Date of this Version
Liu, Xiaohui. "Ferroelectric polarization dependent interface effects." PhD dissertation, University of Nebraska, 2014.
Utilization of the switchable spontaneous polarization of nanometer scale ferroelectric materials offers a promising avenue for future nanoelectronic devices. In this dissertation, we use density-functional calculations and phenomenological modeling to explore the effects of interface termination on thin-film heterostructures, the effects of electron doping in bulk ferroelectric materials on ferroelectric stability, and the effects of ferroelectric polarization switching on the electronic and transport properties of interfaces.
For SrRuO3/BaTiO3/SrRuO3 epitaxial heterostructures grown on SrTiO3, we find that the built-in dipole at the BaO/RuO2 terminated interface leads to a strong preference for one polarization. We predict that this unfavorable interface dipole effect can be alleviated by deposition of a thin layer of SrTiO3 at the BaO/RuO2 interface. Our theoretical prediction is confirmed by the results of experimental studies performed by our colleagues at University of Nebraska.
While ferroelectric materials are normally considered as insulators, ferroelectricity and conductivity can coexist in electron-doped BaTiO3 (n-BaTiO3). We demonstrate that ferroelectric displacements persist up to the critical concentration of 0.11 electron per unit-cell volume consistent with experiment. Our investigations show that the ferroelectric instability requires only a short-range portion of the Coulomb force with an interaction range of the order of the lattice constant, thus providing a new insight into the origin of ferroelectricity in displacive ferroelectrics.
The effects of ferroelectric polarization on the electronic and transport properties are explored for the SrRuO3/n-BaTiO3(001) heterojunction. Ferroelectric polarization controls the accumulation or depletion of electron charge at the interface and thus determines the electron and spin transport regime. First, we find that the interface exhibits a Schottky tunnel barrier for one polarization orientation, whereas an Ohmic contact is present for the opposite polarization orientation. This leads to a five orders of magnitude change in the interface resistance with polarization reversal. Second, by taking into account the fact that SrRuO3 is a ferromagnetic metal below 160 K, we find that the interface transmission is negatively spin-polarized. In the high doping regime, we predict that the ferroelectric polarization reversal alters the transport spin-polarization from -65% to -98%, whereas in the low doping regime, the spin-polarization of transmission across the SrRuO3/n-BaTiO3(001) interface changes sign.
Adviser: Evgeny Y. Tsymbal