The Study of the Polarization Domains of MBE-Grown Barium Titanate Thin Films and Nanodots Using PFM
Date of Graduation
12-2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Physics (PhD)
Degree Level
Graduate
Department
Physics
Advisor/Mentor
Gregory Salamo
Committee Member
Churchill, Hugh
Second Committee Member
Fu, Huaxiang
Keywords
Ferroelectrics, Molecular Beam Epitaxy, Piezoresponse Force Microscopy
Abstract
Polarization domains are the fundamental elements in ferroelectric materials that contribute to their properties. A natural polarization domain, which possesses a specific polarization direction or state, typically ranges in size from one unit cell to a few micrometers. Therefore, studying polarization domains requires the fabrication of materials and the measurement of ferroelectric properties at the nanoscale. In this study, molecular beam epitaxy (MBE) is employed to fabricate barium titanate (BTO) crystals, and piezoresponse force microscopy (PFM) is used to measure the properties of polarization domains at the nanoscale. In the beginning, this study identifies the reflection of high-energy electron diffraction (RHEED) oscillations for the growth of stoichiometric BTO crystals. Then, it demonstrates the MBE growth of (1) single crystal BTO thin films with thicknesses of 2, 10, and 40 nm using the shuttered RHEED method and (2) self-assembled BTO nanodots through a novel two-step method using the Stranski-Krastanov (SK) and droplet epitaxy (DE) techniques. These crystal structures are characterized using X-ray photoemission spectroscopy (XPS) to identify their compositions, atomic force microscopy (AFM) to assess the quality of the film surfaces, and X-ray diffraction (XRD) to determine the lattice parameters and strain and quality of the crystal structures. PFM uses the electric field generated by an AFM tip to manipulate and measure the direction of polarization domains on the surface of ferroelectric crystals. To conduct quantitative studies with PFM, I have established an analytical expression for the electric field beneath the AFM tip. This analytical expression for the electric field is derived and then compared with simulation and experimental results. The AFM tip is modeled as a sphere and a cone, with the sphere being substituted by two image point charges and the cone by a linear charge density that characterizes the electric field. The expressions for the image point charges depend on the thickness and dielectric constant of the film, radius of curvature, and cone half angle of the tip, while the linear charge density is a function of the cone half angle of the tip. The derived electric field expression will be employed to quantitatively investigate the expansion of polarization domains using the electric field under the AFM tip. The dynamic of the expansion of the polarization domains in BTO thin film follows Mrez’s laws. The material constants for BTO thin films in Merz’s law are found and show dependence on strain but not on the thickness and the applied voltage. The effect of temperature on the polarization domain as a function of thickness is also studied. It is found that wall motion and stability of polarization domains are thickness-dependent. This can be attributed to the electric field induced by screening charges in thin films, which is dependent on the thickness. The self-assembled BTO nanodots with a width of 20-40 nm and height of 2-5 nm exhibit a natural downward polarization at their edges, indicating the presence of a skyrmion state for their polarization. This polarization state can be switched by applying an electric field using an AFM tip. This nanodot structure shows great potential as an effective means to enhance the capacity of ferroelectric memory devices.
Citation
Zamani-Alavijeh, M. (2023). The Study of the Polarization Domains of MBE-Grown Barium Titanate Thin Films and Nanodots Using PFM. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/5133