Crystal structure and band gap determination of HfO2 thin films

Valence electron energy loss spectroscopy (VEELS) and high resolution transmission electron microscopy (HRTEM) are performed on three different HfO2 thin films grown on Si (001) by chemical vapor deposition (CVD) or atomic layer deposition (ALD). For each sample the band gap (Eg) is determined by low-loss EELS analysis. The Eg 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 HfO2 ALD film capped with poly-Si exhibits the largest band gap (Eg = 5.9±0.5?eV), as a consequence of its nanocrystallized orthorhombic structure. The large grains with a monoclinic structure formed in the HfO2 ALD film capped with Ge and the carbon contamination induced by the precursors in the HfO2 CVD film capped with Al2O3 are identified to be the main features responsible for lower band gap values (Eg = 5.25±0.5 and 4.3±0.5?eV respectively)