The Role of Oxygen Vacancy Mobility at the Oxide Bulk and Electrode Interfaces for Ceria-Based Memristive Devices
Authors: Nenning Andreas , Schmitt Rafael , Korobko Roman , Rupp Jennifer L.M. 
Published: February 2020

Resistive switching is a recently extensively studied phenomenon for use in non-volatile memories as promising building blocks of future electronics. In such devices transition metal oxides are operated at high electric field strengths to initiate non-linear and hysteretic current-voltage relations that can be addressed in their various resistance states by voltage pulse amplitude or polarity. Despite the known importance of oxygen vacancy defects in metal-oxide memristive devices, systematic strategies on tuning their concentration and mobility in the transition metal oxide are rare.
In this work, we utilized the fluorite-type ceria-gadolinia binary system Ce1-xGdxO2-x/2 (GDC) to tune the oxygen vacancy concentration in order to correlate structure and defect chemistry to the memristive properties. The mixed conduction of GDC is well investigated at high temperature, and it is possible to extrinsically dope over the wide range of 3 to 30 mol% Gd; however, has only recently been reported for ambient and high electric fields1. Therefore, we can tune the oxygen vacancy concentration, migration and association energy by the acceptor dopant as a model system to directly investigate the influence on resistive switching.
For this purpose Pt-GDC-Pt sandwich structures were fabricated as resistive switching devices with cross-bar electrodes. Based on hysteretic current-voltage profiles, a maximum in resistive switching (with ROFF/RON up to 100) is found for the best ion conducting2 20 mol% Gd-doped CeO2. In-operando Raman spectroscopy revealed a homogeneous reduction of the ceria film during electroforming, together with a strong increase in conductivity. Additional high energy XPS measurements we observe the electrostatic potential drop at the electrode-oxide interface.
These experimental results directly prove that the electronic charge carriers generated by the formation and redistribution of oxygen vacancies at high electric fields is the origin of memristance in doped ceria films, and that the switching happens in a rather homogeneous manner. The importance of oxygen vacancy migration in our model is further verified by the doping study. Doping levels of 10-20 mol% resulted in good switching properties, whereas for low (<10 mol%) and high (>20 mol%) concentrations no resistive switching is observed. In both cases the number of mobile oxygen vacancies is not sufficient, either because of the low extrinsic doping level, or because of the formation of bixbyite-like domains with ordered or clustered "immobile" oxygen vacancies. This clustering is observed by the appearance of new vibrational modes in Raman spectroscopy due to the reduced symmetry in a defective crystal. Excitingly, one can study based on the findings the direct implication of "free" oxygen vacancies at low and high mobilites and also of "trapped" clustered vacancies on both, resistive switching and also local structural symmetry breaks3. In conclusion, we observed a clear correlation of oxygen vacancy mobility and switching characteristics. We suggest that a high concentration of mobile oxyen vacancies is of general importance when designing oxide materials for future computing applications.

References

[1]Schweiger, S., Kubicek, M., Messerschmitt, F., Murer, C., & J.L.M. Rupp (2014). ACS nano, 8(5), 5032-5048.
[2] T. Zhang, J. Ma, H. Cheng, S. Chan, Mater. Res. Bull. 41 (2006) (3) 563.
[3] R. Schmitt, R. Korobko, J. Spring, J.L.M. Rupp, “Design of Oxygen Vacancy Configuration for Memristive Systems”, in reveiw