Infoscience

Thesis

Nanoionics Phenomena and Devices with Oxide Thin Films

The number of studies on solid state ionic thin films (Nanoionics) has increased dramatically over last decades due to their high potential in micro and nanoscale technologies. Micro solid oxide fuel cells (micro-SOFC), and resistive switching memories (ReRAMs) are the most promising topics in this field, having impacts on portable energy sources, and future non-volatile memories. One can also add the new discovery of giant electrostriction (GES) in Gd-doped ceria (CGO), proposing ion conducting thin films as a new electromechanical family. While very astonishing results have been obtained, there are still some remaining challenges that are needed to be addressed: a) Mechanical stability in micro-SOFC, b) performance study of classically known fast ion conducting -i.e. yttrium stabilized zirconia (YSZ) and CGO- in ReRAMs, and c) Investigating and understanding of the acting mechanism leading to novel properties in CGO thin films. In micro-SOFC, we have proposed a ring-shaped corrugated structure that can result in more stable membranes up to millimeters wide openings. Stable liberation borders, low mechanical buckling, increased area density in given volume, and high surface utilization are the main characteristics of such membranes. The electrochemical performance of a 500 um wide micro-SOFC cell has been measured at 350 °C and a power density of 12.5 mW/cm2 has been obtained. This value is close to the reported performance in the literature at the same temperature. An open circuit voltage of 0.7 V was achieved. The optimal membrane diameter in a single PEN cell (Positive electrode-Electrolyte-Negative electrode) was identified as 0.5 mm. For upscaling to several Watts of electrical output, it is proposed to fabricate a planar array of such cells. Furthermore, we have studied the potential of CGO and YSZ thin films to be used as switching layer in ReRAMs. Both films have shown outstanding electrical characteristics comparable to high-performance switching materials with high stability and endurance. CGO thin films are able to operate at V(set)=1 V and V(reset)=-0.6 V over multiple cycles. YSZ thin films switch between V(set)=1.1 V and V(reset)=-0.6 V. Both cells have shown stable response over 10000 cycles with ON/OFF ratio of 100 and 5 for CGO and YSZ, respectively. It turned out that the switching mechanism in CGO films is rather of multi-filamentary mechanism as the low resistance state (LRS) and forming voltage decreases with increasing the device size. In contrast, in the YSZ-based memories, less filaments are expected to be formed since the scaling effect was not clearly observed. Finally, we have investigated the GES in CGO thin films and we succeeded to reproduce the giant response, which was found to take place only at low frequency. We have shown that the giant electrostriction is related to the defects which are produced during rather specific deposition parameters such as low deposition temperature, an optimal ion bombardment, and high deposition rate. It has been found that a higher oxygen vacancy concentration than introduced by Gd doping is needed. Additional oxygen vacancies are most likely compensated by the reduction of Ce(4+) to Ce(3+). A large dielectric constant of over 1000 was observed at low frequency reminding at the phenomenon of the colossal dielectric constant. Moreover, the films are characterized by a strong nonlinear I-V behavior with a threshold voltage of about 10 V, very much like a varistor.

Fulltext

  • Thesis submitted - Forthcoming publication

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EPFL authors