Two different behavior patterns of photoluminescence depending on Mg doping rate in p-type GaN epilayers

Abstract

Mg-doped p-type GaN epilayers grown by metalorganic chemical vapor deposition exhibit two different types of photoluminescence (PL) characteristics depending on Mg doping rate; the existence of critical Mg concentration is found where both electrical and optical characteristics show an abrupt change in their behavior. In samples with relatively low Mg concentration, a band edge emission, a peak associated with shallow donor-acceptor pair recombination, and a 2.8 eV blue band emission appear in the PL spectrum. Intensities of all three PL peaks decrease with increasing annealing temperature; the blue band quenches most rapidly. In contrast, in samples grown with high ratio of Mg/Ga flow rate, only a strong blue emission band is observed and the intensity of the blue band increases with the annealing temperature suggesting a different origin from the blue band of lightly doped samples. From the annealing temperature dependence of the blue band intensity, we speculate that the emission mechanism of the blue band in relatively lightly Mg-doped GaN epilayers involves Mg-related deep level complexes such as substitutional/interstitial Mg-related complexes. On the other hand, a model involving a formation of pyramidal defects and the presence of Mg-rich inclusions which modifies the dominant optical transition can be a possible explanation for the origin of the blue band in heavily doped ones.