Morphology and roughness of high-vacuum sublimed oligomer thin films

We present an atomic force microscopy study on the morphology and roughness of sexithienyl (T6) thin films evaporated on mica in high vacuum. The effects of two thermal processes are investigated: (i) temperature of the substrate during evaporation, and (ii) annealing temperature in high vacuum. The former yields a good control on the extension of grain boundaries and the aggregate ordering, but does not affect surface roughness. The latter is characterized by a threshold temperature (∼ 175 °C) above which a smooth continuous surface is formed. The roughness scaling behaviour is analysed by the power spectrum of the topographical profiles. T6 surface is self-affine over 1-2 orders of magnitude of the spatial frequencies. However, films prepared at substrate temperatures above 200 °C or annealed above the critical temperature exhibit an extended self-affine behaviour. The roughness scaling factor suggests a growth process of the Kardar-Parisi-Zhang universality class.