Effects of additive hydrogen gas on the instability due to air exposure in ZnO-based thin film transistors

The effect of adding hydrogen gas (H2) when depositing a zinc oxide (ZnO) thin film in a thin film transistor (TFT) using the ZnO as the channel layer on the electrical characteristics of the ZnO-TFTs, particularly the change in the characteristics according to long-term exposure to air, was investigated. As the amount of added H2 gas was increased, the resistivity of ZnO films was monotonously decreased and their crystallinity was weakened. Compared with the TFT using a ZnO without H2 addition, the threshold voltage (Vth) decreased and the on/off current ratio (Ion/Ioff) greatly increased, if the amount of H2 entry was small (≤ 0.3 sccm). However, when an excessive (≥ 0.5 sccm) amount of H2 was added, the TFT's properties deteriorated. In addition, the ZnO TFTs showed a positive Vth-shift with increased air exposure time. The analysis using the X-ray photoelectron spectroscopy (XPS) confirmed that this was attributed to the reduction of oxygen vacancies due to air exposure. It was noticed that the TFTs that were manufactured using ZnO films with H2 addition showed significant suppression of the Vth-shift according to air exposure.