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Structural, dc electrical conductivity, switching phenomenon, ac electrical conductivity, dielectric and optical properties were studied for Te81Ge15Bi4 films. The amorphous Te81Ge15Bi4 composition was prepared in bulk form by quenching the molten materials in an iced water after synthesis according to a special regime in an evacuated silica tube and then thin film form was prepared from bulk composition by thermal evaporation technique.
X-ray diffraction (XRD) analysis revealed the amorphous nature of the prepared samples in thin film form.
Differential thermal analysis (DTA) for Te81Ge15Bi4 in powder form revealed that the values of glass transition temperature T_g, crystallization temperature T_c and melting temperature T_m are 408 K, 496 K and 666 K, respectively.
Energy dispersive X-ray analysis (EDX) indicates that the real percentage of the constituent elements in the investigated composition are Te81Ge15Bi4 which close to that prepared.
The temperature dependence of the dc electrical conductivity σ_dc was studied for Te81Ge15Bi4 films in the temperature range (303 - 393 K) and in the thickness range (143 - 721 nm), showed that σ_dc increases linearly with increasing temperature and decreases with increasing film thickness. The obtained results showed that the conduction activation energy 〖∆E〗_σ has a single value ( 0.32 eV) and is independent of film thickness through the considering range of temperature for thin film samples of the studied composition, which can be interpreted according to Mott and Davis model.
The obtained results of the static I-V characteristic curves for Te81Ge15Bi4 films of different thicknesses in the range (143 - 721 nm) and in the temperature range (303 - 373 K) revealed that, these curves are typical for a memory switches. The mean value of the threshold voltage V ̅_th increases linearly with increasing film thickness (d) and decreases exponentially with increasing temperature in the studied ranges of thickness and temperature. The obtained value of threshold voltage activation energy ε_th is independent of film thickness in the investigated range and its average value is equal to 0.156 eV. The obtained mean value of ε_th⁄(〖∆E〗_σ≈) 0.484 for the studied films agrees with that obtained theoretically on the basis of an electrothermal model (0.5) for switching process. Additionally, values of the temperature difference between the inside of the film and that of its surface (〖∆T〗_breakdown), which calculated on the basis of the electrothermal model are in the same order with those obtained previously for other amorphous chalcogenide glasses. Therefore, the switching process in the Te81Ge15Bi4 films can be explained according to the electrothermal model.
Ac electrical conductivity σ_ac (ω), dielectric constant ε_1 (ω), dielectric loss ε_2 (ω), the real M_1 (ω) and imaginary M_2 (ω) parts of electric modulus for Te81Ge15Bi4 films were studied as a function of temperature in the range (303 – 393 K), frequency in the range (100Hz - 1MHz) and thickness in the range (143 - 721 nm).
The obtained results showed that the ac conductivity σ_ac (ω) depends on temperature, frequency and the thickness of film sample. The frequency dependence of σ_ac (ω) of the studied films satisfies the power low σ_ac (ω)=Aω^S, where S is the frequency exponent. The average values of the frequency exponent S ̅ decreases with increasing temperature in both frequency ranges [100 Hz - 100 KHz] and [100 KHz - 1 MHz], in accordance with the correlated barrier hopping (CBH) model. The temperature dependence of σ_ac (ω) is linear with a single activation energy. The ac conduction activation energy ΔE_σ (ω) is independent on the film thickness in the studied range and decreases with increasing frequency. The calculated values of the maximum barrier height W_M decreases with temperature in both frequency ranges [100 Hz - 100 KHz] and [100 KHz - 1 MHz]. This decrease may be due to increase in the degree of overlap between potential wells of sites in the structure of the studied samples. The density of localized states at Fermi level N(E_F) in the structure of the investigated composition increases with increasing temperature and frequency. This may be attributed to the increase of σ_ac (ω) with increasing temperature and frequency.
It is found that both dielectric constant ε_1 (ω) and dielectric loss ε_2 (ω) of Te81Ge15Bi4 thin films increased with temperature and decreased with frequency in the investigated ranges of temperature and frequency. The real M_1 (ω) and imaginary M_2 (ω) parts of electric modulus were studied for the investigated films as a function of temperature in the studied range. The calculated value of the activation energy for relaxation process or hopping activation energy of charge carries ∆E_r is thicknesses independent in the investigated range.
The optical properties of Te81Ge15Bi4 films are studied in the thickness range (143 - 721 nm) and in the wavelength range (400 - 2500 nm). Transmittance T(λ) was measured at room temperature for the studied films in the considered ranges of thickness and wavelength to calculate the optical constants (refractive index n and absorption index k) using Swanepoel’s method. Analysis of absorption index k data for the investigated films indicates that the allowed optical transitions were found to be indirect transitions and the values of the optical band gap E_g^opt and the width of the tail of localized states E_e are 0.652 eV and 0.089 eV, respectively. Also, analysis of the obtained values of the refractive index n at normal dispersion region yielded the optical dispersion parameters (E_o and E_d), the static refractive index n_o, the optical high frequency dielectric constant ε_∞, the average inter-band oscillator wavelength λ_o, the average oscillator strength S_o, the lattice dielectric constant ε_L, the ratio of free charge-carriers concentration and its effective mass N⁄m^* and plasma frequency ω_p. The dependence of the dielectric constant ε_(1 ), dielectric loss ε_(2 ), dissipation factor tanδ, dielectric relaxation time τ , volume energy loss function (VELF), surface energy loss function (SELF) and optical conductivity 〖 σ〗_opt on photon energy hν for all investigated films were studied also in the fundamental region and near the fundamental edge. Values of linear optical susceptibility χ^((1)), 3rd order nonlinear optical susceptibility χ^((3)) and the nonlinear refractive index n_2 are increased with increasing photon energy hν. The calculated values of χ^((3)) and n_2 at (hν⟶0) are
6.04 × 10-11 esu and 6.96 × 10-10 esu, respectively for Te81Ge15Bi4 thin films.