Evidence for anisotropic dielectric properties of monoclinic hafnia using high-resolution TEM valence electron energy-loss spectroscopy and ab initio time-dependent density-functional theory

TL;DR

This study demonstrates that monoclinic hafnia exhibits anisotropic dielectric properties, with experimental measurements and ab initio calculations showing direction-dependent plasmon behavior, which is crucial for nanoelectronic applications.

Contribution

The paper combines high-resolution TEM VEELS measurements with TDDFT calculations to reveal and quantify the anisotropic dielectric properties of monoclinic hafnia.

Findings

Evidence of dielectric anisotropy in m-HfO₂

Direction-dependent bulk plasmon oscillator strength

Good agreement between experimental and theoretical dielectric properties

Abstract

The effect of nanocrystal orientation on the energy loss spectra of monoclinic hafnia (m-HfO$_2$) is measured by high resolution transmission electron microscopy (HRTEM) and valence energy loss spectroscopy (VEELS) on high quality samples. For the same momentum-transfer directions, the dielectric properties are also calculated ab initio by time-dependent density-functional theory (TDDFT). Experiments and simulations evidence anisotropy in the dielectric properties of m-HfO$_2$, most notably with the direction-dependent oscillator strength of the main bulk plasmon. The anisotropic nature of m-HfO$_2$ may contribute to the differences among VEELS spectra reported in literature. The good agreement between the complex dielectric permittivity extracted from VEELS with nanometer spatial resolution, TDDFT modeling, and past literature demonstrates that the present HRTEM-VEELS device-oriented methodology is a possible solution to the difficult nanocharacterization challenges given in the International Technology Roadmap for Semiconductors.