Physics and technology of thin-film transistors based on zinc oxide and indium-gallium-zine oxide

Abstract

Thin-film transistors (TFTs) based on metal oxides (MOs), especially zinc oxide (ZnO) and indium-gallium-zinc oxide (IGZO), are promising alternatives to silicon-based TFTs in future flat-panel displays. Undoped ZnO and IGZO are thermally annealed and the dependences of their resistivity (ρ) on the heat-treatment conditions and cover configurations are studied. The physical mechanism of the annealing-induced ρ variation is investigated and ascribed to the generation or annihilation of intrinsic donor-like defects, analyzed in greater detail using photoluminescence and X-ray photoelectron spectroscopy. When MOs are doped with extrinsic dopants, the resulting ρ is highly sensitive to both dopants and MO compositions, suggesting different doping efficiencies, and further depends on the annealing conditions, deriving from the dopant activation or annihilation. The effects of thermal processing on the characteristics of ZnO and IGZO TFTs with either gas-permeable or impermeable gate-stack were studied and compared. Thermal oxidization of the TFT channel, allowed by a gas-permeable gate-stack, results in a low-defect channel, significantly improving the transistor characteristics – e.g. eliminating the hysteresis and increasing the field-effect mobility. In contrast, the characteristics of a TFT heat-treated in a non-oxidizing ambience or under a sealed configuration degrade with the increasing temperature. Such a difference is attributed to the annealing-dependent generation and annihilation of defects in MOs, suggesting a general guideline on the thermal processing of MO TFTs. With the thermal diffusion of dopants from source/drain into the channel, the annealing-induced degradation could be even more serious. Based on these understandings, a MO TFT with annealing-induced homojunctions is proposed, with the source/drain junctions and the channel region capped respectively by impermeable and permeable covers. During a subsequent oxidizing annealing, the low-ρ source/drain and high-ρ low-defect channel are simultaneously formed. Compared to traditional MO TFTs, one photolithography mask for the etch-stopper is saved, significantly cutting down the cost.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology