## 3 Results and Discussion
### THz Conductivity and Permittivity  
The THz waveforms were measured for Bi\({}_{1-x}\)Sb\({}_{x}\) solid solutions with Bi concentration \(x\) of 5, 8, 12 and 15 % and thickness of 150 nm and 70 nm, which were placed on the surface of mica (21 \(\upmu\)m) and polyimide (42 \(\upmu\)m) substrates (see Figure 3). The corresponding amplitude spectra are also shown in this Figure for 0.2-0.8 THz frequency range. Using Equations. (10)-(12), the film conductivity and permittivity dispersions (Figure 4 and 5) were extracted from the measured data. As seen in Figure 3 both mica and PI substrates almost do not absorb the THz radiation (the decrease in transmittance is associated with Fresnel losses). On the contrary, the substrate/film structures transmit relatively small part of the radiation. This is due to both high reflectance of Bi\({}_{1-x}\)Sb\({}_{x}\) films which are placed on the top side of the substrate and to high absorption in these semimetallic films. The transmittance is higher for the thinner films and is also slightly higher for the case of PI substrate. The latter effect is due to the lower conductivity of the films on PI substrate, as it will be shown below. It should be mentioned that the used substrate strongly affects the morphology of Bi\({}_{1-x}\)Sb\({}_{x}\) polycrystalline films due to the morphology of a substrate surface (mica is a crystal, but PI is a polymer) and the difference between the thermal expansion coefficients of a substrate and a film. This difference plays an important role because such films are produced by thermal spraying method and are deformed in a cooling process. The substrate morphology affects the characteristic size of the crystallites, which makes the main contribution to a scattering of charge carriers and reduction of conductivity. The change of amplitude is more noticeable for the thinner films (70 nm) and is also higher in case of PI substrate. Figure 2: The dependencies of Bi\({}_{1-x}\)Sb\({}_{x}\) sheet resistivity (**a**), magnetoresistivity (**b**) and Hall coefficient (**c**) on Sb concentration \(x\) for 150 nm and 70 nm films on mica substrate. (**d**) The same for carrier concentration in 150 nm films on mica and PI substrates and for 70 nm film on mica substrate. (**e,f**) The same for electron and hole mobility in 150 nm Bi\({}_{1-x}\)Sb\({}_{x}\) films on PI and mica substrates. Figure 4: The dynamical complex conductivity spectra of Bi\({}_{1-x}\)Sb\({}_{x}\) films in 0.2–0.8 THz frequency range for the cases of mica (**a**,**b**) and PI (**c**,**d**) substrates and for 150 nm (**a**,**c**) and 70 nm (**b**,**d**) film thicknesses. Different colors represent different Sb concentrations. The solid and dashed lines represent the real and imaginary parts of conductivity, respectively. Figure 3: The temporary dependencies of a relative electric field amplitude of THz wave (THz waveforms) transmitted through the air, mica (**a**,**b**) and PI (**c**,**d**) substrates and full substrate/film structures for film thicknesses of 150 nm (**a**,**c**) and 70 nm (**b**,**d**) and in cases of various Sb concentrations. The corresponding amplitude spectra are shown in the insets for 0.2–0.8 THz frequency range. As shown in Figure 4 the conductivity is higher in case of thicker films and for mica substrate usage (due to a lower scattering rate of charge carriers). However, both real and imaginary parts of conductivity show better tunability (when varying the Sb concentration) in case of thinner samples (for 70 nm films). With the THz signal frequency increasing, the real part of film conductivity tends to decrease for all presented cases, while the imaginary part has a certain maximum value. This behavior is in a good agreement with a classical Drude model for charge carriers due to the semimetallic properties of the Bi\({}_{1-x}\)Sb\({}_{x}\) films. It should be mentioned that the conductivity of these thin-film solid solutions non-linearly depends on the Sb concentration \(x\) due to the complex behavior of their band structure.