Method for manufacturing ScAlMgO4 single crystals and method for manufacturing substrates for epitaxial film deposition.

Abstract

To provide a method for manufacturing ScAlMgO4 single crystals with excellent manufacturing efficiency. [Solution] A method for producing ScAlMgO4 single crystals by melt growth, wherein the composition of the melt used for melt growth satisfies the following conditions based on the total amount of the composition. Sc2O3:Al2O3=27.10mol%:22.90mol%~26.90mol%:23.10mol% 42 mol%

Description

Technical Field

[0001] The present disclosure relates to a method for producing a ScAlMgO4 single crystal and a method for producing a substrate for epitaxial film formation.

Background Art

[0002] A ScAlMgO4 single crystal is known as an epitaxial growth substrate material for GaN / InGaN semiconductors. Non-Patent Document 1 discloses a method for producing a ScAlMgO4 single crystal by bringing a seed crystal into contact with a melt in a furnace container and pulling the seed crystal, wherein the composition of the melt at the start of pulling is deviated from the stoichiometric ratio of ScAlMgO4, and after the start of pulling, a solution that substantially satisfies the stoichiometric ratio of ScAlMgO4 is supplied to the container.

Prior Art Literature

Non-Patent Literature

[0003]

Non-Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0004] Non-Patent Document 1 describes that each raw material was weighed to obtain a melt satisfying 27.0 mol% ≤ Sc2O3 ≤ 30 mol%, 26.0 mol% ≤ Al2O3 ≤ 29.0 mol%, and 44.0 mol% ≤ MgO ≤ 46.5 mol%, and ScAlMgO4 crystals were grown from this melt by the Czochralski method (CZ method). However, in Non-Patent Document 1, because the composition of the grown crystal is different from that of the melt, which is a so-called non-congruent state, as the crystal grows, the crystal composition inside the crystal and inside the melt gradually changes. If the crystal is grown to a size larger than a certain size, a ScAlMgO4 single crystal cannot be obtained and the crystal becomes polycrystalline.

[0005] Figure 1 is a photograph of the appearance of a crystal produced using a conventional method. Specifically, the figure is a photograph of a crystal produced by the CZ method using the melt composition described in Non-Patent Literature 1. As shown in the figure, when the solidification rate is 19%, a ScAlMgO4 single crystal is obtained from the crystal growth initiation area (upper part) to the crystal growth completion area (lower part), whereas when the solidification rate is 22%, the crystal growth completion area (lower part) is polycrystalline. Here, the solidification rate (weight %) is the ratio of the weight of the grown crystal to the total weight of the raw materials put into the crucible. Thus, when ScAlMgO4 crystals are grown using a conventional method, when crystallization progresses to a solidification rate of approximately 20%, the compositional change in the melt remaining in the crucible becomes large, and it becomes impossible to grow a ScAlMgO4 single crystal. A manufacturing method in which polycrystalline crystals are produced when the solidification rate is high cannot be said to be efficient in terms of manufacturing.

[0006] This disclosure is made in view of the above circumstances and aims to provide a method for manufacturing ScAlMgO4 single crystals with excellent manufacturing efficiency. Furthermore, this disclosure aims to provide a method for manufacturing a substrate for epitaxial film deposition. [Means for solving the problem]

[0007] The inventors have diligently researched the melt growth method, a manufacturing technique for ScAlMgO4 single crystals, and have been able to discover a congluent composition in which the melt and the crystal grown from that melt have the same composition. A melt composition that is congluent to the crystal composition allows for the efficient growth of single crystals to a high solidification rate. This disclosure is based on this finding.

[0008] This disclosure relates to a method for producing ScAlMgO4 single crystals by melt growth, The composition of the melt used in melt growth, based on the total amount of the composition, Sc2O3:Al2O3=27.10mol%:22.90mol%~26.90mol%:23.10mol% 42 mol% <MgO≦50mol% Regarding a manufacturing method that satisfies the requirements.

[0009] Furthermore, this disclosure relates to a method for manufacturing an epitaxial film-forming substrate, comprising the step of cutting a GaN or InGaN epitaxial film-forming substrate from a ScAlMgO4 single crystal produced by the above manufacturing method. [Effects of the Invention]

[0010] According to this disclosure, a method for manufacturing ScAlMgO4 single crystals with excellent manufacturing efficiency is provided. Furthermore, this disclosure provides a method for manufacturing a substrate for epitaxial film deposition. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a photograph of the appearance of a crystal produced using a conventional method. [Figure 2] Figure 2 shows the range of melt compositions in the ternary phase diagram (composition ratio of Sc2O3, Al2O3, and MgO) of the ScAlMgO4 crystal, comparing the range of melt compositions in this disclosure with that of Non-Patent Literature 1. [Figure 3] Figure 3 shows the results of ICP analysis (Sc / Al / Mg composition ratio) of crystals prepared using the conventional method. [Figure 4] Figure 4 shows the distribution of Sc content in ScAlMgO4 obtained by calcining raw materials with different amounts of Sc2O3. [Figure 5] Figure 5 is a schematic diagram of the crystal growth method using the Slow Fz method. [Figure 6] Figure 6 shows an image of the ScAlMgO4 crystal obtained by the micro-PD method. [Figure 7] Figure 7 shows the distribution of the Sc / Al ratio in the longitudinal direction of crystals grown by the microPD method using starting materials consisting of Sc2O3: 27.00 mol%, Al2O3: 23.00 mol%, and MgO: 50.00 mol%. [Figure 8]FIG. 8 shows the distribution of the Sc / Al ratio in the length direction of a crystal grown by the Slow FZ method using raw materials of Sc₂O₃: 27.00 mol%, Al₂O₃: 23.00 mol%, and MgO: 50.00 mol%. [Figure 9] FIG. 9 shows the distribution of the Sc / Al ratio in the length direction of a crystal grown by the micro-PD method using raw materials of Sc₂O₃: 27.10 mol%, Al₂O₃: 22.90 mol%, and MgO: 50.00 mol%. [Figure 10] FIG. 10 shows the distribution of the Sc / Al ratio in the length direction of a crystal grown by the Slow FZ method using raw materials of Sc₂O₃: 24.00 mol%, Al₂O₃: 26.00 mol%, and MgO: 50.00 mol%. [Figure 11] FIG. 11 shows the distribution of the Sc / Al ratio in the length direction of a crystal grown by the Slow FZ method using raw materials of Sc₂O₃: 26.00 mol%, Al₂O₃: 24.00 mol%, and MgO: 50.00 mol%. [Figure 12] FIG. 12 shows the distribution of the Sc / Al ratio in the length direction of a crystal grown by the Slow FZ method using raw materials of Sc₂O₃: 27.50 mol%, Al₂O₃: 22.50 mol%, and MgO: 50.00 mol%. MODE FOR CARRYING OUT THE INVENTION

[0012] Hereinafter, preferred embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments.

[0013] The present disclosure relates to a method for producing a ScAlMgO₄ single crystal by a melt growth method, wherein the composition of the melt used for melt growth, based on the total amount of the composition, Sc₂O₃:Al₂O₃ = 27.10 mol%:22.90 mol% to 26.90 mol%:23.10 mol% 42 mol% < MgO ≦ 50 mol% the present invention relates to a production method that satisfies the above conditions.

[0014] This disclosure stems from the inventors' discovery of a melt composition in which the composition of the prepared melt matches the composition of the crystal to be grown. According to this disclosure, high-quality ScAlMgO4 single crystals can be efficiently grown. Such a characteristic melt composition was determined by the analysis described below.

[0015] The most preferable melt composition is one in which the content ratio of Sc2O3 to Al2O3 is 27.10(Sc2O3):22.90(Al2O3) to 26.90(Sc2O3):23.10(Al2O3), with an MgO content of 50 mol%, based on the total amount of composition (100 mol%). For each individual raw material, the most preferable melt composition is as follows: 27.10 mol% ≥ Sc2O3 ≥ 26.90 mol% 22.90 mol% ≤ Al₂O₃ ≤ 23.10 mol% MgO = 50 mol%

[0016] However, if the content ratio of Sc2O3 to Al2O3 is as described above, the MgO content may be less than 50 mol%.

[0017] In other words, the melt composition is such that, with the total amount of composition as the base (100 mol%), the content ratio of Sc2O3 to Al2O3 is 27.10(Sc2O3):22.90(Al2O3) to 26.90(Sc2O3):23.10(Al2O3), and the MgO content may be in the range of 50 mol% to 42 mol%, or in the range of 50 mol% to 45 mol%.

[0018] If the amount of MgO is within this range, it is thought that the ScAlMgO4 melt, ScAlMgO4 crystals, and MgO can coexist near the crystal growth interface, and even if the total amount of MgO in the melt is small, crystal growth with a congluent composition is possible near the growth interface. From the viewpoint of increasing the solidification rate and improving manufacturing efficiency, it is preferable that the amount of MgO be greater than 42 mol%, and more preferably greater than 45 mol%. Furthermore, when the amount of MgO is greater than 42 mol%, crystalline phases other than ScAlMgO4, such as spinel, are less likely to appear, and single crystals are more easily obtained.

[0019] Figure 2 shows the range of melt compositions in the ternary phase diagram (composition ratios of Sc2O3, Al2O3, and MgO) of the ScAlMgO4 crystal, comparing the range of melt compositions in this disclosure with that in Non-Patent Literature 1. As shown in the figure, the two composition ranges are different.

[0020] With the melt composition of this disclosure, even if the solidification rate is increased, it is possible to grow ScAlMgO4 single crystals while suppressing polycrystallization. Therefore, large crystals can be grown efficiently in a single crystal growth process, and manufacturing costs can be reduced.

[0021] Furthermore, in this disclosure, since crystal growth is performed from a melt with a congluent composition that matches the crystal composition, the compositional variation within the crystal is extremely small. Therefore, when using a ScAlMgO4 single crystal as, for example, a substrate for epitaxial growth of GaN / InGaN, the uniformity of the lattice constant is increased, and a high-quality GaN / InGaN epitaxial thin film can be formed.

[0022] Melt growth is a method in which a substance is heated above its melting point to melt it, and then the temperature is lowered to crystallize it. In this disclosure, examples of melt growth methods include the Czochralski method (CZ method) as shown in Non-Patent Document 1, as well as the Slow FZ (Floating Zone) method and the MicroPD method.

[0023] For example, when using the CZ method as the melt growth method, according to this disclosure, ScAlMgO4 single crystals can be obtained by weighing Sc2O3, Al2O3, and MgO to the above composition range, melting them in a crucible, and then bringing a seed crystal of ScAlMgO4 into contact with the melt in the crucible and growing it. Since the obtained ScAlMgO4 single crystal is grown from a melt in which Sc2O3, Al2O3, and MgO are in a congruent composition, the variation in Sc content (variation in composition) is extremely small regardless of where the single crystal is cut.

[0024] The growth conditions for ScAlMgO4 single crystals using the CZ method are not particularly limited, but for example, the melt heating method may be resistance heating, infrared concentrated heating, high-frequency induction heating, etc., the growth atmosphere may be an inert gas atmosphere (e.g., nitrogen gas) with an oxygen concentration of less than 1.0 mass%, the seed crystal rotation speed may be less than 10 rpm, and the pulling speed may be less than 1 mm / h.

[0025] This disclosure also relates to a method for manufacturing an epitaxial film deposition substrate, comprising the step of cutting a GaN or InGaN epitaxial film deposition substrate from a ScAlMgO4 single crystal produced by the above manufacturing method.

[0026] In recent years, a technique using ScAlMgO4 single crystals as an epitaxial growth substrate to epitaxially grow GaN / InGaN thin films has attracted attention as a means of resolving dislocations in GaN / InGaN thin films. ScAlMgO4 single crystals, as an epitaxial growth substrate material for GaN / InGaN thin films (semiconductors), exhibit small lattice mismatch with the thin film and small differences in thermal expansion coefficients, enabling the fabrication of high-quality thin films. [Examples]

[0027] The present disclosure will be described in more detail below based on examples and comparative examples, but the present disclosure is not limited in any way to the following examples.

[0028] (Consideration of Non-Patent Document 1) As shown in Figure 1, there was a 0.2 mol% difference in the Sc content, as determined by X-ray diffraction, between the crystal growth initiation point (1 mm from the contact point between the seed crystal and the melt) and the crystal growth completion point (1 mm from the bottom of the grown crystal) of a crystal with a solidification rate of 19%. In other words, the Sc content was higher in the crystal growth initiation point. From the viewpoint of crystal production, a solidification rate of 20% is not good for crystal production efficiency, and a preferred solidification rate is 50% or higher, more preferably 70% or higher.

[0029] Furthermore, variations in composition within the grown crystal can cause fluctuations in the lattice constant and an increase in crystal defects. This poses a challenge in epitaxial deposition of high-quality GaN / InGaN thin films, thus requiring growth techniques with a melt composition congruent to the crystal composition. From the viewpoint of crystal quality, the difference in Sc content between the crystal growth start area and the crystal growth completion area is preferably less than 0.2 mol%, and more preferably less than 0.1 mol%.

[0030] To narrow down the convergent composition of the melt, approximately 20 ScAlMgO4 crystals were grown using the CZ method with the melt composition shown in Non-Patent Literature 1. The results of the ICP (Inductively Coupled Plasma) analysis of each crystal, which analyzed the Sc, Mg, and Al composition, are shown in Figure 3. In other words, Figure 3 shows the results of the ICP analysis (Sc / Al / Mg composition ratio) of crystals produced using the conventional method.

[0031] The figure shows that, regarding the ratio of Sc to Al, an increase in Sc leads to a decrease in Al, and a decrease in Sc leads to an increase in Al. The amount of Mg, 33.3 at%, is equivalent to 50 mol% in moles of MgO, and this remained approximately constant regardless of the ratio of Sc to Al. From these results, it can be concluded that the amount of MgO in the ScAlMgO4 crystal is stable at 50 mol%, and that the amount of MgO in the melt composition of the congluent is also 50 mol%.

[0032] Next, the raw material powders of Sc2O3, MgO, and Al2O3 were weighed to the ratios (mol%) shown in Table 1, mixed in a mixer and mortar and pestle, solidified with a cold isostatic press, and then calcined at 1600°C for 100 hours. The calcined material was crushed and mixed, and the Sc content ratio was determined from the results of evaluation by X-ray diffraction. Figure 4 shows the distribution of Sc content in ScAlMgO4 obtained by calcining raw materials with different amounts of Sc2O3. In other words, Figure 4 shows the distribution of the Sc content ratio of the calcined ScAlMgO4 relative to the Sc2O3 ratio (mol%) when the raw material powders were mixed. The amount of MgO was kept constant at 50.00 mol% for all samples. The Sc content was determined by X-ray diffraction. From Figure 4, it can be seen that the Sc content is constant when the Sc2O3 is less than 25.00 mol%, while the Sc content increases when the Sc2O3 is 25.00 mol% or higher. This indicates that while it is possible for Sc to be present in greater quantities than Al within a ScAlMgO4 crystal, the reverse is not possible, meaning that the amount of Sc2O3 in the melt composition of the congluent is likely to be greater than 25.00 mol%.

[0033] [Table 1]

[0034] To further narrow down the congruent composition range of the melt, crystal samples were prepared using the Slow FZ (Floating Zoon) method, which allows for simple crystal growth without the use of a crucible, and the MicroPD method, which enables the growth of small, high-quality crystals in a short time. The compositional distribution was then evaluated. Table 2 shows the melt composition and crystal growth method for each example.

[0035] [Table 2]

[0036] (Preparation of raw materials) The raw material powders of Sc2O3, MgO, and Al2O3 were weighed to the ratios (mol%) shown in Table 2, mixed in a mixer and mortar and pestle, solidified with a cold isostatic press, and then calcined at 1600°C for 100 hours. For the Slow FZ method, raw materials for each composition were prepared as raw material rods with a diameter of φ5 mm and a length of 15 mm. On the other hand, the raw materials for micro-PD were crushed into powder form.

[0037] (Slow Fz method) The Slow FZ method was implemented with reference to "Isamu Shindo, Phase Equilibrium Study by Floating Zone Slow Cooling Method (I), JACG 8(1981)1".

[0038] Figure 5 shows the composition of the sample and melt formed by the Slow Fz method (A), its approximate temperature distribution at that time, and the state of the solidified rod when the melt solidifies by the Slow Fz method (B). In the Slow FZ method, as shown in Figure 5, the temperature drops sharply from the highest temperature point of the melt, and a large temperature gradient is formed as you move away from this point. A key feature of this method is that the temperature of the solid even slightly away from the solid in contact with the liquid is lower than the interface temperature of the solid in contact with the liquid, and therefore does not participate in the reaction with the liquid. The temperature of the system is slowly lowered while maintaining the temperature gradient as shown in Figure 5. As the temperature decreases, the solid crystallizes, and crystal growth occurs from the upper and lower raw material rods. During this time, the upper and lower shafts rotate the sample rod in opposite directions, stirring the melt. To form a stable melt shape, the upper and lower shafts are slowly separated, and the separation speed is adjusted so that the diameter of the melt and the length of the upper and lower shafts are always approximately the same. Thus, when all the molten material has solidified, a solidified rod with a pointed tip is obtained, as shown in Figure 5(B).

[0039] (MicroPD method) For the implementation of the micro-PD method, we referred to "Fukuda, Tsuguo; Shimamura, Kiyoshi; Uda, Satoshi, "Fabrication of Micro Single Crystals," Materia Japan, Vol. 37, pp. 56-60 (1998)."

[0040] The micro-PD method is a technique for growing crystals with a diameter of several millimeters to several hundred micrometers by supplying molten raw material to a seed through a small hole at the bottom of a crucible and then pulling down the seed. Because the crystals form directly from the molten material into wires, it is possible to obtain crystals with a shape suitable for measuring the changes in composition from the start to the end of growth.

[0041] (Compositional analysis of the sample) Crystals grown using the Slow Fz method or micro-PD method were surface-polished to allow for compositional analysis by SEM-EDX. The polished samples were then placed in the SEM-EDX apparatus.

[0042] Since the samples grown using the Slow FZ method were not perfect single crystals, we selected large crystal grains and evaluated the compositional distribution of those grains from the start to the end of growth. For crystals grown using the microPD method, the composition of crystals that crystallize at the touch interface was analyzed. Since the entire crystal is a single crystal, the compositional distribution from the start to the end of growth was evaluated.

[0043] (Example 1) Crystal growth was performed using the micro-PD method with a raw material composition of Sc2O3 (27.00 mol%), Al2O3 (23.00 mol%), and MgO (50.00 mol%), and the composition of the obtained crystals was analyzed. Figure 6 is a photograph of the appearance of the ScAlMgO4 crystal obtained by the micro-PD method in this example. In the growth using the micro-PD method, a ScAlMgO4 single crystal was obtained with a solidification rate of 100%. As shown in Figure 7, the Sc composition remained generally constant from the start of growth to the point of 95% solidification, and the difference in variation between the crystal growth start point and the crystal growth completion point was 0.15 mol%.

[0044] (Example 2) Crystal growth was performed using the Slow FZ method with a raw material composition of Sc2O3 (27.00 mol%), Al2O3 (23.00 mol%), and MgO (50.00 mol%), and the resulting crystals were analyzed for composition. As shown in Figure 8, the Sc composition was generally constant from the start to the end of crystal growth for the evaluated crystal grains.

[0045] (Example 3) Crystal growth was performed using the micro-PD method with a raw material composition of Sc2O3 (27.10 mol%), Al2O3 (22.90 mol%), and MgO (50.00 mol%), and the composition of the obtained crystals was analyzed. As shown in Figure 9, the Sc composition remained generally constant from the start of growth to the 80% solidification point, and the difference in variation between the crystal growth start point and the crystal growth completion point was 0.16 mol%.

[0046] (Comparative Example 1) Crystal growth was performed using the Slow FZ method with a raw material composition of Sc2O3 (24.00 mol%), Al2O3 (26.00 mol%), and MgO (50.00 mol%), and the resulting crystals were analyzed for composition. As shown in Figure 10, the amount of Sc decreased significantly from the start to the end of crystal growth in the evaluated crystal grains, with a large variation of 7.90 mol%.

[0047] (Comparative Example 2) Crystal growth was performed using the Slow FZ method with a raw material composition of Sc2O3 (26.00 mol%), Al2O3 (24.00 mol%), and MgO (50.00 mol%), and the resulting crystals were analyzed for composition. As shown in Figure 11, the amount of Sc decreased significantly from the start to the end of crystal growth in the evaluated crystal grains, with a large variation of 1.11 mol%.

[0048] (Comparative Example 3) Crystal growth was performed using the Slow FZ method with a raw material composition of Sc2O3 (27.50 mol%), Al2O3 (22.50 mol%), and MgO (50.00 mol%), and the resulting crystals were analyzed for composition. As shown in Figure 12, the evaluated crystal grains exhibited unstable behavior, with the Sc content initially increasing before decreasing from the start to the end of crystal growth. The difference in the decrease was large, at 2.26 mol%. [Industrial applicability]

[0049] The ScAlMgO4 single crystal obtained by this disclosure exhibits extremely little variation in composition and lattice constant throughout the entire crystal. Furthermore, because a melt with a congruent composition to the crystal composition is used, theoretically there is no limit to the solidification rate, and recharging is infinitely possible, allowing for efficient and low-cost production of single crystals. Substrates processed from the ScAlMgO4 single crystal obtained by this disclosure are low-cost, have almost no variation in composition, and possess a uniform lattice constant, making them promising as substrates for epitaxial deposition of GaN / InGaN thin films.

[0050] The main points of this disclosure are summarized below. [1] A method for producing ScAlMgO4 single crystals by melt growth, The composition of the melt used in melt growth, based on the total amount of the composition, Sc2O3:Al2O3=27.10mol%:22.90mol%~26.90mol%:23.10mol% 42 mol% <mgo≦50mol%A manufacturing method that satisfies the requirements. [2] The composition of the melt is, based on the total amount of the composition, Sc2O3:Al2O3=27.10mol%:22.90mol%~26.90mol%:23.10mol% 45 mol% <mgo≦50mol%A manufacturing method according to [1] that satisfies the requirements. [3] The composition of the melt is such that, based on the total amount of the composition, Sc2O3:Al2O3=27.10mol%:22.90mol%~26.90mol%:23.10mol% MgO = 50 mol% A manufacturing method according to [1] or [2] that satisfies the following conditions. A method for manufacturing an epitaxial film-forming substrate, comprising the step of cutting a GaN or InGaN epitaxial film-forming substrate from a ScAlMgO4 single crystal manufactured by any one of the manufacturing methods described in [1] to [3].

Claims

1. ScAlMgO grown by melt growth method ４ A method for manufacturing single crystals, The composition of the melt used in melt growth, based on the total amount of the composition, Sc ２ O ３ :Al ２ O ３ =27.10mol%:22.90mol%~26.90mol%:23.10mol% 42mol%<MgO≦50mol% A manufacturing method that satisfies the requirements.

2. The composition of the aforementioned melt is, based on the total amount of the composition, Sc ２ O ３ :Al ２ O ３ =27.10mol%:22.90mol%~26.90mol%:23.10mol% 45mol%<MgO≦50mol% A manufacturing method according to claim 1, which satisfies the requirements.

3. The composition of the aforementioned melt is, based on the total amount of the composition, Sc ２ O ３ :Al ２ O ３ =27.10mol%:22.90mol%~26.90mol%:23.10mol% MgO=50mol% A manufacturing method according to claim 1, which satisfies the requirements.

4. ScAlMgO produced by the manufacturing method described in any one of claims 1 to 3 ４ A method for manufacturing an epitaxial film-forming substrate, comprising the step of cutting a GaN or InGaN epitaxial film-forming substrate from a single crystal.

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