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

The authors report results of micro-Raman spectroscopy investigation of mechanically exfoliated single-crystal bismuth telluride films with thickness ranging from a few-nanometers to bulk limit. It is found that the optical phonon mode A(1u), which is not-Raman active in bulk Bi2Te3 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 Bi2Te3 crystals with the thickness of few-quintuple layers important for topological insulator and thermoelectric applications.