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Though phase-change materials (PCMs) are regarded as constituent part of next-generation phase-change memory and other unfolding optoelectronic applications, the kinetics study of glass transition and crystallization, significant property in switching remains obscure in high-temperature region. Hence, this paper reports the investigation of glass transition and crystallization kinetics of Te (1-x) (GeSe 0.5)Y x (TGSY) chalcogenide glass using differential scanning calorimetry (DSC) at heating rates 5, 10, 15, 20 °C/min. The effect of increasing Y quantity has been described by connecting structural relaxation kinematics during glass transition phenomena and devitrification while crystallization in chalcogenide glasses with their varied physicochemical characteristics. The inclusion of Y causes a discernible rise in the crystallization rate. The system is compatible with a strong glass-forming liquid, according to the Te (1-x) (GeSe 0.5) Y x material fragility index. According to the average values of the kinetic exponent factors, there is heterogeneous nucleation for the investigated composition, which is subsequently followed by a two- or three-dimensional crystal growth phenomenon. Different glass stability criteria have been discussed based on the relationship between the characteristic temperatures. The C ZW and C LX criteria were found to be inappropriate for discussing glass stability (GS) for the studied compositions based on the heating rate and composition dependencies of all criteria. The C YL criteria of Yuan et al. is the finest and it demonstrates the optimal ability for evaluating the GS, according to the data that was extracted from several GS criteria and their relative change parameters. By adding Y, thermal stability parameter and enthalpy emitted during the transition of glass and crystalline phases have been found to have an inverse relationship. All studied properties can conclude the application of studied material in phase-change material devices. [ABSTRACT FROM AUTHOR]

Though phase-change materials (PCMs) are regarded as constituent part of next-generation phase-change memory and other unfolding optoelectronic applications, the kinetics study of glass transition and crystallization, significant property in switching remains obscure in high-temperature region. Hence, this paper reports the investigation of glass transition and crystallization kinetics of Te (1-x) (GeSe 0.5)Y x (TGSY) chalcogenide glass using differential scanning calorimetry (DSC) at heating rates 5, 10, 15, 20 °C/min. The effect of increasing Y quantity has been described by connecting structural relaxation kinematics during glass transition phenomena and devitrification while crystallization in chalcogenide glasses with their varied physicochemical characteristics. The inclusion of Y causes a discernible rise in the crystallization rate. The system is compatible with a strong glass-forming liquid, according to the Te (1-x) (GeSe 0.5) Y x material fragility index. According to the average values of the kinetic exponent factors, there is heterogeneous nucleation for the investigated composition, which is subsequently followed by a two- or three-dimensional crystal growth phenomenon. Different glass stability criteria have been discussed based on the relationship between the characteristic temperatures. The C ZW and C LX criteria were found to be inappropriate for discussing glass stability (GS) for the studied compositions based on the heating rate and composition dependencies of all criteria. The C YL criteria of Yuan et al. is the finest and it demonstrates the optimal ability for evaluating the GS, according to the data that was extracted from several GS criteria and their relative change parameters. By adding Y, thermal stability parameter and enthalpy emitted during the transition of glass and crystalline phases have been found to have an inverse relationship. All studied properties can conclude the application of studied material in phase-change material devices. [ABSTRACT FROM AUTHOR]

Non-volatile memory, which can simultaneously store data and compute in memory, is a promising candidate to break through "memory bottleneck". However, its operation speed and thermal stability are still contradictory and difficult to meet the requirement of in-memory computing especially in high-temperature applications. In this work, optoelectronic hybrid phase-change memory based on Cr doped Ge 2 Sb 2 Te 5 material is proposed, where the picosecond laser is used to perform the information recording/erasing operation based on the reversible phase-change characteristics while resistance signal is detected to realize the information readout. Results imply that the Cr doped Ge 2 Sb 2 Te 5 film with Cr concentration of 10.7 at. % has high crystallization temperature (289 °C) and ten-year data-retention temperature (221 °C), low density-change rate (2.3%) and small resistance drift coefficients (0.022 at 85 °C, 0.056 at 120 °C, 0.070 at 150 °C, and 0.00131 for the laser induced SET state). The corresponding memory device has the SET/RESET operation speeds of as fast as 1820 ps/13 ps accompanied by the RESET/SET resistance ratio of higher than two orders of magnitude. The structural analyses indicate that Cr doping can suppress the growth of grains and reduce the grain size, improving the thermal stability of amorphous Ge 2 Sb 2 Te 5 thin film while the octahedral/defective octahedral motif ensures its fast phase-change speed. Therefore, Cr doped Ge 2 Sb 2 Te 5 based optoelectronic hybrid phase-change memory with superior thermal stability and ultrafast operation speed may be one of the promising solutions for in-memory computing. [ABSTRACT FROM AUTHOR]

[Display omitted] Interfacial Phase Change Memory (iPCM) retrench unnecessary power consumption due to wasted heat generated during phase change by reducing unnecessary entropic loss. In this study, an advanced iPCM (GeTe/Ti-Sb 2 Te 3 Superlattice) is synthesized by doping Ti into Sb 2 Te 3. Structural analysis and density functional theory (DFT) calculations confirm that bonding distortion and structurally well-confined layers contribute to improve phase change properties in iPCM. Ti-Sb 2 Te 3 acts as an effective thermal barrier to localize the generated heat inside active region, which leads to reduction of switching energy. Since Ge-Te bonds adjacent to short and strong Ti-Te bonds are more elongated than the bonds near Sb-Te, it is easier for Ge atoms to break the bond with Te due to strengthened Peierls distortions (R long /R short) during phase change process. Properties of advanced iPCM (cycling endurance, write speed/energy) exceed previous records. Moreover, well-confined multi-level states are obtained with advanced iPCM, showing potential as a neuromorphic memory. Our work paves the way for designing superlattice based PCM by controlling confinement layers. [ABSTRACT FROM AUTHOR]

In order to validate the flexible storage potential, [Ge 2 Sb 2 Te 5 (3 nm)/Sb(7 nm)] 5 superlattice-like phase change films on flexible polyetheretherketone substrates were prepared by magnetron sputtering. The phase transition behavior and the effect of different bending cycles on the properties were investigated. Compared with conventional rigid Si/SiO 2 substrate, [Ge 2 Sb 2 Te 5 (3 nm)/Sb(7 nm)] 5 films based on a flexible substrate possessed better thermal stability and surface adhesion. The superlattice-like structure was confirmed by transmission electron microscopy. PCM devices based on polyetheretherketone substrates were fabricated and the electrical conversion characteristics before and after bending were verified and compared. [ABSTRACT FROM AUTHOR]

Previous research on phase change memory (PCM) devices shows that correct modeling of the RESET melting process requires including the Gibbs–Thomson melting temperature for the solid–liquid interface. This additional physical effect results in substantially different predicted scaling properties and melting behavior than shown in previous research on conventional PCM devices scaled to nanometer dimensions. The computations also reveal that the thermodynamic Gibbs–Thomson effect gives rise to two steady state classes of liquid pools over a range of applied voltages. At any given voltage both states appear. This research characterizes these two classes of pool shapes by analyzing the linear stability of each pool shape class to elucidate the time-dependent, dynamic properties of the liquid pools. The stability analysis uses a novel numerical approach, required due to the large thermal and potential gradients within the system and the lack of geometrical symmetry of the steady base states. Results of the analysis show that at all given applied voltages, the smaller liquid pool shape class is linearly unstable and the larger pool shape class is linearly stable. The stability characteristics of the classes of liquid pools justify the designation of the nucleus state for the smaller liquid pool and the melting limit state for the larger liquid pool. The results imply that a system with a liquid pool larger than the nucleus state will eventually grow to the melting limit state which is a stable fixed point. Comparison with a spherically symmetric PCM model system reveals that in the isothermal limit the nucleus state exhibits precisely classical nucleation behavior found for a thermodynamically unstable critical nucleus. The theory used to identify the class of critical nuclei steady state shapes in this analysis is also novel, because it avoids the need to identify the extremum of a thermodynamic potential that is used in most treatments that attempt to identify the initial formation of new phases in inhomogeneous, evolving systems. [Display omitted] • Analysis of melting during the phase change memory RESET process. • Discovery of two classes of steady states of the RESET melting process. • One nucleus-like state and another state representing the final melted state. • Linear stability analysis performed on steady states. • Nucleus-like state is unstable and melting limit state is stable. [ABSTRACT FROM AUTHOR]

Multi-level phase-change (MLPC) memory can not only further raise the storage density of high-density phase-change memory, but also have significant applications in neuromorphic computing. However, acquisitions of MLPC materials are very challenging. MLPC is only observed in a small number of phase-change materials and can't be designed determinately because the mechanism of MLPC emergence has been understood incompletely. Here, we investigate phase-change behaviors of six Ge 2 Sb 2 Te 5 (x nm)/Sb 7 Se 3 (60-x nm) bilayer films with x = 5, 10, 15, 20, 35, and 50. Three-level phase-change memory is observed for five bilayer films with x < 50, but not for one with x = 50, revealing the dependence of MLPC memory on the thickness ratio of two constituent layers composing bilayer films. A parallel resistance model is proposed to describe the measured sheet resistance of bilayer films and can be used to explain the thickness-ratio dependence of three-level phase-change memory very well. Subsequently, a design rule for MLPC memory films is given out unambiguously. Based on the parallel resistance model, MLPC memories are simulated for bilayer and trilayer films. Three-level memory indeed appears in bilayer films only when the design rule is met, confirming the validity of our design rule. Four-level memory is predicted in trilayer films as the design rule is obeyed. [Display omitted] • Multi-level phase-change behaviors of six bilayer phase-change films, Ge 2 Sb 2 Te 5 (x nm)/Sb 7 Se 3 (60-x nm) with x = 5, 10, 15, 20, 35 and 50, were studied by the measurement of sheet resistance as a function of heating temperature. • Appearance of three-level phase-change behaviors depended on the thickness ratio of Ge 2 Sb 2 Te 5 and Sb 7 Se 3 layers. Similar phenomena were reported previously in the literatures, but not understood and explained. • A parallel resistance model was proposed to describe the sheet resistance of bilayer films. Based on the model, the thickness-ratio dependence of three-level phase-change behaviors could be explained very well. • A design rule of multi-level phase-change multi-layer films was given out for the first time. Multi-level phase changes were simulated for bilayer films based on the sheet resistance model. It was found that three-level phase changes indeed emerged only when our design rule was met totally, conforming the validity of our design rule. Furthermore, a scheme of trilayer films was proposed. Its multi-level phase changes were simulated. Four-level phase changes were predicted only if the design rule was obeyed totally. • Multi-level phase-change films could be designed determinately under the guidance of the design rule, no longer be encountered and observed accidentally. Our results pave the way for determinate design of desired multi-level phase-change films. [ABSTRACT FROM AUTHOR]

Rewritable optical storage is dominated by alloys of a small number of elements, particularly Ge, Sb, and Te, and Ge/Sb/Te alloys in the composition range (GeTe) 1− x (Sb 2 Te 3) x (0 ≤ x ≤ 1) have been the materials of choice: all have metastable rock salt structures that change little over decades at archival temperatures, and all contain vacancies (cavities). The special status arises from the close similarity of the valence orbitals of Ge, Sb, and Te, which arises from the irregular changes in atomic orbitals and properties as the atomic number increases ("secondary periodicity"). The instability of cubic (metallic) Bi to a (semimetallic) rhombohedral structure (H. Jones, 1934) can be adapted to Ge/Sb/Te alloys and explains the crucial metastable (rock salt) structures of these compounds. Vacancies almost always occur next to Te atoms, which form one sublattice of the rock salt structure. The crystallization of amorphous bits in phase change materials (e.g. DWDRW, BD-RE) is astonishingly rapid (of the order of nanoseconds) and is a rare case where parameter-free density functional simulations can follow the phase change on the physical time scale. Alloys of Ge, Sb, and Te are the favourites: the atoms have the same size and have average valence 5, and the crucial formation of a metastable rock salt can be understood in terms of a straightforward, 90-year-old theory (H. Jones). [Display omitted] • Alloys of Ge, Sb, Te containing vacancies are optimal phase change memory materials. • These alloys (including vacancies) have an average valence-electron occupancy of 5 s,p-electrons per site. • Simple nearly-free electron explanation of structure in terms of Jones zones. • Existence of accessible metastable rock-salt structure crucial. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pulsed laser deposition of GeTe-Sb 2 Te 3 based heterostructures. • Influence of systematically varied deposition parameters on the microstructure. • Alternation of chemical composition and morphology - formation of Ge-Sb-Te layer. • Amorphous GeTe and Ge-Sb-Te, crystalline Sb 2 Te 3 layers independent of parameters. • Sb 2 Te 3 crystallites of different sizes featured by defects, and multiple phases. Chalcogenide based heterostructures turned out to be a promising approach to further improve outstanding properties of phase change memory materials. For future applications, a deep understanding of the interplay of local structure and properties is crucial. Here, GeTe-Sb 2 Te 3 based heterostructures (HSs) are grown by pulsed laser deposition onto SiO 2 /Si substrates at room temperature. By combining advanced transmission electron microscopy with X-ray measurement techniques, the influence of systematically varied deposition parameters on the microstructure is analyzed in-depth. Experimental results reveal that the growth rate of GeTe and Sb 2 Te 3 layers depends on laser pulse energy. Nanoscale spectroscopic characterization of the HSs shows interdiffusion of Ge/Sb into Sb 2 Te 3 /GeTe layers, respectively. The in-diffusion depends on the layer thickness and deposition parameters. For the lowest laser energy, it leads to the formation of ternary Ge-Sb-Te layer. Whereas GeTe and Ge-Sb-Te are found to be amorphous within the HSs, Sb 2 Te 3 is crystalline in all thin films although some disordered regions are found additionally. Moreover, Sb 2 Te 3 is formed in cubic, vacancy ordered and trigonal structures and contains bilayer as well as twin defects. Overall, this work provides insights into the growth and microstructure of the HSs, which give the reasons for the outstanding performance of Sb 2 Te 3 -GeTe HSs. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) is an exceptional universal memory innovation that offers qualities like fast speed and non-volatility. But major obstacle still exists in achieving better thermal stability and rapid switching at the same time. Therefore, it is crucial to investigate new materials with enhanced properties. To overcome the contrast of access time between storage systems and memories, storage class memory concept based on emerging nonvolatile memory technology has been introduced. Resistive switching has gained much awareness in last decade due to its application in PCM technologies. Investigation of resistive switching on novel Te (1−x) (GeSe 0.5)Sc x (x = 0, 0.05, 0.1, 0.15) quaternary alloy has been analyzed in this paper. Material has revealed electrical resistivity contrast of ∼103 orders of magnitude that offers better thermal stability than the majority of other phase transition alloys. Additionally, current-voltage (I-V) properties and voltage dependent threshold switching support an incredibly fast threshold switching operation. Switching is observed at 18–30 V and threshold voltage has been noticed to decrease (10–12 V) with increasing temperature. From XPS study, it can be observed that binding energy is shifting towards lower energies by 0.25 eV. Sc makes the GeTeSe parent material appealing for next-generation high-speed, non-volatile, and improved thermal stability memory applications. [Display omitted] • Electrical resistivity contrast of ∼103 orders of magnitude. • Better thermal stability than the majority of other phase transition alloys. • Switching is observed at 18-30 V and threshold voltage has been noticed to decrease (10-12 V) with increasing temperature. • The barrier-cluster model is used to interpret compensating effect in thermally controlled resistive switching and d.c. conduction. • Sc makes the GeTeSe appealing for next-generation high-speed, non-volatilememory applications. [ABSTRACT FROM AUTHOR]

Via in situ transmission electron microscopy, we observed on nanoscale unconventional structural evolution during two-dimensional (2D) phase transitions in a Sb 2 Te 3 /TiTe 2 phase-change heterostructure (PCH), which reduces amorphous relxation and crystallization stochasiticity to facilitate rapid and reliable memory programming. [Display omitted] Phase-change heterostructure (PCH) holds great promise for overcoming the low-precision bottleneck that limits multibit storage and parallel computing in conventional phase-change random-access memory. However, the origin of high-accuracy control of electrical resistance achieved in programming PCH memory devices has yet to be established. Via in situ transmission electron microscopy, here we unveil the unusual microscopic processes during the order–disorder phase transitions driven by electrical pulse in a Sb 2 Te 3 /TiTe 2 PCH architecture. The template-modulated phase transition is confirmed to be two-dimensional (2D) in nature. The structural transformation path and dynamics in the confined Sb 2 Te 3 sublayers are found to be profoundly changed with respect to those in bulk monolithic Sb 2 Te 3 , leading to markedly suppressed amorphous relaxation and substantially reduced crystallization stochasticity, both highly desirable for swift and accurate device operations. Our atomic-scale observations provide direct evidence of, and much-needed insight into, the working mechanisms that may enable superior 2D phase-change electronic devices. [ABSTRACT FROM AUTHOR]

The phase change material Sb 2 Te 3 (ST) exhibits a fast-switching speed and poor thermal stability because the four-fold rings are dominant and homo-polar bonds are absent in the amorphous state. Many studies have been performed on the improvement of phase change properties. Chemical doping methods have been widely used because additional chemical bonds are formed between the doped element and host atoms, leading to the modification of the local structure and electronic band structure. Previous reports have indicated the importance of the bond length and lattice mismatch in element doped ST. In this study, the electronegativity of the doped elements was investigated because it determines the strength of the bond via charge transfer between two atoms. A strong correlation between the electronegativity of the doped element and the phase change properties of the doped ST was observed. An ideal electronegativity range, which corresponds to some elements doped ST with excellent thermal stability, was found in this investigation. Nb atoms were selected as the optimal dopant to stabilize the amorphous phase of ST because Nb was in the ideal range. Additional homo-polar bonds and Nb-Te bonds can stabilize the amorphous phase because additional energy is required for ring reconstruction during crystallization. Consequently, the thermal stability and resistance difference of the Nb doped ST were significantly improved. In addition, the phase change speed of ST was not affected by Nb doping. This study highlights a new approach for pursuing better properties of phase change materials by chemical doping methods. • Electronegativity of doped element plays a crucial role in tailoring the Sb 2 Te 3 (ST) properties. • An ideal electronegativity range corresponds to the elemental doping of ST. • Nb was selected as the optimal dopant to stabilize the amorphous phase of ST. • The Nb-ST material is a potential candidate for PCM applications. • This work provides a new approach for pursuing better properties by doping methods. [ABSTRACT FROM AUTHOR]

Ambient humidity has an important impact on the performance of flexible electronic devices, and wearable electronic devices will inevitably involve a high humidity environment. To investigate the impact of humidification on flexible phase-change memory, we prepared Sb phase-change film and memory device unit with polyether ether ketone as substrate, and studied the effect of humidification on the properties of the film and device. The results show that the effective transition from an amorphous to a crystalline state can still be achieved by the film after soaking in water for 55 min, and the phase transition temperature increases with the increase of soaking time. Water molecules refine the film grains, resulting in increased crystalline resistance. Sb is a hydrophilic material, and its hydrophilicity decreases after crystallization. The reversible conversion of SET-RESET can be realized under different pulse widths even after the phase change memory device is soaked in water and bent many times. This study shows that antimony-based flexible phase change memory has certain deformation self-healing ability and moisture resistance, which provides a way to expand its application environment. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, the phase transformation properties of C/Sb composite multilayer films based on flexible substrates were studied. After different times of bending, the flexible films can still achieve amorphous to crystalline phase transition, but the crystalline resistance increases to different degrees. When the bending times are less than 15000 times, the deformation makes the grain fine, the surface of the film tends to be flat, the surface electric field effect is weakened, and the resistance drift is reduced. When the bending times are higher than 15000 times, cracks begin to appear in the film, and the cracks' depth and width increase with the bending times. The distribution of C and Sb elements at the crack was analyzed by scanning electronic microscope energy spectrum. The phase change memory device of C/Sb composite multilayer film based on flexible PEEK substrate has been made, and the rapid reversible resistance transition (∼10 ns) in various complex states has been successfully achieved. This research provides a new idea for flexible information storage. • The addition of carbon increases the phase transition temperature and improves the thermal stability of pure antimony. • Complete the phase transition in the case of external bending and internal bending. • After 80,000 bending, the phase transition can still be completed, showing good bending resistance. • The device made of composite material realizes the SET-RESET process at the pulse width of 10ns. [ABSTRACT FROM AUTHOR]

[Display omitted] • [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 Superlattice-like film exhibits high comprehensive performance in phase change memory. • C3/SS3 SLL film has a high crystallization temperature (∼240 ℃) and a ten-year data retention temperature (∼170 ℃). • The PCM based on C3/SS3 SLL film has ultrafast operating speed (∼6 ns) and low power consumption (∼2.18 × 10-11 J). • The addition of carbon nanolayers has formed C-Sb and C-Se bonds, which have been verified by experiments and simulations. The C/Sb 7 Se 3 superlattice-like film was prepared by magnetron sputtering technology, and the effects of carbon nanolayers on the film and device properties were studied. The C/Sb 7 Se 3 superlattice-like films show excellent thermal stability and data retention, especially the [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 film. The analysis of transmission electron microscopy and x-ray diffraction reveal that the high-quality multilayer structure of [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 is maintained. The segmentation of phase change material by amorphous carbon layer reduces the size of crystalline region, thus accelerates the crystallization rate. Element mapping indicates that C atoms and Se atoms in the annealed [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 films show obvious interlayer diffusion. The results of x-ray photoelectron spectroscopy and density functional theory manifest that atomic diffusion results in the formation of strong C-Sb and C-Se bonds in the phase transition layer and C atom prefers to occupying interstitial site in the center of the periodic structural unit of Sb 2 Se 3 phase. Carbon atoms can effectively suppress grain growth and gradually move to the boundary of the grain, refining grain size, thus improving the stability of the amorphous phase. This study indicates that carbon nanolayers play an important role in improving the thermal stability and operation speed, reducing power consumption and resistance drift in Sb 7 Se 3 films. [ABSTRACT FROM AUTHOR]

In this paper, the flexible phase change behavior of Ge 2 Sb 2 Te 5 /Mg 35 Sb 65 superlattice-like memory based on PEEK substrate is investigated in detail. The flexible memory has good thermal stability (T c ∼270.4 °C), resistance to bending (more than 2 ×104 bending times), low resistance drift (less than 25% of the Ge 2 Sb 2 Te 5 film), stable amorphous to crystalline resistance ratio (∼2 orders of magnitude) and ultra-fast information access speed (∼5 ns). Bending refines the grain of the film and widens the band gap. This study provides a potential option for flexible information storage electronics. • Ge 2 Sb 2 Te 5 /Mg 35 Sb 65 superlattice-like films on PEEK substrates possess good thermal stability and mechanical bending durability. • After several bending cycles, the FPCM can even complete the ultrafast phase transition within 5 ns. [ABSTRACT FROM AUTHOR]

• We propose a photonic device composed of metal ring-wrapped PCMs based on plasmonic effect. • The device achieves a high optical contrast of 28.5% in ultra-small volumes (4.465×10-4 µm3). • It exhibits crystalline and amorphous states with insertion losses of 0.3 dB and 1.7 dB, respectively, at a wavelength of 1550 nm. • The write and erase operations can be completed using optical pulses of 1.5 ns–1 mW and 3 ns–3 mW, respectively. • The proposed device demonstrates the multilevel storage capability. • The photonic neural morphologic network based on the device achieves a recognition accuracy of 95% on the MNIST dataset. The combination of photonic devices and chalcogenide phase change materials (PCMs) provides a potential solution for photonic storage and computing. However, there are still some issues that need to be addressed for currently integrated phase-change photonic memory such as low speed, high energy consumption, and large footprint. To enable stronger interaction between light and PCMs, we utilize the plasmonic enhancement effect to amplify the local electric field, resulting in high energy efficiency in photonic storage and computation. This work proposes a nanoscale elliptical plasmonic nanoantenna that dramatically improves switching speed and energy by placing an Ag elliptical ring wrapped with Ge 2 Sb 2 Te 5 (GST) on a silicon waveguide. The device exhibits crystalline and amorphous states with insertion losses of 0.3 dB and 1.7 dB, respectively, at a wavelength of 1550 nm. Write and erase operations can be completed by optical pulses of 1.5 ns–1 mW and 3 ns–3 mW, respectively. An ultra-small device volume of only 4.465 × 10−4 (µm3) is capable of achieving a switching contrast ratio of 28.5 %. Moreover, the non-volatile storage of multi-level intermediate states facilitates a wide weight-variation range and allows for precise weight updates, enhancing the overall performance and accuracy of the system. In the handwritten digit recognition task of the three-layer perceptron, the device demonstrates a recognition accuracy rate of 95 %, indicating that our approach can enable a more efficient neuromorphic photonic computing network. [ABSTRACT FROM AUTHOR]

Ge-Sb-Te (GST) thin films are emerging materials for various non-volatile memory applications. The compounds contain a huge number of vacancies that play important roles in structural and electronic transitions. Control of disorder and understanding the structural transformations in GST thin films are of great importance for the design and reliability of phase change memory devices. In this work, in situ heating atomic-resolution transmission electron microscopy is used to observe structural transformations from vacancy disordered (fcc) to layered ordered (vacancy-ordered (vo)) and van der Waals-bonded trigonal (vdW t)) structures. Starting from 160 °C, randomly distributed vacancies in fcc grains gradually accumulate into vacancy layers. Further heating to 220 and 240 °C results in the formation of vdW t-domains within the vo-GST grains and the formation of large <001>-oriented t-grains, in a good agreement with outcomes on in situ XRD heating of epitaxial vo-GST thin film. Moreover, vo and fcc GST structures can coexist with vdW t-structure in one grain. Full transformation to 〈 11 2 ¯ 0 〉 -oriented t-grains occurs at 280 °C. Unlike other grains, the 〈001〉 t-grains show abnormal grain growth behaviour, while the electron beam irradiation accelerates the growth. Detailed characterizations of vo-grains reveal transient structures that indicate shear transformation mechanism and different defects as well as chemical changes during the transformations. Overall, this study provides new insights to understand the phase transition mechanisms and to optimize the microstructure in thin GST layers. [Display omitted] [ABSTRACT FROM AUTHOR]

Sb 2 Te thin film exhibits remarkable crystallization speed, which allows rapid data conversion in phase change memory (PCM). Thermal stability is a key challenge to enable reliable data retention. Meanwhile, it is necessary to study the transition energy in the process of crystallization when the thermal stability of the amorphous state is improved. Herein, we demonstrated an alloying strategy to optimize thermal stability and transition energy. Zn was screened to be a promising alloying element. Nano-composite Zn 0.75 Sb 2 Te thin film was designed, which exhibited excellent thermal stability and optimized transition energy. The data retention temperature for 10 years (T 10-yr) increased by ∼ 218 %, and the transition energy consumption decreased by ∼ 27 % compared with Sb 2 Te thin film. The enhancement of these properties originated from Zn-modified local chemical bonding. Zn induced the formation of tetrahedron with strong bonds (Zn–Te and Zn–Sb bonds) and dragged the atomic diffusion in amorphous phase to improve thermal stability. The structural disorder and complexity of the first and second nearest neighbors of atoms were stimulated by Zn alloying, which inhibited the carrier transports, increased the amorphous resistance, and reduced the transition energy in the process of crystallization. [Display omitted] • An alloying strategy is proposed to improve the thermal stability. • Transition energy could also be reduced by the alloying strategy. • Zn induces the formation of tetrahedrons in amorphous Sb 2 Te film. • Nano-composite Zn–Sb 2 Te film is prepared. • Microstructure and phase change mechanism of Sb 2 Te film depend on the Zn content. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) of metallic components has recently become a viable option for series production. In this, the powder of various metallic materials such as steel and aluminum can be processed layer-by-layer to produce dense parts with excellent properties. One of the major challenges in this process is the occurrence of residual stresses, which negatively affect the strength and functionality of the produced components. Understanding the mechanism of distribution of these stresses and the detrimental deformations is one of the main themes covered in this work. In the numerical treatment with the finite element method, a phase-field model (PFM) for phase-change materials is utilized together with a thermo-elastoplastic model to simulate the multi-layer AM process and to evaluate the occurring residual stresses. Using the PFM allows tracking the diffusive melting front and, thus, distinguishing between the melted (soft) and the unmelted (hard) states of the material. One of the novel contributions is the definition of a phase-field history variable, which can capture the irreversible process of metallic powder melting. Additionally, phase-field-dependent material properties are proposed. The coupled governing equations are solved in the open-source FE package FEniCS Project, where three-dimensional initial-boundary-value problems are introduced and the results are compared with reference data from the literature. • Thermodynamically-consistent PFM to describe melting-solidification-re-melting in AM. • A phase-change memory variable captures the irreversible changes during AM processes. • Phase-field-dependent material parameters as an alternative to temperature-dependent. • Solve the multi-field coupled problem in the open-access package FEniCS Project. • Model verification through comparison with benchmark solutions from the literature. • Particular emphasis is on thermal analysis and on the shape of the melt pool. [ABSTRACT FROM AUTHOR]

The ultimate goal of memory researchers and developers is to devise a universal memory to replace all memories in complex hierarchies of memory systems. Phase change memory is considered by the International Semiconductor Industry Association as the most likely emerging storage technology to achieve this ambitious goal due to its excellent performance. However, the relatively slow operation speed and poor cycle endurance are stumbling blocks for this purpose. Here, we propose a new doping strategy to construct stable bonding by introducing atoms with large electronegativity differences from the matrix Te atoms enhancing the capture ability and the pinning effect to Te atoms, which facilitates fast nucleation and suppresses migration of Te atoms to achieve significant performance improvement. Using ab initio molecular dynamics simulations, we explore and compare the atomic structures, chemical bonding, and dynamics properties of Hf doped Sb 2 Te 3 (HfST), La doped Sb 2 Te 3 (LaST), Rb doped Sb 2 Te 3 (RbST) and Sr doped Sb 2 Te 3 (SrST). In the XST (X refers to Hf, Rb, La and Sr atoms) systems, the increase of the more stable ring structure fluctuation and the stronger atom capture capability of rings enable more and faster formation of nuclei during crystallization. Higher Peierls-like distortions suggest better thermal stability. The motility of Te atoms is suppressed due to the pinning effect. High coordination of atoms and the concentration reduction of the lone-pair electron and void enable small density difference. Our results show that fast universal memory with good endurance can be achieved by the construction of strong bonding by doping atoms with large electronegativity differences, providing an important guidance for experiments and industries. • Constructing stable bonding accelerates the crystallization rate of the system. • Constructing stable bonding improves the cycle endurance of the Sb 2 Te 3 system. • COHP analysis corroborate the strong atom capture ability after doping. • MSD analysis of the systems confirm the pinning effect of the doped atoms. • ELF and LED demonstrate the reduction of density difference after doping. [ABSTRACT FROM AUTHOR]

Phase Change Memory (PCM) has attracted widespread attention for its high-speed reading and writing capabilities and high-density non-volatile storage. However, reducing the reset current of PCM can result in an increase in its set resistance, which affects the programming or readout performances. Therefore, further optimization of PCM under this trade-off remains a significant challenge. In this paper, multi-objective genetic algorithm (MOGA) is utilized to address this problem through the optimization of PCM's geometry. An enhanced PCM finite element model, considering the Seebeck, Peltier, and Thomson effects, is employed as the fitness function of MOGA to improve the accuracy of PCM's reset arguments and obtain precise solution. The PCM geometry solution set with optimal performance in different technology nodes are presented in this paper. Specifically, for the 22-nm node, the optimization result shows that the reset current can be reduced by up to 8.7 % when the PCM is employed for low-power applications, such as neuromorphic computing, and the read latency time can be reduced by up to 22.2 % when the PCM is utilized for fast reading purposes, such as in PCM-DRAM main memory. This proposed optimization scheme has great potential in improving the performance of PCM. [ABSTRACT FROM AUTHOR]

Memristor devices are promising nanoscale low-power components, which are expected to significantly reduce power consumption and improve speed of deep learning accelerators if the longitudinal and transverse structure is used. However, memristor devices such as resistive random access memory and phase change memory are difficult to manufacture in small batches because they are prone to various device nonideality. It is particularly important to simulate the performance of memristor deep learning systems and test the effects of nonideal characteristics before circuit level implementation. In this paper, a large-scale memristor-based neural network based on residual neural networks is built to study the influence of non-ideal characteristics of memristors. The influence of four types of non-ideal characteristics on deep learning models is analyzed. The corresponding solutions are discussed. The non-ideal characteristics are tested on the constructed memristor-based neural network. The recognition accuracy of the neural network based on two ideal memristor models reach 64.84% and 64.63% respectively. The further test results show that with the increase of non-ideal characteristics, the recognition accuracy decreases, which prove that the non-ideal characteristics of memristors have a great influence on the practical application of deep learning models, and provide guidance for the practical application of memristors. [ABSTRACT FROM AUTHOR]

• The crystallization kinetics of Sb 70 Se 30 phase change thin films is studied. • The kinetic triplet of Sb 70 Se 30 is determined using model-free and -fitting methods. • Data processing time and data storage time of Sb 70 Se 30 thin films are estimated. We investigate the non-isothermal crystallization kinetics of Sb 70 Se 30 thin films using differential scanning calorimetry and determine the kinetic triplet of Sb 70 Se 30 thin films at various heating rates by combining model-free and model-fitting approaches. The results indicate that the crystallization activation energy decreases with increasing crystallization fraction α , confirming the presence of multi-step crystallization mechanism. In accordance with the local Avrami index n (α), the crystallization mechanism shifts from three-dimensional growth with an increased nucleation rate at 0.05 ≤ α ≤ 0.8 to three- and two-dimensional growth with a reduced nucleation rate at α ＞0.8. Afterwards, we deduce that the Sb 70 Se 30 thin films follow a second-order reaction model within the heating rate range of 20 to 40 K/min. Moreover, the Sb 70 Se 30 thin film shows excellent data retention capability when predicting data processing and data storage time based on phase change memory units. These findings provide valuable insights into the crystallization kinetics of Sb 70 Se 30 thin films, contributing to the advancement of phase change materials. [ABSTRACT FROM AUTHOR]

The superlattice-like Sn 15 Sb 85 /Ti phase change films were fabricated by magnetron sputtering, and the effects of Ti layer on the phase change properties and crystal structure of the Sn 15 Sb 85 films were systematically researched. With the addition of Ti layers, the crystallization temperature of the films increased, indicating their thermal stability and data retention were improved. X-ray diffraction results reflect that the presence of Ti layers restrains grain growth and refines grain size. X-ray photoelectron spectroscopy analysis reveals that the incorporation of Ti layers enhances the interatomic binding energy and improves the stability. X-ray reflectivity analysis and atomic force microscopy indicate that the Sn 15 Sb 85 /Ti films have smaller volume change and smoother surface than pure Sn 15 Sb 85 films, implying the better interfacial property and reliability. The T-type phase change memory cell based on Sn 15 Sb 85 /Ti thin film has faster operation speed (100 ns) and lower power consumption (2.6 ×10−12 J) than the conventional Ge 2 Sb 2 Te 5 material. The results show that the superlattice-like Sn 15 Sb 85 /Ti film will be a potential candidate with outstanding properties in thermostability and power consumption. • Proposing the Sn 15 Sb 85 /Ti thin film with superlattice-like structure. • Performance optimization and structural analysis of film. • PCM cells based on Sn 15 Sb 85 /Ti film was prepared and tested. [ABSTRACT FROM AUTHOR]

[Display omitted] • The stacked films enabling up to four resistive states by adjusting the thickness. • Resistance variation is caused by the crystallization of each layer. • Interfaces can affect the crystallization behavior of MnTe films layer. • The reversible switch between three resistance states can be achieved with a 50 ns electrical pulse in MnTe/GeTe -based PCM cells. With substantial data storage requirements, enhancing the storage density of emerging non-volatile memories is a common challenge for achieving commercial applications. Phase change memory is capable of multi-level data storage in a storage cell, which can be applied in neural-inspired and all-photonic memory computing. In this study, we propose a MnTe/GeTe stacked thin film structure suitable for multi-level phase change memories and systematically study on the correlation between structure and performance. As the temperature rises, the stacked thin film sequentially crystallizes into form MnTe 2 , GeTe and Te phases, resulting in three changes in resistance. By utilizing different resistance states, the phase change memory cell based on the MnTe (15 nm)/GeTe (35 nm) stacked thin film can achieve multi-level storage with a 50 ns electrical pulse. The results show that the amorphous thermal stability of the stacked thin films is improved and that the minimal thickness variation before and after crystallization enhances its reliability. Therefore, the MnTe/GeTe stacked thin film is a phase change memory material with favorable structural properties and potential for multi-level storage applications. [ABSTRACT FROM AUTHOR]

PCMs are Non-Volatile Memories (NVMs) that store data using phase-change semiconductors, such as silicon-chalcogenide glass. In addition to increased integration density, PCMs have high durability and data transfer rates and consume less power during read/write operations. Compared with flash memory, PCM operates at much faster speeds and is close to the performance of DRAM. As a result, it is one of the most popular semiconductor memories which is used in various applications. However, PCM cells are limited to a certain number of write operations, and because of that, values cannot be changed when this limit is reached. Hence, PCMs do not last for a long time. The limited lifespan of PCM memory has led to considerable research in recent years. Several architectural levels and methods have been offered and examined to overcome this challenge. In this study, functional and practical techniques are identified and analyzed to extend the longevity of PCMs. These schemes can also be used to prolong the life of other NVM technologies since they share similar characteristics with PCMs. Furthermore, we will present some improvements to PCM performance to make it competitive with common NVMs. In conclusion, this survey will be helpful to both researchers who are considering starting PCM projects and those already proficient in PCM. [ABSTRACT FROM AUTHOR]

Phase Change Memory (PCM) is considered a promising replacement for DRAM due to its superior performance characteristics such as low leakage power, high integration density, byte addressability and non-volatility. However, PCM's limited write endurance significantly hinders its wide application. For example, PCM wears out quickly with traditional dynamic memory allocation policy in embedded systems which aggregates lots of writes in few memory blocks. To extend the lifespan of PCM, some wear-aware dynamic memory allocators have been proposed, which generally depend on some fixed parameters to limit the wear of PCM. However, these allocators can be inflexible as it is difficult to specify appropriate values for the required parameters in different scenarios. In this paper, we propose a Self-Adaptive Generational Wear-Aware Allocator (GWalloc). GWalloc divides memory blocks into two generations: the young and the old generation, according to their number of allocation times. GWalloc also dynamically adjusts the system's wear threshold during allocations so that it can effectively balance the wear degree of PCM and the consumed memory space. The wear threshold restricts the upper wear limit of young memory blocks. Experimental evaluations show that compared with the state-of-the-art wear-aware dynamic memory allocators (NVMalloc , Walloc and UWLalloc), GWalloc improves PCM wear-leveling (evaluated by CV, a wear leveling indicator) by 38.6%, 39.1% and 38.3%, and saves 62.1%, 22.2% and 37.2% memory space overhead. [ABSTRACT FROM AUTHOR]

A new simultaneous acquisition software for Precession Electron Diffraction (PED) and Energy Dispersive X-ray Spectroscopy (EDX) mapping has been developed. This approach has been used to study the RESET operation in Ge-rich Ge-Sb-Te (Ge-rich GST) non-volatile Phase-Change Memory (PCM). Such an operation consists in the local amorphization of the device's active volume, which defines the memory state. Using PED patterns indexing, we were able to segment the amorphous area, and access its composition through EDX quantification. Moreover, both the spatial distribution and the chemical composition of the two crystalline phases overlapped in the chalcogenide layer were retrieved by using the spectral unmixing approach and PED patterns indexing. It evidences that Ge and Ge 2 Sb 1 Te 2 (GST-212) crystals are separated in the studied devices. Despite this, we show that Ge-enrichment is preserved in the active volume of a functional device, which is crucial to guarantee the RESET state stability at high temperature. We also link the origin of the failure of the RESET operation observed in a dysfunctional device to the larger Ge-depleted area. [ABSTRACT FROM AUTHOR]

High quality van der Waals chalcogenides are important for phase change data storage, thermoelectrics, and spintronics. Using a combination of statistical design of experiments and density functional theory, we clarify how out-of-equilibrium van der Waals epitaxial deposition methods can improve the crystal quality of Sb 2 Te 3 films. We compare films grown by radio frequency sputtering and pulsed laser deposition (PLD). The growth factors that influence the crystal quality for each method are different. For PLD grown films a thin amorphous Sb 2 Te 3 seed layer most significantly influences the crystal quality. In contrast, the crystalline quality of films grown by sputtering is rather sensitive to the deposition temperature and less affected by the presence of a seed layer. This difference is somewhat surprising as both methods are out-of-thermal-equilibrium plasma-based methods. Non-adiabatic quantum molecular dynamics simulations show that this difference originates from the density of excited atoms in the plasma. The PLD plasma is more intense and with higher energy than that used in sputtering, and this increases the electronic temperature of the deposited atoms, which concomitantly increases the adatom diffusion lengths in PLD. In contrast, the adatom diffusivity is dominated by the thermal temperature for sputter grown films. These results explain the wide range of Sb 2 Te 3 and superlattice crystal qualities observed in the literature. These results indicate that, contrary to popular belief, plasma-based deposition methods are suitable for growing high quality crystalline chalcogenides. [ABSTRACT FROM AUTHOR]

The single layer Zn 44 Sb 39 Se 17 thin film, being fabricated by Sb 70 Se 30 and Zn using co-sputtering method, exhibits double phase change transition. The double phase change processes are mainly attributed to the crystallization of the Sb and ZnSb phases. The potential operating temperature for ten years of two phase change processes are 64 °C and 179 °C, of which the good stability of the second phase change can be sufficient for the autoelectronic applications. Meanwhile we also discovered the Zn 44 Sb 39 Se 17 thin film presents apparent low power consumption in comparison with Ge 2 Sb 2 Te 5 and other reported Sb-Se based phase change materials. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• The Se dopant into Ge-Sb-Te ternary system increases its thermal stability. • Modifying the content of Ge enhances the temperature endurance of GSST alloy. • Increased thermal stability was reflected by changed crystallization process. The GSST (Ge-Sb-Se-Te) alloy has been investigated to be a competitive material choice for high density phase-change memory with robust data retention by introducing the doped Se and the excess Ge in Ge-Sb-Te ternary system. The Se dopants are found to increase the crystallization temperature and reduce the grain size. And the excess Ge atoms play a vital role in promoting the thermal stability of doped alloy. The reversible memory switching capability evidences that the GSST alloy is a potential reliable phase-change material in storage application for automobile systems. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) layered GeTe-Sb 2 Te 3 superlattices (GST-SL) have attracted enormous attentions due to the ultralow power consumption for nonvolatile phase change memory. In this paper, effects of biaxial strain on interfacial intermixing and local structures in strain engineered GST-SL were studied. Highly (0 0 l) textured GST-SL with thickness-varied Sb 2 Te 3 sublayers were fabricated by magnetron sputtering and investigated with multiple analyses on surfaces and interfaces. The appearance of Ge-Sb-Te alloys indicated the interfacial Ge/Sb intermixing in strain engineered GST-SL, which actually were composed of Sb 2 Te 3 quintuple layers, Ge-Sb-Te vdW layers and isolated GeTe bilayers. Grazing incidence x-ray diffraction (GID) and x-ray photoelectron spectroscopy (XPS) results revealed that biaxial strain not only facilitated the interfacial intermixing, but also caused the reconfiguration of Ge-Sb-Te vdW layers. Raman spectra indicated that vertical and in-plane vibrations of GeTe layers were not affected by the interfacial intermixing and the newly-formed Ge-Sb-Te layers had similar vibration characteristics as that of GeTe layers. A modified switching mechanism of GST-SL, relating to both the amorphous-crystalline phase transition of GeTe and Ge-Sb-Te layers incorporated into the Sb 2 Te 3 matrix, was then presented. The present studies shed new light on strain engineering for GST-SL and indicate their potential optoelectronic applications. • A modified switching mechanism of GeTe-Sb 2 Te 3 superlattices (GST-SL) was presented. • Effects of biaxial strain on atomic intermixing and local structures of GST-SL were studied. • Biaxial strain promotes the formation of Ge-Sb-Te van der Waals layers. • Strain engineering enables the tunable optoelectronic properties of GST-SL. [ABSTRACT FROM AUTHOR]

The unique properties of phase change materials (PCMs) in the amorphous and crystalline phase, and the feasibility of manipulating the phase transition process, have enabled the application of PCMs in optics and electronics systems, for data storage and neuromorphic computing. As the semiconductor device employing PCMs scales down with the complementary metal oxide semiconductor (CMOS) technology node, the impact of interface on the crystallization of PCMs becomes increasingly significant, that is of great importance for the system performance, e.g. phase change memory (PCM). In this work, crystallization of Ge 2 Sb 2 Te 5 (GST) on passivated SiO 2 surface is triggered by thermal pulses, in-situ observed in transmission electron microscope. A distinguishing heterogenous nucleation dominated crystallization behavior of GST has been verified at the GST/SiO 2 interface. Meanwhile, in the in-situ prepared reactive GST/SiO 2 interface, the interfacial covalent interaction between atoms in GST and SiO 2 is believed to hamper the GST nucleation at the interface. Interfaces consist of the same materials but with different reactivity will lead to distinct crystallization behaviors of PCMs. By forming passive interface between PCMs and surrounding dielectric materials, promotion in nucleation and enhancement in operation speed can be expected in PCM with decreasing cell size. [ABSTRACT FROM AUTHOR]

In this study, the multilayer Zn 15 Sb 85 /SiO 2 materials were investigated for phase change memory application with the intention of improving thermal stability and switching speed. The Zn 15 Sb 85 /SiO 2 materials exhibited a high crystallization temperature (T c ∼230 °C), good data retention (T 10 ∼ 160 °C), small density change (∼3.3%), flat surface and rapid amorphization speed (∼1.34 ns). T-shaped phase change devices based on Zn 15 Sb 85 /SiO 2 materials achieved low RESET voltage (V RESET ∼ 1.3 V) in programming operation. The results showed that the multilayer Zn 15 Sb 85 /SiO 2 material was a promising phase change material with high thermal ability, low power consumption and rapid switching speed. • The films were prepared by alternate sputtering Zn 15 Sb 85 and SiO 2 materials. • The fast phase change speed and good thermal stability were achieved. • The adding of SiO 2 interlayers restrained the crystallization. • The SiO 2 interlayers made the thin film more compact. • The PCM devices based on Zn 15 Sb 85 /SiO 2 had a lower power consumption. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three resistance states with clear distinction are obtained in indium selenide phase-change thin films. • Amorphous indium selenide thin films crystallize at two different temperatures to form two crystalline phases. • Reversible switch between three resistance states as fast as 60 ns is implemented in In 2 Se 3 -based PCM cells. • Successive crystallization in indium selenide thin films is applicable to multi-level phase-change memory. Increasing the storage density of phase-change memory is a challenging subject towards the universal memory commercialization. Multi-level data storage can be implemented in phase-change memory cells by designing multiple stages of crystallization of phase-change materials, which is favorable to push forward the application of phase-change memory in storage-class memory and the emerging in-memory computing. Here, the structure and properties of In 2 Se 3 phase-change thin films with multi-level phase transitions are investigated. The In 2 Se 3 thin films changes resistance states twice upon heating with high crystallization temperature and good thermal stability. The amorphous In 2 Se 3 thin films undergo partial crystallization of the Se phase followed by crystallization of the γ-In 2 Se 3 phase, with successive stages of crystallization accounting for the three distinct levels of resistivity. The phase-change memory cell based on In 2 Se 3 thin films shows a continuous switching process of high resistance state, intermediate resistance state and low resistance state with low drift coefficient in SET-RESET operations at a speed as fast as 60 ns, and the RESET power is as low as 311 pJ, indicating that the In 2 Se 3 thin films have good performance of three-state storage property and can effectively improve the memory density. [ABSTRACT FROM AUTHOR]

Intrinsic robustness against electrostatic discharge (ESD) like HBM or CDM is a major concern for the reliability and functionality in advanced CMOS FD-SOI technology and especially for non-volatile memory (eNVM). And it is well known that device characterizations are a first step of an ESD investigation. Thus, this R&D study the experimental results from 100 ns to 1 ns transmission line pulse (TLP-VFTLP) on 1T1R phase change memory (PCM) at 300 mm wafer level and at room temperature are reported here. Two types of devices are fabricated and characterized in the standard fully depleted (FD) silicon-on-insulator (SOI) with ultra-thin body and BOX (UTBB). These devices have no additional technological step and are embedded in the final memory cell topology with its selector NMOS transistor. The idea is to evaluate the behaviour of the 1T1R device under transmission line pulse stress with different stress durations. TLP/VFTLP I-V curves are reported and discussed along with post-DC responses to determine the magnitude of the phase change generated by the ESD pulse. These characterizations are important for automotive, micro-controllers, neuromorphic computing and spatial applications depending on their dedicated mission profiles at room or cryogenic temperature. • Introduction on eNVM PCM cells (2 topologies) in 28 nm UTBB FD-SOI CMOS technology • For the first time, TLP and VFTLP measurements on all devices and at room temperature • For the first time, identification on a new resistance gap • Presentation on the analogue behaviour according to the ESD pulses • Possible extension for synapses (AI) and quantum applications at cryo temperature [ABSTRACT FROM AUTHOR]

Chalcogenide Ge-Sb-Te alloys are used in a new generation of nonvolatile memories named Phase Change Memories which can be thermally switched from low resistivity to high resistivity states by feeding the memory cells with appropriate electrical pulses. During their lifetime, the heterogeneous redistribution of the elements within the active region of the cells is at the origin of the drift of their electrical characteristics and, ultimately, of their failure. Up to now, little is known about the relative diffusivities of the atomic elements that compose the alloys, what hampers the modeling and optimization of the materials and devices. Here, we report an experimental study of the thermal diffusion of Ge, Sb and Te through Ge 2 Sb 2 Te 5. For this purpose, specific structures consisting of a layer of Ge, Sb or Te sandwiched between two Ge 2 Sb 2 Te 5 layers have been fabricated and annealed at various temperatures and times. Using transmission electron microscopy based techniques, we evidence that Te diffusion can be activated from about 160 °C while Ge and Sb needs much higher temperatures, 220 °C and 350 °C, respectively. These results are in disagreement with previous theoretical simulations predicting that Ge is the most diffusive element while Te is the least. [ABSTRACT FROM AUTHOR]

To improve the heating efficiency and reduce the reset current of phase change memory, the volume-confined cells have been widely studied by employing highly conformal deposition techniques. In this work, a scalable confined structure with sidewall phase change material spacer is investigated for the integration in 12-inch wafer using physical vapor deposition and the subsequent anisotropic plasma etching back process. The finite element simulations reveal a remarkable 80% increase in heating efficiency by taking the confined structure. The isolated and compact trenches with the aspect ratio ranging from 0.40 to 1.04 are designed to accommodate the sidewall phase change material. The scaling down of the critical dimension of the rail-like spacer is observed as the aspect ratio increases. The spacer width of around 34 nm is well beyond the lithography capability used in this study. These results provide a promising path to fabricate the low-power and high-density memory device in a way fully compatible with the manufacturing technology. [ABSTRACT FROM AUTHOR]

• Sn 22 Sb 76 Sc 2 thin films were prepared by SnSb 4 and Sc co-sputtering. • The thermal stability of Sn 22 Sb 76 Sc 2 can meet the need of electric vehicle. • The reversible SET/RESET operations based on Sn 22 Sb 76 Sc 2 is realized at 20ns. • The thermal stability of Sn 22 Sb 76 Sc 2 derives from the formation of Sc-Sn bond. • The phase change speed is not deteriorated with the enhancement of thermal stability. To address the challenge of reconciling the trade-off between phase change speed and thermal stability in phase change materials, we synthesized Sc-doped SnSb 4 thin films with a composition of Sn 22 Sb 76 Sc 2 and investigated their phase change properties. Our findings demonstrate that the Sn 22 Sb 76 Sc 2 thin film exhibits enhanced thermal stability and faster phase change speed than Ge 2 Sb 2 Te 5. The Sc-Sn bond formed during crystallization contributes to an improved data retention capability of up to 166 °C. Notably, the phase change speed of the Sn 22 Sb 76 Sc 2 thin film on the basis of device test can reach up to 20 ns. Additionally, the power consumption of the Sn 22 Sb 76 Sc 2 thin film (1.1 × 10−11 J) is nearly two orders of magnitude lower than that of Ge 2 Sb 2 Te 5 (9.7 × 10−10 J). These results provide compelling evidence that the Sn 22 Sb 76 Sc 2 thin film can overcome the "inverted relation" between high thermal stability and fast phase change speed, achieving a balanced performance. [ABSTRACT FROM AUTHOR]

Ovonic threshold switching (OTS) selector is a key technology for high-density crossbar memory array. The basic characteristics of GeSe OTS material including structural information, film properties, microscopic properties, and electrical threshold switching behaviors are investigated by combining ab-initio molecular dynamics and experiments. Amorphous GeSe contains numerous Ge Se covalent bonds and defective structure motifs. The original resistance is extremely high compared to phase change materials, leading to lower leakage currents in device cells. The structure of GeSe remains stable at ∼350 °C and transforms to orthorhombic phase at higher temperature up to 450 °C. The severe phase separation of 500°C-annealed GeSe is observed with Se crystal grains. The high threshold voltage (>4 V) and holding voltage (>1 V) with large fluctuation (>1 V) are presented in the OTS cells with GeSe material. Due to the comprehensive investigations of GeSe material, we can improve the thermal stability and device performance of GeSe as a base material to find promising candidates for crossbar memory. Image 1 • A full investigation on GeSe (an important base material in OTS selectors). • Film properties, microstructure, and electrical behaviors are studied. • We find the shortcomings of GeSe base material and provide solutions. [ABSTRACT FROM AUTHOR]

Abstract The RESET current (I rst) is a key parameter to characterize power consumption and reliability of phase change memory (PCM) device. And the Optimization of I rst is a classic issue in the investigation of PCM targeting to meet various demands. In this paper, a RESET current optimization method, based on the relationship between the RESET current and the sub-threshold slope (STS), has been presented. Different from the conventional way which uses resistance values and bit yields, the new method that adopting the STS extracted from the V–I characteristics is simple, efficient and non-destructive. Here, the experimental results collected from sample cells of a PCM array confirm that the STS can be an important indicator for RESET current optimization. Furthermore, the voltage stress and endurance performance of the PCM devices operated by different RESET current are also measured, which further prove that the optimal RESET current determined by the new method is beneficial for both power consumption and reliability. [ABSTRACT FROM AUTHOR]

Abstract Phase change materials along the GeTe-Sb isoelectronic tie line are proved to be promising host material for optimization, based on which the well-known golden composition is developed. Here, Sc doped Ge 2 Sb 1 Te 2 has been proposed for phase change memory (PCM) application, showing higher thermal stability and faster operation speed than those of the golden composition. The fast speed of 40 ns, high 10-year data retention of 160 °C, and good endurance of 6×105 cycles have made Sc 0.2 Ge 2 Sb 1 Te 2 a promising candidate for PCM application. The impact of Sc on the microstructure is believed to be essential for those improvements in PCM. [ABSTRACT FROM AUTHOR]

Highlights • The enhanced thermal stability is high enough for application. • SET/RESET operation of 5 ns is realized with large sensing margin. • The role of Cu in suppressing crystallisation was proved by XRD and TEM results. Abstract In this study, Ti-Sb-Te (TST) alloy doped with Cu has been proposed to enhance the phase change and electrical properties of TST. The formation of Cu-Te bond was suggest to be responsible for the improved thermal stability of the Cu 0.28 (Ti 0.09 Sb 0.38 Te 0.53) 0.72 (Cu 0.28 TST) film. Cu 0.28 TST-based phase change memory cell can be triggered by a 5 ns electric pulse. Such fast speed was ascribed to its growth-dominated crystallization mechanism and the precipitation of Sb in the crystallization process. Furthermore, an endurance cycles up to 105 could be achieved in the memory cell. [ABSTRACT FROM AUTHOR]

In order to reduce static energy consumption, emerging Non-Volatile Memory (NVM) technologies such as Spin Transfer Torque Magnetic RAM (STT-MRAM), Spin-Hall Effect Magnetic RAM (SHE-MRAM), Phase Change Memory (PCM), and Resistive RAM (RRAM) are under intense research. Additionally, there is a demand for more reliable circuits as the technology scales due to increased error rates caused by the increased impact of Process Variation (PV). In order to combat PV-induced reliability problems, a novel approach is proposed herein that improves the reliability of read and write operations in emerging NVMs. In the proposed design, which is called the Self-Organized Sub-bank (SOS) approach, two Sense Amplifiers (SAs) have been adopted, one with improved reliability and one with improved energy efficiency profiles, in order to increase the performance of the read operation. In particular, based on the result of a Power-On Self-Test (POST), which detects PV-impact on sub-banks, SOS chooses between a reliable and an energy-efficient SA and assigns a preferred SA to each sub-bank. Furthermore, in order to increase the performance of the write operation, SHE-MRAM is replaced with STT-MRAM to provide better write energy profile. Additionally, SOS design is once implemented with a reliable write scheme and once with an energy-efficient write scheme and results are compared and analyzed. Based on the preliminary observation in our case study, 21.5% of read operations are extremely vulnerable to PV impacts. Our results indicate that the proposed SOS approach reduces the vulnerability of the read operation by 40% on average, hence reducing the fault propagation. In particular, the SOS alleviates Vulnerable False Data Sensing (VFDS) by 82% on average, while enhancing True Data Sensing (TDS) from 72.5% to 95% across all workloads studied herein compared to LLC with conventional STT-MRAM. Additionally, SOS using the reliable write circuit provides 161% improved Energy Delay Product (EDP) on average compared to SOS with conventional STT-MRAM, while providing less than 8% write current variation. On the other hand, SOS using energy-efficient write circuit offers 39% improved EDP on average compared to the SOS using reliable write circuit and 62% EDP improvement over conventional STT-MRAM. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we propose a subsystem architecture under 3D-Storage Class Memory (3D-SCM), termed SuS, to solve the memory and power wall problems. Placing the processing unit under the 3D-SCM achieves high performance. We evaluate SuS using gem5 and the GPGPU-Sim simulator. The simulation results for the central processing unit (CPU)-based SuS architecture reveal a 100% performance improvement and a 73% energy reduction compared to the CPU architecture using dual in-line memory modules and a 4% performance improvement with a 27% energy reduction compared to the CPU architecture using hybrid memory cube. Moreover, the graphics processing unit (GPU)-based SuS architecture simulation results on the neural network benchmark demonstrate performance improvements of 17% and 7% compared to GPU architectures with graphics double data rate series memory and high bandwidth memory, respectively. [ABSTRACT FROM AUTHOR]

Abstract Chalcogenide alloys in pseudobinary line between Sb 2 Te 3 and GeTe are extensively utilied in phase change memories for optical and electronic contrast between crystalline and amorphous phases. Different from the crystalline structure, the understanding of amorphous structures is still insufficient due to disorder and distortion. By employing first-principle molecular dynamics simulations and atomistic cluster alignment analysis, the short-range orders and the associated distortions of amorphous Sb 2 Te 3 , Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 3 Sb 2 Te 6 and GeTe are investigated to explore the origin of nature difference. The results reveal that Ge Ge and Sb Sb bonds present a notable competitive mechanism with GeTe content. The GeTe content has a great influence on Ge- and Te-centered short-range orders but little influence on the structures of Sb-centered clusters, especially for the octahedral sites and unidentified structures. The tetrahedrons in Ge-centered clusters of each alloy show a close proportion, but the fraction of tetrahedrons in total clusters increases with the increasing GeTe content due to the increase in the ratio of Ge atom. As for the distortions of clusters, the distorted tetrahedrons in Ge 2 Sb 2 Te 5 are closest to the standard tetrahedron, the Peierls distortion of Ge-centered 6-fold octahedron reduces with an increase in GeTe content whereas that of Sb-centered 6-fold octahedron shows a little change. Additionally, it is found that GeTe content can inhibit the formation of nanocavity but contribute to the formation of the odd rings. Our findings deepens the understanding of amorphous structures, which can promote the design and application of phase-change materials. Highlights • A newly atomistic cluster alignment method is used to explore the local structures of amorphous Ge-Sb-Te alloys. • The fractions of defective octahedrons contribute to understand the structural difference. • The GeTe content affects the Ge- and Te-centered short-range orders notably. • The GeTe content inhibits the formation of nanocavity but promotes the formation of the odd rings. [ABSTRACT FROM AUTHOR]

Abstract Phase change memory (PCM) is a promising alternative to conventional DRAM main memories, due to its read performance, density, and nonvolatility and resulting low static energy. Unfortunately, reliability is still a significant challenge as limited write endurance, exacerbated by process variation, leads to increasing numbers of stuck-at faults over the memory's lifetime. This includes a significant number of stuck-at faults that appear early in the memory's service. Error-correcting Pointers (ECP) is a popular proposal to mitigate stuck-at faults in PCM by recording the addresses and the values of faulty bits in order to extend the lifetime of the memory. We propose a method to extend the effectiveness of ECP coverage called Yoda, which utilizes a small number of additional encoding bits in order to dramatically improve the effectiveness and fault correction capability of ECP. By adding one additional bit to ECP which corrects f faults, Yoda can correct 2 f +1 faults. Further improvements are possible introducing small numbers additional bits. Our simulation results demonstrate that Yoda has a 3.0× improvement in fault coverage compared to a fault-aware ECP with a similar overhead, while also providing a 2.5–3.0× improvement over state-of-the-art schemes with comparable complexity. Furthermore, Yoda provides a method to protect the auxiliary bits, also with a small overhead. By adding one auxiliary bit to protect the auxiliary bits, Yoda can achieve extra improvement. Highlights • Extending pointers with partition and flip methods improve guaranteed correction capabilities. • Using small additional computational overhead decreases the number of required encoding bits. • Applying partition and flip to the auxiliary bits can allow them to operate reliably in faulty memory. • Yoda improves tolerated faults by 2.5–3.0× compared to state-of-the-art designs. [ABSTRACT FROM AUTHOR]