Patent Publication Number: US-8981398-B2

Title: Semiconductor light emitting device and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 13/450,063, filed Apr. 18, 2012, which is a continuation of U.S. Ser. No. 12/874,475, filed Sep. 2, 2010, now U.S. Pat. No. 8,178,891, and claims the benefit of priority from prior Japanese Patent Application No. 2010-46905 filed on Mar. 3, 2010 in Japan, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate to a semiconductor light emitting device and a method for manufacturing the semiconductor light emitting device. 
     BACKGROUND 
     To achieve high efficiencies and high outputs, nitride semiconductor light emitting devices (hereinafter also referred to as LEDs (Light Emitting Diodes)) designed for white lighting devices are being improved in crystalline structures and device structures, and higher internal quantum efficiencies and higher light extraction efficiencies are being realized. 
     Where a nitride semiconductor is crystal-grown, a sapphire substrate is often used, because it is inexpensive, and stable in high temperature. A crystal growth with high crystallinity can be performed on a sapphire substrate with a low-temperature buffer. However, being an insulator, a sapphire substrate does not have conductive properties and is low in thermal conductivity. Therefore, electrodes cannot be formed on the back face side of a sapphire substrate, and p- and n-electrodes need to be formed on the nitride semiconductor side. Therefore, the tendency to cause higher series resistance and the low heat release properties during a high-power operation become problems in achieving even higher efficiencies and outputs. 
     A thin-film nitride semiconductor LED is known as one of the LED structures that eliminate the above problems and improve luminous efficiencies and outputs. Such a thin-film nitride semiconductor LED transfers LED structural crystals grown on a sapphire substrate onto another supporting substrate such as a Si substrate, a copper substrate, or a gold substrate. As devices are formed after the transfer onto a supporting substrate having conductive properties and high thermal conductivity, the current spread becomes larger by vertical energization, and the electric conductive properties are improved. Further, the heat release properties are also improved. Also, by forming a structure that has an n-layer as an upper face through a transfer and extracts light from the n-layer side, a transparent electrode for diffusing current becomes unnecessary for the n-layer having lower resistance than a p-layer. Since light is not absorbed by a transparent electrode, the light extraction efficiency becomes higher. 
     This process of transfer used here includes a process to bond crystals (epitaxial crystals) formed through an epitaxial growth to the supporting substrate, and a lift-off process to detach the epitaxial crystals from the sapphire substrate. The bonding process may involve a plating technique or a joining technique utilizing weight and heat, and the lift-off process may involve a laser lift-off technique utilizing thermolysis of an interface caused by a laser or a chemical lift-off technique. 
     In such a thin-film LED structure, the difference in refractive index between the surface of a GaN substrate and the external air is as large as 2.5 times where only a laser lift-off process has been carried out, and the light reflection from the boundary face lowers the light extraction efficiency. 
     To counter this problem, a technique of producing concavities and convexities on the surface of each chip has been suggested. The concavities and convexities are formed by regrowing, polishing, and etching an n-type nitride semiconductor layer. According to a method for simple formation, concavities and convexities are formed by roughening the surface through alkaline etching performed on the n-layer on the upper face of a GaN substrate on a supporting substrate. In this manner, the light extraction efficiency is made higher. To sufficiently increase the light extraction efficiency, it is necessary to subject each device containing epitaxial crystals to processing in an alkaline solution for a sufficiently long period of time. Therefore, formation of a protection film that protects the epitaxial crystals and prevents short-circuiting and leakage is critical. 
     However, if a conventional protection film is subjected to long-time processing with a high-density alkaline solution, not only the surface of the subject n-layer but also the side faces of the epitaxial crystals and the active layer are etched, resulting in luminous efficiency degradation, leakage, and short-circuiting. Also, cracks might be formed in the protection film and the epitaxial crystals due to the load and the variation in temperature during the joining process and the large impact of the gas pressure or the like caused by the laser lift-off process performed when thin-film LEDs are formed. Therefore, the challenge is to obtain a highly-reliable semiconductor light emitting device that has such a rough surface as to achieve higher light extraction efficiency in the surface of a nitride semiconductor, and has fewer defects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a ) through  1 ( c ) are cross-sectional views showing procedures for manufacturing semiconductor light emitting devices according to an embodiment; 
         FIGS. 2(   a ) and  2 ( b ) are cross-sectional views showing procedures for manufacturing semiconductor light emitting devices according to the embodiment; 
         FIGS. 3(   a ) through  3 ( c ) are cross-sectional views showing procedures for manufacturing semiconductor light emitting devices according to the embodiment; 
         FIG. 4  is an electron micrograph of the surface having concavities and convexities formed thereon in a semiconductor light emitting device according to the embodiment; 
         FIGS. 5(   a ) and  5 ( b ) are electron micrographs of cross sections of semiconductor light emitting devices according to the embodiment and a comparative example; 
         FIG. 6  is a plan view of the supporting substrate prior to the division into respective devices; 
         FIG. 7  is a cross-sectional view of a semiconductor light emitting device according to the embodiment; and 
         FIG. 8  is a graph showing the light extraction efficiencies of semiconductor light emitting devices according to the embodiment and the comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     Certain embodiments provide a semiconductor light emitting device including: a substrate having a first face and a second face opposed to the first face; a first metal layer having a lower face facing to the first face of the substrate and an upper face; a stack film including a p-type nitride semiconductor layer having a lower face facing to the upper face of the first metal layer and an upper face, an active layer provided on the upper face of the p-type nitride semiconductor layer and including a multiquantum well structure of a nitride semiconductor, and an n-type nitride semiconductor layer having a lower face facing to the active layer and an upper face, the stack film having a tapered shape in cross-section, with an area of a film plane gradually increasing from the n-type nitride semiconductor layer toward the p-type nitride semiconductor layer; an n-electrode provided in a partial region of the upper face of the n-type nitride semiconductor layer; a p-electrode provided on the second face of the substrate; a contact electrode provided in a partial region of the lower face of the p-type nitride semiconductor layer; a second metal layer having a lower face facing to the upper face of the first metal layer and an upper face facing to the lower face of the p-type nitride semiconductor layer, the second metal layer covering the contact electrode and being in contact with the contact electrode and the first metal layer, and the second metal layer having a minimum diameter that is smaller than a minimum diameter of the upper face of the first metal layer but is larger than a minimum diameter of the lower face of the p-type nitride semiconductor layer; and a protection film protecting an outer circumferential region of the upper face of the n-type nitride semiconductor layer, side faces of the stack film, a region of the upper face of the second metal layer other than a region in contact with the p-type nitride semiconductor layer, and a region of the upper face of the first metal layer other than a region in contact with the second metal layer, wherein concavities and convexities are formed in a region of the upper face of the n-type nitride semiconductor layer, the region being outside the region in which the n-electrode is provided and being outside the regions covered with the protection film. 
     The following is a detailed description of an embodiment, with reference to the accompanying drawings. 
     Referring to  FIGS. 1(   a ) through  4 , semiconductor light emitting devices according to the embodiment are described.  FIGS. 1(   a ) through  3 ( c ) show the procedures for manufacturing semiconductor light emitting devices according to the first embodiment. 
     First, nitride semiconductor layers are sequentially grown on a substrate (a wafer) for growing nitride semiconductor crystals or a sapphire substrate  10  by metal organic chemical vapor deposition (MOCVD), for example. More specifically, a GaN layer  12  to be a buffer layer, an n-type GaN layer  14 , an active layer  16  of a multiquantum well structure made of InGaN, and a p-type GaN layer  18  are sequentially grown in this order on the sapphire substrate  10  ( FIGS. 1(   a )). 
     P-electrodes (reflecting contact electrodes)  20  are then formed with stack films of Ni and Ag on the p-type GaN layer  18  ( FIG. 1(   b )). The p-electrodes  20  are formed for respective semiconductor light emitting devices. An adhesive metal film  22  having Ti, Pt, and Au films that are to serve as adhesive metals and are stacked in this order is formed over the nitride semiconductor crystal films  12 ,  14 ,  16 , and  18 , to cover the p-electrodes  20  ( FIG. 1(   b )). With this arrangement, the portions of the adhesive metal film  22  in the regions where the p-electrodes  20  are formed are turned into convex portions, and the portions of the adhesive metal film  22  in the regions where the p-electrodes  20  are not formed are turned into concave portions ( FIG. 1(   b )). Patterning is then performed on the adhesive metal film  22  by a known lithography technique. After that, patterning is further performed on the stack film (the nitride semiconductor crystal films) including the p-type GaN layer  18 , the active layer  16 , the n-type GaN layer  14 , and the GaN layer  12  ( FIG. 1(   c )). 
     Through the patterning, the nitride semiconductor crystal films on the wafer are turned into a mesa having a tapered shape in cross-section, with the area of the film plane gradually increasing from the area of the film plane of the p-type GaN layer  18  to that of the GaN layer  12 . Here, the “film plane” means the upper plane of each of the layers. When patterning is performed on the stack film, a patterned adhesive metal film may be used as a mask. Alternatively, patterning may be performed on the stack film before the adhesive metal film  22  is formed, and after the patterning, the adhesive metal film  22  may be formed. 
     Meanwhile, an Au—Sn layer  32  to be an adhesive metal film is formed on a Si substrate  30  to be a supporting substrate ( FIG. 2(   a )). The adhesive metal film  22  on the sapphire substrate  10  and the adhesive metal film  32  on the Si substrate  30  are placed to face each other, and pressure is applied to them at a high temperature of 250° C. or higher over a certain period of time, so that the adhesive metal film  22  on the sapphire substrate  10  and the adhesive metal film  32  on the Si substrate  30  are bonded to each other. In this bonding, the contact electrodes  20  are buried into the adhesive metal film  32 , since the melting-point temperature of the contact electrodes  20  is much higher than the melting-point temperature of the adhesive metal film  32  ( FIG. 2(   a )). 
     As shown in  FIG. 2(   b ), pulse irradiation is then performed with a UV (Ultra-Violet) laser or a KrF laser of 248 nm in wavelength from the side of the sapphire substrate  10 , for example, so as to detach the sapphire substrate  10  from the nitride semiconductor crystal films  12 ,  14 ,  16 , and  18 . The surface of the GaN layer  12  exposed at this point is the surface to be subjected to wet etching. 
     Patterning is then performed on the nitride semiconductor crystal films  12 ,  14 ,  16 , and  18  by a known lithography technique, to divide the nitride semiconductor crystal films  12 ,  14 ,  16 , and  18  into semiconductor light emitting devices. At this point, patterning is not performed on the adhesive metal film  22 , and the adhesive metal film  22  is left exposed among the nitride semiconductor crystal films divided into the semiconductor light emitting devices. The patterned nitride semiconductor crystal films are turned into mesas each having a tapered shape in cross-section, with the area of the film plane gradually increasing from the area of the film plane of each GaN layer  12  to that of each p-type GaN layer  18  ( FIG. 3(   a )). 
     A SiO 2  film  40  as a protection film is then formed to cover the surfaces of the nitride semiconductor crystal films of tapered shapes and the exposed adhesive metal films  22  and  32 , for example ( FIG. 3(   b )). The nitride semiconductor crystal films form mesa structures, the minimum diameter of each lower face of the nitride semiconductor crystal films in contact with the adhesive metal film  22  is smaller than the minimum diameter of the upper face of the adhesive metal film  22 , and the minimum diameter of the lower face of the adhesive metal film  22  in contact with the adhesive metal film  32  is smaller than the minimum diameter of the upper face of the adhesive metal film  32 . In other words, the structure has a folding-fan shape. Accordingly, the adhesive metal film  22  is in tight contact with the peripheral end region of each lower portion of the nitride semiconductor crystal films each having a mesa shape, and the protection layer  40  without a step separation can be formed, without a void formed between the protection layer  40  and the adhesion metal films  22  and  32 . 
     The protection layer  40  covering the upper face of each semiconductor light emitting device is then removed. However, the protection layer  40  remains on the outer circumferential region of the upper face of each semiconductor light emitting device (the upper face of each GaN layer  12 ). With this arrangement, the upper face of each semiconductor light emitting device is exposed, except for the outer circumferential region of each upper face ( FIG. 3(   c )). N-electrodes  44  are then formed at the center portions of the exposed upper faces of the GaN layers  12  ( FIG. 3(   c )). As the material of the n-electrodes  44 , it is preferable to use an alkali-resistant electrode material. It is particularly preferable to use a material containing one of the following metals: Pt, Au, Ni, and Ti. By using such a material, the sizes (height differences) of the concavities and convexities formed in the upper faces of the GaN layers  12  by the later described alkaline etching can be made larger. 
     After that, etching is performed on the exposed upper faces of the GaN layers  12  with the use of an alkali solution, to roughen the exposed upper faces of the GaN layers  12 . In this manner, the GaN layers  12  are turned into GaN layers  12   a  each having concavities and convexities formed in its exposed upper face ( FIG. 3(   c )). This is supposedly because electrons or holes travel between the surfaces of the GaN layers  12  and the n-electrodes  44  at the time of etching, and an electrochemical reaction is caused in each surface, accelerating the etching. In this embodiment, a potassium hydroxide solution of 1 mol/l in density and 70° C. in temperature is used as the alkaline solution, and etching is performed for 15 minutes. As the etching smoothly progresses, the surface becomes clouded. While being immersed in the potassium hydroxide solution, the concavities and convexities are exposed to UV rays, and can be made even larger as a result. The concavities and convexities can also be made larger by performing etching while a voltage of is intermittently applied between the n-electrodes  44  and the GaN layers  12 . The sizes of the concavities and convexities are several hundreds of nanometers to several micron meters. 
       FIG. 4  shows an electron micrograph of the upper face of a GaN layer  12  having concavities and convexities formed in the above described manner. As can be seen from  FIG. 4 , the concavities and convexities vary in size. Accordingly, the reflection from the boundary surface between each GaN layer  12  and the air becomes smaller, and the light extraction efficiency can be made higher. 
     In this embodiment, the minimum diameter of the lower face in contact with the adhesive metal film  22  is smaller than the minimum diameter of the upper face of the adhesive metal film  22 , and the minimum diameter of the lower face of the adhesive metal film  22  in contact with the adhesive metal film  32  is smaller than the minimum diameter of the upper face of the adhesive metal film  32 . Because of this folding-fan shape, part of the upper face and the side faces of the nitride semiconductor crystal films, and the joined portions between the side faces and the adhesive metal are covered with the protection layer  40  without a step separation. Accordingly, even if the upper face of the nitride semiconductor crystal films is roughened with the use of an alkaline solution, the active layer and the reflecting contact electrodes  20  can be thoroughly protected. Thus, higher reliability and higher light extraction efficiency are achieved. Also, reflecting contact electrodes  20  each having a large area can be formed, and the reflectivity can be made higher. Furthermore, a decrease in operating voltage can be expected, since large reflecting contact electrodes can be formed. Also, since the protection layer  40  without a step separation is formed, leakage and short-circuiting in devices due to metal adherence or the like during the manufacturing procedures can be prevented. Further, since the protection layer  40  without a step separation is formed, the process to manufacture thin-film semiconductor light emitting devices can be tolerated, even though the process involves intensified impacts from the transfer, the bonding, and the laser lift-off technique, for example. Also, cracks and the likes are not formed in the protection layer  40 . 
     As a comparative example of this embodiment, a semiconductor light emitting device is formed. This semiconductor light emitting device is manufactured in the same manner as in this embodiment, except that the area of the lower face of the nitride semiconductor crystal films in contact with the adhesive metal film  22  is the same as the area of the upper face of the adhesive metal film  22 .  FIGS. 5(   a ) and  5 ( b ) show electron micrographs of sections of this embodiment and the comparative example, respectively. As can be seen from  FIGS. 5(   a ) and  5 ( b ), no step separations are formed in the protection layer in this embodiment, but a step separation is formed in the peripheral end region of the lower portion of the nitride semiconductor crystal films in a mesa shape in the comparative example. If a step separation is formed in a protection layer, cracks and the likes are easily formed in the protection layer during the manufacturing process. Also, if a protection layer having a step separation is formed as in the comparative example, the alkaline etching solution enters the device through the step separation and cracks, and the active layer and the reflecting contact electrode are etched. As a result, current leakage occurs, and outputs are degraded. 
       FIG. 6  is a plan view of semiconductor light emitting devices seen from the side of the n-electrodes  44  after the concavities and convexities are formed. As can be seen from  FIG. 6 , undivided devices are placed on the Si substrate  30 . After that, a p-electrode  46  is formed on the face of the silicon substrate  30  on the opposite side from the side on which the n-electrodes  44  are formed, as shown in  FIG. 3(   c ). 
     After the procedures shown in  FIG. 3(   c ) are completed, dicing is performed on the protection layer  40 , the adhesive metal films  22  and  32 , and the Si substrate  30 , to divide them into respective semiconductor light emitting devices. In this manner, the semiconductor light emitting device shown in  FIG. 7  is completed. The semiconductor light emitting devices of the above described comparative example are also divided by dicing. The light extraction efficiency of each semiconductor light emitting device of this embodiment manufactured in the above described manner is 1.5 times higher than the light extraction efficiency of each semiconductor light emitting device of the comparative example, and is 1.3 times higher than the light extraction efficiency observed in a case where alkaline etching is not performed, as shown in  FIG. 8 . 
     As described so far, this embodiment can provide semiconductor light emitting devices having high light extraction efficiency, and a method for manufacturing such semiconductor light emitting devices. 
     The supporting substrate may be a silicon substrate, a silicon carbide substrate, a substrate formed by bonding germanium to a silicon substrate, or a substrate formed by plating a silicon substrate with a metal such as copper. The silicon substrate may be a substrate that has a plane orientation of (111), (110), or (100), and also has an off angle. 
     As for the protection layer, it is preferable to use a material that contains silicon dioxide, silicon nitride, zirconium oxide, niobium oxide, or aluminum oxide. 
     The chemical solution used in the alkaline etching may be tetramethylammonium hydroxide, other than potassium hydroxide. 
     As the reflecting contact electrodes, it is desirable to use aluminum, other than silver. 
     The adhesive metal film  22  preferably contains titanium, platinum, gold, or tungsten. 
     As the adhesive metal film  32 , it is possible to use a low-melting-point metal that is a metal eutectic such as Au—Si, Ag—Sn—Cu, or Sn—Bi, or a non-solder material such as Au, Sn, or Cu, other than Au—Sn. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the sprit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and sprit of the invention.