Terahertz ellipsometry study of the soft mode behavior in ultrathin SrTiO3 films

TL;DR

This study uses terahertz ellipsometry to investigate the temperature-dependent soft mode behavior in ultrathin SrTiO3 films, revealing strain effects and the impact of annealing on phonon dynamics.

Contribution

It demonstrates the sensitivity of terahertz ellipsometry for probing soft modes in ultrathin films and compares strain relaxation effects in different film thicknesses.

Findings

Terahertz ellipsometry effectively detects soft mode changes in SrTiO3 films.

Annealing reduces the soft mode frequency in thicker films but not in ultrathin films.

Ultrathin films show minimal strain relaxation after annealing.

Abstract

We present a combined study with conventional far-infrared and time-domain terahertz ellipsometry of the temperature dependent optical response of SrTiO3 thin films (85 and 8.5 nm) that are grown by pulsed-laser deposition on LSAT substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO3. We show that for the 85 nm film the eigenfrequency of the soft mode is strongly reduced by annealing at 1200 C, whereas for the 8.5 nm film it is hardy affected. For the latter, after annealing the mode remains at 125 cm-1 at 300 K and exhibits only a weak softening to about 90 cm-1 at 10 K. This suggests that this ultrathin film undergoes hardly any relaxation of the compressive strain due to the LSAT substrate.