High concentrations of aluminum (Al) and nitrogen (N) dopants of 6H SiC have been achieved by a fast sublimation growth process. The Al–N co-doped 6H-SiC layer exhibits a strong light-blue photoluminescence emission at low temperatures due to emissions from DI centers and donor acceptor pairs (DAP). The photoluminescence quenching mechanisms of those emissions are different. The decrease of free carrier capture cross-section as temperature increases according to the cascade capture process causes quenching of the photoluminescence emission form DI centers. Emission from Al–N DAP centers exhibits an exponential quenching with activation energy of (95 ± 10) meV. This is attributed to a competing hole capture by non-radiative defect in a multiphonon emission process.