Optical properties of Ln2Ti2O7 (with Ln = La to Lu) thin films grown on (110)-SrTiO3 substrates by pulsed laser deposition

Abstract

Ln2Ti2O7 thin films with Ln = La to Lu were successfully grown on (110)-SrTiO3 substrates by pulsed laser deposition. The microstructure and optical properties of the entire Ln2Ti2O7 series were investigated by X-Ray diffraction, atomic force microscopy, scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy and spectroscopic ellipsometry. For large ionic radii (i.e. Ln3+ = La3+ to Eu3+), the monoclinic/layered-perovskite structure is obtained with epitaxial growth. For smaller ionic radii (i.e. Ln3+ = Tb3+ to Lu3+), the cubic/pyrochlore structure is privileged. We have also observed the growth of a Gd2Ti2O7 polymorphic phase between the stability limit of the two previously described structures. The optical band gap increases when the ionic radius of Ln3+ decreases. Refractive index (n) and extinction coefficient (k) have been calculated over a wavelength range of 260–1500 nm. The low value of the extinction coefficient indicates that these materials are transparent in the visible range and can therefore be used in electro-optics systems in the case of the layered-perovskite structure and more widely as emitting materials in solid-state lasers optically pumped.