Very large-bandgap insulating monolayers of ODS on SiC

In the present work we describe the electronic properties of octadecylsiloxane (ODS) adsorbed on 6H-SiC(0 0 0 1). A quantitative analysis of the C 1s region of the functionalized samples by X-ray photoelectron spectroscopy (XPS) revealed a surface coverage of about one monolayer. The highest occupied molecular orbital (HOMO) of the organic film is located about Eb = −5.3 eV below the Fermi level as determined by ultraviolet photoemission spectroscopy (UPS). Inverse photoemission (IPE) determined the energetic position of the lowest unoccupied molecular orbital (LUMO) about Eu = 3.7 eV above the Fermi level. Thus the HOMO–LUMO energy gap is determined to about 9 eV for the present system. A comparison between the respective electronic states of the substrate and the adsorbate revealed barrier heights for charge transport from the substrate into the adsorbate, 3.3 eV and 2.7 eV for electrons and holes, respectively. The time evolution of the collected IPE spectra indicates that the LUMO is mainly attributed to antibonding image bonds.