Recently proposed novel neural network hardware designs imply the use of memristors as electronic synapses in 3D cross-bar architecture. Atomic layer deposition (ALD) is the most feasible technique to fabricate such arrays. In this work, we present the results of the detailed investigation of the gradual resistive switching (memristive) effect in nanometer thick fully ALD grown TiN/HfO2/TiN stacks. The modelling of the I-V curves confirms interface limited trap-assisted-tunneling mechanism along the oxygen vacancies in HfO2 in all conduction states. The resistivity of the stack is found to critically depend upon the distance from the interface to the first trap in HfO2. The memristive properties of ALD grown TiN/HfO2/TiN devices are correlated with the demonstrated neuromorphic functionalities, such as long-term potentiation/depression and spike-timing dependent plasticity, thus indicating their potential as electronic synapses in neuromorphic hardware.
The plasma-enhanced atomic layer deposition (PEALD) process using Ta(OCH) as a Ta precursor and plasma-activated hydrogen as a reactant for the deposition of TaO films with a controllable concentration of oxygen vacancies (V) is reported herein. The V concentration control was achieved by varying the hydrogen volume fraction of the hydrogen-argon mixture in the plasma, allowing the control of the leakage current density in the tantalum oxide films within the range of 5 orders of magnitude compared with the TaO film grown via thermal ALD using the identical Ta precursor and HO. Temperature-dependent current-voltage measurements combined with Poole-Frenkel emission modeling demonstrated that the bulk trap depth decreases with the increasing hydrogen volume fraction, which could be attributed to the increase of the V concentration. The possible chemical change in the PEALD TaO films grown under different hydrogen volume fractions was confirmed by the in situ X-ray photoelectron spectroscopy (XPS) measurements of the Ta 4f core and valence band spectra. The comparison of the XPS-measured nonstoichiometry and the secondary ion mass spectrometry analysis of the hydrogen content allowed this study to conclude that the nonstoichiometry is largely related to the formation of Ta-V sites rather than of Ta-H sites. Such oxygen-deficient TaO layers were studied for application as an oxygen-deficient layer in a resistance switching random access memory stack (TaO/TaO) where the actual switching occurred within the stoichiometric TaO layer. The bilayer memory stack showed reliable resistance switching up to ∼10 switching cycles, whereas the single-layer TaO memory showed only several hundred switching cycles.
Lithium‐ion batteries based on single‐crystal LiNi1−x−yCoxMnyO2 (NCM, 1−x−y ≥ 0.6) cathode materials are gaining increasing attention due to their improved structural stability resulting in superior cycle life compared to batteries based on polycrystalline NCM. However, an in‐depth understanding of the less pronounced degradation mechanism of single‐crystal NCM is still lacking. Here, a detailed postmortem study is presented, comparing pouch cells with single‐crystal versus polycrystalline LiNi0.60Co0.20Mn0.20O2 (NCM622) cathodes after 1375 dis‐/charge cycles against graphite anodes. The thickness of the cation‐disordered layer forming in the near‐surface region of the cathode particles does not differ significantly between single‐crystal and polycrystalline particles, while cracking is pronounced for polycrystalline particles, but practically absent for single‐crystal particles. Transition metal dissolution as quantified by time‐of‐flight mass spectrometry on the surface of the cycled graphite anode is much reduced for single‐crystal NCM622. Similarly, CO2 gas evolution during the first two cycles as quantified by electrochemical mass spectrometry is much reduced for single‐crystal NCM622. Benefitting from these advantages, graphite/single‐crystal NMC622 pouch cells are demonstrated with a cathode areal capacity of 6 mAh cm−2 with an excellent capacity retention of 83% after 3000 cycles to 4.2 V, emphasizing the potential of single‐crystalline NCM622 as cathode material for next‐generation lithium‐ion batteries.
Atomic‐layer deposition (ALD) technique in combination with in vacuo X‐ray photoelectron spectroscopy (XPS) analysis has been successfully employed to obtain fully ALD‐grown planar TiN/HfO2/TiN metal–insulator–metal structures for resistive random access memory (ReRAM) memory elements. In vacuo XPS analysis of ALD‐grown TiN/HfO2/TiN stacks reveals the presence of the ultrathin oxidized layers consisting of TiON (∼0.5 nm) and TiO2 (∼0.6 nm) at the bottom TiN/HfO2 interface (i); the nonoxidized TiN at the top HfO2/TiN interface (ii); the oxygen deficiency in the HfO2 layer does not exceed the XPS detection limit (iii). Electroformed ALD TiN/HfO2/TiN stacks reveal both conventional bipolar and complementary types of resistive switching. In the complementary resistive switching regime, each programming sequence is terminated by a reset operation, leaving the TiN/HfO2/TiN stack in a high‐resistance state. The observed feature can avoid detrimental leaky paths during successive reading operation, which is useful in the passive ReRAM arrays without a selector element. The bipolar regime of resistive switching is found to reveal the gradual character of the SET and RESET switching processes. Long‐term potentiation and depression tests performed on ALD‐grown TiN/HfO2/TiN stacks indicate that they can be used as electronic synapse devices for the implementation of emerging neuromorphic computation systems.
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