layer  27   b  may be made of (Al x In y Ga (1-x-y) )N (0≦x&lt;1, 0≦y≦1 and 0≦x+y≦1), and the amounts of Al and In are selected depending on light with a required wavelength. The active layer  27   b  may be a single quantum well formed as a single layer or a multi quantum well in which substances with different bandgaps are alternately formed. 
         [0053]    The first semiconductor nanorod  27   a , the active layer  27   b  and the second semiconductor nanorod  27   c  may be consecutively grown using a vapor-liquid-solid (VLS) process or using an MOCVD, MBE or MOHVPE technique. If the MOCVD, MBE or MOHVPE technique is used, a nanorod may be grown without using a catalytic metal or a template. 
         [0054]    The nanorods  27  are formed while being spaced apart from one another, and are preferably formed at an interval of 200 nm or less on the average. 
         [0055]    Referring to  FIG. 4 , an upper electrode layer  29  is deposited on the nanorods  27 . The upper electrode layer  29  may be made of metal or a conductive oxide film. Particularly, the upper electrode layer  29  may be deposited with a transparent material, e.g., Ni/Au or ITO. 
         [0056]    The upper electrode layer  29  may be deposited using a physical vapor deposition, chemical vapor deposition, electroplating or electroless plating technique. Particularly, the upper electrode layer may be deposited using a physical vapor deposition technique such as sputtering, a thermal evaporation method or an electron beam evaporation method. The deposition technique for the upper electrode layer  29  is selected in consideration of its stepcoverage characteristic. 
         [0057]    Referring to  FIG. 5 , the upper electrode layer  29  and the nanorods  27  are patterned to expose the lower electrode layer  25 . The upper electrode layer  29  and the nanorods  27  may be patterned using photolithographic and etching processes. 
         [0058]    Referring to  FIG. 6 , lower and upper electrode pads  31   a  and  31   b  are formed on an exposed portion of the lower electrode layer  25  and the upper electrode layer  29 , respectively. The pads  31   a  and  31   b  may be formed using a lift-off process and may be formed through an identical process or different processes. Further, the upper electrode pad  31   b  may be formed in advance before the lower electrode layer  25  is exposed. 
         [0059]      FIG. 7  is a sectional view illustrating a method of fabricating a light emitting diode according to other embodiment of the present invention. 
         [0060]    Referring to  FIG. 7 , a buffer layer  23 , a lower electrode layer  25 , nanorods  27  and an upper electrode layer  29  are formed on a substrate  21 , as described with reference to  FIGS. 2 to 4 . In this embodiment, the substrate  21  is a conductive substrate, and the buffer layer  23  and/or the lower electrode layer  25  can be omitted. Thus, the nanorods  27  can be grown directly on the substrate  21 . Further, a process of patterning the upper electrode layer  29  and the nanorods  27  is omitted. 
         [0061]    Meanwhile, an upper electrode pad  51   b  is formed on the upper electrode layer  29  using a lift-off process. Further, a lower electrode pad  51   a  is formed on a bottom surface of the substrate  21 . The lower electrode pad  51   a  may be formed in a step of preparing the substrate  21  or in any step thereafter. 
         [0062]    According to this embodiment, it is not necessary to pattern the upper electrode layer  29  and the nanorods  27 . Thus, there is provided a light emitting diode having a relatively large number of nanorods  27  within the same dimension as the light emitting diode described in the first embodiment, thus resulting in the increase of light output.