Hot-carrier and optical-phonon ultrafast dynamics in the topological insulator Bi 2 Te 3 upon iron deposition on its surface
Résumé
Topological insulators (TIs) are promising materials for future spintronic applications such as emerging spin-to-charge conversion (SCC) devices, possibly working at GHz-THz frequency for ultrafast data processing. These devices will rely on hybrid nanostructures composed, for example, of a ferromagnetic layer deposited on the topological insulator. The efficiency of spin-to-charge conversion will depend on the quality of the interface, including chemical (interfacial chemical reactions) and physical (band bending effect, Fermi pinning) aspects. This paper presents a complete study of electronic structures and photoexcited carrier dynamics in topological insulators capped with iron and iron oxide. We combine static and time-resolved angle-resolved photoemission spectroscopies (ARPES, TR-ARPES) with time-resolved optical methods (transient optical reflectivity and transmission). Both single crystal and thin films of Bi2Te3 are studied. We show that monolayers of iron and iron oxide significantly affect the electronic band structure at the interface by shifting the Fermi level into the conduction band, which we explain by a band bending effect, and is confirmed by in situ XPS measurements. This modified interfacial electronic structure offers a new channel for relaxation of hot carriers, illustrated by a drastic decrease of their characteristic decay time after optical excitation. These results might have a potential impact in the future development of TI-based SCC devices.