Patent Publication Number: US-2022231194-A1

Title: Light emitting apparatus and projector

Description:
The present application is based on, and claims priority from JP Application Serial Number 2021-004794, filed Jan. 15, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a light emitting apparatus and a projector. 
     2. Related Art 
     Semiconductor lasers are expected as a high-luminance, next-generation light source. Among the semiconductor lasers, semiconductor lasers using nanocolumns are expected to achieve high power light emission at small radiation angles based on the photonic crystal effect provided by the nanocolumns. 
     For example, JP-A-2009-152474 describes a compound semiconductor light emitting device including a plurality of GaN nanocolumns formed by sequentially laminating an n-type GaN layer, a light emitting layer, and a p-type GaN layer on each other. 
     Due to the presence of dangling bonds on the side surfaces of the nanocolumns described above, carrier recombination in the vicinity of the side surfaces of the nanocolumns is likely to be non-light-emission recombination. 
     SUMMARY 
     A light emitting apparatus according to an aspect of the present disclosure includes a laminated structure including a plurality of columnar portions. The plurality of columnar portions each includes a first semiconductor layer, a second semiconductor layer different from the first semiconductor layer in terms of conductivity type, and a light emitting layer provided between the first semiconductor layer and the second semiconductor layer. The second semiconductor layer has a first section, and a second section that surrounds the first section in a plan view along a lamination direction in which the first semiconductor layer and the light emitting layer are laminated structured on each other and has a bandgap wider than a bandgap of the first section. The second section forms a side surface of each of the columnar portions. 
     A projector according to another aspect of the present disclosure includes the light emitting apparatus according to the aspect of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view diagrammatically showing a light emitting apparatus according to an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view diagrammatically showing a columnar portion of the light emitting apparatus according to the embodiment. 
         FIG. 3  is a plan view diagrammatically showing the columnar portion of the light emitting apparatus according to the embodiment. 
         FIG. 4  is a cross-sectional view diagrammatically showing one of the steps of manufacturing the light emitting apparatus according to the embodiment. 
         FIG. 5  is a cross-sectional view diagrammatically showing a columnar portion of the light emitting apparatus according to a first variation of the embodiment. 
         FIG. 6  is a cross-sectional view diagrammatically showing a columnar portion of the light emitting apparatus according to a second variation of the embodiment. 
         FIG. 7  is a cross-sectional view diagrammatically showing a columnar portion of the light emitting apparatus according to a third variation of the embodiment. 
         FIG. 8  diagrammatically shows a projector according to the embodiment. 
         FIG. 9  is an STEM image in an experimental example. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A preferable embodiment of the present disclosure will be described below in detail with reference to the drawings. It is not intended that the embodiment described below unduly limits the contents of the present disclosure described in the claims. Further, all configurations described below are not necessarily essential configuration requirements of the present disclosure. 
     1. Light Emitting Apparatus 
     1.1. Overall Configuration 
     A light emitting apparatus according to the present embodiment will first be described with reference to the drawings.  FIG. 1  is a cross-sectional view diagrammatically showing a light emitting apparatus  100  according to the present embodiment. 
     The light emitting apparatus  100  includes, for example, a substrate  10 , a laminated structure  20 , a first electrode  40 , and a second electrode  42 , as shown in  FIG. 1 . The light emitting apparatus  100  is, for example, a semiconductor laser. 
     The substrate  10  is, for example, an Si substrate, a GaN substrate, a sapphire substrate, or an SiC substrate. 
     The laminated structure  20  is provided at the substrate  10 . In the illustrated example, the laminated structure  20  is provided on the substrate  10 . The laminated structure  20  includes, for example, a buffer layer  22  and columnar portions  30 . 
     The present specification will be described on the assumption that in a lamination direction of the laminated structure  20  (hereinafter also simply referred to as “lamination direction”), the direction from a light emitting layer  34  as a reference toward the second electrode  42  is called an “upward direction” and the direction from the light emitting layer  34  as the reference toward the substrate  10  is called a “downward direction”. The directions perpendicular to the lamination direction are also called an “in-plane direction”. The “lamination direction of the laminated structure  20 ” refers to the direction in which a first semiconductor layer  32  and the light emitting layer  34  of each of the columnar portions  30  are laminated structured on each other. 
     The buffer layer  22  is provided on the substrate  10 . The buffer layer  22  is, for example, an n-type GaN layer having been doped with Si. A mask layer  50  for forming the columnar portions  30  is provided on the buffer layer  22 . The mask layer  50  is, for example, a silicon oxide layer, a titanium layer, a titanium oxide layer, or an aluminum oxide layer. 
     The columnar portions  30  are provided on the buffer layer  22 . The columnar portions  30  each have a columnar shape protruding upward beyond the buffer layer  22 . In other words, the columnar portions  30  protrude upward from the substrate  10  through the buffer layer  22 . The columnar portions  30  are also called, for example, nanocolumns, nanowires, nanorods, and nanopillars. The columnar portions  30  each have, for example, a regular hexagonal planar shape or any other polygonal planar shape or a circular planar shape. 
     The columnar portions  30  each have a diameter, for example, greater than or equal to 50 nm but smaller than or equal to 500 nm. When the diameter of each of the columnar portions  30  is smaller than or equal to 500 nm, a high-quality-crystal light emitting layer  34  can be produced, and distortion intrinsically present in the light emitting layer  34  can be reduced. The light generated in the light emitting layer  34  can thus be efficiently amplified. 
     In a case where the columnar portions  30  each have a circular planar shape, the term “the diameter of the columnar portions” refers to the diameter of the circular shape, and when the columnar portions  30  each have a non-circular planar shape, the term refers to the diameter of a minimum circle containing the non-circular shape therein. For example, when the columnar portions  30  each have a polygonal planar shape, the diameter of the columnar portions  30  is the diameter of a minimum circle containing the polygonal shape therein, and when the columnar portions  30  each have an elliptical planar shape, the diameter of the columnar portions  30  is the diameter of a minimum circle containing the elliptical shape therein. 
     The columnar portions  30  are located at a plurality of locations. The distance between adjacent columnar portions  30  is, for example, greater than or equal to 1 nm but smaller than or equal to 500 nm. The plurality of columnar portions  30  are arranged in predetermined directions at predetermined pitches when viewed in the lamination direction. The plurality of columnar portions  30  are arranged, for example, in a triangular lattice. The plurality of columnar portions  30  are not necessarily arranged in a specific pattern and may be arranged in a square lattice. The plurality of columnar portions  30  can express the photonic crystal effect. 
     The “pitch of the columnar portions” is the distance between the centers of columnar portions  30  adjacent to each other in any of the predetermined direction. In the case where the columnar portions  30  each have a circular planar shape, the term “the center of each of the columnar portions” refers to the center of the circle, and when the columnar portions  30  each have a non-circular planar shape, the term refers to the center of the minimum circle containing the non-circular shape therein. For example, when the columnar portions  30  each have a polygonal planar shape, the center of each of the columnar portions  30  is the center of a minimum circle containing the polygonal shape therein, and when the columnar portions  30  each have an elliptical planar shape, the center of each of the columnar portions  30  is the center of a minimum circle containing the elliptical shape therein. 
     The columnar portions  30  each include the first semiconductor layer  32 , the light emitting layer  34 , and a second semiconductor layer  36 . 
     The first semiconductor layer  32  is provided on the buffer layer  22 . The first semiconductor layer  32  is provided between the substrate  10  and the light emitting layer  34 . The first semiconductor layer  32  is, for example, an n-type semiconductor layer having been doped with Si. 
     The light emitting layer  34  is provided on the first semiconductor layers  32 . The light emitting layer  34  is provided between the first semiconductor layer  32  and the second semiconductor layer  36 . The light emitting layer  34  generates light when current is injected thereinto. The light emitting layer  34  includes, for example, well layers  33  and barrier layers  35 . The well layers  33  and the barrier layers  35  are each an i-type semiconductor layer having been intentionally doped with no impurities. The light emitting layer  34  has a multiple quantum well (MQW) structure formed of the well layers  33  and the barrier layers  35 . In the illustrated example, three well layers  33  and four barrier layers  35  are provided. 
     The numbers of well layers  33  and barrier layers  35 , which form the light emitting layer  34 , are not each limited to a specific number. For example, only one well layer  33  may be provided, and the light emitting layer  34 , in this case, has a single quantum well (SQW) structure. 
     The second semiconductor layer  36  is provided on the light emitting layer  34 . The second semiconductor layer  36  is a layer different from the first semiconductor layer  32  in terms of conductivity type. The second semiconductor layer  36  is, for example, a p-type semiconductor layer having been doped with Mg. The first semiconductor layer  32  and the second semiconductor layer  36  form a cladding layer having the function of confining the light in the light emitting layer  34 . 
     Although not shown in the figures, an optical confinement layer (OCL) may be provided between the first semiconductor layer  32  and the light emitting layer  34 . An electron blocking layer (EBL) may further be provided between the light emitting layer  34  and the second semiconductor layer  36 . 
     In the light emitting apparatus  100 , the p-type second semiconductor layers  36 , the i-type light emitting layers  34 , which have been doped with no impurities, and the n-type first semiconductor layers  32  form pin diodes. In the light emitting apparatus  100 , when a forward bias voltage for the pin diodes is applied to the space between the first electrodes  40  and the second electrode  42 , current is injected into the light emitting layers  34 , whereby the electrons and holes recombine with each other in the light emitting layers  34 . The recombination causes light emission. The light generated in the light emitting layers  34  propagates in the in-plane direction and forms a standing wave because of the photonic crystal effect provided by the plurality of columnar portions  30 , and the standing wave receives gain in the light emitting layers  34  to undergo laser oscillation. The light emitting apparatus  100  then outputs positive first order diffracted light and negative first order diffracted light as laser light in the lamination direction. 
     Although not shown in the figures, a reflection layer may be provided between the substrate  10  and the buffer layer  22  or below the substrate  10 . The reflection layer is, for example, a distributed Bragg reflector (DBR) layer. The reflection layer can reflect the light generated in the light emitting layers  34 , and the light emitting apparatus  100  can emit the light only via the side facing the second electrode  42 . 
     The first electrode  40  is provided on the buffer layer  22 . The buffer layer  22  may be in ohmic contact with the first electrode  40 . The first electrode  40  is electrically coupled to the first semiconductor layers  32 . In the illustrated example, the first electrode  40  is electrically coupled to the first semiconductor layers  32  via the buffer layer  22 . The first electrode  40  is one of the electrodes for injecting the current into the light emitting layers  34 . The first electrode  40  is, for example, a laminated structure of a Cr layer, an Ni layer, and an Au layer laminated structured on each other in the presented order from the side facing the buffer layer  22 . 
     The second electrode  42  is provided on the second semiconductor layers  36 . 
     The second electrode  42  is electrically coupled to the second semiconductor layers  36 . The second semiconductor layers  36  may be in ohmic contact with the second electrode  42 . The second electrode  42  is the other one of the electrodes for injecting the current into the light emitting layers  34 . The second electrode  42  is made, for example, of an indium tin oxide (ITO). 
     1.2. Detailed Configuration of Columnar Portion 
       FIG. 2  is a cross-sectional view diagrammatically showing one of the columnar portions  30 .  FIG. 3  is a plan view diagrammatically showing one of the columnar portions  30 .  FIG. 2  is a cross-sectional view taken along the line II-II in  FIG. 3 . 
     The first semiconductor layer  32  includes a low bandgap section  32   a  and a high bandgap section  32   b , as shown in  FIG. 2 . The first semiconductor layer  32  is an AlGaN layer containing Al (aluminum), Ga (gallium), and N (nitrogen). 
     The bandgap of the high bandgap section  32   b  is wider than that of the low bandgap section  32   a . The high bandgap section  32   b  surrounds the low bandgap section  32   a  in the plan view along the lamination direction (hereinafter also simply referred to as “in the plan view”). The high bandgap section  32   b  forms a side surface  31  of the columnar portion  30 . The side surface  31  connects the upper surface of the buffer layer  22  to the lower surface of the second electrode  42 . The angle of the side surface  31  with respect to the upper surface of the substrate  10  is greater than or equal to 60° but smaller than or equal to 90° and is, in the illustrated example, 90°. The atomic concentration (at %) of Al in the high bandgap section  32   b  is higher than that in the low bandgap section  32   a.    
     The barrier layers  35  of the light emitting layer  34  each include a low bandgap section  35   a  and a high bandgap section  35   b . The barrier layers  35  are each an AlGaN layer. The well layers  33  are, for example, each an InGaN layer. 
     The bandgap of the high bandgap section  35   b  is wider than that of the low bandgap section  35   a . The high bandgap section  35   b  surrounds the low bandgap section  35   a  in the plan view. The high bandgap section  35   b  forms the side surface  31  of the columnar portion  30 . The atomic concentration (at %) of Al in the high bandgap section  35   b  is higher than that in the low bandgap section  35   a.    
     The second semiconductor layer  36  includes a low bandgap section  36   a  and a high bandgap section  36   b . The second semiconductor layer  36  is an AlGaN layer. 
     The bandgap of the high bandgap section  36   b  is wider than that of the low bandgap section  36   a . The high bandgap section  36   b  surrounds the low bandgap section  36   a  in the plan view, as shown in  FIG. 3 . In the illustrated example, the columnar portion  30  has a regular hexagonal planar shape. The high bandgap section  36   b  forms the side surface  31  of the columnar portion  30 , as shown in  FIGS. 2 and 3 . The atomic concentration of Al in the high bandgap section  36   b  is higher than that in the low bandgap section  36   a . In the columnar portion  30 , Al is unevenly distributed or concentrated at the side surface  31 . 
     The high bandgap sections  32   b ,  35   b , and  36   b  are portions where the atomic concentration of Al is higher than that of Ga. In the high bandgap sections  32   b ,  35   b , and  36   b , the ratio of the atomic concentration of Al to the sum of the atomic concentrations of Al and Ga (hereinafter also referred to as “Al ratio”) is greater than 0.5 and may be greater than or equal to 0.8. Furthermore, in the high bandgap sections  32   b ,  35   b , and  36   b , the Al ratio may be 1.0, in which case the high bandgap sections  32   b ,  35   b , and  36   b  are made of MN. 
     The low bandgap sections  32   a ,  35   a , and  36   a  are portions where the atomic concentration of Al is lower than or equal to that of Ga. In the low bandgap sections  32   a ,  35   a , and  36   a , the Al ratio is lower than or equal to 0.5 and may be lower than or equal to 0.4. 
     The atomic concentrations of Al and Ga can be measured by scanning transmission electron microscope—energy dispersive X-ray spectroscopy (STEM-EDS). 
     1.3. Effects and Advantages 
     In the light emitting apparatus  100 , the second semiconductor layers  36  each include the low bandgap section  36   a  as a first section and the high bandgap section  36   b  as a second section, which surrounds the low bandgap section  36   a  in the plan view and has a bandgap wider than that of the low bandgap section  36   a , and the high bandgap section  36   b  forms the side surface  31  of each of the columnar portions  30 . Therefore, in the light emitting apparatus  100 , the current flowing at the side surface  31  of each of the columnar portions  30  can be reduced as compared, for example, with a case where the first and second sections have the same bandgap. The current resulting in non-light-emission recombination can thus be reduced. As a result, the current injection efficiency can be increased. 
     In the light emitting apparatus  100 , the second semiconductor layers  36  are each an AlGaN layer, and the atomic concentration of Al in the high bandgap section  36   b  is higher than that in the low bandgap section  36   a . Therefore, in the light emitting apparatus  100 , the bandgap of the high bandgap section  36   b  can be readily made higher than that of the low bandgap section  36   a  by growing the second semiconductor layers  36  under the condition that Al is unevenly distributed or concentrated at the side surfaces  31  of the columnar portions  30 . 
     In the light emitting apparatus  100 , the first semiconductor layers  32  each include the low bandgap section  32   a  as a third section and the high bandgap section  32   b  as a fourth section, which surrounds the low bandgap section  32   a  in the plan view and has a bandgap wider than that of the low bandgap section  32   a , and the high bandgap section  32   b  forms the side surface  31  of each of the columnar portions  30 . Therefore, in the light emitting apparatus  100 , the current flowing at the side surface  31  of each of the columnar portions  30  can be further reduced. 
     In the light emitting apparatus  100 , the first semiconductor layers  32  are each an AlGaN layer, and the atomic concentration of Al in the high bandgap section  32   b  is higher than that in the low bandgap section  32   a . Therefore, in the light emitting apparatus  100 , the bandgap of the high bandgap section  32   b  can be readily made higher than that of the low bandgap section  32   a  by growing the first semiconductor layers  32  under the condition that Al is unevenly distributed or concentrated at the side surfaces  31  of the columnar portions  30 . 
     In the light emitting apparatus  100 , the light emitting layers  34  each include the well layers  33  and the barrier layers  35 . The barrier layers  35  each include the low bandgap section  35   a  as a fifth section and the high bandgap section  35   b  as a sixth section, which surrounds the low bandgap section  35   a  in the plan view and has a bandgap wider than that of the low bandgap section  35   a , and the high bandgap section  35   b  forms the side surface  31  of each of the columnar portions  30 . Therefore, in the light emitting apparatus  100 , the current flowing at the side surface  31  of each of the columnar portions  30  can be further reduced. 
     In the light emitting apparatus  100 , the barrier layers  35  are each an AlGaN layer, and the atomic concentration of Al in the high bandgap section  35   b  is higher than that in the low bandgap section  35   a . Therefore, in the light emitting apparatus  100 , the bandgap of the high bandgap section  35   b  can be readily made higher than that of the low bandgap section  35   a  by growing the barrier layers  35  under the condition that Al is unevenly distributed or concentrated at the side surfaces  31  of the columnar portions  30 . 
     The light emitting apparatus  100  is not limited to a laser and may instead be an LED (light emitting diode). 
     2. Method for Manufacturing Light Emitting Apparatus 
     A method for manufacturing the light emitting apparatus  100  according to the present embodiment will next be described with reference to the drawings.  FIG. 4  is a cross-sectional view diagrammatically showing one of the steps of manufacturing the light emitting apparatus  100  according to the present embodiment. 
     The buffer layer  22  is epitaxially grown on the substrate  10 , as shown in  FIG. 4 . Examples of the method for the epitaxial growth may include a metal organic chemical vapor deposition (MOCVD) method and a molecular beam epitaxy (MBE) method. 
     The mask layer  50  is then formed on the buffer layer  22 . The mask layer  50  is formed, for example, by film formation using electron beam evaporation or sputtering, and patterning. The patterning is performed by photolithography and etching. The mask layer  50  has a thickness of, for example, about 5 nm. 
     The mask layer  50  is used as a mask to epitaxially grow the first semiconductor layers  32 , the light emitting layers  34 , and the second semiconductor layers  36  in the presented order on the buffer layer  22 , as shown in  FIG. 1 . Examples of the method for the epitaxial growth may include the MOCVD method and the MBE method. A plurality of columnar portions  30  can be formed by carrying out the steps described above. 
     The epitaxial growth of the first semiconductor layers  32 , the barrier layers  35 , and the second semiconductor layers  36  is performed under the condition that Al is unevenly distributed or concentrated at the side surfaces  31  of the columnar portions  30 . Specifically, the first semiconductor layers  32 , the barrier layers  35 , and the second semiconductor layers  36  are epitaxially grown at a growth temperature higher than or equal to 830° C. but lower than or equal to 870° C., preferably at 850° C. Al can thus be unevenly distributed or concentrated at the side surfaces  31  of the columnar portions  30 . 
     Thereafter, the first electrode  40  is formed on the buffer layer  22 , and the second electrode  42  is formed on the second semiconductor layers  36 . The first electrode  40  and the second electrode  42  are formed, for example, by a vacuum evaporation method. The first electrode  40  and the second electrode  42  are not necessarily formed in a specific order. 
     The light emitting apparatus  100  can be manufactured by carrying out the steps described above. 
     3. Variations of Light Emitting Apparatus 
     3.1. First Variation 
     The light emitting apparatus according to a first variation of the present embodiment will next be described with reference to the drawings.  FIG. 5  is a cross-sectional view diagrammatically showing one of the columnar portions  30  of a light emitting apparatus  200  according to the first variation of the present embodiment. 
     In the following description of the light emitting apparatus  200  according to the first variation of the present embodiment, a member having the same function as that of a constituent member of the light emitting apparatus  100  according to the present embodiment described above has the same reference character and will not be described in detail. The same holds true for the light emitting apparatuses according to second to fourth variations of the present embodiment shown below. 
     In the light emitting apparatus  100  described above, the barrier layers  35  each include the low bandgap section  35   a  and the high bandgap section  35   b , as shown in  FIG. 2 . The barrier layers  35  are each an AlGaN layer. 
     In contrast, in the light emitting apparatus  200 , the barrier layer  35  does not include the low bandgap section  35   a  or the high bandgap section  35   b , as shown in  FIG. 5 . The barrier layer  35  is, for example, a GaN layer. 
     In the light emitting apparatus  200 , the current flowing at the side surfaces  31  of the columnar portions  30  can be reduced, as in the light emitting apparatus  100  described above. 
     Furthermore, in the light emitting apparatus  200 , in which neither the low bandgap section  35   a  nor the high bandgap section  35   b  is formed in each of the barrier layers  35 , which form the light emitting layer  34  having a complicated structure, the columnar portions  30  can be readily formed. 
     3.2. Second Variation 
     The light emitting apparatus according to a second variation of the present embodiment will next be described with reference to the drawings.  FIG. 6  is a cross-sectional view diagrammatically showing one of the columnar portions  30  of a light emitting apparatus  300  according to the second variation of the present embodiment. 
     In the light emitting apparatus  100  described above, the first semiconductor layers  32  each include the low bandgap section  32   a  and the high bandgap section  32   b , as shown in  FIG. 2 . Furthermore, the barrier layers  35  each include the low bandgap section  35   a  and the high bandgap section  35   b . The first semiconductor layers  32  and the barrier layers  35  are each an AlGaN layer. 
     In contrast, in the light emitting apparatus  300 , the first semiconductor layer  32  does not include the low bandgap section  32   a  or the high bandgap section  32   b , as shown in  FIG. 6 . Furthermore, the barrier layer  35  does not include the low bandgap section  35   a  or the high bandgap section  35   b . The first semiconductor layer  32  and the barrier layer  35  are each a GaN layer. 
     In the light emitting apparatus  300 , the current flowing at the side surfaces  31  of the columnar portions  30  can be reduced, as in the light emitting apparatus  100  described above. 
     3.3. Third Variation 
     The light emitting apparatus according to a third variation of the present embodiment will next be described with reference to the drawings.  FIG. 7  is a cross-sectional view diagrammatically showing one of the columnar portions  30  of a light emitting apparatus  400  according to the third variation of the present embodiment. 
     In the light emitting apparatus  100  described above, the first semiconductor layers  32  each include the low bandgap section  32   a  and the high bandgap section  32   b , as shown in  FIG. 2 . Furthermore, the barrier layers  35  each include the low bandgap section  35   a  and the high bandgap section  35   b . The first semiconductor layers  32  and the barrier layers  35  are each an AlGaN layer. Furthermore, the first semiconductor layers  32  are provided under the light emitting layers  34  and are each an n-type semiconductor layer. Moreover, the second semiconductor layers  36  are provided on the light emitting layers  34  and are each a p-type semiconductor layer. 
     In contrast, in the light emitting apparatus  400 , the first semiconductor layer  32  does not include the low bandgap section  32   a  or the high bandgap section  32   b , as shown in  FIG. 7 . Furthermore, the barrier layer  35  does not include the low bandgap section  35   a  or the high bandgap section  35   b . The first semiconductor layer  32  and the barrier layer  35  are each a GaN layer. 
     The first semiconductor layer  32  is provided on the light emitting layer  34 . The first semiconductor layer  32  is provided between the light emitting layer  34  and the second electrode  42 . The first semiconductor layer  32  is a p-type semiconductor layer. 
     The second semiconductor layer  36  is provided below the light emitting layer  34 . The second semiconductor layer  36  is provided on the buffer layer  22 . The second semiconductor layer  36  is provided between the substrate  10  and the light emitting layer  34 . The second semiconductor layer  36  is an n-type semiconductor layer. 
     In the light emitting apparatus  400 , the current flowing at the side surfaces  31  of the columnar portions  30  can be reduced, as in the light emitting apparatus  100  described above. 
     In the light emitting apparatuses  300  and  400  described above, the barrier layers  35  may each be an AlGaN layer or may each include the low bandgap section  35   a  and the high bandgap section  35   b.    
     3.4. Fourth Variation 
     The light emitting apparatus according to a fourth variation of the present embodiment will next be described. 
     In the light emitting apparatus  100  described above, the first semiconductor layers  32 , the barrier layers  35 , and the second semiconductor layers  36  are each an AlGaN layer. 
     In contrast, in the light emitting apparatus according to the fourth variation of the present embodiment, the first semiconductor layers  32 , the barrier layers  35 , and the second semiconductor layers  36  are each a BGaN layer containing B (boron), Ga, and N. 
     The atomic concentration of B in the high bandgap section  32   b  of each of the first semiconductor layers  32  is higher than that in the low bandgap section  32   a . The atomic concentration of B in the high bandgap section  35   b  of each of the barrier layers  35  is higher than that in the low bandgap section  35   a . The atomic concentration of B in the high bandgap section  36   b  of each of the second semiconductor layers  36  is higher than that in the low bandgap section  36   a . In the columnar portions  30 , B is unevenly distributed or concentrated at the side surfaces  31 . 
     The high bandgap sections  32   b ,  35   b , and  36   b  are portions where the atomic concentration of B is higher than that of Ga. In the high bandgap sections  32   b ,  35   b , and  36   b , the ratio of the atomic concentration of B to the sum of the atomic concentrations of B and Ga (hereinafter also referred to as “B ratio”) is, for example, greater than 0.5 and may be greater than or equal to 0.8. Furthermore, the B ratio may be 1.0, in which case the high bandgap sections  32   b ,  35   b , and  36   b  is made of BN. 
     The low bandgap sections  32   a ,  35   a , and  36   a  are portions where the atomic concentration of B is lower than or equal to that of Ga. In the low bandgap sections  32   a ,  35   a , and  36   a , the B ratio is lower than or equal to 0.5 and may be lower than or equal to 0.4. The atomic concentration of B can be measured by STEM-EDS. 
     The epitaxial growth of the first semiconductor layers  32 , the barrier layers  35 , and the second semiconductor layers  36  is performed under the condition that B is unevenly distributed or concentrated at the side surfaces  31  of the columnar portions  30 . 
     In the light emitting apparatus according to the fourth variation of the present embodiment, the current flowing at the side surfaces  31  of the columnar portions  30  can be reduced, as in the light emitting apparatus  100  described above. 
     4. Projector 
     A projector according to the present embodiment will next be described with reference to the drawings.  FIG. 8  diagrammatically shows a projector  900  according to the present embodiment. 
     The projector  900  includes, for example, the light emitting apparatus  100  as a light source. 
     The projector  900  includes an enclosure that is not shown, and a red light source  100 R, a green light source  100 G, and a blue light source  100 B, which are provided in the enclosure and output red light, green light, and blue light, respectively. In  FIG. 8 , the red light source  100 R, the green light source  100 G, and the blue light source  100 B are simplified for convenience. 
     The projector  900  further includes a first optical element  902 R, a second optical element  902 G, a third optical element  902 B, a first light modulator  904 R, a second light modulator  904 G, a third light modulator  904 B, and a projection apparatus  908 , which are provided in the enclosure. The first light modulator  904 R, the second light modulator  904 G, and the third light modulator  904 B are each, for example, a transmissive liquid crystal light valve. The projection apparatus  908  is, for example, a projection lens. 
     The light outputted from the red light source  100 R enters the first optical element  902 R. The first optical element  902 R collects the light outputted from the red light source  100 R. The first optical element  902 R may have another function in addition to the light collection function. The same holds true for the second optical element  902 G and the third optical element  902 B, which will be described later. 
     The light collected by the first optical element  902 R is incident on the first light modulator  904 R. The first light modulator  904 R modulates the light incident thereon in accordance with image information. The projection apparatus  908  then enlarges an image formed by the first light modulator  904 R and projects the enlarged image on a screen  910 . 
     The light outputted from the green light source  100 G enters the second optical element  902 G. The second optical element  902 G collects the light outputted from the green light source  100 G. 
     The light collected by the second optical element  902 G is incident on the second light modulator  904 G. The second light modulator  904 G modulates the light incident thereon in accordance with image information. The projection apparatus  908  then enlarges an image formed by the second light modulator  904 G and projects the enlarged image on the screen  910 . 
     The light outputted from the blue light source  100 B enters the third optical element  902 B. The third optical element  902 B collects the light outputted from the blue light source  100 B. 
     The light collected by the third optical element  902 B is incident on the third light modulator  904 B. The third light modulator  904 B modulates the light incident thereon in accordance with image information. The projection apparatus  908  then enlarges an image formed by the third light modulator  904 B and projects the enlarged image on the screen  910 . 
     The projector  900  can further include a cross dichroic prism  906 , which combines the light outputted from the first light modulator  904 R, the light outputted from the second light modulator  904 G, and the light outputted from the third light modulator  904 B with one another and guides the combined light to the projection apparatus  908 . 
     The red light modulated by the first light modulator  904 R, the green light modulated by the second light modulator  904 G, and the blue light modulated by the third light modulator  904 B enter the cross dichroic prism  906 . The cross dichroic prism  906  is formed by bonding four right-angled prisms to each other, and a dielectric multilayer film that reflects the red light and a dielectric multilayer film that reflects the blue light are disposed at the inner surfaces of the combined prisms. The dielectric multilayer films combine the red light, the green light, and the blue light with one another to form light representing a color image. The combined light is then projected by the projection apparatus  908  on the screen  910 , whereby an enlarged image is displayed. 
     The red light source  100 R, the green light source  100 G, and the blue light source  100 B may instead directly form images in a configuration in which none of the first light modulator  904 R, the second light modulator  904 G, and the third light modulator  904 B is used but the light emitting apparatuses  100  corresponding to the light sources are controlled as the pixels of the images in accordance with the image information. The projection apparatus  908  may then enlarge the images formed by the red light source  100 R, the green light source  100 G, and the blue light source  100 B and project the enlarged images on the screen  910 . 
     In the example described above, transmissive liquid crystal light valves are used as the light modulators, and light valves based not on liquid crystal materials or reflective light valves may be used. Examples of such light valves may include reflective liquid crystal light valves and digital micromirror devices. The configuration of the projection apparatus is changed as appropriate in accordance with the type of the light valves used in the projector. 
     The present disclosure is also applicable to a light source apparatus of a scanning-type image display apparatus including a light source and a scanner that is an image formation apparatus that displays an image having a desired size on a display surface by scanning the screen with the light from the light source. 
     The light emitting apparatus according to the embodiment described above can be used in other applications in addition to a projector. Examples of the applications other than a projector may include an indoor or outdoor illuminator, a backlight of a display, a laser printer, a scanner, an in-vehicle light, a sensing instrument using light, and a light source of a communication instrument. The light emitting apparatus according to the embodiment described above can also be used as minute light emitters of an LED display including the light emitters arranged in an array to display images. 
     5. Experimental Example 
     An n-type GaN layer, an n-type AlGaN layer, and an n-type GaN layer were epitaxially grown in this order to produce columnar portions. The MBE method was used as the method for the epitaxial growth. The temperature at which the AlGaN layer was grown was set at 850° C. 
     The columnar portions produced as described above were observed under an STEM, and the distribution of Al was examined by EDS.  FIG. 9  is an STEM image showing the distribution of Al in the columnar portions. In  FIG. 9 , an area where a smaller amount of Al is present is shown in a darker color, and an area where a larger amount of Al is present is shown in a lighter color. 
       FIG. 9  shows that the epitaxial growth of the AlGaN layer at the growth temperature of 850° C. allowed Al to be unevenly distributed or concentrated at the side surfaces of the columnar portions. 
     The embodiment and the variations described above are presented by way of example, and the present disclosure is not limited thereto. For example, the embodiment and the variations can be combined with each other as appropriate. 
     The present disclosure encompasses substantially the same configuration as the configuration described in the embodiment, for example, a configuration having the same function, using the same method, and providing the same result or a configuration having the same purpose and providing the same effect. Furthermore, the present disclosure encompasses a configuration in which an inessential portion of the configuration described in the embodiment is replaced. Moreover, the present disclosure encompasses a configuration that provides the same effects and advantages as those provided by the configuration described in the embodiment or a configuration that can achieve the same purpose as that achieved by the configuration described in the embodiment. Furthermore, the present disclosure encompasses a configuration in which a known technology is added to the configuration described in the embodiment. 
     The following contents are derived from the embodiment and variations described above. 
     A light emitting apparatus according to an aspect of the present disclosure includes a laminated structure including a plurality of columnar portions. The plurality of columnar portions each includes a first semiconductor layer, a second semiconductor layer different from the first semiconductor layer in terms of conductivity type, and a light emitting layer provided between the first semiconductor layer and the second semiconductor layer. The second semiconductor layer has a first section, and a second section that surrounds the first section in a plan view along a lamination direction in which the first semiconductor layer and the light emitting layer are laminated structured on each other and has a bandgap wider than a bandgap of the first section. The second section forms a side surface of each of the columnar portions. 
     In the light emitting apparatus, the current flowing at the side surfaces of the columnar portions can be reduced. The current resulting in non-light-emission recombination can thus be reduced. As a result, the current injection efficiency can be increased. 
     In the light emitting apparatus according to the aspect described above, the second semiconductor layer may be an AlGaN layer, and an atomic concentration of Al in the second section may be higher than an atomic concentration of Al in the first section. 
     In the light emitting apparatus described above, the bandgap of the second section can be readily made higher than the bandgap of the first section. 
     In the light emitting apparatus according to the aspect described above, the first semiconductor layer may have a third section, and a fourth section that surrounds the third section in the plan view along the lamination direction and has a bandgap wider than a bandgap of the third section. The fourth section may form the side surface of each of the columnar portions. 
     According to the light emitting apparatus described above, the current flowing at the side surfaces of the columnar portions can be further reduced. 
     In the light emitting apparatus according to the aspect described above, the first semiconductor layer may be an AlGaN layer, and an atomic concentration of Al in the fourth section may be higher than an atomic concentration of Al in the third section. 
     In the light emitting apparatus described above, the bandgap of the fourth section can be readily made higher than the bandgap of the third section. 
     In the light emitting apparatus according to the aspect described above, the light emitting layer may include a well layer and a barrier layer. The barrier layer may have a fifth section, and a sixth section that surrounds the fifth section in the plan view along the lamination direction and has a bandgap wider than a bandgap of the fifth section. The sixth section may form the side surface of each of the columnar portions. 
     According to the light emitting apparatus described above, the current flowing at the side surfaces of the columnar portions can be further reduced. 
     In the light emitting apparatus according to the aspect described above, the barrier layer may be an AlGaN layer, and an atomic concentration of Al in the sixth section may be higher than an atomic concentration of Al in the fifth section. 
     In the light emitting apparatus described above, the bandgap of the sixth section can be readily made higher than the bandgap of the fifth section. 
     A projector according to another aspect of the present disclosure includes the light emitting apparatus according to the aspect described above.