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

Abstract

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 ∘C in ambient conditions. Film annealed at 200 ∘C is amorphous, but further annealing at 400 ∘C crystallizes it into the monoclinic structure. Amorphous films exhibit a higher bandgap (≥ 5.8 eV) than the monoclinic film (≈ 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 ∘C annealed films are estimated as 22.35, 22.64, and 20.57, respectively. The increment in the dielectric constant at 200 ∘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 hafnium oxide as gate insulator show almost similar threshold voltage of ≈ −1 V. The transistor with hafnium oxide annealed at 200 ∘C shows the highest on current (1.04 × 10−6 A) followed by as-deposited (6.82 × 10−7 A) and 400 ∘C annealed device (5.86 × 10−7 A), respectively. Moreover, the interface state density increases monotonically from 3.98 × 1012 to 1.11 × 1013 cm−2 for as-deposited and 400 ∘C, respectively. The increment in the interface state density is due to the grain boundary formation due to the crystallization of HfO2.