Development of a low-cost zinc oxide-based material hybrid memristor

Gray, Robert James

Abstract

In recent years, there has been a resurgence of interest in two-terminal resistive devices as a new universal memory, which has the speed of static random access memory (SRAM), simplicity of dynamic random access memory (DRAM) and non-volatile storage of Flash memory. This has arisen from a report in 2008 that links switching in TiO2 memristor (dubbed the HP memristor) with Leon Chua’s memristor theory from 1970. The work importantly helped establish a framework for understanding and developing the field forward, which combined with the potential to scale devices down beyond the limits of complementary metal-oxide semiconductors has stimulated progress, enabling significant improvement in the technology.
In this thesis, we have developed a new material-hybrid zinc oxide (ZnO) nanorod- polymethyl methacrylate (PMMA) memristor fabricated using a novel microwave-based technique that enables devices to be made in a simple, quick and low-cost manner. A study into the growth of the nanowires identifies the significance of a uniform and aligned seed layer in producing dense distributions of aligned and homogeneous nanowire arrays, which is necessary for controlling the properties of the spacing layer and avoiding short-circuits. The rate of heating affects the nanowire growth significantly by inducing the formation of larger crystallites when heating at high power for very short durations, while low-power heating over larger durations reduces the formation of these larger particles.
We also find that the transport mechanisms are dependent on the device configurations. Through electrical I-V measurements, we observe that devices with gold (Au) top and bottom electrodes produce on/off resistance ratios (referring to the ratio of the low resistance state to the high resistance state) of typically around 5 - 8 and exhibit electrical behaviour typical of Poole-Frenkel emission and space charge limited conduction (SCLC). Devices with an aluminium (Al) top electrode and Au bottom electrode typically exhibit on/off ratios of approximately 10 (one order of magnitude), and their on/off ratios are larger than the best achieved with both Au electrodes; these devices typically exhibit Schottky emission behaviour, but do not exhibit clear Poole-Frenkel or SCLC behaviour. Devices using indium tin oxide (ITO) as a bottom electrode typically have on/off ratios of ~5 and appear to be dominated by Schottky emission behaviour.
The introduction of a PMMA layer affects the behaviour of all configurations. None of the transport models clearly fit the data for the Au/ZnO/PMMA/Al configuration; Poole-Frenkel emission and SCLC behaviour can be observed in ITO/ZnO/PMMA/Al devices, with many SCLC regimes being clearly identifiable that were not present in the configuration without PMMA. The additional PMMA layer is observed to affect the SCLC behaviour in Au/ZnO/PMMA/Au configurations, as the device no longer produces the higher-order trap charge limited conduction regime observed in the device without PMMA; the Poole-Frenkel behaviour is also affected, as the change in gradient for the Poole-Frenkel plot indicates that the data fits the “modified Poole-Frenkel effect” model. This suggests that additional trap centres may be created with the addition of a PMMA layer.
This thesis concludes that, while the devices require further optimization (particularly in terms of endurance and retention) to become commercially viable, the technique has much potential for future application.

Citation

Gray, R. J. (2017). Development of a low-cost zinc oxide-based material hybrid memristor. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4224289

• Thesis Type: Thesis
• Deposit Date: Sep 1, 2022
• Publicly Available Date: Feb 24, 2023
• Keywords: Physics
• Public URL: https://hull-repository.worktribe.com/output/4224289
• Additional Information: Department of Physics and Mathematics, The University of Hull
• Award Date: Jul 1, 2017