Multi-level, Forming Free, Bulk Switching Trilayer RRAM for Neuromorphic Computing at the Edge

Resistive memory-based reconfigurable systems constructed by CMOS-RRAM integration hold great promise for low energy and high throughput neuromorphic computing. However, most RRAM technologies relying on filamentary switching suffer from variations and noise leading to computational accuracy loss, increased energy consumption, and overhead by expensive program and verify schemes. Low ON-state resistance of filamentary RRAM devices further increases the energy consumption due to high-current read and write operations, and limits the array size and parallel multiply & accumulate operations. High-forming voltages needed for filamentary RRAM are not compatible with advanced CMOS technology nodes. To address all these challenges, we developed a forming-free and bulk switching RRAM technology based on a trilayer metal-oxide stack. We systematically engineered a trilayer metal-oxide RRAM stack and investigated the switching characteristics of RRAM devices with varying thicknesses and oxygen vacancy distributions across the trilayer to achieve reliable bulk switching without any filament formation. We demonstrated bulk switching operation at megaohm regime with high current nonlinearity and programmed up to 100 levels without compliance current. We developed a neuromorphic compute-in-memory platform based on trilayer bulk RRAM crossbars by combining energy-efficient switched-capacitor voltage sensing circuits with differential encoding of weights to experimentally demonstrate high-accuracy matrix-vector multiplication. We showcased the computational capability of bulk RRAM crossbars by implementing a spiking neural network model for an autonomous navigation/racing task. Our work addresses challenges posed by existing RRAM technologies and paves the way for neuromorphic computing at the edge under strict size, weight, and power constraints

Fast Proton Transport through High Diffusivity Domain Boundaries in VO2 Thin Films

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Anisotropic ionic transport-controlled synaptic weight update by protonation in a VO2 transistor

Ionic–electronic coupling in a lattice strongly influences the memory and learning process by synaptic weight update in electrochemical synaptic transistors. In particular, anisotropic crystal symmetry offers a highly anisotropic diffusion process, which leads to facilitated ion migration and efficient coupling in synaptic devices. Here, we report all-solid-state VO2 synaptic transistors in which the proton diffusion under gate bias can be tuned by utilizing different crystal facets in anisotropic VO2 channels. Synaptic weight update (i.e., excitatory post-synaptic current) by a proton (H+) in the VO2 channel was sensitively tuned depending on the empty tunnel alignment of VO2 layers. By emulating synaptic functions using diffusion-pathway-controlled transistors, the alignment of a facile ionic pathway with gating direction increases the retention of H+ in VO2 lattices by locating H+ into the deep regions from the interfaces, and thus strengthens long-term memory in artificial synaptic devices. These results demonstrate that the control of field-driven ionic redistribution guided by crystal anisotropy provides an opportunity to manipulate the learning and forgetting behavior in artificial synaptic devices.11Nsciescopu

High-throughput Roll-to-Roll Fabrication of Flexible Thermochromic Coatings for Smart Windows with VO2 Nanoparticles

VO2-based ‘nanothermochromics’ that utilizes the dispersion of VO2 nanoparticles in passive host matrix has been evaluated as a economic strategy of energy-saving “smart” windows to reduce energy consumption for heating and air conditioning in building. Here, we demonstrate a high-throughput roll-to-roll fabrication of thermochromic coatings for smart windows that can adapt their optical properties in accordance with external temperature. A large quantity (250 g) of VO2 nanoparticles (NPs) was synthesized at a time by controlled thermal treatment of bead-milled V2O5 NPs as a fast and inexpensive method. The amorphous nature of bead-milled V2O5 NPs combined with nanometer size kinetically facilitates uniform synthesis of high-quality VO2 NPs even under the less-reducing condition than that used to obtain bulk VO2. This mass production of VO2 NPs could be used to fabricate the largest VO2/PVP composites thermochromic coatings (12 cm × 600 cm) yet produced with excellent infrared modulation ability (~ 45%). This scalable and continuous production of large coating with thermochromic NPs will accelerate the commercialization of thermochromic coatings for smart windows, which will contribute to a large reduction in energy consumption to heat or cool buildings.1

Nanoscaffold WO3 by Kinetically Controlled Polymorphism

Several metastable polymorphs, diverse crystal structures with the same chemical formula, could be accessed through epitaxial interfaces with low energy between epitaxial film and symmetry-matched substrates. Here, we fabricated tungsten oxide (WO3) nanoscaffolds composed of hexagonal WO3 (h-WO3)/monoclinic WO3 (m-WO3) polymorphs, and modulated the proportions of these phases by tuning the W arrival rate during epitaxial growth. The WO3 nanoscaffold is vertically aligned with coherent interphase boundaries between h-WO3 and m-WO3; this structure leads to the persistence of h-WO3 despite its metastable characteristic. The coherent interphase boundaries give rise to the unexpected six-fold m-WO3 phases and a distinct lattice response with intercalated hydrogen defects. This WO3 nanoscaffold with different fractions of two polymorphs would be a good model system to systematically study intercalation chemistry and its application with WO3 polymorphs.11Nsciescopu

Synchrotron x-ray study of hydrogen-induced phase transition in VO2 epitaxial thin films

Phase transition by band filling control is one of the core concepts in correlated electronic systems. Unlike the substitutional dopants, hydrogen, the smallest and the lightest atom, plays a key role in effectively filling significant amount of carriers in the empty narrow d band by reversibly adding it into interstitial sites and supplying carriers. Vanadium dioxide (VO2), typical correlated oxide with 3d1 electronic configuration, can also reversibly incorporate hydrogen atoms into its interstitial sites and simultaneously occurs phase transition by its 3d band filling. Here, we demonstrate that as many as two hydrogen atoms can be incorporated into each VO2 unit cell, and that hydrogen is reversibly absorbed into, and released from, VO2 without destroying its lattice framework due to the low temperature annealing process. This hydrogenation process demonstrates twostep insulator (VO2) – metal (HxVO2) – insulator (HVO2) phase modulation during inter-integer d-band filling. Moreover, HVO2 can be thermodynamically stabilized regardless of facet direction of VO2 epi-layer, but remarkable discrepancy in kinetics of phase modulation was clearly visualized depending on the crystal facet. Based on in situ XRD, XPS and NEXAFS in synchrotron, the unprecedented insulating HVO2 with 3d2 configuration is attributed to highly doped electrons via hydrogenation process in conjunction with huge lattice expansion. Our finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO2 and opens up the potential application in proton-based Mottronics and novel hydrogen storage.2

Self-assembly of correlated(Ti,V02) superlattices with tunable lamella period by kineticallt-enhanced spinodal decomposition

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Systematic Tuning of Hydrogen-Induced Phase Transition in VO2 Epitaxial Thin Films.

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Systematic Tuning of Hydrogen-Induced Phase Transition in VO2 Epitaxial Thin Films.

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VO2 에피택셜 박막에서 수소에 의해 유도된 상전이의 체계적 튜닝

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