Lanthanide elements doped IZO semiconductor targets sputtered thin films as channel layers in high mobility thin film transistors

Abstract

Three novel rare-earth-doped indium-zinc oxide (RE-IZO) semiconductor targets, specifically PrIZO, NdIZO, and TbIZO, were synthesized via a defect-engineered sintering process. These targets were subsequently employed in magnetron sputtering to deposit high-mobility thin films, which functioned as channel layers in thin-film transistors (TFTs). Firstly, the mechanism of doping elements' influence on the phase regulation and grain refinement of these three targets during the densification process was systematically investigated, combined with first-principles calculations. The formation of PrInO₃ and NdInO₃ secondary phases during sintering significantly enhanced grain boundary migration resistance. This phenomenon stemmed from the significant mismatch in ionic radii between Pr³⁺ (0.99 Å)/Nd³⁺ (0.98 Å) and In³⁺(0.80 Å), which exceeded the mismatch in ionic radii between Tb³⁺ (0.92 Å) and In³⁺. The results showed that the densities of PrIZO and NdIZO targets were 99.7 % and 99.8 %, respectively, with grain sizes of 2.6 μm and 3.9 μm, and electrical conductivities of 2.2 mΩ cm and 5.2 mΩ cm, respectively, outperforming TbIZO target (density 95.1 %, grain size 4.4 μm, resistivity 8.1 mΩ cm). Subsequently, amorphous-structured, ultra-smooth-surface (R_a < 0.5 nm), and highly uniform PrIZO, NdIZO, and the TbIZO transparent conductive films were prepared via optimized magnetron sputtering. As channel layers for thin-film transistors (TFTs), these films exhibit excellent electrical properties: I_on/I_off >0.9, μ_sat >21 cm²/V·s, threshold voltage <2 V, and subthreshold swing <0.3 V/dec. NdIZO TFTs exhibit the smallest threshold voltage shift (ΔV_th = 0.6 V) under positive bias stress (PBS), demonstrating excellent bias stability. The results showed that the better the overall performance of the targets, the better the performance of the TFT devices. This work provided an effective research strategy for developing channel layer materials for high-performance TFT devices.