  In this work, we use photoluminescence spectroscopy (PL) to monitor changes
in the UV, UV', blue, and green emission bands from n-type (010) Ga2O3 films
grown by metalorganic vapor phase epitaxy (MOVPE) induced by annealing at
different temperatures under O2 ambient. Annealing at successively higher
temperatures decreases the overall PL yield and UV intensity at nearly the same
rates, indicating the increase in formation of at least one non-radiative
defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV
increase, suggesting that defects associated with these emissions increase in
concentration with O2 annealing. Utilizing the different absorption
coefficients of 240 and 266 nm polarization-dependent excitation, we find an
overall activation energy for the generation of non-radiative defects of 0.69
eV in the bulk but 1.55 eV near the surface. We also deduce activation energies
for the green emission-related defects of 0.60 eV near the surface and
0.89-0.92 eV through the films, whereas the blue-related defects have
activation energy in the range 0.43-0.62 eV for all depths. Lastly, we observe
hillock surface morphologies and Cr diffusion from the substrate into the film
for temperatures above 1050 oC. These observations are consistent with the
formation and diffusion of VGa and its complexes as a dominant process during
O2 annealing, but further work will be necessary to determine which defects and
complexes provide radiative and non-radiative recombination channels and the
detailed kinetic processes occurring at surfaces and in bulk amongst defect
populations.
