Abstract:
A nitride light-emitting diode is provided including a current spreading layer. The current spreading layer includes a first layer having a plurality of distributed insulating portions configured to have electrical current flow therebetween; and a second layer including interlaced at least one substantially undoped nitride semiconductor layer and at least one n-type nitride semiconductor layer configured to spread laterally the electrical current from the first layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of PCT/CN2011/083620 filed on Dec. 7, 2011, which claims priority to Chinese Patent Application No. 201010616969.3 filed on Dec. 31, 2010. The disclosures of the above applications are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    Developments in semiconductor lighting technologies, particularly with respect to the epitaxy and processing technologies of nitride semiconductor materials, have led to improved light emission efficiency of light-emitting diodes (LEDs). 
       SUMMARY 
       [0003]    In an aspect, a nitride LED is provided including: a current spreading layer including: a first layer comprising a plurality of distributed insulating portions configured to have electrical current flow therebetween; and a second layer comprising interlaced at least one substantially undoped nitride semiconductor layer and at least one n-type nitride semiconductor layer configured to spread laterally the electrical current from the first layer. 
         [0004]    In some implementations, the distributed insulating portions are separated by predetermined intervals. 
         [0005]    In some implementations, the distributed insulating portions are formed by ion implantation. 
         [0006]    In some implementations, the current spreading layer further includes a gradient-doped n-type nitride semiconductor layer disposed between the first and second layers and configured to repair defects in the first layer caused by the ion implantation. 
         [0007]    In some implementations, the current spreading layer has a thickness of about 1000 Å˜20000 Å. 
         [0008]    In some implementations, the gradient-doped n-type nitride semiconductor layer has a thickness of about 200 Å˜5000 Å. 
         [0009]    In some implementations, the gradient-doped n-type nitride semiconductor layer is formed by secondary growth of epitaxy, and has a silicon doping concentration that gradually changes from about 1×10 17  cm −3  to about 5×10 19  cm −3 . 
         [0010]    In some implementations, the gradient-doped n-type nitride semiconductor layer is formed by secondary growth of epitaxy, and has a silicon doping concentration that gradually changes from about 5×10 18  cm −3  to about 1×10 18  cm −3 . 
         [0011]    In some implementations, the second layer has a thickness of about 700 Å˜10,000 Å, the substantially undoped nitride semiconductor layer and the n-type nitride semiconductor layer have a thickness ratio of &gt;0.8, and a period of the interlacing layers is 1-20. 
         [0012]    In some implementations, the second layer has a thickness of about 1800 Å˜3600 Å, the thickness ratio is 5:1, and the period is 3. 
         [0013]    In some implementations, the at least one substantially undoped nitride semiconductor layer has a silicon doping concentration of less than about 5×10 17  cm −3 , the at least one n-type nitride semiconductor layer has a silicon doping concentration greater than about 1×10 18  cm −3 . 
         [0014]    In some implementations, the first layer has a thickness of about 100 Å˜5000 Å. 
         [0015]    In some implementations, the LED further includes: a sapphire substrate; an n-side layer; a p-side layer; and a nitride semiconductor light-emitting layer disposed between the n-side layer and the p-side layer; wherein the n-side layer comprises: a buffer layer; and an n-layer, wherein the current spreading layer is formed in the n-layer, and wherein the current spreading layer is coupled to the light-emitting layer. 
         [0016]    In another aspect, a method of making an LED is provided, the method including: forming a current spreading layer by: forming a first layer including a plurality of distributed insulating portions to allow electrical current to flow therebetween; and interlacing at least one substantially undoped nitride semiconductor layer and at least one n-type nitride semiconductor layer to form a second layer, wherein the second layer is configured to spread laterally the electrical current received from the first layer. 
         [0017]    In some implementations, said forming a first layer comprises forming the distributed insulating portions in an n-layer by ion implantation. 
         [0018]    In some implementations, the method further includes forming a gradient-doped n-type nitride semiconductor layer between the first and second layers and to repair defects in the first layer caused by the ion implantation. 
         [0019]    In another aspect, a light-emitting system including a plurality of nitride LEDs is provided, each LED including: a current spreading layer including: a first layer comprising a plurality of distributed insulating portions configured to have electrical current flow therebetween; and a second layer comprising interlaced at least one substantially undoped nitride semiconductor layer and at least one n-type nitride semiconductor layer configured to spread laterally the electrical current from the first layer. 
         [0020]    In some implementations, the distributed insulating portions are separated by predetermined intervals. 
         [0021]    In some implementations, the distributed insulating portions are formed by ion implantation. 
         [0022]    In some implementations, the current spreading layer further includes a gradient-doped n-type nitride semiconductor layer disposed between the first and second layers and configured to repair defects in the first layer caused by the ion implantation and to guide electrical current received from the first layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  illustrates a structure of a conventional nitride LED and example current paths therein. 
           [0024]      FIG. 2  illustrates a profile of a nitride LED according to some of the disclosed implementations. 
           [0025]      FIG. 3  is a schematic illustration of a current spreading in the nitride LED according to some of the disclosed implementations. 
           [0026]      FIG. 4  illustrates a profile of a nitride LED according to some other implementations. 
           [0027]      FIG. 5  is a schematic illustration of a current spreading in the nitride LED of  FIG. 4 . 
           [0028]      FIG. 6  illustrates light output power of an example LED according to some of the disclosed implementations as compared with a conventional LED. 
           [0029]      FIG. 7  illustrates a pass rate vs. static breakdown voltage of an LED according to some of the disclosed implementations as compared with a conventional LED. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  illustrates the structure of a conventional nitride LED and example current path therein. In this structure, over the sapphire substrate  100 , a plurality of layers are epitaxially grown. These layers include a buffer layer  101 , an n-type nitride semiconductor layer  102 , a light-emitting layer  104 , a p-type cladding layer  105 , a p-type nitride semiconductor layer  106 , a p-type contact layer  107  formed over the p-type nitride semiconductor layer  106 , a p-electrode  108  formed over the p-type contact layer  107 , and an n-electrode  109  formed over the n-type nitride semiconductor layer  102 . 
         [0031]    Because the electrical current tends to flow in a shorter path between the n electrode  109  and the p electrode  108 , the current density may become overly high in some portions of the LED. This may cause current crowding, and thus limit the light output efficiency. 
         [0032]    According to some of the disclosed implementations, a compound dual current spreading layer may be provided in a nitride LED to substantially improve the light output efficiency of the LED. 
         [0033]    In an example, the LED with the compound dual current spreading layer may include: a sapphire substrate; an n-side layer and a p-side layer formed with nitride semiconductor; and a light-emitting layer between the n-side layer and the p-side layer. 
         [0034]    The n-side layer may include a plurality of layers, including a buffer layer, an n-type nitride semiconductor layer, and the compound dual current spreading layer. The compound dual current spreading layer may include a first current spreading layer and a second current spreading layer. The first current spreading layer may be a distribution insulating layer formed in the n-type nitride semiconductor layer. The second current spreading layer may be formed by an interlacing u-type nitride semiconductor layer and an n-type nitride semiconductor layer. The compound dual current spreading layer may connect respectively to the n-type nitride semiconductor layer and the active layer. 
         [0035]    In some implementations, the distribution insulating layer may include insulating portions separated by predetermined intervals. 
         [0036]    In some implementations, the distribution insulating layer may be formed by ion implantation. 
         [0037]    In some implementations, a gradient-type silicon doped n-type nitride semiconductor layer may be further included between the first current spreading layer and the second current spreading layer. 
         [0038]    In some implementations, the compound dual current spreading layer has a thickness of about 1000 Å˜20000 Å. 
         [0039]    In some implementations, the first current spreading layer has a thickness of about 100 Å˜5000 Å. 
         [0040]    In some implementations, the gradient-type silicon doped n-type nitride semiconductor layer has a thickness of about 200 Å˜5000 Å. 
         [0041]    In some implementations, the gradient-type silicon doped n-type nitride semiconductor layer is formed by secondary growth of epitaxy, wherein the silicon doping concentration gradually changes from about 1×10 17  cm −3  to about 5×10 19  cm −3 . 
         [0042]    In some implementations, the gradient-type silicon doped n-type nitride semiconductor layer is formed by secondary growth of epitaxy, wherein the silicon doping concentration gradually changes from about 5×10 18  cm −3  to about 1×10 18  cm −3 . 
         [0043]    In some implementations, the second current spreading layer has a thickness of about 700 Å˜10,000 Å, the u-type nitride semiconductor layer and the n-type nitride semiconductor layer has a thickness ratio of &gt;0.8, and the period of the stacked layers is about 1-20. 
         [0044]    In some implementations, the second current spreading layer has a thickness of about 1800 Å˜3600 Å, the u-type nitride semiconductor layer and the n-type nitride semiconductor layer has a thickness ratio of about 5:1, and the period of stacked layers is about 3. 
         [0045]    In some implementations, in the second current spreading layer, the silicon doping concentration in the u-type nitride semiconductor layer is less than about 5×10 17  cm −3 , the silicon doping concentration in the n-type nitride semiconductor layer is greater than about 1×10 18  cm −3 . 
         [0046]    The “u-type nitride semiconductor” may generally refer to a low-doped nitride semiconductor. For example, the doping concentration may be less than 5×10 17  cm −3 . 
         [0047]    In some embodiments, the first current spreading layer in the compound dual current spreading layer forms an insulation layer in the n-type nitride semiconductor layer, and can force a uniform current distribution, forming a uniformly distributed point-like current source (see, e.g.,  FIG. 3  and  FIG. 5 ). The second current spreading layer may be formed by interlacing u-type nitride semiconductor layer and n-type nitride semiconductor layers. The second current spreading layer may, for the uniformly distributed point-like electrical current sources formed by the first current spreading layer, through the interlaced u-type layer and n-type layer, force a horizontal expansion of the point-like current sources. As such, the current may extend to the entire light-emitting area with improved uniformity, and current crowding may be reduced. 
         [0048]    In some implementations, a gradient-doped n-type layer is added between the first current spreading layer and the second current spreading layer. This gradient-doped n-type layer may, through its gradient in the silicon doping concentration, repair the surface defects caused by the first current spreading layer, particularly the formation of the distributed insulating portions. As such, the quality of the nitride semiconductor layer lattice after the secondary epitaxy may be improved. In addition, the gradient-doped n-type layer may serve as a current guide layer for the second current spreading layer. 
         [0049]    Advantages of the disclosed implementations may include, for example, that the current spreading layer can distribute the current to the entire light-emitting area substantially uniformly, reduce current crowding, and thus can effectively improve the LED&#39;s light output efficiency. In addition, the static breakdown voltage may be increased. 
         [0050]      FIG. 2  illustrates an LED structure according to some implementations. The structure may include a plurality of layers stacked over the sapphire substrate  100 . For example, a buffer layer  101  may be included, which may comprise GaN, AlN, or GaAlN, and have a thickness of about 200 Å˜500 Å. 
         [0051]    An n-type nitride semiconductor layer  102  may be included, which may comprise Si-doped GaN, and have a thickness of about 20,000 Å to 40,000 Å. 
         [0052]    A compound dual current spreading layer  103  may include a first current spreading layer and a second current spreading layer, and have a thickness of about 1000 Å˜20,000 Å. The first current spreading layer  103   a  may be a distributed insulating layer formed by ion implantation in the n-type nitride semiconductor layer  102 . The distributed insulating layer may comprise insulation portions, which may be different in sizes, or substantially in size. These portions may be separated by predetermined intervals, which may be periodic in the lateral direction, or may be uneven or non-uniform intervals. 
         [0053]    The second current spreading layer  103   c  may be formed by interlacing undoped u-type nitride semiconductor layers and n-type nitride semiconductor layers. The u-type nitride semiconductor layer may have a silicon doping concentration of, for example, about 5×10 16  cm −3 . The n-type nitride semiconductor layer may have a silicon doping concentration of for example, about 1×10 19  cm 3 . The u-type nitride semiconductor layer and the n-type nitride semiconductor layer may have a thickness ratio of for example, about 5:1. The period of the stacked u-type/n-type layers may be, for example, 3. 
         [0054]    A multi-quantum well structure light-emitting layer  104  may have an InGaN layer as a well layer, and a GaN layer as a barrier layer. The well layer may have a thickness of about 18 Å˜30 Å, for example, and the barrier layer may have a thickness of 80 Å˜200 Å, for example. 
         [0055]    A p-type cladding layer  105  may comprise AlInGaN doped with Mg, and have a thickness of about 100 Å˜600 Å, for example. 
         [0056]    The p-type layer  106  and the p-type contact layer  107  may be formed by GaN, InGaN, or another GaN-based material. The thickness of the p-type layer  106  may be about 1000 Å˜3000 Å, for example, and the thickness of the p-type contact layer  107  may be about 50 Å to 200 Å, for example. 
         [0057]    The p-side and n-side electrodes may be formed as illustrated in  FIG. 2 . In a corner of the LED, a portion from the p-type contact layer  107  to the n-type nitride semiconductor layer  102  may be removed by etching. The n-type nitride semiconductor layer  102  may be exposed, and n Ohmic electrode  109  may be formed on the exposed n-type nitride semiconductor layer  102 . In addition, the p Ohmic electrode  108  may be formed over almost the entire surface of the p-type contact layer  107 , and the pad  110  may be formed over a portion of the p Ohmic electrode  108 . 
         [0058]    In the example illustrated in  FIG. 2 , the first current spreading layer  103   a  may form a distributed insulating layer in the n-type nitride semiconductor layer through ion implantation. As such, the electrical current may be forced to be more evenly distributed. This may be viewed as forming a plurality of laterally distributed point-like current sources (see, e.g.,  FIG. 3 ). 
         [0059]    The second current spreading layer  103   c  in the current spreading layer  103  may be formed by interlacing a plurality of u-type nitride semiconductor layers and n-type nitride semiconductor layers. Through the interlaced u-type layers and n-type layers, the point-like current sources formed by the first current spreading layer may be further forced to have a two-dimensional lateral expansion. Therefore, the current can be more evenly distributed to substantially the entire light-emitting area. 
         [0060]    Variations in the design parameters may be implemented to achieve improved results. For example, the shapes, sizes, and distribution densities of the insulation portions (e.g., islands) of the first current spreading layer may be varied, and the insulation portions may have non-even distributions in sizes, locations, distributions, shapes, etc., or may be substantially uniform. 
         [0061]    In the second current spreading layer, the thickness ratio of the u-type layer and the n-type layers, the number of layer cycles (e.g., the period), the doping concentrations, etc. may also be varied, and may be varied according to the parameters of the first current spreading layer. For example, if the distribution density of insulation portions in the first current spreading layer is high, then the number of stacked layer cycle needed for the second current spreading layer may be smaller. Conversely, if the distribution density of the first current spreading layer insulation layer is low, then the number of stacked layer cycles needed for the second current spreading layer may be larger. 
         [0062]    This can help the current to be more evenly distributed to almost the entire light-emitting area, and thus can effectively improve the light output efficiency of the LED, and increase its electrostatic breakdown voltage. For example, an improved LED may be 10%˜20% more luminous than a conventional LED. This may be attributed to the current spreading, as illustrated in  FIG. 3 , wherein illustrative current paths may be compared with those of  FIG. 1  (without a current spreading layer). 
         [0063]      FIG. 4  shows a cross-sectional view of the structure of the nitride LED according to some other implementations. As compared with the structure illustrated in  FIG. 2 , the current spreading layer has an additional gradient-doped n-type layer  103   b  between the first current spreading layer  103   a  and the second current spreading layer  103   c . The gradient-doped n-type layer  103   b  may have a thickness of about 200 Å˜5000 Å. This layer may comprise an n-type nitride semiconductor layer, in which the silicon doping concentration gradually changes from a low doping level (e.g., about 1×10 17  cm −3 ) to a high doping level (e.g., about 1×10 19  cm −3 ). This layer may be formed by a secondary epitaxy process. 
         [0064]    Through the gradient doping, the surface defects caused by ion implantation in the first current spreading layer may be repaired. Therefore, the quality of the nitride semiconductor layer lattice may be substantially maintained or improved even after the secondary epitaxy process. In addition, gradient-doped n-type layer  103   b  can also serve as a current guide layer for the second current spreading layer. 
         [0065]    In some experiments, two types of samples are fabricated based on the disclosed embodiments and the conventional techniques, respectively (i.e. with or without the current spreading layer), to evaluate their light output power and electrostatic breakdown voltage characteristics. 
         [0066]    Some parameters of the two samples are listed in Table 1, and some measurement results are illustrated in  FIGS. 6 and 7 . 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Thickness (Å) and structure 
                   
               
               
                   
                 of the various layers in the 
                 Thickness (Å) and structure 
               
               
                 Semiconductor 
                 LED according to the 
                 of the various layers of a 
               
               
                 layer 
                 disclosed embodiments 
                 conventional LED 
               
               
                   
               
             
             
               
                 Buffer layer 
                  300 
                  300 
               
               
                 101 
               
               
                 n-type 102 
                 25000 
                 25000 
               
               
                 Compound 
                 Thickness of 103a: 1500; 
                 None 
               
               
                 dual current 
                 Thickness of 103b: 1500; 
               
               
                 spreading 
                 Thickness of 103c: 3500; 
               
               
                 layer 103 
                 (total thickness of layer 
               
               
                   
                 103: 6500) 
               
               
                 Light-emitting 
                 GaN(140)/InGaN(25) × 10 
                 GaN(140)/InGaN(25) × 10 
               
               
                 layer 104 
                 periods (the last one being 
                 periods (the last one being 
               
               
                   
                 the GaN layer) 
                 the GaN layer) 
               
               
                 p-type limiting 
                  600 
                  600 
               
               
                 layer 105 
               
               
                 p-type 
                  2000 
                  2000 
               
               
                 layer 106 
               
               
                 p-type contact 
                  100 
                  100 
               
               
                 layer 10 
               
               
                   
               
             
          
         
       
     
         [0067]    As shown in  FIG. 6 , the light output power  601  of the LED according to the disclosed embodiments is about 20% higher than the light output power  602  of the conventional LED of comparison. 
         [0068]    As shown in  FIG. 7 , in the form of a pass rate vs. the static breakdown voltage, the LEDs according to the disclosed embodiments can have a higher electrostatic breakdown voltage than that of the conventional LEDs of comparison, as illustrated in the curve  701  for the LEDs according to the disclosed embodiments as compared with the curve  702  for conventional LEDs. 
         [0069]    Although specific embodiments have been described above in detail, the description is merely for purposes of illustration. It should be appreciated, therefore, that many aspects described above are not intended as required or essential elements unless explicitly stated otherwise. Various modifications of and equivalent acts corresponding to, the disclosed aspects of the exemplary embodiments, in addition to those described above, can be made by a person of ordinary skill in the art, having the benefit of the present disclosure, without departing from the spirit and scope of the disclosure defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent structures.