Turbostratic multilayer graphene synthesis on CVD graphene template toward improving electrical performance

Figure 1 shows a fabrication process of multilayer graphene devices. The pristine monolayer graphene as a growth template was synthesized by a CVD method using a Cu foil via the same process as previously reported.¹⁸⁾ The monolayer graphene was transferred from the Cu foil onto a fused quarts substrate¹⁵⁾ as shown in Fig. 1(a). Subsequently, the growth of graphene layers was conducted by a CVD method using the infrared radiation furnace. A typical graphene growth procedure is as follows: (1) sample was introduced into a furnace quartz tube^19,20) and preheated at 600 °C for 5 min in air under atmospheric pressure to remove residual resist films used in the transferring process. (2) Sample was introduced into the infrared radiation furnace and heated up to 1300 °C for 4 min. (3) The growth of graphene on the monolayer graphene template was carried out by introducing the mixture of ethanol vapor as a carbon feedstock and Ar as a carrier gas under total pressure of 590 Pa. The flow rates of ethanol vapor and Ar gas were 0.5 and 200 standard cubic centimeters per minute, respectively. Figure 1(b) shows an optical microscope image of graphene grains after the growth. We can see a change in the contrast of the graphene grains between before and after growth in the optical images, indicating that the film thickness of the graphene grains becomes to be thicker by the thermal process. Finally, the graphene device for Hall-effect measurement was fabricated by a conventional photolithographic technique. Au (40 nm)/Ni (10 nm) as the electrode was deposited onto the graphene film. Subsequently, the unnecessary part of graphene for van der Pauw geometry was removed by plasma etching to form the square pattern as shown in Fig. 1(c).

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To determine the number of grown graphene layers, Raman spectroscopy was carried out at room temperature using a laser excitation of 532 nm with a 100× objective lens (typical spot diameter: ∼1 μm). The laser power was kept at a constant value of 0.28 mW, so that the heating effects on the frequencies of the Raman signal can be ignored. The details of the evaluation method of the layer numbers from the Raman spectra are described in appendix. The surface morphology on the grown graphene layers was evaluated by atomic force microscope (AFM) observations using HITACHI AFM5100N with the dynamic force mode. Hall-effect measurements were performed under the current of 10 μA and the magnetic field of 3200 G. It should be noted that the samples tested in Raman, AFM and Hall-effect measurements are different.

Figure 2 shows the Raman spectra of grown graphene layers prepared by different growth time. The spectra are normalized by a peak intensity (I_sub.(obs.)) from the fused quartz substrate (∼460 cm⁻¹) as indicated by the arrow in Fig. 2.²¹⁾ Three characteristic peaks can be observed in the Raman spectra: (1) the G-band (∼1580 cm⁻¹) is caused by the double degenerate phonon mode at the Brillouin zone center. (2) The D-band (∼1350 cm⁻¹) mode originates from the sp²-hybridized disordered carbon materials.²²⁾ (3) The two-dimensional (2D)-band is the overtone of the D-band, which is used to determine the number of layers and the stacking structures in few-layer graphene sheets.^23–25) The number of grown graphene layers efficiently increases to 2, 6 and 11 layers by the thermal treatment using ethanol vapor as shown in Fig. 2(a). The intensity ratio (I_D/I_G) of the D-band and G-band peaks, and the full width at half maximum (FWHM; Γ_D) of the D-band are evaluated to be ∼0.36 and ∼ 49 cm⁻¹ for the 6-layer graphene. The grain size evaluated from the I_D/I_G is about 60 nm.²⁵⁾ The values of the I_D/I_G and Γ_D are smaller than those values (I_D/I_G = ∼0.75, Γ_D = ∼201 cm⁻¹) observed from the multilayer graphene (4 layers) grown by a relatively low temperature (720 °C), as previously reported.¹⁷⁾ Since these values reflect the crystallinity of graphene, this indicates that the higher process temperature is effective to improve the crystallinity of grown graphene layers.

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The stacking structure in multilayer graphene can be divided into two types: turbostratic and order stacking (e.g. AB stacking). The turbostratic ratios (R') of the multilayer graphene were evaluated by the peak fitting analysis in the 2D-band region.²⁶⁾ The peak shape in the 2D-band was fitted by three Lorenz curves with the center positions at 2680, 2700 and 2720 cm⁻¹, as noted by I_G'3DA, I_G'2D and I_G'3DB. The turbostratic ratio (R') can be given by

$$\begin{eqnarray}&&{R}^{\prime} =1-{R}=1-\displaystyle \frac{{{I}}_{{\rm{G}}^{\prime} 3{\rm{DB}}}}{{{I}}_{{\rm{G}}^{\prime} 3{\rm{DB}}}+{{I}}_{{\rm{G}}^{\prime} 2{\rm{DA}}}},\end{eqnarray} \tag{ 1 }$$

where the R means the order stacking ratio. The ratios of turbostratic stacking in grown multilayer graphene with 2, 6 and 11 layers are about 92%, 80% and 69%, which decrease with increasing the number of layers. Since the growth time increases with increasing the number of layers, it is likely that the change in the ratio of turbostratic stacking was affected by thermal annealing time.

Figures 3(a)–3(c) show the AFM images observed from the grown graphene surface with different film thicknesses. Figures 3(e) and 3(f) are the height profiles along L–L', M–M' and N–N' lines in each surface and the root mean square (RMS) roughness as a function of the number of layers. When the number of layers is less than 5 layers, the RMS roughness roughly corresponds to the single graphene step height (∼0.34 nm), indicating that the grown graphene forms atomically flat surfaces. Here we define the growth with the low RMS roughness less than 5 layers as an initial stage. Figure 3(d) shows an enlarged AFM image of the grown graphene islands at the initial growth stage. The graphene islands form the flat terrace structure with the single step height as indicated by the height profile along the P–P' line in Fig. 3(g). This means that the step edges of graphene islands after the nucleation proceed by the lateral growth mode [Fig. 3(h)]. The typical diameter of the 2D islands exceeds 100 nm, which is larger than the grain size (∼60 nm) evaluated from the Raman spectra. This is probably because the I_D used for the evaluation of the grain size also reflects the small defects in the grown graphene islands and the domain boundaries formed by the coalescence among islands.

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After the further growth, the three-dimensional (3D) islands with extremely high height are observed as indicated by the arrows in Fig. 3(c). The center of every island tends to contain the defective structure with large roughness as compared with the flat terrace site formed by the lateral growth. The trend is enhanced by increasing the number of layers. Since the defective sites with many dangling bonds promote the incorporation of carbon species, the 3D-islands are formed by the enhancement of the local growth rate [Fig. 3(i)]. As a result, the surface roughness increases with increasing the number of layers [Fig. 3(f)].

Figure 4 shows the dependence of the number of layers on the sheet resistance (R_s) of grown multilayer graphene measured by van der Pauw geometry. The R_s decreases with increasing the number of layers. This result means that the current flows in the grown graphene layers as the additional channels. Note that the change in the R_s is almost linear with respect to the number of layers. This indicates that the R_s of every layer in the turbostratic structure is similar to that of the pristine monolayer graphene used as the growth template. We observed that the carrier mobility (∼1280 cm² V⁻¹ s⁻¹) in the grown graphene with 2 layers is higher than that obtained from the pristine monolayer graphene (∼940 cm² V⁻¹ s⁻¹) at room temperature. It has been reported that the carrier mobility in the few-layer graphene with the order stacking structure degrades as compared with that in monolayer graphene,⁶⁾ which is oppositely different from our grown graphene with 2 layers. The improvement of the carrier mobility is caused by the turbostratic stacking effect that kept the linear dispersion. Unfortunately, the carrier mobility obtained from the grown graphene with 6 layers showed the lower value (∼520 cm² V⁻¹ s⁻¹) compared with that from the pristine graphene. This suggests that the carrier mobility decreases as the defect increases as indicated by the Raman spectrum [Fig. 2(a)]. From these results, we believe that the multilayer graphene grown on the graphene template is effective to improve the carrier transport properties in the graphene sheet.

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