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

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 PrInO 3 and NdInO 3 secondary phases during sintering significantly enhanced grain boundary migration resistance. This phenomenon stemmed from the significant mismatch in ionic radii between Pr 3+ (0.99 Å)/Nd 3+ (0.98 Å) and In 3+ (0.80 Å), which exceeded the mismatch in ionic radii between Tb 3+ (0.92 Å) and In 3+ . 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 cm 2 /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. Herein, three kinds of lanthanum-doped IZO (PrIZO, NdIZO, TbIZO) targets had higher densification, finer grain size and better conductivity (99.7 %/2.6 μm/2.2 mΩ cm and 99.8 %/3.9 μm/5.2 mΩ cm, 95.1 %/4.4 μm/8.1 mΩ cm) and applied as the channel layers of TFTs demonstrated excellent electrical performance ( I on / I off = 10 9 ; μ e >21 cm 2 /V·s; V th <2 V, SS < 0.3 V/dec.). Higher-quality targets were conducive to the improvement of device performance: the electrical performance of PrIZO TFT device was the best ( I on / I off = 10 9 ; μ FE = 25.9 cm 2 /V·s; V th = 0.8 V, SS = 0.18 V/dec), due to the best overall performance of PrIZO target (higher density of 99.7 %, lowest grain size of 2.6 μm and lowest resistivity of 2.2 mΩ cm). ● The formation energies of the secondary phases PrInO 3 , NdInO 3 and TbInO 3 were calculated by first principles as 0.322 eV, 0.470 eV and 0.511 eV respectively. Additionally, the phase composition of PrIZO was In 2 O 3 and Zn 3 In 2 O 6 phases with trace PrInO 3 phase and NdIZO targets was In 2 O 3 and Zn 3 In 2 O 6 phases with trace NdInO 3 phase, while, the TbIZO target was composed of In 2 O 3 and Zn 3 In 2 O 6 phases without TbInO 3 phase due to the highest formation energy. ● PrIZO and NdIZO targets had higher densification, finer grain size and better conductivity(99.7 %/2.6 μm/2.2 mΩ cm and 99.8 %/3.9 μm/5.2 mΩ cm) than TbIZO target (95.1 %/4.4 μm/8.1 mΩ cm). It indicated that the new PrInO 3 /NdInO 3 phases formed in the sintering process leading to a significant increase in grain boundary migration resistance, and then decreased the grain size of PrIZO and NdIZO targets. ● The electrical performance of TFTs based on PrIZO, NdIZO and TbIZO targets demonstrated excellent electrical performance ( I on / I off = 10 9 ; μ e >21 cm 2 /V·s; V th <2 V, SS < 0.3 V/dec.).