Speaker: Dr. Mehmet Dogan(Yale University, Department of Physics)
Title: Ab initio studies of ferroelectric thin ﬁlms
Date/Time:Thursday 13:40, July 6th
Place: FENS G029
Epitaxial interfaces between metal oxides and semiconductors have been of signiﬁcant research interest due to their potential use in electronic device applications. 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 which determines the electronic transport properties of the semiconductor. This thesis presents theoretical studies of a number of metal oxide on semiconductor systems using ﬁrst principles electronic structure 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 any of its bulk phases. 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 single 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 band properties in a manner comparable to available experimental results. We have developed a discrete lattice model to predict domain behavior in these monolayer 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.
Contact: İnanç Adagideli