Comparison of structural and electrical properties of Lu₂O₃ and Lu₂TiO₅ gate dielectrics for α-InGaZnO thin-film transistors

We compared the structural properties and electrical characteristics of high-κ Lu₂O₃ and Lu₂TiO₅ gate dielectrics for amorphous indium-gallium-zinc oxide (α-InGaZnO) thin-film transistor (TFT) applications. The Lu₂O₃ film has a strong Lu₂O₃ (400) peak in the X-ray diffraction pattern, while the Lu₂TiO₅ sample shows a relatively weak Lu₂TiO₅ (102) peak. Atomic force microscopy reveals that the Lu₂O₃ dielectric exhibits a rougher surface (about three times) than Lu₂TiO₅ one. In X-ray photoelectron spectroscopy analysis, we found that the intensity of the O 1s peak corresponding to Lu(OH)_x for Lu₂O₃ film was higher than that of Lu₂TiO₅ film. Furthermore, compared with the Lu₂O₃ dielectric, the α-InGaZnO TFT using the Lu₂TiO₅ gate dielectric exhibited a lower threshold voltage (from 0.43 to 0.25 V), a higher I_on/I_off current ratio (from 3.5 × 10⁶ to 1.3 × 10⁸), a smaller subthreshold swing (from 276 to 130 mV/decade), and a larger field-effect mobility (from 14.5 to 24.4 cm²/V s). These results are probably due to the incorporation of TiO_x into the Lu₂O₃ film to form a Lu₂TiO₅ structure featuring a smooth surface, a low moisture absorption, a high dielectric constant, and a low interface state density at the oxide/channel interface. Furthermore, the stability of Lu₂O₃ and Lu₂TiO₅ α-InGaZnO TFTs was investigated under positive gate-bias stress (PGBS) and negative gate-bias stress (NGBS). The threshold voltage of the TFT performed under NGBS is more degradation than that under PGBS. This behavior may be attributed to the electron charge trapping at the dielectric-channel interface under PGBS, whereas the oxygen vacancies occurred in the InGaZnO under NGBS.