Increasing the integration density of memory chips is critical for high-capacity storage and parallel computing, which demands that selector components in 3D stacked cross-point architectures possess ultra-low leakage current and appropriate threshold voltage. Currently, 3D phase-change memory primarily uses ovonic threshold switching (OTS) selectors to control the on/off state of memory cells, and the access performance of these selectors directly affects chip area efficiency and cost. An ideal selector should achieve picoamp-level sub-threshold leakage current at half-bias voltage while exhibiting extremely high switching nonlinearity, with the threshold voltage maintained between 1.8 and 2.5 volts to be compatible with low-voltage logic devices in advanced technology nodes. However, existing material systems and structural designs often struggle to simultaneously meet these stringent parameter requirements, constraining the development of larger-scale memory arrays.

The challenge lies in the fact that traditional selectors face a conflict of physical properties. Although reducing leakage current by increasing the crosslinking density of the material or inserting a dielectric buffer layer is possible, this often leads to an inevitable and significant increase in threshold voltage. Recent attempts to lower the threshold voltage by reducing the conduction activation energy have resulted in leakage currents generally still above the nanoamp level with limited switching ratios. Furthermore, to withstand the thermal budget of back-end-of-line integration processes in integrated circuits, amorphous materials must possess extremely high thermal stability, which typically widens the electronic bandgap and raises the turn-on threshold. Another core difficulty is that the filamentary conduction mechanism of selectors has long been controversial, lacking direct real-space observational evidence.

To address the above issues, a team from Shenzhen University, Zhejiang University, and other institutions conducted systematic research using ZEPTOOLS' in-situ TEM measurement system. By designing a Ge₂Se₃ amorphous nanoparticle-assembled nanolayer with excellent thermal stability, they significantly suppressed the sub-threshold leakage current of the selector through extremely small electrode contact area, and for the first time internationally, directly confirmed the nanoscale filamentary conduction mechanism using in-situ electron holography.

Title: Ultralow‐Leakage‐Current and Filament‐Conductive Amorphous Germanium Selenide Nanoparticle Ovonic Threshold Switching Selectors
Journal: Advanced Materials
URL: doi.org/10.1002/adma.2024

The research team developed a novel hot-injection synthesis technique and successfully prepared amorphous spherical Ge₂Se₃ nanoparticles, filling the research gap regarding nanoparticle morphology in this material system. This material is designed with a simplified binary composition free of arsenic, effectively lowering the technical barriers for large-scale production. Experimental results show that the nanoparticles remain amorphous even when heated to 550°C under an argon atmosphere, with a crystallization temperature significantly higher than that of most existing ovonic threshold switching thin-film materials. This excellent thermal stability makes them fully compatible with the thermal budget of back-end-of-line processes in semiconductor manufacturing, laying a solid material foundation for high-performance embedded memory applications.

By spin-coating these amorphous nanoparticles onto a substrate, the researchers constructed a macroscopically dense nano-assembled layer and integrated it into a T-shaped selector device. Benefiting from the extremely small effective contact area between the nanoparticles and the electrode, the device achieved an ultra-low sub-threshold leakage current of approximately 1 picoamp while maintaining a threshold voltage within a reasonable range of 2.2 to 2.6 volts. This achievement directly resulted in an on/off selectivity ratio as high as 10⁸, comprehensively surpassing existing thin-film-based selectors in key performance metrics. This design approach, which resolves the conflict in electrical properties through physical morphology innovation, provides a viable path for further scaling of 3D cross-point architectures.

To deeply investigate the physical mechanism during the switching process, the research team used the in-situ TEM sample holder provided by ZEPTOOLS to perform real-time observation of the structural and electric field evolution of a single nanoparticle under voltage driving. Using this in-situ sample holder, the researchers were able to apply voltage bias while capturing high-resolution microscopic images with an aberration-corrected electron microscope. The application of this precision instrument enabled the researchers to precisely control the current compliance value, thereby observing the internal conduction evolution within the particle while maintaining its amorphous state. The experimental process strictly adhered to the low electron dose principle, minimizing potential beam-induced damage to the sample and ensuring the authenticity and high reliability of the observed results.

Using in-situ off-axis electron holography, the research team directly observed, for the first time internationally, the formation of nanoscale conductive filamentary paths inside a single nanoparticle. In the on-state of the device, electron holograms clearly revealed a low-potential sensing region extending outward from the tiny contact point, in stark contrast to the uniformly high potential distribution in the off-state. The study confirmed that as the driving current increases, these conductive paths undergo a process from edge-initiated rooting to gradual inward expansion, ultimately forming effective channels that support high-current transport. This discovery not only provides critical experimental support for the long-debated physical origin of ovonic threshold switching but also offers important theoretical guidance for the future development of ultra-small-scale phase-change memory cells.

ZEPTOOLS, as a Chinese high-end precision instrument company, is a leading supplier of in-situ electron microscopy characterization solutions. The PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for major research achievements of users both domestically and internationally. The figure below shows the ZEPTOOLS in-situ TEM sample holder product used in this research achievement: