Organic Dielectrics Influence the Crystallographic Structure of Pentacene Thin Films

Full text: X-ray diffraction as well as atomic force microscopy experiments have been performed to investigate thin films of pentacene. The films were deposited on thermally grown SiO2 pre-covered by different organic layers. Modifying substrates that the substrate pre-treatments have a strong impact on the performance of the device as well as on the growth of the active layer. However, there are few reports about the influence on the crystalline properties of pentacene. In this work three different systems have been investigated. For one sample series vacuum deposited polymeric Parylene C - with varying thicknesses - was used as the dielectric layer. A second series of dielectric layers was prepared by spin coating a photoreactive polymer (PBHND [poly(bicyclo[2.2.1]hept-5-ene-2,3-(2- nitrobenzyl)dicarboxylate)]) onto the wafer. Subsequently the samples were exposed to UV-light for different time spans. For the third class of systems, a self assembled film of T-SC/SA [4-(2-(trichlorosilyl)ethyl)benzene-1-sulfonyl chloride (TSC), 30% sulfonic acid T-SA] was used to modify the SiOx surface. From the obtained x-ray data we find that the investigated pentacene films are polymorphic and consist of the two commonly observed crystal phases, namely the thin film phase and the Campbell phase. On weakly interacting substrates, these phases are typically oriented with their (001) lattice planes parallel to the substrate surface. Yet in the present investigation it is found that for some dielectric layers the (001) planes of the thin film phase are tilted approximately 3o and of the Campbell phase about 10o with respect to the substrate surface. These small deviations in the structure have a large influence to the in-plane diffraction patterns. Therefore, the changes in the patterns can be unambiguously attributed to the change of preferred orientation. (author)