Unoccupied Electron States and the Formation of Interface between Films of Dimethyl-Substituted Thiophene–Phenylene Coolygomers and Oxidized Silicon Surface

Abstract

The unoccupied electron states and the boundary potential barrier during deposition of ultrathin films of dimethyl-substituted thiophene–phenylene coolygomers of the type of CH₃–phenylene–thiophene–thiophene–phenylene–CH₃ (CH₃–PTTP–CH₃) 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 CH₃–PTTP–CH₃ 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 CH₃–PTTP–CH₃ 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/SiO₂/CH₃–PTTP–CH₃ is accompanied by the decrease in the surface work function from 4.2 ± 0.1 to 4.0 ± 0.1 eV as the organic coating thickness increases to 3 nm. The ratio of atomic concentrations C: S in the CH₃–PTTP–CH₃ films well corresponds to the chemical formula of CH₃–PTTP–CH₃ molecules. The roughness of the CH₃–PTTP–CH₃ coating surface was not higher than 10 nm on the ~10 × 10 μm areas as the total CH₃–PTTP–CH₃-layer thickness was about 100 nm.