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A variety of resistive memories have been proposed in recent years. Among these emerging technologies, phase change memory (PCM) has received the most research attentions since it has the advantages of high scalability, non-volatility, fast access, strong data retention, low cost, and low power consumption. It is also considered as the most promising alternative of DRAM. In order to conquer the inevitable endurance problem of PCM cells which causes serious reliability and yield threats, hard repair and ECC (Error correction code) techniques are widely adopted. However, since soft errors are not a main threat for PCM, incorporating ECC for each data word is not a cost-effective technique since a lot of memory space is required for storing the check bits. In this paper, the progressive ECC techniques including the local progressive ECC (LPE) technique and the global progressive ECC (GPE) technique are proposed to solve this dilemma. The key innovation is to equip ECC for a data word when its first faulty cell is detected. In other words, we only equip fault detection code for data words such that the original code rate can be increased significantly. An ECC DRAM and an ECC CAM are used for storing check bits and accessing purposes, respectively. Hardware architectures for implementing the proposed GPE and LPE techniques are also provided. A simulator is developed for evaluating repair rate, reliability, yield, and hardware overhead. According to experimental results, the degradation of repair rate and reliability are almost negligible. However, the hardware overhead is at least 80% lower than the original ECC technique while maintaining the original reliability and yield levels.

Phase change memory (PCM) is regarded as a promising candidate for next-generation data storage devices. However, the contradiction between crystallization speed and amorphous thermal stability is still a great challenge. In this paper, we demonstrate PCM based on Ru-doped Sb2Te (RST) alloy, showing 112 °C 10-year data retention, 229 °C crystallization temperature, 6 ns Set speed, and 85% reduced power consumption compared to Sb2Te(ST)-based device. The good performance attributes to the grain refinement of the crystalline films with inclusion of Ru, making it a promising material for high-speed and good thermal stability phase change memory applications.

Nonvolatile phase change random access memory (PCRAM) is regarded as one of promising candidates for next-generation memory in the era of Big Data. The phase transition mechanism of phase change materials is the key scientific issue to be addressed for phase change memory. Moreover, obtaining homogeneous phase change materials with high speed, low power consumption, long life and good thermal stability is still the ultimate challenge for high-density three-dimensional (3D) PCRAM. In this paper, starting from the octahedral structure motifs (octahedrons) which are considered as the “gene” of phase change materials, a new view on the phase transition mechanism is proposed. Based on this mechanism, a homogeneous phase change material is developed by constructing three matched octahedrons, which achieved an overall improvement in performance, showing 180 °C ten-year data retention, 6 ns SET speed, one order of magnitude longer life time and 75% reduced power consumption compared with traditional Ge2Sb2Te5 (GST) devices. It is of great significance to use it in 3D PCRAM chip and multi-level brain-inspired computing chip in the future.

This research is to design a history table-based linear analysis method for a DRAM-PCM (phase change memory) hybrid memory system supporting irregular and complex memory accessing patterns in graph processing applications. This linear analysis method can analyze complex memory access patterns in real time through our history table structure, classifying address values requested from the LLC (last-level cache) as the prefetch candidates. Also, the proposed method is to support a prefetching operation from the storage onto the hybrid main memory, causing significant latency losses. Specifically, we proposed a new memory pattern analysis method, which is called a linear analysis based on a history table, to detect any linear memory access patterns from workloads showing irregular accessing characteristics. For the DRAM-PCM hybrid memory, dynamic data that are frequently accessed are stored in the DRAM. Thus, our linear analysis not only categorizes memory access patterns in the form of line patterns but also separates dynamic and static data so that the hybrid memory systems can be managed adaptively and efficiently. Our experiments show that our model can improve performance and energy consumption by 60% and 30%, respectively, compared to those of the traditional DRAM-only memory model. Moreover, our approach can enhance the entire system performance and energy consumption by 11.3%, 6.8%, compared to streamlined prefetcher-based models.

Phase change memory (PCM) based on chalcogenide compounds is considered to be an excellent candidate for next-generation memory because of its high speed, low energy consumption, high-density information storage, and durable stability. This has become a topic of general interest during the last two decades. Various systems have been proposed to explore more suitable compositions for PCM applications such as Ge-Te and Sb-Te binary alloys, GeTe-Sb2Te3 pseudo-binary alloys, and novel Te-free Sb-based alloys. In spite of this, the comprehensive performance of these pure systems still cannot fully satisfy the need for commercialization. Modification of alloys is an effective approach to enhance performance; this has been investigated significantly. Nevertheless, there are relatively few reports that directly provide integrated data on the effect of various alloying modifiers on the properties of PCM materials to obtain the relevant information conveniently, understand the role of chemical tailors on the properties of PCM materials deeply, and select modifiers matching the matrix systems efficiently. To achieve these goals and facilitate studies on PCM materials, this overview begins by summarizing and analyzing the role of alloying tailors on the performance of representative GeTe, Sb2Te3, Ge2Sb2Te5, and Sb-based systems containing Te (Sb2Te, Sb3Te, Sb4Te) and without Te (Sb-M), followed by comparing the comprehensive performance of optimized systems.

Phase change memory (PCM) has reached the level of mass production. The first step in mass production is determining the proper pulse conditions of high-resistance (HR) and low-resistance (LR) states to realize the best performance of PCM chips on the basis of longer endurance characteristics. However, due to the neglect of each of the relations as well as the square term of each relationship for pulse conditions, the standard screening method for pulse conditions cannot accurately determine the optimal pulse conditions. A new statistical prediction method based on regression analysis is presented in this work. The method can model and predict the optimal pulse conditions of PCM chips on the basis of longer endurance characteristics. In the method, the parameter estimates, model equations and surface plot are generated by the least-mean-square (LMS) method for the regression analysis; the prediction model is established by monitoring the distributions of the resistance values collected from a 4 Kbit block of the 4Mbit PCM test chips in 40 nm complementary metal oxide semiconductor (CMOS) process.

Current features of phase change memory (PCM) based on Ge2Sb2Te5 (GST) as a non-volatile memory is insufficient to meet the needs of storage class memory (SCM) such as extremely high-density, ultra-fast operating speed and long life cycle. In this study, Ti0.28Sb2Te3 (TST) device was fabricated based on atomic layer deposition (ALD) method and used as a phase change material. Due to the fast crystallization and low melting temperature of TST, the set speed can be reduced to 6 ns while the reset voltage can be decreased by 20% compared with GST-based device with the same cell structure. In addition, the ALD-deposited TST alloy showed a good gap-fill capability. These results indicate that the ALD-deposited TST film is a fast and scalable phase-change material applied to SCM.

The phase transition properties of MgSb/Sb multilayer thin films were studied systematically. After composited with MgSb layer, MgSb/Sb thin film had better amorphous stability and higher resistance. One-dimensional growth dominated mechanism made MgSb/Sb have ultra-fast phase change speed. The grain growth and interface stress resulted in a little change on surface morphology during crystallization. The multilayer structure was confirmed by element distribution on cross section. The reversible resistance switching was achieved on [MgSb(7 nm)/Sb(3 nm)]5-based device. This work showed that MgSb/Sb multilayer film was a potential material with fast speed and low power consumption for phase change memory application.

Neuromorphic computing using analog non-volatile memory (NVM) devices has been the subject of various studies due to its potential ability to achieve extremely low power consumption less than that of traditional von Neumann architecture. However, using NVM devices, such as phase change memory (PCM) and resistive-RAM devices, presents various challenges, such as limitations in the number of conductance steps and device variability resulting from process variation and electro/thermo-dynamics. Limitations in the number of conductance steps and device variability could reduce the accuracy of neural network training. It is necessary to quantitatively analyze the effect of the number of conductance steps and synaptic device variability on the accuracy of neural network training and assess requirements for NVM devices to make NVM-based neuromorphic computing successful. We conducted the analysis using simulations focusing on a spiking neural network (SNN) based restricted Boltzmann machine (RBM) with PCM devices using the PCM-refresh method. The results of our quantitative simulation, which used the MNIST dataset, showed that having more than 500 conductance steps achieves comparable performance to that when there are more than 1000 conductance steps. We also found that less than 10% conductance update variation in the synaptic devices is required to achieve the comparable accuracy with the no variation case. These results can provide guidelines for designing and optimizing a synaptic device for realizing NVM-based neuromorphic computing.

The phase change behavior of Ga40Sb60/Sn15Sb85 multiple interfacial film was studied systematically for its application in phase change memory. The thermal stability of amorphous state was evaluated using the Arrhenius formulas. The evolution of phase structure indicates that the Sb phase exists in the Sb-rich Ga40Sb60/Sn15Sb85, Ga40Sb60 and Sn15Sb85 films. The change in density and thickness during crystallization was obtained by X-ray reflectance. The nanoscale change in roughness of the Ga40Sb60/Sn15Sb85 film was confirmed by atomic force microscopy. The rate for crystallization and amorphization transformation was measured by a picosecond laser pump-probe system. The phase change memory device based on [Ga40Sb60 (7 nm)/Sn15Sb85 (3 nm)]5 thin film was fabricated. An ultra-fast switching and low power were achieved for Ga40Sb60/Sn15Sb85 multiple interfacial film.

High-current switching performance of ovonic threshold switching (OTS) selectors have successfully enabled the commercialization of high-density three-dimensional (3D) stackable phase-change memory in Intel’s 3D Xpoint technology. This bridges the huge performance gap between dynamic random access memory (DRAM) and Flash. Similar to phase-change memory, OTS uses chalcogenide-based materials, but whereas phase-change memory reversibly switches between a high-resistance amorphous phase and a low-resistance crystalline phase, OTS freezes in the amorphous phase. In this article, we review recent developments in OTS materials and their performance in devices, especially current density and selectivity. Advantages and challenges of OTS devices in the integration with the phase-change memory are discussed. We introduce the evolution of theoretical models for explaining the OTS behavior, including thermal runaway, field-induced nucleation, and generation/recombination of charge carriers.

The cycling endurance of phase-change memory is one of the last hurdles to overcome to enable its adoption in the larger market for persistent memory products. Phase-change memory cycling endurance failures, whether they are stuck-SET (caused by elemental segregation) or stuck-RESET (caused by void formation), are caused by atomic migration. Various driving forces responsible for the atomic migration have been identified, such as hole-wind force, electrostatic force, and crystallization-induced segregation. We introduce several strategies to improve cycling endurance based on an understanding of driving forces and interactions among them. Utilizing some of these endurance-improving techniques, record-high phase-change memory cycling endurance at around 1012 cycles has been recently reported using a confined phase-change memory cell with a metallic liner.

Thermoelectric effects can be harnessed in a vertical-pillar phase-change memory cell by the inclusion of geometry-level and material-level asymmetry to reduce the programming current. However, the trade-off between the programming current and SET resistance of a phase-change memory cell makes their optimization vital for a predictive overall performance. We propose herein an analytical model to include thermoelectric effects in a phase-change memory and optimize the cell using a nature-inspired multiobjective constrained optimization algorithm. The model is then analyzed for a wide range of geometries and material parameters to study the collective effect on the cell performance. Furthermore, we suggest an optimal configuration for the hybrid phase-change memory cell.

As a highly integrated non-volatile memory device, phase change memory (PCM) has attracted considerable attention. In this paper, SbSe/ZnSb (SS/ZS) stacked thin films were developed for high density phase change memory applications. SS/ZS stacked thin films show two pronounced resistance steps with the increase in temperature and possess excellent thermal stability. The X-ray diffraction reveals that Sb, Sb2Se3 phases crystallize first, hexagonal ZnSb phase forms then at higher temperatures. The stacked thin films exhibit subtle variations in thickness and roughness after crystallization. Ultrafast crystallization speed (9.68 ns) in the SS(25 nm)/ZS(25 nm) thin film was validated by picosecond laser pump-probe system. Phase-change memory cells based on SS(25 nm)/ZS(25 nm) stacked thin film can realize multi-level data storage and the whole SET and RESET operations can be implemented with a 20 ns electrical pulse. Thus, SS/ZS stacked thin films have advantages for multi-level data storage capability, better thermal stability, and fast phase change speed, and are therefore good candidates for high density PCM device.

The contradictory nature of increasing the crystallization speed while extending the amorphous stability for phase-change materials (PCMs) has long been the bottleneck in pursuing ultrafast yet persistent phase-change random-access memory. Scandium antimony telluride alloy (ScxSb2Te3) represents a feasible route to resolve this issue, as it allows a subnanosecond SET speed but years of reliable retention of the RESET state. To achieve the best device performances, the optimal composition and its underlying working mechanism need to be unraveled. Here, by tuning the doping dose of Sc, we demonstrate that Sc0.3Sb2Te3 has the fastest crystallization speed and fairly improved data nonvolatility. The simultaneous improvement in such ‘conflicting’ features stems from reconciling two dynamics factors. First, promoting heterogeneous nucleation at elevated temperatures requires a higher Sc dose to stabilize more precursors, which also helps suppress atomic diffusion near ambient temperatures to ensure a rather stable amorphous phase. Second, however, enlarging the kinetic contrast through a fragile-to-strong crossover in the supercooled liquid regime should require a moderate Sc content; otherwise, the atomic mobility for crystal growth at elevated temperatures will be considerably suppressed. Our work thus reveals the recipe by tailoring the crystallization kinetics to design superior PCMs for the development of high-performance phase-change working memory technology. Optimizing the composition of an alloy provides a route to ultrafast and stable non-magnetic electronic memories according to research from China. The atoms in many materials can stably exist in either an organized crystalline or a disorganized amorphous arrangement. Phase-change materials, which quickly and reversibly switch between the two states, are being developed as digital memories: small regions of the material are encoded in crystalline or amorphous states just as ones and zeros are stored in a magnetic memory. Feng Rao from Shenzhen University and co-workers developed an approach for creating phase-change memory materials that change phase quickly and retain a phase for a long time. The team identified the optimum scandium content in the alloy scandium antimony telluride and showed that this led to faster crystallization and improved data stability. In this work, we reveal the recipe for the design of superior phase-change materials to enable ultrafast and persistent memory application.

In this paper, the V2O5/Ge8Sb92 multilayer thin films were prepared and investigated. Compared with Ge8Sb92, V2O5/Ge8Sb92 film had a higher crystallization temperature and a larger conductivity activation energy. The surface roughness for V2O5/Ge8Sb92 film was small before and after crystallization. The crystallization of V2O5/Ge8Sb92 was inhibited and the grain size was only 5.0 nm. A higher stability and smaller grain made V2O5/Ge8Sb92 multilayer film a potential application in high-density phase change memory.

Sb65Se35/Sb multilayer composite thin films were prepared by depositing the Sb65Se35 and Sb layers alternately. In situ resistance vs. temperature was measured and the crystallization temperature increased with thickening the Sb65Se35 layer in Sb65Se35/Sb thin films. The data retention temperature of 10 years increased greatly from 14 °C of pure Sb to 103 °C of [Sb65Se35(3 nm)/Sb(7 nm)]3. Also, the band gap was broadened and the surface became smoother. X-ray diffraction patterns for the studied materials revealed that Sb and Sb2Se3 phases coexisted in Sb65Se35/Sb thin films. Absorbing the advantages of the fast phase change for Sb, the [Sb65Se35(1 nm)/Sb(9 nm)]5 multilayer thin film had an ultrafast amorphization speed of 1.6 ns. The results indicated that Sb65Se35/Sb multilayer thin film was a potential phase change material for fast speed and good stability.

Phase change memory (PCM) is one of promising technology to replace DRAM with its attractive features such as zero leakage power and high scalability. In PCM, a SET operation needs much more time than a RESET operation. A typical write request concurrently writes 64 bytes to a PCM memory line. Therefore, write latency is mainly determined by SET operations. Previously, PreSET has been proposed to improve PCM performance by exploiting asymmetry in write time. A PreSET operation pro-actively SETs all the bits in the memory line before a dirty cache line is written to PCM . Later, when a write request is processed, only RESET operations are actually performed. Consequently, PreSET reduces write latency and improves system performance. However, such PreSET operations are conducted only at a very coarse-grained level, which reduces the endurance of PCM. Through empirical study, we observe that in most applications the number of dirty words in a dirty line is actually quite limited. If we only SET those dirty words, instead of the whole cache line, we would significantly extend the lifetime of PCM while still achieving desirable performance. Inspired by this observation, we propose a scheme called Partial-PreSET which balances performance and endurance of PCM. The core idea of this scheme is to SET those dirty bits of a cache line in a fine-grained fashion. Our experiments show that the proposed Partial-PreSET scheme significantly improves the average lifetime of PCM system, up to 2.79X, while incurring only 2% system performance loss, compared with the state-of-the-art scheme (i.e., PreSET).

For the phase change memory (PCM) application, it is very desirable to possess better comprehensive performance in phase change materials, such as high stability, ultra-faster phase change and low power consumption. In this work, Er doped Sb70Se30 phase change materials were synthesized by magnetron sputtering technique. Compared with pure Sb70Se30 films, it was found that the crystallization temperature (T c) increased from 207 to 221 °C by Er doping, revealing the improvement of thermal stability. In addition, the increasing resistance of amorphous and crystalline state indicated much lower power consumption for PCM. More importantly, the Er doping significantly inhibits the formation of Sb–Se phase, which is to result in much faster phase change speed. Therefore, this work elucidated that the SbSe materials with Er doping were promising candidates possessing better data retention, lower power consumption and faster phase change speed for PCM application.

To close the increasing performance gap between disk storage and processors, flash based solid-state devices (FSSDs) have been adopted within the memory hierarchy to improve the performance of hard disk drive (HDD) based storage system. However, FSSDs still suffer from erase-before-write restriction, coarse access granularity and limited write endurance. Recently, the cutting-edge non-volatile memory technologies are merging, e.g., phase-change memory and resistive memory, which offer us new storage alternatives with faster access, byte-addressability and better endurance. In order to address the imperative data intensive computing issues, we propose to leverage PCM as disk write cache for constructing a hybrid PCM+HDD storage architecture in this paper. First, we develop a novel hash-based write caching scheme called HALO to improve both spatial and temporal locality on hard disks, thus addressing the limitations of traditional LRU caching algorithms and rendering better I/O performance. To deal with the limited durability of PCM devices and reclaim the degraded spatial locality in previous wear-leveling techniques, we further propose novel PCM wear leveling algorithms that provide effectively uniform writes while maximizing access parallelism. We have evaluated this PCM-based hybrid storage architecture using applications with a diverse set of I/O access patterns. Our experimental results demonstrate that the HALO caching scheme leads to an average reduction of 36.8% in execution time compared to the LRU caching scheme, and that the space filling curve based wear leveling extends the lifetime of PCM by a factor of 21.6.

In this study, we propose a Se-incorporated Ge10Sb90 as a phase-change material for phase-change memory (PCM) with high reliability and low operation power. We investigated the effect of the Se concentration on the thermal and electrical properties of Se-doped Ge10Sb90 films by varying the Se concentration from 0 to 20 at.%. The crystallization temperature, crystallization activation energy, and maximum ten-year data retention temperature increased with the increasing Se, thus demonstrating the improved thermal stability of Se-doped Ge10Sb90 films with higher Se contents. More Se also increased the rate factor, band gap, threshold voltage, and load resistance. In addition, the crystallization speed, programming window, and resistances of both the amorphous and crystalline states increased with the increasing Se concentration. In contrast, the reset current decreased with the increasing Se concentration. These results demonstrate that Se-doped Ge10Sb90 is a highly promising material for PCM applications.

Cu–Sn–Se material was prepared by magnetron sputtering to investigate its potential application in phase change memory. The amorphous-to-crystalline transition was studied by in situ film resistance measurements. Cu–Sn–Se material had lower activation energy for crystallization (1.60 eV) and higher crystallization resistance than SnSe. The amorphous Cu–Sn–Se had more narrow band gap compared to SnSe. After the adding of Cu, the crystallization of Cu–Sn–Se material was inhibited and the grain structure became more compact. The picosecond laser pulse measurement indicated that Cu–Sn–Se material had a fast phase change speed (3.36 ns). The results demonstrated that Cu–Sn–Se material was a promising phase change material which had low power and high speed application in phase change memory.

In this work, we studied temperature dependences of the resistivity and current-voltage characteristics of amorphous thin films based on the materials of a Ge–Sb–Te system of compositions GeSb4Te7 (GST147), GeSb2Te4 (GST124), and Ge2Sb2Te5 (GST225) applied in the phase change memory devices. The effect of changes in the composition of thin films on the crystallization temperature, resistivity of films in amorphous and crystalline states, and on the activation energy of conductivity is determined. It is found that the peculiarity of these materials is the mechanism of two-channel conductivity where the contribution to the conductivity is made by charge carriers excited into localized states in the band tails and by carriers of the delocalized states in the valence band.

Although phase change memory technology has developed drastically in the past two decades, the cognition of the key switching materials still ignores an important member, the face-centered cubic Sb2Te3. Apart from the well-known equilibrium hexagonal Sb2Te3 crystal, we prove the metastable face-centered cubic Sb2Te3 phase does exist. Such a metastable crystal contains a large concentration of vacancies randomly occupying the cationic lattice sites. The face-centered cubic to hexagonal phase transformation of Sb2Te3, accompanied by vacancy aggregation, occurs at a quite lower temperature compared to that of Ge2Sb2Te5 alloy. We prove that the covalent-like bonds prevail in the metastable Sb2Te3 crystal, deviating from the ideal resonant features. If a proper doping technique is adopted, the metastable Sb2Te3 phase could be promising for realizing reversibly swift and low-energy phase change memory applications. Our study may offer a new insight into commercialized Ge–Sb–Te systems and help in the design of novel phase change materials to boost the performances of the phase change memory device.

Sb2Se3 alloy is used as a dopant for Ti–Sb–Te phase change material aiming to improve the thermal stability of Ti–Sb–Te based phase change memory cell. In this paper, the thermal and electrical properties of Sb2Se3-doped Ti0.32Sb2Te3 are respectively investigated. Compared with Ti0.32Sb2Te3 material, (Sb2Se3)0.28(Ti0.32Sb2Te3)0.72 has better thermal stability with crystallization temperature of 203 °C and estimated 10-year data retention of 102 °C. For the (Sb2Se3)0.28(Ti0.32Sb2Te3)0.72-based PCM cell, it inherits the advantage of fast switching speed and good endurance of TST based one with wider SET/RESET window. The effects of Sb2Se3 doping on the structure of Ti0.32Sb2Te3 are also systemically studied. In crystalline (Sb2Se3)0.28(Ti0.32Sb2Te3)0.72 film structure, Se is prone to substitute Te and bond with Sb and Te in the Sb2Te3 lattice structure which may decrease the crystal lattice constant.

Phase change behavior in GeSb9 thin films with different thickness were investigated by utilizing in situ resistance measurements. It is found that the crystallization temperatures and resistances increase with decreasing of film thickness. The analysis of X-ray diffraction indicated that the grain size decreases and the crystallization is suppressed by decreasing film thickness. The surface roughness of thin films was measured by atomic forced microscopy. The obtained values of Avrami indexes indicate that the nucleation rate decreases with decreasing the film thickness. The phase change memory devices based on GeSb9 thin films were fabricated and the lower RESET power consumption was observed for thinner film.

Emerging byte-addressable non-volatile memory technologies, such as phase change memory (PCM) and spin-transfer torque RAM (STT-RAM), offer both the byte-addressability of memory and the durability of storage, thus making it feasible to build single-level store systems. To ensure the consistency of persistent data structures in the presence of power failures or system crashes, it requires flushing cache lines to persistent memory frequently, thus incurring non-trivial synchronization overhead. To mitigate this issue, we propose two techniques. First, we use non-volatile STT-RAM as scratchpad memory on chip to store recovery information, thereby eliminating synchronization cost in the logging phase due to the avoidance of off-chip logging operations. Second, we present an adaptive synchronization policy based on caching modes in terms of data access patterns, thereby eliminating unnecessary synchronization cost in the checkpoint phase. Evaluation results indicate that the two techniques improve the overall performance from 2.15x to 2.39x compared with conventional transactional persistent memory.

The influence of different amounts of Bi, Ti and In on the thermal properties and crystallization kinetics of Ge2Sb2Te5 thin films for phase change memory devices was investigated. Temperatures and heat effects of crystallization were evaluated for all investigated compositions. Joint utilization of model-free Ozawa–Flynn–Wall and model-fitting Coates-Redfern methods allowed to estimate effective activation energies and pre-exponential factors for crystallization processes of amorphous films as functions of conversion, and determine reaction models. It was found that crystallization process most adequately can be described by the second- or third-order reaction. Storage and data processing times of the phase change memory cells on the basis of investigated materials were calculated with using of determined kinetic triplets of crystallization processes. Calculations showed that crystallization time decreases nearly on the order of magnitude for Ge2Sb2Te5+1 mass% In in comparison with undoped Ge2Sb2Te5. On the other hand, compositions with 0.5 and 3 mass% In allow to increase sufficiently storage time. Introduction of Ti does not significantly affect data processing time of phase change memory cell; however, it decreases storage time. Ge2Sb2Te5+0.5 mass% Bi composition have the most suitable kinetic parameters for phase change memory among the studied thin films.

There is pressing need in computation of a universal phase change memory consolidating the speed of RAM with the permanency of hard disk storage. A potentiated scanning tunneling microscope tip traversing the soliton separating a metallic, ABA-stacked phase and a semiconducting ABC-stacked phase in trilayer graphene has been shown to permanently transform ABA-stacked regions to ABC-stacked regions. In this study, we used density functional theory (DFT) calculations to assess the energetics of this phase-change and explore the possibility of organic functionalization using s-triazine to facilitate a reverse phase-change from rhombohedral back to Bernal in graphene trilayers. A significant deviation in the energy per simulated atom arises when s-triazine is adsorbed, favoring the transformation of the ABC phase to the ABA phase once more. A phase change memory device utilizing rapid, energy-efficient, reversible, field-induced phase-change in graphene trilayers could potentially revolutionize digital memory industry.

With the help of first-principles molecular dynamics calculations, we obtained the atomic picture of amorphous AlSb(a-AlSb) for phase-change memory application. Generally, a-AlSb shows sp 3 bonding network, which is the intrinsic characteristic for its good thermal stability. Significant wrong(homogenous) Al-Al bonds can also be observed from the pair correlation function. This hints the amorphous phase may consist of Al cluster and Sb-rich Al-Sb alloy. Recent experiment has observed the Sb-rich region of AlSb alloy can be switched to crystal, on the basis of which, combined with our calculations, we thus propose that on the one hand such a Sb-rich region in a-AlSb can retain the rapid crystallization like pure Sb solid and on the other hand some Al atoms play the important role of stabilizing Sb rich network with sp 3 bonding. The present study offers a microscopic view to understand the phase change mechanism of AlSb alloy for information storage device.

To date, the unpleasant trade-off between crystallization speed and thermal stability for most phase change materials is detrimental to achieve phase change memory (PCM) with both features of high-speed and good-retention. However, it is proved that Al doping in Sb2Te, served as storage media in PCM, favors both a high writing speed (6 ns) and a good retention (103 °C), as well as a low power consumption. Judging by experimental and theoretical investigations, doped Al atoms prefer to replace Sb in Sb2Te lattice, strongly bonded with 6 Te atoms, to form a homogeneous phase. While in amorphous Al doped Sb2Te (AST), Al atoms are in tetrahedral environment, firmly bonded with four Sb/Te atoms. The strong bonding in Al centered tetrahedron in amorphous AST can obstruct the collective motion of Sb atoms near the matrix boundary, leading to the improvement in thermal stability and the confinement in grain size.

In the paper, the Ce doping is proposed to improve the thermal stability and increase the resistance in ZnSb thin film for phase-change memory (PCM) application. By Ce doping, it is proved that the resistance of amorphous and crystalline state, crystallization temperature (Tc), 10-year lifetime temperature (Tten) and activation energy (Ea) were significantly increased in ZnSb films. The improvement of thermal stability and the reduction of reset current were achieved. Through the experimental investigation, the new formation of Ce–Sb bonding by Ce doping was found in the Sb hexagonal structure, which might be the key reasons for the enhanced performance.

Phase change random access memory (PCRAM) has been successfully applied in the computer storage architecture, as storage class memory, to bridge the performance gap between DRAM and Flash-based solid-state drive due to its good scalability, 3D-integration ability, fast operation speed and compatible with CMOS technology. Focusing on phase change materials and PCRAM for decades, we have successfully developed 128 Mb embedded PCRAM chips, which can meet the requirements of most embedded systems. 3D Xpoint (3D PCRAM), invented by Intel and Micron, has been regarded as a new breakthrough in the last 25 years since the application of NAND in 1989, which represents state-of-the-art memory technology. This technology has some remarkable features, such as the confined device structure with 20 nm size, the metal crossbar electrodes to reduce the resistance variations in PCRAM arrays, and the ovonic threshold switching selector that can provide a high drive current and a low leakage current. A good understanding of phase change mechanism is of great help to design new phase change materials with fast operation speed, low power consumption and long-lifetime. In this paper, we firstly review the development of PCRAM and different understandings on phase change mechanisms in recent years, and then propose a new view on the mechanism, which is based on the octahedral structure motifs and vacancies. Octahedral structure motifs are generally found in both amorphous and crystalline phase change materials. They are considered to be the basic units during phase transition, which are severely defective in the amorphous phase. These configurations turn into more ordered ones after minor local rearrangements, the growth of which results in the crystallization of rocksalt (RS) phase with a large amount of vacancies in the cation sites. Further driven by thermodynamic driving force, these vacancies move and layer along certain directions; consequently, the metastable RS structure transforms into the stable hexagonal (HEX) structure. Based on our results, we find that reversible phase transition between amorphous phase and RS phase, without further changing into HEX phase, would greatly decrease the required power consumption. Robust octahedra and plenty of vacancies in both amorphous and RS phase, respectively avoiding large atomic rearrangement and providing necessary space, are crucial to achieve the nanosecond or even sub-nanosecond operation of PCRAM.

In this article, Ge10Sb90/SiO2 multilayer thin films were prepared to improve thermal stability and data retention for phase change memory. Compared with Ge10Sb90 monolayer thin film, Ge10Sb90 (1 nm)/SiO2 (9 nm) multilayer thin film had higher crystallization temperature and resistance contrast between amorphous and crystalline states. Annealed Ge10Sb90 (1 nm)/SiO2 (9 nm) had uniform grain with the size of 15.71 nm. After annealing, the root-mean-square surface roughness for Ge10Sb90 (1 nm)/SiO2 (9 nm) thin film increased slightly from 0.45 to 0.53 nm. The amorphization time for Ge10Sb90 (1 nm)/SiO2 (9 nm) thin film (2.29 ns) is shorter than Ge2Sb2Te5 (3.56 ns). The threshold voltage of a cell based on Ge10Sb90 (1 nm)/SiO2 (9 nm) (3.57 V) was smaller than GST (4.18 V). The results indicated that Ge10Sb90/SiO2 was a promising phase change thin film with high thermal ability and low power consumption for phase change memory application.

Structural phase-change materials are of great importance for applications in information storage devices. Thermally driven structural phase transitions are employed in phase-change memory to achieve lower programming voltages and potentially lower energy consumption than mainstream nonvolatile memory technologies. However, the waste heat generated by such thermal mechanisms is often not optimized, and could present a limiting factor to widespread use. The potential for electrostatically driven structural phase transitions has recently been predicted and subsequently reported in some two-dimensional materials, providing an athermal mechanism to dynamically control properties of these materials in a nonvolatile fashion while achieving potentially lower energy consumption. In this work, we employ DFT-based calculations to make theoretical comparisons of the energy required to drive electrostatically-induced and thermally-induced phase transitions. Determining theoretical limits in monolayer MoTe2 and thin films of Ge2Sb2Te5, we find that the energy consumption per unit volume of the electrostatically driven phase transition in monolayer MoTe2 at room temperature is 9% of the adiabatic lower limit of the thermally driven phase transition in Ge2Sb2Te5. Furthermore, experimentally reported phase change energy consumption of Ge2Sb2Te5 is 100–10,000 times larger than the adiabatic lower limit due to waste heat flow out of the material, leaving the possibility for energy consumption in monolayer MoTe2-based devices to be orders of magnitude smaller than Ge2Sb2Te5-based devices. Theoretical calculations reveal that the electrostatically driven phase transition of monolayer MoTe2 is accompanied by little heat waste. Phase change materials have enormous potential for thermal and information storage applications, as they can switch between different phases (and properties) with external stimuli. The amount of heat dissipated during these transitions can however limit the operation and further miniaturization of devices. A team from Stanford University has now modeled heat dissipation during the phase transition in electrostatically driven MoTe2 and thermally driven Ge2Sb2Te5: MoTe2 could consume 100‐10,000 times less energy per unit volume than Ge2Sb2Te5, as in the latter case most of the energy becomes heat waste. The small amount of heat that enters and leaves the system in the case of MoTe2 makes it very promising for low‐energy and low‐dimension devices.

In comparison to Ge2Sb2Te5 (GST) and pure Sb70Se30 (SbSe) thin films, superlattice-like (SLL) Ge/Sb70Se30 (Ge/SbSe) has a higher crystallization temperature, larger crystallization activation energy, better data retention and lower power consumption. SLL Ge/SbSe thin films with different thickness of Ge and SbSe layers were prepared by magnetron sputtering system. The amorphous-to-crystalline transitions of SLL Ge/SbSe thin films were investigated through in situ film resistance measurement. The crystallization activation energy of SLL Ge/SbSe thin films was calculated from a Kissinger plot. The data retention time was estimated through isothermal time-dependent resistance measurement by Arrhenius equation. The phase structure of the thin films annealed at different temperatures was investigated by using X-ray diffraction. Phase change memory cells based on the SLL [Ge(8 nm)/SbSe(5 nm)]4 thin films were fabricated to test and evaluate the switching speed and operation consumption.

The new emergence of data-intensive applications raises a huge requirement on the capacity of memory. However, an obvious increase of memory makes the corresponding energy consumption unacceptable in practice. To address this question, any effort only to reduce the energy consumption of dynamic random access memory (DRAM) is ineffective. Recently, combining DRAM and phase change memory (PCM) to construct a hybrid main memory is recognized as a promising solution to distinctly reduce the energy consumption. In this paper, we propose a new page-level energy management strategy to optimize the energy consumption of the hybrid main memory. The proposed strategy records pages' local and global access information by a new data structure, and then classifies pages by the access history, at last adaptively places PCM or DRAM pages according to the memory characteristics and remaps the migrated pages. Our experimental results show that our strategy can achieve 9.4 % of energy saving and 9.6 % of performance improvement at most compared with APG and PDRAM, which were proposed respectively by conferences RACS'12 and DAC'09.

The carrier concentration of Sb-rich phase SnSb4, Ga3Sb7 and superlattice-like [SnSb4(3.5 nm)/Ga3Sb7(4 nm)]7 (SLL-7) thin films as a function of annealing temperature was investigated to explain the reason of resistance change. The activation energy for crystallization was calculated with a Kissinger equation to estimate the thermal stability. In order to illuminate the transition mechanisms, the crystallization kinetics of SLL-7 were explored by using Johnson–Mehl–Avrami theory. The obtained values of Avrami indexes indicate that a one-dimensional growth-dominated mechanism is responsible for the set transition of SLL-7 thin film. X-ray diffractometer and Raman scattering spectra were recorded to investigate the change of crystalline structure. The measurement of atomic force microscopy indicated that SLL-7 thin film has a good smooth surface. A picosecond laser pump-probe system was used to test and verify phase-change speed of the SLL-7 thin film.

Nitrogen doping is applied to improve the thermal stability and power consumption of Sb70Se30 phase change thin film. Comparing to un-doped Sb70Se30 thin film, N-doped Sb70Se30 thin film has a higher crystallization temperature and better data retention. The measurement of atomic force microscopy indicated that the crystallization is inhibited and the surface of thin films becomes smoother after N doping. The analysis of X-ray diffraction proved that nitrogen doping can suppress the grain growth of the films and limit the grain size. The phase transition speed between the amorphous and crystalline state was investigated by the picosecond laser pulses. Phase change memory devices based on N-doped thin films were fabricated to test and evaluate the electrical properties. The results indicate that nitrogen-doped Sb70Se30 films have the potential in phase change memory application.

The amorphous to crystalline phase change of nitrogen-doped Ge10Sb90 thin films were investigated by in␣situ film resistance measurements. The thermal stability and data retention increased with the increase of N doping concentration. Compared with Ge10Sb90, a higher crystalline resistance of N-doped Ge10Sb90 thin films was obtained, which is beneficial for the reduction of RESET operation consumption of phase change memory. The analysis of x-ray diffractomer indicated that nitrogen doping can refine the grain size. The measurement of atomic force microscopy revealed that the crystallization was inhibited and the surface of thin films became smoother after N doping. A reversible phase transition was realized by the picosecond laser pulses and the switching speed of crystallization was measured.

We show that phase change media based on the gallium and lanthanum chalcogenides can outperform the well know benchmark performance of germanium antimony telluride devices in two key specifications thermal stability and power consumption. Through the fabrication and characterisation of identical nanoscale gallium lanthanum sulphide and germanium antimony telluride based devices, we show that the former compounds offer lower current consumption, while still exhibiting a relatively high resistance in both phases and high resistance contrast between phases, comparable to an equivalent germanium antimony telluride device. We also demonstrate that these sulphide-based devices continue to display a measurable threshold and retain information at temperatures above 500 °C, considerably outperforming conventional telluride based devices.

Available energy becomes a critical design issue for the increasingly complex real-time embedded systems. Phase Change Memory (PCM), with high density and low idle power, has recently been extensively studied as a promising alternative of DRAM. Hybrid PCM-DRAM main memory architecture has been proposed to leverage the low power of PCM and high speed of DRAM. In this paper, we propose energy-aware real-time task scheduling strategies for hybrid PCM-DRAM based embedded systems. Given the execution time variation when a task is loaded into PCM or DRAM, we re-design the static table-driven scheduling for a set of fixed tasks, as well as the Rate-Monotonic (RM) and Earliest Deadline First (EDF) scheduling policies for periodic task sets. Furthermore, since the actual execution time can be much shorter than the worst-case execution time in the actual execution, we propose online schedulers which migrates the tasks between PCM and DRAM to optimize the energy consumption by utilizing the slack time resulted from the completed tasks. All the proposed algorithms minimize the number of task migrations from PCM to DRAM by ensuring that aperiodic tasks are not migrated while each periodic task instance can be migrated at most once. Experimental results show our proposed scheduling algorithms satisfy the real-time constraints and significantly reduce the energy consumption.

Phase change memory (PCM) is one of the most promising candidates for the next-generation non-volatile memory, of which phase change material as the storage media is the key factor. In this paper, we proposed a novel phase change material CrxSb2Te1, which exhibits faster phase transition speed, better thermal stability and higher reliability compared with traditional Ge2Sb2Te5. With the increase of doped-Cr, the grain sizes are refined and the volume change rate decreases. What’s more, in the composition range studied, the crystal structure of CrxSb2Te1 films is single hexagonal phase without phase separation. For preferred Cr0.25Sb2Te1 and Cr0.41Sb2Te1 films, the resistance ratios between amorphous and crystalline states are up to two orders of magnitude or more; the temperatures for 10-year-data-rentation are 102.7 and 128.4 °C, respectively, showing good thermal stability performance. For the PCM cells based on the preferred Cr0.25Sb2Te1 and Cr0.41Sb2Te1 films, the threshold current/threshold voltage are about 4 μA/1.3 V and 4 μA/1.2 V, respectively; and when the voltage pulse width is short as 30 ns, the SET/RESET voltages are 1.7/3.3 and 1.9/4.5 V, respectively, showing high speed application potential for PCM.

The crystallization characteristic, electrical, and optical properties of ZnSb-doped Sb3Te thin films have been systematically investigated. The results show that the crystalline phase, grain size, and the preferred orientation are influenced by ZnSb addition. Meanwhile, ZnSb doping can effectively increase the thermal stability of Sb3Te films such as crystallization temperature and crystallization activation energy, and maintain fast crystallization speed. Among ZnSb-doped Sb3Te films, Zn5.3Sb72.7Te22.0 film exhibits both shorter complete crystallization time (135 ns at 70 mW) and appropriate thermal stability (keeping the amorphous state at 107.2 °C for 10 years).

In this letter, a phase change random access memory (PCRAM) chip based on Ti0.4Sb2Te3 alloy material was fabricated in a 40-nm 4-metal level complementary metal-oxide semiconductor (CMOS) technology. The phase change resistor was then integrated after CMOS logic fabrication. The PCRAM was successfully embedded without changing any logic device and process, in which 1.1 V negative-channel metal-oxide semiconductor device was used as the memory cell selector. The currents and the time of SET and RESET operations were found to be 0.2 and 0.5 mA, 100 and 10 ns, respectively. The high speed performance of this chip may highlight the design advantages in many embedded applications.

Phase change memory (PCM) is a promising technology for future memory thanks to its better scalability and lower leakage power than DRAM (dynamic random-access memory). However, adopting PCM as main memory needs to overcome its write issues, such as long write latency and high write power. In this paper, we propose two techniques to improve the performance and energy-efficiency of PCM memory systems. First, we propose a victim cache technique utilizing the existing buffer in the memory controller to reduce PCM memory accesses. The key idea is reorganizing the buffer into a victim cache structure (RBC) to provide additional hits for the LLC (last level cache). Second, we propose a chip parallelism-aware replacement policy (CPAR) for the victim cache to further improve performance. Instead of evicting one cache line once, CPAR evicts multiple cache lines that access different PCM chips. CPAR can reduce the frequent victim cache eviction and improve the write parallelism of PCM chips. The evaluation results show that, compared with the baseline, RBC can improve PCM memory system performance by up to 9.4% and 5.4% on average. Combing CPAR with RBC (RBC+CPAR) can improve performance by up to 19.0% and 12.1% on average. Moreover, RBC and RBC+CPAR can reduce memory energy consumption by 8.3% and 6.6% on average, respectively.

As the scaling of applications increases, the demand of main memory capacity increases in order to serve large working set. It is difficult for DRAM (dynamic random access memory) based memory system to satisfy the memory capacity requirement due to its limited scalability and high energy consumption. Compared to DRAM, PCM (phase change memory) has better scalability, lower energy leakage, and non-volatility. PCM memory systems have become a hot topic of academic and industrial research. However, PCM technology has the following three drawbacks: long write latency, limited write endurance, and high write energy, which raises challenges to its adoption in practice. This paper surveys architectural research work to optimize PCM memory systems. First, this paper introduces the background of PCM. Then, it surveys research efforts on PCM memory systems in performance optimization, lifetime improving, and energy saving in detail, respectively. This paper also compares and summarizes these techniques from multiple dimensions. Finally, it concludes these optimization techniques and discusses possible research directions of PCM memory systems in future.

Phase change memory (PCM) is a promising nonvolatile memory to reform current commercial computing system. Inhibiting face-centered cubic (f-) to hexagonal (h-) phase transition of Ge2Sb2Te5 (GST) thin film is essential for realizing high-density, high-speed, and low-power PCM. Although the atomic configurations of f- and h-lattices of GST alloy and the transition mechanisms have been extensively studied, the real transition process should be more complex than previous explanations, e.g. vacancy-ordering model for f-to-h transition. In this study, dynamic crystallization procedure of GST thin film was directly characterized by in situ heating transmission electron microscopy. We reveal that the equilibrium to h-phase is more like an abnormal grain growth process driven by surface energy anisotropy. More specifically, [0001]-oriented h-grains with the lowest surface energy grow much faster by consuming surrounding small grains, no matter what the crystallographic reconfigurations would be on the frontier grain-growth boundaries. We argue the widely accepted vacancy-ordering mechanism may not be indispensable for the large-scale f-to-h grain growth procedure. The real-time observations in this work contribute to a more comprehensive understanding of the crystallization behavior of GST thin film and can be essential for guiding its optimization to achieve high-performance PCM applications.

The scaling-down of phase change memory cell is critical to achieve high-performance and high-density memory devices. Herein, we report that Ge2Sb2Te5 nanoparticles along the [1 1 1] direction were synthesized without templates or etching in a molecular beam epitaxy system. Under non-stoichiometric Ge:Sb:Te beam ratio condition, the growth of high-density Ge2Sb2Te5 nanoparticles was achieved by Zn-doping. The average diameter of the nanoparticles is 8 nm, and the full width at half maximum of the size distribution is 2.7 nm. Our results suggest that the size and shape modifications of Ge2Sb2Te5 nanoparticles could be induced by Zn-doping which influences the nucleation in the growth process. In addition, the bonding states of Zn and Te verified by X-ray photoelectron spectroscopy proved that Zn atoms located in the Ge2Sb2Te5 matrix. This approach exemplified here can be applied to the sub-20 nm phase change memory devices and may also be extendable to be served in the design and development of more materials with phase transitions.