EP4681250A1 - Diamond-based electrical components - Google Patents

Abstract

A device comprising a diamond substrate, wherein: the substrate comprises a device track formed by modifying the structure of carbon within the substrate; and electrical conductivity along the device track is asymmetric. A method of manufacturing a device, the method comprising: forming a device track having asymmetric electrical conductivity along the device track by modifying the structure of carbon within a diamond substrate.

Description

DIAMOND-BASED ELECTRICAL COMPONENTS

Carbon is one of the most abundant elements on Earth and forms a rich assortment of allotropes, with well-known examples of diamond and graphite in addition to more exotic structures such as graphene, fullerene and carbon nanotubes. A common factor linking the various allotropes is their importance for both technological application and scientific discovery. This is exemplified by diamond, where natural stones are an important geological resource. Recent advances in laboratory-based diamond growth have driven forward applications where the properties of diamond as a wide band gap semiconductor with extreme electronic, optical and thermal properties can be exploited. In addition to electronics, electrochemical devices and biomedical devices fabricated from laboratory-grown diamond have become of significant interest. Recently, the stable nature of NV- and other defect centres within diamond has generated great interest in diamond as a platform for quantum technology and its many applications.

As the hardest known natural material, it can be challenging to effectively process the raw diamond substrate to manufacture devices. Over the past decade, laser processing with sub-picosecond duration pulses has become established as a useful technique for internal modification of diamond. Such laser processing has largely focused on the fabrication of electrically conductive vias that pass through a diamond wafer. Device applications have mainly focused on radiation detectors, but recent advances in optical control have enabled a new generation of laser-written diamond devices for quantum technology. Previously the laser- written tracks were broadly defined as ‘graphitic’ based upon data from Raman spectroscopy, and efforts were made to engineer the tracks to have as high electrical conduction as possible [1], However, studies with low-resolution transmission electron microscopy (TEM) [2] and scanning electron microscopy (SEM) [3] have indicated that the internal structure might be more complex, while very recent reports have suggested a complicated relationship between the delivered laser dose and ohmic conductivity [4],

Conventional diamond electronics are primarily based on either boron-doped diamond (BDD), or hydrogen terminated (H-term) substrates, both of which have their own distinct drawbacks. BDD substrates are complicated to manufacture, requiring multiple discrete growth steps, and is incredibly susceptible to variations in temperature. H-term diamond, while simpler and cheaper to produce, is at the mercy of its environment and is often short-lived or lacking in reproducibility between neighbouring devices. Electronics based on novel, laser-written nano-carbon network (NCNs), however, make use of readily available diamond substrates, with permanent laser-written structures formed in isolation from the local environment [5], It would be desirable to expand the range of devices that can be fabricated in diamond substrates to provide for further development of diamond electronics and its applications.

According to a first aspect of the invention, there is provided a method of manufacturing a device, the method comprising: forming a device track having asymmetric electrical conductivity along the device track by modifying the structure of carbon within a diamond substrate.

Forming a track with asymmetric electrical conductivity opens up a whole new device paradigm for diamond-based electrical devices, allowing the manufacture of extremely robust active electronic components. Diamond-based devices have applications for high-voltage power electronics and devices in harsh environments, such as nuclear or aqueous. Three-dimensional, electronically-active pathways fabricated within bulk diamond present a new avenue for all-carbon electronic devices, which will benefit both individual device design as well as future integrated electronic systems.

In some embodiments, the modifying of the structure of carbon comprises modifying a pre-existing precursor track, the precursor track having symmetric electrical conductivity along the precursor track. The formation of tracks in diamond with symmetric conductivity has been known for some time. Modifying these symmetric tracks to create tracks with asymmetric conductivity offers a promising manufacturing method for active devices in diamond.

In some embodiments, the precursor track is formed by modifying the structure of carbon within the substrate, optionally wherein the method further comprises forming the precursor track. Forming the precursor track by modifying the structure of carbon means that similar processing equipment can be used both to form the precursor track and to modify the precursor track to obtain the device track.

In some embodiments, the forming of the precursor track comprises modifying the structure of the carbon within the substrate by delivering energy to the substrate according to a first regime; the forming of the device track comprises modifying the precursor track by delivering energy along the precursor track according to a second regime; the first regime is such that delivering energy to a previously unprocessed diamond substrate according to the first regime forms a conductive track; and the second regime is such that delivering energy to a previously unprocessed diamond substrate according to the second regime forms an insulating track. The response of unprocessed diamond to delivered energy differs between the two regimes, forming either conductive or non-conductive tracks. The combination of these two processes provides a mechanism to generate tracks having asymmetric conductivity.

In some embodiments, the forming of the device track comprises a unidirectional process starting at a first end of the device track and ending at a second end of the device track. This allows tracks to be drawn in a continuous manner through the bulk of the diamond substrate.

In some embodiments, the device track is formed between a first surface of the substrate and a second surface of the substrate, optionally wherein the second surface is opposite to the first surface. This allows electrical contact to be made between the device track and other components outside of the bulk of the diamond substrate, for example on its surface.

In some embodiments, the device track is formed entirely within the substrate, optionally wherein the method further comprises forming one or more conductive tracks connecting the device track to a surface of the substrate. The can allow active electrical components to be formed entirely within the substrate, where they can be protected from extreme conditions.

In some embodiments, modifying the structure of the carbon comprises forming graphite. Graphite is a conductive allotrope of carbon, unlike bulk diamond. Forming carbon having a graphite-like structure is important in the fabrication of device tracks having asymmetric conduction properties.

In some embodiments, modifying the structure of the carbon comprises forming one or more regions comprising multi-phase carbon, optionally wherein the one or more regions include: a) a region comprising graphite within which there are multi-phase nanocarbon composite regions; and/or b) a region comprising graphite within which there are diaphites. Microscopy studies suggest that more complex carbon structures incorporating multiple different carbon structures are responsible for the conductive properties of the device tracks.

In some embodiments, modifying the structure of the carbon comprises using a laser. Lasers have been shown to be an effective way to deliver energy to transparent substrates such as diamond, where high intensities allow absorption of energy via non- linear processes. Using a laser also allows for focussing at a spot to be processed, thereby allowing for processing at arbitrary points within the transparent diamond substrate.

In some embodiments, the laser is a pulsed laser. A pulsed laser allows for very high intensities of laser radiation to be achieved in controlled bursts, thereby promoting absorption of energy in transparent substrates such as diamond via non-linear absorption processes.

In some embodiments, a duration of pulses of the laser is at most lOps, optionally at most Ips, optionally at most 5OOfs, optionally at most 200fs. These pulse lengths are achievable with current short-pulse lasers and deliver an appropriately high energy density with each pulse.

In some embodiments, an energy of each pulse of the laser is between lOnJ and lOpJ, optionally between 50nJ and 200nJ. These pulse energies are appropriate to cause the desired modification of diamond when concentrated into a volume typical for pulsed lasers of the type used for this application.

In some embodiments, a pulse rate of the laser during the forming of the device track is between 10kHz and 10MHz, optionally between 100kHz and 5MHz. In some embodiments, the modifying of the structure of carbon comprises modifying a pre-existing precursor track having symmetric electrical conductivity along the precursor track, and a pulse rate of the laser during the modifying of the precursor track is between 10kHz and 10MHz, optionally between 100kHz and 5MHz. These pulse rates correspond to the second regime that allows formation of the device track with typical parameters of a pulsed laser used for this type of application.

In some embodiments, the method further comprises forming a precursor track having symmetric electrical conductivity along the precursor track, and a pulse rate of the laser during the forming of the precursor track is between 100Hz and 30kHz, optionally between 500Hz and 10kHz. These pulse rates correspond to the first regime that allows formation of conductive tracks in diamond with typical parameters of a pulsed laser used for this type of application.

In some embodiments, the laser has a wavelength of between 340nm and 1600nm, optionally between 500nm and 1070nm, optionally about 515nm. In some embodiments, the laser is a Yb or Ti:Sapphire laser. Lasers of this type have been found to be suitable for generating tracks in transparent substrates such as diamond.

In some embodiments, modifying the structure of the carbon comprises scanning a focal point of the laser through the substrate, optionally at a rate of between Ipm/s and lOOpm/s, optionally between 5pm/s and 50pm/s. Scanning the focal point at this rate allows for manufacture of continuous tracks over a suitable timescale.

In some embodiments, the laser is a pulsed laser and during the forming of the device track, the focal point of laser is scanned through the substrate by between Inm and 10pm between pulses. These distances between pulse deliveries are appropriate to create an energy density that corresponds to the second regime that allows formation of the device track with typical parameters of a pulsed laser used for this type of application.

In some embodiments, the laser is a pulsed laser, the method further comprises forming a precursor track having symmetric electrical conductivity along the precursor track, and during the forming of the precursor track, the focal point of laser is scanned through the substrate by between lOOnm and Inm between pulses. These distances between pulse deliveries are appropriate to create an energy density that corresponds to the first regime that allows formation of conductive tracks in diamond with typical parameters of a pulsed laser used for this type of application.

In some embodiments, modifying the structure of the carbon comprises focussing the laser inside the substrate. Focussing the laser inside the substrate allows for processing at arbitrary points within the bulk of the substrate without affecting other regions of the substrate.

In some embodiments, the laser has a focal point that is diffraction limited, optionally wherein the focal point has a diameter of at most 5 pm, optionally at most 2pm. This constrains the processing volume as far as possible, thereby increasing energy density and allowing finer tracks to be written inside the substrate.

In some embodiments, the laser is passed through a spatial light modulator before reaching the substrate, optionally wherein the spatial light modulator is used to compensate for optical aberrations such that the focal point remains diffraction limited during scanning of the laser beam. This allows for optical aberrations to be compensated for during processing.

According to a second aspect of the invention, there is provided a device comprising a diamond substrate, wherein: the substrate comprises a device track formed by modifying the structure of carbon within the substrate; and electrical conductivity along the device track is asymmetric.

A diamond substrate device comprising tracks with asymmetric electrical conductivity opens up a whole new device paradigm for diamond-based electrical devices, allowing the manufacture of extremely robust active electronic components. Diamondbased devices have applications for high-voltage power electronics and devices in harsh environments, such as nuclear or aqueous. Three-dimensional, electronically-active pathways fabricated within bulk diamond present a new avenue for all-carbon electronic devices, which will benefit both individual device design as well as future integrated electronic systems.

In some embodiments, the device track has a first end and a second end, and a first electrical resistance in respect of current flowing from the first end to the second end is different from a second electrical resistance in respect of current flowing from the second end to the first end. Asymmetric conductivity allows for the manufacture of active electronic components in diamond.

In some embodiments, the device track has a first end and a second end, and a first barrier potential opposing current flowing from the first end to the second end is different from a second barrier potential opposing current flowing from the second end to the first end, optionally wherein the first barrier potential is at least 100% greater than the second barrier potential, optionally at least 200%, optionally at least 400%. These values are suitable to allow for the manufacture of active components.

In some embodiments, the device track extends within an interior of the substrate. This can allow for components to be formed inside the substrate, leading to more complex devices produced compactly within the substrate.

In some embodiments, the device track extends entirely within the interior of the substrate, optionally wherein the device track is connected to a surface of the substrate using a conductive track. This can allow active electrical components to be formed entirely within the substrate, where they can be protected from extreme conditions.

In some embodiments, the device track extends between a first surface of the substrate and a second surface of the substrate, optionally wherein the second surface is opposite to the first surface. This allows electrical contact to be made between the device track and other components outside of the bulk of the diamond substrate, for example on its surface.

In some embodiments, the device track comprises graphite. Graphite is a conductive allotrope of carbon, unlike bulk diamond. Forming carbon having a graphitelike structure is important in the fabrication of device tracks having asymmetric conduction properties.

In some embodiments, the device track comprises one or more regions comprising multi-phase carbon, optionally wherein the one or more regions include: a) a region comprising graphite within which there are multi-phase nanocarbon composite regions; and/or b) a region comprising graphite within which there are diaphites. Microscopy studies suggest that more complex carbon structures incorporating multiple different carbon structures are responsible for the conductive properties of the device tracks.

In some embodiments, the device is manufactured using a method according to the first aspect. The methods above provide suitable and efficient means of manufacturing devices of this type.

In some embodiments, the substrate comprises single-crystal diamond or polycrystalline diamond, optionally formed by chemical vapour deposition. These materials have uniform and predictable properties and are readily available, since they are already used for existing diamond-based electronics.

In some embodiments, the substrate comprises boron-doped diamond or hydrogen- terminated diamond. These diamond types are particularly suited for diamond-based electronics, and are readily available.

In some embodiments, the device track is formed along the [100] direction of the substrate. Due to the arrangement of the crystal structure in diamond, forming tracks along this direction is easier and provides more consistent results.

According to further aspects of the invention, there are provided a diode comprising a device according to the second aspect and a transistor comprising a device according to the second aspect. These are active electronic components that will be particularly useful in creating a new generation of diamond-based components.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols represent corresponding parts, and in which

Fig. l is a schematic diagram of a device according to an embodiment;

Fig. 2 is a flowchart of a method of forming devices;

Fig. 3 is a schematic of an arrangement used to manufacture devices using a laser;

Fig. 4 shows more details of an exemplary laser setup used for manufacturing devices;

Fig. 5 shows electrical measurements of fabricated devices; Fig. 6 shows electrical measurements of fabricated devices;

Fig. 7 shows optical microscope images of fabricated devices;

Fig. 8 shows further optical microscope images of fabricated devices;

Processing of diamond substrates, for example using a laser, offers the ability to write conductive structures embedded inside diamond, or passing through diamond wafers. The mechanism for forming nanocarbon networks (NCNs) displaying ohmic conduction in diamond has long been identified. When focused inside the diamond, the laser breaks down the diamond lattice by non-linear absorption of the light. The disruption of the diamond lattice creates some sp2 (graphite-like) bonded carbon that can create conductive pathways. This has been established over the past decade when using pulsed lasers with repetition rates of 1kHz.

Such ohmic ‘wires’ or tracks within diamond have been used for the fabrication of a range of devices, and in particular for 3D radiation detector structures. It is only very recently that investigations of processing at different pulse repetition rate (PRR) have been reported. It has been observed that increasing the laser dose (number of laser pulses interacting with each volume of material, which is equivalent to increasing the PRR at fixed translation speed) impedes the electrical conduction of the NCN [4], It has also been shown that a barrier potential can exist for the NCN when laser parameters were not optimised, but the barrier was symmetric with regard to voltage polarity [1],

The present inventors have determined that by carefully controlling the conditions under which the diamond is processed, for example by controlling the PRR of a laser used for processing, it is possible to induce further, different modifications inside a diamond substrate. This can lead to insulating, semi-metallic (conducting) and semi-conducting NCNs. These structures can be created at will in 3D within bulk diamond, optionally with contacts extending to one or both faces of the substrate. In particular, a two-stage approach of writing tracks and then overwriting them under different processing conditions offers the ability to fabricate connections which do not display simple, linear conduction characteristics, but rather show behaviour like an electrical diode. This permits the manufacture of new devices, where NCNs are controllably manufactured within a diamond substrate, providing not only buried conductive wires, but also active device junctions.

The present invention concerns devices comprising device tracks with asymmetric electrical conductivity, and methods that allow for manufacturing such devices by controllably creating tracks in a diamond substrate showing different electrical conduction mechanisms. The tracks formed in the diamond substrate may also be referred to as nanocarbon networks (NCNs). Fig. 1 shows an example of such a device 2.

The device 2 comprises a diamond substrate 10. Any suitable type of diamond may be used. For example, the substrate 10 may comprise single-crystal diamond or polycrystalline diamond. Where the substrate 10 comprises single-crystal diamond, the device track 22 may be formed along the [100] direction of the substrate 10.

The substrate 10 is typically an artificial diamond, since these have lower cost and more predictable physical properties. For example, the substrate 10 may be formed by chemical vapour deposition (CVD). The substrate 10 may comprise boron-doped diamond or hydrogen-terminated diamond. The substrate 10 used for the example devices discussed in more detail below was a 4x4x0.3 mm, single crystal electronic-grade CVD diamond (Chenguang Machinery, China).

The substrate 10 comprises a device track 22. Electrical conductivity along the device track 22 is asymmetric. The asymmetric conduction is due to the formation of an asymmetric barrier potential along the device track 22. As shown in Fig. 1, the device track 22 has a first end 26 and a second end 28. A first barrier potential opposing current flowing from the first end 26 to the second end 28 is different from a second barrier potential opposing current flowing from the second end 28 to the first end 26. Depending on factors such as the desired application and the configuration of the device 2, the first barrier potential is at least 100% greater than the second barrier potential, optionally at least 200%, optionally at least 400%.

The asymmetric conduction can also be expressed in terms of asymmetric electrical resistance. For example, a first electrical resistance in respect of current flowing from the first end 26 to the second end 28 is different from a second electrical resistance in respect of current flowing from the second end 28 to the first end 26.

The device track 22 extends within an interior of the substrate 10. This means that active components formed inside the substrate 10 based on the device track 22 can be protected from extreme conditions outside the substrate 10 by the highly resilient physical properties of the diamond.

In the example device shown in Fig. 1, the device track 22 extends entirely within the interior of the substrate 10. Both the first end 26 and the second end 28 of the device track 22 are within the interior of the substrate 10. To enable electrical connection to the device track 22, the device track 22 is connected to a surface of the substrate 10 using a conductive track 24. In Fig. 1, the device track 22 is connected to both the first surface 12 and the second surface 14 of the substrate 10 by conductive tracks 24. The second surface 28 is opposite to the first surface 26 in Fig. 1, but this is not essential. Contacts 18 are provided at the ends of the conductive tracks 24 to allow for easier connection of the device track 22 to other components outside the substrate 10.

As an alternative to the embodiment of Fig. 1, the device track 22 itself may extend between surfaces of the substrate 10, such as the first surface 26 of the substrate 10 and the second surface 28 of the substrate 10. A combination of both situations may also be used, wherein the first end 26 of the device track 22 is connected to the first surface 12 of the substrate 10 by a conductive track 24, and the second end 28 of the device track 22 extends to the second surface 14 (or vice versa). The contacts 18 would be connected to whichever track extends to the surface, whether that is the conductive track 24, the device track 22, or both.

The device 2 in Fig. l is a very simple example of a device comprising a device track 22. More complex devices may comprise plural device tracks 22, which may be connected to one another by conductive tracks 24. In such embodiments, one or more of the conductive tracks 24 may extend entirely within the interior of the substrate 10 between different device tracks 22.

The device 2 may be used as part of the manufacture of electronic components that rely on asymmetric conduction. For example, a diode or a transistor may be manufactured comprising the device 2.

The device track 22 is formed by modifying the structure of carbon within the substrate 10. Fig. 2 shows a method of manufacturing a device 2 such as that shown in Fig. 1. The method comprises forming S20 a device track 22 having asymmetric electrical conductivity along the device track 22 by modifying the structure of carbon within a diamond substrate 10. Modifying the structure of the carbon comprises forming graphite, such that the device track 22 comprises graphite.

As discussed above, it has been known for some time that tracks with symmetric conduction properties can be formed inside a diamond substrate by modifying the structure of carbon in the diamond. However, the present method is based on the realisation that multiple different process steps can be used to produce a device track 22 having asymmetric conduction properties.

The method comprises a two-step process in which the device track 22 is formed by delivering energy to the substrate 10 in the same region of the substrate 10 according to two different regimes in two sequential steps. The two regimes differ in the amount and rate of energy delivery to the substrate 10.

More specifically, as shown in Fig. 2, the method comprises forming S10 a precursor track. The modifying of the structure of carbon in the step of forming S20 the device track 22 then comprises modifying the pre-existing precursor track. The precursor track has symmetric electrical conductivity along the precursor track. The precursor track is also formed by modifying the structure of carbon within the substrate, although according to a different regime of energy delivery then is used for modifying of the structure of carbon in the step of forming S20 the device track 22.

This means that the forming of the precursor track comprises modifying the structure of the carbon within the substrate 10 by delivering energy to the substrate 10 according to a first regime, and the forming of the device track 22 comprises modifying the precursor track by delivering energy along the precursor track according to a second regime.

The first regime is such that delivering energy to a previously unprocessed diamond substrate according to the first regime forms a conductive track. This corresponds to known processes for forming conductive tracks in diamond, such as those discussed above, which produce conductive tracks having normal ohmic conduction that is symmetric along the track. The second regime is such that delivering energy to a previously unprocessed diamond substrate according to the second regime forms an insulating track.

Different modifications of the structure of carbon within the diamond substrate 10 occur when energy delivery to the substrate 10 is below or above a critical threshold. When energy delivery to the substrate 10 is below the critical threshold (i.e. in the first regime), electrically conductive tracks are formed. The mechanism for this is relatively well-understood, as discussed above. When energy delivery to the substrate 10 is above the critical threshold (i.e. in the second regime), tracks are formed that are superficially similar in appearance to those formed below the threshold, but which are electrically insulating. Performing two processing steps on the same region of the substrate 10, one below the critical threshold in the first regime and a second subsequent step above the critical threshold in the second regime, unexpectedly leads to tracks that display asymmetric conduction.

Without wishing to be bound by theory, it appears that when delivering energy above the critical threshold in the second regime, the diamond surrounding the point of energy delivery places the graphite that begins to form under significant pressure. The extra energy dose increases local heating such that some of the regions return to sp3 bonded diamond and form a high-pressure-high-temperature (HPHT) pathway. When applied to a previously unprocessed diamond substrate, this breaks the continuous conductive track leading to insulating behaviour.

However, in the two-step method where the device track 22 is formed by modifying the conductive precursor track, it is theorised that forming the precursor track creates a clear conductive pathway which is subsequently difficult to fully break down. However, the second step of delivering energy along the precursor track according to the second regime alters the microscopic structure to introduce some very small breaks which generate an asymmetric barrier potential (voltage) that needs to be overcome before electrical conduction can proceed. This creates the asymmetric electrical conduction properties of the device track 22.

This means that modifying the structure of the carbon comprises forming one or more regions comprising multi-phase carbon, such that the device track 22 comprises one or more regions comprising multi-phase carbon. A region comprising multi-phase carbon here means that the region includes carbon in multiple different phases, or allotropes. The multiple phases may display different crystal structures to normal diamond and to one another.

The multiple phases may include graphite, as mentioned above, but may also include more unusual or exotic forms of carbon. The one or more regions may include a region comprising graphite within which there are nanocarbon composite regions. Nanocarbon composite refers to regions containing a variety of (i.e. more than one) carbon allotropes in close proximity (sub-micrometer level). For example, the one or more regions may include a region comprising graphite within which there are diaphites. Diaphites are graphene-like regions that separate graphite and diamond, containing ordered graphene layers at the interface. Alternatively, diaphites might correspond to highly ordered graphene layers forming a barrier between two regions of sp3 bonded carbon (diamond). In general, nanocarbon composites might contain additional carbon bonding configurations such as lonsdaleite, amorphous carbon, carbon nanotubes, fullerenes or other recognised carbon allotropes.

It should be noted that the conductive tracks 24 connected to the device track 22, mentioned above and shown in Fig. 1, could be created by forming S10 a precursor track and then modifying only a portion of the precursor track during the forming S20 of the device track 22. The remaining parts of the precursor track that are not modified will then remain to form the conductive tracks 24. Alternatively, the method may comprise a further, separate step of forming S25 conductive tracks 24, as shown in Fig. 2. In this step S25, the conductive tracks 24 may be formed in substantially the same way as the precursor track.

Fig. 3 shows schematically an embodiment of the method in which modifying the structure of the carbon comprises using a laser. In the state shown in the schematic, the precursor track 20 has already been formed, and the laser is being used to modify the precursor track 20 to form the device track 20.

Using a laser is preferable for controlled machining, because the laser beam can be focussed through the transparent substrate 10 to a focal point 50 to perform processing at a particular point within the substrate 10 without affecting other neighbouring regions of the substrate 10.

Suitable laser wavelengths include between 340nm and 1600nm, optionally between 500 and 1070nm, optionally about 515nm. Suitable lasers include Yb or Ti: Sapphire lasers. The laser may be frequency doubled before entering the substrate, such that a laser with a base wavelength of 1030nm may be doubled to produce a laser beam that enters the substrate 10 having a wavelength of 515nm.

Modifying the structure of the carbon (both when forming the precursor track 20 and when modifying the precursor track 20 to form the device track 22) comprises scanning the focal point of the laser through the substrate 10. In Fig. 3, the substrate 10 is mounted on a stage 40 that allows the substrate 10 to be translated in three dimensions thereby scanning the focal point 50 of the laser through the substrate 10. In other embodiments, the laser beam may be moved to scan the focal point 50 through the substrate. Suitable rates of scanning include between Ipm/s and lOOpm/s, optionally between 5pm/s and 50pm/s.

As discussed in relation to Fig. 1, the device track 22 may be formed between a first surface 12 of the substrate 10 and a second surface 14 of the substrate 10. The second surface 14 may be opposite to the first surface 12. Alternatively, the device track 22 may be formed entirely within the interior of the substrate 10, such that the device track 22 does not extend to any surface of the substrate 10. The forming S20 of the device track 22 may comprise a unidirectional process starting at the first end 26 of the device track 22 and ending at a second end 28 of the device track 22. This can be arranged by controlling the scanning of the focal point 50 of the laser. The direction of the asymmetry of the conduction properties of the device track 22 is determined by the direction in which the focal point 50 is scanned. To ensure uniform directionality, the scanning is preferably performed in this way such that the focal point 50 does not pass through the same point along the precursor track 20 more than once during the modifying of the precursor track 20 to form the device track 22.

The laser may be a pulsed laser. Pulsed lasers are particularly convenient for carrying out the method, because they can be used to deliver very short pulses of relatively high energy, thereby delivering very high energy density over short timescales. This high energy density causes energy absorption via non-linear processes, such that even transparent substrates such as diamond can be made to absorb significant amounts of energy from the laser. A duration of pulses of the laser may be at most lOps, optionally at most Ips, optionally at most 500fs, optionally at most 200fs. An energy of each pulse of the laser may be between lOnJ and lOpJ, optionally between 50nJ and 200nJ. These values provide suitable energy densities to allow for absorption in the diamond substrate 10.

The pulse rate of the laser during the forming of the device track 22 (i.e. during the modifying of the precursor track 20 to form the device track 22) may be between 10kHz and 10MHz, optionally between 100kHz and 5MHz, for example about 1MHz. The pulse rate of the laser during the forming of the precursor track may be between 100Hz and 30kHz, optionally between 500Hz and 10kHz, for example about 1kHz. These pulse rates are preferable when the focal point of the laser is scanned at approximately lOpm/s.

The pulse rate of the pulsed laser (also referred to as a repetition rate) combined with the rate of scanning of the focal point 50 can be varied to control the the spacing of the regions in which energy is delivered and the overall rate of energy delivery over a longer timescale along the tracks as they are formed.

During the forming of the device track 22 (i.e. during the modifying of the precursor track 20 to form the device track 22), the focal point 50 of the laser may be translated by between Inm and 10pm between pulses. During the forming of the precursor track 20, a focal point 50 of the laser may be translated by between lOOnm and Inm between pulses. As discussed above, modifying the structure of the carbon comprises focussing the laser inside the substrate 10. A narrow focus of the laser is beneficial in concentrating the laser energy and allowing highly controlled processing. Preferably, the laser has a focal point 50 that is diffraction limited. The focal point 50 may have a diameter of at most 5pm, optionally at most 2pm.

In general, during laser processing, strong spherical aberrations are introduced by refraction at the diamond interface which can distort the laser focus and lead to reduced performance [6], To avoid this, the laser may be passed through a spatial light modulator (SLM) before reaching the substrate 10. The SLM is used to compensate for optical aberrations such that the focal point 50 remains diffraction limited during scanning of the laser beam. The SLM can dynamically compensate for optical aberrations introduced by refraction at the diamond interface by pre-distorting the phase of the input laser beam to cancel the spherical aberrations. This ensures diffraction limited performance throughout the diamond layer [6],

A specific example of fabrication of devices is now discussed in more detail. The nanocarbon networks (corresponding to the precursor and device tracks) were laser machined inside a high purity single crystal diamond wafer substrate.

The system used is shown in more detail in Fig. 4. The laser was an ultrashort pulsed Yb:KGW laser (Light Conversion Pharos SP-06-1000-pp) with a central wavelength of 515nm, pulse duration 170fs and maximum repetition rate 1MHz. The pulse energy from the was controlled using a rotatable half-wave plate and polariser. The diamond substrate was mounted on 3D precision translation stages (Aerotech ABL10100 (x-y); ANT95-3-V (z)). All the lenses used were achromatic doublets, with focal lengths as indicated next to each element in Fig. 4.

The laser beam was expanded and directed onto a phase-only liquid crystal spatial light modulator (SLM, Hamamatsu XI 0468), which was imaged in 4f configuration onto the back of a microscope objective lens (Zeiss 20* 0.5NA). In this manner, the laser could be focused inside the diamond substrate down to a diffraction limited spot size of 0.6X/NA = 0.6pm in the transverse plane and 2nk/NA² = 9.8 pm axially (inside the diamond, where the refractive index n = 2.4). The SLM was only used to remove spherical aberration components of the interface aberration, whilst not compensating for the refocusing effect of the interface [7], After compensating for system aberrations by minimising the fabrication threshold on the upper surface of the diamond sample, the aberration correction was done in a predictive manner using the position feedback of the axial translation stage.

Using this setup, writing tracks at 1kHz repetition rate of the laser results in the precursor tracks displaying ohmic electrical conduction. Writing tracks at 1MHz repetition rate onto previously unprocessed substrate results in tracks with no electrical conduction. Writing first at 1kHz and then overwriting at 1MHz creates device tracks which show asymmetric conduction behaviour like an electrical diode.

Columnar tracks of NCN were fabricated by focusing the laser on the back surface of the diamond substrate and scanning the laser focus through to the top surface at a translation speed of lOpm/s. The laser focal point was scanned along the optical axis, and the substrate was arranged such that this corresponded to the [100] direction in the diamond crystal. This scanning rate is the speed v_s at which the substrate physically moves. The laser focal point moves faster inside the diamond because of refraction at the interface. When focusing at NA = 0.5, the speed v_a inside the diamond is well approximated by n*v_s = 24 pm/s.

The substrate 10 was illuminated from beneath by a red LED. A dichroic mirror separated the LED illumination from the laser, before passing through a tube lens to a CCD, forming a microscope to allow imaging of the diamond sample during fabrication.

The electrical characteristics of example devices after each of three types of processing were measured by impedance spectroscopy. The three types of processing were 1) forming of a precursor track only, under the conditions of the first regime, 2) processing a previously unprocessed diamond wafer under the conditions of the second regime, 3) the full processing method, whereby a precursor track is formed under the conditions of the first regime and then modified under the conditions of the second regime.

The laser pulse energy of 120nJ and sample translation speed of lOum/s were constant throughout, so the pulse repetition rate (PRR) was determinative of which regime the processing fell into, and thereby the electrical characteristics of the written NCN wires. Since the experiments herein were conducted by varying the PRR, the three types of processing are referred to by their PRRs, namely as PRR- Ik, PRR- IM, and PRR-lklM respectively.

In order to perform impedance spectroscopy measurements, Ti-Pt-Au contacts were deposited (20-100-200nm, Edwards A500 - FL500 Electron Beam Evaporator) and annealed to form a reliable ohmic contact. I-V measurements were recorded using a Tektronix Keithley 4200-A, and impedance measurements were taken using a Solartron 1260 A Frequency Response Analyser with a 1296 Dielectric Interface Unit attached. Measurements were made at temperatures from 25 to 400 °C.

Electrical analysis of the NCNs fabricated under each of the three types of laser processing showed distinct electrical conduction characteristics, as seen in Fig. 5 and Fig. 6.

Processing types 1) and 2) were dictated by the laser pulse repetition rate (PRR). When set to 1kHz PRR, the process yielded NCNs with ohmic conduction. The DC conduction of these devices is shown as PRR-lk in Fig. 5A. If the PRR was increased to 1MHz the NCN became insulating. The DC conduction of these devices is shown as PRR- 1M in Fig. 5A.

Additionally, a hybrid configuration could be accessed using processing type 3), comprising initially forming the NCN column with a single pass of the laser at 1kHz PRR and then overwriting at 1MHz. This resulted in NCNs with semi-conducting properties and showing diode-like behaviour. Fig. 5B shows the DC conduction of these device, and Fig. 6 shows Arrhenius plots illustrating the semiconducting behaviour.

These results demonstrate that a combination of two differing regimes of energy delivery applied in sequence, in this case accessed by varying the PRR, can create conditions that lead to an asymmetric potential barrier. This creates conduction in one direction only at a given threshold voltage.

As mentioned above, it is theorised that the asymmetric behaviour of the device track 22 arises due to the distinct nature of the interacting NCN tracks formed. To understand the mechanism for the different conduction properties of the NCNs and how they depend on the energy delivery conditions, high resolution imaging of the NCN structure was performed. Imaging was performed using optical microscopy and high resolution transmission electron microscopy (HRTEM).

A range of structures were identified containing composite sp3- and sp2-bonding patterns, which appear closely related to the variety of ‘diaphite’ structures formed by shock impact in meteoritic diamond. This suggests that the processing replicates the extreme pressure and temperature conditions needed for formation of these exotic carbon nanostructures by delivering laser energy into a micrometre scale volume in a picosecond timeframe, with the surrounding diamond matrix providing a high-pressure environment.

Fig. 7 shows images from a transmission optical microscope of the NCN at the initiating side for each of the three types of processing. In the upper panel, ‘ 11’ indicates processing type 1), ‘21’ indicates processing type 2), and ‘31’ indicates processing type 3). More highly magnified images are shown in the lower panels, in which the scale bars 10pm long.

Further optical microscope images of the device are shown in Fig. 8. Each of the three rows corresponded to one of the three processing types, i.e. (i) 1kHz PRR, (ii) 1MHz PRR and (iii) 1kHz PRR initial scan and 1 MHz PRR overwrite. The polarizing microscope shows a lack of strain induced birefringence around the NCN columns produced under processing type 2).

Even though the NCN tracks show different electrical conduction properties, they appear similar in the optical images. The tracks have a column diameter of 5-6pm, showing dark contrast against the bright background from the surrounding diamond. The focal point of the laser is smaller than these dark columns, being approximately 1 m in diameter.

References

[1] “Barrier potential for laser written graphitic wires in diamond” I Haughton, IL Paz, M McGowan, A Oh, A Porter, PS Salter, O Allegre; Diamond and Related Materials 111, 108164 (2021)

[2] P. S. Salter, M. J. Booth, A. Courvoisier, D. A. J. Moran, and D. A. MacLaren, “High resolution structural characterisation of laser-induced defect clusters inside diamond,” Applied Physics Letters, vol. I l l, no. 8, p. 081103, 2017.

[3] K. Ashikkalieva, T. Kononenko, E. Obraztsova, E. Zavedeev, A. Khomich, E. Ashkinazi, V. Konov, Direct observation of grapheme nanostructures inside femtosecondlaser modified diamond, Carbon 102 (2016) 383-389.

[4] Tokunaga, D., Sato, M., Itoh, S. et al. Focus movement distance per pulse dependence of electrical conductivity and diameter of diamond internal modification induced by picosecond laser. S ci Rep 12, 17371 (2022). https://doi.org/10.1038/s41598- 022-21432-9

[5] B. Sun, P. S. Salter, M. J. Booth, Proc. SPIE 9736, 973612 (4 March 2016) https://doi.org/10.1117/12.2212686

[6] Richard D. Simmonds, Patrick S. Salter, Alexander Jesacher, and Martin J. Booth, "Three dimensional laser microfabrication in diamond using a dual adaptive optics system," Opt. Express 19, 24122-24128 (2011). [7] P. S. Salter, M. Baum, I. Alexeev, M. Schmidt, and M. J. Booth, "Exploring the depth range for three-dimensional laser machining with aberration correction," Opt. Express 22, 17644-17656 (2014).

Claims

1. A method of manufacturing a device, the method comprising: forming a device track having asymmetric electrical conductivity along the device track by modifying the structure of carbon within a diamond substrate.

2. The method of claim 1, wherein the modifying of the structure of carbon comprises modifying a pre-existing precursor track, the precursor track having symmetric electrical conductivity along the precursor track.

3. The method of claim 2, wherein the precursor track is formed by modifying the structure of carbon within the substrate, optionally wherein the method further comprises forming the precursor track.

4. The method of claim 3, wherein: the forming of the precursor track comprises modifying the structure of the carbon within the substrate by delivering energy to the substrate according to a first regime; the forming of the device track comprises modifying the precursor track by delivering energy along the precursor track according to a second regime; the first regime is such that delivering energy to a previously unprocessed diamond substrate according to the first regime forms a conductive track; and the second regime is such that delivering energy to a previously unprocessed diamond substrate according to the second regime forms an insulating track.

5. The method of any preceding claim, wherein the forming of the device track comprises a unidirectional process starting at a first end of the device track and ending at a second end of the device track.

6. The method of any preceding claim, wherein the device track is formed between a first surface of the substrate and a second surface of the substrate, optionally wherein the second surface is opposite to the first surface.

7. The method of any of claims 1 to 5, wherein the device track is formed entirely within the substrate, optionally wherein the method further comprises forming one or more conductive tracks connecting the device track to a surface of the substrate.

8. The method of any preceding claim, wherein modifying the structure of the carbon comprises forming graphite.

9. The method of any preceding claim, wherein modifying the structure of the carbon comprises forming one or more regions comprising multi-phase carbon, optionally wherein the one or more regions include: a) a region comprising graphite within which there are multi-phase nanocarbon composite regions; and/or b) a region comprising graphite within which there are diaphites.

10. The method of any preceding claim, wherein modifying the structure of the carbon comprises using a laser.

11. The method of claim 10, wherein the laser is a pulsed laser.

12. The method of claim 11, wherein: a) a duration of pulses of the laser is at most lOps, optionally at most Ips, optionally at most 500fs, optionally at most 200fs; and/or b) an energy of each pulse of the laser is between lOnJ and lOpJ, optionally between 50nJ and 200nJ.

13. The method of claim 11 or 12, wherein a pulse rate of the laser during the forming of the device track is between 10kHz and 10MHz, optionally between 100kHz and 5MHz.

14. The method of claim 11 or 12, wherein the modifying of the structure of carbon comprises modifying a pre-existing precursor track having symmetric electrical conductivity along the precursor track, and a pulse rate of the laser during the modifying of the precursor track is between 10kHz and 10MHz, optionally between 100kHz and 5MHz.

15. The method of any of claims 11 to 14, wherein the method further comprises forming a precursor track having symmetric electrical conductivity along the precursor track, and a pulse rate of the laser during the forming of the precursor track is between 100Hz and 30kHz, optionally between 500Hz and 10kHz.

16. The method of any of claims 10 to 15, wherein: a) the laser has a wavelength of between 340nm and 1600nm, optionally between 500nm and 1070nm, optionally about 515nm; and/or b) the laser is a Yb or Ti: Sapphire laser.

17. The method of any of claims 10 to 16, wherein modifying the structure of the carbon comprises scanning a focal point of the laser through the substrate, optionally at a rate of between Ipm/s and lOOpm/s, optionally between 5pm/s and 50pm/s.

18. The method of claim 17, wherein the laser is a pulsed laser and during the forming of the device track, the focal point of laser is scanned through the substrate by between Inm and 10pm between pulses.

19. The method of claim 17 or 18, wherein the laser is a pulsed laser, the method further comprises forming a precursor track having symmetric electrical conductivity along the precursor track, and during the forming of the precursor track, the focal point of laser is scanned through the substrate by between lOOnm and Inm between pulses.

20. The method of any of claims 10 to 19, wherein modifying the structure of the carbon comprises focussing the laser inside the substrate.

21. The method of claim 20, wherein the laser has a focal point that is diffraction limited, optionally wherein the focal point has a diameter of at most 5 pm, optionally at most 2pm.

22. The method of claim 21, wherein the laser is passed through a spatial light modulator before reaching the substrate, optionally wherein the spatial light modulator is used to compensate for optical aberrations such that the focal point remains diffraction limited during scanning of the laser beam.

23. A device comprising a diamond substrate, wherein: the substrate comprises a device track formed by modifying the structure of carbon within the substrate; and electrical conductivity along the device track is asymmetric.

24. The device of claim 23, wherein the device track has a first end and a second end, and a first electrical resistance in respect of current flowing from the first end to the second end is different from a second electrical resistance in respect of current flowing from the second end to the first end.

25. The device of claim 23 or 24, wherein the device track has a first end and a second end, and a first barrier potential opposing current flowing from the first end to the second end is different from a second barrier potential opposing current flowing from the second end to the first end, optionally wherein the first barrier potential is at least 100% greater than the second barrier potential, optionally at least 200%, optionally at least 400%.

26. The device of any of claims 23 to 25, wherein the device track extends within an interior of the substrate.

27. The device of claim 26, wherein the device track extends entirely within the interior of the substrate, optionally wherein the device track is connected to a surface of the substrate using a conductive track.

28. The device of any of claims 23 to 26, wherein the device track extends between a first surface of the substrate and a second surface of the substrate, optionally wherein the second surface is opposite to the first surface.

29. The device of any of claims 23 to 28, wherein the device track comprises graphite.

30. The device of any of claims 23 to 29, wherein the device track comprises one or more regions comprising multi-phase carbon, optionally wherein the one or more regions include: a) a region comprising graphite within which there are multi-phase nanocarbon composite regions; and/or b) a region comprising graphite within which there are diaphites.

31. The device of any of claims 23 to 30, wherein the device is manufactured using a method according to any of claims 1 to 22.

32. The method or device of any preceding claim, wherein the substrate comprises: a) single-crystal diamond or poly-crystalline diamond, optionally formed by chemical vapour deposition; and/or b) boron-doped diamond or hydrogen-terminated diamond.

33. The method or device of any preceding claim, wherein the substrate comprises single-crystal diamond and the device track is formed along the [100] direction of the substrate.

34. A diode or a transistor comprising a device according to any of claims 23 to 33.