Phase Change Memory Materials Research Articles

Thermal conductivity of (Ge2Sb2Te5)1−xCx phase change films

Germanium–antimony–telluride has emerged as a nonvolatile phase change memory material due to the large resistivity contrast between amorphous and crystalline states, rapid crystallization, and cyclic endurance. Improving thermal phase stability, however, has necessitated further alloying with optional addition of a quaternary species (e.g., C). Here, the thermal transport implications of this additional species are investigated using frequency-domain thermoreflectance in combination with structural characterization derived from x-ray diffraction and Raman spectroscopy. Specifically, the room temperature thermal conductivity and heat capacity of (Ge2Sb2Te5)1−xCx are reported as a function of carbon concentration (x≤0.12) and anneal temperature (T≤350°C) with results assessed in reference to the measured phase, structure, and electronic resistivity. Phase stability imparted by the carbon comes with comparatively low thermal penalty as materials exhibiting similar levels of crystallinity have comparable thermal conductivity despite the addition of carbon. The additional thermal stability provided by the carbon does, however, necessitate higher anneal temperatures to achieve similar levels of structural order.

Recipe for ultrafast and persistent phase-change memory materials

The contradictory nature of increasing the crystallization speed while extending the amorphous stability for phase-change materials (PCMs) has long been the bottleneck in pursuing ultrafast yet persistent phase-change random-access memory. Scandium antimony telluride alloy (ScxSb2Te3) represents a feasible route to resolve this issue, as it allows a subnanosecond SET speed but years of reliable retention of the RESET state. To achieve the best device performances, the optimal composition and its underlying working mechanism need to be unraveled. Here, by tuning the doping dose of Sc, we demonstrate that Sc0.3Sb2Te3 has the fastest crystallization speed and fairly improved data nonvolatility. The simultaneous improvement in such ‘conflicting’ features stems from reconciling two dynamics factors. First, promoting heterogeneous nucleation at elevated temperatures requires a higher Sc dose to stabilize more precursors, which also helps suppress atomic diffusion near ambient temperatures to ensure a rather stable amorphous phase. Second, however, enlarging the kinetic contrast through a fragile-to-strong crossover in the supercooled liquid regime should require a moderate Sc content; otherwise, the atomic mobility for crystal growth at elevated temperatures will be considerably suppressed. Our work thus reveals the recipe by tailoring the crystallization kinetics to design superior PCMs for the development of high-performance phase-change working memory technology.

Experimental observation of two-stage crystallization of Ge2Sb2Te5 amorphous thin films under the influence of a pulsed laser

The nature of crystallization in telluride based phase change memory materials is being actively discussed, and we present new experimental results on the effect of a pulsed laser treatment on the amorphous thin film crystallization of composition Ge2Sb2Te5 (GST225). It is shown that there are two ways of crystalline phase formation under femtosecond pulses excitation. At the moderate value of laser fluence, crystalline fraction forms inside the solid amorphous phase. This process is apparently two-stage, and this leads to formation of fine-grained polycrystalline material. The second way of crystallization proceeds at higher levels of the laser fluence, closer to the beam axis. In this case, larger grains are produced closer to the beam axis, and eventually no new nuclei can be formed due to high cooling velocity leading to inhibition of nucleation. The article also presents the results on the effect of the substrate type (metallic or oxide) and film thickness on crystallization, which is due to the light penetration depth and reflection from the film-substrate interface.

Characterization of low temperature crystal growth parameters of the growth-dominated phase change materials GeSb6Te

Phase change memory materials are semiconducting alloys used in optical- and resistivity-based memory devices that have large differences in optoelectronic properties between the amorphous and crystalline states. Alloys for memory applications must have a low crystallization rate at service temperatures so the written memory remains stable for long times. GeSb6Te may be attractive for use because of its rapid crystallization at high temperatures, but its low temperature crystallization behavior has not been fully assessed. Here, the crystallization parameters of amorphous films of GeSb6Te are characterized using in-situ hot stage optical microscopy under isothermal conditions for temperatures ranging from 185 °C to 195 °C. Individual grain sizes were tracked as well as the percent of material crystallized, from which the direct measurement of the crystal growth rate and Johnson-Mehl-Avrami-Kolmogorov parameters were calculated. GeSb6Te was found to have comparable growth and crystallization rates in the low temperature region as other growth-dominated phase change materials.

Vacancy formation energy and its connection with bonding environment in solid: A high-throughput calculation and machine learning study

The generation of the vacancy involving the bond breaking/re-formation occurs naturally in the material. Here, we present a framework for automatically computing the vacancy-formation energy (Ef) and for analyzing the bonding environment concealed in the Ef by using an artificial neural network (ANN). The ‘effective’ bonding that determines the energy of the system and the Ef will be clarified. The phase-change memory material GeTe is used as a case study. Firstly, 791 Ge-vacancy containing GeTe structures are studied and a large data set of the formation energy of the Ge-vacancy is obtained, which is helpful to understand the vacancy-induced issue of the amorphous GeTe including the resistance drift, etc. By using the ANN fitting based on the large energy data set, a bonding picture that is applicable to both the crystalline and the amorphous state of GeTe is predicted. In terms of the contribution to the formation energy of the vacancy, the weight ratio of the bond with length of 3.0–3.6 Å and 3.6–4.5 Å can be approximated as 6:1. The bonding information is further confirmed by using the first-principles electronic structure analysis on the randomly chosen samples. The bonding analysis using the ANN method based on a large vacancy-formation-energy data set is demonstrated to be a novel alternative technique to understand the bonding in the material. The proposed framework can be applied to a wide range of materials.

Grazing-incidence X-ray fluorescence analysis of thin chalcogenide materials deposited on Bragg mirrors

Advanced chalcogenide materials are key for state-of-the art memories, energy harvesting materials and photonics. The properties of thin chalcogenide layers are highly driven by their chemical composition, chemical depth-profiles and surface/interface effects. The combination of Grazing-Incidence X-ray Fluorescence (GIXRF) and X-ray Reflectometry (XRR) techniques allows for non-destructive access to such information, in the lab or at dedicated beamlines. However, the accuracy of the GIXRF-deduced composition slightly degrades with depth, as the enhancement of the XRF signal by the X-ray Standing Wave (XSW) field is usually limited to the first nanometers at the surface of the probed sample. In this paper, we suggest the use of (Mo/Si)*N Bragg mirrors, rather than bare silicon substrates for thin chalcogenide deposition, to improve the sensitivity of GIXRF/XRR analysis to small process-driven modifications. The aim of such multilayered substrates is to maintain high values of X-ray reflected intensity even at angles significantly higher than the critical angle, therefore generating an XSW-induced enhancement of the XRF signal not only at the surface but also in the depth of the layer of interest. The simulation of GIXRF-XRR data, collected at SOLEIL Metrology beamline and in the lab on Rigaku SmartLab diffractometer, on arsenic-free Ovonic Threshold Switch materials (Ge, Se, Sb) for advanced Phase Change memory (PCRAM) applications illustrates the interest of this approach, which is fully-compatible with numbers of PCRAM materials elaborated with low-temperature physical vapor deposition process.

Quench-rate and size-dependent behaviour in glassy Ge2Sb2Te5 models simulated with a machine-learned Gaussian approximation potential

Phase-change memory materials are promising candidates for beyond-silicon, next-generation non-volatile-memory and neuromorphic-computing devices; the canonical such material is the chalcogenide semiconductor alloy Ge2Sb2Te5. Here, we describe the results of an analysis of glassy molecular-dynamics models of this material, as generated using a newly developed, linear-scaling (O(N)), machine-learned, Gaussian approximation potential. We investigate the behaviour of the glassy models as a function of different quench rates (varied by two orders of magnitude, down to 1 K ps−1) and model sizes (varied by two orders of magnitude, up to 24 300 atoms). It is found that the lowest quench rate studied (1 K ps−1) is comparable to the minimum cooling rate needed in order completely to vitrify the models on quenching from the melt.

Y-Doped Sb2Te3 Phase-Change Materials: Toward a Universal Memory.

The disadvantages of high power consumption and slow operating speed hinder the application of phase-change materials (PCMs) for a universal memory. In this work, based on a rigorous experimental scheme, we synthesized a series of YxSb2-xTe3 (0 ≤ x ≤ 0.333) PCMs and demonstrated that Y0.25Sb1.75Te3 (YST) is an excellent candidate material for the universal phase-change memory. This YST PCM, even being integrated into a conventional T-shaped device, exhibits an ultralow reset power consumption of 1.3 pJ and a competitive fast set speed of 6 ns. The ultralow power consumption is attributed to the Y-reduced thermal and electrical conductivity, while the maintained crystal structure of Sb2Te3 and the grain refinement provide the competitive fast crystallization speed. This work highlights a novel way to obtain new PCMs with lower power consumption and competitive fast speed toward a universal memory.

Electronic mechanism for resistance drift in phase-change memory materials: link to persistent photoconductivity

‘Phase-change’ memory materials, such as the canonical composition Ge2Sb2Te5, are being actively researched for non-volatile resistive random-access memory applications. In these devices, ultra-rapid reversible transformations between metastable highly electrically conducting (degenerate-semiconducting) crystalline and more electrically resistive (semiconducting) glassy phases are produced by the application of appropriate voltage pulses. Multilevel programming, wherein more than two metastable resistance states can be stored in the memory material as different proportions of partially glassy/crystalline regions, allows more than one bit to be stored per memory cell. However, this route to increasing data density, without recourse to device-size down-scaling, is threatened by the phenomenon of ‘resistance drift’, wherein the electrical resistance of the glassy phase slowly increases with time, following a weak power-law dependence, after being written with a voltage pulse. In this paper, we propose an intrinsic electronic mechanism for the resistance drift, particularly valid at ambient temperature and below, by identifying it with the phenomenon of persistent photoconductivity that is commonly observed in a wide range of disordered semiconductors. We develop a model for it in terms of the long-time, deep-trap release and subsequent recombination of charge carriers, akin to that which is believed to be responsible for the long-time photocurrent decay in amorphous semiconductors, such as hydrogenated amorphous silicon. In this case, the parameters controlling the resistance drift are the widths of the (localized) valence- and conduction-band tails in the vicinity of the bandgap. Hence, there is the potential for mitigating resistance drift in the amorphous state of phase-change memory materials by suitable material engineering (e.g. via compositional or fabricational control) to control the extent of band-tailing, thereby facilitating the future introduction of multistate memory.

Nonequilibrium ab initio molecular-dynamics simulations of lattice thermal conductivity in irradiated glassy Ge2Sb2Te5

An analysis of thermal transients from nonequilibrium ab initio molecular-dynamics simulations can be used to calculate the thermal conductivity of materials with a short phonon mean-free path. We adapt the approach-to-equilibrium methodology to the three-dimensional case of a simulation that consists of a cubic core region at higher temperature approaching thermal equilibrium with a thermostatted boundary. This leads to estimates of the lattice thermal conductivity for the glassy state of the phase-change memory material, Ge2Sb2Te5, which are close to previously reported experimental measurements. Self-atom irradiation of the material, modeled using thermal spikes and stochastic-boundary conditions, results in glassy models with a significant reduction of diffusive thermal transport compared to the pristine glassy structure. This approach may prove to be useful in technological applications, e.g., for the suppression of thermal cross talk in phase-change memory and data-storage devices.

Normal-to-topological insulator martensitic phase transition in group-IV monochalcogenides driven by light

A material potentially exhibiting multiple crystalline phases with distinct optoelectronic properties can serve as a phase-change memory material. The sensitivity and kinetics can be enhanced when the two competing phases have large electronic structure contrast and the phase change process is diffusionless and martensitic. In this work, we theoretically and computationally illustrate that such a phase transition could occur in the group-IV monochalcogenide SnSe compound, which can exist in the quantum topologically trivial Pnma-SnSe and nontrivial Fmbar 3m-SnSe phases. Furthermore, owing to the electronic band structure differences of these phases, a large contrast in the optical responses in the THz region is revealed. According to the thermodynamic theory for a driven dielectric medium, optomechanical control to trigger a topological phase transition using a linearly polarized laser with selected frequency, power and pulse duration is proposed. We further estimate the critical optical electric field to drive a barrierless transition that can occur on the picosecond timescale. This light actuation strategy does not require fabrication of mechanical contacts or electrical leads and only requires transparency. We predict that an optically driven phase transition accompanied by a large entropy difference can be used in an “optocaloric” cooling device.

Phase change memory materials: Rationalizing the dominance of Ge/Sb/Te alloys

Rewritable optical storage is dominated by alloys of a small number of elements, overwhelmingly Ge, Sb, and Te. For over 30 years, Ge/Sb/Te alloys in the composition range ${(\mathrm{GeTe})}_{1\ensuremath{-}x}{({\mathrm{Sb}}_{2}{\mathrm{Te}}_{3})}_{x}\phantom{\rule{0.28em}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}1)$ have been the materials of choice in commercial devices: all have metastable rock-salt structures that change little over decades at archival temperatures, and all contain vacancies (cavities). The special status of Ge/Sb/Te alloys arises from the close similarity of their valence orbitals as measured by the orbital radial moments, so that bonds are stronger than in other combinations of elements of groups 14--16 with appropriate valences. The orbital similarity arises from the irregular changes in atomic orbitals and properties as the atomic number increases (``secondary periodicity''). Jones showed [P. R. Soc. London, Ser. A 147, 396 (1934)] that the simple cubic structure of (metallic) Bi (valence configuration $6{s}^{2}6{p}^{3}$) is unstable to a distortion to a (semimetallic) rhombohedral structure. This picture can be adapted to Ge/Sb/Te alloys to explain the metastable structure of the above family of compounds where vacancies almost always occur next to Te atoms, which form one sublattice of the rock-salt structure. The disorder in the Ge/Sb/vacancy sublattice is not random.

A New High Selectivity Acidic Slurry for Chemical Mechanical Planarization of Carbon-Doped Ge2Sb2Te5 Film

During the industrial application of the new phase change memory material carbon-doped Ge2Sb2Te5 (GSTC), it is necessary to perform planarization for the confined memory cell structure through the chemical mechanical polishing (CMP) process. The goal of CMP is to acquire a high material removal rate and selectivity while ensuring that the polished surface is free of scratches and corrosion pits. A new acidic slurry added with potassium persulfate (OXONE) as an oxidizer was developed. The acidic slurry added with 5000 ppm OXONE has a amorphous GSTC film material removal rate of 5200 Å min−1, a high selectivity between GSTC and Si3N4 films of 950:1, and no corrosion-induced pit. In order to clarify the mechanism of GSTC CMP with OXONE, the Energy Dispersive Spectrometer, potentiodynamic polarization curve and X-ray photoelectron spectroscope were adopted to analyze the surface of GSTC. It is found that the chemical reaction mechanism is significantly different from that of hydrogen peroxide (H2O2). Finally, the mechanism of GSTC during the CMP with OXONE in the acidic slurry is depicted.

Nanoscale amorphous interfaces in phase-change memory materials: structure, properties and design

Phase-change memory (PCM) is one of the emerging technologies for the next generation of electric and optical memories. In a typical PCM device, a nanoscale amorphous interface can usually be observed. These nano amorphous interfaces are crucial because they could affect the phase transition characteristics of PCM materials, especially when the PCM devices are scaling down. In this work, according to results of our previous first-principles calculations and transmission electron microscope characterizations, we revisit the effects of three typical nano amorphous interfaces of PCM materials between different phases. We conclude that the bonding network and electronic features of the PCM material in these interfaces will be changed significantly depending on the material next to it. Different types of interface could either enhance the stability of amorphous (i.e. amorphous Si/amorphous Sb2Te3 interface) and metastable crystalline phases (i.e. amorphous GeTe/cubic Sb2Te3 interface) or promote crystallization (i.e. amorphous and crystalline Ge2Sb2Te5 interface) of the PCM materials. Therefore, a nano amorphous interface can be used to control the performance of PCM devices, such as data retention ability and crystallization speed. The present work also offers a feasible way to design novel nanoscale PCM materials based on the interface effects.

Bonding similarities and differences between Y–Sb–Te and Sc–Sb–Te phase-change memory materials

The bonding contrast between Y–Sb–Te and Sc–Sb–Te phase-change memory materials, rendering the two alloys for different types of memory applications, is clarified through orbital-based chemical bonding analyses and structural similarity kernels.

Chemical understanding of resistance drift suppression in Ge–Sn–Te phase-change memory materials

The degree of Peierls distortion and the fraction of tetrahedral units in amorphous Ge–Sn–Te phase-change memory materials are gradually reduced as the Sn content gets richer.

Reduction in thermal conductivity of Sb2Te phase-change material by scandium/yttrium doping

Binary Sb2Te is considered as a potential recording material for phase-change memory due to its higher crystallization speed than the widely investigated ternary Ge-Sb-Te alloys. To further improve the performance of Sb2Te, various dopants such as scandium (Sc) have been explored. Yet, the thermal conductivity of the pristine and doped Sb2Te is unknown, even though the thermal conductivity (κ) is an essential parameter for phase change memory devices as thermal conductivity determines the exchange of energy with surroundings and heat transport in the SET/RESET process. In this work, by means of ab initio calculations, we have calculated the thermal conductivity of Sb2Te, scandium (Sc)-doped and yttrium (Y)-doped Sb2Te. The calculated lattice thermal conductivity values at 300 K are 3.76 Wm−1K−1, 3.22 Wm−1K−1 and 3.35 Wm−1K−1 for pristine Sb2Te, Sc-doped Sb2Te, and Y-doped Sb2Te, respectively, showing a reduction in lattice thermal conductivity by doping. Further analysis of the electron localization function and Bader charge distribution suggest that the increased bonding strength of the dopants with the surrounding atoms contributes to the reduction in lattice thermal conductivity of the doped Sb2Te. On the other hand, the electronic thermal conductivity of Sb2Te should also decrease due to the metal to semiconductor transition by Sc/Y doping as analyzed by the calculated density of states. Overall, the thermal conductivity of Sb2Te decreases to some extent by doping Sc and Y, which can lower the energy consumption and improve the efficiency of energy utilization in the data SET/RESET process and is thus desirable for phase-change memory devices.

Crystallization kinetics with fragile-to-strong crossover in Zn-Sb-Te supercooled phase-change liquids

Understanding crystallization kinetics is essential to select the high-performance materials for phase-change memory. By ultrafast differential scanning calorimetry, we found the distinct fragile-to-strong crossover crystallization kinetics in ZnSb and Zn28Sb54Te18 supercooled liquids. Zn28Sb54Te18 inherits the excellent thermal stability around glass transition from ZnSb and exhibits faster crystal growth rate close to melting temperature (Umax is 9.1 m s−1) and larger crossover magnitude f (2.3), compared to the typical fragile-to-strong crossover material Ag-In-Sb2Te. Such a material with a distinct fragile-to-strong crossover is helpful to improve their thermal stability nearby glass transition temperature and accelerate the phase transition speed close to melting temperature.

Current-voltage curves of eutectic In-Sb-Te thin films for phase change memory devices

PurposeThe purpose of this paper is to investigate electrical properties of eutectic In8Sb8Te84 and In10Sb51Te39 as made thin films to evaluate their potential for non-volatile phase-change memories, once the thermal measurements are very optimistic.Design/methodology/approachThe films were deposited by pulse laser deposition technique. By using a very simple home-made cell, transversal current-voltage curves films were measured involving both voltage controlled-pulses generator and current controlled-pulses generator, employing different pulse shapes: triangular and sine shaped.FindingsThe memory effect, characteristic of a typical phase-change memory material, was observed in both materials under research. For higher tellurium content in the film, lower is the value of threshold voltage.Research limitations/implicationsFurther studies on endurance, scaling and SET/RESET operations are needed.Practical implicationsThe values of the key parameters, threshold voltage and hold voltage are comparable with those of Ge2Sb2Te5, GeTe and Sb2Te being considered to date as the main compounds for PCM devices.Originality/valueThe conduction mechanism in the amorphous regime is agreed with Poole–Frenkel effect in deep traps.

Eutectic Sb7.4Te92.6 thin film for non-volatile phase-change memories

PurposeThe purpose of this paper is to investigate the structure and electrical properties of eutectic Sb7.4Te92.6 as made thin films to evaluate their potentiality for application to non-volatile phase-change memories.Design/methodology/approachThe films were prepared by the pulsed laser deposition technique. The films were characterized by using X-ray diffraction in grazing-incident geometry, differential scanning calorimetry, Raman spectroscopy and transversal current–voltage curves.FindingsThe memory effect state, characteristic of a typical phase-change memory material, was observed. The temperature of crystallization was about 100ºC.Research limitations/implicationsFurther studies on endurance, scaling and SET/RESET operations are needed.Practical implicationsOne of the main characteristic values, the hold voltage and the threshold voltage values, were about 0.85 and 1.2 V, respectively, in a line with those of Ge2Sb2Te5, GeTe and Sb2Te being considered to date as the main compounds for phase-change memory devices.Originality/valueThe conduction mechanism in the amorphous regime is highly agreed with the Poole–Frenkel effect in deep traps.