Structural Characterization of Defects in the Topological Insulator Bi₂Se₃ at the Picometer Scale

Topological insulators are a class of materials that are semiconducting or insulating in their bulk but possess topologically protected gapless states at their boundaries. Bi₂Se₃ is a promising material for applications due to its large band gap and single surface state Dirac cone. Pure electric conduction exclusively via the topological surface state is, however, hampered due to an n-type doping caused by the presence of native point defects, especially Se vacancies. Here, we apply high-resolution atomic force microscopy for real-space imaging and determination of the polarity of surface defects in Bi₂Se₃. We observe surface defects ranging from a single missing Se atom to defects composed of multiple missing Se atoms in the surface layer and find a positive polarity for all Se vacancies, confirming them as electron donors. Our work links to existing STM findings and adds precise structural information provided by the additional AFM channel, opening the possibility to more accurately determine the physical properties of defects in topological insulators.