Graphitic carbon growth on crystalline and amorphous oxide substrates using molecular beam epitaxy
© Jerng et al; licensee Springer. 2011
Received: 26 July 2011
Accepted: 26 October 2011
Published: 26 October 2011
We report graphitic carbon growth on crystalline and amorphous oxide substrates by using carbon molecular beam epitaxy. The films are characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The formations of nanocrystalline graphite are observed on silicon dioxide and glass, while mainly sp 2 amorphous carbons are formed on strontium titanate and yttria-stabilized zirconia. Interestingly, flat carbon layers with high degree of graphitization are formed even on amorphous oxides. Our results provide a progress toward direct graphene growth on oxide materials.
PACS: 81.05.uf; 81.15.Hi; 78.30.Ly.
Keywordsgraphite molecular beam epitaxy Raman oxide
Graphene growth on Ni or Cu by chemical vapor deposition [CVD] is now well established. However, the CVD graphene needs to be transferred onto insulating substrates for application, which may degrade the quality and bring complications to the manufacturing process. This is why direct graphene growth on insulator is still intensively being studied. Notably, the growth on oxide is of great interest because graphene is expected to face current metal-oxide semiconductor [MOS] technology through an oxide layer. Recent studies have shown some accomplishments toward this goal by using CVD [1–3].
Here, we attempt molecular beam epitaxy [MBE] of carbon onto several oxide substrates to figure out the potential of graphene growth. So far, carbon MBE has been applied mostly on group IV semiconductors [4–7], where graphitic carbon growth was observed. We have shown previously that nanocrystalline graphite [NCG] can be formed on sapphire (Al2O3) and observed a Dirac-like peak for the first time in MBE-grown NCGs . In this study, we expand the subject to include various crystalline and amorphous oxides. We observe that graphitic carbon or NCG can be grown by carbon MBE on amorphous SiO2, the most important oxide in the MOS technology. We also obtain similar results on glass (Eagle 2000™, Corning Inc., Corning, NY, USA). In contrast, carbons on amorphous TiO2 or Ta2O5 do not seem to form graphitic structures. Among the crystalline oxides, mainly sp 2 amorphous carbons are observed on SrTiO3(100) and yttria-stabilized zirconia [YSZ] (100).
Materials and film fabrication
Samples were fabricated in a home-made ultra-high-vacuum MBE system. Carbons were sublimated from a heated pyrolytic graphite filament. The pressure of the chamber was kept below 1.0 × 10−7 Torr during the growth with the help of liquid nitrogen flowing in the shroud. Details about the growth procedure can be found elsewhere . Both crystalline and amorphous oxide substrates were purchased from commercial vendors (AMS Korea, Inc., Sungnam, Gyeonggi-do, South Korea; INOSTEK Inc., Ansan-si, Gyeonggi-do, South Korea). The growth temperature (T G) was in the range of 900°C to approximately 1,000°C, based on our previous study with sapphire. The typical thickness of carbon film, determined by measuring the step height after lithography, was 3 to approximately 5 nm.
Raman-scattering measurements were performed by using a McPherson model 207 monochromator with a 488-nm (2.54 eV) laser excitation source. The spectra recorded with a nitrogen-cooled charge-coupled device array detector. X-ray photoelectron spectroscopy [XPS] measurements to analyze carbon bonding characteristics were done by using a Kratos X-ray photoelectron spectrometer with Mg Kα X-ray source. C1s spectra were acquired at 150 W X-ray power with a pass energy of 20 eV and a resolution step of 0.1 eV. Atomic force microscopy [AFM] images were taken by a commercial system (NanoFocus Inc., Seoul, South Korea) in a non-contact mode.
Results and discussion
Raman-scattering measurements have become a powerful, non-destructive tool in the study of sp 2 carbons (carbon nanotube, graphene, and graphite). The well-known G peak is observed in all sp 2 systems near 1,600 cm-1. With the advent of graphene, the so-called 2D peak, which occurs near 2,700 cm-1, has become important. Single-layer graphene is characterized by the sharp and large 2D peak. This 2D peak is actually the second order of D peak. The typical position of D peak is 1,350 cm−1, one half of the 2D peak position. The D peak is absent in a perfect graphene sheet or graphite because of symmetry and increases as defects or disorders in the honeycomb structure increases. However, it should be noted that the D peak also disappears in amorphous carbon. That is, Raman D peak does indicate the presence of sixfold aromatic rings as well as sp 2 bonds. It is from A1g symmetry phonons in which the D peak becomes Raman active by structural disorders in the graphene structure.
Fitting results of the Raman spectra for various samples
Peak (D) (cm−1)
Peak (G) (cm−1)
I D /I G
I 2D /I G
FWHM (G) (cm−1)
FWHM (2D) (cm−1)
The crystalline ordering is worse than that of graphitic carbon grown at the same T G on a sapphire crystal, where a 2D peak is easily identified . In the previous study, we observed that the crystal orientations of sapphire substrates did not affect the quality of NCG grown on them and speculated that the lattice constants and the substrate symmetry were not critical parameters in the NCG growth by MBE . Then, we expect similar growth on cubic SrTiO3 and YSZ, contrary to what we observe. One possible explanation is that the optimum T G depends on the material. In fact, the Raman spectra in Figure 1 are similar to those of NCG on sapphire grown at 600°C, far lower than the optimum T G of 1,100°C . Because of the difference in the sticking coefficient of carbon to the substrate and/or the diffusion constant of carbon on the surface, the optimum growth temperature may depend on the substrate. Further experiments of carbon growth on SrTiO3 or YSZ at different temperatures might prove this assumption.
Now that the carbon films grown on SiO2 and glass by MBE are identified as NCGs, it is informative to calculate the crystallite size from Ferrari and Robertson's model applied to stage 2 . According to the model, the average size L a is related to I D /I G as I D /I G = C L a 2, where C = 0.0055 and L a in Å. From I D /I G = 1.8~1.9 (Table 1), we get L a = 18.1~18.6 Å. In addition, the position of G peak at 1,598 cm−1 is in accordance with the identification of NCG of insignificant doping .
In summary, we have grown graphitic carbon on crystalline and amorphous oxides by using carbon MBE. Notably, the graphitic carbons on amorphous SiO2 and on glass show a relatively high degree of graphitization, evidenced by well-developed D, G, and 2D Raman peaks. The C1s spectra from XPS measurements confirm the dominance of sp 2 carbon bonding. In addition, the surfaces are almost as flat as the substrates, which may play an important role in the integration with the existing technology.
atomic force microscopy
chemical vapor deposition
molecular beam epitaxy
X-ray photoelectron spectroscopy
This research was supported by the Priority Research Centers Program (2011-0018395), the Basic Science Research Program (2011-0026292), and the Center for Topological Matter in POSTECH (2011-0030046) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST). This work was also supported in part by the General R/D Program of the Daegu Gyeongbuk Institute of Science and Technology (DGIST) (Convergence Technology with New Renewable Energy and Intelligent Robot).
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