Process for producing group III element nitride crystal and apparatus for producing group III element nitride crystal

A group III element nitride single crystal is grown on a template immersed in a raw material liquid retained in a crucible and containing a group III material and one of an alkali metal and an alkali earth metal. The raw material liquid remaining after the growth of the single crystal is cooled and solidified, and by feeding a hydroxyl group-containing solution into the crucible, the solidified raw material is removed from around the template, and thus the group III element nitride single crystal is taken out from inside the solidified raw material. The template is disposed at a position away from the bottom of the crucible.

TECHNICAL FIELD

The present invention relates to a process for producing a group III element nitride crystal and an apparatus for producing a group III element nitride crystal.

BACKGROUND ART

Among compound semiconductors, group III element nitrides (hereinafter, referred to as group III nitrides or group III nitride semiconductors, as the case may be) such as gallium nitride (GaN) are attracting attention as the materials for blue light or ultraviolet light-emitting semiconductor elements. Blue laser diodes (LDs) are applied to high-density optical discs or high-density displays, and blue light-emitting diodes (LEDs) are applied to displays or illumination. Ultraviolet LDs are expected to be applied to biotechnology and the like, and ultraviolet LEDs are expected to be applied as ultraviolet light sources for fluorescent lamps.

The substrates made of the group III nitride semiconductors (such as GaN) for use in LDs and LEDs are usually formed on sapphire substrates by heteroepitaxially growing group III nitride single crystals with vapor phase epitaxial growth methods. Examples of the vapor phase growth methods include the metal organic chemical vapor deposition method (MOCVD method), the hydride vapor phase epitaxy method (HVPE method) and the molecular beam epitaxy method (MBE method).

Alternatively, instead of vapor phase epitaxial growth, methods for growing crystals in liquid phase have also been investigated. The nitrogen equilibrium vapor pressure at the melting point of the single crystal of a group III nitride such as GaN or AlN is ten thousands atm or more. Accordingly, it is generally accepted that, for the purpose of growing gallium nitride in the liquid phase, known techniques require the conditions set at 1200° C. and 8000 atm (8000×1.01325×105Pa). In contrast, recently it has been shown that the use of an alkali metal such as Na enables the synthesis of GaN at a relatively low temperature of 750° C. and a relatively low pressure of 50 atm (50×1.01325×105Pa).

Recently, in an ammonium-containing nitrogen gas atmosphere, a mixture composed of Ga and Na was melted at 800° C. and 50 atm (50×1.01325×105Pa), and single crystals having a maximum crystal size of 1.2 mm have been obtained by using the resulting molten liquid, with a growth time of 96 hours (for example, JP2002-293696A).

There has also been proposed a method in which after a GaN crystal layer is formed as a film on a sapphire substrate with the metal organic chemical vapor deposition (MOCVD) method, a single crystal is grown with the liquid phase epitaxy (LPE) method (for example, JP2005-263622A).

FIG. 15shows a schematic configuration of a known production apparatus for growing a GaN crystal with the liquid phase epitaxy method. Reference numeral100denotes a heating growth furnace, in the interior of which an air-tight pressure-resistant heat-resistant vessel103is disposed. Reference numeral104denotes a lid of the vessel103. Reference numeral101denotes a raw material gas feeder for feeding a raw material gas109, namely, nitrogen gas, and the raw material gas feeder101is connected to the pressure-resistant heat-resistant vessel103through a connecting pipe114. The connecting pipe114is equipped with a pressure regulator102, a leak valve106, a joint108and a stop valve105. The growth furnace100is constructed as an electric furnace equipped with a heat insulator111and a heater112, and the temperature of the growth furnace100is controlled with a thermocouple113. The growth furnace100as a whole is capable of being swung about a horizontal shaft center A.

Inside the pressure-resistant heat-resistant vessel103, a crucible107is disposed. A high-temperature raw material liquid110is held inside the crucible107, and a template201is immersed in the raw material liquid110. The template201is prepared by forming as film a semiconductor layer composed of GaN on a sapphire substrate and is used as a seed crystal. The template201is prepared by supplying trimethyl gallium (TMG) and ammonia (NH3) onto a sapphire substrate having been heated so as to reach 1020° C. to 1100° C. The raw material liquid110is a molten substance prepared by melting metallic gallium and Na as raw materials at a high temperature.

When a crystal is produced in the production apparatus having such a configuration as described above, first in the outside of the production apparatus, the template201is disposed in the crucible107so as to lie along and to be oriented parallel to the bottom of the crucible107. Further, metallic gallium and Na as the raw materials are weighed to predetermined amounts and set in the crucible107.

Then, the crucible107is inserted into the air-tight pressure-resistant heat-resistant vessel103, and the pressure-resistant heat-resistant vessel103is set in the growth furnace100and connected to the raw material gas feeder101through the connecting pipe114. The growth temperature is set at 850° C. and the nitrogen atmosphere pressure is set at 50 atm (50×1.01325×105Pa), and while the growth furnace100is being swung about the shaft center A, nitrogen gas is dissolved in the Ga/Na molten liquid as the raw material liquid110to grow the GaN single crystal on the template201.

On completion of the growth of the GaN single crystal, the raw material liquid110is cooled and solidified in the pressure-resistant heat-resistant vessel103. Then, the crucible107is taken out from the pressure-resistant heat-resistant vessel, the raw material cooled and solidified in the crucible107is subjected to a dissolution treatment with ethanol or the like, and the template201with the GaN single crystal grown thereon is taken out.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, while the cooled and solidified raw material is being treated with ethanol or the like after the growth of the GaN single crystal, the raw material liquid gets into and is solidified in the clearance between the template201and the bottom of the crucible107because the template201is disposed so as to lie along and to be oriented parallel to the bottom of the crucible107as described above. The solidified raw material in such a clearance is hardly treated even with an ultrasonic device and hence requires a long time for the treatment thereof, and thus disadvantageously it takes a long time to take out the crystal.

Following the treatment of the solidified raw material, hydrogen gas is generated. The generated hydrogen gas is accumulated between the template201and the bottom of the crucible107, and exerts pressure to the template201from beneath the template201. Consequently, an upward stress is generated in the template201, and the stress causes a distortion in the GaN single crystal; when the distortion is large, the cracking of the GaN single crystal occurs.

In view of the above-described problems, an object of the present invention is to provide a process for producing a group III element nitride crystal which process enables, after the group III element nitride crystal such as a GaN single crystal is grown in the high-temperature raw material liquid, the taking out of the crystal from inside the cooled and solidified raw material in a short time in a manner suppressing the cracking of the crystal.

Means for Solving the Problems

For the purpose of solving the above-described problems, the production process of the present invention is a process wherein: a template, a group III element material and one of an alkali metal and an alkali earth metal are placed in a crucible, and a raw material gas is fed into the crucible; by heating the interior of the crucible, the group III element material and one of the alkali metal and the alkali earth metal are liquefied to produce a raw material liquid, and the template is immersed in the raw material liquid; the raw material liquid and the raw material gas are reacted with each other to grow a group III element nitride single crystal on the template in the raw material liquid; the raw material liquid remaining after the growth of the single crystal is cooled and solidified to be a solidified raw material; and by feeding a hydroxyl group-containing solution into the crucible containing the solidified raw material, the solidified raw material is removed from around the template, and thus the group III element nitride single crystal is taken out from inside the solidified raw material, wherein said process comprises disposing the template at a position away from the bottom of the crucible.

According to the above-described process, the template is disposed at a position away from the bottom of the crucible, and hence after the treatment with a hydroxyl group-containing solution progresses to some extent, the amount of the solidified raw material remaining between the template and the bottom of the crucible is an amount corresponding to the position of the template disposed away from the bottom of the crucible. As compared to the case where the template is disposed directly on the bottom of the crucible as in known techniques, the remaining solidified raw material is larger in the area in contact with the hydroxyl group-containing solution, and hence the treatment with the hydroxyl group-containing solution is performed rapidly. As compared to known techniques, the separation between the template and the bottom of the crucible is wider, and hence the hydrogen gas generated following the treatment escapes easily and the cracking of the single crystal on the template hardly tends to occur.

For the purpose of disposing the template at a position away from the bottom of the crucible, the template is preferably supported with protrusions formed on the bottom of the crucible, or the template is preferably supported with a spacer disposed between the bottom of the crucible and the template.

The template is preferably supported so as to take a position parallel to the bottom of the crucible. Specifically, the template is preferably disposed at a position away from the bottom of the crucible by 1 mm or more.

Advantages of the Invention

According to the present invention, by disposing the template at a position away from the bottom of the crucible, after the group III element nitride crystal is grown on the template, the crystal can be taken out from inside the solidified raw material in a short time and in a manner suppressing the cracking of the crystal.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the embodiments of the present invention are described with reference to the accompanying drawings.

FIG. 1illustrates a schematic configuration of the apparatus for producing a group III element nitride crystal in the embodiment 1 of the present invention.

Reference numeral100denotes a heating growth furnace, in the interior of which an air-tight pressure-resistant heat-resistant vessel103is disposed. Reference numeral104denotes a lid of the vessel103. Reference numeral101denotes a raw material gas feeder for feeding the raw material gas, namely, nitrogen gas, and the raw material gas feeder101is connected to the pressure-resistant heat-resistant vessel103through connecting pipes114and115. The connecting pipe114and the connecting pipe115are communicatively connected to each other with a joint108in a manner separable from each other. The connecting pipe114is equipped with a pressure regulator102and a leak valve106. The connecting pipe115is equipped with a stop valve105.

The raw material gas feeder101is only required to be able to pressurize a raw material gas109at a predetermined pressure, and can control this pressure in a range from normal pressure (1×1.01325×105Pa) to 100 atm (100×1.01325×105Pa). The pressure regulator102, the stop valve105and the leak valve106installed in the connecting pipes114and115are electrically linked to each other, and hence the pressure for feeding to the pressure-resistant heat-resistant vessel103can be maintained at a predetermined pressure. As the raw material gas109, nitrogen gas, ammonia gas, a mixed gas composed of nitrogen gas and ammonia gas or the like is used.

The pressure-resistant heat-resistant vessel103is only required to be capable of housing in the inside thereof the crucible107, and maintaining air-tightness at high temperatures and high pressures respectively falling in a range from normal temperature to 1100° C. and in a range from normal pressure (1×1.01325×105Pa) to 100 atm (100×1.01325×105Pa). Specifically, as the pressure-resistant heat-resistant vessel103, there can be used a vessel produced by using a stainless steel material such as SUS 316 specified by JIS, or a high temperature-resistant and high pressure-resistant material such as Inconel, Hastelloy or Incolloy (these three are all registered trade marks). In particular, a material such as Inconel, Hastelloy or Incolloy also has a resistance to the oxidation at high temperatures under high pressures, can be used even in an atmosphere other than an inert gas, and hence is preferable from the viewpoints of recycling and durability.

The growth furnace100is constructed as an electric furnace equipped with a heat insulator111and a heater112. As the heater112, there can be used an induction heater (high frequency coil), a resistance heater (a heater using nichrome, kanthal, SiC, MoSi2or the like), or the like. Preferable among these is the induction heater, because the induction heater is small in generation of impurity gases at high temperatures. The growth furnace100is equipped with the thermocouple113for the temperature control and the like, and is designed to be controllable in temperature in a range from normal temperature to 1100° C. This is because it is preferable, from the viewpoint of preventing the “agglomeration” of the raw material liquid110in the crucible107, to control the temperature of the pressure-resistant heat-resistant vessel103in such a way that the temperature of the pressure-resistant heat-resistant vessel103is uniformly maintained. The growth furnace100is equipped with a pressure regulator (not shown) for regulating the atmospheric pressure in the furnace, and the atmospheric pressure in the furnace can be controlled in a range of 100 atm (100×1.01325×105Pa) or less. For the purpose of stirring the raw material liquid110in the crucible107, the growth furnace100as a whole is capable of being swung about a horizontal shaft center A. Also for the purpose of stirring the raw material liquid110, the upper section and the lower section of the growth furnace100are made different in temperature from each other to enable the thermal convention to be generated in the raw material liquid110.

For the crucible107, there can be used materials hardly reactive with the group III elements and the alkali metals, such as alumina (Al2O3), sapphire (Al2O3), yttria (Y2O3), magnesium oxide (MgO), calcium oxide (CaO), boron nitride (BN) and tungsten (W). In the crucible107, a plurality of protrusions210are formed on the internal surface of the bottom, and hence the internal surface of the bottom of the crucible107has a protruded and recessed shape. The protrusions210are each only required to have a shape capable of supporting the template201. Although here the protrusions210are simply drawn as ridge-shaped protrusions which are laterally aligned triangular prisms each having a triangular transverse cross section, the details of the shape of the protrusions are described below.

The template201is only required to be a member capable of functioning as a seed crystal of the group III element nitride crystal. Preferably, examples of such a member include: a template in which a semiconductor layer represented by a composition formula AluGavIn1-u+vN is formed on a substrate made of sapphire or the like; a template in which a single crystal represented by the same composition formula AluGavIn1-u+vN is formed on a substrate made of sapphire or the like; a template which is a self-supported semiconductor represented by the same composition formula AluGavIn1-u+vN; and a template which is a single crystal represented by the same composition formula AluGavIn1-u+vN. It is to be noted that in the composition formula, 0≦u≦1, 0≦v≦1, and 0≦u+v≦1. It is also to be noted that the “template” as referred to in the present description means the templates described herein.

By using such a template201as described above as the seed crystal, a thick-film single crystal can be grown on the template201, and a large-area single crystal can be easily grown.

The process for producing a group III element nitride single crystal by using the above-described production apparatus is described below.

As shown inFIG. 2A, a flat-plate template201is placed and held on the top of the protrusions210in the crucible107so as to take a position parallel to the bottom of the crucible107.

Onto the template201, a group III element material215and alkali metal214as raw materials are fed. As the group III element material215, gallium, aluminum or indium can be used. As the alkali metal214, lithium, sodium, potassium or the like can be used. In place of the alkali metal214, an alkali earth metal such as calcium, strontium, barium, radium, beryllium or magnesium may also be used. These alkali metals214and alkali earth metals may be used each alone or in combinations of two or more thereof. The weighing and handling of the group III element material215and the alkali metal214are preferably performed in a glove box replaced with nitrogen gas, argon gas, neon gas or the like, for the purpose of avoiding the oxidation of the alkali metal214and moisture adsorption.

Next, as shown inFIG. 2B, the crucible107is inserted into the pressure-resistant heat-resistant vessel103, the lid104is closed, the stop valve105, integrated with the lid104, of the pipe115is closed, and the pressure-resistant heat-resistant vessel103is taken out as it is from the glove box.

Then, the pressure-resistant heat-resistant vessel103is fixed in the growth furnace100as shown inFIG. 1, the pipe115of the pressure-resistant heat-resistant vessel103is connected to the raw material gas feeder101, the stop valve105is opened, and the raw material gas109is injected from the raw material gas feeder101into the pressure-resistant heat-resistant vessel103. In this case, preferably the raw material gas109is injected and the air in the pressure-resistant heat-resistant vessel103is replaced with the raw material gas109after the interior of the pressure-resistant heat-resistant vessel103is evacuated to vacuum with a not-shown pump such as a rotary pump or a turbo pump.

Subsequently, while the temperature of the growth furnace100and the pressure of the growth atmosphere are being controlled with the thermocouple113and the pressure regulator102, the growth of the group III element nitride single crystal is performed.

In this case, the interior of the growth furnace100is preferably filled with an inert gas such as argon gas, helium gas, neon gas or nitrogen gas. This is because when the pressure-resistant heat-resistant vessel103is maintained in air atmosphere at a high temperature, even the pressure-resistant heat-resistant vessel103itself is oxidized so as to be hardly reused.

The raw material melting conditions and the growth conditions for producing the group III element nitride single crystal are dependent on the component of the group III element material215and the component of the alkali metal214as raw materials, and the component and the pressure of the raw material gas. For example, as the temperature, relatively low temperatures of 700° C. to 1100° C. and preferably 700° C. to 900° C. are applied, and as the pressure, the pressures of 20 atm (20×1.01325×105Pa) or more, and preferably 30 atm (30×1.01325×105Pa) to 100 atm (100×1.01325×105Pa) are applied.

By increasing the temperature to the growth temperature, the molten liquid of the group III element material215/the alkali metal214, namely, the above-described raw material liquid110are formed in the crucible107. Then, the raw material gas109dissolves into the raw material liquid110, the group III element material215and the raw material gas109react with each other, and thus the group III element nitride single crystal216is grown on the template201as shown inFIG. 3A.

The reasons for the fact that the template201is disposed so as to be parallel to the bottom of the crucible107as described above are such that, as shown inFIG. 3A, consequently the distance to the surface of the raw material liquid110is uniform over the entire surface of the template201, and the dissolved amount of the raw material gas109into the raw material liquid110in the vicinity of the surface of the raw material liquid110comes to be uniform, so as to enable the uniform growth of the group III element nitride single crystal216. The template201is not required to be perfectly parallel to, but may be approximately parallel to the bottom of the crucible107.

On the other hand, when a case is considered where the template201is disposed so as to be perpendicular to the bottom of the crucible107as shown inFIG. 3B, the surface level of the raw material liquid110in the crucible107has to be made higher as compared to the case ofFIG. 3A. Accordingly, the raw material gas109more easily dissolves into the upper portion, closer to the gas phase, of the raw material liquid110, and on the other hand, the raw material gas109more hardly dissolves into the lower portion, far away from the gas phase, of the raw material liquid110. Thus, the dissolved amount of the raw material gas109is different in the upper portion from in the lower portion of the raw material liquid110. Consequently, the growth of the group III element nitride single crystal216is faster on the upper portion of the template201and slower on the lower portion of the template201to result in an uneven thickness of the group III element nitride single crystal216as shown in the figure.

After a predetermined time has elapsed and the growth of the group III element nitride single crystal216has been completed, the raw material liquid110is cooled and solidified in the growth furnace100and the pressure-resistant heat-resistant vessel103. Then, the crucible107is taken out from the growth furnace100and the pressure-resistant heat-resistant vessel103. The template201with the group III element nitride single crystal216integrally formed thereon is embedded inside the cooled and solidified raw material in the crucible107. Therefore, the solidified raw material is treated for the purpose of taking out the template201with the group III element nitride single crystal216integrally formed thereon from inside the cooled and solidified raw material.

In the raw material liquid110after the growth, namely, in the solidified raw material, the group III element material215shown in either ofFIG. 2AandFIG. 2Bremains in about 5 to 30% of the original amount. Most of the solidified raw material is composed of the alkali metal214. Accordingly, a hydroxyl group (—OH)-containing optional solution such an alcohol such as ethanol, methanol or isopropyl alcohol, or water is injected into the crucible107. Thus, the solidified raw material is immersed into the solution, and the metal alkoxide (when water is used, the metal hydroxide) dissolved in the injected solution and hydrogen are produced. In this way, the solidified raw material is removed from around the template201with the group III element nitride single crystal216integrally formed thereon.

In this case, the solidified raw material above and around the template201disposed parallel to the bottom of the crucible107can be relatively rapidly treated. On the other hand, it is generally difficult to treat the solidified raw material due to the solidification of the raw material liquid110having gotten into under the template201. However, when the crucible107having protrusions and recesses on the bottom thereof is used as described above and the template201is disposed on the protrusions210, the solidified raw material110aunder the template201remains to such an extent corresponding to the height of the protrusions210as shown inFIG. 4.

The thus remaining solidified raw material110ais more satisfactorily brought into contact with the treatment solution such as ethanol218as compared to the case where a template is disposed in a crucible107having neither protrusions nor recesses on the bottom thereof as in the case of known techniques, and accordingly the treatment of the remaining solidified raw material110acan be performed rapidly. Additionally, the separation between the template201and the bottom of the crucible107is wider, and hence the hydrogen gas217generated due to the treatment easily escapes. Accordingly, the cracking of the group III element nitride single crystal216on the template201due to the retention of the hydrogen gas217is made difficult to occur.

When the ethanol218and the solidified raw material110aare at high temperatures, a severe reaction occurs, and hence it is preferable to perform this treatment at normal temperature to about 50° C. After the completion of the treatment of the solidified raw material110a, the template201with the group III element nitride single crystal216integrally formed thereon is taken out.

The details of the protruded and recessed shape of the bottom in the crucible107are described. The protrusions210are only required to have a shape capable of supporting the template201; as shown inFIGS. 1 to 4, the protrusions may be laterally aligned triangular prisms, namely, a plurality of ridge-shaped protrusions aligned and disposed with appropriate separations therebetween. Alternatively, as schematically shown inFIGS. 5A to 5D, the protrusions may be laterally aligned square prisms (FIG. 5A), laterally aligned triangular prisms with no separation therebetween (FIG. 5B), laterally aligned semicircular columns and protrusions with a wave-shaped transverse cross section (FIGS. 5C and 5D, respectively) or the like.

Yet alternatively, as shown inFIGS. 6A and 6B, three or more small-protrusions210may be integrally formed on the bottom in the crucible107in a non-collinear manner so as to be made capable of supporting the template201with the ends of these small-protrusions210. The shape of the small-protrusions210may be a shown column (a square column as shown, or a circular column or the like), and additionally may be a circular cone, a triangular pyramid, a square pyramid, a semi-sphere or the like. However, the shape of the small-protrusions is not limited to these.

The height211of the protrusion210shown inFIG. 7, namely, the distance from the bottom of the crucible107to the end of the protrusion210is only required to be higher than the height of the raw material liquid110(about 0.5 mm) getting into under the template201when the bottom of the crucible107is flat. Preferably, when the height211is 1 mm or higher, the time required to treat the solidified raw material with the treating solution is reduced to a short time and the occurrence of the crystal cracking can be suppressed. More preferably, by using an ultrasonic wave, the raw material having gotten into under the template201and having been solidified can be effectively treated. In such a case, the height211is preferably larger than half the wavelength of the ultrasonic wave. The frequency of the higher frequency ultrasonic wave is generally about 200 kHz, and accordingly, when such an ultrasonic wave is used, the height211is preferably 2 mm or more.

In the case where a plurality of the ridge-shaped protrusions210are formed, when the period213(the pitch of the positions supporting the template201) of the protrusions210is 1 mm or more, the treatment time can be reduced to half as compared to the case where the period213is less than 1 mm. Alternatively, when the period213is a distance equal to or less than the radius of the template201, the template201can be always supported with two or more lines of the protrusions210, and hence an inclined disposition of the template201can be prevented.

The amount of the raw material liquid110is preferably such that the distance212from the surface of the template201to the surface of the raw material liquid110is 5 mm or more. When the distance212is less than 5 mm, the dissolved amount of the raw material gas109in the raw material liquid110in the vicinity of the surface of the template201is increased, the crystal growth on the template201cannot meet the dissolution rate of the raw material gas109, and inferior crystals are generated in the boundary between the crucible107and the raw material liquid110.

Examples of the process for forming the protrusions210in the crucible107include: a process in which the recesses are formed in the mold for forming the crucible107; and a process in which the protrusions are formed by cutting in the crucible107having a flat bottom. Alternatively, similar effects can also be achieved by disposing as shown inFIG. 8a plate-shaped spacer190having the protrusions210on the flat bottom107aof the crucible107, or by disposing as shown inFIG. 9a plate-shaped spacer190having a large number of through-holes195.

Hereinafter, a specific example of the process for producing a group III element nitride crystal of the present invention is described.

As the pressure-resistant heat-resistant vessel103shown inFIG. 1, a stainless steel vessel formed of SUS 316 in terms of the JIS material symbols was prepared. As the crucible107, a crucible of 70 mm in inner diameter and 50 mm in depth, made of an alumina material (purity: 99.99%) was prepared. The crucible107was designed to have three protrusions210(circular cones of 3 mm in height) arranged on the bottom thereof in a non-collinear manner as shown inFIG. 10A, namely, in a manner such that the ends of the three protrusions210formed a plane. For comparison, another crucible107was prepared which was formed in the same manner as the crucible107shown inFIG. 10Aexcept that neither protrusions nor recesses were disposed on the bottom of the crucible as shown inFIG. 10B.

As the template201, a template was prepared according to the following manner: a sapphire substrate of 2 inches (51 mm) in diameter was heated to from 1020° C. to 1100° C., and then trimethyl gallium (TMG) and ammonia (NH3) were fed to the atmosphere, and thus a semiconductor layer composed of GaN was formed as a film on the substrate. As the raw material shown in either ofFIGS. 10A and 10B, gallium was prepared as the group III element material215and sodium was prepared as the alkali metal214. As the raw material gas109shown inFIG. 1, nitrogen gas was prepared.

Then, in a glove box, the template201was disposed in the crucible107so as to be parallel to the bottom of the crucible107as shown inFIGS. 10A and 10B, 30 g of sodium as the alkali metal214and 30 g of gallium as the group III element material215were placed on the template201. Then, the crucible107was inserted into the pressure-resistant heat-resistant vessel103shown inFIG. 1, the lid104was closed, the stop valve105was closed, and subsequently the pressure-resistant heat-resistant vessel103was taken out from the glove box.

The air in the glove box had been replaced with argon gas, and the glove box was capable of controlling the moisture content and the oxygen concentration. The moisture content is preferably −80° C. or lower in terms of the dew point and the oxygen concentration is preferably 1 ppm or less, and hence the moisture content and the oxygen concentration were controlled so as to satisfy these conditions. In such an environment, when sodium is cut, remarkable discoloration of the cut surface and the like are not observed.

Next, the pressure-resistant heat-resistant vessel103was disposed in the growth furnace100as shown inFIG. 1, the pressure regulator102was set at 40 atm (40 ×1.01325×105Pa), and nitrogen gas as the raw material gas109was fed to the pressure-resistant heat-resistant vessel103from the raw material gas feeder101.

In this case, the interior of the pressure-resistant heat-resistant vessel103was beforehand evacuated to a vacuum of the order of 10−2Pa, then the step of injecting the raw material gas109into the pressure-tight heat-resistant vessel103and replacing the gas in the pressure-resistant heat-resistant vessel103with the raw material gas109was performed. The pressure-resistant heat-resistant vessel103is formed of SUS 316, and hence the interior of the growth furnace100was made to be a nitrogen gas atmosphere for the purpose of preventing the oxidation of the pressure-resistant heat-resistant vessel103.

When the pressure of the interior of the pressure-resistant heat-resistant vessel103reached the growth pressure, namely, 40 atm, the growth furnace100was heated to the growth temperature, namely, 860° C., thus Ga/Na were mixed together to form the molten raw material liquid110in the crucible107, and the gallium nitride single crystal produced by the reaction between the nitrogen gas dissolved in the raw material liquid110and Ga was grown on the template201. The growth time was set at 100 hours.

On completion of the growth, the pressure-resistant heat-resistant vessel103was taken out after cooling from the growth furnace100, and then the crucible107was taken out from the pressure-resistant heat-resistant vessel103. The raw material liquid110taken out from the growth furnace100was solidified by cooling to result in a condition that the template201was embedded, together with the single crystal, inside the solidified raw material liquid110. Accordingly, for the purpose of taking out the gallium nitride single crystal, the solidified raw material110ain the crucible107was treated.

When the solidified raw material110awas treated, ethanol218was injected into the crucible107in such a way that the solidified raw material110awas immersed in ethanol218as shown inFIGS. 11A and 11B. It is to be noted that the crucible107may also be immersed in ethanol218. Following the reaction between the solidified raw material110aand ethanol218, hydrogen gas217was generated. When the hydrogen gas217, generated following the reaction between the raw material liquid110around the template201with the single crystal216formed thereon and the solidified raw material110a, came to be scarcely generated, the reaction concerned was promoted with ultrasonic wave, and when the reaction came to proceed slowly, the treatment of the solidified raw material110awas further promoted by slowly adding water. In this case, the treatment temperature was maintained at 50° C. or lower, for the purpose of avoiding the cracking of the gallium nitride single crystal216caused by the stress exerting on the gallium nitride single crystal216. When the treatment temperature tended to be raised, ethanol218or water was added.

The treatment of the solidified raw material110awas completed after making sure that the hydrogen gas217following the reaction between solidified raw material110aaround the template201and ethanol218was no longer generated, and the template201with the gallium nitride single crystal216integrally formed thereon was able to be moved with tweezers. Then, the template201(grown single crystal body) was taken out from the crucible107. Under similar conditions and with similar procedures, the growth of the single crystal216and the treatment of the solidified raw material110awere performed a plurality of times, and an examination was made on the results for the crucibles107with the protrusions210as shown inFIGS. 10A and 11Aand the results for the crucibles107without protrusions so as to have a flat bottom shape as shown inFIGS. 10B and 11B.

The gallium nitride single crystal216was grown in a thickness of 2 mm during the growth time of 100 hours with any of the crucibles107having the shapes shown inFIGS. 10A,11A,10B and11B. When the crucibles107having the protrusions210on the bottom thereof shown inFIGS. 10A and 11Awere used, no cracking occurred in the obtained gallium nitride single crystals216. On the contrary, when the crucibles107having no protrusions on the bottom thereof and having a flat bottom shape, shown inFIGS. 10B and 11Bwere used, cracking occurred in some pieces of the obtained gallium nitride single crystals216. The times needed for treating the solidified raw materials110ain the crucibles107were as shown in Table 1.

As shown in Table 1, the solidified raw material on the upper surface of the template201was able to be treated in about 2 hours for both of the crucibles107having the protrusions210on the bottom thereof shown inFIGS. 10A and 11Aand the crucibles107having no protrusions on the bottom thereof and having a flat bottom shape shown inFIGS. 10B and 11B. However, the treatment times of the solidified raw materials110ahaving gotten into under the template201were largely different. Specifically, when the crucibles107having the protrusions210on the bottom thereof were used, the treatment was completed in 3 hours; on the contrary, when the crucibles107having no protrusions on the bottom thereof were used, the treatment took a time longer by a factor of 5, namely, 15 hours. The individual treatment times were somewhat varied depending on the effects of the temperature and the like of the treatment environment; however, this tendency remained unchanged.

As is clear from the above-described results, by using the crucible107having the protrusions210on the bottom thereof, the group III element nitride single crystal216was able to be satisfactorily grown, and additionally, the grown single crystal216was able to be taken out from the solidified raw material in a short time and in a manner suppressing the cracking of the single crystal216due to the gas generated from the solidified raw material.

FIG. 12shows a schematic configuration of the apparatus for embodying the process for producing a group III element nitride single crystal of the embodiment 2 of the present invention. The apparatus of the embodiment 2 is different from the apparatus of the embodiment 1 in that the apparatus of the embodiment 2 is equipped with a raw material liquid discharger301. The raw material liquid discharger301is disposed outside the growth furnace100, and equipped with a discharge pipe302. The discharge pipe302penetrates through the growth furnace100and the air-tight pressure-resistant heat-resistant vessel103, and extends into the interior of the crucible107, preferably, to the vicinity of the bottom of the crucible107. Otherwise the configuration is the same as in the apparatus shown inFIG. 1.

Most of the raw material liquid110remaining in the crucible107after the completion of the growth of a single crystal is the alkali metal; however, there is a possibility that an alloy between the alkali metal and the group III element material is present. Accordingly, these are melted by actively heating the interior of the growth furnace100to a temperature falling in a range from the melting point of the alkali metal to the melting point of the alloy between the alkali metal and the group III element material; and these are sucked and removed, as they are melted, by using the discharge pipe302. It is to be noted that the raw material liquid110can also be removed under the conditions that the interior of the growth furnace100is not heated and additionally not actively cooled after the completion of the growth.

For the purpose of satisfactorily performing the above-described operations, in the raw material liquid discharger301, the discharge pipe302preferably has a movable and extensible structure. It is to be noted that when cooling occurs during the discharge of the raw material liquid110, “agglomeration” may occur, and hence it is preferable to maintain the whole portions brought into contact with the raw material liquid110, in the raw material liquid discharger301, at temperatures equal to or higher than a predetermined temperature by disposing heaters or the like in the concerned whole portions.

Specifically, the details are as follows. In the same manner as in Embodiment 1, a single crystal is grown on the template201, and the raw material liquid110in the crucible107is treated after the completion of the growth, for the purpose of taking out the single crystal. For that purpose, first the raw material liquid110before solidification is discharged through the raw material liquid discharger301.

In this case, the group III element material remains in an amount of about 5 to 30% of the original amount thereof in the raw material liquid110in the crucible107; however, as described above, there is a possibility that an alloy between the alkali metal and the group III element material is present. Accordingly, the raw material liquid110is heated to a temperature falling in a range from the boiling point of the alkali metal to the boiling point of the alloy between the alkali metal and the group III element material, and the raw material liquid110is maintained in a liquid state. The preferable heating temperature is 100 to 600° C.

In this case, even in the case where the discharge pipe302extends close to the bottom of the crucible107as described above, when the end of the pipe302is located at a position higher than the height of the end of the protrusions210in the crucible107as shown inFIG. 13, the whole raw material liquid110in the crucible107cannot be sucked and removed, and hence the raw material liquid110remains under the template201. In such a case, the raw material liquid110is treated with ethanol or the like according to the same procedures as in Embodiment 1.

As shown inFIG. 14, when the end of the discharge pipe302is located at a position lower than the height of the end of the protrusions210of the crucible107, it can be accepted, in principle, that all the raw material liquid110in contact with the template201can be sucked and removed. However, the raw material liquid110adheres to the underside of the template201through the surface tension as the case may be. In such a case, according to the same procedures as in Embodiment 1, the raw material liquid110is treated with ethanol or the like. When the template201comes to be movable with tweezers by performing the treatment, it is recognized that the raw material liquid110does not adhere to the underside of the template210and the treatment of the raw material liquid110is completed, and the template201is taken out as it is. In any of the above-described cases, the presence of the protrusions210in the crucible107enhances the suction and removal effects due to the discharge pipe302.

In any case, after the completion of the treatment of the raw material liquid110, the substrate with a single crystal formed on the template201(grown single crystal body) is taken out from the crucible107.

According to the above-described process, the step of sucking and removing the raw material liquid110is included, and hence the treatment of the raw material liquid110can be performed more easily and in a shorter time as compared to Embodiment 1.

Consequently, according to the process of Embodiment 2, a single crystal can be satisfactorily grown similarly to Embodiment 1, and additionally, the grown single crystal can be taken out from inside the raw material liquid in a short time with the suppression of the cracking of the single crystal due to the gas generated from the raw material liquid.

Industrial Applicability

The process for producing a group III element nitride crystal according to the present invention has an advantage that the grown single crystal can be taken out from the raw material liquid in a short time with suppression of the cracking of the single crystal, and is useful for the production of gallium nitride and the like.