WO2022013111A1 - Single crystal metal layers, methods of production and uses thereof - Google Patents

Abstract

The present invention relates to a method for production of a single crystal metal layer, to single crystal metal layers produced according to the method, to use of said single crystal metal layers as catalyst layers in a 2D material growth process, and to 2D material layers grown via such a process. The method comprises steps of: providing a metal source material, providing a single crystal deposition substrate having a selected crystallographic orientation, arranging these in a closely-spaced relationship, and heating the metal source material and deposition substrate under vacuum to cause sublimation of the metal source material and heteroepitaxial deposition of sublimed metal atoms onto the deposition substrate, to thereby form a single crystal metal layer on the deposition substrate. Single crystal metal layers produced according to this method are found to be of high quality, in particular with respect to surface roughness and defect density of the layer.

Description

Single crystal metal layers, methods of production and uses thereof

Field of the Invention

The present invention relates to single crystal metal layers, methods of production and uses thereof and particularly, although not exclusively, to production of single crystal metal layers suitable for use as catalysts for 2D material growth.

Background

Crystal growth lies at the heart of integrated solid state device technology and the rise of 2D layered materials, such as graphene, is driving the need for wafer-scale, atomically thin single crystals of this new class of device materials. While bulk 2D material crystals remain very limited in size/in-plane crystallinity, which in turn limits any exfoliation approach to typically sub-mm domain sizes, there has been significant progress in the chemical vapour deposition (CVD) of large-area, mono-layer graphene films. Graphene CVD is most efficient and controllable when a catalytic substrate is used, with the use of Cu and Cu alloys being most widespread due to the large parameter window to control graphene nucleation density and graphene domain orientation, thus enabling large (>cm) single-crystal graphene growth^1-3. While the early focus of the field had been on the understanding of graphene crystal growth, this has now shifted to the challenge of cost-efficient and scalable metal substrate preparation. Bulk single crystal metal substrates are limited in size and costly. Thus the two current main approaches are thermal crystallisation of commercial poly-crystalline metal foils^4-6 and epitaxial metal film growth^7-9.

Grain growth in foils of face centred cubic (fee) metals like Cu has been shown to allow large-area fabrication of dominantly Cu(111 ) crystal foils^4-6. However, such crystallisation closely links to purity and initial texture of the foil, which for commercial Cu foils tend not to be well controlled, and the typically large surface roughness (>100 nm rms) and thermal expansion behaviour of such foils remain known challenges, increasing for instance graphene wrinkling¹⁰.

Epitaxial Cu film growth via physical vapour deposition methods such as sputtering is well established and has a number of potential benefits, for example epitaxial Cu(111) films have been recently shown to produce graphene without wrinkling¹⁰ _’ ¹¹. However, such epitaxial films of fee metals often tend to show twinning and the thinness (often < 500 nm) of metal film means that there is risk of dewetting during subsequent graphene CVD processes.

A number of prior art processes are known for producing single crystal of non-metal materials. For example, JP5330349B2 discloses a method of preparing single crystal Si thin films. EP0573943B1 discloses methods for the manufacture of large single crystals of diamond, cubic boron nitride, and silicon carbide. However, such methods are not applicable to production of single crystal metal layers.

Whilst some prior art processes are directed to production of metal layers, often these involve difficult technical set-ups, or do not produce layers of suitable quality for use as catalysts for 2D material (e.g. graphene) growth. For example, US4325776A discloses a method for preparing coarse-crystal or single crystal metal films. This method includes steps of providing a cooled substrate maintained at a temperature below about -90° C, precipitating a metal selected from the group consisting of tantalum, tungsten, copper, cobalt, aluminium, aluminium alloys and titanium-vanadium alloys as an amorphous layer on the cooled substrate, and subsequently heating the substrate to about room temperature to produce large-scale crystals of a diameter of larger than about 50 pm.

The present invention has been devised in light of the above considerations.

Summary of the Invention

The present inventors have advantageously realised that by employing ‘close-space’ sublimation-based deposition techniques rather than conventional physical deposition techniques such as sputtering, it is possible to overcome or reduce a number of the above problems.

Accordingly, in a first aspect, the present invention provides a method for production of a single crystal metal layer, the method comprising steps of: providing a metal source material; providing a single crystal deposition substrate having a selected crystallographic orientation; arranging the metal source material and the deposition substrate in a closely-spaced relationship; and heating the metal source material and deposition substrate under vacuum to cause sublimation of the metal source material and heteroepitaxial deposition of sublimed metal atoms onto the deposition substrate, to thereby form a single crystal metal layer on the deposition substrate.

The present inventors have found that the use of ‘close-space’ sublimation allows for the production of a single crystal epitaxial metal layer (also referred to herein as a single crystal metal film) to be formed on the deposition substrate within a fast process time, with the resulting layer being of high quality, in particular with respect to surface roughness and defect density of the layer. This method allows for the cost-efficient preparation of different single crystal film orientations and offers versatility to be expanded to a wide selection of materials. The method can be performed in a conventional CVD reactor, which allows for subsequent CVD processes utilizing the resulting single crystal layer to be performed without the need to transfer the single crystal metal layer between separate pieces of apparatus.

In the method according to the present invention, the metal source material and the deposition substrate are arranged in a closely-spaced relationship. In other words, the metal source material and the deposition substrate are arranged at a distance to thereby allow for redisposition of the sublimed metal source material onto the deposition substrate. The precise spacing necessary to allow for this redisposition of sublimed material will depend on a number of factors, including e.g. the identity of the metal source materials and its corresponding vapour pressure, the vacuum pressure under which the method of performed, and the temperature to which the metal source material and deposition substrate are heated. The metal source material and the deposition substrate may be spaced such that the theoretical mean free path length of the sublimed metal atoms during deposition is greater than the distance between the metal source and the substrate. The theoretical mean free path length of the sublimed metal atoms is herein calculated as the approximate theoretical mean free path, as is based on ideal gas law: J. Phys. Chem. 1964, 68, 3, 441-451

(1 ) l = k * T / (V2 * TT * d² * p) wherein l is the mean free path expressed in the length units, T is the temperature of the gas, p is the pressure of the gas, d is the diameter of a particle, and k is the Boltzmann constant k = 1 .380649^*1 O ²³ J / K.

By providing a spacing which is less than the theoretical mean free path length of the sublimed metal atoms during deposition, a large proportion of the sublimed metal atoms will travel in a line-of-sight manner to be deposited on the deposition substrate.

In practice, the metal source material and the deposition substrate may be arranged at a distance of 5 mm or less, 4 mm or less, 3 mm or less, 2 mm or less, 1 mm or less or 0.5 mm or less. In theory there is no lower limit on the spacing at which the method may be performed other than that the spacing should be greater than 0 mm (i.e. there should be some measurable spacing). In practice, the metal source material and the deposition substrate may be spaced at a distance of at least 0.01 mm or more from one another, or 0.1 mm or more from one another. The spacing may be controlled by placing one or more spacer between the metal source material and the deposition substrate. Where such an arrangement is used, the spacing between the metal source material and the deposition substrate may be substantially equal to the thickness of the one or more spacers.

Any suitable heating arrangement may be used for heating of the metal source material and deposition substrate. In some arrangements, a single heater may be used to heat both the metal source material and deposition substrate. One preferred arrangement is a dual-heater arrangement. In this arrangement, a first heater and a second heater may be provided, with the first and second heaters being disposed on opposite sides of the closely-spaced metal source material and deposition substrate. The first and second heaters may alternatively be referred to as ‘bottom’ and ‘top’ heaters. The first and second heaters may be individually controllable. Alternatively or additionally, the first and second heaters may be arranged to be simultaneously controllable. In one implementation of the method, one of the first and second heaters may be supplied with a fixed amount of power, whilst the other of the first and second heaters is supplied with a variable amount of power. This can help to ensure precise temperature control during implementation of the method.

The metal source material may be heated to a predetermined temperature Tsource at which at least some of the metal source material undergoes sublimation. Tsource may be a temperature in the range of 0.7T_m to 0.99Tm, Tm being the melting temperature of the metal source material at standard atmospheric pressure. For example, where the metal source material is copper, copper having a melting temperature of about 1084 °C, Tsource may be a temperature in the range of 759 °C to 1073 °C. More preferably, Tsource is at least 0.8Tm, or at least 0.9T_m. Heating the metal source material to a higher predetermined temperature may encourage faster sublimation and deposition of the material. The deposition substrate may be heated to a predetermined deposition temperature Tdep, this being a temperature suitable to allow heteroepitaxial deposition of sublimed metal atoms onto the deposition substrate, to thereby form a single crystal metal layer on the deposition substrate. In some implementations of the method, Tdep may be substantially equal to Tsource. In other implementations of the method, Tdep and Tsource may be different. Preferably, Tdep is within 200°C of Tsource, more preferably within 100°C of Tsource. Tdep and Tsource may be measured directly by appropriate location of temperature measurement means (e.g. thermocouples) during implementation of the method. Tdep and Tsource may be varied independently by respective control of first and second heaters located at different positions in a deposition apparatus configured to implement the method of the present invention, for example by control of power supplied to each of the first and second heaters. Alternatively, the metal source material and the deposition substrate may be arranged at different locations with respect to a single heater in a deposition apparatus configured to implement the method of the present invention, with Tdep and Tsource being controlled by disposing the metal source material and the deposition substrate at selected respective distances from this single heater, and selecting the power of operation of the single heater.

The step of heating the metal source material and deposition substrate under vacuum may be performed for predetermined deposition time tdep of 60 minutes or less. Lower deposition times may be possible for the production of smaller layers. For example, a 1x1 cm² area can be deposited in a time of around 45 minutes or less. This timescale allows for efficient high-throughput processing.

The step of heating the metal source material and deposition substrate under vacuum may be performed at a vacuum pressure in the range of 10¹ to 10³ mbar. Whilst there is no lower limit on the pressure at which the method may be performed, one advantage of the present method is that, contrary to other conventional physical deposition processes such as sputtering, it is not necessary to provide high vacuum conditions (e.g. vacuum pressures lower than 10³ mbar) to provide deposited layers of suitable quality. This is due to the closely-spaced relationship between the metal source material and the deposition substrate. This can reduce both cost and complexity of the manufacturing method in comparison to conventional deposition processes.

The identity of the metal source material is not particularly limited. In some arrangements, the metal source material may comprise an FCC metal; that is, the metal source material may comprise a metal having a face centred cubic (FCC) crystalline structure. The metal source material may comprise, or may consist essentially of a metal selected from: aluminium, copper, gold, iridium, lead, nickel, platinum, silver tungsten or molybdenum. A metal source material comprising or consisting essentially of copper or nickel may be advantageous where the single crystal layer is intended for use as a catalyst for growth of 2D material layers.

The metal source material may be provided in any convenient format. In some arrangements, the metal source material is provided as a source layer, for example as a foil layer. Preferably the metal source material is provided as a source layer that is substantially flat. Where the metal source material is provided as a source layer, the source layer and the deposition substrate may be arranged substantially parallel to one another. This can help to ensure even deposition of sublimed metal atoms on the deposition substrate. Preferably the surface area of the metal source material is equal or greater than the area of the deposition substrate. This helps to ensure even deposition of the metal source material onto the deposition substrate.

The identity and/or crystallographic orientation of the single crystal deposition substrate may be selected based on the desired resulting single crystal metal layer, preferably to provide close epitaxial matching between the deposition substrate and the desired single crystal metal layer. In particular, the crystallographic orientation of the deposition substrate may be selected based on the desired crystallographic orientation of the single crystal metal layer. A wide range of materials may be suitable for use as a deposition substrate. In preferred arrangements, the deposition substrate may be selected from c-plane sapphire, MgO, Si or mica (i.e. a phyllosilicate having a formula -teWeZsC OH, F)4,in which Xis K, Na, Ca, Ba, Rb, or Cs; Xis Al, Mg, Fe, Mn, Cr, Ti, Li, etc.; Zis chiefly Si or Al, but also may include Fe³⁺ or Ti).

The method may include a step of cleaning the metal source material and/or the deposition substrate prior to the heating and deposition steps. For example, the metal source material and/or the deposition substrate may be sonicated in an appropriate solvent for a predetermined time. Appropriate solvents may include e.g. acetone, isopropyl alcohol (IPA) or a mixture thereof. The predetermined time may be between 1 and 60 minutes, more preferably about 10 minutes. Cleaning the metal source material and or the deposition substrate prior to heating and sublimation/deposition may help to improve the purity of the resulting single metal crystal layer.

One further advantage of the present invention is that it is not necessary to employ extensive pre treatments of the deposition substrate that are often necessary in prior art processes. For example, for deposition on sapphire substrate, some documents in the literature report the need for pre-treatments such as hot acid treatment, or lengthy high temperature oxygen annealing, to provide a suitably ‘clean’ surface for deposition. The present method offers advantages in terms of reduced complexity of procedure, and reduced need for use of potentially harmful reagents in cleaning of the deposition substrate, whilst allow for production of a suitably high quality single crystal metal layer.

In a second aspect, the present invention provides a single crystal metal layer produced according to the first aspect of the invention.

The present inventors have found that a single crystal metal layer produced according to the first aspect of the invention may be distinguished from single crystal metal layers obtainable by conventional methods (for example by sputtering, or by annealing of commercially available poly-crystalline metal films).

The single-crystal nature of the metal layer/film may be confirmed by any suitable technique, for example by using electron backscatter diffraction (EBSD) or X-ray diffraction (XRD).

The area of the single crystal metal layer (i.e. the area of one continuous single crystal forming said layer) is preferably at least 0.25 cm², more preferably at least 1 cm², more preferably at least 5 cm², more preferably at least 10 cm². It is possible to produce a single crystal metal layer having an area of 20 cm² or more. Generally, the area it is possible to produce is limited only by the size of the apparatus available. For example, the apparatus may be configured to receive a 2” diameter circular deposition substrate (area = 20.27 cm²). The area of the single crystal metal layer is measured as the plan view area of the sheet, measurable for example using TEM, SEM AFM, or optical methods.

The thickness of the single crystal metal layer may be 1 pm or more, 5 pm or more, 10 pm or more, 20 pm or more, or 30 pm or more. For example, the thickness may be in a range of from about 5 pm to about 40 pm. Greater thicknesses may be achieved by longer deposition times, and there is no particular limit on the thicknesses possible, other than those imposed by the configuration of the deposition apparatus, and the amount of source material present. The single crystal metal layer may therefore have a thickness of e.g. 100 pm or more where long deposition times are used. This is thicker than layers produced by e.g. conventional sputtering processes, which are typically on the order of <500 nm in thickness. This increased film thicknesses achievable by methods discussed herein provide the advantage that the single crystal metal layer is less likely to undergo dewetting during subsequent processing steps, or during use e.g. as a catalyst for 2D material deposition.

A single crystal metal layer according to the present invention may have particularly low surface roughness compared to single crystal metal layers produced according to conventional methods. The single crystal metal layer may have an ‘as-grown’ surface roughness value R_a of 5 nm or less, more preferably 3 nm or less, most preferably 2 nm or less. R_a referred to in the present disclosure is the average surface roughness R_a measured from a 10x10 pm² (100 pm²) atomic force microscopy (AFM) topography map sample of the single crystal metal layer. Preferably, at least an area of the single crystal metal layer having an area of 500 pm² (for example, as measured over five separate 10x10 pm² samples) has an ‘as-grown’ surface roughness value R_a of 5 nm or less, more preferably 3 nm or less, most preferably 2 nm or less.

‘As-grown’ surface roughness is used herein to describe the surface roughness immediately after conclusion of deposition. In other words, it is the measure surface roughness of the layer before any further processing steps (for example polishing) have been performed.

A single crystal metal layer according to the present invention may have a carbon bulk normalized ion count of 10⁴ or less, and/or an oxygen bulk normalized ion count of 10³ or less, as measured using TOF- SIMs. This is significantly lower than the carbon and oxygen bulk normalized ion counts seen in commercially available foils used for 2D material growth. Such conventional foils are known to have carbon bulk normalized ion counts typically greater than 2x10⁴, and oxygen bulk normalized ion counts typically greater than 10³. Single crystal metal layers according to the present invention therefore have higher purity than other commercially available foils.

As discussed above, single crystal metal layers according to the present invention find particular implementation in methods of production of 2D materials. In a third aspect, the present invention therefore provides the use of a single crystal metal layer of the second aspect as a catalyst layer in a process of 2D material growth.

In a fourth aspect, the present invention provides a method for production of a 2D material including steps of performing the method of the first aspect, and including further steps of heating the deposition substrate and single crystal metal layer in a selected gaseous atmosphere at a predetermined temperature T_grawth to thereby provide growth of the 2D material on the single crystal metal layer.

That is, a method for production of a 2D material, the method comprising steps of: providing a metal source material; providing a single crystal deposition substrate having a selected crystallographic orientation; arranging the metal source material and the deposition substrate in a closely-spaced relationship; heating the metal source material and deposition substrate under vacuum to cause sublimation of the metal source material and heteroepitaxial deposition of sublimed metal atoms onto the deposition substrate, to thereby form a single crystal metal layer on the deposition substrate; and t at a predetermined temperature T_gr0wth to thereby provide growth of the 2D material on the single crystal metal layer.

The method may be performed in a conventional CVD reactor. Preferably, the method is performed without intermediate transfer or handling of the deposition substrate and single crystal metal layer. In this way, contamination of the single crystal metal layer due to e.g. substrate handing or exposure to ambient air can be reduced or avoided. This can result in production of higher-quality 2D material layer than conventional methods.

The 2D material layer may be a material selected from hexagonal boron nitride (h-BN), a transition metal dichalcogenide (TMDC) with the formula MX2 where M represents a metal atom (M = W, Mo etc.) and X represents a chalcogen (X = S, Se, etc.), and/or graphene. In preferred methods, the 2D material layer comprises or consists essentially of graphene.

The step of heating the deposition substrate and single crystal metal layer in a selected gaseous atmosphere at a predetermined temperature T_gr0wth to thereby provide growth of the 2D material on the single crystal metal layer may a step performed in line with the general teachings and disclosure of earlier work by the present inventors as described in the following reference, which is herein incorporated by reference:

Burton, O. J. et at., “The Role and Control of Residual Bulk Oxygen in the Catalytic Growth of 2D Materials”. J. Phys. Chem. C 123, 16257-16267 (2019).

As described in this reference, the gaseous atmosphere may be selected based on the desired 2D material. For example, where the desired 2D material is graphene, the gaseous atmosphere may comprise a CH4/H2 mixture.

An optional annealing step may be performed on the single crystal metal layer prior to the step of heating the deposition substrate and single crystal metal layer in a selected gaseous atmosphere to provide growth of the 2D material on the single crystal metal layer. Such an annealing step may be performed in a hydrogen (H2) atmosphere, optionally at a pressure of around 50 mBar.

The predetermined temperature T_gr0wth may be selected as appropriate given the identity of the single crystal metal layer, and the desired 2D material. Preferably T_gr0wth is not higher than the melting point of the single crystal metal layer at standard temperature and pressure, to avoid damage to the single crystal metal layer during growth of the 2D material. In one example, where graphene is grown on a copper single crystal metal layer, T_grawth may be selected to be in a range of from about 700 °C to about 1084 °C, preferably in a range of about 1000 °C to about 1084 °C.

The deposition substrate and single crystal metal layer may be heated in the selected gaseous atmosphere for a time t_grawth of 10 minutes or more, for example 20 minutes or more, 30 minutes or more, 40 minutes or more, 50 minutes or more or 60 minutes or more. Deposition times of 10 minutes or more may be preferred, as they may result in formation of complete 2D material monolayers across the single crystal metal layer. Deposition times of 60 minutes or less may be preferred for reasons for economy of procedure.

The method may include a further step of removing the 2D material layer from the single crystal metal layer. In this way, the single crystal metal layer can be re-used for further 2D material growth processes or other CVD processes. The single crystal metal layer may be cleaned before re-use, for example by cleaning in a suitable solvent such as acetone, IPA and/or Dl water.

In one implementation, the 2D material layer may be transferred to a destination substrate (for example, a Si/SiC>2 wafer) by a process including steps of: allowing oxidation of the metal :2D material interface; applying a polymeric support layer to an exposed surface of the 2D material layer such that the 2D material layer adheres to the polymeric support layer; removing the polymeric support layer and adhered 2D material layer from the single crystal metal layer; transferring the polymeric support layer and adhered 2D material layer to a destination substrate; and removing the polymeric support layer.

The step of allowing oxidation of the metal:2D material interface may include exposing the single crystal metal layer supporting the 2D material layer to an oxidising atmosphere (e.g. a water bath). The oxidation of the interface ‘decouples’ the 2D material from the single crystal metal surface, reducing the adhesion such that the 2D material sticks more to the support polymer than the surface of the growth catalyst, thereby allow removal of the 2D material layer.

The polymeric support layer may be any suitable polymer. Polyvinyl acetate (PVA) has been found to be particularly suitable due to its low cost, ease of availability and solubility in water. This allows removal of the PVA by dissolving the PVA in water. Where other polymers are used as the polymeric support layer, they may be removed by dissolving the polymeric support layer in a suitable solvent.

In a fifth aspect, the present invention provides a 2D material layer grown using a single crystal metal layer produced according to first aspect as a catalyst layer.

The present inventors theorise that 2D material layers grown using such single crystal metal layer may have lower defect density than 2D material layers grown using other, commercially available single crystal metal layers. For example, a graphene layer grown using a single crystal metal layer produced according to first aspect as a catalyst layer may have a D/G ratio as measured using Raman spectroscopy of less than 0.05, more preferably 0.025 or less, more preferably 0.02 or less. The present inventors have found that portions of a graphene layer grown using a single crystal metal layer produced according to first aspect as a catalyst layer may have a D/G ratio as low as 0.01 , or measured to be as low as 0, indicating that the measurable D peak is lower than the error margin of the apparatus, i.e. that said portion of the graphene layer has an extremely low defect density or has an absence of defects.

Furthermore, a 2D material layer grown using a single crystal metal layer produced according to first aspect as a catalyst layer may be optically free of wrinkles and/or fold, when viewed under an optical microscope.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

Figure 1 shows a schematic side-view of a deposition apparatus 100 suitable for implementing a method according to the present invention;

Figure 2 shows a schematic process diagram of a method according to the present invention, showing changes in temperature and pressing within the deposition apparatus over time;

Figure 3 shows (a) Electron backscatter diffraction (EBSD), (b) XRD and (c) AFM characterisation of an approximately 10 pm thick Cu single crystal metal layer according to the present invention, measured in 5 regions of the layer (B,R,T,C,L);

Figures 4(a)-(i) show AFM and EBSD measurements, and a computer generated schematic illustration based on the AFM and EBSD measurements, exploring the evolution of the Cu film growth and texture with increasing tde_P;

Figure 5 shows a comparison of TOF-SIMs bulk ion oxygen and carbon count of the Cu source foil used for deposition (before deposition, labelled as ‘Foil (pre-CVD)’), a Cu single crystal metal layer according to the present invention (labelled as ‘Film (post-CVD)’) and a conventional foil used for 2D material growth after annealing (labelled as ‘Foil (post-CVD)’);

Figure 6 shows an example 3D depth profile of the samples tested using TOF-SIMs, showing surface carbon and oxygen atmospheric contamination of samples, and identification of the bulk region, located beyond the saturation of the ion count with depth.

Figure 7 shows (a) a height map of a sapphire deposition substrate before deposition of the single crystal metal layer; (b) a height map of the sapphire after a deposition of tdep = 60 min, with the Cu having been etched away to expose the sapphire structure; and (c) a graph of height distribution of the height map shown in Fig. 7 (b); Figure 8 shows a comparison of a Cu single crystal layer (a), (b), (c) and a Cu source foil (d), (e), (f) before and after a 2D material layer growth process is performed;

Figure 9 shows schematically a process for dry peeling of a 2D material layer from the single crystal metal layer, and transfer of the 2D material layer to a destination substrate;

Figure 10 shows (a) an optical microscopy image of a graphene layer produced according to the present invention; (b) an optical microscopy image of a graphene layer produced according to a conventional method; (c) a histogram of Raman spectra fitted D to G peak ratios from a 1x1 mm² map of a graphene layer produced according to the present invention; and (d) a histogram showing angular measurements of 798 hexagonally holes etched in a 1x1 cm² area of a graphene layer produced according to the present invention;

Figure 11 shows (a) an optical microscopy image of graphene growth on a single crystal metal layer according to the present invention; (b) an SEM image of holes (light colour) etched into a continuous graphene layer (dark colour) produced according to the present invention; (c) a larger zoomed out image of Fig. 10(a); (d) a map of 2D to G peak intensity ratio over a 1x1 mm² area a graphene layer produced according to the present invention; and (e) an example Raman spectrum (black points) with Lorentzian fits of the D, G and 2D peaks in purple, blue and green respectively.

Figure 12 shows: (a) & (b) The EBSD z-IPF Map and (powder)-XRD spectrum of MgO(111) after deposition and graphene growth according to the present invention; (c) & (d) The EBSD z-IPF Map and (powder)-XRD spectrum of MgO(110) after deposition and graphene growth according to the present invention; and (e) & (f) the EBSD z-IPF Map and (powder)-XRD spectrum of MgO(100) after deposition and graphene growth according to the present invention.

Figure 13 shows a schematic model of the geometry for sublimative deposition from a first “source” disc, to a second “drain” or target disc (i.e. deposition substrate) a distance D apart from another.

Figure 14 shows a graph of calculated Flux Let against radius Wd of a single crystal metal film produced according to the present invention, as calculated from the processed results of WLI interferometry measurements.

Detailed Description of the Invention

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Fig. 1 shows a schematic side-view of a deposition apparatus 100 suitable for implementing a method according to the present invention. In this embodiment, the deposition apparatus comprises a first heater 1 , also referred to as a ‘bottom heater’, and a second heater 3, also referred to as a ‘top heater’. A metal source material 5 is disposed adjacent the first heater. Conveniently, the metal source material is provided as a metal source foil layer. First and second spacers 7 a,b are arranged to support a deposition substrate 9 in a closely spaced relationship with the metal source material 5, at a distance indicated as ‘x’. Conveniently, the deposition substrate is provided as a single crystal wafer. The second heater is arranged at a distance indicated in Fig. 1 as distance ‘y’ from the deposition substrate 9. The power supplied to the first and second heaters is independently and respectively controlled to control the temperature of the metal source material and the deposition substrate. A thermocouple 11 is provided at the first heater for monitoring of the temperature within the apparatus. The temperature measured using said thermocouple will herein be referred to as T_c. It will be understood that due to the close locational relationship between the thermocouple and the metal source material, T_c can be considered to be substantially equal to the temperature of the metal source material during the deposition process. The temperature of the deposition substrate can be measured in an analogous way by appropriate location of a second thermocouple. Alternatively, the temperature of the deposition substrate can be calculated from Tcby e.g computational modelling.

The arrangement shown in Fig. 1 may be referred to as a ‘substrate sandwich’ arrangement, because the resulting single crystal metal layer forms between the parallel-spaced metal source material and deposition substrate. In the arrangement shown in Fig. 1 , the set-up is open, i.e. processes gases present in the apparatus are free to travel between the metal source material and the deposition substrate.

Example 1: Deposition of a Cu single crystal layer on a Sapphire substrate , subseauent growth of graphene on the Cu single crystal layer

A deposition apparatus was provided in a commercial, cold-wall CVD reactor (Aixtron Black Magic Pro 4”, base pressure of 4.2 x 10² mbar) with a dual top and bottom heater configuration. A Cu foil (99.99 % Alfar Aesar 127 pm thick roll and 99.9% Goodfellow 100 pm thick pre-cut 2 “ circle) was placed on the bottom heater and a sapphire wafer (Alfar Aesar, 50.8 mm diameter, 0.432 mm thick, one side polished, c-plane ± 0.3 °) was placed on top of quartz spacers to hold it approximately 1 .1 mm above the foil with the epi-ready side facing the foil. The quartz spacers protruded approximately 1 mm into the edge of the sapphire substrate, covering an approximately 1x10 mm² area at the edge of the film. Prior to the deposition procedure, the Cu foil and sapphire wafer were cleaned, unless otherwise stated, by sonication in acetone for 10 min followed by sonication in IPA for 10 min. The Cu foil and sapphire wafer were then dried with N2 immediately before the deposition procedure. For some of the experimental results discussed below, the sapphire wafer was cleaved into 1x1 cm² squares and the 127 pm Cu foil cut into complementary 0.9x2 cm² rectangles prior to cleaning.

Fig. 2 shows a schematic process diagram of this example process over time. The process can be divided into five separate stages: (i) temperature ramp up; (ii) deposition of single crystal metal layer, (iii) introduction of gaseous atmosphere, (iv) growth of 2D material layer; and (v) temperature ramp down.

The method for production of a single crystal metal layer is performed as stages (i)-(ii) of this example.

As discussed below, it is possible to remove the single crystal metal layer at the end of stage (ii), without performing subsequent stages (iii)-(v). Stages (iii)-(v) are a method for production of a 2D material using the single crystal metal layer produced in stages (i)-(ii).

In step (i), the CVD reactor was heated up from room temperature with a base pressure of 4.2x10² mbar to the target deposition temperature at a ramp rate of approximately 30 °C min^-1. The thermocouple was in direct contact with the bottom heater structure. This temperature ramp up stage was performed for a fixed time of 30 minutes to allow the system to reach thermal equilibrium.

In step (ii), the metal source material (Cu film) and deposition substrate (sapphire wafer) were heated for predetermined deposition time tde_P at a base pressure of approximately 5.2x10² mbar (i.e. under medium vacuum). The temperature shown in Fig. 2 is T_c, i.e. the temperature measured from a thermocouple arranged adjacent the first heater. Due to the close locational relationship between the thermocouple and the metal source material, T_c can be considered to be substantially equal to the temperature of the metal source material during heating (i.e. Tsource can be considered to be equal to T_c for this arrangement).

The temperature within the deposition apparatus (and thereby of the metal source material and of the deposition substrate) was controlled by keeping the top heater at a fixed power (2300 W) and varying the power supplied to the bottom heater. Higher deposition rates were observed at higher temperatures. However, very high temperatures approaching the Cu melting point (and thus high deposition rates) were found to cause an increase in the roughness of the single crystal metal layer. Lower deposition rates for temperatures of < 1000 °C were also found to increase the Cu surface roughness, likely due to decreased thickness (earlier stage of Volmer-Webber growth mode) coupled with lower Cu mobility at lower temperatures.

It was found that heating the Cu film to a temperature Tsource of approximately 1065 °C was optimum to minimise the surface roughness of the deposited Cu film for tdep = 60 min. In this example, Tsource is therefore approximately 0.98T_m, T_m being the melting temperature of the metal source material at standard atmospheric pressure (the melting point of Cu being about 1084 °C).

Tdep (the temperature of the sapphire deposition substrate) was estimated by computer modelling based on the value of T_c and the configuration of the deposition apparatus to be around 100 °C lower in temperature than Tsource. The inventors theorise that this temperature differential may help to drive net deposition of the Cu film onto the sapphire deposition substrate.

The Cu single crystal layer deposition can be well fitted with a simple line-of-sight sublimation model (see section below titled ‘Deposition Modelling’) taking into account the Cu vapour pressure (in the range 10² to 10¹ Pa), meaning that the Cu mean free path at base pressure is much larger ( > 1 cm) than the 1.1 mm Cu foil - sapphire separation.

In steps (iii) and (iv), a gaseous atmosphere is introduced into the CVD reactor, resulting in an observed change in pressure within the apparatus over a period of about 20 minutes. The deposition substrate and single crystal metal layer were heated to approximately 1065 °C at 50 mBar for t_grawth = 60 min, the gaseous atmosphere comprising CH4, H2 and Ar in a flow rate ratio 0.32:64:576 seem. A 2D graphene layer results. The higher growth pressure serves to suppress Cu sublimation, thus reducing the likelihood of the Cu surface roughening during graphene domain growth. In step (v), the reactor heaters were turned off and the system was cooled down under vacuum to less than 200 °C within 1 h at base pressure.

In this example, the critical process times, tdep and t_grawth, were both < 60 min, giving a total process time from input of sapphire to retrieval of graphene on single crystal Cu of less than 3.5 h. This shows the advantages of the present method as regards the opportunity for efficient high-throughput processing, with time scales that are economic for commercial graphene production from a single crystal metal layer.

Characterisation of single crystal metal layers

To establish that stage (ii) of the above example results in a clean, single-crystal epitaxial Cu film, characterisation was performed on an approximately 10 pm thick Cu film deposited on a 2” sapphire wafer as discussed above, with tdep = 60 min, but with the reactor cooled immediately after deposition under vacuum, i.e. without the subsequent gaseous atmosphere introduction or graphene growth performed in stages (iii) and (iv) above.

On deposition, the single crystal metal layer was visually observed to be highly reflective, indicative of its thickness homogeneity and low roughness to the naked eye. The 2” single crystal metal layer was then divided into 5 representative regions for analysis, each region being assigned a letter: B, R, T, C, and L. C denotes a central region of the layer. B, R, T and L each denote different peripheral regions of the layer.

Fig. 3 shows (a) Electron backscatter diffraction (EBSD), (b) XRD and (c) AFM characterisation of the approximately 10 pm thick Cu single crystal metal layer.

Fig. 3(a) shows EBSD z-IPF maps and pole figures generated from the respective EBSD maps. The EBSD maps were made with a FEI Nova NanoSEM at 30 kV with a 500 pm aperture. The sample was tilted to 70 ° approximately 17 mm from the pole piece, with the EBSD detector screen approximately 20- 25 mm from the sample. The EBSD was calibrated and optimized for Cu patterns to ensure a successful fit rate of close to 100 %.

The EBSD inverse pole figure (IPF) maps show a consistent and homogeneous blue colour indicating that the Cu is completely (111) oriented in each of the > 2x2mm regions measured. The {110} pole figures generated from the EBSD measurements show a well aligned central point indicating that there is little to no angular deviation from the (111) orientation and three points at a fixed radius from the centre 120 ° apart, indicating that the Cu(111) consists of only one orientation. The combination of the pole figures and the IPF maps highlight that the Cu is single crystal in the regions measured.

Fig. 3(b) shows pole figures from X-ray diffraction in the same 5 regions of the same Cu layer. Texture maps of the Cu(111) reflection were recorded by X-ray diffraction (XRD) using a Phillips X’pert MRD diffractometer with a Cu K a1 X-ray source (l= 1.5405974 A), an asymmetric 4-bounce Ge(111) monochromator and a single point detector (with a 1 ° slit). The angles were set up to select the Cu(111 ) reflection, i.e. at 2Q = 43.31 °. The spot size on the wafer was approximately 5 mm by 15 mm at this incidence angle. The three highlighted spots in each of these XRD pole figures represent three Cu(111 ) reflections representative of a single crystal Cu(111). In films produced using existing processes not according to the present invention, it is common for the films to be twinned so as to have two orientations of Cu(111), which would show up as an additional three Cu 111 reflections 30 ° from those shown in Fig. 3(b). The lack of twinning shown by these XRD pole figures is consistent with the EBSD result discussed above, indicating that over each of the ca. 15 x 5 mm² spots illuminated by the X-ray source the Cu layer is mono-crystalline.

Fig. 3 (c) shows AFM height maps (10x10 pm²) with measured average roughness (R_a) for each of the 5 regions (B, R, T, C, L). AFM was conducted on a Dimension 3100 system in tapping mode using tips with a resonance frequency of approximately 300 kHz. After laser alignment and tuning, the sample was approached with the tip and the scan parameters were optimised. After the scan, plane subtractions and a second order polynomial fit in both x and y was used to account for a non-perpendicular surface and the effects of scanner bow. Roughness calculations were made after this data correction process to provide consistent estimations of roughness. To ascertain film thicknesses a white light interferometer was used. It was calibrated before measurements and a plane subtraction was applied to the height maps (relative to the exposed flat sapphire where available).

R_a was measured to be 1 .77 nm, 1 .41 nm, 2.17 nm, 1 .92 nm and 1 .52 nm in regions B, R, T, C, and L respectively, with a mean R_a of 1 .76 nm across all 5 10x10 pm² regions. This shows that single crystal metal layers produced according to the present invention have low surface roughness. Indeed, R_a values found in the literature for optimised epitaxial metal films prepared by alternative processes are generally around 10 nm²¹’³². Furthermore, the observed roughness of the single crystal metal layer is far lower than that of the starting Cu foil (the metal source material), which has a roughness R_a well above > 100 nm as measured across a single facet.

Fig. 4 shows AFM and EBSD measurements exploring the evolution of the Cu film growth and texture with increasing tdep. For this exploration of the parameter space, 1x1 cm² sapphire deposition substrates were used. Fig. 4 (a)-(d) show AFM height maps during temperature ramping, the start of the defined deposition, tdep = 30 min and tdep = 60 min, respectively. AFM was performed as discussed above for Fig. 3. Fig. 4 (e)-(h) show false coloured EBSD x-IPF maps showing the Cu orientations at different deposition times. Fig. 4(i) shows a schematic of the epitaxial Cu film formation, showing gradual thickness increase and improvements in continuity and crystallinity as reflected by the data in Fig. 4 (a)- (h).

The AFM height map and EBSD IPF map of the Cu film during the temperature ramping stage (Figs. 4(a) and (e)) highlight that the Cu deposition already starts at T< 800 °C with individual islands of Cu(111) orientations forming. The contrasting colours indicate two different Cu(111 ) orientations. Note that the EBSD IPF map in Fig. 4(e) carries many mischaracterised (black) measurements due to the relatively low accumulation time of the electron backscatter patterns required to combat the effect of charging (due to the insulating sapphire). The observed island formation is consistent with a Volmer-Weber growth mode, whereby the formation of three-dimensional adatom clusters or islands reflects the overall lowest surface energy. The prevalence of Cu(111 ) orientations already at the earliest stage highlights the well-known epitaxial effect of c-plane sapphire.¹⁸’¹⁹’³⁵ Fig. 4(b) and (f), and Fig. 4 (c) and (g) show that with increasing tdep the Cu islands grow, merge into a network of partially connected domains and then evolve into a continuous Cu film. After a predetermined deposition time, here tdep = 60 min, corresponding to a Cu film thickness of roughly > 10 pm, a flat single crystal Cu film structure is observed (Fig. 4(d) and (h)), resulting from grain boundary mobility in the deposited metal layer which drives full crystallisation to a single orientation.

Not only the crystallinity and surface roughness, but also the chemical purity of a single crystal metal layer is important, especially when intended for use as a catalyst layer for graphene CVD where impurities such as carbon and oxygen are known to have a significant detrimental effect.⁷ Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was therefore used to analyse and compare the purity of the Cu source foil (before and after the CVD process) to the freshly deposited epitaxial Cu film.

Fig. 5 shows a comparison of ToF-SIMs bulk ion oxygen and carbon count of the Cu source foil used for deposition (before deposition, labelled as ‘Foil (pre-CVD)’), a Cu single crystal metal layer according to the present invention (labelled as ‘Film (post-CVD)’) and a conventional foil used for 2D material growth after annealing (labelled as ‘Foil (post-CVD)’, see Burton et al. for more details⁷). Note that the ion counts of the initial surface sputtering are disregarded (see Fig. 6, which shows an example 3D depth profile highlighting carbon and oxygen atmospheric contamination. The bulk ion count is defined as that measured from a bulk region of the sample, i.e. as measured beyond the saturation of the ion count with depth. The 3D depth profile was constructed by interleaving the vertically resolved surface area images acquired during depth profiling). This allows focus on the relevant “bulk” concentrations which are not influenced by post-growth sample transfer and resulting environmental contamination resulting from e.g. air-induced oxidisation or deposition of hydrocarbons on the sample surface.

Fig. 5 clearly shows that the deposited Cu single crystal metal layer (‘Film’) on sapphire exhibits an order of magnitude lower concentrations of bulk oxygen (O ) and carbon (C ) as compared with the metal source material (‘Foil (pre-CVD)’), and with a conventional foil used for 2D material growth after annealing. It can be seen that the single crystal metal layer according to the present invention may have a carbon bulk normalized ion count of 10⁴ or less, and/or an oxygen bulk normalized ion count of 10³ or less as measured by ToF-SIMs. This is significantly lower than the carbon and oxygen bulk normalized ion counts seen in commercially available foils used for 2D material growth - indeed, a single crystal metal layer according to the present invention may have a carbon bulk normalized ion count and/or a carbon bulk normalized ion count an order of magnitude lower than the metal source material used for deposition of the single crystal metal layer. Such conventional foils are known to have carbon bulk normalized ion counts typically greater than 2x10⁴, and oxygen bulk normalized ion counts typically greater than 10³, as measured by ToF-SIMs. Single crystal metal layers according to the present invention therefore have higher purity than other commercially available foils. This purity is important to warrant reproducible high quality graphene growth, avoiding the variations seen for typical commercial foils and also avoiding the necessity of lengthy pre-treatments to balance for such impurities.⁷

The ToF-SIMS measurements were performed using a ToF-SIMS IV instrument (ION-TOF Gmbh) at a base pressure <5x10^-9 mbar. Each depth profile was acquired by analysing a 150x150 pm² surface area (128 x 128 pixels, randomly rastered) centred within a 400x400 pm² sputtered region in a non-interlaced mode (alternating data acquisition and sputtering cycles). Elemental analysis was carried out using 25 keV Bi³⁺ ions from a liquid metal ion gun (LMIG), with a spot size less than 5 pm in spectroscopy mode, with a cycle time of 100 ps, and an ion current of 0.1 pA. For sputtering, 10 keV CS+ ions with a current of 30 nA oriented at 45 ° to the sample were employed. 3D profiles were constructed by interleaving the vertically resolved surface area images acquired during depth profiling. All depth profiles were normalized to the total ion intensity, using a point-to-point normalization, allowing the most consistent comparison between the samples. The spectra were calibrated using both low and high mass elements and the peaks were assigned in good agreement with theoretical vs experimental isotope identification.

To calculate the bulk ion concentrations the material was sputtered until there was no change in oxygen (O ) and carbon (C ) ion concentrations, at which point the subsequent collected data was binned and used to calculate the boxplots seen in. Fig 5.

Characterisation of sapphire deposition substrate after growth of single crystal metal layer

AFM characterisation of the sapphire deposition substrate was performed, both immediately before deposition, and after deposition once the Cu single crystal metal layer had been removed by ammonium persulfate etchant.

Fig. 7 shows (a) a height map of the sapphire before deposition of the single crystal metal layer; (b) a height map of the sapphire after a deposition of tdep = 60 min, with the Cu having been etched away to expose the sapphire structure; and (c) a graph of height distribution of the height map shown in Fig. 7 (b). The inset roughness of height maps (a) and (b) was after plane subtraction to allow for consistent comparison.

It can be seen that the unused sapphire (cleaned with acetone and IPA), Fig. 7 (a), has a sub-nanometer roughness and no obvious reconstructions at the surface, typical of well-polished epi-ready surfaces.⁶ In contrast, Fig. 7 (b) shows the height map of the sapphire wafer after deposition and Cu etching to be well structured, with regular step edges at regular heights. Fig 7 (c) further characterises the step characteristics, showing consistent step height of around 0.466 nm. This height is approximately one third of the sapphire unit cell (perpendicular to the c-plane), c/3 = 0.433 nm, implying that these step plateaux correspond to the three ‘sub-planes’ of the sapphire. This stepping-type reconstruction is typically what is achieved after many hours of high temperature (>1000 C) annealing in oxygen.⁴’⁵ Whilst the surface termination of the sapphire was not identified, the literature has found that oxygen terminations result in Cu(111 ) with two orientations³. This implies that the sapphire is likely oxygen- terminated during the deposition/restructuring of the Cu single crystal meal layer. The in-situ restructuring of the sapphire surface during deposition again highlights a further benefit of the method according to the present invention: producing in one short deposition qualities that are characteristic of samples that have undergone many hours of pre-processing and optimised deposition using conventional techniques. Characterisation of sinale crystal metal layer and metal source material after 2D material arowth

Fig. 8 shows a comparison of the Cu single crystal layer (a), (b), (c) and the Cu source foil (d), (e), (f) before and after graphene growth as described above in Example 1. Fig. 8 (a) and (d) are optical microscope images of the Cu single crystal layer and Cu source foil before graphene growth, oxidized so that the grain structure of the Cu is clearly visible. Fig. 8 (b) and (e) are AFM height maps and (c) and (f) are white light interferometry (WLI) height maps of the Cu single crystal layer and Cu source foil after the CVD graphene growth process.

R_a roughness values are given as inset figures. The roughness is characterised both by AFM over 10 x 10 pm² area and by WLI over a much larger 1.2 x 0.9 mm² area, to capture both atomistic and macroscopic roughness effects. It can be seen that the R_a measured by AFM remains below 2 nm for the epitaxial single crystal Cu layer also after graphene growth. Advantageously, the roughness of the single crystal Cu layer is of order of 10 nm (R_a=12 nm) over large (mm) areas as measured by WLI. In contrast even just one facet of the Cu source foil is significantly rougher after deposition, with a WLI measured R_a of 414 nm. It should be noted that measurements across Cu grain boundaries of the poly-crystalline film would result in significantly larger R_a values than shown here. This roughness is a well-known issue with the usage of Cu foils for graphene growth ⁵, and again highlights one of the advantages of the present invention in provided low surface roughness single crystal metal layers, which maintain their low roughness even after use as a catalyst for 2D material deposition. This maintenance of low roughness means that the single crystal metal layers can be reused for subsequent reposition processes more easily than conventional films, where the roughening during 2D material deposition may render them unsuitable for further use.

Characterisation of graphene grown using a single crystal metal layer

Graphene grown on a 1x1 cm² single crystal Cu film as described above in Example 1 was characterised, both in-situ on the Cu single crystal layer, and also after removal of the graphene from the single crystal Cu layer by a process shown schematically in Fig. 9.

Interfacial oxidation of the Cu is required to decouple the graphene from the Cu growth template before it can be mechanically peeled off.³⁹’⁴⁰ Cu(111 ) is more difficult to oxidize than other Cu surface orientations,⁴¹ however it was found that a 48 h submersion at 80 °C was sufficient to oxidize the graphene-Cu interface to allow for the transfer. To remove the graphene layer from the single crystal metal layer, as shown in Fig. 9, the graphene 15 on copper 13 on sapphire 9 was therefore placed in 80 °C water 17 for 48 h to oxidize the interface between the graphene and the copper, to thereby allow oxidation of the metal :2D material interface, creating a CuOx layer 19 at the metal :2D material interface.

A polymeric supporting layer 21 was formed by mixing 7g PVA (8000-10000 MW, 80% hydrolysed; Sigma Aldrich) and 3g PVA (85000-124000 MW, 87-89 % hydrolysed; Sigma Aldrich) with 40 ml_ Dl water and stirred at 80 °C until fully dissolved. Approximately 0.1 ml cnr² was placed on a removable support comprising a glass layer 23 and a PDMS layer 25 and dried at room temperature in a cleanroom environment to form a PVA film. The PVA film (‘polymeric support layer’) 21 was then placed into contact with the dried graphene/Cu/sapphire at 120 °C, allowing adherence of the graphene 15 to the PVA film 21 . The PVA/graphene was removed from the Cu 13 at room temperature and placed onto a Si/SiC>2 destination substrate (comprising a top S1O2 layer 27, and a bottom Si layer 29) at 120 °C and left for 1 min. Once the PVA was cool, the PVA/graphene/substrate was placed in Dl water at 80 °C for 24 h to dissolve the PVA, leaving the graphene layer on the Si/Si02 destination substrate.

The described transfer process has particular advantages in that it is possible to preserve the single crystal metal layer for subsequent reuse. Furthermore, as the transfer step comprises a dry mechanical peeling step, it is possible to avoid ionic contamination of the graphene film which often results from conventional wet-etching techniques for graphene removal.³⁸

Fig. 10 (a) is an optical microscopy image of the graphene transferred by peeling onto 280 nm S1O2 on Si as described above, showing minimal cracks over > 100x100 pm² areas and no visible wrinkling/folding (i.e. the layer is optically free of major wrinkles and folds). The homogeneous contrast indicates a high level of homogeneity of the graphene layer.

For comparison Fig. 10 (b) is an optical microscopy image of graphene grown on a typical polycrystalline Cu foil (growth method detailed in Burton et al.⁷), transferred by peeling onto 280 nm SiO² on Si, showing some visible wrinkles/folds as lines of darker contrast throughout the image.

Fig. 10 (c) is a histogram of raman spectra fitted D to G peak ratios from a 1x1 mm² map of the peeled graphene from the single crystal catalyst. To obtain Raman data, a Renishaw inVia system was used. Unless otherwise stated a laser with an excitation wavelength of 532 nm was used with approximately 1 mW output power. The Raman spectra were fit with separate Lorentzians for the D, G and 2D bands⁵⁰. For the D/G ratio, the mean measured value (p) was calculated to be 0.024. The standard deviation (s) was calculated to be 0.01 . The D/G peak ratios seek here demonstrate a very low defect density over a 1x1 mm² mapped area of transferred graphene. The low D/G ratio combined with our deposition and growth geometry of catalyst could indicate a potential lack of amorphous carbon present in the graphene layer.⁴⁸ _’ ⁴⁹

A further highly sensitive characterisation method for the quality of as-grown graphene is its post-growth etching, e.g. in a hydrogen atmosphere.⁷ The crystallinity of the graphene is thereby revealed by the angular distribution of the typically hexagonally shaped etched holes,⁴³ whereas the ease of etching and density of etch holes is indicative of the defect density of the graphene film.⁷’⁸’⁴⁴’⁴⁵

Graphene etching was carried out at 1065 °C for 20 minutes in a H2 and Ar (170:470 seem) gas mixture. In the first case this was conducted immediately after the graphene growth phase such as in previous literature⁷, however no etching was observed for up to 1 h long exposures to the H2/Ar gas mixture. To promote etching the catalyst was removed after deposition, artificially contaminated with abrasive compound (Brasso), oxidized on a hot plate for 30 min at 250 °C before subsequent graphene growth immediately followed by etching for 20 minutes. Fig. 10 (d) is a histogram showing angular measurements of 798 hexagonally etched holes in the graphene film over a 1x1 cm² area. The mean (m) angle was calculated to be 25.7 °. The standard deviation (s) was calculated to be 2.2 °. It was determined that >97 % of the etched holes are within 5° of the mean value, evidencing a single orientation of graphene produced on Cu(111 ) as expected from epitaxial growth.⁴⁶’⁴⁷ It is interesting to note that the as-grown graphene film did not etch under our standard parameters (up to 1 h long exposures to the FE/Ar gas mixture), indicative of a low intrinsic defect density,⁷ and further processing (artificial contamination with an abrasive compound and oxidation as discussed above) was needed to facilitate etching.

Results of further graphene characterisation are shown in Fig. 11 , which shows (a) an optical microscopy image of the Cu film when the graphene growth has been stopped prior to full coverage (tgrowth = 5 min);

(b) an example SEM image of holes (light colour) etched into a continuous graphene layer (dark colour) on the Cu(111 ) single crystal layer, the sample being oxidized to show contrast between the graphene and the Cu; (c) a larger zoomed out image of Fig. 10(a), showing continuity of the transferred film over a larger scale; (d) a map of 2D to G peak intensity ratio over a 1x1 mm² area of the graphene layer, showing that the Raman 2D/G ratio is relatively consistent over a 1x1 mm² area with a value of 2.6±0.2, indicating that the graphene is a monolayer; and (e) an example Raman spectrum (black points) with Lorentzian fits of the D, G and 2D peaks in purple, blue and green respectively.

A low nucleation density of graphene domains under standard growth conditions of approximately 120 mm^-1 after tgrowth = 5 min as shown in Fig. 11 (a) further evidences a lack of contamination on the surface of the catalyst⁵. This low nucleation density is also likely a result of the closely spaced relationship between the Cu/sapphire to the Cu source foil, which would function in the same way as the pocketing method which has been associated with a higher quality of graphene.²⁶ The uniformity of the film can be seen to some degree optically, showing a uniform contrast with no pitting of grain boundaries. The alignment of the graphene domains can be readily seen and indicates that the graphene when merged will be highly crystalline.

Example 2: MpO as a deposition substrate

Whilst the above examples and characterisation primarily relates to formation of single crystal Cu(111 ) on c-plane sapphire, and subsequent growth of graphene on this single crystal layer, methods according to the present invention are more generally application to a wide variety of material systems where heteroepitaxial deposition of a metal onto another material is possible.

Magnesium Oxide (MgO) can be purchased as single crystal wafers and provides an epitaxially ideal crystal structure for fee metals such as Cu and Ni. Maintaining the same methodology and sample geometry as with the sapphire substrate used in Example 1 , MgO(001 ), MgO(011 ) and MgO(111 ) (SurfaceNet, 10 x 10 x 1 mm³, one-side epi-polished) were deposited on using the same parameters used to deposit the Cu (111) single crystal in Example 1 . Graphene was then grown on the resulting Cu single crystal layers using the same methodology as for Example 1 . Fig. 12 shows: (a) & (b) The EBSD z-IPF Map and (powder)-XRD spectrum of MgO(111) after deposition and graphene growth; (c) & (d) The EBSD z-IPF Map and (powder)-XRD spectrum of MgO(110) after deposition and graphene growth; and (e) & (f) the EBSD z-IPF Map and (powder)-XRD spectrum of MgO(100) after deposition and graphene growth.

The EBSD data in Fig. 12 (a), (c) and (e) shows that the MgO substrates produce the corresponding crystallographic orientation of Cu, and indicates that only one orientation of Cu is present in each sample, implying that the Cu films are single crystals. The monocrystalline nature is further supported over larger areas by the (powder) XRD spectra in in Fig. 12 (b), (d) and (f) where each crystallographic orientation shows only the corresponding correct Cu orientation and no others, with the measurement area approximately covering the entire crystal.

Deposition Modelling

The geometry used for sublimative deposition can be modelled by considering two discs, one source and one “drain” or target (i.e. deposition substrate) a distance D apart from another, as shown schematically in Fig. 13.

Using a line-of-sight deposition model the Cu flux can be calculated and tested by using a 2" Cu disk as the evaporation source directly underneath a 2" wafer. Using two circles simplifies the calculation significantly and yields an analytical solution:

Flere Let is the flux incident at a point at radius Xd, n_max is the calculated flux from the Hertz-Knudson equation, D is the distance between the Cu source and the sapphire, R is the radius of the source and n counter (a constant) is the flux from the Cu on the sapphire due to it also being at a high temperature.

Fig. 14 shows the processed results of WLI interferometry measurements over the 2" (25 mm radius) wafer, measuring the average height of the Cu surface relative to the sapphire after some Cu was mechanically removed as a function of the radius of the wafer. The flux was calculated from the thickness of Cu at each radial position on the wafer and the total deposition time taken to be tdep = 60 min.

By using established vapour pressure calculations combined with the Hertz Knudsen equation, the data was fitted using a non-linear least squares method, with fitted variables of D = 0.539 mm, R = 25.5 mm, Tsource = 1020 °C and Tdep = 886 °C, represented by the plotted line in Fig. 14 lying just below the plotted flux values at Xd=0.

For reference another fit (represented by the plotted line in Fig. 14 lying just above the plotted flux values at Xd=0) was made with fixed foil radius and fixed source-target distance to the experimentally measured values, giving temperatures of Tsource = 1048 °C and Tdep = 968 °C. The model reproduces the experimental data reasonably well, estimating the temperature of the source foil to be within 5 % of the thermocouple measurement T_c. It should be noted that there is significant variation in the possible fitting possible to within the errors noted for Let and Xd, however a common factor in the fitting of the experimental data is a consistently lower temperature (approximately 100 °C) for the sapphire compared to the source foil.

This work demonstrates the wafer scale production of single crystal metal layers on various deposition substrates, which can be applied to a wide range of epitaxial system, and which herein is exemplified by production of Cu single crystal metal layers on g c-plane sapphire and MgO in various crystallographic orientations.

Through use of close-spaced sublimation deposition, the method allows for versatile, fast and high quality layer deposition inside commercially available CVD equipment. The use of commercially available CVD equipment for deposition of the single crystal metal layer allows for subsequent CVD processes utilizing the resulting single crystal layer (e.g. 2D material layer growth) to be performed without the need to transfer the single crystal metal layer between separate pieces of apparatus.

A single crystal metal layer according to the present invention has high purity and high flatness, which provides a direct route to production of high quality and large area epitaxial 2D material layers grown using the single crystal metal layer.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +/- 10%.

References

A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

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Claims

Claims:

1. A method for production of a single crystal metal layer, the method comprising steps of: providing a metal source material; providing a single crystal deposition substrate having a selected crystallographic orientation; arranging the metal source material and the deposition substrate in a closely-spaced relationship; and heating the metal source material and deposition substrate under vacuum to cause sublimation of the metal source material and heteroepitaxial deposition of sublimed metal atoms onto the deposition substrate, to thereby form a single crystal metal layer on the deposition substrate.

2. The method according to claim 1 wherein the metal source material and the deposition substrate are spaced such that the theoretical mean free path length of the sublimed metal atoms during deposition is greater than the distance between the metal source and the substrate.

3. The method according to claim 1 or claim 2 wherein the distance between the metal source and the substrate is less than 2 mm.

4. The method according to claim 1 wherein the metal source material is heated to a predetermined temperature Tsource at which sublimation of at least some of the metal source material occurs, wherein Tsource is a temperature in the range of 0.8T_m to 0.99T_m, T_m being the melting temperature of the metal source material at standard atmospheric pressure.

5. The method according to any one of the preceding claims wherein the step of heating the metal source material and deposition substrate under vacuum is performed for predetermined deposition time tdep of 60 minutes or less.

6. The method according to any one of the preceding claims wherein the step of heating the metal source material and deposition substrate under vacuum is performed at a vacuum pressure in the range of 10¹ to 10³ mbar.

7. The method according to any one of the preceding claims wherein the crystallographic orientation of the deposition substrate is selected based on the desired crystallographic orientation of the single crystal metal layer.

8. The method according to any one of the preceding claims wherein the deposition substrate is selected from c-plane sapphire or MgO.

9. The method according to any one of the preceding claims wherein the metal source material is provided as a source layer, and the source layer and the deposition substrate are arranged substantially parallel to one another.

10. The method according to any one of the preceding claims wherein the metal source material comprises a metal having a face-centred cubic, FCC, crystal structure.

11. The method according to claim 10 wherein the metal source material comprises, or consists essentially of a metal selected from: aluminium, copper, gold, iridium, lead, nickel, platinum, silver tungsten or molybdenum..

12. The method according to any one of the preceding claims wherein the method includes a step of cleaning the metal source material and/or the deposition substrate prior to heating and deposition steps.

13. The method according to claim 12 wherein the step of cleaning the deposition substrate includes sonicating the metal source material and/or the deposition substrate in acetone and/or IPA each for a predetermined time period.

14. A method for production of a 2D material including steps of performing the method of any one of claims 1 to 13, and including further steps of heating the deposition substrate and single crystal metal layer in a selected gaseous atmosphere at a predetermined temperature T_grawth to thereby provide growth of the 2D material on the single crystal metal layer.

15. The method for production of a 2D material according to claim 14 wherein the method is performed in a conventional CVD reactor, without intermediate transfer or handling of the deposition substrate and metal layer.

16. The method for production of a 2D material according to claim 14 or claim 15 wherein the gaseous atmosphere comprises a CH4/H2 mixture, and the resulting 2D material is graphene.

17. A single crystal metal layer produced according to the method of any one of claims 1 to 13, wherein the single crystal metal layer has an as-grown surface roughness value R_a of 5 nm or less.

18. A single crystal metal layer produced according to the method of any one of claims 1 to 13, wherein the single crystal metal layer has a carbon bulk normalized ion count of 10^-4 or less, and/or an oxygen bulk normalized ion count of 10³ or less, as measured using TOF-SIMs.

19. Use of a single crystal metal layer according to claim 17 or claim 18 as a catalyst layer in a process of 2D material growth.

20. A 2D material layer grown using a single crystal metal layer produced according to the method of any one of claims 1 to 13 as a catalyst layer.