Growth and structural characterization of strained epitaxial $\mathrm{H}{\mathrm{f}}_{0.5}\mathrm{Z}{\mathrm{r}}_{0.5}{\mathrm{O}}_2$ thin films

Ferroelectricity was recently reported in thin films with several compositions in the $\mathrm{Hf}{\mathrm{O}}_2\text{−}\mathrm{Zr}{\mathrm{O}}_2$ system with orthorhombic crystal structure. In the present paper we study the growth by pulsed laser deposition and the structural characterization of strained epitaxial $\mathrm{H}{\mathrm{f}}_{0.5}\mathrm{Z}{\mathrm{r}}_{0.5}{\mathrm{O}}_2$ films on $\left(001\right)$-oriented yttria-stabilized zirconia (YSZ) substrates. We have determined the conditions for the coherent growth and correlated the deposition parameters with the films structure and microstructure studied through a combination of x-ray diffraction, electron backscatter diffraction, and scanning transmission electron microscopy. In the range of experimental parameters explored, all the films show monoclinic structure with distorted lattice parameters relative to bulk.

Figure 1

Symmetric θ/2θ XRD patterns around the YSZ (002) reflection, measured in 10-nm-thick HZO films grown at 10 mTorr and different substrate temperatures. The dashed line is a guide to the eye.

Figure 2

Reciprocal space maps measured around the YSZ (113) reflection for a 10-nm-thick film grown at $550{}^{\circ }\mathrm{C}$ (a) and for a 90-nm-thick film grown at $750{}^{\circ }\mathrm{C}$ (b).

Figure 3

Dependence of the 2θ angle of the HZO films reflection (left) and the corresponding $c$ lattice parameter (right) with the deposition temperature for the samples shown in Fig. 1. The continuous lines are eye guides and the dashed lines show the respective values of bulk HZO.

Figure 4

Schematic illustration showing the possible orientations of twin domains in the monoclinic HZO films grown epitaxially on cubic YSZ.

Figure 5

EBSD maps of the 30-nm-thick HZO film prepared at $850{}^{\circ }\mathrm{C}$ (a), 90-nm-thick HZO film prepared at $850{}^{\circ }\mathrm{C}$ (b), and 30-nm-thick HZO film prepared at $450{}^{\circ }\mathrm{C}$ (c). A zoom of the central part of the $\langle 001\rangle$-HZO pole figure is shown in (d). The color scheme used for the maps corresponds to the orientation of the different domains in the $\langle 001\rangle$-HZO pole figure (up: red; right: yellow; left: blue; bottom: green), and is the same as in Fig. 4. The $X\text{−}Y$ axes of the three maps and of the pole figure are parallel to the [100] and [010] axes of the YSZ substrate, respectively, as indicated in (a).

Figure 6

(a) Aberration corrected HAADF-STEM image of a 10-nm-thick HZO film grown on YSZ (001) at 850 °C. (b) Detail of the atomic structure. (c) Geometrical phase analysis (GPA) of a low magnification image of the same film (top image), including the in-plane deformation $({\varepsilon }_{xx}$, central panel), and the out-of-plane deformation $({\varepsilon }_{zz}$, lower panel) of the film's lattice parameter with respect to that of the substrate.