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Abstract:

Organic ferroelectric field-effect transistors (FeFET) have received much attention in the last few years as non-volatile re-programmable memory devices in organic electronics. In such a FeFET, the gate dielectric of the transistor is replaced by a ferroelectric dielectric. Ferroelectric materials show a hysteresis in their displacement versus electric field characteristics and as a consequence of this behavior, FeFETs exhibit a stable and reproducible hysteresis in their source-drain current. FeFETs effectively show two on-set voltages, depending on the history of the applied gate voltage and can therefore be used as binary memory devices. The exact mechanism behind this bi-stable operation is, however, still under debate. More specifically, discussion has risen whether the ferroelectric layer is polarized or depolarized when the FeFET has been programmed to the low source-drain current (OFF) state. Additionally, literature reports can be found of FeFETs with the two on-set voltages either symmetrical or non-symmetrical around zero gate voltage with no clear answer to this difference. In this work, the origin of the memory window (the difference between the two on-set voltages) and the ferroelectric polarization in the OFF state are investigated using Scanning Kelvin probe microscopy (SKPM), in conjunction with other electrical characterizations. Pentacene is used as the organic semiconductor in a bottom gate - top contact FeFET architecture. In this configuration, the FeFET shows a remarkable feature: depending on the amplitude of the applied positive gate voltage, the two on-set voltages are either both positive (asymmetric case) or have a different sign (symmetric case). Hence, the FeFET in this work shows both types of memory windows that can be found in literature and is therefore used to study the origin of the memory window in organic FeFETs. Performed SKPM measurements show that the ferroelectric layer in the channel region of the FeFET remains unpolarized in both symmetric and asymmetric cases. SKPM also reveals that an injection barrier blocks majority charge carriers from entering the channel in the symmetric case which causes a larger memory window than that in the asymmetric case. These new insights in the mechanism of the bi-stable operation are useful to improve the design and architecture of organic FeFETs.