Determining indices of refraction for ThO2 thin films sputtered under different bias voltages from 1.2 to 6.5 eV by spectroscopic ellipsometry

We used spectroscopic ellipsometry to determine the optical constants of seven ThO2 thin-film samples, thickness ranging between 28 and 578 nm, for the spectral range of 1.2 to 6.5 eV. The samples were deposited by biased radio-frequency sputtering at DC bias voltages between 0 and − 68 V. The index of refraction, n, does not depend on bias voltage, sputter pressure, deposition rate, or thickness. Specifically, the value of n at 3 eV is 1.86 ± 0.04 for the unbiased samples and 1.86 ± 0.04 for the biased samples. The average value of n at 3 eV for the thicker samples (d ≥ 50 nm) was 1.87 ± 0.05, and 1.85 ± 0.02 for the thinner samples (d ≤ 50 nm).