Crystallization kinetics and optical band gap studies of Se96In4 glass before and after slow neutron irradiation

Results of differential scanning calorimetry (DSC) under non-isothermal condition on Se₉₆In₄ semiconducting chalcogenide glass before and after slow neutron irradiation, for different exposure times, have been reported and discussed. Some of Sn atoms have been injected into the glass by nuclear transmutation processes and the binary glass is converted into a ternary. This is accompanied by an increase in the activation energy of crystallization, E_c, and in the glass transition temperature, T_g and a decrease in the glass transition activation energy, E_t, in the onset crystallization temperature, T_c and in the peak temperature of crystallization T_p. Optical band gap measurements have also been carried out, before and after irradiation, on identical thin pellets of Se₉₆In₄ glass. The energy band gap, E_g, is found to decrease upon irradiation. These effects have been attributed to a structural change upon doping and to irradiation induced defects.     

Optical absorption in vitreous GeSb2Se4

Optical absorption of vitreous GeSb₂Se₄ was studied in spectral region 0.7–25 μm. At low absorption levels near the edge the absorption coefficient K depends exponentially on energy. At high absorption levels the quadratical energy dependence of K is observed. In the present work we determined the optical energy gap E^opt_g = 1.29 eV and discussed the temperature dependence of the absorption coefficient. Measured reflectivity curves were used to estimate the value of the refractive index of GeSb₂Se₄ ($\text{n = 3.13–3.56}\text{at}\text{h}\text{?}\text{ω = 1.00–1.70}\text{eV}$). Vitreous GeSb₂Se₄ is also transparent in the spectral interval 2–15.7 μm.     

Refractive-index dispersion of glassy semiconductors in the pseudo-binary As2Se3–SbSI system

The bulk chalcogenide glasses in the pseudo-binary system (As₂Se₃)_100?x(SbSI)_x (with x = 20, 30, 50, 70 and 80 at.%) were prepared from high-purity elemental components by melt-quenching technique. It is a system with the variable ratio of classical amorphous compound As₂Se₃ and the molecule of antimony-sulphoiodide, SbSI, which in the monocrystal form is characterized as a ferroelectric. The refractive-index behaviour (magnitude and spectral dispersion) of investigated glasses was determined by the prism method and analyzed. To calculate and discuss the parameters of dispersion in the band gap region three different approaches were used (Cauchy, Sellmeier and Wemple–DiDomenico single-oscillator model). The comparison of the results shows good agreement between all applied models. It was found that the value of the refractive-index shows normal dispersion behaviour and increases with the increase of Sb (or SbSI) content in the investigated system, whereas the optical gap of these glasses E_g slightly decreases.     

Photoinduced optical changes in amorphous Se and Ge–Se films

Films of amorphous Se, Ge₁₅Se₈₅, and Ge₂₅Se₇₅ were prepared by thermal evaporation. Measurements of photodarkening at 78 K showed that the optical gap (E_g) and the slope of optical absorption edge (B) are intercorrelated. The correlation between the photoinduced changes of E_g and B is discussed within both the model of Davis and Mott and Tauc's model for optical transitions in amorphous solids.     

γ-ray irradiation induced multiple effects on Ge–As–Se chalcogenide glasses

In this work, multiple effects of γ-ray irradiation on properties of bulk Ge–As–Se chalcogenide glasses were studied. Increased density (ρ), thermal expansion coefficient (α) and decreased optical band gap (E_opt) were observed after irradiation, depending on glass compositions. Glasses with stoichiometric (GeSe₂)_100?x(As₂Se₃)_x compositions showed linear correlations between As₂Se₃ proportion x and irradiation sensitivity, which is expressed by Δρ/ρ, Δα/α and ΔE_opt/E_opt. Nonstoichiometric glasses (Ge₂Se₃)_100?x(As₂Se₃)_x exhibited irregular variations. The phenomena are discussed in terms of chemical bonds transition and structural evolution under γ-ray irradiation.     

Electronic properties of vitreous and liquid AsSe alloys (I). Solid alloys

Measurements of the temperature dependence of the d.c. conductivity, therm?opower and optical absorption (near the fundamental edge) for five vitreous AsSe alloys containing from 30% to 50% As are presented. A conductivity maximum at the stoichiometric composition, As₂Se₃, corresponding to minima in the activation energies for conduction and thermopower, is accompanied by a minimum in the optical gap at this composition. These data suggest a minimum mobility gap at As₂Se₃, and a Fermi level independent of composition. Chemical bonding and bond edge state localization arguments are put forward to justify this proposal, and also to indicate why other amorphous semiconducting binary alloys behave differently at stoichiometric compositions.     

On the compositional dependence of the optical gap in amorphous semiconducting alloys

It is found that the optical gap E_AB for amorphous A-B alloys can be determined by the energy gap E_A for element A and E_AB for the element B in the equation E_AB(Y) = YE_A + (1?Y) E_B where Y is the volume fraction of element A. Calculations based on a random bond network agree with experiments for SiGe, SbSe, and AsTe films (class A). Calculations based on a chemically ordered bond network which tends to form microscopic molecular species gree with experimental results for the AsSe, AsS, GeTe and Sb₂Se₃As₂Se₃ systems (class B). In contrast to the above systems, agreement with experiment is not obtained for the TeSe, As₂Te₃As₂Se₃ and GeTe₂GeSe₂ systems which contain atoms of both Te and Se (class C). The classification into three types (classes A, B and C) is consistent with the calculation based on effective medium percolation theory which interprets the compositional dependence of the conductivity of chalcogenide glasses.     

Optical properties of thin films of system (As2Se3)80−x(As2Te3)x(SnTe)20 prepared by PLD

Thin amorphous films from system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ were prepared by pulsed laser deposition (PLD) from their bulk glasses and their optical properties were studied by spectral ellipsometry. Spectral dependencies of refractive index, absorption and extinction coefficient and optical gap (1.41–1.66 eV for (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 20 resp. x = 0) were calculated from optical tansmittance, from ellipsometric data by Tauc method. High values of refractive index n₀ (2.49–2.60) and of non-linear χ⁽³⁾ coefficient of index of refraction (4.9–7.5 × 10⁻¹² esu for the glass (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 0 resp. x = 20) made studied thin films of system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ promising candidates for application in optics and optoelectronics.     

The structure of Hg-doped As2Se3 glasses

The structure of the semiconducting glassy As₂Se₃Hg_x system was investigated in a composition range x = 0.005?0.12. An explanation of the anomalous behaviour of the macroscopic density is proposed, based on the analysis of radial electron density distribution curves. A formula is given which correlates quantitatively the magnitude of coordination spheres with the experimental macroscopic density.     

Photoluminescence and optical absorption in glassy AsxSe1−x

The photoluminescence and optical absorption spectra in glassy As_xSe_1−x with 0.08 ? x ? 0.40 are essentially independent of x. The PL spectra peak at approximately half the optical gap and the exponential slopes of the optical absorption edges are all approximately 75 meV. For x > 0.4, the PL peaks shift to higher energies, the widths of the PL spectra increase, and there is a strong component to the optical absorption well below the optical gap. Comparisons with ESR experiments in the As_xS_1−x system suggest the possibility of two PL peaks. The second PL peak and the optical absorption below the optical band gap for x > 0.4 are attributed to the presence of As-As bonds.     

Structural transformation on Se0.8Te0.2 chalcogenide glass

Using X-ray diffraction and differential scanning calorimetry (DSC), the structure and the crystallization mechanism of Se_0.8Te_0.2 chalcogenide glass has been studied. The structure of the crystalline phase has been refined using the Rietveld technique. The crystal structure is hexagonal with lattice parameter a = 0.443 nm and c = 0.511 nm. The average crystallite size obtained using Scherrer equation is equal 16.2 nm, so it lies in the nano-range. From the radial distribution function, the short range order (SRO) of the amorphous phase has been discussed. The structure unit of the SRO is regular tetrahedron with (r₂/r₁) = 1.61. The Se_0.8Te_0.2 glassy sample obeys the chemical order network model, CONM. Some amorphous structural parameters have been deduced. The crystallization mechanism of the amorphous phase is one-dimensional growth. The calculated value of the glass transition activation energy (E_g) and the crystallization activation energy (E_c) are 159.8 ± 0.3 and 104.3 ± 0.51 kJ/mol, respectively.     

Optical characterization of As2Se3-Ag4SSe-SnTe amorphous thin films

Amorphous thin films from the system As₂Se₃-Ag₄SSe-SnTe were prepared by thermal vacuum evaporation from the corresponding bulk glassy samples. The film structure and surface morphology were investigated by scanning electron microscopy and atomic force microscopy; the results revealed uniform, smooth and homogeneous coatings. The amorphous chalcogenide films are transparent in a wide spectral range as shown by transmission and reflection measurements in the VIS and NIR regions. The optical band gap was determined and its compositional dependence is discussed in terms of structural considerations and the formation of charged defect centers.     

Growth and Characterization of CdAl2S4 and CdAl2Se4 Single Crystals

Single crystals of CdAl₂S₄ and CdAl₂Se₄ showing high transparency were grown by the chemical vapour transport method. Their composition was proven by microprobe analysis. Structural investigations were done by Rietveld refinements and are in good agreement with known structure data. From transmittance and reflectance measurements the energy of the band gap WBS estimated. Assuming a direct nature of the corresponding optical transition the following values were obtained: E_g = 3.82 eV (RT), E_g = 3.94 eV (85 K) for CdAl₂S₄ and E_g = 2.95 eV (RT), E_g = 3.07 eV (85 K) for CdAl₂Se₄ respectively.     

Optical Properties of Bi2Se3‐xAsx Single Crystals

Bi₂Se_3‐xAs_x single crystals with the As content of c_As = 0 to 2.0x10¹⁹ atoms/cm³ prepared from the elements of 5N purity by means of a modified Bridgman method were characterized by measurements of infrared reflectance and transmittance. Values of the plasma resonance frequency omega_p, optical relaxation time tau, and high‐frequency permittivity were determined by fitting the Drude‐Zener formulas to the reflectance spectra. It was found that the substitution of As atoms for Se atoms in the Bi₂Se₃ crystal lattice leads to a decrease in the omega_p values. This effect is accounted for by a model of point defects in the crystal lattice of Bi₂Se_3‐xAs_x. The dependences of the absorption coefficient K on the energy of incident photons were determined from the transmittance spectra. The optical width of the energy gap is found to decrease with increasing As content. The values of the exponent b from the relation of K ∼ lamda^b for the long‐wavelength absorption edge range within the interval 2.0 to 2.3, i.e. the dominant scattering mechanism of free current carriers in Bi₂Se_3‐xAs_x crystals is the scattering by acoustic phonons.     

ESR study of structural changes in chalcogenide glasses

Structural changes in chalcogenide glasses induced by various processes such as annealing, illumination or application of a high pressure were investigated from a microscopic viewpoint by measuring changes of the ESR spectrum from Mn doped as a probe. Here, Mn incorporated in the glass network exhibits a characteristic ESR signal with g = 4.3 having a hyperfine structure. It is found that the above-mentioned processes cause a change of the lineshape of the hyperfine line and a change of the hyperfine structure constant A, both of which reflect the bonding characteristics around Mn. One of the annealing effects for bulk glasses is the increase of the A-value. The change of the A-value for well-annealed As₂Se₃ and As₄Se₅Ge₁ films occurs reversibly in sequential illumination and annealing cycles, as does the shift of the optical gap. The reversible change of the lineshape is observed for well-annealed As₂Se₃ films in the illumination and annealing cycles. Application of a high pressure to bulk glasses causes changes similar to those by illumination for annealed films. From these results it is concluded that the randomness of amorphous structure decreases by annealing and increases by illumination or application of high pressure.     

Glass formation and properties of chalcogenide systems XXVI: Permittivity and the structure of glasses AsxSe1−x and GexSe1−x

Correlation between the real part of the dielectric constant and the structure of glasses in the system As_xSe_1?x and Ge_xSe_1?x is reported. The mole polarization is calculated using the Sellmeyer approximation neglecting the Lorentz field. The vibrationally caused part of the permittivity which is obtained by subtraction of the mole refraction reflects ordered states in the investigated series. Besides the known crystalline compounds As₂Se₃ and As₄Se₄ the formation of the vitreous AsSe₃ and As₃Se₂ from the liquid state has to be supposed. In the system Ge_xSe_1?x the formation of the compounds GeSe₂ and Ge₂Se₃ is completed by GeSe₄ which as Ge₂Se₃ obviously only exists in the non-crystalline state. GeTe₄ has been reported as a metastable crystalline phase. The temperature dependence of ?_r of vitreous As₂Se₃ is tentatively interpreted in terms of the dipole orientation caused by conversion of the charge in valence alternation pairs.     

The electronic properties of glasses in the Cux(As0.4Se0.6)100−x system with x between 5 and 30

Data are presented on the dc conductivity, thermopower and optical absorption of glasses in the CuAs₂Se₃ system. The electronic properties of the alloys differ markedly from those of As₂Se₃, but variations in composition do not introduce significant changes in properties, until the atomic percent of copper is greater than 25. The results are interpreted in terms of small polaron transport.     

Electron diffraction RDF analysis of amorphous As2Se3,AAs2Se2Te,As2SeTe2 and As2Te3 films

The local order in amorphous films of As₂Se₃, As₂Se₂Te, As₂SeTe₂, and As₂Te₃ has been examined by scanning electron diffraction with direct recording of the intensity of the elastically scattered electrons. The radial distribution functions indicate that there is a systematic increase in mean nearest neighbor distance as the Te concentration is increased, butthe mean coordination number increases slightly around 2.4. Pair function calculation of models shows that the 3-aand 2-fold coordinations of arsenic and chalcogens are retained in these glasses and the interatomic distances are close to those predicted from the Pauling covalent atomic radii of the constituent atomic species. The short range order appears to be similar in amorphous and crystalline As₂Se₃, but different in the case of As₂Te₃ as found by previous workers on bulk materials.     

Experimental characterization of amorphous As–Se–Sb alloys

Thin films were thermally evaporated from ingot pieces of the As₃₀Se_70−xSb_x (with 2.5 ⩽ x ⩽ 17.5 at.%) glasses under vacuum of ∼10⁻⁵ Torr. Increasing Sb content was found to affect the thermal and optical properties of these films. Non-direct electronic transition was found to be responsible for the photon absorption inside the investigated films. The chemical bond approach has been applied successfully to interpret the decrease of the glass optical gap with increasing Sb content. Decreasing the thermal stability of the As₃₀Se_70−xSb_x specimen by increasing Sb content is responsible for occurring the amorphous–crystalline process at lower temperatures. Binary As₂Se₃ and Sb₂Se₃ phases are the main components of the stoichiometric As₃₀Se₆₀Sb₁₀ composition.     

Studies on the AsSbSe glass system

Results are reported of measurements of the glass transition temperature (T_g), conductivity and density (d) for glasses of the AsSbSe system, with compositions (As, Sb)₄₀Se₆₀ and As_xSb₁₅Se_85?x. The results are compared with glasses of similar compositions of the As_xSe_100?x about compositions in the Ge_xSb₁₅Se_85?x and As_xSe_100?x glasses, in the case of As_xSb₁₅Se_85?x glasses, the As-rich compositions exhibit higher values of T_g and d compared to the stoichiometric composition As₂₅Sb₁₅Se₆₀. These results are discussed in the light of a chemically ordered structural arrangement in these glasses.