NL2038302B1 - Method for annealing a diamond substrate that comprises a colour centre - Google Patents

Abstract

The present disclosure relates to a method for annealing a diamond substrate that comprises a colour centre which is preferably configured to be used in quantum computing, quantum communications and/or quantum sensing. The method comprises: - providing a diamond substrate that comprises a first surface; - arranging a protective cover, for instance a protective slab or a thin film, on the first surface of the diamond substrate; and - annealing the diamond substrate While the protective cover is arranged on the first surface.

Description

METHOD FOR ANNEALING A DIAMOND SUBSTRATE THAT COMPRISES A COLOUR

CENTRE

The present disclosure relates to a method for annealing a diamond substrate that comprises a colour centre which is preferably configured to be used in quantum computing, quantum communications and/or quantum sensing. The present disclosure further relates to a diamond substrate obtained by the method and a quantum system comprising the diamond substrate.

The process of diamond nanofabrication regarding the systematic and reliable generation of colour centres, wherein the colour centres are not native impurities in diamond, relies on the incorporation of foreign atomic species of atoms in the diamond. Efforts towards the incorporation of non-native impurities in diamond are based on ion implantation. The known method of ion implantation comprises the acceleration of the ion species at high acceleration voltages towards a diamond substrate surface, with subsequent implantation at a depth with respect to the surface that is dictated by the energy of the accelerated ions. In case one implants a tin (Sn) ion with this high energy implantation method, a typical acceleration voltage that is used may be 350 keV, with an implanted depth predicted by Stopping Range of Ton in Matter (SRIM) simulations of around 88 nanometres. Such high implantation depth is required for optimal integration and coupling of such colour centres to nanophotonic structures that may be fabricated in, at, or on the diamond substrate.

Itis noted that the acceleration voltage may depend on the ion that is implanted and the desired implanted depth.

Following the implantation process, the defect activation must follow via a high temperature thermal annealing step. The thermal annealing step purpose is two-fold. The first purpose is that the crystal damage due to the implantation step is partially healed, thus reducing the possibility of crystal defects in the near vicinity of the implanted ions. The second purpose is that at high temperature the vacancies diffuse in the diamond crystal. Specifically, the annealing step in current state of the art methods occurs at a temperature of 1100 degrees Celsius, with reported higher temperatures of 1200 degrees Celsius, over a time span of several hours (from a minimum of 2 hours to 10 hours).

However, in conditions of high temperature and low pressure, the diamond crystal is in a metastable solid phase, with the result that the surface on the diamond substrate graphite may be formed. As the presence of graphite on the outer surface of the diamond substrate is detrimental to the spectral properties of the diamond substrate, the graphite has to be removed. In general two techniques are used to remove the graphite, namely wet etching and dry reactive ion etching. Wet etching generally comprises applying inorganic cleaning fluid on the diamond surface, wherein the most common methods are based on a mixture of nitric, perchloric and sulfuric acids, also known as a triacid solution. Typically, these acids have a high temperature when applied on the diamond substrate. Reactive ion etching generally comprises the application of ions of a plasma to the diamond surface.

The thickness of the superficial amorphized layer of diamond material, namely the graphite, is suspected to be dependent on the annealing process parameters, such as temperature and temperature increase per minute, as well as vacuum pressure and duration of the process. This in turn represents a challenge in the predictability of the thickness of the amorphized layer.

Therefore, making use of the graphite removal methods described above translates into an unpredictable amount of removed graphite. This subsequently causes an unpredictable implanted depth of the colour centres. After cleaning of the graphitized layer, either via wet or dry cleaning methods, the graphitized layer, which can be (tens of) nanometres thick, is removed, thus effectively reducing the depth of the implanted ions. The location of the colour centres may therefore be different than the optimal depth position in the nanophotonic structures.

In the case of nanophotonic structures, it is expected that the coupling of the emitted light from the colour centres to the guiding modes of the structures is less optimal, with the consequence that the overall emitted light (both zero phonon line and phonon side band) is less.

In the case of devices for sensing applications, the non-optimal position of the colour centre with respect to the designed one will cause a lower performance of the devices.

In general the non-optimal depth reduces the coupling of the emitted light to the designed devices. Besides, in case of quantum sensing for devices the annealing of the substrate might extend up to the shallow (typically) implanted depth of the colour centres, therefore compromising the creation of the colour centre to start with.

To summarize, with the techniques described above the desired depth of the colour centre is not obtained, which has the consequence that the designed characteristics of the colour centre are not achieved when the device comprising the diamond substrate is in use.

It is an object for the present disclosure to obviate or at least reduce the aforementioned problem. In particular, it may be an object of the present disclosure to prevent the graphitization of the surface of the diamond substrate during annealing.

This object is achieved by a method for annealing a diamond substrate that comprises a colour centre which is preferably configured to be used in quantum computing, quantum communications and/or quantum sensing, comprising: - providing a diamond substrate that comprises a first surface; ~ arranging a protective cover, for instance a protective slab or a thin film, on the first surface of the diamond substrate; and - annealing the diamond substrate while the protective cover is arranged on the first surface.

An advantage of the present disclosure is that the protective cover, which is arranged on the first surface of the diamond substrate during annealing, prevents the graphitization of the first surface. Wet inorganic cleaning, such as triacid cleaning, or dry etching to remove the graphite after annealing is thus not necessary anymore if the method according to the present disclosure is used

A further advantage of preventing the graphitization of the diamond substrate during annealing is that the depth of the colour centre with respect to the first surface does not change.

This ensures that the distance between the colour centre and the first surface stays optimal, in other words, remains as per the designed distance. This gives a higher coupling factor of the colour centre to e.g. nanophotonic waveguides. This increases efficient collection of total emitted light from the colour centre.

Another advantage is that the prevention of surface modification of diamond substrate, thus providing an absence of a graphite layer on the diamond substrate, provides for a diamond substrate with the desired surface termination. This in turn gives a desired spectral stability and provides an absence of diffusion.

An even further advantage is that with the method substantially identical diamond substrates can be manufactured, as the diamond substrates are not influenced by a graphite layer which growth is in general irregular.

Another advantage of the method is that the annealing of the diamond substrate can also be performed post-fabrication of nanophotonic structures. In the method according to the prior art when triacid cleaning is used, due to the high temperature of the boiling triacid solution, high mechanical stress is applied on the nanophotonic structures. This gives the risk of breaking of the nanophotonic structures arranged in, at, or on the diamond substrate during the triacid cleaning step. The protective cover prevents graphitization, thereby obviating the need for triacid cleaning, and therefore a priori removing the risk of breaking of the structures.

Additionally, the present method can also be performed on thin film diamond membranes yielding full graphite free thin film membranes. The thin film is optionally covered on both sides.

The method according to the present disclosure can be performed on diamond substrates regardless of the thickness of the diamond substrate. For example, the method can be applied on diamond substrates having a thickness of 0.1 nanometres to 1000 micrometers. Therefore ‚the method can be applied to thin film substrates, micrometer substrates and bulk substrates (typically being 500 micrometres thick).

The step of annealing the diamond is preferably performed by positioning the diamond substrate in an annealing chamber and raising the temperature of the annealing chamber.

The diamond substrate which is obtained by the method can be used in quantum computing, wherein the colour centres can be used as qubits in a quantum computer or quantum network. The diamond substrate obtained by the method may also be used for quantum sensing, for example for sensing magnetic fields, or for nanoscale imaging.

The first surface may be the surface of interest, meaning that the first surface may be the surface on which the nanophotonic structures are present or will be manufactured afterwards.

The method according to the present disclosure can be applied to any diamond substrate, independent on how the colour centre in the diamond substrate is obtained. In particular, the method can be applied regardless of the acceleration voltage used to implant the ion in the diamond substrate.

In an embodiment the protective cover has a surface roughness that is comparable to the surface roughness of the diamond substrate.

An advantage of the surface roughness of the protective cover being comparable to the surface roughness of the diamond substrate is that the Van der Waals forces between the protective cover and the first surface minimizes the physical distance between the cover and the diamond substrate. This minimal physical distance prevents the diffusion of residual gasses from the vacuum annealing chamber towards the first surface. It is believed that the residual gasses cause the graphitization of the first surface. In this way, the comparable surface roughness prevents the graphitization of the first surface. In other words, the comparable surface roughness between the substrate and the cover translates into a better contact between the substrate and the cover, thereby leaving less volume whereto the residual gases can diffuse between the substrate and the cover. As aresult, substantially complete physical shielding from the residual oxygen atoms, which are suspected to induce graphitization, is achieved.

The surface roughness of the protective cover and/or the first surface can be determined by determining the roughness average (Ra) and/or the root mean square roughness average (Rg), which are profile measurements of the surface roughness. In the context of the present disclosure, better parameters for the surface roughness are the area surface roughness parameters. Therefore, more suitable parameters to quantify the roughness of interest are Sa (extension of Ra to an area) and Sq (extension of Rq to an area). Sa and Sq are described in 1SO-standard ISO 25178. Other possible relevant parameters that can be used to characterize the surface roughness are Sz (maximum height), Sp (maximum peak height), and Sv (maximum pit depth).

In an embodiment the surface roughness of the protective cover and the surface roughness of the first surface of the diamond substrate differ by at most 50%, preferably differ by at most 30%, and most preferably differ by at most 10%.

The difference in surface roughness may for example be determined by the Sq (root mean square average of profile area deviations from the mean line) parameter. Alternatively or additionally, the parameters Sa, Rq, or Ra may be used to determine the percentage difference of the surface roughness.

In an embodiment the protective cover comprises diamond.

The protective cover optionally is substantially made of diamond, i.e. the protective cover being a diamond cover. Diamond has the advantage that it can easily acquire the same surface roughness as the diamond substrate, thereby reducing the chance of graphitization. 5 The protective cover may alternatively or additionally comprise molybdenum, silicon nitride (SiNy), aluminium oxide (AlQy) diamond-like-carbon (DLC), silicon dioxide (S102), hafnium oxide (HFO,), silicon oxynitride (SiON), aluminum nitride (AIN), silicon carbide (SiC) and/or barium titanate (BTO).

DLC may be deposited via plasma enhancedchemical vapour deposition (PECVD). Due to

DLC being highly chemically stable, a dry etching method should be used to clean the DLC after annealing. A suitable dry etching method may be dry ashing, as disclosed in Li, Z., Wang, R.N.,

Lihachev, G. er al. “High density lithium niobate photonic integrated circuits.” Nat Commun 14, 4856 (2023).

Silicon nitride (SiN) can be deposited via chemical vapour deposition (CVD) and classes thereof, such as APCVD (atmospheric pressure), PECVD (plasma enhanced), micro-PCVD (micro pressure CVD), LPCVD (low pressure). Further possible deposition methods are atomic layer deposition (ALD), sputtering, and/or evaporation. Aluminium oxide {AlO,) may be deposited via

CVD, physical vapour deposition (PVD), ALD, sputtering, high power impulse magnetron sputtering (HiPIMS), and/or pulsed laser deposition (PLD). Silicon dioxide (SiO) may be deposited by CVD, PECVD, LPCVD, sputtering and/or evaporation. Hafnium oxide (HfOx) may be deposited via ALD, evaporation and/or sputtering. Silicon oxynitride (SixOyN,) may be deposited via CVD, PECVD, LPCVD, sputtering and/or evaporation. Aluminum nitride (AIN) may be deposited via sputtering and/or CVD. Silicon carbide (SiC) may be deposited via CVD and/or

PVD. Barium titanate (BTO) may be deposited via plasma enhanced atomic layer deposition (PEALD), pulsed laser deposition and/or CVD. The materials can be cleaned or removed from the diamond substrate via wet cleaning, for example with (strong) inorganic acid solutions (e.g. HF), or dry etching.

In an embodiment the protective cover is a thin film, preferably comprising silicon nitride, aluminium oxide, DLC, silicon dioxide (8102), hafnium oxide (HfO,), silicon oxynitride (SixO;N;), aluminum nitride (AIN), silicon carbide (SiC) and/or barium titanate (BTO).

An advantage of the thin film is that the film can be easily deposited conformally in all grooves of the diamond substrate. In this way, a substantially perfect covering of the diamond substrate can be achieved. All surfaces of the diamond substrate can be covered, regardless of their orientation. A possible way to apply the thin film is through atomic layer deposition, evaporation and/or chemical vapour deposition (CVD). Conformal deposition is especially guaranteed if these techniques are used.

In the case of post fabrication of nanophotonic structures, the thin film has the additional advantage of not needing any prior machining of a recess in the protective cover. In fact, deposition of silicon nitride or aluminum oxide by atomic layer deposition on post fabricated devices ensures conformal coating with the deposited materials, meaning that the devices are fully covered in contact on all facets. This ensures full coverage post fabrication, with consequent prevention of graphite formation. Removal of such deposited coverage is rather easy. Such materials can for example be removed post annealing process via selective wet inorganic acid cleaning, such as 40% HF (for both aluminum oxide and silicon nitride). In an embodiment the protective cover is in physical contact with the first surface during the step of annealing the diamond substrate.

Due to the physical contact of the protective cover with the first surface any diffusion of residual gas in the annealing chamber is prevented, thereby preventing residual gas from coming into contact with the first surface.

In an embodiment the protective cover comprises a protection surface that is configured to be arranged on or at the first surface of the diamond substrate.

The protection surface is the surface of the protective cover that is arranged or positioned on the first surface of the diamond substrate during the annealing.

In an embodiment the protection surface comprises a recess, further comprising: - arranging the recess over a nanophotonic structure that is arranged in, on, or at the first surface of the diamond substrate.

The recess of the protection surface can in the context of the present disclosure also be understood as a carve-out or pocket of the protection surface.

An advantage of the recess is that the recess can be arranged or positioned over any nanophotonic structures that are present in, at, or on the first surface of the diamond structure. The recess prevents any physical contact and/or interaction between the protection surface and the first surface and/or the nanophotonic structures at the position of the recess. This prevents any breaking of the nanophotonic structures.

The recess may have a height of 300 to 500 nanometres. In other words, the distance between the first surface and/or the nanophotonic structure to the ceiling of the recess may be 300 to 500 nanometres when the protective cover is arranged on the first surface of the diamond substrate. Suitable ranges for the height of the recess may be 50 — 2000 nanometers, preferably 100 — 1000 nanometres, and most preferably 300 — 500 nanometres.

The protection surface preferably fully encloses the recess, such that when the protective cover is arranged on the first surface there is physical contact between the protective surface and the first surface. The protection surface preferably encloses the recess as seen in a direction perpendicular to the protection surface.

In an embodiment the first surface of the diamond substrate and the protection surface of the protective cover are substantially planar surfaces.

An advantage of the first surface and the protection surface being planar surfaces is that there is a large area of physical contact, thereby reducing the chance of diffusion of residual gasses.

In an embodiment the protective cover partially covers the first surface of the diamond substrate.

In an embodiment the protective cover substantially fully covers the first surface of the diamond substrate.

In an embodiment the first surface of the diamond substrate comprises a photon emitting element that is configured to increase the number of photons being emitted from the diamond substrate.

The photon emitting elements can be micro-fabricated structures (which are on micron scale dimensions), such as solid immersion lenses. The solid immersion lenses are configured to minimize the internal reflection at the diamond surface. The photon emitting elements may also be nanophotonic structures (which are on nanometer scale dimensions), such as nanopillars or nanophotonic waveguiding elements such as waveguides or beam-splitters. The nanophotonic waveguiding elements do not modify the far field emission. The nanopillars are configured to modify the far-field emission. Another photon emitting element may be resonant cavity structures, which increase the emission via the effect of Purcell enhancement.

It is noted that the colour centre is the source of the photons. The photon emitting element in the context of the present invention must be understood as the element that increases the number of photons being emitted from the diamond substrate.

In an embodiment annealing the diamond substrate further comprises: - positioning the diamond substrate in an annealing chamber; and - raising the temperature of the annealing chamber to a first temperature range, the first temperature range preferably being in the range of 400 — 2000 degrees Celsius, more preferably being in the range of 1000 — 1300 degrees Celsius, and most preferably being in the range of 1100-1200 degrees Celsius.

The abovementioned temperatures provide a sufficient activation of the colour centres that are introduced in the diamond substrate.

In an embodiment the method further comprises: - raising, before raising the temperature of the annealing chamber to the first temperature range, the temperature of the annealing chamber to a second temperature range, wherein the second temperature range preferably is in the range of 200-1100 degrees Celsius, more preferably in the range of 400-800 degrees Celsius.

By raising the temperature to a second temperature range before raising it to the first temperature range the diamond substrate can effectively be annealed.

In an embodiment the method further comprises: - keeping the temperature of the annealing chamber at a first temperature for a predetermined time period, wherein the predetermined time period is 1-40 hours, preferably for a time span of 2-10 hours, most preferably for a time span of 4-8 hours.

The first temperature may be any temperature inside the ranges of the first temperature range and/or the second temperature range.

In an embodiment the method further comprises: - gradually raising the temperature of the annealing chamber to the first temperature, the first temperature range, and/or the second temperature range during a time span of 1- 40 hours, preferably for a time span of 2-10 hours, most preferably for a time span of 4-8 hours.

In an example embodiment, the annealing step may comprise the following steps: - gradually raising the temperature of the annealing chamber to 400 degrees Celsius during a time span of 4 hours; - keeping the annealing chamber at 400 degrees Celsius for 8 hours; - gradually raising the temperature of the annealing chamber to 800 degrees Celsius during a time span of 12 hours; - keeping the annealing chamber at 800 degrees Celsius for 8 hours; - gradually raising the temperature of the annealing chamber to 1100 degrees Celsius during a time span of 12 hours; and - keeping the annealing chamber at 1100 degrees Celsius for 4 hours.

It is clear for the skilled person that any other temperature (range) and/or time span which fall into the temperature (range) and/or time span mentioned in the present disclosure can be combined into a specific embodiment, and fall under the scope of the present disclosure.

In an embodiment the method further comprises: - adding a gas, for example argon, to the annealing chamber before or during the step of annealing the diamond substrate.

By adding a gas to the annealing chamber any residual gasses being present in the annealing chamber are flushed out. This further prevents any graphitization of the diamond substrate from occurring.

In an embodiment the method further comprises: - providing nanophotonic structures in, on, or at the diamond substrate.

The nanophotonic structures may be nanophotonic waveguide, beam-splitters and/or photonic crystal cavities. Additionally or alternatively, the nanophotonic structures may comprise all other possible nanoelectromechanical systems (NEMS) and/or microelectromechanical systems (MEMS).

In an embodiment the step of providing the nanophotonic structures is performed after the step of annealing the diamond substrate.

In an embodiment the step of providing the nanophotonic structures is performed before the step of annealing the diamond substrate.

In an embodiment the method further comprises: - removing the protective cover from the first surface after the step of annealing has been completed.

Optionally, the protective cover can be re-used for use in a further annealing step of a different diamond substrate. Optionally, the diamond cover is cleaned by boiling in triacid to remove any graphite on its outer surface. An inorganic cleaning, e.g. triacid, or HNO3, or HF 40%, or Piranha {mixture of sulfuric acid and hydrogen peroxide), or a combination of thereof is beneficial for removal of organic contamination from handling therefore reducing the risk of debris on the contact surface. In principle the protection surface will stay free of graphite as the protection surface is in physical contact with the first surface during annealing.

In an embodiment the method further comprises: - activating the colour centre of the diamond substrate due to performing the annealing of the diamond substrate.

In an embodiment the method further comprises: - arranging a second protective cover, for instance a protective slab or a thin film, on the second surface of the diamond substrate.

In this way, both the first surface and the second surface can be covered, such that on both surfaces graphitization is prevented. This in particular advantageous when through both the first surface and the second surface an ion has been implanted to create a colour centre.

The present disclosure further relates to a diamond substrate obtained by the method according to any one of the foregoing embodiments.

The diamond substrate can be distinguished from a diamond substrate that is obtained by a method according to the prior art by inspecting the surface of the diamond substrate with topography analysis or chemical analysis, for example an atomic force microscopy, Raman spectroscopy or X-ray photoelectron spectroscopy (XPS) analysis. In an example, starting with two diamond substrates with comparable surface roughness, the area root mean square area roughness (Sq) of a diamond substrate according to the present disclosure may be 700 picometres, while the area root mean square area roughness (Sq) of a diamond substrate according to a method that uses triacid cleaning may be 1,5 nanometres.

Additionally, in an experiment a diamond substrate was partially covered, thereby dividing the diamond substrate in a covered substrate part and an uncovered substrate part. At this stage, the surface roughness of the covered substrate part and the uncovered substrate part are identical (as they are the same substrate). Then, the diamond substrate was annealed. After annealing, Raman spectroscopy was performed on both the covered substrate part and uncovered substrate part. The

Raman spectrum of the covered substrate part showed no graphite peak, while the Raman spectrum of the uncovered substrate part clearly showed a graphite peak which indicates that graphite was formed on the uncovered substrate part.

Afterwards, diacid cleaning (mixture of perchloric acid and sulfuric acid, at high temperature) was performed on the diamond substrate to remove the graphite that was formed on the uncovered substrate part. Measurements with an atomic force microscope showed that the root mean square area roughness (Sq) of the covered part was 685.5 picometres, while the root mean square area roughness (Sq) of the uncovered part ranged from 1.172 nanometres to 1.543 nanometres. This shows that the surface roughness of a diamond substrate annealed according to the prior art is roughly at least twice the surface roughness of the diamond substrate annealed according to the present disclosure. These measurements clearly indicate a change in the surface properties of the diamond sample upon annealing, namely that the uncovered region undergoes graphitization, which, upon diacid cleaning removal of it, it leaves the surface roughness of the diamond surface modified i.e. increased.

The present disclosure further relates to a quantum system comprising a diamond substrate according to the aforementioned embodiment.

Further advantages, features and details are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, wherein: - figure 1A depicts a first example of a diamond substrate; - figure 1B depicts a first example of a diamond substrate with a partial protective cover; - figure 1C depicts a first example of a diamond substrate after annealing; - figure 2A depicts a second example of a diamond substrate; - figure 2B depicts a second example of a diamond substrate with a protective cover; - figure 2C depicts a second example of a diamond substrate after annealing; - figure 3 depicts a third example of a diamond substrate with a nanophotonic structure; - figure 4 depicts a side view of a cover with a recess arranged on the third example of the diamond substrate; - figure 5 depicts Raman spectra of the diamond substrate; - figure 6 depicts a first example of a method according to the present disclosure; and - figure 7 depicts a second example of a method according to the present disclosure.

Diamond substrate 2 (figure 1A) has a rectangular cuboid shape. Diamond substrate 2 comprises first surface 4, which can be characterized as the upper surface. First surface 4 is the surface of interest as the colour centre is positioned in close vicinity to first surface 4. Therefore, first surface 4 is also the surface on which the nanophotonic structures are present or will be manufactured afterwards. Diamond substrate 2 further comprises first side surface 6, second side surface 8, third side surface 10, and fourth side surface 12. First side surface 6 is positioned opposite of third side surface 10 and second side surface 8 is positioned opposite of fourth side surface 12. Second surface 14 is positioned opposite of first surface 4. Inside diamond substrate 2 colour centre 16 is arranged. In this illustrated embodiment, colour centre 16 is positioned substantially in the middle with respect to the side surfaces 6, 8, 10 and 12. Normally, colour centre 16 is positioned at the desired position for the working of the devices. With respect to first surface 4 colour centre 16 is positioned at a distance of 88 nanometres. In the illustrated embodiment the colour centre comprises an Sn colour centre which was created by introducing a

Sn ion through first surface 4 of diamond substrate 2. As the introduction of the Sn ion caused defects in the crystal structure of diamond substrate 2, and to activate the colour centre, thermal annealing of diamond substrate 2 is necessary.

It is clear for the skilled person that the distance of colour centre 16 with respect to first surface 4 depends on the specific colour centre. The present disclosure is not limited to any specific colour centre, and can be generalised to any Group IV colour centre. The depth of colour centre 16 is dictated by the design of the nanophotonic structures to be then fabricated around them, as a trade-off between best coupling to these and the least possible implantation damage in the crystal.

Cover 18 (figure 1B) is arranged on first surface 4 of diamond substrate 2. In the illustrated embodiment, cover 18 is made of diamond. Cover 18 comprises protection surface 20 which is arranged on first surface 4 of diamond substrate 2. Protection surface 20 has a surface roughness which is comparable to the surface roughness of first surface 4. The area root mean square (Sg, as defined by the ISO 25178 standard) of protection surface 20 is in the illustrated embodiment 10% less than the area root mean square of first surface 4. The comparable surface roughness ensures that the Van der Waals forces between protection surface 20 and first surface 4 prevent the diffusion of residual gasses towards the area of first surface 4 which is covered by cover 18. Cover 18 does not fully cover first surface 4. Non-covered surface 22 is the part of first surface 4 which is not covered by cover 18.

After first surface 4 of diamond substrate 2 is partially covered by cover 18, diamond substrate 2 can be annealed. The annealing can be performed in an annealing chamber. In the annealing chamber a vacuum may be created to remove as much as possible residual gasses, as the residual gasses can cause the graphitization process of the diamond surface. The annealing chamber is brought to a temperature in which diamond substrate 2 will anneal. For example, the annealing chamber may be brought to a temperature of 1100 degrees Celsius for a time period of 8 hours.

After annealing, cover 18 can be removed from diamond substrate 2 (figure 1C). Covered first surface 24 is the part of first surface 4 which was covered by cover 8 during the annealing.

Covered first surface 24 does not comprise any graphite, in other words, is free of any graphite.

This has been ensured by the protection of cover 18 that prevented any residual gasses from coming into contact with covered first surface 24, Non-covered first surface 22 of first surface 4 has been graphitized. In other words, non-covered first surface 22 comprises graphite. It may be the case that on border area 26, between non-covered first surface 22 and covered first surface 24, there is a small region of covered first surface 24 that comprises a little bit of graphite. However, covered first surface 24 is substantially completely free of graphite.

Diamond substrate 102 (figure 2A) has a similar design as diamond substrate 2 that is illustrated in figure 1A. Diamond substrate 102 also comprises first surface 104 and oppositely positioned second surface 114. Diamond substrate 102 further comprises first side surface 106, second side surface 108, third side surface 110, and fourth side surface 112. First side surface 106 is positioned opposite of third side surface 110 and second side surface 108 is positioned opposite of fourth side surface 112. Inside diamond substrate 102 colour centre 116 is arranged.

Cover 118 (figure 2B) covers substantially the whole of first surface 104. This means that the whole area of first surface 104 is in physical contact with protection surface 120 of cover 118.

This prevents any graphitization on first surface 104. When cover 118 is positioned on first surface 4, the annealing step can be performed.

After annealing, cover 118 is removed and covered first surface 124 is free of graphite. In this illustrated embodiment, covered first surface 124 and first surface 4 coincide. Diamond substrate 102 can be used for applications in quantum computing, for example as a qubit in a quantum communications.

Although not illustrated, colour centres 16 and 116 in diamond substrate 2 and 102 respectively are still present in figures 1C and 2C respectively.

Diamond substrate 202 (figure 3) comprises first surface 204. In first surface 204 photonic nanostructure 228 is etched. Diamond substrate 202 needs to be annealed while photonic nanostructure 228 is already present on first surface 204.

Cover 218 (figure 4) comprises protection surface 220. Protection surface 220 is provided with recess 230. Recess 230 is arranged over photonic nanostructure 228. Due to recess 230, there is space 232 between recess 230 and photonic nanostructure 228. In this way, any physical contact between protection surface 220 of cover 218 and photonic nanostructure 228 is prevented. The skilled person understands that recess 230 can also be arranged over other structures that are positioned in, on, or at first surface 204. For example, recess 230 can be arranged over micro- fabricated, such as solid immersion lenses, or nanophotonic structures, such as nanopillars. This is advantageous, as physical contact between protection surface 220 and photonic nanostructure 228 could damage photonic nanostructure 228. When diamond substrate 202 is covered by cover 218, diamond substrate 202 can be annealed.

It is clear for the skilled person that the method according to the present disclosure can be performed on diamond substrates 2, 102, 202 regardless of the thickness of diamond substrate 2, 102, 202. For example, the method can be applied on diamond substrates having a thickness of 0.1 nanometres to 1000 micrometers. Therefore ‚the method can be applied to thin film substrates, micrometer substrates and bulk substrates (typically being 500 micrometres thick).

Figure 5 depicts the Raman spectra of the regions A and B as illustrated in figure 1C. The diagram shows the Raman shift on the x-axis and the counts on the y-axis. Region A is positioned on non-covered first surface 22 and region B is positioned on covered first surface 24. Line 334 corresponds to the Raman spectrum of region A and line 336 correspond to the Raman spectrum of region B. Line 334 shows that graphite is present in region A, as line 334 comprises peak 335 at 1600 cm’, which indicates the presence of graphite. Line 336 is flat at 1600 cmt, which indicates that region B is free of graphite. A reference for graphite peaks in Raman spectra can be found in “J.N, Rouzaud, A. Oberlin, C. Beny-Bassez, Carbon films: Structure and microtexture (optical and electron microscopy, Raman spectroscopy), Thin Solid Films, Volume 105, Issue 1, 1983, Pages 75-96”. The mechanism of the graphite formation upon annealing of diamond is described in

Khmelnitsky, R. A., & Gippius, A. A. (2013). Transformation of diamond to graphite under heat treatment at low pressure. Phase Transitions, 87(2), 175-192, wherein it is described that the basic cause of graphite formation seems to be the interaction of the diamond surface with oxygen.

In an embodiment according to the present disclosure first step 438 (figure 6) comprises providing diamond substrate 2 that comprises first surface 4. In second step 440 protective cover 18, for instance a protective slab, is arranged on first surface 4 of diamond substrate 2. In third step 442 diamond substrate 2 is annealed while protective cover 18 is arranged on first surface 4.

In an embodiment according to the present disclosure first step 544 (figure 7) comprises positioning diamond substrate 2 in an annealing chamber. The annealing chamber is brought into a vacuum of le-6 mbar, i.e. 1*10% mbar. In second step 546 the temperature in the annealing chamber is gradually raised to a first temperature. The first temperature of the annealing chamber can be 400 degrees Celsius. Step 546 may take 4 hours, meaning that the annealing chamber is gradually raised during a time span of 4 hours. In step 548 the annealing chamber is kept at the first temperature, which in the illustrated embodiment amounts to keeping the annealing chamber at 400 or 800 degrees Celsius. The annealing chamber can be kept at the first temperature for a predetermined time period. For example, the annealing chamber can be kept at the first temperature for a time period of 8 hours. In fourth step 550 the temperature in the annealing chamber is gradually raised to a second temperature. The second temperature of the annealing chamber can be 800 or 1100 degrees Celsius. Step 550 may take 12 hours, meaning that the annealing chamber is gradually raised during a time span of 12 hours. In step 552 the annealing chamber is kept at the second temperature, which in the illustrated embodiment amounts to keeping the annealing chamber at 800 or 1100 degrees Celsius. The annealing chamber can be kept at the second temperature for a predetermined time period. For example, the annealing chamber can be kept at the first temperature for a time period of 4, 8 or 12 hours.

The present disclosure is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following clauses within the scope of which many modifications can be envisaged.

Claims (25)

CONCLUSIONS 1. A method for annealing a diamond substrate comprising a colour centre preferably adapted to be used in quantum computing, quantum communication and/or quantum detection, comprising: - providing a diamond substrate comprising a first surface; - applying a protective covering, e.g. a protective plate or a thin film, to the first surface of the diamond substrate; and - annealing the diamond substrate while the protective covering is applied to the first surface. The method of claim 1, wherein the protective cover has a surface roughness comparable to the surface roughness of the diamond substrate. A method according to claim 2, wherein the surface roughness of the protective cover and the surface roughness of the first surface of the diamond substrate differ by at most 50%, preferably by at most 30%, and most preferably by at most 10%. A method according to any preceding claim, wherein the protective cover comprises diamond. 5. A method according to any preceding claim, wherein the protective covering is a thin film, preferably comprising silicon nitride, aluminium oxide, diamond-like carbon, silicon dioxide, hafnium oxide, silicon oxynitride, aluminium nitride, silicon carbide and/or barium titanate. 6. A method according to any preceding claim, wherein the protective cover is in physical contact with the first surface during the step of annealing the diamond substrate. 7. A method according to any preceding claim, wherein the protective cover comprises a protective surface adapted to be applied to or on the first surface of the diamond substrate. 8. The method of claim 7, wherein the protective surface comprises a recess, further comprising: - providing the recess over a nanophotonic structure disposed in, on or near the first surface of the diamond substrate. 9. A method according to claim 7 or 8, wherein the first surface of the diamond substrate and the protective surface of the protective cover are substantially planar surfaces. 10. A method according to any preceding claim, wherein the protective cover partially covers the first surface of the diamond substrate. A method according to any preceding claim, wherein the protective cover substantially completely covers the first surface of the diamond substrate. 12. A method according to any preceding claim, wherein the first surface of the diamond substrate comprises a photon emitting element configured to increase the number of photons emitted by the diamond substrate. A method according to any preceding claim, wherein annealing the diamond substrate further comprises: - positioning the diamond substrate in an annealing chamber; and - raising the temperature of the annealing chamber to a first temperature range, the first temperature range preferably being in the range of 400-2000 degrees Celsius, more preferably in the range of 1000-1300 degrees Celsius, and most preferably in the range of 1100-1200 degrees Celsius. 14. The method of claim 13, further comprising: - before raising the temperature of the annealing chamber to the first temperature range, raising the temperature of the annealing chamber to a second temperature range, wherein the second temperature range is preferably in the range of 200-1100 degrees Celsius, more preferably in the range of 400-800 degrees Celsius. A method according to claim 13 or 14, further comprising: - maintaining the temperature of the annealing chamber at a first temperature for a predetermined time period, the predetermined time period being 1-40 hours, preferably for a time period of 2-10 hours, most preferably for a time period of 4-8 hours. A method according to claims 13 to 15, further comprising: - gradually increasing the temperature of the annealing chamber to the first temperature, the first temperature range and/or the second temperature range over a time period of 1-40 hours, preferably over a time period of 2-10 hours, most preferably over a time period of 4-8 hours. 17. A method according to any one of claims 13 to 16, further comprising: - adding a gas, e.g. argon, to the annealing chamber before or during the step of annealing the diamond substrate. 18. A method according to any preceding claim, further comprising: - providing nanophotonic structures in, on or at the diamond substrate. The method of claim 18, wherein the step of providing the nanophotonic structures is performed after the step of annealing the diamond substrate. The method of claim 18, wherein the step of providing the nanophotonic structures is performed before the step of annealing the diamond substrate. 21. A method according to any preceding claim, further comprising: - removing the protective covering from the first surface after the annealing step is completed. 22. A method according to any preceding claim, further comprising: - activating the color center of the diamond substrate by performing annealing of the diamond substrate. 23. A method according to any preceding claim, further comprising: - applying a second protective covering, e.g., a protective plate or a thin film, to the second surface of the diamond substrate. 24. Diamond substrate obtained by the method according to any of the preceding claims. 25. A quantum system comprising a diamond substrate according to claim 24.