Center for Functional Nanomaterials Seminar

The electrical and physical structures of interfaces play a determinative role in the function of electronic devices. In this talk I will describe a comprehensive program of functional interfaces grown by molecular beam epitaxy and characterized and designed using a combination of first principles theory, synchrotron x-ray diffraction and transmission electron microscopy. We apply this approach to epitaxial oxides grown on semiconductors, where a pronounced change in bonding and symmetry occurs at the oxide semiconductor interface. We can understand the origin of resulting interface structures using first principles theory by examining how the substrate couples to soft lattice modes characteristic of the oxide thin film.
A simple epitaxial oxide, such as BaO/Si, responds to the change from diamond cubic to rocksalt symmetry by simple atomic displacements, which have the symmetry of the silicon surface. The application of epitaxial strain promotes a non-switchable, out-of-plane polarization for SrTiO3 on silicon that is dominated by the chemistry of the interface. For the case of BaTiO3 on Ge we have discovered that the tendency of the truncated Ge (001) surface to dimerize couples to rotation of the oxygen octehedra in BaTiO3 in a complex manner.
The results of our work on oxide-semiconductor interfaces have motivated the design of a new, monolayer ferroelectric with potential applications for non-volatile memory and logic devices. Using x-ray photoelectron spectroscopy, XPS, we have demonstrated that a monolayer of ZrO2 sets up a dipole at a silicon-Al2O3 interface, and we have shown that this dipole can be switched using an applied electric field. One current challenge is to determine the microscopic origin of the switching behavior and measure its dynamics.