Anisotropic optical properties of rhombohedral and tetragonal thin film BiFeO$_3$ phases

TL;DR

This study investigates the anisotropic optical properties of rhombohedral and tetragonal BiFeO$_3$ thin films, revealing strain-induced effects, phase-dependent optical shifts, and polarization orientations using Mueller matrix ellipsometry and optical modeling.

Contribution

It provides detailed dielectric tensors for both phases and uncovers how strain influences optical transitions and polarization directions in BiFeO$_3$ thin films.

Findings

Strain causes significant shifts in optical transition energies.

The polarization in BiFeO$_3$ on LaAlO$_3$ is tilted by about 7 degrees.

Spectral weight analysis highlights the need for improved theoretical models.

Abstract

The anisotropic optical properties of multiferroic BiFeO$_3$ thin films have been determined with Mueller matrix ellipsometry at room-temperature. The full dielectric tensors of tetragonal-like and rhombohedral-like BiFeO$_3$ phases epitaxially grown on LaAlO$_3$ and SrTiO$_3$ single crystal substrates, respectively, within the spectral range of 0.6 and 6.5 eV are reported. Strain-driven anisotropy changes and transition shifts are observed as well as evidence of sub-band gap many-particle excitations are found. The transition shifts, mostly to higher energies for the highly strained tetragonal-like BiFeO$_3$ phase on LaAlO$_3$, are indicative of band structure differences. Additionally, optical modelling, confirmed by piezoelectric force microscopy studies, revealed that the average polarization direction of bivariant BiFeO$_3$ on LaAlO$_3$ is not parallel to the crystallographic [001] direction but tilted by about $7^{\circ}$. Spectral weight analyses reveal phase-dependent differences underlining that theoretical calculations of optical spectra need further improvement to appropriately account for electronic and excitonic correlations to fully understand multiferroic BiFeO$_3$.