Patent Publication Number: US-2015060761-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. 2013-179836, filed Aug. 30, 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 preferred. 
     In nitride semiconductor light emitting devices, it is common to provide a p-side electrode and an n-side electrode on one side of a semiconductor laminate in which a step portion is formed, and then use the other side of the laminate as a light emitting surface. 
     When charge carriers are intensively injected into a narrow peripheral 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 high optical output cannot be obtained, and the operating voltage also becomes higher. 
     Further, when light emitting areas are concentrated in the peripheral area of the laminate, the ratio of light which is emitted from the side surface of the chip increases. For this reason, chromaticity is different between the central portion and peripheral portion of the chip, and color irregularity becomes likely to occur due to optical path length differences. 
    
    
     
       DESCRIPTION OF THE 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 taken 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. 
         FIG. 3A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a comparative example, and  FIG. 3B  is a schematic plan view taken along a line A-A of  FIG. 3A . 
         FIG. 4A  is a graph illustrating dependence of optical output on current, and  FIG. 4B  is a graph for comparing light distribution patterns. 
         FIG. 5A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second embodiment, and  FIG. 5B  is a schematic plan view taken along a line A-A of  FIG. 5A . 
         FIG. 6  is a graph illustrating the optical output of the nitride semiconductor light emitting device according to the second embodiment. 
         FIG. 7A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a third embodiment, and  FIG. 7B  is a schematic plan view taken 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 fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments provide a nitride semiconductor light emitting device having less color irregularity and higher optical output. 
     In general, according to one embodiment, a nitride semiconductor light emitting device includes a first layer of a first-conductivity type layer, first and second protrusions each disposed on a first side of the first layer and extending from the first layer in a first direction and spaced apart from each other in a second direction perpendicular to the first direction, a first electrode disposed on the first side of the first layer and between the first and second protrusions, a phosphor layer disposed on a second side of the first layer that is opposite the first side, and a second electrode disposed on each of the first and second protrusions on a side opposite the first layer. The first and second protrusions each includes a second layer having a second-conductivity type, and a light emitting layer disposed between the first layer and the second layer. 
     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  10 , a first electrode  60 , a second electrode  50 , and a phosphor layer  40 . Further,  FIG. 1A  is a schematic cross-sectional view taken along a line B-B of  FIG. 1B . 
     The laminate  10  includes a first layer  20  including a first-conductivity type layer  22  (a doped layer), a second layer  30  including a second-conductivity type layer, and a light emitting layer  12  is provided between the first layer  20  and the second layer  30  and contains a nitride semiconductor. The laminate  10  includes at least two ridge portions having step portions formed from the surface  30   a  of the second layer opposite to the light emitting layer  12  up to the first-conductivity type layer  22  of the first layer  20 . 
     The first electrode  60  is formed on a surface  22   b  (base surface) between a first ridge portion  14  and a second ridge portion  16 . As depicted in  FIG. 1B , the portions of first electrode  60  between ridge portions are connected to each other by another portion of first electrode  60  that is not directly between the ridge portions and extends in a direction parallel to a direction crossing the ridge portions formed on the first-conductivity type layer  22 . The portion of first electrode  60  formed between first ridge portion  14  and second ridge portion  16  is formed so as to be adjacent to a step portion  14   a  formed in the longitudinal direction (e.g., the top-bottom page direction of  FIG. 1B ) of the first ridge portion  14  and a step portion  16   a  formed in the longitudinal direction of the second ridge portion  16 . Further, the second electrode  50  is formed on the surface  30   a  of the second layer  30 . In  FIG. 1B , the first electrode  60  has five stripe portions (portions between ridge portions), and is open toward one direction (downward in  FIG. 1B ); however, the tip portions of the stripe portions may also be connected to each other with an additional portion of electrode  60 , or similar conductive material (s). 
     The phosphor layer  40  is formed on the surface  20   a  of the first layer  20  opposite to the light emitting layer  12 —that is, the surface  20   a  is on the layer face of first layer  20  that is not in contact with the light emitting layer  12 . The phosphor layer  40  has a side surface  40   b  provided outside (that is, beyond the outside edges) of the first ridge portion  14  and the second ridge portion  16 . Thus, in  FIG. 1B , the outside edge of phosphor layer  40  (which is the projection of the side surface  40   b  on to the page plane of  FIG. 1B ) is beyond the outside edges of the depicted ridge portions. Although at least two ridge portions are provided in the present embodiment, in  FIGS. 1A and 1B , a case where four ridge portions are provided is shown. The phosphor layer  40  can be formed by mixing Yttrium-Aluminum-Garnet (YAG) phosphor particles or the like in transparent resin liquid, applying the mixture, and performing thermal curing or the like. 
     The surface of the phosphor layer  40  opposite to the laminate  10  becomes a light emitting surface  40   a  for light from the light emitting layer  12 . The phosphor layer  40  can serve to absorb at least some portion of emitted light from light emitting layer and then emit light at a wavelength longer than the wavelength of the emitted light from the light emitting layer. For example, in a case where the emitted light from light emitting layer  12  is blue light, and when the phosphor layer  40  contains a yellow phosphor, the blue light is converted to a yellow light that can be mixed to form white light. Furthermore, various phosphors can be included in phosphor layer  40  as required by various potential applications, such that the phosphor layer  40  can contain a green (to yellow) phosphor and a red phosphor, such that blue light emitted light from the light emitting layer  12  can be mixed with green (to yellow) light and red light, provided by the various included phosphors, to form white light. 
     In the first embodiment, a horizontal distance Dh from the center of the light emitting layer  12  to the center of a corresponding first electrode  60  can be decreased as the number of ridge portions increases. Therefore, it is possible to provide an approximately uniform carrier density distribution inside the light emitting layer  12 . Further, when the planar shape of each of at least two ridge portions is set to a rectangular shape, it is possible to make the horizontal distance Dh along the shorter-side direction of the corresponding rectangular ridge portion shorter than that in a case where the planar shape is a square shape having the same area. Therefore, a luminous area EL (shown by a dotted line) of the inside of the light emitting layer  12  uniformly broadens, and the ratio of blue light BL to longer wavelength light (e.g., yellow light) emitted from the side surface  40   b  of the phosphor layer  40  decreases, and color irregularity is suppressed. 
       FIGS. 2A to 2D  are schematic views illustrating a process of manufacturing the nitride semiconductor light emitting device according to the first embodiment. That is,  FIG. 2A  is a schematic cross-sectional view illustrating a laminate that has been formed on a substrate for crystal growth,  FIG. 2B  is a schematic cross-sectional view illustrating a structure in which a metal layer  51  has been formed on the surface of the laminate  10 ,  FIG. 2C  is a schematic cross-sectional view illustrating a structure in which ridge portions have been formed, and  FIG. 2D  is a schematic cross-sectional view illustrating a structure in which a first electrode  60  has been formed. 
     As shown in  FIG. 2A , on a substrate  100  for crystal growth formed of sapphire, a semiconductor, or the like, a nitride semiconductor is crystallized by metal-organic chemical vapor deposition (MOCVD) or the like, whereby the laminate  10  is formed. The first layer  20  of the laminate  10  includes the first-conductivity type layer (doped layer)  22 , an undoped superlattice layer  24 , and the like. The first-conductivity type layer  22  may be composed of an n-type GaN cladding layer and have a donor concentration of 1×10 19  cm −3  and a thickness of 4 μm. The undoped superlattice layer  24  may be formed by alternately stacking 30 pairs of well layers formed of InGaN/InGaN and having thicknesses of 1 nm and barrier layers having thicknesses of 3 nm. 
     The light emitting layer  12  of the laminate  10  may be composed of an InGaN/InGaN undoped multi-quantum well (MQW) structure. The MQW structure may be formed by alternately stacking three well layers having thicknesses of 3 nm and four barrier layers having thicknesses of 10 nm, that is the well layers are disposed between barrier layers. 
     The second layer  30  of the laminate  10  may be formed by sequentially stacking, in the following order: an overflow preventing layer  32  (having an acceptor concentration of 1×10 20  cm −  and a thickness of 5 nm) comprising p-type AlGaN, a cladding layer  34  (having an acceptor concentration of 1×10 20  cm −3  and a thickness of 100 nm) comprising p-type GaN, a contact layer (having an acceptor concentration of 1×10 20  cm −3  and a thickness of 5 nm) comprising p + -type GaN. The thickness of the laminate  10  can be set, for example, in a range of 200 nm to 600 nm. 
     As shown in  FIG. 2B , a metal layer  51  to be the second electrode  50  is formed on the entire upper surface of the contact layer  36 . Next, an upper portion of the laminate  10  and a portion of the metal layer  51  are removed by etching, such that the upper portion of the laminate  10  is formed in to at least two ridge portions (i.e., first ridge portion  14  and second ridge portion  16 ). As a result, the step portions  14   a  and  16   a  are formed from the contact layer  36  of the surface  30   a  of the second layer  30  to the base surface  22   b  of the first layer  20 . The first and second ridge portions  14  and  16  each include a portion  22   a  of the first-conductivity type layer  22 , the undoped superlattice layer  24 , the light emitting layer  12 , the overflow preventing layer  32 , the cladding layer  34 , and the contact layer  36 . Further, the second electrode  50  is disposed on the contact layer  36 . 
     Further, the first electrode  60  is disposed on the exposed base surface  22   b  of the exposed first-conductivity type layer  22 . It may be preferable to provide the base surface  22   b  inside of the first-conductivity type layer  22  rather than at the interface between the undoped superlattice layer  24  and the first-conductivity type layer  22 . The reason is that it is possible to reduce contact resistance between the first electrode  60  and the first-conductivity type layer  22 . The width of one stripe portion of the first electrode  60  can be set, for example, within a range of 5 μm to 15 μm. 
     As shown in  FIG. 1A , a support-substrate-side first electrode (first connection electrode)  74  and a support-substrate-side second electrode (second connection electrode)  72  are formed on the surface of an insulating support substrate  70 . The first electrode  60  and the support-substrate-side first electrode (first connection electrode)  74  are bonded to each other, and the second electrode  50  and the support-substrate-side second electrode (second connection electrode)  72  are bonded to each other. Thereafter, the substrate  100  for crystal growth can be removed, and the surface  22   a  of the first-conductivity type layer  22  is roughened—that is surface irregularities are introduced in the surface  22   a  such that surface  22   a  is not optically flat. The roughening of surface  22   a  may be referred to as a “frost” processing or the formation of “a plurality of concave-convex structures.” The roughening makes serves to improve light-extraction efficiency. On the roughened surface  22   a , the phosphor layer  40  is formed. 
     In some embodiments, the conductive types of the layers in laminate  10  may be reversed. In order to keep the mechanical strength as a light emitting device, it is typically preferable to set the thickness of the insulating support substrate  70  to a large value such as, for example, 50 μm to 400 μm. 
       FIG. 3A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a comparative example, and  FIG. 3B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 3A . 
     The nitride semiconductor light emitting device has a laminate  110 , a first electrode  160 , a second electrode  150 , and a phosphor layer  140 . 
     The laminate  110  has a first layer  120  including a first-conductivity type layer  122 , a second layer  130  including a second-conductivity type layer, and a light emitting layer  112  provided between the first layer  120  and the second layer  130 , and contains a nitride semiconductor. The laminate  110  includes a single ridge portion  114  formed from the surface  130   a  of the second layer  130  opposite to the light emitting layer  112  up to the base surface of the first-conductivity type layer  122  of the first layer  120 . 
     A horizontal distance DDh from the center of the light emitting layer  112  to the center of the first electrode  160  has a length of approximately ½ of the width of the chip (i.e., the depicted structure in  FIG. 3A ). Therefore, current is concentrated in a peripheral area (shown by a dotted line and labeled “EL”) close to the first electrode  160  and the second electrode  150 . In this narrow peripheral area in which current is concentrated, the carrier density becomes high, and thus carrier overflow and Auger non-radiative recombination are likely to occur. Therefore, the luminous efficiency is likely to decrease. Further, at the central area of the ridge portion  114 , since the carrier density decreases, the luminous efficiency is likely to decrease. A decrease in the luminous efficiency causes the rate of increase in the optical output to slow so as to approach saturation even when current increases. 
     Of emitted light, light emitted from the vicinity of a side surface  140   b  of the ridge portion  114  passes through a shorter path in the phosphor layer  140 , and thusly is less absorbed by the phosphor layer  140 . Therefore, blue light BL emitted from this peripheral region is relatively strong (the ratio of blue light to longer wavelength light is high). Meanwhile, from the central area (non-peripheral region), the blue light is relatively weak (a lower ratio of blue light to longer wavelength light) because of the longer path length in the phosphor layer  140 . Therefore, between the light emitted from the peripheral area and the central area color irregularity exists. 
       FIG. 4A  is a graph illustrating dependence of optical output on current, and  FIG. 4B  is a graph for comparing light distribution patterns. 
     In  FIG. 4A , the vertical axis represents optical output (mW) and the horizontal axis represents current (mA), and the optical outputs and light distribution patterns of light emitting devices each having a chip size of 1 mm by 1 mm are obtained by simulations. 
     A case where the number of ridge portions is one is the comparative example, and cases where the number of ridge portions is two, four, or eight are examples of the first embodiment. When the current is 1,000 mA, in the case where the number of ridge portions is two, the optical output is 970 mW, and in the case where the number of ridge portions is four, the optical output is 1,030 mW, and in the case where the number of ridge portions is eight, the optical output is 1,020 mW. In the case where the number of ridge portions is four, the optical output is highest. In contrast to this, in the comparative example in which the number of ridge portions is one, the optical output is 810 mW which is low. 
     In this manner, as the number of ridge portions increases, the horizontal distance Dh between the center of the light emitting layer  12  and the center of the first electrode  60  is reduced, and it is possible to suppress concentration of carriers while narrowing the width of the area where the carrier density decreases. As a result, the luminous area EL broadens, and carrier overflow and Auger non-radiative recombination are suppressed, and it is possible to keep the luminous efficiency high. Further, the ratio of blue light which is emitted from the side surface  40   b  of the phosphor layer  40  decreases, and color irregularity decreases. 
     Further, in a case where the first electrode  60  is set as the n-side electrode, even when a distance from the first electrode  60  up to the light emitting layer  12  is long, it is easy to spread electrons, since electrons have a mobility higher than that of holes, into a wider range of the light emitting layer  12 . Furthermore, since the second electrode (the p-side electrode) is provided to widely cover the surfaces of the ridge portions and has a short running distance up to the light emitting layer  12 , the second electrode  50  can spread holes having mobility lower than that of electrons, into the light emitting layer  12 . Therefore, it is possible to further improve the luminous efficiency. 
     In  FIG. 4B , the light distribution pattern of the first embodiment having four ridge portions is shown by a solid line, and the light distribution pattern of the comparative example having one ridge portion is shown by a dashed line. 
     The vertical axis Y represents a vertical coordinate relative to a light emitting surface, and the horizontal axis X represents a horizontal coordinate relative to the light emitting surface. The upper portion of the light distribution pattern of the present embodiment is wider than that of the light distribution pattern of the comparative example. In this manner, since optical output directed upward is high, optical output directed to a horizontal direction relatively decreases, and color irregularity may decrease. 
       FIG. 5A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a second embodiment, and  FIG. 5B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 5A . 
     The nitride semiconductor light emitting device includes a laminate  10 , a first electrode  60 , a second electrode  50 , and a phosphor layer  40 . 
     The laminate  10  which is formed of a nitride semiconductor includes a first layer  20  including a first-conductivity type layer  22 , a second layer  30  including a second-conductivity type layer, and a light emitting layer  12  provided between the first layer  20  and the second layer  30  and containing a nitride semiconductor. The laminate  10  has at least two ridge portions (first ridge portion  14  and second ridge portion  16 ) having steps formed from the surface  30   a  of the second layer  30  opposite to the light emitting layer up to the surface (base surface)  22   b  of the first-conductivity type layer  22  of the first layer  20 . 
     The nitride semiconductor light emitting device of the second embodiment includes insulating layers  62  provided on the side surfaces of the step portion  14   a  of the first ridge portion  14  and the side surfaces of the step portion  16   a  of the second ridge portion  16 , and reflective portions  64  provided between the insulating layers  62  and the first electrode  60 . Further, the width of the first ridge portion  14  and the width of the second ridge portion  16  increase toward the base surface  22   b  of the first-conductivity type layer  22 . 
     The reflective portions  64  may be formed of a metal having high reflectance for even a blue light wavelength, such as silver or aluminum. When the base angle α of each ridge portion is set to, for example, about 45 degrees, it is possible to efficiently reflect emitted light BL from the light emitting layer  12  toward the phosphor layer  40 . 
       FIG. 6  is a graph chart illustrating the optical output of the nitride semiconductor light emitting device according to the second embodiment. 
     When the current is 1,000 mA, the optical output (shown by a solid line) of the second embodiment having reflective portions  64  formed on four ridge portions is 1,230 mW. Meanwhile, the optical output of the first embodiment having four ridge portions and the same laminate composition is 1,030 mW. That is, when the reflective portions  64  are formed on the side surfaces of the step portions  14   a  and  16   a  of the ridge portions of the laminate  10  whose base angles are 45 degrees, with the insulating layers interposed therebetween, it is possible to further improve the optical output. 
       FIG. 7A  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a third embodiment, and  FIG. 7B  is a schematic plan view illustrating the laminate side as seen along a line A-A of  FIG. 7A . 
     In this third embodiment, each ridge portion may be a portion of a ring shape, such as, for example, depicted in  FIG. 7B . In the third embodiment, two ridge portions  14  and  16  are adjacent to each other. Each of the two ring-shaped ridge portions  14  and  16  is partially disconnected (that is, the two ridge portions do not form a fully closed ring, but rather each separately forms a “C”-shaped structure or a partial ring shape), and in the disconnected radial area (the opening in the ring shape), it is possible to provide an area  60   a  for connecting neighboring portions of the first electrode  60 . Further, in a case where each ridge portion has a ring shape, the longitudinal direction of a corresponding step portion may be the direction of the tangent to the ring. 
     Further,  FIG. 7A  is a schematic cross-sectional view taken along a line B-B of  FIG. 7B . For example, when a first electrode area  60   b  provided at a central portion of the ring shape is connected to a support-substrate-side first electrode  74   a , it is possible to connect the first electrode area  60   b  to a power supply through the insulating support substrate  70 . 
       FIG. 8  is a schematic cross-sectional view illustrating a nitride semiconductor light emitting device according to a fourth embodiment. 
     A laminate  10  and a phosphor layer  40  need not be bonded to an insulating support substrate. In the structure of  FIG. 2D , the plurality of ridge portions, the first electrode  60 , and the second electrode  50  are covered by an insulating layer  80  or the like. Further, openings are formed to expose the surfaces of the first electrode  60  and the second electrode  50 . 
     Thereafter, for example, a photoresist or the like is used as a mask to form a first pillar electrode  61 , which is formed of copper or the like and is connected to the first electrode  60 , and a second pillar electrode  51 , which is formed of copper or the like and is connected to the second electrode  50 , with plating or the like. Next, the photoresist or the like is removed, and a reinforcing resin layer  82  or the like is filled therein. 
     When the thicknesses of the first pillar electrode  61 , the second pillar electrode  51 , and the reinforcing resin layer  82  are set to, for example, 50 μm to 300 μm, it is possible to improve mechanical strength. Therefore, it is possible to remove the substrate for crystal growth, and to provide a phosphor layer  40  on the exposed surface of the first-conductivity type layer  22 . That is, even when bonding to an insulating supporting substrate is not performed, it is possible to perform packaging at a wafer level. Further, the reinforcing resin layer  82  may have a light blocking property (that is, layer  82  may be opaque or partially opaque to emitted light). 
     According to the first to fourth embodiments, nitride semiconductor light emitting devices having less color irregularity and higher optical output may be provided. These nitride semiconductor light emitting devices may be widely used in illuminating devices, displays, signals, and so on. 
     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 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.