Phase‐Change and Ovonic Materials (Second Edition)

It is our great pleasure to introduce this second edition of the focus issue on phase-change and ovonic materials. This special aims summarize recent progress in rapidly developing field materials their numerous applications. provides a snapshot state-of-the-art, both experimental theoretical, for experienced young researchers interested subject area. Since its very first 2019, we expect publish yearly within framework European Phase-Change Ovonic Symposium (E\PCOS). E\PCOS was born Switzerland 2001, with aim providing platform discuss promote basic science (PCMs). goal also included applications rewritable optical disks, which were at that time commercialized as CDs. Later on, DVD Blu-ray disk formats successfully developed lastly 2017 nonvolatile memories introduced market, defining further milestone. had grown out symposium Optical Storage technology (PCOS) based Japan 1990, thanks Professor Masahiro Okuda, who advisor from beginning. In years, has diversified tremendously. While scientific technological footprints founding father field, late Stanford Ovshinsky, are still clearly visible, number topics addressed increased significantly. To date, become leading international conference exciting dynamic topic. However, 2020 worldwide COVID-19 crisis constrained program committee cancel meeting supposed be held Oxford usual early September. Therefore, sounds unprecedented all us. By showcasing latest results well outlining ongoing challenges can raised by active community, hope it will help maintain strong link tremendous debates between main actors players academic industry, waiting hopefully better days. Besides, mark history an large papers published, covering rich variety beyond memories. illustrated through review paper PCM devices physics Yanyun Ren et al. (article 2000394), showing promising features neuromorphic applications, Minh Anh Luong 2000471), describing some unique uncommon Ge-Rich GeSbTe alloys compatible high-temperature environments those embedded addition, 2000443) provide more details impact nitrogen crystallization microstructure such amazing alloys. A journey emphasizes once again, despite present wide use many daily underlying physical mechanisms origin properties portfolio (PCM) opportunities advance understanding Understanding bonding PCMs been seen opportunity unravel unconventional crystalline PCMs. Tae Hoon Lee 2000516) pursue approach conclude multicenter hyperbonding, i.e., electron configuration where three atoms together four electrons (3c/4e) straightforward explanation striking material properties, high Born effective charges or contrast amorphous phase. similar motivated work presented Yudong Cheng 2000482). Yet, these authors invoke different mechanism, about one located adjacent atoms. They make another step forward map identifies employ mechanism separate well-known textbook cases ionic, metallic, covalent bonding. Molecular simulations electronic structure analyses density functional theory (DFT) used Nian-Ke Chen 2000441) study formation crystal nuclei matrix Ge2Sb2Te5 (GST). insights atomic movements, evolution motifs during nucleation GST. Yu-Xing Zhou 2000403) development, machine learning supercooled liquid PCMs, focusing demonstrate potential superior length scales compared molecular-dynamics-based DFT calculations. The same utilized Felix Mocanu 2000485), chemical Sb2Te3. determination arrangement classification states vicinity Fermi level enables them kinetics. networks topic Matthieu Micoulaut 2000490), GeTe2 find evidence tetrahedrally bonded motifs, revisiting vivid debate shaped community last decade, when competing models discussed. novel compound discussed Dario Baratella 2000382), Ga4Sb6Te3 reveal octahedral coordination, combining calculations genetic algorithm. Computing Raman spectra environments, they offer pathway discriminate among possible arrangements. spectroscopy, direct simple nondestructive method demonstrated Eugenio Zallo 2000434) quantify “van der Waals” lamellae epitaxially films, highlighting advantage MBE growth technique precisely controlling film thickness. Jean-Pierre Gaspard 2000536) focuses facet materials, anharmonic properties. He concludes interatomic explain vibrational including Grüneisen parameter transverse modes, leads low thermal conductivity, prerequisite energy efficiency. For characterization major remains thin films. Koichi Shimakawa 2000411) show grain boundaries significantly affect THz DC conductivities (distorted rock-salt structure) GST temperatures >300 K, suggesting intra- intergrain transport jointly control Magnetic behavior studied Chao 2000425), Fe48Mo14Cr15C15B6Y2 exhibits spin-glass behavior, whereas ferromagnetic, rendering suitable magnetic material. Martina Tomelleri 2000451) GeSe1−xTex GeSe not useful PCM, but replacing least 1/6 Se Te atoms, becomes resistivity stability (up 10 years 272 °C) alloy without significant phase separation upon crystallization, making highly automotive Although being important characterizing glass transition Tg generally elusive, because obscured crystallization. Julian Pries 2000478) new assess GeTe selective pure sub-Tg annealing Ge15Te85, gradual change composition surroundings, toward GeTe. devoted device physics. Yi Shuang 2000415) nitrogen-doped Cr2Ge2Te6 detail, unusual resistance generated bulk, rather only interface tungsten electrodes. Alin Velea 2000475) describe multilevel memristive devices, realize intermediate values. Using space-charge-limited conduction model reproduce measured Mozhikunnam Sreekrishnan Arjunan 2000354) less material, In3SbTe2. high-stability low-noise switching employing laser pulses just 5 ns. exploring could open memory instance, Nadim Kanan 2000422) propose innovative programming strategy cross-talk coupling achieve logic functions decrease complementary metal–oxide–semiconductor (CMOS) footprint necessary high-density arrays. Further improvement reliability challenge under scope Hwanwook 2000419), investigate stuck reset failure means finite element simulation stress. context, faced technology, deals reduction required switch two states. aim, optimizing architecture has, up now, order increase hence Joule heating efficiency cell pulses. using approach, Shogo Hatayama 2000392) go one-step alloy, namely Cr2Ge2Te6, electrode Indeed, LaB6 instead commonly W permits contact resistivity, reducing total operation device. Similarly, Damien Térébénec 2000538) successful integration oriented [(GeTe)2/(Sb2Te3)m]n superlattices (SLs) scanning transmission microscopy imaging cell, unambiguously show, time, SL-based RESET state following melting–quenching than standard polycrystalline alloys, local amorphization SL. current lower improved increasing m value 2 8, corresponds Sb2Te3 quintuple layers (QLs) 1 nm each. result cross-plane confinement SLs role played stacking defects bilayer ones. Such SLs, latter, sometimes called iPCMs (for interfacial memories), attention papers. longstanding processes behind performance again widely investigated. Hisao Nakamura 2000393) thermoelectric phonon iPCM first-principles theoretical (GeTe/)2/Sb2Te3 models. simulated SL consisted Ge, Sb, planes types cation exchange. Again atomistic containing planes, Paul Fons 2000412) performed ab initio molecular dynamics explore electric effects proposed structures. Unlike previous speculation literature, effect electrical fields Ge shown negligible and, rather, strongly van Waals gaps call deep revision related mechanism. simulations, Valentin Evang 20000457) alternative amorphous–crystalline transitions ultrathin QLs. made various substrates 2D layered emphasized case topological since considered prototypical insulator (TI). TI property Alexander Kolobov 2000418) disappearance so-called Dirac cones along certain side surfaces. correspond surface characterized metallic linear dispersion. Finally, Yuta Saito 2000414) sequences Bi–Te compounds replace Sb–Te ones move most Ge–Sb–Te Ge–Bi–Te SLs. Moreover, BiTe/SbTe heterostructures DFT-based purpose future achievement devices. conclude, you enjoy “grand cru” research We wish keep us next planned September 2021. Despite large-scale vaccination programs having started, organizing regular currently too risky. Postponing year option therefore online preparation. dreamed scenario, let lively possible, celebrating 20 anniversary E\PCOS, may start decade prosper, come inspiring venues, keeping decades discussions. Pierre Noé, Bart J. Kooi, Matthias Wuttig Guest Editors B.J.K. P.N. acknowledge funding Union's Horizon innovation programme grant agreement No. 824957 (“BeforeHand”: Boosting Performance Phase Change Devices Hetero- Nano-structure Material Design). support ANR projects MAPS ANR–20–CE05 OCTANE ANR–20–CE24–0019 Auvergne-Rhône-Alpes region “Pack Ambition Recherche 2019”-NanoCHARME project. Lastly, appreciate efforts contributors Focus Issue. ovonics, chalcogenides, EPCOS Received: February 6, 2021 Revised: Published online: Noé joined department CEA 2000 engineer Si-based nanostructured microelectronics, photonics spintronics (CEA-DRFMC then CEA-INAC after 2008, Grenoble, France). 2011, he became permanent researcher CEA-Leti received Ph.D. degree 2013 Grenoble-Alpes University. started his own group Advanced Chalcogenide Materials Silicon Technologies division Leti. His interest elaboration investigations chalcogenides frontier knowledge (memories, photonics, thermoelectricity…). Kooi obtained 1995 Delft University Technology, Netherlands. then, worked Groningen (The Netherlands) assistant, associate, full professor, starting group, Nanostructured Interfaces, Zernike Institute 2009. interests nanostructure–property relations, advanced microscopy, interfaces, transformations, tellurium- antimony-based 1988 RWTH Aachen/Forschungszentrum Jülich. visiting professor several institutions Lawrence Berkeley Laboratory, CINaM (Marseille), University, Hangzhou IBM Almaden, Bell Labs, DSI Singapore, Chinese Academy Sciences Shanghai. 1997, appointed Full Aachen. heading collaborative center resistively (SFB 917), funded German Science Foundation (DFG).