Regensburg 2016 – wissenschaftliches Programm
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HL: Fachverband Halbleiterphysik
HL 35: Poster I
HL 35.19: Poster
Dienstag, 8. März 2016, 15:00–19:00, Poster A
Optical properties of compensated topological insulators — •Alessandro Revelli1, Nick Borgwardt1, Jonathan Lux2, Zhiwei Wang1,3, Ignacio Vergara1, Malte Langenbach1, Achim Rosch2, Yoichi Ando1,3, Paul van Loosdrecht1, and Markus Grüninger1 — 1II. Physikalisches Institut, Universität zu Köln — 2Institut für theoretische Physik, Universität zu Köln — 3Institute of Scientific and Industrial Research, Osaka University, Japan
We report on the optical properties of bulk-insulting BiSbTeSe2 for frequencies from 6 meV to 5.5 eV. Combining transmittance data for different sample thicknesses with normal incidence reflectivity data as well as ellipsometric results, we obtain a detailed view on the optical conductivity of the bulk, ranging from σ1(ω) = 0.3 (Ωcm)−1 below the gap to 104 (Ωcm)−1 at 2eV. At 50 K, we find a nearly fully compensated semiconductor with a carrier density N 4· 1016 cm−3. The intrinsic band gap Δ shows a strong temperature dependence, it shifts from 0.26 eV at 5 K to 0.18 eV at 300 K. Below the gap, the optical conductivity σ1(ω) reaches values lower than 0.3 (Ωcm)−1 at 50 K. These are the lowest values of σ1(ω) reported thus far for the entire tetradymite family. Above 50 K, we observe activated behavior of free carriers with an activation energy EA ≈ 26 meV, in agreement with transport data. Upon cooling below 50 K, σ1(ω) rises again due to the formation of puddles of localized carriers which arise from the random potential fluctuations caused by charged defects in a fully compensated semiconductor. Monte Carlo simulations show a screening effect arising from thermally activated carriers, explaining the experimental observations.