Raman and interband optical spectra of epitaxial layers of the topological insulators Bi 2 Te 3 and Bi 2 Se 3 on BaF 2 substrates

We report results of Raman and ellipsometric spectroscopy of the topological insulators Bi 2 Te 3 and Bi 2 Se 3 grown by molecular beam epitaxy on BaF 2 (111) substrates. Surfaces and interfaces of the films are probed by Raman scattering from the front and back sides of the samples, which is possible owing to the transparent substrate. Surface modifications induced by intense illumination with exciting laser light have been detected, with excess tellurium at the surface during and after exposure. We also report data for thin epilayers containing a fractional number of unit cells and/or incomplete Bi 2 Te 3 and Bi 2 Se 3 quintuples. We have used spectroellipsometric measurements to obtain response functions and have derived the penetration depth of light in the 1.0–6.5 eV range.