Influence of thickness on the optical properties of amorphous GeSe2thin films: analysis using Raman spectra, Urbach energy and Tauc parameter
TL;DR: An analysis of thickness-induced defects in amorphous GeSe2 thin films deposited by the vacuum evaporation technique is reported in this article, showing that increasing the thickness promotes tetrahedral and bi-tetrahedral bonding through the reduction in bonding defects.
Abstract: An analysis is reported of thickness-induced defects in amorphous GeSe2 thin films deposited by the vacuum evaporation technique. X-ray diffraction studies confirmed the amorphous nature of the thin films. Optical absorption measurements revealed an indirect transition with an energy gap that increases with film thickness. A blue shift in optical transmittance edges was observed in annealed GeSe2 thin films. The obtained lower values of Urbach energy (E U) indicate that as thickness increases more ordered films can be produced. Raman spectra suggest that annealing promotes corner-sharing GeSe4/2 tetrahedra and edge-sharing Ge2Se8/2 bi-tetrahedra bonding and leads to the reduction in disorder in bonding network, which is amply supported by the way of increase in band gap, increase in Tauc parameter (B 1/2) and reduction in E U from the analysis of transmittance spectra. Increasing the thickness promotes tetrahedral and bi-tetrahedral bonding through the reduction in bonding defects.
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TL;DR: In this article, high resolution Raman spectra of GeSe 2 glass were measured and fitted using individual Gaussian components and the structural origin of the components were interpreted using the results of ab initio density functional theory calculations performed on Ge n Se m nanoclusters.
Abstract: High resolution Raman spectra of GeSe 2 glass were measured and fitted using individual Gaussian components The structural origin of the components were interpreted using the results of ab initio density functional theory calculations performed on Ge n Se m nanoclusters (n = 2–6, 12; m = 6–9, 12, 14–16, 30) which represent the local structure of GeSe 2 glass and on some “defect” Ge n Se m clusters that are thought to be related to the inhomogeneity of the structure at the nanoscale The calculated vibrational properties of Ge n Se m nanoclusters and their couplings with the short- and medium-range order structure formations in GeSe 2 glass are analyzed and discussed
27 citations
TL;DR: In this article, the optical properties of evaporated Ge 30 Se 70 thin films and their relation to thickness were calculated using the O'Leary, Johnson and Lim (OJL) model, based on joint density of states functions.
Abstract: In order to calculate the optical properties of evaporated Ge 30 Se 70 thin films and their relation to thickness, amorphous Ge 30 Se 70 thin films of different thicknesses were deposited by thermal evaporation at a base pressure of 7.5 × 10 −6 Torr at room temperature. The optical transmission and reflection spectra of all films were measured in the wavelength range of 0.2–2.5 μm. Efficient parameterization of the spectral dependence of the optical constants of amorphous Ge–Se thin films was obtained by applying a suitable dielectric function model. The O’Leary, Johnson and Lim (OJL) model, based on joint density of states functions, was used to analyze the optical spectra. The best fit was obtained by configuring the film as two layers, the top layer consisting of bulk Ge 30 Se 70 material embedded in air medium containing different volumes of voids. Therefore, the OJL model coupled with the Bruggeman effective-medium approximation model were used to determine the optical constants of the Ge 30 Se 70 thin films. The photon energy dependence of the dielectric function, ɛ = ɛ r − iɛ i of the investigated films is presented. The film thickness, absorption coefficient α , refractive index n , extinction coefficient k , static refractive index n ( 0 ) and optical band gap Eg were deduced. We found that increasing the film thickness increased the direct optical energy gap and decreased the refractive index.
20 citations
TL;DR: In this paper, the optimal geometries, total energies and their derivatives as well as the electronic properties of GenSem nanoclusters were calculated using traditional DFT method and the TD-DFT method has been applied to calculate the electronic band gaps of the clusters.
Abstract: Ab initio DFT calculations were performed on GenSem nanoclusters (n = 2, 3, 5, 6, 12; m = 6–9, 14, 16, 30) that represent the local structure of GeSe2 glass and on some ‘defect’ GenSem clusters that are thought to be related to the inhomogeneity of the structure at the nanoscale. The optimal geometries, total energies and their derivatives as well as the electronic properties of GenSem nanoclusters were calculated using traditional DFT method. In addition, the TD-DFT method has been applied to calculate the electronic band gaps of the clusters. The calculated physico-chemical properties of GenSem nanoclusters and their couplings with the local-and medium-range order structure formations in GeSe2 glass are analysed and discussed.
19 citations
TL;DR: In this paper, photo-induced effects on the optical and structural properties of ternary Ge30Se70−xBix (x = 5, 10) thin films illuminated with 532 nm green laser light were reported.
Abstract: The paper reports the photo-induced effects on the optical and structural properties of ternary Ge30Se70−xBix (x = 5, 10) thin films illuminated with 532 nm green laser light. The material exhibits photo-bleaching nature when exposed to laser light for a prolonged time. The amorphous nature sustains after laser irradiation as detected by X-ray diffraction. The chemical composition of the deposited thin film was examined by energy dispersive X-ray analysis. Field emission scanning electron microscopy investigation showed that the surface morphology was influenced by the laser irradiation. The transmission spectra were collected from UV–Vis-NIR spectroscopy which shows the films exhibit indirect allowed transition. The other optical parameters were calculated from the transmission spectra. The linear optical properties were influenced by the laser-induced phenomena. The photobleaching is explained on the basis of homopolar bond breaking and formation of heteropolar bonds with photon energy. The Raman spectra provided the evidence of photo structural changes in the films.
13 citations
TL;DR: In this article, the electrical switching, structural, optical and photoacoustic analysis have been undertaken on chalcogenide GeSe1.5S0.5 thin films of various thicknesses prepared by vacuum evaporation technique.
Abstract: Investigations on the electrical switching, structural, optical and photoacoustic analysis have been undertaken on chalcogenide GeSe1.5S0.5 thin films of various thicknesses prepared by vacuum evaporation technique. The decrease of band gap energy with increase in film thickness has been explained using the `density of states model'. The structural units of the films are characterized using Raman spectroscopy and the deconvoluted Raman peaks obtained from Gaussian fit around 188 cm(-1), 204 cm(-1) and 214 cm(-1) favors Ge-chalcogen tetrahedral units forming corner and edge sharing tetrahedra. All the thin films samples have been exhibited memory-type electrical switching behavior. An enhancement in the threshold voltages of GeSe1.5S0.5 thin films have been observed with increase in film thickness. The thickness dependence of switching voltages provide an insight into the switching mechanism and it is explained by the Joule heating effect. (C) 2014 Elsevier B.V. All rights reserved.
12 citations
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Book•
01 Jan 1940
TL;DR: The Fermi Glass and the Anderson Transition as discussed by the authorsermi glass and Anderson transition have been studied in the context of non-crystalline Semiconductors, such as tetrahedrally-bonded semiconductors.
Abstract: 1. Introduction 2. Theory of Electrons in a Non-Crystalline Medium 3. Phonons and Polarons 4. The Fermi Glass and the Anderson Transition 5. Liquid Metals and Semimetals 6. Non-Crystalline Semiconductors 7. Tetrahedrally-Bonded Semiconductors - Amorphous Germanium and Silicon 8. Aresnic and Other Three-Fold Co-ordinated Materials 9. Chalcogenide and Other Glasses 10. Selenium, Tellurium, and their Alloys
8,188 citations
TL;DR: In this article, the optical constants of amorphous Ge were determined for the photon energies from 0.08 to 1.6 eV, and the absorption is due to k-conserving transitions of holes between the valence bands as in p-type crystals.
Abstract: The optical constants of amorphous Ge are determined for the photon energies from 0.08 to 1.6 eV. From 0.08 to 0.5 eV, the absorption is due to k-conserving transitions of holes between the valence bands as in p-type crystals; the spin-orbit splitting is found to be 0.20 and 0.21 eV in non-annealed, and annealed samples respectively. The effective masses of the holes in the three bands are 0.49 m (respectively 0.43 m); 0.04 m, and 0.08 m. An absorption band is observed below the main absorption edge (at 300 °K the maximum of this band is at 0.86 eV); the absorption in this band increases with increasing temperature. This band is considered to be due to excitons bound to neutral acceptors, and these are presumably the same ones that play a decisive role in the transport properties and which are considered to be associated with vacancies. The absorption edge has the form: ω2ϵ2∼(hω−Eg)2 (Eg = 0.88 eV at 300 °K). This suggests that the optical transitions conserve energy but not k vector, and that the densities of states near the band extrema have the same energy-dependence as in crystalline Ge. A simple theory describing this situation is proposed, and comparison of it with the experimental results leads to an estimate of the localization of the conduction-band wavefunctions.
8,184 citations
TL;DR: The use of regression diagnostics combined with nonlinear least-squares to refine cell parameters from powder diffraction data, presenting a method which minimizes residuals in the experimentallydetermined quantity.
Abstract: We discuss the use of regression diagnostics combined with nonlinear least-squares to refine cell parameters from powder diffraction data, presenting a method which minimizes residuals in the experimentallydetermined quantity (usually 20hkt or energy, Ehkt). Regression diagnostics, particularly deletion diagnostics, are invaluable in detection of outliers and influential data which could be deleterious to the regressed results. The usual practice of simple inspection of calculated residuals alone often fails to detect the seriously deleterious outliers in a dataset, because bare residuals provide no information on the leverage (sensitivity) of the datum concerned. The regression diagnostics which predict the change expected in each cell constant upon deletion of each observation (hkl reflection) are particularly valuable in assessing the sensitivity of the calculated results to individual reflections. A new computer program, implementing nonlinear regression methods and providing the diagnostic output, is described. I~YWORDS: powder diffraction, regression diagnostics, lattice parameters, computer program.
1,248 citations
TL;DR: The effect of thermal and structural disorder on the electronic structure of hydrogenated amorphous silicon is investigated by measurement of the shape of the optical absorption edge as a function of temperature and thermal evolution of hydrogen as discussed by the authors.
Abstract: The effect of thermal and structural disorder on the electronic structure of hydrogenated amorphous silicon is investigated by measurement of the shape of the optical absorption edge as a function of temperature and thermal evolution of hydrogen. The data are consistent with the idea that the thermal and structural disorder are additive, and suggest that the disorder, rather than the hydrogen content, is the fundamental determining factor in the optical band gap.
974 citations
TL;DR: In this paper, the authors provide an update of glass preparation in bulk, fibre and film form; optical and thermal properties, and potential applications of chalcogenide glasses.
Abstract: The author provides an update of: glass preparation in bulk, fibre and film form; optical and thermal properties, and potential applications of chalcogenide glasses.
433 citations