Self-ordering of silicon polymer thin film grown on indium tin oxide surface investigated by X-ray absorption spectroscopy

The ordering and molecular orientation of polydimethylsilane (PDMS) films grown on indium tin oxide (ITO) surface has been assigned. The film has been synthesized by vacuum evaporation heated by electron bombardment. The orientation of the samples as well as that of the annealed sample was studied by Si K-edge near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy using linearly polarized synchrotron radiation. The Si K-edge NEXAFS spectra for the thick film have at least four resonance peaks of which two peaks are found to be strongly polarization dependent. The lower-energy resonance peak at 1842.0 eV has short-axis polarization corresponding to the resonance excitation from Si 1s → σ* p yz orbital of Si–C bond and the higher-energy resonance peak at 1843.2 eV has long-axis polarization due to the excitation of Si 1s → σ* p x orbital of Si–Si bond. On the basis of the polarization dependences, it is revealed that the backbones of the PDMS films are perpendicularly oriented to the ITO surface. The angle between surface and the Si–Si bonds for multilayered film calculated from the helical structure of PDMS molecule was found to be 41.5° which was almost coincided with the experimentally obtained angle of 40°. The polarization dependency disappeared for the annealed sample at about 80 °C which suggests that the PDMS was almost parallel and/or randomly oriented to the ITO surface after annealing.