Gallium arsenide single crystal substrate, method for manufacturing gallium arsenide single crystal, and method for manufacturing gallium arsenide single crystal substrate

Abstract

The present invention provides a gallium arsenide single crystal substrate that can reduce performance degradation due to radiation exposure and other factors. [Solution] The gallium arsenide single crystal substrate has an electron-donating conductivity type. The gallium arsenide single crystal substrate has a circular main surface. The gallium arsenide single crystal substrate contains an electron-donating dopant. The value of the infrared absorption coefficient at a wavelength of 1 micron in the gallium arsenide single crystal substrate, obtained by measuring after electron beam irradiation under the conditions of an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy, is 95% or more and 100% or less of the value of the infrared absorption coefficient at a wavelength of 1 micron in the gallium arsenide single crystal substrate, obtained by measuring before the electron beam irradiation.

Description

Technical Field

[0001] The present disclosure relates to a gallium arsenide single crystal substrate, a method for manufacturing a gallium arsenide single crystal, and a method for manufacturing a gallium arsenide single crystal substrate.

Background Art

[0002] Japanese Patent Application Publication No. 2011-527280 (Patent Document 1) discloses a method for manufacturing a gallium arsenide single crystal substrate (hereinafter, also referred to as "GaAs single crystal substrate") having a small optical absorption coefficient in the near infrared region. International Publication No. 2012 / 160781 (Patent Document 2) discloses a GaAs single crystal substrate having a high carrier concentration and high crystallinity. Japanese Unexamined Patent Application Publication No. 2004-115339 (Patent Document 3) relates to the manufacture of a gallium arsenide single crystal (hereinafter, also referred to as "GaAs single crystal") doped with silicon (Si) as an impurity by a vertical boat method, and discloses a method for manufacturing a GaAs single crystal in which the generation of dislocations caused by the precipitation of boron arsenide is prevented.

Prior Art Documents

Patent Documents

[0005] When considering the use of optical devices in a space environment, reliability is required to ensure stable operation under exposure to cosmic radiation. For example, NanotechJapan Bulletin Vol.13, No.1, 2020, P1-8 (Non-Patent Literature 1) evaluates the degradation of gallium nitride-based devices in response to 2 MeV electron beams caused by solar flares. Phys.ReV.B, Vol.52, No.11, (1995), 10932 (Non-Patent Literature 2) evaluates the introduction and recovery of point defects in a Si-doped GaAs single crystal substrate manufactured using the horizontal Bridgman method when irradiated with a 1.5 MeV electron beam. However, a GaAs single crystal substrate with superior reliability characteristics for use in the above optical devices has not yet been realized. Therefore, the development of a GaAs single crystal substrate that satisfies the above requirements is highly desirable.

[0006] In view of the above circumstances, the purpose of this disclosure is to provide a GaAs single crystal substrate that can reduce performance degradation due to radiation exposure, a method for manufacturing a GaAs single crystal, and a method for manufacturing a GaAs single crystal substrate. [Means for solving the problem]

[0007] The gallium arsenide single crystal substrate according to this disclosure is a gallium arsenide single crystal substrate having an electron-donating conductivity type. The gallium arsenide single crystal substrate has a circular main surface. The gallium arsenide single crystal substrate contains an electron-donating dopant. The value of the infrared absorption coefficient at a wavelength of 1 micron in the gallium arsenide single crystal substrate, obtained by measuring after electron beam irradiation of at least one of the main surfaces under the conditions of an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy, is 95% or more and 100% or less of the value of the infrared absorption coefficient at a wavelength of 1 micron in the gallium arsenide single crystal substrate, obtained by measuring before the electron beam irradiation. [Effects of the Invention]

[0008] This disclosure provides a GaAs single crystal substrate in which performance degradation due to radiation exposure and the like can be reduced, a method for manufacturing a GaAs single crystal, and a method for manufacturing a GaAs single crystal substrate. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is an explanatory diagram illustrating the GaAs single crystal substrate according to this embodiment, and the measurement area (measurement region) for measuring the infrared absorption coefficient at a wavelength of 1 micron on the substrate. [Figure 2] Figure 2 is an explanatory diagram illustrating a sample for Hall measurement prepared using the central portion of the GaAs single crystal substrate according to this embodiment, in order to measure the carrier concentration in the substrate. [Figure 3] Figure 3 is a schematic flowchart showing the method for manufacturing a GaAs single crystal and a GaAs single crystal substrate according to this embodiment. [Figure 4] Figure 4 is an explanatory diagram illustrating the process of growing a GaAs single crystal using a single crystal manufacturing apparatus, relating to the method for manufacturing a GaAs single crystal according to this embodiment. [Modes for carrying out the invention]

[0010] [Description of Embodiments in this Disclosure] First, an overview of the embodiments of this disclosure will be described. Through diligent study, the inventors have arrived at this disclosure, which can solve the above-mentioned problems. Specifically, in a GaAs single crystal substrate having an electron-donating conductivity (hereinafter also referred to as "n-type"), the inventors focused on reducing the fluctuation in the activation rate of the n-type dopant before and after exposure to radiation. With this focus in mind, the vertical boat method was used, and the above-mentioned GaAs single crystal was manufactured by controlling the amount of n-type dopant added, the arsenic (As) partial pressure in the crucible, etc., so that the activation rate of the n-type dopant in the GaAs single crystal was around 50%. In this case, it was found that the GaAs single crystal substrate obtained from the above-mentioned GaAs single crystal showed little fluctuation in the absorption coefficient of infrared radiation at a wavelength of 1 micron before and after irradiation with an electron beam under predetermined conditions. As a result, a GaAs single crystal substrate that can reduce performance degradation due to radiation exposure was conceived, and this disclosure was completed.

[0011] The embodiments of this disclosure will now be described by listing them. [1] A GaAs single crystal substrate according to one aspect of the present disclosure is a GaAs single crystal substrate having an n-type conductivity. The GaAs single crystal substrate has a circular main surface. The GaAs single crystal substrate contains an n-type dopant. The value of the infrared absorption coefficient at a wavelength of 1 micron in the GaAs single crystal substrate, obtained by measuring after electron beam irradiation of at least one of the main surfaces under the conditions of an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy, is 95% or more and 100% or less of the value of the infrared absorption coefficient at a wavelength of 1 micron in the GaAs single crystal substrate, obtained by measuring before the electron beam irradiation. A GaAs single crystal substrate having such characteristics can reduce performance degradation due to radiation exposure, etc. Therefore, the GaAs single crystal substrate can be used as a substrate for optical devices intended for use in space environments.

[0012] [2] The GaAs single crystal substrate described in [1] is 3.5 × 10 18 cm -3 The above 5.0 × 10 18 cm -3It may have the following carrier concentration. The above GaAs single crystal substrate may have an activation rate of the above n-type dopant of 40% or more and 60% or less. In this case, a GaAs single crystal substrate with reduced performance degradation due to radiation exposure or the like can be provided with good yield.

[0013] [3] The GaAs single crystal substrate according to [2] may have an activation rate of the above n-type dopant of 45% or more and 55% or less. In this case, a GaAs single crystal substrate with reduced performance degradation due to radiation exposure or the like can be provided with good yield.

[0014] [4] The GaAs single crystal substrate according to any one of [1] to [3] may have an atomic concentration of the above n-type dopant of 5.8×10 18 cm -3 or more and 1.3×10 19 cm -3 or less. In this case, it is possible to prevent excessive decrease in the activation rate of the n-type dopant.

[0015] [5] The GaAs single crystal substrate according to any one of [1] to [4] may contain boron (B). The above GaAs single crystal substrate may have an atomic concentration of B of 4.0×10 18 cm -3 or more and 6.×10 18 cm -3 or less. In this case, a GaAs single crystal substrate with reduced performance degradation due to radiation exposure or the like can be provided with good yield.

[0016] [6] In the GaAs single crystal substrate according to any one of [1] to [5], the value of the absorption coefficient of infrared rays with a wavelength of 1 micron in the above GaAs single crystal substrate, measured before performing the above electron beam irradiation, may be 3.6 cm -1 or more. In this case, a GaAs single crystal substrate having excellent optical properties can be provided.

[0017] [7] In the GaAs single crystal substrate described in any one of [1] to [6], the n-type dopant may be at least one selected from the group consisting of silicon (Si), tellurium (Te), and tin (Sn). In this case, a GaAs single crystal substrate in which performance degradation due to radiation exposure, etc., can be reduced can be provided with a high yield.

[0018] [8] The GaAs single crystal substrate described in any one of items [1] to [7] is 1000 cm -2 The following dislocation densities may be present. In this case, a GaAs single crystal substrate with excellent optical properties can be provided.

[0019] The GaAs single crystal substrate described in [9] and [8] may contain at least Si as the n-type dopant. The GaAs single crystal substrate is 2.0 × 10 18 cm -3 The above-mentioned atomic concentration of Si may be present. The above-mentioned GaAs single crystal substrate may have a 100 cm² -2 The following dislocation densities may be present. In this case, an nGaAs single crystal substrate with excellent optical properties can be provided.

[0020]

[10] A method for manufacturing a GaAs single crystal according to one aspect of the present disclosure is a method for manufacturing a GaAs single crystal having n-type conductivity using the vertical boat method. The above manufacturing method includes the steps of: preparing a single crystal growth apparatus comprising at least a cylindrical crucible and a heating device arranged to surround the outer circumference of the crucible; arranging a seed crystal formed from the GaAs single crystal at the bottom of the crucible, and arranging an n-type dopant, boron oxide (B2O3), metallic arsenic, and a massive GaAs polycrystalline material above the seed crystal in the crucible; heating the crucible with the heating device to sublimate the metallic arsenic, melt the B2O3 into a B2O3 melt, melt a portion of the seed crystal and the GaAs polycrystalline material into a GaAs melt, and obtaining a first melt in which the n-type dopant is dissolved in the GaAs melt, and bringing the first melt into contact with the remainder of the seed crystal; and obtaining the GaAs single crystal by growing a crystal on the remainder of the seed crystal from the first melt. The above single crystal growth apparatus includes a branch tube section for controlling the arsenic partial pressure in the crucible. In the process of obtaining the GaAs single crystal, the end faces that do not come into contact with the remainder of the seed crystal in the first melt are covered with the B2O3 melt by 10% to 50% of the area. In the process of obtaining the GaAs single crystal, the GaAs in the first melt has a stoichiometric ratio in which As exceeds 1 relative to gallium (Ga). A manufacturing method having these characteristics yields a GaAs single crystal in which performance degradation due to radiation exposure and the like is reduced.

[0021]

[11] The method for producing GaAs single crystals described in

[10] is such that the crucible does not need to be rotated and the first melt does not need to be stirred in the step of obtaining the GaAs single crystal. In this case, it is possible to obtain a GaAs single crystal in which the activation rate of the n-type dopant is around 50%.

[0022]

[12] A method for manufacturing a GaAs single crystal substrate according to one aspect of the present disclosure includes the step of obtaining a GaAs single crystal substrate having a circular main surface by processing the GaAs single crystal obtained by the method for manufacturing a GaAs single crystal described in

[10] or

[11] . A manufacturing method having such characteristics reduces performance degradation due to exposure to radiation, etc., thereby providing a GaAs single crystal substrate suitable as a substrate for optical devices intended for use in space environments.

[0023] [Details of the embodiments of this disclosure] One embodiment of the present disclosure (hereinafter also referred to as "this embodiment") will be described in further detail below, but the present disclosure is not limited thereto. Drawings may be referenced in the following description, and the same or corresponding elements in this specification and the drawings will be denoted by the same reference numerals, and the same description will not be repeated. Furthermore, the scale of the drawings has been adjusted as appropriate to facilitate understanding of each component, and the scale of the components shown in the drawings does not necessarily correspond to the scale of the actual components.

[0024] In this specification, the notation "A~B" means an upper and lower limit of a range (i.e., greater than or equal to A and less than or equal to B). If no unit is specified for A, but a unit is specified only for B, the units for A and B are the same. Furthermore, when compounds are represented by chemical formulas in this specification, unless the atomic ratio is particularly limited, the compound includes all conventionally known atomic ratios and should not necessarily be limited to the stoichiometric range. For example, when "GaAs" is written, unless otherwise specified, the ratio of the number of atoms constituting GaAs is not limited to Ga:As=1:1, but includes all conventionally known atomic ratios. This also applies to the description of compounds other than "GaAs".

[0025] In this specification, the “main surface” of a GaAs single crystal substrate means both of the two circular surfaces of the substrate. In a GaAs single crystal substrate, if at least one of these two surfaces satisfies the claims of this disclosure, it falls within the scope of the present invention. An epitaxial film may be disposed on the “main surface” of the GaAs single crystal substrate. Also, in this specification, the term “in-plane” means “main surface.” Furthermore, when the diameter of a GaAs single crystal substrate is stated as “70 mm,” it means the diameter is approximately 70 mm (about 70 to 76.5 mm) or 3 inches. When the diameter is stated as “100 mm,” it means the diameter is approximately 100 mm (about 95 to 105 mm) or 4 inches. When the diameter is stated as “150 mm,” it means the diameter is approximately 150 mm (about 145 to 155 mm) or 6 inches. When the diameter is stated as "210mm" above, it means that the diameter is approximately 210mm (around 195-210mm), or that it is 8 inches. The diameter is measured using a conventionally known outer diameter measuring instrument such as a caliper.

[0026] In this specification, "dislocation" refers to a type of crystal defect in which a linear atomic displacement occurs within the crystal lattice (a linear defect). A dislocation is identified as a dislocation line generated at the boundary between a region that slips (shifts) and a region that does not slip when a region within the crystal experiences slip (shift). The magnitude and direction of the slip associated with a dislocation are represented by a Burgers vector.

[0027] In this specification, "yield" refers to the proportion of the mass of the ingot formed from GaAs single crystals grown in the crucible that is evaluated as being of good quality when processed into GaAs single crystal substrates. A higher yield (larger value) indicates that more GaAs single crystal substrates can be obtained from the GaAs single crystals grown in the crucible.

[0028] In this specification, crystallographic descriptions are indicated by [] for individual orientations, <> for collective orientations, () for individual planes, and {} for collective planes. Furthermore, while negative crystallographic exponents are usually indicated by a "-" (bar) above the number, in this specification, the negative sign is placed before the number.

[0029] [GaAs single crystal substrate] The GaAs single crystal substrate according to this embodiment is an n-type conductive GaAs single crystal substrate. The GaAs single crystal substrate has a circular main surface. The GaAs single crystal substrate contains an n-type dopant. The value of the infrared absorption coefficient at a wavelength of 1 micron in the GaAs single crystal substrate, obtained by measuring after electron beam irradiation at an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy on at least one of the main surfaces, is 95% to 100% of the value of the infrared absorption coefficient at a wavelength of 1 micron obtained by measuring before electron beam irradiation. Because the fluctuation rate of the infrared absorption coefficient before and after electron beam irradiation is small, the degradation of performance due to radiation exposure can be reduced. Therefore, the GaAs single crystal substrate can be used as a substrate for optical devices intended for use in space environments.

[0030] The reason why performance degradation due to radiation exposure can be reduced in the GaAs single crystal substrate according to this embodiment is based on the following characteristics. In other words, the performance degradation of single crystal substrates made of so-called compound semiconductor single crystals in the space environment is generally attributed to the disruption of the structure (e.g., crystal lattice structure) of the material forming the substrate due to the ejection of atoms that occurs when the substrate is irradiated with cosmic radiation. Generally, compound semiconductors exhibit either n-type or p-type properties depending on the occupational position of impurity elements called dopants in the crystal. When the ejection of atoms occurs due to radiation irradiation, the occupational position of the dopants in the crystal changes, which can adversely affect various physical properties such as electrical characteristics. Therefore, even if events such as radiation exposure occur in the substrate, if the fluctuation of the occupational position in the substrate is reduced before and after the event, it means that the degradation of the substrate due to radiation exposure will be reduced.

[0031] The GaAs single crystal substrate according to this embodiment may have characteristic carrier concentrations and n-type dopant atomic concentrations as described later. The GaAs single crystal substrate may also contain impurity elements other than n-type dopants, such as B, at predetermined atomic concentrations. Furthermore, as will be clearly shown in the manufacturing method described later, the GaAs single crystal used in the GaAs single crystal substrate is obtained by vertical boat processing, where the upper part of the molten material in the crucible is partially coated with B2O3, and the partial pressure of As in the crucible is controlled during crystal growth. In this case, the activation rate of the n-type dopant in the GaAs single crystal substrate obtained by processing the GaAs single crystal may be around 50%.

[0032] In the above-described GaAs single-crystal substrate, if the activation rate of the n-type dopant is around 50%, the amount of fluctuation in the activation rate due to atomic ejection in response to radiation exposure in the GaAs single-crystal substrate will be smaller than the amount of fluctuation in the GaAs single-crystal substrate where the activation rate of the n-type dopant is close to 100%. For example, when the main surface of the above-described GaAs single-crystal substrate is irradiated with an electron beam under the conditions of an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy, the fluctuation rate of the infrared absorption coefficient at a wavelength of 1 micron before and after the irradiation is small, within 5%. In other words, the above-described GaAs single-crystal substrate with an n-type dopant activation rate close to 50% can reduce the degradation of the substrate due to radiation exposure, etc. From the above, the GaAs single-crystal substrate according to this embodiment can reduce the degradation of performance due to radiation exposure, etc., and can therefore be used as a substrate for optical devices intended for use in space environments.

[0033] <Main surface> Figure 1 is an explanatory diagram illustrating a GaAs single crystal substrate according to this embodiment, and a measurement area (measurement region) for measuring the absorption coefficient of infrared radiation at a wavelength of 1 micron on the substrate. As shown in Figure 1, the GaAs single crystal substrate 1 has a circular main surface 11. In this specification, the term "circular shape" used to describe the shape of the main surface 11 includes not only a geometrically circular shape, but also a shape in which the main surface 11 does not form a geometrically circular shape due to the formation of at least one of a notch, orientation flat (hereinafter also referred to as "OF") or index flat (hereinafter also referred to as "IF") on the outer periphery of the main surface 11. Here, "a shape in which the main surface does not form a geometrically circular shape" means a shape in which, among line segments extending from any point on the outer periphery of the main surface 11 to the center O of the main surface 11, the length of the line segment extending from any point on the notch, OF, and IF to the center O of the main surface 11 is shortened. Furthermore, the term "shape when the main surface does not form a geometric circle" also includes shapes where the lengths of all line segments extending from any point on the outer circumference of the main surface 11 to the center O of the main surface 11 are not necessarily the same, due to the shape of the GaAs single crystal that is the raw material for the GaAs single crystal substrate 1. In this case, the center O of the main surface 11 refers to the position of the centroid. The diameter of the GaAs single crystal substrate 1 refers to the length of the longest line segment that extends from any point on the outer circumference of the GaAs single crystal substrate 1, passing through the center O of the main surface 11 and to another point on the outer circumference.

[0034] (Off-angle) The main surface 11 may be the {100} plane of the GaAs single crystal. For example, the GaAs single crystal substrate 1 shown in Figure 1 may have a main surface 11 that is the (100) plane. Furthermore, the GaAs single crystal substrate 1 may have a main surface 11 that has an off-angle greater than 0° and less than or equal to 15° from the {100} plane.

[0035] In the case where the main surface 11 of the GaAs single crystal substrate 1 is a {100} plane, and in the case where the main surface 11 has an off-angle greater than 0° and 15° or less from the {100} plane, the main surface 11 of the GaAs single crystal substrate 1 is a plane having an orientation that is excellent in electrical properties and optical properties. As a result, in this embodiment, a GaAs single crystal substrate 1 is provided having a main surface 11 that facilitates the processing of rectangular or square elements using the cleavage of the {100} plane of GaAs, and therefore the GaAs single crystal substrate 1 can be applied to optical devices. The GaAs single crystal substrate 1 may have a main surface 11 that is a {100} plane, or a main surface 11 that has an off-angle greater than 0° and 10° or less from the {100} plane. The GaAs single crystal substrate 1 may have a main surface 11 that is a {100} plane, or a main surface 11 that has an off-angle greater than 0° and 5° or less from the {100} plane.

[0036] <diameter> The diameter of the GaAs single crystal substrate 1 may be between 95 mm and 205 mm. Specifically, the GaAs single crystal substrate 1 with a diameter between 95 mm and 205 mm may have a diameter of 100 mm, 150 mm, or 200 mm, in other words, a diameter of 4 inches, 6 inches, or 8 inches. This reduces performance degradation due to radiation exposure, etc., in large-diameter GaAs single crystal substrates 1 with a diameter between 95 mm and 205 mm. As described above, the diameter of the GaAs single crystal substrate 1 is measured using a conventionally known outer diameter measuring instrument such as a caliper.

[0037] <Conductivity type is n-type> As described above, the GaAs single crystal substrate 1 has an n-type conductivity. The GaAs single crystal substrate 1 contains an n-type dopant. In other words, the GaAs single crystal substrate 1 exhibits an n-type conductivity as described above due to the inclusion of an n-type dopant. When an optical device is formed on the main surface 11, for example by laminating an epitaxial layer, the GaAs single crystal substrate 1 may have a light-emitting layer (active layer) directly formed on the main surface 11, or, since the conductivity is n-type, an n-type cladding layer, a DFB (Distributed Feedback) layer, or a DBR (Distributed Bragg Reflector) layer may be formed.

[0038] (n-type dopant) The GaAs single crystal substrate 1 contains an n-type dopant as described above. The n-type dopant may be at least one selected from the group consisting of Si, Te, and Sn. If the n-type dopant contained in the GaAs single crystal substrate 1 is, for example, Si, then the Si can be introduced into both Ga sites where Ga may exist and As sites where As may exist in the GaAs crystal lattice structure of the GaAs single crystal substrate 1. Therefore, the activation rate of the n-type dopant in the GaAs single crystal substrate 1 can be controlled by adjusting the number of Si present in the Ga sites and the number of Si present in the As sites. The GaAs single crystal substrate 1 may contain two types of elements as the n-type dopant, such as Si and Te, Si and Sn, or Te and Sn, or it may contain three types of elements, such as Si, Te, and Sn.

[0039] (atomic concentration of n-type dopant) GaAs single crystal substrate 1 is 5.8 × 10 18 cm -3 The above 1.3 × 10 19 cm -3 The following atomic concentrations of the above n-type dopant may be present. The GaAs single crystal substrate 1 has 6.0 × 10⁻¹⁶ atoms. 18 cm -3 The above 1.15 × 10 19 cm -3 The following atomic concentrations of the above n-type dopant may be present: 7.0 × 10 18 cm-3 The above 1.0 × 10 19 cm -3 The GaAs single crystal substrate 1 may have the following atomic concentrations of the n-type dopant. When the atomic concentration of the n-type dopant is within the range described above, the GaAs single crystal substrate 1 can prevent the activation rate of the n-type dopant from decreasing too much. If the activation rate of the n-type dopant decreases too much in the GaAs single crystal substrate 1, it becomes difficult to provide it as a substrate for forming high-quality optical devices. When the GaAs single crystal substrate 1 contains two or more types of the n-type dopant, the "atomic concentration of the n-type dopant" described above means the total atomic concentration of the two or more types of n-type dopant.

[0040] The type and atomic concentration of n-type dopants contained in the GaAs single crystal substrate 1 are measured using glow discharge mass spectrometry (GDMS). GDMS is a technique in which a glow discharge plasma is generated with the analytical sample as the cathode in a high-purity argon atmosphere, and the surface of the analytical sample is sputtered within the plasma, thereby measuring the constituent elements in the ionized analytical sample with a mass spectrometer. This allows for the qualitative and quantitative determination of impurity elements other than Ga and As contained in the GaAs single crystal substrate, including n-type dopants.

[0041] The above GDMS is performed, for example, in the following manner. First, a GaAs single crystal substrate 1 is obtained by the manufacturing method described later. Furthermore, the GaAs single crystal substrate 1 is cleaved, etc., to prepare a strip-shaped analytical sample measuring 2 mm square and 20 mm in length. Next, the analytical sample is placed in the sample placement section attached to the GDMS apparatus. At this point, the sample placement section may be cleaned according to a conventional method to prevent contamination and remove foreign matter, and may also be pre-sputtered for 60 minutes.

[0042] Next, GDMS is performed on the above-mentioned sample under the following conditions. The semi-quantitative value of the n-type dopant element in the above-mentioned sample is calculated by correcting the ionic intensity ratio of Ga and As with a relative sensitivity factor (RSF). The relative sensitivity factor used is the value built into the software attached to the instrument described below. For example, the measurement conditions for GDMS targeting GaAs single crystal substrate 1 are as follows. Equipment: Glow discharge mass spectrometer (product name (model number): VG-9000, manufactured by VG Elemental) Ion source: Pin-type cell (cooled with liquid nitrogen during analysis) Discharge area: diameter 10mm Discharge gas: High-purity argon (6N grade) Discharge conditions: 2mA, 1kV (constant current mode) Detectors: Faraday cup and multiplier Mass resolution: m / Δm or higher (high resolution mode)

[0043] <Career density> GaAs single crystal substrate 1 is 3.5 × 10 18 cm -3 The above 5.0 × 10 18 cm -3 The following carrier concentrations may be present: GaAs single crystal substrate 1 has a carrier concentration of 3.6 × 10⁻⁶. 18 cm -3 The above 4.9 × 10 18 cm -3 The carrier concentration may be as follows: 3.7 × 10 18 cm -3 The above 4.8 × 10 18 cm -3 The carrier concentrations may be as follows. When the carrier concentrations are within the range described above, the activation rate of the n-type dopant in the GaAs single crystal substrate 1 can be easily controlled to around 50% in relation to the atomic concentration of the n-type dopant.

[0044] The carrier concentration of the GaAs single crystal substrate 1 is measured by Hall measurement at 25°C using the Van der Pauw method. The procedure for obtaining the carrier concentration will be specifically described with reference to FIGS. 1 and 2. First, as shown in FIG. 1, for example, for a GaAs single crystal obtained based on the manufacturing method described later, a known processing method is applied to obtain one GaAs single crystal substrate 1 to be measured. From the central portion of this one GaAs single crystal substrate 1, a rectangular slice 11a with a size of 4 mm in length × 4 mm in width centered on its center (for example, the center of the main surface 11) (for example, a thickness of 650 μm) is produced. Next, as shown in FIG. 2, electrodes 21 made of an alloy containing gold, nickel, and germanium are formed at the four corners of the rectangular slice 11a (the measurement surface), thereby obtaining a sample for Hall measurement. Here, the shape of the electrode 21 is not limited to the illustrated rectangle, and may be fan-shaped or circular. For the rectangular slice 11a provided with such electrodes 21, Hall measurement by the Van der Pauw method is applied in an atmosphere of 25°C to obtain the carrier concentration. In this specification, the carrier concentration obtained based on measuring the above-described rectangular slice is defined as the carrier concentration of the GaAs single crystal substrate.

[0045] <Activation rate of n-type dopant> The GaAs single crystal substrate 1 may have an activation rate of the n-type dopant of 40% or more and 60% or less. The GaAs single crystal substrate 1 may have an activation rate of the n-type dopant of 45% or more and 55% or less. In this case, due to the reasons described above, a GaAs single crystal substrate 1 with reduced performance degradation due to radiation exposure or the like can be provided with good yield.

[0046] The activation rate of the n-type dopant is obtained from the ratio of the above-described carrier concentration of the GaAs single crystal substrate 1 to the atomic concentration of the n-type dopant, and is expressed as a percentage. Specifically, it is obtained by substituting the carrier concentration of the GaAs single crystal substrate 1 and the atomic concentration of the n-type dopant into the formula: (activation rate of n-type dopant, %) = (carrier concentration) / (atomic concentration of n-type dopant) × 100.

[0047] <Boron (B)> The GaAs single crystal substrate 1 may contain B. The GaAs single crystal substrate 1 is 4.0 × 10 18 cm -3 The above 6.0 x 10 18 cm -3 The atomic concentration of B may be as described above. Since B is introduced into the Ga sites in the crystal lattice of the GaAs single crystal, it competes with the n-type dopant introduced into the Ga sites. Therefore, when B is included in the GaAs single crystal substrate 1 within the atomic concentration range described above, the n-type dopant is more likely to enter the As sites. In this case, B contributes to lowering the carrier concentration in the GaAs single crystal substrate 1, and the activation rate of the n-type dopant in the GaAs single crystal substrate 1 is controlled.

[0048] B originates from the B2O3 melt that partially coats the first melt, in which an n-type dopant is dissolved in the GaAs melt, in the GaAs single crystal manufacturing method described later. B has the effect of suppressing the ejection of atoms during radiation exposure in order to improve the strength of the GaAs single crystal. However, the atomic concentration of B is 6.0 × 10⁻⁶. 18 cm -3 If the concentration exceeds this value, the presence of boron arsenide in the GaAs single crystal substrate 1 may adversely affect its optical properties. The atomic concentration of B above can be determined by the measurement method using GDMS described above.

[0049] <Absorption coefficient of infrared light with a wavelength of 1 micron> In a GaAs single crystal substrate 1, the value of the infrared absorption coefficient at a wavelength of 1 micron in the GaAs single crystal substrate 1 (hereinafter also referred to as the "near-infrared light absorption coefficient after electron beam irradiation"), obtained by measuring after electron beam irradiation of at least one of the main surfaces 11 under the conditions of an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy, is 95% or more and 100% or less of the value of the infrared absorption coefficient at a wavelength of 1 micron in the GaAs single crystal substrate 1 (hereinafter also referred to as the "near-infrared light absorption coefficient before electron beam irradiation"), obtained by measuring before the above electron beam irradiation. In this case, the value of the near-infrared light absorption coefficient before electron beam irradiation is 3.6 cm -1 The above is acceptable. The value of the near-infrared light absorption coefficient before electron beam irradiation is 4.2 cm. -1 The following may apply: The value of the near-infrared light absorption coefficient after the above electron beam irradiation is 3.6 cm. -1 More than 4.4cm -1 The following is acceptable: If the value of the near-infrared light absorption coefficient before electron beam irradiation is within the range described above, for example, if the optical device is used as a light sensor by transmitting light through the GaAs single crystal substrate 1, light absorption by the GaAs single crystal substrate 1 will be suppressed, which may reduce the amount of grinding required on the back surface of the substrate and mitigate problems such as damage during back surface grinding.

[0050] The absorption coefficient described above can be determined by conventionally known methods from the transmittance and reflectance of light for a material measured using an ultraviolet-visible-infrared spectrophotometer or the like. However, when obtaining the transmittance and reflectance of light for the GaAs single crystal substrate 1 using an ultraviolet-visible-infrared spectrophotometer or the like, it is necessary to consider that multiple reflections occur between two parallel main surfaces. Therefore, the absorption coefficient can be determined by the following method. That is, an ultraviolet-visible-infrared spectrophotometer is used, and near-infrared light with a wavelength of 1 micron is incident on the center O of the main surface 11 of the GaAs single crystal substrate 1 at an angle of 85 degrees, thereby measuring the transmittance and reflectance of the near-infrared light in the GaAs single crystal substrate 1. Furthermore, by substituting the values ​​of the transmittance and reflectance, as well as the thickness of the GaAs single crystal substrate 1, into the following equations 1, 2, and 3, the absorption coefficient can be determined along with the reflectance of the near-infrared light at one reflection in the GaAs single crystal substrate 1. R+R(1-R) 2 exp(-2αd) / (1-r)=R * formula 1 (1-R) 2 exp(-αd) / (1-r)=T * formula 2 r=R 2 exp(2αd) Equation 3 In equations 1, 2, and 3 above, R * This represents the reflectance considering multiple reflections measured by the above-mentioned ultraviolet-visible-infrared spectrophotometer, and the unit of the above reflectance is dimensionless. T * This represents the transmittance considering multiple reflections as measured by the above-mentioned ultraviolet-visible-infrared spectrophotometer, and the unit of the above transmittance is dimensionless. α represents the light absorption coefficient, and the unit of the light absorption coefficient is cm. -1 That is the case. R represents the reflectance of the GaAs single crystal substrate 1 during one reflection of the near-infrared light, and the unit of the reflectance during one reflection is dimensionless. d represents the thickness of the GaAs single crystal substrate 1, and the unit of thickness is cm.

[0051] The procedure for determining the absorption coefficient described above will be explained below with reference to Figure 1. First, two GaAs single crystal substrates 1 to be measured are obtained by applying a conventionally known processing method to a GaAs single crystal obtained by, for example, the manufacturing method described later. From these two GaAs single crystal substrates 1, two rectangular sections 11a (for example, 650 μm thick) measuring 20 mm vertically x 20 mm horizontally are prepared, centered on the center O of the main surface 11, and these are used as samples for absorption coefficient measurement. Furthermore, the center of the surface of one of these samples (corresponding to the center O of the main surface 11 on the GaAs single crystal substrate 1) is irradiated with an electron beam under the conditions of an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy. The angle of the electron beam incident on the above surface is set to perpendicular incidence toward the center O.

[0052] Next, an ultraviolet-visible-infrared spectrophotometer (product name (part number): "U-4000", manufactured by Hitachi High-Tech Corporation) and its attached absolute reflectance measurement unit are used to direct near-infrared light with a wavelength of 1 micron to the center of the surface of both the sample irradiated with an electron beam and the sample not irradiated with an electron beam, at an angle of 85 degrees, which is 5 degrees tilted from perpendicular incidence to the main surface 11. This measures the transmittance and reflectance of the near-infrared light on the GaAs single crystal substrate 1 before and after electron beam irradiation. The transmittance and reflectance are obtained as measured values ​​after multiple reflections between the two parallel main surfaces 11 of the GaAs single crystal substrate 1 (i.e., the front and back surfaces). Finally, the absorption coefficient is obtained by substituting the transmittance, reflectance, and the thickness of the rectangular section into equations 1, 2, and 3, which take multiple reflections into account, and performing iterative numerical calculations. The absorption coefficient is obtained together with the reflectance at the time of one reflection on the main surface 11 of the GaAs single crystal substrate 1. The "reflectance at a single reflection" mentioned above refers to the reflectance determined based on the initial transmission and reflection that occur at the main surface 11 of the GaAs single crystal substrate 1 in response to incident near-infrared light. In this specification, the absorption coefficient obtained by measuring the rectangular section 11a described above is defined as the absorption coefficient of the GaAs single crystal substrate 1 for near-infrared light with a wavelength of 1 micron. Note that equations 1 and 2 above, which consider multiple reflections, assume an infinite number of reflections and determine the convergence value of the infinite geometric series.

[0053] <Dislocation density> GaAs single crystal substrate 1 is 1000 cm² -2 The following dislocation densities may be present. The GaAs single crystal substrate 1 is 500 cm². -2 The following dislocation densities may be present. In particular, the GaAs single crystal substrate 1 contains at least Si as the n-type dopant and has a dislocation density of 2.0 × 10⁻⁶. 18 cm -3 If the above-mentioned atomic concentration of Si is present, then 100 cm -2 The following dislocation densities may be present. In this case, the GaAs single crystal substrate 1 has a dislocation density of 100 cm². -2 It may have a dislocation density of less than 50 cm -2The following dislocation densities may be present. This allows for the provision of a GaAs single crystal substrate 1 that has excellent optical properties due to its low dislocation density.

[0054] In this specification, "dislocation density" means the etch pit density (EPD) measured by the following method. That is, the above "dislocation density" refers to the 1 cm² of corrosion holes (etch pits) formed on the main surface 11 in an etching test in which a GaAs single crystal substrate 1 is immersed in molten potassium hydroxide at 500°C for 10 minutes. 2 This refers to the number of hits. The measurement method for EPD described above is roughly as follows. First, a GaAs single crystal substrate 1 to be measured is obtained, for example, based on the manufacturing method described later. Next, the GaAs single crystal substrate 1 is mirror-polished, and then immersed in a potassium hydroxide (KOH) melt at 500°C for 10 minutes to form the etch pits on the main surface 11. Here, in order to clearly show the etch pits on the mirror-polished main surface 11, at least one of the following may be performed before immersion in the potassium hydroxide (KOH) melt for 10 minutes: a pretreatment using sulfuric acid and hydrogen peroxide, or a pretreatment using ammonia water and hydrogen peroxide. Although the etch pit density is not academically synonymous with dislocation density, in this technical field it is considered equivalent to dislocation density (an industrial intermediate index).

[0055] Furthermore, a lattice is formed on the entire surface of the main surface 11 of the GaAs single crystal substrate 1 extracted from the potassium hydroxide molten state, by arranging 5 mm x 5 mm squares in the most parallel arrangement without overlapping, thereby dividing the main surface 11 into segments. For example, the lattice is formed by directly writing on the substrate with a pen or the like. Next, each square constituting the lattice on the main surface 11 is considered one field of view, and the number of etch pits present in each field of view is counted using a known optical microscope (for example, product name: "ECLIPSE(registered trademark) LV150N", manufactured by Nikon Corporation). Finally, the number of etch pits present in each field of view is counted in 1 cm². 2 This is converted to the number of units. 2Since the number of etch pits per unit is equal to the number of squares that make up the grid, the above 1cm 2 The value obtained by dividing the sum of the number of etch pits per square by the number of squares mentioned above is the EPD, or dislocation density, of the GaAs single crystal substrate 1. Note that if the outer periphery and the area outside of the main surface 11 appear within the aforementioned field of view, that area is excluded from the calculation of the EPD. This is because the region near the outer periphery of the GaAs single crystal substrate 1 exhibits significant variation in the number of etch pits from substrate to substrate and is not typically used as a material for optical devices.

[0056] <Application> The GaAs single crystal substrate 1 according to this embodiment can be used as a substrate for optical devices such as semiconductor lasers (e.g., vertical-resonance surface-emitting lasers, photodetectors, etc.). In particular, based on the features described above, it can be used as a substrate for optical devices intended for use in environments that are easily exposed to radiation, such as the space environment.

[0057] [Method for manufacturing GaAs single crystals] The method for manufacturing a GaAs single crystal according to this embodiment may be, for example, a method for manufacturing a GaAs single crystal used in the GaAs single crystal substrate described above. That is, the above manufacturing method may be a method for manufacturing a GaAs single crystal with n-type conductivity using the vertical boat method. The above manufacturing method includes the steps of: preparing a single crystal growth apparatus comprising at least a cylindrical crucible and a heating device arranged to surround the outer circumference of the crucible; arranging a seed crystal formed from the GaAs single crystal at the bottom of the crucible, and arranging an n-type dopant, B2O3, metallic arsenic (As), and a bulk or powdered GaAs polycrystalline material above the seed crystal in the crucible; heating the crucible with the heating device to sublimate the metallic As, melt the B2O3 into a B2O3 melt, melt a portion of the seed crystal and the GaAs polycrystalline material into a GaAs melt, obtain a first melt in which the n-type dopant is dissolved in the GaAs melt, and bringing the first melt into contact with the remainder of the seed crystal; and obtaining the GaAs single crystal by growing a crystal from the first melt on the remainder of the seed crystal. Hereafter, the upward and downward directions along the axis of the crucible will simply be referred to as the upward and downward directions, and when describing the position or arrangement of the components, the position in the upward direction will be referred to as the upper part, and the position in the downward direction will be referred to as the lower part.

[0058] The above single crystal growth apparatus includes a branch tube section (hereinafter also referred to as the "As partial pressure control section") for controlling the partial pressure of As in the crucible. In the process of obtaining the GaAs single crystal, the end face that does not come into contact with the remainder of the seed crystal in the first melt is covered with the B2O3 melt by 10% to 50% of the area. In the process of obtaining the GaAs single crystal, the GaAs in the first melt has a stoichiometric ratio in which As exceeds 1 relative to Ga. In particular, in the method for producing the GaAs single crystal, the crucible does not need to be rotated and the first melt does not need to be stirred in the process of obtaining the GaAs single crystal.

[0059] In the method for manufacturing GaAs single crystals according to this embodiment, in the process of manufacturing GaAs single crystals using the vertical boat method, the amount of n-type dopant added, the partial pressure of As in the crucible, etc., are controlled so that the activation rate of the n-type dopant in the GaAs single crystal is around 50%, thereby manufacturing the GaAs single crystal. In particular, in the process of manufacturing the GaAs single crystal, the end face that does not come into contact with the remainder of the seed crystal in the first melt is covered with B2O3 melt by 10% to 50% of the area. Furthermore, the partial pressure of As in the crucible is controlled by the As partial pressure control unit provided in the single crystal growth apparatus, so that the required amount of metallic As precipitated after sublimation is dissolved in the first melt, the GaAs in the first melt has a stoichiometric ratio of As to Ga that exceeds 1. If the end face is completely covered with B2O3 melt, the partial pressure of As control by the As partial pressure control unit provided in the single crystal growth apparatus becomes unable to control the partial pressure of As. If the end face is not covered at all with the B2O3 melt, the As partial pressure control unit can control the As partial pressure, but B, which improves crystal strength, is not added. According to the manufacturing method of GaAs single crystals of this embodiment, partial covering of the end face with the B2O3 melt and As partial pressure control are performed simultaneously, thereby controlling the occupied positions of Si and B in the first melt and in the GaAs single crystal, increasing their added concentrations, and reducing the ejection of atoms due to radiation exposure. A manufacturing method having such characteristics can be obtained in which performance degradation due to radiation exposure and the like can be reduced.

[0060] The method for manufacturing a GaAs single crystal and a GaAs single crystal substrate containing the same according to this embodiment will be specifically described below with reference to Figures 3 and 4. Figure 3 is a schematic flowchart of the method for manufacturing a GaAs single crystal and a GaAs single crystal substrate according to this embodiment. Figure 4 is an explanatory diagram illustrating the process of growing a GaAs single crystal using a single crystal manufacturing apparatus, relating to the method for manufacturing a GaAs single crystal according to this embodiment.

[0061] In this embodiment, a vertical boat method is used to manufacture a GaAs single crystal having an n-type conductivity. The vertical boat method includes, for example, the vertical Bridgeman (VB) method, the vertical temperature gradient solidification (VGF) method, and a hybrid method combining the VB method and the VGF method. Below, a method for manufacturing a GaAs single crystal using the VB method will be described. The above manufacturing method is, for example, a method for manufacturing a GaAs single crystal using the VB method, and may include a step (preparation step) S10 of preparing a single crystal growth apparatus that includes at least a cylindrical crucible and a heating device arranged to surround the outer circumference of the crucible, as shown in Figure 3, and a step S20 of growing a GaAs single crystal from a seed crystal by crystal growth using the single crystal growth apparatus.

[0062] Step S20 for growing a GaAs single crystal may include: Step S21 (raw material containment step) of containing a seed crystal formed from the GaAs single crystal at the bottom of the crucible, and containing an n-type dopant, B2O3, metallic As, and a bulk or powdered GaAs polycrystalline material above the seed crystal in the crucible; Step S22 (raw material melting and contact step) of heating the crucible with the heating device to sublimate the metallic As, melt the B2O3 into the B2O3 melt, melt a portion of the seed crystal and the GaAs polycrystalline material into the GaAs melt, obtain a first melt in which the n-type dopant is dissolved in the GaAs melt, and bring the first melt into contact with the remainder of the seed crystal; and Step S23 (raw material melting and contact step) of growing a crystal on the remainder of the seed crystal from the first melt to obtain the GaAs single crystal. As shown in Figure 3, the method for manufacturing a GaAs single crystal substrate according to this embodiment, which will be described later, can be constructed from the preparation step S10 and the step S20 for growing a GaAs single crystal, which are steps included in the above manufacturing method, and the step S30 for obtaining a GaAs single crystal substrate by processing the GaAs single crystal obtained by the above manufacturing method.

[0063] <Preparation process> In the preparation step S10 for preparing the single crystal growth apparatus, a single crystal growth apparatus 10 is prepared, for example, equipped with a crucible 5 and a heating device 73 as shown in Figure 4. Furthermore, in the preparation step S10, along with the single crystal growth apparatus 10, seed crystals 8a, such as a lump of metallic As, and a lump or powdered polycrystalline body of GaAs, which will be used as materials for growing the GaAs single crystal 81, are prepared. The seed crystals 8a are formed from a GaAs single crystal. The seed crystals 8a and the polycrystalline body of GaAs may be prepared by known methods or by obtaining commercially available materials. The seed crystals 8a may have a cylindrical shape that can be accommodated in the seed crystal housing section 51 of the crucible 5, which will be described later. The polycrystalline body of GaAs may have a size that can be accommodated in at least the straight body section 53 of the crucible 5, which will be described later.

[0064] (Single crystal growth apparatus) 1) Crucible As shown in Figure 4, the single crystal growth apparatus 10 includes a crucible 5. The crucible 5 may include a cylindrical seed crystal housing section 51 for housing a seed crystal 8a formed from a GaAs single crystal, a diameter-increasing section 52 connected to the seed crystal housing section 51, and a straight body section 53 connected to the diameter-increasing section 52. The seed crystal housing section 51 has a hollow section that opens at the position where it is connected to the diameter-increasing section 52 and has a bottom wall formed at the position opposite to the diameter-increasing section 52. The seed crystal housing section 51 can house and hold the seed crystal 8a in the hollow section. The diameter-increasing section 52 has a frustoconical shape that expands in diameter upwards in the crucible 5 and is connected to the seed crystal housing section 51 on the smaller diameter side of the diameter-increasing section 52. The straight body section 53 has a hollow cylindrical shape and is connected to the larger diameter side of the diameter-increasing section 52. The diameter-increasing section 52 and the straight section 53 have the function of holding a polycrystalline GaAs material within them, which will be used to obtain a GaAs single crystal 81 by crystal growth. The diameter-increasing section 52 and the straight section 53 have the function of growing a GaAs single crystal by solidifying the first melt 82 containing GaAs melt and n-type dopant, as will be described later. The crucible 5 can be made of various materials that can withstand the temperature of the first melt 82. For example, the material of the crucible 5 is pyrolysis boron nitride (PBN).

[0065] The maximum inner diameter of crucible 5 is 100 mm or more. More specifically, the inner diameter of the straight body section 53 of crucible 5 may be 100 mm or more. The inner diameter of the straight body section 53 of crucible 5 may be 150 mm or more. There is no particular upper limit to the maximum inner diameter of crucible 5, but for example, it is 210 mm.

[0066] 2) Holding stand The single crystal growth apparatus 10 includes a holder 71 for holding the diameter-increasing portion 52 of the crucible 5. The holder 71 may have, for example, a cylindrical appearance. The holder 71 may be formed from a single material. The material may be, for example, quartz, alumina, or silicon carbide. The material may be formed from quartz. In particular, the material may be opaque when considering heat conduction in the crucible and the GaAs melt. The outer diameter of the holder 71 may correspond to or be the same as the outer diameter of the straight body portion 53.

[0067] 3) Heating device The single crystal growth apparatus 10 includes a heating device 73 for heating the crucible 5. The heating device 73 may consist of two bodies. In this case, the two bodies are arranged to surround the outer circumference of the crucible 5. Each heating device 73 is divided into multiple parts perpendicular to the axis of the crucible. Thus, each heating device 73 is configured in multiple stages. Furthermore, the output of the heating device 73 can be controlled independently for each body and each part. Therefore, the temperature at the interface between the GaAs single crystal 81 and the first melt 82 can be precisely controlled. As the heating device 73, for example, a known electric heater may be used.

[0068] The single crystal growth apparatus 10 can include a thermocouple capable of measuring the temperature of the crucible 5 heated by the heating device 73. A plurality of thermocouples may be arranged outside the crucible 5 and along the axial direction. In this case, a plurality of thermocouples, at least, may be arranged near the height corresponding to the interface between the GaAs single crystal 81 and the first melt 82 in the crucible 5 on the outside and in the axial direction of the crucible 5. Based on the temperatures measured by each of the thermocouples, the temperatures at various locations of the GaAs single crystal 81 growing in the crucible 5 are estimated. In particular, based on the temperatures measured by each of the thermocouples, the radial temperature distribution at the above interface can be estimated.

[0069] 4) Branch pipe part (As partial pressure control part) The single crystal growth apparatus 10 includes an As partial pressure control part 9 for controlling the As partial pressure in the crucible 5. For example, the As partial pressure control part 9 is arranged at a position opposite to the seed crystal accommodation part 51 in the axial direction. The As partial pressure control part 9 controls the As partial pressure in the crucible 5 in the step S23 of obtaining the GaAs single crystal. Specifically, by the action of the As partial pressure control part 9, in the step S23 of obtaining the GaAs single crystal, the temperature in the crucible 5 is controlled to be the lowest at the As partial pressure control part 9. As a result, the metallic As 9a sublimated with the heating of the crucible 5 in the raw material melting and contact step S22 and deposited on the side wall, upper wall, etc. of the crucible 5 is dissolved in the first melt 82 in an amount such that the As pressure becomes 101 kPa at 1238 °C, for example, in the step S23 of obtaining the GaAs single crystal. At this time, the stoichiometric ratios of both As in the first melt 82 containing the GaAs melt and the GaAs single crystal 81 exceed 1 with respect to Ga. As shown in FIG. 4, in order to contribute to the temperature control in the crucible 5 by the As partial pressure control part 9, a heat insulating material 91 may be arranged adjacent to the As partial pressure control part 9. As the heat insulating material 91, for example, a known heat insulating material used for temperature control of a crucible or the like in the vertical boat method can be adopted. Thereby, since the n-type dopant in the first melt 82 is easily introduced into the Ga sites in the crystal lattice of the GaAs single crystal, the activation rate of the n-type dopant can be controlled. The As partial pressure control part 9 can be formed of, for example, the same material as the crucible 5.

[0070] <Step of growing a GaAs single crystal> Next, in the above manufacturing method, step S20 is performed to grow a GaAs single crystal from a seed crystal by crystal growth using the above single crystal growth apparatus. Specifically, in step S20 for growing the GaAs single crystal, the above-described single crystal growth apparatus 10 and seed crystal 8a are used to grow a GaAs single crystal 81. Step S20 for growing the GaAs single crystal includes the following raw material receiving step S21, raw material melting and contact step S22, and step S23 for obtaining the GaAs single crystal. Step S20 for growing the GaAs single crystal is performed in this order.

[0071] (Full raw material processing) In the raw material containment step S21, a seed crystal 8a made of a GaAs single crystal is contained in the hollow part of the seed crystal containment section 51 of the crucible 5. A known method can be used to contain the seed crystal 8a in the seed crystal containment section 51. Multiple GaAs polycrystalline bodies are also contained and stacked in the diameter-increasing section 52 and the straight body section 53 of the crucible 5. Along with the GaAs polycrystalline bodies, at least one metal As selected from the group consisting of Si, Te, and Sn as an n-type dopant for arsenic pressure control is added to the diameter-increasing section 52 and the straight body section 53 in the crucible 5. Furthermore, solid B2O3 is contained on top of the GaAs polycrystalline bodies. In this case, the atomic concentration of the n-type dopant in the GaAs single crystal 81 obtained by crystal growth is 5.8 × 10⁻¹⁶. 18 cm -3 The above 1.3 × 10 19 cm -3 The following amounts may be added: When solid B2O3 is converted into B2O3 melt 6 in step S23, which is described later, the amount of B2O3 that does not come into contact with the remainder of the seed crystal 8a in the first melt 82 may be covered by the B2O3 melt 6 by 10% to 50% of its surface area.

[0072] (Raw material melting and contact process) In the raw material melting and contact process S22, a seed crystal 8a and a first melt 82 in which n-type dopant is dissolved in melted GaAs are brought into contact with each other for the purpose of obtaining a GaAs single crystal 81. In the raw material melting and contact process S22, a crucible 5 containing the seed crystal 8a, GaAs polycrystalline material, n-type dopant, metallic As, and solid B2O3 is supported on a holder 71. Then, current is supplied to the heating device 73 and the crucible 5 is heated. As a result, the GaAs polycrystalline material melts, and the n-type dopant dissolves into it to form the first melt 82. The lump of metallic As placed near the GaAs polycrystalline material sublimes. Subsequently, metallic As 9a precipitates on the side walls, top walls, etc. of the crucible 5 where the temperature is lower. Furthermore, a portion of the seed crystal 8a also melts, and the remaining portion of the seed crystal 8a comes into contact with the first melt 82 at its interface. The solid B2O3 also melts to form melted B2O3 6. Due to the difference in specific gravity, melted B2O3 6 is located above the first melt 82.

[0073] (Process for obtaining GaAs single crystals) In step S23, which is the process for obtaining a GaAs single crystal, the crucible 5 is gradually lowered (towards the seed crystal containment section 51) relative to the heating device 73, for example, thereby creating a temperature gradient in the crucible 5 such that the temperature on the seed crystal 8a side is lower and the temperature on the first melt 82 side is higher. As a result, the first melt 82 in contact with the seed crystal 8a solidifies, and the GaAs single crystal 81 continuously grows from the first melt 82 on the remainder of the seed crystal 8a. The speed at which the crucible 5 is lowered is not particularly limited, but is, for example, 1 to 5 mm / hour.

[0074] In step S23, which is the process for obtaining a GaAs single crystal, the end face of the first melt 82 that does not come into contact with the remainder of the seed crystal 8a is covered by the B2O3 melt 6 by 10% to 50% of the area. Specifically, when the first melt 82 is heated by the heating device 73, the end face becomes convex upward, and the top of the end face is exposed from the B2O3 melt 6. As a result, only the peripheral area around the top is covered by the B2O3 melt 6. Covering the end face by the B2O3 melt 6 by 10% to 50% of the area means that the peripheral area of ​​the end face is covered by 10% to 50% of the area, and the top is covered by 50% to 90% of the area.

[0075] Furthermore, in step S23, which is the step for obtaining a GaAs single crystal, the amount of As in the first melt 82 is controlled via the top portion, which is 50% to 90% of the surface area that is not covered by the B2O3 melt 6 at the end face. Specifically, the temperature of the As partial pressure control unit 9 is set to the lowest possible temperature within the crucible 5, so that an As pressure corresponding to the temperature of the As partial pressure control unit 9 acts toward the end face in the first melt 82. As a result, of the metallic As 9a deposited on the side walls, top walls, etc. of the crucible 5 as described above, the amount that has an arsenic pressure of 101 kPa at 1238°C due to the action of the As partial pressure control unit 9 is dispersed in the crucible 5 and dissolved in the first melt 82. The GaAs in the first melt 82 is made to have a stoichiometric ratio in which As exceeds 1 relative to Ga, making it easier for the n-type dopant in the first melt 82 to be introduced into the Ga site in the crystal lattice of the GaAs single crystal 81, and the activation rate of the n-type dopant can be controlled.

[0076] In the step S23 of obtaining a GaAs single crystal, as the crucible 5 is pulled downward with respect to the heating device 73, the crystal growth of the GaAs single crystal 81 continues until the solidification of the first melt 82 remaining in the straight body portion 53 of the crucible 5 is completed. Here, in the step S23 of obtaining a GaAs single crystal, from the viewpoint of retaining B, which is a light element in the first melt 82, at the upper part of the first melt 82 by gravity segregation, the crucible 5 does not need to be rotated, and the first melt 82 does not need to be stirred. This is because if the concentration of B mixed from the B2O3 melt 6 into the first melt 82 becomes too high, boron arsenide may precipitate in the first melt 82, which may deteriorate the crystallinity of the GaAs single crystal 81. Thereby, the atomic concentration of B in the GaAs single crystal 81 can be controlled to be 4.0×10 18 cm -3 or more and 6.0×10 18 cm -3 or less. As described above, a GaAs single crystal 81 (for example, its ingot or the like) is obtained. The GaAs single crystal 81 in the crucible 5 is then taken out of the crucible 5.

[0077] <() 〔Method for manufacturing a GaAs single crystal substrate〕 <Step S30 of obtaining a GaAs single crystal substrate> The method for manufacturing a GaAs single crystal substrate according to the present embodiment is a method for manufacturing a GaAs single crystal substrate having an n-type conductivity type. As shown in FIG. 3, the manufacturing method includes a step S30 of obtaining a GaAs single crystal substrate having a circular main surface by processing the GaAs single crystal obtained by the method for manufacturing a GaAs single crystal. Specifically, the step S30 of obtaining a GaAs single crystal substrate includes a cutting step of obtaining a GaAs single crystal substrate precursor by cutting the GaAs single crystal 81 obtained by the method for manufacturing a GaAs single crystal into a disk shape, and an outer peripheral grinding step of obtaining a GaAs single crystal substrate having a circular main surface by processing such as grinding the outer periphery of the GaAs single crystal substrate precursor. By such a manufacturing method, a GaAs single crystal substrate suitable as a substrate for an optical device assumed to be used in a space environment can be obtained because deterioration of performance due to exposure to radiation or the like is reduced.

[0078] Step S30 for obtaining a GaAs single crystal substrate includes the following cutting step, outer edge grinding step, and, if necessary, a polishing step described later, and these steps may be carried out in this order.

[0079] The cutting step is a process of slicing the ingot formed from the GaAs single crystal 81 removed from the crucible 5 into wafers of a predetermined thickness in order to obtain a disc-shaped GaAs single crystal substrate precursor. Furthermore, the outer edge grinding step is a process of grinding the outer edge of the GaAs single crystal substrate precursor to obtain a GaAs single crystal substrate having a circular main surface. Known cutting methods and outer edge grinding methods may be used for the cutting step and outer edge grinding step. Furthermore, the polishing step is a process of mirror-finishing the main surface. Known polishing methods may be used for the polishing step. [Examples]

[0080] The present disclosure will be described in more detail below with reference to examples, but will not be limited thereto. In this example, the inventors used a single crystal growth apparatus as shown in Figure 4 and the VB method <100> A single crystal of GaAs was grown using the direction as the growth direction. In the following description, Samples 1 to 4 are examples, and Samples A and B are comparative examples.

[0081] [Manufacturing of GaAs single crystal substrates] <Sample 1> (preparation process) First, a single crystal growth apparatus as shown in Figure 4 was prepared. Furthermore, GaAs seed crystals formed from GaAs single crystals and massive GaAs polycrystalline bodies were prepared by known methods. The GaAs seed crystals had a cylindrical shape that could be accommodated in the seed crystal housing section of the crucible. The GaAs polycrystalline bodies were of a size that could be accommodated in the straight section. The inner diameter of the straight section of the crucible included in the single crystal growth apparatus was 154 mm.

[0082] (Process for growing GaAs single crystals) In accordance with the raw material receiving process, raw material melting and contact process, and process for obtaining a GaAs single crystal described above, a process of growing a GaAs single crystal from a GaAs seed crystal by crystal growth using the single crystal growth apparatus described above was carried out. In the raw material receiving process, along with the GaAs polycrystalline material, an n-type dopant was added, for example, Si, which was 7.0 × 10⁻⁶ 18 cm -3 The arsenic was added to the crucible at the specified atomic concentration. Furthermore, for the purpose of controlling the arsenic pressure, an amount of metallic As was placed in the crucible such that the arsenic pressure at 1238°C was 101 kPa. Additionally, 10 g of solid B2O3 was placed in the crucible. During the raw material melting and contact process, the entire amount of metallic As sublimated. In the process of obtaining the GaAs single crystal, the edges of the first melt, in which the n-type dopant was dissolved in the GaAs melt, that did not come into contact with the GaAs seed crystal, were covered with B2O3 melt at a peripheral area corresponding to 10% of their area. Here, if we let T1 be the temperature of the first melt not covered by B2O3, P1 be the arsenic pressure in equilibrium with the GaAs (first melt) at temperature T1, T2 be the temperature of the branch tube, and P2 be the vapor pressure of metallic As in the branch tube at temperature T2, then the higher the temperature T2 of the branch tube, the greater the vapor pressure P2 of metallic As. In accordance with the above facts, in the process of obtaining a GaAs single crystal, the temperature T2 of the branch tube was adjusted so that it was the lowest temperature in the crucible when all of the GaAs had melted, thereby controlling the partial pressure of As in the crucible. As a result, the stoichiometric ratio of GaAs in the first melt was set so that As was greater than 1 relative to Ga. For example, the arsenic pressure P1 in equilibrium with GaAs at 1238°C is approximately 91 kPa, which is the sum of As4 (a tetraatomic molecule of As) and As2 (a diatomic molecule of As). In this case, if the temperature T2 of the branch tube is 613°C, the vapor pressure P2 of metallic As is 101 kPa. That is, P2 > P1, so the stoichiometric ratio of GaAs in the first melt is set so that As is greater than 1 relative to Ga.

[0083] In the process of obtaining the GaAs single crystal, the crucible was gradually lowered from the heating device at a speed of 4 mm / hour. The crystal growth of the GaAs single crystal was then continued until the solidification of the molten material remaining in the straight section of the crucible was complete. As a result, a GaAs single crystal ingot with a diameter of 154 mm and a thickness of 200 mm was produced.

[0084] (Process for obtaining a GaAs single crystal substrate) Following the process for obtaining the GaAs single crystal substrate described above from the GaAs single crystal ingot, the required number of GaAs single crystal substrates of Sample 1, which has an n-type conductivity and a circular main surface, were manufactured. The GaAs single crystal substrate of Sample 1 had a diameter of 150 mm and a thickness of 650 μm.

[0085] <Sample 2 to Sample 4> Except for the change in the amount of Si added as an n-type dopant during the raw material handling process, as shown in Table 1, the required number of GaAs single crystal substrates for Samples 2 to 4 were manufactured using the same procedure as for obtaining the GaAs single crystal substrate for Sample 1.

[0086] <Sample A> A GaAs single crystal substrate of sample A was manufactured using the known horizontal Bridgman method (HB method). In this case, the atomic concentration of Si as an n-type dopant added to the GaAs melt was as shown in Table 1. B2O3 was not used in the manufacture of the GaAs single crystal substrate of sample A.

[0087] <Sample B> Except for the fact that the amount of Si added as n-type dopant in the raw material handling process was changed as shown in Table 1, and in the process of obtaining the GaAs single crystal, the entire surface of the first melt in which the n-type dopant was dissolved in the GaAs melt was covered with B2O3 melt, and the As partial pressure in the crucible was not actively controlled in the As partial pressure control unit, the required number of GaAs single crystal substrates for sample B were manufactured in the same manner as the manufacturing method for sample 1.

[0088] [Characterization of GaAs single crystal substrates] For GaAs single crystal substrates of Samples 1 to 4, Sample A, and Sample B, the atomic concentration of Si, atomic concentration of B, carrier concentration, activation rate, dislocation density (EPD), absorption coefficient of infrared radiation at a wavelength of 1 micron before and after electron beam irradiation, and the absorption coefficient ratio (absorption coefficient of infrared radiation at a wavelength of 1 micron after electron beam irradiation / absorption coefficient of infrared radiation at a wavelength of 1 micron before electron beam irradiation × 100) were determined by applying the measurement and calculation methods described above. The results are shown in Table 1.

[0089] [Table 1]

[0090] [Consideration] According to Table 1, the GaAs single crystal substrates of Samples 1 to 4 exhibit a ratio of 95-100% of the infrared absorption coefficient at a wavelength of 1 micron before and after electron beam irradiation compared to Samples A and B, indicating reduced performance degradation due to radiation exposure. Therefore, it is suggested that the GaAs single crystal substrates of Samples 1 to 4 can serve as substrates for optical devices intended for use in space environments.

[0091] As described above, embodiments and examples of this disclosure have been explained, but it is also intended from the outset that the configurations of each of the embodiments and examples described above may be combined as appropriate.

[0092] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than the embodiments and examples described above, and all modifications within the scope of the claims are intended to be included in the meaning of equivalents and within the scope. [Explanation of Symbols]

[0093] 1 GaAs single crystal substrate 11 Main surface 11a Rectangular section O center OF Orientation Flat 21 electrodes 10 Single crystal growth apparatus 5 Crucible 51 Crystal housing section 52 Diameter increase section 53 Straight body part 6 B2O3 melt 71 Holding stand 73 Heating device 8a seed crystal 81 GaAs single crystal 82 First melt 9. Branch pipe section (As pressure control section) 9a Metal As 91 Insulation S10 Preparation process Process for growing S20 GaAs single crystals S21 Raw material containment process S22 Raw material melting and contact process S23 Process for obtaining GaAs single crystal Process for obtaining S30 GaAs single crystal substrate

Claims

1. A gallium arsenide single crystal substrate whose conductivity type is electron-donating, The gallium arsenide single crystal substrate has a circular main surface, The gallium arsenide single crystal substrate contains an electron-donating dopant. A gallium arsenide single crystal substrate, wherein the value of the infrared absorption coefficient at a wavelength of 1 micron in the gallium arsenide single crystal substrate, obtained by measuring after electron beam irradiation of at least one of the main surfaces under the conditions of an acceleration voltage of 2 MeV and an irradiation dose of 150 kGy, is 95% or more and 100% or less of the value of the infrared absorption coefficient at a wavelength of 1 micron in the gallium arsenide single crystal substrate, obtained by measuring before the electron beam irradiation.

2. The gallium arsenide single crystal substrate is 3.5 × 10 １８ cm －３ The above 5.0 x 10 １８ cm －３ Having the following carrier concentrations, The gallium arsenide single crystal substrate according to claim 1, wherein the gallium arsenide single crystal substrate has an activation rate of the electron-donating dopant of 40% or more and 60% or less.

3. The gallium arsenide single crystal substrate according to claim 2, wherein the gallium arsenide single crystal substrate has an activation rate of the electron-donating dopant of 45% or more and 55% or less.

4. The gallium arsenide single crystal substrate is 5.8 × 10 １８ cm －３ The above 1.3 x 10 １９ cm －３ A gallium arsenide single crystal substrate according to any one of claims 1 to 3, having the following atomic concentrations of the electron-donating dopant.

5. The gallium arsenide single crystal substrate contains boron, The gallium arsenide single crystal substrate has a boron atomic concentration of 4.0×10 １８ cm －３ or more and 6.0×10 １８ cm －３ or less, and is the gallium arsenide single crystal substrate according to any one of claims 1 to 3.

6. The value of the infrared absorption coefficient at a wavelength of 1 micron in the gallium arsenide single crystal substrate, obtained by measurement before the electron beam irradiation, is 3.6 cm². －１ The gallium arsenide single crystal substrate according to any one of claims 1 to 3.

7. The gallium arsenide single crystal substrate according to any one of claims 1 to 3, wherein the electron-donating dopant is at least one selected from the group consisting of silicon, tellurium, and tin.

8. The gallium arsenide single crystal substrate is 1000 cm² －２ A gallium arsenide single crystal substrate according to any one of claims 1 to 3, having the following dislocation density.

9. The gallium arsenide single crystal substrate contains at least silicon as the electron-donating dopant, The gallium arsenide single crystal substrate is 2.0 × 10 １８ cm －３ Having the above-mentioned atomic concentration of silicon, The gallium arsenide single crystal substrate is 100 cm² －２ A gallium arsenide single crystal substrate according to claim 8, having the following dislocation density.

10. A method for producing a gallium arsenide single crystal having an electron-donating conductivity type using the vertical boat method, The manufacturing method includes the steps of preparing a single crystal growth apparatus comprising at least a cylindrical crucible and a heating device arranged to surround the outer circumference of the crucible, The process involves placing a seed crystal formed from the gallium arsenide single crystal at the bottom of the crucible, and placing an electron-donating dopant, boron oxide, metallic arsenic, and a massive polycrystalline gallium arsenide above the seed crystal in the crucible. The process involves heating the crucible with the heating device to sublimate the metallic arsenic, melting the boron oxide into the boron oxide melt, melting a portion of the seed crystal and the gallium arsenide polycrystalline material into the gallium arsenide melt, obtaining a first melt in which the electron-donating dopant is dissolved in the gallium arsenide melt, and bringing the first melt into contact with the remainder of the seed crystal. The process includes the step of obtaining the gallium arsenide single crystal by growing a crystal on the remainder of the seed crystal from the first melt, The single crystal growth apparatus includes a branch tube section for controlling the arsenic partial pressure in the crucible. In the process of obtaining the gallium arsenide single crystal, the end face that does not come into contact with the remainder of the seed crystal in the first melt is coated with the boron oxide melt by 10% to 50% of the area. A method for producing a gallium arsenide single crystal, wherein, in the step of obtaining the gallium arsenide single crystal, the gallium arsenide in the first melt has a stoichiometric ratio in which arsenic exceeds 1 relative to gallium.

11. The method for producing a gallium arsenide single crystal according to claim 10, wherein, in the step of obtaining the gallium arsenide single crystal, the crucible is not rotated and the first melt is not stirred.

12. A method for manufacturing a gallium arsenide single crystal substrate, comprising the step of obtaining a gallium arsenide single crystal substrate having a circular main surface by processing the gallium arsenide single crystal obtained by the method for manufacturing a gallium arsenide single crystal according to claim 10 or claim 11.

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