Two charge states of the CN acceptor in GaN: Evidence from photoluminescence

We have found a photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the BLC band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free-electron concentration of about 10(18) cm(-3)). The electron- and hole-capture coefficients for this defect-related band are estimated as 10(-9) and 10(-10) cm(3)/s, respectively. The BLC band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and BL c bands likely originate from the C-N defect, namely, from electron transitions via the -/0 and 0/+ thermodynamic transition levels of the C-N. The BLC band appears only at high excitation intensities in n-type GaN samples codoped with Si and C, and it can be found in a wide range of excitation intensities in semi-insulating (presumably p-type) GaN samples doped with C. The properties and behavior of the YL1 and BLC bands can be explained using phenomenological models and first-principles calculations.