Phase Change Memory Applications Research Articles

Long-Range Forces in Rock-Salt-Type Tellurides and How they Mirror the Underlying Chemical Bonding.

Chemical bonding in main-group IV chalcogenides is an intensely discussed topic, easily understandable because of their remarkable physical properties that predestine these solid-state materials for their widespread use in, for instance, thermoelectrics and phase-change memory applications. The atomistic origin of their unusual property portfolio remains somewhat unclear, however, even though different and sometimes conflicting chemical-bonding concepts have been introduced in the recent years. Here, it is proposed that projecting phononic force-constant tensors for pairs of atoms along differing directions and ranges provide a suitable and quantitative descriptor of the bonding nature for these materials. In combination with orbital-based quantitative measures of covalency such as crystal orbital Hamilton populations (COHP), it is concluded that the well-established many-center and even n-center bonding is an appropriate picture of the underlying quantum-chemical bonding mechanism, supporting the recent proposal of hyperbonded phase-change materials.

Unraveling the Atomic Structure of Bulk Binary Ga-Te Glasses with Surprising Nanotectonic Features for Phase-Change Memory Applications.

Binary Ge-Te and ternary Ge-Sb-Te systems belong to flagship phase-change materials (PCMs) and are used in nonvolatile memory applications and neuromorphic computing. The working temperatures of these PCMs are limited by low-T glass transition and crystallization phenomena. Promising high-T PCMs may include gallium tellurides; however, the atomic structure and transformation processes for amorphous Ga-Te binaries are simply missing. Using high-energy X-ray diffraction and Raman spectroscopy supported by first-principles simulations, we elucidate the short- and intermediate-range order in bulk glassy GaxTe1-x, 0.17 ≤ x ≤ 0.25, following their thermal, electric, and optical properties, revealing a semiconductor-metal transition above melting. We also show that a phase change in binary Ga-Te is characterized by a very unusual nanotectonic compression with the high internal transition pressure reaching 4-8 GPa, which appears to be beneficial for PCM applications increasing optical and electrical contrast between the SET and RESET states and decreasing power consumption.

Investigation of Ru-doped Sb2Te alloy for high-speed and good thermal stability phase change memory applications

Investigation of Ru-doped Sb2Te alloy for high-speed and good thermal stability phase change memory applications

Crystallization mechanism and switching behavior of In–S–Sb phase change thin films

The crystallization mechanism of In2S3-doped Sb thin films is studied in detail to verify the potential application in phase change memory. Here, we observe directly that two different crystallization behaviors can exist in In–S–Sb thin layers by using aberration-corrected scanning transmission electron microscopy. The difference between Sb53.3(In2S3)46.7 and Sb30.9(In2S3)69.1 materials is induced by phase separation. The crystallization mechanism of the Sb53.3(In2S3)46.7 material is related to the formation of the nanocomposite structure with continuous precipitation of Sb nanocrystals. The crystallization characteristic of the Sb30.9(In2S3)69.1 material originates from the diffusion-driven In–S/In–S–Sb interface formation that acts as a “n–p” heterojunction, thereby resulting in the “depletion layer effect” and decreasing the carrier density to 7.42 × 1020 cm−3 at 280 °C. Sb30.9(In2S3)69.1 shows good bipolar-type resistance switching characteristics as the conventional Ge2Sb2Te5. This work provides clear experimental evidence to deepen the understanding of the crystallization mechanism for indium chalcogenides alloyed with Sb films, contributing to the improved control of the phase change behavior to establish high-performance multi-level nonvolatile memory and neuromorphic synaptic systems.

Crystallization Properties of Al-Sb Alloys for Phase Change Memory Applications

Material properties of Al-Sb binary alloy thin films deposited under ultra-high vacuum conditions were studied for multi-level phase change memory applications. Crystallization of this alloy was shown to occur in the temperature range of 180 °C–280 °C, with activation energy >2 eV. X-ray diffraction (XRD) from annealed alloy films indicates the formation of two crystalline phases, (i) an Al-doped A7 antimony phase, and (ii) a stable cubic AlSb phase. In-situ XRD analysis of these films show the AlSb phase crystalizes at a much higher temperature as compared to the A7 phase after annealing of the film to 650 °C. Mushroom cell structures formed with Al-Sb alloys on 120 nm TiN heater show a phase change material resistance switching behavior with reset/set resistance ratio >1000 under pulse measurements. TEM and in situ synchrotron XRD studies indicate fine nucleation grain sizes of ∼8–10 nm, and low elemental redistribution that is useful for improving reliability of the devices. These results indicate that Te-free Al-Sb binary alloys are possible candidates for analog PCM applications.

Carrier type induced magnetoresistance upending in assembly of sulfur doped Sb2Se3 nanotubes

Materials showing low field induced switch over in magnetoresistance is investigated heavily nowadays for promising power electronics and phase change memory applications. In this work, morphological, topographical, magnetoresistance (MR) and thermoelectric properties of nanobulk assembly of sulfur doped Sb2Se3nanotubes synthesized by novel microwave assisted solvothermal process is investigated and reported. Structural properties indicate that the sample crystallizes in orthorhombic phase while morphology study confirms the formation of nanotubes of length 1–5 μm and diameter 100–150 nm. Electrical resistivity measurements indicates 2D variable range hopping at low temperature (2–50 K), nearest neighbor hopping at intermediate temperature (50–200 K) and thermal activation behavior with activation energy of 2.3 meV from 200 K to 300 K. Interestingly the positive magnetoresistance observed in low magnetic field and at constant low temperatures makes a transition to negative magnetoresistance above 20 K. Bipolaron model along with wave function shrinkage models were invoked to understand the MR measurements while low temperature thermoelectric measurement univocally elucidates that the carrier type switching at 13 K dictates the conduction mechanism as demonstrated by the complex crossover in magnetoresistance.

Pulsed laser deposited stoichiometric GaSb films for optoelectronic and phase change memory applications

Phase-change memory (PCM) holds great potential in realizing the combination of DRAM-like speeds with non-volatility and large storage capacity for future electronic devices including in-memory computing. However, various (reliability) issues related to e.g. too high programming current (power consumption), resistance drift, data retention (low crystallization temperature), phase separation and density change upon switching stand in the way to make PCM really attractive. GaSb thin films have interesting optical and electrical properties which are attractive for optoelectronic and PCM applications but so far reported stoichiometric GaSb compositions are Sb-rich which produced reliability issues in PCM devices. In this study, we managed to deposit stoichiometric GaSb thin films using pulsed laser deposition (PLD) by varying deposition parameters and conditions. Using electron microscopy, the morphology of deposited films and target surface and the compositional deviation from exact stoichiometry have been investigated. We show that the directional nature of laser-target interaction is directly responsible for film quality in PLD in which particulates with high number density are generated due to directional pillar formation. Suppressing this pillar formation, by a simple 180° target rotation, showed an increase in deposition yield by 60%, exact stoichiometric transfer from target to substrate, and large reduction in particulate density. Moreover, from XRD analysis, we show that exact stoichiometric transfer from target to substrate is crucial for structural integrity of the produced films. Temperature induced structural transformation from resistivity vs. temperature measurements show a high crystallization temperature of 250 °C for stoichiometric GaSb thin film. We believe the exact stoichiometric GaSb thin films with reduced particulate densities and favorable structural and (opto)electronic properties are attractive for future PCM devices.

Designing artificial carbon clusters using Ge2Sb2Te5/C superlattice-like structure for phase change applications

Resistance drift and heat loss are two challenges in phase change memory. In this work, superlattice-like (SLL) Ge2Sb2Te5/C film was fabricated, which was designed to suppress the resistance drift and heat loss of the device. Ab initio molecular dynamics simulations were used to evaluate the evolution of C nanofilms at the atomic scale and most C atoms were found accumulated spontaneously as nanometer-scale carbon clusters. Long-range atomic migrations were restrained because of the artificial carbon clusters, which contributed to the resistance drift suppression. The fabricated carbon nanolayers were also believed to serve as an artificial thermal barrier, and an energy-inexpensive amorphization process based on the device has been achieved. With two-dimensional finite element analysis, we found that the suppressed heat dissipation in the programming regions were related to the low interfacial thermal conductance between the C and Ge2Sb2Te5. The Ge2Sb2Te5/C SLL thin film demonstrates potential for high-performance phase change memory applications.

Non-isothermal crystallization kinetics of Ge-Cu-Te thin films for phase-change memory

We investigated the crystallization kinetics of Ge-Cu-Te ternary system phase-change thin films using non-isothermal differential scanning calorimetry and estimated the kinetic triplet (activation energy, pre-exponential factor, and reaction model) by the combined application of isoconversional and model-fitting approaches at different heating rates over a range of 10–40 K/min for GeCu2Te2 thin films. The results indicate a multi-step crystallization process where the activation energy of crystallization is not constant but decreases via three recognizable stages with crystallization fraction α. The local Avrami exponents suggest that two crystallization mechanisms containing three-dimensional growth with increasing nucleation rates in the range of crystallization fraction α ≤ 0.4 and three- and two-dimensional growths at a decreasing nucleation rate over the wide range of α > 0.4 are responsible for the whole crystallization process. Furthermore, the reaction model displays dependence on the heating rates for the GeCu2Te2 thin films, where heating rates of 10–30 K/min drive a second-order reaction while a three-halve order reaction occurs for a heating rate of 40 K/min, which is different from the second-order reaction for the GeCu2Te3 thin films. Also, both GeCu2Te2 and GeCu2Te3 thin films provide better data retention capability than the Ge2Sb2Te5 thin films without sacrificing the crystallization time. The results indicate that Ge-Cu-Te ternary system thin films have great potentiality in phase-change memory applications.

Structural transformation and phase change properties of Se substituted GeTe

GeTe1−xSex (0 ≤ x ≤ 1.0) alloys have been prepared both in bulk and thin film forms to study the effect of selenium (Se) substitution for tellurium (Te) on the phase change properties. It is observed that with increasing Se substitution in GeTe, the structure transforms from rhombohdral structure to orthorhombic structure. Rietveld Refinement analysis support the phase transformation and show that the short and long bond lengths in crystalline GeTe decrease with increasing Se substitution but the rate of reduction of shorter bond length is more than the longer bond length. The GeTe1−xSex thin films undergo amorphous to crystalline phase change when annealed at high temperatures. The transition temperature shows an increasing trend with the Se substitution. The contrast in electrical resistivity between the amorphous and crystalline states is 104 for GeTe, and with the Se substitution, the contrast increases considerably to 106 for GeTe0.5Se0.5. Devices fabricated with thin films show that the threshold current decreases with the Se substitution indicating a reduction in the power required for WRITE operation. The present study shows that the crystalline structure, resistance, bandgap, transition temperature and threshold voltage of GeTe can be effectively controlled and tuned by the substitution of Te by Se, which is conducive for phase change memory applications.

Scalability of Sulfur‐Based Ovonic Threshold Selectors for 3D Stackable Memory Applications

A two‐terminal selector is an essential element for high‐density 3D stackable memory arrays. Suppressing sneak current, the device also needs to provide extremely high current density to melt‐quench the storage layer in phase‐change memory (PCM). Recently, an ovonic threshold switching (OTS) selector based on amorphous GeS demonstrated a large current density, which seems to be promising for 3D stackable PCM application. Herein, the scalability of GeS OTS selectors is investigated as the device scales down from 200 to 60 nm. Interestingly, the ON/OFF current, threshold voltage, and switching speed hardly change as the device scales down, whereas the ON current density exponentially grows. A large current density, ≈35.4 MA cm−2, is achieved in 60 nm‐sized devices. The high current density endows the GeS selector with great potential for use in high‐density 3D stackable memory applications.

Kinematics of glass to crystal phase transformation in novel multi-component glassy Se–Te–Sn–M (M = Sb, In, Cd) alloys

The kinetics of the thermally induced glass / crystal phase transformation of chalcogenide glasses plays an important role in determining their candidacy for optical phase change memory applications. The rate of crystallization and the corresponding activation energy are the two crucial kinetic parameters that reflect the durability and quality (i.e., storage properties) of phase change materials. This script deals with metal-induced effects on thermally regulated non-isothermal crystallization in a new glass alloy of Se-Te-Sn using calorimetric measurements. The elements Antimony (Sb), Cadmium (Cd) and Indium (In) have been used as structural modifiers for this purpose. The crystallization and glass transition kinetics of these glass alloys have been investigated by thermal analysis of several kinetic parameters such as the parameter of order n, the maximum crystallization temperature Tc, the crystallization rate K and the consequent activation energy Ec). A DSC is used in non-isothermal mode for the present studies. The values of the activation energy Ec are determined using the data obtained from the displacement of the exothermic peaks of crystallization in non-isothermal DSC plots at various heating rates. The role of the additives Sb, Cd and In in the variation in the rate of crystallization K of and the Avrami index (n) for each glass alloy is also examined. Detailed thermal analysis of the kinetic data confirms the superiority of Cd over the other two additives (In and Sb) for optimization of the kinetic properties of the main SeTeSn glass.

Exploring Phase‐Change Memory: From Material Systems to Device Physics

To deal with the growing demand for data storage and processing, phase‐change memory (PCM) provides one of the most promising candidates for next‐generation nonvolatile data storage and neuromorphic computing applications. A lot of effort has been made toward optimizing the materials and device design; thus, excellent device performances have been achieved including high density, fast switching speed, great endurance, and retention. In addition, the widely tunable optical characteristics of PCMs are irresistibly attractive for optoelectronic or all‐optical applications with unprecedented bandwidth, low energy consumption, and multilevel data storage. Herein, the materials system and switching mechanisms on experimental and modeling methods for PCM designs and applications are discussed. Electric‐domain and optical‐domain PCM‐based artificial synapses/neurons and their applications in neuromorphic computing are also reviewed. Finally, the future prospects and challenges of PCM‐based applications on materials, devices, algorithms, and system levels are highlighted.

Effect of Mo doping on phase change performance of Sb2Te3 * *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFB0701703 and 2017YFA0206101), the National Natural Science Foundation of China (Grant No. 61874151), and the Science and Technology Council of Shanghai, China (Grant Nos. 19JC1416801 and 19JC1416802).

Mo, as a dopant, is doped into SbTe to improve its thermal stability. It is shown in this paper that the Mo-doped Sb2Te3 (Mo0.26Sb2Te3, MST) material possesses phase change memory (PCM) applications. MST has better thermal stability than Sb2Te3(ST) and will crystallize only when the annealing temperature is higher than 250 °C. With the good thermal stability, MST-based PCM cells have a fast crystallization time of 6 ns. Furthermore, endurance up to 4 × 105 cycles with a resistance ratio of more than one order of magnitude makes MST a promising candidate for PCM applications.

Influence of Cu Doping in Si–Te-Based Chalcogenide Glasses and Thin Films: Electrical Switching, Morphological and Raman Studies

To understand the electrical switching behavior of Si15Te $_{85-{x}}$ Cu x ( $1\le {x} \le10$ ) series, ${I}$ – ${V}$ characterization has been performed on bulk as well as amorphous thin films of the as-prepared samples. Both the bulk glasses and amorphous thin films are found to manifest memory-type switching behavior. The threshold voltages of thin-film devices are found to be much lower than the bulk counterparts and hence could find application for phase change memory (PCM). The composition analyses of both have divulged the existence of intermediate phase (IP) in the composition range of $2 \le {x} \le6$ . To examine the probability of the given glass for PCM application, Set–Reset studies have been performed on the glasses with a triangular pulse of 6 mA for set operation and rectangular pulse of 12 mA for the reset operation. The study has revealed a continuous repetition of few Set–Reset cycle by the Si–Te–Cu series. Raman studies carried out on the bulk glasses report the occurrence of blue shift over the composition in a regular manner. Further, SEM studies have been carried out on Si–Te–Cu samples to understand the morphological changes that would have occurred during switching. Additionally, thickness dependence of threshold voltage of representative Si15Te80Cu5 and Si15Te76Cu9 glasses has been carried out to reveal the relationship between threshold voltage and thickness.

Rhenium doped Sb2Te phase change material with ultrahigh thermal stability and high speed

Sb2Te material is an important base material for phase change memory or optical data storage, but the poor amorphous stability of Sb2Te severely limits its application. In this work, we proposed a superhard and infusible Rhenium (Re) doped Sb2Te (RST) phase change material to improve the properties of Sb2Te. The data retention of RST was remarkably increased to 152 °C@10 years, which is much higher than those of traditional Ge2Sb2Te5 (87 °C@10 years) and Sb2Te (56 °C@10 years). The operation speed of the RST based device is as fast as 10 ns and the resistance ratio is more than 102 times. The density change of RST (4.2%) is smaller than those of Ge2Sb2Te5 (6.5%) and Sb2Te (5.7%). Furthermore, the operation voltage and power consumption of RST based device are much lower than those of Ge2Sb2Te5 and Sb2Te based devices. The results indicate that the RST is a promising material for high performance phase change memory or optical data storage applications.

Overview of the Role of Alloying Modifiers on the Performance of Phase Change Memory Materials

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-field modeling of the non-congruent crystallization of a ternary Ge–Sb–Te alloy for phase-change memory applications

The ternary alloy of germanium, antimony, and tellurium (GST) is widely used as a material for phase-change memories. In particular, the stoichiometric compound Ge2Sb2Te5 exhibits a rapid congruent crystallization. To increase the temperature at which spontaneous crystallization erases the stored information, alloys that are enriched in germanium have been investigated. Their crystallization is accompanied by segregation and eventually the nucleation of a new, germanium-rich phase. In order to model the redistribution of alloy components and the time evolution of the microstructure during device operations, we develop a multi-phase-field model for the crystallization of GST that includes segregation and couple it with orientation fields that describe the grain structure. We demonstrate that this model is capable to capture both the emergence of a two-phase polycrystalline structure starting from an initially amorphous material, and the melting and recrystallization during the SET and RESET operations in a memory cell of the “wall” type.

Crystallization properties of melt-quenched Ge-rich GeSbTe thin films for phase change memory applications

The crystallization process of melt quenched Ge-rich GeSbTe films, with composition optimized for memory applications, has been studied by optical reflectance measurements. The optical properties have been related to the structure and composition by means of the effective medium approximation. The compositional variations have been investigated by transmission electron microscopy and electron energy loss spectroscopy. Amorphous materials prepared by melt-quenching with different laser energy densities have been studied. For the energy density of 1.5 J cm−2, a uniform amorphous layer, with embedded Ge crystalline grains, is obtained. The film exhibits a crystallization temperature of 275 °C and no relevant phase separation during crystallization. For a lower energy density of 1 J cm−2, only half of the film thickness is quenched to the amorphous phase, with Ge depletion. The crystallization temperature of the Ge depleted film is 245 °C, and a partial phase separation occurs.

Sn tuned microstructure and phase-change characteristics of GeTe nanowires

Sn-doped GeTe (SGT) nanowires (NWs) were investigated systematically for use in phase-change memory (PCM) applications. Composition and microstructure characterizations indicate that SGT with ∼3.0% Sn (SGT_3.0) NWs preserves the GeTe rhombohedral (R) structure, whereas SGT with a Sn content of ∼25.0% (SGT_25.0) NWs exhibits a cubic (C) structure. R–C structural conversion of SGT NWs is revealed with increasing Sn content. According to ab initio calculations, optimizing doping leads to a decrease in density of states near the Fermi level and reduces electrical conductivity, and thereby, SGT_3.0 is more applicable for PCM than SGT_25.0, which is attributed to Sn-induced structural change that brings about a diversity in the electrical properties. Experimentally, SGT_3.0 NWs have two significant threshold switchings and ideal high/low resistance ratio (∼105). Compared with undoped GeTe, SGT_3.0 NWs experience an increase in crystalline resistance, in agreement with our theoretical calculations, perfectly satisfying the requirement of low programming currents for PCM.