Mid-infrared complex permittivity in Bi2-Sb Te3-Se thin films synthesized using a combinatorial method

Successful employment of plasmonics-inspired solutions in the mid-infrared frequency range would benefit a number of highly important technologies. As a plasmonic material beyond gold and silver, Bi-based chalcogenide semiconductor alloys exhibit a wide tunability in the spectral dispersion of relative permittivity by varying the composition of their ternary and quaternary compounds. In our investigation, a combinatorial materials library of Bi2-xSbxTe3-ySey thin films were synthesized. The complete solid solubility and the in-depth chemical composition profile of thin film pixels in the library were confirmed using micro-area X-ray diffraction and secondary ion mass spectroscopy, respectively. The stoichiometries of thin film pixels were acquired using the energy dispersive X-ray analysis. The mid-infrared spectral transmittance was measured using a Fourier-transform infrared spectrometer. The complex permittivity ε(ω) in the spectral range from 2.5 to 15 μm was derived by fitting spectral transmittance to the Drude–Lorentz equation. It can be demonstrated that the negative values of real part of permittivity, εr, emerge at the shorter wavelengths. The crossover wavelength, at which εr crosses zero from negative to positive, can shift from 5.346 μm for Bi2Se3 to 9.232 μm for Sb2Te3 with the increasing of x and the deceasing of y. Therefore, an exceptionally wide and potentially interesting mid-infrared plasmonic property, which is compositionally adjustable, can be carried out for evaporation-deposited Bi2-xSbxTe3-ySey solid-solution thin films.