(551b) Atomic Layer Deposition of Rare Earth Ion Co-Doped Oxides for Optical Applications

Rare earths (REs) exhibit unique shielded f-electrons giving rise to sharp spectral transitions. These transitions are dictated by the RE identity and can be manipulated by engineering the interactions of multiple RE ions. In this work, radical enhanced atomic layer deposition (RE-ALD) is used to design complex metal oxides with multiple dopants, whose concentration variation and spatial distribution control enable the synthesis of a wide range of multifunctional materials with tunable properties including magnetic, spectral, and electronic. Specifically, the control of sensitizer proximity and concentration is used to enhance amplification at 1.54 µm (Er ⁴I_13/2 → ⁴I_15/2) for compact planar optical amplifier applications and promote Er upconversion at 535 nm (²H_11/2, ⁴S_3/2 → ⁴I_15/2 ) and at 670 nm (⁴F_9/2 → ⁴I_15/2 ). The spatial distributions between Er³⁺ and RE (RE =Yb³⁺, Eu³⁺, Ce³⁺) are investigated with 1.54 µm emission promotion via direct energy transfer for Yb sensitizers and via cross relaxation for Eu and Ce sensitizers. Polycrystalline thin films are synthesized by sequential radical-enhanced ALD of Y₂O₃, Er₂O₃, Yb₂O₃, and Eu₂O₃ at 350°C using 2,2,6,6-tetramethyl-3,5-heptanedionato analog of the corresponding metal (e.g. RE(TMHD)_x) and reactive oxygen atoms from a plasma. The composition, microstructure, cation distribution, local chemical bonding and optical properties of the as-synthesized thin films are determined by x-ray and Rutherford backscattering spectroscopies, electron microscopy and photoluminescence measurements. The effect of concentration is examined using a 8:1:x Y:Er:RE cycle sequence with x = 0, 1, 3, 5, 7, while the spatial distribution is investigated using a y:5:y:5 Y:Er:Y:RE ratio with y = 0, 2, 4, 6, 8, 10. High resolution transmission electron microscopy on thin films deposited on nanotubes verify the construction of nanolaminates by changing the deposition sequence. Extended x-ray absorption fine structure Yb L2 edge scans show that the Yb local environment possesses more 2nd nearest neighbor Yb ions as the Yb cycles increases. Photoluminescence (PL) spectra using both 488 nm and 980 nm laser excitations show sharp characteristic Er intra 4f peaks with peak intensity centered at 1535 nm at low pump powers (~50 mW for 980 nm excitation). Approximately 8x luminescence enhancement is achieved using a 8:1:3 Y:Er:Yb cycle ratio, while a slight decrease in PL intensity is apparent as the spacing between Er and Yb increases. Individual PL studies of Eu and Ce codoped Er:Y₂O₃ samples were found to require higher laser excitation powers due to the lower absorption cross sections compared to Yb doped samples, while tri-doped samples (Yb:Eu and Yb:Ce codoped Er:Y₂O₃) are currently under investigation to study further attainable PL enhancement and upconversion efficiency.