Resonance Raman Scattering of Topological Insulators Bi₂Te₃ and Bi_2 − xSb_xTe_3 − ySe_y Thin Films

Abstract

Phonon occupation is temperature (T) dependent; it may participate in scattering with electronic states of topological insulators (TIs). Longitudinal optical (LO) phonons and in Bi2 − xSbxTe3 − ySey (BSTS) was resonantly excited by a photon energy (Ep) 2.33 eV due to electronic transition of unoccupied conduction band. The intensity of these modes was enhanced in Bi2Te3 film at Ep 1.87 eV, which is close to the electronic transition of unoccupied Dirac states. Fröhlich coupling strength was the main mechanism for higher intensity of and modes. At 300 K, the intensity of the mode was significantly decayed in both the BSTS and Bi2Te3 at Ep 2.33 and 1.87 eV due to the anharmonic coupling. However, at similar value of T, spectral profile of and modes was not affected because of the lower probability of decay rate of these phonons. In resonant condition, well‐resolved Raman forbidden surface modes were observed at T of 50 K. At 300 K, more asymmetric Fano profile of surface phonon was observed due to the anharmonic coupling. The study indicated that at high T, mode and anharmonic coupling may become primary cause for scattering with electronic states of the TIs. However, at low T, primarily, both the LO phonons participated in the scattering.