Interplay of Ga vacancies, C impurities, and yellow luminescence in GaN

We have applied positron annihilation spectroscopy to study the role of gallium vacancies in the yellow luminescence of gallium nitride. We measured the Ga vacancy concentrations of a set of wurtzite GaN layers grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire and containing different amounts of C and exhibiting different intensities in the yellow range of the photoluminescence spectrum. Interestingly, the relationship between the Ga vacancies, C impurities and yellow luminescence depends on the electrical conductivity of the material. In semi-insulating samples, we observe a correlation between the yellow luminescence and the carbon concentration, while there is anti-correlation between these two and the gallium vacancy concentration. In contrast, in the n-type samples both the yellow luminescence and the Ga vacancy concentration are independent of the carbon content. These results support the view that the gallium vacancy is not the only defect related to yellow luminescence observed in GaN, but that another, carbon-related mechanism is involved as well.

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