Unoccupied electron states of ultrathin quaterphenyl films on the surfaces of layered CdS and oxidized silicon

The results of a study of unoccupied electronic states and the formation of a boundary potential barrier during thermal vacuum deposition of ultrathin films of 4-quaterphenyl oligophenylene on the surface of CdS and on the surface of oxidized silicon are presented. Using X-ray photoelectron spectroscopy (XPS) it was determined, that the atomic concentrations of Cd and S were equal in the surface layer of a 75-nm-thick CdS film formed by atomic layer deposition (ALD). The electronic properties of 4-quaterphenyl films up to 8 nm thick were studied during their deposition on the surface of the CdS layer and on the surface of oxidized silicon using total current spectroscopy (TCS) in the energy range from 5 eV to 20 eV above $E_{\mathrm{F}}$. The energetic position of the main maxima of the fine structure of the total current spectra (FSTCS) of 4-quaterphenyl films was determined. The location of the maxima was reproducible when two selected substrate materials were used. A minor decrease in the work function, from 4.2 eV to 4.1 eV, was registered during the thermal deposition of 4-quaterphenyl onto the CdS surface. During the deposition of a 4-quaterphenyl film on the oxidized silicon surface, an increase in the work function from 4.2 eV to 4.5 eV was found. Possible mechanisms of the physicochemical interaction between the 4-quaterphenyl film and the surface of the investigated substrates, which lead to a difference in the observed values of the work function of the films on these substrates, are discussed.