Motivated by the occurrence of superconductivity transition in Cu-doped topological insulator Bi2Se3, we
perform a combined study of low temperature scanning tunneling microscopy/spectroscopy and angle-resolved
photoemission spectroscopy experiments and of ab initio density functional theory to clarify the doping nature
of Cu atoms in Bi2Se3 films. By measuring the structural and electronic properties of the Cu-doped Bi2Se3 films
at different doping temperatures, we find that Cu atoms behave as donors at intercalated and interstitial sites in
Bi2Se3 films. Only the interstitial defect density plays an important role in the observation of Landau quantization
of the topological surface states in Bi2Se3.