High Performance Thin Film Transitor with Nitrogen-doped ZnO as an Active Layer Grown at Low Temperature by Atomic Layer Deposition

Abstract

Many research groups have studied on oxide semiconductors as active channel materials for transparent TFTs. Especially, ZnO is considered as a promising candidate of oxide semiconductors because of absence of environmental concerns, and moderate mobility. In addition, TFTs with the ZnO thin films show high field effect mobility comparing to a-Si (>1cm2/Vs) and better transparency because of the wide bandgap. However, carrier concentration in ZnO is so high (>1018cm-3) that ZnO thin films are very conductive even without applied gate voltage, which leads to degradation of the performance such as poor on/off current ratio which leads to problems in switching modulation.Causes of excessive carrier concentration in ZnO thin films are considered to be due to zinc interstitials, and oxygen vacancies. Many research groups tried to reduce those defects, controlling the deposition conditions such as oxygen pressure ratio in sputtering and pulsed laser deposition. In addition, they studied on doping systems to develop high field effect mobility ZnO, adopting Ⅲ-Ⅴ elements such as Ga and In.In this study, we studied nitrogen-doped ZnO (ZnO:N) thin films using ALD method. We investigated the change of microstucture and electrical properties of the thin films by using X-ray diffraction, scanning electron microscope, X-ray photoemission spectroscope. We present ZnO:N thin films deposited at low temperature (less than 100℃) for high performance thin film transistors. We could obtain the high performance TFTs on the flexible plastic substrate at low temperature using the ALD method: greater than 30cm2/Vs of field effect mobility, >108 of on-off current ratio, 0.5V/decade of swing slope, and less than 1V of threshold voltage.