Abstract
An understanding of the aging and oxidation of the (0001) surface of Bi2Se3 is critical to a comprehensive physical picture of its topologically protected surface states. Here, the authors
contribute new experimental observations about the aging and oxidation process. The authors find
that surface aging in ambient conditions occurs in two major steps. Within 2 h of exfoliation, a
series of ~3.2A ° high islands are observed by atomic force microscopy over approximately 10% of
the surface. Subsequently, patch growth stops, and oxidation begins after the 2 h and continues
until one quintuple layer has been oxidized. X-ray photoelectron spectroscopy shows no sign of
oxidation before ~120 min of exposure to air, and the oxygen 1 s peak, as well as oxidized Se 3d
and Bi 4d peaks, are clearly present after ~190 min of ambient exposure. Variable angle spectroscopic
ellipsometry indicates that the oxidation of a full quintuple layer occurs on the time scale of days. These results are in good agreement with the time dependent changes observed in the surface
crystal structure by second harmonic generation. In addition to providing the ability to nondestructively
measure oxide on the surface of Bi2Se3 crystals, ellipsometry can be used to identify the thickness of Bi2Se3 flakes. With these methods, the authors have constructed a consistent, experimentally based model of aging process at the surface of Bi2Se3.
