Tailoring structural and optical properties of Ta2O5 thin films via radio frequency magnetron sputtering for high-refractive index transparent materials
Résumé
This work investigates the impact of growth conditions, oxygen flow, sputtering power density, and annealing temperature on the optical performance of Ta2O5 thin films grown on Silicon (Si) substrate by RF sputtering. The objective is to determine whether the amorphous or crystalline phase provides the optimal trade-off between high refractive index (n), low optical loss (k), and broad optical transparency window (Esize). A near-stoichiometric Ta/O ratio of 0.38, close to the ideal 0.40 was achieved at a power density of 3.29 W/cm² and oxygen flow of 8 sccm. Crystallization begins at TA=650 °C, resulting in a predominant orthorhombic and a minor hexagonal phase as demonstrated by XRD and confirmed by TEM. This crystallization was accompanied by a densification of 7.85 g.cm-3 at 750 °C, but showed signs of structural degradation at 850 °C. Ellipsometry shows an increase in n during crystallization to 2.24 at 750 °C, before decreases at 850 °C. The onset of k redshifts from 4 to 3 eV, and the optical band gap (Eg) dropped from 4.14 eV to 2.5 eV following crystallization. Transmittance (T), calculated using the transfer matrix method (TMM), showed an average of 80% over 1-4 eV range for amorphous films and narrowed below 3 eV in crystalline films due to defect-induced mid-gap states. Our findings underscore a critical balance where crystallization enhances the refractive index of Ta2O5 thin films at the expense of a restricted transparency window in the NIR-visible range.
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