band gap, optical constant, thickness measurement, thorium oxide, spectroscopic ellipsometry, thoria, refractive index, DC-bias, sputtering, x-ray diffraction


We use spectroscopic ellipsometry to determine the optical constants of seven thin-film ThO2 samples deposited by radio-frequency sputtering, thickness ranging between 24 and 578 nm, for the spectral range of 1.2 to 6.5. We used a hollow-cathode light source and vacuum monochromator to measure constants at 10.2 eV. None of the deposition parameters studied including DC-bias voltages successfully increased the n of (that is, densify) thoria films. The value of n at 3.0 eV is 1.86 ± 0.04. We find compelling evidence to conclude that the direct bad gap is at ~5.9 eV, clarifying the results of others, some of whom observed the absorption edge below 4 eV. the edge in the two thickest films is of a narrow feature (FWHM = 0.4 eV) with modest absorption (α~ 6µm-1, K~0.1). Absorption may go down briefly with increasing energy from 6.2 to 6.5 eV). But at 10.2 eV absorption is very high and index low as measured by variable angle reflectometry, α= 47.3 ± 5.5 µm-1 and K= 0.48 ± 0.05, and n= 0.87 ± 0.12.

Original Publication Citation

William R. Evans, Sarah C. Barton, Michael Clemens and David D. Allred, "Understanding DC-Bias Sputtered Thorium Oxide Thin Films Useful in EUV Optics [6317-37]," in: Advances In X-Ray/EUV Optics, Components, And Applications, edited by Ali M. Khounsary, and Christian Morawe, Proceedings of SPIE, 6317, 631711-1 to 8 (26). [][].

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Peer-Reviewed Article

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Society of Photo Optical Instrumentation Engineers (SPIE)




Physical and Mathematical Sciences


Physics and Astronomy