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Growth and structure of yttrium sesquioxide epitaxial films Webster, Scott Elliott


The use of molecular beam epitaxy as a method for producing solid state host crystals for planar waveguide lasers has been investigated. Single crystal yttrium sesquioxide with a very high degree of structural order has been grown on R-plane sapphire substrates. The (01Ῑ2) Al₂O₃ substrates were annealed in air at 1150°C to generate atomically smooth surfaces with parallel atomic steps. This process was important for maximizing structural quality and minimizing surface roughness of the grown Y₂O₃ film. A critical-thickness-like phenomenon was discovered, where the Y₂O₃ would grow in regions with near structural perfection at the beginning of growth. In thicker films, the x-ray diffraction peaks became wider, indicating less crystalline uniformity. The maximum equivalent “critical thickness” achieved was 7 nm for a film grown at 800°C with a growth rate of 20 nm/hr. The highly ordered material may be present in one uniform layer or distributed in smaller regions throughout the thin film. Y₂O₃ films on Al₂O₃ were annealed in air at temperatures up to 1400°C to study interdiffusion. By analyzing x-ray diffraction measurements, we found that Al migrated from the substrate into the Y₂O₃ film with an approximate activation energy for bulk diffusion of 3.0 eV. Diffusion on the Y₂O₃ surface was estimated to have an activation energy of (0.5 ± 0.3) eV from atomic force microscopy images. After annealing, the presence of Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ phases was confirmed using x-ray diffraction and photoluminescence measurements. Attempts were made to use molecular hydrogen gas and gallium as surfactants during growth to improve film properties. No conclusive benefit was observed. Y₂O₃ film surface roughness was observed to increase roughly proportionally to the square root of film thickness. A 600 nm thick waveguide layer grown under optimal conditions had a root-mean-square roughness of 5.8 nm. This level of roughness could cause scattering loss at the waveguide core-cladding interface that is problematic for practical applications.

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