Abstract:
A light-emitting diode (LED) and a method for manufacturing the same are described. The light-emitting diode has a metal substrate, a first transparent conductive layer, a first contact layer, and an illuminating epitaxial structure stacked in sequence. An ohmic contact layer is located on a portion of the illuminating epitaxial structure. A thickness of the metal substrate is greater than 40 μm. The first contact layer is a doped strained-layer-superlattices (SLS) structure. Additionally, the light-emitting diode can further be a reflective layer located between the metal substrate and the first transparent conductive layer.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is based on, and claims priority from, Taiwan Application Serial Number 93139452, filed Dec. 17, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a light-emitting diode (LED) and a method for manufacturing the same, and, more particularly, to a light-emitting diode with high efficiency and a method for manufacturing the same.  
       BACKGROUND OF THE INVENTION  
       [0003]     In general, for a light-emitting diode, the light output depends on the quantum efficiency of the active layer and the light extraction efficiency. The higher the quantum efficiency of the active layer, the higher the light output of the light-emitting diode. Generally, the quantum efficiency of the active layer is increased by improving the quality of the epitaxial structure and the structural design of the active layer. In addition, as the light extraction efficiency increases, the light output of the light-emitting diode is enhanced. In order to improve the light extraction efficiency, efforts are made to overcome the significant photon loss resulting from total reflection inside the light-emitting diode after emission from the active layer, the absorbing of the substrate, and the light shielding of the metal. Generally speaking, the problem may be overcome by using a flip-chip LED or by using a transparent substrate.  
         [0004]     Besides, since the use of LEDs for illumination has become increasingly popular in recent years and most applications of LEDs as illumination are of high efficiency, the substrate needs to be made of a material with a good heat-dissipating property. The flip-chip LED not only may overcome the photon loss resulting from the light shielding of the metal, but also may achieve the goal of high efficiency and high power by using materials with good heat-dissipating property, such as Si or AlN, as submount substrate. However, as far as the present production technique is concerned, the processes of the flip-chip LED are complicated and the yield is low.  
         [0005]     Furthermore, although metal has an excellent heat conduction property and conductivity, the metal substrate is not suitable for an epitaxial substrate. Hence, wafer bonding is generally used to replace the original epitaxial substrate with a substitute substrate. However, the processes are also complicated and the yield is low, especially the yield loss resulting from the step of wafer bonding.  
       SUMMARY OF THE INVENTION  
       [0006]     Therefore, one objective of the present invention is to provide a light-emitting diode and a method for manufacturing the same in which a light-emitting diode with a metal substrate is formed, and thus, the heat dissipation is so good that the light extraction efficiency is greatly increased and further, the light output is raised.  
         [0007]     Another objective of the present invention is to provide a light-emitting diode and a method for manufacturing the same, in which the processes of wafer bonding is not needed, and thus the processes are simplified and the yield is raised.  
         [0008]     Still another objective of the present invention is to provide a light-emitting diode and the method for manufacturing the same in which the thermal processing is not needed, so that the cracking of the interface may be prevented and the light output of the light-emitting diode may also be raised.  
         [0009]     According to the aforementioned objectives, the present invention provides a light-emitting diode comprising a metal substrate, a first transparent conductive layer located on the metal substrate, a first contact layer located on the first transparent conductive layer, an illuminating epitaxial structure located on the first contact layer, and an ohmic contact layer located on part of the illuminating epitaxial structure.  
         [0010]     According to an embodiment of the present invention, the light-emitting diode further comprises a reflective layer located between the metal substrate and the first transparent conductive layer. The first contact layer is a doped strained layer superlattice (SLS) structure. In the embodiment of the present invention, the light-emitting diode further comprises a second contact layer and a second transparent conductive layer located between the illuminating epitaxial structure and the ohmic contact layer, in which the second contact layer is located beneath the second transparent conductive layer.  
         [0011]     According to another objective, the present invention provides a light-emitting diode comprising a metal substrate, an illuminating epitaxial structure located on the metal substrate, a first contact layer located on the illuminating epitaxial structure, a first transparent conductive layer located on the first contact layer, and an ohmic contact layer located on part of the first transparent conductive layer.  
         [0012]     According to another embodiment of the present invention, the light-emitting diode further comprises a metal layer located between the metal substrate and the illuminating epitaxial structure, in which the metal layer is used as a reflective and ohmic contact layer. The first contact layer is a doped strained layer superlattice (SLS) structure. In the embodiment of the present invention, the light-emitting diode further comprises a second contact layer and a second transparent conductive layer located between the illuminating epitaxial structure and the metal substrate, in which the second contact layer is located on the second transparent conductive layer.  
         [0013]     According to the objectives, the present invention provides a method for manufacturing a light-emitting diode, comprising the following steps. First, an epitaxial substrate is provided, and an illuminating epitaxial structure is formed on the epitaxial substrate. Then, a first contact layer is formed on the illuminating epitaxial structure, and a first transparent conductive layer is formed on the first contact layer. Afterwards, a metal substrate is formed on the first transparent conductive layer. Then, the epitaxial substrate is removed to expose a surface of the illuminating epitaxial structure. Then, an ohmic contact layer is formed on part of the illuminating epitaxial structure.  
         [0014]     According to an embodiment of the present invention, the present invention further comprises forming a reflective layer located on the first transparent conductive layer between the steps of forming the first transparent conductive layer and forming the metal substrate. In the embodiment of the present invention, the thickness of the metal substrate is more than about 40 um. Besides, the first contact layer is a doped strained layer superlattice structure.  
         [0015]     According to the objectives, the present invention provides a method for manufacturing a light-emitting diode comprising the following steps. First, an epitaxial substrate is provided, and a first contact layer is formed on the epitaxial substrate. Then, an illuminating epitaxial structure is formed on the first contact layer, and a metal substrate is formed on the illuminating epitaxial structure. Afterwards, the epitaxial substrate is removed to expose a surface of the first contact layer. Then, a first transparent conductive layer is formed on the first contact layer. Then, an ohmic contact layer is formed on part of the first transparent conductive layer.  
         [0016]     According to an embodiment of the present invention, the present invention further comprises forming a metal layer located on the illuminating epitaxial structure between the steps of forming the illuminating epitaxial structure and forming the metal substrate, in which the metal layer is used to be as a reflective and ohmic contact layer. In the embodiment of the present invention, the present invention further comprises forming a second contact layer, a second transparent conductive layer and a reflective layer located on the illuminating epitaxial structure in sequence between the steps of forming the illuminating epitaxial structure and forming the metal substrate. In the embodiment of the present invention, the metal substrate is formed by evaporation, sputtering, electroless plating, chemical electroplating, or electroforming. The epitaxial substrate is removed by polishing, chemical etching, or laser stripping. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0018]      FIGS. 1   a - 1   f  are schematic flow diagrams showing the process for manufacturing a light-emitting diode in accordance with an embodiment of the present invention;  
         [0019]      FIGS. 2   a - 2   f  are schematic flow diagrams showing the process for manufacturing a light-emitting diode in accordance with another embodiment of the present invention; and  
         [0020]      FIGS. 3   a - 3   f  are schematic flow diagrams showing the process for manufacturing a light-emitting diode in accordance with another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     The present invention discloses a light-emitting diode and a method for manufacturing the same by which a light-emitting diode and a reflector with a metal substrate may be made. Thus, the heat dissipating is so good that it is suitable for high power use. The light extraction efficiency is greatly increased, and, further, the light output is raised. In order to make the illustration of the present invention more explicit and complete, the following description is stated with reference to  FIGS. 1   a - 1   f,    FIGS. 2   a - 2   f,  and  FIGS. 3   a - 3   f.    
         [0022]     Reference is made to  FIGS. 1   a - 1   f,  which are schematic flow diagrams showing the process for manufacturing a light-emitting diode in accordance with an embodiment of the present invention. First, an epitaxial substrate  100  is provided, in which a material of the epitaxial substrate  100  is, for example, sapphire (Al 2 O 3 ), silicon carbide (SiC), silicon (Si) or gallium arsenide (GaAs). Then, an illuminating epitaxial structure composed of a cladding layer of a first type  102 , an active layer  104  and a cladding layer of a second type  106  is sequentially formed on the epitaxial substrate  100 . The cladding layer of a first type  102  can be a III-V semiconductor material of N- or P- type, in which the material may be, for example, Al x In y Ga l-x-y N (x,y≧0; 0≦x+y&lt;1; x&gt;c) or Al x In y Ga l-x-y P (x,y≧0; 0≦x+y&lt;1; x&gt;c) of N-type or P- type. The material of the active layer  104  may be a doped or undoped quantum well (QW) structure composed of Al a In b Ga l-a-b N/Al c In d Ga l-c-d N (a,b≧0; 0≦a+b&lt;1; c,d≧0; 0≦c+d&lt;1; x&gt;c&gt;a) or Al a In b Ga l-a-b P/Al c In d Ga l-c-d P (a,b≧0; 0≦a+b&lt;1; c,d≧0; 0≦c+d&lt;1; x&gt;c&gt;a), and the dopant may be N-type or P-type. The cladding layer of a second type  106  can be a III-V semiconductor material of P- or N- type, in which the material may be, for example, Al x In y Ga l-x-y N (x,y≧0; 0≦x+y&lt;1; x&gt;c) or Al x In y Ga l-x-y P (x,y≧0; 0≦x+y&lt;1; x&gt;c) of P-type or N-type.  
         [0023]     Then, a metal organic chemical vapor deposition (MOCVD) technique is used to form a contact layer  108  on the cladding layer of a second type  106 , and a structure as shown in  FIG. 1   a  is formed. The contact layer  108  is preferably a doped strained layer superlattice (SLS) structure, and more preferably a periodic and modulated doped semiconductor material, such as Al u In v Ga l-u-v N/Al x In y Ga l-x-y N (u,v≧0; 0≦u+v≦1; x,y≧0; 0≦x+y&lt;1; x&gt;u) or Al u In v Ga l-u-v P/Al x In y Ga 1-x-y P (u,v≧0; 0≦u+v≦1; x,y≧0; 0≦x+y&lt;1; x&gt;u), and the dopant may be N-type or P-type. Since the contact feature of the strained layer superlattice structure and a transparent conductive layer  110  subsequently formed is good, the conductivity it supplies is high.  
         [0024]     Then, the transparent conductive layer  110  is formed on the contact layer  108 . The material of the transparent conductive layer  110  may be a thin metal, such as Ni/Au, TiN, and Pd/Au/Pt/Au, a transparent conductive oxide (TCO) of N-type, such as indium tin oxide (ITO), cadmium tin oxide (CTO), ZnO doped with Al (ZnO:Al; AZO), AgInO 2 :Sn, and In 2 O 3 :Zn (IZO), or a transparent conductive oxide of P-type, such as CuAlO 2 , LaCuOS, NiO, CuGaO 2  and SrCu 2 O 2 .  
         [0025]     A feature of the present invention is the use of the strained layer superlattice structure and the transparent conductive layer. Since thermal processing is not needed, the cracking of the interface is avoided, and, further, the light output of the light-emitting diode is not affected.  
         [0026]     Then, a reflective layer  112  is formed on the transparent conductive layer  110  by, for example, evaporation, sputtering, electroplating, or electroless plating, and a structure as shown in FIG. I c is formed. The material of the reflective layer  112  is, for example, a reflective material, such as Au, Ag, Al, In, Sn, Pt, Ti, Zn, Pb, AuBe, Ni, PbSn, or AuZn. Besides, the reflective layer  112  may also be a distributed Bragg reflector composed of different kinds of transparent conductive oxide layer stacked on each other in multiple films, such as indium tin oxide/AlZnO (ITO/AZO). After forming the reflective layer  112 , a metal substrate  114  is formed on the reflective layer  112  by, for example, evaporation, sputtering, electroless plating, chemical electroplating, or electroforming, and a structure as shown in  FIG. 1   d  is formed. The thickness of the metal substrate  114  is preferably more than about 40 um, and the material of the metal substrate  114  is metal such as, for example, Cu, Al, Ag, Au, Cr, Pt, W, Pb, Sn, Ni or a related alloy suitable for chemical electroplating or electroforming.  
         [0027]     A feature of the present invention is the use of the chemical electroplating or electroforming to form the metal substrate. Since the thermal processing is also not needed, cracking of the interface is avoided.  
         [0028]     Besides, since the heat conduction of the metal substrate  114  of the present invention is excellent, the heat dissipating property of the light-emitting diode is greatly increased, and further achieves the goal of enhancing the performance of the light-emitting diode. In the present invention, if the material used to form the metal substrate  114  has good reflectivity, formation of the reflective layer  112  may be omitted.  
         [0029]     Then, as shown in  FIG. 1   e,  the epitaxial substrate  100  is removed by, for example, polishing, chemical etching, or laser stripping, to expose a surface of the cladding layer of a first type  102 . The polishing technique may be chemical mechanical polishing (CMP), and the chemical etching technique may be dry etching or wet etching.  
         [0030]     Then, the cladding layer of a first type  102  and the structure thereon is turned over, and an ohmic contact layer  116  is formed on part of the cladding layer of a first type  102 , and a structure as shown in  FIG. 1   f  is formed. The material of the ohmic contact layer  116  is, for example, Al, Au, Pt, Ti, Cr, Be, Zn, Mg, Ni, Ge, or any arbitrary alloy thereof. These are materials known to those skilled in the semiconductor art.  
         [0031]     When the manufacturing of a light-emitting diode structure of an embodiment of the present invention is finished, the process continues on to the cutting of the dies of the light-emitting diode to finish the manufacturing of a light-emitting diode device. The materials and techniques used in the embodiment may also be applied in other embodiments described below.  
         [0032]     Reference is made to  FIGS. 2   a - 2   f,  which are schematic flow diagrams showing the process for manufacturing a light-emitting diode in accordance with another embodiment of the present invention. In the embodiment, a contact layer  202  is first formed on the epitaxial substrate  200 , and then an illuminating epitaxial structure, composed of a cladding layer of a first type  204 , an active layer  206 , and a cladding layer of a second type  208 , is sequentially formed, as shown in  FIG. 2   a.  Then, a metal layer  210  is formed on the illuminating epitaxial structure, in which the metal layer  210  is used as a reflective and ohmic contact layer. Afterwards, a metal substrate  212  is formed on the metal layer  210 , and a structure as shown in  FIG. 2   c  is formed. Then, the epitaxial substrate  200  is removed to expose a surface of the contact layer  202 , as shown in  FIG. 2   d.  Then, the contact layer  202  and the structure thereon is turned over, and a transparent conductive layer  214  is formed on part of the contact layer  202 . Then, an ohmic contact layer  216  is formed on part of the transparent conductive layer  214 , and a structure as shown in  FIG. 2   f  is formed. When the manufacturing of a light-emitting diode structure of another embodiment of the present invention is finished, the process continues on to the cutting of the dies of the light-emitting diode to finish the manufacturing of a light-emitting diode device.  
         [0033]     In still another embodiment of the present invention, a first contact layer  302  is first formed on a epitaxial substrate  300 , and then an illuminating epitaxial structure, composed of a cladding layer of a first type  304 , an active layer  306 , and a cladding layer of a second type  308 , is sequentially formed. A second contact layer  310  is formed on the illuminating epitaxial structure, and a structure as shown in  FIG. 3   a  is formed. Afterwards, a first transparent conductive layer  312  is formed on the second contact layer  310 , and a reflective layer  314  is formed on the first transparent conductive layer  312 , as shown in  FIG. 3   c.  Then, a metal substrate  316  is formed on the reflective layer  314 , and a structure as shown in  FIG. 3   d  is formed. Then, the epitaxial substrate  300  is removed to expose a surface of the first contact layer  302 . The first contact layer  302  and the structure thereon are turned over, and then a second transparent conductive layer  318  is formed on the surface of the first contact layer  302 . An ohmic contact layer  320  is formed on part of the second transparent conductive layer  318 , and a structure as shown in  FIG. 3   f  is formed. When manufacturing of a light-emitting diode structure of the embodiment of the present invention is finished, the process continues on to the cutting of the dies of the light-emitting diode to finish the manufacturing of a light-emitting diode device.  
         [0034]     According to the embodiments of the present invention, there are some advantages in applying the present invention. First, the light-emitting diode of the present invention has a metal substrate, and thus the heat dissipating ability of the light-emitting diode is greatly raised. Furthermore, the light-emitting diode of the present invention has a reflective layer, so that the light extraction efficiency of the light-emitting diode is increased, and the light output is also enhanced. Besides, the process of wafer bonding is not needed in the light-emitting diode of the present invention, and therefore, the processes are simplified and the yield is raised. On the other hand, the thermal processing is also not needed in the light-emitting diode of the present invention, so cracking of the interface is avoided and the light output of the light-emitting diode is also raised.  
         [0035]     As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.