Effect of O-2-Plasma Treatment of Mo on the Crystal Growth Mode of Pentacene of Organic Thin-Film Transistors

Abstract

We report the change in the crystallinity of pentacene film on O-2-plasma-treated molybdenum (Mo) and the electrical properties of pentacene thin-film transistors using Mo as source-drain electrodes. The surface energy of the Mo film was increased with O-2-plasma treatment. Synchrotron radiation photoemission spectroscopy results revealed that the valence band split from the metallic Fermi level as the surface of the Mo was treated with O-2 plasma. This provides evidence that the surface of the Mo film changed to Mo oxide (MoOx). Grazing-angle incidence X-ray diffraction results showed that pentacene molecules on O-2-plasma-treated Mo has well-aligned features in the vertical direction as well as in the lateral direction, leading to a high crystalline film growth. The growth mode of pentacene changed from a tilted bulklike structure to a thin-film phase type by the treatment. This change in the growth mode is attributed to the reduction in interaction energy between pentacene and Mo via the formation of oxide at the Mo/pentacene interface. Secondary electron emission spectra showed that the work function increased by 0.55 eV after O-2-plasma treatment on the Mo surface. Thus, the Mo oxides lowered the potential barrier for the hole injection from Mo to pentacene, increasing the drain current of bottom-contact organic thin-film transistors.