THE EFFECTS OF HfO2 BUFFER LAYER ON Al2O3-PASSIVATED INDIUM-GALLIUM-ZINC OXIDE THIN FILM TRANSISTORS

Abstract

In recent years, a new class of high-performance thin-film transistors (TFTs) has emerged comprising amorphous oxide channel materials composed of heavy-metal cations (HMCs) with (n - 1)d10ns0 (n ≥ 4, where ‘n’ refers to the row of the periodic table) electronic configuration. This thesis is devoted to the characterization of TFTs employing amorphous oxide channel: indium zinc gallium oxide (IGZO). There are substantial interests in the use of transparent oxide semiconductor as an active layer of thin film transistor. Among these semiconductors materials, amorphous Indium-Gallium-Zinc-Oxide (IGZO) has been emerged as a promising solution for high performance backplane and investigated its realistic possibility. In spite of their many merits, the electrical reliability of oxide semiconductor based TFT devices remains a very important and critical issue. Recently, several studies have reported that exposure of the active channel layer to environmental conditions causes charge trapping, charge injection, and channel defect creation, leading to the electrical instability of oxide semiconductor based TFTs. Passivation is one of the promising processes to protect the active channel layer from contamination caused by environmental conditions. Therefore, various passivation layers such as SiO2, Si3N4, and Al2O3 are widely used to address the instability problems. However, exposure of the active channel layer to various reactant gases during the passivation process is unavoidable in the case of bottom-gate type TFTs. Namely, the passivation process can adversely affect the electrical properties and stability of TFTs under these conditions. In this study, the effects of HfO2 buffer layer between an Al2O3 passivation layer and an indium-gallium-zinc-oxide (IGZO) thin film transistor (TFT) were investigated to enhance the environmental stability and electrical properties of the IGZO TFTs. When applying the HfO2 buffer layer, the formation of HfAlxOy at the Al2O3/HfO2 interface reduces the water vapor transmission rate (WVTR) due to the densification of the passivation layer. Consequently, by adding the HfO2 buffer layer, WVTR of the TFT were highly improved, moreover, the electrical characteristics changes can be prevented compared to both the non-passivated and Al2O3-passivated TFTs.