الفهرس 
Introduction and Aim of the WorkOnly 14 pages are availabe for public view
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Abstract
Samples belonging to the system Se80Ge20-xBix (where x = 0, 5, 10, 15 and 20 at. %) were prepared in the amorphous state by means of well known melt quenching technique. All alloys were amorphous as checked by X-ray diffraction. The energy dispersive X-ray (EDX) analysis indicates the absence of impurities in the studied alloys. The thermograms of differential thermal analysis (DTA) yield the characteristic fixed points of the glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm). Thin films of Se80Ge20-xBix (x = 0, 5, 10, 15 and 20 at. %) were prepared by thermal evaporation technique. The absence of sharp peaks and appearance of humps in the X-ray diffraction (XRD) patterns of the asdeposited thin films confirms the amorphous nature of these films. The transmittance (T) and the reflectance (R) of the system Se80Ge20-xBix were recorded in spectral range 400 nm to 2500 nm. The T and R as functions of the wavelength (λ) were nonlinear. The optical constants; refractive index (n), absorption coefficient (α) and extinction coefficient (k) were determined from the detected transmittance (T) and reflectance (R) in the same wavelength range. The replacing of Ge by Bi in the Se80Ge20-xBix system tends to decrease the optical energy gap (Eg) from 1.86 eV to 1.25 eV as the Bi content increases from 0 to 20 at. %. The optical data suggests that the addition of Bi produces localized states within the band edges, so that the electric transport is due to the hopping of electrons. The electrical properties of the thin films have been monitored by the dc conductivity measurements over a temperature range of 301 K to 433 K. The dc conductivity increases exponentially with increasing the temperature. Additionally, for each composition, there are two linear parts of different slopes. This indicates that there are two conductionmechanisms contributing to the dc conductivity. At high temperatures, the dominant mechanism is the band conduction through the extended states. Hopping conduction via localized states is responsible for the conduction at low temperatures. In the high temperature region, the electrical activation energy decreases from 1.077 eV to 0.535 eV with increasing Bi content from 0 to 20 at. %. This suggests that the addition of Bi produces localized states within the band edge so that the electric transport is due to electrons hopping. The D.C. conductivity at low temperature region (T < 376 K) are used to calculate the density of localized states (DOLS) at Fermi level, N(EF). These results are used to calculate the hopping distance (Rav), the hopping energy (W) and the conduction electrons (N). It is found that both N(EF) and N increase with increasing Bi content, while Rav and W decrease in the same sense. The effects of thermal annealing below the crystallization temperature, Tc and above Tg on the structural, optical and electrical properties of Se80Ge20-xBix (x = 0, 5, 10, 15 and 20 at. %) was investigated. The samples were annealed at 573 K and 673 K for 90 min. The X-ray analysis shows that the amorphous (as-prepared) films have been transformed from the disordered state into the ordered one. Additionally, it was observed that the increase in the annealing temperature leads to increase in the number and size of the crystallized particles. Also it is found that the optical energy gap Eg decreases with increasing annealing temperature. The electrical activation energies seem to decrease with increasing the annealing temperature as well as Bi content. The room temperature conductivity, σRT (Ω-1cm-1), increases with both annealing and Bi content. The density of localized states N(EF) increases with increasing the annealing temperature as will as Bi content. The optical energy gap was found to be twice the electrical energy gap for both the green samples and the thermally treated samples.