Optical and chemical properties of mixed-valent rhenium oxide films synthesized by reactive DC magnetron sputtering

• We report on the spectral optical constants for mixed-valent ReOx from 380 to 1700 nm.
• Optimal oxygen flow ratios are selected via in situ spectroscopic ellipsometry.
• Films consist of ReO3 with additional contributions from ReO2, Re2O3, and Re2O7.
• Films exhibit environmental sensitivity correlated to formation of Re2O3 and Re2O7.

Mixed-valent rhenium oxide thin films were deposited using reactive magnetron sputtering employing a metallic rhenium target within an oxygen–argon environment. The oxygen and argon flow rates were systematically varied, while the extinction coefficient, k, of the deposited layers was monitored using in situ spectroscopic ellipsometry. In situ monitoring was used to identify absorption features specific to ReO3, namely, the minimization of k brought on by the gap between interband absorption features in the UV at 310 nm and the onset of free electron absorption at wavelengths above 540 nm. Based on these results, oxygen flow ratios of 50% and 60% were shown to produce films having optical properties characteristic of ReO3, and thus, were selected for detailed ex situ characterization. Chemical analysis via X-ray photoelectron spectroscopy confirmed that all films consisted largely of ReO3, but had some contributions from Re2O3, ReO2 and Re2O7. Additional monitoring of the chemistry, as a function of environmental exposure time, indicated a correlation between structural instability and the presence of Re2O3 and Re2O7 in the films.

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