The present invention relates to a method of forming a graphene layer structure, in particular by CVD on specific growth surfaces of substrates. In a particularly preferred aspect of the present invention, the growth surface is formed of yttrium stabilised zirconium oxide (yttria stabilised zirconia; YSZ). The present invention also provides a graphene substrate, in particular wherein a graphene layer structure is directly on a specific layer of material, preferably, YSZ.

Graphene has received much attention as a two-dimensional material in view of its unique electronic properties and its applications in electronic devices. It is common in the art for graphene to be manufactured by techniques such as exfoliation or by CVD on catalytic metal substrates such as copper. The graphene produced by such methods is then transferred to electronic device compatible, insulative or semiconducting substrates.

It is also known in the art that graphene may be synthesised, manufactured, formed, directly on non-metallic surfaces of substrates. These include silicon, sapphire and lll-V semiconductor substrates. The present inventors have found that the most effective method for manufacturing high-quality graphene, especially directly on such non-metallic surfaces, is that disclosed in WO 2017/029470, as well as GB 2585842. This publication discloses methods for manufacturing graphene; principally these rely on heating a substrate held within a reaction chamber to a temperature that is within a decomposition range of a carbon based precursor for graphene growth, introducing the precursor into the reaction chamber through a relatively cool inlet so as to establish a sufficiently steep thermal gradient that extends away from the substrate surface towards the point at which the precursor enters the reaction chamber such that the fraction of precursor that reacts in the gas phase is low enough to allow the formation of graphene from carbon released from the decomposed precursor. Preferably the apparatus comprises a showerhead having a plurality of precursor entry points or inlets, the separation of which from the substrate surface may be varied and is preferably less than 100 mm. The method of WO 2017/029470 is ideally performed using an MOCVD reactor. Whilst MOCVD stands for metal organic chemical vapour deposition due to its origins for the purposes of manufacturing semiconductor materials such as AIN and GaN from metal organic precursors such as AIMe3 (TMAI) and GaMe3 (TMGa), such apparatus and reactors are well known and understood to those skilled in the art as being suitable for use with non-metal organic precursors. MOCVD may be used synonymously with metal organic vapour phase epitaxy (MOVPE).

Whilst the method of WO 2017/029470 enables the production of high-quality graphene with excellent uniformity and a constant number of layers (as desired) across its whole area on the substrate without additional carbon fragments or islands, the strict requirements in the art of electronic device manufacture means that there remains a need to further improve the electronic properties of the graphene and to provide methods that are more reliable and more efficient for the industrial manufacture of graphene, particularly large area graphene on non-metallic substrates. US 2012/181505 A1 discloses forming a carbon-based material on a substrate comprising calcium fluoride.

CN 105355702 B discloses transferring graphene grown by CVD onto a dielectric layer.

Karamat et al. "Growth of nano-graphene on SrTiCb (110) substrates by chemical vapour deposition" Materials Chemistry and Physics 2017, 200, 187-195 discloses growth of multilayer nanographene domains on SrTiC>3.

US 2018/0323406 A1 discloses growth of graphene by CVD on a metal film of a substrate or by growth on a metal foil substrate and transferring graphene.

CN 212162092 U relates to a tuneable terahertz absorber and teaches using photolithographic exposure techniques to fabricate a graphene pattern and deposit a graphene layer.

The present inventors sought to overcome the problems in the prior art and were surprised to find that particular non-metallic materials provide superior growth surfaces for the formation of high quality graphene and graphene substrates suitable for use in electronic device fabrication.

In accordance with a first aspect of the present invention, there is provided a method of forming a graphene layer structure, the method comprising: providing a growth substrate having a growth surface; and forming a graphene layer structure on the growth surface by CVD; wherein the growth surface is formed of a material selected from the group consisting of:

YSZ, MgAI ₂ 0 ₄ , YAIOs, CaF ₂ , and LaF ₃ .

In a further aspect there is provided a graphene substrate comprising: a CVD-grown graphene layer structure grown directly on a first layer, wherein the first layer is formed of a material selected from the group consisting of: YSZ, MgAI ₂ C>4, YAIO3, CaF ₂ and LaF3.

The present disclosure will now be described further. In the following passages, different aspects/embodiments of the disclosure are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention relates to a method of forming a graphene layer structure by CVD growth of graphene on a growth surface of a substrate (which may be referred to as a growth substrate). The method thereby forms a graphene substrate and accordingly, the present invention also provides a graphene substrate per se. Forming may be considered synonymous with synthesising, manufacturing, producing and growing. Graphene is a very well-known two-dimensional material referring to an allotrope of carbon comprising a single layer of carbon atoms in a hexagonal lattice. Graphene, as used herein, refers to one or more layers of graphene. Accordingly, the present invention relates to the formation of a monolayer of graphene as well as multilayer graphene (which may be termed a graphene layer structure). Graphene, as used herein, refers to a graphene layer structure, preferably having from 1 to 10 monolayers of graphene. In many subsequent applications of a graphene substrate, one monolayer of graphene is particularly preferred. Accordingly, the graphene layer structure is preferably a graphene monolayer. Nevertheless, multilayer graphene may be preferable for certain applications and 2 or 3 layers of graphene may be preferred. As described herein, the method of forming a graphene layer structure comprises forming graphene by CVD directly onto specific growth surfaces.

A graphene substrate will be understood as a substrate comprising graphene and suitable for subsequent use. Particularly, the graphene substrate is suitable for use in preparing graphene based electronic devices. As used herein, the term substrate may be used to refer to a material suitable for the deposition of another layer thereon. The term substrate is typically synonymous with a wafer. Accordingly, each of the support layer and metal oxide can each independently be referred to as a substrate.

The method comprises providing a growth substrate having a growth surface. Substrates suitable for the growth of layers on their surface are well known. Substrates may also be referred to in the art as wafers and may consist of a single material or layers of multiple materials. As will be appreciated, a substrate, and a growth surface thereof, is formed of crystalline material as is known in the art. Accordingly, substrates and wafers provide a planar growth surface, preferably formed of a single crystal, and do not include powders or nano-crystalline materials. Typically, substrates have a diameter of at least 1 inch (25 mm), preferably at least 2 inches (51 mm).

The growth substrate for use in the present method is provided with a growth surface wherein the growth surface is formed of a material selected from the group consisting of: yttrium stabilised zirconium oxide (YSZ), magnesium aluminate (MgAbO-t), yttrium aluminium perovskite (YAIOs or YAP), calcium difluoride (CaF2) and lanthanum trifluoride (LaF3). In one embodiment, the growth substrate consists of one of said materials. Preferably, the thickness of the substrate is at least 250 pm, preferably at least 400 pm. However, preferably, the growth substrate further comprises a support layer which preferably comprises silicon or sapphire. As will be appreciated, a silicon support layer, for example, includes a “pure” silicon wafer (essentially consisting of silicon, doped or undoped) or what may be referred to as a CMOS wafer which includes additional associated circuitry. The thickness of the material used to form the growth surface of such a substrate may be much thinner. Preferably, the thickness is at least 5 nm, preferably at least 10 nm. The upper limit is not particularly restricted though a “thin film” growth surface provided on a support substrate is generally less than 50 pm thick, such as less than 10 pm or even less than 5 pm.

The present inventors found that growth surfaces for the formation of graphene by CVD being made of YSZ, MgAl2C>4, YAICb, CaF2 or LaF3 were surprisingly advantageous. Without wishing to be bound by theory, the inventors believe that these materials may have a low carbon solubility at high temperatures (relative to known growth substrate materials) such that during the high temperatures of CVD, high-quality uniform graphene may be grown without the defects which can be present when grown directly on other known growth surfaces. For example, it is known that growth surfaces formed of materials such as silicon or lll-V semiconductors can give rise to covalent bonding to the carbon atoms during growth resulting in graphene defects. Accordingly, the use of the materials described herein provides advantages for CVD grown graphene in terms of the resulting electronic properties, i.e. improved mobility, sheet resistance and Flail sensitivity.

As will be appreciated, the stoichiometries of the materials need not be precise. As is known in the art, the stoichiometry of such materials may vary. In particular, it is known that the oxygen stoichiometry may vary. By way of example only, magnesium aluminate, may be referred to as MgA Ox wherein x is about 4.

Preferably, the growth surface is formed of a material selected from the group consisting of YSZ, MgA CU, YAIO3, and CaF2, even more preferably from the group consisting of YSZ, YAIO3, and CaF2. It is preferred that the growth surface is formed of YSZ or CaF2 as the inventors have found that these materials were surprisingly the most effective of those of the present invention in providing high quality graphene by CVD. It is particularly preferred that the growth surface is formed of YSZ. The crystallographic orientation of the YSZ or the CaF2 growth surface may preferably be <100>, <110>, or <111 >, more preferably <100> or <111 >, and most preferably <111 >.

CVD refers generally to a range of chemical vapour deposition techniques, each of which involve vacuum deposition to produce thin film materials such as two-dimensional crystalline materials like graphene. Volatile precursors, those in the gas phase or suspended in a gas, are decomposed to liberate the necessary species to form the desired material, carbon in the case of graphene.

CVD as described herein is intended to refer to thermal CVD such that the formation of graphene from the decomposition of a carbon-containing precursor is the result of the thermal decomposition of said carbon-containing precursor.

Preferably, the method involves forming graphene by thermal CVD such that decomposition is a result of heating the carbon-containing precursor. Preferably, the temperature of the growth surface during CVD is from 700°C to 1350°C, preferably from 800°C to 1250°C, more preferably from 1000°C to 1250°C. The inventors have found that such temperatures are particularly effective for providing graphene growth directly on the materials described herein by CVD. Preferably, the CVD reaction chamber used in the method disclosed herein is a cold-walled reaction chamber wherein a heater coupled to the substrate is the only source of heat to the chamber.

In a particularly preferred embodiment, the CVD reaction chamber comprises a close-coupled showerhead having a plurality, or an array, of precursor entry points. Such CVD apparatus comprising a close-coupled showerhead may be known for use in MOCVD processes. Accordingly, the method may alternatively be said to be performed using an MOCVD reactor comprising a close-coupled showerhead. In either case, the showerhead is preferably configured to provide a minimum separation of less than 100 mm, more preferably less than 25 mm, even more preferably less than 10 mm, between the surface of the substrate and the plurality of precursor entry points. As will be appreciated, by a constant separation it is meant that the minimum separation between the surface of the substrate and each precursor entry point is substantially the same. The minimum separation refers to the smallest separation between a precursor entry point and the substrate surface (i.e. the surface of the metal oxide layer). Accordingly, such an embodiment involves a “vertical” arrangement whereby the plane containing the precursor entry points is substantially parallel to the plane of the substrate surface (i.e. the growth surface).

The precursor entry points into the reaction chamber are preferably cooled. The inlets, or when used, the showerhead, are preferably actively cooled by an external coolant, for example water, so as to maintain a relatively cool temperature of the precursor entry points such that the temperature of the precursor as it passes through the plurality of precursor entry points and into the reaction chamber is less than 100°C, preferably less than 50°C. For the avoidance of doubt, the addition of precursor at a temperature above ambient does not constitute heating the chamber, since it would be a drain on the temperature in the chamber and is responsible in part for establishing a temperature gradient in the chamber.

Preferably, a combination of a sufficiently small separation between the substrate surface and the plurality of precursor entry points and the cooling of the precursor entry points, coupled with the heating of the substrate to with a decomposition range of the precursor, generates a sufficiently steep thermal gradient extending from the substrate surface to the precursor entry points to allow graphene formation on the substrate surface. As disclosed in WO 2017/029470, very steep thermal gradients may be used to facilitate the formation of high-quality and uniform graphene directly on non-metallic substrates, preferably across the entire surface of the substrate. The substrate may have a diameter of at least 5 cm (2 inches), at least 15 cm (6 inches) or at least 30 cm (12 inches). Particularly suitable apparatus for the method described herein include an Aixtron® Close-Coupled Showerhead® reactor and a Veeco® TurboDisk reactor.

Consequently, in a particularly preferred embodiment wherein the method of the present invention involves using a method as disclosed in WO 2017/029470, forming the graphene layer structure on the growth surface by CVD comprises: providing the growth substrate on a heated susceptor in a close-coupled reaction chamber, the close-coupled reaction chamber having a plurality of cooled inlets arranged so that, in use, the inlets are distributed across the growth surface and have constant separation from the substrate; cooling the inlets to less than 100°C (i.e. so as to ensure that the precursor is cool as it enters the reaction chamber); introducing a carbon-containing precursor in a gas phase and/or suspended in a gas through the inlets and into the close-coupled reaction chamber; and heating the susceptor to achieve a growth surface temperature of at least 50°C in excess of a decomposition temperature of the precursor, to provide a thermal gradient between the substrate surface and inlets that is sufficiently steep to allow the formation of graphene from carbon released from the decomposed precursor; wherein the constant separation is less than 100 mm, preferably less than 25 mm, even more preferably less than 10 mm.

The most common carbon-containing precursor in the art for graphene growth is methane (CPU). The inventors have found that it is preferable that the carbon-containing precursor used to form graphene is an organic compound, that is, a chemical compound, or molecule, that contains a carbon-hydrogen covalent bond, which comprises two or more carbon atoms. Such precursors have a lower decomposition temperature than methane which advantageously allows the growth of graphene at lower temperatures when using the method described herein which is particularly advantageous for growth on such non-metallic surfaces. Preferably, the precursor is a liquid when measured at 20°C and 1 bar of pressure (i.e. under standard conditions according to lUPAC). Accordingly, the precursor has a melting point that is below 20°C, preferably below 10°C, and has a boiling point above 20°C, preferably above 30°C. Liquid precursors are simpler to store and handle when compared to gaseous precursors which typically require high pressure cylinders. Due to their relatively reduced volatility when compared to gaseous precursors, they present a lower safety risk during large scale manufacture. Increasing the molecular weight of the compounds beyond about Cio, particularly beyond about C12, typically reduces their volatility and suitability for CVD growth of graphene on non-metallic substrates (though graphene can be produced from solid organic compounds). Preferably, the organic compound consists of carbon and hydrogen and, optionally, oxygen, nitrogen, fluorine, chlorine and/or bromine.

As discussed above, the method described herein preferably uses a carbon-containing precursor that is an organic compound comprising two of more carbon atoms, i.e. a C2 ₊ organic compound. Preferably, the carbon-containing precursor is a C3-C12 organic compound consisting of carbon and hydrogen and, optionally, oxygen, nitrogen, fluorine, chlorine and/or bromine. As described herein, a Cn organic compound refers to one comprising “n” carbon atoms and optionally one or more further hetero atoms oxygen, nitrogen, fluorine, chlorine and/or bromine. Preferably, the organic compound comprises at most one heteroatom as such organic compounds are typically more readily available in high purity, for example ethers, amines, and haloalkanes. The carbon-containing precursor is preferably a C3-C10 organic compound consisting of carbon and hydrogen and, optionally, oxygen, nitrogen, fluorine, chlorine and/or bromine, even more preferably a C6-C9 organic compound. In a preferred embodiment, the precursor does not comprise a heteroatom, such that the precursor consists of carbon and hydrogen. In other words, preferably the carbon- containing precursor is a hydrocarbon, preferably an alkane.

It is also preferable that the organic compound comprise at least two methyl groups (-CH3). Particularly preferred organic compounds for use as carbon-containing precursors, and methods of forming graphene therefrom by CVD, are described in UK Patent Application No. 2103041 .6, the contents of which is incorporated herein in its entirety. The inventors have found that when forming graphene directly on non-metallic substrates, precursors beyond the traditional hydrocarbons methane and acetylene allow for the formation of even higher quality graphene. Preferably, the precursor is a C4-C10 organic compound, more preferably the organic compound is branched such that the organic compound at least three methyl groups.

Without wishing to be bound by theory, the inventors believe that heavier organic compounds (i.e. those greater than C12, or greater than C10, and/or those which are solid under standard conditions) provide a “less pure” source of CH3 radicals. With an increase in size and complexity of the organic compound there is an increase in the number of decomposition pathways and the possibility of a greater range of byproducts which can lead to graphene defects. The organic compounds as described herein provide a balance of being large enough to deliver the required, and a desirably high fraction of, methyl groups under pyrolysis. The organic compounds are however small enough to be simple to purify, particularly where the precursor is liquid, and have a relatively simple pyrolysis chemistry with limited decomposition pathways. Furthermore, unlike heavier compounds, they do not so readily condense within the reactor plumbing which is a particular disadvantage for the industrial production of graphene due to the greater risk of reactor downtime.

The present invention also provides a graphene substrate comprising a CVD-grown graphene layer structure grown directly on a first layer, the first layer formed of a material selected from the group consisting of YSZ, MgA C , YAIO3, CaF2 and LaF3. That is, preferably, the graphene substrate is obtainable, more preferably obtained, by the method disclosed herein. The graphene being that grown directly on the specific materials disclosed herein by CVD therefore avoids physical transfer processing. The physical transfer of graphene, usually from copper substrates, introduces numerous defects which negatively impacts the physical and electronic properties of graphene. As such, a person skilled in the art can readily ascertain whether a graphene layer structure, and by extension a graphene substrate as described herein, is one comprising a CVD-grown graphene layer structure that has been grown directly on the specific materials using conventional techniques in the art such as atomic force microscopy (AFM), energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS) and preferably time-of-flight secondary ion mass spectrometry (ToF-SIMS). The graphene layer structure is devoid of copper contamination and devoid of transfer polymer residues by virtue of the complete absence of these materials in the process of obtaining the graphene substrate. Furthermore, such processing is not suitable for large scale manufacture (such as on CMOS substrates in fabrication plants). Unintentional doping, particularly from the catalytic metal substrates together with the etching solutions, also results in the production of graphene which is not sufficiently consistent from sample to sample as is required for commercial production.

Accordingly, it is preferred that first layer is formed of a material selected from the group consisting of YSZ, MgA C , YAIO3, and CaF2, more preferably from the group consisting of YSZ, YAIO3, and CaF2. Even more preferably, the first layer is YSZ or CaF2, preferably YSZ. Preferably, the first layer is directly on a support layer, preferably wherein the support layer comprises sapphire or silicon.

It is preferred that the growth surface is formed of YSZ or CaF2 as the inventors have found that these materials were surprisingly the most effective of those of the present invention in providing high quality graphene by CVD. It is particularly preferred that the growth surface is formed of YSZ. The crystallographic orientation of the YSZ or the CaF2 growth surface may preferably be <100>, <110>, or <111 >, more preferably <100> or <111 >. Specifically, the inventors were surprised to find that the growth mechanism of graphene by CVD differed for various substrate orientations which unexpectedly gave rise to different improvements in the graphene properties.

Advantageously, <100> and <111 > of YSZ produced graphene having particularly beneficial electrical properties resulting from the combination of mobility and charge carrier density which the inventors found were correlated with the ratio of the peak areas A2D to AG in the Raman spectrum of the graphene. A higher ratio is indicative of the reduced “substrate interaction” with the graphene layer structure. Accordingly, it is preferred that the ratio A2D/AG of the graphene layer structure as measured by Raman spectroscopy is greater than 3, preferably greater than 3.5, more preferably greater than 4. On the other hand, <110> was unexpectedly advantageous in that graphene having a lower charge carrier density is formed when compared to growth on <100> and <111 >. It is also preferred that the ratio AD/AG is less than 0.8, preferably less than 0.6, more preferably less than 0.4.

Furthermore, the inventors were surprised to find that the doping type of the graphene may be influenced by the substrate orientation. CaF2 having a crystallographic orientation of <100> or <110> was found to provide n-type graphene, whereas CaF2 having a crystallographic orientation of <111 > was found to provide p-type graphene. Moreover, CaF2 <111 > gave rise to graphene having higher mobility than <100> or <110> and a higher A2D/AG ratio resulting from the reduced substrate interaction. This was evidenced by the greater wrinkle density of graphene grown on the <100> face of CaF2 (relative to the diagonal of <111 >). The cubic influence of the substrate can be seen in the wrinkle pattern in Figures 2B and 13B. However, graphene grown on CaF2 <100> unexpectedly had markedly lower sheet resistance than graphene grown on <110> or <111 >. In accordance with a further aspect of the present invention, there is provided an electrical device comprising the graphene substrate as described herein. As will be appreciated, the graphene layer structure of the graphene substrate may be patterned using known techniques and electrical contacts may be provided so as to enable the substrate to be incorporated into an electrical device. That is, an electrical device may be manufactured from a graphene substrate thereby incorporating a graphene layer structure directly on a metal oxide layer, said metal oxide layer optionally directly on a support layer as described herein. Further steps for forming electrical devices are known in the art and may include patterning, such as by photolithography, laser and/or plasma etching, and/or deposition of additional layers and materials such as dielectric layers and/or metal ohmic contacts. The electrical device may be diced from an array of devices formed simultaneously from a larger graphene substrate.

An electrical device comprising such a graphene substrate (i.e. comprising graphene grown directly on a first layer) may be improved over prior art devices in view of the advantageous properties afforded by the graphene, in particular an improvement in carrier mobility. For example, an electro optic modulator is one preferred electrical device which may benefit from greater carrier mobility. In particular, an electro-optic modulator comprising the graphene substrate may operate with greater bandwidth. Other preferred electric devices include transistors (i.e. graphene transistors) such as radio frequency graphene field effect transistors (RF GFETs) which rely on high carrier mobilities to switch “on” and “off” at such high frequencies. Biosensors are also preferred electrical devices which benefit from the higher mobility of graphene due to the associated reduction in sheet resistance which reduces the power required for operation. Another particularly preferred electrical device is a Hall effect sensor. The sensitivity of such devices may be improved with higher carrier mobilities.

The inventors have also found that graphene may be grown by CVD on a substrate having a growth surface formed of magnesium oxide (MgO), strontium titanate (SrTiCb) or yttrium aluminium garnet (Y3AI5O12 or YAG). Other preferred fluorides include magnesium difluoride (MgF2) or barium difluoride (BaF2). Accordingly, there is also described herein a graphene substrate comprising a graphene layer structure directly on a first layer, wherein the first layer is formed of MgO, SrTi03, YAG, MgFå or BaF2. In these instances, all passages of the description herein which refer to YSZ, MgAl204, YAIO3, CaF2 and LaF3 should be construed as applying equally for MgO, SrTi03, YAG, MgFå and BaF2.

Figures

The present invention will now be described further with reference to the following non-limiting Figures, in which:

Figure 1 A is a Raman spectrum of graphene grown on a substrate having a YSZ growth surface. Figure 1 B is an AFM image of the graphene on YSZ. Figure 2A is a Raman spectrum of graphene grown on a substrate having a CaF2 growth surface. Figure 2B is an AFM image of the graphene on CaF2.

Figure 3A is a Raman spectrum of graphene grown on a substrate having a YAIO3 growth surface. Figure 3B is an AFM image of the graphene on YAIO3.

Figure 4A is a Raman spectrum of graphene grown on a substrate having an SrTiCb growth surface. Figure 4B is an AFM image of the graphene on SrTiCb.

Figure 5A is a Raman spectrum of graphene grown on a substrate having an MgA C growth surface. Figure 5B is an AFM image of the graphene on MgAl2C>4.

Figure 6A is a Raman spectrum of graphene grown on a substrate having an MgO growth surface. Figure 6B is an AFM image of the graphene on MgO.

Figure 7 is a Raman spectrum of graphene grown on a substrate having an LaF3 growth surface.

Figure 8A is a Raman spectrum of graphene grown on a substrate having a YAG growth surface. Figure 8B is an AFM image of the graphene on YAG.

Figures 9A, 9B and 9C are Raman spectra of graphene grown on YSZ <100>, <110>, and <111 >, respectively.

Figure 10 is a plot of mean step height against the ratio of A2D/AG for graphene grown on YSZ <100>, <110>, and <111>.

Figure 11 is a plot of the carrier densities against mobility obtained for graphene grown on YSZ <100>, <11 >, and <111 >.

Figure 12A is a Raman spectrum of graphene grown on CaF2 <111 >. Figure 12B is an AFM image of the graphene on CaF2 <111 >.

Figure 13A is a Raman spectrum of graphene grown on CaF2 <100>. Figure 13B is an AFM image of the graphene on CaF2 <100>.

Figure 14A is a Raman spectrum of graphene grown on CaF2 <110>. Figure 14B is an AFM image of the graphene on CaF2 <110>.

Examples A substrate is positioned upon a silicon carbide-coated graphite susceptor within an MOCVD reactor chamber. The reactor chamber itself is protected in an inert atmosphere within a glovebox. The reactor is then sealed closed using a vacuum cavity which separates the reactor interior from the glovebox ambient by a double O-ring. The reactor is purged under a flow of nitrogen, argon or hydrogen gas at a rate of 10,000 to 60,000 seem. The susceptor is rotated at a rate of 40 to 60 rpm. The pressure within the reactor chamber is reduced to 30 to 100 mbar. An optical probe is used to monitor the wafer reflectivity and temperature during growth - with the substrate still in its unheated state, it is rotated under the probe in order to establish a baseline signal. The substrate is then heated using resistive heater coils positioned beneath the susceptor to a setpoint of from 1 ,100 to 1 ,350°C at a rate of 0.5 to 2.0 K/s. The wafers are optionally baked under flow of hydrogen gas for from 10 to 60 min, after which the ambient gas is switched to nitrogen or argon and the pressure is reduced to 30 to 50 mbar. The wafer is annealed at the growth temperature and pressure for a period of from 5 to 10 min, after which a hydrocarbon precursor is admitted to the chamber. This is transported from its liquid state in a bubbler by passing a carrier gas (nitrogen, argon or hydrogen) through the liquid which is held under constant temperature and pressure. The vapour enters a gas mixing manifold and proceeds to the reactor chamber through a showerhead via a multitude of small inlets commonly referred to in the art as plenums/plena, which guarantees uniform vapour distribution and growth across the growth surface. The substrate is exposed to the hydrocarbon vapour under constant flow, pressure and temperature for a duration of 1 ,800 to 10,800 s at which point the precursor supply valve is shut off. The substrate is then cooled under continuing flow of nitrogen, argon or hydrogen gas at a rate of from 2 to 4 K/min. Once the substrate temperature reaches below 200°C, the chamber is pumped to vacuum and purged with inert gas. The rotation is stopped and the heaters are shut off. The reactor chamber is opened and the graphene substrate is removed from the susceptor once the heater temperature reaches below 150°C.

The table below provides Raman data of various graphene substrates.

The present example demonstrates the particularly advantageously low A(D) / A(G) ratio observed for monolayer graphene manufactured on a growth surface in accordance with the present invention. Similarly, the inventors observed an advantageously low GFWHM (full width at half maximum of the Raman G peak) even when compared to growth of graphene using substrates and precursor known in the Applicant’s own prior art (R-plane sapphire substrates and 2,2,3-trimethylbutane) to improve quality of CVD grown graphene.

Further experiments were conducted for graphene growth on MgAl2C>4, YAIO3, SrTiCb, MgO, CaF2, LaF3 and Y3AI5O12. The inventors found that sheet resistances of 700 W/a (Ohm/sq) and lower can be achieved. Graphene grown on CaF2 was observed to have a sheet resistance of less than 600 W/a. Additionally, graphene may be grown on CaF2 at substantially reduced temperatures due to significantly enhanced mass transport mobility of carbon species during growth compared with oxide surfaces such as sapphire. Growth surfaces such as MgAl204 have the advantage of providing a particularly low roughness. The inventors have found that YAIO3 can provide the graphene with a charge transport anisotropy due to the thermal expansion anisotropy of the growth surface. Accordingly, the inventors have found that each of the growth surfaces provide particular advantages for the formation of graphene by CVD. Various Raman and AFM data for the graphene obtained on these growth surfaces is shown in Figures 1 A to 8B.

It was found that graphene grown directly on these growth surfaces (in particular YSZ, MgAl204, YAIO3, CaF2 and LaFe) formed without undesirable carbon dissolution into the growth surface during growth and with a reduction in the formation of covalent bonds between the graphene and growth surface which otherwise result in graphene defects. Accordingly, such surfaces are particularly suitable for providing a growth surface for the growth of monolayer graphene due to the lack of carbon dissolution and subsequent precipitation of carbon either as defects or areas of multi-layer graphene. Equally, the lack of covalent bonding provides for the formation of graphene rather than other carbon allotropes such as carbon nanotubes.

The inventors also investigated the influence of the substrate crystallographic orientation. The growth process was repeated for each of YSZ <100>, <110> and <111 > and growth was halted prior to complete formation of a graphene monolayer. The “step height” of the graphene grain edges were measured by AFM. That is, the difference in height between the substrate surface and the adjacent graphene surface was measured, and the results provided in the table below.

The results are also plotted in Figure 10 demonstrating the strong correlation between mean step height and the resulting A2D/AG ratio. That is, the inventors were surprised to find that different orientations gave rise to different strengths of interaction with the graphene thereon. YSZ <111 > was found to have the least interaction with the graphene as evidence by the greatest mean change in height which in turn resulted in graphene having a higher A2D/AG ratio. The Raman spectra of the graphene after complete monolayer growth on each substrate are shown in Figures 9A to 9C.

The inventors also investigated the resulting electronic properties of the graphene grown on each of the crystal orientations and were equally surprised to find that there were distinct differences. As shown by the plot in Figure 11 of carrier density (cm ^-2 ) against mobility (cm ² /Vs), YSZ <111 > consistently gave rise to higher carrier mobilities across a range of carrier densities. YSZ <100> gave rise to consistently larger carrier densities compared to YSZ <110>.

The inventors also investigated the influence of CaF2 crystallographic orientation. The growth process was repeated for CaF2 <111 >, <100>, and <110>, the resulting data for which is shown in Figures 12A and B, 13A and B, and 14A and B, respectively, and the following table.

The inventors were surprised to find that the different orientations gave rise to different doping types in the graphene grown thereon as evidence by the charge carrier density. It is also particularly unexpected that whilst less “substrate interaction” was observed when grown in CaF2 as shown by the larger A2D/AG ratio, a higher defect density was also observed such that CaF2 <100> gave rise to graphene have improved electrical properties over <111 >.

As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.