Center for Functional Nanomaterials Seminar

Optimizing microelectronics devices, such as transistors, requires detailed knowledge of electronic states at various interfaces. Electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) provides a way to probe, with atomic precision, the local chemical environment near such boundaries. With this approach, we investigated systems containing (1) crystalline, and (2) amorphous oxide layers. In the first instance, several stacks of SrTiO3, SrRuO3, and BiFeO3 are compared. In these systems where the oxide layers are crystalline, some interfaces are atomically abrupt, whereas others display element intermixing and nearby defects. We link these observations to the instability of certain interfaces, as well as growth dynamics. In the second case, HfO2 layers doped with nitrogen to ensure an amorphous configuration are viewed before and after annealing. A spatially-resolved analysis of fine structures in EELS demonstrates that in situ capping with amorphous Si was necessary to retain nitrogen in the layer. In such a gate stack, we then explore low energy excitations (<50 eV) in loss spectra. Using a monochromated STEM, spectral features in the visible/near-infrared domain were also analyzed. We first demonstrate the spatial resolution limit of bandgap mapping, and show instead that low-loss EELS is particularly well suited to probe interface plasmons and optical modes in nanostructures.