posted on 2023-11-28, 18:07authored bySamantha T. Jaszewski, Sebastian Calderon, Bishal Shrestha, Shelby S. Fields, Atanu Samanta, Fernando J. Vega, Jacob D. Minyard, Joseph A. Casamento, Jon-Paul Maria, Nikolas J. Podraza, Elizabeth C. Dickey, Andrew M. Rappe, Thomas E. Beechem, Jon F. Ihlefeld
Phase
identification in HfO2-based thin films is a prerequisite
to understanding the mechanisms stabilizing the ferroelectric phase
in these materials, which hold great promise in next-generation nonvolatile
memory and computing technology. While grazing-incidence X-ray diffraction
is commonly employed for this purpose, it has difficulty unambiguously
differentiating between the ferroelectric phase and other metastable
phases that may exist due to similarities in the d-spacings, their low intensities, and the overlapping of reflections.
Infrared signatures provide an alternative route. However, their use
in phase identification remains limited because phase control has
overwhelmingly been accomplished via substituents, thereby convoluting
infrared signatures between the substituents and the phase changes
that they induce. Herein, we report the infrared optical responses
of three undoped hafnium oxide films where annealing conditions have
been used to create films consisting primarily of the ferroelectric
polar orthorhombic Pca21, antipolar orthorhombic Pbca, and monoclinic P21/c phases, as was confirmed via transmission electron microscopy
(TEM), UV–visible optical properties, and electrical property
measurements. Vibrational signatures acquired from synchrotron nano-Fourier
transform infrared spectroscopy (nano-FTIR) are shown to be capable
of differentiating between the phases in a nondestructive, rapid,
and nanoscale manner. The utility of nano-FTIR is illustrated for
a film exhibiting an antiferroelectric polarization response. In this
sample, it is proven that this behavior results from the Pbca phase rather than the often-cited tetragonal phase. By demonstrating
that IR spectroscopy can unambiguously distinguish phases in this
material, this work establishes a tool needed to isolate the factors
dictating the ferroelectric phase stability in HfO2-based
materials.