Electrostatic gating is a common approach to alter the properties of thin films. It is utilized in field effect transistors in which controlled and reversible changes in the carrier concentration of the channel material are induced by applied electric fields. The magnitude of the electric fields supported by conventional gate dielectrics, however, is limited by their dielectric properties; and the maximum possible field strength is often not sufficient to tune the charge carrier concentrations over a wide range to, for instance, induce phase transitions. This motivated gating experiments involving ionic liquids (IL). By replacing the insulating solid-state gates with an IL, much higher electric fields can be implemented due to the formation of an electric double layer at the IL–channel interface that supports higher fields. Consequently, field effect transistor devices that are based on ILs can, in principle, electrostatically induce much higher charge carrier densities in the transistor channel. These high carrier densities, in turn, can lead to the formation of novel phases with interesting properties.
It has been long assumed that a purely electrostatic charge accumulation is responsible for the IL gate-induced changes. However, many recent studies show that the observed gating effects in various oxide materials are not in agreement with a purely electrostatic picture, but rather related to electrochemical processes. The exact mechanisms are still being discussed and many contradictory results are being published. One important reason underlying these controversies might be a lack of purity in the experiments. Most ILs are known to be hygroscopic - they easily absorb water from the atmosphere which can considerably change their properties and significantly shrink the electrochemical window of the ILs. Thus, an extremely careful handling of the ILs is required. In most studies, however, at least the critical IL device preparation is done ex-situ.
The Department Physics of Correlated Matter has vast expertise in the growth (molecular beam epitaxy) and characterization of high quality thin films all under ultra-high vacuum (UHV) conditions. The goal of this project is to facilitate and perform a true all in-situ/UHV IL gating study on oxide thin films to achieve a better understanding of the mechanisms governing the gate-induced effects.