| AVS 58th Annual International Symposium and Exhibition | |
| Graphene and Related Materials Focus Topic | Monday Sessions |
| Session GR-MoM |
| Session: | Graphene Growth |
| Presenter: | D. Kurt Gaskill, U.S. Naval Research Laboratory |
| Authors: | V.D. Wheeler, U.S. Naval Research Laboratory G.G. Jernigan, U.S. Naval Research Laboratory N.Y. Garces, U.S. Naval Research Laboratory L.O. Nyakiti, U.S. Naval Research Laboratory R.L. Myers-Ward, 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) enables wafer-scale production needed to realize graphene-based technologies. Since monolayer graphene is all surface, any impurities or adsorbed atoms can alter the graphene by doping, acting as scattering sites which decrease the mobility, and creating additional resistances that degrade device performance. Also, impurities and resist residues can increase ohmic contact resistance and impact the ability to deposit uniform gate dielectrics. Achieving pristine graphene surfaces allows ultimate control over future interface formations and thus improves device performance. In this work, an in-situ H2 anneal is explored to attain the pristine epitaxial graphene required to fully realize the advantages of this material for future electronic applications.
EG was grown on semi-insulating, on-axis, (0001) 6H-SiC substrates in an Aixtron VP508 CVD reactor from 1575 to 1650°C for 60 to 180 min. Initially, samples were grown and cooled in a 100 mbar Ar ambient. Chemical analysis, using x-ray photoelectron spectroscopy (XPS), of the as-grown graphene surfaces showed the presence of both O and excess C impurities. These impurities can dope the EG and reduce the mobility, suggesting the need for methods to obtain pristine surfaces.
To address this need, several post-growth treatments were studied. Unlike exfoliated graphene1, uncontaminated EG surfaces could not be obtained using a 400 °C Ar/H2 anneal, and wet chemical cleans reduced the surface impurities. A pristine post-growth EG surface was only accomplished with a 1300 °C UHV anneal for 30 min.
Ex-situ UHV anneals are not practical for manufacturing graphene devices, so we explored a new technique incorporating a H2 anneal during cool down from growth temperature. First, EG samples were cooled in Ar from growth temperature to 1000°C. Next, Ar was evacuated and H2 introduced at constant pressure. Samples were annealed at 1000°C for 30 min., cooled in H2 to 700°C, and then the chamber was evacuated. XPS analysis showed that the EG samples have no O impurities and reduced excess C impurities compared to Ar cooled samples. Also, the signal from the interfacial layer2 is reduced, and both the graphene and SiC peaks are shifted to lower binding energies, suggesting a reduction in strain between the EG and the SiC substrate. These samples also had a more inert surface, showing only a small amount of adsorbed O (< 2%) after 3 days in atmosphere. Further, van der Pauw Hall measurements revealed a 2X increase in mobility over Ar cooled samples with no change in the sheet concentration magnitude.
1. Ishigami, et. al. Nano Letters 7(6) 1643 (2007)
2. Jernigan, et. al. Nano Letters 9(7) 2605 (2009)