Erbium doped single crystal diamond

Abstract

A rare earth doped single-crystal diamond showing photoluminescence using off-resonant excitation at 853nm and having a full width at half maximum spectral linewidth at 1508nm of less than 1.0nm and a

Description

This invention was made with government support under Grant No. DE-SC0012704 awarded by the Department of Energy and FA9550-17-1-0158 awarded by the Air Force of Scientific Research. The government has certain rights in the invention.” FIELD OF THE INVENTION The invention relates to the field of doped single crystal diamond, and to methods of preparing erbium doped single crystal diamond. BACKGROUND For applications in quantum communication, the telecom C-band is highly favourable to minimize propagation loss across long distances through existing optical fibre-based communication networks. Rare earth ions are attractive candidates for quantum networks by virtue of their narrow optical transitions in the telecom band, with Er3* being the only reported atomic defect that emits in the telecom C-band as described in, for example Dibos et al, Physical Review Letters 120 243601 (2018). In infrared optical communications, C-band refers to wavelengths in the range of 1530-1565 nm. The C-band is located between the short wavelengths band (1460-1530 nm) and the long wavelengths band (1565-1625 nm). “C” stands for “conventional” and is sometimes referred to as the “erbium window”. A problem is that the spin coherence times are limited by magnetic noise in most of the host crystals for Er3* studied. Diamond as a host crystal has a low magnetic noise environment due to low natural abundance of 13C. Further advantages of diamond in this application are a large optical transparency window and a large bandgap. Diamond is chemically inert and biocompatible, and synthetic diamond can be manufactured to a high level of purity. A wide range of colour centres have been studied extensively in diamond, spanning visible to near infrared emission, including silicon-vacancy defects (Si-V), silicon divacancy 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. Another recently reported point defect is the O-band emitter (Mukherjee et al., Nano Lett. 2023 23, 7, 2557-2562). Such colour centres in synthetic diamond material, particularly quantum spin defects and / or optically active defects, have been proposed for use in various sensing, detecting, and quantum communication and 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. Previous work has demonstrated erbium ion implantation in diamond with broad emission bands >20 nm (for example, Cajzl et al., Phys. Chern. Chern. Phys. 19, 6233 2017). SUMMARY Implanting ions in diamond mostly damages the crystal lattice which means the properties of the defects either do not behave as would be expected, or that each defect behaves differently due to changes in the local environment. This is exacerbated as the atom gets larger as damage area around the defect increases. An object of the invention is to provide an optically accessible rare earth defect in a single crystal diamond lattice. According to a first aspect, there is provided a rare earth doped single crystal diamond showing photoluminescence using off-resonant excitation at 853 nm and having a full width half maximum spectral linewidth at 1508 nm of less than 1.0 nm. As an option, the rare earth doped single crystal diamond has spectral linewidth at 1508 nm is selected from any of less than 0.5 nm, less than 0.2 nm and less than 0.1 nm. As an option, the photoluminescence has a lifetime of less than 4 ps. As an option, the rare earth is erbium. The single crystal diamond optionally comprises a concentration of doped rare earth in a range of 20 ppm to 80 ppm. As an option, the rare earth doped single crystal diamond has a single substitutional nitrogen concentration as measured by electron paramagnetic resonance of no more than 10 ppb. Providing a high purity diamond reduces the chances of other impurities interfering with the rare earth dopant. As an option, the rare earth metal is disposed in a layer below a surface of the single crystal diamond, the layer having a thickness of no more than 500 nm. As a further option, the layer is disposed within 500 nm, 200 nm, 100 nm, 50 nm, 30 nm, 10 nm, or 5 nm of the surface. Optionally, at least a portion of the surface has a surface roughness Ra selected from any of no more than 10 nm, 5 nm, 2 nm, 1 nm, or 0.5 nm. The portion of the surface which has said surface roughness Ra optionally has an area selected from any of at least 100 nm2, 500 nm2, 1 pm2, 20 m2, 25 pm2, 100 pm2, 200 pm2, 400 pm2, 900 pm2, 2500 pm2, 10,000 pm2, 0.25 mm2, or 1 mm2. As an option, the rare earth doped single crystal diamond has at a temperature of 20°C a low optical birefringence, indicative of low strain, such that in a sample measured over an area of at least 3 mm x 3 mm, for 98% of the area analysed, the sample remains in first order (5 does not exceed tt / 2), and the maximum value of An[aVerage], the average value of the difference between the refractive index for light polarised parallel to the slow and fast axes averaged over the sample thickness does not exceed 5 x 10'5. The lower strain reduces the effect of strain on the predicted response of the rare earth dopant. According to a second aspect, there is provided a method of producing rare earth doped single crystal diamond. A single crystal diamond is provided, and a rare earth metal is implanted into the single crystal diamond using ion implantation. The implanted single crystal diamond is then annealed at a pressure of at least 5 GPa and a temperature of at least 1600°C. As an option, prior to implanting the rare earth into the provided single crystal diamond, as surface of the provided single crystal diamond is etched using inductively coupled plasma etching. As an option, the pressure is selected from any of at least 5.5 GPa, at least 6 GPa and at least 6.5 GPa. The temperature is optionally selected from any of at least 1700°C, at least 1800°C, at least 1850°C, at least 1900°C and at least 1950°C. As an option, the provided single crystal diamond has a single substitutional nitrogen concentration as measured by electron paramagnetic resonance of no more than 10 ppb. The rare earth metal is optionally erbium. According to a third aspect, there is provided a device comprising the optically accessible rare earth doped single crystal diamond described above in the first aspect. Typically, the device is a quantum communications device. 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 photoluminescent (PL) spectra of samples 1 and 2 at 10 K showing the principal peak at 1508 nm; Figure 2 is a graph of PL intensity of sample 1 at different temperatures; Figure 3 shows time resolved PL measurements on sample 1; Figure 4 shows time resolved PL measurements on sample 2; Figure 5 is a flow diagram showing exemplary steps for producing rare earth metal doped diamond; and Figure 6 illustrates schematically in a block diagram a device that utilises the rare earth metal doped diamond. Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity. DETAILED DESCRIPTION The inventors have surprisingly found that by implanting rare earth metals into a diamond crystal lattice and subsequent annealing, the damage around the defect is reduced thereby creating a more stable and consistent defect. This can be further improved by ICP etching the surface of the diamond prior to implantation to reduce strain in the diamond crystal lattice. Using the techniques described herein, optically active Er3* with narrow optical linewidths has been implanted into single crystal diamond. Erbium incorporation into single crystal diamond is challenging, because its ionic radius is very large compared to the small lattice constant of diamond, making strain and lattice damage mitigation particularly crucial. Furthermore, ultrahigh purity diamond has very low dopant concentrations, making it difficult to stabilize ions with high charge, like the 3+ charge state required for optically active erbium. High pressure high temperature (HPHT) annealing of diamond has been found to preserve near-surface layers of implanted defects. Two single crystal diamonds with a nitrogen content of no more than 10 ppb as measured by electron paramagnetic resonance were provided, and designated sample 1 and sample 2. Sample 2 was boron-doped with a boron concentration of approximately 1 x 1019 cm-3. Each sample was prepared for ion implantation by polishing, reactive ion etching (RIE), thermal annealing, and surface cleaning in a method similar to that described in Sangtawesin et al., Phys. Rev. X 9, 031052 (2019). The RIE step was inductively couple plasma reactive ion etching (ICP-RIE), which is described in WO 2008 / 090511. This allows the surface of the diamond to be treated to provide low subsurface damage. ICP etching is a largely chemical process in which a plasma is used to break down the etching gases into a mixture of free radicals (i.e. neutral species) and ions (i.e. charged species). The plasma is remote from the substrate being etched. Between the plasma and the diamond being etched, the vast majority of the ions generated in the plasma are removed. Thus the majority of species that reach the diamond are neutral. The resulting etching is therefore largely chemical (e.g. surface reactions leading to volatile products), rather than physical (e.g. sputtering from the surface by ions from the plasma). Since atoms in a higher energy state in the substrate, such as those in a region with extended lattice imperfections (e.g. a damaged region), are easier to etch, then this type of etch generally preferentially etches the regions of extended lattice imperfections. 166Er ions were implanted into both samples with a total fluence of 1014 cm-2 and an energy range of 10 to 350 keV. The implantation energy and dose can be controlled to control the depth, thickness, and concentration of implanted rare earth metal within the high purity single crystal diamond. Factors such as channelling mean that it can be desirable to control the angle of implantation with respect to the orientation of the as-grown crystal face. That is, the rare earth metal may be implanted into the surface of the single crystal diamond at an acute angle relative to the as-grown growth face. The exact implantation depth and concentration of erbium will depend on the required characteristics of the diamond component in an end application. The rare earth metal may be implanted into the as-grown growth face of the single crystal CVD diamond layer to a depth of no more than 1 pm, 500 nm, 100 nm, 50 nm, 30 nm, 10 nm or 5 nm. Typically, the implantation dose will be at least 105 N / cm2, 106 N / cm2, 107 N / cm2, 108 N / cm2, 109 N / cm2, 1010 N / cm2, or 1011 N / cm2 and / or no more than 1014 N / cm2 or 1013 N / cm2. In certain circumstances for reasons of yield, it can be desirable to control the temperature of the diamond material during implantation, for example by heating or cooling the sample during implantation. Both samples had an erbium concentration in the implanted layer of around 50 ppm calculated from the implantation dose. Implantation can cause undesirable damage within the diamond crystal structure which can detrimentally affect the properties of the implanted defects. The subsequent annealing process described below is thought to reduce some of that damage. After implantation, the samples were cleaned using a mixture of nitric, perchloric and sulfuric acids. X-ray photoelectron spectroscopy (XPS) was then performed to ensure that the samples had minimal surface contamination. Sample 1 was thermally annealed at 1200°C at low pressure. Subsequently, both samples 1 and 2 were HPHT annealed at a pressure of 6 GPa and a temperature of 1850°C. Photoluminescence spectra were obtained for both samples 1 and 2. As shown in Figure 1, bulk PL spectra on the two Er-implanted diamonds with 850 nm excitation show narrow lines in the telecom band with an ensemble linewidth of -0.75 nm at 10 K, the 1508 nm transition showing the highest intensity, hypothesized as the Zi —> Yi transition. The principal PL peak of sample 1 was observed to be around four times brighter than that of sample 2. Temperature-dependent spectra, shown in Figure 2, reveal additional lines (Figure 2 inset) around the 1508 nm transition at higher temperatures originating from transitions between different crystal field levels of the ground and excited states. This was accompanied by a linewidth broadening and a decrease in the intensity of the principal peak. The spectral linewidth was less than 1 nm. Additional lines were observed at higher temperatures. References to showing photoluminescence refer to a photoluminescence being observed over any noise at a specific wavelength in a photoluminescence spectrum. The spectral linewidth refers to the width of a photoluminescence line measured at full width half maximum (FWHM) in the photoluminescence spectrum. Turning now to Figures 3 and 4, using an off-resonant excitation of 853 nm, a photoluminescence lifetime of -3.5 ps and -3 ps was measured in samples 1 and 2 respectively by collecting emission above 1400 nm. The measurements show that using the techniques described above, optically addressable rare earth ion defects in diamond for quantum applications can be produced. By thermal annealing under a stabilising pressure, other defects are suppressed that would otherwise create relaxation mechanisms that suppress the emission from erbium. Figure 5 is a flow diagram showing exemplary steps for producing rare earth metal doped diamond. The following numbering corresponds to that of Figure 5. S1. A single crystal diamond is provided. As described above a high purity single crystal diamond is preferable, for example one having a single substitutional nitrogen concentration as measured by electron paramagnetic resonance of no more than 10 ppb. S2. ICP is used to etch a surface of the single crystal diamond. S3. A rare earth metal, such as erbium, is implanted a rare earth metal into the surface using ion implantation; S4. The diamond is then annealed at a diamond stabilising pressure and temperature, such as a pressure of at least 5 GPa and a temperature of at least 1600°C. Pressures of at least 5.5 GPa, at least 6 GPa and at least 6.5 GPa can be used. Temperatures may be selected from any of at least 1700°C, at least 1800°C, at least 1850°C, at least 1900°C and at least 1950°C. As described above, rare earth ion defects in diamond for quantum applications can be produced. This enables quantum communications devices to be developed that use the rare earth ion defects in diamond. Figure 6 illustrates schematically in a block diagram such a device 1. The device comprises a rare earth doped single crystal diamond 2 that shows photoluminescence using off-resonant excitation at 853 nm and having a spectral linewidth at 1508 nm of less than 1.0 nm, and a microprocessor 3 for controlling the device. 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 rare earth doped single crystal diamond showing photoluminescence using off-resonant excitation at 853 nm and having a full width at half maximum spectral linewidth at 1508 nm of less than 1.0 nm.

2. The rare earth doped single crystal diamond according to claim 1, wherein the spectral linewidth at 1508 nm is selected from any of less than 0.5 nm, less than 0.2 nm and less than 0.1 nm.

3. The rare earth doped single crystal diamond according to claim 1 or claim 2, wherein the photoluminescence has a lifetime of less than 4 ps.

4. The rare earth doped single crystal diamond according to any one of claims 1 to 3, wherein the rare earth is erbium.

5. The rare earth doped single crystal diamond according to any one of claims 1 to 4, wherein the single crystal diamond comprises a concentration of doped rare earth in a range of 20 ppm to 80 ppm.

6. The rare earth doped single crystal diamond according to any one of claims 1 to 5, wherein the rare earth doped single crystal diamond has a single substitutional nitrogen concentration as measured by electron paramagnetic resonance of no more than 10 ppb.

7. The rare earth doped single crystal diamond according to any one of claims 1 to 6, wherein the rare earth metal is disposed in a layer below a surface of the single crystal diamond, the layer having a thickness of no more than 500 nm.

8. The rare earth doped single crystal diamond according to claim 7, wherein the layer is disposed within 500 nm, 200 nm, 100 nm, 50 nm, 30 nm, 10 nm, or 5 nm of the surface.

9. The rare earth doped single crystal diamond according to claim 7 or claim 8, wherein at least a portion of the surface has a surface roughness Ra selected from any of no more 10 nm, 5 nm, 2 nm, 1 nm, or 0.5 nm.

10. The rare earth doped single crystal diamond according to claim 9, wherein the portion of the surface which has said surface roughness Ra has an area selected from any of at least 100 nm2, 500 nm2, 1 pm2, 20 m2, 25 pm2, 100 pm2, 200 pm2, 400 pm2, 900 pm2, 2500 pm2, 10,000 pm2, 0.25 mm2, or 1 mm2.

11. The rare earth doped single crystal diamond according to any one of claims 1 to 10, having at a temperature of 20°C a low optical birefringence, indicative of low strain, such that in a sample measured over an area of at least 3 mm x 3 mm, for 98% of the area analysed, the sample remains in first order (5 does not exceed tt / 2), and the maximum value of An^rage], the average value of the difference between the refractive index for light polarised parallel to the slow and fast axes averaged over the sample thickness does not exceed 5x1 O'5.

12. A method of producing rare earth doped single crystal diamond, the method comprising:providing single crystal diamond;implanting a rare earth metal into the provided single crystal diamond using ion implantation;annealing the rare earth doped single crystal diamond at a pressure of at least 5 GPa and a temperature of at least 1600°C.

13. The method according to claim 12, further comprising, prior to implanting the rare earth metal into the provided single crystal diamond, etching a surface of the provided single crystal diamond using inductively coupled plasma etching.

14. The method according to claim 12 or claim 13, wherein the pressure is selected from any of at least 5.5 GPa, at least 6 GPa and at least 6.5 GPa.

15. The method according to any one of claims 12 to 14, wherein the temperature is selected from any of at least 1700°C, at least 1800°C, at least 1850°C, at least 1900°C and at least 1950°C.

16. The method according to any one of claims 12 to 15, wherein the provided single crystal diamond has a single substitutional nitrogen concentration as measured by electron paramagnetic resonance of no more than 10 ppb.

17. The method according to anyone of claims 12 to 15, wherein the rare earth metal is erbium.

18. A device comprising the rare earth doped single crystal diamond according to any one of claims 1 to 11.

19. The device according to claim 18, wherein the device is a quantum communications device.

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