Impact of film thickness on the structural, linear and non-linear optical properties of ferroelectric Bi2FeCrO6 perovskite thin films

Abstract

Bi2FeCrO6 thin films were grown by PLD with different thicknesses. X-ray diffraction shows that the samples are epitaxial. The Williamson-Hall equation indicates that the microstrain diminishes with increasing film thickness, i.e., the upsurge in relaxation. The chemical composition and the distribution of elements inside the films were investigated using TEM-EDS. The measurements suggest the existence of cationic (Fe, Cr) ordered regions with relaxed lattices while the Fe–Cr disordered regions are strained due to the substrate. The valences of Fe and Cr determined by XPS measurements indicate that both cations have mainly a 3+ valence. The gap band energy decreases from 2.25 to 1.85 eV with the rise of thickness, which was attributed to the film relaxation. To validate the Eg decrease with relaxation, we used ab initio calculations based on DFT. Its shows that Eg indeed depends on the variation of the lattice parameter and decreases with increasing lattice parameter in line with relaxation. Urbach energy, refractive index, dielectric coefficient, and optical conductivity were extracted and linked to structure relaxation. We determined the non-linear optical parameters such as the energy of the oscillator, dispersion energy, static refraction index and third-order nonlinear optical susceptibility as a function of film thickness.