Diamond spin sensor and method for manufacturing same

Abstract

A diamond spin sensor according to the present invention includes a diamond including an NV- center having an electron spin. When the lateral relaxation time of the electron spin measured by the Hahn echo method is T2 μsec and the fluorescence intensity of the fluorescence emitted from the diamond when irradiated with microwaves and laser light is represented by a current value InA output from an Si-PIN diode that receives the fluorescence, the product of T2 and I is larger than 3000, the wavelength of the microwaves is 2.07 GHz to 3.67 GHz, the wavelength of the laser light is 520 nm to 540 nm, the power of the laser light is 3 mW, and the light reception sensitivity of the Si-PIN diode is 0.36 A / W to 0.44 A / W at a wavelength of 600 nm and 0.40 A / W to 0.50 A / W at a wavelength of 660 nm.

Description

Diamond spin sensor and its manufacturing method

[0001] The present disclosure relates to a diamond spin sensor and a method for manufacturing the same. This application claims priority to Japanese Application No. 2024-027998, filed February 28, 2024, the entire contents of which are incorporated herein by reference.

[0002] A diamond spin sensor using the NV center of diamond (hereinafter referred to as NV center) is known. When the NV center, which is composed of nitrogen (i.e., N) in the substitution position of carbon (i.e., C) in diamond and a vacancy (i.e., V: Vacancy) adjacent to the nitrogen, becomes negatively charged, its ground state becomes a triplet state (i.e., spin S=1). The negatively charged NV center is NV － Although the term "NV center" is used in the present specification, it will be abbreviated as "NV center" below for convenience. When a charged NV center is excited by laser light with a wavelength of approximately 530 nm (i.e., green light), it emits fluorescence with a wavelength of approximately 635 nm (i.e., red light). The intensity of the fluorescence changes depending on the spin state of the NV center, and the spin state of the NV center changes due to magnetic resonance between a magnetic field applied to the NV center and microwaves or radio waves, so it can be used as a magnetic sensor.

[0003] The detection system uses a diamond substrate containing NV centers, which is a diamond spin sensor, an optical system that transmits excitation light from a light source and irradiates the NV centers, a transmission line and microwave circuit that transmit microwaves from a power source and irradiate the NV centers, and an optical system that collects fluorescence from the NV centers and transmits it to a photodetector.

[0004] Known is the method of reducing the distortion and impurity of diamond crystal, and making high-purity diamond.For example, in the following Patent Document 1, it is disclosed that, for the nitrogen element that is the largest impurity, by adding getter material such as titanium (that is, Ti) to raw material, it can make diamond with less nitrogen.In the following Patent Document 2, it is disclosed that, as the diamond of seed crystal, cut out the part with less crystal defects from the diamond of 3mm or more size to 1mm or less size, and use this as seed substrate to grow, it can reduce the distortion of crystal.

[0005] Japanese Patent Application Laid-Open No. 7-148426 Japanese Patent Application Laid-Open No. 9-165295 International Publication No. 2022 / 210696 International Publication No. 2022 / 210723 International Publication No. 2022 / 209512 International Publication No. 2016 / 013588

[0006] The diamond spin sensor according to one aspect of the present disclosure is a NV having electron spin. － The diamond includes a center, and when the transverse relaxation time of electron spin measured by the Hahn echo method is T2 μsec, and the fluorescence intensity of fluorescence emitted from the diamond by irradiating the diamond with microwaves and laser light is expressed in terms of a current value InA output from a Si-PIN diode that receives the fluorescence, the product of T2 and I is greater than 3000, the microwave wavelength is 2.07 GHz or more and 3.67 GHz or less, the laser light wavelength is 520 nm or more and 540 nm or less, the laser light power is 3 mW, and the light receiving sensitivity of the Si-PIN diode is 0.36 A / W or more and 0.44 A / W or less at a wavelength of 600 nm, and 0.40 A / W or more and 0.50 A / W or less at a wavelength of 660 nm.

[0007] FIG. 1 is a perspective view showing a diamond spin sensor according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing the crystal planes and orientations of diamond. FIG. 3 is a block diagram showing the configuration of an apparatus used for measurements using the diamond spin sensor shown in FIG. 1. FIG. 4 is a sequence diagram showing the timing of irradiation of excitation light and electromagnetic waves, and the timing of measurement of synchrotron radiation during measurements using the diamond spin sensor shown in FIG. 1. FIG. 5 is a graph showing the relationship between the observed signal intensity (i.e., fluorescence intensity) and the frequency of the electromagnetic wave (i.e., microwave). FIG. 6 is a schematic diagram showing the NV center of diamond. FIG. 7 is a perspective view showing the optical path of fluorescence emitted from the NV center. FIG. 8 is a sequence diagram showing the timing of irradiation of excitation light and electromagnetic waves, and the timing of measurement of synchrotron radiation, for measuring the transverse relaxation time T2 of the diamond spin sensor shown in FIG. 1. FIG. 9 is a graph showing the transverse relaxation time T2. FIG. 10 is a schematic diagram showing a diamond synthesis apparatus. FIG. 11 is a schematic diagram showing a method for producing a seed crystal used in diamond synthesis. Figure 12 is a schematic diagram showing a method for synthesizing diamond using a diamond cut out from the synthetic diamond shown in Figure 11 as a seed crystal. Figure 13 is a schematic diagram showing a method for synthesizing diamond using a seed crystal larger than the seed crystal shown in Figure 12. Figure 14 is a plan view showing the synthetic diamond shown in Figure 13. Figure 15 is a plan view showing a conventional synthetic diamond. Figure 16 is a diagram showing the manufacturing conditions of experimental samples in table format. Figure 17 is a diagram showing the experimental results in table format. Figure 18 is a graph showing the experimental results.

[0008] [Problem to be Solved by the Present Disclosure] With regard to sensors using diamond NV centers (also called color centers), the longer the transverse spin relaxation time T2, the higher the sensor sensitivity, which is preferable. That is, the longer the transverse relaxation time T2, the longer the time that the resonant electromagnetic waves are applied, thereby increasing the sensor sensitivity. Furthermore, the stronger the emission intensity, the easier it is to detect signals, and the higher the sensor sensitivity. If the concentration of NV centers is increased to increase the emission intensity, many spin-bearing nitrogen atoms get in the way, shortening the transverse relaxation time T2 (fast signal decay). On the other hand, if the concentration of NV centers in diamond is reduced to increase the transverse relaxation time T2, the number of NV centers, which are the source of fluorescence, decreases, and the emission intensity weakens. As such, because there is a trade-off between the transverse relaxation time T2 and the fluorescence intensity, it has been difficult to increase the fluorescence intensity without decreasing the transverse relaxation time T2. It has also been difficult to increase the transverse relaxation time T2 without decreasing the fluorescence intensity.

[0009] As disclosed in Patent Document 1 and Patent Document 2, it is easy to make high-purity diamond crystals that are almost free of impurities and defects.However, in order to use diamond that contains NV centers that require nitrogen and defects (vacancies) as a sensor, it is necessary to remove the factors that cause scattering of fluorescence in the state where a trace amount of strain and impurities remains.That is, it is desired to find a method that reduces other impurities and strain while leaving a trace amount of nitrogen and defects (vacancies) that are impurities.

[0010] Therefore, an object of the present disclosure is to provide a diamond spin sensor and a method for manufacturing the same that can increase the fluorescence intensity without decreasing the transverse relaxation time, or that can increase the transverse relaxation time without decreasing the fluorescence intensity.

[0011] [Effects of the Present Disclosure] According to the present disclosure, it is possible to provide a diamond spin sensor and a manufacturing method thereof that can increase the fluorescence intensity without decreasing the transverse relaxation time, or that can increase the transverse relaxation time without decreasing the fluorescence intensity.

[0012] [Description of Embodiments of the Present Disclosure] The contents of the embodiments of the present disclosure will be listed and described below. At least some of the embodiments described below may be combined in any combination.

[0013] (1) The diamond spin sensor according to the first aspect of the present disclosure is a NV having electron spin. － The diamond includes a center, and when the transverse relaxation time of electron spin measured by the Hahn echo method is T2 μsec, and the fluorescence intensity of fluorescence emitted from the diamond by irradiating the diamond with microwaves and laser light is expressed in terms of the current value InA output from a Si-PIN diode that receives the fluorescence, the product of T2 and I is greater than 3000, the microwave wavelength is 2.07 GHz or more and 3.67 GHz or less, the laser light wavelength is 520 nm or more and 540 nm or less, the laser light power is 3 mW, and the Si-PIN diode has a light receiving sensitivity of 0.36 A / W or more and 0.44 A / W or less at a wavelength of 600 nm, and 0.40 A / W or more and 0.50 A / W or less at a wavelength of 660 nm. This allows the fluorescence intensity to be increased without decreasing the transverse relaxation time, or the transverse relaxation time to be increased without decreasing the fluorescence intensity.

[0014] (2) In the above (1), the product of T2 and I may be greater than 10,000. This allows the fluorescence intensity to be further increased without decreasing the transverse relaxation time, or allows the transverse relaxation time to be further increased without decreasing the fluorescence intensity.

[0015] (3) The diamond spin sensor according to the second aspect of the present disclosure is a NV having electron spin. － Contains a diamond center, NV － The center concentration is 0.02 ppm to 10 ppm, and the average phase difference over the entire surface of the diamond is 6 nm / mm or less, which allows the fluorescence intensity to be increased without decreasing the transverse relaxation time, or the transverse relaxation time to be increased without decreasing the fluorescence intensity.

[0016] (4) In any one of (1) to (3) above, the average phase difference over the entire surface of the diamond may be 4 nm / mm or less, thereby increasing the product of the transverse relaxation time T2 and the fluorescence intensity I, and realizing a sensor with higher sensitivity.

[0017] (5) In any one of (1) to (4) above, NV － The concentration of the center may be 0.02 ppm or more and 1.2 ppm or less, which increases the product of the transverse relaxation time T2 and the fluorescence intensity I, thereby realizing a sensor with higher sensitivity.

[0018] (6) The diamond spin sensor according to the third aspect of the present disclosure is a NV having electron spin. － Contains a diamond center, NV － The center concentration is 0.02 ppm to 10 ppm, and the half-width of the rocking curve measured by the double crystal method is 8 seconds or less, which allows the fluorescence intensity to be increased without decreasing the transverse relaxation time, or the transverse relaxation time to be increased without decreasing the fluorescence intensity.

[0019] (7) In any one of (1) to (4) and (6) above, NV － The center concentration may be 0.02 ppm or more and 2 ppm or less, and the half-width of the rocking curve of X-ray diffraction by the double crystal method may be 6 seconds or less, thereby enabling the fluorescence intensity to be increased further without decreasing the transverse relaxation time, or enabling the transverse relaxation time to be increased further without decreasing the fluorescence intensity.

[0020] (8) In any one of (1) to (7) above, NV － The ratio of the concentration of isolated vacancies to the concentration of centers may be 10% or less, which increases the product of the transverse relaxation time T2 and the fluorescence intensity I, thereby realizing a sensor with higher sensitivity.

[0021] (9) In the above (8), NV － The ratio of the concentration of isolated vacancies to the concentration of centers may be 1% or less, which increases the product of the transverse relaxation time T2 and the fluorescence intensity I, thereby realizing a sensor with higher sensitivity.

[0022] (10) A method for manufacturing a diamond spin sensor according to a fourth aspect of the present disclosure includes a synthesis step of synthesizing a diamond single crystal under a pressure of 5 GPa or more by a temperature difference method using a seed crystal having a rectangular surface of a (001) crystal face or a (111) crystal face as a seed surface for synthesizing diamond; an irradiation step of irradiating a cut diamond cut from the diamond single crystal synthesized in the synthesis step with an electron beam having an energy of 500 KeV or more and 1 MeV or less; and a method for manufacturing a diamond spin sensor according to the fourth aspect of the present disclosure. and an annealing step of annealing the seed crystal at a temperature of 1200°C to 1400°C for 0.1 to 0.5 hours to produce a diamond spin sensor, wherein the seed crystal has a nitrogen concentration of 0.1 ppm to 30 ppm and 10 or fewer linear dislocation defects, and one side of the rectangle is parallel to the <100> or <010> direction if the surface is a (001) crystal plane, or parallel to the <1-10>, <10-1>, or <01-1> direction if the surface is a (111) crystal plane. This makes it possible to produce a diamond spin sensor with increased fluorescence intensity without decreasing the transverse relaxation time, or with increased transverse relaxation time without decreasing the fluorescence intensity.

[0023] (11) In the above (10), a nitrogen getter is used in the synthesis step, and the nitrogen getter contains any of titanium, zirconium, hafnium, aluminum, gallium, copper, silver, and gold. This increases the product of the transverse relaxation time T2 and the fluorescence intensity I, thereby realizing a sensor with higher sensitivity.

[0024] (12) In the above (10) or (11), the cut diamond may include only one sector boundary or may not include any sector boundary, thereby increasing the product of the transverse relaxation time T2 and the fluorescence intensity I, and realizing a more sensitive sensor.

[0025] [Details of the embodiments of the present disclosure] In the following embodiments, the same components are denoted by the same reference numerals, and their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0026] Referring to FIG. 1 , the diamond spin sensor 100 according to the embodiment of the present disclosure is a rectangular parallelepiped having a first surface 102 and a first edge 104. The diamond spin sensor 100 may be a rectangular parallelepiped with equal side lengths, i.e., a cube. The diamond spin sensor 100 is formed of a single crystal diamond containing an NV center composed of nitrogen (N) and vacancies (V). The first surface 102 is a crystal plane, such as the (001) or (111) plane. When the first surface 102 is a (001) plane, the first edge 104 is formed along the <100> or <010> direction. When the first surface 102 is a (111) plane, the first edge 104 is formed along the <1-10>, <10-1>, or <01-1> direction. Note that the notation "-1" corresponds to the notation of a crystal orientation in which a bar (horizontal bar) is placed above the number 1.

[0027] Although FIG. 1 shows a rectangular parallelepiped diamond spin sensor 100, the shape is not limited to this. The shape of the first surface 102 is not limited to a rectangle and can be any shape. The first surface 102 may be, for example, a triangle. In this case, the first side 104 corresponds to one side of the triangle. The three-dimensional shape of the diamond spin sensor 100 is not limited to a rectangular parallelepiped and may be a pyramid (such as a pyramid or a cone). For example, the diamond spin sensor 100 may be a tetrahedron (e.g., a corner cube with right-angled sides) in which the first surface 102 is a triangle (e.g., an equilateral triangle).

[0028] 2, among the crystal planes of diamond, the (001) plane is a plane defined by points A5 to A8 (i.e., a plane passing through these four points). When the first surface 102 is a (001) plane as described above, the diamond spin sensor 100 is realized as a cube with vertices from points A1 to A8, for example. The <100> direction and the <010> direction, which are directions that the first edge 104 can take, are the direction from point A1 to point A2 and the direction from point A1 to point A4, respectively. For example, if the first surface 102 is a plane with vertices from points A5 to A8, the first edge 104 corresponds to the line segment connecting points A5 and A6, or the line segment connecting points A5 and A8.

[0029] The (111) plane is a plane defined by points A5, A2, and A4. As described above, the first surface 102 may be the (111) plane. In this case, the <1-10> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B1. That is, the first side 104 corresponds to the line segment connecting points A2 and A4. The <10-1> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B2. That is, the first side 104 corresponds to the line segment connecting points A5 and A2. The <01-1> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B3. That is, the first side 104 corresponds to the line segment connecting points A5 and A4.

[0030] Measurement using the diamond spin sensor 100 is performed, for example, by the device shown in Fig. 3. The control unit 230 includes a CPU (Central Processing Unit) and a storage unit (neither of which are shown). The processing performed by the control unit 230 is realized by the CPU reading and executing a program stored in advance in the storage unit.

[0031] The excitation light generating unit 210 generates excitation light for exciting the NV centers of the diamond spin sensor 100 under the control of the control unit 230. The control unit 230 supplies, for example, a voltage to the excitation light generating unit 210 at a predetermined timing to cause the excitation light generating unit 210 to emit light. The excitation light 204 is green light (i.e., wavelength 490 nm to 560 nm). The excitation light 204 is, for example, laser light, and the excitation light generating unit 210 is, for example, a semiconductor laser (e.g., emitted light wavelength 532 nm).

[0032] The filter 212 is an element for separating the excitation light 204 incident from the excitation light generating unit 210 and the light (i.e., fluorescent light) emitted from the diamond spin sensor 100. For example, the filter 212 is a filter that cuts off (i.e., reflects) light with wavelengths equal to or less than a predetermined wavelength and passes light with wavelengths greater than the predetermined wavelength, or a bandpass filter that passes light with wavelengths within a predetermined wavelength range and cuts off (i.e., reflects) light with wavelengths outside the predetermined wavelength range. Generally, excitation light has a shorter wavelength than fluorescent light, so such a configuration is preferable. For example, the filter 212 is a dichroic mirror with this function.

[0033] The focusing element 214 focuses the excitation light 204 input from the filter 212. The focusing element 214 is, for example, a spherical lens. The focusing element 214 inputs as much of the excitation light diffused and output from the excitation light generating unit 210 as possible into the end of the optical waveguide 216. The optical waveguide 216 includes a medium for transmitting light and transmits light in both directions. That is, the optical waveguide 216 has a first end and a second end, and transmits the excitation light 204 incident on the first end to the second end located near the diamond spin sensor 100. The optical waveguide 216 also transmits the emitted light (i.e., fluorescence) from the diamond spin sensor 100 incident on the second end to the first end and outputs it. The optical waveguide 216 is, for example, an optical fiber.

[0034] The LPF (Long Pass Filter) 218 ​​is a long-pass filter that passes light with wavelengths equal to or greater than a predetermined wavelength and cuts (e.g., reflects) light with wavelengths smaller than the predetermined wavelength. The fluorescent light 206, which is the emitted light of the diamond spin sensor 100, is red light and passes through the LPF 218, but the excitation light 204 output from the excitation light generator 210 has a shorter wavelength and does not pass through the LPF 218. This prevents the excitation light 204 emitted from the excitation light generator 210 from being detected by the photodetector 220 and becoming noise, which reduces the detection sensitivity of the fluorescent light 206, which is the emitted light of the diamond spin sensor 100. The photodetector 220 generates and outputs an electrical signal corresponding to the incident light. The photodetector 220 is, for example, a photodiode. The output signal from the photodetector 220 is acquired by the control unit 230.

[0035] The electromagnetic wave irradiating unit 202 irradiates the diamond spin sensor 100 with electromagnetic waves (e.g., microwaves). The electromagnetic wave irradiating unit 202 is, for example, a coil or microwave resonant circuit formed including an electric conductor. The electromagnetic waves are supplied from the electromagnetic wave generating unit 232 to the electromagnetic wave irradiating unit 202 by, for example, a coaxial cable. The irradiation of the excitation light and electromagnetic waves to the diamond spin sensor 100 is controlled by the control unit 230, and is performed, for example, at the timing shown in FIG. 4.

[0036] Referring to Fig. 4, the control unit 230 controls the excitation light generating unit 210 to output excitation light for a predetermined period (e.g., time interval t1) at a predetermined timing. The control unit 230 controls the electromagnetic wave generating unit 232 to output electromagnetic waves for a predetermined period (e.g., time interval t2) at a predetermined timing. Any appropriate pulse sequence may be used during the time interval t2. This allows the excitation light and the electromagnetic waves to be irradiated onto the diamond in a temporally and spatially combined manner. The control unit 230 captures the output signal of the light detecting unit 220 at a predetermined timing (e.g., time interval t3) and stores it in the memory unit.

[0037] The NV center transitions from the ground state to an excited state when exposed to green light with a wavelength of 490 nm to 560 nm (e.g., laser light of 532 nm), and then emits red light with a wavelength of 630 nm to 800 nm (e.g., fluorescent light of 635 nm) and returns to the ground state. The NV center captures one electron (i.e., NV － ) the magnetic quantum number m ｓ forms spin triplet states of -1, 0, and +1, and in the presence of a magnetic field, m ｓ The energy level of the state of = ±1 splits depending on the magnetic field strength (i.e., Zeeman splitting). ｓ = 0 state is m ｓAfter transitioning to the state of π = ±1 (i.e., electron spin resonance), the electron spin resonance is excited by irradiating it with green light. As a result, the transition back to the ground state includes a transition that does not emit light (i.e., fluorescence), and the intensity of the observed emitted light decreases. Therefore, a valley (i.e., a drop in the signal) is observed in the ODMR (Opticaly Detected Magnetic Resonance) spectrum.

[0038] As described above, the control unit 230 controls the excitation light generating unit 210 and the electromagnetic wave generating unit 232 to measure a spectrum such as that shown in FIG. 5. The frequency difference Δf, which is the distance between the two observed valleys, depends on the magnetic field strength at the position of the diamond spin sensor 100 (corresponding to Zeeman splitting). The control unit 230 can calculate the magnetic field from the frequency difference Δf. Referring to FIG. 6, the magnetic field detected by the NV center is a component in the direction of the axis passing through N and V of the NV center formed in the diamond (hereinafter referred to as the NV axis). That is, where φ is the angle formed by the magnetic field vector B and the NV axis, the diamond spin sensor 100 detects a change in signal intensity (i.e., fluorescence intensity) according to B cos φ.

[0039] As explained above, the magnetic field can be calculated from the change in the ODMR spectrum, but it is known that the center frequency of the two resonant frequencies of the NV center has temperature dependence in the range from 120 K to 700 K. Therefore, by using the diamond spin sensor 100 to measure the change in the center frequency split into Δf, it is possible to measure the temperature.

[0040] In diamond spin sensor 100, the transverse relaxation time of the electron spin of the NV center measured by the Hahn echo method described below is T2 μsec, and the fluorescence intensity emitted from the diamond is InA (nanoamperes), and the product α (α = T2 × I) of T2 (transverse relaxation time) and I (fluorescence intensity) is greater than 3000. As a result, when diamond spin sensor 100 is used as a sensor, the fluorescence intensity I can be increased without decreasing the transverse relaxation time T2, or the transverse relaxation time T2 can be increased without decreasing the fluorescence intensity I. Therefore, a sensor with higher sensitivity than conventional sensors can be realized.

[0041] The fluorescence intensity emitted from the diamond is measured as follows: Diamond spin sensor 100 is irradiated with microwaves (wavelength 2.87±0.8 GHz (i.e., 2.07 GHz or more and 3.67 GHz or less)) and excitation laser light (wavelength 530±10 nm (i.e., 520 nm or more and 540 nm or less) and power 3 mW). The fluorescence intensity emitted is detected as a current value (in nA units) using a Si PIN photodiode (model name S6967, manufactured by Hamamatsu Photonics K.K.). Diamond spin sensor 100 is, for example, a diamond processed into a rectangular plate, with the top and bottom surfaces nearly parallel (parallelism (the angle between the two planes) within 1 degree) and polished so that the surface roughness Ra is 6 nm or less. In this state, the above laser light is focused onto a 400 μm diameter area and irradiated onto one side (top or bottom) of the diamond spin sensor 100, and the fluorescence emitted from the irradiated area is detected by a Si PIN photodiode, and the output current InA is defined as the fluorescence intensity.

[0042] Referring to FIG. 7 , fluorescence is emitted in all directions (solid angle 4π (sr)) from NV centers within the laser light irradiation area 240 (diameter 400 μm). The proportion of the amount of fluorescence emitted from the surface on which the Si PIN photodiode is located (e.g., the top surface 100a of the diamond spin sensor 100) is constant relative to the total amount of fluorescence emitted. That is, of the fluorescence emitted from the irradiation area 240, fluorescence (e.g., fluorescence L1) whose angle of incidence on the top surface 100a is smaller than angle θ (the critical angle of the diamond spin sensor 100 placed in air) is emitted from the top surface 100a into the air. The output area 242 is a spherical area centered on the irradiation area 240, and fluorescence traveling from the irradiation area 240 toward the output area 242 is emitted from the top surface 100a. Other fluorescence (e.g., fluorescence L2 and L3) is reflected by the top surface 100a and is not emitted into the air. Considering the refractive index n (n = 2.4) of diamond spin sensor 100, angle θ is approximately 24.6°, and the solid angle of output area 242 is calculated to be approximately 4.5% of the total solid angle. Furthermore, if the transmittance of diamond spin sensor 100 is 83%, the proportion of the amount of fluorescence emitted from top surface 100a is a constant proportion of approximately 3.7% of the total amount of fluorescence. Therefore, as described above, by detecting the fluorescence emitted from top surface 100a with a Si PIN photodiode, the degree of fluorescence emission from diamond spin sensor 100 can be evaluated from the fluorescence intensity described above, without having to detect the fluorescence emitted in all directions.

[0043] Microwaves may be irradiated onto the diamond spin sensor 100 using a microwave resonant circuit (see, for example, Patent Document 3). The Si PIN photodiode used to detect fluorescence is not limited to the one described above. A Si PIN photodiode with the model name S6967 has a photosensitivity of 0.40 (A / W) at a wavelength of 600 nm and 0.45 (A / W) at a wavelength of 660 nm, and any photosensitivity equivalent thereto may be used. For example, a Si PIN photodiode (e.g., model name S6775 (manufactured by Hamamatsu Photonics K.K.)) with a photosensitivity of 0.36 (A / W) to 0.44 (A / W) at a wavelength of 600 nm and 0.40 (A / W) to 0.50 (A / W) at a wavelength of 660 nm may be used. The photosensitivity is calculated by dividing the magnitude of the photocurrent (A) by the energy of the incident light (W).

[0044] The transverse relaxation time T2 is measured, for example, using the pulse sequence shown in FIG. 8 . A method for observing signals using such a pulse sequence is called the Hahn echo method or the spin echo method. Pulses P1 and P3 tilt the electron spin of the NV center by 90° (π / 2). Pulse P2 flips the electron spin of the NV center by 180° (π). Pulses P1, P2, and P3 are applied to the diamond spin sensor 100 with the same time interval τ. The electron spin tilted by pulse P1 tends to return to its original state over time, resulting in a relaxation phenomenon (a phenomenon in which the phase velocity variation among multiple electron spins increases). In contrast, when pulse P2 is applied, electron spins with a fast phase velocity move to a position where the phase velocity is delayed, and electron spins with a slow phase velocity move to a position where the phase velocity is advanced. Therefore, the state of multiple electron spins changes in a direction in which the phases of the electron spins become aligned. Subsequently, when pulse P3 is applied and a signal is detected, the signal increases.

[0045] When ESR (Electron Spin Resonance) signal measurements using the pulse sequence shown in FIG. 8 are repeated and the signal intensity is plotted against time, a graph such as that shown in FIG. 9 is obtained. In FIG. 9, the solid line schematically shows the measured values. The vertical axis is expressed in arbitrary units (au (arbitrary unit)). The horizontal axis is 2τ (twice the time interval τ). The dashed line is a graph obtained by fitting the graph of the measured values ​​using an exponential function. The transverse relaxation time T2 is the value of time τ when the value of the exponential function becomes 1 / e of the initial value. In other words, the transverse relaxation time T2 represents the time over which the measurement signal is sustained, and the longer the transverse relaxation time T2, the longer the signal can be measured.

[0046] With diamond spin sensors, the observed fluorescence intensity increases with the number of NV centers that generate fluorescence. However, if defects, strain, and impurities other than nitrogen that constitute the NV centers are present in the diamond single crystal, the emitted fluorescence is scattered and absorbed by them. Therefore, the greater the number of defects, strain, and impurities (other than nitrogen), the lower the measured fluorescence intensity. In other words, the measured fluorescence intensity I is considered to be inversely proportional to the amount of defects, strain, and impurities (other than nitrogen) (I ∝ 1 / X, where X represents the degree of defects, strain, or impurities (other than nitrogen)). Note that the fluorescence intensity also depends on the surface roughness Ra of the diamond spin sensor, but the effect of surface roughness Ra can be eliminated by polishing the surface.

[0047] The transverse relaxation time T2 mentioned above increases as the number of NV centers decreases. Like the fluorescence intensity, the transverse relaxation time T2 is also affected by defects, strain, and impurities (other than nitrogen) in the crystal. Defects, strain, and impurities (other than nitrogen) in the crystal act as disturbances on the transverse relaxation time T2, and it is thought that the transverse relaxation time T2 is inversely proportional to the amount of defects, strain, and impurities (other than nitrogen) (T2 ∝ 1 / X). Therefore, the product α (T2 × I) of the transverse relaxation time T2 and the fluorescence intensity I increases as the number of defects, strain, and impurities (other than nitrogen) in the diamond crystal decreases. If the product α can be increased, a more sensitive sensor can be realized.

[0048] The examples described below show that a diamond spin sensor with a larger product α of the transverse relaxation time T2 (unit: μsec) and the fluorescence intensity I (unit: nA) can be realized. That is, the product α of the transverse relaxation time T2 and the fluorescence intensity I can be greater than 6000. This allows for a sensor with higher sensitivity to be realized. The product α can be greater than 10000, greater than 15000, or greater than 20000. The larger the product α, the more sensitive the sensor can be realized. That is, in a diamond spin sensor, the fluorescence intensity I can be increased more without decreasing the transverse relaxation time T2, or the transverse relaxation time T2 can be increased more without decreasing the fluorescence intensity I.

[0049] As mentioned above, the product α of the transverse relaxation time T2 and the fluorescence intensity I depends on the number of NV centers, i.e., the concentration (density). In the diamond spin sensor 100 shown in FIG. 1, if the concentration of the NV centers (the ratio of the number of NV centers to the number of carbon atoms) is 0.02 ppm or more and 10 ppm or less, a larger product α can be realized than in the past. Furthermore, the average phase difference over the entire surface of the diamond spin sensor 100 can be made 6 nm / mm or less. This makes it possible to increase the fluorescence intensity I in the diamond spin sensor without decreasing the transverse relaxation time T2, or to increase the transverse relaxation time T2 without decreasing the fluorescence intensity I. Therefore, a sensor with higher sensitivity than in the past can be realized.

[0050] The concentration of the NV center may be 0.02 ppm or more and 1.2 ppm or less. This allows a larger product α to be realized, resulting in a sensor with higher sensitivity. The concentration of the NV center may be 0.04 ppm or more and 5 ppm or less. The concentration of the NV center may be 0.08 ppm or more and 0.5 ppm or less.

[0051] The concentration of NV centers in diamond can be calculated from the measured value by, for example, electron spin resonance method. Also, in the case of low concentration, it can be measured by observing with a fluorescence microscope and counting single NV centers. In the case of high concentration, for diamonds containing low concentration of NV centers, the conversion ratio between concentration and fluorescence intensity is obtained, and using this, the concentration can be calculated by converting from the fluorescence intensity ratio. Furthermore, the concentration of NV centers can also be calculated from the absorption coefficient at 637 nm in the absorption spectrum measured by ultraviolet-visible absorption spectroscopy.

[0052] The average phase difference will now be explained. Diamonds inherently have an isotropic crystal structure and an isotropic refractive index (dielectric constant). However, in reality, defects and distortions exist within single-crystal diamonds, resulting in birefringence. When circularly polarized light is irradiated onto a diamond with birefringence, a phase difference occurs between two orthogonal polarized light beams (linearly polarized light), resulting in the output of elliptically polarized light. The optical axis and phase difference can be determined from the orientation of the major and minor axes of the ellipse in the elliptically polarized light output from the diamond, as well as the ratio of the lengths of the major and minor axes. The measured phase difference is a value integrated in the direction in which light passes through the diamond (e.g., the thickness direction of the diamond). Therefore, the phase difference is normalized by the thickness of the diamond, i.e., expressed as a phase difference converted to a thickness of 1 mm (unit: nm / mm). The phase difference is measured locally and distributed two-dimensionally within the measurement surface. Therefore, the phase difference is expressed as an average value within the measurement surface (hereinafter referred to as the average phase difference). The term "average value" does not mean the phase difference per area, but rather means the average value of the phase differences obtained by performing local measurements multiple times over the plane, i.e., the average of the frequency distribution of the phase difference over the plane.

[0053] The average phase difference over the entire surface of the diamond spin sensor may be 4 nm / mm or less. This allows a larger product α to be realized, resulting in a more sensitive sensor. The average phase difference over the entire surface of the diamond spin sensor may also be 3 nm / mm or less, 2 nm / mm or less, or 1 nm / mm or less. This allows an even larger product α to be realized, resulting in a more sensitive sensor.

[0054] The crystallinity of the diamond spin sensor 100 can be evaluated by its rocking curve. In the X-ray diffraction using the double crystal method described below, another diamond crystal cut from the original crystal from which the diamond spin sensor 100 shown in FIG. 1 was cut is used as the first crystal, and the diamond spin sensor 100 is used as the second crystal. The diamond spin sensor 100 is measured with CuKα radiation in a (004) plane parallel arrangement. The half-width of the rocking curve of the diamond spin sensor 100 obtained from the measurement is 8 seconds or less. This allows for a larger product α than conventional diamond spin sensors to be achieved, and the fluorescence intensity I can be increased without decreasing the transverse relaxation time T2, or the transverse relaxation time T2 can be increased without decreasing the fluorescence intensity I. Therefore, a sensor with higher sensitivity than conventional diamond spin sensors can be realized. Note that the half-width refers to the full width at half maximum.

[0055] The crystallinity of diamond can be evaluated by measuring the rocking curve in X-ray diffraction. The smaller the half-width of the X-ray diffraction rocking curve, the higher the crystallinity of the diamond and the fewer crystal defects and distortions. To improve detection accuracy, the double-crystal method is used. In the double-crystal method, two crystals are used: an analyzing crystal (first crystal) and a sample crystal (second crystal) to be evaluated. X-rays output from an X-ray source are irradiated onto the first crystal, the resulting diffracted X-rays are incident on the second crystal, and the angle of the second crystal is changed to measure the diffracted X-rays with a detector. The angle of incidence of the diffracted X-rays on the second crystal (the angle between the second crystal and its plane) is represented by ω, and the angle between the direction of the detector and the direction of incidence of the diffracted X-rays on the second crystal is represented by 2θ. By fixing the first crystal and the detector and placing the center of rotation on the surface of the second crystal and rotating the second crystal, the diffracted X-rays are measured by fixing the angle 2θ and varying only the angle of incidence ω. This allows the measurement of the diffraction intensity distribution on a spherical surface centered on the origin in reciprocal lattice space. A rocking curve is obtained by plotting the detected values ​​of the measurement results on the vertical axis with the incident angle ω on the horizontal axis, and the width of the rocking curve is proportional to the degree of fluctuation in the plane orientation. By using crystals cut from the same single diamond crystal for the first and second crystals, the half-width of the rocking curve sensitively reflects the quality of the crystal being measured.

[0056] In the diamond spin sensor 100, the half-width of the X-ray diffraction rocking curve obtained by the double crystal method may be 7 seconds or less. This allows a larger product α to be realized, resulting in a sensor with higher sensitivity. The half-width of the X-ray diffraction rocking curve may also be 6 seconds or less, or may be 5 seconds or less. This allows a larger product α to be realized, resulting in a sensor with higher sensitivity.

[0057] In the diamond spin sensor 100 shown in FIG. － The ratio of the concentration of isolated vacancies to the concentration of the center may be 10% or less. An isolated vacancy means a vacancy in a state where no nitrogen exists around the vacancy. This makes it possible to realize a larger product α than conventionally, and a sensor with higher sensitivity than conventionally. The density (concentration) of vacancies is determined by the ratio of the number of neutral isolated vacancies (V ０) and the density of negatively charged isolated vacancies (V － The density of each isolated vacancy is calculated by the integrated absorption of light at wavelengths of 741 nm and 394 nm (wavelength integral of absorption coefficient: unit: meV × cm) at liquid nitrogen temperature. －１ It can be calculated from the integral absorption A and density β (unit cm －３ ) and the proportionality coefficient k (k = A / β) is 1.2 × 10 －１６ , and 4.8 × 10 －１６ In addition, when the vacancy density cannot be calculated using the absorption coefficient in the visible region, it can be calculated using the positron annihilation method. The relative density value obtained by the positron annihilation method can be converted into density even in low concentration regions by calibrating (proportional calculation) using the value in the region where density can be obtained using the absorption coefficient as a reference.

[0058] In the diamond spin sensor 100, NV － The ratio of the concentration of isolated vacancies to the concentration of centers may be 3% or less. － The ratio of the concentration of isolated vacancies to the concentration of centers may be 1% or less, or may be 0.3% or less. － By reducing the ratio of the concentration of isolated vacancies to the concentration of centers, a larger product α can be realized than before, and a sensor with higher sensitivity than before can be realized.

[0059] (Manufacturing Method of Diamond Spin Sensor) A method for manufacturing the diamond spin sensor 100 shown in Fig. 1 will now be described. Using granular diamond as a seed crystal, a synthetic diamond is produced by a temperature difference method under high pressure, and a portion of the diamond is selected and cut out to be used as a seed crystal in the subsequent process.

[0060] Figure 10 shows the configuration of an apparatus for synthesizing diamond by the temperature gradient method under high pressure. In the temperature gradient method, crystals are grown by utilizing the difference in solubility of diamond in a solvent caused by a temperature difference. Referring to Figure 10, a vertical temperature gradient is created within a pressure medium 250 equipped with a graphite heater 252 and an insulating member 254. The insulating member 254 is placed in the high-temperature section, and a seed crystal 300 is placed in the low-temperature section, with a solvent metal 258 placed between them. Single-crystal diamond is grown on the seed crystal 300 by maintaining conditions above the pressure at which diamond becomes thermally stable at a temperature above the temperature at which the solvent metal 258 melts. Diamond powder is preferably used as the carbon source 256. Alternatively, graphite or pyrolytic carbon may be used as the carbon source 256. The solvent metal 258 is one or more metals selected from iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), etc., or an alloy containing these metals. Pressure at which diamond is thermodynamically stable and temperature conditions at which solvent metal 258 is eutectic with carbon are achieved by applying pressure with pressure medium 250 receiving external force from a high-pressure generator (not shown) and heating with graphite heater 252. Carbon dissolves into solvent metal 258 from carbon source 256 in the high-temperature section and is transported by diffusion to the low-temperature section below solvent metal 258, whereupon a crystal grows on seed crystal 300 to form synthetic diamond 302.

[0061] (Step 1) First, a single crystal diamond with a nitrogen concentration of 30 ppm or less is prepared. In the above-mentioned high-pressure single crystal diamond synthesis method, a single crystal diamond with a nitrogen concentration of 30 ppm or less can be produced by adding a nitrogen getter to the solvent metal 258. This allows for the cutting out of a seed crystal with a single growth sector and reduced defects, with 10 or fewer dislocation defects detected by X-ray topography, in the second step described below. The growth sector can be confirmed by a two-dimensional image (surface distribution image) of PL (photoluminescence: light emission image due to ultraviolet irradiation) or CL (cathodoluminescence: light emission image due to electron beam irradiation) fluorescence. In other words, it can be determined by whether the boundary between regions with different fluorescence intensities is linear (see Patent Document 4).

[0062] Dislocations are measured, for example, by an etching test (see Patent Document 5). The etching test is carried out as follows: A single crystal diamond is immersed in an etching solution of potassium nitrate (KNO ３ The diamond is immersed in the melt and heated to 600°C in a platinum crucible for 1 hour (etching). After slow cooling, the single crystal diamond is removed and the surface is observed under an optical microscope at 50x magnification. The number of point-like etch pits is counted within a rectangular measurement area of ​​1000µm x 1000µm, and the number of pits is counted within 1mm. ２ The number of point-like etch pits per 1 mm can be obtained. Point-like etch pits refer to point-like depressions present on the surface of a single crystal diamond. Point-like etch pits correspond to dislocation defects. Point-like depressions are square, square with rounded corners, or approximately circular on the (100) face of a single crystal diamond, and are triangular, triangular with rounded corners, or approximately circular on the (111) face. The diameter of the point-like depressions is approximately 1 μm to 50 μm. ２ By multiplying the number of punctate etch pits per cm by 100, ２ The number of point-like etch pits (dislocation density) per 1000 nm is calculated. Note that linear etch pits may be observed on the surface of the single crystal diamond after etching, along with point-like etch pits. Linear etch pits are derived from stacking faults in the single crystal diamond. The number of linear etch pits is not included in the measurement of dislocation defects.

[0063] Dislocations can also be detected by X-ray topography (see Patent Document 6). When measuring in transmission using synchrotron X-rays, for example, X-rays with a wavelength of 0.71 Å (0.071 nm) are used, and (220) diffraction at a diffraction angle 2θ = 32.9° is used. When measuring in reflection, for example, X-rays with a wavelength of 0.96 Å (0.096 nm) and (113) diffraction at a diffraction angle 2θ = 52.4° may be used. Images may be taken by changing the wavelength of the X-rays and the diffraction angle 2θ. Laboratory X-ray diffraction equipment may also be used for measurement; for example, a Mo source may be used to observe (111) diffraction, and a Cu source may be used to observe (113) diffraction. While a CCD (Charge Coupled Device) camera can also be used for measurement, it is preferable to use a nuclear emulsion plate to increase resolution. After developing the nuclear emulsion, dislocations can be identified and quantified by capturing images with an optical microscope.

[0064] In the synthesis of single-crystal diamond using the temperature difference method, for example, the composition of the solvent metal 258 is Fe / Co = 10 / 90 to 90 / 10 (mass ratio), and titanium (Ti) or aluminum (Al) is added to the solvent metal as a nitrogen getter at 1.5 mass% to 3 mass%. The temperature gradient is adjusted so that the temperature difference between the carbon source 256 and the seed crystal 300 is 10°C to 25°C, and a pressure of 5.0 GPa to 5.5 GPa and a temperature of 1300°C to 1350°C are maintained for 80 hours to 250 hours. As a result, referring to Figure 11, synthetic diamond 302 is synthesized from the seed crystal 300. If the temperature difference exceeds 25°C, crystal growth becomes disrupted and vicinal growth marks tend to become obscured. If the temperature difference is lower than 10°C, it takes a long time to grow a crystal of a predetermined size, which increases production costs. Furthermore, the temperature change during the holding period is controlled to within 3°C. This further improves the crystallinity. If the temperature change is more than 3°C, the growth becomes unstable, and crystal defects, distortion, and inclusions occur, resulting in a decrease in crystallinity.

[0065] (Step 2) From the single crystal diamond synthesized in Step 1, a cut diamond 304 (see FIG. 11) is cut out to be used as a seed crystal in Step 3 described later. The seed surface of the cut diamond 304 used as the seed crystal is preferably, but not limited to, a square or octagon. The size of the seed surface (e.g., the length of the opposite side) is preferably 0.3 mm or more and 3 mm or less. The seed crystal is cut out by laser processing into a plate with a thickness of about 0.5 mm to 1.0 mm, and the surface of the plate is polished to a surface roughness Ra of 20 nm or less. Then, it is preferably cut out into a plate with a size of about 0.3 mm x 0.3 mm x 0.3 mm to 3.0 mm x 3.0 mm x 1.0 mm by laser cutting. A larger size is preferable because it is easier to avoid dislocation defects and obtain a single sector. If the cut diamond 304 is rectangular and the plane serving as the seed surface is a (001) crystal plane, one side of the rectangle serving as the seed surface is parallel to the <100> direction or the <010> direction. If the plane serving as the seed surface is a (111) crystal plane, one side of the rectangle serving as the seed surface is parallel to the <1-10> direction, the <10-1> direction, or the <01-1> direction. This allows the cutting margin to be small, resulting in a seed crystal with a seed surface that is less damaged.

[0066] In single crystal diamond synthesized by the temperature difference method, there are many dislocation defects in the direction of the main surface of the seed crystal and in the direction of the opening angle of about X° from that direction.For example, when the main surface direction is the <001> direction, there are many dislocation defects in the four directions of the <112> direction, the <-112> direction, the <1-12> direction and the <-1-12> direction, which have an opening angle of about 35° (i.e., X=35).In addition, when the main surface direction is the <111> direction, there are many dislocation defects in the three directions of the <112> direction, the <121> direction and the <211> direction, which have an opening angle of about 19.5° (i.e., X=19.5).In the above description, the opening angle is the X° direction, because when the main surface direction and the <001> direction or the <111> direction do not coincide, the opening angle will be slightly different, so it is expressed as X°.When they do not coincide, the opening angle will be an angle corrected from 35° or 19.5°. Excluding these opening angles and the ±5° angle in the direction of the principal plane, a single sector contains a high-quality crystal with few crystal defects. Furthermore, near the boundaries between different sectors, such as the boundary between the {001} sector and the {111} sector, there is a lot of crystal distortion and defects are likely to occur. As the seed substrate becomes larger, the interval between the opening angles starting from the seed substrate becomes wider, and the boundaries between different sectors are also biased toward the edges, allowing for a larger size of high-quality crystal. Here, the {abc} sector refers to a region grown on the {abc} plane in a stacked manner and exposed at the top surface. For example, the {001} sector refers to a region grown on the {001} plane in a stacked manner and exposed at the top surface. Therefore, in this second step, a seed crystal with few defects can be obtained by cutting out a cut diamond 304 from a single growth sector of the principal plane growth of the synthetic diamond 302, excluding ±5° angle in the principal plane growth direction of the seed crystal.

[0067] As the seed substrate becomes larger, the size of the high-quality crystal can also be increased. For example, a seed crystal with reduced defects, with 10 or less dislocation defects detected by X-ray topography, can be cut out. By using this seed crystal, the single crystal diamond synthesized in the subsequent process will have fewer defects and further reduced distortion. The single growth sector of the primary surface growth of the synthetic diamond 302, excluding ±5° from the primary surface growth direction of the seed crystal, is, for example, the {001} sector or the {111} sector. In addition, any other crystallographically possible sector (such as the {113} sector, the {115} sector, the {110} sector, or the {135} sector) may be used as long as it is a single growth sector of the primary surface growth, excluding ±5° from the primary surface growth direction of the seed crystal. When cutting out the cut diamond 304 used as the seed crystal from the synthetic diamond 302, it may be cut out from within one sector, or it may be cut out to include two or more sectors. That is, the cut diamond 304 may include zero or one sector boundary. In a single crystal diamond, the region of each sector can be identified by an luminescence image obtained by irradiating ultraviolet light (ultraviolet-excited luminescence image).

[0068] (Step 3) Diamond is synthesized by the temperature difference method as described above using the seed crystal cut out in Step 2. That is, referring to Fig. 12, cut diamond 304 is used as seed crystal 310 to synthesize synthetic diamond 312. This makes it possible to obtain a single crystal diamond with reduced crystal defects and strain.

[0069] Specifically, referring to FIG. 10 , diamond powder is used as the carbon source 256. Iron or cobalt, which has high solubility and affinity for carbon, is used as the solvent metal 258. Depending on the synthesis conditions, trace amounts of nickel or manganese may be incorporated into the diamond. The amount of boron impurity contained in the carbon source 256 and the solvent metal 258 is controlled to 1 ppm or less. This allows the boron (B) atomic number content of the single crystal diamond to be 0.1 ppm or less. Titanium is added to the solvent metal 258 as a nitrogen getter. The concentration of the added titanium is 1.5 mass% or more and 3 mass% or less. This allows the nitrogen content of the single crystal diamond to be 0.1 ppm or more and 10 ppm or less, based on the atomic number. Aluminum may also be added as a nitrogen getter. In this case, an Fe-Al alloy may also be used for the solvent metal 258.

[0070] Conditions for the temperature difference method include, for example, adjusting the temperature gradient so that the temperature difference between the carbon source 256 and the seed crystal 300 is 10°C or more and 25°C or less, and maintaining a pressure of 5.0 GPa or more and 5.5 GPa or less and a temperature of 1300°C or more and 1350°C or less for 80 hours or less and 300 hours or less. If the temperature difference exceeds 25°C, crystal growth becomes somewhat disturbed, and vicinal surface growth often becomes impossible to observe. Furthermore, by controlling the temperature change during maintenance to within 3°C, crystallinity is further improved.

[0071] (Step 4) Cut diamonds 318 are cut from the synthetic diamond 312 (see FIG. 12) synthesized in Step 3. The cut diamonds 318 are located within a single growth sector (e.g., sector 316) that does not include the sector boundary 314, and are regions with 10 or fewer dislocations detected by X-ray topography. The cut diamonds 318 are cut using laser processing in the same manner as in Step 2 described above.

[0072] (Step 5) The cut diamond 318 cut from the synthetic diamond 312 in step 4 is irradiated with an electron beam having an energy of 300 KeV or more and 1.2 MeV or less. This ionizes the orbital electrons of carbon in the cut diamond 318, ejecting carbon nuclei and forming vacancies in the cut diamond 318. Alternatively, an electron beam having an energy of 500 KeV or more and 1 MeV or less may be irradiated. The electron beam irradiation dose is determined based on the amount of NV desired to be generated. － 1 x 10 depending on the amount １８ cm －２ From 4 x 10 １９ cm －２ The temperature can be varied in the range of

[0073] (Step 6) The cut diamond 318 that has been subjected to Step 5 is annealed in a vacuum at a temperature of 1100° C. to 1400° C. for 0.1 to 0.5 hours, thereby moving the nitrogen in the cut diamond 318 and forming NV centers consisting of nitrogen and vacancies.

[0074] As a result of the above, it is possible to manufacture a diamond spin sensor 100 that is larger in size than conventional ones and has an increased fluorescence intensity I without decreasing the transverse relaxation time T2, or an increased transverse relaxation time T2 without decreasing the fluorescence intensity I.

[0075] The irradiation step in the fifth step and the annealing step in the sixth step may be repeated two or more times to form a desired amount of NV centers in the cut diamond 318.

[0076] As described above, a nitrogen getter is used in the third step. The nitrogen getter can include any of titanium (Ti), zirconium (Zr), hafnium (Hf), aluminum (Al), gallium (Ga), copper (Cu), silver (Ag), and gold (Au). This can increase the product of the transverse relaxation time T2 and the fluorescence intensity I, thereby achieving a sensor with higher sensitivity.

[0077] As mentioned above, the cut diamond 318 may include only one or no sector boundary, which can increase the product of the transverse relaxation time T2 and the fluorescence intensity I, resulting in a more sensitive sensor.

[0078] 13, the above synthesis steps may be performed using a larger seed crystal 320. For example, in a rectangular parallelepiped seed crystal 320, the length L of one side of the rectangle of the (001) crystal face or the (111) crystal face is 3 mm or more. This allows the sector boundary 324, which is prone to impurities, to be separated from the center of the sector 326. Therefore, a larger cut diamond 328 can be cut out from the sector 326 of the synthetic diamond 322 grown from the seed crystal 320. The cut cut diamond 328 is then subjected to electron beam irradiation (step 5) and annealing (step 6) as described above. This allows a larger diamond spin sensor 100 to be manufactured.

[0079] A diamond synthesized as described above is shown schematically in FIG. 14 . FIG. 14 is a plan view of the (001) plane. The left-right direction in FIG. 14 corresponds to the <100> direction. The triangular sectors 332 at the four corners represent (111) sectors. The boundaries between different sectors are sector boundaries with different impurity concentrations (see dot pattern), forming distortions and introducing impurities. As shown in FIG. 13 , by enlarging the seed crystal, the impurity-infused region can be positioned more toward the periphery from the center. Therefore, a larger region can be cut out and a larger diamond spin sensor can be fabricated. Referring to FIG. 14 , a diamond spin sensor can be fabricated by cutting out the shaded portion of one sector 330 and using it as a seed crystal. Alternatively, as shown by the dashed lines, a larger seed crystal can be fabricated by cutting out a diamond including dislocation defects and two sector regions, and then using that to fabricate a diamond spin sensor. Furthermore, the entire (111) sector 330 can be used as a seed crystal to fabricate a diamond spin sensor.

[0080] As a comparative example, a synthetic diamond for a conventional NV spin sensor is shown in Figure 15. Figure 15 is a plan view similar to Figure 14. Sector 342 is a (111) sector. In the synthetic diamond of Figure 15, a large sector boundary (see dot pattern) with different impurity concentrations is formed around the periphery of the largest (001) sector 340, and it can be seen that it is not possible to cut out an area large enough for a diamond spin sensor. This is because different sectors such as the (001) sector and the (111) sector form a boundary.

[0081] The effectiveness of the diamond spin sensor of the present disclosure will be demonstrated below by examples. The fluorescence intensity I and transverse relaxation time T2 were measured using multiple diamond spin sensors fabricated by the above-mentioned manufacturing method. The manufacturing conditions are shown in Figure 16, and the measurement results are shown in Figures 16 and 17.

[0082] In Figure 16, diamond seed crystals with sizes of 3mm x 3mm x 0.8mm, 0.7mm x 0.7mm x 0.5mm, and 0.9mm x 0.9mm x 0.5mm were cut from the synthetic diamond prepared as described above using a laser processing machine (see step 2), and are shown as Samples 1 to 8. Sample 9 is a commercially available product (DNV-B14 manufactured by ElementSix) produced by CVD (Chemical Vapor Deposition). The diamond seed crystal was cut from within the sector shown in the "Seed Crystal Cutting Sector" column of Figure 16. In the "Seed Crystal Cutting Sector" column, "(100)" means that it was cut from the (100) sector. Furthermore, "(100) + (111)" means that it was cut to include the (100) sector and the (111) sector. The presence or absence of defects in the resulting diamond seed crystal was confirmed using X-ray topography images. The number of dislocation defects confirmed is shown in the "Dislocation Defects" column in Figure 16.

[0083] Next, a diamond crystal was grown on the diamond seed crystal by the temperature difference method to obtain a single crystal diamond for each sample (see step 3). This is indicated by HPHT (High-Pressure High-Temperature) in Figure 16. Diamond powder containing 100 ppm to 200 ppm of nitrogen and 0.5 ppm to 1 ppm of boron as impurities was used as the carbon source. High-purity iron (Fe) and cobalt (Co) were used as the solvent metal, with a solvent composition of Fe:Co = 55:45 (weight ratio). 1.75 mass% titanium was added to the solvent metal. The temperature difference method for each sample was performed under the following conditions: titanium was used as a nitrogen getter, the temperature difference between the high-temperature section (carbon source) and the low-temperature section (seed crystal) was 23°C, the pressure was 5.3 GPa, and the holding temperature of the low-temperature section was 1350°C. The holding time was 150 hours.

[0084] Thereafter, the above-mentioned fourth to sixth steps were carried out to fabricate a plurality of diamond spin sensors. The diamonds cut by the fourth step are shown in the "Method for cutting out sensor material" column in Figure 16. The half-width (arcsec unit) of the measured X-ray diffraction rocking curve is shown in the "Xrc" column. Regarding the "position of growth sector," "above" represents the shaded area in Figure 14, and "above + adjacent" represents the dashed area in Figure 14. The measured nitrogen concentration (ppm unit) and NV － The center concentration (ppm) is shown in the "N concentration" and "NV" columns, respectively. － The concentration is shown in the "Concentration" column.

[0085] The fluorescence intensity I and transverse relaxation time T2 were measured using the fabricated diamond spin sensor. The results are shown in Figure 17. Samples 1 to 9 are the same as those in Figure 16. In Figure 17, the fluorescence intensity I is expressed in nA units, and the transverse relaxation time T2 is expressed in μsec units. The phase difference represents the average phase difference (unit: nm / mm) over the entire surface. (V ０ +V － ) / NV － is NV －The product α means the product of the fluorescence intensity I and the transverse relaxation time T2 (T2 × I). － The meanings of concentration and Xrc are the same as in FIG.

[0086] Both the prototyped Sample 1 and Sample 6 achieved a larger product α than Samples 7 to 9, and also had a longer transverse relaxation time T2. The fluorescence intensity of Sample 1 was greater than that of Samples 7 to 9. The fluorescence intensity of Samples 2 to 6 was comparable to that of Samples 7 to 9. NV － When comparing Sample 1 to Sample 3, which have the same concentration of NV, it can be seen that the smaller the phase difference and Xrc, the greater the fluorescence intensity I, transverse relaxation time T2, and product α. When comparing Sample 1 and Sample 6, which have similar values ​​of phase difference and Xrc, it can be seen that － It can be seen that the product α increases by decreasing the concentration of NV － By decreasing the concentration of , the fluorescence intensity decreases.

[0087] Figure 18 shows a graph plotting the transverse relaxation time T2 and fluorescence intensity I for Sample 1 to Sample 9 shown in Figure 17. In Figure 18, the horizontal axis represents the logarithm of the fluorescence intensity I (unit: nA), and the vertical axis represents the logarithm of the transverse relaxation time T2 (unit: μsec). The white circles labeled S1 to S9 correspond to Sample 1 to Sample 9 in Figure 17.

[0088] The multiple straight lines shown in Figure 18 represent graphs where the product α is 2300, 3000, 6000, 10000, and 15000, respectively, from the bottom. In Figure 18, since both axes are expressed in logarithms, graphs where the product α of T2 and I is constant are represented by straight lines. In Figure 18, S9 is approximately on the line where the product α = 2300. In comparison, S1 to S6 all have product α greater than 3000. Therefore, it can be seen that a diamond spin sensor with fewer factors causing fluorescence absorption and scattering has been fabricated. To suppress fluorescence absorption and scattering, it is preferable to select a material with a small phase difference, a material with little influence from sector boundaries, a material with few entrapped impurities, a material with few excess point defects (i.e., vacancies), and a material with little interstitial nitrogen. That is, phase difference, nitrogen concentration, NV － By setting the center concentration, dislocation defects, and rocking curve half-width Xrc within the above ranges, a diamond spin sensor can be realized in which the absorption and scattering of fluorescence is suppressed.

[0089] Although the present disclosure has been described above by explaining the embodiments, the above-described embodiments are merely examples, and the present disclosure is not limited to only the above-described embodiments. The scope of the present disclosure is defined by the claims in the scope of the claims, taking into consideration the description of the detailed description of the invention, and includes all modifications within the meaning and scope equivalent to the wordings described therein.

[0090] 100 Diamond spin sensor 100a Upper surface 102 First surface 104 First edge 202 Electromagnetic wave irradiation unit 204 Excitation light 206 Fluorescence 210 Excitation light generation unit 212 Filter 214 Light-collecting element 216 Optical waveguide 218 LPF 220 Light detection unit 230 Control unit 232 Electromagnetic wave generation unit 240 Irradiation area 242 Output area 250 Pressure medium 252 Graphite heater 254 Insulating member 256 Carbon source 258 Solvent metal 300, 310, 320 Seed crystal 302, 312, 322 Synthetic diamond 304, 318, 328 Cut diamond 314, 324 Sector boundary 316, 326, 330, 332, 340, 342 Sector A1, A2, A3, A4, A5, A6, A7, A8 Point C Carbon L Length N Nitrogen P1, P2, P3 Pulse t, t1, t2, t3, τ Time interval V Hole X, Y, Z axis Δf Frequency difference φ, θ Angle

Claims

1. NV with electron spin － a diamond spin sensor including a diamond including a center, wherein when the transverse relaxation time of the electron spin measured by the Hahn echo method is T2 μsec and the fluorescence intensity of fluorescence emitted from the diamond by irradiating the diamond with microwaves and laser light is expressed as a current value InA output from a Si-PIN diode that receives the fluorescence, the product of T2 and I is greater than 3000, the microwave has a wavelength of 2.07 GHz or more and 3.67 GHz or less, the laser light has a wavelength of 520 nm or more and 540 nm or less, the laser light has a power of 3 mW, and the Si-PIN diode has a light receiving sensitivity of 0.36 A / W or more and 0.44 A / W or less at a wavelength of 600 nm, and 0.40 A / W or more and 0.50 A / W or less at a wavelength of 660 nm.

2. The diamond spin sensor of claim 1, wherein the product of T2 and I is greater than 10,000.

3. NV with electron spin － The NV includes a diamond containing a center. － A diamond spin sensor, wherein the concentration of the center is 0.02 ppm or more and 10 ppm or less, and the average phase difference over the entire surface of the diamond is 6 nm / mm or less.

4. A diamond spin sensor according to any one of claims 1 to 3, wherein the average phase difference over the entire surface of the diamond is 4 nm / mm or less.

5. The above NV － 5. The diamond spin sensor according to claim 1, wherein the concentration of the center is 0.02 ppm or more and 1.2 ppm or less.

6. NV with electron spin － The NV includes a diamond containing a center. － A diamond spin sensor having a center concentration of 0.02 ppm or more and 10 ppm or less, and an X-ray diffraction rocking curve half-width of 8 seconds or less by a double crystal method.

7. The above NV － 7. The diamond spin sensor according to claim 1, wherein the concentration of the center is 0.02 ppm or more and 2 ppm or less, and the half-width of the rocking curve of X-ray diffraction by the double crystal method is 6 seconds or less.

8. The above NV － 8. The diamond spin sensor according to claim 1, wherein the ratio of the concentration of isolated vacancies to the concentration of centers is 10% or less.

9. The above NV － 9. The diamond spin sensor according to claim 8, wherein the ratio of the concentration of isolated vacancies to the concentration of centers is 1% or less.

10. A method for producing a diamond spin sensor, comprising: a synthesis step of synthesizing a diamond single crystal under a pressure of 5 GPa or more by a temperature difference method using a seed crystal having a rectangular surface of a (001) crystal plane or a (111) crystal plane as a seed surface for synthesizing diamond; an irradiation step of irradiating an electron beam having an energy of 500 KeV to 1 MeV onto a cut diamond cut from the diamond single crystal synthesized in the synthesis step; and an annealing step of annealing the cut diamond that has been subjected to the irradiation step at a temperature of 1200°C to 1400°C for 0.1 to 0.5 hours, thereby producing a diamond spin sensor, wherein the seed crystal has a nitrogen concentration of 0.1 ppm to 30 ppm and has 10 or less linear dislocation defects, and one side of the rectangle is parallel to the <100> direction or the <010> direction if the surface is a (001) crystal plane, If the surface is a (111) crystal plane, it is parallel to the <1-10> direction, the <10-1> direction or the <01-1> direction.

11. The method for producing a diamond spin sensor according to claim 10, wherein a nitrogen getter is used in the synthesis step, and the nitrogen getter includes any one of titanium, zirconium, hafnium, aluminum, gallium, copper, silver and gold.

12. A method for manufacturing a diamond spin sensor according to claim 10 or 11, wherein the cut diamond includes only one sector boundary or no sector boundary.

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