Dissertation Defense: Mehmet Dogan, Yale University, “Ab initio studies of ferroelectric thin films”

Thin films of metal oxides can display many functional physical properties, an important example of which is ferroelectricity. Ferroelectric thin metal oxide films grown on semiconductors can enable non-volatile transistors, where the state of the device is encoded in the polarization state of the oxide. This thesis presents theoretical studies of a number of metal oxide on semiconductor systems using first principles methods. We have studied the BaTiO3/Ge interface as a candidate of a ferroelectric oxide/semiconductor system. In one set of studies of this interface, we have shown how cross-interfacial structural couplings can create atomic-scale structural motifs in the metal oxide that do not exist in bulk. Separately, we have found that multiple polarization states in the BaTiO3 film are possible and, in principle, that one can switch between them by the application of an external electric field. Unfortunately, the overall direction of the polarization is pinned by the interface chemistry in this system. In order to modify the interface chemistry to promote ferroelectricity, we have proposed the usage of a buffer layer between the oxide and the semiconductor, such as a monolayer of zirconia. We have explored the possible stable configurations of monolayers of ZrO2 on Si and found that multiple polarization states are indeed stabilized. We have found that ferroelectric switching between these two structures would lead to modifications of the Si electronic properties in a manner comparable to available experimental results. We have developed a discrete lattice model to predict domain behavior in these films at finite temperatures. In a final set of works, we have conducted a study of thin films of doped hafnia which have recently shown ferroelectric behavior. We have focused on strain effects in doped HfO2 to explain some of the experimental observations from a structural point of view. Our findings provide an understanding for the stabilization of ferroelectricity in hafnia based thin films.