Amorphous Chalcogenide Research Articles

Microscopic Origin of Polarity-Dependent VTH Shift in Amorphous Chalcogenides for 3D Self-Selecting Memory.

Ovonic threshold switching (OTS) selectors based on amorphous chalcogenides can revolutionize 3D memory technology owing to their self-selecting memory (SSM) behavior. However, the complex mechanism governing the memory writing operation limits compositional and device optimization. This study investigates the mechanism behind the polarity-dependent threshold voltage shift (ΔVTH) through theoretical and experimental analyses. By examining the physical principles of threshold switching and conducting defect state analysis, the ΔVTH as a memory window is confirmed to be attributed to the dynamics of charged defects and their gradient near electrodes, influenced by the nonuniform electric field after threshold switching. This study provides critical insights into the operational mechanism of OTS-based SSM, known as selector-only memory, highlighting its advantages for developing high-density, low-cost, and energy-efficient memory technologies in the artificial intelligence era.

The incident photons cut-off energy, the optical linear equations and the electron phonon interactions in ZnO thin films annealed at different temperatures

The using RF-magnetron sputtering, the ZnO thin films were deposited on glass substrates at room temperature. Then using an electrical furnace in the presence of argon gas, they were annealed at different temperatures (400–600 °C). It was found that with taking Nc = 4, Za = 2, Ne = 8 for ZnO films for covalently bonded crystalline and amorphous chalcogenides, the constant β has values of about 0.37 ± 0.04 and for halides and most oxides that have ionic structure the constant β has values of about 0.26 ± 0.04 eV. It can be seen that with increasing annealing temperature absorption edge these films have a shifting behavior towards larger wavelength. Due to shifting behavior of defects distribution of atoms into films, the values of the cut-off energy, E cut-off and the λ cut-off of these films were about 3.48 eV and 355 nm, respectively. The ZnO films annealed at 500 °C specially above 3 eV have maximum value of optical density. The different linears fitting of ln (⍺) for films were obtained as y = Ex + F where 10<E < 12.5 and 14<F < 16. The ZnO films annealed at 600 °C have minimum value of electron phonon interaction (Ee-p) in a bout of 0.858 eV. The optical band gap and disordering energy plots of these films can be fitted by linear relationship EU and Eg = 0.0989–0.148 EU. We found that as deposited ZnO films have minimum value of steepness parameter σ in about of 30.93 × 10 −2eV.

The role of band-tail states on the electric properties of amorphous chalcogenides: A simulative approach

The role of band-tail states on the electric properties of amorphous chalcogenides: A simulative approach

Off-Current Analysis with Temperature-Dependent Subthreshold Conduction in SiTe Based Amorphous Chalcogenides for Ovonic Threshold Switching Selector Application

Off-Current Analysis with Temperature-Dependent Subthreshold Conduction in SiTe Based Amorphous Chalcogenides for Ovonic Threshold Switching Selector Application

Local bandgap narrowing in the forming state of threshold switching materials

Threshold switching (TS) materials, such as amorphous chalcogenide, have received significant attention for their application in storage class memory and in-memory computing. These materials contribute to efficient data processing and reduced power consumption in data centers. The initial switching process after fabricating a TS device, known as “forming,” has a profound impact on its subsequent TS behavior. However, it remains unclear how TS materials undergo changes in their atomic and electronic structures during the forming process. Consequently, the key factors that govern TS behavior remain obscure, necessitating a deeper understanding of the underlying physics behind TS phenomena. In this Letter, we investigated the forming state of the TS material AlTeN by combining scanning internal photoemission microscopy (SIPM) and ab initio calculations. Thanks to nondestructive evaluation by SIPM measurements, we observed local bandgap narrowing of AlTeN after its forming process. This is an experimental demonstration showing the presence of nuclei of the conductive filament formed in its ON state. Moreover, we conducted an ab initio calculation to reveal the origin of bandgap narrowing. We applied strong electrothermal stresses to the AlTeN model by ab initio molecular dynamics simulation with high electronic and lattice temperatures. By quenching from the electrothermal stress conditions, we reproduced an experimentally observed forming state with a narrowed bandgap. Analysis of the electronic structures of the forming state revealed that the origin of bandgap narrowing is the generation of the valence band top and conduction band bottom stemming from the increased homopolar bonds.

Mechanism for Local-Atomic Structure Changes in Chalcogenide-based Threshold-Switching Devices.

Threshold-switching devices based on amorphous chalcogenides are considered for use as selector devices in 3D crossbar memories. However, the fundamental understanding of amorphous chalcogenide is hindered owing to the complexity of the local structures and difficulties in the trap analysis of multinary compounds. Furthermore, after threshold switching, the local structures gradually evolve to more stable energy states owing to the unstable homopolar bonds. Herein, based on trap analysis, DFT simulations, and operando XPS analysis, it is determined that the threshold switching mechanism is deeply related to the charged state of Se-Se homopolar defects. A threshold switching device is demonstrated with an excellent performance through the modification of the local structure via the addition of alloying elements and investigating the time-dependent trap evolution. The results concerning the trap dynamics of local atomic structures in threshold switching phenomena may be used to improve the design of amorphous chalcogenides.

Transient photodarkening in amorphous and liquid chalcogenides: fractional dynamics in rheological point of view

Transient photodarkening in amorphous and liquid chalcogenides: fractional dynamics in rheological point of view

Praseodymium-Doped Ge20In5Sb10Se65 Films Based on Argon Plasma Cosputtering for Infrared-Luminescent Integrated Photonic Circuits.

In this paper, we report on the infrared luminescence of amorphous praseodymium-doped Ge20In5Sb10Se65 waveguides, which can be used as infrared sources in photonic integrated circuits on silicon substrates. Amorphous chalcogenide thin films were deposited by radiofrequency magnetron cosputtering using an argon plasma whose deposition parameters were optimized for chalcogenide materials. The micropatterning as ridge waveguides of the chalcogenide cosputtered films was performed using photolithography and plasma-coupled reactive ion etching techniques. The influence of the rare earth concentration within those thin films on their optical properties and rare earth spectroscopic properties was investigated. Using an excitation wavelength of 1.55 μm, the mid-infrared luminescence of Pr3+ ions from 2.5 to 5.5 μm was clearly demonstrated for studied chalcogenide materials. A wide range of waveguide widths and doping ratios were tested, assessing the ability of the cosputtering technique to preserve the luminescence properties of the rare earth ions initially observed in the bulk glass through the thin-film deposition and patterning process.

Photo‐Induced Evolution of Randomly Rough Surfaces of Amorphous Chalcogenide Films

Photoinduced (PI) evolution of statistically rough surfaces of amorphous chalcogenide films As20Se80 at room temperature has been studied by measuring the angular dependence of the intensity of light scattered from a surface illuminated by CW laser (λ = 660 nm). The interpretation of the scattering data based on the resonant scattering theory enables to confirm unequivocally the diffusion mechanism of PI mass transfer. It is detected that the change of the amplitude of a spatial harmonic in the roughness spectra strongly depends on its period ∧ . During illumination, the amplitude increases at ∧ &gt; ∧*, whereas harmonics with ∧ &lt; ∧* decreases by ∧*, which corresponds to zero evolution rate, is found to be 6.7 μm. In accordance with our theoretical prediction, both growth and decrease are exponential with the rates depending on ∧. As the result, the roughness with initial rms height of 50–70 nm transforms into quasiperiodic surface grating with the average amplitude of about 400 nm and average period close to 15 μm. From the kinetics of time variation of the scattered intensity, the PI diffusion coefficient D is calculated. When the laser intensity changes from 5.6 to 14 W cm−2, D is found in the range 1 × 10−13–3.4 × 10−13 m2 s−1.

Optical red shift spectra in CuxGe32S68-x films for infrared and solar cell window applications

The physical characteristics of Cu-doped ternary glasses have been analyzed. The CuxGe32S68 − x (x = 0.0, 4.0, 7.0, 10.0 and 13.0 at.%) chalcogenide alloys have been manufactured by the conventional melt quenching method. The glass densities (ρ) increase with the Cu amounts, whereas the main atomic volume (Vm) decreases. Transmission and reflectance were measured in the wavelength range of 0.35–2.5 μm. The generated envelops (RM and Rm) of reflectance spectra were used to calculate the film thickness (t), the refractive index (n), and the extinction coefficient (k). With the increase in Cu contents from 0.0 to 13.0 at.%., the n values increased from 2.07 to 2.43, whereas the optical gap (Eg) decreased from 2.81 eV to 2.42 eV. The obtained results were explained in terms of electronic polarizability, plasma frequency, metallization parameter, and refraction losses, which have been calculated to understand the need for amorphous chalcogenide compositions that can be applied for optical device purposes. The effect of Cu content on the non-linear index of refraction (n2) and third-order susceptibility (χ (3)) has been studied. The value of n2 increased from 13.88 × 10−12 to 60.2 × 10−12 esu, whereas the value of χ (3) rose from 7.6 × 10−11 to 38.83 × 10−13 esu with the addition of denser and higher atomic radius elements Cu in the CGS system. Overall, the composition of Cu13Ge32S55 is suitable, as all-optical parameters show the maximum values that can be used for optical device purposes.

Evaluation of dynamics of charge accumulation and dissipation processes in Ge15Se85 thin film under electron beam irradiation by mapping surface potential distribution

The interaction of Ge15Se85 amorphous chalcogenide films with an electron beam has been studied. Three ranges of electron irradiation doses, in which a surface relief of various shapes is formed, are found. The presence and persistence of the surface potential for a long time after film irradiation have been established using the Kelvin probe force microscopy (KPFM). It is shown that the formation of surface relief is result of the charge accumulation in the interaction region. Based on the mapping of surface potential distribution in irradiated regions using KPFM we propose models of electric field distribution in the irradiated spots and suggest mechanism of electron-induced mass transfer. It has been suggested that the formation of a high surface relief structures is due to the electron-induced plastic effect. It is shown that the appearance of asymptotically sharp Taylor cones in chalcogenide films is due to charge collapse in the interaction region when the volume charge density and internal electric field strength exceed their critical values.

Chalcogenide Perovskite Thin Films with Controlled Phases for Optoelectronics

AbstractChalcogenide perovskites have emerged as promising semiconductor materials due to their appealing properties, including tunable bandgaps, high absorption coefficients, reasonable carrier lifetimes and mobilities, excellent chemical stability, and environmentally benign nature. However, beyond the well‐studied BaZrS3, reports on chalcogenide perovskite thin films with diverse compositions are scarce. In this study, the realization of four different types of chalcogenide perovskite thin films with controlled phases, through CS2 annealing of amorphous chalcogenide precursor films deposited by pulsed laser deposition (PLD), is reported. This achievement is guided by a thorough theoretical investigation of the phase stability of chalcogenide perovskites. Upon crystallization in the distorted perovskite phase, all materials exhibit photoluminescence (PL) with peak positions in the visible range, consistent with their expected bandgap values. However, the full‐width‐at‐half‐maximum (FWHM) of the PL spectra varies significantly across these materials, ranging from 99 meV for SrHfS3 to 231 meV for BaHfS3. The difference is attributed to the difference in kinetic barriers between local structural motifs for the Sr and Ba compounds. The findings underscore the promise of chalcogenide perovskite thin films as an alternative to traditional halide perovskites for optoelectronic applications, while highlighting the challenges in optimizing their synthesis and performance.

Atom Probe Tomography Advances Chalcogenide Phase‐Change and Thermoelectric Materials

Main‐group chalcogenides show outstanding performance for phase‐change data storage and thermoelectric energy conversion applications. A common denominator for these different property requirements is ascribed to the metavalent bonding (MVB) mechanism. Atom probe tomography (APT) provides a unique way to distinguish MVB from other bonding mechanisms by determining the bond‐breaking behavior. Specifically, an unusually high probability to dislodge several fragments upon one successful laser pulse (probability of multiple events [PME]) is found in metavalently bonded crystalline phase‐change and thermoelectric materials. In contrast, amorphous phase‐change materials and poor thermoelectrics usually show lower PME values. This indicates that the large optical and electrical contrast between the crystalline and amorphous chalcogenides is attributed to a transition of chemical bonding. A strong correlation between high thermoelectric performance and large PME is also established. Besides, APT can investigate structural defects on the subnanometer scale. These characteristics reveal the interdiffusion of elements in interfacial phase‐change materials and revisit its switching mechanism. The complex role of structural defects such as grain boundaries in tuning the thermoelectric properties can also be unraveled by investigating the local composition and bonding mechanism at defects. This review demonstrates that APT is a powerful technique for designing phase‐change and thermoelectric materials.

Four-Layer Surface Plasmon Resonance Structures with Amorphous As2S3 Chalcogenide Films: A Review.

The paper is a review of surface plasmon resonance (SPR) structures containing amorphous chalcogenide (ChG) films as plasmonic waveguides. The calculation method and specific characteristics obtained for four-layer SPR structures containing films made of amorphous As2S3 and As2Se3 are presented. The paper is mainly based on our previously obtained and published scattered results, to which a generalized point of view was applied. In our analysis, we demonstrate that, through proper choice of the SPR structure layer parameters, we can control the resonance angle, the sharpness of the SPR resonance curve, the penetration depth, and the sensitivity to changes in the refractive index of the analyte. These results are obtained by operating with the thickness of the ChG film and the parameters of the coupling prism. Aspects regarding the realization of the coupling prism are discussed. Two distinct cases are analyzed: first, when the prism is made of material with a refractive index higher than that of the waveguide material; second, when the prism is made of material with a lower refractive index. We demonstrated experimentally that the change in reflectance self-induced by the modification in As2S3 refractive index exhibits a hysteresis loop. We present specific results regarding the identification of alcohols, hydrocarbons, and the marker of E. coli bacteria.

Understanding Charge Storage Mechanisms for Amorphous MoSnSe1.5S1.5 Nanoflowers in Alkali‐Ion Batteries

AbstractTransition metal sulfides/selenides have been reported as promising materials for alkali‐ion batteries owing to their high pseudocapacitive effects and large capacities. However, these materials undergo large volume expansion, which results in poor cycling retention. Hence, in this study, an amorphous bimetallic chalcogenide, MoSnSe1.5S1.5 (MSSS), is synthesized to mitigate the volume expansion. By introducing an amorphous structure, MSSS can reach high capacities of 805 mAh g‒1 in Li‐ion batteries (LIBs) and 526 mAh g‒1 in Na‐ion batteries (NIBs) at a current density of 0.1 A g‒1. Moreover, amorphous MSSS can tolerate a high current density of 20 A g‒1 and possess high percentages of capacitance contributions in both LIBs and NIBs. To explore fundamentals, in situ/operando measurements, such as X‐ray absorption spectroscopy, transmission X‐ray microscopy, and transmission electron microscopy, are utilized to investigate real‐time phenomena and consequently establish the reaction mechanisms for amorphous MSSS electrodes in alkali‐ion batteries.

Optical and optoelectronic properties of (Ge2S8)100-xTex thin films for IR optical device fabrication

A detailed study of the optical and optoelectronic properties of (Ge2S8)100-xTex thin films for optical device fabrication is described in the present paper. Thin films of melt-quenched chalcogenide samples were deposited, on ultrasonically cleaned glass substrates, employing thermal evaporation technique. Transmission spectra were used to calculate the refractive index (n) and extinction coefficients (k). The refractive index increases from 2.187 to 2.329 by adding Te = 0 to 12. Dispersion parameters are evaluated using the WDD single oscillator model. The dielectric parameters real (εr) and imaginary (εi), electric modulus (M) and dielectric loss (tan δ) are used to evaluate the relaxation process in different compositions of (Ge2S8)100-xTex thin films. To further grasp the importance of amorphous chalcogenide compositions, we compute their optical conductivity (σ*op), electronic polarizability (αp), plasma frequency (ωp), metallization parameter (M), and refraction losses (RL) in optical devices. The dependency of Te content on the nonlinear refractive index (n2) and third-order susceptibility (χ(3)) is also calculated by an empirical formula, and it is found that the value of n2 increases from 11.938 × 10−12 to 20.615 × 10−12 esu, and the value of χ(3) rises from 6.8 × 10−13 to 12.472×10−13 esu with the addition of denser and higher atomic radius elements Te in the Ge-S system. The overall composition of Ge17.6S70.4Te12 is suitable as all-optical parameters show the maximum values that can be used for optical device purposes.

Atomistic Modeling of Charge‐Trapping Defects in Amorphous Ge‐Sb‐Te Phase‐Change Memory Materials

Understanding the nature of charge‐trapping defects in amorphous chalcogenide alloy‐based phase‐change memory materials is important for tailoring the development of multilevel memory devices with increased data storage density. Herein, hybrid density‐functional theory simulations have been employed to investigate electron‐ and hole‐trapping processes in melt‐quenched glassy models of four different Ge‐Sb‐Te compositions, namely, GeTe, Sb2Te3, GeTe4, and Ge2Sb2Te5. The calculations demonstrate that extra electrons and holes are spontaneously trapped, creating charge‐trapping centers in the bandgap of the amorphous materials. Over‐ and undercoordinated atoms, tetrahedral and “see‐saw” octahedral‐like geometries, fourfold rings, homopolar bonds, near‐linear triatomic configurations, and chain‐like motifs comprise the range of the defective atomic environments that have been identified in the structural patterns of the charge‐trapping sites inside the glassy networks. The results illustrate that charge trapping corresponds to an intrinsic property of the glassy Ge‐Sb‐Te systems, show the impact of electron and hole localization on the atomic bonding of these materials, and they may have important implications related to the operation of phase‐change electronic‐memory devices.

Amorphous Ge-Se-S chalcogenide alloys via post plasma sulfurization of atomic layer deposition GeSe for ovonic threshold switch applications

For the future scaling of 3D cross-point (X-point) memory, it is necessary to implement atomic layer deposition (ALD) of chalcogenides for ovonic threshold switch (OTS) applications. We investigated ternary Ge-Se-S amorphous chalcogenide alloys based on ALD process, motivated by the expectation of low off-current and stable OTS behavior by partially replacing S in binary Ge-Se. Ge-Se-S alloys were synthesized by post-deposition sulfurization of ALD GeSe2 thin films. Especially, we investigated the growth characteristics and film properties of ALD GeSe2 using HGeCl3 precursors with Se(SiMe3)2 together with density-functional theory (DFT) calculations. By changing the temperature and the low-temperature plasma sulfurization time, the compositions of 10-nm-thick Ge-Se-S thin films were controlled along the GeSe2-Ge2S pseudo-binary line in ternary phase diagram. It was confirmed that the Ge5Se3S2 alloys maintained an amorphous phase and excellent step coverage, similar to ALD GeSe2. Finally, we compared the OTS electrical characteristics of 10-nm-thick ALD GeSe2 with Ge5Se3S2 amorphous chalcogenide thin films in a mushroom-type device with a 50-nm bottom electrode. The novel Ge5Se3S2 had a slightly larger threshold voltage (Vth) drift than GeSe2 but exhibited the advantages of a higher threshold field, lower off-current, and smaller Vth fluctuation up to 106 cycles. Data availabilityThe data that has been used is confidential.

Amorphous As2S3 Doped with Transition Metals: An Ab Initio Study of Electronic Structure and Magnetic Properties.

Crystalline transition-metal chalcogenides are the focus of solid state research. At the same time, very little is known about amorphous chalcogenides doped with transition metals. To close this gap, we have studied, using first principle simulations, the effect of doping the typical chalcogenide glass As2S3 with transition metals (Mo, W and V). While the undoped glass is a semiconductor with a density functional theory gap of about 1 eV, doping results in the formation of a finite density of states (semiconductor-to-metal transformation) at the Fermi level accompanied by an appearance of magnetic properties, the magnetic character depending on the nature of the dopant. Whilst the magnetic response is mainly associated with d-orbitals of the transition metal dopants, partial densities of spin-up and spin-down states associated with arsenic and sulphur also become slightly asymmetric. Our results demonstrate that chalcogenide glasses doped with transition metals may become a technologically important material.

Toward the Speed Limit of Phase-Change Memory.

Phase-change memory (PCM) is one of the most promising candidates for next-generation data-storage technology, the programming speed of which has enhanced within a timescale from milliseconds to sub-nanosecond (≈500 ps) through decades of effort. As the potential applications of PCM strongly depend on the switching speed, namely, the time required for the recrystallization of amorphous chalcogenide media, the finding of the ultimate crystallization speed is of great importance both theoretically and practically. Inthiswork, through systematic analysis of discovered phase-change materials and ab initio molecular dynamics simulations, elemental Sb-based PCM is predicted to have a superfast crystallization speed. Indeed, such cells experimentally present extremely fast crystallization speeds within 360ps. Remarkably, the recrystallization process is further sped up as the device shrinks, and a record-fast crystallization speed of only 242ps is achieved in 60nm-size devices. These findings open opportunities for dynamic random-access memory (DRAM)-like and even cache-like PCM using appropriate storage materials.