R&D: Five Articles on Phase Change Technologies and Memories

R&D: Intermediate resistance and capacitance states in Ge-RICH GeSbTe phase change memory
Authors investigate intermediate states of Ge-rich GeSbTe phase-change memory cells by electrical impedance spectroscopy after partial SET and RESET programming

HAL Science Ouverte has published an article written by Adrien Delpoux, Laboratoire de Physique & Chimie des Nano-Objets (LPCNO), Toulouse, France, Sijia Ran, Centre d’Elaboration de matériaux & d’Etudes Structurales (CEMES-CNRS), Toulouse, France, Laura Pecourt, Roberto Simola, STMicroelectronics, Rousset, France, Daniel Benoit, Simon Jeannot, STMicroelectronics, Crolles, France, Alain Claverie, Centre d’Elaboration de matériaux & d’Etudes Structurales (CEMES-CNRS), Toulouse, France, and Jérémie Grisolia, Laboratoire de Physique & Chimie des Nano-Objets (LPCNO), Toulouse, France.

Abstract: “We investigate intermediate states of Ge-rich GeSbTe phase-change memory cells by electrical impedance spectroscopy after partial SET and RESET programming. The impedance response is well described by a series resistor and a parallel RC circuit, allowing extraction of state-dependent resistance and capacitance values. A resistance contrast of and a capacitance reduction of are observed between RESET and SET states. While resistance can be continuously tuned, capacitance major variation remains confined near the SET state, with crystalline-dominated cells exhibiting the highest values. Technology Computer-Aided Design simulations confirm the equivalent circuit and reveal that conduction mainly occurs along the amorphouscrystalline interface. Cells with embedded Ge or Sb grains display the largest capacitances, as these inclusions provide extra conduction pathways and localized charge storage sites that jointly influence the device’s resistance and capacitance. Overall, this work demonstrates that the electrical properties of inhomogeneous multiphase PCMs are governed by the complex network of nanoscale heterostructures present in their active regions. It also demonstrates that impedance spectroscopy is a suitable, non-destructive technique for characterizing PCM devices.“

R&D: Enhancing non-volatile memory and neuromorphic computing, integration of PRAM and OTS for scalable, energy-efficient architectures
Paper investigates the integration of Phase-Change Random Access Memory (PRAM) and Ovonic Threshold Switch (OTS) devices, emphasizing their ability to advance non-volatile memory (NVM) technologies, neuromorphic computing architectures, and energy-efficient systems

Journal of Physics D: Applied Physics has published an article written by Seoyoung Park, Division of Electronics and Electrical Engineering, Dongguk University, Seoul 04620, Republic of Korea, Minsuk Koo,Department of AI Semiconductor, School of Advanced Cross-Disciplinary Studies, University of Seoul, Seoul 02504, Republic of Korea, and Sungjun Kim, Division of Electronics and Electrical Engineering, Dongguk University, Seoul 04620, Republic of Korea.

Abstract: “This paper investigates the integration of Phase-Change Random Access Memory (PRAM) and Ovonic Threshold Switch (OTS) devices, emphasizing their ability to advance non-volatile memory (NVM) technologies, neuromorphic computing architectures, and energy-efficient systems. OTS devices’ nonlinear threshold switching effectively mitigates sneak currents in high-density crossbar arrays, while challenges like resistivity drift and structural relaxation are addressed through advanced modeling and experimental analysis. The study highlights innovations in phase-change materials, such as oxygen-doped Sb₂Te₃ and Sb₂Te₃-GeTe superlattices and leverages Atomic Layer Deposition (ALD) to enhance thermal stability, switching speed, and cycling endurance. PRAM-OTS hybrid systems demonstrate exceptional performance in spiking and multi-layer neural networks, replicating neuronal behaviors such as integrate-and-fire dynamics and spike-timing-dependent plasticity for low-latency, energy-efficient processing in artificial intelligence, robotics, and IoT applications. These scalable and reliable systems provide a robust framework for next-generation high-performance computational platforms, addressing key challenges in scalability, energy efficiency, and operational longevity.“

R&D: Impact of Te network connectivity in governing the threshold switching dynamics of amorphous GeTe and GeTe₆ devices
Results elucidate the influence of the network connectivity and its impact on local phonons in enabling extraordinarily faster TS in chalcogenide-based OTS selectors (GeTe₆) and PCM (GeTe) devices

ACS Applied Electronic Materials has published an article written by Sakthikumaran Panneerselvam, Salman Khan, Advanced Memory and Computing Research Centre, Phase Change Memory Group, Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India, Bellarmine Francis, M.S. Ramachandra Rao, Materials Science Research Centre, Nano Functional Materials Technological Centre, Department of Physics, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India, and Anbarasu Manivannan, Advanced Memory and Computing Research Centre, Phase Change Memory Group, Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India.

Abstract: “The threshold switching (TS) in amorphous chalcogenides is a critical factor governing the performance of phase-change memory (PCM) and Ovonic Threshold Switching (OTS) selector devices. While its technological relevance is well-established, the dynamics of resistance breakdown during TS, especially its relationship to the atomic-scale network, remains unclear. In this study, we investigate the influence of amorphous network connectivity and rigidity percolation on the TS dynamics of GeTe (rigid) and GeTe₆ (floppy) devices belonging to the binary Ge_xTe_100-x glass system. Time-resolved electrical measurements were performed on the as-deposited amorphous GeTe and GeTe₆-based devices to precisely capture the current evolution from subthreshold to superthreshold regimes. GeTe devices exhibit a relatively longer delay time of ∼14 ns at the threshold voltage (V_T) and require more than 70% overvoltage above V_T to reduce the delay time to an order lower value. In contrast, GeTe₆ devices switch significantly faster within a relatively shorter delay time of ∼2.6 ns at V_T and the delay time reduced to ∼800 ps within a minimal overvoltage of 26% above V_T. The voltage-dependent delay time characteristics validate a significantly sharper exponential reduction in the delay time for GeTe₆ compared to GeTe devices. Confocal Raman spectroscopic measurements were carried out on GeTe and GeTe₆ thin films, validating the role of Te–Te network connectivity in shaping the vibrational landscape and rigidity of the amorphous Ge_xTe_100-x glass systems. Together, these findings corroborate the floppy (GeTe₆) network that possesses longer Te–Te linkages, promoting higher transient phonon interactions and governing subnanosecond TS for a lower applied voltage above V_T. On the other hand, rigid (GeTe) networks dominated by Ge–Te bonding with damped Te–Te vibrational modes exhibit a delayed TS process. These results elucidate the influence of the network connectivity and its impact on local phonons in enabling extraordinarily faster TS in chalcogenide-based OTS selectors (GeTe₆) and PCM (GeTe) devices.“

R&D: Thickness-dependent optical and structural properties of chalcogenide phase-change memory thin films grown at room temperature by pulsed laser deposition
Results demonstrate how film thickness strongly influences the microstructure and optical behavior of PCMs, providing valuable insights for tailoring their properties in future electronic and optoelectronic applications

Applied Physics Letters has published an article written by Kostiantyn Shportko, V.E. Lashkaryov Institute of Semiconductor Physics of NAS of Ukraine, Nauki av., 41, Kyiv, Ukraine, Sonja Cremer, and Andriy Lotnyk, Leibniz Institute of Surface Engineering (IOM), Permoserstr. 15, 04318 Leipzig, Germany.

Abstract: “Phase-change materials, such as GeTe and Sb₂Te₃ alloys, are of great interest for applications in photonics, data storage, and computing due to their ability to rapidly and reversibly switch optical properties during phase transitions. This study investigates the thickness-dependent optical and structural properties of GeTe and Sb₂Te₃ thin films, deposited on amorphous substrates at room temperature using pulsed laser deposition. GeTe films, measured over a thickness range of 1.8–72.4 nm, show notable changes below 20 nm, with the high-frequency dielectric constant (ε_inf) increasing from 8.9 to 14.0 and the optical bandgap (E_g) decreasing from 0.95 to 0.74 eV. These variations are primarily attributed to quantum confinement effects in the amorphous GeTe structure. Sb₂Te₃ thin films develop with thickness, from mostly amorphous at 3.4 nm to partially amorphous-crystalline at 5.1–10 nm and finally to fully polycrystalline at larger thicknesses. Thinner films exhibit up to a 1.5% reduction in the c-lattice parameter, while thicker films show a clear change in crystal orientation. Correspondingly, ε_inf rises from 14.1 to 41.0, and E_g decreases from 0.69 to 0.41 eV. These results demonstrate how film thickness strongly influences the microstructure and optical behavior of PCMs, providing valuable insights for tailoring their properties in future electronic and optoelectronic applications.“

R&D: Deciphering the bonding in metavalent materials for next-generation memory
Refined understanding of metavalent bonding provides a foundational framework for the rational design of novel PCMs, paving the way for advancements in phase change memory technology

Journal of Physical Chemistry C has published an article written by Gaurav Jhaa, Mithu Kumari, and Priyakumari Chakkingal Parambil, Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Mohali 140 306, India.

Abstract: “Phase change materials (PCMs) have garnered significant attention due to their applications in phase change memory (PCM) technology, which has the potential to surpass the von Neumann bottleneck via in-memory computing. This is particularly useful in artificial intelligence applications requiring rapid data transfer. Therefore, identifying the bonding mechanism in such materials is crucial, as it will aid in designing better materials for PCM applications. In this context, a new bonding mechanism named metavalency, indicating a chemical bond in between metallic and covalent, was introduced recently to describe the bonding in phase change materials. Metavalent materials are characterized by nearly half bond order, large Born effective charge (BEC), low band gap, moderate electrical conductivity, and large optical dielectric constant. However, their bonding features overlap significantly with those of the well-known hypervalent and partial σ-bonded systems, raising fundamental questions about whether metavalency constitutes a truly distinct bonding mechanism. Here, we employ molecular orbital theory and DFT calculations to show that the known metavalent compounds are electron-sufficient/rich hypercoordinate systems with partial σ bonds formed mainly by p–p overlap and have low ionicity, low band gap, and consequently large BEC. This implies that they belong to a distinct class of compounds, though the bonding mechanism is not new. This refined understanding of metavalent bonding provides a foundational framework for the rational design of novel PCMs, paving the way for advancements in phase change memory technology.“