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We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1 × 10 16 cm-3 to 2 × 10 18 cm-3 in samples with a high C content, strongly correlated with the Si doping level in the samples ranging from 1 × 10 17 cm-3 to 7 × 10 18 cm-3. On the other hand, we find predominantly neutral cation vacancies with concentrations above 5 × 10 18 cm-3 in samples with a low C content. The cation vacancies are important as compensating centers only in material with a high C content at high Si doping levels.