JP2009212284A - Gallium nitride substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a GaN substrate with which a device having superior characteristics is manufactured with good reproducibility. <P>SOLUTION: The gallium nitride substrate is characterized in that a first intensity at a first peak of a wavelength corresponding to a band gap of gallium nitride in a spectrum of cathode luminescence obtained through irradiation with an electron of ≥13 kV in acceleration voltage is ≥2 times as large as a second intensity at a second peak observed on a longer-wavelength side than the first peak. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a gallium nitride substrate. In particular, the present invention relates to a gallium nitride substrate capable of manufacturing a device having excellent characteristics with high reproducibility.

  2. Description of the Related Art Conventionally, attempts have been made to improve damage in a shallow portion from the surface of a gallium nitride substrate in order to produce an electronic device having excellent characteristics by forming a predetermined nitride semiconductor on the gallium nitride substrate. As a method for evaluating the damage on the surface of the gallium nitride substrate, for example, a damage evaluation method for performing photoluminescence (PL) measurement on the surface of the compound semiconductor substrate has been proposed (see, for example, Patent Document 1).

  In the damage evaluation method described in Patent Document 1, in the emission spectrum obtained by photoluminescence measurement, the half width of the peak at the wavelength corresponding to the band gap of the compound semiconductor substrate is used, and several tens of nm from the surface of the compound semiconductor substrate. The damage of depth is evaluated.

JP 2006-339605 A

  However, even in the case of a gallium nitride (GaN) substrate that has been successfully evaluated by the damage evaluation method described in Patent Document 1, the characteristics of the epitaxial layer grown on the substrate cannot be used for device fabrication. There is a case.

  Therefore, an object of the present invention is to provide a GaN substrate capable of manufacturing a device having excellent characteristics with high reproducibility.

  In order to achieve the above object, the present invention provides a first peak of a first peak at a wavelength corresponding to a band gap of gallium nitride in a spectrum of cathodoluminescence obtained when electrons having an acceleration voltage of 13 kV or more are irradiated. The gallium nitride substrate is provided whose intensity is at least twice the second intensity of the second peak observed on the longer wavelength side than the first peak.

  In order to achieve the above object, the present invention provides a first peak at a wavelength corresponding to the band gap of gallium nitride in the spectrum of cathodoluminescence obtained when electrons having an acceleration voltage of 13 kV or higher are irradiated. A gallium nitride substrate is provided in which the first intensity is five times or more the second intensity of the second peak observed on the longer wavelength side than the first peak.

  According to the gallium nitride substrate of the present invention, it is possible to provide a GaN substrate capable of manufacturing a device having excellent characteristics with good reproducibility.

[Embodiment]
When manufacturing a nitride-based high-performance device, a nitride-based semiconductor layer and / or an electrode are formed on a gallium nitride (GaN) substrate. Device characteristics (for example, electrical characteristics) are strongly influenced by the interface between the GaN substrate and the nitride-based semiconductor layer and / or the interface between the GaN substrate and the electrode. Therefore, the characteristics of the surface of the GaN substrate, particularly the degree of damage near the surface of the GaN substrate is important. Since a GaN substrate is manufactured through a plurality of manufacturing processes, damage caused in each process is included in the GaN substrate.

  For example, when the surface of the GaN substrate is rough, that is, when the surface roughness is a predetermined value or more, defects and the like are inherited in the semiconductor layer grown on the GaN substrate. When defects or the like are inherited in the grown semiconductor layer, the device characteristics are deteriorated due to the inherited defects or the like, so that the GaN substrate surface is subjected to polishing treatment or etching treatment. At this time, damage such as scratches and / or processing distortion occurs on the surface of the GaN substrate and in the vicinity of the surface. When a device is manufactured by growing an epitaxial film such as a nitride semiconductor on the surface of the GaN substrate where such damage exists on the surface, the device is caused by damage existing at the interface between the GaN substrate and the epitaxial film. Characteristics deteriorate.

  Here, the present inventor found that the damage in the region having a depth of about 1 μm from the surface of the GaN substrate greatly affects the characteristics of the nitride compound semiconductor layer formed on the GaN substrate, that is, the device characteristics. The knowledge that it is. And the knowledge that a device with excellent reproducibility and excellent characteristics can be manufactured by using a GaN substrate with reduced damage in a region having a depth of about 1 μm from the surface of the GaN substrate was obtained. In addition, whether it is a GaN substrate according to the present embodiment as described above can be specified by cathodoluminescence (CL) measurement.

(Structure of GaN substrate)
In the GaN substrate according to the present embodiment, the first intensity of the first peak at the wavelength corresponding to the band gap of GaN out of the CL spectrum obtained when electrons having an acceleration voltage of 13 kV or higher are irradiated. , 2 times or more, or 5 times or more of the second intensity of the second peak observed on the longer wavelength side than the first peak.

  The GaN substrate according to the present embodiment is used for process steps of manufacturing electronic devices including light emitting elements such as light emitting diodes and laser diodes. For example, a GaN substrate according to this embodiment is used as a base substrate, and a predetermined compound semiconductor, for example, a predetermined nitride-based compound semiconductor layer is epitaxially grown on the GaN substrate as the base substrate. Thereby, a device structure layer having a predetermined structure is formed on the GaN substrate according to the present embodiment. The nitride compound semiconductor layer is formed by using a vapor phase growth method such as an organic metal growth method (MOVPE method), a molecular beam vapor phase growth method (MBE method), or a hydride vapor phase growth method (HVPE method). It can be grown by epitaxial growth on it.

(Method for manufacturing GaN substrate)
The GaN ingot from which the GaN substrate according to the present embodiment is cut out is, for example, a low-temperature buffer growth method, an epitaxial lateral overgrowth (ELO) method, a facet-initiated epitaxy lateral overgrowth (FIERO) method, or a displacement eligibility method. It can be grown using an inverted-pyramidal pits (DEEP) method or the like.

  The low temperature buffer growth method includes a step of forming a low temperature deposition buffer layer (buffer layer) on a heterogeneous substrate such as sapphire and a step of forming an epitaxial growth layer on the buffer layer. More specifically, in the low temperature buffer growth method, a low temperature buffer layer deposition step of depositing an aluminum nitride (AlN) layer or a GaN layer on a substrate such as sapphire at around 500 ° C., and a low temperature buffer layer deposition step, A continuous film forming step of forming an amorphous film or a continuous film of GaN containing polycrystalline in a part, and the formed continuous film is heated to around 1000 ° C. to evaporate or crystallize a part of the continuous film A crystal nucleation step for forming high-density crystal nuclei, and a GaN film growth step for growing a GaN film using the crystal nuclei as growth nuclei.

  The ELO method is a growth method including a mask forming step of forming a mask having an opening on a base substrate such as a sapphire substrate and a lateral growth step of laterally growing GaN from the opening of the mask. In the ELO method, GaN is formed on a sapphire substrate and then removed by etching or the like.

  The FIELO method is common to the ELO method in that a silicon oxide mask is used to form a mask having an opening on a base substrate such as a sapphire substrate and is selectively grown. However, the FIELO method reduces the threading dislocations reaching the upper surface of the epitaxial growth layer by changing the propagation direction of dislocations generated during crystal growth to a predetermined direction by forming facets at the opening of the mask during selective growth. It differs from the ELO method in that it is a growth method. In the FIELO method, for example, a thick GaN layer is grown on a base substrate such as sapphire. Subsequently, the base substrate is removed. Thereby, a high-quality GaN single crystal with few crystal defects is formed.

  The DEEP method includes a patterning process for patterning a mask made of silicon nitride or the like on a gallium arsenide (GaAs) substrate, and a growth process for growing GaN after the patterning process. In the DEEP method, first, a plurality of pits surrounded by facet planes are intentionally formed on the surface of a GaN crystal by growing GaN using a mask. When GaN continues to grow, dislocations accumulate at the bottom of the pit. Thereby, in the DEEP method, a region having a lower dislocation density than the bottom of the pit can be formed.

  After manufacturing the GaN ingot, the GaN ingot is sliced to manufacture a GaN substrate (before surface treatment). Subsequently, after slicing the surface of the GaN substrate obtained by slicing, polishing treatment with a predetermined abrasive having a predetermined average particle diameter and hardness and / or predetermined etching processing (dry etching processing) (And / or wet etching treatment), the GaN substrate according to the present embodiment can be manufactured.

  As an example, the surface treatment includes a step of performing wet etching after performing dry etching on the surface of the GaN substrate after rough polishing and polishing. Another example of the surface treatment includes a step of polishing the surface after rough polishing with a predetermined abrasive having a predetermined particle size and then subjecting the surface to a wet etching process with a predetermined etchant. Further, in the polishing of the surface after the rough polishing, the polishing is performed from the surface of the GaN substrate after the rough polishing to a predetermined depth (for example, around 10 μm). Further, as an example, the surface treatment may employ a cleaning process using an organic solvent such as isopropyl alcohol on the surface of the GaN substrate after rough polishing and polishing.

  Whether or not the substrate is a GaN substrate according to the present embodiment can be determined by measuring damage near the surface of the GaN substrate using cathodoluminescence measurement (CL measurement).

  In the CL measurement, the depth from the surface of the GaN substrate as a measurement object to the end of the region where damage can be evaluated is the depth of penetration as the depth at which the irradiated electron beam enters from the surface of the GaN substrate. It is decided accordingly. The penetration depth of the electron beam is determined according to the acceleration voltage of electrons of the electron beam to be irradiated and the atomic weight, atomic number, and density of the measurement object. Specifically, the electron penetration depth is determined by the following “Equation 1” (see, for example, Jpn. J. Appl. Phys., Vol. 40 (2001), pp. 476-479).

Here, Re is the electron penetration depth (μm), A is the atomic weight of the measurement object, ρ is the density (g / cm ³ ) of the measurement object, Z is the atomic number of the measurement object, Eb Is the acceleration voltage (kV) of electrons.

  In the CL measurement, damage existing in the GaN crystal from the surface of the GaN substrate to a predetermined depth is evaluated. In “Formula 1”, the atomic weight, density, and atomic number are values inherent to the measurement object. Therefore, if the measurement object is determined, these values are uniquely determined. Therefore, the depth at which electrons enter from the surface of the GaN substrate is determined according to the acceleration voltage of the electrons applied to the GaN substrate. Then, CL measurement is performed by irradiating the GaN substrate with an electron beam having a predetermined acceleration voltage. For example, from the CL spectrum, the intensity of the wavelength peak (first peak) corresponding to the band gap of the GaN crystal constituting the GaN substrate and the longer wavelength side than the first peak are observed, and the band gap between GaN And the intensity of the peak (second peak) attributed to the level generated in.

  That is, in the CL measurement, the degree of damage at a depth that cannot be measured by the photoluminescence (PL) measurement from the surface of the GaN substrate is evaluated. For example, the peak intensity of the second peak measured in the CL measurement changes depending on the degree of damage that causes the light emission contained in the GaN substrate. Therefore, the intensity of the first peak and the intensity of the second peak By evaluating the intensity ratio, it is possible to grasp the extent of damage in a region from the GaN substrate surface to a predetermined depth (for example, a depth of about 1 μm).

  FIG. 1 shows an outline of a method for evaluating substrate damage by CL measurement.

  In the CL measurement, the intensity I1 of the first peak 50 at the wavelength λ1 corresponding to the band gap of the measurement object and the intensity I2 of the second peak 52 at the wavelength λ2 longer than the wavelength λ1 are detected. Subsequently, an intensity ratio (I1 / I2) between the detected intensity I1 and intensity I2 is calculated, and crystallinity is evaluated based on the calculated intensity ratio. For example, the intensity ratio between the intensity of the first peak and the intensity of the second peak tends to decrease as the degree of damage in the region from the GaN substrate surface to a predetermined depth increases.

  In CL measurement, the intensity of the first peak 50 at a wavelength corresponding to the band gap of GaN is longer than that of the first peak 50 in the CL spectrum obtained when an electron having an acceleration voltage of 13 kV or higher is irradiated. When the intensity of the second peak 52 observed on the wavelength side is 2 times or more, or 5 times or more, the GaN substrate according to this embodiment can be specified.

  FIG. 2 shows the relationship between the penetration depth of electrons into the GaN substrate and the acceleration voltage of electrons.

  In the GaN substrate as the measurement object, the relationship between the electron penetration depth into the GaN substrate and the acceleration voltage can be obtained from “Expression 1” in the above description. Referring to FIG. 2, when the acceleration voltage of electrons is 13 kV or more, the penetration depth of electrons to the GaN substrate is 1 μm or more. Therefore, in the CL measurement, when the acceleration voltage of the irradiated electrons is set to 13 kV or more, damage in a region having a depth of 1 μm or more from the surface of the GaN substrate can be evaluated. Can be identified.

  In this case, the peak near the wavelength 365 nm corresponding to the band gap of GaN becomes the first peak 50 described in FIG. 1, and the peak near the wavelength 500 nm to 700 nm is the second peak 52 described in FIG. It becomes.

  In the emission spectrum obtained by CL measurement of the surface of the GaN substrate when the acceleration voltage of the electrons to be irradiated is set to 13 kV or more, the GaN substrate according to the present embodiment has an intensity I1 of the first peak 50 of the second peak. The peak 52 has an intensity I2 of 2 times or more, more preferably 5 times or more. Such a GaN substrate according to the present embodiment can be used as a GaN substrate capable of producing a device having good characteristics with good reproducibility with little damage to the crystal when the depth from the substrate surface is about 1 μm.

(Effect of embodiment)
Since the GaN substrate according to the embodiment of the present invention has little damage at a depth of about 1 μm from the substrate surface, an epitaxial layer in which defects caused by damage at a depth of about 1 μm from the substrate surface are grown on the substrate. It is possible to reduce the occurrence (or propagation) of the inside. Thereby, it is possible to provide a GaN substrate capable of manufacturing a high-quality device with high reproducibility.

[Examples and Comparative Examples]
In Examples and Comparative Examples described below, different surface treatments were applied to GaN single crystal substrates (diameter: 2 inches) obtained by slicing GaN single crystal ingots obtained by a predetermined method. And CL measurement and substrate damage evaluation were implemented about each of each GaN single crystal substrate after surface treatment. Furthermore, after a predetermined GaN thin film was formed on each GaN single crystal substrate according to the example and the comparative example, the surface roughness was also measured. Note that some examples and some comparative examples were also evaluated by photoluminescence. Details will be described below.

First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the rough polished surface was polished by 10 μm using diamond abrasive grains having a particle size of 0.1 μm. Further, the polished surface was dry etched using a reactive ion etching method (Reactive Ion Etching method: RIE method). The RIE conditions are: etching gas: Cl ₂ gas, supply etching gas flow rate: 50 sccm, supply power: 200 W, chamber pressure: 10 Pa, etching time: 60 minutes.

After RIE, the GaN single crystal substrate was immersed in a 5% NH ₄ OH solution at 80 ° C. for 60 minutes for the purpose of removing damage on the etched surface, and wet etched. As a result, a GaN single crystal substrate according to Example 1 was obtained.

The GaN single crystal substrate according to Example 2 was obtained as follows. First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the surface was polished by 10 μm using diamond abrasive grains having a particle diameter of 1 μm. Further, the surface was washed with isopropyl alcohol. Subsequently, the GaN single crystal substrate was wet etched by immersing it in a 5% NH ₄ OH solution at 80 ° C. for 900 minutes. Thus, a GaN single crystal substrate according to Example 2 was obtained.

The GaN single crystal substrate according to Example 3 was obtained as follows. First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the surface was polished by 10 μm using diamond abrasive grains having a particle diameter of 1 μm. Further, the surface was washed with isopropyl alcohol. Subsequently, the GaN single crystal substrate was wet-etched by immersing it in a 5% NH ₄ OH solution at 80 ° C. for 780 minutes. Thus, a GaN single crystal substrate according to Example 3 was obtained.

The GaN single crystal substrate according to Example 4 was obtained as follows. First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the surface was polished by 10 μm using diamond abrasive grains having a particle diameter of 1 μm. Further, the surface was washed with isopropyl alcohol. Subsequently, the GaN single crystal substrate was wet-etched by immersing in a 5% NH ₄ OH solution at 80 ° C. for 660 minutes. In this way, a GaN single crystal substrate according to Example 4 was obtained.

The GaN single crystal substrate according to Example 5 was obtained as follows. First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the surface was polished by 10 μm using diamond abrasive grains having a particle diameter of 1 μm. Further, the surface was washed with isopropyl alcohol. Subsequently, the GaN single crystal substrate was wet etched by immersing it in a 5% NH ₄ OH solution at 80 ° C. for 540 minutes. In this way, a GaN single crystal substrate according to Example 5 was obtained.

The GaN single crystal substrate according to Example 6 was obtained as follows. First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the rough polished surface was polished by 10 μm using diamond abrasive grains having a particle diameter of 1 μm. Further, the surface was washed with isopropyl alcohol. Subsequently, the GaN single crystal substrate was wet etched by immersing it in a 5% NH ₄ OH solution at 80 ° C. for 480 minutes. In this way, a GaN single crystal substrate according to Example 6 was obtained.

(Comparative Example 1)
The GaN single crystal substrate according to Comparative Example 1 was obtained as follows. First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the rough polished surface was polished 10 μm using diamond abrasive grains having a particle diameter of 0.8 μm. Subsequently, the surface was washed with isopropyl alcohol. Thereby, a GaN single crystal substrate according to Comparative Example 1 was obtained.

(Comparative Example 2)
A GaN single crystal substrate according to Comparative Example 2 was obtained in the same manner as Comparative Example 1 except that diamond abrasive grains having a particle diameter of 1 μm were used instead of diamond abrasive grains having a particle diameter of 0.8 μm.

(Comparative Example 3)
A GaN single crystal substrate according to Comparative Example 3 was obtained in the same manner as Comparative Example 1 except that diamond abrasive grains having a particle diameter of 3 μm were used instead of diamond abrasive grains having a particle diameter of 0.8 μm.

(Comparative Example 4)
First, the surface of a GaN single crystal substrate obtained by slicing a GaN single crystal ingot was roughly polished. Next, the roughly polished surface was polished by 10 μm using diamond abrasive grains having a particle size of 3 μm. Subsequently, the surface was washed with isopropyl alcohol. Next, this substrate was annealed to obtain a GaN single crystal substrate according to Comparative Example 4. The annealing conditions are normal pressure, substrate temperature 1,100 ° C., N ₂ gas 9 L / min, NH ₃ gas 1 L / min, and the annealing time is 50 hours.

(Cathodeluminescence measurement)
FIG. 3 shows an example of the cathodoluminescence emission spectrum of the GaN single crystal substrate according to the embodiment of the present invention.

  For the cathodoluminescence measurement of the GaN single crystal substrate according to this example, an apparatus in which a device for detecting and spectroscopically analyzing an emission spectrum obtained by cathodoluminescence was used in a scanning electron microscope. Specifically, S-3000N manufactured by Hitachi High-Technologies was used as the scanning electron microscope, and MP-32M manufactured by HORIBA, Ltd. was used as the spectrum detection apparatus as a device for detecting and spectrally analyzing the emission spectrum. The emission voltage of each GaN single crystal substrate according to the example and the comparative example was obtained by setting the acceleration voltage of the electron beam (electron beam) of the scanning electron microscope to 13 kV.

  FIG. 3 specifically shows an emission spectrum obtained from the GaN single crystal substrate according to Example 4. The vertical axis represents the cathodoluminescence (CL) intensity, and the horizontal axis represents the wavelength (unit: nm). The CL intensity in FIG. 3 is a relative value where the intensity I1 of the first peak 50 near 365 nm is 1. In addition, a broad second peak 52 was observed near 500 nm to 700 nm on the longer wavelength side than 365 nm. Here, the intensity of the second peak 52 is I2.

  Table 1 shows the intensity ratio I1 / I2 between the first peak 50 and the second peak 52 in the emission spectrum of CL obtained from the GaN single crystal substrates according to Examples and Comparative Examples of the present invention. Shown for each acceleration voltage. The acceleration voltage was set to 13 kV, 21 kV, and 30 kV, and CL measurement was performed.

(Photoluminescence measurement)
Table 2 shows the peak intensity ratio between the first peak and the second peak in the emission spectrum of the photoluminescence obtained from the GaN single crystal substrates according to some examples and some comparative examples of the present invention. .

  For photoluminescence measurement, RPM2000 manufactured by Nanometrics was used. PL emission spectra of the GaN single crystal substrates of Example 6 and Comparative Example 4 were obtained using a He—Cd laser light source. The peak intensity of the first peak near the wavelength 365 nm corresponding to the band gap of the GaN substrate is I3, and the peak intensity of the second peak located on the longer wavelength side than the peak of wavelength 365 nm corresponding to the band gap of the GaN substrate is I4. Table 2 shows the results of obtaining the peak intensity ratio I3 / I4 in the emission spectra obtained from the GaN single crystal substrates of Example 6 and Comparative Example 4.

(Surface roughness measurement)
Table 3 shows the results of measuring the roughness of the substrate surface after the GaN thin film was formed on the GaN single crystal substrates according to Examples and Comparative Examples of the present invention.

First, a GaN thin film having a thickness of 1 μm was formed on the surfaces of the GaN single crystal substrates of Examples 1 to 6 and Comparative Examples 1 to 4 using the Halide Vapor Phase Epitaxy (HVPE) method. The raw materials used for the growth of the HVPE method were NH ₃ and GaCl, and a mixed gas of 5% H ₂ and 95% N ₂ was used as a carrier gas. The growth conditions were normal pressure and a substrate temperature of 1050 ° C. The partial pressure of NH ₃ in the supply gas was 5 × 10 ⁻³ atm, and the partial pressure of GaCl in the supply gas was 1 × 10 ⁻³ atm. And after forming a GaN thin film, the surface roughness (RMS: root mean square roughness) of the GaN thin film was measured with the atomic force microscope (AFM).

  As can be seen by referring to Tables 1 and 3, the GaN unit according to Examples 1 to 4 in which the intensity ratio I1 / I2 is 5 or more when the acceleration voltage of the electron beam is 13 kV, 21 kV, or 30 kV. According to the crystal substrate, the root mean square roughness of the surface of the GaN thin film grown thereon was 0.5 nm or less.

  In addition, when the acceleration voltage of the electron beam is 13 kV, 21 kV, or 30 kV, the GaN single crystal substrate according to Examples 1 to 6 in which the intensity ratio I1 / I2 is 2 or more grows thereon. The root mean square roughness of the surface of the GaN thin film was 1 nm or less.

  From the above, it can be seen that the GaN single crystal substrates according to Examples 1 to 6 have little surface roughness of the GaN thin film even when the GaN thin film is formed on the surface thereof. That is, it was shown that the GaN single crystal substrates according to Examples 1 to 6 can be sufficiently used practically as a base substrate for electronic devices and the like.

  On the other hand, in the GaN single crystal substrates according to Comparative Examples 1 to 4, as shown in Tables 1 and 3, the intensity ratio is obtained when the electron beam acceleration voltage is 13 kV, 21 kV, and 30 kV. According to the GaN single crystal substrates according to Comparative Examples 1 to 4 in which I1 / I2 is less than 2, the root mean square roughness of the surface of the GaN thin film formed thereon is 1 nm or more. It was shown that it was difficult to use in practical use. This is presumably because the extent of damage in the region having a depth of about 1 μm from the surface of the GaN single crystal substrate according to Comparative Examples 1 to 4 is large.

  Further, referring to Table 2, in the GaN single crystal substrate according to Comparative Example 4, the intensity ratio of I3 / I4 in the photoluminescence measurement is 5 or more, but in the GaN single crystal substrate according to Example 6, The intensity ratio of I3 / I4 in the photoluminescence measurement was 2 or less. Then, referring to Table 3, the root mean square roughness of the surface of the GaN thin film formed on the GaN single crystal substrate according to Comparative Example 4 is 2.34 nm, that is, 1 nm or more. This is presumably because the extent of damage in a region having a depth of about 1 μm from the surface of the GaN single crystal substrate according to Comparative Example 4 is large.

Here, the depth from the surface of the GaN substrate that can be evaluated by the PL measurement is determined by the penetration depth of light applied to the GaN substrate to be measured. This penetration depth is generally determined by the wavelength of the light source to be irradiated and the absorption coefficient of the substrate to be evaluated. Here, as a light source usually used in PL measurement, there are a He-Cd laser (wavelength 325 nm) and a YAG laser (wavelength 266 nm), and the absorption coefficient of GaN of the He-Cd laser having a wavelength of 325 nm and the YAG laser having a wavelength of 266 nm is 12.5 × 10 ⁴ cm ⁻¹ (He—Cd laser) and 18.5 × 10 ⁴ cm ⁻¹ (YAG laser).

Since the reciprocal of the absorption coefficient is the penetration depth, the penetration depth of the He-Cd laser with respect to the GaN substrate is 1 / 12.5 / 10 ⁴ × 10 ⁷ = 80 nm, and the penetration depth of the YAG laser with respect to the GaN substrate is 1 / 18.5 / 10 ⁴ × 10 ⁷ = 54 nm. As a result, both the penetration depths are 0.1 μm or less, and the depth from the GaN substrate surface evaluated by PL measurement is also very shallow, 0.1 μm or less. Therefore, in the PL measurement of the GaN substrate, damage in a very shallow region from the surface is evaluated.

  Therefore, even a GaN substrate that is judged to have a small degree of damage on the outermost surface by PL measurement that evaluates only a very shallow region, the damage existing to a depth of about 1 μm from the surface cannot be grasped. Even when a GaN substrate that is well evaluated by PL measurement is used, good device characteristics may not be obtained with good reproducibility, not only on the surface, but also on damage at a depth of about 1 μm from the surface. This is because damage such as defects is propagated to the epitaxial layer formed on the GaN substrate.

(Effect of Example)
In an emission spectrum obtained by CL measurement performed with an acceleration voltage of irradiated electrons of 13 kV or higher, the peak intensity at the wavelength corresponding to the band gap of GaN is located on the longer wavelength side than the wavelength corresponding to the band gap. A GaN substrate according to an embodiment of the present invention having a peak intensity of 2 times or more, more preferably 5 times or more, has sufficient performance as a substrate used in a device. By using the GaN substrate according to such an example for manufacturing an electronic device, a device with good performance can be obtained with good reproducibility.

  While the embodiments and examples of the present invention have been described above, the embodiments and examples described above do not limit the invention according to the claims. It should be noted that not all combinations of features described in the embodiments and examples are necessarily essential to the means for solving the problems of the invention.

It is an outline figure of a method of evaluation of substrate damage. It is a figure which shows the relationship between the penetration depth of an electron with respect to a GaN substrate, and the acceleration voltage of an electron. It is a figure which shows an example of the cathodoluminescence emission spectrum of the GaN single crystal substrate which concerns on an Example.

Explanation of symbols

50 First peak 52 Second peak

Claims (2)

  Of the cathodoluminescence spectrum obtained when electrons having an acceleration voltage of 13 kV or higher are irradiated, the first intensity of the first peak at a wavelength corresponding to the band gap of gallium nitride is longer than that of the first peak. A gallium nitride substrate that is at least twice the second intensity of the second peak observed on the wavelength side.   Of the cathodoluminescence spectrum obtained when electrons having an acceleration voltage of 13 kV or higher are irradiated, the first intensity of the first peak at a wavelength corresponding to the band gap of gallium nitride is longer than that of the first peak. A gallium nitride substrate that is at least five times the second intensity of the second peak observed on the wavelength side.

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