Phase Change Memory Devices Research Articles

Crystallization kinetics of Sn doped Ge20Te80−xSnx (0 ≤ x ≤ 4) chalcogenide glassy alloys

Chalcogenide semiconductors have evolved as multifunctional materials due to their fascinating thermal, optical, electrical and mechanical properties. In this report, Ge20Te80−xSnx (0 ≤ x ≤ 4) glassy alloys are systematically studied in order to understand the effect of variation of Sn content on the thermal parameters such as glass transition (Tg), onset crystallization (Tc), peak crystallization (Tp), melting temperature (Tm), activation energy of glass transition (Eg), and crystallization (Ec). The values of Eg are calculated from the variation of Tg with the heating rate (α), according to Kissinger and Moynihan model, while the values of Ec are calculated from the variation of Tp with the heating rate (α), according to Kissinger, Takhor, Augis-Bennett and Ozawa model. Thermal stability and glass forming ability (GFA) are discussed for understanding the applicability of the synthesized materials in phase change memory (PCM) applications. Thermal parameters are correlated with the electrical switching studies to get an insight into the phase change mechanism. The results of the calculated thermal parameters reveal that the GFA of the synthesized Ge20Te80−xSnx (0 ≤ x ≤ 4) glassy alloys has a synchronous relationship with their thermal properties studied through differential scanning calorimetry, indicating their potential for phase-change memory device applications.

Scalable CGeSbTe-based phase change memory devices employing U-shaped cells

Phase change memory (PCM) that is operated on resistance changes caused by joule heating has been suggested as the next-generation memory for scaling since its programming current scales linearly. We propose a U-shaped cell design to further reduce the reset current in PCM devices, which enables easier and more efficient scaling than conventional PCMs. Simulation studies of heat transfer demonstrated that our U-shaped design with a dashed heater has a higher thermal efficiency of 4.97K/μA compared to 3.36K/μA in a lance cell with a ring heater for the same storage node. The reset current can be better scaled proportionate to k2.0 in which the exponent is higher than the lance cell of k1.5 in non-isotropic scaling. This better scalability is attributed to the small programming volume of the U-shaped cell, which was verified by transmission electron microscopy analysis. Furthermore, the cyclic endurance of the U-shaped cell was enhanced by 1 order of magnitude compared to a lance cell and the thinner CGeSbTe films reduced the reset current further. Our results show that a U-shaped cell is a highly promising design to scale reset current in next-generation PCM devices.

(Invited) Synaptic Plasticity in a Memristive Device below 500mV

While machine learning algorithms are being successfully employed for a wide variety of high level cognitive tasks, their wide-spread adoption on mobile platforms faces challenges because of the large network sizes and extremely long training times involved in learning underlying features of user-dependent data. In this paper, we first discuss the requirements and target specifications of nanoscale devices that could enable hardware platforms supporting on-chip, real-time learning based on third generation spiking neural networks. We then survey some recent developments from the field of emerging non-volatile memory technologies such as chalcogenide based phase change memory and other memristive devices that could be used to build cognitive learning systems. We will then discuss how we have implemented synaptic plasticity in a Cu/SiO2/W memristive device at voltages below 500 mV using bio-mimetic waveforms. These devices hold promise to herald the next generation of information processing systems that are inspired by the brain.

Electrochemical Nucleation and Growth of Binary Antimony Telluride Compounds

Whereas antimony telluride based compound materials have been demonstrated to be suitable for phase change memory devices,[1] among other applications, and are well suited to be fabricated via an electrochemical route, the ability to electrodeposit high quality thin films of these materials requires a good understanding of the nucleation and growth behavior that defines the initial stages of growth of these films.[2-4] Although electrochemical nucleation has been studied for decades,[5] little attention has been paid to the behavior of codeposition systems, where two or more materials are deposited simultaneously from a single electrolyte bath. In this study, the nucleation of Sb-Te compound is studied on gold and glassy carbon electrodes, to determine how the nucleation behavior of each element contributes to that of the overall system. Additionally, the effect of applied potential and electrolyte concentrations on the nucleation behavior and deposit composition is studied. Cyclic voltammetry coupled with quartz crystal microbalance measurements are used to extensively study the deposition processes that occur during compound electrodeposition. It is seen that deposited Te is further reduced to a soluble species near the potentials required to deposit Sb, however the presence of Sb in the deposited film seems to hinder the further reduction to a soluble species, enabling compound film growth (figure 1). Additionally, the free energy released by compound formation allows the compound to be formed at potentials positive to those normally required for Sb deposition. Chronoamperometry, along with TEM-EDS analysis of the composition gradient of individual nucleation sites will be presented and discussed in conjunction with the CVs to understand the nucleation and growth of binary Sb-Te compounds. REFERENCES 1. Raoux, S.; Wuttig, M., Phase Change Materials: Science and Applications. New York : Springer Dec. 2008: 2008. 2. Huang, Q.; Kellock, A. J.; Raoux, S., Electrodeposition of SbTe Phase-Change Alloys. Journal of The Electrochemical Society 2008, 155(2), D104-D109. 3. Catrangiu, A. S.; Sin, I.; Prioteasa, P.; Cotarta, A.; Cojocaru, A.; Anicai, L.; Visan, T., Studies of antimony telluride and copper telluride films electrodeposition from choline chloride containing ionic liquids. Thin Solid Films 2016, 611, 88-100. 4. Jung, H.; Myung, N. V., Electrodeposition of antimony telluride thin films from acidic nitrate-tartrate baths. Electrochimica Acta 2011, 56(16), 5611-5615. 5. Gunawardena, G.; Hills, G.; Montenegro, I.; Scharifker, B., Electrochemical nucleation. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1982, 138 (2), 225-239. Figure 1

Molecular beam epitaxial growth of oriented and uniform Ge2Sb2Te5 nanoparticles with compact dimensions

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.

First-principles calculation of lattice thermal conductivity in crystalline phase change materials: GeTe, Sb2Te3 , and Ge2Sb2Te5

Thermal transport is a key feature for the operation of phase change memory devices which rest on a fast and reversible transformation between the crystalline and amorphous phases of chalcogenide alloys upon Joule heating. In this paper we report on the ab initio calculations of bulk thermal conductivity of the prototypical phase change compounds Ge 2 Sb 2 Te 5 and GeTe in their crystalline form. The related Sb 2 Te 3 compound is also investigated for the sake of comparison. Thermal conductivity is obtained from the solution of the Boltzmann transport equation with phonon scattering rates computed within density functional perturbation theory. The calculations show that the large spread in the experimental data on the lattice thermal conductivity of GeTe is due to a variable content of Ge vacancies which at concentrations realized experimentally can halve the bulk thermal conductivity with respect to the ideal crystal. We show that the very low thermal conductivity of hexagonal Ge 2 Sb 2 Te 5 of about 0.45 W m −1 K −1 measured experimentally is also resulting from disorder in the form of a random distribution of Ge/Sb atoms in one sublattice.

Electrophysical Properties of Ge–Sb–Te Thin Films for Phase Change Memory Devices

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.

Inverse heat conduction problem in a phase change memory device

An invers heat conduction problem is solved considering the thermal investigation of a phase change memory device using the scanning thermal microscopy. The heat transfer model rests on system identification for the probe thermal impedance and on a finite element method for the device thermal impedance. Unknown parameters in the model are then identified using a nonlinear least square algorithm that minimizes the quadratic gap between the measured probe temperature and the simulated one.

Synthesis, Characterization, and Catalytic Performance of Sb2Se3 Nanorods

Antimony selenide has many potential applications in thermoelectric, photovoltaic, and phase-change memory devices. A novel method is described for the rapid and scalable preparation of antimony selenide (Sb2Se3) nanorods in the presence of hydrazine hydrate and/or permanganate at 40°C. Crystalline nanorods are obtained by the addition of hydrazine hydrate in a reaction mixture of antimony acetate and/or chloride and sodium selenite in neutral and basic media, while amorphous nanoparticles are formed by the addition of KMnO4 in a reaction mixture of antimony acetate/chloride and sodium selenite. The powder X-ray diffraction pattern confirms orthorhombic phase crystalline Sb2Se3 for the first and second reactions with lattice parameters a=1.120 nm, b=1.128 nm, and c=0.383 nm and amorphous Sb2Se3 for the third reaction. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) images show the diameter of nanorods for the first and second reactions to be in the order of 100 nm to 150 nm and about 20 nm particles for the third reaction. EDX and XPS suggest that the nanorods are pure Sb2Se3. The UV-vis analysis indicates a band gap of 4.14 and 4.97 eV for the crystalline and amorphous Sb2Se3, respectively, corresponding to a blue shift. The photocatalytic study shows that the decolorization of Rhodamine in solution by nanoparticles is slightly greater than nanorods.

Material functionalities from molecular rigidity: Maxwell’s modern legacy

Abstract

Nanosecond Switching in Superlattice-Like GeTe/Sb Thin Film for High Speed and Low Power Phase Change Memory Application

Superlattice-like GeTe/Sb thin films were investigated for their potential application in phase change memory. Compared with monolayer GeTe thin film, GeTe/Sb thin film has lower crystallization temperature (208°C), crystallization activation energy (2.38 eV) and thermal conductivity (0.15 W/(m·K)). In GeTe/Sb thin film, GeTe and Sb phases exist independently. The superlattice-like structure is observed even after annealing at 250°C for 10 min. The shorter phase change time is achieved for GeTe/Sb thin film (4.3 ns). The GeTe/Sb-based phase change memory devices show an operation speed of as fast as 9 ns and a low RESET operation power of 4.09 × 10−13 J.

Can conventional phase-change memory devices be scaled down to single-nanometre dimensions?

The scaling potential of ‘mushroom-type’ phase-change memory devices is evaluated, down to single-nanometre dimensions, using physically realistic simulations that combine electro-thermal modelling with a Gillespie Cellular Automata phase-transformation approach. We found that cells with heater contact sizes as small as 6 nm could be successfully amorphized and re-crystallized (RESET and SET) using moderate excitation voltages. However, to enable the efficient formation of amorphous domes during RESET in small cells (heater contact diameters of 10 nm or less), it was necessary to improve the thermal confinement of the cell to reduce heat loss via the electrodes. The resistance window between the SET and RESET states decreased as the cell size reduced, but it was still more than an order of magnitude even for the smallest cells. As expected, the RESET current reduced as the cells got smaller; indeed, RESET current scaled with the inverse of the heater contact diameter and ultra-small RESET currents of only 19 μA were achieved for the smallest cells. Our results show that the conventional mushroom-type phase-change cell architecture is scalable and operable in the sub-10nm region.

Superlattice-like Ga40Sb60/Sb films with ultra-high speed and low power for phase change memory application

Superlattice-like (SLL) Ga40Sb60/Sb (GSS) is proposed for its ultra-high speed, low power for phase change application. The excellent properties derives from the properly designed SLL structure of Sb layers with high speed and low crystallization temperature (Tc) inserting high Tc Ga40Sb60. Appropriate Tc ~ 220 °C and ultra-long data retention (151 °C for 10 years) is obtained in the SLL [Ga40Sb60(5 nm)/Sb(4 nm)]5 thin films. X-ray diffraction analysis shows that only Sb phase exists in the SLL GSS thin films and the low surface roughness is confirmed by AFM measurement. Moreover, the phase change memory devices based on Ge2Sb2Te5 (GST) and SLL [Ga40Sb60(5 nm)/Sb(4 nm)]5 thin films were fabricated and the set and reset operation was studied. The current–voltage measurement for set operations shows the threshold switching voltage (2.0 V) is smaller than the GST (4.4 V). The resistance–voltage tests indicate the reversible phase change could be realized by a pulse as short as 10 ns and the reset power of the [Ga40Sb60(5 nm)/Sb(4 nm)]5 cell calculated as 2.3 × 10−11 J is 6.5 times lower than the GST cell 1.5 × 10−10 J.

Thermally Tunable Ultrasensitive Infrared Absorption Spectroscopy Platforms Based on Thin Phase-Change Films

The thermal tunability of the optical and electrical properties of phase-change materials has enabled the decades-old rewritable optical data storage and the recently commercialized phase-change memory devices. Recently, phase-change materials, in particular, Ge2Sb2Te5 (GST), have been considered for other thermally configurable photonics applications, such as active plasmonic surfaces. Here, we focus on nonplasmonic field enhancement and demonstrate the use of the phase-change materials in ultrasensitive infrared absorption spectroscopy platforms employing interference-based uniform field enhancement. The studied structures consist of patternless thin GST and metal films, enabling simple and large-area fabrication on rigid and flexible substrates. Crystallization of the as-fabricated amorphous GST layer by annealing tunes (redshifts) the field-enhancement wavelength range. The surfaces are tested with ultrathin chemical and biological probe materials. The measured absorption signals are found to be compa...

Low power phase change memory switching of ultra-thin In3Sb1Te2 nanowires

We report on the fabrication and electrical characterization of phase change memory (PCM) devices formed by In3Sb1Te2 chalcogenide nanowires (NWs), with diameters as small as 20 nm. The NWs were self-assembled by metal organic chemical vapor deposition via the vapor–liquid–solid method, catalyzed by Au nanoparticles. Reversible and well reproducible memory switching of the NWs between low and high resistance states was demonstrated. The conduction mechanism of the high resistance state was investigated according to a trap-limited model for electrical transport in the amorphous phase. The size of the amorphized portion of the NW and the critical electric field for the transition to the low resistance state were evaluated. The In3Sb1Te2 NW-based devices showed very low working parameters, such as RESET voltage (∼3 V), current (∼40 μA), and power (∼130 μW). Our results indicated that the studied NWs are suitable candidates for the realization of ultra-scaled, high performance PCM devices.

Training a Probabilistic Graphical Model With Resistive Switching Electronic Synapses

Current large scale implementations of deep learning and data mining require thousands of processors, massive amounts of off-chip memory, and consume gigajoules of energy. Emerging memory technologies such as nanoscale two-terminal resistive switching memory devices offer a compact, scalable and low power alternative that permits on-chip co-located processing and memory in fine-grain distributed parallel architecture. Here we report first use of resistive switching memory devices for implementing and training a Restricted Boltzmann Machine (RBM), a generative probabilistic graphical model as a key component for unsupervised learning in deep networks. We experimentally demonstrate a 45-synapse RBM realized with 90 resistive switching phase change memory (PCM) elements trained with a bio-inspired variant of the Contrastive Divergence (CD) algorithm, implementing Hebbian and anti-Hebbian weight updates. The resistive PCM devices show a two-fold to ten-fold reduction in error rate in a missing pixel pattern completion task trained over 30 epochs, compared to untrained case. Measured programming energy consumption is 6.1 nJ per epoch with the resistive switching PCM devices, a factor of ~150 times lower than conventional processor-memory systems. We analyze and discuss the dependence of learning performance on cycle-to-cycle variations as well as number of gradual levels in the PCM analog memory devices.

Extracting the temperature distribution on a phase-change memory cell during crystallization

Phase-change memory (PCM) devices are enabled by amorphization- and crystallization-induced changes in the devices' electrical resistances. Amorphization is achieved by melting and quenching the active volume using short duration electrical pulses (∼ns). The crystallization (set) pulse duration, however, is much longer and depends on the cell temperature reached during the pulse. Hence, the temperature-dependent crystallization process of the phase-change materials at the device level has to be well characterized to achieve fast PCM operations. A main challenge is determining the cell temperature during crystallization. Here, we report extraction of the temperature distribution on a lateral PCM cell during a set pulse using measured voltage-current characteristics and thermal modelling. The effect of the thermal properties of materials on the extracted cell temperature is also studied, and a better cell design is proposed for more accurate temperature extraction. The demonstrated study provides promising results for characterization of the temperature-dependent crystallization process within a cell.

Pulse voltage induced phase change characteristics of the ZnxSbyTez phase-change prototype device

ZnxSbyTez thin films are deposited on quartz or glass substrates by the electron beam evaporation technique in an ultra-high vacuum. A prototype phase change memory device using the ZST (ZnxSbyTez) thin film is fabricated. The current–voltage test results of the device show the threshold voltage of ZST531 (Zn5.18Sb3.75Te1.10 at.%) is 2.4 V, which is similar to that of the device based on pure Ge2Sb2Te5. It is shown that the phase-change device with the ZST film is able to perform several reading and writing cycles and the off/on resistance ratio is nearly 10 under pulse voltage. The switching performance of the device is also investigated. Most importantly, the results of the in situ resistance measurements show that the increase of crystallization temperature and the higher 10-year data retention temperature are as high as 300 °C and 191 °C, respectively. This indicates that the ZnxSbyTez material is quite stable, and thus appropriate for use in phase-change memory.

Atomic Migration Induced Crystal Structure Transformation and Core-Centered Phase Transition in Single Crystal Ge2Sb2Te5 Nanowires.

A phase change nanowire holds a promise for nonvolatile memory applications, but its transition mechanism has remained unclear due to the analytical difficulties at atomic resolution. Here we obtain a deeper understanding on the phase transition of a single crystalline Ge2Sb2Te5 nanowire (GST NW) using atomic scale imaging, diffraction, and chemical analysis. Our cross-sectional analysis has shown that the as-grown hexagonal close-packed structure of the single crystal GST NW transforms to a metastable face-centered cubic structure due to the atomic migration to the pre-existing vacancy layers in the hcp structure going through iterative electrical switching. We call this crystal structure transformation "metastabilization", which is also confirmed by the increase of set-resistance during the switching operation. For the set to reset transition between crystalline and amorphous phases, high-resolution imaging indicates that the longitudinal center of the nanowire mainly undergoes phase transition. According to the atomic scale analysis of the GST NW after repeated electrical switching, partial crystallites are distributed around the core-centered amorphous region of the nanowire where atomic migration is mainly induced, thus potentially leading to low power electrical switching. These results provide a novel understanding of phase change nanowires, and can be applied to enhance the design of nanowire phase change memory devices for improved electrical performance.

Time-temperature-transformation and continuous-heating-transformation diagrams of GeSb2Te4 from nanosecond-long ab initio molecular dynamics simulations

We use ab initio molecular dynamics (AIMD) simulations to build an amorphous model of GeSb2Te4 and nanosecond-long annealing AIMD simulations to construct its Time-Temperature-Transformation (TTT) and Continuous-Heating-Transformation (CHT) diagrams. The critical cooling rate for amorphous GeSb2Te4, derived from the “nose” of the TTT diagram, is in the range 470–1100 K/ns. The Kissinger analysis of the non-isothermal crystallization kinetics suggests that it follows a non-Arrhenius crystallization behavior in which the crystallization temperature is upshifted by the heating rates. These results, from unprecedentedly long AIMD simulations, provide a detailed picture of the complex crystallization of GST alloys at the extremely small time and length scales relevant to phase change memory devices.