•A HfOx based multi-bit nonvolatile logic devices is proposed.•Multi-level resistive switching is demonstrated on the Gd:HfOx device.•The devices are demonstrated to exhibit both multi-bit computing and self-data storage functions.•Innovative operation schemes are developed to achieve the functions of computing and data storage.•A two-bit nonvolatile adder is demonstrated based on the device.A material-oriented methodology based on the crystal defect theory is proposed to design the hafnium oxide based multi-bit nonvolatile logic devices (NLD). The designed devices are demonstrated to exhibit both multi-bit computing and self-data storage functions. Innovative operation schemes are developed to achieve the functions of computing and data storage on the NLD. A two-bit nonvolatile adder is demonstrated based on the NLD, indicating the potential application of the designed NLD for complex functions and higher density integration in a simple system.

Neuromorphic computing is an attractive computation paradigm that complements the von Neumann architecture. The salient features of neuromorphic computing are massive parallelism, adaptivity to the complex input information, and tolerance to errors. As one of the most crucial components in a neuromorphic system, the electronic synapse requires high device integration density and low-energy consumption. Oxide-based resistive switching devices have been shown to be a promising candidate to realize the functions of the synapse. However, the intrinsic variation increases significantly with the reduced spike energy due to the reduced number of oxygen vacancies in the conductive filament region. The large resistance variation may degrade the accuracy of neuromorphic computation. In this work, we develop an oxide-based electronic synapse to suppress the degradation caused by the intrinsic resistance variation. The synapse utilizes a three-dimensional vertical structure including several parallel oxide-based resistive switching devices on the same nanopillar. The fabricated three-dimensional electronic synapse exhibits the potential for low fabrication cost, high integration density, and excellent performances, such as low training energy per spike, gradual resistance transition under identical pulse training scheme, and good repeatability. A pattern recognition computation is simulated based on a well-known neuromorphic visual system to quantify the feasibility of the three-dimensional vertical structured synapse for the application of neuromorphic computation systems. The simulation results show significantly improved recognition accuracy from 65 to 90% after introducing the three-dimensional synapses.Keywords: 3D integration; memory; metal oxide; neuromorphic computation; resistive switching; synapse; synaptic device

A novel complex nanostructured TiO2 electrode and fabrication process were proposed and demonstrated to improve the performance of dye-sensitized solar cells (DSSCs). In the proposed process, a nanoporous TiO2 layer was firstly fabricated on the FTO (fluorine-doped tin oxide) conducting substrate by an anodization process, then a nanoparticulate TiO2 film was deposited on the nanoporous TiO2 layer by the screen printed method to form the complex nanostructured TiO2 electrode. The experiments demonstrated that the nanoporous TiO2 layer can enhance the light scattering, decrease the contact resistance between TiO2 electrode and FTO, and suppress the recombination of I3− ion with the injected electrons of FTO. The process variables are crucial to obtain the optimized performance of DSSCs. By adopting the optimized process, improved conversion efficiency of DSSCs was achieved at AM 1.5 sunlight.

Resistive Switching (RS) phenomenon in Metal–Insulator–Metal (MIM) structures with polycrystalline HfO2 layers as dielectric has been studied at the nanoscale using Conductive Atomic Force Microscope (CAFM). The CAFM measurements reveal that (i) the conductive filaments (CFs) created at very small areas are the origin of the RS phenomenon observed at device level and (ii) RS conductive filaments are primarily formed at the grain boundaries, which exhibit especially low breakdown voltage. CAFM images obtained on MIM structures at the Low and High Resistive states also show that, although the current in the Low Resistive State is mainly driven by a completely formed single CF, the cell area dependence of the conductivity in the High Resistive State could be explained by considering the presence of multiple partially formed CFs.