Surface phonon and valence band dispersions in graphite overlayers formed by solid-state graphitization of6H−SiC(0001)

Abstract

Solid-state graphitization of $\mathrm{Si}$-terminated $6\mathrm{H}\text{\ensuremath{-}}\mathrm{SiC}(0001)$ polar surfaces under thermal treatment in ultra-high-vacuum in the temperature region $500--1400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ has been investigated by Auger electron spectroscopy (AES), photoemission (PE) and high-resolution electron energy loss spectroscopy (HREELS). Up to annealing temperatures of $900--950\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ core level $\mathrm{C}1s$ and $\mathrm{Si}2p$ photoemission spectra and corresponding phonon modes in HREEL spectra show the structure characteristics for $\mathrm{SiC}$. Further heating up to temperatures above $1050\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ causes intense graphitization of the surface layer with the formation of a well-ordered surface graphite phase at temperatures of about $1350--1400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ with clearly distinguished characteristic graphite-derived phonon modes and a valence band electronic structure. Using angle-resolved HREELS and valence band photoemission the surface phonon mode and valence band dispersions of the formed graphite overlayer have been determined in the $\overline{\ensuremath{\Gamma}}\overline{M}$ direction over the whole energy range and the whole Brillouin zone. The graphitized layer shows four dispersive features (LA, LO, ZA, and ZO modes) and corresponding branches of valence band $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ states which both agree with those characteristic of the natural monocrystalline graphite surface; an analysis of the measured dispersions allows us to conclude that at least 2 monolayers of well-ordered epitaxial graphite on top of the system have formed.