Abstract:
A number of light-emitting layer structures for the GaN-based LEDs that can increase the lighting efficiency of the GaN-based LEDs on one hand and facilitate the growth of epitaxial layer with better quality on the other hand are provided. The light-emitting layer structure provided is located between the n-type GaN contact layer and the p-type GaN contact layer. Sequentially stacked on top of the n-type GaN contact layer is the light-emitting layer containing a lower barrier layer, at least one intermediate layer, and an upper barrier layer. That is, the light-emitting layer contains at least one intermediate layer interposed between the upper and lower barrier layers. When there are multiple intermediate layers inside the light-emitting layer, there is an intermediate barrier layer interposed between every two immediately adjacent intermediate layers.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a division of U.S. application Ser. No. 10/939,689, filed Sep. 11, 2004 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the gallium-nitride (GaN) based light emitting diode (LED), and in particular to the structure of the light-emitting layer of the GaN-based LED. 
     2. The Prior Arts 
     LEDs have long been widely used as indicators or light sources in various electronic consumer devices due to their features including low power consumption, low heat dissipation, and long operation life. In recent years, researches have been focused on the development of LEDs with various colors and LEDs with high luminance. Among these researches, highly efficient and illuminant blue-light LEDs that can be put to practical use receive the most attention. In October 1995, Nichia Corporation, Japan, announced the successful production of highly illuminant blue-light LEDs based on the indium-gallium-nitride (InGaN) material. This breakthrough has led the world&#39;s optoelectronic industry to invest tremendous capitals and resources in the gallium-nitride (GaN) based, such as GaN, aluminum-gallium-nitride (AlGaN), indium-gallium-nitride (InGaN), etc., LEDs. 
       FIG. 1  is a schematic diagram showing the structure of a GaN-based LED according to prior arts. As shown in  FIG. 1 , the conventional structure of a GaN-based LED contains a substrate  10  made of sapphire. Then, on one side of the sapphire substrate  10 , the GaN-based LED further contains an n-type GaN contact layer  11 , an InGaN light-emitting layer  12 , and a p-type GaN contact layer  13 , sequentially stacked from bottom to top in this order. In addition, there are a positive electrode  14  and a negative electrode  15  stacked upon the p-type GaN contact layer  13  and the n-type GaN contact layer  11  respectively. Within this conventional GaN-based LED structure, the light-emitting layer  12  usually has a multi-quantum well (MQW) structure made of In x Ga 1-x N (0≦x≦1). The electrons and holes are joined with each other within the In x Ga 1-x N (0≦x≦1) potential well and photons are thereby released. Please note that the epitaxial growth of the In x Ga 1-x N (0≦x≦1) requires a very high temperature to obtain epitaxial layer with better quality. On the other hand, to increase the possibility of forming the electron-hole pairs and thereby the lighting efficiency, the growing temperature of the In x Ga 1-x N (0≦x≦1) cannot be higher than 850° C. so that multiple localized states can be formed from the characteristics of the In x Ga 1-x N (0≦x≦1) such as indium segregation and phase separation. This is a dilemma requiring an appropriate solution. 
     SUMMARY OF THE INVENTION 
     To overcome the foregoing disadvantages, the present invention provides a number of light-emitting layer structures for the GaN-based LEDs that can increase the lighting efficiency of the GaN-based LEDs on one hand and facilitate the growth of epitaxial layer with better quality on the other hand. 
     The light-emitting layer structure provided by the present invention is located between the n-type GaN contact layer and the p-type GaN contact layer. Sequentially stacked on top of the n-type GaN contact layer in the following order, the light-emitting layer contains a lower barrier layer, at least one intermediate layer, and an upper barrier layer. That is, the light-emitting layer contains at least one intermediate layer interposed between the upper and lower barrier layers. When there are multiple intermediate layers inside the light-emitting layer, there is an intermediate barrier layer interposed between every two immediately adjacent intermediate layers. 
     The upper and lower barrier layers have higher band gaps than that of the intermediate layer so that the electrons and the holes have a higher possibility joining with each other within the intermediate layer, which in turn increases the lighting efficiency of the GaN-based LED. The barrier layers each have a thickness between 5 Å and 300 Å, and a growing temperature between 400° C. and 1000° C. 
     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 
         FIG. 1  is a schematic diagram showing the structure of a conventional GaN-based LED. 
         FIGS. 2(   a ),  2 ( b ), and  2 ( c ) are schematic diagrams showing the GaN-based LED structures according to a first embodiment of the present invention. 
         FIGS. 3(   a ),  3 ( b ), and  3 ( c ) are schematic diagrams showing the GaN-based LED structures according to a second embodiment of the present invention. 
         FIGS. 4(   a ) and  4 ( b ) are schematic diagrams showing the GaN-based LED structures according to a third embodiment of the present invention. 
         FIGS. 5(   a ) and  5 ( b ) are schematic diagrams showing the GaN-based LED structures according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 2(   a ),  2 ( b ), and  2 ( c ) are schematic diagrams showing the GaN-based LED structures according to a first embodiment of the present invention. As shown in  FIGS. 2(   a ),  2 ( b ), and  2 ( c ), the GaN-based LED structures use sapphire as the substrate  20 . Then, sequentially from bottom to top on the sapphire substrate  20 , the GaN-based LED structures contain a n-type GaN contact layer  21 , a lower barrier layer  22  made of un-doped aluminum-gallium-indium-nitride (Al 1-x-y Ga x In y N, 0≦x, y≦1, x+y≦1), at least an intermediate layer  23 , an upper barrier layer  24  made of un-doped Al 1-p-q Ga p In q N (0≦p, q≦1, p+q≦1), and a p-type GaN contact layer  25 . The GaN-based LED structures further contain a positive electrode  26  and a negative electrode  27  on top of the p-type GaN contact layer  25  and the n-type GaN contact layer  21  respectively. 
     As shown in  FIG. 2(   a ), the intermediate layer  23  further contains, from bottom to top, an ultra-thin quantum-dot layer  231  made of indium-nitride (InN) and a quantum-well layer  232  made of un-doped Al 1-m-n Ga m In n N (0≦m, n≦1, m+n≦1). 
     As shown in  FIG. 2(   b ), the intermediate layer  23  can further contain an optional InN ultra-thin quantum-dot layer  231 ′ on top of the un-doped Al 1-m-n Ga m In n N (0≦m, n≦1, m+n≦1) quantum-well layer  232 . 
     As shown in  FIG. 2(   c ), when there are multiple intermediate layers, every two immediately adjacent intermediate layers  23  and  23 ′ have an intermediate barrier layer  28  made of un-doped Al 1-i-j Ga i In j N (0≦i, j≦1, i+j≦1) interposed therebetween. 
     The upper, intermediate, and lower barrier layers  24 ,  28 , and  22  each have a total thickness between 5 Å and 300 Å, and a growing temperature between 400° C. and 1000° C. The ultra-thin quantum-dot layers  231  and  231 ′ each have a thickness between 2 Å and 30 Å, and a growing temperature between 400° C. and 1000° C. The quantum-well layer  232  has a thickness between 5 Å and 100 Å. Even though the quantum-well layer and the barrier layers are all made of aluminum-gallium-indium-nitrides, their compositions are not required to be identical. That is, the (x, y), (p, q), (m, n), (i, j) parameters in the foregoing molecular formulas are not necessarily the same. 
       FIGS. 3(   a ),  3 ( b ), and  3 ( c ) are schematic diagrams showing the GaN-based LED structures according to a second embodiment of the present invention. The second embodiment and the foregoing first embodiment of the present invention actually have identical structures. The difference lies in the materials used for the respective intermediate layers. As shown in  FIG. 3(   a ), the intermediate layer  33  further contains, from bottom to top, an ultra-thin layer  331  made of InN and quantum-well layer  332  made of un-doped Al 1-m-n Ga m In n N (0≦m, n≦1, m+n≦1). 
     As shown in  FIG. 3(   b ), the intermediate layer  33  can further contain another optional InN ultra-thin layer  331 ′ on top of the un-doped Al 1-m-n Ga m In n N (0≦m, n≦1, m+n≦1) quantum-well layer  332 . 
     As shown in  FIG. 3(   c ), when there are multiple intermediate layers, every two immediately adjacent intermediate layers  33  and  33 ′ must have an intermediate barrier layer  38  made of un-doped Al 1-i-j Ga i In j N (0≦i, j≦1, i+j≦1) interposed therebetween. 
     The upper, intermediate, and lower barrier layers  34 ,  38 , and  32  each have a thickness between 5 Å and 300 Å, and a growing temperature between 400° C. and 1000° C. The ultra-thin layers  331  and  331 ′ each have a thickness between 2 Å and 10 Å, and a growing temperature between 400° C. and 1000° C. The quantum-well layer  332  has a thickness between 5 Å and 100 Å. Even though the quantum-well layer and the barrier layers are all made of aluminum-gallium-indium-nitrides, their compositions are not required to be identical. That is, the (x, y), (p, q), (m, n), (i, j) parameters in the foregoing molecular formulas are not necessarily the same. 
       FIGS. 4(   a ) and  4 ( b ) are schematic diagrams showing the GaN-based LED structures according to a third embodiment of the present invention. The third embodiment and the previous two embodiments of the present invention actually have identical structures. The difference lies in the materials used for the respective intermediate layers. As shown in  FIG. 4(   a ), the intermediate layer  43  is a supper lattice well layer further containing at least an InN ultra-thin monolayer  431  and a GaN ultra-thin monolayer  432 . Within the intermediate layer  43 , the monolayers are sequentially stacked and interleaved with each other. For one example, from the lower barrier layer  42  up, there are InN ultra-thin monolayer  431 , GaN ultra-thin monolayer  432 , then another InN ultra-thin monolayer  431 ′, and then another GaN ultra-thin monolayer  432 ′, and so on. For another example, from the lower barrier layer  42  up, there are GaN ultra-thin monolayer  432 , InN ultra-thin monolayer  431 , then another GaN ultra-thin monolayer  432 ′, and then another InN ultra-thin monolayer  431 ′, and so on. The monolayers each have a thickness between 2 Å and 20 Å, and a growing temperature between 400° C. and 1000° C. Within the intermediate layer  43 , there are at least one InN ultra-thin monolayer  431  and one GaN ultra-thin monolayer  432 , making the total number of monolayers at least two. On the other hand, within the intermediate layer  43 , there are at most five InN ultra-thin monolayers  431  and five GaN ultra-thin monolayers  432 , making the total number of monolayers at most ten. 
     As shown in  FIG. 4(   b ), when there are multiple intermediate layers, every two immediately adjacent intermediate layers  43  and  43 ′ must have an intermediate barrier layer  48  made of un-doped Al 1-i-j Ga i In j N (0≦i, j≦1, i+j≦1) interposed therebetween. 
     The upper, intermediate, and lower barrier layers  44 ,  48 , and  42  each have a total thickness between 5 Å and 300 Å, and a growing temperature between 400° C. and 1000° C. Even though the barrier layers are all made of aluminum-gallium-indium-nitrides, their compositions are not required to be identical. That is, the (x, y), (p, q), (i, j) parameters in the foregoing molecular formulas are not necessarily the same. 
       FIGS. 5(   a ) and  5 ( b ) are schematic diagrams showing the GaN-based LED structures according to the fourth embodiment of the present invention. The fourth embodiment and the third embodiment of the present invention actually have identical structures. The difference lies in the materials used for the upper, intermediate, and lower barrier layers. As shown in  FIGS. 5(   a ) and  5 ( b ), the intermediate layer  53  has a structure and materials identical to the intermediate layer  43  of the third embodiment, and each barrier layer  52 ,  54 ,  58  also has a structure identical to that of the intermediate layer  43  but with different materials. Specifically, each barrier layer is a supper lattice barrier layer further containing at least an In-doped, AlN ultra-thin monolayer  521  and an In-doped, GaN ultra-thin monolayer  522 . Within each the barrier layer, the monolayers are sequentially stacked and interleaved with each other, similar to the intermediate layer  43 . The monolayers each have a thickness between 2 Å and 20 Å, and a growing temperature between 400° C. and 1000° C. Within each barrier layer, there are at least one AlN ultra-thin monolayer  521  and one GaN ultra-thin monolayer  522 , making the total number of monolayers at least two. On the other hand, within each barrier layer, there are at most five AlN ultra-thin monolayer  521  and five GaN ultra-thin monolayer  522 , making the total number of monolayers at most ten. The upper, intermediate, and lower barrier layers  54 ,  58 , and  52  may contain different numbers of monolayers respectively. However, the barrier layers each have a thickness between 5 Å and 300 Å, and a growing temperature between 400° C. and 1000° C.  FIG. 5(   b ) shows that there are multiple intermediate layers, every two immediately adjacent intermediate layers  53  and  53 ′ must have an intermediate barrier layer  58  interposed therebetween. 
     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.