Thermal Cleaning Step S₃₁₀ First, thermal cleaning step S₃₁₀ can be performed on the GaAs single crystal substrate. Thermal cleaning step S₃₁₀ is a step of performing thermal cleaning under a convention-ally known condition such as a heating treatment at 600° C. for 10 minutes. Even under such a condition, since the separation temperature of the oxide film is low in the GaAs single crystal substrate, the oxide film can be effectively removed by the thermal cleaning.

Epitaxial Film Growth Step S₃₂₀ In addition, a step of growing an epitaxial film on the main surface of the GaAs single crystal substrate subjected to thermal cleaning step S₃₁₀ can be performed (epitaxial film growth step S₃₂₀). Through this step, a GaAs single crystal substrate in which an epitaxial film with a reduced number of LPDs is formed on the main surface can be obtained. For example, the number of LPDs present on the epitaxial film and each having a major axis of 18 µm or more can be 5 or less per cm2 of the main surface, and preferably 2 or less per cm2 of the main surface. The lower limit of the number of LPDs each having a major axis of 18 µm or more is zero per cm2 of the main surface. Thus, the GaAs single crystal substrate allows device characteristics to be improved. In epitaxial film formation step S300, as a method of forming an epitaxial film on the main surface of the GaAs single crystal substrate, a conventionally known method can be used. The epitaxial film may be, for example, a compound film formed of Al₁-y-zGayInzAs, where the y may be 0 to 1, the z may be 0 to 1, and the sum of the y and the z may be 0 to 1. In other words, in the present embodiment, the compound film formed of Al₁-y-zGayInzAs (0≤y≤1, 0≤z≤1, 0≤y+z≤1) can be applied as the epitaxial film formed on the main surface of the GaAs single crystal substrate. Further-more, the epitaxial film can be a compound film of AlxGa₁-x As (0≤x≤1) or Al₁-y-zGayInzP (0≤y≤1, 0≤z≤1, 0≤y+z≤1). The epitaxial film is formed so as to have a thickness of 0.5 to 10 µm, for example. When the thickness of the epitaxial film is in the above-mentioned range, the epitaxial substrate can be applied to a wide range of applications. The number of LPDs each having a major axis of 18 µm or more in the epitaxial film disposed on the main surface of the GaAs single crystal substrate can be determined by a conventionally known surface foreign matter inspection B₂ apparatus (for example, trade name: “Surfscan 6220”, manu-factured by KLA-Tencor Corporation) in the same manner as in the method of measuring the number of particles described above. A specific measurement method is the same as the above-described measurement method of the number of particles, and thus redundant description thereof will not be repeated. <Method of Manufacturing Second GaAs Single-Crystal substrate> The second method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including performing a new acid cleaning step (hereinafter also referred to as “second acid cleaning step”) using an acid solution to which an additive such as a surfactant is added, instead of the oxidation treatment step and the subsequent acid cleaning step, in the first method of manufacturing a GaAs single crystal substrate. Hereinafter, the second acid cleaning step will be described with refer-ence to FIG. 7. In the second method of manufacturing a GaAs single crystal substrate, the steps other than a second acid cleaning step S₂₃₁ in cleaning step S₂₀₀ (which replaces oxidation treatment step S₂₂₀ and subsequent acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Second Acid Cleaning Step S₂₃₁) Second acid cleaning step S₂₃₁ is a step of turning the alkali-cleaned surface into an acid-cleaned surface by clean-ing the alkali-cleaned surface with an acid cleaning liquid. In particular, the acid cleaning liquid contains both or one of an alcohol and a surfactant. Through second acid cleaning step S231, impurities in the alkali cleaning liquid attached to the alkali-cleaned surface of the GaAs single crystal substrate precursor and the gallium oxide film can be removed by an oxidation reaction (etching of the alkali-cleaned surface) with the acid cleaning liquid. Furthermore, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove particles from the alkali-cleaned surface, and facilitate removal of organic compo-nents in pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass to 5% by mass. An acid contained in the acid cleaning liquid (hereinafter also referred to as “second acid”) is hydrochloric acid, hydrofluoric acid, or nitric acid. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the second acid in the acid cleaning liquid is less than 2% by mass, the function of modifying the alkali-cleaned surface is reduced. When the acid concentration of the second acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or the main surface) tends to vary due to the function of the second acid. The second acid contained in the acid cleaning liquid is hydrochloric (HCl), hydroflu-oric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious secondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant is expected to have an effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor. In particular, since the hydrophilic group of an anionic surfactant is an anion (negative ion) and most of the particles on the surface of the GaAs single crystal substrate precursor are negatively charged (zeta potential is negative), it is expected that the particles are less likely to adhere to the surface by controlling a potential of the surface to be negative. The addition of the alcohol is expected to have the effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor as well. Since the alcohol gen-erally has a high purity, there is also an advantage that an impurity concentration of the surface can be easily reduced. In second acid cleaning step S231, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the alkali-cleaned surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after second acid cleaning step S231, preferably immediately after second acid cleaning step S231, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manu-facturing a GaAs single crystal substrate, redundant descrip-tion thereof will not be repeated. <Method of Producing Third GaAs Single Crystal substrate> The third method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including, instead of the acid cleaning step in the first method of manufacturing a GaAs single crystal substrate, performing a new acid cleaning step (hereinafter also referred to as “third acid cleaning step”) with an acid solution to which an additive such as a surfactant is added to be able to increase the acid concentration of the acid cleaning liquid used in the acid cleaning step. Hereinafter, the third acid cleaning step will be described with reference to FIG. 8. In the third method of manufacturing a GaAs single crystal substrate, the steps other than the third acid cleaning step S₂₃₂ (which replaces acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) in cleaning step S₂₀₀ are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Third Acid Cleaning Step S₂₃₂) The third acid cleaning step S₂₃₂ is a step of turning the oxidized surface into an acid-cleaned surface by cleaning the oxidized surface with an acid cleaning liquid. Through the third acid cleaning step S232, the oxide film on the oxidized surface can be modified into an oxide film having a lower content of the gallium oxides. In particular, in the third acid cleaning step S232, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove impurities and particles remaining on the oxidized surface, and facilitate removal of organic components in the pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass B₂ to 5% by mass. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the acid in the acid cleaning liquid is less than 2% by mass, the number of particles to be removed is small, and the effect of the acid cleaning liquid containing both or one of the alcohol and the surfactant may not be fully obtained. When the acid con-centration of the acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or main surface) tends to vary due to the function of the acid. The acid contained in the acid cleaning liquid is hydrochloric (HCl), hydrofluoric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious sec-ondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant and the addition of the alcohol respectively have the same effects as the addition of the surfactant and the addition of the alcohol described for second acid cleaning step S₂₃₁ in the second method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. In third acid cleaning step S232, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the oxidized surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after third acid clean-ing step S232, preferably immediately after third acid clean-ing step S232, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, redundant description thereof will not be repeated. <Function and Effect> By the method of manufacturing a GaAs single crystal substrate according to the present embodiments (the first method of manufacturing a GaAs single crystal substrate, the second method of manufacturing a GaAs single crystal substrate, and the third method of manufacturing a GaAs single crystal substrate), a GaAs single crystal substrate in which an oxide film having a low content of gallium oxides can be formed on the main surface and the number of particles present on the main surface is small can be obtained. In such a GaAs single crystal substrate, since the separation temperature of the oxide film is low, the oxide film can be effectively removed by thermal cleaning, and failures in epitaxial film caused by the particles can also be reduced, so that an epitaxial film with a reduced number of LPDs can be formed thereon. Example Hereinafter, the present disclosure will be described in more detail with Examples, but the present disclosure is not limited thereto. Among the GaAs single crystal substrates from Sample 1 to Sample 18 described below, Sample 4, Sample 7 to Sample 10, and Sample 12 to Sample 18 are examples of the present disclosure, and Sample 1 to Sample 3, Sample 5, Sample 6, and Sample 11 are comparative examples. [Production of GaAs Single Crystal substrate] [Sample 1] <Preparation Step> A silicon (Si) atom-doped conductive GaAs single crystal grown by the vertical Bridgman (VB) method was sliced with a wire saw, and the edge portion was ground to prepare a GaAs single crystal substrate precursor. The main surface of the GaAs single crystal substrate precursor was ground with a surface grinding machine, and then the main surface was polished with a hard polishing cloth including a mixture of a chlorine-based polishing agent and silica powders in a clean room. Subsequently, the main surface was mirror-finished by polishing with an INSEC NIB polishing agent (manufactured by FUJIMI INCORPORATED). Further-more, the main surface was roughly cleaned by performing ultrasonic cleaning with isopropyl alcohol (IPA). As described above, a required number of GaAs single crystal substrate precursors each having a diameter of 6 inches (150 mm) and a thickness of 680 µm were prepared. <Cleaning Step> (Alkali Cleaning Step) The GaAs single crystal substrate precursors were immersed in a 1 mass % choline aqueous solution by a vertical-batch method. At the same time, ultrasonic waves were applied to the entire surfaces of the GaAs single crystal substrate precursors for 5 minutes under a condition of a frequency of 500 kHz and a sound pressure of 10 mV. In this manner, the surfaces of the GaAs single crystal substrate precursors were alkali-cleaned. Next, the alkali-cleaned sur-faces of the GaAs single crystal substrate precursors were cleaned with pure water having a dissolved oxygen concen-tration (DO) of 1 ppb by mass for 5 minutes. The total organic carbon (TOC) of this pure water was 0.5 ppb by mass. (Acid Cleaning Step) The GaAs single crystal substrate precursors were placed in a single-wafer cleaning apparatus, and the alkali-cleaned surfaces were subjected to acid cleaning with an acid cleaning liquid containing 0.05% by mass of hydrofluoric acid while being rotated at a rotation speed 500 rpm. In the acid cleaning, at room temperature (25° C.), 1 liter of the acid cleaning liquid was supplied to the alkali-cleaned surfaces of the GaAs single crystal substrate precursors for 1 minute. Furthermore, the GaAs single crystal substrate precursors were rinsed with the same ultrapure water as used in the alkali cleaning step for 3 minutes at a supply rate of 1 L/min. Through this step, the alkali-cleaned surfaces were regarded as acid-cleaned surfaces. (Drying Step) The acid-cleaned surfaces of the GaAs single crystal substrate precursors were rotated at 2500 rpm for 15 to 30 seconds to be dried, and a required number of GaAs single crystal substrates for Sample 1 having a main surface having a circular shape were obtained. <Epitaxial Film Formation Step> The GaAs single crystal substrate was subjected to ther-mal cleaning in a metal-organic vapor phase epitaxial growth furnace under a condition of 600° C. and 10 minutes in an atmosphere in which arsine gas was added to hydrogen gas (thermal cleaning step). Furthermore, subsequently to the thermal cleaning step, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface of the GaAs single crystal substrate by a metal-organic vapor phase epitaxial growth method B₂ (MOVPE method) (epitaxial film growth step; the GaAs single crystal substrate having the epitaxial layer grown on the main surface is hereinafter also referred to as “epitaxial substrate”). Thus, epitaxial substrate for Sample 1 was obtained. When the epitaxial layer was grown, the GaAs single crystal substrate was heated to 550° C. [Sample 2] A required number of GaAs single crystal substrates for Sample 2 were obtained in the same manner as in Sample 1 except that the following oxidation treatment step was performed between the alkali cleaning step and the acid cleaning step in the cleaning step. Furthermore, for one of the GaAs single crystal substrates, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface thereof in the same manner as in Sample

A gallium arsenide single crystal substrate comprising a main surface having a circular shape, wherein the gallium arsenide single crystal substrate has 60 a first analytical value or a second analytical value, the first analytical value and the second analytical value are each obtained by determining a spectrum of a detected intensity of a 3d electron of each of arsenic and gallium with respect to binding energy of a pho-toelectron emitted to an outside of the gallium arsenide single crystal substrate based on X-ray photoelectron B₂ spectroscopy in which a center of the main surface is irradiated with X-rays under each of the following five different conditions and subjecting the spectrum to a maximum smoothness method which is a mathematical analysis method, the first analytical value is a value representing a content of gallium oxides present as digallium monoxide and digallium trioxide in a region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 0.6 nm or less, the second analytical value is a value representing a ratio of the content of the gallium oxides to a content of arsenic oxides present as diarsenic pentoxide and diarsenic trioxide in the region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 1.4 or less, the number of particles present on the main surface and each having a major axis of 0.16 µm or more is 2 or less per cm2 of the main surface, and the five different conditions are the following condition 1, condition 2, condition 3, condition 4, and condition 5, Condition 1: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 30° Condition 2: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 45° Condition 3: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 85° Condition 4: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 45° Condition 5: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 85°.

Thermal Cleaning Step S₃₁₀ First, thermal cleaning step S₃₁₀ can be performed on the GaAs single crystal substrate. Thermal cleaning step S₃₁₀ is a step of performing thermal cleaning under a convention-ally known condition such as a heating treatment at 600° C. for 10 minutes. Even under such a condition, since the separation temperature of the oxide film is low in the GaAs single crystal substrate, the oxide film can be effectively removed by the thermal cleaning.

Epitaxial Film Growth Step S₃₂₀ In addition, a step of growing an epitaxial film on the main surface of the GaAs single crystal substrate subjected to thermal cleaning step S₃₁₀ can be performed (epitaxial film growth step S₃₂₀). Through this step, a GaAs single crystal substrate in which an epitaxial film with a reduced number of LPDs is formed on the main surface can be obtained. For example, the number of LPDs present on the epitaxial film and each having a major axis of 18 µm or more can be 5 or less per cm2 of the main surface, and preferably 2 or less per cm2 of the main surface. The lower limit of the number of LPDs each having a major axis of 18 µm or more is zero per cm2 of the main surface. Thus, the GaAs single crystal substrate allows device characteristics to be improved. In epitaxial film formation step S300, as a method of forming an epitaxial film on the main surface of the GaAs single crystal substrate, a conventionally known method can be used. The epitaxial film may be, for example, a compound film formed of Al₁-y-zGayInzAs, where the y may be 0 to 1, the z may be 0 to 1, and the sum of the y and the z may be 0 to 1. In other words, in the present embodiment, the compound film formed of Al₁-y-zGayInzAs (0≤y≤1, 0≤z≤1, 0≤y+z≤1) can be applied as the epitaxial film formed on the main surface of the GaAs single crystal substrate. Further-more, the epitaxial film can be a compound film of AlxGa₁-x As (0≤x≤1) or Al₁-y-zGayInzP (0≤y≤1, 0≤z≤1, 0≤y+z≤1). The epitaxial film is formed so as to have a thickness of 0.5 to 10 µm, for example. When the thickness of the epitaxial film is in the above-mentioned range, the epitaxial substrate can be applied to a wide range of applications. The number of LPDs each having a major axis of 18 µm or more in the epitaxial film disposed on the main surface of the GaAs single crystal substrate can be determined by a conventionally known surface foreign matter inspection B₂ apparatus (for example, trade name: “Surfscan 6220”, manu-factured by KLA-Tencor Corporation) in the same manner as in the method of measuring the number of particles described above. A specific measurement method is the same as the above-described measurement method of the number of particles, and thus redundant description thereof will not be repeated. <Method of Manufacturing Second GaAs Single-Crystal substrate> The second method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including performing a new acid cleaning step (hereinafter also referred to as “second acid cleaning step”) using an acid solution to which an additive such as a surfactant is added, instead of the oxidation treatment step and the subsequent acid cleaning step, in the first method of manufacturing a GaAs single crystal substrate. Hereinafter, the second acid cleaning step will be described with refer-ence to FIG. 7. In the second method of manufacturing a GaAs single crystal substrate, the steps other than a second acid cleaning step S₂₃₁ in cleaning step S₂₀₀ (which replaces oxidation treatment step S₂₂₀ and subsequent acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Second Acid Cleaning Step S₂₃₁) Second acid cleaning step S₂₃₁ is a step of turning the alkali-cleaned surface into an acid-cleaned surface by clean-ing the alkali-cleaned surface with an acid cleaning liquid. In particular, the acid cleaning liquid contains both or one of an alcohol and a surfactant. Through second acid cleaning step S231, impurities in the alkali cleaning liquid attached to the alkali-cleaned surface of the GaAs single crystal substrate precursor and the gallium oxide film can be removed by an oxidation reaction (etching of the alkali-cleaned surface) with the acid cleaning liquid. Furthermore, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove particles from the alkali-cleaned surface, and facilitate removal of organic compo-nents in pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass to 5% by mass. An acid contained in the acid cleaning liquid (hereinafter also referred to as “second acid”) is hydrochloric acid, hydrofluoric acid, or nitric acid. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the second acid in the acid cleaning liquid is less than 2% by mass, the function of modifying the alkali-cleaned surface is reduced. When the acid concentration of the second acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or the main surface) tends to vary due to the function of the second acid. The second acid contained in the acid cleaning liquid is hydrochloric (HCl), hydroflu-oric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious secondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant is expected to have an effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor. In particular, since the hydrophilic group of an anionic surfactant is an anion (negative ion) and most of the particles on the surface of the GaAs single crystal substrate precursor are negatively charged (zeta potential is negative), it is expected that the particles are less likely to adhere to the surface by controlling a potential of the surface to be negative. The addition of the alcohol is expected to have the effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor as well. Since the alcohol gen-erally has a high purity, there is also an advantage that an impurity concentration of the surface can be easily reduced. In second acid cleaning step S231, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the alkali-cleaned surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after second acid cleaning step S231, preferably immediately after second acid cleaning step S231, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manu-facturing a GaAs single crystal substrate, redundant descrip-tion thereof will not be repeated. <Method of Producing Third GaAs Single Crystal substrate> The third method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including, instead of the acid cleaning step in the first method of manufacturing a GaAs single crystal substrate, performing a new acid cleaning step (hereinafter also referred to as “third acid cleaning step”) with an acid solution to which an additive such as a surfactant is added to be able to increase the acid concentration of the acid cleaning liquid used in the acid cleaning step. Hereinafter, the third acid cleaning step will be described with reference to FIG. 8. In the third method of manufacturing a GaAs single crystal substrate, the steps other than the third acid cleaning step S₂₃₂ (which replaces acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) in cleaning step S₂₀₀ are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Third Acid Cleaning Step S₂₃₂) The third acid cleaning step S₂₃₂ is a step of turning the oxidized surface into an acid-cleaned surface by cleaning the oxidized surface with an acid cleaning liquid. Through the third acid cleaning step S232, the oxide film on the oxidized surface can be modified into an oxide film having a lower content of the gallium oxides. In particular, in the third acid cleaning step S232, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove impurities and particles remaining on the oxidized surface, and facilitate removal of organic components in the pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass B₂ to 5% by mass. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the acid in the acid cleaning liquid is less than 2% by mass, the number of particles to be removed is small, and the effect of the acid cleaning liquid containing both or one of the alcohol and the surfactant may not be fully obtained. When the acid con-centration of the acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or main surface) tends to vary due to the function of the acid. The acid contained in the acid cleaning liquid is hydrochloric (HCl), hydrofluoric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious sec-ondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant and the addition of the alcohol respectively have the same effects as the addition of the surfactant and the addition of the alcohol described for second acid cleaning step S₂₃₁ in the second method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. In third acid cleaning step S232, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the oxidized surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after third acid clean-ing step S232, preferably immediately after third acid clean-ing step S232, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, redundant description thereof will not be repeated. <Function and Effect> By the method of manufacturing a GaAs single crystal substrate according to the present embodiments (the first method of manufacturing a GaAs single crystal substrate, the second method of manufacturing a GaAs single crystal substrate, and the third method of manufacturing a GaAs single crystal substrate), a GaAs single crystal substrate in which an oxide film having a low content of gallium oxides can be formed on the main surface and the number of particles present on the main surface is small can be obtained. In such a GaAs single crystal substrate, since the separation temperature of the oxide film is low, the oxide film can be effectively removed by thermal cleaning, and failures in epitaxial film caused by the particles can also be reduced, so that an epitaxial film with a reduced number of LPDs can be formed thereon. Example Hereinafter, the present disclosure will be described in more detail with Examples, but the present disclosure is not limited thereto. Among the GaAs single crystal substrates from Sample 1 to Sample 18 described below, Sample 4, Sample 7 to Sample 10, and Sample 12 to Sample 18 are examples of the present disclosure, and Sample 1 to Sample 3, Sample 5, Sample 6, and Sample 11 are comparative examples. [Production of GaAs Single Crystal substrate] [Sample 1] <Preparation Step> A silicon (Si) atom-doped conductive GaAs single crystal grown by the vertical Bridgman (VB) method was sliced with a wire saw, and the edge portion was ground to prepare a GaAs single crystal substrate precursor. The main surface of the GaAs single crystal substrate precursor was ground with a surface grinding machine, and then the main surface was polished with a hard polishing cloth including a mixture of a chlorine-based polishing agent and silica powders in a clean room. Subsequently, the main surface was mirror-finished by polishing with an INSEC NIB polishing agent (manufactured by FUJIMI INCORPORATED). Further-more, the main surface was roughly cleaned by performing ultrasonic cleaning with isopropyl alcohol (IPA). As described above, a required number of GaAs single crystal substrate precursors each having a diameter of 6 inches (150 mm) and a thickness of 680 µm were prepared. <Cleaning Step> (Alkali Cleaning Step) The GaAs single crystal substrate precursors were immersed in a 1 mass % choline aqueous solution by a vertical-batch method. At the same time, ultrasonic waves were applied to the entire surfaces of the GaAs single crystal substrate precursors for 5 minutes under a condition of a frequency of 500 kHz and a sound pressure of 10 mV. In this manner, the surfaces of the GaAs single crystal substrate precursors were alkali-cleaned. Next, the alkali-cleaned sur-faces of the GaAs single crystal substrate precursors were cleaned with pure water having a dissolved oxygen concen-tration (DO) of 1 ppb by mass for 5 minutes. The total organic carbon (TOC) of this pure water was 0.5 ppb by mass. (Acid Cleaning Step) The GaAs single crystal substrate precursors were placed in a single-wafer cleaning apparatus, and the alkali-cleaned surfaces were subjected to acid cleaning with an acid cleaning liquid containing 0.05% by mass of hydrofluoric acid while being rotated at a rotation speed 500 rpm. In the acid cleaning, at room temperature (25° C.), 1 liter of the acid cleaning liquid was supplied to the alkali-cleaned surfaces of the GaAs single crystal substrate precursors for 1 minute. Furthermore, the GaAs single crystal substrate precursors were rinsed with the same ultrapure water as used in the alkali cleaning step for 3 minutes at a supply rate of 1 L/min. Through this step, the alkali-cleaned surfaces were regarded as acid-cleaned surfaces. (Drying Step) The acid-cleaned surfaces of the GaAs single crystal substrate precursors were rotated at 2500 rpm for 15 to 30 seconds to be dried, and a required number of GaAs single crystal substrates for Sample 1 having a main surface having a circular shape were obtained. <Epitaxial Film Formation Step> The GaAs single crystal substrate was subjected to ther-mal cleaning in a metal-organic vapor phase epitaxial growth furnace under a condition of 600° C. and 10 minutes in an atmosphere in which arsine gas was added to hydrogen gas (thermal cleaning step). Furthermore, subsequently to the thermal cleaning step, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface of the GaAs single crystal substrate by a metal-organic vapor phase epitaxial growth method B₂ (MOVPE method) (epitaxial film growth step; the GaAs single crystal substrate having the epitaxial layer grown on the main surface is hereinafter also referred to as “epitaxial substrate”). Thus, epitaxial substrate for Sample 1 was obtained. When the epitaxial layer was grown, the GaAs single crystal substrate was heated to 550° C. [Sample 2] A required number of GaAs single crystal substrates for Sample 2 were obtained in the same manner as in Sample 1 except that the following oxidation treatment step was performed between the alkali cleaning step and the acid cleaning step in the cleaning step. Furthermore, for one of the GaAs single crystal substrates, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface thereof in the same manner as in Sample

A gallium arsenide single crystal substrate comprising a main surface having a circular shape, wherein the gallium arsenide single crystal substrate has 60 a first analytical value or a second analytical value, the first analytical value and the second analytical value are each obtained by determining a spectrum of a detected intensity of a 3d electron of each of arsenic and gallium with respect to binding energy of a pho-toelectron emitted to an outside of the gallium arsenide single crystal substrate based on X-ray photoelectron B₂ spectroscopy in which a center of the main surface is irradiated with X-rays under each of the following five different conditions and subjecting the spectrum to a maximum smoothness method which is a mathematical analysis method, the first analytical value is a value representing a content of gallium oxides present as digallium monoxide and digallium trioxide in a region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 0.6 nm or less, the second analytical value is a value representing a ratio of the content of the gallium oxides to a content of arsenic oxides present as diarsenic pentoxide and diarsenic trioxide in the region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 1.4 or less, the number of particles present on the main surface and each having a major axis of 0.16 µm or more is 2 or less per cm2 of the main surface, and the five different conditions are the following condition 1, condition 2, condition 3, condition 4, and condition 5, Condition 1: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 30° Condition 2: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 45° Condition 3: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 85° Condition 4: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 45° Condition 5: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 85°.

Thermal Cleaning Step S₃₁₀ First, thermal cleaning step S₃₁₀ can be performed on the GaAs single crystal substrate. Thermal cleaning step S₃₁₀ is a step of performing thermal cleaning under a convention-ally known condition such as a heating treatment at 600° C. for 10 minutes. Even under such a condition, since the separation temperature of the oxide film is low in the GaAs single crystal substrate, the oxide film can be effectively removed by the thermal cleaning.

Epitaxial Film Growth Step S₃₂₀ In addition, a step of growing an epitaxial film on the main surface of the GaAs single crystal substrate subjected to thermal cleaning step S₃₁₀ can be performed (epitaxial film growth step S₃₂₀). Through this step, a GaAs single crystal substrate in which an epitaxial film with a reduced number of LPDs is formed on the main surface can be obtained. For example, the number of LPDs present on the epitaxial film and each having a major axis of 18 µm or more can be 5 or less per cm2 of the main surface, and preferably 2 or less per cm2 of the main surface. The lower limit of the number of LPDs each having a major axis of 18 µm or more is zero per cm2 of the main surface. Thus, the GaAs single crystal substrate allows device characteristics to be improved. In epitaxial film formation step S300, as a method of forming an epitaxial film on the main surface of the GaAs single crystal substrate, a conventionally known method can be used. The epitaxial film may be, for example, a compound film formed of Al₁-y-zGayInzAs, where the y may be 0 to 1, the z may be 0 to 1, and the sum of the y and the z may be 0 to 1. In other words, in the present embodiment, the compound film formed of Al₁-y-zGayInzAs (0≤y≤1, 0≤z≤1, 0≤y+z≤1) can be applied as the epitaxial film formed on the main surface of the GaAs single crystal substrate. Further-more, the epitaxial film can be a compound film of AlxGa₁-x As (0≤x≤1) or Al₁-y-zGayInzP (0≤y≤1, 0≤z≤1, 0≤y+z≤1). The epitaxial film is formed so as to have a thickness of 0.5 to 10 µm, for example. When the thickness of the epitaxial film is in the above-mentioned range, the epitaxial substrate can be applied to a wide range of applications. The number of LPDs each having a major axis of 18 µm or more in the epitaxial film disposed on the main surface of the GaAs single crystal substrate can be determined by a conventionally known surface foreign matter inspection B₂ apparatus (for example, trade name: “Surfscan 6220”, manu-factured by KLA-Tencor Corporation) in the same manner as in the method of measuring the number of particles described above. A specific measurement method is the same as the above-described measurement method of the number of particles, and thus redundant description thereof will not be repeated. <Method of Manufacturing Second GaAs Single-Crystal substrate> The second method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including performing a new acid cleaning step (hereinafter also referred to as “second acid cleaning step”) using an acid solution to which an additive such as a surfactant is added, instead of the oxidation treatment step and the subsequent acid cleaning step, in the first method of manufacturing a GaAs single crystal substrate. Hereinafter, the second acid cleaning step will be described with refer-ence to FIG. 7. In the second method of manufacturing a GaAs single crystal substrate, the steps other than a second acid cleaning step S₂₃₁ in cleaning step S₂₀₀ (which replaces oxidation treatment step S₂₂₀ and subsequent acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Second Acid Cleaning Step S₂₃₁) Second acid cleaning step S₂₃₁ is a step of turning the alkali-cleaned surface into an acid-cleaned surface by clean-ing the alkali-cleaned surface with an acid cleaning liquid. In particular, the acid cleaning liquid contains both or one of an alcohol and a surfactant. Through second acid cleaning step S231, impurities in the alkali cleaning liquid attached to the alkali-cleaned surface of the GaAs single crystal substrate precursor and the gallium oxide film can be removed by an oxidation reaction (etching of the alkali-cleaned surface) with the acid cleaning liquid. Furthermore, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove particles from the alkali-cleaned surface, and facilitate removal of organic compo-nents in pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass to 5% by mass. An acid contained in the acid cleaning liquid (hereinafter also referred to as “second acid”) is hydrochloric acid, hydrofluoric acid, or nitric acid. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the second acid in the acid cleaning liquid is less than 2% by mass, the function of modifying the alkali-cleaned surface is reduced. When the acid concentration of the second acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or the main surface) tends to vary due to the function of the second acid. The second acid contained in the acid cleaning liquid is hydrochloric (HCl), hydroflu-oric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious secondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant is expected to have an effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor. In particular, since the hydrophilic group of an anionic surfactant is an anion (negative ion) and most of the particles on the surface of the GaAs single crystal substrate precursor are negatively charged (zeta potential is negative), it is expected that the particles are less likely to adhere to the surface by controlling a potential of the surface to be negative. The addition of the alcohol is expected to have the effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor as well. Since the alcohol gen-erally has a high purity, there is also an advantage that an impurity concentration of the surface can be easily reduced. In second acid cleaning step S231, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the alkali-cleaned surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after second acid cleaning step S231, preferably immediately after second acid cleaning step S231, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manu-facturing a GaAs single crystal substrate, redundant descrip-tion thereof will not be repeated. <Method of Producing Third GaAs Single Crystal substrate> The third method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including, instead of the acid cleaning step in the first method of manufacturing a GaAs single crystal substrate, performing a new acid cleaning step (hereinafter also referred to as “third acid cleaning step”) with an acid solution to which an additive such as a surfactant is added to be able to increase the acid concentration of the acid cleaning liquid used in the acid cleaning step. Hereinafter, the third acid cleaning step will be described with reference to FIG. 8. In the third method of manufacturing a GaAs single crystal substrate, the steps other than the third acid cleaning step S₂₃₂ (which replaces acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) in cleaning step S₂₀₀ are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Third Acid Cleaning Step S₂₃₂) The third acid cleaning step S₂₃₂ is a step of turning the oxidized surface into an acid-cleaned surface by cleaning the oxidized surface with an acid cleaning liquid. Through the third acid cleaning step S232, the oxide film on the oxidized surface can be modified into an oxide film having a lower content of the gallium oxides. In particular, in the third acid cleaning step S232, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove impurities and particles remaining on the oxidized surface, and facilitate removal of organic components in the pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass B₂ to 5% by mass. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the acid in the acid cleaning liquid is less than 2% by mass, the number of particles to be removed is small, and the effect of the acid cleaning liquid containing both or one of the alcohol and the surfactant may not be fully obtained. When the acid con-centration of the acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or main surface) tends to vary due to the function of the acid. The acid contained in the acid cleaning liquid is hydrochloric (HCl), hydrofluoric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious sec-ondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant and the addition of the alcohol respectively have the same effects as the addition of the surfactant and the addition of the alcohol described for second acid cleaning step S₂₃₁ in the second method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. In third acid cleaning step S232, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the oxidized surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after third acid clean-ing step S232, preferably immediately after third acid clean-ing step S232, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, redundant description thereof will not be repeated. <Function and Effect> By the method of manufacturing a GaAs single crystal substrate according to the present embodiments (the first method of manufacturing a GaAs single crystal substrate, the second method of manufacturing a GaAs single crystal substrate, and the third method of manufacturing a GaAs single crystal substrate), a GaAs single crystal substrate in which an oxide film having a low content of gallium oxides can be formed on the main surface and the number of particles present on the main surface is small can be obtained. In such a GaAs single crystal substrate, since the separation temperature of the oxide film is low, the oxide film can be effectively removed by thermal cleaning, and failures in epitaxial film caused by the particles can also be reduced, so that an epitaxial film with a reduced number of LPDs can be formed thereon. Example Hereinafter, the present disclosure will be described in more detail with Examples, but the present disclosure is not limited thereto. Among the GaAs single crystal substrates from Sample 1 to Sample 18 described below, Sample 4, Sample 7 to Sample 10, and Sample 12 to Sample 18 are examples of the present disclosure, and Sample 1 to Sample 3, Sample 5, Sample 6, and Sample 11 are comparative examples. [Production of GaAs Single Crystal substrate] [Sample 1] <Preparation Step> A silicon (Si) atom-doped conductive GaAs single crystal grown by the vertical Bridgman (VB) method was sliced with a wire saw, and the edge portion was ground to prepare a GaAs single crystal substrate precursor. The main surface of the GaAs single crystal substrate precursor was ground with a surface grinding machine, and then the main surface was polished with a hard polishing cloth including a mixture of a chlorine-based polishing agent and silica powders in a clean room. Subsequently, the main surface was mirror-finished by polishing with an INSEC NIB polishing agent (manufactured by FUJIMI INCORPORATED). Further-more, the main surface was roughly cleaned by performing ultrasonic cleaning with isopropyl alcohol (IPA). As described above, a required number of GaAs single crystal substrate precursors each having a diameter of 6 inches (150 mm) and a thickness of 680 µm were prepared. <Cleaning Step> (Alkali Cleaning Step) The GaAs single crystal substrate precursors were immersed in a 1 mass % choline aqueous solution by a vertical-batch method. At the same time, ultrasonic waves were applied to the entire surfaces of the GaAs single crystal substrate precursors for 5 minutes under a condition of a frequency of 500 kHz and a sound pressure of 10 mV. In this manner, the surfaces of the GaAs single crystal substrate precursors were alkali-cleaned. Next, the alkali-cleaned sur-faces of the GaAs single crystal substrate precursors were cleaned with pure water having a dissolved oxygen concen-tration (DO) of 1 ppb by mass for 5 minutes. The total organic carbon (TOC) of this pure water was 0.5 ppb by mass. (Acid Cleaning Step) The GaAs single crystal substrate precursors were placed in a single-wafer cleaning apparatus, and the alkali-cleaned surfaces were subjected to acid cleaning with an acid cleaning liquid containing 0.05% by mass of hydrofluoric acid while being rotated at a rotation speed 500 rpm. In the acid cleaning, at room temperature (25° C.), 1 liter of the acid cleaning liquid was supplied to the alkali-cleaned surfaces of the GaAs single crystal substrate precursors for 1 minute. Furthermore, the GaAs single crystal substrate precursors were rinsed with the same ultrapure water as used in the alkali cleaning step for 3 minutes at a supply rate of 1 L/min. Through this step, the alkali-cleaned surfaces were regarded as acid-cleaned surfaces. (Drying Step) The acid-cleaned surfaces of the GaAs single crystal substrate precursors were rotated at 2500 rpm for 15 to 30 seconds to be dried, and a required number of GaAs single crystal substrates for Sample 1 having a main surface having a circular shape were obtained. <Epitaxial Film Formation Step> The GaAs single crystal substrate was subjected to ther-mal cleaning in a metal-organic vapor phase epitaxial growth furnace under a condition of 600° C. and 10 minutes in an atmosphere in which arsine gas was added to hydrogen gas (thermal cleaning step). Furthermore, subsequently to the thermal cleaning step, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface of the GaAs single crystal substrate by a metal-organic vapor phase epitaxial growth method B₂ (MOVPE method) (epitaxial film growth step; the GaAs single crystal substrate having the epitaxial layer grown on the main surface is hereinafter also referred to as “epitaxial substrate”). Thus, epitaxial substrate for Sample 1 was obtained. When the epitaxial layer was grown, the GaAs single crystal substrate was heated to 550° C. [Sample 2] A required number of GaAs single crystal substrates for Sample 2 were obtained in the same manner as in Sample 1 except that the following oxidation treatment step was performed between the alkali cleaning step and the acid cleaning step in the cleaning step. Furthermore, for one of the GaAs single crystal substrates, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface thereof in the same manner as in Sample

A gallium arsenide single crystal substrate comprising a main surface having a circular shape, wherein the gallium arsenide single crystal substrate has 60 a first analytical value or a second analytical value, the first analytical value and the second analytical value are each obtained by determining a spectrum of a detected intensity of a 3d electron of each of arsenic and gallium with respect to binding energy of a pho-toelectron emitted to an outside of the gallium arsenide single crystal substrate based on X-ray photoelectron B₂ spectroscopy in which a center of the main surface is irradiated with X-rays under each of the following five different conditions and subjecting the spectrum to a maximum smoothness method which is a mathematical analysis method, the first analytical value is a value representing a content of gallium oxides present as digallium monoxide and digallium trioxide in a region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 0.6 nm or less, the second analytical value is a value representing a ratio of the content of the gallium oxides to a content of arsenic oxides present as diarsenic pentoxide and diarsenic trioxide in the region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 1.4 or less, the number of particles present on the main surface and each having a major axis of 0.16 µm or more is 2 or less per cm2 of the main surface, and the five different conditions are the following condition 1, condition 2, condition 3, condition 4, and condition 5, Condition 1: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 30° Condition 2: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 45° Condition 3: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 85° Condition 4: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 45° Condition 5: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 85°.

Thermal Cleaning Step S₃₁₀ First, thermal cleaning step S₃₁₀ can be performed on the GaAs single crystal substrate. Thermal cleaning step S₃₁₀ is a step of performing thermal cleaning under a convention-ally known condition such as a heating treatment at 600° C. for 10 minutes. Even under such a condition, since the separation temperature of the oxide film is low in the GaAs single crystal substrate, the oxide film can be effectively removed by the thermal cleaning.

Epitaxial Film Growth Step S₃₂₀ In addition, a step of growing an epitaxial film on the main surface of the GaAs single crystal substrate subjected to thermal cleaning step S₃₁₀ can be performed (epitaxial film growth step S₃₂₀). Through this step, a GaAs single crystal substrate in which an epitaxial film with a reduced number of LPDs is formed on the main surface can be obtained. For example, the number of LPDs present on the epitaxial film and each having a major axis of 18 µm or more can be 5 or less per cm2 of the main surface, and preferably 2 or less per cm2 of the main surface. The lower limit of the number of LPDs each having a major axis of 18 µm or more is zero per cm2 of the main surface. Thus, the GaAs single crystal substrate allows device characteristics to be improved. In epitaxial film formation step S300, as a method of forming an epitaxial film on the main surface of the GaAs single crystal substrate, a conventionally known method can be used. The epitaxial film may be, for example, a compound film formed of Al₁-y-zGayInzAs, where the y may be 0 to 1, the z may be 0 to 1, and the sum of the y and the z may be 0 to 1. In other words, in the present embodiment, the compound film formed of Al₁-y-zGayInzAs (0≤y≤1, 0≤z≤1, 0≤y+z≤1) can be applied as the epitaxial film formed on the main surface of the GaAs single crystal substrate. Further-more, the epitaxial film can be a compound film of AlxGa₁-x As (0≤x≤1) or Al₁-y-zGayInzP (0≤y≤1, 0≤z≤1, 0≤y+z≤1). The epitaxial film is formed so as to have a thickness of 0.5 to 10 µm, for example. When the thickness of the epitaxial film is in the above-mentioned range, the epitaxial substrate can be applied to a wide range of applications. The number of LPDs each having a major axis of 18 µm or more in the epitaxial film disposed on the main surface of the GaAs single crystal substrate can be determined by a conventionally known surface foreign matter inspection B₂ apparatus (for example, trade name: “Surfscan 6220”, manu-factured by KLA-Tencor Corporation) in the same manner as in the method of measuring the number of particles described above. A specific measurement method is the same as the above-described measurement method of the number of particles, and thus redundant description thereof will not be repeated. <Method of Manufacturing Second GaAs Single-Crystal substrate> The second method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including performing a new acid cleaning step (hereinafter also referred to as “second acid cleaning step”) using an acid solution to which an additive such as a surfactant is added, instead of the oxidation treatment step and the subsequent acid cleaning step, in the first method of manufacturing a GaAs single crystal substrate. Hereinafter, the second acid cleaning step will be described with refer-ence to FIG. 7. In the second method of manufacturing a GaAs single crystal substrate, the steps other than a second acid cleaning step S₂₃₁ in cleaning step S₂₀₀ (which replaces oxidation treatment step S₂₂₀ and subsequent acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Second Acid Cleaning Step S₂₃₁) Second acid cleaning step S₂₃₁ is a step of turning the alkali-cleaned surface into an acid-cleaned surface by clean-ing the alkali-cleaned surface with an acid cleaning liquid. In particular, the acid cleaning liquid contains both or one of an alcohol and a surfactant. Through second acid cleaning step S231, impurities in the alkali cleaning liquid attached to the alkali-cleaned surface of the GaAs single crystal substrate precursor and the gallium oxide film can be removed by an oxidation reaction (etching of the alkali-cleaned surface) with the acid cleaning liquid. Furthermore, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove particles from the alkali-cleaned surface, and facilitate removal of organic compo-nents in pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass to 5% by mass. An acid contained in the acid cleaning liquid (hereinafter also referred to as “second acid”) is hydrochloric acid, hydrofluoric acid, or nitric acid. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the second acid in the acid cleaning liquid is less than 2% by mass, the function of modifying the alkali-cleaned surface is reduced. When the acid concentration of the second acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or the main surface) tends to vary due to the function of the second acid. The second acid contained in the acid cleaning liquid is hydrochloric (HCl), hydroflu-oric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious secondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant is expected to have an effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor. In particular, since the hydrophilic group of an anionic surfactant is an anion (negative ion) and most of the particles on the surface of the GaAs single crystal substrate precursor are negatively charged (zeta potential is negative), it is expected that the particles are less likely to adhere to the surface by controlling a potential of the surface to be negative. The addition of the alcohol is expected to have the effect of reducing adhesion of the particles due to a lower surface tension and an increased liquid permeability between the particles and the surface of the GaAs single crystal substrate precursor as well. Since the alcohol gen-erally has a high purity, there is also an advantage that an impurity concentration of the surface can be easily reduced. In second acid cleaning step S231, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the alkali-cleaned surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after second acid cleaning step S231, preferably immediately after second acid cleaning step S231, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manu-facturing a GaAs single crystal substrate, redundant descrip-tion thereof will not be repeated. <Method of Producing Third GaAs Single Crystal substrate> The third method of manufacturing a GaAs single crystal substrate is a method of manufacturing a GaAs single crystal substrate including, instead of the acid cleaning step in the first method of manufacturing a GaAs single crystal substrate, performing a new acid cleaning step (hereinafter also referred to as “third acid cleaning step”) with an acid solution to which an additive such as a surfactant is added to be able to increase the acid concentration of the acid cleaning liquid used in the acid cleaning step. Hereinafter, the third acid cleaning step will be described with reference to FIG. 8. In the third method of manufacturing a GaAs single crystal substrate, the steps other than the third acid cleaning step S₂₃₂ (which replaces acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate) in cleaning step S₂₀₀ are the same as those in the first method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. (Third Acid Cleaning Step S₂₃₂) The third acid cleaning step S₂₃₂ is a step of turning the oxidized surface into an acid-cleaned surface by cleaning the oxidized surface with an acid cleaning liquid. Through the third acid cleaning step S232, the oxide film on the oxidized surface can be modified into an oxide film having a lower content of the gallium oxides. In particular, in the third acid cleaning step S232, the acid cleaning liquid containing both or one of an alcohol and a surfactant can further remove impurities and particles remaining on the oxidized surface, and facilitate removal of organic components in the pure water cleaning after the acid cleaning described later. An acid concentration in the acid cleaning liquid is 2% by mass B₂ to 5% by mass. The alcohol is isopropyl alcohol or ethanol. The surfactant is an anionic surfactant. When the acid concentration of the acid in the acid cleaning liquid is less than 2% by mass, the number of particles to be removed is small, and the effect of the acid cleaning liquid containing both or one of the alcohol and the surfactant may not be fully obtained. When the acid con-centration of the acid in the acid cleaning liquid exceeds 5% by mass, the chemical composition of the acid-cleaned surface (or main surface) tends to vary due to the function of the acid. The acid contained in the acid cleaning liquid is hydrochloric (HCl), hydrofluoric (HF) or nitric (HNO₃) acid. As a result, the acid cleaning liquid has a high cleaning power, is free of elements (for example, metal elements and other elements) which affect electrical characteristics, and the acid component is evaporated together with water when droplets of the acid cleaning liquid are scattered in the equipment, thereby suppressing occurrence of serious sec-ondary contamination and equipment deterioration. A concentration of the surfactant contained in the acid cleaning liquid is preferably 1 ppm by mass to 1000 ppm by mass. A concentration of the alcohol contained in the acid cleaning liquid is preferably 0.01% by mass to 1% by mass. The addition of the surfactant and the addition of the alcohol respectively have the same effects as the addition of the surfactant and the addition of the alcohol described for second acid cleaning step S₂₃₁ in the second method of manufacturing a GaAs single crystal substrate, and thus redundant description thereof will not be repeated. In third acid cleaning step S232, as in acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, the acid cleaning liquid may be supplied to the oxidized surface while the GaAs single crystal substrate precursor is rotated at 100 to 800 rpm with the main surface thereof held horizontal. Furthermore, after third acid clean-ing step S232, preferably immediately after third acid clean-ing step S232, the acid-cleaned surface of the GaAs single crystal substrate precursor is preferably cleaned with pure water. Since the characteristics of the pure water are the same as those of the pure water used after acid cleaning step S₂₃₀ in the first method of manufacturing a GaAs single crystal substrate, redundant description thereof will not be repeated. <Function and Effect> By the method of manufacturing a GaAs single crystal substrate according to the present embodiments (the first method of manufacturing a GaAs single crystal substrate, the second method of manufacturing a GaAs single crystal substrate, and the third method of manufacturing a GaAs single crystal substrate), a GaAs single crystal substrate in which an oxide film having a low content of gallium oxides can be formed on the main surface and the number of particles present on the main surface is small can be obtained. In such a GaAs single crystal substrate, since the separation temperature of the oxide film is low, the oxide film can be effectively removed by thermal cleaning, and failures in epitaxial film caused by the particles can also be reduced, so that an epitaxial film with a reduced number of LPDs can be formed thereon. Example Hereinafter, the present disclosure will be described in more detail with Examples, but the present disclosure is not limited thereto. Among the GaAs single crystal substrates from Sample 1 to Sample 18 described below, Sample 4, Sample 7 to Sample 10, and Sample 12 to Sample 18 are examples of the present disclosure, and Sample 1 to Sample 3, Sample 5, Sample 6, and Sample 11 are comparative examples. [Production of GaAs Single Crystal substrate] [Sample 1] <Preparation Step> A silicon (Si) atom-doped conductive GaAs single crystal grown by the vertical Bridgman (VB) method was sliced with a wire saw, and the edge portion was ground to prepare a GaAs single crystal substrate precursor. The main surface of the GaAs single crystal substrate precursor was ground with a surface grinding machine, and then the main surface was polished with a hard polishing cloth including a mixture of a chlorine-based polishing agent and silica powders in a clean room. Subsequently, the main surface was mirror-finished by polishing with an INSEC NIB polishing agent (manufactured by FUJIMI INCORPORATED). Further-more, the main surface was roughly cleaned by performing ultrasonic cleaning with isopropyl alcohol (IPA). As described above, a required number of GaAs single crystal substrate precursors each having a diameter of 6 inches (150 mm) and a thickness of 680 µm were prepared. <Cleaning Step> (Alkali Cleaning Step) The GaAs single crystal substrate precursors were immersed in a 1 mass % choline aqueous solution by a vertical-batch method. At the same time, ultrasonic waves were applied to the entire surfaces of the GaAs single crystal substrate precursors for 5 minutes under a condition of a frequency of 500 kHz and a sound pressure of 10 mV. In this manner, the surfaces of the GaAs single crystal substrate precursors were alkali-cleaned. Next, the alkali-cleaned sur-faces of the GaAs single crystal substrate precursors were cleaned with pure water having a dissolved oxygen concen-tration (DO) of 1 ppb by mass for 5 minutes. The total organic carbon (TOC) of this pure water was 0.5 ppb by mass. (Acid Cleaning Step) The GaAs single crystal substrate precursors were placed in a single-wafer cleaning apparatus, and the alkali-cleaned surfaces were subjected to acid cleaning with an acid cleaning liquid containing 0.05% by mass of hydrofluoric acid while being rotated at a rotation speed 500 rpm. In the acid cleaning, at room temperature (25° C.), 1 liter of the acid cleaning liquid was supplied to the alkali-cleaned surfaces of the GaAs single crystal substrate precursors for 1 minute. Furthermore, the GaAs single crystal substrate precursors were rinsed with the same ultrapure water as used in the alkali cleaning step for 3 minutes at a supply rate of 1 L/min. Through this step, the alkali-cleaned surfaces were regarded as acid-cleaned surfaces. (Drying Step) The acid-cleaned surfaces of the GaAs single crystal substrate precursors were rotated at 2500 rpm for 15 to 30 seconds to be dried, and a required number of GaAs single crystal substrates for Sample 1 having a main surface having a circular shape were obtained. <Epitaxial Film Formation Step> The GaAs single crystal substrate was subjected to ther-mal cleaning in a metal-organic vapor phase epitaxial growth furnace under a condition of 600° C. and 10 minutes in an atmosphere in which arsine gas was added to hydrogen gas (thermal cleaning step). Furthermore, subsequently to the thermal cleaning step, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface of the GaAs single crystal substrate by a metal-organic vapor phase epitaxial growth method B₂ (MOVPE method) (epitaxial film growth step; the GaAs single crystal substrate having the epitaxial layer grown on the main surface is hereinafter also referred to as “epitaxial substrate”). Thus, epitaxial substrate for Sample 1 was obtained. When the epitaxial layer was grown, the GaAs single crystal substrate was heated to 550° C. [Sample 2] A required number of GaAs single crystal substrates for Sample 2 were obtained in the same manner as in Sample 1 except that the following oxidation treatment step was performed between the alkali cleaning step and the acid cleaning step in the cleaning step. Furthermore, for one of the GaAs single crystal substrates, an Al0.4Ga0.6As layer having a thickness of 4 µm was grown as an epitaxial layer on the main surface thereof in the same manner as in Sample

A gallium arsenide single crystal substrate comprising a main surface having a circular shape, wherein the gallium arsenide single crystal substrate has 60 a first analytical value or a second analytical value, the first analytical value and the second analytical value are each obtained by determining a spectrum of a detected intensity of a 3d electron of each of arsenic and gallium with respect to binding energy of a pho-toelectron emitted to an outside of the gallium arsenide single crystal substrate based on X-ray photoelectron B₂ spectroscopy in which a center of the main surface is irradiated with X-rays under each of the following five different conditions and subjecting the spectrum to a maximum smoothness method which is a mathematical analysis method, the first analytical value is a value representing a content of gallium oxides present as digallium monoxide and digallium trioxide in a region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 0.6 nm or less, the second analytical value is a value representing a ratio of the content of the gallium oxides to a content of arsenic oxides present as diarsenic pentoxide and diarsenic trioxide in the region from the main surface of the gallium arsenide single crystal substrate to a depth of 2 nm and is 1.4 or less, the number of particles present on the main surface and each having a major axis of 0.16 µm or more is 2 or less per cm2 of the main surface, and the five different conditions are the following condition 1, condition 2, condition 3, condition 4, and condition 5, Condition 1: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 30° Condition 2: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 45° Condition 3: incident X-ray energy of 150 eV, and pho-toelectron take-off angle of 85° Condition 4: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 45° Condition 5: incident X-ray energy of 600 eV, and pho-toelectron take-off angle of 85°.