JP3700786B2 - GaN substrate manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a GaN substrate.

The GaN substrate fabrication methods that have been attempted so far can be broadly divided into four. One is the melt growth method. This is a method for growing a bulk crystal of GaN from a GaN melt at a high pressure, and has not yet been technically established. It is unclear whether large crystal substrates of 1 inch or more can be grown in the future.
Second is the solution growth method. This is a method in which GaN is dissolved in an appropriate solvent (for example, sodium) to prepare a solution, from which a bulk crystal is grown, and this has not been technically established yet.

A third method is to grow a GaN thick film on a sapphire substrate by CVD or MOCVD, and then peel the GaN film by lift-off using a laser. According to this method, since the substrate size is determined by the sapphire substrate, a large substrate can be formed, but a low yield is a bottleneck.
The fourth method is a method for producing a GaN substrate by growing a GaN thick film on a GaAs substrate by CVD or MOCVD, and then dissolving the GaAs substrate with an acid or the like. This can also form a large substrate, but the high cost is a bottleneck.

Fifth, after forming a ZnO layer on a sapphire substrate, a GaN layer is generated on the ZnO layer. Then, the ZnO layer is melted and the GaN substrate is taken out.
At this time, it has been found that the use of an MgO buffer layer on sapphire is effective in improving the surface shape of the ZnO layer grown on the buffer layer (see Non-Patent Documents 1 and 2).

Chen Y, Ko HJ, Hong SK, and Yao T. Layer-by- layer growth of ZnO epilayer on Al2O3 (0001) by using a MgO buffer layer., Appl. Phys. Lett., 2000; 76: 559. Chen Y, Hong SK, Ko HJ, Kirshner V, Wenisch H, Yao T, Inaba K, and Segawa Y. Effects of an extremely thin buffer on heteroepitaxy with large lattice mismatch., Appl Phys Lett, 2001; 78: 3352.

  An object of the present invention is to provide a method for producing an inexpensive large-scale and high-quality GaN substrate.

In order to achieve the above-mentioned object, the present invention forms a MgO buffer layer on a high-quality substrate, anneals the MgO buffer layer, and forms a Zn-polar ZnO layer on the annealed MgO buffer layer. The GaN substrate is manufactured by growing a GaN substrate, preventing the nitridation of the surface of the ZnO layer, growing a Ga-polar GaN layer, and dissolving the ZnO film .
For the GaN layer, a high temperature GaN may be grown after forming a low temperature GaN buffer layer.

  By the production method of the present invention, a high-quality GaN substrate can be obtained.

In the present invention, an MgO buffer is grown on a high-quality substrate, a ZnO single crystal film is formed thereon, and a GaN single crystal film is grown thereon, but the GaN single crystal film is grown by controlling the polarity. Then, the ZnO is melted and the GaN single crystal substrate is peeled off.
Thereby, a Ga-polar high-quality GaN single crystal can be obtained.
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

As an example, a procedure for producing a GaN substrate using a sapphire substrate as a high-quality substrate will be described in detail below with reference to FIGS.
(1) Clean the sapphire substrate (C surface or A surface) 110. The cleaning method may be a well-known method (see FIG. 1A).
(2) An MgO single crystal thin film (buffer layer) 120 having a thickness of 2 nm or less is deposited. The growth conditions are as follows: substrate temperature: 300 to 800 ° C., growth rate: 10 nm / min or less. In the examples described later, the MBE method was used, but the growth method was HVPE, MOCVD, or plasma CVD. Either may be used (see FIG. 1B).
(3) The MgO buffer layer is annealed in a temperature range of 800 ° C. or higher and 1200 ° C. or lower for 25 minutes or more in an oxygen atmosphere, an oxygen plasma atmosphere, or an ultrahigh vacuum. (See FIG. 1 (c)).
(4) ZnO 130 is grown to a thickness of 0.1 μm or more. Although the MBE method was used in the examples described later, the growth method may be any of the HVPE method, the MOCVD method, and the plasma CVD method (see FIG. 2D).
(5) GaN is grown on the ZnO surface while preventing initial nitridation. That is, the growth of GaN is started without exposing the ZnO surface to ammonia or nitrogen plasma for 30 seconds or more (see FIG. 2E). For this purpose, for example, an inert gas atmosphere may be used.
(6) GaN is grown by a predetermined thickness (see FIG. 2 (f)). Although the MBE method is used in the embodiment, the growth method may be any of the HVPE method, the MOCVD method, and the plasma CVD method.
(7) By dipping in an acid solution such as hydrochloric acid, the GaN film is peeled off to form a self-supporting substrate (see FIG. 2G). In the examples, hydrochloric acid was heated to 80 degrees. However, any acid such as sulfuric acid or buffered hydrofluoric acid is possible. Also, any alkali such as ammonium fluoride may be used.

The points on the above process are (3) annealing treatment and (5) GaN growth start. First, by annealing the MgO buffer layer, the polarity of the ZnO film grown thereon becomes Zn polarity, and when GaN is grown while preventing the initial nitridation of the surface of the ZnO film, the GaN thereon also becomes Ga polarity, High quality GaN grows. If the annealing process is not performed, the ZnO film becomes O-polar.
When the surface of the ZnO film is exposed to O-plasma, oxygen / ozone, or to NH ₃ or nitrogen plasma, GaN grown on the surface becomes Ga polarity or N polarity. This makes it possible to control the crystal polarity. This control of crystal polarity will be described in detail later.
In the step (6), a thin buffer layer (for example, about 100 nm) of low-temperature GaN (LT-GaN) is first grown, and then the high-temperature GaN is grown, whereby crystallinity can be improved. This will also be described in detail later.
The effect of the annealing treatment on the MgO buffer layer is also in the reduction of defect density in the ZnO layer and the improvement in quality. The defect density of the GaN layer grown on the ZnO layer having a low defect density naturally decreases.

First, the reduction in the defect density and the improvement in quality of the ZnO layer by annealing will be described below with reference to FIGS.
<Annealing treatment>
The ZnO layer was grown on the LT-MgO (low-temperature magnesium oxide) buffer layer or the annealed LT-MgO buffer layer by P-MBE on c-sapphire. The substrate is cleaned with an ultrasonic cleaner with acetone and methanol, and then chemically etched in a solution of H ₂ SO ₄ (96%): H ₃ PO ₄ (85%) = 3: 1 at 160 ° C. for 15 minutes. did.
Prior to growth, the substrate was thermally cleaned at 750 ° C. for 1 hour in a preparation chamber. The substrate was then treated with oxygen plasma in a growth chamber at 650 ° C. for 30 minutes to create an oxygen-terminated c-sapphire surface.
The composition of the prepared sample is as follows. First, the MgO buffer layer is continuously formed on the cleaned c-sapphire at 700 ° C. and 490 ° C. Next, after the LT-ZnO buffer layer is grown at 490 ° C., it is annealed at 750 ° C. for 3 minutes. Thereafter, an HT—ZnO layer is grown at 700 ° C. In the case of a ZnO layer grown on an annealed MgO buffer, the LT-MgO buffer layer is annealed at 800 ° C. for 25 minutes. The thickness of the HT-ZnO layer is about 800 nm.

  FIG. 3 is a photograph of an atomic force microscope (AFM) of (a) a ZnO layer grown on an LT-MgO buffer and (b) a ZnO layer grown on an annealed LT-MgO buffer. Show. The values of the root mean square (rms) roughness in the 2.5 μm × 2.5 μm scan region are (a) 1.1 nm and (b) 0.76 nm, respectively. The average of the step height (the size of the terrace) is (a) 0.21 nm (94 nm) and (b) 0.25 nm (245 nm), respectively. These photos can increase the surface migration of adsorbed atoms by hot annealing to the LT-MgO buffer, resulting in larger terrace formation and smoother surface morphology on the surface.

  As shown in FIG. 4, the full width at half maximum (FWHM) (arc seconds) of the omega scan of (0002) ω and (10-11) ω is as follows on the LT-MgO buffer. 18 (1079) and 22 (843), respectively, for the ZnO layer grown on the annealed LT-MgO buffer. That is, similar low FWHM values for the two samples in the (0002) ω scan indicate that both have a low screw dislocation density. Compared to (0002) ω, a significant spread of (10-11) ω reflection indicates the presence of a large density of edge-type dislocations. All types of dislocations (edge, spiral, mixed) do not (10-11) broaden the reflection. [0002] Reflections are only sensitive to spiral and mixed dislocations. In addition, the FWHM value of the (10-11) omega scan of the ZnO layer grown on the annealed MgO buffer is smaller than that grown on the LT-MgO buffer, indicating a smaller edge dislocation density. Yes.

The type and density of the dislocation can be seen from a photograph of a bright-field two-beam cross-sectional TEM, and FIG. 5 shows the photograph. FIGS. 5A and 5B show a ZnO layer grown on the LT-MgO buffer. The dislocations woven are 15% helical (Burgers vectors b = [0001]), 67% edge (Burgers vectors b = 1/3 <11-20>), and 18% mixed (Burgers vectors b = 1/3 <11-23>). 5C and 5D show a ZnO layer grown on the annealed LT-MgO buffer. The dislocations that are woven are distributed as 90% edge type and 10% spiral type. Here, the majority of dislocations woven along the c-axis are edge-type dislocations in both ZnO samples. Edge-type dislocations can be greatly reduced by the annealed LT-MgO buffer. All dislocation densities are 1.5 × 10 ⁹ cm ⁻² and 5.2 × 10 ⁸ cm ⁻² for the ZnO layers grown on LT-MgO and annealed LT-MgO buffers, respectively. The result of a transmission electron microscope (TEM) confirms that the dislocation density of the ZnO layer decreases due to the annealing of the LT-MgO buffer layer. These results are consistent with HRXRD described above.
As described above, the dislocation density of the ZnO layer fabricated in this way can be reduced as compared with the case where the MgO buffer layer is not used. The GaN layer grown on this ZnO layer has a similar dislocation density, and can reduce the dislocation density.

<Crystal polarity control>
Next, crystal polarity control by starting GaN growth on the ZnO layer will be described. This will be described in detail with reference to FIGS.
First, a method of generating the actually used GaN layer will be described below.
The epilayer growth of GaN was performed by ammonia-assisted MBE on ZnO (˜1.3 μm) / MgO (˜3 nm) / c-sapphire template grown with P-MBE. However, MgO is not annealed at high temperature. Solid Ga and NH ₃ gas were used as the source of Ga and N, respectively. Similar results were obtained when nitrogen plasma was used as the N supply source. The substrate temperature for GaN growth is 800 ° C. The GaN growth rate is 1 μm / h, the Ga beam pressure at that time is 2.8 × 10 ⁻⁷ Torr (3.7 × 10 ⁻⁵ Pa), and the flow rate of NH ₃ is 10 sccm.
Two sets of samples were prepared which were treated differently just before GaN growth.
Sample 1: GaN epilayer grown on ZnO template pre-exposed to O-plasma Sample 2: GaN grown on ZnO template pre-exposed to NH ₃
These ZnO thin films were grown on a c-sapphire (Al ₂ O ₃ (0001)) substrate by P-MBE. After the growth of the ZnO template, the pattern of the ZnO layer by reflective high energy electron diffraction (RHEED) is striped by specular reflection points, suggesting a smooth surface. The measured value of the root mean square (rms) of the roughness of the surface performed by an atomic force microscope (AFM) is less than 1 nm for an area of 5 μm × 5 μm.

In order to confirm the polarity of the ZnO template, CBED (convergent electron diffraction) was used. CBED patterns were obtained on several parts of the sample with a 20 nm probe.
FIG. 6 shows an AFM image (a) of the surface of GaN (sample 1) grown on ZnO pre-exposed to O-plasma and the surface of GaN (sample 2) grown on ZnO pre-exposed to NH ₃ . An AFM image (b) is shown. The values of the mean square roughness of Sample 1 and Sample 2 are 97.6 nm and 23.8 nm, respectively. This value indicates that GaN grown on ZnO pre-exposed to NH ₃ has a smoother surface. It has been reported that a smooth surface is obtained on a Ga-polar GaN layer by observing the surface morphology by AFM, but a rough surface due to a hexagonal ridge is observed on the N-polar GaN surface. Etching on two GaN samples was performed in a 0.5 M KOH (10%) solution. The etching rates of Sample 1 and Sample 2 are 3.9 nm / min and 0.1 nm / min or less, respectively. It has been found that the etching rate for the Ga-polar GaN layer is very low compared to the etching rate for the N-polar GaN layer. These results show that GaN layer grown on Sample 1, ie ZnO pre-exposed to O-plasma, has N polarity, and GaN grown on Sample 2, ie ZnO pre-exposed to NH ₃ , has Ga polarity. It is suggested that you have.

The CBED technique is widely used to determine the polarity of Wurtzite materials. When the polarity changes, the symmetry of the CBED pattern with respect to the c-axis is reversed. FIGS. 7 (a) and 7 (b) show the zone axis [10] when (a) obtained for O-polar ZnO with a thickness of 28 nm and (b) obtained with N-polar GaN with a thickness of 58 nm. -10] shows a CBED pattern by simulation. FIGS. 7C and 7D show the measured CBED patterns for the zone axis [10-10] of the (c) ZnO template and (d) GaN layer of Sample 1. FIG. As shown in FIGS. 7 (a) and 7 (c), the CBED pattern of O-polar ZnO by simulation is the same as that measured, indicating that the ZnO template has O-polarity. Show. As shown in FIGS. 7B and 7D, the CBED pattern of N-polar GaN by simulation is very similar to that measured. This shows that the GaN layer grown on the O-plasma-treated ZnO has N polarity. It appears that O-plasma exposure to O-polar Zn 2 O can retain the O-polar ZnO (anionic polarity) surface and assist in the growth of N-polar GaN (anionic polarity).

FIGS. 8A and 8B show the zone axis [11-20] when calculated for (a) O-polar ZnO having a thickness of 26 nm and (b) determined for N-polar GaN. The CBED pattern by simulation is shown. FIGS. 8C and 8G show the measured CBED patterns for the same zone axis of the (c) ZnO template and (d) GaN layer of sample 2. FIG. When the CBED pattern of O-polarity ZnO by simulation (FIG. 8A) is compared with the measured one (FIG. 8C), the ZnO template of Sample 2 has O-polarity. The measured GaN CBED pattern shown in FIG. 8 (d) is inverted along the c-axis as compared to the simulated N-polar GaN CBED pattern shown in FIG. 8 (b). It is concluded that This inverted CBED pattern indicates that the GaN layer grown on the O-polar ZnO template exposed to NH ₃ has Ga polarity. That is, exposure of NH ₃ to the anion polarity ZnO template reverses polarity from anion polarity to cation polarity.

The bonding sequence at the GaN / O-polar ZnO interface found from the above will be described. FIG. 9 shows a schematic diagram at the interface. FIG. 9A shows a GaN layer grown on ZnO pre-exposed to O-plasma, and FIG. 9B shows a GaN layer grown on ZnO pre-exposed to NH ₃ .
After exposing O -plasma to O-polar ZnO, the surface of ZnO is O-terminated. When GaN growth starts on the anion-terminated ZnO surface, Ga (cation) adatoms are stably fixed, and thereafter N (anion) adatoms are bonded to Ga. As shown in FIG. 9A, the bonding sequence is as follows:... -Zn—O—Ga—N— at the interface of GaN / O-plasma exposure / O-polar ZnO.
In the case of GaN grown on NH ₃ exposed O-polar ZnO, the Zn 2 O surface is modified to form an interface layer such as Zn ₃ N ₂ . Zn ₃ N ₂ has a cubic structure (Ia3) with a lattice constant of 0.978 nm and has inversion symmetry. The interface layer having such inversion symmetry can invert the crystal polarity. The (111) Zn ₃ N ₂ surface is composed of Zn atoms. The space between the Zn faces along [111] of Zn ₃ N ₂ is 0.339 nm. Zn surface can be attached to O of Ga-polar GaN N and O-polar Zn O, the bond _{sequence, ... -Zn-O- (Zn-} Zn 3 N 2 -Zn) -N-Ga- ... in Lead. This inverts the crystal polarity from anion polarity to cation polarity.
In the above description, the polarity control is described using O-plasma and NH _3. However, exposure to oxygen or ozone instead of O-plasma or exposure to N-plasma instead of NH ₃ is also possible. Polarity can be controlled.

<Low-temperature GaN buffer>
When growing GaN on a ZnO film, low-defect GaN can be obtained by growing a low-temperature GaN buffer and then growing GaN at a high temperature. This will be described below with reference to FIG.
First, GaN is grown on ZnO at a low temperature. FIG. 10 is a comparison of XRD (0002) omega rocking curves of GaN grown at a high temperature of 800 ° C. on GaN grown at a substrate temperature of 500 ° C. and a GaN layer grown without a low-temperature GaN layer. is there. When there is no LT-GaN, the FWHM is 23.1 arcmin, and when LT-GaN is present, it is almost halved to 11.9 arcmin.
Thus, high-quality GaN can be obtained by growing low-temperature GaN on ZnO and then growing GaN at high temperature.

<Other embodiments>
As an extension of the above-described GaN production methods (1) to (7), the following is also possible.
In the step (5), a GaN film thickness of about 10 μm is formed. Then, SiO ₂ is deposited to a thickness of about 100 nm. For this SiO ₂ layer, the SiO ₂ film is patterned into lines and stripes using a lithography technique. The SiO _{2 at the} stripe is removed by etching. At this time, the line direction is aligned with the <11-20> direction or <1-100> direction of the GaN crystal. Next, GaN is grown by HVPE and MOCVD. Here, a high quality GaN film with a small defect density is grown by properly selecting the growth conditions.
An embodiment using a silicon substrate as a substrate for growing GaN will be described below. In this case, CaF ₂ is used for the buffer layer.
(1) Clean the silicon substrate and grow a CaF ₂ thin film on it.
This can be done with already established techniques.
(2) An electron beam is irradiated on CaF ₂ to remove F atoms on the surface.
(3) Next, ZnO is grown. This corresponds to (4) in the GaN substrate manufacturing method procedure using the sapphire substrate described above. Thereafter, the GaN substrate can be manufactured by the procedure (5) and below.
Instead of the above (2), it is also possible to perform hydrogen treatment on the surface of CaF ₂ at a temperature of 500 ° C. or higher. Either hydrogen plasma irradiation or atomic hydrogen irradiation may be used. Combination with electron beam processing is also possible.
Further, instead of the above (2), oxygen plasma treatment may be performed on the CaF ₂ surface at a temperature of 500 ° C. or higher. It can also be used in combination with electron beam processing.

The above-mentioned annealing treatment of MgO buffer, GaN polarity control by ZnO surface treatment, GaN defect reduction by low-temperature GaN buffer, etc. can be independently applied to GaN substrate fabrication. By applying these, each effect can be obtained.
The GaN substrate manufacturing method of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the present invention. For example, in the above-described embodiment, a sapphire substrate or a silicon substrate is used as a high-quality substrate. However, for example, SiC, CaF ₂ , GaAs, MgO, MgAl ₂ O _4, or the like may be used.

It is a figure which shows the preparation method of a GaN substrate. It is a figure which shows the continuation of the preparation method of the GaN substrate of FIG. It is an AFM photograph for demonstrating the effect of annealing treatment. It is a figure which shows the result by an omega scan. It is a transmission electron micrograph of a ZnO layer showing a dislocation distribution. It is an AMF photograph of the ZnO layer surface of O plasma treatment (sample 1) (a) and NH ₃ treatment (sample 2) (b). It is a figure which compares what measured the CBED pattern of the sample 1, and simulation. It is a figure which compares what measured the CBED pattern of the sample 2, and simulation. It is a figure explaining the crystal polarity of GaN. It is a figure explaining the effect of a low-temperature GaN buffer layer.

Claims (2)

Form a buffer layer of MgO on a high quality substrate,
Annealing the MgO buffer layer;
A Zn-polar ZnO layer is grown on the annealed MgO buffer layer;
Preventing nitridation of the surface of the ZnO layer and growing a Ga-polar GaN layer;
A method for producing a GaN substrate, comprising: dissolving the ZnO film to produce a GaN substrate. In the GaN substrate manufacturing method according to claim 1,
The GaN layer is produced by forming a low-temperature GaN buffer layer and then growing high-temperature GaN.

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