Low-voltage, oxide-based thin-film electronic devices

Oxide semiconductors have drawn a lot of attention particularly since the first report of InGaZnO (IGZO) in 2004. For battery-powered electronic devices incorporating integrated thin-film transistors (TFTs), a low operating voltage, ideally at or below 1 V, is required in order to minimize the power consumption. Until now it has been challenging to achieve simultaneously a high current on/off ratio and low operating voltage for IGZO TFTs. Therefore, the first objective of this project was to develop high performance IGZO TFTs capable of operating at a low voltage on both rigid and flexible substrates. Given that for applications where a fixed current is required, a higher mobility allows a reduced size of the device, the next objective was to improve mobility through modifying the dielectric/channel interface where charges play a crucial role. The top surface of an oxide TFT also plays an important role in its performance. This especially applies to a device with a very thin channel layer. Therefore another goal of the project was to find a suitable encapsulation layer for further improving the device performance and reducing cost through decreasing the channel thickness. Finally, beyond the discrete device investigations and development, the application of IGZO TFTs to low-voltage circuits was demonstrated. In this project, low-voltage high-performance IGZO TFTs were fabricated and characterized using several types of gate dielectrics. The first dielectric was sputtered porous SiO2, which enabled low-voltage operation owing to a large gate capacitance arising from an electric-double-layer (EDL). Alternatively, IGZO TFTs with a low operating voltage at or below 1 V were demonstrated using solution-processed, ultra-thin gate dielectrics (i.e., AlxOy and HfOx). Devices were then modified by a self-assembled monolayer (SAM) at the dielectric/channel interface. Due to the creation of better interface states with less surface energy, the performance of these devices, including mobility, current on/off ratio and bias stress stability was significantly improved. A SAM treatment was also carried out on the top surface, showing reduced water adsorption-desorption effects and again, improved performance. With the SAM encapsulation, IGZO TFTs were shown to maintain their high performance even when the thickness of the channel was reduced to 5 nm. Finally, low-voltage full-swing inverters were demonstrated using the high-performance IGZO TFTs and showed a large voltage gain with noise margins close to the theoretical maximum.