JP2009018975A - Manufacturing method of nonpolar plane group iii nitride single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preparing a self-standing substrate composed of a nonpolar plane group III nitride single crystal having a good quality with high productivity. <P>SOLUTION: The method comprises forming a ground film 2 composed of a nonpolar plane group III nitride on a substrate 1 by a vapor phase deposition method, forming a metal film on the ground film 2, providing a metal nitride film 4 having voids by nitriding the metal film, forming a seed crystal film 5 on the metal nitride film 4 by a vapor phase deposition method, and growing a nonpolar plane group III nitride single crystal 6 on the seed crystal film 5 by a flux process. The single crystal 6 can easily be peeled from the substrate 1 along the metal nitride film 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for growing a nonpolar plane group III nitride single crystal.

Conventional technology

  Gallium nitride (GaN) thin film crystals are attracting attention as excellent blue light emitting devices, put into practical use in light emitting diodes, and expected as blue-violet semiconductor laser devices for optical pickups. In particular, when a nonpolar plane GaN substrate is applied, an increase in defects can be suppressed in manufacturing a green LED using InGaN having a high In composition ratio, and high efficiency can be achieved because there is no generation of a piezoelectric field. In addition, it is expected that a highly efficient LD can be realized.

A method has been reported in which a seed substrate of GaN or AlN is deposited on a single crystal substrate such as sapphire to obtain a template substrate, and a GaN single crystal is grown on the template substrate (Patent Document 1).
JP 2000-327495 A

Patent Document 2 discloses a so-called void-separated separation (VAS) method. That is, titanium or nickel having a GaN decomposition catalytic action is deposited on a sapphire substrate after GaN epitaxial growth, and heat treatment is performed in an atmosphere such as ammonia gas to form titanium nitride and nickel nitride having voids. When GaN is grown on the nitride layer having voids, GaN grows while filling the voids, and crystallinity is improved. Then, after the GaN single crystal is grown, the sapphire substrate and the GaN single crystal are separated from each other in the gap layer, so that a GaN single crystal free-standing substrate is obtained.
JP 2002-343728 A

  In the method described in Patent Document 1, for example, it is difficult to peel a GaN single crystal film from an underlying template substrate, and it is difficult to produce a GaN single crystal free-standing substrate.

  In the VAS method described in Patent Document 2, a polar surface (c-plane) GaN film is formed on a sapphire substrate by MOCVD. Then, after depositing a titanium film, the titanium film is heat-nitrided, and a c-plane GaN single crystal is grown on the titanium film by HVPE. However, the present inventors tried to form a nonpolar plane (a-plane / m-plane) GaN single crystal on the titanium nitride film by this method. However, it has been found that this method cannot be used industrially because the growth rate of the GaN single crystal is extremely slow and the defect density of the obtained GaN single crystal is high.

  An object of the present invention is to provide a method for obtaining a high-quality free-standing substrate made of a single crystal of a nonpolar plane group III nitride having a good quality.

The method according to the present invention comprises:
A base film forming step of forming a base film made of a non-polar group III-nitride on the substrate by vapor deposition;
A metal film forming step of forming a metal film on the base film;
A nitriding step of nitriding the metal film to provide a metal nitride film having voids;
A step of forming a seed crystal film made of a nonpolar group III nitride on the metal nitride film; and a crystal growth step of growing a single crystal of the nonpolar group III nitride on the seed crystal film by a flux method It is characterized by having.

The inventor studied the application of the method described in Patent Document 2 to a nonpolar plane group III nitride single crystal. In this method, a GaN film is formed on a sapphire substrate by vapor deposition, and then a metal film is deposited and then nitrided is used as a seed substrate. A GaN single crystal was grown on this seed substrate by the HVPE method. First, in the case of a c-plane GaN single crystal, a growth rate of 100 μm / h and a defect density of 10 ⁷ cm ⁻² were obtained, and both the growth rate and crystallinity were relatively good. However, in the case of the a-plane GaN single crystal and the m-plane GaN single crystal, the growth rate was as extremely low as 3 μm / h and the defect density was as high as 10 ⁸ cm ⁻² , which proved unpractical.

For this reason, the present inventor formed a nonpolar plane (a-plane / m-plane) GaN film on a sapphire substrate by vapor deposition, then deposited a metal film, nitrided, and again a nonpolar plane GaN film An attempt was made to form a nonpolar plane group III nitride single crystal on the substrate by a flux method. As a result, for example, when a GaN single crystal was formed, a growth rate of 10 μm / h and a defect density of 10 ⁶ cm ⁻² were obtained on the c-plane, a-plane, and m-plane. That is, the present inventors have found that a growth rate and crystallinity equivalent to or higher than those of c-plane group III nitrides can be obtained even in the case of nonpolar plane group III nitrides.

  Moreover, when the temperature is lowered after the completion of crystal growth by the flux method, due to the difference in thermal expansion coefficient between the sapphire substrate and the nonpolar plane group III nitride single crystal, the substrate and the single crystal Has been found to peel off spontaneously and single crystals become self-supporting. Since this peeling eliminates stress due to warping, cracks are unlikely to occur during temperature drop after crystal growth. Moreover, since it peels naturally, it does not require a peeling process such as pulse laser or substrate polishing, and there is no thermal or mechanical damage to the self-supporting substrate.

Hereinafter, the present invention will be described in detail with appropriate reference to the drawings.
As shown in FIG. 1A, a base film 2 made of a nonpolar plane III-nitride is formed on a surface 1a of a substrate 1. Next, as shown in FIG. 1B, a metal film 3 is formed on the base film 2 by a vapor phase method such as sputtering or vapor deposition. Next, the metal film 3 is nitrided to form a metal nitride film 4 (FIG. 1C). During this nitridation, many fine voids are generated in the metal nitride film 4. Also, the base film 2 tends to generate many fine voids or recesses. Next, as shown in FIG. 1D, a seed crystal film 5 made of a nonpolar plane group III nitride is formed on the metal nitride film 4.

  Next, as shown in FIG. 2A, a single crystal 6 of non-polar plane group III nitride is epitaxially grown by a flux method. In this state, when the nonpolar plane group III nitride single crystal 6 becomes thick, it becomes easy to peel off from the substrate 1 along the metal nitride film 4. Therefore, as shown in FIG. 2B, the single crystal 6 was easily peeled from the substrate 1 along the metal nitride film 4 to obtain a self-supporting substrate.

In the present invention, the substrate is not particularly limited as long as non-polar plane group III nitride can be grown. Examples thereof include sapphire, silicon single crystal, α-SiC single crystal, γ-LiAlO ₂ and LiGaO ₂ .

  In the present invention, the base film and the seed crystal film are formed of nonpolar plane group III nitride, and the nonpolar plane group III nitride single crystal is grown by a flux method. These nonpolar group III nitrides preferably have the same composition, but may be different from each other as long as epitaxial growth is possible.

  Each III-nitride wurtzite structure has a c-plane, a-plane and m-plane. Each of these crystal planes is defined crystallographically. The nonpolar plane group III nitride means a film or a single crystal substrate in which the substrate surface is a group-III nitride having an a-plane or m-plane. The growth direction of the base film, the seed crystal film, and the nonpolar plane group III nitride single crystal grown by the flux method may be the normal direction of the a plane or the normal direction of the m plane. Good.

  Each of these nonpolar plane group III nitrides is preferably a nitride of one or more metals selected from Ga, Al, and In, and GaN, AlN, GaAlN, GaAlInN, and the like are particularly preferable. Further, these nitrides may contain an unintended impurity element. Moreover, in order to control electroconductivity, you may include dopants, such as Si, Ge, Be, Mg, Zn, Cd added intentionally.

  The formation method of the base film is a vapor phase growth method, and examples thereof include a metal organic chemical vapor deposition (MOCVD) method, a hydride vapor phase growth (HVPE) method, an MBE method, and a sublimation method.

  In the present invention, the metal film is not particularly limited as long as the metal film has a catalytic action for promoting the decomposition of the metal nitride. The metal is preferably made of titanium, titanium alloy, nickel or nickel alloy, but Fe, Zr, Hf, W, and Pt can also be used.

  The thickness of the metal film is desirably 500 nm or less. The metal film after nitriding may be thicker than the metal film before nitriding, but here, the thickness of the metal film before nitriding shall be said. This is because if the thickness of the metal film is greater than 500 nm, when the metal film is nitrided, it becomes difficult to form fine holes and it becomes difficult to bury and grow the underlying GaN layer. The lower limit of the thickness of the metal film is not particularly limited, but is preferably 50 nm or more from the viewpoint of the present invention.

  The heat treatment temperature for nitriding the metal film and forming voids in the base film is not limited, but 700 ° C. or higher is preferable from the viewpoint of promoting decomposition. Moreover, 1200 degreeC or less is preferable from a viewpoint of suppressing deterioration of a base film or a board | substrate.

  The atmosphere for nitriding the metal film only needs to contain a nitrogen atom-containing gas, specifically nitrogen, ammonia, or the like. Gases other than the nitrogen-containing gas in the atmosphere are not limited, but hydrogen gas, argon, and helium are particularly preferable.

  The porosity of the metal nitride film having voids is preferably 10% or more, and more preferably 30% or more. This improves the effect of reducing the dislocation density of the nonpolar group III nitride. Further, from the viewpoint of promoting the formation of nonpolar group III nitride, the porosity is preferably 90% or less, and more preferably 70% or less.

  The metal film nitriding step is preferably performed continuously in the same furnace until the next step, which is the seed crystal film generation.

The seed crystal film can be formed of the materials described for the underlayer and can be formed by the method described above. However, the seed crystal film and the base film may be made of group III nitrides having the same composition, or may be made of group III nitrides having different compositions. The seed crystal film formation method may be the same as or different from the underlayer film formation method.

  In the present invention, the nonpolar group III nitride is grown by the flux method. At this time, the type of flux is not particularly limited as long as a group III nitride single crystal can be generated. In a preferred embodiment, a flux containing at least one of an alkali metal and an alkaline earth metal is used, and a flux containing sodium metal is particularly preferred.

  In the flux, the raw material of the target group III nitride single crystal is mixed and used. The raw materials constituting the flux are selected according to the target group III nitride single crystal.

  For example, as a gallium source material, a gallium simple metal, a gallium alloy, or a gallium compound can be applied, but a gallium simple metal is also preferable in terms of handling. As the aluminum raw material, an aluminum simple metal, an aluminum alloy, and an aluminum compound can be applied, but an aluminum simple metal is also preferable in terms of handling. As the indium raw material, indium simple metal, indium alloy, and indium compound can be applied, but indium simple metal is preferable from the viewpoint of handling.

  The growth temperature of the group III nitride single crystal and the holding time at the time of growth in the flux method are not particularly limited, and are appropriately changed according to the type of target single crystal and the composition of the flux. In one example, when growing a GaN single crystal using a sodium or lithium-containing flux, the growth temperature can be set to 800 to 1000 ° C.

  In the flux method, a single crystal is grown in an atmosphere containing a gas containing nitrogen atoms. This gas is preferably nitrogen gas, but may be ammonia. The total pressure of the atmosphere is not particularly limited, but is preferably 10 atm or more, and more preferably 30 atm or more from the viewpoint of preventing evaporation of the flux. However, since the apparatus becomes large when the pressure is high, the total pressure of the atmosphere is preferably 2000 atmospheres or less, and more preferably 500 atmospheres or less. A gas other than nitrogen in the atmosphere is not limited, but an inert gas is preferable, and argon, helium, and neon are particularly preferable.

Example 1
A nonpolar a-plane GaN single crystal was grown in accordance with the method described with reference to FIGS.
(1) Formation of base film The r-plane sapphire substrate 1 having a diameter of 2 inches is placed in a MOCVD furnace (metal organic chemical vapor deposition furnace) and heated at 1150 ° C. for 10 minutes in a hydrogen atmosphere to clean the surface. It was. Next, the substrate temperature was lowered to 500 ° C., and a GaN film was grown to a thickness of 0.03 μm using TMG (trimethylgallium) and ammonia as raw materials. Next, the substrate temperature was lowered to 1100 ° C., and a GaN film 2 was grown to a thickness of 0.5 μm using TMG (trimethylgallium) and ammonia as raw materials (FIG. 1A).

(2) Metal deposition / nitriding treatment A titanium film 3 was deposited on the substrate to a thickness of 0.2 μm (FIG. 1B). Subsequently, when heated at 1100 ° C. for 60 minutes in an atmosphere of hydrogen gas and ammonia gas, it was confirmed that fine holes were generated in the entire surface of the titanium layer. When X-ray diffraction measurement was performed, a TiN diffraction peak was observed, confirming the formation of titanium nitride 4 (FIG. 1C).

(3) Formation of Seed Crystal Film This substrate was put back into the MOCVD furnace, and the GaN film 5 was grown to a thickness of 2 μm using TMG (trimethylgallium) and ammonia as raw materials at a substrate temperature of 1100 ° C. ( FIG. 1 (d)). When the defect density of the GaN film 5 was measured, it was 10 ⁹ pieces / cm ² .

(4) Flux method The GaN crystal 6 was grown by the Na flux method using this substrate as a seed substrate. The raw materials used for the growth are metallic gallium, metallic sodium and metallic lithium. An alumina crucible was filled with 45 g of metallic gallium, 66 g of metallic sodium, and 45 mg of metallic lithium, respectively, and a GaN single crystal was grown at a furnace temperature of 900 ° C. and a pressure of 50 atm for about 100 hours. When taken out from the crucible, a transparent single crystal was grown, and GaN was deposited on the substrate surface to a thickness of about 1 mm.

  The sapphire substrate peeled off naturally during cooling, and no cracks were observed. Peeling occurred due to the difference in thermal expansion between sapphire and GaN in the voids formed during nitriding of the titanium film. When the same process was repeated 10 times, the results were the same for all 10 times.

The GaN free-standing substrate thus obtained was flattened by polishing with diamond abrasive grains, and a GaN single-crystal free-standing substrate having a diameter of 2 inches was obtained. When the defect density of the GaN single crystal substrate was measured, it was 10 ⁶ pieces / cm ² or less, which was significantly smaller than that of the seed crystal film. The half width of the (11-20) ω scan by XRD was 50 seconds.

(Example 2)
A nonpolar m-plane GaN single crystal was grown according to the method described with reference to FIGS.
(1) Formation of base film A 2 inch diameter m-plane sapphire substrate 1 is placed in a MOCVD furnace (metal organic chemical vapor deposition furnace) and heated in a hydrogen atmosphere at 1150 ° C. for 10 minutes to clean the surface. It was. Next, the substrate temperature was lowered to 500 ° C., and a GaN film was grown to a thickness of 0.03 μm using TMG (trimethylgallium) and ammonia as raw materials. Next, the substrate temperature was lowered to 1100 ° C., and the GaN film 2 was grown to a thickness of 0.5 μm using TMG (trimethylgallium) and ammonia as raw materials.

(2) Metal vapor deposition / nitridation treatment / generation of seed crystal film Titanium film 3 was vapor-deposited to a thickness of 0.2 μm on this substrate, placed again in the MOCVD furnace, and then in a hydrogen gas and ammonia gas atmosphere 1100 The substrate was heated at 60 ° C. for 60 minutes, the metal layer 3 on the substrate surface was annealed, and nitriding was performed. It was confirmed that fine holes were generated on the entire surface of the obtained nitride layer. When X-ray diffraction measurement was performed, a TiN diffraction peak was observed, confirming the formation of titanium nitride 4. Next, the GaN film 5 was grown to a thickness of 2 μm using TMG (trimethylgallium) and ammonia as raw materials at a substrate temperature of 1100 ° C. (FIG. 1D). When the defect density of the GaN film 5 was measured, it was 10 ⁹ pieces / cm ² .

(3) Flux method The GaN crystal 6 was grown by the Na flux method using this substrate as a seed substrate. The raw materials used for the growth are metallic gallium, metallic sodium and metallic lithium. An alumina crucible was filled with 45 g of metallic gallium, 66 g of metallic sodium, and 45 mg of metallic lithium, respectively, and a GaN single crystal was grown at a furnace temperature of 900 ° C. and a pressure of 50 atm for about 100 hours. When taken out from the crucible, a transparent single crystal was growing, and an m-plane GaN single crystal film 5 was deposited on the substrate surface with a thickness of about 1 mm.

  The sapphire substrate peeled off naturally during cooling, and no cracks were observed. Peeling occurred due to the difference in thermal expansion between sapphire and GaN in the voids formed during nitriding of the titanium film. When the same process was repeated 10 times, the results were the same for all 10 times.

The GaN free-standing substrate thus obtained was flattened by polishing with diamond abrasive grains, and a GaN single-crystal free-standing substrate having a diameter of 2 inches was obtained. When the defect density of the GaN single crystal substrate was measured, it was 10 ⁶ pieces / cm ² or less, which was significantly smaller than that of the seed crystal film. The half width of the (1-100) ω scan by XRD was 50 seconds.

(Comparative Example 1)
(MOCVD method)
The r-plane sapphire substrate 1 having a diameter of 2 inches was placed in a MOCVD furnace (metal organic chemical vapor deposition furnace) and heated at 1150 ° C. for 10 minutes in a hydrogen atmosphere to clean the surface. Next, the substrate temperature was lowered to 500 ° C., and a GaN film was grown to a thickness of 0.03 μm using TMG (trimethylgallium) and ammonia as raw materials. Next, the substrate temperature was lowered to 1100 ° C., and the GaN film 2 was grown to a thickness of 0.5 μm using TMG (trimethylgallium) and ammonia as raw materials. The growth rate at this time was almost the same as when the c-plane sapphire substrate was grown.

(Metal vapor deposition, nitriding, seed crystal film generation)
A titanium film 3 is deposited to a thickness of 0.2 μm on this substrate, and is again put in the MOCVD furnace, and heated in a hydrogen gas and ammonia gas atmosphere at 1100 ° C. for 60 minutes to form the metal layer 3 on the substrate surface. Annealing treatment and nitriding treatment were performed. It was confirmed that fine holes were generated on the entire surface of the obtained nitride layer. When X-ray diffraction measurement was performed, a TiN diffraction peak was observed, confirming the formation of titanium nitride 4. Next, the GaN film 5 was grown to a thickness of 2 μm using TMG (trimethylgallium) and ammonia as raw materials at a substrate temperature of 1100 ° C. (FIG. 1D). When the defect density of the GaN film 5 was measured, it was 10 ⁹ pieces / cm ² .

(HVPE method)
Following this, the substrate was placed in an HVPE furnace and hydrogen chloride (HCl
) And metal gallium (Ga), gallium chloride (GaCl), ammonia and hydrogen as raw materials, and the substrate is heated to a temperature of 1050 ° C. The ammonia gas and hydrogen gas flow rates were 1 slm and 5 slm, respectively. After the temperature became stable, HCl was allowed to flow over metal Ga at a flow rate of 0.05 slm to generate GaCl. When a-plane GaN was grown on the substrate for 100 hours by such a method, the film thickness was 300 μm, and the growth rate was very slow. Many cracks were generated in the GaN single crystal, and the sapphire substrate was not peeled off. When the same process was repeated 10 times, the results were the same for all 10 times.

An attempt was made to remove the sapphire substrate from this a-plane GaN single crystal by polishing with diamond abrasive grains. However, the GaN single crystal was cracked and divided. When the defect density of some of the divided portions was measured, it was 10 ⁸ pieces / cm ² or more. This was more than the a-plane GaN single crystal substrate obtained by the Na flux method, and the dislocation reduction effect by the titanium nitride film was small.

(Comparative Example 2)
(MOCVD method)
The m-plane sapphire substrate 1 having a diameter of 2 inches was placed in a MOCVD furnace (metal organic chemical vapor deposition furnace) and heated at 1150 ° C. for 10 minutes in a hydrogen atmosphere to clean the surface. Next, the substrate temperature was lowered to 500 ° C., and a GaN film was grown to a thickness of 0.03 μm using TMG (trimethylgallium) and ammonia as raw materials. Next, the substrate temperature was lowered to 1100 ° C., and the GaN film 2 was grown to a thickness of 0.5 μm using TMG (trimethylgallium) and ammonia as raw materials. The growth rate at this time was almost the same as when the c-plane sapphire substrate was grown.

(Metal vapor deposition, nitriding, seed crystal film generation)
On this substrate, a titanium film 3 is deposited to a thickness of 0.2 μm, and is again put in the MOCVD furnace, and heated in a hydrogen gas and ammonia gas atmosphere at 1100 ° C. for 60 minutes to anneal the metal layer on the substrate surface. And nitriding was performed. It was confirmed that fine holes were generated on the entire surface of the obtained nitride layer. When X-ray diffraction measurement was performed, a TiN diffraction peak was observed, confirming the formation of titanium nitride 4. Next, the GaN film 5 was grown to a thickness of 2 μm using TMG (trimethylgallium) and ammonia as raw materials at a substrate temperature of 1100 ° C. (FIG. 1D). When the defect density of the GaN film 5 was measured, it was 10 ⁹ pieces / cm ² .

(HVPE method)
Following this, the substrate was placed in an HVPE furnace and hydrogen chloride (HCl
) And metal gallium (Ga), gallium chloride (GaCl), ammonia and hydrogen as raw materials, and the substrate is heated to a temperature of 1050 ° C. The ammonia gas and hydrogen gas flow rates were 1 slm and 5 slm, respectively. After the temperature became stable, HCl was allowed to flow over metal Ga at a flow rate of 0.05 slm to generate GaCl. When a GaN single crystal was grown on the substrate for 100 hours by such a method, the film thickness was 300 μm, and the growth rate was very slow. Many cracks were generated in the GaN single crystal, and the sapphire substrate was not peeled off. When the same process was repeated 10 times, the results were the same for all 10 times.

Attempts were made to remove the sapphire substrate from this m-plane GaN single crystal by polishing with diamond abrasive grains. However, the GaN single crystal was cracked and divided. When the defect density of some of the divided portions was measured, it was 10 ⁸ pieces / cm ² or more. This was more than the m-plane GaN single crystal substrate obtained by the Na flux method, and the dislocation reduction effect by the titanium nitride film was small.

  As described above, according to the present invention, a high-quality nonpolar plane GaN single crystal substrate having a low dislocation density and stacking fault density can be easily manufactured using conventional equipment.

(A) is sectional drawing which shows the state which formed the base film 2 on the board | substrate 1, (b) is sectional drawing which shows the state which formed the metal film 3 on the base film 2, (c) These are sectional drawings which show the state 4 which nitrided the metal film 3, (d) is sectional drawing which shows the state which formed the seed crystal film 5 on the metal nitride film 4. FIG. (A) is sectional drawing which shows the state which formed the single crystal 6 of the nonpolar surface group III nitride on the seed crystal film 5, (b) shows the state which peeled the single crystal 6 from the board | substrate. It is sectional drawing.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Substrate 2 Base film 3 Metal film 4 Metal nitride film 5 Seed crystal film 6 Nonpolar plane group III nitride single crystal

Claims (4)

A base film forming step of forming a base film made of a non-polar group III-nitride on the substrate by vapor deposition;
A metal film forming step of forming a metal film on the base film;
A nitriding step of nitriding the metal film to provide a metal nitride film having voids;
Forming a seed crystal film made of a nonpolar group III nitride on the metal nitride film by a vapor deposition method; and forming a single crystal of the nonpolar group III nitride on the seed crystal film by a flux method. A method for producing a nonpolar plane group III nitride single crystal, comprising a crystal growth step for growing.   The method according to claim 1, wherein the metal film is made of one or more metals selected from the group consisting of titanium, nickel, and alloys thereof.   3. The claim 1, wherein the nonpolar group III nitride is made of a nitride of at least one group III element selected from the group consisting of gallium, aluminum and indium. The method according to item.   The method according to any one of claims 1 to 3, wherein a self-supporting substrate is obtained by peeling off the nonpolar group III nitride grown in the crystal growth step from the substrate. .

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