As-As Bonds Research Articles

Study of SiGeAsTe and SiGeAsSe chalcogenide thin films by Raman spectroscopy and understanding of their OTS properties

In this paper, a range of sputtered ternary and quaternary (Si)(Ge)AsxTe3 layers (x: 2 or 5) and (Si)(Ge)As2Se3 layers are examined using Raman spectroscopy. The results are linked to the Ovonic Threshold Switching properties of these materials when incorporated in selector devices, as observed in separate studies. In case of both the As-rich and As-poor tellurides, a large amount of homopolar bonds are present as the spectra are dominated by peaks associated with As–As and Te–Te bonds. Such homopolar bonds are commonly linked with drift in OTS properties. In the case of the selenides the spectra are dominated by modes associated with heteropolar As–Se bond vibrations. Adding Ge as an alloying element has a significant impact on the bond structure for both material systems. In contrast, Si has a much less pronounced impact and will mostly bond with itself. Time-resolved Raman measurements were also performed to determine the stability of the layers under micro-Raman laser excitation. At the regular exposure settings, no significant changes in the spectra were observed during the measurement. At elevated exposure settings, however, persistent changes in the bond structure can be induced for certain compositions.

Electrochemical Li intercalation in b-AsyP1−y alloys: In-situ Raman spectroscopy study

Structural evolution of b-AsyP1−y alloys with varying As concentration, y was studied during Li-intercalation using in-situ Raman spectroscopy. A monotonic redshift of all the vibrational modes corresponding to P-P bonds (A1g, A2g, B2g), As-As bonds (A1g, A2g, B2g), and As-P bonds of all the samples was observed during the initial stages of the intercalation process due to the softening of each mode caused by the intercalation driven donor-type charge-transfer from Li to b-AsyP1−y. The emergence of a new peak identified as the Eg mode of gray As, above an intercalation threshold, is indicative of the presence of an intercalation-driven structural phase segregation process. Further, A1 g mode of gray As emerging after this intercalation threshold lies in close proximity of Ag2 Raman mode of b-AsyP1−y. All the peaks beyond the intercalation threshold show an upshift due to the co-existence of gray As with b-AsyP1−y alloys causing strain and thus hardening of the phonon modes. In the sample with the highest As concentration phase segregation takes place during the synthesis process. Computational modeling reveals the co-existence of gray As in the b-AsyP1−y alloys with high As concentrations. It also confirms the existence of a local structural segregation taking place at a critical Li concentration during the intercalation process. This work shows the significance of intercalation on structural changes in the search for new phases of b-AsyP1−y alloys.

The effect of the structure on the physical properties in GexAs10Se90-x glasses

We have prepared a group of GexAs10Se90-x glass(x=5, 10, 15, 20, 25, 30, and 35 at. %) and investigated their structure and physical properties. It was found that, the minimum refractive index and maximum optical bandgap occur in Ge25As10Se65 glass that is chemically stoichiometric. Analysis of Raman spectra of the glasses indicated that the number of the Ge-Ge, As-As, and Se-Se homopolar bonds is closely related to the bandgap, because the band-tails formed by homopolar bonds could reduce the optical bandgap. The transition behavior of the structural units and physical properties of the glasses occurs at the glass with the chemically stoichiometric composition, and thus the chemical composition dominates physical properties of GexAs10Se90-x chalcogenide glasses

Binding, Release and Functionalization of Intact Pnictogen Tetrahedra Coordinated to Dicopper Complexes.

The bridging MeCN ligand in the dicopper(I) complexes [(DPFN)Cu2(μ,η1 : η1‐MeCN)][X]2 (X=weakly coordinating anion, NTf2 (1 a), FAl[OC6F10(C6F5)]3 (1 b), Al[OC(CF3)3]4 (1 c)) was replaced by white phosphorus (P4) or yellow arsenic (As4) to yield [(DPFN)Cu2(μ,η2 : η2‐E4)][X]2 (E=P (2 a–c), As (3 a–c)). The molecular structures in the solid state reveal novel coordination modes for E4 tetrahedra bonded to coinage metal ions. Experimental data and quantum chemical computations provide information concerning perturbations to the bonding in coordinated E4 tetrahedra. Reactions with N‐heterocyclic carbenes (NHCs) led to replacement of the E4 tetrahedra with release of P4 or As4 and formation of [(DPFN)Cu2(μ,η1 : η1‐MeNHC)][X]2 (4 a,b) or to an opening of one E−E bond leading to an unusual E4 butterfly structural motif in [(DPFN)Cu2(μ,η1 : η1‐E4 DippNHC)][X]2 (E=P (5 a,b), E=As (6)). With a cyclic alkyl amino carbene (EtCAAC), cleavage of two As−As bonds was observed to give two isomers of [(DPFN)Cu2(μ,η2 : η2‐As4 EtCAAC)][X]2 (7 a,b) with an unusual As4‐triangle+1 unit.

Effect of Se on Structure and Electrical Properties of Ge-As-Te Glass.

The Ge-As-Te glass has a wide infrared transmission window range of 3–18 μm, but its crystallization tendency is severe due to the metallicity of the Te atom, which limits its development in the mid- and far-infrared fields. In this work, the Se element was introduced to stabilize the Ge-As-Te glass. Some glasses with ΔT ≥ 150 °C have excellent thermal stability, indicating these glasses can be prepared in large sizes for industrialization. The Ge-As-Se-Te (GAST) glasses still have a wide infrared transmission window (3–18 μm) and a high linear refractive index (3.2–3.6), indicating that the GAST glass is an ideal material for infrared optics. Raman spectra show that the main structural units for GAST glass are [GeTe4] tetrahedra, [AsTe3] pyramids, and [GeTe4Se4−x] tetrahedra, and with the decrease of Te content (≤50 mol%), As-As and Ge-Ge homopolar bonds appear in the glass due to the non-stoichiometric ratio. The conductivity σ of the studied GAST glasses decreases with the decrease of the Te content. The highest σ value of 1.55 × 10−5 S/cm is obtained in the glass with a high Te content. The activation energy Ea of the glass increases with the decrease of the Te content, indicating that the glass with a high Te content is more sensitive to temperature. This work provides a foundation for widening the application of GAST glass materials in the field of infrared optics.

Pressure-densified new rhombohedral phase of EuSn2As2

The magnetic topological insulator ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ is known to crystallize in a ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$-type layered structure in rhombohedral symmetry at ambient conditions and undergo a structural phase transition toward a monoclinic $\ensuremath{\beta}$ phase under increasing pressure. Here, we identify by ab initio calculations a new high-pressure rhombohedral structure (termed the $\ensuremath{\gamma}$ phase) in $R\overline{3}m$ symmetry comprising Sn-As-As-Sn linear chains along the $z$ axis via the formation of As-As bonds in the AsSn/SnAs bilayers with strong anisotropic lattice distortion along the [001] orientation. This pressure-densified new rhombohedral phase has a metallic A-type antiferromagnetic ground state and is energetically more favorable than the original rhombohedral structure and the monoclinic $\ensuremath{\beta}$ phase above 11 GPa. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak around $14.{7}^{\ensuremath{\circ}}$ found in experiments for ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ above 13 GPa. These findings offer key insights into a new type of high-pressure phase of layered magnetic topological insulators, laying a foundation for further exploration of this intriguing class of materials.

Cluster Modeling of Network-Forming Amorphization Pathways in AsxS100−x Arsenicals (50 ≤ x ≤ 57) Diven by Nanomilling

Complete hierarchy of network amorphization scenarios initiated in AsxS100-x nanoarsenicals within As4S4-As4S3 cut-Sect. (50 ≤ x ≤ 57) is reconstructed employing materials-computational approach based on ab-initio quantum-chemical modeling code (CINCA). Under nanostructurization due to high-energy mechanical milling, the inter-crystalline transformations to nanoscopic β-As4S4 phase accompanied by appearance of covalent-network amorphous matrix are activated. General amorphization trend under nanomilling obeys tending from molecular cage-like structures to optimally-constrained covalent-bonded networks compositionally invariant with parent arsenical. The contribution of amorphization paths in nanoarsenicals is defined by their chemistry with higher molecular-to-network barriers proper to As4S3-rich alloys. The generated amorphous phase is intrinsically decomposed, possessing double-Tg relaxation due to stoichiometric (x = 40) and non-stoichiometric (x > 40) sub-networks, which are built of AsS3/2 pyramids and As-rich arrangement keeping (i) two separated As-As bonds derived from realgar-type molecules, (ii) two neighboring As-As bonds derived from pararealgar-type molecules or (iii) three neighboring As-As bonds in triangle-like geometry derived from dimorphite-type molecules. Compositional invariance of nanoamorphous phase is ensured by growing sequence of network-forming clusters with average coordination numbers Z in the row (As2S4/2,Z = 2.50) – (As3S5/2, Z = 2.55) – (As3S3/2, Z = 2.67). Diversity of main molecular-to-network amorphizing pathways in nanoarsenicals is reflected on the unified potential energy landscape specified for boundary As4S4 and As4S3 components.

Uniaxial compression of [001]-oriented CaFe2As2 single crystals:the effects of microstructure and temperature on superelasticity Part I: Experimental observations

Micropillar compression experiments on [001]-oriented CaFe2As2 single crystals have recently revealed the existence of superelasticity with a remarkably high elastic limit of over 10%. The collapsed tetragonal phase transition, which is a uni-axial contraction process in which As-As bonds are formed across an intervening Ca-plane, is the main mechanism of superelasticity. Usually, superelasticity and the related structural transitions are affected strongly by both the microstructure and the temperature. In this study, therefore, we investigated how the microstructure and temperature affect the superelasticity of [001]-oriented CaFe2As2 micropillars cut from solution-grown single crystals, by performing a combination of in-situ cryogenic micromechanical testing and transmission electron microscopy studies. Our results show that the microstructure of CaFe2As2 is influenced strongly by the crystal growth conditions and by subsequent heat treatment. The presence of Ca and As vacancies and FeAs nanoprecipitates affect the mechanical behavior significantly. In addition, the onset stress for the collapsed tetragonal transition decreases gradually as the temperature decreases. These experimental results are discussed primarily in terms of the formation of As-As bonds, which is the essential feature of this mechanism for superelasticity. Our research outcomes provide a more fundamental understanding of the superelasticity exhibited by CaFe2As2 under uni-axial compression.

Chemical and Structural Variety in Sodium Thioarsenate Glasses Studied by Neutron Diffraction and Supported by First-Principles Simulations.

Sodium-conducting sulfide glasses are promising materials for the next generation of solid-state batteries. Deep insight into the glass structure is required to ensure a functional design and tailoring of vitreous alloys for energy applications. Using pulsed neutron diffraction supported by first-principles molecular dynamics, we show a structural diversity of Na2S-As2S3 sodium thioarsenate glasses, consisting of long corner-sharing (CS) pyramidal chains CS-(AsSS2/2)k, small AspSq rings (p + q ≤ 11), mixed corner- and edge-sharing oligomers, edge-sharing (ES) dimers ES-As2S4, and isolated (ISO) pyramids ISO-AsS3, entirely or partially connected by sodium species. Polysulfide S-S bridges and structural units with homopolar As-As bonds complete the glass structure, which is basically different from structural motifs predicted by the equilibrium phase diagram. In contrast to superionic silver and sodium sulfide glasses, characterized by a significant population of isolated sulfur species Siso (0.20 < Siso/Stot < 0.28), that is, sulfur connected to only mobile cations M+ with a usual M/Siso stoichiometry of 2, poorly conducting Na2S-As2S3 alloys exhibit a modest Siso fraction of 6.2%.

Structure of As-Se glasses by neutron diffraction with isotope substitution.

The method of neutron diffraction with selenium isotope substitution is used to measure the structure of glassy As0.30Se0.70, As0.35Se0.65, and As0.40Se0.60. The method delivers three difference functions for each sample in which the As-As, As-Se, or Se-Se correlations are eliminated. The measured coordination numbers are consistent with the "8-N" rule and show that the As0.30Se0.70 network is chemically ordered, a composition near to which there is a minimum in the fragility index and a boundary to the intermediate phase. Chemical ordering in glassy As0.35Se0.65 and As0.40Se0.60 is, however, broken by the appearance of As-As bonds, the fraction of which increases with the arsenic content of the glass. For the As0.40Se0.60 material, a substantial fraction of As-As and Se-Se defect pairs (∼11%) is frozen into the network structure on glass formation.

Uniaxial Compression of [001]-Oriented CaFe 2As 2 Single Crystals: The Effects of Microstructure and Temperature on the Superelasticity, Part I: Experimental Observations

Micropillar compression experiments on [001]-oriented CaFe 2 As 2 single crystals have recently revealed the existence of superelasticity with a remarkably high elastic limit over 10%. The collapsed tetragonal phase transition, which is a uni-axial contraction process in which As-As bonds are formed across an intervening Ca-plane, is the main mechanism of superelasticity. Usually, superelasticity and the related structural transitions are affected strongly by both the microstructure and the temperature. In this study, therefore, we investigated how the microstructure and temperature affect the superelasticity of [001]-oriented CaFe 2 As 2 micropillars cut from solution-grown single crystals, by performing a combination of in-situ cryogenic micromechanical testing and transmission electron microscopy studies. Our results show that the microstructure of CaFe 2 As 2 is influenced strongly by the crystal growth conditions and by subsequent heat treatment. The presence of Ca and As vacancies and FeAs nanoprecipitates affect the mechanical behavior significantly. In addition, the onset stress for the collapsed tetragonal transition decreases gradually as the temperature decreases. These experimental results are discussed primarily in terms of the formation of As-As bonds, which is the essential feature of this mechanism for superelasticity. Our research outcomes provide a more fundamental understanding of the superelasticity exhibited by CaFe 2 As 2 under uni-axial compression.

Mixed Valence and Unusual Germanium Coordination in SrGe8As10, BaGe8As10, and BaGe7P12.

The new Zintl-compounds AEGe8As10 (AE = Sr, Ba) and BaGe7P12 were synthesized via solid-state reactions, and their structures were determined by single crystal and powder X-ray diffraction. SrGe8As10 and BaGe8As10 crystallize in the space group Cmce and show complex 3D networks composed of three different Ge-As motifs and As-As bonds with mixed valence of germanium in the oxidation states +2, + 3, and +4. Mixed valences of germanium +3 and +4 occur in BaGe7P12, which crystallizes in the space group R3̅ with a 3D network built up of Ge2P6 dumbbells and P-P bonds. An exceptional 6-fold coordinated germanium resides in the center of a GeP6 trigonal antiprism. High temperature X-ray diffraction shows thermal stabilities up to 923-953 K. UV-Vis and resistivity measurements reveal a semiconducting nature with small indirect band gaps between 0.02 and 1.6 eV. Electronic band structure calculations confirm the semiconducting state and indicate covalent bonds within the Ge-Pn polyanions.

Pressure-Driven Chemical Disorder in Glassy As2S3 up to 14.7 GPa, Postdensification Effects, and Applications in Materials Design.

A small difference in energy between homopolar and heteropolar bonds and the glass-forming ability of pure chalcogens leads to unexpected trends in densification mechanisms of glassy chalcogenides compared to vitreous oxides. Using high-precision compressibility measurements and in situ high-energy X-ray diffraction up to 14.7 GPa, we show a new densification route in a canonical glass As2S3. After the first reversible elastic step with a maximum pressure of 1.3 GPa, characterized by a strong reduction of voids and cavities, a significant bonding or chemical disorder is developed under higher pressure, reaching a saturation of 30% in the population of As-As bonds above 8-9 GPa. The pressure-driven chemical disorder is accompanied by a remarkable structural relaxation and a strongly diminished optical gap and determines structural, vibrational, and optical properties under and after cold compression. The decompressed recovered glass conserves a dark color and exhibits two relaxation processes: (a) fast (a few days) and (b) slow (months/years at room temperature). The enhanced refractive index of the recovered glass is promising for optical applications with improved functionalities. A nearly permanent red shift in optical absorption after decompression can be used in high-impact-force optical sensors.

Robust magnetoresistance in TaAs2 under pressure up to about 37 GPa

The extremely large magnetoresistance (XMR) in nonmagnetic semimetals has inspired growing interest owing to both intriguing physics and potential applications. We report the results of synchrotron X-ray diffraction and electrical transport measurements on TaAs2 under pressure up to ∼37 GPa, which revealed an anisotropic compression of the unit cell, formation of unusual As-As bonds above 9.5 GPa, and enhancement of metallicity. Interestingly, the MR of TaAs2 under pressure changed gently, which at 1.7 GPa is 96.6% and at 36.6 GPa is still 36.7%. The almost robust MR under pressure could be related to the nearly stable electronic structure unveiled by the ab initio calculations. The discovery would expand the potential use of XMR even under high pressure.

Effects of La content in ZrLaON gate dielectric on the interfacial and electrical properties of GaAs metal-oxide-semiconductor devices

GaAs MOS capacitors with ZrLaON as the high-k dielectric layer and LaON as the interfacial passivation layer (IPL) are fabricated and the effects of the La content in ZrLaON on the device performance are investigated. Experimental results show that ZrLaON with a moderate or high La content induces serious moisture absorption, thus deteriorating the interfacial and electrical properties of the device, even with the LaON IPL. By adjusting the La content to a suitable value, e.g. a La/(La + Zr) atomic ratio of 13%, good interfacial and electrical properties can be obtained because the passivating role of La on the defects in ZrON can over-compensate the performance degradation induced by the moisture absorption. With the LaON IPL, further improvements in interfacial and electrical properties can be achieved because the LaON IPL can effectively passivate the As-O, Ga-O and As-As bonds at/near the high-k/GaAs interface.

X-ray photoelectron spectra of Ge-As-Te glasses

Ternary Ge10AsxTe90-x glasses with a mean coordination number (MCN) from 2.3 to 2.8 were prepared, and their physical and structural properties were characterized. It was found that, the density of the glass decreases but glass transition temperature Tg increases, and the near infrared transmission edge shifts to shorter wavelength with increasing As content. The Ge, As, and Te 3d spectra were decomposed into different doublets that correspond to different structural units and the results showed that, the numbers of Te-Te-Te trimmers and Te-Te-As(Ge) structural units decrease and finally disappear, while the perfect AsTe3/2 pyramidal and GeTe4/2 tetrahedral structure in Te-rich samples gradually transferred to defect structures including As-As and Ge-Ge homopolar bonds with increasing As concentration. No threshold behaviour can be found in the structural evolution of Ge10AsxTe90-x glasses due to a large atomic contrast between As and Te, and no any change in the chemical coordination of Te can be observed even in Te-poor glasses.

A novel method for synthesis of arsenic sulfide films employing conversion of arsenic monosulfide in a plasma discharge

A novel plasma-based method for synthesis of arsenic sulfide films with different structural units and stoichiometries is demonstrated. For the first time As-S films have been prepared via direct conversion of arsenic monosulfide (As4S4) as a single precursor in a low-temperature non-equilibrium RF plasma discharge at low pressure. The interplay between composition, structure and properties of the chalcogenide materials prepared has been studied. Quantum-chemical calculations have been performed to gain insight into the films structure and the mechanism of its formation in the plasma discharge. Raman spectroscopy proves that with the As-content increase in the As-S films the intensity of the line, corresponding to vibrations of homopolar As-As bonds in the realgar As4S4 units and in the S2As-AsS2 bridges increases. The last ones are responsible for the appearance of a photoluminescence phenomenon in chalcogenide glasses.

Collapsed tetragonal phase as a strongly covalent and fully nonmagnetic state: Persistent magnetism with interlayer As–As bond formation in Rh-doped Ca0.8Sr0.2Fe2As2

A well-known feature of CaFe$_{2}$As$_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As-As bond. Using detailed X-ray scattering and spectroscopy, we find that Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_{2}$As$_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As-As bonds. Our density functional theory calculations reveal that the Fe-As bond geometry is critical for stabilizing magnetism and that the pressure-induced drop in the $c$ lattice parameter observed in pure CaFe$_{2}$As$_{2}$ is mostly due to a constriction within the FeAs planes. These phenomena are best understood using an often overlooked explanation for the equilibrium Fe-As bond geometry, which is set by a competition between covalent bonding and exchange splitting between strongly hybridized Fe $3d$ and As $4p$ states. In this framework, the collapsed tetragonal phase emerges when covalent bonding completely wins out over exchange splitting. Thus the collapsed tetragonal phase is properly understood as a strong, covalent phase that is fully nonmagnetic with the As-As bond forming as a byproduct.

Structural transformations on the As&lt;sub&gt;X&lt;/sub&gt;S&lt;sub&gt;100-X&lt;/sub&gt;: X-ray photoelectron and electron microscopy investigations

Background: The research of chalcogenide glassy (ChG) materials formed a ge-neral understanding of electronic phenomena in disordered structures. The numerous investigations of their fundamental physical and chemical properties have been already studied. Unique structural, electronic and optical properties determined their various applications. Photosensitivity is the main feature of chalcogenide glasses for fabrication phase-change memory, tailoring direct waveguides and grating patterning. Materials and methods: Amorphous As x S 100-x (x = 40, 45, 50) were prepared by thermal vacuum evaporation from appropriate bulk glass powders onto silicon wafers. The surface chemical composition and structure of material were investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) on as-deposited ChG thin film samples and samples being illuminated by near bandgap laser (l = 532 nm, photon energy ~ 2.4 eV, p = 25 mW), under ambient conditions. Results: All spectra of S 2p electron core level were fitted by two components: S-As 2 and S-SAs s.u. The As 3d core level spectra of As 40 S 60 and As 50 S 50 nanolayers contains As-S 3 , As-S 2 As and As-SAs 2 s.u. As 45 S 55 structure is characterized by the presence of the fourth, As-As 3 component apart from previous mentioned three. Near-bandgap laser light illumination of As x S 100-x (x = 40, 45, 50) samples causes decreasing of the contribution of components with the homopolar S-S bonds in all samples, decreasing of components with homopolar As-As bonds in the structure of As 40 S 60 and As 50 S 50 nanolayers and increasing of concentration of components with As-As bonds in As 45 S 55 films. Conclusion: The local and molecular structure of the surface layers of As x S 100-x thin films (x = 40, 45, 50) and their structural transformations induced by coherent laser irradiation with energy close to band gap energy, with using XPS and SEM were investigated. An essential difference between the surface stoichiometry and the composition of the appropriate glasses has been found. Established that this behavior caused by peculiarities of the molecular component of the gas phase during deposition. This indicates that the type of molecules in a pair plays a decisive role in forming composition of the film. Laser irradiation reduces the concentration of homopolar S-S bonds in the structure of all As x S 100-x nanolayers (x = 40, 45, 50). However a decreasing of the As-As homopolar bond concentration induced by laser light was observed only on samples of As 40 S 60 and As 50 S 50 films. During illumination significant contribution of As-S 2 As and As-SAs 2 s.u. and the appiarence of As-enriched As-As 3 s.u. was seen in the case of As 45 S 55 thin film sample. SEM photos of the surface of the films confirm the ordering of the structure of the films during processing. Keywords: X-ray photoelectron spectroscopy, amorphous film, laser irradiation, scanning electron microscopy, structural units.

Using Ge-Doped La-Oxynitride as Interfacial Passivation Layer for GaAs Metal-Oxide-Semiconductor Capacitors

The GaAs MOS capacitors using ZrON as high-k gate dielectric with LaGeON or LaON as an interfacial passivation layer (IPL) are fabricated and their electrical and interfacial properties are investigated. Compared to their counterpart without IPL, introducing LaGeON or LaON as IPL results in a large improvement of interface quality and electrical properties of the GaAs MOS devices, for example, improved capacitance-voltage behavior, reduced interface-state density and gate leakage current, and enhanced device reliability. Moreover, because of Ge incorporation, the LaGeON IPL becomes more stable and can more efficiently block the Ga/As out-diffusion and O in-diffusion to reduce the defect-related Ga/As-O and As-As bonds at the GaAs surface, thus exhibiting the best interface quality and electrical properties, and high device reliability.