ABSTRACT Chalcogenide ferroelectric Rashba semiconductors (FERSCs) offer a promising route to ultralow‐power spin–orbit devices, yet their integration into microelectronics has been hindered by the lack of high‐quality films grown using industry‐compatible methods. Here, we demonstrate CMOS‐compatible synthesis of rhombohedral α ‐GeTe(111) thin films on 200/300 mm Si wafers using an industrial deposition process based on van der Waals epitaxy, applied to this material class for the first time. Scanning Transmission Electron microscopy and advanced simulations of anomalous X‐ray diffraction measurements at synchrotron reveal that films are intrinsically self‐poled, exhibiting a robust upward out‐of‐plane ferroelectric polarization. The growth strategy is universal, enabling high‐quality α ‐GeTe(111) on metals and insulators without compromising structural or ferroelectric performance. Piezoresponse force microscopy confirms reversible 180° ferroelectric switching with performance comparable to molecular‐beam‐epitaxy benchmarks. This substrate‐independent growth strategy represents a decisive step toward the future large‐scale integration of FERSCs into functional spin‐orbit architectures, with the hope of bridging the gap between Rashba's fundamental physics and microelectronic implementation.

GeTe has been widely studied and exploited as a phase-change material in non-volatile memory technology. Upon alloying with GeSe, which is not a phase-change material, the phase-change contrast between amorphous and crystalline phases of GeSe1-x Tex compounds progressively vanishes as x decreases. In this work, we focus on the properties of the amorphous phase of GeSe1-xTex alloys. Using ab initio molecular dynamics (AIMD) together with the r2SCAN+rVV10 exchange-correlation functional, we generate amorphous models which correspond nicely to the available experimental data. We observe that the main effect of alloying GeSe with GeTe is due to an increase of homopolar Ge-Ge bonds. In all cases, distorted octahedral environments are dominant, and the amorphous phase is shown to possess regular iono-covalent bonds.

Herein, the linear and nonlinear optical properties, together with the remarkable thermal stability and structural properties of thin films of nitrogen‐doped GeSe 1− x Te x chalcogenide materials, are unveiled. These alloys are obtained by reactive magnetron cosputtering technique in a 300 mm wafer industrial tool, and they are promising candidates for high‐temperature nonvolatile resistive memories due to unprecedented crystallization temperatures among all known congruent phase‐change materials (PCMs). Besides, their uncommon optical properties, measured by spectroscopic ellipsometry, make them highly attractive candidates for reconfigurable and nonlinear photonic applications in the infrared range. They exhibit a reversible phase transformation between the amorphous and crystalline phases, displaying an unprecedented contrast in electronic and optical properties between these two phases. Unlike most known PCMs, they maintain high transparency in the mid‐infrared range in both phases. By tailoring nitrogen and tellurium contents in N‐doped GeSe 1− x Te x thin films, it is shown here how it becomes feasible to customize the linear/nonlinear optical properties and thermal stability of such PCM thin films, enabling their optimization for various future optical and photonic applications.

Phase change memories (PCMs) are at the heart of modern memory technology, offering multi-level storage, fast read/write operations, and non-volatility, bridging the gap between volatile DRAM and non-volatile Flash. The reversible transition between amorphous and crystalline states of phase-change materials such as GeTe or Ge2Sb2Te5 is at the basis of PCM devices. Despite their importance, PCM devices face challenges including high power consumption during the RESET operation. Current research efforts focus on improving device architecture and exploring alternative phase-change materials such as GeTe/Sb2Te3 super-lattices (SLs), for which a reduced programming power consumption is observed compared with standard PCMs. Herein, by combining X-ray diffraction and scanning transmission electron microscopy imaging of SL thin films with the study of the same SL in PCM devices, it is shown that it is possible to significantly decrease RESET energy of the device, without modifying the SL composition, by reducing the amount of structural defects through annealing treatment. The best device properties are obtained after transforming the SL into a defect-free, highly out-of-plane oriented rhombohedral phase. These results offer a promising avenue for further improving the performance of SL-based PCM devices through structural optimization.

In this work, the experimental evidence of glass-like phonon dynamics and thermal conductivity in a nanocomposite made of GeTe and amorphous carbon is reported, which is of interest for microelectronics, and specifically phase change memories. It is shown that, the total thermal conductivity is reduced by a factor of three at room temperature with respect to pure GeTe, due to the reduction of both electronic and phononic contributions. This latter, similarly to glasses, is small and weakly increasing with temperature between 100 and 300 K, indicating a mostly diffusive thermal transport and reaching a value of 0.86(7) Wm-1K-1 at room temperature. A thorough investigation of the nanocomposite's phonon dynamics reveals the appearance of an excess intensity in the low energy vibrational density of states, reminiscent of the Boson peak in glasses. These features can be understood in terms of an enhanced phonon scattering at the interfaces, due to the presence of elastic heterogeneities, at wavelengths in the 2-20 nm range. The findings confirm recent simulation results on crystalline/amorphous nanocomposites and open new perspectives in phonon and thermal engineering through the direct manipulation of elastic heterogeneities. An investigation of thermal transport in nanocomposites made of crystalline GeTe and amorphous carbon is reported. The thermal conductivity is low and weakly increasing with temperature, like in glasses. It is shown that this is due to a strong diffusion of the main heat carriers, phonons, at the interfaces, which leads to a glass-like phonon dynamics. image

Scanning transmission electron microscopy (STEM) techniques are used to improve the understanding of out‐of‐plane oriented Sb 2 Te 3 thin films deposited by sputtering on SiO 2 and Si substrates. Nanobeam precession electron diffraction, energy‐dispersive X‐ray spectroscopy, and high‐angle‐annular dark‐field imaging show that the presence of 1–2 atomic planes of Te on top of the substrate is a crucial factor for successful growth of such films, which can be achieved by optimizing cosputtering of Te and Sb 2 Te 3 targets. The formation of an actual van der Waals (vdW) gap between the substrate and the first Sb 2 Te 3 quintuple layer allows for vdW epitaxy. This gap is larger than those separating Te planes in the pseudo‐2D Sb 2 Te 3 structure. HAADF image analysis provides detailed information on the atomic arrangement such as interplanar distances, vdW gaps, and Debye–Waller coefficients, all these with a few pm precision. For the anisotropic atomic displacements, a new methodology is introduced based on the statistical analysis of atomic column positions that provides information on the low‐frequency phonon modes. Ab initio calculations are used to support our results. Overall, this study provides quantitative STEM tools particularly well suited for nonperiodic pseudo‐2D materials, such as Sb 2 Te 3 /GeTe superlattices.

Driving a spin‐logic circuit requires the production of a large output signal by spin‐charge interconversion in spin‐orbit readout devices. This should be possible by using topological insulators, which are known for their high spin‐charge interconversion efficiency. However, high‐quality topological insulators have so far only been obtained on a small scale, or with large scale deposition techniques that are not compatible with conventional industrial deposition processes. The nanopatterning and electrical spin injection into these materials have also proven difficult due to their fragile structure and low spin conductance. The fabrication of a spin‐orbit readout device from the topological insulator Sb 2 Te 3 deposited by large‐scale industrial magnetron sputtering on SiO 2 is presented. Despite a modification of the Sb 2 Te 3 layer structural properties during the device nanofabrication, a sizeable output voltage is measured that can be unambiguously ascribed to a spin‐charge interconversion process. The results pave the way for the integration of layered van der Waals materials in spin‐logic devices.

Engineering of chalcogenide phase-change materials at the nanoscale is required to improve the performances of ultimate size memory devices and reduce their power consumption.

2022 International Conference on Solid State Devices and Materials ,Breaking the Thermal Stability Limit of Phase-Change Materials for Embedded Memory thanks to Innovative N-doped GeSe1-xTex Alloys

Innovative nanocomposites consisting of [(GeTe) 4 nm/ C 1 nm ] 10 multilayers (MLs) deposited by magnetron sputtering are integrated in phase‐change memory (PCM) test devices with a “wall structure.” Scanning transmission electron microscopy (STEM) shows that an ML structure, with crystallized GeTe layers, is kept after integration in as‐fabricated devices and also after an additional annealing of the devices at 425 °C. The programming current (RESET current) required to reach the high resistance state of [(GeTe) 4 nm /C 1 nm ] 10 ML devices decreases by 45% after annealing at 425 °C. The reduction in RESET current is 55% and the reduction in drift coefficient is about 40% in ML devices annealed at 425 °C compared to similar devices incorporating Ge 2 Sb 2 Te 5 . STEM imaging, coupled with nano‐beam electron diffraction and electron energy loss spectroscopy, of ML devices in the high resistance state shows that the RESET current reduction after annealing is correlated to a reduction of the amorphized volume.

Certains verres de chalcogénures, alliages contenant au moins un des éléments chalcogènes (soufre, sélénium, tellure), ont suscité une attention croissante au fil des ans en raison de leur large éventail d’applications, allant de l’optique infrarouge aux mémoires non volatiles optiques et résistives. Ces dernières utilisent la capacité de certains chalcogénures à commuter rapidement et de manière réversible entre une phase amorphe fortement résistive et une phase cristalline métallique, lorsqu’on leur applique des impulsions électriques qui chauffent localement le matériau. À partir de l’analyse du fonctionnement d’une mémoire résistive à changement de phase utilisant deux types de verres de chalcogénures, nous présentons les propriétés physiques de ces derniers ainsi que des recherches menées actuellement pour poursuivre leur optimisation.

GeTe/Sb 2 Te 3 superlattices (SLs) obtained by sputtering are integrated in phase‐change memory (PCM) devices with a “wall structure”. The high structural quality of SLs deposited on TiN or SiN x layers, used as metallic bottom heater and dielectric bottom layer in PCM devices, is established by X‐ray diffraction, for as‐grown SLs and after an annealing corresponding to the maximum thermal budget during the integration process. Scanning transmission electron microscopy (STEM) images of SLs within PCM cells confirm that the SL structure is kept after integration. A robust statistical analysis on a large number of devices demonstrates unambiguously that the RESET current is lower in SL devices than in GeTe reference devices and decreases when the Sb 2 Te 3 layer thickness in the SL increases from 2 to 8 nm. STEM imaging of a PCM cell incorporating an SL demonstrates that switching from the low‐ to the high‐resistance state occurs through a melting–quenching process and is not due to crystal–crystal transition or defect reorganization in the SL, in contrast to what is commonly stated in the literature on interfacial phase‐change memories (iPCMs). The origin of the improved switching performance of SL‐based PCM devices is discussed, linked with the impact of swapped bilayers.

Sb2Te3 is a layered material with outstanding properties leading to applications in interfacial phase-change memories, spintronic and thermoelectric devices. For successful integration in devices, controlling the orientation of the atomic planes of Sb2Te3 deposited by sputtering on various materials used for electrodes and on dielectric layers is required. We have succeeded in depositing Sb2Te3 thin films (thickness in the range 10-100 nm) by sputtering in industrial deposition equipments on WSi, TiN, amorphous Si as well as on native and thermal silicon oxide layers. The structure and orientation of the films were studied by x-ray diffraction. The Sb and Te planes are found parallel to the substrate, whatever the nature of the bottom material, provided that the sputtering conditions avoid a Te deficiency in the deposited film. These results show that deposition of Sb2Te3 with out-of-plane orientation on silicon oxide is actually possible, in contrast with previous literature results. Scanning transmission electron microscopy images of the interface between the Sb2Te3 film and the bottom material allow to elucidate the growth mechanism. The formation of a surface layer containing a few Te planes on top of the bottom material is mandatory for the subsequent growth of an out-of-plane oriented Sb2Te3 film by van der Waals epitaxy.

Herein, the local structure of [(GeTe)2/(Sb2Te3)m]n chalcogenide super-lattices (SLs), which are at the basis of emerging interfacial Phase-Change Memory (iPCM), is studied by x-ray absoprtion spectroscopy at the Ge-K edge. The quantitative analysis of the first coordination shells reveals that the SLs possess a structure very similar to that of thin film of the canonical Ge2Sb2Te5 (GST225) phase-change alloy. By comparing experimental data with ab initio molecular dynamics simulations of the extended x-ray absorption fine structure spectra, we show that chemical disorder is mandatory in order to reproduce the experimental data in the full spectral range. As a result, we can unambiguously conclude that Ge/Sb intermixing resulting from inter-diffusion of the GeTe and Sb2Te3 layers within SLs is inherent to SLs and is not induced by sample preparation method nor by interaction with the electron beam of electron microscopes used in all the previous studies that were suggesting such a phenomenon. We further evidence that the short Ge-Te distance is the same in GeTe and GST225 films, as well as in SLs. The main difference is the impact of disorder in GST225 and SLs. Intermixing being definitively present in [(GeTe)2/(Sb2Te3)m]n SLs, this parameter must be considered in future models aiming at going further in the understanding and the development of iPCM technology. This seems mandatory in order to allow such technology to emerge in the near future on the non-volatile memory market.

The electrical, optical, and structural properties of GeSe1−xTex phase‐change materials thin films with 0.16 ≤ x ≤1 prepared by cosputtering of GeSe and GeTe targets are studied. The crystallization temperature of the films increases significantly when the Te content decreases. Se‐rich films show an extremely large electrical contrast between their amorphous and crystalline states. A high polarizability of the crystalline phase is observed in the entire x range and is related to the presence of metavalent bonds. This is explained by the persistence of a rhombohedral crystalline phase, isostructural to GeTe, in the GeSe1−xTex films down to x = 0.16. Hence, the substitution of only 16 at% of the Se atoms by Te atoms transforms the covalent GeSe into a phase‐change material with a huge and unprecedented contrast of resistivity (up to 11 orders of magnitude) and a very high thermal stability (up to 10 years at 272 °C) for an alloy exhibiting no phase separation upon crystallization. This outstanding combination of properties makes Se‐rich GeSe1−xTex thin films extremely promising for integration in memory devices requiring a very high data retention such as automotive and embedded applications.

The outstanding properties of chalcogenide phase-change materials (PCMs) led to their successful use in innovative resistive memory devices where the material is switched between its amorphous and crystalline phases. However, PCMs are easily oxidized at interfaces. Oxidation is detrimental to device performances. In particular, it reduces the data retention time since oxidized PCMs crystallize at a lower temperature than nonoxidized ones. The aim of this study is to investigate how oxidation affects the crystallization process of germanium telluride (GeTe), a prototypical PCM. By using advanced scanning transmission electron microscopy (STEM) techniques, including spatially resolved correlations between composition maps measured by energy-dispersive X-ray (EDX) spectroscopy and structural information deduced from electron diffraction patterns and high-resolution X-ray photoelectron spectroscopy, we obtained a thorough description of the local chemistry and structure of an oxidized GeTe thin film, partly crystallized by heating an initially amorphous film at -180 degrees C. Under an oxide layer consisting of amorphous GeOx and TeOx, the upper part (similar to 30-40 nm thick) of the film consists of segregated amorphous GeOx, crystalline GeTe, and, strikingly, pure Te crystallites. The bottom part of the film, in which no oxygen has penetrated, stayed amorphous. This study reveals why oxidation promotes crystallization of GeTe through segregation of Te regions and heterogeneous nucleation. These results explain why oxidation at the surface or interfaces reduces the crystallization temperature of GeTe (by 50 degrees C with respect to a nonoxidized material) and shed light on the major impact of interface chemistry on the crystallization mechanism of PCMs used in resistive memory devices.

The phase change from the amorphous to crystalline state which occurs upon thermal annealing in prototypical Ge2Sb2Te5 and nitrogen-doped Ge2Sb2Te5 phase-change-materials (PCM) thin films is studied by concomitant, complementary and combined in situ and ex situ X-ray diffraction (XRD) and X-ray reflectivity (XRR) techniques. It is demonstrated that combined in situ X-ray scattering techniques allow accurate investigation and clarification of the structural, morphological and mechanical variations occurring in the films upon crystallization. The crystallization process is correlated with volume shrinkage (densification and thickness reduction) and with structural change with a tensile strain build-up. The comparison of Ge2Sb2Te5 and nitrogen-doped Ge2Sb2Te5 reveals a significant slowdown of the crystallization process, induced by the incorporation of nitrogen. However, the mechanisms involved in the phase change are not strongly modified by the incorporation; rather, the crystallization process is inhibited because of the presence of nitrogen. In this way, different stages of the crystallization process can be observed. The combined XRD/XRR analysis gives new insights on the stress components built up in phase-change materials. First, at the early stage of crystallization, a large hydrostatic tensile stress builds up in the PCM thin film. Afterwards, concomitant grain growth, viscous flow, densification and thickness accommodation are observed, which lead to a partial stress relaxation in the PCM films. This combined characterization technique offers a new approach that may further our understanding of the phase change involved.

Chalcogenide phase-change materials (PCMs), such as Ge-Sb-Te alloys, have shown outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, recently, in non-volatile resistive memories. The latter, known as PCM memories or phase-change random access memories (PCRAMs), are the most promising candidates among emerging non-volatile memory (NVM) technologies to replace the current FLASH memories at CMOS technology nodes under 28 nm. Chalcogenide PCMs exhibit fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties leading to a unique set of features for PCRAMs, such as fast programming, good cyclability, high scalability, multi-level storage capability, and good data retention. Nevertheless, PCM memory technology has to overcome several challenges to definitively invade the NVM market. In this review paper, we examine the main technological challenges that PCM memory technology must face and we illustrate how new memory architecture, innovative deposition methods, and PCM composition optimization can contribute to further improvements of this technology. In particular, we examine how to lower the programming currents and increase data retention. Scaling down PCM memories for large-scale integration means the incorporation of the PCM into more and more confined structures and raises materials science issues in order to understand interface and size effects on crystallization. Other materials science issues are related to the stability and ageing of the amorphous state of PCMs. The stability of the amorphous phase, which determines data retention in memory devices, can be increased by doping the PCM. Ageing of the amorphous phase leads to a large increase of the resistivity with time (resistance drift), which has up to now hindered the development of ultrahigh multi-level storage devices. A review of the current understanding of all these issues is provided from a materials science point of view.

Van der Waals layered GeTe/Sb2Te3 superlattices (SLs) have demonstrated outstanding performances for use in resistive memories in so-called interfacial phase-change memory (iPCM) devices. GeTe/Sb2Te3 SLs are made by periodically stacking ultrathin GeTe and Sb2Te3 crystalline layers. The mechanism of the resistance change in iPCM devices is still highly debated. Recent experimental studies on SLs grown by molecular beam epitaxy or pulsed laser deposition indicate that the local structure does not correspond to any of the previously proposed structural models. Here, a new insight is given into the complex structure of prototypical GeTe/Sb2Te3 SLs deposited by magnetron sputtering, which is the used industrial technique for SL growth in iPCM devices. X-ray diffraction analysis shows that the structural quality of the SL depends critically on its stoichiometry. Moreover, high-angle annular dark-field-scanning transmission electron microscopy analysis of the local atomic order in a perfectly stoichiometric SL reveals the absence of GeTe layers, and that Ge atoms intermix with Sb atoms in, for instance, Ge2Sb2Te5 blocks. This result shows that an alternative structural model is required to explain the origin of the electrical contrast and the nature of the resistive switching mechanism observed in iPCM devices.