Description Title of Invention METHOD OF MANUFACTURE FOR NITRIDE SEMICONDUCTOR LIGHT EMITIING ELEMENT, WAFER, AND NITRIDE SEMICONDUCTOR LIGHT EMITTING ELEMENT Technical Field [0001] The present invention relates to: a method of manufacture for a nitride semiconductor light emitting element; and a wafer; and a nitride semiconductor light emitting element. Background Art [0002] A nitride semiconductor light emitting element that emits light in a wavelength region of visible to ultraviolet light holds potential of application in a wide range of fields, such as in health, medicine, industry, illumination, precision machinery and the like, because of the advantageous in terms of its low power consumption and small size. A Nitride semiconductor light emitting element for partial wavelength regions, for instance blue light wavelength region, is already in commercial use. [0003] However, as to the nitride semiconductor light emitting element, not limited to the nitride semiconductor light emitting element that emits blue light (hereafter, referred to as "blue light-emitting diode"), itisdesired for enhanced emission efficiency and light output. In particular, at present, the practical use of a nitride semiconductor light emitting element that emits light in ultraviolet wavelength region (hereafter, referred to as "ultraviolet light-emitting diode") is hampered by the problem of its considerably poorer external quantum efficiency and light output as compared with the blue light-emitting diode. The low efficiency of light-emitting layer (hereafter, referred to as "internal quantum efficiency") is one of the causes underlying the significantly poor external quantum efficiency and light output. [0004] The internal quantum efficiency of the light emitting layer formed of a nitride semiconductor crystal is influenced by threading dislocations. In a case of high dislocation density of the threading dislocations, non-radiative recombination is dominant, and therefore a drop in the internal quantum efficiency is caused. [0005] In a case where a substrate made of a material such as sapphire or the like causing a significant lattice mismatch with a nitride semiconductor is used as a substrate for epitaxial growth, the aforementioned threading dislocations are likely to occur in particular at a growth interface. Therefore, in order to obtain a nitride semiconductor crystal layer having a low threading dislocation density, it is extremely important to control the behavior of each of the constituent elements in the early stages of growth. In particular, in growth of a nitride semiconductor crystal layer containing Al (particularly A1N layer), a diffusion length of the constituent element composed of a group III atom is shorter than that of a nitride semiconductor crystal layer that does not contain Al (particularly GaN layer). Therefore, a plurality of nuclei are generated in a relatively high density at~the early stage of growths Then, ithas been known that most of threading dislocations are likely to occur on an interface between adjacent nuclei when the adjacent nuclei are combined. Moreover, with using MOCVD equipment (Metal Organic Chemical Vapor Deposition equipment) as a manufacturing apparatus, trimethylaluminum (TMA1) gas and an ammonia (NH3) gas, which are respectively a typical group III material and a typical group V material, react with each other undesirably in a gas phase to give particles (nanoparticles) having a size in a nanometer order. The nanoparticles present on a surface of the substrate may inhibit growth in an A1N crystal. Therefore, an ultraviolet light-emitting diode that includes Al as the constituent element in the nitride semiconductor crystal layer has more threading dislocations present in the nitride semiconductor crystal layer than a blue light-emitting diode that does not include Al as the constituent element. Therefore, the ultraviolet light-emitting diode has lower emission efficiency than the blue light-emitting diode. [0006] For the aforementioned problem, a method for producing a semiconductor structure including a step of forming an A1N buffer layer capable of emitting light having an emission wavelength in a range of 230 to 350 nm in an ultraviolet region and suitable for a light-emitting device structure (Patent Document l) was proposed. An A1N high-quality buffer growth structure described in Patent Document 1 includes a sapphire (0001) substrate as well as an A1N nucleation layer, a pulsed supplied A1N layer, and a continuous growth A1N layer which are successively formed on the sapphire substrate. [0007] The A1N nucleation layer, the pulsed supplied A1N layer, and the continuous growth A1N layer are formed with MOCVD equipment. The A1N nucleation layer is grown in an initial nucleation mode, which is a first growth mode, by means of an NH3 pulsed supply method. The pulsed supplied A1N layer is formed by using NH3 pulsed supply in a slow growth mode, which is a second mode. The continuous growth A1N layer is grown in a fast vertical growth mode. Patent Literature 1 discloses that the second mode is a mode for increasing a grain size and reducing dislocations, and can make the uneven A1N nucleation layer flat. Moreover, Patent Literature 1 discloses that the fast vertical growth mode is a mode for more improving the flatness and suppressing cracks and do not use the A1N growth method by means of the NH3 pulsed supply. The A1N growth method by means of the NH3 pulsed supply method is a method in which a TMA1 gas, which is an Al source, is continuously supplied while a NH3 gas, which is an N source, is supplied in a pulsed manner. [0008] For example, Japanese patent application publication No. 2009-54780 (Patent Document l) discloses growth temperatures of the A1N nucleation layer, the pulsed supplied A1N layer, and the continuous growth A1N layer are selected to 1300°C, 1200'C, and 1200°C, respectively. [0009] Patent Document 1 also discloses that a deep ultraviolet LED with an emission wavelength of 250 nm includes an A1N buffer layer formed on a sapphire substrate and a LED structure formed on the A1N buffer layer. The LED structure includes a MQW (multiple-quantum well) of a Si doped n-type Alo.75Gao.25N layer and an Alo.75Gao.25N/Alo.60Gao.40N-layer, an electron blocking layer made of a Mg doped Alo.95Gao.05N, a Mg doped Alo.75Gao.25N"layer, and a Mg doped p-type GaN layer which are arranged in this order starting from the A1N buffer layer side. Furthermore, a first electrode is formed on the Mg doped p-type GaN layer, and a second electrode is formed on the Si doped n-type Alo.75Gao.25N layer. Summary of Invention [0010] The inventors of the present invention prepared an A1N buffer layer under growth conditions described in Patent Document 1, and evaluated the quality of the A1N buffer layer. The result confirms an excellent quality, for example, a small full width at half maximum (FWHM), which is 500 arcsec, of a rocking curve obtained by X-ray diffraction, and distinct atomic steps observed with an atomic force microscope (AFM). However, by further observation with an optical microscope, the inventors of the present invention detected many protrusions having a hexagonal shape in a cross-section present on a surface of the A1N buffer layer. Besides, the inventors of the present invention frepared trial ultraviolet Ught-emitting diodes having a chip size of 350 um per square and an A1N buffer layer formed under the same conditions and measured current-voltage characteristics of the ultraviolet light emitting diode. The inventors of the present invention revealed that some of the ultraviolet light-emitting diodes showed great leak currents, and were short-circuited and failed to emit light when a drive current is increased. [0011] The present invention has been achieved in view of circumstances, and an object thereof is to propose: a method of manufacture for a nitride semiconductor light emitting element capable of increasing reliability of electrical characteristics; and a wafer capable of increasing reliability of electrical characteristics of the nitride semiconductor light emitting element; and the nitride semiconductor light emitting element capable of increasing reliability of electrical characteristics. [0012] The present invention relates to a method of manufacture for a nitride semiconductor light emitting element. The nitride semiconductor light emitting element includes a monocrystalline substrate; and an A1N layer provided on a surface of the monocrystalline substrate! and a first nitride semiconductor layer of a first electrical conductivity type provided on the A1N layer; and a light emitting layer made of an AlGaN-based material and provided on an opposite side of the first nitride semiconductor layer from the A1N layer," and a second nitride semiconductor layer of a second electrical conductivity type provided on an opposite side of the light emitting layer from the first nitride semiconductor layer. The method of manufacture for a nitride semiconductor light emitting element includes a step of forming A1N layer on the monocrystalline substrate prepared and set in a reactor. The step includes: a first step of supplying an Al source gas and an N source gas into the reactor to generate a group of A1N crystal nuclei having Al-polarity to be a part of the A1N layer on the surface of the monocrystalline substrate; and a second step of supplying the Al source gas and the N source gas into the reactor to form the A1N layer on the surface of the monocrystalline substrate, after the first step. [0013] In an embodiment in accordance with the present invention, in the first step, a substrate temperature, which is a temperature of the monocrystalline substrate, is selected to be a ; first predetermined temperature where growth of A1N crystals having N-polarity is suppressed. [0014] .-.,...,..,..,,_,,..,.,,....,. In an embodiment in accordance with the present invention, the monocrystalline substrate is a c-plane sapphire substrate and the first predetermined temperature falls within a range of 1000°C to 1150°C inclusive. [0015] In an embodiment in accordance with the present invention, the monocrystalline substrate is a c-plane sapphire substrate and the first predetermined temperature falls within a range of 1000°C to 1100"C inclusive. [0016] An embodiment in accordance with the present invention includes a third step of increasing a size of the A1N crystal nucleus generated through the first step, between the first step and the second step. [0017] In an embodiment in accordance with the present invention, in the third step, the Al source gas and the N source gas are supplied to increase the size of the A1N crystal nucleus and the substrate temperature is selected to be a second predetermined temperature higher than tHb first predetermined temperature. [0018] In an embodiment in accordance with the present invention, in the third step, the Al source gas and the N source gas are supplied in such a manner that the Al source gas is supplied continuously while the N source gas is supplied intermittently. [0019] In an embodiment in accordance with the present invention, the second step includes: a first sub-step of supplying the Al source gas continuously while supplying the N source gas intermittently; and a second sub-step of continuously supplying each of the Al source gas and the N source gas after the first sub-step. [0020] . The present invention relates to a wafer including a plurality of nitride semiconductor light emitting elements. Each of the nitride semiconductor light emitting elements includes: a monocrystalline substrate! and an A1N layer provided on a surface of the monocrystalline substrate; and a first nitride semiconductor layer of a first electrical conductivity type provided on the A1N layer,' and a light emitting layer made of an AlGaN-based material and provided on an opposite side of the first nitride semiconductor layer from the A1N layer; and a second nitride semiconductor layer of a second electrical conductivity type provided on an opposite side of the light emitting layer from the first nitride semiconductor layer. In this wafer, a density of the A1N crystals having N-polarity in the A1N layer is 1000 crystals/cm2 or less.' and a full width at half maximum (FWHM) of an X-ray rocking curve obtained by o-scan X-ray diffraction on an A1N (10-12) plane of the A1N layer is 500 arcsec or less. [0021] The present invention relates to a nitride semiconductor light emitting element including: a monocrystalline substrate; and an A1N layer provided on a surface of the monocrystalline substrate," and a first nitride semiconductor layer of a first electrical conductivity type provided on the A1N layer; and a light emitting layer made of an AlGaN-based material and provided on an opposite side of the first nitride semiconductor layer from the A1N layer,' and a second nitride semiconductor layer of a second electrical conductivity type provided on an opposite side of the light emitting layer from the first nitride semiconductor layer. In the nitride semiconductor light emitting element, a density of A1N crystals having N-polarity in the A1N layer is 1000 crystals/cm2 or less, and a full width at half maximum (FWHM) of an X-ray rocking curve obtained by u-scan X-ray diffraction on an A1N (10-12) plane of the A1N layer is 500 arcsec or less. [0022] The method of manufacture for a nitride semiconductor light emitting element in accordance with the present invention provides an increased reliability of electrical characteristics of the nitride semiconductor light emitting element. [0023] The wafer in accordance with the present invention provides an increased reliability of electrical characteristics of the nitride semiconductor light emitting element. [0024] The nitride semiconductor light emitting element in accordance with the present invention provides an increased reliability of electrical characteristics. Brief Description of Drawings [0025] FIG. 1A is a schematic cross-sectional view of an example of a nitride semiconductor light-emitting element in accordance with an embodiment; FIG. IB is a schematic diagram illustrating a method of manufacture for the nitride semiconductor light-emitting element in accordance with the present invention.' FIGs. 2A to 2D are surface morphology diagrams taken with an optical microscope showing A1N layers of samples where substrate temperatures are varied in the first step in a method of manufacture for the semiconductor light-emitting element in accordance with the present invention! FIG. 3A is a bird's-eye view SEM image of the wafer including the comparative example of the nitride semiconductor light emitting element! FIG. 3B is a cross-sectional STEM image of the wafer including the comparative example of the nitride semiconductor light emitting element! FIG. 4 is a X-ray rocking curve diagram of an sample in which the A1N layer is formed after appropriate steps! and FIG. 5 is a light emission spectrum of an embodiment of a nitride semiconductor light-emitting element in accordance with the present invention. Description of Embodiments [0026] A nitride semiconductor light emitting element in accordance with the present invention will be first described with reference to FIGs. 1A and IB, and thereafter a method of manufacture for the nitride semiconductor light emitting element in accordance with the present invention will be described. [0027] The nitride semiconductor light emitting element D includes: a monocrystalline substrate l! and an A1N (i.e., Aluminum nitride) layer 2 provided on a surface 101 of the monocrystalline substrate 1! and a first nitride semiconductor layer 3 of a first electrical conductivity type provided on an opposite surface 201 of the A1N layer 2 from the monocrystalline substrate 1! and a light emitting layer 4 made of an AlGaN (aluminum gallium nitride) -based material and provided on an opposite surface 301 of the first nitride semiconductor layer 3 from the A1N layer 2; and a second nitride semiconductor layer 6 of a second electrical conductivity type provided on an opposite surface 401 of the light emitting layer 4 from the first nitride semiconductor layer 3. [0028] Besides, the nitride semiconductor Ught emitting elementD may include a first electrode 13, which is connected electrically to the first nitride semiconductor layer 3, and a second electrode 16, which is connected electrically to the second nitride semiconductor layer 6. [0029] Additionally, in the nitride semiconductor Ught emitting element D, the first electrical conductivity type of the first nitride semiconductor layer 3 is n-type, and the second electrical conductivity type of the second nitride semiconductor layer 6 is p-type. On an opposite surface 601 of the second nitride semiconductor layer 6 from the light emitting layer 4, a p-type contact layer 7 is disposed. On a part of an opposite surface 601 of the second nitride semiconductor layer 7 from the second nitride semiconductor layer 6, the second electrode 16 is disposed. Namely, in the nitride semiconductor light emitting element D, the second electrode 16 is electrically connected to the second nitride semiconductor layer 6 via the p-type contact layer 7. The nitride semiconductor light emitting element D preferably includes an electron blocking layer 5 between the Ught emitting layer 4 and the second nitride semiconductor layer 6. Besides, the nitride semiconductor Ught emitting element D includes a mesa structure, and the first electrode 13 is provided on a part of a surface 303 of the first nitride semiconductor layer 3, with the surface 303 facing the A1N layer 2. This nitride semiconductor Ught emitting element D is enabled to emit ultraviolet Ught produced by the Ught emitting layer 4, through another surface 102 of the monocrystalline substrate 1 [0030] In the nitride semiconductor Ught emitting element, an AlGaN-based material is used as a material for the Ught emitting layer 4. The nitride semiconductor Ught emitting element is an ultraviolet Ught-emitting diode capable of emitting Ught having an emission wavelength in a range of 210 nm to 360 nm, that is, in a ultraviolet region. [0031] Each component of the nitride semiconductor Ught emitting element D wul be described in detail as foUows. [0032] The monocrystalline substrate 1 is a substrate for epitaxial growth. This monocrystalline substrate 1 is a sapphire substrate with the surface 101 being a (0001) plane, that is, a c-plane sapphire substrate. The c-plane sapphire substrate preferably has an off-angle relative to the (0001) plane in a range of 0 degree to 0.2 degrees. Accordingly, at the time of formation of a group of A1N crystal nuclei 2a having Al-polarity onthe surface 101 of the monociystalHne substrate 1, it is possible to reduce a density of the A1N crystal nuclei 2a, and therefore it is possible to improve the quality of the A1N layer 2. This is because atoms supplied so as to form the A1N crystal nuclei 2a are likely to diffuse over the surface 101 of the monocrystalline substrate 1 and contribute to crystal growth at stable locations, and because the smaller the off-angle of the monocrystalline substrate 1, the longer a terrace width is, and therefore the density of the A1N crystal nuclei 2a is more likely to be reduced. [0033] The A1N layer 2 is a buffer layer provided for the purpose of reducing threading dislocations and residual strain in the first nitride semiconductor layer 3. In a step of forming the A1N layer 2, an Al (Aluminum) source gas and a N (Nitrogen) source gas are supplied to form the group of the A1N crystal nuclei 2a that have Al-polarity and constitute a part of the A1N layer 2 on the surface 101 of the monocrystalline substrate 1, and subsequently the Al source gas and the N source gas are supplied to form the entire A1N layer 2. Therefore, the A1N layer 2 preferably has such a thickness that a surface of the A1N layer 2 is flat, and the thickness of the A1N layer 2 is 4 um in this embodiment, for example. However, the thickness of the A1N layer 2 is not particularly limited as far as the surface of the A1N layer 2 is made flat. In view of preventing occurrence of cracks due to lattice mismatch, the thickness of the A1N layer 2 is preferably 10 um or less. [0034] The light emitting layer 4 functions as converting injected carriers (herein holes and electrons) into light, and has a quantum well structure. The quantum well structure includes a barrier layer 4a and a well layer 4b. In an example as shown in FIG. LA, the barrier layer 4a and the well layer 4b are alternately stacked so that the stack has two well layers 4b. However, the number of the well layers 4b in the stack is not limited. In short, the aforementioned quantum well structure may be a multiple quantum well structure or a single quantum well structure. Thicknesses of the well layer 4b and the barrier layer 4a are not particularly limited. Note that in the light emitting layer 4 with a too thick well layer 4b, electrons and holes injected into the well layer 4b are separated spatially from each other due to piezoelectric field caused by a lattice mismatch in the quantum well structure. Consequently, the light-emitting efficiency is decreased. On the other hand, the light emitting layer 4 with a too thin well layer 4b lowers an effect of carrier confinement, thereby decreasing the light-emitting efficiency. Therefore, the thickness of the well layer 4b preferably falls in a range of about 1 run to about 5 nm, and more preferably falls in a range of about 1.3 nm to about 3 nm. The thickness of the barrier layer 4a is preferably in a range of about 5 nm to about 15 nm, for example. In this embodiment, the thickness of the well layer 4b is 2nm, and the thickness of the barrier layer 4a is 10 nm, but the thicknesses of the well layer 4b and the barrier layer 4a are not limited to these thicknesses. [0035] The well layer 4b is formed to include Al at a predetermined composition so that the light emitting layer 4 emits ultraviolet light with a desired emission wavelength. In the light emitting layer 4 made of an AlGaN-based material, the Al composition of the well layer 4b is changed to give the desired wavelength as described above, and the emission wavelength (emission peak wavelength) can be arbitrarily set in a range of 210 to 260 nm by changing the Al composition of the well layer 4b. For example, when the desired emission wavelength is set close to 265 nm, the Al composition may be selected to 0.50 parts with respect to 1 part of all amounts of metal composition of the well layer 4b. Furthermore, the light emitting layer 4 may have a single layer structure, and the light emitting layer 4 with the single layer structure may serve to form a double-hetero structure together with adjacent layers (i.e., a n-type nitride semiconductor layer and a p-type nitride semiconductor layer), which are adjacent to the light emitting layer 4 in a thickness direction of the light emitting layer 4. [0036] The first nitride semiconductor layer 3 is a n-type nitride semiconductor layer when the first electrical conductivity type is the n-type. The n-type nitride semiconductor layer serves as transporting electrons into the light emitting layer 4. A thickness of the n-type nitride semiconductor may be set 2 pm, but is not particularly limited to 2 um. The n-type nitride semiconductor is a n-type AUGai-xN (0