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 10¹⁸0.16em0excm^-3). The electron- and hole-capture coefficients for this defect-related band are estimated as 10^-9 and 10^-100.16em0excm³/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 BLC bands likely originate from the CN defect, namely, from electron transitions via the -/0 and $0/+$ thermodynamic transition levels of the CN. 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.