The visible ultra-violet absorption and cathodoluminescence spectra of phase-pure epitaxial BiFeO3 thin films grown on SrTiO3(001) substrates by ultra-high vacuum sputtering revealed a band-gap of 2.69 to 2.73eV for highly strained ~70nm-thick BiFeO3 films. This band-gap value agreed with theoretical calculations and recent experimental results on epitaxial BiFeO3 films, demonstrating only minimal band-gap changes with lattice distortion. Both absorption and cathodoluminescence spectra revealed defect transitions at 2.20 and 2.45eV, of which the latter could be attributed to defect states due to O vacancies.

Characterization of Electronic Structure and Defect States of Thin Epitaxial BiFeO3 Films by UV-Visible Absorption and Cathodoluminescence Spectroscopies. A.J.Hauser, J.Zhang, L.Mier, R.A.Ricciardo, P.M.Woodward, T.L.Gustafson, L.J.Brillson, F.Y.Yang: Applied Physics Letters, 2008, 92[22], 222901