JP7636737B2 - How diamonds are made - Google Patents

Description

The present invention relates to a method for producing diamond that contains nitrogen-vacancy centers in its crystal structure.

Patent Document 1 describes a single crystal CVD diamond component having a nitrogen vacancy defect (nitrogen-vacancy center, hereafter also referred to as NV center). This NV center (Nitrogen Vacancy center) consists of a pair of a carbon vacancy (a position where a carbon atom is missing) and an adjacent substitutional nitrogen atom (a nitrogen atom that exists in place of a carbon atom) in the diamond crystal lattice.

As described in Patent Document 1, the negatively charged NV center (NV ⁻ ) has various desirable characteristics. For example, the electron spin state of NV ⁻ can be arranged in a specific electron spin state even at non-cryogenic temperatures (e.g., room temperature of about 300 K). In other words, the quantum state of the NV center can be artificially changed from the outside.

In addition, the electronic structure of ^NV- contains emissive and non-emissive electron spin states, making it possible to read the electron spin state of the defect through photons. In other words, the quantum state of the NV center can also be read from the outside. Since the quantum state can be artificially changed and read, the NV center can be used as a quantum bit whose quantum state can be written/read.

Furthermore, the electron spin state of NV ^- has an extremely long coherence time T ₂ (also called the transverse relaxation time, spin coherence time, or decoherence time), which allows stable writing and reading of the quantum state. For this reason, it has been proposed that the NV center can be used for a variety of purposes.

For example, diamonds containing NV centers can be used as quantum information elements in solid-state quantum information processing by applying the above-mentioned use as quantum bits. They can also be used in measuring devices and sensor elements such as magnetometers, nuclear magnetic resonance devices, and magnetic resonance imaging devices.

Patent Document 1 gives an example of a method for forming such NV centers, in which nitrogen ions are implanted after the synthesis of the diamond material. This provides NV centers near the surface of the diamond.

JP 2016-539900 A

Patent Literature 1 discloses that NV centers with long coherence times can be provided by positioning the NV centers near a surface with low surface roughness and low surface damage in a growth state. However, as a specific example, Patent Literature 1 discloses that the _T2 for NV centers formed by implantation into a surface with low roughness and low damage in a growth state is about 65.1 μs. Although this is longer than the _T2 of only about 16.6 μs for NV centers formed by implantation into a polished surface, it cannot be said that it is suitable for use in quantum information devices, etc. Thus, in practice, it has been difficult to obtain a long _T2 .

In view of these problems , an object of the present invention is to provide a method for producing diamond that improves the T2 of NV centers (nitrogen-vacancy centers) that are incorporated into the crystal structure of diamond by ion implantation _.

In order to solve the above problems, a method of manufacturing diamond as one example of an embodiment of the present invention involves doping phosphorus to n-type when manufacturing diamond by chemical vapor deposition to produce phosphorus-doped n-type diamond, then implanting nitrogen ions into the phosphorus-doped n-type diamond to include nitrogen-vacancy centers in the crystal structure of the phosphorus-doped n-type diamond so that the concentration of nitrogen-vacancy centers relative to the concentration of nitrogen atoms is 5% or more , and doping phosphorus when manufacturing the phosphorus-doped n-type diamond by chemical vapor deposition is performed so that the phosphorus concentration contained in the phosphorus-doped n-type diamond is 1×10 ¹⁵ cm ^-3 or more and 1×10 ¹⁸ cm ^-3 or less .

In this method for producing diamond, when nitrogen ions are implanted into the phosphorus-doped n-type diamond, it is preferable to use ¹⁵ N ions as the nitrogen ions.

In addition, in this method for manufacturing diamond, when nitrogen ions are implanted into the phosphorus-doped n-type diamond, it is preferable to adjust the acceleration energy of the nitrogen ions so that the nitrogen ions are implanted to a depth of 1 μm or less from the crystal surface of the phosphorus-doped n-type diamond.

Diamonds produced by the method for producing diamond of the present invention exhibit a longer coherence time _T2 than conventional diamonds even at room temperature. Such diamonds exhibit excellent properties as sensor elements in terms of optimum operating temperature range and sensitivity, and are also suitable for use in quantum information elements, measuring devices, and the like.

FIG. 1 is a diagram showing the flow of each step in a method for producing diamond as one embodiment of the present invention. FIG. 2 is a diagram showing the concentration distribution of nitrogen atoms in a phosphorus-doped n-type diamond produced by the diamond production method shown in FIG. 1 . 2 is a graph showing the _T2 value measurement results of phosphorus-doped n-type diamond produced by the diamond production method shown in FIG. 1 . 2 is a table showing a comparison of the _T2 values of phosphorus-doped n-type diamonds produced by the diamond production method shown in FIG. 1 with the _T2 values of conventional non-doped diamonds.

Figure 1 shows the flow of each process in a diamond manufacturing method as an example of an embodiment of the present invention. As shown in Figure 1, the manufacturing method of this embodiment includes a phosphorus-doped CVD process S1, an ion implantation process S2, and a heat treatment process S3.

In the phosphorus-doped CVD process S1, diamond doped with phosphorus to the n-type, that is, phosphorus-doped n-type diamond, is produced. In the phosphorus-doped CVD process S1 of this embodiment, a chemical vapor deposition (CVD) method is performed to deposit a diamond film on the surface of a diamond crystal substrate, which serves as a substrate, in an atmosphere containing a carbon compound (e.g., methane) and a phosphorus compound (e.g., phosphine). Here, being phosphorus-doped to the n-type means that the diamond is in a state in which phosphorus atoms are distributed in the crystal structure to an extent sufficient for the diamond to function as an n-type semiconductor (a state in which the mobility of electrons as carriers is sufficient).

The phosphorus-doped n-type diamond produced by the phosphorus-doped CVD process S1 described above contains a predetermined concentration of phosphorus according to the conditions of CVD method (concentration of phosphorus compound, etc.).In this embodiment, the phosphorus-doped CVD process S1 is carried out so that the phosphorus concentration contained in the phosphorus-doped n-type diamond is 1×10 ¹⁵ cm ⁻³ or more and 1×10 ¹⁸ cm ⁻³ or less.

Then, the phosphorus-doped n-type diamond produced by the phosphorus-doped CVD process S1 is subjected to ion implantation process S2. In the ion implantation process S2 of this embodiment, the ^15N ions are given acceleration energy and implanted into the phosphorus-doped n-type diamond, so that nitrogen atoms are incorporated into the phosphorus-doped n-type diamond. Here, it is preferable to adjust the acceleration energy so that nitrogen ions are implanted to a depth of 1 μm or less from the crystal surface of the phosphorus-doped n-type diamond. More preferably, the acceleration energy of the nitrogen ions is adjusted so that the nitrogen atoms are distributed at a concentration of 1.0×10 ¹¹ /cm ³ or more and 1.0×10 ¹⁸ /cm ³ or less at a depth of 30 nm or less from the crystal surface.

Then, the phosphorus-doped n-type diamond into which the nitrogen atoms have been incorporated by the ion implantation process S2 is subjected to a heat treatment process S3 so that the incorporated nitrogen atoms are accommodated at the lattice points of the diamond crystal lattice. In the heat treatment process S3 of this embodiment, an annealing process is performed. As a result, nitrogen-vacancy centers (NV centers) are included in the crystal structure of the phosphorus-doped n-type diamond.

The diamond produced in this manner is n-type phosphorus-doped and contains nitrogen-vacancy centers (NV centers) in the crystal structure, with the phosphorus atoms distributed over a wider range than the nitrogen atoms. Specifically, the phosphorus atoms are distributed evenly throughout the diamond, while the nitrogen atoms are distributed more locally than the phosphorus atoms. In this embodiment, the nitrogen atoms are distributed to a depth of 1 μm or less from the crystal surface. Here, with ion implantation, it is possible to include NV centers in the diamond crystal structure so that the concentration of NV centers relative to the concentration of nitrogen atoms is 5% or more.

The NV center contained in the diamond thus manufactured shows a longer T2 even at room temperature compared with the NV center formed by implanting nitrogen ions into a diamond not doped with phosphorus (non-doped). In addition, the NV center formed by nitrogen atoms incorporated by ion implantation is located near the surface of the diamond (within the range of nm order), so it reacts with high sensitivity to changes in the external environment (e.g., magnetic field). Since _{the T2} of the NV center near the surface is long, the diamond manufactured by this embodiment is more suitable for use in measuring devices and sensor elements than conventional ones. Furthermore, if the concentration of NV centers relative to the concentration of nitrogen atoms near the crystal surface is 5% or more, many NV centers will react in one detection, and the sensitivity of the sensor element or measuring device will be higher. In contrast, when the NV center is included in the diamond crystal structure only by the CVD method, the concentration of NV centers relative to the concentration of nitrogen atoms is only about 3%. Therefore, by incorporating NV centers into the crystal structure of phosphorus-doped n-type diamond by ion implantation, it is possible to obtain diamond that is more suitable for use in sensor elements and measuring devices than when only CVD is performed.

An example of a method for producing diamond according to the present invention will now be described in more detail. First, a diamond film is homoepitaxially grown on the (111) face of a diamond substrate of type ^IIa and having a density of 3.513 g/cm3 while doping with phosphorus by chemical vapor deposition (CVD) (phosphorus-doped CVD step S1).

In the phosphorus-doped CVD process S1, diamond film is grown under an atmosphere containing carbon compound (here, methane CH ₃ ) and phosphorus compound (here, phosphine PH ₃ ), thereby producing phosphorus-doped n-type diamond.The phosphorus compound is made small in amount relative to the carbon compound, so that phosphorus atoms are distributed to such an extent that the phosphorus-doped n-type diamond produced functions as an n-type semiconductor.Here, the phosphorus-doped CVD process S1 is carried out so that the phosphorus concentration contained in the phosphorus-doped n-type diamond is 5×10 ¹⁶ cm ⁻³ (one example of 1×10 ¹⁵ cm ⁻³ or more and 1×10 ¹⁸ cm ⁻³ or less).

When the phosphorus-doped n-type diamond film on the diamond substrate has grown to a sufficient thickness (700 nm in this example), the phosphorus-doped CVD process S1 is terminated and the ion implantation process S2 is started. In the ion implantation process S2, the implantation energy of the nitrogen ions is adjusted so that the nitrogen atoms in the phosphorus-doped n-type diamond have a concentration distribution as shown in Figure 2 (the vertical axis is a logarithmic scale). By adjusting the implantation energy at this time, the concentration of NV centers relative to the concentration of nitrogen atoms in the final diamond crystal structure is set to 5% or more.

Specifically, ^15N ions are used and implanted with an implantation energy of 10 keV and an implantation dose of 1.0× ¹⁰⁹ atoms/ ^cm2 . As a result, as shown in FIG. 2, 15N atoms are distributed at a concentration of 1.0× ¹⁰¹¹ atoms/ ^cm3 (1E+11 atoms/ ^cm3 ) to 1.0× ¹⁰¹⁵ atoms/ ^cm3 (within a range of 1.0× ¹⁰¹¹ atoms/ ^cm3 to 1.0× ¹⁰¹⁸ atoms/ ^cm3 ) at a depth of ¹ nm to 30 nm from the crystal surface. Note that, if limited to a depth of 1 nm to 20 nm, the concentration of ^15N atoms is 1.0× ¹⁰¹³ atoms/ ^cm3 to 1.0× ¹⁰¹⁵ atoms/ ^cm3 .

The reason for using ^15N ions is to artificially incorporate ^15N atoms, which have a low natural abundance, into diamond, so that it is possible to distinguish whether the nitrogen-vacancy center (NV center) that will ultimately be included in the crystal structure is naturally generated or artificially generated by ion implantation. Whether the NV center is due to ^14N atoms, which have a high natural abundance, or due to artificially implanted ^15N atoms can be distinguished by the difference in optical/electromagnetic properties of the NV center. In addition, since it is possible to measure each NV center separately, it is also possible to compare and measure artificially generated NV centers and naturally generated NV centers.

When the ^15N ions have been sufficiently implanted, the ion implantation step S2 is terminated and the heat treatment step S3 is started. Typically, annealing is performed at 600° C. or higher.

By the phosphorus-doped CVD step S1, the ion implantation step S2, and the heat treatment step S3, phosphorus-doped n-type diamond containing NV centers in the crystal structure is manufactured. When the echo intensity of this diamond (one of the NV centers contained in it) was measured by the Hahn Echo method to check the _T2 value, the results shown in Figure 3 were obtained. For the sake of simplicity, the plot of the actual measured values is omitted, and only the fitting curve obtained based on the actual measured values is shown.

Since the diamond produced in this embodiment contains ¹³ C atoms, the echo intensity shown in Figure 3 shows the effect of the Rabi oscillation of ¹³ C nuclear spin. Specifically, the echo intensity is measured as a damped oscillation, but the actual damping of the echo intensity can be expressed as a curve (exponential function shown by the two-dot chain line in Figure 3) that passes through the peak value of the oscillation. The value of _T2 obtained from this curve is about 476.74 μs, as shown in Figure 3.

Figure 4 shows a table comparing the _T2 value when nitrogen ions are implanted into phosphorus-doped n-type diamond and the _T2 value when nitrogen ions are implanted into non-phosphorus-doped (non-doped) diamond. This is the result of investigating _T2 for five of the NV centers in each sample for phosphorus-doped n-type diamond sample and non-doped diamond sample. Here, for the sake of simplicity, only the estimated amount based on the measurement result is shown in the table, and the standard deviation (estimation error) of the estimated amount is omitted. Note that Figure 3 shows the measurement result for the measurement object "3" in Figure 4.

As shown in Fig. 4, the _T2 value of non-doped diamond is less than 200μs for all measurement objects (NV center). On the other hand, the _T2 value of phosphorus-doped n-type diamond is 400μs or more for all measurement objects (NV center). Thus, the diamond manufactured by the manufacturing method of this embodiment shows a _T2 value that is dramatically improved compared to non-doped diamond. All of these measurements were performed at room temperature (about 300K).

Diamonds with such a long _T2 are said to have a stable quantum state and are suitable for use as quantum information devices at room temperature. In addition, the magnetic field sensitivity (detection limit and resolution) of the NV center is inversely proportional to the square root of _T2 (proportional to _T2-1 ^/2 ), so diamonds with a longer _T2 are more suitable for use as magnetic field sensor devices.

In addition, the diamond produced by the manufacturing method of this embodiment has nitrogen ions implanted near the crystal surface by ion implantation, so the NV centers are present near the crystal surface. Therefore, these NV centers have high sensitivity to environmental changes (e.g. magnetic field changes) outside the crystal, making them suitable for use in sensor elements and measuring devices. Furthermore, the magnetic field sensitivity of the entire sensor element is inversely proportional to the square root of the number of NV centers that react in one detection. Therefore, diamonds with a high concentration of NV centers relative to the concentration of nitrogen atoms of 5% or more have a large number of NV centers that react in one detection, making them more suitable for use in sensor elements and measuring devices.

In this embodiment, ^15N atoms, which have a low natural ratio, are incorporated near the crystal surface of diamond by ion implantation, so that ^15N atoms are distributed locally near the crystal surface. Since diamonds with such a distribution do not occur unless they are artificially manufactured, it is possible to confirm whether the diamond is manufactured by the manufacturing method of this embodiment by examining the distribution of ^15N atoms near the crystal surface. Specifically, diamonds in which ^15N atoms are distributed at a concentration of 1.0× ¹⁰¹¹ atoms/cm3 or ^more and 1.0× ¹⁰¹⁵ atoms/ ^cm3 or less at a depth of 1 nm or more and 30 nm or less from the crystal surface and are doped with phosphorus in n-type are considered to be diamonds manufactured by the manufacturing method of this embodiment. In addition, phosphorus-doped n-type diamonds in which the concentration of NV center is 5% or more with respect to the concentration of nitrogen atoms, not limited to ^15N atoms, even if they are ^14N atoms, are considered to have been doped with phosphorus by CVD and then ion implanted.

In addition, the NV center incorporated into the diamond crystal structure by ion implantation does not have the axis connecting the nitrogen atom and the vacancy (nitrogen-vacancy axis, NV axis) oriented perpendicular to a specific orientation plane (it does not orient in a specific direction). More specifically, each NV center randomly takes one of the four possible directions for the NV axis. In other words, the NV axis faces one of the four directions with a probability of about 25% each. In contrast, the NV axis of the NV center incorporated into the crystal structure by the CVD method tends to be oriented in a specific direction. Specifically, it is known that in homoepitaxial growth on the (111) face of diamond, the NV axis of the NV center included in the crystal structure faces the (111) direction with a probability of nearly 100%, and in homoepitaxial growth on the (110) face, the NV axis faces only two of the four directions with a probability of about 50% each. Therefore, diamonds with NV centers whose NV axes are not oriented in a specific direction (pointing in four directions with a probability of approximately 25% each) are considered to be N-ion implanted diamonds.

In addition, when NV center is incorporated into diamond crystal structure by ion implantation, it is possible to make hydrogen hardly be incorporated into diamond crystal structure.Specifically, when the diamond is analyzed by secondary ion mass spectrometry (SIMS), the amount of remaining hydrogen can be suppressed to a degree that hydrogen is only confirmed below background level (for example, below 2×10 ¹⁷ cm ⁻³ ).In contrast, when NV center is incorporated into diamond crystal structure by CVD method, hydrogen is introduced into the crystal structure, so that more remaining hydrogen than the background level in SIMS is contained in diamond.Therefore, diamond that has NV center and the amount of remaining hydrogen contained in the crystal structure is below the background level in secondary ion mass spectrometry is considered to be diamond that has been implanted with N ions.

By the way, since phosphorus has an electronic spin, it was previously believed that phosphorus atoms in diamond crystals become magnetic noise for NV centers by phosphorus doping, shortening _T2 . However, as described above, the _T2 of the phosphorus-doped n-type diamond produced by the present invention is improved compared to the conventional one. This is thought to be because phosphorus atoms play a role in reducing magnetic noise.

One of the magnetic noise sources is a "multiple vacancy defect" consisting of multiple vacancies. If a large number of these multi-vacancy defects exist, the magnetic noise source increases, and _T2 is shortened. Here, in phosphorus-doped n-type diamond, a single vacancy, that is, a single vacancy, carries a (negative) charge. Since the single vacancy carries a charge, the single vacancies repel each other due to Coulomb force, and it is difficult for multiple vacancies to gather together. Therefore, it is considered that the generation of "multiple vacancy defects" is suppressed, and as a result, the magnetic noise source is reduced.

At the end of the ion implantation step S2, the phosphorus-doped n-type diamond contains phosphorus atoms, nitrogen atoms, and vacancies in addition to carbon atoms. However, since monovacancies are annealed out at 600°C, if annealing is performed at, for example, 900°C as the heat treatment step S3, ultimately no monovacancies will remain in the phosphorus-doped n-type diamond. On the other hand, complex defects due to nitrogen-vacancy centers (NV centers) are stable even at 1500°C, so in the crystal structure of the phosphorus-doped n-type diamond, the magnetic noise source is ultimately reduced and the NV centers remain.

In the above embodiment, the method for manufacturing diamond is comprised of phosphorus-doped CVD step S1, ion implantation step S2, and heat treatment step S3, but other steps may also be included. For example, since part of the diamond may be graphitized in the heat treatment step S3, a step of cleaning with acid and removing graphite by annealing in oxygen may be performed.

In the above embodiment, the diamond substrate was type IIb, but it may be type Ia, type Ib, or type IIb. The surface on which the diamond film is grown is not limited to the (111) surface, but may be, for example, the (001) surface.

In the above embodiment, the nitrogen ions implanted in the ion implantation step S2 are ^15N ions, but this is not limited thereto, and if there is no need to distinguish the artificially implanted nitrogen atoms from others, ^14N ions may be implanted.

S1 Phosphorus doped CVD process S2 Ion implantation process S3 Heat treatment process

Claims (3)

When producing diamond by chemical vapor deposition, phosphorus is doped to n-type to produce phosphorus-doped n-type diamond,
implanting nitrogen ions into the phosphorus-doped n-type diamond to include nitrogen-vacancy centres in the crystal structure of the phosphorus-doped n-type diamond such that the concentration of nitrogen-vacancy centres relative to the concentration of nitrogen atoms is 5% or more ;
The method for producing diamond, wherein the phosphorus doping is carried out when producing the phosphorus-doped n-type diamond by chemical vapor deposition so that the phosphorus concentration contained in the phosphorus-doped n-type diamond is 1×10 ¹⁵ cm ⁻³ or more and 1×10 ¹⁸ cm ⁻³ or less . 2. The method for producing diamond according to claim 1 , wherein when nitrogen ions are implanted into the phosphorus-doped n-type diamond, ^15N ions are used as the nitrogen ions. 3. The method for producing diamond according to claim 1 or 2, wherein when nitrogen ions are implanted into the phosphorus-doped n-type diamond, the acceleration energy of the nitrogen ions is adjusted so that the nitrogen ions are implanted to a depth of 1 μm or less from the crystal surface of the phosphorus-doped n-type diamond.

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