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Low-temperature Raman fingerprints for few-quintuple layer topological insulator Bi2Se3 films epitaxied on GaAs

Abstract : Topological insulators (Bi2Se3) 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 coupling. The results are discussed in terms of stacking-induced changes in inter-and intralayer bonding and/or the presence of long-range Coulombic interlayer interactions in topological insulator Bi2Se3. This work demonstrates that Raman spectroscopy is sensitive to changes in film thickness over the critical range of 9-to 4-QLs, which coincides with the transition between a gapless topological insulator (occurring above 6-QLs) to a conventional gapped insulator (occurring below 4-QLs).
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Submitted on : Monday, November 16, 2015 - 1:39:10 PM
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Mahmoud Eddrief, Paola Atkinson, Victor H. Etgens, Bernard Jusserand. Low-temperature Raman fingerprints for few-quintuple layer topological insulator Bi2Se3 films epitaxied on GaAs. Nanotechnology, Institute of Physics, 2014, 25 (24), pp.245701 ⟨10.1088/0957-4484/25/24/245701⟩. ⟨hal-01229143⟩

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