JP2023165572A - Production method of gan self-standing substrate of high quality and low cost - Google Patents

Abstract

To provide a production method of a GaN self-standing substrate in a larger diameter with better crystal quality and better productivity at lower cost than before in producing GaN self-standing substrate by a HVPE method.SOLUTION: A GaN template on which a GaN thin film is directly grown without a buffer layer on a ScAlMgO4(SAM) by using a MBE device, and GaN is grown at 1000°C or more by a HVPE device on the template, and a GaN self-standing substrate is produced with spontaneous peeling in cooling time, in a high quality and low cost production method of the GaN self-standing substrate.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing a high-quality, low-cost GaN free-standing substrate.

The characteristics of typical growth methods for thick-film GaN that have already been reported: (a) ammonothermal method, (b) Na flux method, and (c) void formation ablation method (AS) are shown schematically in Figure 1. Explain using diagrams.
Since the ammonothermal method uses a pressure of over 1,000 atmospheres, large-scale devices cannot be manufactured due to restrictions imposed by the law regarding pressure vessels (pressure x volume). On the other hand, the proposed method using HVPE does not require a pressure vessel, so it is easy to increase the size of the device (enables larger diameter and growth of a large number of layers). moreover. Another advantage is that the growth rate is higher than other growth methods. In the Na flux method, since the crystal growth rate in the stacking direction is low, large crystals are grown by growing from a large number of growth nuclei formed on a GaN template as a base point. In this case, it is considered that many threading dislocations occur at the meeting positions of islands (island crystals) grown from crystal nuclei, making it difficult to improve the quality of the entire surface of the substrate. In the void formation peeling method, it is difficult to increase the diameter because there is a large difference in thermal expansion coefficient between the GaN and sapphire substrates. In the method proposed herein using a ScAlMgO ₄ (hereinafter abbreviated as SAM) substrate as the base substrate, the peeling properties of the SAM substrate are used to separate the GaN from the base substrate. It is easy to increase the diameter.
Currently, HVPE is the only method for growing GaN substrates suitable for device applications. A base substrate is required for HDPE growth, and the base substrates used in the production of GaN substrates are sapphire substrates and GaAs substrates, and recently, GaN substrates produced by the Na flux method are being considered as base substrates. Table 1 shows a comparison of the superiority of the base substrates. The present invention utilizes the SAM substrate of the present invention as a base substrate when comparing the separability between HVPE-grown GaN and the base substrate, crystal quality, larger diameter, and expectations for cost reduction by reusing the base substrate. The technological superiority is considered to be significant.

FIG. 2 shows an example in which GaN was grown on a SAM substrate using the HVPE growth technique originally developed at Yamaguchi University, and a GaN crystal with a thickness of 1 mm and a diameter of 50 mm was obtained by natural exfoliation. When GaN is grown on a sapphire substrate by HVPE, there are times when GaN crystals are obtained through spontaneous separation, but the probability of this happening is low. The technical issues of HVPE growth of GaN on a SAM substrate that should be solved during this development period are clear, and the feasibility of this proposed technology and the commercialization scenario described below is high.

Japanese Patent Application Publication No. 2013-058741

An object of the present invention is to provide a method for manufacturing a GaN free-standing substrate that has better crystal quality, better productivity, lower cost, and can be made to have a larger diameter than conventional methods.

In the invention according to claim 1, a GaN template is prepared by directly growing a GaN thin film on a ScAlMgO ₄ (SAM) substrate without a buffer layer using an MBE device, and GaN is grown on the GaN template at a temperature of 1000° C. or higher using an HVPE device. This is a method for manufacturing a high-quality, low-cost GaN free-standing substrate, in which the GaN free-standing substrate is grown at a temperature of , and is naturally peeled off upon cooling.
The invention according to claim 2 is the GaN according to claim 1, wherein the MBE apparatus has a circular heater that heats the substrate 1.2 to 1.5 times as many times as the SAM substrate in growing one sheet. This is a method for manufacturing a self-supporting substrate.
The invention according to claim 3 provides that when the heater diameter of the MBE apparatus is larger than 1.5 times that of the SAM substrate, there is a space between the substrate and the heater that is 1.2 times to 1.5 times the diameter of the substrate. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein a heat shielding plate with holes is provided.
The invention according to claim 4 provides the GaN according to claim 1, in which when a plurality of SAM substrates are installed on one molybdenum susceptor to grow a thin film, a heat shielding plate having the same number of holes as the SAM substrates to be installed is installed. This is a method for manufacturing a self-supporting substrate.
In the invention according to claim 5, the SAM substrate is produced by processing a crystal produced by a CZ method, and the composition ratio of the starting materials used as the melt is:
27%≦ _Sc2O4 ≦28%
45%≦MgO≦46%
26% _{≦ Al2O3} ≦28%
and
The composition ratio of the crystal is
26%≦ _Sc2O4 ≦28%
49%≦MgO≦50%
23% _{≦ Al2O3} ≦24%
A method for manufacturing a GaN free-standing substrate according to claim 1.
The invention according to claim 6 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein the surface of the SAM substrate is a c-plane ±0.01° plane.
The invention according to claim 7 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate is 0.5° off from the c-plane.
The invention according to claim 8 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate has a dislocation density of 10 3 /cm 3 or less.
The invention according to claim 9 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein the surface roughness of the SAM substrate is Ra<0.2 or less.
The invention according to claim 10 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate size is 2 inches or more in diameter and 300 μm or more in thickness.
The invention according to claim 11 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate is colorless and transparent and has absorption in the infrared region (4 to less than 5 μm).
The invention according to claim 12 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein an InGaN film is formed on the SAM substrate.
The invention according to claim 13 is the production of the GaN free-standing substrate according to claim 1, wherein the Ga flux amount at the time of preparing the GaN template is 3 × 10 7 to 8 × 10 7 Torr, and hexagonal GaN with mirror surface and c-axis orientation is grown. It's a method.
The invention according to claim 14 is that in the GaN template, the GaN film thickness is 300 to 2 μm, or the In0.17Ga0.83N film thickness is 50 to 200 μm, and the GaN film thickness is 300 to 2 μm. 1. This is a method for manufacturing a GaN free-standing substrate according to Item 1.
The invention according to claim 15 is the method of manufacturing a GaN free-standing substrate according to claim 1, wherein the surface of the GaN template is in-plane uniform, mirror-like, and flat on an atomic level.
The invention according to claim 16 provides that the HVPE apparatus includes a film-forming chamber, a heater disposed outside the film-forming chamber, and an inner tube disposed within the film-forming chamber,
The inner tube has an outlet on the downstream side, and the inner tube is a storage part for the Ga raw material and a passage for HCl, and is arranged to form an ammonia passage between it and the inner wall of the film forming chamber. has been
2. The method of manufacturing a GaN self-supporting substrate according to claim 1, wherein the film forming chamber includes a holding member for holding the GaN template on the exit side.
The invention according to claim 17 is characterized in that the holding member includes a susceptor, a carbon ring that presses a peripheral edge of a GaN template placed on the susceptor, and a susceptor cover that presses the carbon ring. This is a method for manufacturing the GaN free-standing substrate described above.
The invention according to claim 18 is the method for manufacturing a GaN free-standing substrate according to claim 17, wherein the susceptor is a carbon susceptor.
The invention according to claim 19 is the method for manufacturing a GaN self-supporting substrate according to claim 18, wherein the holding member is capable of holding a plurality of GaN templates, and a plurality of templates can be formed into films at the same time.
The invention according to claim 20 is the method for manufacturing a GaN free-standing substrate according to claim 1, wherein the surface of the SAM crystal after peeling is polished by 10 to 20 μm and reused.

For epitaxial growth of GaN on a foreign substrate, an amorphous or polycrystalline buffer layer is formed at a low temperature of approximately 600°C using the MOVPE method, and approximately 3 μm of GaN is grown on top of the buffer layer at a high temperature of approximately 1000°C or higher to form a GaN template. Create.
A template is produced in which GaN or lattice-matched In0.17Ga0.83N is grown without a buffer layer using the MBE method, which allows growth in an ultra-high vacuum and low temperature environment.
The method for producing a free-standing GaN crystal substrate of the present invention involves growing a highly crystalline GaN thin film on a ScAlMgO ₄ (SAM) base substrate by MOVPE or MBE, and then growing a dielectric material such as SiO _{2 .} After forming a mask with a pattern shape such as a stripe or hexagonal shape, high-speed GaN crystal growth using the HVPE method suppresses the occurrence of cracks and fractures and reduces defect density (dark spot density observed in CL). ) is obtained. Furthermore, due to the cleavability of the SAM substrate, thick film GaN crystals can easily be naturally peeled off from the SAM substrate, and a self-supporting GaN substrate can be easily obtained, and the naturally peeled SAM substrate can be reused as a substrate by re-polishing. .

Note that the dark spot density observed in the above CL is a physical property value that serves as an index for indicating dislocation defects (threading dislocations) exposed on the crystal surface, and is a physical property value that is an index for indicating dislocation defects (threading dislocations) exposed on the crystal surface. It is measured using The accelerating voltage during measurement is 5 kV, and the observation range is 20 μm x 20 μm. At this time, the scotoma density is calculated from the number of scotomas observed within the observation range.

[SAM board]
Conventionally, a sapphire substrate has been used as a base substrate for GaN crystal growth, but the base substrate used in the present invention is the above-mentioned SAM substrate, and the (0001) plane is used. It is characterized by the cleavage property of the (0001) plane. The off angle (misforce angle) of the SAM substrate is optimized to maximize the crystallinity of the GaN.
The SAM substrate is usually disk-shaped with a thickness of 0.4-1 mm and a diameter of 50-300 mm. It is desirable that the diameter be about 10 mm larger than the diameter of the target GaN substrate.

[Creation of GaN template by MOVPE method]
A GaN free-standing substrate using the HVPE method is produced by homoepitaxial growth on a GaN template. First, a GaN template is produced by growing a thin layer of GaN on a SAM substrate by MOVPE.
In order to protect the SAM from thermal damage and to suppress diffusion of SAM-derived impurities such as Mg into the GaN layer, a silicon oxide film or a silicon nitride film is formed on the back and side surfaces of the SAM substrate. Thereafter, a single layer of GaN buffer is grown on the exposed (0001) plane at a low temperature by MOVPE, and then a GaN film of about 1-3 μm is formed. Although the dislocation density is 10 ⁴ to 10 ⁵ /cm ² , the present invention uses a low-dislocation SAM substrate, preferably a non-dislocation SAM substrate, to improve the quality of the GaN template and reduce dislocations.

As a method to clarify the effectiveness of dislocation-free SAM in developing high-quality GaN templates, we will grow GaN films by RF-MBE. We will investigate the reduction of dislocations and the inclusion of Mg impurities, and consider the possibility of creating high-quality GaN templates. In the conventionally used MOVPE and HVPE methods for GaN growth on SAM substrates, the required high-temperature conditions of nearly 1000°C and the high-temperature gas atmosphere during growth may deteriorate the SAM substrate surface or destroy substrate constituent elements such as Mg. Problems such as desorption and mixing into the GaN film still remain. In contrast, RF-MBE using active nitrogen plasma. 1 method, it is possible to grow GaN in a high vacuum state of 10 ^-10 Torr or more at around 700°C, which is several hundred degrees lower than in the MOCVD method, so the problem of contamination with impurities such as Mg is suppressed. there is a possibility.

[Natural peeling of GaN crystal and underlying substrate using separation SAM]
After growing a thick-film GaN crystal by the HVPE method, separating the thick-film GaN crystal from the underlying substrate is an important issue that affects the production yield of GaN substrates. When a sapphire substrate is used as the base substrate, it is separated from the base substrate by a known method such as laser single irradiation separation (laser lift-off), polishing method, or VAS method to obtain a free-standing GaN substrate.
However, methods such as single laser irradiation separation and polishing may cause damage to the GaN crystal, and the VAS method has a complicated process and is a cost saving factor, so it is not necessarily an efficient separation method.
After growing a GaN crystal with a film thickness of 100 μm or more, preferably 300 μm or more using the SAM substrate, the substrate is cooled, and the thermal stress during cooling is reduced without applying any special mechanical stress. Due to the cleavability of SAM, natural peeling occurs at the GaN/SAM interface or within the SAM crystal near the interface, resulting in separation. Cooling may be performed forcibly by supplying low-temperature gas or the like, or may be performed by natural cooling. The temperature of the GaN crystal layer is lowered to 20°C to 150°C by cooling. The cooling rate is, for example, 1 to 100° C./min.
In the present invention, since the SAM base substrate allows separation of GaN crystals by a simple operation of cooling after GaN crystal growth, it is useful for improving the production yield of free-standing GaN crystals.

[Mask pattern]
Film forming means for mask patterning are known per se, and include vacuum evaporation, sputtering, and CVD (Chemical
_A SiO ₂ film _{, SiN} One method is to do so. The thickness of the mask is usually about 0.001 to 3 μm, the width of the mask pattern can be selected from 3 μm to 1000 μm, and the opening width can be selected from 3 μm to 1000 μm.

The mask pattern is formed using a conventional photolithography technique.
The shape of the mask formed on the surface of the GaN crystal layer is not particularly limited, and various shapes such as a stripe mask, a lattice mask, a dot mask, and a hexagonal (honeycomb) mask can be adopted. The lattices of the lattice mask may be orthogonal or may cross at a predetermined angle. There is no particular restriction on the shape of the dots, and specific examples include circular, triangular, square, and hexagonal shapes. In addition, various shapes such as a triangular lattice or a square lattice are adopted as dot arrangement patterns.
It is preferable that the units constituting the mask, for example, the lines in the case of the striped mask, and the dots in the case of the dotted mask, are equally spaced from each other. Further, the above unit may be a single unit or may be a plurality of aggregates. For example, the striped mask may be composed of one line or a combination of a plurality of lines.
This mask no i-turn has the effect of suppressing the occurrence of cracks in GaN grown as a thick film.

[HVPE method]
As a conventional HVVP device, for example, the device described in Patent Document 1 is generally known. The HVPE apparatus according to the embodiment of the present invention includes a film forming chamber, a heater arranged outside the film forming chamber, and an inner pipe arranged inside the film forming chamber,
The inner tube has an outlet on the downstream side, and the inner tube is a storage part for the Ga raw material and a passage for HCl, and is arranged to form an ammonia passage between it and the inner wall of the film forming chamber. The film forming chamber includes a holding member for holding the GaN template on the exit side.

The holding member includes a susceptor, a carbon ring that presses the periphery of the GaN template placed on the susceptor, and a susceptor cover that presses the carbon ring.
The HVPE method is a method in which GaN crystal is epitaxially grown on a base substrate by causing a gas phase reaction between gallium chloride and ammonia, which serves as a nitrogen source, and is a well-known method in itself.
The HVPE apparatus used for crystal growth mainly consists of a reaction tube, a heating system, a gas supply system, and a gas exhaust system. The gas supply system allows precise control of the gas supply amount using a mass flow controller. The heating system uses a hot wall method in which a quartz reaction tube is covered with a resistance heating type heater to heat the reaction tube, the substrate, susceptor, and metal raw materials installed inside the reaction tube. By using the hot wall method, the raw material gas can be sufficiently heated and supplied to the substrate surface, and the saturated vapor pressure of the supplied raw material can be increased. As a result, a large amount of raw material can be supplied and high-speed growth can be achieved.

The inside of the reaction tube is roughly divided into a metal raw material section and a substrate heating section. Ga metal is placed in the metal raw material section, and by supplying HCl gas at high temperature, the Ga metal and HCl gas react to generate GaCl. The generated GaCI gas passes through a quartz pipe and is led to the substrate heating section. A susceptor made of carbon or SiC is disposed in the substrate heating section perpendicularly to the gas flow, and the susceptor has an autorotation mechanism or an autorevolution mechanism. Then, a template substrate in which a GaN crystal is grown on a SAM base substrate is set on the susceptor, and the GaCI gas and NH ₃ gas generated in the metal source react on the substrate, thereby growing the GaN crystal. progresses.

For crystal growth, NH ₃ gas as a nitrogen raw material and HC1 gas for generating GaCl as a Ga source are used as raw material gases. Cl ₂ gas may be used instead of HCl gas to generate GaCl. Further, Ga metal, which is a Ga raw material, is installed in the device. In addition to the raw material gases NH ₃ and HCI gas, a carrier gas such as H ₂ or N ₂ is used.

It is possible to manufacture a GaN free-standing substrate that has better crystal quality, better productivity, lower cost, and can be made larger in diameter than conventional methods.

Explanatory drawings showing growth methods of various GaN thick films. Exterior photographs of (a) a naturally separated GaN free-standing substrate, (b) a SAM, (c) a naturally separated GaN free-standing substrate, and (d) a sapphire substrate. A photograph showing a crystal of GaN directly grown on a SAM substrate by the MBE method. FIG. 3 is a conceptual diagram of an MBE substrate holder according to an embodiment. Conceptual diagram of a substrate holder of an RF-MBE device according to another embodiment Conceptual diagram of a substrate holder of an RF-MBE device according to another embodiment According to an embodiment, a ternary phase diagram of SAM FTIR diagram in Example. FIG. 3 is a photographic diagram of the GaN side immediately after growth by HVPE in an example. FIG. 4 is a photographic diagram of the SAM side immediately after growth by HVPE according to an example. A photographic diagram of the appearance of the MO-GaN template. External photographic diagram of MBE-GaN template.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to these Examples in any way. Furthermore, not all combinations of features described in the embodiments are essential to the solution of the present invention.

(Example 1)
[Formation of GaN crystal layer] [GaN template MOVPE method]
A (0001) plane SAM just substrate was subjected to CMP processing and cleaning treatment to prepare an epi-ready SAM substrate. Although the MOVPE method, MBE method, etc. are selected for forming the GaN crystal layer, the MOVPE method was used here. The SAM substrate was placed on a quartz tray with the (0001) side facing upward, and set in the MOVPE reactor.
After setting the quartz tray carrying the SAM substrate in MOVPE, the SAM substrate was heated to 1100°C, the pressure inside the reaction vessel was set to 100 kPa, and H ₂ was flowed into the reaction vessel as a carrier gas at 10 L/min. The SAM substrate was thermally cleaned by maintaining this state for 10 minutes.

Next, the temperature of the SAM substrate was lowered to 450°C, and while the pressure inside the reaction vessel was maintained at 100 kPa, carrier gas H ₂ was flowed into the reaction vessel at a flow rate of 5 L/min and N ₂ gas at a flow rate of 5 L/min. While flowing, a group V element supply source (NH ₃ ) and a group Ill element supply source (TMG) were supplied at a supply rate of 5 L/min and 5.5 μmol/min, respectively, and an amorphous layer was formed on the SAM substrate. About 25 nm of GaN was deposited. Subsequently, the supply of TMG was stopped, and the temperature of the substrate was raised to 1100° C. while maintaining the pressure inside the reaction vessel at 100 kPa. During this temperature raising process, crystallization of a portion of the amorphous-like GaN deposited on the substrate progresses, and crystal growth nuclei for the next GaN growth process are formed.

Subsequently, the temperature of the substrate was set to 1100°C, the pressure inside the reaction vessel was set to 100 kPa, and the carrier gas flowing in the reaction vessel was _H2 , and while it was flowing at a flow rate of 10 L/min, Group V was added thereto. By flowing an element supply source (NH ₃ ) and a group III element supply source (TMG) for 90 minutes at a supply rate of 5 L/min and 30 μmol/min, respectively, and growing GaN, the main component of the SAM substrate is grown. (0001) plane GaN was grown to a thickness of about 3 μm on the surface, and a GaN/SAM template was prepared.

[Formation of SiO ₂ mask pattern]
A resist was spin-coated on the GaN crystal layer (growth surface) of the prepared GaN/SAM template. Next, a striped or hexagonal pattern was formed in the a-axis or m-axis direction of GaN using photolithography.
Next, a 200 nm thick film of SiO ₂ was formed by sputtering. After the film formation, a striped or hexagonal SiO ₂ pattern was formed on the GaN crystal layer by washing and removing the resist.

[Formation of GaN thick film] HVPEGaN (GaN template MOVPE method)
The GaN template with the SiO ₂ mask pattern formed on its surface was set in a carbon sample fixing jig (susceptor) in the HVPE apparatus. Thereafter, N ₂ gas was flowed into the reaction tube for 30 minutes to create an N 2 gas atmosphere inside the reaction tube. The reaction tube was heated to 850° C. in the metal raw material portion and 1050° C. in the substrate heating portion, and held for 25 minutes after reaching the set temperature. At this time, N2 gas was passed through the reaction tube until the temperature of the substrate heating section reached 500°C, and _H2 gas and _NH3 gas were passed above 500°C.

Next, HCl gas was passed through the Ga metal installed in the reaction tube at a rate of 0.8 L/min. Further, NH ₃ gas was flowed at 8 L/min and H ₂ gas as a carrier gas was flowed at 4.1 L/min to grow a GaN crystal for 720 minutes. The film thickness of the GaN crystal was 2000 μm.
Thereafter, the flow of HCl gas was stopped, growth was completed, and the substrate was cooled. Cooling was performed by natural cooling while circulating gas. During cooling, NH ₃ gas was flowed at 5 L/min until the substrate temperature became 600° C. or lower, and N 2 gas was flowed at 37 L/min at 600° C. or lower.

When the substrate temperature became 150° C. or less, the substrate was taken out of the apparatus. At that point, the SAM substrate and the GaN crystal layer had already peeled off naturally over the entire surface, and a self-supporting GaN substrate was obtained. Natural; peeling yield was 100%.
The characteristics of the obtained GaN crystal free-standing substrate were measured by the following method.

[scotoma density evaluation]
The surface of the obtained GaN crystal free-standing substrate was observed using a scanning electron microscope/cathode luminescence (SEM/CL) device (JSM-7600F manufactured by JEOL Ltd./MonoCL4 manufactured by Gatan Corporation). Ta. The accelerating voltage at this time was 5 kV, the observation range was 20 μm x 20 μm, and the dark spot density was calculated from the total number of dark spots observed within the observation range.

[Crack number evaluation]
The obtained free-standing GaN crystal substrate was irradiated with high-intensity halogen light, and the number of cracks present in the crystal was measured.

〔Surface condition〕
The surface state of the obtained GaN free-standing substrate was evaluated using a Nomarski differential interference microscope.

The obtained GaN crystal had naturally peeled off from the SAM substrate when taken out from the HVPE furnace, and was a free-standing GaN substrate having a (0001) plane as a main surface and having a thickness of about 2 mm. No cracks were observed on the board. The peeled surface on the back side was a mirror surface on both the GaN crystal side and the SAM substrate side, and the SAM substrate could be recovered without breaking.
The probability of natural peeling was 100%, and the recovery rate of the SAM substrate was 80%.
The full width at half maximum of 0002XRC of the naturally exfoliated GaN crystal was 120 arcsec, and the dark spot density by CL measurement was 1.0×10 ⁶ pieces/cm ² .

[MBE GaN template production method]
Next, we will discuss improvements to the MBE GaN template manufacturing method to suit the SAM substrate, and an example of the GaN on SAM growth method.
When nitride is grown on a ScAlMgO _{4 (} SAM) substrate by the MBE method, the lattice mismatch in the (000l) plane is relatively small (there is no lattice mismatch in In _0.17 Ga _0.63 N), but ( 0001) The step heights in the direction perpendicular to the plane vary widely. Therefore, a nitride having a cubic crystal structure is easily formed due to the anisotropy of the step height. In order to avoid this, it is necessary to make the terrace wide and the off angle in the (0001) plane needs to be small. For small substrates, the off-angle can be processed uniformly within the plane, but as the diameter increases, the off-angle within the plane will vary due to distortion within the bulk crystal and warping during slicing. If there is such non-uniformity, cubic crystals are likely to occur where the off-angle is large, making it difficult to form a hexagonal single phase over the entire surface.
(2) In large-diameter substrates, warpage usually occurs concentrically from the center to the periphery. Therefore, it is necessary to further reduce the off-angle, especially at the center.
(3) The step height differs greatly between the nitride and SAM substrates. Therefore, unless the step is stable on the surface during processing of the SAM substrate, cubic crystals are easily generated and a hexagonal single phase cannot be obtained.
(4) In the MOCVD method, when growing a nitride semiconductor on a sapphire substrate, it is said to be effective to nitride the surface. Although it is easy to ensure in-plane uniformity for small-diameter substrates, in-plane dispersion increases for large-diameter substrates. In order to solve this problem, the substrate temperature is maintained at a high temperature for a certain period of time in a high vacuum state in an MBE growth apparatus while rotating the substrate, and then the substrate temperature is changed and the radicalized nitrogen gas is applied to the substrate for a certain period of time. Surface treatment is performed while rotating. Thereafter, it is maintained at the growth temperature to perform crystal growth. As a result, the surface condition of the substrate is made uniform, and a hexagonal single-phase heavy compound can be produced.

(Example 3)
Growth of GaN on a 50 mm diameter c-plane SAM substrate by MBE method In the Mercure Beam Epitaxy (MBE) method, a growth substrate is placed in the upper part of a high vacuum container with the growth surface facing downward. A crucible containing Ga metal is placed at the bottom of the vacuum container and heated by a heater to generate metal vapor. Similarly, nitrogen gas is sprayed from the nitrogen gas introduction hole at the bottom of the vacuum chamber, and the Ga vapor and nitrogen gas are combined on the substrate to grow a GaN single crystal on the substrate. The raw material Ga metal has a purity of 6N, and the amount of Ga vapor supplied is determined by the temperature of the heater. Nitrogen gas is supplied through the plasma cell for activation at a purity of 5N. The supply amount is determined by the gas flow rate, and the activation degree is determined by the power of the plasma cell.
Growth conditions Atmosphere: Vacuum, 1x10 ^-6 Pa or less Growth temperature: 700°C
Substrate rotation: 10rpm
Ga flux amount: ^6x10-7 Torr
Nitrogen gas flow rate: 2sccm
Plasma power for nitrogen radicals: 120w
Growth time: 4 hours

Growth was performed while observing with RHEED. As a result, we succeeded in growing a GaN single crystal film with a uniform thickness over the entire surface of the SAM substrate. The film thickness was 1.0 μm, and the film thickness was measured by non-destructive film thickness measurement using a spectrophotometer.

As shown in FIG. 3, it was demonstrated that GaN was grown directly on the SAM substrate without using a buffer layer. A GaN plate with a thickness of about 1 μm was formed on a SAM substrate with a diameter of 50 mm without a buffer layer.
Figure 4 shows the internal structure of the device actually used. A substrate with a diameter of 50 mm or more was used as the SAM substrate.
When growing a nitride semiconductor using the MBE method, a single crystal growth method was used that included a step of leaving the substrate in vacuum for a certain period of time after raising the temperature of the substrate, and a step of irradiating radicalized nitrogen gas.

1. In the previous section, since the substrate diameter is 50 mm or more, the temperature rise rate of the molybdenum susceptor and the SAM substrate are different. Therefore, after reaching a predetermined substrate temperature, the temperature is maintained at that temperature for a certain period of time to make the temperature distribution of the SAM substrate uniform.
2. Thereafter, in order to keep the substrate surface constant, radicalized nitrogen gas is irradiated for a certain period of time.
3. Immediately after irradiation with nitrogen gas, heat is removed from the substrate surface temperature, resulting in temperature non-uniformity. To avoid this, the process described in the previous section is performed, and then metal particles are irradiated to generate an underground film.

When the diameter of the heater is larger than 1.5 times, MBE growth is performed in which a heat shield plate with holes 1.2 to 1.5 times the diameter of the substrate is installed between the substrate and the heater. Make it into a device. FIG. 5 shows an MBE growth apparatus.

In the previous section, if the diameter of the heater is larger than 1.5 times that of the SAM board to be installed, install a heat shield plate with a hole between the heater in the previous section and the SAM board, and make sure that the diameter of the hole is equal to or larger than the diameter of the board. It has increased from 1.2 times to 1.5 times.

Since the radiant heat from the heater is blocked by the heat shield plate, the heater diameter has the same effect as in the previous section when viewed from the board side.
When growing thin films by installing a plurality of SAM substrates on one molybdenum susceptor, use an MBE apparatus that is structured to install a heat shield plate with the same number of holes as the substrates to be installed, as in the previous section. The apparatus is shown in FIG.

(Example 4)
Growth of lnGaN on a c-plane SAM substrate with a diameter of 50 mm by MBE method In the same configuration as in the above-described example, an ln metal source was installed in addition to the Ga metal source, and lnGaN crystal growth was performed. The purity of ln metal is 5N.

Growth conditions Atmosphere: Vacuum, 1x10 ^-6 Pa or less Growth temperature: 600°C
Substrate rotation: 10rpm
ln+Ga flux amount: 4.3x10 ^-7 Torr
ln/(ln+Ga) ratio: 0.4
Nitrogen gas flow rate: 2sccm
Plasma shower power for nitrogen radicals: 110W
Growth time: 4 hours

Growth was performed while observing with RHEED. As a result, a ln _0.17 Ga _0.83 N single crystal film with a uniform thickness was successfully grown over the entire surface of the SAM substrate. The film thickness was 0.85 μm, and the film thickness was measured using a non-destructive film thickness meter using a spectrophotometer.
GaN template ln _0.17 Ga _{0.1 by the method of the present invention by MBE method.} 83, the SAM substrate had a diameter of 50 mm or more and the (0001) plane (c plane) was used, but the following conditions are also desirable.

(1) In the (0001) ScAIMgO ₄ single-crystalline product used for vacuolide semiconductor growth by the MBE method, in order to uniformly generate a hexagonal vacuolide semiconductor film over the entire surface of a substrate with a diameter larger than 50 mm, (000l) ) A single-piece substrate characterized in that the off-angle of the ScAIMgO ₄ substrate is within 0.5° over the entire surface of the substrate.
(2) In the above substrate, the off angle is within 0.1° at the center.
(3) The surface of the substrate is in a state where steps can be observed by AFM.
(4) When growing a nitride semiconductor using the MBE method, the SAM substrate is a single-product growth method that includes a step of leaving the substrate in vacuum for a certain period of time after raising the temperature of the substrate, and a step of irradiating radicalized nitrogen gas. , was created by the CZ method, and the composition ratio (mass ratio) of the starting materials used as the melt is as shown in Figure 7.
27% _{≦ Sc2O4} ≦28%
45%≦MgO≦46%
26% _{≦ A12O3} ≦28%
and
The composition ratio of the product is
26% _{≦ Sc2O4} ≦28%

49%≦MgO≦50%

23%≦A1 ₂ O ₃ ≦24%

It is preferable that

(Example 5)
SAM single crystals were grown using a high-frequency induction heating Czochralski furnace.
An Ir ring with an outer diameter of 150 mm and an inner diameter of 20 mm was installed on the top of an Ir crucible with an outer diameter of 50 mm and a height of 50 mm, and 4N (99.99%) scandium, aluminum, and magnesium were added as starting materials to Sc:A1:Mg. 6500g of raw materials mixed at =27.8%:26.7%:45.5% were charged. At this time, a gap was created in the refractory material at the bottom of the crucible, and the configuration was adjusted to enhance heat dissipation from the bottom of the crucible.

The crucible containing the raw materials is placed in the growth furnace, the inside of the furnace is evacuated, and then _N2 gas is introduced, and when the inside of the furnace reaches atmospheric pressure, heating of the device is started until it reaches the melt. , heated for 24 hours. After that, when the raw material became a melt, O ₂ gas was mixed with N ₂ gas at a ratio of 0.5%. A SAM single crystal processed into a cylindrical shape with <001> orientation was used as a seed, and the seed was lowered to near the melt. This seed crystal is gradually lowered while rotating at 5 rpm, and after the tip of the seed crystal comes into contact with the melt, necking treatment is performed, and then, while the temperature is gradually lowered, the pulling speed is 0.5 mm/ Seed crystal growth was performed by raising the seed crystal at a rate of Hr. As a result, a crack-free single crystal with a diameter of 70 mm and a straight section length of 50 mm was obtained.

The composition of the obtained single crystal was analyzed by ICP-MS, and it was confirmed that Sc:A1:Mg=26.8%:23.6%:49.5%. Thereafter, the crystal was evaluated for dislocation using XRT, and the threading dislocation of the crystal was 10 ³ /cm ² or less. The created crystal was processed into a wafer and used as a substrate for growing a GaN thin film. At that time, the weno was mirror polished, the surface roughness Ra was 0.2 nm or less, and the surface orientation tolerance was 0.01 degree or less. A GaN template was fabricated by directly growing a GaN thin film on the processed wafer without a buffer layer.

(Example 6)
Single SAM products were grown using a high-frequency induction heating Czochralski furnace.
An Ir ring with an outer diameter of 50 mm and an inner diameter of 120 mm was installed on the top of an Ir crucible with an outer diameter of 150 mm and a height of 150 mm, and 4N (99.99%) scandium, aluminum, and magnesium were added as starting materials to Sc:A1:Mg. 6500g of raw materials blended at =27.2%:27.2%:45.6% were charged. Crystal growth was performed under the same configuration and conditions as in Example 1.
As a result, as in Example 1, a single piece with no cracks and a diameter of 70 mm and a straight body portion length of 50 mm was obtained.

When the composition of the obtained single crystal product was analyzed by ICP-MS, it was confirmed that Sc:Al:Mg=27.5%:23.0%:49.5%. Thereafter, the crystal was evaluated for dislocations using XRT in the same manner as in Example 1, and it was found that the threading dislocations observed in the crystal were less than 10 ³ /cm ^{2 .} It was used for thin film growth. A GaN template was fabricated by directly growing a GaN thin film on the processed wafer without using a buffer.

(Example 7)
The SAM substrate used for producing the GaN template in Example 3 is colorless and transparent and has an absorption edge in the infrared region of 6 μm or more.

(Sapphire, etc.) When epitaxially growing a nitride semiconductor on a transparent substrate such as sapphire, the peak wavelength of radiant energy is 4 μm to 2.5 μm at a growth temperature of 450° C. to 900° C. (723 K to 1173 K) in the MBE method. Since the infrared absorption edge of sapphire is 5 to 6 μm (depending on thickness), heating efficiency is poor, and the substrate temperature tends to become uneven during heating. Therefore, in the case of epitaxial growth on a substrate size of 2 inches or more, a metal such as Ti is often vapor-deposited on the back side of the growth surface, or a molybdenum plate is installed as an object to be cured near the substrate.

(SAM) The SAM substrate is colorless and transparent, but it absorbs in the infrared region (2 to 10 μm), so it can be made of high quality by simply installing a heater directly facing the substrate. Epitaxial growth of 2 inch size is possible.

(Evaluation of infrared absorption characteristics)
A crystal prepared with a specified composition was processed into a shape with a length of 10 mm, a width of 10 mm, and a thickness of 0.5 mm, and two transmitting surfaces were optically polished to prepare a sample for spectrum measurement.
Measurement was performed in a vacuum atmosphere using a Fourier transform infrared spectrophotometer (FTIR), and transmittance was calculated.
A comparison with sapphire is shown in FIGS. 8(a) and 8(b).

(Example 8)
[Formation of SiO ₂ mask pattern]
A resist was spin-coated on the GaN crystal layer (growth surface) of the prepared GaN/SAM template. Next, a striped or hexagonal pattern was formed in the a-axis or m-axis direction of GaN using photolithography.

Next, a 200 nm thick film of SiO ₂ was formed by sputtering. After film formation, the resist was washed away to form a striped or hexagonal SiO ₂ layer pattern on the GaN crystal layer.

[Formation of GaN thick film]
The GaN template with the SiO2 mask pattern formed on its surface was set in a carbon sample fixing jig (susceptor) in the HVPE apparatus. Thereafter, N ₂ gas was flowed into the reaction tube for 30 minutes to create an N ₂ gas atmosphere inside the reaction tube. The reaction tube was heated so that the metal raw material part was at 850°C and the substrate heating part was at 1050°C, and after reaching the set temperature, the temperature was maintained for 25 minutes. At this time, N ₂ gas was passed through the reaction tube until the temperature of the substrate ripening section reached 500°C, and H ₂ gas and NH ₃ gas were passed above 500°C.

Next, HC1 gas was passed through the Ga metal installed in the reaction tube at a rate of 0.8 L/min. In addition, 8 L/min of NH ₃ gas and 4 L/min of H ₂ gas as a carrier gas were added. A GaN crystal was grown for 720 minutes by flowing IL/min. The film thickness of the GaN crystal was 2000 μm.
Thereafter, the flow of HCl gas was stopped, growth was completed, and the substrate was cooled. Cooling was performed by natural cooling while circulating gas. During cooling, NH ₃ gas was flowed at 5 L/min until the substrate temperature became 600° C. or lower, and N ₂ gas was flowed at 37 L/min at 600° C. or lower.

When the substrate temperature dropped to below 150°C, the substrate was removed from the apparatus, and at that point, the SAM substrate and the GaN crystal layer had already peeled off naturally over the entire surface, and a self-supporting GaN substrate was obtained. . The yield of natural peeling was 100%.
The characteristics of the obtained GaN crystal free-standing substrate were measured by the following method.

[scotoma density evaluation]
The surface of the obtained GaN crystal free-standing substrate was observed using a scanning electron microscope/force sword luminescence (SEM/CL) device (JSM-7600F manufactured by JEOL Ltd./MonoCL4 manufactured by Gatan Corporation). I did it. The accelerating voltage at this time was 5 kV, the observation range was 20 μm x 20 μm, and the dark spot density was calculated from the total number of dark spots observed within the observation range.

[Crack number evaluation]
The obtained free-standing GaN crystal substrate was irradiated with high-intensity halogen light, and the number of cracks present in the crystal was measured.

〔Surface condition〕
The surface state of the obtained GaN crystal free-standing substrate was evaluated using a Nomarski differential interference microscope.
The obtained GaN crystal had naturally peeled off from the SAM substrate when taken out from the HVPE furnace, and was a free-standing GaN substrate having a (0001) plane as a main surface and having a thickness of about 2 mm. No cracks were observed on the board. The peeled surface on the back side was a mirror surface on both the GaN crystal side and the SAM substrate side, and the SAM substrate could be recovered without breaking.
The probability of natural peeling was 100%, and the recovery rate of the SAM substrate was 80%.
The full width at half maximum of 0002XRC of the naturally exfoliated GaN crystal was 120 arcsec, and the dark spot density by CL measurement was 1.0 x 10 ⁵ pieces/cm ² .

(Example 9)
Simultaneous production of multiple GaN sheets using HVPE method HVPE growth was performed by arranging five GaN/SAM templates each having a diameter of 2 inches using an HVPE device that can produce 6 inches or more in diameter. The templates used were those created in Example 1 (MOVPE) and Example 3 (MBE).
The HVPE GaN growth conditions were almost the same as those specified in Example 2.
The experiment was conducted using three GaN templates made from MOVPE and two GaN templates made from MBE.
FIG. 9 shows a photograph of the GaN side immediately after HVPE growth. A photograph of the SAM side immediately after HVPE growth is shown in FIG.

Further, FIG. 11 shows a naturally separated MO-GaN template, and FIG. 12 shows a naturally separated MBE-GaN template.
As shown in FIGS. 11 and 12, it was confirmed that the MBE template also spontaneously separates like the MOVPE template.
In the case of MOVPE, the thickness of GaN is about 3 μm, but with MBE, natural separation was possible even with a thickness of about 1 μm, and the surface of the SAM substrate after separation was clean and flat.

Claims (20)

A GaN template was prepared by directly growing a GaN thin film on a ScAlMgO ₄ (SAM) substrate without a buffer layer using an MBE device, and then GaN was grown on the template at a temperature of 1000°C or higher using an HVPE device. A method for producing a high-quality, low-cost GaN free-standing substrate using natural peeling. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the MBE apparatus has a circular heater that heats the substrate 1.2 to 1.5 times as many times as the SAM substrate during growth of one substrate. When the diameter of the heater of the MBE device is larger than 1.5 times that of the SAM substrate, a heat shield plate having a hole 1.2 to 1.5 times the diameter of the substrate is provided between the substrate and the heater. 2. The method for manufacturing a GaN free-standing substrate according to claim 1, wherein: 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein when a plurality of SAM substrates are installed on one molybdenum susceptor for thin film growth, a heat shield plate having the same number of holes as the SAM substrates to be installed is installed. The SAM substrate was fabricated by processing a crystal produced by the CZ method, and the composition ratio of the starting materials used as the melt was as follows:
27% _{≦ Sc2O4} ≦28%
45%≦MgO≦46%
26% _{≦ Al2O3} ≦28%
and
The composition ratio of the crystal is
26% _{≦ Sc2O4} ≦28%
49%≦MgO≦50%
23% _{≦ Al2O3} ≦24%
The method for manufacturing a GaN free-standing substrate according to claim 1. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the surface of the SAM substrate is a c-plane ±0.01°. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate is 0.5° off from the c-plane. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate has a dislocation density of 10 ³ /cm ³ or less. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate has a surface roughness of Ra<0.2 or less. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate size is 2 inches or more in diameter and 300 μm or more in thickness. The method for manufacturing a GaN free-standing substrate according to claim 1, wherein the SAM substrate is colorless and transparent and has absorption in the infrared region (4 to less than 5 μm). 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein an InGaN film is formed on the SAM substrate. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the Ga flux amount at the time of preparing the GaN template is set to 3×10 ⁷ to 8×10 ⁷ Torr, and hexagonal GaN with mirror surface and c-axis orientation is grown. The GaN according to claim 1, wherein in the GaN template, the GaN film thickness is 300 to 2 μm, or the In _0.1 7Ga _0.83 N film thickness is 50 to 200 μm, and the GaN film thickness is 300 to 2 μm. A method for manufacturing a free-standing substrate. 2. The method of manufacturing a GaN free-standing substrate according to claim 1, wherein the surface of the GaN template is uniform in the plane, mirror-like, and flat at the atomic level. The HVPE apparatus includes a film-forming chamber, a heater placed outside the film-forming chamber, and an inner tube placed inside the film-forming chamber,
The inner tube has an outlet on the downstream side, and the inner tube is a storage part for the Ga raw material and a passage for HCl, and is arranged to form an ammonia passage between it and the inner wall of the film forming chamber. has been
2. The method for manufacturing a GaN self-supporting substrate according to claim 1, wherein the film forming chamber includes a holding member for holding the GaN template on the exit side. 17. The method for manufacturing a GaN free-standing substrate according to claim 16, wherein the holding member includes a susceptor, a carbon ring that presses a peripheral edge of a GaN template placed on the susceptor, and a susceptor cover that presses the carbon ring. . 18. The method for manufacturing a GaN free-standing substrate according to claim 17, wherein the susceptor is a carbon susceptor. 19. The method for manufacturing a GaN self-supporting substrate according to claim 18, wherein the holding member is capable of holding a plurality of GaN templates, and is capable of simultaneously forming a film on a plurality of GaN templates. The method for producing a GaN free-standing substrate according to claim 1, wherein the surface of the SAM crystal after peeling is polished by 10 to 20 μm and reused.

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