Structural, optical and dielectric characterization of Au/HfO2/(Pt,TiN) capacitors

Highlights: • HfO₂ shows a mixture of monoclinic and orthorhombic phases. • HfO₂ displays 13 Raman-active modes. • Both direct and indirect band-gaps are expected. • Photoluminescence spectra consist of five emissions. • Electrode polarization process governs the low-frequency behavior. - Abstract: Structural, optical and dielectric properties of atomic layer deposited HfO₂ were studied in close relation with the bottom electrode nature. We used XRD, HRTEM and AFM to study the internal microstructure and the surface roughness of the films. The HfO₂/Pt sample shows a mixture of monoclinic and orthorhombic phases whereas the HfO₂/TiN sample presents few crystallites in orthorhombic structure dispersed in a badly crystallized or amorphous phase. Raman spectroscopy shows that between 100 and 800 cm⁻¹, the HfO₂/Pt device displays 13 Raman-active modes whereas the HfO₂/TiN device reveals 10 Raman-active modes. Using the UV–Vis reflectance spectra, we expect both direct and indirect electronic transitions with optical band-gap energies of 5.11 and 4.61 eV, respectively. Photoluminescence spectroscopy allows the identification of localized states in the HfO₂ band-gap. PL emission spectra consist of five sub-peaks, centered at around 1.64–1.92 eV, 2.10 eV and 2.33 eV. These energies are close to the trap levels calculated for oxygen vacancies, involved during the film deposition. As the temperature is increased up to 380 °C, an obvious dispersion of the real permittivity takes place in the low frequency domain. This behavior, highly enhanced by the oxygen vacancies mobility, is typical for the electrode polarization process.