Revealing the Substrate Dependent Ultrafast Phonon Dynamics in Bi2Se3 Thin Films.

The bulk carrier excitation influences the phonon dynamics, which can alter and modulate the surface charge density of topological insulators such as bismuth selenide (Bi₂Se₃). This work investigates the charge carrier and phonon dynamics in Bi₂Se₃ grown on various substrates. The orientation of the substrate, the size of the crystallites, and the misfit of the lattice affect the carrier and phonon dynamics. Bi₂Se₃ thin films are grown under the same growth conditions on SiO2, Si(111), and SiN. Bi₂Se₃ overlayers exhibit substrate- dependent charge carrier relaxation channels and phonon dynamics. It is evident from Raman spectroscopy and ultrafast transient absorption spectroscopy that the heterointerface interactions of all three samples affect the vibration modes of Bi₂Se₃ and coherent acoustic phonon oscillations in the NIR range. At 13.6, 41.2, and 34.4 GHz, the vibration modes of SiO2, Si(111), and SiN are equivalent. The propagation depth of phonon waves is shown by measuring the speed of sound in Bi₂Se₃ overlayers on SiO2, Si(111), and SiN. This study demonstrates that the surface and bulk-bound charge carriers of a topological insulator determine the frequency and velocity of the generated sound. [ABSTRACT FROM AUTHOR]