Stabilization of phase-pure rhombohedral $\mathrm{HfZr}{\mathrm{O}}_4$ in pulsed laser deposited thin films

Controlling the crystalline structure of hafnium zirconate and its epitaxial relationship to a semiconducting electrode has high technological interest, as ferroelectric materials are key ingredients for emerging electronic devices. Using pulsed laser deposition, a phase-pure, ultrathin film of $\mathrm{HfZr}{\mathrm{O}}_4$ is grown epitaxially on a GaN(0001)/Si(111) template. Since standard microscopy techniques do not allow us to determine with certitude the crystalline structure of the film due to the weak scattering of oxygen, differentiated differential phase contrast scanning transmission electron microscopy is used to allow the direct imaging of oxygen columns in the film. Combined with x-ray diffraction analysis, the polar nature and rhombohedral $R3$ symmetry of the film are demonstrated.

Figure 1

RHEED diffraction patterns of GaN (a), (b) and HZO (c), (d) along the [10–10] direction of GaN (a), (c) and along the [11–20] (b), (d), showing the epitaxy of HZO on GaN. The blue lines are guides to the eye.

Figure 2

(a) Symmetric XRD scan of a $\mathrm{HfZr}{\mathrm{O}}_4$ film with a thickness of 5.9 nm. The plain arrow indicates the HZO (111) diffraction peak and the dashed arrow a satellite Laue fringe. The inset shows a rocking curve around the HZO diffraction peak. (b) Pole figures of the same film with six peaks at $\chi \approx {71}^{\circ }$ labeled P1–P6. (c) Black: symmetric XRD scans of the 111 peak, red, blue, green: GIXRD scans of peaks P2, P4, P6.

Figure 3

(a) Cross-sectional HAADF-STEM image of a 5.9-nm $\mathrm{HfZr}{\mathrm{O}}_2$ sample. The different layers are labeled with their respective material. In the HZO layer, the R1 and R2 domains have been indicated. (b), (c) show the Fourier transforms of the R1 and R2 domains, in which the diffraction spots are labeled. (d) HAADF-STEM image of a R2 domain. The inset shows the simulated HAADF image for the r phase.

Figure 4

(a) dDPC image of a HZO film, scale 1 nm. The blue box is enlarged in (b). The blue (red) spots represent the position of the Hf/Zr (oxygen) atoms, with an overall displacement on 8.6 pm. (c) Simulated dDPC images for the $R3m$ and $R3$ phases. The yellow and red lines emphasize the best match between experimental data and the $R3$ phase. (d) Simulated $R3$ unit cell.