Development of Tungsten Oxides based Memristor Devices for Artificial Synaptic Applications

Abstract

The advance in conventional silicon-based semiconductor industry is now facing an uncertain future as it still following the road of device down scaling, resulting in the presence of various practical issues including short channel effect, limited storage capacity and higher processing expense. It is usually acknowledged that transistor based memory devices will be reaching the end of scaling in 2018. Therefore, one of the most important challenges today in semiconductor industry is the need of a new memory technology which is able to infuse the best characterises of current devices. The memristors thus gains great attentions because of their specific nonlinear electrical performance, non-volatile properties, and high storage capacity. More importantly, such behaviour shows an analogous transmission characteristic with the synapse in biology and thus for building artificial neural networks. Therefore, studies of synapses biomimetic devices based on memristors not only may satisfy the increasing demands for high-performance memory applications but also will likely bring a great research prospect in artificial synapse emulations.

Tungsten oxides (WOx) exhibits many essential properties to be nominated as an appropriate candidate for memristive device such as accredited write/erase endurance, complimentary metal-oxide-semiconductor (CMOS) process compatibility and configurable electrical properties in non-volatile rectification, memorization and learning functions. However, up to date, studies on chemical-proceed tungsten oxides based memristors remain rarely exploited owing to the difficulties in high volatile precursors, impurities formation along with synthesis process and mesoporous nature of as-deposited films.

Therefore, to fill the blank for developing a novel solution towards a chemical processed memristors, the aim of this research project is to take tungsten oxides as the research object material, and to develop nano-scaled WOx based memristors by various chemical synthesis routines. Throughout this dissertation, we produced solution derived tungsten oxides based memristors through different chemical synthetic methods including sol-gel synthesis and electrodeposition. By employing inkjet printing technique, high quality of Ag top electrodes was deposited which not only overcomes the abovementioned technical challenges but also leads to an important step towards the development of high performance memristor at low cost and ambient pressure. The prepared sample devices feature a stacked three-layer structure of Ag/WOx/FTO with extraordinary continuous resistive switching behaviours. In addition, a series of mimicking behaviours for biological synaptic plasticity and memory functions has been effectively demonstrated in such devices with superb performance including spike-timing dependent plasticity (STDP), short-term plasticity (STP) and long-term plasticity (LTP). The possible mechanism of the device operation can be possibly attributed to the formation/dissolution of Ag percolation paths between two electrodes. With regard to these promising features of tungsten oxide in analog synaptic devices, this work promotes a great potential possibility of using tungsten oxides materials for the future development of bio-inspired neuromorphic computing applications.