Abstract:
This invention provides an optoelectronic semiconductor device having a rough surface and the manufacturing method thereof. The optoelectronic semiconductor device comprises a semiconductor stack having a rough surface and an electrode layer overlaying the semiconductor stack. The rough surface comprises a first region having a first topography and a second region having a second topography. The method comprises the steps of forming a semiconductor stack on a substrate, forming an electrode layer on the semiconductor stack, thermal treating the semiconductor stack, and wet etching the surface of the semiconductor stack to form a rough surface.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims the right of priority based on Taiwan Application Ser. No. 096131320, filed Aug. 23, 2007, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     This invention relates to a semiconductor optoelectronic device having a rough surface and the manufacturing method thereof. 
     BACKGROUND OF THE DISCLOSURE 
     Surface roughening is one of the efficient ways to improve light extraction efficiency of a light-emitting device. Roughening the substrate or the upmost semiconductor layer into irregular protrusions/depressions to scatter incident light impinging on the roughened surface is an example to improve light extraction efficiency. Roughening surface can be achieved by known processes like mechanically polishing or reactive-ion-etching (RIE). Another feasible way is performed by wet etching the wafer immersed in an etching solution for certain duration. The surface is roughened by different etching rates of the etching solution versus different exposed crystal planes of the surface. The roughened surface of the light-emitting device, as shown in  FIG. 1 , is formed by wet etching. The light-emitting device comprises a growth substrate  11 , an n-type semiconductor layer  12 , an active layer  13 , a p-type semiconductor  14 , a p-side conductive pad, and an n-side conductive pad. The surface of the p-type semiconductor layer  14  is wet-etched to form a roughened surface. An undercut  17  is formed due to lateral etching in the border between the p-side conductive pad  16  and the p-type semiconductor layer  14 . The contact area between the p-side conductive pad  16  and the p-type semiconductor layer  14  is therefore reduced such that the device reliability is easily failed or the p-side conductive pad  16  is easily peeled off by the stress. Besides, the uniformity of the roughened surface formed by the conventional wet etching method is not good enough to keep the product stable. 
     Another conventional way to prevent the light-emitting device from reliability failure or pad peeling is to form the roughened surface before forming the conductive pad, but the contact resistance between the conductive pad and the roughened surface of the underlying layer becomes high and therefore downgrade the device performance. Besides, the resulted surface of the conductive pad is uneven and therefore obstructs the wire-bonding yield. 
     SUMMARY OF THE DISCLOSURE 
     One aspect of the present invention is to provide an optoelectronic semiconductor device comprising a substrate; a semiconductor stack further comprising a first semiconductor layer of a first conductivity, an active layer, and a second semiconductor layer of a second conductivity; and an electrode layer formed on the second semiconductor layer; wherein the first and/or second semiconductor layer having a rough surface comprising a first region having a first topography and a second region having a second topography. 
     Another aspect of the present invention is to provide a method for forming a rough surface on a semiconductor layer of an optoelectronic semiconductor device. The method comprises the steps of forming a semiconductor stack on a substrate; forming an electrode layer on the semiconductor stack; heat treating the semiconductor stack and the electrode layer; and wet etching the surface of the semiconductor stack to form a rough surface. 
     The optoelectronic semiconductor device comprises light-emitting device or photovoltaic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a light-emitting device according to a conventional structure. 
         FIG. 2A  to  FIG. 2B  are schematic diagrams showing one embodiment of a light-emitting device in accordance with the present invention. 
         FIG. 2C  is a schematic diagram showing the top view of the light-emitting device in  FIG. 2A  or  FIG. 2B . 
         FIG. 3A  is an SEM picture showing the first topography of the rough surface formed in accordance with the present invention. 
         FIG. 3B  is an SEM picture showing the second topography of the rough surface formed in accordance with the present invention. 
         FIG. 4  is a flow chart showing the manufacturing method in accordance with the present invention. 
         FIG. 5A  to  FIG. 5C  are schematic diagrams showing a light-emitting device comprising a bonding structure in accordance with the present invention. 
         FIG. 6  is a schematic diagram showing a light-emitting device comprising a lateral structure in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2A  to  FIG. 2C  show a light-emitting device  2  in accordance with the present invention.  FIG. 2C  is a top view of the light-emitting device  2 , and  FIG. 2A  and  FIG. 2B  are the cross-section views along with AA′ and BB′ respectively. As shown in  FIG. 2A  and  FIG. 2B , the light-emitting device  2  comprises a substrate  21  having a top surface and a bottom surface; a first semiconductor layer  22  of a first conductivity overlaying the substrate; an active layer  23  overlaying the first semiconductor layer  22 ; a second semiconductor layer  24  of a second conductivity comprising a rough surface overlaying the active layer  23 ; an extended electrode layer  25  overlaying the second semiconductor layer  24 ; a first conductive pad  26  overlaying a portion of the extended electrode layer  25  and a portion of the second semiconductor layer  24 ; and a second conductive pad  27  underlying the bottom surface of the substrate  21 . The material of the first semiconductor layer  22 , the active layer  23 , or the second semiconductor layer  24  comprises n-type or p-type Al p Ga q In (1-p-q) P or Al x In y Ga (1-x-y) N (0≦p, q, x, y≦1; (p+q)≦1; (x+y)≦1). The first semiconductor layer  22  comprises a first conductivity type cladding layer, and the second semiconductor layer  24  comprises a second conductivity type cladding layer. The extended electrode layer  25  is a current spreading layer having a pattern extended toward the surroundings of the light-emitting device for spreading current as shown in  FIG. 2C . The first conductive pad  26  is a bonding pad covering and electrically coupled to a portion of the extended electrode layer  25 . The material of the extended electrode layer  25  comprises single or multilayer of metal or metal alloy such as Ge/Au. The material of the conductive pad  26  comprises multilayer of metal such as Cr/Au. In one embodiment, the light-emitting device  2  further comprises an ohmic contact layer  28  interposed between the second semiconductor layer  24  and the extended electrode layer  25 . The ohmic contact layer  28  comprises a semiconductor layer with a higher doping concentration than that of the second semiconductor layer  24 , such as GaAs having Si-doping concentration higher than 10 18 cm −3 . The conductivity type of the ohmic contact layer  28  can be the same as or different to the second semiconductor layer  24  and form an ohmic contact with the second semiconductor layer  24 . In a preferred embodiment, the ohmic contact layer  28  is formed only under the region covered by the extended electrode layer  25 . The first conductivity type comprises n-type or p-type, and the second conductivity type is different from the first conductivity type. 
     The rough surface comprises a first region  241  having a first rough topography and a second region  242  having a second rough topography. The first region and the second region comprise a plurality of depressions and protrusions. The dimension of the first rough topography is smaller than that of the second rough topography. In an embodiment of the present invention, the distance between the neighboring depressions or the distance between the neighboring protrusions of the first rough topography is around 0.1 to 0.5 micron. The depth of at least one of the depressions or the height of at least one protrusion of the first rough topography is around 0.1 to 0.5 micron. The distance between the neighboring depressions or the distance between the neighboring protrusions of the second rough topography is around 1 to 10 microns. The depth of at least one of the depressions or the height of at least one of the protrusions of the second rough topography is around 0.5 to 2 microns. The first rough topography as shown in  FIG. 3A  is a randomly rough surface having a plurality of depressions and protrusions. The second rough topography as shown in  FIG. 3B  is a wave-shape surface. The second region  242  having the second topography is adjacent to and surrounds the extended electrode layer  25 , and therefore separating the extended electrode layer  25  from the first region  241  having the first rough topography. The level, i.e. the average altitude of the surface, of the second region  242  is about 0.5 to 2 microns lower than that of the first region  241 . The cross-sectional shape of the second region  242  is a curve such that the interface between the extended electrode layer  25  and the second region  242  is outward oblique from the extended electrode layer  25  to prevent from undercut and further improve the reliability of the device and the peeling-off issue of the electrode layer. 
       FIG. 4  discloses a manufacturing method for forming the above-mentioned light-emitting diode  2 . The method comprises the steps of:
         1. providing a growth substrate  21 ;   2. forming a first semiconductor layer  22 , an active layer, and a second semiconductor layer  24  sequentially on the growth substrate;   3. forming an extended electrode layer  25  on the second semiconductor layer  24 ;   4. forming a first conductive pad  26  on a portion of the extended electrode layer  25  and the second semiconductor layer  24 ;   5. proceeding a thermal treatment step, such as rapid thermal annealing (RTA);   6. wet etching the second semiconductor layer  24  by an etching solution comprising HF,HNO 3 ,CH 3 COOH, and iodine under a low temperature condition from room temperature to 60° C. to form a rough surface thereon, wherein the features of the rough surface is described in the previous embodiments, such as  FIG. 3A  and  FIG. 3B ;   7. forming a second conductive pad  27  on the other side of the substrate  21 .       
     The structure formed by the method is shown in  FIG. 2A  or  FIG. 2B . 
       FIG. 5A  shows another embodiment of the present invention. The light-emitting device  5   a  as shown in  FIG. 5A  is similar to the light-emitting device  2 . The distinction is that the substrate  51  of the light-emitting device  5   a  is a conductive substrate. The substrate  51  can also be a transparent or opaque conductive substrate. It is also preferred that the substrate is a conductive substrate comprising a material having high thermal conductivity not lower than 100 W/cm·° C., such as Si, Cu, or diamond. The conductive substrate  51  is coupled to the first semiconductor layer  22  by a conductive connecting layer  52 . The conductive connecting layer  52  comprises a transparent conductive layer or a conductive adhesive layer. The material of the transparent conductive layer comprises transparent conductive oxide, such as indium tin oxide (ITO), zinc oxide (ZnO), or thin metal. The material of the conductive adhesive layer comprises silver paste or solder metal. The light-emitting device  5   a  further comprises a reflecting layer  53  formed between the conductive connecting layer  52  and the first semiconductor layer  22  for reflecting the light emitted from the active layer  23  and preventing from being absorbed by the conductive substrate  51  if the conductive substrate  51  is opaque. 
     The manufacturing method for forming the light-emitting device  5   a  comprises the steps of:
         1. providing a growth substrate (not shown);   2. forming a first semiconductor layer  22 , an active layer  23 , and a second semiconductor layer  24  sequentially on the growth substrate;   3. bonding a temporary substrate (not shown) to the second semiconductor layer  24 ;   4. removing the growth substrate to expose one surface of the first semiconductor layer  22 ;   5. forming a reflecting layer  53  on the exposed surface of the first semiconductor layer  22 ;   6. forming a conductive connecting layer  52  on a conductive substrate  51 ;   7. bonding the conductive substrate  51  with the conductive connecting layer  52  to the reflecting layer  53 ;   8. removing the temporary substrate to expose one surface of the second semiconductor layer  24     9. forming an extended electrode layer  25  on the exposed surface of the second semiconductor layer  24 ;   10. forming a first conductive pad  26  on a portion of the extended electrode layer  25  and the second semiconductor layer  24 ;   11. proceeding a thermal treatment step, such as rapid thermal annealing (RTA);   12. wet etching the second semiconductor layer  24  to form a rough surface thereon, wherein the features of the rough surface is described in the previous embodiments, such as  FIG. 3A  and  FIG. 3B ;   13. forming a second conductive pad  27  on the other side of the conductive substrate  51 .       

       FIG. 5B  shows another embodiment of the present invention. The light-emitting device  5   b  as shown in  FIG. 5B  is similar to the light-emitting device  5   a  mentioned above. The distinction is that the rough surface is formed on the first semiconductor layer  22 . 
     The manufacturing method for forming the light-emitting device  5   b  comprises the steps of:
         1. providing a growth substrate (not shown);   2. forming a first semiconductor layer  22 , an active layer  23 , and a second semiconductor layer  24  sequentially on the growth substrate;   3. forming a reflecting layer  53  on the second semiconductor layer  24 ;   4. forming a conductive connecting layer  52  on a conductive substrate  51 ;   5. bonding the conductive substrate  51  with the conductive connecting layer  52  to the reflecting layer  53 ;   6. removing the growth substrate to expose one surface of the first semiconductor layer  22 ;   7. forming an extended electrode layer  25  on the exposed surface of the first semiconductor layer  22 ;   8. forming a first conductive pad  26  on a portion of the extended electrode layer  25  and the first semiconductor layer  22 ;   9. proceeding a thermal treatment step, such as rapid thermal annealing (RTA);   10. wet etching the first semiconductor layer  22  to form a rough surface thereon, wherein the features of the rough surface is described in the previous embodiments, such as  FIG. 3A  and  FIG. 3B ;   11. forming a second conductive pad  27  on the other side of the conductive substrate  51 .       

       FIG. 5C  shows another embodiment of the present invention. The distinction between the light-emitting device  5   c  as shown in  FIG. 5C  and the above-mentioned embodiment is that the rough surface is formed inside the light-emitting device  5   c  instead of on the outer surface of the light-emitting device. 
     The manufacturing method for forming the light-emitting device  5   c  comprises the steps of:
         1. providing a growth substrate (not shown);   2. forming a first semiconductor layer  22 , an active layer  23 , and a second semiconductor layer  24  sequentially on the growth substrate;   3. forming an extended electrode layer  25  on the second semiconductor layer  24 ;   4. proceeding a thermal treatment step, such as rapid thermal annealing (RTA);   5. wet etching the second semiconductor layer  24  to form a rough surface thereon, wherein the features of the rough surface is described in the previous embodiments, such as  FIG. 3A  and  FIG. 3B ;   6. forming a reflecting layer  53  on a conductive substrate  51 ;   7. forming a conductive connecting layer  52  on the reflecting layer  53 ;   8. bonding the extended electrode layer  25  and the second semiconductor layer  24  to the reflecting layer  53  by the conductive connecting layer  52 ;   9. removing the growth substrate to expose one surface of the first semiconductor layer  22 ;   10. forming a first conductive pad  26  on a portion of the exposed surface of the first semiconductor layer  22 ;   11. forming a second conductive pad  27  on the other side of the conductive substrate  51 .       

       FIG. 6  shows a light-emitting device having a lateral structure as another embodiment of the present invention. The light-emitting device  6  and the light-emitting device  5   c  each comprises a rough surface inside the device. The distinction is that the light-emitting device  6  further comprises a transparent conductive layer  63  for electrically coupling the first conductive pad  26  to the second conductive pad  27  and bonding to a transparent substrate  61  by a transparent adhesive layer  62 . At least one of the transparent adhesive layer  62  and the transparent substrate  61  comprises an insulating layer or an insulating structure for electrically isolating the transparent substrate  61  from the transparent conductive layer  63 , such as forming an insulating layer on a transparent conductive substrate. The material of the transparent conductive layer  63  comprises indium tin oxide (ITO), zinc oxide (ZnO) or thin metal. The material of the transparent adhesive layer  62  comprises polyimide (PI), benzocyclobutene (BCB), perfluorocyclobutane (PFCB), epoxy resin, or silicone. The material of the transparent substrate  61  comprises sapphire, glass GaP, SiC, or CVD diamond. The light-emitting device  6  comprises a transparent bonding structure and a transparent substrate to be capable of extracting light from the substrate, and then improve the light extraction efficiency. 
     The manufacturing method for forming the light-emitting device  6  comprises the steps of:
         1. providing a growth substrate (not shown);   2. forming a first semiconductor layer  22 , an active layer  23 , and a second semiconductor layer  24  sequentially on the growth substrate;   3. forming an extended electrode layer  25  on the second semiconductor layer  24 ;   4. proceeding a thermal treatment step, such as rapid thermal annealing (RTA);   5. wet etching the second semiconductor layer  24  to form a rough surface thereon, wherein the features of the rough surface is described in the previous embodiments, such as  FIG. 3A  and  FIG. 3B ;   6. forming a transparent conductive layer  63  the extended electrode layer  25  and the second semiconductor layer  24 ;   7. forming a transparent adhesive layer  62  on a transparent substrate  61 ;   8. bonding the transparent conductive layer  63  to the transparent substrate  61  by the transparent adhesive layer  62 ;   9. removing the growth substrate to expose one surface of the first semiconductor layer  22 ;   10. removing a part of the first semiconductor layer  22 , the active layer  23 , the second semiconductor layer  24 , and the transparent conductive layer  63  to expose a portion of the transparent conductive layer  63 ;   11. forming a first conductive pad  26  on the first semiconductor layer  22 ;   12. forming a second conductive pad  27  on the exposed portion of the transparent conductive layer  63 .       

     Another alternative embodiment for step  10  to step  12  of the method for forming the light-emitting device  6  is to remove a part of the first semiconductor layer  22 , the active layer  23 , the second semiconductor layer  24  to expose a portion of the second semiconductor layer  24 , and form the second conductive pad  27  on the exposed portion of the second semiconductor layer  24 . 
     According to the various embodiments described as above, it is still under the scope of the present invention to form the featured rough surface on both of the first semiconductor layer and the second semiconductor layer to further enhance the light extraction efficiency. 
     It should be noted that the proposed various embodiments are not for the purpose to limit the scope of the invention. Any possible modifications without departing from the spirit of the invention may be made and should be covered by the invention.