Arsenic Chalcogenides Research Articles

Investigation of the optical constants of arsenic chalcogenide films in the wavelength range 0.5–2.5 μm

Thin films of chalcogenide glasses deposited on quartz glass substrates by thermal evaporation in vacuum have been investigated. The dependences n(λ) and k(λ) for films of different composition have been determined from the transmission spectra. Expressions of the n = A + BL + CL 2 + Dλ 2 + Eλ 4 type (L = (λ 2 − 0.028)−1 and A, B, C, D, and E are constants) for calculating the refractive indices of As2Se3, AsSe4, AsS4, and AsS16.2Se16.2 films in the wavelength range from 0.5 to 2.5 μm are reported.

Photoinduced volume change in arsenic chalcogenides by band-gap light

Photoinduced structural changes in molecular semicrystalline ${\mathrm{As}}_{4}{\mathrm{Se}}_{3}$ films have been characterized by extended x-ray absorption fine structure and x-ray diffraction under in situ band-gap laser illumination. Transient and permanent changes in the local and long-range structure have been determined quantitatively. In contrast to fully amorphous arsenic selenide films, a photoinduced contraction has been observed in semicrystalline ${\mathrm{As}}_{4}{\mathrm{Se}}_{3}$ films. A comprehensive phenomenological model is proposed for the photoinduced volume change in As-based chalcogenides. This model is based on two competing processes viz. intermolecular interaction through Coulombic repulsive force that results in photoexpansion, and cross-linking of molecular units through bond switching that causes photocontraction. A guideline is presented for selecting chalcogenide glass composition with specific photoinduced volume change characteristics.

Amorphous arsenic chalcogenide films modified using rare-earth complexes

The optical absorption and current–voltage characteristic of amorphous arsenic chalcogenide As2X3 (X=S, Se) films modified by rare-earth complexes with organic mixed-ligands have been studied. The following two types of complexes were used: europium dipivaloylmethanate Eu(thd)3 and lanthanide diethyldithiocarbamates Ln(ddtc)3 (Ln=Pr, Eu). It was shown that the use of rare-earth mixed-complexes with similar volatility to the chalcogenide volatility permits the deposition of amorphous films by thermal evaporation. The decrease in absorption coefficient at the Urbach’s edge after introduction of the europium dipivaloylmethanate complexes containing oxygen in arsenic selenide has been revealed. The type of organic ligands, incorporated in the amorphous matrix, determines the shape of current–voltage characteristic. The observed results have been discussed on the basis of the different rigidity of the structure of amorphous arsenic chalcogenides.

The properties of amorphous arsenic chalcogenide films modified by rare-earth complexes

The optical and electrical properties of thin arsenic chalcogenide As-Se and As-S films modified by rare-earth complexes with various organic ligands (europium trisdipivaloylmethanate Eu(thd)3 and lanthanide diethyldithiocarbamate Ln(ddtc)3 (Ln = Pr, Eu)) have been studied. It is shown that the use of mixed rare-earth complexes possessing saturated vapor pressures close to chalcogenide vapor pressures allows thermal evaporation growth of films that are promising for producing planar optical waveguides. It is found that the introduction of Eu(thd)3 complexes that incorporate oxygen into arsenic selenide decreases the absorptivity in the Urbach edge region and significantly decreases the conductivity activation energy, which is not characteristic of materials of this type. These results are attributed to the effect of oxygen on the medium-range order of the structure matrix.

Structural and photostructural properties of chalcogenide glasses

Tight binding calculations of the electric field gradients at As sites in crystalline arsenic and arsenic chalcogenides are in reasonable agreement with experimental values obtained from nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) studies. This agreement suggests that one may use this tight binding approach to obtain useful structural information for amorphous As and the arsenic chalcogenide glasses.

Self-organization in simulated arsenic chalcogenide networks

Polymerization–depolymerization processes induced by bandgap light irradiation with alternating high and low intensity light control the nano-scale extension of disordered layer-like clusters in arsenic rich chalcogenides. These processes resulting in a fractal morphology of the clusters were simulated in the frame of a Monte-Carlo-Metropolis method. The fractal model for photo-darkening and photo-bleaching phenomena in arsenic chalcogenides is discussed.

Structural and photostructural properties of chalcogenide glasses: recent results from magnetic resonance measurements

We review nuclear quadrupolar resonance (NQR) and high-field (up to 30 T) nuclear magnetic resonance (NMR) measurements of 75As in crystalline and glassy samples of arsenic chalcogenides. The results, which yield asymmetries in the electric field gradient at the arsenic sites, provide detailed information on the local structural order and the photoinduced structural changes in these glasses and crystals – order that cannot easily be obtained from other techniques such as X-ray or neutron scattering. In crystalline and glassy As2Se3 and As2S3 these results disagree with those inferred from previous low field Zeeman-perturbed NQR studies, and they are inconsistent with the predictions of a common model (Raft Model) for the structure of glassy As2Se3 and other chalcogenide glasses. Results on optically irradiated and annealed, glassy As2S3 place severe restrictions on the possible changes in local and intermediate range order that can occur on photodarkening. The high-field, NMR technique should be useful for many other solids that contain one of the 20 elements whose nuclei have spin 3/2 and relatively large quadrupole moments.

Photoluminescence lifetime distributions of chalcogenide glasses obtained by wide-band frequency resolved spectroscopy

Photoluminescence (PL) lifetime distributions for amorphous arsenic chalcogenides g-As2Se3 and g-As2S3, and amorphous selenium a-Se, are obtained for the first time using a quadrature frequency resolved spectroscopy (QFRS) technique modified for nanosecond resolution. The g-As2S3 and a-Se chalcogenides exhibit double-peak lifetime distributions, whereas the lifetime distribution of g-As2Se3 peaks uniquely at around 10−4 s, which is consistent with earlier results. PL fatigue is found to reduce the intensity of PL but not affect the observed PL lifetimes. A self-trapped exciton model is adopted to explain the experimental results, providing reasonable mechanisms for the two-component lifetimes and associated phenomena. For a-Se, singlet–triplet exchange energy of ≈160 meV is estimated.

Photoinduced changes in the optical properties of obliquely deposited a-As 2S 3 thin films

The photoinduced changes in the optical properties of obliquely deposited a-As 2S 3 thin films are followed. It is established that the increase of the refractive index and absorption coefficient as a result of illumination is highest in the sample prepared at 70° vapour incidence angle. In addition, the refractive index dispersion has been analyzed according to the Wemple–Didomenico single oscillator model and the effective As coordination number in non-illuminated and illuminated samples has been evaluated. On the basis of the results obtained the role of microstructure in the photoinduced transformations in vacuum deposited arsenic chalcogenide thin films is discussed.

Semiconductor–metal transition in liquid arsenic chalcogenides by ab initio molecular-dynamics simulations

The microscopic mechanism of the semiconductor–metal transition in the liquid arsenic chalcogenides, As 2S 3, As 2Se 3, and As 2Te 3, with increasing temperature and pressure is studied by means of ab initio molecular-dynamics simulations. It is shown that in the liquids As 2S 3 and As 2Se 3 the network structure is transformed into a two-fold chain-like structure at higher temperatures, while in the liquid As 2Te 3 the four-fold coordinated As and the three-fold coordinated Te increase in the network structure with increasing temperature. For the metallization of the liquids As 2S 3 and As 2Se 3, the electronic states around the lower-coordinated atoms are essential. On the other hand, the electronic states around the Te atoms contribute importantly to metallization in the liquid As 2Te 3.

Electronic Structure of Glassy ChalcogenidesAs4Se4andAs2Se3: A Joint Theoretical and Experimental Study

We present an interpretation of the x-ray absorption spectra of arsenic chalcogenide glasses, As4Se4 and As2Se3, from a first-principles calculation. Our calculation identifies the atomistic origins of the observed photoemission data. The importance of structural "building blocks" present in a particular glass to the electron states is emphasized. The effects of disorder on the electronic spectra are clearly demonstrated by a significant change in the electronic density of states, originating in the breakdown of long-range order coherence in the bonding states of the building blocks. We discuss the relation between observed in situ light-induced changes and the electronic structure.

Optical spectra of arsenic chalcogenide glasses over a wide energy range of fundamental absorption

The complete sets of the spectra of optical fundamental functions for vitreous arsenic chalcogenides in the energy range 0–40 eV are compared for the first time. The general features of the spectra and their dependence on the nature of the chalcogen anion are revealed.

Application of the free volume concept to glasses in the Ge-As-S system

The energy of hole formation Eh, the hole volume Vh, and the fraction fg of fluctuation free volume for glasses in the Ge–As–S system are calculated from the data on the elastic moduli, the microhardness H, and the glass transition temperature Tg. It is shown that the fraction of fluctuation free volume in ternary glasses is considerably larger than that in arsenic chalcogenide glasses. For the glasses studied, the Eh energies fall in the range 13–18 kJ/mol and the hole volumes are equal to (7–84) × 10–6 m3/mol. The concentration dependences of the elastic moduli and the parameters of the free volume theory for glasses in the As2S3–GeS2 system are less pronounced than those for glasses in the As–S system with small coordination numbers of glass-former ions. This is explained by the topological transition observed in glasses of this system at a high germanium content. It is demonstrated that the parameters of the fluctuation free volume theory are related to the characteristics of the boson peak in the Raman spectra.

Transient photodarkening in amorphous chalcogenides

Illumination of amorphous arsenic chalcogenides with band-gap light induces photodarkening and volume expansion. In addition to metastable changes, which persist after the light is switched off, transient effects occur during and after illumination. We present measurements of these metastable and transient effects in evaporated films of a-As 2 S 3, a-As 2 Se 3 and a-Se. The results are discussed in terms of a model in which, on photoexcitation, layer- or chain-like clusters of atoms expand and slip relative to each other.

Ab initio molecular-dynamics simulation method for complex liquids

The atomic structure and the electronic states of complex liquids, such as liquid chalcogens, which consist of the chain- and/or the ring-molecules, are investigated by means of the ab initio molecular-dynamics simulation. We clarify the microscopic mechanism of the semiconductor-metal transition in liquid alkali tellurides and liquid arsenic chalcogenides.

Influence of the microstructure on the photoinduced transformations in vacuum deposited As 2S 3 thin films

The photoinduced transformations in As 2S 3 thin films with amorphous and crystalline microstructure are followed. It is found that upon actinic radiation of both samples the transmission edge shifts to longer wavelengths, i.e. a typical effect of photodarkening is observed. The effect is less pronounced in the thin films with crystalline microstructure. Besides, it is established that a phase separation process occurs in all samples under the action of light. On the basis of the results obtained the role of the microstructure in photoinduced transformations in arsenic chalcogenides is discussed.

Anharmonic vibrations and grüneisen parameter of the quasi-lattice of glassy arsenic chalcogenides in the free-volume approach

It is shown that the free-volume approach offers the possibility of taking into account the anharmonicity of vibrations and nonlinearity of interatomic interactions in glassy arsenic chalcogenides. The Gruneisen parameters calculated by different formulas are compared, and the bond-breaking strain is correlated with the maximum possible bond elongation. The elementary atomic displacement in the structure of the glasses is evaluated. The ratio of the fraction of fluctuation-induced free volume to the lattice Gruneisen parameter in glassy arsenic chalcogenides is shown to differ from that in silicate glasses. The parameters of the free-volume model are correlated with those of the models relying on the present-day views of the structure of chalcogenide glasses.

Pulsed neutron diffraction study of the short range structure in amorphous arsenic chalcogenides

The pair distribution functions G( r)’s of As 2S 3 and As 2Se 3 glasses were obtained from the structure factors S( Q)’s. The structural parameters were refined by applying the correlation method in the ( Q, Q·i( Q)) space. Although the structural parameters by PND were somewhat different from those by XRD, the peak shape in PND was clearer than in XRD. The local structure of the glasses was similar to that of orpiment (As 2S 3 crystal) with AsS 3 pyramids. However, the nearest neighbor distances of As–S pair were 2.29 Å in PND, 2.27 Å in XRD, and 2.25 Å (average value of As–S bonds in crystal), implying the little expansion of AsS 3-type pyramids and the variation in linkage of pyramids.

Copper-induced changes in the properties of arsenic chalcogenides

The influence of the presence of Cu in the amount between 1% and 6% in arsenic chalcogenide glasses is examined through a study of the electronic energy levels by means of x-ray photoelectron and x-ray emission spectroscopy, through an investigation of the low-energy tunneling systems by means of phonon echoes at 0.37 K, and through an examination of the photodarkening and the photoinduced dichroism caused by polarized Ar+ laser irradiation. Possible links between the various effects are examined. The Cu atoms become an integral part of the amorphous lattice structure and strongly influence the photodarkening, but they do not have a significant effect on the tunneling systems or the dichroism. It is concluded that the tunneling levels and the dichroism involve only local configurations, while the photodarkening involves larger-scale areas of the lattice.

Asymmetry in the local structural order in arsenic chalcogenide glasses: High field NMR in crystalline and glassy As 2Se 3

Nuclear magnetic resonance (NMR) of 75As in crystalline and glassy samples of As 2Se 3 has been studied at magnetic fields up to 22 T. The results, which yield asymmetries in the electric field gradient at the arsenic sites that disagree with those inferred from previous low field studies, are inconsistent with the predictions of a common model (Raft Model) for the structure of glassy As 2Se 3 and other chalcogenide glasses. This technique should be useful for many other solids that contain one of the twenty elements whose nuclei have spin 3 2 and relatively large quadrupole moment.