Erbium-ytterbium co-doped aluminum oxide thin films: Co-sputtering deposition, photoluminescence, luminescent lifetime, energy transfer and quenching fraction

Erbium-ytterbium co-doped aluminium oxide (Al2O3:Er3+:Yb3+) thin films were deposited on thermally oxidized silicon wafers using reactive radio frequency co-sputtering deposition. The effects of deposition parameters (including oxygen flow rate, substrate temperature and substrate bias) were investigated to obtain low loss Al2O3 films (~0.15 dB/cm at 1550 nm). A set of singly- and co-doped Al2O3 films (surface roughness: ~144.8 p.m.) with various Er3+ concentrations (1.4, 2.0 and 2.5 × 1020 cm−3) and/or Yb3+ concentrations (1.3, 2.1, 3.2, 4.3 and 5.8 × 1020 cm−3) were deposited and investigated. The Er3+ photoluminescence (PL) spectrum around 1550 nm exhibits a pump power-independent full width at half maximum (FWHM) of ~40 nm with and without the presence of Yb3+ as a sensitizer. The luminescent lifetime of Er3+ (4I13/2, 6.52–7.14 m s) exhibits a power-dependent characteristic, while that of Yb3+ (2I5/2, 0.514–0.716 m s) is power-independent. A power-dependent role of energy transfer upconversion is concluded from both the power-dependent Er3+ PL intensity trend and its power-dependent PL decay rate. Yb3+-Er3+ ion energy transfer also exhibits power-dependent characteristics. The energy transfer coefficient (up to ~3.2 × 10−17 cm3s−1) and energy transfer efficiency (up to ~80%) depend on the Yb3+ and Er3+ concentrations. The PL intensity trend and power-dependent absorption of Er3+ and Er3+-Yb3+ under 1469-nm excitation can be well described by using a three-energy-level model. The derived Er3+ quenching fraction based on the fittings of the measured absorption and PL intensity, decreases with the presence of Yb3+ ions. In addition, the Yb3+ quenching fraction was derived based on a two-energy-level model for 973-nm absorption. This work provides parametric references and insights regarding deposition, characterization, and design of the evolving rare-earth-doped Al2O3 platform for on-chip near-infrared applications.