Cathodoluminescence and transmission electron microscopy study of the influence of crystal defects on optical transitions in GaN

Defect related states and excitonic transitions in epitaxial GaN have been studied by combining cathodoluminescence and transmission electron microsco py. A series of deep revels with energies at about 2.4, 2.6 and 2.8 eV has been found by low temperature cathodoluminescence on freestanding 150 mu m thick epitaxial GaN. These deep levels are characterised by a high recombin ation efficiency. They are radiative from 5 to 70 K and undergo a nonradiat ive transition at 70 K. These levels completely quench the near band edge a nd the conventional yellow emissions. We discuss the structural origin of t hese defects in terms of formation of V-Ga-Si-Ga and V-Ga-O-N complexes. Th e consequences of our model with respect to non radiative transitions at th reading dislocations are also presented. An excitonic transition at 3.41 eV close to the near band edge line on differently grown epitaxial GaN has be en correlated to stacking faults. This line can be explained by a model bas ed on the concept of excitons bound to SFs that form a quantum well of cubi c material in the wurtzite lattice of the layer.