Tailoring structural and optical properties of Ta2O5 thin films via radio frequency magnetron sputtering for high-refractive index transparent materials

This work investigates the impact of growth conditions, oxygen flow, sputtering power density, and annealing temperature on the optical performance of Ta 2 O 5 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 (E size ). 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 T A =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 (E g ) 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 Ta 2 O 5 thin films at the expense of a restricted transparency window in the NIR-visible range.