Thickness dependence of phase stability in epitaxial (HfxZr1−x)O2 films

Abstract

The thickness dependence of the crystal structure in epitaxial $({\mathrm{Hf}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}){\mathrm{O}}_{2}$ $(x=0,0.5,1)$ films is demonstrated. X-ray diffraction measurements suggest that the crystal phase changes from the monoclinic to the orthorhombic and tetragonal phases sequentially as film thickness decreases in $\mathrm{Hf}{\mathrm{O}}_{2}$ and $({\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}){\mathrm{O}}_{2}$ films. Both films are very thin at the ferroelectric orthorhombic phase $(<5\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$, but the $({\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}){\mathrm{O}}_{2}$ films are thicker than the $\mathrm{Hf}{\mathrm{O}}_{2}$ films. In $\mathrm{Zr}{\mathrm{O}}_{2}$ films, the monoclinic phase changes into the tetragonal phase as film thickness decreases. This means that the ferroelectric orthorhombic phase was not observed in the present study. However, ferroelectricity was observed for the present $\mathrm{Zr}{\mathrm{O}}_{2}$ films in the film thickness around the thickness boundary where the tetragonal and monoclinic phases stabilize. We confirmed the field-induced phase transition from the tetragonal to the orthorhombic phase in these $\mathrm{Zr}{\mathrm{O}}_{2}$ films by micro-x-ray diffraction measurement, which suggested that the phase transition takes place due to the small free energy difference between the ferroelectric orthorhombic phase and paraelectric tetragonal phase. Our experimental results agree with theoretical reports of the thickness dependence of the crystal structure in epitaxial films with a surface energy effect.