Phase Change Memory Research Articles

GeN Underlayer for SegregationTolerant Ge-Rich GeSbTe-Based PhaseChange Memory Featuring Low Current, Low SET Drift, and HighTemperature Data Retention

GeN Underlayer for SegregationTolerant Ge-Rich GeSbTe-Based PhaseChange Memory Featuring Low Current, Low SET Drift, and HighTemperature Data Retention

Structure, bonding and electronic characteristics of amorphous Se.

Ovonic threshold switching (OTS) selectors can effectively improve the storage density and suppress the leakage current of advanced phase-change memory devices. As a prototypical OTS material, amorphous GeSe is widely investigated. But the attention paid to amorphous Se (i.e., the functional constituent in amorphous GeSe) has been very limited up to now. Here we have explored the structure, bonding and electronic characteristics of amorphous Se using ab initio molecular dynamics simulations. The results reveal that the Se atoms in amorphous Se tend to form 2-coordinated configurations, and they connect with each other to form long chains. The fraction of the vibrational density of state located in the high frequency range is relatively large, and the formation energy of the Se-Se bond is as large as 4.44 eV, hinting that the Se-Se bonds in chains possess high stability. In addition, the mid-gap state related to the OTS behavior is also found in amorphous Se despite the small proportion. Our findings enrich the knowledge of amorphous Se, which aids the applications of Se-based OTS selectors.

Multi-Bit Self-Rectifying Synaptic Memristor Having Tri-Layer Structure for Quantization Aware Training of Quantized Neural Network

Recently, a cross-point synaptic memristor arrays have been employed to implement synaptic cores of various ANNs, i.e., deep neural networks (DNNs), spiking neural networks (SNNs), and convolutional neural networks (CNNs). The most of reported studies conducted training and inference of ANNs using analog synaptic weight modulation of memristors such as resistive random access memory (ReRAM), phase change random access memory (PCRAM), and ferroelectric random access memory (FeRAM). In other words, no case has been reported so far in which a synaptic memristor with quantized multi-bit is utilized onto a synaptic core of a quantized neural network (QNN).In this study, for the first time, we introduced the multi-bit self-rectifying synaptic memristor having tri-layer structure being composed of oxygen-rich AlOx rectifying layer, oxygen-deficient HfOx top switching layer, and oxygen-rich HfOy bottom switching layer for quantization aware training of QNN. The resistive switching and self-rectifying mechanism of the multi-bit self-rectifying synaptic memristor was evidently proven by precisely investigating migration of oxygen ions and vacancies in resistive switching layers via ToF-SIMS and XPS depth profiles depending on the resistance states (i.e., pristine, set, and reset). The designed multi-bit self-rectifying synaptic memristor presented linear, discrete, and quantized 4-bit (i.e., 16-level) conductance level depending on incremental write pulse number, which was 4-bit self-rectifying synaptic memristor for the first time. In addtion, the quantization aware training (QAT) was conducted using 4-bit quantized conductance level of the desgiend multi-bit self-rectifying synaptic memristor via stright through estimator (STE). Finally, three different iris datasets were successfully classified using a quantized neural network designed via SPICE circuit simulation. The conductance mechanism of the self-rectifying synaptic memristor and its application of QNN will be presented in detail. Acknowledgement This research was supported by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT(2021M3F3A2A01037733) Figure 1

A Study on Film Properties Control of Atomic Layer Deposition Amorphous Carbon for PRAM Electrode Application

As the amount of data usage increases with the development of technologies such as artificial intelligence, Internet of Things (IoT), and big data, the need for development of high-performance memory devices and improvement of density is increasing. Phase change memory is a next-generation memory device that stores and erases information by the resistance difference generated by reversibly and rapidly changing the crystal structure of the memory cell. Recently, research on the material engineering aspect to increase thermal efficiency by using various electrode materials and the structural aspect to increase the degree of integration by making a 3D structure is attracting attention. As an electrode material to increase the thermal efficiency of PRAM devices, carbon, which has various resistances (conductor (graphite, sp2 bond) – insulator (diamond, sp3 bond)) depending on the state of atomic bonding is an attractive material. Therefore, research using carbon as an electrode by using PVD and CVD processes has been conducted, but there is a problem that it is difficult to use it for devices with a complex 3D structure with the existing process. Therefore, in this study, an a-C ALD process that can be uniformly deposited in a 3D structure was developed using the CBr4 precursor. And by analyzing the bonding state and physical properties of the deposited thin film, the mechanism of how the a-C thin film grows according to the thin film deposition conditions is studied.

Phase-Change Memory from Molecular Tellurides.

Phase-change memory (PCM) is an emerging memory technology based on the resistance contrast between the crystalline and amorphous states of a material. Further development and realization of PCM as a mainstream memory technology rely on innovative materials and inexpensive fabrication methods. Here, we propose a generalizable and scalable solution-processing approach to synthesize phase-change telluride inks in order to meet demands for high-throughput material screening, increased energy efficiency, and advanced device architectures. Bulk tellurides, such as Sb2Te3, GeTe, Sc2Te3, and TiTe2, are dissolved and purified to obtain inks of molecular metal telluride complexes. This allowed us to unlock a wide range of solution-processed ternary tellurides by the simple mixing of binary inks. We demonstrate accurate and quantitative composition control, including prototype materials (Ge-Sb-Te) and emerging rare-earth-metal telluride-doped materials (Sc-Sb-Te). Spin-coating and annealing convert ink formulations into high-quality, phase-pure telluride films with preferred orientation along the (00l) direction. Deposition engineering of liquid tellurides enables thickness-tunable films, infilling of nanoscale vias, and film preparation on flexible substrates. Finally, we demonstrate cyclable and non-volatile prototype memory devices, achieving performance indicators such as resistance contrast and low reset energy on par with state-of-the-art sputtered PCM layers.

Superlattice-like films with high thermal stability and ultralow power consumption via adding graphite carbon nanolayers

The effect of graphite carbon nanolayers on the phase change performance and microstructure of Zn15Sb85 film was systematically studied. After the addition of graphite carbon nanolayers, superlattice-like films exhibit better thermal stability and data retention, especially [C(4 nm)/Zn15Sb85(10 nm)]5 superlattice-like film. The phase change memory based on [C(4 nm)/Zn15Sb85(10 nm)]5 has fast operating speed and ultralow power consumption. During the aging process, the electronic structure of [C(4 nm)/Zn15Sb85(10 nm)]5 superlattice-like film remains almost unchanged. The results of x-ray diffraction and transmission electron microscopy indicate that the addition of graphite carbon nanolayers reduces the grain size. The formation of new C–Sb and C–Zn bonds was speculated through energy dispersion spectroscopy and x-ray photoelectron spectroscopy. The results indicate that graphite carbon nanolayers play an important role in improving the thermal stability, operating speed, reducing power consumption, and resistance drift of Zn15Sb85 films.

Non-isothermal crystallization kinetics and memory switching properties of Tl-Se-Ge-Sb chalcogenide semiconductor alloy for thermally stable phase change memory applications

Quaternary (Tl20Se70Ge10)0.85Sb0.15 chalcogenide glass was synthesized using melt-quenching and thermal evaporation techniques for bulk and thin film samples, and its amorphous character was confirmed by XRD. Based on different models of non-isothermal crystallization kinetics, the average values of the glass transition and the crystallization activation energies are 121.33 and 82.94 kJ mol−1, respectively. The crystallization mechanism in the examined composition is 2D nucleation growth, according to Avrami exponent Thermal parameters of the studied quaternary glass, like fragility fluctuation free volume reduced glass transition temperature thermal stability and glass formation ability were investigated and indicated that the (Tl20Se70Ge10)0.85Sb0.15 ChG exhibits a good ability of glass formation and thermal stability. The obtained results of the dc electrical conductivity are found to increase with temperature and decrease with film thickness. The electrical conduction activation energy (0.583 ± 0.004 eV) is thickness independent and the predominant conduction mechanism is the hopping of charge carriers in the band tail localized states. The () curves of (Tl20Se70Ge10)0.85Sb0.15 chalcogenide glass film samples were analyzed and shown to be appropriate for a memory switch. The mean value of the threshold voltage decreases exponentially as temperature increases, whereas it increases linearly as film thickness increases. The values of threshold voltage and threshold resistance activation energies are 0.282 ± 0.003 and 0.521 ± 0.004 eV, respectively. The obtained switching characteristics data were discussed in view of the electrothermal model motivated by current channel Joule heating. These findings highlight the suitability of the investigated composition for various optoelectronic applications, such as memory switching devices, optical data storage, phase-change memories and optical fibres sensing devices.

OML-PCM: optical multi-level phase change memory architecture for embedded computing systems

Unlike Dynamic Random Access Memory (DRAM), Phase Change Memory (PCM) offers higher density, longer data retention, and improved scalability because of its non-volatility and low leakage power. However, Electrically-Addressable PCM (EPCM) has a higher dynamic power and long latency than DRAM. To address these issues, scientists have developed Optically-Addressable PCM (OPCM), which uses 5-level cells instead of 2-level cells in EPCM. A silicon photonic link allows optical signals to reach OPCM cells at a high speed. Hence, OPCM can achieve a higher density while maintaining better performance at multi-level cells and consuming less power per access. However, OPCM is not suitable for general use since the photonic links do not provide an electrical interface to the processor. The aim of this paper is to present a hybrid OPCM architecture based on the use of novel multi-bank clusters with distinctive properties. Electrical-Optical-Electrical conversion (EOE) allows OPCM cells to be randomly accessed by using DRAM-like circuitry. The proposed hybrid design with multi-core processing and OPCM achieves a 2.13x speedup over previous approaches while consuming less Central Processing Unit (CPU) power. It is important to note that the proposed design offers 97 units fewer power-consistent bits than EPCM. In addition, the proposed architecture provides comparable performance and power to DDR4, as well as improved bandwidth density, space efficiency, and versatility. The Gem5 simulator was used to evaluate the design. Based on the outcomes of the analysis, the proposed architecture offers 2.08x and 2.14x better evaluations and density performance than EPCM. Furthermore, the execution time has been reduced by 2.13x, the analysis time by 1.23x, and the composition time by 4.60%.

Metric and fault-tolerant metric dimension for GeSbTe superlattice chemical structure.

The concept of metric dimension has many applications, including optimizing sensor placement in networks and identifying influential persons in social networks, which aids in effective resource allocation and focused interventions; finding the source of a spread in an arrangement; canonically labeling graphs; and inserting typical information in low-dimensional Euclidean spaces. In a graph G, the set S⊆V(G) of minimum vertices from which all other verticescan be uniquely determined by the distances to the vertices in S is called the resolving set. The cardinality of the resolving set is called the metric dimension. The set S is called fault-tolerant resolving set if S\{v} is still a resolving set of G. The minimum cardinality of such a set S is called fault-tolerant metric dimension of G. GeSbTe super lattice is the latest chemical compound to have electronic material that is capable of non-volatile storing phase change memories with minimum energy usage. In this work, we calculate the resolving set (fault tolerant resolving set) to find the metric dimension(fault-tolerant metric dimension) for the molecular structure of the GeSbTe lattice. The results may be useful in comparing network structure and categorizing the structure of the GeSbTe lattice.

Air‐Stable Atomically Encapsulated Crystalline‐Crystalline Phase Transitions in In2Se3

AbstractThe layered semiconductor In2Se3 has a low temperature crystalline–crystalline (α → β) phase transformation with distinct electrical properties that make it a promising candidate for phase change memory. Here, using scanning tunneling microscopy, correlative in situ micro‐Raman, and electrical measurements, it is shown that the β phase can persist in bulk crystals at room temperature in non‐oxidative environments. Of particular note, the stability of β phase crystals in ambient conditions under encapsulation of graphene and similar passivation layers, is reported for the first time. The strategy of encapsulation to ensure the persistence of β phase overlaps with efforts to passivate switching materials. It is further demonstrated that degradation from the elevated temperatures required for the phase change is slowed through examination of Raman signatures. These results demonstrate an alternative method of phase manipulation with a new stabilization of β‐In2Se3 in ambient conditions potentially extendable to other polymorphic materials, and the importance of passivation in In2Se3 memory devices.

Calorimetric studies of yttrium doped non-conventional phase-change materials for improved performance

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) (GeSe0.5)Yx (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) (GeSe0.5) Yx 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 CZW and CLX 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 CYL 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.

In situ insight into temperature-dependent microstructure evolution of carbon doped phase change materials

Carbon-doped Ge2Sb2Te5 (CGST) is a potential candidate in phase change random access memory (PCRAM) with superb thermal stability and ultrahigh cycle endurance. Direct observation of the microstructure evolution of CGST is desirable to uncover the phase transformation mechanism on the relationship of nucleation/crystalline behaviors of the crystalline phase at elevated temperatures and the pristine amorphous phase at room temperature. Here, we investigate the structural evolution of CGST using combined in situ techniques. Our in situ x-ray diffraction and ellipsometry results demonstrate that CGST exhibits a much higher phase transition temperature than undoped one. Temperature-dependent in situ transmission electron microscopy observations further reveal that carbon doping plays a critical role in tailoring the properties of GST by tuning the stochasticity of nucleation/crystallization, stabilizing amorphous and crystalline GST via isolating and refining the grain size at room temperature and elevated temperature. Our work provides detailed information for understanding the microscopic origin of crystallization kinetics of carbon-doped phase change materials toward high-performance PCRAM.

Atomic insights into device-scale phase-change memory materials using machine learning potential

Atomic insights into device-scale phase-change memory materials using machine learning potential

High speed and high reliability phase transition via constrained crystallization in ultrathin a-C/Sb2Te multilayer nanostructures

Operation speed, data retention, thickness variation, and resistance drift are main shortages for conventional Ge2Sb2Te5 based phase change memory (PCM). In this Letter, we propose a type of ultrathin amorphous carbon (a-C)/Sb2Te multilayer nanostructures with semiconductor process compatibility, which has high speed and excellent reliability in terms of data retention (131 °C for 10-year), thickness variation (1.52%), and resistance drift (0.0095). The PCM devices based on [a-C (1 nm)/Sb2Te (7 nm)]5 multilayer nanostructures exhibit a fast operation speed of 5 ns. The SET/RESET resistance ratio remains constant over 105 cycles by more than two orders of magnitude. The excellent performance of a-C/Sb2Te multilayer nanostructures makes it a promising storage medium to expand the applications of PCM, such as automotive electronics.

Exploring the potential of GeTe for the application in Thermophotovoltaic (TPV) cell

Germanium telluride (GeTe) having a direct bandgap of 0.6 eV has mainly been used in phase change memory and thermoelectric power generation. In this article, we study the electronic structure of the GeTe by first-principles calculations. The theoretical direct bandgap of GeTe was found to be 0.69 eV which is very close to the experimental value. Then, we demonstrated a single-junction GeTe thermophotovoltaic (TPV) cell based on device transport model with np structure at the black body and cell temperature of 1775 and 300 K, respectively. The device was optimized for the higher performance of the TPV cell. The GeTe TPV cell exhibited an efficiency of 7.9% with JSC = 16.16 A cm−2, VOC = 0.360 V and FF = 75.51%, respectively. These results indicate that GeTe could be a promising material for the fabrication of efficient TPV cell.

A-C/GeTe superlattices: Effect of interfacial impedance adaptation modeling on the thermal properties

Recently, nanostructuration has been proposed to improve the performance of phase change memories. This is the case of superlattices composed of amorphous carbon and crystalline germanium telluride, which we have investigated by molecular dynamics. For this, a modified Stillinger–Weber potential is adapted to reproduce their stiffness contrast/impedance ratio. In order to study the effect of the interface interaction, two sets of parameters are used to model the interfaces with different interactions between the two materials using the properties of the softer material or the average properties between the two creating an adaptation of impedance across the layers. The effects of interface roughness and carbon diffusion at grain boundaries are studied. Using equilibrium molecular dynamics as well as the propagation of wave-packets, we show first that without impedance adaptation, the anisotropy is high, and the roughness has a marked impact on the properties. However, the introduction of impedance adaptation destroys those effects on the thermal conductivity. Finally, we show that the periodic texturing of the interface increases the transmission of in-plane transverse phonons.

Influence of samarium modification on the phase-change performance and phase structure of tin antimonide

This work presents the optimization of the crystallization behavior and reliability of Sn15Sb85 thin films by doping Sm element. The phase transition behaviors induced by thermal were investigated by in situ resistance measurement. With the addition of Sm element, Sn15Sb85 film exhibits the superior crystallization temperature (232 °C) and data conservation (172.32 °C for 10 years), larger activation energy of crystallization (4.91 eV) and crystalline resistance (∼103 Ω), which contributes to the increased thermal stability of the amorphous state and decrease in the programming energy. The Sm-doping can broaden the energy band gap from 0.55 to 1.07 eV. The amorphous Sm and Sn compositions could retard grain growth and refine grain size from 21.13 to 11.13 nm, combining with x-ray diffraction and x-ray photoelectron spectroscopy. The surface morphology of Sn15Sb85 film becomes smoother after Sm doping as determined by atomic force microscopy images, resulting in the improved interfacial reliability. Phase change memory devices based on Sm0.095(Sn15Sb85)0.905 films can successfully achieve the complete SET and RESET reversible operation process with high operating speed (200 ns) and low power consumption (1.6 × 10−10 J). The results suggest that doping the proper concentration of Sm element will be an effectual solution to adapt and optimize the crystallization properties of Sn15Sb85 phase change material.

Ultrafast SET/RESET operation for optoelectronic hybrid phase-change memory device cells based on Ge2Sb2Te5 material using partial crystallization strategy

Combination of nonvolatile storage and in-memory computing promises to break through the “memory bottleneck” that computing device adopts von Neumann architecture with individual computing and memory unit. Thus, the advanced nonvolatile memory device with ultrafast operation speed is urgently required. Here, the optoelectronic hybrid phase-change memory based on the Ge2Sb2Te5 material is proposed, where the picosecond laser induced reversible phase-change is utilized to write and erase the information while the resistance difference is adopted to realize the accurate information readout. Due to the significant difference in resistance between crystalline and amorphous states, a partial crystallization strategy can be adopted to achieve ultrafast SET operation. Results indicate that SET operation speed of the Ge2Sb2Te5 film and device unit can be as fast as 52 and 130 ps, respectively, while the RESET speed reaches 13 ps. In parallel, the resistance ratio of RESET to SET state is still as high as two orders of magnitude. By using partial crystallization strategy, the phase-change induced by picosecond laser only occurs from amorphous to face-centered-cubic crystalline state with low crystallinity and the defective octahedral motif is observed in the Ge2Sb2Te5 film, which is beneficial to achieve the ultrafast operation speed. At the same time, the ordered clusters existed in the as-deposited and picosecond laser induced RESET films can accelerate the nucleation process of the Ge2Sb2Te5 film, which is one of the important reasons for achieving ultrafast SET speed. The optoelectronic hybrid phase-change memory with ultrafast operation speed may be one of the promising solutions for the in-memory computing.

GWalloc: A self-adaptive generational wear-aware allocator for non-volatile main memory

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.

Research status of phase change memory and its materials

Phase change memory technology is a new technology in non-volatile memory technology. Phase change memory technology has many advantages, such as non-volatility, high reading and writing speeds, better data retention, and strong compatibility with CMOS technology, and has been paid attention to by many researchers. Phase change materials are mainly chalcogenide compound materials. Researchers have done a lot of research on Ge-Sb-Te, Ge-Te, and Sb-Te-based phase change materials and developed some new phase change material systems. In addition, researchers have carried out in-depth research on the phase transition mechanism, but the existing atomic umbrella jump theory, multiple ring theory, resonance bond theory, octahedral structure theory, etc. have not formed a unified understanding of the phase transition mechanism. At present, phase change memory is mainly prepared by magnetron sputtering of phase change material targets, which results in fast deposition and high purity of the prepared film. In this paper, based on the development of phase change storage materials, the system of phase change materials and its phase change mechanism, the phase transition mechanism of phase change memory, the preparation and characterization methods of phase change films, the industrialization progress, and other research work are reviewed.