Dynamics of the Bulk-to-Topological State Scattering of Photoexcited Carriers in Bi2Se3 Thin Films

Carrier dynamics in polycrystalline Bi2Se3 topo- logical insulator thin films were investigated by femtosecond transient absorption spectroscopy (FTAS) at 77 K, by using an infrared pump photon of 0.62 eV energy and a white super- continuum probe ranging from the near infrared to ultraviolet regions (0.9-3.5 eV). The Bi2Se3 samples were grown by vapor solid deposition, a quick, inexpensive, and easy-to-control growth technique to obtain films of different thicknesses, endowed with topological properties. FTAS spectra present several absorption bleaching signals, which can be attributed to electronic transitions involving both bulk and surface states present in the complex Bi2Se3 band structure. We observe clear differences in the rise times of several bleaching signals, differences that can be attributed to different band filling dynamics. Fast rise times are observed for transitions only involving bulk states, while a delayed onset of the bleaching signal has been observed for transitions involving surface topological states, which are more efficiently populated by carrier-phonon scattering of bulk electrons and holes, rather than by direct photoexcitation. The observed features shed fresh insights into the properties that allow these materials to be employed as innovative, low-cost, and wide-range photodetectors.