Patent Publication Number: US-11393958-B2

Title: Light emitting device to improve the extraction efficiency

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase of International Patent Application No. PCT/JP2017/010199 filed on Mar. 14, 2017, which claims priority benefit of Japanese Patent Application No. JP 2016-067490 filed in the Japan Patent Office on Mar. 30, 2016. Each of the above-referenced applications is hereby incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present technology relates to a light emitting device using a semiconductor material. 
     BACKGROUND ART 
     Low (light) extraction efficiency is one significant problem of a semiconductor light emitting device. The extraction efficiency is a ratio of light emitted from an active layer of the light emitting device and light exited from the light emitting device. At an interface between a material having a high refractive index (semiconductor) and a material having a low refractive index (for example, air or resin), light incident at an angle greater than the critical angle is totally reflected. If the total reflection occurs at the extraction surface of the light emitting device, the light is confined within the light emitting device, is absorbed on an electrode, and is internally absorbed by the material of the light emitting device. As a result, the extraction efficiency is lowered. 
     A semiconductor light emitting device described in Patent Literature 1 includes a side surface oblique to an upper surface (extraction surface) and a bottom surface of the device (for example, see Patent Literature 1, paragraph [0032]). Yet, the semiconductor light emitting device is susceptible to further improvement in order to increase the extraction efficiency. 
     Patent Literature 2 discloses a semiconductor light emitting device in which a side surface of a semiconductor layer is formed in a curved shape (for example, see Patent Literature 2, paragraph [0013]). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 1998-341035 
     Patent Literature 2: Japanese Patent Application Laid-open No. 2006-196694 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     However, the manufacturing costs of the semiconductor light emitting device of Patent Literature 2 are increased because the side surface is entirely processed to have a curved shape. 
     An object of the present disclosure is to provide a light emitting device capable of decreasing the manufacturing costs and improving light extraction efficiency. 
     Solution to Problem 
     In order to achieve the object, a light emitting device according to an embodiment of the present technology includes a semiconductor layer having a light extraction surface and side surfaces. 
     The semiconductor layer includes a cladding layer and an active layer. 
     The cladding layer has the extraction surface and a cladding layer side surface of the side surfaces, the cladding layer side surface being arranged at a first angle to the extraction surface. 
     The active layer has an active layer side surface of the side surfaces, the active layer side surface being arranged at a second angle different from the first angle to the extraction surface. 
     With this structure, from the light reflected by the active layer side surface, the amount of the light reflected by the extraction surface can be decreased. With this simple structure of the light emitting device, the manufacturing costs can be decreased and the extraction efficiency can be improved. 
     The extraction surface may be flat. 
     Thus, even if the extraction surface is flat, high extraction efficiency can be provided as long as the second angle satisfies the following appropriate condition. 
     The second angle may be set such that light generated at the active layer and reflected by the active layer side surface enters the extraction surface at an angle smaller than a critical angle. 
     High extraction efficiency can be provided as long as the second angle satisfies the predetermined expression. 
     The second angle may be set such that light generated at the active layer and reflected by the active layer side surface is reflected by the cladding layer side surface and exits from the extraction surface. 
     The extraction surface may have a concavo-convex part. 
     Thus, in a case where the extraction surface has a concavo-convex part, high extraction efficiency can be provided as long as the second angle satisfies the following appropriate condition. 
     The second angle may be set such that light generated at the active layer and reflected by the active layer side surface enters an internal surface of the concave part at an angle smaller than a critical angle. 
     A light emitting device according to another embodiment includes a semiconductor layer having a light extraction surface and side surfaces. 
     The semiconductor layer includes a cladding layer and an active layer. 
     The cladding layer has the extraction surface. 
     The active layer has an active layer side surface of the side surfaces, the active layer side surface being arranged in a convex shape curved outside the light emitting device. 
     Advantageous Effects of Invention 
     As described above, according to the present technology, the manufacturing costs can be decreased and the light extraction efficiency can be improved. 
     It should be noted that the effects described here are not necessarily limitative and may be any of effects described in the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view schematically showing a structure of a light emitting device according to a first embodiment of the present technology.  FIG. 1B  is an enlarged view showing a side surface of a semiconductor layer of the light emitting device. 
         FIG. 2  shows a simulation image of an illuminance distribution about a light component reflected by an active layer side surface in a light emitting device according to a reference example. 
         FIG. 3  is a table showing appropriate angle of an active layer side surface in the light emitting device formed of a material that emits red, blue or green light in the first embodiment. 
         FIG. 4  is a graph showing a simulation result of each extraction efficiency of a semiconductor device (device that emits red light) of Example 1 and that of a reference example. 
         FIG. 5  is a cross-sectional view schematically showing a structure of a light emitting device according to a second embodiment of the present technology and a side surface of a semiconductor layer is enlarged. 
         FIG. 6  is a table showing appropriate angle of an active layer side surface in the light emitting device formed of a material that emits red, blue or green light in the second embodiment. 
         FIG. 7  is a graph showing a verification result of each extraction efficiency by a light emitting device (device that emits red light) according to Example 2 and a light emitting device (device that emits red light) according to the first embodiment. 
         FIG. 8  shows a simulation result in a case where a is changed between 45 and 90 in a light emitting device according to Example 2. 
         FIG. 9A  is a cross-sectional view schematically showing a structure of a light emitting device according to a third embodiment of the present technology.  FIG. 9B  is a view showing an enlarged side surface of a semiconductor layer of the light emitting device. 
         FIGS. 10A and 10B  are views for explaining values x, h, and d in Expression 10. 
         FIG. 11  is a cross-sectional view schematically showing a structure of a light emitting device according to a fourth embodiment of the present technology. 
         FIG. 12A  is a cross-sectional view schematically showing a structure of a light emitting device according to a fifth embodiment of the present technology.  FIG. 12B  is a view showing an enlarged side surface of a semiconductor layer of the light emitting device. 
         FIG. 13  is a view showing reflected light beam in a case where an extraction surface is flat. 
         FIGS. 14A, 14B, and 14C  each show a shape of cladding layer of the semiconductor layer in a variety of forms. 
         FIGS. 15A and 15B  each show a package arranged around a light emitting device. 
         FIG. 16  shows a light emitting device where a cladding layer having an extraction surface has a plurality of side surfaces arranged at different angles. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present technology will be described with reference to the drawings. In the following description, with reference to the drawings, in order to point out the positions or the directions of the device and the components thereof, words such as “up”, “down”, “left”, “right”, “depth”, “width”, “horizontal”, “vertical”, and the like may be used and are for convenience of explanation. In other words, these words may be often used for easy understanding of explanation and may not be matched with the position or the direction where the device is actually produced or used. 
     1. First Embodiment 
     1. 1) Structure of Light Emitting Device 
       FIG. 1A  is a cross-sectional view schematically showing a structure of a light emitting device according to a first embodiment of the present technology. 
     A light emitting device  100 A is a semiconductor light emitting device, i.e., an LED (Light Emitting Diode). The light emitting device  100 A includes a semiconductor layer  30 . The semiconductor layer  30  includes a light extraction surface (upper surface)  302 , side surfaces  303 , and a bottom surface  301 . For example, electrodes  11  and  12  are formed at the bottom surface  301  and the extraction surface  302 , respectively. 
     The semiconductor layer  30  includes cladding layers  35  and an active layer  33 . The cladding layers  35  include a first conductive layer  31  having the bottom surface  301  and a second conductive layer  32  having the extraction surface  302 . Typically, the first conductive layer  31  is a p type conductive layer and the second conductive layer  32  is an n type conductive layer, and vice versa. The active layer  33  is arranged between the first conductive layer  31  and the second conductive layer  32 . 
     The side surfaces  303  of the semiconductor layer  30  include cladding layer side surfaces  35   s  that are the side surfaces of the cladding layers  35  and an active layer side surface  33   s  that is the side surface of the active layer  33 . 
     The extraction surface  302  is at least flat. In addition, the extraction surface  302  and the bottom surface  301  are substantially in parallel. An angle of inclination α (°) (first angle) of the cladding layer side surface  35   s  with respect to the extraction surface  302  is set such that the area of the extraction surface  302  is greater than the area of the bottom surface  301  in planer view. 
     In this embodiment, the angle of a side surface  31   s  of the first conductive layer  31  and the angle of a side surface  32   s  of the second conductive layer  32  are substantially the same a. Hereinafter, an angle of inclination (second angle) of the active layer side surface  33   s  with respect to the extraction surface  302  is referred to as β (°). According to the present technology, these angles α and β are set to be different. 
     As shown in  FIG. 1A , light generated at the active layer  33  and exited from the extraction surface  302  mainly includes a light component L 1  and a light component L 2 . The light component L 2  is a component of light that is generated at the active layer  33 , is reflected by the active layer side surface  33   s , and enters the extraction surface  302  at an angle smaller than a critical angle. The light component L 1  is a component of light that is generated at the active layer  33 , is reflected by the side surface  32   s  of the second conductive layer  32 , and enters the extraction surface  302  at the angle smaller than the critical angle. 
     According to the present technology, the light component L 2  is focused.  FIG. 2  shows a simulation image of an illuminance distribution about the light component L 2  reflected mainly by the active layer side surface  33   s  in a light emitting device according to a reference example. The light emitting device according to the reference example includes the structure similar to the semiconductor light emitting device described in Patent Literature 1, for example. The angle of the side surface  313  of the semiconductor layer  130  is fixed, i.e., α=β. Note that  FIG. 2  is a gray scale image, but original one is a color image. 
     As shown in  FIG. 2 , since most of the light generated in a width direction (horizontal direction) at the active layer  133  enters the top surface, i.e., the extraction surface  312 , at an angle greater than the critical angle, the light reflected by the extraction surface  312  is easily gathered on the center of the cladding layers  35 . Thus, the amount of light exited from the extraction surface  312  is decreased and extraction efficiency is not improved. 
     According to the idea of the present inventor, by setting the value of β to an appropriate value, the light incident on the extraction surface  302  at the angle smaller than the critical angle is increased. Specifically, in this embodiment, the β is set such that light generated at the active layer  33  and reflected by the active layer side surface  33   s  enters the extraction surface  302  at the angle smaller than the critical angle. 
     The present inventor verified by a simulation of a relationship between the angle β and the angle of the light exited from the extraction surface  302  (incident angle with respect to extraction surface  302 ) of the light emitting device  100 A shown in  FIG. 1A .  FIG. 1B  is an enlarged view showing the side surface  303  of the semiconductor layer  30  of the light emitting device  100 A. 
     For example, in a case where the angle β is represented by the following Expression 1, a high extraction efficiency can be provided. 
     
       
         
           
             
               
                 
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     n 1 : refractive index of semiconductor layer  30   
     n 2 : refractive index of material (insulator (resin) or air) around semiconductor layer  30   
     Hereinafter, a method of deriving the angle β will be described. As shown in  FIG. 1B , angles ε (°), δ (°), and θ c  (°) are defined as follows: 
     ε: incident angle to active layer side surface  33   s    
     δ: incident angle to extraction surface  302   
     θ c : critical angle of extraction surface  302   
     By the trigonometry, the angles of incidence ε and δ are represented by the following Expressions 2 and 3.
 
[Math. 2]
 
ε=90−β  Expression 2
 
[Math. 3]
 
δ=90−2 ε=−90+2β  Expression 3
 
     δ=0 is the condition for maximizing the light exited from the extraction surface  302 . Here, it assumes that the light entering the active layer side surface  33   s  almost propagates in the horizontal direction of the active layer  33 . Then, β=45 is the condition for maximizing the light exited from the extraction surface  302 . In other words, when β=45, the light generated at the active layer  33  propagates in the horizontal direction and is reflected by the active layer side surface  33   s  in the vertical direction, whereby a highest extraction efficiency is thus provided. 
     The critical angle θ c  is represented by the following Expression 4. 
     
       
         
           
             
               
                 
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     From a conditional expression for exiting the light from the extraction surface  302 , δ=±θ c  and Expression 3, −90+2β=±θ c  is provided. From this expression and the Expression 4, β is provided as shown in Expression 1. 
     As described above, the light emitting device  100 A according to this embodiment has a greatest feature that α and β are different. (Note that Expression 1 is held irrespective of α.) In a case where the angle β of the active layer side surface  33   s  of the light emitting device  100 A has the condition of Expression 1, the extraction efficiency is improved. To be more specific, there is no need to form the curved side surfaces of the semiconductor layer as described in Patent Literature 2, the manufacturing costs can be decreased, and the extraction efficiency can be improved. 
     Note that the angles of inclination α and β can be controlled by changing etching parameters upon etching. Examples of the etching parameters include a gas type, a gas pressure, a gas amount, power, and the like. In particular, the angle can be controlled with high precision by ICP (Inductively Coupled Plasma)-RIE (Reactive Ion Etching). 
     1. 2) Example 1 and Effect Verification 
     For example, it assumes that α=62.5 and β=45. A material of the semiconductor layer  30  is an AlGaInP-based material that emits red light (refractive index n 1 =3.3) and a sealing material arranged around the semiconductor layer  30  is resin (refractive index n 2 =1.5). At this time, β=32 to 59. More preferably, β=36 to 54. 
       FIG. 3  is a table showing appropriate β of a light emitting device formed of a material that emits red light, a light emitting device formed of a material that emits blue light, and a light emitting device formed of a material that emits green light. The material that emits blue light of the semiconductor layer  30  is a GaN-based material. The material that emits green light of the semiconductor layer  30  is a GaP-based material. 
       FIG. 4  is a graph showing a simulation result of each extraction efficiency of the semiconductor device (device that emits red light) of Example 1 according to this embodiment and that of the reference example. The light emitting device of the reference example has β=α=62.5°, and the material and the refractive index, both of which are similar to those of this embodiment. If the extraction efficiency of the reference example is 1.00, the extraction efficiency of this embodiment is 1.19. 
     2. Second Embodiment 
     2. 1) Structure of Light Emitting Device 
       FIG. 5  is a cross-sectional view schematically showing a structure of a light emitting device according to a second embodiment of the present technology and a side surface  303  of the semiconductor layer  30  is enlarged. Hereinafter, components substantially similar to the component, the functions, and the like of the light emitting device  100 A according to the first embodiment are denoted by the same reference signs, and description thereof will be omitted or simplified. Different points will be mainly described. 
     A relationship between the angle α of the cladding layer side surface  35   s  of the light emitting device  100 B according to this embodiment and the angle β of the active layer side surface  33   s  is represented by the following Expression 5. 
     
       
         
           
             
               
                 
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     In the first embodiment, β is specified irrespective of α. In this embodiment, β is specified with respect to α. Specifically, as shown in  FIG. 5 , the angle β is such that light generated at the active layer  33  and reflected by the active layer side surface  33   s  is reflected (for example, one time) by the side surface  32   s  of the second conductive layer  32  that is the cladding layer side surface  35   s  and exits from the extraction surface  302 . 
     On the condition that the following Expression 6 is satisfied, Expressions 7 and 8 are provided by using the trigonometry of the triangle shown by the reference sign H.
 
[Math. 6]
 
α&gt;2β  Expression 6
 
[Math. 7]
 
γ=α−2β  Expression 7
 
[Math. 8]
 
δ=90−2α+2β  Expression 8
 
     Similar to the first embodiment, δ=0 is the condition for maximizing the light exited from the extraction surface  302 . Also, β=α−45 is the condition for maximizing the light exited from the extraction surface  302 . Accordingly, from these conditions, Expressions 7 and 8, and Expression 4 of the critical angle θ c , Expression 5 is derived. 
     2. 2) Example 2 and Effect Verification 
     For example, it assumes that α=62.5 and β=45. A material of the semiconductor layer  30  is an AlGaInP-based material that emits red light (refractive index n 1 =3.3) and a sealing material arranged around the semiconductor layer  30  is resin (refractive index n 2 =1.5). At this time, β=2 to 29. 
       FIG. 6  is a table showing appropriate β of a light emitting device formed of a material that emits red light, a light emitting device formed of a material that emits blue light, and a light emitting device formed of a material that emits green light. α is set to 60, for example. 
       FIG. 7  is a graph showing a verification result of the extraction efficiency by the light emitting device  100 B (device that emits red light) according to Example 2 of this embodiment and the light emitting device  100 A (device that emits red light) according to Example 1 of the first embodiment. 
     Here, a length on a side of the extraction surface  302  in a square shape is 150 μm, a height of the light emitting device (here, mainly semiconductor layer  30 ) is 70 μm (first conductive layer  31  is 30 μm, active layer  33  is 10 μm, and second conductive layer  32  is 30 μm), for example. α=62.5 and β is a variable. As described above, taking the condition for maximizing the light exited from the extraction surface  302  into consideration, the optimal β is β=45 in Example 1 and β=17.5 in Example 2.  FIG. 7  shows this. 
       FIG. 8  shows a simulation result in a case where a is changed between 45 and 90 in the light emitting device  100 B according to Example 2. As apparent from Expression 1, even if a is any value, β is always 45°, which shows the first embodiment. Note that according to the present technology, since a α≠β, when β=45°, it employs a setting of a α≠45°. 
     On the other hand, the optimal β of the second embodiment (Example 2) varies together with a as shown in Expression 5. 
     Note that β in the first embodiment is determined irrespective of α (note that α≠β) and β in the second embodiment can be considered as a subordinate concept of the first embodiment. Accordingly, as shown in  FIG. 8 , both concepts can be drawn as the same line. 
     3. Third Embodiment 
       FIG. 9A  is a cross-sectional view schematically showing a structure of a light emitting device according to a third embodiment of the present technology.  FIG. 9B  is a view showing an enlarged side surface  403  of a semiconductor layer  40  of a light emitting device  100 C. 
     The extraction surface  402  of the light emitting device  100 C has a pattern (concavo-convex part) including a concave part  48  formed. In other words, the extraction surface  402  includes a surface (upper surface  412 ) perpendicular to a lamination direction of the respective semiconductor layers (up and down directions in  FIGS. 9A and 9B ) and an internal surface  48   a  of the concave part  48 . In  FIGS. 9A and 9B , γ(°), δ(°), ζ(°) are defined as follows: 
     γ: angle of internal surface  48   a  of concave part with respect to upper surface  412   
     δ: incident angle to internal surface  48   a  of concave part 
     ζ: exit angle to upper surface  412   
     The β is set such that light generated at the active layer  43  and reflected by the active layer side surface  43   s  enters the internal surface  48   a  of the concave part  48  at the angle smaller than the critical angle. The β is represented by the following Expression 9 using γ. 
     
       
         
           
             
               
                 
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     Expression 9 is derived from the following expressions 10 and 11. Expression 11 represents ζ using Expression 4 of the critical angle θ c  and γ. 
     
       
         
           
             
               
                 
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     For example, γ=62.5. In this case, if β=64.4, δ is optimal and light exits to the upper surface  412  in the vertical direction (ζ=0). This is the condition that the light exited from the extraction surface  402  is maximized. 
     Note that with respect to the arrangement of the concave part  48  in the extraction surface  402 , the following Expression 12 should be satisfied. 
     
       
         
           
             
               
                 
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       FIGS. 10A and 10B  are views for explaining the values x, h, and d in Expression 10. The distance x is from a point c to an edge of the internal surface  48   a  of the concave part  48  (boundary between internal surface  48   a  and upper surface  412 ). The point c is a point of the upper surface  412 , the point of the upper surface  412  corresponding to a light incident point of the active layer side surface  43   s , i.e., an intersection point of a perpendicular line passing through the incident point and the upper surface  412 . The h is a depth of the concave part  48 , i.e., a distance from the upper surface  412  to bottom surface  48   b  of the concave part  48 . The d is a depth from the upper surface  412  to the center of the active layer  43 . 
     For example, if h=0.2 (mm) and d=1 (mm), the range of the x (mm) is 0.08≤x≤0.8 from Expression 12. 
     As described above, in a case where the extraction surface  402  has the pattern of the concavo-convex part, a high extraction efficiency is achieved by appropriately setting β. 
     4. Fourth Embodiment 
       FIG. 11  is a cross-sectional view schematically showing a structure of a light emitting device according to a fourth embodiment of the present technology. An extraction surface  502  of a light emitting device  100 D includes a random concavo-convex part  58 . The angle γ of an internal surface of the concave part (or angle of external surface of convex part) with respect to the surface perpendicular to the respective semiconductor layers in the lamination direction has a distribution. There is a peak at a certain angle γ. Accordingly, in Expression 9, the γ may be replaced with γ a  representing an average of the γ distribution. For example, if γ a =45, β=58 at ζ=0. 
     5. Fifth Embodiment 
       FIG. 12A  is a cross-sectional view schematically showing a structure of a light emitting device according to a fifth embodiment of the present technology.  FIG. 12B  is a view showing an enlarged side surface  603  of a semiconductor layer  60  of a light emitting device  100 E. 
     An active layer  63  according to this embodiment has an active layer side surface  63   s  arranged in a convex shape curved outside. The incident angle ε to the active layer side surface  63   s  has a distribution and an influence on Expression 2. Thus, as shown in  FIG. 13 , if β=45 and it is flat, the incident angle δ to the extraction surface  302  also changes (Expression 3) and moves away from an ideal status of δ=0. 
     As shown in  FIG. 12B , a position incident on the active layer side surface  63   s  of the active layer  63  is different for each light beam. Accordingly, by forming a convex curve with δ=0 for each position at the active layer side surface  63   s , the extraction efficiency can be increased. 
     6. Other Various Embodiments 
     The present technology is not limited to the above-described embodiments, and other various embodiments may be implemented. 
     For example, as shown in  FIGS. 14A to 14C , the shapes of the cladding layers of the semiconductor layer  30  may take a variety of forms. 
     For example, as shown in  FIGS. 14A, 14B, and 14C , the shapes of the cladding layers of the semiconductor layer  30  may take a variety of forms. 
     As shown in  FIG. 14B , an angle of a lower cladding layer side surface  351   s  may be different from an angle of an upper cladding layer side surface  352   s . In this example, the angle of the lower cladding layer side surface  351   s  is substantially a right angle. 
     As shown in  FIG. 14C , angles of the upper cladding layer side surface  352   s  and the lower cladding layer side surface  351   s  are substantially right angles. 
     Alternatively, as shown in  FIGS. 15A and 15B , each package  10  arranged around a light emitting device  200 A or  200 B may have a surface  10   s  having an angle β that receives light from the active layer  33 . 
     Alternatively, as shown in  FIG. 16 , there are cladding layers  751  and  752 . The cladding layer  752  having an extraction surface  702  may have a plurality of side surfaces arranged at different angles. In this embodiment, the cladding layer  752  has a first side surface  752   a  having an angle α and a second side surface  752   b  having an angle γ. Note that the second side surface  752   b  has the angle γ that a width (horizontal width) of the cladding layer  752  is increased closer to the active layer  33 . By appropriately setting the α, β, and γ, the second side surface  752   b  functions as the extraction surface. 
     Examples of the light emitting devices according to the first to fifth embodiments include a light emitting device using an inorganic semiconductor. Also, the present technology is applicable to a light emitting device using an organic semiconductor such as organic EL (Electro-Luminescence) and the like. 
     Although the above-described embodiments mainly describe that sealing resin is arranged around the semiconductor layer, no resin may be arranged, i.e., air may be arranged around the semiconductor layer. 
     The light emitting device according to each embodiment has the structure that the electrodes are formed on the both surfaces (upper and bottom surfaces) of the semiconductor layer, respectively. Note that the present technology is applicable to a flip-chip type light emitting device having two electrodes on one surface. 
     It is possible to combine at least two features of the respective embodiments described above. 
     REFERENCE SIGNS LIST 
     
         
           30 ,  40 ,  60  semiconductor layer 
           33 ,  43 ,  63  active layer 
           33   s ,  43   s ,  63   s  active layer side surface 
           35 ,  351 ,  352 ,  751 ,  752  cladding layer 
           35   s ,  351   s ,  352   s  cladding layer side surface 
           48  concave part (part of concavo-convex part) 
           48   a  internal surface 
           58  concavo-convex part 
           100 A,  100 B,  100 C,  100 D,  100 E,  200 A light emitting device 
           301  bottom surface 
           302 ,  402 ,  502 ,  702  extraction surface 
           303 ,  403 ,  603  side surface (of semiconductor layer) 
           351   s  cladding layer side surface 
           752   a  first side surface 
           752   b  second side surface