  Atomically flat thin films of topological insulator Bi2Se3 have been grown on
double-layer graphene formed on 6H-SiC(0001) substrate by molecular beam
epitaxy. By a combined study of reflection high energy electron diffraction and
scanning tunneling microscopy, we identified the Se-rich condition and
temperature criterion for layer-by-layer growth of epitaxial Bi2Se3 films. The
as-grown films without doping exhibit a low defect density of 1.0\pm
0.2x1011/cm2, and become a bulk insulator at a thickness of 10 quintuple
layers, as revealed by in situ angle resolved photoemission spectroscopy
measurement.
