Guidelines for the stabilization of a polar rhombohedral phase in epitaxial Hf_0.5Zr_0.5O₂ thin films

Abstract

The unconventional Si-compatible ferroelectricity in hafnia-based systems, which becomes robust only at nanoscopic sizes, has attracted a lot of interest. While a metastable polar orthorhombic (o-) phase ( Pca21) is widely regarded as the responsible phase for ferroelectricity, a higher energy polar rhombohedral (r-) phase is recently reported on epitaxial HfZrO4 (HZO) films grown on (001) SrTiO3 (R3m or R3), (0001) GaN (R3), and Si (111). Armed with results on these systems, here we report a systematic study leading towards identifying comprehensive global trends for stabilizing r-phase polymorphs in epitaxially grown HZO thin films (6 nm) on various substrates (perovskites, hexagonal and Si).

In situ heating studies on temperature-induced phase transitions in epitaxial Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 heterostructures

“…While the orthorhombic (o-) phase (Pca2 1 ) is the least energetic polar phase and is commonly observed through various synthesis procedures, a higher energy rhombohedral phase (R3m, R3) was also shown to be stabilized under specific conditions in epitaxial Hf 0.5 Zr 0.5 O 2 systems (HZO). 6,7 Surface energy and size effects, epitaxial strain, thermal and inhomogeneous stress effects, oxygen vacancies, and dopants have all been explored as possible ways of stabilizing the polar phases. 2,8 The nature of these polar phases and the investigations of polarization switching continue to reveal intriguing details, confirming the unconventional nature of this ferroelectricity.…”

“…In the perovskite phase, c 0 measured across 20 consecutive unit cells of LSMO is uniform (3.86 Å 6 0.05 Å ), with perturbations most likely arising from growth-induced V€ o . 6 In the BM-precursor phase (at 150 C), c 0 varies in a disorderly fashion between 3.55 and 4.27 Å . In a BM phase, at 300 C, there is an alternating oscillation of the c 0 parameter between 3.79 and 4.43 Å (60.05 Å ), resulting in a doubling of the original perovskite unit cell along the c 0 -axis [ Fig.…”

“…In Fig. 2(c), the domains D1 and D2 correspond to viewing the BM structure along the [1][2][3][4][5][6][7][8][9][10] pc and [110] pc zone axes, respectively. In D1, back-to-back tetrahedra in the ZA:[110] pc can be clearly visualized as Mn doubling in the tetrahedral Mn planes [ Fig.…”

“…La(Sr)O and MnO 2 planes alternate along the c-axis, which is the out-of-plane (growth) direction. Along the in-plane[1][2][3][4][5][6][7][8][9][10], antiphase (a À a À a À ) tilts of oxygen octahedra manifest as antiphase d tilts with a zigzag arrangement of Mn-O-Mn bonds 26. Figure 1(f) shows an iDPC-STEM image of the LSMO layer in the virgin state.…”

“…(b) iDPC-STEM image of the LSMO layer stabilized at 300 C for $1 h. It is in a BM phase with ZA:[1][2][3][4][5][6][7][8][9][10] pc . The inset shows the schematic of the BM phase with ZA:[1][2][3][4][5][6][7][8][9][10] pc green: La, green: Mn, red: O: brown (c) c 0 parameter variation in 18 psuedocubic unit cells of LSMO at various temperatures. (d) iDPC-STEM image of the LSMO layer stabilized at 400 C for $1 h. Domains with ZA [110] (D1) and [1-10] pc (D2) are imaged.…”