Optical band gap of $BiFeO3$ grown by molecular-beam epitaxy

$BiFeO3$ thin films have been deposited on (001) $SrTiO3$ substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase $BiFeO3$ 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 $ω$ rocking curve full width at half maximum values as narrow as $29arcsec$ (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at $2.74eV$ for stoichiometric as well as 5% bismuth-deficient single-phase $BiFeO3$ films.