Abstract:
A high-brightness vertical light emitting diode (LED) device includes an outwardly located metal electrode having a low illumination side and a high illumination side. The LED device is formed by: forming the metal electrode on an edge of a surface of a LED epitaxy structure using a deposition method, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), evaporation, electro-plating, or any combination thereof; and then performing a packaging process. The composition of the LED may be a nitride, a phosphide or an arsenide. The LED has the following advantages: improving current spreading performance, reducing light-absorption of the metal electrode, increasing brightness, increasing efficiency, and thereby improving energy efficiency. The metal electrode is located on the edge of the device and on the light emitting side. The metal electrode has two side walls, among which one side wall can receive more emission light from the device in comparison with the other one.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of Ser. No. 12/939,142 filed Nov. 3, 2010. 
     
    
     BACKGROUND 
       [0002]    Currently, LED is widely applied in our daily life due to its characteristics of low production cost, easy fabrication, small size, low power consumption and high efficiency, for example, in the fields of cell phones, electric boards, electric torches, traffic lights and so on. Nevertheless, improvements in the luminous efficiency and brightness of a LED are pursued continuously. 
         [0003]    Recently, high-brightness LEDs using nitrides and phosphides have been developed, which can not only emit red, green and blue light but also produce light in various colors and white light. At present, LED lighting applications are developed enthusiastically by industry. In the early stage of manufacturing development, multiple LEDs were combined to form an array, so as to achieve high output power. However, in terms of manufacturing, a LED device including a LED array is more complicated than a single LED device with high output power. Therefore, the cost of manufacturing the LED array device is higher and the stable reliability is less likely to be achieved. 
         [0004]    One method to increase the power and luminous flux of a LED is to increase the size and luminous surface area thereof However, the semiconductor material layer used in a conventional LED usually has poor conductivity, such that electric current cannot be spread effectively and uniformly over an active layer from a contact. Therefore, some areas inside the LED can produce high electric current density phenomenon, thereby affecting the whole brightness, even leading to early deterioration in the proximity of the active layer. As a result, the service life of the LED is reduced significantly. 
         [0005]      FIG. 1A  is a top view of a configuration of a conventional small-size vertical LED device  100 .  FIG. 1B  is a cross-sectional view of the configuration of the LED device  100  shown in  FIG. 1A .  FIG. 2  is a top view of a configuration of a conventional large-size vertical LED device  200 . With reference to  FIG. 1B , the configuration of the conventional small-size LED device  100  typically includes a first electrode  109 , a conductive substrate layer  108  formed on the first electrode  109 , a reflective mirror layer  106  formed on the conductive substrate layer  108 , a first conductivity type semiconductor layer  104  formed on the reflective mirror layer  106 , an active layer  103  (or referred as an emission layer) formed on the first conductivity type semiconductor layer  104 , a second conductivity type semiconductor layer  102  formed on the active layer  103 , and a second metal electrode  101  formed on the second conductivity type semiconductor layer  102 . As shown in  FIG. 1A , in the small-size vertical LED device  100 , the second metal electrode  101  is located on the center of the second conductivity type semiconductor layer  102 . Furthermore, additional metal wires are not required due to the small size and the good current spreading performance of the LED device  100 . 
         [0006]    For a conventional large-size vertical LED device, a major reason of affecting the luminous efficiency of the LED device is the failure to spread electric current uniformly, so it is contemplated to increase the thickness of a semiconductor material layer so as to increase the conductivity. For the small-size LED (less than about 0.25 mm 2 ) shown in  FIGS. 1A and 1B , its brightness and current spreading performance can certainly be improved by this method. However, the increased thickness of the semiconductor material layer may not only increase production costs but also lead to stress problems. Therefore, it is impossible to unlimitedly increase the thickness of the semiconductor material layer to comply with the current spreading performance requirement of a large-size LED device. As a result, for the large-size device shown in  FIG. 2 , a satisfactory performance cannot be achieved merely by increasing the thickness of the semiconductor material layer. This is because when the size of a LED device is increased, it becomes more unlikely to uniformly spread electric current over the semiconductor material layer from an n-type contact or a p-type contact. It can be seen that the size of a LED is substantially limited by the current spreading characteristic of the semiconductor material layer. 
         [0007]    As shown in  FIG. 2 , in a conventional large-size vertical LED device  200 , a second metal electrode pad area  210  is located on the center of a second conductivity type semiconductor layer  202 , which generally utilizes radial metal electrodes  201  to increase the current spreading performance. However, most of the outlines of common LED devices are squares or rectangles, therefore it is difficult not only to place each radial metal wire on an emission layer such that the best current spreading performance can be achieved, but also to ensure that the adjacent radial metal wires have constant interval therebetween. In addition, both sides of the metal electrode are high illumination sides, which tend to absorb emission light, thereby decreasing brightness. As shown in  FIGS. 3A and 3B , for other conventional large-size vertical LED devices  200 A and  200 B, both sides of their metal electrodes are high illumination sides, which also tend to absorb emission light, thereby decreasing brightness. Therefore, conventional LED devices still generally have the following problems including uneven current density, low light extraction efficiency, unsatisfactory brightness, unsatisfactory efficiency, short service life, and so on, which are to be solved. 
       SUMMARY 
       [0008]    In view of the above problems, an improved vertical LED device is provided, which has the higher output brightness and efficiency in comparison with conventional LED devices. In addition, the LED device can fully fulfill the contemporary demand for high energy efficiency without increasing production costs. Furthermore, the manufacturing method of the present LED device involves no complicated technique, which means the method is economically beneficial. 
         [0009]    In order to solve the above problems and achieve the above goals, an LED device is provided having improved current spreading performance and reduced light-absorption of a metal electrode. 
         [0010]    One aspect is to provide a vertical light emitting diode (LED) device having an outwardly located metal electrode, the LED device including: a first electrode, a conductive substrate layer formed on the first electrode, a reflective mirror layer formed on the conductive substrate layer, a first conductivity type semiconductor layer formed on the reflective mirror layer, an active layer formed on the first conductivity type semiconductor layer, a second conductivity type semiconductor layer formed on the active layer, a second metal electrode formed on the second conductivity type semiconductor layer and being located on an edge of the second conductivity type semiconductor layer, two sides of the second metal electrode being a high illumination side and a low illumination side respectively, wherein the low illumination side is located beyond the width scope of the reflective mirror layer. 
         [0011]    The current spreading performance of a vertical LED device can be optimized and the light-absorption of a metal electrode can be reduced by applying an outwardly located metal electrode, thereby increasing the brightness, efficiency and service life of the LED device, and reducing energy costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the accompanying drawings, like elements are represented by like numerals. 
           [0013]      FIG. 1A  shows a top view of a conventional small-size vertical LED device; 
           [0014]      FIG. 1B  shows a cross-sectional view of conventional small-size vertical LED device; 
           [0015]      FIG. 2  shows a top view of a conventional large-size vertical LED device; 
           [0016]      FIG. 3A  shows both of a top view and a detailed cross-sectional view of a conventional large-size vertical LED device, both sides of the metal electrode thereof being high illumination sides; 
           [0017]      FIG. 3B  shows both of a top view and a detailed cross-sectional view of another conventional large-size vertical LED device, both sides of the metal electrode thereof being high illumination sides; 
           [0018]      FIG. 4  shows a top view of a large-size vertical LED device according to one embodiment, wherein the die size is 1 mm 2 ; 
           [0019]      FIG. 5  shows both of a top view and a cross-sectional view of the large-size vertical LED device shown in  FIG. 4 ; 
           [0020]      FIG. 6  illustrates a three dimensional view of the large-size vertical LED device shown in  FIG. 4 ; 
           [0021]      FIG. 7  shows both of a top view and a detailed cross-sectional view of a large-size vertical LED device according to one embodiment, wherein the die size is 1 mm 2 ; 
           [0022]      FIG. 8  shows both of a top view and a detailed cross-sectional view of a large-size vertical LED device according to another embodiment, wherein the die size is 1 mm 2 ; 
           [0023]      FIG. 9  shows both of a top view and a detailed cross-sectional view of a large-size vertical LED device according to yet another embodiment, wherein the die size is 1 mm 2 ; 
           [0024]      FIG. 10  shows both of a top view and a detailed cross-sectional view of a large-size vertical LED device according to another embodiment, wherein the die size is 1 mm 2 ; 
           [0025]      FIG. 11  shows a top view of a large-size vertical LED device according to another embodiment, wherein the die size is 0.6 mm 2 ; 
           [0026]      FIG. 12  shows both of a top view and a cross-sectional view of the large-size vertical LED device shown in  FIG. 11 ; 
           [0027]      FIG. 13  shows both of a top view and a cross-sectional view of a small-size vertical LED device according to one embodiment, wherein the die size is 0.1 mm 2 ; 
           [0028]      FIGS. 14A-14F ,  15 A- 15 F,  16 A- 16 F,  17 A- 17 F,  18 A- 18 F, and  19 A- 19 F respectively show top views of large-size vertical LED devices according to other embodiments, wherein their die sizes are more than 0.3 mm 2 ; 
           [0029]      FIGS. 20A-20D  respectively show top views of vertical LED devices according to other embodiment, wherein their die sizes are less than 0.3 mm 2 ; 
           [0030]      FIGS. 21A-21I  respectively show top views of vertical LED devices having a rectangular die shape according to other embodiments; 
           [0031]      FIGS. 22A-22B  show side views of the large-size vertical LED devices;  FIGS. 23A-23B  show side views of the small-size vertical LED shown in  FIG. 13 ; 
           [0032]      FIGS. 24A-24B  show side views of the vertical LED device having a rectangular die shape shown in  FIG. 21A ; and 
           [0033]      FIG. 25  shows comparison results between a large-size (1 mm 2 ) vertical nitride-based (gallium nitride) blue LED device  300  and four LED devices with prior art designs A, B, C and D in terms of their brightness. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    The preferred embodiments are described hereinafter, including various embodiments of vertical LED devices in which the current spreading performance of a semiconductor layer and the light-absorption property of a metal electrode have been modified, thereby achieving better brightness, efficiency and service life in comparison with conventional LED devices. 
         [0035]      FIG. 4  shows a top view of a large-size vertical GaN-based (gallium nitride) LED device  300 .  FIG. 5  shows both of a top view and a cross-sectional view of the LED device  300  shown in  FIG. 4 .  FIG. 6  illustrates a three dimensional view of the LED device  300  shown in  FIG. 4 . In this embodiment, the size of the n-type (second conductivity type) semiconductor layer  302  is 1 mm 2 . The large-size vertical LED device  300  includes a first electrode  309 , a conductive substrate layer  308  formed on the first electrode  309 , a reflective mirror layer  306  formed on the conductive substrate layer  308 , a p-type (first conductivity type) semiconductor layer  304  formed on the reflective mirror layer  306 , an active layer  303  (also referred as “an emission layer”) formed on the p-type (first conductivity type) semiconductor layer  304 , an n-type (second conductivity type) semiconductor layer  302  formed on the active layer  303 , and a second metal electrode  301  formed on the n-type (second conductivity type) semiconductor layer  302 , in which the second metal electrode  301  is provided on an edge of the n-type semiconductor layer  302 , and two sides of the second metal electrode  301  are a high illumination side  301 ′ and a low illumination side  301 ″, respectively. The low illumination side  301 ″ is located beyond the width scope W of the reflective mirror layer  306 . In other words, the low illumination side  301 ″ is not covered by the reflective mirror layer  306 . Three metal electrode wires are provided inwardly to connect with the second metal electrode  301 . It should be noted that the numbers of the inwardly provided metal electrode wires can be adjusted to comply with the outline and size of the entire LED device or to meet a request. Partial areas of a surface of the second conductivity type semiconductor layer can be patterned to improve light extraction efficiency. In addition, the LED device  300  further includes a metal pad area  310 , as shown in  FIGS. 4 and 6 , used as an electrical contact. It should be noted that the metal pad area  310 , used as the electrical contact, shown in the drawings is intended for purposes of illustration only and is not intended to limit the scope of the claims. The numbers of the metal pad area  310  can be adjusted according to actual demands. Furthermore, the LED device  300  may include a conductive transparent layer (not shown), which is provided between the second conductivity type semiconductor layer  302  and the second metal electrode  301 . 
         [0036]      FIG. 7  shows both of a top view and a cross-sectional view of a large-size vertical GaN-based (gallium nitride) LED device  400  according to another embodiment. In the LED device  400 , a surface of the second conductivity type semiconductor layer  302  near the high illumination side is roughed to increase light extraction efficiency.  FIG. 8  shows both of a top view and a cross-sectional view of a large-size vertical LED device  400 ′ according to yet another embodiment. In the LED device  400 ′, the entire surface of the second conductivity type semiconductor layer  302  is roughed to further increase light extraction efficiency. A surface of the second conductivity type semiconductor layer  302  can be roughed by using domes/beads or using a wet/dry etching technique, but not limited to this. 
         [0037]      FIG. 9  shows both of a top view and a cross-sectional view of a large-size vertical LED device  500  according to another embodiment. The LED device  500  further includes a protective layer  311 , which is used to protect the reflective mirror layer  306 , so as to prevent the reflective mirror layer  306  from being oxidized and resulting in brightness decrease. A material of the protective layer  311  may be at least one material selected from the group consisting of Ni, W, Mo, Pt, Ta, Rh, Au, V, WTi, TaN, SiO 2 , SiN X , Al 2 O 3 , AN, ITO and Ni-Co. The protective layer  311  can be formed by using at least one of the following methods: PVD, CVD, evaporation, sputtering, electro-plating, electroless plating, coating, printing, or any combination thereof. Although only the surface of the second conductivity type semiconductor layer  302  near the high illumination side is roughed according to  FIG. 9 , the entire surface of the second conductivity type semiconductor layer  302  can be roughed if necessary. 
         [0038]      FIG. 10  shows both of a top view and a cross-sectional view of a large-size vertical LED device  600  according to another embodiment. In the LED device  600 , an optical transparent layer  312  is provided between a reflective mirror layer  314  and the first conductivity type semiconductor layer  304  to form an omni-directional reflector. The reflective mirror layer  314  can be a high-reflectivity metal layer, or a distributed Bragg reflector (DBR), so as to increase external quantum efficiency. The method of manufacturing the reflective mirror layer  314  can be a conventional method, such as PVD, CVD, evaporation, sputtering, electro-plating, electroless plating, coating, printing, or any combination thereof In one embodiment, a reflective mirror layer can have a single- or multi-layer structure. In addition, a material of the reflective mirror layer may be one metal selected from the following: Ag/Ni, Ni/Ag/Ni/Au, Ag/Ni/Au, Ag/Ti/Ni/Au, Al, Ti/Al, Ni/Al, Au, any combination of at least two thereof, or an alloy thereof containing Ag, Au, Ni, Cr, Pt, Pd, Rh, Cu, W, In, Pd, Zn, Ge, Bi, AlSi, or Al. A material of the distributed Bragg reflector may be, for example, SiO 2 , TiO 2 , MgO, Al 2 O 3 , ITO, ZnO, SiN x , or any combination of at least two thereof. A material of the omni-directional reflector may be, for example, SiO 2 , TiO 2 , MgO, Al 2 O 3 , ITO, ZnO, SiN X , or any combination of at least two thereof. The conductive substrate layer may be a metal or a semiconductor material, such as silicon, GaP, SiC, GaN, AN, GaAs, InP, AlGaAs, and ZnSe, or any combination of at least two thereof. Likewise, the conductive substrate layer can be formed by using a conventional method, such as PVD, CVD, evaporation, sputtering, electro-plating, electroless plating, coating, printing, wafer bonding, or any combination thereof; its thickness may be from 10 μm to 1000 μm based on various requests. Although only the surface of the second conductivity type semiconductor layer  302  near the high illumination side is roughed according to  FIG. 10 , the entire surface of the second conductivity type semiconductor layer  302  can be roughed if necessary. 
         [0039]      FIG. 25  shows comparison results between a large-size (1 mm 2 ) vertical nitride-based (gallium nitride) blue LED device  300  according to one embodiment and four LED devices with prior-art designs A, B, C and D in terms of their brightness (light output power). The five LED devices with various designs are made from the same epitaxy wafer, using identical frames, and finally packed by silica gels via totally identical procedures to obtain the products. In Table 1, the brightness (light output power) was measured by an integrating sphere, which is well known to those skilled in the art, and thus detailed descriptions thereof are omitted here. As shown in Table 1, the LED device has higher output power in comparison with other prior-art LED devices. 
         [0040]      FIG. 11  shows a top view of a large-size (0.6 mm 2 ) vertical GaN-based (gallium nitride) LED device  700  according to another embodiment.  FIG. 12  shows both of a top view and a cross-sectional view of the LED device  700  shown in  FIG. 11 . The LED device  700  includes a second metal electrode  701 , a second conductivity type semiconductor layer  702 , an active layer (emission layer)  703 , a first conductivity type semiconductor layer  704 , a reflective mirror layer  706 , a conductive substrate layer  708 , and a first electrode  709 , in which the size of the second conductivity type semiconductor layer  702  is 0.6 mm 2 , and the second metal electrode  701  is provided on an edge of the second conductivity type semiconductor layer  702 . Two sides of the second metal electrode  701  are a high illumination side  701 ′ and a low illumination side  701 ″ respectively, wherein the low illumination side  701 ″ is located beyond the width scope W of the reflective mirror layer  706 . In other words, the low illumination side  701 ″ is not covered by the reflective mirror layer  706 . Furthermore, in this embodiment, a metal pad area  710  used as an electrical contact is provided. 
         [0041]      FIG. 13  shows both of a top view and a cross-sectional view of a small-size vertical GaN-based (gallium nitride) LED device  800  according to one embodiment. The LED device  800  includes a second metal electrode  801 , a second conductivity type semiconductor layer  802 , an active layer (emission layer)  803 , a first conductivity type semiconductor layer  804 , a reflective mirror layer  806 , a conductive substrate layer  808 , and a first electrode  809 . In this embodiment, the size of the second conductivity type semiconductor layer  802  is 0.1 mm 2 . The small-size vertical LED device  800  includes a first electrode  809 , a conductive substrate layer  808  formed on the first electrode  809 , a reflective mirror layer  806  formed on the conductive substrate layer  808 , a first conductivity type semiconductor layer  804  formed on the reflective mirror layer  806 , an active layer  803  (also referred as “an emission layer”) formed on the first conductivity type semiconductor layer  804 , a second conductivity type semiconductor layer  802  formed on the active layer  803 , and a second metal electrode  801  formed on the second conductivity type semiconductor layer  802 , in which the second metal electrode  801  is provided on an edge of the second conductivity type semiconductor layer  802 . Two sides of the second metal electrode  801  are a high illumination side  801 ′ and a low illumination side  801 ″ respectively, wherein the low illumination side  801 ″ is located beyond the width scope W of the reflective mirror layer  806 . In other words, the low illumination side  801 ″ is not covered by the reflective mirror layer  806 . 
         [0042]    Preferably, the first conductivity type semiconductor layer ( 304 ,  704 , and  804 ) is p-type, and the second conductivity type semiconductor layer ( 302 ,  702 , and  802 ) is n-type. An n-type semiconductor layer has better conductivity, and thus less numbers of metal electrodes are required, so as to reduce shading and increase brightness. Furthermore, preferably, doping levels may range from 1×10 15  cm −3  to 1×10 22  cm −3 , and a thickness of the semiconductor layer may be 0.3 μm to 100 μm. In one embodiment, a first conductivity type semiconductor layer, a second conductivity type semiconductor layer and an active layer may be formed by using a conventional method, such as metal-organic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), and molecular beam epitaxy (MBE), which are well known to those skilled in the art and need not be described in further detail. A configuration of the active layer may be selected from the group consisting of double-hetero and quantum-well structures containing aluminum gallium indium nitrides ((Al x Ga 1-x ) y In 1-y N; 0 x 1; 0 y 1), or selected from the group consisting of double-hetero and quantum-well structures containing aluminum gallium indium phosphides ((Al x Ga 1-x ) y In 1-y P; 0 x 1; 0 y 1), or from the group consisting of double-hetero and quantum-well structures containing aluminum gallium arsenides (Al x Ga 1-x As; 0 x 1). The second metal electrode ( 301 ,  701 , and  801 ) and the first electrode ( 309 ,  709 , and  809 ) may be formed by using a conventional method, such as PVD, CVD, evaporation, sputtering, electro-plating, electroless plating, coating, printing, or any combination thereof. For example, the second metal electrode may have a single- or multiple-layer structure containing one of the following materials: Cr/Au, Cr/Al, Cr/Pt/Au, Cr/Ni/Au, Cr/Al/Pt/Au, Cr/Al/Ni/Au, Al, Ti/Al, Ti/Au, Ti/Al/Pt/Au, Ti/Al/Ni/Au, Ti/Al/Pt/Au, WTi, Al/Pt/Au, Al/Pt/Al, Al/Ni/Au, Al/Ni/Al, Al/W/Al, Al/W/Au, Al/TaN/Al, Al/TaN/Au, Al/Mo/Au, or a alloy consisting of at least two thereof, or other suitable conductive materials. 
         [0043]    The width of the second metal electrode may be 1 μm to 50 μm, preferably 3 μm to 30 μm. Although a broader metal electrode wire may spread electric current more effectively, it can obstruct or absorb more emission light from an n-type layer. One solution for this is to provide a current blocking structure configured to prevent the emission light from the n-type layer from being obstructed or absorbed by the metal electrode wire. However, if the broader metal electrode wire is employed, the size of the current blocking structure is required to be increased accordingly, thereby reducing the emission area of the active layer, and thus decreasing the amount of light through the active layer. A space between the second metal electrode wires may be 50 μm to 600 μm. The current spreading performance becomes better when the space is adequate. However, a contact area can be reduced when the space between the metal electrode wires is larger, thereby adversely affecting the operation voltage. Preferably, a total surface area of the second metal electrode occupies less than 25% of a surface area of the second conductivity type semiconductor layer, and a contact area between the reflective mirror layer and the first conductivity type semiconductor layer occupies more than 75% of a surface area of the first conductivity type semiconductor layer. A thickness of the second metal electrode wire may be 0.1 μm to 50 μm, preferably 1 μm to 10 μm. A thicker second metal electrode has a lower series resistance, but the corresponding manufacturing time and costs are inevitably increased. 
         [0044]    It should be noted that the aforesaid materials of the second metal electrode are intended for purposes of illustration only and are not intended to limit the scope of the claims. 
         [0045]      FIGS. 14A-14F ,  15 A- 15 F,  16 A- 16 F,  17 A- 17 F,  18 A- 18 F, and  19 A- 19 F respectively show top views of large-size vertical LED devices according to other embodiments, wherein their die sizes are more than 0.3 mm 2 .  FIGS. 20A-20D  respectively show top views of vertical LED devices according to other embodiments, wherein their die sizes are less than 0.3 mm 2 .  FIGS. 21A-21I  respectively show top views of vertical LED devices having a rectangular die shape according to other embodiments.  FIGS. 22A-22B  show side views of the large-size vertical LED devices, such as the LED devices shown in  FIGS. 4-12 ,  14 A- 14 F,  15 A- 15 F,  16 A- 16 F,  17 A- 17 F,  18 A- 18 F, and  19 A- 19 F.  FIGS. 23A-23B  show side views of the small-size vertical LED shown in  FIG. 13 .  FIGS. 24A-24B  show side views of the vertical LED device having a rectangular die shape shown in  FIG. 21A . 
         [0046]    The present device is characterized in that a metal electrode of a vertical LED device is provided on a semiconductor layer to form an outwardly located metal electrode. The current spreading performance of the vertical LED device having a cube or rectangular shape can be optimized and the light-absorption of the metal electrode can be reduced via the configuration of providing the metal electrode on the edge, thereby increasing the brightness, efficiency, and service life of the LED devices, and thus displaying a superior performance over other prior-art LED devices. 
         [0047]    It should be understood by those skilled in the art that the foregoing description only shows the preferred embodiments, the same is to be considered as illustrative and not restrictive in character. Various equivalent changes and modifications can be made without departing from the spirit and scope of present disclosure, which are therefore intended to be embraced in the appended claims.