Scanning Tunneling Microscopy of Defect States in the Semiconductor Bi${}_2$Se${}_3$

Scanning tunneling spectroscopy images of Bi${}_2$Se${}_3$ 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 Bi${}_2$Se${}_3$ 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.