Study On Infrared Optical Properties Of Tellurium Elemental Semiconductor

Tellurium(Te),as a narrow-bandgap semiconductor,has become the research hotspot in optoelectronics,thermoelectric and spintronic fields,due to the complex band structure and unique chiral properties.However,most researchers have predicted the band structure of Te and its complex structural changes and potential topological properties under strain regulation by theoretical calculations.In this paper,mid-infrared transmission(370-7500 cm^-1)spectroscopy is utilized to investigate fine band structure characteristics of Te,and the temperature evolution of lattice vibration is explored by variable-temperature far-infrared transmission(30-680 cm-1)and Raman spectra,which is of great significance for the study of the optoelectronic and thermal properties of Te-based optoelectronic devices.The major contents are summarized as follows:(1)Variable-temperature mid-infrared transmission spectroscopy of Te single crystal is performed by Fourier transform infrared spectrometer to obtain the connection between the intrinsic absorption and band structure.The results of mid-infrared absorption spectra show that the energy values between the valence-band(H₄,H₅ and H₆)maximum and the conduction-band minimum of Te are about 0.333 e V,0.42 e V and 0.76 e V respectively at T=5 K.In addition,the Tauc plot and the first-order derivative spectrum of absorption coefficient of Te single crystal verify that there are both direct transition and indirect transition near the absorption edge.According to the energy difference between the extracted direct gap(0.333 e V)and indirect gap(0.325 e V)of Te from the first derivative spectrum,suggests that the energy between the extremums of the camel's back H₄ band is about 8 me V.Finally,the non-monotonic temperature dependence of band gap can be simulated and analyzed by the modified Manoogian-Leclerc model.(2)The results of variable-temperature far-infrared transmission spectroscopy show that the one-phonon absorption peaks V₀,V₁ and V₂ are dominated by three Raman phonon vibrational modes,including the bond-bending mode E₁,the chain expansion mode A₁ and antisymmetric bond-stretching mode E₂,and their shift varies with temperature differently.The results of variable-temperature Raman spectroscopy indicate that the Raman phonon vibrational modes E₁,A₁ and E₂undergo redshifts with increasing temperature.According to the first-order linear fitting of phonon frequencies with temperature,the vibration of Te atoms in the vertical direction of the helical chain is more sensitive to temperature than the parallel direction,revealing the anisotropic thermal expansion of Te.(3)The rod-shaped Te single crystal is prepared by physical vapor deposition(PVD)method and the basically characterization is carried out.The mid-infrared transmission spectra,Raman spectra and I-V test of Te rods and Te single crystal flake are compared to illustrate the effects of shape and size on the optical properties,lattice vibration and electrical properties of Te.The results emphasize the thickness dependence of band gap of Te single crystal.