Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (E F) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO$_2$)Ge). The concentration of oxide on the (GeO$_2$)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6–31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO$_2$)Ge surface to 6 nm was studied. At energies below 9 eV above $E_{\mathrm{F}}$, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by $\pi^*$ molecular orbitals. The higher density maxima of unoccupied states are of $\sigma^*$ nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO$_2$)Ge surface is accompanied by an increase in the work function of the surface, $E_{\operatorname{vac}}$–$E_{\mathrm{F}}$, from 4.6 $\pm$ 0.1 to 4.9 $\pm$ 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.