Two anomalous broad bands are usually found in the Raman spectrum of bulk and 2D Te-based chalcogenides, which include binary compounds, like ZnTe, CdTe, HgTe, GaTe, GeTe, SnTe, PbTe, GeTe₂, As₂Te₃, Sb₂Te₃, Bi₂Te₃, NiTe₂, IrTe₂, and TiTe₂, as well as ternary compounds, like GaGeTe, SnSb₂Te₄, SnBi₂Te₄, and GeSb₂Te₅. Many different explanations have been proposed in the literature for the origin of the anomalous broad bands in tellurides, usually located between 119 and 145 cm⁻¹. They have been attributed to the intrinsic Raman modes of the sample, to oxidation of the sample, to the folding of Brillouin-edge modes onto the zone center, to the existence of a double resonance, like that of graphene, or to the formation of Te precipitates. In this paper, we provide arguments to demonstrate that such bands correspond to clusters or precipitates of trigonal Te in the form of nanosize or microsize grains or layers that are segregated either inside or at the surface of the samples. Several mechanisms for Te segregation are discussed and sample heating caused by excessive laser power during Raman scattering measurements is emphasized. Besides, we show that anomalous Raman modes related to Se precipitates also occur in selenides, thus providing a general vision for better characterization of selenides and tellurides by means of Raman scattering measurements and for a better understanding of chalcogenides in general.

The relation between the concentration of free charge carriers and the concentration of copper atoms in Bi₂Te₃ single crystals doped with copper over a wide range of concentrations has been investigated, with the aim of clarifying the existence of inactive Cu ions. Changes in the concentration of free charge carriers arising from Cu-doping of the melt with that induced by electrochemical intercalation of copper are compared. Models of possible defect structures are proposed for both doped and intercalated single crystals of Bi₂Te₃.

While the bulk, stoichiometric Bi${}_2$Te${}_3$ single crystals often exhibit $p$-type metallic electrical conduction due to the Bi${}_{\mathrm{Te}}$-type antisite defects, doping by thallium (Bi${}_{2-x}$Tl${}_x$Te${}_3$, $x=0-0.30$) progressively changes the electrical conduction of single crystals from $p$ type (0 ≤ $x$ ≤ 0.08) to $n$ type (0.12 ≤ $x$ ≤ 0.30). This is observed via measurements of both the Seebeck coefficient and the Hall effect performed in the crystallographic (0001) plane in the temperature range of 2–300 K. Since any kind of substitution of Tl on the Bi or Te sublattices would result in an enhancement of the density of holes rather than its decrease, and because simple incorporation of Tl at interstitial sites or in the van der Waals gap is unlikely as it would increase the lattice parameters which is not observed in experiments, incorporation of Tl likely proceeds via the formation of TlBiTe${}_2$ fragments coexisting with the quintuple layer structure of Bi${}_2$Te${}_3$. At low levels of Tl, 0 ≤ $x$ ≤ 0.05, the temperature-dependent in-plane ($I\perp c$) electrical resistivity maintains its metallic character as the density of holes decreases. Heavier Tl content with 0.08 ≤ $x$ ≤ 0.12 drives the electrical resistivity into a prominent nonmetallic regime displaying characteristic metal-insulator transitions upon cooling to below ∼100 K. At the highest concentrations of Tl, 0.20 ≤ $x$ ≤ 0.30, the samples revert back into the metallic state with low resistivity. Thermal conductivity measurements of Bi${}_2$Te${}_3$ single crystals containing Tl, as examined by the Debye-Callaway phonon conductivity model, reveal a generally stronger point-defect scattering of phonons with the increasing Tl content. The systematic evolution of transport properties suggests that the Fermi level of Bi${}_2$Te${}_{3,}$ which initially lies in the valence band (for $x$ = 0), gradually shifts toward the top of the valence band (for 0.01 ≤ $x$ ≤ 0.05), then moves into the band gap (for 0.08 ≤ $x$ ≤ 0.12), and eventually intersects the conduction band (for 0.20 ≤ $x$ ≤ 0.30).