Vol.9 No.3 2017
12/74

Research paper : High quality and large-area graphene synthesis with a high growth rate using plasma-enhanced CVD (M. Hasegawa et al.)−125−Synthesiology - English edition Vol.9 No.3 (2017) In this study, plasma is used, and by establishing plasma cleaning of the surface of the copper substrate, cleaning and synthesis can be carried out continuously in situ. For this reason, it is possible to prevent the recontamination of the substrate. In many processes using plasma, an inert gas is added to the discharge gas in order to illuminate stably, and argon is commonly used as an inexpensive inert gas. On the other hand, from the point of view of sputtering that causes release of impurities from the reaction chamber, it is desirable to use lighter inert gas.[19]–[21] Thus, we attempted plasma CVD synthesis of graphene using helium, which is expected to be effective for preventing impurity incorporation because of it being the lightest inert gas.Figure 1 shows a schematic illustration of the surface-wave microwave plasma CVD equipment.[13] The waveguide for microwave propagation is connected to the reaction chamber. The waveguide is equipped with a slot antenna that emits the microwaves into the reaction chamber through a quartz window. In the case of surface-wave microwave plasma, high-density plasma is excited along the surface of the quartz window. High-density plasma which exceeds the cutoff density of 7.4 × 1010 cm-3 is excited along the surface of the quartz window by 2.45 GHz microwaves.[22]–[27] Microwaves cannot penetrate through the high density plasma, and the copper foil substrate is not exposed to the microwaves directly. Therefore, temperature control of the substrate is easy over a wide range from low-temperature to high-temperature.[10][11][14]A tough-pitch copper foil (purity: 99.7 %) with a thickness of 33 μm was used for graphene synthesis. We compared plasma pretreatment of copper foil using two kinds of gas mixtures, Fig. 1 Schematic illustration of the surface-wave microwave plasma CVD equipment[13] Copyright (2014) The Japan Society of Applied Physics Ar/H2 and He/H2. The copper foil substrate was placed 50 mm away from the quartz window and the substrate temperature was kept in the range of 350–400 °C. The duration of pretreatment was 1 min. The cleaning effect for the copper foil surface by the plasma pretreatment was confirmed by X-ray photoelectron spectroscopy (XPS) (ULVAC Phi ESCA 5800X, AlKα). XPS measurement was performed ex situ after the cleaning. Quantitative evaluation was considered difcult owing to the oxidation of the copper foil surface by air exposure after cleaning. Therefore, we conducted XPS measurement under the same ex situ condition for all samples and compared the results in terms of the changes in the spectra. Subsequent to the plasma pretreatment of the copper foil substrate in the reaction chamber, synthesis of graphene by plasma CVD was performed using two kinds of gas mixtures, He/H2/CH4 and Ar/H2/CH4. The synthesis time was 20 min. The synthesized graphene lms were evaluated by Raman scattering spectroscopy (HORIBA XploRa, spot size 1 μm and wavelength 638 nm), energy-dispersive X-ray spectroscopy (EDS) (JEOL-2100F with EDS detector JED-2300F; acceleration voltage, 200 kV), and XPS. For cross-sectional transmission electron microscope (TEM) observation, an amorphous carbon thin film was deposited on an as-deposited graphene surface in order to make the sample structure robust for gallium focus ion beam etching. TEM observation was performed at an acceleration voltage of 300 keV. Figure 2 shows the XPS survey spectrum of the as-received tough-pitch copper foil substrate. In this spectrum, the peaks corresponding to Cu 3d, 3p, 3s, 2p, and 2s and Cu Auger were observed.[28] The peaks of C 1s and O 1s were also observed together with the low-intensity peaks of Si 2p and N 1s. It was suggested that these were because organic silicon and hydrocarbon compounds containing nitrogen were coated on the surface of the as-received tough-pitch rolled copper foil. In this study, we examined the removal of these impurities from the copper foil substrate surface by the plasma pretreatment. In Fig. 3, we compared the XPS high-resolution spectra from the copper foil substrate before and after the plasma pretreatment. First, the removal of copper oxide on the copper foil surface by plasma pretreatment was examined using O 1s signals. O 1s binding energy spectra are shown in Fig. 3(a). In the case of Ar/H2 plasma pretreatment, the O 1s peak was still observed, which indicates that oxygen was not removed efciently by this plasma treatment. Moreover, the peaks were separated more clearly, which suggests that a certain amount of oxygen was newly formed during the Ar/H2 plasma treatment. In contrast, the O 1s peak disappeared with the He/H2 plasma pretreatment, which indicates that the oxygen on the surface was removed efciently by this plasma Surface wave plasmaQuartz windowSubstrateSlot antennaMWWaveguideVacuum pumpReaction gasStage

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