Voltage-controlled electro-chemical devices exploiting proton doping in correlated oxide

Abstract

Correlated transition metal oxides have attracted considerable attention due to a remarkable variety of functionalities that originate from the strong correlations between the localized transition metal valence d electrons. One representative functionality is an abrupt Mott phase transition under various external stimuli. The unique phenomenon observed in these materials exhibits sensitivities that cannot be achieved by using conventional semiconductor alone, and thus the control of the rich electronic phases in the correlated oxides may open up an important arena for future electronics to overcome the limitation of current electronic devices. In this regards, this thesis focused on a voltage-induced electrochemical switching devices exploiting proton (H+) doping in correlated oxide. The first topic is about a realization of H+-based correlated memory resistor with multilevel states in epitaxial NNO heterostructure devices by exploiting an asymmetry of H+ concentration. The resistivity of RNiO3 thin films can be reversibly modulated up to eight orders of magnitude by H+ doping, thereby realizing a colossal MIT. Due to the extreme sensitivity of NNO resistivity to H+, resistive switching in the memory resistor occurs near the hydrogenated interface, and the extent of resistance modulation in our protonic devices can be controlled by adjusting the magnitude of the external bias. Exploitation of resistive switching by H+ migration in correlated oxides provides a strategy to design energy-efficient electronic devices by overcoming the fundamental bottleneck conventional resistive switching devices, and provides an important step toward the development of correlated electronics, such as two-terminal neuromorphic devices. The second theme is about an all-solid-state three-terminal artificial synapse using extremely sensitive metal−insulator transition in a NNO channel by a H+ pump from the gate electrolyte. Gate-voltage-controlled H+ intercalation induced nonvolatile and stable multilevel analogue states in the NNO channel, accompanied by significant modulation of the out-of-plane lattice parameters. Using this correlated transistor operated by a H+ injection/extraction, persistent long-term potentiation and depression were demonstrated with nonvolatile and distinct multilevel analogue switching, high energy efficiency and stability to temperatures up to 150 ℃. The sensitive response of correlated materials with the ionic motion of H+ and thermal stability in these correlated synaptic transistors offers an opportunity to develop energy-efficient and scalable neuromorphic devices. The third subject is about the speed and retention time enhancement of H+ pump gating VO2 synaptic transistor by intrinsic oxygen vacancy incorporation. VO2, a class of correlated oxides, exhibited facile phase transition from insulator-to-metal-to-insulator depending on the H+ doping concentration. Exploiting a H+ doping-induced VO2 electrochemical transistor can exhibit a reversible phase transition accompanying with resistance switch. In this regards, H+ doping-induced electrochemical transistor with oxygen deficient VO2-δ channel were demonstrated that showed much sensitive response to gate bias as well as longer response time compared to the stoichiometric VO2 channel. The oxygen vacancy acted as a shallow donor that improved the intrinsic electrical conductivity of the VO2-δ contributing to a faster electron supply from source electrode during the H+ doping, and the expanded H+ diffusion path lowered the activation energy of H+ hopping. This result accounted for the correlation of the oxygen vacancy in the VO2 electrochemical transistor and H+ doping dynamics as well as the resultant improved device's synaptic properties. The present studies will (i) guide the development of energy-efficient switching devices for non-volatile memory and neuromorphic applications, (ii) development of thermally-stable solid-state electronic artificial synapses that operate at low voltage, and (iii) provides the correlation between the oxygen vacancy in the VO2 electrochemical transistor and H+ doping dynamics opening up the improvements of electrochemical device's properties.