AVS 59th Annual International Symposium and Exhibition
    Graphene and Related Materials Focus Topic Monday Sessions
       Session GR+EM+NS+PS+SS+TF-MoM

Paper GR+EM+NS+PS+SS+TF-MoM3
Impact of Growth Parameters on Uniformity of Epitaxial Graphene

Monday, October 29, 2012, 9:00 am, Room 13

Session: Graphene Growth
Presenter: L.O. Nyakiti, U.S. Naval Research Laboratory
Authors: L.O. Nyakiti, U.S. Naval Research Laboratory
V.D. Wheeler, U.S. Naval Research Laboratory
R.L. Myers-Ward, U.S. Naval Research Laboratory
J.C. Culbertson, U.S. Naval Research Laboratory
A. Nath, George Mason University
N.Y. Garces, U.S. Naval Research Laboratory
J. Howe, Oak Ridge National 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) offers a facile method for attaining large area graphene for device applications. Since wafer uniformity and thickness control is vital, a systematic study of the parameters affecting the EG growth process was performed and the optimal conditions for obtaining uniform morphology and high electronic quality were determined. EG was synthesized in a low pressure Ar flowing ambient on 8x8mm2 6H-SiC(0001) substrates that were offcut 0.8o from the basal plane, using an Aixtron VP508 reactor. The samples were placed on a rotating ~100 mm diameter susceptor and excellent EG layer uniformity and run-to-run reproducibility were obtained. The investigation focused upon the critical synthesis parameters of temperature (T) (1520-1660oC) and time (t) (15-60 min), an in-situ H2 etch conditions (1520-1600oC for 10-30min). Morphology, layer thickness, chemical analysis, and strain variations across the samples were characterized using electron microscopy, AFM, XPS and µ-Raman spectroscopy. Large-area van der Pauw Hall effect was performed to quantify the graphene mobility (µ), and carrier density. Results show that growth T and t had the most significant impact on EG electronic and morphological properties. For example, synthesis at 1660oC for 30min resulted in 4-8 monolayers (ML) and a step-bunched morphology with high concentration of wrinkles originating from the step-edge and pinned at the nearest terrace edge. Other morphological features were pits primarily located at the step edges having a depth ~20nm and density 6.4x106 cm-2. In contrast, EG synthesis at 1520oC for 30min results in uniform ML coverage along the terrace width that is devoid of pits and wrinkles. Mobility was found to have a drastic dependence on graphene thickness. Under optimal conditions, 1-2 ML were obtained and µ as high as 1240 cm2V-1s-1 was achieved; in contrast, for EG with >2 ML µ ~ 550 cm2V-1s-1, presumably due to interlayer interaction and electronic screening. XPS C1s and Raman 2D spectra of EG grown on substrates after undergoing in-situ H2 etch at different times did not show shifts in peak position/intensity suggesting lack of etch time dependence on EG electronic or structural quality. Yet etch conditions affect the final morphology, as EG synthesis performed after an in-situ H2 etch at 1600oC resulted in step-bunched morphology with step heights 5-10nm, whereas, substrates etched at 1520oC had EG with step-heights 10-15nm. In addition other growth parameters investigated were found to be of secondary importance, including: Ar pressure, flow rates, and sample cool down conditions.