Abstract:
A GaN-based LED structure is provided so that the brightness and lighting efficiency of the GaN-based LED are enhanced effectively. The greatest difference between the GaN-based LEDs according to the invention and the prior arts lies in the addition of a thin layer on top of the traditional structure. The thin layer could be formed using silicon-nitride (SiN), or it could have a superlattice structure either made of layers of SiN and undoped indium-gallium-nitride (InGaN), or made of layers SiN and undoped aluminum-gallium-indium-nitride (AlGaInN), respectively. Because of the use of SiN in the thin layer, the surfaces of the GaN-based LEDs would be micro-roughened, and the total internal reflection resulted from the GaN-based LEDs&#39; higher index of refraction than the atmosphere could be avoided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. application Ser. No. 10/987,518, filed on Nov. 12, 2004, now U.S. Pat. No. 7,180,097, issued on Feb. 20, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to gallium-nitride based light emitting diodes and, more particularly, to the high-brightness gallium-nitride based light emitting diodes having a micro-roughened surface. 
     2. The Prior Arts 
     Gallium-nitride (GaN) based light-emitting diodes (LEDs), as various colored LEDs can be developed by controlling the GaN-based material&#39;s composition, have been the research and development focus in the academic arena and in the industries as well in recent years. One of the research directions regarding GaN-based LEDs lies in the further understanding of the light emitting characteristics of GaN-based LEDs. Based on this knowledge, then, methods for enhancing GaN-based LEDs&#39; lighting efficiency and brightness can be developed and discovered. These high-efficiency and high-brightness GaN-based LEDs would soon find their widespread applications in outdoor display panels and automobile lamps. 
     The lighting efficiency of a GaN-based LED is mainly determined by the GaN-based LED&#39;s internal quantum efficiency and external quantum efficiency. The former relates to the probability of recombination of electrons and holes, thereby causing photons to be released, within the GaN-based LED&#39;s active layer. The more easily the electrons and holes are recombined, the more photons are released, and the higher the lighting efficiency of the GaN-based LED will be. The latter, on the other hand, relates to the probability of photons&#39; successful escape from the GaN-based LED without being absorbed or trapped inside. The more photons escape from the GaN-based LED, the higher the external quantum efficiency is, and the higher the lighting efficiency of the GaN-based LED will be. 
     The GaN-based LED&#39;s external quantum efficiency would, therefore, be affected by its index of refraction. Generally, the index of refraction of GaN-based LEDs is 2.5, higher than that of the atmosphere (which is 1). As such, total internal reflection would happen and photons released from the active layer would be trapped inside the GaN-based LEDs, significantly reducing the external quantum efficiency. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention provides an epitaxial structure for the GaN-based LEDs so that the limitations and disadvantages in terms of lighting efficiency and external quantum efficiency from the prior arts can be obviated practically. 
     The greatest difference between the GaN-based LEDs according to the present invention and the prior arts lies in the addition of a thin layer on top of the traditional structure. The thin layer could be formed using silicon-nitride (SiN), or it could have a short-period superlattice structure either made of layers of SiN and undoped indium-gallium-nitride (InGaN), or made of layers of SiN and undoped aluminum-gallium-indium-nitride (AlGaInN), alternately stacked upon each other. Because of the use of SiN in the thin layer, the surfaces of the GaN-based LEDs would be micro-roughened, and the total internal reflection resulted from the GaN-based LEDs&#39; higher index of refraction than the atmosphere could be avoided. The GaN-based LEDs according to the present invention therefore have superior external quantum efficiency and lighting efficiency. 
     The improvement in the GaN-based LEDs&#39; lighting efficiency could be easily seen from  FIG. 1 .  FIG. 1  is a characteristics graph showing, under different amount of injection current, the brightness measured from GaN-based LEDs according to the present invention and the prior arts. As shown in  FIG. 1 , the GaN-based LEDs with the aforementioned thin layer having a superlattice structure made of SiN and undoped In 0.2 Ga 0.8 N apparently have a superior lighting efficiency than the GaN-based LEDs according to prior arts. 
     In addition, as the thin layer has a lower band gap than that of the traditional doped contact layer, the interposition of the thin layer between the contact layer below and the metallic electrode and transparent conductive layer above would have additional benefits, such as the resistivity between the thin layer and the electrode and the transparent conductive layer above is lower and, therefore, ohmic contact is easer to form. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanied drawings are provided to illustrate the various embodiments of the present invention as described in this specification, so as to achieve better understanding of the major objectives of the present invention. 
         FIG. 1  is a characteristics graph showing, under different amount of injection current, the brightness measured from GaN-based LEDs according to the present invention and the prior arts. 
         FIG. 2  is a schematic diagram showing a GaN-based LED device according to a first embodiment of the present invention. 
         FIG. 3  is a schematic diagram showing a GaN-based LED device according to a second embodiment of the present invention. 
         FIG. 4  is a schematic diagram showing a GaN-based LED device according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, detailed description along with the accompanied drawings is given to better explain preferred embodiments of the present invention. Please be noted that, in the accompanied drawings, some parts are not drawn to scale or are somewhat exaggerated, so that people skilled in the art can better understand the principles of the present invention. 
       FIG. 2  is a schematic diagram showing a GaN-based LED device according to a first embodiment of the present invention. As shown in  FIG. 2 , the GaN-based LED has a substrate  10  made of C-plane, R-plane, or A-plane aluminum-oxide monocrystalline (sapphire), or an oxide monocrystalline having a lattice constant compatible with that of nitride semiconductors. The substrate  10  can also be made of SiC (6H—SiC or 4H—SiC), Si, ZnO, GaAs, or MgAl 2 O 4 . Generally, the most common material used for the substrate  10  is sapphire or SiC. An optional buffer layer  20  made of a GaN-based material whose molecular formula could be expressed as Al a Ga b In 1-a-b N (0≦a,b&lt;1, a+b≦1) having a specific composition is then formed on an upper side of the substrate  10 . On top of the buffer layer  20 , a first contact layer  30  made of a GaN-based material having a first conduction type (e.g., it could be p-typed or n-typed GaN) is formed on the buffer layer  20 . On top of the first contact layer  30 , an active layer  40  made of a GaN-based material such as indium-gallium-nitride (InGaN) is formed on top of the first contact layer  30 . 
     On top of the active layer  40 , an optional cladding layer  50  made of a GaN-based material having a second conduction type opposite to that of the first contact layer  30 . In other words, for example, if the first contact layer  30  is made of an n-typed GaN-based material, then the cladding layer  50  is made of a p-typed GaN-based material. Then, on top of the cladding layer  50  or the active layer  40  (if there is no cladding layer  50 ), a second contact layer  60  made of a GaN-based material having the second conduction type opposite to that of the first contact layer  30  is formed. Again, on top of the second contact layer  60 , a micro-roughened thin layer  70 , which is the major characteristic to the present invention, is formed. In the present embodiment, the micro-roughened thin layer  70  is made of a group-IV nitride Si d N e  (0&lt;d, e&lt;1) having a specific composition. The micro-roughened thin layer  70  has a thickness between 2 Å and 50 Å and is formed at a growing temperature between 600° C. and 1100° C. The micro-roughened thin layer  70 , thus formed, contains multiple randomly distributed micro-clusters of Si d N e  on the second contact layer  60 . It is important to note that the multiple randomly distributed micro-clusters of the micro-roughened thin layer can reduce the total reflection of light inside the GaN-based LED structure and improve external quantum efficiency because of the randomness in the distribution of the micro-clusters. In other words, the micro-clusters are un-uniformly distributed as shown in  FIG. 2 . 
     Up to this point, the epitaxial structure of the present invention has been completed. To package the epitaxial structure into a LED device, the electrodes for the LED device have to be formed. Conventionally, the epitaxial structure is appropriately etched to expose a portion of the first contact layer  30  and, then, a first electrode  42  made of an appropriate metallic material is formed on top of the exposed first contact layer  30 . 
     On the other hand, on top of the micro-roughened thin layer  70 , an optional transparent conductive layer  82  could be formed. The transparent conductive layer  82  can be a metallic conductive layer or a transparent oxide layer. The metallic conductive layer is made of one of the materials including, but not limited to, Ni/Au alloy, Ni/Pt alloy, Ni/Pd alloy, Pd/Au alloy, Pt/Au alloy, Cr/Au alloy, Ni/Au/Be alloy, Ni/Cr/Au alloy, Ni/Pt/Au alloy, Ni/Pd/Au alloy, and other similar materials. The transparent oxide layer, on the other hand, is made of one of the materials including, but not limited to, ITO, CTO, ZnO:Al, ZnGa 2 O 4 , SnO 2 :Sb, Ga 2 O 3 :Sn, AgInO 2 :Sn, In 2 O 3 :Zn, CuAlO 2 , LaCuOS, NiO, CuGaO 2 , and SrCu 2 O 2 . A second electrode  80  is formed on top of the transparent conductive layer  82  or besides the transparent conductive layer  82  as shown in the accompanied drawings. The second electrode  80  is made of one of the materials including, but not limited to, Ni/Au alloy, Ni/Pt alloy, Ni/Pd alloy, Ni/Co alloy, Pd/Au alloy, Pt/Au alloy, Ti/Au alloy, Cr/Au alloy, Sn/Au alloy, Ta/Au alloy, TiN, TiWN x  (x≧0), WSi y  (y≧0), and other similar metallic materials. 
       FIG. 3  is a schematic diagram showing a GaN-based LED device according to a second embodiment of the present invention. As shown in  FIG. 3 , the present embodiment has an identical structure as in the previous embodiment. The only difference lies in the material and structure used for the micro-roughened thin layer. In the present embodiment, the micro-roughened thin layer  72  has a short-period superlattice structure formed by interleaving at least a SiN thin layer  721  and at least an InGaN thin layer  722 . Each of the SiN thin layers  721  is made of Si f N g  (0&lt;f, g&lt;1) having a specific composition, and has a thickness between 2 Å and 20 Å, and is formed at a growing temperature between 600° C. and 1100° C. In addition, the Si f N g  composition (i.e., the parameters f, g of the foregoing molecular formula) of each SiN thin layer  721  is not necessarily identical. On the other hand, each of the InGaN thin layers  722  is made of undoped In h Ga 1-h N (0&lt;h≦1) having a specific composition, and has a thickness between 2 Å and 20 Å, and is formed at a growing temperature between 600° C. and 1100° C. Similarly, the In h Ga 1-h N composition (i.e. the parameters h of the foregoing molecular formula) of each InGaN thin layer  722  is not required to be identical. 
     Within the superlattice structure of the micro-roughened thin layer  72 , the bottommost layer (i.e., the one immediately above the second contact layer  60 ) could be a SiN thin layer  721 . Then, on top of the bottommost SiN thin layer  721 , an InGaN thin layer  722 , another SiN thin layer  721 , another InGaN thin layer  722 , and so on, are sequentially and alternately stacked in this repetitive pattern. Alternatively, the bottommost layer could be an InGaN thin layer  722 . Then, on top of the bottommost InGaN thin layer  722 , a SiN thin layer  721 , another InGaN thin layer  722 , another SiN thin layer  721 , and so on, are sequentially and alternately stacked in this repetitive pattern. The number of repetition is at least one (i.e., both the number of the SiN thin layers  721  and the number of the InGaN thin layers  722  are at least one). The overall thickness of the micro-roughened thin layer  72  should be no more than 200 Å. 
       FIG. 4  is a schematic diagram showing a GaN-based LED device according to a third embodiment of the present invention. As shown in  FIG. 4 , the present embodiment has an identical structure as in the previous embodiment. The only difference lies in the material and structure used for the micro-roughened thin layer. In the present embodiment, the micro-roughened thin layer  74  has a short-period superlattice structure formed by interleaving at least a SiN thin layer  741  and at least an AlInGaN thin layer  742 . Each of the SiN thin layers  741  is made of Si i N j  (0&lt;i, j&lt;1) having a specific composition, and has a thickness between 2 Å and 20 Å, and is formed at a growing temperature between 600° C. and 1100° C. On the other hand, each of the AlInGaN thin layers  742  is made of undoped Al m In n Ga 1-m-n N (0&lt;m, n&lt;1, m+n&lt;1) having a specific composition, and has a thickness between 2 Å and 20 Å, and is formed at a growing temperature between 600° C. and 1100° C. Similarly, the S i N j  and Al m In n Ga 1-m-n N composition of each SiN thin layers  741  and AlInGaN thins layers  742  are not required to be identical. 
     Within the micro-roughened layer  74 , the bottommost layer (i.e., the one immediately above the second contact layer  60 ) could be a SiN thin layer  741 . Then, on top of the bottommost SiN thin layer  741 , an AlInGaN thin layer  742 , another SiN thin layer  741 , another AlInGaN thin layer  742 , and so on, are sequentially and alternately stacked in this repetitive pattern. Alternatively, the bottommost layer could be an AlInGaN thin layer  742 . Then, on top of the bottommost AlInGaN thin layer  742 , a SiN thin layer  741 , another AlInGaN thin layer  742 , another SiN thin layer  741 , and so on, are sequentially and alternately stacked in this repetitive pattern. The number of repetition is at least one (i.e., both the number of the SiN thin layers  741  and the number of the AlInGaN thin layers  742  are at least one). The overall thickness of the micro-roughened thin layer  74  should be no more than 200 Å. 
     In aforementioned preferred embodiments of the present invention, the development of the SiN material within the micro-roughened thin layer would cause the surfaces of the GaN-based LEDs to be micro-roughened. As such, the total internal reflection resulted from the GaN-based LEDs&#39; higher index of refraction than the atmosphere could be avoided. The GaN-based LEDs according to the present invention therefore have superior external quantum efficiency and lighting efficiency. 
     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.