Chemical Composition Dependent Raman Scattering Spectroscopy of MBE Grown Manganese - Based Bismuth Telluride Topological Insulator Thin Films

Chemical composition dependent vibrational modes of manganese-based bismuth telluride (MBT) magnetic topological insulators thin films grown by molecular beam epitaxy were studied by Raman spectroscopy and compared with the vibrational modes of Bi₂Te₃ films. Intensity of $E_g^3$ TO mode was much stronger in Bi efficient MBT film. In contrast, vibrational cross-section of $A_{1g}^2$ LO mode was stronger in Bi deficient MBT compound. $E_g^3$ TO mode corresponds to in-plane vibrations of Bi-Te atomic layers. Deficiency of Bi atoms in the MBT indicated that the vibrational degree of freedom of Bi-Te in the septuple unit cell was less probable. Antisymmetric out-of-phase vibration of Te-Te atoms resulted in $A_{1g}^2$ LO mode in which Bi atom was stationary, therefore vibrational cross-section of this mode for Bi deficient MBT film was stronger. A resonantly excited E_g and A_1g modes was observed at excitation energy 1.57 eV associated to deformation potential and Frohlich electron–phonon coupling, respectively.