Effect of sputtering power on properties of ZnO thin film transistors with Bi_1.5Zn_1.0Nb_1.5O₇ gate insulator

ZnO based thin film transistors (ZnO-TFTs) exhibit high field effect mobility, high drive capability, low operating voltage, low sub-threshold swing and low gate-leakage current. In this work, ZnO-TFTs with Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) gate insulator were fabricated by radio frequency magnetron sputtering at low temperature (500 °C). Pyrochlore BZN thin films were deposited onto Pt/TiO₂/SiO₂/Si substrates under different sputtering powers from 110 W to 190 W. The influence of sputtering power on dielectric properties of BZN thin film has been investigated. The capacitance density and dielectric constant increase from 165 nF/cm² to 230 nF/cm² and from 48 to 66, respectively, while the loss tangent reduces from 1.7% to 0.8% with increasing sputtering power. BZN thin films are amorphous in nature and have dense and smooth surface. The roughness of BZN thin films decreases from 6.70 nm to 3.67 nm with increasing sputtering power. The influence of sputtering power on electrical properties of ZnO-TFTs with BZN gate insulator has also been studied. The field effect mobility, current on/off ratio and saturation current increase from 0.026 cm²/Vs to 0.18 cm²/Vs, from 10³ to 10⁵ and from 0.1 μA to 2.79 μA, respectively. While the sub-threshold swing decreases from 689 mV/dec. to 384 mV/dec., threshold voltage is less than 4 V. ZnO-TFTs with BZN gate insulator exhibit the optimized electrical properties at sputtering power of 150 W, which also result in the formation of a smooth surface at the oxide/channel interface.