Beeinflussung der (Sauerstoff-) Stöchiometrie korrelierter Materialien mit Hilfe von Steuerelektroden realisiert durch ionische Flüssigkeiten
Zusammenfassung der Projektergebnisse
In ionic liquid (IL)-based devices, large changes in the charge carrier concentration of the channel material can be induced by applying relatively small gate voltages. This is related to the formation of an electrochemical double layer at the IL-channel interface which supports larger electric fields compared to transistor structures using conventional solid-state dielectric materials. The origin of the IL gate-induced effects in oxide materials is known to be not purely electrostatic. The exact electrochemical mechanisms like the creation of oxygen vacancies or the intercalation of hydrogen, however, are still under discussion. Since the ILs are hygroscopic and their properties change upon exposure to air, also an influence of water contamination is suspected which is especially important since most IL gating studies involve at least a short exposure of sample or IL to air. In the framework of this project, we have designed and implemented an all under ultra-high vacuum setup for IL gating experiments allowing for ultra-clean IL gating experiments. We observed that the liquids do not wet the channel material but rather form small droplets or accumulate at the gold electrodes after deposition or injection of the baked ILs onto the device under ultra-high vacuum conditions. This indicates that water might play indeed an important role in the IL gating experiments. To overcome the wetting problem, we had to modify the setup such that a baked Teflon frame can be transferred in-situ onto the freshly prepared device forcing the IL to cover the channel and gate areas. Having established the instrumentation, we studied the gating response of SrTiO3, TiO2, EuO, and Fe3O4. While the expected IL-gating behavior was observed for SrTiO3 and TiO2, proving the functionality of our all in-situ setup, EuO was found to be too sensitive for IL gating studies. Even under ultra-clean conditions, it reacts chemically with the ILs as indicated by an increase of the resistance after the injection of the IL independent of the applied gate voltage. The preliminary results on Fe3O4 show that the Verwey transition is not considerably influenced by IL gating using moderate gate voltages. Only for large gate voltages which might already exceed the electrochemical window of the ILs, a considerable change in the resistance of the Fe3O4 device is observed. Our results suggest that, while IL gating shows large effects in materials that readily lose oxygen like SrTiO3 and TiO2, IL gating may be not strong enough to induce oxygen vacancies in EuO or Fe3O4. However, further experimental work is needed to understand the mechanisms governing the IL-gate induced effects in more detail. Our in-situ setup provides a good basis for unraveling the existing controversies.
