Structural, optical and electronic properties of the wide bandgap topological insulator Bi$_{1.1}$Sb$_{0.9}$Te$_2$S

Y. E. Khatchenko, M. Yakushev, C. Seibel, H. Bentmann, M. Orlita, V. Golyashov, Y. Ponosov, N. Stepina, A. Mudriy, K. Kokh, O. Tereshchenko, F. Reinert, R. Martin, and T. Kuznetsova

Abstract

Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi$_{1.1}$Sb$_{0.9}$Te_2S crystals and examined their elemental composition, structural, optical and electronic properties as well as the electronic band structure. The high structural quality of the grown crystals was established by X-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy demonstrated a near parabolic character of the valence and conduction bands and a direct bandgap of 0.36 eV. The dispersion relations also revealed a Dirac cone, confirming the topological insulator nature of this material, with the position of the Dirac point being 100 meV above the valence band maximum. Far infrared reflectivity spectra revealed a plasma edge and two phonon dips. Fitting these spectra with theoretical functions based on the Drude-Lorentz model allows determination of the high frequency dielectric constant (41.3), plasma frequency (936 cm−1) and the frequencies of two infrared phonons (177.7 cm−1 and 77.4 cm−1).

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