Patent Publication Number: US-2013228741-A1

Title: Light emitting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 101106753, filed on Mar. 1, 2012. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     1. Technical Field 
     The technical field relates to a light emitting diode (LED), and more particularly to an LED capable of enhancing the luminous efficiency. 
     2. Related Art 
     A light emitting diode (LED) is a semiconductor device constituted mainly by group III-V compound semiconductor materials, for example. Since such semiconductor materials have a characteristic of converting electricity into light, when a current is applied to the semiconductor materials, electrons therein would be combined with holes and release excessive energy in a form of light, thereby achieving an effect of luminosity. 
     Generally speaking, since the lattice mismatch between gallium nitride (GaN) and sapphire substrate is approximately 16%, a large quantity of defects are generated at the lattice interface, and thus causing a drastic decay in the light emitting intensity. The amount of defects is unavoidable during the growth process of LED. However, when the emitted wavelength of light from the LED is 450 nm, it is conventionally known that lattice stress is released around the defects and forms self-assembled indium-riched regions. Therefore, when carriers move to the defects, the carriers are likely to capture by the self-assembled indium-riched regions, thus forming the so-called localized effect. Since the quantum confinement effect of the self-assembled indium-riched regions is capable of increasing the carrier recombination efficiency, therefore, even though the GaN LED is limited by the high defect density, a certain degree of luminous efficiency is still maintained at the 450 nm wavelength of light. 
     However, when the emission wavelength of the LED gradually shifts from blue to the ultraviolet wavelengths of light, due to the concentration of indium decreasing gradually in the active layer, the self-assembled indium-riched regions are also correspondingly lessened. Consequently, the carriers in the LED are likely to move to the defect areas, thereby drastically decreasing the luminous efficiency of the LED at the ultraviolet wavelengths. Therefore, many people try to enhance the luminous efficiency for the ultraviolet LEDs. 
     SUMMARY 
     A light emitting diode (LED) is provided in the disclosure. By having the layer number of the quantum barrier layers doped with n-type dopants satisfying a specific proportion, the luminous efficiency of the LED at the 222 nm-405 nm wavelength range can be enhanced. 
     Another LED is provided in the disclosure. By having the lowest doping concentration at the quantum barrier layer doped with n-type dopants that is closest to the p-type semiconductor, the luminous efficiency of the LED at the 222 nm-405 nm wavelength range can be enhanced. 
     An LED is provided in the disclosure. By having the doping concentrations of the quantum barrier layers doped with n-type dopants satisfying a specific relationship, the luminous efficiency of the LED at the 222 nm-405 nm wavelength range can be enhanced. 
     The disclosure provides an LED, including a substrate, a n-type semiconductor layer, an active layer, a p-type semiconductor layer, a first electrode, and a second electrode. The n-type semiconductor layer is disposed on the substrate. The active layer has an active region with a defect density DD, in which DD≧2×10 7 /cm 3 . The active layer is disposed on a portion of the n-type semiconductor layer, and a wavelength of light emitted by the active layer is 222 nm≦λ≦405 nm. The active layer includes i quantum barrier layers and (i−1) quantum wells. Each of the quantum wells is disposed between any two quantum barrier layers, and i is a natural number greater than or equal to 2, in which a n-type dopant is doped in at least k layers of the quantum barrier layers, k being a natural number greater than or equal to 1, when i is an even number, k≧i/2, and when i is an odd number, k≧(i−1)/2. The p-type semiconductor layer is disposed on the active layer. The first electrode is disposed on a portion of the n-type semiconductor layer, and the second electrode is disposed on a portion of the p-type semiconductor layer. 
     The disclosure provides another LED, including a substrate, a n-type semiconductor layer, an active layer, a p-type semiconductor layer, a first electrode, and a second electrode. The n-type semiconductor layer is disposed on the substrate. The active layer has an active region with a defect density DD, in which DD≧2×10 7 /cm 3 . The active layer is disposed on a portion of the n-type semiconductor layer, and a wavelength λ of light emitted by the active layer is 222 nm≦λ≦405 nm. The active layer includes i quantum barrier layers and (i−1) quantum wells. Each of the quantum wells is disposed between any two quantum barrier layers, and i is a natural number greater than or equal to 2, in which a n-type dopant is doped in at least k layers of the quantum barrier layers, k being a natural number greater than or equal to 1, when i is an even number, k≧i/2, and when i is an odd number, k≧(i−1)/2. The p-type semiconductor layer is disposed on the active layer, and a doping concentration of the quantum barrier layer in the k quantum barrier layers nearest to the p-type semiconductor layer is less than or equal to the doping concentration of the other quantum barrier layers in the k quantum barrier layers. The first electrode is disposed on a portion of the n-type semiconductor layer, and the second electrode is disposed on a portion of the p-type semiconductor layer. 
     The disclosure provides another LED, including a substrate, a n-type semiconductor layer, an active layer, a p-type semiconductor layer, a first electrode, and a second electrode. The active layer has an active region with a defect density DD, in which DD≧2×10 7 /cm 3 . The n-type semiconductor layer is disposed on the substrate. The active layer is disposed on a portion of the n-type semiconductor layer, and a wavelength λ of light emitted by the active layer is 222 nm≦λ≦405 nm. The active layer includes i quantum barrier layers and (i−1) quantum wells. Each of the quantum wells is disposed between any two quantum barrier layers, and i is a natural number greater than or equal to 2, in which a n-type dopant is doped in at least k layers of the quantum barrier layers, k being a natural number greater than or equal to 1, when i is an even number, k≧i/2, when i is an odd number, k≧(i−1)/2, and a doping concentration of the k quantum barrier layers is from 5×10 17 /cm 3  to 1×10 19 /cm 3 . The p-type semiconductor layer is disposed on the active layer. The first electrode is disposed on a portion of the n-type semiconductor layer, and the second electrode is disposed on a portion of the p-type semiconductor layer. 
     In summary, in the LED according to the embodiments of the disclosure, by having a number of quantum barrier layers of the active layer doped with n-type dopants, in which the layer number of the doped quantum barrier layers satisfies a specific relationship, or by having the lowest doping concentration at the quantum barrier layer doped with n-type dopants closest to the p-type semiconductor, or by having the doping concentrations of the quantum barrier layers doped with n-type dopants satisfying a specific relationship, the n-type dopants can compensate for the effect which defects have on the carriers. Accordingly, the carrier recombination rate of the LED can be enhanced. Therefore, by employing any one of the afore-described techniques, the luminous efficiency of the LED in the disclosure can be drastically increased at the 222 nm-405 nm wavelength range. 
     Several exemplary embodiments accompanied with figures are described in detail below to further describe the disclosure in details. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings constituting a part of this specification are incorporated herein to provide a further understanding of the disclosure. Here, the drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
         FIG. 1  is a schematic cross-sectional view of an LED according to an exemplary embodiment. 
         FIG. 2A  is a schematic cross-sectional view of a single quantum well active layer in an LED according to an exemplary embodiment. 
         FIG. 2B  is a schematic cross-sectional view of a multi-quantum well active layer in an LED according to an exemplary embodiment. 
         FIG. 3  is an enlarged schematic cross-sectional view of an active layer in an LED according to an exemplary embodiment. 
         FIG. 4A  depicts a comparative example of LEDs according to an exemplary embodiment. 
         FIG. 4B  depicts a comparative example of LEDs according to an exemplary embodiment. 
         FIGS. 5A-5D  respectively represents simulation diagrams of the LED electron concentration when the number of doped quantum barrier layers in  FIG. 3  is adjusted. 
         FIGS. 6A-6D  respectively represents simulation diagrams of the LED hole concentration when the number of doped quantum barrier layers in  FIG. 3  is adjusted. 
         FIGS. 7A-7D  respectively represents simulation diagrams of the LED electron-hole recombination rate when the number of doped quantum barrier layers in  FIG. 3  is adjusted. 
         FIGS. 8A-8D  respectively represents simulation diagrams of the LED non-radiative recombination rate when the number of doped quantum barrier layers in  FIG. 3  is adjusted. 
         FIG. 9A  is a relational diagram depicting the impact different number of doped layers in the quantum barrier layers of an LED has on the current-output power curve. 
         FIG. 9B  is a relational diagram depicting the impact different number of doped layers in the quantum barrier layers of an LED has on the current-voltage curve. 
         FIG. 10A  is a relational diagram depicting the impact different doping concentrations in the quantum barrier layers of an LED has on the current-output power curve. 
         FIG. 10B  is a relational diagram depicting the impact different doping concentrations in the quantum barrier layers of an LED has on the current-voltage curve. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a schematic cross-sectional view of an LED according to an exemplary embodiment. 
     Referring to  FIG. 1 , an LED  200  includes a substrate  210 , a n-type semiconductor layer  220 , an active layer  230 , a p-type semiconductor layer  240 , a first electrode  250 , and a second electrode  260 . The substrate  210  is, for example, a sapphire substrate. Specifically, the stacking layers of a nitride semiconductor capping layer  212  (e.g. un-doped GaN), a n-type semiconductor layer  220 , an active layer  230 , the active layer  230  and the p-type semiconductor layer  240  are formed in sequence on a surface of the sapphire substrate  210 . The active layer  230  is disposed between the n-type semiconductor layer  220  and the p-type semiconductor layer  240 . The n-type semiconductor layer  220  may include the stacking layers of a first n-type doped GaN layer  222  and a second n-type doped GaN layer  224  disposed sequentially on the nitride semiconductor capping layer  212 . The p-type semiconductor layer  240  may include the stacking layers of a first p-type doped GaN layer  242  and a second p-type doped GaN layer  244  disposed sequentially on the active layer  230 . Moreover, a difference between the first n-type doped GaN layer  222  and the second n-type GaN layer  224 , or a difference between the first p-type doped GaN layer  242  and the second p-type doped GaN layer  244  may be in thickness or in doping concentration. Furthermore, a material of the n-type semiconductor layer  220  and the p-type semiconductor layer  240  may be AlGaN, for example. According to requirements in practice, people skilled in the art may select the thickness, doping concentration, and the aluminum concentration for growth of the nitride semiconductor capping layer  212 , the first n/p-type doped GaN layers  222  and  242 , the second n/p-type doped GaN layers  224  and  244  grown, although the disclosure is not limited thereto. 
     Specifically, as shown in  FIG. 1 , the nitride semiconductor capping layer  212  (e.g. un-doped GaN), the first n-type doped GaN layer  222  and the second n-type doped GaN layer  224 , the active layer  230 , the first p-type doped AlGaN layer  242  and the second p-type doped GaN layer  244  are formed in sequence on the sapphire substrate  210 . Moreover, the first electrode  250  and the second electrode  260  are respectively formed on a portion of the second n-type doped GaN layer  224  and the second p-type doped GaN layer  244 , so that the first electrode  250  is electrically connected to the n-type semiconductor layer  220 , and the second electrode  260  is electrically connected to the p-type semiconductor layer  240 . It should be appreciated that, a nitride buffer layer may also be added between the sapphire substrate and the n-type semiconductor, although the disclosure is not limited thereto. 
     The composition of the active layer  230  may be as shown in  FIGS. 2A and 2B , with a single quantum well active layer  230 A or a multi-quantum well active layer  230 B.  FIG. 2A  is a schematic cross-sectional view of a single quantum well active layer in an LED according to an exemplary embodiment, and  FIG. 2B  is a schematic cross-sectional view of a multi-quantum well active layer in an LED according to an exemplary embodiment. Generally speaking, the active layer includes i quantum barrier layers and (i−1) quantum wells. Moreover, each of the quantum wells is disposed between any two quantum barrier layers, and i is a natural number greater than or equal to 2. For example, as shown in  FIG. 2A , the single quantum well active layer  230 A may be formed by two quantum barrier layers  232  and a quantum well  234  sandwiched therebetween, constituting quantum barrier layer  232 /quantum well  234 /quantum barrier layer  232 . Taking the LED  200  with an emitted wavelength of 222 nm-405 nm as an example, a material of the quantum barrier layers  232  may be Al x In y Ga 1-x-y N, in which 0≦x≦1, 0≦y≦0.3, and x+y&lt;1. Moreover, a material of the quantum well  234  may be Al m In n Ga 1-m-n N, in which 0≦m&lt;1, 0≦n≦0.5, m+n≦1, x&gt;m, and n≧y. According to requirements in practice such as different emitted wavelengths, people skilled in the art may select the concentrations of m and n, or x and y for growth, although the disclosure is not limited thereto. 
     On the other hand, the composition of the active layer may be as shown by the multi-quantum well active layer  230 B in  FIG. 2B . As shown in  FIG. 2B , the multi-quantum well active layer  230 B may be formed by at least two pairs of stacking quantum barrier layers  232  and the quantum wells  234 , for example as depicted by the repeating three pairs of the stacking quantum barrier layer  232 /quantum well  234 . 
     It should be noted that, in the LED  200  of the disclosure, a n-type dopant doping process is performed on the quantum barrier layers  232  in the active layer  230 , so as to adjust a layer number of doped quantum barrier layers  232  in the quantum barrier layers  232 , a doping concentration in the quantum barrier layers  232 , and a doping concentration distribution in different doped quantum barrier layers  232  in order to enhance the luminous efficiency of the LED  200  at the 222 nm-405 nm wavelengths. Specifically, although GaN growth techniques are limited by a certain amount of defect density inherent in fabrication, however, even when the active layer  230  in the LED  200  has a defect density on the order of 10 7 /cm 3 , the effect of the defect density in the active region on the carriers can be lowered by intentionally doping n-type dopants through adjusting the layer number and the doping concentrations of the doped quantum barrier layers  232 , thereby enhancing the luminous efficiency. Particularly, the enhancement effect is especially pronounced for the emitted light from the active layer  230  having a wavelength range from 222 nm to 405 nm. 
     The effects of the LED  200  in the disclosure are further illustrated with support from the experimental results described below. In the embodiments hereafter, silicon is used as the n-type dopant as an exemplary scope for implementation, although people skilled in the art may also use other elements in the same group IVA as silicon to implement the embodiments in the disclosure by substituting the silicon. 
       FIG. 3  is an enlarged schematic cross-sectional view of an active layer in an LED. As shown in  FIG. 3 , the active layer in the present embodiment includes six quantum barrier layers and five quantum wells, and each quantum well is sandwiched between any two quantum barrier layers. Counting from the n-type semiconductor side, the quantum barrier layers are, sequentially,  232   a ,  232   b ,  232   c ,  232   d ,  232   e , and  232   f . The quantum wells are, sequentially,  234   a ,  234   b ,  234   c ,  234   d , and  234   e , counting from the n-type semiconductor side. 
       FIG. 4A  is an optical simulation diagram of an LED comparison example according to an exemplary embodiment, and  FIG. 4B  is an optical simulation diagram of an LED according to an exemplary embodiment, in which the defect density in  FIGS. 4A and 4B  is set as 1×10 8 /cm 3 . Please refer first to  FIG. 4A ,  FIG. 4A  is a relational diagram between adjustments to the layer number of doped quantum barrier layers in the quantum barrier layers  232   a - 232   f  and the emission intensities of an emission wavelength around 450 nm for an LED according to an exemplary embodiment. Referring both to  FIGS. 3 and 4A , the horizontal axis represents the emission wavelength (unit: nm), and the vertical axis represents the emission intensity (unit: a.u.). Moreover, the numerals before and after the slanted line of the different lines A, B, C, and D respectively represents the layer numbers of doped/un-doped quantum barrier layers in the quantum barrier layers  232   a - 232   f . The layer numbers of the doped layers are counted from the n-type semiconductor layer  220  side. For example, 6/0 in the line A represents all six of the quantum barrier layers  232   a - 232   f  are doped. 4/2 in the line B represents four quantum barrier layers  232   a - 232   d  near the n-type semiconductor layer  220  side are doped quantum barrier layers, and two layers are un-doped quantum barrier layers  232   e - 232   f.  2/4 in the line C represents two quantum barrier layers  232   a - 232   b  near the n-type semiconductor layer  220  side are doped quantum barrier layers, and four layers are un-doped quantum barrier layers  232   c - 232   f . On the other hand, 0/6 in the line D represents all six of the quantum barrier layers  232   a - 232   f  are un-doped. As shown in  FIG. 4A , the results show that increasing the layer number of doped quantum barrier layers instead decreases the luminous efficiency of the LED around 450 nm. 
     By contrast, when the layer number of doped quantum barrier layers is increased, the emission intensity of the LED at the 222 nm-405 nm wavelength range can be effectively enhanced. Specifically,  FIG. 4B  is a relational diagram between adjustments to the layer number of doped quantum barrier layers in the quantum barrier layers and the emission intensities of an emission wavelength around 365 nm. In  FIG. 4B , the definitions of the horizontal axis, the vertical axis, and the lines are similar to  FIG. 4A , but  FIG. 4B  represents an emission wavelength range of 222 nm-405 nm having a main peak of around 365 nm. As shown in  FIG. 4B , the results show that increasing the layer number of doped quantum barrier layers  232  promotes the enhancement of the luminous efficiency of the LED at the 222 nm-405 nm wavelength range. 
     When the emission wavelength from the LED is near 450 nm, it can inferred from the results presented in  FIGS. 4A and 4B  that, due to the comparatively strong localized effect in the quantum wells, the carriers are not easily influenced by the defect density. Therefore, doping the quantum barrier layers with n-type dopants cannot effectively enhance the emission intensity near 450 nm. On the other hand, too much doping results in the carrier overflow phenomenon and thus lowers the emission intensity, as shown in  FIG. 4A . However, for the LED having an emission wavelength around 365 nm, the effect of doping the quantum barrier layers with n-type dopants has a completely inverse effect from the LED emitting near 450 nm. 
     Specifically, as shown in  FIG. 4B , when the emission wavelength range of the LED near the main peak of 365 nm is 222 nm-405 nm, due to the weakened localized effect in the quantum wells, the carriers experience comparatively stronger influence from the defect density, and therefore doping the available quantum barrier layers with n-type dopants (e.g. Si) helps compensate for the effect of the defect density on the carriers. In other words, the n-type dopants can also provide radiative recombination for the electrons, thereby effectively enhancing the luminous efficiency of the LED at the 222 nm-405 nm emission wavelength range. The n-type dopants referred here in the disclosure may be dopants from group IV capable of replacing the group III elements and provided from an external source. As shown in  FIG. 4B , the emission intensity of the emission wavelength range from 222 nm-405 nm increases as the layer number of the doped quantum barrier layers increases. The enhancement effect of the luminous efficiency is especially pronounced when a layer number k of the doped quantum barrier layers and a total number i of the quantum barrier layers satisfy the following formula: when i is an even number, k≧i/2; and when i is an odd number, k≧=(i−1)/2. 
     In order to further verify the deductions arrived at above,  FIGS. 5A-8D  respectively represents for an LED with an emission wavelength range of 222 nm-405 nm, simulation diagrams of the LED electron concentration, hole concentration, electron-hole recombination rate, and non-radiative recombination rate when the number of doped quantum barrier layers  232  in  FIG. 3  is adjusted. In  FIGS. 5A-8D , the horizontal axis represents a distance from a substrate surface (unit: nm), and the numerals before and after the slanted line in  FIGS. 5A-8D  respectively represents the layer numbers of doped quantum barrier layers and un-doped quantum barrier layers, with definitions thereof being the same as the lines A-D in  FIGS. 4A and 4B , and so further elaboration is omitted. 
     As shown by the electron concentration simulation diagrams from  FIGS. 5A-5D , when the layer number of the doped quantum barrier layers increases, the electron concentration thereof gradually increases. As shown by the hole concentration simulation diagrams from  FIGS. 6A-6D , when the layer number of the doped quantum barrier layers increases, the hole concentration thereof gradually decreases. Moreover, the overall hole concentration is highest when all of the quantum barrier layers are un-doped. As shown from the simulation diagrams of the electron-hole recombination rate in  FIGS. 7A-7D , although the overall hole distribution is more even when all of the quantum barrier layers are doped, theoretically the electron-hole recombination rate in  FIG. 7D  for the LED having all un-doped quantum barrier layers should comparatively high. However, as shown by the trend in  FIGS. 7A-7D , the highest electron-hole recombination rate occurs in  FIG. 7A  when all of the quantum barrier layers  232  are doped, and conversely, the lowest electron-hole recombination rate occurs when all of the quantum barrier layers  232  are un-doped. Therefore,  FIGS. 7A-7D  can also verify that n-type dopants can provide radiative recombination for electrons, and accordingly it can be deduced that the luminous efficiency of the LED with emission wavelength range of 222 nm-405 nm is effectively enhanced. Moreover, as shown by the simulation diagrams of the electron-hole non-radiative recombination rate in  FIGS. 8A-8D , the lowest non-radiative recombination rate occurs in  FIG. 8A  when all of the quantum barrier layers are doped, and the highest electron-hole non-radiative recombination rate occurs in  FIG. 8D  when all of the quantum barrier layers are un-doped. Combining the results from  FIGS. 7A-7D  and  FIGS. 8A-8D , doping the quantum barrier layers with n-type dopants can provide electrons to increase the electron-hole radiative recombination rate, thereby effectively enhancing the luminous efficiency. At the same time, the non-radiative recombination rate of electrons and holes which results in non-light emitting states such as heat is lowered, and this can also verify the deduced result that the n-type dopants are capable of enhancing the emission intensity of the LED at the 222 nm-405 nm emission wavelength range. 
     Table 1 records the emission intensity results under different currents of the LED having the active layer structure shown in  FIG. 3 . Table 1 also records the forward voltages which change with the layer numbers of the doped quantum barrier layers and the un-doped quantum barrier layers. In the experiments tabulated in  FIG. 1 , the doping concentrations C 1 , C 2 , . . . C k  are all 2×10 18 /cm 3 , for example. Moreover, in an embodiment where the emission wavelength is 365 nm, a material of the quantum wells is In c Ga 1-c N, in which 0≦c≦0.05, and a material of the quantum barrier layers is Al d Ga 1-d N, where d is between 0.13 to 0.30. In the present embodiment, a preferable aluminum concentration is 0.16-0.25, and a thickness of the quantum barrier layer is, for example, 5 nm-15 nm. The preferable thickness is 8 nm-12 nm in the present embodiment. Additionally, the results of Table 1 are illustrated in  FIGS. 9A and 9B .  FIG. 9A  is a relational diagram depicting the impact different number of doped layers in the quantum barrier layers of an LED has on the current-output power curve.  FIG. 9B  is a relational diagram depicting the impact different number of doped layers in the quantum barrier layers of an LED has on the current-voltage curve. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Total Quantum Barrier 
                   
                 Forward 
               
               
                   
                 (QB) Layers i = 6 
                 Output Power 
                 Voltage 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Doped 
                   
                 Doped 
                 (mW) 
                 (V) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 LED 
                 QB 
                 Un-Doped 
                 Concen- 
                 at 
                 at 
                 at 
               
               
                 200 
                 Layers k 
                 QB Layers 
                 tration 
                 350 mA 
                 700 mA 
                 350 mA 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A 
                 0 
                 6 
                 N.A. 
                 9.5 
                 23.2 
                 4.36 
               
               
                 B 
                 2 
                 4 
                 2 × 10 18   
                 10.6 
                 24.9 
                 4.29 
               
               
                 C 
                 4 
                 2 
                   
                 17.0 
                 36.3 
                 4.27 
               
               
                 D 
                 5 
                 1 
                   
                 24.2 
                 49.0 
                 4.13 
               
               
                 E 
                 6 
                 0 
                   
                 31.1 
                 58.4 
                 4.14 
               
               
                   
               
            
           
         
       
     
     As shown in the results of Table 1 and  FIG. 9A , the output powers of the LEDs  200 A- 200 E increase as the number of doped quantum barrier layers grow in the quantum barrier layers available. To be specific, firstly, when the quantum barrier layers are not doped with n-type dopants, the doping concentration thereof is 0, but the GaN material has a background doping concentration that is different according to different epitaxial techniques or different epitaxy quality. In the present embodiment, since the background doping concentration cannot be measured, therefore the un-doped concentration is represented by N.A. At this time when six layers of quantum barrier layers are all un-doped with n-type dopants (e.g. Si), the output power is 9.5 mW (LED  200 A). When two layers in the six layers of quantum barrier layers are doped with n-type dopants (e.g., purposely doping the two quantum barrier layers  232   a - 232   b  in the quantum barrier layer  232   a - 232   f  depicted in  FIG. 3  closest to the n-type semiconductor layer  220 ), the output power of the LED  200 B can be increased from the un-doped 9.5 mW to 10.6 mW. Preferably, when there are four doped quantum barrier layers  232  in the six quantum barrier layers  232  (e.g. purposely doping the four quantum barrier layers  232   a - 232   d  depicted in  FIG. 3  closest to the n-type semiconductor layer  220 ), the output power of the LED  200 C can be drastically increased from the un-doped 9.5 mW to 17.0 mW, which is a twofold enhancement. Therefore, when the layer number k of the doped quantum barrier layers  232  is greater than or equal to half of the total number of quantum barrier layers  232 , the luminous efficiency of the LED  200 C can be effectively increased. Moreover, when five of the quantum barrier layers are doped, the output power of the LED  200 D is 24.2 mW. When all of the quantum barrier layers  232  are doped (e.g., purposely doping all six quantum barrier layers  232   a - 232   f  in  FIG. 3 ), the output power of the LED  200 E can be increased to 31.1 mW, which is close to a threefold enhancement. 
     Furthermore, as shown by the results of Table 1 and  FIG. 9B , by doping n-type dopants in the quantum barrier layers, besides effectively increasing the luminous efficiency of the LED  200 A, the resistance value of the quantum barrier layers can be further lowered, thereby reducing the forward voltage of the LED. For example, a forward voltage of 4.36 V when all of the quantum barrier layers are un-doped is lowered to 4.14V when all of the quantum barrier layers are doped. The foregoing results represent that by increasing the number of doped layers in the quantum barrier layers, the effect defect density has on the luminous efficiency of the LED at the 222 nm-405 nm wavelength range (main peak at around 365 nm) can be compensated. In other words, the n-type dopants injected in the quantum barrier layers can effectively provide electrons for radiative recombination, and lower energy release from non-radiative recombination such as heat. Accordingly, the luminous efficiency can be effectively enhanced, and the foregoing experimental results have verified the simulation results shown in  FIGS. 5-8 . 
     Therefore, in light of the above, the LED in the disclosure has a number of quantum barrier layers of the active layer doped with n-type dopants, in which the layer number of the doped quantum barrier layers satisfies a specific proportion, and accordingly the luminous efficiency of the LED at the 222 nm-405 nm wavelength range is effectively enhanced. When the layer number k of the doped quantum barrier layers is greater than or equal to half of the total number i of quantum barrier layers, the luminous efficiency enhancement effect is specifically pronounced. Specifically, when i is an even number, k≧i/2; and when i is an odd number, k≧(i−1)/2. 
     In the disclosure below, the effect that the doping concentration of the n-type dopant in the quantum barrier layers has on the luminous efficiency of the LED at the 222 nm-405 nm wavelength range is further discussed. 
     Table 2 records experiments of an LED having the active layer structure as depicted in  FIG. 3 , and the four quantum barrier layers  232   a - 232   d  closest to the n-type semiconductor layer are fixedly doped. Therefore, the number of doped quantum barrier layers  232  in each experiment of Table 2 is four layers, and the quantum barrier layers  232   e - 232   f  closest to the p-type semiconductor layer are un-doped. Table 2 represents the impact different doping concentrations in the doped quantum barrier layers of the LED has on the emission intensity and the forward voltage performance. Additionally, the results of Table 2 are illustrated in  FIGS. 10A and 10B .  FIG. 10A  is a relational diagram depicting the impact different doping concentrations in the quantum barrier layers of an LED has on the current-output power curve.  FIG. 10B  is a relational diagram depicting the impact different doping concentrations in the quantum barrier layers of an LED has on the current-voltage curve. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Total QB Layers 
                   
                   
               
               
                   
                 232 i = 6 
                   
                 Forward 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Doping 
                 Output Power 
                 Voltage 
               
               
                   
                 Doped 
                   
                 Concen- 
                 (mW) 
                 (V) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 LED 
                 QB 
                 Un-Doped 
                 tration 
                 at 
                 at 
                 at 
               
               
                 200 
                 Layers k 
                 QB Layers 
                 (cm −3 ) 
                 350 mA 
                 700 mA 
                 350 mA 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 A 
                 0 
                 6 
                 N.A. 
                 9.5 
                 23.2 
                 4.36 
               
               
                 F 
                 4 
                 2 
                 ~8 × 10 17   
                 11.8 
                 26.1 
                 4.14 
               
               
                 G 
                   
                   
                 ~2 × 10 18   
                 17.0 
                 36.3 
                 4.27 
               
               
                 H 
                   
                   
                 ~4 × 10 18   
                 19.1 
                 38.9 
                 4.14 
               
               
                 I 
                   
                   
                 ~6 × 10 18   
                 21.5 
                 45.3 
                 4.09 
               
               
                   
               
            
           
         
       
     
     As shown in the results of Table 2 and  FIG. 10A  and by referring to  FIG. 3 , the output powers of the LED increases as the doping concentration grows. For example, as described earlier, when no n-type dopants are doped, since the background doping concentration cannot be measured, therefore the un-doped concentration is represented by N.A, and the output power thereof is 9.5 mW (LED  200 A). When the doping concentration of four doped quantum barrier layers  232   a - 232   d  is 8×10 17  cm −3 , the output power of the LED  200 F is increased from the un-doped 9.5 mW to 11.8 mW. Preferably, when the doping concentration is 2×10 18  cm −3 , the output power of the LED  200 G can be drastically increased from the un-doped 9.5 mW to 17.0 mW, which is a twofold enhancement. Moreover, when the doping concentration is 4×10 18  cm −3 , the output power of the LED  200 H is 19.1 mW, and when the doping concentration is 6×10 18  cm −3 , the output power of the LED  200 E can be enhanced to 21.5 mW. Therefore, it can deduced from Table 2 and  FIG. 10A  that in the quantum barrier layers of the LED, when the number of doped layers is over half of the total layers, and the doping concentration is 5×10 17 /cm 3  to 1×10 19 /cm 3 , the luminous efficiencies of the LEDs  200 E-2001 can be effectively enhanced. 
     Moreover, as shown by the results of Table 2 and  FIG. 10B , when the doping concentration in the four doped quantum barrier layers is between 5×10 17 /cm 3  to 1×10 19 /cm 3 , besides effectively increasing the luminous efficiency of the LED, the n-type dopants can also lower the resistance value of the quantum barrier layers, thereby reducing the forward voltage of the LED. For example, the forward voltage of the LED is reduced from 4.36 V when the doping concentration is 0 to 4.09 V when the doping concentration is 6×10 18 /cm 3 . The foregoing results represent that by increasing the doping concentration of the n-type dopants (e.g. Si) in the quantum barrier layers  232 , the effect defect density has on the luminous efficiency of the LED at the 222 nm-405 nm wavelength range can be effectively compensated. In other words, the n-type dopants injected in the quantum barrier layers can effectively provide electrons for radiative recombination, and lower energy release from non-radiative recombination such as heat. Accordingly, the luminous efficiency can be effectively enhanced, and the foregoing experimental results have verified the simulation results shown in  FIGS. 5-8 . 
     It should be noted that, according to the embodiments of the LEDs  200 B- 2001  in the disclosure, at least one element in the group IVA may also be used as the n-type dopant to provide electrons for radiative recombination, and thereby enhance the luminous efficiency. Moreover, besides the doping concentrations in the doped quantum barrier layers being equal to the values tabulated in Table 1 and 2, the doping concentrations may also have a laddered variation. As an example, the total number of quantum barrier layers is six, and four of the six layers are doped quantum barrier layers. The doping concentrations of the four doped quantum barrier layers are C 1 , C 2 , . . . C k , where C k ≦C k-1 , counting sequentially from the n-type semiconductor side. For example, the doping concentrations of the four doped quantum barrier layers  232   a - 232   d  are 6×10 18  cm −3 , 5×10 18  cm −3 , 4×10 18  cm −3 , and 3×10 18  cm −3  in sequence. In other words, the doping concentrations of the doped quantum barrier layers vary by gradually decreasing from the first quantum barrier layer  232   a  closest to the n-type semiconductor side to the fourth layer  232   d  closest to the p-type semiconductor side. Accordingly, the n-type dopants injected can also effectively provide electrons for radiative recombination, and thereby enhance the luminous efficiency. 
     Additionally, the laddered variation of the doping concentrations C 1  to C k  in the doped quantum barrier layers may also be 6×10 18  cm −3 , 7×10 18  cm −3 , 8×10 18  cm −3 , and 6×10 18  cm −3  in sequence counting from the n-type semiconductor side. In other words, the variation of the doping concentrations may be in a state where the doping concentrations of the middle layers are greater than the doping concentrations of the layers nearest to the n-type semiconductor and the p-type semiconductor. Moreover, the laddered variation of the doping concentrations in the doped quantum barrier layers may also be 6×10 18  cm −3 , 5×10 18  cm −3 , 8×10 18 cm −3 , and 6×10 18  cm −3  in sequence counting from the n-type semiconductor side. To sum up, as long as the doping concentration of the doped quantum barrier layer nearest to the p-type semiconductor layer is less than or equal to the doping concentrations of the other quantum barrier layers in the k doped quantum barrier layers, the injected n-type dopants can effectively provide electrons for radiative recombination, and thereby enhance the luminous efficiency. 
     In view of the foregoing, in the LED according to the embodiments of the disclosure, by having a number of quantum barrier layers of the active layer doped with n-type dopants, in which the layer number of the doped quantum barrier layers satisfies a specific relationship, or by having the lowest doping concentration at the quantum barrier layer doped with n-type dopants that is closest to the p-type semiconductor, or by having the doping concentrations of the quantum barrier layers doped with n-type dopants satisfying a specific relationship, the n-type dopants can compensate for the effect which defects of GaN have on the carriers. Accordingly, the carrier recombination rate of the LED can be enhanced. Therefore, by employing any one of the afore-described techniques, the luminous efficiency of the LED in the disclosure can be drastically increased at the 222 nm-405 nm wavelength range. 
     Moreover, the LED of the disclosure is not limited to the embodiments depicted above. The LED may be configured with horizontal electrodes or vertical electrodes, both of which can implement the disclosure but should not be construed as limiting the disclosure. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents.