Unoccupied electron states and the formation of interface between films of dimethyl-substituted thiophene–phenylene coolygomers and oxidized silicon surface

The unoccupied electron states and the boundary potential barrier during deposition of ultrathin films of dimethyl-substituted thiophene–phenylene coolygomers of the type of CH$_3$–phenylene–thiophene–thiophene–phenylene–CH$_3$ (CH$_3$–PTTP–CH$_3$) on an oxidized silicon surface have been studied. The electronic characteristics have been measured in the energy range from 5 to 20 eV above the Fermi level using total current spectroscopy (TCS). The structure of the CH$_3$–PTTP–CH$_3$ film surfaces has been studied by atomic force microscopy (AFM), and the atomic compositions of the films have been studied by X-ray photoelectron spectroscopy (XPS). The changes in the maximum intensities measured by the TCS method obtained from the deposited CH$_3$–PTTP–CH$_3$ film and from the substrate during increasing in the organic coating thickness to 6 nm is discussed. The formation of the boundary potential barrier in the $n$-Si/SiO$_2$/CH$_3$–PTTP–CH$_3$ is accompanied by the decrease in the surface work function from 4.2 $\pm$ 0.1 to 4.0 $\pm$ 0.1 eV as the organic coating thickness increases to 3 nm. The ratio of atomic concentrations C: S in the CH$_3$–PTTP–CH$_3$ films well corresponds to the chemical formula of CH$_3$–PTTP–CH$_3$ molecules. The roughness of the CH$_3$–PTTP–CH$_3$ coating surface was not higher than 10 nm on the $\sim$10 $\times$ 10 $\mu$m areas as the total CH$_3$–PTTP–CH$_3$-layer thickness was about 100 nm.