Phase‐Change and Ovonic Materials (Fourth Edition)

Abstract

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). We have to admit that the 2022 edition of E\PCOS had a special spirit as it marked the long-awaited return to a face-to-face on-site symposium after a two-year hiatus due to the Covid crisis. Last September, the 2022 edition of E\PCOS was the most successful in terms of attendance in the history of E\PCOS. This is no coincidence and once again, in this editorial, Harish Bhaskaran, Luci Bywater and the Oxford team are sincerely thanked on behalf of the entire E\PCOS community for making this success possible even if unfortunately, some of the E\PCOS major actors could not join us at the Wolfson College in Oxford this year. As in the three previous editions, this special issue again aims to summarize recent and innovative scientific and technological achievements in the field of phase-change materials, as well as their possible new fields of application. In addition to recent advances in this field, the objective is also to present emerging interests in neuromorphic computing, phase-change and nonlinear photonics or plasmonics. This special issue thus provides an overview of the state of the art, both experimental and theoretical, for experienced and young researchers interested in these topics. As usual, let us first recall, for the younger and newer members of our ever-evolving community, that E\PCOS was born in Switzerland in 2001, with the aim to provide a platform to discuss and promote the fundamental science of phase-change materials (PCM). This goal also included their applications in rewritable optical discs (e.g., first with CDs and later with the successfully developed DVD and Blu-ray Disc formats) and thus initially PCOS referred to phase-change optical storage (which was diversified in 2005 to phase-change and ovonic science). 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. In recent years, the field has diversified considerably. While the scientific and technological fingerprints of the field’s founding father, the late Stanford Ovshinsky, are still very recognizable, the number of topics covered has continued to grow significantly with applications including non-volatile electronic memories, optoelectronics, photonics, and neuromorphic computing. The 2022 edition of E\PCOS has confirmed that E\PCOS is the premier international conference on this exciting and evergreen topic. This 2022 edition, which follows the 2021 virtual edition, was somewhat of a challenge for the E\PCOS community. However, its unprecedented success confirmed the close ties between key players in the field, both academic and industrial. By again covering a rich variety of topics beyond phase-change memories, this fourth special issue will again mark the history of E\PCOS. The paper from Park et al. on the use of Sb2Te3/TiTe2 heterostructure to replace the conventional and canonical GST (Ge2Sb2Te5) alloy for storage class memory and neuromorphic computing hardware is a first illustration [pssr.202200451]. Indeed, for these recently introduced novel phase-change memory (PCM) applications, faster SET speed and lower RESET energy than those obtained with the usual GST225 alloy are required. In this study, PCM devices based on multilayers made of amorphous Sb2Te3 and TiTe2 nanolayers deposited by sputtering exhibit fast SET speed ( 30 ns), RESET energy reduction of more than 80% compared to the GST-based reference PCM with also lower resistance drift in the high resistance state. These very promising results will deserve future work in the community, for example to evaluate the endurance of this novel type of heterostructure during programming cycles. 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. They concluded that their GST alloy-based model could account for the impacts of dynamic changes in crystallinity during device operation and that their results were consistent with experimental observations, providing a better understanding of device dynamics. This model would allow any device geometry to be studied to explore the effect of programming pulse and material engineering, as well as device architecture on device performance. For example, the simulation results predict the impact of thermoelectric effects on RESET current requirements and the significant role of heater height on heat loss and thus RESET current. One of the other challenges of PCM technology for storageclass memory applications is related to improving storage density by using of multilevel cells (MLCs), as shown by Zhao et al. P. Noé Univ. Grenoble Alpes CEA, Leti F-38000 Grenoble, France E-mail: pierre.noe@cea.fr B. J. Kooi Zernike Institute for Advanced Materials University of Groningen Nijenborgh 4, NL-9747 AG Groningen, The Netherlands M. Wuttig I. Physikalisches Institut (IA) RWTH Aachen University Sommerfeldstraße, 52074 Aachen, Germany M. Wuttig JARA-Institut Green IT JARA-FIT Forschungszentrum Jülich GmbH and RWTH Aachen University 52056 Aachen, Germany