Silicon-Compatible Ferroelectric Tunnel Junctions with a SiO2/Hf0.5Zr0.5O2 Composite Barrier as Low-Voltage and Ultra-High-Speed Memristors

The big data era requires ultrafast, low-power, and silicon-compatible materials and devices for information storage and processing. Here, ferroelectric tunnel junctions (FTJs) based on SiO2/Hf0.5Zr0.5O2 composite barrier and both conducting electrodes are designed and fabricated on Si substrates. The FTJ achieves the fastest write speed of 500 ps under 5 V (2 orders of magnitude faster than reported silicon-compatible FTJs) or 10 ns speed at a low voltage of 1.5 V (the lowest voltage among FTJs at similar speeds), low write current density of 1.3 × 104 A cm-2, 8 discrete states, good retention > 105 s at 85 °C, and endurance > 107. In addition, it provides a large read current (88 A cm-2) at 0.1 V, 2 orders of magnitude larger than reported FTJs. Interestingly, in FTJ-based synapses, gradually tunable conductance states (128 states) with high linearity (<1) are obtained by 10 ns pulses of <1.2 V, and a high accuracy of 91.8% in recognizing fashion product images is achieved by online neural network simulations. These results highlight that silicon-compatible HfO2-based FTJs are promising for high-performance nonvolatile memories and electrical synapses.

Luminescence in Anion-Deficient Hafnia Nanotubes

Hafnia-based nanostructures and other high-k dielectrics are promising wide-gap materials for developing new opto- and nanoelectronic devices. They possess a unique combination of physical and chemical properties, such as insensitivity to electrical and optical degradation, radiation damage stability, a high specific surface area, and an increased concentration of the appropriate active electron-hole centers. The present paper aims to investigate the structural, optical, and luminescent properties of anodized non-stoichiometric HfO2 nanotubes. As-grown amorphous hafnia nanotubes and nanotubes annealed at 700 °C with a monoclinic crystal lattice served as samples. It has been shown that the bandgap Eg for direct allowed transitions amounts to 5.65 ± 0.05 eV for amorphous and 5.51 ± 0.05 eV for monoclinic nanotubes. For the first time, we have studied the features of intrinsic cathodoluminescence and photoluminescence in the obtained nanotubular HfO2 structures with an atomic deficiency in the anion sublattice at temperatures of 10 and 300 K. A broad emission band with a maximum of 2.3-2.4 eV has been revealed. We have also conducted an analysis of the kinetic dependencies of the observed photoluminescence for synthesized HfO2 samples in the millisecond range at room temperature. It showed that there are several types of optically active capture and emission centers based on vacancy states in the O3f and O4f positions with different coordination numbers and a varied number of localized charge carriers (V0, V-, and V2-). The uncovered regularities can be used to optimize the functional characteristics of developed-surface luminescent media based on nanotubular and nanoporous modifications of hafnia.

ALD HfO2 Films for Defining Microelectrodes for Electrochemical Sensing and Other Applications

Microelectrodes are used in a wide range of applications from analytical electrochemistry and biomolecular sensing to in vivo implants. While a variety of insulating materials have been used to define the microelectrode active area, most are not suitable for nanoscale electrodes (<1 μm²) due to the limited robustness of these films when the film thickness is on the order of the nanoelectrode dimension. In this study, we investigate atomic layer deposited hafnium dioxide (ALD HfO₂) as an insulating film to coat planar platinum microelectrodes, with the active areas being defined where the HfO₂ is etched. Thermally grown films with thicknesses between 10 and 60 nm were deposited by 100 to 550 ALD cycles and were initially characterized by measuring their standard electrical properties and imaging incipient texture development. Electrochemical measurements on the structures were made, including linear sweep voltammetry and electrochemical impedance spectroscopy, which identified the presence of pinholes in films deposited over the range of 100 to 350 cycles, resulting in leakage. These measurements also suggest a lower limit to the size of microelectrodes below which the electrochemical current detected is no longer dominated by that through the exposed active area. A bilayer insulator comprising ALD HfO₂ coated with parylene-C was investigated to minimize the pinhole leakage. Steady-state currents were measured for different electrode areas, qualitatively agreeing with the theory for areas down to ∼1 μm². For sub-square micrometer electrode areas, bilayer-insulated devices with parylene-C apertures that exposed the smallest microelectrode area showed measured currents that were consistent with extrapolations, indicating that it reduces leakage through HfO₂.

Lanthanide-Doped Hafnia Nanoparticles for Multimodal Theranostics: Tailoring the Physicochemical Properties and Interactions with Biological Entities

High-Z metal oxide nanoparticles hold promise as imaging probes and radio-enhancers. Hafnium dioxide nanoparticles have recently entered clinical evaluation. Despite promising early clinical findings, the potential of HfO₂ as a matrix for multimodal theranostics is yet to be developed. Here, we investigate the physicochemical properties and the potential of HfO₂-based nanoparticles for multimodal theranostic imaging. Undoped and lanthanide (Eu³⁺, Tb³⁺, and Gd³⁺)-doped HfO₂ nanoparticles were synthesized and functionalized with various moieties including poly(vinylpyrrolidone) (PVP), (3-aminopropyl)triethoxysilane (APTES), and folic acid (FA). We show that different synthesis routes, including direct precipitation, microwave-assisted synthesis, and sol-gel chemistry, allow preparation of hafnium dioxide particles with distinct physicochemical properties. Sol-gel based synthesis allows preparation of uniform nanoparticles with dopant incorporation efficiencies superior to the other two methods. Both luminescence and contrast properties can be tweaked by lanthanide doping. We show that MRI contrast can be unified with radio-enhancement by incorporating lanthanide dopants in the HfO₂ matrix. Importantly, ion leaching from the HfO₂ host matrix in lysosomal-like conditions was minimal. For Gd:HfO₂ nanoparticles, leaching was reduced >10× compared to Gd₂O₃, and no relevant cytotoxic effects have been observed in monocyte-derived macrophages for nanoparticle concentrations up to 250 μg/mL. Chemical surface modification allows further tailoring of the cyto- and hemocompatibility and enables functionalization with molecular targeting entities, which lead to enhanced cellular uptake. Taken together, the present study illustrates the manifold properties of HfO₂-based nanomaterials with prospective clinical utility beyond radio-enhancement.

Atomic Scale Modulation of Self-Rectifying Resistive Switching by Interfacial Defects

Higher memory density and faster computational performance of resistive switching cells require reliable array-accessible architecture. However, selecting a designated cell within a crossbar array without interference from sneak path currents through neighboring cells is a general problem. Here, a highly doped n++ Si as the bottom electrode with Ni-electrode/HfO x /SiO2 asymmetric self-rectifying resistive switching device is fabricated. The interfacial defects in the HfO x /SiO2 junction and n++ Si substrate result in the reproducible rectifying behavior. In situ transmission electron microscopy is used to quantitatively study the properties of the morphology, chemistry, and dynamic nucleation-dissolution evolution of the chains of defects at the atomic scale. The spatial and temporal correlation between the concentration of oxygen vacancies and Ni-rich conductive filament modifies the resistive switching effect. This study has important implications at the array-level performance of high density resistive switching memories.