JP2005008444A - Method for preparing nitride crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preparing a high quality nitride crystal, especially, a gallium nitride crystal hardly containing impurities under the industrially applicable condition of a relatively low pressure. <P>SOLUTION: The method for preparing the nitride crystal is characterized in that a raw material is charged into a reaction vessel equipped with a valve, then a nitrogen-containing solvent is introduced into the reaction vessel through the valve without being exposed to ambient air, thereby obtaining the crystal. In this method, impurities are suppressed from being mixed into the reaction vessel, thus a high crystallinity and high quality bulk nitride crystal can easily, safely and efficiently be obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a nitride crystal, and more particularly to a method for producing a high-quality bulk crystal of a periodic table group 13 element nitride represented by gallium nitride.
[0002]
[Prior art]
Gallium nitride (GaN) is useful as a material applied to electronic devices such as light emitting diodes and laser diodes. The most common method for producing this gallium nitride crystal is a method of performing vapor phase epitaxial growth on a substrate such as sapphire or silicon carbide by MOCVD (Metal-Organic Chemical Vapor Deposition). However, since this method uses heteroepitaxial growth in which the substrate and GaN have different lattice constants and thermal expansion coefficients, lattice defects are likely to occur in the obtained GaN, and it is difficult to obtain a quality that can be applied with a blue laser or the like. There is a problem that there is.
[0003]
Therefore, in recent years, there has been a strong demand for establishment of a new manufacturing technique for high-quality gallium nitride massive single crystals for homoepitaxial substrates, which can replace the above method. Various methods have been proposed for producing such a new gallium nitride crystal.
S. Porowski et al. Reacts nitrogen and Ga under a high pressure of about 2000 MPa to obtain a gallium nitride crystal (see Non-Patent Document 1).
In addition, R.A. Dwilinski et al. Used KNH as a mineralizing agent for crystallization using ammonia in a supercritical state as a solvent under a high pressure of 100 to 500 MPa. ₂ Is used to obtain a gallium nitride crystal (see Non-Patent Document 2).
On the other hand, H. Yamane et al. Have obtained a gallium nitride crystal from metal gallium and dissolved nitrogen using metal sodium or metal lithium as a flux (see Non-Patent Document 2).
[0004]
[Non-Patent Document 1] Crystal Growth 178 (1997) 174-178
[Non-Patent Document 2] ACTA PHYSICA POLONICA A Vol. 88 (1995) 833-836
[Non-patent Document 3] Crystal Growth 218 (2000) 7-12
[0005]
[Problems to be solved by the invention]
However, the method of growing a nitride crystal by the above method has not been established industrially, and is only a rudimentary study. There is no clear method for obtaining large-sized large single crystals of high quality, ensuring high safety by requiring very expensive equipment for high-pressure equipment or using highly reactive substances as flux. Therefore, it is difficult to obtain high production efficiency. Generally speaking, the growth method of gallium nitride by the solution growth method using ammonia as a solvent is limited to the operation and equipment in the production of oxide crystals by the hydrothermal synthesis (growth) method using water as a solvent. It will be only a rudimentary condition study in Japan. Therefore, the details of the crystal growth mechanism are unknown, and no method has been established for obtaining a high-quality single crystal having a large size and a small amount of impurities, high crystallinity, and low yield. It is enough. In addition, in order to carry out nitride crystal growth, safety issues must be fully considered when ammonia, which is more toxic and dangerous than water, is used as a solvent.
[0006]
[Means for Solving the Problems]
In view of the above problems, the present inventor has intensively studied a method capable of producing high-quality nitride crystals by a method that can be applied industrially, and as a result, the conditions for supplying the nitrogen-containing solvent to the reaction vessel In consideration of the fact that the outside air outside the reaction system, especially the incorporation of a small amount of water or oxygen into the reaction system, has a greater negative impact on the crystal growth rate and crystal quality of the nitride than expected. As a result, the present invention was reached. That is, the present invention is characterized in that after a raw material is filled in a reaction vessel attached with a valve, a nitrogen-containing solvent is introduced into the reaction vessel through the valve without touching the outside air to obtain a crystal. The present invention relates to a method for producing a nitride crystal.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail. In the method for producing a nitride crystal of the present invention, the type of nitride crystal to be produced is not particularly limited, but is mainly nitriding a single metal of a group 13 element such as B, Al, Ga, and In The present invention relates to a method for manufacturing a crystal of a material (for example, GaN, AlN) or a crystal of a nitride of an alloy group (for example, GaInN, GaAlN), and particularly targets a crystal of gallium nitride.
As a raw material for producing such a nitride crystal, a nitride polycrystalline powder raw material (hereinafter sometimes referred to as a polycrystalline raw material) is used, and this is crystal-grown in a nitrogen-containing solvent. There is no particular need for such a nitride polycrystalline material to be a complete nitride, and it may be preferable to include a metal component in a metal state (that is, zero valence) depending on conditions. The reason for this is not clear, but perhaps when a small amount of oxygen is mixed in the reaction system, the metal component plays a role as an oxygen trapping agent that prevents the oxygen from diffusing into the nitrogen-containing solvent. Presumed to be. Further, the ratio of the metal in the metal state is not particularly limited, but it is necessary to consider that the generation of hydrogen during the growth of the nitride crystal cannot be ignored if it is too much.
[0008]
There are no particular restrictions on the method for producing the polycrystalline raw material that is used as the raw material, but, for example, many nitrides produced by reacting metal, oxide, or hydroxide with the ammonia in a reaction vessel in which ammonia gas is circulated. Crystals can be used. In addition, as a metal compound material having higher reactivity, a halide, an amide compound, an imide compound, a compound having a covalent MN bond such as galazane, or the like can be used. Furthermore, it is possible to use a nitride polycrystal produced by reacting a metal such as Ga with nitrogen at a high temperature and a high pressure.
[0009]
As for the above-mentioned polycrystalline raw material, in order to obtain a high quality crystal by growing the crystal, contamination of water and oxygen should be avoided as much as possible. Therefore, the amount of oxygen atoms in the polycrystalline raw material is usually 5% by weight or less, preferably 2% by weight or less, particularly preferably 0.5% by weight or less. The ease with which oxygen is mixed into the polycrystalline raw material is related to the reactivity with water or the absorption capacity. For example, the worse the crystallinity of the polycrystalline raw material, the more active groups such as NH groups exist on the surface, etc., and this may react with water to generate some oxides and hydroxides. is there. Therefore, it is usually desirable to use a polycrystalline material having as high a crystallinity as possible, and the crystallinity can be estimated by the half-value width of powder X-ray diffraction. The preferred polycrystalline material is not particularly concerned with the crystal form, but the half-value width of the (100) diffraction line (2θ = about 32.5 ° for hexagonal gallium nitride) is usually 0.25 ° or less, preferably 0 20 ° or less, particularly preferably 0.17 ° or less.
[0010]
The grain size of the polycrystalline raw material is preferably in the range of 1 to 20 μm. The smaller the particle size, the larger the specific surface area and the higher the dissolution rate in the solvent, which is preferable. However, if the particle size is too small, the particles are transported to the crystal growth section of the reactor by thermal convection, and if a seed crystal is used, there is a risk of adhering to the seed crystal, so it is necessary to take measures.
In addition, by using two types of polycrystalline raw materials having different average particle diameters, Ga (( Insufficient supply of ions and the like to the crystal growth section can be suppressed, and as a result, particularly when a seed crystal is used, it is possible to suppress the disadvantage of the growth of the bulk single crystal, elution of the seed crystal.
On the other hand, the shape of the polycrystalline raw material is not particularly limited, but in consideration of the uniformity of dissolution in a solvent, a spherical material is usually preferable.
[0011]
The polycrystalline raw material as described above is usually subjected to a crystallization step after being mixed with an additive called a mineralizer. Such mineralizers are believed to enhance the solubility of polycrystalline raw materials in solvents. In addition to using one kind of mineralizer, another kind may be used as a co-mineralizer or a mixture of two or more kinds may be used. For example, in the case of GaN, the ratio of the polycrystalline raw material to the mineralizer can be selected as a mineralizer / Ga molar ratio, usually in the range of 0.01-10, and the optimum ratio is the raw material, mineralizer Etc., and the size of the target crystal.
The mineralizer is usually a halogen atom or a compound containing an alkali metal, alkaline earth metal, or rare earth metal. Also preferred are those containing nitrogen atoms in the form of ammonium ions or amides. Examples containing halogen atoms include ammonium halides, hydrogen halides, ammonium hexahalosilicates, and hydrocarbyl ammonium fluorides, tetramethylammonium halides, tetraethylammonium halides, benzyltrimethylammonium halides, dipropylammonium halides. And alkyl ammonium salts such as isopropyl ammonium halide, alkyl metal halides such as alkyl sodium fluoride, and the like are exemplified, and ammonium halide and hydrogen halide are preferable, and ammonium halide is particularly preferable. These substances are highly soluble in solvents and have nitriding ability by decomposition.
[0012]
As the curing agent compound containing an alkali metal, alkaline earth metal, or rare earth metal, those containing a nitrogen atom in the form of ammonium ion or amide in addition to an alkali halide, alkaline earth, rare earth halide or the like are preferable. Alkali, alkaline earth, and rare earth oxoacid salts can also be used, but are not preferred mineralizers from the viewpoint of preventing the generated crystals from containing oxygen. Such curing agents include sodium amide (NaNH ₂ ), Potassium amide (KNH) ₂ ), Lithium amide (LiNH ₂ ), Lithium diethylamide ((C ₂ H ₅ ) ₂ NLi)) and other alkali metal amides, and Mg (NH ₂ ) ₂ Alkalinity metal amides such as La (NH ₂ ) ₃ Terrestrial amides such as Li ₃ N, Mg ₃ N ₂ , Ca ₃ N ₂ , Na ₃ Examples thereof include alkali metal nitrides such as N or alkaline earth metal nitrides. Also, chromium nitride (CrN), niobium nitride (NbN), silicon nitride (Si ₃ N ₄ ), Zinc nitride (Zn ₃ N ₂ ) Or NH ₂ NH ₃ Salts of hydrazines such as Cl, ammonium carbonate ((NH ₄ ) ₂ CO ₃ ), Ammonium carbamate (NH ₂ COONH ₄ ).
[0013]
Thus, the mixed polycrystalline raw material and the additive such as mineralizer are filled in the reaction vessel, but if not particularly required, the polycrystalline raw material and one or more additives such as mineralizer are reacted separately. The container may be filled. Depending on the types of additives such as raw materials and mineralizers, the reaction vessel may be introduced as a liquid after the lid of the reaction vessel is closed, and at that time, it may be passed through a valve attached to the reaction vessel of the present invention. . When additives such as polycrystalline raw materials and mineralizers are required for moisture absorption, it is preferably used to dry sufficiently by heating and degassing before filling. Furthermore, when it is necessary to mix and fill the mineralizer with high decomposability and the polycrystalline raw material, it is promptly performed in an atmosphere in which oxygen and moisture are eliminated as much as possible. For example, in a container or a room filled with an inert gas, the inside of the reaction vessel is sufficiently replaced with the inert gas and then introduced. After the polycrystalline raw material and an additive such as a mineralizer are mixed and filled into the reaction vessel, or separately filled into the reaction vessel, the lid is closed. Thereafter, it is also effective to heat and deaerate the reaction vessel and the piping section through a valve attached to the reaction vessel of the present invention. In addition, it is also preferable to bring a substance (for example, a metal piece such as titanium) that serves as a scavenger that selectively absorbs oxygen and moisture into the reaction vessel.
Additives such as raw materials and mineralizers are usually packed so as to fit in the lower part of the reactor. This is because by giving a temperature difference between the lower part of the reactor and the upper part of the reactor, dissolved crystals can be deposited on the upper part of the reactor. Thus, by obtaining crystals through the process of dissolution and precipitation of raw materials, it is possible to obtain high-quality, high-quality bulk crystals with high crystallinity.
[0014]
In addition, by preparing a seed crystal in the upper part of the reactor, the generation of a single crystal can be promoted and a larger single crystal can be obtained. The seed crystal is normally charged after filling with additives such as raw materials and mineralizers. If necessary, it is also effective to heat and deaerate after charging into the reaction vessel.
Although it is desirable to use a single crystal of the target nitride as the seed crystal, it may not necessarily be the same nitride as the target, and may be an oxide single crystal in some cases. However, in that case, it is consistent with the target nitride, has a suitable lattice constant, crystal lattice size parameter, or is coordinated to ensure heteroepitaxy (ie, coincidence of the crystallographic position of some atoms). It is necessary to be composed of a single crystal material piece or a polycrystalline material piece. For example, in the case of gallium nitride (GaN), in addition to a single crystal of GaN, a single crystal of zinc oxide (ZnO), a single crystal of silicon carbide (SiC), lithium gallate (LiGaO) ₂ ), Zirconium diboride (ZrB) ₂ ) And the like.
The seed crystal should be determined considering the solubility in the nitrogen-containing solvent used and the reactivity with the mineralizer. As a seed crystal of GaN, a single crystal obtained by epitaxial growth on a dissimilar substrate such as sapphire by MPCVD method or HVPE method and exfoliation, or crystal growth of Na, Li, or Bi from metal Ga as a flux was obtained. A single crystal and a homocrystal / heteroepitaxially grown single crystal obtained by using the LPE method, a single crystal produced based on a solution growth method including the method of the present invention, and a crystal obtained by cutting them are used.
[0015]
The crystal growth reaction is performed by closing the lid of the reaction vessel into which the polycrystalline raw material, mineralizer and the like are introduced, and further introducing a nitrogen-containing solvent into the reaction vessel through a valve. The nitrogen-containing solvent is not limited as long as it can dissolve the raw material in the reaction as described above. The nitrogen-containing solvent dissociates itself into a cation and an anion, and is ionically solvated with respect to the raw material. It may be one that dissolves, or one that dissolves raw materials or the like by intermolecular force. For example, ammonia (NH ₃ ), Hydrazine (NH ₂ NH ₂ ), Primary amines such as methylamine, secondary amines such as dimethylamine, tertiary amines such as trimethylamine, diamines or melamines such as ethylenediamine, and imidazole and pyridine. And other nitrogen-containing solvents that do not impair the stability of the nitride III-V such as cation salts of these nitrogen-containing compounds and ionic liquids thereof. be able to.
[0016]
Further, a substance that becomes liquid at a high temperature such as ammonium halide, alkali nitride, or alkali amide can be used as the nitrogen-containing solvent. However, in view of avoiding the incorporation of carbon or the like into the resulting nitride crystal, the nitrogen-containing solvent is preferably ammonia or hydrazine, ammonium halide, alkali nitride or alkali amide, and avoiding the incorporation of alkali into the produced crystal. In view of safe and safe handling and removal of the solvent, ammonia is more preferable.
Also, from the viewpoint of impurities, there is a concern that oxygen present in the atmosphere will be taken in by the crystals to be generated. Especially when ammonia is used as a solvent, the affinity between ammonia and water is high, so the system There is a possibility that oxygen derived from water is easily brought in, the amount of mixed oxygen in the generated crystal is increased, and the crystallinity of the nitride may be deteriorated. It is desirable to reduce the amount of water and oxygen contained in the solvent, preferably 1000 ppm or less, particularly preferably 100 ppm or less. These solvents can be used singly or in combination of plural kinds.
[0017]
These solvents are preferably used in a subcritical state or even in a supercritical state during nitride crystal synthesis or growth. A supercritical fluid means a concentrated gas that is maintained above its critical temperature, and the critical temperature is a temperature at which the gas cannot be liquefied by pressure. Supercritical fluids generally have lower viscosities and are more easily diffused than liquids, but have solvating power similar to liquids. Of course, unlike water used as a solvent in hydrothermal synthesis (growing) methods, it is difficult to say that the properties of nitrogen-containing solvents have been clarified. The reason why nitride formation and dissolution precipitation are promoted cannot be determined, but if the concept of ionic product known in water is applied to a nitrogen-containing solvent, it increases with increasing temperature, and hydrolysis in water There is also a possibility that an increase in the action such as amylolysis equivalent to
When used in a supercritical state, the reaction mixture is generally maintained at a temperature above the critical point of the solvent. When ammonia is used as a solvent, the critical point is a critical temperature of 132 ° C. and a critical pressure of 11.35 MPa. However, if the filling rate of the reaction vessel is high, the pressure far exceeds the critical pressure even at a temperature below the critical temperature. The supercritical state here includes a state exceeding the critical pressure. Since the reaction mixture is enclosed within a constant volume (container volume), an increase in temperature increases the pressure of the fluid. In general, if T> Tc (critical temperature of one solvent) and P> Pc (critical pressure of one solvent), the fluid is in a supercritical state.
[0018]
In fact, since the solubility of the nitride polycrystalline raw material in the solvent is very different between the subcritical state and the supercritical state, a sufficient growth rate of the nitride crystal can be obtained under the supercritical condition. The reaction time depends in particular on the reactivity of the mineralizer or co-mineralizer and on the thermodynamic parameters, ie temperature and pressure values. During nitride crystal synthesis or growth, the reaction vessel is maintained in a pressure range of about 5 MPa to 2 GPa. The pressure is set according to the type and temperature of the nitrogen-containing solvent and the filling rate of the solvent volume with respect to the reactor volume. Originally, if a nitrogen-containing solvent is selected, the rest is uniquely determined by temperature and filling rate, but in reality, additives such as raw materials, mineralizers, and reaction vessel temperature heterogeneity, and It differs slightly due to the presence of unheated parts (called dead volume) such as the pressure sensor and the pipe to the valve. Further, for example, when ammonia is used as a nitrogen-containing solvent, the dissociation equilibrium is greatly inclined to nitrogen and hydrogen at a high temperature, so that a change in pressure due to this may not be negligible at a high temperature. Generally, the dissociation reaction is catalyzed by a metal component, and there is a possibility of reaching an equilibrium depending on the kind of additives such as raw materials and mineralizers. However, even in consideration of these, after charging the raw material into the reaction vessel attached to the valve, and introducing and introducing the nitrogen-containing solvent into the reaction vessel through the valve without touching the outside air, the valve is closed, The lower limit of the temperature range in the reaction vessel is usually 150 ° C or higher, preferably 200 ° C or higher, particularly preferably 300 ° C or higher, and the upper limit is usually 800 ° C or lower, preferably 700 ° C or lower, particularly preferably 650 ° C or lower. A range is desirable. Then, it is desirable to maintain the pressure range in the reaction vessel at a lower limit of usually 20 MPa or more, preferably 30 MPa or more, particularly preferably 50 MPa or more, and an upper limit of usually 500 MPa or less, preferably 400 MPa or less, particularly preferably 200 MPa or less. .
[0019]
The injection rate of the nitrogen-containing solvent is the free volume of the reaction vessel, that is, when a raw material and a seed crystal containing a polycrystalline raw material are used in the vessel, and when a structure and a baffle plate for installing it are installed. Subtract the volume of the baffle plate from the reaction vessel, and the liquid density in the standard state of the nitrogen-containing solvent, and in the case of gas in the standard state, the liquid density at the boiling point is the standard, and the packing rate is usually 20 to 95%, preferably 40 to 90%, more preferably 50 to 80%.
The nitrogen-containing solvent is introduced into the reaction vessel through a pipe from the tank, through a valve attached to the reaction vessel, and without contact with outside air. At that time, the nitrogen-containing solvent is usually introduced as a liquid. When introducing the nitrogen-containing solvent as a liquid, if additives such as raw materials and mineralizers are sufficiently soluble in the nitrogen-containing solvent, they can be dissolved and introduced simultaneously. In addition, when the boiling point of the nitrogen-containing solvent is low, introduction as a gas is also preferably used. When introducing as gas or liquid, it is also possible to provide a flow control device in the middle and introduce a preset amount. When introducing a nitrogen-containing solvent, it is generally more time to introduce a gas than a liquid to fill the reaction vessel with a solvent. However, when introducing the gas into the reactor as a gas, impurities originating from piping or other sources are introduced into the reaction vessel. Since it is possible to avoid bringing it in, and it is possible to replace the reaction vessel once as a gas, it is possible to fill a nitrogen-containing solvent with extremely high purity.
[0020]
The reaction is generally carried out by closing the lid of the reaction vessel into which the polycrystalline raw material and mineralizer have been introduced, and then adding a nitrogen-containing solvent, but degassing the reaction vessel and the piping before adding the nitrogen-containing solvent. It is also preferable to use an inert gas such as nitrogen. Here, it is possible to introduce the nitrogen-containing solvent safely and reliably by using a reaction vessel attached with a valve. That is, when the reaction vessel is degassed after closing the lid, the connection with the side that is evacuated and then evacuated can be shut off by closing at least one valve, and then a nitrogen-containing solvent is introduced. This is because the nitrogen-containing solvent can be safely introduced by opening the valve on the side where the outside air does not leak. In addition, even when the reaction vessel is circulated with an inert gas after closing the lid, oxygen or water is mixed by opening a pipe with a valve that introduces a nitrogen-containing solvent in advance. Without introducing a nitrogen-containing solvent. Normally, if a reaction vessel with a valve is not used, even if the inside of the reaction vessel is once degassed or inert gas is circulated, the lid is opened or opened again when the nitrogen-containing solvent is introduced. It will be introduced from the mouth, and it is inevitable that outside air is mixed in, and oxygen and water. In particular, when ammonia is used as a nitrogen-containing solvent, the reaction vessel itself is cooled by the latent heat, and it is inevitable that moisture in the air is condensed and mixed into the reaction vessel. If this is to be avoided, the reaction vessel must be filled with an inert gas in advance or put in or put into a room, and then a nitrogen-containing solvent will be introduced into it, which is a very dangerous and large reaction. In the case of a container, the efficiency is low, especially when using a highly volatile and harmful solvent such as ammonia, it is impossible for the person performing the work to enter the room and the solvent component leaked into the room after use It is also difficult to safely remove and process.
[0021]
If a reaction vessel with a valve is used, it is also suitable to put a nitrogen-containing solvent while circulating an inert gas. Also, by attaching one or more pipes with a valve, for example, it can be connected to the absorption tower via a valve different from the valve for introducing a nitrogen-containing solvent, and the valve is connected at the time of solvent introduction or solvent removal. By opening it, it is possible to safely recover the excess solvent and the removal solvent. When introducing the nitrogen-containing solvent, the reaction vessel may be cooled below the boiling point of the nitrogen-containing solvent. However, when ammonia is not used, even if the boiling point is below room temperature, the latent heat is large. It is possible to fill the liquid without cooling.
[0022]
FIG. 1 shows a schematic view of a reaction vessel with a valve that can be suitably used in the present invention, but the method for producing a nitride of the present invention is not limited to this. The upper part of the vertically arranged long cylindrical reaction vessel is connected to a pressure control device and a pressure sensor by piping, and at least one piping for introducing or removing solvent is attached. They can be closed by at least one or more valves. Although the end of the pipe on the side opposite to the reaction vessel of the valve is not shown, it is often branched and is often connected to a tank of a nitrogen-containing solvent, a vacuum pump, or the like via a further valve. As a result, continuous gas degassing, replacement exchange, solvent introduction and removal without contact with the outside air can be performed. In addition, a joint is provided at the end of the pipe on the side opposite to the reaction vessel of the valve, and the degassing of the gas is performed while the reaction vessel is closed at a time other than the degassing, replacement exchange, introduction of the solvent, and removal. It is also possible to separate the various reaction equipment from the equipment related to substitution exchange and the introduction and removal of the solvent. Furthermore, at least the container is provided in or can be housed in a furnace which encloses the container over its entire height and can apply a temperature gradient to the container along the container axis.
In fact, in the valved reaction vessel that can be used in the present invention, the valved pipe may be attached to the reaction vessel main body or the lid portion. The valve may be arranged in series with one pipe connected to the reactor, or may be arranged in parallel with one or more pipes connected to the reaction vessel. It is preferable that the so-called dead volume of the piping portion from the valve closest to the reaction vessel cavity to the reaction vessel cavity is smaller.
[0023]
In addition, it is preferable to cover all or part of the inner surface of the reaction vessel, the inner surface of the pipe portion to the attached valve, and the inner surface of the valve with a surface composed of Group 4, 5, 6 or 10, 11 elements of the periodic table. Used. Or the inner cylinder produced with the said metal can also be installed in reaction container. For example, the Group 10 metals in the periodic table are nickel (Ni), palladium (Pd), and platinum (Pt). In this invention, the method of sealing this reaction container inner cylinder and installing in a reaction container can also be used as needed. As described above, contamination of impurities from the reaction vessel material into the generated nitride crystal can be suppressed.
[0024]
Further, when a seed crystal is provided at the upper part of the reaction vessel, for example, the reaction vessel is divided into two overlapping zones, that is, a lower raw material filling part and an upper growing part separated by a baffle plate, and these two zones are divided. The temperature gradient ΔT during the period is usually 10 to 100 ° C.
The seed crystal is held by a metal wire, and can be fixed by fastening the metal wire to a metal frame. The metals here are preferably those of periodic table 4, 5, 6 or 10, 11 group, for example, nickel (Ni), tantalum (Ta), titanium (Ti), palladium (Pd), platinum (Pt). Gold (Au) and niobium (Nb).
In addition, a baffle plate can be installed in the container and divided into a raw material filling part filled with a raw material made of a polycrystalline raw material and a crystal growth part in which a seed crystal is arranged. The baffle plate preferably has a porosity of 2 to 5% (excluding 5%), and the surface material of the baffle plate is nickel (Ni), tantalum (Ta), titanium ( Ti), palladium (Pd), platinum (Pt), gold (Au), niobium (Nb) and the like are preferable. By controlling the aperture ratio of the baffle plate, it becomes easy to appropriately control the degree of supersaturation in the crystal growth portion under the solution growth conditions. In some cases, a polycrystalline raw material can be additionally supplied onto the baffle plate. Thus, by further interposing a polycrystalline raw material on the baffle plate, that is, between the raw material filling part and the seed crystal arranging part, the transition speed of the crystal growing part to the supersaturated state can be increased, and the seed crystal It is possible to prevent various disadvantages in elution.
[0025]
Further, the reaction vessel may be provided with a precipitate collection net and a precipitation preventing umbrella plate above the location where the seed crystal is placed, that is, near the convergence point of the solvent convection. The role of the precipitate collection net and the umbrella plate for preventing precipitation is as follows. That is, as it goes to the upper part of the reaction vessel, the solvent convection, that is, the solute transport flow, goes to a lower temperature region, but the solute that is supersaturated in such a low temperature part is only on the seed crystal. In addition, there is a problem that the metal wire that suspends the seed crystal, the metal frame that fastens the metal wire, and the inner wall of the reaction vessel are deposited as precipitates. In such a case, by providing a precipitate collection net or a precipitation prevention umbrella plate in the vicinity of the convection converging point, the remaining solute that could not be deposited on the seed crystal can be reversed downward by the top inner wall. In the collection net, the microcrystals or precipitates in the transport stream can be captured, and the microcrystals can be selectively deposited on the collection net. In addition, a structure for preventing crystal precipitation can be provided at the piping port leading to the attached valve above the reaction vessel and at the piping port connected to the pressure sensor. As a result, the piping up to the pressure sensor and the piping used to remove the nitrogen-containing solvent are blocked by the precipitated crystals, or additives such as mineralizers dissolved in the solvent when the nitrogen-containing solvent is removed are precipitated. It is possible to avoid blocking. As the material of the collection net, the precipitation prevention umbrella plate, and the piping port precipitation prevention structure, nickel (Ni), tantalum (Ta), titanium (Ti), palladium (Pd), platinum (Pt), gold ( Au) or niobium (Nb) is preferred. The umbrella plate for preventing precipitation may be provided with an opening.
[0026]
In the present invention, in order to appropriately control the degree of supersaturation as described above, it is preferable to set the ratio of the crystal growth part volume to the raw material filling part volume within a range of 1 to 5 times. If the degree of supersaturation usually exceeds 1.5, the rate of precipitation on the seed crystal is too high, so that the consistency inside the grown crystal is deteriorated and defects are introduced, which is not preferable. Moreover, since the amount deposited on the inner wall of the growth vessel and the frame increases, it is not preferable that the precipitates are enlarged and come into contact with the GaN single crystal to inhibit the growth of the single crystal.
Here, “supersaturation” refers to a state in which the amount of dissolution has increased more than the saturated state. Further, the “supersaturation degree” refers to the ratio between the dissolved amount in the supersaturated state and the dissolved amount in the saturated state. In the solution growth method, the amount of nitride dissolved in the crystal growth portion that is supersaturated due to the transport of nitride by thermal convection from the raw material filling portion is compared to the amount of nitride dissolution in the saturation state of the crystal growth portion. Say ratio.
The degree of supersaturation as described above can be controlled by appropriately changing and selecting the density of polycrystalline nitride, the baffle plate opening ratio, the temperature difference between the raw material filling portion and the crystal growth portion, and the like.
[0027]
The crystal growth reaction in the reaction vessel with a valve as described above is maintained in a subcritical state or a supercritical state of the nitrogen-containing solvent by heating and heating the inside of the reaction vessel using an electric furnace or the like. Is done. The method for heating the reaction vessel and the rate of temperature increase to a predetermined reaction temperature are not particularly limited, but it is usually carried out over several hours to several days. If necessary, it is also preferable to perform multi-stage temperature rise or change the temperature rise speed in the temperature range. Further, the reaction vessel can be partially heated with a temperature difference, or partially heated while being cooled.
[0028]
The reaction temperature referred to here is measured by a thermocouple provided so as to be in contact with the outer surface of the reaction vessel, and is not strictly an actual temperature inside the reaction vessel. The temperature gradient inside the reaction vessel varies depending on the shape of the reaction vessel, the shape of the furnace in which it is stored, and the heating and thermal insulation conditions represented by the positional relationship, but some of the temperature gradient was opened inward from the outer surface of the reaction vessel for thermocouples. The temperature inside the reaction vessel is estimated by extrapolating or extrapolating the temperature gradient toward the inside of the reaction vessel using a hole that does not penetrate to the cavity inside the reaction vessel. Similarly, the temperature in the vertical direction of the reaction vessel also varies depending on the shape of the reaction vessel, the shape of the furnace in which it is stored, and the heating and heat retention conditions represented by the positional relationship. Therefore, it is desirable to measure the temperature at several points above and below the outer surface of the reaction vessel and to control the temperature after estimating the temperature inside the reaction vessel at each position. Depending on the shape and temperature of the reaction vessel, even if the temperature of the outer surface of the reaction vessel is the same up and down, or even when the upper part is several tens of degrees Celsius, the temperature inside the reaction container may be several tens of degrees Celsius lower. In addition, as described above, in order to promote dissolution and precipitation of crystals, when a temperature gradient is provided in advance during the reaction, the temperature at several points on the outer surface of the reaction vessel is measured, and a heater divided into multiple stages is used. It is also effective to perform temperature control mainly divided in the vertical direction of the reaction vessel.
[0029]
The reaction time after reaching a predetermined temperature is usually several hours to several hundred days, although it varies depending on the type of nitride crystal, the raw material used, the nitrogen-containing solvent, and the size and amount of the crystal to be obtained. During the reaction, the reaction temperature may be constant or may be gradually raised or lowered. After a reaction time to produce the desired crystals, the temperature is lowered. The method for lowering the temperature is not particularly limited, but the heating of the heater may be stopped and the reaction vessel may be left as it is in the furnace for cooling, or the reaction vessel may be removed from the furnace and air cooled. If necessary, quenching with a refrigerant is also preferably used. In order to prevent partial precipitation of crystals and certain mineralizers such as mineralizers when the temperature drops, the reaction vessel can be partially cooled with a temperature difference or partially heated slightly. It can also be cooled.
[0030]
After the temperature of the outer surface of the reaction vessel or the estimated temperature inside the reaction vessel is lower than the predetermined temperature, the reaction vessel is opened as follows. The predetermined temperature at this time also depends on the type of nitrogen-containing solvent. Open the valve attached to the reaction vessel by opening the valve that is not the reaction vessel side of the valve attached to the reaction vessel of the present invention by opening a valve that is different from the valve of the pipe that is connected to the vessel filled with water or the like. . At this time, an inert gas may be used. When the boiling point of the nitrogen-containing solvent is low, the nitrogen-containing solvent moves as a gas from the reaction vessel to the vessel and is absorbed by water or the like. In this case, when the heat of evaporation of the nitrogen-containing solvent is large, it is also preferable to heat the reaction vessel in order to shorten the movement time. It is also preferable to cool the container on the moving side without filling water or the like. When a method of absorbing in a solvent such as water is not used, the recovered nitrogen-containing solvent can be easily reused. When the boiling point of the nitrogen-containing solvent is high, the solvent may be removed directly by a pump or the like after opening the valve attached to the reaction vessel of the present invention. By these methods, additives such as mineralizers and unreacted raw materials can be removed simultaneously when removing the nitrogen-containing solvent.
[0031]
After removing the nitrogen-containing solvent, the lid will be opened to take out the produced nitride crystals and unreacted raw materials and additives such as mineralizers, but a valve attached to the reaction vessel of the present invention is provided. In this case, in order to completely remove the residual gas of the nitrogen-containing solvent before that, it becomes possible to replace the reaction vessel with an inert gas or the like, or to perform heat degassing.
After the operation as described above, the reaction vessel is safely opened and the produced crystals and unreacted raw materials are taken out.If there is no valve attached to the reaction vessel of the present invention, in order to take them out, Since the lid is first opened, in the case of a nitrogen-containing solvent having a low boiling point, it is dangerous to open the lid unless the reaction vessel is cooled below the boiling point, and the production efficiency is lowered. Further, in the case of a harmful nitrogen-containing solvent, the lid must be opened in a container or room that has been previously shielded from the atmosphere, resulting in low production efficiency. Furthermore, depending on the raw material, since nitride synthesis and crystal growth are performed simultaneously, for example, when a raw material containing a metal is used, hydrogen is generated in the container. In this case, no matter how much the reaction vessel is cooled, the pressure may remain, and it is expected that it is dangerous to open the lid without releasing the pressure.
[0032]
As described above, the method for producing a nitride crystal of the present invention has been described by taking as an example a case where a nitride polycrystal is used as a raw material. Alternatively, it is possible to carry out the above method using a similar compound and a precursor that can be converted to the compound as a raw material. Compounds similar to or similar to them, and precursors that can be converted to them, include pure metals mentioned as potential raw materials, compounds having a covalent MN bond such as galazane, Ga (NH ₂ ) ₃ Metal amides such as KGa (NH ₂ ) ₄ Alkali metal amides, metal imides, GaCl ₃ And halogen salts such as ammonium halide adducts and ammonium halogallates. In addition, it should not be used positively in the sense of avoiding contamination of impurities, but it is not impossible to use hydroxides, oxides, oxoacid salts, or the like.
[0033]
When using these raw materials that are not nitride polycrystals themselves to obtain bulk nitride crystals, it is necessary to simultaneously perform nitride synthesis and dissolution and precipitation of nitrides in a nitrogen-containing solvent. Therefore, stricter control of reaction conditions is required. When it is very difficult and it is desired to obtain larger lump crystals, it is preferable to use a multistage production method. That is, by using a production method using a valve attached to the reaction vessel of the present invention, a compound similar to or similar to the nitride polycrystalline raw material as described above and a precursor that can be converted to the raw material are used as a raw material. A polycrystalline nitride is first produced according to the reaction conditions, and then a bulk nitride crystal is grown using the same as a raw material and a production method using a valve attached to the reaction vessel of the present invention. In the case of using such a raw material, it is easy to produce a massive nitride crystal by this multi-stage method. At this time, the reaction divided into multiple stages may be performed in the same reactor without removing the nitrogen-containing solvent or the like, or may be performed by replacing with the same or another nitrogen-containing solvent or mineralizer. The synthesized nitride polycrystalline raw material may be taken out once, subjected to treatment such as washing, and then filled into the same reactor or another reactor to grow nitride crystals. At that time, it is also preferable to use a seed crystal as described above.
[0034]
As described above, the nitride crystal can be efficiently and safely manufactured by the manufacturing method using the valve attached to the reaction vessel of the present invention. The bulk nitride crystals obtained in the present invention can be prepared by using hydrochloric acid (HCl), nitric acid (HNO, if necessary). ₃ ) And the like. Also, the reaction vessel after removing the produced crystals and unreacted raw materials and additives such as mineralizers can be similarly washed when necessary.
[0035]
【Example】
Hereinafter, specific embodiments for carrying out the present invention will be described with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist.
Example 1
After mounting a Pt inner cylinder made so as to be almost in close contact with the inner wall in a 30 ml (+6 ml dead volume) reactor whose inner surface is coated with Pt, vacuum degassing and inert gas replacement are repeated and dried. The lid of the reaction vessel is opened in an inert gas atmosphere, and the half width (2θ) of the (100) diffraction line (2θ = about 32.5 °) by XRD is 0.17 degrees or less as a raw material at the bottom of the inner cylinder Then, 1.0 g of a sufficiently dried polycrystalline h-GaN (hexagonal gallium nitride) raw material having an oxygen content of 0.2 wt% measured with an oxygen / nitrogen analyzer TC-436 manufactured by LECO Co. was added. Further fully dry NH ₄ The lid was closed after adding 0.2 g of Cl. Subsequently, the pipe was connected to a vacuum pump through a valve attached to the reaction vessel. After opening the valve and vacuum degassing, the valve was once closed and the reaction vessel was cooled with a dry ice ethanol solvent. NH ₃ After operating to reach the tank, the valve is opened again and NH is continuously exposed to the outside air. ₃ Was introduced into the reaction vessel. Based on flow control, NH ₃ Is introduced as a liquid corresponding to 60% of the reaction vessel cavity (NH at −33 ° C. ₃ After conversion, the valve was closed, and the reaction vessel was placed in an electric furnace composed of a heater that was divided into two parts up and down. The temperature was raised so that the temperature of the outer surface of the lower part of the reaction vessel reached 530 ° C. over 6 hours. After the temperature of the outer surface of the lower part of the reaction vessel reached 530 ° C., the temperature was maintained for 72 hours. The pressure in the reaction vessel was about 130 MPa. The temperature fluctuation during the holding was plus or minus 10 ° C. or less. Then, the heating of the furnace heater was stopped, and it was naturally cooled in the furnace. After confirming that the temperature of the outer surface of the lower part of the reaction vessel has dropped to almost room temperature, let inert gas flow through the piping part that is not on the reaction vessel side of the valve attached to the reaction vessel so that it passes through the absorption tank filled with water. After the operation, the valve is opened and the NH inside the reaction vessel ₃ Was purged. NH in the reaction vessel ₃ After removing the gas, the valve was once closed, and this time, the valve was operated again so that it could be connected to the vacuum pump. ₃ Was almost completely removed. Thereafter, the lid was opened and the inside was confirmed, and about 0.3 g of massive nitride crystals were deposited on the upper part of the reaction vessel. The nitride crystal was taken out, pulverized, and powder X-ray diffraction was measured. The crystal form was a hexagonal type, and the half width (2θ) of the (100) diffraction line (2θ = about 32.5 °) was 0.18 °. The oxygen amount measured by LECO oxygen / nitrogen analyzer TC-436 was 0.4 wt%.
[0036]
Example 2
NH as in Example 1 except that 2.0 g of metal Ga having a purity of 99.9999% was used as a raw material and 0.48 g of sufficiently dried NH 4 Cl was used. ₃ Introduction (NH3 liquid density at −33 ° C., filling rate 60%), temperature rise, reaction, and NH ₃ A removal operation was performed. When the lid was opened and the inside was confirmed, about 0.3 g of massive nitride crystals were deposited on the top of the reaction vessel. The nitride crystal was taken out, pulverized, and powder X-ray diffraction was measured. The crystal form was a hexagonal type, the half width (2θ) of the (100) diffraction line was 0.2 °, and the oxygen content was 0.4 wt%.
Moreover, since a gray substance was confirmed at the bottom of the reaction vessel, it was taken out, pulverized, and measured for powder X-ray diffraction. As a result, it was h-GaN. The half width (2θ) of the (100) diffraction line was 0.17 °, and the oxygen content was 1.8 wt%.
[0037]
Comparative Example 1
As raw materials, the same h-GaN polycrystal and NH used in Example 1 were used. ₄ To demonstrate the effect of using a valve with Cl, NH ₃ With the reactor lid partially open, ₃ NH was introduced in the same manner as in Example 1 except that ₃ Introduction (NH at -33 ° C ₃ 60% liquid density fill rate), temperature rise, reaction, and NH ₃ A removal operation was performed. When the lid was opened and the inside was confirmed, about 0.2 g of crystals were deposited on the top of the reaction vessel. The crystals were taken out and pulverized to measure powder X-ray diffraction. The crystal form was a hexagonal type, the half width (2θ) of the (100) diffraction line was 0.25 °, and the oxygen content was 4.2 wt%.
[0038]
Comparative Example 2
As a raw material, a fully-dried polycrystal h having an XRD (100) diffraction line with a half-width of 0.24 and an oxygen amount of 5.2 wt% measured with an oxygen-nitrogen analyzer TC-436 manufactured by LECO -NH sufficiently dried using 3.0 g of GaN raw material ₄ NH as in Comparative Example 1 except that 0.6 g of Cl was used. ₃ Introduction (NH at -33 ° C ₃ 60% liquid density fill rate), temperature rise, reaction, and NH ₃ A removal operation was performed. When the lid was opened and the inside was confirmed, about 0.2 g of crystals were deposited on the top of the reaction vessel. The crystal was taken out, pulverized, and measured for powder X-ray diffraction. The crystal was found to be α-Ga. ₂ O ₃ The O / N atomic ratio exceeded 1.
From the results of the above examples and comparative examples, the nitride crystals obtained by the method of the present invention of the examples have higher crystallinity and higher quality than those of the conventional method which is a comparative example. Recognize. Moreover, the gallium nitride crystal obtained by the method of the present invention usually has the following characteristics.
1) XRD (100) diffraction line half-width (2θ) is 0.2 ° or less
2) Oxygen content is 4wt% or less
[0039]
【The invention's effect】
According to the method for producing a nitride crystal of the present invention, a high quality massive nitride crystal having good crystallinity can be efficiently obtained by a simpler and safer method than before. According to the present invention, it is possible to avoid the incorporation of oxygen into the reaction system as much as possible, so that the solubility of the nitride raw material in the nitrogen-containing solvent, the ion transportability in the solvent convection, and the recrystallization factor to the seed crystal. Control can be facilitated, and oxygen impurities can be prevented from accumulating in the massive nitride crystal to be produced, and deterioration of crystallinity can be avoided. Moreover, in the reactor system as used in the present invention, the temperature and pressure conditions are relaxed, and since the raw material gas in the reaction vessel does not leak, safety is high and industrial implementation is easy.
[Brief description of the drawings]
FIG. 1 shows a conceptual diagram of an example of a reaction vessel used in a method for producing a nitride crystal according to the present invention.
[Explanation of symbols]
1 Valve
2 Pressure gauge
3 reaction vessels
4 Crystal growth department
5 Raw material filling section
6 Electric furnace
7 Thermocouple

Claims (9)

A method for producing a nitride crystal, comprising filling a reaction vessel with a valve with a raw material and then introducing a nitrogen-containing solvent into the reaction vessel through the valve without touching outside air to obtain a crystal. . 2. The method for producing a nitride crystal according to claim 1, wherein after the introduction of the nitrogen-containing solvent, the valve is closed and the temperature is raised. The method for producing a nitride crystal according to claim 2, wherein the temperature is raised and the nitrogen-containing solvent is maintained in a subcritical state or a supercritical state. 4. The method for producing a nitride crystal according to claim 2, wherein the temperature is raised to 200 ° C. or higher and 700 ° C. or lower. The method for producing a nitride crystal according to any one of claims 2 to 4, wherein the temperature is raised and the pressure is maintained in a range of 20 MPa to 500 MPa. 6. The method for producing a nitride crystal according to claim 1, wherein the nitrogen-containing solvent is ammonia. The method for producing a nitride crystal according to any one of claims 1 to 6, wherein the oxygen content in the raw material is 5% by weight or less. The method for producing a nitride crystal according to any one of claims 1 to 7, wherein gallium nitride is contained as a raw material. The method for producing a nitride crystal according to any one of claims 1 to 8, wherein metal gallium is contained as a raw material.

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