Nonvolatile Phase Change Research Articles

Electric-Field-Driven Topological Phase Switching and Skyrmion-Lattice Metastability in Magnetoelectric Cu2OSeO3

Due to their topological protection and nanometric size, magnetic skyrmions are anticipated to form components of new high-density memory technologies. In metallic systems skyrmion manipulation is achieved easily under a low density electric current flow, although the inevitable thermal dissipation ultimately limits the energy efficacy of potential applications. On the other hand, a near dissipation-free skyrmion and skyrmion phase manipulation is expected by using electric \emph{fields}, thus meeting better the demands of an energy-conscious society. In this work on an insulating chiral magnet Cu$_{2}$OSeO$_{3}$ with magnetoelectric coupling, we use neutron scattering to demonstrate directly i) the creation of metastable skyrmion states over an extended range in magnetic field and temperature, and ii) the in-situ electric field-driven switching between topologically distinct phases; the skyrmion phase and a competing non-topological cone phase. For our accessible electric field range, the phase switching is achieved in a high temperature regime, and the remnant (E=0) metastable skyrmion state is thermally volatile with an exponential lifetime on hour timescales. Nevertheless, by taking advantage of the demonstrably longer-lived metastable skyrmion states at lower temperatures, a truly non-volatile and near dissipation-free topological phase change memory function is promised in magnetoelectric chiral magnets.

A Magnetoresistance Induced by a Nonzero Berry Phase in GeTe/Sb2Te3 Chalcogenide Superlattices

AbstractThe chalcogenide alloy Ge–Sb–Te (GST) has not only been used in rewritable digital versatile discs, but also in nonvolatile electrical phase change memory as a key recording material. Although GST has been believed for a long time not to show magnetic properties unless doped with magnetic impurities, it has recently been reported that superlattices (SLs) with the structure [(GeTe)L(Sb2Te3)M]N (where L, M, and N are usually integers) have a large magnetoresistance at room temperature for particular combinations of L and M. Here it is reported that when [(GeTe)L(Sb2Te3)M]N chalcogenide SL films are thermally annealed at 470 K and cooled down to room temperature under an external magnetic field accompanied by current pulse injections, a large magnetoresistance change (>2500 Ω) is induced. This study shows that the phenomenon has a strong correlation with the GeTe thickness and the periodic structure of the SL films, and that it is induced by the structural phase transition between electrically nonpolar and polar phases in the GeTe layers in the SLs. This study proposes that the relationship between the polar (ferroelectric) phase and the Berry curvature in the SLs is responsible for the magnetoresistance change.

Element-specific amorphization of vacancy-ordered GeSbTe for ternary-state phase change memory

Abstract GeSbTe alloys have the ability of rapidly transforming between amorphous and crystalline phases. Therefore, they can be used in the non-volatile phase change memory. Recently, a vacancy-ordered cubic Ge 2 Sb 2 Te 5 ( VOC GST) phase change material where the vacancies are highly ordered in the (111) plane, has been experimentally demonstrated by STEM. However, studies are mainly on the structural characterization, rather than on the phase change behavior and possible applications of the VOC GST. Here, using first-principles molecular dynamic simulations, we study the melt-quenched amorphization process and its possible applications. We find that the VOC GST exhibits a quasi-two-dimensional amorphization process that is triggered by the diffusion of Ge atoms but not others. A partial amorphous ( P-amor ) phase is obtained, which can act as an intermediate state between the pure amorphous and pure crystalline phases for possible ternary-state data storage.

Properties of Ti–Sb–Te doped with SbSe alloy for application in nonvolatile phase change memory

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

Tunable split-ring resonators using germanium telluride

We demonstrate terahertz (THz) split-ring resonator (SRR) designs with incorporated germanium telluride (GeTe) thin films. GeTe is a chalcogenide that undergoes a nonvolatile phase change from the amorphous to crystalline state at approximately 200 °C, depending on the film thickness and stoichiometry. The phase change also causes a drop in the material's resistivity by six orders of magnitude. In this study, two GeTe-incorporated SRR designs were investigated. The first was an SRR made entirely out of GeTe and the second was a gold SRR structure with a GeTe film incorporated into the gap region of the split ring. These devices were characterized using THz time-domain spectroscopy and were heated in-situ to determine the change in the design operation with varying temperatures.

CHF3/O2-Based Plasma Reactive Ion Etching of GeTe for Nonvolatile Phase Change Memory

The reactive-ion-etching (RIE) characteristics of phase change material GeTe in CHF3/O2 plasma for back end of nonvolatile phase change memory devices were investigated in this paper. The etch rate and surface root-mean-square (RMS) roughness of crystalline GeTe and amorphous GeTe films were studied with various etching parameters, such as gas mixture ratio, chamber gas pressure, and power. It is found that the etch rate first increases with the increasing concentration of O2 and get at peak value 68.5 nm/min at 12% O2/(CHF3+O2), then decreases from 12% to 20% of O2 fraction. The surface RMS roughness slightly increases with the increasing oxygen content. By changing the gas pressure, the etch rate of GeTe films increases approximately linearly with increasing gas pressure up to 50 mTorr. In addition, the etch rate increases approximately linearly with increasing power below 400 W. The XPS spectra shows that some non-volatile GeFx, GeOx, TeFx, and TeOx by-products were left on the etched film surface. From these results of etched TiN films and GeTe films, the result of RIE with one step recipe shows the formation of under-cut on the profile of etched film. However, the result of RIE with two steps recipe shows better vertical characteristic and few residues were left on the profile of etched film.

Influence of indium doping on the electrical properties of Ge2Sb2Te5 thin films for nonvolatile phase change memory devices

In this article the influence of different amounts of In (0, 0.5, 1 and 3 wt. %) on the thermal and electrical properties of Ge2Sb2Te5 thin films for nonvolatile phase change memory devices is investigated. Crystallization temperature, resistivity, width of the optical band gap, Urbach energy, activation energy and type of conductivity are estimated for all investigated compounds. Storage and data processing times of the PCM cells on the basis of investigated materials were calculated. Nonmonotonic concentration dependences of properties were observed.

B12-O-05Towards Dynamic Electron Holographic Analysis of Solid State Electrochemical Devices at Operating Condition

Chalcogenide glasses have been used in non-volatile phase change random access memory owing to their property of rapid change from amorphous phase to crystalline. These materials generally exhibit a threshold switching behavior, which is reversible transition between two states. It is of great importance to choose suitable chalcogenide material because threshold switching controls switching voltage, and reading and writing speed. There are two predominant models to explain threshold switching mechanism. One is electrical transition model [1] and the other is structural transition model [2]. In this study, crystallization behavior of carbon doped Ge2Sb2Te5 (C-GST) under electrical pulse was analyzed by in-situ electrical probing transmission electron microscopy (TEM). The TEM sampling of thin multi-layer (TiN/C-GST/TiN) film was carried out with lift-out technique of focused ion beam. The current-voltage curves were acquired with electrical sweep of 0~4.5 voltage. Direct observation of the microstructure transition with electrical biasing provides a clear understanding of the threshold switching characteristics.

The role of atomic vacancies on phonon confinement in α-GeTe

Atomic defects and their dynamics play a vital role in controlling the behavior of non-volatile phase change memory materials used in advanced optical storage devices. Synthesis and structural analysis by XRD and Raman spectroscopy on α-GeTe single crystal with different sizes are reported. The spectroscopic measurements on micron and nano sized α-GeTe single crystal reveal the evolution of phonon confinement with crystal sizes of few hundred nanometers. The characteristic vibrational modes of bulk α-GeTe structure are found to downshift and asymmetrically broaden to lower frequency with decreasing the single crystal size. We attribute the observed downshift of Raman lines in α-GeTe is largely due to the presence of high concentration of atomic vacancies. The crystal size and temperature dependent Raman spectra provide explicitly the dynamics of vacancies on optical phonon confinement in α-GeTe structure. Thus, the observed large concentration of vacancies and their size dependency might influence the phase change phenomenon in GeTe based alloys.

Heterogeneous Crystallization of the Phase Change Material GeTe via Atomistic Simulations

Phase change materials are the active compounds in optical disks and in nonvolatile phase change memory devices. These applications rest on the fast and reversible switching between the amorphous and the crystalline phases, which takes place in the nano domain in both the time and the length scales. The fast crystallization is a key feature for the applications of phase change materials. In this work, we have investigated by means of large scale molecular dynamics simulations the crystal growth of the prototypical phase change compound GeTe at the interface between the crystalline and the supercooled liquid reached in the device upon heating the amorphous phase. A neural network interatomic potential, markedly faster with respect to first-principles methods, allowed us to consider high-symmetry crystalline surfaces as well as polycrystalline models that are very close to the actual geometry of the memory devices. We have found that the crystal growth from the interface is dominant at high temperatures while it is competing with homogeneous crystallization in the melt at lower temperatures. The crystal growth velocity markedly depends on the crystallographic plane exposed at the interface, the (100) surface being kinetically dominant with respect to the (111) surface. Polycrystalline interfaces, representative of realistic conditions in phase change memory devices, grow at significantly slower pace because of the presence of grain boundaries.

RESET Distribution Improvement of Phase Change Memory: The Impact of Pre-Programming

Some nonvolatile phase change memory (PCM) cells with 80-nm heating electrodes are found very difficult to RESET at 3 mA, which directly affects the RESET distribution of the PCM. The large crystal grains with hexagonal structure in the active phase change area, discovered by transmission electron microscope, are the major reason. One preprogramming testing method is introduced, and the resistance distributions of the PCM cells before and after preprogramming are presented. Results show that the large hexagonal crystal grains have been eliminated, thus the resistance distributions have been greatly improved after preprogramming.

Role of the nano amorphous interface in the crystallization of Sb2Te3 towards non-volatile phase change memory: insights from first principles.

The nano amorphous interface is important as it controls the phase transition for data storage. Yet, atomic scale insights into such kinds of systems are still rare. By first-principles calculations, we obtain the atomic interface between amorphous Si and amorphous Sb2Te3, which prevails in the series of Si-Sb-Te phase change materials. This interface model reproduces the experiment-consistent phenomena, i.e. the amorphous stability of Sb2Te3, which defines the data retention in phase change memory, and is greatly enhanced by the nano interface. More importantly, this method offers a direct platform to explore the intrinsic mechanism to understand the material function: (1) by steric effects through the atomic "channel" of the amorphous interface, the arrangement of the Te network is significantly distorted and is separated from the p-orbital bond angle in the conventional phase-change material; and (2) through the electronic "channel" of the amorphous interface, high localized electrons in the form of a lone pair are "projected" to Sb2Te3 from amorphous Si by a proximity effect. These factors set an effective barrier for crystallization and improve the amorphous stability, and thus data retention. The present research and scheme sheds new light on the engineering and manipulation of other key amorphous interfaces, such as Si3N4/Ge2Sb2Te5 and C/Sb2Te3, through first-principles calculations towards non-volatile phase change memory.

Thermally efficient and highly scalable In2Se3 nanowire phase change memory

The electrical characteristics of nonvolatile In2Se3 nanowire phase change memory are reported. Size-dependent memory switching behavior was observed in nanowires of varying diameters and the reduction in set/reset threshold voltage was as low as 3.45 V/6.25 V for a 60 nm nanowire, which is promising for highly scalable nanowire memory applications. Also, size-dependent thermal resistance of In2Se3 nanowire memory cells was estimated with values as high as 5.86×1013 and 1.04×106 K/W for a 60 nm nanowire memory cell in amorphous and crystalline phases, respectively. Such high thermal resistances are beneficial for improvement of thermal efficiency and thus reduction in programming power consumption based on Fourier's law. The evaluation of thermal resistance provides an avenue to develop thermally efficient memory cell architecture.

Phase change memory based on SnSe4 alloy

Abstract A phase change alloy has been synthesized and characterized. The reversible phase transitions between amorphous and crystalline states of SnSe 4 films have been studied using variable electrical pulses and X-ray diffraction. Temperature dependent sheet resistance measurements have shown two distinct resistivity states of more than two orders of magnitude. This high electrical contrast makes the alloy suitable for nonvolatile phase change memory applications. X-ray diffraction has attributed the large electrical contrast to an amorphous–crystalline phase transition. The nonvolatile memory cells have been fabricated using a simple sandwich structure (metal/chalcogenide thin film/metal). A threshold voltage of 3.71 V has been determined for this phase change random access memory cell. Memory switching was initiated using the voltage pulses of 3.71 V, 90 ns, 1.3 V and 26 μs, for the crystallization and amorphization process, respectively.

Electrical conduction in chalcogenide glasses of phase change memory

Amorphous chalcogenides have been extensively studied over the last half century due to their application in rewritable optical data storage and in non-volatile phase change memory devices. Yet, the nature of the observed non-ohmic conduction in these glasses is still under debate. In this review, we consolidate and expand the current state of knowledge related to dc conduction in these materials. An overview of the pertinent experimental data is followed by a review of the physics of localized states that are peculiar to chalcogenide glasses. We then describe and evaluate twelve relevant transport mechanisms with conductivities that depend exponentially on the electric field. The discussed mechanisms include various forms of Poole-Frenkel ionization, Schottky emission, hopping conduction, field-induced delocalization of tail states, space-charge-limited current, field emission, percolation band conduction, and transport through crystalline inclusions. Most of the candidates provide more or less satisfactory fits of the observed non-linear IV data. Our analysis calls upon additional studies that would enable one to discriminate between the various alternative models.

Understanding on the current-induced crystallization process and faster set write operation thereof in non-volatile phase change memory

We experimentally demonstrate that the crystallization process of Ge-Sb-Te crystallites during the set operation in non-volatile phase change memory commences after threshold switching event. It is also shown that the nucleation and growth rates have opposite behaviors with the increase of set operation power: the incubation time in nucleation stage can be minimized at higher power, whereas the percolation time in growth stage is smaller at lower power. Based on these results, we introduce a two-step set pulse of high-power nucleation and low-power growth making the set write operation much faster than conventional simple rectangular or slow-quenched form.

Atomic Layer Deposition of Antimony and its Compounds Using Dechlorosilylation Reactions of Tris(triethylsilyl)antimony

For the first time an element other than a metal was deposited by atomic layer deposition (ALD). Pure and conformal thin films of elemental antimony were prepared by ALD using SbCl3 and (Et3Si)3Sb as precursors. In situ reaction mechanism studies showed that the dehalosilylation reactions involved are very efficient in eliminating the ligands from the growing surface enabling the use of low growth temperatures down to 95 °C. Various antimony compounds, such as GeSb, Sb2Te, GaSb, and AlSb, can also be deposited by reacting (Et3Si)3Sb with other metal halides or mixing Sb growth cycles with other ALD processes. The new antimony ALD process is a major step in the realization of non-volatile phase change random access memories (PCRAM) and ALD of III−V compounds.

Nanosecond switching in GeTe phase change memory cells

The electrical switching behavior of GeTe-based phase change memory devices is characterized by time resolved experiments. SET pulses with a duration of less than 16 ns are shown to crystallize the material. Depending on the resistance of the RESET state, the minimum SET pulse duration can even be reduced down to 1 ns. This finding is attributed to the increasing impact of crystal growth upon decreasing switchable volume. Using GeTe or materials with similar crystal growth velocities, hence promises nonvolatile phase change memories with dynamic random access memorylike switching speeds.

Nanoscaling of Phase Change Memory Cells for High Speed Memory Applications

We have explored a different method to increase the phase switching speed for the nanocell of the non-volatile phase change random memories. The correlation between the nanocell size and the switching speed has been investigated theoretically, and the ultrafast phase switching has been demonstrated experimentally. The ultrafast switching mechanisms are discussed which are due to the contribution of free carriers and defects at the material interface when the cell dimension is sufficiently small. The nanoscaling effects of the phase change materials not only reduce the programming power, but also provide a new approach to enhance switching speed, which is highly essential for the realization of universal memory devices.

Low Temperature Deposition of Ge Thin Films for Phase Change Memory

The deposition of Ge-Sb-Te (GST) chalcogenide alloys in high aspect-ratio trench structures via CVD and ALD-like processes has attracted great attention for their application in next generation non-volatile Phase Change Random Access Memory (PRAM) devices. Several Ge (II) silylamido precursors were synthesized and characterized, and their behaviors in thermal CVD processes explored. Of these, a novel Ge precursor - bis (2,2,5,5-tetramethyl-2,5-disila-1-azacyclopentanide) germanium (II) was found to give a deposition rate of 1 Aå/min at 320 ºC by using NH3 as a co-reactant gas. Deposition rates below 0.1 Aå/min at these temperatures were obtained in the presence of H2 or an inert gas, which means this precursor exhibits the co-reactant limited deposition which is desirable for good step-coverage and uniform film growth in ALD. Data will be presented on deposition rates vs. temperature using this precursor and compared with two other bis(silylamido) germanium precursors. Preliminary results on the deposition of GeTe binary films will also be presented.