JP7712166B2 - Method for manufacturing epitaxial growth substrate and epitaxial growth substrate - Google Patents

Description

The present invention relates to a method for manufacturing a substrate for epitaxial growth and a substrate for epitaxial growth.

A single crystal represented by the general formula _RAMO4 (wherein R represents at least one trivalent element selected from the group consisting of Sc, In, Y and lanthanoid elements, A represents at least one trivalent element selected from the group consisting of Fe(III), Ga and Al, and M represents at least one divalent element selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn and Cd) is known. For example, a _ScAlMgO4 substrate is used as a substrate for epitaxial growth of nitride semiconductors such as GaN (see Non-Patent Document 1).

Takuya Iwabuchi, "Research on the fabrication of InGaN-based LEDs on cleaved ScAlMgO4 substrates and their characteristic improvement," Tohoku University Institutional Repository, http://hdl.handle.net/10097/00127477

As described in Non-Patent Document 1, the polished substrate of ScAlMgO ₄ has many steps, and three types of flat terraces of the c-plane are mixed corresponding to the three cleavage planes existing in the unit cell. Therefore, when a single crystal such as GaN is epitaxially grown on the polished substrate, many threading dislocations are generated at the boundary between the different types of terraces. On the other hand, since the cleaved substrate of ScAlMgO ₄ has very few steps, when a single crystal such as GaN is epitaxially grown on the polished substrate, the crystal growth nuclei are insufficient, and island-shaped uneven crystals grow.

The present invention has been made in consideration of the above circumstances, and aims to provide a method for manufacturing an epitaxial growth substrate that is more suitable for the epitaxial growth of single crystals. Another aim of the present invention is to provide an epitaxial growth substrate that is more suitable for the epitaxial growth of single crystals.

A method for producing an epitaxial growth substrate according to one aspect of the present invention includes the steps of cleaving a single crystal represented by the general formula _RAMO4 to obtain a substrate having the cleavage plane as an epitaxial growth plane, and bombarding the cleavage plane with energetic charged particles to form a periodic step-terrace structure on the cleavage plane. The width of the terrace, which is the distance between the steps, is 0.2 to 100 μm.
In the general formula, R represents at least one trivalent element selected from the group consisting of Sc, In, Y, and lanthanoid elements, A represents at least one trivalent element selected from the group consisting of Fe(III), Ga, and Al, and M represents at least one divalent element selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd.

In one aspect of the above-mentioned manufacturing method, the collision of the charged particles with the cleavage plane may be performed by irradiating the cleavage plane with an ion beam. In this case, the ion beam is preferably an Ar ion beam, and the ion beam is preferably irradiated so as to satisfy at least one of the following conditions:
The number of particle collisions of Ar ions is 10 ⁸ /cm ² or less.
The particle energy of the Ar ions is 1 KeV or less.
The beam current value of Ar ions is 0.05 μA or less.

According to one aspect of the present invention, a substrate for epitaxial growth has, as an epitaxial growth surface, a cleavage surface of a single crystal body represented by the general formula _RAMO4 , the cleavage surface has a periodic step-terrace structure, and the width of the terrace, which is the distance between the steps, is 0.2 to 100 μm.

In one embodiment of the substrate, the height of the steps is preferably an integer multiple of the unit cell length of the single crystal body.

In one embodiment of the above substrate, the terrace width is preferably 0.3 to 30 μm.

According to the present invention, it is possible to provide a manufacturing method for an epitaxial growth substrate that is more suitable for the epitaxial growth of a single crystal. Furthermore, according to the present invention, it is possible to provide an epitaxial growth substrate that is more suitable for the epitaxial growth of a single crystal. The epitaxial growth substrate can be obtained by the above manufacturing method. According to the present invention, it is possible to provide an excellent epitaxial growth substrate that can suppress the occurrence of threading dislocations and suppress the growth of the single crystal in an island shape when epitaxially growing a single crystal such as GaN.

FIG. 1 is a diagram showing step-flow growth using a substrate for epitaxial growth. FIG. 2 is an AFM image of the terrace portion of the cleaved surface. FIG. 3 is an AFM image of a step portion of the cleaved surface. FIG. 4 is an AFM image of the step-terrace structure formed on the cleaved surface by ion beam irradiation. FIG. 5 is a cross-sectional view of an AFM image of a step-terrace structure formed by ion beam irradiation on a cleavage plane. FIG. 6 is a cross-sectional view of an AFM image of a step-terrace structure on the polished surface.

The following describes in detail the embodiments of the present invention, with reference to the drawings where necessary. However, the present invention is not limited to the following embodiments.

<Method for manufacturing epitaxial growth substrate>
The method for manufacturing an epitaxial growth substrate includes a step (cleavage step) of cleaving a single crystal represented by the general formula _RAMO4 to obtain a substrate having the cleavage plane as an epitaxial growth plane, and a step (step-terrace formation step) of bombarding the cleavage plane with energetic charged particles to form a periodic step-terrace structure on the cleavage plane.

(cleavage process)
Examples of the single crystal represented by the general formula _RAMO4 (wherein R represents at least one trivalent element selected from the group consisting of Sc, In, Y and lanthanoid elements, A represents at least one trivalent element selected from the group consisting of Fe(III), Ga and Al, and M represents at least one divalent element selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn and Cd) include _ScAlMgO4 , _LuGaMgO4 , _InGaMgO4 , _YFeFeO4 , _YFeMnO4 , etc. The type of single crystal can be selected from the viewpoint of lattice constant matching with the crystal to be grown.

The general structure of RAMO ₄ will be explained using ScAlMgO ₄ as an example. ScAlMgO ₄ has a structure in which ScO ₂ layers with a rock salt structure and AlMgO ₂ layers with a hexagonal graphite structure (h-BN) are alternately laminated. Since the bonding force between the three AlMgO ₂ layers present in the unit lattice is weak, the ScAlMgO ₄ single crystal can be cleaved perpendicular to the c-axis between the AlMgO ₂ layers. The unit lattice length of ScAlMgO ₄ is 25.160 Å.

A substrate having a cleavage plane as an epitaxial growth surface can be obtained by cutting a high-quality RAMO4 _single crystal having no small-angle grain boundaries, which is obtained by a known method such as a pulling method, into a block shape, and then applying pressure to a corner of the block with a blade parallel to the c-plane to cleave the block.

(Step/terrace formation process)
By uniformly bombarding the cleavage surface of the substrate obtained in the cleavage process with energetic charged particles, the crystal structure of the cleavage surface can be reconstructed. Terraces, which are unstable and flat on the atomic scale, are generated on the cleavage surface obtained by mechanical force. Thermally stable steps can be formed by bombarding these terraces with energetic charged particles under appropriate conditions. The purpose of bombarding the charged particles is to increase the temperature of the cleavage surface and form unevenness that will become steps on the flat cleavage surface. When energetic charged particles hit the surface of a crystal, atoms on the surface are generally ejected or the surrounding atoms are energized and heated, causing the atoms to move. When bombarding the charged particles, plasma or laser light irradiation may be used in combination to heat the cleavage surface.

In addition to the method using charged particles, methods for modifying the substrate surface include simply heating the substrate in air and chemically etching the substrate using hot phosphoric acid. However, with the former method, the crystal becomes cloudy at 1400°C, and it is difficult to increase the number of steps at temperatures below this. Furthermore, with the latter method, only the dislocations and steps are deeply etched, making it difficult to form uniform growth nuclei.

The charged particles are collided with the cleavage surface so that the terrace width, which is the distance between steps, is 0.2 to 100 μm. This results in the formation of steps at an appropriate density on the cleavage surface, which suppresses the occurrence of threading dislocations and the growth of single crystals in an island shape. From this perspective, it is more preferable that the terrace width is 0.3 to 30 μm. The terrace width can be adjusted by the energy intensity of the colliding charged particles, the number of collisions, etc. The terrace width of the cleavage surface of a cleavage substrate with very few steps is generally more than 100 μm. The terrace width of the polished surface of a polished substrate with very many steps is generally 0.15 μm or less.

The charged particles are preferably bombarded with the cleavage plane so that the step height is an integer multiple of the unit lattice length. When growing a crystal epitaxially on a substrate, the crystal grows parallel to the terrace from the steps as growth nuclei, and then grows vertically when the terrace is covered (step-flow growth). As it grows vertically, it collides with the crystal layer grown on the adjacent terrace, but if the step height at the boundary between the terraces is an integer multiple of the unit lattice length, the lattice mismatch can be reduced and the occurrence of threading dislocations can be suppressed. The step height can be adjusted by changing the energy intensity of the charged particles.

Figure 1 shows step-flow growth using an epitaxial growth substrate. In the figure, H is the step height, W is the terrace width, the arrow Gp indicates the growth direction of the crystal on the terrace, and the arrow Gv indicates the growth direction of the crystal after covering the terrace.

It is preferable to irradiate the energetic charged particles so as to reduce the three types of steps originally present on the cleavage plane, each of which has a length that is an integer multiple of 1/3 of the unit lattice length, and to increase the number of steps that are an integer multiple of the unit lattice length. By minimizing the total number of the three types of steps originally present, the occurrence of threading dislocations can be further suppressed. The total number of steps originally present on the cleavage plane can be adjusted by changing the energy intensity of the charged particles.

Methods for bombarding the cleavage plane with energetic charged particles include, for example, irradiating the cleavage plane with an ion beam, or treating the cleavage plane with plasma (wherein the charged particles such as ions in the plasma are adjusted to the same conditions (energy intensity, number of collisions, etc.) as the charged particles in the ion beam irradiation). In either method, the desired step-terrace structure can be formed by treating the cleavage plane surface under very weak conditions.

There is no particular restriction on the ion species used in the ion beam as long as it does not chemically react with the substrate. Examples of ion species include Ar, _N2 , He, _O2, etc. The type of particles is not important as long as it is possible to heat the surface and destroy the surface by particle collision, which are necessary to achieve the purpose of forming steps of an integer multiple of the lattice constant at a uniform density on an atomically flat surface.

The ions contained in the ion beam are not particularly limited as long as the ion beam can be irradiated with a uniform density, and may be single ions or cluster ions (gas cluster ion beam). By appropriately adjusting the ion species of the irradiated ions, the energy of the ion beam, the number of particles of the irradiated ions, the irradiation density, etc., it is possible to form homogeneous single-type steps with the desired density and terraces with the desired width on the cleavage surface.

As the ion beam, an Ar ion beam can be used from the viewpoint that the ions are inactive and have a certain degree of mass. In addition, it is preferable to irradiate the ion beam so as to satisfy at least one of the following conditions.
The number of particle collisions of Ar ions is 10 ⁸ /cm ² or less.
The particle energy of the Ar ions is 1 KeV or less.
The beam current value of Ar ions is 0.05 μA or less.
All of these conditions mean that the cleavage plane surface is treated with an ion beam under very weak conditions. To adjust the step height or reduce the number of steps that originally exist on the cleavage plane, various conditions can be adjusted within the above ranges.

The ion beam irradiation conditions can be set, for example, as follows, although there are no particular limitations as long as the steps can be formed.
The number of particle collisions of Ar ions is 10 ³ /cm ² or more.
The particle energy of the Ar ions is 10 eV or more.
The beam current value of the Ar ions is 1 pA or more.

In this disclosure, a method is used in which energetic charged particles are bombarded onto the cleavage plane to form step irregularities on the flat cleavage plane surface, but the cleavage plane surface may also be treated by other methods that can roughen a flat surface at the atomic level, such as chemical etching, thermal etching, or laser ablation.

<Epitaxial growth substrate>
The substrate for epitaxial growth has a cleavage plane of a single crystal represented by the general formula _RAMO4 as an epitaxial growth plane, the cleavage plane having a periodic step-terrace structure, and the width of the terrace, which is the distance between the steps, is 0.2 to 100 μm.

The epitaxial growth substrate can be obtained by the above-mentioned method for manufacturing an epitaxial growth substrate.

Crystals of III-V group semiconductors, II-VI group semiconductors, and other compound semiconductors can be grown on the epitaxial growth substrate. III-V group semiconductors include AlN, GaN, InN, and solid solutions thereof. II-VI group semiconductors include ZnO, ZnS, and other compound semiconductors include Ga ₂ O ₃ .

For example, epitaxial growth substrates obtained by processing _ScAlMgO4 are particularly suitable for GaN crystal growth from the viewpoint of lattice matching. _ScAlMgO4 has a lattice mismatch rate with GaN that is 10 times smaller than that of sapphire, and can significantly reduce the threading dislocation density.

For example, MOVPE can be used to grow GaN crystals, as described in Non-Patent Document 1. In MOVPE, gaseous organometallic (Ga) and ammonia (NH ₃ ) react near the surface of a substrate heated to a high temperature, and GaN is adsorbed onto the substrate. Atomic GaN moves across the substrate surface and deposits on steps, and epitaxially grows laterally parallel to the substrate along the terraces.

The present invention will be described in detail below using examples, but the present invention is not limited to the following examples.

(Growth of ScAlMgO4 _single crystal)
As raw materials, scandium oxide Sc ₂ O ₃ , magnesium oxide MgO, and aluminum oxide Al ₂ O ₃ powders were weighed and mixed to obtain a mixed powder.
The resulting mixed powder was placed in an alumina crucible, fired at 1400° C., and then cooled, and the fired powder was placed in an Ir crucible in a single crystal growth furnace.
The Ir crucible was heated by high-frequency heating to melt the sintered powder in nitrogen containing a small amount of oxygen, and a seed was placed on top of the melt to solidify it, pulling up a single crystal. The grown single crystal was separated from the melt, cooled, and then removed from the furnace.
In this manner, a ScAlMgO4 _single crystal was obtained.

(Preparation of cleaved substrates of ScAlMgO4 _single crystal)
The ScAlMgO4 _single crystal obtained as described above was cut into a block shape. A blade was then applied to the corner of the block parallel to the c-plane, and pressure was applied to obtain a cleaved substrate of the ScAlMgO4 _single crystal. No steps observable by the naked eye were observed on the cleaved surface of the cleaved substrate.

AFM images of the cleaved surface of the cleaved substrate are shown in Figures 2 and 3.

Figure 2 is an AFM image of the terrace portion of the cleavage surface. As shown in the figure, it is clear that the cleavage surface has terraces that are flat at the atomic level.

Figure 3 is an AFM image of the step portion of the cleavage plane. Most of the cleavage plane is composed of terraces, but some of them have steps. The step height is about 8 Å (0.8 nm), and in the figure, it forms the steps between the terraces that are arranged in an orderly fashion in several steps. The step height is about 1/3 of the unit lattice, which is consistent with the fact that it has been pointed out that there are three planes (cleavage planes) with weak bonds in the unit lattice.

(Ion beam irradiation to cleaved substrate)
The cleaved surface of the cleaved substrate obtained as described above was irradiated with an Ar ion beam using an irradiation device manufactured by Showa Vacuum Co., Ltd. In this device, low-pressure Ar gas ionized by a high-frequency electric field is converted into an ion beam by a direct current electric field. In this example, the beam diameter was narrowed by an aperture and then irradiated onto the cleaved surface. Irradiation with the ion beam allowed a step-terrace structure having periodicity to be formed on the cleaved surface. In the portion where the desired steps were formed, the number of particle collisions of the Ar ions was 10 ⁸ /cm ² or less. The particle energy of the Ar ions at that time was 1 KeV or less, and the beam current value of the Ar ions was 0.05 μA or less.

Figure 4 is an AFM image of a step-terrace structure formed by ion beam irradiation on the cleaved surface of the cleaved substrate. Figure 5 is a cross-sectional view of an AFM image of a step-terrace structure formed by ion beam irradiation on the cleaved surface of the cleaved substrate. The height of the steps in the formed step-terrace structure was an integer multiple of about 25 Å. No steps with a height of about 8 Å (1/3 of the unit cell c-axis) were found. The width of the terrace was 10 μm.

For comparison, an epi-ready polished substrate of ScAlMgO4 _single crystal is shown. Figure 6 is a cross-sectional view of an AFM image of a step-terrace structure on the polished surface of the polished substrate. The step height in the step-terrace structure was about 8 Å (1/3 of the unit cell c-axis). The width of the terrace was about 0.14 μm.

The substrate obtained as described above can be said to be an epitaxial growth substrate suitable for epitaxial growth of single crystals such as GaN.

The epitaxial growth substrate according to the present disclosure has an appropriate number of steps, fewer than an epi-ready polished substrate, and more than a simple cleaved substrate. Therefore, by using the epitaxial growth substrate according to the present disclosure, it is possible to reduce threading dislocations while suppressing the growth of island-shaped single crystals during epitaxial growth of the single crystal. This makes it possible to obtain nitride semiconductors such as GaN with low defect density.

Claims (5)

A step of cleaving a single crystal represented by the general formula _RAMO4 to obtain a substrate having the cleavage surface as an epitaxial growth surface;
a step of forming a periodic step-terrace structure on the cleavage surface, the step having steps at boundaries between terraces, the width of the terraces being the distance between the steps being 0.2 to 100 μm, and the height of the steps being an integer multiple of a unit lattice length of the single crystal body, by colliding the cleavage surface with charged particles having energy, the step being performed by irradiating the cleavage surface with an ion beam .
[In the general formula, R represents at least one trivalent element selected from the group consisting of Sc, In, Y, and lanthanoid elements, A represents at least one trivalent element selected from the group consisting of Fe(III), Ga, and Al, and M represents at least one divalent element selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd.] the ion beam is an Ar ion beam,
The method according to claim 1 , wherein the ion beam is irradiated so as to satisfy at least one of the following conditions:
The number of particle collisions of Ar ions is 10 ⁸ /cm ² or less.
The particle energy of the Ar ions is 1 KeV or less.
The beam current value of Ar ions is 0.05 μA or less. 3. The method according to claim 1, wherein the width of the terrace is 0.3 to 30 μm. The cleavage plane of a single crystal body represented by the general formula _RAMO4 is used as an epitaxial growth plane,
a cleavage surface having a periodic step-terrace structure in which steps are provided at boundaries between terraces, the width of the terraces, which is the distance between the steps, is 0.2 to 100 μm, and the height of the steps is an integer multiple of a unit lattice length of the single crystal body .
[In the general formula, R represents at least one trivalent element selected from the group consisting of Sc, In, Y, and lanthanoid elements, A represents at least one trivalent element selected from the group consisting of Fe(III), Ga, and Al, and M represents at least one divalent element selected from the group consisting of Mg, Mn, Fe(II), Co, Cu, Zn, and Cd.] The substrate according to claim 4 , wherein the terrace has a width of 0.3 to 30 μm.