Oxide bicrystals provide a convenient object for local SPM studies. In many cases, quantitative information on grain boundary properties can be obtained. Imaging can be performed on static grain boundary to image space charge layer at the interface. Imaging of the laterally dc biased interface allows current-voltage characteristic to be reconstructed. Finally, using recently developed Scanning Impedance Microscopy, ac transport properties can be quantified.
Static grain boundary
Transport properties of ceramic materials are strongly influenced by grain boundary structure and topology. Dopant or vacancy segregation as well as intrinsic interface states result in the interface charge compensated by the band bending and formation of adjacent depletion regions. Such structures referred to as Double Schottky Barrier (DSB) give rise to non linear I-V (varistor) behavior and constitute the basis of numerous industrial applications. Properties of grain boundaries vary strongly depending on relative crystallographic orientation of the adjacent grains, presence of second phase wetting layers, etc. Scanning probe microscopy based techniques are able to detect the stray fields above DSB structure and thus be used to identify grain boundary structure and properties. Shown below are crystallographic structure of 2 simple grain boundaries in SrTiO3. Note that surface potential contrast is different for different grain boundaries. We have shown that the presence of mobile surface charges alters experimental contrast; nevertheless, position and type of grain boundary in some cases can be determined.
Biased grain boundary
Shown below is surface topography and surface potential on grounded, forward and reverse biased SrTiO3 bicrystal surface. Surface topography is essentially flat. Grain boundary is associated with a number of pores that renders its detection in transmission optical microscope possible. Surface potential of grounded surface exhibits a small feature associated with screened double Schottky barrier at the grain boundary. Application of lateral bias shows that grain boundary resistivity is significantly less than that of the bulk.
Extremely simple equivalent circuit in this measurements allows to reconstruct the non-linear transport characteristic of grain boundary fro SSPM measurements. Shown below is potential drop at the grain boundary as a function of external bias and equivalent dc circuit for the grain boundary. From the mathematical analysis of the data we estimate the nonlinearity coefficient as ~3 and ohmic part of grain boundary conductivity as ~250 Ohm.