Atomic force microscopy analysis of morphology of thin pentacene films deposited on parylene-C and benzocyclobutene

This paper reports on an atomic force microscopy (AFM) investigation of the surface of pentacene films deposited onto parylene-C and benzocyclobutene (BCB), two polymers dedicated to organic field effect transistors (OFETs). A thorough grain analysis was carried out by power spectral density (PSD) on pentacene films of different thicknesses (ca. 15–60 nm). PSD spectra of experimental AFM images were analyzed by using fractal, ABC and superstructure models. These approaches permitted to explain the difference of the pentacene morphology on the two substrates and to illustrate structural specificities related to change in the growth mode. Superstructures larger for the pentacene on BCB than for the pentacene on parylene were revealed. On both substrates, larger superstructures occurred for 30 nm thick pentacene layers. This thickness also corresponded to the highest field effect mobility in top gate OFETs parylene/pentacene deposited on BCB. The superstructure model applied on the PSD curves showed that the increase of mobility for a 30 nm thickness was not related to an increase of the grain size but to an increase of the island size.