Ion Gel Gating of Oxide Semiconductor Films: Gating Mechanisms and Electronic Transport

Abstract

Electrolyte gating using electric double layer transistors based on electrolytes is a powerful technique to tune charge carrier densities and thus material properties. Due to a large capacitance (10 – 100 µF cm^-2), electrolyte gating can induce carrier density up to 10^14 – 10^15 cm^-2, enabling electric field control of phase transitions, including insulator-metal-superconductor transitions, magnetic order modulation, and more. Important questions related to electrostatic vs. electrochemical gating mechanism and electrolyte/channel interfacial electronic disorder, however, remain to be answered. In this thesis, electrolyte gating based on ion gels is applied to two high mobility transparent conducting oxides, BaSnO_3 (BSO) (mainly) and CdO, in thin film form, which are of high interest for electronic and optical devices. This modulates the electronic properties, also addressing open electrolyte-gating-related questions. In ion-gel-gated BSO films, we demonstrate that electrostatic gating mechanisms dominate over an exceptional gate voltage window (approaching ±4 V), even at 300 K, supported by reversible transport response and operando synchrotron x-ray diffraction experiments. This is in stark contrast with many complex oxides and is attributed to the exceptionally small diffusivity of oxygen vacancies (Vo) in BSO. Wide-range voltage control of resistance is then demonstrated in a series of undoped and initially chemically n-doped BSO films, spanning a strong to weak localization crossover. Two-channel conduction analysis is combined with electron density profile calculation to extract accumulation layer electron densities (n) and mobilities (µ), demonstrating gate voltage (Vg)-induced 2D electron densities approaching 10^14 cm^-2. The µ-n relation is probed in a wide range, from ~10^18 cm^-3 to >10^20 cm^-3, where µ increases rapidly with n before decreasing above ~10^20 cm^-3. Modeling of phonon, ionized impurity, charged dislocation, interface roughness, and electrolyte scattering reveals the dominance of charged dislocations and ionized impurities at low and moderate n, crossing over to electrolyte-limited mobility at high n. The end result is Vg-induced μ enhancement over 2000% and a maximum μ of 140 cm^2/(Vs), comparable to the highest mobilities ever reported in BSO films. In ion-gel-gated CdO films, we determine a reversible Vg window of -3.5 – 2 V for control of electronic transport properties. Above 2 V, large irreversible resistance increases, out-of-plane lattice expansions, and surface pitting are induced, due to electrochemical reactions possibly involving Vo formation or H+ intercalation. -1 V ≤ Vg ≤ 2 V can induce a 2D electron density change up to 1.6 × 10^14 cm^-2, with the 3D electron density varied between ~ 1 × 10^20 cm^-3 and 6 × 10^20 cm^-3. Resistance change approaching 4 orders of magnitude is achieved, with a crossover from a metallic to insulating state. These results lay the groundwork for understanding the gating mechanism and open up the opportunity for electric-field-control of optical and photonic properties of CdO films.