S.S. Fouad

The relationship between optical gap and chemical composition in SbxSe1−x system

Abstract The optical constants of vacuum deposited SbxSe1−x amorphous thin films (x = 0.075, 0.15 and 0.20 at%) of different thicknesses (90–150 nm) were studied in the wavelength range 550–800 nm. It was found that both the refractive index n and the absorption index k are independent of the film thickness, whilst there is a distinct dependence of n and k on the Sb content. The allowed optical transition was found to be non-direct, the corresponding optical gaps Egopt are determined. The width of the band tail Ee is also determined. As a result, Egopt decreases while Ee increases with increasing Sb contents. The variation of Egopt with x for SbxSe1−x agree with the optical gap Eo predicted theoretically in terms of random bond network proposed by Shimakawa. The relationship between the optical gap and chemical composition in SbxSe1−x amorphous system is discussed in terms of the average heat of atomization Hs and the average coordination number m. The results of these calculations can be used to rationalize the observed optical properties of these materials.

Derivation of a relation between the conduction mechanism and chemical bonding of amorphous Ge15Se85−xAgx alloys

Abstract A critical analysis of the existing theories of AC conduction in amorphous Ge15Se85−xAgx films with x=0, 10 and 20 has been made. Measurements of the conductivity and the dielectric constant have been made at various frequencies from 100xa0Hz to 100xa0kHz and at various temperatures from 303 to 393xa0K. The AC conductivity of these alloys is found to be temperature dependent and to obey the Aωs law. The conductivity and dielectric behavior of these glasses have been explained on the basis of the correlated barrier hopping model, as well as the effect of Ag atoms on the homonuclear bonds. The effect of both the chemical composition and the nature of chemical bonding of amorphous Ge–Se–Ag alloyed samples is analyzed using a simple consideration based on the coordination number Nco. The coordination number has been used for the estimation of the cohesive energies of these glasses, assuming simple additivity of the bond energies. It was found that there is a good correlation between experimental and theoretical results.

Optical and other physical characteristics of amorphous Ge–Se–Ag alloys

Abstract Measurements of optical properties and differential thermal analysis have been made on alloyed samples of Ge and Se with Ag, prepared under vacuum to evaluate their glass-forming tendencies. Optical absorption measurements showed that the fundamental absorption edge is a function of composition. The optical absorption is due to direct transition and the energy gap increases with the increase of Ag content. The average heat of atomization and related parameters such as the average coordination number N co , the number of constraints per atom, N con , were also calculated. It was found that there is a correlation between the glass-forming ability and the number of lone-pair electrons L . An attempt has been made to evaluate the correlation according to the Liang criterion. Our experimental results support this criterion.

On the glass transition temperature and related parameters in the glassy GexSe1−x system

Abstract High-purity GexSe1−x amorphous samples of different composition (0.05⩽x⩽0.30) were prepared. An empirical relation between the glass transition temperature (Tg) and the coordination number Nc is proposed by modifying the Gibbs–Di Marzio equation for glass transition temperature of a cross-linked polymer as a function of cross-linked density. The utility of the Gibbs-Di Marzio relation was achieved by estimating Tg theoretically. The Tg results were also compared with an arbitrary defined optical gap (Eo) where Tg provided an index of atomic mobility, and Eo an index of covalent bond strength. From the previous calculations the Tg–Eo correlation is interpreted in terms of a proposed fluidity equation for covalent liquids with 2.1⩽Nc⩽2.7. The relationship between Eo and the chemical composition in the vacuum prepared GexSe1−x is discussed in terms of the average heat of atomization (Hs) and the average coordination number (Nc). The effect of both the chemical composition and the nature of the chemical bonding on the efficiency of the structural modification (ΔΦ) of the GexSe1−x system is also analyzed.

Memory switching of ZnGa2Te4 thin films

Electrical and switching property of amorphous defect chalcopyrite ZnGa2Te4 thin films prepared by thermal evaporation technique has been studied. The elemental chemical compositions of the prepared bulk as well as the as-deposited film were determined by means of energy dispersive X-ray spectrometry. X-ray diffraction pattern revealed that the powder compound is polycrystalline and the as-deposited and the annealed films at taxa0≤xa0548xa0K have the amorphous phase, while that the annealed at txa0≥xa0573xa0K are polycrystalline with a single phase of a defect chalcopyrite structure similar to that of the synthesized material. The great advantage of this material is the capability to appear in two different phases, the amorphous and the crystalline phases, with rather different electrical properties. Both dynamic and static I–V characteristics and the switching phenomenon at 601xa0nm are investigated. The threshold switching mechanism was explained by a thermal model of switching, i.e., joule heating with an electrically conducting channel. ZnGa2Te4 is good candidate in phase change memory device.

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.

Topology of covalent non-crystalline solids: Short-range order in SeyGexSb1−x−y alloys

Abstract The pronounced glass-forming tendencies of alloys of Se and Ge with Sb are discussed topologically in terms of the chemical bonds expected to be present in these materials. Using simple considerations based on coordination numbers (m) and bond energies, the average number of near neighbors of each type expected to surround the atom has been estimated. These average number of bonds have been used to estimate the cohesive energies (CE) of these glasses, assuming simple additivity of bond energies. An attempt has also been made to explain the previously published data of threshold voltage (Vth), optical properties (E04) and glass transition temperature (Tg) of these materials. On the basis of our calculations, the correlation between Tg and E04 has been interpreted in terms of a proposed fluidity equation for covalent liquids with 2.3 ⩽ m ⩽ 2.49.

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.

The influence of indium on the structural modification of the amorphous Ge–In–Se system

Abstract The effect of both the chemical composition and the nature of the chemical bonding of amorphous alloyed samples of GexInySe100−x−y prepared under vacuum with x=20, 4⩽y⩽15 on the efficiency of the structural modification Δφ is analysed using a simple consideration based on the coordination number Z, and the number of topological constraints Nco. The previously obtained parameters have been used for the determination of the number of continuous deformation (i.e. zero-frequency modes fxa0), and for the estimation of the cohesive energies of these glasses, assuming simple additivity of bond energies. A trial has been made to correlate the results of this paper with the previously published data of glass transition temperature Tg, activation energy e0 and the shift of the K-absorption edge (ΔEK) of the composition of vacuum prepared GexInySe100−x−y. It was found that there is a correlation between the lone-pair electrons and the stability of the vitreous state. According to the criterion of Liang, the above correlation has been interpreted in terms of Δφ.

The optical properties of manganese thin films

Abstract The optical constants n and k of thin manganese films of different thicknesses ranging from 20 up to 50 nm were determined in the spectral range 3.5–20 μm. The results were compared with the predictions of the Drude free electron theory and anomalous skin effect theory. The previous theories were used to evaluate some micro characteristics of Mn thin films such as the free charge concentration N , the relaxation time τ, the static conductivity σ s , and the electron velocity at Fermi surface V F and the value of the effective area of Fermi surface A F . Using the optical constants, the optical conductivity ( σ = σ 1 + i σ 2 ) was also estimated. It was found that the contribution of the effective absorption A eff equals 69%, i.e. the absorption due to bound electrons may be 31%.