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Phase change memories (PCRAM) are often made of chalcogenide alloys in the form of multilayer systems (MLS). The mostly used alloys are Ge2Sb2Te5 and Ge‐rich Ge2Sb2Te5. The current article reports on the thermal characterization of very thin (<5 nm) Ge‐rich Ge2Sb2Te5/Ge2Sb2Te5 MLS by modulated photothermal radiometry (MPTR). The MPTR method allows for the investigation of such samples by determining, with an inverse method, the total thermal resistance of the stack deposited on the substrate. With the measurement of the total thermal resistance, it is possible to determine the thermal conductivity of the deposit and the interfacial thermal resistances between layers. The interfacial thermal resistance between Ge‐rich Ge2Sb2Te5/Ge2Sb2Te5 is characterized, which is an important parameter to reduce the energy cost of the PCRAM functioning. It is also possible to highlight a decrease in interface quality inside the MLS after the beginning of the phase transition around 250 °C. [ABSTRACT FROM AUTHOR]

Understanding the nature of charge‐trapping defects in amorphous chalcogenide alloy‐based phase‐change memory materials is important for tailoring the development of multilevel memory devices with increased data storage density. Herein, hybrid density‐functional theory simulations have been employed to investigate electron‐ and hole‐trapping processes in melt‐quenched glassy models of four different Ge‐Sb‐Te compositions, namely, GeTe, Sb2Te3, GeTe4, and Ge2Sb2Te5. The calculations demonstrate that extra electrons and holes are spontaneously trapped, creating charge‐trapping centers in the bandgap of the amorphous materials. Over‐ and undercoordinated atoms, tetrahedral and "see‐saw" octahedral‐like geometries, fourfold rings, homopolar bonds, near‐linear triatomic configurations, and chain‐like motifs comprise the range of the defective atomic environments that have been identified in the structural patterns of the charge‐trapping sites inside the glassy networks. The results illustrate that charge trapping corresponds to an intrinsic property of the glassy Ge‐Sb‐Te systems, show the impact of electron and hole localization on the atomic bonding of these materials, and they may have important implications related to the operation of phase‐change electronic‐memory devices. [ABSTRACT FROM AUTHOR]

The phase change memory (PCM) device has spotlighted as a candidate group for storage class memory devices and neuromorphic devices. However, the conventional GST‐based PCM has problems of relatively slow set speed and high reset energy consumption. In this study, we fabricate an amorphous Sb2Te3/TiTe2 heterostructure using a sputtering process by inserting multiple TiTe2 nanolayers inside the Sb2Te3 layer. The fabricated amorphous Sb2Te3/TiTe2 heterostructure film is confirmed as a phase change material with excellent properties through temperature‐dependent crystallinity change and resistance change analysis. Also, the Sb2Te3/TiTe2 heterostructure is integrated into a conventional T‐shape phase change memory with a bottom electrode diameter of 200 nm to analyze electrical switching characteristics. The Sb2Te3/TiTe2 heterostructure‐based PCM exhibited a faster set speed (≈30 ns) than the conventional GST‐based PCM, and the reset energy consumption is also reduced by more than 80% compared with the GST‐based PCM. In addition, the resistance drift coefficient is also reduced to 1/10 to improve the resistance drift characteristics. This study confirmed the excellent characteristics of the Sb2Te3/TiTe2 heterostructure as a phase change material and as a next‐generation PCM. [ABSTRACT FROM AUTHOR]

Models of the amorphous phase of the GeTe compound have been generated by depositing individual atoms on a substrate by molecular dynamics simulations. This material is of interest for applications in phase change memories. The kinetic energy of the atoms impinging on the surface is in the range 1.5–10 eV which is typical of magnetron sputtering growth. The simulations with up to 8000 atoms exploit a machine learning potential devised previously by the group. The structural properties of the films depend very little on the energy of the impinging atoms in the range 1.5–10 eV and they are also very similar to those of amorphous models generated by quenching from the melt, but for a larger fraction of GeGe bonds and Ge atoms in tetrahedral geometries in the films. The crystal nucleation and growth are then simulated at 600 K which is close to the temperature of maximal crystal growth velocity exploited in the memory devices. Despite the structural differences mentioned above, the kinetics of crystallization at 600 K is very similar for the as‐deposited and melt‐quenched amorphous models. [ABSTRACT FROM AUTHOR]

Herein, a finite element simulation framework for phase‐change memory devices that simultaneously solves for current continuity, electrothermal heating, and crystallization–amorphization dynamics using electrothermal models and dynamic material parameters that are functions of electric field and temperature is described. In this latest model, an electric field‐ and temperature‐dependent electrical conductivity model of stable amorphous Ge2Sb2Te5 (GST) obtained from experiments performed on GST line cells to study Read, Reset, and Set operations of mushroom cells is incorporated. The effects of current polarity, heater height, Reset pulse rise and fall times, access device configuration, and ambient temperature are analyzed. The simulation results predict a 2x change in Reset current requirements with different current polarity due to thermoelectric effects. Heater height plays a significant role in thermal losses; ≈16% decrease in Reset current for 4x increase in the heater height is obtained. Increase in the ambient temperature results in a linear decrease in the Reset power required to achieve the same Reset/Set resistance contrast. [ABSTRACT FROM AUTHOR]

Keywords: chalcogenide; EPCOS; ovonic; phase-change materials EN chalcogenide EPCOS ovonic phase-change materials 1 3 3 08/07/23 20230801 NES 230801 Once again, it is our great pleasure to continue the E\PCOS tradition by presenting this fourth edition of the special issue on Phase-Change and Ovonic Materials that is published each year as part of the European Symposium on Phase-Change and Ovonic Sciences (E\PCOS). A similar goal has also motivated the work presented by Kashem et al. [pssr.202200419] in which they proposed a finite element simulation framework combining amorphization-crystallization dynamics and electro-thermal effects to better describe RESET-SET-READ operations of PCM nanoscale devices. In fact, E\PCOS was born from the first PCOS symposium held in Japan in 1990, thanks to Professor Masahiro Okuda, who was the advisor of E\PCOS during its early years. [Extracted from the article]

Discovery of electrical switching in chalcogenide glasses by S.R. Ovshinsky paves a new path for developing high‐speed nonvolatile electronic memory and high‐performance computing solutions. This article presents a review on the systematic understanding of threshold switching (TS) properties in various chalcogenide materials, Ovonic threshold switching (OTS) and Ovonic memory switching (OMS), the nature of TS, voltage‐dependent transient characteristics, and the role of TS in governing the programming speed based on research efforts over the last six decades. Furthermore, realization of TS in picosecond timescale, the commonalities between OTS and OMS, and the possible underlying mechanism has been explored. Furthermore, a scheme of material classification based on TS dynamics for ultrafast yet energy‐efficient programming has been proposed for phase‐change memory with SRAM‐like programming speed for future electronics. [ABSTRACT FROM AUTHOR]

To passivate Si−SiO2 dangling bonds, metal–oxide–semiconductor field‐effect transistor devices are usually treated with hydrogen. Herein, the effects of such a treatment on the crystallization behavior on N‐doped, Ge‐rich Ge2Sb2Te5 phase‐change materials for memory applications are investigated using synchrotron X‐ray diffraction (XRD) in situ during heat treatment. Uniform thin films, and laterally confined, metallized ones (simulating devices of different complexity) of initially amorphous N‐doped GGST are investigated. The specimens are heated up to 450–500 °C at a rate of 2 °C/min. Some of the specimens are treated with H/D; the equivalent untreated specimen is investigated for each of them. Crystallization onsets are estimated by quantification of the crystallized quantity during material transformation from the XRD patterns. In thin films, the hydrogen treatment results in lowered crystallization temperatures of the emerging cubic, metastable Ge2Sb2Te5 phase. Its trigonal, thermodynamically stable polymorph always forms, but its crystallization temperature is unchanged. Patterned and metallized samples show less differences, are strongly textured, and no trigonal phase is observed. It is shown that certain questions might only be answered at large‐scale facilities where high energy photons are available at high flux, allowing data acquisition during the annealing process with a temperature resolution sufficient for a fine description of the sample transformation. [ABSTRACT FROM AUTHOR]

A phase‐change material (PCM) exhibiting a significant difference in resistance between the amorphous and crystalline phases can be used for phase‐change random access memory (PCRAM). Reduction of the energy to operate is one of the major challenges in PCRAM technology. One strategy for energy reduction is to increase the resistance of the memory device in the crystalline state of the PCM. Cr2Ge2Te6 (CrGT) shows p‐type semiconductor characteristics in both the amorphous and crystalline phases. A CrGT‐based memory device shows a contact resistance–dominant behavior, suggesting that the resistance of a CrGT‐based memory device can be increased by changing the electrode material. The contact resistivity (ρc) of a CrGT/electrode increases with a decrease in the work function of the electrode material in both amorphous and crystalline phases, as with general p‐type semiconductor materials. The highest ρc is observed for a LaB6 electrode. The resistance of the CrGT‐based device with a LaB6 electrode (LaB6 device) is three or four orders of magnitude greater than that of a device with a W electrode (W device). The LaB6 device is indicated to require much smaller operation energy than the W device. [ABSTRACT FROM AUTHOR]

The Ga4Sb6Te3 compound on the GaSb–Sb2Te3 pseudobinary tie‐line is proposed in the literature as a phase change material with high crystallization temperature. Herein, the crystal structure of this compound is uncovered by means of a genetic algorithm and electronic structure calculations based on density functional theory. As opposed to the parent GaSb compound which crystallizes in the zincblende structure, the Ga4Sb6Te3 compound features an octahedral‐like coordination for Ga as well as for Sb and Te atoms. Other structures close in energy to the ground state are also proposed, including some with a tetrahedral‐like coordination of Ga atoms. Raman spectra computed within density functional perturbation theory and an empirical Bond Polarizability Model are shown to be able to discriminate among the different possible local environments of Ga atoms. [ABSTRACT FROM AUTHOR]

Herein, eight uniform optical states (3 bit) are demonstrated by irradiating nanosecond laser pulses on thin In3SbTe2 films having high stability (260 °C), revealing at least 1% reflectivity contrast between any two consecutive states with strikingly low noise variation of 0.18% at each level, which is almost a 50% lower value compared to Ge2Sb2Te5 and AgInSbTe materials, revealing the two times enhanced signal‐to‐noise ratio of the In3SbTe2 material. Furthermore, a systematic structural evolution during multilevel switching is investigated using confocal Raman spectroscopic studies. The experimental findings demonstrate low‐noise yet highly stable multilevel switching toward the development of reliable phase change photonic memory devices. [ABSTRACT FROM AUTHOR]

Using a computational high-throughput screening method, 29 doping elements have been investigated for improving the thermal and electrical characteristics of In3SbTe2 (IST) phase-change material. Among the 29 dopants, it is found that Y offers largest distortion in the lattice structure of IST with negative doping formation energy while Y substitutes the In site. The atomic lattice images clearly show that the In site is substituted by Y and the distortion angles of the Y-doped IST (Y-IST) are well matched with the calculated results of density functional theory (DFT). Set/reset speed of the Y-IST phase-change memory is faster than IST and Ge2Sb2Te5 (GST) devices, which is strongly related with the fast and stable phase transition due to the larger lattice distortion. The power consumption of the Y-IST device is also less than a fourth of that of the GST device. [ABSTRACT FROM AUTHOR]

Phase‐change materials (PCMs) based on Ge–Sb–Te alloys are a strong contender for next‐generation memory technology. Recently, PCMs in the form of GeTe–Sb2Te3 superlattices (CSLs) have shown superior performance compared to ordinary PCM memory, which relies on switching between amorphous and crystalline phases. Although detailed atomic structure switching models have been developed with the help of ab‐initio simulations, there is still fierce scientific debate concerning the experimental verification of the actual crystal structures pertaining to the two CSL memory states. One of the strongest techniques to provide this information is (scanning) transmission electron microscopy ((S)TEM). The present article reviews the analyses of CSLs using TEM‐based techniques published during the last seven years since the seminal 2011 Nature Nanotechnology paper of Simpson et al., showing the superior performance of the CSL memory. It is critically reviewed what relevant information can be extracted from the (S)TEM results, also showing the impressive progress that has been achieved in a relatively short time frame. Finally, an outlook is given including several open questions. Although debate on actual switching mechanism in CSL memory is clearly not settled, still there is consensus in this field that CSL research has a bright future. Chalcogenide superlattices (CSLs) based on GeTe–Sb2Te3 have shown superior performance compared to ordinary phase‐change memory relying on GeSbTe alloys. Here, the authors critically review the structural characterization of CSLs that has been performed in recent years using transmission electron microscopy (TEM). Undoubtedly TEM‐based techniques will finally resolve the actual structures holding for the two CSL memory states. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) device physics comprehension represents an important chapter of future development of the PCM‐based architectures and their placement into the storage class memory (SCM) segment of the memory hierarchy. Here, a reduction of SET and RESET currents by more than 60% with respect to conventional GeTe–Sb2Te3 (GST) alloys is demonstrated by using phase change memory cells containing (GeTe–Sb2Te3)/Sb2Te3 super‐lattices (SL). Further, it is demonstrated that our SL PCM devices have similar characteristics in terms of the memory transition as conventional memory cells based on GST, even though showing reduced power consumption, indicative of an efficiency augmented SET‐to‐RESET transition. The reduced power consumption may be attributed to an increased thermal resistance of the SL with respect to the bulk GST alloy. This demonstrates that it is possible to engineer PCM with enhanced performance by employing SL structures, enlarging the possibility of employing SL as SCM players. A 60% reduction of currents in phase change memory (PCM) with super‐lattice (SL) active stacks is reported with respect to bulk stacks. Similarities between the electrical properties of SL and bulk PCM are pointed out in terms of programming curves and SET behavior, paving the way for a thermal‐based working principle in SL PCM as in the bulk counterpart. [ABSTRACT FROM AUTHOR]

Focusing on meeting the requirements for embedded applications of faster switching and sensing, lower‐power consumption, higher reliability, longer cycling endurance, and higher temperature retention, a lot of progress in the phase change memory (PCM) field has been made, including materials improvement, process optimization, new circuits design, and better operation algorithms. Finally, a 128 MB embedded PCM chip has been demonstrated in 40 nm node. Using carbon‐doped Ge2Sb2Te5 phase change material and 3 nm thick heating bottom electrode contact (BEC) with nitride coating, a resistance ratio of more than two orders of magnitude has been achieved in Reset and Set states. Combined with the parasitic matched sensing circuit, optimized ramp‐down SET pulses, adaptive pre‐read and write‐verify methods, and automatic configure and test procedures, the chip exhibits excellent data retention and endurance characteristics with minimum program time of 200 ns, and the Reset/Set resistance ratio becomes even larger after 260 °C soldering test. The estimated data retention time is 10 years at 128 °C, and in a 128 MB full integrated chip the endurance over 109 cycles is achieved. It is confirmed that this PCM technology is suitable for embedded applications, especially for those with crucial requirements of high access speed, high thermal stability, and cycling endurance. This review summarizes the authors' main progress in phase change memory (PCM) technology and presents a PCM device based on carbon‐doped Ge2Sb2Te5 (CGST) as a 128 MB chip, demonstrating the following key properties: minimum Read/Write time of 10 ns/200 ns, over 1011 cycles endurance of the TiN blade bottom electrode contact with nitride coating, Reset/Set ratio over two orders of magnitude, 10‐year 128 °C data retention, and over 109 cycles endurance of the device. It has great potential for embedded applications. [ABSTRACT FROM AUTHOR]

Phase‐change materials (PCMs) used in data storage devices have unique structural features and transition properties by thermal heating. Pressure, as another important thermodynamic tool, can also induce a series of interesting phase transitions in PCMs, accompanied by the altering of bonding nature and physical properties. Here, the structure transition as well as property change of prototypical phase‐change material Ge–Sb–Te (GST) under hydrostatic pressure has been reviewed. The high‐pressure behavior of some other relevant chalcogenides such as GeTe, Sb2Te3, and GeSe, is also discussed. The revealing of structure and property changes due to high pressure sheds light on the underlying physics of many fascinating properties of PCMs, and therefore it will have profound implications on various applications of phase change materials in memory and other fields. The recent progress in studying the high‐pressure behavior of phase‐change materials (PCMs) has been reviewed in this article. The unique structure and property transitions of these materials, which are widely used in data storage and other electronic devices, reveal rich physics under high pressure, allowing researchers to fully understand the transition mechanism of PCMs. [ABSTRACT FROM AUTHOR]