Growth of Graphene by Gas-source Molecular Beam Epitaxy
Fumihiko Maeda and Hiroki Hibino
Materials Science Laboratory
Because of the excellent electrical performance of graphene, such as a highest carrier mobility at room temperature, it is recognized as one of the most promising candidates for the next-generation of electronics materials. However, for its practical application, a remaining challenge is the formation of graphene films with high quality and wafer-scale size to achieve compatibility with a large-scale manufacturing process. To meet this challenge, we propose a new approach for forming wafer-scale graphene, which is based on gas-source molecular beam epitaxy (MBE). In this method, the substrate is not restricted to specific ones as it is in other promising methods, such as graphene formation by means of epitaxy on SiC  or on transition metals . We report here a demonstration of the gas-source MBE to show the feasibility of our new approach for the formation of graphene .
In this experiment, about three atomic layers of graphene, prepared by high-temperature annealing of 6H-SiC(0001) under an ultra-high vacuum, were used as a substrate for the homoepitaxial growth. This substrate was heated to 620ºC and cracked ethanol was supplied as a growth material. After this growth, in situ x-ray photoelectron spectroscopy (XPS) measurements indicated that about four monolayers of graphitic material were grown. The cross-sectional transmission electron microscope image in Fig. 1 shows that this grown graphitic material has a layered structure. These results show that graphene was grown. Meanwhile, in the Raman spectra of the samples with and without the growth [Fig. 2], we observed changes in the peak shape of the 2D-band and an increase in the intensity of the G-band. This result supports the increase of graphene thickness by the growth. However, a remarkable D-band peak was also observed after the growth, indicating that the domain size of the graphene is small. These results indicate that our new approach is feasible for the formation of wafer-scale graphene and that it would be compatible with device fabrication processes, although improvement in the crystal quality is needed.
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Fig. 1. Cross-sectional transmission electron microscope
image after MBE growth.
Fig. 2. Raman spectra of the samples (a) with and
(b) without the overlayer growth.
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