Synergistic impact of Al₂O₃ capping layer and deposition temperature for enhancing the ferroelectricity of undoped-HfO₂ thin films

Synergistic effects of Al₂O₃ capping layer and deposition temperature, ranging from 220 to 280 °C, on the ferroelectric properties of undoped HfO₂ thin films were investigated. The use of an Al₂O₃ capping layer during atomic layer deposition (ALD) process was verified to induce a ferroelectric orthorhombic phase in undoped HfO₂ thin films, leading to a net remnant polarization (2P_r) of >20 μC/cm² when deposited <260 °C. The HfO₂ thin film deposited at 240 °C showed a low leakage current density of 5.7 × 10⁻⁶ A/cm² at an electric field of 2 MV/cm, accompanied by a high 2P_r and an endurance of >10⁷ cycles. Furthermore, the switching time for ferroelectric polarization reversal was estimated to range from 2.1 to 1.7 μs in the voltage range of 3.5–4.5 V. The results of this work demonstrated the impact of deposition temperature conditions on the ferroelectric properties of the HfO₂ thin films by means of the Al₂O₃ capping process.