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

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 A2u and Eu 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 A1g modes changed in width, frequency, and intensity for decreasing numbers of QL, while the in-plane Eg 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).