Crystal Symmetry Breaking in Few-Quintuple Bismuth Telluride Films: Applications in Nanometrology of Topological Insulators

TL;DR

This study uses micro-Raman spectroscopy to reveal crystal symmetry breaking in thin bismuth telluride films, enabling thickness identification crucial for topological insulator applications.

Contribution

It demonstrates that Raman signatures can identify few-quintuple layer bismuth telluride, revealing symmetry breaking effects not present in bulk crystals.

Findings

A1u mode appears in thin films due to symmetry breaking.

Raman intensity ratios correlate with film thickness.

Raman signatures enable nanometrology of topological insulators.

Abstract

We report results of micro-Raman spectroscopy investigation of the "graphene-like" mechanically exfoliated single-crystal bismuth telluride films with the thickness ranging from a few-nm-range to bulk limit. It is found that the optical phonon mode A1u, which is not-Raman active in bulk bismuth telluride crystals, appears in the atomically-thin films due to crystal-symmetry breaking. The intensity ratios of the out-of-plane A1u and A1g modes to the in-plane Eg mode grow with decreasing film thickness. The evolution of Raman signatures with the film thickness can be used for identification of bismuth telluride crystals with the thickness of few-quintuple layers, which are important for topological insulator and thermoelectric applications.