الفهرس 
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Abstract
The present thesis is devoted to study the structural identification, dc electrical conductivity, switching phenomenon, ac electrical conductivity, dielectric and optical properties of Te81Ge15Bi4 thin films.
Structural identification for Te81Ge15Bi4 in thin film form is confirmed by X-ray diffraction (XRD) analysis, energy dispersive X-ray (EDX) spectroscopy and differential thermal analysis (DTA).
Dc conductivity σ_dc measurements of the investigated composition in thin film form were carried out as a function of thickness in the range (143 - 721 nm) and temperature in the range (303 - 393 K). The dc conductivity σ_dc for Te81Ge15Bi4 films increases with increasing temperature and decreases with increasing film thickness. The obtained results showed that the value of the conduction activation energy 〖∆E〗_σ is independent on the film thickness and the conduction occurs by hopping of charge carriers through the localized states in the investigated range of temperature and thickness.
The I-V characteristic curves of Te81Ge15Bi4 films were investigated in the temperature range (303 - 373 K) and in the thickness range (143 - 721 nm). The obtained I-V curves are typical for a memory switch. It is found that the mean value of the threshold switching voltage increases with increasing film thickness while it decreases with increasing temperature. The obtained mean value of the ratio ε_th⁄〖∆E〗_σ (0.484) is attributed to an electrothermal model initiated by Joule heating.
Ac conductivity σ_ac (ω) and dielectric measurements were carried out for the investigated composition at different frequencies in the range (100 Hz - 1 MHz) and in the temperature range (303 - 393 K). The ac conductivity σ_ac (ω) for the investigated composition is temperature dependent, it obeys A ω^(S ) law, 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]. The obtained results are explained on the basis of correlated barrier hopping (CBH) model. The ac conduction activation energy ΔE_σ (ω) is independent on the film thickness and decreases with increasing frequency in the studied range. 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]. The density of localized states N(E_F) in the structure of the investigated composition increases with increasing temperature and frequency. Both dielectric constant ε_1 (ω) and dielectric loss ε_2 (ω) for the studied composition 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 and the value activation energy for relaxation process ∆E_r is thicknesses independent in the investigated range.
The optical properties of Te81Ge15Bi4 films were studied using spectrophotometric measurements of transmittance T(λ). The optical constants (refractive index n, absorption index k) were determined from the transmittance T(λ) data using Swanepoelʼs method. The optical band gap E_g^opt and the absorption edge E_e are estimated from the values of the absorption coefficient α using the indirect transition model by Tauc’s extrapolation procedure. The optical dispersion parameters (E_o and E_d), the 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^* were calculated from the analysis of the obtained data of the refractive index n in the normal dispersion region. The dependence of dielectric constant ε_1, dielectric loss ε_2, dissipation factor tanδ, 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 using the values of n and k 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 the 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 thin Te81Ge15Bi4 films.