JPH0840799A - Substrate for single crystal growth - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a new and excellent single crystal growth substrate capable of synthesizing a high-quality aluminum nitride thin film with extremely few defects. [Configuration] A main surface on which an aluminum nitride single crystal is vapor-grown is composed of a single crystal belonging to any one of a cubic system, a trigonal system and a hexagonal system, and a lattice plane spacing of the main surface is set. When the integral multiple is A and the integral multiple of the lattice spacing in the growth direction of the aluminum nitride single crystal is B, | AB
The feature is that the value of | / A is 0.08 or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single crystal growth substrate suitable for an aluminum nitride single crystal thin film applied to a surface acoustic wave filter used in a microwave region, an optical device utilizing a nonlinear optical effect, and the like. .

[0002]

2. Description of the Related Art A single crystal of aluminum nitride (AlN) is a III-V group compound semiconductor excellent in various physical properties such as electric insulation, thermal conductivity, heat resistance, and light transmission. Therefore, it is expected to be applied as an electronic material for passivation or packaging, or as a functional material such as a piezoelectric device or an optical device utilizing excellent piezoelectricity and nonlinear optical characteristics.

By the way, aluminum nitride has a large N ₂ vapor pressure during synthesis and a high melting point.
It is extremely difficult to manufacture it by the usual melting method. Therefore, it has been synthesized mainly by a vapor phase growth method such as MOCVD method, and many researchers have made efforts to obtain a high quality aluminum nitride thin film with few defects (for example, J.Vac.Scl. Technol.B, vol10, No4, Jul / Aug1
992, P1237 to 1266).

In order to obtain such a high quality single crystal thin film of aluminum nitride, it has been conventionally emphasized that the substrate for growing the single crystal thin film is stable even at a high temperature, and sapphire (Al ₂ O ₃ ), gallium arsenide (GaA
s), silicon (Si), spinel, and various glasses have been used as substrate materials.

Among these substrate materials, especially sapphire single crystal has the same hexagonal crystal system as aluminum nitride and is stable even at high temperature (about 1000 ° C.) during vapor phase growth. It is currently most commonly used because it is relatively easy.

[0006]

Generally, when a junction having a constant crystallographic relationship exists between two single crystals, this is called epitaxy, and another crystal is fixed on a certain crystal plane. Epitaxial growth is a growth mode that grows in an azimuth relationship. In the case of such directional growth, it is important that both crystals have crystal chemical affinity or that the structural periods of the two crystals are the same or similar. Is. In particular, the misfit rate, which is indicated by the percentage difference between the crystal lattice constants of the two, is used as a measure of the possibility of epitaxy, and it is well known that the smaller the misfit rate, the more frequently the epitaxy relationship is established.

However, even if the above-mentioned conventional substrates made of various materials are used, it cannot be said that a crystallographically high quality thin film that sufficiently satisfies the device characteristics has been synthesized. It was not an excellent material that conformed to the lattice constant and the coefficient of thermal expansion. That is, since the conventional substrate and the aluminum nitride thin film grown in contact therewith have a difference in crystal structure, thermal expansion coefficient, etc. between them, a considerable number of defects occur in the aluminum nitride thin film during the vapor phase growth, It is recognized that the characteristics of the device deteriorate due to the cause.

Therefore, it is an object of the present invention to provide an excellent single crystal growth substrate capable of synthesizing a high-quality aluminum nitride thin film which cannot be grown on a conventional substrate.

[0009]

In order to achieve the above object, in the single crystal growth substrate of the present invention, the main surface on which an aluminum nitride single crystal is grown is a cubic system, a trigonal system or a hexagonal system. Of the above, and the misfit ratio to the structural period of the aluminum nitride single crystal (that is, an integer multiple of the lattice spacing of the main surface is A, an integer of the lattice spacing in the growth direction of the aluminum nitride single crystal). When doubled as B, | AB− × 100 / A%) is 8% or less of a single crystal.

The reason why the misfit rate is limited to 8% or less is that if it exceeds 8%, it becomes difficult to heteroepitaxially grow an aluminum nitride single crystal, and it becomes impossible to grow an aluminum nitride single crystal on a substrate. Because it will be.

The substrate is not limited to a particular shape such as a flat plate as long as it can grow an aluminum nitride single crystal, and at least the surface layer may have the above structure.

[0012]

[Table 1]

In particular, if the single crystal growth substrate is a cubic crystal garnet structure single crystal, as shown in Table 1,
Aluminum Nitride (Hexagonal system, a = 3.112 Å, c = 4.98
It becomes possible to make an integer multiple of the a-axis length or c-axis length of 2 Å) almost equal to an integer multiple of the lattice spacing dhkl of the (hkl) plane of garnet, which makes ideal epitaxial growth possible. It was confirmed that a good quality aluminum nitride single crystal was obtained using such a substrate.

The thermal expansion coefficient of garnet in the c-axis direction (about 6.0 × 10 ⁻⁶ / ° C.) is compared with that of sapphire in the c-axis direction (about 9.0 × 10 ⁻⁶ / ° C.). The coefficient of thermal expansion of aluminum nitride in the c-axis direction (about 5.0 × 10 ^-6 /
Since it is closer to (° C), stable growth is possible even at high temperature during the growth of aluminum nitride. Furthermore, garnet can have a wide range of lattice constants, and if the lattice constant a [Å] is in the range of 12.19 <a <12.95, it is possible to form a garnet solid solution and to obtain a desired structural period. It is possible to obtain a single crystal growth substrate having the same.
In addition, the optimum crystallographic orientation of the main plane of garnet is, as garnet is a cubic system, clear from Table 1, equivalent planes {100}, {110}, {120}, {112}
Alternatively, it may be parallel to the {111} crystal plane.

It was also confirmed that a good quality aluminum nitride single crystal can be obtained by using a lithium niobate or lithium tantalate single crystal as the main surface on which the aluminum nitride single crystal is grown. Further, the main surface on which the aluminum nitride single crystal is grown is (001), (100), (116), (01
It was also confirmed that a higher quality aluminum nitride single crystal can be obtained when it is parallel to the crystal plane of (8), (110), (122), (211), (214) or (202).

[0016]

As described above, in the case of oriented growth such as epitaxial growth, it is important that the crystals in contact with each other have a crystal chemical affinity, or the structural periods of the crystals match or approximate each other. Yes,
The smaller the misfit rate, the more frequently the epitaxy relationship is established. Other factors include the temperature condition and the surface condition of the substrate crystal, but the most important necessary condition is considered to be the approximation of the structural period (period of lattice plane intervals). Therefore, by forming the single crystal growth substrate in the heteroepitaxial growth as described above, an excellent quality aluminum nitride single crystal film with very few defects can be obtained.

[0017]

【Example】

Example 1 By the Czochralski (CZ) method, (GdCa) ₃ (G) of a garnet solid solution single crystal having a lattice constant of 12.448Å
The aMgZr) ₅ O ₁₂ was grown, and further, the grown single crystal was subjected to wafer processing to fabricate a 2-inch wafer having a (111) plane as a main surface.

As shown in Table 1, this wafer can have a matching relationship in which four times the lattice constant in the a-axis direction of aluminum nitride matches the interplanar distance d100 of the (100) plane of the garnet solid solution. This wafer is used as a single crystal growth substrate for epitaxial growth (hereinafter, also referred to as a substrate) M
By the OCVD method, the film thickness is about 1 at the substrate temperature of 1000-1180 ℃.
An aluminum nitride single crystal thin film of about μm was grown.

When the grown single crystal thin film was observed by electron beam diffraction, it was confirmed that the typical RHEED pattern had a strong streak shape, and that it was a good single crystal film. In addition, the surface shape by SEM observation is smooth,
The full width at half maximum of the X-ray rocking curve was also within 1 '(minute).

On the other hand, the misfit rate of an aluminum nitride film formed under the same conditions as above using the C surface (00.1) of sapphire, which has been used most frequently, as a substrate, is about 15%.
%, The RHEED pattern had a spot-like elongated grain shape with orientation, and the full width at half maximum of the X-ray rocking curve was about 1 ° (degrees), which was an extremely large value.

[Example 2] Ho ₃ Ga ₅ O ₁₂ garnet (1 having a lattice constant of 12.293Å grown by the CZ method and processed (1
10) LPE (Liquid Phase) using the substrate whose main surface is
Epitaxy method yields Y ₃ (FeAl) ₅ O with a lattice constant of 12.329Å
_Twelve 200 μm-thick garnet single crystal films were prepared. table
As shown in 1, the interplanar distance d of the (110) plane of this LPE film
Four times 110 is in agreement with seven times the lattice constant of aluminum nitride in the c-axis direction.

Then, using this LEP film as a substrate for epitaxial growth, a thin film of aluminum nitride was grown in the same manner as in Example 1. The S of the single crystal thin film thus obtained
According to EM observation, it is very smooth and the full width at half maximum of the X-ray rocking curve is as small as 1.0 '.
It was confirmed from the RHEED pattern that it was a good quality single crystal film.

On the other hand, a sapphire having a misfit rate of 10.5% was used as a substrate whose main surface was the R-plane (01.2) and was formed under the same conditions as above. RHEE of this aluminum nitride film
The D pattern had a spot shape, the surface shape was a mound shape, and a coarse grain shape, and the half width of the X-ray rocking curve showed an extremely large value of 3.6 °.

[Embodiment 3] The lattice constant is 12.2 by the CZ method.
A 00Å garnet solid solution single crystal Y ₃ (AlGa) ₅ O ₁₂ was grown and further processed into a wafer to obtain a 0.5 mm thick (2
11) A 2-inch wafer whose main surface is the plane was prepared.

As shown in Table 1, this wafer can have a matching relationship in which the lattice constant of aluminum nitride in the c-axis direction matches the interplanar distance d211 of the (211) plane of the garnet solid solution. Using this wafer as a substrate for epitaxial growth, an aluminum nitride film was formed under the same conditions as in Example 1.

The RHEED pattern of the grown single crystal thin film showed a strong streak pattern, and the Kikuchi line was also observed, confirming that the single crystal film was good. Further, the surface shape observed by SEM was very smooth.

On the other hand, the RHEED pattern of the growth film when the (111) plane of Si, which has a misfit rate of about 5.9% with respect to the c-axis of aluminum nitride, is used as a substrate, has a streak shape, and the surface observed by SEM observation. The shape was a relatively smooth grain, but the film quality was inferior as compared with the above-mentioned examples.

The misfit rate of the substrate used in Examples 1 to 3 with respect to the structural period of the aluminum nitride single crystal was almost zero, and ideal epitaxial growth was possible. Also, the coefficient of thermal expansion of these substrates in the c-axis direction is about
6.0 × 10 ^-6 / ℃, which is about 9.0 × 10 ^-6 / ℃ of sapphire
Compared with aluminum nitride, it is close to about 5.0 × 10 ^-6 / ℃ of aluminum nitride, so the stress strain of the film during c-axis orientation is small, and it is possible to obtain a high quality single crystal aluminum nitride film with few defects at low cost. it can.

In these examples, three types of crystal planes and garnet solid solution were taken as examples, but the present invention is not limited to these, and equivalent planes {120}, {112} or {214}.
If it is parallel to the plane, and if it is a garnet solid solution having a lattice constant having a matching relationship shown in Table 1, it is possible to obtain a high-quality aluminum nitride film excellent in growth on conventional substrate materials such as sapphire and Si. You can Further, the garnet material may be a bulk or a thin film and can be appropriately selected.

[Embodiment 4] A lithium niobate (LiNbO ₃ ) single crystal grown by the CZ method and having a thickness of about 0.5 mm (001)
A 3-inch wafer with the (116) plane as the main plane is prepared. A wafer whose main surface is the (001) plane has a misfit rate of about 7.2% with respect to the c-axis length of aluminum nitride. The wafer having the (116) plane as the main surface has a misfit rate of about 3.4% with respect to the c-axis length of aluminum nitride.

Next, these wafers were used as substrates for epitaxial growth of aluminum nitride single crystals, and reduced pressure MOC was performed.
An aluminum nitride film having a film thickness of about 1 μm was formed at a substrate temperature of about 1180 ° C. by the VD method (atmospheric pressure of about 60 Torr). R of this film
The HEED pattern had a weak streak shape, and the surface shape under SEM observation was smooth, indicating a good quality film.

On the other hand, the misfit rate of the aluminum nitride film formed under the same conditions as above using the C-plane (0001) of sapphire, which has been used most frequently, is about 15%.
Therefore, the RHEED pattern had a spot-like elongated granular shape with orientation, and the film quality was poor.

[Embodiment 5] In the same manner as in Embodiment 4, (110)
Planes and a lithium tantalate wafer having a thickness of 0.4 mm parallel to the (018) plane were prepared as substrates for epitaxial growth of aluminum nitride. The misfit rate of the wafer whose main surface is the (018) plane to the c-axis of aluminum nitride is about 3.3%, and the misfit rate of the wafer whose main surface is the (202) plane is to the a-axis of aluminum nitride. It was about 2.9%.

Then, in the same manner as in Example 4, a thin film of aluminum nitride was grown on each wafer.
The RHEED pattern of these films was streaky, the surface morphology was fairly smooth and finely grained.

On the other hand, a film having a misfit ratio of 10.5% and having a sapphire R-plane (01.2) as the main surface was used and the film was formed under the same conditions as described above. RH of this aluminum nitride film
The EED pattern is spot-shaped, the surface shape is a mound,
It was coarse-grained.

[Example 6] A thickness parallel to the (211) plane prepared from a lithium niobate single crystal in the same manner as in Example 4.
A 0.5 mm optical mirror-finished wafer was used as a substrate for epitaxial growth of aluminum nitride. The misfit rate for the c-axis in this case is about 0.8%.

Then, in the same manner as in Example 4, a thin film of aluminum nitride was grown on this wafer. The typical RHEED pattern after growth showed a strong streak pattern and the Kikuchi line was observed, indicating a good single crystal film. As a result of SEM observation, the surface shape was extremely smooth.

On the other hand, for comparison, an RHEED pattern of an aluminum nitride film formed under the same conditions using a Si substrate having a (111) face as a main face and a misfit ratio of 5.9% is streak-like, The surface shape was a relatively smooth grain, but the film quality was poor.

In Examples 4 to 6, four types of crystal planes were taken as an example, but the (122), (100), (202) or (214) plane is of high quality aluminum nitride equivalent to or better than that of the sapphire substrate. It was confirmed that a film was obtained. In addition, especially when lithium niobate or lithium tantalate is used, the coefficient of thermal expansion in the c-axis direction is higher than that of sapphire of 8.7 × 10 ^-6 / ° C.
Since it has a value close to 3.17 × 10 ^-6 / ℃ of aluminum nitride, that is, 7.5 × 10 ^-6 / ℃ for lithium niobate and 4.1 × 10 ^-6 / ℃ for lithium tantalate. It is possible to obtain a high-quality single crystal aluminum nitride film with few defects and low cost such as a small stress strain of the film.

[0040]

As described in detail above, according to the single crystal growth substrate of the present invention, a single crystal structure similar to that of an aluminum nitride single crystal having a misfit rate of 8% or less with respect to the structural period of the aluminum nitride single crystal is used. Since it is made of crystal, optimal heteroepitaxial growth is possible, and an excellent quality aluminum nitride single crystal film with extremely few defects can be obtained.

In particular, if the single crystal growth substrate is a cubic garnet structure single crystal, the misfit rate of the aluminum nitride single crystal with respect to the structural period becomes almost zero, and ideal epitaxial growth becomes possible. A good quality aluminum nitride single crystal can be obtained. Garnet has a coefficient of thermal expansion in the a-axis direction (approximately 6.0
× 10 ^-6 / ℃) is the coefficient of thermal expansion of sapphire in the c-axis direction (approx.
The thermal expansion coefficient of aluminum nitride in the c-axis direction (about 5.0 × 10 ^-6 / ℃) is close to that of 9.0 × 10 ^-6 / ℃), so that stable growth is possible even at high temperatures during the growth of aluminum nitride. It will be possible. Further, in this case, if the lattice constant a [Å] of garnet is in the range of 12.19 <a <12.95, it is possible to form a garnet solid solution and to obtain a single crystal growth substrate having a desired structural period. .

Also, a good quality aluminum nitride single crystal can be obtained by using a lithium niobate or lithium tantalate single crystal as the main surface on which the aluminum nitride single crystal is grown. The coefficient of thermal expansion in the axial direction is closer to 5.17 × 10 ^-6 / ℃ of aluminum nitride, compared to about 9.0 × 10 ^-6 / ℃ of sapphire, that is, 7.5 × 10 ^-6 / ℃ for lithium niobate, Since lithium tantalate has a value of 4.1 × 10 ⁻⁶ / ° C., it is possible to obtain a high-quality single crystal aluminum nitride film with few defects at low cost such as a small stress strain of the film at the time of c-axis orientation.

Claims (4)

[Claims] 1. A main surface of a substrate on which an aluminum nitride single crystal is vapor-grown is made of a single crystal belonging to any one of a cubic system, a trigonal system and a hexagonal system, and A single crystal growth substrate satisfying the following formula (1), where A is an integer multiple of the lattice spacing and B is an integer multiple of the lattice spacing in the growth direction of the aluminum nitride single crystal. ｜ AB ｜ / A ≦ 0.08 ・ ・ ・ (1) 2. The single crystal growth substrate according to claim 1, wherein a main surface on which the aluminum nitride single crystal is vapor-grown is a cubic single crystal having a garnet structure. 3. The single crystal growth substrate according to claim 2, wherein the cubic single crystal having the garnet structure has a lattice constant a [Å] in the range of 12.19 <a <12.95. 4. The single crystal growth substrate according to claim 1, wherein the main surface on which the aluminum nitride single crystal is vapor-phase grown is made of a lithium niobate or lithium tantalate single crystal.