IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Monday Sessions
       Session SS2-MoP

Paper SS2-MoP14
Photoemission from Graphite Overlayers on SiC(0001)

Monday, October 29, 2001, 5:30 pm, Room 134/135

Session: Surfaces and Interfaces Poster Session
Presenter: T. Balasubramanian, Lund University, Sweden
Authors: T. Balasubramanian, Lund University, Sweden
T. Kihlgren, Chalmers University of Technology, Sweden
L. Walldén, Chalmers University of Technology, Sweden
R. Yakimova, Linköping University, Sweden
Correspondent: Click to Email
For lack of large graphite crystals with well defined termination overlayers of the material prepared by heating SiC can provide an attractive alternative to natural crystals or to samples of highly oriented pyrographite, which is often used as substrate for adsorption and absorption studies. Using LEED, STM and UPS (20 to 140 eV photon energy) for sample characterization we find that heating (2200 K, 1/2 h) an off axes (8 degrees) 4H-SiC(0001) produces a well ordered, multilayer thick overlayer, which shows a 3-fold LEED pattern and gives quite well resolved photoemission spectra. We use the latter to monitor the dispersion of valence electrons, photoemission line widths and cross sections. Particularly narrow emission lines are observed for the Fermi level electrons near K-H in the Brillouin zone and for the upper @sigma@ state at the zone center. Of interest for adsorption studies is that the latter gives a line width on par with that of the C 1s core level.For an on axes 6H-SiC(0001) crystal a similar preparation procedure yields a 6@sr@3x6@sr@3 LEED pattern and photoemission spectra with no dispersion along the c-axes for the @pi@ state, which indicates that the overlayer in this case is a single graphene layer. Such a layer in vacuum should be a zero gap semiconductor with a band crossing at K defining the Fermi level. For the adsorbed monolayer we find that the bands do not cross but form a 0.2 eV gap centered 0.35 eV below the Fermi energy. The gap splits of a thin electron gas with a filled band width of 0.25 eV and an estimated surface density corresponding to 0.06 electrons per C atom.For both mono- and multilayers the photoemission cross sections are oscillatory with nearly out of phase variations for the @pi@ and @sigma@ states when plotted versus the kinetic electron energy and the curves shifted to around 5 eV higher energy for the monolayer.