PhD defense of Thoai-Khanh KHUU - 29/06/2023

To overcome the constraints that the traditional von Neumann computing architectures and the end of Moore’s law pose, neuromorphic computing is one of the promising solutions. The construction of these brain-inspired systems requires specific electronic devices capable of artificially producing synaptic behavior. Valence change memories (VCM) with analogue resistive switching (RS) capabilities are one of the exciting candidates for artificial synapse applications, as their resistance (or conductance), which represents the synaptic weight, can be modulated in a continuous manner. In oxide-based VCMs, oxygen drift involves a redox reaction, which triggers the change in resistance of the device.

In this context, lanthanum nickelate (La₂NiO_4+δ, L2NO4), a mixed ionic electronic oxide well known for its ability to store and transport oxygen thanks to its interstitial oxygen, is deposited by pulsed-injection metal-organic chemical vapor deposition (PI-MOCVD) and studied as a memristive layer. In this thesis, we first managed to transfer the Ti\L2NO4/Pt planar memristive devices from single-crystal substrates to Si-based substrates, i.e., using Si₃N₄/SiO₂/Si. Such devices require very high operating voltages and an additional heating step (rapid thermal annealing, RTA) to activate the switching. Then, Ti/L2NO4/Pt memristive devices were constructed in vertical configuration for the first time. The L2NO4 films were deposited on another Si-based substrate, i.e., platinized substrate (Pt/TiO₂/SiO₂/Si). The deposition of L2NO4/Pt was optimized to get a pure L2NO4 phase, as well as flatter L2NO4 thin films and to avoid Pt dewetting. L2NO4 (optimized or non-optimized)-based memristors combined with Ti electrodes show better high-resistance-state/low-resistance-state (HRS/LRS) ratios, in a range from 6 to 10, compared to the planar configuration with dynamic resistance relaxation. The formation of a TiO_x interlayer at the Ti/L2NO4 interface was observed by transmission electron microscope (TEM), which is assumed to play a crucial role in the switching of these devices.

In the second stage of the thesis, TiN/L2NO4/Pt vertical memristive devices were built for the first time. After the ‘soft-forming’ step, they exhibit bipolar RS with gradual SET and RESET transitions. A HRS/LRS ratio close to 40 was measured with good reproducibility. Highly multilevel resistance states can be obtained by using different voltage amplitudes, pulse amplitudes (pulse height) or pulse duration (pulse length). Potentiation/depression of biological synapse was artificially reproduced by applying many sweeps/pulses in the same polarity in such device with data retention characteristic up to (at least) 6 hours, opening the door to use TiN/L2NO4/Pt devices as the long-term artificial synapses.

Finally, the RS mechanism was investigated by X-ray absorption near-edge spectroscopy (XANES) and TEM in different modes and conditions. Operando XANES at Ni-K edge and in situ TEM equipped with electron energy loss spectroscopy (EELS) at the Ti-L_2,3 edges allowed us to get better insights into the underlying switching mechanisms in TiN/L2NO4/Pt devices. A TiN_xO_y interlayer is spontaneously formed during the microfabrication of the TiN top electrode. Changes in the Ni oxidation state and the oxidation/reduction of the TiN_xO_y interlayer could be reproducibly measured during operation, as the device was cycled in both voltage polarities, confirming a valence-change mechanism taking place. A simplified switching model is proposed based on the coexistence of filamentary and interfacial switching in these TiN/L2NO4/Pt memristive devices.