Dopants Promoting Ferroelectricity in Hafnia: Insights From A Comprehensive Chemical Space Exploration

TL;DR

This study uses first principles calculations to explore how nearly 40 dopants influence the phase stability of hafnia, identifying elements that can promote ferroelectricity by lowering the energy of the polar phase, despite not stabilizing it as the ground state.

Contribution

The paper provides a comprehensive chemical space analysis of dopants in hafnia, revealing chemical trends and identifying the most promising dopants for ferroelectricity promotion.

Findings

Ca, Sr, Ba, La, Y, and Gd significantly lower the polar phase energy.

Larger ionic radii and lower electronegativity favor the polar phase.

Trivalent dopants stabilize the polar phase at lower strains.

Abstract

Although dopants have been extensively employed to promote ferroelectricity in hafnia films, their role in stabilizing the responsible ferroelectric non-equilibrium Pca21 phase is not well understood. In this work, using first principles computations, we investigate the influence of nearly 40 dopants on the phase stability in bulk hafnia to identify dopants that can favor formation of the polar Pca21 phase. Although no dopant was found to stabilize this polar phase as the ground state, suggesting that dopants alone cannot induce ferroelectricity in hafnia, Ca, Sr, Ba, La, Y and Gd were found to significantly lower the energy of the polar phase with respect to the equilibrium monoclinic phase. These results are consistent with the empirical measurements of large remnant polarization in hafnia films doped with these elements. Additionally, clear chemical trends of dopants with larger ionic radii and lower electronegativity favoring the polar Pca21 phase in hafnia were identified. For this polar phase, an additional bond between the dopant cation and the 2nd nearest oxygen neighbor was identified as the root-cause of these trends. Further, trivalent dopants (Y, La, and Gd) were revealed to stabilize the polar Pca21 phase at lower strains when compared to divalent dopants (Sr and Ba). Based on these insights, we predict that the lanthanide series metals, the lower half of alkaline earth metals (Ca, Sr and Ba) and Y as the most suitable dopants to promote ferroelectricity in hafnia.