TW201817035A - Light-emitting element and method for manufacturing light-emitting element - Google Patents

Abstract

The present invention pertains to a light-emitting element having a transparent substrate bonded to a light-extraction surface side thereof, the light-emitting element being characterized in that a transparent film, having a refractive index lower than that of the transparent substrate, is provided on the outer surface on the light-extraction surface side of the transparent substrate, and the outer surface of the transparent film is roughened. Consequently, there is provided: the light-emitting element formed by bonding the transparent substrate, wherein a light distribution angle is increased and a luminous efficiency is increased; and a method for manufacturing the light-emitting element.

Description

Light-emitting element and manufacturing method of light-emitting element

The present invention relates to a light-emitting element to which a transparent substrate is bonded and a manufacturing method thereof.

Among the light-emitting elements made of AlGaInP, a light-emitting element having a shape of one electrode and two electrodes was exposed. Among the light-emitting elements having such a shape, a light emission angle is divided into a narrow application and a wide application. In a case where the light emission angle is wide, it is necessary to provide a rough surface on the light extraction surface.

Although Patent Document 1 discloses a technology for bonding light-emitting elements on ITO provided on a glass substrate and roughening the surface of the glass substrate, this technology is because it is difficult to provide a rough substrate for a sapphire substrate. Surface sake.

On the other hand, Patent Document 2 discloses a method of providing a rough surface directly on a substrate side in a light-emitting element having a single-sided two-electrode shape. This is the reason for choosing a material that can provide a rough surface on the substrate. Although it can be made of GaP, which can be etched as a material that can roughen the surface, it is difficult to make GaP crystals large in diameter regardless of epitaxial growth or block substrates, and it is not suitable for the production of large-scale light-emitting devices. .

On the other hand, Patent Document 3 discloses a technique of bonding transparent substrates. Since this method is irrespective of the material of the base material to be joined, it can be made larger in diameter. [Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent No. 5372766 [Patent Document 2] International Publication No. 2016/072050 [Patent Document 3] Japanese Patent Laid-Open No. 2008-205468

[Problems to be Solved by the Invention] Among the materials of the transparent substrate, oxides are obtained at a low price. However, as a substrate material capable of processing precision to the extent that it can pass a semiconductor step, it is limited to a sapphire substrate. However, the sapphire substrate is a difficult-to-etch material, and there is a problem that it is difficult to obtain a rough surface by a wet etching method.

In view of the problems as described above, an object of the present invention is to provide a light-emitting element and a method for manufacturing a light-emitting element in which light-emitting elements are formed by joining transparent substrates with an increased light emission angle and an increased light-emitting efficiency. [Technical means to solve problems]

In order to achieve the above object, according to the present invention, a light-emitting element is provided. A transparent substrate is attached to a side of a light extraction surface, and a transparent film is disposed on a surface of the transparent substrate on a side of the light extraction surface. The refractive index of the film is lower than that of the transparent substrate, and the surface of the transparent film is surface roughened.

In this way, since the surface of the transparent film is surface-roughened, regardless of the material of the transparent substrate, it can be a light-emitting element having an increased light emission angle and an increased light emission efficiency. Since a lower refractive index material is formed on the light extraction surface to generate a total reflection angle, the luminous efficiency can be further improved.

At this time, the transparent substrate is a sapphire substrate, and the transparent film is preferably a SiO ₂ film.

In this way, a sapphire substrate can be suitably used for the transparent substrate, and a SiO ₂ film can be suitably used for the transparent film.

Furthermore, according to the present invention, there is provided a method for manufacturing a light-emitting element. The light-emitting element is bonded to a transparent substrate on a side of a light extraction surface, and the manufacturing method is on a surface on a side of the light extraction surface of the transparent substrate. A transparent film is laminated, the refractive index of the transparent film is lower than that of the transparent substrate, and the surface of the laminated transparent film is surface roughened by chemically-treated matte processing.

In this way, it is easy to roughen the surface regardless of the material of the transparent substrate, and it is easier to manufacture a light-emitting element having an increased light emission angle and an increased light emission efficiency. Since a lower refractive index material is formed on the light extraction surface to generate a total reflection angle, a light emitting element with higher light emitting efficiency can be manufactured.

At this time, the transparent substrate is a sapphire substrate, and the transparent film is preferably a SiO ₂ film.

In this way, as the transparent substrate, a sapphire substrate using a material that is difficult to be chemically processed by frosting is used. By making the transparent film a SiO ₂ film, the surface of the transparent film can be easily processed by chemically frosting.

Furthermore, at this time, the matte processing is preferably a surface roughening treatment of the surface of the transparent film by an etching treatment with a mixed liquid of a fluoric acid and a monovalent to tetravalent inorganic acid or organic acid.

With such a method, the surface of the transparent film can be reliably roughened.

In this case, as the inorganic acid, at least one of sulfuric acid, hydrochloric acid, and phosphoric acid is used, and as the organic acid, at least one of malonic acid, acetic acid, citric acid, and tartaric acid is preferably used.

As the inorganic acid or the organic acid, as described above, unevenness can be more reliably formed on the surface of the transparent film.

[Contrast with the effect of the prior art] The light-emitting element of the present invention can be a light-emitting element having an increase in light emission angle and an increase in light emission efficiency, regardless of the material of the transparent substrate, because the surface of the transparent film is surface roughened. Since a lower refractive index material is formed on the light extraction surface to generate a total reflection angle, the luminous efficiency can be further improved. Then, according to the present invention, a sapphire substrate having a low cost and high processing accuracy can be used as the transparent substrate.

Furthermore, according to the method for manufacturing a light-emitting element of the present invention, it is easy to roughen the surface surface regardless of the material of the transparent substrate, and a light-emitting element having an increased light emission angle and an increased light emission efficiency can be easily manufactured. Since a lower refractive index material is formed on the light extraction surface to generate a total reflection angle, a light emitting element with higher light emitting efficiency can be manufactured.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited thereto.

As described above, the sapphire substrate is a difficult-to-etch material, and there is a problem that it is difficult to obtain a rough surface by a wet etching method.

Here, the present inventors have repeatedly reviewed to solve such problems. As a result, it is thought that a transparent film having a refractive index lower than that of the transparent substrate is provided on the surface of the light extraction surface of the transparent substrate among light emitting elements with a transparent substrate bonded to the light extraction surface. When the surface of the transparent film is It is a roughened surface, regardless of the material of the transparent substrate. It can be a light-emitting element with an increase in light emission angle and an increase in light emission efficiency. It is further produced by forming a low refractive index material on the side of the light extraction surface. Total reflection angle, which can further improve luminous efficiency. Then, the best mode for carrying out these was precisely examined, and the present invention was completed.

[First Embodiment] FIG. 1 shows the first embodiment of the light-emitting element of the present invention. As shown in FIG. 1, a light-emitting element 100 according to a first embodiment of the present invention has a transparent substrate 110 bonded to a side of a light extraction surface 115. Furthermore, a transparent film 180 having a refractive index lower than that of the transparent substrate 110 is provided on the surface of the light emitting element 100 on the light extraction surface 115 of the transparent substrate 110, and the surface of the transparent film 180 is a surface roughened material. The roughness of the rough surface is preferably Ra (arithmetic average roughness) = 0.3 μm or more.

In this case, a transparent substrate 110 made of, for example, sapphire, on which a transparent film 180 made of, for example, SiO ₂ is formed on the surface can be suitably used. In addition, a second dielectric film 121 having a thickness of approximately 100 nm made of, for example, SiO ₂ can be formed on the transparent substrate 110 on the side opposite to the transparent film 180.

As described above, the surface of the transparent film 180 is roughened to form irregularities. The transparent film 180 uses a material having a lower refractive index than the transparent substrate 110. In this way, when a transparent film 180 having a refractive index lower than that of the transparent substrate 110 is provided, and unevenness is formed on the surface of the transparent film 180, the surface can be easily roughened regardless of the material of the transparent substrate 110, and can have a light emitting angle. And a light-emitting element having an increased light emission efficiency. Moreover, a low-refractive-index material is formed on the side of the light extraction surface 115 to generate a total reflection angle, which can further improve the luminous efficiency.

In addition, the transparent adhesive layer 125 can be formed on the surface of the second dielectric film 121. The transparent adhesive layer 125 can be formed of a plurality of layers of the first adhesive layer 125A and the second adhesive layer 125B, for example. Furthermore, the transparent adhesive layer 125 is formed with a first dielectric film 120 made of, for example, SiO ₂ and having a thickness of about 100 nm. The surface of the first dielectric film 120 can be formed with a thickness of 0.5 to 20 μm, for example. A current propagation layer 107 made of AlGaAs, GaAsP, GaP, or the like.

In addition, the second electrode 151 may be formed on a part (second surface) of the surface of the current propagation layer 107, and the buffer layer 106 may be formed in a region (first surface) where the second electrode 151 is not formed.

When the second electrode 151 is n-type, the second electrode 151 contains at least one or more materials selected from Au, Ag, Al, Ni, Pd, Ge, Si, and Sn, and has a film thickness of 100 nm or more. When the second conductivity type is a p-type, it contains at least one type of material of at least one of Au, Be, Mg, and Zn, and has a film thickness of 100 nm or more.

A buffer layer 106 for reducing lattice mismatch can be formed on the surface of the current propagation layer 107. In this case, when the current propagation layer 107 is formed with GaAs _x P _1-x (0 ≦ x <1), the buffer layer 106 is most preferably formed of InGaP or AlInP. Due to the lattice mismatch between GaAs _x P _1-x (x ≠ 1) and AlGaInP-based materials or AlGaAs-based materials, GaAs _x P _1-x (x ≠ 1) has high density strain and penetration difference. row. The through-drain density can be adjusted according to the composition x.

A second semiconductor layer 105 made of AlGaInP or AlGaAs and having a thickness of 0.5 to 1.0 μm can be formed on the surface of the buffer layer 106. An active layer 104 having a thickness of 0.1 to 10 μm can be formed on the surface. The active layer 104 can be (Al _x Ga _1-x ) _y In _1-y P (0 ≦ x ≦ 1, 0.4 ≦ y ≦ 0.6) or Al _z Ga _1-z As (0 ≦ z ≦ 0.45). In the case of visible light illumination, AlGaInP is suitable, and in the case of infrared illumination, AlGaAs or InGaAs is appropriate. However, the design of the active layer 104 is not limited to the above-mentioned materials because the wavelength can be adjusted to a wavelength other than the wavelength due to the material composition by using a superlattice or the like.

A first semiconductor layer 103 made of AlGaInP or AlGaAs and having a thickness of 0.5 to 1.0 μm can be formed on the surface of the active layer 104. A first electrode 150 can be formed on the surface of the first semiconductor layer 103. In this case, if necessary, a desired layer such as a buffer layer 116 may be provided between the first semiconductor layer 103 and the first electrode 150.

The first electrode 150, when the first conductivity type is n-type, contains at least one type of material of Au, Ag, Al, Ni, Pd, Ge, Si, and Sn, and has a film thickness of 100 nm or more. When the first conductivity type is a p-type, at least one type of material including Au, Be, Mg, and Zn is included, and the film thickness is 100 nm or more.

Although in the aspect of the present embodiment, the case where the second electrode 151 is provided in the current propagation layer 107 is exemplified, the second electrode 151 may be provided in the second semiconductor layer 105.

Next, a method for manufacturing a light-emitting element according to a first embodiment of the present invention will be described with reference to FIGS. 2 to 8.

Initially, as shown in FIG. 2, a substrate 101 is prepared as a starting substrate. As the substrate 101, it is preferable to use the substrate 101 whose crystal axis is inclined from the [001] direction to the [110] direction. In addition, as the substrate 101, GaAs or Ge can be suitably used. If the substrate 101 is selected from the above materials, since the material of the active layer 104 described later can be epitaxially grown in a lattice matching system, the quality of the active layer 104 can be easily improved, and the brightness and life characteristics can be improved.

Next, on the substrate 101, a first semiconductor layer 103 (for example, having a thickness of 0.5 to 1.0 μm) and an active layer 104 of a first conductivity type that are substantially the same as the lattice constant of the substrate 101 can be sequentially formed by epitaxial growth. (For example, the thickness is 0.1 to 1.0 μm), the second semiconductor layer 105 of the second conductivity type (for example, the thickness is 0.5 to 1.0 μm), the buffer layer 106 and the current propagation layer 107 (for example, the thickness is about 2.0 μm). Alternatively, a selective etching layer 102 for removing the substrate 101 may be interposed between the substrate 101 and the first semiconductor layer 103. The selective etching layer 102 is composed of two or more layer structures, and preferably has at least a first selective etching layer 102A connected to the substrate 101 and a second selective etching layer 102B connected to the first semiconductor layer 103. The first selective etching layer 102A and the second selective etching layer 102B may be made of different materials or compositions.

At this time, specifically, on the substrate 101 (in the case where the selective etching layer 102 is provided, on the selective etching layer 102), for example, MOVPE method (organic metal vapor deposition method), MBE (molecular wire) Epitaxial method) or CBE (chemical line epitaxial method), and is fabricated on a light-emitting portion 108 composed of a first semiconductor layer 103 of a first conductivity type, an active layer 104, and a second semiconductor layer 105 of a second conductivity type. The epitaxial substrate 109 is epitaxially grown in the order of the buffer layer 106 and the current propagation layer 107.

The active layer 104 can be (Al _x Ga _1-x ) _y In _1-y P (0 ≦ x ≦ 1, 0.4 ≦ y ≦ 0.6) or Al _z Ga _1-z As (0 ≦ z ≦ 0.45) formation. In the case of visible light illumination, AlGaInP is suitable, and in the case of infrared illumination, AlGaAs or InGaAs is appropriate. However, the design of the active layer 104 is not limited to the above-mentioned materials because the wavelength can be adjusted to a wavelength other than the wavelength due to the material composition by using a superlattice or the like.

The first semiconductor layer 103 and the second semiconductor layer 105 are selected from AlGaInP or AlGaAs, and a material ratio with a wider energy gap than that of the first semiconductor layer 103 is selected. It should be noted that the material selection may not necessarily be the same as that of the active layer 104.

In this embodiment, although the case where the first semiconductor layer 103, the light emitting layer 104, and the second semiconductor layer 105 which are the simplest structures are AlInGaP of the same material is exemplified, the first semiconductor layer 103 or the first semiconductor layer 103 The second semiconductor layer 105 is for improving the characteristics, and it is normal to include a plurality of layers in each layer. The second semiconductor layer 105 is not limited to a single layer.

Furthermore, the first semiconductor layer 103 is composed of a layer composed of two or more types of Al, and can be a second layer 103B on the side near the active layer 104 and a first layer with a low Al composition on the side near the substrate 101. Composition of layer 103A. The second layer 103B is a functional layer having the function of a cladding layer, and does not refer to a single composition or a single condition layer.

As the current propagation layer 107, AlGaAs, GaAsP, or GaP can be suitably used. When the current propagation layer 107 is formed of GaAs _x P _1-x (0 ≦ x <1), the buffer layer 106 is most preferably formed of InGaP or AlInP. Due to the lattice mismatch between GaAs _x P _1-x (x ≠ 1) and AlGaInP-based materials or AlGaAs-based materials, GaAs _x P _1-x (x ≠ 1) has high density strain and penetration difference. row. The through-drain density can be adjusted according to the composition x.

Next, as shown in FIG. 3, a first dielectric film (first SiO ₂ film) 120 is deposited on the current propagation layer 107 in the epitaxial substrate 109. The first dielectric film 120 can be formed by a photo-CVD method, a sputtering method, or a PECVD method.

Next, a transparent adhesive layer 125 is formed on the first dielectric film 120 so as to serve as the first bonding substrate 126. The transparent adhesive layer 125 can be selected from BCB (benzocyclobutene) or epoxy resin. The formation method is appropriately selected from materials that can be formed by a dip coating method or a spin coating method.

Next, a second dielectric film (second SiO ₂ film) 121 is deposited on the transparent substrate 110 to form a second bonding substrate 131. The second dielectric film 121 can be formed by a photo CVD method, a sputtering method, or a PECVD method. In addition, the same effect can be obtained by providing a transparent adhesive layer on the second bonding substrate 131.

Next, the first bonding substrate 126 and the second bonding substrate 131 are provided so that the transparent adhesive layer 125 and the second dielectric film 121 face each other and do not contact each other, and a vacuum atmosphere of 10 Pa or less is provided. After the vacuum atmosphere, the transparent adhesive layer 125 is brought into contact with the second dielectric film 121 and controlled to a pressure between 5000N and a temperature between 100 and 200 ° C for 5 minutes or more, and then heat is applied at 100 ° C or more to A bonding substrate 126 is pressed against the second bonding substrate 131 to form a bonding substrate 140.

Next, as shown in FIG. 4, the substrate 101 is removed from the bonded substrate 140 by etching. During the etching, etching can be performed with a mixed solution of ammonia water and hydrogen peroxide water. By using the etching stop layer (the first selective etching layer 102A) as a material different from that of the substrate 101, it is possible to selectively stop the etching using a mixed solution of ammonia water and hydrogen peroxide water. As the first selective etching layer 102A, AlInP can be used.

After the substrate 101 is removed, the first selective etching layer 102A is removed. Since the etching stop layer 102A is made of AlInP, the removal is removed using hydrochloric acid. Since the second selective etching layer 102B stops etching by hydrochloric acid, GaAs can be used.

Next, a first electrode 150 is formed in contact with the first semiconductor layer 103. The first electrode 150 can contain at least one material of Au, Ag, Al, Ni, Pd, Ge, Si, and Sn when the first conductivity type is n-type, and has a film thickness of 100 nm or more. In the case where the first conductivity type is a p-type, it can contain at least one type of material of Au, Be, Mg, and Zn, and has a film thickness of 100 nm or more. In addition, the second selective etching layer 102B may be left.

Next, as shown in FIG. 5, a pattern in which the first semiconductor layer 103 and the active layer 104 of the region 160 are cut off is formed by etching using a dry method or a wet method. Although an example of cutting up to the current propagation layer 107 is shown in FIG. 5, stopping the etching with the second semiconductor layer 105 or the buffer layer 106 exposed has the same function. The area other than the area 160 is not limited to a flat surface, and the area other than the area 160 may be a rough surface or an uneven surface.

Next, as shown in FIG. 6, an insulating layer 170 that covers at least a portion of the first semiconductor layer 103 can be formed. The insulating layer 170 can be selected from SiO ₂ and SiNx.

Next, as shown in FIG. 7, a light-emitting element substrate 171 in which a second electrode 151 is formed in a part of the region 160 is formed. In the case where the second conductivity type is an n-type, it can contain at least one type of material of Au, Ag, Al, Ni, Pd, Ge, Si, and Sn, and has a film thickness of 100 nm or more. In the case where the second conductivity type is a p-type, it can contain at least one or more materials of Au, Be, Mg, and Zn, and has a film thickness of 100 nm or more.

Next, as shown in FIG. 8, on the surface on the light extraction surface 115 side of the transparent substrate 110 of the light-emitting element substrate 171, a transparent film 180 having a lower refractive index than the transparent substrate 110 is laminated. Then, the surface of this laminated transparent film 180 is surface-roughened by a chemically-processed sanding process. The roughness of the rough surface is preferably Ra (arithmetic average roughness) = 0.3 μm or more.

At this time, the transparent substrate 110 is preferably a sapphire substrate, and the transparent film 180 is preferably a SiO ₂ film. In this way, as the transparent substrate 110, a sapphire substrate that is inexpensive and has high processing precision, but which is difficult to process with chemically processed frosted materials, and the transparent film 180 is a SiO ₂ film can be easily aligned. The surface of the transparent film 180 is subjected to a chemically-treated matte process.

At this time, the surface of the SiO ₂ film which is the transparent film 180 is a matte process with a mixed solution of a fluoric acid and a monovalent to tetravalent inorganic acid or organic acid, and the surface of the SiO ₂ film can be produced with an uneven layer. A matte-processed substrate 182 of 181. According to such a method, the surface of the transparent film 180 can be surface-roughened, and unevenness can be formed.

Here, the inorganic acid can be composed of at least one of sulfuric acid, hydrochloric acid, and phosphoric acid, and the organic acid can be composed of at least one of malonic acid, acetic acid, citric acid, and tartaric acid. When an inorganic acid or an organic acid is used as described above, unevenness can be more reliably formed on the surface of the transparent film.

Next, the matte-processed substrate 182 is divided into individual crystal grains by an invisible cutting method or a blade cutting method, and the crystal grains are fixed to a bracket, so that a light-emitting diode sealed with an epoxy resin can be manufactured.

In this way, since the surface of the transparent film 180 laminated on the transparent substrate 110 is subjected to a surface roughening treatment, the surface of the transparent substrate 110 can be easily roughened regardless of the material of the transparent substrate 110, and it is relatively easy to manufacture a light emitting angle A light-emitting element having an increased light emission efficiency. Furthermore, by forming a low-refractive-index material on the side of the light extraction surface 115 to generate a total reflection angle, the light emission efficiency can be further improved.

[Second Embodiment] FIG. 9 shows a second embodiment of the light-emitting element of the present invention. As shown in FIG. 9, a light-emitting element 200 in a second embodiment of the present invention has a transparent substrate 210 bonded to a side of a light extraction surface 215. Furthermore, the light emitting element 200 is provided on the surface on the side of the light extraction surface 215 of the transparent substrate 210 with a transparent film 280 having a refractive index lower than that of the transparent substrate 210, and the surface of the transparent film 280 is roughened. The roughness of the rough surface is preferably Ra (arithmetic average roughness) = 0.3 μm or more.

In this case, a transparent substrate 210 made of, for example, sapphire or the like on which a transparent film 280 made of, for example, a SiO ₂ film is formed on the surface can be suitably used. In addition, a second dielectric film 221 made of, for example, SiO ₂ and having a thickness of about 100 nm can be formed on the side opposite to the transparent film 280 of the transparent substrate 210.

As described above, the surface of the transparent film 280 is roughened to form irregularities. The transparent film 280 uses a material having a lower refractive index than the transparent substrate 210. In this way, when a transparent film 280 having a refractive index lower than that of the transparent substrate 210 is provided, and unevenness is formed on the surface of the transparent film 280, the surface of the transparent substrate 210 can be easily roughened regardless of the material of the transparent substrate 210, and can be A light-emitting element having an increase in light emission angle and an increase in light emission efficiency. Furthermore, a low-refractive-index material is formed on the side of the light extraction surface 215 to generate a total reflection angle, which can further improve the luminous efficiency.

Furthermore, in the aspect of the second embodiment, the surface on the side of the light extraction surface 215 of the transparent substrate 210 is also uneven. The transparent substrate 210 is polished and polished in order to adjust the thickness. However, since polishing has a long processing time, it is cost effective to perform polishing only. Although the surface that is polished only becomes a concave-convex surface, the Ra (roughness) of the concave-convex cannot be controlled due to processing constraints, and the unevenness is small. By forming the transparent film 280 to form rough unevenness on the surface, it becomes a thing having a roughness which is favorable for light extraction.

Furthermore, a transparent adhesive layer 225 can be formed on the surface of the second dielectric film 221. The transparent adhesive layer 225 can be formed of a plurality of layers such as a first adhesive layer 225A and a second adhesive layer 225B. Furthermore, the transparent adhesive layer 225 can be formed with a first dielectric film 220 made of, for example, SiO ₂ and having a thickness of about 100 nm, and formed on the surface of the first dielectric film 220 with a thickness of 0.5 to 20 μm. For example, a current propagation layer 207 made of AlGaAs, GaAsP, GaP, or the like.

In addition, the second electrode 251 can be formed on a part (the second surface) of the surface of the current propagation layer 207, and the buffer layer 206 can be formed in a region (the first surface) where the second electrode 251 is not formed.

The second electrode 251, when the second conductivity type is n-type, contains at least one or more materials of Au, Ag, Al, Ni, Pd, Ge, Si, and Sn, and has a film thickness of 100 nm or more. When the second conductivity type is a p-type, it contains at least one type of material of at least one of Au, Be, Mg, and Zn, and has a film thickness of 100 nm or more.

A buffer layer 206 for reducing lattice mismatch can be formed on the surface of the current propagation layer 207. In this case, when the current propagation layer 207 is formed with GaAs _x P _1-x (0 ≦ x <1), the buffer layer 206 is most preferably formed of InGaP or AlInP. Due to the lattice mismatch between GaAs _x P _1-x (x ≠ 1) and AlGaInP-based materials or AlGaAs-based materials, GaAs _x P _1-x (x ≠ 1) has high density strain and penetration difference. row. The through-drain density can be adjusted according to the composition x.

A second semiconductor layer 205 made of AlGaInP or AlGaAs and having a thickness of 0.5 to 1.0 μm can be formed on the surface of the buffer layer 206. An active layer 204 having a thickness of 0.1 to 10 μm can be formed on the surface. The active layer 204 can be (Al _x Ga _1-x ) _y In _1-y P (0 ≦ x ≦ 1, 0.4 ≦ y ≦ 0.6) or Al _z Ga _1-z As (0 ≦ z ≦ 0.45). In the case of visible light illumination, AlGaInP is suitable, and in the case of infrared illumination, AlGaAs or InGaAs is appropriate. However, the design of the active layer 204 is not limited to the above-mentioned materials because the wavelength can be adjusted to a wavelength other than the wavelength due to the material composition by using a superlattice or the like.

A first semiconductor layer 203 made of AlGaInP or AlGaAs and having a thickness of 0.5 to 1.0 μm can be formed on the surface of the active layer 204. A first electrode 250 can be formed on a surface of the first semiconductor layer 203. In this case, if necessary, a desired layer such as a buffer layer 216 may be provided between the first semiconductor layer 203 and the first electrode 250.

The first electrode 250 includes at least one material selected from Au, Ag, Al, Ni, Pd, Ge, Si, and Sn when the first conductivity type is n-type, and has a film thickness of 100 nm or more. When the first conductivity type is a p-type, at least one type of material including Au, Be, Mg, and Zn is included, and the film thickness is 100 nm or more.

Although a case where the second electrode 251 is provided in the current propagation layer 207 is exemplified in the aspect of this embodiment, the second electrode 251 may be provided in the second semiconductor layer 205.

Next, a method for manufacturing a light-emitting element according to a second embodiment of the present invention will be described with reference to FIGS. 10 to 16.

Initially, as shown in FIG. 10, a substrate 201 is prepared as a starting substrate. As the substrate 201, it is preferable to use a substrate 201 whose crystal axis is inclined from the [001] direction to the [110] direction. In addition, as the substrate 201, GaAs or Ge can be suitably used. If the substrate 201 is selected from the above materials, since the material of the active layer 204 described later can be epitaxially grown in a lattice matching system, the quality of the active layer 204 can be easily improved, and the brightness and life characteristics can be improved.

Next, a first semiconductor layer 203 (for example, having a thickness of 0.5 to 1.0 μm) and an active layer 204 of a first conductivity type that are substantially the same as the lattice constant of the substrate 201 can be sequentially formed on the substrate 201 by epitaxial growth. (For example, the thickness is 0.1 to 1.0 μm), the second semiconductor layer 205 of the second conductivity type (for example, the thickness is 0.5 to 1.0 μm), the buffer layer 206 and the current propagation layer 207 (for example, the thickness is about 2.0 μm). Alternatively, a selective etching layer 202 for removing the substrate 201 may be interposed between the substrate 201 and the first semiconductor layer 203. The selective etching layer 202 is composed of two or more layer structures, and preferably has at least a first selective etching layer 202A connected to the substrate 201 and a second selective etching layer 202B connected to the first semiconductor layer 203. The first selective etching layer 202A and the second selective etching layer 202B may be made of different materials or compositions.

At this time, specifically, on the substrate 201 (in the case where the selective etching layer 202 is provided, on the selective etching layer 202), for example, MOVPE method (organic metal vapor deposition method), MBE (molecular wire) Epitaxial method) or CBE (chemical line epitaxial method), and is fabricated on a light-emitting portion 208 composed of a first semiconductor layer 203 of a first conductivity type, an active layer 204, and a second semiconductor layer 205 of a second conductivity type. The epitaxial substrate 209 is epitaxially grown in the order of the buffer layer 206 and the current propagation layer 207.

The active layer 204 can be (Al _x Ga _1-x ) _y In _1-y P (0 ≦ x ≦ 1, 0.4 ≦ y ≦ 0.6) or Al _z Ga _1-z As (0 ≦ z ≦ 0.45) formation. In the case of visible light illumination, AlGaInP is suitable, and in the case of infrared illumination, AlGaAs or InGaAs is appropriate. However, the design of the active layer 204 is not limited to the above-mentioned materials because the wavelength can be adjusted to a wavelength other than the wavelength due to the material composition by using a superlattice or the like.

The first semiconductor layer 203 and the second semiconductor layer 205 are selected from AlGaInP or AlGaAs, and a material ratio with a wider energy gap than that of the first semiconductor layer 203 is selected. It should be noted that this material selection need not be the same as that of the active layer 204.

In the aspect of this embodiment, although the case where the first semiconductor layer 203, the light emitting layer 204, and the second semiconductor layer 205 which are the simplest structures are AlInGaP of the same material is exemplified, the first semiconductor layer 203 or the first semiconductor layer 203 The second semiconductor layer 205 is for improving characteristics, and it is normal to include a plurality of layers in each layer. The second semiconductor layer 205 is not limited to a single layer.

In addition, the first semiconductor layer 203 is composed of a layer composed of two or more types of Al. The first semiconductor layer 203 can be a layer having a second layer 203B on the side near the active layer 204 and a first having a low Al composition on the side near the substrate 201. Structure of layer 203A. The second layer 203B is a functional layer having the function of a cladding layer, and does not refer to a single composition or a single condition layer.

As the current propagation layer 207, AlGaAs, GaAsP, or GaP can be suitably used. When the current propagation layer 207 is formed with GaAs _x P _1-x (0 ≦ x <1), the buffer layer 206 is most preferably formed of InGaP or AlInP. Due to the lattice mismatch between GaAs _x P _1-x (x ≠ 1) and AlGaInP-based materials or AlGaAs-based materials, GaAs _x P _1-x (x ≠ 1) has high density strain and penetration difference. row. The through-drain density can be adjusted according to the composition x.

Next, as shown in FIG. 11, a first dielectric film (first SiO ₂ film) 220 is deposited on the current propagation layer 207 in the epitaxial substrate 209. The first dielectric film 220 can be formed by a photo CVD method, a sputtering method, or a PECVD method.

Next, a transparent adhesive layer 225 is formed on the first dielectric film 220 to serve as a first bonding substrate 226. The transparent adhesive layer 225 can be selected from BCB (benzocyclobutene) or epoxy resin. The formation method is appropriately selected from materials that can be formed by a dip coating method or a spin coating method.

Next, a second dielectric film (second SiO ₂ film) 221 is deposited on the transparent substrate 210 to form a second bonding substrate 231. The second dielectric film 221 can be formed by a photo-CVD method, a sputtering method, or a PECVD method. In addition, the same effect can be obtained by providing a transparent adhesive layer on the second bonding substrate 231.

Next, the first bonding substrate 226 and the second bonding substrate 231 are provided so that the transparent adhesive layer 225 and the second dielectric film 221 face each other and do not contact each other, and a vacuum atmosphere of 10 Pa or less is provided. After the vacuum atmosphere, the transparent adhesive layer 225 is brought into contact with the second dielectric film 221, and is controlled at a pressure between 5000 N and a temperature between 100 and 200 ° C for 5 minutes or more. A bonding substrate 226 is pressed against the second bonding substrate 231 to form a bonding substrate 240.

Among the bonded substrates, in order to process the second bonded substrate 231 to a desired thickness, a thin film is processed to a predetermined thickness by grinding, flat grinding, or sandblasting, and on a surface opposite to the mirror surface 211 of the transparent substrate 210 That is, a side of the light extraction surface 215 of the transparent substrate 210 forms an uneven surface 212. Although the concave-convex surface 212 can be mirror-finished by processing such as polishing, it is necessary to perform processing for a long time. In order to prevent damage to the epitaxial substrate 209, it is better not to perform it here. Moreover, it is advantageous to cost by carrying out only the polishing.

Next, as shown in FIG. 12, the substrate 201 is removed from the bonded substrate 240 by etching. During the etching, etching can be performed with a mixed solution of ammonia water and hydrogen peroxide water. By using the etching stop layer (the first selective etching layer 202A) as a material different from that of the substrate 201, it is possible to selectively stop the etching by the mixed solution of ammonia water and hydrogen peroxide water. As the first selective etching layer 202A, AlInP can be used.

After the substrate 201 is removed, the first selective etching layer 202A is removed. Since the etching stop layer 202A is made of AlInP, the removal is removed using hydrochloric acid. Since the second selective etching layer 202B stops the etching by hydrochloric acid, GaAs can be used.

Next, a first electrode 250 is formed in contact with the first semiconductor layer 203. The first electrode 250 can contain at least one material of Au, Ag, Al, Ni, Pd, Ge, Si, and Sn when the first conductivity type is n-type, and has a film thickness of 100 nm or more. In the case where the first conductivity type is a p-type, it can contain at least one type of material of Au, Be, Mg, and Zn, and has a film thickness of 100 nm or more. In addition, the second selective etching layer 202B may be left.

Next, as shown in FIG. 13, a pattern in which the first semiconductor layer 203 and the active layer 204 of the region 260 are cut off is formed by dry or wet etching. Although an example of cutting up to the current propagation layer 207 is shown in FIG. 13, stopping the etching with the second semiconductor layer 205 or the buffer layer 206 exposed has the same function. The area other than the area 260 is not limited to a flat surface, and the area other than the area 260 may be a rough surface or an uneven surface.

Next, as shown in FIG. 14, an insulating layer 270 that covers at least a portion of the first semiconductor layer 203 can be formed. The insulating layer 270 can be selected from SiO ₂ and SiNx.

Next, as shown in FIG. 15, a light-emitting element substrate 271 in which a second electrode 251 is formed in a part of the region 260 is formed. In the case where the second conductivity type is an n-type, it can contain at least one type of material of Au, Ag, Al, Ni, Pd, Ge, Si, and Sn, and has a film thickness of 100 nm or more. When the second conductivity type is a p-type, it can contain at least one of Au, Be, Mg, and Zn, and has a film thickness of 100 nm or more.

Next, as shown in FIG. 16, on the surface on the light extraction surface 215 side of the transparent substrate 210 of the light-emitting element substrate 271, a transparent film 280 having a lower refractive index than the transparent substrate 210 is laminated. Then, the surface of this laminated transparent film 280 is surface-roughened by a chemically-treated matte process. The roughness of the rough surface is preferably Ra (arithmetic average roughness) = 0.3 μm or more.

At this time, the transparent substrate 210 is preferably a sapphire substrate, and the transparent film 280 is preferably a SiO ₂ film. In this way, as the transparent substrate 210, a sapphire substrate that is inexpensive and has high processing precision, but which is difficult to process with chemically processed frosted materials, and the transparent film 280 is a SiO ₂ film can be easily aligned. The surface of the transparent film 280 is subjected to a chemically-treated matte process.

At this time, the surface of the SiO ₂ film that is the transparent film 280 is blasted with a mixed solution of a fluoric acid and a monovalent to tetravalent inorganic acid or organic acid, so that the surface of the SiO ₂ film can have an uneven layer. 281's matte processing substrate 282. According to such a method, the surface of the transparent film 280 can be surface-roughened, and unevenness can be formed.

Here, the inorganic acid can be composed of at least one of sulfuric acid, hydrochloric acid, and phosphoric acid, and the organic acid can be composed of at least one of malonic acid, acetic acid, citric acid, and tartaric acid. When an inorganic acid or an organic acid is used as described above, unevenness can be more reliably formed on the surface of the transparent film.

Next, the matte-processed substrate 282 is divided into individual crystal grains by an invisible cutting method or a blade cutting method, and the crystal grains are fixed to a bracket, so that a light-emitting diode sealed with an epoxy resin can be manufactured.

In this way, since the surface of the transparent film 280 laminated on the transparent substrate 210 is subjected to surface roughening treatment, the surface surface can be easily roughened regardless of the material of the transparent substrate 210, and the light emitting angle can be manufactured relatively easily. And a light-emitting element having an increased light emission efficiency. Furthermore, by forming a low-refractive-index material on the side of the light extraction surface 215 to generate a total reflection angle, the luminous efficiency can be further improved. Furthermore, in the appearance of the second embodiment, since the unevenness is also formed on the surface of the transparent substrate 210, the roughened unevenness is formed on the surface of the transparent substrate 210 by laminating the transparent film, so that it is more advantageous. Light extraction of roughness.

In addition, in the method for manufacturing a light emitting device according to the first and second embodiments described above, after bonding the transparent substrate and the first bonding substrate, the surface on the light extraction surface side of the transparent substrate is laminated. The case where a transparent film having a refractive index lower than that of a transparent substrate is subjected to surface roughening treatment has been described, but the present invention is not limited to this. For example, a transparent film is laminated on the transparent substrate before bonding, and the surface of the laminated transparent film is subjected to a surface roughening treatment, and then may be bonded to the first bonding substrate.

[Examples] Hereinafter, the present invention will be described more specifically by showing examples and comparative examples of the present invention, but the present invention is not limited to these.

[Example 1] As shown in FIG. 2, as a starting substrate, a GaAs substrate (substrate 101) having a crystal axis inclined from a [001] direction to a [110] direction was prepared. Next, on the GaAs substrate 101, an MOVPE method (organic metal vapor deposition method) was used to sequentially make an n-type cladding layer (first semiconductor layer 103) made of AlGaInP and having a thickness of 1.0 μm, and an active layer. The layer 104, the p-type cladding layer (second semiconductor layer 105) with a thickness of 1.0 μm are epitaxially grown, and the buffer layer 106 composed of InGaP is sequentially formed by the epitaxial growth, and the A current propagation layer 107 made of GaP. Between the GaAs substrate and the n-type cladding layer, a selective etching layer 102 (also referred to as an etch stop layer) composed of an AlInP layer and a GaAs layer is formed.

Furthermore, the first semiconductor layer 103 is composed of a layer composed of two or more types of Al composition, and a first layer 103A having a low Al composition is formed on the side near the substrate 101.

Next, as shown in FIG. 3, a first SiO ₂ film (first dielectric film 120) is formed on the current propagation layer 107 made of GaP by using TEOS and O ₂ as raw materials and by a PECVD method.

Next, a transparent adhesive layer 125 is formed on the first dielectric film 120 to form a first bonding substrate 126. The transparent adhesive layer 125 was CYCLOTENE dropped and spin-coated at a rotation speed of 1,000 rpm. After the spin coating, the solvent was maintained at 100 ° C. for 60 seconds on a hot plate to evaporate the solvent.

Next, a sapphire substrate was prepared as the transparent substrate 110, and a second SiO ₂ film (second dielectric film 121) was deposited on the transparent substrate 110 to form a second bonding substrate 131. The second dielectric film 121 is formed by using TEOS and O ₂ as raw materials and by a PECVD method.

The first bonding substrate 126 and the second bonding substrate 131 are provided so that the transparent adhesive layer 125 and the second dielectric film 121 face each other and do not contact each other, and a vacuum atmosphere of 10 Pa or less is provided. After the vacuum atmosphere, the transparent adhesive layer 125 is brought into contact with the second dielectric film 121, and the pressure is maintained at 5000 N and the temperature of 100 ° C. is maintained for 5 minutes or more. Then, the first bonding substrate 126 is applied with heat of 100 ° C. or more. The second bonding substrate 131 is pressure-bonded to form a bonding substrate 140. Thereafter, in order to achieve a desired thickness, the surface of the sapphire substrate was polished and polished.

Next, as shown in FIG. 4, the self-bonded substrate 140 is removed by etching with a mixed solution of ammonia water and hydrogen peroxide water. After the substrate 101 is removed, the first selective etching layer 102A is removed. Since the first selective etching layer 102A uses AlInP, hydrochloric acid is used for the removal. Next, a first electrode 150 made of an AuGeNi alloy and having a thickness of 500 nm is formed in contact with the first semiconductor layer 103.

Next, as shown in FIG. 5, a pattern in which the first semiconductor layer 103, the active layer 104, the second semiconductor layer 105, and the buffer layer 106 of the region 160 are cut off is formed by dry or wet etching. Next, as shown in FIG. 6, an insulating layer 170 that covers the first semiconductor layer 103, the active layer 104, the second semiconductor layer 105, and the buffer layer 106 can be formed. The insulating layer 170 is formed by a PECVD method using TEOS and O ₂ as raw materials. The film thickness is 100 nm. Next, as shown in FIG. 7, a second electrode 151 made of an AuBe alloy and having a thickness of 500 nm is formed in a part of the region 160 to form a light-emitting element substrate 171.

Next, as shown in FIG. 8, a SiO ₂ film as a transparent film 180 is formed on the surface on the light extraction surface 115 side of the transparent substrate 110 of the light-emitting element substrate 171. A frosted surface was applied to the transparent film 180 with a mixed solution of hydrofluoric acid and acetic acid to produce a frosted substrate 182 having an uneven layer 181 on the surface of the transparent film 180.

Next, the matte-processed substrate 182 is divided into individual crystal grains by an invisible cutting method, and the crystal grains are fixed to a holder, so that a light-emitting diode sealed with an epoxy resin can be produced.

[Example 2] It was the same as Example 1 except that the sapphire substrate was bonded to the sapphire substrate, and then the film was processed to a predetermined thickness by grinding, and then the surface of the sapphire substrate was roughened without polishing. The method produces a light-emitting diode.

[Comparative Example] A light-emitting diode was produced in the same manner as in Example 1 except that a SiO ₂ film was not formed on the surface of the sapphire substrate.

The light-emitting characteristics of the light-emitting diodes produced in Examples 1, 2, and Comparative Examples were compared. FIG. 17 shows the difference in light-emitting characteristics of the light-emitting diodes produced in the first, second, and comparative examples. As shown in Fig. 17, the light emission angles of ± 60 degrees in Examples 1 and 2 have a relative light emission intensity of 50% or more relative to the light emission angles of ± 30 degrees in the comparative example, and It was learned that the light emission angle became wider.

FIG. 18 is a graph showing current-luminance characteristics of the light-emitting diodes produced in Examples 1, 2, and Comparative Examples. Compared with the comparative example, the first embodiment and the second embodiment are generally not related to high luminance, and the current-luminance relationship is kept linear.

In addition, the present invention is not limited to the embodiment described above. The above embodiment is an example, and anyone who has substantially the same structure and produces the same effect as the technical idea described in the patent application scope of the present invention is included in the technical scope of the present invention no matter what.

100‧‧‧Light-emitting element

101‧‧‧ substrate

102‧‧‧Select Etching Layer

102A‧‧‧The first choice of etching layer (etch stop layer)

102B‧‧‧Second Selective Etching Layer

103‧‧‧First semiconductor layer

103A‧‧‧First floor

103B‧‧‧Second floor

104‧‧‧active layer (light emitting layer)

105‧‧‧Second semiconductor layer

106‧‧‧ buffer layer

107‧‧‧ current propagation layer

108‧‧‧Lighting Department

109‧‧‧Epimorph substrate

110‧‧‧ transparent substrate

115‧‧‧light extraction surface

116‧‧‧Buffer layer

120‧‧‧ the first dielectric film (the first SiO ₂ film)

121‧‧‧Second dielectric film (second SiO ₂ film)

125‧‧‧ transparent adhesive layer

125A‧‧‧First Adhesive Layer

125B‧‧‧Second Adhesive Layer

126‧‧‧First bonding substrate

131‧‧‧Second bonding substrate

140‧‧‧Board

150‧‧‧first electrode

151‧‧‧Second electrode

160‧‧‧area

170‧‧‧ Insulation

171‧‧‧Light-emitting element substrate

180‧‧‧ transparent film

181‧‧‧ Bump layer

182‧‧‧Frosted processed substrate

200‧‧‧Light-emitting element

201‧‧‧ substrate

202‧‧‧Select Etching Layer

202A‧‧‧The first choice of etching layer (etch stop layer)

202B‧‧‧Second Selective Etching Layer

203‧‧‧First semiconductor layer

203A‧‧‧First floor

203B‧‧‧Second floor

204‧‧‧active layer (light emitting layer)

205‧‧‧Second semiconductor layer

206‧‧‧Buffer layer

207‧‧‧Current Propagation Layer

208‧‧‧Lighting Department

209‧‧‧Epimorph substrate

210‧‧‧ transparent substrate

211‧‧‧Mirror

212‧‧‧convex surface

215‧‧‧light extraction surface

216‧‧‧Buffer layer

220‧‧‧ the first dielectric film (the first SiO ₂ film)

221‧‧‧Second dielectric film (second SiO ₂ film)

225‧‧‧Transparent adhesive layer

225A‧‧‧First Adhesive Layer

225B‧‧‧Second Adhesive Layer

226‧‧‧First bonding substrate

231‧‧‧Second bonding substrate

240‧‧‧Board

250‧‧‧first electrode

251‧‧‧Second electrode

260‧‧‧area

270‧‧‧ Insulation

271‧‧‧Light-emitting element substrate

280‧‧‧ transparent film

281‧‧‧ Bump layer

282‧‧‧ matte processing substrate

FIG. 1 is a schematic diagram showing the appearance of a first embodiment of a light-emitting element according to the present invention. FIG. 2 is an explanatory diagram showing an epitaxial substrate in which a selective etching layer, a light emitting portion, a buffer layer, and a current propagation layer are grown on a substrate in a first embodiment of a method for manufacturing a light emitting element according to the present invention. FIG. 3 is an explanatory diagram showing a bonded substrate in which a first bonded substrate and a second bonded substrate are bonded together in a first embodiment of the method for manufacturing a light emitting element according to the present invention. FIG. 4 is an explanatory view showing a light-emitting element substrate on which a first electrode is formed in a first embodiment of a method for manufacturing a light-emitting element according to the present invention. FIG. 5 is an explanatory diagram showing a light-emitting element having a pattern in which a first semiconductor layer and an active layer are cut out of a first embodiment of a method for manufacturing a light-emitting element according to the present invention. FIG. 6 is an explanatory view showing a light-emitting element substrate having an insulating layer covering at least a part of the first semiconductor layer in the first embodiment of the method for manufacturing a light-emitting element according to the present invention. FIG. 7 is an explanatory diagram showing a light-emitting element substrate having a second electrode formed therein in a first embodiment of the method for manufacturing a light-emitting element according to the present invention. FIG. 8 is an explanatory view showing a matte-processed substrate that has been subjected to a surface roughening treatment on the surface of the transparent film in the first embodiment of the method for manufacturing a light-emitting element according to the present invention. Fig. 9 is a schematic view showing a second embodiment of the light emitting device of the present invention. FIG. 10 is an explanatory view showing an epitaxial substrate in which a selective etching layer, a light emitting portion, a buffer layer, and a current propagation layer are grown on a substrate in a second embodiment of the method for manufacturing a light emitting element according to the present invention. FIG. 11 is an explanatory view showing a bonded substrate to which a first bonded substrate and a second bonded substrate are bonded in a second embodiment of the method for manufacturing a light emitting element according to the present invention. FIG. 12 is an explanatory diagram showing a light-emitting element substrate having a first electrode formed therein in a second embodiment of the method for manufacturing a light-emitting element according to the present invention. FIG. 13 is an explanatory diagram showing a light-emitting element having a pattern in which a first semiconductor layer and an active layer are cut out in a second embodiment of the method for manufacturing a light-emitting element according to the present invention. FIG. 14 is an explanatory view showing a light-emitting element substrate having an insulating layer covering at least a part of the first semiconductor layer in the second embodiment of the method for manufacturing a light-emitting element according to the present invention. Fig. 15 is an explanatory view showing a light-emitting element substrate having a second electrode formed therein, in a second embodiment of the method for manufacturing a light-emitting element according to the present invention. FIG. 16 is an explanatory view showing a matte-processed substrate that has been subjected to a surface roughening treatment on the surface of the transparent film in the second embodiment of the method for manufacturing a light-emitting element according to the present invention. Fig. 17 is a graph showing the light-emitting characteristics of the light-emitting diodes produced in Examples 1 and 2 and Comparative Examples. Fig. 18 is a graph showing the current-luminance characteristics of the light-emitting diodes produced in Examples 1 and 2 and Comparative Examples.

Claims (6)

A light emitting element is attached to a transparent substrate on a side of a light extraction surface, wherein a transparent film is disposed on a surface of the transparent substrate on the side of the light extraction surface, and the refractive index of the transparent film is lower than that of the transparent substrate. The surface of the transparent film is roughened. The light-emitting element according to claim 1, wherein the transparent substrate is a sapphire substrate, and the transparent film is a SiO ₂ film. A method for manufacturing a light-emitting element, the light-emitting element is bonded to a transparent substrate on a side of a light extraction surface, wherein the manufacturing method is to laminate a transparent film on a surface of the transparent substrate on a side of the light extraction surface, the transparent The refractive index of the film is lower than that of the transparent substrate, and the surface of the laminated transparent film is surface-roughened by chemically-treated matte processing. The method for manufacturing a light emitting device according to claim 3, wherein the transparent substrate is a sapphire substrate, and the transparent film is a SiO ₂ film. The method for manufacturing a light-emitting device according to claim 4, wherein the frosting process is performed by etching the liquid with a mixed solution of hydrofluoric acid and a monovalent to tetravalent inorganic acid or organic acid, so that the surface of the transparent film Surface roughening treatment. The method for manufacturing a light emitting device according to claim 5, wherein as the inorganic acid, at least one of sulfuric acid, hydrochloric acid and phosphoric acid is used, and as the organic acid, at least one of malonic acid, acetic acid, citric acid and tartaric acid is used .