ABSTRACT: 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 (0 1 1 -2) 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 activation energy for bulk diffusion of (5.5 ± 0.6) 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.