22B06
Growth and structure control of graphene by plasma CVD
プラズマCVDによるグラフェン合成と構造制御
Toshiaki Kato and Rikizo Hatakeyama
加藤 俊顕, 畠山 力三
Department of Electronic Engineering, Tohoku University, Aoba 6-6-05, Aramaki-Aza, Aoba-Ku, Sendai 980-8579,
Japan
東北大学 大学院工学研究科 仙台市青葉区荒巻字青葉6-6-05
A transfer-free method for growing carrier-density controlled graphene directly on a SiO2 substrate has
been realized for the first time by plasma chemical vapor deposition (PCVD). Using this method,high-quality single-layer graphene sheets can be selectively grown between a Ni film and the SiO2substrate. Systematic investigations reveal that the relatively thin Ni layer and PCVD are critical to thesuccess of this unique method of graphene growth.

1. General
Graphene is a monolayer carbon sheet
including high carrier mobility, flexibility, andhigh optical transmittance [1,2]. These properties
are advantageous if graphene is to be used as acomponent in electrical devices such as field
effect transistors, solar cells, and various gas andchemical sensors. Chemical vapor deposition(CVD) is one of the most promising methods ofgrowing graphene, which can produce large,relatively high-quality graphene sheets. Agraphene layer on an insulating layer, especiallyon an insulating SiO2 substrate, is an important
structural arrangement used in the fabrication ofgraphene-based electrical devices. Currently,graphene layers on SiO2 substrates are fabricatedby the following process. First, graphene is grownon Ni, Cu, or Co surfaces. These surfaces arenecessary for the growth of graphene as they
decompose hydrocarbon gases and promote thenucleation of graphene. The graphene layer isthen transferred to a SiO2 substrate using polymercapping and chemical etching techniques. This
process can fabricate relatively large-scalegraphene sheets on an insulating substrate.
However, it is difficult to transfer specific andfine graphene structures, such as graphene
nanoribbons or nanoscale patterned graphene, tospecific points on a device. This precise
placement of graphene is important when
fabricating many widely used silicon-based
electrical devices. The direct growth of grapheneon the insulating substrate is an alternative
approach to the above method. It is a challenging
method, however, and only a few studies about
the direct growth approach have so far beenreported. Several critical issues relating to theyield, layer number control, large scale growth,and tunable carrier doping are still to be resolved
before the direct growth method will be suitableforindustrial
processes
togenerategraphene-based electronics.
Here we report a novel, simple, and scalable
method for the direct growth of graphene on aninsulating substrate with a tunable carrier doping.
It is revealed that by adjusting the growthparameters using plasma CVD (PCVD) [3-6], thegraphene layer can be made to grow along theinterface of the Ni layer and the SiO2 substrateinstead of on top of the Ni layer.
2. Experimental
A homemade plasma CVD system was used togrow single-layer graphene. Inductively coupledplasmas were generated by supplying a radio
frequency (RF; 13.56 MHz) signal to the coils,
which were positioned outside a quartz tube. An
electrical furnace was heated to a desired
temperature prior to moving a substrate fromoutside the quartz tube into its center. A 9:1 mixtureof methane and hydrogen gas was blown over thearea after the substrate reached a desiredtemperature, after which the RF power wasswitched on to generate the plasma. After thedesired growth time, the RF power supply wasswitched off and the substrate was cooled rapidly
by the ambient room temperature.
3. Results and Discussions
Scanning
electronmicroscope(SEM)
observations and Raman mapping measurements
are carried out for the produced material. It is foundthat hole-like structures are formed on the entire
surface of the Ni film (Fig. 1a). Figure 1b-d show
the optical microscope image (Fig. 1b) and theintegrated Raman intensity mappings of the Si-peak

abb
310 m

BA

Intensity [arb. units]

2 m2D/GSiA
B

D

12DG

1500

2000

2500

Raman shift [cm-1]

Fig. 1. (a) SEM and (b) optical microscope
images of the Ni surface after PCVD. (c,d)
The integrated Raman intensity mapping ofthe Ni surface after PCVD of the (c) Si-peakand (d) peak ratio of 2D to G. (e) The raw
Raman scattering spectra taken at theposition A and B shown in (c).

(Fig. 1c) and the peak ratio of 2D to G (Fig. 1d). It
should be noted that Fig. 1b-d are described at thesame scale. Since the strong Si-peak appears only at
the hole-like structures, the Ni film should bepartially evaporated and the SiO2 surface appears at
the hole-like structures (Fig. 1c). The G-peak and2D-peak are detected only at the hole-like structures,and the intensity of the 2D-peak is much higher
than that of the G-peak at the hole-like structures(Fig. 1d). This is also confirmed with the raw
Raman scattering spectra taken at the position Aand B, which are described at the hole-like structureand the top of the flat Ni film, respectively (Fig. 1e).
The spectrum of A in Fig. 1e shows the low D-peak,sharp G-peak, and high and sharp 2D-peak withhigher 2D/G intensity ratio (~3). These features are
consistent with those of high -quality single-layer
graphene sheet.
After removing the Ni film by the chemicaletching, interestingly, it is found that high-quality
graphene are also grown at the inter layer between
the Ni film and SiO2 substrate. Figure 2a-c are theoptical microscope image (Fig. 2a), the integrated
Raman intensity mapping of the 2D/G (Fig. 2b),251

42D/GcIntensity [arb. units]

4dce2 mIntensity ratio

2 m

3Intensity ratio

a0.2123451000 1000

1600
1600

2200
2200

Raman shift [cm-1]

2800
2800

Fig. 2. (a) The optical microscope and (b)integrated Raman intensity mapping for
2D/G of substrate after removing the Nifilm. (c) The raw Raman scattering spectraof graphene taken at the position 1-5 shownin (b).

and the raw Raman spectra (Fig. 2c) of the substrate
after the Ni etching. Although certain variation ofthe 2D/G intensity ratio is observed depending on
the position, high-quality single- or few-layer
graphene are grown on the entire substrate area.
4. Conclusion
We have realized the easy and scalable method
for the growth of high-quality graphene directly on
a SiO2 substrate. Based on the systematicinvestigations, it is revealed that the relatively thin
Ni layer with PCVD is critical elements for theselective growth of graphene at the inter layer
between the Ni film and SiO2 substrate.
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