STEM Characterization of Atomic and Ferroelectric Microstructure in Barium Titanate Thin Films

Abstract

Thin film barium titanate is a promising material platform for photonic devices that allow for on-chip laser physics. These devices take advantage of the electro-optic effect, where the speed of light through a material varies with an applied electric field, and thus these devices must be fabricated on a material that has a high electro-optic coefficient. Barium titanate is one such strongly electro-optic material; however high-quality thin film barium titanate is challenging to fabricate due to its low Curie temperature that makes it prone to cracking. Thin film barium titanate can be grown from the bottom up with molecular beam epitaxy (MBE) and detailed characterization of MBE-grown thin films is needed to optimize growth. Additionally, the high degree of control that MBE affords means that films can be grown intentionally off-composition in order to inform the kinds of defects that may arise in films grown by other methods with less precise stoichiometry control. In this dissertation, I present scanning transmission electron microscopy (STEM) characterization of MBE-grown thin film barium titanate. STEM is used because it is an atomic-scale imaging technique that enables picometer-scale polar domain mapping as well as highly localized defect characterization. Using STEM, I demonstrate the high quality of films achievable with metal-organic hybrid MBE. I also investigate the effect of stoichiometry on atomic and ferroelectric structures in conventional-MBE grown thin film barium titanate. I identify an asymmetry in how the barium titanate lattice accommodates off-stoichiometry composition; excess titanium leads to disordered polar structures whereas excess barium forms a water-soluble surface layer on an otherwise well-ordered film.Engineering and Applied Sciences - Applied Physic