| AVS 56th International Symposium & Exhibition | |
| Graphene Topical Conference | Tuesday Sessions |
| Session GR+TF-TuA |
| Session: | Graphene: Characterization, Properties, and Applications |
| Presenter: | J.L. Tedesco, U.S. Naval Research Laboratory |
| Authors: | J.L. Tedesco, U.S. Naval Research Laboratory R.L. Myers-Ward, U.S. Naval Research Laboratory G.G. Jernigan, U.S. Naval Research Laboratory J.A. Robinson, The Pennsylvania State University J.C. Culbertson, U.S. Naval Research Laboratory P.M. Campbell, U.S. Naval Research Laboratory J.K. Hite, U.S. Naval Research Laboratory C.R. Eddy, Jr., U.S. Naval Research Laboratory D.K. Gaskill, U.S. Naval Research Laboratory |
| Correspondent: | Click to Email |
Epitaxial graphene (EG) grown in vacuo on semi-insulating, on-axis 4H- and 6H-SiC substrates has demonstrated morphological problems [1], such as pitting in C-face substrates during in vacuo growth. In order to reduce the nonuniformities present in the EG, films were grown in an Ar ambient and compared to in vacuo films using substrates sourced from the same wafers. The growths took place in a commercial Aixtron VP508 hot-wall epitaxial SiC growth reactor. The Ar growths were performed using temperatures ranging from 1,500°C to 1,600°C and pressures ranging from 50 to 200 mbar. The in vacuo growths took place for temperatures ranging from 1,225°C to 1,700°C and pressures ranging from 10-3 to 10-6 mbar. The resulting films were characterized with Nomarski and atomic force microscopies, Raman and x-ray spectroscopies, and Hall effect measurements.
Si-face EG films did not form under Ar at temperatures ≤ 1,500°C, yet in vacuo films formed for temperatures above ~1,400°C. At temperatures > 1,500°C under Ar, graphene was successfully grown and, while the morphology was not significantly different from in vacuo films, Raman spectroscopy measurements indicated that EG films with improved thickness and strain uniformity were realized. At an Ar growth pressure of 50 mbar, resulting Si-face EG was mostly bilayer graphene, while at 200 mbar, the resulting EG films were predominantly monolayer graphene. X-ray photoelectron spectroscopy measurements indicated that argon-grown EG was thinner than in vacuo EG grown at the same temperature.
The morphology of argon-grown C-face films was markedly different from in vacuo films. Under Ar, graphene did not grow below 1,500°C, while in vacuo films began to grow at 1,225°C. For growths at temperatures > 1,500°C, the density of substrate pits was reduced significantly and was eliminated in half of the argon-grown films. Furthermore, at temperatures between 1,500°C and 1,600°C, ambient-controlled films were incomplete, and the percentage of the substrate covered by graphene increased with growth temperature.
Growth under Ar improved the electrical properties for films on both faces. Mobilities increased by up to 4 times relative to in vacuo films, while carrier densities decreased by ~10 times. For both in vacuo and argon-grown EG, the carrier densities of Si-face EG were at least an order of magnitude less than those of C-face EG films. Furthermore, Hall effect measurements showed that, while in vacuo EG films on both faces were divided between n-type and p-type, argon-grown Si-face EG was generally n-type while argon-grown C-face EG was generally p-type.
[1] B.L. VanMil, et al., Mater. Sci. Forum 615-617, 211 (2009).