Improved ferroelectric properties in lanthanide-doped HfO2 thin films epitaxially grown on yttria-stabilized zirconia substrates

Hafnium oxides are important ferroelectric materials that can be compatible with current silicon-based technology. However, the polar orthorhombic (o) phase is a metastable state, which is hard to control and results in low effective polarizations. In this work, we report improved ferroelectric properties in lanthanide-doped HfO2 thin films with controllable epitaxial growth on (001)-orientated yttria-stabilized zirconia substrates. Ce, Nd, and Eu elements are selected as the doping cations and the ferroelectric o phase is found to be stabilized in the (100)o family of crystal planes in Nd- and Eu-doped HfO2 thin films. In addition, at the Eu concentration of 7.5 at. %, a (001)o-orientated texture—the polar axis direction of o phase—is achieved in the HfO2-based heterostructure, yielding well-saturated ferroelectric hysteresis loops and improved remanent polarizations (Pr) of ∼37 μC/cm2. The optimized Eu-doped HfO2 also exhibits good reproducibility, long-term retention with the bistable polarizations up to 10 years, and strong fatigue resistance with a large readout margin (2Pr) of ∼60 μC/cm2 observed above 1.0 × 108 fatigue-free cycles. Even after 1.0 × 1010 cycles, the Pr is decreased by only 25%. These performances are outstanding in ferroelectric HfO2-based films and shed light on the material designs of high-performance nonvolatile memories.