(Invited) MLD of Metal-Organic Thin Films with Tunable Conductance for Neuromorphic Computing Applications
Authors: Nisula Mikko , Karttunen Antti J. , Detavernier Christophe 
Published: December 2020

Artificial neural networks have revolutionized the field of artificial intelligence with human-like performance in fields such as computer vision and speech recognition. As the tasks for AI are increasingly demanding, significant gains in terms of speed and power consumption could be achieved by moving from software-based AI-systems to neuromorphic hardware that directly emulates the functionality and interconnectivity of the biological neural networks with layers of artificial neurons connected to each other via synaptic elements characterized by a weight factor.

However, purely CMOS-based neuromorphic circuits are ultimately impractical for the implementation of large networks, as mimicking the operation dynamics and behavior of even a single synapse or neuron can take tens of transistors each to implement. Therefore, a key challenge is to develop new types of hardware, that is capable of achieving directly the in-memory processing of brain-like computation.

For instance, single-element synapses can be achieved with memristors, in which the synaptic weight is emulated by their history-dependent variable conductance. Typically, 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 are the device-to-device and cycle-to-cycle variability caused by the stochastic nature of the filament formation. Additionally, the typically high conductance in the ON-state results in high energy consumption.

Here we explore metal-organic coordination polymers deposited by atomic/molecular layer deposition (ALD/MLD) as an interesting alternative for this challenge. The most attractive feature of coordination polymers is their unparalleled tunability towards different functionalities arising from practically limitless number of metal and ligand combinations available. Yet, poor processability restricts their applicability towards nanotechnology applications. However, MLD, an extension to ALD, expands the deposition of thin films with superior quality to hybrid organic-inorganic materials with the implementation of volatile organic molecules as the co-reactant.

We show that in high-quality coordination polymer thin films, post-synthetic control of their oxidation state is akin to tuning the doping level in terms band structure, and allows for tunable conductivity spanning over 8 orders of magnitude (10-10 – 10-2 S cm-1). On device level, this redox-reaction based operation can be harnessed for low variability as the switching voltage is governed by the materials’ intrinsic reduction potential in contrast to the stochastic filament formation of the inorganic counterparts.

Aside from the analog conductance tuning, the redox reaction rate, and thus 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). As the redox-reaction is also coupled with the motion of an associated counter ion, the interplay of the electron and ion mobilities apparently allows for the realization of concurrent short- and long-term memory in a single device. As such, ALD/MLD enabled coordination polymer thin films are attractive for use in more bio-realistic AI technology such as spiking neural networks.