Electronic Materials Conference location:Santa Barbara, USA date:26/06/02
Organic thin-film transistors (OTFT) are promising devices for low-cost large-area electronic applications. Nowadays, pentacene is amongst the most popular oligomer semiconductors for use in OTFT's. We used purified pentacene grown on thermally oxidized silicon wafers with a backside gate contact and shadow-mask evaporated Au top contacts to make OTFT's. The substrate was kept at room temperature, while deposition fluxes varied between 0,01 and 0,45 Å/s. By applying different substrate cleanings, it is possible to vary the surface energy and therefore the growth. Substrates cleaned with aceton and isopropylalcohol had a contact angle of 26,5°. Pentacene grown on those surfaces formed dendritic grains approximately 5μm in size. Pentacene grown on surfaces which were in addition cleaned with an oxygen-plasma (contact cangle 14,5°), formed compact grains ~0.5-1 μm in size. Hole mobilities in the latter film were of order 1E-3 to 1E-2 cm^2/Vs, while the mobilities in the film with dendritic grains were around 1E-1 to 1E+0 cm^2/Vs. The difference in mobility is attributed to grain boundary effects which are more pronounced in films with compact grains. It is interesting to compare those results with films grown using as-received pentacene. Different films were grown with varying flux and substrate temperature. Due to the substrate cleaning applied, only the compact grain morphology was observed. The films show a definite trend towards higher mobility with increasing positive threshold voltage. Even for these films with compact grains, mobilities as high as 1E-1 to 1E+0 cm^2/Vs were readily obtained, albeit with impractically high threshold voltages. We tentatively explain the results by assuming that deposition conditions with lower pentacene supersaturation favour higher concentrations of dopant impurities to become incorporated in the pentacene film. More dopant impurities not only lead to higher threshold voltages, but also provide sufficient free carrier densities to mask grain boundary effects. This explains the high mobilities, despite the less favourable compact grain morphology.