November 3, 2010
FRONTIERS IN MATERIALS SCIENCE
VINSE COLLOQUIUM SERIES
Professor Terry Alford
School of Mechanical, Aerospace, Chemical, and Materials Engineering
Arizona State University, Tempe, AZ
"Zinc-Oxide Based Thin Film Transistors and ZnO/Metal/ZnO Electrodes for Future Flexible Electronics"
Abstract. Flexible electronic devices are low profile, light weight, small, and conformal. These attributes make flexible electronics ideal for a wide range of applications. However, advancement in two important areas are needed to make flexible electronics a ubiquitous technology: thin film transistors (TFTs), used as switching components in the active-matrix; and transparent conducting oxides (TCOs), a class of direct wide band-gap semiconductors that are used as electrodes in solar cells and organic light emitting diodes (OLED). The first part of the presentation will address recent advances in TCO technology. This will include the electrical and optical properties of indium-tin oxide (ITO) grown on flexible polyethylene naphthalate (PEN) substrates and the motivation to transition to zinc oxide (ZnO) based TCOs. We also show work on multilayer ZnO/metal/ZnO structures that exhibit improved electrical properties (~10-5 Ohm-cm) without degradation of optical properties when compared to ITO. Moreover, we show that ZnO/metal/ZnO electrodes have the potential to improve OLED device performance compared to current devices. The second part of the presentation we will discuss work on the electrical and mechanical reliability of indium-zinc oxide and amorphous Si (a-Si) TFTs fabricated by a low-temperature process on PEN. Conventional polycrystalline Si TFTs and a-Si TFTs have low charge carrier mobility and require high process temperatures (350 °C) making these devices incapable of being used for high resolution displays on flexible plastic substrates. Metal oxide based TFTs (i.e., ZnO) have at least 1 order of magnitude higher mobility (10~50 cm2/Vs) than a-Si TFTs or organic TFTs (<1 cm2/Vs) and can be fabricated using relatively low temperatures. The high mobility of metal-oxide based TFTs makes them viable for high resolution displays and motivates our transition to metal oxide TFTs. We have also improved the performance and stability (both room temperature and elevated temperatures) of these TFTs by novel low temperature post-anneals. The stability improvement after extended anneals is found to be due to annihilation of defects present at the channel/insulator interface and in the channel layer. Mechanical and electromechanical stress investigation reveals that the TFTs do not degrade even after an extended duration of stress.