Superlattice Phase Change Memory Research Articles

A Review of Advances in Deposition Methods and Material Properties of Superlattice Phase-Change Memory

A Review of Advances in Deposition Methods and Material Properties of Superlattice Phase-Change Memory

Understanding Interface-Controlled Resistance Drift in Superlattice Phase Change Memory

Resistance drift in phase change memory (PCM) reduces the accuracy of analog computing applications such as neural network inference. Recently, PCMs based on superlattice (SL) phase change layers have shown low resistance drift, however the origin of this low drift remains unexplored. Here, we uncover that resistance drift in SL-PCM based on alternating layers of Sb2Te3 and Ge2Sb2Te5 (GST) is controlled by the number of SL interfaces as well as the degree of SL intermixing. Temperature-dependent measurements reveal smaller and more stable activation energy upon annealing (thus suppressed structural relaxation) in our SL-PCM vs. control GST devices, accounting for the low resistance drift. By controlling SL interfaces, we achieve low resistance drift coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${v} &lt; 0.01$ </tex-math></inline-formula> in these SL-PCMs, maintained after extensive cycling and at various read voltages and intervals - showing robustness required for analog computing with PCM.

Energy Bandpass Filtering in Superlattice Phase Change Memories

We propose energy bandpass filtering employed using the idea of anti-reflection heterostructures as a means to reduce the energy requirements of a superlattice phase change memory based on GeTe and Sb$_{2}$Te$_{3}$ heterostructures. Different configurations of GeTe/Sb$_{2}$Te$_{3}$ superlattices are studied using the non-equilibrium Green's function approach. Our electronic transport simulations calculate the coupling parameter for the high resistance covalent state, to $97 \%$ that of the stable low resistance resonant state, maintaining the ON/OFF ratio of 100 for a reliable read operation. By examining various configurations of the superlattice structures we conclude that the inclusion of anti-reflection units on both sides of the superlattice increases the overall ON/OFF ratio by an order of magnitude which can further help in scaling down of the memory device. It is also observed that the device with such anti-reflection units exhibits 32$\%$ lesser RESET voltage than the most common PCM superlattice configurations and 27$\%$ in the presence of elastic dephasing. Moreover, we also find that the ON/OFF ratios in these devices are also resilient to the variations in the periodicity of the superlattice.

(Invited) Sputter Growth of Chalcogenide Superlattice Films for Future Phase Change Memory Application

Ge-Sb-Te ternary alloys are key materials for phase-change random access memory (PCRAM). In PCRAM, data recording relies on reversible switching between amorphous and crystalline phases by electrical pulse induced Joule heating. We have proposed and developed GeTe/Sb2Te3 superlattice phase change memory, which is known as interfacial phase change memory (iPCM), and have demonstrated a significant reduction in switching energy and much longer endurance compared to conventional alloy-type phase change memory. Since this superlattice is composed of atomically aligned GeTe and Sb2Te3 multilayers, fabricating a highly-oriented film is crucial. Furthermore, in order to utilize such material for novel devices in an industrial setting, reliable processes that allow large area deposition of uniform films is critical. Sputtering is one of the most useful, technologically friendly, and reliable methods for thin film fabrication. In this work, the growth mechanism of layered chalcogenide films is discussed. The superlattice films were grown on various substrate materials by radio-frequency magnetron sputtering. Ge-Te and Sb-Te alloy targets were used. Before the growth of the film, Ar sputter cleaning was carried out to remove a native oxide from the substrate surface. An Sb2Te3 seed layer was grown initially at room temperature followed by alternate deposition of GeTe and Sb2Te3 layers at high temperature. The crystal structure of the films as well as the degree of orientation were evaluated by x-ray diffraction (XRD) while the microstructure was observed by transmission electron microscopy (TEM). We found there is a strong dependency on substrate materials in terms of out-of-plane orientation of the superlattice film. On the basis of the obtained results, we will discuss the growth mechanisms of layered chalcogenide films and propose optimal growth conditions for future phase change memory applications. This work was supported by JST-CREST (JPMJCR14F1).

(Invited) Sputter Growth of Chalcogenide Superlattice Films for Future Phase Change Memory Applications

Ge-Sb-Te ternary alloys are key materials for phase-change random access memory (PCRAM). In PCRAM, data recording relies on reversible switching between the amorphous and crystalline phases by means of electrical pulse induced Joule heating. We proposed and developed GeTe/Sb2Te3 superlattice phase change memory, also known as interfacial phase change memory (iPCM), and have demonstrated a significant reduction in switching energy and much longer endurance compared to devices fabricated from conventional alloy-type phase change memory. In this work, we discuss the growth mechanisms of layered chalcogenide films and propose optimal growth conditions for future phase change memory applications.

Atomic Layering, Intermixing and Switching Mechanism in Ge-Sb-Te based Chalcogenide Superlattices.

GeSbTe-based chalcogenide superlattice (CSLs) phase-change memories consist of GeSbTe layer blocks separated by van der Waals bonding gaps. Recent high resolution electron microscopy found two types of disorder in CSLs, a chemical disorder within individual layers, and SbTe bilayer stacking faults connecting one block to an adjacent block which allows individual block heights to vary. The disorder requires a generalization of the previous switching models developed for CSL systems. Density functional calculations are used to describe the stability of various types of intra-layer disorder, how the block heights can vary by means of SbTe-based stacking faults and using a vacancy-mediated kink motion, and also to understand the nature of the switching process in more chemically disordered CSLs.

Investigation of switching region in superlattice phase change memories

We investigated superlattice phase change memories (PCMs) to clarify which regions were responsible for switching. We observed atomic structures in a superlattice PCM film with a stack of GeTe / Sb2Te3 layers using atomically resolved EDX maps, and we found an intermixed region with three atom species of the Ge, Sb and Te around the top GeTe layer under the top electrode. We also found that a device with a GeTe layer on an Sb2Te3 layer without superlattice structure had the same switching characteristics as a device with a superlattice PCM, that had the same top GeTe layer. We developed and fabricated a modified superlattice PCM that attained ultra low Reset / Set currents under 60 μA.

Morphology and Electric Conductance Change Induced by Voltage Pulse Excitation in (GeTe)2/Sb2Te3 Superlattices.

Chalcogenide superlattice (SL) phase-change memory materials are leading candidates for non-volatile, energy-efficient electric memory where the electric conductance switching is caused by the atom repositioning in the constituent layers. Here, we study the time evolution of the electric conductance in [(GeTe)2/(Sb2Te3)1]4 SLs upon the application of an external pulsed electric field by analysing the structural and electrical responses of the SL films with scanning probe microscopy (SPM) and scanning probe lithography (SPL). At a low pulse voltage (1.6–2.3 V), a conductance switching delay of a few seconds was observed in some SL areas, where the switch to the high conductance state (HCS) is accompanied with an SL expansion under the strong electric field of the SPM probe. At a high pulse voltage (2.5–3.0 V), the HCS current was unstable and decayed in a few seconds; this is ascribed to the degradation of the HCS crystal phase under excessive heating. The reversible conductance change under a pulse voltage of opposite polarity emphasised the role of the electric field in the phase-transition mechanism.

Logic gates realized by nonvolatile GeTe/Sb2Te3 super lattice phase-change memory with a magnetic field input

Nonvolatile memory devices or circuits that can implement both storage and calculation are a crucial requirement for the efficiency improvement of modern computer. In this work, we realize logic functions by using [GeTe/Sb2Te3]n super lattice phase change memory (PCM) cell in which higher threshold voltage is needed for phase change with a magnetic field applied. First, the [GeTe/Sb2Te3]n super lattice cells were fabricated and the R-V curve was measured. Then we designed the logic circuits with the super lattice PCM cell verified by HSPICE simulation and experiments. Seven basic logic functions are first demonstrated in this letter; then several multi-input logic gates are presented. The proposed logic devices offer the advantages of simple structures and low power consumption, indicating that the super lattice PCM has the potential in the future nonvolatile central processing unit design, facilitating the development of massive parallel computing architecture.

Modeling of switching mechanism in GeSbTe chalcogenide superlattices.

We study the switching process in chalcogenide superlattice (CSL) phase-change memory materials by describing the motion of an atomic layer between the low and high resistance states. Two models have been proposed by different groups based on high-resolution electron microscope images. Model 1 proposes a transition from Ferro to Inverted Petrov state. Model 2 proposes a switch between Petrov and Inverted Petrov states. For each case, we note that the main transition is actually a vertical displacement of a Ge layer through a Te layer, followed by a lateral motion of GeTe sublayer to the final, low energy structure. Through calculating energy barriers, the rate-determining step is the displacive transition.

Non-Melting Phase Change Memory – Topological-Switching RAM (TRAM)

Superlattice phase change memory (PRAM) of the repeated films of [Sb2Te3/GeTe], we call this memory "TRAM", Topological-switching RAM, was studied to show some evidences of non-melting features compared to that of the conventional alloy PRAM, e.g. Ge2Sb2Te5. One evidence was very fast switching with the shorter and the rectangular pulses in both SET and RESET. Other evidence was direct observation of the TRAM using TEM after 10 million switching cycles.

GeTe sequences in superlattice phase change memories and their electrical characteristics

We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb2Te3] stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 200 °C and 240 °C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.

Investigation of multi-level-cell and SET operations on super-lattice phase change memories

This paper gives the optimum SET pulse with the investigation on SET current delay and the multi-level-cell (MLC) operation for super-lattice phase change memories (SL-PCMs). From the investigation, the voltage, or the electric field triggers RESET/SET transition of SL-PCM. The induced energy is also essential for changing the resistance state. In this paper, the MLC operation is also verified with RESET pulse, 1-step SET pulse and 2-step SET pulse. The measurement results indicate the 2-step SET pulse is the best for the MLC function, which realizes the precise resistance controlling. Additionally, the retention-time is measured to evaluate the reliability of MLC SL-PCM. The features of SL-PCM are not only small RESET/SET current, but also MLC operation and the SL-PCM technology provides a potential for next generation non-volatile memories.

Theoretical and Experimental Understanding of Charge-Injection GeTe/Sb2Te3Superlattice Phase Change Memory

ABSTRACTA charge-injection GeTe/Sb2Te3 superlattice phase change memory (PCM or PRAM) has been developed as a candidate for a non-volatile memory that replaces NAND flash memory. It differs from PRAM with the conventional material of GeSbTe, and is therefore named “TRAM (topological switching random access memory)”. First principle calculations showed a resistance change in the GeTe/Sb2Te3 superlattice was enhanced by charge injection. The fabrication and analyses of a one-resistor TEG revealed that the superlattice structure was maintained after 1M endurance, which proved the occurrence of non-melting resistance change in TRAM. The reset current of TRAM was found to be less than 1/5 of that of conventional PRAM. Furthermore, TRAM enables a set -speed of 10 ns and reset -operation by DC-sweep to be achieved, which experimentally proved the atomic movement in TRAM can be enhanced by charge injection.

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.