Current Controlled Relaxation Oscillations in Ge_2Sb_2Te_5-Based Phase Change Memory Devices

The relaxation oscillation of the phase change memory （PCM） devices based on the Ge2Sb2Te5 material is investigated by applying square current pulses. The current pulses with different amplitudes could be accurately given by the independently designed current testing system. The relaxation oscillation across the PCM device could be measured using an oscilloscope. The oscillation duration decreases with time, showing an inner link with the shrinking threshold voltage Vth. However, the relaxation oscillation would not terminate until the remaining voltage Von reaches the holding voltage Vh. This demonstrates that the relaxation oscillation might be controlled by Von. The increasing current amplitudes could only quicken the oscillation velocity but not be able to eliminate it, which indicates that the relaxation oscillation might be an inherent behavior for the PCM cell.

Threshold switching uniformity in In_2Se_3 nanowire-based phase change memory

The uniformity of threshold voltage and threshold current in the In2 Se3 nanowire-based phase change memory （PCM） devices is investigated. Based on the trap-limited transport model, amorphous layer thickness, trap density, and trap depth are considered to clarify their influences upon the threshold voltage and threshold current through simulations.

Thermal effect of Ge_2Sb_2Te_5 in phase change memory device

In the fabrication of phase change random access memory （PRAM） devices, high temperature thermal processes are inevitable. We investigate the thermal stability of GezSb2Te5 （GST） which is a prototypical phase change material. After high temperature process, voids of phase change material exist at the interface between Ge2Sb2Te5 and substrate in the initial open memory cell. This lower region of GezSb2Te5 is found to be a Te-rich phase change layer. Phase change memory devices are fabricated in different process conditions and examined by scanning electron microscopy and energy dispersive X-ray. It is found that hot-chuck process, nitrogen-doping process, and lower temperature inter-metal dielectric （IMD） deposition process can ease the thermal impact of line-GST PRAM cell.

A review of compact modeling for phase change memory

Phase change memory(PCM)attracts wide attention for the memory-centric computing and neuromorphic comput-ing.For circuit and system designs,PCM compact models are mandatory and their status are reviewed in this work.Macro mod-els and physics-based models have been proposed in different stages of the PCM technology developments.Compact model-ing of PCM is indeed more complex than the transistor modeling due to their multi-physics nature including electrical,thermal and phase transition dynamics as well as their interactions.Realizations of the PCM operations including threshold switching,set and reset programming in these models are diverse,which also differs from the perspective of circuit simulations.For the purpose of efficient and reliable designs of the PCM technology,open issues and challenges of the compact modeling are also discussed.

Characteristics of Sb6Te4/VO2 Multilayer Thin Films for Good Stability and Ultrafast Speed Applied in Phase Change Memory

The Sb6 Te4/VO2 multilayer thin films are prepared by magnetron sputtering and the potential application in phase change memory is investigated in detail. Compared with Sb6 Te4, Sb6 Te4/VO2 multilayer composite thin films have higher phase change temperature and crystallization resistance, indicating better thermal stability and less power consumption. Also, Sb6 Te4/VO2 has a broader energy band of 1.58 eV and better data retention （125℃ for 103/）. The crystallization is suppressed by the multilayer interfaces in Sbf Te4/VO2 thin film with a smaller rms surface roughness for Sbf Te4/VO2 than monolayer Sb4Te6. The picosecond laser technology is applied to study the phase change speed. A short crystallization time of 5.21 ns is realized for the Sb6Te4 （2nm）/VO2 （8nm） thin film. The Sb6 Te4/VO2 multilayer thin film is a potential and competitive phase change material for its good thermal stability and fast phase change speed.

Crystallization Process of Superlattice-Like Sb/SiO_2 Thin Films for Phase Change Memory Application

After compositing with SiO_2 layers, it is shown that superlattice-like Sb/SiO_2 thin films have higher crystallization temperature（~240°C）, larger crystallization activation energy（6.22 e V）, and better data retention ability（189°C for 10 y）. The crystallization of Sb in superlattice-like Sb/SiO_2 thin films is restrained by the multilayer interfaces. The reversible resistance transition can be achieved by an electric pulse as short as 8 ns for the Sb（3 nm）/SiO_2（7 nm）-based phase change memory cell. A lower operation power consumption of 0.09 m W and a good endurance of 3.0 × 10~6 cycles are achieved. In addition, the superlattice-like Sb（3 nm）/SiO_2（7 nm） thin film shows a low thermal conductivity of 0.13 W/（m·K）.

A Survey of Phase Change Memory Systems

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.

Endurance characteristics of phase change memory cells

The endurance characteristics of phase change memory are studied. With operational cycles, the resis- tances of reset and set states gradually change to the opposite direction. What is more, the operational conditions that are needed are also discussed. The thilure and the changes are concerned with the compositional change of the phase change material. An abnormal phenomenon that the threshold voltage decreases slightly at first and then increases is observed, which is due to the coaction of interthce contact and growing active volume size changing.

Crystallization kinetics of Sn_(40)Se_(60) thin films for phase change memory applications

The crystallization kinetics of Sn4oSe6o thin films has been successfully investigated using sheet resis- tance versus temperature measurements. Thermal evaporation was used to deposit the films on ordinary glass sub- strates. The crystallization temperature for Sn4oSe60 thin film was found to be 156.6 -t- 0.3 ~C. In the as-deposited state, the sheet resistance was found to be 195 Mf2/[2, this value declined to 1560 f2/[2] upon annealing. The value of activation energy obtained from the Kissinger plot was 0.62 ＋ 0.07 eV. From the results obtained, SnaoSe60 is a promising alloy for PCM application because of its high electrical contrast, high crystallization temperature, and relatively high activation energy.

Improving the Performance and Energy Efficiency of Phase Change Memory Systems

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.

Three-Dimensional Simulations of RESET Operation in Phase-Change Random Access Memory with Blade-Type Like Phase Change Layer by Finite Element Modeling

An optimized device structure for reducing the RESET current of phase-change random access memory （PCRAM） with blade-type like （BTL） phase change layer is proposed. The electrical thermal analysis of the BTL cell and the blade heater contactor structure by three-dimensional finite element modeling are compared with each other during RESET operation. The simulation results show that the programming region of the phase change layer in the BTL cell is much smaller, and thermal electrical distributions of the BTL cell are more concentrated on the TiN/GST interface. The results indicate that the BTL cell has the superiorities of increasing the heating efficiency, decreasing the power consumption and reducing the RESET current from 0.67mA to 0.32mA. Therefore, the BTL cell will be appropriate for high performance PCRAM device with lower power consumption and lower RESET current.

Investigation and solution of low yield problem for phase change memory with lateral fully-confined structure

This paper mainly focuses on solving the low yield problem for lateral phase change random access memory with a fully confined phase change material node. Improper over-etching and bad step-coverage of physical vapor deposition were the main reasons for the poor contact quality, which leads to the low yield problem. Process improvement was carried out to better control over-etching within 10 nm. Atomic layer deposition process was used to replace physical vapor deposition to guarantee good step coverage. Contrasting cross-sectional photos taken by scanning electron microscopy showed great improvement in contact quality. The atom layer deposition process was demonstrated to have good prospects in nano-contact for phase change memory application.

Set Programming Method and Performance Improvement of Phase Change Random Access Memory Arrays

A novel slow-down set waveform is proposed to improve the set performance and a 1 kb phase change random access memory chip fabricated with a 13nm CMOS technology is implemented to investigate the set performance by different set programming strategies based on this new set pulse. The amplitude difference （I1 - I2） of the set pulse is proved to be a crucial parameter for set programming. We observe and analyze the cell characteristics with different I1 - I2 by means of thermal simulations and high-resolution transmission electron microscopy, which reveal that an incomplete set programming will occur when the proposed slow-down pulse is set with an improperly high I1 - I2. This will lead to an amorphous residue in the active region. We also discuss the programming method to avoid the set performance degradations.

Polarization readout analysis for multilevel phase change recording by crystallization degree modulation

Four different states of Si15Sb85 and Ge2Sb2Te5 phase change memory thin films are obtained by crystallization degree modulation through laser initialization at different powers or annealing at different temperatures. The polarization characteristics of these two four-level phase change recording media are analyzed systematically. A simple and effective readout scheme is then proposed, and the readout signal is numerically simulated. The results show that a high-contrast polarization readout can be obtained in an extensive wavelength range for the four-level phase change recording media using common phase change materials. This study will help in-depth understanding of the physical mechanisms and provide technical approaches to multilevel phase change recording.

650 ps SET speed in Ge_(2)Sb_(2)Te_(5)phase change memory induced by TiO_(2) dielectric crystal plane

Crystallization speed of phase change material is one of the main obstaclesfor the application of phase change memory(PCM)as storage classmemory in computing systems,which requires the combination ofnonvolatility with ultra-fast operation speed in nanoseconds.Here,wepropose a novel approach to speed up crystallization process of the onlycommercial phase change chalcogenide Ge_(2)Sb_(2)Te_(5)(GST).By employingTiO_(2)as the dielectric layer in phase change device,operation speed of650 ps has been achieved,which is the fastest among existing representativePCM,and is comparable to the programing speed of commercialdynamic random access memory(DRAM).Because of its octahedralatomic configuration,TiO_(2)can provide nucleation interfaces for GST,thus facilitating the crystal growth at the determinate interface area.Ti–O–Ti–O four-fold rings on the(110)plane of tetragonal TiO_(2)is critical forthe fast-atomic rearrangement in the amorphous matrix of GST thatenables ultra-fast operation speed.The significant improvement of operationspeed in PCM through incorporating standard dielectric materialTiO_(2)in DRAM paves the way for the application of phase change memoryin high performance cache-type data storage.

Comprehensive study of the ultrafast photoexcited carrier dynamics in Sb_(2)Te_(3)–GeTe superlattices

Chalcogenide superlattices Sb_(2)Te_(3)-GeTe is a candidate for interfacial phase-change memory(iPCM) data storage devices.By employing terahertz emission spectroscopy and the transient reflectance spectroscopy together,we investigate the ultrafast photoexcited carrier dynamics and current transients in Sb_(2)Te_(3)-GeTe superlattices.Sample orientation and excitation polarization dependences of the THz emission confirm that ultrafast thermo-electric,shift and injection currents contribute to the THz generation in Sb_(2)Te_(3)-GeTe superlattices.By decreasing the thickness and increasing the number of GeTe and Sb_(2)Te_(3) layer,the interlayer coupling can be enhanced,which significantly reduces the contribution from circular photo-galvanic effect(CPGE).A photo-induced bleaching in the transient reflectance spectroscopy probed in the range of~1100 nm to~1400 nm further demonstrates a gapped state resulting from the interlayer coupling.These demonstrates play an important role in the development of iPCM-based high-speed optoelectronic devices.

Phase transformation in Mg Sb_3Te thin films

Mg-doped Sb3Te films are proposed to improve the performance of phase-change memory （PCM）. We prepare Mg- doped Sb3Te films and investigate their crystallization behaviors, structural, optical and electrical properties. We find that Mg-doping can increase the crystallization temperature, enhance the activation energy, and improve the 10-year data retention of Sb3Te. Especially Mg25.19（Sb3Te）74.81 shows higher Tc （~ 190℃） and larger Ea （~ 3.49 eV）, which results in a better data retention maintaining for 10 yr at ~ 112 ℃. Moreover Ra/Rc value is also improved. These excellent properties make Mg-Sb-Te material a promising candidate for the phase-change memory （PCM）.

Exploiting write power asymmetry to improve phase change memory system performance

Phase change memory （PCM） is a promising can- didate to replace DRAM as main memory, thanks to its bet- ter scalability and lower static power than DRAM. However, PCM also presents a few drawbacks, such as long write la- tency and high write power. Moreover, the write commands parallelism of PCM is restricted by instantaneous power con- straints, which degrades write bandwidth and overall perfor- mance. The write power of PCM is asymmetric： writing a zero consumes more power than writing a one. In this paper, we propose a new scheduling policy, write power asymme- try scheduling （WPAS）, that exploits the asymmetry of write power. WPAS improves write commands parallelism of PCM memory without violating power constraint. The evaluation results show that WPAS can improve performance by up to 35.5%, and 18.5% on average. The effective read latency can be reduced by up to 33.0%, and 17.1% on average.

The“gene”of reversible phase transformation of phase change materials:Octahedral motif

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℃ten-year data retention,6 ns SET speed,one order of magnitude longer life time and 75%reduced power consumption compared with traditional Ge_(2)Sb_(2)Te_(5)(GST)devices.It is of great significance to use it in 3D PCRAM chip and multi-level brain-inspired computing chip in the future.

Direct observation of metastable face-centered cubic Sb2Te3 crystal

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 memorv device.