Single- and few-layer graphene sheets were fabricated by selective chemical reactions between Co film and SiC substrate. A rapid cooling process was employed. The number of layers and crystallinity of graphene sheets were controlled by process parameters. The formation mechanism of graphene was highly sensitive to carbon diffusion. Free carbon precipitated and then moved across the product layer that was composed mainly of cobalt-silicides. The graphene layer formed homogeneously on the surface and then transferred to the other substrate. This could provide a method for high-quality fabrication of wafer-sized graphene sheets.
A highly crystalline material, graphene is represented as a two-dimensional (2D) building block of carbon allotropes with honeycomb lattice [
Currently, graphene sheets are prepared by different methods such as mechanical cleavage of graphite using adhesive tapes [
This paper presents a route for graphene preparation by using SiC substrate reaction with Co film instead of Ni. Co can also act as a catalyst for graphitization of SiC. However SiC decomposes at higher temperature by catalyzed by Co, which could lower defects in formation of graphenes, compared with that by Ni. Additionally, unlike in fast heating (cf. [
Commercial grade n-type 6H-SiC single-crystal wafers (surface roughness < 0.5 nm, TankeBlue Semconductor Company) were used as substrates. Co layers (200–300 nm thick, 99.99 wt.% pure) were deposited by AJA ATC-1800F magnetron sputtering on a polished SiC silicon-terminated surface. Then, samples were subjected to thermal annealing in a vacuum greater than 5 × 10−5 Pa or in ambient pressure under an Ar environment. Sample annealing temperatures were within 900–1000°C. Water bath was used to cool quickly the annealed samples to room temperature.
Graphene on the surface was transferred to the other substrate. The transferring process used PMMA film as support. Prior transferring, a layer of PMMA (MicroChem, 950,000 MW, 9–6 wt.% in anisole) was spin-coated on the sample surface. Afterwards, detachment of the PMMA/graphene layer from the initial surface was done by etching partially the Co/silicides layer with a HNO3 (14 wt.%) solution for more than 24 h. Upon release, the substrate was placed in distilled water at room temperature where manual peeling completely detached the PMMA/graphene membrane from the substrate. Finally, the PMMA membrane was dissolved with acetone. Graphene settled on another SiC substrate. After the transfer, the original SiC pieces were polished for subsequent use.
The whole preparation process for graphene sheets is illustrated in Figure
Schematic diagram of graphene formation process.
X-ray diffraction (XRD) method was used to identify the phase formation of the Co/SiC contact by a Philips X’PERT Pro X-ray ploycrystal diffractometer, where Cu K
Identification of the interfacial reaction products obtained after each annealing was performed by reference to powder diffraction files. The XRD pattern is given in Figure
XRD patterns of the Co/SiC sample annealed under 900°C for 10 min.
SEM images of (a) surface morphology of produced layers. (b) The cross-section of the sample. (c) The corresponding Raman spectra (532 nm excitation) recorded from light contrast region. Intensity ratio of
It is well known that the most striking differences of Raman features in bulk graphite and graphene sheets are as follows: (1) 2D intensity over G intensity ratio (
Evolution of D, G, and 2D peak of our samples as a function of number of graphite layers for 532 nm excitations.
Raman mapping was performed to obtain further information on film uniformity. Figure
Raman mapping data for an area of 20 × 20
The topography imaging of transferred graphene sheet was performed by AFM measurement as shown in Figure
AFM images of the transferred graphene sheet on SiC substrate: (a) interface edge of graphene contacting with SiC, (b) surface morphology of graphene film. The size of the selected area is 10 × 10
The current-voltage (I-V) characteristics of Co/SiC contact annealed at 1000°C for 1 min and transferred graphene film on SiC substrate were performed by four-probe method as shown in Figure
Current-voltage characteristics of the samples: (a) Co/SiC contact annealed at 1000°C for 1 min and (b) transferred graphene sheet on SiC substrate.
Co/SiC sample annealed at 1000°C
Transferred graphene sheets on SiC substrate
To understand better the formation mechanism of graphene sheets using Co as catalyst for SiC decomposition, element distribution across the Co/6H-SiC contacts was measured using various annealing parameters. The AES depth of elemental distribution across the contact annealed at 1000°C for 10 min is shown in Figure
AES depth profile of the Co/SiC contact annealed for 10 min under 1000°C.
In a thick (>100 nm) metal/SiC contact, the diffusion of species is the rate-determining step for the interfacial process [
Based on the proposed mechanism, the number of layers and graphene sheet crystallinity were tuned according to process parameters. Annealing duration and cooling rate are critical in carbon layer formation and efficient graphene layer separation from the substrate. With increasing reaction time, the 2D peaks of Raman spectra broadened, suggesting that thickness of the carbon layers diffused to the surface has become larger. A fast cooling step after annealing would decrease carbon diffusion rate and result in fewer amounts of carbon moving outward the reaction zone to form single- and few-layer carbons. The Raman spectra dependence on the cooling process after thermal annealing is illustrated in Figure
Raman spectra of segregated carbon with different cooling rates. Bulk graphite is used as a reference.
Single- and few-layer graphene sheets were successfully synthesized using a Co film that was selectively reacted with a SiC single-crystal substrate at 900–1000°C, and assisted by rapid cooling. Graphene formation was based on controlling the reaction between Co and Si from the SiC substrate. The process produced carbon diffusion throughout the Co/Co silicide layers. The formation mechanism reveals that optimal conditions for single- and few-layer graphenes resulted from tuning Co film thickness, annealing temperature/duration, and cooling rate. By using PMMA films, graphene could be transferred to the other substrate. The 1–4 layers of graphene, with a few defects or impurities, were obtained. Further optimization of the growth process should focus on higher uniformity of graphene sheets. The use of multilayer transition metals like (Co + Au) could provide better block layers for the available carbon, apart from ensuring better crystal orientation in the graphene sheets. It is expected to provide a simple and efficient strategy to prepare high-quality graphene with wafer size.
Financial support was provided by the National Natural Science Foundation of China (Grant No. 50772012 and 50972010) and the Fundamental Research Funds for Central Universities (FRF-TP-09-021B). Professor Liu Zhongfan from Peking University is grateful for the help of graphene transferring.