Effect of the number and distribution of Al₂O₃ atomic layer deposition cycles within HfO₂ layer on ferroelectric characteristics

We comprehensively analyze the effects of the number and distribution of Al₂O₃ atomic layer deposition (ALD) cycles into a 10-nm-thick HfO₂ matrix on the ferroelectric switching behavior. An ALD cycle containing one pulse for Hf (or Al) precursor and one pulse of water as reactant is repeated 150 times for the given thickness of 10 nm. Spontaneous remnant polarization (P_r) is enabled through the formation of crystalline Al-doped HfO₂ (Al:HfO₂) by incorporating at least two Al₂O₃ ALD cycles evenly into the HfO₂ film under annealing at 600 °C for 3 min following W top electrode (TE) deposition. When more than four Al₂O₃ cycles are used, the Al elements function as leakage sources rather than stressors, resulting in an open hysteresis loop and a weak endurance of 10⁵ cycles. Notably, an improved 2 P_r of ∼9 μC/cm² is achieved when the Al₂O₃ layers are concentrated near the lower region of the HfO₂. On the other hand, as the Al₂O₃ layers are intensively located in the upper region of the HfO₂, a dielectric response is observed in the polarization-voltage and current-voltage measurements. Our results indicate that the two mechanical stresses induced by the Al dopant with a size smaller than Hf and the difference in the thermal expansion coefficient between TE and Al:HfO₂ effectively activate both the lower and upper sites. Therefore, many dipoles are observed to participate in the polarization owing to the stresses that are applied evenly throughout the Al:HfO₂ layer to form the orthorhombic phase.