Effect of ${\rm O}_{2}$ Flow Rate During Channel Layer Deposition on Negative Gate Bias Stress-Induced $V_{\rm th}$ Shift of a-IGZO TFTs

Abstract

The effect of O2 flow rate during the sputtered deposition of channel layer on the negative gate-bias stress (NGBS)-induced threshold voltage (Vth) instability of a-IGZO TFTs is investigated. It is shown that the negative gate-bias stress results in a negative Vth shift of the a-IGZO TFTs, and the shift amount decreases with the increase in O2 flow rate. It is proposed that the Vth shift originates from the electron-detrapping from the oxygen vacancy-related donor-like states at the channel/dielectric interface. As the O2 flow rate increases, the density of donor-like states is decreased and the distribution of neutral donor-like states below EF is also reduced. Therefore, the amount of Vth shift caused by the positively charged trap states and electrons injecting into the channel from the donor-like states decreases with the O2 flow rate increase. It is also shown experimentally that while the electrical characteristics of the a-IGZO TFTs are generally improved with the O2 flow rate increase, they are degraded if an excess O2 is introduced. An optimal O2/Ar flow rate ratio of about 5 sccm/45 sccm is suggested to make a trade-off between the electrical performances and the gate-bias stress-induced Vth instability.