Phase‐Change and Ovonic Materials (Sixth Edition)

It is with great pleasure that we introduce the sixth edition of the special issue on phase-change and ovonic materials, published annually in conjunction with the European Symposium on Phase Change and Ovonic Science (E\PCOS). Like its predecessors, this latest edition reflects the enduring strength and vitality of our community, built over 23 years of E\PCOS conferences. The 2024 edition, held last September in Leipzig (Germany), once again demonstrated the dynamic and flourishing state of research in this field. As with previous editions, this issue presents recent scientific and technological breakthroughs in phase-change and ovonic materials, while also exploring promising new applications. In addition to covering core topics, it highlights emerging interests in areas such as neuromorphic computing, nonlinear optics, and phase-change photonics. The issue aims to offer a broad and up-to-date overview—both experimental and theoretical—that is valuable to both established experts and new researchers entering the field. For those newer to our evolving community, it's worth revisiting the origins of E\PCOS. Founded in Switzerland in 2001, the symposium was originally created to advance the fundamental science of phase-change memory (PCM) materials, particularly in rewritable optical storage technologies such as CDs, DVDs, and Blu-ray Discs. Back then, it was known as the PCOS symposium. In 2005, its scope expanded to include the broader field of ovonic and phase-change science. E\PCOS can trace its roots even further back to the first PCOS symposium held in Japan in 1990, thanks to the vision and guidance of Professor Masahiro Okuda, who served as an early advisor to E\PCOS. In recent years, the field has grown considerably, branching into new and exciting directions. While the pioneering contributions of the late Stanford Ovshinsky remain central to the field, current research now spans nonvolatile electronic memories, optoelectronics, photonics, energy applications, and neuromorphic systems. The 2024 E\PCOS conference reaffirmed its role as a leading international forum for innovation and collaboration. Following the success of the 2023 edition in Rome, the 2024 event in Saxony brought fresh challenges and reaffirmed the strong ties between academia and industry. By capturing this growing diversity, the sixth special issue continues to play an important role in documenting and shaping the evolution of E\PCOS and the field at large. PCM remains a prominent focus in the E\PCOS community, due to its critical role in enabling 3D memory architectures when paired with an Ovonic selector based on a chalcogenide glass—especially in crossbar applications. The paper from Boniardi and Redaelli [pssr.202500052] is an excellent introduction to the history of the field of PCM. This review traces the evolution of PCM technology, from Stanford Ovshinsky's foundational work on ovonic threshold switching and ovonic memory switching effects to ongoing advancements in cell architecture and material research. PCM is a leading emerging memory technology, valued for its ability to switch between high-conductivity and high-resistivity phases in chalcogenide phase-change materials to store data. Material innovation has been pivotal, first in optical disk technology and now in enabling PCM's integration into embedded applications like smart cards and microcontrollers. Looking ahead, PCM is poised to play a key role in artificial intelligence accelerators for edge computing and broader Internet of Things applications. The search for new phase-change materials beyond traditional GeTe–Sb2Te3 alloys continues to drive research efforts worldwide, with the aim of improving PCM performance and addressing technological challenges such as reducing energy consumption or improving endurance, as well as reducing resistance drift during ageing, with a view to reliable multilevel storage. In that context, the field of GeSbTe (GST) super-lattices (SLs) is still promising, as shown by Zhu et al. In contrast to traditional high-temperature deposition methods, a low-temperature atomic layer deposition approach at 90 °C is shown to enable the growth of highly crystalline SL films with favorable electrical characteristics like multilevel behavior and low resistance drift. This point is also examined by Ritarossi et al. using model simulations of Sb/TiTe2 superlattices, demonstrating that the Sb layer can undergo a complete switching cycle—amorphization (RESET) and recrystallization (SET)—without melting the TiTe2 confining layer. However, achieving this requires extremely high quenching rates to prevent unwanted recrystallization, and TiTe2 nanoconfinement does not significantly enhance the stability of amorphous Sb. The findings highlight the need for precise control over growth conditions and switching protocols, as real devices may face structural degradation due to imperfections and thermal stress. Melting-free phase-change materials are also promising candidates for reducing the energy required for switching in PCM devices. Tsuruta et al. successfully fabricated CrTe thin films using magnetron cosputtering and demonstrated a crystalline-to-crystalline transition through annealing. Devices based on these films showed a large, reversible resistance change, primarily driven by variations in contact resistance due to changes in carrier concentration during phase transition. These findings highlight CrTe's potential for PCM applications and underscore the importance of optimizing device architecture for low-power, high-speed operation. Polymorphic phase-change behavior has also been explored in VTe thin films. Orihara et al. [pssr.202400430] studied the crystallization of amorphous VTe films and found modest resistance changes upon the formation of two different crystalline phases upon annealing. Yet, the small size of the resistance change was identified as a challenge for future applications as a memory device. Shportko et al. [pssr.202400390] investigated the optical properties of glassy Ga–Ge–Te alloys in an attempt to increase the number of unconventional glasses, i.e., glasses that differ significantly in their optoelectronic properties from their crystalline counterparts. Indeed, these Ga containing GeTe compounds reveal an Urbach edge, which is clear evidence for significant disorder. It increases with increasing Ga content, while only modest changes of the optical dielectric constant ε∞ are reported. Fernandes et al. [pssr.202400392] also tailor the stoichiometry of PCM films by studying Ge-rich GST (Ge2SbTe5). To study fast crystallization and amorphization processes, ultrafast laser excitation is combined with synchrotron-based XRD. These studies enable the investigation of Ge-rich GST, and show how the crystallization and amorphization are affected by nitrogen. Wang et al. [pssr.202500118] add another dimension to phase change materials. They explore the potential of magnetic switching phenomena. To do so, they explore crystallization in CrGeTe3, where a major change in the magnetic properties is identified. To increase the effect of disorder, CrSiTe3 has been explored for comparison. Indeed, it shows a higher coercive force and hence stronger pinning in amorphous CrSiTe3, which leads to a more robust spin glass state. The large magnetic contrast between the amorphous and crystalline phases makes CrSiTe3 a candidate for magnetic switching applications. For automotive applications, PCM must meet strict data retention standards, including stability for 10 years at temperatures up to 150 °C and resistance to BEOL soldering reflow at 260 °C for several minutes. Ge-rich GST alloys have proven to be well-suited for meeting these demanding conditions. Remondina et al. explore how nitrogen and hydrogen doping—individually and combined—affect the crystallization behavior of Ge-rich GST alloys, using various characterization techniques [pssr.202500035]. Nitrogen enhances thermal stability and grain refinement, while hydrogen, when co-doped, alters crystallization pathways by counteracting nitrogen's effects through NH bonding and promoting heterogeneous nucleation. These findings reveal the complex interplay between dopants and highlight opportunities to fine-tune phase-change materials for improved performance in PCM applications. The optical properties of phase-change materials remain a major area of interest, now approached with a renewed focus and perspective. (GeSe)1-x(GeTe)x, or alternatively described as GeSe1-xTex chalcogenide films, are another intriguing material class since such films have significantly lower losses for sufficiently low × values, due to a larger bandgap. Albanese et al. [pssr.202400413] studied how the crystallization temperature can be further increased if these films are doped with nitrogen. They demonstrate that this enables the realization of chalcogenide films suitable for on-chip low-loss reconfigurable photonics and nonlinear device applications. Remarkably, these films maintain a high transparency in the mid-infrared regime in both the amorphous and crystalline states. In, Hoff et al. [pssr.202500125] investigate coherent phonons in GeTe thin films using femtosecond laser excitation, revealing that phonon behavior—particularly the A1g and Eg modes—varies significantly with film thickness and excitation intensity. The findings highlight thickness-dependent phenomena such as phonon mode softening and hardening, attributed to metavalent bonding and bond confinement effects, which alter vibrational and optical properties. Additionally, this research demonstrates that coherent phonons can be actively manipulated through pump pulse polarization and timing, offering pathways for ultrafast control in optoelectronic and quantum applications. Traditional phase-change materials like Ge2Sb2Te5 exhibit considerable absorption, even in the telecom wavelength range (1250–1750 nm). Therefore, for specific optical applications so-called low-loss PCM materials like Sb2Se3 and Sb2S3 have come into play. In [pssr.202400311] this research is extended to arsenic sesqui-chalcogenides using density functional theory (DFT) calculations and DFT-based ab initio molecular dynamics (AIMD) simulations. Indeed, the extinction coefficient reduces dramatically when going from crystalline As2Te3 to As2Se3 or As2S3. However, at the same time, the optical contrast between the amorphous and crystalline phases vanishes for As2Se3 and As2S3, but remains large for As2Te3. This behavior is explained well by the presence of metavalent bonding in crystalline As2Te3 and still covalent bonding in crystalline As2Se3 and As2S3 (and all the amorphous chalcogenides). Keeping DFT calculations and DFT-based AIMD simulations as the main research tool, in [pssr.202500037] they are used to study the effect of irradiation on the structure of amorphous GST225. PCM is considered a radiation-hard memory technology, but more research is needed to understand the effect of irradiation on the structure and properties of PCM materials. Moreover, irradiation is a potential tool to further tailor the properties of PCM materials, e.g., to reduce the thermal conductivity of the amorphous phase even more. In [pssr.202500037] particularly the development and relaxation of liquid-like states in the glassy structure are studied, providing insights into the structure dynamics and the radiation tolerance of this amorphous material. Staying within the amorphous GST225 phase, in [pssr.202400416] Gokirmak et al. model conductive filament formation in metal-insulator-metal structures with as insulator the amorphous GST by 2D COMSOL multiphysics simulations using an intricate model for the relevant electron transport phenomena. The average carrier activation energy (EA) as a function of temperature for hopping transport between traps in the GST is obtained from experiments, but then assuming local variations in EA in the 2D model allows for instabilities and thermal runaway effects leading to local filament formation that can nicely explain the insulating to conducting transition (with I-V ‘snapback’) observed in PCM and OTS devices. Advanced nanoscale confined PCM architectures are explored to optimize the performance of PCM, in particular, reducing RESET current (IRESET) and switching power. However, in cylindrical-based confined devices, variations in the ellipticities of the heater and PCM on top can lead to considerable variations in IRESET for nanoscale devices. In [pssr.202400424] Mannivanan et al. focus on nanotube PCM devices, where variations in the ellipticities of both the inner and outer radii of the PCM material are possible. They use 3D-TCAD simulations to systematically calculate the effect of these varying ellipticities on IRESET and again find that these effects will play an important role, particularly in the smaller technology nodes. In conclusion, we are delighted to present the sixth and most recent issue of E\PCOS 2024, continuing our tradition of highlighting cutting-edge research in phase-change and ovonic science. As in previous years, we hope this edition fosters connections within our community as we look ahead to the next E\PCOS meeting, returning to Marseille from September 23 to 26, 2025 [https://epcos2025.fr]. We eagerly anticipate welcoming you to this exciting event that will be hosted in the beautiful Jardin du Pharo, where we will mark 24 years of E\PCOS and celebrate the continued vitality of research in phase-change and Ovonic science. Pierre Noé, Bart J. Kooi and Matthias Wuttig Guest editors P.N. thanks the support of the French National Research Agency (ANR) through projects OCTANE ANR–20–CE24–0019 and PEPR Electronique through the EMCOM project, as well as the European Union's Horizon Europe research and innovation programme under grant agreement No. 101070238 (‘NEUROPULS’: Neuromorphic Energy-Efficient Secure Accelerators Based On Phase-Change Materials Augmented Silicon Photonics). P.N. also thanks the French research group “GdR CHALCO” and the French Chalcogenide family (https://gdrchalco.cnrs.fr/). M.W. acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG) via the collaborative research center Nanoswitches (SFB 917). At last, authors highly appreciate again the time and efforts of all contributors to this focus issue.