ZnO thin-film transistor grown by rf sputtering using Zn metal target and oxidizer pulsing

Abstract

We studied ZnO films grown by rf sputtering using Zn metal targets. During the growth the metal target can be at a metal or at an oxide mode, depending on oxidation of the target surface. At a metal mode the target surface is free of oxide, and the sputtering yield is higher, but deposited ZnO films show poor transistor characteristics. ZnO films deposited at an oxide mode show better transistor characteristics, but the sputtering yield is lower. In order to solve these problems, we supplied oxidizer gas as pulses during the growth. We hoped that the target condition could be controlled by varying parameters of the pulses. Our ZnO was grown at 450°C using CO₂ or O₂ as an oxidizer. After sputtering growth ZnO films were annealed in mixture of CO₂ and H₂ at 400°C. With these methods, bottom-gate ZnO thin-film transistor showed 6.5 cm²/Vsec mobility, 5 × 10⁶ on/off ratio, and −5 V threshold voltage.