Low-temperature Raman fingerprints for few-quintuple layer topological insulator Bi 2 Se 3 films epitaxied on GaAs

Topological insulators (Bi 2 Se 3 ) of single- and few-quintuple-layer (few-QLs) films were investigated by Raman spectroscopy and epitaxied on a GaAs substrate. At a measurement temperature of 80 K, we observed the emergence of additional A 2u and E u modes (Raman inactive in the bulk crystal) below 9-QLs film thicknesses, assigned to the crystal-symmetry breakdown in ultrathin films. Furthermore, the out-of-plane A 1g modes changed in width, frequency, and intensity for decreasing numbers of QL, while the in-plane E g mode split into three Raman lines, not resolved in previous room temperature experiments. The out-of-plane Raman modes showed a strong Raman resonance at 2.4 eV for around 4-QLs film thickness, and the resonant position of the same modes shifted to 2.2 eV for 18-QLs-thick film. The film thickness-dependence of the phonons frequencies cannot solely be explained within models of weak van der Waals interlayer ...

Mahmoud Eddrief, Paola Atkinson, Victor Etgens and Bernard Jusserand

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