Patent Publication Number: US-11049999-B2

Title: Template, nitride semiconductor ultraviolet light-emitting element, and method of manufacturing template

Description:
TECHNICAL FIELD 
     The present invention relates to a template including a sapphire substrate and a method of manufacturing the template, and a nitride semiconductor ultraviolet light-emitting element including the template. In particular, the present invention relates to a template for a nitride semiconductor ultraviolet light-emitting element that has a peak emission wavelength in an ultraviolet region and a method of manufacturing the template, and the nitride semiconductor ultraviolet light-emitting element. 
     BACKGROUND ART 
     As a template of a nitride semiconductor ultraviolet light-emitting element including an AlGaN-based nitride semiconductor in an active layer, a template in which an AlN layer is epitaxially grown on a main surface of a sapphire substrate is often used. 
     In a semiconductor light-emitting element produced by epitaxially growing a plurality of semiconductor layers sequentially, the crystallinity of an underlying semiconductor layer will have a great influence on the crystallinity of a semiconductor layer thereon. Therefore, the crystallinity of a surface of the template, which has an influence on the crystallinity of the overall semiconductor light-emitting element, is particularly important. As the crystallinity of the surface of the template improves, the crystallinity of each semiconductor layer (in particular, active layer) included in the semiconductor light-emitting element also improves, and recombination of electrons and holes causing light emission becomes difficult to be inhibited by crystal defects, leading to better characteristics such as light emission efficiency. 
     However, there is a problem that in the template produced by epitaxially growing the AlN layer on the main surface of the sapphire substrate, it is difficult to obtain the AlN layer with good crystallinity for reasons such as lattice mismatch between sapphire and AlN, and difficulty in migration of Al atoms. 
     Regarding this problem, Patent Documents 1 and 2 and Non-Patent Document 1 propose a method of manufacturing a template in which the crystallinity of an AlN layer is improved by devising the supply timing of source gases at a time of growing the AlN layer. Here, the method of manufacturing a template proposed in Patent Documents 1 and 2 and Non-Patent Document 1 will be described with reference to the drawings.  FIG. 14  is a schematic diagram illustrating a conventional method of manufacturing a template and is part of  FIG. 3  described in a special feature article of Mr. Hideki Hirayama, who is one of inventors of Patent Documents 1 and 2 and one of authors of Non-Patent Document 1 (pages 2 to 5 of RIKEN NEWS June, 2011). 
     The method of manufacturing a template proposed in Patent Documents 1 and 2 and Non-Patent Document 1 is, after first forming several AlN crystalline nuclei on a main surface of a sapphire substrate (first stage of  FIG. 14 ), to supply an N source gas (ammonia) in pulses while supplying an Al source gas continuously, thereby growing an AlN layer that fills a space between the AlN crystalline nuclei (second stage of  FIG. 14 ). The AlN layer grown by such a method has less threading dislocations than the AlN layer grown in a form of a film on the main surface of a sapphire substrate. Then, after that, a film thickness of the AlN layer is increased and a surface is made flat (third stage of  FIG. 14 ). Note that the method of manufacturing a template proposed in Patent Document 1 and Non-Patent Document 1 supplies, while increasing the film thickness of the AlN layer, the source gases by a method similar to the second stage of  FIG. 14 , thereby promoting lateral growth of the AlN crystals (fourth stage of  FIG. 14 ). Accordingly, as the AlN layer grows, the threading dislocations that propagate upward are bent laterally, resulting in improvement in the crystallinity in the final surface of the AlN layer. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent Application Publication No. 2009-54780 
         Patent Document 2: WO 2013/005789 
       
    
     Non-Patent Document 
     
         
         Non-Patent Document 1: Physica Status Solidi, A206, No. 6, 1176-1182 (2009) 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, the method of manufacturing a template proposed in Patent Documents 1 and 2 and Non-Patent Document 1 does not significantly change a growth mode of AlN crystals from the conventional mode, and is limited to reduction in the threading dislocations, which has been conventionally considered to cause decrease in the crystallinity of the AlN layer, not leading to a dramatic improvement in the crystallinity of the AlN layer. 
     Therefore, an object of the present invention is to provide a template that dramatically improves the crystallinity of the AlN layer by greatly changing the growth mode of AlN crystals from the conventional mode and a method of producing the same, and a nitride semiconductor ultraviolet light-emitting element including the template. 
     Means for Solving the Problem 
     To achieve the above object, the present invention provides a template comprising: a sapphire substrate with one of a (0001) plane and a plane inclined by a predetermined angle with respect to the (0001) plane as a main surface; and an AlN layer formed directly on the main surface of the sapphire substrate and composed of AlN crystals having an epitaxial crystal orientation relationship with respect to the main surface, wherein an average particle diameter of the AlN crystals of the AlN layer at a thickness of 20 nm from the main surface is 100 nm or less. 
     Furthermore, in the template, the average particle diameter of the AlN crystals of the AlN layer at a thickness of 20 nm from the main surface may be 75 nm or less and may be 70 nm or less. Furthermore, in the template, the average particle diameter of the AlN crystals of the AlN layer at a thickness of 300 nm from the main surface may be 300 nm or less. 
     Furthermore, in the template, the main surface of the sapphire substrate may be a plane inclined by 0.2° or more with respect to the (0001) plane. With this template, the AlN crystals having a small average particle diameter as described above can be easily obtained. 
     Furthermore, in the template, the AlN crystals of the AlN layer at a thickness of 300 nm from the main surface may be +C-axis oriented upward of the sapphire substrate. With this template, the crystallinity of the AlN layer can be further improved. 
     Furthermore, the present invention provides a nitride semiconductor ultraviolet light-emitting element comprising: the template; and an element structure part including a plurality of AlGaN-based semiconductor layers stacked on the template. 
     Furthermore, the present invention provides a method of manufacturing a template, the method comprising a step of epitaxially growing AlN crystals directly on a main surface of a sapphire substrate with one of a (0001) plane and a plane inclined by a predetermined angle with respect to the (0001) plane as the main surface to form an AlN layer, wherein the step includes epitaxially growing the AlN crystals under a growth condition that an average particle diameter of the AlN crystals on a surface of the AlN layer epitaxially grown from the main surface to a thickness of 20 nm is 100 nm or less. 
     Furthermore, the present invention provides a method of manufacturing a template, the method comprising a step of epitaxially growing AlN crystals directly on a main surface of a sapphire substrate with one of a (0001) plane and a plane inclined by a predetermined angle with respect to the (0001) plane as the main surface to form an AlN layer, wherein the step includes epitaxially growing the AlN crystals under a growth condition that an average particle diameter of the AlN crystals on a surface of the AlN layer is 100 nm or less when the AlN layer covering 90% or more of the main surface is epitaxially grown to a thickness of 20 nm. 
     Furthermore, the step of the method of manufacturing a template may include epitaxially growing the AlN layer under a growth condition that the average particle diameter of the AlN crystals on the surface of the AlN layer epitaxially grown from the main surface to a thickness of 300 nm is 300 nm or less. 
     Furthermore, the step of the method of manufacturing a template may include epitaxially growing the AlN layer under a growth condition that an RMS value of surface roughness of the AlN layer epitaxially grown from the main surface to a thickness of 20 nm is equal to or less than an RMS value of surface roughness of the AlN layer epitaxially grown from the main surface to a thickness of 300 nm. For example, the step of the method of manufacturing a template may include epitaxially growing the AlN layer under a growth condition that the RMS value of surface roughness of the AlN layer epitaxially grown from the main surface to a thickness of 20 nm is 5 nm or less, and may include epitaxially growing the AlN layer under a growth condition that the RMS value of surface roughness of the AlN layer epitaxially grown from the main surface to a thickness of 300 nm is 10 nm or less. 
     Furthermore, the step of the method of manufacturing a template may include epitaxially growing the AlN layer under a growth condition that the AlN crystals on the surface of the AlN layer epitaxially grown from the main surface to a thickness of 300 nm are +C-axis oriented. By this method of manufacturing a template, the crystallinity of the AlN layer can be further improved. 
     Furthermore, the step of the method of manufacturing a template may include setting a growth temperature of the AlN layer at 1150° C. or higher and 1300° C. or lower. By this method of manufacturing a template, the AlN crystals can be epitaxially grown suitably on the main surface of the sapphire substrate. 
     Effects of the Invention 
     With the template and the method of manufacturing the template, the crystallinity of the AlN layer epitaxially grown on the main surface of the sapphire substrate can be dramatically improved. Also, the nitride semiconductor ultraviolet light-emitting element including this template, which improves the crystallinity of the element structure part, can improve characteristics such as light emission efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a main part schematically illustrating one example of a structure of a nitride semiconductor ultraviolet light-emitting element according to an embodiment of the present invention. 
         FIG. 2  is a plan view schematically illustrating one example of the structure of the nitride semiconductor ultraviolet light-emitting element illustrated in  FIG. 1  when viewed from an upper side of  FIG. 1 . 
         FIG. 3  is an AFM image of an AlN layer having a thickness of 20 nm and grown on a main surface of a sapphire substrate with an off angle of 0.2°. 
         FIG. 4  is an AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 0.5°. 
         FIG. 5  is an AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 1.0°. 
         FIG. 6  is tables illustrating a particle diameter of AlN crystals measured from the AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 0.2°. 
         FIG. 7  is tables illustrating the particle diameter of the AlN crystals measured from the AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 0.5°. 
         FIG. 8  is tables illustrating the particle diameter of the AlN crystals measured from the AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 1.0°. 
         FIG. 9  is tables illustrating measurement results of the particle diameter of the AlN crystals illustrated in  FIGS. 6 to 8  together with the particle diameter of the AlN crystals and RMS values of surface roughness of the AlN layer measured with an AFM device. 
         FIG. 10  is an AFM image of the AlN layer having a thickness of 300 nm and grown on the main surface of the sapphire substrate with the off angle of 0.2°, and a table illustrating the particle diameter of the AlN crystals and the RMS value of surface roughness of the AlN layer measured with the AFM device. 
         FIG. 11  is an AFM image of the AlN layer having a thickness of 300 nm and grown on the main surface of the sapphire substrate with the off angle of 1.0°, and a table illustrating the particle diameter of the AlN crystals and the RMS value of surface roughness of the AlN layer measured with the AFM device. 
         FIGS. 12A and 12B  are diagrams illustrating a comparison between the AlN layer in the template according to the embodiment of the present invention, and the AlN layer in the conventional template described in Patent Document 1 and Non-Patent Document 1. 
         FIGS. 13A and 13B  are tables illustrating measurement results by the XRC method on the (0002) plane of the AlN layer in the template according to the embodiment of the present invention. 
         FIG. 14  is a schematic diagram illustrating a conventional method of manufacturing a template. 
         FIG. 15  is a table illustrating a difference of altitude in a measurement region of the AlN layer with a thickness of 300 nm in the template according to the embodiment of the present invention. 
         FIG. 16  is a table illustrating the difference of altitude in the measurement region of the AlN layer with a thickness of 20 nm and a height of a cumulative frequency of 90% in the template according to the embodiment of the present invention. 
         FIGS. 17A to 17C  are diagrams illustrating one example of a height profile and a height histogram of the AlN layer with a thickness of 20 nm in the template according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     When describing an embodiment of the present invention, a nitride semiconductor ultraviolet light-emitting element, which is a light-emitting diode, and a method of manufacturing the same will be illustrated below. The light-emitting diode includes a template including a sapphire substrate, and an element structure part including a plurality of AlGaN-based semiconductor layers stacked on the template. The light-emitting diode emits light having a peak emission wavelength of 365 nm or less (ultraviolet) by turning on electricity. Note that the AlGaN-based semiconductor that is a material constituting each of the AlGaN-based semiconductor layers included in the element structure part refers to AlGaN, AlN, or GaN, or a semiconductor of AlGaN, AlN, or GaN containing a small amount of impurities (e.g., Si, Mg, In, and the like). Hereinafter, subscripts used for Al and Ga as necessary represent relative composition ratios of Al and Ga (e.g., Al X Ga 1-X N). 
     However, since the present invention mainly relates to a template, the structure of the element structure part on the template may be any structure, and is not limited to the structure exemplified in the following section “Nitride semiconductor ultraviolet light-emitting element.” 
     &lt;Nitride Semiconductor Ultraviolet Light-Emitting Element&gt; 
     First, one example of the structure of the nitride semiconductor ultraviolet light-emitting element according to the embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a cross-sectional view of a main part schematically illustrating one example of the structure of the nitride semiconductor ultraviolet light-emitting element according to the embodiment of the present invention.  FIG. 2  is a plan view schematically illustrating one example of the structure of the nitride semiconductor ultraviolet light-emitting element illustrated in  FIG. 1  when viewed from an upper side of  FIG. 1 . Note that since  FIG. 1  schematically illustrates thicknesses of a substrate, AlGaN-based semiconductor layers, and electrodes (vertical lengths in the figure) for convenience of illustration,  FIG. 1  does not necessarily agree with an actual dimensional ratio. Also, in the following description, an AlGaN-based semiconductor having description of neither p-type nor n-type is an undoped semiconductor, but even the undoped semiconductor can contain a small amount of impurities that is unavoidably mixed. 
     As illustrated in  FIGS. 1 and 2 , a nitride semiconductor ultraviolet light-emitting element  1  according to the embodiment of the present invention includes a template  10  including a sapphire substrate  11 , and an element structure part  20  including a plurality of AlGaN-based semiconductor layers  21  to  24  and electrodes  25  and  26 . This nitride semiconductor ultraviolet light-emitting element  1  is mounted (flip-chip mounted) with an element structure part  20  side (upper side in the figure in  FIG. 1 ) facing a mounting base, and a light extraction direction is a template  10  side (lower side in the figure in  FIG. 1 ). 
     The template  10  includes the sapphire substrate  11  with the (0001) plane or a plane inclined by a predetermined angle (off angle) with respect to the (0001) plane as a main surface, and an AlN layer  12  directly formed on the main surface of the sapphire substrate  11 . The AlN layer  12  is composed of AlN crystals epitaxially grown from the main surface of the sapphire substrate  11 , and these AlN crystals have an epitaxial crystal orientation relationship with respect to the main surface of the sapphire substrate  11 . Specifically, for example, the AlN crystals grow such that the C axis direction of the sapphire substrate  11  (&lt;0001&gt; direction) and the C axis direction of the AlN crystals are aligned. Note that the AlN crystals constituting the AlN layer  12  may contain a small amount of Ga and other impurities. In addition, a layer composed of an Al α Ga 1-α N (1&gt;α&gt;0)-based semiconductor may further be formed on an upper surface of the AlN layer  12 . 
     The element structure part  20  has a structure in which an n-type cladding layer  21 , an active layer  22 , an electron blocking layer  23 , and a p-type contact layer  24  are epitaxially grown and stacked in order from the template  10  side. 
     The n-type cladding layer  21  is composed of an n-type Al X Ga 1-X N (1≥X&gt;0)-based semiconductor. The active layer  22  has a single or multiple quantum well structure obtained by alternately laminating one or more well layers composed of an Al Y1 Ga 1-Y1 N-based semiconductor (X&gt;Y1≥0) and one or more barrier layers composed of Al Y2 Ga 1-Y2 N (X≥Y2&gt;Y1). The electron blocking layer  23  is composed of a p-type Al Z Ga 1-Z N (1≥Z≥Y2)-based semiconductor. The p-type contact layer  24  is composed of a p-type Al Q Ga 1-Q N (Z&gt;Q≥0)-based semiconductor. 
     Furthermore, the element structure part  20  includes the p-electrode  25  composed of, for example, Ni/Au and formed on the upper surface of the p-type contact layer  24 , and the n-electrode  26  composed of, for example, Ti/Al/Ti/Au and formed on the upper surface of the n-type cladding layer  21  in certain region where the n-type cladding layer  21  is exposed. When electric power is applied such that holes are supplied from the p-electrode  25  and electrons are supplied from the n-electrode  26 , the supplied holes and electrons reach the active layer  22  and recombine to emit light. 
     Next, one example of a method of manufacturing the nitride semiconductor ultraviolet light-emitting device  1  illustrated in  FIG. 1  will be described. 
     First, by a well-known epitaxial growth method such as metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE), the AlN layer  12  included in the template  10  and the AlGaN-based semiconductor layers  21  to  24  included in the element structure part  20  are epitaxially grown and stacked sequentially on the sapphire substrate  11 . At this time, for example, Si is doped as a donor impurity into the n-type layer, and for example, Mg is doped as an acceptor impurity into the p-type layer. 
     Next, by a well-known etching method such as reactive ion etching, some region of the stacked semiconductor layers as described above is selectively etched to expose the n-type cladding layer  21  of the region. Then, by a well-known film formation method such as an electron beam evaporation method, the p-electrode  25  is formed on the p-type contact layer  24  within a region that is not etched, and the n-electrode  26  is formed on the n-type cladding layer  21  within the etched region. Note that after the formation of one or both of the p-electrode  25  and the n-electrode  26 , heat treatment may be performed by a well-known heat treatment method such as rapid thermal annealing (RTA). 
     &lt;Template&gt; 
     Next, the above-described template  10  will be described. Note that the template  10  according to the embodiment of the present invention is characterized by the AlN layer  12  formed on the main surface of the sapphire substrate  11 , and any sapphire substrate can be used for the sapphire substrate  11  as long as the AlN layer  12  can be epitaxially grown (cab be particularly grown in the C-axis direction). 
     The template  10  according to the embodiment of the present invention is largely different from the template proposed in Patent Documents 1 and 2 and Non-Patent Document 1 in that a particle diameter of the AlN crystals formed on the main surface of the sapphire substrate  11  is made as small as possible. The particle diameter of the AlN crystals is determined depending on various growth conditions, such as, for example, the off angle and a growth temperature (substrate temperature) of the sapphire substrate  11 , a supply amount and a supply ratio of a raw material (V/III ratio), and a supply amount of a carrier gas, and the particle diameter can vary depending on a film forming device to use. Note that as in the template proposed in Patent Documents 1 and 2 and Non-Patent Document 1, for forming AlN crystalline nuclei on a sapphire substrate and growing an AlN layer so as to fill the space between the AlN crystalline nuclei, a special growth method (pulse supply of ammonia) is required when growing the AlN layer; however, for making the particle diameter of individual AlN crystals as small as possible as in the template  10  according to the embodiment of the present invention, no special growth method is required, and it is required at least to suitably explore, by experiments or the like, and employ appropriate growth conditions suitable for the film forming device to use. However, for growing the AlN layer particularly by the MOVPE method, from the viewpoint of epitaxially growing the AlN crystals suitably on the main surface of the sapphire substrate  11 , it is preferable to set the growth temperature at 1150° C. or higher and 1300° C. or lower, and more preferably higher than 1200° C. and lower than 1300° C. 
       FIGS. 3 to 5  are atomic force microscope (AFM) images of the surface of the AlN layer grown to a thickness of 20 nm in the template according to the embodiment of the present invention.  FIG. 3  is an AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 0.2°.  FIG. 4  is an AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 0.5°.  FIG. 5  is an AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 1.0°. 
       FIGS. 6 to 9  are tables illustrating the particle diameter of the AlN crystals measured from the AFM images of the surface of the AlN layer grown to a thickness of 20 nm in the template according to the embodiment of the present invention.  FIG. 6  is tables illustrating the particle diameter of the AlN crystals measured from the AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 0.2°.  FIG. 7  is tables illustrating the particle diameter of the AlN crystals measured from the AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 0.5°.  FIG. 8  is tables illustrating the particle diameter of the AlN crystals measured from the AFM image of the AlN layer having a thickness of 20 nm and grown on the main surface of the sapphire substrate with the off angle of 1.0°.  FIG. 9  is tables illustrating measurement results of the particle diameter of the AlN crystals illustrated in  FIGS. 6 to 8  together with the particle diameter of the AlN crystals and root mean square (RMS) values of surface roughness of the AlN layer measured with the AFM device. 
     The measurement results illustrated in  FIGS. 6 to 8  are results of dividing an AFM image measurement region having a size of 500 nm×500 nm into 25 small regions of 100 nm×100 nm, and measuring one by one the particle diameter of the AlN crystals included in each small region. Note that the AlN crystal positioned on a boundary of the small region is distributed to a small region in which a portion equal to or greater than half of the AlN crystal is included, whereas the AlN crystal positioned on a boundary of the measurement region is neglected because the particle diameter cannot be measured. In addition, since the AlN crystal in the AFM image is generally considered circular or elliptical (strictly speaking, the AlN crystal is considered to have a shape close to a hexagon, and sides (facets) are visible in some particles, but the AlN crystal appears to be circular or elliptical for reasons such as the limitation of performance of the AFM device), an average value of a major axis length and a minor axis length is defined as the particle diameter. In addition, as illustrated in  FIGS. 6 to 8 , for each of three types of samples (wafers) having the off angle of the sapphire substrate  11  of 0.2°, 0.5°, and 1.0°, the particle diameter of the AlN crystals is measured from each AFM image of different measurement regions  1  to  3  on the wafer. 
     In  FIG. 9 , the average particle diameter and standard deviation of “individual measurement values” refer to values obtained by measuring the AlN crystals one by one in the AFM images as illustrated in  FIGS. 6 to 8 . Meanwhile, the average particle diameter, standard deviation, and RMS value of surface roughness of “device measurement values” are values measured with the AFM device (probe station: NanoNavills, scanning probe microscope unit: NanoCute, and software: NanoNaviStation ver 5.6B). The AFM device regards, as one particle, one closed region in which a height at each measurement point in the measurement region is equal to or higher than a predetermined threshold height (e.g., an intermediate value such as average and median), detects the number of particles and the particle gross area, and calculates a circle diameter that has an average particle area obtained by dividing the particle gross area by the number of particles as the average particle diameter. Furthermore, this AFM device calculates standard deviation of the particle area. The value obtained by converting the standard deviation of the particle area into the standard deviation of the circle diameter is the standard deviation of the particle diameter of “the device measurement value” in  FIG. 9 . In addition, the RMS value of surface roughness is a value of Rq calculated by the following formula (1). Note that in the following formula (1), Z (i) is the height at each measurement point in the measurement region, n is the number of measurement points in the measurement region, and Ze is the average value of the height at each measurement point in the measurement region. 
     
       
         
           
             
               
                 
                   
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     As illustrated in  FIGS. 3 to 5  and  FIGS. 6 to 8 , in any of samples in which the off angles of the sapphire substrate  1  are 0.2°, 0.5° and 1.0°, the AlN crystals having a particle diameter of approximately 20 nm or more and 100 nm or less are densely packed. In addition, as illustrated in  FIG. 9 , the average particle diameter of the AlN crystals obtained by measuring the AlN crystals in the AFM image one by one, and the average particle diameter of the AlN crystals measured with the AFM device have similar sizes, and it can be said that the average particle diameters of the AlN crystals measured by either method are appropriate values. 
       FIGS. 10 and 11  are diagrams each illustrating the AFM image of the AlN layer grown to a thickness of 300 nm and the particle diameter and the RMS value of surface roughness measured with the AFM device in the template according to the embodiment of the present invention.  FIG. 10  is an AFM image of the AlN layer having a thickness of 300 nm and grown on the main surface of the sapphire substrate with the off angle of 0.2°, and a table illustrating the particle diameter of the AlN crystals and the RMS value of surface roughness of the AlN layer measured with the AFM device.  FIG. 11  is an AFM image of the AlN layer having a thickness of 300 nm and grown on the main surface of the sapphire substrate with the off angle of 1.0°, and a table illustrating the particle diameter of the AlN crystals and the RMS value of surface roughness of the AlN layer measured with the AFM device. 
     Here, with reference to the drawings, a comparison result will be described between the AlN layer in the template according to the embodiment of the present invention and the AlN layer in the conventional template described in Patent Document 1 and Non-Patent Document 1.  FIGS. 12A and 12B  are diagrams illustrating a comparison between the AlN layer in the template according to the embodiment of the present invention and the AlN layer in the conventional template described in Patent Document 1 and Non-Patent Document 1. Note that  FIG. 12A  is an AFM image of the AlN layer in the conventional template described in Patent Document 1 and Non-Patent Document 1.  FIG. 12B  is a diagram illustrating a comparison, in the same size (2 μm×2 μm), between the AFM image of the AlN layer in the template according to the embodiment of the present invention (a figure in which the plurality of AFM images of  FIG. 3  are arranged, and part of the AFM image of  FIG. 10 ), and the AFM image of the AlN layer in the conventional template described in Patent Document 1 and Non-Patent Document 1 (part of the AFM image of  FIG. 12A ). 
     The AFM image of the AlN layer in the conventional template described in Patent Document 1 and Non-Patent Document 1 illustrated in  FIG. 12A  is an AFM image of an AlN nucleation layer formed on the main surface of the sapphire substrate (layer in an initial stage in the AlN layer having a thickness of 300 nm and first formed on the main surface of the sapphire substrate), and corresponds to a state of the template in  FIGS. 3 to 5  (thickness: 20 nm) according to the embodiment of the present application. In the AFM image illustrated in  FIG. 12A , some AlN crystals are combined and grow huge to about several micrometers, and it is difficult to measure the particle diameter before the combination, but a relatively small AlN crystal that is considered not combined also has an average particle diameter of about 1000 nm. 
     As is apparent from comparison results of the AFM images illustrated in  FIG. 12B , the average particle diameter of the AlN crystals in the AlN layer of the template according to the embodiment of the present invention is significantly smaller than the average particle diameter of the AlN crystals in the AlN layer of the template described in Patent Document 1 and Non-Patent Document 1. 
     Specifically, in the template described in Patent Document 1 and Non-Patent Document 1, the average particle diameter of the AlN crystals in a growth start stage of the AlN layer is about 1000 nm. Furthermore, in the template described in Patent Document 1 and Non-Patent Document 1, when the thickness of the AlN layer becomes 300 nm, the plurality of AlN crystals is completely combined into a film state, in which individual crystals cannot be observed. In contrast, in the template according to the embodiment of the present invention, the average particle diameter of the AlN crystals in the growth start stage of the AlN layer (thickness: 20 nm) is only about 50 nm. Furthermore, in the template according to the embodiment of the present invention, even when the thickness of the AlN layer becomes 300 nm, individual crystals are sufficiently observable, and the average particle diameter is only about 200 nm. 
     Also, in the template described in Patent Document 1 and Non-Patent Document 1, it is presumed that the RMS value of surface roughness in the growth start stage of the AlN layer is 21.4 nm, and that the RMS value of surface roughness in the stage when the thickness of the AlN layer becomes 300 nm is a value between 21.4 nm and 8.2 nm (see FIG. 4B of Patent Document 1). Meanwhile, in the template according to the embodiment of the present invention, the RMS value of surface roughness in the growth start stage of the AlN layer (thickness: 20 nm) is about 3 nm, and the RMS value of surface roughness in the stage when the thickness of the AlN layer becomes 300 nm is about 5 nm. Therefore, the RMS value of surface roughness in the AlN layer of the template according to the embodiment of the present invention is significantly smaller than the RMS value of surface roughness in the AlN layer of the template described in Patent Document 1 and Non-Patent Document 1. 
     In particular, in the template described in Patent Document 1 and Non-Patent Document 1, relatively large AlN crystalline nuclei stand out everywhere in the growth start stage of the AlN layer, and the film-like AlN layer is formed so as to fill the space between the AlN crystalline nuclei, and thus the RMS value of surface roughness of the AlN layer becomes relatively large. After that, since the space between individual AlN crystalline nuclei are completely filled with the film-like AlN layer, the RMS value of surface roughness of the AlN layer simply decreases with the growth of the AlN layer (increase in thickness). Meanwhile, in the template according to the embodiment of the present invention, since the fine initial AlN crystals are grown in large quantity and high density, the RMS value of surface roughness in the growth start stage of the AlN layer becomes relatively small. After that, since individual AlN crystals are combined or become coarse afterwards, the RMS value of surface roughness of the AlN layer in this stage (stage when the thickness of the AlN layer becomes 300 nm) is equal to or greater than the RMS value of surface roughness of the AlN layer in the growth start stage. 
     As described above, the growth mode of the AlN crystals in the growth start stage of the AlN layer is fundamentally different between the template described in Patent Documents 1 and 2 and Non-Patent Document 1 and the template according to the embodiment of the present invention, and the difference therebetween is shown in the average particle diameter of the AlN crystals and the RMS value of surface roughness of the AlN layer. Note that even with the template according to the embodiment of the present invention, when the AlN layer is further grown (thickness is made larger than 300 nm, for example, 1 μm or more, preferably 2 μm or more), individual AlN crystals are gradually combined, and finally the film-like AlN layer is obtained. 
     Next, the crystallinity of the AlN layer in the template according to the embodiment of the present invention will be described with reference to the drawings.  FIGS. 13A and 13B  are tables illustrating measurement results by the X-ray rocking curve (XRC) method on the (0002) plane of the AlN layer in the template according to the embodiment of the present invention. Note that  FIG. 13A  is a table illustrating the measurement result of ω scan,  FIG. 13B  is a table illustrating the measurement result of 2θ-ω scan, and numerical values described in each table are average values of the full width at half maximum (FWHM) of the peak corresponding to the (0002) plane. Also, in measurement of ω scan illustrated in  FIG. 13A , if axis alignment is performed to align the C axis of a sample (C axis of the sapphire substrate and the AlN layer) with the X-ray specular reflection axis, specular reflection of the X-ray by the sapphire substrate is measured, and it becomes difficult to measure the full width at half maximum of the AlN layer, and thus measurement is carried out without performing such axis alignment. 
     As illustrated in  FIGS. 13A and 13B , in the measurement results of both ω scan and 2θ-ω scan, when the thickness of the AlN layer is 20 nm, the full width at half maximum of the (0002) plane is about 1000 arcsec. Also, as illustrated in  FIG. 13B , in the measurement result of 2θ-ω scan, when the thickness of the AlN layer is 300 nm, the full width at half maximum of the (0002) plane is about 100 arcsec. 
     Usually, the full width at half maximum of the (0002) plane in the AlN layer grown on the (0001) plane of the sapphire substrate by about several micrometers without particularly restricting the particle diameter of the AlN crystals is about 2000 arcsec. In contrast, Non-Patent Document 1 reports that the full width at half maximum of the (0002) plane in the AlN layer grown to a thickness of 4.8 μm has improved to about 200 arcsec, by forming a small number of AlN crystalline nuclei in the growth start stage of the AlN layer, growing the film-like AlN layer filling the AlN crystalline nuclei, and further promoting lateral growth during the growth of the AlN layer. 
     Compared with these conventional techniques, in the template according to the embodiment of the present invention, the full width at half maximum of the (0002) plane in the growth start stage of the AlN layer (thickness: 20 nm) is already as small as 1000 arcsec, and the full width at half maximum of the (0002) plane in the AlN layer grown to a thickness of 300 nm is smaller, as small as 100 arcsec. That is, by merely growing the AlN layer to only about 300 nm, the template according to the embodiment of the present invention can achieve the crystallinity equal to or higher than the crystallinity that cannot be achieved without growing the AlN layer to a thickness of 4.8 μm in the template described in Patent Document 1 and Non-Patent Document 1. In addition, in the template according to the embodiment of the present invention, further improvement in the crystallinity is expected by growing the AlN layer more thickly. 
     As described above, by making the average particle diameter of the AlN crystals epitaxially grown on the main surface of the sapphire substrate sufficiently smaller than the average particle diameter of the AlN crystals in the AlN layer of the template described in Patent Document 1 and Non-Patent Document 1 (furthermore, Patent Document 2 in which the AlN layer is formed by a similar method), the crystallinity of the AlN layer epitaxially grown on the main surface of the sapphire substrate can be dramatically improved. 
     Note that considering a range and deviation of the average particle diameter and variation in the particle diameter of the AlN crystals in the template according to the embodiment of the present invention illustrated in  FIGS. 6 to 12 , the divergence of the particle diameter of the AlN crystals in each of the template according to the embodiment of the present invention and the template described in Patent Document 1 and Non-Patent Document 1, and the like, it is considered that the above effect can be obtained by setting the average particle diameter of the AlN crystals at 100 nm or less in the growth start stage of the AlN layer (thickness: 20 nm). In particular, in  FIGS. 6 to 8 , since the average value of the maximum value in the particle diameter of the AlN crystals is about 75 nm at the most, it is preferable to set the average particle diameter of the AlN crystals at 75 nm or less, and more preferable at 70 nm or less. Also, it is preferable to set the average particle diameter at 20 nm or more, and in particular, in  FIGS. 6 to 8 , since the average value of the minimum value in the particle diameter of the AlN crystals is about 28 nm, it is more preferable to set the average particle diameter of the AlN crystals at 28 nm or more. Similarly, it is considered that the above effect can be obtained by setting the average particle diameter of the AlN crystals in the AlN layer having a thickness of 300 nm at 300 nm or less, and it is more preferable to set the average particle diameter at 250 nm or less. Also, it is preferable to set this average particle diameter at 150 nm or more. 
     Also, considering the RMS value of surface roughness of the AlN layer in the template according to the embodiment of the present invention illustrated in  FIGS. 9 to 12  and the divergence of the RMS value of surface roughness of the AlN layer in each of the template according to the embodiment of the present invention and the template described in Patent Document 1 and Non-Patent Document 1, it is considered that the above effect can be obtained by setting the RMS value of surface roughness of the AlN layer in the growth start stage of the AlN layer (thickness: 20 nm) at 5 nm or less, and it is more preferable to set this RMS value at 4 nm or less. Also, it is preferable to set this RMS value at 2 nm or more. Similarly, it is considered that the above effect can be obtained by setting the RMS value of surface roughness of the AlN layer having a thickness of 300 nm at 10 nm or less, and it is more preferable to set the RMS value at 6 nm or less. Also, it is preferable to set this RMS value at 4 nm or more. 
     Note that AlN, which has a wurtzite structure, has a crystalline structure asymmetric in the C-axis direction (vertically asymmetric crystalline structure when the C axis is selected as the vertical direction), the +C-axis direction ([0001] direction) and the −C-axis direction ([000-1] direction) are not equivalent, and the +C plane ((0001) plane: A1 polarity plane) and the −C plane ((000-1) plane: N polarity plane) are also not equivalent. When the AlN crystals are epitaxially grown on the (0001) plane of the sapphire substrate, the AlN crystals growing in the +C-axis direction and the AlN crystals growing in the −C-axis direction can be mixed. 
     Regarding this, if the AlN crystals constituting the AlN layer are +C-axis oriented toward the upper side of the substrate (if the main growth direction of the AlN crystals is the +C axis direction and the entire surface or most part (for example, 80% or more, preferably 90% or more) of the surface of the AlN layer is the +C plane), it is preferable because the crystallinity of the AlN layer can be further improved. 
     As one example of a method of making the AlN crystals +C-axis oriented, for example, a method employed in Applied Physics Express 4 (2011) 092102 is considered. In addition, for example, when the AlN layer is epitaxially grown on the sapphire substrate by the MOVPE method, by starting supply of an Al source gas (e.g., TMA: TriMethylAluminium) at the same time or earlier than the N source gas (e.g., ammonia), a method of suppressing excessive nitriding of the main surface of the sapphire substrate and making the AlN layer +C-axis oriented is considered. Note that the template according to the above-described embodiment is obtained by growing the AlN layer with the latter method, at least 50% of the surface of the AlN layer grown to a thickness of 20 nm from the main surface of the sapphire substrate is the +C plane, and at least 80% of the surface of the AlN layer grown to a thickness of 300 nm is the +C plane (+C-axis oriented). 
     Also, in the embodiment described above, description has been made of the template having the off angle of the sapphire substrate of 0.2°, 0.5°, and 1.0°; however, as long as AlN crystals similar to the above-described embodiment can be obtained, the off angle of the sapphire substrate is arbitrary. However, it is preferable to set the off angle at 0.2° or more because the AlN crystals similar to the embodiment described above can be easily obtained. 
     &lt;Difference Between AlN Crystalline Nuclei in Patent Documents 1 and 2 and Non-Patent Document 1, and Fine AlN Crystals Constituting AlN Layer in the Embodiment of the Present Invention&gt; 
     Here, a difference will be described between the AlN crystalline nuclei formed before the growth of the AlN layer in Patent Documents 1 and 2 and Non-Patent Document 1, and the fine AlN crystals constituting the AlN layer in the embodiment of the present invention. 
     First, as described with reference to  FIG. 14 , in Patent Documents 1 and 2 and Non-Patent Document 1, after the AlN crystalline nuclei are formed on the main surface of the sapphire substrate (first stage of  FIG. 14 ), the AlN layer is formed so as to fill the space between the AlN crystalline nuclei (second stage of  FIG. 14 ). At this time, since the AlN crystalline nuclei are only scattered on the main surface of the sapphire substrate as illustrated in the first stage of  FIG. 14 , in this stage, the AlN crystalline nuclei have not become “a layer” that sufficiently covers the main surface of the sapphire substrate. 
     For example, paragraph [0060] of the specification of Patent Document 2 describes that “AlN crystalline nuclei having a diameter of 20 to 50 nm and a height of 20 to 40 nm are formed at a density of about 200/μm 2 .” Here, even though a practically impossible situation is assumed in which the coverage of the AlN crystalline nuclei is maximized, that is, all 200 AlN crystalline nuclei are contained in a region of 1 μm 2  without lack, and all the AlN crystalline nuclei are circular in a plan view with a diameter of 50 nm, the total area occupied by the AlN crystalline nuclei is less than 0.4 μm 2 , and the coverage is less than 40%. Therefore, in the state where only the AlN crystalline nuclei are formed in Patent Documents 1 and 2 and Non-Patent Document 1, obviously “the layer” has not been formed. 
     In Patent Documents 1 and 2 and Non-Patent Document 1, as illustrated in the second stage of  FIG. 14 , when the film-like AlN crystals filling the space between the AlN crystalline nuclei are formed, “the layer” that sufficiently covers the main surface of the sapphire substrate is formed. Then, when reaching the second stage of  FIG. 14 , since the film-like AlN crystals having an extremely large average particle diameter and fused with the AlN crystalline nuclei are formed as illustrated in  FIG. 12 , the average particle diameter of the AlN crystals constituting the AlN layer is as extremely large as about 1000 nm. 
     Also, paragraph [0071] of the specification of Patent Document 2 describes that “It has been confirmed that the AlN crystalline nuclei  2   a  have been combined with each other and the AlN layer on the one surface side of the single crystal substrate  1  has been flattened to some extent. Here, the density of the exposed AlN crystalline nuclei  2   a  has decreased to about 100/μm 2  and that the diameter of the AlN crystalline nuclei  2   a  has also increased to about 50 to 100 nm.” This means that in the second stage of  FIG. 14 , the thickness of the film-like AlN crystals has increased and part of the AlN crystalline nuclei are filled with the film-like AlN crystals, and the top of the remaining AlN crystalline nuclei are exposed. Then, in this state, since the film-like AlN crystals having a large average particle diameter and fused with the AlN crystalline nuclei are present as illustrated in  FIG. 12 , the average particle diameter of the AlN crystals constituting the AlN layer is as extremely large as about 1000 nm. 
     In contrast, in the template according to the embodiment of the present invention, as is apparent from the AFM images illustrated in  FIGS. 3 to 5 , “the layer” that sufficiently covers the main surface of the sapphire substrate has already been formed while the fine AlN crystals constituting the AlN layer have been formed. This will be specifically described below with reference to the drawings. 
       FIG. 15  is a table illustrating a difference of altitude in a measurement region of the AlN layer with a thickness of 300 nm in the template according to the embodiment of the present invention.  FIG. 16  is a table illustrating the difference of altitude in the measurement region and a height of a cumulative frequency of 90% where a thickness of the AlN layer is 20 nm in the template according to the embodiment of the present invention. Here, the difference of altitude in the measurement region is a difference between a height of the highest mountain in the measurement region and a height of the lowest valley in the measurement region in the measurement with the above-mentioned AFM device. In addition, the height of the cumulative frequency of 90% is a height when the cumulative total reaches 90% in a case where the height in the measurement region is counted in decreasing order of height. Note that in  FIG. 15 , the differences of altitude in the measurement region of the AlN layer with a thickness of 300 nm are values obtained from two measurement regions described in the respective tables of  FIGS. 10 and 11 . In addition, in  FIG. 16 , the differences of altitude in the measurement region and the heights of the cumulative frequency of 90% of the AlN layer with a thickness of 20 nm are values obtained from two measurement regions out of three measurement regions described in the respective tables of  FIGS. 6 to 8 . 
       FIGS. 17A to 17C  are diagrams illustrating one example of a height profile and a height histogram of the AlN layer with a thickness of 20 nm in the template according to the embodiment of the present invention, and illustrating details of the measurement region of a first row in the table illustrated in  FIG. 16 . Note that  FIG. 17A  is an AFM image,  FIG. 17B  is a height profile along the line L in the AFM image illustrated in  FIG. 17A , and  FIG. 17C  is a height histogram of the measurement region of the AFM image illustrated in  FIG. 17A . Note that the measurement region of the AFM image illustrated in  FIG. 17A  is the same as the measurement region of the AFM image illustrated in  FIG. 3 . Also, in  FIG. 17C , a thin line F 1  represents frequency distribution, whereas a dark line F 2  represents cumulative frequency distribution. 
     As illustrated in  FIG. 15 , the difference of altitude in the measurement region of the AlN layer with a thickness of 300 nm is about 40 nm, which is sufficiently smaller than the thickness of the AlN layer. In this case, it can be said that the AlN crystals with a sufficient thickness are also present in the deepest valley in the measurement region. Therefore, in the 300 nm-thick AlN layer, it can be said that the entire surface of the main surface of the sapphire substrate is covered with the AlN crystals and the coverage of the main surface of the sapphire substrate by the AlN crystals is 100%. 
     Meanwhile, as illustrated in  FIG. 16 , the difference of altitude in the measurement region of the AlN layer with a thickness of 20 nm is about 20 nm, which is similar to the thickness of the AlN layer. Therefore, there is a possibility that the AlN crystals are not present in the deepest valley in the measurement region. However, as illustrated in  FIG. 16 , the height of the cumulative frequency of 90% of the AlN layer with a thickness of 20 nm is about 5 nm. Here, considering that the height of a step on the main surface of the sapphire substrate having an off angle larger than 0 is 0.22 nm and that the average roughness Ra of the main surface is 0.1 nm or less, even if the AlN crystals are not present in the deepest valley in the measurement region, it can be said that there is a high possibility that the AlN crystals are present in a portion having a height of 1 nm or more, and that there is an extremely high possibility that the AlN crystals are present in a portion having a height of 2 nm or more. Therefore, in the template according to the embodiment of the present invention, it can be said that the coverage of the main surface of the sapphire substrate by the AlN crystals at the time when the AlN layer having a thickness of 20 nm is formed is 90% or more, and that “the layer” that sufficiently covers the main surface of the sapphire substrate has been formed. 
     Actually, in the example illustrated in  FIGS. 17A to 17C , the valley bottom is not flat in the height profile of  FIG. 17B . Also, in the height histogram illustrated in  FIG. 17C , the height of the cumulative frequency of 90% is as high as 5.30 nm, and the cumulative frequency of up to 1 nm (or even 2 nm) in height is a value extremely close to 100%. Also, this tendency applies not only to the measurement region illustrated in  FIGS. 17A to 17C  (first-row measurement region in the table illustrated in  FIG. 16 ), but also to the other measurement region in the table illustrated in  FIG. 16  (measurement region in the second to sixth rows in the table illustrated in  FIG. 16 ). Therefore, in the template according to the embodiment of the present invention, even if the AlN crystals are not present in the deepest valley in the measurement region, it can be said that the AlN crystals sufficiently cover the main surface of the sapphire substrate. 
     Furthermore, in the template according to the embodiment of the present invention, as described above, the AlN crystals are “the layer” sufficiently covering the main surface of the sapphire substrate, and as is apparent from the AFM images illustrated in  FIGS. 3 to 5  and the height profile illustrated in  FIG. 17B , adjacent AlN crystals already collide at this point. For this reason, in the template according to the embodiment of the present invention, even if the AlN crystals grow from the state illustrated in  FIGS. 3 to 5  (state where the AlN layer is grown by only a thickness of 20 nm), the average particle diameter of the AlN crystals at a thickness of 20 nm from the main surface of the sapphire substrate does not vary much. Therefore, in the template according to the embodiment of the present invention, the average particle diameter of the AlN crystals at a thickness of 20 nm from the main surface of the sapphire substrate is 100 nm or less (furthermore, 75 nm or less). 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used for a template including a sapphire substrate and a method of manufacturing the same, and a nitride semiconductor ultraviolet light-emitting element including the template. In particular, the present invention is suitable for use in a template for a nitride semiconductor ultraviolet light-emitting element that has a peak emission wavelength in an ultraviolet region and a method of manufacturing the same, and the nitride semiconductor ultraviolet light-emitting element. 
     DESCRIPTION OF SYMBOLS 
     
         
         
           
               1  nitride semiconductor ultraviolet light-emitting element 
               10  template 
               11  sapphire substrate 
               12  AlN layer 
               20  element structure part 
               21  n-type cladding layer 
               22  active layer 
               23  electron blocking layer 
               24  p-type contact layer 
               25  p-electrode 
               26  n-electrode