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 microscopy. 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 recombination efficiency. They are radiative from 5 to 70 K and undergo a nonradiative transition at 70 K. These levels completely quench the near band edge and the conventional yellow emissions. We discuss the structural origin of these defects in terms of formation of V-Ga-Si-Ga and V-Ga-O-N complexes. The consequences of our model with respect to non radiative transitions at threading dislocations are also presented. An excitonic transition at 3.41 eV close to the near band edge line on differently grown epitaxial GaN has been correlated to stacking faults. This line can be explained by a model based on the concept of excitons bound to SFs that form a quantum well of cubic material in the wurtzite lattice of the layer.