Temperature dependence of the electronic structure ofC60films adsorbed onSi(001)−(2×1)andSi(111)−(7×7)surfaces

Abstract

We report here the temperature-dependent measurements of the valence spectra, the C 1s and the Si 2p core level spectra of the one monolayer ${\mathrm{C}}_{60}$ film adsorbed on Si(001)-$(2\ifmmode\times\else\texttimes\fi{}1)$ and Si(111)-$(7\ifmmode\times\else\texttimes\fi{}7)$ surfaces, using photoelectron spectroscopy. At 300 K, most ${\mathrm{C}}_{60}$ molecules are physisorbed with the coexistence of minority chemisorbed species on both Si(001)-$(2\ifmmode\times\else\texttimes\fi{}1)$ and Si(111)-$(7\ifmmode\times\else\texttimes\fi{}7)$ surfaces. After annealing the samples at 670 K, ${\mathrm{C}}_{60}$ molecules change the bonding nature to a chemisorption that has both covalent and ionic characters. The covalent bonding orbital is observed at a binding energy of 2.10 eV on both Si surfaces. The amount of charge transfer is estimated to be 0.19 electrons per ${\mathrm{C}}_{60}$ molecule on the Si(001) surface, and to be 0.21 electrons per molecule on the Si(111) surface. We consider the origin of the change in bonding nature to the different distance between two dangling bonds that results from the rearrangement of the surface Si atoms. After annealing at 1070 K, ${\mathrm{C}}_{60}$ molecules decompose and the SiC formation takes progress at the interface. On the Si(001) surface, the molecular orbitals (MO's) disappear at 1120 K and the binding energies of peaks observed in the valence spectra indicate the formation of SiC islands at this temperature. On the Si(111) surface, the disappearance of MO's and the formation of SiC islands are verified at 1170 K. The difference in formation temperature is attributed to the different surface structure.