Method for exfoliating and transferring graphene from a doped silicon carbide substrate to another substrate

Abstract

The present invention refers to a method for exfoliating and transferring graphene from a doped silicon carbide substrate to another substrate, the method being based on exfoliation induced by hydrogen bubbles produced in the electrolysis of water.

Claims

1. A method for transferring graphene deposited on a substrate, consisting of a layer of doped silicon carbide, to another substrate characterised in that it comprises the following steps: a) obtaining at least one system comprising graphene the layer of doped silicon carbide and a polymeric layer,  wherein the graphene is deposited between the layer of doped silicon and the polymeric layer, b) applying a potential between the cathode and the anode of an electrochemical cell between 6 V and 20 V; wherein the system obtained in step (a) acts as a cathode and a reference electrode acts as an anode and wherein the electrolytic solution of said electrochemical cell comprises water and an electrolyte; c) depositing the system obtained in step (b) comprising the graphene deposited in the polymeric layer, on the substrate of interest, wherein the graphene is deposited between the polymeric layer and the substrate of interest; and d) removing the polymeric layer from the system obtained in the step by dissolution; with the condition that the substrate of interest is insoluble in the dissolution of step (d).

2. The method according to claim 1, wherein the substrate of interest is selected from SiO.sub.2, Si on SiO.sub.2 and any combinations thereof.

3. The method according to claim 1, wherein the system obtained in step (a) in turn comprises the following steps: (a1) obtaining graphene deposited on a doped silicon carbide substrate by chemical decomposition of the silicon carbide crystal, comprising the following steps: (i) removing impurities from the surface of the doped silicon carbide substrate, and (ii) heat treating the substrate obtained in (i) under vacuum and at a temperature between 1300° C. and 2000° C. for a time between 0.5 h and 2 h; and (a2) depositing a polymeric layer on the graphene obtained in (a1).

4. The method according to claim 1, wherein the applied potential between the cathode and the anode in step (b) is between 10 V and 13 V.

5. The method according to claim 1, to wherein the electrolyte used in step (b) in the electrochemical dissolution is selected from an acid, a base or a salt.

6. The method according to claim 5, wherein the electrolyte is a base.

7. The method according to claim 6, wherein the base is NaOH or KOH.

8. The method according to claim 1, wherein the substrate is silicon carbide doped with nitrogen.

9. The method according to claim 1, wherein the polymeric layer of step (a) is selected from polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polyethylene (PE), polyphenyl ether (PPE) and any combinations thereof.

10. The method according to claim 1, wherein the solvent used in the solution of step (d) is selected from a polar aprotic solvent.

11. The method according to claim 10, wherein the solvent is acetone or acetone followed by isopropanol.

12. The method according to of claim 1, wherein the rinse of the product obtained in step (d) is carried out with water.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 Three-dimensional representation of the topography characterisation of the SiC surface completely covered with epitaxial graphene. It corresponds to the sample used for the experiment described in FIG. 1, corresponding to graphene exfoliated with the electrochemical method and transferred to SiO.sub.2 substrate. The image has been taken with an atomic force microscope, corresponds to the so-called topography signal, and sweeps an area of 1.7×1.7 μm.sup.2.

(2) FIG. 2 Raman spectra in three different positions of the same sample for graphene that is exfoliated and transferred to an SiO.sub.2 substrate once the layer of PMMA has been removed in the acetone and isopropanol bath.

(3) FIG. 3 Two Raman spectra of the remaining SiC substrate doped with nitrogen, when the present electrochemical bubbling technique is applied to separate the epitaxial graphene from the substrate.

(4) FIG. 4 Sequence of photographs of the electrochemical exfoliation of epitaxial graphene in doped silicon carbide.

EXAMPLES

(5) The invention is illustrated below by means of assays conducted by the inventors that demonstrate the effectiveness of the method of the invention.

(6) Preparation of the Samples

(7) First a pre-treatment of a highly doped substrate (n+) of 4H—SiC was carried out, that is to say, the cleaning and removal of native oxide from substrates. Highly doped commercial SiC (Intrinsic SC) was used in the present invention. SiC is a wide-bandgap semiconductor type material, 3.23 eV for 4H—SiC. SiC can be doped intentionally to modify its band structure, that is, to generate additional levels between the valence band and the conduction band. With high doses of doping, for example with boron atoms, aluminum or nitrogen atoms, metallic conduction can be achieved. In the present example a substrate highly doped with nitrogen (n+) of 4H—SiC is used, with a resistivity of ˜0.05 ohm/cm.sup.2.

(8) The initial substrates are samples of 1.4×1.4 cm.sup.2 cut from a 5.08 centimetre (2 inches) wafer.

(9) The graphene growth was then carried out by decomposition at a high temperature. The epitaxial growth of a few layers of graphene was performed on the Si face of 4H—SiC. The cut of the wafers with respect to the crystalline planes of the SiC is of the off-axis type (4°). The growth was carried out at a high temperature, at a temperature of 1850° C. and under vacuum. Under these conditions, graphene covers the entire SiC surface doped with nitrogen. It should be noted that the substrate itself is also the source of atomic C and therefore the precursor of graphene. As a result of the decomposition of SiC substrate doped with nitrogen, the surface of the SiC substrate is reconstituted forming characteristic graphene terraces of typical width between hundreds of nanometres up to 1-2 microns; the length of the terraces is greater than several millimetres, typically greater than 3 mm based on the determination with optical microscopy. The steps are of variable height, between about 10-100 nm. The layers of graphene cover the topography of the SiC surface doped with nitrogen in a conformal and continuous manner in the steps (FIG. 1).

(10) For transfer handling, a thin polymeric layer covering the graphene previously obtained was used. A commercial product (MicroChem) of polymethyl methacrylate was used, PMMA 950 MW available in solution that was deposited on the face of graphene by the spin coating method. When deposited at a spinning speed of 1500 rpm, the nominal thickness of the polymeric layer is 800 nm thick.

(11) Method for Transferring Graphene from One Substrate to Another

(12) The separation of the graphene and the substrate was induced by the formation of H.sub.2 bubbles at the graphene-SiC doped with nitrogen interface. H.sub.2 bubbles are formed in the electrolysis of water according to:
2H.sub.2O(l)+2e.sup.−.fwdarw.H.sub.2(g)+2OH.sup.−(aq) Cathode E°.sub.red=−0.828V
O.sub.2(g)+2H.sub.2O+4e.sup.−.fwdarw.4OH.sup.−(aq) Anode E°.sub.red=0.401V

(13) The electrochemical cell where the electrolysis of water was carried out was formed by a pair of electrodes: a platinum reference electrode (anode) and an electrode consisted in the sample, that is to say, by the PMMA-graphene-SiC system doped with nitrogen (cathode) described in the previous section. The electrochemical solution was made up of an aqueous solution containing 0.25 M NaOH. With an external source, a potential difference of between 10 V and 13 V between the electrodes was applied, which caused the electrolysis of water and sufficient bubbling at the graphene-SiC interface to break the bond or overcome the cohesion forces at the interface. The separation occurred between 30 seconds and 10 minutes of bubbling.

(14) Once the PMMA-graphene bilayer is dettached, it is manually transferred to an SiO.sub.2 on silicon substrate with tweezers. The PMMA, which formed the support layer, was removed by an acetone bath followed by a bath in isopropanol, and then a rinse was carried out in pure water.

(15) The verification and structural analysis of graphene transferred to SiO.sub.2 was performed by Raman spectroscopy.

(16) FIG. 2 shows the Raman spectrum obtained for the system formed by a layer of graphene in physical contact with the support layer of PMMA. Specifically, the distinctive Raman G band (˜1585 cm.sup.−1) and 2D band (˜2690 cm.sup.−1) of graphitic materials can be observed. The characteristics of the peaks of the three spectra indicate that it is monolayer type graphene: ratio of intensities between the 2D and G peaks greater than one and 2D peak with a profile that can be fit to a single Lorentzian equation with full width at half height (FWHM) ˜35 cm.sup.−1. As demonstrated by the structural characterisation by Raman spectroscopy of the graphene material, the membrane of transferred material is of a high crystalline quality. The eventual presence of the D peak is relatively common in this type of polymeric layer-assisted transfer techniques. It is attributed, for example, to polymer residues or structural stress due to an imperfect planarisation (presence of wrinkles, folds, corrugation, etc).

(17) FIG. 3 shows the Raman spectrum of the substrate remaining after exfoliation, which consists of SiC doped with nitrogen, where only the characteristic peaks of the SiC crystal are observed. In the displayed frequency range (1200 cm.sup.−1 to 3000 cm.sup.−1), the resonant bands of the SiC crystal at 1516 cm.sup.−1, 1705 cm.sup.−1 and 1711 cm.sup.−1 stand out. No graphene is detected since the G and 2D bands of graphene are not present. Since the characteristic G and 2D bands of graphene are not present, an efficient/effective exfoliation of graphene is included/demonstrated.

(18) This includes the complete pickling of the layer(s) of graphene, in the order of the chip/substrate area, or the exfoliation of graphene tapes. Under the high-temperature atomic silicon sublimation deposition process conditions used for the demonstrator, layers of graphene are obtained covering the entire surface of the SiC doped sample and in a reproducible manner. The epitaxial graphene continuously covers the terraces and steps of SiC (FIG. 4) either by coalescence or superposition of layers of graphene. The pickling materials have a correspondence with the surface morphology determined with atomic force microscopy of epitaxial graphene, insofar as said terraces and steps can be recognised in the transferred graphene.

(19) FIG. 4 shows a sequence of photographs of the electrochemical exfoliation process of epitaxial graphene in doped silicon carbide. As can be seen in FIG. 4, the dettachment occurs spontaneously in less than 20 seconds and without the help of mechanical traction.

(20) In the case of obtaining transferred graphene tapes, for example, observed by optical microscopy, these have a high ratio in proportions: typical width 300 nm-1.5 μm and length of hundreds of micrometres—a few millimetres, determined by the morpho-structural features of the high-temperature deposition products, and therefore coinciding with the dimensions of the terraces or features of the steps.

(21) Comparative Test 1:

(22) A non-doped PMMA-graphene-SiC system was prepared as indicated above and exfoliation was carried out in the electrolysis conditions described in the previous example. Because non-doped SiC is insulating, the flow of electrical current between the electrodes, which is necessary for electrolysis, and therefore bubbling, does not take place.