Phase-change Random Access Memory Devices Research Articles

High performance antimony-rich RexSb3Te for phase-change random access memory applications

Phase-change random access memory (PCRAM) demands for high-performance phase change materials, including operation speed, data retention, resistance drift etc. In this work, a high performance PCRAM device based on antimony-rich RexSb3Te material was put forward and studied. Excellent data retention, fast operation speed, low resistance drift, low power consumption and good cycle characteristics were achieved. The lattice information, bonding properties and surface morphology of the films were characterized by transmission electron microscopy, Raman spectroscopy and atomic force microscopy. Density functional theory calculations show that the Re dopant is more inclined to replace Sb and forms chemical bonds with Te, and the Re-Te bonds are more stable than the previous Sb-Te bonds, which is conducive to the formation of stable precursors to accelerate nucleation. The research shows that the antimony-rich Re0.18Sb3Te material has good overall performance, which provides a strategy for developing high-performance memory.

Phase-change heterostructure with HfTe2 confinement sublayers for enhanced thermal efficiency and low-power operation through Joule heating localization

Although phase-change random-access memory (PCRAM) is a promising next-generation nonvolatile memory technology, challenges remain in terms of reducing energy consumption. This is primarily because the high thermal conductivities of phase-change materials (PCMs) promote Joule heating dissipation. Repeated phase transitions also induce long-range atomic diffusion, limiting the durability. To address these challenges, phase-change heterostructure (PCH) devices that incorporate confinement sublayers based on transition-metal dichalcogenide materials have been developed. In this study, we engineered a PCH device by integrating HfTe2, which has low thermal conductivity and excellent stability, into the PCM to realize PCRAM with enhanced thermal efficiency and structural stability. HEAT simulations were conducted to validate the superior heat confinement in the programming region of the HfTe2-based PCH device. Moreover, electrical measurements of the device demonstrated its outstanding performance, which was characterized by a low RESET current (∼1.6 mA), stable two-order ON/OFF ratio, and exceptional cycling endurance (∼2 × 107). The structural integrity of the HfTe2 confinement sublayer was confirmed using X-ray photoelectron spectroscopy and transmission electron microscopy. The material properties, including electrical conductivity, cohesive energy, and electronegativity, substantiated these findings. Collectively, these results revealed that the HfTe2-based PCH device can achieve significant improvements in performance and reliability compared with conventional PCRAM devices.

Suppressing Structural Relaxation in Nanoscale Antimony to Enable Ultralow-Drift Phase-Change Memory Applications.

Phase-change random-access memory (PCRAM) devices suffer from pronounced resistance drift originating from considerable structural relaxation of phase-change materials (PCMs), which hinders current developments of high-capacity memory and high-parallelism computing that both need reliable multibit programming. This work realizes that compositional simplification and geometrical miniaturization of traditional GeSbTe-like PCMs are feasible routes to suppress relaxation. While to date, the aging mechanisms of the simplest PCM, Sb, at nanoscale, have not yet been unveiled. Here, this work demonstrates that in an optimal thickness of only 4nm, the thin Sb film can enable a precise multilevel programming with ultralow resistance drift coefficients, in a regime of ≈10-4 -10-3 . This advancement is mainly owed to the slightly changed Peierls distortion in Sb and the less-distorted octahedral-like atomic configurations across the Sb/SiO2 interfaces. This work highlights a new indispensable approach, interfacial regulation of nanoscale PCMs, for pursuing ultimately reliable resistance control in aggressively-miniaturized PCRAM devices, to boost the storage and computing efficiencies substantially.

Set/Reset Bilaterally Controllable Resistance Switching Ga‐doped Ge2Sb2Te5 Long‐Term Electronic Synapses for Neuromorphic Computing

AbstractLong‐term plasticity of bio‐synapses modulates the stable synaptic transmission that is quite related to the encoding of information and its emulation using electronic hardware is one of important targets for neuromorphic computing. Ge2Sb2Te5 (GST) based phase change random access memory (PCRAM) has become a strong candidate for complementary‐metal‐oxide‐semiconductor (CMOS) compatible integrated long‐term electronic synapses to cope with the high‐efficient and low power consumption data processing tasks for neuromorphic computing. However, the performance of PCRAM electronic synapses is still quite limited due to the challenges in linear and continuous conductance regulation, which originates from the fast and uncontrollable resistance switching characteristic of conventional PCRAM for the data storage application. Here an in‐depth study is reported on the impact of gallium (Ga) doping on GST (GaGST) structural properties and on the corresponding 0.13 µm CMOS technology fabricated PCRAM integrated devices with a mushroom structure. The Ga doping effectively retarded the crystallization process of GST by augmenting the local disorder of GeTe4‐nGen tetrahedron, which subsequently leads to the Set/Reset bilaterally controllable resistance switching of corresponding PCRAM devices. The optimized 6.5%GaGST electronic synapses demonstrate gradual resistance switching characteristics and a good multilevel retention feature and eventually exhibit outstanding long‐term synaptic plasticity like potentiation/depression and spiking time dependent plasticity in four forms. Such long‐term electronic synapses are applied to handwritten digits recognition (96.22%) and CIFAR‐10 image categorization (93.6%) and attain very high accuracy for both tasks. These results provide an effective method to achieve high performance PCRAM electronic synapses and highlight the great potential of GaGST PCRAM as a component for future high‐performance neuromorphic computing.

The Effect of Carbon Doping on the Crystal Structure and Electrical Properties of Sb2Te3.

As a new generation of non-volatile memory, phase change random access memory (PCRAM) has the potential to fill the hierarchical gap between DRAM and NAND FLASH in computer storage. Sb2Te3, one of the candidate materials for high-speed PCRAM, has high crystallization speed and poor thermal stability. In this work, we investigated the effect of carbon doping on Sb2Te3. It was found that the FCC phase of C-doped Sb2Te3 appeared at 200 °C and began to transform into the HEX phase at 25 °C, which is different from the previous reports where no FCC phase was observed in C-Sb2Te3. Based on the experimental observation and first-principles density functional theory calculation, it is found that the formation energy of FCC-Sb2Te3 structure decreases gradually with the increase in C doping concentration. Moreover, doped C atoms tend to form C molecular clusters in sp2 hybridization at the grain boundary of Sb2Te3, which is similar to the layered structure of graphite. And after doping C atoms, the thermal stability of Sb2Te3 is improved. We have fabricated the PCRAM device cell array of a C-Sb2Te3 alloy, which has an operating speed of 5 ns, a high thermal stability (10-year data retention temperature 138.1 °C), a low device power consumption (0.57 pJ), a continuously adjustable resistance value, and a very low resistance drift coefficient.

Effect of vacancy ordering on the grain growth of Ge2Sb2Te5 film

Ge2Sb2Te5 (GST) is the most widely used matrix material in phase change random access memory (PCRAM). In practical PCRAM device, the formed large hexagonal phase in GST material is not preferred, especially when the size of storage architecture is continually scaling down. In this report, with the aid of spherical-aberration corrected transmission electron microscopy (Cs-TEM), the grain growth behavior during the in situ heating process in GST alloy is investigated. Generally, the metastable face-centered-cubic (f-) grain tends to grow up with increasing temperature. However, a part of f-phase nanograins with {111} surface plane does not grow very obviously. Thus, the grain size distribution at high temperature shows a large average grain size as well as a large standard deviation. When the vacancy ordering layers forms at the grain boundary area in the nanograins, which is parallel to {111} surface plane, it could stabilize and refine these f-phase grains. By elaborating the relationship between the grain growth and the vacancy ordering process in GST, this work offers a new perspective for the grain refinement in GST-based PCRAM devices.

The Relationship between Electron Transport and Microstructure in Ge2Sb2Te5 Alloy.

Phase-change random-access memory (PCRAM) holds great promise for next-generation information storage applications. As a mature phase change material, Ge2Sb2Te5 alloy (GST) relies on the distinct electrical properties of different states to achieve information storage, but there are relatively few studies on the relationship between electron transport and microstructure. In this work, we found that the first resistance dropping in GST film is related to the increase of carrier concentration, in which the atomic bonding environment changes substantially during the crystallization process. The second resistance dropping is related to the increase of carrier mobility. Besides, during the cubic to the hexagonal phase transition, the nanograins grow significantly from ~50 nm to ~300 nm, which reduces the carrier scattering effect. Our study lays the foundation for precisely controlling the storage states of GST-based PCRAM devices.

Cr-doped Sb2Te materials promising for high performance phase-change random access memory

Transition metal doping is an effective strategy to improve the properties of phase-change random access memory (PCRAM), while the underlying mechanism remains to be sufficiently investigated. Herein, a Cr-doped Sb2Te film is fabricated by co-sputtering and the enhancement mechanism of Cr-doping is deeply investigated by experiments and first-principles calculations. The CrxSb2Te film exhibits excellent performances involving high crystallization temperature (238.9 °C) and good data retention (10 years @161.7 °C) for Cr0.56Sb2Te, low density change rate (3.8%) and high reversible switching speed (5 ns) for Cr0.29Sb2Te, providing a variety of options for different applications. The calculated results show that Cr atoms preferentially substitute the Sb1 site and the CrTe bonds display a stronger ionic bond character in contrast to the covalent bonded SbTe, thus improving the crystalline phase stability and crystallization temperature. Moreover, ab initio molecular dynamics simulations demonstrate that in the amorphous phase Cr atoms locate in defective octahedral coordination. Meanwhile, the homopolar SbSb wrong bond increased with Cr doping and the Cr atoms tend to form tight cluster in the amorphous phase which could benefit for the fast phase-change speed. Our present findings of Cr0.29Sb2Te PCRAM device clearly reveal the enhancement mechanism by Cr doping which provides guidance for developing high-performance PCRAM devices.

Boolean Logic Function Realized by Phase-Change Blade Type Random Access Memory

Phase-change random access memory (PCRAM) has been considered as one of the most promising candidates for future nonvolatile storage applications. Recent advent of the blade-type PCRAM further shed a light on its commercial prospect due to its much smaller energy consumption than the conventional Lance-type PCRAM. However, the logic computing capability of the blade-type PCRAM remains vague up-to-date. To deeply study its function versatility, we developed a comprehensive three-dimensional model based on the classical nucleation-growth theory to mimic the crystallization and amorphization processes of the blade-type PCRAM device. According to this well-established model, the device resistance after phase transformation was calculated with respect to different crystallization and amorphization pulses. By pertaining high and low pulse magnitudes and device resistances to appropriate input and output logic levels, a variety of Boolean logic functions, including “ NAND,” NOR,” AND,” OR,” and “ NOT,” were successfully achieved on one single PCRAM cell. The feasibility of simultaneously realizing storage and logic computing functions on the designed PCRAM device was therefore demonstrated. Such intriguing merit, combined with its superb storage traits, makes the proposed blade-type PCRAM cell suitable for current in-memory computing and neuromorphic circuits as the key components of future digital computers operated on non-von Neumann mode.

The origin of hexagonal phase and its evolution process in Ge2Sb2Te5 alloy

Ge2Sb2Te5 (GST) is the most important material for phase change random access memory (PCRAM) applications, while the formation of hexagonal (h-) phase results in low switching speed, large energy consumption, and worse endurance performance. Uncovering the formation mechanism of h-phase is beneficial for the further improvement of GST-based PCRAM devices. In this work, through advanced spherical aberration corrected transmission electron microscopy and transmission electron back-scattered diffraction technique, the mechanism of h-phase microstructure evolution is clearly clarified. We find that the vacancy ordering is more likely to appear around the grain boundary in a face-centered-cubic (f-) phase grain, which is the starting point for the generation of h-phase. More specifically, all the atoms in f-phase undergo a gradual shift into h-lattice positions to complete the f-to-h structural transition. By introducing an elemental dopant, for instance, carbon (C), the aggregation of C clusters prefers to distribute in the grain boundary area, which is the essential reason for postponing the generation and expansion of h-phase and greatly improving the thermal stability of C-GST material. In short, clarification of the origin of h-structure incubated from f-phase guides the optimization strategy of GST-based PCRAM devices.

Impact of Thermal Boundary Resistance on Thermoelectric Effects of the Blade-Type Phase-Change Random Access Memory Device

Phase-change random access memory (PCRAM) is widely regarded as one of the most promising candidates to replace Flash memory as the next generation of non-volatile memories due to its high-speed and low-power consumption characteristics. Recent advent of the blade-type PCRAM with low programming current merit further confirms its prospects. The thermoelectric effects existing inside the PCRAM devices have always been an important factor that determines the phase-transformation kinetics due to a fact that it allows PCRAM to have electric polarity dependent characteristics. However, the potential physics governing the thermoelectric effects for blade-type PCRAM device still remains vague. We establish a three-dimensional (3D) electro-thermal and phase-transformation model to study the influences of thermal boundary resistance (TBR) and device scaling on the thermoelectric effects of the blade-type PCRAM during its “RESET” operation. Our research shows that the presence of TBR significantly improves the electric polarity-dependent characteristics of the blade-type PCRAM, and such polarity-dependent characteristic is found immune to the scaling of the device. It is therefore possible to optimize the thermoelectric effects of the blade-type PCRAM through appropriately tailoring the TBR parameters, thus further lowering resulting power consumption.

Nano-composite phase-change antimony thin film for fast and persistent memory operations

Elemental Sb phase-change material (PCM) has to be diminished to nanometer in thickness to guarantee non-volatile memory storage. Integrating ultrathin Sb films poses great challenges in manufacturing high-density phase-change random-access memory (PCRAM) chips. Here, we introduce another feasible approach of utilizing nano-composite Sb–SiO2 PCM, for which we demonstrate superior phase-change properties of faster crystallization speed and better data retention ability over the conventional Ge2Sb2Te5 compound. These two seemingly conflicting features of the Sb–SiO2 are reconciled by an obvious fragile-to-strong crossover in crystallization kinetics of the supercooled Sb liquids via nanosize confinements, enabling the more suffocated atomic diffusion near room temperature and the comparable crystal growth at elevated temperatures to the Ge2Sb2Te5. The nanosize confinements also catalyze heterogeneous nucleation remarkably, therefore speeding up the SET operation of the Sb–SiO2-based PCRAM device by over two orders of magnitude as compared to the Ge2Sb2Te5-based one. In addition, owning the potentials to accomplish low-drift and long-life memory programming, the nano-composite Sb–SiO2 PCMs hold great promise to be employed for working memory and neuromorphic computing applications.

Reliable 2D Phase Transitions for Low-Noise and Long-Life Memory Programming

Extending cycling endurance and suppressing programming noise of phase-change random-access memory (PCRAM) are the key challenges with respect to the development of nonvolatile working memory and high-accuracy neuromorphic computing devices. However, the large-scale atomic migration along electrical pulse direction in the unconstrained three-dimensional phase transitions of the phase-change materials (PCMs) induces big resistance fluctuations upon repeated programming and renders the classic PCRAM devices into premature failure with limited cycling endurance. Previous efforts of superlattice-like and superlattice PCM schemes cannot effectively resolve such issues. In this work, we demonstrated that, through fine-tuning the sputtering techniques, a phase-change heterostructure (PCH) of Sb2Te3/TiTe2 can be successfully constructed. In contrast to its superlattice-like counterpart with inferior crystal quality, the well-textured PCH architecture ensures the reliable (well-confined) two-dimensional phase transitions, promoting an ultralow-noise and long-life operation of the PCRAM devices. Our study thus provides a useful reference for better manufacturing the PCH architecture and further exploring the excellent device performances and other new physics.

The importance of contacts in Cu2GeTe3 phase change memory devices

Cu2GeTe3 (CGT) is a promising phase change material for phase change random access memory (PCRAM) applications because of its high thermal stability in the amorphous phase and its capability to undergo rapid phase change. In this paper, the electrical conduction mechanism of a CGT memory device fabricated using W electrodes (W/CGT) was investigated using current–voltage (I–V) measurements and angle resolved hard x-ray photoelectron spectroscopy (AR-HAXPES). The I–V characteristics of the W/CGT memory device were found to display non-linear behavior in the RESET (amorphous) state, while linear behavior was observed in the SET (crystalline) state, indicating that the W/CGT memory device exhibited Schottky conduction in the RESET state, but Ohmic conduction in the SET state. The effective Schottky barrier height was found to increase linearly as the ideality factor decreased to unity with the ideal W/CGT Schottky barrier height in the RESET state estimated to be 0.33 eV, a value in good agreement with the directly measured Schottky barrier height of 0.35 eV between W and amorphous CGT by AR-HAXPES measurements. These results suggest that the interface between the metal electrode and the phase change material plays an important role in PCRAM devices, and its comprehensive understanding is necessary for future application development.

Monatomic 2D phase-change memory for precise neuromorphic computing

Phase-change random-access memory (PCRAM) is the leading candidate to overcome the intrinsic bottleneck in current von Neumann architecture by unifying computing with storage, enabling its prospect as cognitive memory. However, strong resistance drift in current Ge2Sb2Te5-based PCRAM devices obstructs the accomplishment of high-accuracy computing tasks that require stable multi-state programming. Moreover, the implementation of synaptic depression still remains highly challenging for current PCRAM devices due to the abrupt amorphization nature. We employed 4 nm-thick monatomic antimony (Sb) film to achieve reliable iterative RESET and cumulative SET operations simultaneously, with considerably lowered resistance drift and programming noise. The 2D Sb-based device is also capable of performing progressive RESET operations. Our work thus demonstrates the potential of monatomic 2D PCRAM for the future development of high-performance neuromorphic computing chips.

Microscopic Mechanism of Carbon-Dopant Manipulating Device Performance in CGeSbTe-Based Phase Change Random Access Memory.

Carbon (C)-doped Ge2Sb2Te5 material is a potential candidate in phase change random access memory (PCRAM) because of its superb thermal stability and ultrahigh cycle endurance. Unfortunately, the role and distribution evolution of C-dopant is still not fully understood, especially in practical industrial devices. In this report, with the aid of advanced spherical aberration corrected transmission electron microscopy, the mechanism of microstructure evolution manipulated by C-dopant is clearly defined. The grain-inner C atoms distinctly increase cationic migration energy barriers, which is the fundamental reason for promoting the thermal stability of metastable face-centered-cubic phase and postponing its transition to the hexagonal structure. By current pulses stimulation, the stochastic grain-outer C clusters tend to aggregate in the active area by breaking C-Ge bonding; thus, grain growth and elemental segregation are effectively suppressed to improve device reliability, for example, lower SET resistance, shorter SET time, and enlarged RESET/SET ratio. In short, the visual distribution variations of C-dopant can manipulate the performance of the PCRAM device, having much broader implications for optimizing its microstructure transition and understanding C-doped material system.

A thermodynamically-consistent multi-physics framework for crystallization of phase-change material

Phase Change Materials (PCMs) have a diverse set of technological applications, including phase-change random access memory devices (PCRAMs), and their behavior can be understood based on a complex set of coupled physical phenomena, e.g., heat transfer, phase transition, and thermoelectric effects. A detailed understanding of these phenomena and their interactions during the operation of a cell results in an efficient and compact design of the devices. This paper reports our development of a coupled, thermodynamically-consistent model to study the interplay among various parameters in PCRAMs, e.g., temperature, electrical current, thermoelectric effect, and crystal nucleation/growth. A Phase-Field (PF) model was developed to model the phase transition (PT). Also, we show that incorporating the heat of crystallization in heat transfer model results in identifying a self-heating mechanism in PCRAMs and consequently self-sustaining crystallization; crystallization continues even after the external voltage is turned off.

The effect of thickness on texture of Ge2Sb2Te5 phase-change films

Phase-change random access memory (PCRAM) has the advantages of nonvolatile, good scalability, high speed, low power, long life and compatibility with standard complementary metal oxide semiconductor (CMOS) process. The key material in PCRAM is phase-change material, which can greatly affect the performance of PCRAM device. Ge2Sb2Te5 (GST) is the most mature phase-change material and has been studied most widely at present. In this paper, GST films with thickness of 500 nm, 100 nm, 50 nm and 20 nm were studied by scanning electron microscope (SEM)–electron backscattering diffraction (EBSD) technique. According to the experiments, it is found that the crystal grain size has a tendency to grow larger and the GST film has a more obvious texture as its thickness decreases. During this process, surface energy plays an increasingly important role with the decrease in GST film thickness. Finally, (0001) plane with the highest work function and lowest surface energy leads to visible Z texture corresponding to film thickness. These results of regularity are helpful as the semiconductor industry today has the need for PCRAM devices with higher density and smaller size.

Exploring Cycle-to-Cycle and Device-to-Device Variation Tolerance in MLC Storage-Based Neural Network Training

A multilevel cell (MLC) memristor that provides high-density on-chip memory has become a promising solution for energy-efficient artificial neural networks (ANNs). However, MLC storage that stores multiple bits per cell is prone to device variation. In this paper, the device variation tolerance of ANN training is investigated based on our cell-specific variation modeling method, which focuses on characterizing realistic cell-level variation. The parameters of cycle-to-cycle variation (CCV) and device-to-device variation (DDV) are extracted separately from the experimental data of a 39-nm, 1-Gb phase-change random access memory (PCRAM) array. A quantized neural network designed for low bit-width (≥6-bit) training is used for simulations to demonstrate the potential of MLC storage. Our results demonstrate that training is more vulnerable to DDV than CCV, and CCV can even compensate for accuracy degradation caused by severe DDV. As a result, for a multilayer perceptron (MLP) on Modified National Institute of Standards and Technology (MNIST) database, 95% accuracy can be achieved with three MLC PCRAM devices per weight, which is a 40% reduction in the number of cells compared with using conventional single-level cells (SLCs). If the size of DDV is reduced by half, then only two cells, that is 60% fewer cells than using SLC, are needed.

Effect of Electrode Material on the Crystallization of GeTe Grown by Atomic Layer Deposition for Phase Change Random Access Memory.

The electrical switching behavior of the GeTe phase-changing material grown by atomic layer deposition is characterized for the phase change random access memory (PCRAM) application. Planar-type PCRAM devices are fabricated with a TiN or W bottom electrode (BE). The crystallization behavior is characterized by applying an electrical pulse train and analyzed by applying the Johnson–Mehl–Avrami kinetics model. The device with TiN BE shows a high Avrami coefficient (>4), meaning that continuous and multiple nucleations occur during crystallization (set switching). Meanwhile, the device with W BE shows a smaller Avrami coefficient (~3), representing retarded nucleation during the crystallization. In addition, larger voltage and power are necessary for crystallization in case of the device with W BE. It is believed that the thermal conductivity of the BE material affects the temperature distribution in the device, resulting in different crystallization kinetics and set switching behavior.