JP2009161398A - Method for manufacturing nitride single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a growing method of a group III nitride single crystal using a flux, by which the growing amount per unit time of the nitride single crystal can be increased and the variation of the quality or the film thickness of the single crystal can be suppressed. <P>SOLUTION: The dissolution of nitrogen into a solution 2 is promoted while holding a growing vessel 1 in a first posture. After that, a nitride single crystal is grown on a seed crystal substrate 4 while holding the growing vessel 1 in a second posture. Thereby, the area B of the gas-liquid interface 2a of the solution 2 in the first posture is made larger than the area A of the gas-liquid interface 2a of the solution 2 in the second posture, and accordingly, it becomes possible to shorten the time until the concentration of nitrogen dissolved in the solution reaches a saturation level. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

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

  Gallium nitride III-V nitride has attracted attention as an excellent blue light emitting device, and has been put to practical use as a material for light emitting diodes and semiconductor laser diodes. Methods for growing group III nitride single crystals using flux have been reported by various organizations.

  In the nitride single crystal growth method using Na flux, since nitrogen is supplied from the gas-liquid interface, the nitrogen concentration of the raw material solution in the vicinity of the gas-liquid interface is the highest among the solutions. Nitrogen dissolved in this solution is transported to the seed crystal substrate, and a single crystal is deposited on the seed crystal substrate. The growth rate of single crystals is limited by the rate of nitrogen penetration at the gas-liquid interface.

In particular, in Patent Document 1, the seed crystal substrate is arranged vertically. However, in such a case, it is necessary to make the height of the solution higher than in the case where the seed crystal substrate is disposed horizontally on the crucible bottom. That is, the amount of the solution raw material increases for the same gas-liquid interface area. For this reason, it takes a long time until the concentration of nitrogen dissolved in the solution reaches saturation and single crystal precipitation starts, and the productivity of the single crystal decreases.
WO 2007/122865 A1

The following methods can be considered to increase the rate of nitrogen dissolution into the solution at the gas-liquid interface.
(1) Increase the nitrogen pressure of the atmosphere,
(2) Increase the gas-liquid interface area of the solution,
(3) Stir the solution well near the gas-liquid interface.

  However, when the nitrogen pressure in the atmosphere is increased, the nitrogen concentration at the gas-liquid interface increases, and natural nuclei are likely to be generated, so that miscellaneous crystals are likely to be generated. The miscellaneous crystals adhere to the single crystal and do not easily peel off from the single crystal.

Patent Document 2 discloses a shape in which the cross-sectional area of the crucible (mixed solution holding container) becomes smaller as it goes downward. By making the crucible into such a shape, the gas-liquid interface area can be increased without increasing the raw material solution so much.
JP 2002-128587 A

In Patent Documents 3 and 4, a crucible containing a solution and a seed crystal substrate is attached to a rotating shaft, and the solution is agitated by swinging during single crystal growth, so that nitrogen dissolves from the gas-liquid interface into the solution. Promotes.
WO 2007/102610 A1 WO 2004/083498 A1

  However, in the crucible shape of Patent Document 2, convection of the solution does not occur smoothly and stagnation is likely to occur. For this reason, the quality of a part of the single crystal is deteriorated, and the single crystal film thickness is likely to vary.

  In the methods described in Patent Documents 3 and 4, since the stirring of the solution during the growth of the single crystal is promoted, the thickness of the single crystal and the variation in the thickness are suppressed. However, when nitrogen is first dissolved in the solution, a long time is required until the nitrogen concentration in the solution is saturated and single crystal precipitation starts.

  An object of the present invention is to increase the amount of nitride single crystal grown per unit time by shortening the time until the nitrogen dissolved in the solution reaches saturation, as well as variations in single crystal quality and film thickness. It is to suppress.

The present invention is a method of immersing a seed crystal substrate in a solution containing a flux and a Group III raw material in a growth vessel and growing a nitride single crystal on the seed crystal substrate in a nitrogen-containing atmosphere,
A nitrogen dissolving step for promoting the dissolution of nitrogen in the solution while holding the growth vessel in the first posture; and the nitride single crystal on the seed crystal substrate while holding the growth vessel in the second posture; A single crystal growth step for performing growth is provided, wherein the area of the gas-liquid interface of the solution in the first posture is larger than the area of the gas-liquid interface of the solution in the second posture.

  According to the present invention, the gas-liquid interface area of the solution in the nitrogen dissolving step is made larger than the gas-liquid interface area of the solution in the single crystal growing step. As a result, the dissolution of nitrogen at the initial solution gas-liquid interface is promoted, and the time required for saturation of the nitrogen concentration of the solution and the start of single crystal precipitation can be shortened. In addition, in the single crystal growth process, the solution gas-liquid interface area is made relatively small, so that the amount of miscellaneous crystals generated can be suppressed, and variations in the quality of single crystals due to adhesion of miscellaneous crystals can be suppressed. it can.

Hereinafter, the present invention will be described in more detail with reference to the drawings as appropriate.
When growing a nitride single crystal by the flux method, a group III material and a flux material are enclosed in a glove box in a non-oxidizing atmosphere, and as shown in FIGS. 1 (a) and 1 (b), the container 1 The inside space 3 is sealed in a non-oxidizing atmosphere. This container may be provided with a lid. In the example of FIG. 1A, the seed crystal substrate 4 is installed horizontally on the bottom wall 1 a of the growth vessel 1. In the example of FIG. 1B, the seed crystal substrate 4 is installed vertically on the bottom wall 1 a of the container 1.

  Next, for example, as schematically shown in FIG. 4, the growth container 1 is placed in an outer container 5 that can be sealed and into which gas can be introduced, the outer container 5 is taken out of the glove box, and then installed in the crystal growth apparatus as it is. To do.

  In the example shown in FIG. 4, the outer container 5 and the growth container 1 are installed in a pressure container 10 of a HIP (hot isotropic pressure press) apparatus. A gas mixture cylinder (not shown) is provided outside the pressure vessel 10. The mixed gas cylinder is filled with a mixed gas having a predetermined composition. The mixed gas is compressed by a compressor to a predetermined pressure, and is supplied into the pressure vessel 10 through the supply pipe 9 as indicated by arrows D and E. Nitrogen in the atmosphere of the pressure vessel 10 serves as a nitrogen source, and an inert gas such as argon gas suppresses evaporation of the flux. This pressure is monitored by a pressure gauge (not shown). Heaters 8A, 8B, and 8C are installed around the outer container 5 so that the growth temperature in the growth container 1 can be controlled.

  When the growth vessel 1 is heated and pressurized in the pressure vessel 10, all the raw materials are dissolved in the vessel 1 to produce a solution 2. Here, if predetermined single crystal growth conditions are maintained, nitrogen is stably supplied into the solution 2 from the space 3 in the growth vessel, and a single crystal film grows on the seed crystal substrate 4.

  Here, as shown in FIG. 1A, when the single crystal is grown with the seed crystal substrate 4 lying sideways, the height h of the solution can be made relatively small. Thereby, transportation of the dissolved nitrogen to the seed substrate can be promoted, and since the ratio of the solution amount to the gas-liquid interface area is small, it is possible to promote that the dissolved nitrogen reaches saturation. However, if the height h of the solution is too small during the growth, the solution is likely to stagnate. For this reason, miscellaneous crystals are likely to occur near the gas-liquid interface 2a, and defects in the single crystal are likely to occur.

  As shown in FIG. 1B, when a single crystal is grown with the seed crystal substrate 4 in the vertical direction, the single crystal can be grown simultaneously on a large number of seed crystal substrates 4. However, since the height H of the solution 2 is increased, the ratio of the solution amount to the gas-liquid interface area is increased, and it takes time for the dissolved nitrogen to reach saturation. For this reason, it takes time to start the growth of the single crystal.

  Here, for example, as shown in FIG. 4, the rotation shaft 6 is attached to the outer container 5 so that the rotation shaft 6 can be rotated by a motor. As a result, the outer container 5 and the growth container 1 can be rotated in the paper surface direction as indicated by an arrow G in FIG.

  First, in the first nitrogen dissolving step, when a predetermined temperature and nitrogen pressure are maintained, a certain concentration of nitrogen is dissolved in the solution. This concentration can be increased or decreased depending on the solution composition and additives. Further, since the solubility increases as the temperature increases, the saturation concentration of nitrogen that can be dissolved in the solution increases. In general, a solution immediately after heating is not saturated because nitrogen has not sufficiently dissolved therein. However, as time elapses, nitrogen dissolves into the solution, and the nitrogen concentration in the solution increases. As a result, the nitrogen concentration eventually reaches a saturation concentration, the solution becomes supersaturated with nitrogen, and the precipitation of miscellaneous crystals begins. Therefore, during the nitrogen dissolving step, the holding time is managed so that the solution is not excessively saturated.

  Here, in this invention, melt | dissolution of nitrogen to a solution is accelerated | stimulated, hold | maintaining a growth container in a 1st attitude | position. Here, the area of the gas-liquid interface of the solution is relatively increased.

  A method for relatively increasing the area of the solution gas-liquid interface is not particularly limited. For example, in the example of FIG. 2A, the growth container 1 is held in a state (first posture) inclined by an angle θ with respect to the horizontal plane. In this state, nitrogen is dissolved in the solution to obtain a supersaturated state. In this position, it is preferable not to start the precipitation of the single crystal.

  Next, the inclination angle θ of the growth container 1 is set to 0 (second posture), and the state shown in FIG. Single crystals are grown in this state. When the area of the gas-liquid interface 2a of the solution 2 in the second posture (FIG. 2B) is A, the area B of the gas-liquid interface 2a of the solution 2 in the first posture (FIG. 2A) is ( A / cos θ). Therefore, by inclining the growth vessel 1 in the nitrogen dissolving step, the amount of nitrogen dissolved in the solution 2 per hour increases, and the time until the start of single crystal precipitation can be shortened.

  In the example of FIG. 3A, the growth container 1A is laid down (first posture) and held. In this state, nitrogen is dissolved in the solution to obtain a supersaturated state. In this position, it is preferable not to start the precipitation of the single crystal.

  Next, the growth container 1 is placed vertically (second posture), and the state shown in FIG. Single crystals are grown in this state. The area of the gas-liquid interface 2a of the solution 2 in the second posture (FIG. 3B) is A, and the area of the gas-liquid interface 2a of the solution 2 in the first posture (FIG. 3A) is B. is there. By elongating this container, the ratio between A and B can be easily set. And by making B larger than A, the amount of nitrogen dissolved in the solution 2 per hour increases, and the time until the precipitation of the single crystal can be shortened.

  Single crystals are grown after the nitrogen melting step. The nitrogen pressure P2 and the temperature T2 in the single crystal growth step may be the same as the nitrogen pressure P1 and the temperature T1 in the nitrogen melting step. Or precipitation of a single crystal can be accelerated | stimulated by making the nitrogen pressure P2 in a single crystal growth process higher than the nitrogen pressure P1 in a nitrogen melt | dissolution process. Alternatively, by setting the temperature T2 in the single crystal growth step to be lower than the temperature T1 in the nitrogen dissolution step, saturation solubility can be lowered and single crystal precipitation can be promoted.

  In the nitrogen dissolving step, it is necessary to dissolve more nitrogen into the solution containing the flux and the group III raw material. Therefore, the temperature T1 and the pressure P1 are set so that the nitrogen is not saturated.

  If the holding time in the nitrogen dissolving step is too short, the amount of nitrogen dissolved in the solution is small, and it becomes difficult to improve the productivity of the single crystal. On the other hand, there is no particular upper limit. However, even without a nitrogen dissolution step, single crystal growth tends to start within several tens to 100 hours from the start of the growth step.

  Since the specific value of T1 varies depending on the flux and the composition ratio of the single crystal, it is appropriately selected. For example, when Na flux is used and a GaN single crystal is grown, 850-1000 ° C. is preferable. When an AlN single crystal is grown using Sn-Mg flux, 1200 to 1500 ° C is preferable. Further, the specific value of P1 varies depending on the temperature, the flux, the composition ratio of the single crystal, the additive, and the like, and therefore is appropriately selected.

  The difference between T1 and T2 is not limited and may be 0 ° C. Alternatively, from the viewpoint of promoting precipitation of nitride single crystals, the temperature is preferably 10 ° C. or higher, and more preferably 30 ° C. or higher. In addition, if this difference is too large, the crystal quality tends to be lowered. From this viewpoint, 100 ° C. or less is preferable.

  In the case where the growth container is inclined between the first posture and the second posture, the inclination angle θ is 30 from the viewpoint of promoting the dissolution of nitrogen by increasing the difference in the solution gas-liquid interface area. It is preferably at least 40 °, more preferably at least 40 °. There is no particular upper limit for θ, and it may be a right angle.

  The ratio B / A between the area B of the solution gas-liquid interface in the first posture and the area A of the solution gas-liquid interface in the second posture is 1.1 or more from the viewpoint of promoting the dissolution of nitrogen into the solution. Is preferable, and 1.2 or more is more preferable. However, if A is too small, miscellaneous crystals are likely to be generated in the crystal growth step. From this viewpoint, B / A is preferably 2 or less, and more preferably 1.8 or less.

  The shape of the growth container is not particularly limited. The planar form of the inner space 3 of the growth container may be a circle or a polygon such as a regular triangle, a square, a rectangle, or a regular hexagon.

  In the single crystal growth apparatus of the present invention, the apparatus for heating the raw material mixture to produce a solution is not particularly limited. This apparatus is preferably a hot isostatic pressing apparatus, but other atmospheric pressure heating furnaces may be used.

  The flux for producing the solution is not particularly limited, but one or more metals selected from the group consisting of alkali metals and alkaline earth metals or alloys thereof are preferable. Examples of the metal include lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, and barium. Lithium, sodium, and calcium are particularly preferable, and sodium is most preferable.

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

  The material of the heater is not particularly limited, but iron-chromium-aluminum and nickel-chromium alloy heating elements, platinum, molybdenum, tantalum, tungsten and other refractory metal heating elements, silicon carbide, molybdenum silicite, carbon, etc. Non-metallic heating elements can be exemplified.

  Using the present invention, a gallium nitride single crystal can be grown using a flux containing at least sodium metal. In this flux, the gallium source material is dissolved. As the gallium source material, a gallium simple metal, a gallium alloy, and a gallium compound can be applied, but a gallium simple metal is also preferable in terms of handling.

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

  A gas other than nitrogen in the atmosphere is not limited, but an inert gas is preferable, and argon, helium, and neon are particularly preferable.

The material of the growth substrate for epitaxially growing the single crystal is not limited, but sapphire, AlN template, GaN template, silicon single crystal, SiC single crystal, MgO single crystal, spinel (MgAl ₂ O ₄ ), LiAlO ₂ , LiGaO _2. And perovskite complex oxides such as LaAlO ₃ , LaGaO ₃ , and NdGaO ₃ . The composition formula _{[A 1-y (Sr 1-} x Ba x) y ] _{[(Al 1-z Ga z)} 1-u · D u ] _O 3 (A is a rare earth element; D is niobium and One or more elements selected from the group consisting of tantalum; y = 0.3-0.98; x = 0-1; z = 0-1; u = 0.15-0.49; x + z = 0 .1 to 2) cubic perovskite structure composite oxides can also be used. SCAM (ScAlMgO ₄ ) can also be used.

Example 1
A square flat bottom crucible 1 shown in FIG. 2 was used as a growth container. The inner dimensions of the crucible were 60 mm wide, 60 mm deep, and 100 mm high. Into this, 150 g of metal Ga and 220 g of metal Na were added as growth raw materials, and three seed crystal substrates 4 (GaN templates having a diameter of 2 inches) were arranged at equal intervals. The GaN template substrate is obtained by epitaxially growing a GaN single crystal thin film on a sapphire substrate by 5 μm. This operation was performed in a glove box having a dew point of -85 ° C and an oxygen concentration of 0.1 ppm. The liquid height H of the raw material in the crucible was about 7 cm.

  This crucible 1 was placed in a container in a glove box, and after sealing the container 5, it was taken out of the glove box and installed in a growth furnace. After raising the temperature and pressure to 870 ° C. and 4.5 MPa, the container 5 was inclined 45 ° with respect to the horizontal plane. The gas-liquid interface area B at this time has increased to 1.41 times the initial value. When the holding time in this inclined state was changed to 72, 96, 120, and 150 hours and the time for starting crystal growth was determined, it was found that the crystal growth started in 70 hours.

  For this reason, after 70 hours from the start of the nitrogen melting step, the inclination of the crucible was returned to the original state, and then maintained for 100 hours, and the crystal growth step was performed. Thereafter, the mixture was gradually cooled to room temperature, and crystals were collected. A GaN crystal of about 0.5 mm was grown on the entire surface of the 2-inch seed substrate. The in-plane thickness variation was small, less than 10%. Also, the average thickness variation of the three sheets was as small as about 10%.

(Example 2)
As a growth container, a square flat bottom crucible 1A shown in FIG. 3 was used. The inner diameter of the crucible 1 was 60 mm wide, 60 mm deep, and 120 mm high. In this, 150 g of metal Ga and 220 g of metal Na were accommodated as growth raw materials, and three GaN templates having a diameter of 2 inches were arranged at regular intervals as a seed crystal substrate. The GaN template substrate is obtained by epitaxially growing a GaN single crystal thin film on a sapphire substrate for 5 microns. This operation was performed in a glove box having a dew point of -85 ° C and an oxygen concentration of 0.1 ppm. The liquid height H of the raw material in the crucible was about 7 cm.

  This was placed in the inner container in the glove box, and after sealing the inner container, it was taken out of the glove box and installed in the growth furnace. At this time, the crucible 1A was installed at an angle of 45 degrees with respect to the horizontal plane. After raising the temperature and pressure to 870 ° C and 4.5 MPa, the growth furnace was tilted 45 degrees in the same direction as the tilt of the crucible. At this time, the crucible is as shown in FIG. The gas-liquid interface area B is twice as large as A, and the liquid height h is half of H. When the holding time in this state was changed to 48, 72, and 96 hours and the time for starting crystal growth was determined, it was found that the crystal growth started in about 50 hours.

  For this reason, after returning to the state of FIG. 3A, the state was returned to the state of FIG. 2B after 50 hours, held for 120 hours thereafter (crystal growth step), and then gradually cooled to room temperature to recover the crystals. A GaN crystal of about 0.6 mm was grown on the entire surface of the 2-inch seed substrate.

(Comparative Example 1)
In the same manner as in Example 1, the raw materials were weighed and accommodated in a crucible to grow crystals. However, as shown in FIG. 2 (a), the crucible 1 was held for 45 hours while being inclined at 45 ° with respect to the horizontal plane, and dissolution of nitrogen into the solution and precipitation of a single crystal were performed. As a result, the thickness of the obtained crystal was thick in the vicinity of the gas-liquid interface 2a and thin at the bottom of the crucible, and the in-plane thickness variation was large. Moreover, the average thickness of the three sheets arranged also varied.

(Comparative Example 2)
The raw materials were weighed in the same manner as in Example 2 and accommodated in the crucible 1A to grow a single crystal. However, the crucible 1A was held for 150 hours while being laid down as shown in FIG. 3 (a), and nitrogen melting and single crystal growth were performed. As a result, of the three seed crystal substrates, the uppermost substrate was not in contact with the raw material and was not grown. The one placed in the middle grew about 1 mm, but it grew three-dimensionally, the gas-liquid interface was not flat, there were inclusions around, and the crystal quality was not good. The substrate at the bottom grew about 0.5 mm, but the gas-liquid interface was not flat as well, and there were inclusions around and some crystal grains were taken in. It was not good.

(Comparative Example 3)
The raw materials were weighed in the same manner as in Example 1 and accommodated in the crucible 1 to grow a single crystal. However, the crucible was not tilted and held in the state of FIG. 2B for 170 hours to dissolve nitrogen and grow a single crystal. The crystal grew only about 0.35 mm. The in-plane thickness variation was small, about 10%. Also, the average thickness variation of the three sheets was as small as about 10%. Estimating the time to start growth with each holding time being 72, 96, 120, it was found to be as long as about 100 hours.

(A) is a cross-sectional view schematically showing a state in which a seed crystal substrate 4 is placed horizontally in the crucible 1 and a single crystal is grown, and (b) is a diagram showing the seed crystal substrate 4 in the crucible 1. It is sectional drawing which shows typically the state which has grown up the single crystal vertically. (A) is sectional drawing which shows the state which inclined the crucible 1 and has melt | dissolved nitrogen in the solution 2, (b) is raising the crucible 1 horizontally and growing a single crystal. It is sectional drawing which shows a state. (A) is sectional drawing which shows the state which hold | maintains the crucible 1A in a horizontally long state, and dissolve | melts nitrogen in the solution 2, (b) is a solution in which the crucible 1A is held in a vertically long state It is sectional drawing which shows the state currently melt | dissolved in 2. FIG. It is sectional drawing which shows typically an example of the apparatus for implementing this invention.

Explanation of symbols

  1, 1A crucible 2 solution 2a gas-liquid interface of solution 3 space in crucible 4 seed crystal substrate 5 outer container 6 rotating shaft 8A, 8B, 8C heater A area of solution gas-liquid interface in second posture B first posture The area of the solution gas-liquid interface at H The height of the solution in the second position θ The inclination angle of the crucible in the first position and the second position

Claims (3)

A method of immersing a seed crystal substrate in a solution containing a flux and a group III material in a growth vessel and growing a nitride single crystal on the seed crystal substrate in a nitrogen-containing atmosphere,
A nitrogen dissolving step for promoting dissolution of nitrogen in the solution while holding the growth vessel in a first posture; and the nitriding on the seed crystal substrate while holding the growth vessel in a second posture; A single crystal growth step for growing a single crystal of the product, wherein an area of the gas-liquid interface of the solution in the first posture is larger than an area of the gas-liquid interface of the solution in the second posture A method for producing a nitride single crystal.   The method according to claim 1, wherein the first posture is a posture for holding the growth container in an inclined state.   The method according to claim 1, wherein an angle formed by a bottom wall of the growth vessel is a right angle between the first posture and the second posture.

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