GB2570127A

GB2570127A - A method of making graphene structures and devices

Info

Publication number: GB2570127A
Application number: GB1800450.7A
Authority: GB
Inventors: Thomas Simon; Guiney Ivor
Original assignee: Paragraf Ltd
Current assignee: Paragraf Ltd
Priority date: 2018-01-11
Filing date: 2018-01-11
Publication date: 2019-07-17
Anticipated expiration: 2038-01-11
Also published as: GB2570127B; GB201800450D0

Abstract

Method for the production of an unsupported graphene layer structure, comprising; i) providing a substrate on a heated susceptor in a reaction chamber, the chamber having a plurality of cooled inlets arranged so that, in use, the inlets are distributed across the substrate and have a constant separation from the substrate and ii) supplying a flow comprising a precursor compound through the inlets and into the reaction chamber to thereby decompose the precursor compound and form graphene on the substrate, wherein the inlets are cooled to less than 100°C, preferably 50 to 60°C, and the susceptor is heated to a temperature of at least 50°C in excess of a decomposition temperature of the precursor, wherein the flow and/or reactor conditions are controlled to grow a plurality of graphene layers on the substrate, whereby a graphene layer structure is obtained comprising at least 15 layers, preferably at least 50 layers, and whereby the graphene layer structure self-separates from the substrate. Optionally, growth conditions may be controlled to prevent the collapse of the graphene layer structure by; a) changing a flow-rate of a precursor compound through the inlets and/or; b) changing a pressure of the flow and/or c) changing the susceptor temperature by from 10oC to 50oC.

Description

The present invention relates to a method of making an unsupported graphene layer structure comprising at least 50 graphene layers. In particular, the method of the invention provides an improved method for making graphene layer structures which can be separated intact from the substrate on which they are formed.

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, March 2007, 183- 191.

WO 2017/029470, the content of which is incorporated herein by reference, discloses methods for producing two-dimensional materials. Specifically, WO 2017/029470 discloses a method of producing two-dimensional materials such as graphene, comprising heating a substrate held within a reaction chamber to a temperature that is within a decomposition range of a precursor, and that allows graphene formation from a species released from the decomposed precursor; establishing a steep temperature gradient (preferably> 1000°C per meter) that extends away from the substrate surface towards an inlet for the precursor; and introducing precursor through the relatively cool inlet and across the temperature gradient towards the substrate surface. The method of WO 2017/029470 may be performed using vapour phase epitaxy (VPE) systems and metal-organic chemical vapour deposition (MOCVD) reactors.

The method of WO 2017/029470 provides two-dimensional materials with a number of advantageous characteristics including: very good crystal quality; large material grain size; minimal material defects; large sheet size; and are self-supporting. However, there remains a need for fast and low-cost processing methods for fabricating devices from the two-dimensional materials.

It is an object of the present invention to provide an improved method for producing graphene layer structures which overcome, or substantially reduce, problems associated with the prior art or at least provide a commercially useful alternative thereto.

Accordingly, the present invention provides a method for the production of an unsupported graphene layer structure, the method comprising:

providing a substrate on a heated susceptor in a reaction chamber, the chamber having a plurality of cooled inlets arranged so that, in use, the inlets are distributed across the substrate and have a constant separation from the substrate, supplying a flow comprising a precursor compound through the inlets and into the reaction chamber to thereby decompose the precursor compound and form graphene on the substrate, wherein the inlets are cooled to less than 100°C, preferably 50 to 60°C, and the susceptor is heated to a temperature of at least 50°C in excess of a decomposition temperature of the precursor, wherein the flow and/or reactor conditions are controlled to grow a plurality of graphene layers on the substrate, whereby a graphene layer structure is obtained comprising at least 15 layers, preferably at least 50 layers, and whereby the graphene layer structure self-separates from the substrate.

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 inventors have found that if graphene is grown under suitably controlled conditions in an MOCVD process such that a sufficiently thick graphene layer structure can be obtained, then the graphene has been found to self-separate from the substrate on which it is formed. This provides a new approach to achieving large intact graphene layer structures for a range of different applications.

The present disclosure relates to a method for the production of a graphene layer structure having at least 15 layers, preferably at least 50 graphene layers, more preferably 75 to 150. The upper limit on the number of layers which could practically be grown in the present reactors was found to be 500 layers, although almost any thickness could be achieved. The thicker the layer formed, the more pronounced the self-delamination process.

Graphene is a well-known term in the art and refers to an allotrope of carbon comprising a single layer of carbon atoms in a hexagonal lattice. The term graphene used herein encompasses structures comprising multiple graphene layers stacked on top of each other. The term graphene layer is used herein to refer to a graphene monolayer. Said graphene monolayers may be doped or undoped. The graphene layer structures disclosed herein are distinct from graphite since the layer structures retain graphenelike properties.

The method comprises a first step of providing a substrate on a heated susceptor in a reaction chamber. The substrate of the present method may be any known MOCVD or VPE substrate. It is preferred that the substrate provides a crystalline surface upon which the graphene is produced as ordered crystal lattice sites provide a regular array of nucleation sites that promote the formation of good graphene 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. Examples of suitable substrates include silicon, nitride semiconductor materials (AIN, AIGaN, GaN, InGaN and complexes thereof), arsenide/phosphide semiconductors (GaAs, InP, AllnP and complexes of), and diamond. The inventors have found that the choice of substrate affects the layers separating, for example growth on sapphire wafers will generally increase layer separation from the substrate. Sapphire is particularly preferred.

Generally it is preferred to have a substrate that is as thin as possible to ensure thermal uniformity across the substrate during graphene production. Suitable thicknesses are 50 to 300 microns, preferably 100 to 200 microns and more preferably about 150 microns. 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. Preferably the substrate has a diameter of at least 2 inches, preferably 2 to 24 inches and more preferably 6 to 12 inches. This substrate can be cut after growth to form individual devices using any known method.

The substrate is provided on a heated susceptor in a reaction chamber as described herein. Reactors suitable for use in the present method are well known and include heated susceptor capable of heating the substrate to the necessary temperatures. The susceptor may comprise a resistive heating element or other means for heating the substrate.

The chamber has a plurality of cooled inlets arranged so that, in use, the inlets are distributed across the substrate and have a constant separation from the substrate. The flow comprising a precursor compound may be provided as a horizontal laminar flow or may be provided substantially vertically. Inlets suitable for such reactors are well known and include Planetary and Showerhead reactors available from Aixtron.

The spacing between the substrate surface upon which the graphene 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 as small as possible. A smaller spacing changes the boundary layer conditions at the substrate surface that in turn promotes uniformity of graphene 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 graphene production on the substrate surface and hence, in most cases, significantly reduced process time.

Experimentation suggests a maximum spacing of about 100mm is suitable. However, ore reliable and better quality two- dimensional crystalline material is produced using a much smaller spacing equal to or less than about 20mm, such as 1 to 5mm; a spacing equal or less than about 10mm 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 10mm is strongly preferred to minimise the time taken for the precursor to reach the substrate.

During the production method, a flow is supplied comprising a precursor compound through the inlets and into the reaction chamber to thereby decompose the precursor compound and form graphene on the substrate. The flow comprising a precursor compound may further comprise a dilution gas. Suitable dilution gases are discussed in more detail below.

Preferably the precursor compound is a hydrocarbon. Preferably a hydrocarbon which is a liquid at room temperature and most preferably a C₅ to C10 alkane. The use of simple hydrocarbons is preferred since this gives a pure source of carbon with gaseous hydrogen as a by-product. In addition, since the hydrocarbons are liquid at room temperature, they can be obtained in a highly pure liquid form at low cost. Preferably the precursor compound is hexane.

The precursor is preferably in the gas phase when passed 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 500mbar. Theoretically, the lower the pressure the better, but the benefit provided by very low pressures (e.g. less than 200mbar) will be offset by very slow graphene 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 600mbar 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 50mbar, 950mbar 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 identified in the first runs as being most suitable. The preferred pressure for hexane is from 50 to 800mbar.

The precursor flow rate can be used to control the graphene 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 graphene 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 graphene formation and/or may contaminate the graphene 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 graphene layer growth rate are linearly related.

Preferably 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 dilution 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.

The susceptor is heated to a temperature of at least 50°C in excess of a decomposition temperature of the precursor, more preferably from 100 to 200°C in excess. 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. The selected temperature is higher than the complete decomposition temperature to promote improved substrate surface kinetics and so encourage formation of graphene with good crystal quality. For hexane, the most preferred temperature is about 1200°C, such as from 1150 to 1250°C.

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. The inlets are cooled to less than 100°C, preferably 50 to 60°C.

For some embodiments it may be desirable to dope the graphene. This may be achieved by 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 graphene. Straightforward empirical experimentation can be used to determine these variables using the guidance described above. This process can be used with or without a dilution gas.

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.

The inventors have found that it is necessary to control the flow and/or reactor conditions to allow the formation of a graphene layer structure at least 15 layers, and especially for 50 layers or more. This is to avoid the collapse of the structure into a graphite structure. The best way to achieve this is to ensure that there are changes in the growth conditions so that the graphene being formed in different portions throughout the thickness of the layer structure changes. This has been found to allow the sufficient layer thickness to be achieved, whereby the graphene layer structure self-separates from the substrate after growth.

Preferably the flow is controlled to prevent the collapse of the graphene layer structure by adjusting the layer growth conditions at least once during the growth of the graphene layer structure. Preferably the layer growth conditions are adjusted by:

(a) changing the precursor compound; and/or (b) changing a flow-rate of the flow comprising a precursor compound through the inlets; and/or (c) changing a pressure of the flow supplied through the inlet; and/or (d) changing the dilution gas; and/or (e) changing the reactor/susceptor temperature.

Further examples of suitable changes include: 1) one or more step changes in the growth rate such as a change of +/- at least 25%; 2) a change in the gas flow turbulence such as can be achieved by a change of +/- at least 25% in the rotation rate of the substrate; 3) a change in the gas flow turbulence such as can be achieved by a change of +/- at least 25% in the flow rate of the gas added; 4) a change in the system energy such as can be achieved by a change of +/-10 to 50°C reactor temperature; 5) a change in the precursor compound from one to another or the introduction of a secondary compound for forming doped layers. All of these changes are within the capability of the skilled person.

Some of the above changes relate to a change of at least 25%. In each case, preferably the change is from +/- 25 to 200%, preferably +/-50 to 100%, within the conditions during the growth cycle. Preferably the changes occur as sharp changes in the conditions, but gradual changes can also be effective.

Furthermore, the inventors have found that they can encourage layers to selfdelaminate by using growth conditions that don’t produce ‘ideal graphene’. That is, by deoptimising the growth conditions, for example by using lower than optimum growth temperatures, the delamination can be encouraged.

The inventors have found that that a change in growth rate, for example faster growth rates generally increase layer separation from the substrate. Preferably the reactor conditions are controlled to achieve growth of the graphene at a rate of at least 10nm/hr. To encourage delamination it is desirable to significantly increase this rate to at least 10x the minimum speed, i.e. at least 100nm/hr. Growth rates are dependent on the precursor and desired graphene layer (doped, multi-layer) and the substrate.

Preferably the layer growth conditions are adjusted at least once every 10 layers during the growth of the graphene layer structure, preferably every 5 to 10 layers.

According to another aspect there is provided a heatsink for an LED or a high-power electronic device comprising a graphene layer structure obtainable by the method described herein. The LED may be formed on the graphene layer structure before it is peeled off the substrate, or afterwards, depending on the device being manufactured. Alternatively the LED or high-power electronic device may be simply adhered to the graphene layer structure.

According to another aspect there is provided an LED or a high-power electronic device comprising a heatsink comprising a graphene layer structure obtainable by the method described herein.

Elements of the above-described method will now be discussed in more detail.

A close coupled reaction chamber provides a separation between the substrate surface upon which the graphene 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 graphene. 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 graphene 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.

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 graphene 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 minimisation of gas phase reactions within the chamber, which are detrimental graphene 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

Through both simultaneously heating the substrate and providing cooling to 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 a significantly lower temperature than the substrate surface itself, largely reducing the probability of precursor reaction, in the gas phase, until the precursor is proximate the substrate surface.

An alternative design of MOCVD reactor is also contemplated which has been demonstrated to be efficient for graphene growth as described herein. This alternative design is a so-called High Rotation Rate (HRR) or “Vortex” flow system. Whereas the close-coupled reactor described above focussed on creating graphene using a very high thermal gradient, the new reactor has a significantly wider spacing between the injection point and growth surface or substrate. Close coupling allowed extremely rapid dissociation of precursors delivering elemental carbon, and potentially other doping elements, to the substrate surface allowing the formation of graphene layers. In contrast, the new design relies on a vortex of the precursors.

In the new reactor design, in order to promote laminar flow over the surface this system utilizes a higher rotation rate to impinge a high level of centrifugal acceleration on the injected gas stream. This results in a vortex type fluid flow within the chamber. The effect of this flow pattern is a significantly higher residency time of the precursor molecules proximate to the growth/substrate surface compared to other reactor types. For the deposition of graphene this increased time is what promotes the formation of elemental layers.

However, this type of reactor does have a couple of parasitic issues, firstly the amount of precursor required to achieve the same amount of growth as other reactors increases due to the reduced mean free path that this flow regime causes, resulting in more collisions of precursor molecules delivering non-graphene growth atomic recombination. However, the use of reagents such as hexane which are relatively cheap means that this problem can be readily overcome. Additionally, the centrifugal motion has varying impacts on atoms and molecules of different sizes resulting in the ejection of different elements at different velocities. While this probably assists graphene growth due to the uniform rate of carbon supply with ejection of unwanted precursor by-products it can be detrimental to desired effects such as elemental doping. It is therefore preferred to use this design of reactor for undoped graphene.

An example of such a reaction system is the Veeco Instruments Inc. Turbodisc technology, K455i or Propel tools.

Preferably the reactor used herein in a high rotation rate reactor. This alternative design of reactor may be characterised by its increased spacing and high rotation rate.

Preferred spacings are from 50 to 120mm, more preferably 70 to 100mm. The rotation rate is preferably from 100rpm to 3000rpm, preferably 1000rpm to 1500rpm.

Figures

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

Figure 1 shows a schematic cross-section of a graphene-layer growth chamber for use in the method described herein.

The reactor of Figure 1 is constructed for the deposition of a graphene layer on a substrate through the method of Vapour Phase Epitaxy (VPE), in which a precursor is introduced to thermally, chemically and physically interact in the vicinity of and on the substrate to form a graphene layer structure having from 1 to 10 graphene layers.

The apparatus comprises a close coupled reactor 1 having a chamber 2 having an inlet or inlets 3 provided through a wall 1A and at least one exhaust 4. A susceptor 5 is arranged to reside within the chamber 2. The susceptor 5 comprises one or more recesses 5A for retaining one or more substrates 6. The apparatus further comprises means to rotate the susceptor 5 within the chamber 2; and a heater 7, e.g. comprising a resistive heating element, or RF induction coil, coupled to the susceptor 5 to heat the substrate 6. The heater 7 may comprise a single or multiple elements as required to achieve good thermal uniformity of the substrate 6. One or more sensors (not shown) within the chamber 2 are used, in conjunction with a controller (not shown) to control the temperature of the substrate 6.

The temperature of the walls of the reactor 1 are maintained at substantially constant temperature by water cooling.

The reactor walls define one or more internal channels and/or a plenum 8 that extend substantially adjacent (typically a couple of millimetres away) the inner surface of reactor walls including inner surface IB of wall 1A. During operation, water is pumped by a pump 9 through the channels/plenum 8 to maintain the inside surface 1B of wall 1A at or below 200°C. In part because of the relatively narrow diameter of the inlets 3, the temperature of the precursor (which is typically stored at a temperature much below the temperature of inside surface 1B), as it passes through inlets 3 through wall 1A into the chamber 1 will be substantially the same or lower than the temperature of the inside surface 1B of wall 1A.

The inlets 3 are arranged in an array over an area that is substantially equal or greater than the area of the one or more substrates 6 to provide substantially uniform volumetric flow over substantially the entirety of surfaces 6A of the one or more substrates 6 that face the inlets 3.

The pressure within the chamber 2 is controlled through control of precursor gas flows through inlet(s) 3 and exhaust gas through exhaust 4. Via this methodology, the velocity of the gas in the chamber 2 and across the substrate surface 6A and further the mean free path of molecules from the inlet 3 to substrate surface 6A are controlled. Where a dilution gas is used, control of this may also be used to control pressure through inlet(s)

3. The precursor gas is preferably hexane.

The susceptor 5 is comprised from a material resistant to the temperatures required for deposition, the precursors and dilution gases. The susceptor 5 is usually constructed of uniformly thermally conducting materials ensuring substrates 6 are heated uniformly. Examples of suitable susceptor material include graphite, silicon carbide or a combination of the two.

The substrate(s) 6 are supported by the susceptor 5 within the chamber 2 such that they face wall 1A with a separation, denoted in Figure 1 by X, of between 1mm 100mm, though, as discussed above, generally the smaller the better. Where the inlets 3 protrude into or otherwise sit within the chamber 2, the relevant separation is measured between the substrate(s) 6 and exit of the inlets 3.

The spacing between the substrate 6 and the inlets 3 may be varied by moving the susceptor 5, substrate 6 & heater 7.

An example of a suitable close coupled reactor is the AIXTRON® CRIUS MOCVD reactor, or AIXTRON® R&D CCS system.

Precursors in gaseous form or in molecular form suspended in a gas stream are introduced (represented by arrows Y) into the chamber 2 through inlets 3 such that they will impinge on or flow over the substrate surface 6A. Precursors that may react with one another are kept separated until entering the chamber 2 by introduction through different inlets 3. The precursor or gas flux/flow rate is controlled externally to the chamber 2 via a flow controller (not shown), such as a gas mass flow controller.

A dilution gas may be introduced through an inlet or inlets 3 to modify gas dynamics, molecular concentration and flow velocity in the chamber 2. The dilution gas is usually selected with respect to the process or substrate 6 material such that it will not have an impact on the growth process of the graphene layer structure. Common dilution gases include Nitrogen, Hydrogen, Argon and to a lesser extent Helium.

After the graphene layer structure having at least 50 graphene layers has been formed, the reactor is then allowed to cool and the substrate 6 is retrieved having the graphene layer structure thereon. The graphene layer generally lifts from the substrate such that it can be gently peeled off to obtain a free self-supporting graphene layer structure of unprecedented size and quality.

Examples

The present invention will now be described further with reference to the following nonlimiting examples.

The following describes example processes using the aforementioned apparatus that successfully produced graphene layer structure having at least graphene layers. In all examples a close coupled vertical reactor of diameter 250mm with six 2(50mm) target substrates were used. For reactors of alternate dimensions and/or different target substrate areas, the precursor and gas flow rates can be scaled through theoretical calculation and/or empirical experimentation to achieve the same results.

Using the method of the invention it has been possible to produce patterned graphene with substantially improved properties over known methods, for example with a grain size greater than 20pm, covering a substrate of 6 inch diameter with 98% coverage, a layer uniformity of >95% of the substrate, sheet resistivity less than 450Q/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/V s 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 (15cm) 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.

Example 1

The reactor was heated to 1200 degrees Celsius in a nitrogen gas flow of 14000 seem, while simultaneously pumping the reactor to a pressure of 50mbar. Dibromomethane was introduced to the reactor at a flow of 900 seem at a precursor pressure of 800mbar and a precursor temperature of 32 degrees Celsius. Growth was allowed to proceed for 15 minutes before the precursor flow was stopped, and the reactor was cooled down to room temperature in a 10 minute ramp. This resulted in graphene which was 40 layers thick. It was possible to pick the 40-layer graphene sheet up off the wafer by using plastic tweezers, as the layer had already delaminated from the substrate.

Example 2

The reactor was heated to a temperature of 1120 degrees Celsius in a nitrogen gas flow of 12000 seem while simultaneously pumping the reactor to a pressure of 150mbar. Dibromomethane was introduced to the reactor at a flow of 300sccm at a precursor pressure of 800mbar and a precursor temperature of 32 degrees Celsius. Growth was allowed to proceed for 5 minutes before the reactor temperature was increased to 1180 degrees Celsius and the precursor flow increased to 420sccm. Growth was allowed to proceed for a further 5 minutes before the reactor temperature was increased to 1220 degrees Celsius and the precursor flow increased to 650sccm, Growth was allowed to proceed for a further 25 minutes before the precursor flow was stopped and the reactor was cooled down to room temperature in a 20 minute ramp. Using this phased temperature and precursor flow rate process growth rate and layer formation can be closely controlled allowing more than 50 layers of graphene to be formed while maintaining continued, smooth, uniform structure on the substrate.

All percentages herein are by weight unless otherwise stated.

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 in 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 10 their equivalents.

Claims

1. A method for the production of an unsupported graphene layer structure, the method comprising:

2. The method according to claim 1, wherein the flow is controlled to prevent the collapse of the graphene layer structure by adjusting the layer growth conditions at least once during the growth of the graphene layer structure.

3. The method according to claim 2, wherein the layer growth conditions are adjusted at least once every 10 layers during the growth of the graphene layer structure.

4. The method according to claim 1, wherein the reactor conditions are controlled to achieve growth of the graphene at a rate of at least 10nm/hr.

5. The method according any of claims 2 to 4, wherein the layer growth conditions are adjusted by:

6. The method according to claim 5, wherein the layer growth conditions are adjusted by:

(b) changing a flow-rate of the flow comprising a precursor compound through the inlets by at least 25%; and/or (c) changing a pressure of the flow supplied through the inlet by at least 25%; and/or (e) changing the reactor/susceptor temperature by from 10 to 50°C.

7. The method according to any of the preceding claims, wherein the precursor compound is a hydrocarbon, preferably a hydrocarbon which is a liquid at room temperature and most preferably a C5 to C10 alkane.

8. The method according to any of the preceding claims, wherein the substrate has a diameter of at least 6 inches.

9. The method according to any of the preceding claims, wherein the substrate is sapphire.

10. A heatsink for an LED or a high-power electronic device comprising a graphene layer structure obtainable by the method of any of the preceding claims.

11. A LED or a high-power electronic device comprising a heatsink comprising a graphene layer structure obtainable by the method of any of the preceding claims.