JP2005197573A - Group iii nitride semiconductor light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a group III nitride semiconductor light emitting element which is high in light output efficiency and reliability. <P>SOLUTION: The light emitting element of group III nitride semiconductor comprises a sapphire substrate 101, and a group III nitride semiconductor 110 formed on the substrate 101. The group III nitride semiconductor 120 has a GaN buffer layer 102 and an n-type GaN layer 103 as group III nitride semiconductor layers formed on a first main surface 101a, and an MQW active layer 104 formed thereon. Holes 101h are formed in the substrate 101 to extend from a second main surface 101b to the first main surface 101a, and reach the GaN buffer layer 102 and the n-type GaN layer 103. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a group III nitride semiconductor light-emitting device, and more specifically, in a nitride-based group III-V semiconductor and device, particularly a group III nitride semiconductor, the group III nitride semiconductor is grown on a sapphire substrate. The present invention relates to a semiconductor light emitting device.

Conventionally, sapphire substrates have been mainly used in group III nitride semiconductor light emitting devices, which have been commercialized. For example, Japanese Unexamined Patent Application Publication No. 2003-92426 discloses a light emitting element. FIG. 11 is a cross-sectional view of a conventional semiconductor light emitting device. Referring to FIG. 11, in the conventional light emitting device, a light emitting device structure including an n-type GaN layer 52, a light emitting region 53, and a p type semiconductor layer 54 is formed on a substrate 51. When manufacturing the light emitting element, the n-type GaN layer 52 is made uneven, thereby providing a stepped portion in the light emitting area. As a result, by increasing the light emitting area per unit area of the entire light emitting element, The light output is improved.
JP 2003-92426 A

  However, laminating the light emitting region after making the n-type GaN layer uneven has to expose the unevenness to the atmosphere once when the light emitting element is stacked. As a result, failure to manage the contamination of the substrate surface will affect the quality of the growth of the light emitting region that will be stacked later.In addition, stacking the light emitting region on the uneven surface will cause the light emitting region to be flat on the conventional flat surface. There was a problem that it was more difficult to stack. Therefore, the present invention has been made to solve the above-described problems, and an object thereof is to provide a high-quality group III nitride semiconductor light-emitting device.

  A group III nitride semiconductor light emitting device according to the present invention is formed on a first main surface, a substrate having a second main surface different from the first main surface, a first main surface, And a group III nitride semiconductor not formed on the main surface. The group III nitride semiconductor includes a group III nitride semiconductor layer formed on the first main surface and a light emitting layer formed on the group III nitride semiconductor layer. The substrate has a hole extending from the second main surface to the first main surface and reaching the group III nitride semiconductor layer.

  In the group III nitride semiconductor light-emitting device configured as described above, the substrate has a hole extending from the second main surface to the first main surface and reaching the group III nitride semiconductor layer. By virtue of the action of the holes, it is possible to provide a group III nitride semiconductor light-emitting device with high light extraction efficiency and high reliability.

  Preferably, the substrate includes a sapphire substrate.

  Preferably, the hole in the substrate is formed by processing the substrate using a solid-state laser.

  Preferably, the substrate transmits light emitted from the light emitting layer.

  Preferably, the group III nitride semiconductor light emitting device further includes a dielectric film in contact with the second main surface.

  Preferably, the dielectric film is a conductive glass.

  Preferably, the dielectric film is excited by light emitted from the light emitting layer and emits light having a wavelength different from that of the light emitted from the light emitting layer.

  Preferably, the group III nitride semiconductor light emitting device further includes a phosphor contained in the dielectric film.

  Preferably, the dielectric film is divided into a plurality of regions, and the group III nitride semiconductor light-emitting device further includes an electrode provided in each region.

  Preferably, a different phosphor is added to each region.

  Preferably, the dielectric film includes first to third divided regions, and light emitted from the first to third regions is mixed to generate white light.

  Preferably, irregularities are formed in the dielectric film.

  Preferably, irregularities are formed on the second main surface.

  The method of manufacturing a group III nitride semiconductor light emitting device according to the present invention improves external light extraction, and makes it possible to obtain one or more emission wavelengths that are excited and emitted by light of the light emitting layer. To do.

  Conventional group III nitride semiconductor light-emitting devices are mainly commercialized using sapphire substrates. However, since the sapphire substrate has high hardness and is insulative, it is difficult to process in the past, so the external light extraction efficiency is improved by processing a part of the group III nitride semiconductor grown on the sapphire substrate. There have been attempts to do so. Therefore, when a group III nitride semiconductor light emitting device is fabricated on a sapphire substrate, the transparent dielectric film formed so as to be in contact with the back surface and the back surface of the substrate is uneven and the n-type GaN layer is formed with a solid laser on the back surface of the substrate. It has become possible to manufacture a gallium nitride-based compound semiconductor light-emitting device capable of improving the efficiency of extraction to the outside by making holes to such an extent.

  Embodiments of the present invention will be described below with reference to the drawings. In the following embodiments, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

(Embodiment 1)
1 is a cross-sectional view of a group III nitride semiconductor light-emitting device according to the first embodiment of the present invention. Referring to FIG. 1, group III nitride semiconductor light-emitting device 1 according to the first embodiment of the present invention includes a first main surface 101a and a second main surface 101b different from first main surface 101a. And a group III nitride semiconductor 120 formed on the first main surface 101a and not formed on the second main surface 101b. The group III nitride semiconductor 120 includes a GaN buffer layer 102 and an n-type GaN layer 103 as a group III nitride semiconductor layer formed on the first main surface 101a, and a light emitting layer formed on the semiconductor layer. In the sapphire substrate 101, a hole 101h extending from the second main surface 101b to the first main surface 101a and reaching the GaN buffer layer 102 and the n-type GaN layer 103 is formed. Yes.

  The hole 101h is formed by processing the sapphire substrate 101 using a solid-state laser.

  The sapphire substrate 101 transmits the light emitted from the MQW active layer 104.

On a sapphire substrate 101 having a thickness of 430 μm, a GaN buffer layer 102, an n-type GaN layer 103, four pairs of MQW active layers 104 made of In _0.08 Ga _0.92 N and GaN, a clad layer 105 made of p-type AlGaN, and p-type GaN A group III nitride semiconductor 120 composed of the contact layer 106 made of is laminated. On this gallium nitride based semiconductor light emitting device, a p-type contact electrode, a Pd translucent electrode 107 as a translucent electrode, and an n-type pad electrode 108 using Al and Ti as n-type contact electrodes are provided. Is formed.

  A hole 101 h is formed in the second main surface 101 b of the sapphire substrate 101, and the hole 101 h reaches the GaN buffer layer 102 and the n-type GaN layer 103. The hole 101h only needs to reach the GaN buffer layer 102 and does not have to reach the n-type GaN layer 103. On the second main surface 101b, an uneven portion 110 is formed in a region different from the hole 101h.

  FIG. 2 is a plan view of the group III nitride semiconductor light-emitting device viewed from the direction indicated by arrow II in FIG. Referring to FIG. 2, the outermost periphery of sapphire substrate 101 is constituted by unprocessed second main surface 101b, and rectangular n-type electrode 109 is provided on the inner side thereof. The n-type electrode 109 is embedded in the hole 101h.

  In a region surrounded by the n-type electrode 109, a comb-shaped uneven portion 110 is formed. The uneven portions 110 extend in one direction and are positioned parallel to each other.

Next, a method for manufacturing the group III nitride semiconductor light emitting device shown in FIGS. 1 and 2 will be described. 3 and 4 are cross-sectional views for explaining a method of manufacturing the group III nitride semiconductor light-emitting device shown in FIG. Referring to FIG. 3, group III nitride semiconductor 120 is stacked on sapphire substrate 101 using MOCVD (metal organic chemical vapor deposition). Specifically, after the sapphire substrate 101 was mounted on a susceptor in the reaction chamber, baking was performed in a mixed atmosphere of NH ₃ , N _2, and H ₂ at a temperature of 1200 ° C. Thereafter, the temperature is quickly lowered and the GaN buffer layer 102 is laminated to a thickness of 30 nm using H ₂ as a carrier gas at a temperature of 450 ° C. and using trimethyl gallium (TMG) and ammonia (NH ₃ ). Thereafter, the temperature was raised quickly and at a temperature of 1100 ° C., using TMG, NH ₃ , and monosilane (SiH ₄ ) as a dopant, the n-type GaN layer 103 having n-type conductivity was 5 μm thick. The layers were laminated in order.

Thereafter, at a temperature of 750 ° C., four pairs of MQW active layers 104 in which the well layer and the barrier layer are composed of In _0.08 Ga _0.92 N and GaN are stacked using trimethylindium (TMI), TMG, and NH ₃ . Each In _0.08 Ga _0.92 N had a thickness of 3 nm and GaN had a thickness of 9 nm. Next, at a temperature of 1100 ° C., a clad layer 105 made of Mg-doped p-type Al _0.08 Ga _0.92 N using trimethylaluminum (TMA), TMG, NH ₃ , and cyclopentadienylmagnesium (Cp ₂ Mg) as a dopant. Were laminated so as to have a thickness of 30 nm. Finally, using TMG, NH ₃ and Cp ₂ Mg at the same temperature, the contact layer 106 made of Mg-doped p-type GaN is stacked in order so that the thickness becomes 120 nm, and after cooling to room temperature, it is taken out to the atmosphere did.

Thereafter, in order to perform p-type activation, heat treatment was performed in a N ₂ atmosphere at a temperature of 800 ° C. for 15 minutes using a heat treatment furnace. Thereafter, a resist was applied to the surface of the light emitting element, and photolithography was performed. Then, a translucent electrode 107 made of Pd corresponding to the translucent electrode was formed on the p-type GaN layer with a p-type contact electrode. Then, after removing the resist using organic cleaning, the resist was applied again and photolithography was performed, and then a pad electrode 108 made of Au was formed.

Referring to FIG. 4, sapphire substrate 101 is attached to a sheet, fixed to a jig, and second main surface 101b of sapphire substrate 101 is processed using a solid-state laser. As the laser, YAG (synthetic garnet of yttrium and aluminum oxide) was used as the light source, and the wavelength was a harmonic of about 266 nm or 355 nm. An energy of 10 μJ / cm ^{2 to} 100 mJ / cm ² was used in a pulsed or continuous state. A hole 101 h was formed in the portion where the n-type electrode was to be formed so as to reach the n-type GaN layer 103 by laser. At the same time, the concavo-convex portion 110 was processed on the sapphire substrate 101. The uneven portion 110 is formed only on the sapphire substrate 101 in FIG. 4, but the uneven portion 110 may reach the buffer layer 102. Furthermore, a groove was formed on the sapphire substrate 101 so as to divide the chip when the uneven portion 110 was produced (the groove is not shown).

Referring to FIG. 1, after removing sapphire substrate 101 from a sheet, performing organic cleaning, applying a photoresist, and performing photolithography, an n-type electrode 109 made of Ti and Al is used as an n-type contact electrode. Formed. Thereafter, a resist was applied again, and SiO ₂ covered the light emitting element and the electrode surface by photolithography (SiO ₂ not shown).

  Thereafter, the sapphire substrate 101 and the group III nitride semiconductor 120 formed on the sapphire substrate 101 were divided using a breaking device in accordance with the groove manufactured in the previous step. As a result, the size of the chip was 350 μm × 350 μm in length × width, and the group III nitride semiconductor light emitting device 1 shown in FIG. 1 could be formed.

  Next, the light output of the group III nitride semiconductor light emitting device according to the above-described embodiment was measured. First, on a sapphire substrate polished and ground to a thickness of about 100 μm by a conventional method, chips were formed by scribing and breaking to form a semiconductor light emitting device. In this conventional light emitting device, when the amount of current (If) injected in the forward direction is 20 mA, the emission wavelength λp is 460 nm, the operating voltage (Vf) is 3.8 V, and the optical output (Po) is 3.0 mW. It was.

  The light-emitting element manufactured in Embodiment 1 had Vf of 3.2 V and Po of 6.0 nW when If was 20 mA. This shows that the group III nitride semiconductor light emitting device according to the present invention has an improved light output. This is because the product of the present invention and the conventional product are compared, and the thick portion of the sapphire substrate 101 is provided with the concavo-convex portion 110 and the hole 101h, and further, the electrodes can be formed on both surfaces of the sapphire substrate. Due to the concavo-convex portion 110 and the hole 101h on the back surface of the sapphire substrate, the light emitted from the light-emitting layer is increased from the side surface of the sapphire substrate 101, and the light extracted from the transparent electrode surface is not blocked by the electrode. Since it was extracted, the light extraction to the outside was improved. As a result, the light output is considered to have increased. Furthermore, if the sapphire substrate is simply thick, the light emitted from the light-emitting layer reflected by the first main surface 101b returns toward the Pd translucent electrode 107, so that light is transmitted by the Pd translucent electrode 107. Loss of light taken out by the sapphire substrate 101 was formed by forming a concavo-convex portion 110 on the sapphire substrate 101 and thinning a part of the substrate. Was able to be reduced.

  In this embodiment, the YAG laser is used for processing. However, the present invention is not limited to the YAG laser, and a laser having a wavelength of about 266 to 355 nm and capable of scribing the sapphire substrate 101 using sublimation or thermal expansion may be used. .

  In this embodiment, the step of forming the concavo-convex portion 110 by processing the sapphire substrate 101 with a laser is performed after the formation of the p-type electrode. However, the group III nitride semiconductor 120 is grown on the sapphire substrate 101. After the p-type annealing, the concavo-convex portion 110 may be formed on the sapphire substrate 101 by a laser.

(Embodiment 2)
FIG. 5 is a cross-sectional view of a group III nitride semiconductor light emitting device according to the second embodiment of the present invention. Group III nitride semiconductor light-emitting device 1 according to the second embodiment of the present invention differs from group III nitride semiconductor light-emitting device 1 according to the first embodiment in that it has a transparent dielectric film 209. The sapphire substrate 201 to the pad electrode 208 in FIG. 5 correspond to the sapphire substrate 101 to the pad electrode 108 of the first embodiment. The group III nitride semiconductor light emitting device 1 includes a transparent dielectric film 209, an uneven portion 210 formed in the transparent dielectric film 209, a hole 201 h and an n-type electrode 211. Further, the thickness of the sapphire substrate 201 of this embodiment is 430 μm.

  Next, a method for manufacturing a group III nitride semiconductor light emitting device according to the second embodiment will be described. FIG. 6 is a cross-sectional view for explaining a method of manufacturing the group III nitride semiconductor light-emitting device shown in FIG. Referring to FIG. 6, GaN buffer layer 202 to Au pad electrode 208 are formed on sapphire substrate 201. This forming method is the same as that of the first embodiment.

A transparent dielectric film 209 made of ITO (conductive glass) was formed on the second main surface 201b so as to have a thickness of 5 μm by sputtering. Thereafter, the film was taken out to the atmosphere, a resist was applied to the transparent dielectric film 209, and after performing photolithography, the transparent dielectric film 209 was etched with FeCl ₃ to form a groove. Thereafter, the concave and convex portion 210 was formed by processing the transparent dielectric film 209 using a laser based on the groove. At the same time, the hole 201h was formed by processing the transparent dielectric film 209 and the sapphire substrate 201. The hole 201 h reaches the n-type GaN layer 203. Further, a groove (not shown) capable of dividing the chip was formed on the sapphire substrate 201.

  Thereafter, an n-type electrode 211 was formed, and the sapphire substrate 201 was divided to form a group III nitride semiconductor light emitting device 1 as shown in FIG.

  The characteristics of the light-emitting element manufactured in this embodiment were such that If was 20 mA, the emission wavelength (λp) was 460 nm, and Po was 5.0 mW, and the same effects as those of Embodiment 1 were obtained. Furthermore, since the dielectric film was formed on the second main surface 201b of the sapphire substrate 201 and the concavo-convex portion 210 was formed thereon, the concavo-convex portion could be easily formed.

  In the first and second embodiments, a sapphire substrate is used as the substrate. However, a spinel substrate or SiC substrate that is transparent and transmits light from the light emitting layer may be used.

In the second embodiment, ITO is formed on the second main surface 201b. However, TiO ₂ and SiN having conductivity by containing impurities may be formed.

(Embodiment 3)
FIG. 7 is a sectional view of a group III nitride semiconductor light-emitting device according to the third embodiment of the present invention. Referring to FIG. 7, sapphire substrate 301 to Au pad electrode 308 are the same as sapphire substrate 101 to Au pad electrode 108 of the first embodiment. N-type transparent dielectric films 309 and 310 are formed on second main surface 301b. The thickness of the sapphire substrate 301 is 430 μm. The sapphire substrate 301 is formed with a plurality of holes 301h as irregularities extending from the second main surface 301b toward the first main surface 301a.

  Next, a method for manufacturing the group III nitride semiconductor light emitting device shown in FIG. 7 will be described. FIG. 8 is a cross-sectional view for explaining the method for manufacturing the group III nitride semiconductor light-emitting device shown in FIG. Referring to FIG. 8, up to Au pad electrode 308 is formed on sapphire substrate 301 as in the first embodiment.

  Next, n-type transparent dielectric films 309 and 310 made of ITO, to which phosphors that are excited by light emission from the MQW active layer 304 and emit light of different wavelengths, are added to the second main surface 301b. Produced.

  The sapphire substrate 301 was processed using a laser in the same manner as in the first embodiment. As a result, a hole 301 h reaching the n-type GaN layer 303 was formed in the sapphire substrate 301 and the GaN buffer layer 302.

  Further, a groove (not shown) for dividing the sapphire substrate 301 was formed.

  Then, after applying a resist on the sapphire substrate 301 and performing photolithography, ITO as an n-type transparent dielectric film was formed by sputtering so as to have a thickness of 5 μm. The ITO is added with a phosphor that is excited by light from the MQW active layer 304 and emits different light. Thereafter, a resist was applied again, and after photolithography, ITO was formed as a film-like conductor so that the thickness of the n-type transparent dielectric film 310 was 5 μm. The ITO is added with a phosphor that is excited by light from the MQW active layer 304 and emits light of different wavelengths.

  Thereafter, after applying a resist on the transparent dielectric film and performing photolithography, a part of the transparent dielectric film was etched with FeCl 3 to expose the sapphire substrate 301. Here, as the phosphor added with the n-type transparent dielectric films 309 and 310, those emitting different wavelengths by the light emission of the MQW active layer 304 were used. In addition, the n-type transparent dielectric films 309 and 310 are not electrically connected, and emit light partially separately. Thereafter, the sapphire substrate 301 was divided along the groove formed earlier using a braking device. As a result, a group III nitride semiconductor light-emitting device 1 in the form of chips having dimensions of length × width of 350 μm × 350 μm could be formed. Further, bonding was performed so that the n-type transparent dielectric films 309 and 310 were electrically operated separately to emit light.

  In this embodiment, the same effect as in the first and second embodiments was obtained. In the light emitting device according to the embodiment of the present invention, the emission wavelength (λp) of the light emitting layer is 460 nm, and an n-type transparent dielectric that is colored red by a phosphor using light having this wavelength. By simultaneously emitting light from the film 309 and the n-type transparent dielectric film 310 emitted in green by the phosphor, white light is obtained, and by emitting separately, blue, red, green monochromatic, or Light emission using one or more of these three colors of light is possible. As the phosphor, La2O2S: Eu3 +, Y2O2S: Eu as red, and ZnS: Cu, Al, ((Ba, Mg) Al10O17: EuMn) as green may be used.

(Embodiment 4)
9 and 10 are plan views of the group III nitride semiconductor device according to the fourth embodiment of the present invention. Note that the semiconductor device shown in FIGS. 9 and 10 corresponds to the plan view shown in FIG. Referring to FIG. 9, in Embodiment 4, the wavelength of light emitted from the light emitting layer is 385 nm to 410 nm, and an n-type doped with a phosphor that is excited by this light and emits light of different wavelengths is added. Transparent dielectric films 404 to 406 were formed. A contact region 407 between the n-type GaN and the dielectric is provided. The contact region 407 was formed by providing the holes shown in the first to third embodiments.

  N-type transparent dielectric films 404, 405, and 406 made of ITO are manufactured, and, for example, a sapphire substrate 301 and a GaN are formed so that a portion reaching the n-type GaN layer is formed by the laser of the first embodiment. A groove was formed in the buffer layer 302, and transparent dielectric films 404, 405, and 406 were formed in this portion to form a contact region 407. As a result, the same effect as in the third embodiment can be obtained, and white light composed of red, blue, green, and these wavelengths can be produced by separately electrically connecting them. In this case, since the three light-emitting colors have an intersecting electrode structure, color unevenness is reduced when wavelengths extracted outside are mixed.

  Referring to FIG. 10, n-type transparent dielectric films 408, 409, and 410 made of ITO may be formed linearly as shown in FIG. In this case, a plurality of holes 301 h are formed in the sapphire substrate 301, and the inside of the holes 301 h is a contact region 411 between the n-type GaN layer and the transparent dielectric. Such a group III nitride semiconductor light emitting device according to the fourth embodiment has the same effects as the group III nitride semiconductor light emitting device according to the first, second and third embodiments.

It is sectional drawing of the group III nitride semiconductor light-emitting device according to Embodiment 1 of this invention. FIG. 2 is a plan view of a group III nitride semiconductor light emitting device viewed from a direction indicated by an arrow II in FIG. 1. FIG. 3 is a cross-sectional view for explaining a first step in the method for manufacturing the group III nitride semiconductor light-emitting device shown in FIG. 1. It is sectional drawing which shows the 2nd process of the manufacturing method of the group III nitride semiconductor light-emitting device shown in FIG. It is sectional drawing of the group III nitride semiconductor light-emitting device according to Embodiment 2 of this invention. It is sectional drawing for demonstrating the manufacturing method of the group III nitride semiconductor light-emitting device shown in FIG. It is sectional drawing of the group III nitride semiconductor light-emitting device according to Embodiment 3 of this invention. FIG. 8 is a cross-sectional view for illustrating the method for manufacturing the group III nitride semiconductor light-emitting device shown in FIG. It is a top view of the group III nitride semiconductor light-emitting device according to Embodiment 4 of this invention. It is a top view of the group III nitride semiconductor light-emitting device according to Embodiment 4 of this invention. It is sectional drawing of the conventional semiconductor light-emitting device.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Group III nitride semiconductor light-emitting device, 101 sapphire substrate, 101a 1st main surface, 101b 2nd main surface, 102 GaN buffer layer, 103 n-type GaN layer, 104 MQW active layer, 105 clad layer, 106 contact layer , 107 Pd translucent electrode, 108 pad type electrode, 109 n type electrode, 110 concavo-convex part, 101h hole.

Claims (13)

A substrate having a first main surface and a second main surface different from the first main surface;
A group III nitride semiconductor device comprising a group III nitride semiconductor formed on the first main surface and not formed on the second main surface,
The group III nitride semiconductor includes a group III nitride semiconductor layer formed on the first main surface, and a light emitting layer formed on the group III nitride semiconductor layer,
A group III nitride semiconductor light emitting device, wherein a hole extending from the second main surface to the first main surface and reaching the group III nitride semiconductor layer is formed in the substrate.   The group III nitride semiconductor light-emitting device according to claim 1, wherein the substrate includes a sapphire substrate.   The group III nitride semiconductor light-emitting device according to claim 1, wherein the hole of the substrate is formed by processing the substrate using a solid-state laser.   The group III nitride semiconductor light emitting device according to claim 3, wherein the substrate transmits light emitted from the light emitting layer.   The group III nitride semiconductor light-emitting device according to claim 3, further comprising a dielectric film in contact with the second main surface.   The group III nitride semiconductor light-emitting device according to claim 5, wherein the dielectric film is made of conductive glass.   The group III nitride semiconductor light-emitting element according to claim 5, wherein the dielectric film is excited by light emitted from the light emitting layer and emits light having a wavelength different from that of light emitted from the light emitting layer.   The group III nitride semiconductor light-emitting device according to claim 5, further comprising a phosphor contained in the dielectric film.   9. The group III nitride semiconductor light-emitting device according to claim 5, wherein the dielectric film is divided into a plurality of regions, and further includes an electrode provided in each region. 10.   The group III nitride semiconductor light-emitting device according to any one of claims 5 to 9, wherein a different light emitter is added to each of the regions.   11. The dielectric film according to claim 5, wherein the dielectric film includes first to third divided regions, and light emitted from the first to third regions is mixed to generate white light. 3. A group III nitride semiconductor light-emitting device according to item 1.   The group III nitride semiconductor light-emitting device according to claim 5, wherein the dielectric film is uneven.   The group III nitride semiconductor light-emitting device according to any one of claims 5 to 12, wherein the second main surface has irregularities.

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