| AVS 58th Annual International Symposium and Exhibition | |
| Graphene and Related Materials Focus Topic | Tuesday Sessions |
| Session GR-TuA |
| Session: | Graphene on Dielectrics, Graphene Transfer to Novel Substrates |
| Presenter: | Sara Rothwell, University of Minnesota |
| Authors: | S. Rothwell, University of Minnesota P.I. Cohen, University of Minnesota M. Kumar, National Physical Laboratory, India |
| Correspondent: | Click to Email |
Large area turbostratic graphene was grown on the (0001) plane of sapphire by thermal decomposition of acetylene. Sapphire is an attractive substrate since it has a symmetry match and close coincidence lattice match to graphene. It is a good insulator, appropriate for electronics applications, and large single crystal wafers are readily available. We have found that only after overcoming nucleation barriers, high quality graphene can be grown directly on sapphire without transfer. The sapphire was first heated to about 1400 C to obtain a reconstructed sqrt 31x31 R9 surface structure. The reconstruction was monitored in real time via reflection high energy electron diffraction, which was possible due to the low Debye-Waller factor. After obtaining a clean reconstructed surface, the sample was cooled to near room temperature and exposed to 10 Torr of acetylene. The sample was then heated to 1400 C in the presence of acetylene, in order to nucleate growth. At these pressures, sufficient acetylene coverage for growth was maintained during the ramp to high temperatures. Continued exposure to acetylene at 1400 C did not result in further growth. We speculate that at high temperature there is not sufficient residence time for incorporation, thus lower temperature is needed for further growth. Controlling pressure and temperature during a cool down phase becomes the fine control for film thickness. For example, 10 nm thick graphene samples were obtained by cooling in 1 – 7 E-7 Torr of acetylene. Transmission electron diffraction showed very sharp, nearly continuous rings, indicating large domains and no preferential azimuthal rotation between planes. X-ray diffraction showed an increased layer separation of 0.345 nm compared to graphite. Electron energy loss spectroscopy showed bulk-like plasmons or interband transitions, indicative of multilayer graphene. Raman spectra showed 2D/G peak intensity ratios of 0.5 to 1, comparable to literature values for turbostratic graphene. The spectra also exhibit the expected broader highly symmetric 2D peak. Thicker films could be easily lifted from the substrate. Films greater than 100 nm thick exhibited macroscopic ripples while 10 nm thick films were flat. Hydrogen was explored as a means to control growth but was found to rapidly etch graphene and to passivate the room temperature reconstructed sapphire surface. The growth was modeled with a simple rate equation analysis. These results offer a route to large area graphene grown directly on single crystal sapphire wafers.
Partially supported by the University of Minnesota IREE and by the National Physical Laboratory, India