On the Origin of the Large Remanent Polarization in La:HfO₂

The outstanding remanent polarization of 40 µC cm^–2 reported for a 10 nm thin La:HfO₂ film in 2013 has attracted much attention. However, up to now, no explanation for this large remanent polarization has been presented. Density functional theory and X-ray diffraction are used to shine light onto three major aspects that impact the macroscopically observed remanent polarization: phase fraction, spontaneous polarization, and crystallographic texture. Density functional theory calculations show that the spontaneous polarization (P_s) of La:HfO₂ is indeed a bit larger than for other HfO₂- or ZrO₂-based compounds; however, the P_s is not large enough to explain the observed differences in remanent polarization. While neither phase fractions nor spontaneous polarization nor strain are significantly different from those in other HfO₂ films, a prominent 020/002 texture distinguishes La doped from other HfO₂-based ferroelectric films. Angular-dependent diffraction data provide a pathway to calculate the theoretically expected remanent polarization, which is in agreement with the experimental observations. Finally, an interplay of the in-plane strain and texture is proposed to impact the formation of the ferroelectric phase during annealing. Further aspects of the special role of La as a dopant are collected and discussed to motivate future research.