Effect of excess Bi₂O₃ on structure and performance of ZnO-based thin film transistors

High-performance ZnO-based thin film transistors (ZnO-TFTs) were fabricated by radio frequency magnetic sputtering using Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) films as the gate dielectrics. BZN films with excess Bi₂O₃ were compared to traditional BZN films to investigate the effects on ZnO-TFT performance. The results showed that excess Bi₂O₃ (5 mol. %) can significantly reduce leakage currents and increase dielectric constants in BZN gate dielectrics. The excess Bi₂O₃ in these films served to compensate for the bismuth loss that typically occurs in BZN films during the deposition process, leading to improved electrical characteristics. At an applied electric field of 250 kV/cm, the leakage current density for the BZN film with excess Bi₂O₃ was approximately 2 orders of magnitude lower than that of the BZN film without excess Bi₂O₃. Moreover, the BZN film with 5 mol. % excess Bi₂O₃ exhibited high capacitance density (233 nF/cm²) and a high dielectric constant (65). The electrical performances of ZnO-TFTs were remarkably enhanced by using BZN films with excess Bi₂O₃ as the gate insulator, with demonstrated field-effect mobility of 0.25 cm²/Vs, on/off ratio of 1 × 10⁵, and low threshold voltage of 2.3 V.