JP6856356B2 - Aluminum nitride single crystal substrate and method for manufacturing the single crystal substrate - Google Patents

Description

The present invention relates to an aluminum nitride single crystal substrate used for manufacturing an ultraviolet device such as a light emitting diode (LED) and a method for manufacturing the single crystal substrate. In particular, the present invention relates to an aluminum nitride single crystal substrate for suppressing variation in the performance of an ultraviolet light emitting element and for producing an ultraviolet light emitting element with a high yield, and a method for producing the single crystal substrate.

Group III nitride semiconductors containing aluminum (Al) (Al _X Ga _Y In _Z N, X + Y + Z = 1, 0 <X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ Z ≦ 1) correspond to wavelengths of 200 nm to 360 nm. Since it has a direct transition type band structure in the ultraviolet region, it is possible to manufacture a highly efficient ultraviolet light emitting device. Such group III nitride semiconductor devices are classified by the metalorganic vapor deposition (MOCVD) method, the molecular beam epitaxy (MBE) method, or the hydride vapor beam epitaxy (HVPE). It is produced by crystal-growning a Group III nitride semiconductor thin film on a single crystal substrate by a vapor phase growth method such as the Epitaxy) method. Among them, the MOCVD method is the most widely used method in the industry at present because the film thickness can be controlled at the atomic layer level and a relatively high growth rate can be obtained.

Further, as the single crystal substrate for crystal growth of the group III nitride semiconductor thin film, a group III nitride single crystal substrate such as GaN or AlN obtained by a known crystal growth method is used. In order to obtain a particularly highly efficient ultraviolet device, it is necessary to suppress the absorption of ultraviolet rays by the substrate, and as a method for producing a group III nitride single crystal substrate having high transparency to ultraviolet light, a group III nitride by the HVPE method is used. A method for producing a single crystal substrate has been proposed so far (see Patent Document 1 or 2).

Patent No. 3803788 Japanese Unexamined Patent Publication No. 2016-09433

In the group III nitride single crystal substrate used for such a group III nitride semiconductor, after producing the group III nitride single crystal layer by the above method, the unevenness of the single crystal layer is usually reduced and the thickness is adjusted. Therefore, after processing such as grinding on both sides, a surface (hereinafter, also referred to as “crystal growth surface”) for crystal growth of a group III nitride semiconductor thin film such as an n-type layer, an active layer, and a p-type layer is colloidal. It is processed into an ultra-flat surface by a chemical mechanical polishing (CMP) method or the like using an abrasive such as silica. By making the crystal growth surface an ultra-flat surface in this way, a group III nitride semiconductor thin film can be easily laminated on the substrate, and a high-quality one can be obtained. Further, the group III nitride semiconductor wafer obtained by crystal growth of the group III nitride semiconductor thin film can obtain an ultraviolet light emitting device by cutting the wafer.

However, when the present inventors manufacture an ultraviolet light emitting device using the group III nitride single crystal substrate obtained by polishing the crystal growth surface, the performance of each obtained ultraviolet light emitting device may vary greatly. It has been found that there is a problem that the ultraviolet light emitting element may not be obtained with good yield. In particular, when an aluminum nitride (AlN) single crystal is used as the group III nitride single crystal substrate, the above-mentioned variation is large, and it has been found that there is a problem from the viewpoint of producing an ultraviolet light emitting element with good yield.

Therefore, an object of the present invention is to provide an aluminum nitride single crystal substrate and the single crystal substrate for producing an ultraviolet light emitting device with good yield by suppressing variations in the performance of the ultraviolet light emitting element.

The present inventors have diligently studied the above-mentioned problems. When the aluminum nitride (AlN) single crystal substrate obtained by polishing only the crystal growth surface was analyzed, it was found that there was a substrate having a large curvature of the crystal lattice surface and the substrate surface. Then, using a substrate having a large curvature of the crystal lattice surface and the substrate surface, a group III nitride semiconductor thin film is crystal-grown on the substrate to produce a group III nitride semiconductor wafer, and the wafer is cut to obtain ultraviolet rays. When the light emitting device was manufactured, it was found that the external quantum efficiency (EQE) of the manufactured ultraviolet light emitting device varied widely and the average value of EQE was also low.

Based on the above findings, the present inventors have diligently studied a method for manufacturing an aluminum nitride single crystal substrate in order to obtain an ultraviolet device in which the EQE variation of the ultraviolet light emitting device is small and the average value of the EQE is high. As a result, it was found that not only the crystal growth surface of the aluminum nitride single crystal substrate but also the back surface of the crystal growth surface is polished to an ultra-flat surface to reduce the curvature of the crystal lattice surface and the substrate surface. Here, the crystal lattice plane of the aluminum nitride single crystal substrate having a hexagonal wurtzite structure has c-plane, m-plane, a-plane, r-plane, etc., and the axis perpendicular to the crystal lattice plane is called the crystal axis. .. The fact that the crystal lattice plane is curved indicates that it is curved. On the other hand, the curvature of the substrate surface means the curvature of the outermost surface of the substrate. Furthermore, when a group III nitride semiconductor thin film was crystal-grown on a substrate obtained by polishing both sides of an aluminum nitride single crystal substrate to produce an ultraviolet light emitting device, it was found that the EQE variation and the average value of EQE were greatly improved. , The present invention has been completed.

That is, the first invention is an aluminum nitride single crystal substrate composed of only an aluminum nitride single crystal obtained by a crystal growth method, in which the crystal growth surface of the single crystal substrate and the back surface of the crystal growth surface are white. It is an aluminum nitride single crystal substrate having a surface roughness Ra of 100 nm or less measured by an interference microscope and a radius of curvature of the crystal growth surface and the crystal lattice surface of the single crystal substrate of 15 m or more. In the aluminum nitride single crystal substrate of the present invention, the linear transmittance of light having a wavelength of 265 nm is preferably 50% or more. Further, it is preferable that the crystal growth surface of the aluminum nitride single crystal substrate is an Al polar surface.

Further, in the second invention, after producing an aluminum nitride single crystal substrate by a crystal growth method, both the crystal growth surface and the back surface of the crystal growth surface of the single crystal substrate are ground, and then both sides are white. This is a method for producing an aluminum nitride single crystal substrate, which comprises polishing until the surface roughness Ra measured by an interference microscope becomes 100 nm or less. In the method for producing an aluminum nitride single crystal substrate of the present invention, it is preferable to produce an aluminum nitride single crystal substrate on a base substrate by a crystal growth method.

A third aspect of the present invention is an ultraviolet light emitting device in which a light emitting element layer is laminated on the aluminum nitride single crystal substrate.

According to the present invention, not only the crystal growth surface on which the nitride semiconductor thin film is crystal-grown but also the surface opposite to the crystal growth surface is polished on the aluminum nitride single crystal substrate obtained by the crystal growth method. The surface roughness Ra of the crystal growth surface and the back surface of the crystal growth surface is 100 nm or less, and the radius of curvature of the crystal growth surface and the crystal lattice surface of the single crystal substrate is 15 m or more, and the curvature is small. A single crystal substrate can be manufactured. By crystal-growing a group III nitride semiconductor thin film on an aluminum nitride single crystal substrate having a small curvature of the crystal lattice surface and the substrate surface to produce an ultraviolet device, the in-plane variation of EQE is small and the average value is small. Can obtain high ultraviolet devices.

It is the schematic which shows an example of the aluminum nitride single crystal substrate of this invention. It is the schematic which shows another example of the aluminum nitride single crystal substrate of this invention. Schematic diagram of an ultraviolet light emitting element wafer viewed from above Schematic diagram of an ultraviolet light emitting device using the aluminum nitride single crystal substrate of the present invention.

(Group III nitride single crystal substrate)
The group III nitride single crystal substrate of the present invention is an aluminum nitride single crystal substrate obtained by the crystal growth method, and is measured by a white interference microscope on the crystal growth surface of the single crystal substrate and the back surface of the crystal growth surface. The surface roughness Ra is 100 nm or less, and the radius of curvature of the crystal growth surface and the crystal lattice surface (hereinafter, also simply referred to as “crystal lattice surface”) in the single crystal substrate is 15 m or more. is there. The crystal growth surface of the aluminum nitride single crystal substrate of the present invention indicates a surface on which aluminum nitride single crystals are laminated by the crystal growth method. FIG. 1 is a schematic view showing the aluminum nitride single crystal substrate of the present invention. In FIG. 1, 12 corresponds to the crystal growth surface and 13 corresponds to the back surface of the crystal growth surface of the aluminum nitride single crystal substrate 1. Further, the aluminum nitride single crystal substrate of the present invention also includes a case where an aluminum nitride single crystal is further grown on a base substrate which is an aluminum nitride single crystal by a crystal growth method. In this case, the aluminum nitride single crystal substrate has a plurality of aluminum nitride single crystal layers. FIG. 2 is a schematic view showing an aluminum nitride single crystal substrate in which an aluminum nitride single crystal 11 is grown by a crystal growth method on a base substrate 14 which is an aluminum nitride single crystal. In this case, 12 is a crystal growth plane. , 13 correspond to the back surface of the crystal growth surface. A group III nitride semiconductor thin film is crystal-grown on the crystal growth surface of the aluminum nitride single crystal substrate to produce an ultraviolet device. By using an aluminum nitride single crystal substrate having such a radius of curvature and a small curvature. , The light emitting element layer can be uniformly laminated. Therefore, in-plane variation in external quantum efficiency (EQE) in the wafer on which the light emitting element layer is laminated can be suppressed, variation in quality of the ultraviolet light emitting element cut from the wafer is suppressed, and the yield is good. It becomes possible to manufacture an ultraviolet light emitting element.

In particular, when an aluminum nitride single crystal substrate is used as the group III nitride single crystal substrate, stress or defects are generated due to bending of the crystal axis or curvature of the outermost surface of the crystal growth surface, so that stress is also generated in the laminated light emitting element layer. And defects occur, so that the in-plane variation of the laminated light emitting element layers tends to be large. Therefore, by using the aluminum nitride single crystal substrate of the present invention, it is possible to manufacture a high-performance ultraviolet light emitting device with good yield.

The radius of curvature of the crystal growth plane and the crystal lattice plane in the aluminum nitride single crystal substrate of the present invention can be obtained by the following method. That is, the radius of curvature of the crystal lattice plane is calculated from the peak position of the X-ray locking curve of the AlN (002) plane measured at two different points in the crystal growth plane. Specifically, after measuring the X-ray locking curve at two different points on the substrate surface using a thin film X-ray diffractometer, the distance between the two points is Δx, and the difference between the two points is the difference in the diffraction peak position. When Δω, the radius of curvature R is calculated by Δx / Δω. On the other hand, the radius of curvature of the crystal growth surface is calculated from the difference between the maximum height and the minimum height of the outermost surface of the crystal growth surface and the diameter measured using a white interference microscope or the like under the assumption of spherical approximation. Specifically, using a white interference microscope, the height information of the substrate is acquired at a magnification of 50 times, and the radius of curvature of the substrate is calculated from the in-plane height distribution under the assumption of spherical approximation. ..

The aluminum nitride single crystal substrate may be convex upward (that is, the crystal growth surface is convex) or convex downward (that is, the crystal growth surface is concave), but in the present invention, it is upper. The convex state was defined as a positive radius of curvature, and the downward convex state was defined as a negative radius of curvature. Therefore, the radius of curvature of the crystal growth plane and the crystal lattice plane of the aluminum nitride single crystal substrate of the present invention is ± 15 m or more. Regardless of whether the radius of curvature of the crystal growth plane and the crystal lattice plane is a positive value or a negative value, it is preferable that the larger the value, the smaller the curvature, but it is industrially efficient. From the viewpoint of being able to do so, it is particularly preferable that the radius of curvature is ± 20 m or more. The upper limit of the radius of curvature of the crystal growth surface can be increased by finely polishing, which will be described later, but it is about several hundred to 1,000 m from the viewpoint of industrial productivity. The upper limit of the crystal lattice plane is usually about 100 m or less, although it depends on the crystal growth conditions.

Further, as the surface roughness Ra of the crystal growth surface and the back surface of the crystal growth surface in the aluminum nitride single crystal substrate of the present invention is smaller, the surface becomes flatter and the light emitting element layers can be uniformly laminated. preferable. In particular, the surface roughness of the crystal growth surface and the back surface of the crystal growth surface is preferably 100 nm or less from the viewpoint of reducing stress in the substrate and reducing defects in the laminated light emitting device layer. The crystal growth surface is more preferably 2 nm or less, still more preferably 0.5 nm or less in order to reduce defects in the laminated light emitting device layer. The back surface of the crystal growth surface is more preferably 10 nm or less, still more preferably 5 nm or less. The lower limit of the surface roughness differs depending on the polishing conditions described later, and can be made smaller as the polishing is performed finer. However, from the viewpoint of industrial productivity, the crystal growth surface is 0.06 nm, and the crystal growth surface is It is about 1 nm on the back surface.

The aluminum nitride single crystal substrate of the present invention is used as a substrate when manufacturing an ultraviolet light emitting element, and from the viewpoint of luminous efficiency in the light emitting element, one having transparency to ultraviolet light is preferable. Specifically, the linear transmittance of light having a wavelength of 265 nm is preferably 50% or more, and more preferably 60% or more. The upper limit of the linear transmittance is 100%, but it is 72% in consideration of the ease of industrial production and the theoretical limit of the linear transmittance.

Further, the thickness of the aluminum nitride single crystal substrate of the present invention is generally 1000 μm or less, and it is more preferable that the thickness is thin from the viewpoint of the cost of the substrate, but 100 μm or more is preferable from the viewpoint of handling.

Generally, it is preferable that a c-plane exposed aluminum nitride single crystal substrate is used for forming the light emitting device layer, the crystal growth surface is an Al polar surface, and the back surface of the crystal growth surface connection is an N polar surface. This is because the Al-polar surface has higher chemical resistance than the N-polar surface, and is stable against the gas used for forming the light emitting element layer, and the thin film laminated is less likely to have defects.

(Manufacturing method of aluminum nitride single crystal substrate)
The above-mentioned aluminum nitride single crystal substrate of the present invention is the crystal after producing an aluminum nitride single crystal substrate by a crystal growth method and then grinding both the crystal growth surface and the back surface of the crystal growth surface of the single crystal substrate. It can be produced by polishing until the surface roughness Ra measured by a white interference microscope on the back surface of the growth surface and the crystal growth surface becomes 100 nm or less.

As a crystal growth method for producing the aluminum nitride single crystal, a known crystal growth method can be adopted. Specific examples of such a crystal growth method include a sublimation method, a MOCVD method, and an HVPE (Hydride Vapor Phase Epitaxy) method. When the MOCVD method or the HVPE method is used as the crystal growth method, it is easy to produce a homogeneous aluminum nitride single crystal layer having a large film thickness. Therefore, an aluminum nitride single crystal layer is formed on the substrate by using a base substrate. It is preferable to grow it. Hereinafter, the method for producing the aluminum nitride single crystal substrate of the present invention will be described in detail, taking as an example the case where the aluminum nitride single crystal substrate is produced on the base substrate by the crystal growth method.

(Base board)
The base substrate used in the production method of the present invention is not particularly limited as long as it is a base substrate capable of crystal growth of the aluminum nitride single crystal layer, and an aluminum nitride single crystal may be used as the base substrate, or a different type of substrate may be used. Although it may be used as a base substrate, it is preferable to use an aluminum nitride single crystal substrate as the base substrate from the viewpoint of growing a homogeneous and large-thickness aluminum nitride single crystal layer. As the aluminum nitride single crystal substrate as the base substrate, an aluminum nitride single crystal substrate manufactured by a known method such as a sublimation method or an HVPE (Hydride Vapor PhaseEpitaxy) method can be used. As the base substrate, few defects (e.g. dislocation density is ¹⁰ 6 ^{cm -2} or less), the thickness was 100μm or 1000μm or less, it is preferable that the crystal growth surface is polished flat to a 2nm or less. The linear transmittance of light having a wavelength of 265 nm of the base substrate itself may be low because it does not affect the linear transmittance of the laminated aluminum nitride single crystal substrate and may be removed by grinding or the like after lamination. When the linear transmittance of light having a wavelength of 265 nm of the aluminum nitride single crystal used as the base substrate is sufficiently high, the laminate itself in which the aluminum nitride single crystal layer is laminated on the base substrate is the aluminum nitride single crystal of the present invention. It can be used as a crystal substrate.

(Manufacturing of aluminum nitride single crystal substrate)
In the production method of the present invention, an aluminum nitride single crystal substrate is laminated on the base substrate by a crystal growth method. As a method for laminating the aluminum nitride single crystal substrate, a known method, for example, a method produced by a crystal growth method such as the HVPE method described in Patent Document 1 or the sublimation method can be used. Among these crystal growth methods, the HVPE method has a slower growth rate than the sublimation method and is not suitable for producing a bulk single crystal, but it has a low impurity concentration that adversely affects the deep ultraviolet light transmittance. , Suitable as a crystal growth method for aluminum nitride single crystal substrates used in ultraviolet light emitting elements.

In the growth of the aluminum nitride single crystal substrate by the HVPE method, the aluminum source gas and the nitrogen source gas, which are the raw material gases, are supplied into the reactor on the heated base substrate, and both gases are reacted on the heated substrate. It is done by letting. As the aluminum source gas, a halogenated aluminum gas such as aluminum chloride gas is preferably used, and as the nitrogen source gas, ammonia gas is preferably used.

The heating temperature of the base substrate, the supply amount of the aluminum source gas and the nitrogen source gas as the raw material gas, and the supply rate are factors that affect the crystal growth rate, and are appropriately determined according to the desired crystal growth rate. Normally, the heating temperature of the base substrate is in the range of 1400 ° C. or higher and 1700 ° C. or lower, the supply amount of the aluminum source gas as the raw material gas is 0.001 to 100 sccm, and the supply amount of the nitrogen source gas is 0.01 to. It may be performed in the range of 1000 sccm.

(Removal of base substrate)
When a substrate having a low linear transmittance of light having a wavelength of 265 nm is used as the substrate, an aluminum nitride single crystal substrate is laminated on the substrate and then removed by grinding described later, and the linear transmittance of light having a wavelength of 265 nm is 50. Leave a portion of% or more. The step of removing the base substrate may be performed after forming the light emitting element layer and the electrodes, which will be described later.

(grinding)
The aluminum nitride single crystal substrate produced by the above crystal growth method is ground on both sides in order to reduce irregularities on the crystal growth surface and the back surface of the crystal growth surface and to adjust the thickness. As a grinding method, the substrate is fixed on a plate such as ceramic with an adhesive or the like, and the grindstone to which the abrasive grains are fixed is rotated and ground against the surface of the substrate, or the free abrasive grains are poured and the metal surface plate is rotated. However, there is a method of scraping the surface of the substrate. The form of the abrasive grains used in processing such as grinding is not particularly limited to the abrasive grains fixed to metal, resin or the like, and the free abrasive grains. The abrasive grains used are generally diamond, silicon carbide, boron carbide and the like. The particle size of the abrasive grains used for grinding is generally 1 μm or more and 100 μm or less, and may be ground at a speed of 1 μm / min or more and 100 μm / min or less. As a guideline for how much to grind, it may be appropriately determined in consideration of the polishing conditions in polishing described later. For example, when the entire surface of the substrate is uniformly scraped and fixed on the plate, the variation in the thickness of the substrate (for example, the maximum and minimum difference of a total of 5 points, 1 point at the center and 4 points at the outer periphery) becomes 5 μm or less. It is enough to go to the extent. After the grinding of one side is completed, the adhesive is removed, the substrate is once peeled off and washed, and then the opposite side is turned up and reattached for grinding. The order of grinding is not particularly limited, and either the crystal growth surface or the back surface of the crystal growth surface may be performed first.

(Polishing)
In the production method of the present invention, the ground aluminum nitride single crystal substrate is polished until the surface roughness Ra measured by a white interference microscope on the crystal growth surface and the back surface of the crystal growth surface becomes 100 nm or less. Is a feature. Generally, polishing refers to a process in which a mirror surface is formed so that turbidity cannot be seen due to roughness such as grinding marks. When forming a light emitting device layer on an aluminum nitride single crystal substrate, the crystal growth surface of the substrate is usually a crystal growth surface, and the light emitting device layer is laminated on the crystal growth surface. Therefore, until now, it has been considered sufficient to polish the crystal growth surface ultra-flat in order to uniformly stack the light emitting device layers. However, the surfaces of the crystal growth surface and the back surface of the crystal growth surface are subject to processing damage due to grinding, but since this damage is different between the crystal growth surface and the back surface of the crystal growth surface, nitriding due to the difference in damage due to processing on both sides. The aluminum single crystal substrate is curved. As described above, the bending of the single crystal substrate cannot be eliminated by polishing only one side, and the light emitting element layers laminated due to the generation of stresses and defects due to the bending of the crystal axis and the bending of the outermost surface of the crystal growth surface. Since stress and defects are also generated inside, it is presumed that the in-plane variation of the laminated light emitting element layers becomes large. Therefore, in order to eliminate the curvature of the aluminum nitride single crystal substrate, it is also necessary to polish the back surface of the crystal growth surface of the single crystal substrate. Further, the inventors of the present application have stated that even if the back surface of the crystal growth surface of the substrate is polished, the curvature of the substrate cannot be sufficiently eliminated unless the surface roughness of the back surface of the crystal growth surface is sufficiently reduced. I found it. For example, an ultra-flat surface having a surface roughness Ra of about 0.1 nm was observed in a ^{4 μm 2} (2 μm × 2 μm) field range with an atomic force microscope (AFM) using a chemical mechanical polishing (CMP) method on the crystal growth surface. In the case of a single-sided polished substrate in which the surface roughness Ra of the back surface (that is, the substrate surface) of the crystal growth surface is 100 nm or more, the surface of the crystal lattice surface and the substrate surface is polished. The radius of curvature is about ± 10 m or less.

Therefore, in order to obtain the aluminum nitride single crystal substrate of the present invention, it is necessary to polish the back surface of the crystal growth surface until the surface roughness Ra measured by a white interference microscope becomes 100 nm or less.

As the polishing method in the manufacturing method of the present invention, a known polishing method used when manufacturing a light emitting element can be adopted as long as the polishing to the above surface roughness is possible. Specific examples of the polishing method include a chemical mechanical polishing (CMP) method, mechanical polishing using diamond abrasive grains, and the like. Further, the same method may be used as the polishing method for the crystal growth surface and the back surface of the crystal growth surface, or different methods may be used for each. The crystal growth surface in the production method of the present invention may be polished to the above surface roughness, but in order to prevent defects from occurring in the laminated light emitting element layer, 4 μm by an atomic force microscope (AFM). ² (2 μm × 2 μm) It is preferable to use an ultra-flat surface having a surface roughness Ra of about 0.1 nm in the visual field range. Therefore, among the above polishing methods, the CMP method is preferable as the method for polishing the crystal growth surface because it can polish a more ultra-flat surface.

For example, when polishing an aluminum nitride single crystal in which the crystal growth surface is an Al polar surface and the back surface of the crystal growth surface is an N polar surface, the Al polar surface as the crystal growth surface is polished by the CMP method. ^{The surface roughness Ra in the 4 μm 2} (2 μm × 2 μm) visual field range by the atomic force microscope (AFM) ^{is about 0.1 nm, and the surface roughness Ra in the 58800 μm 2} (280 μm × 210 μm) visual field range by the white interference microscope. Polishing of 100 nm or less is possible.

Further, as a method for polishing the N-polar surface as the back surface of the crystal growth surface, not only CMP but also mechanical polishing using a metal surface plate and diamond particles having a particle size of about 0.1 to 5 μm may be used. Since the N-polar surface as the back surface of the crystal growth surface has low chemical resistance ^{, the surface roughness in the 4 μm 2} (2 μm × 2 μm) visual field range by the atomic force microscope (AFM) in both the CMP method and the mechanical polishing method is used. Ra is about 1 to 5 nm. ^{The surface roughness Ra in the 58800 μm 2} (280 μm × 210 μm) visual field range measured by a white interference microscope can be polished to 100 nm or less.

The method of polishing by the CMP method is not particularly limited, and a known method can be adopted. Specifically, the aluminum nitride single crystal substrate is rotated on a non-woven fabric or a suede type pad in which a slurry having a pH of 2 to 11 containing 20 to 45% by mass of colloidal silica having a primary particle size of 20 to 80 nanometers is dropped. While pressurizing, polishing may be performed.

Generally, since an aluminum nitride single crystal substrate with an exposed c-plane is used to form the light emitting element layer, the nitridation of the present invention is described in which the crystal growth surface is an Al polar plane and the back surface of the crystal growth plane is an N polar plane. When an aluminum single crystal substrate is used, the N-polar surface has low chemical resistance, and the surface roughness becomes large especially when it comes into contact with an alkaline liquid. Therefore, when polishing the N-polar surface, which is the back surface of the crystal growth surface, it is preferable to use an acidic or neutral abrasive.

In the manufacturing method of the present invention, the polishing order of the crystal growth surface and the back surface of the crystal growth surface may cause minute scratches on the contact surface when the substrate is fixed on a plate such as ceramic with an adhesive or the like. Therefore, it is preferable to first polish the back surface of the crystal growth surface and then polish the crystal growth surface. This is because the presence of scratches on the crystal growth surface causes defects in the laminated light emitting device layers.

The polished substrate is simply washed with running water, then washed with diluted hydrofluoric acid (DHF cleaning), which is generally known as semiconductor cleaning, and with a mixed solution of sulfuric acid and hydrogen peroxide (SPM). Foreign substances such as colloidal silica abrasives, metals, and organic substances are removed by cleaning) and cleaning with a mixed solution of hydrochloric acid and hydrogen peroxide (SC-2 cleaning).

(Ultraviolet light emitting device using the aluminum nitride single crystal substrate of the present invention)
An external quantum efficiency (EQE) is obtained by laminating a light emitting device layer composed of a group III nitride semiconductor thin film on the aluminum nitride single crystal substrate of the present invention to manufacture a wafer, and then separating the ultraviolet light emitting device from the wafer. ) Is small, and an ultraviolet light emitting device having a high EQE average value can be obtained. Hereinafter, the method for manufacturing the ultraviolet light emitting device in the present invention will be described in detail.

(Light emitting element layer)
The light emitting element layer is formed on the substrate 22 as shown in FIG. 4, and the n-type layer 23, the active layer 24, and the p-type layer 25 (layer composed of the p-type clad layer and the p-type contact layer) are laminated in this order. Being done.

Each layer constituting the light emitting device layer is a _{group III nitride semiconductor composed of Al X} Ga _Y In _Z N, X + Y + Z = 1, 0 <X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ Z ≦ 1. Is preferable. Further, it may be preferably a layer containing impurities. Examples of impurities include Si, Ge and the like in the n-type layer, and Mg and the like in the p-type layer. The active layer 4 has a laminated structure of a well layer and a barrier layer having a bandgap energy larger than that of the well layer. The p-type layer 25 is composed of a p-type clad layer and a p-type contact layer. Each layer can be manufactured by the MOCVD method.

(N electrode and p electrode)
As shown in FIG. 3, the n-electrode 27 is formed on the exposed surface of the n-type layer 22, and the p-electrode 28 is formed on the p-type layer 25 of the mesa structure 26. Patterning of both electrodes can be performed using the lift-off method. Examples of the method for depositing both electrode metals include vacuum deposition, sputtering, chemical vapor deposition, and the like. The material used for both electrodes can be selected from known materials. For example, the n electrode includes Ti, Al, Rh, Cr, In, Ni, Pt and Au, and the p electrode includes Ni, Cr, Au, Mg, Zn, Pd and Pt. The n-electrode may have a single-layer or multi-layer structure containing alloys or oxides of these metals.

(Manufacturing of wafers including multiple configurations of ultraviolet light emitting elements and evaluation of their characteristics)
A wafer includes a plurality of configurations of an ultraviolet light emitting element as described above. FIG. 3 shows a drawing of a wafer including a plurality of configurations of ultraviolet light emitting elements when viewed from above. A light emitting element layer (n-type layer, active layer, and p-type layer) is formed on one surface of the substrate obtained by the above processing method by the MOCVD method. A plurality of mask patterns having the same shape are formed at equal intervals on the light emitting element layer. This single mask pattern constitutes a single ultraviolet light emitting element in a later step. Then, by etching, the p-type layer and the active layer in the region where the mask is not formed are etched. As a result, a plurality of light emitting element layers (composed of a p-type layer and an active layer) having the same shape are formed on the wafer surface in a plateau shape. Next, after removing the etching mask, an n-electrode is formed on the n-type layer and a p-electrode is formed on the p-type layer. Through the above steps, a plurality of semiconductor light emitting device configurations are formed on the substrate, in which the n electrode to the p electrode are one semiconductor light emitting device. Although it depends on the area of the substrate used for manufacturing the wafer, in the case of a circular wafer having a diameter of 1 inch, 100 to 1000 ultraviolet light emitting elements are usually formed on one wafer.

When a substrate in which a single crystal portion of aluminum nitride having a wavelength of 265 nm and a linear transmittance of 50% or more is laminated on a seed substrate having a low transmittance is used for manufacturing a wafer on which an ultraviolet light emitting element is formed. After the formation, the ultraviolet light emitting element can be manufactured by removing the seed substrate portion having low transmittance by mechanical polishing.

Hereinafter, the present invention will be further described with reference to Examples, but the present invention is not limited to these Examples. The methods for measuring the radius of curvature of the crystal lattice surface, the crystal growth surface, and the surface roughness of the ground surface and the polished surface in the following Examples and Comparative Examples were as follows.

[Measurement method of radius of curvature of crystal lattice plane]
The radius of curvature of the crystal lattice plane of the aluminum nitride single crystal substrate is calculated from the peak position of the X-ray locking curve of the AlN (002) plane measured at two different points in the crystal growth plane. The distance between the two points after measuring the X-ray locking curve at two different points in the crystal growth plane using a thin film X-ray diffractometer (X'Pert MRD manufactured by PANalytical) with the crystal growth plane as the measurement plane. Is Δx, and the difference between the diffraction peak positions between the two points is Δω, the radius of curvature R is calculated by Δx / Δω. The two points to be measured this time were 8 mm to the left and 8 mm to the right from the center of the φ1 inch substrate. The upwardly convex state was defined as a positive radius of curvature, and the downwardly convex state was defined as a negative radius of curvature.

[Measurement method of radius of curvature of crystal growth surface]
For the radius of curvature of the crystal growth plane, a white interference microscope (NewView7300 manufactured by Zygo) was used to obtain the height information of the substrate at a magnification of 50 times, and the height distribution in the plane was based on the assumption of spherical approximation. The radius of curvature of the substrate was calculated in. As with the radius of curvature of the crystal lattice plane, the state of being convex upward was defined as a positive radius of curvature, and the state of being convex downward was defined as a negative radius of curvature.

[Measurement method of surface roughness Ra of ground surface]
The surface roughness Ra of the ground surface before polishing is set between two points, 7.5 mm to the left and 7.5 mm to the right from the center of the φ1 inch substrate, using a stylus type surface roughness meter (Surfcom manufactured by Tokyo Seimitsu Co., Ltd.). I asked.

[Measurement method of surface roughness Ra of polished surface]
The surface roughness Ra of the polished surface ^{was determined by observing a 4 μm 2} (2 μm × 2 μm) visual field range with an AFM. ^{Further, the surface roughness Ra in a wider range was determined by observing a 58800 μm 2} (280 μm × 210 μm) visual field range at a magnification of 50 times using a white-white interference microscope (NewView7300 manufactured by Zygo).

Example 1
(Preparation of aluminum nitride single crystal substrate by crystal growth method)
It is a c-plane aluminum nitride single crystal substrate obtained by performing crystal growth by the HVPE method described in Patent Document 2 on a commercially available aluminum nitride single crystal substrate of φ1 inch manufactured by a sublimation method.

(Manufacturing conditions of HVPE method)
The aluminum nitride single crystal substrate was placed on a BN-coated graphite susceptor in the HVPE apparatus so that the Al polar side of a commercially available aluminum nitride single crystal substrate of φ1 inch manufactured by the sublimation method was the growth surface.

As a group III raw material gas, 6N grade high-purity aluminum was placed on a quartz glass boat, heated to 400 ° C., and then hydrogen chloride gas was added together with a mixed carrier gas in which hydrogen and nitrogen were mixed at a ratio of 7: 3. Aluminum chloride gas was generated by supplying 8 sccm. Further, 1.1 sccm of hydrogen chloride gas is supplied to the aluminum chloride gas to make a total of 1800 sccm of mixed gas including 1782.1 sccm of hydrogen-nitrogen mixed carrier gas, and the mixed gas is a reactor with a susceptor on which a base substrate is installed. Introduced in.

Further, as a nitrogen source gas, a total of 200 sccm of ammonia gas 31 sccm, hydrogen chloride gas 3.1 sccm, and hydrogen carrier gas 165.9 sccm was supplied to the reaction region. In addition, 6500 sccm of hydrogen-nitrogen mixed carrier gas in which hydrogen and nitrogen were mixed at a ratio of 7: 3 was flowed as a gas for flushing the entire reactor. Further, as a barrier gas for preventing the group III raw material gas and the nitrogen source gas from reacting before reaching the substrate, 1500 sccm of nitrogen gas was supplied from between the group III raw material gas nozzle and the nitrogen source gas nozzle. As described above, the total flow rate of the gas supplied into the reactor was set to 10000 sccm. In addition, the pressure in the growing system was maintained at 0.99 atm.

The base substrate placed on the BN-coated graphite susceptor was heated to 1450 ° C., grown at a growth rate of 25 to 30 μm / h for 11 hours while flowing the above gas, and then the gas supply was stopped and cooled to room temperature. did.

(Processing of aluminum nitride single crystal substrate)
As a substrate for measuring the linear transmittance in advance, a c-plane aluminum nitride single crystal substrate (thickness of sublimation method portion 510 μm, thickness of HVPE method portion 280 μm) was produced by crystal growth by the HVPE method. The sublimation method portion of this substrate is completely removed by grinding, and both the crystal growth surface and the back surface of the crystal growth surface of the HVPE method portion are mechanically polished to prepare a substrate for measuring linear transmittance with a thickness of 150 μm. The linear light transmittance was evaluated using a double-beam type ultraviolet / visible spectrophotometer (JASCO spectrophotometer V-7300) and found to be 62% at a wavelength of 265 nm. The sublimation substrate used as the seed substrate was similarly mechanically polished on both sides of the crystal growth surface and the back surface of the crystal growth surface, and the linear transmittance with a thickness of 150 μm was measured and found to be 35% at a wavelength of 265 nm.

Metal # 1200 diamond particles are used on both the crystal growth surface and the back surface of the crystal growth surface of the c-plane aluminum nitride single crystal substrate (thickness of sublimation method portion 530 μm, thickness of HVPE method portion 270 μm) produced by the above-mentioned HVPE method. The back surface (N-polar surface) of the crystal growth surface was polished to a thickness of 40 μm by the CMP method using a colloidal silica abrasive (particle size 20 nm, pH 7.3) and a suede pad. .. After polishing, the mixture was rinsed with pure water for 5 minutes (flow rate: 1.8 L / min), a 1% aqueous hydrofluoric acid solution was added to a Teflon (registered trademark) beaker, a substrate was placed therein, and the mixture was immersed for 10 minutes. The obtained substrate was rinsed with pure water for 1 minute under running water (flow rate: 1.8 L / min), immersed in isopropanol for 1 minute, and spin-dried at 6000 rpm for 30 seconds. The surface roughness Ra of the polished N-polar surface ^{was 4 nm in the 4 μm 2} (2 μm × 2 μm) visual field range by AFM ^{and 63 nm in the 58800 μm 2} (280 μm × 210 μm) visual field range by the white interference microscope.

Next, the crystal growth surface (Al polar surface) was polished to a thickness of 35 μm using a colloidal silica abrasive (particle size 20 nm, pH 7.3) and a non-woven fabric pad, and then the pad was further changed to a suede pad. A thickness of 5 μm was polished. The thickness after double-side polishing was 585 μm. After the above-mentioned cleaning, the surface roughness Ra of the polished Al polar surface was measured and found to be ^{0.1 nm in a 4 μm 2} (2 μm × 2 μm) visual field range by AFM. Further, by measuring the crystal growth plane (Al polar plane), the radius of curvature of the crystal lattice plane was -22.1 m, and the radius of curvature of the substrate plane was +129.9 m.

(Manufacturing and characterization of wafers including multiple semiconductor light emitting device configurations)
Immediately before the MOCVD step, fine particles were removed by scrubbing with polyurethane foam and Sunwash MD-3041 (manufactured by Lion Co., Ltd.) diluted 20-fold, followed by a mixture of phosphoric acid and sulfuric acid heated to 90 ° C. (phosphorus). The natural oxide film on the surface was removed by immersing in (acid: sulfuric acid = 1: 3 (volume ratio)) for 10 minutes. Next, using the MOCVD method, an n-type layer (Al _0.7 Ga _0.3 N), an active layer (well layer: Al _0.5 Ga _0.5 N, barrier layer) are placed on the Al polar surface of the substrate. : Al _0.7 Ga _0.3 N), Al N layer, p-clad layer (Al _0.8 Ga _0.2 N) and p-GaN layer were formed.

Next, the obtained semiconductor wafer was activated and annealed. Next, after forming a metal mask on the p-GaN layer, dry etching was performed to form a mesa structure as shown in FIG. Then, an n electrode (Ti20 nm / Al200 nm / Au5 nm) was formed on the n-type layer, and a p-electrode (Ni20 nm / Au50 nm) was formed on the p-type layer. By the above operation, a wafer including a plurality of semiconductor light emitting devices (emission peak wavelength 265 nm) as shown in FIG. 3 was manufactured.

Next, the sublimation method portion having a low linear transmittance was removed by mechanical polishing to prepare a semiconductor light emitting device wafer containing an HVPE method aluminum nitride single crystal having a thickness of 110 μm and a high linear transmittance. With respect to the light emitting elements in the semiconductor wafer manufactured in this manner, characteristics such as light emission intensity, light emission wavelength, and voltage for 370 pieces were evaluated using a probe tester. The actual current value of this semiconductor light emitting device was set to 400 mA. Then, chips were cut out from the wafer by laser scribe, and 15 chips were mounted on a polycrystalline AlN carrier to complete the LED. The emission intensity and emission peak wavelength of the 15 LEDs produced were measured using a 2-inch integrating sphere (Zenis coating manufactured by Sphere Optics) and a multi-channel spectroscope (USB4000 manufactured by Ocean Photonics) to calculate EQE. went. Next, a calibration curve of emission intensity measured with 15 EQEs and a probe tester was prepared. Using this calibration curve, EQE was calculated from the emission intensities of all the light emitting elements in the wafer measured using a probe tester, and statistical processing was performed. As a result, the average value of EQE was 2.00% and the standard deviation was 0. It was 12%.

Comparative Example 1
(Preparation of aluminum nitride single crystal substrate by crystal growth method)
A c-plane aluminum nitride single crystal layer was grown on a φ1 inch commercially available aluminum nitride single crystal substrate manufactured by the sublimation method in the same manner as in Example 1 to prepare an aluminum nitride single crystal substrate (sublimation). The thickness of the method portion is 510 μm, and the thickness of the HVPE method portion is 290 μm).

(Processing of aluminum nitride single crystal substrate)
Both the crystal growth surface and the back surface of the crystal growth surface of the produced c-plane aluminum nitride single crystal substrate were ground with a grindstone in which diamond particles of # 1200 were immobilized on metal. The back surface (N-polar surface) of the crystal growth surface was ground by grinding. Since it is not a mirror surface, it was impossible to measure the surface roughness with an AFM or a white interference microscope as in Example 1. The surface roughness Ra was 149 nm as measured by a stylus type surface roughness meter.

Next, the crystal growth surface (Al polar surface) was polished by the same method as in Example 1. The thickness after single-side polishing was 600 μm. After performing the same cleaning as in Example 1, the surface roughness Ra of the polished Al polar surface was measured and found to be ^{0.1 nm in a 4 μm 2} (2 μm × 2 μm) visual field range by AFM. Further, by measuring the crystal growth plane (Al polar plane), the radius of curvature of the crystal lattice plane was -4.9 m, and the radius of curvature of the substrate plane was -4.1 m.

When a semiconductor light emitting device was manufactured by the same method as in Example 1 and the EQE of the entire number of wafers was calculated, the average value of EQE was 1.64% and the standard deviation was 0.20%.

Comparative Example 2
(Preparation of aluminum nitride single crystal substrate by crystal growth method)
A c-plane aluminum nitride single crystal layer was grown on a φ1 inch commercially available aluminum nitride single crystal substrate manufactured by the sublimation method in the same manner as in Example 1 to prepare an aluminum nitride single crystal substrate (sublimation). The thickness of the method portion is 540 μm, and the thickness of the HVPE method portion is 310 μm).

(Processing of aluminum nitride single crystal substrate)
Both the crystal growth surface and the back surface of the crystal growth surface of the produced c-plane aluminum nitride single crystal substrate were ground with a grindstone in which diamond particles of # 1200 were immobilized on metal. The back surface (N-polar surface) of the crystal growth surface was ground by grinding. Since it is not a mirror surface, it was impossible to measure the surface roughness with an AFM or a white interference microscope as in Example 1. The surface roughness Ra was 148 nm as measured by a stylus type surface roughness meter.

Next, the crystal growth surface (Al polar surface) was polished by the same method as in Example 1. The thickness after single-side polishing was 600 μm. Cleaning after polishing The difference between Example 1 and Comparative Example 1 is that after cleaning with hydrofluoric acid, a mixed solution of sulfuric acid and hydrogen peroxide solution heated to 120 ° C. (sulfuric acid: hydrogen peroxide solution = 150: SPM cleaning by immersing in 1 (volume ratio) for 10 minutes is added. The surface roughness Ra of the polished Al polar surface was measured and found to be ^{0.1 nm in a 4 μm 2} (2 μm × 2 μm) visual field range by AFM. Further, by measuring the crystal growth plane (Al polar plane), the radius of curvature of the crystal lattice plane was -6.9 m, and the radius of curvature of the substrate plane was -9.6 m.

When the semiconductor light emitting device was manufactured by the same method as in Example 1 and the EQE of the total number in the wafer was calculated, the average value of EQE was 1.89% and the standard deviation was 0.22%.

The results of the above Examples and Comparative Examples are summarized in Table 1.

1: Aluminum nitride single crystal substrate 11: Aluminum nitride single crystal 12: Crystal growth surface 13: Back surface of crystal growth surface 14: Base substrate 21: Wafer 22: Substrate 23: n-type layer 24: Active layer 25: p-type layer 26 : Mesa structure 27: n electrode 28: p electrode 29: ultraviolet light emitting element

Claims (7)

An aluminum nitride single crystal substrate composed of only an aluminum nitride single crystal obtained by the crystal growth method, and the surface roughness Ra measured by a white interference microscope on the crystal growth surface and the back surface of the crystal growth surface in the single crystal substrate. Is 100 nm or less, and the radius of curvature of the crystal growth surface and the crystal lattice surface of the single crystal substrate is 15 m or more. The aluminum nitride single crystal substrate according to claim 1, wherein the linear transmittance of light having a wavelength of 265 nm is 50% or more. The aluminum nitride single crystal substrate according to claim 1 or 2, wherein the crystal growth surface of the aluminum nitride single crystal substrate is an Al polar surface. After producing an aluminum nitride single crystal substrate by the crystal growth method, both the crystal growth surface and the back surface of the crystal growth surface of the single crystal substrate are ground, and then the surface roughness Ra measured by a white interference microscope on both sides. A method for producing an aluminum nitride single crystal substrate, which comprises polishing until the thickness becomes 100 nm or less. The method for producing an aluminum nitride single crystal substrate according to claim 4, wherein the aluminum nitride single crystal substrate is produced on the base substrate by a crystal growth method. An ultraviolet light emitting device in which a light emitting element layer is laminated on the aluminum nitride single crystal substrate according to any one of claims 1 to 3. A method for manufacturing an ultraviolet light emitting device, wherein an aluminum nitride single crystal substrate is obtained by the manufacturing method according to claim 4 or 5, and then a light emitting element layer is laminated on the obtained aluminum nitride single crystal substrate.

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