Regensburg 2007 – scientific programme
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DS: Fachverband Dünne Schichten
DS 17: Contact Properties (SYOE 10)
DS 17.3: Talk
Wednesday, March 28, 2007, 17:45–18:00, H32
Influence of film thickness on contact resistance in nanoscale oligothiophene field effect transistors — •Arne Hoppe1, Torsten Balster1, Günther Götz2, Peter Bäuerle2, and Veit Wagner1 — 1School of Engineering and Science, International University Bremen, Campus Ring 8, 28759 Bremen, Germany — 2Department Organic Chemistry II, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany
Nanoscale organic transistors for high switching frequencies are often limited by contact resistances. We report on a study of those contact resistances in nanoscale organic field-effect transistors (OFETs) by a systematic variation of the bandgap of the organic semiconductor. We use high mobility alkyl-substituted oligothiophenes (Dihexyl-n-thiophenes, DHnT) with n, the number of thiophene rings, ranging from four to seven. The bandgap variation also shifts the HOMO-level by almost 0.5 eV, which is expected to modify contact properties. Generally we find increasing mobilities up to 0.12 cm2/Vs towards larger molecules and at the same time decreasing contact resistances. We find for all materials a systematically decreasing mobility with decreasing channel length while the contact resistance remains fairly constant with channel length. We also report on thickness dependent in-situ electrical measurements on those nano OFETs, which are in agreement with the fact, that one monolayer is sufficient for electronic transport. Furthermore, we demonstrate monolayer-oscillations not only in the mobility, but also on the contact resistance. In contrast to the mobility, the contact-resistance shows minimal values with any closed monolayer.