JP2008297189A - Method for producing nitride single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a nitride single crystal such as a GaN single crystal having excellent crystallinity without using a seed crystal substrate in which an expensive GaN thin film is deposited. <P>SOLUTION: In this method, the nitride single crystal is produced by growing a nitride single crystal 8 expressed by chemical formula XN (wherein, X is at least one kind selected from Ga, Al and In) on the surface of a seed crystal substrate 4 by a flux method. In the seed crystal substrate 4, the growth surface used for growing the nitride single crystal 8 is terminated with nitrogen. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a nitride single crystal by growing a nitride single crystal such as GaN or AlGaN using a seed crystal substrate, and in particular, a light emitting element such as a light emitting diode (LED) or a semiconductor laser (LD), or a transistor. The present invention relates to a method for producing a nitride single crystal applied to electronic devices such as power devices.

A nitride single crystal (nitride semiconductor) such as GaN or AlGaN has a high melting point and a high equilibrium vapor pressure of N, so that it is difficult to produce a bulk type single crystal. Therefore, a thin film made of nitride single crystal is grown on a dissimilar material substrate (substrate made of a material different from the growing single crystal) such as sapphire (Al ₂ O ₃ ) or silicon carbide (SiC), and the thin film Is used for various devices.

  However, a substrate such as sapphire has a large lattice constant difference from a thin film such as GaN or AlGaN grown on the substrate. For example, since sapphire has a lattice mismatch of 13.7% with GaN, many dislocations are generated in the grown thin film. In particular, high brightness is required for LEDs and high power is required for electronic devices, so it is necessary to reduce dislocations in crystals. In addition, in order for these devices to become general-purpose products, it is essential to reduce the cost of the substrate.

Thus, recently, a flux growth method of GaN single crystal using sodium has been proposed (see, for example, Patent Documents 1 and 2). In this growth method, a seed crystal obtained by growing a GaN layer on a sapphire substrate by the MOVPE method or the like is used (for example, see Patent Document 3).
JP 2002-128586 A International Publication No. 2004/013385 Pamphlet JP 2000-327495 A

  However, the actually produced GaN single crystal uses a seed crystal obtained by growing a GaN layer on a sapphire substrate by a MOVPE method or the like, so that it is higher than a GaN single crystal grown by a MOVPE method or the like. It is cost. This is because the surface of the sapphire substrate is terminated with oxygen having a low affinity for Ga, so that the GaN single crystal does not grow directly on the sapphire substrate by the flux growth method. Therefore, in the flux growth method, a seed crystal having a GaN thin film formed on its surface is required.

  Accordingly, the present invention has been completed in view of the above-mentioned conventional problems, and its purpose is to use a GaN single crystal having excellent crystallinity without using a seed crystal substrate on which an expensive GaN thin film is deposited. It is to obtain a single crystal of nitride.

  In the method for producing a nitride single crystal of the present invention, a nitride single crystal represented by the chemical formula XN (where X is at least one selected from Ga, Al and In) is fluxed on the surface of the seed crystal substrate. A method for producing a nitride single crystal grown by a method, wherein the seed crystal substrate has a growth surface for growing the nitride single crystal terminated with nitrogen.

  In the method for producing a nitride single crystal of the present invention, preferably, the seed crystal substrate is nitrided in an atmosphere containing a nitrogen source to form a nitride layer on the growth surface, thereby forming the seed crystal substrate. The growth surface is terminated with nitrogen.

In the nitride single crystal manufacturing method of the present invention, preferably, the seed crystal substrate is made of a single crystal of Al ₂ O ₃ , GaAs, ZrB ₂ , LiGaO ₂ , LiAlO ₂ , Ga ₂ O ₃ or SiC. It is characterized by.

  In the method for producing a nitride single crystal of the present invention, a nitride single crystal represented by the chemical formula XN (where X is at least one selected from Ga, Al and In) is fluxed on the surface of the seed crystal substrate. The seed crystal substrate is represented by the chemical formula XN on the growth surface of the seed crystal substrate because the growth surface for growing the nitride single crystal is terminated with nitrogen. It is possible to easily grow a nitride single crystal. Accordingly, when a gallium nitride compound semiconductor is grown on the manufactured nitride single crystal substrate, the quality of the gallium nitride compound semiconductor crystal is improved and the gallium nitride compound semiconductor is formed. The characteristics of various devices are improved.

  In the method for producing a nitride single crystal according to the present invention, the growth surface of the seed crystal substrate is preferably formed by nitriding the seed crystal substrate in an atmosphere containing a nitrogen source to form a nitride layer on the growth surface. Since it is terminated with nitrogen, it is possible to easily align nitrogen atoms on the outermost surface of the growth surface of the seed crystal substrate.

In the method for producing a nitride single crystal according to the present invention, the seed crystal substrate is preferably made of a single crystal of Al ₂ O ₃ , GaAs, ZrB ₂ , LiGaO ₂ , LiAlO ₂ , Ga ₂ O ₃ or SiC. Even a seed crystal substrate made of a material having a large difference in lattice constant from that of a nitride single crystal can be used, and the degree of freedom in manufacturing can be increased, and a nitride single crystal can be manufactured at a low cost.

An embodiment of the method for producing a nitride single crystal of the present invention will be described below. In this embodiment, as a seed crystal substrate, a sapphire (Al ₂ O ₃ ) substrate, a nitride single crystal represented by the chemical formula XN (where X is at least one selected from Ga, Al, and In) is used. This will be described in detail by taking GaN as an example.

  In the method for producing a nitride single crystal of the present invention, a nitride single crystal represented by the chemical formula XN (where X is at least one selected from Ga, Al and In) is fluxed on the surface of the seed crystal substrate. The seed crystal substrate has a structure in which a growth surface for growing a nitride single crystal is terminated with nitrogen.

FIG. 1 is a cross-sectional view showing a main part in a single crystal growth furnace S of a high pressure LPE (Liquid Phase Epitaxy) apparatus which is a nitride single crystal manufacturing apparatus. In the single crystal growth furnace S, a container 2 made of boron nitride (BN) or Al ₂ O ₃ is disposed in a heater 1, and a mixed solution 3 of sodium and gallium is contained in the container 2. It is filled at a temperature of 700 to 1000 ° C. The mixing ratio of the mixed solution 3 of sodium and gallium is preferably sodium: gallium = 0.5: 1.0 (mol%) to 2.5: 1.0 (mol%). When the mixing ratio of sodium is less than 0.5 mol%, the degree of supersaturation of nitrogen is low and the crystal is difficult to grow. When it exceeds 2.5 mol%, sodium is mixed as an impurity in the nitride single crystal, and the crystal quality is deteriorated.

  Also, a disc-shaped sapphire seed crystal substrate 4 having a diameter of 50.8 mm and a thickness of 0.3 mm for growing a GaN single crystal is disposed in the container 2. The seed crystal substrate 4 is heat-treated at 400 ° C. or higher in an atmosphere composed of ammonia gas as a nitrogen source, or heat-treated at a temperature of 400 ° C. or higher in nitrogen gas excited by a high voltage (about 500 kHz, about 200 V). It is preferable to use those subjected to nitriding treatment. Thereby, a nitride layer is formed on the growth surface of the seed crystal substrate 4, and the growth surface of the seed crystal substrate 4 is terminated with nitrogen.

  When the seed crystal substrate 4 is heat-treated at 400 ° C. or higher in an atmosphere made of ammonia gas, it may be heat-treated for about 1 to 10 minutes. If it is less than 1 minute, the nitriding reaction does not proceed sufficiently, and if it exceeds 10 minutes, the surface state of the seed crystal substrate 4 saturates and does not change, which is useless.

  When the seed crystal substrate 4 is heat-treated at a temperature of 400 ° C. or higher in nitrogen gas excited by a high voltage, it may be heat-treated for about 1 to 15 minutes. If it is less than 1 minute, the nitriding reaction does not proceed sufficiently, and if it exceeds 15 minutes, the surface state of the seed crystal substrate 4 is saturated and no change occurs, which is useless.

  The thickness of the nitride layer formed on the growth surface of the seed crystal substrate 4 is preferably about 10 nm or less. When the thickness exceeds 10 nm, defects in the nitride layer caused by a mismatch in material properties between the seed crystal substrate 4 and the nitride layer increase.

  The nitride layer (7) formed on the growth surface of the seed crystal substrate 4 does not have to be formed on the entire growth surface of the seed crystal substrate 4, as shown in FIG. it can. For example, it is preferably formed as a regular repeating pattern such as a stripe shape, a lattice shape, or an island shape. In this case, the growth of the GaN single crystal on the growth surface of the seed crystal substrate 4 promotes the crystal growth in the two-dimensional direction, so that it grows more than when the nitride layer is formed on the entire growth surface. The transition density of the GaN single crystal is reduced and the crystallinity is improved.

  The vessel 2 is installed in the pressure vessel 5 and is filled with nitrogen gas at 2 to 10 MPa as an atmospheric gas in the pressure vessel 5 and serves as a nitrogen source for growing a GaN single crystal.

  In such an apparatus configuration, the temperature of the mixed solution of sodium and gallium in the container 2 is kept constant (about 780 ° C.) or is gradually cooled, so that as shown in FIG. A GaN single crystal 8 can be grown on the nitride layer 7 whose outermost surface is terminated with nitrogen. In the case of gradual cooling, the gradual cooling may be performed at a temperature gradient of about 2 ° C./day from about 785 ° C. to about 775 ° C.

The surface of a disc-shaped GaN single crystal having a diameter of 50.8 mm and a thickness of 0.8 mm grown on the seed crystal substrate 4 is roughly polished using diamond abrasive grains and SiC abrasive grains, and then mirror-finished using colloidal silica. Polishing is performed to produce a substrate made of a GaN single crystal having a diameter of 50.8 mm and a thickness of 0.4 mm. The produced GaN single crystal has a dislocation density of 2 × 10 ⁴ cm ⁻² on the surface, and becomes a crystal having a large diameter and high quality that has not been conventionally obtained.

  As described above, a substrate made of a GaN single crystal suitable for epitaxial growth of a gallium nitride compound semiconductor can be manufactured.

As the seed crystal substrate 4, a substrate made of a single crystal of GaAs, ZrB ₂ , LiGaO ₂ , LiAlO ₂ , Ga ₂ O ₃ or SiC in addition to Al ₂ O ₃ can be used. These are made of a material having a large lattice constant difference from the nitride single crystal, but such a seed crystal substrate can also be used. Therefore, the degree of freedom in manufacturing is increased, and a nitride single crystal can be manufactured at low cost.

  The nitride single crystal is represented by the chemical formula XN (where X is at least one selected from Ga, Al, and In), for example, a single crystal of GaN, AlN, InN, or Ga, Al , In mixed crystal single crystal. The nitride single crystal is used as a substrate for forming a semiconductor layer constituting a light emitting element or an electronic element, for example.

  When a semiconductor layer made of a gallium nitride compound semiconductor is formed on a substrate made of a nitride single crystal to constitute a light emitting element, the structure is as follows.

For example, a gallium nitride compound semiconductor layer is formed on a substrate made of GaN single crystal via a buffer layer made of GaN or the like. Note that the buffer layer can be omitted. For example, the gallium nitride compound semiconductor layer is a gallium nitride compound semiconductor represented by the chemical formula Ga _1-x1-y1 In _y1 Al _x1 N (where 0 ≦ x1 + y1 ≦ 1, x1 ≧ 0, y1 ≧ 0). represented by a first conductivity type gallium nitride-based compound semiconductor layer composed of the chemical formula _{Ga 1-x2-y2 in y2} Al x2 N ( provided, however, that 0 ≦ x2 + y2 ≦ 1, x2 ≧ 0, y2 ≧ 0.) between the second conductive type gallium nitride-based compound semiconductor layer composed of a gallium nitride-based compound semiconductor, the chemical formula _{Ga 1-x3-y3 in y3} Al x3 N ( However, 0 ≦ x3 + y3 ≦ 1 , x3 ≧ 0, y3 ≧ 0 And a structure in which a light emitting layer made of a gallium nitride compound semiconductor is sandwiched and joined (where (x3, y1, y2) <(x1, x2), (x3, y1, y2)) ≦ y3).

  For example, the first conductive gallium nitride compound semiconductor layer and the second conductive gallium nitride compound semiconductor layer are a p-type gallium nitride compound semiconductor layer and an n-type gallium nitride compound semiconductor layer, respectively. In order to make a gallium nitride compound semiconductor a p-type semiconductor, magnesium (Mg), which is an element of Group 2 in the periodic table, may be mixed into the gallium nitride compound semiconductor as a dopant. In order to make the gallium nitride compound semiconductor an n-type semiconductor, silicon (Si), which is a group 4 element in the periodic table, may be mixed into the gallium nitride compound semiconductor as a dopant.

  Both the first conductivity type gallium nitride compound semiconductor layer and the second conductivity type gallium nitride compound semiconductor layer are made of a gallium nitride compound semiconductor containing aluminum (Al), and both are included in the light emitting layer. It is better to increase the content than aluminum. In this case, the forbidden band widths of the first and second conductivity type gallium nitride-based compound semiconductor layers are both larger than the forbidden band width of the light emitting layer, so that electrons and holes are confined in the light emitting layer. Electrons and holes can be efficiently recombined to emit light with strong emission intensity. The first and second conductivity type gallium nitride compound semiconductor layers are made of gallium nitride compound semiconductor containing aluminum, so that the forbidden bandwidths of the first and second conductivity type gallium nitride compound semiconductor layers are compared. The absorption of light on the short wavelength side such as ultraviolet light in the first and second conductivity type gallium nitride compound semiconductor layers can be reduced. The first and second conductivity type gallium nitride compound semiconductor layers may be made of a gallium nitride compound semiconductor containing aluminum.

  The first conductivity type gallium nitride compound semiconductor layer and the second conductivity type gallium nitride compound semiconductor layer may be an n-type gallium nitride compound semiconductor layer and a p-type gallium nitride compound semiconductor layer, respectively.

  In addition, a conductive layer (electrode) for injecting a current into the light emitting layer is formed in each of the first conductivity type gallium nitride compound semiconductor layer and the second conductivity type gallium nitride compound semiconductor layer. Thereby, a light emitting element such as an LED or a semiconductor laser (LD) is formed.

  Further, the composition of the gallium nitride compound semiconductor forming the light emitting layer may be set to an appropriate one that can obtain a desired emission wavelength. For example, if the light-emitting layer is made of GaN containing neither aluminum nor indium, the forbidden band width is about 3.4 electron volts (eV), and ultraviolet light having an emission wavelength of about 365 nanometers (nm) is obtained. The light emitting layer can emit light. When the emission wavelength is shorter than this, the emission layer is made of a gallium nitride compound semiconductor containing aluminum, which is an element for increasing the forbidden bandwidth, in an amount set according to the emission wavelength. And it is sufficient.

  In addition, the light emitting layer may contain indium (In), which is an element for reducing the forbidden band width, and a composition of aluminum, indium, and gallium by adding more aluminum so as to obtain a desired emission wavelength. The ratio may be set as appropriate. The light-emitting layer is a multi-quantum quantum well structure (MQW: Multi Quantum) which is a superlattice in which a quantum well structure composed of a barrier layer having a wide forbidden band and a well layer having a narrow forbidden band is regularly stacked multiple times. Well).

  Such a light emitting device operates as follows. That is, a bias current is applied to a gallium nitride compound semiconductor including a light emitting layer to generate ultraviolet light to near ultraviolet light having a wavelength of about 350 to 400 nm in the light emitting layer, and the ultraviolet light to near ultraviolet light is emitted outside the light emitting element. Operates to take out.

  The light-emitting element can be applied to a semiconductor laser as a light source for an optical pickup of an optical recording medium such as a CD and a DVD, and high recording is achieved by using ultraviolet light to near ultraviolet light having a wavelength of about 350 to 400 nm or purple light. An optical recording medium capable of recording / reproducing for a long time with a density can be manufactured and used. Such an optical pickup may have a well-known configuration, for example, a light emitting element and a beam splitter, a polarizing beam splitter, a prism, a reflecting mirror, a diffraction grating, and the like installed on the optical axis of light emitted from the light emitting element. It can be easily configured by combining a slit, a condensing lens, and the like.

  The light-emitting element can be used for a lighting device, and the lighting device includes a light-emitting element and at least one of a phosphor and a phosphor that emit light by receiving light emitted from the light-emitting element. is there. With this configuration, a lighting device with high luminance and illuminance can be obtained. The lighting device may be configured so that the light emitting element is covered or encapsulated with a transparent resin or the like, and a phosphor or phosphor is mixed in the transparent resin or the like. ~ Near ultraviolet light can be converted into white light or the like. In addition, a light reflecting member such as a concave mirror can be provided in a transparent resin or the like in order to improve the light collecting property. Such an illuminating device consumes less power than a conventional fluorescent lamp or the like, and is small in size. Therefore, the illuminating device is effective as a small and high-luminance lighting device.

  Note that the present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the scope of the present invention.

  Examples of the method for producing a nitride single crystal of the present invention will be described below.

  As the seed crystal substrate 4, a sapphire substrate having a diameter of 50.8 mm and a thickness of 0.3 mm and having one side (growth surface) mirror-polished, the growth surface is in an ammonia gas atmosphere at 400 ° C. or higher. Heat treated for 1 minute to 5 minutes.

  As a comparative example, a seed crystal substrate made of an untreated sapphire substrate was prepared.

  As for each of the seed crystal substrates of Example 1 and Comparative Example, the surface was subjected to secondary ion mass spectrometry. As a result, the seed crystal substrate of Example 1 was found to detect nitrogen, and was subjected to heat treatment in an ammonia gas atmosphere. Was found to be nitrogen terminated. On the other hand, nitrogen was not detected on the surface of the seed crystal substrate of the comparative example.

  Next, the sapphire substrate was accommodated in the cylindrical container 2 made of boron nitride (BN) having an inner diameter of 60 mm and a depth of 15 mm installed in the pressure vessel 5 of the single crystal growth furnace S. The container 2 was filled with a mixture of 16.5 g sodium and 100 g gallium.

  Next, the container 2 is placed in the center inside the cylindrical heater 1, and the pressure vessel 5 is filled with nitrogen gas as an atmospheric gas 6 at 2 to 10 MPa, and the heater 1 is energized, whereby the inside of the container 2 is filled. The mixed solution 3 was heated up to a temperature of 700 to 1000 ° C.

  Next, a GaN single crystal was grown on the sapphire substrate for 40 hours by keeping the temperature of the mixed solution 3 constant (780 ° C.) while keeping the atmospheric gas pressure in the pressure vessel 5 constant (4 MPa). .

  On the sapphire substrate of Example 1 whose surface was heat-treated in an ammonia gas atmosphere, as shown in FIG. 2, a thickness of about 0. An 8 mm GaN single crystal 8 was grown.

  On the other hand, formation of a GaN single crystal was not observed on the sapphire substrate of the comparative example whose surface was not heat-treated in an ammonia gas atmosphere, and the growth surface of the sapphire substrate remained a mirror surface.

  From the above, it was confirmed that GaN single crystal can be grown by nitriding the surface of the seed crystal substrate and terminating the outermost surface with nitrogen.

Next, the surface of the GaN single crystal 8 of Example 1 was roughly polished using diamond abrasive grains and SiC abrasive grains, and then mirror-polished using colloidal silica to obtain a GaN single crystal having a diameter of 50.8 mm and a thickness of 0.4 mm. A disk-shaped substrate made of crystals was produced. This substrate was analyzed by a transmission electron microscope image, and it was found that the dislocation density on the surface of the crystal was 2 × 10 ⁴ cm ⁻² , which was an unprecedented low dislocation density.

Even when the surface was nitrided using a single crystal of GaAs, ZrB ₂ , LiGaO ₂ , LiAlO ₂ , Ga ₂ O ₃ , or SiC as the seed crystal substrate, the same result as above was obtained. .

  From the above, it has been found that a GaN single crystal that could not be grown directly on a sapphire substrate can be grown with high quality by the manufacturing method of the present invention.

  As shown in FIG. 3, nitride layers 7 were formed in various patterns on the surface of the seed crystal substrate 4. In FIG. 3, 12 is an oxygen termination surface.

  (A) shows a case in which the nitride layer 7 is formed in a stripe pattern having a spacing and a width of 10 μm.

  (B) shows a structure in which the nitride layer 7 is formed in a lattice pattern having a large number of 10 μm × 10 μm square non-formed portions.

  (C) shows a case where the nitride layer 7 is formed in a pattern in which a large number of 10 μm × 10 μm square islands are formed.

(A) to (c) are formed with a thickness of 1 μm by combining a photolithography method, a vapor deposition method and a sputtering method using a mask made of SiO ₂ or Ti.

  In the same manner as in Example 1, each of the seed crystal substrates (a) to (c) was heat-treated at 400 ° C. or higher in an ammonia gas atmosphere.

After the heat treatment, the mask was removed with HF to prepare a sapphire substrate on which the nitride layer 7 having the patterns (a) to (c) was formed. Using these, a GaN single crystal 8 was grown in the same manner as in Example 1, and mirror-polished to produce a disk-shaped substrate made of a GaN single crystal having a diameter of 50.8 mm and a thickness of 0.4 mm. This substrate has a dislocation density of 10 ⁴ cm ^-2 or less in the portion where the mask pattern on the surface of the crystal was found by analysis using a transmission electron microscope image. Was confirmed.

It is sectional drawing of the single-crystal growth furnace for manufacturing a nitride single crystal. It is sectional drawing which shows the nitride single crystal grown on the seed crystal substrate. (A)-(c) is a top view of the seed crystal substrate in which the nitride layer was formed in various patterns on the growth surface.

Explanation of symbols

4: Seed crystal substrate 7: Nitride layer 8: Nitride single crystal

Claims (3)

  A nitride single crystal manufacturing method in which a nitride single crystal represented by the chemical formula XN (where X is at least one selected from Ga, Al and In) is grown on the surface of a seed crystal substrate by a flux method. A method for producing a nitride single crystal, wherein the seed crystal substrate has a growth surface for growing the nitride single crystal terminated with nitrogen.   2. The growth surface of the seed crystal substrate is terminated with nitrogen by nitriding the seed crystal substrate in an atmosphere containing a nitrogen source to form a nitride layer on the growth surface. The manufacturing method of the nitride single crystal of description. _3. The nitride single crystal according to claim 1, wherein the seed crystal substrate is made of a single crystal of Al ₂ O ₃ , GaAs, ZrB ₂ , LiGaO ₂ , LiAlO ₂ , Ga ₂ O ₃ or SiC. Production method.

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