Phase Change Memory Devices Research Articles

Phase-change behaviors of Sb80Te20/SbSe nanocomposite multilayer films

The crystallization temperatures and 10 year data retention temperatures of the Sb 80 Te 20 /SbSe nanocomposite multilayer films can be modulated by varying the thickness ratio between Sb 80 Te 20 and SbSe layers. The crystallization dynamics of the films induced by nanosecond laser pulses are studied using in situ reflectivity measurement. The set and reset operation for phase change memory device based on [Sb 80 Te 20 (4 nm)/SbSe (10 nm)] 7 multilayer film can be achieved by an electric pulse as short as 50 ns.

$\hbox{1}/f$ Noise in 45-nm RESET-State Phase-Change Memory Devices: Characterization, Impact on Memory Readout Operation, and Scaling Perspectives

In this letter, we study the low-frequency noise behavior of RESET-state Phase-Change Memory (PCM) devices belonging to a 45-nm technology node. Furthermore, by dealing with a typical case study, we calculate the fluctuation of the cell readout current induced by <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{1}/f$</tex></formula> noise and provide predictions for estimating its effect in RESET-state PCM of future technology nodes.

Modeling of Threshold-Voltage Drift in Phase-Change Memory (PCM) Devices

The amorphous phase of the chalcogenide material in phase-change memory (PCM) devices is sensitive to temperature-activated crystallization and structural relaxation (SR). The latter leads to a change of device/material properties, such as the mobility band gap, the resistance, and the threshold voltage <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> for threshold switching. In this paper, we present a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> drift model based on physical descriptions of the electrical transport, the threshold switching, and the SR. We introduce an analytical formula describing the relation between the drift slopes of resistance and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> via the subthreshold slope STS of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> curve. A numerical model predicting the time evolution of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> for different programmed states in the PCM multilevel cell is finally presented and compared with experiments.

Addition of HfO2 interface layer for improved synaptic performance of phase change memory (PCM) devices

In this work, we will focus on the use of phase change memory (PCM) to emulate synaptic behavior in emerging neuromorphic system-architectures. In particular, we will show that the performance and energy-efficiency of large scale neuromorphic systems can be improved by engineering individual PCM devices used as synapses. This is obtained by adding a thin HfO2 interface layer to standard GST PCM devices, allowing for the lowering of the Set/Reset currents and the increase of the number of intermediate resistance states (or synaptic weights) in the synaptic potentiation characteristics. The experimentally obtained potentiation characteristics of such PCM devices are used to simulate a 2-layer ultra-dense spiking neural network (SNN) and to perform a complex visual pattern extraction from a test case based on real world data (i.e. cars passing on a 6-lane freeway). The total power dissipated in the learning mode, for the pattern extraction experiment is estimated to be as low as 60μW. Average detection rate of cars is found to be greater than 90%.

Investigation on nitrogen-doped Ge2Sb2Te5 material for phase-change memory application

Nitrogen-doped Ge2Sb2Te5 thin film is proposed to present the feasibility for electrical phase-change memory application. The effect of nitrogen doping on the structure is studied by in situ sheet resistance measurement and X-ray diffraction method. The temperature upon crystallization from amorphous to cubic structure is increased by nitrogen addition, while the phase transformation to hexagonal phase occurs at a lower temperature compared with the nitrogen-free Ge2Sb2Te5 material. The X-ray diffraction test reveals that the grain size has been refined significantly by the incorporation of nitrogen atoms in the film. Phase-change memory device based on nitrogen incorporated Ge2Sb2Te5 material is fabricated using complementary metal oxide semiconductor technology. A complete crystallization of the phase-change cell is realized in the current–voltage sweeping process. Nitrogen-doped Ge2Sb2Te5 with higher sheet resistance in the crystalline state is favored for phase-change memory application due to the decreased reset voltage and promoted resistance ratio of the reset and set states.

Triangle Current Controlled Relaxation Oscillations in Ge2Sb2Te5 Based Phase Change Memory Devices

Triangle Current Controlled Relaxation Oscillations in Ge2Sb2Te5 Based Phase Change Memory Devices

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.

W-Sb-Te phase-change material: A candidate for the trade-off between programming speed and data retention

W-Sb-Te phase-change material has been proposed to improve the performance of phase-change memory (PCM). Crystallization temperature, crystalline resistance, and 10-year data retention of Sb2Te increase markedly by W doping. The Wx(Sb2Te)1−x films crystallize quickly into a stable hexagonal phase with W uniformly distributing in the crystal lattice, which ensures faster SET speed and better operation stability for the application in practical device. PCM device based on W0.07(Sb2Te)0.93 shows ultrafast SET operation (6 ns) and good endurance (1.8 × 105 cycles). W-Sb-Te material is a promising candidate for the trade-off between programming speed and data retention.

Memory switching of ZnGa2Te4 thin films

Electrical and switching property of amorphous defect chalcopyrite ZnGa2Te4 thin films prepared by thermal evaporation technique has been studied. The elemental chemical compositions of the prepared bulk as well as the as-deposited film were determined by means of energy dispersive X-ray spectrometry. X-ray diffraction pattern revealed that the powder compound is polycrystalline and the as-deposited and the annealed films at ta ≤ 548 K have the amorphous phase, while that the annealed at t ≥ 573 K are polycrystalline with a single phase of a defect chalcopyrite structure similar to that of the synthesized material. The great advantage of this material is the capability to appear in two different phases, the amorphous and the crystalline phases, with rather different electrical properties. Both dynamic and static I–V characteristics and the switching phenomenon at 601 nm are investigated. The threshold switching mechanism was explained by a thermal model of switching, i.e., joule heating with an electrically conducting channel. ZnGa2Te4 is good candidate in phase change memory device.

Physical aspects of low power synapses based on phase change memory devices

In this work, we demonstrate how phase change memory (PCM) devices can be used to emulate biologically inspired synaptic functions in particular, potentiation and depression, important for implementing neuromorphic hardware. PCM devices with different chalcogenide materials are fabricated and characterized. The asymmetry between the potentiation and depression behaviors of the PCM is stressed. Detailed multi-physical simulations are performed to study the underlying physics of the synaptic behavior of PCM. A versatile behavioral model and a multi-level circuit-compatible model are developed for system and circuit-level neuromorphic simulations. We propose a unique low-power methodology named the 2-PCM Synapse, to use PCM devices as synapses in large scale neuromorphic systems. To show the strength of our proposed solution, we efficiently simulated fully connected feed-forward spiking neural network capable of complex visual pattern extraction from real world data.

Analysis and Modeling of Internal State Variables for Dynamic Effects of Nonvolatile Memory Devices

Hybrid integration of CMOS and nonvolatile memory (NVM) devices has become the foundation for emerging nonvolatile memory-based computing. The primary challenge to validate hybrid memory system with both CMOS and NVM devices is to develop a SPICE-like simulator that can simulate the dynamic behavior accurately and efficiently. Since memristor, spin-transfer-toque magnetic-tunneling-junction (STT-MTJ) and phase-change-memory (PCM) devices are the most promising candidates of next generation of NVM devices, it is under great interest in including these new devices in the standard CMOS design flow. The previous approaches either ignore dynamic effect without consideration of internal states for dynamic behavior, or need complex equivalent circuits to represent those devices. This paper proposes a new modified nodal analysis for NVM devices with identified internal state variables for dynamic behavior. As such, compact SPICE-like implementation can be derived for all three new NVM devices in the design of large-scale memory circuits. As demonstrated by a number of examples on hybrid memory circuits with both CMOS and NVM devices, our newly developed SPICE-like simulator can capture dynamic behaviors of memristor, STT-MTJ and PCM devices, and can also reduce CPU runtime by 20-69 times when compared to the previous equivalent circuit based approaches.

Improved thermal and electrical properties of Al-doped Ge2Sb2Te5 films for phase-change random access memory

Alx(Ge2Sb2Te5)100−x materials with different Al contents are systemically studied for applications in phase-change random access memory (PRAM) devices. Al-doped Ge2Sb2Te5 (GST) films show better thermal stability than GST because they do not have phase transformation from face-centred cubic (fcc) to hexagonal at high annealing temperatures. As the Al content increases, the resistance in both amorphous and crystalline phases improves and there is four to five orders of magnitude difference in the resistance between the amorphous and crystalline phases, all of which are helpful in achieving a higher On/OFF ratio for PRAM. In addition, the introduction of Al into the GST films can increase the optical band gap that is favourable to decrease the threshold current of PRAM devices. Raman spectra show that a significant change in the local bonding arrangement around Sb atoms has occurred due to the phase transformation from fcc to hexagonal in the GST film but this can be suppressed by Al addition during the crystallization process. All these results confirm that Al-doped GST films are suitable for use in PRAM.

Topological Insulating inGeTe/Sb2Te3Phase-Change Superlattice

$\mathrm{GeTe}/{\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ superlattice phase-change memory devices demonstrated greatly improved performance over that of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$, a prototype record media for phase-change random access memory. In this work, we show that this type of $\mathrm{GeTe}/{\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ superlattice exhibits topological insulating behavior on the basis of ab initio calculations. The analysis of the band structures and parities as well as ${Z}_{2}$ topological invariants unravels the topological insulating nature in these artificial materials. Furthermore, the topological insulating character remains in the $\mathrm{GeTe}/{\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ superlattice under small compressive strains, whereas it is not observed as more ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ building blocks introduced in the superlattice. The present results show that multifunctional data storages may be achieved in the $\mathrm{GeTe}/{\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ superlattice. Such kinds of artificial materials can be used in phase-change random access memory, spintronics, and quantum computing.

Progress in understanding the Ovshinsky Effect: Threshold switching in chalcogenide amorphous semiconductors

AbstractIn the nearly 45 years since Stan Ovshinsky's original publication [Phys. Rev. Lett. 21, 1450 (1968)] announcing the discovery of electric‐field‐induced threshold switching in chalcogenide alloy semiconductor glass thin films, the field of amorphous semiconductor physics has developed remarkably. Many commercial applications of these materials have been developed, including X‐ray image intensifiers, photocopier photoreceptors, and optical and electrical phase change memory devices. Despite these developments, a universally agreed upon, comprehensive model for the Ovshinsky Effect threshold switch still has not emerged. This paper is a brief review of the present state of understanding of this fascinating solid state physics phenomenon.

Role of potential fluctuations in phase‐change GST memory devices

AbstractThe long range potential fluctuations (LRPFs) arising from the defects and heterogeneities in disordered semiconductors are important for understanding their atomic and electronic properties. Here, they are measured in GeXSbYTe1−X−Y (GST) chalcogenide glasses used in rewritable phase change memory (PCM) devices. It is found that the most commonly used composition Ge2Sb2Te5 has the smallest LRPF amongst its nearby compositions. This finding may be useful in the search for better PCM materials.

High temperature reliability of μtrench Phase-Change Memory devices

In this paper, we investigate the reliability of μtrench Phase-Change Memory (PCM) devices at high operating temperature (up to 180°C). We show that the high temperature strongly impacts the programming curves, by linearly decreasing the threshold voltage and decreasing the RESET current. Furthermore, the cyclability of the cell significantly increases up to more than 109 cycles. The decreasing of the threshold voltage at high operating temperatures, reduces the thermal stress in the cell during the SET operations. The increased lifetime of the cell at high operating temperatures is explained by means of physical simulations.

Threshold switching via electric field induced crystallization in phase-change memory devices

Phase-change devices exhibit characteristic threshold switching from the reset (off) to the set (on) state. Mainstream understanding of this electrical switching phenomenon is that it is initiated electronically via the influence of high electric fields on inter-band trap states in the amorphous phase. However, recent work has suggested that field induced (crystal) nucleation could instead be responsible. We compare and contrast these alternative switching “theories” via realistic simulations of device switching both with and without electric field dependent contributions to the system free energy. Results show that although threshold switching can indeed be obtained purely by electric field induced nucleation, the fields required are significantly larger than experimentally measured values.

Very low angle annular dark field imaging in the scanning transmission electron microscope: A versatile tool for micro- and nano-characterization

The author shows how very low angle annular dark field (VLAADF) imaging in the scanning transmission electron microscope (STEM) is used to delineate regions of different orientation or structural phase in the same material. First, the author describes the settings used to obtain images in which only very low indexed beams contribute to the image formation. These settings can be implemented easily on any commercial STEM/TEM by choosing the appropriate camera length and objective aperture. The author then demonstrates how VLAADF was used to perform detailed grain size studies in Cu interconnects, and show how the technique can be used to outline the amorphous region within the crystalline matrix in a phase change memory device. The author finishes by addressing and discussing the dependence of the “orientation contrast” on the main imaging parameters.

Fast crystal nucleation induced by surface oxidation in Si-doped GeTe amorphous thin film

Fast crystallization in the phase change materials (PCMs) used for optical and electrical phase change memory improves their data recording rate. In the present work, it was found that the preferential surface oxidation of Si shortens the nucleation time of Ge46.55Te46.55Si6.9 amorphous films. The nucleation time of a surface-oxidized film was approximately 20% faster than that of a non-oxidized film. This was due to the formation of inhomogeneous nucleation sites at the film surface. These results suggest that preferential surface oxidation of PCM is an effective method to enhance the data recording rate of phase change memory devices.

Observation and modeling of polycrystalline grain formation in Ge2Sb2Te5

The relationship between the polycrystalline nature of phase change materials (such as Ge2Sb2Te5) and the intermediate resistance states of phase change memory (PCM) devices has not been widely studied. A full understanding of such states will require knowledge of how polycrystalline grains form, how they interact with each other at various temperatures, and how the differing electrical (and thermal) characteristics within the grains and at their boundaries combine through percolation to produce the externally observed electrical (and thermal) characteristics of a PCM device. We address the first of these tasks (and introduce a vehicle for the second) by studying the formation of fcc polycrystalline grains from the as-deposited amorphous state in undoped Ge2Sb2Te5. We perform ex situ transmission electron microscopy membrane experiments and then match these observations against numerical simulation. Ramped-anneal experiments show that the temperature ramp-rate strongly influences the median grain size. By truncating such ramped-anneal experiments at various peak temperatures, we convincingly demonstrate that the temperature range over which these grains are established is quite narrow. Subsequent annealing at elevated temperature appears to change these established distributions of grain sizes only slightly. Our numerical simulator—which models nuclei formation through classical nucleation theory and then tracks the subsequent time- and temperature-dependent growth of these grains—can match these experimental observations of initial grain distributions and crystallization temperature both qualitatively and quantitatively. These simulations show that the particular narrow temperature range over which crystallization occurs shifts as a function of temperature ramp-rate, which allows us to quantify the lower portions of the time-temperature-transformation map for Ge2Sb2Te5. Future experiments and extensions of the simulator to investigate temperature-dependent interactions between neighboring grains, and to study nucleation from within the melt-quenched amorphous state, are discussed.