Uniformity and bias‐temperature instability of bottom‐gate zinc oxide thin‐film transistors (ZnO TFTs)

Abstract

Abstract— Short‐range uniformity and bias‐temperature (BT) instability of ZnO TFTs with SiOx/SiNx stacked gate insulators which have different surface treatments have been investigated. The short‐range uniformity of ZnO TFTs was drastically improved by N2O plasma treatment of the gate insulator. The variation in the gate voltage where a drain current of 1‐nA flows (Vgs at an Ids of 1 nA) was dramatically reduced from ±1.73 V to ±0.07 V by N2O plasma treatment of the gate insulator. It was clarified that the variations in the subthreshold characteristics of the ZnO TFTs could be reduced by N2O plasma treatment of the gate insulator due to a decrease in the variation of trap densities in deep energy levels from 0.9–2.0 × 1017 to 1.2–1.3×1017 cm−3‐eV−1. From the BT stress tests, a positive shift of Vgs at an Ids of 1 nA could be reduced by N2O plasma treatment of the gate insulator due to a decrease in the charge traps in the gate insulator. When the gate‐bias stress increases, state creation occured in the ZnO TFTs in addition to the charge trapping in the gate insulator. However, N2O plasma treatment of the gate insulator has little effect on the suppression of the state creation in ZnO TFTs under BT stress. The surface treatment of the gate insulator strongly affects the short‐range uniformity and the BT instability of Vth in the ZnO TFTs.