  Here we report a method to fabricate high quality Bi2Se3 thin films using
molecular beam epitaxy with a radio frequency cracker cell as an atomic
selenium source. With rates close to exact stoichiometry, optimal
layer-by-layer growth of high quality Bi2Se3 thin films with smooth surfaces,
has been achieved on CaF2(111) substrates and Si(111) substrates with a thin
CaF2 buffer layer(CaF2/Si). Transport measurements show a characteristic weak
antilocalization mangnetoresistance, with emergence of weak localization in the
ultrathin film limit. Quantum Oscillations attributed to the topological
surface states have been observed, including in films on CaF2/Si.
