Sergiu Clima

The long-standing challenge in resolving the atomic-scale threshold switching mechanism in amorphous chalcogenides, fundamental constraint on further development of promising memory technologies, stems from their intrinsic structural disorder. Here, we overcome this pivotal challenge by capturing electric-field-driven dipolar ordering in amorphous GeSe through combined atomic-resolution angstrom-beam electron diffraction and field-coupled ab initio molecular dynamics. Electric fields induce anti-parallel displacements of Ge ( + 0.23 Å) and Se ( − 0.21 Å) atoms within picoseconds, aligning dipoles into one-dimension chains. These polarity-locked chains, evidenced by two distinct diffraction spots (1.95 Å spacing), guide conductive filament growth perpendicular to chain alignment. This mechanism enables direct harnessing of dipole-originated threshold voltage asymmetry in selector-only memory, achieving dual functionality through single-material engineering. This field-induced non-Arrhenius process squashes thermal activation barriers, enabling dipolar-order-driven switching within the picosecond regime thus breaking the thermal speed limit for resistive switching. Our findings establish a pathway to atomic-scale dipole control for ultrafast storage-class memory.

Arsenic‐free ovonic threshold switch (OTS) materials are important for enabling sustainable selector devices in high‐density crossbar memory arrays. Guided by ab initio design, we investigate Si‐doped GeS ₂ as a promising candidate, offering improved thermal stability and reduced leakage compared to alternative arsenic‐free materials. Thin films with up to 10% Si content were deposited by cosputtering and demonstrated thermal stability above 500°C. Device electrical characterization revealed good I _OFF performance comparable to reference SiGeAsSe devices. These results position Si‐doped GeS ₂ as a viable candidate for environmentally friendly, arsenic‐free OTS selectors, with further optimization needed to address endurance limitations.

In this study, we investigated the effect of pulse falling time (Tfall) on the electrical characteristics of SiGeAsSebased selector-only memory (SOM) devices. Our experimental results demonstrate that increasing the Tfall leads to an increased threshold voltage (Vth) and reduced Vth drift in SiGeAsSe devices. The optimized devices exhibit a remarkable memory window (> 1 V) and significantly suppressed drift characteristics ( 10 mV/dec.). Electrical measurements at high temperatures demonstrate that Tfall is one of the important factors in material relaxation, and these improvements are attributed to the intentionally induced reconfiguration of the chalcogenide film. Furthermore, our results reveal that a suitable Tfall can effectively mitigate the degradation of the memory window at high temperatures. These findings afford valuable insights into the role of material relaxation in SOM devices, potentially aiding the development of high-performance memory devices.

The unique concept of a non-volatile capacitive memory window (CMW) in ferroelectric (FE) capacitors (FeCAP) allows for a non-destructive read operation (NDRO). NDRO decouples the read- from the write-endurance which is ideal for random access memory and compute-in-memory applications. However, the optimal strategy to further improve the CMW remains uncertain. In this work, we first identify potential pathways to improve the CMW by studying the dependence of the relative dielectric permittivity ( $\epsilon _{\text {r}}$ ) of a FE on the remnant polarization ( ${P}_{\text {R}}$ ) and coercive field ( ${E} _{\text {C}}$ ). Results show that a FE stack having a higher ${P}_{\text {R}}$ and/or a lower ${E}_{\text {C}}$ leads to an improved $\epsilon _{\text {r}}$ and CMW for a given read-bias. Next, we experimentally demonstrate this and show that a trilayer FeCAP with an enhanced $2{P}_{\text {R}}$ achieves a high CMW $_{\epsilon }$ of ~20, which is ~1.5x higher than earlier best. Finally, we demonstrate a pulse-based NDRO confirming a robust 1.5x enhancement in CMW even after $10^{{11}}$ NDRO cycles.

In this work, we experimentally validate our theoretical framework for the ab initio screening of new, sustainable materials for ovonic threshold switching (OTS) applications, avoiding elements, such as As and Se that are associated with a high health and safety risk. We demonstrate that the newly designed Sn-S-based OTS ternary alloys are electrically functional and investigate the links between material properties and electrical performance. Results indicate that the Sn-S material system is a promising candidate for OTS selector and memory applications. Our findings open new research paths for further composition optimization aimed at bridging the performance gap with the state-of-the-art materials based on As and Se. The experimental results will enable further refinement of the theoretical model and improve its prediction capabilities.

Integration density is one of the major criteria that ferroelectric (FE) capacitors must meet before they can be deployed in FeRAM memory arrays that can replace the conventional DRAM memories. In this work we demonstrate, for the first time, back-end-of-line compatible 3D-trench FE La doped hafnium zirconium oxide (HZO) based capacitors fabricated with a foot-print area of 200nm x 200nm for FeRAM applications. The capacitors are based on a trilayer stack (TiO2/La doped HZO/Nb2O5) with TiN as top and bottom electrodes. The initial wake-up of the films is dominated by domain de-pinning effect followed by other mechanisms. Pre-cycling scheme is demonstrated to wake-up the capacitor using higher bias (2.85V, 833KHz, $10^{{5}}$ cycles). Post wake-up, the device survives $10^{{12}}$ cycles with 2P $_{\text {R}}26~\mu \text{C}$ /cm2 till the end of endurance cycling. Retention measurements performed at $85^{\text {o}}\text{C}$ show retention of >70% of the initial polarization at the end of $10^{{5}}\text{s}$ and ~30% polarization is projected to remain at the end of 10 years. The retention was found to be limited by the imprint effect. Our demonstration brings HZO based FE capacitors one step closer to nonvolatile FeRAM applications.