Nanoscale behaviour and applications of insulator-metal transition materials

The computing industry increasingly struggles to improve the performance, area, and efficiency of traditional computational hardware. To make advances and enable emerging bio-inspired systems, which hold promise for efficient machine learning, new materials with unique functionality will be needed. Insulator-metal transition (IMT) materials display unusual electronic behaviour due to an abrupt change in resistivity at a critical temperature, becoming metallic when heated and insulating when cooled, and can be made into nanoscale resistive switching devices. This is accompanied by changes in crystal structure, as well as thermal and optical properties. However, the IMT mechanism has been a topic of much debate. In this work, we investigate the origins and applications of the IMT in vanadium dioxide (VO2) and niobium dioxide (NbO2) devices. In the first part of this thesis, we investigate IMT behaviour in VO2 at nanoscale dimensions, including the interplay of the electrical and structural transitions associated with the IMT. Using synchrotron x-ray spectromicroscopy, we map both of these transitions across the IMT with high spatial (~20 nm) and thermal resolution (~0.1 K). The results show islands of percolating phase coexistence during both transitions, and that the electronic transition precedes the change in crystal structure. Then, we use ~1 nm diameter metallic carbon nanotubes (CNT) as a ultra-narrow heaters to locally induce the IMT in VO2 devices. Using in situ Kelvin Probe Microscopy (KPM) and Scanning Thermal Microscopy (SThM), we map the electric fields and heating within devices, then compare to electrothermal finite element models. The results shed light on the nature of the IMT and highlight the role of heating in triggering the IMT in nanoscale devices. In the second part of this thesis, we develop oscillatory device applications and study how the static and dynamical electrical behaviour of IMT devices change with device size. we find that the power consumption of devices decreases with volume, and efficiency can further be doubled by locally concentrating heating using a CNT. We also show that these devices can exhibit periodic current spiking under certain bias condi-tions, acting as highly compact artificial neurons. By adding a CNT heater, in addition to reducing the IMT device size, the spiking frequency increases dramatically by > 1000x. This Section presents design considerations for incorporating IMT devices into brain-inspired computing hardware. In the final Section of this thesis, we evaluate the use of IMT snapback devices based on VO2 and NbO2 for electrostatic discharge protection (ESD). ESD is a major cause of failure in commercial chips, requiring significant chip area and design costs to protect against. We find that IMT-based protection devices can survive extremely high currents up to 10 A in the metallic state during a ~100 ns discharge, and still recover to their insulating state once an ESD event is over. Furthermore, we show how the snap-back voltage and performance can be engineered via device geometry. The results suggest that IMT materials could be promising for use in on-chip ESD protection, with the potential to be implemented in the back end (upper layers) of the chip to save area. Overall, the work presented in this thesis improves the understanding of nanoscale IMT devices and demonstrates two applications for them on the path to next-generation computing

Keywords: Electrical Engineering.
Advisor: Pop Eric 
Fan Jonathan Albert 
Kumar Suhas