Crystal symmetry breaking in few-quintuple Bi2Te3 films: Applications in nanometrology of topological insulators

Abstract

The authors report results of micro-Raman spectroscopy investigation of mechanically exfoliated single-crystal bismuth telluride films with thickness ranging from a few-nm-range to bulk limit.It is found that the optical phonon mode A 1u , which is not-Raman active in bulk Bi 2 Te 3 crystals, appears in the atomically-thin films due to crystal-symmetry breaking. The intensity ratios of the out-of-plane A 1u and A 1g modes to the in-plane E g mode grow with decreasing film thickness.The evolution of Raman signatures with the film thickness can be used for identification of Bi 2 Te 3 crystals with the thickness of few-quintuple layers important for topological insulator and thermoelectric applications.

Surface phonons in the topological insulators Bi2Se3 and Bi2Te3

“…The observation of extra Raman lines and, in particular, the lines we assign to surface phonons have been prevalently attributed in the literature to surface-induced symmetry breaking, which seemingly allows for scattering of nominally Raman forbidden, infrared-active modes [18,31,32,33,34,35,36,38,40]. For various reasons, we find such an interpretation to be incorrect.…”

“…The peak we attribute to a surface phonon is the weak feature which appears between the 2 and 1 2 modes at 159 cm -1 and 114 cm -1 for Bi2Se3 and Bi2Te3, respectively. This peak has been previously observed in thin films and few-monolayer samples of both compounds [16,18,31,32,33,34,35,36,37,38,39] and in single crystals of Bi2Se3 [39,40], but never 4 | P a g e before in bulk Bi2Te3. Note that these prior studies detected numerous other weak features in both compounds that were not observed in the present work.…”

“…Also, weak features observed in Raman spectra were attributed to surface effects unrelated to the topological surface states [18].…”

“…Because of its crucial relevance to their surface conductivity properties, the study of electron-phonon coupling [13] and, moreover, the search for vibrations localized at the surface have been the subject of many studies in recent years [14,15,16,17]. In particular, inelastic helium scattering [14,15], surface enhanced Raman scattering [16] and time-resolved photoemission measurements [17] in Bi2Se3 and Bi2Te3 show features that were attributed to surface modes as well as strong electron-phonon coupling at the surface.Also, weak features observed in Raman spectra were attributed to surface effects unrelated to the topological surface states [18].Here, we present experimental results on the temperature-and excitation-wavelength (λL−) dependence of Raman scattering, as well as first-principles phonon calculations for bulk and fewquintuple-layer Bi2Se3 and Bi2Te3. Other than the expected, and previously reported Raman-active bulk modes [19,20], we find weak peaks at low temperatures in both compounds, which we ascribe 2 | P a g e to surface vibrational modes.…”

“…Their origin remains controversial. Spectra of few-monolayer Bi2Te3 show lines at ~ 93 cm −1 and 114 cm −1 [18,31,32,33,34,35] and at ~ 160 cm −1 in Bi2Se3 [36,37], which were assigned to Raman-forbidden, infrared modes resulting from surface-induced symmetry breaking, while thin-film studies of Bi2Te3 reveal five additional peaks, one of which, at ~ 93 cm -1 , was assigned to a surface mode [38]. In bulk, single crystal of Bi2Se3, additional peaks at 68, 125, 129 and 160 cm -1 were observed and also assigned to Raman-forbidden polar modes [40].…”