JP4583060B2 - Method for manufacturing single crystal sapphire substrate and method for manufacturing nitride semiconductor light emitting device - Google Patents

Description

  The present invention relates to a single crystal sapphire substrate and a nitride semiconductor using the same, and more particularly to a single crystal sapphire substrate suitable for crystal growth for use in an optical device such as a light emitting diode.

  Nitride semiconductors have been put into practical use as light-emitting devices such as LEDs (Light Emitting Diode), and electronic device applications that make use of features excellent in heat resistance and environmental resistance. The nitride semiconductor layer is often grown on a single crystal sapphire substrate, and the main surface of the single crystal sapphire substrate is often mirror-finished. Hereinafter, a single crystal sapphire substrate in which only one main surface is mirror-finished is referred to as a single-sided mirror surface.

  A single crystal sapphire substrate for growing a nitride semiconductor layer is required to have a stable quality in terms of shape, surface state, crystallinity, and the like in order to stabilize the emission wavelength and luminance of the nitride semiconductor layer after film formation. Within this quality, the condition of the back surface of a single-crystal sapphire substrate with a single-sided mirror is the cause of variations in temperature distribution between the substrates during film formation and in the substrate surface, and variations in light extraction efficiency during substrate inspection after film formation. Therefore, stability is required.

FIG. 6 shows an example of a light-emitting element using a nitride semiconductor. As shown in FIG. 6, this light emitting device includes a buffer layer 62 on a single crystal sapphire substrate 61, and a group 3-5 semiconductor multi-layer 63 constituting the light emitting device on the buffer layer 62. The multi-layer 63 includes an n ⁺ layer 64 made of a Si-doped n-type GaN layer provided on the entire surface of the buffer layer 62, an electrode 65 provided on the n ⁺ layer 64, and a portion other than the electrode 65. N layer 66 made of Si-doped Al _0.1 Ga _0.9 N layer, active layer 67 made of silicon-doped GaN layer, p-layer 68 made of magnesium-doped Al _0.1 Ga _0.9 N layer, and and a electrode 70 including the window portion of the SiO ₂ layer 79 and the SiO ₂ layer 69 for covering the.

  In a light-emitting element using a nitride semiconductor, a layer as shown in FIG. 6 is formed on a single crystal sapphire substrate, and then ranking is performed according to the emission wavelength and luminance. Thereafter, the single crystal sapphire substrate 61 is back-grinded from the back side using a grindstone, diamond abrasive grains, etc., and after the single crystal sapphire substrate 61 is thinned, a step of cutting the single crystal sapphire substrate 61 into chips for each element using a scriber. It is common to take.

  However, since the single crystal sapphire substrate 61 is a transparent body, it has light transmittance, and light from the light emitting layer at the time of the above rank classification may leak from the single crystal sapphire substrate 61 side (Patent Literature). 1).

  In order to prevent light from the light emitting layer from leaking from the single crystal sapphire substrate side, it has been proposed to provide irregularities on the back side of the single crystal sapphire substrate after forming the nitride semiconductor layer (Patent Document). 2).

In addition, a measure has been proposed in which the single crystal sapphire substrate is provided with at least one of metallic elements such as titanium, vanadium, chromium, manganese, iron, cobalt, and nickel, thereby providing light absorption for a specific wavelength ( (See Patent Document 3).
Japanese Patent Laid-Open No. 9-237934 JP 2002-368261 A JP 2000-332420 A

  Light emitting devices using nitride semiconductors are ranked according to luminance after forming a light emitting layer on a single crystal sapphire substrate, but the conventional problem is that the luminance decreases when evaluation is performed after deposition of nitride semiconductor light emitting devices. was there. In addition, there is a problem in that the luminance varies when the nitride semiconductor light emitting element is formed.

FIG. 7 is a diagram showing light leakage when a nitride semiconductor light emitting element is formed on a single crystal sapphire substrate 71. Light generated from the nitride semiconductor layer is emitted over the entire surface of the substrate. This also applies to the single crystal sapphire substrate 71 side. If the transmittance of the single crystal sapphire substrate 71 at this time is large, light leakage 74 from the back side of the substrate, that is, from the single crystal sapphire substrate 71 side occurs. The method of forming irregularities on the back surface of the substrate according to Patent Document 2 is intended to improve the luminance of the substrate after the light emitting layer is formed, and at the time of backgrinding after the nitride semiconductor layer is formed on the single crystal sapphire substrate. This defines the surface state. Therefore, when evaluating a substrate after forming a light emitting element using a nitride semiconductor, the state of the conventional single crystal sapphire substrate is reflected as it is, so that it is difficult to determine the ranking. In addition, the generation of stress for the formation of new irregularities cannot be avoided, and there is concern about the risk of affecting the warpage accuracy of the substrate and causing problems in scribing and the like.

  In addition, the method including the metal element of Patent Document 3 eliminates leakage of light generated from a light emitting element using a nitride semiconductor from the single crystal sapphire substrate side, but the single crystal sapphire substrate itself has a light emitting layer. The generated light may be absorbed, leading to a decrease in light emission luminance.

  Accordingly, an object of the present invention is to prevent a decrease in luminance at the time of evaluation after the formation of a nitride semiconductor light emitting device, and to suppress variation in luminance at the time of forming a nitride semiconductor light emitting device. And to provide a nitride semiconductor light emitting device using the same.

  As a result of researches to solve the above problems, the present inventor has a single crystal sapphire substrate that suppresses transmittance and reduces variation, and prevents a decrease in luminance at the time of evaluation after film formation. It has been found that it is possible to suppress variations in luminance when forming a light emitting element.

  The single crystal sapphire substrate of the present invention is characterized by having a transmittance of 10% or less with respect to light in the wavelength region of 200 to 600 nm.

  In addition, the transmittance variation within the substrate surface at each wavelength is 5% or less.

  Moreover, one main surface is a mirror surface, and the arithmetic mean roughness Ra of the other main surface is 0.4 to 1.5 μm.

  Further, the variation in arithmetic average roughness within the substrate surface is 0.4 μm or less.

Furthermore, after forming a buffer layer of Al _X Ga _Y N (X + Y = 1, X ≧ 0, Y ≧ 0) on the main surface of the single crystal sapphire substrate, a nitride semiconductor crystal layer is grown thereon. A nitride semiconductor light emitting device having an element structure in which a semiconductor crystal layer including a light emitting layer is stacked therein.

  Since the single crystal sapphire substrate of the present invention has a transmittance of 10% or less in a wavelength region of 200 to 600 nm, a light emitting layer using a nitride semiconductor is formed on the single crystal sapphire substrate, and then light is emitted from the light emitting layer. Leakage of light to the single crystal sapphire substrate side is reduced, and the luminance of the light emitting element can be stabilized.

  Furthermore, since the variation in the wavelength region of the single crystal sapphire substrate is 5% or less, it is possible to reduce the variation in leakage of light from the light emitting layer using the nitride semiconductor to the single crystal sapphire substrate side. It is possible to reduce the luminance variation of the element.

  Further, the single crystal sapphire substrate of the present invention is single-sided mirror-finished, and a nitride semiconductor layer is formed on the mirror side. On the back surface side, the arithmetic average roughness Ra is 0.4 to 1.5 μm, and the variation of the arithmetic average surface roughness Ra is 0.4 μm or less, so that it is in the wavelength range of 200 to 600 nm. It becomes easy to control the transmittance to 10% or less, and the variation in the transmittance with respect to the wavelength range can be set to 5% or less.

Furthermore, after forming a buffer layer of Al _X Ga _Y N (X + Y = 1, X ≧ 0, Y ≧ 0) using the single crystal sapphire substrate of the present invention, a nitride semiconductor crystal layer is grown thereon. When a device structure in which a semiconductor crystal layer including a light emitting layer is laminated is formed, a nitride semiconductor light emitting device having excellent luminance characteristics can be obtained.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram in which a nitride semiconductor light emitting element is formed on a main surface 12 of a single crystal sapphire substrate 11 obtained by the present invention. One main surface 12 of the single crystal sapphire substrate 11 is mirror-finished, and the other main surface 13 has an arithmetic average surface roughness Ra of 0.4 to 1.5 μm, and an arithmetic average roughness within the substrate surface. The variation in thickness Ra is 0.4 μm or less. Thus, by making the other main surface 13 moderately rough, the single crystal sapphire substrate 11 has a transmittance of 10% or less for light in the wavelength region of 200 to 600 nm, and the transmittance for each wavelength is within the substrate surface. The variation in is less than 5%.

Further, after forming a buffer layer 14 of Al _X Ga _Y N (X + Y = 1, X ≧ 0, Y ≧ 0) on the main surface 12 of the single crystal sapphire substrate 11, a nitride semiconductor crystal layer 15 is formed thereon. And an element structure in which a semiconductor crystal layer including the light emitting layer 16 is stacked.

  Light generated from the light emitting layer 16 is emitted to the entire surface of the substrate. The same applies to the single crystal sapphire substrate 11 side. If the transmittance of the single crystal sapphire substrate 11 at this time is large, light leaks from the other main surface 13 side of the single crystal sapphire substrate. In the single crystal sapphire substrate 11, the transmittance of 10% or less is required in the wavelength region of the transmittance of 200 to 600 nm. When the transmittance is greater than 10%, leakage of light emitted from the light emitting layer 16 from the other main surface 13 side of the single crystal sapphire substrate increases, and evaluation after the light emitting layer 16 is formed and evaluation after actual device formation is performed. This makes a difference and cannot be evaluated properly. In particular, an LED on a single crystal sapphire substrate in a nitride semiconductor is desired to have a low transmittance in the wavelength range from 380 to 450 nm in the ultraviolet to blue region.

  The transmittance of the single crystal sapphire substrate 11 is measured using a spectrophotometer, and light is incident from the main surface 12 side of the single-sided mirror surface of the sapphire substrate, and light is extracted from the other main surface 13 side. It is defined by the ratio of light intensity. By measuring the wavelength of 200 to 600 nm, it is possible to evaluate the variation of the state at the light emitting element wavelength of the red to ultraviolet light region after the single crystal sapphire substrate is formed.

  Further, it is preferable that the transmittance variation in the substrate surface is as small as possible, and it is necessary that the transmittance is 5% or less in the substrate surface, and particularly preferably 3% or less. When this variation is large, particularly when it is greater than 5%, the variation in which light generated from the light emitting layer 16 is transmitted to the other main surface 13 becomes large, resulting in variation in light emitting element luminance. Further, if it is 3% or less, it is possible to further reduce variations in the luminance of the light emitting element when the light emitting layer 16 is formed.

  Here, the transmittance variation in the substrate plane represents the difference between the maximum value and the minimum value when the central portion and the outer peripheral portion of the substrate are measured at two or more points as the variation.

  In addition, one main surface 12 of the single crystal sapphire substrate 11 needs to be mirror-finished to be in a surface state where a nitride semiconductor layer can be formed, but the other main surface 13 is in a lapping processed surface state. It is necessary to suppress the transmission of light.

  If the arithmetic mean roughness Ra of the other principal surface 13 of the single crystal sapphire substrate 11 is smaller than 0.4 μm, light leakage increases and the transmittance exceeds 10%, which is not preferable. When it is larger than 1.5 μm, the roughness of the other main surface 13 affects the mirror surface state of the substrate surface during mirror processing, and the substrate accuracy is significantly deteriorated. Further, regarding the variation of the arithmetic mean roughness Ra in the substrate surface, if it exceeds 0.4 μm, the transmittance of the single crystal sapphire substrate 11 varies, which is not preferable.

  The measurement conditions for the arithmetic average roughness Ra of the single crystal sapphire substrate 11 are a measurement length of 4 mm, a cut-off value of 0.8 mm, and a scan speed of 0.1 mm with a contact-type R2 μm stylus. Further, the variation is represented by the difference between the maximum value and the minimum value when the central portion and the outer peripheral portion of the substrate are measured at two or more points, similarly to the transmittance.

  A nitride semiconductor light emitting device can be formed on the single crystal sapphire substrate 11 as described above.

  FIG. 2 shows a blue LED structure formed using the single crystal sapphire substrate 11. First, the single crystal sapphire substrate 11 according to the present invention was mounted on a MOVPE apparatus, and heated to 1100 ° C. in a nitrogen gas main component atmosphere to perform thermal cleaning. Thereafter, the temperature was lowered to 500 ° C., trimethylgallium (hereinafter referred to as TMG) was flown as a Group 3 raw material of the periodic table, and ammonia was flowed as an N raw material to grow an AlGaN low temperature buffer layer 21 having a thickness of 30 nm. Subsequently, the temperature was raised to 1000 ° C., and TMG and ammonia were flown as raw materials, and silane was flown as a dopant to grow the n-type GaN contact layer 22. Subsequently, an n-type AlGaN cladding layer 24, a GaN-based semiconductor layer 25, a p-type AlGaN cladding layer 26, and a p-type GaN contact layer 27 are formed in this order, and an etching process for exposing the n-type contact layer, n Electrode 23 and p-type electrode 28 were formed.

  The luminance of the nitride semiconductor light emitting device on the sapphire substrate 11 after the above formation is not reduced because the light leakage from the back surface of the substrate is small. Thereafter, each of the blue LEDs after the back surface is ground and the device is separated. There is no variation in luminance between elements.

  Here, the manufacturing method of the single crystal sapphire substrate 11 of the present invention will be described.

The single crystal sapphire substrate 11 is cut into a desired shape from an ingot or a flat single crystal sapphire, and then cut using a band saw, a wire saw, or the like so as to have a predetermined thickness. Thereafter, a lapping process for defining the roughness of the back surface of the substrate is performed.

FIG. 3 is a diagram schematically showing a double-sided lapping process of the single crystal sapphire substrate 11 of the present invention. Wrapping while letting. Regarding the loose abrasive grains 31, since the sapphire substrate has a high hardness, it is necessary to use a material having a hardness as high as possible. Specifically, at least one of SiC, B ₄ C, glass beads, alumina, and diamond abrasive grains is used. Since the arithmetic average roughness Ra of the surface obtained by lapping is defined by the particle size of the free abrasive grains, the arithmetic average roughness Ra of the single-sided mirror single crystal sapphire substrate in the present invention is 0.4 to 1.5 μm, In order to obtain an arithmetic average roughness Ra variation of 0.4 μm or less in the substrate surface, it is necessary to use abrasive grains having an average particle diameter of 1 to 80 μm, and abrasive grains having an abrasive grain diameter variation of 30 μm or less. Must be used. When the abrasive grain size varies, the arithmetic average roughness Ra of the surface obtained by the double-sided lapping process, that is, the back surface of the single-sided mirror single crystal sapphire substrate, varies.

  After carrying out the above double-sided lapping processing, after removing abrasive grains, processing chips, surface plate components, etc. adhering to the substrate surface by washing, heat treatment is carried out for the purpose of removing processing strain. This heat treatment can be performed in any atmosphere of oxidation, reduction, and vacuum, but depending on each treatment atmosphere, a difference occurs in the surface state of the back surface, which affects the transmittance. Even under the same heat treatment conditions, a difference occurs in the back surface state of the substrate due to the atmospheric conditions of the heat treatment, which affects the transmittance. Further, even under the same atmosphere, a difference occurs in the surface state depending on the processing conditions, and similarly, a difference occurs in the transmittance. For this reason, in order to reduce the transmittance and reduce variations, it is necessary to appropriately set the heat treatment atmosphere and conditions after lapping.

  After the above heat treatment step, one main surface is mirror-finished using an abrasive such as colloidal silica and precision cleaning is performed for the purpose of removing contamination on the surface. A sapphire substrate 11 can be obtained.

  As an example of the present invention, a single-crystal sapphire substrate having a diameter of 2 inches and a thickness of 0.430 mm and having a single-sided mirror surface and having a transmittance of 200 to 600 nm is 6% ± 0.5%. Was prepared as follows.

First, a single crystal sapphire substrate cut into a predetermined plane orientation and trimmed in outer shape was subjected to double-sided lapping with free abrasive grains. At this time, B ₄ C abrasive grains having an average particle diameter of 65 μm were used as the abrasive grains, and processing of 60 μm was performed.

  After the lapping process, the single crystal sapphire substrate was washed with an alkaline detergent / pure water and heat-treated in a vacuum furnace. Thereafter, the main surface was subjected to rough polishing using a diamond or the like, and then subjected to mechanical chemical polishing using colloidal silica.

  The arithmetic average roughness Ra of the other principal surface 13 of the single-sided mirror single-crystal sapphire substrate 11 thus obtained is within a range of Ra 0.8 ± 0.15 μm between the substrates and within the substrate surface. The rate was around 6% at 200 to 600 nm, and the in-plane variation was ± 0.5% or less.

  A single crystal sapphire substrate 11 obtained by this processing method and a single crystal sapphire substrate 71 having a transmittance of 15% and an in-plane variation of transmittance of 12%, which are produced by another method as a comparative example, are used. The general LED structure shown in 2 was formed, and the luminance was evaluated.

  As shown in FIG. 4A, the luminance distribution of the LEDs on the single crystal sapphire substrate 11 of the inventive example, the average luminance was 48.3 mcd, and the in-plane variation was within 3%. On the other hand, as shown in FIG. 4B, the luminance distribution of the LED formed on the single crystal sapphire substrate 71 of the comparative example is 30.3 mcd on average, and the result is that the in-plane variation exceeds 15%. became. Next, the single crystal sapphire substrates 11 and 71 on which the light emitting layer was formed were back-ground, formed into chips, and evaluation of luminance and luminance variation was performed again.

  As shown in FIG. 5 (a), the luminance distribution of the LEDs on the sapphire substrate 11 of the embodiment of the present invention has an average luminance of 45.4 mcd, and the in-plane variation is within 3%. On the other hand, FIG. 5B shows an average so as to indicate the luminance of the LED prepared on the single crystal sapphire substrate 71 having a transmittance of 15% and an in-plane variation of the transmittance of 12%, which is created by another method of the comparative example. The luminance was 40.1 mcd, and the in-plane variation was more than 15%.

  As described above, by using the single crystal sapphire substrate of the present invention, it is possible to prevent a decrease in luminance when determining luminance after film formation, and to reduce variation in luminance when forming a nitride semiconductor light emitting layer.

  A single crystal sapphire substrate suitable for a nitride semiconductor light emitting device can be provided.

It is sectional drawing which shows the nitride semiconductor element using the single crystal sapphire substrate of this invention. It is sectional drawing which shows the structure of the blue LED element which is an example of the nitride semiconductor light-emitting element of this invention. It is a schematic diagram which shows the double-sided lapping process of a single crystal sapphire substrate. (A) (b) is a figure which shows the brightness | luminance when a nitride semiconductor light-emitting device is formed in a single crystal sapphire substrate. (A) and (b) are the figures which show the brightness | luminance after forming the nitride semiconductor light-emitting device in a single crystal sapphire substrate, and making it into a device. It is sectional drawing which shows the structure of the conventional nitride semiconductor light-emitting device. It is sectional drawing which shows the light leakage of the nitride semiconductor light-emitting device formed on the single crystal sapphire substrate.

Explanation of symbols

11: single crystal sapphire substrate 12: one main surface 13: the other main surface 14: buffer layer 15: nitride semiconductor crystal layer 16: light emitting layer 21: low temperature buffer layer 22: n-type GaN contact layer 23: n-type electrode 24: n-type AlGaN cladding layer 25: GaN-based semiconductor layer 26: p-type AlGaN cladding layer 27: p-type GaN contact layer 28: p-type electrode 31: loose abrasive 32: lapping platen 61: single crystal sapphire substrate 62: Low temperature buffer layer 63: Multiple layer 64: n-type GaN layer 65: n-type electrode 66: Si-doped Al _0.1 Ga _0.9 Nn layer 67: active layer 68: magnesium-doped Al _0.1 Ga _0.9 Np layer 69: SiO ₂ layer 70: p-type electrode 71: single crystal sapphire substrate 72: low-temperature buffer layer 73: nitride semiconductor crystal layer 74: light leakage

Claims (3)

The arithmetic average roughness Ra of one main surface is 0.4 to 1.5 μm, the in-plane variation of the arithmetic average roughness of the one main surface is 0.4 μm or less, and a wavelength region of 200 to 600 nm A method for producing a single crystal sapphire substrate having a light transmittance of 10% or less,
A lapping step of lapping both main surfaces of the single crystal sapphire substrate using abrasive grains;
A removal step of removing abrasive grains from the two main surfaces of the single crystal sapphire substrate;
A heat treatment step of heat treating the single crystal sapphire substrate;
A polishing step of mirror-polishing the other main surface of the single crystal sapphire substrate;
Have
The method for producing a single crystal sapphire substrate, wherein the average grain size of the abrasive grains is 1 to 80 μm, and the variation of the average grain diameter of the abrasive grains is 30 μm or less. The abrasive grains are SiC or B ₄ The method for producing a single crystal sapphire substrate according to claim 1, which is C. A method for manufacturing a nitride semiconductor light emitting device, comprising:
The arithmetic average roughness Ra of one main surface is 0.4 to 1.5 μm, the in-plane variation of the arithmetic average roughness of the one main surface is 0.4 μm or less, and a wavelength region of 200 to 600 nm A substrate manufacturing process for manufacturing a single crystal sapphire substrate having a light transmittance of 10% or less;
A buffer layer forming step of forming an AlGaN buffer layer on the other main surface of the single crystal sapphire substrate;
A crystal layer forming step of forming a nitride semiconductor crystal layer on the buffer layer;
Have
The substrate manufacturing process includes
A lapping step of lapping both main surfaces of the single crystal sapphire substrate using abrasive grains;
A removal step of removing abrasive grains from the two main surfaces of the single crystal sapphire substrate;
A heat treatment step of heat treating the single crystal sapphire substrate;
A polishing step of mirror-polishing the other main surface of the single crystal sapphire substrate;
The average particle size of the abrasive grains is 1 to 80 μm, and the variation of the average particle size of the abrasive grains is 30 μm or less.

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