Scanning tunneling microscopy of defect states in the semiconductor Bi 2 Se 3

Scanning tunneling spectroscopy images of Bi2Se3 doped with excess Bi reveal electronic defect states with a striking shape resembling clover leaves. With a simple tight-binding model, we show that the geometry of the defect states in Bi2Se3 can be directly related to the position of the originating impurities. Only the Bi defects at the Se sites five atomic layers below the surface are experimentally observed. We show that this effect can be explained by the interplay of defect and surface electronic structure. Understanding the electronic properties of defects and the ability to control them will be crucial for the performance of the future microelectronic devices. 1 Scanning tunneling microscopy ~STM! represents a unique tool for the studies of defects as it combines atomic scale resolution with local spectroscopic capability. However, STM observation and analysis of defect states in semiconductors are complicated by surface effects such as in-gap surface states and reconstruction. These effects are avoided at the ~110! surfaces of a number of III-V semiconducting systems, 2 attracting extensive research. 3‐ 8 A number of point defect types have been observed. However, the positions of these defects with respect to the surface plane could be inferred only from indirect observations. The interpretation of such observations is complicated by the drastic effect the surface proximity may have on the defect states. 9 Modeling STM measurements of defects in semiconductors is not straightforward: Approximation of the STM images by maps of the local surface electronic density of states 10 is justified only if the charge relaxation rates of defect states significantly exceed the tunneling rate of electrons between the tip and the sample. 11 Tip-induced effects also need to be taken into account. These may include both local band bending, 3 and charging of the defect states by the tunneling current, resulting in bias voltage-dependent lattice relaxation in the vicinity of the defect atoms. 8 Careful analysis is necessary to clearly separate these effects from the intrinsic defect properties, and the bulk features of the observed defect states from the surface effects.