Band Structure and Lattice Vibration of Elemental Tellurium Investigated by Temperature-Dependent Mid-and-Far Infrared Transmission and Raman Spectroscopy

Tellurium (Te) recently attracts much attention owing to its peculiar band structure arousing high-efficiency thermoelectricity and layered crystal structure suitable for band engineering. Herein, the temperature evolution of band structure and lattice vibration in Te single crystal are studied by mid-and-far infrared transmission (7500–370 and 680–30 cm⁻¹) and Raman spectra measurements. Mid-infrared absorption spectra confirm Te is a narrow indirect gap semiconductor, in which the direct (indirect) bandgap is about 0.333 (0.325) eV, the two upper valence bands (H₄ and H₅) and the third valence band (H₆) being about 0.333, 0.42, and 0.76 eV, respectively, below the conduction-band minima at 5 K. Meanwhile, the direct and indirect gaps of Te reveal non-monotonic variation from 5 to 290 K due to the competition between electron–phonon interaction and lattice thermal expansion, which is well described by the modified Manoogian–Leclerc model. Moreover, the temperature evolution of Raman and far-infrared transmission spectra proves there are two kinds of phonons in Te, which originates from the different thermal expansion of Te atoms along parallel and vertical directions of spiral chain. Thus, the anisotropic thermal expansion in Te is indicated and a reference for studying thermoelectricity of Te-based optoelectronic devices is provided.