Patent Publication Number: US-2015076540-A1

Title: Nitride semiconductor light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-191196, filed Sep. 13, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a nitride semiconductor light emitting device. 
     BACKGROUND 
     Nitride semiconductor light emitting devices are widely used in illuminating devices, video displays, signals transmission, and so on. 
     In these applications, semiconductor light emitting devices having low operating voltages and high optical outputs are generally demanded. 
     In nitride semiconductor light emitting devices, it is typical to provide a p-side electrode and an n-side electrode on one surface side of a semiconductor laminate on which a step portion has been formed, and use the other surface side of the laminate as a light emitting surface. 
     When carriers are intensively injected into a narrow area of a light emitting layer close to the p-side electrode and the n-side electrode, Auger non-radiative recombination and carrier overflow increase. For this reason, the luminous efficiency decreases, and thus it is not possible to obtain the high optical output, and the operating voltage also becomes higher for a given output level. 
     Also, the directional characteristic of emitted light from the light emitting layer and the directional characteristic of wavelength converted light (that is light from the light emitting layer that has been absorbed then re-emitted by a phosphor element) are generally different. For this reason, at the outer peripheral portion of the light emitting surface, chromaticity is different from the center portion of the light emitting surface, and color irregularity will occur in the light from the light emitting device. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a first embodiment. 
         FIG. 1B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 1A . 
         FIGS. 2A to 2D  are schematic views illustrating a process of manufacturing the nitride semiconductor light emitting device according to the first embodiment up to a wafer bonding. 
         FIG. 3A to 3F  are schematic views illustrating the process of manufacturing the nitride semiconductor light emitting device according to the first embodiment after the wafer bonding. 
         FIG. 4A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a first modification of the first embodiment 
         FIG. 4B  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second modification of the first embodiment. 
         FIG. 5A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a first comparative example. 
         FIG. 5B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 5A . 
         FIG. 6A  is a graph illustrating light distribution characteristics for various light emitting devices obtained by simulation. 
         FIG. 6B  is a graph illustrating a dependence of optical output on operating current for various light emitting devices obtained by simulation. 
         FIG. 7A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second embodiment. 
         FIG. 7B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 7A . 
         FIG. 8  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a modification of the second embodiment. 
         FIG. 9  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second comparative example. 
         FIG. 10A  is a graph illustrating light distribution characteristics for various light emitting devices obtained by simulations. 
         FIG. 10B  is a graph illustrating a dependence of optical output on operating current for various light emitting devices obtained by simulation. 
         FIG. 11A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a third embodiment. 
         FIG. 11B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 11A . 
         FIG. 12A  is a graph illustrating light distribution characteristics for various light emitting devices obtained by simulation. 
         FIG. 12B  is a graph illustrating a dependence of optical output on operating current for various light emitting devices obtained by simulation. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of a nitride semiconductor light emitting device provides a higher axial luminous intensity and less color irregularity across the light emitting device. 
     In general, according to one embodiment, a nitride semiconductor light emitting device includes a laminate body, a first electrode, a second electrode, and a phosphor layer having a light emitting surface. The laminate body includes a first layer having a first-conductivity-type, a first portion of a second layer having a second-conductivity-type, and a light emitting layer containing a nitride semiconductor between the first layer and the second layer. The first electrode is formed on a surface of the first layer. The second electrode is formed on a surface of a second portion of the second layer that is between the laminate body and the phosphor layer. At least one of the laminate body, the second portion of the second layer, and the phosphor layer has a lateral width that increases toward the light emitting surface. 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. 
       FIG. 1A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a first embodiment, and  FIG. 1B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 1A . 
     The nitride semiconductor light emitting device includes a laminate  16 , a first electrode  24 , a second electrode  20 , and a phosphor layer  40 . 
     The laminate  16  includes a first layer  14  including a first-conductive-type layer, a second layer  10  including a second-conductive-type layer, and a light emitting layer  12  provided between the first layer  14  and the second layer  10 , the light emitting layer  12  includes a nitride semiconductor material. The outer peripheral portion of the laminate  16  includes a step portion  16   m  formed from a surface of the first layer  14  in to a portion of the second layer  10 . Further, the laminate  16  has a cross section that widens between a base surface  10   c  of the step portion  16   m  and a surface  10   e  (which in  FIG. 1A  is an upper surface of the second layer  10 ) of the second layer  10 . That is, second layer  10  has a side surface (e.g., an edge surface of portion  10   a  of the second layer  10 ) and this side surface and the base surface  10   c  are at an angle greater than 90 degrees (obtuse angle) to each other. 
     The first electrode  24  is provided on the surface of the first layer  14 , and reflects a portion of emitted light from the light emitting layer  12 . Further, the second electrode  20  is provided on the second layer  10  on the base surface  10   c  of the step portion  16   m.    
     The phosphor layer  40  is provided on the surface  10   e  of the second layer  10  on the opposite side to the light emitting layer  12 . Further, a surface of the phosphor layer  40  placed on the opposite side to the light emitting layer  12  is a light emitting surface  40   a . In  FIG. 1A , the light emitting surface  40   a  is an upper surface of the phosphor layer  40 . 
     In the first embodiment, it is assumed that the phosphor layer  40  has a widening cross section, for example, such as, an inverted, truncated square pyramid. The inclination angle of the side surface of the phosphor layer  40  is substantially the same as the inclination angle of the side surface of the second layer  10  of the laminate  16 ; however, these side surface inclination angles may be different from each other in some embodiments. 
     The phosphor layer  40  absorbs light emitted from the light emitting layer  12 , and then emits “wavelength converted” light having a wavelength longer than the wavelength of the emitted light. For example, in a case where the emitted light is blue light, then the phosphor layer  40  can contains a yellow phosphor, a green phosphor, a red phosphor, and the like, the phosphor layer  40  can emit white light or a colored light as mixed light. 
     So the second layer  10  of the laminate  16  can have a predetermined thickness, the second layer  10  placed can be thinned by etching or the like. It is typically preferable to form interfacial irregularities on the etched surface  10   e  because it is possible to improve light-extraction efficiency. When the phosphor layer  40  is formed on the surface  10   e  having irregularities (surface roughness which may sometimes referred to as “concave-convex” structures), it is possible to form irregularities on both surfaces of the phosphor layer  40 , and to further improve the light-extraction efficiency. 
     The nitride semiconductor light emitting device may further include a support  30 . The support  30  has, for example, a third electrode (first connection electrode)  30   a  and a fourth electrode (second connection electrode)  30   b . The first electrode  24  of the surface of the laminate  16  and the third electrode  30   a  of the support  30  are bonded, and the second electrode  20  and the fourth electrode  30   b  of the support  30  are bonded. The support  30  may be formed of Si, SiC, or the like. 
       FIGS. 2A to 2D  are schematic views illustrating a process of manufacturing the nitride semiconductor light emitting device according to the first embodiment, up until wafer bonding. 
       FIG. 2A  is a schematic cross-sectional view illustrating a wafer in which the laminate  16  has been formed on a crystal growth substrate  90  that comprises sapphire, silicon, or the like. The laminate  16  can be formed on the crystal growth substrate  90  by metal-organic chemical vapor deposition (MOCVD) or other methods. The laminate  16  includes the second layer  10 , the light emitting layer  12 , and the first layer  14  stacked on the crystal growth substrate  90  in the stated order. In this example, the first layer  14  includes a p-type layer and the second layer  10  includes an n-type layer; however, the present disclosure is not limited to that specific arrangement. 
     The second layer  10  includes, for example, an n-type GaN cladding layer (where a donor concentration is 5×10 18  cm −3  and whose thickness is 4 μm)  10   a , and a superlattice layer (such as, 30 pairs of well layers with thicknesses of 1 nm and barrier layers with thicknesses of 3 nm)  10   b  which is formed of InGaN/InGaN. The superlattice layer  10   b  may be an undoped layer. Provision of the superlattice layer  10   b  can reduce lattice-mismatch defects in the nitride semiconductor layers. 
     The light emitting layer  12  may be formed of an InGaN/InGaN undoped multi-quantum well (MQW) layer (such as 3.5 pairs of well layers having thicknesses of 3 nm and barrier layers having thicknesses of 5 nm). In this case, emitted light from the light emitting layer  12  may have a wavelength in a bluish purple (violet) to blue range. 
     The first layer  14  includes, for example, a p-type AlGaN overflow preventing layer  14   a  (where an acceptor concentration is 1×10 20  cm −3  and whose thickness is 5 nm), a p-type cladding layer  14   b  (where an acceptor concentration is 1×10 20  cm −3  and whose thickness is 100 nm), a p-type contact layer  14   c  (where an acceptor concentration is 1×10 21  cm −3  and whose thickness is 5 nm), and the like. 
     Subsequently, as shown in  FIG. 2B , on the first layer  14 , the first electrode  24  is formed. The first electrode  24  may be formed of gold (Au), a metal multi-layer film containing Au, a multi-layer film containing silver (Ag) at its surface, or the like. It is typically preferable that the first electrode  24  have Ag at its surface because it is possible to obtain high reflectance even with respect to a short wavelength of bluish purple to blue. 
     Subsequently, as shown in  FIG. 2C , etching or the like is performed on the laminate  16 , whereby a step portion  16   m  is formed from the surface of the first layer  14  up to a portion of the second layer  10 . The base surface  10   c  of the step portion  16   m  may protrude to the n-type GaN cladding layer ( 10   a ) side. 
     Subsequently, as shown in  FIG. 2D , on the base surface  10   c  of the step portion  16   m , the second electrode  20  is formed. The second electrode  20  may be formed of, for example, Au, or a metal multi-layer film containing Au. 
     Meanwhile, on the support  30 , the electrodes  30   a  and  30   b  are formed to contain Au or the like at their surfaces. The support  30  and the laminate  16  on the crystal growth substrate  90  are bonded by wafer bonding using heating, pressing, and the like, such that the electrode  30   a  and the first electrode  24  are bonded and the electrode  30   b  and the second electrode  20  are bonded. 
       FIGS. 3A to 3F  are schematic views illustrating the process of manufacturing the nitride semiconductor light emitting device according to the first embodiment after the wafer bonding. 
     By wafer bonding, it is possible to obtain a structure shown in  FIG. 3A . After bonding, as shown in  FIG. 3B , the crystal growth substrate  90  is removed. Next, as shown in  FIG. 3C , the second layer  10  is thinned (e.g., by polishing or grinding) to a predetermined thickness, then the phosphor layer  40  is formed on the second layer. The phosphor layer  40  may be formed by mixing Yttrium-Aluminum-Garnet (YAG) phosphor particles or the like in a transparent pre-resin liquid, applying the mixture, and then performing thermal curing or the like to set the resin. 
     Subsequently, as shown in  FIG. 3D , unnecessary layer portions are removed such that the support  30  has a predetermined size. 
     Subsequently, as shown in  FIG. 3E , the cladding layer  10   a  of the second layer  10  is removed by etching or the like, such that the cladding layer  10   a  has a predetermined size and its outer edge surface has a predetermined inclination angle. As shown in  FIG. 3F , division of the light emitting elements is performed by etching or dicing in a manner such that the resulting outer edge surface of the phosphor layer  40  has a predetermined inclination angle. This division process is not limited to etching or dicing methods and the outer surface of phosphor layer  40  may be formed in the predetermined inclination angle in a separate step from a primary etching/dicing step. 
     The planar shape of the nitride semiconductor light emitting device may be set to have a 0.5 mm first side length L1 and a second side length L2 equal to L1. L1 and L2 need not be equal and the planar shape of the light emitting may have a rectangular or other shape. 
     Optionally the second layer  10  may be etched up to the middle thereof (that is, a portion of the second layer from a surface level to an interior level may be removed by etching), and the edge portion side surface of the second layer  10  may be etched such that its side surface (lateral surface) is inclined, and the support  30  may be partially removed, and the division may be performed by etching and dicing such that the side surface of the phosphor layer  40  is inclined. 
     The operational effects of the first embodiment will be described. The first electrode  24  being formed to widely cover the surface of the light emitting layer  12  and provides a relatively short travel distance for charge carriers to the light emitting layer  12 , and thus is likely to widely spread carriers into a luminous area ER of the light emitting layer  12 . For this reason, it is possible to lower the probability of Auger non-emitting (non-radiative) recombination and carrier overflow, and to thereby improve luminous efficiency. Auger recombination dissipates energy generated by recombination to other carriers rather than emitting photons, thereby causing non-emitting recombination, resulting in a reduction in the luminous efficiency. The probability of Auger recombination increases as an electron concentration or a hole concentration increases. As a result, a reduction in the luminous efficiency in a high-current operation is suppressed, and it is possible to further improve the optical output. 
     When the second electrode  20  is set as the n-side electrode, it is possible to spread electrons, which have mobility higher than that of holes, into the luminous area ER of the light emitting layer  12 . But since the first electrode (the p-side electrode) is provided to widely cover the surface of the light emitting layer  12  and has a relatively short travel distance to the light emitting layer  12 , the first electrode  24  is likely to spread holes, which having mobility lower than that of electrons, into the luminous area ER of the light emitting layer  12 . Therefore, it is possible to further improve the luminous efficiency. As a result, it is possible to further improve the optical output in a high-current operation. 
     In the first embodiment, at the outer surface  10   g  of the laminate  16  and the outer surface  40   b  of the phosphor layer  40 , it is possible to reflect emitted light g1, which is initially directed outward, inward toward the central axis of the light emitting device. Thus, in the vicinity the central axis of the nitride semiconductor light emitting device, the light intensity (luminous intensity) of wavelength converted light and emitted light from the light emitting layer  12  is improved, and the percentage of emitted light passing through the outer peripheral portion of the phosphor layer  40  decreases. As a result, color irregularity of mixed light at the outer peripheral portion of the nitride semiconductor light emitting device improves (decreases). 
       FIG. 4A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a first modification of the first embodiment, and  FIG. 4B  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second modification of the first embodiment. 
     As shown in  FIG. 4A , the side surface of the phosphor layer  40  need not be inclined, and only the side surface of the laminate  16  is inclined. Alternatively, as shown in  FIG. 4B , an inclined surface can be formed up to the middle of the side surface and then cutting may be performed by dicing or the like, whereby device division may be performed. The inclined surface thus formed may be a curved surface, as depicted in  FIG. 4B . 
       FIG. 5A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a first comparative example, and  FIG. 5B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 5A . 
     In the nitride semiconductor light emitting device according to the first comparative example, the side surface of a phosphor layer  140  and the side surface of a laminate  116  are perpendicular to the surface of a support  130 . A large proportion of light gg emitted laterally (left-right page direction) from the a light emitting layer  112  is emitted from the side surface of the edge portion side surface or the side surface of a step portion  116   m  to the outside. For this reason, it is difficult to improve optical output. As depicted in  FIG. 5B , the surface  110   c  of laminate  116  has a projected planar area equal to a projected planar area of the interface between phosphor layer  140  and laminate  116 . 
       FIG. 6A  is a graph illustrating light distribution characteristics obtained by simulation, and  FIG. 6B  is a graph illustrating dependence of optical output on operating current obtained by simulation. In  FIG. 6A , the X-axis corresponds to the relative horizontal position (left-right page direction) in the cross-sectional views, such as in  FIG. 1A ,  FIG. 4A ,  FIG. 4B , and  FIG. 5A . The Y-axis in  FIG. 6A  corresponds to the relative vertical position in the cross-sectional views. 
     As simulated, the first embodiment has improved (greater) luminous intensity in vicinity of central area of the phosphor layer  40  when compared to the first comparative example. Therefore, it is possible to reduce color irregularity at the outer peripheral portion of the phosphor layer  40 . The axial luminous intensity of the modification of the first embodiment is between the first embodiment and the first comparative example. Further, as shown in  FIG. 6B , it is possible to make the overall optical output intensity of the first embodiment and a modification of the first embodiment equal to or greater than that of the first comparative example. Therefore, it is possible to make device brightness increase even as the axial luminous intensity increases. 
       FIG. 7A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second embodiment, and  FIG. 7B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 7A . As depicted in  FIG. 7B , the second electrode  20  contacts base surface  10   c  of the second layer  10  at central portion of the second layer such that the second electrode is between outer edges of the laminate  16 . 
     The laminate  16  has a step portion (recess)  16   m  formed from the surface of the first layer  14  up to a portion of the second layer  10  at the central portion of base surface  10   c . As depicted, a portion  10   b  of second layer  10  may be at a level below base surface  10   c . The outer surface  16   j  of the laminate  16  is inclined such that the cross section of the laminate  16  widens toward the light emitting surface  40   a  of the phosphor layer  40  as shown in  FIG. 7A . In this case, a portion g2 of emitted light from the light emitting layer  12  toward the outer surface  16   j  is reflected by the inclined outer surface  16   j . Therefore, it is possible to improve light-extraction efficiency at the light emitting surface  40   a  in an upward direction. In this example, the cross-section of the phosphor layer  40  also widens toward the light emitting surface  40   a.    
       FIG. 8  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a modification of the second embodiment. 
     In this modification of the second embodiment, only the outer surface  16   j  of the laminate  16  is inclined and the cross-section of the phosphor layer  40  does not widen with distance from the interface between laminate  16  and phosphor layer  40 . Even in this modification of the second embodiment, it is possible to emit a portion of light reflected from the outer surface  16   j  upward from the light emitting surface  40   a.    
       FIG. 9  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second comparative example. 
     The outer edge surfaces of the phosphor layer  140  and the outer edge surfaces of laminated layers, which are on support  130  surrounding a second electrode  120 , are substantially perpendicular to an emitting surface  140   a , and thus it is difficult to concentrate laterally emitted light (e.g., g4) toward the central axis direction of the phosphor layer  140 . 
       FIG. 10A  is a graph illustrating light distribution characteristics obtained by simulations, and  FIG. 10B  is a graph illustrating dependence of optical output on operating current obtained by simulation. 
     In  FIG. 10A , the horizontal axis X corresponds to the relative horizontal position of the cross-sectional view of  FIG. 7A ,  FIG. 8 , or  FIG. 9 . Further, the vertical axis Y corresponds to the relative vertical position in the cross-sectional view of  FIG. 7A ,  FIG. 8 , or  FIG. 9 . 
     As shown in  FIG. 10A , the luminous intensity in the vicinity of an axial area of a nitride semiconductor light emitting device according to a modification is higher than the luminous intensity in the vicinity of an axial area of the second comparative example. Further, it is possible to make the luminous intensity in the vicinity of an axial area of the second embodiment higher than that of the modification. That is, when an inclined surface is formed, the luminous intensity in the vicinity of an axial area is improved. 
     As shown in  FIG. 10B , when operating current is 1,000 mA, the optical output of the second comparative example is about 810 mW. And when operating current is 1,000 mA, the optical output of the second embodiment is about 930 mW, which is about 15% greater than that of the second comparative example. 
       FIG. 11A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a third embodiment, and  FIG. 11B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 11A . 
     The laminate  16  includes a step portion (recess)  16   m  at the central portion of the laminate  16 —that is, step portion  16   m  is formed in the center, as viewed from a direction perpendicular to surface  10   c , of laminate  16 . The inner surface  16   k  of the step portion  16   m  is inclined such that the width of the laminate  16  widens toward the phosphor layer  40 . Light g5 emitted from the light emitting layer  12  and directed toward the inner surface  16   k  is reflected by the inner surface  16   k  upward towards the light emitting surface  40   a.    
       FIG. 12A  is a graph illustrating light distribution characteristics obtained by simulation, and  FIG. 12B  is a graph illustrating dependence of optical output on operating current obtained by simulation. 
     As shown in  FIG. 12A , it is possible to make the luminous intensity in the vicinity of an axial area of the third embodiment higher than the luminous intensity in the vicinity of an axial area of the second comparative example shown in  FIG. 9 . That is, when an inclined surface is formed on the surface of step portion  16   m , the luminous intensity in the vicinity of an axial area of the device can be improved relative to the second comparative shown in  FIG. 9 . Further, as shown in  FIG. 12B , when operating current is 1,000 mA, the optical output of the third embodiment is about 870 mW which is about 9% greater than the optical output of the second comparative example, which is 800 mW. 
     According to the first to third embodiments, it is possible to provide a nitride semiconductor light emitting device having higher axial luminous intensity and less color irregularity in mixed light emitted from the device. Such nitride semiconductor light emitting devices may be used in a wide variety of applications such as in illuminating devices, displays, signals transmission, and the like. 
     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 embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.