Optical constants, band gap, and infrared-active phonons of (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) from spectroscopic ellipsometry

Abstract

Using spectroscopic ellipsometry, the authors determined the optical constants (complex dielectric function) for (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) from 0.01 to 6.5 eV. Above 0.5 eV, the data were described with a sum of two Tauc-Lorentz oscillators and two poles. A direct gap of 5.8 ± 0.1 eV was found. An Urbach tail extends to even lower photon energies and makes the crystal opaque above 4.8 eV. Using Fourier-transform infrared ellipsometry, the lattice dynamics was studied. Nine pairs of transverse/longitudinal phonons were found and attributed to disorder in the La/Sr sublattice, ordering in the Al/Ta sublattice, and two-phonon absorption.