Abstract- Monochromated aberration-corrected STEM-VEELS (Valence Electron Energy-Loss Spectroscopy) measurements are performed on monoclinic hafnia. The maximum energy-loss anisotropy is observed for the  orientation, in agreement with ab-initio TDDFT simulations. From the fine structure of the energy loss spectra, the hypothesis of a possible Fano effect on the Hf 5p edge may be investigated.
Nanoscopy inside a transmission electron microscope opens a large field of applications, in particular in the semiconductor industry. VEELS has been developed to map the opto-electronic properties at a subnanometer scale inside a HR(S)TEM. A dielectric anisotropy in m-HfO2 has recently been observed1 by energy-filtered TEM. The agreement with ab initio simulations is nearly quantitative2 but the anisotropy along  and the finest structures of the spectra have not been published yet. The motivation of this presentation is to compare STEM-VEELS spectra recently recorded at high energy resolution with their ab initio simulations.
STEM measurements are performed with the FEI Titan Ultimate aberration-corrected STEM in monochromated mode with an energy resolution of 0.17 eV. The convergence and collection angles are 15.6 and 20 mrad respectively. The 25 nm-thick samples are prepared with a dedicated3 FIB, following a procedure optimized for VEELS measurements. The time-dependent density-functional theory (TDDFT) simulations are generated with the DP4 code using the random phase approximation including local field effects.
- Results and discussion
The structure of monoclinic hafnia is usually complex because of the presence of small overlapping nanocrystals separated by intermixed defective interfaces. Therefore it is rather difficult to find an isolated HfO2 grain with the well-defined  orientation, as checked by HRTEM focal series and associated JEMS simulations (fig. 1). The agreement between experimental and simulated diffraction patterns is also nearly perfect. The highest anisotropy in EELS spectra, is observed for the  orientation around the plasmon peak position (~ 16 eV, see fig. 2), when compared to random orientation spectra and with the reference spectrum of Couillard et al.5[A1] . The agreement with ab-initio simulations is also remarkable (fig. 3), especially in terms of predicted peak positions, although measurements realized on a perfect single crystal of m-HfO2 would improve the comparison. With the high energy resolution of our instrument, the Fano effect suggested to be responsible for the spectral dip associated with the excitation of the Hf 5p electrons in m-HfO2 can be more finely investigated. The associated asymmetric line shape can be identified around 42.3 eV, with a resonance width of 0.8 eV and a coupling factor of 0.6. The optical absorption spectra deduced from a Kramers-Krönig analysis also confirm the occurrence of such an effect in m-HfO2.
Monochromated STEM experiments demonstrate a clear anisotropy between  and other crystal orientations in m-HfO2. This anisotropy is mostly visible around the plasmon energy (16 eV). The statistical analysis of thousands of EELS spectra confirms the existence of a Fano effect around 42.3 eV to be associated with the excitation of the Hf 5p electrons.
Acknowledgments: This work has been carried out in the nanocharacterisation platform (PFNC) of MINATEC.
 C. Guedj et al., Appl. Phys. Lett. 105, 222904 (2014)
 L. Hung et al., submitted to Phys. Rev. B (2016)
 C. Guedj et al., submitted to IPFA conference (2016)
 M. Couillard et al., Phys. Rev. B 76, 165131 (2007)
To cite this abstract:Cyril Guedj, Nicolas Bernier, Christian Colliex, Valerio Olevano; Measurement of energy-loss anisotropy along  in monoclinic hafnia and comparison with ab-initio simulations. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/measurement-of-energy-loss-anisotropy-along-001-in-monoclinic-hafnia-and-comparison-with-ab-initio-simulations/. Accessed: October 21, 2021
EMC Abstracts - https://emc-proceedings.com/abstract/measurement-of-energy-loss-anisotropy-along-001-in-monoclinic-hafnia-and-comparison-with-ab-initio-simulations/