Direct Evidence for the Role of Oxygen Vacancy Configuration for Oxide-Based Resistive Switching Devices
Authors: Schmitt Rafael , Spring Jonathan , Korobko Roman , Rupp Jennifer L. M. 
Published: February 2020

The miniaturization of standard silicon-based memory technologies reaches physical limits in terms of size and power dissipation. Novel memory and computing architectures based on resistive switches are promising for future electronics beyond Von Neumann computing architecture. In oxide-based resistive switches, the memristance is associated with the movement of ionic carriers through oxygen vacancies in an oxide film at high electric field strength via structural defect modifications that are still poorly understood. Here, we newly employ an oxide solid solution by systematic extrinsic doping to probe the role of oxygen vacancy configuration either set as free, immobile or clustered to directly answer the fundamental question how the mobility of oxygen vacancies affects the switching characteristics of the devices at high electric fields. Till date strategies to tailor systematically oxygen vacancy concentration and configuration for oxide-based resistive switches are still to be explored.

For this, we design resistive switching devices of Ce1-xGdxO2-y as a novel resistive switching oxide constituent. The active oxide layer of the devices, consisting of a 500 nm thick CeO2 film with Gd3+ doping concentrations of 3 to 30 mol% deposited by Pulsed Laser Deposition (PLD), is sandwiched between micro-fabricated Pt-crosspoint electrodes. Cyclovoltammetry measurements show a strong dependence of the ROFF/RON
-resistance ratio on the effective gadolinia concentration per bit. At low gadolinia doping concentration < 10 mol%, a purely capacitive behaviour prevails. By increasing up to 20 mol% a significant hystereis in the current-voltage profile is measured with resistance ROFF/RON
-ratios up to 100. Turning to high gadolinia concentrations from 20 to 30 mol%, decreases the hysteretic current-voltage profile. To analyse the structural changes related to the oxygen vacancy concentration and configuration, changes opposed by the extrinsic gadolinia dopant in the ceria lattice of the resistive switching devices, we turn to Raman Spectroscopy. From these results we conclude that for moderate gadolinium concentrations up to 20 mol% the concentration of mobile oxygen vacancies steadily increases and is available as "free" carriers for the resistive switches; in this state the oxide is in a fluorite structure type. Further increase in doping, for which we see a reduction of switching characteristics, leads to the formation of oxygen vacancy clusters in the structure, which are simply as a defect configuration "immobile". For this state we confirm a phase change to a more bixbyite structure.

Unequivocally the experiments together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol% Gd3+ doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching.

A clear correlation between memristance and oxygen vacancies’ mobility was seen in this defect chemical model experiment on gadolinium doped ceria device structures. Extrinsic doping allows to tune the configuration, concentration and mobility of the oxygen vacancies in a much more systematic and controlled manner than what can be achieved through oxygen-deficient deposition or annealing steps as being the state-of-the-art in resistive switches. The research demonstrates guidelines for engineering of the oxide`s solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the memristance for future nonvolatile memory and logic applications.