Study of the blue luminescence in unintentional doped GaN films grown by MOCVD

We studied the blue luminescence at about 2.9 eV in unintentional doped GaN films, which were grown on (0 0 0 1) oriented sapphire substrates by MOCVD. The intensity ratio of the blue luminescence to the band-edge emission is large in high compensation ratioʹs GaN films, and it decreases with the decrease of the compensation ratio. The FWHM of double-crystal X-ray diffraction obviously increases with the increase of the intensity ratio of the blue luminescence to the band-edge emission. Our results indicate that blue luminescence is the transition from the free electron to acceptor levels that are due to some intrinsic defects. We also observed that the peak position of the blue luminescence shifted to higher energy with decreasing excitation density in a high background electron carrier concentration GaN sample. We consider that this phenomenon is due to the screening effects of the free carriers.