Ultra-high-density phase-change storage and memory

Phase-change storage is widely used in optical information technologies (DVD, CD-ROM and so on), and recently it has also been considered for non-volatile memory applications. This work reports advances in thermal data recording of phase-change materials. Specifically, we show erasable thermal phase-change recording at a storage density of 3.3 Tb inch(-2), which is three orders of magnitude denser than that currently achievable with commercial optical storage technologies. We demonstrate the concept of a thin-film nanoheater to realize ultra-small heat spots with dimensions of less than 50 nm. Finally, we show in a proof-of-concept demonstration that an individual thin-film heater can write, erase and read the phase of these storage materials at competitive speeds. This work provides important stepping stones for a very-high-density storage or memory technology based on phase-change materials.     

Antimony selenide phase-change nanowires for memory application

Synthesis and device characteristics of highly scalable antimony selenide nanowire-based phase transition memory are reported. Antimony selenide nanowires prepared using the metal-catalyst-free approach are single-crystalline and of high-purity. The nanowire memory can be repeatedly switched between high-resistance (approximately 10 Momega) and low-resistance (approximately 1 komega) states which are attributed to amorphous and crystalline states, respectively.     

Application of phase-change materials in memory taxonomy

Abstract

Phase-change materials are suitable for data storage because they exhibit reversible transitions between crystalline and amorphous states that have distinguishable electrical and optical properties. Consequently, these materials find applications in diverse memory devices ranging from conventional optical discs to emerging nanophotonic devices. Current research efforts are mostly devoted to phase-change random access memory, whereas the applications of phase-change materials in other types of memory devices are rarely reported. Here we review the physical principles of phase-change materials and devices aiming to help researchers understand the concept of phase-change memory. We classify phase-change memory devices into phase-change optical disc, phase-change scanning probe memory, phase-change random access memory, and phase-change nanophotonic device, according to their locations in memory hierarchy. For each device type we discuss the physical principles in conjunction with merits and weakness for data storage applications. We also outline state-of-the-art technologies and future prospects.     

Highly scalable non-volatile and ultra-low-power phase-change nanowire memory

The search for a universal memory storage device that combines rapid read and write speeds, high storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phase-change materials, which can be reversibly switched between amorphous and crystalline states, are promising in this respect, but top-down processing of these materials into nanostructures often damages their useful properties. Self-assembled nanowire-based phase-change material memory devices offer an attractive solution owing to their sub-lithographic sizes and unique geometry, coupled with the facile etch-free processes with which they can be fabricated. Here, we explore the effects of nanoscaling on the memory-storage capability of self-assembled Ge2Sb2Te5 nanowires, an important phase-change material. Our measurements of write-current amplitude, switching speed, endurance and data retention time in these devices show that such nanowires are promising building blocks for non-volatile scalable memory and may represent the ultimate size limit in exploring current-induced phase transition in nanoscale systems.     

Chalcogenide phase-change memory nanotubes for lower writing current operation

We report the synthesis and characterization of Sb-doped Te-rich nanotubes, and study their memory switching properties under the application of electrical pulses. Te-rich nanotubes display significantly low writing currents due to their small cross-sectional areas, which is desirable for power-efficient memory operation. The nanotube devices show limited resistance ratio and cyclic switching capability owing to the intrinsic properties of Te. The observed memory switching properties of this new class of nanostructured memory elements are discussed in terms of fundamental materials properties and extrinsic geometrical effects.     

Minimum voltage for threshold switching in nanoscale phase-change memory

The size scaling of the threshold voltage required for the amorphous-to-crystalline transition in phase-change memory (PCM) is investigated using planar devices incorporating individual GeTe and Sb2Te3 nanowires. We show that the scaling law governing threshold switching changes from constant field to constant voltage scaling as the amorphous domain length falls below 10 nm. This crossover is a consequence of the energetic requirement for carrier multiplication through inelastic scattering processes and indicates that the size of PCM bits can be miniaturized to the true nanometer scale.     

Crystallization kinetics of amorphous Ga-Sb films extended for phase-change memory

Performance of phase-change materials based on Ga-Te-Sb was found getting better with decreasing Te content in our earlier studies. We concerned much properties of Te-free, Sb-rich binary Ga-Sb, which has been known to possess extremely fast crystallization behavior. Non-isothermal and isothermal crystallization kinetics of amorphous Sb-rich Ga-Sb films were explored by temperature dependent electrical resistance measurements. The crystallization temperature (183 to 261 degrees C) increases with decreasing Sb content (91 to 77 at%). The activation energy and rate-factor vary with Sb contents and reach the maximum at Ga19Sb81. The kinetic exponent is smaller than 1.5 at Sb < 85 at% denoting that the mechanism is one-dimensional crystal-growth from nuclei. The temperature corresponding to 10-year data-retention, evaluated from films, is 180 degrees C (Ga19Sb81) and 137 degrees C (Ga13Sb87), respectively. We verified memory performance using test-devices made of Ga16Sb84 working at voltages with 100 ns pulse-width.     

Impact of thermoelectric phenomena on phase-change memory performance metrics and scaling

The coupled transport of heat and electrical current, or thermoelectric phenomena, can strongly influence the temperature distribution and figures of merit for phase-change memory (PCM). This paper simulates PCM devices with careful attention to thermoelectric transport and the resulting impact on programming current during the reset operation. The electrothermal simulations consider Thomson heating within the phase-change material and Peltier heating at the electrode interface. Using representative values for the Thomson and Seebeck coefficients extracted from our past measurements of these properties, we predict a cell temperature increase of 44% and a decrease in the programming current of 16%. Scaling arguments indicate that the impact of thermoelectric phenomena becomes greater with smaller dimensions due to enhanced thermal confinement. This work estimates the scaling of this reduction in programming current as electrode contact areas are reduced down to 10 nm × 10 nm. Precise understanding of thermoelectric phenomena and their impact on device performance is a critical part of PCM design strategies.     

Ultrafast switching in nanoscale phase-change random access memory with superlattice-like structures

Phase-change random access memory cells with superlattice-like (SLL) GeTe/Sb(2)Te(3) were demonstrated to have excellent scaling performance in terms of switching speed and operating voltage. In this study, the correlations between the cell size, switching speed and operating voltage of the SLL cells were identified and investigated. We found that small SLL cells can achieve faster switching speed and lower operating voltage compared to the large SLL cells. Fast amorphization and crystallization of 300 ps and 1 ns were achieved in the 40 nm SLL cells, respectively, both significantly faster than those observed in the Ge(2)Sb(2)Te(5) (GST) cells of the same cell size. 40 nm SLL cells were found to switch with low amorphization voltage of 0.9 V when pulse-widths of 5 ns were employed, which is much lower than the 1.6 V required by the GST cells of the same cell size. These effects can be attributed to the fast heterogeneous crystallization, low thermal conductivity and high resistivity of the SLL structures. Nanoscale PCRAM with SLL structure promises applications in high speed and low power memory devices.     

Microscopic origin of the fast crystallization ability of Ge-Sb-Te phase-change memory materials

Ge-Sb-Te materials are used in optical DVDs and non-volatile electronic memories (phase-change random-access memory). In both, data storage is effected by fast, reversible phase changes between crystalline and amorphous states. Despite much experimental and theoretical effort to understand the phase-change mechanism, the detailed atomistic changes involved are still unknown. Here, we describe for the first time how the entire write/erase cycle for the Ge(2)Sb(2)Te(5) composition can be reproduced using ab initio molecular-dynamics simulations. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rocksalt structure, form during melt cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge(2)Sb(2)Te(5). As this simulation procedure is general, the microscopic insight provided on crystal nucleation should open up new ways to develop superior phase-change memory materials, for example, faster nucleation, different compositions, doping levels and so on.     

Unveiling the structural origin to control resistance drift in phase-change memory materials

The global demand for data storage and processing is increasing exponentially. To deal with this challenge, massive efforts have been devoted to the development of advanced memory and computing technologies. Chalcogenide phase-change materials (PCMs) are currently at the forefront of this endeavor. In this Review, we focus on the mechanisms of the spontaneous structural relaxation – aging – of amorphous PCMs, which causes the well-known resistance drift issue that significantly reduces the device accuracy needed for phase-change memory and computing applications. We review the recent breakthroughs in uncovering the structural origin, achieved through state-of-the-art experiments and ab initio atomistic simulations. Emphasis will be placed on the evolving atomic-level details during the relaxation of the complex amorphous structure. We also highlight emerging strategies to control aging, inspired by the in-depth structural understanding, from both materials science and device engineering standpoints, that offer effective solutions to reduce the resistance drift. In addition, we discuss an important new paradigm – machine learning – and the potential power it brings in interrogating amorphous PCMs as well as other disordered alloy systems. Finally, we present an outlook to comment on future research opportunities in amorphous PCMs, as well as on their reduced aging tendency in other advanced applications such as non-volatile photonics.     

Simulation study on heat conduction of a nanoscale phase-change random access memory cell

We have investigated heat transfer characteristics of a nano-scale phase-change random access memory (PRAM) cell using finite element method (FEM) simulation. Our PRAM cell is based on ternary chalcogenide alloy, Ge2Sb2Te5 (GST), which is used as a recording layer. For contact area of 100 x 100 nm2, simulations of crystallization and amorphization processes were carried out. Physical quantities such as electric conductivity, thermal conductivity, and specific heat were treated as temperature-dependent parameters. Through many simulations, it is concluded that one can reduce set current by decreasing both electric conductivities of amorphous GST and crystalline GST, and in addition to these conditions by decreasing electric conductivity of molten GST one can also reduce reset current significantly.     

Influence of chalcogenide glasses electro physical parameters on threshold voltage for phase-change memory

The simple equations have been derived for main setting parameters of phase-change memory (PCM) devices. The calculations of threshold voltage and delay time are carried out based on emission model for an initial stage of electric breakdown in amorphous semiconductors. The temperature dependences of threshold voltage agree with the experimental data for GST material.     

Characterizations and thermal stability improvement of phase-change memory device containing Ce-doped GeSbTe films

Phase-transition temperature of GeSbTe (GST) chalcogenide film was drastically increased from 159 to 236 °C by cerium (Ce) doping (up to 8.6 at.%) without altering the resistivity property of GST. Grain refinement via the solid-solution mechanism and the amplification of p-type semiconducting behavior in Ce-doped GST were observed. They were correlated with the enhancement of thermal stability and data retention property of GST as revealed by exothermal and isothermal analyses. Phase-change memory (PCM) device characterized at various temperatures revealed an effective thermal stability improvement on the threshold voltage of PCM device by Ce doping.     

Investigation of phase transition behaviors of the nitrogen-doped Sb-rich Si-Sb-Te films for phase-change memory

The phase transformation properties of the nitrogen-doped Sb-rich Si-Sb-Te films were investigated in detail. It was found that the addition of N atoms into the Si-Sb-Te films increases the temperature for phase transition from the amorphous phase to a stable hexagonal structure and enhances the sheet resistance of the films following grain refinement. The surface topography of the crystalline films was improved by doping nitrogen atoms. The activation energy for crystallization of the films was increased from 1.84 to 2.89 eV with the increased nitrogen content from 0 to 21 at.%, which promises an improved thermal stability. A prolonged data lifetime up to 10 years at 149.4 °C was realized. From the device performance point of view, the N-doped Si-Sb-Te film with a moderate nitrogen content was preferable for the phase-change memory applications due to its advantage of higher reliability.     

Binary semiconductor In2Te3 for the application of phase-change memory device

Nonvolatile phase-change memory devices with 500 nm contact hole based on In₂Te₃ were successfully fabricated by using focused ion beam, pulsed laser deposition, and dc magnetic sputtering techniques. In₂Te₃ films were characterized by using differential thermal analysis, X-ray diffraction, and UV–vis diffuse absorption spectroscopy, respectively. The devices can be switched between high and low resistance states repeatedly with the programmed voltage pulses. The reset operation (crystalline to amorphous) was done by the voltage pulse with a magnitude of 3.5 V and a duration of 30 ns, and the set operation (amorphous to crystalline) was done by the voltage pulse with a magnitude of 1.4 V and a duration of 100 ns. A dynamic resistance switching ratio (OFF/ON ratio) of 3.2 × 10³ has been obtained.     

Finite-element model for phase-change recording

The finite-element method is applied to model phase-change recording in a grooved recording stack. A rigorous model for the scattering of a three-dimensional focused spot by a one-dimensional periodic grating is used to determine the absorbed light in a three-dimensional region inside the phase-change layer. The optical model is combined with a three-dimensional thermal model to compute the temperature distribution. Land and groove recording and polarization dependence are studied, and the model is applied to the Blu-ray Disc.     

Thermal properties and structure of TeGa2Sb7 thin films for phase-change memory

We disclose in this paper properties of a composition TeGa₂Sb₇ selected from the pseudo-tie line Sb₈₀Te₂₀-GaSb in Te-Ga-Sb system. TeGa₂Sb₇ film possesses crystallization temperature 236 °C and activation-energy of crystallization 5.76 eV, by Kissinger's peak-shift method. The study on failure-time versus isothermal temperatures fits in an Arrhenius plot, which can be extrapolated to the 10 year data retention at 200 °C. Grazing-incident X-ray diffraction shows that crystallized TeGa₂Sb₇ films compose of a single HCP phase. Electrical resistance decreases by four orders-of-magnitude upon crystallization. Memory switching is verified using a bridge-memory-cell. TeGa₂Sb₇ is a potential for phase-change memory anticipating high thermal stability.     

Disorder-induced localization in crystalline phase-change materials

Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.     

Synthesis and characterization of phase-change nanowires

Phase-change memory materials have stimulated a great deal of interest although the size-dependent behaviors have not been well studied due to the lack of method for producing their nanoscale structures. We report the synthesis and characterization of GeTe and Sb(2)Te(3) phase-change nanowires via a vapor-liquid-solid growth mechanism. The as-grown GeTe nanowires have three different types of morphologies: single-crystalline straight and helical rhombohedral GeTe nanowires and amorphous curly GeO(2) nanowires. All the Sb(2)Te(3) nanowires are single-crystalline.