Abstract
We have investigated the optical properties of thin films of topological insulators , , and their alloys on substrates by a combination of infrared ellipsometry and reflectivity in the energy range from 0.06 to 6.5 eV. For the onset of interband absorption in , after the correction for the Burstein-Moss effect, we find the value of the direct band gap of meV at 10 K. Our data support the picture that has a direct band gap located at the point in the Brillouin zone and that the valence band reaches up to the Dirac point and has the shape of a downward-oriented paraboloid, i.e., without a camel-back structure. In , the onset of strong direct interband absorption at 10 K is at a similar energy of about 200 meV, with a weaker additional feature at about 170 meV. Our data support the recent band-structure calculations suggesting that the direct interband transition does not occur at the point but near the line of the Brillouin zone. In the alloy, the energy of the onset of direct interband transitions exhibits a maximum near (i.e., the composition of ), suggesting that the crossover of the direct interband transitions between the two points in the Brillouin zone occurs close to this composition.
2 More- Received 25 September 2017
DOI:https://doi.org/10.1103/PhysRevB.96.235202
©2017 American Physical Society