Onset of electronic conductivity in nanometer thick films of yttria stabilized zirconia (YSZ) at high electric fields

Electrochemical cells were prepared on a silicon wafer consisting of a 100 nm thick solid electrolyte of yttria stabilized zirconia (YSZ) prepared by atomic layer deposition (ALD), a bottom Ruthenium and an upper Gold electrode. Constant voltages were applied to the cells and currents were measured at temperatures between 120 °C and 170 °C. After an incubation time stationary currents were measured corresponding to conductivities well above the ionic one of the YSZ. After the polarization an open circuit voltage remained, which was reduced to zero by passing a constant depolarization current through the cell. All results were explained qualitatively and quantitatively assuming reactions with the electrodes and internal reactions within the YSZ. The latter lead to virtual electrodes with high electronic conductivity between which unreacted YSZ exists, i.e. a n/i/p junction forms. A stationary flux of single charged vacancies through the unreacted YSZ is assumed leading to the final stationary current. The calculated characteristic time for reaching this steady state is shown to be the incubation time, which in agreement with the experiment is inverse proportional to both the second power of electric field strength and the diffusion coefficient of oxygen vacancies.