HEAD TO HEAD AND TAIL TO TAIL 180◦ DOMAIN WALLS IN PbTiO3/PbTiO3 FERROELECTRIC THIN FILMS: AN AB INITIO STUDY

Abstract

Charged domain walls (DWs) in ferroelectric materials have gained a lot of interest for a decade now. Large conductivity has been reported at the DWs of insulating ferroelectric materials, and it has lots of applications in actuators, sensors and capacitors. In the prior studies, the origination of this conductivity at DWs and the stabilization procedures of the charged domains remain controversial. Plenty of studies have called for the employment of dopants, oxygen vacancies or other external potentials to screen the polarization discontinuity at the DWs. Unfortunately, these kind of computations in literature cannot be used to answer questions about; the intrinsic critical thickness for stabilizing the two-dimensional conducting layers for the screening of the polarization charges, the spatial extension of the hole and electron gases, or the size of the polarization that would result from the induction of such DWs in an ideal slab. This thesis seeks to elucidate the stabilization of 180◦ head to head (HH) and tail to tail (TT) DWs in an ideal slab. Once this stabilization is achieved, queries on the origin of the conductivity seen in at the DWs and the stabilization of such domains will be a thing of the past. The electronic and structural characteristics of charged domain walls (DWs) in PbTiO3 thin films were determined using first-principles simulations as prescribed by the SIESTA method. At first, it was important to implement a strategy that would take into account the Hubbard (U) and spin-orbit coupling in the SIESTA method. This was important so as to accurately describe the physics of the DWs in this study. To benchmark the strategy implemented herein, calculations were performed on tetragonal IrO2. The lattice constants of IrO2 were found to be in a perfect agreement to experimental results. A difference of the order of 1.5% on the lattice constants was seen in comparison to other well known methodologies like VASP and QUANTUM ESPRESSO. The band structures of IrO2 generated by the implementation in here, VASP and QUANTUM ESPRESSO were identical. This gave the author confidence to use the implementation on the complex HH and TT domains. The polarization ( e PZ j ) in the DWs under this study was computed using the effective Born charges. The polarization was then used to obtain the spatial extension of the hole and electron gases at the DWs. To understand better on how much charge was transferred from the free-surfaces to the DWs in order to screen the depolarizing fields, the author plotted the planar average of the free charge (ρfree(z)). This thesis demonstrates how the formation of two-dimensional electron and hole gases at the free surface and at the DWs may stabilize the highly energetic 180◦ head to head (HH) and tail to tail (TT) DWs in free-standing slabs. The walls’ breadth, ≈ 7 unit cells for TT DW and ≈ 6 for HH DW, was found to be significantly more than what is seen when the domains are configured in their neutral state. With a perfect balance between the bound charges (divergence of polarization) and the screening charges, the distribution of the free charge was electrostatically associated with structural distortions. Examining the DWs in an intrinsic manner, the global charge neutrality expected, was observed. Also to note, was that at the interior of the domains, the polarization profile was astonishingly flat and was found to be 57.26 μC/cm2 for HH and 45.72 μC/cm2 for TT structures respectively. From a practical standpoint, this study provides an alternate explanation for the intriguing discovery of extremely high conductivities at the DWs between two insulating polar materials.