Structural properties and electrical characteristics of Ho ₂O₃ and HoTi_xO_y gate dielectrics for a-InGaZnO thin-film transistors

In this study, we developed high-κ Ho₂O₃ and HoTi_xO_y gate dielectrics for amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistor (TFT) applications. X-ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy were used to study the structural, morphological and chemical features of Ho₂O ₃ and HoTi_xO_y dielectric films. Compared with the Ho₂O₃ dielectric, the a-IGZO TFT incorporating the high-κ HoTi_xO_y gate dielectric exhibited very good electrical characteristics, such as a high I_on/off ratio of 1.1 × 10⁸, a high field effect mobility of 20.6 cm² V^-1 s^-1, a low threshold voltage of 0.23 V, and a low subthreshold swing of 183 mV decade^-1. These results are probably due to the incorporation of Ti into the Ho₂O₃ film, resulting in the formation of a smooth surface and a low density of interface states at the oxide/channel interface. In addition, the stability of high-κ Ho ₂O₃ and HoTi_xO_y a-IGZO TFTs was investigated under positive gate-bias stress (PGBS) and negative gate-bias stress (NGBS). The electron charge trapping at the dielectric-channel interface resulted from the PGBS, whereas the oxygen vacancies occurred in the a-IGZO under the NGBS.