M. Fadel

Electrical and thermal properties of chalcogenide glass system Se75Ge25?x Sb x

The dc and thermal conductivities of five different compositions of the chalcogenide glass system Se75Ge25−xSbx have been studied in a temperature range below Tg. The dc conductivity results indicate that each composition has a single activation energy in the considered temperature range. The coefficient of thermal conductivity increased linearly with temperature below Tg for the compositions investigated. The increase of Sb content in the chalcogenide glass system leads to an increased coefficient of electrical conductivity σ, an increased coefficient of thermal conductivity ψ, and to a decreased activation energy Eσ and pre-exponential factor σ0. The observed compositional dependencies of σ and Eσ have been correlated with the increase of weak bond density and the decrease of covalent bond density in the structure of the compositions investigated with increasing Sb content at the expense of Ge content. The decrease in σ0 and the increase in ψ has been, respectively, correlated with the decrease in mobility and the increase in phonon velocity.

Some optical properties of Se-Ge-As amorphous chalcogenide glasses

The effects of composition, film thickness, substrate temperature, and annealing of amorphous thin films of Se75Ge25−xAsx (5⩽x⩽20) on their optical properties have been investigated. X-ray diffraction revealed the formation of amorphous films. The absorbance and transmission of vacuum-evaporated thin films were used to determine the band gap and refractive index. Optical absorption measurements showed that the fundamental absorption edge is a function of glass composition and the optical absorption is due to indirect transition. The energy gap increases linearly with increasing arsenic content. The optical band gap,Eopt, was found to be almost thickness independent. The shapes of the absorption edge of annealed samples displayed roughly the same characteristic as those of the unannealed films, but were shifted towards shorter wavelengths; as a result,Eopt increased andEe, the width of the band tails, decreases. The increase inEopt is believed to be associated with void removal and microstructural re-arrangement during annealing. The influence of substrate temperature on the optical parameters is discussed.

Theoretical and experimental study of the conduction mechanism in Sb2 Se3 alloy

To understand the nature of Sb2 Se3 semiconductor as a bulk sample, the X-ray diffraction study was carried out to identify the sample structure. Dc and ac electrical properties measured in the temperature range 290–370 K, and frequency range 102–105 Hz were the subject of the present work. Properties such as dielectric constant ζ\, loss function tanδ, and electrical conductivities σac as a function of both frequency and temperature are reported. Different models of electrical conduction in semiconductor materials have been used to explain the observed results. A trail has been made to correlate the results of this paper with the average heat of atomization HS and the average coordination number M. The experimental and theoretical investigation is in accordance with Liang criterion.

Effect of composition, film thickness and annealing on the optical properties of Bi-Sb-Se thin films

Thin films of Bi10SbxSe90−x (x≥35, 40, 45) of different composition and thickness, were deposited on glass substrates by vacuum evaporation. Optical absorption measurements show that the fundamental absorption edge is a function of glass composition, film thickness and annealing temperature. The optical absorption is due to indirect electronic transitions. The value of the optical band gap was found to increase with thickness and decreasing the antimony content and with increasing temperature of heat treatment. The validity of the Urbach rule was investigated and the respective parameters estimated. X-ray diffraction was used to obtain an insight into the structural information.

Electrical and thermal properties of Ge40S60 films

Films of the composition Ge40S60 have been studied in the temperature range of 313–423 K for electrical conductivity, and 293–373 K for thermal conductivity. The dc conductivity results indicate a single value activation energy of 0.863 eV for the conductivity in the applied temperature range. The thermal conductivity coefficient increases linearly with temperature at a thickness of d=0.311 cm. It was found that the investigated samples show a memory effect. The threshold switching voltage was found to increase linearly with film thickness. Moreover, the threshold voltage decreases exponentially with temperature. The data are analysed using a thermal model for the switching process.

AC conductivity of Se-Ge-As glassy system in relation to rigidity percolation

Measurements of conductivity (ac & dc) and dielectric constant (∈) have been made for amorphous alloyed samples of Se0.75Ge0.25−xAsxwith x= 0.05, 0.10, 0.15 and 0.20 at different temperature (289 to 389 K) and various frequencies (102to 105Hz). The conductivity and the dielectric constant of these glasses have been explained on the basis of the correlated barrier hopping (CBH) model. Recent progress in applying percolation theory to explain properties and glass forming ability of chalcogenide glasses is critically reviewed. Percolation theory is shown to be relevant to the liquid-state behavior of glass-forming ability of the Se0.75Ge0.25−xAsxchalcogenide system. The relationship between the optical gap (ΔEg) and chemical composition is also discussed in terms of the average heat of atomization (Hs) and the average coordination number (r). These findings provide to some extent an important link between experimental and theoretical results.

Investigation of solid-state reaction in Ag/Sn nanostructured thin films at room temperature

Diffusion intermixing processes in nanostructured Ag/Sn thin-film system at room temperature were investigated by means of secondary neutral mass Spectrometry depth profiling technique. As it was confirmed by X-ray diffraction too, the reaction started already in the as-deposited sample. Since the bulk diffusion was frozen at room temperature, the Ag3Sn phase was formed along the grain boundaries (GBs), gradually consuming the interior of grains, and was grown perpendicular to the GBs. At the same time, formation and growth of a small compact reaction layer near the interface were observed and the shift of the bordering parallel interfaces was controlled by GB diffusion. From the kinetics of the diffusion process in the above two mechanisms, both the interface velocity in the diffusion-induced grain boundary motion regime as well as the coefficient of parabolic growth in the planar growth regime were determined.

Preparation and investigation on glasses in the Te46As32Ge10Si12 and Te41As37Ge10Si12 systems

Thin films of Te46−xAs32+xGe10Si12 (x=0,5) of different thicknesses are deposited on glass substrate by vacuum evaporation. X-ray diffraction revealed the formation of amorphous films. The value of the optical band gap, Eg, is found to increase with the thickness of the films and with increasing As content. The films are heat treated at different elevated temperatures from 298 to 423 K. The values of Eg are found to decrease with increasing temperature of heat treatment. The band tail, Ee, obey Urbachs empirical relation.

Temperature dependence of electrical and thermal properties of Te82.2Ge13.22Si4.58 glassy alloy

Thin film and bulk samples were prepared from synthesized amorphous Te82.2Ge13.22Si4.58 chalcogenide glassy alloy by the thermal evaporation technique. X-ray analysis shows that both synthesized material and thin film forms have an amorphous nature. The electrical and thermal conductivities for the bulk sample were studied as a function of temperature in a range below Tg (Tg = 423 K). The obtained results for electrical conductivity are explained in accordance with the Mott and Davis model. The switching effects in amorphous films were also investigated. The switching phenomenon for this composition was of memory type. The mean value of the threshold voltage th was found to increase linearly with increasing film thickness (98.9-250.1 nm), while it decreased exponentially with increasing temperature (below Tg). The results obtained are explained in accordance with the electrothermal model for the switching process.

Characterization of Ge–Bi–S glass by thermal, electrical, switching and optical measurements

A study of the synthesized Ge22.5Bi7S70.5 glassy system has been carried out. Differential thermal analysis data indicate the retention in the as-quenched sample of two amorphous phases. Thermal conductivity, ψ, measurements on bulk sample reveal that the main contribution to ψ is due to phonon thermal conductivity. Thermal evaporation of the synthesized ingot gives films with Ge20.7Bi6.8S72.5 as composition. The values of the activation energy and the pre-exponential factor calculated from the direct current electrical conductivity above 53 °C suggest that carrier conduction occurred between extended states in these films. The I–V characteristics in the off-state and the switching phenomenon are investigated. A memory switch with a threshold voltage decreasing with temperature is detected for the studied films. Optical parameters such as absorption coefficient, optical gap and refractive index are also determined. Comparison with binary Ge–S glass reveals that the addition of Bi introduces additional absorbing states at band edges.