In situ measurement of spatial variations in electronic properties of individual oxide interfaces

The continued miniaturization and hybridization of integrated circuits necessitates an understanding of interfacial phenomena as device dimensions approach the length scales of interfacial effects. Scanning Surface Potential Microscopy provides the energy and spatial resolution to probe such phenomena, and has been uniquely applied to investigate polycrystalline oxide structures during in situ biasing as when configured for commercial applications (e.g. as a varistor or 'surge protector'). Using this technique, a novel approach is developed to determine otherwise inaccessible electronic properties of individual grain boundaries. The voltage dependence of the interface potential barrier and the energy dependence of the interface density of states are each measured with nanometer scale spatial resolution. Limitations of the technique are addressed by comparing experiments of a model Sigma3 SrTiO3 bicrystal grain boundary with SSPM simulations using two dimensional finite element analysis. A dispute in the oxide community regarding the sign of the interface charge for this model grain boundary is resolved by direct measurements of the boundary potential barrier. The variation in properties for individual interfaces of polycrystalline systems, isolated by fabrication, is also compared for several commercially important oxide compositions. Equilibrium potential barriers differ by a factor of two, and both the local voltage dependence of the potential barrier and the spatially resolved interface density of states are each interface specific. The potential barrier of an individual interface was determined to smoothly vary by 0.5% over a 2 um length of grain boundary in the first measurement of this kind. Monophase NiO interfaces in ZnO-NiO PTCR devices are directly observed to be resistive relative to the NiO bulk, confirming indirect macroscopic measurements. Finally, in the first in situ variable temperature SSPM study, an individual grain boundary of a BasrTiO3 PTCR device is determined to be only weakly temperature dependent. The novel in situ scanning surface potential microscopy technique presented in this work is clearly a valuable tool for measuring and understanding individual interface properties at length scales of importance for future microelectronic device development.