Nanoscale electric phenomena at oxide surfaces and interfaces by scanning probe microscopy.

摘要

Strong coupling between mechanical, electrical and magnetic properties in oxide materials, heterostructures and devices enable their widespread applications. Achieving the full potential of oxide electronics necessitates quantitative knowledge of material and device properties on the nanoscale level. In this thesis, Scanning Probe Microscopy is used to study and quantify the nanoscale electric phenomena in the two classes of oxide systems, namely transport at electroactive grain boundaries and surface behavior of ferroelectric materials.; The groundwork for the application of SPM for the determination of interface I-V characteristics avoiding contact and bulk resistivity effects is established. Scanning Impedance Microscopy (SIM) is developed to access ac transport properties. SIM allowed the interface capacitance and local C-V characteristic of the interface to be determined thus combining the spatial resolution of traditional SPMs with the precision of conventional electrical measurements. SPM of SrTiO₃ grain boundaries in conjunction with variable temperature impedance spectroscopy and I-V measurements allowed to find and theoretically justify the effect of field suppression of dielectric constant in the vicinity of the electroactive interfaces in strontium titanate. Similar approaches were used to study ferroelectric properties and ac and do transport behavior in a number of polycrystalline oxides.; Polarization-related chemical properties of ferroelectric materials were investigated and quantified, leading to the discovery of the effects of potential retention above Curie temperature and temperature induced potential inversion. The origins of these phenomena were traced to the interplay between fast polarization and slow screening charge dynamics. Piezoresponse Force Microscopy (PFM) was used to study the polarization dynamics. An extensive description of contrast mechanisms in PFM conveniently represented in the form of “Contrast Mechanism Maps” was developed to relate experimental conditions such as tip radius and indentation force with the dominant tip-surface interactions. This topic was further developed to study the photochemical activity on ferroelectric surfaces as a function of domain orientation and use PFM to create predefined domain structures paving the way for photochemical assembly of metallic nanostructures on ferroelectrics.