Electronic band structure vs intrinsic antisite doping in the MBE grown films MnTe · Bi(2−x)Te3(1−x/2) (0 ≥ x < 2): Evidence from spectroscopic ellipsometry and infrared studies

The intrinsic antisite defects, which cause doping in the antiferromagnetic topological insulators of the MnTe ·n Bi2Te3 (n = 1, 2, 3, …,) family, prevent the exploration of the Dirac states affecting the Fermi level (EF) position and magnetic properties. In the present study, the MnTe · Bi(2−x)Te3(1−x/2) films grown by the MBE technique onto Si(111) substrates with increasing the Bi and Te contents from MnTe to MnBi2Te4 were investigated by 0.5–6.5 eV spectroscopic ellipsometry. In addition, the infrared (IR) reflectance and transmittance spectra were examined. The measured ellipsometric angles, Ψ(ω) and Δ(ω), were simulated in a two- or three-layer Gaussian models. As a result, the spectra of the complex dielectric function, ε̃(ω)=ε1(ω)+iε2(ω), the complex index of refraction, ñ(ω)=n(ω)+ik(ω), and the optical conductivity σ1(ω) were determined. We found that the absolute values of the ε1(ω) and ε2(ω) increased with increasing the Bi and Te contents from MnTe to MnBi2Te4, while the ε2(ω) maximum progressively shifts to lower photon energies from ∼3.7 to ∼1.2 eV, peculiar of the end point compounds. At the same time, the stoichiometric MnBi2Te4 film exhibits the emergent Drude-type contribution in the far-IR range associated with the intrinsic antisite doping. However, the charge carrier contribution is suppressed in the MnTe · Bi(2−x)Te3(1−x/2) films with the reduced Bi and Te stoichiometry, the latter being also responsible for the electronic band structure reconstruction and pronounced redistribution of the optical spectral weight.