Field of Expertise: Advanced Material Science

Initial steps of rubicene film growth on silicon dioxide
Boris Scherwitzl
Institute of Solid State Physics
15:20 - 17:30 Thursday 24 October 2013 Foyer Alte Technik

Studies of the initial stages of organic film growth are of great relevance for the use of organic materials in microelectronic devices, which became of increasing importance in a variety of applications. This contribution describes the behavior of rubicene (C₂₆H₁₄) film growth on silicon dioxide with respect to sample treatment under UHV conditions. Physical vapor deposition from a Knudsen cell was used to create films with varying thickness, from sub-monolayer up to multilayer range. A number of different analytical methods were used to examine film and substrate properties, namely Auger Electron Spectroscopy (AES) for chemical analysis, Thermal Desorption Spectroscopy (TDS) to determine desorption behavior and Atomic Force Microscopy (AFM) for a subsequent ex-situ analysis of the sample surface. It could be shown that the initial film formation of rubicene molecules on SiO2 exhibits a quite unusual behavior. First a monolayer of flat lying molecules is formed, followed by a second layer. Further deposition of rubicene leads to a destabilization of this bilayer and to a dewetting process with a subsequent formation of bulk-like 3D islands. Different desorption energies suggest that the binding of the organic molecules to the substrate material is not as strong as among the molecules themselves as shown in Figure 1. This is the reason for the strong tendency to dewet. Another reason for a dewetting of the bilayer is the exposure of such a layer to atmospheric conditions. This has been shown by TDS and AFM measurements. Furthermore, for the island-like film under ambient conditions Ostwald ripening was observed, leading to films composed of few, large crystalline islands. In the case of very thin films most of the material in the small islands even disappeared by vaporization within several days. A further peculiarity of the adsorption system rubicene/SiO2 is related to the sticking coefficient. Contrary to common belief the initial sticking coefficient is not unity, even at low substrate temperatures. We measured an initial sticking coefficient of only 0.2 ± 0.05, depending on the substrate conditions. This low value can be related to the relatively weak interaction energy between the molecules and the substrate, as well as to other reasons such as orientational hindering. Only a weak dependence of the adsorption and desorption behaviour on the substrate chemical composition was observed. Accumulation of carbon on the surface, due to rubicene dissociation during sample heating, does neither significantly influence the shape of the desorption spectra nor the coverage dependence of the sticking coefficient.