Figure a,d shows the ratio of 2D to G peak intensities (I2D
) at two different locations for a sample that was grown under 23 sccm CH4
for 120 s. In each case, the I2D
ratio is in the range of 0.9 to 1.6 over 96% of the total 200 μm
200 μm area. This suggests that the BLG is grown over a larger percentage of area on polycrystalline Ni film [21
]. The I2D
plots over these locations are shown in Figure b,e and Figure c,f, respectively. These plots show a uniform intensity distribution for the G and D peaks, which further implies the graphene sample uniformity.
Figure 1 Two-dimensional Raman intensity map for bilayer graphene. (a) I2D/IG ratio (ratio of 2D to G peak intensities). (b) I2D (intensity of 2D peak). (c) IG (intensity of G peak). (d), (e) and (f) show I2D/IG, I2D and IG, respectively, for a different area. (more ...)
Next, the growth time was varied to study the effect on the number of layers, uniformity and defect density of the synthesized graphene. Figure a shows the Raman spectra of the samples that were treated under 23 sccm of CH4 for 50, 60 and 120 s. The I2D/IG ratio is close to unity with these varying growth times. This quenching method inhibits the precipitation of extra C on the Ni surface and thus controls the number of layers and the uniformity of the graphene for various growth times. Furthermore, after turning the CH4 off, if there is some residual C inside the furnace, the ultra-fast cooling suppresses its further segregation. Moreover, the effect of the CH4 flow rate was also studied for constant growth time. Figure b shows the Raman spectra of samples for which the growth time was 120 s with CH4 flow rates of 6, 12 and 23 sccm. It shows that the BLG growth is consistent for a wide range of flow rates.
Raman spectra for various growth conditions. (a) Increasing the growth time decreases the D peak intensity for 23 sccm of CH4. (b) BLG quality is uniform over wide CH4 flow rates for 120-s growth time.
Another important observation is that the intensity of the D peak decreases as the growth time increases from 50 to 120 s, as shown in Figure a. The ratio of D to G peak intensities (ID/IG ratio) was taken over 20 locations for the samples grown under 23 sccm of CH4 for 50, 60 and 120 s. The mean and standard deviation of ID/IG ratio are plotted in the error bar graph shown in Figure . The average ID/IG ratio for the sample grown under 23 sccm of CH4 for 120 s is 0.1 with a standard deviation of 0.05, which suggests a low defect density of BLG for these parameters. Moreover, Figure also shows that the average defect density decreases with the increasing growth time. Figure b also shows that the defect density of BLG is small for the samples grown under wide CH4 flow rates for 120 s.
ID/IGratio (ratio of D to G peak intensities). The average defect density decreases with increasing growth time.
We find that quenching the samples from the hot region of the furnace helps in reducing the non-equilibrium precipitation of extra carbon on the Ni surfaces during the cooling process, and that the main growth mechanism is diffusion of carbon on Ni surface due to the decomposed CH4. With fast cooling, the reduced sample temperature stops further segregation of carbon due to any residual carbon inside the furnace, even after CH4 flow was turned off. The thickness of the graphene is almost constant even with a wide range of CH4 flow rates (6 to 23 sccm), which shows that the segregation process is rather self-limiting. Furthermore, the growth temperature is high due to high decomposition temperature of CH4 that supports the uniform carbon diffusion over the Ni surface. This helps in growing uniform BLG with less defect density. Moreover, as the growth time is decreased, the average intensity of the D peak increases, which indicates incomplete growth. This further verifies the self-limiting equilibrium segregation of carbon on Ni surface, with reduced out-diffused carbon atoms from the C-Ni solution due to fast cooling. To verify the proposed growth mechanism, graphene was grown on 300 nm Ni film, with 23 sccm CH4 flow rated for 120 s, cooling the samples within the furnace. Due to slow cooling, the precipitation of carbon on Ni surface from the C-Ni solution is a dominant process.
Yet, another way to reduce the precipitation of extra carbon is to reduce the thickness of Ni film as less thick films would absorb less carbon and thus contribute to further decrease in out-diffused carbon. To characterize this effect, the growth was performed on 200 and 100 nm thick Ni films, with 23 sccm CH4 flow rate for 120 s. For the 200 nm Ni film, the I2D/IG ratio is close to unity, and the area uniformity is similar to the 300 nm thick films. However, growth on 100 nm Ni film results in increased surface roughness. Although the I2D/IG ratio is still around unity in this process, surface coverage is only 50%.