Periodic Reporting for period 1 - RESWITCH (Redox-Controlled Resistive Switching in Hybrid Metal-Organic Thin Films towards Neuromorphic Computing)
Reporting period: 2019-04-01 to 2021-03-31
Neuromorphic computing presents a potential solution to these challenges by emulating the functionality and interconnectivity of the biological neural networks directly on hardware level. However, purely CMOS based neuromorphic circuits are impractical for the implementation of large networks, as even a single synapse or neuron can take tens of transistors each to implement. To accelerate the development of neuromorphic circuits, a next-generation device technology is needed. In practice, this equates to emulating the key features of the neurons and synapses directly on the materials’ level.
For the synaptic connections, a potential new technology are memristors that can emulate the synaptic weight through their history-dependent variable conductance. Various inorganic materials have been employed for these resistive switching devices that rely on the formation/dissolution of a conductive filament within an insulating matrix. The main challenges preventing widescale implementation are the device-to-device and cycle-to-cycle variability arising from the stochastic nature of the filament formation. Additionally, the typically high conductance in the ON-state results in high energy consumption.
The aim of the RESWITCH project is to enable a new type of resistive switching device concept based on metal-organic coordination polymer thin films. With redox-active ligands, the emulation of the synaptic weight can be based on the oxidation-state dependent properties of these thin films as the electronic conductivity can be modulated electrochemically with dynamic operation arising from the concurrent counter-ion motion. The interplay of the metal and organic constituents allows for precise control of the electric/electrochemical properties, but poor processability has been limiting their applicability for nanotechnology applications. The key element in RESWITCH is to implement a new thin film -based approach for conductive coordination polymers using Molecular Layer Deposition (MLD), a vapor phase thin film deposition method derived from the Atomic Layer Deposition (ALD) technique. As with ALD, it is defined by the sequential, self-saturating exposure of vapor-phase precursors onto surfaces. This monolayer accuracy in process control leads to sub-nanometer range precision in layer thickness control and in excellent uniformity over large area substrates,
The Objectives of the Project are three-fold:
1. Establishing novel MLD-chemistries based on redox-active organic ligands.
2. Gaining detailed understanding on the link between the thin film composition and it’s redox-properties and electrical conductivity.
3. Integration of the newly developed materials in a resistive switching device demonstrator with artificial synapse -like functionality.
The applicability of these materials towards resistive switching devices was demonstrated. The memristive properties of arise from the changing oxidation states of the adjacent layers when the device is operated in a nanobattery-like fashion. This redox-reaction based operation could be harnessed for excellent cycle-to-cycle performance as the switching threshold voltage is governed by the materials’ intrinsic reduction potential in contrast to the stochastic filament formation in the inorganic counterparts. The relatively low overall conductance of the materials resulted in appreciably high device resistance for both the Off- and On-states, highly desirable for low-power operation. Furthermore, the device design allows for tailoring the switching threshold voltage, the On and Off -state resistance, and the volatility of the memory state by combining different material pairs. Aside from the conductance tuning, the redox reaction rate, and thus the rate of change in the device conductance is dependent on the excitation frequency and amplitude, thereby emulating the time-dependent dynamics of biological synapses such as spike-rate and spike timing dependent plasticity (SRDP and STDP). The mixed ion/electron conduction in these materials allowed for the realization of concurrent short- and long-term memory in a single device.