JPH07165498A - Gan single crystal and its production - Google Patents

Abstract

PURPOSE:To obtain a high-quality GaN single crystal having enough thickness to be usable as a substrate singly. CONSTITUTION:This GaN single crystal is such that the full width at half maximum in its double crystal X-ray rocking curve stands at 5-250sec and its thickness >=80mum. This single crystal can be obtained by film formation, as buffer layers, of a substance good in the lattice conformity with the GaN single crystal on a substrate with at least its surface representing GaN single crystal and then by crystal growth of the aimed GaN. There are two alternatives in this production method depending on the method of removing the buffer layers: one method is as follows: the above-mentioned crystal growth cycle is repeated desired times and laminates are formed on the substrate P0 and then the respective buffer layers B1-Bn are removed at a time to obtain GaN single crystals P1-Pn; the other method is as follows: a buffer layer is removed at every cycle and the above-mentioned crystal growth cycle is repeated for a GaN single crystal singly at all times. These two alternative methods may be combined with each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high quality GaN single crystal which can be suitably used as a GaN single crystal substrate for blue light emitting diodes and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Due to the demand for multi-color in light-emitting displays and the demand for higher data density in communications and recording, the emergence of semiconductor devices capable of emitting light in a short wavelength range from blue to the ultraviolet wavelength region is strongly demanded. ing. As a semiconductor material for this blue to ultraviolet light emitting device, III-
Attention has been paid to a GaN-based single crystal, which is a nitride having the widest band gap in the V-based compound semiconductor. Since GaN has a direct transition type band structure, highly efficient light emission is possible, and since the band gap at room temperature is as large as about 3.4 eV, it exhibits blue to ultraviolet light emission, which is suitable for the requirements of the above semiconductor device. It is a material. However, since GaN has a high crystal growth temperature and a high equilibrium vapor pressure of nitrogen near the crystal growth temperature, it is extremely difficult to produce a high-quality large single crystal from a melt. Therefore, the growth of the GaN-based single crystal is performed by using a sapphire substrate or SiC that has excellent heat resistance.
MOVPE (Metal Organic Vapor Phase) on the substrate
Epitaxy) or MBE (Molecular Beam Epitaxy). On the other hand, in recent years, ZnO has been used as a buffer layer on a sapphire substrate and GaN
Methods of growing single crystals are disclosed (eg, Applied
physics letter Vol.61 (1992) p.2688), it became possible to fabricate GaN substrates. Ga of the same quality on this GaN substrate
By growing the N-based single crystal thin film, the quality of the GaN-based single crystal thin film was improved as compared with the direct crystal growth on the sapphire substrate.

[0003]

However, the above ZnO
In the conventional method of using as a buffer layer, since the ZnO film is formed on the sapphire substrate by the sputtering method, the ZnO layer does not become a high-quality single crystal,
There is a problem in that a high quality GaN single crystal cannot be obtained because the quality of this crystal structure affects the next GaN single crystal layer. In addition, the crystal quality of a conventionally known GaN single crystal, even if it is the highest quality, is the full width at half-maximum of the two-crystal method X-ray rocking curve.
the double-crystal X-ray rocking curve) is 100
sec, mobility at room temperature is 600 cm ² / VS
However, since the method of growing the crystal is MOVPE, the film thickness is only about 5 μm and it is very thin. Therefore, the GaN single crystal is separated from the original substrate, for example, as a substrate of a semiconductor light emitting device. , Was difficult to use alone. Therefore, when a GaN single crystal is used, it has been forced to use it as it is on the original substrate. Hereinafter, in the present specification, "full width at half maximum of X-ray rocking curve of two-crystal method" is simply referred to as "full width at half maximum".
Say. Further, as described above, H on the ZnO buffer layer
Although a GaN single crystal grown by VPE (Hydride Vapor Phase Epitaxy) has a sufficient thickness as a substrate, its full width at half maximum is 300 sec.
It was of low quality. That is, there was no GaN single crystal having both good quality and sufficient thickness at the same time.

An object of the present invention is to provide a GaN single crystal which is a high quality single crystal and has a sufficient thickness to be used as a substrate by itself. Another object of the present invention is Ga, which is a high-quality single crystal and has a sufficient thickness so that it can be used alone as a substrate.
An object of the present invention is to provide a method for producing a GaN single crystal that enables the production of N single crystal.

[0005]

The inventors of the present invention have found that whether a GaN single crystal has a low quality crystal structure produced by a conventional manufacturing method, or a crystal other than GaN such as a sapphire substrate. Using these as the first crystal substrate, a material having good lattice matching with the GaN single crystal is grown as a thin film on this to form a buffer layer, and GaN is crystal-grown on this to obtain a higher quality GaN single crystal. And that the GaN single crystal is used as a new substrate, and the buffer layer / GaN single crystal is grown again on the new substrate, so that the buffer layer material and the GaN are alternately epitaxially grown. The inventors have completed the present invention by finding that the quality of GaN single crystal can be improved and the GaN single crystal can be formed with a sufficient thickness.

The GaN single crystal and the manufacturing method thereof according to the present invention have the following features. (1) The full width at half maximum of the X-ray rocking curve of the two-crystal method is 5 to 2
G of 50 sec and a thickness of 80 μm or more
aN single crystal. (2) Ga at least on the substrate whose surface is GaN single crystal
A method for producing a GaN single crystal, which comprises a step of growing a substance having a good lattice matching with an N single crystal into a buffer layer to grow a GaN crystal to obtain a GaN single crystal. (3) Ga on a substrate whose surface is at least GaN single crystal
The step of obtaining a GaN single crystal by growing a thin film of a material having a good lattice matching with the N single crystal to form a buffer layer is a single crystal growth cycle. At least one cycle of the above crystal growth cycle is repeated to form a laminate, and then each buffer layer is removed to obtain a GaN single crystal.
Method for producing aN single crystal. (4) Ga at least on the substrate whose surface is GaN single crystal
A process of obtaining a GaN single crystal by growing a GaN crystal on a material having a good lattice match with the N single crystal as a buffer layer is defined as one crystal growth cycle. The above-mentioned crystal growth cycle is repeated for at least one cycle as a new substrate, and the buffer layer is removed every cycle to obtain a GaN single crystal.

[0007]

The GaN single crystal of the present invention is formed by repeating the film formation of the buffer layer and the GaN crystal growth on the substrate alternately and cyclically. Many dislocations, stacking faults and the like existing in the substrate disappear in the GaN crystal growth layer, the buffer layer, the interface between the substrate and the buffer layer, or the interface between the buffer layer and the GaN crystal growth layer. . Therefore, each time the crystal growth cycle is performed, the quality of the GaN single crystal structure is improved, and if this is repeated infinitely, it will asymptotically converge to the single crystal structure defined by the growth conditions. Seem. The GaN single crystal obtained by the production method of the present invention has good quality and can be formed to a thickness of 80 μm or more if necessary, which is particularly preferable as a substrate for a semiconductor light emitting device.

[0008]

The present invention will be described in more detail with reference to examples. As described above, the GaN single crystal of the present invention is of high quality having a full width at half maximum of 5 to 250 sec, and has a thickness of 80 μm or more sufficient to be used alone as a substrate. It is a thing. In the present invention, the X-ray diffraction method is used as a method for numerically expressing the quality of the GaN single crystal, and the value of the full width at half maximum indicated by this method is defined as the quality of the GaN single crystal. The X-ray diffraction method is a method that utilizes the diffraction of X-rays irradiated on the crystal. Among them, in the present invention, in order to improve the measurement accuracy, the measurement was performed by a method using two crystals. The X-ray diffraction method using two crystals is a method of precisely evaluating the lattice constant of a sample and evaluating the crystal perfection from the half width. In the quality evaluation of the GaN single crystal in the present invention, the X-rays incident from the X-ray source are highly monochromated by the first crystal, and the GaN single crystal of the sample that is the second crystal is irradiated and diffracted from this sample. FWHM (full width at half-maximum) centering on the X-ray peak was measured. Cukα ₁ is used as the X-ray source, and 30 kV, 10 mA
To generate X-rays. The first crystal for monochromatization has G
e (400) was used. The measurement is performed on the diffraction peak of GaN (0002), and the measurement step interval is 0.0
It was supposed to be performed at 02 °.

Experimental values of the quality of the GaN single crystal according to the present invention will be described later.
A manufacturing method capable of obtaining an aN single crystal will be described. FIG. 1 is a diagram schematically showing an example of a step of forming a GaN single crystal by the manufacturing method of the present invention. The simplest method for producing a GaN single crystal of the present invention is as shown in Step 1 of the figure.
A material having a good lattice matching with the GaN single crystal is deposited on the first substrate P ₀ having at least the surface of the GaN single crystal to form the buffer layer B _1, and then GaN is crystal-grown to form G.
This is to obtain an aN single crystal.

In the method for producing a GaN single crystal of the present invention, as shown in step 2 in the figure, the lattice matching with the GaN single crystal is excellent on the substrate P _{1 of the} laminated body formed in step 1. The material is grown in a thin film to form a buffer layer B ₂ and G
AN is crystal-grown to form a substrate P ₂ made of a GaN single crystal. Such a process is counted as the first process, and is repeated n times, so that the GaN single crystal P _n is grown on the uppermost layer, and then each buffer layer accumulated until then is removed at once to obtain the GaN single crystal. This is a method of separating the crystals P _{1 to} P _n into a large number of GaN single crystals. In the above method, the crystal quality of GaN obtained improves as the number of cycles increases, but after the improvement in crystal quality reaches an equilibrium state, it becomes rather useful as a technique for producing a large number of substrates.

In the method for producing a GaN single crystal of the present invention, further, as shown in step 3 of the figure, the buffer layer B _{1 of} step ₁
Is removed to separate the GaN single crystals P ₀ and P ₁ to obtain a GaN single crystal single substrate P ₁ , and a thin film of a substance having a good lattice matching with the GaN single crystal is grown on the substrate P _1. After forming the buffer layer B ₂ , GaN is crystal-grown and the buffer layer B ₂ is removed to separate the GaN single crystals P ₁ and P ₂ . A GaN single crystal P _n can be obtained by repeating the above-mentioned steps cyclically n times, counting step 1 as the first time. That is, in this method, each time a newly obtained GaN single crystal grows, the buffer layer which is the basis of the crystal growth is removed every cycle, and the newly obtained GaN single crystal is always made of GaN single crystal. It is a method of making a crystal. Also in this method, as in the case of the above step 2, after the improvement of the crystal quality reaches the equilibrium state, the two GaN single crystals separated by removing the buffer layer are used as product materials.
Each may be reused as a substrate for the next GaN crystal growth cycle.

As a method of appropriately combining the two methods shown in steps 2 and 3, a method of removing the buffer layer every arbitrary number k of crystal growth cycles can be considered. In this case, the arbitrary number of times k may be freely selected.

As described above, the method for producing a GaN single crystal according to the present invention is characterized by repeating the crystal growth cycle.
The quality of the GaN crystal structure improves each time the crystal growth cycle is performed, and an extremely high quality GaN single crystal P _n can be obtained at the desired number n times.

A known film forming method or crystal growing method is used for thin film growth of the buffer layer, and a film forming method capable of epitaxial growth is particularly preferable for improving the quality of the obtained GaN single crystal. Also, as for the method of crystal-growing GaN on the buffer layer, a film-forming method capable of epitaxial growth is preferable for quality improvement, like the thin film growth of the buffer layer.

Epitaxial growth is a method in which the same substance or substance having the same crystal structure is grown on a crystal substrate as a single crystal whose crystal axes are aligned with the crystal axes of the substrate. In the present invention, a film forming method of epitaxially growing GaN or a substance to be a buffer layer is most preferable, and particularly VPE (Vapor Phase Epitaxy),
HVPE, MOVPE, MBE, GS-MBE (Gas So
urse MBE), CBE (Chemical beam Epitaxy) and the like.

Number of repetitions n of the crystal growth cycle
Is not particularly limited, and the number of cycles may be determined according to the desired quality of the GaN crystal and the required number of GaN crystal substrates, but a GaN crystal substrate for a normal semiconductor device is used. To use as twice
Sufficient crystal quality is obtained by about 5 times.

The above-mentioned method of removing the buffer layer which is the basis of the GaN crystal growth may be any method as long as it can separate the obtained GaN single crystal. Chemical removal methods are effective.

The material used for the buffer layer is Ga
A material having a good lattice matching with the N single crystal is used. G
A substance having a good lattice compatibility with an aN single crystal is a wurtzite type crystal structure in which the lattice constant of the a-axis in the crystal lattice is within ± 10%, preferably within ± 5% of that of a GaN single crystal. Also say what you have. ZnO is mentioned as a preferable example of the substance which satisfies this. ZnO a
The lattice constant of the axis (length of unit lattice) is 3.2496Å, which is + with respect to the lattice constant of the a-axis of GaN of 3.189Å.
It has a very close lattice constant of 1.9%, which is desirable because good GaN crystal growth can be performed. Also,
ZnO has a good acid removability by etching, and in this respect also, it is suitable as a substance used for the buffer layer. The thickness of the buffer layer is preferably about 0.01 μm to 100 μm.

The first substrate P ₀ has at least a Ga surface.
A single crystal of N is used. That is, the whole is substantially G
a single substrate of GaN single crystal consisting only of aN, or
A substrate having a GaN single crystal layer on the surface on the buffer layer forming side and having only a GaN single crystal surface.
In the latter case, as the base material supporting the GaN single crystal layer, the growth temperature of the GaN single crystal (1000 to 1100 ° C.)
It is desirable that it has a good heat resistance to, for example, a sapphire crystal substrate, a Si substrate, a crystal, a ZnO substrate, a SiC substrate and the like. The GaN single crystal layer can be formed on these base materials by a heteroepitaxial growth method based on nonequilibrium reaction such as MOVPE method and MBE method.

[Manufacturing Experiment of GaN Single Crystal and Quality Confirmation Experiment] Next, the result of actually manufacturing a GaN single crystal by the method for manufacturing a GaN single crystal of the present invention and confirming its quality will be shown. Experimental Example 1 In this experimental example, as a method of repeating the crystal growth cycle in the method for producing a GaN single crystal of the present invention, as shown in step 2 in FIG. 1, a buffer layer and a GaN single crystal were initially formed on the substrate P _0. Was sequentially grown and laminated, and finally each buffer layer was removed at once to separate the GaN single crystal. As the first substrate P ₀ , a substrate obtained by epitaxially growing a GaN single crystal layer on the sapphire crystal base material by the MOVPE method was used. The buffer layer had a thickness of 0.2 μm and was made of ZnO. The crystal growth cycle was repeated 5 times. The GaN single crystals P _{1 to} P ₅ formed in each crystal growth cycle each have a thickness of 300 μm.
Was the goal. When the full width at half maximum of the finally obtained GaN single crystal P ₅ was measured, it was 29 sec, and its thickness was 305 μm.

Experimental Example 2 In this experimental example, instead of the method of repeating the crystal growth cycle in Experimental Example 1 described above, as shown in Step 3 in FIG. 1, every time a GaN single crystal was epitaxially grown, the buffer layer before it was grown. Is removed, and a new substrate is always used as one G
A GaN single crystal was manufactured in exactly the same manner as in Experimental Example 1 except that a single crystal of aN alone was used. Regarding the quality of the finally obtained GaN single crystal P ₅ , the full width at half maximum was 28 sec and the thickness thereof was 289 μm.

Experiment Example 3 In this experiment example, a sapphire substrate, a buffer layer of AlN (aluminum nitride), and a three-layer substrate made of GaN single crystal were used as the first substrate in the above experiment example 2. A GaN single crystal was manufactured in exactly the same manner as in Experimental Example 2. A process of manufacturing a three-layer substrate will be briefly described. On a sapphire crystal substrate with a thickness of 300 μm and an area of 5 cm × 5 cm, AlN was epitaxially grown as a buffer layer by MOVPE to a thickness of 500 Å, and the material gas was changed in that state, and the same MOVPE method was used to grow a GaN single crystal to a thickness of 2 μm. Epitaxial growth was performed to form a surface layer, and a substrate having a three-layer structure with a total thickness of about 302 μm, which includes a sapphire crystal substrate, an AlN buffer layer, and a surface layer of a GaN single crystal, was obtained. The quality of the GaN single crystal P ₅ finally obtained by this experiment had a full width at half maximum of 25 sec and a thickness of 295 μm.

Experimental Example 4 In this Experimental Example, the first substrate P ₀ in Experimental Example 2 was used.
As a material of the buffer layer in each cycle when a GaN single crystal growth cycle is repeated using the same three-layer substrate as in Experimental Example 3, (BeO) _0.13 (ZnO)
Except for using _0.87 , a GaN single crystal was manufactured in exactly the same manner as in Experimental Example 2. G obtained at the end of this experiment
Regarding the quality of the aN single crystal P ₅ , the full width at half maximum was 28 sec, and the thickness thereof was 301 μm.

Experimental Example 5 In this experimental example, in order to compare with the quality of the GaN single crystal according to the present invention, the quality of the GaN single crystal according to the conventional manufacturing method was examined. ZnO was formed on a sapphire crystal substrate having a thickness of 300 μm and an area of 5 cm × 5 cm by a sputtering method.
A buffer layer having a thickness of 0.6 μm is formed by using as a material, and a GaN single crystal having a thickness of 250 μm is formed thereon by HVPE.
Was grown epitaxially. The full width at half maximum of the quality of this GaN single crystal was 420 sec.

As is clear from the above experimental results, the GaN single crystal manufacturing method of the present invention has a high quality G which has never been obtained by the conventional method.
It is possible to produce an aN single crystal, and GaN
It was confirmed that it is possible to manufacture a single crystal in a thickness sufficient for use as a single substrate.

Such a high quality and thick GaN single crystal obtained by the manufacturing method of the present invention is used for a semiconductor light emitting device such as a light emitting diode (LED), a laser diode (LD), a super luminescence diode, and an electronic device. It is preferably used for. In the semiconductor light emitting device, by using the GaN single crystal of the present invention as a substrate, it is possible to manufacture an LED, an LD, or the like having the same electrode structure as a conventional red LED or the like. Of these, those that emit blue light are particularly important. Further, the efficiency of light emission of the semiconductor light emitting element becomes higher.

[LE Using GaN Single Crystal According to the Present Invention
Quality Confirmation Experiment of D] An LED using the GaN single crystal obtained by the manufacturing method of the present invention as a substrate was actually manufactured and the quality thereof was confirmed. Further, LEDs having conventional GaN single crystals and sapphire crystals as substrates were manufactured and compared with the quality of LEDs having the GaN single crystals of the present invention as substrates. As a conventional quality GaN single crystal, a GaN single crystal having a full width at half maximum of 300 sec was used. LED quality was evaluated for initial brightness and lifetime. Life is 85 ℃
The luminance after continuously emitting light for 20 hours at a current of 20 mA in an atmosphere of 85% humidity was measured, and the reduction rate of the luminance with respect to the initial luminance was calculated. A reduction rate of less than 2% was A and a reduction rate of 2 to less than 5 % For B, and a reduction rate of 5-10% for C
It was divided into three ranks. In the structure of the LED, a GaN single crystal obtained by the manufacturing method of the present invention is used as a substrate, and an n-AlGaN clad layer and an undoped I layer are formed on the substrate.
A double heterojunction type structure was formed by sequentially growing an nGaN active layer and a p-AlGaN cladding layer. The substrate quality of the GaN single crystal according to the present invention has a full width at half maximum of 30 se.
There are three types: c, 100 sec, and 250 sec. All thicknesses are 280 μm. In addition, the active layer of InGaN
The composition ratio of In _0.15 Ga _0.85 N and In _0.25 Ga _0.75 N
There are two types of N, and InGaN of each composition ratio,
A light emitting device was prepared and an experiment was conducted. The results of this experiment are shown in Tables 1 and 2 below.

[0028]

[Table 1]

[0029]

[Table 2]

As shown in Tables 1 and 2, the LED using the high quality GaN single crystal according to the present invention as a substrate is superior to the conventional one in terms of initial brightness and life. It was

Regarding LD, the following phenomenon was confirmed. In a conventional LD using a sapphire crystal as a substrate, since the sapphire crystal is a substance in which a cleavage surface is difficult to form, the substrate surface does not have a preferable mirror surface state, and the GaN compound semiconductor layer formed on the substrate surface. Since the state of the surface of (1) follows the state of the surface of the substrate, it was not possible to form a reflecting surface preferable for the LD. However, G according to the present invention
Since aN single crystal is of high quality and has sufficient thickness, G
Using the aN single crystal as a substrate, it became easy to obtain a cleavage plane. In addition, in the LD using the conventional GaN-based compound semiconductor, the crystal quality is inferior, so that stimulated emission by current injection cannot be achieved. However, a high-quality GaN single crystal according to the present invention is used as a substrate. When a stripe laser with a Perot type cavity was constructed and tested,
Stimulated release was confirmed at room temperature.

[0032]

As described above in detail, the GaN single crystal of the present invention has a crystal quality and a sufficient thickness which have not been available in the past. Further, the manufacturing method of the present invention can suitably provide a GaN single crystal having such a high quality and a sufficient thickness at the same time. Therefore, it is possible to provide a GaN single crystal substrate suitable for obtaining an LED that emits blue light with high efficiency, an ultraviolet laser diode, or a semiconductor device having good heat resistance. In addition, the manufacturing method of the present invention is capable of not only improving the crystal quality of a GaN single crystal and obtaining a thickness, but also efficiently manufacturing a large number of high-quality GaN single crystals, which is extremely industrially important. Technology.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of steps of a method for producing a GaN single crystal according to the present invention.

[Explanation of symbols]

P ₀ First substrate P _{1 to} P _n GaN single crystal B _{1 to} B _n buffer layer

Claims (4)

[Claims] 1. A GaN single crystal having a full width at half maximum of a 2-crystal X-ray rocking curve of 5 to 250 sec and a thickness of 80 μm or more. 2. A GaN single crystal is obtained by growing a thin film of a substance having a good lattice matching with the GaN single crystal on a substrate having at least the surface of the GaN single crystal to form a buffer layer and then crystallizing GaN. A method of manufacturing a GaN single crystal having a step. 3. A GaN single crystal is obtained by growing a thin film of a substance having a good lattice matching with the GaN single crystal on a substrate having at least the surface of the GaN single crystal to form a buffer layer and then crystallizing GaN. The step is one crystal growth cycle, and the crystal growth cycle is repeated at least once on the obtained GaN single crystal to form a laminate, and then each buffer layer is removed to obtain a GaN single crystal. And a method for producing a GaN single crystal. 4. A GaN single crystal is obtained by growing a thin film of a substance having a good lattice matching with the GaN single crystal on a substrate having at least the surface of the GaN single crystal to form a buffer layer and then crystallizing GaN. The process is performed once as a crystal growth cycle, and the obtained GaN single crystal is used as a new substrate to repeat the above crystal growth cycle for at least one cycle, and 1
A method for producing a GaN single crystal, which comprises removing the buffer layer every cycle to obtain a GaN single crystal.