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Valence electron energy loss spectroscopy (VEELS) and high resolution transmission electron microscopy (HRTEM) are performed on three different Hf O 2 thin films grown on Si (001) by chemical vapor deposition (CVD) or atomic layer deposition (ALD). For each sample the band gap (E g) is determined by low-loss EELS analysis. The E g values are then correlated with the crystal structure and the chemical properties of the films obtained by HRTEM images and VEELS line scans, respectively. They are discussed in comparison to both experimental and theoretical results published in literature. The Hf O 2 ALD film capped with poly-Si exhibits the largest band gap (E g= 5.9±0.5 eV)⁠, as a consequence of its nanocrystallized orthorhombic structure. The large grains with a monoclinic structure formed in the Hf O 2 ALD film capped with Ge and the carbon contamination induced by the precursors in the Hf O 2 CVD …