Amorphous Indium-Gallium-Zinc Oxide Planar Nanodiodes

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Abstract

In this thesis work, novel planar nanodiodes (PNDs) using an amorphous indiumgallium- zinc oxide (IGZO) film as the active layer have been electrically characterised for the first time. Simulation techniques and experimental methods, such as e-beam lithography (EBL) and nanoimprint lithography (NIL), have been explored for these devices. In addition, a novel approach was realized that produced self-aligned contacts for the nanostructured devices. A preliminary parameter space for experimentation of the PNDs was ascertained by simulating the devices using a technology computer aided design (TCAD) simulator. In this study Silvaco’s ATLAS default IGZO material system was adopted. These simulations showed device performance to be heavily dependent on the carrier concentration of the film, owing to the high leakage current during the off-state of device operation. Furthermore, device geometry had a significant influence on the device’s electrical response. Channel width, length and trench width were all examined.Experimental characterisation of PNDs were attained by fabricating devices using EBL. These devices are the first to exhbit diode-like DC electrical response from an IGZO-based PND. Full current rectification was obtained with a rectification ratio of 10^4 for devices with a long, narrow channel with a width of 50nm and a length of 4μm. This particular device geometry had a turn-on voltage, Von, of 2.2V and did not breakdown within the −10V bias range tested. An output drive current of 0.1μA at 10V was obtained by the single PND device. It was also demonstrated that by increasing the channel width, Von could be reduced; however, rectificationalso diminished. It is reasoned that the exposed IGZO surface was subject to contamination from the ambient which changed the device’s electrical response after 17 days.An ultraviolet NIL (UV-NIL) technique was developed to produce the PNDs. This fabrication method offers a suitable route towards high-volume manufacture of these nanodevices, which is critical for them to be incorporated into a low-cost RF energy harvester. A novel NIL process was established in which the contact pads were self-aligned to within ~ 200nm of the channel by patterning both metal and semiconductor layers with a single imprint. DC electrical characterisation of the imprinted PNDs produced high rectifications ratios at a lower Von. The greater number of devices tested allowed a coarse parameter space for channel width and length to determined. PNDs with a channel aspect ratio (length divided by width) of more than 20 exhibited the greatest DC rectification of 10^4. An alumina capping layer was found to eliminate hysteresis in the electrical response; however, the greater permittivity value had no noticeable effect on device performance.Finally, a large-signal RF analysis is carried out on a device which suggest no deterioration in device perfromance up to at least 1GHz.

Keyword(s)

High Rectification; Indium-Gallium-Zinc Oxide; Nanoimprint; Planar Nanodiode; Self-Switching Diode; Self-aligned contacts

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