Electronic localization on the structural inhomogeneities formed due to Bi and Te deficiency in the MBE grown films of AF topological insulator MnBi2Te4: Evidence from spectroscopic ellipsometry and infrared studies

The intrinsic substitutional and antisite defects cause unintentional doping and a shift of the EF position above the conduction band minimum in the AF topological insulator MnBi2Te4. This prevents measurements of the quantum anomalous Hall effect and the investigation of the topological Dirac states. In the present study, the Mn-Bi-Te films grown by the MBE technique onto Si(111) substrates with decreasing Bi and Te contents and increasing Mn content were investigated by 0.5-6.5 eV spectroscopic ellipsometry. In addition, the 0.004-0.9 eV infrared transmittance spectra were examined. An effective medium model was used to reproduce the measured ellipsometric angles, Ψ(ω) and Δ(ω), of the Mn-Bi-Te films in terms of the constructed model, including film thickness, surface roughness, and volume fractions of two (MnTe and Bi2Te3) or three constituents, the latter being associated with the structural inhomogeneities contribution. The results obtained for the inhomogeneous Mn-Bi-Te films using the three-phase effective medium approximation model reveal that the defect-associated optical response systematically shifts to higher photon energies from ∼1.95 to ∼2.43 eV with decreasing Te and Bi contents and increasing Mn content, indicating that the electrons become more deeply localized in the formed structural inhomogeneities. The obtained results indicate that the structure of the non-stoichiometric Mn-Bi-Te films is not continuous but represented by regions of a nearly stoichiometric MnBi2Te4 phase, which includes hollows or quantum anti-dots. The measured far-infrared transmittance spectra for the non-stoichiometric Mn-Bi-Te films show substantially reduced (or absent) contributions from free charge carriers, which supports the relevance of localization effects.