High Quality Ultrathin Bi2Se3 Films on CaF2 and CaF2/Si by Molecular   Beam Epitaxy with a Radio Frequency Cracker Cell
  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. 
Li Zhang, Robert Hammond, Merav Dolev, Min Liu, Alexander Palevski,   Aharon Kapitulnik
cond-mat.mtrl-sci cond-mat.mes-hall quant-ph