Understanding the induced structural and electronic changes in amorphous InGaZnO by low temperature annealing for flexible electronic applications

Abstract

The expansion of the thin-film transistors (TFTs) market has been closely tied to the advancement of liquid-crystal displays (LCD). However, the demand for higher resolutions and refresh rates has rendered the mobility of hydrogenated amorphous silicon (a-Si:H) inadequate for driving these evolving technologies. This has led researchers to seek alternative materials. Amorphous oxide semiconductors (AOS), specifically amorphous indium-gallium-zinc oxide (a-InGaZnO), has emerged as a strong candidate due to the improved mobility and compatibility with standard flat-panel display processes. The excellent properties of InGaZnO in the amorphous phase have garnered interest for wider electronics applications involving flexible substrates, which are typically polymer-based and thus impose limitations on processing temperature. Consequently, understanding the effects of low-temperature annealing on the structural and electronic characteristics of a-InGaZnO is crucial to harness its potential across a diverse range of applications. This thesis centers on discerning the influence of low-temperature annealing exclusively on the a-InGaZnO semiconductor layer, excluding subsequent layer processing. By employing X-ray reflectivity and Hall-effect techniques for structural and electronic evaluation, respectively, this study identifies two distinct temperature regimes. Below 200°C, a gradual enhancement in film conductivity corresponds to a slight rise in carrier concentration. In contrast, temperatures above 200°C result in substantial elevations in both carrier concentration and mobility, accompanied by an approximately linear increase in film density. Furthermore, the literature frequently presents X-ray photoelectron spectroscopy (XPS) characterizations of a-InGaZnO, attributing variations in device electronic performance to alterations in the O 1s orbital of the thin-film. However, conflicting interpretations have arisen and this project offers a comprehensive review of XPS analysis applied to a-InGaZnO, covering the disputes and discrepancies among different research findings. Separately, a protocol for focused ion beam preparation of lamellae, on polymeric substrates for TEM cross sectional imaging of thin-films and devices was developed and preliminary imaging is presented.