H. El-Zahed

Influence of composition on the electrical and optical properties of Ge20BixSe80−x films

Abstract The optical gap and electrical resistivity of a series of Ge20BixSe80−x glasses (x=0, 5, 9 10, 15, 20, 25) was measured. In order to determine the optical gap, Eg, the absorption spectra of films with different Bi content, (denoted by x), were analyzed. The optical band gap showed a sharp decrease from 2.1 to 1.65 eV as the Bi content increased to 10 at.%. A slight change was observed for Bi >10 at.%. The electrical activation energy of the films was determined by investigating the temperature dependence of resistivity. A large decrease in the electrical activation energy from 0.97 to 0.09 eV was observed as the Bi content was increased up to 10 at.%. The optical and electrical data suggest that the addition of Bi produces localized states near the conduction band edge so the electrical transport is due to the hopping of electrons after being excited into localized states at the conduction band edge.

Optical studies of [N(CH3)4]2CoCl4, [N(CH3)4]2MnCl4 single crystals in the normal paraelectric phase

Abstract The absorption edge in the main crystallographic directions of [N(CH 3 ) 4 ] 2 CoCl 4 and [N(CH 3 ) 4 ] 2 MnCl 4 single crystals were studied in the normal paraelectric phase (at 303xa0K). The optical transmittance T and absorbance A were measured and applied to deduce the absorption coefficient α and optical band gap E g . The analysis of the data revealed the existence of two optical transition mechanisms. The calculated values of the direct band gap E gd for both crystals were estimated to be between 4.553 and 4.667xa0eV for the Co compound; and between 5.351 and 5.383xa0eV for the Mn compound. The allowed indirect transition data were analyzed and interpreted in terms of the two valence bands which were suggested to be originated from the spin–orbit interaction and crystal-field splitting. The momenta E p were attributed to the energy difference between the heavy and the split valence bands. The anomalous behavior of the polar c -axis for [N(CH 3 ) 4 ] 2 CoCl 4 crystals was attributed to the presence of domain walls in the paraelectric phase. The refractive index n , and both the real e r and imaginary parts e i of the dielectric permittivity were calculated and the results are discussed.

Optical absorption studies on AgInSe2 and AgInTe2 thin films

Abstract The analysis of optical spectra of AgInSe 2 and AgInTe 2 thin films has been discussed. These ternary chalcopyrite compounds show an allowed direct transition near the fundamental absorption edge (E g1 ) in addition to a transition originating from crystal-field split levels (E g2 ). The calculated band tail in the first region (E e1 ) and (E e2 ) in the second region revealed that the crystallinity of AgInTe 2 was higher than that of the AgInSe 2 composition. The extinction coefficient ( k ) and refractive index ( n ) were also calculated for both compositions.

Effect of heat treatment on some of the optical parameters of Cu9Ge11Te80 films

Abstract Some optical parameters of as-deposited and annealed Cu 9 Ge 11 Te 80 films were studied in the visible spectral range using both absorbance and transmittance measurements. The absorption coefficient was computed and the analysis of the data revealed the existence of two optical transition mechanisms. The direct optical energy gap E gd decreases from 1.74 to 1.58xa0eV as the film annealed to 520xa0K. Such behavior is attributed at present to unsaturated defects, which increase the density of localized states. The width of the tails of localized states E e were calculated and found to be increased at high annealing temperatures. Films annealed at high temperature ( T a >420xa0K) show a splitting of valence band by the spin orbit interaction. One may conclude that at high concentrations of Cu ( x =9), it is likely that the glass forming ability decreases. Other optical parameters ( e i , e r , n and k ) were calculated at different annealing temperatures.

Influence of composition and heat treatment on the structure of Se-Te films

Abstract Thin films of Se 1 − x Te x (composition x = 0, 0.2, 0.4, 0.6 and 0.8) were obtained by direct thermal evaporation of the bulk compounds. X-ray diffraction and electron diffraction microscopy investigations were carried out on Se 1 − x Te x films. From X-ray analysis and electron diffraction measurements it was found that the films with x x > 0.6 the as-deposited films were crystalline and the crystallinity increased with increasing Te content. It has been reported that the binary system Se 1− x Te x has a hexagonal structure. The lattice parameters a and c and the grain size for the annealed films of Se 1 − x Te x are calculated.

The effect of γ-doses on the optical band gap of AgInSe2 films

Abstract Some optical properties of AgInSe 2 thin films are studied as a function of γ-doses. In order to determine the optical energy gap E g , the absorbance spectra of the films of different γ-doses are analyzed. The compound shows an allowed direct transition near the fundamental absorption edge E g1 , in addition to transition originating from the crystal field split level E g2 . The direct energy gap E g1 increases from 1.21xa0eV for an unirradiated film to 1.31xa0eV for an irradiated film (0.7xa0Mrad) and then decreases to 1.17xa0eV for higher doses (3.0xa0Mrad). E g2 exhibits an opposite trend of E g1 , i.e. with the first dose (0.7xa0Mrad) it decreases from 1.76xa0eV for the unirradiated film to 1.66xa0eV and then increases to 1.75xa0eV as the γ-dose increases to 3.0xa0Mrad. The calculated band tails in the first region ( E e1 ) and in the second region ( E e2 ) revealed that the crystal field reduces the crystallinity of the composition. The influence of γ-irradiation on the optical gap was explained in terms of diminution of disorder, defects in the structural bonding and crystal field mechanism. From the absorbance spectra, the relationship between the extinction coefficient ( k ) and refractive index ( n ) at different γ-doses were derived.

Optical studies of K2ZnCl4 single crystals doped with Cs+ in the ferroelectric phase

Abstract The optical band gaps of K2ZnCl4 single crystals doped with Cs+ in the three crystallographic directions at different temperatures were calculated in both direct and indirect transitions. The data on the allowed indirect transition was analyzed and interpreted in terms of valence band initiated by spin-orbit interaction and crystal field splitting. The energy difference Ep between Eg1, the first valence and conduction band and Eg2 for the second valence band was calculated. Analysis of absorption spectra indicates that for a- and c-axes, the indirect Egi and direct Egd energy gaps decreased with increasing temperature. The anomalous behavior of the band gap in the b-axis was attributed to the relatively weak electrostatic interactions between the cations and the stacks. The results are discussed in comparison with our previous work on pure and Mn2+ doped K2ZnCl4 single crystals.

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

Abstract The dielectric permittivity ( ϵ ) of TMA-ZC single crystals was measured along the mean crystallographic axes a , b and c , in a temperature range from 273 to 340xa0K. The ϵ - T relationship exhibited peak values at T i =296xa0K and T c1 =279xa0K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j - E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.

Optical studies of (NH4)2SO4 single crystal in the paraelectric phase

Abstract Transmittance and absorbance spectra of (NH 4 ) 2 SO 4 single crystals along [010] direction were measured at different temperatures (296, 308, 318, 328 and 348 K) in the paraelectric phase. The absorption coefficient was computed and the analysis of the data revealed the existence of two optical transitions in (NH 4 ) 2 SO 4 single crystals. The direct and indirect band gaps were shifted towards the longer wavelength with increasing temperature. The data on the allowed indirect transition was analyzed and interpreted in terms of two valence bands originated by spin orbit interaction and crystal field splitting. The momenta E p were calculated as the difference between E g1 , the first valence to conduction band, and E g2 for the second valence band at different temperatures. The results of extinction coefficient ( k ), the refractive index ( n ), and dielectric constants ( e ) were also discussed and calculated as a function of wave length ( λ ). The heat treatment of the crystals proved that the variations of these optical parameters can be consequence of the internal microstructure changes caused by annealing.

Optical parameter studies of as-deposited and annealed Se1 − xTex films

Abstract Thin films of Se1 − xTex (x = 0.2, 0.4, 0.6) are deposited on a glass substrate by thermal evaporation under vacuum. The optical gaps (Eg) are determined from the absorbance and transmittance measurements in the visible and near IR spectral range (500–1100 nm), and are found to decrease with the tellurium concentration. After annealing at different elevated temperatures (423 K and 473 K), the values of optical gaps of Se0.8Te0.2 films, as a representative example, are also found to increase with temperature from 1.91 eV at room temperature to 1.97 eV at 473 K. This effect is interpreted in terms of the density-of-state model of Mott and Davis. The optical constants (extinction coefficient k, and refractive index n) were also determined. It was found that both n and k depend markedly on composition as well as the temperature of heat treatment. The valence band density of states of Se1 − x Tex films is calculated from the optical absorption data.