Simultaneous in-situ real-time measurements of X-ray reflectivity and optical spectra of organic semiconductor thin film during growth

The relation between optical and structural properties of organic semiconductors in thin films is crucial for their fundamental understanding as well as their application in electronic devices. Here we present first results of simultaneous in-situ real-time measurements of X-ray reflectivity (XRR) and differential reflectance spectroscopy (DRS) of perfluorinated copper phthalocyanine (F16CuPc) thin films grown on SiO2/Si wafers. Using DRS we determine the optical absorption spectra of the thin films starting from monolayer coverage whereas real-time XRR provides structural information about the film growth. After a rapid decrease of the reflectivity in the monolayer regime we observe intensity oscillations in time at constant qz with a strong damping. By calibrating film thickness d(t), we found oscillation period of 1.45 nm at 1/2q Bragg, which correspond to the lattice spacing of standing F16CuPc molecules. This behaviour is characteristic for layer-growth with a finite roughness. In the monolayer regime the DRS signal shows a broad absorption peak at ∝2.0 eV, while for coverages of more than one monolayer an additional and relatively sharp peak appears at ∝1.6 eV. These results indicate that the film structure in the monolayer regime is different from the layer-growth regime.