Optical properties of a-HfO2 thin films

Amorphous hafnium oxide (a-HfO2) thin films were grown on silicon and quartz substrates by RF reactive magnetron sputtering at temperature < 52 °C. X-ray diffraction revealed that the thin films grown on the substrates are amorphous. The optical constants of a-HfO2 films were obtained by analysis of the measured ellipsometric spectra in the wavelength range 200–1400 nm, using the Cauchy–Urbach and Sellmeier models. Refractive indices and extinction coefficients of the films were determined to be in the range 1.86–2.15 and 0.07–2.6 × 10− 5, respectively. The absorption coefficients, α, of a-HfO2 has been determined by spectroscopic ellipsometry and spectrophotometric methods over the energy range 0.88–6.2 eV. Analysis of α shows the bandgap energy of the films to be 5.68 ± 0.09 eV. Measurement of the polarized optical properties reveals a high transmissivity (80%–97%) and low reflectivity (< 15%) in the visible and near infrared regions at angles of incidence between 10° to 80°.