Abstract:
An epitaxial structure for GaN-based LEDs to achieve better reverse withstanding voltage and anti-ESD capability is provided herein. The epitaxial structure has an additional anti-ESD thin layer as the topmost layer, which is made of undoped indium-gallium-nitrides (InGaN) or low-band-gap (Eg&lt;3.4 eV), undoped aluminum-indium-gallium-nitrides (AlInGaN). The anti-ESD thin layer could also have a superlattice structure formed by interleaving at least an undoped InGaN thin layer and at least a low-band-gap, undoped AlInGaN thin layer. 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:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a division of U.S. application Ser. No. 11/266,415, filed on Nov. 3, 2005, which is a continuation-in-part of U.S. application Ser. No. 10/964,350, filed on Oct. 12, 2004, now U.S. Pat. No. 7,180,096, issued on Feb. 20, 2007. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     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.  
         [0004]     2. The Prior Arts  
         [0005]     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.  
         [0006]     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.  
         [0007]     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 degraded.  
         [0008]     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.  
         [0009]     Accordingly, the present invention is directed to overcome the foregoing disadvantages of conventional GaN-based LEDs of the prior arts.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides an epitaxial structure for the GaN-based LEDs so that the limitations and disadvantages in terms of their anti-ESD capability from the prior arts can be obviated practically.  
         [0011]     The most significant difference between the GaN-based LEDs according to the present invention and those of 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) beneath the transparent conductive layer of traditional GaN-based LEDs. The anti-ESD thin layer could also have a superlattice structure formed by interleaving a plurality of InGaN thin layers and a plurality of low-band-gap, undoped AlInGaN thin layers. 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.  
         [0012]     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.  
         [0013]     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 traditional n-type or 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 layer above.  
         [0014]     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  
       [0015]     FIGS.  1 ( a ) and  1 ( b ) illustrate the maximum ESD voltage and the reverse withstanding voltage of a GaN-based LED according to the present invention versus the thickness of the GaN-based LED&#39;s anti-ESD thin layer.  
         [0016]      FIG. 2  is a schematic diagram showing a GaN-based LED device according to a first embodiment of the present invention.  
         [0017]      FIG. 3  is a schematic diagram showing a GaN-based LED device according to a second embodiment of the present invention.  
         [0018]      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  
       [0019]     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.  
         [0020]      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 device 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  is formed and made of a GaN-based material having a first conduction type (e.g., it could a p-typed GaN or n-typed GaN). Then, on top of the first contact layer  30 , an active layer  40  made of a GaN-based material such as InGaN is formed on top of the first contact layer  30 .  
         [0021]     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 active layer  40  (if there is no cladding layer  50 ) or the 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, 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 major characteristic of the present invention. In this first embodiment of the present invention, the anti-ESD thin layer  70  is made of undoped (i.e., without having any n-typed or p-typed impurities) 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 is formed at a growing temperature between 600° C. and 1100° C.  
         [0022]     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 .  
         [0023]     On the other hand, on top of the anti-ESD 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.  
         [0024]      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 , 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. In this embodiment, the anti-ESD thin layer  72  is made of undoped, low-band-gap (Eg&lt;3.4 eV) Al e In f Ga 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.  
         [0025]      FIG. 4  is a schematic diagram showing a GaN-based LED according to a 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. In this embodiment, the anti-ESD thin layer  74  has a superlattice structure formed by interleaving one or more InGaN thin layers  741  with one or more AlInGaN thin layers  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 is formed at 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 is formed at 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.  
         [0026]     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 alternately 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 alternately 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 one (i.e. there are at least one layer of the InGaN thin layer  741  and at least one layer of the AlInGaN thin layer). The total thickness of the anti-ESD thin layer  74  is at most 200 Å.  
         [0027]     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.