Patent Publication Number: US-9425361-B2

Title: Light-emitting device

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. patent application, Ser. No. 14/681,291, which claims the right of priority based on TW application Serial No. 103112952, filed on Apr. 8, 2014, and the contents of which are hereby incorporated by references in their entireties. 
    
    
     TECHNICAL FIELD 
     The application relates to a light-emitting device, and more particularly, to a light-emitting device comprising a reflective layer. 
     DESCRIPTION OF BACKGROUND ART 
     The lighting theory of light-emitting diodes (LEDs) is that electrons and holes between an n-type semiconductor and a p-type semiconductor are combined in the active layer to release light. Due to the difference of lighting theories between LEDs and incandescent lamps, the LED is called “cold light source”. An LED has the advantages of good environment tolerance, a long service life, portability, and low power consumption so it is regarded as another option for the lighting application. LEDs are widely adopted in different fields, for example, traffic lights, backlight modules, street lights, and medical devices and replace conventional light sources gradually. 
     An LED has a light-emitting stack which is epitaxially grown on a conductive substrate or an insulative substrate. The so-called “vertical LED” has a conductive substrate and includes an electrode formed on the top of a light emitting layer; the so-called “lateral LED” has an insulative substrate and includes electrodes formed on two semiconductor layers which have different polarities and exposed by an etching process. The vertical LED has the advantages of small light-shading area for electrodes, good heat dissipating efficiency, and no additional etching epitaxial process, but has a problem that the conductive substrate served as an epitaxial substrate absorbs light easily and is adverse to the light efficiency of the LED. The lateral LED has the advantage of radiating light in all directions due to a transparent substrate used as the insulative substrate, but has disadvantages of poor heat dissipation, larger light-shading area for electrodes, and smaller light-emitting area caused because of the epitaxial etching process. 
     The abovementioned LED can further connect to/with other components for forming a light-emitting device. For a light-emitting device, the LED can connect to a sub-carrier by the substrate side or by soldering material/adhesive material between the sub-carrier and the LED. Besides, the sub-carrier can further comprise a circuit electrically connected to electrodes of the LED via a conductive structure, for example, a metal wire. 
     SUMMARY OF THE APPLICATION 
     A light-emitting device comprises: a light-emitting stack including a first surface and a second surface opposite to the first surface, wherein the light-emitting stack emits a light having a wavelength between 365 nm and 550 nm; and a first electrode formed on the first surface and including a first metal layer and a second metal layer alternating with the first metal layer, wherein the first electrode has a reflectivity larger than 95% for reflecting the light, and the second metal layer has a higher reflectivity relative to the light than that of the first metal layer. 
     A light-emitting device comprises: a light-emitting stack comprising a first surface and a second surface opposite to the first surface, wherein the light-emitting stack emits a light having a wavelength between 365 nm and 550 nm, and the first surface comprises a first portion having a first conductivity and a second portion having a second conductivity; a first electrode, comprising a first electrode pad and a reflective stack comprising a first metal layer and a second metal layer alternating with the first metal layer, wherein the reflective stack is electrically connect to the first portion of the first surface and has a reflectivity larger than 95% for reflecting the light, and the second metal layer has a higher reflectivity relative to the light than that of the first metal layer; a second electrode, comprising a second electrode pad and an ohmic contact layer formed on the second portion of the first surface; and a carrier comprising a first contact pad electrically connected to the first electrode pad and a second contact pad electrically connected to the second electrode pad. 
     A light-emitting device comprises a light-emitting stack comprising a first surface and a second surface opposite to the first surface; a first electrode formed on the second surface of the light-emitting stack; a current blocking layer formed on the first surface of the light-emitting stack and corresponding to a location of the first electrode; and a second electrode covering the current blocking layer and comprising a plurality of first metal layers and a plurality of second metal layers alternating with the plurality of first metal layers, wherein the plurality of first metal layers is discontinuous. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1E  illustrate a manufacturing method of a light-emitting device in accordance with a first embodiment of the present application; 
         FIG. 1F  illustrates a light-emitting stack in accordance with the first embodiment of the present application; 
         FIG. 2  illustrates a light-emitting device in accordance with a second embodiment of the present application. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1A to 1E , a manufacturing method of a light-emitting device in accordance with a first embodiment of the present application is disclosed. 
     As shown in  FIG. 1A , a buffer layer  103  and a light-emitting stack are epitaxially grown on a growth substrate  101 . The growth substrate  101  can comprise transparent substrate such as sapphire, or conductive substrate such as SiC. The buffer layer  103  can comprise an un-intentionally doped AlN, AlGaN or GaN, and the light-emitting stack  108  can comprise GaN. The buffer layer  103  can reduce the defect resulted from the lattice mismatch between the growth substrate  101  and the light-emitting stack  108 . The light-emitting stack  108  can comprise a first semiconductor layer  102 , an active layer  104 , and a second semiconductor layer  106 . The first semiconductor layer  102  and the second semiconductor layer  106 , for example, can be cladding layer or confinement layer, capable for providing electrons and holes, and the electrons and holes can be combined in the active layer  104  to emit light. The first semiconductor layer  102 , the active layer  104 , and the second semiconductor layer  106  can comprise III-V group semiconductor material such as Al x In y Ga (1-x-y) N, 0≦x, y≦1; (x+y)≦1. In accordance with the material of the active layer  104 , the emitted light thereof can be green light having a wavelength between 530 nm and 570 nm, blue light having a wavelength between 450 nm and 490 nm, or ultraviolet light having a wavelength between 365 nm and 405 nm. The first semiconductor layer  102  can comprise an n-type semiconductor layer and the second semiconductor layer  106  can be a p-type semiconductor layer. 
     As shown in  FIG. 1B , a patterned current-blocking layer  110  is formed on the first surface  108   a  of the light-emitting stack  108 , that is, the patterned current-blocking layer  110  is formed on the second semiconductor layer  10 . The current blocking layer can be insulating oxide such as SiO 2  or TiO 2 , or can be nitride such as SiN x . 
     As shown in  FIG. 1C , a first electrode  112  can be formed on the first surface  108   a  of the light-emitting stack  108  and cover the current blocking layer  110 . Then a barrier layer  114  comprising a first barrier layer  114   a  and a second barrier layer  114   b  can be formed on the uncovered region of the first surface  108   a  and the first electrode  112 . The current blocking layer  110  is entirely covered by the first electrode  112 , and on the first surface  108   a  the first electrode  112  is narrower than the barrier layer  114 . 
     The first electrode  112  can be a reflective stack comprising a first metal layer  112   a  and a second metal layer  112   b  alternating with the first metal layer  112   a , and the thermal stability of the first metal layer  112   a  is better than that of the second metal layer  112   b , and the reflectivity of the second metal layer  112   b  is higher than that of the first metal layer  112   a . For example, the first metal layer  112   a  can be Al and the second metal layer  112   b  can be Ag. Further referring to  FIG. 1F , the first metal layer  112   a  and a second metal layer  112   b  can alternate with each other for 2 to 12 times. In the embodiment, the first electrode  112  comprises a first metal layer  112   a  directly contacting the first surface  108   a . The barrier layer  114  can comprise an alloy or a stack comprising Ti, W, Pt, and Ni. The thickness of the first metal layer  112   a  can be between 1˜10 Å, and the thickness of the second metal layer  112   b  can be between 100˜700 Å. To be more specific, the thickness of the first metal layer  112   a  can be approximately 3 Å, wherein the first metal layer  112  may be discontinuous or embedded in the second metal layer  112   b , and the total thickness of the first electrode  112  can be between 1400 Å and 1500 Å, or even thicker than 1500 Å. To make the first electrode  112  ohmically contact the second semiconductor layer  106  of the light-emitting stack  108 , a Rapid Thermal Annealing (RTA) process can be proceeded under a condition of 500° C. and 40 minute after the first electrode  112  is formed. For example, when the second metal  112   b  is Ag and the second semiconductor layer  106  is p-type GaN, a high temperature annealing for Ag and p-type GaN is proceeded, and the first metal layer  112   a  can stabilize the second metal layer  112   b  when the high temperature annealing is performed. Beside pure Al, the first metal layer  112   a  can be an alloy or stack comprising Al, Ti, W, Pt or Ni. 
     Referring to  FIG. 1D , a conductive substrate  118  is provided to attach to the light-emitting stack  108  via a conductive bonding layer  116 . The conductive bonding layer  116  is between the conductive substrate  118  and the barrier layer  114  and comprises metal such as Au, In, Ni or the alloy thereof. The light-emitting stack  108  comprises a first semiconductor layer  102 , an active layer  104 , and a second semiconductor layer  106 , and is between the growth substrate  101  and the conductive substrate  118 . A laser (not shown) can be provided to decompose the buffer layer  103  so as to remove the growth substrate  101 , and residues of the buffer layer  103  can be cleaned by dry etching and wet etching. 
     Please refer to  FIG. 1E , the light-emitting stack  108  can expose a second surface  108   b  after the removal of the buffer layer in  FIG. 1D . The second surface  108   b  serves as a primary light-extraction surface and is also a surface of the first semiconductor layer  102 , and the second surface  108   b  can be a roughing surface to increase light-extraction efficiency. A second electrode  120  can be formed on the second surface  108   b  and corresponds to the location of the current blocking layer  110 . 
     When a driving current is injected into the light-emitting stack  108  via the second electrode  120  and the conductive substrate  118 , the active layer  104  can emit light L resulted from the combination of electrons and holes, and the light L can be reflected by the first electrode  112  and extracted out from the second surface  108   b . In the embodiment, when the wavelength of the light is between 365 nm to 550 nm, the reflectivity of the first electrode  112  can be higher than 95%, and can be even up to 98% to 100%. In the embodiment, the first electrode  112  is composed of the first metal layer  112   a  having high thermal stability and the second metal layer  112   b  having high reflectivity so the problem of substantially reduced reflectivity caused by the high temperature annealing of high reflective metal (e.g. Ag) and the semiconductor layer in the conventional art is relieved. The origin of the problem is that the high reflectivity metal such as Ag is unstable after high temperature annealing. Moreover, when the light-emitting stack of conventional art receives a high current larger than 350 mA, the high reflectivity metal becomes more unstable and the reflectivity thereof is further decreased. In the embodiment, the first metal layer  112   a  has a high reflectivity close to that of the second metal  112   b  and has a better ohmic contact with the second semiconductor layer  106 , and the first metal layer  112   a  has better thermal stability than that of the second metal layer  112   b , therefore the first metal layer  112   a  can keep the second metal  112   b  stable under high temperature annealing to avoid the reflectivity from dramatically reducing after high temperature annealing. Besides, in one experiment, the reflectivity of the first electrode  112  is not obviously decreased even a current higher than 350 mA is provided to the light-emitting stack  108 . 
     Referring to  FIG. 2 , a light-emitting device in accordance with a second embodiment of the present application is illustrated. A light-emitting device  200  comprises: a light-emitting stack  210  comprising a first surface  210   a  and a second surface  210   d  opposite to the first surface  210   a , and the light-emitting stack  210  emits a light L 1  having wavelength equal to that of the light L of the first embodiment, and the first surface  210   a  comprises a first portion  210   b  having a first conductivity and a second portion having a second conductivity, a first electrode  217  comprising a first electrode pad  226  and a reflective stack comprising a first metal layer  214   a  and a second metal layer  214   b  alternating with the first metal layer  214   a , wherein the reflective stack is electrically connected to the first portion  210   b  of the first surface  210   a  and having a reflectivity lager than 95% relative to light L 1  so the light L 1  is emitted out the light-emitting stack  210  from the second surface  210   d , a second electrode  250  comprising a second electrode pad  224  and an ohmic contact layer  231  formed on the second portion  210   c  of the first surface  210   a , and a carrier  218  comprising a first conductive pad  222  electrically connected to the first electrode  217  and a second conductive pad  220  electrically connected to the second electrode pad  224 . The light-emitting stack  210  comprises a first semiconductor layer  204  having two sides on which the second portion  210   c  of the first surface  210   a  and the second surface  210   d  are formed on, respectively, an active layer  206 , and a second semiconductor layer  208  comprising the first portion  210   b  of the first surface  210   a . The second portion  210   c  of the first surface  210   a  is formed by removing a part of the second semiconductor layer  208  and the active layer  206 . An insulating layer  203  is formed on the first surface  210   a  of the light-emitting stack  210  and a trench can be formed by etching process, and a metal can be filled into the trench to form a conductive channel. The first electrode  217  can further comprise a barrier layer  216  covering the reflective stack formed by the first metal layer  214   a  and the second metal layer  214   b  and a first conductive channel  228  penetrating through the insulating layer  203  wherein the two ends of the first conductive channel  228  are electrically connected to the barrier layer  216  and the first electrode pad  226 . The first metal layer  214   a  and the second metal layer  214   b  comprise the same material as those in the first embodiment. The first metal layer  214   a  can directly contact the second semiconductor layer  208 , and in the embodiment the first metal layer  214   a  and the second metal layer  214   b  can alternate with each other for 2 to 12 times, therefore further raising the reflectivity over 95% relative to the light L 1 , even up to 98% to 100%. In another embodiment, a metal oxide (not shown) can be formed between the first electrode  217  and the second semiconductor layer  208  to promote current-spreading effect. The second electrode  250  further comprises a second conductive channel  230  having two ends connected to the ohmic contact layer  231  and the second electrode pad  224 . A transparent substrate  202  can be formed on the second surface  210   d  of the light-emitting stack  210 , and the transparent substrate  202  can be a growth substrate such as sapphire for epitaxially growing the light-emitting stack  210 . In another embodiment, the transparent substrate  202  can be removed and the second surface  210   d  can be a roughing surface by an etching process. The first electrode pad  226 , the second electrode pad  224 , the first conductive channel  228  and the second conductive channel  230  can be a stack composed of metals such as Ni, Au and/or Ti. The ohmic contact layer  231  of the first electrode  250  can be a stack composed of metals such as Cr, Pt, and/or Au. Areas of the first electrode pad  226  and the second electrode pad  224  can be larger than the cross-sectional areas of first conductive channel  228  and the second conductive channel  230 , respectively, and both of the first electrode pad  226  and the second electrode pad  224  are extended on the surface of the insulating layer  203  for receiving a high current from the carrier  218 . 
     Although the present application has been explained above, it is not the limitation of the range, the sequence in practice, the material in practice, or the method in practice. Any modification or decoration for present application is not detached from the spirit and the range of such.