Oxygen annealing induced changes in defects within beta-Ga2O3 epitaxial films measured using photoluminescence

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.