  Oxygen is the most common unintentional impurity found in GaN. We study the
interaction between substitutional oxygen (ON) and the gallium vacancy (VGa) to
form a point defect complex in GaN. The formation energy of the gallium vacancy
is largely reduced in n-type GaN by complexing with oxygen, while thermodynamic
and optical transition levels remain within the band gap. We study the
spectroscopy of this complex using a hybrid quantum-mechanical
molecular-mechanical (QM/MM) embedded-crystal approach. We reveal how a single
defect center can be responsible for multiband luminescence, including the
ubiquitous yellow luminescence signature observed in n-type GaN, owing to the
coexistence of diffuse (extended) and compact (localized) holes.
