Thickness-dependent optical properties in compressively strained BiFeO3/LaAlO3 films grown by pulsed laser deposition

Bismuth ferrite (BiFeO3) films with various thicknesses were epitaxially grown on LaAlO3 substrates by pulsed laser deposition. The X-ray diffraction and Raman scattering spectra reveal that the films were highly ( 1     1 ¯     1 ) oriented with the single phase. With increasing the thickness, the compressive strain decreases and the strain ratios between the film and bulk crystal are evaluated to be 1.75, 1.57, and 1. Moreover, the compressive strain induces band gap shrinkage from 2.7 to 2.65 eV, while the charge transfer transition energy increases from 3.5 to 4.1 eV. It could be due to the shift of O 2p states and the variation of local Fe3+ crystal field.