Phase transformation of sputtered hafnium oxide by post annealing treatment and its effect on the amorphous Si-In-Zn-O thin film transistor

The effect of post annealing treatment on the phase transformation and the electrical properties of the sputtered hafnium oxide (HfO2) is investigated. As-deposited HfO2 films were annealed at 200 and 400 degrees C in ambient conditions. Film annealed at 200 degrees C is amorphous, but further annealing at 400 degrees C crystallizes it into the monoclinic structure. Amorphous films exhibit a higher bandgap (>= 5.8 eV) than the monoclinic film (similar to 5.65 eV). The electrical characteristics of the Metal-Insulator-Metal devices reveal a nonlinear capacitance with respect to the voltage. The nonlinearity is predominant in the as-deposited film due to the oxygen vacancy related defects formed during the deposition. However, excess Hf-O bond formation occurs with annealing at higher temperatures, reducing the capacitance-voltage curve's nonlinearity. The dielectric constant of the as-deposited, 200, and 400 degrees C annealed films are estimated as 22.35, 22.64, and 20.57, respectively. The increment in the dielectric constant at 200 degrees C is due to its amorphous phase and reduced defects. Amorphous Si-In-Zn-O thin film transistors fabricated using the as-deposited and annealed haf-nium oxide as gate insulator show almost similar threshold voltage of similar to -1 V. The transistor with hafnium oxide annealed at 200 degrees C shows the highest on current (1.04 x 10-6 A) followed by as-deposited (6.82 x 10-7 A) and 400 degrees C annealed device (5.86 x 10-7 A), respectively. Moreover, the interface state density increases monotonically from 3.98 x 1012 to 1.11 x 1013 cm-2 for as-deposited and 400 degrees C, respectively. The increment in the interface state density is due to the grain boundary formation due to the crystallization of HfO2. (c) 2022 Elsevier B.V. All rights reserved.