Event Date and Time:
Tue, 2012-02-28 12:30 - 15:30
The Penthouse, Graduate Student Centre
Physics and Astronomy, UBC
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
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.