Optical band gap of BiFeO3 grown by molecular-beam epitaxy

Abstract

$\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin films have been deposited on (001) $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with $\omega$ rocking curve full width at half maximum values as narrow as $29\mathrm{arc}\sec$ (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at $2.74\mathrm{eV}$ for stoichiometric as well as 5% bismuth-deficient single-phase $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ films.