The
crystal structure and ferroelectric properties of 12- to 18
nm-thick epitaxial YO1.5-HfO2 films with 5–9%
YO1.5 on (111)ITO//(111)YSZ substrates are investigated
to clarify the formation mechanism of the ferroelectric phase. The
ferroelectric orthorhombic phase can be obtained by transformation
from the higher symmetric tetragonal phase by surmounting a relatively
low energy barrier. The orthorhombic phase is obtained for 6% and
7% YO1.5-doped HfO2 films by heat treatment
at 1000 °C. Although the 5% YO1.5-doped HfO2 film heat-treated at 1000 °C is in a monoclinic phase, the
orthorhombic phase was increased by heat treatment at 1200 °C
because the high temperature promotes the phase transition from the
monoclinic phase in as-deposited films to the tetragonal phase. The
8% and 9% YO1.5-doped HfO2 films have a tetragonal
structure without the transition to the orthorhombic phase. Nevertheless,
the 8% YO1.5-doped HfO2 film exhibits ferroelectricity
by polarization-electric field hysteresis measurement. A microarea
X-ray diffraction study reveals that the electric-field-induced phase
transition can take place in an 8% YO1.5-doped HfO2 film. The comprehensive study of high-temperature X-ray diffraction
measurements implies that the tetragonal phase in 8% YO1.5-doped HfO2 is a supercooled state. Therefore, external
stimulation, such as application of an electric field, induces the
transition from the tetragonal to the orthorhombic phase. The supercooled
tetragonal phase can also be reduced by a slower cooling rate. These
results reveal that the formed phase in YO1.5-doped HfO2 epitaxial film is not governed by the simple difference in
the formation energy; rather, the kinetics is more important for obtaining
the ferroelectric orthorhombic phase.
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.
The effect of composition on the ferroelectric properties was investigated using epitaxial x%YO 1.5 −(100−x%)Hf 1−y Zr y O 2 films. The tetragonal phase, with a composition near the phase boundaries with the orthorhombic or monoclinic phase, exhibits ferroelectricity through the field-induced phase transition from the tetragonal to the orthorhombic phase. Remarkably, the orthorhombic phase generated via the fieldinduced phase transition has a lower coercive field (E c ). The E c could be controlled from 1400 (5%YO 1.5 −95%(Hf 0.50 Zr 0.50 )O 2 ) to 2400 kV/cm (5%YO 1.5 − 95%(Hf 0.75 Zr 0.25 )O 2 ) with the same remnant polarization (P r ) value of ∼17 μC/cm 2 . These results are important for ferroelectric device applications with low-voltage drives.
Ferroelectricity facilitated by field‐induced phase transition is demonstrated by Takao Shimizu, Hiroshi Funakubo, and co‐workers in article number http://doi.wiley.com/10.1002/pssr.202000589. Their X‐ray diffraction and scanning transparent electron microscopy studies show the as‐deposited paraelectric tetragonal phase transforms to the ferroelectric orthorhombic phase in the Y‐doped Hf0.5Zr0.5O2 film by applying a strong electric field.
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