BACKGROUND

The present invention relates to a method of producing a two-dimensional material particularly but not exclusively graphene and silicene. The invention also relates to a method of producing heterostructures comprising two-dimensional materials.

Graphene is a well-known material with a plethora of proposed applications driven by the material's theoretical extraordinary properties. Good examples of such properties and applications are detailed in ‘The Rise of Graphene’ by A. K. Geim and K. S. Novoselev, Nature Materials, vol. 6, Mar. 2007, 183-191.

Nevertheless, to achieve these desired material properties and applications, it is well understood that it is essential for the graphene to have a number of characteristics including:

Very good crystal quality, namely that the graphene structure lattice is extremely uniform about all axes, highly repeatable in symmetry throughout the full monolayer and exhibits minimal lattice malformations;

Large material grain size, whereby the grain structure of grown graphene exhibits individual grain dimensions ≥10 μm×10 μm;

Minimal material defects, whereby defects include crystal lattice breaks, interruptions, atomic or molecular contamination to the crystal from other elements, or poor graphene monolayer surface condition through, for example oxidation;

Large sheet size, i.e. greater than 3 cm×3 cm and preferably in the order of 10 s of centimeters; and

Be self supporting so that a complete sheet of the size given in iv above can be removed intact from the substrate upon which it was produced.

Conventional graphene production methodologies to date have been unable to manufacture graphene with all of the aforementioned properties. Consequently, the predicted performance properties and device applications of graphene have yet to be realized.

Several conventional methods of graphene production exist and are in wide use;

examples are described in:

US 20130156678 Al13solution based electrophoresis of graphene on metallic substrates or films, whereby an electrical potential is applied to a conductive substrate immersed in carbon comprising solution. The result is carbon transport, due to the applied field, to the substrate surface at which point self alignment of the carbon into graphene ‘sheets’ occurs;

US 8147791 B2—a graphene oxide reduction mechanism whereby graphene oxide is introduced to a water and solvent solution and heated to moderate (<300° C.) temperature resulting in the dissociation of oxygen and allowing carbon amalgamation, potentially in graphene crystal structure configuration;

WO 2014110170 A1—a catalysis driven chemical vapor deposition (CVD) technique whereby a heated copper substrate is used as a catalytic surface in a standard CVD chamber for the decomposition of hydrocarbons resulting in carbon being left on the metallic surface.

Beyond the current inability to achieve the important material properties described above, further limitations exist to these conventional techniques.

The need of special sacrificial metal catalyst substrates to promote graphene formation introduces limitations to the production process parameters. Examples of these limitations include the need to use temperatures that do not affect phase changes in the metal substrates and the need to use non-reducing gases and precursors that will not degrade the metallic substrate surface. This inflexibility in production variables has lead, in certain processes, to difficulty to achieve good graphene growth and the inability to remove unintended contamination or dopants.

In current processes, the graphene grains formed are not sufficiently bonded to maintain sheet form once removed from the substrate. Consequently, graphene is most commonly available in the form of flakes or powder, or sheets that comprise a protective fixant that retains the grains together. The protective fixant renders the sheet unsuitable for electronic device construction.

Two further problems with the conventional production methodologies derive from the need to:

a) remove the produced graphene from the equipment to make an electronic device, thus exposing the graphene to the external environment resulting in surface contamination that adversely affects the further processes required to produce the electronic device; and

b) separation of the graphene from the catalyst metal base substrate which requires chemical or physical processes that contaminate the graphene material.

Since the proposal of graphene, additional two dimensional (2D) layers, often termed single layer materials, have been speculated and are now being researched extensively at a rate that now exceeds new research into graphene. Such materials include Silicene, Phosphorene, Borophene, Germanene and Graphyne allotropes of Silicon, Phosphor, Boron, Germanium and Carbon respectively. As with graphene these materials will theoretically exhibit extraordinary properties particularly suited to next generation electronics as outlined in “Electronics based on two-dimensional materials”, Nature Nanotechnology, 2014 (9), 768-779.

In all cases, the realization and hence efficient manufacturing of these materials still remains theoretical although several approaches have been potentially identified, as described in ‘Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene’ (ACS Nano, 2013, 7 (4), 2898-2926. The issues are largely similar to those detailed above for graphene production, however, other 2D materials, unlike graphene, are inherently unstable in air which demands production in inert environments.

Until now, no techniques exist, beyond delaminating single monolayers from bulk materials in nitrogen environments, to produce other 2D materials and no single layers or structures have been successfully produced that survive outside of the controlled inert environment.

The main target applications for 2D materials lie with the combination of these monolayers with semiconductor or dielectric materials in electronic and photonic structures and devices. A plethora of potential inventions have been theorized and speculated, as well detailed in “Science and Technology Roadmap for Graphene, Related Two-Dimensional Crystals, and hybrid systems”, Nanoscale 11, 2015.

Several prototype structures have been realized physically through the manual combination of very small samples, less than 1 cm2, of individual graphene and semiconductor material samples, however the performance of these structures is well below the predicted properties due to the poor quality of the graphene), the manual combination techniques and the inherent contamination that occurs due to the assembly procedure.

The present invention was conceived with the aim of overcoming or at least ameliorating the above problems.

BRIEF SUMMARY

According to a first aspect of the invention there is provided a method of producing a two-dimensional crystalline material, the method may comprise: providing a substrate with a surface having nucleation sites within a reaction chamber. The method may also comprise introducing a precursor into the reaction chamber, the precursor being in a gas phase and/or suspended in a gas; and heating the substrate to a temperature that is within a decomposition range of the precursor, and that allows two-dimensional crystalline material formation from a constituent released from the decomposed precursor. The method preferably comprises cooling the precursor entry point. The reaction chamber may be a close coupled reaction chamber. A separation may be provided between the substrate surface upon which the two-dimensional crystalline material is formed and the point at which the precursor enters the reaction chamber that is sufficiently small, and a thermal gradient between the substrate surface and the point at which the precursor enters the chamber that is sufficiently steep, that the fraction of precursor that reacts in the gas phase within the reaction chamber is low enough to allow the formation of the two-dimensional crystalline material.

According to a further aspect of the invention there is provided a method of producing a two-dimensional crystalline material, the method comprising: providing a substrate having nucleation sites within a close coupled reaction chamber;

introducing a precursor into the close coupled reaction chamber, the precursor being in a gas phase and/or suspended in a gas; and

heating the substrate to a temperature that is within a decomposition range of the precursor, and that allows two-dimensional crystalline material formation from a species released from the decomposed precursor.

A close coupled reaction chamber provides a separation between the substrate surface upon which the two-dimensional crystalline material is formed and the entry point at which the precursor enters the close coupled reaction chamber that is sufficiently small that the fraction of precursor that reacts in the gas phase within the close coupled reaction chamber is low enough to allow the formation of two-dimensional crystalline material. The upper limit of the separation may vary depending upon the precursor chosen, substrate temperate and pressure within the close coupled reaction chamber.

Compared with the chamber of a standard CVD system, the use of a close coupled reaction chamber, which provides the aforementioned separation distance, allows a high degree of control over the supply of the precursor to the substrate; the small distance provided between the substrate surface on which the two-dimensional crystalline material is formed and the inlet through which the precursor enters the close coupled reaction chamber, allows for a steep thermal gradient thereby providing a high degree of control over the decomposition of the precursor.

In many instances the separation between the substrate surface upon which the two-dimensional crystalline material is formed and the chamber wall directly opposite the substrate surface will be substantially equal to the separation between the substrate surface and the point at which the precursor enters the close coupled reaction chamber. Nevertheless, this may not necessarily be the case, for example, where the close coupled reaction chamber comprises an inlet for precursor that sits/extends within the chamber or where a horizontal introduction type chamber is used.

The relatively small separation between the substrate surface and the chamber wall provided by a close coupled reaction chamber, compared with the relatively large separation provided by a standard CVD system allows:

1) a steep thermal gradient between the precursor's entry point and the substrate surface;

2) a short flow path between the precursor entry point and the substrate surface; and

3) a close proximity of the precursor entry point and the point of two-dimensional crystalline material formation.

These benefits enhance the effects that deposition parameters including substrate surface temperature, chamber pressure and precursor flux have on the degree of control over the delivery rate of the precursor to the substrate surface and the flow dynamics across the substrate surface.

These benefits and the greater control provided by these benefits enable minimization of gas phase reactions within the chamber, which are detrimental to two-dimensional crystalline material deposition; allow a high degree of flexibility in the precursor decomposition rate, enabling efficient delivery of the species to the substrate surface; and gives control over the atomic configuration at the substrate surface which is impossible with standard CVD techniques.

The refined control of the environment at and around the substrate surface available through use of a close coupled reaction chamber enables the deposition of two-dimensional crystalline material using, for example vapor phase epitaxy (VPE). This, together with the benefits provided by a steep thermal gradient lead to the ability to deposit two-dimensional crystalline material without the need to use a metallic catalytic substrate.

By removing the need for a metallic catalytic substrate there is provided increased flexibility in the process conditions that can be used to produce two-dimensional crystalline material. This in turn offers the possibility of limiting unintentional doping and increasing grain size. It also allows the choice of using substrates to which the two-dimensional crystalline material adheres less well, i.e. is poorly bonded, facilitating separation of the two-dimensional crystalline material by more straightforward and quicker processes that result in minimal or no contamination of the two-dimensional crystalline material.

In one preferred embodiment the two-dimensional crystalline layer is graphene, the precursor is a carbon comprising precursor and the species is carbon.

Using the method of the invention it has been possible to produce graphene with substantially improved properties over known methods, for example with a grain size greater than 20 μm, covering a substrate of 6 inch diameter with 98% coverage, a layer uniformity of >95% of the substrate, sheet resistivity less than 4500/sq and electron mobility greater than 2435 cm2/Vs. The most recent tests on a graphene layer produced using the method of the invention have demonstrated electron mobility >8000 cm2/Vs across the full layer tested at standard conditions for temperature and pressure. The method has been able to produce graphene layers across a substrate of 6 inches (15 cm) having undetectable discontinuity, measured by standard Raman and AFM mapping techniques to micron scale. The method has also shown to be able to produce a uniform graphene monolayer and stacked uniform graphene layers across the substrate without formation of additional layer fragments, individual carbon atoms or groups of carbon atoms on top of the or uppermost uniform monolayer.

In another embodiment the two-dimensional crystalline layer is silicene, the precursor is a silicon comprising precursor and the species is silicon.

The preferred temperature to which the substrate is heated is dependent upon the precursor selected. The temperature selected needs to be high enough to allow at least partial decomposition of the precursor in order to release the species, but preferably not so high as to promote increased recombination rates in the gas phase away from the substrate surface and hence production of unwanted by-products. A complete decomposition temperature, whereby 100% of precursor is decomposed, is typically available from the precursor supplier or can be found in one of many online databases. Recombination rates can be theoretically calculated once other process conditions have been selected, for example reaction chamber pressure and precursor flow rate, using well known methods known to the person skilled in the art. Notwithstanding, the selected temperature may be higher than the complete decomposition temperature to promote improved substrate surface kinetics and so encourage formation of two-dimensional crystalline material with good crystal quality. There can be a trade off in selecting a temperature that provides good crystal quality at the cost of a higher recombination rate and thus lower two-dimensional crystalline material growth rate. It is possible to determine a best temperature, or preferred temperature range, for a specific precursor through straightforward empirical experimentation.

The temperature range for deposition of two-dimensional crystalline material in a close coupled reactor is dependent on the chosen substrate material, the chosen species comprising (and/or where applicable dopant) precursor and the desired final graphene properties. VPE two-dimensional crystalline material deposition temperature can range from 200° C. when using highly volatile precursors with low decomposition temperatures, up to 1500° C. for other precursor compounds.

It is tacit that in order for there to be a thermal gradient between the substrate surface and the introduction point for precursor, the inlet will need to be of a lower temperature than the substrate. For a fixed separation a greater temperature difference will provide a steeper temperature gradient. As such it is preferred that at least the wall of the chamber through which the precursor is introduced, and more preferably the walls of the chamber are cooled. Cooling may be achieved using a cooling system, for example, using fluid, preferably liquid, most preferably water, cooling. The reactor's walls may be maintained at constant temperature by water cooling. The cooling fluid may flow around the inlet(s) to ensure that the temperature of the inner surface of the reactor wall through which the inlets extend, and thus of the precursor itself as it passes through the inlet and into the reaction chamber, is substantially lower than the substrate temperature, favorably equal to or under 200° C., 170° C. or below being more preferred.

In arrangements in which the inlet is defined by a conduit that protrudes into the chamber by a meaningful amount, it is likely to be necessary to provide fluid cooling by running cooling fluid across and/or though the protruding conduit wall in order that the precursor within the protrusion is kept cool until it passes out of the inlet. As a consequence of the added complexity caused by this, an arrangement including a protruding conduit is not preferred.

Through both simultaneously heating the substrate and providing cooling the wall of the reactor directly opposite the substrate surface at the inlet, a steep thermal gradient can be formed whereby the temperature is a maximum at the substrate surface and drops rapidly towards the inlet. This ensures the reactor volume above the substrate surface has significantly lower temperature than the substrate surface itself, largely reducing probability of precursor reaction, in the none useful gas phase, until the precursor is proximate the substrate surface.

Although the temperature profile across the spacing is not in actuality linear, it is preferred that the temperature difference between the substrate and the precursor inlet is equivalent to a linear gradient greater than approximately 1000° C. per meter, (ts-t,)/s>1000° Cm−1, where isis the temperature of the substrate surface, t, is the temperature at the inlet and s is the spacing in meters.

Such a gradient could be obtained, for example, using a temperature drop of 30° C. across a ˜30 mm spacing between the inlet and the substrate. More preferably the temperature difference is equal or greater than 3000° C. per meter, for example using a substrate temperature of 500° C. inlet temperature of 200° C. (difference 300° C.) with a spacing of 100 mm. Even more favorably the difference is over 10,000, which would be achieved, for example using a substrate temperature of 1100° C., inlet temperature of 200° C. and a spacing of 60 mm—equating to ˜14,500° C. m−1, or a substrate temperature of 500° C. inlet temperature of 200° C. (difference 300° C.) with a spacing of 10 mm (30,000° C. m−1).

In a preferred embodiment the method comprises passing the precursor, which may be in a gas phase, over the heated substrate. There are two variables to be considered: pressure within the close coupled reaction chamber and the gas flow rate into the chamber.

The preferred pressure selected depends upon the precursor chosen. In general terms, where precursors of greater molecular complexity are used, improved two-dimensional crystalline material quality and rate of production is observed using lower pressures, e.g. less than 500 mbar. Theoretically, the lower the pressure the better, but the benefit provided by very low pressures (e.g. less than 200 mbar) will be offset by very slow two-dimensional crystalline material formation rates.

Conversely for less complex molecular precursors, higher pressures are preferred. For example where methane is used as a precursor for graphene production, a pressure of 600 mbar or greater may be suitable. Typically, it is not expected to use pressures greater than atmospheric because of its detrimental impact on substrate surface kinetics and the mechanical stresses placed on the system. A suitable pressure can be selected for any precursor through simple empirical experimentation, which may involve for example, five test runs using respective pressures of 50 mbar, 950 mbar and three others of equidistance intervals between the first two. Further runs to narrow the most suitable range can then be conducted at pressures within the interval indentified in the first runs as being most suitable.

The precursor flow rate can be used to control two-dimensional crystalline material deposition rate. The flow rate chosen will depend upon the amount of the species within the precursor and the area of the layer to be produced. Precursor gas flow rate needs to be high enough to allow coherent two-dimensional crystalline material layer formation on the substrate surface. If the flow is above an upper threshold rate, bulk material formation, e.g. graphite, will generally result or increased gas phase reactions will occur resulting in solid particulates suspended in the gas phase that are detrimental to two-dimensional crystalline material formation and/or may contaminate the two-dimensional crystalline material layer. The minimum threshold flow rate can be theoretically calculated using techniques known to the person skilled in the art, by assessing the amount of the species required to be supplied to the substrate to ensure sufficient atomic concentrations are available at the substrate surface for a layer to form. Between the minimum and upper threshold rates, for a given pressure and temperature, flow rate and two-dimensional crystalline material layer growth rate are linearly related.

The initial nucleation of the species on the target substrate, a pre-former for the eventual production of the two-dimensional crystalline material layer, can require different surface conditions from the conditions required to achieve the eventual target two-dimensional crystalline material, depending on substrate and precursor selection. It is often desirable to have very different surface kinetics on the target substrate to encourage initiation of the species' adsorption to the substrate surface. The surface kinetics, precursor decomposition and surface reaction rate may be readily controlled by the substrate temperature, reactor pressure, precursor flow rate and the presence of a dilution gas.

As such, a preferred method comprises providing a first set of reactor conditions to promote initial species adsorption to the substrate, followed by, and preferably without removing the substrate from the reaction chamber, providing a second set of reactor conditions to promote formation and coalescence of a two-dimensional crystalline material layer. In the simplest form, this embodiment is a two stage process whereby a set of conditions: e.g. a first pressure within reaction chamber, a first temperature of substrate and a first precursor flow over the substrate, is used to promote initial species adhesion to the substrate and then a second set of conditions, e.g. a second pressure, second temperature and second flow rate are used to promote formation and coalescence of the two-dimensional crystalline material layer from the initial species locations on the substrate surface. It might, for example be necessary to change only one property, e.g. only temperature, in other instances, it may be desirable to change multiple properties. Additionally, optimized two-dimensional crystalline material layers have been produced by using a preferred variant method that comprises cycling between two or more sets of reactor conditions.

In further refined processes, it may be beneficial to include a further step(s) using a further set of reactor conditions. It may also be beneficial to include a further step to treat, e.g. anneal, the two-dimensional crystalline material after formation.

In a preferred embodiment, the method includes a substrate preparation process before introducing the precursor to improve the surface condition of the substrate prior to two-dimensional crystalline material production. The precise preparation requirement is dependent on the selected substrate material and its surface condition when placed into the reaction chamber. Most commonly the substrate preparation process involves heat treatment of the substrate, to remove common surface contamination such as native oxides and/or hydro carbons, in combination with reducing the pressure within the reaction chamber to below atmospheric pressure and/or providing a reducing environment within the reaction chamber, for example a low pressure process in a hydrogen ambient. In other cases it may be preferential to change the termination of the substrate surface to provide a more suitable state for the deposition of two-dimensional crystalline material. This may be achieved by exposing the substrate, in the reaction chamber, to a gas or precursor that will change the surface state of the substrate prior to the two-dimensional crystalline material deposition process, for example using ammonia to nitrogen terminate a sapphire substrate. It is envisaged that additional preparation processes may be used to those mentioned.

In a preferred embodiment, the method comprises pulsing the flow of precursor over the heated substrate. The periods of reduced, e.g. below a minimum flow rate needed for appreciable two-dimensional crystalline material growth, or zero flow rate of precursor over the substrate aids the surface kinetics and promotes atomic surface diffusion to align the species at the substrate surface in a preferred monolayer arrangement.

The combination of one “on” period in which precursor is caused to flow over the substrate followed by an “off” and/or “reduced flow” period is defined as a cycle. The number of cycles required to improve the deposition process may vary depending upon the precursor, substrate and desired final properties of the two-dimensional crystalline material layer. Initial experimentation showed an improved result using anything between 2 to 22 cycles. Further experimentation has extended this known range to 35 cycles, though it is expected that this upper figure could be greater for certain precursors and/or reaction chamber conditions. In certain conditions is thought that as many as 100 cycles may still be beneficial.

Similarly the preferred length of the on and off times will also vary depending upon the precursor and substrate. Based on experimentation to date using a limited number of precursors for producing graphene, it is speculated that a preferred “on” time is at least ten seconds and a preferred “off” and/or “reduced flow” time is at least five seconds. These times may differ for the production of other two-dimensional layers for example the production of borophene using borane as a precursor may require significantly less than 10 seconds.

It may also be preferred to employ different process conditions for the “on” period and “off” or “reduced flow” period, for example by changing the substrate surface temperature, and/or reaction chamber pressure and/or precursor flow rate. Further it may also be preferable to use different process conditions from cycle to cycle including, for example, modifying the precursor flow rate.

In a further alternative embodiment, a purge gas may be introduced into the reaction chamber during precursor “off” periods to actively remove precursor or precursor by-products from the substrate surface which may otherwise cause a barrier to carbon surface diffusion on the substrate during precursor “off” periods. Non limiting examples of suitable purging gases include hydrogen and/or nitrogen.

In an alternative embodiment the method comprises sealing the close coupled reaction chamber after introducing the precursor to minimize and/or prevent flow of the precursor into or out of the close coupled reaction chamber. This limits the exposure of substrate surface to precursor which may aid two-dimensional crystalline material formation by reducing currents of the species from the decomposed precursor proximate the substrate surface. The amount of the species available at the surface, which will determine the quality of the two-dimensional crystalline material crystal, may be controlled by controlling the pressure within the close coupled reaction chamber and/or through use of a dilution gas within the reaction chamber. The substrate may be heated either before, after or while introducing the precursor.

The spacing between the substrate surface upon which the two-dimensional crystalline material is formed and the wall of the reactor directly above the substrate surface has a significant effect on the reactor thermal gradient. It is preferred that the thermal gradient is as steep as possible which correlates to a preferred spacing that is a small as possible. A smaller spacing changes the boundary layer conditions at the substrate surface that in turn promotes uniformity of two-dimensional crystalline material layer formation. A smaller spacing is also highly preferred as it allows refined levels of control of the process variables, for example reduced precursor consumption through lower input flux, lower reactor and hence substrate temperature which decreases stresses and non-uniformities in the substrate leading to more uniform two-dimensional crystalline material production on the substrate surface and hence, in most cases, significantly reduced process time.

Experimentation suggests that a spacing of about 100 mm, between the substrate surface upon which the two-dimensional crystalline material is formed and the precursor entry point (which may equate to the separation between the substrate surface and a wall of the reactor directly above the substrate surface), is approaching the upper limit able to provide the conditions necessary for graphene formation; this may be increased slightly to around 110 mm where the inlet for the precursor protrudes beyond the wall and into the chamber so as to be spaced from the substrate surface by around 100 mm. However, more reliable and better quality two-dimensional crystalline material is produced using a much smaller spacing equal or less than about 20 mm; a spacing equal or less than about 10 mm promotes the formation of stronger thermal currents proximate the substrate surface that increase production efficiency.

Where a precursor is used that has a relative low decomposition temperature such that there is likely to be a more than negligible degree of decomposition of the precursor at the temperature of the precursor inlet, a spacing below 10 mm is strongly preferred to minimize the time taken for the precursor to reach the substrate.

Suitable reaction chambers for this method include vertical introduction systems whereby gas is injected towards the subject from inlets directly opposite the substrate, and to a lesser extent horizontal introduction systems in which the flow is introduced into the chamber at a point laterally away from the substrate. Common examples of suitable apparatus that can be used to perform the method are vapor phase epitaxy (VPE) systems and metal-organic chemical vapor deposition (MOCVD) reactors.

Irrespective of the form of the apparatus used, it preferably comprises means to maintain the chamber wall directly opposite the substrate surface upon which two-dimensional crystalline material is to be formed, and more favorably all walls of the chamber, at a temperature substantially below the temperature of the heated substrate in order to provide a steep thermal gradient. This may be achieved, for example, by water cooling.

It is preferred that the reactor is a cold-walled reactor, in that the heater coupled to the substrate is the primary and preferably only source of heat to the chamber.

In respect of vertical introduction systems, and although less preferred, where the introduction point for precursor extends into the chamber, the difference in spacing between the end of the inlet directly opposite the substrate and the wall of the chamber directly opposite the substrate may be no more than about 10 mm.

In the case of horizontal reactor configurations, it is possible to use significantly greater spacing between the precursor introduction point and the substrate surface compared with the spacing between the chamber wall directly opposite the substrate and substrate, and still produce two-dimensional crystalline material on the substrate surface. It has been experimentally proven that a horizontal spacing between substrate surface and precursor introduction point of up to about 400 mm can produce two-dimensional crystalline material on the substrate surface.

Usually the ceiling of the reactor will be the wall that is directly opposite the substrate surface upon which two-dimensional crystalline material is produced; however, it will be appreciated that the substrate may be arranged in the reactor such that this is not the case.

It is preferred that the substrate provides a crystalline surface upon which the two-dimensional crystalline material is produced as ordered crystal lattice sites provide a regular array of nucleation sites that promote the formation of good two-dimensional crystalline material crystal overgrowth. The most preferred substrates provide a high density of nucleation sites. The regular repeatable crystal lattice of substrates used for semiconductor deposition is ideal, the atomic stepped surface offering diffusion barriers.

Notwithstanding, with modified growth conditions it is possible to use non-crystalline, poly crystalline or amorphous materials as substrates for two-dimensional crystalline material growth. Although the process may be less efficient, such substrates may be beneficial in other ways, for example cost, ease of layer removal etc.

Non-crystalline substrates may offer suitable nucleation sites through surface irregularities, surface morphology or defects. Further, substrates may be made more preferable for material deposition through surface modification such as profiling or patterning, e.g. using wet or dry etching techniques. Alternatively, or in addition, certain substrates such as, for example, plastics and ceramics may be pre-formed with a desirable surface finish that provides the nucleation sites.

Although the process may be used with a metal substrate, this is not preferred. Rather it is preferred that the substrate provides a non-metallic surface upon which the two-dimensional crystalline material is produced. This avoids the process condition limitations associated with metal substrates and circumvents the problems associated with removal of the formed two-dimensional crystalline material layer from such substrates.

Non limiting examples of suitable substrates include:

Semiconductor single crystal wafers, for example silicon (Si), silicon carbide (SiC), gallium arsenide (GaAs), Indium phosphide (InP), gallium nitride (GaN), zinc oxide (ZnO) or indium antimonide (InSb);

Insulating materials, for example sapphire (Al2O3), silica (SiO2);

Compound semiconductor homo and heterostructures structures, for example InP/CdTe, GaN/InGaN/AlGaN, Si/AlN/GaN, GaAs/AlInGaP, GaN/BN, silicon on insulator (SOI);

Ceramics, for example Zirconium dioxide, aluminosilicates, silicon nitride (Si3N4), boron carbide (B4C);

Glasses, for example quartz, fused silica glass, borofloat;

Plastics and polymers, for example high performance plastics such as Poly ether ketone (PEK), polyetherether ether ketone (PEEK), polyamide-imides (PAI), polyphenylene sulfides (PPS);

Composite materials, for example fibre reinforced polymers, glass reinforced matrices and carbon composites;

Nanomaterials, for example nanotubes and nanoparticles; and

Organic bases, for example organic polymers such as polyethylene terephthalate (PET) or polycarbonate (PC).

Generally it is preferred to have a substrate that is as thin as possible to ensure thermal uniformity across the substrate during two-dimensional crystalline material production. The minimum thickness of the substrate is however determined in part by the substrate's mechanical properties and the maximum temperature at which the substrate is to be heated.

The maximum area of the substrate is dictated by the size of the close coupled reaction chamber.

A wide variety of compounds can be used as a precursor, the minimum requirement being that it comprises the desired species, can be delivered into the close coupled reaction chamber in the gas phase and/or suspended by a gas, and that it will decompose at a temperature below or equal to the maximum temperature that the reactor can operate. For many commercially available reactors the maximum temperature is between 1200° C. and 1500° C. though it is conceived that future reactors will provide the ability of using higher temperatures.

For the production of graphene, the precursor may include one or more compounds from any one or more of the groups: hydrocarbon, hydride, halocarbon, including haloalkane and halo amides, metallocene, metalorganic, amine including alkylamines, organic solvents and azo compounds, and also optionally azides, imides, sulphides and phosphides. These groups provide precursors with non-carbon decomposition by products that can be removed from the reaction chamber without incorporation within the graphene or interfering with the deposition process. It is possible that the precursor includes multiple carbon-comprising compounds from one or more of the above groups.

The precursor favorably is comprised from and more favorably consists of one or more compounds from any one or more of the groups: halocarbon, hydrocarbon, azo and metallocene, and optionally metalorganic, because they are comparatively easy to handle and are widely available.

The precursor is most favorably comprised from or consists one or more of bromomethane, methane, ethane, cyclopentadienylmagnesium, carbon tetra bromide, azomethane, azoethane and/or acetylene.

Methane, ethane, and acetylene are particularly suitable because they are commercially available in high purity form and have a molecular structure that comprises the wanted carbon; the other constituent, hydrogen (and in the case of the azo compounds, nitrogen), does not interfere with the graphene deposition process.

Azomethane and azoethane further have a molecular structure that provides a free radical carbon group from which it is easy to liberate carbon.

Halcocarbons are considered to be particularly suitable precursors because they are generally volatile making them easy to deliver to the reactor in the gas phase and readily dissociate to release carbon and the halogen, which itself is volatile and so easy to evacuate from the chamber.

Bromomethane is particularly favored because its high volatility provides two advantages. Under one set of process conditions, in which a pressure within the chamber that is lower than the vapor pressure of bromine, bromine can be readily removed from the reaction chamber without interaction with the graphene layer. In a second set of process conditions, in which the pressure within the reaction chamber is greater than the vapor pressure of bromine, bromine can be incorporated into the graphene as a dopant.

It is speculated that other compounds within the halocarbon group will exhibit similar properties making them suitable as both a pure graphene source and a doped graphene source. Examples of speculated compounds within this group include bromoethane, methyliodide and methylchoride. It is also believed that certain non-halocarbon compounds may be suitable as both a pure graphene source and a doped graphene source, such as for example cyclopentadienylmagnesium, carbon tetra bromide and optionally also triethylborane.

For the production of silicene the precursor may include one or more compounds from the silane, silicon comprising metalorganic or organosilicon molecular groups. These groups provide precursors that comprise the desired silicon and non-silicon containing decomposition by-products that can be readily removed from the reaction chamber without affecting the silicene growth process. Preferred precursor compounds include one or more of but are not limited to: silane, disilane, methylsilane, silicon tetrachloride and tetramethyl or tetraethyl-orthosilicate. It is possible that the precursor includes multiple silicon-comprising compounds from one or more of the above groups.

For the production of borophene the precursor may include one or more compounds from the borane, organoboron or boron comprising metalorganic molecular groups. These groups provide precursors that comprise the desired boron and non-boron containing decomposition by-products that can be readily removed from the reaction chamber without affecting the borophene growth process. Preferred precursor compounds include one or more of; borane, diborane, trimethyl and triethyl-boron. It is possible that the precursor includes multiple boron-comprising compounds from one or more of the above groups.

For the production of germanene the precursor may include one or more compounds from the germane or metalorganic molecular groups. These groups provide precursors that contain the desired germanium and non-germanium containing decomposition by-products that can be readily removed from the reaction chamber without affecting the germanene growth process. Preferred precursor compounds include one or more of, germane, (C2H5)4Ge and (n-C4H9)4Ge. It is possible that the precursor includes multiple germanium-comprising compounds from one or more of the above groups.

In certain preferred embodiments, a mixture of the precursor with a dilution gas is passed over the heated substrate within a close coupled reaction chamber. The use of a dilution gas allows further refinement of the control of the carbon supply rate.

It is preferred that the dilution gas includes one or more of hydrogen, nitrogen, argon and helium. These gases are selected because they will not readily react with a large number of available precursors under typical reactor conditions, nor be included in the graphene layer. Notwithstanding, hydrogen may react with certain precursors. Additionally, nitrogen can be incorporated into the graphene layer under certain conditions. In such instances one of the other carrier gases can be used.

In spite of these potential problems, hydrogen and nitrogen are particularly preferred because they are standard gases used in MOCVD and VPE systems.

In one preferred variant embodiment, the method comprises heat treating the formed two-dimensional crystalline material (often referred to as ‘annealing’ within the semiconductor production field) within the close coupled chamber. Generally it is expected that the annealing temperature will be equal to or greater than the formation temperature, though may, in certain cases, be below. For example, a temperature greater than 1100° C. has been shown to cause lattice reordering providing improved graphene structure. For silicene a heat treatment temperature as low as ˜150° C. has been shown to induce dehydrogenation of the layer resulting in improved electrical properties. The annealing process may comprise multiple stages, in which the two-dimensional crystalline material is held at different temperatures, for example a first stage at a lower temperature to facilitate removal of impurities, and a second stage at a higher temperature to improve lattice structure.

As alluded to above, the flexibility in the reaction conditions provided by the invention allows for the production of controllably doped two-dimensional crystalline material structures. To this end, a favorable further embodiment of the invention comprises introducing a doping element into the close coupled reaction chamber and selecting a temperature of the substrate, a pressure of the reaction chamber and a gas flow rate to produce a doped two-dimensional crystalline material. Straightforward empirical experimentation can be used to determine these variables using the afore described guidance. This process can be used with or without a dilution gas.

In one variant of this method the precursor molecule used for two-dimensional crystalline material growth includes the doping element.

In an alternative variant the precursor-comprising the species and a second precursor comprising the doping element is introduced to the substrate within a close coupled reaction chamber; the second precursor being a gas or suspended in gas. In certain embodiments, the flow of the second precursor is pulsed to allow time for the two-dimensional crystalline material layer to preferentially form on the substrate surface. In a further variation, more than one doping element may be introduced either for example by including a third precursor and/or through using a first precursor that includes a doping element with the second precursor.

There is no perceived restriction as to doping element that may be introduced. Commonly used dopant elements for the production of graphene include silicon, magnesium, zinc, arsenic, oxygen, boron, bromine and nitrogen. For silicone favorable doping elements include oxygen, copper, silver, gold, iridium and platinum. In the case of borophene favorable doping elements include carbon and nitrogen.

A wide variety of compounds can be used as a dopant precursor, the minimum requirement being that it can be delivered into the close coupled chamber in the gas phase or suspended in a gas flow, and that it will decompose to release the required dopant in the conditions used to grow two-dimensional crystalline material with the selected precursor.

Commonly available compounds that may be suitable as dopant precursor sources include those within following groups: hydride, metalorganic, metallocene and halocarbon.

The flexibility in the reaction processes allowed by the invention provides the ability to alter the reactor conditions during and/or between layer formation. This provides the possibility of depositing multiple layers and/or layers with varying properties from layer to layer. As such, according a further preferred embodiment, the method may additionally comprise the step of changing the reactor conditions e.g. the temperature of the substrate and/or the pressure of the reaction chamber or flow rate of precursor to form a further two-dimensional crystalline material layer or doped two-dimensional crystalline material layer over (e.g. on top of) the first layer of two-dimensional crystalline material or doped two-dimensional crystalline material resulting in a two-dimensional crystalline material heterostructure. Alternatively it may comprise introducing a second precursor or changing the precursor between formation of the first and second stacked two-dimensional crystalline material layers such that the second layer has a different material property to the first layer.

Combining two-dimensional material layers opens the possibility of producing heterostructures with extraordinary structure properties, provided continuous discrete two-dimensional layers can be produced cumulatively without cross layer contamination or interlayer molecular or elemental diffusion. With considered control of the growth parameters, evaluating the deposition properties of the current two-dimensional layer growth in conjunction with the conditions required to keep the preceding two-dimensional layer stable two-dimensional material heterostructures may be produced.

The invention also provides the possibility of producing heterostructures, and thus as a corollary, electronic devices comprising at least one two-dimensional crystalline material layer (doped or otherwise) with at least one non-two-dimensional crystalline material layer, e.g. a layer of semiconductor material and/or a dielectric material, in situ within the close coupled reaction chamber, i.e. without the need to remove the first formed layer from the chamber before adding the next layer. This overcomes the problems of ambient environment contamination suffered by prior art methods that require the structure to be transferred between different deposition chambers to provide the different process conditions required for formation of each layer of the heterostructure.

Combination of the above techniques may be used to form heterostructures of any desired configuration, for example, simple two or three stacked junction devices through to complex superlattice structures.

It is believed that the above described method allows the manufacture of graphene material with a novel structure and therefore according to a second aspect of the invention there is provided a two-dimensional crystalline material (preferably graphene) sheet having a mean grain size equal or greater than 20 micrometers.

Because of the significantly greater mean grain size compared with existing graphene materials, the mechanical strength of the graphene sheet is sufficiently increased such that it is self supporting, and thus can be removed from a substrate upon which it was formed with no or little disintegration.

According to a further aspect of the invention there is provided a method of manufacturing a heterostructure that includes a two-dimensional crystalline material having an interface with a second layer, the method comprising: using a first set of reactor conditions to produce a two-dimensional crystalline material on a substrate within a close coupled reaction chamber; and

introducing a second precursor under a second set of reactor conditions to form the second layer on the substrate.

The two-dimensional crystalline material may be formed and the second layer deposited thereon or vice-versa. This allows which ever layer is formed first to remain within the chamber, and thus remain free of contaminate before the next layer is deposited.

The second layer may be formed directly on top of the first layer or the first layer may be formed directly on top of the second layer.

The second layer may be another two-dimensional crystalline material layer or a non-two-dimensional crystalline material layer. The second layer may for example be a semiconductor.

In one example, the second layer comprises at least one of the following: GaN, BN, AlN AlGaN, SiN.

In order to promote the best formation of the individual layer, it is preferred that the separation between the substrate and the precursor inlet (which may equate to the separation between the substrate and a ceiling of the reactor chamber directly above the substrate) is changed between forming the two-dimensional crystalline material layer and the non-two-dimensional crystalline layer.

According to a further aspect of the invention there is provided a heterostructure material that includes a two-dimensional crystalline material having an interface with a second layer, wherein the interface is continuous across the material. Preferably the heterostructure has a width greater than 1 cm squared.

