Composite diamond material

Abstract

A composite diamond material 1 having a single crystal diamond 2 with a major surface aligned substantially in a {100} crystallographic plane 3. There are a plurality of trenches within the major surf

Description

FIELD OF THE INVENTION The invention relates to the field of composite diamond materials, and to methods of making diamond materials. BACKGROUND Point defects in synthetic diamond material, particularly quantum spin defects and / or optically active defects, have been proposed for use in various sensing, detecting, and quantum processing applications including magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; spin resonance imaging devices for magnetic resonance imaging (MRI); and quantum information processing devices such as for quantum computing. In addition, radio frequency (RF) sensors can also be developed using optically active defects in diamond. Many point defects have been studied in synthetic diamond material including: silicon containing defects such as silicon-vacancy defects (Si-V), silicon di-vacancy defects (Si-V2), silicon-vacancy-hydrogen defects (Si-V:H), silicon di-vacancy hydrogen defects (S-V2:H); nickel containing defect; chromium containing defects; and nitrogen containing defects such as nitrogen-vacancy defects (N-V), di-nitrogen vacancy defects (N-V-N), and nitrogen-vacancy-hydrogen defects (N-V-H). These defects are typically found in a neutral charge state or in a negative charge state. It will be noted that these point defects extend over more than one crystal lattice point. The term point defect as used herein is intended to encompass such defects but not include larger cluster defects, such as those extending over ten or more lattice points, or extended defects such as dislocations which may extend over many lattice points. The nitrogen-vacancy (NV-) defect in synthetic diamond material has attracted a lot of interest as a useful quantum spin defect because it has several desirable features including: (i) Its electron spin states can be coherently manipulated with high fidelity owing to an extremely long coherence time (which may be quantified and compared using the transverse relaxation time T2); (ii) Its electronic structure allows the defect to be optically pumped into its electronic ground state allowing such defects to be placed into a specific electronic spin state even at non-cryogenic temperatures. This can negate the requirement for expensive and bulky cryogenic cooling apparatus for certain applications where miniaturization is desired. Furthermore, the defect can function as a source of photons which all have the same spin state; and (iii) Its electronic structure comprises emissive and non-emissive electron spin states which allows the electron spin state of the defect to be read out through photons. This is convenient for reading out information from synthetic diamond material used in sensing applications such as magnetometry, spin resonance spectroscopy and imaging. Furthermore, it is a key ingredient towards using the NV' defects as qubits for long-distance quantum communications and scalable quantum computation. Such results make the NV' defect a competitive candidate for solid-state quantum information processing (QIP). The NV' defect in diamond consists of a substitutional nitrogen atom adjacent to a carbon vacancy. Its two unpaired electrons form a spin triplet in the electronic ground state (3A), the degenerate ms = ± 1 sublevels being separated from the ms = 0 level by 2.87 GHz. The ms = 0 sublevel exhibits a high fluorescence rate when optically pumped. In contrast, when the defect is excited in the ms = ± 1 levels, it exhibits a higher probability to cross over to the non-radiative singlet state (1A) followed by a subsequent relaxation into ms = 0. As a result, the spin state can be optically read out, the ms = 0 state being “bright” and the ms = ± 1 states being dark. When an external magnetic field is applied, the degeneracy of the spin sublevels ms = ± 1 is broken via Zeeman splitting. This causes the resonance lines to split depending on the applied magnetic field magnitude and its direction. SUMMARY An object of the invention is to improve the surfaces of devices. One application is to improve the resolution of devices by orienting spin centres along certain crystallographic directions below the surface. Locating spin centres near a surface makes them easier to query. Low damage surfaces are important as damage around the spin centres can be deleterious to their properties. Catalytic etching of diamond surfaces using transition metals is a known technique that has been used for structuring diamond surfaces. High temperature water vapour-assisted nickel etching one way to achieve anisotropically etched diamond surfaces (Nagai, et al. Sci. Rep. 8 (2018), 6687). Ni etching of diamond can produce well-defined, micron-scale structures on diamond with smooth {111} walls. According to a first aspect, there is provided a composite diamond material comprising: a single crystal diamond having a major surface aligned substantially in a {100} crystallographic plane; at least a portion of the major surface comprising a plurality of trenches, the trenches having surfaces aligned substantially in a {111} crystallographic plane; a diamond layer on the surface of the trenches Optional aspects are described in the appended dependent claims. According to a second aspect, there is provided a method of manufacturing a diamond material, the method comprising: providing a single crystal diamond having a major surface aligned substantially in a {100} crystallographic plane; introducing at least one trench into at least a portion of the major surface, the trench having surfaces aligned substantially in a {111} crystallographic plane; and applying an inductively coupled plasma etch to the surfaces of the trench. The inductively coupled plasma etch has been found to smooth the surfaces of the trench and make it suitable for overgrowth of further diamond material containing spin defects. Optional aspects are described in the appended dependent claims. According to a third aspect, there is provide a method of manufacturing the composite material described above in the first aspect, the method comprising: providing a single crystal diamond having a major surface aligned substantially in a {100} crystallographic plane; introducing a plurality of trenches into at least a portion of the major surface, the trenches having surfaces aligned substantially in a {111} crystallographic plane; and growing a diamond layer on the surface of the trenches. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates schematically a mechanism of catalytic etching of diamond using nickel as an exemplary etchant; Figure 2 is a photographic image of a {100} surface of a single crystal diamond material after patterning with direct laser lithography and subsequent sputtering of nickel and liftoff; Figure 3 is four Scanning Electron Microscopy images of nickel- etched trenches with {111} walls before (a-b) and after (c-d) surface processing; Figure 4 shows ODMR spectra from a {100} surface and a {111} trench, showing the preferential orientation of spin centres; Figure 5a shows Hahn echo decay of NV ensembles measured in a {111} trench; Figure 5b shows Hahn echo decay of NV ensembles measured at a {100} surface; Figure 6 is a flow diagram showing exemplary steps for preparing diamond with {111] trenches; Figure 7 is a flow diagram showing exemplary steps for preparing composite diamond material; and Figure 8 illustrates schematically a cross section view of an exemplary diamond surface with an overgrown layer; and Figure 9 illustrates schematically in a block diagram an exemplary device that uses the diamond of Figure 8. The figures are not drawn to scale. Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity. DETAILED DESCRIPTION Turning to Figure 1, a mechanism of catalytic etching of diamond is shown. In the example of Figure 1, nickel is used as an exemplary etchant. Nickel is deposited on a surface of a single crystal diamond. The surface lies substantially on a {100} crystallographic plane. In step a, the nickel is oxidised at elevated temperature and in the presence of water according to equation 1: Ni (s) + H2O (gq) -> NiO (s') + H2 (g) Eq 1 This leads to a layer of nickel oxide on a surface of the nickel opposite to the surface of the diamond. Carbon from the diamond diffuses to the nickel / nickel oxide interface. There is a redox reaction between the nickel oxides and the carbon according to equation 2. AW (.$) + C (s) Afi (s) + CO(,g) Eq 2 The diffusion of the carbon causes trenches to be etched in the diamond underneath the nickel. The etching is anisotropic. The etch is preferentially in the {110} and {100} planes which results in etched trenches having {111} terminated walls surfaces. This technique provides etch rates of the order of microns per minute, compared to nanometres per minute for dry etching techniques. This technique can also achieve atomically flat {111} surfaces, and can be used as a route to produce high quality {111} pores. Once the {111} trenches have been produced in the surface of the diamond, it has been found that an inductively coupled plasma reactive ion etch (ICP-RIE) smooths the surfaces of the {111} trenches, making them suitable for overgrowth. A further layer of diamond material can then be overgrown on the surface of the trenches using a CVD technique. The overgrown diamond layer may contain a higher nitrogen content then the single crystal diamond, and subsequent irradiation and annealing causes the formation of NV' centres. As the overgrown layer grows homoepitaxially on the surface of the trenches, it adopts the same crystallographic structure, and so will also lie substantially in a {111} plane. The NV' centres formed in this overgrown layer are therefore similarly oriented. NV' centres have trigonal symmetry and hence there are four different possible orientations within the diamond lattice, corresponding to the four equivalent <111> directions. In the case of building an ensemble sensor using such defects in diamond it may be advantageous to promote the formation of only a subset of these orientations, as this can help increase sensitivity. This can be understood by the contrast (on / off resonance difference in luminescence intensity) being distributed amongst a smaller number of resonance lines. This has previously been demonstrated by the growth of NV-containing layers on {110} oriented diamond substrates, {113} oriented substrates, or {111} oriented substrates. In these examples the entire growth surface of the chosen substrates had a common orientation, i.e. they were non-structured, and the degree of preferential alignment was controlled by the growth-face orientation. In the following description, diamond with oriented NV- defects is described. However, the skilled person understands that the same techniques can be applied to diamond containing other types of spin centre. Examples of other types of spin centre are silicon containing defects; nickel containing defects; chromium containing defects; germanium containing defects; tin containing defects; and nitrogen containing defects. Example Single crystal diamond substrates with <100 edges and {100} surfaces were patterned by direct laser writing lithography followed by sputtering Ni (150 nm) and lift off. The single crystal diamond had a low nitrogen content. The structures were aligned relative to the sample edges. Figure 2 shows a {100} surface of a single crystal diamond material after patterning with direct laser lithography and subsequent sputtering of nickel and liftoff in a <110> direction. The samples were annealed at temperatures of greater than 600°C in a sealed quartz tube in an atmosphere containing water vapour for 4 hours. Different times and temperatures were used. This caused the etching of the {111} trenches as described above. Figures 3a and 3b show the nickel- etched trenches with {111} surfaces. The samples were then cleaned in aqua regia at 50°C for at least one hour before etching. An ICP-RIE was applied to the surface of the diamond using ICP / RF power of 500 W1200 W for 50 minutes (although it has been found that much shorted times as low as 5 to 10 minutes also etch the trenches) using a platen temperature of 5 °C and in an atmosphere of Ar (10 seem) and Ch (20 seem). Figures 3c and 3d show the nickel- etched trenches with {111} surfaces after etching. It can be seen visually that the surfaces of the trench have been smoothed by the etching process. The surfaces were then overgrown with a layer of diamond containing a higher nitrogen content to form a composite diamond material. The layer had a thickness of approximately 1.5 pm. The composite diamond material was then irradiated and annealed using the conditions described in WO2020 / 201208 to convert Ns° in the diamond layer to NV'. For some applications where NV' centres are required, the presence of 13C can be detrimental to the properties of the diamond as it has a non-zero nuclear spin. It therefore may be preferred to grow the diamond layer using a carbon containing gas in the source gas that in which at least 99% of the carbon is 12C, at least 99.9% of the carbon is 12C or at least 99.99% of the carbon is 12C. Turning now to Figure 4, optically detected magnetic resonance (ODMR) spectra were obtained from a {100} surface and a {111} trench, showing the preferential orientation of NV' centres in the overgrown diamond layer. It can be seen the diamond layer overgrown on the nickel-etched {111} walls with subsequent irradiation and annealing resulted in almost perfect crystallographic alignment of NV' defects. Figures 5a and 5b shows Hahn echo decay of NV ensembles measured in a {111} trench and at a {100} surface. It can be seen that there is a difference in the T2 times inside the {111} trenches compared to the {100} surface of the sample. The provision of diamond with highly oriented spin centres can be used to create devices with increases sensitivity. In the case of producing composite materials that combine thin-layers (for example, less than 0.1 mm) of high-[NV] diamond on a diamond substrate (commonly used for wide-field sensing applications where high-spatial resolution is desired) it is important to utilize a high-purity substrate with a nitrogen concentration of significantly below100 ppb, as described in WO 0196633. This is necessary to allow the luminescence from the active high-[NV] layer to be isolated, without a background contribution from the substrate beneath. Such high-purity substrates are typically produced by CVD and have a main {100} face. This is due the reduced uptake in defects such as nitrogen when growing on substrates with this orientation and the relative ease of production of low-birefringence {100} substrates with suitable areas. Production of {110} or {111} substrates with sufficient top-face area from such a high-purity growth process is challenging as this necessitates growth of a sufficiently-thick layer such that slices can be extracted at the appropriate angle and growth conditions that deliver such high purities typically result in a low growth rate, limiting the scalability of such approaches. The approach described above of creating {111} trenches allows the production of highly oriented NV' centres in a diamond that has a bulk surface orientation of {100}, thereby avoiding the production problems of growing on diamond with a {110} or {111} orientation. Figure 6 is a flow diagram showing exemplary steps for preparing diamond with {111] trenches. The following numbering corresponds to that of Figure 6: S1. Single crystal diamond with a low nitrogen content and having a major surface aligned substantially in a {100} crystallographic plane is provided. S2. A trench is introduced into at least a portion of the major surface, the trench having surfaces aligned substantially in a {111} crystallographic plane. As described above, this may be by applying a transition metal such as nickel, iron or cobalt using laser lithography, and subsequent sputtering of the metal. Suitable metals include nickel, iron and cobalt, although nickel was used in the examples described above. S3. An inductively coupled plasma etch is applied to the surfaces of the trench. This may be, for example, and ICP-RIE etch. This has the effects of smoothing the surfaces of the trench, making it suitable of overgrowth of a further layer of diamond. Figure 7 is a flow diagram showing exemplary steps for preparing composite diamond material. The following numbering corresponds to that of Figure 7: S4. Single crystal diamond with a low nitrogen content and having a major surface aligned substantially in a {100} crystallographic plane is provided. S5. A plurality of trenches is introduced into at least a portion of the major surface, the trench having surfaces aligned substantially in a {111} crystallographic plane. As described above, this may be by applying a transition metal such as nickel, iron or cobalt using laser lithography, and subsequent sputtering of the metal. Suitable metals include nickel, iron and cobalt, although nickel was used in the examples described above. S6. A further diamond layer is overgrown on the surface of the trenches. As described above, the further layer of CVD diamond comprises a plurality of spin defects substantially aligned with the {111} surface. S7. If the overgrown diamond layer contains a higher nitrogen content, it is irradiated and annealed to create NV' centres in the diamond layer. A concentration of spin defects is selected from any of equal to or greater than: 1 x 1013 defects / cm3; 1 x 1014 defects / cm3; 1 x 1015 defects / cm3; 1 x 1016 defects / cm3; 1 x 1017 defects / cm3; and 1 x 1018 defects / cm3, and any of equal to or less than: 4 x 1018 defects / cm3; 2 x 1018 defects / cm3; 1 x 1018 defects / cm3; 1 x 1017 defects / cm3; and 1 x 1016 defects / cm3. Turning now to Figure 8, there is illustrated schematically a cross section view of an exemplary composite diamond 1. The composite diamond 1 has a single crystal diamond 2 with a major surface oriented substantially in a {100} plane 3. A plurality of trenches having {111} surfaces are introduced into the major surface 3 of the diamond 2. A further diamond layer 5 is overgrown onto the diamond surface, or optionally the diamond surface is masked to expose only the trenches before overgrowth. Figure 9 illustrates schematically in a block diagram an exemplary device that uses the diamond of Figure 8, for example a widefield imaging device. The device 6 is provided with the composite diamond 1 described and shown in Figure 8 for sensing, and a microprocessor 7 for controlling and reading signals from the composite diamond 1. While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A composite diamond material comprising:a single crystal diamond having a major surface aligned substantially in a {100} crystallographic plane;at least a portion of the major surface comprising a plurality of trenches, the trenches having surfaces aligned substantially in a {111} crystallographic plane;a diamond layer on the surface of the trenches2. The composite diamond material according to claim 1, wherein the diamond layer comprises a plurality of spin defects substantially aligned with the {111} surface.

3. The composite diamond material according to claim 2, wherein the spin defects are selected from any of:silicon containing defects;nickel containing defects;chromium containing defects;germanium containing defects;tin containing defects; and nitrogen containing defects.

4. The composite diamond material according to claim 2 or 3, wherein the spin defects are negatively charged nitrogen-vacancy defects NV'.

5. The composite diamond material according to any one of claims 2 to 3 wherein a concentration of spin defects is selected from any of equal to or greater than: 1 x 1013 defects / cm3; 1 x 1014 defects / cm3; 1 x 1015 defects / cm3; 1 x 1016 defects / cm3; 1 x 1017 defects / cm3; and 1 x 1018 defects / cm3.

6. The composite diamond material according to any one of claims 2 to 5, wherein a concentration of spin defects is selected from any of equal to or less than: 4 x 1018 defects / cm3; 2 x 1018 defects / cm3; 1 x 1018 defects / cm3; 1 x 1017 defects / cm3; and 1 x 1016 defects / cm3.

7. The composite diamond material according to any one of claims 2 to 6, wherein the spin defects are disposed within 500 nm, 200 nm, 100 nm, 50 nm, 30 nm, 10 nm, or 5 nm of the surface.

8. The composite diamond material according to any one of claims 2 to 7, wherein the thickness of diamond layer is selected from any of no more than 100 pm, no more than 50 pm, no more than 10 pm, no more than 5 pm, no more than 1 pm, no more than 500 nm, no more than 200 nm and no more than 100 nm.

9. The composite diamond material according to any one of claims 1 to 8, wherein a width of the trenches is selected from any of 1 to 100 pm, 5 to 50 pm and 10 to 40 pm.

10. The composite diamond material according to any one of claims 1 to 8, wherein a depth of the trenches is selected from any of 1 to 100 pm, 5 to 50 pm and 10 to 40 pm.

11. The composite diamond material according to any one of claims 1 to 9, wherein the single crystal diamond has a single substitutional nitrogen concentration selected from any of no more than 300 ppb, 200 ppb, 100 ppb, 80 ppb, 60 ppb, 40 ppb, 20 ppb, 10 ppb, 5 ppb, or 1 ppb.

12. The composite diamond material according to any one of claims 1 to 11, wherein the single crystal diamond has at least one lateral dimension selected from any of at least 0.5 mm, at least 1 mm, at least 2 mm, at least 3 mm, at least 4 mm, and at least 5 mm.

13. A method of manufacturing a diamond material, the method comprising: providing a single crystal diamond having a major surface aligned substantially in a {100} crystallographic plane;introducing at least one trench into at least a portion of the major surface, the trench having surfaces aligned substantially in a {111} crystallographic plane; and applying an inductively coupled plasma etch to the surfaces of the trench.

14. The method according to claim 13, wherein the inductively coupled plasma etch is an inductively coupled plasma reactive ion etch.

15. The method according to claim 13 or 14, wherein the trench is introduced by applying a metal to a portion of the {100} major surface and heating at a temperature of at least 500°C in the presence of water.

16. The method according to claim 15, wherein the metal is selected from any of nickel, iron and cobalt.

17. The method according to claim 15 or 16, wherein the metal is applied using laser lithography, and subsequent sputtering of the metal.

18. The method according to any one of claims 13 to 17, further comprising growinga further layer of CVD diamond over the surfaces of the trench.

19. The method according to claim 18, wherein the further layer of CVD diamond comprises a plurality of spin defects substantially aligned with the {111} surface.

20. The method according to claim 19, wherein the plurality of spin defects is selected from any of:silicon containing defects;nickel containing defects;chromium containing defects;germanium containing defects;tin containing defects; and nitrogen containing defects.

22. The method according to any one of claims 19 to 20 wherein a concentration of spin defects is selected from any of equal to or greater than: 1 x 1013 defects / cm3; 1 x 1014 defects / cm3; 1 x 1015 defects / cm3; 1 x 1016 defects / cm3; 1 x 1017 defects / cm3; and 1 x 1018 defects / cm3.

23. The method according to any one of claims 19 to 22, wherein a concentration of spin defects is selected from any of equal to or less than: 4 x 1018 defects / cm3; 2 x 1018 defects / cm3; 1 x 1018 defects / cm3; 1 x 1017 defects / cm3; and 1 x 1016 defects / cm3.

24. The method according to claim 18, further comprising irradiating and annealing the further layer of CVD diamond.

25. A device comprising the composite material according to any one of claims 1 to 12.

26. The device according to claim 25, wherein the device is a widefield imaging device.

27. A method of manufacturing the composite material according to any one of claims 5 1 to 12, the method comprising:providing a single crystal diamond having a major surface aligned substantially in a {100} crystallographic plane;introducing a plurality of trenches into at least a portion of the major surface, the trenches having surfaces aligned substantially in a {111} crystallographic plane; and10 growing a diamond layer on the surface of the trenches.

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