الفهرس 
Theoretical background and literature review.Only 14 pages are availabe for public view
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Abstract
Summary
The structural, thermal kinetics, dc electrical conductivity, switching phenomenon, ac conductivity, dielectric properties and optical properties of Se88Te12 and Se88Te8Zn4 compositions are studied. The investigated compositions were prepared by melting quenching technique in bulk form. The thermal evaporation technique is used to prepare our samples in thin film form.
X-ray diffraction analysis (XRD) revealed that the studied samples in powder and thin film forms have an amorphous structure. Energy dispersive X-ray spectroscopy analysis (EDX) indicated that the real percentages of the constituent elements in the studied compositions are near the same as that prepared with average deviation 0.01% for binary system and 0.14% for ternary system.
Thermal kinetics analysis was studied using differential thermal analysis (DTA) under non-isothermal conditions at different heating rates. The shifting of the characteristic transition temperatures with heating rates was used to determine the activation energy of glass transition Eg and crystallization Ec. The glass transition activation energy is found decreased while the crystallization activation energy and thermal stability increase with Zn addition to Se88Te12 composition. Analysis of Avarni exponent (n) indicates that the nucleation in our sample is change from three dimensional to two dimensional growths with Zn addition. New parameter (Relative change parameter PGR) is used to choose the more suitable and sensitive glass stability criterion and the less one as well for the studied compositions.
The temperature dependence of the dc electrical conductivity σdc is studied in temperature range below glass transition temperature Tg and thickness range (221-698 nm) and (172-780 nm) for Se88Te12 and Se88Te8Zn4 films respectively. The dc conduction activation energy (ΔEdc) was found Zn addition dependent while thickness independent.
The current-voltage (I-V) characteristic curves for Se88Te12 and Se88Te8Zn4 films revealed that they are typical for a memory switch type. The threshold voltage (Vth), resistance (Rth), power (Pth), field (Eth) are determined and studied as a function of temperature, thickness and Zn addition. The temperature and thickness dependence of threshold voltage Vth indicated that the application of electro thermal model to explain the switching process for the studied films.
The AC conductivity σac was measured for Se88Te12 and Se88Te8Zn4 films of different thickness in the frequency range (100 Hz-130 KHz). The frequency dependence of σac for the studied films is found satisfies the relation ( ), where s is the frequency exponent. The values of exponent s and its temperature dependence is found accordance with the Correlated Barrier Hopping (CBH) model between centers forming non-intimate valence alternation pairs (NIVP,s) for Se88Te12films while the hopping of carriers occurs between intimate valence alternation pairs (IVP,s) for Se88Te8Zn4 films. The temperature dependence of σac is also studied and the activation energy (ΔEac) is determined as well.
Both the dielectric constant (ɛ1) and the dielectric loss (ɛ2) increase with increasing temperature and have an opposite behavior with increasing frequency for Se88Te12 and Se88Te8Zn4 films. The frequency dependence of the dielectric loss (ɛ2) is discussed on the basis of (Guntini) equation to obtain the maximum barrier height (Wm). The obtained Wm values are found agree with theory of charge carriers over a potential barrier suggested in case of chalcogenide glasses.
Transmittance and reflectance measurements in the wavelenght range (500-2500 nm) for Se88Te12 and Se88Te8Zn4 films of different thickness are used to calculated the optical constants such as refractive index n, absorption index k. Murmann’s exact equations conjunction with a special relative computer programme can be used to determine the optical constants. The allowed optical transitions were found to be indirect transitions with an optical energy gap of 1.54 eV for Se88Te12 and 1.79 eV for Se88Te8Zn4. Moreover some of opto electric parameters like dielectric relaxation time τ, dissipation factor, and optical conductivity are determine.
Summary
The structural, thermal kinetics, dc electrical conductivity, switching phenomenon, ac conductivity, dielectric properties and optical properties of Se88Te12 and Se88Te8Zn4 compositions are studied. The investigated compositions were prepared by melting quenching technique in bulk form. The thermal evaporation technique is used to prepare our samples in thin film form.
X-ray diffraction analysis (XRD) revealed that the studied samples in powder and thin film forms have an amorphous structure. Energy dispersive X-ray spectroscopy analysis (EDX) indicated that the real percentages of the constituent elements in the studied compositions are near the same as that prepared with average deviation 0.01% for binary system and 0.14% for ternary system.
Thermal kinetics analysis was studied using differential thermal analysis (DTA) under non-isothermal conditions at different heating rates. The shifting of the characteristic transition temperatures with heating rates was used to determine the activation energy of glass transition Eg and crystallization Ec. The glass transition activation energy is found decreased while the crystallization activation energy and thermal stability increase with Zn addition to Se88Te12 composition. Analysis of Avarni exponent (n) indicates that the nucleation in our sample is change from three dimensional to two dimensional growths with Zn addition. New parameter (Relative change parameter PGR) is used to choose the more suitable and sensitive glass stability criterion and the less one as well for the studied compositions.
The temperature dependence of the dc electrical conductivity σdc is studied in temperature range below glass transition temperature Tg and thickness range (221-698 nm) and (172-780 nm) for Se88Te12 and Se88Te8Zn4 films respectively. The dc conduction activation energy (ΔEdc) was found Zn addition dependent while thickness independent.
The current-voltage (I-V) characteristic curves for Se88Te12 and Se88Te8Zn4 films revealed that they are typical for a memory switch type. The threshold voltage (Vth), resistance (Rth), power (Pth), field (Eth) are determined and studied as a function of temperature, thickness and Zn addition. The temperature and thickness dependence of threshold voltage Vth indicated that the application of electro thermal model to explain the switching process for the studied films.
The AC conductivity σac was measured for Se88Te12 and Se88Te8Zn4 films of different thickness in the frequency range (100 Hz-130 KHz). The frequency dependence of σac for the studied films is found satisfies the relation ( ), where s is the frequency exponent. The values of exponent s and its temperature dependence is found accordance with the Correlated Barrier Hopping (CBH) model between centers forming non-intimate valence alternation pairs (NIVP,s) for Se88Te12films while the hopping of carriers occurs between intimate valence alternation pairs (IVP,s) for Se88Te8Zn4 films. The temperature dependence of σac is also studied and the activation energy (ΔEac) is determined as well.
Both the dielectric constant (ɛ1) and the dielectric loss (ɛ2) increase with increasing temperature and have an opposite behavior with increasing frequency for Se88Te12 and Se88Te8Zn4 films. The frequency dependence of the dielectric loss (ɛ2) is discussed on the basis of (Guntini) equation to obtain the maximum barrier height (Wm). The obtained Wm values are found agree with theory of charge carriers over a potential barrier suggested in case of chalcogenide glasses.
Transmittance and reflectance measurements in the wavelenght range (500-2500 nm) for Se88Te12 and Se88Te8Zn4 films of different thickness are used to calculated the optical constants such as refractive index n, absorption index k. Murmann’s exact equations conjunction with a special relative computer programme can be used to determine the optical constants. The allowed optical transitions were found to be indirect transitions with an optical energy gap of 1.54 eV for Se88Te12 and 1.79 eV for Se88Te8Zn4. Moreover some of opto electric parameters like dielectric relaxation time τ, dissipation factor, and optical conductivity are determine.