Physical properties of the As2 (Se,Te)3 glasses (Review article)

Recent interest in the properties of amorphous chalcogenides has resulted in a great expansion in the literature on this subject. The present work reviews the physical properties of one of the most carefully studied alloy systems: the As₂ Se₃ − As₂ Te₃ glasses. The review emphasises experimental results and suggests areas of research which need to be more closely examined.     

Spectra of optical parameters in bulk and film amorphous alloys of the Se<Subscript>95</Subscript>As<Subscript>5</Subscript> system containing samarium (Sm) impurities

Reflectance spectra of bulk and film amorphous alloys of the Se₉₅As₅ system containing samarium (Sm) impurities are studied in the energy range of 1–6 eV. Spectral dependences of optical constants and derivatives of optical dielectric functions are calculated by the Kramers-Kronig method. Changes in spectra of optical parameters depending on the content of impurities introduced into Se₉₅As₅ and conditions of their preparation are explained based on the cluster model. According to the latter, changes in the electron density of states depends on changes in atomic configurations in clusters, i.e., short-range order changes.     

Reversible Amorphous‐to‐Amorphous Transitions in Chalcogenide Films: Correlating Changes in Structure and Optical Properties

Structural transitions in materials are accompanied by appreciable and exploitable changes in physical‐chemical properties. Whereas reversible optically‐driven atomistic changes in crystal‐to‐amorphous transitions are generally known and exploited in applications, the nature of the corresponding polyamorphic transitions between two structurally distinct meta‐stable amorphous phases is an unexplored theme. Direct experimental evidence is reported for the nature of the atomistic changes during fully reversible amorphous‐to‐amorphous switching between two individual states in the non‐crystalline As₅₀Se₅₀ films prepared by pulsed‐laser deposition and consequent changes in optical properties. Combination of surface sensitive X‐ray photoelectron spectroscopy and spectroscopic ellipsometry show that the near‐bandgap energy illumination and annealing induce reversible switching in the material's structure by local bonding rearrangements. This is accompanied by switching in refractive index between two well‐defined states. Exploiting the pluralism of distinct structural states in a disordered solid can provide new insights into the data storage in emerging optical memory and photonic applications.     

缺陷相互作用对HfO2基阻变存储器件均匀性的影响：第一性原理研究

The physical mechanism of doping effects on switching uniformity and operation voltage in Al-doped HfO₂ resistive random access memory（RRAM） devices is proposed from another perspective:defects interactions, based on first principle calculations.In doped HfO₂,dopant is proved to have a localized effect on the formation of defects and the interactions between them.In addition,both effects cause oxygen vacancies（V_O） to have a tendency to form clusters and these clusters are easy to form around the dopant.It is proved that this process can improve the performance of material through projected density of states（PDOS） analysis.For V_O filament-type RRAM devices, these clusters are concluded to be helpful for the controllability of the switching process in which oxygen vacancy filaments form and break.Therefore,improved uniformity and operation voltage of Al-doped HfjO₂ RRAM devices is achieved.     

The effect of amorphization on the local structure of arsenic chalcogenides

The effect of amorphization on the symmetry of the local environment of chalcogen atoms in As₂S₃, As₂Se₃, and As₂Te₃ compounds has been investigated by ¹²⁹Te(¹²⁹I) Mössbauer spectroscopy. Three states of triply coordinated tellurium atoms are indistinguishable in the Mössbauer spectra of crystalline As₂Te₃. Amorphization of As₂Te₃ decreases the local symmetry of triply coordinated states of Te atoms and leads to the formation of doubly coordinated states in -As-Te-Te-As- chains. In the structure of crystalline As₂S₃ and As₂Se₃, two states of doubly coordinated chalcogen atoms X in -As-X-As- chains manifest themselves in the broadening of the Mössbauer spectra. Amorphization of As₂S₃ and As₂Se₃ is not accompanied by a change in the local symmetry of doubly coordinated chalcogen atoms in -As-X-As- chains; however, doubly coordinated states of S and Se atoms in -As-X-X-As chains are formed in the amorphous material.     

Spectra of nuclear quadrupole resonance in vitreous semiconductors

The method of nuclear quadrupole resonance is used to study the chalcogenide semiconductors with compositions As₂Se₃ and As₁₄Sb₄Se₂₇. It is shown that partial crystallization occurs in a sample of vitreous As₂Se₃ kept for a long time at room temperature; as a result, a change in the shape of the spectrum of the nuclear quadrupole resonance is observed. The ⁷⁵As spectrum in vitreous As₁₄Sb₄Se₂₇ at a temperature of 77 K is measured for the first time. It is assumed that the ¹²¹Sb or ¹²³Sb nuclei can contribute to the broad line of the quadrupole resonance. It is shown that the use of the nuclear spin-echo Fourier-transform mapping spectroscopy for reconstruction of very broad lines of the nuclear quadrupole resonance provides no advantages compared to the method of reconstruction based on the points in the integrated intensity of the echo signals.     

Photoluminescence and composition of amorphous As<Subscript>2</Subscript>Se<Subscript>3</Subscript> films modified with Er(<Emphasis Type="Italic">thd</Emphasis>)<Subscript>3</Subscript> complex compound

The photoluminescence and composition of amorphous As₂Se₃ films modified with an Er(thd)₃ complex compound have been studied. A band centered at 1.54 μm, characteristic of photoluminescence from Er embedded in amorphous matrices, has been revealed at room temperature. The composition of thin amorphous As₂Se₃ films modified with an Er(thd)₃ complex compound has been examined by methods of nuclear microanalysis: Rutherford backscattering and nuclear resonant reactions. Dependences of the concentrations of Er ions, oxygen, and carbon on the growth conditions of the films are obtained. It is shown that the Er concentration in a thin film varies nonlinearly as the relative concentration of the starting complex compound increases. In addition, the increase in the Er content of a film is accompanied by a simultaneous rise in the content of such light elements as oxygen and carbon. Comparative analysis of the nuclear microanalysis data and IR spectra demonstrates that, in modification of As₂Se₃ with the Er(thd)₃ complex compound by the given method, the nearest environment of Er in the complex compound is partly preserved.     

Resistive switching behavior of Sb2S3 thin film prepared by chemical bath deposition

The chalcogenides are the excellent memristor materials. Here we report the resistive switching properties of an amorphous Sb₂S₃ thin film. Sb₂S₃ films were deposited on FTO glass using a low-temperature (10 °C) chemical bath deposition technique. SEM and XRD results indicate that the as-grown Sb₂S₃ film is dense and amorphous with uniform thickness and smooth surface. The Ag/Sb₂S₃/FTO memristor shows typical bipolar switching behavior with low operating voltage, high resistance ratio, long retention time, and good endurance. The electrical tests demonstrate that the switching behavior of the amorphous Sb₂S₃ film is based on electrochemical metallization mechanisms.     

Accelerated Ionic Motion in Amorphous Memristor Oxides for Nonvolatile Memories and Neuromorphic Computing

Memristive devices based on mixed ionic–electronic resistive switches have an enormous potential to replace today's transistor‐based memories and Von Neumann computing architectures thanks to their ability for nonvolatile information storage and neuromorphic computing. It still remains unclear however how ionic carriers are propagated in amorphous oxide films at high local electric fields. By using memristive model devices based on LaFeO₃ with either amorphous or epitaxial nanostructures, we engineer the structural local bonding units and increase the oxygen‐ionic diffusion coefficient by one order of magnitude for the amorphous oxide, affecting the resistive switching operation. We show that only devices based on amorphous LaFeO₃ films reveal memristive behavior due to their increased oxygen vacancy concentration. We achieved stable resistive switching with switching times down to microseconds and confirm that it is predominantly the oxygen‐ionic diffusion character and not electronic defect state changes that modulate the resistive switching device response. Ultimately, these results show that the local arrangement of structural bonding units in amorphous perovskite films at room temperature can be used to largely tune the oxygen vacancy (defect) kinetics for resistive switches (memristors) that are both theoretically challenging to predict and promising for future memory and neuromorphic computing applications.     

Effect of thermal oxide on the crystallization of the anodic Ta2O5 film

SEM and TEM investigations of the crystallization process have been performed on amorphous Ta₂O₅ films grown by electrochemical oxidation of Ta foils. It was found that kinetics of crystallization and final structure of the anodic Ta₂O₅ film depend strongly on the thickness of the thermal oxide layer on the surface of the original Ta substrate. Two different modes of crystallization were detected for the substrate with native surface oxide and with the thermal oxide grown at elevated temperatures. Aborting of the crystallization was shown to be possible using short heating of the Ta₂O₅/Ta san dwiches which cuts crystalline inclusions grown into the amorphous matrix of the anodic Ta₂O₅ film from the Ta surface.     

Physical mechanism of resistance switching in the co-doped RRAM

The physical mechanism of the resistance switching for RRAM with co-doped defects (Ag and oxygen vacancy) is studied based on the first principle calculations and the simulation tool VASP. The interaction energy, formation energy and density of states of Ag and oxygen vacancy defect (V_O) are calculated. The calculated results reveal that the co-doped system is more stable than the system only doped either Ag or V_O defect and the impurity energy levels in the band gap are contributed by Ag and V_O defects. The obtained partial charge density confirmed further that the clusters are obvious in the direction of Ag to Hf ions, which means that it is Ag but V_O plays a role of conductive paths. For the formation mechanism, the modified electron affinity and the partial charge density difference are calculated. The results show that the ability of electron donors of Ag is stronger than V_{m O} In conclusion, the conductivity of the physical mechanism of resistance switching in the co-doped system mainly depends on the doped Ag.     

Impact of Bonding on the Stacking Defects in Layered Chalcogenides

Phase‐change materials for high‐density data storage traditionally exploit the amorphous‐to‐crystalline phase transition. A number of these compounds are organized in blocks, separated by van der Waals‐like gaps. Such layered chalcogenides are attracting interest due to their unique material properties and the possibility to change their properties upon local rearrangements at the gap, giving rise to novel applications. To better understand the behavior of layered chalcogenides, the connection between structural defects, physical properties, and the bonding situation is highlighted here using electron microscopy, X‐ray diffraction, and density functional theory. In particular, stacking defects in hexagonal Ge₄Se₃Te, GaSe, and Sb₂Te₃ are characterized experimentally, followed by an investigation of the influence of observed and hypothetical stacking defects on optical and electronic properties by theoretical means. Then, a connection between observed defects and the bonding situation in these materials is drawn and related to the presence of van der Waals and metavalent bonding in chalcogenides. Finally, additional experiments are performed to validate the conclusions for other metavalently bonded layered chalcogenides. Transmission electron microscopy provides a powerful tool for direct detection of defects and, when combined with diffraction experiments and ab initio theory, it facilitates the precise investigation of the bonding mechanisms in layered chalcogenides.     

The phase relations in the system Tl2Se-As2Se3 and the crystal growth of Tl3AsSe3

The phase diagram for the system T12Se-As2Se3 was studied by quenching and thermal analysis experiments, with emphasis on the composition range 15 to 35 mol % As₂Se₃, so as to determine the melting relations of Tl₃AsSe₃, a useful compound for optical and acousto-optical devices. Tl₃AsSe₃ melts congruently at 311 ± 2‡C, and the only other pseudobinary compound, TlAsSe₂, melts congruently at 272 ± 2‡C. Eutectics lie between Tl₂Se and Tl₃AsSe₃ (∿ 21 mol % As₂Se₃, 302 ± 2‡), between Tl₃AsSe₃ and TlAsSe₂ (∿ 32 mol % As₂Se₃, 238 ± 3‡), and between TlAsSe₂ and As₂Se₃ (∿ 72 mol % As₂Se₃, 249 ± 3‡). The maximum-melting Tl₃AsSe₃ composition lies at 24.62 + 0.13 mol % As₂Se₃. Crystals of Tl₃AsSe₃ were grown from three pseudobinary compositions with the best crystals obtained from melts containing 24.62 mol % As₂Se₃. This research was sponsored by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Materials Laboratory, LP, under Contract No. F33615-72-C-1976     

Rear‐Passivated Ultrathin Cu(In,Ga)Se2 Films by Al2O3 Nanostructures Using Glancing Angle Deposition Toward Photovoltaic Devices with Enhanced Efficiency

In this work, for the first time, the addition of aluminum oxide nanostructures (Al₂O₃ NSs) grown by glancing angle deposition (GLAD) is investigated on an ultrathin Cu(In,Ga)Se₂ device (400 nm) fabricated using a sequential process, i.e., post‐selenization of the metallic precursor layer. The most striking observation to emerge from this study is the alleviation of phase separation after adding the Al₂O₃ NSs with improved Se diffusion into the non‐uniformed metallic precursor due to the surface roughness resulting from the Al₂O₃ NSs. In addition, the raised Na concentration at the rear surface can be attributed to the increased diffusion of Na ion facilitated by Al₂O₃ NSs. The coverage and thickness of the Al₂O₃ NSs significantly affects the cell performance because of an increase in shunt resistance associated with the formation of Na₂Se_X and phase separation. The passivation effect attributed to the Al₂O₃ NSs is well studied using the bias‐EQE measurement and J–V characteristics under dark and illuminated conditions. With the optimization of the Al₂O₃ NSs, the remarkable enhancement in the cell performance occurs, exhibiting a power conversion efficiency increase from 2.83% to 5.33%, demonstrating a promising method for improving ultrathin Cu(In,Ga)Se₂ devices, and providing significant opportunities for further applications.     

Nanocrystal V2O5 thin film as hole-extraction layer in normal architecture organic solar cells

Nanocrystal V₂O₅ dispersion processed thin films are introduced as efficient hole extraction interlayer in normal architecture P3HT:PCBM solar cells. Both thin and rather thick interlayers are studied and demonstrated to work properly in organic photovoltaic. Nanocrystal V₂O₅V₂O₅ layers effectively block electrons and effectively extract holes at the ITO anode. Very constant and high V_OC (above 0.56 V) are easily achieved. Comparable J_SC and PCE are demonstrated for nanocrystal dispersion-processed devices when compared with amorphous sol–gel processed devices. The excellent functionality of nanocrystal V₂O₅ interlayers in Si-PCPDTBT:PCBM devices further demonstrates the broad application potential of this material class for photovoltaic applications.     

Plasma-enhanced chemical vapor deposition and structural characterization of amorphous chalcogenide films

We describe the preparation of layers of amorphous Se, As_xS_1−x , As_xSe_1−x , Ge_xS_1−x , and Ge_xSe_1−x by plasma-enhanced chemical vapor deposition using the hydrides of the elements as precursor gases. We discuss the influence of the gas ratios and the deposition conditions (pressure, rf power input) on the chemical composition and the homogeneity of the binary systems. Information concerning the structure of the films was obtained from infrared and Raman spectroscopy. Fiz. Tekh. Poluprovodn. 32, 958–963 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Synthesis of the lead,arsenic, and bismuth chalcogenides with liquid encapsulation

The possibility of synthesis of the lead, arsenic, and bismuth chalcogenides with liquid encapsulation has been investigated. The conditions and results of the synthesis of PbSe, PbS, PbTe, Bi2Te3, and As₂Te₃, as well as solid solutions in the systems PbSe-CdSe and Bi₂Te₃-Bi₂Se₃ are reported. The obtained materials have been used for fabrication of thin-film photodetectors and interference filters for infrared radiation. Recommendations concerning the choice of fluxes for liquid encapsulation and the temperature conditions of synthesis are given.     

Potential Driven Volume Diffusion in Nanoporous AgBi at Room Temperature

Volume diffusion is crucial for crystal growth and interface manipulation in electron devices and catalysts. Its kinetics only becomes vigorous at high temperature, in which case grain boundary diffusion and surface diffusion dominate usually. In this study, volume diffusion of Bi atoms in nanoporous AgBi can be driven by electrochemical potential at room temperature due to ligament expansion. Bi diffusion induces conformal covering of amorphous Bi₂O₃ shell onto the spatially interconnected skeletons without ligament coarsening. Growth kinetics of amorphous shell demonstrates that the potential dependence of Bi volume diffusion activation energy corresponds to 0.932 eV V⁻¹. Compared with temperature dependence, potential supply is confirmed to be more feasible to modulate volume diffusion rate and shell thickness, and Bi enrichment increases 3.4 times when potential raises from −0.85 to −1.05 V. Potential driven volume diffusion at room temperature reported here knobs one door to manipulate local composition of functional nanostructures and nano-devices at atomic scale.     

Optical response of gamma irradiated-arsenic selenide thin films

Gamma(γ)-radiation induced effects in thin films of arsenic selenide (As₂Se₃) were investigated for possible applications in γ-radiation environment and/or γ-radiation dosimetry purposes. Thin film samples were fabricated using thermal evaporation. Optical properties of As₂Se₃ thin films were studied before and after irradiation. As a result of γ-irradiation, photodarkening of samples was observed for doses up to 10⁴ Gy (10 kGy). Absorption coefficient was found to increase with increase of irradiation dose. Optical energy gap was found to decrease while the localized band tail width was found to increase with dose. SEM was used to examine irradiated films; small nucleation of the crystalline phase could be distinguished. Nucleation increases with dose indicating some local structural changes due to irradiation. Values of absorbance or absorption coefficient in the absorption edge region are suitable control parameters of the proposed dosimeter material. The observed changes in the optical properties suggest that As₂Se₃ thin film may be considered as an effective material for room temperature real time γ-radiation dosimetry for the relatively high dose industrial applications such as sterilization.     

Bipolar threshold switching in vanadate glasses

I-Vcharacteristics are found for switching devices prepared from two-component vanadate glasses. Besides V₂O₅ they contain an oxide of a transition element or an oxide of an other two-valent, three-valent or four-valent element as second component. It is established that using high concentrations of V₂O₅ (above 50 mol%) is possible to prepare threshold switches which possess stable characteristics. It is shown that by increasing the temperature, the threshold power P_th decreases linearly and tends to zero at approximately the temperature T_t of the metal-semicoiidnctor phase transition. An attempt is made to describe the switching using a model in which the electrostatic field increases the number uf nuclei of the metal-like: phase and when the nuclei concentration reaches the percolation concentration the device switches on.