Broadband High Temperature Impedance Spectroscopy

Lietuviškai


  Laboratory of Solid State Ionics has received funding from Lithuanian-Swiss cooperation programme to reduce economic and social disparities within the enlarged European Union under project agreement No CH-3-ŠMM-02/06.

Name of the project: Broadband impedance study of memristor oxide films

Project acronym: BISMOF

Beginning of project: 1 January 2016

Project ends: 30 September 2016

Total budget: 151 229 EUR 24 cnt.

This project is executed by VILNIUS UNIVERSITY (project leader T. Šalkus) and EIDGENÖSSISCHE TECHNISCHE HOCHSCHULE ZÜRICH (prof. J.L.M. Rupp).

Summary of the project

   The project is devoted to the production of oxide thin films by PLD method and their detailed analysis by impedance spectroscopy. The project will be implemented by four secondments of Lithuanian researchers to Switzerland, where they, under supervision of Swiss partners, will produce strontium titanate and gadolinium doped ceria thin films and prepare the electrodes for electrical characterization. The impedance spectroscopy measurements will be performed in Lithuania with the modified broadband impedance spectrometer at high frequencies up to 10 GHz.
   The films can find a potential application in the new generation memory elements, the so called memristors. The understanding of physical reasons for memristor element switching is the main aim of the project. Besides, the training of Vilnius University group researchers to work with PLD system and film characterization in Switzerland will be beneficial for Lithuanian scientists. The final discussion of the results will be in Vilnius during the common scientific seminar at the end of the project.

Achievements

   Strontium titanate (STO) and ceria gadolinia (CGO) thin films were grown by PLD method in Switzerland. Platinum electrodes of a special geometry were formed by using photolithography. The films with the applied electrodes behave as memristors.


Memristor prototype produced during the current project (on the right hand side) compared to a standard 1.6 mm SMD resistor.

   Further on the electrical properties were investigated. The film response to the triangular-shape voltage change in time shows memristive behaviour, hence their I-V curves have a hysteresis.


Cyclic voltammetry curves of CGO film. Blue poits show voltage increase and red points - voltage decrease. The profiles are typical for memristors.

   The impedance spectroscopy investigation at high frequencies on oxygen solid electrolyte films was performed for the first time. A semicircle was observed in the complex plane plot of impedance, and it is related to oxygen ion relaxation process in the film. Temperature dependences of CGO film conductivity was deduced from impedance spectra. Arrhenius graph (logarithm of conductivity vs. inverse temperature) shows similar conductivity values compared to the bulk conductivity of ceramics with the same chemical composition.


Impedance spectrum in complex plane plot of the CGO thin film (on the left hand side) and temperature dependence of its conductivity (on the right hand side).

Conclusions

   Lithuanian scientists learned the technological conditions of STO and CGO thin film growth by PLD.

Individual STO and CGO thin film samples have been produced, and they were suitable for high frequency impedance spectroscopy investigation.

Cyclic voltammetry results on STO and GDC thin films with crossbar electrode configuration showed memristor behaviour.

High frequency impedance spectrometer has been modified, allowing usage of DC bias when measuring impedance.

Relaxation type electrical parameter dispersion was observed in CGO thin film impedance spectra. The conductivity of CGO film was nearly identical to the conductivity of ceramics bulk.