M.A. Afifi

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.

Optical properties of amorphous GeSeTl system films

Abstract The optical properties of Ge Se (9– x ) Tl x thin-film samples of different thicknesses are studied as a function of the thallium contents. Transmittance and reflectance measurements in the range of (400–2600 nm) of various thicknesses range of (114–618.3 nm) were used to calculate the optical band gap E g opt , the width of localised states and optical constants n and k. It is shown that the optical band-gap decreases and the width of the tail increases with the increase in the Tl content. The allowed optical transitions were found to be indirect transitions. From the transmission spectra, the relationship between the refractive index n and the wavelength (λ) at different Tl contents is derived. The increase in the refractive index with the increase in the Tl content is attributed to the increase in the valency density of the current carriers. Analysis of the refractive indices has yielded optical dielectric constant (e ∞ ).

The switching phenomenon in amorphous In2Te3 thin films

Abstract Investigation of the switching phenomenon in amorphous ln2Te3 films revealed that it is typical for a memory switch. The thickness dependence of the mean value of the switching voltage Vth was linear in the investigated range and V th decreased exponentially with a temperature rise from 298 to 373 K. The switching voltage activation energy (ϵ) calculated from the temperature dependence of V th is about 0.25 eV. The conduction activation energy (Eσ) obtained from the temperature dependence of film resistance was found to be 0.51 eV. The agreement between the obtained value of the ratio ϵ E σ (0.49) and those of the temperature difference between that inside the film and that of its surface with their values obtained before suggests that the switching phenomenon in the investigated In2Te3 films may be explained according to an electrothermal model for the switching process.

Effect of annealing on the electrical properties of ln2Se3 thin films

Abstract Thin film samples of In 2 Se 3 were prepared from synthesized polycrystalline ln 2 Se 3 by a thermal evaporation technique. X-ray analysis show that the synthesized material has α-phase structure, while the as-deposited films have an amorphous structure. The films have β-phase structure after annealing at temperature ⩾ 523 K. The conduction activation energy ( E σ ) of the as-deposited samples, as obtained from the temperature dependence of the electrical conductivity, is 0.391 eV. The obtained value of E σ was explained according to a previously postulated band structure of the investigated material. The electrical conductivity of the investigated samples behaves differently with time after annealing at different elevated temperatures. On one hand, E σ decreased for samples after annealing at 423 and 473 K to 0.313 and 0.262 eV respectively; this may be due to the increase of the degree of ordering in the investigated compound with annealing. On the other hand, E σ increased for samples after annealing at 523 K to 0.787 eV; this is due to its crystallization to the β-phase, as confirmed by its X-ray diffraction pattern.

Determination and analysis of optical constants for Ga2Se3 films near absorption edge

Abstract Ga2Se3 has been prepared in bulk and thin film forms. The composition of films has been checked using energy dispersive X-ray spectroscopy technique. X-ray diffraction measurements have shown that Ga2Se3 films evaporated at room temperature substrates were amorphous. The transmittance (T) of Ga2Se3 thin films has been measured over the wavelength range (400– 900 nm ). The optical constants, the refractive index (n) and the absorption index (k) have been determined from the analysis of the transmittance data. Analysis of the refractive index (n) yields the values of the long wavelength dielectric constant (e∞), the average oscillator wavelength (λo), average oscillator strength (So), average oscillator energy Eo, the refractive index dispersion parameter (Eo/So) and the dispersion energy (Ed). Analysis of absorption index (k) yields both direct and indirect allowed transitions with optical energy gaps of 2.65 and 2.056 eV , respectively. The effect of annealing at different temperatures on optical constants is also investigated.

Threshold switching in Te46-xAs32+xGe10Si12 chalcogenide glass system

Abstract Five different compositions of Te46 − xAs32+xGe10Si12 chalcogenide glass system were prepared with the atomic percentages 0, 1, 2, 4 and 5. I–V characteristic curves for thin film samples of the investigated compositions were typical for threshold switching behaviour. It was found that the threshold voltage increased linearly with film thickness in the investigated range (100–800 nm) for all compositions. However, threshold voltage Vth decreased exponentially with temperature. The threshold voltage activation energy (e) was obtained for the investigated compositions. The electrical conduction activation energy (Eσ) for the investigated compositions was obtained from the corresponding temperature dependence of film sample resistance. The ratio ( e E σ ) was obtained theoretically on the basis of an electrothermal breakdown process. A good agreement was obtained between theoretical and experimental values of ( e E σ ). The lifetime for threshold switch devices on the basis of the investigated compositions was measured also. The compositional dependence of Vth, e and the lifetime was also investigated.

Effect of annealing on the structural and electrical properties of In2Te3

Amorphous In2Te3 was prepared in both bulk form, by quenching the molten material, and thin-film form, by the thermal evaporation technique. X-ray diffraction analysis showed that the prepared samples in bulk and as-deposited thin-film forms were in the amorphous state. β- and α-phases of In2Te3 were prepared by annealing bulk samples at 615 and 813 K, respectively. Films annealed at 573 K give β-phase polycrystalline structure. The electrical conductivity for the as-deposited In2Te3 films increases with increasing film thickness. The conduction activation energy, ΔEσ, of the as-prepared bulk and thin film samples were found to be 0.516 and 0.521 eV. The corresponding values of room-temperature electrical conductivity, σRT, for these samples are 1.1×10-6 and 7.15×10-7Ω-1m-1, respectively. The observed change in the value of σRT may be due to the difference in the structure of bulk and thin-film samples. The increase of ΔEσ with annealing temperature for both bulk and thin-film samples is interpreted in terms of the density of states model proposed by Mott and Davis.

Effect of thallium on the optical properties of amorphous GeSe2 and GeSe4 films

The optical properties of amorphous GeSe2, GeSe4, GeSe2Tl and GeSe4Tl films, prepared by thermal deposition from their bulk glasses, have been studied. The respective effect of adding thallium to a-GeSe2 and a-GeSe4 results in increasing the density, d, from 4.16 to 6.2 g cm-3 and from 4.34 to 5.72 g cm-3, the refractive index, n, from 2.42 to 2.78 and from 2.43 to 2.62, whereas decreasing the glass transition temperature, Tg from 395 to 192 degrees C and from 165 to 134 degrees C, and the optical energy gap, Eg, from 2.07 to 1.54 eV and from 2.02 to 1.48 eV. The obtained results are discussed in the light of the proposed structural models of amorphous Ge-Se. The decrease in the energy gap with the introduction of thallium involves a change in the microstructure accompanied by a decrease in the chemical ordering of the stoichiometric compound a-GeSe2 and an increase in the amount of chemical disorder in the composition of a-GeSe4. The pronounced decrease of the gap (about 25%) is attributed to an alloying effect rather than to a hypothetical increase of the gap state density.

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.

Electrical resistivity of crystalline Sb2Se3

Abstract Resistivity variations of bulk and crystallized Sb 2 Se 3 films of different thicknesses were studied at different temperatures. The dark electrical resistivity decreases with increasing film thickness. For substrate temperatures below the crystallization temperature, resistivity variations were characteristic of amorphous films and for greater temperatures an anomalous initial resistivity increase was observed. At higher working temperatures the transition region was not observed. Positive decreasing TCR values were obtained up to 340 K after which a negative constant value was attained for polycrystalline films. While the resistivity and the conduction activation energy decreased with increasing film thickness, the free charge carrier concentration increased and gave a value of the order 10 12 −10 16 cm −3 for the bulk material.