Defect engineering for enhanced stability and spontaneous polarization in rhombohedral HfO2: First-principles insights

The rhombohedral R3-HfO2 with ferroelectric polarization as large as 41 μC/cm2 has attracted intensive attention for potential applications in memory and logic devices. Here, we demonstrate by first-principles calculations that its stability and polarization are very sensitive to the formation of intrinsic point defects and thus can be enhanced through defect engineering. No matter under an Hf-poor or O-poor condition, a high concentration of point defects can form in R3-HfO2, so a moderately Hf-rich and O-rich condition is required for suppressing the defect formation and fabricating high-quality and stable R3-HfO2 thin films. Its strong ferroelectricity can be maintained only when the formed defects are located on the body diagonal of the unit cell without breaking the R3 symmetry, while the off-diagonal defects can break the R3 symmetry, making R3-HfO2 relax into a more stable but paraelectric ground state structure. Among the on-diagonal defects, VO is the dominant donor, and Oi and VHf are the dominant acceptors. Interestingly, the formation of Oi leads to a 24.4% increase in the polarization, while VHf leads to a 66.5% decrease. These results indicate that defect engineering in R3-HfO2 is critical for fabricating high-quality R3-HfO2 thin films with good stability and large polarization.