Refractive-index variation with rare-earth incorporation in amorphous Al2O3 thin films

Rare-earth-doped amorphous aluminum oxide (Al ₂O ₃:RE ^{3 +} ) thin films are attractive materials for near-IR amplifiers and lasers that can be integrated with silicon-on-insulator waveguides or deposited onto CMOS-fabricated integrated optical structures. We characterize reactively co-sputtered Al ₂O ₃:Tm ^{3 +} films on silicon by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The refractive index is systematically studied for different Tm ^{3 +} concentrations. The resulting increase of refractive index is explained by analyzing the mechanism of incorporating Tm ^{3 +} ions into the amorphous Al ₂O ₃ network. Sellmeier dispersion formulas are presented.