Effects of Substrate Orientations on Phase Stability of Epitaxial HfO₂ Thin Films

The discovery of ferroelectricity in both pure and doped HfO₂ thin films have revitalized the interest in using ferroelectrics for nanoscale device applications. Previous studies on epitaxial strain engineering of ferroelectricity have mostly focused on thin films grown on (001)-oriented substrates. There is increasing interest in applying biaxial strain along other crystallographic planes such as (101) and (111) by growing thin films on substrates of different cuts. In this work, we focus on the effects of film orientations on the phase stability of HfO₂ thin films. We start by examining the ideal transition barriers between different phases of HfO₂ with first-principles methods. We find that the three phases, P2₁/c, Pca2₁ , and Pbca are separated by relatively large barriers. We calculate the energetics of different polymorphs of hafnia with epitaxial constraints imposed on the {100}, {110}, and {111} planes. Our calculations suggest that the two orthorhombic phases can become the ground state in (111)-oriented thin films. This work suggests the possibility to better stabilize the ferroelectric phase in HfO₂ thin films through substrate orientation engineering.