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

Ferroelectric HfO2 have attracted much attention owing to their superior ferroelectricity at an ultra-thin thickness and good compatibilities 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 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 have been reported. Structural analysis has suggested that non-polar 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 stabilized 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.