Adsorption-controlled molecular-beam epitaxial growth of $BiFeO3$

$BiFeO3$ thin films have been deposited on (111) $SrTiO3$ single crystal substrates by reactive molecular-beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth overpressure and utilizing the differential vapor pressures between bismuth oxides and $BiFeO3$ to control stoichiometry. Four-circle x-ray diffraction reveals phase-pure, untwinned, epitaxial, (0001)-oriented films with rocking curve full width at half maximum values as narrow as $25arcsec$ (0.007°). Second harmonic generation polar plots combined with diffraction establish the crystallographic point group of these untwinned epitaxial films to be $3m$ at room temperature.