Stoichiometric Control and Optical Properties of BaTiO3 Thin Films Grown by Hybrid MBE

TL;DR

This paper demonstrates a scalable hybrid molecular beam epitaxy method for growing high-quality BaTiO3 thin films with controlled stoichiometry, and correlates optical properties with film quality and defect states.

Contribution

It introduces a self-regulated growth window for epitaxial BaTiO3 films and links optical properties to stoichiometry and defect levels.

Findings

Optical properties closely match bulk BaTiO3 when grown within the optimal window.

Degradation in optical properties indicates defect formation and non-stoichiometry.

Optical measurements serve as a sensitive probe for film stoichiometry.

Abstract

BaTiO3 is a technologically relevant material in the perovskite oxide class with above room temperature ferroelectricity and a very large electro optical coefficient, making it highly suitable for emerging electronic and photonic devices. An easy, robust, straightforward, and scalable growth method is required to synthesize epitaxial BaTiO3 thin films with sufficient control over the film stoichiometry to achieve reproducible thin film properties. Here we report the growth of BaTiO3 thin films by hybrid molecular beam epitaxy. A self regulated growth window is identified using complementary information obtained from reflection high energy electron diffraction, the intrinsic film lattice parameter, film surface morphology, and scanning transmission electron microscopy. Subsequent optical characterization of the BaTiO3 films by spectroscopic ellipsometry revealed refractive index and extinction coefficient values closely resembling those of stoichiometric bulk BaTiO3 crystals for films grown inside the growth window. Even in the absence of a lattice parameter change of BaTiO3 thin films, degradation of optical properties was observed, accompanied by the appearance of a wide optical absorption peak in the infrared spectrum, attributed to optical transitions involving defect states present. Therefore, the optical properties of BaTiO3 can be utilized as a much finer and more straightforward probe to determine the stoichiometry level present in BaTiO3 films.