Development of Zn-based oxide thin film transistors via novel diverse structural modifications

Abstract

This doctoral dissertation describes an experimental study on the electrical features and corresponding mechanisms of Zn-based metal oxide semiconductors by approaching various structural modifications. In the first place, solution-based synthesis of Li-doped ZnO TFTs was presented. In particular, my experiment adopted a carbon-free synthesis method without using hydrocarbon complexes that can easily degrade the electrical properties of TFTs. The structural and electrical properties of Li-doped ZnO TFTs were analyzed by varying the concentrations of Li dopants, together with those of un-doped ZnO TFTs for comparison. Experimental observations suggest that the incorporation of Li dopants on the ZnO TFTs by using a carbon-free synthesis method was clearly enhanced the field effect mobility and electrical stabilities of TFTs. Secondly, electrical and structural features of solution-processed ZnO thin film transistors (TFTs) grown via a zinc ammonia complex with an ultraviolet (UV) annealing step at processing temperatures below 200oC. This approach effectively reduced the processing temperature from 300°C to 200°C, along with the improved electron mobility and operational stability. UV spectroscopy and X-ray photoelectron spectroscopy (XPS) were performed for further investigation of structural and elemental composition features of the samples. Thirdly, I examined a simple approach for highly stable solution-based zinc oxide (ZnO) TFTs by simply evaporating Al on the back channel layer without any additional chemical or plasma process for passivation. In particular, control and manipulation of Al nanoparticle (NP) formation represents one of the key approaches in this work. The possible sketch of the improved nature is proposed, along with various structural and electrical analyses. Fourthly, I studied the effect of simple Al evaporation on solution-processed ZnO TFTs through systematic structural/electrical measurements. Furthermore, I proposed a mechanism based on Al metal evaporation-driven reduction of trap states that convincingly explains the unique features of the solution-processed ZnO TFTs obtained in this work. Lastly, I propose a simple approach to obtain highly enhanced electrical features of InGaZnO (IGZO) TFTs by placing a suitable ultrathin TaOx buffer layer into the channel/gate insulator interface. Precise control of this TaOx buffer layer is critical for fabricating high performance TFTs that have good field effect mobility and outstanding stability. Extensive analyses of structural and electric characteristics were conducted to identify the possible origins for the improved TFT features.