Carbon as a source for yellow luminescence in GaN: Isolated CN defect or its complexes

We study three carbon defects in GaN, isolated C-N and its two complexes with donors C-N-O-N, and C-N-Si-Ga, as a cause of the yellow luminescence using accurate hybrid density functional calculation, which includes the semi-core Ga 3d electrons as valence electrons and uses a larger 300-atom supercell. We show that the isolated C-N defect yields good agreement with experiment on the photoluminescence (PL) peak position, zero-phonon line, and thermodynamic defect transition level. We find that the defect state of the complexes that is involved in the PL process is the same as that of the C-N defect. The role of the positively charged donors (O-N or Si-Ga) next to C-N is to blue-shift the PL peak. Therefore, the complexes cannot be responsible for the same PL peak as isolated C-N. Our detailed balance analysis further suggests that under thermal equilibrium at typical growth temperature, the concentration of isolated C-N defect is orders of magnitude higher than the defect complexes, which is a result of the small binding energy in these complexes. (C) 2015 AIP Publishing LLC.