Resonance Raman Scattering of Topological Insulators Bi2Te3 and Bi2 − xSbxTe3 − ySey Thin Films

Phonon occupation is temperature (T) dependent; it may participate in scattering with electronic states of topological insulators (TIs). Longitudinal optical (LO) phonons $A_{1g}^1$ and $A_{1g}^2$ in Bi_2 − xSb_xTe_3 − ySe_y (BSTS) was resonantly excited by a photon energy (E_p) 2.33 eV due to electronic transition of unoccupied conduction band. The intensity of these modes was enhanced in Bi₂Te₃ film at E_p 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 $A_{1g}^1$ and $A_{1g}^2$ modes. At 300 K, the intensity of the $A_{1g}^2$ mode was significantly decayed in both the BSTS and Bi₂Te₃ at E_p 2.33 and 1.87 eV due to the anharmonic coupling. However, at similar value of T, spectral profile of $A_{1g}^1$ and $E_g^2$ 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, $A_{1g}^1$ 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.