Intensity ratios of characteristic deep cathodoluminescence (CL) bands at 4.6, 3.8, and 3.1 eV to the near-band-edge emissions at 11 K of AlN epilayers grown by N H3 -source molecular beam epitaxy were correlated with the change in the S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 and 3.8 eV bands increased remarkably with lowering supply ratio of N H3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl -O complexes. The VAl concentration could be decreased by adjusting V/III ratio and Tg, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of A exciton was determined to be 48 meV.
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