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

Carrier dynamicsin polycrystalline Bi2Se3 topological insulatorthin films were investigated by femtosecondtransient absorption spectroscopy (FTAS) at 77 K, by using an infraredpump photon of 0.62 eV energy and a white supercontinuum probe rangingfrom the near infrared to ultraviolet regions (0.9-3.5 eV).The Bi2Se3 samples were grown by vapor soliddeposition, a quick, inexpensive, and easy-to-control growth techniqueto obtain films of different thicknesses, endowed with topologicalproperties. FTAS spectra present several absorption bleaching signals,which can be attributed to electronic transitions involving both bulkand surface states present in the complex Bi2Se3 band structure. We observe clear differences in the rise times ofseveral bleaching signals, differences that can be attributed to differentband filling dynamics. Fast rise times are observed for transitionsonly involving bulk states, while a delayed onset of the bleachingsignal has been observed for transitions involving surface topologicalstates, which are more efficiently populated by carrier-phononscattering of bulk electrons and holes, rather than by direct photoexcitation.The observed features shed fresh insights into the properties thatallow these materials to be employed as innovative, low-cost, andwide-range photodetectors.