WO2025137161A1

WO2025137161A1 - Diamond materials with reduced hydrogen-passivated defects and associated fabrication methods

Info

Publication number: WO2025137161A1
Application number: PCT/US2024/060850
Authority: WO
Inventors: David Awschalom; Jonathan MARCKS; Joseph Heremans; Nazar DELEGAN; Alex Martinson; Jessica Jones
Original assignee: University of Chicago; UChicago Argonne LLC
Current assignee: University of Chicago; UChicago Argonne LLC
Priority date: 2023-12-19
Filing date: 2024-12-18
Publication date: 2025-06-26
Anticipated expiration: 2026-06-19

Abstract

A method includes growing an initial diamond layer by performing chemical vapor deposition with at least methane gas and hydrogen gas. The method also includes heating the initial diamond layer to degas hydrogen from the initial diamond layer, thereby creating a degassed diamond layer. The method also includes processing the degassed diamond layer to create vacancies therein, thereby creating a vacancy-containing diamond layer. The method also includes annealing the vacancy-containing diamond layer to create nitrogen-vacancy centers therein. The resulting diamond layer, with nitrogen-vacancy centers therein, contains fewer hydrogen-passivated defects, such as NVH_x centers, than diamond layers fabricated without the step of hydrogen degassing.

Description

PATENT Client Ref. UCHI 24-T-064 Attorney Docket No. UOCH.P2027WO/00627131 DIAMOND MATERIALS WITH REDUCED HYDROGEN-PASSIVATED DEFECTS AND ASSOCIATED FABRICATION METHODS RELATED APPLICATIONS [0001] This application claims priority to U.S. Provisional Patent Application No. 63/612,119, filed December 19, 2023, which is incorporated herein by reference in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0002] This invention was made with government support under grant number FA9550- 19-1-0358, awarded by the Air Force Office of Scientific Research, and grant number DE-FOA- 0002253, awarded by the U.S. Department of Energy. The government has certain rights in the invention. BACKGROUND [0003] The negatively charged nitrogen-vacancy (NV) center in diamond is a defect- based spin qubit that may be used for many quantum applications. SUMMARY [0004] Both single nitrogen-vacancy (NV) centers, used for high-spatial resolution quantum sensing and quantum communication, and ensembles of NV centers, used for high- sensitivity sensing (e.g., magnetometry), may be synthesized with controlled densities and spatial localization via plasma-enhanced chemical vapor deposition (PECVD). PECVD diamond growth occurs in a plasma environment that is mostly hydrogen. Hydrogen ions from plasmas interstitially diffuse into diamond, which may, in turn, passivate NV centers by converting them into nitrogen-vacancy-hydrogen (NVH) centers that are difficult to dissociate. [0005] The present embodiments include methods for fabricating diamond samples that include NV centers. As compared to prior-art techniques and methods, the present method embodiments include a degassing step that occurs after PECVD diamond growth has completed. This degassing step purges the diamond of interstitial hydrogen that becomes trapped in the diamond during CVD. The present embodiments also include diamond materials fabricated with the present method embodiments. [0006] PECVD diamond is grown in a majority-hydrogen plasma atmosphere, PATENT Attorney Docket No. UOCH.P2027WO/00627131 suggesting that during crystal growth, hydrogen diffuses into the crystal [1]. Hydrogen, specifically the hydrogen ion H⁺, is mobile in diamond [2, 3], with an estimated diffusion constant of 6×10⁵ nm²/s at 775°C [1], although this number varies in the literature [2, 3]. Hydrogen can form defect complexes with vacancies and NV centers [4, 5] during NV- activation annealing, precluding NV-center formation via one of two interfering processes: (i) capturing vacancies and converting them to unwanted VHx centers [4] and (ii) passivating NV centers to form unwanted NVHx centers, which are optically dark spin defects that are stable up to 1800°C [6] and thus cannot be removed using conventional annealing. Both of these two processes are of concern with regards to NV-center conversion efficiency [1, 7] as the crystal contains substitutional nitrogen (Ns) vacancies and hydrogen after growth and before annealing. [0007] The present embodiments use high-temperature baking to degas the diamond of interstitial hydrogen, thereby minimizing the formation of unwanted NVHx centers during subsequent high-temperature annealing. For in-situ nitrogen doping performed during PECVD, this degassing step may be performed after the doping and before vacancy creation. For nitrogen-ion implantation, the degassing step may be performed after PECVD (i.e., after growth of the undoped diamond) and before implantation. While VH_x complexes can form, NH_x complexes do not appear in the literature [5] beyond theoretical investigations [8]. Therefore, it should be possible to substantially reduce the amount of interstitial hydrogen in the diamond before introducing vacancies. [0008] As an example of the present embodiments, the degassing step was demonstrated experimentally to increase the yield of near-surface in-situ doped NV centers. These experimental results, described in more detail below, also show evidence of improved NV creation efficiency in characteristic near-surface NV center samples. Incorporating this process step into PECVD diamond growth for quantum applications will improve NV center creation by reducing background dark spin defects and improving device yields. BRIEF DESCRIPTION OF THE FIGURES [0009] FIG. 1 is a flowchart of a method for fabricating a diamond layer with NV centers and reduced hydrogen-passivated defects, in embodiments. [0010] FIG. 2 is a flowchart of a method for fabricating a diamond layer with NV centers and reduced hydrogen-passivated defects, in embodiments. [0011] FIG. 3 is a flowchart of a method for fabricating a diamond layer with NV centers and reduced hydrogen-passivated defects, in embodiments. [0012] FIG. 4 is a plot of pressure versus time that shows the partial pressures of PATENT Attorney Docket No. UOCH.P2027WO/00627131 different gasses while the diamond sample was heated to a temperature of 850°C. [0013] FIGS. 5A and 5B are photoluminescence maps that show NV centers in a first diamond sample (FIG.5A) and a second diamond sample (FIG. 5B) that were grown under identical conditions, except that the second diamond sample was degassed of hydrogen, in embodiments. DETAILED DESCRIPTION [0014] FIG. 1 is a flowchart of a method 100 for fabricating a diamond layer with NV centers and reduced hydrogen-passivated defects, in accordance with some of the present embodiments. In step 102 of the method 100, an initial diamond layer is grown by performing chemical vapor deposition with at least methane gas (CH₄) and hydrogen gas (H₂). As shown in FIG. 1, the output of step 102 is the initial diamond layer. In some embodiments, the initial diamond layer is grown using PECVD during step 102. The methane gas may contain carbon with natural isotopic abundance or isotopically pure carbon. The initial diamond layer may be single-crystal or polycrystalline. In some embodiments, the initial diamond layer is grown on a substrate, such as a diamond substrate or a substrate composed of another material. [0015] In step 104 of the method 100, the initial diamond layer is heated to degas hydrogen. As shown in FIG. 1, the output of step 104 is a degassed diamond layer. Step 104 occurs before the initial diamond layer is processed to create vacancies therein (e.g., see step 206 in FIG.2 or step 306 in FIG.3). Step 104 occurs while the initial diamond layer is located in a high-vacuum environment. For example, step 104 may occur within a vacuum chamber. As part of step 104, degassed hydrogen is removed from the vacuum chamber (e.g., with an active pump, passive non-evaporable getters, or both) concurrently with the heating, thereby maintaining the high-vacuum environment while hydrogen degassing continues. In some embodiments, the initial diamond layer is heated to a maximum temperature (e.g., in the range of 500–600°C). The initial diamond layer may be kept at the maximum temperature for a fixed duration (e.g., three hours or more). Alternatively, the initial diamond layer may be heated for a duration that depends on the background hydrogen partial pressure (e.g., as measured with a residual gas analyzer or mass-spectrometer-based vacuum quality monitor system). For example, heating (and therefore degassing) may stop when the background hydrogen partial pressure has fallen below a predetermined threshold (e.g., 2×10^-8 torr). [0016] In step 106 of the method 100, the degassed diamond layer is processed to create vacancies therein. As shown in FIG.1, the output of step 106 is a vacancy-containing diamond layer. In step 108 of the method 100, the vacancy-containing diamond layer is annealed (e.g., PATENT Attorney Docket No. UOCH.P2027WO/00627131 by heating under vacuum) to create NV centers therein. As shown in FIG.1, the output of step 108 is a diamond layer with NV centers. Step 108 may be performing using annealing techniques known in the art. In some embodiments, the method 100 includes an additional step (not shown in FIG.1) of growing a cap layer over the diamond layer with NV centers. This cap layer may be, for example, a layer of undoped diamond without NV centers. [0017] FIG. 2 is a flowchart of a method 200 for fabricating a diamond layer with NV centers and reduced hydrogen-passivated defects, in accordance with some of the present embodiments. The method 200 is an embodiment of the method 100 of FIG. 1. In step 202 of the method 200, a nitrogen-doped diamond layer is grown by performing chemical vapor deposition with at least nitrogen gas, methane gas, and hydrogen gas. Step 202 is one example of step 102 of the method 100. As shown in FIG.2, the output of step 202 is the nitrogen-doped diamond layer, which is an example of the initial diamond layer of FIG.1. In step 204 of the method 200, the nitrogen-doped diamond layer is heated to degas hydrogen. Step 204 is one example of step 104 of the method 100. [0018] In step 206 of the method 200, the degassed diamond layer is irradiated with electrons to form vacancies in the degassed diamond layer. Step 206 is one example of step 106 of the method 100. Step 206 may be performed using irradiation techniques known in the art. As an alternative to step 206, the degassed diamond layer may be bombarded with carbon ions (or another species of ion) to form vacancies. In any case, the method 200 then continues with step 108 of FIG.1 (annealing) to create NV centers in the vacancy-containing diamond layer. [0019] FIG. 3 is a flowchart of a method 300 for fabricating a diamond layer with NV centers and reduced hydrogen-passivated defects, in accordance with some of the present embodiments. The method 300 is an embodiment of the method 100 of FIG. 1. In step 302 of the method 300, an undoped diamond layer is grown by performing chemical vapor deposition using only methane and hydrogen gases (i.e., without nitrogen gas). Step 302 is one example of step 102 of the method 100. As shown in FIG.3, the output of step 302 is the undoped diamond layer, which is an example of the initial diamond layer of FIG.1. In step 304 of the method 300, the undoped diamond layer is heated to degas hydrogen. Step 304 is one example of step 104 of the method 100. [0020] In step 306 of the method 300, the degassed diamond layer is bombarded with nitrogen ions to form both nitrogen defects and vacancies in the degassed diamond layer. Step 306 is one example of step 106 of the method 100. The method 100 then continues with step 108 (annealing) to create NV centers in the vacancy-containing diamond layer. PATENT Attorney Docket No. UOCH.P2027WO/00627131 Experimental Results [0021] As an experimental demonstration of the method 100 of FIG. 1, a PECVD- fabricated diamond sample was mounted inside a 12-inch spherical vacuum chamber that was pumped to high vacuum (i.e., pressure 10^-6 torr). Inside the vacuum chamber and while under vacuum, the diamond sample was heated to 500–600°C and baked for at least three hours before being removed for further processing. [0022] FIG.4 is a plot of pressure (in torr) versus time (in minutes) that shows the partial pressures of different gasses in the vacuum chamber while the diamond sample was heated to a temperature of 850°C. FIG. 1 demonstrates that hydrogen gas is, in fact, degassed from the diamond sample as it is heated. A 300-L/s pump pumped continuously on the vacuum chamber during the entire duration of the measurement. In FIG.4, the partial pressure of hydrogen (H2) gas is shown as a dotted line while the sum of the partial pressures of oxygen (O2), nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2) gasses are shown as a solid line. Five temperatures are indicated by black arrows in FIG. 4. The partial pressure of H2 significantly increases when the sample temperature is raised to 500°C, as this mobilizes the hydrogen [2], causing it to degas into the chamber. At the same time, there is no significant change in the other partial pressures. The partial pressure of H2 remains elevated for about three hours. These temperatures are exceeded during growth (e.g., step 102 of the method 100 of FIG. 1), but during growth the hydrogen-containing plasma is on and so any equilibrium retains hydrogen in the diamond. [0023] A lower bound on the density of hydrogen present in the diamond sample may be calculated from the peak partial pressure. At 166.19 minutes, the peak partial pressure of _{1.37 torr corresponds to 3.89×10 11 mol at 515°C. Assuming that hydrogen has diffused} uniformly throughout the diamond sample, the estimated lower-bound density of hydrogen is 49.3 ppb in the diamond sample (post-growth). Relativistic electron irradiation produces a vacancy density in the diamond sample of 1.1×10 ppb. At a typical dose of approximately _{1×1014 cm 2, the vacancy density is exceeded by the estimated lower-bound hydrogen density.} This indicates that the first hydrogen-interfering process discussed above—the capture of vacancies and conversion to VHx centers—likely plays a role in reducing NV-center formation. [0024] Typical in-situ nitrogen doping (e.g., delta-doping) generates nitrogen-doped layers in diamond with thicknesses in the range of 2–500 nm and nitrogen densities up to tens of ppm. These thicknesses are relatively thin compared to the substrate, whose thickness may be up to several hundred microns, or more. Thus, the overall nitrogen density in the diamond sample is also exceeded by the hydrogen density. This indicates that the second hydrogen- PATENT Attorney Docket No. UOCH.P2027WO/00627131 interfering process discussed above—the passivation of NV centers to form NVH_x centers— also likely plays a role in reducing NV-center formation. [0025] FIGS. 5A and 5B are photoluminescence maps that show NV centers in a first diamond sample (FIG.5A) and a second diamond sample (FIG. 5B) that were grown under identical conditions, except that the second diamond sample was degassed of hydrogen. Both samples used a nominally 62-nm-thick, isotopically purified PECVD overgrowth layer with an in-situ nitrogen-doped layer located 10–12 nm below the surface of the PECVD overgrowth layer. The degassing step described above (i.e., the step 104 of the method 100 of FIG.1) was _{performed on the second sample only. Both samples were irradiated with a 2×1014-cm 2 dose} of 2-MeV electrons, annealed for two hours at 850°C with a 10°C/min ramp rate, and tri-acid cleaned. The samples were then imaged in a confocal microscope with a 532-nm laser beam while monitoring fluorescence in the NV center emission band. [0026] The photoluminescence maps of FIGS. 5A and 5B display two main features. The first feature is the appearance of several donut-shaped spots, which arise from out-of-focus NV centers in the growth substrate. These out-of-focus NV centers, which are not relevant because they lie outside of the nitrogen-doped layer, are ignored. The second feature is the appearance of in-focus, circular, or ovular bright spots, which are NV centers that lie within the laser beam’s depth-of-field. This depth-of-field is larger than the thickness of the doped nitrogen layer and therefore these spots may be either 10-nm-deep NV centers arising from the in-situ nitrogen doping or substrate NV centers that happen to fall within the depth of field. These two cases are distinguished via optically detected magnetic resonance of the NV center electron spin hyperfine spectrum. The in-situ doped nitrogen is less abundant (¹⁵N, natural abundance 0.4%), with a nuclear spin of 1/2, while substrate nitrogen is more abundant (¹⁴N, natural abundance 99.6%), with a nuclear spin of 1. The difference in nuclear spin leads to a different hyperfine spectrum. By isotopically tagging the in-situ doped nitrogen with the less abundant isotope, it can be determined with high accuracy that an NV center is located in the PECVD overgrowth layer. [0027] The photoluminescence maps of FIGS. 5A and 5B are characteristic of their respective diamond samples. In the 30 m × 30 m areas shown, one near-surface NV center (i.e., an NV center within the in-situ doped layer) was identified in the first sample (FIG. 5A), as compared to six near-surface NV centers in the second sample (FIG.5B). These near-surface NV centers are identified in FIGS.5A and 5B with white circles. Thus, the hydrogen degassing step of the present embodiments increased the yield of NV-center formation in the in-situ doped layer by approximately a factor of six. PATENT Attorney Docket No. UOCH.P2027WO/00627131 [0028] The higher yield of NV-center formation in the second diamond sample (FIG. 5B), as compared to the first sample (FIG.5A), is attributed to the reduction of hydrogen, which in turn prevents passivation that converts some NV centers into NVH_x centers. Thus, it is inferred from FIGS. 5A and 5B that the six-fold increase in near-surface NV centers in the second sample comes with a corresponding reduction in the number of NVH_x centers. From this, it is estimated that the first sample has, on average, 5 NVHx centers within the 30 m × 30 m area shown. Assuming that the nitrogen-doped layer has a thickness of 2 nm, the corresponding volume of the nitrogen-doped layer shown in each of FIGS. 5A and 5B is approximately 1.8 m³. For 5 NVHx centers within this volume, the corresponding NVHx-center volume-number density in FIG. 5A is estimated to be 2.8 m^-3. By comparison, the NV-center volume-number density in FIG.5A is approximately 0.6 m^-3. [0029] The present embodiments include diamond-based compositions of matter that feature various combinations of (i) a lower bound on the volume number density of NV centers and (ii) an upper bound on the volume number density of NVH_x centers. Any of these compositions of matter may be fabricated using the methods disclosed herein (e.g., the method 100 of FIG. 1). Those skilled in the art will recognize how various steps of the fabrication process may be adjusted or modified to achieve a target or specified NV-center density. However, these steps usually change the NVH_x-center density as well. Specifically, increasing the NV-center density increases the NVH_x-center density, thereby introducing a tradeoff between these two quantities. The present embodiments break this tradeoff by identifying how to increase the NV-center density with minimal, if any, increase in NVHx-center density. The present compositions of matter include diamond materials that access these previously unattainable combinations of NV-center density and NVHx-center density. [0030] In some embodiments, the lower bound on the volume number density of NV centers is 0.1 m^-3. In some embodiments, the lower bound on the volume number density of NV centers is 0.5 m^-3. In some embodiments, the lower bound on the volume number density of NV centers is 1.0 m^-3. In some embodiments, the lower bound on the volume number density of NV centers is 5.0 m^-3. In some embodiments, the lower bound on the volume number density of NV centers is 10.0 m^-3. In some embodiments, the lower bound on the volume number density of NV centers is 50.0 m^-3. In some embodiments, the lower bound on the volume number density of NV centers is 100 m^-3. However, the lower bound on the volume number density of NV centers may have another value (e.g., less than 0.1 m^-3 or greater than 100 m^-3) without departing from the scope hereof. [0031] In some embodiments, the upper bound on the volume number density of NVH_x PATENT Attorney Docket No. UOCH.P2027WO/00627131 centers is 0.1 m^-3. In some embodiments, the upper bound on the volume number density of NVH_x centers is 0.5 m^-3. In some embodiments, the upper bound on the volume number density of NVH_x centers is 1.0 m^-3. In some embodiments, the upper bound on the volume number density of NVH_x centers is 5.0 m^-3. In some embodiments, the upper bound on the volume number density of NVH_x centers is 10.0 m^-3. In some embodiments, the upper bound on the volume number density of NVHx centers is 50.0 m^-3. In some embodiments, the upper bound on the volume number density of NVHx centers is 100 m^-3. However, the upper bound on the volume number density of NVHx centers may have another value (e.g., less than 0.1 m^-3 or greater than 100 m^-3) without departing from the scope hereof. [0032] In one embodiment, a composition of matter includes diamond having (i) a volume number density of NV centers greater than 0.6 m^-3 and (ii) a volume number density of NVHx centers less than 2.8 m^-3. In this embodiment, the lower bound on the volume number density of NV centers is 0.6 m^-3 and the upper bound on the volume number density of NVH_x centers if 2.8 m^-3. In other embodiments, the composition of matter alternatively has one or both of (i) a different value for the lower bound on the volume number density of NV centers and (ii) a different value for the upper bound on the volume number density of NVH_x centers. Thus, it should be understood that any of the above-discussed lower bounds on the volume number density of NV centers may be combined with any of the above-discussed upper bounds on the volume number density of NVH_x centers. [0033] In other embodiments, a composition of matter includes diamond having both a volume number density of NV centers and a volume number density of NVHx centers, where the volume number density of NV centers exceeds the volume number density of NVHx centers. In some of these embodiments, the volume number density of NV centers exceeds the volume number density of NVHx centers by a multiplicative factor of two or more. In some of these embodiments, the volume number density of NV centers exceeds the volume number density of NVHx centers by a multiplicative factor of three or more. In some of these embodiments, the volume number density of NV centers exceeds the volume number density of NVHx centers by a multiplicative factor of five or more. In some of these embodiments, the volume number density of NV centers exceeds the volume number density of NVH_x centers by a multiplicative factor of ten or more. [0034] Given that the nitrogen-doped layer (i.e., the diamond layer with NV centers in FIGS. 1, 2, and 3) is thin relative to its two other dimensions, the lower bound on the volume number density of NV centers and the upper bound on the volume number density of NVH_x centers may be alternatively expressed as surface-area densities that ignore the thickness of the PATENT Attorney Docket No. UOCH.P2027WO/00627131 nitrogen-doped layer. Thus, the present embodiments also include diamond-based compositions of matter that feature various combinations of both (i) a lower bound on the area number density of NV centers and (ii) an upper bound on the area number density of NVH_x centers. [0035] In some embodiments, the lower bound on the area number density of NV centers is 10^-4 m^-2. In some embodiments, the lower bound on the area number density of NV centers is 5×10^-4 m^-2. In some embodiments, the lower bound on the area number density of NV centers is 10^-3 m^-2. In some embodiments, the lower bound on the area number density of NV centers is 5×10^-3 m^-2. In some embodiments, the lower bound on the area number density of NV centers is 10^-2 m^-2. In some embodiments, the lower bound on the area number density of NV centers is 5×10^-2 m^-2. In some embodiments, the lower bound on the area number density of NV centers is 10^-1 m^-2. However, the lower bound on the area number density of NV centers may have another value (e.g., less than 10^-4 m^-2 or greater than 10^-1 m^-2) without departing from the scope hereof. [0036] In some embodiments, the upper bound on the area number density of NVH_x centers is 10^-5 m^-2. In some embodiments, the upper bound on the area number density of NVH_x centers is 5×10^-5 m^-2. In some embodiments, the upper bound on the area number density of NVH_x centers is 10^-4 m^-2. In some embodiments, the upper bound on the area number density of NVH_x centers is 5×10^-4 m^-2. In some embodiments, the upper bound on the area number density of NVH_x centers is 10^-3 m^-2. In some embodiments, the upper bound on the area number density of NVHx centers is 5×10^-3 m^-2. In some embodiments, the upper bound on the area number density of NVHx centers is 10^-2 m^-2. However, the upper bound on the area number density of NVHx centers may have another value (e.g., less than 10^-5 m^-2 or greater than 10^-2 m^-2) without departing from the scope hereof. [0037] In one embodiment, a composition of matter includes diamond having (i) an area number density of NV centers greater than 1.1×10^-3 m^-2 and (ii) an area number density of NVHx centers less than 5.6×10^-3 m^-2. In this case, the lower bound on the area number density of NV centers is 1.1×10^-3 m^-2 and the upper bound on the area number density of NVHx centers is 5.6×10^-3 m^-2. However, the composition of matter may alternatively have one or both of (i) a different value for the lower bound on the area number density of NV centers and (ii) a different value for the upper bound on the area number density of NVH_x centers. Thus, it should be understood that any of the above-discussed lower bounds on the area number density of NV centers may be combined with any of the above-discussed upper bounds on the area number density of NVH_x centers. [0038] In other embodiments, a composition of matter includes diamond having both PATENT Attorney Docket No. UOCH.P2027WO/00627131 an area number density of NV centers and an area number density of NVH_x centers, where the area number density of NV centers exceeds the area number density of NVH_x centers. In some of these embodiments, the area number density of NV centers exceeds the area number density of NVH_x centers by a multiplicative factor of two or more. In some of these embodiments, the area number density of NV centers exceeds the area number density of NVH_x centers by a multiplicative factor of three or more. In some of these embodiments, the area number density of NV centers exceeds the area number density of NVHx centers by a multiplicative factor of five or more. In some of these embodiments, the area number density of NV centers exceeds the area number density of NVHx centers by a multiplicative factor of ten or more. [0039] The present embodiments also include devices composed of any one or more of the composition-of-matter embodiments disclosed herein. For example, in some embodiments a diamond sample includes an NV-diamond layer composed of any of the diamond compositions discussed above (e.g., diamond having a volume number density of NV centers greater than 0.6 m^-3 and a volume number density of NVH_x centers less than 2.8 m^-3). The NV-diamond layer may have a thickness in the range of 2–500 nm. In some of these embodiments, the diamond sample further includes a base layer located beneath the NV- diamond layer. The base layer may be composed of conventional diamond. In other embodiments, the diamond sample further includes a cap layer located over the NV-diamond layer. The cap layer may be composed of conventional diamond. Combinations of Features [0040] Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following examples illustrate possible, non-limiting combinations of features and embodiments described above. It should be clear that other changes and modifications may be made to the present embodiments without departing from the spirit and scope of this invention: [0041] (A1) A method includes growing an initial diamond layer by performing chemical vapor deposition with at least methane gas and hydrogen gas; heating the initial diamond layer to degas hydrogen from the initial diamond layer, thereby creating a degassed diamond layer; processing the degassed diamond layer to create vacancies therein, thereby creating a vacancy-containing diamond layer; and annealing the vacancy-containing diamond layer to create nitrogen-vacancy centers therein. [0042] (A2) In the method denoted (A1), growing the initial diamond layer includes growing a nitrogen-doped diamond layer by performing chemical vapor deposition with at least PATENT Attorney Docket No. UOCH.P2027WO/00627131 nitrogen gas, methane gas, and hydrogen gas. Furthermore, heating the initial diamond layer includes heating the nitrogen-doped diamond layer to create the degassed diamond layer. [0043] (A3) In the method denoted (A2), processing the degassed diamond layer includes irradiating the degassed diamond layer with electrons. [0044] (A4) In the method denoted (A2), processing the degassed diamond layer includes bombarding the degassed diamond layer with carbon ions. [0045] (A5) In any of the methods denoted (A2) to (A4), growing the initial diamond layer includes performing chemical vapor deposition with isotopically pure nitrogen gas. [0046] (A6) In the method denoted (A1), growing the initial diamond layer includes growing an undoped diamond layer. [0047] (A7) In any of the methods denoted (A1) to (A6), heating the initial diamond layer includes heating the initial diamond layer to a maximum temperature between 500°C and 600°C. [0048] (A8) In the method denoted (A7), heating the initial diamond layer includes holding the initial diamond layer at the maximum temperature for at least three hours. [0049] (A9) In any of the methods denoted (A1) to (A8), growing the initial diamond layer includes performing plasma-enhanced chemical vapor deposition. [0050] (A10) In any of the methods denoted (A1) to (A8), growing the initial diamond layer includes performing chemical vapor deposition with carbon-isotopically pure methane gas. [0051] (A11) In any of the methods denoted (A1) to (A10), the initial diamond layer is single-crystal diamond. [0052] (A12) In any of the methods denoted (A1) to (A10), the initial diamond layer is polycrystalline diamond. [0053] (A13) In any of the methods denoted (A1) to (A12), growing the initial diamond layer includes growing the initial diamond layer on a substrate. [0054] (A14) In the method denoted (A13), the substrate is composed of diamond. [0055] (A15) In any of the methods denoted (A1) to (A14), heating the initial diamond layer occurs prior to any irradiation or ion bombardment of the initial diamond layer. [0056] (A16) In any of the methods denoted (A1) to (A15), heating the initial diamond layer occurs while the initial diamond layer is under high vacuum. [0057] (A17) In any of the methods denoted (A1) to (A16), the method further includes growing a cap layer over the vacancy-containing diamond layer. [0058] (A18) In the method denoted (A17), the cap layer is composed of undoped PATENT Attorney Docket No. UOCH.P2027WO/00627131 diamond. [0059] (B1) A composition of matter includes diamond fabricated with any of the methods denoted (A1) to (A18). [0060] Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween. References [1] A. Stacey et al., “Depletion of nitrogen-vacancy color centers in diamond via hydrogen passivation,” Appl. Phys. Lett.100, 071902 (2012). [2] C. Saguy et al., “Diffusion of hydrogen in undoped, p-type and n-type doped diamonds,” Diam. Relat. Mater.12, 623 (2003). [3] D. J. Cherniak, E. B. Watson, V. Meunier, and N. Kharche, “Diffusion of helium, hydrogen and deuterium in diamond: Experiment, theory and geochemical applications,” Geochim. Cosmochim. Acta 232, 206 (2018). [4] C. V. Peaker, J. P. Goss, P. R. Briddon, A. B. Horsfall, and M. J. Rayson, “The vacancy– hydrogen defect in diamond: A computational study,” Phys. Status Solidi A 212, 2431 (2015). [5] M. N. R. Ashfold, J. P. Goss, B. L. Green, P. W. May, M. E. Newton, and C. V. Peaker, “Nitrogen in Diamond,” Chem. Rev.120, 5745 (2020). [6] R. U. A. Khan et al., “Colour-causing defects and their related optoelectronic transitions in single crystal CVD diamond,” J. Phys. Condens. Matter 25, 275801 (2013). [7] S. Chakravarthi et al., “Window into NV center kinetics via repeated annealing and spatial tracking of thousands of individual NV centers,” Phys. Rev. Mater.4, 023402 (2020). [8] J. P. Goss, R. Jones, M. I. Heggie, C. P. Ewels, P. R. Briddon, and S. Öberg, “Theory of hydrogen in diamond,” Phys. Rev. B 65, 115207 (2002).

Claims

PATENT Attorney Docket No. UOCH.P2027WO/00627131 CLAIMS What is claimed is: 1. A method, comprising: growing an initial diamond layer by performing chemical vapor deposition with at least methane gas and hydrogen gas; heating the initial diamond layer to degas hydrogen from the initial diamond layer, thereby creating a degassed diamond layer; processing the degassed diamond layer to create vacancies therein, thereby creating a vacancy-containing diamond layer; and annealing the vacancy-containing diamond layer to create nitrogen-vacancy centers therein. 2. The method of claim 1, wherein: said growing the initial diamond layer comprises growing a nitrogen-doped diamond layer by performing chemical vapor deposition with at least nitrogen gas, methane gas, and hydrogen gas; and said heating the initial diamond layer comprises heating the nitrogen-doped diamond layer to create the degassed diamond layer. 3. The method of claim 2, wherein said processing comprises irradiating the degassed diamond layer with electrons. 4. The method of claim 2, wherein said processing comprises bombarding the degassed diamond layer with carbon ions. 5. The method of claim 2, wherein said growing comprises performing chemical vapor deposition with isotopically pure nitrogen gas. 6. The method of claim 1, wherein: said growing the initial diamond layer comprises growing an undoped diamond layer; said heating the initial diamond layer comprises heating the undoped diamond layer to create the degassed diamond layer; and PATENT Attorney Docket No. UOCH.P2027WO/00627131 said processing comprises bombarding the degassed diamond layer with nitrogen ions to create nitrogen defects and vacancies therein. 7. The method of claim 1, wherein said heating comprises heating the initial diamond layer to a maximum temperature between 500°C and 600°C. 8. The method of claim 7, wherein said heating comprises holding the initial diamond layer at the maximum temperature for at least three hours. 9. The method of claim 1, wherein said growing comprises performing plasma-enhanced chemical vapor deposition. 10. The method of claim 1, wherein said growing comprises performing chemical vapor deposition with carbon-isotopically pure methane gas. 11. The method of claim 1, the initial diamond layer comprising single-crystal diamond. 12. The method of claim 1, the initial diamond layer comprising polycrystalline diamond. 13. The method of claim 1, wherein said growing comprises growing the initial diamond layer on a substrate. 14. The method of claim 13, the substrate is composed of diamond. 15. The method of claim 1, wherein said heating occurs prior to any irradiation or ion bombardment of the initial diamond layer. 16. The method of claim 1, wherein said heating occurs while the initial diamond layer is under high vacuum. 17. The method of claim 1, further comprising growing a cap layer over the vacancy- containing diamond layer. 18. The method of claim 17, the cap layer being composed of undoped diamond.