Molecular Beam Epitaxy of Highly Oriented Pentacene Thin Films on an Atomically Flat Sapphire Substrate

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Abstract
The crystal quality of pentacene thin films was found to be markedly improved by the use of atomically flat sapphire substrates. In general, the crystallinity of the thin film depends strongly on the surface condition of the substrate or the underlying layer. However, there are few reports on the flatness effect on the underlying layer to organic thin films grown on dielectric materials such as SiO2 and Al2O3, where it is necessary to fabricate organic thin films for the practical configuration of bottom-gated organic field-effect transistors. The surface of heat-treated sapphire shows step structures with atomically flat terraces, and is an ideal model of the surface of dielectric layers. The grain size and the X-ray diffraction intensity of a pentacene film deposited on this atomically flat substrate increases markedly in comparison with the films deposited on as-polished sapphires. In addition, the photoconductivity of pentacene films was also improved using atomically flat sapphire substrates. This result suggests that the surface roughness of the substrate and the underlying layer plays a important role in the crystallinity and electrical property of pentacene thin films.