Probing the Surface Polarization of Ferroelectric Thin Films by X-ray Standing Waves

TL;DR

This study demonstrates that X-ray standing wave techniques can directly probe surface polarization in ferroelectric thin films with atomic precision, revealing how surface adsorbates influence polarization stability.

Contribution

It introduces a method combining X-ray standing waves and spectroscopy to analyze surface polarization and adsorbates in ferroelectric thin films at atomic scale.

Findings

X-ray standing wave technique measures atomic positions near the surface.

Surface adsorbates affect local polarization amplitude and orientation.

Surface polarization stability is linked to oxygen-containing species.

Abstract

Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, it is demonstrated that the X-ray standing wave technique allows the polarization near the surface of a ferroelectric thin film to be probed directly. The X-ray standing wave technique is employed to determine, with picometer accuracy, Ti and Ba atomic positions near the surface of three differently strained $\mathrm{BaTiO_3}$ thin films grown on scandate substrates, with a $\mathrm{SrRuO_3}$ film as bottom electrode. This technique gives direct access to atomic positions, and thus to the local ferroelectric polarization, within the first 3 unit cells below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface, upon exposure to ambient conditions, is obtained. The combination of structural and spectroscopic information allows us to conclude on the most plausible mechanisms that stabilize the surface polarization in the three samples under study. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to the type, amount and spatial distribution of the oxygen-containing adsorbates.