Abstract
Switching of the resistance in thin ferroelectric films is a complex phenomenon involving an influence of polarization states, point defects, processes of electron injection from electrodes and peculiarity of polarization charges screening. In order to clarify the mechanism of surface charge screening under ferroelectric polarization switching and its correlation with the local resistive properties in thin ferroelectric films, we studied the changes in the surface potential under dc bias voltage application in SrTiO3/La0.7Sr0.3MnO3/BaTiO3 heterostructures using different modes of atomic force microscopy (Kelvin probe, piezoresponse, and conductive) at ambient conditions. It is found that the peculiarity of surface charge screening under polarization reversal gives a strong electrostatic contribution in the measurements of local resistive properties at the ambient conditions and could completely veil the real current distribution over the film surface. Remarkably, scanning tunnelling microscopy measurements under ultra-high vacuum conditions show evidence of oxygen vacancy formation in thin ferroelectric films after poling, revealing an increase of the film thickness due to the chemical expansion.
Highlights
We suppose that the specificity of transport mechanisms in BaTiO3 ultrathin films is defined by the ratio of film thickness and the mean free path of injected electrons
An analysis of experimental I-V curves for ferroelectric thin films with thicknesses in the range of 3.5-12 nm suggests an influence of space charge limited currents on a transport mechanism through the structures
After 10-15 switching cycles in UHV, the voltage at which I-V curve hysteresis appeared is stabilized to a certain value
Summary
We suppose that the specificity of transport mechanisms in BaTiO3 ultrathin films is defined by the ratio of film thickness and the mean free path of injected electrons. We conclude that for thin epitaxial ferroelectric films below the certain thickness (less than ≈ 3.5 nm) a dominant contribution in a current through the structure arose from elastic tunneling. I-V characteristics of these structures are nonlinear, symmetrical without hysteresis [1].
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