Research Highlights

Tailor-made graphene for spintronic and nanoelectronic
applications
S. Entani, Y. Matsumoto, M. Ohtomo, P. V. Avramov, H. Naramoto, and S. Sakai
R. G. for Molecular Spintronics, ASRC, JAEA

Graphene proved interesting for nanoelectronics andspintronics. There have been plenty of attempts to fabricate
graphene-based devices. To realize the spintronic andnanoelectronic devices using graphene, particular concerns
should be addressed to the following two issues: The first one
is to form the well-defined interface between graphene andmetal electrodes. The elucidation and control of the injection/detection processes of spin-polarized carriers through thegraphene/metal interface are essential for designing device
properties [1]. The second one is to synthesize graphene filmwith large area and uniform number of carbon layer. The
most conventional and commonly used fabrication method ofgraphene is the micromechanical exfoliation of graphite [2].
However, this process cannot satisfy above requests due to thefollowing reasons: limited sizes with large distribution in shapeand layer number as well as the contamination and impurities
introduced during the fabrication process. Ultrahigh vacuum
chemical vapour deposition (UHV-CVD) [3] can be a promisingalternative method. The UHV-CVD growth makes it possible
to prepare an epitaxial graphene film on the metal surface withlarge area and uniform layer-number of carbon sheet. In UHVCVD, it is considered that graphene grows by the dissociationand polymerization of hydrocarbon molecules as precursors
on the catalytic metal surfaces. This allows us to control thecrystallinity by the growth condition.
In this study, the process of the UHV-CVD growth ofgraphene on Ni(111) thin films and with benzene as a precursor
are investigated using in-situ reflection high energy electron
diffraction (RHEED) and Auger electron spectroscopy (AES)[4]. Figure 1 shows changes in the RHEED specular beam
intensity and the Auger intensity ratio, IC/INi, of the C KLL peak,IC, and Ni LMM peak, INi as a function of benzene exposure. It
is successfully shown that SLG and bilayer graphene (BLG) canbe synthesized by the control of benzene exposure in the range
in 10−105 langmuirs (L, 1 L = 10−6 Torr·sec) reflecting a change
in the growth rate by three orders of magnitude in between thefirst and second graphene layer on the Ni(111) surface.
The electrical states of the transferred graphene sheets onthe SiO2 substrate are examined from the positions of the G
peak and the 2D peak (Pos(G) and Pos(2D), respectively) in theRaman spectra. Figure 2 shows a plot of Pos(2D) vs. Pos(G)obtained from the randomly selected areas on the graphenesheets prepared with different benzene exposures and on theexfoliated SLG sheet. The linear increase of Pos(2D) withPos(G) as seen in exfoliated SLG is reflective in the degree ofhole doping, and the wide distribution of the peak positions,
1580−1592 cm−1 in Pos(G) and 2687−2704 cm−1 in Pos(2D),
is due to the non-uniformity of the unintentional hole doping[1]. In case of the graphene sheets prepared by UHV-CVD,
the Pos(G)-Pos(2D) relationship deviates from the linear one
and there are rough tendencies to shift the G peak position tolower energy and the 2D peak position to higher energy withincreasing exposure. These changes of the G and 2D peakpositions with exposure are attributed to the increase of the layer
3 ASRC

number. As a striking feature, it is found that the distribution ofPos(G) and Pos(2D) becomes extremely small in SLG and BLG
sheets (62 L and 1.1 × 105 L, respectively). These graphenesheets are presumed to be hole-doped judging from Pos(G) andPos(2D) which are rather close to the linear relationship in theexfoliated SLG sheet. It can be said that the uniformity of thedoping degrees in graphene can be improved remarkably byusing UHV-CVD under the well-controlled exposure conditions
instead of micromechanical exfoliation.
The present results demonstrate that the UHV-CVD method
enables a tailor-made growth of graphene which would benecessary for controls of spin transport properties including
realization of a long spin diffusion length in the graphene-based
spintronicdevices beyond the limits of exfoliated graphene.

Fig. 1 (upper) Change in the RHEED specular intensitiesand (lower) plot of the AES intensity ratio, IC/INi, as afunction of benzene exposure.

Fig. 2 Pos (2D) as a function of Pos (G). The symbols ■ , ● ,▲ , ▼ and ◆ denote the data obtained from the different
areas on the graphene sheets prepared at 25 L, 62 L, 5.6
× 104 L, 1.1 × 105 L exposures and on the exfoliated SLG
sheet, respectively.
References[1] S. Entani et al., J. Phys. Chem. C 122, 345 (2010).
[2] K. S. Novoselov et al., Proc. Natl. Acad. Sci. USA 102,10451 (2005).
[3] C. Oshima et al., J. Phys. Condens. Mater. 9, 1 (1997).
[4] S. Entani et al., J. Appl. Phys. 111, 064324 (2012).