Mehmet Dogan

Mehmet Dogan's picture
Graduate Student Year 7 (Ismail-Beigi)
Ph.D. 2017, Yale University
Field of Study: 
Theoretical Condensed Matter Physics
Thesis Advisor: 
Sohrab Ismail-Beigi
Dissertation Title: 
Ab initio studies of ferroelectric thin films
Dissertation Abstract: 

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 BaTiO_{3}/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 BaTiO_{3} 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 ZrO_{2} 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 HfO_{2} 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.

Degree Date: 
May, 2017