Hybrid co-deposition of molybdenum doped niobium pentoxide (NbxMoyOz) thin films

• We report on the spectral optical constants for mixed-valent NbxMoyOz thin films.
• Spectral absorption is controlled through variation of Mo5+ content from 0 to 3 at.%.
• Film optical properties are systematically tailored as a function of applied power.
• NbxMoyOz thin films exhibit band gaps able to be tailored between 3.05 and 3.48 eV.

Mixed valent thin films, consisting of optically absorbing metallic cations within a dielectric matrix, possess a variety of optical and electronic properties that can be tailored through variation of dopant concentration and chemistry. This work details the reactive magnetron co-deposition of mixed valent NbxMoyOz thin films, systematically doped with optically absorbing Mo5+ cations, through adjustment of the molybdenum source power. Control over the concentration the Mo5+ valence state was achieved through decreases in the oxygen partial pressure facilitated via oxygen chemisorption by the molybdenum sputter flux at power levels of 0, 20, 40, 60, 80, and 100 W, while the Nb source was operated concurrently at a constant sputter power of 200 W. The resulting films, roughly 100 nm in thickness, were characterized using variable angle spectroscopic ellipsometry as well as x-ray photoelectron spectroscopy in order to derive a process-property relationship among molybdenum source power and its effects on the refractive index, extinction coefficient, optical band gap, as influenced by the relative concentrations of Mo5+, Mo6+, and Nb5+ cations. Increases in the molybdenum source power from 0 to 100 W allow for tailorability of the refractive index, 2.29 ≤ n550 ≤ 2.34, extinction coefficient, 0.00 ≤ k550 ≤ 0.02, and optical band gap, 3.05 ≤ Eg ≤ 3.48 eV. The co-deposition procedure used within this work affords the ability to systematically adjust the content of the absorbing Mo5+ cations, from 0 to 3 at. %, within a dielectric matrix comprised of MoO3 and Nb2O5, presenting several opportunities in increasing the design-space for both optical coatings and electrochromic devices.

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