MORPHOLOGY OF ALPHA-HEXATHIENYL THIN-FILM-TRANSISTOR FILMS

We have studied the morphology of thin films of alpha-hexathienyl (alpha-6T), a hexamer of thiophene that is a very promising material for thin-film-transistor applications. Using electron- and atomic-force microscopies, we found that on both rigid (Si/SiO2 and glass) and flexible (polyimide) substrates, evaporated films show an apparently random, polycrystalline morphology. The crystals are lamellar, ca. 100-200 nm in lateral dimensions and 15-30 nm in thickness, and exhibit irregular boundaries. Nevertheless, electron-diffraction evidence from such films indicates that the constituent molecules are deposited preferentially end-on and assume a normal or neatly normal orientation with respect to their substrates. Rapid high-temperature annealing causes growth of much larger (mu m-sized) crystalline grains and a partial transformation to a high-temperature polymorph; however, this process leads to formation of gaps in the film, which may cause deterioration of electronic performance.