Atomic-scale fabrication of diamond quantum computers

Abstract

The present disclosure relates to a method for fabricating a plurality of optically addressable qubits in diamond, the method comprising: providing a diamond substrate with a passivated surface; removing passivating atoms from the passivated surface to create a plurality of depassivated sites from which the passivating atoms are removed; exposing the plurality of depassivated sites to a nitrogen-containing compound to adsorb nitrogen at the plurality of depassivated sites of the diamond substrate; overgrowing the plurality of depassivated sites with diamond by chemical vapor deposition (CVD) at a diamond growth rate related to temperature and pressure, where the diffusion or desorption of nitrogen at the plurality of depassivated sites is avoided to incorporate nitrogen into the diamond; converting the incorporated nitrogen into a plurality of nitrogen vacancies; and charging the plurality of nitrogen vacancies with a negative charge.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from Australian Provisional Patent Application No. 2021903915, filed December 3, 2021, and Australian Provisional Patent Application No. 2022902826, filed September 29, 2022, the contents of which are incorporated herein by reference in their entireties.

[0002] The present disclosure relates to the fabrication of diamond quantum computers, and in particular to the fabrication of nitrogen vacancies in diamond computers. [Background technology]

[0003] In quantum computing, a qubit, or quantum bit, is the basic unit of quantum information. In classical systems, a bit can be in one state or the other. In contrast, quantum mechanics allows a qubit to be in a coherent superposition of both states at the same time, a property essential to quantum mechanics and quantum computing.

[0004] The scalable architecture of the diamond quantum microprocessor consists of an array of processor nodes. Each processor node consists of an NV center and a cluster of nuclear spins (unique nitrogen nuclear spin and 0-4 13C nuclear spin impurities). The nuclear spins act as qubits for the microprocessor, while the NV centers act as quantum buses mediating qubit initialization and readout, as well as multiqubit operations within and between nodes. Quantum computation is controlled via integrated electrical, optical, magnetic, and classical computing systems.

[0005] One aspect of the realization of a scalable architecture is the precision fabrication of arrays of NV centers separated by approximately 5-10 nm with tolerances of less than 1 nm. Such precision is useful for magnetically coupling the electron spins of NV centers such that they may mediate multi-qubit operations between nodes. However, such precision fabrication is difficult to achieve using "top-down" nitrogen (N) ion implantation techniques due to limitations in the fabrication of implant masks and scattering of the implanted ions.

[0006] Any discussion of documents, acts, materials, devices, articles or the like which has been included in this specification should not be construed as an admission that any or all of such matters formed part of the prior art document or were common general knowledge in the field relevant to the present disclosure as they existed prior to the priority date of each of the appended claims.

[0007] Throughout this specification the word "comprise" or variations such as "comprises" or "comprising" will be understood to mean the inclusion of a stated element, component or step, or group of elements, components or steps, but not the exclusion of any other element, component or step, or group of elements, components or steps. Summary of the Invention [Means for solving the problem]

[0008] A method for fabricating multiple optically addressable qubits in diamond comprises: Providing a diamond substrate with a passivated surface; removing passivating atoms from the passivated surface to create a plurality of depassivated sites from which the passivating atoms have been removed; exposing the plurality of depassivated sites of the diamond substrate to a nitrogen-containing compound to adsorb nitrogen at the plurality of depassivated sites; overgrowing the plurality of depassivated sites with diamond by chemical vapor deposition (CVD) at a diamond growth rate related to temperature and pressure, wherein diffusion or desorption of nitrogen at the plurality of depassivated sites is avoided in order to incorporate nitrogen into the diamond; converting the incorporated nitrogen into a plurality of nitrogen vacancies; and charging the plurality of nitrogen vacancies with a negative charge.

[0009] It is an advantage that the low temperature and / or pressure and / or diamond growth rate avoids the diffusion and / or desorption that would occur when using CVD optimized for maximum diamond growth. As a result, the method can be used to encapsulate nitrogen atoms arranged with atomic precision without affecting the arrangement of the nitrogen atoms.

[0010] In some embodiments, any one or more of the diamond growth rate, temperature, and pressure are low enough to avoid diffusion or desorption of nitrogen at the depassivated sites.

[0011] In some embodiments, the nitrogen in the plurality of depassivated sites is bonded to the diamond substrate by a covalent bond between the nitrogen and a carbon atom of the diamond substrate, the covalent bond being defined by a bond energy.

[0012] In some embodiments, any one or more of the diamond growth rate, temperature, and pressure are low enough to preserve covalent bonds.

[0013] In some embodiments, the relative rate of sample etching, controlled by sample temperature and reactive species, is significantly slower than the growth rate, such that nitrogen at the depassivated sites does not desorb and diffuse out before or during diamond overgrowth.

[0014] In some embodiments, the covalent bond is sp 3 It is a bond.

[0015] In some embodiments, the method further includes encapsulating nitrogen at the plurality of depassivated sites with a protective layer.

[0016] In some embodiments, the method further comprises forming a protective layer by special chemical vapor deposition overgrowth according to one or more options in Table 1.

[0017] In some embodiments, the method further includes forming the protective layer by molecular beam epitaxy.

[0018] In some embodiments, the method further comprises preparing the diamond substrate to create atomically smooth patches on the diamond substrate.

[0019] In some embodiments, the method further comprises preparing the diamond substrate at a substrate surface misorientation angle relative to the nominal surface orientation to create atomically smooth patches.

[0020] In some embodiments, preparing the diamond substrate with a substrate surface misorientation angle includes creating step edges that define atomically smooth patches between adjacent step edges.

[0021] In some embodiments overgrowing diamond comprises growing the crystal lattice from a step edge.

[0022] In some embodiments, the substrate surface misorientation angle is between 0.1 and 3.4 degrees.

[0023] In some embodiments, converting the incorporated nitrogen to nitrogen vacancies comprises carbon ion irradiation and annealing.

[0024] A method for fabricating multiple optically addressable qubits in diamond comprises: providing a diamond substrate with a passivated surface, the diamond substrate including doped regions for introducing delocalized charge carriers into the diamond substrate and / or for providing a base for injected carriers; removing passivating atoms from the passivated surface to create a plurality of depassivated sites where the passivating atoms have been removed, the removing of the passivating atoms comprising: moving a tip of a scanning tunneling microscope (STM) across the passivated surface with atomic precision; and creating a pulsed voltage drop between the tip and the diamond surface to remove the passivating atoms; exposing the plurality of depassivated sites of the diamond substrate to a nitrogen-containing compound to adsorb nitrogen at the plurality of depassivated sites; overgrowing a plurality of depassivated sites with diamond; converting the incorporated nitrogen into a plurality of nitrogen vacancies; and charging the nitrogen vacancies with a negative charge.

[0025] An advantage is that STM allows the removal of passivating atoms with atomic precision, meaning that the final location of the nitrogen is also atomically precise. Because quantum properties change significantly with changes in the location of the nitrogen, the method enables quantum architectures with precise and reproducible properties, such as inter-qubit coupling.

[0026] In some embodiments, the method further comprises preparing the diamond substrate to create atomically smooth patches on the diamond substrate.

[0027] In some embodiments, moving the STM tip further includes imaging the passivated surface to locate the atomically smooth patch.

[0028] In some embodiments, the method further comprises preparing the diamond substrate at a substrate surface misorientation angle relative to the nominal surface orientation to create atomically smooth patches.

[0029] In some embodiments, preparing the diamond substrate with a substrate surface misorientation angle includes creating step edges that define atomically smooth patches between adjacent step edges.

[0030] In some embodiments overgrowing diamond comprises growing the crystal lattice from a step edge.

[0031] In some embodiments, the substrate surface misorientation angle is between 0.1 and 3.4 degrees.

[0032] In some embodiments, the method further includes using STM imaging to confirm removal of passivating atoms from the passivated surface after moving the STM tip and prior to exposing the depassivated site to the nitrogen-containing compound.

[0033] In some embodiments, the method further comprises confirming the adsorption of the nitrogen-containing compound on the diamond substrate using STM imaging.

[0034] In some embodiments, verifying the adsorption of the nitrogen-containing compound further comprises verifying that the nitrogen-containing compound adsorbed on the diamond substrate has a desired orientation relative to the diamond substrate.

[0035] In some embodiments, (a) the diamond substrate has {100} faces and the desired orientation is an orientation relative to the diamond substrate that provides four sp3 bonds across two adjacent surface dimers, or (b) the diamond substrate has {111} faces and the desired orientation is an orientation relative to the diamond substrate that provides three sp3 bonds to three surface carbon atoms.

[0036] In some embodiments, the method further comprises, upon determining that the nitrogen-containing compound has an undesired orientation relative to the diamond substrate, using an STM to desorb the nitrogen-containing compound from the depassivated sites.

[0037] In some embodiments, removing passivating atoms by STM may be performed at a rate of 1×10 -11 Torr~1×10 -9 Torr pressure.

[0038] In some embodiments, removing the passivating atoms by STM further comprises a current pulse in the range of 1 ms to 10 ms, with a voltage in the range of 2.7V to 7V and a current in the range of 1 nA to 50 nA.

[0039] In some embodiments, overgrowing the depassivated sites with diamond is carried out by chemical vapor deposition.

[0040] In some embodiments, converting the incorporated nitrogen to nitrogen vacancies comprises carbon ion irradiation and annealing.

[0041] A method for fabricating multiple optically addressable qubits in diamond comprises: Providing a diamond substrate with a passivated surface; removing passivating atoms from the passivated surface to create a plurality of depassivated sites from which the passivating atoms have been removed; exposing the plurality of depassivated sites of the diamond substrate to a nitrogen-containing compound that includes reactive nitrogen groups to adsorb nitrogen from the reactive nitrogen groups at the plurality of depassivated sites of the diamond substrate; overgrowing a plurality of depassivated sites with diamond; converting the incorporated nitrogen into a plurality of nitrogen vacancies; and charging the nitrogen vacancies with a negative charge.

[0042] Exposing the depassivated sites to nitrogen-containing compounds has the advantage that it does not destroy the passivation in the unexposed areas. This means that nitrogen will only adsorb where the passivating atoms have been removed. This allows for the precise implantation of nitrogen into diamond, which in turn allows for the reproducible fabrication of devices with desired quantum properties, such as inter-qubit coupling.

[0043] In some embodiments, the nitrogen of the reactive nitrogen group forms a bond with a carbon atom of the diamond substrate at the depassivated site.

[0044] In some embodiments, the reactive nitrogen group is a functional group and is bonded to a non-reactive group.

[0045] In some embodiments, the non-reactive group comprises a hydrocarbon.

[0046] In some embodiments, adsorbing the nitrogen-containing compound at the depassivated sites further comprises removing non-reactive groups by heating after the exposure.

[0047] In some embodiments, the post-exposure heating is carried out at a temperature that (a) maintains bonds between the carbon atoms of the diamond substrate at the depassivated sites and the nitrogen of the reactive nitrogen groups, and (b) breaks bonds between the reactive nitrogen groups and the non-reactive groups.

[0048] In some embodiments, the nitrogen-containing compound is a nitrile.

[0049] In some embodiments, the nitrogen-containing compound is an aziridine.

[0050] In some embodiments, the nitrogen-containing compound comprises an aromatic ring.

[0051] In some embodiments, the nitrogen-containing compound comprises a nitrogen atom bonded to three carbon atoms.

[0052] In some embodiments, the nitrogen forms a lone pair.

[0053] In some embodiments, the nitrogen-containing compound contains three or four double carbon bonds.

[0054] In some embodiments, exposing the plurality of depassivated sites to a nitrogen-containing compound further comprises isotope control of the adsorbed nitrogen to control the spin of the adsorbed nitrogen.

[0055] In some embodiments, the nitrogen-containing compound is 13 C isotope and exposing multiple depassivated sites to a nitrogen-containing compound. 13 This involves doping the diamond substrate with a C isotope.

[0056] In some embodiments, the diamond substrate 13 The C isotope forms a qubit.

[0057] In some embodiments, during use, 13 Qubits formed by C isotopes perform quantum data operations, and nitrogen vacancies act as quantum buses.

[0058] In some embodiments, the method further comprises confirming the adsorption of the nitrogen-containing compound on the diamond substrate using scanning tunneling microscope (STM) imaging.

[0059] In some embodiments, verifying the adsorption of the nitrogen-containing compound further comprises verifying that the nitrogen-containing compound has a desired orientation relative to the diamond substrate.

[0060] In some embodiments, the desired orientation is one relative to the diamond substrate that provides four sp3 bonds across two adjacent surface dimers.

[0061] In some embodiments, the method further comprises, upon determining that the nitrogen-containing compound has an undesired orientation relative to the diamond substrate, using an STM to desorb the nitrogen-containing compound from the depassivated sites.

[0062] In some embodiments, the method further comprises preparing the diamond substrate to create atomically smooth patches on the diamond substrate.

[0063] In some embodiments, the method further comprises preparing the diamond substrate at a substrate surface misorientation angle relative to the nominal surface orientation to create atomically smooth patches.

[0064] In some embodiments, preparing the diamond substrate with a substrate surface misorientation angle includes creating step edges that define atomically smooth patches between adjacent step edges.

[0065] In some embodiments overgrowing diamond comprises growing the crystal lattice from a step edge.

[0066] In some embodiments, the substrate surface misorientation angle is between 0.1 and 3.4 degrees.

[0067] In some embodiments, overgrowing the depassivated sites with diamond is carried out by chemical vapor deposition.

[0068] In some embodiments, converting the incorporated nitrogen to nitrogen vacancies comprises carbon ion irradiation and annealing.

[0069] It should be noted that an optional feature provided with respect to one of the above methods is equally an optional feature of the other methods. [Brief description of the drawings]

[0070] [Figure 1a]1 illustrates multiple optically addressable qubits in diamond. [Figure 1b] 1 illustrates a method for fabricating multiple optically addressable qubits in diamond. [Figure 2a] 1 illustrates the preparation of diamond samples and surfaces. [Figure 2b] Illustrates atomically precise diamond hydrogen desorption lithography using a scanning tunneling microscope (STM). [Figure 2c] 1 illustrates exposure of a depassivated site to a nitrogen-containing compound. [Figure 2d] 1 illustrates the inclusion of nitrogen prior to diamond growth. [Figure 2e] 1 illustrates the creation of nitrogen vacancies via ion implantation and annealing. [Diagram 3] FIG. 1 illustrates a schematic of diamond preparation for atomic scale fabrication techniques. [Figure 4a] Illustrates the {100}H terminated diamond surface after HDL. [Figure 4b] Illustrates the {111}H terminated diamond surface after HDL. [Diagram 5] FIG. 1 illustrates a schematic diagram of STM-based desorption. [Figure 6] Illustrates the expected adsorption configuration of acetonitrile on a smoothly terminated {100} diamond surface. [Figure 7] Illustrates step-flow growth during chemical vapor deposition (CVD), nucleated through C-C dimers adsorbed on a fresh surface layer. [Figure 8a] One possible adsorbate configuration of aziridine on exposed {111} faces is illustrated. [Figure 8b] FIG. 8a illustrates further bonding of the adsorbed species of FIG. 8a with CH2 groups during CVD. [Figure 8c] Illustrates nuclei for new layer growth during {111} CVD overgrowth. [Figure 9]FIG. 1 illustrates an XPS scan of a partially hydrogen-terminated diamond surface after exposure to 1000 Langmuir of acetonitrile at room temperature and subsequent heat treatment. [Figure 10] 13 shows simulated constant current (approximately 5 nA) STM images of acetonitrile adsorbates in the A1, A2, and B1 configurations. [Figure 11] Illustrated are examples of nitrogen-containing aromatic compounds for industrial scaling of the methods disclosed herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0071] The present disclosure provides a method for atomic-scale fabrication of nitrogen-vacancy (NV) centers in diamond. "Atomic scale" in this context means that the location of the NV center is determined on the length scale of atomic spacing. This is important because diamond quantum computers use interactions between NV centers to enable operations on qubits, and this interaction depends on the distance between the qubits. The greater the interaction (the closer the NV centers are), the higher the speed and fidelity of the qubit operations. However, the NV centers should not be too close to each other, as this would hinder their respective stability and distinguishability. In some instances, even changes in the distance of a few atomic lattice sites in a diamond crystal cause significant degradation of quantum operations. Thus, the advantage of the disclosed method is that atomic precision allows for the fabrication of multi-qubit quantum computers with reliable interactions between qubits.

[0072] Specifically, the present disclosure provides techniques for atomic-scale fabrication of NV centers that allow for the creation of NV centers with atomic precision, which is important for their function in quantum computing. As discussed above, atomic-scale positioning of NV centers is difficult to achieve with "top-down" approaches, such as by ion implantation into diamond substrates. More specifically, the energy required for ions to penetrate diamond is very high, with a standard deviation of ion location of about 5 nm. This is the lower limit of positioning precision and is still too large for the fabrication of a functional multi-qubit quantum computer.

[0073] Thus, the present disclosure provides a "bottom-up" approach in which nitrogen is bonded to the diamond surface with atomic-scale precision, and then diamond is grown on the nitrogen. The binding sites are defined by atomic-scale hydrogen depassivation lithography (HDL) as disclosed herein. HDL can be performed using scanning tunneling microscopy (STM) or other techniques. Furthermore, the disclosed atomic-scale fabrication process uses multiple processes to suppress the desorption and diffusion of nitrogen defects, allowing the achievement of atomic-scale precision alignment of NV centers.

[0074] In one example, the disclosed fabrication technique uses a scanning tunneling microscope (STM) to find suitable sites on the diamond surface suitable for introducing nitrogen defects, depassivates some hydrogen terminated sites with atomic precision based on the intended device requirements, and verifies that the depassivated sites have been created. It is noted that hydrogen passivation is used as an example herein, but other examples may use surfaces passivated by other molecules or elements, such as fluorine. Furthermore, techniques other than STM can be used to desorb the passivating hydrogen, thus performing HDL, such as using electrons or x-rays to desorb the hydrogen.

[0075] The diamond surface is then exposed to a nitrogen-containing compound, and the nitrogen in the nitrogen-containing compound adsorbs to the depassivated sites. If the nitrogen-containing compound is in the desired orientation relative to the diamond surface, the covalent bond between the nitrogen and carbon on the diamond surface can be strong enough to withstand the variable conditions of diamond overgrowth and prevent nitrogen migration and desorption. STM imaging can confirm the orientation of the nitrogen-containing compound adsorbed on the diamond surface. If the nitrogen-containing compound is misoriented, it can be removed using thermal desorption or other methods. After the nitrogen is introduced into the diamond sample, chemical vapor deposition (CVD) is performed at a sufficiently low temperature and pressure. This allows diamond to grow on the nitrogen without it diffusing into the diamond sample or desorbing from the surface. Other examples use molecular beam epitaxy (MBE) or atomic layer deposition (ALD) instead of or in addition to CVD.

[0076] In addition, the fabrication process may include forming a protective layer around the adsorbed nitrogen, which further prevents the nitrogen from mitigating and desorbing in the diamond. Furthermore, atomic layer deposition (ALD) can deposit high quality, homogeneous material with precise control of layer thickness, which may provide a trigger to initiate overgrowth by deposition of a few monoatomic carbon layers. Initiation of ALD growth on depassivated diamond can be a challenge due to the lack of out-of-plane active bonds, but ALD growth can occur on defect sites or grain boundaries where dangling bonds or functional groups exist. ALD can be performed on diamond in any of the following embodiments: 1) use of a seed layer such as a self-assembled monolayer, 2) creation of functional groups on the diamond surface, for example, by fluorine, ozone, and plasma treatment, and 3) conditioning the underlying substrate to enhance nucleation. Finally, overgrowth of diamond can also be achieved by molecular beam epitaxy using mass-selected carbon ions at low gas pressure under high vacuum, for example, by evaporation of carbon-60 from an effusion cell, using a molecular beam epitaxy reactor.

[0077] FIG. 1a illustrates a plurality of optically addressable qubits in diamond 100, also referred to as an array of qubits. A plurality of optically addressable qubits 101 are created by arranging nitrogen on atomically flat surfaces 102 (called "terraces") on the surface of a diamond substrate 103, and are defined by step edges 104 that are the boundaries between adjacent terraces. The step edges 104 are typically just one atom high, as they naturally exist due to the miscut angle. The nitrogen is incorporated into the diamond through diamond overgrowths 105, which are created using chemical vapor deposition (CVD) to form the optically addressable qubits 101. Note that the step edges 104 are shown for illustrative purposes only, and that after the overgrowths, the step edges 104 are no longer present, as they are incorporated into the diamond crystal. The thick lines (dashed and solid) show the final device (including the qubits 101). The thin lines show the terraces and step edges 104 that are not present in the final device.

[0078] This also means that the optically addressable qubits 101 will not themselves be on different terraces. This is because once the nitrogen has been converted into a qubit, the terraces no longer exist and it is incorporated into the diamond. The net result is that qubits originating from nitrogen on different terraces will have slightly different heights after overgrowth. As the steps are only one atom high, such different heights should not degrade the functionality of the final device.

[0079] Furthermore, the qubits 101 interact with each other to perform quantum operations, thereby forming a quantum computer. In this regard, it is noted that depending on the location of the qubits 101, qubits on different terraces interact with each other as well as qubits located on the same terrace. The qubits 101 are optically addressable because they are embedded in diamond, which is transparent to light. This is in contrast to other architectures such as silicon, which is not transparent to light. In diamond, the qubits 101 can be individually addressed by a laser in combination with a magnetic field gradient and a frequency-selective microwave pulse. There may also therefore be a microwave source, magnets, and photodetectors (not shown) for controlling and reading out the quantum information.

[0080] FIG. 1b illustrates a method 150 for atomic-scale fabrication of nitrogen vacancy (NV) centers in diamond. The fabrication process includes providing a diamond substrate with a passivated surface (151) and removing hydrogen atoms from the passivated surface to create depassivated sites where the hydrogen atoms have been removed (152). The next step is to expose the depassivated sites to a nitrogen-containing compound to adsorb nitrogen at the depassivated sites of the diamond substrate (153) and overgrow the depassivated sites with diamond by CVD (154). After overgrowth of diamond using CVD, the fabrication process includes converting the incorporated nitrogen to nitrogen vacancies (155) (either by an explicit processing step or by overgrowing diamond as outlined below) and charging the nitrogen vacancies with a negative charge (156). The incorporated nitrogen may be converted to NV centers during CVD overgrowth, in which case steps 154 and 155 are performed in the same fabrication step. The fabrication process can be described in five successive steps, which are explained in more detail with reference to Figures 2a to 2e.

[0081] Figure 2a illustrates the preparation of a diamond sample and surface, corresponding to step 151 of method 150. At this stage, a diamond sample 201 is prepared that includes a diamond surface, making it compatible with STM and associated hydrogen desorption lithography (HDL). The process uses sufficiently large, atomically smooth, well-ordered patches of atoms (known as terraces) with low surface roughness, e.g. 10 nm x 10 nm, as suitable sites for N defects. In some examples, the minimum size of a patch is three dimers wide, i.e., 1.5 nm. The carbon atoms on the surface of the terraces are hydrogen terminated 202, with a low density of unterminated sites across the diamond surface. The creation of such a surface uses an atomically smooth surface, which means that there is a relatively small misorientation angle 203 relative to the nominal surface orientation. Such an angle may be relative to a given low index crystal plane, such as {100} or {111}. In one example, such a misorientation angle applies only to a portion of the surface where the NV is made (local miscut), rather than the entire sample surface (global miscut), which may have a different average misorientation angle. The diamond sample 201 also includes doped regions 204 that provide mobile charge carriers to the diamond sample 201, which allows for the implementation of an STM.

[0082] FIG. 2b illustrates atomically precise diamond HDL using a scanning tunneling microscope, corresponding to step 152 of method 150. The tip 221 of the scanning tunneling microscope is used to desorb hydrogen 222 atoms from the diamond surface, leaving behind depassivated carbon sites 223. The STM is used in conjunction with the depassivation process to both identify adsorption sites and to verify successful molecular adsorption by using STM imaging modes. The depassivation process leaves behind unpaired (i.e., non-bonded) valence electrons that readily form bonds with highly reactive nitrogen atoms from nitrogen-containing molecules. Thus, the depassivated sites may also be referred to as "reactive sites."

[0083] After depassivation and before nitrogen exposure, the carbon atoms on the depassivated {111} faces are dangling sp 3 On the depassivated {100} faces, the orbitals are simply sp 2 or sp 3 and the two depassivation orbitals of the dimer may not be strictly dangling, since they form weak bonds. For completeness, note that in both surface orientations (and other orientations) the depassivation site is highly reactive.

[0084] FIG. 2c illustrates the exposure of the depassivated sites to a nitrogen-containing compound, corresponding to step 153 of method 150. After depassivation of the terminal hydrogen, the diamond substrate is exposed to a nitrogen-containing compound 241. The high reactivity of the depassivated carbon sites chemically adsorbs the nitrogen atoms of the nitrogen-containing compound 241 to the depassivated sites. Ideally, the nitrogen-containing compound adsorbed to the diamond surface 242 is strongly bound to withstand the variable conditions of subsequent CVD for diamond overgrowth. Undesirable adsorbate configurations can be desorbed using post-exposure heating of the diamond sample after exposure to the nitrogen-containing compound 241. The adsorption of the nitrogen-containing compound to the diamond surface 242 can be confirmed through STM imaging.

[0085] FIG. 2d illustrates the inclusion of nitrogen before diamond growth, corresponding to step 154 ​​of method 150. Inclusion of nitrogen 261 means that subsequent diamond overgrowth 262 can occur without causing nitrogen migration or desorption. There are two methods to inclusion of nitrogen: special CVD overgrowth and in situ growth of molecular beam epitaxy (MBE) diamond "capping" layers. Both methods introduce a protective layer with a thickness of one or more (1-100) atoms that protects the nitrogen during etching with reactive gas species (such as atomic hydrogen, or carbon-containing species) and thermal desorption. The layer does not have to be a complete "layer" (i.e., nitrogen can be located in holes in the "protective layer").

[0086] FIG. 2e illustrates the conversion of nitrogen to nitrogen vacancies via radiation damage and annealing, corresponding to step 155 of method 150. Ion implantation, electron beam, and pulsed laser are examples of radiation damage mechanisms. Nitrogen defects 281 in diamond structure 282 are converted to nitrogen vacancy (NV) centers 283 through carbon ion irradiation and annealing 284. There is a non-zero probability that adsorbed N is converted to NV centers 283 during CVD overgrowth instead of substitutional N defects. The conversion of nitrogen to NV centers can occur during growth (i.e., this can be a feature of {111} growth, which also leads to alignment results). Therefore, step 5 can occur simultaneously with step 4 if that growth method is applied. NV centers can be formed by the presence of nearby electron donors, which can lead to the formation of NV centers. - The charge state is converted to a negatively charged NV center (i.e., a negatively charged NV center), which can be enhanced by applying an external voltage near the NV center. In some examples, the device design may include electrodes to further stabilize the charge state.

[0087] The diamond surface can be provided with one of two surface orientations: {100} plane orientation and {111} plane orientation. The notations {100} and {111} are Miller indices that indicate the planes of the diamond unit cell in which the surface carbon atoms lie.

[0088] Here, the key process steps of each of the above stages 151-156 will be described with the relevant technical details and scientific rationale.

[0089] Step 1: Preparation of the diamond sample and surface The result of step 1 (item 151 of method 150) is the preparation of a diamond sample (including its surface) that is compatible with HDL and subsequent steps in the fabrication process.

[0090] Figure 3 illustrates a schematic diagram of the preparation of diamond for atomic scale fabrication techniques.

[0091] A single crystal diamond substrate is provided having an orientation of {100} or {111} planes. The orientation of {100} and {111} planes is compatible with step 3. Both planes allow for the adsorption of nitrogen-containing molecules. Although the examples herein relate to the surface preparation and chemistry of the {100} planes, the {111} planes may be more compatible with atomic-scale fabrication processes due to their favorable reaction chemistry (see step 3 as shown in FIG. 2c), stability and preferential alignment of NV defect axes during overgrowth (see step 4 as shown in FIG. 2d).

[0092] Atomically flat terraces of 10 nm x 10 nm (at least 1.5 nm wide) and low surface roughness can be achieved by growth or etching. The atomic scale fabrication process is performed by atomic manipulation of hydrogen-terminated diamond surfaces, i.e., fabrication has location accuracy on the scale of individual atoms. The surface exhibits sufficiently large, atomically smooth, well-ordered patches (known as terraces). Creation of such surfaces uses controlled low substrate surface misorientation angles relative to the nominal surface orientation (also referred to in the scientific literature and industry as the "surface miscut angle") and careful ex-situ preparation. Performing HDL in a reliable manner uses atomically flat terraces on the scale of 10 nm x 10 nm, corresponding to a maximum surface miscut of, for example, 0.5 degrees. There can also be a minimum miscut angle of 0.1 degrees, for example, to provide sufficient step edges for crystal growth. The miscut angle can also depend on the surface orientation (i.e., {100} or {111}). In some examples, the miscut angle is between 0.1 degrees and 3.4 degrees.

[0093] The miscut direction also influences the shape and structure of the terraces. Surface defects make accurate STM imaging and associated HDL difficult, so a low roughness (i.e., each terrace is flat and has few defects) is used. Preparation of the diamond surface at a surface misorientation angle also produces step edges that act as boundaries between adjacent terraces. These step edges are typically just one atom high. These step edges also provide control of the overgrowth of the CVD diamond, as diamond growth emanates from the step edges.

[0094] It should be noted that the methods disclosed herein can be used to fabricate multiple qubits. That is, the HDL removes hydrogen atoms at multiple sites, and each of these multiple sites is later used to create one qubit. The interaction between these qubits depends on the distance between the depassivated sites and can be designed to suit a particular quantum computing application. Thus, there can be more than one depassivated site for nitrogen atoms per terrace, provided that the sites are appropriately spaced so that the qubits formed later also interact with the qubits formed by the nitrogen on different terraces. For example, there can be 2×2 qubits on a 10 nm×10 nm terrace, spaced 5 nm from the nitrogen, or there can be 10×10 qubits on a 100 nm×100 nm terrace, spaced 5 nm from the nitrogen. In one example, the distance between the qubits is 5 nm, with a tolerance of + / - 1 nm.

[0095] The disclosed method provides for the creation of a diamond surface that is ideally fully hydrogen terminated, but can tolerate a low density of uncontrolled / random / parasitic unterminated sites on the diamond surface. The uncontrolled / random / parasitic unterminated sites on the surface act as false adsorption sites for nitrogen-containing molecules. Hydrogen termination is known to provide an effective resist for lithography. The process of hydrogen termination using microwave plasma is part of a multi-step process that results in a surface with a suitable terraced morphology for subsequent steps. The remaining steps of this multi-step process are polishing, etching of the surface, and / or growth on the surface.

[0096] The disclosed method uses high purity diamond with less than 1 ppb N content and less than 0.3% 13C isotope. The coherence time of NV centers corresponds to diamond purity, as does the quantum computing performance. Nitrogen defects can be inadvertently converted to NV centers during annealing and generate background signals in fabricated quantum processors that degrade qubit initialization and readout performance.

[0097] The method uses a doped region to activate defects with a concentration of 10 16 ~10 20 cm -3 This produces n or p doping, where . Activated defects are defined as acting as either donors or acceptors. Two possible dopants are substitutional boron (p-type) or substitutional phosphorus (n-type). This doped region introduces delocalized charge carriers into the diamond substrate. These delocalized charge carriers provide an electrical current through the diamond, thus enabling the implementation of an STM.

[0098] The doped regions are far enough away (on the order of micrometers, e.g., 1-100 μm) from the fabrication point to prevent NV decoherence effects in the final device, which limit the performance of quantum computing. This is limited by the sharpness of the interface between the doped and intrinsic regions. As shown in Figure 3, STM operation (in conventional imaging mode) uses a depth (D) of the doped region that is larger than the distance (d) between the fabrication point and the doped region. For STM operation (in hot electron injection imaging mode, see step 2 as illustrated in Figure 2b), d can be up to about 100 μm.

[0099] The method creates a low resistance contact, for example an ohmic contact using palladium, on the diamond surface above the doped region. The contact provides a source or sink of charge carriers during scanning tunneling microscopy.

[0100] The procedure of this step is as follows. 1. Obtain an undoped, high purity single crystal diamond sample with {100} or {111} plane orientation from a supplier. 2. Surface preparation using a combination of mechanical polishing, chemical mechanical polishing (CMP), reactive ion etching (RIE) and wet chemical cleaning (e.g., acid clean) to achieve threshold root mean square (RMS) and miscut angles and minimize subsurface damage. 3. Post-polishing cleaning step. 4. Patterned dopant implantation through a mask and subsequent annealing results in n-type or p-type doping (10 16 ~10 20 cm -3 This can also be accomplished by lithography and CVD. 5. Acid cleaning and other cleaning. 6. Use hydrogen plasma or add carbon to the plasma to create atomically flat terraces in multiple plasma recipe steps to ensure high quality surface. Other plasma additives such as argon / oxygen / nitrogen / fluorine may also be used. 7. Hydrogen plasma treatment to achieve near uniform hydrogen surface coverage. 8. Fabrication of low resistance contact pads on the diamond surface above the doped regions using electron beam evaporation in vacuum.

[0101] There is a close relationship between the miscut of the diamond surface and the growth morphology of CVD diamond. Therefore, there can be a trade-off between the geometrically limited maximum terrace size and the CVD growth morphology. The miscut angle is optimized to ensure that the adsorbed species do not undergo desorption during CVD growth.

[0102] In some experiments, the exemplary preparation protocol produces a typical average terrace width of 3 nm, with isolated individual terraces of 10 nm. Further optimization of the surface treatment technique is used to produce terraces with an average size of 10 nm x 10 nm.

[0103] Preparation of the {111} diamond surface may also involve planarization of the surface via a thermo-chemical reaction process. This etching process can be used to produce {111} diamond surfaces exhibiting roughness of 0.3 nm over a length scale of 10-15 mm. The process involves: Acid cleaning of the {111} diamond substrate. Deposition of 500 nm thick transition metal (e.g. Ni, or NiCr) films on {111} diamond substrates via electron beam evaporation. Annealing the substrate at high temperature (900°C) in a quartz tube furnace in a water-rich environment at atmospheric pressure by bubbling nitrogen gas through water and flowing it through the quartz tube at 10 sccm. - Acid clean the board to remove any residual material. Repeat the process until the desired surface is achieved.

[0104] Step 2: Hydrogen desorption lithography Hydrogen desorption lithography (HDL) with an STM tip, achieved through a series of voltage pulses, is the controlled, deterministic passivation of single atomic sites (i.e., with atomic-scale precision) on a hydrogen-terminated diamond surface. The depassivated carbon sites on the surface are desorbed by unbonded sp 3 configuration (also called a "dangling" bond). The unpaired electron is a highly reactive site for molecular adsorption and readily forms bonds with incident N-containing molecules (see step 3 as illustrated in Figure 2c). In contrast, the hydrogen-depassivated diamond surface is largely unreactive due to the strong C-H bond energy.

[0105] The proposed specifications for fabricating a single NV centre are as follows: for {100} diamond, a patch of six adjacent hydrogen atoms is removed across three adjacent dimers, as determined based on theoretical calculations with potential candidate nitrogen-containing molecules; for {111} diamond, a patch of three adjacent hydrogen atoms arranged in a triangle is removed.

[0106] Figure 4a illustrates a diamond surface that is {100}H terminated after HDL, and Figure 4b illustrates a diamond surface that is {111}H terminated after HDL. The resulting smoothly terminated patches are thermally stable and serve as adsorption sites for nitrogen-containing molecules.

[0107] The above specifications describe the minimum amount of lithography required to fabricate a single NV center with lattice site precision. Even if the depassivated area is larger, nitrogen-containing molecules can adsorb on the surface with almost the same chemical properties, as presented in step 3.

[0108] STM imaging identifies the adsorption sites and verifies successful molecular adsorption. Conventional STM imaging of diamond uses boron doping to achieve p-type conductivity throughout the entire sample. However, the presence of dopants near the fabrication sites is undesirable as it would lead to decoherence effects in the final device. Therefore, STM imaging of the fabrication sites is performed in essentially insulating (i.e., locally undoped) diamond. There are two methods to achieve this: (1) resonant electron injection, and (2) conventional imaging using nearby doped contact regions.

[0109] Resonant electron injection is achieved by injecting electrons from the STM tip into the diamond surface through a standing wave resonance established in the vacuum gap between the tip and the sample. The energy of the resonant state varies with the surface potential at the site of the tip. Thus, when the tip is biased at a fixed voltage, the injection (tunneling) current is modulated by the local change in surface potential as the tip is scanned across the surface. This allows imaging of the surface to identify suitable sites for performing lithography, and imaging of sites after lithography to verify hydrogen desorption.

[0110] Conventional STM imaging may also be achievable on insulating surfaces by drawing charge carriers from a highly n- or p-doped region beneath an ohmic contact located near the fabrication point. Because the doped region is near the fabrication point, the charge carriers can be drawn in for use in STM imaging. Thermal ionization of the dopants creates delocalized carriers, which localize beneath the STM tip when the tip is appropriately biased. The presence of carriers enables conventional STM imaging on insulating surfaces by creating a significant voltage drop between the STM tip and the surface and providing an unoccupied / occupied density of states. If n-type doping is used, the same dopant region may be used to stabilize the NV charge states during device operation. In one example, conventional imaging is performed in the region D>>d, as shown in FIG. 3.

[0111] The procedure of this step is as follows. 1. Introduce the hydrogen-terminated diamond into an ultra-high vacuum environment containing an STM and appropriate sample preparation tools. 2. An in-situ annealing process, such as 450°C annealing for 1 hour, is used to remove atmospheric contamination and adventitious carbon. 3. Using STM imaging mode (resonant injection or conventional), identify terraces (10 nm x 10 nm) suitable for lithography. 4. Position the STM tip over the site where hydrogen desorption is desired and apply a current / voltage pulse until the hydrogen is removed. Repeat for all hydrogen atoms at the desired depassivated site. 5. Hydrogen desorption, indicated by the presence of local dangling bond features, is verified using STM imaging mode.

[0112] Figure 5 illustrates a schematic of STM-based desorption. STM-based HDL on H-terminated diamond surfaces uses four parameters: tip radius, tunneling current (which depends on bias voltage and tip height), and tunneling duration, as well as precise control of tip location. In particular, sharper tips can be used to obtain higher positional accuracy and precision, such as better than 1-2 atomic sites. The tip radius can be 1 nm or less.

[0113] Lithography in boron-doped diamond is 2×10 -11 1×10 Torr etc. -9 Hydrogen depassivation in diamond can be carried out with current pulses ranging from 1 ms to 10 ms, voltages ranging from 2.7 V to 7 V, and currents ranging from 1 nA to 50 nA.

[0114] Step 3: Exposure to nitrogen-containing molecules Step 3 (item 153 of method 150) may follow immediately after step 2 in situ. The desired result is the chemisorption of a single N-containing molecule onto the depassivated sites created using HDL. Ideally, the adsorbed molecule has a strong covalent bond (binding energy of 1 eV or greater) with the surface, with the nitrogen portion of the molecule directly bonded to one or more of the depassivated sites. This is to survive the variable (high temperature / high particle flux) conditions of CVD during overgrowth (see step 4 as illustrated in FIG. 2d).

[0115] In some examples, nitrogen-containing compounds are used, including molecules synthesized with specific nitrogen isotopes, which provide a choice between spin-1 and spin-1 / 2 qubit systems, with different advantages and disadvantages for different roles. 13 C and / or 14 N or 15 There is also the possibility to seed a C qubit during step 3 by synthesizing the molecule using a N precursor. The deliberate inclusion of isotopes in the molecule allows the fabrication of additional qubits associated with each NV center. In one example, 13 By manipulating the positions of the C isotopes, their location on the diamond surface can be deterministically controlled through adsorption chemistry. Thus, after overgrowth, 13 The C qubit can be positioned relative to the NV centre, which is fabricated with atomic-scale precision. {100} side

[0116] Figure 6 illustrates the expected adsorption configuration of acetonitrile on a smoothly terminated {100} diamond surface. Different classes of molecules may be suitable for adsorption depending on the surface geometry. In the case of the {100} geometry, the surface may be exposed to a molecule containing a nitrile group (-CN). In one example, the molecule is hydrogen cyanide (HCN) or acetonitrile (C2H3N), but other molecules with nitrile groups should also work well, since the nitrile group is what determines the reaction with the diamond surface in the desired geometry. Therefore, it should be irrelevant what shape the rest of the molecule takes.

[0117] Quantum chemical calculations on two exemplary molecules predict that the nitrile group can adsorb to the depassivated surface in one of two orientations, parallel to the dimer rows (denoted A1) or perpendicular to them (denoted B1). In the case of the A1 adsorbate, it is energetically feasible and favorable for a second surface reaction to occur in which the acetonitrile molecule is repositioned to the center of the dimer, designated A2.

[0118] Similar bonding occurs in larger molecules that also have nitrile functional groups. This is because the nitrile groups are the active bonding regions of the molecule, and other components of the molecule may not participate in bonding if they are not chemically reactive. The active bonding regions can also be considered reactive nitrogen groups, since they are the components of the molecule that contain nitrogen and introduce it to the diamond substrate by reacting with "dangling" bonds at depassivated sites. The remaining components of the molecule form non-reactive groups that are bonded to the reactive nitrogen groups. In some instances, the number of single-bonded carbons bonded to nitrile groups in an alkane chain can be arbitrarily large, since they do not react with the diamond surface. Thus, most molecules with nitrile groups (and no additional functional groups) bond to {100} faces in the manner described above.

[0119] FIG. 7 illustrates step-flow growth during CVD that is nucleated through C-C dimers adsorbed on a new surface layer. Because of the geometric and electronic similarity to the known structure of diamond layer growth, the A2 adsorbate configuration is preferred for step 4. Step-flow growth during CVD is nucleated through C-C dimers adsorbed on a new surface layer. The A2 adsorbates form an adsorbed dimer, specifically, a C-C dimer with four sp across two adjacent surface dimers. 3 The A2 adsorbates have a geometric similarity to the bond and can therefore be efficiently incorporated into the bulk diamond structure. {111} side

[0120] Atomic-scale fabrication on {111} planes may be desirable because NV centers naturally align along a common spin axis during CVD growth, and center alignment is desirable for the function of quantum computing devices.

[0121] As with {100} diamond, it may be advantageous to use molecular adsorbates that have geometric and chemical similarities to the structures observed during CVD growth, primarily to allow for the incorporation of nitrogen into the subsequently grown encapsulation layer. Candidates for {111} diamond that fit these requirements include aziridine and indolizine.

[0122] FIG. 8a illustrates one possible adsorbate configuration of aziridine on an exposed {111} surface. FIG. 8b illustrates further bonding of the adsorbate of FIG. 8a with CH2 groups during CVD. FIG. 8c illustrates the initiation of new layer growth during {111} CVD overgrowth. The aziridine functional group consists of a nitrogen atom and two carbon atoms bonded together in a triangular shape. Due to the high distortion of such a molecular structure, aziridine is highly reactive. In the case of the simplest aziridine (also called "aziridine", (CH2)2NH), quantum chemical calculations demonstrate that the molecule can adsorb on the {111} surface in the manner shown in FIG. 8a. During CVD overgrowth, further bonding with CH2 groups can result in the structure shown in FIG. 8b, which shows strong geometric and chemical similarity to FIG. 8c and is believed to be the initiation of new layer growth during {111} CVD overgrowth.

[0123] Other potential candidates for adsorption on the {111} face include indolizines and molecules containing isocyanide functionality. Similar to nitriles, aziridine and molecules with only isocyanide functionality should bind to the {111} face in a similar manner to all other molecules with only that functionality. Other examples include the use of pyridine derivatives, which may be more practical to use.

[0124] The procedure of this step is as follows. 1. Expose the surface to nitrogen-containing molecules (either the candidates listed above or other molecules) under UHV conditions at a temperature that allows for molecular adsorption. 2. Undesirable adsorbates / adsorbate configurations are desorbed by heating the sample after exposure. 3. Use STM in imaging mode to confirm molecular adsorption. Non-STM (i.e. lithography performed using electron / x-ray or other forms of lithography) relies on the non-deterministic nature of chemical processes. 4. Repeat steps 1 to 3 until the molecule is well adsorbed.

[0125] Both the nitrile and aziridine functional groups in the corresponding compound groups act as reactive nitrogen groups in this process, which serve to introduce nitrogen into the diamond substrate by reacting with the "dangling" bonds at the depassivated sites.

[0126] A nitrile group consists of a molecule containing a nitrile functional group bonded to a non-reactive group, which consists of any molecular structure that does not contain additional reactive functional groups. The non-reactive group may include, but is not limited to, a carbon chain of any length having any one or more of an alkane, alkene, alkyne, or aromatic ring.

[0127] Similarly, an aziridine group consists of a molecule containing an aziridine functionality attached to a non-reactive group, which includes a three-membered heterocycle with an amine and two methylene bridges, but may also include any other molecular structure that is not an additional reactive functional group.

[0128] While the above examples relate to reactive groups attached to non-reactive groups which are then removed, other examples may use other molecules. For example, the method may involve attaching the tail of the molecule in a particular way (e.g., two nitrogen-attached aryl groups). 13 Slightly more complex molecules may be used, such as to engineer the reaction to break (leaving the C atom but removing everything else). This may include molecules with more heteroatoms. In other words, 13 There may be one or more extended functional groups that aid in C doping. Additionally, there may be molecules that are potentially more industrially scalable, i.e., the nitrile group may not be the only part of the molecule that remains bound to the surface. For example, 13 There may be an extending group to assist C doping.

[0129] In a sense, it can be said that there is a competitive reaction in which the nitrogen in the reactive nitrogen group bonds more favorably to diamond than other atoms in the nitrogen-containing compound, which can include all nitrogen-containing compounds that form substituted or unsubstituted nitrogen-containing aromatic groups.

[0130] As mentioned above, the nitrogen-containing compound may contain one, two or more aromatic rings. In the case of the double ring example, the nitrogen-containing compound may contain triple coordinated nitrogen, i.e., nitrogen is bonded to three carbon atoms from the carbon surface. This can provide adsorption stability. Furthermore, the nitrogen-containing compound may contain nitrogen that forms a chemically stable lone pair. Such a pair is useful for the formation of a nitrogen vacancy. More specifically, the lone pair prevents further bonding of N to additional elements, which strengthens the pairing of N with the vacancy, thus aiding in the formation of a nitrogen vacancy. Furthermore, the nitrogen-containing compound contains a nitrogen lone pair that is substantially aligned in a predictable direction achieved by the bond angle involved in the triple coordination.

[0131] Nitrogen-containing compounds may have three or four double carbon bonds in the molecule, providing low energy for adsorption to the diamond surface at high surface coverage. In some instances, double bonds adsorb efficiently on the surface as opposed to single bonds. The latter is used with bond breaking, a high energy process that may limit adsorption. Nitrogen-containing compounds may contain five or six carbon member rings, as naturally observed in diamond, facilitating the overgrowth process to incorporate nitrogen into the diamond surface. Nitrogen-containing compounds may be aromatic and may contain cyclic resonances that stabilize and maintain the lone pair electronic state of nitrogen. The lone pair electronic state of nitrogen facilitates the formation of NV pairs by preventing additional bonding of nitrogen to carbon atoms during the overgrowth process. The nitrogen in the nitrogen-containing compound may be part of a five member ring, providing the largest known resonance for the stabilization of the lone pair electronic state.

[0132] A candidate identified through modeling and theoretical exploration, validated by computational chemistry, is indolizine. Indolizine is a heterocyclic compound with the formula CHN, which is an isomer of indole with the nitrogen located at the ring fusion position, meaning that the nitrogen in indolizine forms a reactive nitrogen group. Indole is a heterocyclic aromatic compound consisting of a pyrrole ring (five-membered ring) linked to a benzene ring (six-membered ring). This compound is highly stable and is found in several natural products, which is an advantage for industrial use. This means that indolizine may support an industrially scalable process so that a large number of devices can be manufactured using existing manufacturing hardware. In some examples, indolizine is used to incorporate nitrogen into {111} diamond surfaces. However, in other examples, indolizine is used to incorporate nitrogen into {100} diamond surfaces.

[0133] Indolizines form a class of compounds and are derived from either: (1) In its pure form, it has two rings (pyrrole and benzene) (2) In pure form, with an additional attachment on the five-membered pyrrole ring (3) In its pure form, with an additional attachment on the six-membered benzene ring

[0134] The process involving (1) is identified as the most effective option. Pure indolizine is commercially available and provides a route to the process. In one example, the purity is at least 95%. However, where unidentified contaminants may be detrimental to the use of this compound, option (3) exposes a large class of compounds that are commercially available at very high purity.

[0135] In addition, the ligands attached to the six-membered ring may be groups found in CVD growth chambers to avoid uncontrolled secondary reactions of the overgrowth process and may contain hydrocarbons. In some examples, the ligands include: (1) Alkyl substituents (CH3, C2H5, C3H7, C4H9, ...) attached to the six-membered indolizine ring, with specific identified examples being 2-tert-butyl-indolizine and 5,8-dimethyl-indolizine. (2) Aryl-substituted (-CHX), 5-methyl-2-phenyl-indolizine

[0136] Halogen groups may also provide a viable route with -F, -Cl, -Br, -I groups such as: (1) 7-Bromo-indolizine (2) 7-Chloro-indolizine (3) 7-(Bromo-methyl)-indolizine (4) 7-(chloro-methyl)-indolizine

[0137] Figure 11 illustrates some examples of commercially available indolizine derivatives containing halogen, alkyl, and aryl substituents. For example, 5-methyl-2-phenylindolizine and 2-tert-butyl-indolizine are available from Molport (molport.com), and 7-bromo-indolizine is available from Biosynth (biosynth.com).

[0138] The reactive nitrogen groups described above, including nitrile, aziridine, and indolizine molecules, as well as others contemplated herein, are useful because they can be adsorbed to the surface of the diamond substrate in a desired orientation relative to the surface. Such a desired orientation is compatible with subsequent CVD overgrowth, such that the molecules are oriented to have similar geometric and chemical similarities to the diamond structure in the overgrowth. More specifically, in the example of a {100} surface, the desired orientation is such that four sps across two adjacent surface dimers. 3 These bonds establish strong adsorption of the nitrogen-containing compound to the diamond substrate, enabling it to withstand the variable conditions of subsequent diamond overgrowth.

[0139] The {111} face may not exhibit a dimer. Rather, it is a 1 × 1 reconstruction. The surface geometry is such that there is a regular honeycomb pattern, with hexagons formed between three adjacent surface carbon atoms and three adjacent surface carbon atoms. As a result, HDL patches on {111} faces are triangles formed from three surface atoms, rather than rectangles as in {100}, due to the different surface geometry. As shown in Figure 4b, the triangular patch should be fabricated over a single hexagonal surface carbon, instead of over three adjacent hexagons, to prevent crystal twinning in the subsequently overgrown layer. In the case of {111}, the adsorbed units form sp3-bonds with the depassivated surface carbon. The resulting structure resembles a new diamond subunit formed on a triangular patch.

[0140] Once the nitrogen from the reactive nitrogen group is adsorbed on the depassivated site, the unwanted or undesired non-reactive group can be removed by a process such as heating or exposure to a reactive gas after the exposure of the diamond sample. The bond between the nitrogen and the carbon atom in the depassivated site is strong enough to withstand the temperature of the heating after the exposure. This temperature is also high enough to break the bond between the reactive nitrogen group and the non-reactive group, causing the unwanted and undesired portion of the nitrogen-containing compound to be removed. This leaves only the nitrogen adsorbed on the diamond substrate at the depassivated site.

[0141] In the case of {100} diamond, a combination of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) experiments determined that hydrogen provides an effective resist for the adsorption of acetonitrile at room temperature (see step 3 as illustrated in Figure 2c).

[0142] Figure 9 illustrates the XPS scan of a smoothly terminated diamond surface after 1000 Langmuir exposure to acetonitrile at room temperature and subsequent heat treatment. Figure 9 also illustrates the Nitrogen 1s (N1s) core level XPS data after acetonitrile exposure. The experimental results further demonstrate the chemisorption of acetonitrile onto the partially depassivated HC{100} surface at room temperature and 1000 Langmuir exposure. Spectra are obtained both at room temperature and after annealing at higher temperatures.

[0143] These results demonstrate that the nitrogen atoms remain bonded to the surface up to 900°C. This indicates a chemical bond as strong as the C-H surface bonds on the {100} faces of diamond substrates, which undergo thermal desorption at about 850°C. Note that these temperatures are also roughly comparable to the sample temperatures during CVD growth. Adsorption of acetonitrile occurs on the surface with fully deactivated C-C dimers (produced via thermal desorption of hydrogen) and partially deactivated C-C dimers produced by x-ray exposure. Further angle-resolved NEXAFS measurements demonstrate the presence of two distinct peaks, suggesting that chemisorption, rather than physisorption, of acetonitrile has occurred. More specifically, the distinct angle-resolved peaks in the N-K edge NEXAFS imply that there is some sort of ordering of the nitrogen bonds (presumably to carbon in this case). This, in and of itself, may not imply that there is chemisorption. However, for a molecule like acetonitrile, no ordering is expected unless the nitrogen is directly bonded to the substrate. This is because the nitrogen would otherwise migrate and the angle dependence would be weaker (or absent).

[0144] The bonding configuration of the adsorbed species can be verified in situ using STM in imaging mode by comparison with simulated images. Different adsorption configurations produce distinct STM images, simulated using ab initio techniques. If less desirable configurations are identified, selective removal means can be used to remove the adsorbed molecules, including selective thermal desorption and resonant optical or electrical techniques (e.g., atomic precision lithography using STM). Nevertheless, the desired outcome of lithography is to reproduce smoothly depassivated patches. This allows step 3 to be repeated to obtain the desired adsorption shape.

[0145] Figure 10 illustrates simulated constant current (~5 nA) STM images of acetonitrile adsorbates in the A1, A2, and B1 configurations. These images demonstrate that it is possible to distinguish three different configurations based on the orientation of the adsorbates. The adsorbates appear to be either parallel (A1, A2) or perpendicular (B1) to the dimer rows. Calculations were performed using best standard techniques of ab initio and STM theory. Simulations were performed using density functional theory with the Vienna ab initio software package (VASP). The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, plane wave basis with an energy cutoff of 600 eV, is employed, optimizing the ionic structure down to a force cutoff of 0.05 eV / Å. The diamond surface is simulated using a periodic slab geometry with dimensions 5 × 4 × 6 (units of the bulk diamond fundamental unit cell). Chen's derivative rule is used to simulate STM images assuming a tip of the s-like feature.

[0146] Step 4: N inclusion through diamond overgrowths The purpose of step 4 (item 154 of method 150) is to encapsulate the adsorbed nitrogen atoms in the diamond to achieve bulk substitutional N defects or aligned NV centers. At least 50 nm of diamond overgrowth is used to obtain bulk-like NV characteristics, where the diamond is overgrown to encapsulate the surface nitrogen without causing either migration or desorption. The amount of overgrowth depends on the desired optical and electronic control element characteristics of the quantum computing device and is arranged at different distances to the NV centers and in specific morphologies depending on the design.

[0147] Currently, there are two proposed methods for encapsulating adsorbates in bulk diamond. The first employs CVD growth under special conditions to minimize the probability of desorption. The second employs a carbon source and controlled sample conditions under UHV to produce a molecular beam epitaxy (MBE) diamond grown encapsulation layer a few atoms thick. This then protects the adsorbates from desorption in the subsequent CVD overgrowth step. Both methods are considered here.

[0148] Special CVD overgrowth Special CVD overgrowth is divided into three distinct phases, a pre-growth surface preparation phase, a special growth phase, and a bulk growth phase. It is noted here that the gases used in CVD include a carbon source, such as methane, and hydrogen. Hydrogen is included because it selectively etches away surface hydrogen and non-diamond carbon. The gases are ionized into chemically active radicals in the growth chamber using microwave power, hot filaments, arc discharge, welding torch, laser, electron beam, or other means. To prevent desorption of adsorbed molecules, growth conditions are first used that slow down the rate of sample etching, for example by increasing the growth-promoting (i.e., carbon-containing) gas species, and protect N defects by encapsulating them in several atomic layers of diamond. More specifically, the process may use an unusually large number of highly volatile carbon species. This means that the method uses a ratio of reactive carbon / growth species to reactive hydrogen / etching species that allows diamond growth while avoiding nitrogen desorption. Avoiding desorption or diffusion in this context means that a suitable number of nitrogen atoms remain in the diamond crystal for quantum devices. This means that some nitrogen atoms may still desorb or diffuse and become unusable, while others remain in the diamond and provide usable quantum devices. It is also possible that the desired yield can be achieved by discarding some diamonds, or discarding some areas in one diamond where there are insufficient nitrogen atoms, i.e. areas where too many nitrogen atoms have desorbed or diffused away from their intended locations. Once hydrogen is desorbed from the diamond surface in intended areas and nitrogen-containing species are adsorbed in those areas, it is not desirable for the nitrogen-containing species to become mobile (desorption = moving away from the diamond surface into the vacuum (z) or diffusion = moving away from sites on the surface (x,y)).

[0149] After encapsulation, conventional CVD is used to grow more diamond such that the substitutional defects or NV centers are far enough away from the surface to stabilize their properties and form an integrated control structure for device operation.

[0150] surface preparation

[0151] A surface coating of carbonaceous / hydrocarbon species to protect the adsorbed species during early stage plasma ignition (and to provide a time-limited carbon-rich source to the plasma during early stage growth).

[0152] Special growth stages [Table 1]

[0153] The micropressure and very low sample temperature of the special growth stage result in the preservation of covalent bonds between carbon atoms at the depassivated sites and the incorporated nitrogen. More specifically, the low pressure can result in relatively small amounts of atomic removal (etching) species, such as atomic hydrogen, and the low sample temperature reduces the reactivity of surface atomic species with atomic hydrogen.

[0154] Bulk Overgrowth The method uses conventional plasma conditions to produce bulk CVD diamond growth selected to minimize NV-->NVH conversion due to hydrogen diffusion into the crystal.

[0155] In situ growth of MBE diamond capping layers This process aims to achieve the growth of a thin (<5 nm) MBE diamond capping layer under ultra-high vacuum (UHV) conditions, in a manner such that there is minimal surface etching, allowing for immediate encapsulation after step 3. This can be achieved by: 1. Producing hydrocarbons (typically light hydrocarbons such as methane) and hydrogen radicals in a UHV chamber using either pyrolysis or a microwave cavity. 2. Exposing the sample to a controlled flux of hydrocarbons and hydrogen radicals. 3. Surface and sp on the surface 3Controlling the sample temperature in such a manner as to permit reactions between the hydrocarbon radicals that promote carbon growth, with the hydrogen radicals acting to promote such reactions, the sample temperature may be in the range of 300°C to 650°C to prevent acetonitrile desorption, and may be below 1200°C to prevent graphitization. 4. After this growth, the sample can be transferred to another fabrication system (such as conventional CVD) for further processing.

[0156] As indicated above, chemical vapor deposition (CVD) is a growth technique for both {100} and {111} faces in which the diamond sample is exposed to a high-temperature plasma of atomic hydrogen and hydrocarbon radicals. In general, the stability of the adsorbed species increases with the number of chemical bonds to the surface. The A2 structure has the greatest possible number of stable bonds to the surface and has a geometry similar to the growth of new layers during CVD (see Figures 6 and 7). It is therefore considered the most viable adsorbed species for atomic-scale fabrication on the {100} face. Similarly, for {111} diamond, molecular adsorbed species resemble new surface growth on the {111} face (see Figures 6a-c).

[0157] There are two main desorption mechanisms during CVD: thermal desorption and β-scission. In general, the probability of thermal desorption increases with temperature. Therefore, low-temperature CVD growth may be a viable option to mitigate thermal desorption. β-scission occurs when it is energetically favorable for the transfer of a surface electron to the adsorbed species, which then drives desorption.

[0158] Both the sample temperature and pressure are low enough during CVD growth to control the growth rate of diamond and ensure that nitrogen defects do not migrate or desorb from the diamond. Temperature affects diamond growth rate because increasing the temperature increases the thermal energy of the system, thereby increasing the kinetic energy of radicals in the CVD plasma. If the kinetic energy of the radicals is greater than the bond energy of the covalent bond between nitrogen and carbon on the diamond surface, the covalent bond may break and nitrogen desorbs from the surface. Pressure affects diamond growth rate because increasing the pressure changes the number of radicals created and therefore the probability that carbon radicals are close enough to bond and form the diamond structure. Low pressure can reduce the probability of radicals colliding with nitrogen bound to the diamond surface, preventing the transfer of kinetic energy to keep the covalent bond.

[0159] Growth on {100} and {111} faces is typically achieved through a step-flow mode characterized by a rate of layer growth exceeding the rate of nucleation. For both {100} and {111} faces, the presence of nitrogen enhances the rate of step-flow growth through nucleating new growth fronts. This is believed to be caused by the stability of surface-bound nitrogen against β-cleavage on the {111} faces. CVD growth on {111} faces is also known to result in preferentially aligned NV centers, with recent studies demonstrating near perfect alignment (99%). Note that "preferentially aligned NV" are defined such that their defect axes are collinear, with the defect axis defined as the direction between the nitrogen and vacancy sites.

[0160] UHV-based thin film diamond growth methods can be applied to materials such as silicon by exposure of the sample to a controlled flux of hydrocarbon and hydrogen radicals. This approach relies on an increase in the chemical reactivity of radical species compared to their "standard" counterparts to allow for alternative reaction pathways to those observed in CVD reactor conditions or atmospheric conditions, thereby enabling the preferred growth of diamond via MBE processes. The low pressure of the UHV environment means that such radical species can be present for a sufficient time to be transported to the sample surface, but at higher pressures these reactive species will not be present for a sufficient time to do so.

[0161] Various designs for UHV compatible radical sources, including methyl (methane) and hydrogen radicals, are possible and can either be purchased commercially or adapted from commercially available equipment. For example, heated gas flow tubes can be used, allowing control of gas flow rate and temperature. Commercial examples include the Focus EFM-H and CreaTec HLC. Such sources are pieces of add-on equipment that can be integrated into UHV instruments. There are three related methods for radical generation: 1. Pyrolysis of gas molecules - i.e., flowing the gas over a heating element that is at the right temperature to "break" the molecules into reactive pieces. 2. Electron ionization of gas molecules. 3. Microwave excitation of molecules.

[0162] Pyrolyzers, the most common radical source for UHV experiments, typically create radicals through processes 1 and 2 because most heating elements, under UHV, generate electron flux due to thermionic emission. Microwave cavity-based approaches to radical generation create radicals via microwave excitation, similar to CVD systems.

[0163] Control of the radical flux reaching the sample is achieved by: · Controlled gas flow through the radical source, typically achieved with mass flow controllers on each gas line. · Controlled temperature of the heating element to change the rate of "decomposition" and affect the rate of ionization of electronic molecules (pyrolyzer). Employing a "suppressor" electrode element on the radical source that can remove some of the radicals before they reach the sample.

[0164] Step 5: NV creation via ion implantation and annealing Post-growth conversion of bulk substitutional nitrogen defects into aligned NV centers (Method 150, item 155).

[0165] The procedure of this step is as follows. 1. Localized clusters of vacancies can be generated near bulk nitrogen defects through carbon ion bombardment. The energy of the ion bombardment depends on the depth of the nitrogen. For example, at 50 nm, the energy is about 50 keV. 2. Annealing at temperatures above 600°C, such as 1400°C, induces the migration of vacancies. When a mobile vacancy and a static substitutional nitrogen defect come into contact, an NV center is subsequently formed.

[0166] There is a non-zero probability that adsorbed nitrogen is converted to NV centers during CVD overgrowth instead of substitutional nitrogen defects. This means that in method 150, steps 154 and 155 are performed in the same fabrication step. Optical microscopy can be used to confirm the presence of NV centers instead of substitutional nitrogen defects after overgrowth before the generation of vacancies. In this case, step 5 may not be performed.

[0167] The NV center, through a charging process, -This charging process can occur via an n-type donor region. The donor region can be introduced in step 1 during the preparation of the diamond sample, or an additional n-type donor region can be introduced in step 5 via ion implantation or doped diamond growth. The n-type dopant in the diamond allows an external voltage to be applied to the NV centre, which changes the position of the Fermi level and charges the NV centre.

[0168] It will be understood by those skilled in the art that many variations and / or modifications may be made to the above-described embodiments without departing from the broad general scope of the present disclosure, and the present embodiments are therefore to be considered in all respects as illustrative and not restrictive.

Claims

1. 1. A method for fabricating a plurality of optically addressable qubits in diamond, said method comprising: providing a diamond substrate with a passivated surface; removing passivating atoms from the passivated surface to create a plurality of depassivated sites from which the passivating atoms have been removed; exposing the plurality of depassivated sites of the diamond substrate to a nitrogen-containing compound to adsorb nitrogen at the plurality of depassivated sites; overgrowing the plurality of depassivated sites with diamond by chemical vapor deposition (CVD) at a diamond growth rate related to temperature and pressure, wherein diffusion or desorption of the nitrogen in the plurality of depassivated sites is avoided in order to incorporate the nitrogen into the diamond; converting the incorporated nitrogen into a plurality of nitrogen vacancies; and charging the plurality of nitrogen vacancies with a negative charge.

2. 2. The method of claim 1, wherein any one or more of the diamond growth rate, the temperature, and the pressure are sufficiently low to avoid diffusion or desorption of the nitrogen at the plurality of depassivated sites.

3. 2. The method of claim 1, wherein the nitrogen at the plurality of depassivated sites is bonded to the diamond substrate by a covalent bond between the nitrogen and a carbon atom of the diamond substrate, the covalent bond being defined by a bond energy.

4. Any one or more of the diamond growth rate, the temperature, and the pressure are sufficiently low to maintain the covalent bond, or the relative rate of sample etching, as controlled by sample temperature and reactive species, is significantly lower than the growth rate so that the nitrogen at the plurality of depassivated sites does not desorb and diffuse before or during diamond overgrowth, or the covalent bond is sp ３ The method of claim 3, wherein the binding is

5. 10. The method of claim 1, further comprising: encapsulating the nitrogen in the plurality of depassivated sites with a protective layer; and forming the protective layer by special chemical vapor deposition overgrowth and by molecular beam epitaxy according to one or more options in Table 1.

6. preparing the diamond substrate to create atomically smooth patches on the diamond substrate, or preparing the diamond substrate at a substrate surface misorientation angle relative to a nominal surface orientation to create the atomically smooth patches; preparing the diamond substrate at the substrate surface misorientation angle includes creating step edges that define the atomically smooth patches between adjacent step edges; overgrowing the diamond includes growing a crystal lattice from the step edges; and the substrate surface misorientation angle is 0.1 to 3.4 degrees; and The method of claim 1 , wherein converting the incorporated nitrogen to nitrogen vacancies comprises carbon ion irradiation and annealing.

7. 1. A method for fabricating a plurality of optically addressable qubits in diamond, said method comprising: providing a passivated surface on a diamond substrate, the diamond substrate including doped regions for introducing delocalized charge carriers into the diamond substrate and / or for providing a base for injected carriers; removing passivating atoms from the passivated surface to create a plurality of depassivated sites where passivating atoms have been removed, wherein removing the passivating atoms comprises moving a tip of a scanning tunneling microscope (STM) across the passivated surface with atomic precision and creating a pulsed voltage drop between the tip and the diamond surface to remove the passivating atoms; exposing the plurality of depassivated sites of the diamond substrate to a nitrogen-containing compound to adsorb nitrogen at the plurality of depassivated sites; overgrowing the plurality of depassivated sites with diamond; and converting the incorporated nitrogen into a plurality of nitrogen vacancies; and charging the nitrogen vacancies with a negative charge.

8. further comprising preparing the diamond substrate to create atomically smooth patches on the diamond substrate; The method of claim 7 , wherein moving the tip of the STM further comprises imaging the passivated surface to locate the atomically smooth patch.

9. 8. The method of claim 7, further comprising using STM imaging to confirm the removal of the passivating atoms from the passivated surface after moving the tip of the STM and before exposing the depassivated site to the nitrogen-containing compound.

10. further comprising confirming the adsorption of the nitrogen-containing compound on the diamond substrate using STM imaging; confirming the adsorption of the nitrogen-containing compound further comprises confirming that the nitrogen-containing compound adsorbed on the diamond substrate has a desired orientation with respect to the diamond substrate; and (a) the diamond substrate has a {100} face and the desired orientation is four sp across two adjacent surface dimers; ３ an orientation relative to the diamond substrate that provides bonding; or (b) the diamond substrate has a {111} surface and the desired orientation is three sp ３ The method of claim 7 , wherein the orientation relative to the diamond substrate provides bonding.

11. Upon determining that the nitrogen-containing compound has an undesired orientation relative to the diamond substrate, the method further comprises using the STM to desorb the nitrogen-containing compound from the depassivated sites; Removing the passivating atoms by the STM is －１１ Torr ~ 1 x 10 －９ Torr pressure; and removing the passivating atoms with the STM further comprises a current pulse in the range of 1 ms to 10 ms at a voltage in the range of 2.7 V to 7 V and a current in the range of 1 nA to 50 nA; The method of claim 10 further comprising:

12. 1. A method for fabricating a plurality of optically addressable qubits in diamond, said method comprising: providing a diamond substrate with a passivated surface; removing passivating atoms from the passivated surface to create a plurality of depassivated sites from which the passivating atoms have been removed; exposing the plurality of depassivated sites of the diamond substrate to a nitrogen-containing compound containing reactive nitrogen groups to adsorb nitrogen from the reactive nitrogen groups at the plurality of depassivated sites of the diamond substrate; overgrowing the plurality of depassivated sites with diamond; and converting the incorporated nitrogen into a plurality of nitrogen vacancies; and charging the nitrogen vacancies with a negative charge.

13. 13. The method of claim 12, wherein the nitrogen of the reactive nitrogen group forms a bond with a carbon atom of the diamond substrate at the depassivated site.

14. the reactive nitrogen group is a functional group and is bonded to a non-reactive group; and The method of claim 12 , wherein the non-reactive group comprises a hydrocarbon.

15. 15. The method of claim 14, wherein adsorbing the nitrogen-containing compound at the depassivated sites further comprises removing the non-reactive groups by heating after exposure.

16. Heating after exposure (a) maintaining the bond between the carbon atom of the diamond substrate at the depassivated site and the nitrogen of the reactive nitrogen group; and 16. The method of claim 15, wherein (b) is carried out at a temperature that breaks the bond between the reactive nitrogen group and the non-reactive group.

17. 13. The method of claim 12, wherein the nitrogen-containing compound is a nitrile, an aziridine, contains an aromatic ring, contains a nitrogen atom bonded to three carbon atoms, contains three or four double carbon bonds, or the nitrogen forms a lone pair of electrons.

18. exposing the plurality of depassivated sites to the nitrogen-containing compound further comprises isotope control of the adsorbed nitrogen to control the spin of the adsorbed nitrogen; the nitrogen-containing compound comprises a C isotope, and exposing the plurality of depassivated sites to the nitrogen-containing compound comprises doping the diamond substrate with the C isotope; 13. The method of claim 12, wherein the 13C isotope in the diamond substrate forms a qubit.

19. 20. The method of claim 18, wherein the nitrogen-containing compound comprises a C isotope, exposing the plurality of depassivated sites to the nitrogen-containing compound comprises doping the diamond substrate with the C isotope, and wherein, in use, the qubits formed by the C isotope perform quantum data operations and the nitrogen vacancies act as a quantum bus.

20. further comprising confirming the adsorption of the nitrogen-containing compound to the diamond substrate using scanning tunneling microscope (STM) imaging by confirming that the nitrogen-containing compound has a desired orientation relative to the diamond substrate; and The desired orientation is such that four sp ３ The method of claim 12, wherein the orientation relative to the diamond substrate provides bonding.

Images