Surface metallization on Si(001) at elevated temperatures studied by angle-resolved photoemission spectroscopy and near-edge x-ray absorption fine structure: Effect of thermal adatoms

We report the metallization of the $\text{Si}(001)2\ifmmode\times\else\texttimes\fi{}1$ surface at elevated temperatures using angle-resolved photoemission spectroscopy (ARPES) and near-edge x-ray absorption fine structure (NEXAFS). A metallic state $({S}_{m})$ over the ${E}_{F}$, which corresponds to the empty $({\ensuremath{\pi}}^{\ensuremath{\ast}})$ state of the $2\ifmmode\times\else\texttimes\fi{}1$ asymmetric dimer model, increases in the ARPES spectra, while the ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ state decreases in the NEXAFS spectra with increasing temperature. Since ${S}_{m}$ is observed even at 400 K, the structural phase transition at $\ensuremath{\sim}900\text{ }\text{K}$ [Phys. Rev. Lett. 91, 126103 (2003); Phys. Rev. Lett. 77, 3869 (1996)] is not related to the metallization. Thermal excitation seems to be too small to detect in ARPES in initial stage of the metallization and cannot account for the different behavior of ${S}_{m}$ and the filled surface state of the up-dimer upon oxidation. We suggest, based on the existence of ${S}_{m}$ even at 400 K and the oxidation behavior, that the metallization is attributed to thermal adatoms.