Structural and optical properties of Ge20SbxSe80-x films

20 Sb_xSe_80-x (where 10≤x≤40 at.%) thin films. The optical absorption results indicate that the absorption mechanism is due to non-direct transition. The optical gap of the as-deposited films was found to decrease monotonically with increasing antimony content, a result which was interpreted on the basis of the chemical-bond approach proposed by Bicerno and Ovshinsky. Annealing of the Ge₂₀Sb₄₀Se₄₀ films at temperatures higher than 450 K was found to decrease the optical gap and increase the refractive index of the investigated film. Increasing the amount of crystalline GeSe₂ and Sb₂Se₃ phases while increasing the annealing temperature could be responsible for the continuous decrease of the optical gap of the Ge₂₀Sb₄₀Se₄₀ film. Received: 4 November 1997/Accepted: 16 December 1997     

Influence of Ge addition on the optical properties of As40Se50Ge10 thin film probed by spectroscopy techniques

The thin films of As₄₀Se₆₀ and As₄₀Se₅₀Ge₁₀ were prepared on glass substrates by thermal evaporation method with thickness 1000 nm. The prepared films were amorphous in nature which was confirmed through X-ray diffraction. The chemical composition and the surface picture were obtained from energy dispersive X-ray analysis and Scanning Electron Microscopy analysis. The transmission data of the two films were collected in the wavelength range 400–1000 nm. The transmission percentage is found to be decreased whereas the absorption coefficient is increased with the Ge addition. The addition of Ge into As₄₀Se₆₀ is found to increase the refractive index and the extinction coefficient of As₄₀Se₅₀Ge₁₀ thin film. The decrease in optical band gap is explained on the basis of increase in density of states and disorderness due to Ge addition. The optical absorption in the film is due to allowed indirect transition, and the homopolar bond density is increased with Ge addition. The Raman shift observed in the two films clearly supports the optical changes due to Ge addition.     

Femtosecond optical Kerr effect study of Ge10As40S30Se20 film

The results of the femtosecond optical heterodyne detection of optical Kerr effect at 805 nm with the 80 fs ultrafast pulses in amorphous Ge₁₀As₄₀S₃₀Se₂₀ film is reported in this paper. The film shows an optical non-linear response of 200 fs under ultrafast 80 fs-pulse excitation, and the values of real and imaginary parts of non-linear susceptibility χ⁽³⁾ were 9.0×10⁻¹² and −4.0×10⁻¹² esu, respectively. The large third-order non-linearity and ultrafast response are attributed to the ultrafast distortion of the electron orbits surrounding the average positions of the nucleus of Ge, As, S and Se atoms. This Ge₁₀As₄₀S₃₀Se₂₀ chalcogenide glass would be expected as a promising material for optical switching technique.     

Effect of replacement of Se by S on structural and physical properties of Ge–Sb–As–Se–S chalcogenide glasses

In the present study, the structural and opto-mechanical properties of Ge–Sb–As–Se–S chalcogenide glasses have been investigated. For this purpose, different bulk glasses of Ge₂₀Sb₅As₁₅Se_60?xS_x (0 ≤ x≤50) were prepared by conventional melt quenching technique in quartz ampoule and different characteristics of prepared glasses such as glass transition temperature, density, hardness, transmittance, optical band gap energy and refractive index were determined. The value of hardness and glass transition temperature of prepared glasses were found to increase with increasing the sulfur content as a result of formation of GeS₄ tetrahedral units and increasing the network connectivity and average bonding energy. The optical energy gap (according to Tauc’s relation), transmittance and refractive index of prepared glasses are in direct relation with sulfur content. In this study, the highest value of transmittance (about 70%) and lowest value of refractive index (2–2.3) was achieved in Ge₂₀Sb₅As₁₅Se₄₀S₂₀ and Ge₂₀Sb₅As₁₅Se₁₀S₅₀ glasses, respectively.     

Effects of Pb addition and heat treatment on optical properties of thermally evaporated Ge1−xSe2Pbx thin films

Thin chalcogenide films of Ge_1−xSe₂Pb_x (x=0, 0.2, 0.4, 0.6 and 0.8) have been prepared by the thermal evaporation technique, from previously synthesized bulk samples. The X-ray diffraction showed the amorphous nature for the as-deposited films and the partially crystalline for the annealed films. The optical constants (the refractive index, n, and absorption index k) were determined for as-deposited and annealed Ge_1−xSe₂Pb_x films of different thicknesses by using spectrophotometeric measurements of the transmittance and reflectance at normal incidence in the spectral range 200-2500 nm. The obtained values of both n and k were found to be independent of the film thickness. The optical absorption edges are described using both the Urbach rule and the indirect transition. In transparent region, the spectral dependences of refractive index were interpreted in the frame of a single oscillator model.     

Effect of Cd and Pb impurities on the optical properties of?fresh?evaporated amorphous (As2Se3)90Ge10 thin films

Transmission spectra (400–1500 nm) of thermally evaporated amorphous [(As₂Se₃)₉₀Ge₁₀]₉₅M₅ thin films have been analyzed to study the effect of impurities (M = Cd and Pb) on their optical properties. The refractive index increases with addition of metal impurities. The dispersion of refractive index has been studied using Wemple–DiDomenico single oscillator model. The optical gap has been estimated using Tauc’s extrapolation and was found to decrease with the addition of metal impurities from 1.46 to 1.36 eV (Cd) and 1.41 eV (Pb) with an uncertainty of ±0.01 eV. The change in optical properties with metal impurities has been explained on the basis of density, polarizability and bond energy of the system.     

Effect of nonlinear refraction and two-photon absorption on the optical limiting in amorphous chalcogenide films

This paper reports on the results of investigations into the nonlinear optical characteristics of chalcogenide films (As₂S₃, As₂₀S₈₀, 2As₂S₃/As₂Se₃, 3As₂S₃/As₂Se₃). The nonlinear refractive indices and two-photon absorption coefficients for these films are measured using the Z-scan technique at wavelengths of a picosecond Nd: YAG laser (λ=1064 and 532 nm). The optical limiting due to Kerr-type nonlinearities is analyzed.     

Photosensitive post tuning of chalcogenide Te20As30Se50 narrow bandpass filters

We present an experimental study on the photosensitive properties of narrow bandpass filters based on a chalcogenide Te₂₀As₃₀Se₅₀ (TAS) spacer. The transmittance curve of single TAS layer was shifted towards long wavelength direction after 2 h exposure by Xenon arc lamp. The refractive index and extinction coefficient were both increased together with a red shift of optical gap. A maximum 1.7% photo-induced effect was observed. Narrow band filters constituted by TAS and cryolite were manufactured by electron beam deposition. The transmittance spectrum of the filter during the exposure by a wide band source was in situ measured and the resonant wavelength was observed to turn longer gradually till saturation. A spatially localized central wavelength change up to 5.7 nm was finally obtained. The stability of the photo-induced effect was studied and some comments were given at the end of this paper.     

Optical properties of polyvinyl alcohol doped (Se80Te20)100–xAgx (0 ≤ x ≤ 4) composites

Polyvinyl alcohol (PVA) doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films were prepared by the spin-coating technique on a quartz substrate. The optical parameters of PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) composites at the same chalcogen concentration (S₀ = 0.1 mg ml^?1) and PVA/(Se₈₀Te₂₀)₉₆Ag₄ composites at three different chalcogen concentrations viz. S₁ = 0.3 mg ml^?1, S₂ = 0.6 mg ml^?1 and S₃ = 1 mg ml^?1 have been studied. The semi-crystalline nature of the as-deposited thin filmsisdetermined by X-ray diffraction. The transmission and reflection spectra of PVA-doped Se–Te–Ag thin films were obtained in a 350–650 nm spectral region. The optical-band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical-band gap increases, but the refractive index, extinction coefficient, and the real and imaginary parts of the dielectric constant decrease, with increase in Agcontent in PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films. Such type of behavior is explained on the basis of decrease in density of the defect states. However, the optical-band gap has been found to be decreased and all other optical parameters show increase in their values with increase in concentration of (Se₈₀Te₂₀)₉₆Ag₄ glass in PVA-doped composites. The results have been explained on the basis of cluster-size formation at the time of dissolution. This study shows that the optical properties of new composites are affected by the change in silver and chalcogen concentration.     

Nonlinear optical absorption in Bi3TiNbO9 thin films using Z-scan?technique

Bi₃TiNbO₉ (BTN) thin films with layered perovskite structure were fabricated on fused silica by pulsed laser deposition. The XRD pattern revealed that the films are single-phase perovskite and highly (00l) textured. Their fundamental optical constants, such as band gap, linear refractive index, and linear absorption coefficient, were obtained by optical transmittance measurements. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The nonlinear optical absorption of the films was investigated by single beam Z-scan method at a wavelength of 800 nm with laser duration of 80 fs. We obtained the nonlinear absorption coefficient β=1.44×10⁻⁷ m/W. The results show that the BTN thin films are promising for applications in absorbing-type optical devices.     

Energy band gap and oscillator parameters of Ga4Se3S single crystals

The optical properties of the Bridgman method grown Ga₄Se₃S crystals have been investigated by means of room temperature, transmittance and reflectance spectral analysis. The optical data have revealed an indirect allowed transition band gap of 2.08 eV. The room temperature refractive index, which was calculated from the reflectance and transmittance data, allowed the identification of the dispersion and oscillator energies, static dielectric constant and static refractive index as 21.08 and 3.85 eV, 6.48 and 2.55, respectively.     

Facile synthesis and phase control of copper chalcogenides with different morphologies

A series of stoichiometric and nonstoichiometric copper–chalcogenide nanocrystallines with different morphologies, e.g., extremely high aspect ratio nanofibers (Cu₉S₈), tubular structure (Cu _x S (x=∼1.86–1.96), nanorods (CuS, Cu₃₁S₁₆), platelets (β-CuSe, Cu₃Se₂), rope-like Cu₃Se₂, as well as spherical nanoparticles (Cu₇Se₄, Cu_2−x Se), have been successfully synthesized in 20 vol% water and 80 vol% organic solvents mixture under mild conditions. The products were characterized by various techniques, including X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electronic diffraction (ED), and high-resolution transmission electron microscopy (HRTEM). The studies of the optical properties revealed that the copper chalcogenides have a wide absorption in the range of about 400–700 nm, with accessional IR band. Systematic studies showed that the mixture of 20 vol% water and 80 vol% organic solvents played a key role in controlling the copper chalcogenides with different morphologies and phases.     

Reversible and athermal photo-vitrification of As50Se50 thin films deposited onto silicon wafer and glass substrates

Photo-vitrification of As₅₀Se₅₀ thin films deposited onto silicon wafer and glass substrates has been studied using X-ray diffraction, far-infrared, and differential infrared spectroscopies. The optical study of this photo-amorphization effect has been carried out by two different methods enabling the determination of the average thickness and refractive index of a wedge-shaped thin film. The refractive-index behaviour of the as-evaporated, crystallized, and photo-vitrified As₅₀Se₅₀ films is analyzed within the single-oscillator approach. The optical-absorption edge is described using the non-direct transition model, and the optical energy gap is calculated. In the course of the vitrification of an As₅₀Se₅₀ thin film deposited on a silicon substrate the photo-oxidation of the film has been additionally detected and arsenic trioxide micro-crystals were formed on the surface of the film. Such oxidation has not been observed with As₅₀Se₅₀ films deposited on glass substrates, which demonstrates that the photo-vitrification phenomenon depends also on the type of substrate. Finally, it is concluded from the optical study that a reversible photo-darkening effect accompanies the photo-induced vitrification phenomenon. Received: 3 August 1998 / Accepted: 13 January 1999 / Published online: 7 April 1999     

Determination of the thickness and optical constants of amorphous Ge–Se–Bi thin films

The effect of varying bismuth concentration on the optical constants of amorphous Ge₂₀Se_{80? x} Bi _x (where x = 0, 3, 6, 9 and 12 at%) thin films prepared by thermal evaporation has been investigated. The transmission spectra T(λ) of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. An analysis proposed by Swanepoel [J. Phys. E: Sci. Instrum. 16 (1983) p.1214], based on the use of the maxima and minima of the interference fringes, was applied to derive the real and imaginary parts of the complex index of refraction and also the film thickness. Increasing bismuth content was found to affect the refractive index and extinction coefficient of the Ge₂₀Se_{80? x} Bi _x films. Optical absorption measurements show that the fundamental absorption edge is a function of composition. With increasing bismuth content, the refractive index increases while the optical band gap decreases.     

Effect of annealing on structure and optical properties of Ga5Se95 films

To investigate the effect of annealing on the structural and optical properties of a binary compound Ga₅Se₉₅, thin films of Ga₅Se₉₅ have been deposited on quartz substrates at room temperature by the thermal evaporation technique. X-ray diffraction patterns showed that the films before and after annealing at 573 K have polycrystalline texture and exhibit tetragonal structure. The dependences of the optical constants, the refractive index n and extinction coefficient k were studied in the spectral range of 200 nm to 2500 nm. The normal dispersion of the refractive index of the films could be described using the Wemple–DiDomenco single-oscillator model. Analysis of absorption index data reveals that as-deposited Ga₅Se₉₅ films has indirect transitions with optical energy gap of 1.685 eV.     

Interrelation between optical, refractometric, and lattice parameters of Cu6P(S1−x Sex)5I crystals

The influence of the compositional disordering on the energy of the fundamental absorption edge and the refractive index dispersion of the crystalline solid solutions Cu₆P(S_1?x Se_x)₅I is studied. It is shown that the concentration dependences of the optical pseudogap width, the refractive index, and the lattice parameter in these crystalline solid solutions are interrelated.     

Compositional dependence of the optical properties of amorphous Se100– x Te x thin films

Thin films of amorphous Se_{100 –x} Te _x with different compositions (x = 10, 20, 30 and 40 at%) were deposited on glass substrates by thermal evaporation. Transmission spectra T(λ) of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. A straightforward analysis proposed by Swanepoel [J. Phys. E: Sci. Instrum. 17 896 (1984)], using of the maxima and minima of the interference fringes, was applied to derive the real and imaginary parts of the complex index of refraction plus film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model [Phys. Rev. B 3 1338 (1971)]. The optical band gap was determined from the spectral dependence of the absorption coefficient using the Tauc formula [in The Optical Properties of Solids, edited by F. Abeles (North Holland, Amsterdam, 1970), p. 227]. The refractive index increases and the optical band gap decreases with increasing tellurium content.     

Effect of incorporating nonlanthanoidal indium on optical properties of ferroelectric Bi4Ti3O12 thin films

Bi₄Ti₃O₁₂ (BTO) and Bi_3.25In_0.75Ti₃O₁₂ (BTO:In) thin films were prepared on fused quartz and LaNiO₃/Si (LNO) substrates by chemical solution deposition (CSD). Their microstructures, ferroelectric and optical properties were investigated by X-ray diffraction, scanning electron microscope, ferroelectric tester and UV-visible-NIR spectrophotometer, respectively. The optical band-gaps of the films were found to be 3.64 and 3.45 eV for the BTO and BTO:In films, respectively. Optical constants (refractive indexes and extinction coefficients) were determined from the optical transmittance spectra using the envelope method. Following the single electronic oscillator model, the single oscillator energy E₀, the dispersion energy E_d, the average interband oscillator wavelength λ₀, the average oscillator strength S₀, the refractive index dispersion parameter (E₀/S₀), the chemical bonding quantity β, and the long wavelength refractive index n_∞ were obtained and analyzed. Both the refractive index and extinction coefficient of the BTO:In films are smaller than those of the BTO films. Furthermore, the refractive index dispersion parameter (E₀/S₀) increases and the chemical bonding quantity β decreases in the BTO and BTO:In films compared with those of bulk.