Thickness dependence of the electro-optic properties in epitaxial Hf0.5Zr0.5O2 thin films: Evident response down to 3 nm

We investigated the dependence of the electro-optic (EO) properties of (111)-epitaxial Hf0.5Zr0.5O2 (HZO) thin films on their thickness in the range of 3–30 nm. HZO films were deposited on (La, Sr)MnO3-bufferred SrTiO3(001) substrates using pulsed laser deposition. Both the ferroelectric orthorhombic and paraelectric monoclinic phases were found to coexist in the 30 nm thick film, and the fraction of the former phase increased with the decrease in thickness down to 5 nm. Although the effective EO coefficient, reff, remained almost unchanged down to 10 nm and decreased with a further decrease in thickness, the evident EO response was observed down to 3 nm, which agrees with the fact that HZO films can maintain ferroelectric properties down to a few monolayers in thickness. The small reff in the HZO films with thickness below 5 nm was attributed to the stabilization of the ferroelectric rhombohedral-like phase.