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

Rare-earth-doped amorphous aluminum oxide (Al2O3:RE3+) 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 Al2O3:Tm3+ 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 Tm3+ concentrations. The resulting increase of refractive index is explained by analyzing the mechanism of incorporating Tm3+ ions into the amorphous Al2O3 network. Sellmeier dispersion formulas are presented.