Low coercive field in quasi-epitaxial Al-doped HfO2 films for energy-efficient ferroelectric memories

Abstract

Unlike ALD (Atomic Layer Deposition) deposited HfO₂, sputtered films enable highly oriented or epitaxial growth and, more importantly, higher doping concentrations in HfO₂, even with dopant atoms significantly different in size or chemistry from Hf, without forming impurity phases. A 90-degree off-axis RF sputtering was used to fabricate a heavily Al-doped HfO₂ film to achieve these merits. The fabrication consisted of an ITO (Indium–Tin-Oxide) bottom electrode and Hf_0.67Al_0.33O_y (HAO) on a YSZ(100) substrate using off-axis RF (Radio Frequency) sputtering and an Al top electrode to fabricate a three-layer MFM (Metal–Ferroelectric–Metal) capacitor. When the dielectric layer thickness was optimized to 20, 16, 12, and 8 nm, the performance of the fabricated capacitors was evaluated, including polarization curves and current–voltage measurements. The dielectric properties of the HAO layer, such as residual polarization and forced voltage, were investigated to assess its suitability as a high-performance dielectric material. We demonstrated that heavy doping of Al on HfO₂ films retains ferroelectric properties with thickness and exhibits superior performance to conventional Zr-doped HfO₂, especially in terms of coercive electric field (E_c). The 0.42 MV cm⁻¹ value obtained is superior to the reported values, suggesting that Al doping effectively enhances ferroelectric properties. The results demonstrated that the ferroelectricity and energy storage capacity of HAO were enhanced owing to an increased number of orthorhombic crystal phases and polarization. The developed Hf_xAl_(1−x)O_y MFM capacitor structure exhibited excellent electrical and dielectric properties and has the potential for several applications, such as power system converters and integrated circuits.