Amorphous Chalcogenide Research Articles

Photonic bandgap amorphous chalcogenide thin films with multilayered structure grown by pulsed laser deposition method

Amorphous chalcogenide thin films were fabricated by the pulsed laser deposition technique. Thereafter, the stacks of multilayered thin films for reflectors and microcavity were designed for telecommunication wavelength. The prepared multilayered thin films for reflectors show good compatibility. The microcavity structure consists of Ge25Ga5Sb10S65 (doped with Er3+) spacer layer surrounded by two 5-layer As40Se60/Ge25Sb5S70 reflectors. Scanning/transmission electron microscopy results show good periodicity, great adherence and smooth interfaces between the alternating dielectric layers, which confirms a suitable compatibility between different materials. The results demonstrate that the chalcogenides can be used for preparing vertical Bragg reflectors and microcavity with high quality.

Thickness-dependent electrical and optical properties of Ge8Sb2Te11 thin films

ABSTRACTThe amorphous Ge8Sb2Te11thin films with varying thickness are thermally deposited on well-cleaned glass substrate from its polycrystalline bulk. Absence of any sharp peak confirms the amorphous nature of deposited films. Thickness-dependent electrical and optical properties including dc-activation energy, sheet resistivity, optical band gap, band tailing parameter, etc. of Ge8Sb2Te11thin films have been studied. The optical parameters have been calculated from transmission, reflection and absorbance data in the spectral range of 200–1100 nm. It has been found that optical band gap and band tailing parameter decreases with the increase in Ge8Sb2Te11thin films thickness. The dc-activation energy and sheet resistivity decreases while the crystallization temperature of the amorphous Ge8Sb2Te11 films increases with the increase in thickness of the films. The decrease of the sheet resistivity has been substantiated quantitatively using the classical size-effect theory. These results have been explained on the basis of rearrangements of defects and disorders in the amorphous chalcogenide system.

High efficiency and stability gratings recorded in GeAsS thin films

Scalar and vector gratings were recorded and studied on amorphous chalcogenide Ge25As30S45 thin films motivated by the promise of their high stability and photosensitivity. Films were prepared by e-beam evaporation technique. The holographic gratings were recorded by using light of 514.5 nm wavelength (near the material band gap). Those gratings have shown significantly better thermal resistance compared to those recorded on As2S3 thin films fabricated by the same method. Very preliminary analyses of possible photo sensitivity mechanisms involved is presented.

Back Cover: Graphene–Amorphous Transition‐Metal Chalcogenide (MoSx, WSx) Composites as Highly Efficient Hybrid Electrocatalysts for the Hydrogen Evolution Reaction (ChemElectroChem 4/2016)

The Back Cover picture illustrates composites based on graphene–amorphous transition-metal chalcogenides of Mo and W, which are highly effective hybrid electrocatalysts towards the hydrogen evolution reaction. More information can be found in the Full Paper by M. Pumera, Z. Sofer and co-workers on page 565 in Issue 4, 2016 (DOI: 10.1002/celc.201500497).

Studies on dielectric properties, opto-electrical parameters and electronic polarizability of thermally evaporated amorphous Cd50S50−xSex thin films

The objective of this work is to study the influence of the addition of more Se on dielectric properties, opto-electrical parameters and electronic polarizability of amorphous chalcogenide Cd50S50−xSex thin films (30 ≤ x ≤ 50 at%). Thin films of thickness 200 nm were synthesized by vacuum deposition at ≈8.2×10−4 Pa. Both refractive index and extinction coefficient were used to obtain all the studied parameters. The high frequency dielectric constant, real and imaginary parts of dielectric constant were discussed. Drude theory was applied to investigate opto-electrical parameters, like optical carrier concentration, optical mobility and optical resistivity. Moreover, other parameters were investigated and studied, e.g. Drude parameters, volume and surface energy loss functions, dielectric loss factor, dielectric relaxation time, complex optical conductivity and electronic polarizability as well as optical electronegativity and third-order nonlinear optical susceptibility. Values of electronic polarizability and nonlinear optical susceptibility were found to be decreased while optical electronegativity increased as Se-content was increased. Increment of Se-content in amorphous Cd50S50−xSex thin films has also led to minimize the energy losses when electromagnetic waves propagate through films as well as optical conductivity and the speed of light increased. The other studied properties and parameters of Cd50S50−xSex films were found to be strongly dependent upon Se-content.

DC Hall-effect measurement for inkjet-deposited films of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) by using microscale gap electrodes

DC Hall-effect measurements were performed for inkjet-deposited p-type poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) films, site-selectively fabricated onto micrometer-scale van der Pauw electrode gap patterns. A commercially-available ink of high conductivity grade PEDOT/PSS was utilized. Carrier concentration and Hall mobility of the PEDOT/PSS containing ethylene glycol were in the order of 1021cm−3 and 0.1cm2/(Vs), respectively, derived from the repeated Hall voltage measurements under reversal of current flow and magnetic field. The Hall voltage showed an n-type behavior, while the thermoelectric measurement indicated p-type. This sign anomaly of the Hall voltage, which is often observed in such low-mobility materials as amorphous silicon and chalcogenide glasses, may be due to hopping carrier transport in the PEDOT/PSS, although non-uniformities in the sample structure should be noted. Nonetheless this study suggests that the combination of microscale electrodes and inkjet deposition may be useful in high-sensitivity detection of Hall voltages for organic conducting films.

Nanoporous chalcogenides for adsorption and gas separation.

The adsorption and gas separation properties of amorphous porous chalcogenides such as GeS2 are investigated using statistical mechanics molecular simulation. Using a realistic molecular model of such amorphous adsorbents, we show that they can be used efficiently to separate different gases relevant to environmental and energy applications (H2, CO2, CH4, N2). In addition to shedding light on the microscopic adsorption mechanisms, we show that coadsorption in this novel class of porous materials can be described using the ideal adsorbed solution theory (IAST). Such a simple thermodynamic model, which allows avoiding complex coadsorption measurements, describes the adsorption of mixture from pure component adsorption isotherms. Our results, which are found to be in good agreement with available experimental data, paves the way for the design of gas separation membranes using the large family of porous chalcogenides.

The First Sharp Diffraction Peak in the Total Structure Function of Amorphous Chalcogenide Glasses: Anomalous Characteristics and Controversial Views

Anomalous structural characteristics of the so-called first sharp diffraction peak (FSDP) that arises in the total static structure functions of network-forming glasses and liquids at around 1-2 A-1 have been reviewed and discussed in details. Unlike other peaks in the static structure functions, the FSDP has anomalous dependencies on temperature, pressure and composition. Despite the fact that the FSDP is considered as a signature of intermediate range order (IRO) in network-forming glasses and liquids, its structural origin remains unclear and till now, it forms a subject of debate. A brief account for some anomalous characteristics of the FSDP followed by the different controversial interpretations about its structural origin has been reviewed and discussed. Some of the interpretations that seem to be inconsistent with recent experimental results have been ruled out. The most likely structural origins for the occurrence of the FSDP have been highlighted and discussed in details.

Application of photoelectron spectroscopy methods for investigation of subsurface layers of amorphous chalcogenide materials

Проаналізовано можливість застосування методів рентгенівської фотоелектронної спектроскопії та фотоелектронної спектроскопії із використанням синхротронного випромінювання для дослідження структури некристалічних напівпровідникових матеріалів, зокрема халькогенідних. Виявлено переваги і недоліки цих методів у випадку дослідження тонких шарів або поверхні халькогенідних стекол.

Work function and AC operating gas‐sensitive films based on quaternary chalcogenides

A study of quaternary alloys of As–S–Ge–Te was performed in order to assess their use in future gas sensors operating at room temperature. To elucidate the effect of tellurium, the quaternary compositions and , with increasing concentration of Te have been considered along with pure tellurium films. SEM, AFM, and X‐ray analysis have shown that the nature of the films was predominantly amorphous. To overcome the sensing disadvantage of DC chalcogenide‐based sensors due to small signal/noise ratio, gas‐sensing measurements were performed using both potential difference (Kelvin probe) and AC methods. The work‐function measurements showed that the amorphous chalcogenides in question are suitable materials for the detection of small concentrations of NO2. The sensing mechanism of NO2 is explained by “strong” chemisorptions via interaction of adsorbed species with lone‐pair electrons, which form the upper part of the valence band of chalcogenide semiconductors. The chemisorption of NO2 molecules results in increases in both work‐function change ΔΦ > 0 and electrical conductivity Δσ > 0 because of the additional charging of the surface and band bending. The impedance spectra, being strongly influenced by gaseous environment, depend on material composition and film microstructure. The frequency‐dependent impedance sensitivity to nitrogen dioxide denotes the competitive influence of carrier transport via states of allowed bands, hopping between localized states in the extended band tails and tunneling (variable‐range hopping) between localized states close to the Fermi level. Impedance sensitivity, being maximal for amorphous , is assumed to be controlled by competition of these charge‐transport mechanisms.

Electron conductivity in GeTe and GeSe upon ion implantation of Bi

This paper presents results on ion implantation of bismuth in GeTe and GeSe films. The conductivity and the thermopower of amorphous chalcogenide films are investigated. Electron conductivity in the films is attained at the Bi implantation doses higher than (1.5–2) × 1016 cm−2. In conjunction with the structural modification in the films as revealed Raman spectroscopy, the results suggest the structural re-arrangement of the amorphous network occurs via weakening the bonds of a lower energy. The onset of electron conductivity is hindered by a stronger bond in an alloy. In GeTe, this is the Ge-Ge bond.

Optical field-induced surface-relief micropatterning of amorphous chalcogenide thin films

The study of direct recording of the surface relief gratings on amorphous chalcogenide thin (2.5 to 5 μm films is presented by three different recording setups. Recording was performed on As2S3 by 532-nm wavelength laser light. Additionally, the evolution of a surface relief in dependence from the recording time and polarization has been investigated in detail and for the first time, the mass transfer process has been explained from the point of view of the photoinduced birefringence. The role of photoinduced plasticity in the formation of surface relief in amorphous chalcogenides during holographic recording has been discussed.

Characterization of optical constants and dispersion parameters of highly transparent Ge20Se76Sn4 amorphous thin film

Amorphous chalcogenide Ge20Se76Sn4 thin films of six different thicknesses (50–350nm) are prepared by the thermal evaporation technique. Optical transmission and reflection spectra, in the wavelength range of the incident photons from 250 to 2500nm, are used to study the effect of the film thickness on some optical properties. It is found that the effect of film thickness leads to increase in the absorption coefficient, refractive index, extinction coefficient and the width of the tails of localized states in the gap region. The decrease in optical band gap energy with increasing the film thickness is attributed to the formation of a band tail which narrows down the band gap. Dispersion analyses of refractive index reveal a decrease in the single-oscillator energy and an increase in the dispersion energy with increase in film thickness.

Experimental investigation of light induced defects in amorphous thin films of Se90X10 (X = Sb, In, Ag)

An attempt is made to observe light induced defects (LID) in amorphous chalcogenides Se90X10 (X=Sb, In, Ag) thin films prepared by vacuum evaporation technique. For the determination of light induced defects quantitatively, space charge limited current (SCLC) measurements have been made in a vacuum ∼10−3Torr before and after exposing amorphous films to white light for different exposure times (0 to 6h). Results indicate that light induced metastable defects are created due to prolonged exposure of light which is explained by a microscopic model for light induced metastable defects creation proposed by Shimakawa and co-workers. Fractional increase in density of defects due to light exposure is found to be least in case of Se90Ag10 indicating that this alloy may be used for optoelectronic applications as degradation upon light soaking is small in this case.

Porous Amorphous Chalcogenides as Selective Adsorbents for Heavy Metals

Porous Amorphous Chalcogenides as Selective Adsorbents for Heavy Metals

Disorder-induced anomalously signed Hall effect in crystalline GeTe/Sb2Te3 superlattice-like materials

Opposite to the almost persistent p-type conductivity of the crystalline chalcogenides along the GeTe-Sb2Te3 tie line, n-type Hall mobility is observed in crystalline GeTe/Sb2Te3 superlattice-like material (SLL) with a short period length. We suggest that this unusual carrier characteristic originates from the structural disorder introduced by the lattice strain and dangling bonds at the SLL interfaces, which makes the crystalline SLLs behave like the amorphous chalcogenides. Detailed structural disorder in crystalline SLL has been studied by Raman scattering, X-ray photoelectron spectroscopy, as well as Variable-energy positron annihilation spectroscopy measurements. First-principles calculations results show that this structural disorder gives rise to three-site junctions that dominate the charge transport as the period length decreases and result in the anomalously signed Hall effect in the crystalline SLL. Our findings indicate a similar tetrahedral structure in the amorphous and crystalline states of SLLs, which can significantly reduce the entropy difference. Due to the reduced entropy loss and increased resistivity of crystalline phase introduced by disorder, it is not surprising that the SLLs exhibit extremely lower RESET current and power consumption.

Low-temperature photo-induced mass transfer in thin As20Se80 amorphous films

We have detected that surface relief gratings (SRG) in amorphous chalcogenide films As20Se80 can be optically recorded at low temperature, such as 77K. A diffusion mechanism of photo-induced (PI) mass transport is proposed. A driving force of PI mass transport is a lateral steady state electric field induced by light interference. The kinetics of PI SRG growth depends on temperature due to temperature dependence of PI diffusion coefficients and concentration of radiation defects. By comparison of low temperature kinetics with that at 300K we estimated diffusion activation energy, which turned out 0.09eV. We present a model that explains low diffusion activation energy.

Nanosecond light-induced transient absorption in As2S3: Self-trapped exciton recombination in amorphous chalcogenides

In this article, we report spectral and temporal dependence of nanosecond (ns) transient absorption (TA) in a-As2S3 thin film when illuminated with the above (355nm) and near (532nm) bandgap laser. Detailed temperature dependent study of TA by 355nm excitation demonstrates the decrease in TA together with faster relaxation kinetics at higher temperatures. Such observation of the change in kinetics and magnitude of TA with temperature is well consistent with the self-trapped exciton recombination mechanism. In addition to this, our experimental results also reveal that the long wavelength component of TA decays faster than the short wavelengths which provides interesting insights that the trapped exciton possesses longer lifetime in deep traps than in shallow traps. The excitation fluence dependent study further reveals that TA exhibits a quadratic dependence on laser dose which yields that the effects are originating from the two-photon process.

Reversible migration of silver on memorized pathways in Ag-Ge40S60 films

Reversible and reproducible formation and dissolution of silver conductive filaments are studied in Ag-photodoped thin-film Ge40S60 subjected to electric fields. A tip-planar geometry is employed, where a conductive-atomic-force microscopy tip is the tip electrode and a silver patch is the planar electrode. We highlight an inherent “memory” effect in the amorphous chalcogenide solid-state electrolyte, in which particular silver-ion migration pathways are preserved “memorized” during writing and erasing cycles. The “memorized” pathways reflect structural changes in the photodoped chalcogenide film. Structural changes due to silver photodoping, and electrically-induced structural changes arising from silver migration, are elucidated using Raman spectroscopy. Conductive filament formation, dissolution, and electron (reduction) efficiency in a lateral device geometry are related to operation of the nano-ionic Programmable Metallization Cell memory and to newly emerging chalcogenide-based lateral geometry MEMS technologies. The methods in this work can also be used for qualitative multi-parameter sampling of metal/amorphous-chalcogenide combinations, characterizing the growth/dissolution rates, retention and endurance of fractal conductive filaments, with the aim of optimizing devices.

Direct surface relief formation on As2S3–Se nanomultilayers in dependence on polarization states of recording beams

Direct one step grating relief formation by laser beam on the surface of amorphous chalcogenide nanomultilayers of the composition As2S3–Se was performed. Successive nanolayers on a glass substrate were prepared by thermal vacuum deposition with cyclic motion of substrate. Optical constants were obtained from transmission spectra data in the range of 450–900nm. The dependence of the surface relief formation on nanomultilayers vs. the polarization states of laser beams was studied. The surface gratings with maximum relief depth were recorded under the two linear polarized beams falling on the sample surface at (+45°)–(−45°). Holographic diffraction gratings with the diffraction efficiency ∼28% value in transmission mode at λ=0.65μm were recorded without additional treatment. This fact is explained by the contribution of mass transport whose direction is defined by the electric vector of laser beams. Recorded grating provides high optical quality of the obtained relief with depth of the grooves of about 100nm at the total structure thickness 2500nm.