JP4968708B2 - Method for producing nitride single crystal - Google Patents

Description

The present invention relates to a method for producing a nitride single crystal .

Gallium nitride III-V nitride has attracted attention as an excellent blue light emitting device, has been put to practical use in light emitting diodes, and is also expected as a blue-violet semiconductor laser device for optical pickup. As a method for growing a gallium nitride single crystal by the Na flux method, in Patent Document 1, the pressure is set to 10 to 100 atm using a mixed gas of nitrogen and ammonia. Also in patent document 2, the atmospheric pressure at the time of a cultivation is 100 atmospheres or less, and is 2, 3, 5 MPa (about 20 atmospheres, 30 atmospheres, 50 atmospheres) in an Example.
On the other hand, the present applicant disclosed in Patent Document 3 a method for efficiently growing a gallium nitride single crystal under specific conditions using a hot isostatic press (HIP) apparatus.

JP 2002-293696 A JP 2003-292400 A Japanese Patent Application No. 2004-103092 Japanese Patent Application No. 2005-70649

  However, it has been found that the following problems newly occur when crystal growth is performed by the flux method using such a heating and pressurizing apparatus. That is, in growth using a conventional muffle furnace, raw materials are weighed in a glove box, filled in a crucible, sealed in a stainless steel sealed container with a valve, and then the sealed container is removed from the glove box. It was not exposed to the atmosphere, and it was possible to prevent oxidation of the raw material and reaction with moisture in the atmosphere due to oxygen in the atmosphere. However, the above-mentioned airtight container with a valve cannot be used in the HIP apparatus. Since the lid of the pressure-resistant container of the HIP device is opened and the crucible is placed directly in the container, the lid is closed, so that there is a problem that the raw material is exposed to the atmosphere during work and is oxidized or moisture-absorbed.

  As a result, the penetration of nitrogen on the flux surface was inhibited, the gallium nitridation rate was lowered, and a black colored gallium nitride single crystal was obtained.

  In addition, in the patent document 4, the present inventor accommodates a raw material of Na flux in a container, hermetically seals the opening of the container, melts the sealant during heating and pressurizing treatment, and opens the opening. Disclosed that the inside of the container communicates with an external non-oxidizing atmosphere.

  However, even with such a method, it has been found by the study of the present inventor that there is still room for improvement in practice. That is, metallic sodium and the like are weighed in a glove box, but the glove box has a dew point of −80 ° C. and is in a low moisture concentration and low oxygen concentration environment with an oxygen concentration of 1 ppm or less. However, even in such an environment, the surface of the sodium metal changes slightly white within a few minutes of weighing. The weighing operation depends on the case, but it takes 30 minutes even if it is short, and several hours when the number of samples is large. In particular, when the number of samples is large, the dew point is gradually deteriorated, and the surface of the sodium metal is likely to be altered (oxidation, moisture absorption). For this reason, when the number of samples is large, as described above, depending on the lot, the penetration of nitrogen on the flux surface is inhibited, the nitridation rate of gallium is lowered, and a gallium nitride single crystal colored black is obtained. I understood.

The object of the present invention is to prevent unnecessary oxidation and moisture absorption before processing the flux and the raw material when growing the nitride single crystal by heat-treating the nitride raw material in a nitrogen-containing atmosphere in a flux such as Na flux. Then, it is to grow a good quality nitride single crystal .

The present invention accommodates a raw material of one or more metals and nitride single crystals selected from the group consisting of alkali metals and alkaline earth metals in a container covered with a metal tin foil, and the container is contained in a nitrogen-containing atmosphere. The metal, the raw material, and the metal tin foil are melted by heat treatment while applying pressure , and a nitride single crystal is grown .

According to the present invention, one or more metals selected from the group consisting of alkali metals and alkaline earth metals and nitride single crystal raw materials are housed in a container covered with a metal tin foil, and the container is non-oxidized. Heat treatment in a sexual atmosphere. At this time, the metal and the raw material are melted together with the metal tin foil surrounding the metal and the raw material to cause a predetermined reaction. Thereby, the said metal and raw material are interrupted | blocked from oxidizing atmosphere and moisture from the time of weighing to the time when metal tin foil fuse | melts and it uses for reaction in nitrogen-containing atmosphere. In addition, in the present invention, the metal tin foil is melted in a container and present in the melt in a non-oxidizing atmosphere, and unlike a simple protective film, it is shielded from oxygen and moisture until the reaction stage. So it is an epoch-making method.

  Hereinafter, the present invention will be described in more detail with reference to the drawings as appropriate.

As shown in FIG. 1, the raw material 20 is enclosed in the glove box of non-oxidizing atmosphere, and is enclosed in the inner space 1a of the container 1 in non-oxidizing atmosphere. The container 1 may be provided with a lid. In this example, the raw material 20 is made of one or more metal and nitride single crystal raw materials selected from the group consisting of alkali metals and alkaline earth metals . For example, 3 is metallic sodium, 4 is metallic gallium, and 5 is metallic lithium. A seed crystal substrate 6 is installed at the bottom of the container 1. The entire raw material 20 is covered with the metal foil 2 without a gap and placed on the seed crystal substrate 6. In this state, the container 1 is taken out from the glove box, and then placed in the crystal growing apparatus as it is.

  For example, in the example shown in FIG. 2, the outer container 21 and the container 1 are installed in the pressure container 12 of the HIP (hot isotropic pressure press) apparatus 11. A gas mixture cylinder (not shown) is provided outside the pressure vessel 12. The mixed gas cylinder is filled with a mixed gas having a predetermined composition. The mixed gas is compressed by a compressor to a predetermined pressure, and is supplied into the pressure vessel 12 through the supply pipe 15 as indicated by an arrow A. Nitrogen in the atmosphere serves as a nitrogen source, and an inert gas such as argon gas suppresses evaporation of the flux. This pressure is monitored by a pressure gauge (not shown). A heater 14 is installed around the outer container 21 and the container 1 so that the growth temperature in the container can be controlled.

  When the container 1 is heated and pressurized in the pressure container 21, as shown in FIG. 3, the mixed raw material 20 is completely dissolved in the container 1, and the metal foil is also dissolved to generate the flux 7. Here, if predetermined single crystal growth conditions are maintained, nitrogen is stably supplied from the inner space 1 a into the flux 7, and the single crystal film 8 is grown on the seed crystal substrate 6.

Here, according to the present invention, the heat treatment reaction is performed in a state where the raw material of one or more metals selected from the group consisting of alkali metals and alkaline earth metals and nitride single crystals is covered with the metal tin foil. It is transported into the apparatus, and the metal foil melts during heating to generate a flux together with the raw materials. Therefore, there is no possibility that the metal and the raw material oxidize and absorb moisture, nitrogen is satisfactorily supplied as indicated by arrow B in the flux, and the nitride single crystal is grown without variation.

  On the other hand, for example, when sodium metal oxidizes and absorbs moisture, as shown in FIG. 4, for example, as shown in FIG. 4, the oxidized raw material gathers in the vicinity of the liquid surface of the flux 10. So as not to melt. For this reason, nitrogen flows on the liquid surface of the flux 10 as shown by an arrow C and is not supplied well into the flux. As a result, a high-quality single crystal film is not formed on the seed crystal 6 with high productivity, and problems such as coloring may occur in the obtained single crystal.

  The nitrogen-containing atmosphere may consist only of nitrogen, but may contain a non-oxidizing gas other than nitrogen, for example, an inert gas such as argon or a reducing gas.

  In the present invention, an apparatus for heating and pressurizing is not particularly limited. This apparatus is preferably a hot isostatic pressing apparatus, but other atmospheric pressure heating furnaces may be used.

  An easily oxidizable and easily hygroscopic substance means a substance that is easily oxidized and absorbed when it comes into contact with the atmosphere at room temperature. For example, a substance that can be oxidized and absorbed within 1 minute. Means. The easily oxidizable and easily hygroscopic substance may be a mixture of powders or a molded body.

In the present invention , an easily oxidizable and easily hygroscopic substance is a flux and a raw material for growing a nitride single crystal. The flux contains at least one metal selected from the group consisting of alkali metals and alkaline earth metals. As this metal, sodium, lithium and calcium are particularly preferable, and sodium is most preferable.

In addition to the metal, for example, the following metals can be added to the flux.
Potassium, rubidium, cesium, magnesium, strontium, barium, tin A small amount of an impurity element can be added as a dopant. For example, silicon can be added as an n-type dopant.

For example, the following single crystals can be suitably grown by the growing method of the present invention.
GaN, AlN, InN, mixed crystals of these (AlGaInN), BN

In the present invention, one or more metals selected from the group consisting of alkali metals and alkaline earth metals and nitride single crystal raw materials are housed in a container covered with a metal tin foil, and the container contains nitrogen. Heat treatment under atmosphere.

  As described above, the present invention is applied to a single crystal growth reaction in a flux.

Metal tin foil is a metal that is difficult to oxidize and has low hygroscopicity, and has a melting point equal to or lower than the heat treatment temperature (melting point 232 ° C.).

  The heating temperature and pressure in the present invention are not particularly limited because they are selected depending on the kind of easily oxidizable and easily hygroscopic substance. The heating temperature can be, for example, 800 to 1200 ° C. Moreover, although there is no upper limit in particular, in the case of the flux method, it can be made into 1500 degreeC or less, for example. Also, the pressure is not particularly limited, but in an embodiment in which the sealing means is destroyed or removed by pressure, the pressure is preferably 1 MPa or more, and more preferably 5 MPa or more. The upper limit of the pressure is not particularly defined, but in the case of the flux method, it can be set to 200 MPa or less, for example.

  The material of the container for carrying out the reaction is not particularly limited as long as it is an airtight material that is durable under the intended heating and pressurizing conditions. Examples of such materials include refractory metals such as tantalum, tungsten, and molybdenum, oxides such as alumina, sapphire, and yttria, nitride ceramics such as aluminum nitride, titanium nitride, zirconium nitride, and boron nitride, tungsten carbide, tantalum carbide, and the like. And pyrolytic products such as p-BN (pyrolytic BN) and p-Gr (pyrolytic graphite).

Hereinafter, more specific single crystals and their growth procedures will be exemplified.
(Gallium nitride single crystal growth example)
Using the present invention, a gallium nitride single crystal can be grown using a flux containing at least sodium metal. This flux is mixed with a gallium source material. As the gallium source material, a gallium simple metal, a gallium alloy, and a gallium compound can be applied, but a gallium simple metal is also preferable in terms of handling.

  This flux can contain metals other than sodium, such as lithium. The usage ratio of the gallium source material and the flux source material such as sodium may be appropriate, but in general, the use of an excess amount of Na is considered. Of course, this is not limiting.

  In this embodiment, a gallium nitride single crystal is grown under an atmosphere composed of a mixed gas containing nitrogen gas under a total pressure of 300 atm or more and 2000 atm or less. By setting the total pressure to 300 atm or higher, a high-quality gallium nitride single crystal could be grown, for example, in a high temperature region of 900 ° C. or higher, more preferably in a high temperature region of 950 ° C. or higher. The reason for this is not clear, but it is presumed that the nitrogen solubility increases as the temperature rises, and nitrogen is efficiently dissolved in the growing solution. Further, if the total pressure of the atmosphere is 2000 atm or more, the density of the high-pressure gas and the density of the growth solution are considerably close, and therefore it is difficult to hold the growth solution in the container for performing the reaction. Absent.

  In a preferred embodiment, the nitrogen partial pressure in the growth atmosphere is set to 100 atm or more and 2000 atm or less. By setting this nitrogen partial pressure to 100 atm or higher, for example, in a high temperature region of 1000 ° C. or higher, dissolution of nitrogen into the flux was promoted, and a high-quality gallium nitride single crystal could be grown. From this viewpoint, it is more preferable that the nitrogen partial pressure of the atmosphere is 200 atm or more. Further, it is preferable that the nitrogen partial pressure is practically 1000 atm or less.

A gas other than nitrogen in the atmosphere is not limited, but an inert gas is preferable, and argon, helium, and neon are particularly preferable. The partial pressure of a gas other than nitrogen is a value obtained by subtracting the nitrogen gas partial pressure from the total pressure.
In a preferred embodiment, the growth temperature of the gallium nitride single crystal is 950 ° C. or higher, more preferably 1000 ° C. or higher, and a high-quality gallium nitride single crystal can be grown even in such a high temperature region. . Moreover, there is a possibility that productivity can be improved by growing at high temperature and high pressure.

  The upper limit of the growth temperature of the gallium nitride single crystal is not particularly limited. However, if the growth temperature is too high, the crystal is difficult to grow. Therefore, the temperature is preferably set to 1500 ° C. or lower. From this viewpoint, the temperature is further set to 1200 ° C. or lower. preferable.

The material of the growth substrate for epitaxially growing the gallium nitride crystal is not limited, but sapphire, AlN template, GaN template, silicon single crystal, SiC single crystal, MgO single crystal, spinel (MgAl ₂ O ₄ ), LiAlO ₂ , LiGaO ₂ , perovskite complex oxides such as LaAlO ₃ , LaGaO ₃ , and NdGaO ₃ . The composition formula _{[A 1-y (Sr 1-} x Ba x) y ] _{[(Al 1-z Ga z)} 1-u · D u ] _O 3 (A is a rare earth element; D is niobium and One or more elements selected from the group consisting of tantalum; y = 0.3-0.98; x = 0-1; z = 0-1; u = 0.15-0.49; x + z = 0 .1 to 2) cubic perovskite structure composite oxides can also be used. SCAM (ScAlMgO ₄ ) can also be used.

(Example of growing AlN single crystal)
The present invention has been confirmed to be effective even when growing an AlN single crystal by pressurizing a melt containing a flux containing at least aluminum and an alkaline earth in a nitrogen-containing atmosphere under specific conditions. .

Example 1
A gallium nitride single crystal film was grown on the seed crystal substrate 6 according to the method described with reference to FIGS.

  Specifically, 30 g of metallic sodium, 20 g of metallic gallium, and 30 mg of metallic lithium were weighed in a glove box. Here, metal gallium 4 and metal lithium 5 were first wrapped with metal sodium 3. After the metal sodium thin film oxidized during this work was scraped off, the entire metal sodium 3 was tightly wrapped without any gaps using Sn foil 2 (thickness 50 microns, length 5 cm, width 10 cm). These raw materials 20 were filled in an alumina container 1 having an inner diameter of φ80 mm.

  As the seed crystal 6, a φ2 inch AlN template substrate, a GaN template substrate, or a GaN single crystal free-standing substrate was used. The substrate 6 was placed horizontally at the bottom of the container 1 such that the single crystal thin film of the template was facing upward or the Ga surface of the GaN single crystal free-standing substrate was facing upward. An AlN template refers to an AlN single crystal epitaxial thin film formed on a sapphire single crystal substrate. A GaN template substrate refers to a GaN single crystal epitaxial thin film grown on a sapphire substrate. The film thickness of the template may be appropriate, but it needs to be greater than the film thickness that melts back at the start of growth. The AlN template is less likely to melt back than the GaN template. For example, the AlN template may have a film thickness of 1 micron or more, and the GaN template may have a film thickness of 3 microns or more.

  Since the entire metal sodium 3 that is an easily oxidizable and easily hygroscopic substance is covered with the Sn foil 2, there is no fear of oxidation. In order to confirm this, the raw material 20 wrapped in the metal foil 2 was taken out of the glove box and exposed to the atmosphere for 10 minutes, and then a container was placed in the growing apparatus 11. Using nitrogen gas, the temperature was raised and pressurized to 900 ° C. and 50 atm for 1 hour, and held at 900 ° C. for 100 hours. Next, after naturally cooling to room temperature, the container 1 was taken out from the growing apparatus 11 and treated in ethanol to dissolve Na and Li. Thereafter, it was put on thin hydrochloric acid to remove the remaining Ga and Sn, and the GaN single crystal 8 was taken out. The size of the GaN single crystal 8 was φ2 inches, the thickness was about 3 mm, and the shape was substantially circular. The color was almost colorless and transparent.

(Comparative Example 1)
A GaN single crystal 8 was grown in the same manner as in Example 1. However, Sn foil 2 was not used. The exposure time of the raw material 20 to the atmosphere was shortened to about 5 minutes. As a result, the GaN single crystal did not grow at all. This seems to be due to the oxidation and moisture absorption of metallic sodium.

It is sectional drawing which shows typically the container 1 which accommodated the raw material 20 wrapped in the metal foil 2. FIG. It is a schematic diagram which shows the state which set the container of FIG. 1 to the HIP apparatus. 2 is a cross-sectional view schematically showing a state where a raw material and a metal foil are melted in a container 1 to grow a single crystal. FIG. It is sectional drawing which shows typically the state which has grown the single crystal using the oxidized raw material.

DESCRIPTION OF SYMBOLS 1 Container 2 Metal foil 3, 4, 5 Easily oxidizable or hygroscopic substance 6 Seed crystal 7 Flux 8 Single crystal thin film 20
Raw material A, B Flow of nitrogen C Flow of oxidized flux components

Claims (5)

One or more metals selected from the group consisting of alkali metals and alkaline earth metals and nitride single crystal raw materials are housed in a container covered with metal tin foil, and the container is heated while being pressurized in a nitrogen-containing atmosphere. A method for producing a nitride single crystal , wherein the metal, the raw material, and the metal tin foil are melted by the treatment to grow the nitride single crystal . It characterized in that the nitride single crystal is gallium nitride, the process of claim 1. It characterized in that the nitride single crystal is aluminum nitride, The method of claim 1, wherein. The method according to claim 1, wherein the metal contains at least sodium. The heat treatment which comprises carrying out in hot isostatic pressing device, a method according to any one of claims 1-4.

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