Chemical Vapor Deposition (CVD) is a process that is increasingly being used to prepare and produce the wonder material graphene. CVD has also been used to synthesize molybdenum disulphide, hexagonal boron nitride (hBN) and other 2D materials. It has also been reported that process temperatures can be considerably reduced through plasma enhanced CVD (PECVD).
In this article, thermal CVD is applied to produce hBN and graphene using nickel foils as catalyst. PECVD methods are also being used to grow vertically grown graphene and nano crystalline graphene (NCG). The variety of 2D nanostructures produced by this method demonstrates the versatility of CVD-based routes for synthesizing these materials.
Materials and Methods
In this analysis, the Nanofab 1000Agile™ from Oxford Instruments is used to synthesize hBN and graphene materials. This is a cold wall parallel plate plasma enhanced CVD (PECVD) tool developed for high-temperature processes (Figure 1).
Figure 1. Nanofab800Agile™ system for growth of carbon nanotubes (CNT).
Three kinds of morphologies were used to synthesize graphene:
- Vertically grown graphene on both metals and dielectrics through PECVD
- Nanocrystallinegraphene via PECVD on SiO2 substrate
- Thermal CVD of graphene on Nickel (Ni) foils
Additionally, thermal CVD on Ni foils was used to synthesize hBN.
CVD of Graphene on Nickel Foils
Using CH4 gas precursor with a H2 gas pretreatment, graphene was grown on Ni foils at 1000°C. An SEM image of a graphene grown on Ni foils is shown in Figure 2, revealing the film’s characteristic wrinkles indicated by arrows.
Figure 2. A SEM image of a graphene grown on Ni foils
Raman spectroscopy revealed that the process leads to the formation of a single graphene layer, which can be deduced from the lorentzian fit of the 2D peak at approximately 2700 cm-1. The low D peak at approximately 1350 cm-1 indicates low defect density in the synthesized graphene (Figure 3).
Figure 3. Raman spectroscopy revealed that the process resulted in the formation of a single layer of graphene.
PECVD Nanocrystalline Graphene and Vertical Graphene
Figure 4 shows a SEM image of NCG on SiO2 deposited through PECVD at above 650°C using CH4+H2 precursors; Figure 5 shows an image 150 mm-wafer with NCG film; and Figure 6 shows standard Raman spectra of as-deposited NCG on sapphire, quartz, and SiO2 with 532 nm laser excitation wavelength. All spectra respectively show D, G and 2D peaks at 1350, 1600, and 2690 cm-1.
Figure 4. A SEM image of NCG on SiO2 deposited via PECVD at temperatures above 650°C using CH4+H2 precursors.
Figure 5. Photo of 150mm wafer with NCG film. (British penny shown for size comparison)
Figure 6. Standard Raman spectra of as-deposited NCG on sapphire, quartz, and SiO2 with 532nm laser excitation wavelength. All spectra clearly show D, G and 2D peaks at 1350, 1600, and 2690cm-1, respectively.
Figure 7 shows a SEM image of vertically grown graphene that is deposited through PECVD at above 650°C using CH4+H2 precursor. Figure 8 shows HRTEM images of vertical graphene with edges of four graphene sheets, and Figure 9 shows an image of 200 mm wafer with vertical graphene.
Figure 7. A SEM image of vertically grown graphene deposited via PECVD
Figure 8. HRTEM images of vertical graphene with edges of 4 graphene sheets
Figure 9. Photograph of 200mm wafer with vertical graphene
CVD of Hexagonal Boron Nitride
B2H6 and NH3 gas precursor with a H2 gas pretreatment was used to grow hBN on Ni foils at 1100ºC.
Raman spectroscopy using 532 nm excitation showed that the process led to the formation of few layer hBN, which could be deduced from the typical E2g peak at ~ 1365 cm-1 (Figure 10). The absence of peaks from the carbonated phase (Bx Cx Nx), cubic phase (c-BN), and BN soot indicates that it is possible to regulate the process with minimal contamination (Figure 11).
Figure 10. Raman spectroscopy using 532 nm excitation
Figure 11. The absence of peaks from the cubic phase (c-BN), carbonated phase (Bx Cx Nx), and BN soot shows that the process can be well controlled with minimal contamination.
At about 1365 cm-1, the Raman intensity map was obtained from a 1 x 1 mm area of a hBN few layer film grown on Ni foil. The process produces an even hBN film (Figure 12).
Figure 12. Raman intensity map taken at ~1365cm-1
The Nanofab1000 Agile™ is an ideal tool for high-temperature PECVD and CVD processes. Options are available for both ALD operation and remote plasma through ICP. CVD process was used to grow graphene and PECVD was used to demonstrate vertical graphene and nanocrystalline graphene. Thermal CVD was also used to deposit hexagonal boron nitride.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
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