Method of producing a two-dimensional material

Abstract

A method of producing graphene or other two-dimensional material such as graphene including heating the substrate held within a reaction chamber to a temperature that is within a decomposition range of a precursor, and that allows two-dimensional crystalline material 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 steep temperature gradient ensures that the precursor remains substantially cool until it is proximate the substrate surface thus minimizing decomposition or other reaction of the precursor before it is proximate the substrate surface. The separation between the precursor inlet and the substrate is less than 100 mm.

Claims

1. A method of producing multiple stacked graphene layers, the method comprising: providing a substrate comprising nucleation sites within a reaction chamber comprising at least one precursor entry point to provide a separation between the substrate surface upon which graphene is formed and the at least one precursor entry point; cooling the at least one precursor entry point; introducing at the at least one precursor entry point 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 graphene formation from carbon released from the decomposed precursor to produce a first layer of graphene or doped graphene; followed by changing the temperature of the substrate and/or changing the pressure of the reaction chamber and/or changing the flow rate of precursor to form a further graphene layer or doped graphene layer on the first layer of graphene or doped graphene; wherein the distance between the surface of the substrate upon which the graphene is produced and the point at which precursor enters the chamber is less than 100 mm.

2. The method according to claim 1 further comprising sealing the reaction chamber after introducing the precursor to minimise or prevent flow of the precursor into or out of the reaction chamber.

3. The method according to claim 1 wherein the distance between the surface of the substrate upon which the graphene is produced and the point at which precursor enters the chamber is less or substantially equal to 20 mm.

4. The method according to claim 1 wherein the substrate provides a crystalline surface upon which the graphene is produced.

5. The method according to claim 1 wherein the substrate provides a non-metallic surface upon which the graphene is produced.

6. The method according to claim 1 wherein a mixture of the precursor with a dilution gas is passed over the heated substrate.

7. The method according to claim 6 wherein the dilution gas includes one or more from the list of hydrogen, nitrogen, argon and helium.

8. The method according to claim 1 further comprising introducing a doping element into the 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.

9. The method according to claim 8 wherein the precursor includes the doping element.

10. A method of producing graphene, the method comprising: providing a substrate comprising nucleation sites within a reaction chamber comprising at least one precursor entry point to provide a separation between the substrate surface upon which the graphene is formed and the at least one precursor entry point; cooling the at least one precursor entry point; introducing at the at least one precursor entry point 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 graphene formation from carbon released from the decomposed precursor; wherein the distance between the surface of the substrate upon which the graphene is produced and the point at which precursor enters the chamber is less than 100 mm, and wherein the precursor includes 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 optionally azides, imides, sulphides and phosphides.

11. A method of manufacturing a heterostructure that includes graphene comprising an interface with a second layer, the method comprising: providing a substrate comprising nucleation sites within a reaction chamber comprising at least one precursor entry point to provide a separation between the substrate surface upon which the graphene is formed and the at least one precursor entry point; cooling the at least one precursor entry point; introducing at the at least one precursor entry point 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 graphene formation from carbon released from the decomposed precursor; wherein the distance between the surface of the substrate upon which the graphene is produced and the point at which precursor enters the chamber is less than 100 mm, and wherein a first set of reactor conditions are used to produce graphene on the substrate within the reaction chamber; and further comprising introducing a second precursor under a second set of reactor conditions to form the second layer on the substrate.

12. The method according to claim 11 wherein the second layer comprises a film.

13. The method according to claim 11 wherein the second layer is a semiconductor.

14. The method according to claim 13 wherein the second layer comprises at least one of the following: GaN, BN, AlN, AlGaN, or SiN.

15. The method according to claim 11 wherein the separation between the substrate and a ceiling of the reactor chamber directly above the substrate is changed between forming the graphene and the second layer.

16. A method of producing graphene, the method comprising: providing a substrate comprising nucleation sites within a reaction chamber comprising at least one precursor entry point to provide a separation between the substrate surface upon which the graphene is formed and the at least one precursor entry point; cooling the at least one precursor entry point; introducing at the at least one precursor entry point 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 graphene formation from carbon released from the decomposed precursor; wherein the distance between the surface of the substrate upon which the graphene is produced and the point at which precursor enters the chamber is less than 100 mm, and wherein the temperature drop between the substrate and the precursor entry point is greater than 10,000° C. per meter.

17. The method according to claim 1 wherein the precursor includes 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 optionally azides, imides, sulphides and phosphides.

18. The method according to claim 11 wherein the precursor includes 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 optionally azides, imides, sulphides and phosphides.

19. The method according to claim 16 wherein the precursor includes 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 optionally azides, imides, sulphides and phosphides.

20. The method according to claim 1 wherein the temperature drop between the substrate and the precursor entry point is greater than 10,000° C. per meter.

21. The method according to claim 10 wherein the temperature drop between the substrate and the precursor entry point is greater than 10,000° C. per meter.

22. The method according to claim 11 wherein the temperature drop between the substrate and the precursor entry point is greater than 10,000° C. per meter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying figures in which like reference numerals refer to identical or functionally similar elements throughout the separate views, and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present disclosure, in which:

(2) FIG. 1 is a schematic of a vertical reactor used to produce a two dimensional material;

(3) FIG. 2 is a Raman spectrum of graphene produced using vapor phase epitaxy within the reactor of FIG. 1 according to the method of example 1; and

(4) FIGS. 3-6 are schematic cross sections of heterostructures, formed on a substrate, comprising one or more two dimensional material layers in combination with one or more semiconductor or dielectric material layers.

DETAILED DESCRIPTION

(5) As required, detailed embodiments are disclosed herein; however, it is to be understood that the disclosed embodiments are merely examples and that the devices and methods described herein can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one of ordinary skill in the art to variously employ the disclosed subject matter in virtually any appropriately detailed structure and function. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description. Additionally, unless otherwise specifically expressed or clearly understood from the context of use, a term as used herein describes the singular and/or the plural of that term.

(6) The terms “a” or “an”, as used herein, are defined as one or more than one. The term “plurality”, as used herein, is defined as two or more than two. The term “another”, as used herein, is defined as at least a second or more. The terms “including” and “having,” as used herein, are defined as comprising i.e., open language. The term “coupled,” as used herein, is defined as “connected,” although not necessarily directly, and not necessarily mechanically.

(7) It will also be understood that when an element such as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements can also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements can be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

(8) Reference in the specification to “one embodiment” or “an embodiment” of the present principles, as well as other variations thereof, means that a particular feature, structure, characteristic, and so forth described in connection with the embodiment is included in at least one embodiment of the present principles. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”, as well any other variations, appearing in various places throughout the specification are not necessarily all referring to the same embodiment.

(9) The reactor of FIG. 1 is constructed for the deposition of one or more two dimensional material layers on a substrate through the method of Vapor 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 single or multi-layer two-dimensional material film.

(10) 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.

(11) The temperature of the reactor's 1 walls are maintained at substantially constant temperature by water cooling.

(12) The reactor walls define one or more internal channels and/or a plenum 8 that extend substantially adjacent (typically a couple of millimeters away) the inner surface of reactor walls including inner surface 1B of wall 1A.

(13) 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.

(14) 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.

(15) 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.

(16) 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.

(17) 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 FIG. 1 by X, of between 1 mm-100 mm, though 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.

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

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

(20) 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.

(21) 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 two-dimensional material. Common dilution gases include Nitrogen, Hydrogen, Argon and to a lesser extent Helium.

(22) The following describes example processes using the aforementioned apparatus that successfully produced two-dimensional material layers and two-dimensional layer heterostructures from one or more two-dimensional layers and one or more other semiconductor or dielectric materials. In all examples a close coupled vertical reactor of diameter 250 mm with six 2″ (50 mm) 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.

Example 1

(23) Graphene monolayer(s) can be produced on a selected substrate within the standard operating parameters of a close coupled reaction chamber via VPE. With careful selection of the graphene precursor and substrate type and matching with suitable reaction chamber parameters it is possible to deposit graphene on the substrate surface.

(24) For example, selecting a metallocene, Cp.sub.2Mg or Cp.sub.2Fe as a process precursor the reactor is heated to a temperature such that the surface of the substrate (here silicon or sapphire) is greater than the required, or complete, decomposition temperature of the precursor, here >500° C. The reactor pressure is lowered to suitable vacuum level to ensure evacuation of unwanted process by-products, for the metallocenes in this example a pressure of <200 mbar has proven successful. The metallocene and a dilution flow of hydrogen are then introduced to the reactor, through the inlets, and hence substrate surface at a suitable flow rate, in this example 700 sccm of metallocene and 1300 sccm of hydrogen is ideal. The precursor flows into the reactor for a period of time that allows the formation of a full, uniform graphene monolayer on the substrate surface, in this example 545 seconds is ideal for a silicon substrate and 380 seconds for a sapphire substrate. After completion of the layer the metallocene flow is ceased and the reactor is cooled under a continuing hydrogen flow of 2000 sccm at continuing low pressure, to preserve the graphene surface until suitably cooled, ideally to <100° C.

(25) A Raman spectrum of the resulting graphene formed by this process is illustrated in FIG. 2.

Example 2

(26) Graphene monolayer(s) production using CH.sub.3Br as a process precursor. The reactor is heated to a temperature such that the substrate, here sapphire, is greater than the complete decomposition temperature of the precursor, here >350° C. The reactor pressure is lowered to a suitable vacuum level to ensure evacuation of unwanted decomposition and reaction by-products and also facilitate high enough residency time of carbon products at the substrate surface to form graphene. For CH.sub.3Br a pressure of 600 mbar has been proven ideal, as the main unwanted by-product Br has vapor pressure higher than this at the selected deposition temperature. The precursor and a dilution flow of nitrogen gas are then introduced to the reactor and hence substrate, through the chamber inlets, at a suitable flow rate, in this example 1000 sccm is ideal for CH.sub.3Br and 2000 sccm for nitrogen. Nitrogen is used in this process so as to limit the possible formation of HBr. The precursor and dilution gas flow through the reactor for a period of time that allows the formation of a full, uniform graphene monolayer on the substrate surface, in this example 320 seconds is ideal. After completion of the layer the precursor flow is ceased and the reaction chamber cooled under continuing Nitrogen flow until the substrate, and graphene layer, are at a suitably low temperature, ideally <100° C.

Example 3

(27) Graphene monolayer(s) production using CH.sub.4 as a process precursor. The reactor is heated to a temperature such that the substrate, here sapphire, is greater than the initial decomposition temperature of the precursor, here >1100° C. The reactor pressure is set to a suitable vacuum level to ensure favorable gas velocity near the substrate surface, for CH.sub.4 a pressure of 800-900 mbar is suitable as the by-products of CH.sub.4 decomposition will not adversely affect the growing material, the advantage here is the increased residency time of precursor material, at higher reactor pressure, promotes high deposition rates, significantly shortening the time required to deposit graphene. The precursor and a dilution gas of hydrogen are then introduced to the reactor and hence substrate surface, through the chamber inlets, at a suitable flow rate, in this example 1000 sccm is ideal for the CH.sub.4 and 2000 sccm for the hydrogen. The precursor flows across the substrate surface for a period of time that allows the formation of a full, uniform graphene monolayer on the substrate surface, in this example 30 seconds is ideal. After completion of the layer the precursor flow is ceased and the hydrogen dilution flow increased to 3000 sccm, the reactor chamber is then cooled under continuing hydrogen flow until the substrate and graphene layer are suitably cool, ideally <100° C.

Example 4

(28) Graphene production on different substrates can be improved through the application of substrate preparation or conditioning techniques prior to graphene growth, ensuring the substrate surface is in the most preferential state before introducing precursors to initiate graphene deposition.

(29) For example when growing graphene on a silicon substrate the quality of a graphene monolayer is significantly improved by conditioning the silicon surface before introducing the precursor. In general, and in this case, heating the silicon substrate to a surface temperature of 1050° C. at a reactor pressure of 100 mbar under a hydrogen gas flow of 5000 sccm unwanted surface contamination on the substrate surface, including native oxide, is removed revealing a pure silicon surface.

(30) Subsequent deposition of graphene can be readily achieved on this prepared surface with process conditions of; reactor temperature 900° C., reactor pressure 200 mbar, Cp.sub.2Mg precursor flow rate of 700 sccm and hydrogen dilution flow rate of 1300 sccm resulting in a graphene crystal structure significantly improved on the graphene achieved without applying the substrate conditioning procedure. Again here, reactor cooling with a hydrogen flow is desirable until the substrate and graphene have reached suitably low temperature, ideally <100° C.

(31) In the following examples the substrate used was either silicon or sapphire. In the case of silicon substrates the conditioning process outlined in Example 4 is applied prior to the graphene deposition process.

Example 5

(32) Modifying the dilution gas flow for the process of graphene deposition can be preferential for certain precursors or substrates, allowing additional control to graphene layer formation while maintaining the same rate of carbon delivery to the substrate surface. Further, this can be critical for ensuring good material formation in some cases.

(33) For example using a precursor of carbon tetrabromide (CBr.sub.4), with a silicon or sapphire substrate at a temperature of 1025° C. and a reactor pressure of 400 mbar a precursor flow rate of 1000 sccm will result in graphene monolayer deposition exhibiting undesirable microstructure with small grain size and high defect levels due, largely, to interstitial point defect formation. Introducing a dilution of hydrogen (H.sub.2) to the precursor flow for example 2000 sccm, ie a ratio of 2:1 H.sub.2:CBr.sub.4, with other process parameters remaining the same, improves graphene layer material markedly. The presence of hydrogen during the deposition process results in the formation of HBr a very high vapor pressure by-product of precursor decomposition and dilution gas reaction that is readily evacuated reducing parasitic interactions at the substrate surface. Increasing the flow ratio up to about 12:1 further improves the graphene layer, for this precursor (CBr.sub.4). Above a ratio of 12:1 the concentration of dilution species, hydrogen, in the reactor adversely affects the ability of carbon to reach the substrate surface inhibiting coherent layer deposition resulting in a situation where graphene cannot be produced.

Example 6

(34) Graphene with varying, or predefined properties is produced by allowing doping of the graphene layer from other atomic species comprised within the precursor to be incorporated in the graphene layer.

(35) For example using a precursor of Cp.sub.2Mg at a substrate temperature of 870° C. with precursor flow rate of 800 sccm at reactor pressure of 300 mbar the precursor is introduced, through the inlets once the substrate has reached the desired temperature and flows for a period of 500 seconds. This allows the stable incorporation of Mg within the graphene lattice producing a doped graphene layer. Modification of the pressure and/or temperature gives control over the doping level, although care has to be exercised to ensure graphene quality is not compromised by applying reactor deposition conditions that are preferential to doping but outside the ranges for good graphene formation. Applying this technique, but allowing for precursor and substrate properties, may achieve graphene performance, electrical and mechanical, as desired.

Example 7

(36) Graphene is produced with varying properties by introducing a second precursor to actively dope the graphene layer.

(37) For example using a precursor of CH.sub.4 with a substrate temperature of 1250° C. at reactor pressure of 720 mbar and precursor flow rate of 1000 sccm to produce graphene a second, preferred dopant, precursor can be introduced to supplement the material construction. For example using TEZn, to zinc dope the graphene layer, at a flow of 25 sccm will produce a uniform, large grain size graphene with preferentially resistive properties.

Example 8

(38) Graphene is produced more efficiently through the control (in this example reduction) of the reactor precursor introduction point to substrate surface spacing.

(39) For example using a precursor of CH.sub.3Br at a flow rate of 800 sccm; a substrate temperature of 1000° C.; a chamber pressure of 650 mbar and a precursor introduction point to substrate separation of 12 mm, graphene can be readily deposited in growth time of 360 seconds.

(40) Reducing the precursor introduction point to substrate separation to a distance of 10 mm allows the same conditions to be applied with a reduced growth time of 315 seconds to achieve the same graphene as would be produced in 360 seconds with a separation of 12 mm.

(41) Alternatively this reduced separation allows the substrate temperature to be reduced to 970° C. whilst maintaining a CH.sub.3Br at a flow rate of 800 sccm a chamber pressure of 650 mbar and deposition time of 360 s to achieve the same graphene. The reduced temperature in this case leads to less substrate deformation, due to reduced thermal expansion, resulting in a more uniform graphene layer over the substrate surface. Similarly, reducing the precursor introduction to substrate surface separation to 5 mm allows further reduction in the surface temperature, in this example to 920° C., whilst maintaining the other process variables at the same values.

(42) It should be noted that this methodology can also be applied to the variation of other parameters, for example using a precursor of CH.sub.3Br at a flow rate of 800 sccm; a substrate temperature of 1000° C.; a chamber pressure of 650 mbar and a ceiling to substrate separation of 12 mm, graphene can be readily deposited in growth time of 360 seconds. Reducing the ceiling to substrate surface separation to 5 mm allows the precursor flow rate to be reduced to 550 sccm whilst maintaining a substrate temperature of 1000° C. under a chamber pressure of 650 mbar and deposition time of 360 s to achieve the same graphene layer result.

Example 9

(43) Graphene layer material properties can be modified through simple flow “pulsing”.

(44) For example using a precursor of Cp.sub.2Mg, a substrate temperature of 1000° C. and reactor chamber pressure of 200 mbar the Cp.sub.2Mg is introduced to the reactor at a flow rate of 1000 sccm for a period of 20 seconds, the flow is then paused for a period of 20 seconds after which the flow is re-initiated for a further 20 seconds, then again paused for the following 20 seconds. Repeating the methodology several times, in this example for 10 cycles, has shown to significantly increase the amount of Mg that can be incorporated into the graphene layer altering the final layer electrical properties.

Example 10

(45) Graphene layer structure can be modified through precursor flow rate ‘pulsing’. A modification to the pulsed flow approach of Example 9 is employed, in this case applying a high flow, low flow procedure whereby the precursor is admitted to the substrate surface for a period at a level above the minimum flow threshold for growth and then reduced to a level below the minimum flow threshold where there the growth rate is close to or substantially zero for a period and repeated for a number of cycles. In this method the low flow step, as opposed to the precursor off period, as in standard pulsing, helps reduce carbon desorption from the surface during this time.

(46) For example using a growth precursor of CH.sub.3Br at a substrate temperature of 850° C. and chamber pressure of 550 mbar the precursor is introduced to the reaction chamber at a flow of 1000 sccm for a period of 15 seconds, the flow is then lowered to a flow of 200 sccm for a period of 20 seconds, the flow is then increased back to the initial 1000 sccm for a further 15 seconds. This stepped flow procedure is repeated a desired number of cycles, typically 5 to 10 periods to achieve good layer ordering. This process has been shown to significantly improve graphene grain size, allowing, by controlling the number of cycles, effective control of graphene material structure.

Example 11

(47) Graphene layer structure properties can be modified through precursor/dilution gas switching or ‘pulsing’. Applying a further modification to the pulsed deposition process of example 9 comprising switching between precursor and non-carbon purge gas comprising non invasive or non reactive inlet gas to remove the precursor rapidly from the substrate surface or vicinity during the period that the precursor is not flowing. In this case precursor is flowed for a period of time then stopped and the purge gas started for a period of time before flow of purge gas is stopped and the flow of precursor re-started, the procedure is repeated for a number of cycles.

(48) For example, using a precursor of methane at a substrate temperature of 1220° C. and chamber pressure of 800 mbar the precursor is introduced to the reaction chamber at a flow of 1000 sccm for a period of 10 seconds, the precursor flow is then stopped and the purge gas flow of 1000 sccm H2 started for a period of 10 seconds, the precursor flow is then reintroduced for a period of 10 seconds and so on for a selected number of cycles. Eight cycles has been shown to reduce defect density of the graphene layer.

Example 12

(49) In an improvement to example 11, a further modification of the pulsed deposition procedure is to flow the dilution gas continuously and only switch the precursor gas on and off ensuring there is a continuous flow over the substrate surface at all times.

(50) For example using a precursor of methane as a carbon source at a substrate temperature of 1220° C. and chamber pressure of 800 mbar the precursor is introduced to the reaction chamber at a flow of 1000 sccm at the same time as a dilution gas flow of H2 at 1000 sccm for a period of 10 seconds, the precursor flow is then stopped and the dilution gas allowed to continue to flow for a period of 10 seconds. This constitutes a cycle. The precursor flow is then reintroduced for a further period of 10 seconds and so on for a selected number of cycles. Using twenty four cycles has been shown to significantly reduce the defect density of the graphene layer.

Example 13

(51) Efficiency of graphene production can be improved through the application of a ‘sealed volume’ process. In this process enough precursor is admitted to the reactor to enable graphene formation on the substrate surface whilst limiting significantly the amount of precursor consumed. The process involves filling the reaction chamber with precursor such that the environment does not cause the decomposition of the precursor, then initiating a reaction by increasing the substrate surface temperature.

(52) For example using a precursor of CH.sub.3Br, the reaction chamber is reduced to a low pressure, typically 1-5 mbar, and the exhaust sealed. Precursor is admitted to the reactor, through the inlets, allowing the volume to refill to a pressure of 900 mbar, creating a precursor rich static environment. The reactor is heated rapidly, 5° C./s is sufficient, so that the substrate reaches a temperature of 900° C. and held at temperature for a period of time, in this example 10 minutes. Precursor circulation is induced via thermal convection and decomposition of the precursor occurs in the vicinity of the substrate allowing graphene to be produced on the substrate surface. After 10 minutes the reactor is cooled as rapidly as possible, by switching the heating element off, to room temperature. Once the temperature of the reactor chamber and substrate have fallen below the decomposition temperature of the precursor, the reactor is evacuated and then purged, using a purge gas, in this example nitrogen.

(53) This process provides an extremely controllable method for the amount of carbon deposited on the target substrate by limiting the maximum amount of precursor available to the substrate surface for the whole deposition period. Molar concentration of precursor can be readily modified through alteration of the initial re-fill pressure. It should be noted that this process can be difficult to perfect and control in standard reaction chambers as ambient chamber cooling affects the process significantly.

Example 14

(54) A minor variation of the sealed volume process of example 13 in which the chamber is evacuated and then purged with a purge gas such as hydrogen at a flow rate of 5000 sccm for a period of 5 minutes following which the reactor is cooled as rapidly as possible to room temperature. This has been shown to aid limitation of graphene surface contamination during the cooling step.

Example 15

(55) A further variation to the sealed volume process of example 13 in which the substrate is first heated to a temperature in excess of the precursor decomposition temperature before the chamber is evacuated and the precursor then introduced. This procedure has been shown to reduce defect density of the graphene monolayer.

Example 16

(56) Graphene is refined through the application of a post deposition processing technique to modify the graphene structure, reducing structural defects and precipitate the ejection of lattice contamination consisting of unwanted atomic and molecular species that would otherwise deform graphene monolayers and adversely affect graphene material properties.

(57) Depositing graphene using a precursor of Cp.sub.2Fe at a flow rate of 750 sccm with substrate temperature 960° C. and reactor pressure 175 mbar has been shown to produce significantly doped graphene within 560 seconds. While this form of graphene is suitable for some applications it can be modified to have different properties by post deposition heat and gas treatment. After completion of the deposition process increasing the substrate temperature to 1200° C. for 30 minutes under a hydrogen gas introduced at a flow rate of 10000 sccm, results in a significant change to the graphene layer.

Example 17

(58) Optimized graphene layers can be produced through the combination of two or more of the aforementioned examples and the addition of further beneficial process steps.

(59) For example, a sapphire substrate is first prepared by heating within the reaction chamber to a substrate surface temperature of 1100° C. under a hydrogen gas flow of 10000 sccm at a pressure of 50 mbar for at least 5 minutes to remove condensed and contaminating materials or species from the substrate surface. The substrate is then cooled to a temperature of 975° C. whereupon NH.sub.3 is introduced at a flow rate of 3000 sccm into the reactor chamber for a period of 60 seconds to nitride, or nitrogen terminate the substrate surface. The flow of NH.sub.3 is stopped and the precursor, in this case CH.sub.4, is introduced at a flow rate of 200 sccm accompanied by a dilution gas flow of H.sub.2 at 7000 sccm for a second period of 60 seconds. The substrate is heated to a temperature of 1220° C., the reactor pressure increased to 700 mbar and the precursor flow increased to a flow rate of 800 sccm accompanied by a dilution gas flow increase to 10000 sccm of H.sub.2. After reaching the target substrate temperature of 1220° C. the introduction of precursor, CH.sub.4, is pulsed through a cycle of 10 seconds on and 5 seconds off for 15 cycles. In each subsequent cycle the precursor flow rate is reduced by 5 sccm. Upon completion of the 15 cycles the reactor temperature is increased to 1250° C. and held for 60 seconds after which the reactor pressure is reduced to 30 mbar and held for 60 seconds. The reactor pressure is then taken back to 700 mbar for a further 60 seconds, cycling between these two pressures, and holding for 60 seconds, is completed 10 times. The reactor is then cooled, as rapidly as possible, under a continued H.sub.2 flow.

(60) The same process structure, modified in terms of pressure, temperature and flow rate, can be applied to any suitable precursor and viable substrate to produce graphene of high quality.

Example 18

(61) Multilayer graphene is produced through the variation of process variables to enable further graphene layers to be deposited on the first monolayer. As graphene is generally self-limiting in lattice construction, when produced in high quality form via VPE, the surface energy state needs to be overcome to form subsequent high quality mono-layers of graphene on the previous layer. This is achieved, by altering the reaction chamber conditions during formation of the further graphene layers.

(62) For example, using a suitable hydrocarbon as the process precursor, here CH.sub.4, at a substrate temperature of 1120° C.; a reactor pressure of 800 mbar and a flow rate 1000 sccm, graphene can be formed readily on a sapphire substrate surface. Continuing with these conditions will not produce high quality multilayer graphene but rather another carbon poly-type such as amorphous carbon. By decreasing the reactor pressure to 600 mbar, and/or increasing the reactor temperature to 1310° C. and lowering the precursor flow to 400 sccm it is possible to facilitate the formation of further graphene layers on the initial graphene monolayer. It is believed this technique overcomes the potential energy barrier of the previous layer surface and further inhibits the formation of bulk carbon such as graphite as opposed to graphene.

Example 19

(63) Silicene monolayer(s) can be produced on a selected substrate within the standard operating parameters of a close coupled reaction chamber via VPE. Carefully selecting the silicene precursor and substrate type and matching with suitable reaction chamber parameters it is possible to deposit silicene on the substrate surface.

(64) For example, selecting silane with concentration of 100 ppm in hydrogen, as the process precursor the reactor is heated to a temperature such that the surface of the substrate (here silicon or sapphire) is greater than the required, or complete, decomposition temperature of the precursor and preferential to promote surface kinetics for the formation of a silicene crystal structure, here ˜920° C. The reactor pressure is lowered to suitable vacuum level to ensure evacuation of unwanted process by-products, for silane in this example a pressure of <500 mbar has proven successful. The silane and a dilution flow of hydrogen are then introduced to the reactor, through the inlets, and hence substrate surface at a suitable flow rate, in this example 2000 sccm of silane and 10000 sccm of hydrogen is ideal. The precursor flows into the reactor for a period of time that allows the formation of a full, uniform silicene monolayer on the substrate surface, in this example 800 seconds is ideal for a silicon substrate and 600 seconds for a sapphire substrate. After completion of the layer the silane flow is ceased and the reactor is cooled under a nitrogen flow of 5000 sccm at continuing low pressure, to preserve the silicene surface until suitably cooled, ideally to <100° C.

(65) The silicene monolayer will remain intact on the substrate surface for the period that it remains within an inert environment, which may be the reactor chamber or an ancillary container, if the material can be transferred from the chamber to the container in an inert environment and the container ambient is inert.

Example 20

(66) To enable use of silicene ex-situ of the reaction chamber the silicene layer can be capped with a material stable in an air ambient. By using a silicon based alloy it is possible to grow a capping layer on top of silicene protecting the silicene layer from exposure to the external ambient.

(67) Using the method described in Example 19 silicene can be produced on a sapphire or silicon substrate. Upon completion of the silicene layer a further silicon nitride (SiN) layer can be produced by changing the reactor parameters and introducing a nitrogen precursor source, here NH.sub.3. After completing the silicene layer the silane precursor is stopped and the reactor temperature changed to a temperature that allows the formation of high quality SiN, here 1050° C., while the reactor pressure is decreased to limit gas phase reactions during the deposition of the SiN layer, in this example 200 mbar. Once reactor conditions have been stabilised the silane is re-introduced to the reaction chamber at the same time as NH.sub.3. The NH.sub.3 flow rate is set to achieve a silane to NH.sub.3 ratio that will enable efficient formation of SiN on the silicene surface, here 3000 sccm. The precursors are allowed to flow for a period of time that results in a continuous SiN layer, on top of the silicene, thick enough to protect and preserve the silicene once it is removed from the reaction chamber, in this case 600 seconds. The precursors are then stopped and the reactor cooled under the continuing hydrogen flow to preserve the SiN surface ideally to a temperature <100° C.

Example 21

(68) It is possible to modify the inherent electrical, thermal and mechanical properties of silicene through the introduction of a dopant element or elements to the 2D silicene crystal structure. This can be readily achieved by introducing a doping chemical source to the deposition process while producing silicone.

(69) For example silicene can be doped using oxygen, as in example 19, selecting silane with concentration of 100 ppm in hydrogen, as the process precursor the reactor is heated to a temperature such that the surface of the substrate (here silicon or sapphire) is greater than the required, or complete, decomposition temperature of the precursor and preferential to promote surface kinetics for the formation of a silicene crystal structure, here ˜920° C. The reactor pressure is lowered to suitable vacuum level to ensure evacuation of unwanted process by-products. Unlike pure silicene production the addition of a doping source to the process has to be considered carefully here water vapor, in the form of H.sub.2O, is used as the oxygen source where a pressure of <250 mbar has proven successful. The silane, doping source and a dilution flow of hydrogen are then introduced to the reactor, through the inlets, and hence substrate surface at a suitable flow rate, in this example 2000 sccm of silane, 150 sccm of H.sub.2O and 10000 sccm of hydrogen is ideal. The precursor, doping source and dilution gas flows into the reactor for a period of time that allows the formation of a full, uniform, doped, silicene monolayer on the substrate surface, in this example 920 seconds is ideal for a silicon substrate and 690 seconds for a sapphire substrate. After completion of the layer the silane and doping flows are ceased and the reactor is cooled under a nitrogen flow of 5000 sccm at continuing low pressure, to preserve the silicene surface until suitably cooled, ideally to <100° C.

Example 22

(70) It is possible to combine graphene and silicene 2D layers to form a 2D multilayer structure in a single production process by, for example, depositing a graphene layer on a substrate surface, here sapphire, using one of the previous detailed example processes, here example 2 using CH.sub.3Br. After completing the graphene layer, the reactor is purged by reducing the pressure to 100 mbar and increasing the nitrogen flow rate to 10000 sccm for a period of 600 seconds, during this period the reactor temperature is lowered to achieve a substrate temperature suitable for the decomposition of the silicene precursor and produce conditions at the surface of the previous graphene layer preferential for the formation of silicene, here 1015° C. After completing the purge, the reaction chamber pressure is changed to achieve a condition preferential for the formation of silicene on the graphene surface and limiting the possibility of gas phase and surface reactions that could disrupt uniform silicene deposition, in this example 575 mbar. The nitrogen flow is then replaced with a hydrogen purge flow of 5000 sccm for a period of 600 seconds. Silane, of concentration 100 ppm in hydrogen, and a dilution gas of hydrogen are introduced to the reaction chamber with flows of 2500 sccm and 12000 sccm respectively. The silane and dilution gas are flowed for a period of time that allows the formation of a layer of silicene on the graphene surface, in this example 540 seconds, during this period the reactor pressure reduces, to a final pressure of 100 mbar. After this period the silane flow is ceased and the reaction chamber cooled under a continuing flow of hydrogen, at low pressure.

(71) With the capability to deposit 2D materials, both in monolayer and multilayer form, using the apparatus of FIG. 1, 2D material layers can be combined with semiconductor and/or dielectric materials to produce a simple heterostructure. FIG. 3 illustrates an example heterostructure formed on a substrate 10 comprising a semiconductor or dielectric layer 11 upon which a 2D material layer 12 is formed.

Example 23

(72) A simple heterostructure of Boron Nitride (BN) and graphene can be produced in a single continuous deposition process. For example, selecting precursors such as triethylboron (TEB) and ammonia gas (NH.sub.3), the reactor is heated to a temperature such that the surface of the substrate (here silicon or sapphire) is greater than the required, or complete, decomposition temperature of the precursor, here >700 C. The reactor pressure is lowered to a suitable vacuum level to ensure evacuation of unwanted process or gas phase reaction by-products. For these precursors, a pressure of <100 mbar has proven successful. The TEB, NH.sub.3 and dilution flows of hydrogen are then introduced to the reactor through the inlets, and hence substrate surface at a suitable flow rate. In this example, 1500 sccm of NH.sub.3, 40 sccm of TEB and 2500 sccm of hydrogen are ideal. The precursors flow into the reactor for a period of time that allows the formation of boron nitride to a desired thickness. In this example, 4 hours is suitable for the deposition of 50 nm of boron nitride.

(73) After completion of the boron nitride layer, the TEB flow and NH.sub.3 flows are stopped, while maintaining the hydrogen flow. A suitable hydrocarbon precursor such as methane (CH.sub.4) is selected for 2D material growth, in this case graphene. The reaction chamber temperature is changed such that the surface of the boron nitride material is greater than the required, or complete, decomposition temperature of the 2D material precursor, here >1100 C. The reactor pressure is also changed to a suitable level to ensure evacuation of unwanted process by-products from the 2D growth procedure. For this hydrocarbon precursor, a pressure of <200 mbar has proven successful. The precursor flow is now set, along with the hydrogen dilution flow, to be 1000 sccm and 2000 sccm, respectively. The precursor and dilution gas flow into the reactor for a period of time that allows for the formation of a full, uniform monolayer of graphene on the boron nitride surface; in this example 450 seconds is ideal.

(74) After completion of the graphene layer, the methane precursor flow is ceased, and the reactor is cooled under a continuing hydrogen flow of 2000 sccm at continuing low pressure, to preserve the graphene surface until suitable cooled, ideally to <100 C.

Example 24

(75) In a similar process to Example 23, 2D material monolayer(s) can be produced on Aluminium Nitride (AlN) on a selected substrate in the same process.

(76) For example, selecting precursors such as trimethylaluminium (TMAl) and ammonia gas (NH.sub.3), the reactor is heated to a temperature such that the surface of the substrate (here silicon or sapphire or silicon carbide) is greater than the required, or complete, decomposition temperature of the precursors, here >700 C. The reactor pressure is lowered to a suitable vacuum level to ensure evacuation of unwanted process byproducts. For these precursors, a pressure of <100 mbar has proven successful. The TMAl, NH.sub.3 and dilution flows of hydrogen are then introduced to the reactor through the inlets, and hence substrate surface at a suitable flow rate. In this example, 50 sccm of NH.sub.3, 50 sccm of TMAl and 10000 sccm of hydrogen are ideal. The precursors flow into the reactor for a period of time that allow the formation of aluminium nitride to a desired thickness. In this example, 1 hour is suitable for the deposition of 300 nm of aluminium nitride.

(77) After completion of the aluminium nitride layer, the TMAl flow and NH.sub.3 flows are stopped while the hydrogen flow is maintained. A hydrocarbon precursor, such as methane (CH.sub.4), is selected for 2D material growth, in this case graphene. The reaction chamber temperature is changed such that the surface of the aluminium nitride material is greater than the required, or complete, decomposition temperature of the precursor, here >1100 C, while the reactor pressure is changed to a suitable level to ensure evacuation of unwanted process by-products. For this hydrocarbon precursor, in this heterostructure process, a pressure of <200 mbar has proven successful. Precursor and hydrogen dilution flow rates of 1000 sccm and 2000 sccm are established, respectively and the precursor and hydrogen introduced to the chamber and hence aluminium nitride surface for a period of time that allows for the formation of a full, uniform monolayer of graphene on the aluminium nitride surface; in this example 450 seconds is ideal.

(78) After completion of the graphene layer, the methane precursor flow is ceased, and the reactor is cooled under a continuing hydrogen flow of 2000 sccm at continuing low pressure, to preserve the graphene surface until suitable cooled, ideally to <100 C.

Example 25

(79) In a similar process to Example 23, 2D material monolayer(s) can be produced on Gallium Nitride (GaN).

(80) For example, selecting precursors such as trimethylgallium (TMGa) and ammonia gas (NH.sub.3), the reactor is heated to a temperature such that the surface of the substrate (here sapphire or free-standing GaN) is greater than the required, or complete, decomposition temperature of the precursors, here >500 C. The reactor pressure is lowered to a suitable vacuum level to ensure evacuation of unwanted process by-products. For these precursors, a pressure of <600 mbar has proven successful. The TMGa, NH.sub.3 and dilution flows of hydrogen are then introduced to the reactor through the inlets, and hence substrate surface at a suitable flow rate. In this example, 5000 sccm of NH.sub.3, 100 sccm of TMGa and 15000 sccm of hydrogen are ideal. The precursors flow into the reactor for a period of time that allows the formation of gallium nitride material to a desired thickness. In this example, 1 hour is suitable for the deposition of 1.5 um of gallium nitride.

(81) After completion of the gallium nitride layer, the TMGa flow is stopped, while the NH.sub.3 flow and hydrogen flow continues. A hydrocarbon precursor, such as methane (CH.sub.4), is selected for 2D material growth, in this case graphene. The reaction chamber temperature is changed such that the surface of the gallium nitride material is greater than the required, or complete, decomposition temperature of the 2D material precursor, here >1100 C. The reactor pressure is also changed to a suitable level to ensure evacuation of unwanted process by-products. For this hydrocarbon precursor in this heterostructure production process a pressure of <200 mbar has proven successful. The precursor flow is now set, along with the hydrogen dilution flow, to be 1000 sccm and 2000 sccm, respectively. The precursor is introduced to the chamber and hence gallium nitride surface for a period of time that allows for the formation of a full, uniform monolayer of graphene on the gallium nitride surface; in this example 320 seconds is ideal.

(82) After completion of the graphene layer, the methane precursor flow is ceased, and the reactor is cooled under a continuing hydrogen flow of 2000 sccm and ammonia flow of 5000 sccm at continuing low pressure, to preserve the graphene surface until suitable cooled, ideally to <100 C.

(83) Using similar methodologies to examples 23-25 graphene can be produced on semiconductor material surfaces using other graphene precursors.

Example 26

(84) Using the same methodology as Example 23 boron nitride can be readily formed on a substrate. Graphene can then be produced on top of the BN surface using a halocarbon precursor, such as CH.sub.3Br.

(85) After completion of the BN layer, hydrogen dilution gas continues to flow into the reaction chamber at the aforementioned 2000 sccm, as in example 23. The reaction chamber temperature is changed such that the surface of the boron nitride material is greater than the required, or complete, decomposition temperature of the precursor, here >350 C. The reactor pressure is also changed to a suitable level to ensure evacuation of unwanted process by-products, and also high enough to facilitate the appropriate residency time of carbon products at the boron nitride surface to form graphene. For this halocarbon precursor, a pressure of 600 mbar has proven successful, as the main unwanted by-product, Br, has a vapor pressure higher than this at the selected decomposition temperature. The dilution gas is switched from hydrogen to nitrogen then the precursor and a dilution gas are then introduced to the reactor and hence substrate, through the chamber inlets, at a suitable flow rate, in this example 1000 sccm is ideal for CH.sub.3Br and 2000 sccm for nitrogen. Nitrogen is used in this process so as to limit the possible formation of HBr. The precursor and dilution gas flow through the reactor for a period of time that allows for the formation of a full, uniform graphene monolayer on the substrate surface, in this example 420 seconds is ideal. After completion of the layer, the precursor flow is ceased and the reaction chamber cooled under continuing nitrogen flow until the substrate and graphene layer are at a suitable low temperature, ideally <100° C.

Example 27

(86) Similarly using the same methodology as Example 26 graphene can be formed on a BN surface using a suitable Metallocene precursor as a carbon source, for example Cp.sub.2Mg or Cp.sub.2Fe. After completion of the BN layer, reactor conditions are changed for graphene deposition, whereby the substrate temperature is set to a level suitable for the decomposition of the precursor and preferential for surface kinetics suitable for graphene formation, here >500° C., and the reaction chamber pressure to a level preferential for graphene formation, here <200 mbar. Precursor and dilution flows are set to 700 sccm and 1300 sccm respectively and then introduced through the gas inlets to the reaction chamber and hence BN material surface. The precursor and dilution gas flow for a period allowing full graphene layer formation, here 380 seconds is ideal after which the precursor flow is stopped. The reaction chamber is cooled under continuing hydrogen flow until the substrate and graphene layer are at a suitably low temperature, ideally <100 C.

Example 28

(87) The same methodology as Example 24 can be used to form aluminium nitride. Graphene can then be produced on top of the AlN surface using a halocarbon precursor, such as CH.sub.3Br.

(88) After completion of the AlN layer, hydrogen dilution gas continues to flow into the reaction chamber at the aforementioned 2000 sccm, as in Example 24. The reaction chamber temperature is changed such that the surface of the aluminium nitride material is greater than the required, or complete, decomposition temperature of the precursor, here >350 C. The reactor pressure is also changed to a suitable level to ensure evacuation of unwanted process by-products, and also high enough to facilitate the appropriate residency time of carbon products at the aluminium nitride surface to form graphene. For this halocarbon precursor, a pressure of 600 mbar has proven successful. The dilution gas is switched from hydrogen to nitrogen then the precursor and a dilution gas are then introduced to the reactor and hence substrate, through the chamber inlets, at a suitable flow rate, in this example 1000 sccm is ideal for CH.sub.3Br and 2000 sccm for nitrogen. The precursor and dilution gas flow through the reactor for a period of time that allows for the formation of a full, uniform graphene monolayer on the substrate surface, in this example 320 seconds is ideal. After completion of the layer, the precursor flow is ceased and the reaction chamber cooled under continuing nitrogen flow until the substrate and graphene layer are at a suitable low temperature, ideally <100° C.

Example 29

(89) The methodology of Example 28 can be used to form graphene on a AlN surface using a suitable metallocene precursor as a carbon source, for example Cp.sub.2Mg or Cp.sub.2Fe. After completion of the AlN layer, reactor conditions are changed for graphene deposition, whereby the substrate surface temperature is set to a level suitable for the decomposition of the precursor and preferential for surface kinetics suitable for graphene formation, here >500° C., and the reaction chamber pressure to a level preferential for graphene formation, here <200 mbar. Precursor and dilution flows are set to 700 sccm and 1300 sccm respectively and then introduced through the gas inlets to the reaction chamber and hence AlN material surface. The precursor and dilution gas flow for a period allowing full graphene layer formation, here 380 seconds is ideal after which the precursor flow is stopped. The reaction chamber is cooled under continuing hydrogen flow until the substrate and graphene layer are at a suitably low temperature, ideally <100 C.

Example 30

(90) The same methodology as Example 25 may be used to form gallium nitride. Graphene can then be produced on top of the GaN surface using a halocarbon precursor, such as CH.sub.3Br.

(91) After completion of the GaN layer, hydrogen dilution gas and NE1.sub.3 continue to flow into the reaction chamber at the aforementioned 15000 sccm and 5000 sccm respectively, as in example 25. The reaction chamber temperature is changed such that the surface of the gallium nitride material is greater than the required, or complete, decomposition temperature of the precursor, here >350 C. The reactor pressure is also changed to a suitable level to ensure evacuation of unwanted process by-products, and also high enough to facilitate the appropriate residency time of carbon products at the gallium nitride surface to form graphene. For this halocarbon precursor, a pressure of 600 mbar has proven successful. The dilution gas flow is reduced to 2000 sccm then the precursor and dilution gas are introduced to the reactor and hence substrate, through the chamber inlets, at a suitable flow rate, in this example 1000 sccm is ideal for CH.sub.3Br. The precursor and dilution gas flow through the reactor for a period of time that allows for the formation of a full, uniform graphene monolayer on the substrate surface, in this example 320 seconds is ideal. After completion of the layer, the precursor flow is ceased and the reaction chamber cooled under continuing dilution gas flow until the substrate and graphene layer are at a suitable low temperature, ideally <100° C.

Example 31

(92) Similarly using the same methodology as Example 30 graphene can be formed on a GaN surface using a suitable metallocene precursor as a carbon source, for example Cp.sub.2Mg or Cp.sub.2Fe. After completion of the GaN layer, reactor conditions are changed for graphene deposition, whereby the reaction chamber temperature is set to a level suitable for the decomposition of the precursor and preferential for surface kinetics suitable for graphene formation, here >500° C., and the reaction chamber pressure to a level preferential for graphene formation, here <200 mbar. Precursor and dilution flows are set to 700 sccm and 1300 sccm respectively and then introduced through the gas inlets to the reaction chamber and hence GaN material surface. The precursor and dilution gas flow for a period allowing full graphene layer formation, here 380 seconds is ideal after which the precursor flow is stopped. The reaction chamber is cooled under continuing hydrogen and NH.sub.3 flow until the substrate and graphene layer are at a suitably low temperature, ideally <100 C.

(93) In a similar manner to Examples 23-30 above, 2D material layers can be produced on top of multilayered semiconductor heterostructures, directly after the production of the multilayered heterostructure, to form the surface layer of a semiconductor device, as shown schematically in FIG. 4 which illustrates a substrate 20 upon which is formed a heterostructure comprising n-time layers of semiconductor or dielectric material 22 with a 2D material surface layer 23. Optionally, a nucleation layer 21 may be provided between the substrate 20 and the first semiconductor or dielectric layer 22. Each individual semiconductor or dielectric material layer may have the same or different properties from its immediate neighbouring layers.

(94) The deposition of the 2D material needs to be considerate of the preceding semiconductor structure, ensuring the process conditions employed to deposit the 2D layer do not detrimentally effect the underlying semiconductor materials or their interfaces.

Example 32

(95) Surface contact layers for solid state light emitting devices can be produced by growing 2D layers on top of the semiconductor structure directly after producing the structure and in the same process.

(96) For example a graphene contact layer can be deposited on a GaN based LED structure directly after completion of the semiconductor device structure. The deposition of GaN based LED structures, on sapphire and silicon substrates, is well known and the methodology widely available thus not described here as it is a lengthy procedure.

(97) After completing the deposition of the GaN based structure it is essentially to preserve the surface of the material, thus an NH.sub.3 flow is maintained to the reaction chamber, in this case of 4000 sccm, to ensure surface stability. Under this continuing NH.sub.3 flow the reaction chamber conditions are changed to be suitable for the deposition of graphene, considerate of the selected graphene precursor. Using a graphene precursor of CH.sub.3Br the reaction chamber pressure is set to 550 mbar and the temperature to provide a structure top surface temperature of 850° C. As in Example 10 graphene is then deposited on the surface of the LED structure using the pulsed growth technique described above whereby CH.sub.3Br and a dilution gas, in this example nitrogen, are admitted to the reactor for a period of 15 seconds at flow rates of 1000 sccm and 5000 sccm respectively. The CH.sub.3Br flow is then paused for a period of 20 seconds, while still maintaining the nitrogen flux. The pulsing is repeated for a number of cycles to allow the formation of a uniform continuous graphene layer, however the number of cycles is highly dependent on the initial condition of the GaN device top layer, which can be highly changeable. Typically 5-8 cycles are good, but this can be substantially more. In-situ surface monitoring of the deposition, for example spectral reflectance measurement, is recommended in combination with real time process modification to achieve the desired result repeatedly. After completion of the graphene layer the reactor is cooled under mixed nitrogen NH.sub.3 flows, at the levels previously stated until a reactor temperature of <450° C. is reached at which point the NH.sub.3 flow is ceased and the reactor cooled to ambient under nitrogen flow only.

Example 33

(98) 2D layers can be employed as thermal dissipation layers for semiconductor devices by depositing the 2D material on top of the final device structure in the same process as the production of the semiconductor device. Consideration has to be applied when depositing the 2D layer to ensure the process does not detrimentally effect the semiconductor device, structure or individual layers.

(99) For example, multi-layer graphene can be applied to a GaN based solid state high power electronic device top surface to act as a heat sink. The deposition of GaN based solid state electronics devices by VPE, on sapphire and silicon substrates, is well known and the methodology widely available thus not described here as it is a lengthy procedure. After completion of the semiconductor device an NH.sub.3 flow is continued, in this example 4000 sccm, to maintain the GaN surface and the pressure and temperature changed to the conditions suitable for the deposition of graphene on the device top surface, in this example 600 mbar and 1150° C., for a precursor of CH.sub.4. As in Example 11 a procedure of pulsing the CH.sub.4 to the reactor is used whereby a flow of 1000 sccm is introduced for a period of 15 seconds, then ceased for 20 seconds, however in the period of CH.sub.4 “off” a purge gas is introduced, in this case hydrogen, at a flow of 5000 sccm. This is repeated for 12 cycles, after which the reactor pressure is decreased to 300 mbar and the CH.sub.4 flow increased to 1500 sccm. The pulse cycles are then repeated a further 12 times allowing the deposition of several graphene layers, in this example 3 layers. The reactor is then cooled under a combined NH.sub.3 and hydrogen mixture to 450° C. at which point the NH.sub.3 is stopped and the reactor cooled to ambient temperature.

(100) This procedure requires modification considerate of the initial semiconductor device top surface condition, the device structure itself, for example are there limitations on temperature required to maintain the device structure and the number of graphene layers optimal for the structure.

(101) FIG. 5 illustrates a variant form of heterostructure that can be produced on a substrate 30 using the method of the invention. The heterostructure comprises a 2D material layer 31 upon which is formed a semiconductor or dielectric material 32.

(102) Semiconductor growth on 2D materials is complicated by the need to promote lateral overgrowth, however applying variations of state of the art techniques used for the deposition of semiconductors on highly mismatched substrates it is possible to produce high quality semiconductor and dielectric materials on 2D layers.

Example 34

(103) Dielectric Boron Nitride may be deposited on a graphene surface through the use of initial surface deposition processes that overcome the lattice mismatch of BN and graphene.

(104) For example, graphene may be produced on a sapphire surface using one of the previous examples, in this case using the method outlined in Example 12. After completing the graphene layer the reaction chamber temperature and pressure are changed to give conditions that are suitable for the deposition of a nucleation process, in this example a substrate surface 1150° C. and 500 mbar respectively. Using the precursors NH.sub.3 and triethylboron (TEB) boron nitride can be successfully deposited through a 3 stage process of nucleation, coalescing and bulk layer growth controlled by the VIII ratio (or NH.sub.3:TEB ratio) of the precursors. Initially NH.sub.3 and TEB are introduced to the (10:1 ratio) reaction chamber with flows of 1000 sccm and 100 sccm respectively for a period of 350 seconds. After which the VIII ratio is increased to 750 for another 350 seconds while also increasing the growth temperature to 1220° C. Subsequently the VIII ratio is increased further to 1500, for a period of 3600 seconds resulting in a BN layer of significant thickness, here ˜25 nm. The precursor flows are stopped and the reactor cooled under a hydrogen purge flow, to maintain the material surface until ambient temperature is reached.

Example 35

(105) In a similar method to Example 34 the semiconductor aluminium gallium nitride (AlGaN) can be deposited on a graphene layer.

(106) For example, graphene is produced using the methodology outlined in Example 12, after completion of the graphene the reaction chamber conditions are changed to those preferential to the production of a semiconductor nucleation layer on the graphene surface, in this example a temperature of 1120° C. and pressure of 250 mbar. AlGaN can be successfully grown by first depositing an AN nucleation, or inter, layer on the graphene. Trimethyl aluminium (TMAl) and NH.sub.3 are introduced to the reaction chamber with a dilution flow of hydrogen at 50 sccm, 50 sccm and 10000 sccm respectively for a period of 330 seconds, allowing a suitable nucleation layer thickness to be deposited, here ˜10 nm. After completing the nucleation layer trimethyl gallium (TMGa) is additionally introduced to the reaction chamber at a flow rate suitable for the production of AlGaN at the desired mole fraction of Al, here 75 sccm TMGa is used. The precursors flow into the reaction chamber for a period allowing desired thickness of AlGaN to be deposited, in this example 7200 seconds to give ˜1 μm of material. The precursors are then switched off and the reactor cooled under a hydrogen purge flow.

Example 36

(107) In a similar method to Examples 34 and 35 the semiconductor gallium nitride can be deposited on a graphene layer.

(108) For example, graphene is produced using the methodology outlined in Example 12, after completion of the graphene gallium nitride can be produced on the graphene through the application of a multistage process. First the reaction chamber conditions are changed to those preferential for the deposition of a wetting layer, in this example a pressure of 400 mbar and temperature of 1050° C. TMAl is then introduced at a flow rate of 20 sccm for a period of 200 seconds, after which an NH.sub.3 flow of 50 sccm is introduced and the temperature increased to 1150° C. for a period of 300 seconds. The reactor is then cooled to 1000° C. and a flow of 100 sccm of TMGa added. The reactor is held at 1000° C. for 600 seconds then the temperature increased to 1050° C. while decreasing the pressure to 100 mbar and ceasing the TMAl flow and increasing the NH.sub.3 flow to 9000 sccm. TMGa and NH.sub.3 are flowed into the reaction chamber for a period to achieve the desired GaN film thickness, in this example 3600 seconds for ˜2 μm. The TMGa is then stopped and the reactor cooled to <450° C. at which point the NH.sub.3 is stopped and cooling completed under a hydrogen ambient.

(109) FIG. 6 illustrates a heterostructure comprising substrate 40 on which is formed a 2D layer 41 with a semiconductor or dielectric layer 42. This arrangement is repeated n-times; the heterostructure comprising a further top 2D layer 41 to form an electronic device such as a HEMT, LED or FET.

(110) Each of the multiple semiconductor or dielectric layers 42 and 2D layers 41 may themselves comprise n-times layers with differing properties from layer to layer.

Example 37

(111) Graphene can be utilized in an GaN based LED device structure to produce high performance contact layers for the final device whereby the graphene is produced as the first and last layer in the structure deposition.

(112) For example, graphene is produced using the methodology outlined in Example 12, after completion of the graphene gallium nitride can be produced on the graphene through the application of a multistage process as outlined in Example 36. However, in this example GaN is deposited for a period of 600 seconds to produce a thin, stable GaN film upon which the following structure can be produced. After completion of this layer an LED structure, combined with graphene top layer, can be readily produced using the methodology outlined in Example 32.

(113) The presence of a graphene lower layer makes removal of the structure from the substrate relatively simple, resulting in an LED device with transparent contact layers at the required electrical interfaces.

(114) There follows a variant to the structure of FIG. 6 in which the order of the 2D layer and the semiconductor/dielectric layer is reversed.

Example 38

(115) Graphene can be employed as an active channel in devices when produced in high quality form as part of a transistor structure.

(116) For example, graphene can be produced on an MN surface using the methodology outlined in Example 29. Boron nitride can then be produced on the graphene surface using the technique outlined in Example 34, resulting in an ideal, graphene channel based transistor device structure.

(117) This methodology has produced a simple transistor with channel properties of sheet resistivity of 45352/sq and Hall mobility of greater than 8000 cm.sup.2/Vs with a carrier concentration of 10′2/cm2.

(118) The Abstract is provided with the understanding that it is not intended be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.

(119) The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description herein has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the examples in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope of the examples presented or claimed. The disclosed embodiments were chosen and described in order to explain the principles of the embodiments and the practical application, and to enable others of ordinary skill in the art to understand the various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the appended claims below cover any and all such applications, modifications, and variations within the scope of the embodiments.

(120) Although specific embodiments of the subject matter have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the scope of the disclosed subject matter. The scope of the disclosure is not to be restricted, therefore, to the specific embodiments, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present disclosure.