Photoinduced effects on the optical constants of a-Ge–Se–Bi chalcogenide glassy thin films

In addition to the conversion from p-type to n-type conductivity that occurs in Ge–Se–Bi thin films when Bi is incorporated in a certain concentration. We found that, when these films were illuminated to UV light, after being annealed at glass transition temperature T _g, the photobleaching is dominant for Ge₂₀Se_80?x Bi _x (x=0, 2.5, and 5 at.%), while for Ge₂₀Se_72.5Bi_7.5 photodarkening is dominant. The photoinduced changes in the optical constants were studied. The refractive index (n) has been analysed according to the Wwmple–DiDominico single oscillator model and the values of E _o and E _d for exposed and unexposed films were determined, respectively. The photostructural effects were discussed in the light of single–double well model proposed by Tanaka and chemical bond approach.     

Some optical properties of Ge–S amorphous thin films

Amorphous Ge_xS_1−x films (x=0.27, 0.32, 0.36 and 0.4) were prepared by thermal evaporation. The values of the refractive index, the optical gap and the parameters of the Wemple–DiDomenico single oscillator model were determined. Using Miller's generalized rule the values of the third-order non-linear susceptibility were also estimated. Thermally induced bleaching was observed for films studied. Photobleaching was observed for sulfur rich and nearly stoichiometric films (x=0.27, 0.32) while for a germanium rich film (x=0.4) no response to the illumination was observed. The differences in photobleaching are attributed to the role of p-lone pair states and to an overall network rigidity of the films.     

On the properties and stability of thermally evaporated Ge–As–Se thin films

Thin films of Ge–As–Se chalcogenide glasses have been deposited by thermal evaporation from bulk material and submitted to thermal treatments. The linear refractive index and optical band-gap for as-deposited and annealed films have been analyzed as function of the deposition parameters, chemical composition and mean coordination number (MCN). The chemical composition of the films was found to be directly affected by deposition rate, with low rates producing films with elevated Ge and reduced As content, whilst at high rates the Ge content was generally reduced and As levels increased compared with the bulk starting material. As a result films with close to the same stoichiometry as the bulk glass could be obtained by choosing appropriate deposition conditions. As-deposited films with MCN in between 2.44 and 2.55 showed refractive indices and optical band-gaps very close to those of the bulk glass whereas outside this range the film indices were higher and the optical gaps lower than those of the bulk glass. Upon annealing at close to their glass transition temperature, high MCN films evolved such that their indices and band-gaps approached the bulk glass values whereas at low MCN films resulted in no changes to the film properties.     

Photoconductivity in amorphous thin films of Ge22Se78−xBix

Temperature and intensity dependence of photoconductivity is studied in amorphous thin films of Ge₂₂Se_78−x Bi_x with x = 0, 2 and 10. Transient photoconductivity measurements have also been made on the same samples. Our results show that photosensitivity decreases as Bi concentration is increased from x = 0 to x = 2. However, at high concentration of Bi(x = 10), photosensitivity again increases. Transient photoconductivity also show a different behaviour at low and high concentration of Bi. Results have been explained in terms of defect states produced due to Bi incorporation in GeSe system.     

Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films

Amorphous thin film Ge₁₅Te_85−x Sn _x (1≤x≤5) and Ge₁₇Te_83−x Sn _x (1≤x≤4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bottom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn concentration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigidity percolation in the composition dependence of switching fields of Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with increasing Sn concentration, which is consistent with the composition dependence of switching fields.     

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.     

Optical constants of quaternary Ge–As–Te–In amorphous thin films evaluated from their reflectance spectra

Thin films of amorphous Ge₉As₂₀Te_{71? x} In _x with different compositions (x = 0, 3, 6 and 9 at. %) were obtained by deposition onto glass substrates by thermal evaporation. The reflection spectra, R(λ), of the films were obtained in the spectral region from 400 to 2500 nm. A straightforward analysis proposed by Ruiz-Perez et al., based on the use of the maxima and minima of the interference fringes, has been applied to derive the real and imaginary parts of the complex index of refraction, the thickness and the thickness variation of the studied films. Increasing In content is found to affect the refractive index and the extinction coefficient of the films. Optical absorption measurements were used to obtain the fundamental absorption edge as a function of composition. With increasing In content, the refractive index decreases, whereas the optical band gap, E_g , increases. The relationship between E_g and the chemical composition of the Ge₉As₂₀Te_{71? x} In _x system is discussed in terms of the cohesive energy, the average heat of atomization, H _s , and the average coordination number, N _r .     

Physical and optical properties of amorphous Ge x As20S80– x thin films

We report the effect of replacement of sulfur by germanium on the optical constants and some other physical parameters of chalcogenide Ge _x As₂₀S_{80– x} (where x?=?0, 5, 10, 15 and 20 at%) thin films. Increasing germanium content affected the average heat of atomization, average coordination number, number of constraints and the cohesive energy. Films with thicknesses 800–820?nm of Ge _x As₂₀S_{80– x} were prepared by thermal evaporation of bulk samples. Transmission spectra, T(λ), of the films at normal incidence were obtained in the region from 400 to 2500?nm. A straightforward analysis proposed by Swanepoel [J. Phys. E Sci. Instrum. 16 (1983) p 1214], based on the use of maxima and minima of the interference fringes, has been applied to derive the real and imaginary parts of the complex index of refraction and also the film thickness. Optical absorption measurements showed that the fundamental absorption edge is a function of composition. Optical absorption is due to allowed non-direct transition and the energy gap decreases while the refractive index increases with increasing germanium content. The chemical-bond approach has been applied to obtain the excess of S–S homopolar bonds and the cohesive energy of the Ge _x As₂₀S_{80– x} system.     

Effect of Ge doping on the electrical properties of amorphous Zn–Sn–O thin films

We report the effect of germanium doping on the active layer of amorphous Zinc–Tin-Oxide (a-ZTO) thin film transistor (TFT). Amorphous thin film samples were prepared by RF magnetron sputtering using single targets composed of Zn₂Ge_0.05Sn_0.95O₄ and Zn₂SnO₄ with variable oxygen contents in the sputtering gases. In comparison with undoped, Ge-doped a-ZTO films exhibited five order of magnitude lower carrier density with a significantly higher Hall-mobility, which might be due to suppressed oxygen vacancies in the a-ZTO lattice since the Ge substituent for the Sn site has relatively higher oxygen affinity. Thus, the bulk and interface trap densities of Ge-doped a-ZTO film were decreased one order of magnitude to 7.047 × 10¹⁸ eV⁻¹cm⁻³ and 3.52 × 10¹¹ eV⁻¹cm⁻², respectively. A bottom-gate TFT with the Ge-doped a-ZTO active layer showed considerably improved performance with a reduced SS, positively shifted V_th, and two orders of magnitude increased I_on/I_off ratio, attributable to the doped Ge ions.     

Characterization of new quaternary chalcogenide As–Ge–Se–Sb thin films

This paper reports the effect of replacement of selenium by antimony on the optical gap and some other physical parameters of new quaternary chalcogenide As₁₄Ge₁₄Se_{72? x} Sb _x (where x = 3, 6, 9, 12 and 15 at%) thin films. Thin films with thickness 200–220 nm of As₁₄Ge₁₄Se_{72? x} Sb _x were prepared by thermal evaporation of the bulk samples. Increasing antimony content was found to affect the average heat of atomization, the average coordination number, number of constraints and cohesive energy of the As₁₄Ge₁₄Se_{72 ?x} Sb _x alloys. Optical absorption measurements showed that the fundamental absorption edge is a function of composition. Optical absorption is due to allowed, non-direct transition and the energy gap decreases with the increasing antimony content. The chemical bond approach has been applied successfully to interpret the decrease in the optical gap with increasing antimony content.     

Annealing induced transformations in structural and optical properties of Ge30Se70−xBix thin films

ABSTRACT

Thin films of Ge₃₀Se_70?xBi_x (x?=?5, 15, 20) were prepared by thermal evaporation method on glass substrates with thickness 800?nm. The films were annealed at 250°C and 320°C for 2?h to study the annealing-induced structural and optical change. The X-ray diffraction characterization revealed the amorphous to crystalline phase transformation with annealing. The indirect optical band gap decreased with annealing which is explained on the basis of phase transformation and density of localized states. The formation of surface dangling bonds around the crystallites during crystallization process reduced the band gap. The Tauc parameter and Urbach energy change show the degree of chemical disorderness in the films. The transmitivity decreased while the absorption coefficient increased with the annealing process. The microstructural study done by Field emission scanning electron microscopy shows the formation of crystallites upon annealing. Atomic force microscopy investigation on these films shows the influence of annealing on surface topography.     

Interpretation of the change in optical constants of different compositions of Ge–Se–In in terms of cohesive energy

Compositional dependencies of the optical and physical properties of as-deposited amorphous Ge_xSe_90?xIn₅ films (with 5≤x≤30 at%), prepared by thermal evaporation have been studied. The optical energy gap $E_g^{opt}$ is derived from Tauc's extrapolation in the strong absorption region in terms of transmission and reflection spectra. The relationship between the optical gap and chemical composition of the Ge_xSe_90?xIn₅ (with 5≤x≤30 at%) amorphous system is discussed in terms of the chemical bond approach. The refractive index, n and film thickness, d have been determined by an envelope method using transmission spectra. It is observed that the refractive index, n of Ge_xSe_90?xIn₅ thin films increases with increasing x, over the entire spectral range, which is related to both the increased density and average coordination number.