Abstract:
An epitaxial structure for GaN-based LEDs to achieve better reverse withstanding voltage and anti-ESD capability is provided. The epitaxial structure has an additional anti-ESD thin layer on top of the p-type contact layer within traditional GaN-based LEDs, which is made of undoped indium-gallium-nitrides (InGaN) or low-band-gap (Eg&lt;3.4 eV), undoped aluminum-indium-gallium-nitrides (AlInGaN). This anti-ESD thin layer greatly improves the GaN-based LEDs&#39; reverse withstanding voltage and resistivity to ESD, which in turn extends the GaN-based LEDs&#39; operation life significantly.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to the gallium-nitride based light-emitting diodes and, more particularly, to an epitaxial structure of the gallium-nitride based light-emitting diodes having a high reverse withstanding voltage and a high resistivity to electrostatic discharge.  
         [0003]     2. The Prior Arts  
         [0004]     Gallium-nitride (GaN) based light-emitting diodes (LEDs), as various color LEDs can be developed by controlling the GaN-based material&#39;s composition, has been the research and development focus in the academic arena and in the industries as well in recent years. Besides being applied in the display of consumer electronic appliances such as digital clocks and cellular handsets, technology breakthroughs in terms of luminance and lighting efficiency has led GaN-based LEDs into applications such as outdoor display panels and automobile lamps.  
         [0005]     To have practical applicability in these outdoor display devices, besides having high luminance and lighting efficiency, GaN-based LEDs must have a rather high reverse withstanding voltage and high resistivity to electrostatic discharge (ESD), so that they can continue to operate for an extended period of time under the harsh, outdoor environment.  
         [0006]     However, for conventional GaN-based LEDs, they have a traditional epitaxial structure by growing GaN-based nitrides on a sapphire substrate. GaN-based nitrides and the sapphire substrate usually have mismatched lattice constants, causing an excessive accumulation of stresses and, thereby, causing the GaN-based LEDs to have an inferior epitaxial quality. The GaN-based LEDs&#39; anti-ESD capability and reverse withstanding voltage are therefore deteriorated.  
         [0007]     The most widely adopted solution in recent years is to use a flip-chip process to combine a GaN-based LED with a Zener diode made of silicon. Although this solution indeed effectively improves the GaN-based LED&#39;s anti-ESD capability, the flip-chip process is much more complicated than the traditional manufacturing process for general GaN-based LEDs.  
         [0008]     Accordingly, the present invention is directed to overcome the foregoing disadvantages of conventional GaN-based LEDs of the prior arts.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides an epitaxial structure for the GaN-based LEDs so that the limitations and disadvantages from the prior arts can be obviated practically.  
         [0010]     The most significant difference between the GaN-based LEDs according to the present invention and according to the prior arts lies in the formation of an anti-ESD thin layer made of undoped indium-gallium-nitrides (InGaN) or low-band-gap (Eg&lt;3.4 eV), undoped aluminum-indium-gallium-nitrides (AlInGaN) on top of the p-type contact layer of traditional GaN-based LEDs. This anti-ESD thin layer greatly improves the GaN-based LEDs&#39; reverse withstanding voltage and resistivity to ESD, which in turn extends the GaN-based LEDs&#39; operation life significantly.  
         [0011]     FIGS.  1 ( a ) and  1 ( b ) of the attached drawings illustrate the maximum ESD voltage and the reverse withstanding voltage of a GaN-based LED according the present invention versus the thickness of the GaN-based LED&#39;s anti-ESD thin layer. As shown in FIGS.  1 ( a ) and  1 ( b ), an anti-ESD thin layer made of undoped In 0.2 Ga 0.8 N obviously provides much higher reverse withstanding voltage and maximum ESD voltage than anti-ESD thin layers made of Si-doped and Mg-doped In 0.2 Ga 0.8 N, when all three anti-ESD layers are of a same thickness between 5 Å and 100 Å.  
         [0012]     Besides the foregoing advantages, due to the low band gap characteristics of undoped InGaN and undoped AlInGaN, an anti-ESD thin layer made of such material, in comparison to the p-type contact layer in a GaN-based LED of the prior art, has a lower resistivity (and, thereby, is easier to form ohmic contact) between the anti-ESD thin layer and the metallic electrode or transparent conductive electrode above.  
         [0013]     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  
       [0014]     FIGS.  1 ( a ) and  1 ( b ) illustrate the maximum ESD voltage and the reverse withstanding voltage of a GaN-based LED according the present invention versus the thickness of the GaN-based LED&#39;s anti-ESD thin layer.  
         [0015]      FIG. 2  is a schematic diagram showing the epitaxial structure of the GaN-based LED according to the first embodiment of the present invention.  
         [0016]      FIG. 3  is a schematic diagram showing the epitaxial structure of the GaN-based LED according to the second embodiment of the present invention.  
         [0017]      FIG. 4  is a schematic diagram showing the epitaxial structure of the GaN-based LED according to the third embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     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.  
         [0019]      FIG. 2  is a schematic diagram showing the epitaxial structure of the GaN-based LED according to the 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. A buffer layer  20  made of 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 , an n-type contact layer  30  is formed and made of a GaN-based material. Then, on top of n-type contact layer  30 , an active layer  40  made of InGaN covers a part of the n-type contact layer  30 &#39;s upper surface. A negative electrode  42 , on the other hand, is on top of another part of the n-type contact layer  30 &#39;s upper surface not covered by the active layer  40 .  
         [0020]     On top of the active layer  40 , a p-type cladding layer  50  made of a GaN-based material, a p-type contact layer  60  made of p-type GaN, and an anti-ESD thin layer  70  are sequentially stacked in this order from bottom to top. The anti-ESD thin layer  70  is the key to the present invention. Within this first embodiment of the present invention, the anti-ESD thin layer  70  is made of undoped In d Ga 1-d N (0&lt;d≦1) having a specific composition. The anti-ESD thin layer  70  has a thickness between 5 Å and 100 Å and a growing temperature between 600° C. and 1100° C.  
         [0021]     Then, on top of the anti-ESD thin layer  70 , there are a positive electrode  80  and a transparent conductive layer  82 , which are not overlapping with each other. The positive electrode  80  is made of one of the materials including 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. 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 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 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 .  
         [0022]      FIG. 3  is a schematic diagram showing the epitaxial structure of the GaN-based LED according to the second embodiment of the present invention. As shown in  FIG. 3 , this embodiment of the present invention has an identical structure as in the previous embodiment. The only difference lies in the material used for the anti-ESD thin layer. Within this embodiment, the anti-ESD thin layer  72  is made of undoped, low-band-gap (Eg&lt;3.4 eV) Al e In f Gal 1-e-f N (0&lt;e,f&lt;1, e+f&lt;1) having a specific composition. The anti-ESD thin layer  72  has a thickness between 5 Å and 100 Å and a growing temperature between 600° C. and 1100° C.  
         [0023]      FIG. 4  is a schematic diagram showing the epitaxial structure of the GaN-based LED according to the third embodiment of the present invention. As shown in  FIG. 4 , this embodiment of the present invention has an identical structure as in the previous embodiments. The only difference lies in the material used and the structure of the anti-ESD thin layer. Within this embodiment, the anti-ESD thin layer  74  has a superlattice structure formed by alternately stacking an InGaN thin layer  741  and an AlInGaN thin layer  742 . Each of the InGaN thin layers  741  is made of undoped In g Ga 1-g N (0&lt;g≦1) having a specific composition, and has a thickness between 5 Å and 20 Å, and a growing temperature between 600° C. and 1100° C. In addition, the In g Ga 1-g N composition (i.e. the parameter g of the foregoing molecular formula) of each InGaN thin layer  741  is not required to be identical. On the other hand, each of the AlInGaN thin layers  742  is made of undoped, low-band-gap (Eg&lt;3.4 eV) Al h In i Ga 1-h-i N (0&lt;h,i&lt;1, h+i&lt;1) having a specific composition, and has a thickness between 5 Å and 20 Å, and a growing temperature between 600° C. and 1100° C. Similarly, the Al h In i Ga 1-h-i N composition (i.e. the parameters h and i of the foregoing molecular formula) of each AlInGaN thin layer  742  is not required to be identical.  
         [0024]     Within the anti-ESD thin layer  74 &#39;s superlattice structure, a InGaN thin layer  741  is at the bottom and, on top of the bottommost InGaN thin layer  741 , a AlInGaN thin layer  742 , another InGaN thin layer  741 , etc., are stacked upon each other in this repetitive fashion. In another variation of this embodiment, it is an AlInGaN thin layer  742  that is at the bottom. Then, on top of the bottommost AlInGaN thin layer  742 , an InGaN thin layer  741 , another AlInGaN thin layer  742 , etc., are stacked upon each other in this repetitive fashion. In other words, the InGaN thin layer  741  and the AlInGaN thin layer  742  are repetitively and alternately stacked. The repetition count is at least two (i.e. there are at least two layers of the InGaN thin layer  741  and at least two layers of the AlinGaN thin layer). The total thickness of the anti-ESD thin layer  74  is at most 200 Å.  
         [0025]     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.