Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films

TL;DR

This study investigates the infrared dielectric responses of compressively stressed SrTiO3 thin films, revealing phonon stiffening in epitaxial films and broadening in polycrystalline films, affecting their permittivity.

Contribution

It provides a quantitative analysis of polar phonon modes in SrTiO3 thin films under strain, highlighting differences between epitaxial and polycrystalline samples.

Findings

Epitaxial films show strongly stiffened phonon responses.

Polycrystalline films exhibit broadening of phonon modes.

Stiffened soft mode leads to lower in-plane permittivity.

Abstract

Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser deposition, of thicknesses down to 40 nm, were studied using infrared transmission and reflection spectroscopy. The complex dielectric responses of polar phonon modes, particularly ferroelectric soft mode, in the films were determined quantitatively. The compressed epitaxial STO films on (100) La0.18Sr0.82Al0.59-Ta0.41O3 substrates (strain 0.9%) show strongly stiffened phonon responses, whereas the soft mode in polycrystalline film on (0001) sapphire substrate shows a strong broadening due to grain boundaries and/or other inhomogeneities and defects. The stiffened soft mode is responsible for a much lower static permittivity in the plane of the compressed film than in the bulk samples.