JP2007254174A - Gallium oxide single crystal and its manufacturing method, and nitride semiconductor substrate and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gallium oxide single crystal excellent in crystallinity and conductivity, its manufacturing method, a nitride semiconductor substrate suitable for epitaxial growth of a nitride semiconductor, and its manufacturing method. <P>SOLUTION: A powder material prepared by adding an additive comprising one or more kinds selected from Sn, Ge, Si and their oxides to a gallium oxide powder is compacted and sintered at 1,400 to 1,600°C to obtain a gallium oxide sintered body; and the obtained gallium oxide sintered body is used as a source rod to grow a gallium oxide single crystal at a growth rate v satisfying 5 mm/h<v<15 mm/h by an FZ method in a mixture gas atmosphere of oxygen and inert gas. The surface of the single crystal is subjected to nitriding process to obtain a nitride semiconductor substrate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a gallium oxide single crystal excellent in both conductivity and crystallinity and a method for producing the same, and a nitride semiconductor substrate obtained using the gallium oxide single crystal and a method for producing the same.

Gallium oxide (β-Ga ₂ O ₃ ) single crystal is transparent in the visible region and has a wide band gap of 4.8 eV. In addition to semiconductor substrates to obtain laser diodes (LD), flat panel displays, optical emitters, transparent conductors used in solar cells, optical materials such as laser light resonant cavities, high-temperature oxygen gas sensor materials, etc. Various applications to various high-performance devices such as various electrodes, substrates, conductors, gas sensors, and optical recording materials have been studied.

Conventionally, sapphire (α-Al ₂ O ₃ ) or silicon carbide (SiC) is used as a substrate when obtaining light emitting elements such as LEDs and LDs. However, since sapphire is transparent but electrically insulating, the structure of the light-emitting element is a so-called horizontal type. In a horizontal element structure, the light emitting layer formed on the substrate must be partially exposed by etching or masking, and an additional electrode must be attached, which increases the number of steps for forming the element. There is a problem. On the other hand, although SiC is conductive, it is colored and difficult to be transparent. Therefore, the substrate absorbs part of the light emitted on the side opposite to the surface from which light is extracted, and the element is There is a problem that the light emission efficiency does not increase.

  Under such circumstances, it has been reported that a light emitting element having a vertical element structure is obtained using a gallium oxide single crystal having excellent transparency as a substrate (see Patent Document 1). Since the gallium oxide single crystal exhibits the behavior as an n-type semiconductor due to the occurrence of oxygen vacancies in the crystal, by providing a compound semiconductor film (light emitting layer) on a substrate made of gallium oxide single crystal, A light emitting element becomes possible. With such a vertical structure, it is not necessary to attach electrodes as in the case of the horizontal type, and the device manufacturing process is shortened, and the process cost can be reduced. In addition, since the substrate is transparent, the luminous efficiency of the entire device can be improved by taking measures such as reflecting light emitted on the side opposite to the light emitting layer.

  By the way, in the light emitting element having the vertical structure as described above, it is important that the substrate has excellent conductivity as well as transparency. The gallium oxide substrate of Patent Document 1 has n-type conductivity and exhibits an electrical resistivity (specific resistance) of about 0.1 Ω · cm. However, in order to use it as a substrate for a light emitting element, an electrical resistivity of about 0.05 Ω · cm or less is desired from the viewpoints of reduction in driving voltage, suppression of heat generation due to reduction in current density, element lifetime, and the like.

  Therefore, for example, when a gallium oxide single crystal is obtained by the floating zone method (FZ method), oxygen deficiency is forcibly created by crystal growth in an atmosphere in which the amount of oxygen is limited, and low electric power. It has been reported that a gallium oxide single crystal having a resistivity can be obtained (for example, see Non-Patent Document 1). According to this document, it is shown that when a single crystal is grown in a mixed gas atmosphere of oxygen and nitrogen, the electrical resistivity of the gallium oxide single crystal is decreased by increasing the ratio of nitrogen in the mixed gas. Yes. However, a single crystal forcibly creating oxygen vacancies is not suitable for manufacturing a high-performance device such as a light-emitting element because of poor crystallinity.

In order to solve the above problem, a gallium oxide single crystal obtained by adding Sn has been reported (for example, see Non-Patent Document 1 and Non-Patent Document 2). In each of these documents, a gallium oxide single crystal is grown by the FZ method using compression molding of a gallium oxide powder added with 3 mol% of SnO ₂ , and using the sintered material as a raw material rod. ing. The resulting gallium oxide single crystal has a reduced electrical resistivity of the single crystal due to the action of Sn doped in the crystal or the oxygen deficiency formed by evaporation of SnO ₂ when the single crystal is grown. It is thought to do.

However, none of the gallium oxide single crystals obtained by adding Sn in these documents can satisfy the electrical resistivity as described above. On the other hand, if SnO ₂ is evaporated and oxygen vacancies are generated, the crystallinity may be inferior as described above, and Sn is doped in the obtained gallium oxide single crystal. In addition to the impurity levels caused by oxygen vacancies in the band gap, the levels caused by Sn are formed, thereby increasing light absorption. Generally, colored gallium oxide single crystals with poor crystallinity can also be secured by annealing, but in such single crystals, oxygen deficiency is again closed by oxygen, and conversely conductive Will be lost. In other words, in an oxide semiconductor, conductivity and crystallinity are contradictory characteristics. If oxygen deficiency is forcibly created to provide conductivity, the crystallinity is inferior, and conversely, an attempt is made to improve crystallinity. Oxygen deficiency decreases and becomes an insulator. In the above-mentioned Patent Document 1, it is suggested that Sn or the like is added to obtain a gallium oxide single crystal by the FZ method. However, the electrical resistivity is the above-described value and is not sufficiently satisfactory. .

By the way, Patent Document 2 discloses an ultraviolet transparent conductive film made of gallium oxide that is transparent from the ultraviolet region to the visible light region and has conductivity, and its electrical resistivity has the value described above. In addition, the conductive film according to this document is obtained by forming a film on a dissimilar substrate such as quartz glass or sapphire, and it is difficult to obtain a crystal quality such as a bulk crystal.
JP 2004-56098 A JP 2002-93243 A Naoyuki Ueda et al. Appl. Phys. Lett. 70 (26), 3561-3563 (1997) Mitsuo Yamaga et al. PHYSHICAL REVIEW B 68, 155207 (2003)

  As described above, a gallium oxide single crystal that satisfies both crystallinity and conductivity at the same time and is suitable for manufacturing a high-performance device such as a light emitting element has been reported to the best of the inventors' knowledge. Absent. Thus, as a result of intensive studies on a method for obtaining a gallium oxide single crystal that simultaneously satisfies both crystallinity and conductivity, the inventors surprisingly found that a gallium oxide powder containing an additive was sintered. When growing the gallium oxide single crystal by the floating zone melting method (FZ method) using the obtained sintered body as a raw material rod, optimization is performed by combining the sintering temperature and the growth rate of the single crystal under predetermined conditions. As a result, it was found that a bulk gallium oxide single crystal having excellent crystallinity and conductivity and satisfying these performances at the same time was obtained, and the present invention was completed.

Accordingly, an object of the present invention is to provide a gallium oxide single crystal having excellent crystallinity and conductivity.
Another object of the present invention is to provide a method for producing a gallium oxide single crystal having excellent crystallinity and conductivity.

Furthermore, another object of the present invention is to provide a nitride semiconductor substrate suitable for epitaxially growing a nitride semiconductor.
Furthermore, another object of the present invention is to provide a method for manufacturing the nitride semiconductor substrate.

That is, the present invention is an oxidation characterized by having an electrical resistivity of 0.05 Ω · cm or less and an internal transmittance of 80% or more in a wavelength region of 300 to 620 nm at a thickness of 0.4 mm. Gallium (β-Ga ₂ O ₃ ) single crystal.

The present invention also compresses a powder material obtained by adding one or more additives selected from Sn, Ge, Si or oxides thereof to gallium oxide powder, and sinters it at 1400 to 1600 ° C. Thus, a gallium oxide sintered body is obtained, and the growth rate v is set to 5 mm / h <v <15 mm / h in a mixed gas atmosphere of oxygen and inert gas by the FZ method using the gallium oxide sintered body as a raw material rod. A method for producing a gallium oxide (β-Ga ₂ O ₃ ) single crystal characterized by growing a gallium oxide single crystal as

First, the gallium oxide single crystal in the present invention will be described.
The gallium oxide single crystal in the present invention has an electrical resistivity (specific resistance) of 0.05 Ω · cm or less, preferably 0.02 Ω · cm or less. If the electrical resistivity is 0.05 Ω · cm or less, for example, a substrate for forming light emitting elements such as LEDs and LDs, as well as transparent materials used for creating high-performance devices such as oxide transparent conductors and high-temperature oxygen gas sensor materials It can be suitably used as a conductive material. As will be described in the following examples, this electrical resistivity can be obtained by measuring, for example, the Hall coefficient, and in the present invention, it means a value at room temperature.

The gallium oxide single crystal of the present invention has an internal transmittance of 80% or more in a wavelength region of 300 to 620 nm at a thickness of 0.4 mm, preferably an internal transmittance of 85% or more in a wavelength region of 300 to 800 nm. More preferably, the internal transmittance in the wavelength region of 300 to 800 nm is 90% or more. Here, the internal transmittance is a transmittance obtained by removing the contribution of light reflection on the surface of the thin film, and using the light transmittance T and the reflectance R measured using an ultraviolet-visible spectrophotometer, the following formula (1 ) (T _int ) calculated according to That is, the percentage of the light incident on the thin film transmitted through the thin film is shown as a percentage.
T _int = 100 × T / (100−R) (1)

  If the internal transmittance in the above-mentioned wavelength region is 80% or more, it is suitable as a substrate for forming a light emitting device having a vertical structure particularly in the visible region, and also for producing high performance devices such as various light emitting devices and sensors. It can be suitably used as a transparent conductive material to be used for.

  The gallium oxide single crystal preferably has a half width of an X-ray rocking curve obtained by X-ray rocking curve measurement of preferably 180 arcsec (0.05 °) or less, more preferably 60 arcsec or less. Since the gallium oxide single crystal having such a half width has a single diffraction peak (that is, a single crystal domain) and has excellent crystallinity with higher completeness, the GaN-based nitride semiconductor film is particularly preferable. This is extremely advantageous in applications such as a substrate for growing a substrate and a laser beam resonant cavity.

  X-ray rocking curve measurement is generally used for evaluation of crystal composition, lattice distortion, domain structure, etc. of a semiconductor single crystal and the like. The crystal to be inspected is rotated in the range and the detection value of the X-ray counter is monitored to obtain a rocking curve which is a spectrum showing the relationship of the X-ray detection value to the diffraction angle. With respect to this rocking curve, information on the lattice constant can be obtained from the angle giving the peak (peak angle: θ). Since this peak angle (θ) is determined by the diffraction surface, the crystal plane tilts when it fluctuates. It will show that. Further, the X-ray intensity at the peak indicates the intensity of diffraction on the surface, and information on crystallinity can be obtained from the peak intensity. Furthermore, since the half width of the peak (FWHM) represents the fluctuation of the angle of the diffraction surface, the smaller the half width, the smaller the misorientation of the lattice plane and the lattice strain. ) Is also smaller.

Next, the manufacturing method of the gallium oxide single crystal in this invention is demonstrated.
First, a gallium oxide (β-Ga ₂ O ₃ ) powder, preferably a gallium oxide powder having a purity of 99.99% or more, is added with one or more additives selected from Sn, Ge, Si or their oxides. The added powder material is prepared, compression molded, and sintered at 1400 to 1600 ° C., preferably 1500 to 1550 ° C., to obtain a gallium oxide sintered body. When the powder material is compression-molded, for example, the powder material is enclosed in a rubber press, and is preferably molded by rubber pressing at a hydrostatic pressure of 50 to 600 MPa for about 1 to 3 minutes. Time is sufficient. The sintering atmosphere can be performed in an air atmosphere.

The additive to be added to the gallium oxide powder is incorporated into the gallium oxide substitution site when the gallium oxide sintered body is formed. When crystals are formed, they evaporate. That is, the additive contained in the sintered body is in a molten state in which the sintered body is heated to a melting point (about 1740 ° C.) or higher of gallium oxide, and the additive evaporates according to its vapor pressure, and the resulting oxidation Oxygen vacancies are formed in the gallium single crystal to impart conductivity. For example, when the additive is SnO ₂ powder, SnO _{2 has} a boiling point of 1850 ° C. (melting point is 1127 ° C.), so SnO ₂ contained in the sintered body is used for growing a gallium oxide single crystal by the FZ method. As a result, the oxygen vacancies are formed in the resulting gallium oxide single crystal.

  By the way, in this invention, in order to obtain the raw material stick | rod used by FZ method, since it sinters at 1400-1600 degreeC which is comparatively high, the bulk density of a raw material stick | rod becomes large and it approaches the density of a single crystal. Then, if this sintered body is used as a raw material rod and single crystal growth is performed by the FZ method, reflection is suppressed during condensing heating and heat is efficiently absorbed, so that the raw material rod is easily melted during single crystal growth. Become. Further, since the gap in the sintered body is reduced, the amount of gas in the gap mixed as bubbles in the melt zone is reduced, so that the fluctuation of the volume and surface area of the melt zone is suppressed, and the melt zone is stabilized. As a result, the unevenness of oxygen vacancies formed in the obtained gallium oxide single crystal is suppressed and exists uniformly. That is, it is possible to suppress a decrease in crystallinity of the gallium oxide single crystal imparted with conductivity as much as possible and obtain a gallium oxide single crystal excellent in crystallinity. When the sintering temperature is lower than 1400 ° C., the density of the obtained sintered body is lowered, and the raw material melted at the interface between the melting zone and the sintered body is sucked into the gaps in the raw material rod by capillary action. In addition, the gas contained in the raw material rod is taken into the melting zone as bubbles, and the volume and surface area of the melting zone fluctuate each time the gas is discharged from the melting zone, and the evaporation of the additive is not constant, Unevenness occurs in the oxygen vacancies in the obtained single crystal, and a homogeneous crystalline single crystal cannot be obtained. On the other hand, when the temperature is higher than 1600 ° C., the additive element evaporates from the sintered body, and there is a possibility that conductivity is not imparted when a single crystal is obtained. If the sintering temperature is 1500 ° C. to 1550 ° C. in particular, it is preferable because a more excellent crystalline and conductive gallium oxide single crystal can be obtained.

  Regarding the addition amount of the additive, the element amount (the total amount when two or more elements are added) is preferably 0.05 to 5 at% with respect to the gallium element. If the amount of the additive is 5 at% or less with respect to the gallium element, a sintered body in which the additive element is taken into the substitution site of the gallium oxide can be obtained, and oxidation with more excellent conductivity and crystallinity. A gallium single crystal can be formed. If it exceeds 5 at%, a part of the added element may not enter the substitution site in the sintered body and may affect the crystallinity of the obtained single crystal. Further, the evaporation of the additive element becomes incomplete during crystal growth, and it becomes difficult to obtain a gallium oxide single crystal having excellent crystallinity. Although this problem can be avoided by reducing the growth rate, it is wasteful that about 10% of the raw material evaporates, and it becomes difficult to control crystal growth. On the other hand, when the addition amount is less than 0.05 at%, it is difficult to obtain the effect of improving the conductivity of the gallium oxide single crystal.

The additive may be added to the gallium oxide powder in the elemental state or may be added in the oxide state, but when the additive is added in the oxide state, the amount of the element is What is necessary is just to make it become the said range. For example, when the additive is SnO ₂ , it is preferable to add SnO ₂ to 0.1 to 10 mol% with respect to the gallium oxide powder. Moreover, it is preferable to use an additive having a purity of 99.99% or more.

  About the gallium oxide sintered compact obtained above, this is made into a raw material stick, and a gallium oxide single crystal is grown by FZ method. At this time, the growth rate v is higher than 5 mm / h and less than 15 mm / h, preferably 10 mm / h or more and less than 15 mm / h, more preferably 10 to 12.5 mm / h, and a gallium oxide single crystal is formed in the <001> direction. Cultivate. Generally, in the FZ method, it is considered that a single crystal having a good quality can be obtained when the growth rate is slow. In the present invention, a gallium oxide single crystal is grown at a relatively fast growth rate as described above. Such a condition has been found as a result of the systematic efforts of the present inventors to obtain a gallium oxide single crystal that simultaneously satisfies conductivity and crystallinity. The reason is presumed as follows. In the present invention, as described above, a relatively high bulk density sintered body is formed by relatively increasing the sintering temperature when obtaining the sintered body, and this sintered body is used as a raw material rod. If the FZ method is performed, the raw material rod is relatively easy to melt. The additive present in a uniform state evaporates, so that oxygen vacancies can be generated without deteriorating the crystal quality. At this time, if the growth rate is 5 mm / h or less, oxygen deficiency is supplemented by oxygen (O) in the atmospheric gas, and the electric resistivity of the resulting gallium oxide single crystal may be increased. On the contrary, if it is 15 mm / h or more, the evaporation of the additive becomes incomplete, and it becomes difficult to obtain a gallium oxide single crystal excellent in crystallinity.

  The atmosphere for growing the gallium oxide single crystal is a mixed gas atmosphere of oxygen and inert gas. By making the atmosphere containing oxygen, evaporation of gallium oxide itself is suppressed, and a gallium oxide single crystal having excellent crystallinity can be obtained. The inert gas is preferably at least one selected from nitrogen, argon, helium and the like. As such a mixed gas, for example, air or dry air can be used.

  On the other hand, for the seed crystal used in the FZ method, it is preferable to use a gallium oxide single crystal obtained separately by the FZ method. Moreover, about the rotational speed of a raw material stick | rod and a seed crystal, it is good that it is 10-60 rpm respectively, and it is preferable to make it rotate mutually reverse.

  The use of the gallium oxide single crystal in the present invention is not particularly limited and can be used as various semiconductor materials for high-functional devices such as a substrate and a gas sensor for obtaining a light emitting element. A nitride semiconductor substrate having a gallium nitride (GaN) layer on the surface layer of a gallium oxide single crystal may be nitrided. Since the gallium oxide single crystal of the present invention is excellent in crystallinity, a high-quality gallium nitride layer with few defects can be obtained by nitriding the surface. If the substrate includes such a gallium nitride layer, for example, as a substrate for growing a nitride semiconductor made of gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), or a mixed crystal thereof. This is advantageous in that it has fewer lattice mismatches compared to existing substrates such as sapphire and silicon carbide, and can epitaxially grow a high-quality nitride semiconductor.

  The means for nitriding is not particularly limited. For example, ammonia gas heat treatment for heating a gallium oxide single crystal in an ammonia gas atmosphere, ECR (Electron Cyclotron Resonance) plasma, or RF (Radio Frequency) plasma. Specific examples include nitrogen plasma treatment using excited nitrogen plasma and nitrogen ion implantation treatment for implanting nitrogen ions.

  Among these, in the ammonia gas heat treatment, the gallium oxide single crystal may be heated at a temperature of, for example, 700 to 1000 ° C. in an ammonia gas atmosphere. At this time, nitrogen gas is preferably included together with ammonia gas. By diluting the ammonia gas with the nitrogen gas, it is possible to suppress the ammonia from reacting with moisture remaining in the container.

On the other hand, in nitrogen plasma treatment, for example, nitrogen gas used as a nitrogen source, ammonia gas, a mixed gas obtained by adding hydrogen to nitrogen is excited by a predetermined magnetic field to generate nitrogen plasma, and the surface of the gallium oxide single crystal is treated. You just have to do it. For example, when plasma processing is performed in a chamber of an ECR-MBE (molecular beam epitaxy) apparatus using nitrogen gas as a nitrogen source, plasma excited by applying a 2.45 GHz magnetic field (875 G) to molecular nitrogen (N ₂ ) is generated. The GaN layer is formed under conditions of microwave power 100 to 300 W, nitrogen flow rate 8 to 20 sccm (standard cc / min), substrate temperature (temperature of the gallium oxide single crystal) 300 to 800 ° C., and processing time 3 to 120 minutes. Can be formed.

In the nitrogen ion implantation process, N ₂ ⁺ cationized from nitrogen may be implanted into the surface of the gallium oxide single crystal. At this time, the ion implantation chamber is evacuated to about 2 × 10 ^{−6 to} 5 × 10 ⁻⁶ Torr, the acceleration energy of nitrogen ions is 10 to 100 keV, and the implantation amount of nitrogen ions is 1 × 10 ¹⁷ to If ion implantation is performed at 1 × 10 ¹⁸ ions / cm ² , a GaN layer can be formed.

  The surface of the gallium oxide single crystal to be nitrided is preferably the (100) plane of the gallium oxide single crystal. Since the (100) plane of the gallium oxide single crystal is a plane parallel to the growth direction of the gallium oxide single crystal, the gallium oxide single crystal is easily cleaved into the (100) plane. For example, a laser such as a semiconductor laser is used. This is suitable when the mirror of the optical resonator used for oscillation is formed by a cleavage plane of the GaN crystal. Further, prior to the nitriding treatment, the surface may be polished. By polishing, a GaN layer can be formed while suppressing defect formation. Generally used means can be used for such polishing, but preferably chemical mechanical polishing in which the mechanical removal action by particles such as abrasive grains and the chemical removal action by the processing liquid are superimposed. (Chemical Mechanical Polishing: CMP).

  The film thickness of the GaN layer formed by nitriding is preferably 1 to 3 nm from the viewpoint of improving the quality of the grown nitride semiconductor film. If the thickness is less than 1 nm, it may be necessary to separately form a buffer layer in order to grow a nitride semiconductor having excellent crystallinity. Conversely, if it is thicker than 3 nm, it is effective as a substrate for crystal growth of the nitride semiconductor. Saturation is disadvantageous in terms of processing time and cost. The film thickness of the GaN layer can be calculated from, for example, depth direction analysis by secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Or it can also calculate from cross-sectional observation with an electron microscope.

  The gallium oxide single crystal in the present invention has both excellent crystallinity and conductivity that are conventionally considered to be difficult to achieve both. That is, in the present invention, when a gallium oxide single crystal is obtained by the FZ method using a sintered body obtained by sintering gallium oxide powder as a raw material rod, a specific additive is added to the gallium oxide powder and sintered. By combining the temperature and the speed during crystal growth within a predetermined range, the oxide semiconductor succeeded in achieving both crystallinity and conductivity, which are considered to be contradictory properties. The gallium oxide single crystal in the present invention has excellent crystallinity and excellent transparency in the visible region, and has low electrical resistivity and excellent conductivity.

  Hereinafter, based on an Example, this invention is demonstrated more concretely.

[Preparation of gallium oxide single crystal]
Weighing so that the purity of 99.99% of SnO ₂ powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.) is 10 mol% with respect to the purity of 99.99% gallium oxide powder (manufactured by Kojundo Chemical Laboratory Co., Ltd.). And mixed to prepare a powder material. This powder material was put into a rubber tube having a diameter of 10 mm, and pressed using a press machine at a hydrostatic pressure of 60 MPa for 3 minutes to be solidified into a cylindrical shape. Next, this cylindrically solidified powder material was taken out from the rubber tube, put in an electric furnace and sintered at 1500 ° C. for 10 hours in the atmosphere to obtain a gallium oxide sintered body.

In order to produce a gallium oxide single crystal, a gallium oxide single crystal was grown by an optical FZ (floating zone: floating zone melting) method using a double elliptical infrared condensing heating furnace.
The gallium oxide sintered body obtained above was installed as a raw material rod on the upper axis of an infrared condensing heating furnace, and the lower axis was installed as a seed crystal so that the [001] direction of the gallium oxide single crystal was in the axial direction. . This seed crystal is obtained by cutting out a single crystal portion obtained by subjecting a gallium oxide polycrystal to light heating zone melting in advance.

The crystal growth atmosphere is a mixed gas of nitrogen and oxygen (N ₂ : 79 vol%, O ₂ : 21 vol%), and this mixed gas is supplied into the transparent quartz tube of the infrared condensing heating furnace at 500 ml / min. did. In addition, the tips of the raw material rod and the seed crystal are moved so as to be in the center of the furnace and brought into contact with melting, and the raw material rod and the seed crystal are rotated in opposite directions at a rotational speed of 20 rpm, respectively, in the <001> direction. The gallium oxide single crystal was grown under 1 atm by moving the vertical axis so that the crystal growth rate was 10 mm / h. Thus, a gallium oxide single crystal having a length of 37 mm, a width of 7 mm, and a thickness of 5 mm was obtained.

[Evaluation of the physical properties]
The gallium oxide single crystal obtained above was subjected to tin composition analysis by hydride generation atomic absorption. In this analysis, hydride was generated using a reduction vaporizer (VGA77 manufactured by Varian) and evaluated by an atomic absorption photometer (SpectrAA-220 manufactured by Varian). Gallium was evaluated using an ICP emission spectroscopic analyzer (SPS 3100 manufactured by SII Nanotechnology). As a result, Sn: 0.006% (m / m), Ga: 77.9% (m / m).

Next, the (100) plane of the gallium oxide single crystal obtained above was cut out, and as shown in FIG. 1, the gallium oxide single crystal sample having a thickness (d) of 0.4 mm and about 8 mm × 7 mm. A substrate 1 was prepared. About this sample board | substrate 1, the internal transmittance | permeability was measured at room temperature using the ultraviolet visible spectrophotometer (Shimadzu Corporation UV-3100PC). The results are shown in FIG.
As can be seen from FIG. 2, the gallium oxide single crystal obtained in Example 1 has an internal transmittance of 80% or more in the wavelength range of 300 to 620 nm in an as grown state, and is excellent in transparency. It was confirmed.

  The Hall coefficient was measured by the Van Der Pauw method using the gallium oxide single crystal substrate 1. The electrical resistivity, carrier concentration, and mobility of the substrate 1 were measured. The results are shown in Table 1.

  As is clear from Table 1, the gallium oxide single crystal obtained in Example 1 has a very low electrical resistivity and was confirmed to be excellent in conductivity.

  FIG. 3 shows the result of X-ray rocking curve measurement for the gallium oxide single crystal obtained above. According to this, there is diffraction with respect to the (400) plane, its half-value width (FWHM) is as small as 162 arcsec, the peak is single, and there is no peak cracking, so this gallium oxide single crystal has a single crystal domain. It was confirmed that the crystallinity was also quite good.

  Furthermore, cathodoluminescence measurement (CL measurement) was performed on the gallium oxide single crystal obtained above. The measurement was performed at room temperature using CL-900 mounted on Shimadzu Corporation EPMA apparatus EPMA-1600. The acceleration voltage of the electron beam was 15 kV, and the measurement wavelength range was 300 to 600 nm. The obtained results are shown in FIG. Blue light emission showing a sharp emission peak at a wavelength of 390 nm was confirmed, and it was read from the spectrum that the crystallinity was excellent.

A powder material was prepared so that the ratio of SnO ₂ powder to gallium oxide powder was 2 mol%, and a sintered body was obtained in the same manner as in Example 1 except that a rubber tube having a diameter of 12 mm was used. Then, the single crystal was grown in the same manner as in Example 1 except that the obtained sintered body was grown at a growth rate of 12.5 mm / h, and a gallium oxide single crystal having a diameter of about 10 mm × length of about 30 mm was obtained. Obtained. The resulting gallium oxide single crystal was subjected to composition analysis, transmittance measurement, Hall coefficient measurement, X-ray rocking curve measurement, and cathodoluminescence measurement in the same manner as in Example 1.

  As a result of composition analysis, Sn contained in the gallium oxide single crystal obtained in Example 2 was 0.003% (m / m), and Ga was 76.0% (m / m). Moreover, the result of the transmittance measurement is as shown in FIG. 2, and the internal transmittance in the wavelength range of 300 to 864 nm exceeds 85% in the as grown state, and the transparency in the visible range is excellent. confirmed. The result of the X-ray rocking curve measurement was a peak half-value width (FWHM) of 43 arcsec corresponding to the (400) plane as shown in FIG. 3, and showed crystallinity equivalent to that of a high-quality single crystal without addition of impurities. Furthermore, the results obtained from the measurement of the Hall coefficient are as shown in Table 1. It was confirmed that the electrical resistivity was a very low value and the conductivity was excellent. Furthermore, the result of the cathodoluminescence measurement is as shown in FIG. 4. Blue light emission showing a sharp emission peak at a wavelength of 390 nm was confirmed, and it was read from the spectrum that the crystallinity was excellent.

[Comparative Example 1]
Gallium oxide was prepared in the same manner as in Example 1 except that a powder material was prepared so that the ratio of SnO ₂ powder to gallium oxide powder was 3 mol% and sintered at 1300 ° C. for 16 hours to obtain a gallium oxide sintered body. After obtaining the sintered body, the FZ method was performed in the same manner as in Example 1 to obtain a gallium oxide single crystal having a diameter of about 9 mm and a length of about 55 mm. The obtained gallium oxide single crystal was colored blue as a whole, and bluish shades were distributed in stripes in the crystal growth direction.

By the way, since the sintered body obtained above had almost no shrinkage due to sintering in appearance, its sintered density was low, and actually the tip of the sintered body used as a raw material rod was condensed and heated to be melted. When this was done, cracks occurred due to the shrinkage directly above the melted part. Further, when the single crystal was grown, the molten zone was sucked up into the sintered body (raw material rod), and the portion expanded to generate cracks. Presuming from this situation, the gas and SnO ₂ contained in the sintered body are taken into the melt zone as bubbles, and the volume and surface area of the melt zone fluctuate greatly each time the bubbles in the melt zone disappear. Therefore, crystallinity and transparency are lowered, and as is clear from Table 1, the conductivity is considered to be inferior.

[Comparative Example 2]
A powder material made of gallium oxide powder having a purity of 99.99% (manufactured by Kojundo Chemical Laboratory Co., Ltd.) was prepared without adding any additives, and a gallium oxide sintered body was obtained in the same manner as in Example 1. Example except that this sintered body was used as a raw material rod, the crystal growth atmosphere was a mixed gas of nitrogen and oxygen (N ₂ : 500 ml / min, O ₂ : 20 ml / min), and the growth rate was 17.5 mm / h. In the same manner as in Example 1, a gallium oxide single crystal was grown to obtain a single crystal having a diameter of about 9 mm and a length of about 50 mm. The obtained gallium oxide single crystal was subjected to internal transmittance measurement (FIG. 2) and Hall coefficient measurement (Table 1) in the same manner as in Example 1. From FIG. 2, the transmittance of the obtained gallium oxide single crystal was an excellent result, but the electrical resistivity was an order of magnitude larger than those in Examples 1 and 2.

  The gallium oxide single crystal in the present invention is a gallium oxide single crystal excellent in crystallinity, excellent in transparency, low in electrical resistivity, and excellent in conductivity, so that it is a light emitting diode (LED) or laser diode (LD). Semiconductor substrates for obtaining various light emitting devices such as flat panel displays, optical emitters, transparent conductors used in solar cells, optical materials such as laser light resonant cavities, high-temperature oxygen gas sensor materials, etc. In addition, it is extremely useful as a semiconductor material for high-performance devices such as various electrodes, substrates, conductors, gas sensors, and optical recording materials. In particular, if a nitride semiconductor substrate is formed by forming a GaN layer on the surface layer portion, for example, nitridation made of gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), or a mixed crystal thereof. It is suitable as a substrate for growing a physical semiconductor, and a light-emitting element having a so-called vertical element structure can be obtained. That is, the substrate for nitride semiconductor can be used as an electrode as it is, the process for producing the device can be simplified, it is industrially extremely useful, and the luminous efficiency can be improved as compared with the conventional one.

FIG. 1 is a perspective explanatory view showing a gallium oxide single crystal substrate according to an embodiment of the present invention. FIG. 2 is a graph showing the transmittance (and reflectance) of a gallium oxide single crystal (when the thickness is 0.4 mm) according to an example of the present invention. FIG. 3 is an X-ray rocking curve measured using synchrotron radiation of a gallium oxide single crystal according to an example of the present invention. FIG. 4 is an emission spectrum (CL measurement) of a gallium oxide single crystal according to an example of the present invention.

Explanation of symbols

  1 Gallium oxide single crystal sample substrate

Claims (9)

  A gallium oxide single crystal having an electrical resistivity of 0.05 Ω · cm or less and an internal transmittance of 80% or more in a wavelength region of 300 to 620 nm at a thickness of 0.4 mm.   The gallium oxide single crystal according to claim 1, wherein the half width of the X-ray rocking curve obtained by the X-ray rocking curve measurement is 180 arcsec or less.   3. A nitride semiconductor substrate comprising a GaN layer provided on a surface layer portion of the gallium oxide single crystal according to claim 1.   A gallium oxide powder is compression-molded by adding a powder material in which one or more additives selected from Sn, Ge, Si or oxides thereof are added, and sintered at 1400 to 1600 ° C. to sinter gallium oxide. Using this gallium oxide sintered body as a raw material rod, a gallium oxide single crystal was grown at a growth rate v of 5 mm / h <v <15 mm / h by an FZ method in a mixed gas atmosphere of oxygen and inert gas. A method for producing a gallium oxide single crystal, characterized by growing. Additives is SnO ₂ powder, oxidation method for producing a single-crystal gallium according to claim 4, adding 0.1 to 10 mol% of the SnO ₂ powder with respect to gallium oxide powder.   The obtained gallium oxide single crystal has an electric resistivity of 0.05 Ω · cm or less, and an internal transmittance in a wavelength region of 300 to 620 nm at a thickness of 0.4 mm is 80% or more. Of producing a single crystal of gallium oxide.   The method for producing a gallium oxide single crystal according to any one of claims 4 to 6, wherein the obtained gallium oxide single crystal has a half width of an X-ray rocking curve obtained by X-ray rocking curve measurement of 180 arcsec or less.   A surface of the gallium oxide single crystal obtained by the method according to claim 4 is nitrided to form a GaN layer on a surface layer portion of the gallium oxide single crystal. A method for manufacturing a substrate.   9. The nitriding treatment is any one of ammonia gas heating treatment for heating in an ammonia gas atmosphere, nitrogen plasma treatment using nitrogen plasma excited by ECR or RF, or nitrogen ion implantation treatment for implanting nitrogen ions. The manufacturing method of the board | substrate for nitride semiconductors of description.

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