K-resolved inverse photoemission of four different 6H-SiC (0001) surfaces

Abstract

We have investigated the unoccupied electronic states of the Si-terminated (0001) surface of hexagonal 6H-SiC. The main problem with these surfaces is the reliable preparation of well defined surface reconstructions. We give reproducible methods to prepare the $(1\ifmmode\times\else\texttimes\fi{}1),$ $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{},$ $(3\ifmmode\times\else\texttimes\fi{}3),$ and the $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ surface by controlled heating of the SiC sample in a Si flux. These surface reconstructions show a characteristic LEED pattern and a characteristic Si/C peak ratio in Auger electron spectroscopy. We present k-resolved inverse photoemission spectra for the $(1\ifmmode\times\else\texttimes\fi{}1),$ $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{},$ and $(3\ifmmode\times\else\texttimes\fi{}3)$ surface. We compare the measured dispersion relations with ab initio local density approximation surface band structure calculations of the $(1\ifmmode\times\else\texttimes\fi{}1)$- and the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$-reconstructed 6H-SiC(0001) surface and with a Mott-Hubbard model of the electronic ground state of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ and $(3\ifmmode\times\else\texttimes\fi{}3)$ reconstruction. The comparison between experiment and theory supports the Hubbard model: The experiment determines a value of $U=2.0\mathrm{eV}$ for the Mott-Hubbard Coulomb interaction parameter for the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction and $U=1.25\mathrm{eV}$ for the $(3\ifmmode\times\else\texttimes\fi{}3)$-reconstructed surface, respectively.