GB2623076A - CVD diamond product - Google Patents

Abstract

Freestanding CVD single crystal diamond with thickness less than 1µm; a major face having a surface area of at least 4 mm2 and an angle of between 0.2° and 5° from a major crystallographic plane; and the major face having a surface roughness of no more than 5nm Ra. The surface roughness may be no more than 1 nm and no more than 0.5 nm. The thickness may be less than 500 nm, less than 200 nm, less than 100 nm, less than 50nm, less than 30 nm and less than 10nm. The surface area may be at least 10mm2, 20mm2, 50mm2 or 100 mm2. The angle of the crystallographic plane may be selected from any of between 0.2° and 3°, 0.2° and 2° and 0.2° and 1.5°. The major crystallographic plane is selected from any of a {100}, {110}, {113}, and {111} plane. The diamond may have a single substitutional nitrogen concentration selected from any of no more than 300 ppb, 200ppb, 100ppb, 80ppb, 60ppb, 40ppb, 20ppb, 10ppb, 5ppb, 1ppb or 01.ppb.

Description

CVD SINGLE CRYSTAL DIAMOND

Field of the Invention

The present invention relates to a freestanding CVD single crystal diamond and methods of producing the freestanding CVD single crystal diamond.

Backuround of the Invention Point defects in synthetic diamond material, particularly quantum spin defects and/or optically active defects, have been proposed for use in various optics, quantum computing, quantum communications, imaging, sensing, and processing applications. These include quantum information processing devices such as for quantum communication and computing; luminescent tags; magnetometers; spin resonance devices such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) devices; and spin resonance imaging devices for magnetic resonance imaging (MR1).

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 (5- V2:H); chromium containing defects; and nitrogen containing defects such as nitrogen-vacancy defects (N-V), di-nitrogen vacancy defects (N-V-N), nitrogen-vacancy-hydrogen defects (N-V-H), nickel vacancy defects (NE4) and nickel vacancy nitrogen defects (NE8). 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.

It has been found that certain defects are particularly useful for sensing and quantum processing applications. For example, the negatively charged nitrogen-vacancy defect (NV) 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 and have an extremely long coherence time (which may be quantified and compared using the transverse relaxation time T2 and/or 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 electronic structure of the NV-defect is such that 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. This dependency can be used for magnetometry by probing the resonant spin transitions using microwaves (MW) and using optically detected magnetic resonance (ODMR) spectroscopy to measure the magnitude and optionally direction of the applied magnetic field. Similarly, photoelectric detection of magnetic resonance (PDMR) can be used to determine the spin state. ;There are many ways that spin defects can be deliberately formed in diamond. Taking the NV-defect in synthetic diamond material as an example, this defect can be formed in a number of different ways including: formation during growth of the synthetic diamond material where a nitrogen atom and a vacancy are incorporated into the crystal lattice as a nitrogen-vacancy pair during growth; formation after diamond material synthesis from native nitrogen and vacancy defects incorporated during the growth process by subsequent annealing the material at a temperature (around 800°C) which causes migration of the vacancy defects through the crystal lattice to pair up with native single (iii) formation after diamond material synthesis from native nitrogen defects incorporated during the growth process by irradiating the synthetic diamond material to introduce vacancy defects and then subsequently annealing the material at a temperature which causes migration of the vacancy defects through the crystal lattice to pair up with native single substitutional nitrogen defects; (iv) formation after diamond material synthesis by implanting nitrogen defects into the synthetic diamond material after diamond material synthesis and annealing the material at a temperature which causes migration of the native vacancy defects through the crystal lattice to pair up with implanted single substitutional nitrogen defects; and (v) formation after diamond material synthesis by irradiating the synthetic diamond material to introduce vacancy defects, implanting nitrogen defects into the synthetic diamond material, and annealing the material at a temperature which causes migration of the vacancy defects through the crystal lattice to pair up with implanted single substitutional nitrogen defects. ;It is known that similar techniques to those described in (i) to (v) above can be used to form other types of spin defects. ;Spin defects can be introduced typically by in-situ growth or ion implantation. For both techniques, a spin defect precursor is often implanted, which can then be further processed (for example by annealing and optionally irradiation) to form the spin defect. For example, nitrogen may be ion implanted as a spin defect precursor into the diamond crystal lattice and subsequent irradiation and annealing can transform at least some of the implanted nitrogen into NV centres. ;For in-situ growth, a spin defect precursor is deliberately added to the reactor gases during CVD growth such that the spin defect precursor is incorporated into the diamond crystal lattice. A problem with this approach is that small variations in the growth rate leads to variations in the location of the defects of interest, which makes this approach difficult to scale. ;Ion implantation of spin defects or spin defect precursors, particularly at higher energies, has challenges due to straggle and damage. Straggle is the distribution of the depths at which the ions are implanted; the spin defects cannot all be guaranteed to be located at a particular depth. Furthermore, ion implantation at higher energies leads to higher damage surrounding the defect, which has a deleterious effect on the properties of the spin coherence time of the defect ;Summary ;It is an object of the invention to provide a large area single crystal diamond material with better control of the quantum spin defects than previously available. ;According to a first aspect of the invention, there is provided a freestanding CVD single crystal diamond, the single crystal diamond having a thickness of less than 1 pm, a major face having a surface area of at least 4 mm2 and an angle of between 0.2°and 5° from a major crystallographic plane, and the major face having a surface roughness of no more than 5 nm Ra. ;As an option, the major face has a surface roughness Ra selected from any of no more than 1 nm, and no more than 0.5 nm. ;The thickness of the freestanding CVD single crystal diamond is optionally selected from any of less than 500 nm, less than 200 nm, less than 100 nm, less than 50 nm, less than 30 nm, and less than 10 nm. ;The major face optionally has a surface area selected from any of at least 10 mm2, at least 20 mm2, at least 50 mm2 and at least 100 mm2 As an option, the angle from the major crystallographic plane is selected from any of between 0.2° and 3°, 0.2 and 2°, and 0.2 and 1.50. ;As an option, the major crystallographic plane is selected from any of a {100} crystallographic plane, a {110} crystallographic plane, a {113} crystallographic plane, and a {111} crystallographic plane. ;The CVD single crystal diamond optionally has a single substitutional nitrogen concentration selected from any of no more than 300 ppb, no more than 200 ppb, no more than 100 ppb, no more than 80 ppb, no more than 60 ppb, no more than 40 ppb, no more than 20 ppb, no more than 10 ppb, no more than 5 ppb, no more than 1 ppb, and no more than 0.1 ppb. ;As an option, a variation of thickness between opposite edges of the major face is selected from any of no more than 50 nm, no more than 20 nm, no more than 10 nm and no more than 5 nm. ;As an option, the freestanding CVD single crystal diamond further comprises at least one spin defect, wherein the spin defect is selected from any of nitrogen-vacancy centres, silicon vacancy centres, germanium vacancy centres, nickel vacancy centres, nickel vacancy nitrogen centres and tin vacancy centres. ;As a further option, a largest photoluminescence intrinsic peak intensity of the spin defect is at least 50 times the largest photoluminescence non-intrinsic peak intensity measured at wavelengths within 10 nm of the of the wavelength of the intrinsic peak. ;The freestanding CVD single crystal diamond optionally further comprising a layer of substrate material on which the CVD single crystal diamond was grown. ;According to a second aspect, there is provided a method of forming a CVD single crystal diamond. The method comprises providing a single crystal diamond substrate, the substrate having a growth face with a mis-cut of between 0.2° and 5° from a major crystallographic plane, the growth face having an area of at least 4 mm2. The single crystal diamond substrate is located in a CVD reactor. Process gases are fed into the reactor, the process gases comprising a carbon-containing gas and hydrogen, wherein the process gas contains no more than 2 vol.% of the carbon-containing gas. A single crystal diamond layer is then homoepitaxially grown on the substrate, a portion of the layer having an as-grown surface roughness Ra of no more than 5 nm. ;As an option, the portion of the layer which has said surface roughness Ra has an area selected from any of at least 4 mm2, at least 10 mm2, and at least 100 mm2. ;As an option, the single crystal diamond layer has a thickness selected from any of less than 200 nm, less than 150 nm, less than 100 nm, less than 50 nm, less than 30 nm, and less than 10 nm. ;As an option, the mis-cut from the major crystallographic plane is selected from any of between 0.2°and 3°, between 0.2 and 2°, and between 0.2 and 1.5°. ;As an option, the major crystallographic plane is selected from any of a {100} crystallographic plane, a {110} crystallographic plane, a {113} crystallographic plane, and a {111} crystallographic plane. ;The method optionally further comprises polishing a surface of the substrate prior to growing the layer on the surface, the polishing being effected along the hard direction. ;As a further option, the method further comprises removing subsurface damage from the surface prior to growing the layer. The subsurface damage is optionally removed by any of argon chlorine etching, oxygen inductively coupled plasma etching, and chemical mechanical polishing, CMP. ;The method optionally further comprises implanting a spin defect precursor element into a surface of the layer. ;As an alternative option, the method the process gases further comprise a spin defect precursor element. ;As an option, the method further comprises annealing the CVD single crystal diamond product to cause formation of the spin defect. ;As an option, the method comprises annealing the CVD single crystal diamond product in caesium salt at a temperature greater than 1750°C and a pressure greater than 4 GPa. ;According to a third aspect, there is provided a freestanding CVD single crystal diamond product having a thickness of less than 1 pm and a major face having a surface roughness of no more than 5 nm Ra, wherein a largest photoluminescence intrinsic peak intensity of a spin defect in the layer is at least 50 times the largest photoluminescence non-intrinsic peak intensity measured at wavelengths within 10 nm of the of the wavelength of the intrinsic peak. ;Brief Description of the Drawings ;Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a flow diagram showing exemplary steps to produce a CVD diamond product; Figure 2 is a flow diagram showing further exemplary steps to produce a CVD diamond product; Figure 3 is a flow diagram showing exemplary manufacturing steps; Figure 4 is an atomic force microscope, AFM, image of a substrate surface after polishing; and Figure 5 is a graph showing the effect of substrate surface mis-cut angle on the formation of hillocks during subsequent overgrowth. ;Detailed Description ;The following description refers to NV-centres in diamond acting as quantum spin defects. It will be appreciated that the same techniques can all apply to other types of quantum spin defect in diamond, such as silicon vacancy centres, germanium vacancy centres, fin vacancy centres and spin defects that include nickel. ;The term "surface roughness Ra" (sometimes referred to as "centre line average" or "c.l.a.") used herein refers to the arithmetic mean of the absolute deviation of surface profile from the mean line measured, for example, by stylus profilometer according to British Standard BS 1134 Part 1 and Part 2. The mathematical description of Ra is: = *i ly(x)kix T. In contrast, a surface roughness Rq refers to the root mean square roughness (sometimes also called the "RMS roughness"). Where Rq is referred to, it is typically measured either using a stylus profilometer or using a scanning probe instrument, such as an atomic force microscope, over an area of a few pm by a few pm (e.g. 1 pm x 1 pm or 2 pm x 2 pm); in the case of an Rq being referred to, the Rq is measured using a stylus profilometer unless it is specifically stated that the Rq is measured using a scanning probe instrument. The mathematical description of Rq is: R = For a surface with a Gaussian distribution of surface heights, Rq = 1.25 Ra.

In the present case, the surface roughness Ra of the as-grown diamond growth face may be measured, for example, using atomic force microscopy (AFM), and averaged over an area of at least 100 nm2, 500 nm2, 1 pm2, 20 pm2, 25 pm2, 100 pm2, 200 pm2, 400 pm2, 900 pm2, 2500 pm2, 10,000 pm2, 0.25 mm2, or 1 mm2.

In summary the present invention uses a combination of a single crystal diamond substrate that has a surface prepared for homoepitaxial growth that has low damage. It is desired to have a diamond material that has a low dislocation density, low levels of impurities, a low surface roughness and low polishing/processing damage on the surface prepared for growth. The single crystal diamond substrate is a substrate, and the subsequent single crystal diamond growth forms a layer. Spin defects are provided in the layer but they are now far enough below the surface of the diamond product to be minimally affected by surface damage and other surface effects. For this process, it is critical that the substrate has a very low surface roughness Ra, with low amounts of surface damage It has been found that single crystal diamond can be grown on surfaces that are slightly mis-cut from a major crystallographic plane. This, in combination with a low methane concentration in the process gases during growth of the layer, encourages step-flow growth rather than the formation of 'hillocks', and is described in more detail in WO 2015/071487.

To synthesize a CVD diamond product with low surface roughness, a combination of pre-synthesis substrate preparation and synthesis methods are employed. The substrate is a single crystal diamond substrate that is suitable for use in homoepitaxial diamond synthesis. The substrate is selected and prepared to have a low level of extended defects and a low level of polishing related subsurface damage. The substrate may be a low birefringence type la or I lb natural diamond or a low birefringence type lb or I la high pressure/high temperature (HPHT) synthetic diamond or a substrate that has be grown using CVD techniques and then vertically cut (i.e. parallel to the original growth direction) to minimise extended defects breaking the surface of the substrate.

The term "low birefringence" is used to describe a substrate which has at least one of the following properties: a) a density of extended defects as characterised by X-ray topography of about 1000 per cm2 or less over an area of about 0.014 cm2 or more; b) an optical isotropy of about 1 x 104 or less over a volume of about 0.1 me or greater; and c) a FWHM ("Full Width at Half Maximum") X-ray rocking curve width for the (004) reflection of about 120 arc seconds or less.

Preferably, the substrate has an extremely low level of birefringence. In diamond, birefringence is typically associated with the presence of large numbers of extended defects (e.g. dislocations, dislocation bundles and stacking faults) that cause high levels of localised strain and consequently birefringence. Preferably the maximum birefringence evaluated by measurements through the thickness of the substrate over 70%, 80%, 90%, 95%, 98% or more of the area of the surface to be grown on, is 1 x 10-or less, 5 x 10 or less, 1 x 10' or less, 5 x 106 or less, or 1 x 10-6 or less. It is advantageous to use diamond material of such low birefringence because this reduces the number per unit area of extended defects propagating from the substrate into the homoepitaxial diamond layer during growth of the homoepitaxial diamond layer; such defects may be "decorated" with impurity atoms that can have non-zero nuclear spin and therefore can reduce the decoherence time 12 of nearby quantum spin defects.

Crystal defect density is most easily characterised by optical evaluation after using a plasma etch or chemical etch optimised to reveal the defects (referred to as a "revealing etch"). Two types of crystal defects can be revealed: (i) Those intrinsic to the substrate material such as dislocations, stacking faults, twin boundaries, etc. In selected synthetic or natural diamond the density of these crystal defects can be 50 defects per mm2 or lower with more typical values being 102 defects per mm2, whilst in other synthetic of natural single crystal diamond materials the defect density can be 106 defects per mm2 or greater.

(ii) Those resulting from polishing including dislocation structures and microcracks in the form of 'chatter tracks' along polishing lines thereby forming a mechanically damaged layer beneath the surface of the substrate.

Polishing methods known in the art, such as scaife or chemical mechanical polishing, may be carefully controlled to prepare the substrate while minimising the level of subsurface damage introduced during processing. Furthermore, the surface of the substrate is advantageously etched in-situ immediately prior to diamond growth thereon.

Low roughness of the surface of the substrate is also critical.

In order to fabricate smooth as-grown single crystal CVD diamond surfaces it has also been found to be useful to provide a substrate surface to be grown on which is slightly angled relative to a crystallographic plane of the single crystal diamond substrate material.

Referring to Figure 1, an exemplary method for producing a diamond product is shown. The following numbering corresponds to that of Figure 1: Si. A substrate is provided that has a surface to be grown on that is mis-cut by between 0.2 and 5° from a major crystallographic plane. In this example, the crystallographic plane is a {100} plane. Other mis-cut angles can be used, for example between 0.2°and 3°, between 0.2 and 2°, and between 0.2 and 1.50. The surface to be grown on has a surface area of at least 4 mm2. Surfaces with larger surface areas may be used, for example at least 10 mm2, at least 20 mm2, at least 50 mm2 or at least 100 mm2 S2. The surface to be grown on of the substrate is polished along the hard direction.

It is known that different crystallographic directions of single crystal diamond are harder to polish than others. The directions are known as 'hard' and 'soft' directions. For example, for a (100) surface, the <100> direction is known to be soft and the <110> direction is known to be hard. The hard and soft directions depend on the crystallographic plane of the surface to be grown on. An advantage of polishing along the hard direction is that it will reduce the surface roughness of the surface to be grown on of the substrate, although this comes at the expense of high damage at the surface. The surface roughness of the surface to be grown on is no more than 100 nm, but may be no more than 80 nm, no more than 50 nm, no more than 20 nm, no more than 10 nm, no more than 5 nm, no more than 2 nm, no more than 1 nm, and no more than 0.5 nm, and no more than 0.1 nm.

S3. After polishing, subsurface damage is removed from the surface to be grown on of the substrate. This is typically performed by etching, for example by inductively coupled plasma (ICP) argon chlorine etching. Advantageously, the surface of the substrate is etched in-situ after location of the substrate in the reactor (step S4 below) immediately prior to diamond growth thereon. Other ways of removing subsurface damage include oxygen inductively coupled plasma etching (IC) or chemical mechanical polishing (CMP) S4. The substrate is located in a CVD reactor and process gases are fed into the reactor. A plasma is struck and a layer of diamond is homoepitaxially grown on the surface of the substrate. The process gases include a carbon-containing gas and hydrogen. It has been found that by using a low amount of carbon-containing gas, and a substrate with a mis-cut surface on which the layer is grown, the formation of hillocks is suppressed and the layer is produced with low-roughness surface. Typically, the carbon-containing gas is methane, and the process gases comprise no more than 2 volume percent methane. It will be appreciated that other common carbon-containing gases may be used. The combination of the mis-cut substrate surface and the low amount of methane provides a layer with a low surface roughness Ra, typically less than 5 nm. The substrate and overgrown layer together have a thickness selected from any of no more than 300 pm, no more than 200 pm, no more than 100 pm, no more than 80 pm, and no more than 60 pm.

After growth of the layer, the layer can be removed from the substrate by cutting, mechanical processing, or ion implantation and subsequent lift-off. This yields a high purity single crystal CVD diamond plate with a smooth, low damage, as-grown front surface and a smooth processed rear surface. This leads to a free-standing layer that may have a thickness selected from any of less than 1 pm, less than 500 nm, less than 200 nm, less than 100 nm, less than 50 nm, less than 30 nm, and less than 10 nm. A variation of thickness between opposite edges of the layer is selected from any of no more than 50 nm, no more than 20 nm, no more than 10 nm and no more than 5 nm.

Considering the ion implantation and subsequent lift-off technique mentioned above, it is known that a combination of ion implantation and etching can be used to produce thin, uniform diamond membranes. An exemplary process is described in VVO 2021/176015.

The lift-off approach involves ion implantation of the diamond to create a damage layer within the diamond. This implantation is followed by annealing, during which the damage layer becomes graphitic. This graphitic layer can then be removed by electrochemical etching, allowing two layers of diamond either side of the damage layer to be separated.

An advantage of this technique is that the ion implantation step creates a damage layer at a controlled and fixed distance beneath the surface, ensuring lift-off of a diamond layer of uniform thickness. In this example, a damage layer is created just below the surface of the substrate by ion implantation (before step S4), and subsequent annealing causes graphitization of the damage layer and allows the layer to be lifted off from the substrate.

An issue that can arise with ion implantation and lift-off is the existence of "pinning points". These are points which extend laterally in the damage layer where the subsequent electrochemical etching does not occur. This can lead the layer to be "pinned" to the implanted substrate at the damage layer and therefore prevents successful lift-off of an intact layer. This may occur owing to the presence of dislocation bundles and/or sub-surface damage in the substrate It is therefore beneficial to select a substrate with a low-dislocation content and carefully prepare the surface on which subsequent growth will occur in order to reduce sub-surface and improve the chances of successful lift-off.

Diamond is known to be difficult to mechanically process owing to its hardness, and problems of mechanical processing to remove the substrate or for other purposes are exacerbated for very thin (sub-micron) diamond layers. The problems become even more difficult for large area diamond layers, as diamond can be brittle. The lift-off technique described above allows a large area, thin layer to be removed from the substrate on which it was grown, which would otherwise be challenging using standard mechanical processing techniques.

In an alternative method, the layer can be retained on the substrate such that a single crystal plate is provided with a smooth, low damage, as-grown, high purity single crystal CVD diamond front face and a rear face formed of the single crystal diamond substrate.

Once the diamond product has been obtained, there are several ways to provide spin defects in the layer; two exemplary ways are by ion implantation and by introducing a spin defect precursor element in the process gases when growing the layer.

In a first exemplary embodiment, ion implantation is used. Figure 2 is a flow diagram illustrating exemplary steps. The following numbering corresponds to that of Figure 2: 55. Spin defect precursor elements (such as nitrogen in the case that the spin defects are NV-centres) are implanted into the layer using ion implantation. During growth of the layer, and prior to ion implantation, care is taken to eliminate impurities as far as possible from the process gases used to grow the layer. This ensures that the likelihood of uncontrolled formation of spin defects is minimized. In this case, prior to ion implantation, the layer has a single substitutional nitrogen concentration of no more than 300 ppb, and preferably selected from any of no more than 200 ppb, no more than 100 ppb, no more than 80 ppb, no more than 60 ppb, no more than 40 ppb, no more than 20 ppb, no more than 10 ppb, no more than 5 ppb, and no more than 1 ppb, and no more than 0.1 ppb. The layer is grown on the substrate in a similar manner to that described, for example, in WO 01/096633. A high purity single crystal CVD diamond layer is desirable to provide a host material which has low background magnetic "noise" into which quantum spin defects can be introduced via ion implantation.

56. The layer is optionally irradiated to introduce vacancies into the diamond crystal lattice.

S7. The layer is annealed to cause migration of vacancy defects within the single crystal CVD diamond layer and formation of a spin defects in the layer from the implanted spin defect precursor and the vacancy defects. By way of example, it is known that nitrogen-vacancy defects form at around 800°C. As such, the annealing comprises an annealing step at a temperature in a range 700 to 900°C for at least 2 hours, 4 hours, 6 hours, or 8 hours. It has also been suggested that treatment at a higher temperature can be advantageous for removing various paramagnetic defects to increase the decoherence time of spin defects. Accordingly, the annealing may comprise a further annealing step at a temperature in a range 1150°C to 1550°C for at least 2 hours, 4 hours, 6 hours, or 8 hours. For example, the further annealing step may be performed at a temperature of at least 1200°C, 1300°C, or 1350°C and/or a temperature of no more than 1500°C, 1450°C, or 1400°C. In addition, prior to the aforementioned annealing steps, an initial annealing step may be performed at a temperature in a range 350 to 450°C for at least 2 hours, 4 hours, 6 hours, or 8 hours.

Such a multi-stage annealing process has been found to significantly improve the spin coherence time, emission line width, and spectral stability of spin defects within a diamond matrix which is already of high purity and relatively low strain. While not being bound by theory, the reasoning behind why this multi-stage annealing process is successful is as follows.

Annealing diamond material changes the nature and distribution of defects within the diamond lattice. For example, multi-atom defects can be split, defects can become mobile and move through the diamond lattice, and defects can combine to form new defect types. The temperature at which these different processes occur varies and will also be dependent on the type of impurities within the diamond lattice.

It should be noted that high annealing temperatures can lead to graphitisation of the diamond material. Accordingly, high temperature annealing steps may be performed under an inert atmosphere and/or under diamond stabilizing pressure to prevent graphitisation. Furthermore, after performing an annealing process the synthetic diamond material may be acid cleaned and then oxygen plasma ashed or annealed in oxygen to remove any residual graphite.

An advantage of ion implantation of spin defect precursors is that spin defects can be accurately located in predetermined areas of the layer. They can therefore be positioned where they are needed depending on the application.

As an alternative to ion implantation, spin defect precursors may be introduced in a controlled manner in the process gases when growing the layer, in a manner described in WO 2010/149775. Optional irradiation and subsequent annealing as described above may be carried out in order to form spin defects. It will be appreciated that other types of spin defect may be introduced into the layer by introducing different elements to the process gases.

Example 1

A CVD single crystal substrate was prepared with a mis-cut angle of 1° from a {100} plane on the surface to be grown on. The surface was scaife polished along the hard direction. The substrate was placed in a CVD reactor and subsurface damage was removed using an argon chlorine plasma etch. Subsequent growth of the layer was carried out in using a low methane gas mixture, comprising 1.1 vol% methane, argon and hydrogen at a nominal temperature of 940°C. The resultant layer was 400 nm in thickness with a surface roughness Ra 01 0.1 nm.

The layer was then ion implanted with nitrogen and subsequently electron irradiated and annealed at 800°C to form NV-centres.

In order to further improve the surface of the layer, the material was annealed at 1800°C and 5 GPa for 20 minutes in a packing material formed from caesium chloride (CsCI).

The choice of CsCI as a packing material is an important one as it enables very high temperatures to be used without causing significant damage to the diamond surface by surface etching. CsCI is not molten at these temperatures and so it prevents surface graphitisation. Furthermore, CsCI is pressure sensitive so allows annealing temperatures conditions in excess of -2000°C by increasing pressure (>7 GPa) Ion implantation, particularly at high energies, can cause damage to the diamond, which can have a deleterious effect on the properties of spin defects such as the decoherence time. It is known, for example from Gbrlitz et. al., "Spectroscopic investigations of negatively charged fin-vacancy centres in diamond", New J. Phys. 22 (2020) 013048 that annealing after ion implantation has an effect on the room temperature photoluminescence spectra of the diamond material. Spin defects will display slightly different emission due to strain and damage in the lattice. At low pressure, low temperature (e.g. below 1500°C) annealing the intrinsic emission lines are obscured by multiple emission lines, which are mainly caused by energy trap states and inhomogeneous distribution of damage. In contrast, high pressure high temperature annealing under diamond stabilising pressures and temperatures in a range of 1800°C to 2400°C can repair some of the damage to the lattice caused by the ion implantation, which in turn removes many of the non-intrinsic emission lines.

After ion implantation, irradiation and annealing to form NV-centres in the layer, and a further annealing step in CsCI as described above, it has been noted that the largest photoluminescence intrinsic peak of an NV-centre in the layer is at least 50 times the largest photoluminescence non-intrinsic peak within 10 nm or the wavelength of the largest photoluminescence intrinsic peak.

Example 2

Figure 3 is a flow diagram showing exemplary manufacturing steps. In this example, a substrate (the substrate) is ion implanted with a damage layer to allow subsequent electrochemical etching to remove the substrate from the overgrown single crystal diamond layer. The following numbering corresponds to that of Figure 3: S8. A substrate is provided that has a mis-cut of between 0.5° and 5° from a major crystallographic plane and a major face having a surface area of at least 4 mm2. The major surface is polished and etched as described above to form a low damage major surface.

S9. The substrate is implanted (for example using helium ions) just below the major face to form a precursor damage layer that will graphitise on subsequent annealing. 30 S10. The substrate is located in a CVD reactor and process gases are fed into the reactor. As described above, the process gases contain no more than 2 vol.% of a carbon-containing gas (in this example, methane). The combination of a low concentration of methane and the mis-cut angle reduces hillock growth and leads to a final surface with a low roughness. Nitrogen is removed from the process gases as far as possible to ensure that subsequent processing does not form spin defects in an uncontrolled manner.

S11. A spin defect precursor element is ion implanted into the surface of the overgrown single crystal diamond layer. Note this this step could be carried out at the end of the process, in some cases with a subsequent annealing to convert the spin defect precursor element into a spin defect. Furthermore, surface processing may be carried out at this step.

S12. The diamond is annealed in CsCI to convert the precursor damage layer into a damage layer. Note that there may be two annealing steps; a first step is performed to convert the implanted spin defect precursors into spin defects, as described above, and a second HPHT annealing step reduces residual damage from the ion implantation process that can lead to spectral instability of the spin defects and additional parasitic spectral lines.

513. The diamond is then electrochemically etched to remove substrate from the overgrown diamond at the damage layer. This allows a very thin layer to be formed, of the order of less than 1 pm, less than 500 nm, less than 200 nm, 150 nm, less than 100 nm, less than 50 nm, less than 30 nm, and less than 10 nm.

As described above, if the overgrown diamond layer has not yet been ion implanted in step S11, if may now be subsequently implanted, although with such a thin layer this introduces handling difficulties.

As mentioned above, in order to achieve a substrate surface with low roughness and low levels of subsurface damage, the substrate is polished along the hard direction. This leaves a surface as shown in Figure 4, with a surface roughness Ra of less than 0.2 nm over an area of 2 pm x 2 pm, although the occasional shallow scratch mark is observed.

ICP argon chlorine etching was then carried out on the surface to remove subsurface damage.

A series of substrates were prepared with different mis-cut angles from the {100} planes. As described above, the combination of the mis-cut angle and low methane encourage step flow growth rather than the formation of 'hillocks'. Step flow growth is desirable because it leads to an as-grown surface of the diamond layer that is much smoother than it would otherwise be. Figure 5 shows the measured hillock density against substrate mis-cut angle grown using a methane concentration of less than 1.5 vol %. It can be seen that mis-cut angles of between 0.5 and 1.1° significantly reduce the formation of hillocks on the surface of the grown diamond layer.

Using a mis-cut angle of around 1% with a carefully prepared substrate surface, and growing the diamond layer with a methane concentration of around 1 vol % has been shown to provide an as-grown surface roughness Ra of the diamond layer or around 0.1 nm.

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 (24)

1. CLAIMS: 1. A freestanding CVD single crystal diamond, the single crystal diamond having: a thickness of less than 1 pm; a major face having a surface area of at least 4 mm2 and an angle of between 0.2°and 5° from a major crystallographic plane; and the major face having a surface roughness of no more than 5 nm Ra.
2. 2. The freestanding CVD single crystal diamond according to claim 1, wherein the major face has a surface roughness Ra selected from any of no more than 1 nm, and no more than 0.5 nm.
3. 3. The freestanding CVD single crystal diamond according to any one of claims 1 or 2, wherein the thickness is selected from any of less than 500 nm, less than 200 nm, less than 100 nm, less than 50 nm, less than 30 nm, and less than 10 nm.
4. 4. The freestanding CVD single crystal diamond according to any one of claims 1 to 3, wherein the major face has a surface area selected from any of at least 10 mm2, at least 20 mm2, at least 50 mm2 and at least 100 mm2
5. 5. The freestanding CVD single crystal diamond according to any one of claims 1 to 6, wherein the angle from the major crystallographic plane is selected from any of between 0.2° and 3°, 0.2 and 2°, and 0.2 and 1.50.
6. 6. The freestanding CVD single crystal diamond according to any one of claims 1 to 5, wherein the major crystallographic plane is selected from any of a {100} crystallographic plane, a {110} crystallographic plane, a {113} crystallographic plane, and a {111} crystallographic plane.
7. 7. The freestanding CVD single crystal diamond according to any one of claims 1 to 6, wherein the CVD single crystal diamond has a single substitutional nitrogen concentration selected from any of no more than 300 ppb, no more than 200 ppb, no more than 100 ppb, no more than 80 ppb, no more than 60 ppb, no more than 40 ppb, no more than 20 ppb, no more than 10 ppb, no more than 5 ppb, no more than 1 ppb, and no more than 0.1 ppb
8. 8. The freestanding CVD single crystal diamond according to any one of claims 1 to 7, wherein a variation of thickness between opposite edges of the major face is selected from any of no more than 50 nm, no more than 20 nm, no more than 10 nm and no more than 5 nm.
9. 9. The freestanding CVD single crystal diamond according to any one of claims 1 to 8, further comprising at least one spin defect, wherein the spin defect is selected from any of nitrogen-vacancy centres, silicon vacancy centres, germanium vacancy centres, nickel vacancy centres, nickel vacancy nitrogen centres and tin vacancy centres.
10. 10. The freestanding CVD single crystal diamond according to claim 9, wherein a largest photoluminescence intrinsic peak intensity of the spin defect is at least 50 times the largest photoluminescence non-intrinsic peak intensity measured at wavelengths within 10 nm of the of the wavelength of the intrinsic peak.
11. 11. The freestanding CVD single crystal diamond according to any one of claims 1 to 10, further comprising a layer of substrate material on which the CVD single crystal diamond was grown.
12. 12. A method of forming a CVD single crystal diamond, the method comprising: providing a single crystal diamond substrate, the substrate having a growth face with a mis-cut of between 0.2° and 5° from a major crystallographic plane, the growth face having an area of at least 4 mm2; locating the single crystal diamond substrate in a CVD reactor; feeding process gases into the reactor, the process gases comprising a carbon-containing gas and hydrogen, wherein the process gas contains no more than 2 vol.% of the carbon-containing gas; and homoepitaxially growing a single crystal diamond layer on the substrate, a portion of the layer having an as-grown surface roughness Ra of no more than 5 nm.
13. 13. The method according to claim 12, wherein the portion of the layer which has said surface roughness Ra has an area selected from any of at least 4 mm2, at least 10 mm2, and at least 100 mm2.
14. 14. The method according to any one of claims 12 or 13, wherein the single crystal diamond layer has a thickness selected from any of less than 200 nm, less than 150 nm, less than 100 nm, less than 50 nm, less than 30 nm, and less than 10 nm.
15. 15. The method according to any one of claims 12 to 14, wherein the mis-cut from the major crystallographic plane is selected from any of between 0.2°and 3°, between 0.2 and 2°, and between 0.2 and 1.5°.
16. 16. The method according to any one of claims 13 to 16, wherein the major crystallographic plane is selected from any of a {100} crystallographic plane, a {110} crystallographic plane, a {113} crystallographic plane, and a {111} crystallographic plane.
17. 17. The method according to any one of claims 12 to 16, further comprising polishing a surface of the substrate prior to growing the layer on the surface, the polishing being effected along the hard direction.
18. 18. The method according to claim 17, further comprising removing subsurface damage from the surface prior to growing the layer.
19. 19. The method according to claim 18, wherein the subsurface damage is removed by any of argon chlorine etching, oxygen inductively coupled plasma etching, and chemical mechanical polishing, CMP.
20. 20. The method according to any one of claims 12 to 19, further comprising implanting a spin defect precursor element into a surface of the layer.
21. 21. The method according to any one of claims 12 to 19, wherein the process gases further comprise a spin defect precursor element.
22. 22. The method according to claim 21 or claim 22, further comprising annealing the CVD single crystal diamond product to cause formation of the spin defect.
23. 23. The method according to any one of claims 12 to 22, further comprising annealing the CVD single crystal diamond product in caesium salt at a temperature greater than 1750°C and a pressure greater than 4 GPa.
24. 24. A freestanding CVD single crystal diamond product having a thickness of less than 1 pm and a major face having a surface roughness of no more than 5 nm Ra, wherein a largest photoluminescence intrinsic peak intensity of a spin defect in the layer is at least 50 times the largest photoluminescence non-intrinsic peak intensity measured at wavelengths within 10 nm of the of the wavelength of the intrinsic peak.