Abstract:
Disclosed is a multi-quantum-well light emitting diode, which makes enormous adjustments and improvements over the conventional light emitting diode, and further utilizes a transparent contact layer of better transmittance efficiency, so as to significantly raise the illuminance of this light emitting diode and its light emission efficiency. The multi-quantum-well light emitting diode has a structure including: substrate, buffer layer, n-type gallium-nitride layer, active light-emitting-layer, p-type cladding layer, p-type contact layer, barrier buffer layer, transparent contact layer, and the n-type electrode layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the gallium-nitride (GaN) based light-emitting-diode (LED) structure, and in particular to the gallium-nitride (GaN) based light emitting diode which achieves the increased light transmittance and enhanced light illuminance by means of its top transmittance contact layer using ITO (indium-tin-oxide) as its material. 
     2. The Prior Arts 
     It is known, the indium-gallium-nitride (InGaN)/gallium-nitride (GaN) multi-quantum well (MQW) LED is usually used in the prior art as the light emitting device. It has been widely utilized in the various functions and applications of static display, such as, in the clocks/watches, display screens, and advertisement panels, etc for displaying digits and/or images. However, its light illuminance and light transmittance efficiency are restricted by the property of its top transparent contact layer, and its light transmittance at present can only reach 62% at most in the visible light spectrum. Therefore, the effectiveness of its application is not very satisfactory. 
     Now, please refer to  FIG. 1  as we explain the structure of this prior art multi-quantum well light emitting diode (MQW LED) and its restrictions. As shown in  FIG. 1 , its structure comprises a substrate  11 , a buffer layer  12 , an un-doped n-type gallium-nitride (GaN) layer  13 , an n-type gallium-nitride (GaN) layer  14 , a multi-quantum well (MQW) layer  15 , a gallium-nitride (GaN) cladding layer  16 , a p-type gallium-nitride (GaN) layer  17 , a Ni/Au transparent contact layer  18 , and a Ni/Al transparent contact layer  19 . 
     In the above-mentioned structure, the lowest layer is the substrate  11  and it is made of Sapphire. The buffer layer  12  is formed on the substrate  11  and is made of low temperature growth gallium-nitride (GaN). The un-doped n-type gallium-nitride (GaN) layer  13  is formed on the buffer layer  12 . Then, the n-type gallium-nitride (GaN) layer  14  is formed on un-doped n-type gallium-nitride (GaN) layer  13 . Afterwards, the multi-quantum well (MQW) layer  15  is formed on the n-type gallium-nitride (GaN) layer  14 , and it is made of InGaN/GaN. Formed on top of the multi-quantum well layer is the gallium-nitride (GaN) cladding layer  16 . Then, p-type GaN layer  17  is formed on gallium-nitride (GaN) cladding layer  16 . Finally, Ni/Au transparent contact layer  18  is formed on p-type GaN layer  17 , and the Ni/Al transparent contact layer  19  is formed on the n-type gallium-nitride (GaN) layer  14 . 
     This sort of prior art multi-quantum-well light emitting diode structure is known as the “n-Down Structure”. Namely, in this structure, the InGaN/GaN multi-quantum well (MQW) active layer is grown on the n-type (n-GaN) cladding layer, and then the p-type GaN cladding layer is grown on the multi-quantum well active layer. The purpose of the light emitting diode produced in this manner is to take advantage of the superb crystal quality of the multi-quantum well (MQW) active layer, so as to achieve better current distribution in the underlying n-GaN layer, and therefore the lower turn-on voltage of the light emitting diode. 
     However, the main purpose of this kind of prior art multi-quantum-well light emitting diode (InGaN/GaN MQW LED) is to make use of the n-type (n-GaN) layer as the contacting layer, and the Ni/Au as the p-type conductive electrode and the transparent contact layer. According to the experiment data of  FIG. 2 , the maximum transmittance (namely, the transmitted percentage of the incident light) of that Ni/Au transparent contact layer in the visible light spectrum is only 62% at 530 nm. Therefore, the light emitting diode of the prior art is restricted by its intrinsic light transmittance property of this transparent contact layer, and as such its light illuminance can not be raised, and that is its major shortcomings and restrictions. 
     The purpose of the present invention is to improve the shortcomings and the restrictions of the afore-mentioned conventional light emitting diode, so as to achieve the purpose and function of increasing its light illuminance and light emission efficiency. 
     SUMMARY OF THE INVENTION 
     Therefore, the purpose of the present invention is to provide a kind of multi-quantum-well light emitting diode, which makes enormous adjustments and improvements over the structure of the conventional light emitting diode, and further utilizes a transparent contact layer of better transmittance efficiency, so as to significantly raise the illuminance of this light emitting diode and its light emission efficiency. 
     The main structure of this kind of multi-quantum-well light emitting diode includes: substrate, buffer layer, n-type gallium-nitride (GaN) layer, active light-emitting-layer, p-type cladding layer, p-type contact layer, barrier buffer layer, transparent contact layer, and the n-type electrode layer. Wherein, the bottom layer of this structure is substrate, which is made of Sapphire. Formed on top of the substrate is the buffer layer, which is made of aluminum-gallium-indium-nitride (Al 1-x-y Ga x In y N; 0≦x,y≦1 and x+y≦1). The n-type gallium-nitride (GaN) layer is formed on the buffer layer. Then, active light-emitting-layer is formed on n-type gallium-nitride (GaN) layer, and which is made of indium-gallium-nitride (InGaN). And next, p-type cladding layer is formed on the active light-emitting-layer, and which is made of Mg-doped aluminum-indium-nitride (Al 1-x In x N). Then, p-type contact layer is formed on p-type cladding layer, and which is made of gallium-nitride (GaN). And next, the barrier buffer layer is formed on p-type contact layer, and which is made of magnesium-nitride (MgN) or undoped indium nitride (InN) or undoped indium gallium nitride (In x Ga 1-x N; 0&lt;x&lt;1) or magnesium-nitride/undoped-indium-nitride (MgN/InN) or magnesium-nitride/undoped-indium-gallium-nitride (MgN/In x Ga 1-x N). Then the transparent contact layer is formed on barrier buffer layer, and which is made of indium-tin-oxide (ITO). And finally, the n-type electrode layer is formed on n-type gallium nitride (GaN) layer, and which is made of Ti/Al or Cr/Au. 
     The special design multi-quantum-well light emitting diode according to the present invention is characterized in that it does not make use of Ni/Au as the material for its top layer as in the prior art, but instead it utilizes ITO as the material for its top transparent contact layer. Therefore, according to the experiment data as shown in  FIG. 2 , the light transmittance of the transparent contact layer using ITO as its material can reach the very ideal level of 95% at 475 nm of the visible light spectrum. Namely, 95% of the light incident upon the ITO transparent contact layer can be transmitted out. Therefore, through the design of the present invention, the light illuminance and light transmittance efficiency of the multi-quantum-well light emitting diode can be significantly increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The related drawings in connection with the detailed description of the present invention to be made later are described briefly as follows, in which: 
         FIG. 1  is a conventional gallium-nitride based light emitting diode structure; 
         FIG. 2  indicates the comparison of the transmittance of the present invention using ITO as the transparent contact layer material vs that of the prior art using Ni/Au as the transparent contact layer material; 
         FIG. 3  is a gallium-nitride based light emitting diode structure according to the first embodiment of the present invention; 
         FIG. 4  is a gallium-nitride based light emitting diode structure according to the second embodiment of the present invention; and 
         FIG. 5  is a gallium-nitride based light emitting diode structure according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The various features and advantages of the present invention can be understood more thoroughly through the following detailed descriptions together with the attached drawings. 
     Now, please refer to the attached drawings as we describe the various Embodiments of the present invention. 
     Embodiment 1 
       FIG. 3  shows a gallium-nitride based light emitting diode structure  30  with enhanced light illuminance according to the first embodiment of the present invention, comprising: substrate  31 , buffer layer  32 , n-type gallium nitride (GaN) layer  33 , active light-emitting-layer  34 , p-type cladding layer  35 , p-type contact layer  36 , barrier buffer layer  37 , transparent contact layer  38 , and n-type electrode layer  39 . 
     The bottom layer of the structure is substrate  31 , and it is made of Sapphire. And next, the buffer layer  32  is formed on substrate  31 , and it is made of aluminum-gallium-indium-nitride (Al 1-x-y Ga x In y N) wherein 0≦x≦1, 0≦y≦1 and x+y≦1. The n-type gallium-nitride (GaN) layer  33  is formed on buffer layer  32 . The active light-emitting-layer  34  is formed on n-type gallium-nitride (GaN) layer  33 , and it is made of indium-gallium-nitride (InGaN). And next, the p-type cladding layer  35  is formed on active light-emitting-layer  34 , and it is made of Mg-doped aluminum-indium-nitride (Al 1-x In x N), wherein 0≦x&lt;1. Then, p-type contact layer  36  is formed on p-type cladding layer  35 , and it is made of gallium nitride (GaN). And then barrier buffer layer  37  is formed on p-type contact layer  36 , and it is made of magnesium-nitride (MgN). And next transparent contact layer  38  is formed on barrier buffer layer  37 , and it is made of indium-tin-oxide. And finally the n-type electrode layer  39  is formed on n-type gallium nitride (GaN) layer  33 , and it is made of Ti/Al or Cr/Au. 
     In the above-mentioned structure, the thickness of the barrier buffer layer (MgN)  37  is between 2 Å and 200 Å, and its growth temperature is between 500° C. and 1200° C. In the embodiment 1, we can also use undoped indium nitride (InN) or undoped indium gallium nitride (In x Ga 1-x N; 0&lt;x&lt;1) instead of magnesium-nitride (MgN) as the barrier buffer layer  37 . The thickness of the barrier buffer layer (InN or In x Ga 1-x N; 0&lt;x&lt;1)  37  is between 2 Å and 200 Å, and its growth temperature is between 500° C. and 1200° C. 
     Embodiment 2 
       FIG. 4  shows a gallium-nitride based light emitting diode structure  40  with enhanced light illuminance according to a second embodiment of the present invention, comprising: substrate  41 , buffer layer  42 , n-type gallium-nitride (GaN) layer  43 , active light-emitting-layer  44 , p-type cladding layer  45 , p-type contact layer  46 , short-period super-lattice barrier buffer layer  47 , transparent contact layer  48 , and n-type electrode layer  49 . 
     The bottom layer of the structure is substrate  41 , and it is made of Sapphire. And next, the buffer layer  42  is formed on substrate  41 , and it is made of aluminum-gallium-indium-nitride (Al 1-x-y Ga x In y N), wherein 0≦x≦1, 0≦y≦1 and x+y≦1. Then, the n-type gallium-nitride (GaN) layer  43  is formed on buffer layer  42 . And then, the active light-emitting-layer  44  is formed on n-type gallium nitride (GaN) layer  43 , and it is made of indium-gallium nitride (InGaN). And next the p-type cladding layer  45  is formed on active light-emitting-layer  44 , and it is made of Mg-doped aluminum-indium-nitride (Al 1-x In x N), wherein 0≦x&lt;1. Then, p-type contact layer  46  is formed on p-type cladding layer  45 , and it is made of gallium nitride (GaN). And then, the short-period super-lattice barrier buffer layer  47  is formed on p-type contact layer  46 , and it is made of magnesium-nitride/undoped-indium-nitride (MgN/InN). And next the transparent contact layer  48  is formed on the short-period super-lattice barrier buffer layer  47 , and it is made of indium-tin-oxide. And finally the n-type electrode layer  49  is formed on n-type gallium-nitride (GaN) layer  43 , and it is made of Ti/Al or Cr/Au. 
     In the above-mentioned structure, the thickness of the respective portions of the short-period super-lattice barrier buffer layer  47  made of (MgN/InN) is between 2 and 200 Å respectively, its number of repetition is 2 or above, and its configuration can be MgN up/InN down or MgN down/InN up, and its growth temperature is between 500° C. and 1200° C. 
     Embodiment 3 
       FIG. 5  shows a gallium-nitride based light emitting diode structure  50  with enhanced light illuminance according to a third embodiment of the present invention, comprising: substrate  51 , buffer layer  52 , n-type gallium nitride (GaN) layer  53 , active light-emitting-layer  54 , p-type cladding layer  55 , p-type contact layer  56 , short-period super-lattice barrier buffer layer  57 , transparent contact layer  58 , and n-type electrode layer  59 . 
     The bottom layer of the structure is substrate  51 , and it is made of Sapphire. And next, the buffer layer  52  is formed on substrate  51 , and it is made of aluminum-gallium-indium-nitride (Al 1-x-y Ga x In y N), wherein 0≦x≦1, 0≦y≦1 and x+y≦1. Then, the n-type gallium nitride (GaN) layer  53  is formed on buffer layer  52 . And then, the active light-emitting-layer  54  is formed on n-type gallium nitride (GaN) layer  53 , and it is made of indium-gallium nitride (InGaN). And next, the p-type cladding layer  55  is formed on active light-emitting-layer  54 , and it is made of Mg-doped aluminum-indium-nitride (Al 1-x In x N), wherein 0≦x&lt;1. Then, p-type contact layer  56  is formed on p-type cladding layer  55 , and it is made of gallium nitride (GaN). And then, the short-period super-lattice barrier buffer layer  57  is formed on p-type contact layer  56 , and it is made of magnesium-nitride/undoped-indium-gallium-nitride (MgN/In x Ga 1-x N). And next, the transparent contact layer  58  is formed on the short-period super-lattice barrier buffer layer  57 , and it is made of indium-tin-oxide. And finally, the n-type electrode layer  59  is formed on n-type gallium nitride (GaN) layer  53 , and it is made of Ti/Al or Cr/Au. 
     In the above-mentioned structure, the thicknesses of the respective portions of the short-period super-lattice barrier buffer layer  57  made of (MgN/In x Ga 1-x N) are between 2 and 200 Å respectively, its number of repetition is 2 or above, and its configuration can be MgN up/In x Ga 1-x N down or MgN down/In x Ga 1-x N up, and its growth temperature is between 500° C. and 1200° C. 
     In the above-mentioned three embodiments, indium-tin-nitride (ITO) is utilized as the material of the transparent conductive layer. However, the material utilized for the transparent conductive layer of the present invention is not restricted to ITO, and it may comprise for example ITO, CTO, ZnO:Al, ZnGa 2 O 4 , SnO 2 :Sb, Ga 2 O 3 :Sn, AgInO 2 :Sn, and In 2 O 3 :Zn for the n-type transparent conductive oxide (TCO) layer, and CuAlO 2 , LaCuOS, NiO, CuGaO 2 , SrCu 2 O 2  for the p-type transparent conductive oxide (TCO) layer. 
     From the three embodiments described in detail above, and the related experimental data as shown in  FIG. 2 , it is evident that the shortcomings and restrictions of the prior art light emitting diode can certainly be improved by means of the light emitting diode structure of the present invention. And in particular the light transmittance of this kind of light emitting diode can be raised from 62% of the prior art to 95% or above of the present invention, and its light illuminance and light transmittance efficiency can also be raised significantly. Therefore, the present invention does have its value of utilization in the industry, and it is in conformity with the patent requirements. 
     The purpose of the preferred embodiment described above is only illustrative, and it is not intended to be construed as to be any restrictions to the present invention. Therefore, any variations or modifications made within the spirit and scope of the present invention can be included in the scope of protection of the attached claims.