Interfacial phase-change materials (iPCMs), composed of superlattice architecture with GeTe and Sb2Te3, show outstanding advancements in phase-change characteristics over alloy-based PCMs. Owing to their strongly aligned anisotropic structures, iPCMs reduce unnecessary entropic loss during phase-change operation. By conducting meticulous structural analysis, this study elucidates the structural and atomic behaviors of the phase-change process within different memory states in iPCM. Moreover, the overall phase-change properties of iPCMs are enhanced with doping Ti by selectively modulating the covalency of interatomic bonds. In addition to the improvements of the conventional phase-change characteristics, suppressed resistance drift and confined multi-level states present the potential of Ti-doped iPCM as a neuromorphic memristor. The bonding characteristics of iPCMs can be precisely tuned by selectively incorporating transition metals into either the even or odd layers of the superlattice, thereby engineering anisotropic covalency at the atomic scale. This study lays the foundation for designing frameworks for phase-change superlattices, which facilitate the development of next-generation neuromorphic memories.

Disordering atomic structures offers a functionality hardly expected in ordered states, including phase-change memory and photonic computing, offering the potential to renovate von Neumann architecture for neuromorphic engineering with low latency. However, significant energy consumption during the disordering compromises the data reliability and integration efficiency, which is traditionally regarded to take place after melting. Here, we investigate time for disordering in isochronal and isochoric manners, challenging the conventional melt-quenching theory. The disordering times of pure Sb, Ag-In-Sb-Te, and In surpass that of InSb by over 50 times, despite a higher melting point and a lower laser absorption rate of Sb compared to InSb. This nontrivial contrast is elucidated by theoretical calculation that delocalized electrons enable flexible modification of bond lengths even below the melting points where undermined bond directionality provides room for atoms to depart from their original positions. Facilitated by delocalized electrons, specifically through metavalent and metallic bonding rather than covalent bonding, atoms can be disordered without undergoing melting, which aligns with the rapid disordering of Sb compared to that of InSb. The results bridge the unaddressed gap between chemical interaction and kinetic behaviors during the disordering and suggest design rules highlighting electron-delocalization rather than solely relying on melting points to improve energy efficiency.

While Te-based ovonic threshold switching (OTS) materials offer advantages such as low-voltage operation and fast switching speed, their relatively low crystallization temperature compared to S- or Se-based counterparts results in poor thermal stability and limited electrical endurance. Various strategies, including element doping and complex composition design, have been explored to address these limitations. In this study, the OTS device properties of BxTe thin films, a simple two-component system, were systematically investigated across a wide range of compositions. Electrical measurements, band structure analysis, and bonding configuration studies revealed that increasing the boron content enhanced the insulating properties in the off-state and improved the thermal stability of the films. Through composition optimization, BxTe devices exhibited significantly lower off-current densities while maintaining excellent switching characteristics, outperforming previously reported Te-based OTS materials. Furthermore, detailed analysis confirmed that the unique bonding configurations and structural features associated with boron incorporation play a critical role in achieving the outperformances in BxTe devices.

Changes in bond types and the reversible switching process between metavalent and covalent bonds are related to the operating mechanism of the phase-change (PC) behavior. Thus, controlling the bonding characteristics is the key to improving the PC memory performance. In this study, we have controlled the bonding characteristics of GeTe/Sb2Te3 superlattices (SLs) via bismuth (Bi) doping. The incorporation of Bi into the GeTe sublayers tailors the metavalent bond. We observed significant improvement in device reliability, set speed, and power consumption induced upon increasing Bi incorporation. The introduction of Bi was found to suppress the change in density between the SET and RESET states, resulting in a significant increase in device reliability. The reduction in Peierls distortion, leading to a more octahedral-like atomic arrangement, intensifies electron-phonon coupling with increased bond polarizability, which are responsible for the fast set speed and low power consumption. This study demonstrates how the structural and thermodynamic changes in phase change materials alter phase change characteristics due to systematic changes of bonding and provides an important methodology for the development of PC devices.

Interfacial phase-change materials (iPCM), which are alternatively stacked with GeTe and Sb2Te3 in the superlattice structure, have been highlighted as next-generation PCM with improved overall phase-change characteristics. However, several studies have reported that a melt-quenching process, whereby the initial superlattice structure is not maintained within the reversible switching process, rather than the initially proposed melting-free phase-change mechanism, occurs during operation. Herein, GeSbTe superlattices are synthesized using molecular beam epitaxy, and the reversible phases of the superlattice obtained by irradiation with an optical pulsed laser (KrF; 280 nm, 25 ns) and re-annealing or by applying different electrical pulses are investigated through careful structural analyses. The results revealed that Te atoms are aligned parallel to the interface regardless of the reversible phase, whereas cations and inherent vacancies are distributed differently during the phase-change process. The stability of memory cells with cycling operations can be enhanced by enriching inherent vacancies, and the switching energy can be reduced by expanding the interspaces via doping engineering. The inherent vacancies play a crucial role in the GeSbTe superlattice. GeSbTe superlattice has different phases among the reversible phases depending on the degree of alignment of these vacancies. The reversible phases of GeSbTe superlattice correspond to stable (hexagonal-close packed) and metastable (cubic) crystal phases, not crystal and amorphous, which is progressed in alloy-based PCM.image (c) 2024 WILEY-VCH GmbH

Ovonic threshold switching (OTS), characterized by a rapid resistance drop in chalcogenide glass, has enabled the realization of memory and selectors. Despite over five decades of development, the challenges in characterizing the transient switching in amorphous materials upon reaching the threshold voltage have hindered the establishment of its underlying mechanism. This study uses femtosecond terahertz spectroscopy and ab initio simulation to elucidate the dynamics of the free-carrier and IR-active phonons involved in OTS. Specifically, in Te-rich amorphous GeTe, the generation of transient phonons is observed within picoseconds, a phenomenon associated with an increased Born effective charge due to the alignment of Te-centered bonds. The findings demonstrate a correlation between the enhancement of polarizability, due to orbital alignment during the disorder-order structural transition while maintaining a macroscopic amorphous structure, and the switching behavior. These results provide valuable insights into the enigmatic OTS phenomenon.

Using a monochromator in transmission electron microscopy, a low-energy-loss spectrum can provide inter- and intra-band transition information for nanoscale devices with high energy and spatial resolutions. However, some losses, such as Cherenkov radiation, phonon scattering, and surface plasmon resonance superimposed at zero-loss peak, make it asymmetric. These pose limitations to the direct interpretation of optical properties, such as complex dielectric function and bandgap onset in the raw electron energy-loss spectra. This study demonstrates measuring the dielectric function of germanium telluride using an off-axis electron energy-loss spectroscopy method. The interband transition from the measured complex dielectric function agrees with the calculated band structure of germanium telluride. In addition, we compare the zero-loss subtraction models and propose a reliable routine for bandgap measurement from raw valence electron energy-loss spectra. Using the proposed method, the direct bandgap of germanium telluride thin film was measured from the low-energy-loss spectrum in transmission electron microscopy. The result is in good agreement with the bandgap energy measured using an optical method.

Interfacial Phase Change Memory (iPCM) retrench unnecessary power consumption due to wasted heat generated during phase change by reducing unnecessary entropic loss. In this study, an advanced iPCM (GeTe/Ti-Sb2Te3 Superlattice) is synthesized by doping Ti into Sb2Te3. Structural analysis and density functional theory (DFT) calculations confirm that bonding distortion and structurally well-confined layers contribute to improve phase change properties in iPCM. Ti-Sb2Te3 acts as an effective thermal barrier to localize the generated heat inside active region, which leads to reduction of switching energy. Since Ge-Te bonds adjacent to short and strong Ti-Te bonds are more elongated than the bonds near Sb-Te, it is easier for Ge atoms to break the bond with Te due to strengthened Peierls distortions (Rlong/Rshort) during phase change process. Properties of advanced iPCM (cycling endurance, write speed/energy) exceed previous records. Moreover, well-confined multi-level states are obtained with advanced iPCM, showing potential as a neuromorphic memory. Our work paves the way for designing superlattice based PCM by controlling confinement layers.

In this study, spin-to-charge conversion (SCC) of various topological materials with ferromagnet is investigated using spintronic terahertz (THz) emission spectroscopy. Compared with other topological materials, significantly large THz emission is observed for topologically nontrivial phases of Bi1-xSbx (x > 0.2) that predominantly originates from the topological surface state. When Bi is superposed above a certain stoichiometry of Bi1-xSbx, it plays a crucial role in generating a highly spin-split state and enhancing the spin-mixing conductance, resulting in colossal THz emission. This proves that improving the SCC efficiency through interface engineering is a useful strategy to design a powerful spintronic device. Collectively, this study proposes a methodology for systematically analyzing SCC efficiency or spin Hall angle using THz emission spectroscopy and offers an efficient structure for future spintronic devices.

Reversible conversion over multimillion times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, an extended category of crystal structures from stable states via redistribution of vacancies, research on kinetic behavior of the vacancies is highly in demand. However, it remains lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies of Sb2Te3 gradually occurs with diverged energy barriers owing to their own activation code: the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Studies on the behavior can be further applied to multiphase superlattices composed of Sb2Te3 (2D) and GeTe (3D) sublayers, which represent superior memory performances, but their operating mechanisms were still under debate due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb2Te3 (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could only be achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies toward vacancy engineering are expected in versatile materials.

Interfacial phase-change memory (iPCM), comprising alternating layers of two chalcogenide-based phase-change materials—Sb2Te3 (ST) and GeTe (GT)—has demonstrated outstanding performance in resistive memories. However, its comprehensive understanding is controversial. Herein, the phase-change characteristic of iPCM is identified using atomic scale imaging, X-ray diffraction, and chemical analysis with first-principles density functional theory (DFT) calculations. By inducing laser pulsing, the ST/GT superlattice structure in the low-resistance state tends to reversibly convert into the modified metastable face-centered cubic (fcc) GeSbTe structure in the high-resistance state. This transition is driven by Ge atom rearrangement to pre-existing vacancy layers and ordered vacancy-layer formation. DFT atomistic modeling shows that the resistance difference of 102 orders between low- and high-resistance states is a direct consequence of the intercalation of Ge atoms into the vacancy layer. These results provide insights into iPCM phase-change mechanisms and phase-change random access memory design with low energy and high speed.

Germanium telluride (GeTe), which has long been used in phase-change devices, has very unique structural characteristics. While atomic movement and related bond-order switching have been believed to determine the ferroelectric characteristics of this compound, structural changes accompanying ferroelectric switching have not been examined. In this study, using transmission electron microscopy and Raman spectroscopy, we directly observed the atomic movement and the related change in the local structure of GeTe during ferroelectric switching under an applied E-field. The crystal symmetry of GeTe could be alternated between two phases (R3m (rhombohedral) and Cm (monoclinic)) by changing the compound's polarization direction, indicating that the ferroelectric switching behavior of GeTe can be controlled. This observation suggests that ferroelectric switching of GeTe may originate from the phase transition between R3m and Cm through the formation of a morphotropic phase boundary (MPB). Since these two phases can generate MPBs with similar energies, the rotation of lone-pair direction during the phase transition can effectively change the polarization direction. The exotic phenomenon of ferroelectric switching, which involves changes in the chemical bonds, can be predicted on the basis of the role of several dopants through DFT calculations. (c) 2021 Published by Elsevier Ltd.

Phase-change memory (PCM) is the most promising candidate for next-generation memory devices to replace both dynamic random-access memory and flash memory. Sb2Te3 is a promising phase-change material because of its fast operation speed; however, it has poor thermal stability. The operation mechanism of PCMs is based on the Joule heating process; consequently, sufficient thermal stability is one of the most important factors for scaling PCMs in commercialized devices. Herein, a remarkable increase in the thermal stability of C-incorporated Sb2Te3 is reported. The crystallization and 10-year retention temperatures of C-incorporated Sb2Te3 increased to 66% and 52%, respectively, while a reliable operation speed was maintained as compared to that of Ge2Sb2Te5, an existing commercialized phase-change material for 3D Xpoint memory. Regions with highly incorporated C were observed in the Sb2Te3 crystal grains by transmission electron microscopy. Ellipsometry and X-ray photoelectron spectroscopy revealed increased electron localization caused by interstitial C atoms located between Sb and Te, which effectively hindered grain growth and significantly increased thermal stability. The thermal stability can be further enhanced by adjusting the C content, although some of the device operation characteristics are slightly degraded. This study suggests that Sb2Te3 can be easily and effectively utilized as a suitable material for practical applications involving PCM devices with high thermal stability.

Although Sb2Te3, as a candidate material for next-generation memory devices, has attractive properties such as higher operation speed and lower power consumption than Ge2Sb2Te5, its poor stability prevents its application to commercial memory devices. Transition metal dopants provide enhancements in its phase change characteristics, improving both thermal stability and operation energy. However, the enhancement mechanism remains to be sufficiently investigated, and standard properties need to be achieved. Herein, the phase change properties of Sb2Te3 are confirmed to be enhanced by the incorporation of a heavy transition metal element such as Ag. The crystallization temperature increases by nearly 40%, and the operation energy is reduced by approximately 60%. These enhancements are associated with the changes in the local Sb2Te3 structure caused by Ag incorporation. As the incorporated Ag atoms substitute Sb in the Sb-Te octahedron, this turns into a Ag-Te defective tetrahedron with a strong Ag-Te bond that induces distortion in the crystal lattice. The formation of this bond is attributed to the electron configuration of Ag and its fully filled d orbital. Thus, Ag-doped Sb2Te3 is a promising candidate for practical phase change memory devices with high stability and high operation speed.