Structure of amorphous GeSe thin films

Amorphous films of GeSe_0.7 and GeSe_2.4 have been examined by energy-filtered scanning electron diffraction and by electron microscopy. Radial distribution analysis of the GeSe_0.7 intensity curves indicates that the local atomic order of these films differs considerably from the distorted rocksalt structure of bulk crystalline GeSe. Radial distribution studies also indicate a change in the structure of GeSe_0.7 and GeSe_2.4 films supported on copper mesh as they are heated with an electron beam. Gross structural features are not observable in the electron micrographs of the amorphous films. This does not eliminate, however, the possibility of the presence of glassy phase separation on a fine scale in the heat-treated films.     

Optical properties of Ge–Se–Te wedge-shaped films by using only transmission spectra

Amorphous Ge₁₀Se_90?xTe_x (with x = 0, 5, 10 and 15 at.%) thin films were prepared by thermal evaporation method. The optical transmission spectra of these films were measured in the wavelength range of 500–2500 nm in order to drive the refractive index and the absorption coefficient of these films. Applying the analytical expressions proposed by Swanepoel, enabling the calculations of optical constants of thin films with non-uniform thickness with high accuracy. Furthermore, the dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. It was found that, the mechanism of the optical absorption follows the rule of the allowed non-direct transition. The optical band gab, E_g, and the oscillator energy, E_o, decrease while the dispersion energy, E_d, increases by increasing Te content. The relationship between the obtained results and the chemical compositions of the Ge₁₀Se_90?xTe_x thin films were discussed in terms of the chemical bond approach, the excess of Se–Se homopolar bonds and the cohesive energy (CE).     

Structural dependence of ultrafast third-order optical nonlinearity of Ge–Ga–Ag–S chalcogenide glasses

Ultrafast third-order optical nonlinearity of Ge–Ga–Ag–S chalcogenide glasses at the wavelength of 820 nm has been measured using femtosecond time-resolved optical Kerr (OKE) technique. The results show that Ge–Ga–Ag–S glasses have large third-order optical nonlinear susceptibility, χ⁽³⁾ and the response time is also subpicosecond, which are predominantly due to the ultrafast distortion of electron cloud surrounding the balanced positions of Ge, Ga, Ag and S atoms. What’s more, a strong dependence of χ⁽³⁾ on the composition and microstructure of these glasses was found which shows that [GeS₄] and [GaS₄] tetrahedra play an important role on the third-order optical nonlinearity. These Ge–Ga–Ag–S chalcogenide glasses would be expected as promising materials applied on all-optical switching devices.     

Multilayer planar structures prepared from chalcogenide thin films of As–Se and Ge–Se systems and polymer thin films using thermal evaporation and spin-coating techniques

We report the preparation of multilayers based on polyamide–imide polymer and As–Se or Ge–Se chalcogenide thin films. Chalcogenide films of As–Se and Ge–Se systems were deposited using a thermal evaporation method periodically alternating with spin-coated Polyamide–imide films. Fifteen layers of PAI + As–Se system and nineteen layers of PAI + Ge–Se system were coated. Optical properties of prepared multilayers have been established using UV–vis–NIR and Ellipsometric spectroscopy. Both, PAI + As–Se and PAI + Ge–Se multilayer systems, exhibited the high-reflection bands centered around 830 nm and 1350 nm, respectively. The shift of the band position of PAI + Ge–Se multilayers to lower energies was caused by higher thickness of Ge–Se films. The bandwidth of reflection band of 8 PAI + 7 As–Se multilayer was ～90 nm while bandwidth of PAI + Ge–Se system decreased to ～70 nm because Ge–Se films have 0.1 lower refractive index against As–Se films. Design of 1D-photonic crystals based on alternating chalcogenide and polymer films is a new opportunity for application of chalcogenide thin films as optical materials for near-infrared region.     

Crystallization kinetics in Se–Te glassy system

Differential scanning calorimetry measurements were used to study crystallization in the Se_yTe_(1 ? y) glassy system under non-isothermal conditions. The examined compositions were y = 0.1, 0.2 and 0.3. The crystallization kinetics was described in terms of the Johnson–Mehl–Avrami nucleation–growth model. It was found that with the addition of tellurium into the selenium matrix the apparent activation energy of the crystallization process monotonically increases and, more importantly, the parameter of the JMA model m decreases. This change in the crystallization mechanism was interpreted as a gradual crossover from surface to bulk process. Both processes are competitive and proceed simultaneously in glasses with low tellurium content, while in the case of increased Te content the surface crystallization mechanism recedes. The transition between mechanisms was discussed in terms of changes in molecular structure of the material.     

Application of time–frequency wavelet analysis in the reflectometry of thin films

Crystallography Reports - The application of time–frequency wavelet analysis for solving the reflectometry inverse problem is considered. It is shown that a simultaneous transform of specular...     

Effect of tellurium deposition rate on the properties of Cu–In–Te based thin films and solar cells

To investigate the effects of tellurium (Te) deposition rate on the properties of Cu–In–Te based thin films (Cu/In=0.30–0.31), the films were grown on both bare and Mo-coated soda-lime glass substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy and X-ray diffraction. The crystalline quality of the films was improved with increase in the deposition rate of Te, and exhibited a single CuIn₃Te₅ phase with a highly preferred (1 1 2) orientation. Te-deficient film (Te/(Cu+In)=1.07) grown with a low Te deposition rate showed a narrow bandgap of 0.99 eV at room temperature. The solar cell performance was affected by the deposition rate of Te. The best solar cell fabricated using CuIn₃Te₅ thin films grown with the highest deposition rate of Te (2.6 nm/s) yielded a total area (0.50 cm²) efficiency of 4.4% (V_oc=309 mV, J_sc=28.0 mA/cm², and FF=0.509) without light soaking.     

Kinetics of self-bleaching in some photodarkened Ge–As–S amorphous thin films

Three amorphous Ge–As–S films with the average coordination numbers of 2.4–2.8 were prepared by thermal evaporation. True relaxation that is self-bleaching of photodarkened state has been studied. Considerable differences in kinetics of relaxation of photodarkened state were observed for Ge₁₂As₁₇S₇₁ (Sa 1) and Ge₁₅As₂₀S₆₅ (Sa 3) amorphous thin films. In both cases, the relaxation (self-bleaching) followed stretched exponential, however, flexible matrix of Sa 1 film relaxed significantly faster than the matrix of Sa 3 film. The amorphous film Ge_25.5As_29.5S₄₅ (Sa 8) was found to be insensitive to illumination. It is suggested that the network rigidity may significantly influence the magnitude of photodarkening and the rate of relaxation of photodarkened state.     

Spectral response of amorphous–nano-crystalline silicon thin films

A representative set of amorphous–nano-crystalline Si thin films was deposited by radio-frequency plasma enhanced chemical vapor deposition using silane highly diluted by hydrogen. By Raman spectroscopy it was found that the variation of silane to hydrogen ratio resulted in films with crystal fraction between 0 and 55 vol.% and individual crystal sizes between 2 and 20 nm with bi-modal, broad size distribution. High resolution transmission microscopy, done on certain number of samples, confirmed the nano-meter size of crystallites and bi-modal size distribution. The optical properties measured by Fourier transform photocurrent spectroscopy and photo thermal deflection spectroscopy correspond to the material with structure between amorphous and crystalline. The spectral distribution of relative quantum efficiency of photovoltaic solar cell made from this material shows ‘blue shift’ with increase of crystal to amorphous fraction. This result is discussed as a possible consequence of quantum effects accompanied with actual size and size distribution of crystals.     

Gaseous Bubble Formation in Bi2Te3?Bi2Se3 Melt

In this paper the mechanism of the formation of gaseous inclusions in the liquid phase on the example of Bi₂Te₃-Bi₂Se₃ system is considered. The thermodynamic analysis for the model scheme of melting has been carried out. It is shown that the time dependence of the pressure and composition of gaseous phase is determined by establishment of equilibrium between liquid and gaseous phases in the system. The results of numerical computation give a quantitative evolution of the size of critical gaseous nucleus in the Bi₂Te₃-Bi₂Se₃ melt with the time and its dependence on the height of melt column.     

Preparation and second-order optical nonlinearity of inorganic–organic hybrid films

The sol–gel technique has been used to prepare a thin film for second-order nonlinear optics (NLO). An azo-dye molecule 2,5-dimethyl-4-(4′-nitrophenylazo)phenol (DMNPAP) was covalently bonded to the silicon oxide network through hydrolysis and co-condensation between the alkoxysilane dye and tetraethyl silicate (TEOS) in relatively high loading densities (43 wt%). The resulting film showed a thermal stability up to 302 °C in thermogravimetric analysis (TGA) thermograms and a flat surface morphology in atomic force microscope (AFM) image. The orientation behavior of the poled film was studied by UV–visible spectroscopy. The NLO activity (d₃₃), which was estimated to be 6.5 pm/v at 1064 nm by in situ second harmonic generation (SHG) measurement, the thermal stability of the second-order nonlinearity was also reported.     

Differential thermal analysis of Ag–Ge–Se,Ge–Se charge materials in the process of their heating and Ag8GeSe6, GeSe2 compound synthesis

This is the first comprehensive study of the processes of phase transformations (PT) and chemical reactions (CR) that accompany Ge–Se and Ag–Ge–Se charge material heating in stoichiometric proportions corresponding to GeSe₂ compounds, Ag₈GeSe₆ argyrodite and elementary selenium. The investigation was carried out by means of the differential thermal analysis (DTA) method. The PT in selenium and the main types of PT and CR of the formation of GeSe₂, Ag₈GeSe₆ compounds have been identified. The characteristic heats and temperature ranges of reaction processes have been determined. Ag₈GeSe₆ and GeSe₂ compounds formation has been experimentally demonstrated due to the DTA and X-ray analysis.     

Study of As―Se―Te glasses by neutron-, X-ray diffraction and optical spectroscopic methods

The atomic structures of amorphous As₄₀Se_(60?x)Te_x (x = 10 and 15) and As₄₀Se₆₀ glasses have been investigated by neutron and high energy X-ray diffraction methods. The two datasets were modeled simultaneously by reverse Monte Carlo (RMC) simulation technique. The RMC simulations revealed a glassy network built-up from As(Se, Te)₃ pyramids in which Te atoms substitute Se atoms. The As―Se correlation function shows a strong and sharp first peak at 2.4 Å and two broad and much less intense peaks at 3.7 and 5.6 Å, related to 1st, 2nd and 3rd neighbor distances of the As―Se bonds, respectively. They are an evidence for existence of short and medium ordering in the studied glasses. The similarity of Θ_Te―As―Te and Θ_Se―As―Se bond distributions suggests that Te atoms have a similar role in the structure formation as Se atoms. The FTIR spectra analysis revealed impurity bonds of Se―H, As―O, Se―O, and Te―O in the glasses which contributed to enhanced absorption in visible spectral range. From the ellipsometric data analysis the optical constants and the energetic parameters of the studied glasses were established. The compositional variation of these parameters is explained in terms of chemical bonds formation and change in the density of charged defects.     

Effect of γ-ray irradiation on structure formation and optical constants of thermally evaporated rhodamine B thin films

Rhodamine B (RhB) is polycrystalline in powder form; it becomes nanocrystalline in thermally evaporated films. Gamma-rays induce nanocrystalline to amorphous transformation of RhB films. The optical constants are independent of film thickness. The type of electronic transition for as-deposited and gamma-ray irradiated RhB film is indirect allowed one. The onset and optical energy gap for as-deposited films are 2 and 3.45 eV, respectively. Exposure to different γ-ray doses did not change the transmittance and reflectance behavior of RhB films. γ-ray irradiation decreases onset and optical energy gap, refractive index and dispersion parameters and increase extinction coefficient, oscillator and electric dipole strengths.     

Concentration dependence of Dy3+:1.3 μm luminescence in Ge–Ga–Sb–Se glasses

The Ge₂₅Ga₅Sb₅Se₆₅ glasses, doped with 0.01, 0.05, 0.1, 0.2, and 0.5 mol% of Dy³⁺ ions are prepared and the concentration dependence of Dy³⁺:1.3 μm luminescence is investigated. Remarkable energy migration between Dy³⁺ ions occurs as its concentration is more than 0.05 mol%. With further increasing amount of Dy³⁺ ions, the decay time of the 1.3 μm fluorescence decreases rapidly. All the decays are simple exponential and possible regimes of the donor decay are discussed.     

Drift mobilities in amorphous AsSeTe

The electron and hole drift mobilities in As_xSe_100?x?yTe_y (0 ? x ? 7.1, 0 ? y ? 20.9 at.%) evaporated thin amorphous films and their temperature dependence were measured by means of the time-of-flight technique. The electron mobility decreases with increasing As content, while the hole mobility decreases upon addition of 6.4 at.% Te, then remains almost similar on further addition of Te. These effects of As and Te on the drift mobilities of carriers were shown to be independent of each other.     

Thermoelectric power of AsSeTe glasses

The result of the measurement of the thermoelectric power of glasses in the As₂Se₃As₂Te₃ system are reported. The results indicate that towards the As₂Te₃ end of the composition, there is an anomalous increase in thermoelectric power, the origin of which is not clear. Polaron hopping seems to contribute to conductivity in tellurium rich glasses. Structural restrictions on polaron hopping, such as the availability of AsTeAsTe chains seems to be important in discussing transport behaviour.     

Effects of tantalum doping on microstructure and ferroelectric properties of Bi4Ti3O12 thin films prepared by a sol–gel method

Tantalum-substituted Bi₄Ti₃O₁₂ (Bi₄Ti_3-x/5Ta_x/5O₁₂, BTTO) thin films were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates by sol–gel technology. The effects of various processing parameters, including Ta content (x=0～0.08) and annealing temperature (500～800 °C), on the growth and properties of thin films were investigated. X-ray diffraction analysis shows that the BTTO thin films have a bismuth-layered perovskite structure with preferred (117) orientation. With the increase of Ta content, the grain size of film decreased slightly, and highly (117)-oriented BTTO films were obtained in the composition of x=0.06. Ta doping on the B-site of Bi₄Ti₃O₁₂ could induce the distortion of oxygen octahedral and decrease the oxygen vacancy concentration by a compensating effect. The highly (117)-oriented BTTO thin films with x=0.06 exhibits the maximum remanent polarization (2P_r) of 50 μC/cm² and a low coercive field (2E_c) of 104 kV/cm, fatigue free characteristics up to ≧ 10⁸ switching cycles.     

Electrical and optical properties of the AsTeIn and GeSeIn chalcogenide systems

The influence of indium on the optical and properties of As_2−xTe_3−xxIn_2x, As_20−xTe_80−xIn_2x and Ge_20−xSe_80−xIn_2x is described. In Te-containing glasses the Fermi level is shifted by 0.05 eV and in Se-containing glasses by 0.2 eV towards the valence band.