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 D-I 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 D-I 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.