Identifying Antisite and Vacancy Defects in n-doped Bi$_{2}$Se$_{3}$ Topological Insulators from Scanning Tunneling Microscopy and First Principles Calculations

Intrinsic defects are the major sources of n-type doping character in Bi$_{2}$Se$_{3}$ topological insulators, but their structural nature remains unsettled; Theoretical calculations predicted that Se$_{\mathrm{Bi}}$ antisite was the most preferred under Se-rich, i.e. molecular beam epitaxy conditions, but there has been no report on its experimental observation. Here, we present our energy-dependent atomic resolution scanning tunneling microscopy (STM) images for intrinsic defects obtained from Bi$_{2}$Se$_{3}$ thin films grown under Se-rich conditions. We observed two types of defects, and identified them as Se$_{\mathrm{Bi}}$ antisite and Bi vacancy located at Bi layer right below surface Se layer, by comparing experimental STM images with the simulated ones obtained from first principles calculations. Our study shows that, in agreement with previous predictions, not Se-vacancy at surface but Se$_{\mathrm{Bi}}$ antisite is the origin of n-type doping in our Bi$_{2}$Se$_{3}$.