Electric‐Field‐Induced Ferroelectricity in 5%Y‐doped Hf_0.5Zr_0.5O₂: Transformation from the Paraelectric Tetragonal Phase to the Ferroelectric Orthorhombic Phase

Abstract

The ferroelectric phase transformation from the tetragonal phase to the orthorhombic phase, induced by an electric field, is demonstrated in a 5%YO1.5‐doped Hf0.5Zr0.5O2 epitaxial film which is grown on Sn‐doped In2O3‐covered (111) yttria‐stabilized zirconia by the pulsed laser deposition method at room temperature and subsequent heat treatment. Although X‐ray diffraction shows the film to consist of a paraelectric tetragonal phase after the heat treatment, polarization–electric field (P–E) measurements reveal a hysteresis loop attributed to the ferroelectricity. To clarify the discrepancy between the crystal structure and electric characteristics, the crystal structure after electric field loading is determined by scanning transmission electron microscopy and synchrotron X‐ray diffraction measurements. Both structural characterizations clearly reveal that the application of an electric field promotes the phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase. This ferroelectric transition is irreversible, as the ferroelectric phase remains after the removal of the electric field. These results facilitate the elucidation of the mechanism by which ferroelectricity is displayed in HfO2‐based fluorite ferroelectric materials and imply unimportance of the orthorhombic phase for as‐prepared films.

Substrate-dependent differences in ferroelectric behavior and phase diagram of Si-doped hafnium oxide

“…Less (100) and (110) signal is observed for the 10:1 sample. While this might be explainable by increased tetragonal phase, this would not match to the electrical results, as other groups have reported that super-cooled tetragonal phase does not result in AFE-like behavior but reduced P R and P S [20,21]. Moreover, the shape of the {200} -peak is slightly changed for the 10:1 sample, as indicated by the smaller TiN-related shoulder and reduced intensity of the (200) shoulder at smaller angles.…”

“…unlikely as an origin for AFE-like behavior, since the operating temperature is far below the Curie temperature [31,32] and recent investigations demonstrated the irreversible nature of this phase transition [20,21]. Internal bias fields and domain wall pinning on the other side would require diffusion/drift processes during wake-up, which are not observed [22].…”

“…Moreover, combined with results from other groups, which reported on a generalized doping dependence for different dopants [44], the here proposed phase diagram can be adapted for other doping elements. In addition, very high doping levels can result in super-cooled tetragonal phase [20,21], which can be converted to orthorhombic phase by electric field cycling and should result in P S increase. Depending on the present stress level, the orthorhombic phase will then either behave AFE-like or ferroelectric.…”

“…Since most thin films are of polycrystalline nature, the presence of small amounts of monoclinic grains in the microstructure cannot be excluded, as recently shown by transmission Kikuchi diffraction [19]. In addition, a tetragonal phase is present for temperatures above the Curie temperature or in a super-cooled state for very high doping concentrations [20,21].…”

Stabilization of the Ferroelectric Phase in Epitaxial Hf1–xZrxO2 Enabling Coexistence of Ferroelectric and Enhanced Piezoelectric Properties

“…Its origin is discussed in more detail below. The second is electric field-induced t–o transition, which is usually considered the cause of the high polarization that some polycrystalline ZrO 2 and Zr-rich Hf 1– x Zr x O 2 films can present. ,,, It has to be noted that field-induced t–o transition in polycrystalline films requires a large number of cycles, ranging from 160 to thousands. , Field-induced t–o transition has been also reported very recently in epitaxial Y-doped HZO, presenting the films’ wake-up effect too, up to about 100 cycles . In contrast, there is no wake-up effect in our epitaxial ZrO 2 films (Supporting Information, Figure S6).…”