Growth related properties of pentacene thin film transistors with different gate dielectrics

We report a study on the influence of dielectric surface properties on the growth, the morphology and ordering of pentacene layers and associated consequences on the performance of organic field effect transistors (OFETs). This work mainly aims at disentangling the respective influences of surface roughness and surface energy on pentacene growth. A range of inorganic high-k oxides as well as polymer dielectrics were compared: Ta₂O₅ deposited by e-beam evaporation or grown by anodic oxidation, polymethylmethacrylate (PMMA) single layer or PMMA/Ta₂O₅ bi-layer. Some complementary results on anodic HfO₂ were also added. Atomic force microscopy, X-ray diffraction and infrared absorption evidenced that hydrophobicity and surface roughness drastically influence pentacene growth mechanisms. Transistors realized with the different dielectrics show characteristics well correlated to pentacene structural properties. In particularly, we point out the relationship between the grain size and the field effect mobility with the surface energy of the dielectric substrate. The general trend is that the bigger the grains, the higher the mobility but that best electrical performances of OFETs are obtained with a dielectric surface energy close to that of pentacene. This work also bears out our former results on the benefit of a polymer/high-k oxide bilayer dielectric configuration to improve the field effect mobility while keeping a low operating voltage.