Interfacial stress engineering toward enhancement of ferroelectricity in Al doped HfO₂ thin films

Ferroelectric HfO₂ has attracted much attention owing to its superior ferroelectricity at an ultra-thin thickness and good compatibility with Si-based complementary metal–oxide–semiconductor (CMOS) technology. However, the crystallization of polar orthorhombic phase (o-phase) HfO₂ is less competitive, which greatly limits the ferroelectricity of the as-obtained ferroelectric HfO₂ thin films. Fortunately, the crystallization of o-phase HfO₂ can be thermodynamically modulated via interfacial stress engineering. In this paper, the growth of improved ferroelectric Al doped HfO₂ (HfO₂:Al) thin films on (111)-oriented Si substrate has been reported. Structural analysis has suggested that nonpolar monoclinic HfO₂:Al grown on (111)-oriented Si substrate suffered from a strong compressive strain, which promoted the crystallization of (111)-oriented o-phase HfO₂ in the as-grown HfO₂:Al thin films. In addition, the in-plane lattice of (111)-oriented Si substrate matches well with that of (111)-oriented o-phase HfO₂, which further thermally stabilizes the o-phase HfO₂. Accordingly, an improved ferroelectricity with a remnant polarization (2P_r) of 26.7 μC/cm² has been obtained. The results shown in this work provide a simple way toward the preparation of improved ferroelectric HfO₂ thin films.