JP7176977B2 - Method for producing gallium oxide - Google Patents

Description

The present disclosure relates to a method for producing gallium oxide.

At present, there is a demand for the development of power-saving technology, and low-loss power devices are expected. Power devices are installed in all kinds of power converters, such as inverters installed in hybrid vehicles and electric vehicles.

In order to realize low-loss power devices, new wide-gap semiconductor materials such as SiC, GaN, and gallium oxide, which can be expected to realize power devices with higher voltage resistance and lower loss than the current silicon (Si), are attracting attention. Research and development is actively progressing. Among them, gallium oxide is expected to have excellent device characteristics such as higher breakdown voltage and lower loss when applied to power devices due to its physical properties represented by a larger bandgap than SiC and GaN. be.

In order to use it as an automotive power device, it is expected to convert gallium oxide single crystal to p-type and to control its characteristics. A method of doping a single crystal with N (nitrogen), which is expected to act as an acceptor element in gallium oxide, has been proposed (Non-Patent Document 1).

APPLIED PHYSICS LETTERS 113 (2018) 102103

However, with the ion implantation method, it is difficult to convert gallium oxide to p-type and to control its characteristics. In particular, it is difficult to obtain highly crystalline gallium oxide doped with nitrogen at a high density.

The acceptor level of N in gallium oxide is deep, and high-concentration N-doping is expected to be necessary to develop hole conduction (p-type characteristics), but high-concentration N-doping was difficult. rice field. In addition, high crystallinity of the doped gallium oxide is required for power device applications, including automotive applications, but high-concentration N doping causes defects and distortion in the gallium oxide crystal. .

Therefore, there is a demand for a method for producing high-density, nitrogen-doped gallium oxide with high crystallinity.

The gist of the present disclosure is as follows.
(1) Gallium oxide doped with N in an atmosphere containing N radicals and O radicals, an atmosphere containing NO gas, or a combination thereof using a pulsed laser deposition method on a gallium oxide substrate. A method of making gallium oxide, comprising depositing
(2) The method for producing gallium oxide according to (1) above, including setting the temperature of the gallium oxide substrate to 500° C. or higher and 900° C. or lower.
(3) The method for producing gallium oxide according to (1) or (2) above, including depositing the N-doped gallium oxide in an atmosphere containing the N radicals and the O radicals.
(4) Gallium oxide containing N of 7×10 ²⁰ /cm ³ or more and having a rocking curve half-value width measured by X-ray of 1.83 degrees or less.

According to the method of the present disclosure, highly crystalline gallium oxide doped with N at a high density can be obtained.

FIG. 1 is a cross-sectional schematic diagram of an example of a PLD device used in the method of the present disclosure. FIG. 2 is a graph showing the relationship between the substrate temperature and the rocking curve half width of the N-doped gallium oxide single crystal produced in the example. FIG. 3 is a graph showing the relationship between the substrate temperature and the nitrogen density of N-doped gallium oxide single crystals produced in Examples. 4 is an asymmetric plane XRD chart of the N-doped gallium oxide produced in Example 1. FIG. 5 is an asymmetric plane XRD chart of the N-doped gallium oxide produced in Example 4. FIG. FIG. 6 shows the emission spectrum of the plasma measured when the NO gas was plasmatized in Example 4, and the plasma emission spectrum measured when the mixed gas of N ₂ gas and O ₂ gas was plasmatized in Example 8. Emission spectrum. FIG. 7 is a graph showing the N/O radical peak intensity ratio when the ratio of the N ₂ gas flow rate and the O ₂ gas flow rate is changed, and the N/O radical peak intensity ratio obtained by plasma treatment of NO gas. is.

The present inventors have found that by using a pulsed laser deposition (PLD) method and using N radicals and O radicals, NO gas, or a combination thereof as the N source, a level of high density not previously reported can be achieved. We succeeded in fabricating N-doped and highly crystalline gallium oxide.

The method of the present disclosure performs N-doped oxidation on a gallium oxide substrate using the PLD method in an atmosphere containing N radicals and O radicals, an atmosphere containing NO gas, or a combination thereof. A method of making gallium oxide is directed that includes depositing gallium.

The PLD method used in the method of the present disclosure can be performed using a pulsed laser deposition apparatus (PLD apparatus) conventionally used for the production of metal oxide thin films. FIG. 1 is a cross-sectional schematic diagram of an example of a PLD device used in the method of the present disclosure.

The PLD method is performed using the PLD apparatus 100 shown in FIG. 1. A target 20 placed in a vacuum chamber 10 is irradiated with an ablation laser beam from the outside of the chamber, thereby peeling off atoms (molecules) from the target 20. This is a film forming method for forming a thin film on a gallium oxide substrate 30 facing a target 20 . An example of the configuration of the PLD method is shown below.

The target 20 is placed on the stage, and the gallium oxide substrate 30 is placed at a predetermined distance from the target 20 in consideration of the distribution of film-forming active species scattered from the target. Ablation laser light is obliquely incident through a quartz window and condensed at an appropriate intensity on the target surface.

As the ablation laser used, a laser with a wavelength that is absorbed by the target can be used, for example, a KrF excimer laser with a wavelength of 248 nm or the like can be used.

As the target 20, a gallium oxide single crystal or a gallium oxide polycrystal obtained by sintering gallium oxide powder into a pellet can be preferably used, and a high-purity gallium oxide single crystal or gallium oxide polycrystal is more preferably used. More preferably, a high-purity gallium oxide single crystal can be used.

In the method of the present disclosure, an N-doped gallium oxide film is formed on the gallium oxide substrate 30 by the PLD method in an atmosphere containing N radicals and O radicals, an atmosphere containing NO gas, or an atmosphere containing a combination thereof. . That is, N radicals and O radicals, NO gas, or combinations thereof are used as N doping sources.

The atmosphere containing NO gas can be formed, for example, by evacuating the chamber 10 and then introducing NO gas, or by evacuating the chamber and removing an inert gas such as Ar, He, _N2 , etc. can be formed by introducing and then introducing NO gas.

The atmosphere containing NO gas also passes through the chamber 10 of the PLD device 100 shown in FIG. The NO gas may be introduced into the chamber 10 at a predetermined flow rate using a device including a device. NO gas can be introduced into the chamber from the NO gas source and the NO gas introduction path through the gas introduction pipe 40 of the PLD device 100 shown in FIG.

The NO partial pressure in the atmosphere containing NO gas is preferably 0.013 Pa or higher, more preferably 0.13 Pa or higher. When the NO partial pressure is within the above range, it is possible to more stably obtain gallium oxide doped with N at high density and high crystallinity. Although the upper limit of the NO partial pressure is not particularly limited, it may be 13 Pa, for example. The atmosphere containing NO gas preferably consists essentially of only NO gas.

The atmosphere containing NO gas may contain other components such as inert gas as long as they do not affect the crystallinity and electrical properties of the target gallium oxide.

When N-doped gallium oxide is deposited in an atmosphere containing substantially only NO gas, doped gallium oxide containing β and γ phases can be obtained.

The method of generating the atmosphere containing N radicals and O radicals is not particularly limited, but preferably, it can be generated using NO gas or a mixed gas of N ₂ gas and O ₂ gas as source gas. A mixture of NO gas and a mixture of N ₂ gas and O ₂ gas may be used.

When NO gas is used as the raw material gas, when the NO gas is introduced into the chamber 10 through the gas introduction pipe 40 shown in FIG. Thus, N radicals and O radicals can be generated. For example, when NO gas is passed through the gas introduction pipe 40 at a flow rate of 1.69×10 ⁻⁴ to 1.69×10 ⁻² Pa·m ³ /s, plasma treatment is performed at an output of 100 to 200 W, N radicals and O radicals can be generated and introduced into the chamber 10 .

The atmosphere containing N radicals and O radicals generated by plasmatizing NO gas may contain NO gas in addition to N radicals and O radicals.

When a mixed gas of N ₂ gas and O ₂ gas is used as the raw material gas, when the mixed gas of N ₂ gas and O ₂ gas is introduced into the chamber 10 through the gas introduction pipe 40 shown in FIG. N radicals and O radicals can be generated by applying a voltage inside the gas introduction pipe 40 for plasma treatment. For example, when a mixed gas of N ₂ gas and O ₂ gas is passed through the gas introduction pipe 40 at a flow rate of 1.69×10 ⁻⁴ to 1.69×10 ⁻² Pa·m ³ /s, the output is 100 to 100. A plasma treatment is performed at 200 W to generate N radicals and O radicals, which can be introduced into the chamber 10 . The flow rate ratio of the _N2 gas and the _O2 gas is preferably 26% or more of the _N2 gas flow rate with respect to 100% of the total flow rate of the _N2 gas and the _O2 gas. Gallium oxide with a higher nitrogen density can be obtained as the proportion of the _N2 gas flow rate in the total flow rate of the _N2 gas and the _O2 gas is greater. The upper limit of the _N2 gas flow rate with respect to 100% of the total flow rate of _N2 gas and _O2 gas is preferably 99% or less, more preferably 90% or less, still more preferably 80% or less, and still more preferably 74% or less. .

A mixed gas of _N2 gas and _O2 gas is supplied from an _N2 gas source and an _O2 gas source (not shown) through an _N2 gas and _O2 gas introduction path connected to the _N2 gas source and the _O2 gas source. (not shown), respectively, may be introduced into the gas introduction pipe 40 at a predetermined flow rate, or N ₂ gas and O ₂ gas may be formed before being introduced into the gas introduction pipe 40. It may be formed by mixing at a predetermined flow rate. Alternatively, a mixed gas source in which N ₂ gas and O ₂ gas are mixed at a predetermined ratio may be used.

The atmosphere containing N radicals and O radicals generated by plasmatizing a mixed gas of N ₂ gas and O ₂ gas may contain N ₂ gas and O ₂ gas in addition to N radicals and O radicals. . Also, the amounts of N radicals and O radicals are equal to the respective amounts of N radicals and O radicals generated by plasmatizing NO gas at a flow rate equal to the total flow rate of N ₂ gas and O ₂ gas. Therefore, when N-doped gallium oxide is deposited in an atmosphere containing N radicals and O radicals, the N doping density can be controlled independently of the substrate temperature by changing the flow ratio of the mixed gas of _N2 gas and _O2 gas. I can.

The radical partial pressure in the atmosphere containing N radicals and O radicals is preferably 7.06×10 ^-5 to 7.73×10 ^-5 Pa. When the radical partial pressure is within the above range, gallium oxide doped with N at a high density and high crystallinity can be obtained more stably.

The atmosphere containing N radicals and O radicals may contain other components such as inert gas as long as they do not affect the crystallinity and electrical properties of the intended gallium oxide.

By depositing N-doped gallium oxide in an atmosphere containing N radicals and O radicals, N-doped gallium oxide substantially containing only the β phase can be obtained. It is preferable to deposit N-doped gallium oxide in an atmosphere containing N radicals and O radicals in that N-doped gallium oxide containing only the β phase, which is the most stable crystal structure of gallium oxide, can be obtained.

Without being bound by theory, the reason why N-doped gallium oxide containing substantially only the β phase can be obtained by depositing N-doped gallium oxide in an atmosphere containing N radicals and O radicals is NO gas or It is thought that this is because the high-energy nitrogen source is injected by plasmatizing the mixed gas of N ₂ gas and O ₂ gas, and this high energy causes the formation of the thermodynamically most stable β phase. be done.

In addition, when N-doped gallium oxide is deposited in an atmosphere containing N radicals and O radicals, both the N doping density and the crystallinity of gallium oxide improve as the substrate temperature increases, and the N doping density and the crystallinity increase depending on the substrate temperature. does not fluctuate greatly, it is possible to more stably deposit gallium oxide with a high N-doping density and excellent crystallinity. It is preferable to deposit N-doped gallium oxide in an atmosphere containing N radicals and O radicals because both the N doping density and the crystallinity can be stably improved depending on the substrate temperature.

The reason why both the N-doping density and crystallinity of gallium oxide with respect to the substrate temperature are improved when depositing N-doped gallium oxide in an atmosphere containing N radicals and O radicals is not bound by theory. However, the strong oxidizing power of the atmosphere containing N radicals and O radicals is thought to stabilize the production reaction of N-doped gallium oxide.

NO gas and N and O radicals exiting the chamber can be treated by neutralization with an alkaline solution.

The temperature of the gallium oxide substrate is preferably 500° C. or higher and 900° C. or lower. By heating the substrate to a temperature of 500° C. or higher, crystal defects can be reduced and the crystallinity can be improved. By setting the substrate temperature to 900° C. or less, the growth rate can be improved, and film formation can be easily performed. Therefore, when the temperature of the substrate is within the above preferred range, it is possible to more stably obtain gallium oxide doped with N at a high density and with high crystallinity.

The temperature of the gallium oxide substrate can be controlled by any method. For example, the substrate can be heated to a desired temperature by irradiating the surface of the substrate opposite to the side on which the N-doped gallium oxide is deposited with an infrared laser.

When N-doped gallium oxide is deposited in an atmosphere that does not contain N radicals or O radicals and consists essentially of NO gas, the higher the substrate temperature in the range of 500° C. or higher, the smaller the rocking curve half width. , the N concentration tends to decrease slightly, but a high value can still be obtained. Therefore, when N-doped gallium oxide is deposited in an atmosphere that does not contain N radicals or O radicals and is substantially composed of only NO gas, the temperature of the substrate is more preferably 500° C. or higher and 800° C. or lower, and still more preferably 500° C. °C or higher and 700 °C or lower.

When N-doped gallium oxide is deposited in an atmosphere containing N radicals and O radicals, the higher the substrate temperature in the range of 500 to 900° C., the higher the N-doped gallium oxide with a higher N concentration and a smaller rocking curve half-value width. be able to. Therefore, when N-doped gallium oxide is deposited in an atmosphere containing N radicals and O radicals, the substrate temperature is more preferably 600° C. or higher and 900° C. or lower, and still more preferably 700° C. or higher and 900° C. or lower.

As the gallium oxide substrate used in the method of the present disclosure, a Ga ₂ O ₃ single crystal substrate produced by any method or a commercially available Ga ₂ O ₃ single crystal substrate can be used. The Ga ₂ O ₃ single crystal substrate can be α-Ga ₂ O ₃ single crystal, β-Ga ₂ O ₃ single crystal, or Ga ₂ O ₃ single crystal with other crystal structure, preferably β- It is a Ga ₂ O ₃ single crystal.

According to the method of the present disclosure, it is possible to obtain a gallium oxide single crystal containing N of 7×10 ²⁰ /cm ³ or more and having a rocking curve half width of 1.83 degrees or less as measured by X-rays. .

The resulting gallium oxide is doped with N at a high concentration of 7.0×10 ²⁰ /cm ³ or higher. It is difficult to dope a gallium oxide single crystal with such a high concentration of N by ion implantation. The gallium oxide is doped with N, preferably at least 1.3×10 ²¹ /cm ³ /cm ³ , more preferably at least 1.5×10 ²¹ /cm ³ . Although the upper limit of the N doping amount is not particularly limited, the upper limit may be, for example, 1.0×10 ²² /cm ³ .

The resulting gallium oxide also exhibits good crystallinity such that the half width of the rocking curve measured by X-rays is 1.83 degrees or less. The half width of the rocking curve of gallium oxide obtained is preferably 0.82 degrees or less, more preferably 0.25 degrees or less, still more preferably 0.21 degrees or less, even more preferably 0.15 degrees or less, and even more preferably 0.15 degrees or less. It is preferably 0.11 degrees or less.

The N density in the gallium oxide single crystal can be obtained by measuring the obtained gallium oxide by secondary ion mass spectrometry (SIMS).

The half width of the rocking curve can be obtained by measuring the obtained gallium oxide using an X-ray diffractometer.

Gallium oxide obtained by the method of the present disclosure is a thin film, preferably having a thickness of 30-300 nm. The gallium oxide obtained by the method of the present disclosure is preferably p-type gallium oxide, more preferably p-type gallium oxide single crystal. Also, the gallium oxide obtained by the method of the present disclosure preferably comprises β-phase and γ-phase, more preferably consists essentially of β-phase.

The present disclosure is also directed to gallium oxide containing N of 7×10 ²⁰ /cm ³ or more and having a rocking curve half-width measured by X-ray of 1.83 degrees or less.

For the configuration of the gallium oxide of the present disclosure, the content described with respect to the above manufacturing method can be applied.

(Example 1)
Using the PLD apparatus shown in FIG. 1, an N-doped gallium oxide film was formed on a gallium oxide substrate under the following conditions:
Laser: frequency 10 Hz, output 0.5 J/cm ² ;
Atmosphere in the chamber: Atmosphere containing N radicals and O radicals generated by plasma treatment of NO gas used as a raw material gas (8.45 × 10 ^-4 Pa·m ³ /s of NO after evacuating the chamber) Plasma treatment of gas at 100 W);
Gallium oxide substrate: ¹⁰ mm long, ⁵ mm wide, and 0.65 mm thick. β-Ga ₂ O ₃ single crystal;
Substrate temperature: 500°C;
Target: A commercially available β-Ga ₂ O ₃ single crystal (manufactured by Novel Crystal Technology Co., Ltd.) having a (100) plane and a silicon (Si) concentration of 1×10 ¹⁷ /cm ³ .

(Example 2)
An N-doped gallium oxide film was formed under the same conditions as in Example 1, except that the substrate temperature was set to 700.degree.

(Example 3)
An N-doped gallium oxide film was formed under the same conditions as in Example 1, except that the substrate temperature was set to 900.degree.

(Example 4)
N-doped gallium oxide was produced under the same conditions as in Example 1, except that 0.13 Pa of NO gas was introduced without plasma treatment after the chamber was evacuated, and the atmosphere in the chamber was changed to 0.13 Pa of NO gas. was deposited.

(Example 5)
An N-doped gallium oxide film was formed under the same conditions as in Example 4, except that the substrate temperature was set to 700.degree.

(Example 6)
An N-doped gallium oxide film was formed under the same conditions as in Example 4, except that the substrate temperature was set to 900.degree.

(Example 7)
A mixed gas of N ₂ gas and O ₂ gas is used as the raw material gas, and the atmosphere in the chamber is an atmosphere containing N radicals and O radicals generated by plasma processing the mixed gas of N ₂ gas and O ₂ gas (chamber After evacuating the inside, a mixed gas of 2.197 × 10 ^-4 Pa·m ³ /s N ₂ gas and 6.253 × 10 ^-4 Pa·m ³ /s O ₂ gas is plasmatized at 100 W) An N-doped gallium oxide film was formed under the same conditions as in Example 1, except for the above.

(Example 8)
Except that a mixed gas of 4.225×10 ⁻⁴ Pa·m ³ /s N ₂ gas and 4.225×10 ⁻⁴ Pa·m ³ /s O ₂ gas was used as the source gas. N-doped gallium oxide was deposited under the same conditions as in 7.

(Example 9)
Except that a mixed gas of 6.253×10 ⁻⁴ Pa·m ³ /s N ₂ gas and 2.197×10 ⁻⁴ Pa·m ³ /s O ₂ gas was used as the source gas. N-doped gallium oxide was deposited under the same conditions as in 7.

(Nitrogen density measurement)
The nitrogen density and film thickness of the obtained gallium oxide were measured by SIMS. The primary ion beam was Cs ⁺ ions, the primary ion beam energy was 14.5 keV, and the standard sample of EAG was used as the N standard sample.

(Half width measurement of rocking curve)
The obtained gallium oxide was evaluated for crystallinity by measuring the half width of the rocking curve. The half width of the rocking curve was measured by irradiating CuKα1 rays (λ=1.54056 Å) using an X-ray diffractometer (manufactured by Rigaku Corporation, SmartLab).

Table 1 shows the atmosphere, the substrate temperature (film formation temperature), and the thickness of the obtained N-doped gallium oxide in Examples and Comparative Examples.

Table 2 and FIG. 2 show the relationship between the substrate temperature and the obtained rocking curve half width of gallium oxide.

Table 3 and FIG. 3 show the relationship between the substrate temperature and the nitrogen density of the obtained gallium oxide.

FIG. 4 shows the result of the asymmetric plane XRD measurement of the N-doped gallium oxide produced in Example 1. As shown in FIG. In the N-doped gallium oxide produced in an atmosphere containing N radicals and O radicals generated by the plasma treatment of NO gas, almost no peak was observed at 2θ = 30°, which indicates the presence of the γ phase, and 2θ = 64°. A large peak was observed only in °, and it was found to be a single crystal containing substantially only the β phase.

FIG. 5 shows the result of the asymmetric plane XRD measurement of the N-doped gallium oxide produced in Example 4. As shown in FIG. It was found that the N-doped gallium oxide produced in an atmosphere containing only NO gas had peaks at 2θ=30° and 2θ=63°, and contained β phase and γ phase.

FIG. 6 shows the emission spectrum of the plasma measured when the NO gas was plasmatized in Example 4, and the plasma emission spectrum measured when the mixed gas of N ₂ gas and O ₂ gas was plasmatized in Example 8. and emission spectra. A multichannel spectrometer (FLAME-S manufactured by Ocean Optics, using a grating capable of measuring a wavelength band of 200 to 850 nm) was used for the measurement of the emission spectrum. The emission spectrum of N radicals and O radicals when NO gas is plasmatized and the emission spectrum of N radicals and O radicals when a mixed gas of N ₂ gas and O ₂ gas is plasmatized are almost the same. there were.

FIG. 7 shows the intensity of the emission spectrum of O radicals at a wavelength of 616 nm versus the intensity of the emission spectrum of N radicals at a wavelength of 747 nm when the plasma treatment was performed by changing the ratio of the N ₂ gas flow rate and the O ₂ gas flow rate in Examples 7 to 9. The N/O radical peak intensity ratio (N radical peak intensity/O radical peak intensity) of the emission spectrum is shown. FIG. 7 also shows the N/O radical peak intensity ratio when the NO gas is plasmatized in Example 1. As shown in FIG. FIG. 7 also shows, as reference data, the case of plasma treatment with an N ₂ gas flow rate of 0 Pa·m ³ /s and an O ₂ gas flow rate of 8.45×10 ⁻⁴ Pa·m ³ /s, and N ₂ gas flow rate. The N/O radical peak intensity ratio is shown when the gas flow rate is set to 8.45×10 ⁻⁴ Pa·m ³ /s and the O ₂ gas flow rate is set to 0 Pa·m ³ /s for plasma treatment.

_{The N} /O radical peak intensity ratio changes in proportion to the mixing ratio of the _N2 gas flow rate and the _O2 gas flow rate. was shown to be controllable. In addition, the N / O radical peak intensity ratio when NO gas is plasmatized in Example 1, and the N / O radical peak when plasmatization is performed on a mixed gas of N ₂ gas and O ₂ gas in Example 8 The intensity ratio was almost the same.

REFERENCE SIGNS LIST 100 PLD device 10 chamber 20 target 30 gallium oxide substrate 40 gas inlet pipe

Claims (5)

N-doped gallium oxide is deposited on a gallium oxide substrate using a pulsed laser deposition method in an atmosphere containing N radicals and O radicals, an atmosphere containing NO gas, or a combination thereof. including
using gallium oxide as a target, and
Plasma treatment of NO gas or plasma treatment of a mixed gas of N _gas and O gas to generate an atmosphere containing the N radicals and O radicals _;
A method for producing gallium oxide. When the NO gas is plasmatized, the flow rate of the NO gas is 1.69 × 10 ^-4 ~1.69 x 10 ^-2 Pa・m ³ /s, and the N ₂ gas and O ₂ When the mixed gas of gases is subjected to plasma treatment, the N ₂ gas and O ₂ The flow rate of the mixed gas of the gas is 1.69 × 10 ^-4 ~1.69 x 10 ^-2 Pa・m ³ /s. 3. The method for producing gallium oxide according to claim 1, comprising setting the temperature of said gallium oxide substrate to 500[deg.] C. or more and 900[deg.] C. or less. 4. The method for producing gallium oxide according to claim 1, comprising depositing said N-doped gallium oxide in an atmosphere containing said N radicals and O radicals. Gallium oxide containing N of 7×10 ²⁰ /cm ³ or more and having a rocking curve half-value width measured by X-ray of 1.83 degrees or less.

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