Defective interfaces in Yttrium-doped Barium Zirconate films, characterization of the transport properties by scanning probe techniques

Yttrium doped Barium Zirconate (BZY) thin films have recently shown surprising electric transport properties(1). Very high values of conductivity, at intermediate temperatures (550oC-600oC) have been recently reported in thin films of BZY grown on (110) NdGaO3 (NGO), with a lattice mismatch between film and substrate very large(2). Experimental investigations conducted mainly by electrochemical impedance spectroscopy show that a consistent part of this BZY conductivity is of protonic nature. These results suggest that heavily strained interfaces may represent a key parameter for tailoring defect densities in thin epitaxial films thus enhancing their conductivity. These results have stimulated a further investigation in transport properties using local unconventional techniques. Electrochemical Strain Microscopy (ESM) has been used to detect electrochemical activity in BZY films with nanoscale resolution. The technique has been also used in a novel cross sectional measuring set-up allowing the visualization of the interface activity (3). The local electrochemical investigation has been compared with structural studies performed by state of art scanning transmission electron microscopy (STEM) analysis. The results show a correlation between the conductivity of the perovskite thin films and the defective interfaces. A proposed physical model based on a misfit dislocation network may explain in part the high values of proton conductivity obtained in BZY thin films. The model gives only an approximated value of the conductivity, which are related to a qualitative correlation between the ESM relaxation signal and the carrier diffusivity(4). This leads us to the 1st scientific question we plan to address: to find a clear correlation of high values of carrier diffusion coefficient with the presence of a defective interface and derive an analytical expression, which relates the ESM relaxation times with carrier diffusivity. A second very important question, which has not yet been clarified, is the presence of electrons/holes in the transport properties of the thin film BZY protonic conductors. Good protonic/ionic conductors must have a negligible presence of electron carriers. The measure of conductivity is usually performed in particular experimental set-up, which does not clarify whether an electronic contribution is present or not. We think that a tool like the microwave probe microscopy can shine more light on the role of electron/hole contribution on transport properties of these zirconate thin films. Thus, the 2nd main scientific question is to find the electron/hole contribution in the electrical conductivity of BZY films.