synthesis of graphene on silicon dioxide by a solid carbon source

Synthesis of Graphene on Silicon Dioxide by aSolid Carbon SourceJens Hofrichter,*,† Bartholomaus N. Szafranek,† Martin Otto,† Tim J. Echtermeyer,†Matthias Baus,† Anne Majerus,‡ Viktor Geringer,‡ Manfred Ramsteiner,§ and Heinrich Kurz†

†Advanced Microelectronic Center Aachen (AMICA), AMO GmbH, Otto-Blumenthal-Strasse 25,D-52074 Aachen, Germany, ‡II. Physikalisches Institut B, RWTH Aachen and JARA FIT, D-52056 Aachen, Germany,and §Paul-Drude-Institut fur Festkorperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany

ABSTRACT We report on a method for the fabrication of graphene on a silicon dioxide substrate by solid-state dissolution of anoverlying stack of a silicon carbide and a nickel thin film. The carbon dissolves in the nickel by rapid thermal annealing. Upon cooling,the carbon segregates to the nickel surface forming a graphene layer over the entire nickel surface. By wet etching of the nickel layer,the graphene layer was allowed to settle on the original substrate. Scanning tunneling microscopy (STM) as well as Raman spectroscopyhas been performed for characterization of the layers. Further insight into the morphology of the layers has been gained by Ramanmapping indicating micrometer-size graphene grains. Devices for electrical measurement have been manufactured exhibiting amodulation of the transfer current by backgate electric fields. The presented approach allows for mass fabrication of polycrystallinegraphene without transfer steps while using only CMOS compatible process steps.

KEYWORDS Graphene, phase segregation, carbon electronics, Raman spectroscopy, scanning tunneling microscopy

Since the discovery of graphene by Geim and No-voselov in 2004,1 the research on graphene hasgained enormous momentum.2 In their experiments,

graphene was obtained by mechanical cleaving of naturalgraphite with adhesive tapes and the successive transfer tothe desired substrate leading to a random deposition ofgraphene flakes with both low surface coverage and repro-ducibility. This method is widely used in research and hasled to the observation of a variety of new physical effects ingraphene,3,4 but it does not lend itself to mass production.There has been considerable effort in the synthesis of large-area graphene sheets in recent years. High-quality single-and few-layer graphene sheets have been grown by decom-position of single-crystal silicon carbide5 and chemical vapordeposition on single-crystal transition metals such as iri-dium6 and ruthenium.7 These methods are expensive dueto the high cost of the substrates and the ultrahigh vacuum(UHV) process conditions used. Furthermore, the size of theavailable substrates is relatively small compared to, e.g.,silicon wafers.

An approach better suited for large-scale application hasbeen ambient-pressure chemical vapor deposition (CVD) ofhydrocarbon gases on polycrystalline transition metals.Large-area graphene sheets have been fabricated in thismanner on nickel8-10 and copper thin films.11 With recentadvances in this method, single-layer and bilayer graphenecoverage of up to 87% of the film area on nickel12 and

∼95% single-layer coverage on copper foils13 has beenachieved. The growth mechanism on nickel is based on gasdecomposition and diffusion of atomic carbon into the nickelwith subsequent carbon precipitation to the nickel surfaceupon cooling. With copper, on the other hand, the growthof graphene is self-limiting and appears to be based on asurface-catalyzed process rather than a precipitation processas with nickel.13 The graphene film can be transferred toarbitrary substrates by wet etching of the underlying metalfilm. To support the thin graphene film during the transfer,a poly(dimethylsiloxane) (PDMS) stamp9 or a supportivecoating, e.g., a poly(methyl methacrylate) (PMMA) film,12 onthe graphene can be used.

A slightly different approach to graphene synthesis onmetal surfaces has been recently published, in which thecarbon is supplied by a solid-phase method using a siliconcarbide substrate as carbon source.14 In this process a 200nm nickel thin film is deposited on SiC via electron beamevaporation. Two kinds of substrates were used: single-crystal 6H-SiC (0001) and 3C-SiC films on Si substrates. Withboth substrates, the samples are rapidly heated in vacuum(∼10-7 mbar) to 750 °C and immediately cooled to roomtemperature. During the heating process both silicon andcarbon dissolve in the nickel, forming nickel silicide satu-rated with atomic carbon. Upon cooling the carbon segre-gates to the surface of the nickel/nickel silicide film, formingsingle- to few-layer graphene.

Here, we present an approach for the synthesis ofgraphene on an oxidized silicon substrate. Instead of single-crystal SiC substrates or multicrystalline SiC films,14 amor-phous SiC thin films were used as a carbon source depositedon a thermally oxidized silicon substrate. In order to reduce

* Corresponding author. Present address: IBM Research GmbH, Zurich ResearchLaboratory, Saumerstrasse 4, CH-8803 Switzerland. E-mail: [email protected]: + 41 44 724 8560.Received for review: 08/6/2009Published on Web: 12/22/2009

pubs.acs.org/NanoLett

© 2010 American Chemical Society 36 DOI: 10.1021/nl902558x | Nano Lett. 2010, 10, 36-42

the carbon concentration in the nickel film deposited on theSiC film, we chose a thin SiC layer (50 nm) and a 500 nmthick nickel film. The graphene film is formed by rapidthermal annealing (RTA) of the sample at ambient pressure.We found that vacuum is not necessary for graphene forma-tion. During the process, high temperatures of 1100 °C arerequired to completely dissolve the SiC layer in the nickel,which is important for controlling the amount of carbondissolved in the nickel. A detailed analysis of the fabricatedgraphene layers was performed, including atomic forcemicroscopy (AFM), scanning tunneling microscopy (STM),Raman spectroscopy, Raman mapping, and a comparisonto mechanically exfoliated graphene. Additionally, electricalcharacterization of micrometer-scaled devices made fromthe produced graphene layers was performed.

We chose thermally grown SiO2 on Si with a thickness of300 nm as the starting material. In our experiment, 50 nmof amorphous silicon carbide (SiC) is deposited onto thesilicon dioxide layer by plasma-enhanced chemical vapordeposition (PECVD). After deposition of a 500 nm thicknickel layer by dc magnetron sputtering, the sample isannealed at 1100 °C for 30 s in ambient pressure and

nitrogen environment in a rapid thermal annealing (RTA)oven. During this time both the silicon and the carbon ofthe SiC are dissolved in the nickel layer. By choosing a nickellayer much thicker than the SiC layer, the silicon from theSiC is only dissolved in nickel or forms substoichiometricsilicides. The SiC thin film is completely consumed by thisprocess. Thus the carbon content in the nickel layer can becontrolled by the ratio of SiC to Ni layer thickness. However,experiments lowering the SiC thickness by inverse sputteringindicate that carboneous films are formed but are not of aclosed shape thus hindering a wet chemical processingneeded for device fabrication and characterization. A certainamount of a solid carbon source has to be present for theformation of a closed layer. The SiO2 substrate has a chemi-cally and thermally stable interface and does not release anySi atoms into the Ni. When the sample is cooled to roomtemperature, the carbon segregates to the surface of the Nilayer forming a graphene layer. Successively, the nickel layerwith the dissolved silicon was wet chemically removed. Thelower chemical stability of substoichiometric silicides allowsfor the usage of hydrogen fluoride free etchants. In ourexperiment, we removed the silicon-nickel layer by a nitric

FIGURE 1. Surface morphology of the produced layers. (a) Optical microscope image. SEM images of the fabricated layers: (b) side view, (c)top view. AFM images of polygraphene: (d) phase contrast imaging, (e) topography imaging, (f) close-up of one graphene grain.


acid and hydrogen peroxide containing etchant. It is possibleto transfer the graphene to arbitrary substrates by methodsdescribed in refs 11 and 12. However, we chose to let thegraphene settle on the original SiO2/Si substrate instead ofusing a transfer process.

Optical microscopy revealed the marmorated nature ofthe produced layers as shown in Figure 1a. Thin grapheneregions are present in the purple areas of the image, whereasthe light blue areas indicate thicker graphitic regions. There-fore the produced material will be referred to as po-lygraphene. Analysis has also been performed by scanningelectron microscopy (SEM). As illustrated in Figure 1b, free-standing regions of a thin film, through which imaging couldbe performed, could be observed at the borders of thecleaved-facet samples. Again, the marmorated nature of theproduced films becomes evident when taking top-view SEMimages as in Figure 1c. Further investigation of the producedlayers has been performed by AFM. Topography imagingreveals a mean film thickness of ∼6.5 nm and ∼15 nm inthe thickest regions. There are also several areas in the imagewhere no discernible height difference to the substrate couldbe measured. These could be either regions with very thinthickness or voids in the film. However, different materialscan be distinguished by phase contrast imaging. The phaseimaging, presented in Figure 1d, shows that the layer is aclosed film except for a small void in the top left corner ofthe image. The darker regions are the carbon-containing filmwhile the lighter yellow region represents a different ma-terial, i.e., silicon dioxide. Hence, the film is either cor-rugated or of varying thickness having thin regions. Figure1f displays also a topography AFM image, here with a higherresolution. The wrinkles are elevated and can be attributedto folding of the graphite induced by the different expansioncoefficients of nickel and graphite during cooling down afterthe high-temperature process.

STM analysis was used to gain further insight into themicroscopic structure of the produced layers and to distin-guish between single- and few-layer graphene by compari-son with exfoliated natural graphite. Figure 2 displays theatomic structure of the several types of graphene. An artist’sview of a single graphene layer on top of an appropriatetransition metal surface is illustrated in Figure 2a. E.g., thenickel (111) surface corresponds to this image.

STM images have been taken in an UHV chamber at roomtemperature for a bilayer of mechanically exfoliated graphene(Figure 2b) and a monolayer of graphene,15 which is de-picted in Figure 2c. Single-layer graphene exhibits the typicalhoneycomb one expects for a perfect hexagonal lattice. Incontrast, several layers of graphene present a triangularlattice structure by a modification of the surface density ofstates by the underlying layers.16 Also, STM imaging hasbeen performed on the layers fabricated by the methodpresented in this paper. Figure 2d illustrates such an imagewhich contains a perfect hexagonal lattice with no visiblelattice defects. The lattice constant of the synthetic graphene

layers has been determined to 2.49 ( 0.07 Å, perfectlymatching the theoretical value of graphene of 2.46 Å andthe lattice constant of nickel being 2.49 Å.

The band structure is the electronic fingerprint of materi-als. Taking advantage of resonance effects, Raman spec-troscopy is an appropriate method to extract characteristicfeatures of the band structure for different materials.17 Inparticular, graphite and graphene can be distinguished easilyand Raman fingerprints for single layers, bilayers, and fewlayers reflect the changes in the band structure (ref 18 andthe references therein). The most intense Raman featuresin graphite and graphene are the G-peak at about 1600 cm-1

and the 2D-peak at about 2700 cm-1. The G-peak is due tofirst-order Raman scattering by the doubly degenerate zone-center optical phonon mode and the 2D-peak is associatedwith second-order scattering by zone-boundary phonons.18

In first-order Raman scattering, the observation of zone-boundery phonons is inhibited by selection rules. However,in defective graphite, the so-called D-peak at about 1350cm-1 due to first-order scattering by zone-boundary phononscan be observed. The relatively low intensity or the absenceof the D-peak in the spectra of graphene layers reflects thesmall concentration of defects. The Raman fingerprint forsingle-layer graphene, however, is connected with the posi-tion and width of the 2D-peak, which reflects the electronicband structure characteristics via a double-resonant Ramanscattering process.18 The position of the 2D-peak dependson the incident laser wavelength and for excitation at 632.8nm a single, relatively narrow and intense 2D-peak around2700 cm-1 is the signature of single-layer graphene. Strain

FIGURE 2. The atomic structure of graphene. (a) Artist view of asingle layer of graphene on top of a nickel (111) surface, the otherfigures present STM images of different types of graphene. (b) Fewlayer exfoliated graphene. (c) Single layer of exfoliated graphene.(d) Graphene single layer produced by the process described in thiswork.


induced by the substrate, however, leads to a shift of the 2D-peak, ruling out the peak position as a measure for thenumber of layers in our case. Lee et al. have shown that thepeak width is decisive for the number of layers of grapheneproduced by the decomposition of silicon carbide (SiC).19

Hence, their approach of analyzing the produced layers hasbeen used as it allows for the identification of the numberof layers while being insensitive to induced stress.19 Accord-ing to ref 19, the lowest observed full width at half-maximum(fwhm) has been assigned to single-layer graphene while thefwhm given in their paper for bulk graphite has beenassociated to films with more than 10 layers.

Spectra of first- and second-order Raman scattering fromour films are shown in parts a and b of Figure 3, respectively.For excitation, a laser wavelength of 632.8 nm has beenused. As a reference, the sprectrum of a mechanicallyexfoliated monolayer of graphene is shown in addition.Figure 3c displays the positions on the sample from whichthe spectra have been aquired. The position “Poly A” is amonolayer region whereas “Poly B” is a multilayer region.Neither the exfoliated monolayer nor our films show asignificant D-peak around 1350 cm-1, but a strong G-peakaround 1600 cm-1 indicating excellent crystallinity. In Figure3b, the fwhm of the 2D-peak has been determined to be 30cm-1 for synthetic graphene and 19 cm-1 for the exfoliatedmonolayer of graphene. The larger fwhm of the 2D-peak ofthe synthetic graphene can be explained by inhomogeneousstress induced by the high-temperature process, which does

not relax upon cooling. This phenomenon is also observedfor graphene grown on SiC (ref 20). To obtain further insightin the nature of our films, Raman mapping has beenperformed with the scattered light collected by a confocalmicroscope. Figure 4 illustrates for an area of 46 × 36 µm2

the Raman intensities of (a) the D-peak around 1350 cm-1,(b) the G-peak around 1600 cm-1, and (c) the 2D-peakaround 2700 cm-1. The peak width (FWHM) of the 2D-peakaround 2700 cm-1 is displayed in Figure 4d and the corre-sponding number of layers according to ref 19 is shown inFigure 4e, where the dark blue regions indicate mono- orbilayer graphene. This finding is also consistent with the ratioof the 2D-peak intensity to the G-peak intensity displayedin Figure 4f. The AFM results, Raman mapping, and AFMconsistently indicate a produced graphene crystallite size inour films of about 5-10 µm. Besides regions consisting ofmultiple layers of graphene, single layer areas are clearlyevident.

Electrical characterization of the produced layers hasbeen performed by contacting a 2.0 µm wide polygrapheneribbon with metal fingers. First, 50 nm thick nickel contactshave been fabricated on top of the polygraphene layers bya lift-off process. Subsequently, a 1.5 µm thick AZ 5214Aresist has been deposited and developed serving as a maskfor the graphene structuring. The patterning has beenperformed in an anisotropic oxygen-argon plasma. Afterthe transfer of the structures, the resist has been removedand the samples baked out for 12 h at 300 °C in a vacuum

FIGURE 3. Raman spectra excited at 632.8 nm for (a) first-order and (b) second-order scattering. The D- and G-peaks are observed around1350 and 1600 cm-1, respectively, and the 2D-peak at about 2700 cm-1. The width of the 2D-peak for exfoliated single-layer graphene andthe minimum 2D-peak width for graphene layers presented in this work are 19 and 30 cm-1, respectively.


chamber. The final device is displayed in Figure 5a. Electricalmeasurements have been performed with an HP 4156Bsemiconductor parameter analyzer. Figure 5b displays thetransfer current characteristics obtained by applying a dcvoltage between two adjacent contact fingers and sweepingthe backgate voltage applied to the Si substrate as depictedin Figure 5d. The curve is similar to transfer current mea-surements of single-layer graphene, also showing a mini-

mum corresponding to the carge neutrality point at whichthe charge carrier density is minimal and switches from n-to p-conducting state.1,21 On the other hand, the currentmodulation is much weaker than in single-layer graphene.In single-layer graphene the current should have a maximumto minimum ratio of about two in the measured range21

while the polygraphene curve shows a modulation value ofup to 17.5% for the best device and typically of a few

FIGURE 4. Raman mapping data for an area of 46 × 36 µm2: (a) D-peak (1350 cm-1) intensity, (b) G-peak (1600 cm-1) intensity, (c) 2D-peak(2700 cm-1) intensity, (d) width of the 2D-peak, and (e) the assigned number of layers according to ref 18. (f) Intensity ratio of the 2D-peakto the G-peak. Note the defect in the center, which can be been in (b) and (c) as a minimum in the G-peak and 2D-peak, respectively.


percent as in Figure 5b. This is due to the fact that thecontacted material consists of a few graphene layers as ingraphite. Thus, the carrier density is only controlled in thelowest graphene layers, while in the upper graphene layersit remains constant because of electronic shielding by thelower layers. Also, transport along grain boundaries is likelyto contribute to the residual conductivity. Furthermore,contact resistances also lower the modulation value; how-ever, their exctraction is diffucult due to the inhomogeneousnature of the layers. The charge neutrality point is shiftedto positive voltages which is attributed to doping effects fromthe contacting metal, since Ni and adsorbed water both leadto p-doping of graphene. Figure 5c shows the Ohmic behav-ior of a typical device at constant gate voltages.

In conclusion, we have developed a simple method togrow graphene on polycrystalline nickel surfaces using anamorphous SiC thin film underneath the nickel layer ascarbon source. The growth process is based on phasesegregation of a carbon containing layer (SiC) and a solidsolvent (Ni). The SiC dissolves in the nickel, completelyconsuming the SiC thin film. By wet-etching the Ni layer, thegraphene can either be transferred to arbitrary substratesor, as was done here, can be allowed to settle on the SiO2

surface of the original substrate. The graphene layers werecharacterized by optical microscope imaging, SEM, AFM,STM, Raman spectroscopy, and Raman mapping. The analy-sis consistently shows that polygraphene layers have beenproduced with regions consisting of monolayers of graphene.Electricalcharacterizationofthelayersdemonstratedgraphene’selectric field effect. Further optimization of the growthprocess to obtain more uniform layers with larger monolayercoverage should be possible by controlling the amount ofavailable carbon via carefully tuning the ratio of SiC to Nifilm thickness in conjunction with an optimized coolingrate,12 which cannot be varied in the present setup. Also theuse of other transition metals, such as ruthenium, might leadto a reduced thickness variation within the graphene layers.Controlling the carbon concentration, the cooling rate, andthe crystal orientation of the metal layer appear to be themost important parameters for highly uniform graphenefilms on arbitrary substrates.

Acknowledgment. The authors acknowledge the fundingby the BMBF project ALEGRA and the European projectGRAND as well as the technical support by D. Stegers andT. Welter.

FIGURE 5. Electrical characterization of the presented polygraphene layers (a) Graphene field effect transistor (GFET), on which themeasurements have been performed. The width of the ribbon is 2.0 µm. (b) Transfer current modulation by backgate fields (c) Modulation ofthe resistance slope for different backgate fields.


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synthesis of graphene on silicon dioxide by a solid carbon source

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